National Technical IntomrtJoiStnta
                     PB-255 769
Municipal Wastewater
Treatment  Plant Sludge
and Liquid  Sidestreams
Camp, Dresser and McKee, Inc.
Praporao Foe
Environmental Protection Agency
June 1976

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SHEET EPA 430/9-76-007
3. Recipient’s Acceanio No.
PB—255 769
4. Title end Subtitle
S. Report Date
June 1976
7. Author(s)
8. Performing Organization Rept.
9. Performing Organization Name and Address
Camp, Dresser, Mckee
1 Center Plaza
Boston, Massachusetts 02108
10. Project/Task/Work Unit No.
11. Coi itr•ct/Grag it No.
12. Sponsoring Organization Name and Address
Office of Water Program Operations
13. Type of Report & Period -
U.S. Environmental Protection Agency
401 14 Street S.W.,
15. Supplementary Notes
16. Abstracts
see PREFACE to document
17. Key Words and Document Analysis. 17.. Descriptors
17b. Idencifiers/Open-Eaded Ter.i
Sludge, sludge disposal, sludge sidestreams, wastewater treatment, wastewater treat-
ment liquid sidestreams
l7c. COSAT! Field/Group
iias SUBJ T O II
18. Availability Statement
Release Unlimited
Security Class (This
ct c
No. of Pages
O M Nl15.1S IR V. 10.71) tN1 tD by AN AND UNE5W.
USCOs M .DC S2S 5.P74


— ,—.-, V
EPA 430/9•76007
S7 4;
1: 4L PRO1 °
JUI E 1976
IPSIWILLD. Yb. 22 181
i b

EPA 43O/9-76 OO7
CoA Et No. SS.O10324
im ivs
P14SN F.1
WAS$umToN.o.c. 20450

This rcport has been reviewed by the Environmental
Protection Agency, and approved for publication.
Approval does not signify that the contents neces-
sarily reflect the views and policies of the Environmental
Protection Agency, nor does mention of trade names or
commercial products constitute endorsement or recom-
mendation for use.
Copies of this report are available for $5.50 each by
submitting a written request and check to:
National Technical Information Service
Department of Commerce
5285 Port Royal Road
Springfield, Virginia 22161
Please indicate the title and the NTIS number of
the publication in the written request.

This technical report t ip .euts a 1974 da of4hi .s,t in umn1 pd
was$cwater tieatmsnt phil shid md IquMi sidesireanis.
An inportasd factor ii determining fbi total u àf a
wastewatcr treatms t s ematlvs is dctermIi”i the compassil
cod of any aidestremu treatmsnt ami dl iosaI system. Woi’
matkm Iruni this report an ha mid as the tsthnical Input in
detcimLihig the cod .cffeâlv’eucm of a wedawater treatmeil’
allcrnativr. The Oid-Effccüwiwu Analysis Gukklhuca (40 CP*’
3S 1 Appendix A, ?W uI Reg te ScpIe er 10, 1973) deiUh
tim bmic methoda y for determining lbs mod cosi’cffectlös
wade tr atment mqnagemeilt system or the mad cod’eff.dks
cam!’on?at part of —y wads treatment m.ungdsát upst i
ptopc icd for. Fadsial comtmctlbn a
Pvoces ing ska by the metbids described bdsM iseduem
liquid aidestreanm. containing different types of polkitanta 3uck-
Mdeslrcams arc either treated separately or recyCind to ioiatep’
stream pert of rho treatment phil. The icon lc and lecini l
factors Invuh’ed in this recycling are delineated, since lbs audited
cods of preventing álestreams from de iJing’tbs dished full’
effluent quality are aftrlbutabli to the shidge headliag syitsm.
Some treatment method, produce liquid sided me hIvI
high ameenirations of .olubki matter. In this , jocsU
the ddcstrcama before final disposal mud be couikismi a part
u i the slud i, procees coaL Vatkxus advanced iamute Srmui .
procesn s an n iate skkittvasuss not amodaled with dultF
hasudhin or processing.
In vonsidurhul sMudge disposal or utIIhautkms aIc Ialhwa, It
is cspcdaUy impaitail to tmsidrr uvirunmeutal fadon The
1edmnival Dulkiin on Munldpal Sludge Masings u.t: EiVitOn
mcuit.sl Factors,” is prepaf.stums at the time of this Writing,
shouki be available i in . I siIcy contained in the i ’chiiimI
bullctin would have precedence orer guidance miulained in thu
technical report.

Section Page No.
1 Introduction 1
2 Descriptions of Terms Used in Report 3
1 3 Discharge of Industrial Wastes to
Municipal Wastewater Treatment Systems . 7
1 4 Types and Characteristics of Sludges
and Liquid Sidestreams Produced 9
Sludge Stabilization ....................... 21
6 Sludge Thickening 29
7 Supernatants From Biological Digesters 33
8 Sludge Conditioning 41
9 Sludge Dewatering and Sidestreams Produced SI
10 Final Disposal 61
11 RecIama ion and Reuse of Sludge
12 Other Tertiary and
Advanced Treatment Processes •81
13 Acknowledgements 87
References 89
Appendix lOt
Subject index — Ills .

,muus I
I _______
I _________ ____ ..- 43 1
3 Typical t. .. d a MuIilpk Ns.i Ica ial.t , -
3 ,l Dlm a. Iii lsu. Wø A O Iss -- --- - 7I

Page No.
1 Characteristics of Sludges Produced
by Various Treatment Processes 10
2 Primary Clarifier Solids Resulting
From Chemical Treatment of Raw Wastewater 15
3 Supernatant Characteristics
From Anaerobic Digesters 34
4 Characteristics of Aerobic
I igester Supernatarits 39

The purpose of this report is to provide background information on the handling and disposal
of sludges and liquid sidestreams produced in municipal wastewater treatment works.
Guidelines on these subjects are presented in separate publications. The intent of this report is
to provide design engineers the information needed to arrive at environmentally sound and
cost-effective methods of sludge handling and disposal.
The term “sludge,” for the purpose of this study, includes all solid and semisolid wastes and
suspensions of solids resulting from the operation of wastewater treatment facilities. The
sludges produced can be entirely organic in nature, inorganic, or a combination of both.
depending on the treatment processes employed in removing the suspended, colloidal, and
dissolved pollutants from the wastewater.
The handling, treatment, and eventual disposal of the various sludges and process liquid
sidestreams can create sonic difficult technical problems involving appreciable costs. The
sidestreams often carry high concentrations of organic and inorganic matter suspended or in
solution, since they are usually produced from methods involving the thickening and
dewatering of sludges resulting from various physical, biological, and chemical treatment
processes. Frequently, this matter has a high oxygen demand or consists of complex
nonbiodegradable organic compounds. The treatment and disposal of wastewater treatment
plant sludges and resultant sidestreams can account for 30 to 50 percent of the total treatment
plant costs (I).
The report (I) prepared by R.S. Burd in 1968 for the FWPCA on sludge handlingand disposal
contains much detailed information which generally is not repeated in this report.
It should be kept constantly in mind that sludges and sidestreams are an integral part of the
entire wastewater treatment operation. They arc not isolated processes oradjuncts to the main
plant, either from a technical or economic standpoint. The handling, treatment, and disposal
of such sludges and sidestreams must be evaluated as part of the entire treatment system. It is
not possible to optimize the design of wastewater treatment works to ensure the required
effluent quality, for the least capital investment and operating cost, without considering the
sludge and sidestream handling and disposal methods as integral with the other processes.

This study encompasses the treatment pro eues presently uSed in municipal wastewater
treatment which are directly or indirectly involved in the production of sludges and liquid
sidestreams. Processes which arc as yet in the development or pilot plant stage arc discussed on
the basis of reported performance.
Certain sludge processing systems generate liquid sidestreams which can have anappisdable
effect on the total plant performance and cost. Such processes are given particular attention,
and the capital and operating costs are identified for several typical treatment plants (see
it is the goal of wastewater treatment works engineers, and the purpose of the.plaats they
design, to dispose of pOllutants with minimal effect on the total envu -- tt. Va,um
pollutants are present in sidestresni , and their eventual safe disposal to the atmosphere,
receiving water, or land can present some dillicuk technical and economical problems.
Since many pollutants which wsic not considersa in the past arc now being rcmovc?d insMges,
and since the types and quan.I,r, odsludges will continue to increase in th.futwe,wscawbs
assured that the sludge handling and disposal problems will also become nose complex and
more costly to solve.
In the past, there was a tendency to minimize or ignore the “sludge problem.” Vs....
temporary or partial solutions, suciras land dumping or lagoonaig, were adopt .’IIowsvsr,
such methods soon pro ed infeasible because ci mounting concern over odor, gvouadn.kf
contamination’ from leaching, and increased toxicity of industrial sludges Ako population
gowth and development preclude the use of land for such purposes.
Currently, there are some wastewater treatment processes eithcrdevclopcd or in the pilot plant
stage which result in the production of minimal quantities of easily dbposablesh ge. These
processes must be given special emphasis, since, as far as wastewater is concerned , one ái thus
ultimate goals is the adoption of treatment methods that do not produce large qiiuiitIc. of
sludges. Reduction in total sludge production—and generation of a type me.esuiiable for
eventual disposal—will reduce wastewater treatment costs significantly.
The primary goal of this report is to provide concise, up-to.date. objective. and comprehcssM
information on presently used sludge and liquid sidestream handling proLtssL& including the
final disposal steps.
j 2;

Though many of the terms in this report are commonly used by experienced engineers, it has
been noted that in some publications the descriptions of certain processes are not consistent
with general usage, A few important definitions are presented below. A more complete list of
definitions can be found in the joint APHA, ASCE, AWWA, and WPCF Glossary: Water and
Was fewaler Control Engineering.
A brine is any concentrated solution of inorganic salts; it need not necessarily be NaC1. Brines
originate when it is necessary to remove soluble inorganic salts, which cannot be easily
removed by chemical precipitation. The processes used include ion exchange and reverse
osmosis, among otheñ.
This is the liquid extracted from a sludge in a centrifuge used either for thickening or
dewatering. Its composition depends on the physical and! or chemical treatment of the sludge,
the centrifugal force used in the unit, and the design of the centrifuge.
To release the liquid from sludges that are flocculant or of the hydroxide type, it usually is
necessary to treat them with various chemicals, subject them to some drastic physical
conditions such as heat or cold, or process them biologically. These processes, an extremely
important part of overall sludge processing, are referred to as “conditioning,” and may be
necessary to accomplish the desired thickening or dewatering.
Dewatering is the removal of additional liquid so that the thickened sludge attains properties

of a solid: that is. it can be shovekd, conveyed on a sloping tick, and handled by typical solids
handling methods. Though there a,e some methods for linal disposal of thickened
sludges—and these will be discussed and evaluated in this report--most dispos l methods
require that the thickened sludge be further dewatered. Such dewatceed sludge is usually in the
form of a “cake,” such as that produced by a centrifuge or vacuum filter. Frequently.
dcwatenng can be accomplished with a high percentage of the solids retained in the cake:
however, some sort of conditioning will usually be necessary.
This is the water resukm from b ckwasbing and removing the solids retained by a giuuu r
media filter. Various types of panulir m md i filters, in which the media can be sand, coal,
activated carbon, or mixteiss, are used to physically remove most of. the ssspe de4 solids,
usually from settled effluçats. Since these filters ire periodically backwashmd to remove she
accumulated solids, the backwash water may contain suspended solids, ua 1 J 1y U
coagulated and compacted, and their conceeUatioe may vary from a few bun4rçd to . swsal
thousand milligrams per liter.
When sludges us dewatered on vacuum filters, filter , ...::::. . and ether devices in which 4 s 1
liquid is separated from the solids by applying a d fersntial force acews a poraps febrIc oeJ
screen, the extracted liquid ia referred tons Illicit..” hscba,acssnicslcs dspe id so lb
conditioningrncthods used, the chemicab applied, the character ofibs Illieriosmedia, md th&
type of sludge being processed.
Sludge, either thickened,dewatsred, bloingically or chemically altered, or redijeed soaiiasbJ y’
incineration, must be returned to the environment. This final disposal must h vp a mipj al
detrimental effect, if any, rn the environment. Final disposal will usually u’ili’ I .d ce,i
some cases air or the ocean. Alas, in some instances the end product resulting from any c i,
several treatment steps, a. wiUbdsscrlbad later in this rspoui,caabe reused by isçyel is’s back
to some other treatment proosse where it will produce no adverse effects and can poceibly bvs
some economic value.
Much of the material that is skimmed from primary clar ler basins ad sbo1, aI clar(icrs is
liquid, such as oil or water with floating grease and other debris It must he disp .4 ci,
properly with the creation of the least amount of and odors. Typi IIy, bauuihd:
with the waste sludge from Its i rme Y

Wash waters originate at various points in the treatment plant. Some of these originate only
when equipment and working areas are washed down; others are more or less constant, such as
the water used to *ash organic matter from grit and scrubber water from incinerators for
reducing particulate emissions below air pollution standards.
This is the liquid that drains out of the sludge (usually digested) which is applied to sand beds
for natural dewatering. Such beds, if properly designed, have a collecting underdrain system
which carries the liquid to a point where it can be properly handled for disposal. it y have a
high concentration of organic matter in solution a d inorganic nitrogen and phosphorus
compounds typical of digested sludge supernatants. lithe sand beds do not have underdrains,
the liquid that flows by gravity into the soil is called leachate.
Supernatant is the liquid that is decanted from an anaerobic or aerobic digester. in domesticS
wastewater treatment works, such a liquid may have a high concentration of suspended and•
dissolved organic matter plus inorganics such as ammonium compounds, phosphates, heavy.
metals, bicarbonates of calcium and magnesium, as well as various types of pathogens.
Various sludges are thickened in gravity-type thickeners before processing. The overflow from
such units is sometimes referred to as a dccant liquid” or “decantate.”
A thickened sludge is one that is semisolid, though it has the gel-like characteristic of becoming
fluid when shaken, stirred, or otherwise disturbed and of again becoming gel-like when
allowed to stand. It can be pumped through pipes and handled essentially as a fluid.
Sludge solids are either originally suspended in wastewater or are generated by chemical
precipitation or growth of biological organisms. Removal of such solids, at least as far as
domestic wastewaters are concerned, is normally accomplished in relatively quiescent basins
which allow the solids to settle out by gravity. This step is referred to as sedimentation or
clarification. Simultaneously with clarification, some thickening of settled sludge frequently 1
takes place. The solids that have settled thicken with time and! or by the aid of some slow-.
stirring mechanical devices such as pickets on scraper arms. The latter devices mechanically:
break up the agglomerated solid particles and release the liquid entrained or enmeshed in them.
This is referred to as gravity thickening. Such thickening is often accomplished in a separate

More thickening than can be attained by pavity ssttlhi is frequently required before feithsr
proces%ing. This is cepecilly true of the ky*oxide type .ledgss- iatsd by somscbsuiical
coagulants and of waits activated sludge. There aw.sve,aImethod. which can be ud to,
accomplish such tbickeningj and theirwili btdiscsed in detail in this ispoit.
The technical and . conomk slpl&aàue .1 sludge thickening is sot always apparent or
appreci ted. Varioul treatment prscsàá (pinin isIthng cbeuaimlF treatment, or biolngic. 1 4
treatment) wilt produce_sludge or saudi whose weight is propoidous& to the weight
pollutants removed. However , the volume depsudi’ oaths wi ’ath of the .olde in tbs
liquid. For example, a slmdj w th%p rcent meimuue ha. twice thevolume ofibe — sludge
thickened to % pórcsnt omheuee, and thee wssl* i quk. twice the apseily in Ike
treatment processing uaiIs saute of the 1qu1 is I. gummed.
A variety of suis can bsv.sd to ismsvS chbse isintiwly large ipseded solds or samlisr
tolids on microstialairs. Such icuUui ire u ly cehtdsuorha aesatlneseasUs.m d
screened water for their clesa1ng The wach omit w contain a folly high coaoewasios ’ ol
suspended solids which mtM be reàsved from wab waist, uomly by slurs to the
treatment plant, bsfOrs I is dWGssd

A detailed discussion of this subject is contained in EPA regulatory information on
Pretreatment of Industrial Wastes. IndustriaL waste streams entering the domestic wastewater
system must be identified. The potential effect of such streams may include: (1) impairment of
biological treatment because of the effects of various heavy metals, toxic chlorinated
hydrocarbons, industrial type detergents, etc.; (2) increased amounts of sludge produced,
because of increased biological synthesis or a greater amount of inert material settling out in
either the primary or secondary clarifiers; (3) production of much larger volumes and poorer
qualities of liquid sidestreams, since various soluble organic compounds in industrial wastes,
though biologically degradable, may produce sludges that do not dewater readily; and (4)
excessive amounts of greases and oils (soluble and insoluble) entering a biological treatment
process and having serious deleterious effects.
The extent to which sludge and sidestream processes at the municipal wastewater treatment
plant will be affected by industrial wastes must be determined by bench-scale laboratory
studies, extended pilot plant studies, or past experience.
Excesswe concentrations of toxicants such as heavy metals, chlorinated hydrocarbons, and
industrial type detergents can seriously interfere with the proper functioning of a biological
treatment process, whether aerobic or anaerobic.
The sludge volume and type produced by either chemical or biological treatment of industrial
wastewaters containing colloidal and soluble inorganic and organic compounds can be vastly
different from those produced from domestic sewage. For example, the biologically
synthesized solids resulting from removing a pound of BOD (biochemical oxygen demand)
from some industrial organic matter can be different in character and quantity from the solids
produced by biological removal of a pound of BOD from normal domestic wastewater.
The presence of certain types of inorganic solids can adversely affect the conditioning of sludge
before dewatering. Any such solids and their possible effects upon the processes must be
known before the sludge handling methods and processes are selected and designed into,
treatment plants.
it is possible for some industrial wastes to contain organic matter which is biodegradable, but
the resulting sludge can have entirely different dewatering characteristics. It is therefore

important that treatment studies be undertaken where necessary before the plant design is
finalized, or before aceeptance of die industrial wastewater for treatment, not only to establish
the treatability of ‘he industrial waste when mixed in the proper proportions with domestic
wastewater but also to determine the amount and characteristics of the combined sludge to be
Oils and grease decrease the density of (i.e and its settling rate in the final chriflers, thus
decreasing the density of the settled sludge. Oils and pease also inhibit the activity of the
organisms by coating their surfaces and reducing oxygen transfer. Experience has shewn that, I
in general, for any biological waslewaler treatment process the total oil and pease
concentration should not exceed on she awiap abcstiOmg/l, sons not to impede the settling
of the solids and their thickciiing(2). II Sects and experience indicate rapid biodegradability, I
higher concentrations are permissible.
Information on pretreatment of industrial waslewaters is contained in the EPA luidelines
developed pursuant to Section 304 of the Federal Water Pollulion Control Act Amendments
of 1972.

This report is concerned with sludges that result from processes employed in municipal
wastewater treatment works in the treatment of domestic wastewater mixed with those types of
industrial wastewaters that are handled in such plants. Many of these methods produce solids
by physical, chemical, or biological means.
The quantity and quaiity of sludges produced are based on the wastewater characteristics and
the combination of treatment processes used. It is apparent that data gaps exist, as faras sludge
production is concerned, in understanding the effects ofcombinedtreatmentprocessessuchas
chemical coagulation with the activated sludge process. Because of the Costs involved in
disposing the solids generated by wastewater treatment methods, a unified design approach
must be used. In the analysis of sludges and of bench or pilot simulation of treatment methods, \
some quantitative figures are of value regarding the amount of solids generated by various
treatment processes and their water content, since dewatering of the solids is an important and
expensive part of solids handling.
Table I shows ranges of amounts of sludgr’ rc duced by various treatment processes (3).
It is significant to note that settled activatcii ludge contains about three to seven times more
water than settled trickling filter sludge. For ;‘ r (‘cesses used in sequence, the volumes shown in
Table I may be additive, depending on when and if the sludges are mixed and decanted. The
range of values, as far as weights of solids anti olumes of sludge produced, can be quite broad
and is dependent on the strength of the waste’water, the BOD loading used in the biological
processes, the type of chemical used for coagulation, and the temperature and pH of the
wastewater. Because of the wide variation in sludges, care and good judgment must be
exercised in using typical values or extrapolating results from one plant to another.
Since most liquid sidestreams are generated when sludges are treated and processed for
disposal, their characteristics and quantities will be discussed in this report in conjunction with
the various methods used to thicken, stabilize, and dewater the sludges produced by various

— -1
Pounds Dry Solids Gallons Sludge per Percentage Water
Treatment I per Million Gallons in
Processes L Million Gallons Wastewater Treated Sludges
Primary settling 9004,200 2,5003,500 93-9k -
Activated sludge 600-900 15,000-20,000 98-99
Trickling filters
(low loading) 400-500 400-700 9395
Trickling filters
(high loading) ‘.600-900 I l,20 -! ,500 96-98
Chemical precipita-
tion of raw waste-
water - 3,000-4,500 4,000-6,000 90-93
The term “primary” describes a plant utilizing screening facilities, grit removal, and settring
The settling basin is used to remove readily settkable solids and floatable materiab such as
oils, greases, and other debris.
Primary treilment will remove about 25 to 35 percent of the ROD, of the raw domestic
wastewater and about 60 to 65 percent of the suspended solids. In a primary plant, the
following sludges and liquids are removed from the main wastewater stream:
I. Grit and similar solids
2. Grit wash water-
3. Solids removed by screening or changed in size by communition equipment
4. Skimmings, usually pumpable
5. Settled sludge—usually 5 to 8 percent solids. However, a high-rate (short
term) primary plant may produce a sludge 0$ only 2 to 3 percent solids. The
solids loading is increased if there is a substantial amount of groufld
garbage. -
6. Waste sludge from secondary clarifiers.
The sludge from a primary settling basin has been called ‘fresh” sludge. Of course, sludge ie
rarely “fresh” since anaerobic conditions rapidly develop, especially in warm climates, and the
sludge can be quite odorous. In fact, many primary basins are the source of the distinctive and
unpleasant odors the public associates with wastewater treatment plants. Many new, and cvcn
some older, activated sludge plants have been built omitting the primary basin and thus
incidently eliminating the associated odor problems. Trickling filter plants using rock media
require primary sedimentaticin to avoid clogging of the media with large solids and stringy
material which get past any screening arrangements.

Manually operated or automatic screens remove solids which, in some plants, may be ground
up and discharged back into the flow, in larger plants such solids are hauled away and disposed
of with municipal garbage and refuse. Comminutors installed in a channel are essentially
disintegrators which reduce the solids that are larger than can pass through, say, a 3/4-in, bar
screen opening. However, comminutorsC n “string out” rags and allow them to pass through.
Skimmings. which may contain an appreciable amount of grease and oil, are conveyed into a
trough or box located near the periphery of a circular clarifier or the end of a rectangular
clarifier. Skimmings can be pumped to a sludge dewatering process such as a vacuum filter or a
digester. They usually do not require any further thickening prior to biological digestion ora
dewatering unit. At small plants such skimmings are disposed of to a landfill site or to an
Preaeration is sometimes provided ahead of a primary clarifier to supply some oxygen to
wastewater which may be septic or on the verge of anaerobiosis. Preaeration tends to delay
septicity in the primary clarifier, produces some flocculation (and thus assists in removing
some of the slower settling solids), and assists in floating greases and oils to the 3urface of the
clarifier. It also scrubs the oil and grease from the other settleable solids. Such treatment is of
considerable effectiveness in reducing odors that emanate from primary basins.
Pressurized-air flotation units have been used in a few cases to handle overflows from
combined stormwater and sanitary sewerage systems. A 24-mgd (million gallons per day) plant
of this type is in operation in San Francisco. The process starts functioning automatically to
handle the excess flow resulting from storm flows(4). Pilot plant tests were run on primary
treatment using partial flow pressurization and flotation for the raw wastewater of Rio de
Janeiro; as a result, a 240-mgd plant is being designed(S). The principal advantage of using
flotation for such primary treatment is the substantial reduction in required land area and the
reduction in capital costs. Operating costs tend to be higher than those forgravity settlingand
skimming(6). The sludges produced are handled in the the same manner as those from primary
settling basins.
Hydraulically cleaned screens have been tested in the field for use on stormwater flows and also
for primary treatment of domestic wastewater. Some tests have shown them capable of
removing most of the floatables and settleable solids and 35 percent of the suspended solids(7).
A full-scale installation is being constructed to handle raw wastewater in Contra Costa
County, California(8). The stream containing the solids is about 5 to 10 percent of the main
flow. Although much more concentrated than raw wastewater , the liquid stream containing
the suspended solids would not usually be considered a sludge, and it would definitely require
further thickening or dewatering. -
Typically, a secondary treatment plant incorporates those processes used in a primary plant
and follows them with a biological process. The objectives of this type of plant are to remove
most of the carbonaceous oxygen demand matter, both soluble and colloidal; to remove most
of the suspended solids; and possibly to Oxidi7e other oxygen-consuming pollutants such as
ammonia. Thc two common secondary treatment methods are trickling filters and the
activated sludge process. However, there are numerous modifications of these general types,

and the specific design used depends on plant size, climatic conditions, the desired emucat
quality, and cost.
Bacterial activity results in cell synthesis which is a significant portion of the total sludge
produced by the process. A portion of the organic matteror any pollutant will be converted by
aerobic biological activity into a solid, and the excess solids, not required for carrying on the
treatment process, must eventually be disposed of in some manner. The amount of solids
produced. as a fraction of the pollutants removed from the wastewater on a weight basis, will
vary considerably depending on the type of biological process used. The volume of waste
sludge produced will amount to about 1/2 to 2 percent of the volume of the raw wastewater
being treated.
In biological treatment, it should be kept in mind that the various aerobic organisms remove
pollutants by two basic processes. One of these is the result of organism growth, which is
referred to as synthesis. Most of the organisms, calied he:erotrophs, obtain the carbon and
other elements for their cell growth from the organic substsnces present in the pollutional
matter. During their growth process, these organisms oxidize a portion of the carbon and
hydrogen present in the organic matter to CO 2 and water. Organic matter is usually absorbed
through the cell wall and then synthesized and oxidized by complex chemical process
controlled by enzymes that the bacteria produce. If the organic matter is in the form of a
colloidal or suspended particle, the cell attaches to the particle and an exoenzyme solubilizes
the solid matter so it can be absorbed through thecell wall. The cell requires energy tocarry on
its life cycle and growth, and obtains this energy by oxidizing a portion of the organic matter
into CO 2 . water, and other oxidation products. In contrast, other bacteria, known as
auioirophs. can use only inorganic carbon (from C02 or bicarbonates). The organisms
responsible for oxidizing ammonia into nitrites and nitrates are autotrophs, for example.
4.3.1 Trickling Filters and Other Fixed Growth Systems
The conventional trickling filter uses rock media on which the biological organisms are
attached. The biological growths adsorb and remove pollutants as the wastewaterflows.down
over the surfaces. The biologic activity in the attached growths is quite complex, and various
theories have been developed to describe the biological kinetics. It appears that somej
anaerobic decomposition may occur in the film at the rock surface, since the diffusion of’
oxygen through the film is largely molecular and thus quite slow, even though the film outer
surface may have a relatively high concentration of dissolved oxygen. This anaerobic action,
and resultant reduction of volatiles in the growths, probably accounts for the increased density
of the sludge from trickling filters.
The excess solids synthesized slough off the rock surfaces; however, this tends to occur
somewhat erratically depending on the factors such as pollutant loading, temperature, and
hydraulic loading. These solids settle out in the final clarifier that follows the biological
treatment process, and the excess matter that must eventually be removed from the treatment
system. The biological and physical characteristics of the solids from trickling filters depend to
a large degree on the BOD loading. Standard rate units generate about 0.25 lb of solids per
pound of BOD removed, while high-rate filters wilt produce 0.50 to 0.85 lb. The solids from
trickling filters will normally thicken in the clarifier to 2(03 percent by weight with the denser
solids resulting from low-loaded filters(P).
lt should be emphasized that trickling filters do not in any sense remove suspended solids by
physical filtration, and the term “filter” is basically a misnomer. In recent years plastic media

usually polyvinyl chloride, have been used. The media consist of plastic sheets which provide
more surface area per unit media volume and considerably more open volume than rock, thus
aiding air movement. Because of their light weight, such media can be used in units 20 ft or
more in height, thus reducing required area (10) (1!). The excess solids produced from such
units are generally comparable to those from Units with rock media. For economic reasons.
such media have been used primarily for high BOD loadings per unit volume, and in such cases
the solids production increases to between 0.5 and 1.0 lb per pound of BOD removed.
A fixed media type unit that has been extensively field tested for both domestic and industrial
wastewaters is the rotating biological contactor (12) (13). The unit was originally developed in
Germany, and there are presently over 700 installations in Europe, most under 1 mgd in size.
There are at least 12 plants installed or under construction (1973) in the U.S. The units consist
of a series of disks mounted r’n a horizontal shaft. The disk.s (made of light plastic material) arc
closely spaced and provide a relatively large area for biological growths. The wastewater flows
through a tank, which may be compartmentalized, in a direction perpendicular to the rotating
disks which are submerged in the slowly moving wastewater to about three-eighths of their
diameter. The alternate contacting of the disks with the wastewater and the atmosphere
provides oxygen for biological removal of biodegradable pollutants from the wastewater.
Removals of 85 to 90 percent BOD and suspended solids from domestic wastewater have been
reported. The solids slough off the disks and are carried with the flow to a final clarifier. The
quantity and physical characteristics of the solids are comparable to those from trickling
Another type of fixed media biological treatment unit that has been tested, but on a pilot plant
scale, is the aerated packed tower(14). Wastewater flows upward together with dispersed air
bubbles, and biological growth occurs on the media surfaces. Units in which the tower is
flooded and anaerobic conditions develop, with growth of the denitrifiers on the media, have
been studied for denitrification of wastewater(/5). A similar unit was recently tested ona pilot
plant scale for nitrification of ammonia in wastewater. The tests included the use of air and
pure oxygen(76).
Fixed media biological reactor units generally produce waste solids that tend to be denser and
faster settling than those from the activated sludge process. This, of course, is desirable for the
collection, handling, and eventual disposal of such waste solids(/7).
4.3.2 Activated Sludge Plants Using Air
This treatment process uses a suspension of aerobic microorganisms to remove soluble and
colloidal organic matter. The organisms can vary widely in concentration, type, and degree of
agglomeration, depending on various physical features designed into the treatment plant, the
pollutant loading, type of pollutants, and degree of pollutant removal. There are many
modifications of the activated sludge process. From the basic definition, this process includes
plants consisting of aerated lagoons—high-rate aerobic processes which have concentrations
of microbial suspensions on the order of 2,000 to 5,000 mg/I in aerated basins and which are
designed primarily for removal of the carbonaceous BOD (5-day)—-and low-rate processes
designed to oxidize ammonia to nitrates. The high-rate processes are characterized by a high
rate of excess solids production, since the removal of carbonaceous BOD at a high rate
involves high synthesis. Nitrification is at the other extreme, since the organisms involved have
a relatively low reproduction rate and therefore the excess solids produced per pound of
ammonia (nitrogenous BOD) oxidized are very small.

The process for removing carbonaceous pollutants can be operated at a relatively low loading
rate (pounds of BOD 5 per pound of MLVSS per day), thus reducing the amountof excess
synthesized solids and the percentage of volatile matter in such solids. This loading rate would
be about 0.05 to 0.13. Such a process is frequently referred to as extended aeration, and
approaches aerobic digestion. The excess solids produced will be about 0.05 to 0.25 lb per
pound of BOD removed, while in a high-rate activated sludge proccis the solids pt oductrnn
may be between 0.75 to 1.0 lb per pound of BOD removed.
The activated sludge process, called “extended aeration” or “total oxidation,” is -practiced
when the BOD loading per pound of mixed liquor volatile suspended solids (MLVSS) in the
aeration basin is below about 0.15 lb/day. For years it was believed by many engineers that
such long periods of aeration would result in oxidation of all the organic mattcrandthat these
would be only inorganic residues remaining. It was discovered that in all such plants there was
some insoluble organic matter which, if not biodegradable, was only very slowly degradable.
Prom a practical standpoint, some of the solids would have to be removed; otherwise, they
would go out with the plant effluent.
It is not the intent of this report to describe all the numerous ways that the activated studge
process can be carried out, since the literature on this subject is quite extensive. The principsi
point that should be i mptWsiaed is that variations in the design’parameters of the process can
have a profound effect on the character and amount of excess solids that are produced. Solids
from these systems are the most difficult to thicken and dewatier of any that arc prOducediu
wastewater treatment plants, and their disposal can account for a large portion of the tout
cost. Therefore, methods to make them thicken faster and dewater more readily; with the
creation of a minimum amount of other problems, should be carefully evaluated by the design,
engineer. Some extra costs in the actual treatment proce. . may produce a more than
comparable reduction of costs in the waste solids handling and disposal.
As was indicated in Section 4.3.1, there appears to be a basic differencein the physical and
biological character of the solids produced when the growths occur on fixedmed.aand when
they arc in an agitated suspension. Many problems that seem to be commonly experienced
with the activated sludge process are unknown in trickling filter plants. One example is the so.
called “bulking” of the sludge, which often occurs under conditions in which the cau.e-cffict
relationships are not very clear.
4.3.3 Activated Sludge Plants Using Pure Oxygen
The use of pure oxygen instead of air for supplying the required oxygen to the microorganisms
in the activated sludge process was studied over 20 years ago. However, only recently hasit
become possibkrto use theprocess practically, because olthevarious technicaIimp ovemenu.
and significant lowering of costs for on-site oxygen production systems However. thele are
some technical limitations and the total costs ofan oxygen system, as compared to a-mOdern
aeration system, are not necessarily always in favor of the oxygen-system (18).
The solids in the aeration basin can be carried at a higherconcentration than with airsyatems.
The concentration of the sludge drawoff from the fmal clarifier in the oxygen system isabout
1.0 to 2.5 percent; in the normal operation ofan activated slãdgesystem usingairthesolidsare
usually not more than about l to 1.5 percent by weight(l9 2O). The settled solids can be
thickened further using a flotation thickener, which can produce 4 to 5 percent solid,.

