PA-660/2-74-034
1AY 1974
Environmental Protection Technology Series
- " * *?t '^ - A"V*'; ^' ^Y* ''*"' -^ '* ' •'' "'
^y^'
T * ^^^iGrT*%i!t v^^Sf^^* ^^^f•?^^^-'^^^ '-t.^T$r^? ^^ wW*^! *li * ^
* '' *• \. °^ ^i1^^^ * If** ^ "** ^ ^ fc * * *t """ "H ^
> .
:^:-:}
' «^Jl ,tr1« >, **>,,'
*>.*-« ";W7ift -f.W, ,<,- .>-«..
' " "
-------
RESEARCH REPORTING SERIES
Research reports of the Office of Research and
Monitoring, Environmental Protection Agency, have
been grouped into five series. These five broad
categories were established to facilitate further
development and application of environmental
technology. Elimination of traditional grouping
was consciously planned to foster technology
transfer and a maximum interface in related
fields. The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL
PROTECTION TECHNOLOGY series. This series
describes research performed to develop and
demonstrate instrumentation, equipment and
methodology to repair or prevent environmental
degradation from point and -non-point sources of
pollution. This work provides the new or improved
technology required for the control and treatment
of pollution sources to meet environmental quality
standards.
EPA REVIEW NOTICE
This report has teen reviewed by the Office of Research and
Development, EPA, and approved for publication. Approval does
not signify that the contents necessarily reflect the views
and policies of the Environmental Protection Agency, nor does
mention of trade names or commercial products constitute
endorsement or recommendation for use.
-------
EPA-660/2-74-034
May 1974
LIQUID AEROBIC COMPOSTING OF CATTLE WASTES
AND EVALUATION OF BY-PRODUCTS
by
Dr. Frank Grant
Mr. Francis Brommenschenkel, Jr.
Project No. S801647
(Formerly 13040HPV)
Program Element 1BB039
Project Officer
Mr. Lynn R. Shuyler
U.S. Environmental Protection Agency
Robert S. Kerr Environmental Research Laboratory
Ada, Oklahoma 74820
idiSvf :-.,- -.. , ;•*''!?
Prepared for
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 - Price 95 cents
-------
ABSTRACT
The Santa Ana River basin of California has a total dairy cow population of approximately
174,000. Most of these cows are confined to a relatively small portion of the basin in the
vicinity of Chino and Corona. The wastes from these cows burden the basin groundwater
resources, and the Santa Ana Regional Water Quality Control Board has issued waste
discharge requirements for the dairies.
The current study was undertaken to determine the technical and economic feasibility of
treating dairy waste in a liquid state by a tandem thermophilic-mesophilic aerobic
stabilization process, more commonly described as liquid composting. It was envisioned that
thermophilic temperatures would speed the stabilization process, biological heat generation
would maintain desired temperatures, and the product would be free of pathogens and weed
seeds because of the elevated operating temperatures. A well stabilized product would also
be free of odors and attractiveness for flies.
Experimental apparatus were set up at an operating dairy and a program was organized to
study the process. The study showed that a large fraction of dairy manure is relatively
resistant to rapid biological degradation even at thermophilic temperatures. Antithetical
requirements of sufficient oxygen for maximum biological activity and minimum air flow to
preclude the need for an external heat source could not be satisfied with the particular
experimental apparatus when utilizing air as the oxygen source. Improved results were
obtained with an oxygen-enriched air supply which pointed out the potential advantage of a
pure oxygen system.
It is suggested that future investigations focus upon the use of pure oxygen as the oxygen
source for the process and upon mechanical aeration equipment as opposed to diffused
aeration equipment to supply the oxygen. This work would confirm the technical feasibility
of the process. Preliminary cost estimates for a liquid composting process to serve 500 cows
were developed within the context of current dairy operation economics. The estimates
showed that the process is considerably more costly than current, conventional, composting
operations and that the cost of the process is substantially above levels which could be
maintained by dairy operations.
-------
CONTENTS
Page
Abstract ii
List of Figures iv
List of Tables v
Acknowledgments vi
Sections
I Conclusions 1
II Recommendations 2
III Introduction 3
IV Aerobic Thermophilic Treatment Process 7
V Experimental Equipment and Procedures 13
VI Experimental Findings and Discussion 21
VII Engineering and Economic Considerations 42
VIII References 45
IX Appendix 48
m
-------
FIGURES
No. Page
1 Conceptual Thermophilic-Mesophilic Treatment Process 11
2 Schematic Diagram of Batch Reactor 14
IV
-------
TABLES
No. Page
1 Summary of Diary Population and Waste Production in
Upper Santa Ana River Basin 4
2 Experimental Design for Continuous Thermophilic Reactions 18
3 Experimental Characterization of Fresh Dairy Cow Manure 22
4 Reported Dairy Manure Characteristics 23
5 Typical Dairy Cow Ration 24
6 Results of Long Term Aeration of Manure 25
7 Oxygen Uptake for Batch Reactions 26
8 Energy Generation in Continuous Reactions 30
9 Reactions Involved at Levels of Independent Variables 32
10 Effect of Variables on Biological Energy Generation 33
11 Reaction Conditions During Weeks 9 and 10 34
12 Characteristics of Continuous Reaction Products 36
13 Effect of Variables on Stabilization of Manure 38
14 Summary of Analysis of Variance 39
15 Conditions and Products of Mesophilic Reactions 41
16 Summary of Cost Estimates for 500 Cow Treatment Process 44
-------
ACKNOWLEDGMENTS
Those individuals closely involved with the project and their affiliations during their
involvement were as follows:
Chino Basin Municipal Water District, San Bernardino County, California
Mr. J. Andrew Schlange, General Manager
Mrs. Marilyn Thompson
Mr. Francis Brommenschenkel
James M. Montgomery, Consulting Engineers, Inc., Pasadena, California
Dr. Maurice Lynch
Dr. Frank Grant
Trans Nuclear Chemical Company, Pasadena, California
Dr. G. N. Tyson
Dr. Robert Austin
Through April, 1973, the research program was directed by Dr. G.N. Tyson, Jr. who with
the assistance of Dr. Robert Austin and Mr. Francis Brommenschenkel assembled the
equipment, established sampling and analytical procedures, and conducted the first phases
of experimentation. Dr. Maurice Lynch monitored administrative details. From May, 1973,
Mrs. Marilyn Thompson served as project director with Dr. Frank Grant assigned
responsibility for all project details. Mr. Francis Brommenschenkel served as principal
operator of the experimental apparatus and assisted Mrs. Thompson and Dr. Grant in
administrative and technical matters. Several other employees of Chino Basin Municipal
Water District provided support in preparing the study site, operating the experimental
apparatus, analyzing samples, and performing administrative details.
The interest and support of Mr. Lewis Aukeman merits special recognition. The donation of
the use of his dairy as the study site is sincerely appreciated.
Financial support is gratefully acknowledged from the Environmental Protection Agency,
Chino Basin Municipal Water District, Western Municipal Water District, and the California
Dairy Environmental Task Force with contributions from the League of California Milk
Producers, O.K. Kruse Grain and Milling Company, Western Consumers Industries, Inc., and
Coast Grain Company.
VI
-------
SECTION I
CONCLUSIONS
Principal conclusions of this study within the constraints of the equipment and aeration
methods are:
1. The digestibility of dairy cattle manure appears to be limited even at thermophilic
temperatures. A portion of the organics is readily digestible but the majority of the
organic material degrades rather slowly even at thermophilic temperatures.
2. A pasteurized product free of odors and probably free of pathogens and weed seeds can
be produced utilizing procedures of this study. However, the product retains a large
percentage of biodegradable material but is appropriate for land spreading as a soil
conditioner with fertilizer and moisture retention values if properly isolated from
surface waters. The total fixed dissolved solids of the product are probably unaffected
by the process and therefore the dissolved solids would remain a potential burden upon
groundwater supplies.
3. Utilizing air as the oxygen source, insufficient heat is generated biologically to
compensate for energy losses and maintain the thermophilic temperatures. A large
energy input from mixing or some other source is needed to maintain thermophilic
temperatures.
4. The necessary substrate-organisms-oxygen contact to produce rapid stabilization of
organic material appears to be limited by the amount of oxygen that can be transferred
from the air supply. Conventional equipment appears to be inadequate at thermophilic
temperatures and high solids content to transfer the oxygen needed to maintain a high
rate of biological activity.
5. Results with an oxygen-enriched air supply indicate that greater biological activity and
reduced energy losses can be achieved with a pure oxygen supply.
6. The diversity of biological organisms in the treatment process appears to be diminished
at elevated temperatures. This may tend to limit the range of organic material that can
be stabilized at thermophilic temperatures.
7. Advantages, if any, of a liquid composting process over a conventional composting
process presently appear to be more than offset by treatment limitations and costs.
8. Estimated costs of a complete treatment process appear to be too high for current dairy
operations.
-------
SECTION II
RECOMMENDATIONS
1. Further pilot-scale work to develop a liquid, thermophilic, aerobic treatment process for
the dairy wastes produced in the Upper Santa Ana watershed should not be actively
supported at the present time.
2. Future research to develop the process should be directed towards achieving efficient
mixing and high oxygen transfer efficiencies such as may be obtained with a pure
oxygen treatment system.
-------
SECTION III
INTRODUCTION
GENERAL
Urbanization and suburbanization of rural lands have reduced the amount of land available
for animal waste spreading. Total animal populations have increased with increased demands
for meat, poultry and animal products. Rising real estate costs and favorable economics of
mechanized feeding, milking, etc., have spurred trends towards large animal populations on
small acreage. In many cases, therefore, a situation exists where profuse quantities of animal
manure are produced with insufficient land for direct land disposal.
High density animal confinement is exemplified by dairies in the Chino-Corona area of the
upper Santa Ana River basin of California. Table 1 summarizes the dairy situation in the
upper basin. Specifically the table shows a total of approximately 174,000 cows in the
upper basin with approximately 126,000 cows confined within the area around Chino and
Corona. This concentrated dairy area consists of approximately 9,300 hectares (36 sq mi).*
A dairy cow is equivalent to approximately 25 people in terms of suspended solids
production. This indicates that 174,000 cows are equivalent to a human population of
several million, and in the Chino-Corona area the cow density is equivalent to several times
the human population density of such high density cities as New York.
A typical 8 hectare (20 acre) dairy in this area may have 400 cows. One third of the 8
hectares may be used for the dairy home, milking parlor and feeding pens, with the
remaining two-thirds devoted to cropland and spray irrigation of washwater from the
milking parlor. The manure produced in the feeding pens is collected and removed
periodically to the cropland, neighboring farms, composting operations, and literally any
other place to which the dairyman has access. Wastes often accumulate and erode,
contaminating water supplies.
The contamination of water resources by animal wastes has been adequately documented in
numerous publications. ^ In recognition of this, the California Regional Water Quality
Control Board, Santa Ana Region, has issued what has been termed Phase I and Phase II
requirements for the dairies in the Santa Ana watershed. The substance of these
requirements is as follows:
1. There shall be no discharge of dairy wastes to lands not owned or controlled by the
discharger, nor to lands for which the discharger has not obtained permission for waste
disposal from the land owner.
2. The discharger shall provide facilities to contain the runoff from manured areas that
would result from 1.3 times the 10-year 24-hour rainfall. (This corresponds to
approximately 1.3 x 11.55=15 cm (5.9 in. of water.)
*The modern metric system of units and symbols is employed throughout this report
followed by customary units and symbols in parenthesis.
-------
Table 1. SUMMARY OF DAIRY POPULATION AND WASTE PRODUCTION
IN UPPER SANTA ANA RIVER BASIN
County Area
San Bernardino Riverside totals
Number of dairies
Milking cows
Dry cows
Total number of equivalent
cowsb
Average number of equivalent
cows per dairy
Gross dairy area (ha)
Disposal area (ha)
Cows per hectare of
gross dairy area
Cows per hectare of
disposal area
Dairy wash water (I/sec)
Daily solids production (t)
292
(203)a
88,300
21,300
114,500
(75,600)
392
3,610
2,550
31.7
44.9
170
(110)
570
(375)
142
(102)
44,800
11,700
59,200
(50,400)
417
2,340
2,210
25.3
26.8
90
(70)
290
(250)
434
(305)
133,100
33,000
173,700
(126,000)
400
5,950
4,760
29.2
36.5
260
(180)
860
(625)
aThe numbers in parentheses pertain to the dairies concentrated in the Chino-Corona
area
Equivalent" is computed by counting heifers as 1/2 cow and calves as 1/5 cow with
respect to the waste production of a mature milk cow.
Source of data is California RWQCB, Santa Ana Region.
-------
3. The discharge of manure waste to lands owned or controlled by the discharger shall not
exceed 3 tons (dry weight) per acre (6.7 metric tons per hectare) per year. This is
equivalent to 1.5 times the amount of manure produced by one cow in one year.
4. Neither the treatment nor the discharge of dairy wastes shall cause a nuisance.
5. The discharge of dairy wastes shall not alter the quality of waters of the state to a degree
which unreasonably affects such waters for beneficial uses, or affects facilities which
serve such beneficial uses.
