BEHAVIOR AND ASSIMILATION OF
ORGANIC AND INORGANIC PRIORITY POLLUTANTS
CODISPOSED WITH MUNICIPAL REFUSE
Volume I
by
Frederick G. Pohland
Department of Civil Engineering
University of Pittsburgh
Pittsburgh, PA 15261
Wendall H. Cross and Joseph P. Gould
School of Civil Engineering
Georgia Institute of Technology
Atlanta, GA 30332
Debra R. Reinhart
Department of Civil Engineering
University of Central Florida
Orlando, FL 32816
EPA Cooperative Agreement CR-812158
Project Officers
Vincent Salotto, Jonathan G. Herrman,
Charles Moench, Jr., and Robert E. Landreth
Risk Reduction Engineering Laboratory
Cincinnati, OH 45268
RISK REDUCTION ENGINEERING LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OH 45268
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DISCLAIMER
The information in this document has been funded wholly or in part by the United States
Environmental Protection Agency under Cooperative Agreement CR-812158 to the Georgia Institute of
Technology. It has been subjected to the Agency's peer and administrative review, and it has been
approved for publication as an EPA document Mention of trade names or commercial products does not
constitute endorsement or recommendation for use.
11
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FOREWORD
Today's rapidly developing and changing technologies and industrial
products and practices frequency carry with them the increased generation of
materials that, if improperly dealt with, can threaten both public health and the
environment. The U.S. Environmental Protection Agency is charged by Congress
with protecting the Nation's land, air, and water resources. Under a mandate of
national environmental laws, the agency strives to formulate and implement
actions leading to a compatible balance between human activities and the ability
of natural systems to support and nurture life. These laws direct the EPA to
perform research to define our environmental problems, measure the impacts and
search for solutions.
The Risk Reduction Engineering Laboratory is responsible for planning
implementing, and managing research, development, and demonstration programs to
provide .an authoritative, defensible engineering basis in support of the
policies, programs, and regulations of the EPA with respect to drinking water
wastewater, pesticides, toxic substances, solid and hazardous wastes, and
Superfund-related activities.
This publication is one of the products of that research that provides the
basis for an innovative approach to accelerate the stabilization of municipal
solid waste landfills. The research clearly identifies how a bioreactor system
can be maintained and restarted if necessary, as well as how to improve the
predictability of the various physicochemical phases of the landfill. The data
provided will_guide the development of full scale operations with the ultimate
goal o± minimizing potential adverse health and environmental impacts.
E. Timothy Oppelt, Director
Risk Reduction Engineering Laboratory
iii
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PREFACE
f
The purpose of this research was to demonstrate and evaluate the capacity
of landfill systems to assimilate and attenuate inorganic and organic priority
pollutants loadings codisposed with municipal refuse, and to determine the fate
and effect of the codisposed pollutants! as landfill stabilization progressed
under conditions of single pass leaching and leachate recycle.
In accordance with these objectives, ten simulated landfill columns,
operated in pairs, with single pass leaching and leachate recycle were
constructed and loaded with shredded municipal refuse (one pair), shredded
municipal refuse and organic priority ; pollutants (one pair), and shredded
municipal refuse, organic priority pollutants and increasing quantities of heavy
metals (three pairs). The loaded columns were sealed, brought to field capacity
by the addition of tap water and then operated for about 1500 days. Both
leachate and gas samples were routinely analyzed for organic and inorganic
priority pollutants as well as for an array of conventional parameters indicative
of the sequential processes of landfill stabilization.
The results of this investigation demonstrated that the columns employing
leachate recycle achieved waste stabilization more rapidly and completely than
the columns operated with single pass leaching. This was evidenced by trends in
fas volumes produced, gas production rates, gas composition and changes in
eachate indicator parameters. Although'the test columns receiving loadings of
inorganic and/or organic priority pollutants exhibited reduced gas production,
gas production rates and methane content of the gas, these effects were more
severe for the single pass columns than for the similarly loaded recycle columns.
Consequently, the stabilization in the recycle columns resulted in greater gas
production, and concomitantly exhibited more complete assimilation of the
priority pollutant than the single pass columns.
Conservative leachate constituents,; such as chloride and sodium, reflected
the effects of the leachate management- techniques employed. Whereas these
constituents were retained within the leachate of the recycle columns, as
evidenced by the relatively constant concentrations, they were removed from the
single pass columns primarily by washout. All other leachate constituents were
affected by these operational strategies, with washout from the single pass
columns serving to reduce concentration profiles. Operations with leachate
recycle did not result in inhibition of: landfill stabilization, although the
applied priority pollutant loadings did cause some retardation. In contrast, the
test columns containing priority pollutant loadings and operated with single pass
leaching and resultant washout of leachatje constituents exhibited inhibition of
the stabilization and attenuation processes. Hence, the recycle columns
possessed greater assimilative capacity for the organic and inorganic priority
pollutants than that afforded by the single pass columns'. This capacity was
expressed in the case of the heavy metals by removal primarily through
precipitation as hydroxides, carbonates ot sulfides, and by reduction and matrix
capture through encapsulation, sorption, ion-exchange and filtration. Similarly,
the organic priority pollutants were attenuated primarily by abiotic and biotic
transformation as well as by sorption within the waste matrix.
iv
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Based on the extensive database developed during the course of these
investigations, it can be concluded that:
1. Controlled leachate containment, collection and recirculation offers
opportunities for more rapid and complete stabilization of landfilled
municipal solid waste, including attenuation of codisposed priority
pollutants, than does single pass leaching operations more common to
traditional landfill practices.
2. Loadings of codisposed priority pollutants in the form of heavy metals
and/or selected classes of toxic organic substances, can cause retardation
0£-the sequential phases of landfill stabilization. However, loading
effects will be more severe with single pass leaching than with leachate
recycle operations , as manifested by relative changes in leachate and gas
characteristics . 6
3. Leachate and gas characteristics, described by various physical and
chemical indicator parameters, can be used to reflect the progress of
waste stabilization in terms of longevity and intensity of the acid
formation and methane fermentation phases.
4. A threshold inhibition level, equivalent to highest inorganic priority
f^n^f?* 1(?^inS' -,was established for recycle operations, whereas
inhibition with single pass leaching was exhibited at the lowest loading
However, these effects would be site-specific and a function of loadilg
and operational techniques employed.
5'
Landfills possess a. finite capacity to attenuate hazardous and
nonhazardous organic and inorganic waste constituents through a wide array
°£< biolPSical and physicochemical mechanisms. These mechanisms
principally include reduction, precipitation, and matrix capture for the
heavy metals, and biotic or abiotic transformation with matrix interaction
through sorption for the organic pollutants.
6. Controlled landfill systems, designed and operated as bioreactors through
irA5S^K*iC?2,tainmSntU c°llecti°n ,and recycle, enhance performance
predictability and thereby minimize potential adverse health and
environmental impacts, while encouraging innovation and associated
regulatory and public acceptance.
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ABSTRACT
I
Organic and inorganic priority pollutants codisposed with municipal solid
waste (MSW) in ten pilot-scale simulated landfill columns, operated under single
pass leaching or leachate recycle, were capable of being attenuated by
microbially-mediated landfill stabilization processes. The results of the
investigation have indicated that columns operated with leachate recycle
stabilized more rapidly and completely than columns operated under single pass
leaching. The behavior and attenuation! of admixed priority pollutant loadings
were reflected by changes in leachate characteristics and in the total gas
production, the gas production rate, and the methane content of the gas produced
by the simulated landfill columns. Leachate constituents were retained in the
recycle columns, but were principally removed from the single pass columns due
to washout. An explicit inhibition threshold for stabilization consequenced by
the priority pollutant loadings was not observed for the recycle columns,
although retardation was evident for the test column most heavily loaded with
heavy metals. In contrast, stabilization in all single pass columns containing
organic and inorganic priority pollutan|t loadings was inhibited. The organic
priority pollutants appeared to have less impact on landfill stabilization than
the heavy metals. ;
i
Both organic and inorganic priority pollutants loadings were assimilated
within the landfill columns. This, assimilation was greater in the recycle
columns than in the single pass columns i Heavy metals (Cd, Cr, Hg, Ni, Pb and
Zn) were subjected to a complex array of attenuation mechanisms within the MSW
matrix, and were largely removed by precipitation as hydroxides, carbonates or
sulfides, and by reduction and matrix capture through sorption, ion-exchange and
encapsulation. The classes of codispose'd organic priority pollutants (aromatic
hydrocarbons, halogenatedhydrocarbons, pesticides, phenols and phthalate esters)
were primarily subjected to both abiotic iand biotic transformation and physical-
chemical sorption within the waste matrix. Reductive dehalogenation appeared to
be a primary transformation mechanism for halogenated compounds, and reduction,
ring cleavage and possible complete mineralization were indicated for the other
codisposed organic compounds.
Collectively, the results of these investigations have established the
efficacy of controlled landfill systems utilizing leachate containment and
recycle for accelerated in situ stabilization of both nonhazardous and hazardous
solid waste constituents. i
VI
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TABLE OF CONTENTS
DISCLAIMER ±±
FOREWORD
PREFACE .................. .............. iv
ABSTRACT ............ .......... ..... _ vi
FIGURES . . ............... ................ vlli
TABLES ................. ................ xiii
ACKNOWLEDGEMENT ........... ... __,..
*"*****••*••••••« X >T i
1'. INTRODUCTION
2. GENERAL HISTORICAL PERSPECTIVE AND CURRENT STATUS OF
LANDFILL PRACTICE ........... .......... _ 3
Municipal solid waste ..... ......... 5
Landfill stabilization . . . ..... '.'.'.'.'.'.'.'.'. 7
Leachate production and management . . . . '. '.'.'..'.'. 16
Codisposal of MSW and hazardous waste ........ '. ', 25
Fundamentals of anaerobic treatment ...... ..!!-! 27
Effects of hydrogen on anaerobic stabilization . '. '. ! .' 41
Effects of heavy metals on methane production and
hydrogen levels ............... .... 44
3. MATERIALS AND METHODS ..... 45
Simulated landfill design . ...'.'.'.'.'.'.'. ..... 45
Simulated landfill loading .......... '!!!'" 49
Simulated landfill closure ........ * ' ..... 50
Simulated landfill operation ....... . . . . . 50
Analytical procedures and methods ......'..'.'.'.[ 58
Sampling procedures ............. ..... g0
Simulated landfill disassembly ..... ........ 61
4. PRESENTATION AND GENERAL DISCUSSION OF RESULTS . ...... 62
External temperature ............ '.'.'.'.'.' 62
Gas analyses .............. !!!!!!'" 63
Water balance .......... \ '.'.'.•'.'.'.'.'.'.'' 76
Leachate analyses ............. !!!!*'" 79
Inhibition levels .... ........ !!!!!.'! *129
Lithium tracer studies ........ !!!!!!!! 138
Simulated landfill column disassembly ..... '.'.'.'. 150
5. EVALUATION OF ORGANIC AND INORGANIC
PRIORITY POLLUTANTS ....... . ........ ' . . . . 153
Behavior of organic priority pollutants .!!!!!!! 153
Behavior of inorganic priority pollutants ....... 163
6 . SUMMARY AND CONCLUSIONS ............... .... 188
7. REFERENCES ................ . ....... 191
vii
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FIGURES
Number ; Page
1 Changes in selected indicator parameters during the phases
of landfill stabilization; 12
2 Generalized pathways for methane fermentation of complex
wastes .........! 13
3 Generalized pathways of anaerobic treatment of complex
wastes I 32
4 Volatile acid utilization by volatile acid sludges 32
5 Volatile acid formation during excessive loading of
propionic and butyric acijis . 34
6 Volatile acid utilization by fatty acid sludges 35
7 Volatile acid utilization by carbohydrate sludges ~. 35
8 Volatile acid utilization by protein sludges 37
9 Volatile acid utilization by sewage sludge 37
t
10 Volatile acid formed during digester unbalance compared
with those utilized by sludges 38
11 Volatile acid intermediate in methane fermentation of
proteins, carbohydrates and fate 38
12 Glycolytic pathway for glucpse metabolism 42
13 Recycle simulated landfill column 47
°
14 Single pass simulated landfill column 47
15 Ambient temperature variations during simulated
landfill investigations .; 63
16 Cumulative gas production during simulated landfill
investigations \ 64
17 Incremental gas production during simulated landfill
investigations 65
18 Relative gas production between simulated landfill
columns with leachate recycle 67
19 Normalized gas composition for Column 1CR 71
viii
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FIGURES (continued)
Number Page
20 Normalized gas composition for Column 2CS 71
21 Normalized gas composition for Column SOS 72
22 Normalized gas composition for Column 40LS 72
23 Normalized gas composition for Column 50MS 73
24 Normalized gas composition for Column 60R 73
25 Normalized gas composition for Column 70LR 74
26 Normalized gas composition for Column 80HS 74
27 Normalized gas composition for Column 90MR 75
28 Normalized gas composition for Column 100HR 75
29 Leachate pH during simulated landfill investigations .... 80
30 Leachate total volatile acids during simulated landfill
investigations 82
31 Leachate individual volatile acids for Column 1CR 86
32 Leachate individual volatile acids for Column 2CS ....... 86
33 Leachate individual volatile acids for Column SOS 87
34 Leachate individual volatile acids for Column 40LS 87
35 Leachate individual volatile acids for Column 50MS 88
36 Leachate individual volatile acids for Column 60R 88
37 Leachate individual volatile acids for Column 70LR 89
38 Leachate individual volatile acids for Column 80HS ...... 89
39 Leachate individual volatile acids for Column 90MR 90
40 Leachate individual volatile acids for Column 100HR 90
4-1 Mass balance on simulated landfill column control volume . . 93
42 Mass of individual volatile acids released and transformed
in Column 1CR 95
43 Mass of individual volatile acids released and transformed
in Column 2CS 95
44 Mass of individual volatile acids released and transformed
in Column SOS 97
45 Mass of individual volatile acids released and transformed
in Column 40LS 97
ix
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FIGURES (continued)
Number l
46 Mass of individual volatile acids released and transformed
in Column 50MS 98
47 Mass of individual volatile acids released and transformed
in Column 60R 98
[
48 Mass of individual volatile acids released and transformed
in Column 70LR ; 99
49 Mass of individual volatile acids released and transformed
in Column 80HS ......; . 99
50 Mass of individual volatile acids released and transformed
in Column 90MR ; 100
51 Mass of individual volatile acids released and transformed
in Column 100HR .....' 100
I
52 Leachate alkalinity during simulated landfill
investigations „ 104
53 Leachate chemical oxygen demand during simulated landfill
investigations [ 105
54 Leachate total organic carbon during simulated landfill
investigations ; 106
I
55 Leachate ORP during simulated landfill investigations .... 108
56 Leachate chloride during simulated landfill investigations . 110
57 Leachate ammonia during simulated landfill investigations . . Ill
58 Leachate sulfate during simulated landfill investigations . . 112
59 Leachate sulfide during simulated landfill investigations . . 114
60 Leachate naphthalene during simulated landfill
investigations ;. 115
61 Leachate dibromomethane durihg simulated landfill
investigations L 116
62 Leachate trichloroethylene during simulated landfill
investigations '. 117
I
63 Leachate dichlorobenzene during simulated landfill
investigations ;. 118
64 Leachate trichlorobenzene during simulated landfill
investigations L 119
65 Leachate nitrobenzene during simulated landfill
investigations ; 120
66 Leachate nitrophenol during simulated landfill
investigations ..... 1 . 121
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FIGURES (continued)
Number page
67 Leachate dichlorophenol during simulated landfill
investigations •;.... 122
68 Leachate bromide during simulated landfill investigations . . 123
69, Headspace vinyl chloride during simulated landfill
investigations 124
70 Leachate sodium during simulated landfill investigations . . 125
71 Leachate potassium during simulated landfill
investigations 126
72 Leachate calcium during simulated landfill investigations . . 127
73 Leachate magnesium during simulated landfill investigations . 128
74 Leachate iron during simulated landfill investigations . . . 130
75 Leachate cadmium during simulated landfill investigations . . 131
76 Leachate chromium during simulated landfill investigations . 132
77 Leachate lead during simulated landfill investigations . . . 133
78 Leachate manganese during simulated landfill investigations . 134
79 Leachate nickel during simulated landfill investigations . . 135
80 Leachate zinc during simulated landfill investigations . . . 136
81 Leachate mercury during simulated landfill investigations . . 137
82 Lithium breakthrough curve for single pass Column 2
(single spike on Day 158) 139
83 Lithium breakthrough curves for single pass Column 3
(double spike on Days 158 and 771) 140
84 Lithium breakthrough curve for single pass Column 4
(single spike on Day 217) . . . 141
85 Lithium breakthrough curves for single pass Column 5
(double spike on Days 158 and 771) 142
86 Lithium breakthrough curve for single pass Column 8
(single spike on Day 217) 143
87 Lithium breakthrough curves for recycle Column 1
(single spike on Day 217) . . . 145
88 Lithium breakthrough curve for recycle Column 6
(single spike on Day 217) 146
89 Lithium breakthrough curves for recycle Column 7
(single spike on Day 217) 147
xi
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FIGURES (continued)
i
I
Number j ^ Page
90 Lithium breakthrough curves jfor recycle Column 9
(single spike on Day 217) \. . 148
91 Lithium breakthrough curves[for recycle Column 10
(single spike on Day 217) 149
92 Leachate conductivity during simulated landfill
investigations j 166
93 pH-pFe(II) distribution diagram 168
94 pH-pMn(II) distribution diagram ..... 169
95 pH-pCr distribution diagram 170
96 Predominance area diagram for the system
pb+vso/ycr/oH- i _ 173
97 Predominance area diagram for the system
Pb+2/S04-ys-y Sulfide (_ pH ^ 5.5, --- pH - 7,5) 174
98 pCl-pe diagram for mercury in absence of sulfide 175
99 pCT,S-pe diagram for mercury in presence of sulfide
(- pH - 5.5; —pH - 7.5) 176
100 Encapsulation of heavy metal sludge solids in
simulated landfills ...... ^ ..... 179
101 Comparison of heavy metal content of supplemental
sludge samples 183
xii
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TABLES
Number Page
1 Early history of municipal solid waste treatment and disposal . 4
2 , Example amounts of municipal solid waste generated by country . 6
3 Municipal solid waste composition .......... ..... 6
4 Municipal solid waste elemental analysis ........... 7
5 Relative percent of MSW managed by different methods ..... 8
6 Municipal landfill stabilization phases ............ 10
7 Landfill leachate constituent concentration ranges arid
their relative significance in terms of phase of landfill
stabilization ....... . ........... ..... 17
8 Representative variations in landfill leachate composition . . 23
9 Number of municipal solid waste landfill units by type
of leachate management strategy and operating status .... 25
10 Volatile organic acid (VGA) conversion to methane ....... 30
11 Free volatile organic acid concentration in a manure
slurry with controlled oxidation- reduction potentials .... 39
12 Redox half-reactions responsible for degradation of
selected organics during anaerobic treatment .... ..... 40
13 Free -energy changes for syn trophic propionate and
butyrate oxidation coupled to methanogens via interspecies
Hj or formate transfer ................... 43
14 Simulated landfill column loading and operation ........ 46
15 Characteristics of shredded municipal solid waste added
to simulated landfill columns ................ 50
16 Organic priority pollutants loaded to simulated landfill
columns ...... .
17 Characteristics of metal finishing waste treatment
sludge loaded to simulated landfill columns ......... 52
18 Metal sludge/metal oxide/sawdust mixture loaded to
simulated landfill columns .......... ....... 52
xiii
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TABLES (continued)
Number
19 Priority pollutant loading tp the simulated landfill
columns ......... j, ................. 53
20 Shredded municipal refuse, oirganic and inorganic
priority pollutants, and sawdust loadings for each
simulated landfill column . .......... ...... .54
21 Summary of anaerobic digester sludge additions to
simulated landfill columns , ................ ,57
22 Summary of analytical methods used during simulated
landfill investigations . ; ................ ,,59
23 Comparison of gas volumes from simulated landfill columns . . „ 66
i
24 Recycle simulated landfill column gas production rates .... 69
i i .
25 Single pass simulated landfill column gas production rates . . 69
26 Comparison of gas composition during simulated landfill
investigations ..... j ................. 76
t
27 Cumulative liquid removed due to gas production ........ 79
28 t Gas production potential lost due to leachate removal
during simulated landfill investigations .......... 85
29 Average leachate individual volatile acids during the acid
formation phase of the simulated landfill investigations . . 91
30 Indicated retention times of ', single pass simulated
landfill columns . . •. . J ....... . ........ 144
31 Indicated retention times of recycle simulated landfill
columns .......... ................ 150
32 Headspace HjS content at column disassembly ......... 151
33 Net settling or compaction in simulated landfill at
column disassembly . . . .! ................ 151
34 Physical and chemical properties of organic priority
pollutants loaded to the simulated landfill columns .... 154
35 Calculation of retention time for selected organic priority
pollutants loaded to the simulated landfill columns .... 155
36 Mass balance summary on organic priority pollutants
for the single pass simulated landfill columns ...... 157
37 Mass balance summary on organic priority pollutants
for the recycle simulated landfill columns ........ 158
38 Significant equilibrium constants for metal complexes .... 172
39 Distribution of lead species in typical leachate ...... .172
XIV1
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TABLES (continued)
Number Page
40 Distribution of mercury species in typical leachate ..... 172
41 Selected analysis of supplemental metal sludge
codisposed in simulated landfill columns 184
42 Toxic metal content of supplemental sludge codisposed
in simulated landfill columns 184
43 Non-toxic metal content of supplemental sludge
codisposed in simulated landfill columns 184
xv
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ACKNOWLEDGMENTS
The research for this project was performed under the direction of Drs. F.
G. Pohland, W. H._ Cross and J. P. Gould, jwith the special assistance of Dr. D.
R. Reinhart, without which the complementary experiments on fate and
transformation of organic priority pollutants codisposed with municipal shredded
refuse could not have been completed.
Project support from the United States Environmental Protection Agency, in
conjunction with the School of Civil Engineering, College of Engineering, Georgia
Institute of Technology and the Department of Civil Engineering, School of
Engineering, University of Pittsburgh, is gratefully acknowledged.
Special thanks is also extended to the following graduate research
assistants, undergraduate assistants and laboratory technicians for their efforts
in constructing, operating and disassembling the simulated landfill columns, as
well as obtaining and analyzing samples!; Basel Al-Yousfi, Gregory Anderson,
Norman P. Belle, Julie Cassels, Gregory S. Dyson, Linda M. Dyson, Daniel M.
Gatti, Thomas M. Ginn, Jr., Kimberleigh R.i Gnoffo, Julie Hillmeyer, Jin Hong Kim,
Byoung Young Lee, Eva M. Long, Tim McCarter, Ayn M. McClendon, Kellye McDonald,
Mark W. McDonald, Camelia Mercati, May A. Mishu, Isabel Palazzolo, Michaell
Roeder, Kimberly Shank, Sarah J. Shealy;, Marie Stratakis, Joannes van Esch,
Ronald R. Walz, Jin Yuan Wang Susana Waters, Dalvaro Weaver and Amy L. Williams.
Ms. Phyllis K. Scoggins, Henrietta Bowmah, and Elaine Sharpe are particularly
acknowledged for secretarial services during the conduct of the research and
preparation of this final report. i
xvi
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SECTION 1
INTRODUCTION
Effective management of increasing amounts of solid waste has become not
only a local government and national challenge, but an international issue as
well. Effective solid waste management includes an integrated multimedia
approach that incorporates waste minimization, reuse/recycling,
treatment/destruction, and ultimate disposal. Of all available solid waste
management option's, disposal in landfills is most frequently employed, primarily
because of its economic advantage. Moreover, regardless of the emphasis on other
solid waste management methods, the land will continue to serve as a final waste
receptor, whether for incinerator ash, discards from recycling initiatives, or
regulated hazardous waste.
Landfills are designed to comply with regulations allowing either "dry" or
"wet" disposal, provided that adverse health and environmental impacts from
potential leachate and/or gas generation are minimized. Therefore, two important
features of modern landfills include leachate and gas management. Design
requirements often specify that the landfills contain a double liner system,
incorporating essentially impermeable natural and/or synthetic materials, and
also a surface cap at closure. The two liners may be separated by a leak
detection system which is intended to capture any leachate that may escape the
first liner and to warn of loss in liner integrity and the potential release of
leachate (or gas) into the surrounding environment. Directly above the innermost
liner and below the disposed refuse, a leachate collection system may be
installed to facilitate leachate management.
One of two fundamental leachate management strategies can be employed at
full-scale landfills; one which strives for limited rainfall infiltration and
single pass leaching, and the other employing leachate containment and in situ
recirculation. During single pass leaching, leachate is collected and removed
from the landfill and treated by separate biological and/or physical-chemical
operations either on-site or off-site. In contrast, with leachate recirculation,
leachate is collected and recycled back through the landfill, thereby converting
the landfill into a large anaerobic biological reactor with attendant
microbially-mediated, physical-chemical treatment capabilities. This latter
approach is a more recent innovation that has been shown to provide cost-
effective leachate treatment, while accelerating the waste conversion and
stabilization processes occurring within the landfill environment.
The landfill cap typically consists of a composited impermeable natural or
synthetic lower liner, an intermediate drainage layer, and an upper vegetative
cover. The natural or synthetic liner is positioned on top of the refuse mass
to limit moisture infiltration. The drainage layer, which is placed above the
impermeable liner, serves' to divert percolating infiltration away from the
landfill. The vegetative cover, which is the uppermost layer, is provided to
promote evapotranspiration and minimize surface erosion.
Landfill gas, produced as a result of the anaerobic biological waste
stabilization occurring within the landfill, consists primarily of methane and
carbon dioxide. Regulations require that the methane and other landfill gases
be controlled in an environmentally sound manner, including flaring, controlled
venting to the atmosphere, or recovery as a fuel source. Therefore, contemporary
.1
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landfill designs and operations include [integrated leachate and gas management
systems. :
Because a landfill is essentially an anaerobic biological waste conversion
and stabilization process during most of its active life, the same process
fundamentals that apply to separate anaerobic treatment also apply to landfills,
although effective retention times an4 opportunities for assimilation and
attenuation of less available substrates associated with these separate anaerobic
treatment processes are different than those provided by the landfills of today.
Therefore, the purpose of this research;was to emphasize this analogy between
separate anaerobic treatment systems and landfills, and to demonstrate the
behavior and assimilation of organic and inorganic priority pollutants codisposed
with municipal refuse. i
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SECTION 2
GENERAL HISTORICAL PERSPECTIVE AND CURRENT STATUS OF LANDFILL PRACTICE
Landfill disposal of solid waste is common, because landfilling has been
considered to be the simplest and most economically attractive of all available
solid waste management options. However, increasing population growth and
societal demands for more conveniences and a higher standard of living have
increased solid waste management challenges, often resulting in a critical need
for expanded landfill capacity. This increase in landfill capacity has aroused
many environmental concerns, to some degree because landfills have historically
been poorly managed and misunderstood, but also because inadequate attention has
been given to protection of public health and the environment. For example, past
landfill practices in the United States (USA) have resulted in 20% of the
Environmental Protection Agency's (EPA) national priority list (NPL) of toxic
waste clean-up sites being former municipal solid waste (MSW) landfills (Carra
and Cossu(1)) . These NPL landfills have contaminated groundwater with organic and
inorganic constituents that have leached from the refuse and migrated into
underlying aquifers.
The early history of land disposal, which dates back approximately 5000
years®, is shown in Table 1. Within the USA, landfill disposal sites, often
called "dumps" until the 1970's, consisted of the placement of solid waste in
unlined excavations or mounds. Volume reduction was often achieved by setting
the refuse on fire, thereby prolonging the useful life of the dump. However, as
a result of the 1976 Resource Conservation and Recovery Act (RCRA), dumps were
declared obsolete and are rapidly being replaced by more properly engineered
Today, landfills may be classified into three basic categories based
largely upon the type of waste being contained. "Sanitary" landfills are the
most common and contain principally municipal solid waste (MSW) or refuse,
"secure" landfills are developed to contain hazardous wastes, and the
controlled" landfills are designed to manage municipal refuse, but in a manner
where both leachate and gas production safeguards are provided. Because it is
recognized that refuse may contain some hazardous waste, this latter controlled
landfill is specifically designed and operated to also provide assimilation of
many of the hazardous waste constituents.'41 Such hazardous wastes may be
codisiposed with the MSW emanating from household or commercial activities, or
from industrial activities; practices which have occurred in the past and may
still occur today even though regulations generally prohibit large quantities of
hazardous wastes from disposal in municipal landfills.
Contemporary landfills are designed and operated according to standards
which vary from country to country. For example, Germany has established
regulations that may require waste encapsulation and "dry" disposal in which
natural and/or synthetic liners are installed in combination with impermeable
caps to minimize leachate generation. In contrast, the USA is also allowing
wet disposal, where leachate is allowed to be produced and is collected and
treated using a variety of techniques, including discharge to a Publicly Owned
Treatment Works (POTW), on-site chemical and/or biological treatment or
recirculation back through the landfill. Practice in Sweden and Canada has
indicated that leaching is an important part of the stabilization process and
helps integrate the landfill into the surrounding environment, which is the
ultimate goal. In the United Kingdom, codisposal of a restricted range of
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TABLE 1. EARLY HISTORY OF MUNICIPAL SOLID WASTE TREATMENT
AND DISPOSAL®
2000-3000
B.C.
3500 B.C.
3500 B.C.
1900 B.C.
900-400
B.C.
900-400
B.C.
494 B.C.
451 B.C.
350 B.C.
1240
1294
1309
1357
1494
1530
1656
Late 19*-
Early 20*
Century
1900
1901
1930
New Stone Age Man
City of Ur Babylonia
Mohenjo Daro,
Harappa , Indus
Valley !
Knossos, Crete
i
Mosaic Law
Jerusalem |
Rome, Italy
Rome, Italy
Pataliptra, India
London, England
Berwick, England
London, England
1
i
London, England
City of Aberdeen
England •
England i
England ;
Brazil [
Budapest, India
City of London |
Kitchen middens -rubbish heaps
Refuse piles
Covered conduits
Circular walled refuse
bioreactors (Kouloura)
Tent/camp refuse accumulation
prohibited
Perpetual refuse fires
Street cleaning established
Litter fines -corporal punishment
introduced
Litter fines
Refuse collection introduced
Litter fines (96 silver pennies)
Surveyors appointed, fines
introduced
Royal order (imprisonment)
Street scavengers appointed
Fermented refuse for gun powder
manufacture
Saltpeter digging banned
Hand sorting/refuse recycling
Refuse destructor commissioned
(steam/ electricity generation)
Salvage works opened
Ministry of health instructions
and controlled tipping
-------
industrial waste with MSW has been encouraged, because the landfill is believed
to possess a capacity to attenuate key components of the industrial waste.'"
The wastes disposed in' a landfill can be pretreated by a variety of
methods, largely "to facilitate refuse volume reduction, and increase landfill
capacity. Moreover, milling, shredding, and pulverizing the MSW can increase
homogeneity and the surface area available for microbial attack, and also provide
for incipient introduction of oxygen which serves to enhance an initial period
of aerobic waste stabilization. Bailing and mechanical compaction increase
refuse density, decrease permeability, and remove trapped air from void spaces
They may also impede moisture and nutrient distribution required for microbially-
mediated stabilization. Sorting of MSW for the recovery of recyclable materials
also reduces waste types and volumes to be landfilled, but probably does not
affect stabilization of the remaining organic constituents unless such a practice
causes adverse physical impacts on moisture and nutrient distribution.
Regardless of pretreatment, active landfills produce gas in composition
reflecting a microbially-mediated succession of stabilization events. Methane is
the primary gas produced, and can be recovered for use as a fuel depending upon
economics and comparative energy prices. When collected and utilized
incremental cost of methane gas recovery can be decreased.
While landfilling is generally considered to be the least expensive method
of MSW management, the true costs are probably significantly higher than have
been generally recognized. The total cost of landfill disposal includes capital
costs, operating expenses, and the cost of establishing and maintaining controls
to prevent contamination from occurring or providing remediation if contamination
does occur. As land costs increase and more stringent environmental controls are
imposed the total cost of landfilling will rise. Moreover, siting of new
landfills has become increasingly difficult, resulting in increased collection
and transportation costs with landfills being located more distant from generator
AT"eA<3 °
areas.
The environmental impacts of improper landfill design and management
practices have increased public awareness and concern. Instead of the
traditional "out of sight, out of mind" approach, municipal solid waste
management has received national (and international) focus, resulting in alarm
over both short- and long-term environmental effects resulting %rom past
mismanagement of MSW, and a resistance to development of new landfill facilities
In addition, population growth has eliminated many potential landfill sites
thereby compounding the siting problem.
2.1 MUNICIPAL .SOLID WASTE
^The residential commercial, industrial, and agricultural sectors
contribute to the total amount of solid wastes disposed in landfills The
combination of regulated residential and commercial refuse, usually defined as
municipal solid waste (MSW), includes all solids, liquids, semi-solids, and
contained gases discarded by a community. This quantity varies among countries,
and as indicated by Table 2, a consistent categorization of MSW frictions does
not exist.
In general, MSW is a diverse mixture of materials that vary in moisture
content, ^size chemical characteristics, density, and composition. The generator
composition of MSW also varies as a function of location, socioeconomic status
time, and other site specific factors. Increased product packaging, household
garbage grinders, and the trend toward expanded use of disposable products have
significantly impacted MSW composition in recent decades. Selected data on the
range of composition of MSW are presented in Table 3, and corresponding elemental
constituency is provided in Table 4.
-------
TABLE 2. EXAMPLE AMOUNTS OF MUNICIPAL SOLID WASTE GENERATED
BY COUNTRY1" i
Country
Austria
Canada"3
Denmark""
Finland
France"0
FRG
Italy
Japan
Netherlands'01
Poland
South Africa
Sweden
Switzerland
UK
USA
Annual Total
(106 totmes/year)
il.7
25.0
1.3-3.4
2.0-3.0
17.;8-49.8
24.0
17.3
41.0
:8.5
.
12.0
2.5
,6.3
18.0
72.0
Per Capita Amount:
(kg/person • day)
0.6
2.7
0.7-1.8
0.5-1.6
0.9-2.5
1.1
0.8
0.9-1.1
1.6
0.6-1.3
1.0
0.8
2.6
0.9
1.6
Notes: aFigures only available for municipal and industrial -
comercial combined.
''Household wastes include household, commercial, and
bulky wastes. !
'Household wastes include both household waste and
industrial waste that is similar to household waste.
"Household, road sweep, etc., office/shop/service
wastes are combined and reported as household waste.
TABLE 3. MUNICIPAL SOLID WASTE COMPOSITION3
Reference Source
Component/Character
Food Wastes
Garden Wastes
Paper
Cardboard
Plastics
Rubber and Leather
Textiles
Plastic Film
Wood
Glass
Me tallies
Tin Cans
Non-ferrous Metals
Ferrous Metals
Dirt, Ashes, Brick
Miscellaneous
Moisture Content
(3)
Avg.
15.0
15.7
35.3
5.0
2.7
1.7
3.8
10.6
1.0
6.9
2.1
(5)
: Avg.
, 10.0
: 10.0
! 43.0
3.0
2.0
3.0
3.0
9.0
1.0
6.0
10.0
t
|
(6)
Avg.
12.0
39.0
7.0
2.0
3.0
2.0
7.0
10.0
8.0
10.0
(7)
Range
6-26
0-20
25-45
3-15
2-8
0-4
0-4
1-4
4-16
2-8
0-1
1-4
0-10
15-40
(7)
Typical
15.0
12.0
40.0
4.0
3.0
2.0
2.0
2.0
8.0
6.0
1.0
2.0
4.0
20.0
Note: "Percent by weight, wet weight basis.
-------
TABLE 4. MUNICIPAL SOLID WASTE ELEMENTAL ANALYSIS8
Reference Source
Component
Carbon
Nitrogen
Hydrogen
Oxygen
Sulfur
Chloride
Water
Inorganics
(2)
Average
25.5
0.5
3.4
20.3
0.2
0.5
25.2
24.4
(3)
Range
15-30
0.2-1.0
2-5
12-24
0.02-0.1
15-35
Note: aPercent by weight.
As the difficulty in siting of new landfills increased, contemporary
landfills have become larger and more sophisticated than those of the past, and *
manjr countries are seeking alternative methods of MSW management. The U.S. EPA
has developed a hierarchy of solid waste management practices in which waste '
minimization at the source is emphasized as a first alternative, followed by i
rec3»-cling and reuse, treatment and destruction, and disposal.® However,
industries and municipalities in the United States continue to favor the least '
cost: method of solid waste management, which continues to be landfill disposal. '
The relative percent of MSW managed by different methods is presented in
Table 5, where MSW management methods employed by each country is shown to vary
considerably. Only 20 to 30% of the MSW was reported landfilled in Switzerland
and Japan, compared to 95% of the MSW generated in Finland, Poland, and Canada.
In contrast, Japan, Sweden, and Switzerland are reported to incinerate 60 to 80% !
of the MSW compared to only 0 to 4% in Finland, Canada, and Poland. Hence, ;
increased utilization of MSW incineration is becoming apparent for densely •
populated areas with decreased land available for new landfills. Continuing
population growth in these areas also aggravates associated siting problems.
Therefore, as the amount of MSW generated continues to escalate, integrated MSW ;
management systems employing waste minimization, recycling and reuse, treatment !
and destruction, and ultimate disposal will become more common, and the relative
costs of these options will eventually need to be reconciled with a desire to
minimize potential adverse effects on both health and the environment.
2.2 LANDFILL STABILIZATION ' , '
2.2.1 Landfill Stabilization Processes '
Stabilization processes occurring within a MSW landfill normally proceed
through a series of physical, chemical and biological transformations. These ;
changes include: biological decay of putrescible material, either aerobically
or anaerobically, with the evolution of gases and liquids; chemical oxidation of i
waste constituents; dissolution and transport of organic and inorganic
constituents by leaching liquids; diffusion and transport of gases; hydraulic
liquid transport; movement of dissolved constituents as a result of concentration
gradients and osmosis; and uneven settlement caused by waste degradation and
consolidation of material into void spaces.w
The significance and longevity of the physical, chemical and biological
stabilization processes are largely determined by climatological conditions,
operational variables, management options, and control factors operative or being
applied either external or internal to the landfill environment.00' Through the
measurement and analysis of certain leachate and gas parameters, these events can
-------
TABLE 5. RELATIVE PERCENT OF MSW MANAGED BY DIFFERENT METHODS0'
Country
Austria8
Canada
Denmark
Finland
France
FRG
Italy
Japan8
Nether-
lands
Poland
South
Africa0
Sweden
Switzer-
land
UK
USA
Land-
filled
64
95
31
95
47.9
74
83.2
29.6
51
99. 9b
69.2
35
20
88
83
Incin-
erated '
20
4
50
2
41.9 ;
24 !
13.9 [
67.6 i
34
j
20.8 :
t
60 '
80
1
11
6 !
Recycled
1
' 18
3
0.6
0.6
15
3.1
5d .
1.0e
1
11
Composted
16
1
8.7
2
2.3
2.8
0.1
3.8
No
Service
3.9
Notes: "Figures do not account for recycling.
blncludes waste disposed in controlled/uncontrolled
dumps . Less than 1% of the dumps are true sanitary
landfills .
<;225 of 564 landfills are 'uncontrolled.
"Separation/composting facilities .
Mostly waste-derived fuel.
-------
be detected and followed. Initially, waste decomposition within the landfill
proceeds aerobically, primarily utilizing the oxygen contained within the void
spaces of the MSW during placement. After available free oxygen is depleted,
stabilization continues anaerobically during the majority of the remaining active
life of the landfill, somewhat analogous to a batch anaerobic digester, receiving
finite inputs of waste and moisture and producing finite outputs of leachate and
gas. However, the effective retention time in a landfill is on the order of
years compared to days for most separate biological treatment systems.00'
Most municipal solid waste landfills have also been shown to evolve through
five relatively discrete and sequential phases of stabilization,(W) starting with
an initial lag or adjustment phase which is prolonged until sufficient moisture
develops to stimulate an active microbiological community and produce leachate.