Chemical treatment of raw wastewater was practiced fairly extensively in the l920’sand 1930’s.
However, as the activated sludge process came more and more into use, it became apparent
that it could produce a higher quality effluent at lower operating cost. A good review of
chemical treatment is given by Culp(21). Chemical treatment is now coming back into use for
the following reasons: (I) it can reduce BOD loads on an existing secondary plant, (2) it can
precipitate phosphorus, (3) it can be combined with a subsequent physical treatment process
(activated carbon) to obtain an effluent comparable to biological treatment with reduced space
requirements and reduced sludge handling costs, and (4) it can take advantage of organic types
of coagulants (polymers). Polymers accomplish coagulation with much less increase in total
solids in comparison with inorganic chemicals.
Inorganic coagulants such as alum, iron salts, lime, or organic coagulants can be used to
capture the finely divided suspended solids and a portion of the colloids in a primary clarifier.
These coagulants are mixed with the raw wastewater which is then flocculated and settled.
With uch treatment, suspended solids removals from domestic was ewatcr of 85 to9O percent
can be obtained and SOD removals of 55 to 70 percent are possible. The sludges resulting from
treatment with inorganic chemicals are frequently dewatered and disposed of in landfills (when
stabilized using lime treatment at pH of at least 11.5) or incinerated with garbage and refuse. If
organic coagulants are used, the sludge volume is considerably less and such sludges can be
digested before dewatering. Sludges produced with alum or iron coagulant treatment can also
be digested. No solubilization of the precipitated aluminum phosphate occurs, though some
may occur when ferric phosphate is reduced to the ferrous form in an anaerobic
digester(22,2324,25). Presently, lime treatment of raw wastewater is frequently used, with the
addition of a coagulant aid such as one of the polymers if needed. The pH is raised to about 9.5
or 11.5, depending on what degree of phosphorus removal is desired and the calcium and
magnesium content in the wastewater. The precipitated calcium carbonate and, if the pH is
above 10.5, some magnesium hydroxide accomplish coagulation àî the suspended and
colloidal matter. The settled sludge is quite dense—around 10 percent by weight at the lower
pH—-and is usually easily dewatered without further conditioning. It must be recognized that
addition of coagulants can substantially increase the weight of solids to be treated.
The amount and density of the solids produced by various chemical treatments of raw
domestic wastewater are shown in Table 2. These values are based on the solids discharged
from primary clarifiers(2O).
Treatment I
Pounds per
Million Gallons
Percentage Solids
Iron coagulant
Aluminum coagulant
Polymer may be desirable.

The amount of sludge produced will dcpe. d on the alkalinity ofthe wastewatcrand thedegres
of phosphorus removal desired.
Biological digestion of sludge which has beeá.treated with lime for phosphorus precip!tatiise
might not be effective. Some experiences reported recently in Ontario. Canada, indicate that
high pH sludge must be added to the digester with some control to prevent digester upset(27)
It was stated that raw wastewater was treated with lime to a pH of 10.5. By keeping tbe 1udg ,
in the bottom of the clarifier for some 1.5 days, the pH dropped to 9.5 and then, by proper
pumping of the sludge to the digester, normal operation otthe digester was maintained at a p11
of about 7.0 to 7.4. The calcium phosphates solubihzed to only a minor degree; there were only
6 to 8 mg I I of phosphorus in the supernatant, which was less than that in the raw wastswater.
The digested sludge had a solids concentration of 10 to II percent.
A 30-mgd plant that will employ lime treatment of raw wastewater in a primary darilier.
followed by an activated sludge-nitrification stage and then denitrification, is being designed
for Contra Costa County in California(28 The sludge from the primary. basin sill her
incinerated and calcined afWr centrifuging. These processes will be discussed ter.bi thb
report. -
The tequirement for reduction of phosphorus in wastewaters to a low level has broug about-
the use of chemical coagulation as a separate or integrated process in wastewater trcatmeifl
Either alum or an iron salt, loch as ferrotis sulfate, will precipitate orthophosphals as a
insoluble aluminum or irona phosphate at normal pH values. Of course, use of these
coagulants, depending ,on the. alkalinity, of the waitewater, results. in an appreciable.
precipitation of either., aluminum or iron hydroxide. Studies have indicated- thai alui .
aluminate, or. iron salt can be addCd?in an activated sludge aeration basin or in a separate
mixing basin or compartment between the aeration basin and the final clarifier.. SIhee the
phosRhates precipitate at pH values.in the range of 6 to 8 when an aluminum orirop.altis
added, no adverse effect on the.biological activity has been noted(29 L The$ydyoxidcs-
produced tend to coagulate the-fuier particles of the activated sludge and to.improve.the.
clarification- and settleability. Although the quantity of solids increaies the.Iotal volume of
sludge is not greatly increased, sincethese settled solids produce a denser sludge. Abe. iest-
have shown that there is no- adverse effect on either anaerobic or aerobic digçstion of the
sludge. It has been indicated by small scale testing that the sludge in the secondary c1ar erwish
the addition of the aluminumsor iron coagulants to the mixed liquor will settle tostotal sol s
concentration of 10 to 2 percint, while without the chemicsladditionit wilhettletosbovtO.5
to 1.0 percent(3O) Presumably, performance would vary with water characteristics.,and ’
amount of coagulant.’
A fulf-edal field test was made at a trickling filtei plant which involved addinLiodlem
aluminate tothe filter influent to accompliih phosphorus precipitation(31). It was not felt that’
adding alum would be desirable because of the poor mixing and essentially “plug-flow” us a.
filter, which in an insufficiently buffered wastewater could cause a drop of pH thatwovldbe.
too low for optimum biOlogical activity. No adverse effect on the performance was nosed when
using alum mate. However, the phosphorusremoval efficiency wasnot as go d as thatwhena
comparable dosage of alum was added to the aeration basin of an activated sludge plant. Tb.
sludge in the final chirifier had the same volume as before the chemical addition. indicating
some densification.

There are several new treatment processes which are coming into use; some have been studied l
sufficiently and are being incorporated into plants now under design, while others are not as
fully developed and some are still in pilot plant stage.
The new processes or combination of existing processes that are of principal significance in
municipal wastewater treatment, especially if organic industrial wastes are also being treated,
The Physicochemical Process: This is a nonbiological process
involving treatment of raw wastewater with Lime, alum, or iron
salts, and possibly polymers in a unit or units including mixing.
flocculation, and settling. Lime has been used because it
removes phosphorus, complexes and precipitates any heavy
metals, and at pH values below about 10.5 produces a dense
sludge that thickens and dewaters readily. There are at present
(December 1972) some 30 municipal plants under design,
varying in capacity from 1 to 60 mgd, which will use the
physicochemical process. After chemical treatment and settl-
ing, the effluent may go directly to activated carbon columns or
to dual-media (coal-sand) filters for suspended solids removal.
if lime is used, recarbonation precedes filtration. When low
hardness waters are treated with lime and the pH is raised to
above I 1, recarbonation precipitates sufficient calcium car-
bonate so that a second chemical reaction unit is used. After
filtration the wastewater is passed through granular activated
carbon columns, which may be of either the downflow or
upflow type. The advantages and disadvantages of each are
discussed in detail in the Iiteratue(32.3334J5).
The total weight and volume of sludge produced in the
physicochemical process will usually be about the same as that
produced in a conventional primary-biological plant. The
sludges will be similar to those described in Section 4.4. These
sludges may require smaller imounts of conditioning
chemicals to dewatcr than biological sludges. However, their
final disposal may or may not be any more simple or more
economical (sec Section 10).
The filters and carbon bed (if of the downflow type), when
backwashed, will produce a liquid sidestrearn that has aboutl
200 to 500 mg/I of suspended solids which will be fairly dense
and settleable, and this stream will have to be recycled to a
clarifier for solids separation.
Concurrent Biological and Activaled Carbon Process. This
process involves the useof powdered carbon mixed with the
activated sludge. Currently (1973), there is_no municipal
wastewater treatment plant utilizing this process. The method
could have merit where a considerable amount of soluble

src matter with bw bi. aàbiIity I preee.t frOm
du rIaI mist 1 ro’us some p olsua cpci*ioa Irmilag
idu ria1 mscu(M 37 . The activated sledge and the ca bss
settle in the clerIRer to 2*. 3 perseut IO ds eZCIS,iVeO(ShS
• carb4 The powdesed caibosi I aol regeacraisder separated
from the osgenic ulei , which dewaisis with attIc or no
oiidit loniag.
heaves oflhelewcrcod.fpowdavdcaibdsisseompared to
the . ecc:jk oru s ’ . aoedidvs with ether I
— ce
Tliç Ipssal of the tiudge from espUc toMe (isplage) a a .o.tlaule probbrn, .spsihUjI.
— at the casatsy where no la* IC S1 — inlet. Septic tanks arUth ö iy
private taak4vuck epsralOin ish. then diupeas of the tiudgela vuiais way,, * I M
without anthod do so.
In a sur y made hi Indlea wywugs(34 mis ir plaMopOmiCi.
‘Z N ibsic polic .(albwI.sg dlechavgeot.spt.ge hit. thelrsmeuIotI ±
pla st 4 euto.e.hotof: Ipies itsd that th yp lsuahdI$diIj. Thu ,
saW sold ths 4W ace permit aibapsactice, but there was aocapbactioS.(wll s * WUP
was dicharget
The ii 1 ge fromsspsleta*clesaIig,Ihs.dIchai sd hit. the WIswóf ftáIi tbb ;cha
naps.. a shock lauding of BOD.dsiaps.d.d .ollds, together witha hjhs i *JTiIiiIàaaf
mousissad chebly wdr.de. Such dleshergus ha ,. upsct dI.p1 ti (irpdui(y
dhuiflers, causododorous conditions, sad advumely affscse dW.SCOndeVY’jb L T IOI
lauding to the digesters can he gaifnasdy i ssmt ThI condition i áfl 1’
many operators of email’ ØleateIssed us arm. where. h* bdr .rA wtiab bL
Uncositrolled dhdsarge of Iqtap lusts em utpsaet ts’tMt
wades I not desirable.
Some nt4nd •11ta 110s c i the dicuisarge of .uch.spttp 1L *
espscaly Wthi ámsont hb,ge - sad she plans it sslath* s L 1 b’fl *cIfle
hiformadosa1lIblwiegeNI pIst.EMuIani of isptags(39j SI N i *ists
that a’. hi vadom dfrEuacrobIc decomposition. If. h a psi dli lu
cowu.dhoWimg teak áuidiheapvmped ala low rats to she eiàimsnt p uss, shVad .ofkna
could be minimiisd. O orñuusa tinsjrom such a hold 1iák c iv bionp bltdiOa laij
degree by addingIlme to reIn thepH to about I I. Lime a ltlài loW 1IWau,
hydroirsulllds sndths odorn vokIlieacid, . hi udditioa,ahighdsiseofp*thöjAt*
inactivation would beobsalned. Thesladge could be asuetedaher lbs li la á k1ua.
The situation it coavnicaeed beonmebsussuclds having sect fl ateermh1áot p i n ot I
mmsicip.lky or psausealiuait which financed the buiIdluisonhsiecbra sy Ostd trs.tmedt
plant. In some areas there its larger population using septic teaks than it cdhA to the
1 sswer age system and public ‘:: : plant. Csnaily, those who hn re septic lacks ssd
private contractors who dma such tasks muss pays foe if theysapocta t,catmsM p it,Iblch.

they did not pay to build or to operate, to handle their septage. This is a matter that may
warrant some sort of regional control. Septage is a serious problem in some areas and requires
prompt and positive action, if existing and even newly built wastewater treatment facilities are
to perform as required at all times.

Before sludge can be disposed of, it must first be treated to reduce any adverse impact on the
receiving land, air, or ocean. The term “sludge stabilization” is used to describe those methods
which will reduce the detrimental impact of sludge disposal, i.e., render the sludge as
innocuous as practical. Though stabilization has a specific technical meaning, in this report it
involves other items. The following are the primary requirements for stabilization:
1. The highly volatile portion of the sludge is either removed or so treated that
rapid decomposition, with resultant rapid oxygen consumption and the
creation of odors, does not occur.
2. Any toxicants are in a form which would not immediately and adversely
affect the environment. For example, organic toxicants, if not degraded,
should have been altered in composition so their toxicity has been largely
eliminated. Heavy metals should be complexed or rendered insoluble in
water. (However, changes in pH after disposal may eventually cause heavy
metal solubilization.)
3. A high degree of kill or inactivation of various types of pathogens should be
I attained.
5.1.1 Anaerobic Digestion
Water Pollution Control Federation Manual of Practice No. 16 should be referred to for a 1
description of this process and the various design and operational considerations.
Anaerobic digestion involves biological decomposition of organic material in an environment
devoid of dissolved oxygen. Decomposition results from the activities of two major groups of
bacteria. One group is the “acid-formers,” many of which are facultative. In the absence of free
dissolved oxygen, they convert carbohydrates, fats, and proteins to organic acids, alcohols.
and CO . Amino acids are broken down to ammonia. The othergroup is the methane bacteria
which convert organic acids and alcohols to methane and CO 2 . These latter bacteria are
somewhat slow-growing and are sensitive to various toxicarits, such as heavy metals and
chlorinated hydrocarbons. They cannot grow in the presence of any free oxygen in the liquid.
and their optimum temperature is between 850 and 95° F. Below about 70° F, their activity
practically ceases. The acid-formers are not nearly as sensitive to an adverse environment.
Preceding page blank 21

Good anaerobic digestion reduces the volatile matter by 40 to 65 percent. The remaining solids
settle out so that-their concentration by weight is not much less than their concentration in the
raw sludge fed to the digester, and is frequently higher. Anaerobic digesters musi be at a
temperature of 85° to 95° F, which requires heating except in tropical climates. In the$st they
have been single stage or two stage. In the two-stage system, the liquid in the first-stage unit,
where the active biological decomposition takes place, is usually continuously nux d by gas-)
lift circulation, pumped recirculation system, or mechanical mixers. I
In the ecbnd-stage unit, there is nodirect requirement for heating or mixing (although the
equipment should be provided for operational flexibility); instead, a quiescent cohdition is
providgd which leads to the settling out of the solids and formation of a supernatant. In
genei’alj the dige$ion proceeds for about :30 to 60 days. After equilibrium, the solids arc
allowed to settle and are periodically removed for dewatering. Under current practice, the
supernatant is normally sent back to the biological treatment plant because it is high in BOD,
tine suspended solids, afld nutrients; however, s jch a practice must not degrade the (lilt
effluent. Othqfwise,.thc supérnatant should .given p .r separate treatment. The settiCable
solids can bede waéed on Sand beds Lthout further:pot ditioning, though for dewatering by
mechanical equipment digested igeis furthçrconditioned by chemicals or heat.
The combined volume of supernatant and settled solids discharged should be somewhat less
than the inflow, since some of the solids arc converted to methane and CO 2 and escape the
digester. However, the weight of solids discharged should be significantly less than that of raw
sludge fed, since an appreciable portion of the original volatile solids has been liqulfiedand’
gasified. Of course, if the digested solids settle properly, as they should undel suitable
operating conditions, their volume should also be less than that of the raw sludge fed to’ the
The destruction of the volktile solids results in the conversion of the organic nitrogen to
ammonia, which remains insolution asammonium carbonate. As a result there isa substantial!
increase in the alkalinity of the liquid. The phosphorus is released into the solution as some’
form of phosphate, together with other inorganic residues.
Anaerobic digesters are su eptiblc to upsets, primarily due to the sensitivity of the methane-
forming organisms to variations in environment and toxicants in the sludge. Heavy metals,
phenolics, and chlorinated hydrocarbons will inhibit the action of these organisms, resulting in
an accumulation of organic acids and a resultant drop in the pH. The kinetics olinacrobic
digestion are discussed b r Pfeffer, es dI. (4I). Hñdin and Dunstan studibd, under controlled
laboratory conditions, the factors that can cause improper operation ofdigesteia(’42 43). The
loading” of a digester can be expressed in two ways: hydraulic loading (detention time). and
solids loading (usually expressed as pounds of volatile solids per cubic foot of digester p rday).
Hindin and Dunstán studied the effect of both by using the same sludge but chinging its
concentration, either by dilution or by centrifuging. The important infârmation they obtained
is as follows.
With a detention time of 33 days, increased solids loading hadthese-cffccts:
I. increase in volatile solids
2. Increase in alkalinity
3. Increase in suspended solids (nonsettleable) in supernatät.
With a constant solids loading of 0.075 lb/cu ft/day of volatile solids, a decrease in detention
time had these effects:

I. Increase in volatile acids f
2. Increase in BOD and suspended solids in supernatant
3. Decrease in pH, alkalinity, and ammonia
4. Increase in odor of supernatant.
It is thus apparent that digester design criteria depend both on sufficient detention time and
proper solids loading. The inflow solids concentration to the digester determines which factor
is critical. The above investigators found that 30 days’ hydraulic detention time was a critical
value and that the loading should not exceed about 0.075 lb/cu ft/day of volatile solids.
There is a natural buffering system in a digester, due to the ammonium carbonate, volatile!
acids, and CO 2 . For example, a decrease in ammonia (because of decreased detention time) can
cause a digester to become acidic. Another important factor which also depends on sufficient
detention time and proper solids loading is the affinity between the suspended solids and the
liquid. The solids are lyophilic due primarily to the proteins and coiloids. With a sufficient!
digestion time, the proteins are degraded and colloids decomposed. The solids are then’
lyophobic—they have a greatly decreased affinity for the liquid and will settle.
High-rate digestion of primary and activated sludge has been successfully practiced, but
operaticns must be strictly controlled. Zablatzky and Baer describe the necessary controls
thoroughly(46), such as complete mixing. uniform feed, and frequent monitoring of volatile
acid, alkalinity, and pH. The loading for high rate digestion is 0.10 to 0.40 lb of volatile solids
per cu ft/day and the hydraulic detention time is 15 to 20 days. The completely mixed contents
are discharged into a second unit for supernatant separation. -
Some field experiences indicate that mixtures of primary and activated sludge are extremely’
difficult to settle and to obtain filterable concentrations of solids after anaerobic digestion
(44X45) . A review of studies involving digestion ofactiva ted sludges invariably indicatcs that a
poor quality of digested sludge was obtained because the basic criteria controlling anaerobic
digestion. as discussed above, were not followed in the design and operation of thedigesters. In
addition, it is important that activated sludge be thickened before being pumped to a digester
(see Section 6). It has been the general e crence that separate thickening of the activated
sludge is preferable to return of the wasic . tr.ated sludge to the primary clarifier.
Stanbridge (47) and others (48) have made tc nsi e tests involving digesting and dewatering
primary sludge, mixtures of primary and . ted sludge, and activated sludge alone. The
results are important, since all the sludges ‘i d he dewatered satisfactorily. It was concluded
that digested activated sludge dewatered 1c s readily than a mixture of primary and activated
and required more chemical conditioner, sep4 rate thickening of the excess activated sludge
ahead of digestion was effective, and a sm Ikr proportion of the volatile matter was destroyed
when activated sludge was digested than when primary sludge was digested. To obtain a well-
settling digested activated sludge, a detention time greater than 20 days was necessary.
From a practical operational standpoint, one of the most common and troublesome problems
of anaerobic digesters is cleaning the grit and other heavy solids that accumulate at the bottom.
lithe solids are not removed, they gradua Dy decrease the digester volume. Good mixing in the
digester will reduce the frequency of cleaning and is desirable in any case. However, the design
engineer should provide properly sized and accessible openings so that settled solids can be
easily removed by high pressure water streams, for instance. This is discussed in detail in
Manual of Practice No. 16 of the WPCF.

Cleaning of the digester may put the unit out of operation for several days. The two-stage
system could provide a means of continuing digestion when there is only one primary digester.
and it is inâpirative. Of course, heating and mixing would have to be provided for the
secondary digester in that case. The two-stage system, in addition to providing, normally, a
long settling time so that the suspended solids can be reduced to a minimum in the supernatant.
can provide more foolproof operation and a “backup” system when the primary unit is not
operating. It has generally been noted that a better quality supernatant and better overall
operation. as far as solids settling is concerned, are obtained if two-stage digestion is used.
Considering the frequent upsets and other problems that digesters are prone to, it isan expense
that is worthwhile, especially for high-rate digester systems.
5.1.2 Aerobic Digestion
Aerobic digestion is less sensitive to toxicity than anaerobic digestion. If sludge temperatures
below 50° F are to be encountered for any length of time, increased retention time should be
sludge in a tank that is usually uncovered and unheated and has a depth of 10 to 20 ft. The
principal operating cost isibe power required for aeration. The sludge is supplied with oxygen
so that a dissolved oxygen concentration of at least I mg/I exists in allportions ofthebasln.
The aeration can be accomplished by means of compressed air and porous diffusers, surface-
type mechanical aeratOrs, or submerged turbines supplied with compressedair (or oxygen).
However, with relatively thick sludges, it may be almost impossible to dissolve and distribute
oxygen throughout the entire sludge unless a mechanical device is used.
Aerobic digestion is less sensitive to toxicity than anaerobic digestion. If sludge temperatures I
below 50° F are to be encountered for any length of time, increased retention time should be
provided. Normally, a 10- to 15-day retention time is sufficient to stabilize the sludge and
accomplish a reduction in volatile solids of about 30 to 55 percent (49,SOS1). If the!
temperature of the liquid djeps to around 40° F, the retention time should be increased to 25 to I
30 days. Aerobic activity of some degree has been observed down to freezing temperature.
Biological oxidation generates heat and, for thick sludges having a high volatile content,
excessive temperatures can be produced if heat loss from the unit is insufficient(S2). Oklahoma 1
State University has carried out and reported on some extensive tests on aerobic digestmn(5i).
Though sometimes unexplained and erratic results were obtained, in general, aerobic digestion.
was effective in improving the drainability of the sludge. The dcwatering characteristics of I
aerobically digested sludge are usually similar to those of anaerobically digested sludge(54
Cameron reported recently on aerobic digestion of waste activated sludge to improve its
fi lterability (5S).
The oxygen requirements arc about 10 ppm/hr/ 1,000 ppm of volatile solidsin the digester.
However, if primary sludge is digested with secondary sludge, the oxygen requirements will
incrcase by 50 to 100 percent above the 10-ppm/hr figure. If compressed air is used with porous-
diffusers, aboul25 to 35 cfS/ minI 1,000 Cu ft of digester volume should be sufficient If aerobic
digesters arc to be used in cold climates, care should be taken to keep the temperatures above
40° F, and the detention time should be about 30 days. Also, in freezing climates, or where it is
difficult to maintain a proper temperature in the digester, surface-type mechanical aerators
should not be used.. Eithercompresscd air with diffusers or a submerged turbine supplied with
compressed air should be used in such cases. In cold climates, the units should be protected
from heat loss. Care should be taken that all solids are kept in suspension and the digester
liquidis well mixed, and that the proper dissolved oxygen level is maintained. There are, of
course, no odors generated from a properly operated aerobic digester.

As with anaerobic digestion, when the liquid portion is returned to the treatment plant, it
results in a pol1utai t load which would not be imposed if the sludge had not been “digested.” In
the case of aerobic digestion, there is very little BOD load imposed, but the nonbiodegraclable,
or poorly biodegradable, organic solubles as measured by COD or TOC are increascd. More
importantly, a large portion of the nitrogen and phosphorus is solubilized and oxidized to
nitrates and phosphates, which results in increased costs if they are to be removed from the
effluent. The fine suspended solids in the supernatant can be fairly high, since prolonged
aeration causes deflocculation.
Mixtures of primary and secondary sludge may be stabilized if sufficient agitation is provided.
With primary sludge, it is possible for grit and other heavy inert solids to enter the unit.
Therefore, the basin must be designed for easy cleaning and removal of the heavy solids which
will not be kept in suspension. It is good practice to have at least two basins so that one could
be out of service for a few days and some digestion achieved in the second basin. Also, the
second basin can serve the same function as the second-stage anaerobic digester tank; that is,
accomplish liquid-solids separation, but be equpped with aeration so that in an emergency it
can perform as a digester.
5.1.3 Composting
Composting is an aerobic, thermophilic, organic sludge stabilizing process. The therznophilic
organisms grow best in the range of 130° to 165° F.
Dewateied sewage sludge—about 79 percent moisture—has been composted in a specialLy
designed unit(56). The reduction in total solids was 30 percentandvolatileswere reduced by47
percent. No pathogens could be detected in the final product, even with massive inoculations of
the raw sewage. The sludge is completely stable and does not attract insects, so it can be readily
disposed of on land or used as a soil conditioner. Since the process is aerobic, almost all the
ammonia is either oxidized to nitrates, or goes off as a gas.
The principal chemical process that can stabilize a sludge in accordance with the previous
definition of stabilization is the application of lime to obtain a pH of 11.5. It has been known
for some time that applying lime to organic matter and raising its pH to above 11 will cause
complex changes in the volatile solid matter, especially when the sludge is dewatered. If placed
on the land, putrefadtion will be suppressed or a coasiderable time and then, as the pH drops,
gradual decomposition of the organic matter will occur with considerably reduced generation
of odors. Furthermore, at pH of 11.5, the destruction of pathogenic organisms is practically
total The evidence is strong that at such pH virus inactivation can occur(57). Although there is
no good evidence as yet with regard to destr’iction of dormant cysts, worms, and spores,
nevertheless pathogen destruction in a few hours is greater than that accomplished by the
anaerobic or aerobic digestion process over a period of several days.
A detailed study was recently reported on lime stabilization of primary organic sludges and
mixtures of organic and inorganic sludges by the AWT Laboratory of the EPA in
Cincinnati(58). To ensure maximum pathogen destruction, it was found desirable to add the
lime to the sludge over a period of time untia the pH stabilized at a out 11.5. It could then be
dewatered on a sand bed or a vacuum filter. No further conditioning of the sludge was
necessary. The amount of lime required wilLdepend on the chemical composition of the wastes

and whether the raw sowage had been treated with alum or an iron ak to increase the solids
capture in the primary basin. For raw sewage sludge, the lime cost would be about $2/tonal
dry solids; if the sludge had been treated with shun the lime cost would be about $4 toss pen
ton of dry solids; with previous lime treatment, the cost would be $23 to $3 per ton. A sludge
cake with such lime treatifleit could be dh osed olin a properly Operated badlilL
Another chemical Ireatm t that has bun used mi satly to stabiliss sludge and enhance its 1
dewatering characteristics is the addition of leigedousges of cidorine. The .quipmeitfor this
proprietary proenis consisid ala prexc tank and recircuinting pump. Thechlorimeb applied’
to the raw sludge, which is pumped Into a ptsswricsd holding tank under about 45 psi and held
the c for 10 to IS minutcs(59).
The studies rs initially med , in the boifltodesof the Pissaic Valley Sewerage Commisuica,
Newark, New Jersey. The mvestiptoi found that, by adding 500 mg/I of eblorlib per such
percentage solids concentration elan organic sludge, he Was able toeliminate odors and the
color of she sludge chang from black to light tan. This chlorine dasageraho increased the
drainability of tire sludge.
C lorination depresses tlre pH, and this should be corrected by addii an aihalL ‘if
hypochiorise is used, no sigisif leant chang in pH OCCWL At a dosage of 500mg/I and. sludge
with I percent solids, 100 lb of dilorlun are needed per ton .ldiy solids
At the outlet of the reactoi the residal habosit l0mg atthe 00 mg/tdosagea.d2O0mg/I
at double this dosage Nohiformation hè$ been tepo(ted rsgei diu hew thin residal changes-
and in what form k exists. Bsçauso .1 lbs high a rnonh-ultiogu. content of lswagr.ludge -
snout of the residual is probably in the form of chIO,amiaes at hint Initially.
The chlorinated sludgeis reportsdto be stable and todswatufrobauaud’bedwithiutglving.ff
any odors. Defore vacuum fikatino orcentrIfo tiOn, tlreuludgeuirey require semsfsrthsu’
chemical condkIàsl j No installations involvlngsuth mechanical dewatcning are km!wn at
present. Is would appear that proper neutralbatlon of thesludge, prabab1y-with me which
could fwthcr coaditicfl It, would be casentlal before meCha ical deiuterin$. It s ma-
reasonable to expect that the liquor resulting from sucbchlOrluue tiCatacat of the*sdgs may
have many types of cbloriiiated orghnlcs in solutiOn, I. addklento chtoramkme The toxicity
to fish life and other aquatic life if ‘this liquor, even In low fl rne*utiosis , must becousideret
Either recycling the liquor to thG plant biological treatmeflt process or treatlngJtbeliquor
separately mireS be carefolly studlif and cishitet Any ddorinc-complexed’os’gaulea may
have to be removed, and close attentiOn must be given to the blodegradibilky of tl liqUor and
its clTCct on the plant.
Disposal of the dswatere4 sledj solids in inndfllb may he paulible, though no kif mation
appears to exist regarding the claulcteristim and cosipodUss of my kachalet from such
sludge. Before ka!y chlãliretksn ussd for sludge treat nt, conelderatlon begiveisto
the plant efihient, the effect of the sludjliquor when returned to the trea1 ncnt plsM,and the
coispositson of inachates from sludge disposal , Ks No lelormation his been found as the
composition of the soluble. a the liquor. Recently, a small treitmant plant was desigeed for
Delhi, New York, which will use this process to treat the sIud . The sludge will be dewaisred
on saód bsds(12M

The only practical sludge stabilization method that can be considered as being essentially
“physical is heat.treatment. However, this term is probably not too accurate, since during the
heat treatment there are changes in the composition of the organic matter—some solids are
solubilized, and in one of the heat treatment processes, which adds a small quantity of air to the
reactor, some oxidation occurs. This process is discussed in more detail in Section 8.2. Since
the temperature of about 350° to 400° F can destroy pathogens and can degrade a large portion
of the volatile solids, the final sludge is considered stabilized and can be disposed of on the land
or in a landfill after dewatermg.
Pasteurization (1500 F for about 1/2 hr) of liquid sludge has been considered in this country
and is practiced at several localities in Germany. It destroys pathogens to a high degree, but
does not stabilize the sludge since it does not reduce the volatile solids.
27 j

Gravity thickening is the most common sludge concentration process in use at wastewater
treatment plants. Suspended solids particles with a sufficiently great settling velocity may be
separated from water by maintaining quiescent conditions. Gravity thickeners usually follow
gravity clarifiers, sometimes in the same unit, but the emphasis is on removing water from
solids rather than solids from water. In thickening, the predominant mechanism is hindered
settling rather than free settling typical of clarification. An advantage of gravity thickening is
its siniplicity. However, gravity thickening doà not produce as highly concentrated a sludge in
some cases as do other thickening processes. - ..
in a primary clarifier, the settled solids can thicken sufficiently under the right conditionsj
without any further treatment. Sometimes a coagulant or an organic polymer is added to aid:
the removal of the finer and colloidal solids that do not normally settle out. Inorganic!
coagulants also can precipitate phosphorus. Primary clarifiers for sewage are usuallydesigned t
for average hydraulic loadings of 1,000 to 1,500 gpd/sq ft. They can be either circular or
rectangular and must be equipped with skimming devices.
The clarifier following a biological treatment process must handle much lighter solids, and!
therefore the hydraulic loading averages only about 600 to 700 gpdfsq ft and should not exceed:
1,200 gpd/sq ft for peak flows, especially if activated sludge is handled. Such clarifiers’
accomplish a certain amount of thickening of the sludge that settles out. The sludge from at
trickling filter plant will normally be concentrated to about 3 to 4 percent by weight. However, I
activated sludge rarely concentrates to more than 0.5 to 1 percent in final clarifiers, except’
systems where pure oxygen is used, and then concentrations of 2 to 3 percent have been
To th cken the secondary or mixture of primary and secondary sludges further, either before
digestion or dewatering, the sludge can be pumped to a separate gravity thickener. These units
have hydraulic loadings of about 200 to 500 gpd/sq ft. Solids loadings are about 8 to 20 lb/sq
ft. The retention time cannot be too long if the liquid temperature is, say, 800 F or higher
because anaerobic conditions quickly develop with resultant gassing and floating of the sludge.
Gravity thickening is not too effective for waste activated sludge alone. Usually, it can only
increase the solids concentration from about I percent to 3 percent by weight or, in oxygen
plants. to about S to 6 percent. Also, if nitrates are present, denitrification could occur with
release of nitrogen gas bubbles, which will result in floating of the biological sludge floe.
Chlorination of the sLudge in the thickener is frequently used to suppress or delay anaerobic
conditions and denitrification, as well as to control odors.
Preceding page blank 29 \

