Anaerobic Waste Treatment Fundamentals I - UCLA Engineering

66 Pages·2003·2.17 MB·English

°y5-
,ing This i% the first of a series of four articles on a process of
in waste treatment that has not been too well understood and con-
in sequently has not been as widely used as it might deserve. Part
pri- 12-
of One discusses the advantages and disadvantages of anaerobic
(lis-
waste treatment, conventional practices and the present concepts
use
of the microbiology and chemistry involved. Parts Two and
1
fter
lip- Three will cover the environmental requirements for achieving
age
control of the anaerobic process and preventing or correcting
lust
by toxicity in the system. Part Four will outline the application
this of these various concepts in treatment plant design.
i ntil
)er-
re-
;low
Anaerobic Waste Treatment Fundamentals
in
and
ach
or- I
ptly PART ONE Chemistry and Microbiology
and I
ntal
filers
ssed PERRY L. McCARTY portant parameters for design, oper- portion converted to cells is not
use Associate Professor of Sanitary ation, and control. This first article actually stabilized, but 'is simply
ner- Engineering is concerned with a general descrip- enanged in form. Although these
pike Stanford University tion, together with the chemistry cells can be removed from the waste
now HE anaerobic process is in many and microbiology of the process. The stream, the biological sludge they
T
tal subsequent three articles will deal produce still presents a significant
ways ideal for waste treatment.
eck- with treatment control and design. disposal problem.
It has several significant advan-
heir tages over other available methods Advantages 1 In anaerobic treatment, the waste
icles is also mixed with large quantities
and is almost certainly assured of
r to increased usage in the future. An- The advantages of anaerobic of microorganisms, but here, air is
lear treatment can best be indicated by excluded. Under these conditions.
aerobic treatment is presently em-
i all comparing this process with aerobic bacteria grow which are capable of
ployed at most municipal treatment
,oval treatment. In aerobic treatment, as converting the organic waste to
plants, and is responsible for the
major portion of waste stabilization represented by the activated sludge carbon dioxide and methane gas.
uld- that occurs there. However, in spite and trickling filter processes, the Unlike aerobic oxidation, the an-
re- waste is mixed with large quanti- aerobic conversion to methane gas
of the present significance and large
ead- ties of microorganisms and air. yields relatively little energy to the
future potential of this process, it
rees, has not generally enjoyed the fav- Microorganisms use the organic microorganisms. Thus, their rate of
e r y orable reputation it truly deserves. waste for food, and use the oxygen growth is slow and only a small
lings The primary obstacle has been a in the air to burn a portion of this portion of the waste is converted
f lice food to carbon dioxide and water to new cells, the major portion of
lack of fundamental understanding
col- of the process, required both to ex- for energy. Since these organisms the degradable waste being con-
lo- obtain much energy from this oxi- verted to methane gas. Such con-
plain and control the occasional up-
ruc- sets which may occur, and to extend dation, their growth is rapid and a version to methane gas represents
and successfully this process to the large portion of the organic waste waste stabilization since this gas is
-tails treatment of a wide variety of in- is converted into new cells. The insoluble and escapes from the
;lice.
dustrial wastes.
-s at
An increasing realization of the
lings
potentials of anaerobic treatment is
the
r of evident from the reporting each year Table 1-Advantages of Anaerobic Treatment
of larger numbers of research
investigations on this process. Al- 1. A high degree of waste stabilization is possible.
lent,
Di- ready, significant advances have 2. Low production of waste biological sludge.
.been made extending the process so
3. Low nutrient requirements.
it can be used successfully on many
hree
fol- more organic wastes. This series of 4. No oxygen requirements.
articles is intended to summarize
ions, 5. Methane is a useful end product.
our present knowledge of anaerobic
cific
treatment and to point out the im-
1964 1 PUBLIC WORKS for September, 1964 107
stages.1 Each stage represents the dicate an unbalance in the acid mediates formed in anaerobic treat-
culmination of growth of a popula- forming organisms. At present, no ment are acetic acid and propionic
tion of methane formers capable of satisfactory method is available to a..c'i,'d The importance of these
fermenting one particular group of determine the relative populations two acids as precursors of methane
compounds. The process is not com- of the bacteria specifically responsi- is indicated in Fig. 3. which shows
pletely operational until all the ble for production of certain acids. the pathways by which mixed com-
groups of methane formers are plex organic materials are con-
Methane Formation
finally established. This may take verted to methane gas. The per-
several weeks if the process is The methane producing bacteria centages shown are based on COD
started without the benefit of have proven to be very difficult to conversion and are for methane
"seed" sludge containing the me- isolate and study. Consequently, fermentation of complex materials
thane formers required for the relatively little is known of their such as municipal waste sludge or
specific acids present. basic biochemistry. The conversion other wastes of similar composition.
While there are many different of organic matter into methane no The percentages would be different
methane forming bacteria, there are doubt proceeds through a long se- for other wastes.
also many different acid forming ries of complex biochemical steps. The complete methane fermenta-
bacteria. Waste ,],r!~"ilization re- Although almost nothing is known tion of complex wastes has been
quires a balance among all these of the individual steps involved, compared to a factory assembly
organisms. The establishment and tracer studies have indicated the line operation8 in that the process-
maintenance of this balance is nor- major sources of methane as shown ing of raw waste material to the
mally indicated by one of the most in Table 3.4.5 One source of me- final methane product requires the
important control tests, that for the thane is the direct cleavage of acetic help of several different workers.
concentration of volatile acids. The acid into methane and carbon di- The raw material must be worked
volatile acids are the short chain oxide. This acid is one of the most on by each group of organisms to
organic acids indicated in Table 2. important volatile acids formed prepare it for handling by the next.
