CA1060126A - Anaerobic waste treatment process employing recycle of purified effluent - Google Patents

Abstract

Abstract of the Disclosure In a process for the biological purification of a waste water containing organic contaminants which comprises passing the waste water through an anaerobic filter or equivalent anaerobic digestion apparatus, a portion of the purified filter effluent is recycled to the filter Inlet and mixed with the waste water entering the filter. The recycle rate is maintained at a level at least high enough that the liquid mixture entering the filter does not contain contaminants in a concentration which is above the level at which they inhibit the life processes of the microorganisms contained in the filter. This avoids destruction or inhibition of the biological process occurring within the filter. In many cases it also eliminates, or sub-stantially reduces, the need for employing alkaline reagents to adjust the pH of the waste water prior to treatment inasmuch as the effluent commonly contains alkaline bicarbonates (produced in the treatment process) which may not be contained initially in the waste water.

Description

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Back~round of the Invention This invention relates to the biological treatment of wastes, such as sewage and, in particular, industrial wastes comprising aqueous streams contaminated with organic pollutants, by subjecting such was~es to the action of anaerobic microorganisms. More particularly it relates to an improved method for carrying out the anaerobic purification of organic ~, ~
waste by passing them in an aqueous liquid stream through any type of anaerobic digestor but especially through an anaerobic filterS which is essentially a tank or vessel containing an arrangement of solid packing which providès surfaces and interstices upon and within which there is maintained an effective amount of a "biomass" consisting essentially of -~
microorganisms capablc of decomposing the pollutants contained ln the waste stream with attendant formation o methane and carbon dioxide as metabolic products of the microorganisms. Specifically, this invention relates to a method for operating an anaerobic digestor, or especially an anaerobic filter, in such a manner as to avoid destruction of the anaerobic microorganisms or the undesired inhibition of their metabolic action under conditions of high pollutant concentration or adverse pH conditions in the waste material being treated.
Although anaerobic digestors and filters are of widely-var~ing designs, the simplest digestors having very little if any provision for supportin~ the biomass, the filters, as distinguished from simple digestors, have designs which range from simple tanks packed with relatively simple support materials such as broken rock to more sophisticated designs com-prising relatively tall, narrow, vessels whlch are packed with an array of slats or more sophisticated packing upon ~hich the biomass attaches itself at least in part and through ~hich ~typically in an upward direction) the liquid being treated is passed. Filters of the latter type are particularly ;:-' ~

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efficient for their siz~ inasmuch as there is little opportunity for short~
circuiting which could result in the channeling of a portion of the liquid being treated through to the filter outlet after insufficient contact with the biomass. ~
It will be understood, of course that a simple anaarobic digestor ; ~ ;
containing no internal structure other than a flocculent amorphous unsup-ported biomass can be used in some applications if desired, but this poses problems of collapse of the mass, rapid elutria~ion of slow-growing biomass components out of the vessel, poor distribution, etc. such that an anaerobic ` `
filter containing a solid support of some type for the biomass is preferred in all but the simplest and least-demanding situations.
Anaerobic decomposition of pollutants taXes place in two basic stages, both of which may exist within a single treatment vessel. Alterna~
t.~vely, multiple vessels can be used in series, with the inven~ion to be described hereinbelow being applicable to both arrangements. Whether the treatment takes place within a single vessel or in multiple vessels, how-ever, the first stage comprises subjecting the pullutant-containing liquid to the action of acid-forming microorganisms which effect a preliminary con- ~;version of the organic pollutant species Csubstrate) to carboxylate moieties.
2Q Any of a large number of microorganisms can effect this preliminary con~er-sion, suitable microorganisms for the digestion of a given substrate being normally "cultivated" in-situ by initially feeding the filter with a micro-organism sludge initially obtained from, for example, the soil or process ~ ' drain lines in the immediate vicinity and then allowing growth and evolu-tionary development of this initial inoculum into a biomass ~hich will be inherently adapted to metabolic conversion of th~ substrate in which the ;inoculum has been developed.
The second stage of the anaerQbic treatment of digestion comprises subjecting ~he effluent from the first, or carboxylate-forming, stage of 3Q treatment to a second, or methane-forming, stage in which the biomass is predominantly composed of microorganisms capable of further converting the carboxylate-containing primary metabolic products comprising predominantly