One factor that can cause poor thickening and even flotation of light biological sludget
particles, especially activated sludge floc, is saturation of the liquid portion of sludge with
nitrogen gas because of exposure of the wastewater to the atmosphere or to aeration. Changes
in pressure or temperature that decrease the solubility of nitrogen can cause it to come out of
solution in the form of small bubbles which attach themselves to the sludge particles. Any
biological denitrification would aggravate this problem.
The recju7ed surface area t the thickener may be estimated from a solids settling rate test
made in a 2,000-mi graduite: The surface area must be large enough that the upward velocity
of liquid leaving the basin is not greater than the settling velocity of the slowest settling particle
which is to be captured. Sparr states that experience has shown that gravity thickeners foe
activated sludge alone should have a solids loading of about S to 8 lb/sq fe/day and 6 to tO
lb/sq (t/daywlien j)rimaryslüdge is mixed with activated sludge(61). These are loadings for
thickeners having stirring pickets on the scraper trusses, and can be increased if chemicals are
added. - —
There are varied opinions regarding whether activated sludge should be gravity thickened
alone or whether the primary sludge should be mixed with the waste activated sh ge.in the
thickener. Sparr (61) reports that experiences of Torpey in New York City show very definitely
that by mixing the two sludges in the thickener n any problems associatedwith thiekeningof
waste activated sludge aloile can be eliminated. The heavier primary solids fill the voids in the
light, flocculant activated sludge. Torpey has obtained solids concentration in the combined..
thickened sludge of 4 to 6 percent by weight.L -
.—, .q .- -
Conventional thickening tanks are usually circular with a cone-shaped bottoniend center
drawoff. The concentrated sludge is removed from the bottom and the overflow liquor is
drawn off forreturn to the plant inlet. The tanksareabout l2to lSftdcep(sometimcs lOfifor
small plants), with inlet facilities that dissipate the entrance velocities, and a single outlet pipe
with short suction connections. ___________
——-—.—- —. _________________
A common thickener consists of a circular tank with rotary collector arms equipped with
vertical “pickets” which gently agitate the sludge. The action of the pickets releases the water
bound in the sludge particles and also any gas bubbles. Blades on the bottom of the collector
arms move the concentrated sludge to a center drawoff point. A skimthing arrangement should
be installed.
The liquid overflow sidestream from sludge thickeners is returned to the plant after the
primary clarifier. The soluble organic and inorganic matter contained in the sidestream should
not be significantly different from that of the biological process effluent, unless tertiary
treatment will be carried out. It is good practice to return this overflow to thehead of the
secondarysystem, since the suspended solids, which may range from 100 to 1,000 mg/I ,are not
settlcabk in the time available in a primary clarifier and the overflow may cause other
problems. Any thickener overflows will, of course, increase the suspended solids and
associated BOD loadings on the secondary plant and must be considered as a cost attributable
to the sludge handling process of the plant.
The quantity of overflow from gravity thickeners appears to vary considerably from plant to
plant, depending on loading and sludge type. At Grand Rapids, Michigan, the overflow
amounts to about 0.3 percent of the plant flow(62), while the overflow at the Bowery Plant in
New York City averages about .5 percent of the plant flow (63). TIle difference in quantities of
overflow is explained by the difference in thickness of inflow sludge. There are instances where
returning thickener overflow has caused operatioria I problems. Such problems can occur when

septic or bulky sludge will not thicken and the resulting overflow contains high concentrations
of suspended solids and BOD. Jordan and Scherer(64) reported that recycling overflow from a
gravity unit that was thickening a bulking activated sludge resulted in a buildup of suspended
solids in the aeration tanks at Amarillo, Texas. with resultant carryover in the final effluent.
Flotation, like gravity settling, can be and has been used for clarification or removal of
suspended solids from the main wastewater stream. Like gravity settling, it has been adopted
for thickening waste sludges, especially organic sludges (such as waste activated sludge) that do
not thicken readily by gravity settling. To accomplish good thickening, and also to have a
relatively clear underfiow, the raw sludge is frequently “conditioned” with either an organic or
inorganic coagulant.
The particular flotation process described here is referred to as dissolved-air or pressurized-air
flotation. A volume of relatively clear water (usually the underfiow) is pressurized to 30 to 70
psi and air is injected into the pressurized liquid so that dissolution occurs. Various schemes
are used to inject a large amount of air into solution, though rarely does it approach about 75
percent of saturation, corresponding to the theoretical value for the pressure and liquid
This pressurized liquid is then released through a specially designed valve at a pressure equal to
the hydrostatic head in the flotation basin (about 3 to 4 psi) and mixed with the raw sludge. The
drop in pressure causes microscopic air bubbles to come out of solution and attach themselves
to the sludge particles or floc, and thus rapid flotation results. Waste activated sludge having a
concentration as it comes from the final clarifier of 0.5 to 1.0 percent by weight of solids can be
readily thickened to 4 to 5 percent. It is then suitable for dewatering with a vacuum filter, I
centrifuge, filter press, etc. The suspended solids capture can range from 83 to 99 percent,
depending on loading and usage of polymers. Such flotation thickening of activated sludge can
also be used ahead of anaerobic digestion
The two important design criteria are: (I) solids loading. on the basis of pounds of dry solids
per square foot of clarification area in the flotation basin, and (2) the air to solids ratio. It has
been found that the weight of air that is dissolved averages about I percent of the dry solids
applied. The solids loading may range from I to 2 lb/hr/sq ft. Some activated sludge may
require a polymer for the higher loading. 1 he hydraulic loading should not exceed about 1.0
gpm/sq ft. The above criteria are for thickening of waste activated sludge (65X66). One of the
advantages of using air flotation for thickening a sludge, especially activated sludge, is that the
system is kept aerobic. This eliminates septic action and “gassing” in thesludge, which
frequently happens in gravity-type thickeners due to anaerobic decomposition.
Flotation can be used to thicken various types of sludges, including inorganic sludges such as
metal hydroxides. Bench-scale test units are available for testing of any sludge to determine the
necessary design parameters for sludges which are different from those normally produced in a
domestic wastewater treatment plant.
It has been reported that flotation thickening without chemical addition has recovered
between 83 and 95 percent of the solids, based on average values for seven treatment plants
31 \

(67). Ettelt (68) also found that polymer addition increased the capture from 92,7 to 99.6
Flotation thickener liquor can be recycled to the secondary plant. Eltelt recorded the solids
concentration in liquor from thickening activated sludge without chemicals as between
and 1 ,000 mg/I. The liquid underfiow, and also the sludge in a pressurized air flotation unit,
has, of course, a high C ncentration of dissolved oxygen, which prevents any septicity.
Centrifuges have been used in the wastewater treatment field primarily for sludge dewatering
(see Section 9.3); however, one type of centrifuge, known as the disk or nozzle type, is coming
into use for thickening of aCtivated sludge. The disk type centrifuge has been used for many
years in the chemical process industry, but its use for sludge thickening in the field of
wastewater treatment is relatively new and only a few installations exist (691 (70). its use at
present is limited to waste activated sludge. The thickeningefficiency of thedisk centrifuge is
comparable to that of the pressurized-air flotation system. The principal advantages of the dilk
centrifuge are compactness, overall lower total costs, and improved solids capture without
chemicals. There is a risk of clogging if proper screening equipment is cot used and miiñtnined
ahead of the centrifuge.
The sludge passes through nozzles about 0 . I in. in diameter due to the pressure produced by a
“g” value of 5,000 and greater. It then flows out of the unit. The sludge entering the unit m t be
degritted by a hydrocyclonc,if effective grit removal does not occur in the treatment plant, and
at least one self-cleaning screen should be installed ahead of the centrifuge to remove any
particles (such as hair or string) in the liquid larger than the smallest openings in the nut. If
there are large amounts of this material, two screens should be provided. The thickened sludge
can go to a digester or to other processing for eventual dewatering. The solids capture’sna disk
centrifuge, when using a polymer, can be on the order of 95 to 98 percent with a moisture
content of about 4 to 5 percent. Capture oilS to 95 percent has been obtained withoutchemical
The centrate can be returncá to the aeration basins if evidence indicates that the solidseaus be
entrained in the main portion of the M LSS(70). The liquid from the self-cleaning screen shouki
be returned to the primary clarifier. If most of these screened solids are not captured in the
primary basin, this sidestream must, be disposed of with the thickened sludge otherwise, the I
screenings will build up in the system. Screenings in the thickened sludge reduce the solids
concentration only slightly, since the stream is only a small percentage of the main lkp .uId
sludge stream.

In the sludge digestion or stabilization process, whether anaerobic or aerobic, the solids
volume is decreased because the volatile solids are destroyed by conversion to gases, water, and
soluble residues. The remaining solids will settle, if the process has been carried out properly
(refer to Section 5.1), leaving a supernatant which must be removed and disposed of in some
manner. These solids should concentrate to approximately the same percentage solids as the
raw influent sludge. The supernatants have a high concentration of soluble and insoluble
poLlutants and a volume which can average about I percent of the raw wastewater treated.
Handling and disposal of supernatants can present some serious problems in the overall plant
Anaerobic digester supernatants vary considera bly from plant to plant. It has been noted that
supernatants vary from clear, through sh.ad of yellow, to black(71). The odor may be
acceptable or very offensive and nauseating It may be relatively weak liquor, or its strength
may be extremely high in terms of the ra . istewater.
Several factors which affect supernatant q i I include the type of sludge treated, the design
of the digester, and the method of operati)rl ‘ .‘). Plants treating primarily domestic wastes I
produce a weaker supernatant than plants t’eating wastes containing large percentages of
organic type industrial wastes. It was also ubserved that a raw sludge with a high volatile
content will produce a supernatant with hig cr solids content than a raw sludge with a low
volatile content. Short digestion periods of 10 to IS days have been found to leave more solids
in the supernatant(73). It has also been noted(74) that supernatant from primary plants is
generally weaker than supernatant from secondary pla.ns, but there is considerable variation
among plants of each type. Indeed, variation in supernatant quality fora particular plant is not
Table 3 presents data on supernatant characteristics according to plant type(75). These data
show that supernatant from primary plants is, on the average, weaker than the other
supernatants, but there is considerable overlap.
The quantity of supernatant varies less than the quality. Generally, the quantity will range
from 5,000 gal. per million gallons (mil gal.) of sewage treated for primary plants to 10,000 gal.
and more for activated sludge plants, with trickling filter plants falling in between. A
conservative figure to use for activated sudge plarL s j rce t of the wastewater flow.
- 33

Actn tcd
Primary Plants
Trickling Filters
Sludge Plants
Suspended solids
Ammonia (NH,)
Total phosphorus asP
300-700 —
I 9ndudes primary uludgi.
ecycle of supernatant to the plant will have no significant effect on the plant hydmuhcs. Such
ecycling can, however, affect other portions of the plant operation, such as aeration
equirements, chemicals iequired for coagulation, phosphorus removal, ammonia removal,
md the fuial effluent quality in general. Plant operation is affected not by the supermata.4
volume but by the mass of constituents in that volume. For some constituents, the amountin
the supernatant is negligible compared with the amount entering the plant, but for other
onstituents the amount of supernatant is substantial. Malina and DiFilippo(75)reportedthat
the amount of nitrogen in the supernatant of the Archibald, Ohio,activated’ sludge plant was
ibout 40 percent of the nitrogen in the plant influent.
Returning digester supernatant to the head of a plant can produce several problems. In some
cases, supernatant return has caused the primary tanks to become septic and emit odors. The
sludge from these tanks also becomes less concentrated. Supernatant return can came
additional problems in secondary plants. Clogging of trickling filters can occur, and odor
problems can arise because of the release of dissolved gases as the primary effluent is applied to
the trickling filters or is aerated in activated sludge plants.
Returning supernatant will increase operating costs. The added load increases the chlorine
demand in prechlorination. Greater quantities of chemicals are required for chemical
coagulation. All such costs are attributable to sludge handling and disposal.
Supernatant addition can cause periodic upsets in biological treatment processes. Also,
supernatants have h gh soluble COD values, a portion of which is not readily degraded
biologically. Because of upsets, and because treatability is affected, discharge of organics to
receiving waters iS increased.
The above problems seem to dictate against recycling supernatant in a treatment plant;
however, some operators have found few problems from such recycling. The effect of recycling
supernatant can be minimized by proper design of digester and adequate allowances in general
plant design.
Many of the problems associated with recycling supernatant seem to be caused by the septicity
of the liquor. Return of the supernatant to preaeration units has been found to be helpful,
although this could cause release of odorous gases and volatile organic compounds.

Even though recycling supernatant does not appear to affect operation of many plants, and
may reduce BOD and solids removal efficiency only slightly, recycling can greatly affect
removal of nutrients by a treatment plant. It was found at the Archibald, Ohio, activated
sludge plant that 57 percent of the nitrogen fed to the digester was returned as supernatanq7s).
It has been noted by Vacker, el aL(76), that digester supernatant should not be recycled directly
to treatment plants removing phosphorus by uptake in aeration tanks. The return of
supernatant which has a high soluble phosphorus content (such as that from an activated
sludge plant) will work against phosphorus removal, since it is the biological sludge which is
the vehicle for removing phosphorus from the wastewater being treated. However, any
phosphorus that is TM tied up” chemically by aluminum or iron will, to a small degree, be released
to the supernatant (see Sections 4.4 and 4.5). The only phosphorus released is that due to
decomposition of the volatile organic matter.
The above discussion indicates that plants can and have been upset due to return of
supernatant; however, there have been cases in which the effect has been minor. Nonetheless,
the costs associated with removing pollutants that are present in the supernatant, and are
limited in the final effluent, are attributable to the sludge handling process in the treatment
A process was developed many years ago by Kraus of Peoria, Illinois, for handling wastes high
in carbon but deficient in nitrogen. In the process, supernatant, which is rich in nitrogen, is
aerated with some digested sludge and with part of the return sludge from the activated sludge
plant and then returned to the aeration tank. The nitrogen content of the supernatant corrects
the nitrogen deficiency, and the solids in the digested sludge improve the settleability of the
mixed solids in the aeration basin. If the plant is designed for nitrification, the aeration nitrifies
the ammonia in the supernatant.
To reduce, or possibly eliminate, the adverse effect of recycling anaerobic digester supernatant
to the treatment plant, there are several techniques or operational modifications that can be
employed. Adequate thickening of sludges in settling tanks or in sludge thickeners ahead of
digesters is important to reduce the volume of supernatant and obtain a more concentrated
digested sludge.
When sludge is pumped into an unmixed digester, some disturbance of the digester contents
occurs. In single-stage units there isa tendency for the solids in the supernatant to increase. It is
generally agreed that pumping sludge to single-stage digesters at a uniform but low rate is
preferable to pumping intermittently at high rates, but this is not always possible at small
plants. The effect of pumping disturbance on the supernatant is substantially reduced in twa-
stage digestion.
High rate digesters can, and very frequently do, cause serious problems with regard to the
quality of the supernatant and the thickening of the solids in the digester. In fact, there have
been instances where the solids remained almost completely dispersed and did not thicken
much above 2 percent by weight, which greatly iracreasea the costs cf dewatering. Such pocr
performance was attributed to the operation of a hydraulically overloaded digester, or,

looking at it in another way, reducing the solids detention time below that required to obtain
proper digestion. Elutriation has been helpfji l in obtaining improved thickening of the solids
from such digesters. Of course, this has been at the expense of handling, usually by chemical
coagulation, the fine solids in the elutriate.
Digester loading affects the quality of supernatant. Data show that digesters loaded atO.40 lb
volatile solids/cu ft/day produce supernatant about twice as strong in terms of suspended
solids and about three times as strong in terms of BOD as digesters loaded at 0.10 lb #oIatile
solids/cu ft/day. Burd(l) reports on a very interesting performance comparison between a
high-rate and a standard-rate digester. The digestion time for the high-rate unit was 16 days
and for the standard-rate unit was 30 days. The reduction in volatile solids was about 65
percent in both units. However, the settled digested solids in the standard-rate unit were 6to9
percent by weight and 3.5 to 4 percent by weight in the high-rate unit. It is not economical to
dewater a thin sludge, as is well known. Short digestion times produce fine lyophilic solids
which remain in the supernatant.
If high-rate digestion is to be practiced. it is essential that the incoming sludge be thickened to
about 4 to 5 percent. The digester should be completely mixed and operation closely
controlled. This means that excess activated sludge must be thickened by either pressurizodair
flotation or the use of a disk’.type centrifuge.
There are several possibilities for treating supernatants. Storage of the supernatant in lagoons
for long periods to allow settling of the suspended solids is simple, but does require
considerable land area and precautions to minimize leachate. The lagoons must be isolated to
ensure that there are no odor nuisances.
Because of fly breeding and odor problems, lagooning is not recommended near residential
areas. Howe(73) found that a detention time of 60 days decreased BOD, suspended solids,
colors, and ammonia by about 85 percent. Hydrogen sulfide was diminished by 94 percent.
Aeration of supernatant lagoons can provide additional treatment, but at the risk of increased
Separate aerobic biological treatment of anaerobic supernatant by trickling filters and the
activated sludge process has been studied. The Greater London Council, in their
investigations(48), studied the treatment of digested sludge liquor using coke as the trickling
filter media. They investigated various dilutions of the supernatant with clarified plant
effluent. A 1:1 dilution gave about 60 percent removal of ammonia and 85 to 90 percent BOD
removal. The ammonia concentration was about 400 to 500 mg/I in the untreated
supernatant. However, more studies are needed to establish precisely some of the design and
control parameters.
Separate treatment of supernatant may be necessary at existing plants which arc overloaded
and producing poor quality effluents. An overloaded plant is likely to have an overloaded
digester with resultant poor quality supernatant. Separate treatment of supernatant could bea
temporary solution before the main plant can be enlarged and upgraded.
Phosphorus can be removed from supernatants by treatment with lime to raise the pH to about
9.0 to 9.5. The precipitates formed are various calcium phosphates and ammonium-
magnesium-phosphate (NH 4 MgPO4, which is very insoluble at a pH above 9. Also, some
magnesium phosphate will be precipitated, depending on the amount of calcium and -
magnesium tations which are present.The solubility of NH 4 MgPOiis only about 160mg/I at
normal pHAppawwatcr, and thislajurther reduced in the digester supernatant because of the

ammonium ions present. This solubility is equivalent to about 20 mg/ 1 expressed as
phosphorus. The concentration of P in supernatants can amount to scveral hundred mg/I as
P. A detailed study made by the Dearborn Chemical Division of W.R. Grace Co.. supported
by the EPA(77), established the various treatments and conditions that are necessary to
precipitate phosphorus from digester supernatants. The total precipitate will be a mixture of
NH 4 MgPO 4 , calcium phosphates, and magnesium phosphates. If the calcium ion is low, the
addition of a magnesium salt such as MgO or MgSOi will produce a precipitate that has high
fertilizing value because of the concentration of nitrogen and phosphorus it contains. Itcan be
used without concern about root or leaf “burning.” since it dissolves very slowly.
‘Because of the low solubility of the 1 H4MgPO 4 in digester supernatant, it forms
supersaturated solutions, especially if there is a change in pH from evolution of CO 2 . and will
precipitate out as a hard scale on pipes and other wetted surfaces. The Hyperiori plant at Los
Angeles has experienced serious problems with such depositions(78 79). Of course, the
problem would only be serious in hard water localities, especially those that have a relatively
high magnesium hardness. The precipitation of the NH 4 MgPO 4 can also be accomplished by
heating the supernatant to about 65° C to decompose the ammonium bicarbonate present in
supernatants, release the C02, and raise the pH.
The addition of lime or magnesium salts to precipitate the orthophosphates will also coagulate
and remove a Large portion of the fine suspended solids present in many supernatants, thus
reducing the load on the treatment plant.
The liquor in anaerobically digested sludge has a very high alkalinity, primarily from the
ammonia generated during the digestion process. It also hasa variety of organic and inorganic
compounds such as phosphates and a high concentration of free CO 2 . methane acids, and
minor amounts of hydrogen sulfide.
In the mid-1930’s, Genter (80X81) proposed hat the amount of inorganic chemicals (usually
ferric chloride and lime or both) used to condition digested sludge before dewatering on a
vacuum filter could be substantially reduced if the alkalinity of the liquor associated with the
sludge could be lowered by the simple process of “diluting out” with fresh water or treatment
plant effluent. He termed the process elutration. Burd(I), in his report on sludge handling,
gives a detailed description of the process and its present status in the wastewater treatment
field. Burd and others point out that Genter may not have fully realized that a major benefit of
elutriation was the washing out of the fine unsettleable solids and not merely the reduction in
alkalinity. Eliminating such fine colloidal-type solids from the settleable solids reduces the
amount of flocculating (conditioning) chemicals required. These fine solids in the elutriate
must be removed either by separate treatment or by recycling to the plant. Settling by itself is
not the answer. Separation can be either by use of additional flocculating chemicals or by
adsorption in the biological system, such as on the activated sludge.
The inability to settle solids inthe digested sludge properly may indicate that elutriationwould
be helpful. However, it should be kept in mind that the nonsettleable solids present in the
elutriate are the same nonsettleabk solids originally present in the digested sludge, only they
are now in a larger volume of liquid. It cannot be definitely assumed that these solids can now
be removed by recycling the elutriate to the treatment plant. There have been many instances
.where such solids, because they could not be coagulated, have passed through a plant and out
- 37 ,t

mnto the effluent lfthey are captured in the plant, the BOD, suspended solids, ammonia,
phosphorus, and any other pollutants that the elutriate contains must be considered s being a
load over and above that imposed on the treatment plant by the raw wastewater. It may be
necessary to treat the elutriate with additional chemicals to coagulate and settle the solids
before the elutriate is recycled, or the chemicals can be added and flocculated and the
suspension returned to the treatment plant to ensure solids capture. However, it should be kept
in mind that mixing a liquid having flocculated solids with another stream of entirely different
pH and other chemical characteristics can cause deflocculation.
Several plants(44X45)lhave experienced serious deterioration of their final effluent from
recycling the elutriate. The fine solids in the recycled elutriate have degraded the effluent and
caused odors. The basic problem is that the digestion process has created poorly settling solids
(see Section 5.1.1). Biological treatment processes remove soluble and suspended pollutantsby
incorporation into settlealile solids, which are then removed from the treated liquid byseuling
in a clarifier. The elutriation process redisperses solids so they become unsettleable, requiring
the addition of chemicals to flocculate and agglomerate them.
Elutriation was incorporated in the Los Angeles Hyperion Plant, which had primary plus high-
rate activated sludge treat nent. The sludge was digested anaerobically. The digested solids
would not settle sufficiently to dewater: to coagulate them required chemicals which cost S /
ton of dry solids (1958). With elutriation, chemical costs were substantially redtaced,butabout
50 percent of the solids were in the elutriate(45). To avoid excessive BOD load on the aeratiOn
basins, the elutriate was discharged with the plant effluent to the ocean. It is possible for
elutriation to be justified both technically and economically. However, it must be evaluated on
the basis of :o:alcosts and not merely on the single fact that it reduces the costs ofchëmicalsto
condition the sludge for dewatering.
There have been reports that activated sludge or a mixture of primary and activated slàdges
cannot be digested to produce solids readily dewaterable without elutriation. There are,
however, hundreds of plants operating in Europe and the USA which are digestifl activated
sludges and dewatering the solids on sand beds or with mechanical devices and which do flot
practice elutriation. It is generally conceded that activated sludge does not Mneflt frOm
anaerobic digestion as much as does primary sludge, and that for activated sludge more
conditioning chemicals will be required to obtain proper dewatering. Elutriationtaay have a
beneficial effect regarding some of the polymers now used for sludge conditioning;since some
of these polymers would be affected by the high concentration of soluble organic matter that
elutriation “washes out.”
There are some plants in which elutriation perf.rms a proper function, both technically and
economically(82). However, no published information can be found on the completeand total
cost evaluation of handling the digested sludge in this manner. In some cases, no’extraecsts
are readily apparent, since at present a plant is probably oversized forthewastewaterloaditis
To summarize: the or ginal intent of elutriation as developed by Genter—to reduce the very -
high alkalinity of digested sludge liquor and thus reduce the required dosages of inorganic
conditioning chemicals—appears technically sound. It does reduce the quantity of chemicals
needed for conditioning, but at the expense of having all the pollutants in the digested sludge
liquor in diluted form in the elutriate, together with ‘insettleable fines. On this basis, it ddes not
appear that elutriation should be considered for general application in the design of flew
plants. It should be used only under properly designed conditions where effective operation is
38 ,

• - - Z7a . • - - • •.j . — . . ... -
- • . -- _
— ... - . - - r .,,‘ — — -e ‘ —.
In some cases, supernatant is not produced from aerobic digestion becaus the digester
contents are kept completely mixed and the digested sludge is hauled away by a tank truck
periodically. This is possible at small plants. In other cases, the air to the aerobic digester is •
. shut off and the sludge allowed to thicken before waste activated sludge is added and
supernatant is removed and recycled to the activated sludge process. The thickened sludge can
be drawn off at intervals to sludge drying beds. Some plants use two-stage aerobic digestion. 1n
these plants, the first stage is completely mixed and the supernatant is produced in the second
stage only.
• —
Ahlberg and BoykoçR3) have analyzed supernatants from seven activated sludge plants
employing aerobic digestion located in Ontario, Canada. Three of the plants were-
‘ conventional activated sludge plants, three used contact-stabilization, and one used extended
, aeration. Three of the plants had single-stage digestion and four had two-stage digestion. The -
characteristics of the digester supernatants for the various plants are summarized in Table 4.. -,
Ahlberg and Boyko noted that the supernatant flow was generally about 1 percentpf the ‘
L,,,,wastewatcr treatment plant flow.
Range of Various
Plant Averages
Range of Data

Kjeldahl N 200 10—400 3-1,300
Total P
• _—l., ..q, , -
• . . . . . - , ... I C .. * ., • ., .
• The data in Table 4 were obtained from relatively small plants: the largest was I .85 ingd Ma ny:
of these were package-type plants. It was noted that the hydraulic detention.timesiii the
digesters were between IS and 60 days. Studies made under controlled conditions indicate that •
about I 0 to 15 days’ detention is sufficient to produce the optimum seulcability of the solids for:
liquid temperatures oF 600 to 70° F. Longer times are needed when temperatures drop to 50° F
and below. Also, when primary sludge is digested the time should be increased to 20 to 30 days.
Many of the digesters shown in Table 4 had very low dissolved oxygen levels—below 0.5 mg/I.
This, of course, inhibited nitrification. The higher values of BOD and COD are undoubtedly
[ . ,. caused by poor setthng of the solids -

The su i oflr pr perly âperated aerobic a “shàul ave tively low DOD,”
since any carbonaceous matter that results from the lysing of microorganisms would be I
immediately assimilated and oxidized by the living organisms. The remaining organic matter
from lysed organisms will be very stable and only very slowly biodegradable. The phosphorus
present in the sludge will be dissolved and in the form of orthophosphates that can be
chemically precipitated in the digester with alum or iron salts. Nitrates normally will be high
‘since ammonia should be zero.
Drier(84) reported some detailed aerobic digestion data obtained at the University of.
Wisconsin Sanitary Engineering Laboratory. Using mixed primary and activated sludge, the
following average data and criteria were obtained: -.
1. Volatile solids were reduced about 40 percent.
2. The hydraulic detention time was 10 days, with a temperature varying from
60° to 68° F.
3. The solids loading was 0.2 lb/cu ft/day of volatiles.
4. The alkalinity of the liquor dropped because it was used by the nitrifi ation
process and because of the stripping of CO 2 by aeration. Adjustment of the
pH to between 7 to 8 was desirable.
5. The supernatant, after 1 hour settling had a DOD of less than 100mg/I and
6. Nkrification was quite sensitive to temperature at the lower detention times
I Drier also presented some data from well operated full-scale plants that confirmed the above
results. It is thus apparent that the package-type plants that were studied by Ahlberg and
Boyko, some at liquid temperatures near freezing, were not being operated under optimum

In this report, conditioning encompasses those processes which involve biological, chemical.
or physical (or combinations of these) treatment to make the separation of water from sludge
easier. Also, it should be kept in mind that certain conditioning processes do more than merely
ii crease the dewaterability of sludge; they may also aJter the sludge chemically, disinfect it.
destroy odors, and may even accomplish a certain amount of destruction of the sludge mass by
liquidation or oxidation in some processes.
Chemical conditioning is a method used to break down the thixotropic and lyophilic and
colloidal-gelatinous nature of sewage sludges which makes it difficult to separate the water
from the solids. By adding certain chemical flocculants, the “bound” water can be separated
from the solids with much Less effort and cost. The inorganic chemicals used for such
conditioning ar alum, ferrous sulfate and ferric chloride, and lime. Alum is used primarily to
aggLomerate the finefloc of an activated sludge to aid thickening. Ferric sulfate is not used
much because of the difficulty of getting it into solution in cold water. Ferrous sulfate is
generally used with lime. The most widely used in organic conditioner is a combination of ferric
chloride and lime. The pH is raised to 10.5 to II 5, and good conditioning is obtained. The high
pH causes the death of many pathogenic organ isms(58),and would be expected to result in the
inactivation of many viruses(57). The precipitated ferric hydroxide is aided in conditioning the
sludge by the precipitation of calcium carbonate from the calcium alkalinity in the water and
the C02, thus adding weight to aid in thickening light sludges.
A great many of the new organic polyelectrolytes, especially of the cationic type, are effective
flocculants and conditioners. Their use typically adds less than I percent to the dry solids.
which can be important in economic considerations for sizing an incinerator. In the Large
primary plant at Kansas City, Missouri, it was found that the optimum conditioning of raw
sludge at the least cost for disposal by incineration was obtained by using a cationic and an
anionic polymer in series ahead of vacuum filters(85).
There have been a few instances in which operators changed from the use of ferric chloride and
lime conditioning to polymers and found, because of the septic nature of their sludge, that the
odors in the vacuum filter room increased markedly. Apparently, the lime treatment, at high
pH, prevented any H 2 S from evolving, and also the lime tied up some of the odorous organic

(i(s i )sIudje i nM’ th i 1d Ft is
filtration, pressing, or centrifugation has the same soluble BOD or COD as the plant emuentj
The amount of suspended solids may be either fairly low or quite high, depending on the
quality of the conditioning and the method.used in extracting the water. Centrates will, in
eneraL, have higher suspended solids than filtrates. Because there is a high concenttation of
fine or colLoid-type solids, the return of the liquid to the head processes may not permit solids
kecapture in the treatment plant. Solids recapture may require adding a coagulation dhemicaL,
especially in a primary plant.
Sometimes chemical treatment of raw sewage ma y eliminate the need for conditioning the
resulting sludge before dewatering. For example: if raw sewage is to be treated with lime, then,.
depending on the alkalinity and magnesium content of the water, a coagulant such as aiwn, an
iron salt, or a polymer may be needed. At a pH above 11.0, good coagulation of the suspended
solids and settling are frequently obtained with lime alone. Such treatment in most cases
produces a fairly dense and’ easily dewaterable sludge, prevents any odors from developing,
and produces a high kill of pathogenic organisms and inactivation of most viruses A huge
proportion of the phosphorus is also precipitated. This sludge can be dewatered, usually on a
vacuum filter or in a centrifuge, without any further conditioning(88g9) though the addition
of a polymer might be of benefit in some cases.
The ease with which the various types of polymers can be fed has made them popular foraludge
conditioning. Frequently they are more effective in increasing solids capture in vacuum filters
and centrifuges than inorMnic chemicals. They are used extensively with centrifuges.
An indication of the polymer to use for filtration can be obtained by tests in the laboratory. The
exact type and dosage may be different from that determined in laboratory tests, bowenr,
since polymer effectiveness is influenced by such items as exact point of addition, intensity -of
mixing, and variations in chemical characteristics of the sludge liquor. There is-no good
laboratory test for determining optimum chemical dosage for centrifuges(89).
8.1.1 Use of Filter Aids
Many sludges that do not dewater readily without a large amount of conditioning chemicals-
can be dewatered easily on vacuum filters by adding a “Alter aid ” such as diatomite; fly ash
from coal-fired power plants, or sludge incinerator ash. The production of fly ash from coal-
fired plants is tremendous. it has been estimated that if all secondary wastewater treatment
plant sludges produced in 1970 were dewatered using such fly ash as an aid, it would only use
up about one-third of the total fly ash production. Such fly ash consists principally ofiihca and
alumina, with varying amounts of iron oxides and carbon.
Laboratory studies made on dewatering undigested activated sludge by vacuum filtration,
using fly ash as an aid, produced a cake having 40 percent dry solids (including the ash): while
without ash the solids were only 25 percent. The ash in the dry cake was about 70 percent—an
increase from 33 percent(90A91). The additional removal of moisture provided sufficient
calorific value for self-sustaining incineration of the cake. The ash removed an appreciable
a mount of COD from the filtrate, reducing it from 90 to 20 mg/I. Also, the phosphates. as P0 4 .
were reduced from 15mg/Ito practicallyzero.ThefiltratewasaLkalinewithapHof8.Sto9.0.
Thus, the use of fly ash on fresh waste activated sludge reduced the release of organic and
inorganic nutrients to the filtrate.