The schedule for compliance includes completion of facilities to meet requirement 2 by
October 1, 1973 (Phase I), and requirement 3 by March 1, 1974 (Phase II). Representatives
of the dairymen have asked the courts to compel a reconsideration of Phase I order and to
issue a stay order to prevent the Regional Board from enforcing Phase I. The request for
stay order was refused and representatives of the dairymen are now (October, 1973)
working with the Regional Board to work out steps for the dairy industry to achieve
compliance with the orders.
A substantial volume of manure needs to be removed from the Santa Ana River Basin to
meet the Regional Board requirements. Based on the assumption that a cow produces an
equivalent of 14 liters (0.5 cu ft) of composted manure per day (this takes into account a
volume reduction from raw to composted manure), the total amount of composted manure
generated is approximately 900,000 cubic meters (32 million cu ft) per year. Ten percent of
this can be spread on dairy disposal land. Additional cropland exists within the upper Santa
Ana River basin and if this land were to accept manure at the rate of 6.7 metric tons per
hectare (or 1,5 cows per acre), at least 15 percent more manure could be disposed of.
However, a major portion would remain to be exported from the upper Santa Ana River
basin or isolated from basin water supplies. A survey of current composting operations
indicates that the amount of manure currently removed from dairies for composting is
equivalent to approximately 300,000 cubic meters (11 million cu ft) per year as compost.
The compost supplies home gardeners, landscaping firms and commercial farmers. A portion
of the compost is spread upon the available cropland within the basin, but the majority of it
is exported. In any case the data indicate a need to develop additional manure treatment
and/or disposal activities in order to export or isolate as much as one half the total manure
production.
In addition to considerations of water quality degradation, the disposal of manure creates a
potential for production of particulate matter, ammonia, flies and malodors. Particulate
matter affects health, irritates eyes, causes soiling and reduces visibility. Ammonia can be
toxic, can discolor fabric dyes, and may be absorbed in bodies of water, thereby increasing
nitrogen concentrations and potential for growth of undesirable aquatic organisms. Flies and
malodors are unquestionably the greatest complaint of residents living in neighborhoods
around dairies. Flies cause a nuisance and carry filth which may contain pathogenic
organisms. Flies have been implicated in spreading diseases to both humans and animals.
Malodors produce mental and physiological effects such as nausea, headache, loss of sleep,
loss of appetite, curtailment of water intake and allergic reactions. ^ Malodors also decrease
personal and community pride, reduce capital improvement and investment in a
community, drive higher socio-economic classes from a community, and reduce tourist
trade, property values, tax revenues, payrolls and sales. 14
-------
NEED FOR STUDY
The current project evolved from the background of increasing urbanization of agricultural
lands in the southern California area and inadequate treatment and disposal methods for the
Chino-Corona area in particular. Many of the dairies in this area came from former dairy
areas in southern California when pressures of urbanization forced them out. In the
Chino-Corona area they are again facing urbanization pressures in addition to the regulatory
activities of the Regional Water Quality Control Board. Adequate treatment and disposal
facilities for dairy wastes would eliminate the large majority of objectionable characteristics
about dairies and thereby reduce pressures which force them out.
As stated in the proposal to EPA for this project, the need for study is twofold: (1) there is
a definite, ever-increasing need for treating livestock wastes, and (2) in view of the limitation
of existing treatment processes, it is evident that better treatment processes are needed. The
current investigation therefore focuses upon the applicability of a specific treatment process
to the stabilization of dairy cattle waste. The process is identified as a liquid,
thermophilic-mesophilic, aerobic composting process.
LOCATION
The study site was adjacent to feeding pens located on the Lewis B. Aukeman Dairy, 8425
East Walnut Avenue, Ontario, California.
OBJECTIVE
The original principle objective of this project was to demonstrate the technical feasibility
and economic potential of a rapid-stabilizing, minimum space-requiring approach to treating
high strength cattle wastes. Specific objectives were:
1. Establish the design and operating parameters of a thermophilic and a tandem
thermophilic-mesophilic aerobic stabilization process.
2. Determine detention times at thermophilic temperatures needed to achieve pathogen
elimination.
3. Test the feasibility of mathematically modeling and simulating on a computer the
thermophilic and thermophilic-mesophilic operations to optimize performance and
facilitate extension of these processes to treatment of other livestock and high strengh
wastes.
4. Show that the heat produced by microbial thermogenesis will maintain thermophilic
temperatures without addition of external thermal energy.
5. Develop design data and operating requirements for a pilot plant capable of handling
wastes from 500 head of cattle.
6. Perform a preliminary economic evaluation of the processes based on extrapolations of
the bench scale results.
These objectives were optimistically presented in the original proposal and formed the basis
for the direction of the experimental work.
-------
SECTION IV
AEROBIC THERMOPHILIC TREATMENT PROCESS
THEORETICAL CONSIDERATIONS
A theoretical development and a literature review of thermophilic aerobic digestion have
been presented by Andrews and Kambhu.16, ^ The thermophilic temperature range is
often considered to be between 49 and 60° C (120-140° F), although some researchers
choose to define a wider thermophilic range, for example between 42 and 65° C (108-149°
F). Certain bacteria thrive at thermophilic temperatures and in many cases produce the heat
needed to maintain high temperatures such as in composting and certain industrial
fermentations.
Among the potential advantages of a thermophilic aerobic process over a corresponding
mesophilic (temperature range below the thermophilic range) process are: increased reaction
rates leading to greater volatile solids destruction in a specified amount of time; increased
fraction of organic solids that can be biologically destroyed; increased destruction of
pathogenic organisms because of their inability to survive high temperatures; destruction of
undesirable weed seeds; and possibly improved solids liquid separation. Such advantages
have been demonstrated in other research work.l->, 18
The basic theoretical framework utilized in the current investigation is that developed by
Andrews and Kambhu. The mathematical model which they developed is based on first
order reaction kinetics; the rate of oxidation of organic material is therefore represented by
the equation:
d(BVS)/dt --KT (BVS) (1)
where (BVS) = concentration of biodegradable volatile solids
Kj = reaction rate coefficient, days"' at temperature T
t = time, days
A material balance for biodegradable volatile solids (BVS) in a completely-mixed,
continuous-flow, steady-state reactor gives the following:
(BVS)1 = (BVS)0/(1+KT9) (2)
where o = subscript denoting influent
1 = subscript denoting effluent
9 = reactor resident time, days
It is recognized that first order reaction kinetics may be a severe simplification of much
more complex biochemical phenomena actually taking place during the stabilization of
cattle manure. The rate of stabilization is visualized to be a function of temperature and
BVS concentration. However, in addition to these two variables the true rate very likely
depends upon mixing, oxygen transfer rate, limiting nutrients, the organic compounds being
oxidized and the concentration and type of microorganisms. The merits of first order
kinetics involve mathematical simplicity and convenience for incorporating oxygen
consumption rates and generation of thermal energy into a mathematical model.
-------
An analysis by Andrews and Kambhu of previous data showed that the ratio of oxygen
utilized to organic solids oxidized ranged from 1 .4 to 1 .6 kg C^/kg of volatile solids (VS)
for typical waste materials in those cases where nitrogenous oxygen demand is insignificant.
This ratio gives the oxygen equivalent (OE) of the BVS. The amount of heat generated in
the oxidation of typical organic materials was found to range from 13,300 to 15,100 kJ/kg
OE (5,700 to 6,500 Btu/lb OE) with an average of 14,200 kJ/kg OE (6,100 Btu/lb OE)
when nitrogenous oxidation is insignificant. These values correspond to an organic material
with a heating value of (14,200 kJ/kg OE x 1.5 kg OE/kg VS =) 21,300 kJ/kg VS (9,150
Btu/lb VS).
Estimates of the rate constant, Kj, for various substrates based on work of previous
investigators range from 0.1 to 0.2 days'1 at 20° C. At 60° C limited data show that the
value increases to the range 0.25 to 0.4 days"1, indicating that there is a 2.5 to 3-fold
increase in the value of the rate constant at 60° C over 20° C.
Cattle excrement is known not to be as pustrescible as that from many other animals and
human beings. The principal explanation is that cattle manure reflects cattle diet and
metabolism and contains a large portion of cellulosic material and lignins which are
characteristically more difficult and slower to degrade than are many other foodstuffs.
Therefore the rate constant for cattle manure oxidation may be considerably lower than
that for other wastes. In fact, a recent study indicates that there may be two separate K
rates with the first being the oxidation of readily degradable materials and the second, the
slower breakdown of other materials.1" These slowly degradable materials may account for
75 percent of the total biodegradable materials and exert a significant oxygen demand over
periods in excess of 20 days at a rate on the order of 1/20 to 1/30 that of the readily
degradable materials. Recognition of this anomalous behavior can be important in
interpreting results of biochemical oxygen demand (BOD) tests and in establishing the
appropriate percentage of cattle manure that may be considered biodegradable. For
example, the 5-day biochemical oxygen demand (BOD^) may represent less than 20 percent
of the ultimate biochemical oxygen demand (BODL).
An energy balance is needed to complete a mathematical model of the aerobic, thermophilic
treatment process. The important energy inputs to the process are energy of mixing,
biological heat production, and direct heat input from heating coils, etc. Energy losses
include heat lost to the surroundings through reactor walls, latent heat of vaporization of
water picked up by the air stream in passing through the reactor, sensible heat increase of
the air stream, and sensible heat increase of the liquid waste stream in passing through the
reactor.
The energy balance may therefore be represented by:
H + H + H) (3)
where Hf = rate of sensible heat increase of the contents of the
reactor, watts
Hm = rate of heat input from mixing and other direct heat
input, watts
H^ = rate of heat production by biological (and chemical)
oxidation of BVS, watts
Hi = rate of heat loss represented by the difference in sensible
heat between the influent and effluent liquid streams, watts
-------
Hy = rate of heat loss as latent heat in water vapor, watts
Hg = rate of heat loss represented by the difference in sensible
heat between the influent and effluent air (or gas) streams, watts
HS = rate of heat loss to the surroundings, watts
Each of these terms can be estimated from physical constants and measurements.
It is pertinent to point out some of the results of Andrews and Kambhu's model for a
complete-mixing, continuous-flow reactor at steady state conditions. Some of the important
assumptions input to the results are:
1. 70 percent of the total solids are volatile and of the volatile solids, 70 percent are
biodegradable.
2. 1.5 kg of oxygen is utilized for each kg of volatile solids destroyed and 14,200 kJ of
heat is released per kg of oxygen utilized with ammonia as a product (6,100 Btu released
per Ib of oxygen utilized).
o
3. Power input for mixing and gas transfer is constant at 26.3 W/m (1.0 hp per 1,000 cu
ft of reactor).
4. The outside air temperature is -6.6° C (20° F), the influent liquid temperature is 4.4° C
(40° F), the effluent streams are at the same temperature as the reactor contents, the
influent gas stream is dry, and the effluent gas stream is saturated with water.
5. Oxygen transfer efficiency is 15 percent and the overall heat transfer coefficient is
0.5684 J/m2 sec ° C (0.10 Btu/sq ft/° F/hr) for a cylindrical reactor with height equal
to diameter.
6. The rate constant Kj is represented by a smooth curve which has values of 0.10, 0.145,
0.21, 0.24, 0.275, 0.295, and 0.29 days'1 at temperatures of 20°, 30°, 40°, 45°, 50°,
55° and 60° C, respectively.
The hypothetical model treats domestic wastewater sludge or mixtures of domestic
wastewater sludge and ground garbage from a city with a population of 10,000. A summary
of observations is as follows:
1. For a contribution of 0.09 kg (0.2 Ib) of total solids per capita (630 kg/day VS) at 3
percent concentration (30 m^/day) and 10-day hydraulic and solids residence time, the
reactor operates at 39° C (102° F) which is below the thermophilic temperature range.
Heat input from mixing is approximately 10 percent of the total heat input from
biological activity and mixing. At the operating temperature of 39° C the heat losses are
51, 17, 7 and 6 kw for Hi, Hy, Hg and HS respectively. The losses would remain
approximately in this same proportion at higher temperatures except for the loss due to
latent heat of vaporization which increases exponentially with temperature.
Approximately 67 percent of the biodegradable volatile solids (or 47 percent of the
total volatile solids) is destroyed.
2. A five-fold increase of mixing energy would raise the reactor temperature to 49° C
(120° F). Approximately 73 percent of the biodegradable volatile solids would be
destroyed at the 10-day residence time.
-------
3. Increasing the amount of total solids to give a concentration of 4.5 percent (no change
in volumetric flow) would raise the reactor operating temperature to 46° C (115° F).
Approximately 71 percent of the biodegradable volatile solids would be destroyed at the
10-day residence time.
4. Increasing the residence time to 15 days would result in approximately 75 percent
destruction of biodegradable volatile solids (or approximately 53 percent destruction of
total volatile solids).
5. The oxygen transfer efficiency is a critical component of the process because this
determines the amount of air that must be supplied and therefore the amount of
emitted water vapor which carries away latent heat of vaporization. For example, if the
efficiency were 0.05 instead of 0.15 the reactor would operate at approximately 27° C
(80° F) which is well below the thermophilic range. Biodegradable volatile solids
destruction, however, would still amount to approximately 57 percent.
6. The use of oxygen would substantially reduce latent heat losses and therefore allow a
higher operating temperature with greater volatile solids destruction for a given
residence time.