The acids shown are the major in- from the decomposition of complex Although each group's contribution
termediates produced by the first organics and is the source of most to the overall processing may be
stage conversion. They represent methane in anaerobic treatment. small, it is still necessary to the
the intermediate compounds of most The methyl carbon of acetic acid, formation of the final product. Thus,
importance in anaerobic treatment, marked with an asterisk in Table 3, if just one group of workers fails to
and most of the methane formed together with its three hydrogen do its job, the final product cannot
from this process results from fer- atoms, are converted intact into me- be formed. For example, 30 percent
mentation of these acids by the thane gas. The carbonyl carbon, of the complex waste shown in Fig.
methane bacteria. shown without an asterisk, is con- 3 becomes propionic acid through
When the system is in balance, verted to carbon dioxide. the action of the methane bacteria,
the methane bacteria use the acid Most of the remaining methane and if these organisms are not func-
intermediates as rapidly as they in anaerobic treatment is formed tioning, this portion cannot be con-
appear. However, if the methane from the reduction of carbon di- verted to methane gas. This is true
bacteria are not present in suitable oxide. Here, hydrogen, which is re- even though the propionic acid bac-
numbers, or are being slowed down moved from organic compounds by teria themselves directly produce
by unfavorable environmental con- enzymes, reduces carbon dioxide only 13 percent of the methane.
ditions, they will not use the acids to methane gas. The carbon dioxide They convert the remainder of the
as rapidly as they are produced by here functions as a hydrogen or propionic acid, or 17 percent. to
the acid formers, and the volatile electron acceptor, just as oxygen acetic acid.
acids will increase in concentration. in aerobic treatment. There is al- The acetic acid fermenting me-
Thus, an increase in acid concen- ways a large excess of carbon di- thane bacteria are also very im-
tration indicates the methane form- oxide available in anaerobic treat- portant, since if they fail, 72 per-
ers are not in balance with the ment, and thus the availability of cent of the waste cannot be con-
acid formers. An analysis for the carbon dioxide for this reduction verted to methane gas. It is interest-
individual acids present will indi- is never a limiting factor in treat- ing to note that acetic acid is formed
cate the particular methane bac- ment of complex materials. by several routes and through the
teria not carrying out their portion action of many different bacteria.
of the treatment. Unfortunately, the Volatile Acid Intermediates Only about 20 percent of the waste
volatile acids analysis does not in- The two major volatile acid inter- is converted directly to acetic acid
'.f
Table 2-Common Volatile Acid Intermediates Table 3_Major Mechanisms of Methane Formation
Acid Chemical Formula
Formic Acid H COO H I. Acetic Acid Cleavage:
Acetic Acid CH3COOH
Propionic Acid CH;,CH-COON C*H:COOH -> C*H4 + C02
Butyric Acid CH5CH_CH•~000H
Valeric Acid CH3CH_CHaCH_000H 11. Carbon Dioxide Reduction:
Isovaleric Acid (CH3)2CHCH2000H
Caproic Acid CH3CH_CH2CH2CH2000H C02 + 8H - CH4 + 2H20
110 PUBLIC WORKS for September, 1964 f
waste stream where it can be col- heating buildings, t unning engines, BOD values greater than 10,000
lected and burned to carbon di- or producing electricity. . mg/L. For less concentrated wastes,
oxide and water for heat. The anaerobic treatment process the disadvantages become more im-
As much as 80 to 90 percent of does have some disadvantages which portant, and may limit the use of
-the degradable organic portion of may limit the use of this process this process. A noted exception is
a waste can be stabilized in anaer- for certain industrial wastes./ The the successful anaerobic treatment
obic treatment by conversion to major disadvantage is that rela- of meat packing wastes with BOD
methane gas, even in highly loaded tively high temperatures are re- concentrations as low as 1,000
systems. This is in contrast to aer- quired for optimum operation; tem- mg/L.1 These wastes are fairly
obic systems, where only about 50 peratures in the range from 85ƒ to warm and the temperature require-
percent of the waste is actually 95ƒ F are preferred. Dilute wastes ment does not present a limitation.
stabilized. even at conventional may not produce sufficient methane
loadings._; for waste heating and this may rep- Process Description
Other advantages of anaerobic resent a major limitation. This limi- In anaerobic treatment, there are
treatment are shown in Table 1. tation suggests a need for more re- two basically different process de-
[Since only a small portion of the search on low temperature anaer- signs. One is the "conventional pro-
waste is converted to cells, the obic treatment, as there are indica- cess" most widely used for the
problem of disposal of excess sludge tions that much lower temperatures treatment of concentrated wastes
is greatly minimized. Also, the re- can be used if the systems are ade- such as primary and secondary
quirements for the nutrients, nitro- quately designed. sludges at municipal treatment
gen and phosphorus, are proportion- Another disadvantage of anaer- plants. The other process is one de-
ately reduced. This is especially im- obic treatment is related to the slow signed to handle more dilute waste
portant in the treatment of indus- rate of growth of the methane pro- and has been termed the "anaerobic
trial wastes which lack these ma- ducing bacteria. ' Because of it, contact process."1.2 Schematic dia-
terials. The sludge produced is quite longer periods of time are required grams of each process are shown in
• stable and will not present a nuis- for starting the process. This slow Fig. 1.
• ance problem. rate of growth also limits the rate The conventional anaerobic treat-
Since anaerobic treatment does at which the process can adjust to ment process consists of a heated
not require oxygen, treatment rates changing waste loads, temperatures, digestion tank containing waste and
are not limited by oxygen transfer. or other environmental conditions. bacteria responsible for anaerobic
_The absence of a need for oxygen The advantages of anaerobic treatment. Raw waste is introduced
also reduces power requirements treatment are quite significant, either periodically or continuously
for treatment. In contrast, the meth- while the disadvantages are rela- and is perferably mixed with the
ane gas produced by anaerobic tively few. The advantages normally digester contents. The mixed treated
treatment is a good source of fuel far outweigh the disadvantages for waste and microorganisms are usu-
energy and is frequently used for more concentrated wastes, with ally removed together for final dis-
I
posal. Sometimes this mixture is in-
troduced into a second tank where
∎ FIGURE 1. The two basic anaerobic process designs are diagrammed below. the suspended material is allowed
to settle and concentrate for more
MIXING
efficient disposal.
As the detention time in the con-
ventional process is reduced, an in-
creased percentage of bacteria are
removed from the tank each day
RAW WASTE with the effluent. The limiting de-
tention time is reached when the
bacteria are being removed from
the system faster than they can re-
produce themselves, occurring after
about three to five days at tempera-
tures of operation of 95ƒF. For prac-
CONVENTIONAL PROCESS
tical control and reliable treatment,
a detention time much above this, or
MIXING about ten to thirty days, is normally
used.