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methane and carbon dioxide. As pre~iously explained, this second stage may ;
be contained in a separate vessel from the first stage, but more typically it is simply a portion of the same anaerobic filter containing the first~
stage biomass; that is, it is typically simply the upper portion of an ~ ;
anaerobic filter the lower portion o which is occupied by the first-stage biomass.
The organisms predominating in the second-stage biomass are normally much more sensitive to environmental changes than are the acid-forming species found in the first stage, and they are much more sens;tive to pH in particular. Their rate of growth is less rapid than that of the acid-forming organisms, and they regenerate themselves much more slowly after being damaged than do the acid-formers. Thus, an early symptom of the onset of adverse conditions in an anaerobic digestor or filter is a ; ;~
diminution in, or complete cessation of, the evolution of methane and carbon dioxide resulting from destruction or inhibition of these sensitive methanogenic organisms which predominate in the second stage.
It is to be understood that there is no sharp demarcation, in the matter of the nature of the contained miCrQorganismS~ between the first and second stages of the filter. Specifically, the first stage biomass contains a substantial quantity of the second-stage methane-forming micro-organisms along with the relatively less sensitive acid-orming species, in a relationship which, if not actually symbiotic, does entail consump- `
tion by the methanogenic organisms of the metabolic products of the acid-forming species.
One factor which is known to be capable of affecting either type of microorganism adversely is the concentration of pollutants in the liquid being treated. This varies with the individual microorganism being employed ~ ;~
and with ~he nature o~ the pollutants. For example, phenolic pollu~ants can be tolerated only in very low concentra~ion as compared with more innocuous pollutan~s such as, for example, starch.
Another factor kno~n to the art as being of basic importance in anaerobic digestions is the pH of ~he pollutant-containing liquid being

- 3 -introduce~ into the filter. This is a relatively important factor even in the first digestion stage, in which some of the contained microorganisms are relatively toleran~ to environmental ~hock, and of even greater importance in the second stage, in which the methane-forming microorganisms are much ~ ;
more demanding in this regard. Specifically, as is known to the art, a pH of approximately 6.0 to 8.0 is typically required in the aqueous liquid being ~;
introduced into the filter, with 6.3 to 7.6 being preferred, and unless suit~
able buffering compounds are adventitiously present in sufficient quantity to maintain it, it is necessary to introduce alkaline ~or, in some cases, acidic) reagents at such a rate as to maintain the desired pH. Under proper conditions of feedstock pH, the first-stage microorganisms then convert the organic pollutants to carboxylate moities which, as their alkali or alkaline earth metal salts, then come into contact with the second-stage microorgani.sms which convert these carboxylates to methane and carbon dioxide while the alkali or ~`
alkaline earth ions are ultimately discharged in the filter ef~luent as the ;
bicarbonate salts along with carbon dioxide. With sufficient alkalinity in ;
the first-stage decomposition product, the second~stage microorganisms then ;~
operate in a self-buffared environment ~hereby their comparatively stringent `~
environmental pH requirements are met.

Failure to maintain the necessary pH in the anaerobic filter feed material results in inhibition or destruction o the microorganisms; as explained above, this is well known to the art, and the approach normally taken i5 to incorporate a buffer, typically ammonia or a basic alkali or alkaline-earth compound, into the liquid entering the filter. Ordinarily alkali metal compounds are employed, particularly sodium compounds, introduc~
ed as, for example, the carbonate or hydroxide. `
Less straightforward is the situation regarding filter upsets resulting from maladjustments in the organic substrate concentration. Parti~
cularly in anaerobic filters which are of a comparatively high length:diameter ratio such that the liquid flows therethrough wlth very little back-mixing, an increase in substrate concentration above the threshold level above which -the microorganisms are inhibited or even destroyed ~esul~sin cassation of , ~' ., .