Studies have recently been made by the EPA laboratory in Cincinnati(92) on the use of sludge
incinerator ash for sludge dewatering. This has been practiced at several installations in
Germany, using one to five parts of ash for each part of sludge solids. The dewatering is done
using vacuum filters or filter presses. There are presently at least two full scale installations of
this type in the USA. it should be much more economical to condition sludge with incinerator
ash, which would be produced at the treatment plant site, than to haul in fly ash (other things
being equal). At Indianapolis, Indiana, 418,000 lb (dry basis) of a mixture of activated and
primary sludge are daily conditioned with from 0.25 to 0.50 lb of sludge incinerator ash per
pound of sludge prior to vacuum filtration. This has permitted indianapolis to increase the
filter output by a factor of 5, decrease the cake moisture by 22 percent, greatly improve cake
release, and drastically cut chemical requirements(93). incinerator ash is also being
successfully used at the 22-mgd trickling filter plant of Cedar Rapids, Iowa, to condition
digested sludge for pressure filtratiun. Akhcugh power plant ash was found to be superior tv
sludge incinerator ash for conditioning. the difference was not sufficient to justify fly ash
transportation cost. Recycling of the ash through the incinerator, as would occur in normal
operation, did not seem to degrade the ability of the ash to improve sludge dewatering(94).
Light and watery sludge, such as activated sludge alone and alum sludge, can be dewatered
(after some chemical conditioning) on a vacuum filter if a filter aid such as diatomite is used as
a precoat. Filter presses with such precoating have recently been used for alum sludges, and
cakes of 30 to 50 percent solids can be obtained. Of course, a large amount of diatomite is
required, which is an appreciable operating expense. The filtrate has a turbidity of about 5
Jtu. The sludge applied to the filter had a concentration of less than 1 percent solids, and the’
feed rate was 4 to Sgal./ sq ft/hr. The precoat costs average about 52/1,000 gal. of sludge. If the
sludge production averages about 1 percent of the liquid treated, then this would amount to
about 2 cents per 1,000gal. of wastewater or about $40 per ton of dry sludge solids processed.
Prethickening of a I percent sludge to, say, 5 percent by use of a disk centrifuge would be most
worthwhile before such a sludge is dewatered on a vacuum filter or a filter press. This is the
system proposed for dewatering alum sludge at the Passaic Valley Water Commission water
treatment plant(’Pi).
An alum sludge dewatering facility is n n in operation at the Atlanta, Georgia, water
treatment plant (96). The alum sludge is g ía ty thickened and conditioned with lime to a pH of
about II. Filter presses operating at about X) psi are precoated with diatomite and then the
conditioned sludge (about 5 percent solids is pumped into the presses. The filter cake has 40 to
50 percent solids, and the filtrate has less ih& 10mg/i of the suspended solids. Alum sludges
can be produced at wastewatertreatment pun is where tertiary treatment is practiced to reduce
the suspended solids in the final effluent and to remove phosphorus. Such sludges would have
the general characteristics of water treatment alum sludges.
The benefits of using a filter aid must be carefully weighed against the disadvantages. In
addition to aiding filtration, bulk dry solids are being added which, of course, makes the sludge
drier, lithe sludge is incinerated, heat must be supplied to raise these solids to the sludge
incineration temperature. Larger dewatering and solids handling eqdipment is required to
handle recycled ash or other filter aids. These factors can affect the cost-effectiveness of total
sludge treatment significantly.

it should beemphasized that this process is notcomparable to thesocalLed “wetairoxidation”
process (discussed in Section 10.2), since the end results are entirely different.
it is generally acknowledged that heat treatment of sludges, especially those containinga large
percentage of organic matter, will greatly improve their dewaterability. Wastewater sludge can
be classified as being to a large degree a colloidal-gel system, and heating allows entrapped
water to escape the gel structure. Some have referred to this as heat syneresis(97X98). Such -
heat treatment of sewage sludge was first introduced in England in the 1930’s.
Basically, this process involves heating partially thickened sludge in a closed reactor to a
temperature of 350 to 4 OO F at a pressure of about 200 to 250 psi, and holding it-under these
conditions for about 30 minutes. The general system, with required appurtenances, is shown in
In a modification of the heat treatment process, some air is injected into the heat exchanger
and reactor. One reported advantage of this technique is that the conditioned sludge may not
emit unpleasant odors. However, the greater production of organic acids and COz-depmsms
the pH to about 4.5 to 5.0, compared to the process without any air where the pH ofthe liquor
is 5.0 to 6.0(1(1)).
Starting in 1968 and continuing to date( 1973), some basic studies have been made in England
of the heat conditioning process and all its ramifications. Several papers have reported the
results of these studies (99, 100, 101). They have originated from university staff studies,
• investigations made by the Water Pollution Control Laboratory in England, and from
operators of actual pilot and full-scale plants. Though the process is being considered seriously
- in the USA, little basic research has been reported on important aspects such as liquor
treatment or recycle and liquor composition in US literature.
It would appear that additional technical and economic consideration is needed regardingthe
removal of various pollutants in the sludge liquor as a result of the heat treatment. The so-
called “cooking” of the organic sludge causes hydrolysis of the carbohydrates, lipids (fats), and
proteins. The lipids hydrolyze to various soluble organic acids; some of the carbohydrates
break down to simple sugars, while a portion are converted to insoluble polysaccharides and
ceLlulosic compounds. The proteins form amino acids, ammonia, and some carbon
Some recent studies at the Stevenage Laboratory in England have shown that anywhere from
35 to 60 percent of the suspended solids in the sludge are solubilized by the heat treatment, the
higher value being for waste activated sludge. This, of course, reduces the volatiles in the
remaining suspended solids. For raw sludge, 24 to 40 percent of the solids was solubilized; for
- digested sludge, about IS to 50 percent was solubil3zed, depending on whether the sludge was
from a primary plant or was a mixture of primary and secondary. The remaining settleable
solids after treatment will thicken by gravity to 4 to 12 percent solids. On vacuum filtration of
the thickened sludge, a ke having 30 to 45 percent solids can be obtained.
Most of the solubilized organic matter appeared in the decant liquor after heat treatment and
in the filtrate or centrate when the sludge was dewatered. The studies were both with and
without injection of air into the reactor, which resulted in no outstanding differences in the
degree of solubilization or total reduction of COD in the sludge(100). The heat treatment

Heat \
exchanger \
Reactor I
Overflow recycledj
through plant /

system that injects a small amount of air into the reactor is not indicatcd to ach vc appreciable
oxidation of the organic matter at the temperatures used in this proccss;ahout5 percent olthc
COD is reportedly removed by the oxidation.
The specific characteristics of the liquor after heat treatment of a mixture of primary and waste
activated sludge are: BOD 3 = 3.000 to 10,000 mg/I; BOD o = 5.000 to 15,000 mgI I: COD
10,000 to 25.000 mg/I; ammonia = 500 to 700 mg/i; and phosphorus as P = I SO to 200
mg/i (99X100..). About 10 to 30 percent of the COD is not biodegradable. at least not in a 30-
day test period. Though some of the studies in which the above data were obtained were made
in laboratory autoclaves. it was verified that continuous pilot plant studies gave essentially
similar results. In one study in England(102) where such liquor was recycled to the head end of
a I .0-mgd plant, there was an overall increase in BOD of over 30 percent, expressed as a load
(pounds per day) applied to the wastewater treatment processes. The removal of the high
concentrations of BOD, COD if necessary, ammonia, and phosphonis would increase total
plant costs significantly. The BOD 5 could be removed to a high degree by separate biological
treatment of the Iiquor(IO1). The nonsettleable suspended solids in the liquor can be anywhere
from 1,000 to 3,000 mg/i. Some of these solids could, of course, be removed during-filtration
or cntrifugation, but most of them are colloidal in nature and require biological orchemical
flocculation for removal. The total volume of the liquor, including the decantate and the
filtrate (or centrate). for an activated sludge plant, will amount to about 0.75 to 1.5 percent of
the wastewater flow. It would appear that this sludge conditioning process generates a higher
concentration of pollutants in the sludge liquor or liquid sidestreams than well operated
ar 1 acrobic digcstion(!03). As with othcr sidestreams, the problem of removing such pollutants
from this liquor, to avoid degrading the final effluent (whether by direct treatment or recycling
to the main treatment plant), requires careful consideration and evaluation in the design.
Only if pollutant loads are expressed in terms that indicate the increased cost of additionai
oxygen required, larger basin capacity, and more chemical or carbon usage can the :o?alcost of
any sludge processing system be correctly evaluated.
The rivno l of the high concent ation of nonbiodegradable COD by use of activated carbon
or ozone- xidation, if required, can be expensive. Some reports have stated that oIIutants
such as B I) and ammonia in the liquor can be readily handled by the main treatment plant,
sin6e such liquors can be stored and fed into the plant when the regular load is relatively low,
such as at night. In designing a treatment plant, the quantity of any pollutant that must be
removed is calculated typically in pounds per day. Usually, the doily loading will det&mine the
si,e of an aeration basin, oxygen required. chemicals used, etc. This applies to a bioIogicar
treatment process, a chemical coagulation process, or an activated carbon adsorption process.
A recent study was reported in the Journal of the In.s:iiute of Water Pollution Control(Gt.
Brit.)(104) which adds considerably to our knowledge of the character and treatabitity of the
heat treatment sludge liquor. The studies were requested by the London Metropolitan Water
Board, because a treatment plant on the Thames River was planning to use heat conditioning
in processing their sludge before dewatenng with filter presses.:The Thames Conservancy
District required that the heat treatment liquor be treated separately, inctuding adsorption on
activated carbon of the COD, before its discharge. The reasons given were that too Iittle’wa as.
yet known about the exact composition of the soluble organic matter in the liquor to permit its 1 ’
recycle to the main activate•d sLudge treatment plant and eventual d ischargc ii ito the Thames ;
which was used as a source of water supply downatream. There
was,. they stated the possible formation of “unnatural” organic
compounds 1 and it was the nonbiodegradable portion that gave
them concern. It had been noted that chlorination of some of
these diluted liquors gave rise to taste and odor problem water

Briefly, the pilot plant study consisted of an extended aeration plant and filtration followed by
activated carbon columns. The raw liquor had a BOD of about 10.000 mg/l.a COD of 17,000
mg/I. and an ammonia concentration of 700 mg/I. After treatment, the final effluent had a
COD of 52 mg/I on the average and the ammonia content was 130 mg/I, which was
considered suitable for discharge into the Thames River. To carry out the treatment, the raw
liquor was first diluted 5:1 with treated sewage and then aerated for 4 days. which reduced the
COD of the diluted mixture to about 330 mg/I. The M LSS was about 5.000 mg,’ I in the
extended aeration plant. Since further aeration did not accomplish any additional reduction in
COD, it was calculated that 2,000 mg/I of the raw liquor COD was nonbiodegradable. The
carbon adsorbed about 500 g of COD per kilogram of activated carbon. The carbon
regenerated satisfactorily in a high temperature furnace.
The total cost, including amortization and operation and maintenance, was about $4.75 per
1,000 gal. (US) of liquor treated. This was appreciably less than distillation, which was the
other alternative considered, and was not regarded as an especially large addition to the total
cost of the wastewater treatment plant. If this liquor is I percent of raw wastewater flow, which
is a typical volume, the cost per 1,000 gal. of wastewater treated would be almost 5 cents. This
figure, of course, does not include the heat treatment plant and associated costs.
Field experiences that are reported in the literature regarding sludge heat treatment are scarce.
Continual formation of hard calcium sulfate scale on the heat exchanger in these units can be a
serious problem in certain hard water areas. Normal cleaning of the heat exchanger surface is
provided for; however, where the water has a high concentration of permanent hardness in the
form of calcium sulfate, a serious problem results because of the inverse solubility of calcium
sulfate with respect to temperature. For instance, CaSO4 solubility in pure water is about 1,700
mg/I at 212° F, which is about the temperature of the sludge during heat treatment. It thus
appears that frequent acid cleaning or some preventive measure must be considered if this
process is used in hard water areas. However, as indicated previously, documented data
regarding such operational problems are not readily available.
Where the sewage has a relatively high concentration of chlorides, such as may occur from
infiltration of sea water, stainless steel heat exchangers could corrode rapidly and special
materials may be needed. This occurred at an installation in Florida.
In England, two relatively small plants (about 3 mgd and 5 mgd) use heat treatment for the
sludge produced by pi imary plus trickling filter treatment(!OS). A filter press is used to dewater
the conditioned sludge. The liquor resulting from the heatconditioning of the sludge isdiluted
with some sewage and treated separately on plastic media filters. The plant has been in
operation for 2 years and the paper reports some operational and mechanical problems.
However, it appears that most have been resolved and the authors (plant manager and
manufacturer’s representative) feel that they have a workable plant. .The problems
encountered were: scaling of heat exchanger surfaces, flotation of solids in the decant
(thickener) tank following the heat reactor, furnace problems, odors, and various mechanical
failures. Total costs are estimated to average $40 per ton of dry solids (based on direct
conversion of English monetary units to US dollars). This does not include cost of disposal.
A recent (1973) pilot study (119) on dewatering digested primary sludge produced at the Los
Angeles County Sanitation District Treatment Plant showed a total cost of $22. 10 per ton for a
300-ton / day facility using vacuum filters after heat conditioning. Vaccum filters plus polymers
gave a cost of $12.70 per ton.
47 \

There is a genera’ report on the heat treatment installation (or the sludge produced at the
:ictivatod sludge treatment plant at Kalama700, Michigan(106). The plant treats domestic
wastewater and the wastewaters from seven paper mills in the area. The facilities are designed
to handle about 100 tons of dry solids per day. The waste solids consist of 77 percent waste
activated sludge and 23 percent raw primary sludge. The reported total costs 4or heat
conditioning, dewatering, and incineration are about $30 per ton of dry solids. Chemical
treatment was estimated to cost SlO per ton more. No detailed data are given relating to
treatment of the liquor produced by the heat conditioning process. However, the solids capture
by the vacuum filters is given as 87 percent, which left about 12,500mg/I ofsuspendcdsolids in
the filtrate.
At Kalamazoo, a major operational problem reported related to the sludge grindersahead of
the heat treatment unit. This was due largely to the presence of grit in the sludge and an unusual
load of industrial rags andipieces of plastic. Plans are underway to provide more effective
removal of these materials. Also, a new type of maccrater will be tried out in place of one of the
grinders. The heat exchanger tubes are cleaned with dilute nitric acid once every 30 days; it is mi
8-hr task.
A detailed study to evaluate conditioning and dewaicring of sewage sludge by freezag was’
sponsored by EPA and cairied out by the Milwaukee, Wisconsin, Sewerage CammiesioeI12&J
The initial conclusion was that the freeze-thaw process has technical merit. Howcverthefsusl
conclusion was that the total cost of the process was greater than existing chemicel
conditioning processes. One disadvantage appeared to be that it was essentially a batch
process. The tests were made on activated sludge having solids concentrations of LI-to 5.3
percent. The filtrate suspended solids varied from 100 to 1,000 mg/I. the higher values bsusg
obtained from the initially thicker sludges. Storage of the sludge before freezing increased the
phosphorus and COD concentration of the filtrate; P04 was about f 50 to200mg/I andCODI
was 3,500 to 4.000mg/I. Storage of the sludge after thawing also increased the PO,andCOD
in the filtrate.
Freezing and thawing have been studied at the Stc 4 T. 1boi -
laboratory scale(107 .L The process was first put to practical use by thcFylde Witir
England for dewatering alum sludge from a waier treatment plant(108). The total coitsf.ar thu
process amounted to about $40 per ton of dry solids. This is for handling sludgehaving6
percent solids. They used an artificial freeze-thaw system, involving considerable equipment.
The testing at Stevenage was on activated sludge, raw sewage sludge.alUm sludge.andother
metal hydroxide sludges. In all instances, except for the alum sludge, the filterability of the
sludge on alight stirring, subsequent to thawing. deteriorated. With all the other sludge... any
agitation seemed to cause a breakup of the floc. The only way a sludge could be handled after
the freeze-thaw step without agitation would be to dewater it in place. thusavoidingtheneed
for pumping or transferring it. This could be done in open drying beds for alum sludgefrem
water treatment plants(1OP).
The investigators at Stevenage concluded that this process was notsuited for organic sludges
unless a chemical conditioner was also used, and this would make the process too costly.

A study was made at Ely, Minnesota. to provide basic information for proper design of
t tcilitics for dcwatering aluminum hydroxide sludges by natural freezing. This technique
oilers economy for conditioning sludges in cold climates(IIO).

All the various treatment processes sludges are subjected to, such as thickening, stabilization,
conditioning, are largely directed towards facilitating the removal of entrained water from the
solids. This “dewatering” is accomplished so that the solids can be more economically and
more readily disposed of in an environmentally acceptable and cost-effective manner.
The chemical and physical characteristics of the sidestreams that result from sludge dewatering
are described in this section. As has been discussed in previous parts .of this report, the
treatment processes preceding dewatering can have a very important effect on the quantity and
quality of these sidestreams.
The most common method of municipal wastewater sludge dewatering is on sand beds.
Although s nd beds are particularly suitable for small installations, they are used at treat-
ment plants of all sizes and in widely varying geographical areas. Many industrial sludges
and water treatment plant sludges are also dewatered on sand beds.
Dewatering on sand beds is by drainage and evaporation. The proportion removed by
drainage may vary from 20 to 85 percent. Normally, most of the drainage is accomplished in
the first 2 days on the bed; evaporation is the principal effect thereafter. After a few days, the
sludge cake shrinks horizontally, producing cracks at the surface which expose additional
sludge surface area and enhance the drainage as well. The liquid draining from the sludge is
often returned to the treatment plant. Though its volume is small, if the sludge being dried has
been digested the drainage contains a high concentration of soluble organic matter,;
ammonium compounds or nitrates, and phosphates. After one or more months of dewatering.
the sludge is removed by a hand shovel or by a mechanical scraper. Only light weight 1
equipment can be used because heavy wheel loads will damage the underdrain system.
The important parameters affecting sand bed design and use are:
Climatic conditions. The amount and intensity of precipitation, percentage of
sunshine, air temperature, relative humidity, and wind velocity can affect sand
bed design. Temperature is very important; in the Midwest, dewatering rates in
summer are about three times as great as in winter. Many plant operators store
sludge in digesters during the winter and apply it to beds only in the warmer
Preceding page blank

months. Alternatively, more favorable conditions are created at plants where
the beds are covered and artificially heated.
Depth of sludge layer. The depth of sludge applied affects the drainage rate
(depth should not exceed $. to 10 in.)..
Sludge characteristics. Sludges containing grit dry fairly rapidly; those
containing grease more slowly; primary sludge dries faster than secondary
sludge; more completely digested sludge dries and cracks relatively fast. It is
important that wastewater sludge be well digested, or stabilized chemically or
by other means, for optimum drying and to preclude serious odor problems. in
anaerobically digested sludge, entrained gases tend to float the sludge solids,
leaving a layer of relatively clear liquid that readily drains through the sand.
The more water remu cd by drainage, the less to be removed by evaporation.
The overall result is rqduced drying time.
Vnderdrain system. A sludge drying bed must have a drainage system set on an
impervious layer of clay or other materiaL On top of this layer perforated tile
should be placed in gravel with collectors so that the liquid can be conducted to
a point of treatment.
Drying beds are inexpensive and simple to operate. Their disadvantages are thearea requ d,.
potential nuisance problems, susceptibility to adverse weather conditions, and the requirement
that sludge be well-digested or conditioned before dewatering(112).
The advantages of enclosed drying beds are: reduced area requirements, protection from rain
and cold, control of odors and insects, and improved appearance. The disadvantages are the
construction and maintenance costs, and the problems regarding the use of any mechanical
equipment in a relatively small enclosure. Good ventilation is essential to promote
evaporation. In warmer climates, covered beds with sides left open have been effective. This
keeps rain from delaying the drying.
The following publications should be referred to for design details on sand beds: Sludge:
Dewa:ering, WPCF Manual of Practice No. 20, and Sewage Treatment Plant Design WPCF
Manual of Practice No. 8. In determining the area for sludge dewatering beds, consideration i
should be given to climatic conditions and the character and volume of the sludge. At4V north
latitude (St. Louis, Missouri), digested sludge from domestic wastewater requires from 1.00
to 2.25 sq ft per capita in open beds, and somewhat less in covered beds or in southern lati-
tudes. The applied sludge should be well distributed across the drying bed. Since thesludge
may be quite viscous and cannot distribute itself well by gravity, the use of multiple dii..:
charge pipes is encouraged. Each pipe should terminate at least 12 in. above the bed surface.:
Splash plates should be provided at the pipe ends to promote even distribution. I
Open sludge drying beds in the USA have loadings of 5.5 to 35 lb/sq ft/year. For covered beds,
the loadings can be near the higher figure(113) (these figures are on a dry solids basis).
After the dried sludge is removed by hand or machine, the drying beds require maintenance.
Small sludge particles and weeds should be removed from the sand surface. Periodically, the
bed should be disked and the top layer of sand replaced. Usually, resanding is advisable when
50 percent of the original sand depth is lost. The resurfacing of sludge beds is perhaps the major
expense in sludge bed maintenance, but there are other factors that should be considered.’

Underdrains occasionally become clogged and have to be cleaned. Valves or sluice gates that
control the flow of sludge to the beds must be kept water-tight to prevent wet sludge from
leaking onto the beds during dry periods. Provision for drainage of lines should be made. Lines
with sludge in them should not be shut off until they are flushed out. The partitions between
beds should be so light that the sludge will not flow from one compartment to another,
especially if the sand surface is taken down too low. The outer walls or banks around the beds I
also should be water-tight. If earth beds are used, grass and other vegetation anthem should be
kept cut.
Sand beds are the most common method of drying sludge, but little has been written about the
quality of the resulting filtrate, although this sidestream is frequently returned to the treatment
plant. Laboratory studies by .Jeffery and Morgan(114) in which digested sludge was applied to
a 6-in.-thick sand bed indicated that a clear, dark amber liquid was produced with a COD of
300 to 400 mg/land a 5-day BOD of less than 66mg/I. The sand removed the suspended solids
very effectively. Kjeldahl nitrogen was high, with the long term BOD of over 2,000 mg/I,
indicating the typically high original content of ammonia in the anaerobically digested sludge
As with other sludge dewatering methods, the quality of the filtrate will depend largely on the
method used for sludge conditioning, as well as type of sludge; however, sand beds are pri-,
manly used for digested sludge. In any case, the filtrate should be returned to the treatment
plant and steps taken to handle the additional pollutional load. Of course, since sludge is
normally applied to sand beds intermittently, the flow of filtrate will not be continuous. Also,
in dry climates there can be appreciable evaporation. Open beds may have diluted filtrates due
to rains.
Reportedly, “wedge-wire” drying beds have been used in England with great success(115).
These consist of a perforated metal sheet laid on top of conventional drying bed media.
Support water is first added to the drying bed to prevent clogging of the media when sludge is
added. As the sludge is applied, and as it forms its own filtering layer, the support water is
slowly removed. This procedure prevents solids from breaking through the wedge wires and
plugging the drying bed. Advantages claimed for wedge-wire beds include: (1) no clogging of j
the media, (2) constant and rapid drainage i3) increased bed capacity because higher loadings
are possible, (4) easy bed maintenance, (5) e s icr dried sludge removal, (6) less susceptibility to
adverse weather, and (7) difficult to de .1 ‘r sludges can be dried. Application of this meth-
od for use in sludge drying beds has just gun in this country (1972).
The rotary drum type of vacuum filter has been widely used for sludge dewatering. Basically,
there are two types: the stainless steel coil filter and the belt filter, which uses a belt of fabric
(usually synthetic) as the filtering medium The chemical and physical character of the filtrate 1
is largely dependent on the sludge conditioning process used. Vacuum filters will, as a rule,:
capture a much higher proportion of suspended solids and produce a drier cake than 1

It is difficult to characterize the filtrate from vacuum filters because of the many variables that
affect filtrate quality. These variables include sludge type, degree and method of conditioning;...
the type of filtec media, the amount of vacuum applied, and the sludge application rates
Wherever possible, filter leaf tests should be madc to determine cake and filtrate dharacter
All types of municipal wastewater sludges—raw, digested, primary, activated, trickling filter,
and mixtures—can be dewatered by vacuum filtration. The benefit of thickening sludgeb fori .
filtration has been discussed (Section 6). In general, it has been observed that sludge fiItratiän
rates (pounds per square foot per hour) increase as solids input concentration increases. This is , !
because the hydraulic loading that is possible per unit of filter area (gallons per square Ioot’per
hour) is generally constant with constant cake thickness. Input solids concenttatiôn should be
no greater than about 10 percent; at a greater value, chemical conditioning and sludge
on the filter drum are hampered.
Sauce vacuum filters, except in the largest p!ants, may only be operated for, say, 8 hours a diy
and maybe 5 or 6 days a week, except in plants having digesters the th ck ned sledge müs tb ë
stored during periods when the vacuum filters are not operating. This is done by inst llIflg
tanks of sufficient capacity between the thickening operation and the vacuum filters. To
prevent septicity and to keçp the solids from settling, such tanks are aerated usinbmei ed (
perforated pipes, with compressed air. Digesters can, in most cases, pràvide the retj Ek*d
sto age, especially if a two stage system is used. Heat conditioning systemsihalvéa éIfII4t
thiâkening tank after the reactor which serves also as a storage tank. Sinèe the shidge iss éril ,
septicity is suppressed.
The above indicated storage facilities may also be needed when centrifuges or filter pre ate
used for dewatering. . - —
Conditioning prior to filtration is undoubtedly the most important single factor lh”tøe
operation of any well-designed vacuum filter installation. Lime. ferric chloride. tèbuii 4i ,
aluminum chloride, and various polyelectrolytes (polymers) have been used sucessfUlIy’
conditioning chemicals. Although several years ago (1964) it tvas estithated that. at ka i
percent of the: filters- then operating used ferric chloride arid Iime, th re is a
towardsuse of polymers. The advantages cited for polymers compared to norganic1IOctilints
Significantly smaller uirements for chemical t andltng equipMeth tid spade
Much less incinerator ash produced
Lower heat requirements to heat added chemicals
Greater filter yie d in many cases
Improved safety and cleanliness. -
To cánvert from inorganic chemical feed to polymer feed is simple, since existingequipmènt
can be used. Polymeric flocculants have shown no corcosiveness or toxic effectsaiidqeave-in
easily cleaned filter medium and filter drum; however, they do not stabilize, disinfect, or
control odors. The use of ferric chloride and lime at a pH of about 11 suppresses odors. If only
polymers are used in conditioning, undigested raw sludges cannot be disposed of on the land or
in landfills, since they are not stabilized. Typical inorganic flocculant dosages are 400 to 450

lb/ton of dry solids, while typical polymerdosagesare less than 20 lb/ton. These figures are for
the sludges which arc more difflcutt to dewater.
The solids content in the dewatered cake will be 15 to 20 percent for activated sludge. 25 to 30
perccnt for raw primary sludges, and 20 to 30 percent for well digested sludges. The filtrate
from vacuum Iilteiihg, as far as suspended solids and BOD are concerned, is almost entirely
dependent on the type of conditioning used. Filtrate flow is normally about 0.5 to 1.0 percent
of the plant flow, depending on the prcthickening of the sludge. The 130 D and suspended solids
will usually be removed by the secondary treatment if they are recycled back to the head end of
the plant. The ammonia content of the filtrate will be comparable to that of the liquid sludge
and will be equal to that of the supernatant for digested sludges. The soluble phosphorus
present in digested sludges will be precipitated and removed in the sludge cake, if the sludge is
conditioned with lime, an iron salt, or a combination of both.
Malina and DeFilippo(1 16) noted that in the filtrates from vacuutii filters suspended solids can
range from 100 to 20,000 mg/I, depending on the variables mentioned above. Daily variation
at a plant can also be considerable. Five samples collected on 5 consecutive days from a plant
dewatenng digested sludge ranged in suspended solids from 1.300 mg/I to 7,120 mg/I. Total
nitrogen varied from 248 to 9,820 mg/I. Vai iatkn in BOD was less—from 300 to 370 mg/I.
In addition to the filter itself, a vacuum filtering system requires a vacuum pump with silencer,
a vacuum receiver to separate air from filtrate, a filtrate pump, blowers, chemical conditioning
equipment, and sludge feed pumps. A normal design would incorporate:
Separate chemical conditioning tanks to provide flash and slow
mix. The tanks should be open to permit observation. Sludge
and additives should be mixed to produce the best flow at
minimum chemical dosage. Conditioning tanks should be adjacent
to the filter and have flume discharges over the lip of the
filter pan to broadly distribute the treated sludge. Filtering
sludge as soon as possible after chemical conditioning is
Sludge and chemical diLution faciLities. Adequate water lines should be
provided to dilute sludge and/or chemical flocculants to optimum solids
concentrations, if necessary.
Variable speed filter pan agitator drives. The stability of sludges and their need
for agitation in the pan vary, so flexibility in the agitation speed is very
Delivery of a uniform sludge feed.
Effective filter media cleaning facilities.
In the choice of construction materials, ccnsidcratioi’ishould be given to the corrosive
properties of the conditioned sludge. lithe sludge is to be conditioned by the use of the iron
salts alone or if an acid sludge is to be dewatercd, stainless steel, rubber covered construction,
or a suitable plastic should be selected. If the conditioped sludge is expected to be alkaline, mild
steel construction, perhaps with suitable coatings, would be satisfactory.
• Tes’ts have recently been made at the Milwaukee. Wisconsin, wastewater treatment plant on
(dewatering chemically conditioned activated sludge using a “top4eed.” drum-type vacuum
filter(1 17). The thickened sludge was fed from a sealed hopper located near the top of the filter.

• ‘ — — — . — -.. —-.. 4. - . - — - - - ..— — — -. ‘w- —
The filter drum was not submerged in the sludge as is normally done. Improved yields and cake
‘-dryness were obtained compared with filters of’ conventional design operating in parallel.
If a treatment plant is in existence, then there is no problem in making tests in the laboratory,
or even with a pilot filter, to determine the amount and type of conditioning chemicals.
maximum possible loading, dryness of cake, filtrate quality, etc. However, fora new treatment
plant, the design engineer must establish the general character of the sludge, depëndingon the
type of wastewater and the treatment, using past experience with similar sludges, If the
wastewater has industrial wastes which will affect the character of the resulting sludge, the
design engineer will have a diflicL 1 It problem to size the vacuum filters (or any other type of
dewatering equipment). Consultation with the equipment manufacturer is important. It may
in some cases be necessary to carry out at least some laboratory treatment which gimulates that
of the proposed plant, to produce some sludge and make dewatering tests on the samples. In
such cases, conservative design and provision for additional space are recommended.
Reports on the effect of recycled filtrate on wastewater treatment are almost nonexistent,
although it is common practice to return the filtrate, which can contain ito 10 percent of the
solids applied to the filter, If the solids can be captured by the chemical or biological treatment
processes in the plant, no deterioration of plant effluent should result. The quantity of the
soluble pollutants will depend on the upstream processing of the sludge. In any case, proper
design must be provided in the plant to ensure capture of any recycled solids or other
pollutants, so that the final effluent is not degraded. In addition, the costs for providing for
such capture must be considered part of the sludge processing costs.
The unit generally used for wastewater sludge dewatering is the horizontal solid bowl
centrifuge. It operates on a continuous basis and can produce a sludge cake of 15 to 30 percent
solids, depending on the type of sludge and the conditioning it receives(118).
The advantages of centrifuges over vacuum filters are less space and, sometimes,Iower overall
costs. However, in general, the solids capture is not as good as with vacuum filters unless
optimum chemical conditioning is used, and then the cake invariably is not as dry. In other
words, a dry cake results in less solids capture The centrate with the suspended sdlids that have
not been removed is typically returned to the head end of the treatment plant.
Centrifuges tend to classify solids; that is, remove the larger, denser solids and leave the finer,
lighter colloidal solids in the centrate. These solids are usually very fine and few will settle in,
for example, a primary clarifier. If their concentration is excessive, such recycle can cause a
buildup of fine solids in the system with eventual discharge in the plant effluent. Of course, if
chemical coagulation is employed either in the primary treatment or in the secondary
treatment, then such solids will become coagulated and settle out with the other solids in the
final clarifier. Biological secondary processes may not capture all such fine solids, and
therefore chemical coagulants may be necessary.
Frequently, the activated sludge or the sludge from trickling filters is mixed with the primary
sludge and the mixture is dewatered in the solid bowl centrifuge. It is very important that the
primary sludge be free of grit, or there will be rapid wearing of the metal lining of the bowl.
Even with a standard grit removal facility, it is common to have a hydroclone to degrit primary
sludge before it is sent to the centrifuge.

A vertical solid bowl centrifuge, which has recently been extensively tested, is known as the
“basket type”(119j. it is batch operated, with intermittent removal of the cake,and hasa high
degree of solids capture. The batch operation can be highly automated. The hori7ontal bowl
and the basket type can be used in series to obtain an excellent solids capture, the basket type
unit being the final one. In the basket type, when the unit is stopped. a knife moves down into
the vertical bowl to cut the cake, which falls to the open bottom of the machine. Lighter sludges
dewatered in this unit produce a cake with 10 to 15 percent solids without chemical
conditioning and 20 to 30 percent with chemicals (usually polymers).
Polymers have been extensively used to increase the solids capture in centrifuges; generally, an
increase in solids capture results in a wetter cake. The normal solids capture with polymers for
primary plus activated sludge is about 85 percent for a cake having 20 to 25 percent solids. In
one type of horizontal bowl unit it is possible to add some of the polymer inside the unit after I
the heavier solids have been removed from the liquid. This increases the solids capture and
does not decrease the solids concentration in the firal cake to the degree that happens when
polymer is added to the total feed (120). Centrifuge manufacturers may have valuable data
which would be useful during detailed design of the plant(/2/).
The effects of returning centrate to the treatment plant vary according to the type of sludge
being dewatered. Surprisingly little has been documented regarding effects of returned
centrate. Keefer and Kratz(122), in early studies on centrifugation of digested sludge,
determined that returning centrate to the plant influent would increase influent BOD by 4.5
percent. Studies were not conducted, however, to determine if continued return would tend to’
increase the fines in the centrate and thus continually increase the solids returned. It has been
found that the return causes recycling of increasing amounts of pollutants because the fine, i
colloidal solids in the centrate cannot be recoagulated. This is especially true if polymeric I
flocculants are used for sludge conditioning and an attempt is made to coagulate the fines in the!
centrate with a polymer. So-called “over-polymeri7ation” can result, so that increases in
dosage without changing the polymer cause red ispersion of solids(!23).
Walters and Ettelt(I24)believed that centrate from dewateringeffluent of the wet airoxidation
process should not be recycled unless chemicals were employed to increase solids recovery.
They noted that the uncaptured fines, which were extremely slow settling, would present a
significant problem in clarification because the solids returned with the centrate would range
from 20 to 40 percent of the solids in the feed to the centrifuge.
Perhaps the simplest way to decrease the suspended solids in a centrate is by the addition of
chemicals to the sludge before centrifuga Lion. increases in solids recovery of 15 to 30 percent or
more can be achieved by addition of polymers or other chemicals. Reducing the feed rate can
also be used Co decrease centrate suspended solids. This subject is discussed in some detail by
Recently, studies on centrate from digested sludge were conducted by the County Sanitation
Districts of Los Angeles County(119). It was found that the centrate from a solid bowl
centrifuge could be further centrifuged in a basket type centrifuge (actually a vertical solid
bowl type) after addition of a cationic polymer. This type of centrifuge gave excellent solids
capture and produced a dry cake; however, it is a batch unit. The treatment was found to
decrease suspended solids, which were about 20,000 mg/I in the centrate from the horizontal
solid bowl centrifuge, to about 1,000 mg/I in the centrate of the basket type centrifuge. This
combination removed about 97.5 percent of the suspended solids from the digested sludge,
which is excellent capture.