With dilute wastes, hydraulic de-
tention times should be very short
if the process is to be economical.
These are possible in the anaerobic
EFFLUENT contact process. Here, the bacteria
are not lost with the effluent, but
are maintained in the system. In
this case, a digester is used. How-
ever, it is followed by a settling tank
which removes the active biological
suspended solids from the effluent
stream for recycle back to the di-
gester. This system is similar in op-
eration to the activated sludge pro-
ANAEROBIC CONTACT PROCESS cess and permits the maintenance of
108 PUBLIC WORKS for September, 1964
a high biological population for ACID METHANE
rapid decomposition, while operat- FORMING FORMING
ing at a relatively low hydraulic COMPLEX BACTERIA ORGANIC BACTERIA IN CH4
detention time. Such a system has ORGANICS V ACIDS 0 C02
been found economical with wastes
S
It having BOD concentrations of about
D 1,000 mg/L and detention times of FIRST STAGE SECOND STAGE
)0 less than 6 to 12 hours.
(WASTE CONVERSION) (WASTE STABILIZATION)
The gas produced in anaerobic
V
treatment makes the suspended
n. particles buoyant and difficult to ∎ FIGURE 2. The two stages of anaerobic treatment consist of waste conversion
settle. Therefore, a degasifier is fre- by acid forming bacteria followed by stabilization with methane forming bacteria.
quently required between the di-
gester and the settling tank in the obic process than by aerobic treat- stage of treatment, it is required to
anaerobic contact process to permit ment. place the organic matter in a form
is
proper settling of the suspended It commonly considered that suitable for the second stage of
solids. A flotation process making anaerobic treatment is only useful treatment__
use of the large quantities of dis- for the destruction of suspended It is in the second stage of
solved gases to float and concentrate solids. This feeling has probably re- methane fermentation that real
the solids for return to there 1g ester sulted from the extensive use of waste stabilization occurs. During
also appears feasible. anaerobic treatment for sludge di- this stage, the organic acids are
The important parameter gov- gestion. However, the process is converted by a special group of
erning the efficiency and operation also well suited to the treatment bacteria termed the "methane
of both the conventional process of soluble wastes. formers" into the gaseous end prod-.-,
and the anaerobic contact process Another common fallacy is that ucts, carbon dioxide and methane.
is the biological solids retention anaerobic treatment is an ineffi- The methane forming bacteria are
time. This is similar to the sludge cient process. This belief is also strictly anaerobic and even small
age concept used in aerobic treat- related to experience with sludge quantities of oxygen are harmful
ment and is defined as follows: digestion, where most of the or- to them. There are several different
ganic material being treated is not groups of methane formers. and
SRT = ML- (1) readily susceptible to biological de- each group is characterized by its
Me gradation, and only about 50 per- ability to ferment a relatively lim-
where, cent reduction in solids is possible. ited number of organic compounds.
However, such wastes cannot be Thus, in the complete methane
SRT = solids retention time,
treated any better by aerobic proc- fermentation of complex materials,
Mt = total weight of suspended
esses. Parameters of waste strength several different methane bacteria
solids in treatment system
such as BOD, which indicate the are required. The methane formers
Me = total weight of suspended biological degradability of the which use materials such as formic
solids leaving the system
waste, should be used to compare acid and methanol grow very rapid-
per day, including both
the two processes on an equal basis. ly and can thrive at sludge reten-
that deliberately wasted
By using such a comparison, it can tion times of less than two days.
and that passing out with
n- be shown the two processes are However, the most important meth-
the plant effluent.
n- quite comparable in efficiency of ane formers, which live on acetic
ire The weight of suspended solids treatment at similar volumetric and propionic acids, grow quite
lay leaving the system per day refers loadings. slowly, and sludge retention times
le- to the sum total of the suspended of four days or longer are required
he solids lost in the effluent plus the Two-Stage Process for their growth. These bacteria
orn suspended solids deliberately re- Anaerobic treatment of complex carry out the major portion of
re- moved as "waste sludge." The SRT organic materials is normally con- waste stabilization. Their slow
ter relates treatment operation to the sidered to be a two-stage process growth and low rate of acid utiliza-
-a- age and quantity of micro- as indicated in Fig. 2. In the first tion normally represents the limit-
ic- organisms in the system, and is a stage, there is no methane produc- ing step around which the anaero-
nt, sound parameter for design. The tion and hence no waste stabiliza- bic treatment process must be de-
or major requirement of both the con- tion. In this stage,' the complex or- signed.
ally ventional process and the anaerobic ganics are changed in form by a The many different methane
contact process is the SRT be at group of facultative and anaerobic forming organisms responsible for
le- least ten days for temperatures of bacteria commonly termed the anaerobic treatment, their different
ort operation of 95ƒF. The required "acid formers." Complex materials sources of food, and their different
cal. SRT is about doubled for each such as fats, proteins, and carbo- rates of growth are responsible for
,bic 20ƒF lower temperature. hydrates are hydrolized, fermented, some confusion as to when good
ria and biologically converted to sim- waste treatment is well under way.
but ple organic materials. For the most For example, during the start-up
Microbiology and Biochemistry
In part, the end products of this first- of the anaerobic treatment process,
)w- It can generally be said that any stage conversion are organic fatty some methane formation is often
.ink waste susceptible to aerobic treat- acids. Acid forming bacteria bring noted during the early stages. How-
ical ment can also be treated anaerobic- about these initial conversions to ever, this is produced only from
.ent ally_IThere are few_ exceptions to obtain the small amounts of energy certain materials that are fer-
di- this statement. In'-addition, there released for growth, and a small mented to methane readily. Signifi-
op- are certain wastes, such as those portion of the organic waste is con- cant methane production does not
)ro- containing cellulose, which are verted to cells. Although no waste occur for several days or weeks,
I
e of more readily treated by the anaer- stabilization occurs during the first and when it does, it comes in
:964 PUBLIC WORKS for September, 1964 109
-
methane production is obtained.