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their metabolic activit~ followed by progressive destruction of the biomass beginning at the inlet of the filter and then continuing on up through it until the entire bio~ass is substantially destroyed or else converted to some different, less satisfactory, type.
Another characteristic of anaerobic filter operation as currently -practiced is that, even assuming that the substrate loading never becomes so high as to damage the biomass as explained above, there is often a ten-dency, particularly when the linear velocity of liquid through the filter is quite low, for the biomass in the lower, or inlet, portion of the filter to become so lush in its gro~th habit as to clog the filter. In extreme cases this clogging has even been known to cause entrapment of the evolving gases with a resulting buildup of gas pressure which finally caused destruction of the filter internals by the explosive gas release which too~ place when the gas pressure became high enough to break thraugh the biomass obstruction.
It is an object of the present invention to provide a method for preventing the destruction, or the inhibition of the metabolic activity, of ~ ~;
the biomass contained in an anaerobic digester or ~ilter resulting from a too-high substrate concentration in the liquid being treated therein. It is an~
other object to provide a method for reducing the consumption of pH-adjustment reagents required for adjusting the pH of the liquid being introduced into an anaerobic digestor or filter. It is another object to provide a method for increasing the uniformity of distribution of the biomass throughout the interior of an anerobic filter whereby not only are mechanical difficulties due to localized over-growth of the biomass alleviated but also, as a result of the more uniform distrlbution o the biomass, the ability of the filter to handle temporary overloads of pollutants is enhanced.
Other objects will be apparent from the following detailed description.
Brief Summary of the Invention : -~
In accordance with this invention there is provided in a process for the anaerobic biological treatment of uaste water contai~ing organic pollutants which are of such a nature as to be amenable to ananerobic diges~

~ )G0~L26 tion, said process comprising passing sald waste water through an anaerobic digestor or filter containing a biomass consisting essentially of ~icro~
organisms capable of digesting said pollutants while converting said waste ~ater to a purified effluent having a reduced content of said pollutants, the improvement which comprises: recycling to the inlet of said anaerobic digestor filter, and admixing into said waste water entering said digestor ~; i filter, a portion of said efflu0nt in such an amount that the content of said pollutants and any biostatic or biocidal species which are present in the resuiting mixture of recycled effluent and waste water is below that ;~
level above which there results inhibition of the growth processes o~ said microorganisms.
In accordance with the present invention the above-sunmlarized objects are attained by recycling a portion of the effluent from the second or methane-formingl stage o the anaerobic digester or filter back to the inlet in such an amount tha~ the resulting mixture of effluent and fresh feedstock contains a substrate concentration which is lower than that at which inhibition or destruction of the biomass caused by an overly-high substrate concentration takes place. This not only obviates the adverse ~ . . .
effect of an overly-high substrate concentration, but it also results in the realization of three additional benefits not obtained in the relevant prior -~
art~
Pirst, as a result of the conversion of the substrate to methane and carbon dioxide by the action of the methane-forming microorganisms, any alkaline ions initially introduced into the digestor or filter, either adventitiously or as a result of the employment of alkaline reagents to `
neutralize feedstocks, are ultimately discharged in the treated effluent as alkaline bicarbonate salts. Thus, when the effluent is recycled to the inlet of the digestor or filter, these bicarbonates are available for `~
acidity neutralization with resulting decrease in the amount of alkaline _ 6 -, ~'; ;:-' :-. ~