It must be emphasized that the observations are extracted from the results of a computer
model of aerobic thermophilic digestion. Andrews and Kambhu's presentation contains
much more detail. However, the abbreviated presentation here serves to indicate reasonable
engineering expectations which need experimental verification for specific wastes such as
cattle manure.
CONCEPTUAL TREATMENT PROCESS
To help direct the experimental program for this study it was deemed important to lay out
an overall treatment flow diagram that would contain the essential elements of a 500 cow
pilot treatment operation. Figure 1 illustrates a conceptual treatment process. This process
consists of thermophilic and mesophilic treatment followed by some method of solids
separation such as vacuum filtration. Solids concentration in the thermophilic step must be
maintained at a high level to maximize biological heat generation per unit volume, and
therefore the feed approaches the composition of fresh manure. Additional wastewater from
dairy washing operations may be added to dilute the raw manure if necessary. Residence
time in the thermophilic stage is adjusted to achieve a reasonable balance between degree of
stabilization and rate of stabilization. Very long residence times give a high degree of
stabilization, but the rate of stabilization - equation (1) - is low because of low BVS
concentration and even further reduced because of the lower reaction rate constant due to
low temperatures from insufficient biological heat generation. A large reactor is needed to
achieve a long residence time. On the other hand, very short residence times give a high rate
of stabilization, but because the material is present in the reactor for a short period of time,
there is little change between influent and effluent BVS.
Material from the thermophilic reactor passes to the mesophilic reactor for further
stabilization. This reactor receives the benefit of a hot influent, but operates with a lower
BVS concentration and a much lower temperature. This may be considered a finishing stage
towards producing a well stabilized material. The final desired degree of stabilization
establishes the residence time in the reactor. Some recycle of solids after the filtration step
10
-------
DAIRY
WASHWATER
MIXING
DAIRY )
MANURE
THERMOPHIL.IC
TREATMENT
AIR AND OR
OXYGEN
MESOPHIUIC
TREATMENT
OVERFUOW TO FURTHER TREATMENT
IF NECESSARY AND DISPOSAL. OR USE
POSSIBLE RECYCUE OF PRODUCT
FIUTER CAKE
TO DRYING,
STORAGE AND USE
FIL.TRATE TO FURTHER
-^TREATMENT IF NECESSARY
AND DISPOSAL. OR USE
Figure 1. Conceptual thermophilic-mesophilic treatment process
-------
may enhance the process by maintaining a higher concentration of microorganisms and BVS
in the mesophilic reactor.
The filter cake from the filtration step is the major product of the process and is suitable for
soil conditioning. In some cases the product could be spread directly; whereas, in other cases
it may be desirable to dry and store the product for a period of time before use. The filtrate
from the process has a high total dissolved solids (TDS) content as does the clarified dairy
washwater. In some areas this water may be suitable for irrigating purposes. In the Santa
Ana watershed, however, this water is undesirable for irrigating purposes, and therefore, in
the absence of other suitable uses, the water has to be exported from the basin via a salt
export facility that is planned to carry highly mineralized wastewaters to the ocean.20* 21
The thermophilic treatment step provides the bulk of the waste stabilization. It is the most
novel part of the overall process and is therefore the primary focus of the current
investigation.
12
-------
SECTION V
EXPERIMENTAL EQUIPMENT AND PROCEDURES
BATCH REACTORS
A reactor was fabricated from a sturdy polyethylene container, 72 cm (28.5 in.) high and
44.5 cm (17.5 in.) inside diameter. Insulation consisted of three layers of fiberglass cloth,
individually attached by resin, followed by a 2.5 cm (1 in.) layer of polyurethane and then
two additional layers of fiberglass. A polyethylene snap-on lid covered the reactor. Holes
were cut into this lid to fit equipment and provide access for feeding and sampling.
A mixer (Lightnin-variable speed, direct drive, continuous duty, 100 to 1725 rpm, 1/4 kW
(1/3 hp), Model No. WS-l-VM with a ten-digit control dial) was supported above the
reactor. A 1.27 cm (1/2 in.) stainless steel shaft passed through the reactor lid and was fitted
with a 8.4 cm (3.3 in.) diameter stainless steel propeller to provide mixing. A bubble cutter
at the surface of the liquid was also fitted to the shaft.
Air was supplied by a Bell and Gossett oilless air compressor [(Model No. SYC 9-1, 0.56 kW
(3/4 hp), 3.1 I/sec (6.57 cfm) displacement, 2.3 I/sec (4.9 cfm) at 345 kN/m2 (50 psi)]. Air
was dispersed through a 30.5 cm by 5.1 cm (12 in. by 2 in.) diameter chromoglass ceramic
air diffuser located on the bottom of the reactor. Airflow from the compressor to the
diffuser was measured by a Brooks Sho-Rate rotameter, calibrated steel ball up to 0.57 I/sec
(1.2 cfm).
A heat exchanger was constructed of 1.27 cm (0.5 in.) aluminum tubing, 4.26 m (14 ft)
long, which was coiled inside the reactor. The heat source consisted of a portable,
two-burner propane stove which heated a 15 1 (4 gal) container of water. A 0.37 kW (1/20
hp) pump circulated heated water from the 15 1 container through the heating coils and
back to the container. Lines to and from the heating coils were sections of ordinary 1.6 cm
(5/8 in.) garden hose. This system was controlled manually. Temperature of the reactor was
measured with a stainless steel dial thermometer with stem that penetrated the reactor lid
and extended into the contents of the reactor. Figure 2 illustrates the batch reactor.
Initially, one reactor as described above and later two such reactors were placed on a 3.7 m
by 4.9 m (12 ft by 16 ft) concrete pad adjacent to a feeding corral. A roof was built over
the pad to provide protection from rain and bright sunlight. A 6 m (20 ft) trailer adjacent to
the corral and concrete pad served as field laboratory and office during the course of
experimentation.
Freshly dropped manure was collected from the feeding corral prior to experimental runs.
This was accomplished with a shovel and plastic buckets. Extreme care was taken to avoid
contamination of fresh manure with dirt, sand or long pieces of straw. During some phases
of the work, manure was comminuted with an Atomic Grinder (used for industrial garbage
disposal). Hot water was added as needed to lubricate the grinding and prevent stoppage.
Manure was poured into the reactor to a level of 19 cm (7.5 in.) from the top of the reactor
for a total of 79.5 1 (21 gal). Maximum total solids content was limited to approximately 12
percent because of inability to adequately mix the contents at higher total solids content.
13
-------
REMOVABLE STOPPER
FOR FILLING AND OBSERVATION
EFFLUENT GAS **
(GAS SAMPLING). H o
VARIABLE
SPEED MIXER
TIGHT-FITTING LID
INSULATED REACTOR
HEATING COIL,
Figure 2. Schematic diagram of batch reactor
14
-------
Batch studies were performed from November, 1972, to May, 1973. In general, the runs
began by filling the reactor with fresh manure, assembling the top to the reactor, turning on
the airflow, mixer and heat exchanger, and monitoring conditions while temperature
increased from ambient to a desired level. It generally required 11/2 to 2 hours to bring the
reactor to temperature. Variables studied over the course of the batch runs included airflow
rate of 0.118 to 0.472 I/sec (0.25 to 1.0 cfm), reaction time of 5 hours to 7 days, mixing
level of 5 to 10 dial position, temperature 45 to 60° C, and solids concentration of 6 to 12
percent total solids.
CONTINUOUS REACTIONS
Following the batch reactions, continuous reactions were initiated beginning in May and
ending in August, 1973. A total of three therm ophilic reactors were set up essentially as
shown for the batch reactor in Figure 2, except that a spigot was added to the side of each
reactor, approximately 30 cm from the bottom, for easy removal of reactor contents.
Two reactors had the Lightnin variable speed mixers. The third reactor had a Model
5K122-1 Dayton continuous duty mixer, 0.37 kW (1/2 hp), 1750 rpm, prior to June 26 and
thereafter a Model 6K375 Dayton Gearmotor mixer, 0.37 kW (1/2 hp), 40 rpm. The
gearmotor mixer was ideal for slow stirring using flat stainless steel blades, 7.6 cm x 23 cm x
0.3 cm (3 in. x 9 in. x 1/8 in.) with a slight twist for lifting action. The heat exchangers for
each reactor were connected in series with the circulating pump and hot water reservoir.
After several incidents of the gas heater blowing out at night it was replaced with a 1650 W,
two-burner electric hot plate. Manual adjustments were made to maintain reactors at desired
temperature.
According to design a specified amount of material was removed from the reactors and fresh
manure added twice daily. During high feed rates especially, the manure was heated in a 191
(5 gal) bucket partially immersed in a heated, 115 1 (30 gal) container of water. This
minimized temperature shocks which would occur with addition of cold feed.
Three mesophilic reactors with contents of 95, 95, and 150 1 (25, 25, and 40 gal)
were fed once per day from the mixed discharge of the thermophilic reactors. There was no
mixing other than that provided by the air supply and occasional manual stirring.
The single compressor used in the batch studies was fitted with a manifold to supply air for
the three thermophilic and three mesophilic reactors. Airflow to each reactor was measured
with a Brooks rotameter with ranges of 0 to 0.57 I/sec (0 to 1.2 cfm) for the thermophilic
reactors and 0 to 7.6 I/sec (0 to 2.0 cfm) for the mesophilic reactors. During some of the
testing oxygen from a cylinder was mixed with the air supply to a particular reactor.
Quantity of oxygen was determined by observing gage changes on the cylinder and by
oxygen measurements after mixing oxygen with a known flow of air.
During the course of the continuous runs, several changes and modifications were made to
the air-diffusing apparatus. A summary of these modifications and changes with discussion is
contained in the Appendix.
ANALYTICAL
Liquid samples for 5-day BOD, COD, total solids, volatile solids and other constituents were
collected in 0.12 1 plastic jars and sent to various laboratories for analyses in accordance
15
-------
with Standard Methods for the Examination of Water and Wastewater. The bulk of the
routine samples was analyzed at Chino Basin Municipal Water District Laboratory. Analyses
for dissolved oxygen and pH were conducted on site with a YSI Model 54 dissolved oxygen
meter and a Beckman Expandomatic Model 76 pH meter. The latter meter was checked
periodically against a Leeds and Northrup Model 7413 pH meter.
Effluent gas samples from the reactors were collected in plastic bags approximately one liter
in size and analyzed at the study site. The openings to the bags were sealed with 4 cm (1.5
in.) diameter corks with a 1.3 cm (1/2 in.) diameter hole bored through each cork. When the
bags were full, another cork was placed over the hole to prevent mixing the contents of the
bag with outside air. Initially samples were collected with the aid of a hand air pump which
withdrew samples at a rate less than the airflow rate entering the reactor. However, it was
soon realized that if the reactor top was sealed tightly enough there was no need for the
hand air pump because the air pressure in the reactor was sufficient to inflate an empty bag.
One sample from each thermophilic reactor was collected at a time, and this was repeated as
often as ten times per day. Samples were allowed to sit for a few minutes to equilibrate to
room temperature while the oxygen probe and meter (Beckman Fieldlab Oxygen Analyses
Model 1008) was adjusted to give a correct reading for a bag of saturated air. The oxygen
probe was inserted through the hole in the cork sealed in the bag opening. The probe
response changed over the course of a day or night, and therefore, it was necessary to
determine the probe's response to changes in oxygen concentration in air. This was
accomplished by diluting a sample of air with carbon dioxide so that the resulting oxygen
concentration was 20 percent versus 21 percent for pure air. If the oxygen probe then
indicated a difference in percent oxygen between the two (for example, 0.8 instead of 1.0),
then subsequent measurements of differences between effluent samples and saturated air
would be increased by the factor 1.0/0.8 and entered in the log book.
In addition to liquid and air-measurements, filtration tests were performed on samples taken
from the reactors. Although some tests were qualitative, most were performed with
quantitative results as the objective. The Eimco Corporation Filter Leaf testing apparatus
was used for these tests.
EXPERIMENTAL DESIGN
The experimental program was initiated by laying out a work schedule for thermophilic
digestion studies in the sequence: construction of facilities, heat balance studies, effect of
temperature, effect of volatile solids concentration, effect of time of digestion, effect of
concentration of oxygen, and effect of feeding and circulation variants. Mesophilic digestion
studies, evaluation of by-products, and engineering development followed the thermophilic
digestion studies in the work schedule, and the schedule showed overlap of the various
elements.
A more detailed testing program was laid out for the continuous runs which began May 7. It
was decided that a factorial type experiment would generate the most information in the
shortest amount of time. Accordingly, after review of batch study results the following
major variables were chosen for study in the thermophilic reactors:
1. feedrate - three levels corresponding to 2 1/2-, 5-, and 10-day hydraulic and solids
residence time;
16
-------
2. air/oxygen - four levels of 0.12, 0.24, 0.35 I/sec (0.25, 0.50, 0.75 cfm) of air, and 0.12
I/sec (0.25 cfm) of oxygen enriched air; and
3. temperature - two levels at 45-50° C and 55-60° C.