Measured values for methane pro-
unic
duction per pound of COD or BOD,
':,ese
stabilization for a wide variety 0'
,anc
wastes varying from pure labora-
lows
tory substrates to complex waste
nm-
sludge have shown the validity o:
;on- e ACID FORMATION this relationship and the close ac-
per- /o
20 curacy with which it can be used tc
predict methane production.
bane
The relationship between methane
rials
production and waste stabilization
e or Wf can also be used in another way
tion. PROPIONIC OTHER in anaerobic waste treatment op-
,rent ACI D 15% INTERMEDIATES eration. Here. the methane produc-
tion can readily be determined.
,nta-
Such a determination gives a direct
been V and rapid measurement of actual
;nbly waste stabilization and permits
cess- 13 ACETIC 35% closely following the efficiency of
the % ACID 15 METHANE waste treatment. For example, if
• the FERMENTATION 1,500 pounds of waste COD are
kers. 72% added to an anaerobic waste treat-
irked ment system per day, and the me-
as to thane production is 5620 cubic feet
next. STP (standard conditions of tem-
ution perature and pressure). 1000 pounds
v be of COD are being stabilized by con-
, the CH4 version to methane gas. Thus, the
Thus, efficiency of waste stabilization is
AS to 67 percent.
annot
•rcent Anaerobic Biological Growth
:1 Fig. ∎ FIGURE 3. Pathways in methane fermentation of complex wastes such as munici- The most important advantages of
rough pal waste sludges. Percentages represent conversion of waste COD by various routes . the anaerobic waste treatment proc-
_teria, esses are the high percentage of
func- during the acid formation stage. A Table 4 to predict the quantity of stabilization obtained and the low
con- much larger portion (52 percent) methane from a knowledge of the
• true is formed from the action of various waste chemical composition. From pmaetrtceern ttaog eb ioolfo gciocnavle rcseilolsn .o fT hoer gsamnailcl
1 bac- methane producing bacteria which this formula, it can be shown that
quantities of sludge growth mini-
oduce ferment propionic acid and other the ultimate oxygen demand of the mizes the problems of biological
thane. intermediates to acetic acid and me- waste being degraded is eequal to sludge disposal, as well as the re-
o(fit,t hteo thFaoner. different industrial wastes, tmheet hualntei mgaatse porxoydguecne dd.e mTahinsd foafct tahle- quirements for the inorganic nu-
the percentages shown in Fig. 3 may lows prediction of methane produc- trients, nitrogen and phosphorus.
me- be different. However the largest tion in another way, that is, from The biological • growth resulting
• im- percentage of methane will still re- an estimate of COD or BODL (ulti- fernotm taypnease roofb iwca sttreesa tamreen ts hoofw nd iinf fFeirg-.
per- sult from acetic acid fermentation, mate BOD) stabilization. The ulti-
con- which is the most prevalent volatile mate oxygen demand of methane 4.10 Resulting biological suspended
solids under anaerobic conditions
erest- acid produced by fermentation of gas is as follows:
ormed carbohydrates, proteins, and fats. : vary considerably from one type of
.,tcwht ae asrtcithiaeed. CTsPitsihirgeoofn npoio irftoomifhnec eiadcrcn a tmra,vab coioailnhrdaley,ty d io rolfadne tu mertaisihnc enoia grdno sdtif, mhe pperarormlor ettthnheaatnioncanudes-g,..l, trorfhe iCtosHhx ,aynf gee+o n ri.2ms 0u C2leoqan-uv+ ie srvChta0oil2nweg+ns t t 2oot nHoce: u t0bwmi.ooc. l.mf oe(ol2e)fst pwceaoasadss ngitineteto i tootin fosb . etta hhlTpeeshr oewen daerisxetlcttate.wte oedsTd th ufrtsereo,onx mgtwt trahahese mt kaeegnl srcoooownwmleti--,hn
of methane per pound of oxygen, growth are represented by fatty
Thus, although many different
the value shown in Table 4 for rela- acid wastes, which produce the low-
organisms are required in anaerobic
tion between waste stabilization and est growth, to carbohydrates, which
treatment, the two groups of me-
thane bacteria which handle acetic
and propionic acids, are the most
important in the methane fermenta-
tion. Unfortunately, they also ap- Table 4-Methods of Predicting Methane Production
pear to be among the slowest grow-
ing methane bacteria and the most I. Prediction from Waste Chemical Composition
sensitive to environmental changes.
• a b n a b n a b
Waste Stabilization CƒH.Ob +jn H2O - lC02 +-+- -- CH4
4 2 ~~ 2 8 +- 4 / 2 8 4
`Waste stabilizationa naienrboic
treatment is directly related to me-
Il. Prediction from Waste Stabilization:
thane production. ~Buswell and co-
tdorkersa gave the formula shown in One pound BODL or COD stabilized = 5.62 cubic feet CH4 (STP)
ar, 1964 PUBLIC WORKS for September, 1964 111
of relatively dilute waste. Although
the microbiology and biochemistry
of the process is complex, it normal-
ly operates quite well with a mini-
mum of control. The bacteria re-
sponsible for this treatment are
widespread in nature and grow well
by themselves when provided with
the proper environment.
This first in a series of three ar-
ticles was intended to give an un-
derstanding of the bacteriology in-
volved in anaerobic waste treatment
and the biochemical steps resulting
in the formation of acetic and
propionic acids as intermediate
M
J products before a waste is finally
converted to methane gas.