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neutralizing compounds ~hich ~ould otherwise have to be added. It is frequently the case, in fact, that there are sufficient sodium salts in the waste water to be treated that no separate addition of neutralizing chemicals is necessary at all when employing the effluent recycle e~en through, on a once-through non-recycle operating basis, alkaline neutralizing agents would be necessary. In other words, on a once through flow basis it is often necessary to add alkaline reagents to buffer the digestor or filter feed to protect, in particular, the sen-sitive methane-forming microoganisms against the acid shock which would otherwise destroy them as an overly-acidic liquid passes through the biomass. At the same time, the action of these microoganisms in convert-ing organic carboxylate moiety to methane and carbon dioxide ultimataly results in the formation of more alkaline buffering material ~e.g., sodium bicarbonate) than is actually re~uired. There is, thus, an initial thres-hold phenomenon due to initial carboxylic acid formation in the system ;~
which has to ba overcome with alkaline moieties while, at the same time, the net overall process is such that alkal mity is produced in what is, in many instances, a quantity more than sufficient for the needs of the system.
By employing the effluent recycle according to the present invention, a portion at least of this internally-produced alk-linity is returned to the ~ ~inlet of the filter or digestor, where it is needed, with a net reduction, ; ~i;or in some cases complete elimination, of the separately-added alkaline reagents. The quantity of alkalinity required to neutralize the feed and buffer the acid formed in the initial stages of the digestion process is sometimes very large and constitutes a substantial operating expense, so that employment of the recycle with its attendant alkalinity content results in a very substantial reduction in net chemical consumption in the process. It will be ~mderstood that there are some cases in which the wast= w~ter is too alkaline, rather than too acid, requlring acidi- ;

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fication rather than alkalization prior to treatment. In these cases the carbon dioxide content of the recycle stream has a beneficial e~fect analogous to that of the alkaline bicarbonates.
Second, by maintaining always a higher total throughput of liquid through the anaerobic digestor or filter than would otherwise be the case on a once-through flow basis, employment oE the recycle minimizes the effect of transitory fluctuations in the substrate loading.
Third, also as a result of the increased linear velocity of liquid through an anaerobic filter operated according to the present method, there `~
results a more even distribution of the biomass throughout the ilter. A ~`
result of this is an enhanced ability of the system to absorb brief periods of shock loading because the biomass has less tendency to be concentrated in the lower part of the filt0r, and there is also less danger of obstruction o the lower portions of the filter with an overly-luxuriant growth of the microorganisms.
Detailed Description and Preferred Embodiments Although it is known to the existing prior art to divert a small part of an anaerobic filter effluent back to the filter feed for such pur~
poses as increasing feedstock pumpability and a}so for preventing stagnation during periods of no net inflow of liquid, such recycles, on the rare occasions ; ;
when they have been used, have been comparatively small in relation to the net ~ ~;
inflow of waste water into the filter. Such a, relatively small, recycle is mentioned in a paper presented by D. W. Taylor and R. J. Burm in 1972 at the 71st national meeting of the American Institute of Chemical Engineers in Dallas, Texas titled "Full Scale Anaerobic Filter Treatment of Wheat Starch Plant Wastes". In the operation described in said paper, which was di~
rected to using anaerobic filters to treat waste from starch-gluten manu-facture, an intermittent effluent recycle was sometimes employed to the .~ ~ . . . .
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extent of recycling from 25% to 50% of the filter effluent back to the filter inlet for purposes which were not explicitly ou~lined. The present improved process, in comparison, contemplates the normal use of comparatively high recycles to realize the unexpected benefits that have now been discovered to ~-~
be attainable. For example, the present process typically entails recycling at least about three parts, by volume, of filter effluent per part of sub-strate-loaded waste water newly introduced into the filter. More typically, ~;
this recycle ratio will be about 5:1 or higher, up to an upper limit which is determined by the linear velocity of combined liquids passing through the filter above which the biomass begins to be swept away from the packing contained in the filter. This is, of course, a simple hydraulic effect, and the attainment of this limiting upper recycle rate can be readily recognized by visually monitoring the filter effluent for the appearance of substantial-ly increased turbidity indicating that the biomass is being washed out of the filter by hydraulic forces.
A more detailed explanation of the determination of the limits within which the recycle rate is to be controlled is set forth below, with it being understood that the upper limit is simply controlled by the easily-understood hydraulic factor as just explained above.
Insofar as operability of either a filter or a simple digestor at / ;
low recycle rates is concerned, the fundamental controlling factor is the ~
concentration of pollutants in the waste stream which is to be treated. ~;-Since there is wide variation among the many species of microorganisms used in anaerobic digestions, and since there is also an extremely wide variation in composition of the infinitely large number of waste streams which it may be desired to subject to anaerobic digestion, it is not possible `~
to present meaningful quantitative data for the maximum allowable pollutant concentration of all potential waste streams with all potential biomasses _ g _ ~
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which may be found in anaerobic treatment system~ (each biomass being the result of individual e~olutionary adaptation to the environment obtaining within its own anaerobic milieu). Also, a given wasts stream may contain a high loading of cornparatively innocuoùs substrates which could be success-5 fully digested in high concentration but for the presence of a very minorportion of some pa~ticularly difficult chemical ~pecies, such as formalde-hyde, phenols, acrolein, chloroform, or various mercury compounds.
Typically, however, organic compounds which have no pronounced blocidal;
or biostatic action in substantially neutral aqueous solutions (e. g., solutions 10 having a pH of about 6. 0 to 8. 0) do not inhibit the n etabolic processe3 of the anaerobic microorganisrns used in these digestions if their concentration in the liquid pas3ing through the biomass is no greater than that at which the chemical oxygen demand of the liquid is about 2000 mg of oxygen per liter.