Thereafter, the phases of landfill stabilization or waste conversion can be
characterized by leachate and gas composition and production rates, and the four
phases after Initial Adjustment may be described to consist of Transition. Acid
Formation. Methane Fermentation. and Final Maturation phases as defined in Table
6. The accompanying changes in,gas production and leachate and gas composition
during accelerated stabilization are illustrated in Figure 1.
Provided that sufficient moisture and nutrients exist during microbially-
mediated stabilization, and toxic materials do not cause inhibition, the five
fhases of stabilization will occur at some time within each portion of a
andfill. However, since the placement of MSW in a landfill occurs at different
times as the component cells are developed, landfill stabilization phases tend
to overlap. Moreover, the rate of stabilization also varies within each cell
according to the physical, chemical and biological environment present. Thus,
no landfill has a single "age", but rather a family of different ages associated
with various compartments or cells within the landfill complex and their
respective progress toward stabilization.(10)
2.2.2 Factors Affecting Landfill Stabilization
A principal aspect of the stabilization processes occurring in a landfill
is the anaerobic microbial conversion of complex organic material to methane and
carbon dioxide. Accordingly, anaerobic conversion of organic constituents may
b.e described in four steps."" During the first step, complex and/or insoluble
organic material is hydrolyzed to a size and form that can permeate into
bacterial cells and be used as energy or nutrient sources.03 During the second
or acidogenic step, organic monomers are converted into simpler intermediates,
such as the medium- and long-chain volatile organic acids and the short-chain
acetic, propionic, butyric, and valeric acids, as well as hydrogen and carbon
dioxide. During the third or acetogenesis step, the higher volatile organic
acids are converted to acetic acid, carbon dioxide, and hydrogen. For all
longer-chain volatile organic acids except propionic, the conversion to acetic
acid occurs via ^-oxidation, whereas the conversion of propionic acid to acetic
acid occurs via a-carboxylation. Methanogenesis, the final step, consists of the
conversion of acetic acid, carbon dioxide and hydrogen to methane. It is during
the methane formation phase that the majority of waste stabilization takes place.
The generalized pathways for this overall conversion are illustrated in Figure
£* *
Microbially-mediated waste stabilization in landfills, as in separate
anaerobic digestion processes, is influenced by a number of factors, including
temperature, pH, the presence of toxic substances, and nutrient availability.
Moreover, anaerobic processes generally occur best within either mesophilic (30
to 38°C) or thermophilic (50 to 60°C) temperature ranges. The optimum temperature
range for mesophilic anaerobic digestion reported by McCarty(l3) is 30-32°C. Henry
and coworkers00 performed experiments using two completely mixed reactors
(CSTR's) operated at 35°C and at 55°C. The results indicated that at the higher
temperature, carbohydrate, protein and organic matter conversion increased as did
the volatile organic acid concentrations. However, the methane yield was lower
-------
TABLE 6. MUNICIPAL LANDFILL STABILIZATION PHASES(IO>
Phase I: Initial Adjustment
Initial waste placement and moisture
accumulation. ]
Closure and initial subsidence of each
landfill compartment or cell.
Changes in environmental parameters are
first detected to reflect the onset of
stabilization.
Phase II: Transition
Indicated field capacity is exceeded
and leachate is generated.
A transition from initial aerobic to
anaerobic micrbbial stabilization
occurs. '
The primary electron acceptor shifts
from oxygen to [nitrates and sulfates
with the disappearance of oxygen and
production of carbon dioxide in the
gas. ;
A trend toward ^reducing conditions is
established.
Measurable intermediates, such as
volatile organic acids, appear and
increase in concentration in the
leachate. •
Phase III: Acid Formation
Intermediary volatile organic acids
become dominant with the continuing
hydrolysis and fermentation of waste
and leachate constituents.
A decrease in pH occurs with
concomitant mobilization and possible
complexation of metal species.
Nutrients, such as nitrogen and
phosphorus, are; released and utilized
in support of microbial biomass growth
commensurate with prevailing substrate
conversion rates.
Hydrogen gas may be detected and affect
the nature and itype of intermediary
metabolism and product formation.
-continued-
10
-------
TABLE 6 (continued)
Phase IV: Methane Fermentation
Nutrient consumption" and conversion
continues.
Intermediary products, primarily
volatile organic acids formed during
the acid formation phase, are converted
principally into methane and carbon
dioxide.
The leachate pH returns from a buffer
level controlled by the volatile
organic acids to one characteristic of
the bicarbonate buffering system as the
volatile organic acids are converted.
Oxidation-reduction potentials (ORP)
are at their lowest reducing levels
with accumulation of reduced sulfur and
nitrogen species.
Complexation and precipitation of metal
species .proceed.
Leachate organic strength is decreased
dramatically in correspondence with
increases in gas production.
Phase V: Final Maturation
Relative dormancy following active
microbial stabilization of the readily
available organic constituents in the
waste and leachate.
Nutrients may become limiting.
Measurable gas production diminishes.
Environmental parameters reflect
conditions of greater stability and
diminished microbially-mediated
activity.
Oxygen and oxidized species may slowly
reappear with a corresponding increase
in oxidation-reduction potential.
Microbially resistant organic materials
may be slowly converted with the
possible production of humic-like
substances capable of complexing with
and remobilizing heavy metals.
11
-------
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-------
than that of the CSTR operated within the mesophilic temperature range. These
results suggested that, while hydrolysis and liquefaction of feed substrate and
acidification of soluble intermediates were improved under thermophilic
conditions, methanogenesis was inhibited. The depressed gas production and
increased acetate concentration in the .thermophilic process indicated that: the
activity of the methanogens responsible: for acetate conversion to methane was
reduced at higher temperatures. Parkin and Owen(12) suggested that a temperature
as close to 35°C as possible be maintained during anaerobic process start-up and
recovery from upset. The rate of methane generation from solid waste disposed
in existing landfills, studied between :the temperature range of 21°C to 48°C,
indicated that* the optimum temperature was 41°C.(M)
Regardless of the operational temperature selected, the maintenance of
uniform temperatures is considered to be fundamental to anaerobic stabilization
process efficiency."21 Since the processes of landfill stabilization occur under
primarily anaerobic conditions, the same factors affecting separate anaerobic
stabilization processes also influence landfill stabilization processes.
However, temperatures within the landfill environment may display greater
fluctuations than those of separate Anaerobic processes, because landfill
temperature is not externally controlled and generally reflects ambient
temperature conditions, and the extent and effectiveness of insulation provided
by the landfill configuration.
Maintenance of pH within an acceptable range is imperative for efficient
anaerobic waste conversion. The generally accepted operational pH range is 6.5
to 7.6, with the optimum pH being between 7.0-7.2."3-131 Hydrolysis,
liquefaction, and gas production efficiencies have been reported improved at near
neutral pH levels.'"' This range of acceptable pH is primarily controlled by the
bicarbonate buffering system, where buffer capacity may be measured in terms of
alkalinity, and the maintenance of an adequate amount of alkalinity helps to
buffer the anaerobic process from potential failure resulting from excess acid
generation. Since the pH of an anaerobic system is a function of both the
volatile organic acids and alkalinity concentrations, as well as the partial
pressure of carbon dioxide gas evolved during stabilization, if the pH decreases
due to an overproduction of acid, the; magnitude of this pH change will be
dependant upon the types of acids and bases present and their ability to displace
the existing buffering system. During conditions when volatile acids accumulate
(Acid Formation Phase), the pH may be loyered as the bicarbonate buffer system
is displaced by the volatile acid buffer system.
i
The microbial stabilization processes occurring in an anaerobic digester
or landfill may also be adversely affected by the presence of inhibitory
substances, such as high concentrations' of ammonia nitrogen, sulfides, heavy
metals, toxic organic constituents, and excess volatile organic acids. The
relative toxicity of any of these is a function of their physical-chemical
nature, their concentration, and the possibility for microbial acclimation. In
addition, hormesis,(16> or the stimulatory effects caused by low levels of
potentially toxic materials, has been observed.(I3) Accordingly, many potentially
toxic substances may stimulate microbial! reaction rates at low concentrations,
however, increased concentrations may re'sult in inhibitory or toxic effects.
Ammonia is produced during the decomposition of waste constituents
containing nitrogen. Ammonia nitrogen mainly exists as either the ammonium ion
at pH less than 7.2, or ammonia at higher pH values. Ammonia is generally
considered inhibitory at a much lower concentration than the ammonium ion.'"1
McCarty reported that ammonia nitrogen; concentrations between 50 and 200 mg/L
can be beneficial to anaerobic processes» because ammonia nitrogen can serve as
an essential nutrient; concentrations between 200 and 1000 mg/L were shown to
have no adverse effects on anaerobic processes; concentrations ranging from 1500-
3000 mg/L were shown to have inhibitory effects at higher pH levels; and
concentrations above 3000 mg/L were toxic.
14.
-------
Alkali and alkali-earth metal salts such as sodium, potassium, calcium, and
magnesium have also been repotted as toxic03' >at concentrations of 0.005 M for
divalent cations and 0.01 M for monovaleht cations,02 Similarly, soluble
sulfides have been reported to cause significant decreases in methane production
at concentrations in excess of 300 mg/L.(17) However, sulfide can be used in
anaerobic systems to precipitate heavy metals as sparingly soluble sulfides which
are riot toxic because they are removed from solution and no longer in effective
contact with the microbial populations inhabiting the landfill environment.
enzyme
Some heavy metals, are required in trace amounts by microbial species for
-.^._,^3 and coenzyme activation and/or functioning. Excess heavy metal toxicity
is due primarily to disruption of enzyme structure and function by the binding
of metals with functional groups on proteins or replacing naturally occurring
metals in ^enzymes. Therefore, the toxicity of heavy metals is influenced by
their speciation and partitioning within the anaerobic environment. Heavy metals
may be precipitated under anaerobic conditions as sulfides, carbonates,
hydroxides, or bound to waste ligands. Additionally, heavy metals may also be
chelated and maintained in solution by conversion products, and may be subjected
to sorption and ion exchange within the refuse matrix. Therefore, heavy metals
that exist in solution as free cations are toxic to microbial life if present
above some threshold concentration.02'
Some organic substances, such as chlorinated hydrocarbons, are also
considered toxic to anaerobic biological systems and are thereby capable of
adversely affecting the landfill stabilization process. However, finite
concentrations of halogenated organic compounds have been shown to be detoxified
under anaerobic conditions, largely through reductive dehalogenation reactions
beneficiated by reducing potentials.00'
During anaerobic digestion, the predominant volatile organic acids (VGA)
produced include acetic, propionic and butyric acids.(13'1
-------
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properties of the percolating water and adjacent soil or cover. For example,
leachate volumes are greater and contain; lower concentrations of the parameters
indicative of the progress of landfill stabilization in moderate temperature and
humid regions that receive high amounts of rainfall than in hot and arid regions
that receive minimal rainfall.00' In | addition, a landfill containing both
municipal and industrial wastes will produce leachate characteristic of both
components. Hence, the leachate constituents originating from any industrial
waste input will be superimposed upon the landfill leachate characteristics from
the municipal solid waste. Representative variations in landfill leachate
composition are presented in Table 8 . !
A landfill will accumulate moisture until field capacity is attained,
develop an active microbiological community commensurate with substrate type and
availability, and release leachate basediupon the amount of moisture percolating
into and through the landfill. The leachate produced will change in quantity and
quality as stabilization proceeds. Initially, the leachate quality is reflective
of the Acid Formation Phase of landfill stabilization, exhibiting a low pH, high
organic content as indicated by COD, BOD5[, total organic carbon (TOG), and total
volatile acids (TVA) , an abundance of mobilized ions, and absence of dissolved
oxygen concentrations. This leachate is characteristic of initial waste
conversion and has a high pollutional potential if it were to escape the landfill
boundaries. During the Methane Fermentation Phase, the leachate exhibits reduced
TVA, increased pH, and the virtual elimination of readily degradable organic
components. The COD of the leachate during the Methane Fermentation Phase is
reflective of the remaining organics that are not easily biodegraded and/or the
presence of humic-like compounds. :
2.3.2 Leachate Management Strategies !
^ fundamental leachate management strategies exist for conventional
landfill operations, they are single pass leaching with leachate containment,
collection and external treatment, and; leachate containment, collection, and
recirculation back into and through the landfill. The former is employed at
nearly all full-scale landfill facilities where leachate is formed and managed.
During single pass leaching, the moisture; influx to the landfill is curtailed and.
the volume of leachate generated is collected and treated to remove the organic
and inorganic contaminants present prior to ultimate disposal. Treatment may
include a biological process to remove the organic fraction, followed by
physical -chemical treatment to remove residual organics, inorganics, color and/or
odor.0* The treatment of landfill leachate by anaerobic biological processes has
the advantage over aerobic biological processes with similar retention times,
because methane is produced which can be used as" an energy source, and the costly
aeration necessary in aerobic systems is essentially eliminated. <10>
During leachate containment, collection, and recirculation, which has been
the subject of extensive investigation and development by Pohland and
coworkers,00-2*-2* the landfill is managed as both a biological and physical-
chemical treatment system. Such leachate recycle has been shown to be an
economical leachate pretreatment option; by utilizing the landfill as a large
anaerobic bioreactor.(1>27) As a result of this in situ leachate treatment, more
effective use of the assimilative capacity of the landfill is attained through
simultaneous attenuation of both hazardous and non-hazardous substances as
biodegradation of organic compounds occurs . Once these substances are
immobilized or converted within the landfill, and thereby removed from the
leachate (or gas) , the potential for adyerse environmental impacts is greatly
reduced should leachate migration from the landfill boundaries occur. In
addition to the possibility of dramatically reducing leachate volumes through
evaporation during leachate recycle by using surface infiltration/" a more rapid
development of a viable population of i anaerobic bacteria capable of waste
conversion is promoted. The increased biological growth can be attributed to a
more uniform availability of necessary nutrients contained within the recycled
leachate, and continuing exposure of tihe microbial populations to leachate
22!
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TABLE 8. REPRESENTATIVE VARIATIONS IN LANDFILL LEACHATE COMPOSITION
Reference Source
Analysis
PH
Hardness (mg/L as CaC03)
Alkalinity (mg/L as CaC03)
COD (mg/L)
BOD, (mg/L)
TOG (mg/L)
TVA (mg/L as CH3COOH)
Nitrate Nitrogen
(mg/L as N)
Ammonia Nitrogen
(mg/L as N)
Total Phosphate (mg/L)
Total Dissolved Solids (mg/L)
Sulfate (mg/L)
Potassium (mg/L)
Sodium (mg/L)
Cadmium (mg/L)
Chromium (mg/L)
Copper (mg/L)
Iron (mg/L)
Lead (mg/L)
Magnesium (mg/L)
Manganese (mg/L)
Nickel (mg/L)
Zinc (mg/L)
(10)
Range
4.7-8.8
.
140-9650
31-71,700
4-57,700
70-27,700
0-18,800
0-51
2-1030
0.2-120
1460-55,300
0-3240
35-2300
20-7600
70-3900
0.02-18
0.005-2.2
4-2200
0.001-1.44
3-1140
0.6-41
0.02-79
0.06-220
(23)
Range
4.9-8.4
30-13,100
100-20,805
246-750,000
5.9-720,000
—
<100-10,000
5-196
0.2-1106
—
1740-11,254
24-1220
28-3770
85-1805
—
—
—
2-1000
^
^
_
—
-
23
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nutrients. As a result of the rapid development of a suitable microbial
community, the time required for landfill stabilization is decreased from several
decades to as few as 2 to 3 years. (10-27) : Pohland04' also found that by employing
leachate recirculation along with pH control and sludge seeding, the time to
achieve landfill stabilization could be further reduced to months rather than
years. Moreover, evidence suggests that the cost of leachate recycle may be as
low as 25% of the costs of corresponding separate treatment,0*1 thus making
leachate containment, collection and recirculation an attractive pretreatment
option. !
During research investigations by Pohland and coworkers,*2" two experimental
simulated landfill columns (one single pass and one recycle) were loaded with
shredded municipal solid waste, and tap water was added to bring the columns to
indicated field capacity. After the attainment of field capacity, leachate was
recirculated daily to the column employing that management strategy, and routine
moisture additions to both columns were continued at intensities based upon local
rainfall intensity. The experimental results indicated that the recycle column
produced more gas than the single pass cplumn, which was attributed directly to
the opportunity provided by recycle to contain and more thoroughly stabilize the
waste and leachate constituents. In contrast, the leachate from the single pass
column, which contained much of the gas production potential in the form of
volatile organic acids, was removed from the system. Hence, leachate was not
discarded from the recycle column, and the gas production potential was not lost.
In addition, if initial recirculation of high-strength leachate was too frequent,
a delay in methane gas production could',result. This effect was attributed to
acid inhibition of the methanogens caused by high volatile organic acid
concentrations in recycled leachate, which shifts the pH towards the pK. of the
volatile organic acid buffer system. For[ this same reason, gas production tended
to occur earlier in the single pass column, because the potential inhibiting
capacity of the volatile organic acids was removed by dilution and washout.
Pohland and coworkers0" also observed that leaching in the recycle cell,
although more uniform at higher liquid!recycle flow rates and reduced short-
circuiting, could adversely affect the:localized microenvironments harboring
methanogens within the refuse mass by penetrating the mass of refuse otherwise
protecting them from high VOA concentrations and low pH. As indicated
previously, because these microenvironments were present in the single pass
column and were able to harbor and protect methanogens, gas production commenced
earlier than in the recycle column. ;As a consequence of the delayed gas
production in the recycle column, it was suggested that leachate be collected and
stored, or the frequency and intensity of leachate recycle reduced, until the
.onset of rapid methane production could ibe promoted.
Pohland110 found that less nitrogen and phosphorus, essential nutrients for
the growth of biological populations, were present in recycled leachate due to
the enhanced opportunity for distribution and utilization provided by this
management strategy. In addition, residual contaminant concentrations in
stabilized leachate were greatly reduced or completely removed due to the
implementation of leachate recirculation, adjustment of the internal physical-
chemical environment, and opportunities for precipitation, sorption, filtration
and complexation within the landfill system.™ Moreover, during the Final
Maturation Phase of landfill stabilization, humic-like substances tend to persist
and exert a remobilizing effect on heavy metals through complexation. Therefore,
leachate recirculation should be discontinued when the Final Maturation Phase is
reached, and the remaining leachate should be collected and removed for ultimate
treatment and/or disposal.0" ;
Few comprehensive reports of leachate recycle as an in situ treatment
option in the United States have been published in the available literature.
However, a demonstration project which may be considered to have been near full-
scale has been conducted in Mountain; View, CA."0) Six field cells were
24;
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' >t; ' ' ' •* *" V&"
constructed, each having an average volume-of #10,500 m3 and refuse mass of 4825
metric tons, and operated under different combinations of water content, sludge
seeding, nutrient levels and buffering. One of the six cells was operated using
leachate recycle, and results illustrated the benefits of pH and moisture control
and the Increased gas volume produced by the implementation of leachate
containment, collection and recirculation.
Information is also available concerning several field-scale landfills in
Germany where leachate recycle is being used.(IO) A two-stage approach was
employed, where leachate was removed from a new landfill section and recirculated
in an older section of the landfill. The results from this experiment indicate
that the two-stage approach could be used to obtain consistent quantities of
methane at minimum costs, since only a portion of the landfill would contain
leachate recirculation and gas collection systems. Investigations conducted by
the United States Environmental Protection Agency (USEPA) revealed that over 200
municipal solid waste landfills employ,leachate recirculation as a leachate
management method.°9 A full-scale leachate recycle study performed at the
Lycoming County, PA MSW landfill indicated that leachate recycle systems promote
a more rapid decomposition of organic waste, enhanced methane production, and
increased stabilization rate.08' The number of MSW landfill units employing
various leachate management strategies is provided in Table 9. These findings
suggest that leachate recycle is a viable option for design, operation and
control of landfill disposal sites.
TABLE 9. NUMBER OF MUNICIPAL SOLID WASTE LANDFILL UNITS BY TYPE OF
LEACHATE MANAGEMENT STRATEGY AND OPERATING STATUS®0
Type of Leachate Management
Practice3 .
Recirculate by Spraying
Recirculate by Injection
Recirculate by Other Means
Land Spreading
Truck to POTW
Discharge to Sewer to POTW
Discharge to Surface Water
Other or Unknown Off -Site Treatment
On- Site Biological Treatment
On- Site Chemical/Physical Treatment
Number of Landfills
Closed
40
10
11
15
48
53
28 '
5
41
34 •
Active
158
36
34
84
76
118
81
21
102
61
Planned
185
16
22
60
245
135
26
23
108
60
Note: aSome facilities have more than one type of leachate
management practice.
2.4 CODISPOSAL OF MSW AND HAZARDOUS WASTE
The disposal of solid wastes in landfills is the most common method of
management employed in the United States. Such landfill disposal has often
included both municipal solid waste and hazardous waste. The hazardous waste
codisposed with the MSW may have originated from industrial or commercial
25
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sources, and from households. The RCRA small quantity generator exclusion has
allowed limited amounts of hazardous waste to be codisposed with MSW. Codisposal
is, however, restricted by regulations based upon a classification system often
examining the behavior or the waste during laboratory testing. Accordingly, a
solid waste is defined as hazardous by the US EPA if it meets one or more of the
following criteria:0" exhibits ignitability, corrosivity, reactivity, or TCLP
toxicity as determined by standard tests; contains any of the toxic constituents
named on published lists as having toxic,'carcinogenic, mutagenic, or teratogenic
effects on humans or other life forms; or, is listed on prescribed lists.
However, a major problem exists in that laboratory test procedures are rarely
indicative of waste behavior in an actual landfill environment. For example, the
TCLP toxicity test employs a 24-hour leaching period, which may not be
representative of an actual landfill circumstance.
I
The subject of codisposal of hazardous waste along with municipal solid
waste has been studied extensively by Pohland and coworkers.<4-3°-M) This research
has indicated that landfills receiving municipal solid waste display a
significant ability to also accommodate hazardous waste, including heavy metals
and organic pollutants, up to a certain threshold. If this threshold is
exceeded, the waste will tend to interfere with or prolong the normal
stabilization processes occurring within the landfill environment. As long as
the hazardous waste loadings remain beloV this level, attenuation mechanisms are
often sufficient to compensate for the inherent toxic nature of the codisposed
hazardous waste.02' :
i
Research reported by Pohland and Gould02' included the development of four
simulated landfill columns operated with leachate containment, collection and
recirculation. Column 1 served as the bontrol column and contained 400 kg of
shredded municipal solid waste, while Columns 2, 3, and 4 received 33.6 kg, 65.8
kg, and 135.2 kg of metal plating sludge, respectively, along with 400 kg of
shredded MSW. Leachate COD and TVA were used to illustrate changes in organic
strength, and zinc (Zn) , cadmium (Cd) , nickel (Ni) , and iron (Fe) were chosen to
demonstrate the effects of the heavy metal loadings and interchange with the
leachate and waste matrix during the experimental period. The control column,
Column 1, produced leachate that was rapidly depleted of COD and TVA, which is
characteristic of normal refuse stabilization. Column 2 displayed a similar but
delayed pattern, suggesting that the corresponding metal sludge loading impeded,
but did not totally inhibit, the stabilization process. In contrast, both
Columns 3 ' and 4 exhibited behavior characteristic of inhibition. Because
relatively uniform concentrations of COD land TVA were maintained in the leachate
of the columns, failure in the normal process of waste stabilization was
indicated. Additionally, results of this experiment indicated that the delay or
inhibition of the natural stabilization process, as measured by COD and TVA,
occurred^in a cyclic fashion characterized by alternating periods of toxicit}^ and
acclimation. The concentrations of Zn, Cd, and Ni were barely detectable in the
control column, while a behavior characteristic of washout followed by
attenuation of mobilized metals was observed in Column 2 with regard to these
heavy metals. Similar behavior was also observed in Columns 3 and 4, but to a
lesser extent. The iron concentrations remained relatively high in all columns.
Iron, which was present in the codisposed metal sludge, was also produced in all
columns as a result of the corrosion of certain iron-containing refuse
constituents. i
Pohland and Gould03 also observed some elevation of the concentrations of
the codisposed heavy metals Zn, Cd, Ni, and Fe in the leachate during the last
or Final Maturation Phase of landfill stabilization. . This increase in
concentration was attributed to mobilization or remobilization of heavy metals
by complexation with humic-like substances produced as a result of the
degradation of more microbially-resis|tant organic materials that became
predominant during the Final Maturation Phase. The effects of humic-like
substances upon heavy metals include decreased toxicity and increased mobility.
26,
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A variety of mechanisms are considered responsible for the reactions that
occur within the landfill environment and affect the ability of the landfill to
attenuate codisposed heavy metals.?&>• Metal solvabilities in leachate increase as
pH decreases, thus, the highest metal concentrations should be observed during
the Acid Formation Phase when pH values are at a minimum. Secondly, heavy metals
can be precipitated as sparingly soluble salts by many inorganic anions. Sulfide
and hydroxide have wide precipitating capabilities, while others such as sulfate
and chloride will combine only with a limited number of heavy metals. Heavy
metals can also form complexes with many organic and inorganic ligands, thus
affecting their solubility. In addition, oxidation-reduction processes influence
metal speciation and behavior by both modifying the metal itself or transforming
other species in the landfill environment. Solid/solute interactions such as
adsorption, ion exchange, interactions with solid phase ligands, and heavy metal
attenuation in interstitial waters provide opportunities for attenuation of
codisposed heavy metals within the landfill. Finally, the mobility of dissolved
heavy metals will be reduced within localized microenvironments of higher pH and
alkalinity than occur in the landfill environment. In addition, sulfide or
carbonate encapsulation of solids within these microenvironments will also reduce
heavy metal mobility. As a result of these collective mechanisms, landfills
receiving municipal solid waste have been found to exhibit a capacity to remove
and minimize the mobility of heavy metals during the first four phases of
landfill stabilization.
In view of the opportunity for significant attenuation and reduction of
contaminant concentrations in leachate from landfills receiving both municipal
and industrial wastes, it is possible that rigid and ultrarestrictive regulations
concerning landfilling of such wastes may not be completely justified.00' The use
of leachate containment, collection and recirculation has not only been
demonstrated to accelerate stabilization processes and establishment of
microbially-mediated reducing conditions favorable for sulfide formation, but
also provides an in situ physical-chemical process for immobilization of heavy
metals and reduction of the potential for external environmental impairment.00
Therefore, MSW landfills have the ability to attenuate and detoxify certain
quantities of hazardous wastes without impeding the process of landfill
stabilization or causing adverse environmental impacts on the immediate
surroundings, and the remaining challenge is linking this ability to site-
specific landfill circumstances.
2.5 FUNDAMENTALS OF ANAEROBIC TREATMENT APPLICABLE TO LANDFILL SYSTEMS
Waste degradation and stabilization within a landfill proceeds primarily
under anaerobic conditions. Thus, the fundamentals of anaerobic waste treatment
which describe other anaerobic processes such as anaerobic digestion, are
applicable to landfill circumstance as well. Anaerobic treatment processes are
conventionally employed for the stabilization of complex sludges, industrial
wastes, and more recently, the treatment of dilute organic wastes. However, in
spite of the capabilities of anaerobic treatment processes, they have not enjoyed
a favorable reputation, largely due to lack of understanding of the associated
fundamental concepts and mismanagement of existing facilities, whether these
processes are utilized as suspended- or attached-growth systems.
Anaerobic biological treatment can have significant advantages over aerobic
biological treatment, including: low organism growth rates, which translates
into _.Low biomass or sludge production requiring ultimate disposal; low nutrient
requirements as a result of low organism growth rates; reduced energy cost,
largely because aeration equipment necessary for aerobic treatment is not
necessary; production of methane gas which can be purified and sold or used as
fuel; tolerance to high organic loadings; precipitation and detoxification of
heavy metals; potential for a high degree of waste stabilization; and,
biodegradation of volatile compounds otherwise partially stripped during aerobic
treatment. &
27
-------
The disadvantages of anaerobic treatment include: an incomplete
understanding of microbiology and some lack of prudent process application; the
sensitivity of the process to pH variations and the presence of high loadings of
toxic compounds; the necessity of longer solid retention times for stable
operation, thus requiring larger reactor, volumes; and generally slower start-up
and recovery from a shock or upset. :
A fundamental knowledge of the anaerobic waste treatment process is
necessary to the understanding of the progression of waste stabilization within
a landfill, because a landfill exists in an anaerobic state for most of its
active life. |
2.5.1 Microbiological Processes During Anaerobic Stabilization
During anaerobic stabilization, complex organic materials are converted to
methane and carbon dioxide by a variety: of microorganisms. The four steps of
anaerobic digestion, described in Section 2.2.2, include hydrolysis,
acidogenesis, acetogenesis , and methanogenesis . Each of these steps is performed
by a separate and distinct microbial population, and successful waste conversion
is dependant upon these microorganisms iperforming their respective functions
separately or in concert.
i
The first step, hydrolysis, involves the transformation of complex
insoluble organic material to less complex soluble material, a form which is then
readily available for microbial utilization. Hydrolysis is primarily
accomplished by extracellular, hydrolytic enzymes produced and excreted by the
bacterial population for this specific! purpose. However, not all organic
material can be hydrolyzed to simpler soluble .compounds capable of being
assimilated by bacteria because of structure, inaccessibility, and complex non-
hydrolytic linkages, among other factors."2" This would suggest that the overall
rate of stabilization can be limited by the ability to hydrolyze complex organic
material such as present in solid wastes!.
Acidogenesis is the formation of medium- and long-chain volatile organic
acids, such as propionic, butyric and valeric acids, from the hydrolysis
products. In addition, acetic acid, carbon dioxide and methane are also formed
during this step. Toerien and coworkers06* found that aerobic and facultative
anaerobic acidogens comprised less thanjone percent of the total acid-forming
bacterial population. Obligate anaerobic acidogens were found in numbers 100 to
200 times greater than aerobic and facultative anaerobic bacteria, confirming the
importance of the establishment and maintenance of anaerobic conditions for
successful waste conversion. j
Chynoweth and Hah07* have demonstrated that if soluble substrates, such as
glucose, are introduced into an anaerobic digestion process, at concentrations
in excess of those that can be readily metabolized, a shift in the biological
population towards organisms that are capable of converting the excess substrate
to acid end products will occur along with the accumulation of volatile organic
acids. This shift in the acidogenic pattern was shown to result from the sudden
selective growth of euryoxic bacteria normally present within anaerobic processes
in low numbers.
1
Acetogenesis is primarily the conversion of longer-chain volatile organic
acids to acetic acid, carbon dioxide and hydrogen. Two types of acetogenic
bacteria may be recognized: the hydrogen-producing acetogens which obtain energy
for growth by completely dissimilating; alcohols of greater complexity than
methanol, and volatile organic acids of !longer chain length than acetic acid,
into acetic acid, hydrogen and occasionally carbon dioxide; and, the hydrogen-
consuming acetogens which catabolize carbohydrates, hydrogen and carbon dioxide
or one-carbon compounds into acetic acid.o*' Hydrogen (or formate) has been shown
to play a significant role in regulating volatile organic acid production and
consumption as further described in Section 2.6. In order for these reactions
28;
-------
to become energetically favorable, the partial pressure of hydrogen must be ,
maintained at low levels by hydrogen-utilizing bacteria, such as the carbon ;
dioxide-reducing methanogens andshydrogen-utilizing acetogens. ;
The final step, methanogenesis, completes the anaerobic waste conversion
with the ultimate production of carbon dioxide and methane. Three groups of
methanogenic bacteria can be recognized: aceticlastic bacteria which produce !
methane and carbon dioxide through acetate cleavage; carbon dioxide-reducing ;
methanogens which utilize hydrogen to reduce carbon dioxide to methane; and a
final group of bacteria that utilize formic acid and methanol to produce methane.
As indicated in Figure 2, approximately 72% of the methane formed in the
anaerobic digestion of wastewater sludges originates from acetate cleavage, and i
the remaining 28% of the methane is produced by carbon dioxide reduction, with -
13% of the 28% originating from propionic acid and 15% of the 28% coming from
other intermediates.03' ;
2.5.2 Volatile Organic Acid Production During Anaerobic Stabilization
_Unbalanced stabilization can result from a rapid change in temperature,
organic loading, or the addition of substances that are toxic to microorganisms i
responsible for waste conversion. Indications of unbalanced stabilization or
digestion include increased volatile organic acid (VOA) concentrations, increased '
percentage of carbon dioxide in the gas phase, decreased pH, and a decrease in '.
waste stabilization as evidenced by reduced methane production."3' According to ;
McCarty and coworkers, Ol'a) the buildup of volatile organic acids is the result, '
not the cause, of unbalanced digestion. During unbalanced digestion, the main !
volatile organic acids that accumulate include acetic, propionic and butyric
indicating that the organisms responsible for their conversion are perhaps the
most sensitive to environmental change.02' If the increase in VOA concentration '•
exceeds the system buffering capacity, pH will decrease as the excess volatile j
organic acids ionize and increase the concentration of hydrogen ions in solution
The increased levels of volatile organic acids could be the result of: the
nutritional inadequacy of the acetogens responsible for their conversion to
acetic acid, carbon dioxide and hydrogen, or a nutritional deficiency in the
hydrogen-utilizing methanogens which must maintain low hydrogen levels to promote
energetically favorable conditions for the conversion of higher volatile organic
acids.03' Pohland and Suidan,0" in discussing pH stability in anaerobic systems,
noted that increased amounts of carbon dioxide gas were produced during
unbalanced digestion conditions at low pH due to inhibition of both hydrogen-
utilizing and acetate-utilizing methanogens and a release of "stored" carbon
dioxide as the pH was decreased. ;
Acetic acid, the most prevalent acid occurring during anaerobic digestion, :
is formed directly from fermentation of proteins, carbohydrates and fats, and
also as an intermediate in the fermentation of longer-chained volatile organic
acids. Propionic acid is formed primarily from carbohydrates, but is also '
produced from proteins containing odd-numbered-carbon amino acids and butyric '
acid is formed during the degradation of proteins and fats.<"°> Jeris and .
McCarty<41' suggested that a sudden increase in the fat concentration entering an
anaerobic digestion process could cause a backlog of volatile organic acids until
the proper microorganisms were present in sufficient numbers to facilitate their ;
removal. ;
Acetogenic bacteria are responsible for the conversion of longer-chained
volatile organic acids to acetic acid, carbon dioxide and hydrogen. B-oxidation, :
consisting of the separation of two-carbon groups from an even-numbered-carbon
volatile organic acid until acetic acid remains, has been shown to be the major '
mechanism of degradation of medium- and long-chain even-numbered-carbon volatile
organic acids.<4I> If the medium- or long-chain VOA is comprised of an odd number !
of carbon atoms, ^-oxidation will proceed until propionic acid remains, which
then is converted to acetic acid by a-carboxylation.<<2> Pohland and Bloodgood<°>
presented evidence that the mechanism for degradation of even-chained volatile
29
-------
organic acids was less affected by adverse environmental changes . than the
degradation of odd-chained volatile organic acids. The conversion of selected
volatile organic acids to methane is shown in Table 10, along with the associated
free energy changes descriptive of relative thermodynamic favorability.
TABLE 10. VOLATILE ORGANIC ACID (VOA) CONVERSION TO METHANE3
Reaction i
I
AG0
(kJ/Mole)
Conversion of Higher VOA to Acetic Acid
Propionic Acid
CH3CH2COO- + 3H,0 ^ CH3COO" + HCO^ + SHj + H+
Butyric Acid
CH3(CH2)2COO- + 2H20 ^ 2CH3COO' + 2H2 + H+
Valeric Acicl
CH3(CH2)3COO- + 2H/) •» CH3CHj'COO- + CH3COO' +
+ 2H2 +.H+
1
Hexanoic Acid
CH3(CH2)4COO- + 2H20 ~ 2CH3CH2COO- -*• 2^ + H*
CH3(CHj)4COO- + 2HjO •» CH3(CH2)2COO' + CH3COO' +
+ 2Hj 4- H+
CH3(CH2)4COO- + 4HJO »» 3CH3CpO' + 4H2 + 2H+
+76.1
+48.1
+48 . 1
+48.1
+48.1
+96.2
Conversion to Methane
Acetic Acid
CH3COO- + H20 »» HCOj + CH4
I
Bicarbonate
I
HCO; + 4H2 + H- ^ CH4 + 3H20
-31.0
-135.6
Note: "Conditions of 25°C, 1 atm, pH 7.
1
Propionic acid has been shown to be an important intermediate produced
during the anaerobic digestion of complek materials. According to Pohland and
Bloodgood,<43) the mechanism for propionic acid degradation is the most sensitive
to retarded conditions. In an experiment by McCarty and Brosseau,®" four
laboratory digesters were initially seeded with digested sludge from a, municipal
treatment plant, and were fed raw primary sludge diluted with an equal volume of
tap water on a daily basis. Removal of digested sludge also occurred daily. The
four digesters were spiked with 6000 mg/L (as acetic acid) of either acetic,
30
-------
propionic or butyric acids that were neutralized to pH 7 using lime. One
digester served as the control and received np acid addition. The results of
this experiment indicated that the digesters receiving acetic and butyric acids
exhibited increased gas production over the control digester, and the excess
acids added were rapidly utilized. The highest gas production occurred in the
digester receiving butyric acid. Alternatively, the added propionic acid had an
initial inhibitory effect on gas production that was attributed to a lack of
acclimation of the bacterial population. During this period of inhibition, no
other volatile organic acids were produced, and the propionic acid concentration
decreased proportionally with the theoretical washout curve, thereby suggesting
that the organisms producing the volatile organic acids rather than those
responsible for their decomposition were inhibited.
A second experiment was performed by McCarty and Brosseau*2" using four
digesters, one control and three others spiked with a propionic acid
concentration of 3000, 6000, or 8000 mg/L as acetic acid, respectively. The
added propionic acid was neutralized to pH 7 using lime. These results indicated
that addition of propionic acid resulted in decreased gas production when
compared to the control digester. As the added propionic acid concentration
increased, longer times were required for the recovery of gas production. This
suggested that, because volatile organic acids did not accumulate and gas
production declined when propionic acid was added, the introduction of propionic
acid inhibited complex organic material degradation to both propionic and acetic
acids. This is illustrated as Pathway A in Figure 3. Gas production was reduced
because the major substrate source for methanogens, acetic acid, was not
produced.
Research by Gorris and coworkers<44) has shown that an increase in organic
loading affects propionate conversion more than butyrate conversion. The
conversion of propionate was almost completely blocked at acetate concentrations
greater than 500 mg/L, strongly inhibited at concentrations of 200 to 500 mg/L
of acetate, and moderately inhibited at concentrations less than 200 mg/L of
acetate. In addition, efficient propionate degradation only occurred after
acetate levels less than 200 mg/L had been maintained for a sufficient time
period. This information is consistent with the reports of Kaspar and Wuhrmann<4S)
in which high acetate concentrations and a high carbon dioxide partial pressure
were found to inhibit propionate degradation by acetogenic bacteria.
Research by McCarty and coworkers"8-*' suggested that identification of
individual volatile organic acids during upset digestion conditions yields clues
as to the reason for the unbalance. An experiment was performed using four six-
liter digesters, each fed one of the following acids, along with the necessary
inorganic nutrients, as the sole substrate source at the rate of 1 g/1 per day
formic acid, acetic acid, propionic acid or butyric acid. The digesters, seeded
with strained and diluted (1:1) digested sludge from a municipal digester had a
30-day detention time and were mixed by gas recirculation. After the digesters
were_ operated for a period of at least two months to purge them of a majority of
the initial seed material, the substrate additions were suddenly increased to 1 5
g/1 per day, which resulted in increased VOA concentrations within the digesters.