0 10
The next article in this series will
BIOLOGICAL SOLIDS RETENTION TIME
be concerned with the control and
∎ FIGURE 4. Biological solids production resulting from methane fermentation. operation of anaerobic treatment
systems and will indicate the en-
vironmental requirements for prop-
produce the highest. Other types of based on the fraction of waste re-
er digestion, indicators of treatment
waste can be expected to vary be- moved during treatment, rather
unbalance and methods for pH con-
tween these two extremes. than on waste added. However, it trol. 000
Fig. 4 shows that the quantity of is better in anaerobic treatment, to
waste converted to biological sus- base such requirements on waste
References
pended solids decreases with in- additions. The reason for this is that
crease in sludge retention time. in highly loaded systems, the first 1. JSochhnrsooenp,f erA,. GS.. ZJi.;e mFkeul.l Nen., R W.,. aJnd.,
When cells are maintained for long stage of acid formation may take Anderson. J. J., "The Anaerobic
periods of time, they consume them- place to a larger extent than the Contact Process as Applied to Pack-
selves for energy, with the result second stage of methane formation inghouse Wastes," Sewage and In-
that the net growths are less. Thus. or stabilization. The first stage bac- dustrial Wastes, 27, 460-486 (1955).
greater waste stabilization and teria would grow and require 2. Steffen, A. J., "The Treatment of
lower biological cell production is nitrogen and phosphorus, even Packing House Wastes by Anaerobic
Digestion; Biological, Treatment of
I obtained at long sludge retention though the waste at this point is not Sewage and Industrial Wastes, Vol.
times. Such retention times also re- being stabilized. Thus, estimates of 11, Reinhold Publishing Co., New
sult in higher efficiencies of treat- growth and nutrient requirements York (1958).
ment. based on stabilization alone. may be 3. Cassell, E. A. and Sawyer, C. N.,
In order for any biological proc- much too low. "A Method of Starting High-Rate
ess to operate, inorganic nutrients It should be noted that the sus- Digesters," Sewage and Industrial
Wastes, 31, 123-132 (1959).
required by the bacteria for their pended solids formed in anaerobic 4. Barker, H. A., Bacterial Fermenta-
growth must be supplied. The in- treatment as indicated by Fig. 4 only tions, John Wiley, New York (1957).
organic materials required in high- represents the growth of new cells. 5. Buswell. A. M. and Sollo, F. W.,
est concentration for this growth Many wastes, notably municipal "The Mechanism of the Methane
are nitrogen and phosphorus. Since sludges, contain large quantities of Fermentation," American Chemical
these materials may be absent in suspended solids which also con- Society Journal, 70, 1778-1780
(1948).
mtaannyt itnod uksntorwi atlh ewa sqtueasn,tiitti eiss iwmhpiocr-h tthrei bduitgee sttoe rt.h eI ns utshpies ncdaesde ,s otlhied ss uisn- 6. JBeiroicsh, eJm.i Sst. rayn do fM cMCeatrhtayn,e PF. eLr.m, e"nTh-e
may have to be added. The require- pended solids for final disposal tation Using C14 Tracers." Proceed-
ments for nitrogen may be deter- would be much higher than indi- ings of 17th Industrial Waste Con-
mined from the cell growth and the cated by Fig. 4. Wastes similar to ference, Purdue University Engi-
neering Extension Series No. 112
fraction of nitrogen in the cells. municipal sludge are quite complex (1963). I
Based on an average chemical for- and the increase in biological solids 7. McCarty, P. L.. Jeris. J. S.. and
mulation of biological cells of which occurs during treatment may Murdoch, W., "Individual Volatile
C5H903N, the nitrogen requirement be far overshadowed by the large Acids in Anaerobic Treatment."
is about 11 percent of the cell vola- changes in waste suspended solids Journal Water Pollution Control
tile solids weight. The requirement occurring during anaerobic treat- Federation, 35, 1501-1516 (1963).
for phosphorus has been found to ment. Fig. 4 is of most value for 8. Sawyer, C. N., Howard, F. S., and
Pershe, E. R., "Scientific Basis for
be about one-fifth that for nitrogen, predicting requirements for nutrient Liming of Digesters," Sewage and
or about 2 percent of the biological deficient wastes, as well as predict- Industrial Wastes, 26, 935-944
solids weight. Thus, if the solids ing suspended solids production for (1954).
production were 0.1 lb./lb. of BODr,, relatively soluble wastes. 9. Buswell, A. M., and Mueller, H. F.
the nitrogen requirement would be "Mechanisms of Methane Fermenta-
11 percent of this or 0.011 lb/lb. Summary tCihoenm,i"stIrnyd,us4t4r,i a5l5 0-a5n5d2 E(n1g9i5n2e)e.ring
of BODL, and the phosphorous re- The anaerobic process has several 10. Speece, R. E. and McCarty, P. L,
quirement would be 2 percent or advantages over aerobic processes "Nutrient Requirements and Bio-
0.002 lb./lb. of BODL. for waste treatment. Use of the logical Solids Accumulation in
Theoretically, the biological anaerobic contact process, or a sim- Anaerobic Digestion," Proceedings
of First International Conference t
sludge; production and nitrogen and ilar modification, permits the use on Water Pollution Research, Lon-
.phosphorus requirements should be of this process for the treatment don (1962).
112 'PUBLIC WORKS for September, 1964
Anaerobic Waste Treatment Fundamentals
I
PART TWO Environmental Requirements and Control
PERRY L. McCARTY efficient and rapid treatment might can proceed quite well with a pH
Associate Professor of Sanitary be obtained. A summary of optimum varying from about 6.6 to 7.6, with
Engineering, environmental conditions for anaer- an optimum range of about _7.0 to
Stanford University obic treatment are listed in Table 1. 7.2. 'Beyond these limits, digestion
At higher temperatures, rates of dan proceed, but with less efficiency .
If reaction proceed much faster, re- At pH values below 6.2, the efficien-
HE ANAEROBIC PROCESS has
T sulting in more efficient operation cy drops off rapidly, and the acidic
many advantages over other and smaller tank sizes. Two opti- conditions produced can become
methods of organic waste treatment. mum temperature levels for anaer- quite toxic to the methane bacteria.