Biological treatment of centrate from digested sludge has not been studied to any great extent.
It would be expected that such treatment would not be entirely effective because the digested
sludge contains some humus materials that are only slowly degradable. The ratio of COD to
SOD can be taken as an index of biodegradability. This ratio is about 1.5 for raw wastewater,
and a ratio higher than about 5 would indicate a material not readily amenable to biological
treatment. -
The standard filter press, or pressure leaf filter as it is sometime called, has been used for many
years in the chemical process industry for dewatering slurries. It consists of leaves” covered
with some type of porous fabric. These leaves or plates form a series of chambers and the sludge
is retained between the fabric on both sides of the leaf. These plates are first pushed together
and compressed by hydraulic or mechanical pressure exerted on the ends of the series of plates
to prevent leakage. Drainage ports are provided in the plates for the liquid to escape. The•
pressure is imposed by pumping in the sludge which is retained between the filter fabrics. The
final pressure can amount to several hundred psi, though usually for wastewater sludge it is
about 100 psi(125X126).
These presses have, in recent years, become highly automated, since they perform basically as 1,
batch operation. After the sludge pumped into the chambers has been retained for a
predetermined time at the maximum pressure, the plates are pulled apart and the cake is
allowed to fall away from the fabric, normally on a conveyor belt below the press. The
automation of the presses has greatly increased their possible application to wastewater sludge
dewatering, especially in the USA where labor costs are high. However, to date (1973) no full
• scale installation of a filter press exists in the USA in a municipal wastewater plant. There is,
however, at least one full scale demonstration installation (Cedar Rapids, iowa), and a similar
• installation at the Atlanta. Georgia, water treatment plant for dewatering alum sludge.
‘The principal advantage of the filter press over either the vacuum filter or centrifuge is that it
can produce a very dry cake—30 to 50 percent solids—even from sludges that are difficult to
de ater. However, to obtain a dry cake and a relatively clear filtrate, chemical conditioning of
the sludge is necessary, which always increases substantially the total solids to be handled.
Filter presseshave been used quiteextensively in Europe forwastewatersiudgedewateringand
are now being tested in the USA. At least three manufacturers in this country now (1973) have
filter presses for wastewater sludge dewatering Some of the best technical information on the
performance, theory of operation, capacity r tung. and testing of filter presses hasappeared in
the publications of the Filtration Society of E ngland(I27 128). All types of sludge, varying
from raw primary to digested activated, have ecn dewatered on filter presses with good results
and low concentrations of suspended solids n the filtrates. However, proper conditioning is
required for efficient operation. Also, possible differences in character of sludges must be
The liquid sidestream from a filter press is generally, for comparable sludges and conditioning.
similar to that from a vacuum filter. Because of the higher pressures used and the finer mesh
fabrics, the percedtage of solids capture in a filter press is generally higher than in a\vacuum:
filter, and thus the suspended solids in the filtrate are fairly low with good sludge conditioning
(frequently below 100 mgf

9. LT ILT_ 3
Several desi 3 of this type of equipnient, which were originally
developed for chemical process slud es and slurries, have been
adopted for de atering of waste activated sludge in urope.
One of these designs was developed in the Netherlands (129)
and another in Germany (130). In both cases, the prethickened
and conditioned sludge is fed onto the top of a ntovlng
horizontal belt. The lower belt is of porous fabric and there
is an impervious belt (usually of rubber) on top after the
sludge feed point that also moves horizontally on rollers and
presses down on the sludge layer, thus squeezing out the water,
Solids concentration on the order of 15 to 25 percent can be
obtained. The German design is presently being licensed to a
USA equipment supplier (131). A somewhat different filter press
belt is offered by another USA equipment supplier. This
consists of a primary screen onto which the thickened and
conditioned sludge is discharged. From this “screen” it moves
to a secondary “screen” where there are rollers for pressing out
the water fro n the sludge layer (132). The screens are finely
woven synthetic fabrics.
9.5.1 Capillary-Squeeze Belt Filter
A unique type of horizontal belt filter, developed in Europe and used for industrial type slurries
and now offered in the USA, depends on the capillary suction action on the water portion of
the conditioned sludge to extract the liquid from the sludge layer formed on top of a sponge-
like beLt(133). The liquid is squeezed out of the “sponge” belt and the cake is further squeezed
between steel rollers for additional dewatering. The sludge cake adheres to one of the rollers
and is doctored off. Tests show that conditioned waste activated sludge having 0.5 to 1 percent I
solids can be dewatered to 18 percent solids. The unit is relatively economical and would be
especially suited for smaller treatment plants. The solids capture with conditioning is good.
EPA tests on this unit are nearing completion (1973).
9.5.2 Gravity-Type Porous Fabric Concentrator
A unit that has been used fairly extens ely and was developed in the USA is the so-called dual
cell gravity concentrator(/34X!35). It has several unique features which produce dewatering of
sludge through a synthetic porous fabric by the action of gravity. The first cell accomplishes the
removal of the major portion of the water from a thickened sludge. and then the sludge mass is
carried over into a second cell where final dewatering is accomplished. The sludge from the
second cell Is not as dry as is obtained from a vacuum filter or filter press. If further dewatering
is desired, the sludge mass is conveyed to a squeezing arrangement consisting of rollers which
produce a cake of 20 to 25 percent solids.
These are devices which are used to remove the larger solids from wastewaters, but which are
not strictly dewatering devices; however, they do produce a sidestream which must be recycled
to the main treatment plant.
Specially designed self-cleaning metal screening devices are used (and are necessary) ahead of
disk (or nozzle) type centrifuge thickeners for waste activated sludge. The liquid coming from
such devices must be returned to a primary clarifier, since it contains an appreciable
59 .\

concentration of solids. If primary treatment is not used, then this liquid with the solids must
be mixed with the thickened sludge after centrifuging, otherwise the screened out solids will be
recycled. However, since the waste liquid amounts to only about 5 percent of the volume of
sludge being proceucd, the thinning out of the thickened sludge is not significant.
A unique automatic mechanical screening device has been recently devioped and used for
treating combined wastewater and stormwater flow to remove a large portion of the suspended
solids(135). It iscalled a “centrifugal wastewaterconcentrator.” Reports indicate that the 05-
micron stainless steel screen in this device removes about 35 percent of the suspended solids
a d concentrates them in 10 percent of the total liquid flow. This concentrated stream, which
would be the sidcstream, is returned to the sanitary sewerage system for further treatment. A
modification of this device has been tested out on a raw wastewater, and a full scale installation
is being built in Califocnia(137) for use in place of a primary clarifier.
An unusual use of fairly coarse horizontal and sloping screens, of the vibra2ory type, is being
made at the Ilyperion Treatment Plant in Los Angeles to remove the larger solids from
digested sludge before it is disposed of. The organic screened out solids are maccrated and
returned to the main sludge stream, while the inert and hard materials are separated and hauled
to a landfilL
Recently, a stationary sloping screen of an unusual hydrodynamic design has been
introduced(I38). The experience with it is still very limited, but it has application for
nonflocculant sludges, such as those from primary clariflers and trickling fs1ters and for
removing fibrous and discrete particles. The simplicity of design and low cost compared to
other systems warrant its being thoroughly tested and considered.

‘This section discusses the various methods currently used to dispose of the sludge to the land,:
air, or oceans. Fundamental decisions on the environmental acceptability of some of these
methods are still being made (1974). In some of the processes, the sludge must be dewatered to
a high degree. In others, it is disposed olin liquid form, usually after some thickening. Some
processes, such as incineration, may dispose of the organic portion but leave the ash and the;
liquid (if removed from the sludge before incineration) for further processing and! or disposal:
The disposal, by methods other than return or recycle to the treatment plant, of the various
liquid sidestreams that can be generated by different processes used. in wastewater treatment
will be indicated. . . . - . -.
Since the municipal wastewater solids and sludge (including grit and sldmmings) that are
generated have a large portion of orgenic matter, the burningfof such sludge should be
considered, particularly in larger metropolitan areas. If sldmminge are incinerated, a mix
tank may be necessary to prevent hot spots and damage to the incinerator. However, an
ash remains for final disposal. Whether the combustion is self-supporting depends on the
calorific value of the solids and the degree of dewatering that was accomplished. Incine-
rators are always provided with auxiliary fuel for use when needed—such as during start-
ups. Domestic wastewater sludge dewatered preferably to solids content greater than 30
percent will permit self-supporting combustion. Although either may be incinerated, raw
sludge is preferable to digested sludge because of its greater calorific value.
Wastewater sludge incineration has been practiced for many years and-is being considered in
urban areas as sludge volumes increase and as land areas for alternative operations become
scarce. Also, the development of greatly improved designs with regard to control of air
pollution and possible recovery of heat has increased the use of incineration. However, it is a
relatively expensive process in terms of both investment and operating cost. The incineration
process must not produce objectionable smoke, odor, or other atmospheric pollutants.
Incineration achieves volume and weight reduction and solids sterilization. The resulting ash
will be 15 to 45 percent qf the original sludge solids weight (depending on the volatile solids
concentration of the feed sludge), and the volume, assuming a 25 percent solids cake as feed,
will be about 10 percent. The two most common types of sludge incinerators are the multiple
hearth and the fluidized bed. A less common type is the flash drying and burning unit(140).
61 \

Incineration of wastewater sludge may be divided conceptually into two major phascs:dryiiig
and combustion. In drying, the sludge cake is heated to above 212° F, the water isevaporited,
and the temperature of the water vapor is increased to that of the incinerator exit gas
temperature. For studge entering with 25 percent dry solids,a typical heat requirement for this
drying operation is about 4,000 Btu/Ib of dry solids. In some incinerators.dryingand burning
occur sequentially, while in others both take place in the same chamber. The lauermethod us
characteristic of the fluidized bed unit and the former is characteristic of the multiple hearth
unit. In combustion, virtually all of the recoverable heat released may be required to meet the
demands of the drying process. Equipment therefore should be designed and operated to
achieve maximum practical combustion efficiency. Usually, auxiliary fuel (gas or oil) us
‘provided for startup and for use if the cake should become too wet.
Combustion of the sludge is followed by cooling and scrubbing of the gases to remove fly ash
and any unburned particles Usually, the ash is quenched with plant effluent. In all incinerators
there is a possibility of a futnace explosion. Most frequently, explosive conditions are created
by allowing unburned fuct and air to accumulate within the furnace, or by (ceding highly
volatile liquids with a large excess of air. Progressive ignition of sludge and air, as they ie
introduced into the furnace is the best insurance against furnace explosions. Proper purging
procedures prior to light-off of a cold furnace are essential measures to prevent the possibility
of an cxpicsion on startup.
The ash resulting from incinerating wastewater sludge will amount to IS to 45 percent of the
original dry solids by weight and about 10 percent of the volume of, say, a 25 percent sludge
cake. The incinerator ash may be disposed olin a landfill and may bean effective fifteraid for
sludge dewatering. Any slag formation is largely due to excessive temperatures. Available data
indicate that, on the average, uncontrolled multiple hearth incinerator gases contain about 0.6
grain of particulate matter per standard cubic foot of dry gas. Uncontrolled fluidized bed
• incinerator gases contain about 1.0 grain of particulate matter per standard cubic foot. Since
particulate emissions standards proposed by the EPA, based on “best control technology.”
• would limit emissions to no more than 0.030 grain per standard cubic foot, 95 to98 percent of
the fly ash must be scrubbed from the flue gas(141). Most systems utilize a variety ofwct
scrubbers, including venturi, baffle plate, packed tower, and impingement modele..Howevet.
the emission standard is based on use of a venturi type. Dry cyclones have been used With
fluidized bed incinerators followed by a wet scrubber. Scrubber water is a liquid sidestream
requiring treatment before disposal, usually by recycling to the main treatment plant. Wet
scrubbers also absorb significant amounts of gases which lower the pH.
Most of the ash from the multiple hearth unit falls out of the unit at the bottom into a quench
tank. Ins fluidized bed unit it is taken out by the dry cyclone. This ash is usually disposed olin
a landfill.
‘Ffl1 gase, can produce odors when organic molecules in the sludge are only partially broken
down and consumed in the incineration process, thus allowing complex gaseous molecules to
escape up (h stack. To date, investigation has resulted in a criterion that combustion
• ,tethperatures must exceed about 1,4000 F (about 760° C) to ensure destruction of all odorous
• äoi ponents. The thermocouple controls must be set no lower than about 1,6000 F to ensure
. that all parts of the burning chamber are above 1,400° F. .
- . v— — - _______ •_• — —

— - —
10.1.1 Multiple Hearth Incinerators
The most commonly used incinerator for wastewater sludge is the multiple hearth unit. A vast I
I amount of operating experience exists, and there are at least three manufacturers of such units’
in the USA. It consists of a circular steel shell with 4 to 12 hearths inside made of refractory
material supported on a central shaft that rotates (see Fig. 2). . -
Dewatered sludge is fed to the upper hearth where drying occurs because of the hot gases
coming up from the lower combustion hearths. The combustion occurs on the middle hearths,
at temperatures of about 1,500° F. The ash is cooled by the incoming air on the lowest hearth
and is then dumped into a quench tank for eventual disposal. The incinerator may be provided
with an afterburner to ensure complete cornbustio of all gaseous orgaiic matter. The as I
finally goes to a wet scrubber for removal of particulates. - -
JO. 12 Fluidized-Bed Incinerators .
LThe first fluidized-bed incinerator for wastewater sludge was insLal led in 1962. There are now
1973) some 40 units in operation in the USA. The basic design involved a bed of sand, which is
fluidized by the upward flow of air, used for combustion. The sludge is fed on top or into the
turbulent bed of hot sand and combustion is rapid and complete. There is a gas or oil burner
below the sand bed to preheat the incoming air if necessary. The unit operates at several inches’
of water pressure above atmospheric pressure.
The sludge can be either a solid or a fluid. When the sludge does not have sufficient c orific
value to support combustion and evaporate the moisture, supplementary fuel is burned with
the sludge. The ability to handle a liquid sludge is one of the principal distinguishing features of
the fluidized bed unit compared to the multiple hearth unit which must be fed with “solid”
Since the burned gases tend to carry out rn s of the ash, efficient particulate removal facilities
are required. Frequently, a dry cyclone is ed ahead of a wet scrubber, which should be no less
efficient than the venturi type. In some ‘:aUations, when handling a sludge of low calorific
value, the air blown into the sand bed a’ e bottom is preheated in a heat exchanger where
some of the heat in the exhaust gases is r -- vered. This increases the overall efficiency of the
unit. However, if such preheaters are usc thead of dry cyclones, which is desirable since the
cyclone is more efficient when handling . ooler gas, there can be severe abrasion in the heat
exchanger from the particles in.the fly ash Such exchangers must be constructed of special 1
inless steels and be designed for easy maintenance. —- -
The fluidized bed unit is competitive with other types of sludge incinerators and is usually of
lower capital cost in smaller sizes, but the operating costs are higher. it has the advantage of
being capable of rapid startup; therefore, it can operate with relatively good efficiency for as
little as 8 hours at a time. As far as incinerating municipal wastewater sludge is concerned,
loperating results compare favorably with the multiple ,I-iearth furnace.
II._ — — — —
(JO. 1.3 Drying and Incineration
Another type of incinerator that has been used for many years. but not in great numbers. is the
so-called “flash drying ahead of burning” unit. It is somewhat more complex than the other
two units described. Briefly, it hasa sludge drying stage which is accomplished by dispersing
/ -
63 -

Coolin 1 air di chars
Flue p.e%o.t
Sksdp. av
at sack hcueth
Cosbuidos sus
Cooliu 7ØSS
Ask diicbarpe

the solids in the hot gases from the combustion unit and blowing the suspension into a cyclone
dryer. The dried sludge is then conveyed to the furnace.
In some instances it is advantageous, for various reasons related to final disposal, to dry the
dewatered sludge cake. Of the commercial dryers designed to handle sludge cake, the simple
rotary type has been widely used. Drying will sterilize sludge and make it much more suitable
for disposal in a landfill, for incineration with municipal refuse and garbage, or for soil
conditioning. It has been found that fresh sludge, when dewatered after proper chemical
conditioning, can be handled more easil3, than digested sludge. The latter requires larger units
and tends to be slimy and thus interferes with heat transfer from the metal surfaces which are
heated with steam or circulating hot oil. The sludge has its moisture content reduced from
about 65 to 75 percent to 25 to 35 percent. Any lower moisture content tends to make the sludge
“dusty.” -
lO.1.4 Incineration of Sludge With Garbage and Refuse
‘At some plants, sludge is dried before it is mixed with other solid wastes for incineration.
• Drying sterilizes the sludge; also, the dried sludge c n be mixed better and it does not detract
I much from the heat value if the heat is recovered. However, it should be kept in mind that, for
‘domestic waste, the ratio of refuse and garbage to wastewater sludge as dry solids wilt be about
20 to I. Burning of sludge, even without drying, in a refuse and garbage incinerator is an
• economical operation if the sludge does not need to be hauled too far. No auxiliary fuel is
required, which is frequently the case when sludge is incinerated alone.
10.1.5 Incinerator Scrubber Wash Water
The handling of the sidestream that is generated by use of flue gas scrubbers has not been given
much attention in the literature. Little information can be found on the effects of recycling this
sidestream to the treatment plant. However, the effects should be slight since most of the
organic material should be oxidized during combustion to innocuous end products. This
sidestream is, of course, only a small portion of the total produced by the incinerator and
associated sludge treatment, and results from the final cleaning of the flue gas.’
• Tench, el aL(143), summarized analyses of scrubber effluent from two 22-ft-diameter multiple-
• hearth furnaces at the Sheffield wastewater works in England, where the final effluent is used
:for scrubbing and is returned to the raw wastewater channel. Suspended solids in the scrubber
effluent ranged from 600 to 7,690 mg/ I and averaged 1,760 mg/I. COD ranged from 110 to
2,600 mg/I and averaged about 520 mg/I. BOD analyses were on the order of 30 to 80 mg/I.
Ammonia nitrogen averaged 90 mg/I and ranged from 30 to 245 mg/I. No influence on plant
‘effluent quality could be detected, since any effects were within the normal range of variations
‘in effluent quality parameters. The ash quench water should be handled in a manner sim ilar to
scrubber water.
10.1.6 Incineration of Concentrated Liquids
Liquid sidestreams having high concentrations of soluble organic matter can be incinerated in
specially designed incinerators used for liquid wastes of high calorific value. If the sidestream
cannot support combustion itself, sufficient auxiliary fuel (gas or oil) can be used to evaporate
the water. Such liquid incinerators are built to handle 30 to 500 gph (gallons per hour) of liquid

wastes, and can handle waste sidestrcams even if they have an appreciable amount of
suspended matter. Of course, if the suspended solids are organic in nature, they will be
‘incinerated with the soluble matter. Liquid incinerators are fairly simple in design. They are
cylindrical in shape, and a cyclonic action is induced by injecting air in a tangential direction.
They are designed for high rate combustion, with waste liquid being sprayed into the
combustion chamber as finely dispersed droplets. Another type of liquid incinerator, the
fluid ized bed incinerator, is widely used for various liquid combustion processes in industry
jand is well suited for handling liquids having organic compounds of high calorific values in
1 solution and in suspension.
All such incinerators are equipped to accomplish complete combustion, and air pollution by
particulates can be controlled with wet scrubbers.
In the wet air oxidation process, organic compounds in the sludge are chemleally oxidized in
the aqueous phase by dissolved oxygen in a specially designed reactor at temperatures ofaboue
500° to 700° F and pressures from 1,000 to 2,000 psii’144). The degree of oxidation achieved in
the process can vary considerably, depending on sludge characteristics, temperature, and
detention time. In practice, the process is designed to reduce COD by 70 to 80 percent. Wet air
oxidation of sludge produces a sterile, stable product that dewaters and filters readily. mel
oxidized sludge is thickened and dewatered, usually by settling, vacuum filtratiDn,l
centrifugation. or a cembination of these methods.
I Wet air oxidation is especially suited to the treatment of dilute waste liquors and sludges which
I are difficult to dewater. This is because no preliminary dewatering or drying is required. in
marked contrast to incineration. Oxidation of most organic compounds is achieved under high
pressure (1,000 to 2,000 psi) at temperatures of 500° to 700° F. The temperatures are relatively!
low compared to temperatures of 1.5000 F or more required for complete conventional
incineration at atmospheric pressure. Air pollution is controlled because the oxidation takes
place in water and no fly ash, sulfur dios ide. or nitrogen oxides are formed. The remaining
liquid is sterile and has a high concentration of inert and organic matter, both in suspension
and solution. The COD of the liquid can vary from 10,000 to 20,000 mg/ I, depending on the
degree of oxidation achieved and the nature of the solids in the raw sludge. This residual
carbonaceous matter’ is present mainly as fatty acids. The organic nitrogen is converted to
ammonia, which is not oxidized and remains in solution, since at the pH in the reactorall of it
j ionized. The concentration of ammonia can vary from 1,000 to 1,800 mg/ 1. The BOD in the
liquor ranges from 20’to 50 percent of the COD, and reported values of BOD range front 2.000
to 10.0 (M) mg/I. Obviously, this liquid sidestream requires treatment.
The gases from wet oxidation can be odorous, and therefore a catalytic burner is
• recommended. Also, for the process to be economical, the large amount of energy supplied to
compress the air to the required high pressure should be recovered in a gas turbine. A flow
diagram of the process is shown in Fig. 3.
Burd concluded in his report that “The wet air oxidation system is one of the most expensive.
$ overall, of any of thedomestic wastewater sludge handling and disposing systems, and requrci
high quality supervision not normally associated with waste treatment plants”(l).

Oxidized sludge I
Grinder I
Reactor I
Air compressor
i generator
Biot’reatrnent 1
(optional) I
(Settling, filtration,
or centrilugation)
(optional) I
[ -•I

Erickson and Knopp(145) found that the concentrated liquors from settling and filtering wet’
oxidation sludge could be treated separately by activated sludge vitho t any dilution. Thcir
laboratorystu4 showvd that BOD removal of greater than-90perásit-could be achie cd at
loading rates higher than normally employed for municipal wastewater. BOD iecrea from
4,400 mg/I to Ies than345 mg/i. COD dç rçased by about 62 percent, fro 8,000 to 2,100
mg/I. nitrogen was sligbt—from I,250 ,to 1l70 mgIlade ccä tonIy6
percent lecreased from a itt O to 20 mg/I thi Iiqubr volume will
amount the sludge oxidized depending on the character and amount of
olids irri _____
-4 ppa • n removed a large por on of the
phosph4tea seed sludge. The aut ra otod that
The nonbiodegradoblerorganic matter (COD), fi e aoLt I3, and
añthtotia represented ditional pollutional;load’on.t ihe plant
±1’ they are not removed prior to recycling the liquor to the
main plant.
Di sdge has been spreØ on agricultural land. Many studicaaosaprcsestly in progress
nihc.vari us aspects of this method of fi naldisposal(146. 147 148). LJntila4ditional data are
liable o ,the. coinRo.ition ‘ f heat e ted 1udge li uor the di po eF tch liquid on
ig çii jra Ia i ouWno con red -
- ‘ i -.‘ . -
Thedistincticm shquld be ii e b4 eep.iand application ofdigested liquid sludge, discussed
belowand land àpPi a1iQft ftrea*$ Mewater, which will not be discussed. There are some
factors common to both p c ics but the-principal differences.are in thc moisture content
and solids composition of the ipp11ed liquid. Whensecofldtit w Stewater is applied to the
(lan 1. the ability of the soil to receive water will ordinarily be thc limiting factor. The volume of
sludge applied is on the order of I percent of the volume of secondary effluent; therefore,
hydraulic acceptance will rarely be thc limiting lactor. The ability of the soil to accept nutrient
‘salts, organic matter, or trace metals will control the rate of application.
Rather than being considered fertilizers, domestic wastewater sludges arc —coAii rcd soil
conditioners or soil builders. Soil conditioning provides agglomeration, soil struettge tability,
pore vo4ume, permeability, air and moisture holding capacity, and the abilitytowithstand
1 crusling, leaching, and crosion. Although sludges provide, s % (tI i* 4e d hcmical
nutrients, conventionalichemical fertilizers will still be required to provid Ctimum
combination of nutrienis for most crops. ‘
Since digestion does not.guarantee destruction of all pathogenic erganisa%s ppriatc
measures (as will be indicated later on) should be taken to prevent health hazards in applying
liquid sludge to land. Other factors of concern are the effects of nitrogen compounds and heavy
metals (Cu, Zn, Pb, Cd, Ni, Mn, Mg); especially zinc, copper, and nickel because they can be
toxic to plant lifc(149).
I - ‘ :-
Nuisance-free disposal rcqui r 1t$4iiy 4igeatit n to reduce the pathogen content and
prevent putrefaction. Pasteurization has occasionally bccn recluired by health authorities in
Europe. Lagooning before distribution on the kind, although not al ays essential, does serve
many usefuL purposes such as:

Storage of sludge near the distribution site so that intermittent transport to the
site from the plant may be scheduled independently of intermittent distribution
Detention time to permit a more complete die-off of pathogens than may be
accomplished by digestion alone
Detention time to permit some nitrogen to escape as nitrogen gas or as
ammonia. If not removed before application to land, nitrogen as nitrates may
pollute the groundwater, and ammonia may inhibit seed germination.
Distribution may be by furrow irrigation or by sprinkler irrigation. Aesthetically, furrowl
irrigation is less objectionable than sprinkler irrigation, which is more visible and which may
generate a mist of suspended droplets of liquid that can be blown away from the application
areas. The sludge may be incorporated deep into the natural soil by burial, deep disking, or 1
rotary tilling. Current research by the Agricultural Research Center of the USDA(150)should
indicate how helpful deep application may be.
The application to agricultural land of anaerobically digested sludge has been extensively
studied and documented. The application of aerobically digested sludge is not well
documented, but one may assume that its applicability to agricultural land is roughly
equivalent to that of the anaerobic sludge. The organic content of the solids is about the same
in the two kinds of sludge. The liquid portion of aerobically digested sludge is quite low in BOD
(although it may be high in COD). it also contains nitrates, which have direct fertilizer value.
However, if there is danger of groundwater contamination by the sludge liquid, nitrates should
be removed at least partially by a denitrifying step prior to application. Comparison of aerobic
sludge application with anaerobic sludge will be possible shortly, from results of the practice of
both by the Springfield, Illinois, Sanitary District(151).
The sludge accumulating on the ground provides good soil structure and some chemical
fertility. A liability is the possible buildup of heavy metals, potentially hazardous to crops
or to those who consume the crops. All types of crops appear to respond favorably to ap-
plication of liquid sludge to land, except when seed germination is inhibited by an exces-
sive ammonium ion concentration (162). At the time of this writing, a technical bulletin
is being developed by the Environmental Protection Agency, Office of Water Programs,
which will 1 provide guidance on safe sludge application procedures.
10.3.1 Present Practices
Placing sludge on agricultural land has been practiced throughout the world for many years.
Some modern, larger-scale operations are described below.
GREAT BRITAIN. Digested sludge from 20 to 30 of Britain’s 1,200 wastewater treatment plants is’
distributed to farmland by tank truck. In some cases, sludge is discharged from the truck to
ho’ding pond, then pumped and sprayed to the land. In other cases, the trucks drive over the
fields, spraying as they travel(152X153). Solids content is between 2 and 5 percent: Reportedly.
heavy metals concentrations have not been a problem; this may be due to relatively low rates of
application (usually not more than 5 tons of dry solids per acre per year). to the Lime in the soil
which complexes the metals, and effective monitoring program. This method of sludge 1
disposal is reportedly very popular among farmers.
GERMANY. un the! region between the Rhine and Maas rivers, the Niersverband regional

treatment plant has, since 1960, supplied sludge to 820 farms, resulting in increased yields of
truck crops, especially beets, and also pasture(1S4). Wet sludge on grassland has had a
favorable lasting effect in reducing evaporation from the soil. Pasteurization before
application has recently been provided to comply with health regulations. In Munich, sludge is
piped from the treatment plant to neighboring communities and spread on the field by a field’
railway system. -
CHICAGO. IWNOIS. This operation lathe majorexample in the USA of larger-scale disposal of
digested sludge to agricultural land. In contrast to Britain, where the practice is popular with
farmers, Chicago has encountered widespread apprehension from farmers, the general public,
and state officials about the proposed disposal of sludge to the land. Plans to spread sludge in
Kaflkakee County relatively near Chicago had to be abandoned because of opposition. Plans
in various stages of impldnentation are for sludge to be sent by rail to Arcola, Douglas County,
illinois, and by barge down the Illinois River to Fulton County, Illinois. These rural disposal
areas are about 200 miles from Chicago.
For both Douglas and Fulton counties, the Chicago Metropolitan Sanitary District (MSD)
proposes eventual replac nent of rail or barge transport by pipeline. Meanwhile, the system is
to pipe digested sludge from the plant to storage basins, then to barge or haul by sank car. At
the disposal area, sludge is unloaded to storage lagoons and aerated to reduce nitrogencontent
by ammonia stripping. From the lagoons, the sludge is pumped to spray nozzles for spreading.
In Fulton County the MSD purchased 7,000 acres of land, in contrast to the British practice of
spreading on private farms. By purchasing the land, on which MSD paysexistingrealestate’
taxes, the district maintains nec ry control over management of application rates and
multiple purpose land-development(ISSL
Land is prepared by leveling to 5 percent grade or less, and building earth berms to control
runoff. Runoff is retained in basins, sampled automatically, discharged to watercourse if
quality permits, or returned to the cropland if quality is unsatisfactory. Hedgerows are to be
planted around the fields to intercept laterally percolating groundwater which may be overly
rich in nutrients.
The barging opcration commenced in the summer of 1971 and spraying began in Fulton
County in the summer of 19fl over 400 acm. Spraying is expected to be operative over 8
months of the year.
10.3.2 Hygienic Aspects
In Germany and Switzerland, there has been concern about the hygienic aspects of the
utilization of wistewatçr sludge in agriculture. Under unfavorable conditions, viruses.
bacteria, parasites, and orm eggs can be disseminated. Health dangers are also present from
salmonellae ascarids, and cattle tapeworm. Pasteurization is recommended and practiced. At
the Niersvcrband regional treatment plant, sludge is pasteurized at 65° C (150° F) for IS
minutes. to comply with health regulations.
Dotson. Dean, and Stern of the EPA laboratory in Cincinnati reported recently(1S6) on
studies regarding pasteurizing liquid sludge in a tank truck using injected steam. The hot
sludge (1600 F) could be cooled by evaporation during spray application to the grassland. They
found a thousand-fold reduction in pathogenic and indicator organisms following 60 minutes
of exposvre of the sludge to a temperature of 160°F.