Daily analysis of volatile organic acids was performed. In addition, 40 ml of
active sludge from each digester was removed and mixed with 10 ml of solution
containing the sodium salt of the different volatile organic acids in a 125-ml
Warburg flask. The gas production from each acid salt was then measured
manometrically. The results of this experiment are shown in Figure 4 These
results indicated that the formic acid sludge, the sludge developed on formic
acid, was not able to use any other acids. A sudden increase in formic acid
concentration resulted in a rapid increase in its utilization and essentially no
buildup of volatile organic acids. Formic acid was also found to be rapidly
utilized by all four sludges developed in this experiment, however, no formic
acid was ever detected in the volatile organic acids obtained from1 the digestion
units. These results indicated that either formic acid was an important
31
-------
ro
AGIO
c>
V
ACETIC
ACIO
AGIO
FORMATION
METHANE
FORMATION
Figure 3. Generalized Pathways o'f Anaerobic Treatment of Complex
Wastes. <2l> ,
so —
so
en
or
Ul
z
s 20
a
£ 0
0
80
so
1 1
PORMIC
• SI.UOGS
PORMATc
HtGHER
a CONTROI.
ACETIC ACIO
SLUDGE
ACETATE
HIGHER ACIDS
PROPIONIC AGIO
SLUDGE
BUTYRIC ACIO
SLUDGE
PROPIOKATS
8UTYRATS
ACSTATS
ACETATE
PRQPIONATE
/ a CONTROL
ft i
20 4O so •• so Q so 40
TIME IN HOURS
so- ao
Figure 4. Volatile Acid Utilization by Volatile Acid Sludges. <*°>
32
-------
Intermediate in the methane fermentation of all substrates and is readily ;
fermented under most conditions, or sludge adapted to all other substrates was !
simultaneously adapted to formic acid. ;
i
The results also indicated that acetic acid sludge could only utilize '
formic acid and acetic acid. A sudden increase in the acetic acid concentration !
produced a gradual buildup of acetic acid. In addition, propionic acid sludge :
not only had the ability to ferment propionic acid, the intermediate acetic acid
and formic acid, but was also able to ferment butyric acid at a high rate
Moreover, all higher volatile organic acids through caproic acid could be ,
utilized at a rate less- than butyric acid. A sudden increase in the propionic
acid concentration yielded not only a buildup of propionic and acetic acids but '
several higher acids from butyric through caproic as well. These latter results
are shown in Figure 5. When the, propionic acid feed to the digester was reduced !
to the original rate of 1 g/1 per day, the volatile organic acid concentration
decreased and the higher acids disappeared.
Figure 5 also indicates that the butyric acid sludge' was capable of
utilizing all even-carbon volatile organic acids, but not propionic acid
Valeric acid, a five-carbon acid, was studied and found to split into acetic and '
propionic acids. The acetic acid was further fermented, while the propionic acid '•
remained in solution. The six- and eight-carbon volatile organic acids were i
utilized at a lower rate than butyric acid. An increase in the butyric acid
concentration in the digester yielded a buildup of butyric and acetic acids, as '<
well as the next higher even-numbered-carbon acid, caproic (hexanoic) acid. '
^ The combined results of these experiments indicated that, in the case of !
propionic and butyric acids, at high volatile organic acid concentrations, not '
only did the acids appear which are believed to be major intermediates, but also
other "side acids" were formed. These "side acids" were considered produced '
during the conversion of the major acids and occurred in lower concentrations '
than the primary acids. It was speculated that the reason butyric acid was '
utilized by propionic acid sludge was because butyric acid was also synthesized ;
by propionic acid sludge. Moreover, during propionic acid fermentation, the
synthesis of all higher acids through caproic acid was explained as synthesis of '
combinations of propionic acid and acetic acid, i.e., butyric acid can be formed '
by joining two acetic acid molecules, valeric acid is composed of one propionic
acid and one acetic acid molecule, and caproic aciji can be synthesized by ioinine
either two propionic acid molecules or three acetic acid molecules. ,
A second experiment was performed by McCarty and coworkers"*'"0' in which
sludges were developed on octanoic and palmitic acids to represent intermediate-
and long-chain volatile organic acids, respectively; glucose, starch and
cellulose to represent carbohydrates of differing complexity; nutrient broth and
leucine to represent proteinaceous material; and sewage sludge to represent a :
complex organic mixture. The organic loading to the digesters varied from 0 3 !
to 1.0 g/l_per day, and the digesters were operated on a batch basis with a 15- '
day detention time and had approximately 0.75-1 capacity. An analysis procedure i
similar to that used in the previously described experiment by the same ;
researchers was employed.
Fermentation of the 8-carbon octanoic acid and 16-carbon palmitic acid are
shown in Figure 6. These acids were degraded by ^-oxidation into other even-
carbon volatile organic acids. All of the even-numbered-carbon volatile organic
acids produced were fermented at a rate slower than butyric acid.
The results of the carbohydrate sludges are shown in Figure 7 along with ;
the results of ethyl alcohol, which is a common intermediate2 in carbohydrate
fermentation. The results for the three carbohydrates are very similar, and the
relative rates of acetic and propionic acid utilization were much the same The
ffSiJ^i ilatl10S ? butyric acid by the starch and cellulose sludges possibly
signified that butyric acid is a significant intermediate in Carbohydrate :
33
-------
2500
2000 =
ISOO —
IOOO —
500 U-
=i 0
5
O
_j 40OO
o
5
3 COO
aooo
IOOO
Tgm/«/d|
BUTYRIC AGIO SLUDGE
t
I Iqm/Vd L
No Feed
ACETIC
•/
\
(i
\
BUTYRIC
CAPROIC
74 76 78 SO 82 <
i CAY Qf OPERATION
ae aa
Figure 5. Volatile Acid formation During Excessive Loading of
Propionic and Butyric Acids.<*ti>
34
-------
e
£ 80
S 40
o
1"
I I I i
OCTANOIC AQO
SUUOCc
III!
PAOHTIC AGIO
SLUDGE
*0
ACiTATS
CONTSOU 4
.PROPIONATE
I/ I '
SO
120 160 0
TlUg IN HOURS
4O
ao
Figure 6. Volatile Acid Utilization by Fatty Acid Sludges.
(to)
a
o
«0
so
40
20
SO
40
20
csu.ut.oss
SUUOGS
8UTTRATS
ALCOHOU
suuooe
ACSTATS
8UTYRATS
SO SO 0 2Q 4Q
TIME IN HOURS
Figure 7. Volatile Acid Utilization by Carbohydrate Sludges.<*0)
35
-------
fermentation. However, no utilization of butyric acid was found to occur in the
glucose sludge. These results were contradictory to the studies on propionic
acid sludge, but the glucose sludge exception may have been the result of the
presence of different propionic acid organisms in the glucose digester than in
the starch, cellulose and propionic acid sludge digesters. Thus, the authors
concluded that butyric acid and higher > acids are not important in carbohydrate
metabolism, and acetic and propionic acids are the major acids formed from
carbohydrate fermentation.
Figure 8 shows the results for the protein sludges. Acetic acid was the
primary acid formed in protein degradation. The butyric acid utilization rate
was also quite rapid, perhaps suggesting the importance of 6-oxidation in
converting amino acids to acetic acid. Propionic acid was also utilized at a
lower rate than acetic acid or butyric acid, suggesting that propionic acid may
be formed as an intermediate in the degradation of odd-numbered-carbon amino
acids present in proteins.
The results for the digested sewage sludge are presented in Figure 9. The
relative rates of utilization of acetic and propionic acids were similar to those
determined for the protein sludges. However, the butyric acid utilization
started slowly and increased significantly after about two days. Valeric and
caproic acids were also utilized by this sludge.
Figure 10 shows the combined results of both experiments. The acids
present in the various sludges are indicated by circles. Those acids that
frequently occurred as major acids are indicated by large circles, and those
which appeared, but always in lower concentrations as "side acids", are indicated
with small circles. If an acid was utilized by the sludge as well, the circle
was shaded. In most cases, acids found were also utilized, and acids which were
not^found were not utilized. The main volatile organic acid intermediates formed
during methane fermentation of proteins; carbohydrates and fats are illustrated
in Figure 11.
McCarty and coworkers concluded the followingi'18-*" acetic and propionic
acids are the most important volatile acids frequently occurring under unbalanced
digestion conditions; acetic aci'd is the most prevalent volatile acid formed
during the methane fermentation of carbohydrates, proteins and fats; propionic
acid is an important intermediate acid fprmed during the methane fermentation of
carbohydrates and proteins; other minor volatile acids frequently occur in low
concentrations as a result of backup orj side biochemical reactions, but are an
unimportant result of upset conditions; formic acid is utilized rapidly by
sludges developed on many different substrates, but usually is not present in
high concentrations during upset conditions; and butyric acid is utilized rapidly
by most sludges, but probably does not occur as a true volatile acid intermediate
in methane fermentation. Experiments indicated that whenever ^-oxidation is
involved in substrate degradation, and in some cases where it is not (propionic
acid fermentation), butyric acid was utilized. Therefore, the fact that butyric
acid may be utilized by a sludge cannot! be considered evidence indicating that
butyric acid is a true intermediate. N*-"0':
Research has also suggested that the oxidation-reduction potential of an
B11SJ?tfHi?f^S?.t:s the type of v°latile organic acids present. Guenzi and
«> studied the VOA concentration in a cattle manure slurry as a function of
oxidation-reduction potential (Ek) . The controlled redox system was sequentially
lowered in 100 mV increments from +300 to -200 mV, and the desired E. was
"fftn 5ed«^ titratinS with oxygen. Pupe oxygen was used to maintain the K, at
+300 and +200 mV, 20% oxygen was used to maintain the E^ at +100 mV and 0 5%
oxygen was used to maintain the E,, at 0,! -100, and -200 mV. The experiment was
conducted at a temperature of 35°C. Individual volatile acid concentrations of
the suspensions were determined and are; presented in Table 11. After a 7-dav
incubation period at +300 mV £„, acetic acid was the only acid detected As the
oxidation-reduction potential was lowered to -100 mV, acetic acid concentrations
increased and propionic acid was detected followed by iso-butyric acid. When the
36
-------
NUTRIENT BROTH
SUIOGS
*O 80 120 160 0 ZO 4O SO 80
Figure 8. Volatile Acid Utilization by Protein Sludges.(:
(40)
80
en
as.
u
4C
u
o
20
ACSTATc
1 t
DIGcSTED ScWAGE
SLUO<3£
. 8UTYRATS
PROPIONATi
4O 6O
TTME IN HOURS
100
Figure 9. Volatile Acid Utilization by Sewage Sludge.(40)
37
-------
TYPE OF
SLUDGE
ACETIC PROPIONIC BUTYRIC HIGHER
Fatty Acids
Acetic
'Prop ionic
Butyric
Octanoic
Palmitic
Proteins
Nutrient Broth
Leucihe
Carbohydrates
Glucose
• Starch
Cellulose
Ethyl Alcohol
e
•
•
0
o
o
0
o
•
•
Key: -Present as Major Acid -Present as Minor Acid
Shaded Circles - Acid Utilized by Sludge
Figure 10. Volatile Acids Formed during Digester Unbalance
Compared with Those Utilized by Sludges/*03
Figure 11. Volatile Acid
Proteins,
Intermediate
Carbohydrates
38
.__ in Methane Fermentation of
and Fate.<*0)
-------
TABLE 11. FREE VOLATILE ORGANIC ACID CONCENTRATIONS IN A MANURE SLURRY
WITH CONTROLLED OXIDATION-REDUCTION POTENTIALS*461
Time
(Days)
1 hr
.7
8
14
15
21
22
28
29
35
36
42
45
ORP
(Efc.
mV)
+300
+300
+200
+200
+100
+100
0
0
-100
-100
-200
-200
-200
Acetic -
Acid
(mg/g)a
3.82
0.40
0.35
0.18
0.19
0.16
0.17
0.16
0.54
2.46
2.98
9.12
19.30
Propionic
Acid
(mg/g)a
0.40
_b
.
.
.
_
•
_
0.07
0.30
0.34
1.16
2.30
Iso-Butyric
Acid
(mg/g)a
0.05
.
«
.
.
.
.
.
.
0.07
0.08
0.12
0.09
Butyric
Acid
(mg/g)»
0.63
»
—
.
m
—
_
^
.
0.02
0.09
0.21
Note: f(mg of acid)/(g of manure)
''None detected.
redox potential reached -200 mV, butyric acid was detected, and acetic,
propionic, butyric and iso-butyric acids accumulated in that order, respectively.
The rate of acid formation was considerably higher at -200 mV than -100 mV,
except for iso-butyric acid which began forming at -100 mV E^, to -200 mV E,,, but
remained consistently low. Decreasing the oxidation-reduction potential to
around -300 mV would promote the conversion of volatile organic acids to methane
and carbon dioxide.
2.5.3 Me thano gene sis
Methane fermentation occurs by two primary mechanisms: cleavage of acetic
acid to yield methane and carbon dioxide; and the reduction of carbon dioxide
using hydrogen as the energy source. Research has shown that approximately 70
to 73% of the methane produced comes from acetate cleavage, and the remaining 27
to 30% originates from carbon dioxide reduction.(12-41-47-") Table 10 and Figure 2
indicate the chemical transformations that occur and the generalized pathways for
me thane-fermentation of complex wastes, respectively. As indicated in Table 10,
one mole of methane gas is formed from one mole of acetic acid, and one mole of
carbon dioxide produces one mole of methane gas. Research suggests that
hydrogen-utilizing methanogens are more numerous than acetic acid-utilizing
me thano gens,(49) although a greater amount of methane is formed via acetic acid
cleavage. In addition, the formation of methane by acetic acid is less
energetically favorable than methane formation from carbon dioxide, as indicated
by the comparison of AG values presented in Table 12.
According to Smith and Mah,(4»> acetic acid is metabolized by an organism
not involved in its production, as evidenced by the large extracellular pool and
rapid turnover rate. Therefore, methane production is a function of the
community of organisms involved in waste conversion, not just the actual
methanogens. Within this consortia of organisms, some require symbiotic
relationships for growth and proliferation.
39
-------
a:
0
• 5
-------
2.6 THE EFFECTS OF HYDROGEN ON ANAEROBIC STABILIZATION
*'i
Hydrogen is produced by acidogenic and acetogenic microorganisms present
in anaerobic systems, and is used by methanogens during carbon dioxide reduction
to methane. Although methane production from hydrogen is not the rate limiting
step in complex waste conversion,"" this process is crucial because the partial
pressure of hydrogen must be maintained at values low enough to permit otherwise
thermodynamically unfavorable reactions to occur. Some redox half-reactions are
presented in Table 12 and suggest that the degradation of propionate and butyrate
is energetically unfavorable at high hydrogen partial pressures.<»-3» Harper and
Pohland(SO) demonstrated that hydrogen at partial pressures greater than 10"4
atmospheres leads to accumulations of propionic and butyric acids, and inhibits
their oxidation by obligate hydrogen-producing acetogens.
As reported by Smith and McCarty,021 longer-chained volatile organic acids
were produced when a continuously stirred tank reactor (CSTR) was perturbed with
hydrogenic substrate. The hydrogen partial pressure increased due to the rapid
utilization of the high-energy substrate, causing the formation of longer-chained
volatile organic acids to become thermodynamically favorable. Smith and
Mccarty^ also found that when the partial pressure of hydrogen declined, the
direction of the equilibrium changed to again favor ^-oxidation and thes
dissipation of longer-chained volatile organic acids. The longer-chained
volatile organic acids produced may be a normal catabolic product of an organism
present at low concentrations that multiplies rapidly when perturbation.creates
favorable growth conditions. The same researchers found that high molecular
weight volatile organic acids were not formed in perturbed acetate enrichments,
because hydrogen is not liberated during acetate cleavage.^
Kaspar and Wuhrmann(47) observed that an increase in the hydrogen partial
pressure caused a linear accumulation of propionate and an accelerated decrease
in acetate concentrations. Heyes and Hall'5" indicated that hydrogen affects the
distribution of acids produced from glucose by inhibiting the formation of Hj
from NADH. This concept, was also presented by Mosey,(S3) as shown in Figure 12.
When, this reaction is inhibited, acidogens form more propionate and butyrate as
electron sinks. Acidogens still produce hydrogen even when the NADH reaction is
inhibited by the cleavage of ferredoxin and pyruvate.
Harper and Pohland<30) have reported that only two types of methanogens have
been proven capable of acetate cleavage. These include the hydrogen-oxidizing
aceticlastic methanogens (HOA) and the non-hydrogen-oxidizing aceticlastic
methanogens (NHOA). HOA are apparently capable of three times the acetate
utilization rate of NHOA at acetate concentrations above several hundred mg/L.
Also, at hydrogen partial pressures greater than 10"4 atm, HOA use hydrogen and
carbon dioxide in favor of acetate, whereas acetate cleavage by NHOA is
unaffected by hydrogen partial pressure.
Research has shown that both sulfate- and nitrate-reducing bacteria,
preseSitLin anaerobic systems, compete with methanogens for hydrogen and acetic
acid.*30-*" Nitrate reduction is the most energetically favorable reaction,
followed by sulfate reduction and, lastly, methane formation from both acetic
acid cleavage and carbon dioxide reduction.'50^
Boone and coworkers(5S) suggest that formate, not t^, is used by some
hydrogen-utilizing methanogens for the reduction of carbon dioxide to methane,
and thus may be an important factor in mechanisms regulating the symbiotic
relationships between anaerobic microorganisms. In most of the co-cultures
studied, the methanogenic partner was able to utilize either formate or hydrogen
as the electron acceptor, therefore, it is not clear which is the interspecies
electron carrier. Free energy changes for syntropic propionate and butyrate
oxidation coupled to methanogens via interspecies hydrogen or formate transfer
are shown in Table 13. Boone and "coworkers**8 calculated the potential hydrogen
and formate diffusion between microbes and found that at hydrogen concentrations
41
-------
AOP ATP
glucose6 phosphate • : ' ' — gtUCOSS
ATP
AOP
" ' fructose 1:6 diphosphate
Zfacetyl CoAJ-
L
2NAOH.2H
2NAQ"—
•ZHSCoA
2 (gtyceraldehyde phosphate!
?taflcian*f
2NAOH .2H°—
2(1:3 diphosphoglyceratel
: • J.—-tAOP
:H$GA H^o-4— tArp
ruvic acidl
•2H"T^U
atel I
2"
2H-
,— {.NAOH^CH
•NAQ'-J
CNAQ
2[ acetic acidl butyric acid
2(propionic acid]
Figure 12. Glycolytic Pathway for Glucose Metabolism.<53)
-------
I
5
2s'
§5;
0**
o o
S-
ill
°r,0
'„§*
+ + c3
§§•0
+ 00
Sgg
o _r^>
O I
a* C
+ CM
o '
O §
* o
.* °
o _
C^CJ
c^o
o
S
%
= =
+ +
3
43
-------
commonly found in nature, hydrogen could pot diffuse rapidly enough to dispersed
methanogenic bacteria to account for the rate of methane synthesis, however,
formate could.
2.7 THE EFFECTS OF HEAVY METALS ON METHANE PRODUCTION AND HYDROGEN LEVELS
Hickey and coworkers(56) found that the addition of heavy metals to an
anaerobic system inhibited methane production. This decrease in methane
production was not considered necessarily indicative of inhibition of the
methanogens, because an accompanying buildup of volatile organic acids at levels
greater than the control was not observed. These results suggested that the
added heavy metals interrupted the substrate flow at other points in the complex
waste degradation sequence other than the terminal or methanogenic reaction.
Hickey and coworkers®® also observed that samples inhibited by heavy metals
showed a decreased rate of hydrogen accumulation in the headspace with increased
inhibition of methane production. When complete inhibition of methane production
occurred, no hydrogen accumulated in jthe headspace, thus, other organisms
involved in the conversion of complex waste to methane were at least as severely
inhibited as the methanogens. Because the decrease in methane production rate
was not accompanied by a concomitant accumulation of volatile organic acids, it
could be presumed that either all acetic acid produced was consumed, or no acetic
acid was produced. This suggested that the acetate catabolizing population was
inhibited by the added heavy metals. Thus, the authors postulated that the
reduced hydrogen accumulation was a' consequence of the added heavy metals serving
as electron acceptors. '
Hickey and coworkers<5*) also indicated that the most significant
consideration of heavy metal toxicity is the operating solids level. Higher
total or volatile solids yielded better^protection from heavy metal toxicity,
because heavy metals could be bound by ligands contained on the cell membrane and
extracellular polymer matrix. Complexation of heavy metals in the aqueous phase
by soluble ligands and sulfides is a major factor determining the level of
inhibition that occurs, as discussed previously in Section 2.2.2.
Collectively, these citations provide a basis for examining and
understanding the microbially-mediated anaerobic processes of waste stabilization
with intermediate formation and gas production in the landfill environment, a
natural system analogous to controlled anaerobic digestion systems. Therefore,
application of principle^ of understanding drawn from these processes should
benefit the diagnosis and further development of controlled landfill systems.
44
-------
SECTION 3
MATERIALS AND METHODS
i v P16 Pr°cess of landfill stabilization, including concomitant changes in
leachate and gas quantity and quality, was investigated under controlled
operating conditions with simulated landfills columns constructed at the Georeia
S^1^6 °f.Technology. In order to assess the progression of the microbialfy-
mediated stabilization events, 10 pilot-scale simulated landfill columns were
prepared and operated as identical pairs. Five columns were operated in the
single pass'leaching mode and the remaining five columns were operated with
leachate containment, collection and recirculation.
„„,,. £1:L ifive Pfirs received equal quantities of shredded municipal refuse. One
£«£ I C°}^fS (one.slnSle,P.afs and °ne recycle) served as controls and received
™iL£c «/ muni°ipal solid waste. The remaining four pairs received equal
amounts of selected organic priority pollutants. Three pairs of columns
containing organic pollutants also received incremental loading? of heavy
--
., - - codisposed with
14 summarizes the loading
3.1 SIMULATED LANDFILL DESIGN
1- *f¥te de.sign features characteristic of the single pass and recycle simulated
landfill columns are presented in Figures 13 and 14, respectively. Slffluj-acea
3.1.1 Steel Jacket
ri/8 f S^ulated.landflll1c°l^ns were fabricated using two sections of 3-mm
,5^rf"f,?i ' A0.'1g1auge) c.°ld-rolled steel. The lower sections had been previously
used in landfill experiments at the Waterways Experiment Station (Vicksburg MS)
aedtSe UPP^ Sfctl0ns were specially constructed to match and extend th! 'lower
KamtSr'of^Tg mWM ^^T consis^\d of a 1-83-m (6-ft) long cylinder with I
oiameter of 0.9-m (3- ft) and a curved bottom mounted on 0.6-m (2 -ft) lone less
3Kf«gSinnr-e f??r.lcfted from 7-6'cm 0-In) angle iron with 6-im (0 25-iI) bSe
fi^3:,10?1"",^;111) square. A coal, tar-base paint had been applied to the
column L?fa he ^Wer sectl°ns to Prevent corrosion and/or leaching from the
column surface. The upper column sections were 0.9-m (3-ft) diameter cylinders
1.2-m (4- ft) long. The interiors of the upper sections were notppainted '
"PPSr find lQWer column segments were joined together by bolting through
*
prlor to
bottom
45
-------
TABLE 14. SIMULATED LANDFILL COLUMN LOADING AND OPERATION
Column
Number
1
2
3
4
5
6
7
8
9
10
Column
Identity3
CR
CS
OS
OLS
OMS
OR
OLR
OHS
OMR
OHR
Operation
Recycle
Single Pass
Single Pass
Single Pass
Single Pass
Redycle
Recycle
Single Pass
Recycle
Recycle
Organics
No
No
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Inorganics
No
No
No
Low
Medium
No
Low
High
Medium
High
Note: "Column Identity
CR Control, Recycle '
CS Control, Single Pass
OS Organics, Single Pass
OLS Organics, Low Inorganics, Single Pass
OMS Organics, Medium Inorganics, Single Pass
OR Organics, Recycle f
OLR Organics , Low Inorganics , Recycle
OHS Organics, High Inorganics, Single Pass
OMR Organics , Medium Inorganics , Recycle
OHR Organics, High Inorganics, Recycle
bulkhead fitting was connected directly to the leachate sampling system by means
of a 0.9-cm (3/8-in) polypropylene tube tOipermit the collection of leachate from
above the HOPE liner as well as between the liner bottom and steel column
jacket.)
3.1.2 Liquid Addition/Distribution System
The single pass columns were designed to permit the addition of water
directly to an internal distribution system through a 2.5-cm (1-in) PVC ball
valve. The liquid distribution system was comprised of a six-armed pipe array
and was constructed of 3.8-cm (1.5-in) perforated PVC pipe containing 0 9-cm
(0.375-in) holes. ;
I
The recycle columns contained the same liquid distribution system as the
single pass columns, with the exception that a leachate recirculation pump was
joined to the liquid addition system by a 2.5-cm (1-in) PVC ball valve, a section
of 2.5-cm (1-in) PVC pipe, and a one-way check valve to prevent the backwards
flow of leachate. The leachate recirculation pump was also connected to the
leachate sampling/collection system as described in Section 3.1.4.
3.1.3 Leachate Recirculation System
The leachate recirculation system included a liquid addition/distribution
system, a leachate recirculation pump, a site glass system, and a leachate
sampling/collection system. The site glass system consisted of a 2.5-cm (1-in)
46
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transparent PVC pipe connecting the leachate sampling/collection system and gas
measuring/sampling system. The site glass was used for determining leachate
volume within the column, and was equipped with a liquid level controller that
could be used to automatically recirculate leachate as it accumulated. The
leachate recirculation pump was equipped with a 1.9-cm (0.75-in) PVC in-line
strainer, mounted on the intake side to protect the pump from any abrasive
particles that may be contained within the leachate.
i
3.1.4 Leachate Sampling/Collection System
The leachate sampling/collection system consisted of four 2.5-cm (1-in)
ball valves connected to a 2.5-cm (1-in) PVC cross. The first ball valve was
connected to the leachate collection sysjtem and served to isolate the external
piping system from the internal, leachate collection system if repairs and/or
modifications were necessary; the second^ball valve was used to facilitate the
collection of leachate samples for analysis; and the third ball valve was
connected to the site glass system previously described. A fourth ball valve was
attached to the leachate recirculation pump on the recycle columns and was not
functional on the single pass columns.
3.1.5 Gas Measuring/Sampling System ;
The gas measuring/sampling system was connected to a set of PVC flanges
located at the top of the column by a 6-mm (0.25-in) tube fitting mounted between
two,_25-cm (1-in) PVC ball valves. The tube fitting was attached to a tee
fitting. One arm of this tee fitting was |used as a sampling port, and the second
arm was connected to a water-sealed U-tube to prevent air from entering the
column head space. The water-sealed U-tube was connected to a volumetric gas
meter. i
3.1.6 HOPE Liner
High density polyethylene (HDPE) liners of 40-mil thickness (Poly-America,
Inc.) were specially fabricated in a-cylindrical shape with a heat-welded flat
bottom to facilitate leachate containment and eventual refuse removal and
analysis at the conclusion of these investigations. The liner also served to
prevent direct contact between the refuse/leachate mass and the interior of the
steel column jacket. The bottom center of the HDPE liner was fitted with a
section of 3.8-cm (1.5-in) PVC pipe mounted on flanges both inside and outside
of the HDPE liner. The 46-cm (18-in) outer pipe section penetrated the opening
in the steel jacket and permitted the transfer of leachate to the leachate
sampling/collection system. A 10-cm (4-in) long inner PVC pipe section was
perforated with 0.6-cm (0.25-in) holes arid equipped with a cap at the upper end
to facilitate the collection of leachate from within the column.
3.1.7 HDPE Liner Placement
Each steel column was filled to a
-------
3.2 SIMULATED LANDFILL LOADING
Shredded municipal refuse, ^primarily '"of ^residential origin, was obtained
from the DeKalb County, GA shredding facility. The refuse was divided into 9.1-
kg (20-lb) portions and placed into plastic bags. During this weighing and
bagging process, samples were obtained for the determination of refuse
characteristics, including moisture content, calorific value, ash content, and
percentage of carbon, hydrogen, and nitrogen. The results of these analyses are
provided in Table 15.
The 10 simulated landfill columns were loaded with a total of 42 individual
9.1-kg (20-lb) bags of refuse (as received) within a period of about eight hours.
The refuse was placed in the columns by loading five bags and manually compacting
the loose refuse using a series of weighted hand tampers. Compaction was
performed after the addition of each five bags [45 kg (100 lb)] of refuse to the
column.
The organic and Inorganic priority pollutants were simultaneously loaded
with, the refuse in the appropriate columns. The organic priority pollutants
added to Columns 30S (Organics, Single Pass), 40LS (Organics, Low Inorganics,
Single Pass), 50MS (Organics, Medium Inorganics, Single Pass"),, 60R (Organics,
Recycle), 70LR (Organics, Low Inorganics, Recycle), 80HS (Organics, High
Inorganics, Single Pass), 90MR (Organics, Medium Inorganics, Recycle), and 100HR
(Organics, High Inorganics, Recycle) included aromatic and halogenated
hydrocarbons, phenols, phthalate esters, and pesticides as indicated in Table 16.
The organic priority pollutants were added to a closed glass container and
completely mixed as a cocktail prior to their addition into the columns. After
the placement of 30 cm (12 in) of compacted municipal refuse, the organic mixture
was removed from the container and rapidly and uniformly distributed over the
exposed refuse_ surface. The container that held the organic pollutants was
rinsed twice with acetone, and the acetone rinsings were similarly distributed
over the surface of the refuse as well, prior to immediate addition of the next
refuse layer.
The inorganic priority pollutants added to Columns 40LS (Organics, Low
Inorganics, Single Pass), 50MS (Organics, Medium Inorganics, Single Pass), 70LR
(Organics, Low Inorganics, Recycle), 80HS (Organics, High Inorganics, Single
Pass), 90MR (Organics, Medium Inorganics, Recycle), and 100HR (Organics, High
Inorganics, Recycle) consisted of a mixture of two alkaline metal finishing waste
treatment sludges with the characteristics shown in Table 17. The metal
finishing sludges were supplemented with reagent-grade divalent metal oxides as
indicated in Table 18. The metal sludges and metal oxides were admixed with
sawdust to enhance leachate contact with the finely divided particulate matter,
and to provide for even distribution of the mixture between the simulated
landfill columns.
The metal sludge/metal oxide/sawdust mixture added to each column was
thoroughly blended by manual mixing and divided into three equal portions for
strategic placement within the column. The first addition of the metal mixture
was made directly above the layer of refuse where the organic priority pollutants
were placed. The second portion of the metal sludge/metal oxide/sawdust mixture
was placed when the columns were approximately half full of manually compacted
refuse, and the third addition of metals mixture was made approximately 30 cm (12
in) below the upper surface of the compacted refuse. Additionally, 100-g
Eortions of the metal sludge/metal oxide/sawdust mixture were each admixed with
0 cc of 20-mesh Ottawa ,sand and placed within nylon mesh bags. Two bags were
placed in each column receiving inorganic priority pollutants. One bag was
placed in the lower layer of inorganic pollutants and the second bag was placed
within the upper layer of inorganic pollutants in each column. In comparison to
the overall mass loading of the metal sludge/metal oxide/sawdust mixture, the two
bags constituted a negligible addition of less than one percent by mixture
weight. The respective loading conditions are presented in Tables 19 and 20.
49
-------
TABLE 15. CHARACTERISTICS OF SHREDDED MUNICIPAL SOLID WASTE
ADDED TO SIMULATED LANDFILL COLUMNS
Sample
Number
la
Ib
2a
2b
3a
3b
4a
4b
5a
5b
6a
6b
7a
7b
8a
8b
AVG
Moisture
Content
(%)
27.3
26.9
33.5
29.5
26.1
26.5
27.2
27.8
27.9
29.2
28.7
26.2
35.0
32.0
39.2
38.1
30.1
Calorific
Value 1
(cal/g)a
4422
4272
4835
4654 i
4279
4458
1
4318 i
4494 ;
4376
4377
4192
4402 .
4264
4379
4409 |
Ash
Content
(%)a
19.3
14.2
13.5
13.4
10.8
15.9
19.0
14.1
14 .-4
16.4
13.6
10.5
15.6
13.0
17.9
13.7
14.7
C
(%)a
35.0
40.0
36.0
36.0
40.0
39.0
48.0
47.0
38.0
40.0
37.0
41.0
37.0
41.0
38.0
39.0
39.5
H
(%)a
7.6
5.2
5.3
5.0
5.3
5.3
7.0
6.8
5.3
5.9
4.8
5.3
5.3
5.9
5.3
5.0
5.6
N
(%)8
BDLb
5.1
0.7
0.7
1.5
0.9
0.91
0.9
2.7
0.9
BDLb
0.9
1.8
4.5
0.9
0.9
1.5
Note: *Dry Weight Basis i
°BDL - Below Detection Limit
3.3 SIMULATED LANDFILL CLOSURE j
Upon completion of the simulated landfill column loading process, a 7.6-cm
(3-in) layer of 1.2-cm (1/2-in) washed'gravel was placed on the upper refuse
surface to assist in the uniform distribution of leachate recycled and/or water
added to the columns. The liquid distribution system was connected to the
landfill covers to extend just above the gravel, and the covers were bolted1 to
the columns and sealed water and gas tight using a silicone rubber sealant
between the flanges. The silicone rubber sealant was also applied on the outside
of the seal and on the bolt and nut heads as well. The total time required from
the onset of loading until final column closure was approximately three days.
3.4 SIMULATED LANDFILL OPERATION
I
The 10 simulated landfill columns were loaded with shredded municipal solid
waste and inorganic and/or organic priority pollutants on project Day 1. The
columns were sealed water and gas tight on project Day 2, and pressure tests were
conducted from project Day 3 through prbject Day 21 to ensure that the sealing
operations were successful. Tap water additions were commenced on project Day
22 in order to attain immediate field ckpacity. Six liters of tap water were
added to each column on project Day 22, and the water addition rate was increased
to 12 liters per day between project Days 23 and 35 in order to expedite the
attainment of field capacity on approximately project Day 35 with the cumulative
addition of 162 liters of tap water to each column. Tap water additions
50
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TABLE 17. CHARACTERISTICS OF METAL FINISHING WASTE TREATMENT SLUDGE
LOADED TO SIMULATED LANDFILL COLUMNS
Constituent
Moisture Content (%) !
Total Volatile Solids (%)
Metals (g/kg dry sludge)
Cadmium (Cd)
Chromium (Cr) |
t
Copper (Cu)
Iron (Fe) '
Lead (Pb) !
Mercury (Hg)
Nickel (Ni) 1
Zinc (Zn)
DIF»
78.7
18.5
7.2
21.6
NDC
204
0.4
NDC
0.3
45.4
SAFb
79.7
14.6
167
0.4
NDC
2.3
NDC
NDC
459
0.3
Note: *DIF - Dixie Industrial Finishing Company
bSAF - Saft America, Inc. ;
CND - None Detected
TABLE 18. METAL SLUDGE/METAL OXIDE/SAWDUST MIXTURE LOADED TO
SIMULATED LANDFILL COLUMNS
Constituent
(as received)
DIF (kg)
SAF (kg)
Cr203 (g)
HgO (g)
PbO (g)
ZnO (g)
Sawdust (kg)
Low
Loading
Level!
5.0 ;
0.8 .
34.0
22.0
113.0
134.0!
6.0
Medium
Loading
Level
10.0
1.6
68.0
44.0
226.0
268.0
6.0
High
Loading
Level
20.0
3.2
136.0
88.0
452.0
536.0 '
6.0
52!
-------
TABLE 19. PRIORITY POLLUTANT LOADING TO THE SIMULATED LANDFILL COLUMNS1
Pol 1 utant
Inorganics:
Cadi urn
Chromi urn
Lead
Mercury
Nickel
Zinc
Organics:
Dibromomethane
1,4-Dichloro-
benezene
2.4-Dichloro-
phenol
Dieldrin
Dioctyl
Phthalate
Hexachl oro-
benzene
1.2,3.4,5,6-
Hexachl oro- '
cyclohexane
Naphtha! ene
Nitrobenzene
2-Nitrophenol
1.2.4-TM-
chl orobenzene
Tr i chl oroethyl ene
Column Identity
1CR
None
None
None
None
None
None
None
None
None
None
None
None
Hone
None
None
None
None
None
2CS
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
30S
None
None
None
None
None
None
0.45
0.45
0.45
0.11
0.45
0.45
0.45
0.45
0.45
0.45
0.45
0.45
40LS
0.13
0.17
0.4
0.076
0.28
0.59
0.45
0.45
0'.45
0.11
0.45
0.45
0.45
0.45
0.45
0.45
0.45
0.45
SONS
0.26
0.35
0.8
0.16
0.56
1.2
0.45
0.45
0.45
0.11
0.45
0.45
0.45
0.45
0.45
0.45
0.45
0.45
60R
None
None
None
None
None
None
0.45
0.45
0.45
0.11
0.45
0.45
0.45
0.45
0.45
0.45
0.45
0.45
70L
0.13
0.17
0.4
0.076
0.28
0.59
0.45
0.45
0.45
0.11
0.45
0.45
0.45
0.45
0.45
0.45
0.45
0.45
Note: *(g pollutant/kg dry shredded municipal refuse)
80HS
0.53
0.7
1.6
0.31
1.1
2.4
0.45
0.45
0.45
0.11
0.45
0.45
0.45;
0.45
0.45
0.45
0.45
0.45
SOMR
0.26
0.35
0.8
0.16
0.56
1.2
0.45
0.45
0.45
0.11
0.45
0.45
0.45
0.45
0.45
0.45
0.45
0.45
100HR
0.53
0.7
1.6
0.31
1.1
2.4
0.45
0.45
0.45
0.11
0.45
0.45
0.45
0.45
0.45
0.45
0.45
0.45
53
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continued at a reduced rate of six liters per day between project Days 36 and 47
to ensure sufficient leachate production for recycle and/or sampling and
analysis. Additionally, six liters of tap water were added on project Days 69,
76, 79, and 82. The two leachate management strategies, leachate recirculation
and single pass leaching, were operative on project Day 104.
3.4.1 Single Pass Columns
To operate the landfill columns according to the changing leaching
conditions of the landfill column, six liters of tap water were added to the five
single pass columns 'approximately once every three days between project Days 104
and 471 on 123 separate occasions. During this time period, the total
accumulated leachate was discarded every three days. The frequency of water
additions was reduced to six liters every nine days from project Day 471 to 664
Between project Days 483 and 971, leachate was discarded at the rate of 1 8
liters every three days.
The columns were also seeded with anaerobic digester sludge to enhance the
onset of methanogenesis after intentionally prolonged acid formation on 23
separate occasions between project Days 667 and 899. (The seeding procedure is
described in Section 3.4.3.) During the seeding period, water additions of six
liters per day were made to each column on project Days 675, 688, 694, and 713.
In addition, 12 liters of tap water were added to Column 2CS on project Days 688
and 694, and four liters of tap water were added to Column 80HS on project Day
After the columns were seeded, tap water additions resumed at a rate of six
liters every nine days from project Day 908 until project Day 954. During the
time period between project Days 955 and 1016, water additions ceased in all
columns except 80HS. Water additions of six liters every nine days resumed
between project Days 1017 and 1518. The total volume of leachate that had
accumulated in each of the five columns was discarded on project Day 974.
Afterwards, the total accumulated leachate was discarded once every three davs
between project Days 977 and 1540.
3.4.2 Recycle Columns
Water additions of six liters per month were made to each of the five
recycle columns between project Days 104 and 645. The recycle columns, like the,
single pass columns, were seeded with anaerobic digester sludge on 23 occasions
between project Days 667 and 899 to enhance the onset of methanogenesis. Tap
water was added to all five recycle columns, with the exception of Column 60R,
on project Day 688 during the seeding period. The final tap water .addition made
to the recycle columns occurred on project Day 1192 when all columns, except
Column 100HR, received six liters of tap water.
