F IG U R E 4.— Culture from calcium acetate ferm entation, showing large Sarcina packets.
both the free acids and their salts and is independent of the pH value. It is somewhat surprising to encounter such a situation, especially since we have come to think of pH as an almost all-important factor in fermentation control. Since it is the total of acid plus its salt which controls the fer
mentation, the addition of alkali is of little use. In fact, the production of acid is stimulated in some cases by the addition of lime.
There is only one way to limit the accumulation of volatile acids. That is to limit the rate at which the sub
strate is added to the fermentation vessel, so that the acids will be fer
mented to CH4 and C0 2 as rapidly as they are formed from the raw sub
strate. Many of the early failures are
due to the “ batch” type of fermenta
tion in which too much substrate was present.
I f a culture has developed too much acid the only remedy is dilution. This has been adapted to a series of stage digestion for certain industrial wastes.
Scum.
A third condition which must be avoided is the formation of any considerable amount of scum. This is sometimes encountered when ferment
ing greasy material. The objection to scum is that it constitutes a zone of high substrate concentration in which a high concentration of acids is likely to develop. In laboratory fermenta
tions, this can be avoided by breaking
up the scum mechanically. In plant
scale fermentations, the power required
for a scum breaker is too great.
Moist-Vol. 19, N o. 1 CO NSIDEB A T IO N S IN SLU DG E D IG E ST IO N — I I
35
a Ü
: . - A . • v
, * ' /
* ■ r . ''t ' 1
1
/ - •% %
^ * - t
V I - 4
i >
- • ”
1 * * * . V + * *
n * *■• * \
J r ’ /i V ' : V
-* ' 7 -* -* -* 1 ‘ „
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-, # r * v . S I i & J k ^ A ?
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9
KF IG U R E S.—Purified culture of M ethano sarcina.
ening the scum w ith liqu id pum ped from beneath it is a v e ry effective and inexpensive rem edy.
Fibrous Material. A fourth difficulty is encountered w ith fibrous m aterial such as paper, shredded cornstalks, etc.
These substances form a tough m at at the top o f the ferm entation vessel. The objection to this mat is, as above, that it fav o rs the accum ulation o f large amounts o f acid. A m at cannot be broken up m echanically nor b y the circulation o f liquor w ith an y success.
I t is necessary to provide the ferm en
tation vessel w ith suitable connections so th at it can be operated altern ately in an u p rig h t and an inverted position.
In this m anner, the m at is broken up b y the ferm en tative action itself a fte r each inversion. A ferm enter
eontain-ing a slow ly ro tatin g drum has been developed fo r large scale operation.
Liming
Since so much has been w ritten con
cerning the advantage or disadvantage o f lim ing digestion tanks, we w ish to emphasize our view point on this m at
ter. A s M r. Schlenz pointed out, our experience has been that such a p rac
tice is to be avoided.
One of the characteristics o f the or
ganism s which produce methane is that they also produce acetic, propionic, and b utyric acids under favo rab le condi
tions. Because these acids have high commercial value, m any attem pts have been made to fa v o r or direct the fe r mentation tow ard acid production. In our laboratory extensive experim ents
3 6 SEW A GE W ORKS JO U R N A L January, 1947
along this line have been carried out and at times it has been possible to build up acid concentrations o f better than 7,000 p.p.m . In order to do this alk a li w as added, either lime or sodi
um hydroxide. On the other hand, when it was desired to produce com
plete ferm entation to methane and car
bon dioxide it was found necessary to
avoid the presence o f a larg e concen
tratio n of alk ali, such as lim e or soda.
The labo rato ry research from a large num ber o f differen t lab oratories and in stitution s confirms M r. S ch len z’s statem ent th at the addition o f alkali tends to fa v o r the fo rm atio n o f vo latile acids, w hile the avoidance o f alkali favo rs the production o f methane.
I M P O R T A N T C O N S I D E R A T I O N S I N S L U D G E D I G E S T I O N — A D IS C U S S I O N *
B y M. W . T a t l o c k
Partner, Ralph L. Woolpert Company, Consulting Engineers, Dayton, Ohio
A n y discussion of sludge digestion is based on the assum ption th at sewage has been treated, and the pollution o f stream s has been prevented b y the re m oval of the objectionable m aterials from the sewage. D r. B usw ell has pointed out th at the chem ist classifies these m aterials as organic m atter, com
posed of carbon (C ) and hydrogen (H ) , which in the presence of su lfate produces foul-sm elling H 2S, and he says th at we have purified sewage when
ever we have rem oved the carbon-hy- drogen compounds from the polluted liquor. I f this is done aerob ically the chief end products are C02 and H ,0, but if the anaerobic process is used the end products are C02 and C H 4. The latte r ty p e of ferm entation has long been recognized as the cheapest method of rem oving carbon and hydrogen, since it is re ad ily applicable to all ranges of organic m atter over 1 per cent concentration. The u pper lim it of its successful application is a m axim um of only 75 per cent purification.