The airborne transmission of pathogens is an aspect of sprinkler distribution of sludge. There
is scant information now (1973) available on this subject. Adams and Spend1o e(I57)report
that, as far as 0.8 mile (1.2 km) downwind of a trickling filter, coliforms were detected in
aerosol samples. There have recently been other studies on the emission of coliforms from
wastewater treatment plants(158). However, there is no evidence that aerosols from any
domestic wastewater treatment plant have transmitted disease. Buffer zones should be
conservative until more information is available. Also, sprinkler distribution should be
restricted to stabilized sludge.
In connection with use of digested sludge on land, studies were conducted on pathogen
destruction by digestion by a research group at the University of Illinois. Meyer, er aI.(159),
studied porcine enterovirus survival in anaerobic digesters. Preliminary results show that a
reduction and Loss of infecti% ity can be expected upon suitable exposure of the virus in the
digester. Molina, el aI.(’160), observed the bactericidal properties of digested wastewater
sludge —the harmless organisms tended to destroy patnogens. Much work has yet w be done
before the mechanism, rates, and necessary conditions for pathogen destruction by digestion
arc properly understood.
10.3.3 Dried Sludge
‘Djied sludgi uch as “Mil i óT’Orgro,”. is available a a
Both activated sludge (Milwaukee) and digested sludge have been dried and processed for use
as fertilizer(/6 1). When packaged and dry, it is the most convenient means ofsupplyingsludge
to the owner of a small plot of land. However, the economics of producing dried sludge and
packaging it are not attractive for new installations. In fact, Milwaukee continues to sell
“Milorganite”only because the cost of the installation has been written off. The nutrient
content of dried sludge is roughly equivalent to that of the solids portion of liquid sludge. Dried
sludge does not contain the nutrients present in the liquid portion of undewatered sludge; !or I
example, a large portion of the nitrogen is in the liquid in the form of ammonia or nitrate and
phosphorus is in solution as various phosphates, all of which are removed when sludge is
dewatered prior to drying.
10.3.4 Current Studies
The application of sludge to farmland is being ,tudied at the University of Illinois. Conclusions
presented in an interim report released in 197 1(162) are:
To further reduce pathogen survival in digested sludge, about 2 weeks of
lagooning before application to land is practiced to permit pathogen die-off.
However, there is considerable doubt whether any significant die-off occurs in
such a lagoon. In any case, handling of such sludge has not proved to be a
health hazard.
Nitrogen contained in digested liquid sludge is the limiting factor to rates of
application. Data indicate that about 2 inches of sludge would satisfy the
nitrogen needs of nonleguminous crops without producing excessive nitrate in
percolated water. In the interest of higher loading rates, reduction of the
nitrogen content of sludge would be desirable.
Heavy metals are ever-present constituents of digested sludge and are usually
in the solid phase. After application to soil, they remain in the plow layer with
the sludge residue. Solubilization is negligible in soil of neutral or higher p11.
71 ‘

Plant uptake of Zn, Mn, and Fe has generally been enhanced by sludge
application. There is evidence that the uptake is a result not of direct metal
addition with the sludge but of induced motility of the metals native to the soil.
Digested sludge has been shown to be a source of nitrogen, phosphorus, and
micronutrients. Crop response to the water content has also been observed.
Sludge residue decreases the bulk density of the soil. Grease contained in
sludge has not proved to be a problem in clogging soils. Organic carbon has
accumulated in sludge treated soils, but has presented no observable prob m.
The rate of infiltration of digested sludge is low, regardless of soil type. Thus,
on sloping land, s iacial precautions should be taken to control the distribution
of sludge applied to the soil surface.
Seed germination is inhibited by freshly digested sludge.
Observations indicate that properly digested sludge will produce no offensive
odors after application to soil.
Since 1965, the Springfield, Illinois, Sanitary District has beenexpenmeuting with disposalof
liquid, anaerobically dige4ted sludge by irrigation of agricultural land. The conclusions they
have reached are:
Although considerable variation in results, because of climatic conditions, may
be expected from one year to another, the disposal method is econoiniealty
Sludge should bç applied to land utilized for growing crops for aubnal
consumption, rather than to uncropped land.
Soil moisture is a limiting factor in this method of sludge disposal.
Although application of sludge to the land by flooding is possible, this math-
od results in numerous problems, and spray application seems more de-
s rable.
Crops grown on the disposal areas should have-a high dtittand for nitrogen
and phosphorus.
Storing of sludge in lagoons for later distribution has not proved practical [ a
result providing some conflict with the recommendation by Hinesly, - el i
aI.(162), cited earfier, indicating that 2 weeks of lagooning were provided to
permit’ pathogen ie-ofl).
Molina, es aI.(163), have Studied the effect of aeration on liquid digested sludge. R duction in
ammonia-nitrogen conteflt of the digested sludge is broughtaboutwithina few days by
aeration or by aging in contact with the air. Reduction in ammonia is important, because it
1 evidently inhibits the germination of certain seeds sown on sludge applied to land.
The most widely recommended application rates in the US for crøpland are from tOto2Ottins
,of dry sludge per acre per year(164X165).1

It is important to distinguish between two related concepts:
Dumping of stabilized and dewatered sludge cake on land, generally without
Sanitary landfill, involving the controlled and systematic burial of stabilized
dewatered sludge beneath a suitable earth cover.
A stable sludge can be the result of biological digestion, treatment with lime to pH of 11, or
conditioning by use of heat treatment. Current (1973) studies by the U.S. Department of
Agriculture are being conducted to determine the greatest amowit of sludge that can be applied
to the land for maximum benefit and minimum hazard. This is sludge dewatered to about 22
percent solids. Most of the sludge is applied by burial in trenches 2 ft wide and 2 or 4 ft deep,
covered by 1 ft of soil. Other incorporation methods, such as deep disking or rotary tilling, will
be tested eventually. A variety of industrial sir 1 d . lomestic wastewater sludges is being used in
varying amounts. Fruit TiId sháde trees, shrubs, fescue grass, alfalfa, corn, soybeans, and
tomatoes are among the crops being tested.
J ’ L.
Dumping is the simplest and least imaginative sludge handling operation possible. Dumping.
long practiced as a means of final sludge disposal, has apparently been satisfactory when:
Sufficient land area is available.
The dump site is sufficiently far from populated areas that odor and
appearance are not a nuisance and the pathogen content is not a hazard.
Runoff to watercourses is controlled.
Percolation of leachate to groundwater is controlled.
In the past, all four of the above conditions have not always been met. The EPA has a stated
policy to convert dumps to well designed and properly managed landfills. Naturally, odors
increase in intensity with increased rates of sludge volumes deposited. Odors from
incompletely stabilized sludge can be reduced, but not satisfactorily eliminated, by soil cover.
Sanitary landfills require no less area but considerably more operational expense per ton of
solids processed than simple dumping, nor can such landfills guarantee elimination of odors
from incompletely digested sludge or sludge incompletely stabilized. Landfilling is the most
common means of disposal of incinerated sludge ash. Incineration reduces an odorous and
pathogenic dewatered sludge cake to odor-iree ash with only 3 to 10 percent of the mass of the
cake. The corresponding decrease in volume from cake to ash often makes incineration and
landfill of ash an attractive alternative to landfill of sludge cake, where available land isscarce
or distant.
For the disposal of municipal refuse and garbage, see the sanitary landfill information
developed by the Office of Solid Waste Management.
A properly decigned and operated sanitary landfill can be made nuisanceiree and acceptable

from an environmental hàalth standpoint. Studies conducted during the past few years
indicate that a considerable potential br groundwater contamination does exist. Most
landfills will cv ntually produce leachate, as well as gases. The quality of leachate depends on
the degree of decomposition activity within the landfill. Adequate digestion or chemical
stabilization of sludge before disposal to a landfill is essential to avoid poor quality of leachate.
The exact required dryness of the sludge cake for landfill disposal depends on the character of
the sludge. The cake must be such that, after covering with earth, compaction equipment can
be driven over it. To minimize leachate quantity, the landfill operation should be kept an
adequate distance above the high groundwater table, and surface runoff from areas tributary
to the landfill should be intercepted in drainage ditches to carry it around the landfill. It is
possible, and may be neóessary, to collect and treat leachate before it reaches a stream or other
fresh surface or groundsivater supply.
Prevention of rainfall p rcolation into the landfill also reduces the pollution potential. This
can be accomplished by adequate surface slopes in combination with impervious surface
ditches, maintenance of the landfill such as filling settlement areas immediately, and the
planting of over crop to consume a large volume of water. The use of a tight cover material
will also decrease the rate of rainfall percolation, but adequate vents should be provided for tlft
gases that are produced in the dc omposition process.
When a landfill is used for land reclamation, it should be remembered that the newcompacted
refuse and sludge will teiid to settle. Cameron(166) developed equations to predict the degree
of settlement and the period required for settlement using centrifuged digested sludge. Factors
affecting settlement were found to include degradation of volatile solids and initial moisture
The EPA Office of Solid Waste Management has cited(!67) the following as advantages ofa t
sanitary landfill:
Where land is available, a sanitary landfill is usually the most economical
method of solid waste disposal.
The initial investment is low, compared with other disposal methods.
It is a complete or final disposal method, compared to incineration and
compósting which require additional treatment or disposal operations fói
residue, quenching Water. and unusable materials.
It can be put into operation within a short period of time.
It can receive a l types of solid wastes, eliminating the necessity of separate
It is flexible, and increased quantities of solid wastes can be disposed of with
little need for additional personnel and equipment.
Submarginal land may be reclaimed for use.
The disadvantages cited include:
In highly, populated areas, suitable land may not be available within
economical hauling distance.

Proper sanitary landfill standards must be adhered to daily, or thc operation
may result in an open dump.
Sanitary landfills located neat residential areas can i esult in public opposition.
A completed landfill will settle and require periodic maintenance.
Special design and construction must be utili7ed for buildings constructed on
completed landfill because of settlement factor.
Methane and other gases produced by decomposition of the wastes may
become a hazard or nuisance problem and interfere with the use of the
completed landfill.
10.5.1 To Surface Water Other Than Ocean
Disposal of wastewater sludge to navigable waters is regulated by EPA under Section 405 of
the Federal Water Pollution Control Act Amendments of 1972.
I In the interior of this country, especially in the west, there exist bodies of saline and brackish
Iwater that could be used for c isposal of concentrated inorganic brine solutions. Since usually
volume of such brine sidestreams is small, pumping or haulage by tank truck or railroad
tank car could be feasible. Of course, for coastal cities, disposal of such brines into the ocean
may be possible (see Section 10.5.2).
In general, land disposal of brines will nol be acceptable due to contamination of the land and
groundwater with the salts present in the hnnes. For example, a sidestream containing an
appreciable concentration of ammonium ‘ Its could not be disposed of on most land areas.
since such salts would eventually be con’ cr1 cd to nitrates by bacterial action, and the increase
of nitrates in groundwater that might e used for domestic purposes must be considered
10.5.2 Ocean Disposal
At the time this report was prepared (1973). EPA policy on ocean disposal of wastewater
sludge was being developed in response to ne ’ legislation. Enactment of P1 92-532. the
Marine Protection, Research, and Sanctuaries Act of 1972 on October 23, 1972, gave EPA
statutory authority to regulate ocean dump:ng of most materials, including wastewater
sludges. Regulations and guidelines for this purpose are now being developed. When avaiLable,
these regulations and guidelines will establish the EPA policy on ocean disposal of sludges.
Deep well or underground injection may be acceptable for inorganic brine only. There are
pftsóntly many deep well injection systems in use in the oil fields for brine disposal and in other
75 1

locaLities for liquid wastes disposal. Such deep well injection is a highly developed technology
and it can, in certain instances, be a relatively simple method for disposal of concentrated
sidestreanis. especially those containing soluble inorganic matter.
Such underground injection involves comprehensive geologic investigations and field testing
to establish that the liquids will not clog the porous formations, are compatible with the water
already in the formation, and there is no danger of pollution or encroachment on underground
water supplies. A factor to be established is that such inject ions will not cause earth movement.
Waite streams to be disposed of by underground injection must be free from suspended
matter, unless it has been established that the waste is to be placed in an underground cavity,
either natural or man-made. Their chemical composition must be such that precipitates will
not form on contact with various rock formations and thus cause rapid clogging.
Any system for the disposal of a Liquid sidestream by underground injection must be designed
by eAperts in this field, since the technology is specialized arid highly developed. The method is
used in the oil fields extensively forso-called “secondary recovery” of oil from porous strataby
injecting waste brine from oil well operations(168).
Pyrolysis is a process involving the heating of organic matter in the absence of oxygen. The
term “destructive distillation” is used when wood is subjected to this treatment to produce
methanol. Depending on the nature of the organic matter, the decomposition of sludge by
pyrolysis at temperatures varying from 900° F to 1,7000 F produces compounds such as: char.
tars, various liquids, and gases such as hydrogen, carbon monoxide and dioxide, methane, and
Most of the studies reported in the literature on pyrolysis have been made on refuse and
garbage. The city of San Diego has been conducting such studies for many years(169). Their
studies showed that the process, once started, is self-sustaining by using one or more of tl c end
products as a fuel. Ingeneral, the volume reduction for such solid wastes is about SQ percent.
The char, for instance, has a Stu content similar to that of semianthracite coal. The Liquids, or
pyroligneous acids, can be used as a base material for various organic compounds. From the
tests on dried wastewater sludge they found that the end products were generally similar to
those from refuse and garbage.
Extensive studies of pyrolysis of municipal and industrial solid wastes have been conducted by
the U.S. Bureau of Mines( 7O). 1
Studies at the University of Western Ontario(/ 71) were made recently on pyrolyzing vacuum
filtered wastewater sludge. Before pyrolysis, the sludge had 85 percent moisture. 47 percent
ash, and 32 percent carbon on a dry basis. The tests were conducted in a multiple hearth type
furnace fed with an oxygen-deficient gas (about 2 percent oxygen): a temperature of about
1,3000 F was maintained. This burned off the volatile gases produced with minimum oxidation
of the carbon residue, referred to as the pyrolysate. The carbon pyrolysate was tested to
determine its ability to adsorb residual organic material in a treated wastewater effluent. The
tests showed that this carbon residue had an adsorptive capacity between that of fly ashand
activated carbon. Thus, the possibility of using pyrolyz:d sludge as an adsorbent for tertiary
treatment of wastewater is indicated.

A study was conducted by Rensselaer Polytechnic institute on so-called partial combustion,
using oxygen deficient air, with true pyrolysis being a limiting case /72). The tests were made in
fluidi ed bed reactors using materials such as ground paper, dried wastewater sludge, and
dried leaves. The partial combustion reformed the complex compounds, and generally the end
products were not too different from when true pyrolysis occurred, and hydrocarbons
predominated when pyrolysis was used. Separation of the mixture of compounds into valuable
products is not a simple problem.
I At present, as far as is known, no continuous full scale operation using either partial
combustion or pyrolysis exists, either for refuse and garbage or for wastewater sludge.
i However, various tests are in progress. A large scale (5 tons per day) pilot plant operation has
been conducted at Mt. Vernon, New York, by the Union Carbide Co. This is a unique
combination of pyrolysis and oxidation. To date they have only studied refuse and garbage.
but dewatered wastewater sludge cake could be mixed with the refuse.
The Garrett Research and Development Corp. of LaVerne, California. and Hercules, inc.. of
Cumberland, Maryland(173), have both described pilot plant studies involving pyrolysis of
refuse in which the economic justification rests on recovery of valuable end products from
various complex organic materials.
As far as wastewater sludge is concerned, since it must be dewatered to a degree comparable to
that required for complete incineration, there does not appear to be any sound economic or
technical justification for using pyrolysis unless a useful byproduct can be recovered, such as a
char which can be used in place of expensive activated carbon for adsorbing a large portion of
the soluble organic matter in clarified wastewater effluent.
This process fortreatingdewatered organic wastewatersludge is related toaerobicdigestion. It I
stabilizes sludge for final disposal. A large portion of the organic matter is oxidi,.ed to carbon
dioxide, water, nitrates, and phosphates. The process involves the destruction and
decomposition of the volatiles in the orga mc sludge by thermophilic aerobes (organisms whose
optimum temperature is 1350 to 1600 F)
There is an immense amount of literature on composting, especially of municipal refuse and
garbage. The method has also been applied to solids originating from vegetable and fruit
canning operations and to processing various manures(!74X175). Composting plants for
garbage and refuse have not been successful in this country- -not for technical reasons, hut
because financing of such plants was based on obtaining a ccrtain price for the compost, which
was pot realized. The process offers many attractive features for preparing organic solids fou
disposal on the land. It is considered a part of the general solids handling and disposal system
that is being engineered for the state of Delaware by IIercules(i73).
1 he composting of wastewater sludge. except for one study which will be discussed later, has
been done by mixing dewatered fresh or digested sludge with refuse and garbage(176)fl77.J.
1 here are many such installations in Europe. The primary reasons for mixing are that
dcwatcred sludge, averaging 75 percent moisture, is too wet to compost by itself, and the
wastewater sludge adds the nitrogen necessary for biological action for the nitrogen deficient

To maintain aerobic conditions, the composting mass must be sufficiently porous so that air
can penetrate it readily. For wastewater sludge, this means a moisture content of about 50
percent. The ratio of carbon to nitrogen and to phosphorus must be in the range required for
any aerobic biological oxidation and synthesis. There is usually an excess of nitrogen and
phosphorus present in raw wastewater sludge; in any case, there is no deficiency. The moisture
content must be maintained at a proper range and if artificial aeration is applied to a
composting mass, as is done in mechanical composters, this air must be monitored and:
controlled so that the optimum temperature is maintained once the thermophilic organisms
have become established. The moisture content cannot be allowed to go too low, since these
organisms require moisture for their activity.
After composting, the organic matter resembLes slightly moist humus (about 25 to 30 percent
moisture), and has an “earthy” consistency and odor. It can be disposed of on the land asa soil
conditioner or an innocuous landfill. Since an average temperature of 150° F is maintaine f
at least 5 days, all pathogenic organisms, including viruses, are kil’ed or mactivated.
- - - --- -- - ---—- -—- - — -— -
In 1967-6$, the Bureau of Solid Waste U.S. Department of Heal
Education and Wel6ie supported a research study on the composting of diwatered sewage .
sludge without mixing it with any other material(178). The sludge studied was a mixture of
primary and secondary sludge from the Salt Lake City wastewater treatment plant Thesludge
was chemically conditioned and dewatered on a vacuum filter. The tests were made in a 40-cu-
ft mechanical composter equipped with a specially designed agitator mechanism. Air was
supplied through a porous bottom of the unit under controlled conditions to maintain the
• proper temperature and amount of moisture. Recycling the final product to the head end of the
unit served to maintain the desired moisture concentration in the composter and also seeded
the incoming fresh sludge with thermophilic organisms. The composting reduced the original
sludge cake volume by about 60 percent and the weight by about 85 percent. The-final product
I had a fertilizer value ol cattle manure and was ftee from viable plant seeds and pathogen
indicators. it could be stored outdoors, uncovered, without causing any odors or
accumulation of flies or other insects.
When the sludge was conditioned with ferric chloride and lime, its pH was about 11.0. This
dropped to 6.5 very rapidly after the sludge entered the composter, because of the large amount
of CO generated. When the sludge was conditioned with polymers, the moisturecontent of the
cake was about 50 percent higher, this made operation of the composter somewhat more
difficult, and a higher recycle was needed. Otherwise, the end product was practically identical,
• except that the iron-lime conditioned sludge gave a fine-grained product.
• It was of interest that the diy solids were reduced by 30 percent, which means that this portion
of the sludge was oxidized. There were no noticeable odors from the composter. The test work
did not permit extrapolation so that costs could be estimated for a full scale plant, since the:
• studies showcd up several design weaknesses in the equipment. However, rough estimates
indicated that such a compostcr and appurtenances would have a total cost no greater than an
anaerobic digester for handling the same amount of dry solids by weight. This assumes that the.
digested sludge would be vacuum filtered; for composting, the raw sludge must also be vacuum
filtered. The operating costs probably would be higher for the composting installation.
One possible problem area In composting, however, is heavy metals. While its true that
composting of sludges appears to reduce metals translocation to crops, care must be taken
that the original sludge is of Mafficiently high quality. At the tirne of this writing a Technical
bulletin is being developed by the Environmental Protection Agency, Office of Water Programs,
which will provide guidance on safe sludge composting procedures.

Methods of processing various sludges so that the whole or portions of them could have a
beneficial use would obviously be of considerable significance with regard to ultimate disposal.
Therefore, such possibilities should be explored and evaluated.
If chemical coagulation with alum and subsequent settling and/or granular media filtration are
,used to reduce the suspended solids in a wastewater treatment plant effluent, the resulting, l
sludge will be light and difficult to dewater. Tests should be made using a deep, coarse media
1 filter to determine if the gravity sedimentation step could be omitted. The solids that are
accumulated in such a filter are densified and agglomerated so that the) can be settled out of
the backwash water very readily, and thus the volume of sludge that must be disposed of is
greatly reduced.
Studies have been made in the USA and other countries on recovering the alum from this
sludge when there is a high percentage of aluminum hydroxide. By acidifying the sludge with
commercial grade sulfuric acid to a pH of about 2.5, the aluminum hydroxide will be dissolved
and converted to aluminum sulfate (alum). The organic and other solids can be settled out of
such a solution; they form a relatively small volume for final disposal after neutralization with
lime. The solution with the recovered alum would, of course, be reused. There are several
wastewater treatment plants in Tokyo which produce water for various industries. The total
capacity of all the plants is 400 mgd. At all these plants, the above described method of alum
recovery is used.
Fhis method is economical at the Japanese plants, arid also greatly simplifies the waste solids
disposal problem(/79), but it has the disadvantage that acidification of the sludge will
redissolve any precipitated phosphates, iron, and manganese. This can be a serious handicap if
‘phosphorus must be low in the treated effluent. The only phosphorus removed in such a
recovery process would be that in solution in the blowdown from the clarifier after
acidiflcation. This would also be true of any precipitated iron or manganese. The problem
could be controlled by increasing the acid solution blowdown or by wasting some of the alum
sludge. but the recovery efficiency would decrease. In either case, more makeup alum would be
needed. Also, it may be necessary to add lime to tl e water being coagulated to provide
sufficient alkalinity. -

L. . - - -‘ :.?.: .i
An alum recovery plant treating a low turbidity water supply (capacity 38 mgd) is in opeiation
Fat Daer. Scotland, and is quite successful and justified economically both from the aspects d
savingalum and simplifying sludge disposal(!80). A water treatment plant in Warsaw, Poland.
also uses the acid recovery system(181)
jAnother method for recovery of alum from sludge is the alkaline process using lime. This
process was originally developed by the Lurgi Co. in Germany. It consists of adding lime to the
alum sludge and stirring vigorously at a pH of 10 to I I. With this treatment, about 50 percent
of the aluminum is recovered as a calcium aluminate, which can be reused as a coagulant
together with fresh alum. The solids residue settles md thickens readily. With this process, the,
phosphates and any iron and manganese precipitates would not redissolve.

If lime is used, with resultant precipitation of calcium carbonate and calcium phosphates, the
‘sludge probably should not be biologically digested, since some of the phosphate will be
solubili7.ed as the pH drops to neutral in the digester from generation of C02. However, some
recent limited studies indicate that digestion is possible, with only partial solubilization of the
calcium phosphate(27). Such sludges can be easily dewatered on, say, a vacuum filter and
disposed of in landfills or by incineration. The multiple hearth furnace has been adapted to
accomplish the dual function of incineration and calcining the precipitated calcium carbonate
to CaO. Because of the difference in density of the ash and the CaO, a fairly high degree of
separation can be achieved in a dry cyclone, and about 50 percent of the required lime can be
recovered. Consideration must be given to the buildup of MgO and various inorganic inert
material. Such recovery is practiced at the Lake Tahoe waste treatment plant(182), which hasa
capacity of 7 mgd. This method would not be economical for small plants.
Hydrolysis of organic materials has been used to make available otherwise undigestable food
ingredients. After hydrolysis, the solubiliied material is concentrated to a thick syrup which is
referred to as “molasses.” (This term is used ir respective of the origin of the syrup, whether it be
from wood. soybeans. sorghum, cane. etc.) It hai value as an animal feed and currently sells for
about 2 to 5 cents per pound.
The simplest method of hydrolysis is by injecting sulfur dioxide gas into a slurry of solids, then
heating it to about 135° C under pressure and holding it at this temperature forabout 3 hours.
Suh studies were made using a concentrated activated sludge under sponsorship of the
EPA(183X184). About 50 percent of the sludge is solubilized. The cooled mixture was readily
filtered to produce a cake having 40 percent solids and a liquor of light yellow color, having 8
percent by weight of dissolved solids and a pH of 3.0. The liquor was concentrated in an
evaporator to 60 percent dissolved solids, which were 18 percent ash and 82 percent organiës.
The fate of trace elements in the hydrolyzed sludge should be checked before feeding to animals
in the human food chain.

I Microstraining with a rotary drum type strainer has been used for removing suspended solids
from final clarifier effluents. The openings in the stainless steel fabric are on the order of 25
microns The flow through the strainer is due to a differential head of a few inches of water. The
screen is continually washed by directing a stream of clean water opposite the direction of flow
during the straining operation. This wash water with the suspended solids will amount to
about 3 to 5 percent of the flow being treated. Wash water is usually returned to the final or
primary clarifier.
Tests of the final cLarifier effluent at a wastewater treatment plant in Baltimore showed that the
suspended solids were reduced on an average from 20 to 12 mg/I and the BOD from 20 to 10
‘mgi 1(185).
1 arge scale tests were made at the Chicago Metro Hanover Plant, and it was shown that the
niicrostrainer. under certain hydraulic loadings, could reduce the suspended solids to 5 mgi I if
the influent solids loading did not exceed I 5 mg/I (/86). It was not effective in removal of
alum-coagulated solids. There are over 30 microstrainer installations in Great Britain for
reducing the suspended solids in final istewater effluents. However, on an average the
strained effluents have suspended solids ot about 15 mg/I. M icrostrainers are not reported to
be effective in removing colloidal solids(/R7).
Various types of gravity and pressure filters with granular media are used for removing
suspended solids of various types and concentrations to reduce them in effluents. The media
may he sand; coal on top of sand; coal, sand, and garnet; mixtures of the above (mixed media
type): activated carbon on sand; or activated carbon alone. When activated carbon is used.
som adsorption of soluble organic compounds is desired and obtained. However, such
adsorption must be relatively low, or the carbon would have to be removed frequently either.
for disposal or regeneration.

In recent years. granular media filters with relatively large granules have been-built much
deeper than the standard 2-to 3-ft filter used for some 50 years in water treatment plants. Such
deep filters, up to 20 It, with large void volumes, are used to remove concentrations of
suspended matter of 100 to 200 mg/I at filtration rates up to 10 gpm/ sq ft and higher from
The Europeans have used modified designs of granular media filters for many years in
wastewater treatment. The so .cailed upflow filter(188) developed in the Netherlands has had
wide application and is now available in the USA. The filters are cleaned of the accumulated
suspended solids by taking the filter out of service and backwashing it with clean water (filtered
emuent usually), at rates which expand and fluidize the bed to about 100 to 125 percent of its
original volume. Frequently, air is bubbled up through the filter to produce a violent scrubbing
action and clean the granules, and then the water is turned on to remove the loosened dirt.
The backwashing of granular media filters produces a liquid sidestream with a relatively high
concentration of suspended solids, varying from 100 to 1,000 mg/I, which may amount to Ito
5 percent of the water filtered.
The volume of backwash water used varies between about 1.0 to 3.5 percent of the volume of
water filtered in studies on fiLtration of activated sludge plant effluent(186). No correlation
with solids loading or removal rates has been observed, but it was found that the ratio of
backwash flow to flow treated decreased with the hydraulic loading rate (in gallons per minute
per square foot) and with increasing head across the filter. The filtered effluent averaged about
3 mg/I in both SOD and suspended solids. The removal of suspended solids averaged about 70
percent and SOD removal averaged about 90 percent. The backwash water volume amounted
to about 2 percent of the wastewater filtered, and had an average SOD of 1,350 mg/I and
suspended solids of about 350 mg/I.
The backwash water volume for upilow sand filters treating trickling filter effluent having
stispended solids of 30 mg/I and a BOD of 20 mg/I was about 1.25 percent of the volume
trcated(189). Suspended solids in the wash water were 2,260 mg/i at I minute after the start of
hackwashing; they increased to 6.525 mg/ I after 2 minutes; and then decreased to 80 mgi I
after 7 minutes.
I he backwAsh water characteristics depend on the type of effluent applied to the filter. for
example. wash water from filters treating the effluent from coagulation or other chemical
treatment would contain alum, lime, iron salts, polymer, or other chemicals in addition to the
pollutant removed.
The backwash water is frequently recycled to the final clarifier; hoitever, storage must be
provided since the badkwash rate is several times the filtration rate 4p1, unlcss there is a
multiple filter installation, the hydraulic load on a clarifier would he ea sivc. With storage.
the backwash water can be recycled over a period of several hours. The geUcnded solids must.
of course. be kept in suspension by some means of agitation.
In sonic cases the backwash water may be settled in a separate clarifier. a the overflow can be
mixed with the plant effluent. An important characteristic of the solids that arc washed oil
granular media filters is that they are compact, dense, and fast settling n though ahead of
th filter they may have been light. small, and flocculant in character. Ii (hcr words, the filter.
•tends to densify and agglomerate the suspended solids in a wastewater effluent, whether tbey
be organic or inorganic in composition. Thus the disposal of this particular liquid sidestream is
usually a relatively simple operation.

• .Q ov the:downflow ty e qu .ba k sh it isu ily on
4aUy ba1is .tó I t ove the cãñ ulated suspended solids and bacterial growths. TlIoii h suc
bäck 1 wa*l i of suspended soLids, it should be returned to
.p edii clarifier.in th b coming fairly c mmon tou e üpflo dolümns
I plevent the accumulatiofi? dE.ana robic biologicat growthi Alsá , ’ efflueñçs: iaviñg somc
t suspended solids can th n reatedrwithoutclôggi g the bed(1 90).
;. uld usually require fi t ti n-i Llie.suspended solids are to be -keoUQw -
i ne UtsposaI 01 brines arid otherconèiñ a edsoLütions c anüsualljbe f It
mbdcrn desalination ncthods. Forsalt concentrations below 2,000 mg / l.,th reeprocesse sho 4
good potential for desalination: .ion exchange, reverse osmosis, and e1ectfodialysi . IonI
ex tha’ (ge in combination with cbe icaL processes has the dual potential of producing potablej
watêr ’huIe at the same time concentrating the solids up to 8 percent by weight. It has’ the
disadvantage of ccntributing solids’ to the salt disposal problem. Reverse os,no. is has the
advantages of being a nonselective, highly-dependable, low operating cost proces’s It has the
disadvant ’age of discharging large volumes of low concentration brine, from which thç umpj
energy must normally be extracted by a turbine for good power usage e or om Elecrodia!ysis
has th &‘dvantages of being fully commercial, having good power economy, and also h ivinga
good c ncencrating factor for the waste brine. Its main disadvantages tare membrane;
poisoning, polariiation , and so-called spalling of the membrane surfaces by chemical scale
when improperly operated. These three processes produce large volumes of waste. brine
containing all of the contaminants removed in rendering the municipal wastewater effluent’
suitable for reuse. Determination of the costs for ultimate disposal of these waste brines ha s
been studied in dctail(191)(!92). -
In general, waste brine disposal might be accomplished by underground injection, land
spreading (with proper consideration for groundwater contamination), or sea discharge by
- pipeline. Evaporation, either to saturation or dryness, can be used ahead of final disposal.
Howe er. in inland areas the cost of brine (or other solution) disposal can be a major factor in
renovation of municipal vastewater. Where solar evaporation can be used, the cost can be as
low as 5 cents per 1.000 gallons and up to 75 cents per 1,000 gallons. Brine injection into deep
• wills ir Arizona has been estimated to cost about 15 cents per 1,000 gallons; however ir other
parts of thc country it can vary from 3 to 35 cents per 1,000 gallons.
- The dispoisi costs are highly dependent on-climatic and geologic conditions. SOme brines,
especially tk.se having fairly high concentrations of organics. will undoubtedly i q re
• pretreatmeNt bdore disposal. - - -
A considerable number of udLe have been made on the use of ion exchange i r tht’ovinj
vari.us nutrients such a nitrate , phosphates, and ammonia from wastewaters. Al Q , ion.
caching. could very lilce!y be use to keep down the concentration of various inozgani salts
which arc dilTicult to rernov*e by recipitation, when a wastewater is bein ’g recycled, or i ’üsed..
Such treatment might done ononly a portibn of the.tot Lwatèi beina reuscd. 2 -

An i n-cx hange procesi that has reccived çonsiderab!estudyi that of ammonia.zegi%bvaL
fr.n wastcwntcr effluents Ccrtiiø zeolites, including the naturally occurring nineral
ellioptilolite. have a high selectivity for the ammonium ion(I9J). After the column of ion.
eashangd fiatjeriaI is exhausted, it, is rcgenerat ed With li ewater containing sodiumchloridç to
pesd up the. regencrat•ion. ‘The ,Juigh pH of the limewater converti the amnionium Ion to
ammonia gas’ in soluti4n. The amn on1a i s all9wed to build up to about. 5flO mg/ I durifig
• r.g.nerstion. The regenerant soh tion:éontaining the high con entration caibeair 4 strijped of
th ammonia. The regenerant alsQ.c9u1d be reacted with waste sulfuric acid an4 a solution of
ammonium sulfate produ cdtwhich, on evaporation, could be converted to :f?rdiiasr, or
perhaps in some places it could be used dkcctty without evaporation. In any case;the disposal
u this sidestream can bea próbL m ‘and tan ‘hlcrthe tile costof, the proccU significantly.
Recently (1972). studies by the BatteHe N irthwest Laborato!y have md I ted that, tisiega high
constniration of NaCI in the . generan solution, electrolysis can be used tb produce free
chlorine and thus destroy the mm nia by breakpoint chlorination. Itowever,strjct coiirolof
the pH is required to avoid ,rpdnpng ‘the obnoxiou?nitrogen trichtoride.
I I, . . .•
I I • . &
• e • .• • • -
When demineralization isth purppse of ion exchahgë, acid resins for remo r ng citions sad
base resins for removing anIoâs,a epuploycd in series. 11w acid resins exchange Wfor the
cations while the base resins xèha uge OH for the anions. When the excl*nge capacity of the
reshis is exhausted, the unit must be u*generatedby passing an acid through the acid rim
columns and a base through thd liase mesiW columns. :flue regcnsra lbn step priit ces
sidestreams for the pr css$. An6thér idestmeam Is: produced when the col m ai p
backwashed to remove p.r*iculate matter. Baqkwulnng is neceua’ryevery ihe’t fiys
regeneration cycles(194.L The practice bea atolecycle the kack!ash watchichInsoaa(
to about4 percent of the prqduttj)o*.
An ionexchange process d Moped by Konm and Haas Co. and cafled the “DeSar.procse’
has been shown to be effective in renovating secondary wastewater efflnsnts(195). Good
removal of nitrates, phâsphates, detergents, etc., is achieved. The anion.cesin an be
regenerated by use of ammona and then brought back to the bicarbonate form with
pressurized carbonated water. Appreciable removal of COD is obtained and organic fouling .1
the particular rcs)n is not reported to be a problem. The regenerant (sidestreamn) Is, oIc ioi.
composed of a solution of nitrates, )bosphates, etc , which must be disposed of. Unless the
plant is near the maccast oe in a desert or awasteland area, such disposal ii a roblem.
I •• . ‘.. •
• •.. • ...
A “concentrating” process su. h as- ion exchange, as far as pollutants are cäàcer & Is net
particularly attractive, since thepollutants originally present inthcwaaterizéalsopf ISUt
in the concentrated sidestream- Ion exchange does have application for removing tertala
nonpolluting soluble inorganic compounds to keep their concentration below a lc*l desired
for. say. reuse of the water. . • • . :
.. I • • • •
Regenerant solutions from ion-exchange systems have high concentrations olsolubles which
present difficuk disposal problemL In some cases, the soluble compounds cair be precipitated
or chemicallyaltered.
U a cationic an anionic membrane are immerse pirallel to.sach other i a three.


