WATER POLLUTION CONTROL RESEARCH SERIES
17070DIV 04/70
STATE OF THE ART
REVIEW ON
SLUDGE INCINERATION PRACTICE
IJ.8. DEPARTMENT OF THE INTERIOR • FEDERAL WATER QUALITY ADMTNTRTRATTfWi
-------
WATER POLLUTION CONTROL RESEARCH SERIES
The Water Pollution Control Research Reports describe the results
and progress in the control and abatement of pollution of our
Nation's waters. They provide a central source of information on
the research, development, and demonstration activities of the
Federal Water Quality Administration, Department of the Interior,
through in-house research and grants and contracts with Federal,
State, and local agencies, research institutions, and industrial
organizations.
Water Pollution Control Research Reports will be distributed to
requesters as supplies permit. Requests should be sent to the
Planning and Resources Office, Office of Research and Development,
Federal Water Quality Administration, Department of the Interior,
Washington, D. C. 20242.
-------
Errata
STATE OF THE AST REVIEW ON SLUDGE INCINERATION PRACTICE
WATER POLLUTION CONTROL RESEARCH SERIES 170?0 DIV 04/70
Prepared for the Federal Water Quality Administration*
U. S. Department of the Interior
April, 1970
*Nov Part of the Environmental Protection Agency
-------
1. On Figures 11, 12, and 13, the ordinates are too high "by a factor
of ten.
2. On page 42, reference is made to a paper "by Owen^ ' inferring that
Owen recommended use of the DuLong formula. The authors may have
obtained this impression from the report, "Sludge Handling and
Disposal"(2), vhere Owen is inadvertently misquoted. Actually,
Owen recommended against use of the formula "but suggested that
heating value be determined in a bomb calorimeter.
A check of limited data reveals that the standard deviation of the
difference between the calorimeter heating value and the value cal-
culated by the DuLong formula is about 5 percent. This result
indicates that the DuLong formula gives a reasonably good approxi-
mation to the calorimeter value. Nevertheless, it should be made
clear that Owen did not recommend use of the DuLong formula.
3. It appears to this writer that the'material on pages 50-61 applies
to fluidized bed incineration. It should not be presumed to apply
to other types of incinerators except in a general sense.
k. On page 50 (see also pages 51 > 56> 64), the statement is made that
the minimum deodorizing temperature for conventional incineration
units has been established .at-1350°F to l400°F. Figure 17 is given
in support of this statement with no reference to its source.
(o) (M
Figure IT was taken from an article by Sawyer and Kabir . Sawyerx
in a communication to principal manufacturers of sludge incinerators,
observed that it had come to his attention that the results reported
in this paper were being quoted out of context. He pointed out that
in their tests, time of exposure to the temperatures indicated was
0.7 second, and that longer contact times would undoubtedly reduce
the minimum deodorizing temperature.
The emphasis placed in this State of the Art Review on the need to
have exit gas temperatures of at least 1350°F is in the view of this
writer, unwarranted. Numerous multiple hearth incinerators are
operated at much lower temperatures without complaints of odors.
For example, the sludge incinerator at South Lake Tahoe is operated
at an exit gas temperature in the vicinity of 700°F to 800°F and
there have been no complaints of odors.
The question of odor is frankly considered in a discussion of a
paper presented by Sebastian and Isheim at the 1970 Incinerator
Conference(5). Isheim presents an explanation for the lack of
odor when sludge is burned in a multiple hearth incinerator. His
explanation is similar to that given by OwenC^-/. He acknowledges
that some odorous materials might leave the incinerator and be
removed in the wet scrubber. He makes the very^convincing point
that he knows of many installations provided with afterburners,
but that he does not know of any such sludge incinerators where
the afterburners are actually used.
-------
The nature of the gas-solid contact in a multiple hearth -in-
cinerator makes it reasonable to think that the stack gases
can be odorous if the gases leaving the incinerator are between
700-800°F. As a result of the foregoing, this writer is con-
vinced of two things: (l) the possibility of odor production
under these conditions can never be categorically rejected, and
(2) odor-free incineration under these conditions can generally
be accomplished.
J. B. Farrell, Ph.D.
Chemical Engineer
February 2, 1971
References
(l) Owen, M. B., J. Sanit. Eng. Div., Proc. A.S.C.E., 1172-1
to U.72-27 (Feb. 1957), "Sludge Incineration".
(2) Burd, R. S., "Sludge Handling and Disposal", U. S. Dept.
Interior, FWQA (Now EPA), Pub. WP-20-4, May 1968.
(3) Sawyer, C. N., and Kahn, P. A., JWPCF, 32, No. 12, 127^-1278
(Dec. 1960), "Temperature Requirements for Odor Destruction
in Sludge Incineration".
(4) Sawyer, C. N., Memorandum to Bartlett-Snow-Pacific, Combustion
Engineering, Dorr-Oliver, Nichols Engineering and Research
(Oct. 26, 1966).
(5) Sebastian, F. P., and Isheim, M. C., "Advances in Incineration
and Resource Reclamation", Discussion by J. B. Farrell and
Response by M. C. Isheim, pp. 15-16 in "Discussions, 1970
National Incinerator Conference", pub. Am. Soc. Mech. Engrs.,
N. Y.
-------
STATE OF THE ART REVIEW ON SLUDGE INCINERATION PRACTICE
by
S. Balakrishnan, Ph.D.
D. E. Williamson, P.E.
R. W. Okey, P.E.
Resource Engineering Associates
Wilton, Connecticut 06897
for the
FEDERAL WATER QUALITY ADMINISTRATION
DEPARTMENT OF THE INTERIOR
Program #17070 DIV
Contract #14-12-499
FWQA Project Officer, B. V. Salotto
Advanced Waste Treatment Research Laboratory
Cincinnati, Ohio
April, 1970
-------
FWQA Review Notice
This report has been reviewed by the Federal
Water Quality Administration and approved for
publication. Approval does not signify that
the contents necessarily reflect the views
and policies of the Federal Water Quality
Administration, nor does mention of trade
names or commercial products constitute
endorsement or recommendation for use.
For sale by the Superintendent of Documents, U.S. Government Printing Office
Washington, D.C. 20402 - Price $1.25
-------
TABLE OF CONTENTS
Page
Table of Contents iii
Abstract v
Summary vi
Introduction 1
Primary Consideration 3
Solids Production 3
Characteristics of Sewage Solids 3
Degritting 6
Sludge Blending 6
Pretreetment - Sludge Thickening 8
Gravity Thickening 8
Flotation Thickening 10
Centrifugation 14
Pretreatment - Sludge Conditioning 17
Heat Treatment - Porteous, Ferrer and Zimpro 17
Chemicals 22
Polymers 25
Pretreetment - Sludge Dewatering 26
Centrifugation 26
Vacuum Filtration 28
Plug Presses 35
Filter Presses 39
Unconventional Methods 41
Heat Value of Sewage Sludge 44
Improvements in the Heat Value of Sludge 47
Auxiliary Fuel Requirements 50
Process Variables 52
Excess Air 52
Preheating and Heat Recovery 52
Solids and Free Moisture 56
Sludge Incineration Systems 60
System Components and Make Up 60
Multiple Hearth Furnaces 61
Fluidized Bed Furnaces 67
Flash Drying and Incineration 73
Cyclonic Reactors 80
Wet Oxidation 81
Atomized Suspension Techniques 91
Considerations in Incinerator Design 95
Plant Size and Capacity 95
Aesthetics and Location of Plant 95
Economic Factors 97
Air Pollution Standards 'and Control 97
Safety Standards 101
in
-------
TABLE OF CONTENTS (Continued)
Operational Aspects 102
Dust Collection and Ash Handling 102
Flexibility and Controls 103
Capital and Operating Costs 104
Incineration of Materials Other Than Municipal Sludges 107
Disposal of Refuse with Sewage Sludge 109
Effect of Incineration on Other Resource Management Problems 110
Attitudes of State Agencies Toward Incineration 111
Attitudes of Consulting Engineers Toward Incineration 114
Sludge Incineration Market - Current Status 116
An Analysis of Needs 126
Cost of Conditioning 126
Redundant Systems 127
Sludge Conveyance 128
Summery «°
References 1%51
-------
ABSTRACT
This report on the "State of the Art Review on Sludge
Incineration Practice" covers the current status of the incineration
art end the cost of incineration. An up-to-date critical review of
the effect of sewage character, methods of capturing and concentra-
ting solids--including the sludge conditioning--and sludge incinerator
systems are presented. This report also includes the primary con-
siderations in the design of incinerators, the attitudes of state
agencies and consulting engineers. The principal areas of discussion
are: sludge thickening, sludge conditioning, sludge dewatering,
sludge incinerator systems and the design and operation of
incinerators.
The report concludes that:
1. Increasing pressures for complete or nearly complete "on
site" disposal of solids are building up when compared to
conventional sludge digestion and disposal on land.
2. The necessary pretreetment steps such as sludge dewatering
and blending and their costs and operational aspects could
be improved for benefits.
3. There ere a number of sludge incinerator systems commercially
available and an appraisal of the capital and operating costs
of each type of system, as well as the cost of pretreetment,
should be considered in selecting a system.
4. Incineration of materials other than municipal sludge
with the sewage sludge could be used for the effective
disposal of the mixture.
5. New approaches to the problem of sludge disposal are needed
and additional research into the practical aspects of
sludge treatment should be encouraged.
This report was submitted in fulfillment of Program No. 17070 DIV,
Contract No. 14-12-499, between the Federal Water Quality Administration
and Resource Engineering Associates.
-------
SUMMARY
Sanitary engineering practices in the United States have followed
markedly conventional patterns since waste management first became an
essential municipal function. Even though there is little change over
an accepted process, the one waste management operation that is under-
going gradual but substantial change is solids handling and disposal.
The accepted procedure for the disposal of solid wastes has been
direct or indirect disposal to the ground. Such practices ere becoming
less acceptable for a variety of reasons and, presently, pressures are
building up for complete disposal of solid wastes. Sludge should be
considered a liability rather than an asset to any waste management;
there is no known technique for making a profit on its collection end
treatment. A system that is acceptable to ell parties and the most
economical is generally preferred.
Considerable developments in sludge disposal procedures have taken
place and sludge handling end disposal is receiving more attention than
in the past. It should be recognized that sludge handling end disposal
is a costly operation end it represents 25 to 50 per cent of the total
capital end operating cost of a wasteweter treatment plant. The prob-
lem of sludge handling is the most annoying end is growing. It has
been estimeted thet the volume of waste sludge will increese 60 to
seventy per cent within the next 15 years.
Sludge hendling processes such as incineration and heat drying
require pretreatment of sludge. The pretreatment steps include grit
removal, blending, thickening, conditioning end dewetering. Grit re-
moval is a necessary step as it protects the pumps end other mechenicel
equipment against plugging, wear and tear. Also, it helps by increasing
the heat value of sludge by increasing the voletile content of sludge.
When different types of sludges are handled, blending of sludge im-
proves the economic operation of the thickening, dewetering and
incineration processes. When chemicals are used, the blending of
sludge permits more efficient use of chemicals due to e predicteble
demand for cheroicels.
Sludge thickening reduces the volume of the sludge to be handled
in addition to equalization end concentration of different sludges.
Reduction in sludge volume results in savings due to the reduction of
plant size, lebor, power and chemicals. Sludge thickening is accom-
plished in any one of the processes such es gravity thickening,
flotation thickening and centrifugation.
The sludge conditioning methods primarily aim at the reduction
of bound and surface weter quantities and these include: heat treat-
ment, chemicals, polymers and some unconventional methods such as
VI
-------
solvent extraction, artificial freezing and sonic vibration. Sludge
dewetering processes include: centrifugation, vacuum filtration,
plug presses and filter presses* Reduction of sludge moisture con-
tent to the extent of about 75% is achieved in these dewatering
processes so that the fuel requirements for sludge incineration can
be minimized.
The heat value of sludge depends on the amount of combustible
elements such as carbon, hydrogen and sulfur present in the sludge.
When chemicals are used in the pretreatment steps, the weight of
the sludge increases by about 1096 end, because of their inert nature,
the heat content of the sludge is reduced.
The various incineration processes are discussed under sludge
incineration systems in detail including their performance end opera-
tional problems. The present state of the art on sludge incineration
is that it is generally more expensive then other sludge disposal
systems. The capital and operating cost of incineration systems
depends on the type and size of incinerator, nature and amount of
sludge, and whether deodorization, dust collection and disposal are
included. Supplemental fuels ere invariably required for sewage
sludge incineration but their requirements fluctuate depending on
the characteristics of the sludge end these are reflected in the
operating costs.
Based on a survey conducted on the attitude of consulting engi-
neers on sludge incineration, the following ere presented:
1. The overall attitude of consulting engineers is acceptance
and even eagerness to employ incineration. Also, there is
no evidence of emotional bias against the incineration.
2. The bulk of the engineers prefer incinerators for popula-
tions over 15,000. Multiple hearth and fluidized bed type
incinerators are the preferred ones when compared to the
others.
3. High capital and operating costs of incinerators, the cost
of pretreatment steps, and air pollution problems ere the
major factors mitigating against incineration.
4. The consultants feel, universally, that greases, oils,
screenings, end organic industrial wastes could best be
disposed of by incineration.
Vll
-------
INTRODUCTION
Sludge dispose! is rapidly becoming one of the most important
factors to be considered in the design of new plants end expansion
of existing systems to meet increasing population end industrial
pollution loads. The cost of the conventional digestion system
constitutes a significant portion of the total cost .of treatment
plants end yet digestion does not provide for the maximum reduction
nor the ultimate destruction of the remaining waste organic solids.
Further, anaerobic reactors are extremely difficult to operate end
frequently cause es many problems as ell the rest of the plant
combined.
It has become increasingly difficult since World Wer II to sell,
give eway or dump raw or pertielly dried sludges. Conventionel
methods of disposel such es dumping into legoons, drying beds or
lend fills have become expensive even though they are not the most
satisfactory methods of disposel. Attempts to sell treeted end en-
riched sludge es a fertilizer or soil conditioner heve met with
feilure or very limited success. Hence, there ere pressures to find
alternate procedures for sludge conditioning end disposel which in-
volve eesier sludge hendling end less troublesome operetion then
anaerobic treatment.
There ere other pressures which ere derived from our society end
the wey it is chenging end growing. First, the quantity of sludge
will increase by some 60 - 7096 by 1980, due both to the increase in
populetion end in the degree of treetment requiring large areas for
land disposel. Secondly, the fectors mitigeting egeinst ground
disposel will increese beceuse of mounting desires to evoid the
indiscriminete disposal of waste due to aesthetic and heelth reasons.
Thirdly, sludge handling by the older techniques frequently represents
twenty - 4036 of the capital end operating cost of the treetment plent.
These methods were evolved when labor was cheap end the situetion now
is dremeticelly different. Now, more sophisticeted operetionel
techniques requiring less lebor ere being evolved.
The on-site disposel of sludge is compatible with ell these
driving forces end combustion seems to be the only practical means
presently known that can eccomplish maximum reduction of waste solids.
The new combustion methods heve renewed the interest in investigeting
means other than digestion for totel sludge disposel end, also, it is
expected that future improvements in the sludge combustion practices
will reduce costs even further. Also, complete conversion of the
wastes into innocuous gases and inert solids is feasible by combustion
to meet the tight air and water pollution control lews. Thus, the
-------
incineration of the sludge is able to meet the ultimate goal of the
efficient disposal of solid waste materiel without causing air or
water pollution or other nuisances to the community.
The present trend seems to be away from digestion on account of
the increased capacity required owing to increased use of detergents
and toward dewatering in vacuum filters or pressure filters and
disposal of the sludge cake on land directly or after composting
with ground-up refuse. There is increased interest in the country
in refuse incineration and these syteras may be utilized to incin-
erate the sludge cake together with the refuse.
-------
PRIMARY CONSIDERATIONS
Solids Production
The characteristics of the domestic sewage end the type of
treatment received have a great influence on the build up of solids.
The solids in the domestic sewage are in two major forms: suspended
end soluble. The suspended solids fraction (60% settleeble end 4096
colloidel) equels 0.20 to 0.25 Ib/cep./day, and the soluble fraction
equals 0.30 to 0.35 Ib/cap./dey. Thus, the total dry solids in the
domestic sewage ranges from 0.5 to 0.6 Ib/cep./dey.
In the priraery treetment without coagulants, ebout 50 - 6096 of
the suspended solids end 30 - 3596 BOD ere removed. In the secondery
treetment, most of the soluble BOD (up to 90 - 9596) is removed end
converted to biological solids. In en ectivated sludge treetment
process, the sludge build up cen be estimated by the following
relationship:
sludge build up (Ibs) = aLr - b Se
where: a = sludge synthesis coefficient
Lr = Ib of BOD removed in the secondery process
Sa = Ib of voletile biologicel solids under aeration
b = endogenous respiretion rete
The solids production for primary end secondery plents is
shown in Figure 1.
Characteristics of Sewage Solids
The composition of sewege sludges veries widely depending on e
complexity of fectors. Primery sludges ere higher in caloric value
then biologicel sludges beceuse of their high greese content. It
is more economical to burn undigested solids than the digested
solids since digestion significantly reduces the heat content of the
remaining solids.
The average characteristics of sewage solids as described by
Owen are summarized in Table I .
Sludge ratios of primary to secondary ere generally 8 - 10 to 1
to insure aerobic conditions in the thickener. The primery to
secondery sludge retio directly effects both deweterebility end heet
value of the sludge. Experience2 shows that primery plus trickling
-------
0
10
20
30
40
50
SOLIDS PRODUCTION IN 1000 LB PER DAY (DRY)
FIGURE 1
COMPARATIVE SOLIDS PRODUCTION FOR PRIMARY AND SECONDARY PLANTS
-------
TABLE I
AVERAGE CHARACTERISTICS OF SEWAGE SLUDGE
Combustibles Ash
Materiel (%) (%} BTU/lb
Grease end scum 88.5 11.5 16,750
Raw sewage solids 74.0 26.0 10,285
Fine screenings 86.4 13.6 8,990
Ground garbage 84.8 15.2 8^245
Digested sewage ^
> 49.6 50.4 8r020
Solids and ground garbage J
Digested sludge 59.6 40.4 5,290
Grit 30.2 69.8 4,000
-------
filter sludge will produce about 7% solids in the thickener end de-
water to about 25% solids, whereas primary and activated sludge will
thicken to 5% solids and dewater to 22% solids. The difference in
terms of auxiliary fuel costs is between $2 - $3 per ton of dry
solids.
Degritting
Grit can be described as smell inorganic solids that are removed
from the westeweter after screening and include send, silt, gravel,
ashes, coffee grounds end like substances. Grit volume is relatively
smeller when compared to other solids collected in the treatment
processes but their characteristics ere such that they plug, wear
out and even break pumps end other mechanical equipment. Further,
they affect the heat value of the sludge to be incinerated consider-
ably by decreasing the volatile content per pound of sludge.
Removal of grit in the conventional grit chambers is achieved
to levels up to about 80 - 90% of 45 - 65 mesh materiel and this
amounts to en average of 4 cubic feet of grit per million gallons
of sewage3. Hence, a very efficient degritting device has to be
employed to improve the volatile content of the sludge. Hydrocyclones
have been used for this purpose end they remove 95% of the plus
20O - 270 mesh inorganics at a specific gravity of 2.65 and increase
the volatile content of the sludge from 70 - 75% to 80 - 85%.
Sludge Blending
When disposing of sludge by incineration, different types of
sludges such as primary, secondary and digested sludge must be
handled. Blending of sludges is an essential and important step as
it gives a uniform mixture for the efficient and economic operation
of the sludge thickening, sludge dewatering and incineration operation.
Blending is usually done before the mechanical dewetering and
incineration steps in primary clerifiers by recycling secondary
sludges. The mixing and blending of the different types of sludges
is aided further by sludge collecting mechanisms and picket thicken-
ing devices. Mechanical mixing and air agitation in storage tanks
provide good blending but cause air pollution problems due to the
liberation of gases and the subsequent odor nuisance. This could,
however, be overcome by covering the tank. Air agitation has been
found to be better than mechanical mixing as it freshens sludge end
lowers filtration costs. Vigorous agitation must, however, be
avoided to prevent deflocculation of the sludge which would increase
the cost of dewetering.
-------
When chemicals are to be used for improving the dewataring
characteristics of sludge, blending of the sludge permits the more
efficient use of chemicals as the blended sludge has a predictable
and uniform demand for chemicals.
-------
PRETREATMENT - SLUDGE THICKENING
This section includes sludge thickening, sludge conditioning
by heat and chemicals and mechanical dewatering processes such as
filtration, centrifugation and pressing.
Sludge thickening is practiced for equalization and concentra-
tion of primary and/or secondary sludge. Thickening helps reduce the
volume of liquid sludge to be handled in the subsequent processes.
Reduction in the volume of sludge brings about savings due to the
reduction in physical plant size, labor, power and chemicals.
The initial composition of the raw wastes end the method of
wastewater treatment are important factors that affect the degree of
concentration of sludge. The other factors that affect the thicken-
ing process include: initial concentration of sludge, the size, shape
and density of the particles, the temperature and age of the sludge
and the ratio of orgenics to inorganics. The biological floes are
bulky and concentrated to a lesser extent than raw primary sludge.
Better thickening is achieved in separate units than in the initial
wastewater clarification units.
Sludge thickening is accomplished in one of the three processes:
1. Gravity thickening
2. Flotation thickening
3. Centrifugation
Even though flotation and centrifugation produce higher percentage
of solids than the gravity thickening, they ere comparatively
expensive.
Gravity Thickening
Gravity thickening is the most common type of sludge concentra-
tion. Even though it does not produce as high a solids concentration
as other thickening processes, it is a simple end inexpensive method.