Between project Days 288 and 661, one "dose" of leachate was recycled to
the top of each column and was allowed to percolate through the refuse mass. The
volume of recycled leachate was not measured at that time, however, it was later
calculated as the amount of water present in the column in exdess of field
capacity during that time period. These volumes differed between the five
simulated landfill columns due to the variance in the amount of water added to
each column and the amount of leachate removed for sampling and analysis. During
the seeding period, leachate was recycled seven to twelve times between project
Days 723 and 778. Four of these recycles occurred prior to seedings four through
eight to provide more substrate to the methanogens and permit their gradual
development.
One to two liters of leachate were recycled 20 times during the time period
between project Days 786 and 807. This leachate was also neutralized with a 150
g/L NajCOj solution to a pH between six and seven to avoid possible acid
inhibition of the developing methanogens. The same volume of leachate was
55
-------
recirculated and neutralized in each of the five recycle columns. Between
project Days 808 and 833, leachate was recycled in all five columns, however, the
frequency and amount varied between columns based upon the volume of accumulated
leachate. Daily recycle of leachate was performed from project Day 834 to
project Day 859, and the volume of recycled leachate varied between columns.
Leachate recycle continued on a daily basis between project Days 860 and 915 with
the recirculation of 12 liters per day in each column.
Beginning on project Day 916, the volume of leachate recycled was
consistent between columns, but was limited to the volume produced by Column 60R.
From project Day 916 until the end of the experimental time period, the volume
of leachate produced by Column 60R gradually declined. This decrease in leachate
was presumed to be the result of increased microbial activity and biomass growth,
as well as a more complete saturation of the refuse mass and possible retention
of leachate in the void spaces. Daily leachate recycle was performed between
project Days 916 and 1063. The frequency of leachate recirculation was reduced
to every other day between project Days 1064 and 1120, and the recirculation
frequency was further reduced to every fourth day from project Day 1120 until the
end of the experimental period on Day 1518.
3.4.3 Anaerobic Digester Sludge Seeding
B.J
After intentionally prolonged' operation in the Acid Formation Phase of
landfill stabilization, the 10 simulated landfill columns received sequential
seeding =of 23 separate additions of anaerobic digester sludge collected from the
R. M. Clayton Wastewater Treatment Plant in Atlanta, GA. Prior to this seeding
with anaerobic digester sludge, the columns were operated to intentionally
maintain the Acid Formation Phase at a; depressed pH and high chemical oxygen
demand (COD) and total volatile acids (TVA) to observe the relative mobility of
the codisposed organic and inorganic priority pollutants. Although methanogenic
bacteria are normally present in the, MSW and soil used for daily cover at full-
scale landfills, the 10 simulated landfill columns were probably deficient in
methanogens after prolonged operation in the Acid Formation Phase. It was,
therefore, deemed appropriate to provide a seed of methanogens, using anaerobic
digester sludge. The anaerobic digester [sludge added had a pH of 7.9, alkalinity
of 3.1 g/L as CaC03, and 2.5% total solids of which 60% were volatile solids.
The 23 anaerobic digester sludge additions were provided between project
Days 667 and 899. A summary of the seeding procedure is presented in Table 21.
The "seed" added consisted of the anaerobic digester sludge followed by an
addition of one liter of tap water to rinse and prevent fouling of the liquid
addition/distribution system. As indicated in Table 20, leachate was neutralized
in some cases with a 150 g/L Na,C03 solutibn and mixed with the anaerobic digester
sludge to help decrease the effects of existing acid inhibition on the
methanogens:
Seedings No. 1 through 8 consisted of the addition of five liters of
anaerobic digester sludge, followed by one liter of tap water to rinse the liquid
addition/distribution line of residual anaerobic digester sludge. Two to 4.5
liters of leachate were recirculated in;Columns 1CR, 60R, 70LR, 90MR and 100HR
prior to seedings No. 4 through 8 to concomitantly provide substrate to the
methanogens. Seedings Nos. 9 through 20 were conducted under a revised
procedure, since the methanogic activity was only developing at a slow rate,
presumably as a consequence of persistent acid inhibition. The new seeding
procedure consisted of the addition of four liters of anaerobic digester sludge
mixed with one liter of neutralized leachate to enhance substrate availability
and utilization by the methanogenic bacteria. One liter of tap water was added
following each seeding mixture.
Due to this revised seeding procedure, leachate was also recycled in the
five single pass columns. However, the Volume of leachate used and recycled in
the single pass columns was not considered significant in the overall assessment
56
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TABLE 21. SUMMARY OF ANAEROBIC DIGESTER SLUDGE ADDITIONS TO SIMULATED LANDFILL COLUMNS
Seeding
Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Date
(Days Since
Loading)
July 16, 1987
(667)
Aug. 3, 1987
(685)
Aug. 21, 1987
(703)
Sept. 11. 1987
(724)
Sept. 28, 1987
(741)
Oct. 7. 1987
(750)
Oct. 19. 1987
(762)
Oct. 28, 1987
(771)
Nov. 2. 1987
(776)
Nov. 19, 1987
(793)
Dec. 1, 1987
(805)
Dec. 10, 1987
(814)
Dec. 19, 1987
(823)
Dec. 30, 1987
(834)
Jan. 8, 1988
(843)
Jan. 5, 1988
(850)
Jan. 22, 1988
(857)
Jan. 29, 1988
(864)
Feb. 5. 1988
(871)
Feb. 12, 1988
(878)
Feb. 19, 1988
(885)
Feb. 26, 1988
(892)
March 4, 1988
(899)
Anaerobic
Digester
SI udge
(L)
5
5
5
5
5
5
5
5
4
2
4
4
4
4
4
4
4
4
4
4
5
5
5
Tap
Water
(L)
1
1
1
1
1
1
1
1
1
3
1
1
1
1
1
1
1
1
1
1
1
1
1
Leachate
(L)
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1 .
1
1
1
1
1
0
0
0
Total
Volume
Added
(L)
6
6
6
6
6
6
6
6
6
6
6 •
6
6
6
6
6'
6
6 .
6
6
6
6
6
Notes
Leachate was neutralized to pH 6-7
using a 150 g/1 Na2C03 solution.
Leachate was neutralized to pH 6-7
using a 150 g/1 Na2C03 solution.
Leachate was neutralized to pH 6-7
using a 150 g/1 Na2C03 solution.
Leachate was neutralized to pH 6-7
using a 150 g/1 Na2C03 solution.
Leachate was neutralized to pH 6-7
using a 150 g/1 Na2C03 solution.
Leachate was neutralized to pH 6-7
using a 150 g/1 Na2C03 solution.
Leachate was neutralized to pH 6-7
using a 150 g/1 Na2C03 solution.
Leachate was neutralized to pH 6-7
using a 150 g/1 Na2C03 solution.
Leachate was neutralized to pH 6-7
using a 150 g/1 Na2C03 solution.
Leachate was neutralized to pH 6-7
using a 150 g/1 Na2C03 solution.
Leachate was neutralized to pH 6-7
using a 150 g/1 Na2C03 solution.
Leachate was neutralized to pH 6-7
using a 150 g/1 Na2C03 solution.
57
-------
of the leachate management strategy. As an additional measure to alleviate acid
inhibition in the five recycle columns^ one to two liters of leachate were
neutralized prior to daily recirculation between project Days 786 and 807.
Therefore, Seedings No. 21 through 23 wejre conducted according to the original
seeding method of five liters of anaerobic digester sludge followed by one liter
of tap water. ;
3.5 ANALYTICAL PROCEDURES AND METHODS I
Leachate is produced in a landfill when field capacity is exceeded as a
result of moisture accumulation from rainfall infiltration, groundwater
intrusion, and waste degradation. The quantity and quality of leachate produced
is influenced by the nature of the refuse,| the state and extent of decomposition,
and the chemical and physical characteristics of the percolating water and the
adjacent soil and cover. In these studies, deliberate additions of tap water
resulted in the attainment of indicated field capacity in the 10 columns on about
project Day 35, after which leachate was produced for analysis and/or
recirculation. (Rapid attainment of indicated field capacity was used as an
experimental expediency which precluded operational delays otherwise incurred by
liquid additions in accordance with natural rainfall events.)
The leachate was routinely analyzed for chemical, physical, and biological
parameters indicative of the five phases of landfill stabilization. The
codisposed organic and inorganic priority pollutants were also monitored to
assess their potential for attenuation and mobility, and the possible generation
of intermediary conversion products. . Leachate samples from the 10 simulated
landfill columns were analyzed for p!H, conductivity, oxidation-reduction
potential (ORP), total volatile acids (TyA), individual volatile acids (acetic,
propionic, butyric, iso-butyric, valeric, iso-valeric and hexanoic), total
organic carbon (TOG), five-day biochemical oxygen demand (BOD5) , chemical oxygen
demand (COD), total alkalinity, conductivity, nitrogen, phosphorus, sulfates,
sulfides, chlorides, bromide, iron, calcium, potassium, magnesium, manganese,
sodium, ammonia, and the added organic aiid inorganic priority pollutants.
i
The total volume of gas produced was recorded, and gas samples were
analyzed for carbon dioxide, oxygen, nitrogen, hydrogen, and methane. Some gas
samples were analyzed for the volatile organic priority pollutants or conversion
products in order to estimate possible losses due to volatilization. With the
exception of the organic priority pollutant and gas analyses, Table 22 presents
the analytical methodologies used in these studies.
Leachate collected from the -10 simulated landfill columns was analyzed for
both semivolatile and volatile compounds.! Semivolatile compounds were analyzed
by solvent extraction and concentration i followed by GC/MS analysis. A known
volume of leachate sample was diluted to a volume of 500 ml with organic free
water, and placed in a liquid-liquid vapor phase extractor (Kontes, Inc.). The
diluted leachate was acidified to a pH less than 2.0 with concentrated
hydrochloric acid, and a methylene chloride solution of the surrogate compounds
was added. The leachate was heated and stirred during the eight-hour continuous
extraction procedure. The methylene chloride extract was dried over anhydrous
sodium sulfate, and a Kuderna-Danish concentrator was used to reduce the volume
of methylene chloride to one ml. A solution of methylene chloride containing the
internal standard was added to the concentrated methylene chloride, and the
mixture was sealed and stored in a refrigerator until analysis with a capillary
column GC/MS.
Volatile compounds were analyzed by purge and trap gas chromatography
followed by mass spectrometric detection:and identification. Known quantities
of the internal standard and surrogate were added to a 5-ml leachate sample. The
mixture was purged for nine minutes with nitrogen, and the purged compounds were
collected and analyzed by capillary column GC/MS.
58:
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TABLE 22. SUMMARY OF ANALYTICAL METHODS USED DURING SIMULATED
LANDFILL INVESTIGATIONS ;?: *
Measurement
Conductivity
:PH
ORP
Alkalinity
BODS
COD
TOG
ci-, so/*,
P0<-3, S'2
NH3-N
CH4, C02l H2
Cadmium
Calcium
Chromium
Iron
Lead
Magnesium
Manganese
Mercury
Nickel
Procedure
EPA 600/4-79-020
Method 120.1
EPA 600/4-79-020
Method 150.1
ASTM Method 1498-99
EPA 600/4-79-020
Method 310.1
EPA 600/4-79-020
Method 405.1
EPA 600/4-79-020
Method 410.1 ,
EPA 600/4-79-020
Method 415.1
Standard Methods
for the Examination
of Water and Wastewater;
Method 429
EPA 600/4-79-020
Method 350.3
Gas Chromatography
EPA 600/4-79-020
Methods 231.1 & 213.2
EPA 600/4-79-020
Method 215.1
EPA 600/4-79-020
Methods 218.1 & 218.3
EPA 600/4-79-020
Method 236.1
EPA 600/4-79-020
Methods 239.1 & 239.2
EPA 600/4-79-020
Method 242.1
EPA 600/4-79-020
Methods 243.1 & 243.2
EPA 600/4-79-020
Method 245.1
EPA 600/4-79-020
Methods 249.1 & 249.2
Precision
(eO
± 6%
± 0.1
-.
± 5%
± 20%
± 10%
- ± 10%
± 10%
± 5%
± 5%
± 10%
± 5%
± 10%
± 10%
± 10%
± 5%
± 10%
±20%
± 10%
Accuracy
95-105%
± ti.l 1
95-105% 1
90-110% I
90-110% 1
90-110% 1
90-110%
90-110%
90-110%
90-110% 1
90-110% 1
90-110% 1
90-110%
90-110%
90-110%
80-120%
90-110%
59
-------
TABLE 22 (continued)
Measurement
Potassium
Sodium
Zinc
Lithium
Solid Waste
Calorific
Value
Solid Waste
Moisture
Content
Volatile
Organic
Acids
Procedure
EPA 600/4- 79 -,020
Method 258.1
EPA 600/4-79-020
Method 273. [1
EPA 600/4-79-020
Methods 289.1 &j289.2
Standard Methods
for the Examination
of Water and Waste -water;
Method 317B
Parr Instruments
Technical Manual
. #130 .
Ohaus Instruments
Technical Manual
i
Direct Aqueous
Injection
Capillary Column
Gas Chromatography
Precision
(
-------
space above the sample. Leachate samples for metal analyses were collected in
acid-washed, screw-capped polyethylene bottles and preserved by digestion with
nitric acid and hydrogen peroxide. The remaining leachate samples 'were collected
in vials of the same type used for the metals analyses. Following collection of
leachate samples, pH, conductivity, alkalinity, and ORP analyses were performed
immediately, while other samples were collected and stored for a maximum of 24
hours at 4°C.
Gas samples were collected from the lysimeter head space for immediate
analysis. Gas-tight syringes were used to collect gas samples from built-in
sampling ports to prevent sample contamination.
All samples collected were logged into a data book by the > analyst. The
information recorded contained details regarding the sampler, type of analysis,
and recipient personnel. The sample collected displayed a label with the
following information: column number, date of sampling, master log book number,
analysis to be performed, sample volume, preservative amount and type, sampler
and general observations.
3.7 SIMULATED LANDFILL DISASSEMBLY
Upon completion of the study, the 10 simulated landfill columns remained
standing for approximately six weeks prior to disassembly. During this time,
leachate produced by each column was allowed to drained, collected, and
discarded. The head-space gases were analyzed for the presence of hydrogen
sulfide before opening the columns. During disassembly, samples of the waste
matrix were retrieved for analysis, and appurtenances were inspected for changes
in character or functionality. •
61
-------
SECTION 4
I
PRESENTATION AND GENERAL DISCUSSION OF RESULTS
During the simulated landfill studies, leachate and gas produced by the 10
simulated landfill columns were analyzed for the parameters reflective of the
five phases of landfill stabilization as well as for the behavior and fate of the
codisposed organic and inorganic priority pollutants. The total gas volume
produced was measured, and gas composition was determined to reflect the
progression of stabilization events within the landfill simulations. Leachate
quantity and quality, which vary with time and extent of waste conversion, were
evaluated as characteristic of the nature and extent of waste conversion. The
chemical characterization of the leachate produced was also used to determine the
potential adverse environmental impacts cpnsequenced by mobilization of leachate
constituents. j
The 10 simulated landfill column^ received tap water additions until
indicated field capacity was attained and leachate production commenced. As a
result, the first phase of landfill stabilization, Initial Adjustment, was
significantly shortened. In addition, prior to project Day 667, the 10 simulated
landfill columns were intentionally operated to maintain the Acid Formation Phase
of landfill stabilization and to observe! its effect on the relative mobility of
the codisposed organic and inorganic priority pollutants. The columns were then
seeded with anaerobic digester sludge on 23 occasions between project Days 667
and 899 to promote the Methane Fermentation Phase of landfill stabilization.
Therefore, the following discussion concentrates on the Acid Formation and
Methane Fermentation Phases of landfill 'stabilization within the 10 simulated
landfill columns.
Leachate and gas data were obtained approximately once every two weeks.
For purposes of consistent data presentation and interpretation, a sampling time
period of every 14 days was developed, and "missing" sample values were
determined Busing linear interpolation. In the event of more than one sample
being obtained during a 14-day period, an average of the sample results was
considered representative of that time period.
4.1 EXTERNAL TEMPERATURE
The temperature of the area in which the 10 simulated landfill columns were
/d Was recorded and is presented in. Figure 15 and Table A-l (Appendix A).
(Table A-l also contains the vapor pressure of water at the measured temperature
as determined using the Handbook of Chemistry and Physics.*5") Internal column
temperature, which was not determined because of a malfunctioning temperature
probe, was assumed to be the same as the measured room temperature. The observed
temperature fluctuated in accordance with! ambient seasonal variations and ranged
from 10.3°C to 31.1°C, with an average temperature of 22.9°C. During the majority
of the experimental period, the temperature was below that characteristic of the
optimum mesophilic range (30 to 38°C) for anaerobic stabilization. This suggests
that some effects on biological conversion were due to temperature variations,
but that these variations probably did not influence the final outcome other thari
to promote more rapid stabilization during higher . temperatures and to reduce
reaction rates during lower temperatures !
62
-------
Temperature (Celcius)
200
400 600 800 1000 1200
Time Since Loading (Days)
1400
1600
Figure 15. Ambient Temperature Variations during Simulated
Landfill Investigations.
4.2 GAS ANALYSES
Gas volume and composition were determined as indications of the
progression of landfill stabilization processes. Methane and carbon dioxide
arethe primary end products of anaerobic biological waste stabilization . and
reflect the rate of biological activity and organic material conversion within
the landfill environment. In addition, differences in gas volume and composition
delineate the impacts of loading and type of leachate management strategy
employed upon the anaerobic systems established within the 10 simulated landfill
columns. '
4.2.1 Gas Production ,
Cumulative gas volumes produced in the recycle and single pass columns' are
shown in Figure 16, and the corresponding data are presented in Table A-2 of
Appendix A. Incremental gas volumes produced are presented in Figures 17 for
recycle and single pass columns, respectively. The supporting data are included
in Table A-3 of Appendix A.
The overall volume of gas produced was much larger in the recycle columns
than in the single pass columns, and the maximum volume of gas produced during
a two-week period was also greater for the recycle columns than for the single
pass columns. This information, which is summarized in Table 23, indicates that
stabilization proceeded more rapidly and thoroughly in the recycle columns than
in the single pass columns. The higher degree of stabilization observed in the
recycle columns can be directly attributed to the leachate recirculation strategy
employed, since decomposable organic material such as the volatile organic acids,
a^prmcipal substrate for methanogens, was removed due to leachate wasting in the
single pass columns but was retained and recirculated as substrate in the recycle
columns. Therefore, methanogens in the recycle columns experienced an increased
63
-------
Gas Volume (cu.m.)
200 400
600
Time Since
800 1000 1200 1400 1600
Loading (Days)
70
60
so
40
30
Gas Volume (cu.m.)
Sing I* Pats
— 2OS
-4- SOS
4OLS
5OMS
8OHS
200
400 600 800 ' 1000 1200 14OO 1600
Time Since Loading (Days)
Figure 16. Cumulative Gas Production during Simulated Landfill
Investigations.
64
-------
Gas Volume (cu.m.)
RecycU
—— 1CR
-4— 6OR
-*- TOLR
-e- SOMR
10OHR
200 400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
4
Gas Volume (cu.m.)
Sing la Pas*
— 2CS
-+- 3OS
-3K- 4OLS
-5- SOMS
-<£- 8OH5
2OO 400 600 800 . 1000 1200 1400
Time Since Loading (Days)
1600
Figure 17. Incremental Gas Production during Simulated Landfill
Investigations.
65
-------
TABLE 23. COMPARISON OF GAS VOLUMES FROM SIMULATED LANDFILL
COLUMNS
Column
1CR
2CS
SOS
40LS
50MS
60R
70LR
80HS
90MR
100HR
I
Cumulative Gas
Volume Produced
(M3)
65.755
21.676
5.454 ;
2.893 ;
3.266 1
57.126
27.872
7.112
30 . 007
27.833
Maximum Incremental
Gas Volume Produced
(M3)
3.680
2.118
0.248
0.193
0.174
3.506
2.128
0.340
2.081
1.427
contact opportunity with the available substrate due to leachate recirculation.
Incremental gas production began tp increase around project Day 793, which
corresponds to the 10th seeding of anaerobic digester sludge added to promote
methanogenesis. Beginning with the 9th seeding, a revised seeding procedure was
adopted. In accordance with this procedure, leachate added along with anaerobic
digester sludge and tap water was neutralized to pH 7, providing methanogens with
a. more acceptable substrate. After initiation of the revised seeding procedure,
gas volume produced by all 10 simulated landfill columns increased dramatically.
Gas production from the control columns, Columns 1CR and 2CS, was greater
than gas production in columns loaded with inorganic and/or organic priority
pollutants, indicating the retarding effects of the applied admix priority
pollutant loadings. For interpretive ; analysis, the experimental period was
divided into the Acid Formation Phase of landfill stabilization, which lasted
until approximately project Day 800, and the Methane Fermentation Phase which
occurred from project Day 800 until the iend of the experimental period. During
both the Acid Formation and Methane Fermentation Phases, the control recycle
column (Column 1CR) displayed the largest gas productivity, as expected, due to
the absence of codisposed inorganic and o'rganic priority pollutants. Column 60R,
the recycle column containing only organic priority pollutants, produced the next
largest gas volume. The recycle columns receiving both organic and inorganic
pollutants all displayed lower gas volumes when compared with Columns 1CR and
60R, suggesting some retarding effect of the added inorganic priority pollutants.
Although the inorganic priority pollutant loadings varied between Columns 70LR,
90MR, and 100HR, the gas volume produced by each of these columns was similar,
which implies that the landfill environment had the ability to accommodate and
detoxify certain amounts of the pollutant loadings.
The relative gas production between the five recycle columns is shown in
Figure 18. In Figure 18, gas volumes produced by columns loaded with inorganic
and/or organic priority pollutants are compared to the volume of gas produced by
the control recycle column, (Column 1CR) and the column with only organic
66
-------
100
Percent of Column 1CR
80-
60-
40-
20-
826 950 1070 1177 1298 1398
Time Since Loading (Days)
1518
6OR
7OLR
19OMR
10OHR
Percent of Column 6OR
826 950 1070 1177 1298 1398
Time Since Loading (Days)
1518
7OLR
9OMR
10OHR
Figure 18. Relative Gas Production between Simulated Landfill
Columns with Leachate Recycle.
67
-------
priority pollutants added (Column 6OR)I Column 60R displayed increasingly
greater gas volume produced in relation to Column 1CR, especially after rapid gas
production commenced. The three recycle columns loaded with both organics and
inorganics, Columns 70LR, 90MR and 100HR, displayed some retardation of gas
production, but total inhibition was not observed. Towards the end of the
experimental period, these columns were |all producing about the same amount of
fas, indicating that the effects of the codisposed inorganics and organics were
eing lessened as biodegradation and attenuation mechanisms became operative.
When the columns loaded with both organic and inorganic priority pollutants
are compared to the column receiving only organic priority pollutants (Column
60R), the effects of the inorganic priority pollutant loading can be observed.
During the early period of the Methane Fermentation Phase, specifically project
Day 826, Columns 90MR and 100HR showed: relatively high gas volumes produced
compared to Column 60R. This may have resulted from some unavoidable waste
heterogeneity or by other physical-chemical phenomena as the environment changed
from the more acid conditions prevailing during the Acid Formation Phase (low pH)
to the more favorable conditions established during the Methane Fermentation
Phase (neutral pH). The column with the highest inorganic loading, Column 100HR,
displayed the lowest gas production with respect to Column 60R, as expected, due
to the high inorganic loading applied. However, as the experiment progressed,
delineation between gas volumes produced from Columns 70LR, 90MR, and 100HR
became increasingly less distinct as the inherent assimilative capacity of the
landfill environment became operative.
The relationships between gas production in the single pass columns were
not as predictable or logical as those in recycle columns. The control column,
Column 2CS, produced the greatest gas volume, as expected, due to the absence of
added organic and inorganic priority pollutants. However, during the Acid
Formation Phase, Column 2CS did not produce the highest gas volume. It was only
oo£in£ the anaeroDi-c digester sludge seeding period between project Days 667 to
899 that the gas production began to significantly increase. Column 2CS had the
smallest leachate volume removed for sampling and wasting of the five single pass
columns. Column 2CS also received the second highest volume of added tap water,
fiving this column the highest liquid volume retained in excess of indicated
ield capacity. (Water balance data are discussed in Section 4.3.) Therefore,
substrates such as the volatile organic; acids produced may have remained in
Column 2CS, albeit to a lesser degree than in the recycle columns, but may have
been diluted to less inhibitory levels. Subsequent to the anaerobic digester
sludge seeding period, Column 80HS displayed the second highest gas volume and
Column 40LS produced the smallest gas volume, which disagrees with expected
results. Therefore, these findings suggest that the nature of the single pass
operation distorted the effects of the inorganic and/or organic loadings due to
washout of the codisposed pollutants.
I
Gas production rates were calculated during selected time intervals for
both recycle and single pass columns. These time intervals differ between the
recycle and single pass columns based Upon variations in the slope of the
respective cumulative gas volume produced figures. Recycle column gas production
rates were determined during three time intervals and are presented in Table 24.
In general, after the onset of the Methane Fermentation Phase, gas production
rates increased initially to their highest: values and decreased concomitantly as
waste conversion progressed. During the time interval between project Days 920
and 1050, Column 1CR displayed the highest gas production rate, followed by
Columns 60R, 70LR, 90MR and 100HR, respectively. During the third time period
between project Days 1398 and 1518, Column 100HR showed the largest gas
production rate, again suggesting the ability of the landfill to attenuate
codisposed organic and inorganic priority pollutants.
Table 25 presents gas production rates for the five single pass columns
during the latter part of the experimental period. Gas production rates for the
single pass columns were much less than those observed for the recycle columns.
68
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TABLE 24. RECYCLE SIMULATED LANDFILL COLUMN GAS PRODUCTION RATES
Column
Identity
1CR
60R
70LR
90MR
100HR
Project Days
920-1050
(M3/Day)
(L/Day)
0.157
156.6
0.142
142.2
0.072
72.2
0.065
64.9
0.054
53.7
Project Days
1050-1398
(M3/Day)
(L/Day)
0.053
53.2
0.050
50.1 ~
0.016
16.2
0.021
20.7
0.022
22.0
Project Days
1398-1518
(M3/Day)
(L/Day)
0.023
22.9
0.023
23 . 1
0.008
8.4
0.007
7.3
0.027
27.2
TABLE 25. SINGLE PASS SIMULATED LANDFILL COLUMN GAS PRODUCTION RATES
Column
Identity
2CS
SOS
40LS
50MS
80HS
Project Days
826-1090
(M3/Day)
(L/Day)
0.023
22.8
Project Days
1268-1518
(M3/Day)
(L/Day)
0.041
41.4
Project Days
1050-1518
(M3/Day)
.(L/Day)
o.oot
1.3
0.002
2.2
0.0004
0.4
0.005
4.6
69
-------
The control column, Column 2CS , showed an increase in gas production rate towards
the end of the experimental period. Column 80HS, which had the second highest
fas volume produced, also had the second largest gas production rate, followed
y Columns 40LS, 305 and 50MS, respectively .
The higher gas production rates determined for the recycle columns further
illustrate the benefits of leachate recirculation as a landfill management option
in promoting a higher degree and more rapid rate of waste stabilization.
i
4.2.2 Gas Composition i
Normalized gas composition for the 10 simulated landfill columns is
presented in Figures 19 through 28 and (Tables A-4 through A- 13 of Appendix A
Oxygen, probably originating from within the refuse void spaces, was present in
all columns until approximately project Day 550 in amounts ranging from 1 to 21%
of the total gas volume. The initial presence and subsequent disappearance of
oxygen is indicative of the transition from an initial aerobic to anaerobic
environment within the columns . After the utilization of oxygen for initial
aerobic waste stabilization and its diisplacement by carbon dioxide the eas
production rate decreased until methanogenesis was promoted by the seeding with
digester sludge. °
Hydrogen was present in all columns, but at less than 15% by volume until
methane production commenced. Hydrogen completely disappeared in the gas from
the recycle columns by project Day 980, knd remained in the gas from the single
pass columns for a longer period of time. Hydrogen eventually disappeared in
columns except Column 80HS. Column 80HS,; the heaviest loaded single pass column
contained hydrogen in the gas phase throughout the entire experimental period!
This information is summarized in Table 26. The presence of hydrogen prior to
active methane production corresponds to the possible impact of excessive
i?S?f?? p*e,s.s,ures °n microbial activities during the Acid Formation Phase of
landfill stabilization, and the disappearance of hydrogen corresponds to the more
favorable conditions prevailing upon onset of methanpgenesis, when hydrogen can
%?™t tS subs£ratt f°l Carbon dioxide-reducing methanogens. This observation is
(Sections0 " 4 2" nd^S) changes in t^es and concentrations of volatile acids
Nitrogen was present in the air entrained in the refuse void spaces during
column loading operations All 10 simulated landfill columns contained nitrogen
?£ r^fa%I° lxTt Produced were large enough to displace the nitrogen, as shtwn
In Table 26. Nitrogen was displaced in the recycle columns by project Day 868
and was removed from Column 2CS by project Day 896. The remaining single pass
C°1M*OS 40LS 50 '
.
m,1 40LS' U50M?' and 80HS' Apparently did not pro'due suffcien
fofnmT^^°t-lSPiaCe the nitr°Se.n originally present. cfe the single pass
columns exhibitin lingering gas nitrogen content, Column 80HS produced the
, uce e
roS R£»* TiTv- .followedby Columns 308, 50MS and 40LS , respectively. Also,
Column 80HS had the lowest nitrogen percentage in the gas during the period from
project Day 910 to 1428. Column ?OS had the next highelt nitrogin gas percentage
as expected however Columns 40LS and 50MS were reversed in the expectid
f«3Ue^S?«aS £" ln 1f 26,' ^The unexPected results obtained from Columns 40LS
»??«„< °fy have resul1t?d fr°m short-circuiting within Column 50MS, thus
allowing nitrogen gas to linger longer in the refuse void spaces.
The data in Table 26 also indicate that methane gas was produced in all
columns at approximately the same time period, with the exception of Column 2CS
"SiS Produced some methane gas at a much earlier time. The early methane gas
production in Column 2CS was attributed to the absence of possible inhibitory
SvS^? °f 0,rSanLC and inorganic priority pollutants not contained within the
£™£S/ SfiVft88 c°lumn- Additionally s, washout of the volatile organic acids
produced, which have been shown to retard methanogenesis at high concentrations
and low pH, provided a more favorable environment for the proliferation of
methanogens, thus methane gas was produced earlier.
70
-------
100
Gas Composition (Percent by Volume)
200
400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
CARBON DIOXIDE
HYDROGEN
OXYGEN
METHANE
NITROGEN
Figure 19. Normalized Gas Composition for Column 1CR.
100
Gas Composition (Percent by Volume)
200
400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
CARBON DIOXIDE
HYDROGEN
OXYGEN
METHANE
NITROGEN
Figure 20. Normalized Gas Composition for Column 2CS,
71
-------
100
80
60
40
20
0
Gas Composition (Percent by Volume)
CARBON DIOXIDE
HYDROGEN
OXYGEN
METHANE
NITROGEN
200 400 600 800 1000 1200 1400 1600
1 Time Since Loading (Days)
Figure 21. Normalized Gas Composition for Column SOS.
100
Gas Composition (Percent by Volume)
200
400 600 800 1000 1200 1400 1600
Time Since Loading (Days)
CARBON DIOXIDE
HYDROGEN
OXYGEN
METHANE
NITROGEN
Figure 22. Normalized Gas Composition for Column 40LS.
72
-------
100
Gas Composition (Percent by Volume)
200
400 600 800 1000 1200
. Time Since Loading (Days)
1400 1600
CARBON DIOXIDE
HYDROGEN
OXYGEN
METHANE
NITROGEN
Figure 23. Normalized Gas Composition for Column 50MS.
100
Gas Composition (Percent by Volume)
200
400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
CARBON DIOXIDE
HYDROGEN
OXYGEN
METHANE
NITROGEN
Figure 24. Normalized Gas Composition for Column 60R.
73
-------
100
80
60
40
20
0
Gas Composition (Percent by Volume)
200 400 600 800 1000 1200
Time Since, Loading (Days)
1400 1600
CARBON DIOXIDE
HYDROGEN
OXYGEN
METHANE
NITROGEN
Figure 25. Normalized Gas Composition for Column 7OLR.
100
Gas Composition (Percent by Volume)
200
400 600 800 1000 1200
Time Since;Loading (Days)
1400 1600
CARBON DIOXIDE
HYDROGEN
OXYGEN
METHANE
NITROGEN
Figure 26. Normalized Gas Composition for Column 80HS,
74
-------
100
Gas Composition (Percent by Volume)
200
400 600 800 1000 1200
Time Since Loading :(Days)
1400 1600
CARBON DIOXIDE
HYDROGEN
OXYGEN
METHANE
NITROGEN
Figure 27. Normalized Gas Composition for Column 90MR.
100
80
60
40
20
Gas Composition (Percent by Volume)
200 400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
CARSON DIOXIDE
HYDROGEN
OXYGEN
METHANE
NITROQEN
Figure 28. Normalized Gas Composition for Column 100HR.
75
-------
TABLE 26. COMPARISON OF GAS COMPOSITION DURING SIMULATED
LANDFILL INVESTIGATIONS
Column
Identity
Project
Day
When N2
Becomes
Small
«5%)
Project
Day
When CH4
Appears
01%)
Average Gas Percentage
: During the
Methane Fermentation Phase
(Project Days 910-1428)
! C02
1
Nj
CH«
Recycle Columns
1CR
60R
70LR
90MR
100HR
826
868
854
868
868
700
742
728
728
714
44.02
45.17
46.07
i 45.42
43 . 54
2.77
0.67
0.71
0.61
0.95
52.81
54.14
5.3 . 20
53.96
55.44
Single Pass Columns
2CS
SOS
40LS
50MS
80HS
896
-
-
.
-
238
700
714
714
700
1 45 . 37
; 42.96
! 39.47
38.21
I 45.81
0.79
21.95
31.23
37.74
18.88
53.74
34.36
28.73
23.77
34.26
Examination of Figures 19 through 28 indicates for all recycle columns and
Column 2CS an increase in methane production was accompanied by a concomitant
decrease in nitrogen and carbon dioxide content. The decrease in nitrogen
content was more pronounced because carbon dioxide as well as methane was
produced during methanogenesis. Within recycle Columns 1CR, 60R, 70LR, 90MR and
100HR, the average percentage of methanei carbon dioxide, and nitrogen present
in the gas between project Days 910 arid 1428 was very similar (Table 26).
Methane comprised approximately 54%, carbon dioxide about 45%, and nitrogen less
than 1% of the gas by volume. I
In_contrast, Columns SOS, 4OLS, 50MS and 80HS all exhibited reduced gas
carbon dioxide and methane percentages, and increased gas nitrogen percentages
as a result of the retardation of stabilization processes arising from the
combination of applied admix loading and the single pass leachate management
strategy employed. In these four single pass columns, increased gas methane
percentages were accompanied by decreased gas carbon dioxide and nitrogen
percentages, with the decrease in nitrogen percentage being more pronounced.
4.3 WATER BALANCE
After the 10 simulated landfill columns were loaded and sealed water and
gas tight, tap water was added until indicated field capacity was attained on or
about project Day 35. Subsequently, water1 additions continued to the single pass
columns in accordance with that management strategy, and water was added to the
76
-------
recycle columns to compensate for leachate removal. Leachate was removed from
recycle columns for sampling purposes only, and^leachate removals from the single
pass columns facilitated sampling and wasting, which was an integral part of the
single pass leaching strategy.
Leachate was also removed from each column as a result of moisture losses
during gas production. Therefore, the amount of moisture lost due to gas
production was calculated for each column based upon the following, assumptions .
Total system pressure (PT) was assumed to be one atmosphere, since the simulated
landfill columns were not operated to allow a pressure buildup; the volume of gas
collected was assumed to be saturated with moisture; the density of water was
assumed to be 1000 g/L; and temperature fluctuations were neglected. Within the
temperature range encountered in the simulated landfill columns (10 to 30°C) , the
variation in water density with temperature was minimal (999.73 g/L at 10°C,
995.67 g/L at SO^).*60' Therefore, 55.6 moles/1 of H20 was taken as constant with
temperature. Finally, all gases were assumed to behave as ideal gases,
therefore, the volume fraction of gas produced was taken as mole fractions.'60
Based upon these assumptions, the volume of moisture lost due to gas
production was calculated using gas density, which is a function of gas
composition, and specific humidity, which is dependant upon total pressure and
water vapor pressure at a given temperature. The volume of liquid lost due to
gas production was calculated as follows:
v = _ g * Pau; _
LOST >) * (55.6 Moles/ L)
where ;
VLOST ~ Volume of liquid lost due to gas production
[L H2p/L gas]
q - Specific humidity [g HjO/g gas]
PGAS - Gas density [g/L]
MWmo - Molecular weight of water (18.015 g/mole]
The specific humidity, which is the mass of water vapor contained within
a unit mass of moist space, can be calculated using the following expression: <62)
0.622 * (VP) (2)
PT - [0.378*{W)] v '
where;
q - Specific humidity [g H20/g gas]
VP - Water vapor pressure at specified temperature
[millibars]
PT - Total system pressure [millibars]
The gas density can be calculated using the ideal gas law, provided that
the molecular weight of the gas is known. The molecular weight of the gas
produced varies with gas composition, and based upon the assumption stated above
was determined using the following relationship:
77
-------
where;
MWOAS
MW,
- Molecular weight of gas
(3)
mixture [g/mole]
MW
02
X,
Molecular weight of each component gas [g/mole]
44.010 g/mole
31.999 g/mole
28.013 g/mole
. 2.016 g/mole
16.043 g/mole
Mole fraction of gas; [moles gas/total moles gas]
I
Therefore, the gas density was determined using the ideal gas law and the result
of Equation (3), or:
* P
R * T
where;
POAS - Gas density [g/L] j
MWOAS — Molecular weight of gas mixture [g/mole]
R - Universal Gas Constant [0.0821 L*atm/°K*mole]
T - Temperature [K] •
' i
Thus, by combining Equations (3) and (4) land substituting along with Equation (2)
into Equation (1), the volume of liquid removed by the gas produced was
determined. Results of these calculations are presented in Tables B.-l through
B-10 of Appendix B and are summarized in .Table 27. As indicated in Table 27, the
columns producing the most gas lost the: highest volume of liquid, as expected.
However, the cumulative volume of liquid jremoved by gas production was determined
to be insignificant in comparison with the volume of liquid removed for sampling
and wasting.
After calculating the volume of moisture lost due to gas production, it was
necessary to determine the volume of moisture retained in each column in excess
of indicated field capacity. The water balance information, presented 'in Tables
B-ll through B-15 for the recycle columns and Tables B-16 through B-20 for the
single pass columns, is included in Appendix B. Leachate recycle volumes are
also included in Tables B-ll through B-15 for those columns employing leachate
recirculation. The amount of moisture retained in each column was calculated as
follows:
V =
VEX
— V — V
VR 'LOST
(5)
where;
Volume of -liquid retained in excess of indicated
field capacity [L]
Volume of fresh water added [L]
Volume of leachate removed for sampling or wasting [L]
Volume of liquid removed by gas production [L]
78
-------
•"*..
TABLE 27. CUMULATIVE LIQUID REMOVED DUE TO GAS PRODUCTION
Column
1CR
2CS
SOS
40LS
50MS
60R
70LR
80HS
90MR
100HR
Cumulative
Gas
Production
(M3)
65.755
21.676
5.454
2.893
3.266
57.126
27.872
7.112
30.007
27.833
Cumulative
Fresh Water
Added
(L)
355.0
1445.0
1415 . 0
1415 . 0
1415.0
345.0
363.0
1460 . 0
351.0
357.0
Volume of
Liquid Removed
by Gas
(L)
1.449
0.469
0.135
0.070
0'.089
1.271
0.623
0.174
0.222
0.568
Cumulative
Leachate
Removed
(L)
184.640
1090.347
1173.921
1191.639
:1217.835
129.522
103.461
1171.679
107.770
141.368
Thus, the amount of moisture retained in each of the 10 simulated landfill
columns in excess of indicated field capacity was determined during each time
interval and is presented in Tables B-ll through B-20 of Appendix B. These
calculations are used later to determine mass fluxes of leachate constituents.