1 potenfi I . ió migrate out ót t e th Iral c ârtiñ ntinio
the central compartment with a lower ionic concentration. Cations and anions migrate out of t
thc ionic solution ihrough the anionic and cationic membranes, respectively. This process has
been studied in the desalination of sea and brackish waters. It has been shown that the proccss
can he applied to demineralization of secondary wastewater effluentO96, 197, 198). It is most
efficient if only a portion of the salts need be removed to keep the concentration within
acceptable limits. The capital cost of electrodialysis equipment is relatively high compared to
other ud anced wastewater treatment processes. The disposal methods for the brine or
concentrated solution are similar to those for ion-exchange brines. In general, the
• electrodialysis unit should be placed at the end of the various wastewatçr treatment processes
here the concentration of organic matter is lowçst and the concentration of inorganics the
highest. Organics cause membrane fouling. - . - 1 - - - -
‘Reverse osmosis has been extensively studied for use in advanced waste treatment and for
desaiination of vater(199X OO). The use of a process that can remove virious, solubles,
especially inorganic types, from wastewatcr is desirabLe iii some locations to deçreasethe
dissolved solids content of the effluent. Studies on theapplication of this process t wastewater
‘treatment have been limited to d&t (1973) largely to bench and pilot plant investijatrons. It is a
process which affords the engineer concerned with iaitewater enovation,a àreat deal
latitude both with regard to the quality of the feed supply and the desired effluent quaLity. It is
• essentially a membrane process, but the membranes can be designed to remove fine suspended
solids in addition to soluble organiq and inorganic matter.
Two streams are produced in the process: the pr duct water,’ or permeate, and the sidestream,
or concentrat 1 Several design variables affect the quality of the concentrate, including
treatment beforç application to the reverse osmosis unit, efficiency of the membrane in
rejecting salt and the percentage of water recovered in the process. Waste treatment before
reverse osmo is has included: settling. c’ernical treatment follOwed by settling, activated
sludge treatment, and activated carbon u a Iment of activated sludge effluent(’2OO,) 2O1).
I • • • .‘ -
Studies have been conducted to determine hecher reverse osmosis conceiitrates can be treated
by the activated sludge process to remo’ e rganics(2O2, . Two of the studies used chemically
flocculated and settled effluent. In one, ferric chloride wis used as the flocculant, and in the
other alum was used. Both studies found t ut greater than 90 percent removal of total organic
carbon could be obtained. .
I ,
Reverse osmosis appears to ha;c considerable potential in the w stewiter ireatment field
because of its great flexibility to remove ions of various nolecularweights , from NaCIto large
• protein molecules having molecular weights of several millior . rhis is made possible by
“tailoring’ the membrane to the particular separation process. Membrane life is the major
operating cost, but great advances have been made in recent ye rs ,to extend this life for various
operating conditions. .

85 \

“ sauzie
,tnovat.os of wa*watses. Studiss iisdiss thet the bits will be ku thee this, of .iassC
cose(203X1P4 Is .eue,a1, k will be applied so sheblu kssa ponies dthi
treated wassswa$sr waaa shot h.bãj issued, wbse lbs dilsaud WIse is aloud wib th ‘
w ie *saa thscsucsstutiss llp will be WPh the ds lued baits pousle evud ‘
the waslsvateu. - . .•:•
• • • ‘4S
• ?r.s,.
The problem .1 dimoslsi lbs sidsiti tdaiii.iiiths h IuI.d.S im. V


• be simple, depsodieg .11W type of asl bs pMcIs udss. kamybs .
sees _wy so cssc i e this sulsitseem. : . •. • r . •. ‘ —
. •. •, :•... .• .

This design manual (June 1974) was prepared for the U.S. Environmenta’ P otecton Agency
Office of Water Program Operations by the firm of Camp Dresser & McKee Inc. Major EPA
contributors were WA. Whittington, C.H. Sutfin, and C.L. Swanson of the Office of Water
Program Operations and J.B. Farrelland i.E. Smith of the U.S. EPA National Environmentall
Research Center, Cincinnati, Ohio. Major CDM contributions to the writers were by CA.
Parthum, R.E. Leffel, and the CDM Design Review Committee.
A.A. Kalinske, A.B. Pincince. and J.A. French.

1. Burd, R. S., A Study of Sludge Handling and Disposal, FWPCA Publication WP-20-4
(May 1968).
2. The Impact of Oily Materials on Activated Sludge Systems, EPA Report 12050-DSH-
03/71 (March 1971).
3. Smith, James E., Jr., Present Technology of Sludge Dewatering, Internal Report of
AWTR Lab., FWQA, Cincinnati, Ohio (October 1970).
4. Levy, R. L., White, R. L., Shea, T. G., “Treatmeflt of Combined and Raw Sewage!
With the Dissolved Air Flotation Process,” Water Research, 6, No. 12, 1487 (1972).
5. Peixoto, E. C. and Volpi, C. A., Marine Disposal at Rio de Janeiro— Water Quality
Evaluations and Design, Proc. of 5th International Water Pollution Research Con-
ference, San Francisco, Calif., p HA—b/i (July 1970).
I Dissolved-Air Flotation Treatment of Combined Sewer Overflows, FWPCA Research
Report PB-189-775 (January 1970).
7 High-Rate, Fine-Mesh Screening for Sanitary/Storm Overflow and Sanitary Peak
Loading, FWPCA Contract 14-12-128, Symposium of CH 2 and M and Sweco, Inc.,
Portland, Ore. (June 1970).
8.! Private Communication From Sweco, Inc. (October 1972).
9. Bruce, A. M. and Boon, A. 0., “Aspects of High Rate Biological Treatment of
Domestic and Industrial Wastewaters,” Jour. of Inst. of Water Poll. Control (Gi.:
Brit.), 70, No. 5, p 500 (1971).
10. Sak, J. C., “Plastic Biological Oxidation Media for Industrial Waste Treatment
Needs,” Proc. 14th Ontario Industrial Water Conference, Niagara Falls, Canada
(June 1967).
I I. Germain, J. E., “Economical Treatment of Domestic Waste by Plastic-Medium
Trickling Filters,” Journal WPCF, 38, No. 2, p 192 (1966).
12. Antonie, R. L. and Welch, F. M., “Preliminary Results of Novel Biological Process
for Treating Dairy Waters,” Proc. 24th Purdue Industrial Waste Conference, p 115
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port, 17070-DAM-lI/il (November 1971).
Wall, D.J. and Baumann, E.R., Field Investigation of PA B-Filter Processfor Tertiary
Wastewater Treatment, Engineering Research Institute. Iowa State University. Ames, C ’
Preprint No. 72070 (March 1972). . 4
preceding page blank - .!9:_ . \

IS. Young, J. t., and McCarty, P. L., “The Anaerobic Filter for Wastewater Treat-
ment,” Journal WPCF, 41, No. 5, p R160 (1969).
16. Haug, R. T. and McCarty, P. L., “Nitrification With Submerged Filters,” Journal
WPCF. 44, No. II, P 2086 (1972).
17. Lindane, R. C., “Plastic Media Biofilters—Their Application in Canada,” Waler and
Pollution Control (Canada), p 34 (November 1972).
18. The Kinetics and Settleability of Activated Sludge Developed Under Pure Oxygen
Conditions, Tech. Report EHE-72-18, University of Texas, Austin (August 1972). I
19. Stamberg, J. B., Bishop, D. F., Hais, A. B., and Bennett, S. M., System Alternatives
In Oxygen Activated Sludge, presented at Annual Conference of WPCF, Atlanta,’
Ga. (October 1972).
20. Robson, C. M., Nickerson, 0. L., and Block, C., Operational Experience of a Corn-:
mercial Oxygen Activated Sludge Plan:, presented at Annual Conference of WPCF,
Atlanta, Ga. (October 1972).
21. CuIp, Gordon, “Chemical Treatment of Raw Sewage,” Water and Wastes &igi-
neering. p 63 (July 1967) and p 54 (October 1967).
22. Brenner, Richard .E.. Combined Biological-Chemical Treatment for Control of
Phosphorus. AWT and Water Reuse Symposium, EPA, DaIlas,Texas(January 1971). i!
23. Grigoropoulos, S. 0., Vedder, R. C., and Max, D. W., “Fate of Aluminum-Prep-
tated Phosphorus in Activated Sludge and Anaerobic Digestion,” Journal WPCF, 43. p
2366(1971). -
24. Malhotra, S. K. et al., “Anaerobic Digestion of Sludge Containing Iron PhosphatCs,”
Journal Sanitary Engineering Division, Proc. Amer. Soc. Civil Engrs., 97, SASS p6 2 9
25. Singer, P. C., “Anaerobic Control of Phosphate by Ferrous Iron,” Journal WPCFI’
44, No. 4, p 663 (1972). I
26 Kreissl, J. F. and Westrick, i. J., “Murncipal Wastewater Treatment by Physical’
Chemical Methods,” Proc. of Conference on Application of New Concepts toPhvskiel-
Chemical Wastewater Treatment, Vanderbilt Univ., Nashville, Tenu., p I (September
27. Van Fleet, 0. L., Barr, 3. R., and Hams. A J., Treatment and Disposal of Chernkal
Phosphate Sludges in Ontario, presented at Annual Conf. of WPCF, Atlanta, Ga.’
(October 1972).
28. Horstkotte, 0. A., Niles, D. 0., Parker, i) S., and CaIdwell, D. H., Full-Scale Tess-,
Ing of a Water Reclamation System, presented at Annual Conf. of WPCF, Atlanta, I
Ga. (October 1972).
29. Brenner, R. 0.. Combined Biological-Chemical Treatment for Control of Phosphor.
us, Advanced Waste Treatment and Reuse Symposium, EPA, Dallas, Texas (Janu-
ary 1971).
30.’ Convery, J. i., The Use of Physical-Chemical Techniques for the Removal of
Phosphorus From Municipal Wastewa:ers, Advanced Wastewater Treatment Semilar.
FWQA, San Francisco (October 1972).
31. Barth, E. F., Jackson, B. N., Lewis, R. F., Brenner, R. C., “Phosphorus Removal

FromWastewater by Direct Dosing of Aluminate to a Trickling Filter,” Journal
WPCF. 4/, p 1932 (1969).
32. Bishop, D. F., O’Farrell, T. P., and Stamberg, J. B., Physical-Chemical Treatment of
Municipal Wasze;vater, Report of AWTR Lab., FWQA, Cincinnati, Ohio (October
1970). -
33. Weber, W. J., Jr., Hopkins, C. B., and Bloom, R., Jr., “Physicochemical Treatment
of Wastewater,” Journal WPCF. 42, p 83 (1970).
34. Weber, W. J., Jr., Physicochemical Systems for Direct Wastewater Treatment (See
Ref. 26), p 13.,
35. Weber, W. J., Jr., Friedman, L. D., and Bloom, R., Jr., Biologically-Extended Phys-
icochemical Treatment, presented at 6th Conference of Intl. Assoc. on Water
Poll. Research, Jerusalem (June 1972).
36. Cousins, W. G. and Mindler, A. B., “Tertiary Treatment of Weak Ammonia Liquor,”
Journal WPCF, 44, No. 4, p 607 (1972).
37. Robertaccio, F. L., Hutton, D. 0., Grulich, G., and Glotzer, H. L., Treatment of Or-
ganic Chemical Plant Wasrewater With the DuPont PACT Process, presented at,
National Meeting of AIChE, Dallas, Texas (February 1972).
38. “News Item,” Journal WPCF, 35, p 264 (1963).
39. Kolega, J. J., Cosenza, B. J., Dewey, A. W., and Leonard, R. L., Septage: Wastes 1
Pumped From Septic Tanks, Paper 71-411, presented at 1971 Annual Meeting,
Amer. Soc. of Agric. Engrs., Pullman, Wash. (June 1971).
40. Smith, S. A. and Wilson, J. C., “Trucked Wastes: More Uniform Approach Needed,”
Water and Wastes Engineering, 10, No. 3, p 48 (March 1973).
41. Pfeffer, J. T., Leiter, M., and Worlund, J. R., “Population Dynamics in Anaerobic
Digestion,” Journal WPCF, 39, p 1305 (1967).
42. Hindin, E. and Dunstan, 0. H., “Some Characteristics of Anaerobic Sludge Diges-
tion,” Sewage and Industrial Wastes Journal, 3!, p 669 (1959).
43. Hindin, E. and Dunstan, 0. H., “Effect of Detention Time on Anaerobic Digestion,”
Journal WPCF, 32, p 930 (1960).
44. DahI, B. W., Zelinski, J. W., and Taylor, 0. W., “Polymer Aids in Dewatering and’
Elutriation,” Journal WPCF, 44, p 201(1972).
45. Bargman, R. D., Garber, W. F., and Nagono, J., “Sludge Filtration and Use of Or-
ganic Coagulants at Hyperion,” Sewage and Industrial Wastes Journal. 30, p 1079
46. Zablatzky, H. R. and Baer, 0. T., “High-Rate Digester Loadings,” Journal WPCFI
43, p 268 (1971).
47. Stanbridge, H. H., “The Consolidation and Digestion of Activated Sludge,” Jour.
Ins:, of Sewage Purqfica:ion (C :. Brit.), Part .5, p 492 (1966).
48. Brown, B. R., Wood, L. B., and Finch, H. J., “Experiments on the Dewatering of
Digested and Activated Sludge,” Jour. Inst. of Water Poll. Control (Gi. Br!:.). 71,
p 61(1972).
49. Lawton, G. W. and Norman, J. D., “Aerobic Digestion Studies,” Journal WPCF, 36,
p 495 (1964).
9’. \

50. Ritter, L. E., “Design and Operating Experiences Using Diffused Aeration for Sludge
Digestion,” Journal WPCF, 42, p 1783 (1970).
51. Smith, A. E., “Aerobic Digestion Gains Favor,” Water and Wastes Engineering, 8. p24
52. Full Scale conversion of Anaerobic Digesters to Heated Aerobic Digesters, EPA
Report No.. R2-72-050, Waste Treatment Plant, Hamilton, Ohio, NTIS Report No. PB-
211-448 (June 1971). - --
53. Aerobic Digestion of Organic Waste Sludge, EPA Report No. 17070-DAU- 12.71,
Oklahoma State University, Stiliwater, Oklahoma (December 1971).
54. Randall, C. W. and Kock, C. T., “Dewatering Characteristics of Aerobically Digested
Sludge,” Journal WPCF. 41, p R238 (1969).
55. Cameron, J. W., Aerobic Digestion of Waste Activated Sludge To Improve
Filterability, presented at 45th Annual Conf. of WPCF, Atlanta, Ga. (October 1972).
56. Coniposting Dewatered Sewage Sludge, Report on Contract No. Ph-86 -67-103, Bureau
of Solid Waste Management, Eimco Corp. (1969).
57. Thayer, S. E. and Sproul, 0. J., “Virus Inactivation in Water Softening Precipitation
Processes,” Jour. Amer. Water Works Assoc., 58, p 1063 (1966).
58. Farrell, J. B., Smith, J. E., Jr., Hathaway, S. W. and Dean, R. B., Lime Stabilization of
Chemical-Primary Sludges, presented at 45th Annual Conf. of WPCF, Atlanta, Ga.
(October 1972).
59. Goldberg, A. S., “A Procedure for Treatment and Disposal of Wastewater Sludge,”
Journal WPCF, 43, p 1912 (1971).
60. Green, J. E., “Sludge Oxidation,” American City, p 94 (October 1972).
61. Sparr, A. E. and Grippi, V., “Gravity Thickeners for Activated Sludge,” Journal
WPCF, 41 p 1886 (1969).
62. Voshel, D., “Sludge Handling at Grand Rapids, Michigan, Wastewater Treatment
Plant,” Journal WPCF, 38, p 1506 (196).
63. Torpey,W. N., “Concentration of Combined Primary and Activated Sludges in
\ Separate Thickening Tanks,” Proc. Amer. Soc. of Civil Engrs., Paper No.443 (May
\1954). ——— — -
64. Jordan, V. J., Jr. and Sherer, C. H., TM Gravity Thickening Techniques at a Water
Reclamation Plant,” Journal WPCF, 42. p 180 (1970).
65. Internal Report on Use of Flotation for Thickening Activated Sludge of Eimco Corp.
(March 1966).
66. Katz, W. J., “Sewage Sludge Thickening by Flotation,” Public Works, p 114 (December
67. Katz, W. J. and Geinopolos, A., “Sludge Thickening by Dissolved-Air Flotation,”
Journal WPCF. 39, p 946 (1967).
68. Ettelt, G. A., “Activated Sludge Thickening by Dissolved-Air Flotation,” Proc. 19th
Waste Conf Purdue Univ., p 210 (1964).
69. Vaughn, D. R. and Reitwiesner, G. A., “Disk-Nozzle Centrifuges for Sludge
Thickening,” Journal WPCF, 44, p 1789 (1972).

70. Stumpf, M. R. and Harper, W. H , Continuous Feed centr /i ge Replaces Flotationfor
Removal of Excess Activated Sludge From a Pharpnaceut lea! Was:ewater Treatment
Plant, Presented at Purdue University md. Waste Conf. (May 1972).
71. Kappe, S. E., “Digester Supernatant: Problems, Characteristics and Treatment,”
Sewage and md. Wastes Jour., 30, p 937 (1958).
72. Kelly, E. M., “Supernatant Liquor: A Separate Sludge Digestion Operating Problem,”
Sewage Works Jour.. 9, p 1038 (1937).
73. Howe, R. H. L., “What To Do With Supernatant,” Wastes Engineering, 30, p 12’
(January 1959).
74. Fischer, A. J., “Digester Overflow Liquor—Its Character and Effect on Plant
Operation,” Sewage Works Jour., 16, p 956 (1934).
75. Malina, J. F., Jr., and DiFilippo, J., “Treatment of Supernatants and Liquids
A sociated With Sludge Treatment,” Water and Sewage Wor’cs, 118, R/30, Ref. No.
76. Vacker, D.,Conn I1,C. H.,and Ve1ls, W. N., “Phosphate Remo alThiough Municipal
Wastewater Treatment at San Antonio, Texas,” Journal WPCF, 39, p 750 (1967).
77. Ultimate Disposal of Phosphate From Wastewater by Recovery as Fertilizer, FWPCA
Report No. 17070-ESJ-0I/70, by Dearborn Chemical Div. of W.R. Grace Co. (January
78. Borgerding, .1., “Phosphate Deposits in Digestion Systems,”Journal WPCF, 44, p 813
____ 1972) .
79. Garber, W. F. and O’Hara, G. T., “Operation and Maintenance Experience in
Screening Digested Sludge,” Journal JVPCF, 44, p 1518 (1972 )
80. Genter, A. L., “Elutriation—How It Aids in Dewatering Sludge,” Public Works. p II
81. Genter, A. L., “Conditioning and Vacuum Filtration of Sludge,” Sewage and md.
Wastes, 28, p 829 (1956).
82. Span, A. E., “Elutriation Experience at the Bay Park Sewage Treatment Plant,”
Sewage and md. Wastes, 26, p 1443 (1954).
83. Ahlberg, N. R. and Boyko, B. I . “Evaluation and Design of Aerobic Digesters,”
Journal WPCF, 44, p 634 (1972).
84. Dreier, D. E., “Aerobic Digestion,” Proc. of 18th Purdue md. Waste Con!., p 123
85. Hopkins, G.J.andJackson, R. L., “Polymersin the FiltrationofRawSludge,”Jou,.naI
WPCF, 43. p 689 (1971).
86. Personal Communication From Superntendent of Wastewater Treatment Plant,
Ogden, Utah.
87. Buz,eIl, J. C. and Sawyer, C. N., “Removal of Algal Nutrients From Raw Wastewater
With Lime,” Journal WPCF, 39, p R16 (1967).
88. Mulbarger, M. C., Grossman, E., Dean, R. B., and Grant, 0. L., “1.ime Clarification,
Recovery, Reuse, and Sludge Dewatering for WastewaterTreatment,”Journal WPCF,
4!, p 2070 (1969).

89. Ray, D. L., Nebiker, J. H., and Adrian, D. D., Sludge Dewatering by Centrifugatlon.
Report No. EVE-I 1-68-5, FWPCA (September 1968).
90. Deb, P. K., Rubin, A. J., Launder, A. W., and Mancy, K. H., “Removal of COD From
Wastewater by Fly Ash,” Proc. of 21st Purdue md. Waste Conf., p 848 (1966).
91. Tenney, M. W. and Cole, 1. G., “The Use of Fly Ash in Conditioning Biological Sludges
for Vacuum Filtration, Journal WPCF, 40 , p R281 (1968).
92. Smith, J. E., Jr., Hathaway, S. W., Farrell, J. B., and Dean, R. B., Sludge Conditloning
- With Incinerator Ash, presented at 27th Purdue md. Waste Conf . (May 1972).
93. Smith, J. E., Jr., Internal EPA-AWTL Memo (January 1973).
94. Gerlich, J. W. and Rockwell, M. D., Pressure Filtration of Wastewater Sludge With
Ash Filter Aid, Report No. EPA-R2-73-231, Office of Research and Monitonng (May
95. Inhoffer, W. R. and Doe, P. W., The Design of Washwa:er and Alum Sludge Disposal
Facilities for Passaic Valley Water Commission, presented at 92nd Annual AWWA
Meeting, Chicago, 111. (June 1972).
96. Weir, P., Atlanta’s Water Treatment Settled Solids Facility, presented at 92nd Annual
AWWA Meeting, Chicago, III. (June 1972).
97. Harrison, 3., Bungay, H. R., and Lord, A. M., “Heat Syneresis of Sewage Sludges,”
Part I, Water and Sewage Works, lii, No. 5, p 217 (1968).
98. Harrison, J., Bungay, H. R., and Lord, A. M., “Heat Syneresis of Sewage Sludges, Part
2,” Water and Sewage Works, uS, No. 6, p 268 (1968).
99. Brooks, R. B., “Heat Treatment of Sewage Sludges,” Jour. Inst. of Water Poll. Control
(Gt. Britj. 67. p 592 (1968).
100. Fischer, W. J. and Swanwick, 3. D., “High-Temperature Treatment of Sewage
Sludges,” Jour. Ins:, of Water Poll. Control (Gt. Bri:.), 70 p 355 (1971).
101. Brooks, R. B., “Heat Treatment of Sewage Sludges,” Jour. Inst. of Water Poll. Control,
(G:. Bri:j, 69, p 221 (l97O).f
102. Private Communication, Ames Crosta Mills (1966).
103. Dean, R. B., “Ultimate Disposal of Wastewater Concentrates to the Environment,”
Environmental Science and Technology. 2. p 1079 (December 1968).
104. Corrie, K. D., “Use of Activated Carbon in the Treatment of Heat-Treatment Plant
Liquor,” Jour. Inst. of Water Poll. Control (Gi. Brit.), 71. p 629 (1972). /
105. Hirst, G., Muihall, K. G., and Hemming, M. L., “The Sludge Heat Treatment and
Pressing Plant at Pudsey: Design and Initial Operating Experiences,” Jour. Inst. of
Water Poll. Control (Gi. Brit.), 71, p 455 (1972). .
(06. Swets, D. H., Pratt, L., and Metcalf, C. C., Combined !ndustrial-MuniØpal Thermal
Sludge Conditioning and Mulzple-Hear:h Incineration for Kalamazoo, Michigan.
presented at 45th Annual Conf. of WPCF, Atlanta, Ga. (October 1972).
107. Baskerville, R. C., “Freezing and Thawing as a Technique for Improving the
Dewaterability of Aqueous Suspensions,” Filtration and Separation (England), p I
(March/April 1971).

108. Doe, P. W., Benn, D., and Bays, L. R., “The Disposal of Washwater Sludge by
Freezing,” Jour. Inst. of Water Engr. (Gi.Brit.) , p 251 (December 1965).
109. Logsdon, G. S. and Edgerly, E., Jr., “Sludge Dewatering by Freezing,” Journal
A WWA, 63, p 734 (1971).
110. Farrell, J. B., Smith, J. E., Jr., Dean, R. B., Grossman, E., and Grant, D. L., “Natural
Freezing of Aluminum Hydroxide Sludges,” Journal A WWA, 62, p 787 (December
111. Evaluation of Conditioning and Dewatering Sewage Sludge by Freezing, EPA Report,
110 10-EVE-O 1/71, Sewerage Commission of City of Milwaukee, Wis. (January 1971).
112. Adrian, D. D., “Dewatering Sludge on Sand Beds,” Chemical Engineering Progress
Symposium Series, 129, AIChE, Water, 69, p 188 (1972).
113. Water Pollution Control Federation, “Sludge Dewatering,” WPCFManualof Practice
No. 20 (1969).
114. Jeffery, E. A. and Morgan, P. F., “Oxygen Demand of Digested Sludge Liquor,”
Sewage and md. Wastes, 31, p 20 (1959).
115. Sparham, V. R. and Wain, J. A., “The Development of the Wedge Wire Bed and
Techniques Associated With It,” Jour. Inst. of Water Poll. Con:rol(Gt. Br!:.), 66, p 476
116. Malina, 3. F., Jr., and DiFilippo, J., “Treatment of Supernatants and Liquids
Associated With Sludge Treatment,” Water and Sewage Works, 118, p R-30 (1971).
117. Leary, R. D., Ernest, L. A., Douglas, G. R., Mason, D. G., and Geinopolos, A., Top-
Feed Vacuum Filtration of Acnvaied Sludge—A Comparison With Conventional
Feed, presented at 45th Annual Conf of WPCF, Atlanta, Ga. (October 1972).
118. Hayakawa, N., Suzuki, T. and Asa ii o. T . Centr fiigal Dewarering of Municipal Sludge,
presented at 45th Annual Conf ol \VPCF, Atlanta, Ga. (October 1972).
119. Pilot Plant Studies on the Dewa i ’ ‘ ç of. Primary Digested Sludge, presented at 45th
Annual Conf. of WPCF, Atlanta (‘a (October 1972).
120. Vandiver, E. C., and Noble, J. A I he Use of Centrifuges in Sludge Dewatering,” The
Georgia Operator, 10, p 33 (Sum” r 1972).
121. White, W. F., Fjfteen Years of xperienci Dewarering Municipal Wastes With
Continuous Centrifuges, Presented at Annual AIChE Meeting. New York City
( November 1972) .
122. Keefer, C. E. and K i7H ”Experirnents on the Dewatering of Sewage Sludge With a
Centrifuge,” Sewage Works-Journal, 1. p 120 (1969).
:‘ t.1
123. Communication fronx: Dr. Robert B. Dean, Chief, Ultimate Disposal Research
Program, AWTR Laboratory, EPA, Cincinnati (November 1972).
124. Walters, W. R. and Ett 1t, b., “Dewatering the Ash By-Product From the Wet
Oxidation Process,” Proc. oft2O:h Purd’Ae Univ. Conf. on md. Wastes, p 551 (May
1965). ‘.1k. ‘
125. Thomas, M., “The Use of Filter Presses for DewateringofSludges,”Journal WPCF,
43, p 93 (1971).

126. Forster, H. W., Sludge Dewatering by Pressure Filtration 1 presented at Annual Meeting
of AIChE, New York City (November 1972).
227. Baskervile, R. C., Komarek, J. A., and Gale, R. S., “Effect of Operating Variables on
Filter Press Performance,” Jour. Inst. of Water Poll. Control (Gt. Br! :.), p400(1971).
228. Holding, J. C., “Experimental Work on Digested Sludge Pressing,”Jour. Ins:, of Waler
Poll. Control (Gi. Br!:.), 67, p 528 (1968).
129. Zeper, J. and Pepping, R., “Handling of Aerobic Mineralired Sludges by Centrifuges
and Belt-Press Filters,” Water Reasearch, Jour, of Intl. Assoc. Water Poll. Research, 6.
p 507 (April/May 1972).
130. Imhoff, K. R., Sludge Dewatering Tests With a Belt Press (see Ref. 129), p 515.
131. Bulletin of Carter Co. on The Automatic Belt-Filter Press (April 1972).
132. Goodman, B. L. and Higgins, R. B., Conc n:ra:ionofShidgesby Gravity and Pressure,
presented at Annual Conf. of WPCF, Boston, Mass. (October 1970).
133. DaVia, P. G., Capillary Action Applied :ä Sludge Diwatèing, presented at Intl. Water
Conf., Pittsbujgh, Pa. (October 1972).
134. The DCG Solid, Concentrator, Permutit Co. Bulletin No. 5161 (1972).
135. Hedenland, L. D., “District Goes Into Reclamation Business,” Water and Wastes
Engineering, 10, p 30 (March 1973).
136. SymposIum on Storm/San itary Pollution Control, sponsored by city of Poriland, Ore.1
and Sweco, Inc., under FWQA Contract No. 14—12-128 (June 1970).
137. Communication and Bulletin on Centrifugal Wastewater Concentrator From Sweco,
Inc. (October 1972).
238. Hydroscreen for Solids Dewaterlng. Bulletin HS-lO0, Hydrocyclonics Corp. (October
2972). -
239. Reeve, D. A. 1). ii nd Harkncss, N., “Sonic Aspcvt of Sludge Incineration,” Jour. Inst.
of Water Poll. Control (Gt. Br!:.), 71, p 628 (1972).
140. State of the Ar* Review on Sludge Incineration PracricejWQA Report No. 27070-
DIV-04/70 Apri1 1970).
141. Background Information for Proposed New Source Performance Standards, EPA.
Office of Air Programs, APTD-1352a (June 1973).
142. Sewage Sludge Incineration, EPA Task Force, PB-2 1 1-323 (March 1972).
243. Tench, H. B., Phillips, L. F., and Swanwick, K. H., “The Sheffield Sludge Incineration
Plant,” Jour. Inst. of Water Poll. Control 1 G. Bril.), 71, p 176 (1972).
144. Report of San. Engr. Research Committee, “Sludge Treatment and Disposal by the
Zimmerman Process,” Jour. San. Engr. Div., ASCE, 85, p 13 (2959).
145. Erickson, A. H. and Knopp, P. V., “Biological Treatment of Thermally Conditioned
Sludge Liquors,” Proc. of 5th Intl. Water Poll. Research Conf. San Francisco, p Il-
33/1 (July 1970). .
146. Hinesly, T. D. and Sosewitz, B., “Digested Sludge Disposal on Crop Land,” Journal
WPCF. 41, p 822 (1969).

l47. Egeland, D. R., “Land Disposal: A Giant Step Backward ,“ Journal WPCF, 45, p 1463
(July 1973).
148. j Thomas, R. E., “Land Disposal: An Overview of Treatment Methods,” Journal WPCF,
p 1476 (July 1973).
149.1 Spotswood, A. and Raymer, M., “Some Aspects of Sludge Disposal on Agricultural
Land,” Jour. Ins:, of Water Poll. Control (G:. Brit.), 72, p 71(1973).
150.’ “Study of Municipal Sludge for Soil Improvement,” Current studies of USDA
Research Center, Clean Air and Water News, 4, p 427 (1972).
151. Troemper, A. P., Disposal of Liquid Digested Sludge by Crop Land frrigat ion,
unpublished paper of Springfield, Ill., Sanitary District (1972).
152. “The British System of Tanker Sludge Disposal,” Compost Science, 12, p 28 (Novem-!
ben December 1972).
153. The West Hertfordshire Main Drainage Authority, General Manager’s Report (1965—
154. von Triebel, W. and Peil, G., “Die Schlammbehandlung aufdeniGruppenklarwerk I
des Niersverbandes,” Das Gas- und Wasserfach, 495, (1967).
155. “Chicago Reclaiming Strip Mines With Sewage Sludge,” Civil Engineering—A SCE,
98 (September 1972).
156. Dotson, 0. K., Dean, R. B., and Stern, 0., The Cost of Dewazering Sludge on the
Land, presented at 65th Annual Meeting of AIChE, New York City (November
157. Adams, A. P. and Spendlove, J. C., “Coliform Aerosols Emitted by Sewage Treatment
Plants,” Science, 169, p 1218 (1970).
158. Thibodeaux, L. J. and Carter, N. J., Col form Emissions From Air-Water Contactors:
A Preliminary Attempt To Establish Maximum Concentrations, presented at 65th
Annual Meeting of AIChE, New York City (November 1972).
159. Meyer. R. C., Hinds, F. C., lsaacson. H. R.,and Hincsly,T. D.,”Porcine Enterovirus
Survival and Anaerobic Sludge Digestion,” Proc. of IntL Symposium on Livestock
Wastes, p 183 (1971).
160. Molina, J. A. E., Braids, 0. C, and Ilinesly, T. D., “Observations on Bactericidal
Properties of Digested Sewage Sludge,” Env. Science and Technology, 6, p 448
161. Manual of Practice No. 2, Utilization of Sewage Sludge as Fertilizer, Federation of
Sewage Works Associations (1946)
162. Hinesly, T. D., Braids, 0. C., and Molina, J. A. E., Agricultural Benefits and
Environmental Changes Resulting From the Use of Digested Sewage Sludge on Field
Crops, Report SW-30d of EPA, Univ. of III. (1971).
163. Molina, J. A. E., Braids, 0. C., Hinesly, T. D., and Cropper, J. B., “Aeraticn In- I
duced Changes in Liquid Digested Sewage Sludge,” Proc. Soil Science Soc. of
America, 35, p 60 (1971).
164. Ewing, B. B. and Dick, R. I., “Disposal of Sludge on Land,” Water Quality
Improvement by Physical and Chemical Processes. Univ. of Texas Press (1970).