, This figure is ofiered by way of general guidancei rather than as a limita- -15 tion of the present invention, inasmuch as it will be unde~stood that some materials, such as sugars, are extremely susceptible to biological diges-tion even at "syrup" concentrations while others, especially when they contain trace quantities of biocides or bio~3tats, are much less so, In the ~;
latter case a very small quantity of a strong biocide or biostat can be the Z0 determining factox aifecting maximum allow~ble substrate loading.
With further reference to the foregoing, it will be understood that , waste strearns are frequently encountered in which there i3 a biostatic or biocidal effect due to the presence oi inorganic contaminants, with the .
organic materials which are present being quite innocuou~ to the biomaas~
25 and not a controlling factor. Heavy metal~, for example, pre~ent a problem of this sort. In these case~ it will be understood that, if it is not feaslble to remove these inorganic materials, as by chernical method~, before the anaerobic treatment, then the limiting factor in determining the minimum dilution ratio to be employ~d with such materials in the present improved treatment procass is the maxim~ allowable concentration of these in-organic species, rather than that of the organic components in the waste stream being treated. As will be explained below, properly controlled anaerobic digestion can actually result in abstraction of heavy metals from the waste water by the biomass.
In determining the maximum allowable concentration o~ pollutant species in the diluted liquid which is to be passed through the anaerobic digestor or filter, there will be, of course, some instances in which it is known from the prior art what level of substrate can be tolerated without affecting the biomass adversely. In other instances, however, as when tho waste water to be proces~qd is one which 15 new to the art, the most practical approach is to actually study the behavior of the new substrate in the laboratory by standard experimental biological techniques. That is, a laboratory-size anaerobic digestor is seeded with a suitable inoculum which is then acclimated with continuously-increasing concentrations of the pollutant substrate under conditions of controlled pH (e.g., around 7.0) and with addition of nutrient salts as commonly e~ployed in the art, until, as a result of continuing increase in the substrate concentration, the evolution of methane and carbon dioxide from the digestor begins to de-crease while the chemical oxygen demand of ~he effluent begins to increase.
This identifies the maximum allowable concentration of pollutant substrate.
As has been explained, these techniques are widely understood such that, although experimentation is of necessity required in determining the digestion characteristics of a given waste water, the determination of the maximum allowable substrate concentration which is the first step in -~
applying the present invention to a new pollutant species is well within the skîll vf the average worker in the field of anaerobic waste treatment.
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' ' ~ ., 1 Once the maximum allowable substrate concentration is determined as explained above, it is, of course, a matter o simple mathematics to ~ ;
determine the minimum recycle ratio required to effect sufficient dilution . ~ ., . ~ .
of the waste water that the substrate concentration does not exceed the maximum allowable level above which action of the biomass is inhibited. ;
A very important aspect of the present invention is its inter~
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relationship with the relatively stringent requirements of the methane-form-ing microorganisms in the matter of pH, which must be taken into account in preparing the mixture of waste water and recycled e~fluent which is to be introduced into the anaerobic filter. Fortunately, in the majority of cases, the only adjustment necessitated by the incorporation of the present process improvement into an existing "once through" anaerobic filter system is to reduce the rate of, or even eliminate entirely, the introduction of any pH-adjusting reagents employed in a given system to neutralize acidity or excess alkalinity in the pollutant-containing stream which is being process-ed. As previously explained, the methane-forming organisms which pre- ii: :': ;
dominate in the biomass in the second stage o the anaerobic filter and which are also present in the first stage require a pH of about 6.0 to 8.0, preferably about 6.3 to 7.6, in almost all cases. Inasmuch as the first-stage microorganisms generate carboxylate moiety in their metabolic process, it is standard practice to buffer the liquid entering the first stage with alkaline materials (or, in some cases, acidic) in order that the liquid entering the second stage have a pH within the desired range. Other-. :.
wise, even though the acid-forming microorganisms which are most active in the first stage may continue living, the methane-forming microorganisms `~which are also in the first stage in admixture with them, and which generate buffering bicarbonate salts in their own part of the digestion process, will die. The result is that the effluent from the first stage becomes distinctly ~` :