Two other important variables, solids concentration and amount of mixing, were held at
relatively constant values. It was reasoned that the higher the solids concentration the
greater the microbial thermogenesis per unit volume, and therefore, the solids concentration
was kept at a reasonable maximum of approximately 10-12 percent total solids. Batch
studies had shown high oxygen uptake rates for high mixing speeds with the propeller type
mixers. Since mixing was going to be an important, major-cost item in the process, it was
felt appropriate to hold mixing speeds to a low level but sufficient to give a good visual
turn-over of the reactor contents. Because of physical constraints and the desire to provide
minimal change in conditions for the microbial cultures, the sequence of runs were not
completely random. Table 2 gives the conditions for the runs. Each run spanned one week
with three runs performed simultaneously because three thermophilic reactors were
available. Prior to starting the runs there was a trial period of three weeks in which the
reactors were brought slowly to temperature. After completion of the runs shown in Table
2, two weeks of additional runs were conducted to verify results.
Three mesophilic reactors, labeled 1A, 2A and 3A, were operated simultaneously with the
thermophilic reactors. The conditions for these remained constant throughout the
experiment. Residence times were 10, 10 and 5 days, respectively, and solids contents were
12, 6 and 6 percent, respectively, for reactors 1 A, 2A and 3A.
Energy balances for the thermophilic reactors were determined from physical measurements
and calculations. During an afternoon when nothing except air was being fed or removed
from the reactors, the supplemental heat supply was turned off and measurements taken to
determine Hf, FL, Hg, Hy, and Hm as defined for Equation 3 (page 8). Hm included only
the direct input from mixing and Hj was zero because no liquid was flowing.
The rate of biological heat production, H^, could then be calculated from Equation 3. In
practice, however, the value of H^ could not be determined with great accuracy and
therefore the total energy input, H^ + Hm, was calculated from Equation 3. An estimate of
H^ itself was obtained from the oxygen utilization data assuming 1.0 kg of oxygen uptake is
equivalent to 14,200 kJ (6,100 Btu) of energy released from biochemical oxidation.
The value of Hr was determined by measuring temperature changes in the reactor contents
over time with an assumption that the specific heat of the contents was 3.9 kJ/kg-° C (0.93
Btu/lb-° F). This is a reasonable estimate based upon the type and concentration of organics
present in the liquid. It was also assumed that the heat capacity of the reactor walls could be
neglected in the heat balance calculations.
With a computer program a straight line was fitted by method of least squares through the
temperature-time points to determine a rate of temperature change. From this
Hr = C x W dT/dt (4)
where Hr = rate of sensible heat increase (or loss if negative), watts
C = specific heat of reactor contents, J/kg-° C
17
-------
Table 2. EXPERIMENTAL DESIGN FOR CONTINUOUS THERMOPHILIC REACTIONS
Week:
Reactor:
Temperature, °C:
Feedrate level:3
Aeration level.
1
1 23
A Z
<4O
1 1 2
234
2
1 23
cri
OU
1 1 2
143
3
1 23
3 1 2
1 3 2
4
1 23
cc
J J
3 1 2
24 1
5
1 23
f.O
ou
3 2 1
34 1
6
1 23
3 2 1
432
7
1 23
/I C
*rj
323
2 1 3
8
1 23
CA
-JU
323
1 24
00
aLevel 1 corresponds to 7.6 liters per day (2 gpd)
Level 2 corresponds to 15.1 liters per day (4 gpd)
Level 3 corresponds to 30.3 liters per day (8 gpd)
"Level 1 corresponds to 0.07 I/sec (0.15 cfm) of air and 0.05 I/sec (0.10 cfm) of oxygen
Level 2 corresponds to 0.12 I/sec (0.25 cfm) of air
Level 3 corresponds to 0.34 I/sec (0.50 cfm) of air
Level 4 corresponds to 0.35 I/sec (0.75 cfm) of air
-------
W = weight of reactor contents, kg
dT/dt = rate of temperature increase (or decrease if negative), ° C/sec
Customary units were utilized in the analyses, and results were later converted to the
modern metric system as presented here. Temperatures were actually measured and
recorded in ° C and converted to ° F only within a computer program set up to perform the
routine heat balance calculations.
Heat losses to the environment through the reactor wall were determined by:
Hs = UxS(Tr-Ts) (5)
where Hg = rate of heat loss to the surroundings, watts
U = overall heat transfer coefficient, J/m^ sec-° C
S = reactor surface area, m
Tr = temperature of reactor, ° C
TS = temperature of surroundings, ° C
In determining the quantity U x S the reactors were filled with 79.5 1 (21 gal) of hot water
and allowed to stand in order to observe temperature loss with time. The water was agitated
periodically to prevent temperature stratification but this provided no significant energy
input. Since all other energy inputs and outputs were zero, HS was equated to Hf and the
quantity U x S determined thereby.
The heat lost with the airstream leaving the reactor consisted of sensible and latent heat.
Airflow rate, ambient temperature, and exit temperature of the air leaving the reactor
provided the necessary physical measurements:
Hg = Qx(Te-Ts)x 1.20 x 1000 (6)
where Hg = rate of sensible heat lost in airstream, watts
Q = flow rate of air standardized to one atmosphere and 21 ° C,
m /sec
Te = exit air temperature leaving the reactor, ° C
TS = temperature of surroundings, ° C
The constants in the formula represent density of air of 1.20 kg/m3 (0.075 Ib/scf) at
standard conditions and specific heat of air of 1000 J/kg-° C (0.24 Btu/lb-° F).
The latent heat of valorization lost from the reactor was:
Hy = Wx 2.35 x 106x 0.744 (7)
where Hy = rate of latent heat lost in water vapor, watts
W = flow rate of water vapor, m^/sec, corrected to standard
conditions and calculated from saturation vapor pressure (a
function of exit air temperature) and flowrate of air.
The constants in formula (7) represent latent heat of vaporization for water of 2.35 x 10°
J/kg (1010 Btu/lb) and density of water vapor of 0.744 kg/m3 (0.0464 Ib/scf) corrected to
standard conditions. The basic formula used in the computer calculations to determine the
vapor pressure of water was:
19
-------
log (Vp) - 0.00648 x T - 3550.44/T - 6.1213 x In (T) + 46.227 (8)
where Vp = vapor pressure of water, mm Hg
T = absolute temperature, ° K
log = logarithm to the base 10
In = natural base logarithm
The energy input from mixing was more difficult to measure. Conceptually it could be
determined as follows:
Hm = P x E x V x A (9)
where Hm = rate of energy input from mixing, watts
P = power factor, expressed as a fraction
E = efficiency, expressed as a fraction
V = voltage, volts
A = amperage, amperes
The limitations of this approach arise from the difficulty in estimating the power factor and
efficiency. An alternate approach of measuring torque and angular speed of the mixer shaft
was rejected because of equipment costs and difficulty in obtaining true torque readings
even with the appropriate equipment. For the type of motors used with the mixers the
maximum value of P x E is approximately 0.3. This then set a maximum energy input from
mixing when the amperage and voltage were measured for each mixer. The two Lightnin
mixers showed reduced amperages at lower speeds, and therefore, it was possible to better
estimate the net energy input to the reactors from mixing. The third mixer remained at a
fixed amperage under different loads, and therefore, it was more difficult to estimate net
energy input. A measure of the wattage indirectly would give a measure of V x A x P but
would still require an estimate of E, and therefore, a wattmeter would contribute towards
obtaining H but could not eliminate the need for making a judgment on an unknown
factor in the above equation.
To summarize the procedure for obtaining an energy balance, the quantities Hp Hj, Hy, Hg,
and HS of Equation 3 were determined without complication. Therefore, the sum of the
quantities Hm and H^ was calculated from Equation 3. A maximum value and an
approximation of Hm was obtained through Equation 9, and this allowed an estimate of H^,
the energy generation from biochemical oxidation. An independent estimate of H^ was
obtained from oxygen utilization data assuming that oxygen utilization is related directly
with energy generation. Therefore, energy balance results were limited by the precision with
which specific components of Equation 3 could be determined and by the accuracy of the
assumption that one kilogram of oxygen utilized results in 14,200 kJ of energy released
during biochemical oxidation.
20
-------
SECTION VI
EXPERIMENTAL FINDINGS AND DISCUSSION
MANURE CHARACTERIZATION
Fresh dairy cow manure is variable in character as evidenced by results of analyses presented
in Table 3. The bulk of the analyses were performed during the period of May through
August, 1973. Averages and variations are shown for various parameters. A small amount of
water was added to the manure collected from the feeding pens in order to aid in mixing the
entire batch before taking a sample. Therefore, the values for percent total solids (TS) and
quantities given in grams per liter are slightly lower than they would be for undiluted
manure without urine mixed in.
The variation shown in volatile solids, 5-day BOD, COD, and other parameters, is due, in
large part, to the variation resulting from sampling and analytical procedures as evidenced
by results of replicate samples. However, much of the variation is also estimated to come
from the inherent variability of manure. This variability is expected to be due to the cow
feed and to environmental factors which affect the metabolism of cows.
Table 4 gives a compilation of dairy cow manure characteristics as reported by other
investigators and substantiates that there is considerable inherent variation in the
characteristics of manure. This variation cannot be disregarded in design of manure
processing facilities.
During this study the feed to the cows varied in composition from time to time. No attempt
was made to control the normal dairy operation. The individual cow ration was
approximately 23 kg (50 Ib) per day on a dry weight basis. This ration consisted of
approximately 11.5 kg (25 Ib) of alfalfa hay and 11.5 kg (25 Ib) of grain. However, tomato
pulp, brewers grain and orange hulls were also fed when available as a partial substitute for
hay and grain, always with the objective of a total ration of approximately 23 kg per cow
per day. The typical dairy cow ration is presented in Table 5.
Table 6 shows analyses of a 38 1 (10 gal) sample of manure that had been aerated for a
seven-week period with no additions except water to maintain a solids concentration of
approximately 6 percent. Tables 3, 4, and 6 indicate some characteristics of dairy cow
manure that relate to biological treatment of the manure. The low ratio of 5-day BOD to
COD, in the range of 0.1 to 0.3, indicates the resistance of the organic fraction of manure to
biological degradation. This resistance is attributed to lignins and cellulosic material. The
simple, extended aeration of a sample for 7 weeks indicated a substantial reduction in 5-day
BOD, some reduction in soluble COD, but no reduction in total COD. Thus giving further
evidence to the resistance of the bulk of the waste to biological treatment. In fact, the
results indicated an increase in total COD which is difficult to explain other than to state
that it may be an error. This result was not duplicated, but a possible explanation may lie
within the COD test itself. Some of the organics may not be thoroughly oxidized when the
COD test is performed on raw manure. After aeration for an extended period of time, these
organics may be partially broken down physically and biochemically and be more
susceptible to the strong oxidizing agent in the COD test. This possibility places an added
burden upon the interpretation of COD results. An appropriate value of BVS to use in the
mathematical model presented in Section III is not readily apparent from waste
21
-------
Table 3. EXPERIMENTAL CHARACTERIZATION OF FRESH DAIRY COW MANURE
Component
Total solids, percent of manure
Volatile solids, percent of solids
Total COD, gms/1
Soluble COD, gms/1
Total BOD5, gms/1
Soluble BOD5, gms/1
pH
Total COD, gm/gm TS
Total COD, gm/gm VS
Soluble COD, gm/gm TS
Total BOD5, gm/gm TS
Total BOD5, gm/gm VS
Soluble BOD5, gm/gm TS
Total BOD5/Total COD, gm/gm
Soluble BOD5/Soluble COD, gm/gm
Total nitrogen (N) percent TS
Soluble phosphorous (P), percent TS
Total potassium (K), percent TS
Heating value, kJ/kg
Average
15.4
86.1
149
33
16.1
9.3
6.2
0.96
1.10
0.22
0.105
0.123
0.062
0.108
0.270
2.8
0.25
19,000
Range
12.9- 19.8
76.7-91.8
81 -284
19-53
8.6-21.5
4.6- 14.4
5.2-6.8
0.56- 1.48
0.66- 1.69
0.11 -0.33
0.06-0.13
0.06-0.16
0.03-0.10
0.04-0.15
0.16-0.41
2.6-2.9
0.17-0.32
0.5 -5
Standard
deviation
2.17
3.0
57
10
3.5
3.0
0.5
0.30
0.34
0.07
0.025
0.030
0.023
0.038
0.075
Coefficient
of variation
0. 14
0.035
0.38
0.30
0.22
0.32
0.08
0.31
0.31
0.33
0.24
0.25
0.36
0.35
0.27
Number
of samples
21
21
17
16
12
11
12
17
17
16
12
12
11
10
9
2
4
7
1
to
-------
Table 4. REPORTED DAIRY MANURE CHARACTERISTICS3
Total solids, percent of manure
Volatile solids, percent of solids
Total COD, gm/gm TS
Total BOD5, gm/gm TS
Total BOD5, gm/gm VS
Total BOD5/Total COD, gm/gm
Nitrogen (N), percent TS
Phosphorous (P), percent TS
Potassium (K), percent TS
Range
10- 16
72-85
0.8-2.1
0.06-0.18
0.13-0.39
0.08-0.23
2.8-5.5
0.4-0.5
0.3-3.0
Typical
13
80
1.0
0.18
0.23
0.17
4
0.5
1.7
aSee references 15, and 22-32
characterization measurements. As indicated in Section III, the complicated biochemical
processes probably cannot be adequately approximated by a single reaction, but requires
two to make a reasonable model. The BVS for an overall fast reaction acting upon readily
biodegradable material appears to be in the vicinity of 15 percent of the volatile solids. The
BVS for the second, slow reaction could be an additional 75 percent of the volatile solids.