4.4 LEACHATE ANALYSES
Leachate, which provides moisture, nutrients and a principal substrate for
biological activity and serves as a transport medium, reflects the progression
of landfill stabilization events. Therefore, through the analysis and
interpretation of certain leachate parameters, an insight could be gained
regarding the extent of biological waste conversion and the behavior of the
codisposed organic and inorganic priority pollutants as well.
4.4.1 EH ' ,
The pH of an anaerobic system such as a landfill is an indication of the
intensity of the prevailing buffer system, and also affects species ionization.
The prevailing pH is dependent upon interactions between volatile organic acids
alkalinity, and partial pressure of evolving carbon dioxide gas. During the Acid
Formation Phase of landfill stabilization, pH values are generally low due to the
presence of volatile organic acids (VGA) and their effect on system pH. During
periods of low pH, an abundance of mobilized ions may appear in the leachate
along with volatile organic acids. When available VGA are converted to methane
and carbon dioxide during the Methane Fermentation Phase, pH usually rises to
values characteristic of the bicarbonate buffering system, and may continue to
rise with excess ammonia generation.
Leachate pH values for the recycle and single pass simulated landfill
columns are presented in Figure 29. Measured leachate pH values are presented in
table A-14 of Appendix A. Recycle column pH, shown in Figure 29, remained
between 5.0 and 5.5 during the Acid Formation Phase, which corresponds to the
period of dominance by the volatile organic acids. As volatile organic acids
began to decrease between project Days 800 to 1000, pH increased to the 5 5 to
6.0 range. After project Day 1000, when VGA began to be reduced to very low
79
-------
pH (Standard Units)
0 . 200 400 600 800 1000 1200 1400 1600
Time Since Loading (Days)
1CR
60R
Recycle
-7(!>LR
90MR
-10OHR
pH (Standard Units)
0 200 400 600 8(30 1000 1200 1400 1600
Time Since Loading (Days) '
2CS
Single Pass
3OS -*- 4OLS
5OMS
8OHS
Figure 29. Leachate pH during Simulated Landfill Investigations.
80
-------
concentrations due to their utilization by the methanogens, pH increased to a
range of 6.4 to 7.5, which is characteristic of a bicarbonate buffering system.
This behavior was observed in all recycle columns except Column 100HR, the
heaviest heavy metal-loaded recycle column. Leachate pH for Column 100HR
remained between 5.0 and 6.0 throughout the experimental period, and total
volatile acid (TVA) concentrations never decreased significantly after project
Day 800. These results suggest that a similarly viable population of methanogens
never developed in Column 10OHR, as evidenced by the absence of a decline in TVA
concentrations and by gas production trends (Table 24).
Single pass column pH, except for Column 2CS, remained between 5.0 and 5.5
during the experimental period. These values are well below the acceptable pH
range of 6.5 to 7.6 for active methanogenesis (Section 2.2.2), suggesting that
for Columns 30S, 40LS, 50MS and 80HS, a transition from the Acid Formation Phase
to the Methane Fermentation Phase of landfill stabilization, as evidenced by
increased leachate pH due to VGA consumption by methanogens, was not evident
This conclusion is supported by the relatively constant TVA concentrations
remaining after the apparent washout of volatile organic acids, and the apparent
washout of buffer capacity from the single pass columns. These concepts are
discussed further in Sections 4.4.2 and 4.4.3, respectively.
The control single pass column, Column 2CS, displayed a behavior similar
to that observed for the recycle columns. A decrease in TVA concentration around
project Day 1000 was accompanied by a concomitant increase in pH as shown in
Figure 29, implying that increased system pH during methanogenesis resulted
primarily from VOA conversion to methane and carbon dioxide and the establishment
of a pH characteristic of the bicarbonate buffering system.
4.4.2 Total Volatile Acids
Volatile organic acids are produced during degradation of organic material
and provide substrate for the methanogens. Total volatile acids (TVA) represent
the combination of individual volatile acids, usually expressed as an equivalent
amount of acetic acid. Leachate TVA concentrations for the recycle and single
pass columns are presented in Figure 30. Measured leachate TVA concentrations
are presented in Table A-15 of Appendix A.
The five recycle columns exhibited similar behavior regarding leachate TVA
concentrations during the first 1000 days of column operation. Leachate TVA
concentrations increased gradually until project Day 800, which corresponds to
the Acid Formation Phase of landfill stabilization. Between project Days 800 and
1000, leachate TVA concentrations began to decline concomitantly with the
development of a viable population of methanogens, as indicated previously by
increased gas production and percent methane. After project Day 1000, leachate
TVA concentrations declined rapidly for Columns 1CR, 60R, 70LR and 90MR in
accordance with the influence of active methanogenesis. However, Column 90MR
showed signs of slight retardation during the Methane Fermentation Phase, as
evidenced by lingering TVA concentrations until approximately project Day 1400
This slight retardation suggests that the loadings to Column 90MR stressed the
anaerobic bacterial population, but could be accommodated with a small lag in
stabilization. Column 100HR, the heaviest heavy metal-loaded recycle column
showed signs of more severe retardation. Leachate TVA concentrations did not
decline during methanogenesis, suggesting that the applied loading more
significantly retarded the growth of organisms responsible for conversion of
volatile organic acids to methane.
Because leachate was contained and recirculated in the five recycle
columns, VOA produced remained in the systems and were removed primarily due to
conversion to methane and carbon dioxide. Additional support for this conclusion
is evidenced by noting that the period of increased gas production and increased
methane percentage in the gas corresponded to the time period when leachate TVA
concentrations declined and system pH increased from values characteristic of the
si :
-------
TVA (g/I as Acetic Acid)
200 400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
~~""~ 1CR ' 6OR
Recycle
-*- 7OLR
i
-s- 9OMR -
-*- 10OHR
TVA (g/i as Acetic Acid)
200 400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
2CS
SOS
Single Pass
-*- 4OLS
5OMS
8OHS
Figure 30. Leachate Total Volatile Acids during Simulated Landfill
Investigations.
82
-------
VGA buffering system to those of the bicarbonate buffering system. Thus,
leachate recirculation provided more intimate contact between the substrate
contained within the leachate and the existing anaerobic bacterial populations,
thereby promoting the development of viable populations of acidogens, acetogens,
and methanogens responsible for anaerobic waste stabilization. This conclusion
is further evidenced by the ability of the recycle columns to accommodate applied
loadings of inorganic and/or organic priority pollutants, as reflected by trends
in gas production, gas composition, pH, and leachate TVA concentrations.
However, increased priority pollutant loadings can cause some retardation of
stabilization, as also evidenced by the behavior of the aforementioned parameters
observed for Columns 90MR and 100HR.
Leachate TVA concentrations observed in the single-pass columns were lower
than those measured in the recycle columns, which was primarily attributed to
dilution and washout effects. The changes in the TVA concentrations for the
control single pass column, Column 2CS, were similar to the behavior observed in
the recycle columns. However, a delay was observed compared to the control
recycle column, Column 1CR. This delay implies that the leachate recirculation
management strategy affords an opportunity for more complete and rapid
stabilization when compared to the single pass leaching option.
_The behavior of Columns SOS, 40LS, 50MS and 80HS was characteristic of
inhibition. Leachate TVA concentrations remained relatively constant throughout
the Methane Fermentation Phase, indicating that applied loadings under conditions
?£,washout adversely affected TVA conversion to methane and carbon dioxide.
(These four single pass columns also had the lowest volume of gas produced and
the smallest percentage of methane in the gas of all 10 simulated landfill
columns.) Leachate TVA concentrations were high during the Acid Formation Phase
between project Days 0 and 300, and decreased slightly between project Days 300
and 800 as dilution and washout became operative. Thus, the volatile organic
acids produced were removed primarily by washout as evidenced by relatively
constant leachate TVA concentrations during the Methane Fermentation Phase, low
gas.volumes produced, and low percent methane in the gas.
Overall, the results of total volatile acids analyses suggest that the
recycle columns were able to accommodate higher inorganic and organic loadings
than the single pass columns. Moreover, because the recycle columns were able
to achieve volatile acid conversion and waste stabilization earlier than
similarly loaded single pass columns, leachate produced and not converted by the
single pass columns exhibited a greater requirement for external treatment and
potential of imposing adverse environmental impacts.
4.4.2.1 Mass of Volatile Acid Lost due to Leachate Removal--
During operations of the recycle and single pass simulated landfill
columns, leachate was removed for sampling and analysis, and leachate was also
wasted from the single pass columns in accordance with that management strategy.
The leachate removed contained, among other things, volatile organic acids that
could serve as substrate for methanogens.
As discussed in Section 4.2.1 and illustrated in Figure 16, the five
recycle columns produced considerably more gas than the five single pass columns.
This difference in cumulative gas production may have resulted because more
substrate (VOA) was removed by washout from the single pass columns than from
recycle columns, thus depriving methanogens in the single pass columns of their
food source, with resultant lower gas production.
In order to determine if the single pass columns had the same gas
production potential as the recycle columns, the potential gas volume lost due
to acid removal was calculated based upon leachate TVA concentrations (Tables C-l
through C-10 of Appendix C) . The mass of volatile acids in the leachate removed
was determined as the product of leachate volume removed and, measured TVA
concentration expressed as acetic acid for each of the 10 simulated landfill
83
-------
columns. This calculation assumed that all volatile acids removed, expressed as
TVA as acetic acid, would have been converted into methane and carbon dioxide.
Also, the hydrogen produced during the conversion of individual volatile acids
to acetic acid was ignored, as was the carbon dioxide produced during the a-
carboxylation of propionic acid to yield acetic acid.
The hydrogen produced during the conversion of individual volatile acid
(IVA) to acetic acid exists both as the hydrogen ion and as hydrogen gas.
Released hydrogen is utilized both in the production of methane from carbon
dioxide and in the condensation of lower molecular weight IVA to form higher
molecular weight IVA. (As indicated previously, the percentage of hydrogen gas
measured by volume decreased to zero as landfill stabilization progressed.)
Carbon dioxide released during propionic acid degradation can be utilized to
produce methane and can exist, as can hydrogen, in either an aqueous or gaseous
state. Because the purpose of this Calculation was to determine the gas
production potential lost due to acids' removal and to compare these results
between the 10 simulated landfill columns, the additional effort required and the
benefits derived by including these calculations was not considered justified and
beyond the scope of these investigations.
Calculations of the potential gas volume lost due to leachate removal were
based upon the relationship that one mole of methane and one mole of carbon
dioxide are produced from one mole of acetic acid. Therefore, the number of
moles of gas removed could be determined by:
* (2 mo^es ffas }
mole CH3COOH (6)
where; !
nRMvo — Moles of gas removed [moles]
MRMVD ~ Mass of TVA as acetic acid removed [g]
MWoocooH " Molecular weight of acetic acid [60.053 g/mole]
The potential gas volume lost coulol then be found using the ideal gas law:
i
I
y , nRHVD * R * T (7)
where;
VLOST ~ Potential gas volume lost due to leachate removal [L]
nRMvn ~ Moles of gas removed [moles]
R - Universal Gas Constant [0.0821 L*atm/K*Mole]
T - Temperature [K]
PT — Total pressure [atm] \
The results of these calculations for the 10 simulated landfill columns are
presented in Tables C-l through C-10 of Appendix C and are summarized in Table
28. As presented in Table 28, the single pass columns lost a greater mass of
acid and greater gas production potential due to leachate removal than the
recycle columns where leachate was retained. In fact, the percentage of
potential gas volume lost due to leachate removal was 24.9 to 72.9% for the
single pass columns compared to 2.2 to 5.4% for the recycle columns.
84
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TABLE 28. GAS PRODUCTION POTENTIAL LOST DUE TO LEACHATE REMOVAL
DURING SIMULATED LANDFILL INVESTIGATIONS
Column
Identity
1CR
2CS
30S
40LS
50MS
60R
70LR
80HS
90MR
100HR
Total
Mass of
Acid
Removed
in
Leachate
(g TVA as
Acetic)
2584
8680
9284
8340
10506
1536
964
10959
1297
1910
Total
Potential
Gas Volume
Lost Due to
Leachate
Removal
(M3)
2.153
7.199
7.746
6.970
8.787
1.281
0.804
9.139
1.077
s 1.579
Measured
Total Gas
Volume
(M3)
65.755
21.676
5.454
2.893
3.266
57.126
27.872
7.112
30.007
27.833
Potential
Total Gas
Volume
(M3)
67.908
28.875
13.200
9.863
12.053
58.407
28.676
16.251
31.084
29.412
Percent
of Total
Gas
Volume
Lost
3.2
24.9
58.7
70.7
72.9
2.2
2.8
56.2
3.5
5.4
In comparing the potential cumulative gas production for the recycle and
single pass columns, calculated values show that the single pass columns
consistently produced less gas than the recycle columns. Therefore, the single
pass columns, by virtue of the leachate management strategy employed, did Hot
exhibit the same gas production potential as the recycle columns. Moreover the
decreased gas production observed in the columns receiving inorganic and/or
organic priority pollutants, compared to the respective recycle and single pass
control columns, was not due to differences in column operation, but was
attributed to loading effects upon the anaerobic stabilization process.
4-4.3 Individual Volatile Organic Acid Concentrations
During the simulated landfill investigations, leachate concentrations of
seven VOA were measured by the methods described in Section 3.5. These volatile
acids included acetic, propionic, butyric, iso-butyric, valeric, iso-valeric and
hexanoic acids The measured leachate concentrations of these seven acids are
included in Tables C-ll through C-17 of Appendix C and are plotted for each
column in Figures 31 through 40. As these figures indicate, the predominant
acids found in each column included acetic, propionic, and butyric acids The
average concentrations of the seven acids between project Days 30 and 623 prior
to anaerobic digester sludge seeding, were calculated and are presented in Table
^y. The five recycle columns exhibited similar trends regarding acid
concentrations in that butyric acid was found in the highest concentration
followed by acetic, propionic, hexanoic, iso-butyric, valeric, arid iso-valeric
acids, respectively. This behavior was also observed for the control single pass
column, Column 2CS.
*»* fln£oeticJa1id Wa/ m°st prevalent in the leachate of single pass Columns 40LS
and SOHS, and butyric acid was found in the highest concentrations in that of
85
-------
Concentration (g/I)
200 400 600 800 1000 1200 1400
Time Since Loading (Days)
1600
Acetic
Iso-Valerlc
Proplonlc
Hexanoic
Iso-ButyrIc
Valerie
Butyric
Figure 31. Leachate Individual Volatile Acids for Column 1CR.
Concentration (g/1)
200 400 600 300 1000 1200
Time Since Loading (Days)
1400 1600
Acetic
Iso-Valerlc
Proplonlc
Hexanoic
Iso-ButyrIc
Valeric
Butyric
Figure 32. Leachate Individual Volatile Acids for Column 2CS.
86
-------
Concentration (g/l)
200
400 600 800 1000 1200
Time Since Loading (Days)
140Q 1600
Acatlc
Iso-Valerlc
Proplonlc
Hexanolc
lao-Butyrlc
Valeric
Butyric
Figure 33. Leachate Individual Volatile Acids for Column SOS,
25
20
15
10
Concentration (g/l)
200 400 600 800 1000 1200 1400 1600
Time Since Loading (Days)
Acetic
Iso-Valeric
Proplonlc
Hexanolc
Iso-Butyrlc
Valeric
Butyric
Figure 34. Leachate Individual Volatile Acids for Column 40LS.
87
-------
Concentration (g/I)
200
400 600 800 1000 1200 1400 1600
Time Since Loading (Days)
Acetic
Iso-Valerlc
Proplonic
Hexanotc:
Uo-ButyrIc
Valeric
Butyric
Figure 35. Leachate Indiyidtial Volatile Acids for Column 50MS
Concentration (g/i)
200
400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
Acetic
Iso-Valeric
Proplonic
Hexanolc
Iso-Butyrlc
Valeric
Butyric
Figure 36. Leachate Individual Volatile Acids for Column 60R.
88
-------
Concentration (g/l)
200 400 600 800 1000 1200 1400 1600
Time Since Loading (Days)
Acetic
Iso-ValerIc
Proplonlc
Hexanolc
Uo-Butyric
Valeric
Butyric
Figure 37. Leachate Individual Volatile Acids for Column 70LR.
Concentration (g/I)
0 200 400 600 800 1000 1200 1400 1600
Time Since Loading (Days) ]
Acetic
I so-Valeric
Proplonlc
Hexanolc
Iso-Butyrlc
Valeric
Butyric
Figure 38. Leachate Individual Volatile Acids for Column 80HS.
89
-------
Concentration (g/1)
200 400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
Acetic
Iso-Valarlc
Proplonie
Hexanolc
Iso-Butyrlc
Valeric
Butyric
Figure 39. Leachate Individual Volatile Acids for Column 9OMR.
I
Concentration (g/l)
200
400 600 800 1000 1200
Time Since'Loading (Days)
1400 1600
Acetic
Iso-Valeric
Proplonie
Hexanole
Iso-Butyrlc
Valeric
Butyric
Figure 40. Leachate Individual Volatile Acids for Column 100HR.
90
-------
TABLE 29. AVERAGE LEACHATE INDIVIDUAL VOLATILE ACIDS DURING THE ACID
FORMATION PHASE OF THE SIMULATED LANDFILL INVESTIGATIONS
Individual
Volatile
Acid
Acetic
Prop ionic
Butyric
Iso -Butyric
Valeric
Iso-Valeric
Hexanoic
1CR
6.328
2.389
12.419
0.512
0.308
0.17.8
0.564
Concentration
(g/L)
2CS
4.408
1.275
6.601
0.366
0.276
0.156
0.686
30S
0.964
0.953
5.088
0.127
0.204
0.118
0.904
40LS
4.366
0.697
3.795
0.181
0.141
0.100
0.616
50MS
4.107
0.815
5.037
0.259
0.134
0.087
0.405
60R
4.970
1.639
9.225
0.202
0.197
0.102
1.006
70LR
4.364
1.319
7.639
0.154
0.127
0.063
0.657
80HS
5.637
1.189
4.769
0.429
0.158
0.091
0.421
90MR
5.164
1.800
8.470
0.392
0.219
0.127
0.623
100HR
5.885
2.271
7.874
0.287
0.269
0.123
0.751
single pass Columns SOS and 50MS. In Columns 30S, 40LS and 50MS, hexanoic acid
was found in higher concentrations than valeric, iso-valeric, and iso-butyric
acids, and hexanoic acid was found in higher concentrations than valeric and iso-
valeric acids in Column 80HS.
Moreover, the behavior of the IVA was reflected in measured TVA
concentrations previously presented in Figure 30. The decline in TVA
concentration for Columns 1CR, 60R, 70LR, 90MR and 2CS was accompanied by a
concomitant decrease in individual volatile acids concentrations, as expected
Elevated TVA concentrations present in Column 100HR during the Methane
Fermentation Phase was due to lingering propionic and acetic acid concentrations
as illustrated in Figure 40. Single pass Columns SOS, 40LS and 50MS all
displayed higher TVA concentrations during the Methane Fermentation Phase which
can be attributed to lingering acetic acid concentrations. The increase in TVA
concentration for Column 80HS towards the end of the experimental period can be
explained by increased acetic acid concentrations.
In general, the recycle columns exhibited higher individual volatile acids
concentrations during the Acid Formation Phase of landfill stabilization than the
corresponding single pass columns. This trend can be attributed to washout and
• iu °n fcids in the single pass columns, which did not significantly occur
in the recycle columns because leachate was not wasted but contained within the
system. Therefore, in order to avoid dilution and washout effects influencine
comparison of results between the 10 simulated landfill columns, the acids
produced were compared on a mass basis instead of employing a concentration-based
comparison. A comparison of the mass of acid produced is discussed in Section
4.4.3.1 Mass Production of Individual Volatile Acids--
The process of anaerobic conversion of organic material to produce methane
and carbon dioxide occurring in a landfill environment is the same as that
occurring in separate anaerobic treatment systems, with the exception that the
effective retention time in a landfill is on the order of years compared to days
tor separate anaerobic processes. As in separate anaerobic systems, VGA are
produced in landfills as a result of waste degradation, and the identification
91
-------
of the type and quantity of IVA produced can provide evidence regarding the
progression or inhibition of landfill stabilization processes.
The production of IVA cannot be measured simply as leachate IVA
concentrations because the effects of dilution, washout, and operational
differences between columns tend to obscure -the results. More correctly, IVA
production should encompass both the mass of IVA transferred to the leachate and
the mass of IVA transformed into methane , gas.0 " The mass of acid transferred to
the leachate should include both the JLeachate removed from the column for
sampling and/or wasting' and the leachate retained in the column in excess of
indicated field capacity. The mass of acid produced can be determined by
performing a mass balance around the column control volume as shown in Figure 41 .
The mass of volatile acids recirculated ;in the five recycle simulated landfill
columns is considered internal to the column control volume. The mass balance
is performed as follows:
1 — = INFLOW - OUTFLOW + PRODUCTION - UTILIZATION W
dt
The change in volatile acids mass jper unit time (dM/dt) is a function of
the mass of acids added, removed, produced and consumed. Because anaerobic
processes progress slowly relative to the sampling time scale, the mass of acids
accumulated from one sampling period to the next will be negligible in comparison
with the mass of acids utilized, lost in [the effluent, or recycled. Thus, dM/dt
can be assumed to be zero. Therefore, for each IVA: i
+ [ cs(i-u^cs(i) ^( 4) } } + M^(i) _MOTIL(I]
where ;
i . — Sampling time period
MPRQ - Mass of IVA produced [g]
Mm^ - Mass of IVA transformed into methane [g]
C, - Concentration of IVA in feed [g/L]
CE — Concentration of IVA in leachate sample [g/L]
V, — Volume of liquid input to column [L]
Vw - Volume of leachate wasted; [L]
VEX — Volume of liquid in excess of indicated field capacity [L]
The third term in Equation (9) represents an average concentration of IVA in the
liquid retained in the column in excess of indicated field capacity, because the
assumption of a uniform concentration throughout the column is not necessarily
valid. Equation (9) can be further simplified because only tap water was added
to each column. Thus the input concentration, C,, was zero. Rearranging
Equation (9) yields:
The first two terms in Equation (10) represent the mass of acids released into
the leachate, and the last term, M^, is the mass of IVA transformed into
methane .
In order to determine HUTJL, the volume of methane produced by each column
92
-------
CH.
L
r- 1
VEX
VR
L, -. -
vw cg
CONTROL
VOLUME
Figure 41 .• Mass Balance on Simulated Landfill Column Control Volumne.
93
-------
was calculated. Because methane exists in both the gaseous and dissolved forms,
the volume of methane gas was calculated as follows:
where;
VMO — Volume of methane gas produced [L]
VQJX - Total volume of gas produced [L]
FCJM ~ Fraction of methane in gas
T - Temperature [°C] j
VP — Water vapor pressure at indicated temperature [mm Hg]
i
The last two terms in Equation (11) are a correction of the gas volume produced
to standard temperature (0°C) and pressure (1 atm). The volume of dissolved
methane was calculated by assuming that methane behaves as an ideal gas and that
the density of water does not change significantly with temperature, as discussed
previously. Thus: . !
R * (273.15+r(i)) * Vggd)
where; H(i) is Henry's constant for methane [atm], which varies with temperature
according to: \
= ALOg(j~ff- + K) (13)
and; ;
AH - 1540 kcal/kraol
R - Universal Gas Constant [1.987 kcal/kmol*K]
T - Temperature [K]
K - 7.22 [dimensionless constant]
The total volume of methane produced (V^) was determined as:
I
The volume of methane produced, computed from Equation (14), can be converted
to COD using the following expression:
U) = VguU) * (2.857
where ; '
CODOH ~ Methane COD [g] |
i
94!
-------
calculation of, the .COD of methane produced is presented for the 10 simulated
landfill columns in Tables C-18 through C-27 of Appendix C.
It was then necessary to determine the fraction of methane produced that
could be attributed to each of the seven individual volatile acids monitored
during these studies This calculation was performed using a ratio of the COD
?f,S V11?^.^1 volatile acid under consideration in the leachate to the total
leachate individual volatile acid COD. This ratio was determined for each of the
^latile acids during each sampling period and was determined
MC(n,i)
IVA(n,i) « COP (a)
) * COD(n)}
^ J
where ;
n -
i -
MC -
IVA -
COD -
Appendx C
Acetic, propionic, butyric, iso-butyric, valeric
iso -valeric, or hexanoic acid
Sampling time period
Fraction of methane attributable to IVA, during
time period i -no
Leachate concentration of IVA. fe/Ll
COD equivalent of IVAn
i °f methane attributable to each IVA in each of the
columns are presented in Tables C-28 through C-37 of
Lastly, the mass of acid transformed into methane
SubstitUtinS the «.«lt into
can be determined
(10) to
* MC(n,i)
COD
The mass of the seven IVA produced in each of the 10 simulated landfill
' *S° figures display the mass of fach of tLlev.n IVA r" tasjd
"11" '"1 MS of "cld "ansformed into m.than"
95
-------
8
Mass (kg)
6-
4-
2-
0
^.
t
42 182 322 482 802 [ 742 832 1022 1182 1302 1428
112 262 392 632 872 812 962 1092 1232 1372
Time Since Loading (Days)
EH Acttle
Valeric
I I Proplonlo
Iso-Valaric
Butyric
Hexanole
Iso-Butyrle
Transformed
Figure 42. Mass of Individual Volatile Acids Released
and Transformed in Column 1CR.
8
Mass (kg)
6-
4-
2-
0
I
42 182 322 482 602 742 882 1022 1182 1302 1428
112 262 392 632 872 812 962 1092 1232 1372
Time Since Loading (Days)
EM3 Acetic
W& Valeric
I — I Proplonlc
Iso-Valerlc
Butyric
Haxanolc
Iso-Butyrle
Transformad
Figure 43. Mass of Individual Volatile Acids Released
and Transformed in Column 2CS.
96
-------
10
Mass (kg)
6-
4-
2-
I
42 182 322 482 802 742 882 1022 1182 1302 1428
112 282 392 632 872 812 962 1092 1232 1372
Time Since Loading (Days)
Acetic
Valeric
L—I Proplonlc
Iso-Valerlc
Butyric
[• Hexanolc
Iso-Butyrlc
Transformed
Figure 44. Mass of Individual Volatile Acids Released
and Transformed "in Column 308.
8
Mass (kg)
6-
4-
2-
^
H
42 182 322 482 802 742 882 1022 1182 1302 1428
112 282 392 632 872 812 962 1O92 1232 1372
Time Since Loading (Days)
Acetic
Valeric
I 1 Proplonlc
Iso-Valerlc
Butyric
Haxanolc
Iso-Butyrlc
Transformed
Figure 45. Mass of Individual Volatile Acids Released
and Transformed in Column 40LS.
97
-------
8
Mass (kg)
e-
4-
2-
42 182 322 482 602 742 882 1022 1162 1302 1428
112 262 392 632 672 812 362 1092 1232 1372
Time Since Loading (Days)
EEO Acatlc
Valarlc
I—I Proplonlc
I«o-Valerlc
Butyric
Hexanole
I«o-Butyrlc
Transforms*!
Figure 46. Mass of Individual Volatile Acids Released
and Transformed in Column 50MS.
Mass (kg)
•• v -
42 182 322 462 602 742 882 1022 1182 1302 1428
112 2S2 392 632 672 812 962 1092 1232 1372
Time Since Loading (Days)
Ac.tlc
Valerte
I — I Proplonlc
!«o-Valerlc
Butyric
Hsxanole
leo-Butyrlc
Tran«form«d
Figure 47. Mass of Individual Volatile Acids Released
and Transformed in Column 60R.
98'
-------
Mass (kg)
6-
4-
2-
42 182 322 482 802 742 882 1022 1182 1302 1428
112 262 392 632 872 812 962 1092 1232 1372
Time Since Loading (Days)
ES3 Acetic
Valerie
I—! Proplonlc
lao-Valarlc
Butyric
Hexanolc
Iso-ButyrIc
Transformed
Figure 48. Mass of Individual Volatile Acids Released
and Transformed in .Column 70LR. !
Mass (kg)
6-
4-
2-
•f i-j n-j, i r-vi lip tup-i rr,|,Yi IVJM r.yi rr;,
42 132 322 482 802 742 882 1022 1182 1302 1428
112 262 392 632 872 812 962 1092 1232 1372
Time Since Loading (Days)
Acetic CD Proplonlo
Valeric H^ Iso-ValeHe
Butyric
Hexanolc
lao-Butyrlc
Tran«formed
Figure 49. Mass of Individual Volatile Acids Released
and Transformed in Column 80HS.
99
-------
8
Mass (kg)
6-
4-
2-
I
1
p r.-f,i rrfi I,T,YI rhp-i nyi i,-.r.i.t.-p .v.,-,. .,•,:,•,•! ..--p IT,-,-,I iryi ny.-.,!-,-)-,, ..-,-
42 182 322 462 602! 742 882 1022 1132 1302 142H
112 262 392 632 872 812 982 1092 1232 1372
Time Since Loading (Days)
MB Acetic
Valerie
I—I Proplonie
I«o-Val«rlc
Butyric
Hexanolc
lao-Butyrlc
Transformed
Figure 50. Mass of Individual Volatile Acids Released
and Transformed in Column 90MR,
Mass (kg)
42 182 322 482 802 I 742 882 1022 1182 1302 1428
112 262 392 632 872 812 962 1092 1232 1372
Time Sinc^ Loading (Days)
Acetle
Valerie
I—I Proplonie,
Iso-Valaric
Butyric
Hexanolc
lao-Butyrlc
Transformed
Figure 51. Mass of Individual Volatile Acids Released
and Transformed ;in Column 100HR.
100
-------
difficult to detect in the leachate. Towards the end of the experimental period
at project Days 1372 and 1428, butyric acid began to appear along with low masses
of acetic and propionic acids. The appearance of these acids probably resulted
from the onset of degradation of more microbially resistant materials.
Additionally, Column 1CR had the highest mass of acids transformed into methane,
signifying the most viable and greatest degree of waste conversion.
Column 60R displayed similar behavior to Column 1CR, however, a delay in
the removal of acids from the leachate occurred. Volatile organic acids were not
significantly removed until project Day 1302, a probable result of the added
organic priority pollutant loadings applied to this column.
Similar to Column 60R, Column 70LR also displayed a delay in the removal
of volatile organic acids from the leachate compared to Column 1CR, but acids
mass was significantly reduced by project Day 1232. Also, Column 70LR showed
smaller masses of acids being initially transformed into methane, providing
evidence of the retarding effects of the added inorganic and organic priority
pollutants. '
Column 90MR exhibited a slow decline in the mass of volatile organic acids
released. However, the VGA were never reduced to the low levels achieved by
Columns 1CR, 60R, and 70LR. This behavior was attributed to the higher inorganic
priority pollutant loading present in Column 90MR.
The control single pass column, Column 2CS, also behaved similarly to the
control recycle column, Column 1CR. The mass of volatile organic acids released
to the leachate was removed by project Day 1372 by both washout and conversion
to methane and carbon dioxide. Although Column 2CS produced methane, the mass
of acid utilized to form this methane was much lower in Column 2CS than in Column
1CR, suggesting that leachate recirculation promoted more thorough waste
conversion by providing a more complete contact between the microorganisms and
usable substrate.
The most heavily loaded recycle column, Column 100HR, exhibited behavior
characteristic of severe retardation due to applied organic and inorganic waste
loadings. The mass of volatile organic acids released increased until'
approximately project Day 882, followed by a slight decline commensurate with the
onset of methane production. However, the mass of volatile organic acids
released remained relatively constant from project Day 1022 until the end of
the experimental period. Because Column 100HR released about the same mass of
acids into the leachate, and had a significantly lower mass of acids transformed
into methane, it is likely that the applied loadings adversely affected the
acetate utilizing methanogenic population.
The remaining four single pass columns, Columns SOS, 40LS, 50MS and 80HS,
displayed lower masses of volatile organic acids released than from Column 2CS.
Additionally, all of these four columns displayed a negligible mass of acids
transformed into methane, and relatively constant mass of acids released
throughout the experimental period after an initial lag of 182 days. These
results suggest that the combination of loading applied and leachate management
strategy employed adversely affected both the acid forming and methane forming
populations. °
In general, the five recycle columns displayed a greater mass of individual
volatile acids transformed into methane than did the single pass columns Also
recycle Columns 60R, 70LR, 90MR and 100HR showed more volatile organic acids
released than the corresponding single pass columns, while the control single
pass column, Column 2CS, had a higher mass of acids released than did the control
recycle column, Column 1CR. These results imply that the environment in the
recycle columns, in general, was more favorable to acid formers and methanogens,
which can be attributed to increased contact opportunities between biomass,
nutrients and substrate offered by this leachate management strategy. Because
101
-------
the single pass columns were more severely affected by the codisposed inorganics
and/or organics, as evidenced by the relatively constant mass of volatile organic
acids released throughout the experimental period, leachate produced in the
single p^ass systems would require external treatment and had a greater
possibility of adversely affecting the ', environment if it were to escape the
landfill boundaries. '
The primary acids released by the 10 simulated landfill columns included
acetic, propionic, and butyric acids. Of these acids, butyric acid was the most
predominant during the Acid Formation Phase, followed by acetic and propionic
acids, respectively. ;
All seven individual volatile acids were released into the leachate of the
10 simulated landfill columns, however, the mass of IVA released differed between
columns. In general, columns that exhibited the progression of anaerobic waste
stabilization processes similar to those displayed in Figure 1 (Columns 1CR, 2CS,
60R, 70LR, 90MR and 100HR) released more IVA into the leachate than those columns
exhibiting signs of retardation of the stabilization process (Columns SOS, 40LS,
50MS, and 80HS). The columns that released the lowest mass of IVA into the
leachate and displayed" signs of retardation were all single pass columns that
received inorganic and/or organic priority pollutant loadings. These findings
suggest that a combination of the single pass leaching management strategy and
priority pollutant loadings inhibited ithe decomposition of soluble organic '
material into volatile organic acids. (Factors affecting the waste stabilization
process were discussed in Section 2.2.2.) Therefore, higher masses of IVA
released and transformed into methane were observed in Columns 1CR, 2CS, 60R,
70LR, 90MR and 100HR, while lower masses of IVA released and transformed were
observed in Columns SOS, 40LS, 50MS and 80HS.
Upon examining Figures 42 through 51, and studying supporting data, some
interesting relationships among the IVA was observed. Probably the most >
significant of these relationships occurred between the mass of hexanoic and
butyric acids released into the leachate. Hexanoic acid was detected in the
leachate of all 10 simulated landfill; columns, however, hexanoic acid was
released into the leachate earlier in single pass columns than in recycle
columns. Additionally, an increase in the mass of butyric acid released into the
leachate was accompanied by an increase in hexanoic acid mass released.
Similar 'results regarding the formation of longer chain IVA due to
increases in lower molecular weight VOA were reported by MeCarty and coworkers("MO)
using laboratory-scale anaerobic digesters. An explanation of hexanoic acid
production during periods of high butyric acid mass is the thermodynamic
favorability of the condensation of butyric and acetic acids to form hexanoic
acid. Once hexanoic acid was produced,\ i,t was present in the leachate until
methane fermentation commenced, which promoted a reduction in the mass of butyric
acid in the leachate. Additionally, the .decline in hexanoic acid mass released
was accompanied by decreases in butyric and acetic acid mass as well. This
relationship may result from the breakdown of hexanoic acid into butyric and
acetic acid, and the breakdown of butyrib acid to form acetic acid.
I
A second interesting behavior was observed between propionic, valeric, and
hexanoic acids. Within all 10 simulated landfill columns during the Acid
Formation Phase of landfill stabilization, it was observed that increases in the
mass of propionic acid released were accompanied by concomitant increases in the
mass of hexanoic and valeric acids released. This relationship may be explained
by the condensation of two moles of propionic acid to form hexanoic (caproic)
acid, and the combination of propionic and acetic acids to produce valeric acid.
These reverse reactions become favorable when the mass of released propionic acid
is high. Propionic acid, which is produced during the anaerobic conversion of
carbohydrates and proteins, may be released in large masses as a result of the
conversion of these constituents during stabilization processes.
.
102
-------
As mentioned previously, the primary acids released in all 10 simulated
landfill columns included butyric,, acetic ,,Baijd propionic acids. Hexanoic acid
masses became significant toward the end fff the Acid Formation Phase and
beginning of Methane Fermentation Phase for those columns exhibiting significant
progress toward complete anaerobic waste stabilization. These columns included
Columns 1CR, 60R, 70LR, 90MR, 100HR and 2CS. Within these columns, hexanoic acid
masses were reduced upon commencement of methane production. However, in single
pass Columns SOS, 40LS, 50MS and 80HS, hexanoic acid masses were significant
throughout the experimental period. These results suggest that the more
sensitive members of the anaerobic microbial consortia were adversely affected
by the leachate management strategy, simulated landfill column pH, and/or
presence of toxic materials. Thus, it appears that the organisms responsible for
both the production (acidogens) and subsequent degradation (acetogens) were
inhibited, as evidenced by lower masses of volatile organic acids produced and
the composition of individual volatile acids produced.
4.4.4 Total Alkalinity
Total alkalinity is a measure of system buffer capacity. The measured
leachate alkalinity concentrations for the recycle and single pass columns are
provided in Figure 52. Measured leachate alkalinity concentrations are presented
in Table A-16 of Appendix A. Alkalinity concentrations for the five recycle
columns closely paralleled the measured TVA concentrations. During the Acid
Formation Phase, leachate alkalinity remained relatively constant which, along
with high TVA concentrations and low pH (5.0 to 5.5), suggested that a volatile
organic acid buffering system predominated. As methanogens utilized the
available VGA as substrate, pH increased and the total alkalinity concentrations
tended to decrease. As previously discussed, Column 100HR was adversely affected
by applied pollutant loadings as evidenced by lingering leachate TVA
concentrations and depressed pH during the Methane Fermentation Phase. Further
evidence of retardation was demonstrated by higher total alkalinity
concentrations for Column 100HR than for the other recycle columns.
Single pass column leachate alkalinity concentrations were high initially
until project Day 150, and decreased thereafter primarily due to washout.
Alkalinity concentrations remained consistently low during the Methane
Fermentation Phase, even though TVA concentrations were relatively stable, which
provides further evidence of washout. Column 2CS exhibited a slightly different
behavior than the other four single pass columns during the Methane Fermentation
Phase, in that alkalinity concentrations declined to lower values around project
Day 1100, which was accompanied by a concomitant decline in TVA concentration.
4.4.5 Organic Strength Indicators
Leachate chemical dxygen demand (COD) and total organic carbon (TOG) were
measured as indicators of organic strength and the potential pollutional impact
that may result if leachate is released. Leachate COD concentrations for the
recycle and single pass columns are presented in Figure 53 and TOG concentrations
for the recycle and single pass columns are provided in Figure 54. Measured
leachate COD and TOG concentrations are presented in Tables A-17 and A-18 of
Appendix A. . •
Recycle column leachate COD and TOC concentrations closely paralleled those
of the TVA, since TVA represent a large portion of COD and TOC especially during
the Acid Formation Phase. Leachate COD and TOC both remained relatively constant
at their highest concentrations during the Acid Formation Phase. After seeding
with anaerobic digester sludge and neutralization promoted the onset of
methanogenesis, leachate COD and TOC concentrations declined in accordance with
the progression of microbially-mediated stabilization processes.: Decreases in
COD and TOC concentrations, as well as TVA concentrations, occurred along with
corresponding increases in gas production. This relationship indicates that much
of the leachate organic content was removed by in situ treatment and conversion
103 i
-------
Alkalinity (g/I as CaCOS)
0 200 400 600 800 1000 1200 1400 1600
Time Since Loading (Days)
1CR
Recycle
-*- 6OR -*- 70LR -e- 90MR
-^- 10OHR
25
20
15
10
I
5
0
Alkalinity (g/I as CaCOS)
200 400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
2CS
3OS
Single Pass
40LS
5OMS
8OHS
Figure 52. Leachate Alkalinity during Simulated Landfill Investigations.