Thickening is generally achieved in two ways. One method is to pro-
vide a deep primary clarifier where primary solids ere collected end
secondary sludge recycled end resettled. As far as the fixed equip-
ment costs ere concerned, this is a leest expensive method. Another
method is to provide a separate thickener to collect the primary end
secondary sludges. This method generally includes cyclones for grit
removal end e sludge disintegrator to insure uniform sludge consis-
tency.
-------
The theory of gravity thickening has been presented by Mencini4
in an exhaustive manner and is not included in this report for the
sake of brevity. Studies by Kynch, Fitch, Telraadge end others5»6,7
developed design parameters for the design of thickeners* Thickeners
ere designed on a Ib dry solids/ft^-dey end the values are the gener-
ally recommended values for sewage sludges:
Primary sludge 22 Ib/ft2-dey
Primery + trickling filter sludge 15 Ib/ft2-dey
Primary + waste activated sludge 8-12 Ib/ft2-dey
Waste activated sludge 4 Ib/ft2-day
Mixing of the primary and secondary sludge and/or digested
sludge is desirable as secondary sludges release their water slowly
end the mixtures respond well to thickening. Sludge ratios of
primary to secondary ere generally 8 - 10 to 1 to insure aerobic
conditions in the thickeners. The primary to secondary sludge ratio
directly effects the deweterebility end heat value of the sludge.
The septicity end gasification interferes with optimum solids con-
centretion and this can be prevented by using chlorine at a dosage to
produce a residual of 0.5 to 1.0 rag/1. Excessive chlorine dosage
disperses biologicel sludges and, therefore, overdosing must be
avoided. Thickeners are circular end about 15 feet deep for better
performance. A minimum detention of 6 hours and an overflow rate
of 400 - 800 gal./ft2-dey are recommended.
To enhance the degree of sludge thickening and reduce odor
nuisence, chemicels and heavy inert agents are used. Rudolfs^
observed that alum and ferric salts did not improve sludge concen-
trations appreciably even after 24 hour compaction. Sulfuric acid,
at a dosage of 600 - 1000 mg/1, was found to improve the compaction
but the cost was prohibitive. Lime, at dosages of 250 to 500 mg/1,
significantly increased the sludge compaction. Inert agents such as
iron oxides, flyash and dietomaceous earth improved the compaction
but only at high dosages.
Use of organic polyelectrolytes as aids to sludge thickening
hes been found to be very successful. Higher dosages of polymers
produce higher degrees of compaction but increase the settled solids
concentration. Filtrate from the vacuum filtration units containing
residual polymers or inorganic flocculent are found to produce bene-
ficial results when it is returned 'to the thickening tank.
Sludge blanket thickness in a thickener is en important parameter
as it affects the ultimate solids concentration. Sludge blanket
-------
depths beyond 3 feet did not seem to increase the solids concentre-
tion^ whereas it was found that underflow solids concentration
decreased as the depth of the compression zone increased^. This
may be due to the increase in the resistance to the flow of water
from the sludge blanket. Further, sludge at greater depths becomes
septic, produces ges and a bulky sludge which is not very conducive
for sludge settling. Increased detention time of solids in the sludge
blanket increases the ultimate solids concentration but a period
of 24 hours is suggested for maximum compaction-'-'-'.
The degree of compaction depends on the type of sludge, and
gentle agitation helps the compaction. Many attempts have been
made to improve the compaction and one such is the use of pickets
with the sludge collection mechanism. The pickets are vertical mem-
bers that move through sludge blanket and create passages for entrained
water and gas to reach the surface as well as aid agglomeration.
The total annual operating costs (capital and operating) for
gravity thickening vary from $1.30 to $5.00 per ton of dry solids
depending on the size of the plant and the local conditions. Gravity
thickening has a future in the handling of westewater solids and
offers a good way to thicken mixed sludges at a low operating cost.
Flotation Thickening
Flotation is best applied to thickening aerobic biological
sludges, especially activated sludge because of higher solids con-
centration and lower initial cost of equipment. Primary sludges
and combinations of primary end trickling filter sludges are more
economically thickened by gravity.
There are four methods of flotation, as listed below:
1. Dispersed air flotation where bubbles are generated by
introducing air through an impeller or porous media.
2. Dissolved air-pressure flotation where air under higher
pressure is put in solution end later released et
atmospheric pressure.
3. Dissolved eir-vaccum flotation in which a vacuum is
applied to westewater aerated et etmospheric pressure.
4. Biological flotation where the gases formed by natural
biological activity ere used to floet solids.
Dissolved air-pressure flotation is the route very often used
compered to the others. The biologicel flotetion is used only et a
10
-------
few sewege treatment plants because of the limited advantages. The
dispersed-eir flotation end dissolved air-vacuum flotation are more
applicable to westewater clarification than thickening because appre-
ciable increases in sludge concentrations are difficult to achieve.
Dissolved air-pressure flotation is used for the separation and
concentration of sludges. The waste flow or a portion of clarified
effluent is pressurized to 40 - 60 psi in the presence of sufficient
air to approach saturation. When this pressurized air-liquid mixture
is released to atmospheric pressure in the flotation unit, minute air
bubbles are released from solution. The sludge floes and suspended
solids are floated by these minute air bubbles which attach themselves
to end become enmeshed in the floe particles. The air-solids mixture
rises to the surface where it is skimmed off.
The major variables for flotation thickening are:
1. Pressure
2. Detention period
3. Air-solids ratio
4. Feed-solids concentration
5. Solids and hydraulic loading rates
6. Type end quality of sludge
7. Recycle ratio
8. Use of chemical aids
Increased air pressure produces greater float solids concen-
tration end a lower effluent suspended solids concentration. Higher
air pressure breaks up fragile floes and, therefore, an upper limit
of 60 psi is used. The recycle of clarified effluent allows a larger
quantity of air to be dissolved because there is more liquid which
dilutes the feed sludge. Recycle ratios of 40% have been found to
be the optimum11 by the Chicago Sanitary District.
The concentration of sludge increases with the increase in
detention period up to 3 hours1 . Beyond 3 hours, no additional
thickening was observed. Air-solids ratio influences the floated
solids and effluent solids concentration. With the increase in air-
solids ratio, an increase in floated solids was observed and a ratio
of 0.02 pound of air per pound of solids was very effective13. Efflu-
ent solids concentration was found to be independent of the air-solids
11
-------
except for very low air input rates or very high solids
loading rates. Variation in influent solids concentration would
alter the air-solids ratio and frequently cause process upset.
Flotation thickening is especially applicable to a mixture of
primary and activated sludges. Design of thickeners is based on rise
rates and these usually range from 1.5 to 4.0 gpm/ft2. A typical
relationship between unit loadings, solids production and recovery
of floated solids as observed in Chicago Sanitary District^ is
shown in Figure 2.
Cationic polyelectrolytes, long-chained high molecular weight
polymerized organic coagulants, ere most often employed as flotation
aids to increase the float solids recovery to as high as 97%. The
use of polyelectrolytes is justified economically because of their
higher activity and subsequent advantages. The normal dosages range
from 1-5 Ib/ton of dry solids end the cost of polyelectrolytes is
in the order of $1 - $5/ton of dry solids. Flotation without aids
generally results in solids eoncentrations about 1% less than with
flotation aids.
Rudolfs, while looking for a chemical that would flocculate end
dehydrate solids, found that calcium hypochlorite was very effective
at a dosage of 364 Ib/ton and increased the solids concentration
from 1.05 to 3.7596 after 6 hours of compaction.
For normal activated sludges, 4% solids concentration (by
weight) is specified as the minimum for design purposes. Attaining
five to 6% solids is generally possible and further concentration
can be achieved in a holding tank. A solids loading of 2 Ib/sq ft
per hour is used for the design of flotation units.
The flotation thickener is normally a prefabricated steel unit
furnished complete with skimming device, drive unit, adjustable
overflow weir, inlet assembly, recirculation pump, retention tank,
flow meters and pressure-reducing valve.
The initial capital cost for flotation is lower then gravity
thickening but the operating cost is higher. The operating cost of
flotation thickening without aids is between $4 and $5 per ton dry
solids end with aids it is between $9 end $11 per ton dry solids.
The total annual cost (including amortization) of air flotation
thickening is between $6 to $15 per ton of dry solids.
Flotation processes are not as simple, consistent or economical
as compared to other thickening processes. However, for thickening
waste activated sludge or low specific gravity, non-activated indus-
trial sludge, flotation is very attractive.
12
-------
13
60
§
I11
>50
40
1O
CO
Q
M
J -
83
Q
M
s
8
10
0
I
FLOTATION UNIT
INLET DESIGN NO. 6
AVERAGE FLOATED SOLIDS OF 4.0%
SI =83
PER CENT 3DLIDS
RECOVERY
SOLIDS
PRODUCTION
UNIT LOADING (TONS/DAY)
FIGURE 2
EFFECT OF LOADING ON FLOATED 9DLIDS PRODUCTION AND RECOVERY
13
-------
Natural biological flotation is successfully used to concentrate
raw sludge at e few sewage treatment plants. Improvements on this
natural flotation technique is achieved by the Laboon processl4 where
control over temperature and detention could be asserted. The
heating of the sludge to 95° F is accomplished in heat exchangers
operated at 15 psi following the disintegration of the raw primary
sludge. Concentration of sludge in tanks is achieved by biological
means for 5 days end the escaping gases buoy and compact the sludge.
The Laboon process is being used to thicken raw primary end waste
activated sludge at Charlotte, North Carolina15. The sewage treatment
plant at Ashland, Ohio, thickens sludge to 15% by biological flota-
tion without heat. The Leboon process at the Allegheny County
treatment plant, Pennsylvania, produced an average sludge concen-
tration of 18% from a feed sludge of 10.7%.
The biological flotation process is fairly expensive because of
the sludge heating, the lengthy detention period and the need to
blend sludges to be used as a feed to the incinerators. The mechani-
cal dewetering step ahead of sludge incineration can be eliminated as
is done in Ashland, Ohio, and Pittsburgh, Pennsylvania. When raw
sludge thickening is practiced, there is a possibility of odor devel-
opment and secondary sludges do not respond well to the treatment.
Thus, biological flotation as a sludge concentration technique seems
limited unless improvements are made in the process. Use of chemical
additives and/or waste heat from incineration units can make the pro-
cess less expensive and more efficient.
Centrifugetion
Centrifugetion is generally used for dewatering rather than for
thickening. The thickened sludge from centrifugation is in a fluid
stage that could be pumped. Centrifuges are e compact and flexible
unit. The capital cost is relatively low but the operation and main-
tenance costs ere high. Solids capture efficiency is very poor when
chemicals are not used. On the whole, there are more advantages then
disadvantages and, therefore, with the recent improvements in machine
design, centrifugation will become more popular for thickening of
primary sludges*
For activated sludge thickening, centrifugation is not as attrac-
tive, whereas flotation thickening would seem to be better suited.
However, when chemicals are required in the flotation operatipn but
not in centrifugation, centrifugation may be less expensive. Chemi-
cals, when used in centrifugetion, could cost $4 to $10 per ton.
A solid-bowl centrifuge9 thickened a feed from 2.5 to 6% and the
14
-------
machine operating parameters were:
average speed - 2300 rpm
pool depth - 2-1/8 in.
Solids capture averaged from 85 - 97%. The relationship between
solids recovery and concentration is shown in Figure 3.
Centrifugal thickening was used very successfully at the Yonkers
sewage treatment plant of Westchester County, New York. Here, the
digested and primary sludges ere thickened prior to ocean barging.
15
-------
100
80
60
>. 40
I
CO
Q
20
8
10 12 14
SOLIDS CONCENTRATION (%)
16
FIGURE 3
DECREASE IN RECOVERY WITH
CORRESPONDING INCREASE IN SOLIDS
CONCENTRATION FROM LOWERING LIQUID
LEVEL IN CENTRIFUGE-ACTIVATED SLUDGE
16
-------
PRETREATMENT - SLUDGE CONDITIONING
A typical biological sludge particle contains a large quantity
of water. This water is contained within the cell as cell water,
around the cell as bound water, and around the bound water as surface
water. The total weight of water (cell water, bound water and sur-
face water) is about 8 to 12 times the weight of the dry solids in
the cell end the break up of this figure is as shown below:
cell water - 2 to 3 times that of dry solids
bound water - 4 to 6 times that of dry solids
surface water - 2 to 3 times that of dry solids.
Thus, it could be seen that a concentration of solids of about 8
to 1296 can be achieved without the use of chemicals.
When chemicals are added, the bound and surface water quantities
will be reduced due to coalescence and subsequent reduction in surface
area. It is possible to concentrate the sludge to about 15% with the
use of chemicals and the chemical cost is in the range of $15 - $30
per ton dry solids.
When the sludge is heated, the cell wall is broken and the con-
tents leek out. In addition, hydrolysis of the bound water structure
occurs. The surface water is also reduced in proportion to the re-
maining area of the cell wall. With all these reductions, the net
effect is that the solids could be concentrated to 40 - 55% solids
range by conventional dewatering methods.
In the following paragraphs, the sludge conditioning by heat
treatment, chemical and polymer addition nethoda is diaeuased and
analyzed*
Heat Treatment
When colloidal gels are heated, thermal activity causes water
to escape from the ordered structure. This phenomenon, known as
syneresis, has been shown to be effective in dewatering municipal
sewage sludge by many researchers-'-^*^ »^®. The heat treatment of
sludge conditions the sludge for easy and efficient handling in
drying, incineration end wet combustion of sludge. Easier handling
of the sludge has implications to the design, costs, and operation
of sludge disposal plants in the United States. The conditioning of
the sludge by heat treatment would result in a saving in labor, space
17
-------
and treatment costs. An additional advantage of heating the sludge
is that ell the organisms ere killed and the sludge so treated can be
handled without health hazard or special precaution. The sludge from
this process can either be dried and used as a totally nonpethogenic
fertilizer or land fill, or used in its wet condition as a fuel in a
heat recovery system.
The British have built three plants in 1939 and 194616*17»18. A
plant for heat treating sludge built in Switzerland in 1965 has pro-
vided a better economic base due to the use of improved technology1^.
A porteous plant has recently been built at Colorado Springs, Colorado,
while several low-pressure Zimpro units (see section on Incineration)
ere also in operation. With the advent of organic flocculating
compounds, improved filtration technology, and modern heat exchange
equipment, it appears that heat treating of U.S. domestic sludge
would find widespread acceptance.
The stability of e colloidal system such as sludge is governed
by two important surface phenomena: electrostatic repulsion end
hydration. A colloidal gel system is e homogenous mess end when
heated, the velocity of the particles increases end overcomes the
electrostatic repulsion resulting in the collapse of a gel structure.
This decreases the hydration end water affinity of the solids.
Heating of the sludge has been shown to increase the filtration
rate of domestic sewage sludge by many folds16*17»19»20. The filtra-
tion has been found to improve appreciably when the temperature was
in excess of 130° C. Complete breakdown of the colloidal structure
occurs when the temperature is raised between 160 and 190° C end held
for 10 to 45 minutes16*!?. This resulted in a sludge that was 200
to 1000 times more filterable than untreated sludge and 15 to 50 times
more filterable than chemically conditioned sludge. The following
data, given in Table II, show the relative dewatering rates of sludge
conditioned by different agents.
While e holding time of about 20 minutes at e temperature of 170°
produced greatest filterability for raw primary sludge^*, a holding
time of 30 minutes at a temperature of 180° C was required for second.
ary sludges to produce the same relative rate of dewetering. Higher
temperatures and longer holding times did not improve the dewater^.
ability of the sludge appreciably. It was also found that heat
treatment solubilized a small fraction of the solid matter of the
sludge and the bulk of the organic nitrogen of the sludge*2.
Using the heat treatment concept of sludge handling, two commer-
cial processes have been developed and these are described below.
1. Porteous Process
18
-------
TABLE II
RELATIVE DEV/ATBRING RATES OF SLUDGE
CONDITIONED BY DIFFERENT CONDITIONING AGENTS
Relative Dewetering Rates
Conditioning Agent
None
o
Sulfuric acid
3
Aluminum sulfete
Ferric sulfete3
o
Ferric chloride
T • 3
Lime
Heat treatment^
Primary Sludge
30
100
200
300
400
1000
6000
Secondary Sludge_
1
2
10
15
20
80
1000
Note; 1. Mixed humus end activated sludge
2. At optimum pH value
3. At optimum dosage
4. One-half hour at 360° F
19
-------
Porteous process is e unique system developed for treatment
of organic waste. This process has been in use since the
early part of this century to treat primary or secondary
sludge in any proportion on e batch basis. Porteous process
reduces moisture to 35 - 70%, and produces a final product
that is sterile, compact, and easy to handle. Presently, it
is an automated continuous process that requires no chemicals
and the total power, fuel and water costs are as low as $2.0O
per ton of dry solids.
The flow diagram of the Porteous process is shown in Figure 4.
Raw sludge (primary or secondary) is stored in storage tank
and after disintegration is pumped to the reaction vessel
through heat exchanger. In the reaction vessel, temperatures
of 350 - 390° F end pressures of 180 - 210 psi ere maintained
and e specially designed steam-jet circulator assures inti-
mate mixing of sludge and steam.
The detention time in the reaction vessel is approximately
thirty minutes and the hot conditioned sludge is passed back
through the heat exchanger, gives up its heat to incoming raw
sludge end enters the decanting vessel with a temperature of
about 90° F. The solid materiel settles rapidly while super.
natent water rises to the top where it is drawn off. The
treated sludge at this point has been reduced to about one
third its original volume. The dense product is passed for
final dewatering to vacuum filters, filter presses or other
mechanical dewetering equipment.
There are eleven installations in Europe23 serving populations
from 10,OOO to 5OO,OOO; however, it is still a new process to
the United States. This system has been proven to be an
efficient and low-cost operation. This process could be added
to almost any installation without changes in existing equip.
ment. It also has variable capacity which could be used to
increase the capacity with some hardware modifications.
2. Ferrer System
The Farrer system of sludge conditioning is basically the seme
as the Porteous system in principle. The Ferrer Company, after.
purchasing Mr. Porteous' patents, modified the process to brin
the following improvements: a) to overcome the odorous steam
release, end b) to prevent short circuiting in the vessel-type
reactor when heat treating continuously. This process has
licensed to Dorr-Oliver (Stamford, Connecticut) in 1969, by
William E. Ferrer, Ltd. (Birmingham, England).
20
-------
BOILER FOR PROCESS STEAM
RAW SLUDGE gDtJOGE RAM PUMP
STORAGE DISINTEGRATOR
AAA,
I
STEAM
I HEAT EXCHANGER REACTION VESSEL
I
I
X) AUTOMATIC DISCHARGE VALVE
THICKENED
SLUDGE
FIGURE 4
PLOW DIAGRAM OF THE PORTEOUS PROCESS
RESIDUAL LIQUORS
I ED SLUDGE
VACUUM FILTER
-------
Heet treatment of sludge is a time-temperature relationship
and the temperature and detention time requirements range
from 350 to 400° F and 20 to 30 minutes, respectively, de-
pending on the nature of the sludge. The Ferrer system is
a continuous sludge conditioning process as compared to the
batch treatment of Porteous process.
The Ferrer system is shown in Figure 5. This system contains
a thickener, a disintegrator, heat exchangers, boilers, de-
canting and storage tank, end a dewatering device. The major
improvement in this system over the Porteous system is that
heating is accomplished in tube-type heat exchanger by indi-
rect heat exchange using hot water through a closed loop.
This technique replaces the injection of steam directly into
the sludge practiced in the Porteous system and has the
following major advantages:
A. Sophisticated deodorizing devices ere not needed as
there is no odorous steam release.
B. The feed volume to the reactor is not increased by
condensed steam.
C. The need for a continuous water supply end treatment is
eliminated as the water is used in a closed loop.
For economy, the heat treatment process is comprised of a two-
stage heat exchanger followed by an economizer. The reactor is
specially designed to eliminate short circuiting and to insure the
desired detention time for sludge conditioning. When used in con-
junction with a Dorr-Oliver fluosolids (FS) system, a waste heat
boiler can be utilized for additional overall economy in operating
costs. In addition to being a complete system in itself, the Ferrer
system may be integrated into the FS disposal system where the ulti-
mate in solids disposal is desired. From an evaluated capitalized
cost point, the addition of the Farrer system for medium to large-
size plants will pay for itself. The capitalized cost evaluation for
primary end activated sludge plants of one through 20 MOD capacity is
shown in Figure 6. Lumb20 reported the total operating cost of the
Porteous process for the plant at Halifax, England, as $6.58 per ton
of dry solids. This cost would be competitive with sludge condition-
ing end dewatering costs in the U.S.A.
Chemicals
Chemicals are used for the conditioning of the sludge as they
increase the maximum efficiency of sludge dewatering. Chemical
22
-------
to
CO
REACTOR
SECOND H3AT
EXCHANGER
PRE-HEATER
CIRCULATING
PUMP
THICKENER
AUTOMATIC
VALVES
(ONE BACK-UP)
DECANTING
AND STORAGE
TANK
CENTRIFUGE
TO PS SOIL LAND FILL
CONDITIONING
GRINDER PUMP
FIGURE 5
FLOW SHEET FOR THE DORR-OLIVER FARRER SYSTEM
-------
to
fc
CO
IX
(X
o
o
H
H
I
100
50
0
CAPITALIZED COST COMPARISON
PS SYSTEM and FS/FARRER SYSTEM
FS
SYSTEM
10 15
FLOW - MGD
FS
AIR PRBHEATER
FS
FARRER
FIGURE 6
THE CAPITALIZED COST EVALUATION
FOR PRIMARY AND ACTIVATED SLUDGE PLANTS OF 1 THROUGH 20 M3D CAPACITY
24
-------
conditioning changes the colloidal structure of the sludge and causes
the particles to coalesce and creates large uniform voids in the sludge
so that water can pass through them.