This process has been widely used
obic treatment have been re- For this reason, it is important that
for the stabilization of municipal
ported,'-'--'1 one in the mesophilic the pH not be allowed to drop be-
waste sludges and has good poten-
range from 85ƒ to 100ƒF, and the low this value for a significant per-
tial for the treatment of many in-
other in the thermophilic range iod of time. Because this parameter
dustrial wastes. In this series of
from 120ƒ to 135ƒF. Although treat- is so important, the control of pH
articles, a summary of the current
ment proceeds much more rapidly will be discussed in more detail in
information on the biochemistry and
at thermophilic temperatures, the a following section.
chemistry as related to process de-
additional neat required to maintain 'A last requirement for successful
sign and control is being presented. such temperatures may offset the anaerobic treatment is that the
The first article in this series' con-
advantage obtained. Therefore, most waste be free from toxic materials.
sidered the basic microbiology and
treatment systems are designed to Normally, concentrated wastes are
biochemistry. This article summar-
operate in the mesophilic range or more susceptible to anaerobic treat-
izes the environmental require-
lower. ment. However, such wastes are also
ments for anaerobic treatment and
Another environmental require- more likely to have high or in-
describes methods of process and
ment for anaerobic treatment is hibitory concentrations of various
pH control.
that anaerobic conditions be main- materials ranging from inorganic
rEnvironmental Requirements tained. Small quantities of oxygen salts to toxic organic compounds.
can be quite detrimental to the With municipal wastes, the major
The methane bacteria, which are
methane-formers and other anaer- problem usually results from heavy
responsible for the majority of waste
obic organisms involved. This re- metals. Industrial wastes, on the
stabilization in anaerobic treatment,
quirement usually necessitates a other hand, may have inhibitory
grow quite slowly compared to aer-
closed digestion tank, which is also concentrations of various common
obic organisms and so a longer time
desirable so the methane gas can salts such as those containing so-
is required for them to adjust to
or be collected for heating. dium, potassium, magnesium, cal-
changes in organic loading, tem-
ns The anaerobic process is depend- cium, ammonium, or sulfide. Heavy
perature or other environmental
S. ent upon bacteria, which require metals may also be a problem. An
conditions.. For this reason, it is
th nitrogen, phosphorus and other ma- understanding of the nature of the
usually desirable in design and op-
.In eration to strive for optimum en- terials in trace quantities for opti- toxicity caused by these materials
of mum growth. Municipal waste and their control is quite important
vironmental conditions so that more
lit sludge normally contains a variety in evaluating the potential of the
Irt of these materials, and thus usually anaerobic process for treatment for
provides an ideal environment for industrial wastes, and will be con-
,I- Table 1-Optimum growth. However, industrial wastes sidered in more detail in the follow-
.4_-
'w are frequently more specific in com- ing article in this issue.
Conditions for Anaerobic
Ild position and biological nutrients
Treatment
to must be added for optimum opera- Indicators of Treatment
tion. For such wastes, it has been Unbalance
Il- 'Optimum Temperatures
lu. found that materials in addition to `Under normal conditions, anaer-
ed Mesophilic Range nitrogen and phosphorus are fre- obic waste treatment proceeds with
85ƒ to 100ƒF
ry quently required.6 In some cases, it a minimum of control. However, if
Ics Thermophilic Range has been found beneficial to add environmental conditions are sud-
I'y 1200 to 135ƒF from 30 to 60 mg/L of iron in the denly changed, or if toxic materials
a- Anaerobic Conditions form of ferric chloride.? In addition, are introduced to the digester, the
Iig the inclusion of domestic wastes process may become unbalanced. An
lic. Sufficient Biological Nutrients along with industrial wastes for "unbalanced digester" is defined as
ed Nitrogen treatment can be of benefit by sup- one which is operating at less than
wd F Phosphorous plying inorganic and organic ma- normal efficiency. In extreme cases,
;Iu3-e Others terials which stimulate growth, re- the efficiency may decrease to al-
sulting in more efficient and rapid most zero, in which case a "stuck"
ta, oOptimum pH-6.6 to 7.6 treatment. digester results. It is important to
ed r
0- Absence of Toxic Materials One of the most important en- determine when a digester first be-
vironmental requirements is that for comes "unbalanced" so that control
nt.
a proper pH.8!Anaerobic treatment measures can be applied before
.)64
PUBLIC WORKS for October, 1964 123
control is lost. A stuck digester is usually results from a high volatile maintaining pH, this condition can
difficult to restart, and, if a supply acid concentration. A significant be prevented. The proper pH can
of seed sludge containing high con- drop in pH, however, does not be maintained either by decreasing
centrations of methane bacteria is usually occur until the digester is the waste feed to the digester, if
not available, this may take several seriously affected, and conditions this is possible; or by addition of 1
weeks. resulting in a "stuck" digester are neutralizing materials such as lime;
There is no single parameter near. or both.
which will always tell of the onset With some types of toxicity, the Once the pH is under control, the
of unbalanced conditions, and sev- first indication is a decrease in total next item is to determine the cause
eral parameters must be watched gas production. However, this para- of the unbalance. The unbalance
for good control. Several of the par- meter is useful as an indicator only may be temporary in nature or it
ameters of importance are listed in when the daily feed is quite uni- may be prolonged, as indicated in
Table 2. form and the daily gas production Table 4. Temporary unbalance can
Of the many parameters, the best does not vary too widely from day be caused by sudden changes in
individual one is that for the con- to day under normal conditions. temperature, organic loading or the
centration of volatile acids. As in- Changes in the percentage of car- nature of the waste. Such un-
dicated in the previous article,' the bon dioxide in the digester gas may balances take place while the bac-
volatile acids are formed as inter- sometimes indicate the onset of un- teria are adjusting to the new con-
mediate compounds during the com- balanced condition,*' as unbalanced ditions. What is needed here is time
plete anaerobic treatment of com- treatment often results in decreased for the adjustment. By providing
plex organic materials. The methane methane production which is ac- optimum environmental conditions
bacteria are responsible for destruc- companied by an increase in carbon and controlling pH, a temporary un-
tion of the volatile acids, and if they dioxide percentage. Another indica- balanced condition will soon correct
become affected by adverse condi- tion of unbalanced conditions is a itself.
tions, their rate of utilization will decrease in efficiency of operation. A prolonged unbalance may be
slow down, and the volatile acid Such a decrease in efficiency may caused by the introduction of toxic
concentration will increase.. A sud- be evidenced from a drop in meth- materials to the digester. It may also
den increase in volatile acid con- ane production per pound of vol- result from an extreme drop in pH
centration is frequently one of the atile solids added, as frequently de- when adequate pH control is not
first indicators of digester unbalance termined for municipal sludge, or maintained, or may result during
and often will indicate the onset of may be indicated by an increase in initial digester start-up when a suf-
adverse conditions long before any effluent COD in the treatment of in- ficient population of methane form-
of the other parameters are affected. dustrial waste. ers is not present. In all cases tiie
It should be noted that a high vol- Although none of the above para- control is much more difficult than if
atile acid concentration is the result meters may be a sure sign of di- the unbalance is only temporary in
of unbalanced treatment and not gester unbalance when used indi- nature. If toxic materials have been
the cause as is sometimes believed.5 vidually, together they give a good introduced, pH control alone will
Thus, a high volatile acid concen- picture of digester operation. The not correct the situation. The toxic
tration in itself is not harmful, but best and most significant individual materials themselves must be re-
indicates that some other factor is parameter, however, is the volatile moved or controlled. However, pH
affecting the methane bacteria. acids concentration, and this should control will prevent a disastrous
Another indicator of digester un- always be closely followed. drop in pH, and may give additional
balance is a decreasing pH, which time to correct the undesirable con-
3
Cause and Control of Treatment dition.