The combination thereby would indicate a total BVS of 90 percent of the volatile solids
which corresponds to the volatile solids reduction noted during a long period of composting.
However, this latter estimate of 75 percent cannot be deduced from the data presented
above and may be substantially in error in a liquid system such as that studied herein, as
opposed to a moist but solid-phase, conventional composting operation. Even with
substantially more BVS to work upon, the slower reaction would not contribute
significantly to biological heat generation because the rate is very much lower than that of
the faster reaction. Therefore, for practical purposes in terms of oxygen utilization and
biological heat generation it appears that the slower reaction can be neglected.
23
-------
Table 5. TYPICAL DAIRY COW RATION
Alfalfa - 11.5 kg (25 Ib) per cow per day
Moisture, percent 9
Crude protein, percent 17
Digestible protein, percent 14
Modified crude protein, percent 25
Crude fiber, percent 25
Total digestible nutrients 52
Molybdenum, ppm 3.8
Copper, ppm 11.0
Feed concentratea - 11.5 kg (25 Ib) per cow per day
Crude protein, percent > 13.5
Crude protein NPN, percent < 4.0
Crude fat, percent > 3.0
Crude fiber, percent < 7.0
Ash, percent < 7.0
Added minerals, percent < 2.0
alngredients include orange pulp, almond hulls, hominy pellets,
cane molasses, urea, sulphur, rolled milo, brewers dried grain,
dairy concentrate, and salt.
An important consequence of the relatively low BVS of dairy manure is that it precludes
achieving a high BVS concentration in the reactor. For example, in order to achieve a BVS
concentration of 3 percent a total solids concentration on the order of 20 percent would be
required. Such a mixture is hardly liquid. In order to maintain a liquid system which can be
mixed without difficulty, the maximum BVS concentration will be limited to
approximately 2 percent of the total liquid. Following the rationale of Andrews and
Kambhul^,!/^ ^ js evident that there would be a definite upper limit to the rate of heat
generation and, therefore, an upper limit of operating temperature to the low thermophilic
range.
BATCH REACTIONS
Data collected from batch reactions invited a challenge for interpretation in terms of
response of dependent variables such as oxygen uptake rate to independent variables such as
temperature, mixing, and airflow rate. Part of the perplexity arose from variations in
percentage oxygen depletion indicated by the oxygen meter for the gas samples. Over
relatively short time periods (on the order of an hour) large changes were often noted in the
24
-------
Table 6. RESULTS OF LONG TERM AERATION OF MANURE3
Volatile solids,
percent of solids
Total COD,
gm/gm TS
Soluble COD,
gm/gm TS
Total 5-day BOD,
gm/gm TS
Soluble 5-day BOD,
gm/gm TS
Average
manure
86.1(21)
0.96(17)
0.22(16)
0.105(12)
0.062(11)
Measured at
start of aeration
86.1(2)
0.77(l)b
0.32(1)
-
-
Measured after
7-weeks aeration
84.9(3)
1.26(2)
0.20(2)
0.03(2)
0.014(2)
aNumbers in parenthesis following the entries in the table show the number
of samples that were averaged to obtain the entry.
"Questionable result.
percent oxygen depleted in the gas samples, and a major enigma throughout the
experimental work was whether such changes reflected true changes or were simply
variations in the response of the oxygen meter. The oxygen probes were replaced
periodically, the instrument was tested by the manufacturer to insure that it worked
properly, and the instrument was checked constantly against a known concentration of
oxygen. In view of these precautions and other precautions to insure reliable samples it was
concluded that at least a substantial portion of the variation was true variation within the
reactor system. Because of large variations the oxygen uptake values shown in Table 7 must
be viewed as crude averages. These oxygen uptake rates were obtained by inspecting a large
number of points and choosing what appeared to be reasonable averages. It did not appear
that a more precise method of obtaining oxygen uptake rates from the oxygen data would
contribute to evaluation of the process.
An explanation for some of the variation may relate to changes in the air distribution
pattern within the reactor. Accumulations of solids on the aerator were noted after every
run. Mixing action was an important factor in achieving higher oxygen uptake rates and it is
reasonable, therefore, that the direction of the airstream relative to the mixing propeller had
a large effect upon getting oxygen into solution which is an important prerequisite to
oxygen utilization.
25
-------
Table 7. OXYGEN UPTAKE FOR BATCH REACTIONS
Date
15 November
21 November
22 November
6 December
12 December
13 December
19 December
20 December
26 December
27 December
2 January
3 January
4- 5 January
10-12 January
16-18 January
18-19 January
23-26 January
Length of
reaction, hrs
6.7
5.8
7
5.7
6.5
6.8
6
7.3
7.3
8.5
7.7
5.2
48
56
47
32
72
Temperature
range3
H
H
H
H
H
H
H
L
L/H
L/H
L
H
L
H/L
H/L
LL
H/L
Total solids,
percent
_b
_b
.b
_b
12
15
10
11
14
15
15
14
15
15
15
—
—
Mixing
speed0
5
5
5
5
5
10
10
10
10
10
10
10
10
10/5
10/5
10
10
Airflow,
I/sec
0.12
0.24
0.24
0.12
0.24
0.24
0.12
0.24
0.24
0.24/0.47
0.47
0.47
0.47/0.24
0.24/0.12
0.24
0.12
0.24
Oxygen
uptake,
ml/sec
2
1
1
2
3
3
1
3
2/3
2/4
4
6
6/4
6/0.5
5/0.5
0.5
4/3
Comment on
feed to reactor
comminuted
comminuted
comminuted and compost
added
comminuted
comminuted
compost added
comminuted and N-source
added
as collected
1/3 preceding product
as collected
1 /3 preceding product
1/3 preceding product
1/3 preceding product
all preceding product
2/3 preceding product
comminuted
all preceding product
to
ON
aTemperature range: H = above 55° C but generally less than 60° C; L = between 45° and 55° C; LL = below 45° C;
HH = between 60° and 70° C.
^Samples not taken but values subsequently estimated to be less than 10 percent total solids.
cMixing speed can be set from 1 to 10 on Lightnin mixer.
"Air enriched with 02 to approximately 32 percent 02-
eAir enriched with 02 to approximately 42 percent 02-
-------
Table 7. (Continued)
Date
30-31 January
7- 10 February
13-16 February
20-24 February
27-14 March
15- 16 March
20-23 March
10-11 April
10-11 April
12-13 April
12-13 April
1 8-20 April
18-20 April
25 April
25 April
Length of
reaction, hrs
24
72
80
96
360
31
72
24
26
30
24
55
55
32
32
Temperature
range3
H
H
HH/H
H
H
H
H/L
HH
HH
HH
HH
HH
HH
HH
HH
Total solids,
percent
—
—
—
13
13
14
14
12
13
16
8
13
13
12
12
Mixing
speed0
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
Airflow,
I/sec
0.24
0.24
0.24
0.24
0.24/0.12
0.12
0.12
0.12
0.24
0.24
0.24
0.12
0.1 2d
0.12
0.1 2e
Oxygen
uptake,
ml/sec
2
4
4/2
2/4
2/1
2
1/0.5
2
4
4
4-8
5
7
6
12
Comment on
feed to reactor
2/3 preceding product
comminuted and 1/3 preceding
product
comminuted and 1 /3 preceding
product each a.m.
comminuted and 1 /3 preceding
product each a.m. on 21 and
22 February
1 /3 preceding product
all preceding product
comminuted and 3.8 1 preceding
product
comminuted
comminuted
as collected
as collected and diluted
as collected
as collected
as collected
as collected
to
aTemperature range: H = above 55° C but generally less than 60° C; L = between 45° and 55° C; LL = below 45° C;
HH = between 60° and 70° C.
"Samples not taken but values subsequently estimated to be less than 10 percent total solids.
cMixing speed can be set from 1 to 10 on Lightnin mixer.
^Air enriched with 02 to approximately 32 percent 02-
eAir enriched with 02 to approximately 42 percent 02-
-------
The average values do not indicate trends observed during the course of the reactions.
Generally, there was greatest oxygen uptake immediately after the reactions were brought
up to temperature. For the longer runs, therefore, the oxygen uptake rates tended to
decrease as the reactions proceeded.
Reactions were of too short duration in general to observe any significant change in COD or
BVS. However, some physical changes were apparent in the manure after the reactions. The
product was more fluid which could be attributed to physical break-down of fibrous
materials as a result of vigorous mixing. For a certain period of time the product also
possessed an innocuous quality as evidenced by lack of odors and lack of attraction for flies.
In fact, the nature of the product suggested that it had been sterilized which lead to
questions about the nature of the reactions, such questions as whether the oxygen uptake
could be due to chemical reactions rather than biochemical reactions. In support of the
predominance of chemical reactions are the following considerations:
1. Pasteurization achieves its effect by raising temperatures rapidly to a level of
approximately 62° C (143° F) and maintaining this temperature for 30 minutes. In the
above batch reactions temperatures were raised at the rate of 15° to 30° C per hour to
reaction temperature and held generally at that temperature for the duration of the run.
Such a procedure would be expected to kill all but the hardiest thermophilic and
thermoduric bacteria.
2. The initial rapid oxygen utilization rate observed when reaction temperatures were
reached belies an expected utilization rate which would occur if there were initially
only a small number of aerobic thermophiles which needed to multiply before a
relatively high utilization rate was attained.
3. Wet chemical oxidation has been observed at temperatures as low as those in the above
batch reactions.33 It appears plausible, therefore, that large energy input through the
propeller blades could create sufficiently high temperatures and pressures adjacent to
the propeller to allow wet chemical oxidation.
4. If bacterial growth were responsible for the oxygen uptake, it would be reasonable to
assume that there would be plenty of bacteria present in the product to induce flies and
cause other nuisances in view of the fact that plenty of BVS remained in the product.
The lack of fly attraction and odors suggests that there was not a large biological
population present. When the material was left to stand for a considerable length of
time, fly attraction and odors began to occur, suggesting that a bacterial population was
being reestablished.
5. For many of the reactions, product from the previous reaction was added at the start.
Also, for two reactions (22 Nov. and 13 Dec.), dry, composted manure was added, and
in another instance (19 Dec.), urea and calcium nitrate were added to the feed of the
reactions. If the oxygen uptake resulted primarily from biochemical activity, then one
would expect substantial differences in oxygen uptake rates as a result of these varying
conditions. Substantial differences could not be identified from the data collected, and
therefore, doubt is cast upon the presence of substantial biochemical activity. However,
it is questionable whether any of these additions to the reactor could influence
biological activity within the short periods of the tests.
28
-------
The data presented in Table 7 provides some indication of apparent reaction rates. For
example, a first order reaction rate constant can be calculated based upon the data with
certain assumptions presented earlier in Section IV. An oxygen uptake rate of 5 ml/sec and
a reactor containing readily degradable material equivalent to approximately 1.8 kg of
oxygen would indicate a first order reaction rate constant of 0.3 day. However, the range
of uptake rates indicate a wide range for the rate constant between 0.06 to 0.7 day. The
lower value is well within the range of biological activity, but the higher value appears to be
out of the range of biological activity and defies an easy explanation unless it be in terms of
chemical oxidation or possibly oxygen absorption which only partially oxidizes the organic
material and does not significantly reduce the weight of volatile solids. This latter suggestion
may help explain why no significant volatile solids reductions were observed even after
several days of apparently high oxidation rates.
CONTINUOUS REACTIONS
Each thermophilic reactor was filled with hot water and allowed to cool for a several-hour
period in order to determine the heat loss through the reactor walls for a differential
temperature between the reactor and the surroundings. The results of these experiments
showed that heat loss amounted to 12.0, 12.6 and 13.0 kJ/hr-° C (6.32, 6.65 and 6.85
Btu/hr-° F) for reactors 1, 2 and 3 respectively. These values and various temperatures were
applied to the energy balance equations discussed in Section V, and thereby an estimate of
the energy or heat (these are used interchangeably here) generated during thermophilic
reactions was obtained. The total energy generation includes biological heat production and
mixing energy input to the reactors and this total is shown in Table 8 for each of the
thermophilic reactions. During the latter weeks of experimentation more than one
determination was made and these are shown in Table 8. Because of the difficulty in
estimating mixing energy input, it was not possible to separate the total energy generation
into biological and mixing components, although considerable effort was expended in
attempting to do so. However, another approach based on heat released per unit of oxygen
utilized, as discussed in Sections IV and V, gave an independent estimate of the biological
component of the total energy generation. During the period that energy balances were
being determined, oxygen utilization measurements were also taken, and the resulting
biological energy generation values estimated from these oxygen measurements are also
shown in Table 8. In three specific cases the biological energy generation exceeded the total
energy generation, giving unreasonable results. In these cases, the biological energy
generation was assumed equal to the total energy generation in subsequent analyses. An
explanation for two of the three values could be simply that because of inherent error of
measurement some overlap is expected, in that biological energy generation exceeds the
total very slightly. In the third situation, a reaction in which pure oxygen was involved, an
explanation is more elusive. Conceivable explanations include:
1. Sampling error for that reactor during oxygen measurements - perhaps there was
substantial mixing with ambient air of gas leaving the reactor;
2. Error in analysis with the oxygen meter although all other analyses appear reasonable;
3. Peculiar absorption of oxygen in the reactor which did not oxidize material to produce
anything near 14,200 kJ/kg (6,100 Btu/lb) of oxygen absorbed.