A wide variety of chemicals have been evaluated for conditioning
sludges and they are: ferric chloride, ferrous chloride, ferric
sulfate, ferrous sulfate, sulfuric acid, nitric acid, hydrochloric
acid, sodium dichromate, aluminum chloride, lime, chromic chloride,
chlorine, sodium chloride, potassium permanganate, cupric chloride,
aluminum chlorohydrate, zinc chloride, titanium tetrachloride, soap,
aluminum sulfate, sulfur dioxide, phosphoric acid, dicalcium phosphate,
and organic polyelectrolytes.
Ferric chloride, lime and cationic polyelectrolytes are the most
popular ones for sludge conditioning in the United States and overseas.
Aluminum chlorohydrate is a common flocculating agent along with lime
and ferric salts. The optimum cost of chemical conditioning is brought
about by suitable combination of ferric salts and lime. Ferric salts
end lime, when added to raw sewage sludge, changes the pH end reduces
the population of the microorganisms. The reduction of microorganisms
is important to control odor problems but it is not possible to pro-
duce a sterile sludge by this process.
Polymers
The advent of synthetic polymeric flocculents has contributed to
major advances in the sludge-handling field. Anionic end cationic
polymers are very effective for raw waste activated sludge because they
bring about charge neutralization and agglomeration of particles.
Physical filter aids are used along, or in conjunction, with
chemicals to increase porosity and filtration rates. These include:
coke, bone ash, peat, paper pulp, ground blast furnace slag, diatoma-
ceous earth, ground garbage, flyash, clay, sawdust, crushed coal,
animal blood and activated carbon.
25
-------
PRETREATMENT - SLUDGE DEWATERING
Sludge dewatering is an important step in the sludge incinera-
tion practice in order to reduce the sludge moisture content to the
required degree. There ere a number of sludge dewatering processes
such as centrifugation, vacuum filtration, plug presses, filter
presses and other miscellaneous processes.
Centrifugation
Centrifuges ere becoming the most popular mechanical device for
dewatering sludge due to their low capital cost, simplicity of opera-
tion and effectiveness with difficult-to-dewater sludges. Centrifuges
separate solids from the liquid through sedimentation end centrifuge!
forces. The machines are of different types: horizontal, cylindrical-
conical, solid bowl, basket end disc. Disc-type machines do a poor job
for dewatering even though they ere good for clarification. The basket
centrifuges, on the other hand, dewater sludges effectively but liquid
clarification obtained is poor. All the other types of centrifuges are
very effective for dewatering sludges.
In a solid bowl centrifuge, sludge concentration is accomplished
by subjecting the thickener underflow to a force of 3000 gravities.
This unit can concentrate the sludge to a moisture content lower than
that achieved by vacuum filtration and without the use of chemical
conditioners. The high speed of the rotor generally produces excep-
tional concentration and capture of the solids. In addition, in Merco
Bowl centrifuges, a pump integrated with the centrifuge returns the
centrate to the thickener feed or influent sewage. Thus, the centrete,
which may be quite odorous, is kept out of contact with the atmosphere.
In a typical continuous centrifuge shown in Figure 7, sludge is
fed through a stationary feed pipe from which it is thrown out through
feed ports into the conveyor hub; the solids are settled out against
the bowl wall by centrifugal force. From the bowl wall they are con-
tinuously conveyed by a screw to the end of the machine at which point
they are discharged. A pool volume is maintained in the machine and
the liquid effluent discharges out of effluent ports after passing the
length of the pool under centrifugal force.
The major variables involved in centrifuge operation ere the speed
of rotation, the liquid throughput, the solids throughput, end the pool
depth. Increasing the pool depth decreases the dreinage beech for the
dewatered solids and reduces the effluent solids. Readily dewetereble
solids require less drainage time so that a higher pool depth can be
maintained. Increasing the liquid flow rate will reduce the recovery
26
-------
DEPOSITED
DBWATERED SOLIDS
DISCHARGE
AND
VOLUME
VARIABLE
to
SLURRY PEED PORT
FIGURE 7
SCHEMATIC OP CONTINUOUS CENTRIFUGE
-------
because of decreased retention in the pool. Increasing the mess flow
rate will reduce the recovery because of the lessened conveyability of
the deposited solids. The general relationships between the operating
parameters such as pool depth, recovery, liquid flow rate and percent
cake solids are shown in Figures 8 and 9.
The use of polyelectrolytes at low dosages increases the recovery
at a given flow rate. The coagulants are usually added to the pool to
minimize turbulence end resulting floe dispersion. However, chemical
treatment usually lowers the cake dryness probably due to the capture
of the fine solids and, therefore, a compromise on dryness versus re-
covery has to be reached. In general, polymers permit higher unit
loadings as well as higher solids recovery.
For biological sludges, the solids capture in the centrifuge is
very poor end the cost of chemicals to improve the recovery is very
high. Further, the maintenance costs ere high in eddition to the pro-
duction of poor quelity centrete. The fine 'solids in the centrete
ere not removed in the settling tenks when recycled and, therefore,
pose a problem. New techniques of handling centrete separately such
as eeretion for stebilizetion, mixing with incinerator ash prior to
filtration, combining with digester supernatant liquor end lime to
produce a liquid fertilizer when fully developed might improve the
situation. Vacuum filters cannot be completely replaced where biologi-
cal sludges are dewatered.
Los Angeles County Senitery District hes reported2** the centrifuge
dewetering cost, which includes capital, power, labor and maintenance,
to be about $4.25 per ton of dry solids. Chemical costs vary from
$6 to $20 per ton of dry solids depending on the type of sludge. The
maintenance of centrifuges is a major operating cost as parts get worn
out regularly. However, the capital costs are about 30% less than the
capital cost of vacuum filters. The dewetering costs, in general, are
more attractive than vacuum filtration except when biological sludges
ere handled. A typical average value for total ennuel costs is $12
per ton of dry solids with a range of $5 to $35 per ton.
Vacuum Filtration
Vacuum filtration is a major mechenicel dewetering step eppliceble
to ell types of sewege sludges. Vecuum filters ere very efficient for
dewetering difficult biological sludges end prove economicel for popu-
lations of 10,000 end greeter. Vecuum filters ere becoming very
popular because of the production of drier cake for incineretion, less
floor spece requirement, good solids capture end flexibility in opere-
tion. There ere ebout 1300 vecuuro filters installed in the United
States for dewetering sewage sludges25.
28
-------
100
40
;90 »-
5
a
—30
H
hJ
8
M
&<
<
U
—20
80
I I [
369
POOL DEPTH (INCHES)
FIGURE 8
CENTRIFUGE OPERATING RELATIONSHIPS
.10
12
29
-------
o
100
MASS-PLOW RATE (Ib/hr)
100 200
300
90.
80
50O
1 I
1000 1500
LIQUID-FLOW RATE (GPH)
FIGURE 9
CENTRIFUGE OPERATING RELATIONSHIPS
20OO
-------
Rotary-drum vacuum filtration is generally used for dewetering
sewage sludges. In this type of filtration, a rotary drum passes
through a sludge slurry tank in which solids are retained on the drum
surface under applied vacuum (See Figure 10). As the drum passes
through the slurry, a cake is built up and water is removed by filtra-
tion through the deposited solids and the filter medium. The drum is
divided internally into drainage compartments which connect to the
filtrate system. A portion of the drum ranging from 20 to 4096 is sub-
merged in the slurry and a sludge mat is formed on the filter media
due to applied vacuum of about 10 to 26 in. of Hg. As the drum ro-
tates, the sludge met is out of submergence end is subjected to
dewatering. At the end of a cycle, before the submergence in the
sludge slurry once again, a knife edge scrapes the filter cake from
the drum to a conveyor. The filter medium is usually washed with
water sprays before it is immersed again in the slurry tank.
The amount of solids which can be dewetered per unit time end
per unit area, end the dryness of the cake formed are dependent upon
the sludge end operating variables. The sludge variables include:
solids concentration, sludge age, temperature, viscosity, compressi-
bility, chemicel composition end the other sludge cherecteristics
such as volatile content, bound water, size, shape and so forth. The
operating variables ere: epplied vecuum, drum submergence, drum
speed, degree of egitation, filter media end conditioning of sludge.
Increesed feed solids concentretion, up to ebout 8 to 1096, eid
in increasing the yield26 and beyond this upper limit chemicel con-
ditioning end sludge distribution becomes difficult. Added edventeges
of having a higher feed solids concentretion are: the chemicals re-
quirement for conditioning ere reduced end a reduction in filter ceke
moisture is obtained. The relationship between feed solids concen-
tration end filter loading is shown in Figure 11.
Ageing of the sludge affects the filterebility of the sludge.
Freshening of the sludge by re-eeretion not only reduces the ceke
moisture but elso reduces the ferric chloride requirement due to e
decrease in elkalinity end to the oxidetion of reducing compounds.
The effect of vacuum is such thet the higher the vecuum, the
greater the yield up to e point end this upper limit eppears to be in
the range of 15 to 20 psi. For very compressible cakes, vecuum fil-
ter design generally incorporates two independent vacuum systems--
one operating to apply moderate vacuum while the ceke is being formed
to prevent medie plugging, end the other operating et high vecuum to
produce e ceke of minimum moisture content. The effect of the in-
creese in epplied vecuum on filter loading is shown in Figure 12.
Drum submergence end speed effects the filter yield end filter
ceke moisture. Increese in drum submergence results in greeter
31
-------
FILTER CAKE
CAKE SCRAPER
WATER SPRAY
NOTEi td -
DRYING CYCLE TIME
FORM CYCLE TIME
LUDOE RESERVOIR
FIGURE 10
TYPICAL MECHANISM OF VACUUM FILTRATION
32
-------
200
v«
s
100
50
I
20
I
I
2 3 4 56789 10
% FEED SOLIDS
FIGURE 11
RELATIONSHIP BETWEEN FEED SOLIDS
CONCENTRATION AND FILTER LOADING
33
-------
100
I ^
fe
5,5O
s
M
O
§
8
5
20i
Nil
10
VACUUM (in Hg)
2O
FIGURE 12
EFFBCT OP INCREASE OP APPLIED VACUUM ON FILTER LOADING
-------
filter yield but also produces a higher cake moisture. An increase
in cycle time decreases the filter cake moisture due to an increase
in drying cycle but the production rate is reduced. The relationship
between form tine and filter loading is shown in Figure 13.
Following chemical conditioning, agitation of the sludge is de-
sirable. Variable speed mixing equipment is usually included with the
vacuum filtration equipment in order to provide violent agitation
while mixing with the chemicals and gentle agitation later on to keep
the solids in suspension.
The maximum efficiency of sludge dewatering is increased by
chemical conditioning. The chemicals and polymers used in sludge
conditioning are discussed in the chapter, Pretreatment - Sludge
Conditioning.
The capital cost of a vacuum filtration system includes the cost
of filters with auxiliaries together with the cost of the building
to house the filter. The cost of filters, including auxiliaries,
range from $95 to $275 per square foot depending on the size of the
installation and the filter media. When the building cost is in-
cluded, the capital outlay may double^ •
The operating cost generally includes the cost of hauling filter
cake to lend fill sites, etc., in addition to the cost of labor, power,
chemicals and maintenance. The total operating cost reported by
Simpson and Sutton28, based on cost surveys of a number of sewage
treatment plants, varied from $5.34 to $30.17 per ton dry solids. The
breakdown of the direct operating cost is given in Table III.
The operating costs reported by Dietz29f based on a survey of
vacuum filtration costs at sewage treatment plants, were $8.20 to
$32.40 per ton with a median of about $20 per ton. The chemical costs
obtained from operating records from about sixty sewage treatment
plants are shown in Table IV.
Plug Presses
Pressing techniques ere limited to a two-stage dewetering system
installed prior to incineration. In order to minimize the need for
chemicals, plug presses taking advantage of free water drainage when
subjected to low pressures are used. The "Roto-Plug" 30 and the "DCG
Solids Concentrator"31 are the two proprietary systems that use this
technique. The two major objectives in these types of presses ere:
1) to avoid the critical pressure that would break the structure of
sludge solids and blind the filter media, and 2) to avoid large dos-
ages of flocculents necessary to build a firm solids structure.
35
-------
3 4 56789 10
FORM TIME (Min.)
FIGURE 13
RELATIONSHIP BETWEEN FORM TIME AND FILTER LOADING
36
-------
TABLE III
DISTRIBUTION OP VACUUM FILTRATION COSTS
Labor and direct supervision 39%
Chemicals end supplies 37%
Electric power 8%
Ma intenance 16%
Total 100%
37
-------
TABLE IV
Sludge Type
CHEMICAL AND OPERATING COSTS -
VACUUM FILTRATION FACILITIES
Smell Plants*
($/ton)
Chemical Total Opera-
Cost ting Cost
Raw primary
Digested primary
Elutriated digested
primary
Raw primary +
filter humus
Raw primary +
activated
Digested primary +
filter humus
Digested primary +
activated
Raw activated
Elutriated digested
primary + activated
$ 7.00
$11.50
$ 4.00
$17.50
$38.70
$10.00
$10.20 $25.50
$21.50 $53.80
$13.00 $32.50
Large Plants
**
Chemical Total Opera-
Cost ting Cost
$ 3.00 $ 7.50
$ 5.50 $13.75
$ 3.50 $ 8.75
$ 6.50 $16.30
$10.50 $26.20
$ 9.50 $23.80
$12.50 $31.25
$ 6.50 $16.30
$ 8.50 $21.28
**
Plow less than 10 MGD
Plow more than 10 MGD
38
-------
The dewetering is accomplished in successive stages with in-
creasing pressure in each stage. Polymers or waste paper pulp are
used for conditioning septic or digested sludges to prevent struc-
tural collapse of the solids whereas such chemicals are found
unnecessary3^ with fresh sludges due to the presence of natural floe.
The Roto-Plug flow diagram is shown in Figure 14 end the process
starts with a thickening step using free drainage of easily separated
water through a nylon cloth under a low pressure of 1 to 1.5 inches
of water. A plug is formed as solids accumulate and squeeze the
water from the sludge due to its own weight. The plug forces the
thickened sludge into the cake formation unit where the sludge is
pressed at about 10 to 15 psi between a wedge-wire drum and rubber
covered rollers. Pressed sludge is incinerated or hauled away to
land disposal.
The manufacturers claim that very little sludge conditioning
is required, power requirements are low, the area required for equip-
ment installation is smell and the equipment is simple and economical,
The pressing techniques, however, are not widely adopted as the re-
sultant cake is not sufficiently dry and the separated water contains
excessive solids.
Filter Presses
Mechanical filter presses are commonly used in Europe for de-
watering sewage sludge, and they use the principle of free water
drainage followed by the application of low pressures. However, in
the U.S., the filter presses are used in industries more then in sew-
age treatment plants for dewetering purposes. The major objections
to this process being used in this country are the high labor and
maintenance costs.
Filter presses ere operated in batches and chemical conditioning
of the sludges is invariably done. The chemicals used include: lime,
aluminum chloride, aluminum chlorohydrete end ferric salts. Flyash
has also been used successfully for precoeting. The only major ad-
vantage of press filters over vacuum filters seems to be the minimum
chemical costs. The conditioned sludge is pressed at about 90 psi
for 3 hours32. The filter cakes formed varied in thickness from 1/2
to 1-1/4 inches with moisture content as low as 40%.
The variations in the filter pressing operations include: leaf
filters, screw and hydraulic leaf filters. These dewater quickly end
require less space. However, filter presses have major disadvantages
as compared to vacuum filter operation due to the high moisture con-
tent in the cake and high operation costs. Screw end hydraulic
presses require a thickened sludge feed of 6 to 8% solids for effective
39
-------
THICKENING CELL
THICKENED
SLUDGE
LIQUID
SLUDGE
NYLON CLOTH
RUBBER COVERED ROLLERS
COMPRESSION UNIT
WF-DGE-WIRE SCREEN
DEWATERED SLUDGE CAKE
MACHINE EFFLUENT
FIGURE 14
ROTO-PLUG FLOW DIAGRAM
40
-------
dewetering and this appears to be a major disadvantage for the
application of sewage sludges.
Unconventional Methods
In an attempt to eliminate the need for chemicals end to in-
crease production rates, a number of unconventional approaches have
been undertaken and these are summarized in the following paragraphs.
The use of electricity for sludge conditioning has been tried by
many researchers33*34»35 in laboratory end pilot-plant scale studies.
Slagle and Roberts, in their laboratory studies using electrodielysis,
found that the filterability of sludge increased following the passage
of a direct current, as shown in Table V.
In their pilot plant, Slagle and Roberts found that the electro-
dialysis reduced the pH of the sludge system to 3.4 and the sludge
could be filtered without the use of chemical conditioners. It was
also found that the sludge settles rapidly and seems to be stabilized
as there was very little gas produced during extended detention. For
a fresh sludge at 6.56% solids, a comparison of the electrodialysis
and chemical conditioning has been found33 per ton of solids as shown
in Table VI.
The roost economical current density was found to be about 0.3 amp
per square foot of anode surface with a potential drop of 4 volts be-
tween the electrodes. For economic comparison, the price of
flocculents and electricity at a particular location must be known.
Based on the typical data , 181 KWH is equivalent to 408 pounds of
ferric chloride end pricing electrical energy at $0.01/KWH, the cost
of electrodialysis appears to be less than the cost for chemical
treatment33 .
Cooling and coworkers reported a process3^ , electro-osmosis, for
conditioning digested sludge. From his experiments, Cooling found
that the quantity of water removed from sludge was proportional to
the electricity transported. An electro-osmosis permeability of
0.006 gallons per square foot per hour per inch per volt end a con-
stant equal to 0.02 gallons per ampere-hour was used. The consumption
of electricity was too high to make the process practical and a high
degree of maintenance was required.
High sludge-drying costs at Chicago provided the incentive to
seek a way to decrease the vacuum filter-cake moisture. Beeudoiir*4 f
working on this problem, obtained best results by conditioning the
filter cake with 25 volts for 2 minutes. Even though the use of elec-
tricity was effective, the process was not economically feasible.
41
-------
TABLE V
INCREASE IN FILTBRABILITY WITH ELBCTRODIALYSIS
Wetcr Removed by
Degree of TreatmentVacuum Filtration
Untreated sludge
Sludge electrodielyzed for 15 min. 43%
Sludge electrodialyzed for 30 min. 65%
TABLE VI
COMPARISON OF ELBCTRODIALYSIS AND CHEMICAL CONDITIONING
Electrodialysis Conditioning Chemical Conditioning
181 KWH expended 89 Ib ferric chloride used
Filter cake moisture - 70% Filter cake moisture - 59.5%
Filter cake solids - 2065 Ib Filter cake solids - 1440 Ib
Filter cake water - 4665 Ib Filter cake water - 2130 Ib
pH - 6.2 pH - 3.4
Note:
1. Comparison is for fresh sludge having a 6.56 percent solids
concentration.
2. The figures given are per ton of dry solids.
42
-------
The dewatering qualities of the sludge ere enhanced when the pH
of the sludge is reduced. When autotrophic sulfur bacilli are added
to digested sludge, acids are produced under aerobic conditions*
This principle has been investigated as a sludge conditioning method
but no data are yet available to describe the performance or economics
of the bacterial process*
Solvent extraction is an interesting approach to sludge de-
watering and has been tested at the Rockford, Illinois, treatment
plant. The process, known as McDonald process36, involves the follow-
ing stepst dewatering by contrifugation, solvent extraction with
carbon tetrachloroethylene and distillation. This process has been
described as impractical.
Artificial freezing of sludge has been found successful by many
researchers37 »38 in promoting rapid dewatering. It is speculated39
that freezing disrupts the cell walls retaining the internal moisture
in sludge and, thereby, allows the water release end drainage. Clements
and co-workers reported that freezing was an effective sludge condition-
ing process for all types of sludges and that the use of flocculents
with freezing was helpful but not necessary. They also reported that
the slow and complete freezing of the total sludge was necessary for
good results and the method of thawing was not Critical as long as it
is not accompanied by vigorous agitation.
The operating cost for freezing includes: power, flocculents and
refrigerants. It has been reported that it takes 28 BTU to lower the
temperature of one pound of sludge from 60° F to 32° P end 142 BTU to
freeze a pound of sludge3®. Clements, et el., have quoted a total op-
erating cost for freezing of $5.60 per ton of dry sludge while others
have quoted40 as high as $32 to $45 per dry ton. The freezing tech-
nique by artificial means undoubtedly aids sludge dewatering but
because of the high operating cost, it may never become practical
unless the economics are improved greatly.
The British laboratories have explored41 the conditioning of
sewage sludges by ultra or supersonic vibration. This process has
not been found successful because ultrasonic vibrations tend to
destroy sludge floes resulting in fine solids that are more difficult
to dewater. The only advantage found in this process is that the
vibrations degasify which aid sludge dewatering.
43
-------
HEAT VALUE OF SEWAGE SLUDGE
The combustible elements of sewage sludge are carbon, hydrogen
and sulfur and these elements are chemically combined in the organic
sludge as grease, carbohydrates end protein. The combustible portion
of sewage sludge has a BTU content equal to that of lignite coal.
Air is added to provide oxygen to support the combustion of the
combustible elements.
The reactions of these elements with oxygen ere as given in
Table VII.
The composition of elements in sewage sludge varies*^ from
plant to plant as shown in Table VIII.
The heat value of sewage sludge can be estimated if its ultimate
analysis is known. DuLong's formula (1) can be used to compute the
heat value:
Q = 14,600 C + 62,000 (H S_) (/)
where: Q = BTU/lb of dried sludge
C = % carbon
H = % hydrogen
0. = % oxygen
A reduction in the thermal value of the sludge occurs when inor-
ganic chemicals are added to aid filtration. These chemicals used
are inert and, therefore, lower the heat content per pound of filter
cake. The other disadvantage is that the weight of the sludge is
increased by 10 to 15% by the addition of chemicals^S. The heat
energy available for sustaining combustion will be reduced if ferric
hydroxide and calcium hydroxide sludges are burned with the sewage
sludge due to the heat used up in the dehydration of these hydrous
sludges.