165. Lyman, B. T., Sosewitj B. and Hinesly, T. D., “Liquid Fertilizer to Reclaim Land and
Produce Crops,” Water Research, 6, p 545 (1972).
166. Cameron, R. D., “Prediction of Settlements in Landfills Constructed From Centri-
fuged DigestecL Sewage Sludge,” Water Poll. Abstracts (G:. Brit.), 44, No. 1646
(August 1971).
167. Guidelines for Local Governments on Solid Waste Management, Public Health Ser-
v;ce Publication No. 2084, GPO (1971).
168. Piper, A. M., “Disposal of Liquid Wastes by Injection Underground, Chem. Progress 1
Synip., 65, No. 97, AIChE, p 5 (1969).
169. Hoffthan, D. A., Pyrolysis of Solid Municipal Wastes, Report on work done by San
Diego, Calif. (July 1967).
170. Conversion of Municipal and Industrial Refuse Into Useful Materials by Pyrolysis,
U.S. Bureau of Mines Report No. 7428 (August 1970).
171. Beekmans, J. M., and Ng, P. C., “Pyrolized Sewage Sludge: Its Production and
Possible Utility,” Env. Science and Technology, 5, p 69 (1971).
172. Partial Oxidation of Solid Organic Wastes, Report on research grant to Rensselaer
Polytechnic Inst., Bureau of Solid Waste Matagement, Public Health Service PubI.’
No. 2133 (1970).
173. Graham, D. H., Evans, P. R., and Riggleman, B. M., The Technology of Solid Waste.
Reclamation, presented at 70th National Meeting of AIChE, Atlantic City, NJ.
(August! September 1971).
174. Mercer, W. A., “Aerobic Composting of Vegetable and Fruit Wastes,” Compost
Science, (Autumn 1962).
175. Wiley, J. S., “A Report on Three Manure Composting Plants,” Compost Science,
5, p 15 (Summer 1965). .
176. Wiley, J. S., “Discussion of Composting of Refuse and Sewage Sludge,” C’ompost
Science, 8, p 22 (1967).
177. Straub, H., “The Sewage Treatment and Compost Plants of City of Baden-Baden,
Germany,” Das Gas- und Was.serfach. Munich, 92 (1956).
178. Composting Dewatered Sewage Sludge. Report No. 3W-12c on Contract No. Ph-
86-67-103 with BSWM of Dept. of HEW by Eimco Corp. (1969).
179, Fujita, H., “Tokyo’s Asaka Purification Plant,” Water and Sewage Works, 114,
p 73 (1967).
180. Webster, J. A., “Operational and Experimental Experience at Daer (Scotland)
Water Works, With Special Reference to Use of Activated Silica and Recovery of
Alum From Sludge,” Jour. Inst. of Water Engrs., (Gt. Brit.), 20, p 167 (May 1966).
181. Chojnacki, A., “The Treatment and Use of Alum Sludge,” Proc. of Intl. Water Sup-
ply Congress, Barcelona, Spain, p QIl (October 1966).
182. Advanced Was:ewater Treatment as Practiced at South Tahoe, EPA Report No.
170 lO-ELQ-08/7! by South Tahoe Public Utility District (August 1971).
183. Bouthilet, R. J., and Dean, R. B., “Hydrolysis of Activated Sludge,” Proc. 5th
Intl. Water Poll; Research Conf., San Francisco, p 1 11-3 1/ I (July 1970).

184. Feasibility of Hydrolysia of Sludge Using Low Pressure Steam With SO as a
Hydrolytic Adjunct and Utilization of i/se Resulting Hvdrolysa:e, by Foster D. Snell
Inc., WPCA Report P B-194-784 (December 1969).
185. Kalin ke, A. A., Discussion of paper on “Applications of Microstrainer to Water
Treatment in Great Britain,” Journal A WWA, p 734 (July 1953).
186. Lyman, B. T., Ettelt, G., and McALoon, T., “Tertiaiy Treatment of Metro Chicago
by Means of Sand Filtration and Microstrainers,” Journal WPCF. 41, p 247 (1969).
187. Oakley, H. R., and Cripps, T., “British Practice in the Tertiary Treatment of Waste-
water,” Journal WPCF. 41, p 36 (1969).
188. Hamann, C. L., and McKinney, R. E., “Upflow Filtration Process,” Journal A WWA,
60, p 1023 (1968). -
189. Michaelson, A. P., “Under the Solids Limit at Ashton-Under-Lyne, ” Jour. Inst. of
Water Poll. Control (G:. BrIt.), 70, 5, p 533 (1970).
190. Weber, W. J., Jr., Hopkins, C. B., and Bloom, R., Jr., “Physicochemical Treatment
of Wastewater,” Journal WPCF, 42, p 83 (1970).
191. Disposal of Brines Produced In Renovation of Municipal Wastewarer, by Burns and
Roe, Inc., FWQA Report No. 17070-DLY-O5/70 (May 1970).
192. Mulbarger, M. C., Sludges and Brines: Handling, Conditioning, Treatment, and Di ,-,
posal, Report of Ultimate Disposal Research Div., AWTR Laboratory, Cincinnati!
193. Ammonia Removal From Agricultural Runoff and Secondary Effluents by Selected
Ion Exchange, Report by Pacific Northwest Battelle Labs. for AWTR Laboratory,
Cincinnati, Report No. TWRC-S (March 1969).
194. Parkhurst, J. D., Chen, C., Carry, C. W., and Marse, A. N., “Demineralization of’
Wastewater by Ion-Exchange,” Proc. 5th Intl. Water Poll. Research Conf., p 1-20/I
(July 1970).
195. Kunin, R. and Downing, D. 0., “New Ion Exchange Systems for Treating Municipal.
Domestic, and Industrial Waste Effluents,’ Proc. Intl. Water Conf., Pittsburgh, Pa.
196. Helfgott, T. and Hunter, J. V., “The Removal of Phosphates From Wastewaters by
Electrodialysis,” Chem. Engr. Progress Symposiwn Series, AIChE, 65, p 218(1969).
197. Electrodialysls in Advanced Waste Treatment, FWPCA Publication WP-20-AWTR-
18, AWTR Laboratory, Cincinnati (February 1967).
198. Mintz, M. S., “Electrodialysis Principles of Process Design,” md. Eng. Chem., Si,
p 18 (1963).
199. Application of Hyperfihtratlon to Treatment of Municipal Sewage Effluents, FWQA
Report No. 17030-EOH-0I/70 by Oak Ridge National Lab. for AWTR Lab., Cincin-
nati (January 1970).
200. Renovation of Municipal Wasiewaler by Reverse Osmosis, EPA Report No. 17040-
05/70 by AWTR Lab. of EPA in Cincinnati (May 1970).
201. Reverse Osmosis Renovation of Primary Sewage, EPA Report No. I 7040.EFQ . .02/7 I
by Envirogenics Co. (February 1971).

202. An enabihsy of Reverse Osmosis Concentrate to Activated Sludge Treatment, EPA
Report No. l7040-EUE-07/71 by Rex Chainbelt Co. (July 1971).
203. Cost of Purifying Munkipal Wasiewater by D&uilla:ion, Report No. AWTR.6 of Pub-
he Health Service, Dept. of HEW (November 1963).
204. Advanced Worse Treatment by Distillallon, Report No. AWTR-7, Public Health See.
vice, Dept. of HEW (March 1964).

This appendix describes pro&d ir which may be useful in the design of systems for
processing sludges and liquid sidestreams in a wastewater treatment plant. Particular emphasis
. is given to recycling sidestreams within the treatment plant itself. .
To present all the f ccts of process performance and interaction needed for compulation ol
required process capacity, it is helpful to use a quantitative flow diagram (QFI)). Individual
processes are represented by labeled circles or recta ngles and flow directions are represented h
A simple example will serve to show the utility of thc QFI) for design purposes. I-or simplicity.
consider a three-process plant consisting of (I) primary clarification. (2) anaerobic digestion
of the sludge, and (3) transport and disposal of the digested sludge to agricultural land.
Digester supérnatant is recycled to the primary clarifiers. It is assumed (though experience
indicaics it unlikely) that the same perccntdge of solids will he removed from the supernatant
as from the raw waste. The average plant how is 10 mgd. with an influent suspended solids
concentration of 250 mg/I. 01 the suspended solids flowing into the clarifier, 6() percent is
removed in the sludge. whose solids concentration is 6 percent or 60.000 mgi I. 1 he sludge
stream goes to the digester. The digester supernatant volume flow is equal to I0.00() gpd
(gallons per day). I he supernatant’s suspended solids concentration is assumed to be 5.0 ( X)
mg I. It is estimated that 25 percent oh the solids entering the digester k ctin eried to gas and
escapes from the system or is soluhili,ed. It is assumed that volume balance is maintained.
which is approximately true, since the loss or g tin of liquid volume h esaiporalion or in
biological metabolism is minor.
I.. . - .. .. . 4C’ • - .. ‘ . . ... .. —.. I
Therefore, one of the baste relations for any unit or process is the volume balance relation. The
other relations are those for mass balance of the pollutants. The volumetric flow of digested
sludge to land disposal, for example, is calculated as follows: First, the blank QFD (Fig. A-la),
:with the primary clarifier and anaerobic digester, and the six associated flows—influent 1
effluent, raw sludge, recycled supernatant, removed solids, and digested sludge—is conj
The heavy lines arQun&digested sludge discharge emphasize that ji is the quatüity tq

• 250
20, 500
I b OOO 1• ’,
Lf 5
5000 Im I

be determined. AU the inLormation given above would then be filled in (Fig. Al-Ib): influent
flow and suspended so1id concentration, supernatant flow and suspended solids concentra-
tion, and raw sludge con entration.
The suspended solids loads in pounds per day for influent and supernatant would then be
computed from the values for discharge and concentration. Next, all suspended solids loads
entering the clarifier would be added: 20.800 pounds per day + 415 pounds per day = 21.215
pounds per day with 60 percent, or 12.730 pounds per day. going to the sludge stream. l ’ rom
this value and the given sludge concentration, the derived volume of the sludge stream is0.03 1
mgd. The volumetric discharge of digested sludge is therefore equal to raw sludge discharge
minus supernatan discharge, or 0.021 mgd.
The appropriate spaces on the QFD should be completed as computations are made(Eig. A-
Ic). As soon as the bold-line box is filled in. the problem is solved, although some of the other
spaces are still blank.
This simple example serves to empha i7e that:
All aspects of plant operation should be set down together on
one diagram.
Although pollutant loads are usuaffy ssecfmn t rT ofi
concentration (mg/I), the relative importance of the various
treams is shown most clearly by comparison of mass flow
values (lb/day). -
It should be noted that:
Various quantities can he computed by a volume-balance and
mass-balance approach: the mass (volume) flow into a process
must cqual the mass (volume) flow out. The ‘ mass in” includes
the mass introduced by all inflows, whether the main plant
stream or a recycled sidestrc.im, and includes mass introduced
by chemical precipitation or biological synthesis. Mass out”
includes mass removed by the main effluent, sludge, liquid
sidestream. off-gassing, or biological synthesis. I3ecausc the
solids that are volatile ar’d converted to a gas cannot be
determined, mass balances around certain processes can only
be estimated.
Computations are not always straightforward. hut may
rçquire a trial and error approach. particularly where rec)cle
streams are involved.
Neither influent volume and composition nor plant perliir-
mance is firmly predictable. It must be remembered that each
of the values assumed in this or any other example is merely
one value from a possible range.
To start the computational procedure, numerous estimates
about plant performance must be made. Application of’ the
103 \

mass-balance approach may show that two or more assump-
tions. although each is typical of plant operation. may not be
mutually compatible. much as a problem in structural analysis
may be ‘overdeterriiined.”
In the short exercise provided, several spaces have been leli
blank. An empty box on a QEL) signifies that: (I) the
parameter represented has been given due consideration, and
(2) ii has proved to be unimportant for the problem worked.
To denote a required process capacity with a bold-line box.
though hardly a necessary procedure in this simple example.
will be useful in more complex examples where values of tO o.
more required capacities are sought.
The QED presented here as a method to assist computations of
process capacity is closely related to the computation scheme
proposed by Smith (I).
Material balances of systems involving sludge and liquid sidestreams will he further illustrated
by four examples which. to a large degree. represent systems presently (1973) being used in I till-
scale plant designs (Figs. A-2 through A-5). The hypothetical effluent quality parameters.
although arbitrarily chosen, do represent some currently specified requirements such as
removal of large proportions of phosphorus and ammonia. Rcmo al of (01) is considered in
Example 4.
MI examples utili,e primary clarification and a two-stage acti atcd sludge process. The first
stage is a high-rate activated sludge unit (HRAS). with provision for alum to be added to
remove phosphorus. Sludge from the H RAS is removed in an intermediate clarifier. The
second stage is a nitrifying activated sludge unit (NAS) for oxidizing ammonia to nitrate.
Solids from NAS arc removed in a final clarifier. Sludge from all three clarificrs is gravity
thickened and ultimately dewatered by a vacuum filter and hauled to landfill. The filtrate is
returned to the HRAS unit.
The first three examples differ only in the handling of sludge between the thickener and the
vacuum filter. In the first example. the thickened sludge is stored for periods of up i I day to
permit intermittent operation of the vacuum filter. Ferric chloride and lime are added as
conditioners before filtration. In the second example. the thickened sludge is anaerobically
digested for 30 days. conditioned with lerric chloride and lime, and vacuum filtered. l)igestcr
supernatant is returned to the H RAS unit. In the third example. the thickened sludge is heat
conditioned. then dewatered on a vacuum filter without the addition of chemical conditioners.
Included in the heat conditioning system is backup chemical feed system (lime and ferric
chloride) so sludge can be conditioned and stabiliied at pH above II for landfill disposal when
the heat conditioner unit is down for repairs or maintenance. l)ecantatc from the heat
conditioner liquid-solids separation unit is returned to the H RAS unit. All vacuum litter
systcms have more than one unit so the plant can operate if one unit is down for repairs.

I ’1
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The first three examples are of plants that provide good removal of suspended solids, BOD,
phosphorus, and ammonia; the fourth example is of a pLant designed to provide a high degree
of COD removal as well. The effluent from a plant identical to that of Example 3 (with heat
conditioning of.the sludge) is put through a dual-media filter for suspended sdlids removal,
then through an-activated carbon adsorption unit. Wash water from the filter is returned to the
final clarifier.
For all the examples. the 24-hour average influent conditions are: 10 mgd of flow. 250 mg/I of
5-day BOD, 250 mg/I of suspended solids. 10 mg/I of soluble phosphorus (P). and 20mg/I ol
ammonia (NH ). It is required that effluent concentrations not exceed: 20mg/I BOD. 20 mg,I
suspended solids. 1.0 mg/I P. and 1.0 mg/I N H 3 . For Example 4. it is required that effluent
COD not exceed 30 mg/I. Steady state conditions are assumed. These influent and effluent
values are summarized in Table A-I.
Also shown in Table A-I are the given performance characteristics assumed for each process.
such as detention time, overflow rate, percentage removal of suspended solids, and soon. The
capacity parameters that ‘have to be evaluated to determine process costs are listed in the
second column of Table A-2. Quantitative flow diagrams for the four examples are presented
in Figs. A-2 through A-S. -
1iie,mputAtion procedure uiing the QFD’s is that describe reviously ith two additional’
assumptions: (I) one may assume that the concentrations of entirely soluble components, P
and NH 3 , are not changed by gravity settling equipment; i.e., the concentrations in underflow
and overflow from a clarifier or sludge thickener are the same, and (2) the mass per day of a!
constituent “removed” from a process is equal to the mass per day of that constituent flowing
into the process minus the mass per day flowing out of the process in the principal stream,
usually a clarifier overflow. The mass is “removed” generally by withdrawalin a sludge stream
or liquid sidestream and by chemical or biological conversion to other matter, as in oxidation
or digestion.
The QFD’s are intended to help estimate the required capacities of the component processes,
taking into account the layout and performance of the plant as a whole. Values of the capacity;
parameters are transferred from the bold-line boxes of the completed QFD’s to Table A-2.
.• —-a—- - : - —- --
EPA regula tidhs 4O CFR Part 35) require that a cost effectiveness analysis be made for
aIternati’Je witeireatment management techniques before a construction grant is awarded.
Systems for sludge processing and liquid sidestream treatment are subject to a cost
effectiveness analysis. The quantitative flow diagram can be a very useful tool in such an
analysis, because it provides a - ‘ru thod of determining the impact of a particular sludge
treatment alternative on tbtal tment system costs. -
— p ipe , - - . - — . —— ..—
Recycle from aprocess mjsigrnjrçanily affect the loading on that process. In Figs. A-2, A-3,
and A-4, for the three eAample that differed only in method of sludge handling after
tl ickening and before .dewatering, hotice the wide range in hydraulic flow and mass suspended
i 9 \

Volume of sludge re-
turned from final clari-
fier = 50% of influent:
retention time = tO hr.
based on influcnt
final clarifier Overflow rate: 600 gpd’
Waste activated
sludge: I’r solids:
example No.4: wash _
water sludge: 6’% sol-
95 4 SS removal
0.75 lb/day BOD
removed per lb
MISS: 0.8 lb th
required per lb
ROD removed
300 mg/I. all but
20% of which can
be removed by ox-
idation and sedi-
30mg/I 1.0 mg/I
Concentrations in
overflow and sludge
stream are e aL
1.0 lb P removed
by biological syn-
thesis per 100 lb
BOD removed:
other P: 21.6 lb
alum lb P. 8 lb
sludge lb P
Concentrations in
overflow and
sludge stream are
iqtua I
Influent 10 mgd (24-hr average ) 250 mg/I
(Cons,dered in
Example 4
Process (Hydraulics)
Solids HOD Only) Phosphorus Ammonia
Primary clarifier 10 mgd
250 mg/I
20 mg/I
30% removal
High rate activated
sludge basin
Iniernediate clan-
Nitrifying activated
sludge basin
Volume of sludge re-
turned from intermedi.
ate clarifier = 50nf or in-
Overflow raIe 700 gpd/
sq ft
20 mg/I
60% removal to thick-
ener; sludge concen-
tration = 6% solids
MISS = 2.500 mg/I:
08 lb SS produced!
lb HO!) iemosed
Overflow 35 mg/I:
sludge 1% solids
MISS = 1.500 mg/I:
0.2 lb SS produced
SOD removed
20 mg/I
1.0 mg/I
Concentrations in
overflow and
sludge stream are
Concentrations in
overflow and
sludge stream arc
Overflow. 35 mg/I
Dual-media filter
(example No.4
Concentrations in
overflow and sludge
stream are equal
(1.0mg I)
Concentrations in
overflow and sludge
stream are equal
S gpm sq it; wash water
= 2°b of tiltrate: rccy-
ckd to final clarifier
20 mg/I remo%ed

Sliwage tank (cx-
ample No. I only)
Anaerobic digester
(example 2 only)
( 1 miles awa )
0.073 mgd to HRAS
Per tOO lb sludge SS:
add 3 lb FeCh (ex-
amples Nos. 1, 2)
8 lb CaO (ex-
ample No. 1)
tO lb CaO (ex-
ample No. 2)
Sludge: 4% solids.
99% solids capture
Sludge out: 4% sot-
lids; 30% of SS volati-
3.000 mg/I
30% of SS to heat
conditioner rendered
soluble; 3.000 mg/I
sludge at 6% solids
Example No. I: SS
= 25.8% of incoming
sludge SS:
Example No. 2: SS
= 18.1% of incoming
sludge SS:
Examples No.. k4i
Cake: 30% solids
Examples Nos. 1,2 :
500 mg/I
Examples Nos.
1.000 mg/I
Overflow: DOD
concentration ap-
proximately equal
to SS concentra-
Example No.
500 mg/I:
Example No. 2:
1.000 mg I:
Examples No .. 3.
4: 10000 mg/I
Concentrations in
overflow and out-
flowing sludge
stream are equal
Concentrations in
overflow and out-
flowing sludge
stream arc equal
Activated carbon
qtpflow coL ( cx-
ample No.4 only)
‘ Gra vity thk ”
Process Hydraulics
(Considered in
Suspended Example 4 —
DOD Only) Phbijiháus —
Detention time 30 days
3.000 mgil
0.000 mg/I
Heat conditioner
— decantate (exam-
ples No.. 3 and 4 only)
FCCI) and lime
Vacuum filter
10.000 mg/I (10%
25.000 mgI I (30%
I: 25.000 mg/I
To HRAS basin
I SO mg/I
ISO mg/I
,ExamplcsNos. 1,2:
Examples Nos. 3, 4:
ISO mg /I
500 mg/I
Examples No. I not
Examples Nos. 2, 3.
4: 500 mg/I
Ili)!Ii r.ik ,eii%. ,i,?II I sd i

____ Capacity Pi n_s b,
_________ Panmsbi(s) E’ p No. Va
Primary c rfisr Ovetfiow. mpl I $0.0
2 10.0
3 $0.0
4 0.0
Nigh rats activabd Poondslday 80$) is - I 12.313
ciud s (HRAS) mo d 2 14Th
3 2 1.015
4 2 1.013
Aoratio. sqwpais.a for PoordaIday DOD is- I 2.303
HRAS hula moied 2 14.775
3 2 1.115
4 2 1 .6 13
Mini Puada/day 4 dnd I 13401
2 14.131
3 13.70 $
4 15.70 $
Iasumediats chillier Overflow. mgd I $0.0
and sludge rots puu a 2 $0.0
3 $0.0
4 $0.0
Nitnifyliug •csh’sted hilkew. mgd I 10 1$
s lssdyhisks$NA%l 3 10$
3 10.0
4 10.0
80$) NH 1
Aeration equipment for Pooudslday DOD no- I .240 3M
NAS basin movsd 2 .241 2.017
Pasadaday Nil 1 is- 3 1.24$ 2.013
moved 4 1.24$ 2.013
‘1 12

Final clarifier and ,IuJ i’ Sff low, mg&
retuTa pumps -
pual-ined.a lilt.,
Activated carbon upflow
Gravity thickener
Sludge handling before
Aerated storage
Anaerobic digestion
Heat conditioning
Heat conditioning
FeCti nd lime (CaO)
Vacuum flithi
F ow, d
l L O ui&a&COD
remove ’ po nds/ day
-. ‘Poiiindsiday.. SS in
Inflow. mgd
Pounds/day SS in inflow
Iiflo mnd):
,• Poundi/day cak 1
Ca cil
“ ‘ Parameter

l. . .4O.2
¶4 8,762
2 38 ,83)
3 47,392
4 48,976
I .
•0117 t”
,•1 ’ ‘
2 ‘

:• 0.061
I. - ”
2 ‘ 97,500. •:
3 . . / ajl

• solids floluo the storage tank, anaerobic digester, or heat conditioner. One expects to find
differences in outflow from different processes, but one all too oftc. neglects she possible
differcnces in bifiow that result directly or indirectly from the sidcstseams recycled from these
proccues” differences in suspended solids Loadings result both from recycled suspended
solids and from recycled DOD and P. whose removal in the activated sludge basin generates I
additional suspended folids
ilecycle from a procets may slgnlflca,itIy alJics Ike IOtding on olkerprocesm. In the examples
çniidered, the primary chntier receives no recycled sidestreams, and thus its capacity aad
costar. the same for all examples. The volume of recycled sidestreams is usually only about I
to3 percent of the plant inflow, so pvocrim whose capacity parameter (forcosting ptwpsses)ss
volumetric flow rate, such as the intermediate and final clariliers and the NAS basin in tbás
ales, are not sgniflcantly affected.
However, mase IeExawp
consider the 3,130 lb/day áIBODr.cj dcd to the RRAS bsn, whiCh is equal to more tha.20
percent C the .a! g e basin ! stream from the prim
clarifier. En Example 3 consid r t ie ,OO0 lb/day of sufpende ’ solids and the 11040 lb/day of
ROD réyclcd to the H i 11 . 1 1i ase4aal to 4 7S pe cent, respectively, Il
the suspended solids and-ROD loidscitesiflg front th pn naiycladfis,. Mibswain Table A
2, the required capacities of tl HRAS basin and ituãadoii eqi3ipiri t, t liealvmdomge,aUd
sius of the pavity th _ i .E affected by pocus.
• râjj igMeJto içgv the pk,,s can qffecs.osherprocwss baàgs . In Example 4, Where
from a plant Identical to that of Example 3.. filtered and passed through a carbon
colurnø Ot removal, there is an additional sidestream: filter waChwaterc.-taviing 1 $14
lb/ solids, recycled to the final clarifier. The volumetric and mass loadiap of
r to be insignificant, yct it is still advisable to consider theirsffect through
lnakc sure. Indeed, the addition of washwatcr solids leads 1* differences in
ttyxequiicd forth. gravity thickener, the heat conditioner, the vaeuumIllier, andibs
1In4rationotoniy a littlemotethan 5 perq nt, not significant consideringth. lowoede,
‘pf aocuriq in the cost estimation Considered. Ik er, note that the mas. load of suspended
i,lids from the fital clarifier to the thickener has more than doubled. There is only a slight
increase in volumetric flow because the wuhwater solids may be assumed’ to gtta. easily to
about Sorö percent concentration, In contrast to the relatively thiowasteacthutsd sludge. The
QFD therefore helps assure oneu to which factors arc insignifieasi$,as well as loakit one as to
which factors arc important.
Without a complete dIa ram andizanalyaLs. ii i : not possible Io1thefeqwbe q,s*vnf
process eqiipment when recrc lint of pollutants bek,g removed hr nwlw .L Thi, is preiliahly
one of the most important design outputs that an analysis such as that desesibed hevci ieill
provide which cannot properly be obtain d by any “estimating” on ipplicatin. .1 arhitrary
“%afcly factors.” A simple example will illustrate this. In determining the solid, hadhlng
capacity of. for example, the heat conditioning equipMent in Process No.3. one could estinsate
it thus: the total solids âenerated in treating the Ill mgd of wastewatcr flow averages about
34.7(X) Ibi day, as shown in Process No. I. The recycled solids in the decant liquor and intihrate
arc estimated to be about MOO lb/day. flicrefisre, the capacity of the heat enudlionuip
equipment should he the sum or about 3L30U lb day. Actually, is can he nosed ir s . . she
analysis that the total solids to be handled will he about 47.000 lb day an “essnating”evnw
of 23 percent. The crude and incomplete analp.s (ailed to sake into account the solids
produced by she other pollutants. such as HOD. ammonis. and phusplwsrus in she rce le.J
114 ..

stream. A complete flow diagram, with all solid and liquid streams identified, precludes serious /
errors in plant and process equipment sizing.
It should be enj, ize6 itiai ihe examples shown with various assumed performai c and
operational parameters for the different unit processes utilize only one set of values. Because
these parameters may have a wide range of values , .any QFD should be examined for variations
of the performanceparameters. The influer(cóof a specific value that characterizes a unit
process performance can be readily c t rinu ed once a QFD has beei draw i up.
I. Smith. Robert. Preliminary Design of Wastewater Systems.” Jaurna! of the Sw,iswi
lflvicion, 4SCE 95. No. SM. p 117 (February 1969).
115 I

Section Page No. 1
1.1 Purpose and Scope .\ I
1.2 Goats of Study 2
2.1 Brines 3
2.2 Centratcs ............ . 3
2.3 Conditioning of SIudges ..... . 3
2.4 I)ewatering. . . .............. 3
2.5 Filter Backwash Waters 4
2.6 Filtrates 4
2.7 Final Disposal ... .. 4
2.8 Liquid Skiiinnings .. ..... ..... 4
2.9 M isceflaneous V/ash ‘aters . 5
2.0 Sand Bed Drainage . . S
2.1 I Supcrnatant . .......... . 5
2.12 ihickener Overflows ..... . . ..... S
2.13 Thickening ..... ... 5
2.14 Wash Waters From Screens and Strainers 6
4.1 General Discussion 9
4.2 Primary Ireatmnent Plants 10
4.3 Secondary Plants . I I
4.3.1 TrIckling Filters and Other
Fixed Growth Systenis 12
‘4.3.2 Activated Sludge Plants
Using Air. ... ;. 13
Preceding page blank

SUB. ECTINDEX(Coetinosd) I
-. ‘: .3.3 Activated Sludge Plants
Using Pure Oxygen. ..... $4
4.4 Chemical Addition to Primary Pbnts .. .. . . .............a............ ..a... IS
4.5 Chemical Addition to ! : oiidary Plaets........................._..__—_—.... 1
44 Sludges From New Trcatmeat Processes ......_.... ........................ 17
4.7 Septic Tank Sludge (Septage) ... ..... .....• ... - . _ ..................... .... . S I
$ SLUDGE S—rAB IL IZATION . .............. .•.•...••.••.••..••, — 25
S. I llolocal Methods .......fl.fl. •sssselesss•,a. .s• .••ns ssIs . _ — _ .............. . 2$
5.1.1 Anaerobic D stmon .... _ U-- — . . II
5.1.2 AerobIc . — ........a.. __ r
5.1.3 Coinpostrag . —. __ — —..... _ 33
5.2 Chemical Mesbods.......................................-—. _ — . __ .. - 33
5.3 Physical Mcthods............ ............_.. ._......... ._........ ......... 27
6 SLUDGE THICKENING ._.............................. -- __ . . -.... . _ . .. .. . 33
6.1 Gf$Vit ..n...nn..onn.nnn.nn.n.n. . . .nnn . . ..n _ - 33
- ____ • _ _ .. __ • - . . Ju
• 6.3 p . .u.p. _ - - — 33
7.1 Anaerobic I) sstcr Soperuatanis - - 33
7.2 Sepernatant Trtmssit—Rcduciag
hi hnpset on Plunt Purformance . ... .. ................ .. _ ,.—— - . . __ _ __ I . 33
7.3 Elutriatma......................... . . ..............- - — —....... — - - —._ ‘37
74 D cstion Snpsrnstanhs . ......... .. .....—. _ . __ . __ • __ -— - ——.... _ 33
$ SLUDGE CONDI’I IONING.... . ............... _ ..... _ .. - _ . ... . 4$
8.1 Chenrical Conditioning .. 4$
8.1.1 Use al Filler Aids .... . ... . ... ... ....., . .......•. .....a..a.. C
1.2 Ikat Conditioning ....,..... . . . .l . . . . . . ... .. . . . . . . . .•••flSSs ....._.......
1.3 Frce7 ig ................................ .......... nn.......n...n............n..ssnlr—r C
- l _ • IUUSS’ SI
9$ Sind Heds..._..... ...._ ..... ......s... ... ...a ... -— — —— SI
9.2 Vacuum Filtration ... —— . .....—..... . 53
9.3 Centrifuption ..
9.4 Filler Pi ___ . ——l.a 33
93 It Filters . . . .fl . ... ..fl . . ......... ...— .— — — .... . 33
9.5.1 ( apiIlary.Squeeze ’ Sell Filter ..............._... — .—....——-- . . 33
9.5.2 GravIty-Type POrous Fabric Csnceiitrassr........ . -- — .. . 33

- SUBJECT INDEX (Continued)
Page No.
9.6 Screens .- 59
10.1 Incineration 61
10.1.1 Multiple Hearth Incinerators 63
10.1.2 Fluidized-Bed Incinerators 63
10.1.3 Drying nd Incineration 63
10.1.4 Incineration of Sludge With
Garbage and Refuse 65
10.1.5 Incinerator Scrubber Wash Water 65
10.1.6 Incineration of Concentrated Liquids
10.2 Wet Air Oxidation of Liquid Sludge 66
10.3 Land Spreading of Sludge 68
10.3.1 Present Practices 69-
10.3.2 Hygienic Aspects 70.
10.3.3 Dried Sludge ‘. 71
10.3.4 Current Studies 71
10.4 Landfill Disposal of Dewatered
and Stabilized Sludge 73
10.5 Ocean and Surface Water Disposal 75
10.5.1 To Surface Water Other
Than Ocean 75
10.5.2 Ocean Disposal 75
10.6 Deep Well or Underground Injection ‘25 ’
10.7 Pyrolysis of Sludge . .. 76
10.8 Composting of Sludge 77
II. !. Alum Recovery 79
11.2 Recalcination of Lime Sludge 80
11.3 Hydrolysis of Organic Sludges 80
12.1 Microstraining 81
12.2 Filtration and Adsorption Using
Granular Media...... .. . 81
12.3 Brines and Concenirated Solutions 83
12.4 Ion-Exchange Treatment 83
12.5 Electrodialysis 84
12.6 Reverse Osmosis... 85
12.7 Distillation 85
.ii .,1

( ,,j WASHINGTO D.C. 20460
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