104
-------
100
Chemical Oxygen Demand (g/l)
200
400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
100
Chemical Oxygen Demand (g/l)
Single Pass
2CS
SOS
-#- 40LS
-B- 5OMS
-£r- 80HS
200
400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
Figure 53. Leachate Chemical Oxygen Demand during Simulated Landfill
Investigations.
105
-------
Total Organic Carbon (g/l)
200
400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
40
35
30
25
20
15
10
5
0
Total Organic Carbon (g/I)
Single Pass
— 2CS
SOS
-*- 40LS
-B- SOMS
80HS
200 400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
Figure 54. Leachate Total Organife Carbon during Simulated Landfill
Investigations. <
106
-------
to methane and carbon dioxide, since leachate was essentially not wasted from the '
recycle columns. Column 100HR continued to show signs of retardation, as
indicated by higher relative COD and TOG concentrations during the Methane
Fermentation Phase. The high removal rate of organic material from the leachate ;
of the recycle columns can be attributed to the nature of leachate recirculation i
management strategy and continuing exposure of anaerobic microorganisms to
available substrate.
Single pass column COD and TOG concentrations exhibited effects i
characteristic of washout as indicated by the rapid decline in concentration
until project Day 400 and the absence of significant methane gas production
during this time period. Although some organic material stabilization occurred |
during the Methane Fermentation Phase, as evidenced by some methane gas
production, the majority of the leachate organic content was removed by washout.
However, Column 2CS, displayed signs of increased biological conversion of :
organic material when compared to other single pass columns, in that both COD and '
TOG concentrations were reduced around project Day 1050 to levels lower than :
those existing in the leaehates of the other four single pass columns.
These results again suggested that the leachate recirculation management '
option provided for more complete organic material conversion than did the single
pass leaching management strategy. Stabilization of organic material released
to the leachate of the recycle columns surpassed that of the single pass columns,
as illustrated by increased gas production, and larger percent methane in the gas
phase, and decreased leachate COD, TOG and TVA concentrations. In addition,
inorganic and/or organic loadings applied to recycle columns were apparently •
accommodated by the landfill environment present, whereas similar loadings '
applied to the single pass columns were inhibitory to 'waste stabilization !
processes.
4.4.6 Oxidation-Reduction Potential
Oxidation-reduction potential (ORP) measured during these studies indicated '
the oxidizing or reducing conditions present in the 10 simulated landfill '
columns. Measured leachate ORP values for recycle and single pass columns are
presented in Figure 55 and the supporting data are included in Table A-19 of
Appendix A.
As indicated by Figure 55, leachate ORP values for the recycle columns
remained characteristic of a reducing environment, however, dramatic fluctuations
during the Acid Formation Phase were observed. After the onset of active methane
production around project Day 800, variations in leachate ORP were dampened, and
ORP values declined to a low of approximately -350 mV on project Day 1348 for all
recycle columns except Column 100HR. After this minimum ORP value was achieved, •
leachate ORP values began to become less negative. Leachate ORP values measured i
in Column 100HR followed a pattern similar to the other four recycle columns, ;
however, the minimum value reached was -181 mV. Despite the analytical
sensitivity of ORP measurements, these results further support the conclusion ;
that methanogens in Column 100HR were adversely affected by the applied loading
because an ORP of at least -300 mV is generally required for proliferation of
methanogens.
Leachate ORP values measured for the single pass columns were generally
less negative (or in some cases positive) than those measured for the recycle
columns. Column 2CS, the control single pass column, displayed a behavior
similar to the recycle columns during the Methane Fermentation Phase, in that ORP
values declined concomitantly with more active methane production and reached
their lowest value of -279 mV on project Day 1327. After achieving this value, !
leachate ORP values increased similarly to the behavior of recycle columns ;
During the Acid Formation Phase, ORP values measured in the single pass columns
were sometimes positive or only slightly negative, which can partially be >
attributed to displacement of air originally present in refuse void spaces. The
107 !
-------
200
ORP (mV)
-200
-400
-600
-800
0 200 400 600 800 1000 1200 1400 1600
Time Since Loading (Days)
1CR
-+-6OR
Recycle
-*-7QLR -3-9OMR
i
-A- 10OHR
200
ORP (mV)
-800
0 200 400 600 800 1000 1200 1400 1600
Time Since Loading (Days)
2CS
Single Pass
-4— SOS -*- 4JOLS -<
3- SOWS -*- 8OHS
Figure 55. Leachate ORP during Simulated Landfill Investigations.
108
-------
remaining four single pass columns, Columns 30S, 40LS, 50MS and 80HS, reached j
their minimum ORP values of approximately ,-160 mV on project Day 1327. The ORP j
values maintained by these columns during the Methane Fermentation Phase were '
less conducive to the establishment of a viable population of methanogens, as
evidenced by low gas production volumes, low percent methane in the gas, and i
lingering leachate TVA, COD, and TOC concentrations. !
4.4.7 Chloride i
In order to observe the effects of leachate management strategies upon I
priority pollutant behavior and fate, leachate chloride was measured as a :
conservative tracer, not affected by biological conversion. Chloride
concentrations for the recycle and single pass columns are presented in Figure ;
56, and measured leachate chloride concentrations are presented in Table A-20 of :
Appendix A. Leachate chloride concentrations in the recycle columns decreased :
slowly due to dilution effects after an initial period of leaching and '
mobilization. In contrast, leachate chloride concentrations for the single pass •
columns exhibited a behavior characteristic of washout of a conservative tracer,
as evidenced by the rapid decline in concentrations. i
i
4.4.8 Ammonia
Ammonia is an essential nutrient derived directly from the waste input or :
produced during decomposition of organic material containing nitrogen. Leachate
ammonia concentrations were measured as an indication of nutrient availability •
and buffer within each of the 10 simulated landfill columns. Within the pH range '•
existing in the columns, ammonia was primarily present as the. ammonium ion, which
is less toxic than ammonia gas produced at a higher pH. Measured leachate '
ammonia concentrations for the recycle and single pass columns are shown in
Figure 57 and corresponding data are presented in Table A-21 of Appendix A.
Ammonia concentrations for the recycle columns remained relatively constant :
throughout the experimental period, with the exception of a decline in all '
columns around project Day 600. Ammonia concentration behavior was attributed
directly to the nature of the leachate recirculation management strategy, whereby
available nutrients are contained and recirculated within the column,-providing i
an increased opportunity for their accumulation and/or removal through biological
assimilation. The range of ammonia concentrations observed for the recycle
columns was between 600 and 1200 mg/L, which has been shown to exert no adverse
effect upon anaerobic processes."3' :
Ammonia concentrations in the leachate from the single pass columns
displayed evidence of washout, although concentrations were maintained above 200 ;
mg/L at all times during the experimental period, thereby indicating availability '•
for establishing a viable microbial population.
4.4.9 Sulfate and Sulfide ,
Sulfate was present in the refuse and the metal sludge mixture added to six i
of the 10 simulated landfill columns. Leachate sulfate concentrations measured ;
for the recycle and single pass columns are shown in Figure 58, and measured
leachate sulfate concentrations are presented in Table A-22 of Appendix A.
Sulfate was present in the recycle columns until methane production began on or
about project Day 800. The decline in leachate sulfate concentration resulted
from the establishment of highly reducing conditions and the ensuing reduction
of sulfate to sulfide. Sulfate was removed from the leachate of single pass !
columns mainly by washout, as evidenced in Figure 58.
The removal of sulfate from the leachate of the recycle and single pass
columns was accompanied by the appearance of sulfide around project Day 800,
which becomes possible at low ORP values (below -200 mV). Moreover, sulfide is !
a potent precipitant of heavy metals, thereby removing the potential toxic
effects of heavy metals on the biological populations. As discussed in Section -
109 !•
-------
Chloride (mg/i)
200
400 600 800 ' 1000 1200
Time Since Loading (Days)
1400 1600
4000
Chloride (mg/l)
200 400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
Figure 56. Leachate Chloride during Simulated Landfill Investigations.
110
-------
1600
Ammonia (mg/l)
200
400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
Recycle
1CR — *— 6OR -*-7OLR ~e~ 9OMR
-*- 10OHR
1600
1400
1200
1000
800
600
400
200
0
Ammonia (mg/l)
200 400 600 800 1000 1200
Time Since Loading (Days)
Single Pass
1400 1600
2CS
sos
5OMS
8OHS
Figure 57. Leachate.Ammonia during Simulated Landfill Investigations.
Ill
-------
Sulfate (mg/1)
Recycle
— 1CR
6OR
-*- 70LR
-a- 9OMR
10OHR
200
400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
4000
Suifate (mg/l)
200 400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
Figure 58. Leachate Sulfate during Simulated Landfill Investigations.
112
-------
4.4.11, heavy metal concentrations were observed to decline concomitantly with
the appearance of sulfide. Sulfide concentrations for the recycle and single
pass columns are presented in Figure 59, and measured leachate sulfide
concentrations are included in Table A-23 of Appendix A.
4.4.10 Organic Priority Pollutants
Twelve organic priority pollutants were added to Columns 30S, 40LS, 50MS,
60R, 70LR, 80HS, 9OMR and 100HR. A summary of the type and amount of organic
priority pollutant added to these eight test columns was presented previously in
Tables 19 and 20. Measured leachate concentrations of eight of the twelve
organic priority pollutants are included in Figures 60 through 67, and measured
leachate concentrations of the organic priority pollutants are included in Table
A-24 through A-31 of Appendix A.
Five non-polar organic priority pollutants were added to the eight columns
and included naphthalene, lindane, dieldrin, dioctyl phthalate and
hexachlorobenzene. As indicated by Figure 60, naphthalene showed significant
mobility, but was also detected in the leachate from the control columns, Columns
1CR and 2CS. Lindane was generally retained within the columns, being detected
in concentrations less than 20 jug/L only near the end of the experimental period.
Hexachlorobenzene, dieldrin, and bis-2-ethylhexyl phthalate were never detected
in the leachate from any column.
Two organics measured by the purge and trap method of analysis,
dibromomethane and trichloroethylene, were also added to the eight simulated
landfill columns and their leachate concentrations are shown in Figures 61 and
62. These compounds were detected early in the leachate and were highly mobile,
particularly during the Acid Formation Phase.
Two extractable organic compounds were also added to the columns, which
included dichlorobenzene and trichlorobenzene. These compounds, shown in Figures
63 and 64, exhibited low mobility as evidenced by their slow evolution and low
leachate concentrations.
Nitrobenzene, nitrophenol and dichlorophenol, three polar semi-volatile
organic priority pollutants, were also added to the eight test simulated landfill
columns. Nitrobenzene and nitrophenol, shown in Figures 65 and 66, exhibited
slow yet distinct but diminishing mobility, especially during the Methane
Fermentation Phase. Dichlorophenol, shown in Figure 67, seemed to be mobilized
during methane fermentation, possibly as a consequence of the more neutral pH.04'
The possible attenuation mechanisms affecting the codisposed organic
compounds included dispersion, fractionation, volatilization, sorption, and
biodegradation. Reductive dehalogenation was suggested by 'the appearance of
elevated leachate bromide concentrations, shown in Figure 68, as dibromomethane
concentrations decreased, and by the detection of vinyl chloride shown in Figure
69, a probable transformation product of trichloroethylene, in the headspace gas
as this compound diminished in leachate concentration. Leachate bromide and
headspace vinyl chloride concentrations are presented in Tables A-25 and A-26 of
Appendix A. A more in-depth analysis of the attenuation mechanisms is provided
by Pohland and coworkers^4' and in Section 5.,1.
When examined relative to effects upon the progress of landfill
stabilization, the applied organic priority pollutant loadings present, in the
absence of admixed heavy metals, were apparently readily accommodated as
evidenced by gas production, gas composition and reductions in TVA
concentrations.
4.4.11 Metals
Metal toxicity is a function of speciation and metal partitioning in the
113
-------
2.5
Sulfide (mg/I)
1.6
Recycl*
— 1CR
—H 6OR
-#- TOUR
-B- 9OMR
10OHR
0.5
200 400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
0.6
0.5
0.4
0.3
0.2
0.1
Sulfide (mg/l)
0 200 400 600 800 1000 1200 1400 1600
Time Since Loading (Days)
Figure 59. Leachate Sulfide during Simulated Landfill Investigations.
114
-------
12
Naphthalene (mg/l)
200
400 ( 600 800 1000 1200
Time Since Loading (Days)
1400 160O
1CR — *— 6OR
Recycle
-*- 7OLR -*- 9OMR -
-*- 10OHR
12
10
8
6
4
2
0
Naphthalene (mg/l)
200 400 600 800 1000 1200
Time Since Loading (Days)
2CS
sos
Single Pass
-*-4OLS
1400 160O
50MS
80HS
Figure 60. Leachate Naphthalene during Simulated Landfill Investigations.
115
-------
250
200
150
. 100
50
Dibromomethane (mg/l)
200 400 600 800 1000 1200
Time Since loading (Days)
1400 1600
250
200
150
100
50
Dibromomethane (mg/l)
200 400 600 :800 1000 1200
Time Since i Loading (Days)
1400 1600
Figure 61. Leachate Dibromomethane during Simulated Landfill
Investigations.
116
-------
Trichloroethylene (mg/l)
200
400 600 800 1000 1200
Time Since Loading (Days)
1400
1600
Trichloroethylene (mg/l)
200
400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
Figure 62. Leachate Trichloroethylene during Simulated Landfill
Investigations.
117
-------
Dichlorobenzene (mg/1)
200
400 600
Time Since
Dichlorobenzene (mg/I)
800 1000 1200
Loading (Days)
200
400 600 800 1000 1200
Time Since Heading (Days)
1400 1600
1400 1600
Figure 63. Leachate Dichlorobenzene during Simulated Landfill
Investigations.
118
-------
Trichlorobenzene (mg/l)
200
400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
Recycle
1CR — +— 6OR -*- 7OLR
•*- 9OMR -*- 1QOHR
Trichlorobenzene (mg/I)
2.5
2
1.5
1
0.5
0
200 400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
2CS
3OS
Single Pass
4OLS
SOWS
8OHS
Figure 64. Leachate Trichlorobenzene during Simulated Landfill
Investigations.
119
-------
30
25
20
15
10
Nitrobenzene (mg/l)
Recycla
— 1CR
6OR
70LR
SOMR
10OHR
200
400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
Nitrobenzene (mg/I)
Single Pass
— 2CS
—I— sos
-*- 4OLS
-B- SOMS
8OHS
200 400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
Figure 65. Leachate Nitrobenzene during Simulated Landfill
Investigations.
120
-------
12
10
8
6
Nitrophenol (mg/l)
200
Recycle
—- 1CR.
—f- 6OR
-*- 70LR
9OMR
10OHR
400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
12
10
8
Nitrophenol (mg/l)
Single Pass
—- 2CS
—f— sos
-*- 4OLS
-S- SOMS
8OHS
200 400 600 800 1000 1200 1400 1600
Time Since Loading (Days)
Figure 66. Leachate Nitrophenol during Simulated Landfill Investigations.
121
-------
Dichlorophenol (mg/l)
0 . 200 400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
Dichlorophenol (mg/l)
Sing I* Pas*
— 2CS
3 OS
-3K- 4OLS
-B- SOMS
80HS
200
400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
Figure 67. Leachate DichloropheTiol during Simulated Landfill
Investigations. ;
122
-------
Bromide (mg/l)
200
400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
400
350
300
250
200
Bromide (mg/l)
Single Pass
—— 2CS
—i— SOS
-*- 40LS
SOUS
8OHS
200
400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
Figure 68. Leachate Bromide during Simulated Landfill Investigations.
123
-------
Vinyl Chloride (ug/I)
l.<*
1.2
1
0.8
0.6
0.4
0.2
o
~
~
"
-
Racyct*
— 1CR
-4- 6OR
-*- 701.R
-S- 9OMR
-£r- 1QOHR
i
tv
/
1" I
I I
I /
I
J f
Ir '
w
J !_ _.l -t i \ , 1 ' 1
200 400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
Figure 69. Headspace Vinyl Chloride during Simulated Landfill
Investigations. ,
anaerobic environment. Metal solubility is a function of numerous factors,
including pH, ORP, dilution, presence of complexing ligands, and the presence of
precipitators such as sulfide and hydroxide, as discussed in Section 2.4.
Analyses for 12 leachate metals included sodium, potassium, magnesium, calcium,
iron, manganese, chromium, cadmium, nickel, zinc, lead and mercury. Six columns
received incremental loadings of heavy metals, and the type and quantity of each
heavy metal added was previously summarized in Tables 19 and 20. Measured
leachate metal concentrations are presented in Tables A-27 through A-28 of
Appendix A. The 12 metals studied were divided and discussed according to their
periodic table groupings.
L
The alkali metals, Group IA, include sodium and potassium. Leachate sodium
concentrations are presented in Figure 70, and leachate potassium concentrations
are illustrated in Figure 71. Both sodium and potassium showed evidence of
washout from the single pass columns. However, leachate sodium and potassium
concentrations in the recycle columns [ exhibited a behavior that suggested
possible increased ion exchange reactions and'dilution effects were predominant.
The alkali-earth metals, Group 'IIA, include calcium and magnesium.
Leachate calcium and magnesium concentrations are presented in Figures 72 and 73.
Both calcium and magnesium are relatively conservative metals, which was
reflected in their behavior within the simulated landfill columns. Both alkali-
earth metals displayed behavior characteristic of dilution in recycle columns and
.washout in single pass columns. Also, the possibility for the precipitation of
calcium existed after project Day 1150 as a result of the rise in pH which would
increase the possibility of a reaction between calcium and available carbon
dioxide.
Selected heavy metals were codispos^ed with the refuse and organic priority
pollutants in Columns 40LS, 50MS, 70LR, 80HS, 90MR and 100HR. These heavy metals
124
-------
2500
Sodium (mg/l)
2000 -
200
400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
1CR -+- 6OR
Recycle
-*- 7OLR
-s- 9OMR -A- 10OHR
2500
Sodium (mg/l)
200
400 600 800 1000 1200
Time Since Loading (Days)
2CS
sos
Single Pass
4OLS
1400 1600
5OMS
8OHS
Figure 70. Leachate Sodium during Simulated Landfill Investigations.
125
-------
3000
Potassium (mg/I)
-B-9OMR -A-10OHR
0 200 400 600 800 1000 1200 1400 1600
Time Since Loading (Days)
3000
Potassium (mg/I)
2500 -:£
2000 -
-------
Calcium (g/I)
200
400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
Calcium (g/I)
0 200 400 600 800 1000 1200 1400 1600
Time Since Loading (Days)
Figure 72. Leachate Calcium during Simulated Landfill Investigations,
127
-------
1000
800
600
400
200
Magnesium (mg/l)
0 200 400 600 800 1000 1200 1400 1600
Time Since Loading (Days)
1000
Magnesium (mg/l)
800 -
600 -
400
200 -
0 200 400 600 ;800 1000 1200 1400 1600
Time Since Loading (Days)
Figure 73. Leachate Magnesium during Simulated Landfill Investigations.
128
-------
consisted of iron, cadmium, chromium, lead, manganese, nickel, zinc and mercury.
Iron was present in the codisposed metal sludges and is also present in municipal
solid waste. .Therefore, iron was detected in leachate originating from all 10
simulated landfill columns. Leachate iron concentrations for the recycle and
single pass columns are presented in Figure 74. Iron was initially present in
the Acid Formation Phase as Fe3*, but was then most likely reduced to the more
soluble Fe2* during the Methane Fermentation Phase due to the highly reducing
conditions established. This change in speciation offers an explanation of the
increasing iron concentration in single pass columns towards the end of the
Methane Fermentation Phase. Recycle iron concentrations decreased during the
Methane Fermentation Phase, which was attributable to precipitation as sulfide.
Cadmium, chromium, lead, manganese, nickel and zinc leachate concentrations
are presented in Figures 75 through 80. Reductions in leachate heavy metal
concentrations in the recycle columns were attributable to precipitation as the
sulfide under reducing conditions, or chromium precipitation as the hydroxide.041
Heavy metal removal occurred in the single pass columns as well, but the primary
mechanisms of removal was washout and, to a lesser degree, precipitation. These
chemical attenuation mechanisms are discussed in more detail in Section 5.2.
Leachate mercury concentrations were also measured and are shown in Figure
81. Mercury removal in the recycle columns probably resulted initially from
precipitation as the sulfide, but later removal may have been accomplished by
conversion to metallic mercury when the landfill environment became highly
reducing, as is also discussed in more detail in Section 4.7. 'Additionally
mercury was removed in the single pass columns, principally by washout as well
as by precipitation.
Superimposed upon the primary heavy metal attenuation .mechanisms were
opportunities for encapsulation, sorption, ion exchange, complexation and void
space containment consequenced by the method of loading, the associated reaction
opportunities, and the physical characteristics of the waste matrix.04'
Precipitation or other mechanisms, including filtration of heavy metals within
the waste matrix in the five recycle columns, apparently lowered leachate total
metal concentrations below an apparent threshold, below which methane.
fermentation could occur. However, since methane was produced in all columns
containing heavy metals, the applied loadings could be accommodated, but at
delayed cumulative gas production which was less as heavy metal loadings
increased. Moreover, gas production was occurring at similar or increasing rates
for all metal-loaded recycle columns at the end of the experimental period
thereby indicating that gas production similar to that of the control recycle
column could be projected to be eventually attained.
4.5 INHIBITION LEVELS
Based upon the behavior of the 10 simulated landfill columns and their
respective progression toward anaerobic waste stabilization, a priority pollutant
loading can be determined above which the stabilization process is inhibited.
This inhibition level differs according to the leachate management strategy
- i_-vN
-------
2500
Iron (mg/I)
2000 -
0 200 400 600 : 800 1000 1200 1400 1600
Time Since Loading (Days)
1CR '
-*-6OR
Recycle
-*-J7OLR
-3- 9OMR -*- 10OHR
2500
Iron (mg/I)
200 400 600 | 800 1000 1200 1400 1600
Time Since Loading (Days)
2CS
30S
Single Pass
4OLS
Figure 74. Leachate Iron during
5OMS
8OHS
Simulated Landfill Investigations.
130
-------
100
Cadmium (mg/I)
200
400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
100
Cadmium (mg/l)
so -
60 -
40 -
20 -r
Single Pass
—— 2CS
-f- SOS
-*- 4OLS
-B- 5OMS
-A- 80HS
200 400 600 800 1000 1200 1400 1600
Time Since Loading (Days)
Figure 75. Leachate Cadmium during Simulated Landfill Investigations.
131
-------
50
40
30
20
10
Chromium (mg/1)
Recycla
— 1CR
—I— 6OR
-*- 70LR
-S- 9OMR
10OHR
0' F*nSg
0 200 400 600 800 1000 1200 1400
Time Since Loading (Days)
1600
50
40
30
20
10
Chromium (mg/l)
0 200 400 600 800 1000 1200 1400 1600
Time Since Loading (Days)
Figure 76. Leachate Chromium during Simulated Landfill Investigations.
132
-------
40
Lead (mg/l)
20 -
10
Recycle
— 1CR
H— 6OR
-*- 70LR
SOMR
10OHR
0 200 400 600 800 1000 1200 1400 1600
Time Since Loading (Days)
40
Lead (mg/l)
so -
20 -
10 -
Single Pass
—- 2CS
-+- sos
4OLS
5OMS
8OHS
200 400 600 800 1000 1200 1400 1600
Time Since Loading (Days)
Figure 77. Leachate Lead during Simulated Landfill Investigations.
133
-------
500
400
300
200
100
Manganese (mg/l)
200
400 600 800 1000 1200
Time Since Leading (Days)
1600
soo
400
300
200
100
Manganese (mg/I)
0 200 400 600 800 1000 1200 1400 1600
Time Since Loading (pays)
i
Figure 78. Leachate Manganese duririg Simulated Landfill Investigate
oils,
134
-------
350
Nickel (mg/l)
200
400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
350
Nickel (mg/l)
Single Pass
——i 2CS
303
-#- 4OLS
-S- SOMS
8OHS
200
400 600 800 1000 1200 1400 1600
Time Since Loading (Days)
Figure 79. Leachate Nickel during Simulated Landfill Investigations,
135
-------
2000
1500 -
1000 -
500
Zinc (mg/I)
0 200 400 600 800 1000 1200 1400
Time Since Loading (Days)
1600
2000
1500r
1000L
500
Zinc (mg/I)
200
400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
Figure 80. Leachate Zinc during Simulated Landfill Investigations.
136
-------
250
200
150
100
50
Mercury (ug/l)
Recycle -
— - 1CR
-#- 70LR
-a- SOMR
10'OHR
!T! t J.' IT t
200 400 600 800 1000 1200
Time Since Loading (Days)
1400 1600
250
Mercury (ug/l)
200 -
150 -
100 -
0 200 400 600 800 1000 1200 1400 1600
Time Since Loading (Days)
Figure 81. Leachate Mercury during Simulated Landfill Investigations.
137
-------
mass of pollutant per mass of municipal solid waste basis.)
A different behavior was observed in the five single pass simulated
landfill columns. The control single pass column, Column 2CS, behaved similar
to the recycle columns in that waste stabilization processes progressed in a
predictable fashion. Single pass coljimns that received priority pollutant
loadings, Columns 308, 40LS , 50MS and 80HS, displayed behavior characteristic of
inhibition as reflected by gas volume produced, gas composition, leachate
constituent analyses, and mass of IVA released and transformed. Therefore, no
priority pollutant loadings applied to single pass simulated landfill columns
during these investigations could be tolerated.
4.6 LITHIUM TRACER STUDIES ;
At various times during the course of these studies, lithium chloride was
added to the columns for the purpose o|f ascertaining the nature of the flow
through the columns and estimating the approximate retention times within the
columns. Data obtained from these studies were analyzed by the following
equation, with elaborations based on the nature of the lithium spiking procedure
and the column operation methodologies as discussed subsequently.
e, -E^O/EC^ <18)
where ;
t, - retention time
Cu — concentration of lithium
t — time at which that concentration has been
measured in days since addition of the lithium
4.6.1 Single Pass Column/Retention Times
The lithium breakthrough curves for the single pass columns (Columns No.
2, 3, 4, 5 and 8) are shown in Figures- 82 through 86. In all cases, simple
unimodal breakthrough curves were obtained. In the cases of Columns 3 and 5, a
second lithium spike was added after the first spike had substantially exited the
columns in an^effort to evaluate the possible impact of time -dependent compaction
on the^retention times. In those cases, the retention time was based on the date
of addition of the second lithium spike taken as day zero of the measurement. The
retention time results of these tracer studies are included in Table 30.
t
As indicated, the initial retention times ranged from 200 to 278 days for
all single pass columns with the exception of Column 8 which had an initial
retention time of 373 days. The cause of this difference is not clear, although
it may have been a consequence of some anomaly in the loading of waste added to
the columns. Hence, these data suggest that, even with attention to the loading
of the columns, their development may proceed in a more heterogeneous manner and
total consistency in retention time behayior may be very difficult to obtain.
The second lithium spikes to Columns 3 and 5 resulted in breakthrough
curves indicative of increases in retention times of greater than 100% in both
cases (102 and 132%, respectively). These increases were consistent with the
trend in additional compaction of the Waste matrix and more active microbial
development with time. With this increase in retention times as column
operations progressed through the phases' of waste stabilization, the ability to
act as a more efficient microbially-mediated treatment medium was likely
enhanced.
138
-------
0
COLUMN 2
SINGLE PASS
bJO
s
1
1
"T^J
JU
25
20
15
10
5
n i
:•
"i
i ".•"•:
&
* •
%• •
• * m m
*•<;»• •
• • •
- .* •
• ^Ifll ^b "%^P^L
I ^ ^B ^^J"%j^3B
500 1000
Time, days
1500
Figure 82. Lithium Breakthrough Curve for Single Pass
Column 2 (Single Spike).
139
-------
0
COLUMN 3
SINGLE PASS
£
WJJ
a
r>
B
S3
^4
£3
UV
50
40
30
20
10
n i
•• i
1 • •
! •
: *
•^ . •
• * »
* •. > ""
• • •!•
- I • • *»
500 1000,
Time, days
1500
Figure 83. Lithium Breakthrough Curves for Single Pass
Column 3 (Double Spike).
140
-------
40
30
20
10
0
COLUMN 4
SINGLE PASS
0
I •
••*
500 1000
Time, days
1500
Figure 84. Lithium Breakthrough Curve for Single Pass
Column 4 (Single Spike).
141
-------
50
40
30
20
10
0
0
CQLUMN 5
SINGLE PASS
500 1000
Time, days
1500
Figure 85. Lithium Breakthrough Curves for Single Pass
Column 5 (Double Spike).
142
-------
30
25
20
15
10
5
0
0
COLUMN 8
SINGLE PASS
•~v.
500 1000
Time, days
1500
Figure 86. Lithium Breakthrough Curve for Single Pass
Column 8 (Single Spike).
143
-------
TABLE 30. INDICATED RETENTION TIMES OF SINGLE PASS SIMULATED
LANDFILL COLUMNS
Column
Identity
2CS
SOS
40LS
50MS
80HS
Average
Retention Time, Days
First
Spike
264
206
278
200
373
264.2
Second
Spike
. .
417
465
. „
--
4.6.2 Recycle Columns/Retention Times:
Analysis of retention times for the recycle columns (Columns No. 1, 6, 7,
9 and 10) posed a problem related to the manner in which the lithium was added.
As a result of being added to the top of the opened recycle lines, the lithium
was not introduced immediately onto the top of the waste matrix, but into the
sump from which samples were drawn for analysis and from which leachate was
recycled to the columns. The results of this mode of addition are clearly
evident in the breakthrough curves from these columns (Figures 87 through 91)
where very high initial lithium concentrations first diminished rapidly with
leachate recycle to levels essentially characteristic of the background
concentrations in all columns. Following this, all columns showed typical
unimo'dal breakthrough curves, except that these curves tended to reach minima at
levels in excess of the leachate background concentrations. Since lithium was
being recycled, this was not unexpected.! The appearance of a second breakthrough
as the first lithium pulse was recycled to the top of the waste in the columns
and re-emerged from the bottom of the column indicated a similarity in magnitude
to the first breakthrough'curve. This Suggested that dispersion of lithium was
limited during passage through the columns, and that for the conservative lithium
tracer or other conservative materials, one pass through the columns would
closely resemble the next. Therefore, in the analysis of these data, the day of
the first, post-spiking minimum was taken as Day 0 for the first breakthrough
and, where the second breakthrough was sufficiently developed to permit analysis,
the minimum point between the completion of the first breakthrough and initiation
of the second was taken as Day 0 for the second breakthrough. The results of
these analyses are presented in Table 31.
As indicated in Table 31, The estimated retention times in the recycle
columns for the first lithium pulse were pharacterized by substantial variability
(256 to 432 days) and averaged about 30% longer than in the single pass columns
(265 to 345 days). The second breakthrough was measured in Columns No. 1 and 6
and found to have increased, but to a significantly smaller extent (23 and 50%,
respectively) than observed in the single pass columns. These variations could
again be accountable to the difference in single pass and recycle operations and
the more rapid progression toward stabilization established in the recycle
columns as a consequence of leachate recycle. However in either case, the
extended time of passage of the liquid transport phase provided a bioreactor
circumstance beneficial to in situ adaptation and assimilation of both solid
waste constituents and admixed loadings of organic and inorganic priority
pollutants.
144
-------
%,- -«
40
S
S3'
30
20
10
0
0
COLUMN 1
RECYCLE
l-
V • "•
•• • 0
•
• A
m
m
500 1000
Time, days
1500
Figure 87. Lithiiom Breakthrough Curves for Recycle
Column 1 (Single Spike).
145
-------
40
S
-------
40
30
s
*t
s
20
10
0
0
COLUMN 7
RECYCLE
• ••;:
•
500 1000
Time, days
1500
Figure 89. Lithium Breakthrough Curves for Recycle
Column 7 (Single Spike).
147
-------
COLUMN 9
RECYCLE
bO
a
e
1
a
DU
50
40
30
20
10
n
:
• •
* ™ •
" * V
: • "•-.% - / .
••• a %
• a • ** % •• •
"w -s-
!*
:V !
0
500 1000
Time, days
1500.
Figure 90. Lithium Breakthrough Curves for Recycle
Column 9 (Single Spike).
148
-------
v^
I
*s
•T—I
nJ
50
40
30
20
10 -
COLUMN 10
RECYCLE
1
0
500 1000
Time, days
1500
Figure 91. Lithium Breakthrough Curves for Recycle
: Column 10 (Single Spike).
149
-------
TABLE 31. INDICATED RETENTION TIMES OF RECYCLE SIMULATED
LANDFILL COLUMNS
Column
Identity
1
6
7
9
10
Retention Time, Days
First
Pass
256
262
432
372
! 407
345.8
Second
Pass
315
399
--
--
4.7 Simulated Landfill Column Disassembly
At the completion of the simulated landfill column operations (~ Day 1518) ,
the 10 simulated landfill columns were allowed to remain dormant for
approximately six weeks (40 days) before column disassembly was commenced.
During this time, the accumulated leachate was drained periodically from the
columns and discarded. However, prior to opening the columns for sampling and
disassembly, head space gases were analyzed for the presence of hydrogen sulfide.
The results of these analyses are included in Table 32 and indicate that H2S was
detected for all simulated landfill columns except for the column with the
highest metal sludge loading (Column No. 100HR). In addition, the highest H2S
level was in the headspace of the control, recycle column (Column No. 1 CR) , with
the recycle columns generally exhibiting more H2S, but moderated by the presence
and/or effects of heavy metals. Therefore, l^S utilization in precipitation
reactions and precipitate production by sulfate reduction were influenced by the
presence and intensity of the heavy metal loadings.
After the tele-thermometers and gas meters were disconnected and the column
tops removed, several additional obsejrvations concerning the ten simulated
landfill columns were made subsequent to their disassembly. First, all columns
had settled uniformly across the surface during the experimental period. The
depth of the stabilized refuse was determined by measuring from the top of the
column to the surface of the gravel layer and adding to this the~ depth of the
gravel. This result was then compared to the measurement originally taken prior
to closure as indicated in Table 33. Based on these measurements, the
refuse/waste mass had settled or compacted an average of 28%. (19 to 36%) , with
the greatest and least settlement exhibited in the recycle control and highest
metal loaded columns, respectively. Secondly, a uniform layer of black sludge,
having an approximate depth of 1.0 to 2.0 cm (7/16 to 8/16 in), covered the
entire gravel surface of the recycle columns. This sludge layer was shallower
in the single pass columns and was confined only to a small portion at the center
of the column. This again indicated a possible distribution of heavy metal
precipitates in the presence of sulfides not only throughout the waste matrix,
but in drainage gravel layers as well. Part of this layer, however, was
attributable to the sludge seeding procedures introduced during landfill
operations to initiate methane fermentation as described previously (Section
3.4.3).
150
-------
TABLE 32. HEADSPACE H,S CONTENT AT COLUMN DISASSEMBLY
Column Identity
1 CR
2 CS
3 OS
4 OLS
5 OMS
6 OR
7 OLR
8 OHS
9 OMR
10 OHR
Gas -Phase
Concentration
50
2
2
2
1
10
1
1
20
BDLa
HjS
, mg/m3
Note: aBelow Detection Limit (<1 mg/m3)
TABLE 33. NET SETTLING OR COMPACTION IN SIMULATED LANDFILL AT
COLUMN DISASSEMBLY
Column
Identity
1 CR
2 CS
3 OS
4 OLS
5 OMS
6 OR
7 OLR
8 OHS
9 OMR
10 OHR
Original Refuse
Depth, cm
' 185
193
188
178
190
183
188
190
185
198
Final Refuse
Depth, cm
119
127
135
130
137
124
130
147
137
160
Change in Refuse
Depth, cm
66
66
53
48
53
59
58
43
48
38
'Settling, %a
36
34
28
27
28
32
31
23
26
19
Note:
a% settling (original refuse depth - final refuse depth)^100
original refuse depth
Core samples of the stabilized waste were obtained from the 10 columns in
order to determine the distribution of the residual organic priority pollutants
codisppsed in the columns. These samples were obtained using a core sampler at
depth increments of about 30 cm, and were extracted with methylene chloride using
a Soxhlet extractor. The concentrated methylene chloride extract was analyzed
using the same GC/MS methods described for the leachate samples. The results of
these analyses are presented and discussed in more detail in Section 5.1, and the
corresponding analyses of supplemental heavy metal sludge samples retrieved from
the columns during disassembly are presented and discussed in Section 5.2.
When the stabilized refuse was removed from the columns, visual
observations were also made with regard to the physical appearance of the refuse
the sand/gravel/geotextile underdrain system within the HOPE liner, and the
sand/geotextile/gravel system between the HPDE liner and the steel containment
shell of the columns. Much of the inert materials such as metal, glass and
plastic were readily recognizable, as was a considerable amount of cardboard and
151
-------
newsprint, some indicating little apparent physical or chemical change. Sections
of newspapers were still readable, although these waste materials appeared
stained from exposure to and reaction with the leachate over the experimental
period. Because the original refuse was shredded, it was difficult to discern
other differences in component characteristics, with exception of the greater
compaction discussed previously.
The-layers of sand, geotextile and gravel composing the underdrain system
appeared to be unchanged from when it was initially placed in the columns. In
a few instances, the materials were slightly discolored by what visually appeared
to be rust stains. These stains were neither uniform nor widespread through the
underdrain systems and there was little indication of either biological growth
or precipitation/cementation within the underdrain materials. Similarly, the
layers of materials contained between the HDPE liner and the steel column shell
were alike in appearance to those same materials within the HDPE liner, with the
exception of the rust stains and apparent moisture content. These materials did
not show the discoloration attributed to iron oxide precipitation noted
previously, but in some instances appeared to be somewhat less moist than the
inner liner contents. Again, there was little evidence of microbial or chemical
interaction within the sand/gravel/geotextile base materials, and all drainage
systems and appurtenances appeared to have retained their functionality.
152
-------
7ft
SECTION 5
EVALUATION OF ORGANIC AND INORGANIC PRIORITY POLLUTANTS i
5.1 BEHAVIOR OF ORGANIC PRIORITY POLLUTANTS
In addition to the previous presentation and discussion of the routine
analytical parameters, particular attention was focused on the behavior of the !
organic priority pollutants codisposed with the municipal refuse in the simulated
landfill columns (Table 16) . (
5.1.1 Chemical and Physical Properties
The chemical and physical properties affecting the potential mobility of
the organic priority pollutants are included in Table 34. Accordingly, the i
expected mobility of the test compounds in the liquid and gas phases associated
with the refuse matrix can be summarized as follows:
Halogenated Aliphatic Compounds- - (trichloroethene , TCE , and dibrombme thane ,
DBM) ; high volatility and mobility in liquid and gas phases as indicated by high
solubility and vapor pressure and low octanol/water partition coefficient
Chlorinated Benzene Compounds-- (1.4- dichlorobenzene, DCB, and 1,2,4-
trichlorobenzene, TCB) ; relatively volatile with low mobility in the liquid phase
as indicated by low solubility and high K,^.
Phenolic and Nitro-Aromatic Compounds-- (2. 4-dichlorophenol, DCP, 2-
nitrophenol, NP, and, nitrobenzene, NB) ; relatively mobile in liquid phase, low
volatility and mobility in the gas phase as indicated by high solubility low
vapor pressure and K^.
Hexachlorobenzene (HCB) . Polvnuclear Aromatic Hydrocarbons- - (naphthalene
NAP), Pesticides (lindane, LIN, and dieldrin, DIEL) , and Phthalic Acid Esters
(Bis(2-ethylhexyl)phthalate,. BEHP) ; low volatility and mobility as indicated by
low solubility and vapor pressure and high K^.