S lu d ge digestion has been an in tegral p a rt o f sewage treatm ent processes fo r m any years, but there are to date no u n iform practices of design and oper
ation. The digesters are too often de
* Presented at 20th Annual Ohio Confer
ence on Sewage Treatment, Akron, Ohio, June 20, 1946.
signed and operated sim p ly as holding tanks or as “ catch -alls” to use in some m anner to aid the other processes of treatm ent and sludge d ryin g . I f the destruction o f organics is poor the op
eration al difficulties are too o ften diag
nosed as “ lack o f c a p a c ity ” and the suggested correction is “ b u ild m ore.”
The best answ er m ight often be “ cor
rect the operational p ro ced u res.”
The p ast w a r h as brou ght to atten tion digestion conditions previou sly thought to be im possible. M un icipal in stallation s h ave handled th eir w a r
tim e loads because lack o f construction m aterials made im possible the addition o f more digester cap acity. M ilita ry in stallations, if m easured b y so-called stan d ard practices, often h ad to take more than the designed load fro m their first d ay of operation. Somehow they were su ccessfu lly operated because the job had to be done. Th e re su lt has been th at we have learned th at even sludge digesters w ill do more than was ever expected o f them i f th ey are treated rig h t, i.e., i f th ey are in te lli
gen tly operated.
The w rite r has recen tly returned from m ilita ry service in E u ro p e where the G erm ans re a lly p racticed sludge digestion because the operation of mo
tor vehicles w as only possible b y the use o f fu e l (m ethane gas) produced in
Vol. 19, N o. 1 C O N SID E B A T IO N S I N SLUDGE D IG E ST IO N — I I
37
the sew age sludge digesters. N ecessityproduced good design and operation.
Sin ce retu rn in g to p riv ate practice the
•ganie m atter into the unobjectionable end products of H 20, C02 and C H 4. environm ental conditions fo r the organ
isms which do the w ork in a d ig e ste r: the optimum operatin g tem perature.
The design must provide not only the means of h eating the digesting mass to this tem perature but must insure th at this tem perature is uniform throughout the entire mass. The proper tem perature should be m aintained same organism s which produce methane also produce acids u n der favorab le conditions, and when acid production from a ferm entation process has been
term ination of vo latile acids calculated to acetic acid w ill fo retell the approach o f retarded digestion, or serve as a
“ w a rn in g ” long before the u sual d i
gestion difficulties indicate th at trouble is at hand. The test is simple and is quickly made. D igestion progresses best at volatile acid values below 2,000
38
SEW A GE W ORKS JO U R N A L J a n u a r y , 1 9 4 7are so high that discharge back into the p lan t b ad ly dam ages the treatm ent processes. The best procedure involves a continuous, slow w ith d raw al o f the liquor w ith the lowest suspended solids. Germ an practice is to pass di
gester su pern atan t through a roughing filter before its discharge into the treatm ent processes. L . S . K ra u s, at P eo ria, has advantageou sly used the nitrogenous m aterial present in this m aterial in secondary treatm ent proc
esses and the w riter knows o f some ad
ditional developm ents in this direction which m ay be reported soon.
M r. Schlenz concluded his paper w ith a sum m ary which described “ de
sign and operation to obtain desired digestion co n tro l.” The yeast w aste treatm ent p lan t which was previou sly re fe rred to as being operated b y the w riter, em ploys these items of design and th ey do make “ controlled opera
tio n ” a re a lity instead of a theory.
H o u rly records o f feed o f ra w w aste, o f tem perature, and of gas production give a grap h ic p ictu re of the v e ry close relationship between the three, and especially dem onstrate the need fo r v e ry close tem perature control i f opti
mum gas production is to be expected.
Continuous circulation of the u pper two-thirds o f the digester contents through the heat exchanger gives the desired u n iform tem perature, which by actu al test shows on ly one-half de
gree variatio n between the bottom sludge cone and the top lay e rs of the tank. The volatile acid test, done d aily, gives a positive and im m ediate pictu re of the conditions u n der which diges
tion is progressin g, and if adju stm en t is n ecessary the p rim a ry digester con
tents are diluted b y low er vo latile acid content m aterial pum ped fro m the secondary unit.