29
-------
Table 8. ENERGY GENERATION IN CONTINUOUS REACTIONS
Week
1
2
3
4
5
6
7
8
9
10
Reactor
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
Reaction
number
14
15
20
13
16
19
9
3
6
10
4
5
11
8
1
12
7
2
22
17
23
21
18
24
—
—
Total energy
generation,
kJ/hr
170
160
370
400
250
320
310
210
250
150
110
110
180
150
90
220/300
270/440
260/130
320/260
740/450
60/60
610
250
220
420/520/470
370/370/330
630/610/520
590/560/510
420/390/300
410/380/390
Biological energy
generation,
kJ/hr
70
60
190
290
120
130
(840)
130
130
80
60
90
130
120
50
110/140
90/160
40/40
110/130
590/220
50/(70)
360
110
70
270/340/270
270/230/210
560/370/370
270/260/240
150/260/(350)
60/ 80/120
30
-------
This latter explanation would be the most disturbing because it affects a basic assumption in
the analysis. Although this isolated result does challenge the basic assumption of
approximately 14,200 kJ/kg of oxygen, the analyses proceeded utilizing this assumption.
An inspection of Table 8 indicates the dependence of biological activity upon the mixing
energy input. Higher rates of biological activity are associated with larger mixing energy
input. Aerobic biological activity depends upon receiving adequate quantities of oxygen,
and the results confirm that mixing is important in the present system for producing
substrate-organism-oxygen contact. Throughout the series of experiments, especially the
first 8 weeks, the mixing input was minimized to provide no more than the amount of
mixing needed to keep the reactor contents well mixed. This was a subjective evaluation
performed for each reaction. It was apparent prior to the start of the continuous
experiments that mixing would be a substantial portion of the overall cost of operation in a
large scale system, so a conscious effort was made to minimize the amount of mixing.
During weeks 9 and 10, following the 8-week, factorial experiment, an increase in mixing
energy was allowed and the results show an increase in biological activity.
In order to examine the effects of aeration, feedrate, and temperature upon biological
activity, the individual runs need to be separated according to the patterns presented in
Table 9. For example, to compare the effects of temperature upon any dependent variable
such as biological heat generation, reactions 1 through 12 are compared with reactions 13
through 24. All the basic 24 reactions conducted during the 8-week period are used to
examine the effects of one independent variable upon any dependent variable such as
biological activity. Table 10 summarizes the analyses for effects of aeration, feedrate and
temperature upon biological energy generation. Table 10 shows that aeration level Aj (0.05
I/sec of pure oxygen and 0.07 I/sec of air) produces far greater energy generation than does
any other aeration level. (The question of statistical significance is addressed later in this
Section.) This result suggests that organism activity is limited by the oxygen that can be
transferred to the solution. The lack of substantial effects among fy, A^ and A4 suggests
that the aeration system employed herein was not effective in transferring proportionately
increased quantities of oxygen from air at airflow rates above the 0.12 I/sec level.
Feedrate produced a smaller effect upon biological activity, but the data do show an
increase in activity between Fj, the lowest feedrate, and ¥^, the intermediate feedrate.
There is essentially no difference between ^2 anc' ^3* the highest feedrate. This result might
indicate that available substrate begins to control biological activity somewhere between 5
and 10 days residence time (hydraulic and solids residence time were identical during the
experiment).
The effect of temperature is indicated by the difference in energy generation between Tj
and 1*2- Apparently a 10° C rise from 45-50° C to 55°-60° C produces approximately a
40-percent increase in energy generation. However, because of the oxygen-limited nature of
many of the reactions, this value probably does not reflect the true increase in biological
activity that would occur with unlimited oxygen.
Following the factional group of experiments, the thermophilic reactions were continued an
additional two weeks. The six reactions were too few to establish definitive conclusions, but
Table 11 shows the conditions of the reactions together with the apparent biological
activity. Feedrate was 11.4 I/day (3 gpd) or 7 days residence time for each reactor except
for reactor 3 which received no feed during the last week. The effect of no feed was reduced
31
-------
Table 9. REACTIONS INVOLVED AT LEVELS OF INDEPENDENT VARIABLES
Independent variable
Reaction numbers involved at level of variable
Aeration level
AI (0.121/sec)a
A2 (0.12 I/sec)
A3 (0.24 I/sec)
A4 (0.35 I/sec)
Feedrateb
F! (7.6 I/day)
F2 (15.1 I/day)
F3 (30.3 I/day)
Temperature
T! (45-500 c)
T2 (55-60° C)
1,5, 9,13,17,21
2,6, 10, 14, 18,22
3,7, 11, 15, 19,23
4,8, 12, 16,20,24
1, 2, 3, 4, 13, 14, 15, 16
5, 6, 7, 8, 17, 18, 19,20
9, 10, 11, 12,21, 11,23,24
1 through 12
13 through 24
a0.07 I/sec of air and 0.05 I/sec of oxygea
bFeedrates Fj, F2, and F3 correspond to 10.5, 5.25 and 2.63 days
retention time, respectively.
biological activity which remained fairly constant for the entire week, and a 4-percent
reduction in the volatile solids fraction was recorded at the end of the week.
Lower solids concentration produced little net effect upon biological activity. This may
indicate a combination of greater solubility of substrate and increased oxygen transfer
efficiencies at lower total solids concentration. Another possibility is that there is some
inherent limitation to the rate at which the organisms can utilize the substrate. It is apparent
that interpretation of results is hampered by the complexity of the system. There are
intimate relationships among organisms, substrate and oxygen. The substrate may be bound
with the manure solids and become available upon comminution of the solids by the mixer.
It is also possible that the propeller mixer tends to destroy bacterial cells at higher mixing
rates. Then too, the relative location and condition of the mixer and aerator could
considerably influence reaction rates.
32
-------
Table 10. EFFECT OF VARIABLES ON BIOLOGICAL ENERGY GENERATION
Variable
Energy generation,
kJ/hr
Aeration level:
A!
A2
A4
Feedrate:
F2
Temperature:
Average for all reactions
251
92
106
115
103
162
157
116
166
141
Considerable effort went towards modifying the aeration equipment and the basic mixing
agitator to increase the biological activity of the thermophilic reactions. Several shapes of
aerators, fabricated from plastic tubing, and several agitators, including some large,
slow-turning types, were experimented with. Any substantial improvement over the basic
diffuser and propeller could not be detected. Some details of these experiments are included
in the Appendix.
Table 12 presents some of the stabilization measures determined during the end of each
reaction just before conditions were changed for a subsequent reaction.
The most significant observation for all of the reactions is that thermophilic treatment
produced very minimal stabilization. The effect of variables upon stabilization, as measured
by the ratios of soluble COD and soluble 5-day BOD to the total solids content, is shown in
Table 13.
33
-------
Table 11. REACTION CONDITIONS DURING WEEKS 9 AND 10
1
Wee^l
9
1
10
Reactor
1
2
3
1
2
3
TS concentration,
percent
10
7
8
10
7
7
Aeration
level
A!
A2
A3
A2
A!
A3
Feedrate,
I/day
11.4
11.4
11.4
11.4
11.4
0.
Temperature
°C
59-61
56-61
57-61
58-63
57-64
44-47
Biological
energy3
kJ/hr
295
235
430
255
255
85
aAverages of values shown in Table 8.
The differences indicated for the various aeration levels were not significantly different. The
differences shown for feedrates were significantly different and provide some insight to
what is occurring in the reactions. The low feedrate showed higher soluble BOD and COD
values in the product than did higher feedrates. This surprising result indicates that the
longer residence time allowed BOD and COD in the reactor to become soluble faster than it
could be biologically degraded. A higher temperature increased the amounts that became
soluble. In fact, there was more soluble COD and BOD in the product on the average than
there was in the raw manure. Also, there was more total COD indicated in the product than
in raw material. This could be explained in terms of limitations of the COD test which
allowed more complete chemical oxidation of organic material after it passed through the
thermophilic reactors.
A summary of an analysis of variance is shown in Table 14. The F test was employed to
determine whether differences measured as effects were significant or could be attributed to
statistical effor. For the biological energy generation analysis, the effects due to aeration,
feedrate, and temperature were significant at approximately the 99-, 85-, and 90-percent
levels, respectively. This means that the observed differences would occur by chance alone
no more than 1,15 and 10-percent of the time, respectively. The differences in stabilization
as affected by aeration levels could be attributed to statistical error. For soluble-COD
differences the effects due to feedrate and temperature were significant at approximately
the 85- and 99.5-percent levels respectively. For soluble-BOD differences the effects due to
feedrate could be attributed to statistical error, but the effects due to temperature were
significant at approximately the 97.5-percent level.
34
-------
Other measurements and observations should be noted regarding the thermophilic reactions.
The pH was higher in the reactors than it was in the raw manure. Values generally ranged
above pH 7.0, and in no case was the pH below 6.6, so pH of the reactions was not
considered a limitation to the biological activity. There was approximately a one quarter
reduction in the total nitrogen values from the raw manure. There were isolated incidents
when an ammonia smell could be detected from the reactors. There was a short period of
time of two or three days at the start of the experimentation when the three thermophilic
reactors began to smell somewhat sour. An increase in mixing energy and a continual
clean-up of the aerators throughout the experimentation prevented this from occurring
again. Odor from the reactors generally smelled of fresh cow manure and was not
objectionable to the operators. However, members of the dairy family at the study site did
comment that they found the odors objectionable, which probably meant that the odors
were somewhat different from the characteristic dairy odor. Foaming in reactors at times
posed a problem which was solved by the use of bubble cutters fastened to the mixer shaft.
At other times there was very little foam. The variation could not be identified with any
operating procedure or parameter and remained an enigma. The bacterial population in the
thermophilic reactors was examined under a microscope and found to be very undiversified
when compared with the bacterial population in the raw manure. The thermophilic bacteria
appeared to be practically all small, gram-positive cocci. The raw manure included many
rods and many gram-negative and gram-positive cocci forms.
35
-------
Table 12. CHARACTERISTICS OF CONTINUOUS REACTION PRODUCTS2
Week
1
2
3
4
5
Reactor
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
Reaction
number
14
15
20
13
16
19
9
3
6
10
4
5
11
8
1
VS/TS
0.826
0.865
0.857
0.838
0.798
0.864
0.845
0.850
0.843
0.867
0.865
0.882
0.886
0.867
0.871
Sol-COD/TS
0.65
0.67
0.68
0.35
0.52
0.39
0.31
0.34
0.43
0.21
0.26
0.29
0.23
0.23
0.20
Sol-BOD/TS
0.26
0.17
0.19
0.08
0.14
0.09
0.04
0.05
0.10
0.04
0.04
0.07
0.07
0.06
0.05
Total COD/TS
1.52
1.57
1.48
1.19
1.62
1.11
0.84
1.08
0.85
0.89
0.97
0.98
0.91
1.54
—
Total-BOD/TS
0.28
0.20
0.09
0.16
0.11
0.06
0.06
0.10
0.09
0.05
0.12
0.12
0.10
0.11
ON
aValues are presented as a ratio without units, e.g., grams/gram.
-------
Table 12. (Continued)
Week
6
7
8
9
10
Reactor
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
Reaction
number
12
7
2
22
17
23
21
18
24
—
—
Average weeks 1-10
Raw manure average
VS/TS
0.855
0.856
0.862
0.882
0.941
0.877
0.854
0.878
0.852
0.856
0.875
0.873
0.883
0.893
0.835
0.863
0.861
Sol-COD/TS
0.15
0.23
0.34
0.28
0.36
0.37
0.37
0.32
0.36
0.15
0.27
0.15
0.31
0.27
0.21
0.33
0.22
Sol-BOD/TS
0.07
0.08
0.07
0.11
0.07
0.11
0.07
0.01
0.06
0.01
0.02
0.02
0.01
0.02
0.01
0.07
0.06
Total COD/TS
1.41
1.74
1.00
1.08
1.19
0.91
0.90
1.09
1.30
0.54
0.94
0.92
1.08
1.11
0.76
1.12
0.96
Total-BOD/TS
0.12
0.15
0.12
0.14
0.15
0.12
0.11
0.02
0.11
0.01
0.02
0.02
0.02
0.02
0.01
0.10
0.105
aValues are presented as a ratio without units, e.g., grams/gram.
-------
Table 13. EFFECT OF VARIABLES ON STABILIZATION OF MANURE
Variable
Aeration level:
A!
A2
A3
A4
Feedrate:
F!
F2
F3
Temperature:
Tl
T2
Average of all reactions during
weeks 1 through 8
Raw manure as feda
Average
Standard deviation
Soluble-COD/TS
0.31
0.37
0.37
0.37
0.42
0.37
0.29
0.27
0.44
0.356
0.22
0.07
Soluble-BOD/TS
0.063
0.098
0.095
0.093
0.108
0.084
0.071
0.062
0.113
0.0875
0.062
0.023
aSee Table 3.