44
-------
TABLE VII
COMBUSTION REACTIONS OP SEWAGE SLUDGE
Reaction
1. Carbon + oxygen
C + 02 —
(1 Ib) (2.67 Ib)
2. Hydrogen + oxygen
2H,
(1 Ib) (7.94 Ib)
3. Sulfur + oxygen
S + 0^
(1 Ib) (1 Ib)
carbon dioxide
-> C0
(3.67 Ib)
water
2H20
(8.94 Ib)
sulfur dioxide
-ft S02
(2 Ib)
Heat Release
(BTU/lb)
14,500
62,000
4,500
45
-------
TABLE VIII
ELEMENTAL
Elemental
Composition
Carbon (96)
Hydrogen (96)
Oxygen (96)
Nitrogen (96)
Sulfur (96)
Volatalite (96)
V.S.S. (BTU/lb)
T.S.S. (BTU/lb)
COMPOSITION OP SEWAGE SLUDGE
No. 1
64.3
8.2
21.0
4.3
2.2
47.9
12,640
6,160
Source*
No. 2
65.6
9.0
20.9
3.4
1.1
72.5
12,510
9,080
No. 3
55.0
7.4
33.4
3.1
1.1
51.4
10,940
5,620
No. 4
51.8
7.2
38.0
3.0
Trace
82.0
8,990
7,380
*Source No. 1 - Cleveland Southerly Plant, 1955
No. 2 - Detroit, Michigan, 1955-56
No. 3 - Minneepolia, Minnesota, 1955
No. 4 - New Rochelle, New York, 1960-62
46
-------
IMPROVEMENTS IN THE HEAT VALUE OF SLUDGE
Improvements in the heat value of sludge can be achieved only
by improving the volatile content of sludge as fed to the combustion
unit as no control can be exercised over the stoichiometry of sludge
combustion. The volatile content of a given sludge may be improved
by a very efficient degritting system. Hydrocyclones4^, used for this
purpose, have shown removals of 9596 of the plus 200 - 270 mesh inor-
ganics at a specific gravity of 2.67 and increases in the volatile
content of the sludge from 70 - 75% to 80 - 85%. The effect of vola-
tile content on the operating cost of sludge combustion is shown in
Figures 15 end 16.
A flocculation process^ used in conjunction with clarification
in the primary treatment area, increases the sludge settling rates
and, therefore, the ratio of primary to secondary sludge. This pro-
cess removes about 7096 suspended solids and 40 - 50% BOD depending
upon the strength of the sewage influent. Assuming an overall re-
moval of 95% for conventional activated sludge, the sludge resulting
from such a process will thicken (7%) and dewater (25%) to the same
degree as primary plus trickling filter. Therefore, we save on fuel
end increase a combustion unit's capacity. The other benefit is the
reduction in the required size of the secondary system, due to the
higher BOD removals in the primary treatment.
Consideration must also be given to materials present which will
react endothermicelly at combustion temperatures. The moisture con-
tent of the calcium carbonate (CeCOs) sludge and the endothermic
decomposition to calcium oxide (CeO) materially increases the thermal
burden. The off gas cleaning requirement would also be increased.
Further, a highly alkaline sludge would be more difficult to dispose
of than the original calcium carbonate, except in some cases where
liming of soils is required. Therefore, the calcium carbonate sludge
should be handled separately and dewetered further, if necessary,
for final disposal by land fill.
47
-------
CO
Q
M
I
SE
B
X
•CO-
H
CO
3
1.5
1.0
0
$1.68/TON
65
% VOLATILE VS AUXILIARY FUEL
SLUDGE <§ 3056 TS
EXIT TEMP. <§ 150O°P
EXCESS AIR 20*
NATURAL GAS @ 1,000 BTU/CF & 4O0/1,OOO CP
70
75
80
$0.16/TON
85
% VOLATILE
@ 10,000 BTU/LB V.S.
FIGURE 15
EFFECT OF VOLATILES IN SLUDGE ON FUEL (NATURAL GAS) COST
48
-------
$3.4O
•*».
<£>
(0
a
2
8
8
o-
•o.
o
Q
2
O
D
8
•4
H
O
09
DESIGN CONDITIONS
SLUDGE FBBD
GAS EXIT TENP-
EXCESS AIR
•3096 T.S.
•15OO F
-2056
$0.80
% VOLATILE IN SLUDGE
-------
AUXILIARY FUEL REQUIREMENTS
The two important factors that affect the auxiliary fuel re-
quirements are the heat value of the sludge and the heat required
for adequate burning. By adequate burning, it is meant the heat re-
quired for complete incineration of the sludge and to raise the
temperature of the gases to a sufficient level to insure odor
control. The magnitude of the temperature requirement depends upon
the nature of the sludge being burnt but the minimum deodorizing
temperature for conventional incineration units has been established
at 1350° F - 1400° F as shown in Figure 17.
The heat required for the incinerator system depends primarily
on the efficiency of burning and the degree of excess air required.
The following constitute the total heat requirements:
1. Heat required in raising the temperature of sludge from
about 60° F to 212° F; evaporating water from sludge;
increasing the water vapor end air temperature of the
gas; end increasing the temperature of dried volatiles
to the ignition point.
2. Heat required to raise the temperature of the exhaust
gas to the deodorizing temperature.
3. Heat required to raise the temperature of the air
supply required for burning plus the excess air.
4. Heat losses due to radiation.
5. Cooling air losses.
6. Heat required for other endotherraic reactions taking
place.
The heat content of the organic sludge solids serves to raise
the end products of combustion along with the moisture content of
the filter cake. The sludge solids drew sufficient heat from the
surroundings to reach kindling temperature before combustion can
start. When the heat released is sufficient to replace the amount
withdrawn, combustion will be maintained. When the quantity of
heat released is insufficient to maintain combustion temperature
at deodorizing level, heat is recovered from the stack gases end
reused, or heat is supplied from an outside source.
50
-------
ANAEROBIC FLOATED
SLUDGE
RAW CHEMICALLY
CONDITIONED
RAW SLUDGE
DIGESTED ELUTRIATED
1150 1200 1250 1300 1350 1400 1450
TEMPERATURE *F
FIGURE 17
RELATIONSHIP OF ODOR LEVEL IN STACK GASES
TO HIGHEST PROCESS TEMPERATURE ENCOUNTERED
51
-------
PROCESS VARIABLES
Excess Air
Because of the normal variations in the organic characteristics
of the sludge and the feed rate, excess air is added to the combus-
tion chamber. The excess air also increases the opportunity of
contact between fuel and oxygen which is necessary if combustion is
to proceed. To insure complete thermal oxidation, it has been neces-
sary to maintain 50 to 100% excess air over the stoichiometric amount
of air required in the combustion zone. This much excess air is un-
desireble in that it quenches the reaction temperature by acquiring 12
to 24% of the input BTU's to heat the excess air. If excess air is
not supplied for this reason, it may be difficult to maintain the mini.
mum deodorizing temperature. Therefore, a closely controlled minimum
excess air flow is desirable for maximum thermal economy. The amount
of excess air required varies with the type of burning equipment, the
nature of the sludge to be burned, and the disposition of the stack
gases. The impact of use of excess air on the cost of fuel in sludge
incineration is shown in Figures 18 end 19.
When the amount of excess air is inadequate, only partial com-
bustion of the carbon occurs, resulting in the formation of carbon
monoxide, soot and odorous hydrocarbons in the stack gases. Further
the heat recovered from the partial burning of the carbon is sub- *
stentially reduced as the heat value of carbon monoxide is only
4400 BTU/lb.
Preheating end Heat Recovery
Preheating of air is an important step in improving the thermal
economy. Air preheat affords en increase in capacity for a given size,
reactor since the combustion ges volume is used most effectively and
since this eliminates the otherwise necessary quantity of auxiliary
fuel. The marked effect of preheating air on the cost of auxiliary
fuel for various solids concentrations to sustain combustion is shown
in Figure 20.
Preheating of air can be avoided in exceptional circumstances
where the following conditions are satisfiedi
1. The excess combustion air volume is maintained at the
minimum required to insure combustion.
2. The grit and inert chemical agents are eliminated.
3. The moisture content is reduced to a point not often
52
-------
o
a:
w
a.
Cu
S
D
O
•t
o
•-(
©
CO
CO
a
I
EXCESS AIR VS AUXILIARY FUEL
SLUDGE @ 30% TS, 7O% VOL & 10,000 BTU/LB
VS
EXIT TEMPERATURE <§ 1500° F
4 .
I
$3.70/TON
$0.92/TON
JL
J.
O
80
100
•J
H
X
20 40 60
% EXCESS AIR FOR SLUDGE
«* EXCESS AIR FOR NATURAL GAS @ 2096 (CONSTANT)
FIGURE 18
THE IMPACT OF EXCESS AIR ON THE COST OF NATURAL GAS IN SLUDGE INCINERATION
53
-------
CO
Q
i
8
I
2
8
B
eu
-co
55
M
I*
e
CO
I
V
o
in
CO
©
DESIGN CONDITIONS
SLUDGE PEED-
EXIT TEMP.—
HEAT CONTENT-
VOLATILE-
•30X T.S.
•1500 P
•95OO BTU/LB
VOL
•75%
100
EXCESS AIR
FIGURE 19
THE IMPACT OF EXCESS AIR ON THE COST OP NO. 2 OIL IN SLUDGE INCINERATION
54
-------
EXIT GAS TEKPSRATURE - 14OO F
WITH HEAT RECOVERY AND AIR PREHEAT - 100O F
6 8 10
FUEL COSTS - $/TON DRY SOLIDS
FIGURE 20 THE EFFECT OF PREHEATING AIR ON FUEL COSTS
-------
attainable by vacuum filtration.
4. The volatile content of the total solids exceeds 70%.
Heat is recovered from stack gases and the advantage of recover-
ing heat is shown in Figure 20. It should be noted that the preheat
exchanger represents a significant capital cost and it is to be
recommended only after a complete economic evaluation of the process.
Solids end Free Moisture
Most of the sludges to be disposed of by incineration will not
support autogenous combustion because of an excessive water content.
Thus, auxiliary fuel becomes a prime factor in process evaluation.
When the sludge feed is drier, a smaller sized combustion unit is
needed end the burning is more efficient. The impact of free mois-
ture on the cost of auxiliary fuel required to sustain combustion
for systems with and without heat recovery is shown in Figures 21
and 22. The importance of obtaining a solids concentration greater
than 30% can be illustrated with Figure 23. For example, at 25%
total solids there is only enough heat available to raise the combus-
tion products end moisture to 900° F and this temperature is far
below the accepted 1350 - 1400° p necessary for deodorizing the stack
gases of a conventional combustion unit.
56
-------
O
6
W Ou
s 2
g §
I
% TOTAL SOLIDS Va AUXILIARY FUEL
EXIT TEMPERATURE @ 1500 »F EXCESS AIR 2036
$2.56/TON
$1.76 AON
$0.92/TON
* 25 27.5 3O
% TOTAL SOLIDS IN SLUDGE
SLUDGE 7596 VOL. & 10,000 BTU/LB V. S.
FIGURE 21
THE EFFECT OF MDISTURE CONTENT ON THE COST OF SLUDGE COMBUSTION
57
-------
10 ,
CO
Q
55
B
•CO-
SB
H
•J
M
D
. 7 •
1
ti 8
m
m
(J
M
o
09
DESIGN CONDITIONS
GAS EXIT TEMP.'
EXCESS AIR
WITH HEAT
RECOVERY AND
PREHEAT^AIR TO
1000* F
•1500°P
•20*
WITHOUT HEAT
RECOVERY
25 30
% TOTAL SOLIDS IN SLUDGE
<§ 7596 VOL AND 9500 BTU/LB. VOLATILE
FIGURE 22
EFFECT OF MDISTURE CONTENT ON THE COST OP
SLUDGE COMBUSTION SYSTEMS WITH AND WITHOUT HBAT RECOVERY
58
-------
NO HEAT
RECOVERY
80O
9OO 1OOO 1100 12OO 1300
TEMPERATURE - °F
1400
15OO 1600
FIGURE 23
EQUILIBRIUM CURVES RELATING COMBUSTION TEMPERATURE
TO CAKE CONCENTRATION
-------
SLUDGE INCINERATION SYSTEMS
System Components and Make Up
Sludge incineration systems include the following components,
in general:
1. Sludge thickener
2. A disintegrating or macerating system
3. Polymer handling and feeding system or other
pretreatment schemes
4. Centrifuge or vacuum filter or any mechanical
dewatering system
5. Incinerator feed system
6. Air pollution control devices
7. Ash handling facilities
8. Complete set of automatic controls such as fail-safe
devices, stack temperature regulator and interlocks to
permit positive control of excess air.
Incineration processes involve two steps: 1) drying, and 2)
combustion. In addition to fuel and air, time, temperature and tur-
bulence are necessary for a complete reaction. The drying step is
different from preliminary dewatering. The drying is achieved by
mechanical means and precedes the incineration process. Sludges
having a solids content of 25% and more are delivered to the most
common types of incinerators* Because of the high moisture content
the heat required to evaporate the water nearly balances the heat
available from combustion of the dry solids44.
Drying and combustion is done in separate pieces of equipment
or successively in the same unit. Manufacturers have developed
widely varying types of sludge drying and combustion equipment.
But the principle variation between manufacturers is in the require-.
ment for heating excess air and the efficiency of utilizing the
waste gases.
The principal types of sludge incineration systems are as
follows:
1. Multiple hearth furnace
60
-------
2. Fluid!zed bed
3. Flesh drying with incineration
4. Wet oxidation (Zimpro Process)
5. Atomized suspension technique
Multiple Hearth Furnaces
The most widely used type of incineration system is multiple
hearth furnace. The multiple hearth type of incineration is very
popular in large cities where alternate final sludge disposal
techniques are inconvenient or too expensive. There are about 120
of these units installed^5. The types of solids incinerated were
very varied and are as follows:
Raw primary sludge Scum
Grit Ground refuse
Grease Activated sludge
Screenings Trickling filter sludge
Skimmings
Multiple hearth units ere popular because they ere simple,
durable and have the flexibility of burning a wide variety of
materials even with fluctuations in the feed rate.
A cross section of a typical multiple hearth incinerator is
shown in Figure 24, and a typical flow diagram for a plant incor-
porating such a system is shown in Figure 25. Multiple hearth units
are available in sizes to handle from 5 to 1250 tons/24 hr. These
units are designed with varying diameters from 6 ft-0 in. to 22 ft-
3 in., and a varying number of hearths--usually between four and
twelve1. Multiple units are often used as it allows flexibility of
operation. The units are capable of burning grit, screenings,
grease end sludge.
The design and operation of multiple hearth units are made
simple so that they cost less compared to the other types of incin-
erators. The multiple hearth furnace consists of a circular steel
•hell surrounding a number of solid refractory hearths (See Figure
24) and a central rotating shaft to which rabble arms are attached.
Each hearth has openings that allow the aludge to be dropped to the
61
-------
WASTE COOLING AIR
TO ATMD SPHERE
CLEAN GASES TO
ATMD SPHERE
^INDUCED DRAFT FAN
BYPASS ON POWER OR
(WATER STOPPAGE
NJ. NERCO-ARCO
r^. >-.—- —.. ^.... __«
r ^^. /^OVr^T fiMTi^ T LI
^V / LyXwUWPIXw *J ul
*» ^^* * — ™ •^ -~ «•
5ATING DAMPER
X. SCRUBBER
GREASE
SKIMMINGS
MAKEUP WATER
TO DISPOSAL
FILTER CAKE
^--SCREEN*
INGS &
GRIT
COMBUSTION ATP
"RETURN ra
ASH PUMP
ASH HOPPER
COOLING AIR
FIGURE 24
TYPICAL SECTION OP MULTIPLE HEARTH INCINERATOR
62
-------
GRIT
CHAMBER
SECONDARY SLUDGE
J RAW PRIMARY CLARIPIBR
GRIT
TRICKLING
FILTER
SLUDGE
& GREASE
AERATED HOLDING TANK
FILTRATE TO PRIMARY
CLARIFIER
EFFLUENT
SECONDARY
CLARIFIER
VACUUM
FILTER
EXHAUST GASES
.MULTIPLE
/HEARTH
INCINERATOR
EXHAUST GASES TO STACK
PLANT WATER
SCRUBBER
V
\ SCRUBBER
ASH
HOPPER
STERILE EFFLUENT
INORGANICS TO PRIMARYCLARIFIER
TO FILL
\
FIGURE 25
FLOW SHEET OF A TYPICAL PLANT WITH MULTIPLE HEARTH INCINERATOR
-------
next lower hearth. The central shaft and rabble arms are cooled
by air supplied in regulated quantity and pressure from a blower
discharging air into a housing at the bottom of the shaft. Rab-
bling is very important to combustion because it breaks up large
sludge particles, thereby exposing more surface area to the hot
furnace gases that induce rapid and complete combustion.
Partially dewatered sludge is continuously fed to the upper
hearths which form a drying and cooling zone. In the drying zone,
vaporization of some free moisture and cooling of exhaust gases occur
by transfer of heat from the hot gases to the sludge. Intermediate
hearths form a high temperature burning zone where all volatile gases
and solids are burned. The lowest hearth of the combustion zone is
the place where most of the total fixed carbon is burned. The bot-
tom hearth of the furnace functions as a cooling and air preheating
zone where ash is cooled by giving up heat to the shaft cooling air
which is returned to the furnace in this zone. The temperatures
range from 600° P at the bottom, 1600 - 1800° P in the middle hearths
and 1000° F on the top hearths. The waste gases from combustion are '
heated to deodorizing temperature so as to guard against odor nui-
sance. Exhaust gases leaving the incinerator at the top are scrubbed
in a wet scrubber to remove flyash.
Auxiliary fuel is invariably used to heat the waste gases be-
fore venting to the atmosphere and the operating cost becomes
expensive due to the fuel cost. A heat recovery device may be
suggested to improve the economy of high temperature deodorization
but this requires expensive deodorization and combustion air pre-
heating equipment.
Incineration of sludge is generally considered to be more expen-
sive than other sludge disposal processes. However, with the increase
in the population served, the economics are favorable expecielly for
large size communities. This could be noted in the Figure 26, de-
veloped from the data of Bartlett-Snow-Pacific , Inc.46. The cost
figures include vacuum filtration equipment, chemicals, power, fuel
and maintenance.
McLaren, in his evaluation of sludge treatment and disposal
systems for Canadian municipalities*7 estimated that the capital
cost of incinerators was between $5 to $10 per ton of dry solids
based on a 30-year amortization at 596 interest. He also estimated
the operating costs to be between $4 to $7 per ton and, thus, the
total annual cost varies from $9 to $17 per ton of dry solids.
Quirk estimated the cost for digested sewage sludge incinera-
tion^7 for a city of about 100,000 contributing 2,530 tons of solids
per year. The total annual operating cost of solids with and with-
out deodorization is summarized in Table IX.
64
-------
500,000
400,000
•CO-
*
B
8
w
300.00O
200,000
100,000
0
10.0
"z.
I
S3
7.5 8
is
M
H
5.O
2.5
25.OOO 50,000 75,OOO 1OO,OOO 125,000 150,OOO 175,000
POPULATION
FIGURE 26
RELATIONSHIP BETWEEN POPULATION SYSTEM COST AND ANNUAL OPERATING COST
2OO,OOO
-------
TABLE IX
TOTAL ANNUAL OPERATING COST FOR SLUDGE INCINERATION
WITH AND WITHOUT DBODORIZATION
(FOR DORR-OLIVER F/S UNIT)
1. Total annual cost without deordorization
A* Capital cost (includes vacuum filtration
but excludes ash disposal facilities)
B. Operating cost
1) vacuum filtration
2) incineration
Total
$11.75/ton
7.91/ton
6.367ton
$26.02/ton
2. Total annual cost with deodorizetion
A. Capital cost (includes vacuum filtration
but excludes ash disposal facilities)
B. Operating cost
1) vacuum filtration
2) incineration
Total
$12.07/ton
7.91/ton
9.SO/ton
$29«48/tnn
66
-------
There is a wide variation in operating costs found in the
literature end this variation could be attributed to the variation
in supplemental fuel requirements. A wide difference between the
fuel required for raw sludge incineration and digested sludge incin-
eration was reported by Schroepfer4® and the additional heat
requirements for raw sludge and digested sludge were found to be
1.75 and 17.996, respectively. This variation in fuel requirements
accounts for a difference in operating cost of $1.06 per ton.
Multiple hearth furnaces may also be operated as sludge dryers.
The flow of sludge and the rabbling action in multiple hearth fur-
naces are identical to incineration procedures. Modifications to
the basic furnace design include fuel burners at the top and bottom
hearths plus down-drafting of the gases. As the solids moved down-
ward through the furnace, the gases became cooler and the solids
became drier. At the point of exit from the furnace, the gas
temperature was about 325° F and the solids temperature about 100° P.
Incineration of sewage sludge in multiple hearth units is pro-
gressively increasing and, therefore, some improvements in the
design and operation would be desirable even though the present
units operate quite satisfactorily. Recovery and reuse of heat
offers one important potential way of improving the economy of in-
cineration* Recovered heat could be used to condition the sludge
to be incinerated* Other areas of improvements could be the de-
velopment of additional instrumentation to control the combustion
process and the development of some beneficial uses for ash, such
as the use of ash as en aid to sludge conditioning.
Pluidized Bed Furnaces
Fluidized bed technology, developed for catalyst recovery in
oil refining by the Standard Oil Development Company, has been ap-
plied to metallurgical roasting, lime mud reburning, spent sulfite
liquor combustion, the incineration of municipal and industrial
sludges and a host of other industrial applications.