To assist in the interpretation of data descriptive of the behavior of the
organic priority pollutants, sorption isotherms were developed in separate
laboratory tests for each of the test compounds with the exception of DIEL, HCB
and BEHP by Reinhart<65), All resulting isotherms confirmed a 'strong linearity
between sorbed and solute concentrations, and from a least squares regression
analysis, the coefficients of linearity or sorption partition coefficients, 1C,
for each compound were calculated and are presented in Table 35. K. could then
be used to calculate a retardation factor, RET, or:
RET = 1 + pKp/Qv (19)
where ;
p - refuse bulk density
0W - moisture content
Since RET is a measure of the rate of movement of the test compound
153
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relative to the bulk water movement, from average leachate flowrates through the
columns and the calculated RET,, the expepted relative retention time, presented
in Table 35 for each organic compound, could be calculated. However, because of
the opportunity for some dispersion and the possibility of shortcircuiting, the
compound may tend to appear in the leachate before and after the calculated
retention time. From predicted retention time ranges (Section 4.6) and in the
absence of bioconversion, only TCE, DBM, NP and NB were thereby expected to elute
during the time period of the investigation. Although DCP ha_d an indicated
retention time of 13 to 20 years, the extremely high aqueous solubility indicated
a probability of appearance in the leachate toward the end of the investigations.
In addition, as already indicated, this physical-chemical behavior was expected
to be modified by microbially-mediated processes of attenuation.
I
5.1.2 General Observations ;
The fate of the organic priority pollutants in the recycle and single pass
columns, based on leachate and waste matrix analysis after the simulated landfill
columns were disassembled, is summarized in Tables 36 and 37, respectively.
Details of the analyses of extracted core samples and mass balances for each
column at the end of the project period iare provided in Tables D-l through D-4
of Appendix D. Mass not accounted for in the leachate or on the waste is assumed
to have been "transformed." Such transformation is not explicitly defined and
is not meant to imply complete mineralization, since, in most cases,, all
potential byproducts were not identified nor quantified. In addition, although
limited gas analysis indicated that volatilization was a minor pathway for
several of the more volatile compounds, because sufficient data were not
available to completely quantify volatilization, mass possibly lost to the gas
phase is also included.in the total mass transformed.
Notwithstanding these limitations, several general conclusions can be drawn
from the indicated mass balances regarding the fate of the organic priority
pollutants. The more hydrophilic compounds (log K^. less than 2.29), including
DBM, TCE, NP and NB (Figures 61, 62, 65 and 66), appeared in the leachate very
early in the investigations at relatively high concentrations (up to 225 mg/L) .
In addition, the order in which the cpmpounds appeared in the leachate was
approximately proportional to their respective K^ values. Thus the more
hydrophobic the compound, the longer it was retained, presumably as a result of
some interaction with the waste matrix. However, appearance of these compounds
was relatively brief and most had either disappeared or dramatically declined in
concentration (to the ng/L range) within the first year. Overall transformation
of these compounds, as indicated in Tables 36 and 37, was very efficient,
approaching 100% of the mass placed in most cases.
Compounds with intermediate hydrophobicity (log K^ between 2.75 and 4.04) ,
such as DCP, NAP, DCB, LIN and TCB (Figures 57, 60, 63 and 64), were detected in
the leachate and, with the exception of DCP, were measured at relatively low
concentrations. Most of these compounds exhibited moderate (30 to 60%) overall
transformation, however, the extent varied significantly from one column to
another. DCP was much more mobile in the liquid phase than the remaining
compounds in this category, particularly at more neutral pH values.
The extremely hydrophobic compounds (K,^, greater than 5), DIEL, HCB and
BEHP, were never detected in the leachate, i.e., DIEL and BEHP were completely
retained or transformed in all columns. In contrast, HCB retention or
transformation varied from zero to 100%.
As indicated previously, volatilization was not well quantified during the
investigations. Only TCE, DCB, and TCB were detected in the gas phase. This
could be expected, since these compounds have Henry's Law constants in excess of
0.117 (Table 34).,
156
-------
TABLE 36. MASS BALANCE SUMMARY ON ORQANIC PRIORITY POLLUTANTS
FOR THE SINGLE PASS SIMULATED LANDFILL COLUMNS8
Compound
Dibromomethane (DBM)
Trichloroethene (TCE)
Nitrobenzene (NB)
2-Nitrophenol (NP)
2,4-Dichlorophenol (DCP)
1,4-Dichlorobenzene (DCB)
Naphthalene (NAP)
Lindane (LIN)
1,2, 4-Trichlorobenzene(TCB)
DIeldrin (DIEL)
Hexachlorobenzene (HCB)
Bis(2-ethylhexyl)phthalate (BEHP)
% Leached
14.1
(6.1-27.4)
10.7
(7.77-14.58)
0.75
(0.02-2.31)
0.31
(0.03-1.16)
10.9
(8.66-11.81)
3.8
(2.53-5.98)
1.2
(1.04-1.34)
0
0.17
(0.08-0.32)
0
0
0
% Retained
0
0
0
0
15.4
(0.74-25.2)
48.4
(30.96-68.63)
46.8
(17.48-59.53)
52.2
(0-100)
39.7
(6.67-60.0)
0
57.1
(0-96.67)
0
% Transformed19
85.9
(72.6-93.9)
89.3
(85.42-92.23)
; 99.25
(97.69-99.98)
99.69
(98.84-99.97)
73.6
(57.71-87.75)
47.8
(28.55-81.27)
52.0
(39.13-81.27)
47.8
(0-100)
60.1
(42.00-93.01)
100
, 42.9
:(3. 33-100)
100
fRanges in parentheses
"Mass not accounted for in the leachate or recovered from the waste
157
-------
TABLE 37. MASS BALANCE SUMMARY ON ORGANIC PRIORITY POLLUTANTS
FOR THE RECYCLE SIMULATED LANDFILL COLUMNS3
Compound
Dibromomethane (DBM)
Irichloroethene (TCE)
Nitrobenzene (NB)
2-Nitrophenol (NP)
2,4-Dichlorophenol (DCP)
1,4-Dichlorobenzene (DCB)
Naphthalene (NAP)
LIndane (LIN)
1 , 2 , 4-Trichlorobenzene (TCB)
Dieldrin (DIEL)
Hexachlorobenzene (HCB)
Bis (2-ethylhexyl)phthalate (BEHP)
% Leached
1.71
(0.12-2.66)
0.57
(0.40-0.83)
0.07
(0.02-0.10)
0.03
(0.01-0.04)
2.55
(0.41-8.73)
1.20
(0.21-3.90)
0.41
(0.09-1.32)
0
0.05
(0.0-0.17)
0
t
0
0
% Retained
0
0
0
0
25.17
(6.50-41.99)
35.37
(18.99-48.89)
48.28
(21.75-63.31)
66.29
(33.75-93.17)
38.15
(32.58-43.75)
0
86.31
(46.42-100)
0
% Transformed15
98.29
(97.34-99.88)
99.43
(99.04-99.60)
99.93
(99.90-99.98)
99.97
(99.96-99.99)
72 . 29
(41.99-94.39)
63.44
(50.79-80.80)
53.00
(21.2-78.16)
33.71
(6.83-66.25)
61.81
(37.42-67.40
100
13.69
(0-53.58)
100
"Ranges in parentheses !
'Mass not accounted for in the leachate or recovered from the waste
158
-------
5.1.3 Behavior of Specific Organic Priority Pollutants
5.1.3.1 Dibromome thane-- ;
Dibromome thane (DBM) appeared in all column leachates early In the
investigations, rapidly reaching a maximum leachate concentration, and then
disappearing within the first year (Figure 61). As would be expected from its
relatively high solubility, concentrations observed for DBM were the highest
measured for all test compounds. Transformation of DBM is expected to produce
bromide, and possibly methane or ethylene, as observed by Wade and Castro(S> Ion'
chromatographic analysis, initiated on Day 700, confirmed the presence of bromide :
in all test column leachates (Figure 68) and accounted for 30 to 100% of DBM
transformed. A close mass balance on DBM was not possible, since bromide data'
were unavailable before Day 700. Moreover, as could be expected, the bromide
concentrations remained relatively constant in the recycled leachate However :
t?r>A leac*\ate of. the single pass column, it decreased significantly after Day
1200 at which point most of the DBM should have eluted, as suggested by the :
calculated retention time (Table 35). &&«»«•«« uy cne ,
Microbially- mediated bromine removal from the DBM molecule appeared to be <
independent of column ORP, unlike dechlorination as is discussed subsequently '-
™™A £ecau?e of Jhe .lesser relative strength of the carbon-bromine bond as ,
compared to carbon- chlorine bond, debromination rates are greater and acclimation
periods shorter than those of dechlorination as reported by Suflita, et al.<<7>.
5.1 ,,3. 2 Trichloroethene--
Trichloroethene (TCE) also appeared in leachates early in the
nRM6S ™gat £S (FlSure 62> • and reached maximum leachate concentration soon after
Ev n-Plo^ -er> concentra.tions of TCE declined and were below detectable levels i
by Day 1026 in columns with ORP values consistently below -200 mV (recycle
columns, excluding Column 10 OHR) . For columns with leachate ORP above -200 mV
* r0Sh0t
for Tc! r^abl*; 3Sr0vSh0Ht ^vf "S^' More,over. the calculated retention time \
tor TCE (Table 35) exceeds the time period of the investigations Thus the
£2*SE£ Presence of TCE would be expected, particularly5 if transformation i
mechanisms were inhibited in some manner.
t-« ^ Like^manyhalogenated aliphatic compounds, reductive dehalogenation appears
S *™%mtj°r Pf^y f°r fhe transformation of TCE under anaerobic conditions.
^ rf- byP"du5Lts include cis- and trans -dichloroethene, vinyl chloride and '•
carbon dioxide (Bower and McCarty,<«>; Wilson,*". Reductive dehalogenation
however, becomes less predominant as the ORP increases™, and ceases for many
aliphatic compounds under aerobic conditions*". Vinyl chloride was measured in i
the gas phases for all test columns containing TCE (Figure 69). !
«,.« While TCE transformation was indicated as nearly complete, the continued !
?"^n£ and Possible intermediary product formation of TCE in some column :
leachates at the end of the investigation suggest that some TCE remained in the
£°^mnf
iLcLt-^5™ ed together. Both compounds appeared at low concentrations in the ;
}!?«^o «!/«*, D6 "ives0t/igaclons began, with NB detected just prior to NP
£jgure£1661and,65>; By Day 200, NP and NB concentrations had decreased below
detectable levels however, the compounds were again detected during subsequent
periods whenever the ORP rose above -200 mV. Afte&r Day 800, neither compound ^was
SeJlaCnate>o^en during Periods when the ORP exceeded -200 mV. It
«'?if- b£D«y.8°0. either all the NP and NB had been transformed, or
within the columns supported transformation despite the elevated ORP
159
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This period of the investigation also coincided with the onset of methanogenesis
in most columns, and a time when pH increased significantly due to the
consumption of the volatile acids (Figures 29 and 30) . It has been observed by
Macalady, e_t al.*7", that the standard potential for reduction of nitro-moieties
is highly pH dependent, i.e. , as the pH increases, the minimum ORP for reduction
appears to decrease. Moreover, increases in pH may have reversed the sorptive
capacity of the waste matrix for phenolic compounds.
While daughter products of NP and NB transformation were not identified,
it is assumed that nitro-groups were reduced to aryl amine groups as has been
widely observed for a variety of compounds by Kobayashi.and Rittman^', Macalady,
et al.0", Weber and Wolfem>. The resulting aryl amine has been reported to
rapidly react with carbonyl and quinone moieties commonly found in humic
substances by Parris174'. Ring cleavage for the mineralization of nitroaromatic
and aryl amine compounds has been reported to be strongly inhibited under
reducing conditions Macalady, et al.*7". Moreover, because of the nitro- and
amine groups, thet aromatic ring has a low electron density and is resistant to
electrophilic attack. In contrast, the presence of the phenolic hydroxyl group
improves the probability of transformation of nitrophenols over nitrobenzenes.
Indeed, NP was detected in column leachates at lower concentrations than NB, and
its presence in the leachate appeared |to be less sensitive to excursions in
leachate ORP (Figure 55).
5.1.3.4 2,4-Dichlorophenol--
Leachate DCP (Figure 67) reached relatively high concentrations by Day 400
and remained at concentrations greater than 10 mg/L throughout the remainder of
the investigations in the single pass columns and Column 10 OHR, where the ORP
was higher than -200 mV. In the remaining recycle columns, leachate DCP
concentrations decreased between Day 400 and Days 1000 to 1300 when they were no
longer detected. By the end of the investigations, only a small fraction of DCP
had eluted from the columns (3 to 10%), 15 to 25% still remained on the refuse,
and the remainder had been transformed in some manner. The delayed appearance
in the leachates and significant retention by the.refuse is consistent with the
pH effect and relatively long calculated retention time (Table 33).
Chlorophenols are degraded through the reductive removal of aryl halogens
followed by the mineralization of the aromatic moiety (Krumme and Boyd1™;
Mikesell and Boyd™, Knoll and Winter177*. In addition, chlorinated phenolic
compounds were observed by Artiola-Fortuny and Fuller™ to react with humic
substances present in leachate. Sawney land Kozloski17", however, reported that
phenolic compounds leached from soils mor£ readily under anaerobic conditions and
suggested that phenolic polymerization ^observed under aerobic conditions was
inhibited. Here again, the change in leachate pH from acidic to more basic
levels as landfill stabilization proceeded from the Acid Formation Phase through
the Methane Fermentation Phase probably affected the relative sorption and
consequent mobility of DCP.
5.1.3.5 Chlorinated Benzene Compounds--
Both TCB and DCB exhibited some displacement up the landfill columns, as
indicated in Tables D-l and D-2 of Appendix D, presumably moving in the gas phase
as a result of their volatility. Both DCB and TCB were detected at low
concentrations in the leachate (Figures 63 and 64) , due in part to background DCB
levels in the refuse as well as volatility-enhanced movement. As indicated in
Tables 36 and 37, DCB was moderately transformed (30 to 50%), while TCB
transformation was as high as 93%. In contrast, HCB, an extremely hydrophobic
compound, was not detected in the column leachates. This limited distribution
also curtailed transformation opportunities, although indicated containment or
transformation ranged up to 100 percent, possibly by conversion into TCB-or DCB
daughter products.
The type and position of a substituent group on an aromatic compound
affects its potential biodegradability. The carboxy-, methoxy-, hydroxy- and
i
160
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bromo-groups apparently facilitate degradation, whereas fluoro-, amino- and
nitro-groups retard degradatio.^ Bpuwer andj-lcgarty080'; Horowitz*0. Oxygen in the
substituent group appears to'be^required' *fof effective anaerobic degradation
Zeyer, ej: al.052', and unsubstituted aromatic compounds are devoid of activating
sites that facilitate cleavage. Therefore, benzene and alkylbenzenes have been
reported to ' be recalcitrant Zeyer, et al.*82'. However, the complete
mineralization of toluene, benzene, ethylbenzene, and o-xylene has been observed
in anaerobic microcosms composed of aquifer materials previously exposed to
benzene by Wilson*6", a period of 120 weeks was required for mineralization of all
compounds except toluene, which required only 40 weeks.
Fathepure00' identified two degradation pathways for the anaerobic
transformation of RGB. One pathway sequentially removed chlorine atoms to
eventually produce a stable trichlorobenzene, while the other involved 1,2,4-
trichlorobenzene and produced three stable dichlorobenzene isomers, including
1,4-dichlorobenzene. Thus, the successful transformation of TCB and DCB during
the simulated landfill investigations is not surprising, particularly since the
extended adaptation and reaction period afforded by the hydrophobicity of TCB and
DCB as well as the- relatively long leachate retention times (Section 4.6)
provided enhanced opportunities for dehalogenation and, possibly, mineralization
to occur.
The extent of containment or transformation of DCB, TCB and HCB (as well
as LIN and NAP as discussed subsequently) varied from one column to another. The
extent in the single pass columns sometimes exceeded that of recycle columns.
Transformation in columns containing medium and high loading levels of heavy
metals was much higher than for columns containing no admixed heavy metals or low
metals loading levels, and was independent of leachate ORP. It is possible that
the metals (or alkaline conditions) took part in a metal and/or hydroxide-
catalyzed reductive and/or abiotic reactions. In addition, in the presence of
localized high concentrations of heavy metals and organic compounds, biological
activity may have been sufficiently inhibited to preclude extensive microbially-
mediated transformations.
5.1.3.6 Naphthalene--
Naphthalene (NAP) was detected in all column leachates, including the
control columns (Figure 60). Therefore, the presence of NAP in the leachates is
assumed to be related to background contamination of the refuse and not to
significant mobility of this fairly hydrophobic compound. NAP transformation
varied significantly among the test columns, with increasing 'transformation
efficiency indicated with increasing metal loading.
Polynuclear aromatic hydrocarbon degradation has been observed to be
strongly affected by the redox environment and the nature of substituent groups
and several laboratory investigations have determined naphthalene to be
recalcitrant at low ORP^-*®. However, in the present investigations, some
transformation apparently occurred again likely as a consequence of long
retention times or the localized conditions described previously.
5.1.3.7 Lindane- -
Lindane (LIN) was not observed in the test column leachates.
Transformation followed the variable patterns described previously for DCB, TCB
and NAP, ranging from low to negligible in columns with no or low levels of heavy
metals to high (100% in one column) for those columns with medium and high metal
waste loadings. The significance of the metal loadings on LIN transformation was
not explicitly established, but may have shunted normal degradative pathways in
favor of partial dehalogenation.
Several investigators have reported anaerobic degradation pathways for
gamma-hexachlorocyclohexane (lindane) Bachman, et al."«; Hill and McCartV8";
Buisson(88); Fries'891, with the production of penta- and tetra-chlorocyclohexane as
well as tri- and tetra-chlorobenzenes. Conversion of lindane has even been
161
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observed to proceed after biological activity was arrested, suggesting an abiotic
pathway. , i
5.1.3.8 Dieldrin--
Dieldrin (DIEL) was not observed in any test column leachates and
experienced consistent and complete transformation in all columns. Buisson*88*
concluded that dieldrin (1,2,3,4,10,10-hexachloro- 6 , 7, - epoxy-1,4,4a,5,6,7,8,8a, -
octahydro-1,2, -endo, exo-5,8-dimethanonaphthalene) was resistant tobiodegradation
during anaerobic digestion of primary and mixed municipal wastewater sludges.
However, other research has indicated biotransformation of dieldrin to
photodieldrin or to several water-soluble metabolites under reducing
conditions1*9-90'.
5.1.3.9 Bis(2-ethylhexyl)phthalate--
Bis(2-ethylhexyl)phthalate (BEHP) was not detected in test column
leachates. Like DIEL, BEHP was apparently consistently and completely
transformed in all columns. Phthalic acid esters have been shown to be
susceptible to anaerobic degradation by! Johnson, et al."0, and the length of the
alkyl side chains profoundly affects the ease and rate of biodegradation, with
short chain groups more readily removed,than long chain groups. Following this
side chain removal, the acid can be decarboxylated and then mineralized, but is
reported to be slowly degraded anaerobically1^.
5.1.4 Malor Findings during Landfill Column Operative and Disassembly
Major findings related to the behavior of organic priority pollutants
loaded to the simulated landfill columns include:
• The mobile, more hydrophilic compounds, including dibromomethane,
trichloroethene, 2-nitrophenol, nitrobenzene and 2,4-dichlorophenol eluted
from the columns in the approximate order of increasing affinity for the
waste matrix.
• The mobile, hydrophilic compounds were assimilated within the landfill
systems, apparently enhanced by microbially-mediated transformation. In
the case of dibromomethane and trichloroethene, the detection of
transformation byproducts confirmed this as a contributing mechanism.
v
• The transformation of certain chlorinated and nitroaromatic compounds
(trichloroethene, 2,4-dichlorophenol, 2-nitrophenol, and nitrobenzene)
appeared to be inhibited by ORP levels more positive than -200 mV.
However, partial transformation of each class of compound was observed.
• The more hydrophobic compounds, including 1,4-dichlorobenzene, 1,2,4-
trichlorobenzene, naphthalene, lindane, dieldrin, bis(2-
ethylhexyl)phthalate, and hexachlbrobenzene, were not readily released to
the leachate, and were apparently retained within the landfill systems as
a result of an association with the solid matrix. Such retention and
consequential delayed migration enhanced opportunities for the in situ
attenuation and assimilation within the simulated landfill columns,
thereby affording a greater possibility for more complete transformation.
• The degree of apparent transformation for 1,4-dichlorobenzene, 1,2,4-
trichlorobenzene, naphthalene, lindane, and hexachlorobenzene was
significantly higher in columns containing medium and high metal waste
loadings as compared to that of columns without or with low metal waste
loadings.
Complete retention and transformation of dieldrin and bis(2-
ethylhexyl)phthaiate were observed for all columns.
162
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5.2 BEHAVIOR OF INORGANIC PRIORITY POLLUTANTS :
5.2.1 Preliminary Considerations . *'* * i
As with the organic Apriority pollutants, a primary component of this
investigation was to examine the behavior of toxic heavy metals codisposed as :
sludges with the municipal refuse. To meet this objective, cadmium (Cd) ''
chromium (Cr) mercury (Hg) , nickel (Ni), lead (Pb) and zinc (Zn) were added as
either actual industrial sludges or as analytical grade metal oxides at the three
column loadings of low, moderate and high to the three pairs of recycle and
single pass simulated landfill columns (Tables 17 and 18). The remaining two '
pairs of simulated landfill columns served as controls with no priority '
pollutants added or with only organic priority pollutants added. With the
exception of the Cr, which was added as chromic oxide (Cr203) with an oxidation
state of +3, all metals were added in the +2 oxidation state. In addition to i
these metals, analyses for concentrations of sodium (Na), potassium (K), calcium
(Ca), magnesium (Mg), iron (Fe) and manganese (Mn), and lithium (Li) tracer
studies (Section 4.6), were also performed after column operations had begun. '.
The complex chemical and physical environment of the landfill columns as
m^^^S1^ £ i5he- obse/ved leachate properties and other characteristics, ,
mediated the behavior of both codisposed and indigenous heavy metals For
®^f??leif the, _leachates were rich in an array of inorganic anions and, in
particular chloride and sulfate which were present in all columns at levels well •
in excess of 1 000 mg/L at the start of leachate management operations (Figures •
56 and 58). Both anions could act as reasonably conservative tracers in the
single pass columns and showed clear evidence of washout. As indicated
previously, sulfate was also subject to microbially-raediated transformations
ySi?? ^suited in its eventual conversion to sulfide with substantial ;
implications in terms of metal behavior. Indeed, its role as a powerful ,
precipitant for metals was one of the most important factors determining the fate
of many of the heavy metals. Sulfate reduction with consequent formation and i
release of sulfide began to a significant degree coincidentally with the onset
of methanogenesis (Figure 59). In addition, tarbon dioxide production provided
^potential source of bicarbonate and carbonate anions once the Acid Formation
Phase was completed and leachate PH levels increased (Figure 29). In those
columns to which dibromomethane (CHJBr,) had been added as an organic priority •
pollutant substantial concentrations of bromide were released into the leachates
fn a^e^ir,tL°,- S?minauion °f the compound (Figure 68). However, with respect ;
to its impact on the behavior of the metals, bromide can be considered to act
essentially identically to the much more abundant chloride and therefore will ;
n^Li = *%Tined ^rtjher- Nitrate nitrite and o-phosphate were not detected at
levels which could impact on metal behavior during this project.
,V,T. The other significant inorganic cation in addition to the metals and
nydrogen ion present during these studies was the ammonium ion (NH4+) (Figure
57). In terms of potential reactivity with the heavy metals, this cation is
larselv nonrpant--fvo jf the pH had i-.r-.-i- •> . . . . _.
a£tVe- • tVpH had *isen hi§h enoh to Pemit signfcan
of ammonium to form ammonia (pK. - 9.3), some complexation might
rr ' esPecially wi<* Cd, Hg, Ni, Zn and Fe. Since%he column pE
(Figure 29) never reached levels which would have resulted in significant
was ™^Lt5d i anoaonxa, its impact on the behavior of metals in the si systeL
Ph^fi?^ ?i S a£S P" levels were cyPical acid during the Acid Formation
loffLina ^ 7> a? f U increased to levels somewhat in excess of neutrality
rollowing the onset of methanogenesis.
Due to problems associated with the maintenance of ORP probes implanted in
S nec?ssary. to measure ORP on leachate samples withdrawn from
F
P r.n H .
Lr^T.?^ ^Ce> **,&**• °f care in makin§ the ORP measurements, the
potentials observed were likely less reducing than those actually present within
the columns. Furthermore, it is probable that some degree of waste matrix
heterogeneity and possible shortcircuiting promotfd deveTopmen? of
163
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microenvironments which may have been very much more reducing than indicated by
ORP levels externally measured in the leachate. The ORP values throughout the
investigations were sufficiently reducing (Figure 55) to permit the reduction of
sulfate to sulfide with eventual complete depletion of leachate sulfate in the
recycle columns.
The column leachates were also characterized by high concentrations of
organic carbon. The nature of this organic matter was necessarily complex,
arising as it did from both washout of soluble materials from the refuse mass and
from products of chemical and biological conversion. In addition, there was a
time dependency which, during the initial acetogenic phase of landfill evolution,
resulted in a major portion of the leachate organic content being contributed by
the volatile organic acids (Figures 30, 53 and 54). The volatile organic acid
homologues (Figures 31 through 40) have fairly uniform strengths with pK, values
in the range of 4.8 to 5.0. As a result, the acids were significantly
dissociated during the acetogenic phase of column evolution with correspondingly
significant concentrations of the corresponding anions being present. However,
with the onset of methanogenesis and increased pH levels, any of the remaining
unionized acids present would have been virtually entirely ionized. The increase
of pH and corresponding rapid and essentially complete consumption of the
volatile acids, diminished the effect of pH on the behavior of the organic acids
during this phase. In contrast, during the acetogenic phase, these acid anions
might have had a significant impact on the behavior of the heavy metals due to
their relative abundance and a possibility of acting as competing ligands. On
the other hand, the formation constants [for complexes between these anions and
the metals of interest are quite low, ^nd the degree of formation of metal-
carboxylate complexes was therefore considered insignificant compared to other
potential interactions of the heavy metals within the waste matrix.
A second group of organic substances of concern was that resulting from
both the leaching and decomposition of components of the waste mass. These
substances are similar to the moderate and high molecular weight compounds found
in all natural waters and characterized as humic-like substances. They are of
poorly defined structures and are considered composed of mixtures of compounds
of varying character and molecular weight. They have, however, been associated
with significant heavy metal complexation capabilities Singer**8, and it is
probable that they may have had some impact on the behavior of metals in these
investigations. Indeed, earlier studies™ on similar simulated landfill systems
had provided some indication that humic-like substances, as quantified by the
Folin-Dennis analysis"4' for aromatic hydroxyl groups, were imparting a possible
mobility to the metals as a consequence of formation of soluble complexes.
However, this suspected remobilization could not be explicitly confirmed and, on
further examination of the Folin-Dennis Method, it was determined that the method
was subject to strong and unavoidable positive interferences resulting from the
presence of high concentrations of reduced iron and manganese in the leachate.
Calculations of the quantitative impact of these metals on the aromatic hydroxyl
measurements led to the conclusion that the analytical response could have been
due to iron and manganese, and that reliable estimates of the presence and
activity of the aromatic hydroxyl groups in the leachate matrix was not possible.
As a consequence, measurements of aromatic hydroxyl groups were suspended, and
no attempt was made to relate such measurements to metal behavior in the final
analysis.
These same studies'*3' examined information on ionic strength and its
potential impact on activity in the leachates by application of the empirical
relationship developed by Lind(95) in studies on groundwaters. This equation
relates the ionic strength, /*, to the conductivity as follows:
H — 1.6 x 10'J x conductivity in /zmhos (20)
164
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Using this expression, and the leachate conductivity measurements indicated in
Figure 92 and Table D-5 of Appendix D, a series of estimates of ionic strength
were obtained and these were in turn used to develop estimates of the activity
coefficients in the leachates by application of the extended DeBye-Huckel
Equation'961, or
In general, it was determined that the activity corrections displayed
virtually no relationship to time of column operation with standard deviations
for this parameter being on the order of 0.02 units for all values of Z (the
charge on the ion under consideration). For monovalent ions such as Na+, K* ,
C1-, etc. the value of Y averaged 0.78 units. Divalent ions such as Ca2+, Ni2+
and SO.,2- had Y values which averaged 0.41 units. Trivalent ions which were
U21£k?jy to be present at any significant levels in these leachates had a Y value
of 0.12 units. Although these values are of some interest in developing ionic
equilibria, considering both the magnitudes of the activity corrections and the
uncertainty associated with their estimation based on conductivity and an
empirical equation derived from groundwater systems, evaluation of suggested
equilibrium behavior of leachate components in terms of these coefficients did
not appear to offer results of sufficient reliability to justify the effort and
adjustments entailed. Moreover, some difficulty was encountered with leachate
conductivity measurements during the respective acid formation and methane
fermetntation phases due to difficulties in instrumental analyses., The analyses
that follow are sufficiently instructive to obviate such a refinement in a matrix
as complex as the column leachates .
In addition to chemical impacts, the physical environment of the landfill
columns could also have influenced the behavior of the metals. Accordingly the
landfill columns might be regarded as large chromatographic partitioners ,
complicated by irregular packing of the potentially adsorbent refuse solids and
by intermittent and potentially nonuniform flow of the eluant water/leachate
In a system of this type, adsorption of the metals would be expected to be
important to leachate mobility, especially in the case of the more polar metals
ot higher _ atomic weights such as mercury and lead. However, even the lower
atomic weight metals will be subject to physical interactions with the refuse
solids, a mechanism not directly assessed during the course of the
investigations.
_ Finally, the simulated landfill systems were devised to be biologically
active with a short-term aerobic condition followed by a much longer -tenured
anaerobic condition as oxygen introduced with waste placement was rapidly
depleted and reducing conditions were established. Hence, while the nature of
the biological processes underwent significant change, anaerobic microbial
activity predominated and was of greatest significance in terms of the behavior
of the heavy metals.
5.2.2 Properties of the Metals
5.2.2.1 Sodium and Potassium- -
Sodium and potassium presented the simplest chemistry of all the metals of
concern. There exists no significant precipitant for these elements, they do not
participate in any considerable way in complexation reactions, and' they exist in
only one oxidation state (+1) in the landfill systems. Thus, their behavior was
expected to be that of a conservative tracer throughout these studies. Indeed
as indicated in Figures 70 and 71, these elements were washed out of the single
pass columns., and concentrated to an essentially constant level in the recycle
columns.
165
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25
Conductivity (1000 umhos)
20-
15-1
10-
5-
0 200 400 600 800 1000 1200 1400 1600
Time Since Loading (Days)
1CR
— *— 6OR '
-*-7OLR
-&- 9OMR
-*- 10OHR
25
Conductivity (1000 umnos)
20-
15-
10-
5-
0 200 400 600 800 1000 1200 1400 1600
Time Since Loading (Days)
2CS
-+-3OS '
-*-4OLS
Q £f*>M<5 •• X' -• fifSUQ
oviviw ov^riw
Figure 92. Leachate Conductivity during Simulated Landfill
Investigations.
166
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5.2.2.2 Calcium and Magnesium--
Calcium and magnesium are only slighply more chemically complicated than
sodium and potassium, with the primary difference being that both have available
and potential precipitants. In the case of calcium, the potential precipitant
is carbonate (CaC03l pK^-8.4), while magnesium is subject to precipitation as the
hydroxide (Mg(OH)2) pK^-11.1). (Neither of these precipitants attained high
concentrations in the landfill columns, although some precipitation to the
corresponding salt was possible.) Otherwise, calcium and magnesium participate
in complexation reactions only to a slightly greater degree than do sodium and
potassium, and each has only the +2 oxidation state. Hence, like sodium and
potassium, calcium and magnesium were expected to act somewhat as conservative
tracers although, due to their higher charges and atomic weights, subject also
to a greater degree of adsorption in the waste matrix.
5.2.2.3 Iron and Manganese--
Iron and manganese present a more complicated circumstance than those
• discussed previously. In particular, each has more than one easily accessible
oxidation state. In the case of iron, the states are the +2 or ferrous state and
the +3 or ferric state. Manganese could be expected to exist in either the +2
or +4 oxidation states. Under strongly reducing conditions, as was the case
during_the majority of the simulated landfill investigations, the +2 oxidati'on
state is the preferred state for both metals. In the +2 oxidation state both
metals are characterized by their fairly high solubilities at pH levels below =9
figures VJ and 94) and the potential for forming sparingly soluble sulfides
^Jreo, pKj^io . o ; MnS, pKK)="14.0).
5.2.2.4 Cadmium, Nickel and Zinc--
Cadmium, nickel and zinc can be treated together due to their considerable
degree of similarity. All three of these metals exist in only the +2 oxidation
state and are subject to precipitation as sparingly soluble sulfides (CdS?
P?""^:1' NlS' P^-24.0; ZnS, pK^-23.8). None of these metals is subject to
significant complexation with any of the important inorganic ligands in the
leachates (The hydroxides of these metals, while sparingly soluble at alkaline
pH level (pK^lT), are unlikely to have controlled solubility at any PH level
observed in the leachates from the simulated landfill columns over the course of
=!!?^,1i1Vefitlga^Lons1V) WltLh the §eneration of significant concentrations of
sulfide through sulfate reduction, little else was present in the columns which
would have otherwise acted to precipitate cadmium, nickel and zinc. £5 active
sultate reduetion/sulfide generation commenced, these elements could be expected
to be removed by precipitation as the respective sulfides and physical entrapment
in tne waste matrix.
5.2.2.5 Chromium--
_ Chromium presents a somewhat different situation from that of the afore-
^r^nm™^111!^^ •cjjro,mium is much like iron in that it can exist in the +2
(chromous) and the +3 (chromic) oxidation states. Unlike iron, however, the
redox; potentials needed to reduce chromium from the chromic state to the chromous
state are more strongly reducing then could be expected to be attained in the
simulated landfill columns (E--0.41V). Thus, while the chemistry of iron in
these systems is that of the ferrous ion, Fe2+, chromium chemistry is entirely
in_ the domain of the chromic ion, Cr3*. One consequence of this is that
among the heavy metals added to the columns, chromium does not form
' n i v,nd wil1 not be removed as a result of the generation of
On the other hand, the chromic ion forms one of the more sparinelv
^Df all hydroxides, Cr(OH)3, with a pK^, of 30.8. This is sufficient to
values (FiSfe95?ant rem°Val °f chromium as the hydroxide, even at acidic pH
5.2.2.6 Lead--
,-r,t-0^aI5? chemistry of lead is complicated by the array of available chemical
i£«££ S ?nS;>, Wh?-lenlead can exist in a +4 oxidation state, the redox potentials
observed in the simulated landfill systems limit lead to the +2 oxidation state
167
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pFe(ll) 3 -
pH
- 3 pFe(ll)
Figtire 93. pH-pFe(II) Distribution Diagram.
168
-------
pH
pMn(ll)
pMn(ll)
Figure 94. pH-pMn(II) Distribution Diagram.
169
-------
pCr
Figure 95. pH-pCr Distribution Diagram,
170
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Lead in that state can form strong complexes with hydroxide, chloride and
sulfate, and five different sparingly soluble salts with these anions and with
sulfide (Table 38). Thus, while the four heavy metals described earlier display
relatively simple chemical behavior, lead is affected by a much more complex
range of controls. With respect to solubility control, an examination of Figure
96 indicates that, prior to the onset of sulfate reduction, the controlling solid
species was most probably the sulfate. Based on this assumption and the
equilibria presented in Table 39, and taking into account the approximate
concentrations of leachate sulfate and chloride, the mole percent distribution
of lead among the expected important complexes is also presented in Table 39.
Based on these results, it was anticipated that the total lead concentration in
the leachates during the acetogenic phase of landfill evolution would be on the
order of 16 times the concentration of free, uncomplexed lead as calculated on
the basis of the indicated concentration of sulfate and the solubility product
of lead sulfate.
\ !
With the onset of sulfate reduction/sulfide production, it was expected
that the very sparingly soluble lead sulfide (PbS with its pY^, >= 28.0) would
become the controlling solid, and that the lead concentrations would rapidly
decrease to very low leachate levels (Figure 97).
5.2.2.7 Mercury--
Mercury was the most complicated of the heavy metals included in the
investigations. Under the reducing conditions extant in the columns, mercury has
ready access to three oxidation states, the +2 or mercuric state, the +1 or
mercurous state, and the neutral or free metal state. In the latter case, this
implies reduction of the ionic form to metallic mercury. The mercuric ion is
subject to unusually strong interactions with many inorganic ligands and, in
particular, with halide ions such a 'chloride (Table 38). Examination of the
equilibria indicates that, under the conditions present prior to significant
sulfide formation, all but a trace of soluble mercury would be present as
chloride complexes (Table 40) . The redox behavior of mercury is also significant
as indicated earlier. A pH-pe diagram was generated using the indicated
conditions expected in the columns during the acetogenic phase (Figure 98)'. On
examination of this diagram, it was clear that, even when complexed with.
chloride, the mercury could be easily reduced to the neutral metal. (It might
be noted incidentally that one conclusion drawn from the computations involved
in the generation of this diagram was that the mercurous- ion, Hg22+, was not
thermodynamically stable in these systems). A similar diagram was generated
assuming the presence of sulfide in the solutions and.the consequent existence
of solid mercuric sulfide, HgS (Figure 99). In this case, while reduction of the
mercury to the neutral metal remains possible under the column conditions, the
situation is not as clear, and it is possible that formation of the sulfide might
hinder reduction of the mercury to the metal. Reduction of mercury to the metal
opens one unusual pathway for the loss of mercury by transport in the vapor
phase; metallic mercury is volatile even at ambient temperatures. Finally,
mercury is capable of forming organomercury compounds under conditions such as
those anticipated to be present. The formation of such substances as
methylmecuric chloride (CH3HgCl) by microorganisms in aquatic systems has been
well documented Hughes*99'; Jernelov000', and the possibility of this occurring in
the simulated landfill investigations could not be dismissed.
5.2.3 Observed Leachate Metal Behavior
5.2.3.1 Sodium, Potassium, Calcium and Magnesium--
As indicated earlier, it was anticipated that no significant precipitants
would be present for sodium, potassium, calcium and magnesium during the
operation of the simulated landfill columns, with primary behavior as
conservative tracers. Examination of Figures 70 through 73 indicates that these
metals 'did indeed behave in a fashion consistent with that expectation. This is
most obvious in the case of sodium and potassium, which demonstrated clear
washout patterns in the single pass columns with net removals of the metals
171
-------
TABLE 38. SIGNIFICANT EQUILIBRIUM CONSTANTS FOR METAL COMPLEXES*97-98)
Solubility Products, log K., !
PbS
PbS04
Pb(OH)2
PbCl2
PbOHCl
Pb2Cl(OH)3
HgS
Hg(OH)2
CdS
NiS
ZnS
Acidic Dissociation
H2S Log K,, -
Log K.2 •
-26.6
•-7.8
-15.5
-4.7
-13.7
-36.1 i
-52.0
-25.7
-27.2
-23.8
-24.8
Constants •
-7.0
• -13.6 !