38
-------
Table 14. SUMMARY OF ANALYSIS OF VARIANCE
I. Biological Energy Generation:
Source of
variation
Aeration
Feedrate
Temperature
Error
Total
Sum of
squares
99,500
16,900
14,200
82,900
213,500
Degrees of
freedom
3
2
1
17
23
Mean
square
33,200
8,500
14,200
4,880
F-ratio
testa
6.8
1.74
2.91
F-ratio
testb
7.8
2.00
3.36
II. Stabilization of Manure:
Source of
variation
Aeration
Feedrate
Tempera-
ture
Error
Total
Sum of squares
Sol-COD/
TS
0.0146
0.0702
0.1837
0.2145
0.4830
Sol-BOD/
TS
0.0048
0.0054
0.0160
0.0438
0.0700
Degrees
of
freedom
3
2
1
17
23
Mean square
Sol-COD/
TS
0.00487
0.0351
0.1837
0.0126
Sol-BOD/
TS
0.0016
0.0027
0.0160
0.0026
F-ratio test
Sol-COD/
TS
0.39
2.78
14.6
Sol-BOD/
TS
0.62
1.04
6.15
aF-ratio test based on error mean square of 4,880.
bp-ratio test based on error mean square of 4,230 derived from replicate
analyses during weeks 6, 7, 9 and 10 as shown in Table 8.
39
-------
MESOPHILIC REACTIONS
Mesophilic reaction conditions remained unchanged throughout the 10 weeks of
experimentation. Product from the thermophilic reactors was fed to the mesophilic reactors.
The only mixing was that produced by the aerators and that done manually to break up any
solids accumulation on a once or twice a day basis. Table 15 gives conditions and some
results of the reactions. Airflow rate was adjusted to 0.24-0.35 I/sec for each reactor.
However, it became necessary at times to reduce this flow because of foaming problems.
The results again show minimal, overall stabilization of the manure. However, significantly
reduced values of total-BOD, soluble-COD and soluble-BOD were recorded. This would be
expected because there was very little mechanical action occurring in the reactors to help
dissolve additional organic material.
FILTRATION EXPERIMENTS
Experiments with the filter leaf testing apparatus were conducted on products from both
the thermophilic and mesophilic reactors. Among 56 recorded tests no substantial
differences were noted between the filterability of the two products. The addition of lime
or alum alone as a filtering aid did not produce as high a filtration rate as did the addition of
lime and alum together or the addition of lime, alum and ferric chloride together. With a
formation time of one minute, a drying time of two minutes, Eimco filter cloth number
PO801HF, and heavy chemical dosages (for example, 15 percent lime, 1 percent alum, and 3
percent ferric chloride on a dry solids basis), it was possible to achieve a filtration rate of
approximately 20 kg/m -hr. The filtering rate was not maximized as a function of filter aid
chemicals, nor was it optimized in terms of costs. The effort to accomplish this was not
justified in view of the treatment and process limitations uncovered in thermophilic and
mesophilic treatment steps.
40
-------
Table 15. CONDITIONS AND PRODUCTS OF MESOPHILIC REACTIONS
Reactor:
Hydraulic and solids residence time, days
Average TS in reactor, percent
Volume of reactor contents, liters
Characteristics of reactor products:
VS/TS
Total-COD/TS
Total-BOD/TS
Soluble-COD/TS
Soluble-BOD/TS
10
11.9
95
0.857
0.90
0.055
0.29
0.035
10
6.3
95
0.856
1.09
0.04
0.24
0.015
5
6.6
151
0.853
1.08
0.04
0.27
0.02
Apparent removals in percent:3
Total-COD
Total-BOD
Soluble-COD
Soluble-BOD
45-60
12-27
50-80
aRemovals based on average characteristics of feed to mesophilic reactors from the
thermophilic reactors.
41
-------
SECTION VII
ENGINEERING AND ECONOMIC CONSIDERATIONS
ENGINEERING CONSIDERATIONS
The successful development of a treatment process incorporating aerobic, thermophilic
digestion of dairy cattle manure is limited by inherent characteristics of the manure and of
the process. Economic considerations provide an additional limitation upon the process.
A large fraction of dairy cattle manure degrades rather slowly under aerobic conditions, and
therefore, complete stabilization of the manure requires more time than does stabilization
of normal domestic sludges. This has been attributed to the presence of greater quantities of
lignins and cellulosic material in cattle manure.
Maximum rates of aerobic biological activity require an abundant supply of oxygen, and in a
liquid system, a large excess of air is needed to obtain sufficient quantities of oxygen in
solution. The transfer of oxygen is aided by an increase in diffusivity or diffusion coefficient
with temperature. On the other hand, thermophilic temperatures and high solids content
result in saturation oxygen concentrations reduced from that of pure water at ambient
O A ^ C
temperatures. As an example of the magnitude of these factors, data^4^5 indicate that the
diffusivity of oxygen in an air-water system increases by approximately 18 percent for a
temperature rise from 25° C to 60° C while the solubility of oxygen in pure water decreases
by approximately 30 percent for the same temperature rise. The effect of high manure
solids concentration is not precisely known but it substantially reduces both the diffusivity
and the solubility of oxygen.36 it is apparent that the net effect of these factors is a
substantial reduction in ability to transfer oxygen to solution under thermophilic digestion
conditions when compared, for example, with an activated sludge process. This is an
unfortunate situation because less oxygen can be supplied precisely when greater biological
activity that is potentially available at higher substrate concentrations and temperatures
requires more oxygen.
A large excess supply of air could provide the necessary oxygen but this imposes an
additional stress upon the process in terms of removing heat from the thermophilic reactor.
This heat loss severely limits the maximum temperature that the reactor can reach without a
separate external source of heat, which is a relatively costly operating item. The major heat
loss with the airstream is latent heat of vaporization of water vapor that saturates the exit
airstream. The quantity of water vapor and, therefore, the heat loss increase exponentially
with temperature. (The sensible heat required to raise the airstream from ambient
temperature to reactor temperature is a relatively minor loss by comparison.) The need to
minimize heat losses precludes the use of large quantities of air at thermophilic temperature
conditions.
Antithetical requirements for quantity of air supply, as indicated in the above discussion,
could not be adequately compromised within limitations of the experimental apparatus used
in the current study. Improved conditions were recorded with an oxygen-enriched airstream,
and this supports the suitability of a pure oxygen system. A pure oxygen system,
particularly under pressure, would substantially increase transfer rates of oxygen and
eliminate a major heat loss item.
42
-------
Other pertinent observations affecting design of a large scale process were foaming and
coating of equipment with solids. The foaming problem was not consistent in the
experiments, and variations could not be identified with observed parameters. Bubble
cutters kept the problem under control and appeared to be a practical solution. The severe
clogging of air diffusers on the liquid side experienced in the study indicates unsuitability of
small-opening air diffusers for service in this particular type operation. Mechanical aerators
would appear to be more suitable toward overcoming the clogging problems.
In summary, indications from the current study are that a technically feasible process would
include a pure oxygen system, mechanical aeration which insures plenty of mixing, and
foam-control equipment such as mechanical bubble cutters. Elements of this visualized
system exist on a commercial scale. In particular, the Union Carbide Corporation, Linde
Division has developed the Unox-System for pure oxygen considerably beyond what was
examined within the current study. The De Laval Separator Company has developed liquid
composting equipment specifically for animal wastes based upon experimental and pilot
work. It is the authors' understanding that the proprietary De Laval equipment is currently
undergoing performance testing for treatment of dairy manure. Other experiences may exist
and a reasonable approach in future work on liquid composting would be to incorporate
into the work the background of all relevant experiences with due allowance for the
proprietary nature of much previous work.
It was apparent during the current study that substantial, temperature control provisions
would be necessary for thermophilic units if air were the oxygen source. With pure oxygen
instead of air as the oxygen source, control is less critical because the rate of heat loss and,
thereby, the potential for cooling in case of upset is less. Observations during experiments
suggested that a separate heating system was unnecessary when heat losses were not
excessive and that sufficient temperature control could be provided by varying energy input
through mixing.
ECONOMIC CONSIDERATIONS
The economics of dairy operation and the cost of alternative treatment processes are vital to
the economic evaluation of a liquid, aerobic composting process. At the present time (end
of 1973), a dairy in the Chino-Corona area has capital investment estimated at
approximately $1,100 per cow. The estimated gross revenue after recent milk price increases
is approximately $1,100 to $1,200 per cow per year. A profit of 10 percent on capital
investment corresponds to a net revenue of approximately $110 per cow per year, and for a
500 cow dairy, the total profit would be approximately $55,000. These estimates, especially
those pertaining to profits, may be high or low for a specific dairy but are reasonable guides
to judge economic consequences of waste treatment costs.
Cost estimates for a thermophilic-mesophilic treatment process, for a 500 cow operation,
including vacuum filtration, are summarized in Table 16. The figures are engineering
judgments of costs for a treatment process with capacities for 5-day and 10-day hydraulic
residence times, respectively, for thermophilic and mesophilic units. The figures do not
include waste collection prior to treatment or product handling after treatment. This data
indicates a treatment cost of $230 per cow per year which is approximately 20 percent of
gross revenue per cow per year and a factor of two greater than estimated profit per cow per
year. This cost appears to be a substantially greater burden than can be absorbed within the
dairy expenses.
43
-------
An alternate treatment method which has been demonstrated successfully in the
Chino-Corona area is conventional composting. The fact that evaporation substantially
exceeds precipitation in this area makes this method particularly attractive because moisture
content of manure can be easily reduced to optimum levels of approximately 50 percent
moisture. The cost of conventional composting is approximately two dollars per cow per
year. The obvious items of cost advantage over liquid composting can be identified with
natural drying in place of filtration and predominantly natural aeration in place of
mechanical aeration.
Table 16. SUMMARY OF COST ESTIMATES FOR 500 COW
TREATMENT PROCESS
Yearly cost
Thermophilic-mesophilic treatment:
Capital $ 22,000
Operation and maintenance 45,000
Cost for oxygen and storage 26,000
Vacuum filtration:
Capital 11,000
Operation and maintenance 12,000
Total annual cost $116,000
Annual cost per cow $ 230
Transportation costs of getting compost to market are a substantial hurdle because the
fertilizer value of compost is insufficient to cover transportation costs to potential market
areas outside the Santa Ana River basin. The cost of trucking compost to many of the hay
fields is estimated to be S20 per cow per year. Dairymen have yet to implement a
composting and trucking program which would cost them as much as S20 to S30 per cow
per year. This fact substantiates the conclusion that the estimated cost of S230 per cow per
year for liquid composting is much too high to be considered economically viable.
44
-------
SECTION VIII
REFERENCES
1. Santa Ana Watershed Planning Agency, Livestock Waste Study Task Order, No. VI-3,
April 1970, 30 p.
2. Adriano, D.C., Pratt, P.P., and Bishop, S.E., "Fate of Inorganic Forms of N and Salt
from Land-Disposed Manures from Dairies," Livestock Waste Management and Pollution
Abatement, Proceedings International Symposium on Livestock Wastes, Ohio State
University, April 19-22, 1971, pp 243-246.
3. Adriano, D.C., Pratt, P.P., and Bishop, S., "Nitrate and Salt in Soils and Ground Waters
from Land Disposal of Dairy Manure," Soil Sci. Soc. Amer. Proc. 35, 1971, pp 759-762.
4. Miner, J.R., Bundy, D., and Christenbury, G., Bibliography of Livestock Waste
Management, Grant No. 13040 FUU, EPA-R2-72-101, Office of Research and
Monitoring of USEPA, December 1972, 137 p.
5. Miner, J.R., and Willrich, T.L., "Livestock Operations and Field-Spread Manure as
Sources of Pollutants," Agricultural Practices and Water Quality, Iowa State University,
1970,231 p.
6. Smith, S.M., and Miner, J.R., "Stream Pollution from Feedlot Runoff," Transactions,
14th Annual Conference on Sanitary Engineering, University of Kansas, 1964, pp.
18-25.
7. Viets, F.G. Jr., "Cattle Feedlot Pollution," Animal Waste Management. Proceedings of
National Symposium on Animal Waste Management, Warrenton, VA, September 28-30,
1971, pp 97-105.
8. Madden, J.M., and Dornbush, J.N., "Measurement of Runoff and Runoff Carried Waste
from Commercial Feedlots," Livestock Waste Management and Pollution Abatement,
Proceedings International Symposium on Livestock Wastes, Ohio State University, April
19-22, 1971, pp 44-47.
9. Edwards, W.M., Chichester, F.W., and Harrold, L.L., "Management of Barnlot Runoff to
Improve Downstream Water Quality," Livestock Waste Management and Pollution
Abatement, Proceedings International Symposium on Livestock Wastes, Ohio State
University, April 19-22, pp 48-50.
10. McCalla, T.M., Ellis, J.R., and Gilbertson, C.B., "Chemical Studies of Solids, Runoff,
Soil Profile and Groundwater from Beef Cattle Feedlots at Mead, Nebraska," Waste
Management Research, Proceedings of the 1972 Cornell Agricultural Waste Management
Conference, pp 211-223.