A typical section of the fluid-bed reactor used for combustion
of sewage sludges is shown in Figure 27* The bed material is com-
posed of graded silica sand. When particles are suspended in an
upward-moving stream of gases, the mixture of particles and gases
behaves much like a fluid. Mixing is an important factor in com-
bustion. The air is supplied as near the surface of the fuel as
practical and thoroughly mixed with the combustible matter in order
that the combustion may be completed in a short time and the combus-
tion space used more effectively. Sufficient air is used to keep
the sand in suspension but not to carry it out of the reactor* The
67
-------
SIGHT GLASS
EXHAUST ^ [j~
SAND PEED
FLUIDIZED
SAND
PRESSURE TAP.
PREHEAT BURNER
ACCESS DOORS
THERMOCOUPLE
FIGURE 27
TYPICAL SECTION OF A FLUID BED REACTOR
(DORR-OLIVER, INC.)
68
-------
intense and violent mixing of the solids and gases results in
uniform conditions of temperature, composition end particle size
distribution throughout the bed. Heat transfer between the gases
and the solids is extremely rapid because of the large surface area
available^.
The heat required for raising the sludge to the kindling point
must come from the combustion zone. While standard combustion units
rely on the heat transfer from the hot gases which contain only
16 BTU/cu ft, the expanded bed of the fluid-bed reactor has 16,000
ETU/cu ft. Because of the enormous reservoir of heat in the bed
and a rapid distribution of fuel and sludge throughout the bed,
optimum contact between fuel and oxygen and rapid transfer of heat
is insured. The sand bed retains the organic particles until
they are reduced to mineral ash and the violent motion of the bed
comminutes the ash material, preventing the build up of clinkers.
The resulting fine ash is constantly stripped from the bed by the
up-flowing gases.
The major advantages of using the fluid-bed reactor include:
1. Ideal mixing of the sludge and the combustion air is
achieved.
2. Drying and combustion take place concurrently within
the bed and, therefore, there is no air pollution problem.
3. The reactor has no moving parts.
4. There are no liquid heat-exchange surfaces to scale end
the operating pressure is as low as 2 psig.
5. The unit can be operated four to eight hours a day with
little reheating when restarting because of the fact that
the sand bed serves as a heat reservoir.
6. Need for a mechanical system for ash removal is eliminated
because ash removal from the reactor is accomplished by
the up-flowing combustion gases.
The flow diagram of the Dorr-Oliver Fluo Solids disposal
system is shown in Figure 28.
The major process steps involved ere listed belowt
1. solids preparation
2* solids dewatering
69
-------
EFFLUENT
GRIT
CENTRATE TO THICKENER
RAW _,
SLUDGfi *
GORATOR DORRCLONE
SULZER FEED
DISINTEGRATOR PUMP
MERCO
BOWL
REACTOR PEEI«R
THICKENER
EXHAUS
SCRUBBER
GAS
02 CONTROLLER
T
PRBH EATER
(OPTIONAL)
I
BLO
J^
PS REACTOR
ASH
RETURNED
TO FR DORRCLONE
FIGURE 28
PLOW DIAGRAM OP DORR-OLIVER'S PS DISPOSAL SYSTEM
-------
3. solids combustion
4. stack gas treatment.
The solids preparation starts with the degritting operation.
Degritting is an important step in order to increase the heat value
of the sludge end to protect the equipment from wear end tear. In
the conventional design of grit chambers practiced in the United
States, usually about 95% of the 48 - 65 mesh particles are removed.
To achieve higher degree removal of about 95% of the 200+ mesh
particles, hydrochlones* are installed and they represent only a
fraction of the installed cost of a gravity unit. Sludge thickening
is the next step for solids preparation as it equalizes the sludge
flow and increases the solids concentration.
The second step is sludge dewetering and is preceded by a solids
disintegrator or comminutor. Sludge dewatering is usually achieved
by either a centrifuge or vacuum filter. Dewatering is a very im-
portant step as it improves the economics of combustion by reducing
the amount of water fed to the reactor.
In the combustion step, the dewatered sludge solids are pumped
into the reactor operating at a pressure of about 2 psi end a tempera-
ture of 1400 - 1500° F. Sludge has to be fed only when the bed
temperature has been raised to 1400° F and this is necessary in order
to insure odor control. Because of the high temperature, the sludge
quickly dries end burns end, thus, helps maintain the bed temperature.
The solids ere reduced to inert ash and removed from the fluidized
bed by the upward flowing combustion gases.
Exhaust gases are scrubbed usually in wet scrubbing equipment
using the treatment plant effluent as the scrubbing medium. Ash
solids are separated from the liquid in a Dorrclone and the liquid
returned to the raw waste stream.
Following the laboratory test work, a complete pilot plant was
Installed at the New Rochelle, New York, sewage treatment plent42.
The initiel pilot plant installation consisted of a thickener, a
sludge storage tank, a sludge transfer pump, a progressing cavity
pump to feed the solids to concentration unit, a screening-type
centrifuge, a fluid-bed reactor and a wet-impingement scrubber. The
recoveries in the screening centrifuge were too low and, therefore,
it was replaced by solid-bowl centrifuge which produced 30 to 40%
solids and a recovery of 85 - 95% of the feed solids. The pilot
plant tests showed that 10 to 15% excess air was adequate for complete
*Dorrclone is the registered trade mark of Dorr-Oliver, Inc., for
hydrocyclones.
71
-------
combustion of the carbon and hydrogen. It was also found that smoke
and odor conditions were eliminated above 1100 - 1150° F, compared
with the required 135O - 1400° F for deodorizetion. However, the
combustion capacity of the unit was reduced because of the reduced
sludge combustion rate. Radiation losses were found to be less
than 496 of the input BTU's when the operating temperature was
1600 - 1700° F.
The first commercial combustion system employing the fluid-bed
technique was installed in the City of Lynnwood, Washington42. Raw
primary sewage sludge was combusted in this unit following gravity
thickening and centrifuge dewataring. The underflow from the thicken.
er varied considerably and the average concentration was in the
range of 10 to 12% total solids.
The scum removed from the primary clarifier is pumped to the
thickener along with the settled solids. The floating material
removed from the thickener is concentrated by removing the subne-
tant liquid and, then, by pumping the conentrated scum into the
thickener sludge blanket via sludge withdrawal pipe. Pneumatic
conveying of the low volatile sludge was found to be impractical
and this system was replaced by a stainless steel lift conveyor
and retractable extrusion screw.
The fluid-bed reactor was designed to receive 220 pounds dry
solids per hour at "75% volatile sludge at about 3596 solids. The
reactor has been operated with 2096 excess air or about 360 scfm at
a sludge feed rate of about 210 Ib dried solids/hr. No. 2 oil was
used for daily reheating and as auxiliary fuel because the reactor
has not been operated continuously. The reheat time and the fuel
required for reheating is a function of the duration of shut down
and this is shown in Figure 29 as observed in the Lynnwood plant.
The feed rate control was automatic and based on the oxygen
content of the stack. It has worked very well. The auxiliary fuel
system is controlled by the bed temperature, and fuel can be added
automatically if the temperature falls below a preset minimum* The
system was designed in such a wey that it will shut down automatic-
ally in case of failure of en item of equipment or instrument.
The scrubber, using about 40 gpm final effluent, was able to
cool the exhaust gases to about 160° F in addition to removing the
ash. The total opereting power for the complete disposal system
has been estimated at 237 KWH/ton dry solids.
The annual operating cost per ton of dry solids varies, de-
pending on the amount of solids, from about $25 to $50. The cost
of operation is greatly reduced by the utilization of automatic
72
-------
o
15
REHEAT TIME - MINUTES
30 45
180
14OO
1200 •
co
M 1OOO
tt
I
w
£
g 800
6OO
4OO
2.5 5.0
#2 OIL REQUIRED - GALS.
NOTE
PREHEAT BURNER ONLY
PREHEAT BURNERS ARE OF IGNITION TYPE
THE INJECTION OF OIL IS DONE TO RE-
HEAT THE FURNACE ONLY WHEN THE TEM-
PERATURE IS UP AROUND 1000*F. THIS
IS A STIPULATION OF THE INSURANCE
COMPANIES.
36
SHUT DOWN HOURS
FIGURE 29
FUEL REQUIREMENTS FOR PREHEATING AND REHEATING THE LYNNVDOD PS REACTOR AFTER VARIOUS SHUT DOWN PERIODS
-------
controls to maintain optimum combustion conditions end, thus,
freeing the plant operator for other duties. The results of the
pilot plant at New Rochelie and the commercial installation at
Lynnwood indicate that high speed centrifugation produces primary
sludge concentrations that are economical to burn without heat
recovery or the use of a sludge-drying stage.
The economy of the fluidized-bed system is a function of the
percentage of the excess air. Therefore, control of the excess air
is very necessary and this is achieved by automatic controls.
An air preheater is an optional piece of equipment which will
reduce the auxiliary fuel cost49. The incoming air for fluidizetion
end combustion is heeted to 1000° F by the hot exhaust gases from the
reactor. Even though preheating reduces the fuel cost considerably
the capitel cost of preheet system is about 15% of the fluidized bed
plant's investment. Further, maintenance cost of the preheeting
system is considerable compared to other auxiliary equipment.
Albertson reported42 the following cost date (See Table X) on
combustion based on the performance of the plant at Lynnwood,
Washington, serving a population of 8,000. These cost figures were
extrepolated to 22,000 population end it was assumed that volatile
matter will not increase above 7096. As can be seen in the Table,
power and fuel accounted for 21.7% of the operating cost at the
8,000-population level, and 38.6% at the 22,000-population level.
However, based on the total operating cost, including amortization
of the capital cost, the figures are more attractive for 22,000
population than 8,000 population.
The capital and operating cost for an alternative system--
single-stage digestion system--has been estimated for the same
plant and this is furnished in Table XI.
Sohr50 has reported a total operating cost of $25.32 per ton of
dry solids for the East Cliff Sanitary District Plant, California.
This figure includes the following:
fuel costs $ 2.50 per ton
power cost: $ 4.47 par ton
labor cost: $18.35 par ton
Feed solids concentration and its volatile content are the
other major factors in the economy of combustion. The higher the
solids concentration from vacuum filtration or centrifugation, the
lesser the cost of combustion and this is shown in Figure 30.
The effect of per cent of volatile solids on the cost of auxiliary
74
-------
TABLE X
COMBUSTION COSTS FOR DORR.QLIVBR P/S UNIT
$/Ton Dry Solids
Details 8.000 Population 22.000 Population
Capital Costs
Combustion system, amor-
tized at 496 interest over
25 years, on basis of de- 15.00 15.00
sign tonnage capacity.
Operating Costs
Power @ 10/KWH 2.37 2.37
Fuel © 12
-------
TABLE XI
SLUDGE DIGESTION COST
Details
Capital Costa
Digestion system*
Operating Costs
Power
Maintenance
Labor (3 man hr/day)
Sludge haulage**
$/Ton Dry Solids
8.000 Population 22.000 Populating
7.50
Total
45.98
7.50
4.48
HA
16.00
18.00
1.80
NA
7.00
18.00
34.30
^Capacity for years 1 through 10 only.
**Besed on hauling 50% of the raw solids weight
at 606 TS and a unit cost of 0.9£/gallon.
76
-------
($10.00
W tJ
Q <
M O
fi I
8 •
5^ I*H
Q£ O
* I
•CO- CQ
Z Q
H Q
ri a
grt
H
4 '
3 -
1 •
20
,25
DESIGN CONDITIONS
GAS EXIT TEMP. —1500°P
EXCESS AIR 2056
30
35
% TOTAL SOLIDS IN SLUDGE
@ 7596 VOL. AND 9500 BTU/LB. VOL.
FIGURE 30
EFFECT OP PftBE IDISTURE OH FUEL COST (WO. Z'OIL)
77
-------
fuel for sludge incineration is shown in Figures 15 end 16. The
process steps, degritting, thickening, sludge dewetering, air pre-
heating and close control of excess air, are very essential for
optimizing the process efficiency*
Vacuum filtration always requires chemical conditioning whereas
centrifugation does not* The operating cost for fuel, power and
chemicals for fluidized-bed systems handling raw primary and
secondary sludge is $15 - $18 per ton whereas it is $5 per ton for
those handling primary raw sludge alone'.
The fluidized-bed systems have been operating satisfactorily
and they are very competitive with other techniques, especially
when the operation is continuous and deodorizetion is required.
When deodorization steps are not required, fluidized-bed systems are
more expensive than multiple hearth furnaces.
Plash Drying and/or Incineration
Flash drying is the instantaneous removal of moisture from sol.
ids by introducing them into a hot gas stream. This process was
first applied to the drying of sewage sludge at the Chicago Sanitary
District in 1932 by the Raymond Division of Combustion Engineering,
Inc.51.
The pictorial flow diagram of the C-E Raymond Flash Drying and
Incineration System is shown in Figure 31. This system is composed
of four distinct cycles which can be combined in different arrange-
ments to give the system maximum flexibility to meet specific
requirements. The first cycle is the flash drying cycle, consisting
of the hot gas duct, cage mill, mixer, uptake duct, cyclone, air
lock, dry divider, and vapor fan. The wet filter cake is blended
with some previously dried sludge in a mixer to make it fit for
pneumatic conveyance. The blended sludge and the hot gases from the
furnace at 130O° F meet ahead of the cage mill and flashing of the
water vapor begins* The cage mill mechanically agitates the mixture
of sludge and gas and the drying is virtually complete by the time
the sludge leaves the cage mill. The sludge, at this stage, is dry
at a moisture content of 8 to 10% and the dry sludge is separated
from the spent drying gases in a cyclone. The dried sludge can be
sent either to fertilizer storage or to the furnace for incinera-
tion.
The second cycle is the fuel-burning cycle. Combustion of fuel
is essential to provide heat for drying the sludge and the fuel con-
sists of gas, oil, coal or sewage sludge, itself. Primary combustion
air, provided by the combustion air fan, is preheated and introduced.
78
-------
RELIEF VENT
VENTED AIR
DUST
COLLECTOR
ASH
RELIEF VENT
FERTILIZER
CYCLONE
FERTILIZER
PICKUP &
AIR INLET
COMBUSTION AIR
PREHEATER
DRYER CYCLONE
DRY SLUDGE
CONVEYOR
VAPOR FAN
SLUDGE
/*BURNER
FERTILIZER
STORAGE BIN
AUTOMATIC
' DIVIDER
WET SLUDGE
~- CONVEYOR
. AUXILIA^
DBODORIZI
I PRSHEATER
MIXER &
/-CONDITIONING
HOT GAS DUCT
AGITATION
CAGE MILL
ASH PUMPX !!;[,' r-VJSLUICE WAY
1)1 /
!u /
L--' FIGURE 31
PICTORIAL FLOW DIAGRAM OF THE C-E RAYMOND FLASH DRYING AND INCINERATION SYSTEM
-------
at a high velocity to promote complete sludge combustion. The sludge
ash accumulates in the furnace bottom end is removed periodically by
a hydraulic sluicing system to an ash lagoon or other disposal area.
The third cycle is the effluent gas cycle or induced draft cycle
consisting of the deodorizing and combustion air preheaters, dust
collector, induced draft fan, and stack. Heat recovery is practiced
to improve the economy of operation. The effluent gases then pass
through a dust collector (dry centrifuge or wet scrubber) and the
induced fan discharges the effluent gases through a stack into the
atmosphere.
The fourth cycle is the fertilizer-handling cycle. Some of the
advantages arising out of flexibility in operation are listed below:
1. Sludge can be dried or incinerated to suit the plant's
immediate requirements.
2. The final moisture content can be automatically very closely
controlled since a relatively small amount of sludge ia in
the system at one time.
3. The system can be started and shut down in a short period
of time and no standby fuel is required when sludge is not
being processed.
The lack of fertilizer market for dried sewage has eliminated
the major advantage of this system, the flexibility of drying or
burning. As an incineration unit, the flash drying system has the
major disadvantages of complexity, potential for explosions and po-
tential for air pollution by fine particles. Even though air
pollution controls ere readily applicable to the flash drying and
incineration systems, in comparative situations it is not equal to
other furnece designs.
Cyclonic Reactors
Cyclonic reactors are ideally suited for effective and economical
sludge disposal in the smaller sewage treatment plants because of
their simplicity in installation and flexibility in operation. The
mechanism in cyclonic reactors is that high velocity air, preheated
with combustion gases, from a burner is introduced tangentielly into
the cylindrical combustion chamber. Concentrated sludge solids are
sprayed radially towards the intensely heated walls of the combustion
chamber. This feed is immediately caught up in the rapid cyclonic
flow of hot gases and combustion takes place so rapidly that no
material adheres to the walls. The ash residue is carried off in the
80
-------
cyclonic flow and passes out of the reactor. Basically, the per-
formance of the cyclonic reactor depends on 1) the cyclonic flow
pattern, 2) the dispersion of the feed, and 3) the temperature of
the combustion chamber walls.
Cyclonic reactors have high efficiency operation and this is
achieved by the cyclonic action. Cyclonic reactors, manufactured by
Dorr-Oliver^, are quite compact and occupy a space of about 3.5 ft x
six feet with a height of about 6 ft. The reactor is brick-lined
steel and weighs less than 2 tons. Sargent (Zurn Industries) also
produces a cyclonic incinerator.
An atomizing-type oil burner serves as the primary heat source
and the hot gases from this burner enter the cyclonic reactor at high
temperature and maintain the reactor walls at a high temperature to
prevent sludge from sticking while burning on the walls. The sludge
is fed into the reactor by a progressive cavity pump and the dispersed
sludge particles burn as soon as they hit the wall of the reactor.
These reactors process combined primary plus secondary sludge at
a nominal rate of 100 to 130 pounds of dry solids per hour or 500
to 650 pounds of wet sludge per hour. The detention time for the
sludge within the reactor is less than 10 seconds. The cyclonic re-
actor oxidizes the sludge, producing inert ash, water and carbon
dioxide (CO2). The trace amounts of sulfur in the sludge and in the
fuel oil are oxidized to sulfur dioxide (SOg) but this amount is
negligible after efficient scrubbing. The temperature is kept
above 1400° F so that the organic matter is burned above the odor
producing level.
Wet Oxidation
The wet oxidation process is based on the discovery that any
substance capable of burning can be oxidized in the presence of liquid
water at temperatures between 250° F and 700° F. The process is
uniquely suited to the treatmeat of difficult-to-dewater waste liquors
and sludges where the solids ere but a few percent of the water
streams. In general, given the proper temperature, pressure, reaction
time and sufficient compressed air or oxygen, any degree of oxidation
desired can be accomplished.
This process has been commercialized and patented as "Zimpro"
process. This process has also been known as wet incineration, wet
combustion and wet air oxidation processes. Wet air oxidation does
got require preliminary dewatering or drying when compared to the
conventional flash combustion. Water can be present up to 9996 in
this process whereas in conventional combustion it must be reduced to
81
-------
about 75% or auxiliary fuel must be added. Far from "quenching"
the reaction, water is essential to wet air oxidation.
Another significant difference is the flamelegs oxidation of
the organics at low temperatures of 300° F to 400 F when compared
to 1500° F to 2700° F in the conventional combustion processes. Air
( pollution is controlled because the oxidation takes place in water
V,- at low temperatures and no flyash, dust, sulfur dioxide or nitrogen
> 'i i oxides are formed.
."* ' i
The general flow diagram of Zimpro continuous wet air oxidation
system is shown in Figure 32. In the continuous process, the sludge
is passed through a grinder which reduces any particles greater
then 1/4 in. to about 1/4 in. size. Sludge and air are then pumped
into the system and the mixture is passed through heat exchangers
and brought to the initiating reaction temperature. As oxidation
takes place in the reactor, the temperature increases. The oxidized
products leaving the reactor are cooled in the heat exchangers
against the entering cold sludge end air. The gases ere separated
from the liquid carrying the residual oxidized solids and released
through a pressure control valve to a catalytic oxidation unit for
complete odor control. Where economic conditions make it attrac-
tive , the gases may be expended in power recovery equipment before
being discharged. The oxidized liquid and remaining suspended solid*
are released through a level control valve and the solids may be
separated by settling and drainage in lagoons or beds, or other
methods depending upon project requirements.
For startup, heat is obtained from an outside source, usually a
small steam generator. With high degree oxidations and high fuel
value sludges, no external heat is needed once the process is started,
Whenever the process is not thermally self-sustaining, steam may be
injected continuously to sustain the reaction temperature.
This process may be applied to any type of sewage sludge—raw
or digested, primary or secondary, plus scum and screenings. No
special thickening beyond conventional settling is needed for appli.
cation of the process. The choice of the degree of organic matter
destruction, of COD reduction, depends on economic factors such as
availability and cost of land, size of plant, power rates, etc.
Typical cost relationships for various degrees of COD reduction are
shown in Figure 33. This process can provide a wide renge of oxida-
tion end products depending upon the requirements of the application.
It can be designed for high oxidation to produce a minimum volume
of inert ash or for low oxidation to produce a residue containing
stabilized organic matter with soil conditioning value.