Cumulative
PbS04
Pb(S04)22
PbCl+
PbCl2
PbCLy
PbCl4-2
HgCl2
HgCl3-
HgCV
Formation Constants
2.8
3.6
1.3
1.7
1 9
J. o •?
1.5
13.9
14.8
15.8
1
TABLE 39. DISTRIBUTION OF LEAD SPECIES IN TYPICAL LEACHATE"
[Pb]r -
Species
Pb«
PbCl+
PbCl2
PbS04
Pb(S04)22
[Pb«] + [PbCl+] + [PbCl2] -
Distribution
6%
5%
' 1%
77%
: 11%
h [PbS04] -i- [Pb(SOJ22] •
- 16 [Pb+2]
"Chloride and sulfate at 1,500 mg/L, pH - 5.3
TABLE 40. DISTRIBUTION OF MERCURY SPECIES IN TYPICAL LEACHATE"
Species
Distribution
HgCl2
HgCl3-
HgCl/2
23%
66%
11%
[Hg]T - [HgCl2] + [HgCL/] + [HgCl/2] - 2 x 10" [Hg«]
"Chloride and sulfate at 1,500 mg/L, pH - 5.3
172
-------
2
i
4
PS04
6
8 10
12
-2
0-
2-
PbCL
Range of values
observed
-0
PbCIOH
-2
Pb2Cl(OH)s
PbSO,
-8
Pb
4-2
1O
o 2
PS04
T~
10
12
Figure 96. Predominance Area of Diagram for the System
+-- o j
173
-------
0.0
2.5
6.0-
10.0-
12.6-
16.0-
5
i
10
t
20
PbS
_l__Extreme sulfate
'levels observed
PbSO.
10 16
PC,
PbJ
25
0.0
- 2.5
- TR
7.6
- 10.0
12-i
1S.O
"r.*
Figure 97. Predominance Area Diagram for the System
Pb2+/S042-/Sulfide (—pH - 5.5, --- pH - 7.5).
174
-------
20
18
18-
17-
16-
16-
13-
12-
11-
10-
° f ? ? f ? • 7 8 i
HgCla
Hg2*
.X^
.X^^ Hg°
s*r •
0 1 2. 3 4 I 0 7 T r~"~ ~
PCI
0
-1200
•1100
•1000
-800
-800
-700
-876
-875
•776
-676 )
676
475
375
Figure 98. pCl-pe Diagram for Mercury in Absence of Sulfide.
175
-------
20
-10
1200
etection limit
for sulfide
-see
•see
•35S
•les jr
-45 mV
-245
-846
Figure 99. pCt s - pe Diagram for Mercury in Presence
of 'Sulfide ( pH - 5.5; ---pH - 7.5).
176
-------
IMOH>2
AC!O PHASE
NEGLIGIBLE SUUROE
I I I I I
HjCO,*.CHjlCHj),,COCH
I \ \\ \ \
'oooo
ooooo
ooooo
ooooo
I I I I I
•:\.\ Hi
METHANE PHASE
SULHD6 PRESENT
HCO—.MS.HS—
MUCH!
\ H I H. \
OOOO
poooo
"'ooooo
OOOO
HCOT
1
H,CO,» I [ RCOOH AOO PHASE
11 pHxSJS
METHANE PHASE
ENCAPSULATION PROCESS
Figure 100. Encapsulation of Heavy Metal Sludge Solids
in Simulated Landfills.
179
-------
observed. Since the onset of methane formation was associated with an increase
in leachate pH, the solubility of chromium would be expected to be further
limited, an expectation borne out by recorded observations.
5.2.3.5 .Mercury--
Upon consideration of the significant equilibrium constants for mercury
(Table 38), two facts are evident. I Sulfide can function as a powerful
precipitant for mercuric ions, even at extremely low equilibrium concentrations,
and any soluble mercury present should be associated in a complex interaction
with the ligand, chloride. The actual concentrations of mercury measured in the
leachates during this study are presented in Figure 81, and chloride
concentrations are shown in Figure 56. The data of Figure 81 give evidence that,
following early increases in the mercury concentrations, leachate levels for all
simulated landfills to which mercury had been added decreased rapidly to «20 yug/L
(ssO.l /iM) at Day 300, and showed only minor variations around that level
thereafter. If it were assumed that the mercury was present in the leachates in
the same oxidation state as that which was placed in the columns initially (+2),
on the basis of equilibria presented in Table 38, mercury speciation would be
dominated by chloro-complexes (Table 40), and the total concentration of soluble
mercury would, at the approximately 1,500 mg/L of chloride in the leachates,
equal approximately 1.8 x 10" times the concentration of "uncomplexed" Hg2*.
Since the leachates contained approximately 0.1 ^M total soluble mercury, this
implies a "free" mercuric concentration of 5 x 10"19 M. Based on mercuric sulfide
solubility, H2S acid dissociation constants, and the leachate pH levels during
the acetogenic phase, this mercury concentration would be in equilibrium with
extremely low levels of total sulfide CslO'20 M). However, since it is not
possible to analyze sulfide at such low concentrations, direct substantiation of
the participation of sulfide to control mercury behavior was not possible.
An alternative rational for the observed behavior of mercury, which is
compelling both on the basis of its relative simplicity and its reasonable
consistency with the observed behavior in these columns, can be developed. Shown
in Figure 98 is a. pCl-pe diagram for the'mercury-chloride system. This diagram
was developed assuming no participation of sulfide in the control of the mercury.
It should also be noted that in developing this diagram, it was found that the
mercurous ion was thermodynamically unstable with respect to the other species
in the system. Based on the oxidation-reduction potentials (ORP) measured in the
leachates as shown in Figure 55, it is clear that the columns had been operating
from the start of the investigations at redox potentials in excess of 500
millivolts more negative than needed to reduce the mercuric chloride complex to
metallic mercury. Therefore, it is proposed that the rapid early decrease in
leachate mercury concentrations was a result of the reduction of divalent mercury
to neutral metallic mercury, and since the rate at which the mercury
concentration decreased is consistent with this premise, it is highly unlikely
that significant sulfate reduction/sulfide formation could have taken place soon
enough to effect the observed decrease iti leachate mercury concentrations. In
addition, it has been reported"00 that metallic mercury has a solubility in water
of 20 to 40 /*g/L, a concentration range consistent with that observed in these
investigations. This transformation of mercury would permit volatile mercury
metal to undergo mobilization in the gases emitted from such landfill systems,
particularly during the methane generation phase when large volumes of gas are
characteristically produced.
In further support of the reduction of mercury to the metallic form, it was
noted that, even after the onset of active sulfide formation, the mercury
concentrations in the leachates remained at the 20 to 40 /ig/L observed
previously. These levels of mercury should not be possible in the presence of
0.1 to 1.0 mg/L of sulfide, unless the mercury was not able to be precipitated
by the sulfide, a condition met uniquely by neutral, metallic mercury. Even at
sulfide concentrations as high as 1 mg/L, the potentials in the columns were
sufficiently reducing to permit mercury reduction (Figure 99).
180
-------
approaching 95% of the metals originally present. In the recycle columns, the
concentrations of sodium and potassium in the leachates decreased approximately
25 to 30% over the duration of the study, a phenomenon which could be ascribed
to removal of leachate during sampling and replacement of these withdrawals with
tap water. Calcium and magnesium behaved in a somewhat more complicated fashion.
In the case of magnesium, washout was apparently the dominant process in the
single pass columns while, in the recycle columns, following a substantial
increase during the first 100 days of operation, concentrations decreased to and
were maintained at essentially uniform levels of approximately 100 mg/L for the
duration of the study. It is likely that this early surge of magnesium was a
result_of concentration of the metal in the initial leachate front, after which
it rapidly became dispersed more uniformly throughout the leachate pool as a
consequence of leachate recirculation. No evidence of any significant chemical
controls on the behavior of magnesium was observed.
In the single pass columns, washout of calcium was again the predominant
process, whereas in the case of the recycle columns the behavior of calcium was
not as obvious. An initial increase in calcium concentrations during the first
100 days similar to that for magnesium was observed, after which calcium
concentrations decreased to approximately 1,500 mg/L and were maintained at that
level until approximately Day 750. Following Day 750, the calcium concentrations
in the leachates from the recycle columns decreased steadily in an apparently
linear fashion for the remainder of the study. The most likely cause for this
behavior_is a combination of a modification of operational methodology and the
progression of the columns to a new phase in their evolution. At a time
approximately coincident with the initiation of the decline in calcium
concentrations, it was decided to augment the onset of methanogenesis by
additions of sodium bicarbonate for pH adjustment. Several such additions were
carried out and, with each addition of sodium bicarbonate, an increment of
carbonate ion equal to approximately one percent of the bicarbonate added was
introduced to the columns in accordance with the acid-base equilibria of the
carbonic acid system. This carbonate could have acted effectively as a
precipitant for the calcium and contributed to its removal from the leachates.
In addition, with the onset of methane formation between Days 900 and 1,000, the
pH of the leachates increased rapidly, thereby favoring further production of
carbonate through dissociation of bicarbonate and dissolution and dissociation
of carbon dioxide released during anaerobic stabilization. Thus, it appears that
carbonate precipitation of calcium played a role in reducing the concentration
and mobility of this metal. :
5.2.3.2 Manganese and Iron--
The behavior of manganese and iron (Figures 74 and 78) could be explained
on the basis of a combination of washout and sulfide precipitation; In the case
of the manganese, washout appeared to dominate in the single pass columns, with
only a slight indication of sulfide precipitation in the control column after Day
1250. In the recycle columns, manganese behaved much as did calcium and
magnesium, with an initial increase in the first 100 days followed by a decrease
to a constant concentration by Day 450, and a further decrease in concentration
approximately coincident with the onset of sulfide production (Figures 58 and 59)
at about Day 750.
In contrast to manganese, the behavior of iron was more .difficult to
interpret. For the single pass columns, little evidence of washout was apparent
In addition, the only period during which sulfide precipitation might have been
important was a brief interval between Days 1,000 and 1,150, and it is not
evident that sulfide was a controlling factor over this interval, except possibly
in the control column. The most reasonable explanation for this behavior relates
to the forms in which iron is available. While present in metal sludges added
to the columns, iron is also present in abundant but slowly releasing forms as
iron minerals in soils and metallic iron present in the waste mass. Thus the
reservoir of iron likely far exceeded both the ability of simple washout to
deplete it in the time interval of this investigation, and the capacity of the
sulfide generated to precipitate it.
177
-------
In the single pass columns, the continued addition of tap water to the
simulated landfill columns resulted in a steady release of iron from the
reservoir sources in quantities unlikely to be much influenced by such removal
mechanisms as sulfide precipitation, except in the case of the control column
where no other significant metal concentrations were present as competition and
where sulfide concentrations were highest of the single pass columns (Figures 58
and 59). On the other hand, the leachate from the recycle columns was nearer
equilibrium with the iron reservoir, and the impact of sulfide was more apparent.
In these columns, the leachate iron concentrations were essentially constant from
the initiation of the investigations until approximately Day 750, at which point
a. steady decrease in iron concentrations commenced. The relationship of this
decrease to the onset of sulfate redxiction/sulfide formation is clear and,
therefore, provides the most obvious explanation for the observed behavior.
5.2.3.3 Cadmium, Nickel and Zinc-- \
Cadmium, nickel and zinc (Figures 75, 79 and 80) in the single pass columns
were present at high concentrations , although in no case at concentrations which
approached those expected ion the basis of considerations of the solubility
equilibria of the corresponding hydroxides . After a period of time , which ranged
from approximately 200 days for zinc to approximately 400 days for cadmium, the
concentrations of the metals decreased rapidly to much lower concentrations and
were maintained low thereafter until the investigations were terminated. In no
case was either washout or sulfide precipitation observed, which would have been
expected to occur as sulfate was reduced. Therefore, it is postulated that the
initial surge of metals was a consequence of the leaching of metals from the
codisposed sludge layers, limited by such factors as incomplete contacting of
the sludge by the leachate, the kinetics of the dissolution of the sludge solids,
the reprecipitation of metal salts in the alkaline microenvironment of the
hydroxide sludge layers and encapsulation of sludge particulates with other
anions such as carbonate and, possibly, some small quantities of sulfide being
produced even during these early phases of the studies. It is also probable that
sorption processes involving the solids of the waste matrix were acting to retard
mobility of the metals, a process which might have been enhanced by complexation
of the metals with high molecular weight and polar humic-like materials in the
leachate.
The rapid decrease in leachate concentrations of the metals in the single
pass columns may reflect the completion! of encapsulation processes , again with
sulfide being the most likely candidate for this encapsulation. In the case of
the recycle columns, the initially high concentrations of these metals were
maintained until the onset of sulfide generation, an event which very clearly led
to precipitation of the metals. In this case, the initial dissolution of the
sludges yielded metal concentrations quite similar to those observed for the
single pass columns, while encapsulation of the sludge solids and sludge mass
precipitation of the metals' acted to limit further dissolution of the metals in
the leachates upon subsequent recycling events. With the onset of sulfide
formation, the metals were precipitated more rapidly than they could be
redissolved from the modified sludge solids , and the metal concentrations
commenced to decline and reach a new equilibrium level at approximately 900 Days ,
a level which was maintained thereafter. These hypothesized mechanisms are
illustrated in. Figure 100 for the Acid Formation and Methane Fermentation Phase
of landfill stabilization, as discussed in more detail in Section 5.2.5.
5.2.3.4 Chromium- -
The behavior of chromium (Figure 76) gave every indication of being
directed largely by hydroxide solubility equilibria, with leachate levels
decreasing rapidly (except for a brief and unexplained concentration increase
around Day 350) in the leachate of the recycle columns from initially high
concentrations to levels below one mg/L in all columns. Neither the operational
mode nor sulfide generation had an impact on the behavior of this metal. It
should be noted here that, at the observed pH of =5.5 in the acetogenic phase of
landfill evolution, the predicted concentration of chromium in equilibrium with
Cr(OH)3 would be about one pM. (0.06 Mg/L) which is consistent with the results
178
-------
While it is possible that some conversion of mercury to alkylmecuric
compounds took place in the environment of the columns, efforts at detecting such
species by cold vapor analysis of undigested .leachates were unsuccessful and
their possible existence was, therefore, not confirmed.
5.2.3.6 Lead-- [
During the acetogenic phase of column operation, leachate lead
concentrations varied, but tended to remain in the range of 1 to 10 mg/L (Figure
77). As discussed earlier, operating conditions were such that the solubility
of lead during that phase, assuming the absence of any sulfide, would have been
dominated by sulfate species and only a small amount of chloride complexation
(Figure 96). Under these conditions, the total soluble lead concentration would
have been equal to approximately 16 times the concentration of free Pb2+ as
indicated previously (Table 39) . On the basis of the solubility product of lead
sulfate (PbS04) and an average leachate concentration of sulfate of 1,500 mg/L
during the Acid Forming Phase, the equilibrium concentration of "uncomplexed"
lead would be expected to have been approximately one micromolar, and the total
soluble (complexed + uncomplexed) lead concentration would have been 16 fM or
approximately 3 to 4 mg/L. Given the analytical complexities associated with
the leachate matrix and the inherent variability of the parameters measured in
this istudy, this is in excellent accord with the concentrations of lead actually
measured in the leachates.
With the onset of active sulfate reduction/sulfide production lead
concentrations decreased in the leachates to levels well below 1 mg/L This
again is consistent with the very limited solubility of lead sulfide (pKj=26 6)
which would yield estimated equilibrium lead concentrations many orders of
magnitude below detectability by conventional analytical methods. Therefore it
could be surmised that lead was highly responsive to removal as the sulfide once
this precipitant began to be formed in the landfill columns coincident with the
onset or metnanogenesis. ,
5.2.4 Mai or Findings During-Landfill Column Operations .
Based on the results obtained during/the simulated landfill operations,
several conclusions concerning the behavior of the codisposed inorganic prioritv
pollutants could be drawn. ,
« The^landfill columns had substantial assimilative capacities for the
codisposed heavy metals.
• The assimilative mechanisms were diverse and dependent on.the chemical
characteristics of the heavy metals as well as the environmental
conditions to which they were exposed.
• Chromium removal was controlled by precipitation of Cr(OH)3 and was
essentially complete very early after column operation commenced As
expected, sulfide generation did not influence the behavior of chromium.
• The behavior of cadmium, nickel, lead and zinc was controlled during the
early phases of column operation by sorptive interactions between
components of the waste matrix and the metal ions. With the onset of
sulfate reduction, removal of these metals was then controlled by of
sulfide generation and precipitation of sparingly soluble sulfides.
« Mercury was very rapidly reduced to metallic mercury, and the subsequent
behavior of the metal was controlled by the properties of the neutral
element. Leachate concentrations of mercury were in excellent agreement
with solubility expected for metallic mercury, and sulfide formation had
little significant impact on residual leachate mercury concentrations.
In this context, vapor phase removal of volatile metallic mercury may
be an important transport mechanism for mercury, particularly during
high generation periods. . ' J &
181
-------
» Once sulfate reduction/sulfide generation had commenced, even the higher
loadings of added metals were assimilated within the simulated landfill
columns. However, the ability of the metals to retard or prevent
entirely the onset of microbially-mediated sulfate reduction or methane
fermentation by exertion of toxic or inhibitory effects must be
considered in any decision regarding acceptable metal loadings to
landfills subjected to the codisposal of heavy metals.
• Based on the observed performance of the simulated landfill columns, it
can be concluded that they could readily assimilated the lowest metal
sludge loading, while the moderate loading was a near limit for the
codisposal method employed without dramatically interfering with the
progress of waste stabilization!
• Leachate containment and recycle enhanced the landfill stabilization
processes associated with assimilation of the heavy metals, and reduced
the potential for uncontrolled discharge to the environment and the need
for separate treatment by fixing the dissolved metals in situ within the
waste matrix. It also acted to increase the opportunities for
beneficial metal-refuse mass and sludge solids interactions, and
tempered the need for codisposal and ultimate discharge control.
5.2.5 Characteristics of Supplemental Metal Sludge Samples
The codisposed metals contained in the test columns were added in the form
of typical alkaline sludges beneficiateid as necessary by metal oxides (Tables 17
and 18). While the primary objective of these additions was to acquire
information regarding the potential for release of metals from these sludges into
the column leachates as landfill stabilization progressed and the relative
impacts on microbial mediation, it was also desired to obtain corresponding data
on the effect of the landfill environment during acid formation and methane
fermentation on the sludges and the implications of resulting changes in their
characteristics and.associated behavior. In order to facilitate the acquisition
of this information, to each test column to which metals were added, supplemental
packets of sludge, contained in mesh bags (nylon stockings) and tied with bright
yellow plastic cord to facilitate recovery during column disassembly, were also
added. These packets were placed in the columns at the same locations as the
sludge loadings and each packet contained the same metal sludge/oxide mixture
added in bulk to the columns with the exception that these sludges contained
clean Ottawa sand rather than sawdust admixed to facilitate permeability to the
leachate. While the results presented in Figure 101 apply to the sludge/metal
oxide/sand mixtures, the values in Tables 41, 42 and 43 have been corrected for
the sand content of the mixture and, hence, apply only to the metal sludge/metal
oxide mix. ,
5.2.5.1 Initial Cons iderations
It has been hypothesized from the beginning of the experimental
investigations that the alkaline metal sludges codisposed in the simulated
landfill columns would react in a fashion affected by the composition and
consequent chemistry of the sludges and their potential to serve as a reaction
site within the waste matrix. In particular, the metal sludges, placed as they
were in three discrete layers rather than distributed uniformly throughout each
test column, were expected to provide regions of high alkalinity which could
locally moderate the acidic nature of the columns during the early phases of
landfill stabilization and, in the process, undergo chemical modification
themselves. Furthermore, by providing such an alkaline microenvironment,
eventual encapsulation of the sludges by sulfides during the methane
formation/sulfate reduction phase of landfill stabilization would strongly
influence the degree of long-term mobility of metals originating from the
sludges. I
The encapsulation process, previously introduced schematically in Figure
100, is a recognized process whereby a particle of metal hydroxide sludge is
impacted during the acid formation phase of landfill stabilization by both
carbonic acid and volatile organic acids with, as the primary effect, the release
182
-------
T3
O
O>
Q
c
LO
C
C
O)
c
o o o o
O O O O
c
_3
o
O
O
w
0)
OT
0!
bO
•O
i-l
03
•U
0)
0)
r-4
o.
I-
CO
c
0)
•U
CT)
4J
r
-------
TABLE 41. SELECTED ANALYSIS OF SUPPLEMENTAL METAL SLUDGE
- CODISPOSED IN SIMULATED LANDFILL COLUMNS
Column
Identity
4 OLS
5 QMS
7 OLR
9 OMR
10 OHR
Sludge
Solids, %a
63.3
61.4
66.5
64.8
60.1
Sulf ide ,
mg/kg (dry)b
56,000
8,600
3,300
1,300
110
Inorganic
Carbon, % (dry)c
8.2
8.0
8.0
8.8
7.8
Note: "Includes Ottawa sand added with sludge in mesh bags.
bOriginal sulfide content <
C0riginal inorganic carbon content 1.9% (dry)
5 mg/kg (dry)
TABLE 42. TOXIC METAL CONTENT OF SUPPLEMENTAL SLUDGE CODISPOSED
IN SIMULATED LANDFILL COLUMNS
Column
Identity
4 OLS
5 OMS
7 OLR
9 OMR
10 OHR
Average
Constituent Toxic Metal3
Cd
mg/g
18.0
12.5
15.6
13.3
17.2
%
110
76
95
81
105
93
Cr
mg/L
37.5
33.6
29.0
38.3
31.3
%
142
127
109
143
118
128
: Hg
mg/g
5.4
4.!6
4.15
5.5
7.iO
%
63
54
53
64
82
63
Ni
mg/g
35.2
37.5
23.5
19.6
28.9
*
110
117
73
61
90
90
Pb
mg/g
70.4
76.6
58.6
61.8
66.5
*
167
181
139
146
157
158
Zn
mg/g
61.0
79.7
54 ..7
53.9
54.7
%
75
98
67
67
68
75
Note:
Original Cd, Cr, Hg, Ni, Pb and Zn added; 16.4, 26.6, 8.6, 32.1,
42.2 and 81.3 mg/g (dry), respectively
% - % of original metal level added.
TABLE 43. NON-TOXIC METAL CONTENT OF SUPPLEMENTAL SLUDGE CODISPOSED
IN SIMULATED LANDFILL COLUMNS
Column
Identity
4 OLS
5 OMS
7 OLR
9 OMR
10 OHR
Na
5.8
1.9
5.4
10.9
9.8
Constituent Non-Toxic Metal, mg/g (dry)
K
0.3
0.5
• 1.0
1.1
1.1
Ca
<0.1
<0.1
0.4
0.3
0.4
Mg
47.7
57.9
36.7
80.5
72.7
Fe
18.8
20.3
27.4
27.4
28.9
Mn
39.1
32.1
50.0
40.7
43.0
184
-------
of some metals to the leachate. Countering this process is a concomitant buildup
of a layer of metal carbonates analogous to ion exchange at the particle surface.
This initial encapsulation process would thereby hinder the further mobilization
of the metals during the acid formation phase of landfill stabilization.
With the initiation of sulfate reduction coincident with the onset of
methane fermentation, a second ion exchange-type process could result in the
replacement of carbonate at the particle surface by sulfide. Given the extremely
low solubilities of most heavy metal sulfides, this second encapsulation would
have the effect of essentially terminating the mobilization of most metals at the
neutral and higher pH levels characteristic of the landfill environments during
the methane phase and thereafter. The corresponding impact of the sludge on the
characteristics of the contacting leachates is illustrated in (See Figure 100).
During the acid formation phase, the conversion of carbonic acid to carbonate ion
and the neutralization of the organic acids would be expected to result in a_
sharp local increases in the pH of the leachate, with a corresponding decreases
in heavy metal solubilities. Consequently, the alkaline environment of the
sludges could be expected to actually hinder the mobilizatidn of the heavy metals
into the leachates during the acid formation phase of operations .(Figure 101),
while, during the methane fermentation phase, the heavy metal concentrations,
already diminished as a consequence of the elevated pH levels characteristic of
the later portions of this phase arid ensuing conditions, would be further
decreased by precipitation as sulfides in the alkaline micro environment of these
sludge layers.
5.2.5.2 Sludge Compositions-- ;
Following termination of the experimental studies, samples of metal sludge
were recovered from test Columns No. 40LS, 50MS, 70LR, 90MR and 100HR. (The
packets contained in Column 80HS could not be found.) The retrieved samples were
analyzed for solids content, sulfide, inorganic carbon, heavy metals and for the
six other metals routinely analyzed in the leachates. The solids in. these sludge
samples were estimated after drying at 107° C. The results are presented in
Table. 41, and varied only slightly between columns, averaging about 63%. These
determinations were then used to express other measurements on a dry weight
basis.
Sulfide levels in the sludge samples were estimated in accordance with
Standard Methods and EPA methods (Table 22) by purging acidified suspensions of
sludge into an alkaline ascorbic acid using nitrogen gas. The sulfide trapped
was then measured by means of a sulfide electrode calibrated against a
standardized (Lead Titration) sulfide solution. The resulting values .were
normalized on a dry weight basis and are also presented in Table 41.
The most striking finding with respect to sludge sujlfide levels was the
wide range of values measured, varying in excess of three 'orders of magnitude.
While the data are too few to draw an unequivocal conclusion regarding trends,
it does appear that the higher the metal sludge loading in a column, the lower
the sludge sulfide level. Moreover, the effect is far too obvious than to be
rationalized simply on the basis of increased capacity for sulfide formation
resulting from heavier sludge loadings, since the range is out of proportion to
the variation in sludge loadings. However, it is possible that the increased
metal loadings hindered the microbially-mediated reduction of sulfate to sulfide
and thereby reduced the amount of sulfide available for encapsulation.
Therefore, encapsulation and its effect on long-term metal mobilities is
considered significant and worthy of further investigations under a variety of
landfill loading and operational conditions.
Inorganic carbon levels were measured in a similar manner to sulfide
levels, except that reagent grade sodium hydroxide was used to trap the C02 and
the inorganic carbon was analyzed using a Beckman Total Carbon Analyzer. No
significant trend in the inorganic carbon levels in the retrieved sludge samples
was observed (Table 41), and all values ranged from 7.8 to 8.8% on a dry weight
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basis as compared to approximately 1.9% for the sludge as added. Therefore, some
encapsulation by inorganic carbon was indicated in accordance with the process
previously described.
The heavy metals (Cd, Cr, Hg, Ni, Pb, Zn) added to .the columns were
analyzed by digestion of the sludges in strong acid followed by flame atomic
absorption spectrometry. The results of these analyses are presented in Table
42 and Figure 101. The results indicate an interesting pattern in terms of metal
accumulations and releases. In the case of cadmium and nickel, these two metals
appear to have been largely conservative, with small average reductions in metal
content in the sludges and somewhat erratic behavior. Nevertheless, the combined
effects of encapsulation and the alkaline sludge environment likely served to
minimize mobilization of cadmium and nickel. In comparison and for reasons which
are not clear, zinc levels in the sludges decreased substantially («25%), which
is somewhat surprising when compared with the behavior of its close chemical
relative, cadmium, which exhibited a minimal decrease in sludge levels.
Apparently, some unknown factor in these systems was acting to enhance zinc
mobility, but the exact nature of the factor cannot be ascertained with the
available data. Likewise, mercury was the most diminished of the metals (=37%),
a finding which was most likely attributable to the loss of mercury as a
consequence of reduction o-f the mercuric ion and volatization of metallic mercury
as discussed previously (Section 5.2.3.5).
In contrast, lead and chromium accumulated significantly (158 and 128%,
respectively) in the sludges, thereby suggesting chemical precipitation within
or on the sludge solids. Precipitation of chromium as hydroxide (Cr(OH)3) and
lead as sulfide and/or sulfate, both sparingly soluble species, in or on the
alkaline sludge mass would be an expected reaction as also discussed previously
(Sections 5.2.3.4 and 5.2.3.6. It should be observed here that this accumulation
results from either single pass or recycle liquid phase transport of metals from ,
higher levels of concentration in the columns
The sludge samples were also analyzed for the group of nontoxic metals
monitored routinely in the leachates (Na, K, Ca, Mg, Fe and Mn) as presented in
Table 43. Most of these metals were present at extremely low levels in the
sludges, thereby reflecting their relative mobilities in the leachate of the
simulated landfills. Indeed, only iron and manganese, both subject to
precipitation as very sparingly soluble sulfides in the +2 oxidation states
expected for both metals, and magnesium, which forms a very sparingly soluble
hydroxide, were present at significant levels. The others of these metals,
deprived of reaction opportunity with sulfide or hydroxide, were found to be
present only at very low levels in the sludge.
5.2.5.3 Implications of Results from Retrieved Sludge Sample Analyses--
Based on the results of these analyses, there is every indication that
metal sludge codisposed with refuse in a landfill will react in a complex
fashion, resulting in modifications of both the localized landfill environment
and the sludge itself. Encapsulation of sludge particles with both carbonate and
sulfide can occur, but elevated metal levels may hinder microbially-mediated
sulfide formation with resultant reduction of both metal complexing capabilities
and encapsulation behavior. Therefore, it may also be concluded that the metal
sludges in the simulated landfill columns actually acted as sinks for metals
transported from other regions of the landfill (Pb and Cr) and that, although no
parallel studies on the chemical constituency and behavior of more homogeneously
distributed sludges were available, layering of the sludge in high concentrations
zones enhanced this beneficial immobilizing behavior. In addition, leachate
recycle facilitated opportunities for both metal deposition in or on the sludge
layers and resultant encapsulation of sludges with mobility retardants such as
carbonate and sulfide. Consequently, it could be concluded that the manner in
which toxic metal sludges are added to !a landfill and the method of landfill
operation can significantly impact mobilities and ultimate behavior of metals
contained in such sludges.
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SECTION 6
SUMMARY AND CONCLUSIONS
The purpose of. this.research was to demonstrate and evaluate landfill
assimilative capacity in terms of the ability to accommodate inorganic and/or
organic priority pollutants codisposed with municipal refuse without inhibiting
the microbially-mediated processes of waste stabilization. In accordance with
this objective, the experimental results can be summarized as follows:
1. In general, simulated landfill columns employing the leachate
recirculation management strategy achieved waste stabilization more
quickly and completely than simulated landfill columns operated with
i single .pass leaching as reflected by trends in gas volumes produced, gas
production rate, gas composition, and leachate indicator parameters. In
addition, a greater mass of volatile organic acids were released into the
leachate during the Acid Formation Phase and transformed into methane
.during the Methane Fermentation Phase in the recycle columns than in the
single pass columns. This more complete and rapid waste stabilization in
the recycle columns was attributed to the more favorable environment that
was developed in the recycle columns in comparison with the single pass
| columns. Within the recycle columns, leachate was contained and
recirculated, thus the biomass generated within the columns had an
enhanced and controlled contact opportunity with substrate, nutrients and
moisture needed for growth and proliferation. In contrast, biomass within
the single pass columns did not have the same contact opportunity, because
leachate was routinely removed from the system. Therefore, the
environment within the recycle columns was more uniform than that within
the single pass columns, because the nutrient-rich leachate was removed
.and lost with the single pass leaching management strategy. Moreover,
leachate produced by facilities employing single pass leaching would incur
greater treatment challenges and pollutional potential in terms of adverse
environmental and health impacts if it were to migrate from landfill
boundaries.
2. The consequences of the applied admix loadings of inorganic and/or organic
priority pollutants were reflected by reduced gas production, gas
production rates, and percentage of methane in the gas. In addition,
simulated landfill columns that received priority pollutant loadings
generally exhibited different trends in leachate indicator parameters than
the control columns, as discussed subsequently. However, the effects of
the organic and inorganic priority pollutant loadings upon the single pass
columns were more severe than for similarly loaded recycle columns as
reflected by differences in gas production, gas composition, and leachate
indicator parameters.
3. The recycle columns displayed greater gas production than single pass
columns and, although increases in priority pollutant loadings in the
recycle columns were accompanied by temporary decreases in gas production,
eventual assimilation of the priority pollutants was achieved. In
contrast, this behavior was not clearly reflected in the single pass
columns because both substrate and priority pollutants were removed by
washout, thereby affecting the assimilative capacity as well as the
quality of the leachates and gas discharges.
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4. Carbon dioxide, methane, oxygen, hydrogen and nitrogen gas were detected
from all ten simulated landfill columns. Oxygen and nitrogen, introduced
during waste loading and contained within refuse void spaces, was removed
in all ten columns when anaerobic conditions predominated, and nitrogen
was displaced by gas production. All five recycle columns and the control
single pass column produced adequate gas volumes to displace the nitrogen,
however, nitrogen was present throughout the entire experimental period in
the remaining single pass columns because of insufficient gas production.
The lack of sufficient gas production in the single pass columns
containing priority pollutants was attributed both to the admix priority
pollutant loadings and washout promoted by the leachate management
strategy employed.
5. Similar behavior was observed between the ten simulated landfill columns
with respect to the leachate indicator parameters of pH, alkalinity, TVA,
COD, TOG and ORP. Recycle Columns 1CR, 60R, and 70LR demonstrated the
greatest degree of waste stabilization as determined by trends in these
indicato'r parameters, and Column 90MR displayed signs of slight
retardation. Column 100HR, however, showed signs of more severe
retardation due to the applied priority pollutant loading as evidenced by
lower pH, lingering TVA concentrations, higher COD and TOG concentrations,
higher alkalinity concentrations, and less negative ORP. The control
single pass column, Column. 2CS, exhibited the greatest degree of waste
conversion of the five single pass columns -as reflected by these, same
parameters. The remaining single pass columns, Columns 308, 40LS, 50MS
and 80HS, showed signs of inhibition consequenced by applied loadings and
washout due to the type of leachate management strategy employed.
6. Leachate chloride and sodium concentrations, as well as lithium tracer
studies, were useful in determining the effects of leachate management
upon leachate constituent behavior. These analyses indicated that
leachate constituents within the recycle columns were retained within the
column, but decreased in concentration somewhat due to dilution. However,
leachate constituents within the single pass columns were removed
primarily by washout, as evidenced by the rapid decline in chloride
concentration. Liquid retention times within the landfill columns
increased as landfill stabilization progressed and was on the order of
several hundred days.
7. The mass of volatile organic acids removed due to leachate withdrawal was
much greater in the single pass columns than in the recycle columns. Most
of the generated leachate was contained within the recycle columns with
only a minor amount removed for sampling. Calculation of the potential
gas volume lost due to the removal of VOA, which serve as the primary
substrate for methanogens, revealed that the single pass columns did not
exhibit the same gas production pbtential as the recycle columns. This
decreased gas production potential observed for the single pass columns
was attributed to the less favorable biological environment present in
these columns, as well as the effects of washout.
8. Based upon calculations of the mass of individual volatile acids (IVA)
released into the leachate of the simulated landfill columns, the primary
IVA released in all columns included acetic, propionic and butyric acids.
These results agree with those reported in the literature concerning
separate anaerobic treatment processes. The mass of IVA released into the
leachate increased until the onset of methanogenesis. Thereafter, IVA
masses released declined commensurate with IVA transformation into methane
in Columns 1CR, 2CS, 60R, 70LR and 90MR. Columns 100HR, 308, 40LS, 50MS
and 80HS were adversely affected by applied priority pollutant loadings,
as evidenced by the relatively constant mass of acid released into the
leachate. Additionally, IVA were released into the leachate of the
control recycle and single pass columns towards the end of the
188
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experimental period, which was attributed to the decomposition of more
microbially resistant organic material. The mass of IVA released into the ;
leachate declined as priority pollutant loadings increased in the four ,
single pass test columns, while the miss of IVA released into the leachate !
of the recycle test columns remained relatively constant as priority ;
pollutant loadings increased. ;
9. The mass of IVA transformed into methane increased commensurate with the :
decrease of the mass of IVA released into the leachate of Columns 1CR,
2CS, 60R, 70LR and 90MR. However, Columns 100HR, 30S, 40LS, 50MS and 80HS ;
displayed a negligible mass of IVA transformed into methane. As priority '-
pollutant loadings increased within the ten simulated landfill columns, :
the transformation of IVA into methane was both delayed and diminished,
however, the effects of priority pollutant loadings were more severe in
the single pass columns than in the recycle columns. ,
10. A relationship between the mass of hexanoic and butyric acid released into
the leachate was observed in the simulated landfill columns, as well as a
relationship between released propionic, hexanoic and valeric acid mass. :
An increase in the mass of butyric acid released was accompanied by ;
increased hexanoic acid mass, and increases in the mass of propionic acid
released occurred concomitantly with increased releases of hexanoic and i
valeric acid mass. Additionally, hexanoic acid was released earlier in
the single pass columns than in the redycle columns.
These results can be explained by the thermodynamic favorability of the
condensation of butyric and acetic acids to form hexanoic acid during
periods of high leachate butyric acid mass, and, similarly, the i
condensation of two moles of propionic acid to form hexanoic acid and the '
condensation of propionic and acetic acids to form valeric acid during !
periods of high leachate propionic acid mass. The mass of hexanoic acid i
released into the leachate remained high until methane production ',
commenced, after which butyric and hexanoic acid mass were observed to
decrease. However, in the single pass test columns, the mass of released
hexanoic acid was significant throughout the experimental period. Similar ;
results regarding the production of higher molecular weight IVA have been
reported and it appears that the microbial populations responsible for
both the production (acidogens) and subsequent degradation (acetogens) of
IVA were inhibited as evidenced by a lower mass and varying composition of :
IVA produced.
11. The inhibition threshold, or priority pollutant loading below which
inhibition of landfill stabilization was not observed, was determined for '•
recycle and single pass simulated landfill columns. The recycle columns >
did not exhibit inhibition of stabilization processes, although the
applied priority pollutant loadings did cause retardation (or a delay) of
stabilization processes. All four single pass columns containing priority
pollutants were inhibited by the applied loadings.
12. Both organic and inorganic priority pollutant loadings were assimilated i
within the landfill columns. This assimilation was greater in the recycle
columns than in the single pass columns. Heavy metals were largely i
removed by precipitation as hydroxides, carbonates or sulfides, although i
reduction and matrix capture by sorption, ion exchange and encapsulation
were also operative. The organic priority pollutants were also
attenuated, largely by abiotic and biotic transformations, as well as by
partitioning within the waste mass. Reductive dehalogenation appeared to i
be a principal mechanism for halogenated compounds,' whereas some evidence
of reduction, ring cleavage and possible complete mineralization was
present. Overall conversion of the organic priority pollutants was more •
evident in the recycle than the single pass columns, and the degree of
conversion was also related to the inhibition or retardation exhibited at
189
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higher heavy metal loadings.
The following conclusions are drawn based upon the experimental results
obtained during the course of these investigations.
1. The leachate recirculation management strategy offers opportunities for
more complete and rapid waste stabilization, including attenuation of
codisposed priority organic and inorganic pollutants, than the single pass
leaching management strategy, thereby providing a controlled and more
favorable environment for accelerated microbiological growth and
concomitant waste stabilization.
2. Admix inorganic and/or organic priority pollutant loadings exhibited
retardation of the landfill stabilization process in both recycle and
single pass columns. However, single pass columns were more severely
affected than- the similarly loaded recycle columns, and greater
attenuation and detoxification was achieved with leachate recycle.
3. Trends in leachate indicator parameters such as pH, alkalinity, TVA, COD,
TOG and ORP, provide insight into the status of the progress of waste
stabilization, 'and can reflect the magnitude of stress imposed upon the
system such as introduced by the applied priority pollutant loadings
and/or the type of leachate management strategy employed.
4. Both acid-forming and methane-forming populations within the recycle
columns were essentially unaffected by the applied priority pollutant
loadings. However, these same populations were adversely affected within
the single pass columns, as evidenced by the type 'and quantity of IVA
released into the leachate and the relative efficiency of their conversion
to gas.
5. The threshold inhibition level for the recycle columns was equivalent to
the inorganic priority pollutant loading applied to the highest loaded
column, whereas the threshold inhibition level for the single pass columns
was determined to be the lowest codisposed priority pollutant loading.
6. Landfills possess a finite capacity to attenuate hazardous and
nonhazardous organic and inorganic constituents through microbially-
mediated physicochemical processes of reduction, precipitation, sorption
and matrix capture of heavy metals, , and fixation and sorptive matrix
capture and both biotic and abiotic conversion of complex organic
substances.
7. Controlled landfill systems, designed and operated as bioreactors with
both leachate and gas management, enhance predictability, minimize
potentials for adverse health and environmental impacts, and encourage
further innovations responsive to regulatory and public concerns.
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