11. White, R.K., and Edwards, W.M., "Beef Barnlot Runoff and Stream Water Quality,"
Waste Management Research, Proceedings of the 1972 Cornell Agricultural Waste
Management Conference, pp 225-235.
12. Air Quality Criteria for Particulate Matter, U.S. Dept. HEW, PHS, NAPCA Publication
No. AP-49, January 1969, 211 p.
45
-------
13. Senn, C.L., et al., Dairy Waste Management Study. Final Report of PHS Grants
1-D01-VI00137-1 and 2-D01-VI00137-02, Office of Solid Waste Management of EPA,
December 1971, 153 p.
14. Sullivan, R.J., Preliminary Air Pollution Survey of Odorous Compounds, U.S. Dept.
HEW, PHS, NAPCA Publication No. APTD 69^2, October 1969, 244 p.
15. Loehr, R.C., Pollution Implications of Animal Wastes - A Forward Oriented Review,
U.S. Dept. of the Interior, FWPCA, Robert S. Kerr Water Research Center, Ada,
Oklahoma, July 1968, pp 39-43.
16. Andrews, J.F., and Kambhu, K., Thermophilic Aerobic Digestion of Organic Solid
Wastes. USDHEW, PHS, Office of Solid Wastes Research Grant Ul 00550, Final Progress
Report, Clemson University, May 1971, 76 p.
17. Kambhu, K., and Andrews, J.F., "Aerobic Thermophilic Process for the Biological
Treatment of Wastes - Simulation Studies," Journal WPCF, 4_[ (5): R 127-R141, May
1969.
18. Shell, G.L., and Boyd, J.L., Composting Dewatered Sewage Sludge. USDHEW, PHS,
Bureau of Solid Waste Management Contract No. Ph86-67-103, PHS Publication No.
1936, 1969, 28 p.
19. McGhee, T.J., Torrens, R.L., and Smaus, R.J., "BOD Determinations on Feedlot
Runoff," Water and Sewage Works, H9:58-61, June 1972.
20. California Regional Water Quality Control Board - Santa Ana Region, Water Quality
Control Plan (Interim), Santa Ana River-Basin 8, June 1971 (Revised April 1973).
21. Santa Ana Watershed Planning Agency, Preliminary Draft of SAWPA Final Report to
EPAT published in several parts, 1971-1973.
22. Dale, A.C., and Day, D.L., "Some Aerobic Decomposition Properties of Dairy-Cattle
Manure," Trans. A.S.A.E., 10:546-548, 1967.
23. Taiganides, E.P., and Hazen, T.E., "Properties of Farm Animal Excreta," Trans.
A.S.A.E.. 9:374-376. 1966.
24. Fogg, C.E., "Livestock Waste Management and the Conservation Plan," Livestock Waste
Management and Pollution Abatement, Proceedings International Symposium on
Livestock Wastes, Ohio State University, April 19-22, 1971, pp 34-35.
25. Hart, S.A., and Turner, M.E., "Lagoons for Livestock Manure," Journal WPCF. 37: 1578-
1596, November, 1965.
26. Hart, S.A., "The Management of Livestock Manure," Trans. A.S.A.E.. 3:78-80. 1960.
27. Jones, D.D., Day, D.L., and Dale, A.C., "Aerobic Treatment of Livestock Wastes," Univ.
of Illinois Agr. Exp. Sta., Bull. 737, May 1970, 55 p.
46
-------
28. Jones, D.D., Converse, J.C., and Day, D.L., "Aerobic Digestion of Cattle Wastes," Trans.
A.S.A.E.. 11:757-761. 1968.
29. Peterson, J.R., McCalla, T.M., and Smith, G.E., "Human and Animal Wastes as
Fertilizers," Fertilizer Technology and Use, 2nd Edition, Soil Science Society of
America, Madison, Wise., 1971, pp 557-596.
30. Witzell, S.A., McCoy, E., Polkowski, L.B., Attoe, O.J., and Nichols, M.S., "Physical,
Chemical and Bacteriological Properties of Bovine Animals," Proceedings of National
Symposium on Animal Waste Management, A.S.A.E. Pub. SP-0366, 1966, pp 10-14.
31. Bhagat, S.K., and Proctor, D.E., "Treatment of Dairy Manure by Lagooning," Journal
WPCF, 41:785-795, May 1969.
32. Murphy, L.S., Wallingford, G.W., Powers, W.L., and Manges, H.L., "Effects of Solid Beef
Feedlot Wastes on Soil Conditions and Plant Growth," Waste Management Research,
Proceedings of the 1972 Cornell Agricultural Waste Management Conference, pp
449-464.
33. Hurwitz, E., and Dundas, W.A., "Wet Oxidation of Sewage Sludge," Journal WPCF,
32:918-929, September 1960.
34. Perry, J.H., ed., Perry's Chemical Engineers' Handbook, 4th Edition, New York,
McGraw-Hill Book Company, 1963, pp 14-20.
35. Treybal, R.E., Mass Transfer Operations, 2nd Edition, New York, McGraw-Hill Book
Company, 1968, p 25.
36. Metcalf and Eddy, Inc., Wastewater Engineering, New York, McGraw-Hill Book
Company, 1972, p 346.
47
-------
SECTION IX
APPENDIX
AERATION EQUIPMENT OBSERVATIONS
1. Types of aeration equipment used during the experiment.
A. Chromoglass Air Diffusers
1. Standard (Round Stone), 5 cm x 30 cm (2" x 12")
2. Standard (Round Stone), 9 cm x 30 cm (3.5" x 12")
B. PVC piping, 1.3 cm and 1.9 cm (1/2" and 3/4")
1. 1.3 cm PVC-T-shaped, 15 cm x 15 cm (6" x 6") with 10 holes, 0.16 cm (1/16")
in diameter.
2. 1.3 cm PVC-circular shaped, 20 cm (8") diameter with 15 holes, 0.16 cm in
diameter.
3. 1.9 cm PVC-rectangular sawtooth pattern with 25 holes, 0.16 cm in diameter.
4. 1.3 cm PVC-straight piece, 15 cm in length with 4 holes, 0.24 cm (3/32") in
diameter; 6 holes, 0.24 cm in diameter; one hole 0.95 cm (3/8") in diameter.
2. Observations with each of the above types.
A. Chromoglass Standard, 5 cm x 30 cm: These diffusers were principally used in the
thermophilic reactors, although one was used in a mesophilic reactor. After three
weeks of continuous running, clogging of small pores caused air to be diffused only
at one end or the other. With air diffusion in a local area of the reactor the mixing
system was unable to disperse the air throughout the reactor, and therefore,
biological activity was diminished. Also when clogging occurred air rates as high as
0.35 I/sec (0.75 cfrn) could not be attained. In addition to the problem of pore
clogging, agglomerations of dried material would build up all along the diffusers
causing restrictions of airflow. Scraping clean this build-up would alleviate this
problem. Higher air rates would cause a greater build-up, amounting to as much as 5
cm (2") in one week's time. After five weeks of continuous operation, airflows
could not be maintained. A cleaning solution of dichromate-sulfuric acid was used in
an attempt to restore these diffusers to their original operation capabilities. After
submerging the diffusers in the cleaning solution for 2 days or more, only one was
satisfactorily cleaned.
In clear water, these air diffusers gave a very fine bubble pattern with excellent air
diffusion. But in the reactors with 12% solids, this very fine bubble pattern was not
attained. Air tended to surface in large "blurps" and not always along the full length
of the diffuser.
48
-------
All in all, this air diffuser appeared to operate better at low air rates, low solids
concentration and with rapid mixing.
B. Chromoglass Standard, 9 cm x 30 cm: These were primarily used in the mesophilic
reactors, where greater airflows were desired due to the absence of mixing. The
bubble size and pattern in water from these diffusers were larger than those of the 5
cm x 30 cm diffusers. These larger diffusers were more suitable for higher air rates,
although they also created large "blurps" rather than small bubbles. In mesophilic
reactors with 6% solids, settling occurred after 8 hours even with the higher air rate
of 0.35 I/sec. Any difference in settling between 0.12 I/sec and 0.35 I/sec airflow
rates was not apparent.
In the thermophilic reactors, the 9 cm x 30 cm diffusers gave greater movement to
the contents than did the 5 cm x 30 cm diffuser when mixing action was held
constant.
The problems with the 9 cm x 30 cm diffusers were the same as with the 5 cm x 30
cm diffusers in that the pores clogged after periods of one month or more. Generally
they passed 0.35 I/sec without difficulty but air diffused out of only a portion of
the diffuser. In all cases agglomerations were very hard, dry, and porous.
C. PVC-T-shaped: This shape was experimented with in thermophilic reactor 3, with
stirring at 40 rpm (Dayton 1/2 hp Gearmotor), turning three 23 cm x 8 cm (9" x
3") flat blade props, each with ten 0.16 cm (1/16") holes. The purpose of this
configuration was to obtain maximum air dispersion directly under the stirring
mechanism and have the blades move the air to the outside of the reactor. A folding
effect was produced. Initially oxygen uptake increased slightly and the bubble
pattern on the top of the reactor appeared much improved in that smaller bubbles
were popping all over the surface of the reactor. However, no substantial
improvement in treatment or oxygen uptake occurred. Agglomerations did not
occur. This aerator was used for about 10 days while a Chromoglass diffuser was
being cleaned.
D. PVC-circular shaped: This was used in a thermophilic reactor for 2 weeks, with a
propeller made of six 10 cm bolts spaced up and down the stirring shaft from about
8 cm from the surface of the liquid to about 15 cm from the bottom. This
combination of aeration and stirring appeared to perform better than most schemes
in terms of oxygen uptake. These props turned at about 250 rpm with a mixer speed
set at 4. Airflow restriction due to clogging of holes was minimal and there was little
agglomeration build-up.
E. PVC-rectangular sawtooth: This was the only aerator used in mesophilic reactor 3A.
This aerator fits flat on the bottom of the reactor with holes drilled uniformly along
the side of the PVC. Aeration appeared to be even throughout, though solids settled
on the bottom. The solids were easily stirred up twice a day. There was a consistent
accumulation of bubbles of 40 cm and more in height on the top of the liquid
surface. Airflow was held at 0.24 I/sec throughout the experiment. There were no
problems with restricted flow and no agglomeration was observed.
49
-------
F. PVC-straight piece: After replacing a particularly clogged chromoglass diffuser with
this diffuser, there was a noticeable increase in oxygen uptake similar to that
described in C. above. The problem with this aerator was restrictions in the small
holes. Whenever 2 or more holes became clogged, desired quantities of air could not
be maintained.
50 1U.S. GOVERNMENT PRINTING OFFICE: 1974 546-319/424 1-3
-------
SELECTED WATER
RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
w
LIQUID AEROBIC COMPOSTING OF CATTLE WASTES AND
EVALUATION OF BY-PRODUCTS
Grant, Frank, and Brommenschenkel, Francis, Jr.
Chino Basin Municipal Water District
P. 0. Box 697
Cucamonga, California 91730
'11,'
rganization
Environmental Protection Agency
S,
6,
8,
rofr Oigaa'-'atioa
S801647
ttt
Environmental Protection Agency report number, EPA-660/2-7^-031*, May
The study was undertaken to determine the technical and economic feasibility of
treating dairy waste in a liquid state by a tandem thermophilic-mesophilic aerobic
stabilization process, more commonly described as liquid composting. Experimental
apparatus were set up at an operating dairy and a program was organized to study
the process. The study showed that a large fraction of dairy manure is relatively
resistant to rapid biological degradation even at thermophilic temperatures.
Antithetical requirements of sufficient oxygen for maximum biological activity
and minimum air flow to preclude the need for an external heat source could not
be satisfied with the particular experimental apparatus when utilizing air as the
oxygen source. Improved results were obtained with an oxygen-enriched air supply
which pointed out the potential advantage of a pure oxygen system. Preliminary
cost estimates for a liquid composting process to serve 500 cows were developed
within the context of current dairy operation economics. The estimates showed
that the process is considerably more costly than current, conventional, composting
operations and that the cost of the process is substantially above levels which
could be maintained by dairy operations.
17st. jDescr/ptoTS
*Farm wastes, *Aerobic treatment, *By-products, economics, Biological oxygen demand,
Chemical oxygen demand
17b. Identifiers
Volatile solids, thermophilic reactor, mesophilic reactor, total dissolved solids
05D
-jwr, jfofe*
Send To:
WATER RESOURCES SCIENTIFIC INFORMATION CENTER
U.S. DEPARTMENT OF THE INTERIOR
WASHINGTON. D. C. 2OZ4O
-------
-------
-------
-------
-------
-------
-------
-------
8 i WN
ana 3iva
-------
RD
ENVIRONMENTAL PROTECTION A6ENCY
Forms and Publications Center
Route 8, Box 116, Hwy. 7O, West
Raleigh, North Carolina 27612
POCTAOE AND FH3 PAID
ENVIRONMENTAL PROTECTION AOBNCY
EPA-335
Official Business
If your address is incorrect, please change on the above label;
tear off; and return to the above address.
If you do not desire to continue receiving this technical report
series, CHECK HERE Q ; tear off label, and return it to the
above address.
------- |