The chemical oxygen demand (COD) of the waste serves as a
valuable parameter for the design of the wet air oxidation process
82
-------
SLUDGE
STORAGE
TANK
SLUDGE
REACTOR
STEAM
GENERATOR
(OPTIONAL)
POWER
RECOVERY
(OPTIONAL)
BIOTREATMENT
(OPTIONAL)
(SETTLING,
FILTRATION OR
CENTPIPUGATION) STERILE
INOFFENSIVE
SOLIDS
COLORLESS
EXHAUST
GAS
SLUDGE
AIR
OXIDIZED SLUDGE
GASES
STEAM
FIGURE 32
FLOW DIAGRAM FOR CONTINUOUS WET AIR OXIDATION
83
-------
to
8
a
Cti
1.6
1.4
1.2
0.8
0.6
!.<•
fc 1.2
8
§ 1.0
M
H
£ °-8
o
ta
M 0.6
H
§
K
O
•
/
20 40 60 80 100 20 40 60 80 loo
% C.O.D. REDUCTION 96C.O.D. REDUCTION
SLUDGE ASSUMED TO BE 5% SOLIDS
FIGURE 33
TYPICAL COST RELATIONSHIPS FOR
VARIOUS DEGREES OP C.O.D. REDUCTION
84
-------
and waste sludges most applicable to the process have COD values
between 25 and 100 grams per liter (25,000 to 100,000 mg/1). "Zimpro"
units are operated either on a continuous or batch operetion basis.
In either cese, the basic principles of oxidation are the same and
the oxidation achieved depends on the temperature, pressure, hold-
ing time in the reactor, and the solids concentration of the sludge
entering the process.
The degree and rate of sludge solids oxidation are significently
influenced by the reactor temperature. With increased tempereture,
B higher degree of oxidation is possible with shorter reaction times.
The relative COD reduction with increase in temperature from 100° C
to 300° C for a number of different sludges es reported by Hurwitz
and co-workers53 is shown in Figure 34.
Oxidation in an aqueous system requires sufficient pressure in
the reactor to prevent weter vaporization because temperatures are
above 212° F. Operating pressures varied from 150 to 3000 psi,
depending on the size of the plant and the degree of oxidation required.
The effect of feed solids concentration on capacity and costs of
wet air oxidation process as observed in the Chicago Sanitary District^*
is shown in Figure 35. As can be seeti from the Figure, the cost can
be reduced considerebly by increasing the feed sludge solids concen-
trations to about 6& and future modifications of the heet exchangers
and pump capacity may reduce the cost further.
The operating costs of the high pressure Zimpro plant, including
labor end maintenance, are about $15.00 per ton of solids processed.
The installed end operating cost^S for high pressure and low pressure
operating units is shown in Figures 36 and 37, respectively. The
building area requirements for the Zimpro units ere shown in Figure 38.
The wet air oxidation process has many advantages in sewage sludge
disposal and they are as follows:
1. Sterile end products low in volume and of special value ae a
soil amendment are produced.
2. Flyash or dust are not produced as the oxidation takes place
in the presence of water. Sulfur dioxide and nitrogen oxides
ere not formed and odor control is assured by use of ges
incineration devices.
3. Wet air oxidation renders sewage sludge easily dewaterable
by filtration or settling without chemical conditioning.
4. This system permits a cleen, sanitary plant without exposure
of operating personnel to obnoxious unsterilized sludge solids.
85
-------
100
K 80
I
25
P 60
. 40
Q
•
O
•
O
20
0
K
ONE HOUR OXI
0 1
, * •
i
If
/y*
-------
18
240
3.0
3-5 4.0 4.5 5.0
PEED SOLIDS 'DNCENTRATION (%)
5.5
14O
6.0
FIGURE 35
EFFECT OF PEED SOLIDS CONCENTRATION ON CAPACITY
AND COSTS OF VET AIR OXIDATION PROCESS AT CHICAGO
87
-------
25OO
PEED:
OPERATION:
P3? FOR MANGE:
6% SOLIDS AND 65% VOLATILE
3OO° P AND 13OO PSI
RBMDVBS 9056 OP INSOLUBLE
VOLATILES
2000
6
8
3
15OO
* iooo • •
5OO ••
8
5OO
1OOO
150O
3OOO
2000 25 OO
CAPACITY, LB/HR
FIGURE 36 ZIHffOt HPO - INSTALLED AND OPERATING COST
3500
4OOO
2
8
Q
W
-co
8
7 §
fe
-------
600
5OO
O
O
O
*4OO
B3OO
200
1,00
FEED: 6% SOLIDS AND 6596 VOLATILE
OPERATION: 30O° F AND 30O PSI
PERFORMANCE: REM3VES 40% OF INSOLUBLE VOLATI
OPERATING COST EXCLUDES LABOR
0
4000
O
5OO
1OOO
FIGURE 37
15OO
2500
3000
2OOO
LB/HR
ZIMPRO LPO-INSTALLED AND OPERATING COST
3500
-------
10,000
8OOO
6000
S
Q
•J
4000
2000
ZIMPRO - LPO
500
1000 1500 2000 2500
CAPACITY, LB/HR
FIGURE 38
ZIMPRO - BUILDING AREA REQUIREMENTS
3000
3500
4000
-------
5. Wet air oxidation has the flexibility of handling any type
of sludge end a wide range of oxidation conditions end end
products is possible.
6. Lend end building area requirements ere minimal end the
Zimpro units can be easily integrated into existing facili-
ties with digesters, vacuum filters or incinerators to
increase the sludge-handling capacity.
There are disadvantages associated with the wet oxidation pro-
cess end the major one is the cost of construction end operation.
This system is generally the most expensive of the processes consid-
ered in the design of sewage treatment plents end the specific cost
depends on the required degree of oxidation which, in turn, depends
on fectors unique to a local situation such as the size of plant,
the availability of land end the cost of power-53.
Odor problems can develop from the off-gases end from lagooning
of the esh-conteining effluent. Though eir pollution caused by the
stack gases can be controlled by catalytic burning at high tempera-
tures, this is an unknown added expense. Another suggested disadvan-
tage of wet combustion systems is the need for high quality supervision
end frequent maintenance due to the use of sophisticated equipment and
controls. A major operational disadvantage is the need to recycle
wet oxidation liquors beck through the westeweter treatment processes.
This may represent a considerable organic load on the system and the
fine ash could plug eir diffusion plates end sludge vacuum filter
media. Therefore, the effluent requires further treatment before
final disposal. It should also be realized here that the wet oxide- .
tion cennot approach the degree of destruction of organic matter as
is achieved in a true incineretion process.
It is possible that wet eir oxidation has the potential of being
the best method for ultimate sludge disposal and further research and
development of this technique could bring down the capital and opera-
ting cost.
Atomized Suspension Technique (AST)
The atomized suspension technique is designed for high tempera-
ture-low pressure thermal processing of westeweter sludges. In-this
system, sludges are reduced to en innocuous ash and bacteria end odors
are destroyed. This system is known under different names such as
spray evaporation, and thermosonic reactor system.
Figure 39 shows the basic components of the system end the unique
features of the atomized sludge incineretion process start with a sonic
atomizer that produces a mist end fine particle spray at the top of
91
-------
RAW SLUDGE
SLUDGE
THICKENER
r
THICKENED —1
SLUDGE
FILTRATE
GRINDER
SONIC
NOZZLE
A-S'T
REACTOR
AIR
PREH EATER
ODOR FREE
GASES
.. STACK
T
— DUST
SEPARATOR
AUXILIARY
FUEL & AIR FEED
REACTOR
PEED PUMP
INERT ASH
FIGURE 39
THERMD SONIC REACTOR SYSTEM FOR TREATMENT AND
DISPOSAL OF RAW SLUDGE
92
-------
the reactor* The process generally includes the following steps:
1. Thickening of the feed sludge to 8% end higher.
2. Grinding the sludge to reduce the particle size to less
than 25 microns.
3. Spraying the sludge into the top of a reactor to form en
"atomized suspension."
4. Drying and burning the sludge in the reactor.
5. Collecting and separating the ash from the hot gases.
The important parameters in the design and performance of
atomized suspension incineration include: sludge type, sludge
solids concentration, amount of excess air used, pressure in the
reactor and sludge particle size. Sewage sludges, in general, are
thermally not self-sufficient unless first dewatered in mechanical
equipment. It has been estimated that a raw sludge having a heating
value of 8,780 BTU per pound of dry solids would have to be thickened
to 14% to be thermally self-sufficient* Fuel consumption has been
related to the raw sewage sludge solids concentration^ as shown in
Figure 40.
Particle size distribution is an important factor to be consid-
ered to prevent sludge stoppages in lines end in the atomizing nozzles,
and to improve the combustion* With the increase in the particle
volume, the rates of evaporation end heat transfer in the reactor are
increased directly proportional to particle volume. The operating
pressure is kept under 30 inches of water to prevent leakage from
the equipment and to insure no inhibition of evaporation end gasifi-
cation.
This sytem has the following advantages: versatility in sludge
handled, small space requirement, rapid conversion of raw sludge to
innocuous ash, steam and CO^, end no nuisance conditions. This
system ia very new and it has been estimated that the cost will be
somewhat higher than conventional incineration processes due to
maintenance and the need for supplemental fuel oil or gas* Capital
costs do not also appear to be less than for other incineration tech-
niques. The AST process has an advantage over the Zimpro process in
that the operating pressures are much lower. The possibility of
incinerating a dilute sludge, thereby eliminating costly dewetering
steps | is very attractive.
93
-------
200 .
150 .
I
(X
w
2
100 .
4 6 8 10 12 14
SLUDGE CONCENTRATION TO AST UNIT - % TOTAL SOLIDS
SLUDGE FLOW = 4 GPM
FIGURE Jj.0
FUEL CONSWPTION AS A FUNCTION OF SLUDGE CONCENTRATION
94
-------
CONSIDERATIONS IN INCINERATOR DESIGN
Plant Size and Capacity
Determining the actual continuous burning capacity of a pro-
posed plant is vital to its ability to satisfy community needs - both
present and future. The usual method of applying a nominal rating
based on theoretical capacity can be misleading and may provide a
plant too small and inflexible for those needs* It is considered
good design to plan for multiple units in the event of operational
failure of one unit. While it costs more to build two plants than a
single plant of the same total capacity, the benefit derived from
having separate units may outweigh the cost, particularly when rating
is based on a continuous burning operation. By borrowing from the
power industry the concept of "firm power"--that capacity remaining
for operation when the largest of multiple units is off the line--
the incinerator plant can be designed with an assured actual continu-
ous burning capacity so that there will always be enough available
capacity to prevent sludge accumulation.
The required incinerator size as a function of population served
end degree of treatment is shown in Table XII.
Aesthetics and Location of Plant
The quality of aesthetic design of the plant is a vital factor
in community acceptance. Incinerator plants can be housed in attrac-
tive buildings with pleasing proportions, colorful or textured facades
and surrounding landscaping. The only element that need distinguish
an incinerator plant is the chimney, and the height of the stack can
be visually minimized. Any unsightly activities or areas such as
storage pits can be screened from view by trees and shrubbery, by
placing them within buildings or by placing them in the lower areas
of the natural configuration of the landscape.
Good design, both outside end inside the plant, can have the
added advantage of creating the kind of environment that will attract
more competent employees to the municipal incinerator. Currently,
employment in an incinerator plant is not considered a desirable
occupation and more desirable competing fields get first choice of
skilled or technically trained people. To attract those with ade-
quate training to handle increasingly sophisticated equipment and
operations, the entire concept of working in an incinerator needs
upgrading in the public mind.
The problems of heat, odor and insects could be avoided by
separating the incinerator from other parts of the plant and air
95
-------
TABLE XII
REQUIRED INCINERATOR SIZE AS A FUNCTION
OF POPULATION SERVED AND DEGREE
Population
250
750
1,750
3,750
7,500
17 ,500
37 ,500
75 ,000
175,000
250,000
Lb/hour Dry Solids
Primary
9
28
66
141
283
660
1,410
2,810
6,260
9,400
OF TREATMENT
(8-hour operation)
Secondary
14*
42*
99*
210*
420
985
2,110
4,220
9,850
14,100
*Small cities, probably under 5,OOO population, will not burn
sludge 7 days a week under normal circumstances. Therefore,
the size of unit required for design populations under 5,000
will be between 1.5 and 2 times the sludge quantity mentioned
here.
96
-------
conditioning the offices; and wherever possible, inclusion of showers
and lockers for employees so that work clothes can be left at the
plant is a greet boon to those who must work in the sludge-handling
areas. It should be noted thet none of these features add materially
to the cost of a modern plant but they can have a considerable effect
on the efficiency of operation.
Careful operation and maintenance of the plant and surrounding
areas are prerequisites to continued aesthetic quality, but any
additional costs incurred in these areas will be more than offset by
the maintenance of the land values not only of the site itself but
in the vicinity. More modern plants burn better end operate better
end the improvement cannot enitrely be credited to technological
improvement in the incineration process.
Economic Factors
The lack of clarity in the economic picture presents a consider-
able challenge to the designer. For against comparatively small
return in costs, the designer must weigh the costs of installing and
maintaining equipment over the plant's useful period of life. It must
also be decided how far to go in selecting automated equipment. Also,
careful consideration must be given not only to initial costs of such
sophisticated installations but to the costs involved in the use of
relatively untested equipment by personnel oftentimes untrained to
the degree necessary to handle the complexity of the job they are
called upon to do^S. Bach plant currently designed must be given
sufficient flexibility to encompass not only the community's exist-
ing needs and to take into consideration already existing facilities,
but also the community's future need for additional facilities.
Air Pollution Standards end Control
Incineration offers the opportunity to reduce sludge to a sterile
land fill and remove offensive odors, but it can be a significant con-
tributor to the air pollution problem in an urban community. The
quantity and size of particulete emission leaving the furnece of en
incinerator varies widely, depending on such factors as the sludge
being fired, operating procedures and completeness of combustion.
Complete combustion to produce the principal end products of
C02» H20 and 803 is costly but too much of SOg emission is not per-
missible due to its toxic end corrosive nature. Incomplete combustion
can be disastrous because the intermediate products formed, such as
hydrocarbon and carbon monoxide, are more objectionable. Smoke and
gases contribute to overall air pollution through reduction in
visibility and through their ability to enter into smog-forming
97
-------
photochemical reactions in the air.
The emission standards for particulate matter very from state
to state* Previous practice usually attempted to control these emis-
sions to 0.85 Ib of flyesh per 1000 Ib of flue gas, adjusted to
excess air (1 Ib per 106 BTU) , as suggested in the "1949 ASME Example
for a Smoke Regulation Ordinance." The ASME published a new suggested
regulation in 1966 entitled "Recommended Guide for the Control of Dust
Emission - Combustion for Indirect Heat Exchangers." It seems reason-
able to assume that this document will receive the same widespread
acceptance that the earlier ordinance did. Thus, future codes can be
expected to lower the allowable emission from 1.0 to 0.80 Ib of flyesh
per million BTU, or to 0.68 Ib of dust per 1000 Ib of gas corrected
to 50% excess air. More congested metropolitan areas or areas with
adverse topography, such as the Los Angeles Basin, may adopt the re-
cent regulations that limit emissions to 0.10 - 0.20 grains per
standard cubic foot at 5096 excess air (O.22 - 0.44 Ib of dust per
million BTU fired) for incinerator capacities of 200 Ib per hour and
larger. The lower standards have been adopted not only in California
but in many parts of the Northeast and Midwest. It is expected to
become the standard.
Most incinerator manufacturers advertise to limit the particulate
matter to 0.20 to 0.28 Ib per 1000 Ib of stack gas at 5O% excess air.
However, in the event of increased air pollution standards, electro-
static precipitation or high efficiency scrubbing may be required.
Such systems are expensive and, hence, the less excess air used, the
lower the cost will be for electrostatic equipment.
It is also important to observe that suggested criteria for
particulate matter in the ambient air as developed by the Department
of Health, Education and Welfare and adopted by various jurisdictions
may affect these discharge limits.
The stack gases must be cooled so that the plume produced will
dissipate upon entry into the atmosphere. Temperatures of up to 160° p
have proven to be quite satisfactory. Care must be taken to prevent
plume condensation which would violate equivalent opacity regulations
even though the plume may be white in color.
Particulate matter can be effectively controlled by centrifugal
dust collectors or wet scrubbers57 . Centrifugal collectors remove
75 to 8096 of the particles and are suitable for exhaust gas tempera-
tures of 650 - 700° F. Water scrubbers are at least equally effective;
they are less sensitive to loadings and gas temperatures and they
collect the condensable portion of The exit gases. In general, the
nature of the emitted particulate matter from sludge incinerators does
not lend itself to centrifugal collection and most systems utilize wet
98
-------
scrubbers of a variety of types including venturi, baffle plate,
packed tower and impingement models. These scrubbers have the added
advantage of absorbing significant amounts of gases including sulfur
oxides and odorous organics.
Air pollution control has assumed enormous importance in ell
waste management fields due to public awareness and the expansion of
urban areas. Generally, environmental problems are interrelated and
the solution of a water pollution and land use problem may cause an
odorous air pollution problem in poorly designed and/or operated
sludge incinerators. Odors generally emanate from raw sludge thicken-
ing or storage tanks, vacuum filtration units, sludge incinerators
and dryers*
Odors can be eliminated at their source or can be prevented from
reaching the atmosphere by control. The basic requirements for pre-
venting odor are good plant design and operation. Septicity of sludge
can be prevented by providing adequate sludge hoppers and flexibility
in pumping schedules. Odors, when emitted, can be controlled by any
one of the following five methods with certain limitations:
1. Combustion
2. Chemical oxidation
3. Adsorption
4. Dilution
5. Masking
Chemical oxidation can be echieved in two ways^Qj i) oxidizing
the gases in a dry environment, or 2) scrubbing the gases with a
liquid-containing oxidant. Ozone is commonly used for odor control
as it is relatively inexpensive. Its effectiveness is open to con-
siderable question. Chemical oxidants such as chlorine (C12) ,
hydrogen peroxide (^03) Bnd hypochlorite (HoCl) have been used in
absorption processes to control odors. Their efficiency depends upon
the chemistry of the situation. Packed scrubbing towers are usually
used for gradual oxidation of the gases.
Odor control can be achieved through the use of adsorption
towers packed with activated carbon. The technique is relatively
expensive and usually limited to cases where organics may be
recovered.
Tell stacks are frequently uaed (especially with chemical or
thermal combustion) to dilute odorous gases and particulates with the
99
-------
outside atmosphere and reduce their concentration but this route has
not been a satisfactory odor control technique due to the existence
of downwind problems. Normal diffusion equations have not been satis-
factory in predicting the necessary dilution to prevent odor detection!
Frequently, odors have been observed 10 - 50 miles downwind of an
incinera tor.
Use of masking agents (often quaternary amines) is not a very
satisfactory odor control measure and, therefore, should be limited to
temporary emergency situations. Sometimes odor masking may produce
intolerable combinations of sludge odors plus masking agent odors; and
masking prevents recognition of a serious community problem. However,
masking agents are perhaps the most frequently utilized for masking
the odor.
While the above methods have some usefulness in the control of
odors, the control of odors from sludge incinerators is generally
limited to two techniques. The main and most successful approach is
to incorporate a means of ensuring that ell gases arising from the
system are raised to and held at a sufficiently high temperature and
for a sufficient time period to ensure complete oxidation of all or-
ganics. It is generally considered that if the gases are held at
1400° F, oxidation will occur in a matter of seconds. Thus, if the
gases ere held at 1400° F for the usual gas phase detention time (10 -
60 seconds), no odors should be present in the gas exhaust. However,
through poor design, operation and/or maintenance, these conditions
are frequently not achieved end a serious odor problem can and does
arise. As sewage treatment plants become surrounded by valuable real
estate (homes, etc.) and as society becomes more aware of olfactory
insults, pressures will increase to ensure non-odorous operation.
A less frequently utilized control technique is to take the off
gases to a secondary incineration chamber. This may be of the flame
type in which the gases ere passed through a natural gas or oil
(usually the former) flame to ensure complete organic destruction.
As an alternative, the gases may be passed over a catalytic system
where the same oxidation takes place but at a lower temperature since
the catalytic surface lowers the oxidation energy "hump." In both
cases, the chemistry is identicel to that described relative to in-
cineration and additional air may be added to ensure complete
combustion.
The detection of odors is generally dependent upon complaints
and control is usually by means of the nuisance clause of most air
pollution codes (or Common Law). However, many communities have
developed trained odor panels to detect odors from a variety of
sources on a regular basis.
In contrast, the emission of particulate matter from incinera-
tors is controlled by many air pollution regulatory agencies by the
100
-------
application of precise technically measurable (See this section
above) limits. Techniques to sample incinerator effluents for
particulate matter are well established although considerable con-
troversy exists concerning the methodology. The variation in
operational conditions cause additional problems to the tester.
The variation of control approach between odors and particu-
late matter means that incinerator operators must have an unusually
broad outlook when dealing with the general public and regulatory
agencies relative to air pollution regulations.
Safety Standards
It is advantageous to control the entire system from a single
instrument panel and to protect against oil burner flame failure,
high and low exhaust temperatures, blower failure and high and low
oil burner inlet temperature* An alarm should ring and shut down
the operation in all these failure modes. The complete instrument
end control package allows the system to be run semi-automatically
after startup. However, periodic checking on sludge level in the
hopper is required to ensure constant feed to the incinerator.
Explosions that damage equipment may occur from the combustion
of grease. For this reason, separate feed openings in the furnace
are to be provided for grease and screenings. In cases where a
unit is used for the incineration of grease and skimmings only, a
parallel flow of feed solids and hot gases is desirable.
101
-------
OPERATIONAL ASPECTS
Dust Collection and Ash Handling
Following the deodorization step, the perticulete matter is
removed from the cooled gases before they pass up the stack end
into the atmosphere. If a centrifugal dust collector is used, the
cooled gases are drawn through the dust collector by the induced
draft fan. The flyesh settles out by the centrifugal action and is
discharged automatically into the furnace bottom. The deodorized
and ash-free gases, along with the moisture from the drying opera-
tion, are vented to the atmosphere without nuisance.
Ash assumes a fine, granular form resembling sand, free from
clinker and unburned organic matter. Any convenient method of dis-
posal may be used but a preferred method involves discharging the
total ash into a water-filled sump and pumping the mixture to land
fill.