Unbalance If the prolonged unbalance is
caused by an extreme drop in pH,
Table 2-Indicators of Digester unbalance must be con-
trolled to prevent the serious con- and no toxic materials are involved,
Unbalanced Treatment
then pH control alone can correct
ditions resulting from a stuck di-
the situation. However, time for ad-
Parameters Increasing gester. Once the start of an un-
justment will be similar to that re-
balance is detected, the steps listed
Volatile Acids Concentration in Table 3 should be observed. quired during initial process start-
COs Percentage in Gas up. This may vary from a few weeks
The first thing to do is control pH
to months, as required to allow a
Parameters Decreasing near neutrality. Unbalance is usu-
ally accompanied by an increase in new population of methane formers
pH volatile acids, which, if allowed to to grow up. I
Total Gas Production Once the cause of the unbalance
go unchecked, may depress the pH
Waste Stabilization below 6. This, in itself, can rapidly is determined and corrected, then
the proper pH should be maintained
result in an inoperable digester, a
until the system can adjust itself
difficult situation to correct. By
and return to a balanced condition.
Because of the various chemical
equilibria existing in a digester, pH
Table 3-Steps to Follow in Controlling Unbalance control can be somewhat difficult
unless the factors affecting pH are
,1. Maintain pH near neutrality. understood. This is discussed in the
following section.
2. Determine cause of unbalance.
pH Control
3. Correct cause of unbalance.
The pH of liquor undergoing an-
4. Provide pH control until treatment returns to normal . aerobic treatment is related to sev-
eral different acid - base chemical
PUBLIC WORKS for October, 1964
equilibria. However, at the near
neutral pH of interest for anaerobic
treatment (between G and 8) the
major chemical system controlling
pH is the carbon dioxide-bicarbon-
ate system, which is related to pH
or hydrogen ion concentration
through the following equilibrium
equation:
[H_C03]
[H+] = Kt (1)
[HC031
The carbonic acid concentration
(H2C03) is related to the percent-
age of carbon dioxide in the di-
gester gas, K, is the ionization con-
stant for carbonic acid, and the bi-
carbonate ion concentration
(HC03) forms a part of the total
alkalinity in the system. Fig. 1
shows the relationship between
these factors for anaerobic treatment
near 95ƒF.
The bicarbonate ion concentration ' 50 500 1000 2500 5000 10,600 25,000
or bicarbonate alkalinity is approx- BICARBONATE ALKALINITY-mg/I AS CaC03
imately equivalent to the total al-
kalinity for most wastes when the ∎ FIGURE 1. Relationship between pH and bicarbonate concentration near 95ƒF.
volatile acid concentration is very
low. When the volatile acids begin for the fact that only 85 percent of be done by the addition of alkaline
to increase in concentration, they
the volatile acid alkalinity is meas- materials such as lime or sodium
are neutralized by the bicarbonate
ured by titration of total alkalinity bicarbonate.
alkalinity, and in its place form vo-
to pH 4. The equation also assumes
latile acid alkalinity.9 Under these Liming a Digester
there is no significant concentration
conditions, the total alkalinity is
of other materials such as phos- Lime is the most widely used ma-
composed of both bicarbonate alka-
phates, silicates, or other acid salts terial for controlling pH in anaer-
linity and volatile acid alkalinity.
which will also produce a significant obic treatment, mainly because it is
Under these conditions, the bicar-
alkalinity. readily available and fairly inex-
bonate alkalinity can be approxi-
Fig. 1 indicates that when the bi- pensive. However, occasionally some
mated by the following formula:
carbonate alkalinity is about 1,000 problems have arisen from its use
BA=TA-(0.85)(0.833) mg/L and the percentage of carbon which are related to the relative
TVA (2) dioxide is between 30 and 40 per- insolubility of some of the calcium
where: cent, the pH will be about 6.7. If the salts which form in the digester.
BA = bicarbonate alkalinity, bicarbonate alkalinity drops below Because of this problem, close con-
mg/L as CaCO3, this value, the pH will drop to un- trol over lime additions is required,
TA = total alkalinity, mg/L desirable levels. Such a low alka- and a knowledge of the solubility
as CaC03, linity does not give much safety problem with lime is helpful
TVA = total volatile acid con- factor for anaerobic treatment, for Control of pH is usually con-
contration, mg/L as a small increase in volatile acids sidered when it appears likely to
acetic acid. will result in a significant decrease drop below 6.5 to 6.6. If lime is
r in bicarbonate alkalinity and diges- then added, it initially increases the
This formula is similar to that used
ter pH. bicarbonate alkalinity by combina-
by Pohland and Bloodgood,e but
On the other hand, a bicarbonate tion with the carbon dioxide present
includes a factor (0.85) to account
alkalinity in the more desirable as follows:
range of 2,500 to 5,000 mg/L pro-
vides much "buffer capacity" so Ca(OH)2 + 2CO2--> Ca (HC03)2 (3)
Table 4-Factors Causing that a much larger increase in vo-
However, the calcium bicarbonate
Unbalanced Treatment latile acids can be handled with a
formed is not very soluble, and
minimum drop in pH.10 This gives
Temporary Unbalance a good factor of safety and allows when the bicarbonate alkalinity
reaches some point between 500 and
Sudden change in temperature. time for control if an upset results.