A sewage sludge combustion unit does not dispose of the solids
completely. It produces an end product that requires further
handling but by virtue of greater solids destruction, the handling
procedures are greatly simplified.
Ash handling can be performed by either "dry11 or wet methods.
Dry handling is never absolutely dry since water sprays are utilized
to prevent dust from scattering. Such systems are generally used
at smaller installations (less than 30,000 populations).
The problem is quite different if wet scrubbers are used. The
ash from the underflow of the scrubbing unit can be handled by
various means. Among these are the following:
1. Settling Basins
Ash has a high settling rate and clarification tests on a
typical ash indicate that 99.9% plus will settle in less
than two hours. The quantity of ash removed from the
scrubber for the purpose of sizing the settling basin need
not be uniform.
2. Mechanical Concentration
If settling basins are not practical for a given application,
the ash can be concentrated in hydrocyclones. Tests have
indicated that underflow concentrations in the range of
20 - 25% total solids can be expected. If further dewetering
is desired, the underflow of the hydrocyclone is fed to a
102
-------
rake classifier or small settling tanks where concentra-
tions of 70 - 80% total solids can be achieved.
Flexibility and Controls
Sludge incineration systems should be designed in such a way
that they could have utmost flexibility in operation. Some of the
advantages accruing from this flexibility are as follows:
1 1. Sludge can be dried or incinerated to suit the plant's
immediate requirements.
2. The final moisture can be very closely controlled during
the sludge-drying operation.
3. The system can be started end shut down in a short period
of time and no standby fuel is required when sludge is not
being processed.
While designing facilities for sludge incineration, flexibility
should be built in so that increasing demands due to the population
growth can be met.
A full set of instruments and controls must be provided to the
operator in order to ensure operation at maximum efficiency at all
times. Recording instruments are generally preferred so that a
permanent record may be kept. Devices such as deodorizing air pre-
heat er must be protected against overheating by an automatic air
damper. When the gas temperature entering the preheater exceeds a
safe figure, this damper automatically opens and permits room air
to enter and reduce the temperature.
Production of sludge of low and uniform moisture is important
in either drying or incineration of sludge and this could be achieved
only by automatic controls. Because of the variations in the mois-
ture and heat content of the incoming wet filter cake, continuous
attention to auxiliary fuel burners is required with manual controls
and this could be avoided by automatic controls.
103
-------
CAPITAL AND OPERATING COSTS
Sludge incineration is generally more expensive than other
sludge dispose! methods. The approximate pricing information on
incineration systems is presented in Table XIII. The capital out-
lay required for incinerator systems by population group is
presented in Table XIV. The capital cost of incineration systems
depends on the type of incinerator end whether deodorization,
dust collection and disposal are included. There are many factors
that affect the cost of sludge incineration end the major ones in-
clude :
1. Size and design of incinerator
2. Nature «f waste sludge
3. Amount and type of chemicals used for sludge conditioning
4. Extent of standby facilities
5. Cost of utilities (fuel, water, power)
6. Air pollution control requirements
The operating costs have been reported to have wide variations
and these variations are partly due to the fluctuations in supple-
mental fuel requirements. A survey on supplemental fuel4® showed a
variation from less then 1% to 35% of the heat value supplied by the
sludge cake itself. There was a wide difference observed between
the fuel required for raw sludge incineration and for digested
sludge incineration. In general, raw sludge units required an
average of 1.7596 additional heat in the form of fuel, while digested
sludge required 17.996 and this variation means a difference in
operating costs of $1.00 per ton*8.
104
-------
TABLE XIII
PRICING INFORMATION ON INCINERATION SYSTEMS
Type
Fluid Solid
Multiple
Hearth
Cyclonic
Reactor
Wet
Oxidation
Flash Dryer
•nd Incin-
erator
Manufacturer
Dorr-Oliver
Nichols
Bartlett-Snow-Pecific
Dorr-Oliver
Sterling Drug
Zinpro Division
Combustion Engineering
Raymond Division
Size
Ub/hr)
200
400
1,000
2,000
500
2,000
4,000
6,000
100
200
1,000
470
400
600
1,000
2,000
5,000
1968 Dollars
180,000
300,000
550,000
825,000
300,000
550,000
700,000
850,000
85,000
120,000
300,000
284,000
300,000*
330,000
375 ,000
460,000
700,000
(Oxidation
unknown)
(High oxi-
dation)
Cycle-Burner Sergent - Zurn
130
70,000
*Pricea included drying but not dewetering equipment.
Delete 20% for special equipment and add 25% for
dewatering•
1. Obtained from the manufacturer's estimating price and bids,
105
-------
TABLE XIV
CAPITAL OUTLAY REQUIRED FOR INCINERATOR
SYSTEMS BY POPULATION GROUP *
Averege Incinerator System
Capital Cost -
Population (1968 Dollars)
250 Not applicable
750 50,OOO1
1,750 70,0001
3,750 120,OOO1
7,500 130,OOO2
17,500 345,OOO2
1Based on one shift operation
2Based on two shift operation
Derived from a composite of estimates and bids.
106
-------
INCINERATION OP MATERIALS OTHER THAN MUNICIPAL SLUDGES
The general opinion obtained from the consulting engineer
questionnaire (See Section on "Attitudes of the Consulting Engineer")
and other data is that waste materials other than sewage sludge may
be disposed of through incineration. The three classes of non-sludge
materials which could be incinerated in the same type of burner,
either in place of or with pumpable sewage sludge, aret
1. Screenings - the materials removed at the sewage treatment
plant headworks by screens.
2. Waste oils, greases and other skimmed material.
3. Industrial waste solids - waste materials from the fruit
and vegetable processing industry.
In larger plants where digestion is still employed, a separate
incinerator for skimmings and screenings may be desirable. However,
it is not likely that smaller plants with two-stage digestion will
purchase a second solids disposal system where incineration of
screening is practical. A separate macerator and pump are needed
to move the screenings to the incinerators. It is generally de-
sirable to mix the ground screenings with the sludge prior to
centrifugation.
While handling skimmings in incinerators, special care should
be taken to avoid slugging of high BTU material. This problem is
solved by mixing the skimmings directly into the thickener sludge
blanket. This has been patented by Dorr-Oliver. This system, however,
has a major disadvantage in that if the sludge is held long or if the
temperature of the sewage is high under normal detention conditions,
substantial breakdown and solubilizetion of the greases occur which
results in the loss of the high caloric value fuel. A satisfactory
solution is to bleed the scum into the system on a continuous basis
from a mixed holding tank and the system would be operated when the
sludge incinerator is functioning. The grease edds 10 to 15% to the
gross BTU value of the sludge and is very desirable from this
standpoint.
Other waste oils constitute a potential market of great magni-
tude. The total waste oil disposal requirements of the gasoline
stations in the United States is 350 x 106 gallons per year. The
total number of stations is 210,000 and the average annual disposal
burden is 140 gallons per month per station. This oil is now being
disposed of in a highly haphazard manner of much concern to both
federal and state officials. For example, in the Westport-Norwalk,
Connecticut, area, there are in excess of 300 gasoline stations}
107
-------
about half of these ere served by sewers, the remainder by other
means of waste disposal. Assuming these are of average size, 32,000
gallons of waste oil a month needs to be handled.
The waste oil constitutes an enormous pollution problem and it
is not possible at this point in time to identify a customer. It is
also not to be a municipal chore to pick up waste oils. However, it
is clear that the 20 to 25 cent differential between buying BTU's at
roughly 15 cents per gallon and picking up the oil at 5 to 10 cents
per gallon is sufficient to make it a satisfactory adjunct to the
municipal incineration of sludge.
It is important to note that in the President's special report39,
"A Report on Pollution of the Nation's Waters by Oil and Other
Hazardous Substances," the proposed action program indicates a com-
plete lack of understanding of the problem of assigning or finding a
responsible party or legal entity to deal with waste oils from
service stations* Therefore, it would appear that little can be ex-
pected in terms of federal pressures in the near future.
A thorough analysis of the waste sludges from the fruit and
vegetable processing industries indicates that while the problem is
large, there appears to b* insufficient fiscal pressure to make
incineration attractive* The National Canners Association states0^*
that waste solids cannot be incinerated at costs comparable to their
present method of disposal* Waste disposal from the food industry,
which amounts to 500,000 to 700,000 tons of wet waste during the four
month campaign in the 13 county delta area surrounding the California
Bay area, is currently barged to sea. This waste cannot be dried much
further than 12 to 18% solids, depending on which fruit or vegetable
has been processed. The operating costs of incineration would ex-
ceed $30 - $40 per ton which is more than $25--the industry's total
investment for barge disposal at this time.
Another area of interest is the marine waste disposal. The
disposal of concentrated sewage from the containment vessels at the
marina pierhead by incineration will be a substantial service
business which can be correlated with fueling and dewatering* Water
from dockside could be employed for scrubbing with direct ash dis-
charge or cyclonic ash separation.
A similar business is possible in connection with the disposal
of septic tank or pit toilet wastes at camps of the United States
Park Service and the United States Forest Service. The device for
such service would have to be mobile but could be operated in many
instances with dry rather than wet stack quality control devices.
108
-------
DISPOSAL OP REFUSE WITH SEWAGE SLUDGE
A number of municipalities are reaching the capacity of their
land fill areas to receive their refuse. The value of lend surround-
ing municipalities is increasing at a rapid rate for residential and
industrial development making new land areas very expensive and often
unattainable. Furthermore, people are becoming sensitive to the exis-
tence of a dumping or land fill area in close proximity to the
residential areas. These factors are giving impetus to the ever
growing popularity of the mixed refuse incinerator for the incinera-
tion of sewage sludge and refuse.
Whenever the location of the sewage treatment plant will permit
reasonable hauls, the installation of a mixed refuse incinerator at
the sewage treatment plant site will permit disposal of the municipal
garbage, refuse and sewege sludge at the same plant site. Such an
installation permits drying or incineration of the sewage sludge with
no auxiliary fuel requirements due to the heat in the waste gases
from the burning of the mixed refuse.
Heat for drying the sewage sludge filter cake is supplied by
the mixed refuse incinerator and the flash dried sludge may be mar-
keted as fertilizer or incinerated at will. The dual disposal of
mixed refuse and sewege sludge at the same plant site affects
economics in both first cost and operating costs of the disposal
equipment. For smaller communities, this system provides modern
disposal facilities whereas the first cost or operational cost of
the separate disposal facilities will be prohibitive. The following
cities have this system in use:
Watervliet, New York
Stamford, Connecticut
Waterbury, Connecticut
Fond du Lac, Wisconsin
Bloomsburg, Pennsylvania
Louisville, Kentucky
Neenah-Menesha, Wisconsin
The success of burning the sludge with refuse depends on the
type of sludge, hauling cost for refuse, etc. However, in all
cases, it is important to give consideration to combined use and
sludge incineration. This system may be particularly useful in small
cities where hauling costs could be reasonable. For larger cities,
centrally located refuse collection and sewage treatment could make
thia system very conducive,. Improved mechanical design of incinera-
tors and development of inexpensive refuse collection technique
would encourage combined incineration.
109
-------
EFFBCT OF INCINERATION ON OTHER RESOURCE MANAGEMENT
The incineration of sewage sludge is usually cited as a prime
example of the interrelationship between the management of land, air
and water resources* It is unfortunate that many sludge disposal
systems (starting at the clarifier) have not, during their concept,
been designed with an eye towards total resource management. Among
the factors which need to be considered in any evaluation of a sludge
disposal system with respect to total resource management ares
1* Effect of the return of recovered liquors from sludge
dewatering steps to the westewater treatment system.
2. Availability of land for solids disposal.
3. Impact of solids disposal upon lend use and ground
and surface waters.
4. Impact of treatment facility upon adjacent land
use and value.
5. Extent of odor problems.
6. Effect of sludge disposal upon ambient air quality.
7. Interrelation between sludge disposal and the potential
disposal of other community solid and high-caloric
liquid wastes.
Only when the total resource management picture is seen can
the municipality be satisfied that an optimum solution be found to
the aludge disposal problem.
110
-------
ATTITUDES OP STATE AGENCIES TOWARD INCINERATION
As a result of the clean water legislation in the United States
Congress, the state regulatory agencies have increased the tempo of
their regulatory activities. As of now, all fifty states have had
their water quality criteria approved by the United States Department
of the Interior, at least in part. This implies obligatory secondary
treatment and in most states the deadline is prior to 1973.
The results available to Resource Engineering Associates from
a survey61 conducted on sludge incineration earlier to the initia-
tion of this project are analyzed and presented in the following
paragraphs.
About half the states in the United States contain plants which
practice some form of sludge incineration* The bulk of these serve
population groups over 10,000 in sice. However, six states—Nevada,
Missouri, New Jersey, Colorado, Washington and Cslifornia—have
incinerators serving population groups under 10,000 people. The
states generally not employing incinerators are located in the deep
South, Southwest, Rocky Mountain area, upper New England and the
Great Plains. Incineration appears to go with urban thinking and
planning and not with the rural community.
States reporting no incineration are the following!
Alaska1 New Hampshire
Arizona New Mexico
Delaware North Dakota
Florida Oklahoma
Georgia Oregon
Idaho2 South Carolina
Iowa South Dakota
Maine Utah
Mississippi Vermont
Montana Wyoming
1Abandoned one
2Boise is installing twin type C/R Reactors (Dorr-Oliver)
111
-------
Most states indicated that they could foresee an increase in
the use of incineration. Fifty-four per cent saw an increase, 29% saw
no increase and 17% had no opinion.
Prom an operational viewpoint, there were comments from only
fourteen states. The most common concerns weret 1) ash disposal,
2) thickening and dewatering, and 3) odors; although others commented
on 4) capacity, 5) temperature control (related normally to grease
incineration), 6) sludge conveyance problems, and 7) smoke. Only
in Texas did 8) vector control also appear to be a problem.
Most states indicated that they felt no reluctance to approve
incineration facilities. Seventy-seven per cent indicated no reluc-
tance, 1756 some reluctance and 6% had no opinion.
Very little incinerator type preference was reported from the
states* Those reporting were split between multiple hearth and fluid
bed reactors at 6 - 10. Only one state, Florida, appeared to favor
cyclonic-type reactors.
In response to the question concerning sice and applicability,
the states responded in the following ratioss
Greatest use over 50,000 persons 10
Greetest use from 10,000 - 50,000 5
Greatest use from 5,000 - 10,000 1
Greatest use from 2,000 - 5,000 0
Grestest use from 1,000 - 2,000 0
Orestest use from 0 - 1,000 0
Generally speaking, the states responded to the questions con-
cerning sludge disposal problems by saying at a ratio of 4 to 1 that
they had no major problem disposing of sludge in their areas. Those
states indicating a major problem weret
Georgia New Jeraey
Iowa Oregon
Maryland Rhode Island
Nevada
112
-------
Thoae states indicating some acute, but localized problems
were:
Idaho New Jersey
Kentucky Rhode Island
Maryland South Dakota
Massachusetts Tennessee
Nevada Washington
It should be pointed out that often cost is not considered a
major problem by state officials. Therefore, there probably are
more true problem areas than are reflected in that answer.
113
-------
ATTITUDES OF THE CONSULTING ENGINEER TOWARD INCINERATION
The following paragraphs are presented based on a survey^l con-
ducted in the latter part of 1966 on the attitudes of consulting
engineers on sludge incineration.
As might be expected, only a portion of the firms questioned
have constructed incinerators; about 35% indicated that they have
built incinerators. Nonetheless, a large majority, 65%, believe in-
cinerator use is increasing, only 6% believe it is decreasing and
29% had no opinion* Almost all of the units constructed, over 9096,
were above 500 Ib/hr, as might be expected.
The bulk of the engineers, or 8096, reported that the operators
prefer to burn sludge less than 8 hours a day. The remaining few
that answered were evenly scattered up to 24 hours of burning.
Roughly half of the consultants had no type preference. In the
larger size • over 15,000 population - 25% preferred the multiple
hearth, 25% the fluid bed, Q% the travelling grate and 42% had no
opinion. Under 15,000 population, 24% preferred the fluid bed, 12%
multiple hearth, 8% cyclonic type, 2% travelling grate, and 54% had
no opinion.
When asked if two-stage digestion would disappear, 39% said yes,
27% said no, end 34% had no opinion.
A good response was obtained to the question dealing with the
factors mitigating against incineration. The response was as follows:
High operating cost 39%
High capital cost 35%
Thickening and dewatering problems 26%
Air pollution 22%
Required operator expertise 18%
General increase of operating problems 12%
Safety 0
Forty-three per cent favor packaged system, 16% built-up, and
41% had no opinion.
Only four consultants have experienced problems in getting
incineration approval. These were located in California and Illinois.
114
-------
Three of the four ere in sensitive air pollution areas.
Forty-three per cent indicated that some of their clients have
a history of sludge disposal problems. Sixteen per cent indicated
no problem end 41% did not respond.
The consultants felt almost universally that greases, oils and
screenings could best be disposed of by incineration. A substantial
fraction also favor disposal of organic industrial wastes by this
technique.
In summary, the overall attitude appears to be one of acceptance
and even eagerness to employ incineration. Certainly there is little
evidence of genuine emotional bias against this mode of sludge dis-
posal.
115
-------
THE SLUDGE INCINERATION MARKET - CURRENT STATUS
The incineration of sewage sludge has been practiced in this
country since 1934. The vast bulk of the incinerators that have
been installed have been of the multiple hearth or flash drying
and incineration types. However, since the beginning of this
decade, two new incinerator concepts have cut into the commercial
lead of the other manufacturers; these are the wet oxidation and
fluid-bed reactors. The major incinerator manufacturers are
shown in Table XV.
The time of construction and releted pertinent information
concerning these incinerators are shown in Tables XVI through XX.
The two manufacturers of systems incinerating pumpeble sludges
in small sizes are both employing cyclonic-type reactors. On the
cost basis, multiple hearth, flesh drying and fluid-bed reactors
are very expensive in the smeller sizes.
116
-------
TABLE XV
MAJOR MANUFACTURERS OF SLUDGE INCINERATORS
Manufacturer
Nichols
Ba rt 1 e tt - Snow-
Pacific
Dorr-Oliver
Dorr-Oliver
Type
Constructed
Multiple
hearth
Multiple
hearth
Fluid-bed
Cyclonic-
reactor
Year of
Entry
1934
1963
1962
1966
System
No
No
Yes
Yes
Other Remarks
81 Constructed
24 Constructed
38 Constructed
1 Constructed
4 Under construction
Raymond
(Combustion
Engineering)
Sterling Drug
Zimpro Divi-
sion
Open furnace 1935 Optional
with drying
column
Wet oxidation 1961 Yes
50 Constructed
19 Drying only
17 Constructed
1 Under construction
117
-------
TABLE XVI
MULTIPLE HEARTH INSTALLATIONS
BY MAJOR UNITED STATES MANUFACTURERS
Capacity
Year Ib/hr Dry Solids
1934
1936
1936
1937
1938
1938
1938
1938
1939
1939
1939
1939
1939
1941
1945
1948
1948
1949
1949
1949
1949
1949
1950
1950
1952
1952
1952
1952
1952
1953
1953
1954
1954
1954
1955
1955
1956
1956
1956
1957
1958
1958
1959
1959
1,000
500
1,000
2,000
11,000
7,000
500
1,000
1,600
23,000
700
700
900
900
1,600
1,300
7,000
5,000
1,000
3,900
2,000
1,600
2,000
4,000
1,000
1,500
1,OOO
500
1,000
1,000
1,000
800
400
3,000
14,000
12,000
3,000
2,000
1,600
6,000
800
300
2,000
1,400
Dewatering Technique
if Known
Remarks
118
-------
TABLE XVI (Continued)
Capacity Dewetering Technique
Year Ib/hr Dry Solids if Known Remarks
1960 1,500
I960 800
1961 1,500
1961 3,000
1961 500
1962 6,100
1962 4,000
1962 2,000
1962 2,700
1962 1,300
1962 6,600
1962 6,600
1963 1,400
1963 400
1963 6,000
1963 11,500
1963 800
1963 5,000
1963 5,400 Vacuum Filter
1963 600 Vacuum Filter
1963 900 None Grease and skimmings
only
1964 1OO
1964 150
1964 5,000
1964 700
1964 5,000
1964 3,600 Centrifuge
1964 2,030 Vacuum Filter
1964 1,700 Vacuum Filter
1965 15,000
1965 25,000
1965 400
1965 2,000
1965 3,000
1965 7,150 Vacuum Filter
1965 500 Vacuum Filter
1965 6,600 Vacuum Filter
1966 7,000
1966 2,200 Vacuum Filter
1966 4,000 Vacuum Filter
1966 25 Vacuum Filter Pilot plant
(Centrifuge)
1966 1,750 Vacuum Filter
1967 3,000
1967 3,000
1967 2,600
1967 8,000
1967 300
1967 1,100 Centrifuge
119
-------
TABLE XVI (Continued)
Capacity Dewatering Technique
Year Ib/hr Dry Solids if Known Remarks
1967 900 Centrifuge
1967 1,500 Centrifuge
1967 1,500 Centrifuge
1967 3,250 Vacuum Filter
1967 450 Vacuum Filter
1967 1,OOO Vacuum Filter
1968 5,000
1968 1,500
1968 3,600
1968 1,200
1968 2,500
1968 1,800 Vacuum Filter
1968 2,000 Centrifuge
1968 2,100 Vacuum Filter (Porteous Plant)
120
-------
TABLE XVII
FLASH DRYING AND INCINERATION SYSTEMS
Year
Capacity
Ib/hr Dry Solids
1935
1938
1939
1940
1940
1941
1943
1944
1946
1950
1950
1950
1951
1951
1952
1953
Unknown
1953
1953
1953
1954
1954
1954
1955
1955
1955
1956
1956
1957
1957
1957
1958
1958
1958
1958
1958
1959
1959
1959
1959
1959
1960
1960
1962
1,667
2,500
5,250
878
420
1,060
1,570
1,500
1 ,353
2,250
3 ,210
7,740
1,000
785
1,170
2,083
420
2,100
890
3,000
1,500
5,025
750
5,250
4,370
1,400
3,000
1,000
2,190
1,075
630
2,000
862
1,224
354
4,000
4,610
2,671
2,694
3,710
1,490
4,300
1,714
2,520
Dewetering Technique
Vacuum Filter
Remarks
Drying only
Drying only
Drying only
Drying only
Drying only
Drying only
Drying only
Drying only
Drying only
Drying only
Drying only
Drying only
Drying only
Drying only
Drying only
Drying only
Drying only
Drying only
121
-------
TABLE XVII (Continued)
Capacity
Year Ib/hr Dry Solids Dewetering Technique Remarks
1963 3,100
1964 700
Unknown 1,820 Drying only
Unknown 5,178 Vacuum Filter
Unknown 4,830
Unknown 3,460
122
-------
TABLE XVIII
WET OXIDATION INSTALLATION LIST
Capacity
Year Ib/hr Dry Solids Dewatering Technique Remarks
1961 25,000 Sedimentation and/or
Thickening
1961 960
1963 125
1964 175
1967 400
1967 330
1967 1,250
1969 1,040
1969 540
1969 117
1969 560
1970 4,200
1970 3,840
123
-------
TABLE XIX
FIJUID-BED REACTORS INSTALLATION LIST
Capecity Dewatering Technique
Year Ib/hr Dry Solids if Known Remarks
1962 22O Centrifuge
1963 500 Centrifuge
1963 1,OOO Centrifuge
1964 220 Centrifuge
1964 420 Centrifuge
1964 500 Vacuum Filter
1964 5,000 Centrifuge
1964 84O Vacuum Filter
1965 490 Centrifuge
1965 2,000 Vacuum Filter
1965 500 Centrifuge
1966 282 Centrifuge
1966 470 Centrifuge
1966 450 Centrifuge
1967 500 Centrifuge
1967 1,215
1967 500 Centrifuge
1967 1,000
1967 350 Centrifuge
1967 500 Centrifuge
1967 875
1967 1,100 Centrifuge
1968 430 Centrifuge
1968 860 Centrifuge
1968 700
1968 425 Centrifuge
1968 700 Centrifuge
1968 3,340 Centrifuge
1968 950 Centrifuge
124
-------
TABLE XX
CYCLONIC-TYPE REACTORS INSTALLATION LIST
Capacity
Year Ib/hr Dry Solids Dewatering Technique Remarks
1966 100 Centrifuge
1968 350 Centrifuge
1968 400 Centrifuge
125
-------
AN ANALYSIS OP NEEDS
A number of system needs have come to light either directly or
indirectly from the analysis of the state of the art on sludge in-
cineration* Some of these, such as the need for cost reduction for
sludge conditioning and the need for a greater ability to concentrate
waste activated sludge have been derived in a driect fashion from the
data and observations contained in this document* However, there
are a number of other items which, because of their absence from
existing systems, the need can only be deduced from an analysis of
processes which might be considered desirable or to be good engineer-
ing practice. Each of the individual items with the pertinent details
will be considered in turn in the following paragraphs.