1,000 mg/L, additional lime additions
Sudden change in organic loading. If an increase in volatile acid con-
result in the formation of the in-
centration drops the bicarbonate
Sudden change in natureofwaste. soluble calcium carbonate as fol-
concentration too low as calculated
lows:
Prolonged Unbalance by equation 2, and a serious drop
Presence of toxic materials. in pH threatens, then the bi- Ca(OH)2+CO2-CaCO3+H20 (4)
carbonate alkalinity should be con-
Extreme drop in pH.
trolled. This may be done by re- Lime additions beyond this point
Slow bacterial growth ducing the feed rate to allow the do not increase the soluble bicar-
during start-up.-
volatile acids to be utilized and de- bonate alkalinity, and so have little
crease in concentration, or it may direct effect on digester pH. Fig. 2
PUBLIC WORKS for October, 1964 125
I
expensive than lime, less quantities
are required because it does not
precipitate from solution. The ease
of control, addition, and handling.
make it a very desirable material
for pH control in digesters. It is
expected this material will be used
more in the future.
N
Conclusion
a
The successful control of the an-
z aerobic treatment process depends
upon a knowledge of the various
environmental factors which affect
the microorganisms responsible for
waste degradation. Of the various
factors, pH is one of the most im-
portant to controls This control de-
pends upon the maintenance of an
adequate bicarbonate buffer system
both to counteract the acidity of
the carbon dioxide and that of or-
ganic acids produced during anaer-
obic treatment. It is also important
to control materials which may pro-
duce an adverse environment for
the anaerobic microorganisms. The
4 toxicity which may be caused by
5 0 1 2 3
common materials as well as their
LIME ADDED (RELATIVE UNITS) control will be discussed in the next
article in this series.
∎ FIGURE 2. The effect of lime additions on pH and carbon dioxide percentage.
References
is an illustration of what happens the pH to about 6.7 to 6.8. Once 1. McCarty, P. L.. "Anaerobic Waste
to the pH and carbon dioxide per- the lime is added, the pH in the Treatment Fundamentals, I. Chem-
centage in the gas when lime is digester must be closely watched. istry and Microbiology," PUBLIC
Tahded edp Ha frteemra itnhsi sb eptowiente ni s6 .r5e aacnhde 7d,. Aofs 6s.o4o nt oa s6 .i5,t addrdoiptsi obnaell olwi mae vaadldiu-e 2. WFaanOidRr K,R SaGt..e S Meo.pf t a.S,nldu1 d9M6go4eo .rDei.g eE.st iWo.,n ." Tainmde
until the CO., concentration has de- tions must be made. If this proce- Their Variation with Temperature."
creased to less than about 10 per- dure is followed, and pH is closely Sewage Works Jour. 6. 3-13 (1934).
cent by reaction with the lime as watched, then lime can serve as a 3. Malina. J. F., "The Effect of Tem-
perature on High-Rate Digestion of
indicated in equations 3 and 4. The cheap and effective method for con- Activated Sludge," Proc. 16th In-
pH then suddenly increases above trolling pH. Good mixing of the dustrial Waste Conf., Purdue Univ.,
7, and approaches 8 largely as a lime is required in the digester and 232-250, (1961).
result of decrease in CO_ percent- caution must be excercised to pre- 4. Miller, F. H. and Barron, W. T..
"The CO-- Alarm in Digester Op-
age as indicated in Fig. 1. After a vent the creation of a vacuum from eration." Water and Sewage Works,
short period of time when biologi- the removal of the carbon dioxide 104. 362-365 (1957).
cal action occurs, the percentage of from the gas by combination with 5. McCarty, P. L. and Brosseau, M. H.,
CO2 in the gas will begin to in- the lime. "Effect of High Concentrations of
Individual Volatile Acids on Ana-
crease again. As soon as it exceeds erobic Treatment," Proc. 18th In-
10 percent. the pH will again drop Sodium Bicarbonate for pH Control dustrial Waste Conf., Purdue Univ.,
below 7. This may occur even with- Sodium bicarbonate, although sel- (1963).
out the formation of any additional dom used, is one of the most effec- 6. McCarty, P. L. and Vath, C. A.,
"Volatile Acid Digestion at High
volatile acids. If lime is then added tive materials for pH control in Loading Rates," Int. Jour. of Air
again, the cycle repeats itself. anaerobic treatment. This material and Water Pollution, 6, 65-73 (1962).
Thus, nothing beneficial is ob- has significant advantages over 7. Speece, R. E. and McCarty, P. L.,
tained if additional lime is added other materials. It is relatively in- "Nutrient Requirements and Bio-
logical Solids Accumulation in Ana-
to raise the pH above 6.7 to 6.8. expensive when purchased in large erobic Digestion," Proc. Inter. Con-
After this point, the lime simply quantities. It does not react with ference on Water Pollution Re-
combines with the carbon dioxide carbon dioxide to create a vacuum search, London (1962) .
in the gas to form insoluble calcium in the digester, and there is little 8. Barker, H. A., "Biological Forma-
carbonate, which precipitates in the danger that it will raise the pH to tneieorni nogf MeCthehmainset."ryI,ndu4s8t,. 1a4n3d8 -E1n4g4i2-
digester. This insoluble calcium car- undesirable levels. It is quite sol- (1956).
bonate is quite ineffective for the uble and can be dissolved prior to 9. Pohland. F. G. and Bloodgood, D. E.,
neutralization of excessive volatile addition to the digester for more "Laboratory Studies on Mesophilic
acids or for raising the pH. effective mixing. This material can Daingde sTthieonr,m"ophJioluirc. AnWaaetreorb iPco lSlluutdigoen
Thus, for effective use of lime, it be added to give alkalinity in the Control Federation. 1, 11-42 (1963).
should not be added until the pH digester of 5,000 to 6,000 mg/L 10. Sawyer, C. N., Howard, F. S.. and
drops below 6.5. A quantity should without producing any adverse or Pershe, E. R., "Scientific Basis for
bedded then sufficient only to raise toxic effects. Although it is more Liming of Digesters." Sewage and
Industrial Wastes. 26, 935-944 (1954).
126
PUBLIC WORKS for October, 1964