Cost of Conditioning
A number of figures for the cost of conditioning the sludge have
been presented. It is obvious from these data and from more casual
references to this particular problem that the cost of conditioning
mixed activated and primary sludges can vary from as little as ten or
twelve dollars a ton of dry solids to as much as fifty or one hundred
dollars a ton of dry solids.
Much of the high cost for sludge conditioning derives from the
condition of enaerobiosis of the sludges when they reach the sludge
conditioning portion of the system. Aneerobiosis results from the
fact that most operators do not like to burn on a twenty-four hour
basis. Hence, there is a necessity to store sludge, probably in
the thickener for some period of time. If the plant is smell, sludge
may be stored for as long as 16 or 18 hours and sometimes it may be
stored over an entire weekend.
Storage has some additional undesirable features in that many
of the items of high BTU fuel in the sludge may be sufficiently
hydrolyzed so they result in the filtrate or centrate at the de-
watering device and ere returned to the system. They then appear as
new biological material rather than being incinerated directly as a
high BTU fuel. Therefore, storage has two significant disadvantages.
Obviously, the heat treatment sludge conditioning concepts
reported on in this document in an extensive fashion are one answer
to the problem of reducing the cost of sludge conditioning. Depend-
ing on the plant size, the overall cost of sludge conditioning can
be reduced to less than ten to fifteen dollars per ton including
the amortization of equipment.
126
-------
In a very special way, heat conditioning of the sludges is also
an answer to the second problera--the one of sludge hold over and the
resulting anaerobiosis and loss of high BTU fuels. That is, that the
normal impact of waste activated sludge on the solids concentration
of the filter or centrifuge cake going to the incinerator is altered.
The cell rupturing effects of heat treatment with the resulting loss
of internally bound water obviously permits a much higher cake con-
centration. Therefore, although additional biological material may
be produced as a result of sludge holding, its total impact on the
entire sludge-handling process is much reduced.
It can be concluded that there is a very real need for an in-
expensive sludge conditioning system which can be incorporated in
small incineration packages. There are some rather obvious features
or attributes which should be included in such a system which can be
drawn by inference from this document. These are as follows:
1. The impact of sludge hold over on both solids content and
the loss of high BTU fuel should be eliminated.
2. Should the system involve heat conditioning maximum use
of waste heat, which is substantial in quantity because of
the need for deodorizetion, should be included.
3. The impact of the physical or physical-chemical character-
istics of the conditioned sludge on the entire system should
be evaluated. For example, it is becoming increasingly
obvious that the physical characteristics of heat-treated
sludge are dramatically different from those which have
not received heat treatment.
Redundant Systems
Because of the basic characteristics employed in the design of
conventional sludge incineration systems, it has not been economic-
ally feasible to provide redundant or multiple incinerators within
the entire solids disposal facility* There is a need for overlapping
capacity to permit the continuous, albeit lowered, rate of sludge
disposal during the time an incinerator is out of service. Experi-
ence has shown that the overell effect of long-term storage is
undesirable. Experience has also shown that the cost of alternate
means of disposal such as trucking, emergency lagooning, and so
forth, is excessive.
Extremely large systems have employed the redundant or multiple
unit concept. There is an equal need to provide or include such
facilities in systems of smaller size, for example, in most systems
for populations under 50,000,
127
-------
Dorr-Oliver, Inc., has, in part, applied this concept in the
design of the multiple Type C/R sludge incineration system. The
most recent unit placed in operation is at Laguna Beach, California.
This plant contains two Type C/R incinerators which will provide
some of the redundant capacity which is believed to be desirable*
Sludge Conveyance
In all of the systems examined, there have been reports of
problems of sludge conveyance particularly associated with either
lack of or improper internal system capacitance. It is believed
that these two problems--that is, conveyance and capacitance--roust
be considered together. It appears from the results of the analysis
that the ideal sludge storage and conveyance system would embody
the following characteristics:
1. A strong capability or provision for internal capacitance.
This implies the need for short-term storage of conditioned
sludge between the sludge dewetering device and the incinera-
tor. There are several reasons as to why system capacitance
at this point is desirable and perhaps necessary.
a) The incinerator can be run for short periods of tine
without the sludge dewetering unit.
b) With internal capacitance, there is no need to balance
the output of the sludge dewatering unit on a continuous
basis with the incinerator burning rate.
c) If the sludge dewatering unit goes off the line for a
short period of time, the incinerator need not be
shut down.
As indicated, it appears necessary to concurrently evaluate
means of conveyance with techniques for providing system capacitance.
A number of conveying methods have been employed. The selection has
been more or less based on the general characteristics of the sludge
cake itself and the methods of sludge injection into the incinerator.
For example, dewatered raw sludge is frequently conveyed to the in-
cinerator with a screw-type device. Mixed, raw and activated sludge
varying in solids concentration between 20 and 25% frequently has
been conveyed from the point of dewatering to the incinerator with a
positive displacement pumping device. It has generally been consid-
ered impractical to attempt to design into the sludge conveying system
• modulating or variable feed property. At the present time, and
this applies usually to fluid bed system, when the fuel tends to run
much higher than autogenous in its net BTU content, the system is
cooled with water sprays. The net result is effective loss of
128
-------
fuel whether it happens to be derived from raeteriel to be burned or
•uxiliery fuel. Therefore, in a way this is a self defeating approach,
There will be also en increase in the average velocities above the
bed due to the steam resulting from the added water.
As heat conditioning becomes a more significant factor in terms
of the number of applications, fuels will tend to be, in general,
more nearly autogenous in character. Therefore, a system for tempera-
ture control within the reactor would appear to be substantial.
While, in a physical sense there would appear to be nothing wrong
with the water cooling as the fuels become increasingly high in net
or effective BTU value, the need for temperature control by other
means will become more acute. It would appear, therefore, to mini-
mise the reactor size end also provide for the optimum or desirable
temperature controls that a means of varying the feed rate should be
included. It may be necessary to control the varying rate feed
device from the stack temperature sensing system. Considering
current practice, it would appear this should be controlled between
1200 or 1300° F and perhaps up to 1600 or 1700° p. While in some
cases it may be possible to employ on/off sludge feeding mechanisms,
those systems {such as the Dorr-Oliver Type C/R incinerator) which
have a fairly high system capacity in terms of pounds of solids
burned per unit volume per unit time will cool quite rapidly should
sludge not be fed. Therefore, a varying rate feed system would
appear to be more desirable than the one based on the on/off concept.
A number of other points which should be considered are the
following s
1. Better techniques for handling skimmings and screenings in
the incinerator need to be evolved. Conveyance of these
materials to the incinerator is also a problem.
2* Continued considerations need to be given to coincineration
of solid wastes and sludges because of the high net BTU
content of the former.
3, Based on the operator complaints, better odor control
around the conditioning end dewatering subsystems needs
to be practiced.
4. Better meens of ash removal, and scrubber water recycle
need to be evolved.
5. Where applicable better techniques for fuel conservation
through the heating of the secondary air need to be
precticed.
129
-------
Summery
To summarize briefly, the following system needs have been noted:
1. The need for better and more economical sludge conditioning
techniques particularly applied to those cases where substan-
tial quantities of waste activated sludge are encountered
and in those instances where sludge storage for periods of
hours or days is a necessary pert of the operation.
2* A need for system redundancy has been noted. It has been
further noted that the Dorr-Oliver Type C/R system, as it
is currently being marketed, to a degree meets the need
for system redundancy. However, further research and
development would appear to be desirable in this particu-
lar area.
3. A need has been noted for system capacitance and a variable
feed sludge conveyance system as sludge conditioing tech-
niques improve and more nearly autogenous sludges are burned.
The conditions make it desirable to control the reactor
temperature by the use of a modulating or variable feed
system. System capacitance between the sludge dewatering
device and the incinerator will aid in the development of
a varying type feed system and also provide for short
periods of sludge incineration during a period that the
sludge dewatering facility may be out of service.
4. Several secondary considerations have been noted such as
coincineration, internal odor control, detritus incinera-
tion, secondary air heating, and ash handling.
130
-------
REFERENCES
1. Owen, M.B., "Sludge Incineration," Proc. of the ASCB.
San. Engr. Div., Proc. Paper No. 1172, 1957.
2. Mi11ward, R.S., Booth, P.B., "Incorporating Sludge Combustion
into a Sewage Treatment Plant," Paper presented at the 39th
Annual Meeting of the New York Water Pollution Control
Association at New York, 1967.
3. Babbitt, H.E., Sewerage and Sewage Treatment. John Wiley & Sons,
Inc., London, 7th Edition, 1952.
4. Mancini, J.L., "Gravity Clerifier and Thickener Design,"
Proc. of the 17th Purdue Industrial Waste Conference.
pp. 267-277, 1962.
5. Kynch, G.J., "A Theory of Sedimentation," Trans, the Faraday
Society. Vol. 48, pp. 166-176, 1952.
6. Fitch, E.B., "Sedimentation Process Fundamentals," Trans.
Society of Mining Engineers. Vol. 223, pp. 129-137, 1962.
7. Fitch, E.B., Telmage, W.P., "Determining Thickening Unit Areas,"
Industrial and Engr. Chemistry, Vol. 47, No. 1, pp. 38-41, 1955.
8. Rudolfs, w., "Concentration of Activated Sludge by Compacting
end Flotation," Sewege Works Journal. Vol. 15, No. 4,
pp. 642-657, 1943.
9. Burd, R.S., "A Study of Sludge Handling and Disposal," Publication
No. WF-2O-4. Federal Water Pollution Control Administration.
U.S. Department of the Interior, p. 29, 1968.
10. Comings, E.W., "Continuous Settling end Thickening," Industrial
and Bngr. Chemistry. Vol. 46, No. 6, pp. 1164-1172, 1954.
11. Ettelt, G.A., "Activated Sludge Thickening by Dissolved Air
Flotation," Proc. of the 19th Purdue Industrial Waste Conf..
pp. 210-244, 1964.
12. Eckenfelder, W.W., McCebe, B.J., Biological Treatment of Sewage
end Industrial Wastes. Vol. II, pp. 222-258, Reinhold Publishing
Corporation, New York, 1958.
13. Mayo, E., "Industrial Applications of Air Flotation," Paper
presented at the 38th WPCF Conference, 1965.
14. Laboon, J.F., "Pittsburgh Plans Unique Project to Abate Stream
Pollution," Civil Engr.. Vol. 24, No. 1, pp. 44-45, 100, 1954.
131
-------
15. Committee on Sewerage and Sewage Treatment, "Advances in Sludge
Disposal in the Period from October 1, 1954, to February 1,
I960," A Progress Report of the ASCE - San. Bngr. Div..
Vol. 88, No. SA2, pp. 13-51, 1962.
16. Evans, S.C., Roberts, F.W., "Heat Treatment of Sewage Sludge,"
The Surveyor. Vol. 106, No. 2867, 1947.
17. Lumb, C., "Heat Treatment as an Aid to Sludge Dewetering - Ten
Year's Full Scale Operation," Jour, of the Institute of Sewage
Purification. Part I, pp. 5-15, 1951
18. Evans, S.C., "Metropolitan and Southern Branch - Sludge Treatment
at Luton," Jour, of the Institute of Sewage Purification.
Part V, 1961.
19. Lancord, P., "Utilisation de Nouvelles Techniques dans les
Installations D'Assainissment du Cento de Geneve," Bulletin
Technique de la Swisse Roraando. Vol. 91, No. 14, 1965.
20. Lumb, C., "Heat Treatment as an Aid to Sludge Dewatering - Ten
Month's Full Scale Operation," The Surveyor, Vol. 98,
No. 2550, 194O.
21. Harrison, J., "Aspects of the Heat Treatment of Domestic Sewage
Sludge," M.S. Thesis, Virginia Polytechnic Institute, 1964.
22. Lumb, C., Hirst, J. , "Experiments on Dewatering Humus and
Activated Sludge," The Surveyor. Vol. 92, No. 2371, 1937.
23. BSP Corporation, "The Porteous Process," U.S. Patent 3,155,611.
24. White, W. F., Burns, T.E., "Continuous Centrifugal Treatment of
Sewage Sludge," Water and Sewage Works, Vol. 109, pp. 384-
386, 1962.
25. Jones, W.H., "Sewage Sludge Dewatering - Selection/Sizing and
Application of Equipment," A Paper Presented at the 9th Great
Plains Sewage Works Design Conf., 1965.
26. Schempman, B.A., Cornell, C.P., "Fundamental Operating Variables
in Sewage Sludge Filtration," Sewage end Industrial Wastes,
Vol. 28, No. 12, pp. 1443-1460, 1956.
27. McLaren, J.W. , "Evaluation of Sludge Treatment and Disposal,"
Canadian Municipal Utilities, pp. 23-33, 51-59, 1961.
28. Simpson, G.D., Sutton, S.H., "Performance of Vacuum Filters" in
"Sludge Concentration Filtration and Incineration," Continued
Education Series 113. U. of Mich., Ann Arbor, pp. 126-138, 1964,
132
-------
29. Dietz, J.C., "Some Factors Influencing the Selection of Sludge
Handling and Disposal Methods for Municipal Sewage Treatment
Plants," Proc« of the 13th Purdue Industrial Waste Conference.
pp. 555-565, 1958.
3O. "Nichols-Roto-Plug Sludge Concentrator," Bulletin No. 254.
Nichols Engr. and Research Corp., New York, 1963.
31. Smith, E.G., "Features of a Mechanical Sludge Concentrator for
Dewetering Sludge," Sewage and Industrial Wastes, Vol. 29,
No. 5, pp. 601-606, 1957.
32. Thompson, J.T., Proctor, J.W., "Filter Pressing of Sludge,"
The Surveyor. Vol. 93, pp. 235-236, 1938.
33> Slagle, E.A., Roberts, L.M., "Treatment of Sewage and Sewage
Sludge by Electrodialysis," Sewage Works Jour. Vol. 14, No. 5,
pp. 1021-1029, 1942.
34. Beaudoin, R.E., "Reduction of Moisture in Activated Sludge Filter
Cake by Electro-Osmosis," Sewage Works Jour. Vol. 15, No. 6,
pp. 1153-1163, 1943. ~~~~~
35. Cooling, L.F., end others, "Dewetering of Sewage Sludge by
Electro-Osmosis," Water and San. Engr.. Vol. 3, No. 7,
p. 246, 1952.
36. Stellery, R.H., Eauth, E.H., "Treatment of Sewage Sludge by the
McDonald Process," Public Works. Vol. 88, p. Ill, 1957.
37. Bruce, A., and others, "Further Work of the Sludge Freezing
Process," The Surveyor. Vol. 112, p. 849, 1953.
3ti. Clements, G.S., and others, "Sludge Dewatering by Freezing with
Added Chemicals," Jour. Inst. of Sew. Purif., Part 4,
pp. 318-337, 1950.
39. Doe, P.W., Beys, L.R., Benn, D., "Sludge Concentration by
Freezing," Water end Sewage Works. Vol. 112, pp. 401-406, 1965.
40. "Sludge Concentration by Freezing," Editors, Water and Sewage
Works. November, 1965.
41. Coeckley, P., "Research on Sewage Sludge Carried Out in the
Civil Engr. Dept. of University College," Jour. Inst. of Sew.
Purif.. Pert I, pp. 59-72, 1955. "
43. Albertson, O.E., "Low Cost Combustion of Sewage Sludges,"
Paper presented at the 1963 Annual Meeting of the Water Pollution
Control Federation in Seattle, Washington, Oct. 8, 1963.
133
-------
43. Chasick, A.H., Burger, T.B., "Using Graded Sand to Test Grit
Removal Apparatus," Vol. 36, No. 7, p. 884, Jour. Water
Pollution Control Federation. July, 1964.
44. Russel, R.A., "Theory of Combustion of Sludge," in "Sludge
Concentration - Filtration and Incineration," Continued Education
Series No. 113. U. of Mich., Ann Arbor, pp. 380-382, 1964.
45* Albertaon, O.E., Personal Communication, Dorr-Oliver, Inc.,
Stamford, Connecticut, 1969.
46,» Bartlett-Snow-Pacific, Inc., Bulletin No. HI-2M-1-65. 1964.
47* Quirk, T.P., "Economic Aspects of Incineration Versus Incineration-
Drying," in "Sludge Concentration - Filtration and Incineration,"
Continued Education Series No. 113. U. of Mich., Ann Arbor,
pp. 389, 1964.
48, Schroepfer, G.J., "Incinerator Problems," Sewage Works Journal.
Vol. 19, No. 4, pp. 559-579, 1946.
49. Welter, L.H., Millward, R.S. , "Sludge Disposal by the F/S System,"
A Paper presented at the 16th U. of Kansas San. Engr. Conf., 1966*
5O. Sohr, W.H., et el., "Fluidized Sewage Sludge Combustion," Water
Works and Wastes Engineer. Vol. 2, No. 9, pp. 90-93, 1965.
51. "The C-E Raymond Flash Drying and Incineration System for Sewage
Sludge Disposal," Bulletin No. PD-60, Combustion Engineering,
Inc., Chicago, 111.
52. Hubbard, P.J., Albertson, O.E., "The Type CR FS Disposal System,11
Dorr-Oliver Technical Reprint No. 607-P. 1967.
53. Hurwitz, E., et al., "Wet Air Oxidation of Sewage Sludge,"
Water end Sewage Works, Vol. 112, No. 8, pp. 298-305,
Aug., 1965.
54. Ettelt, G.A., Kennedy, T.J., "Research and Operation Experience
in Sludge Dewatering," Jour. Water Pollution Control Federation,
Vol. 38, No. 2, p. 250, 1966.
55* Pinder, K.L. , Gauvin, W.H., "Applications of the Atomized Suspension
Technique to the Treatment of Waste Effluents," Proc. of the
12th Purdue Industrial Waste Conference, pp. 217-249, 1957.
56. Campbell, H.J. , "The Changing Pattern of Municipal Incinerator
Design," Consulting Engineer Magazine. November, 1965.
57. Nickerson, R.D., "Sludge Drying and Incineration," Jour. Water
Pollution Control Federation, Vol. 32, No. 1, pp. 90-98, 1960*
134
-------
58f. Jaffe, T., "Odor Control in Sewage Treatment," Water and
Sewage Works. Vol. 104, No. 4, pp. 175-178, 1957.
59^ A Report on Pollution of the Nation's Waters by Oil and Other
Hazardous Substances, Department of the Interior and Secretary
of Transportation, 1968.
60. "The Canning Industry," National Canners Association Publication,
1963.
61. Balakrishnan, S., Personal Communication, Resource Engineering
Associates, Wilton, Conn., 1968.
135
ft U. S. GOVERNMENT PRINTING OFFICE : 1970 O - 405-436
------- |