acidic and then, upon entering the second stage, causes a progressive destruction of the biomass there, beginning at the bottom and progressing upward through the second stage until the methane-forming biomass has been completely destroyed. If, on the other hand, the methane-forming microorganisms are protected from such a "p~l shock", they generate ~;
increasing amounts of alkali or alkaline ear~h (e.g., calcium or sodium) ;~bicarbonates to form an effluent which is buffered by these alkali or alkaline earth bicarbonates. Thus, if the pH of the liquid entering the second stage in particular is maintained within the desired range, the con~
}0 tained microorganisms will then metabolically maintain the desired pH while forming additional bicarbonate throughout the remainder of the second stage and produce a final efluent liquid rich in buffering capabilities.
The connection bet~een the above-explained buffering properties . . .
of the methane-forming microorganisms and the present invention is that (a) from pH considerations there is no upper limit on the proportion of ~ ;
effluent which can be recycled to the anaerobic filter inlet ~since it is inherently buffered at the desired pH range); and (b) any alkalin reagents . . .
which were being added at the inlet of the filter to overcome initial acidity can, upon.ithe adoption of the effluent recycle, be reduced by an amount stoichiometrically equivalent to the alkali or alkaline earth bi~car~
bonate ttypically sodium bicarbonate) which is contained in the recycle.
I'his has three beneficial effects. ~irst, the rate at which alkaline ;
reagents from external sources are added to the waste water entering the ...
filter can be reduced in many cases, i not eliminated entirely (as when, as is often the caseJ there is already a substantial concentration of adventi- ~-tious alkali ions). This is especially important because the common buffer-ing ions ~e.g. Na, Ca, K~ NH4) themselves have an undesirable biostatic effect at conc`entrations above about 2000 mg per liter. Second, the dis- ~;
solved solids content . ~' hO60~26 ;;
- of the fllter ef1uent is reducad by an amount -corresponding- to the r~ion ~-i~ exterl~ally-3upplied alkali treatrnent with resulting ecological benefits in any body of water to which the effluent is discharged. Third, since at least a substantial portion of the alkali being introduced into the filter is being S introduced automatically and on a continuing basis in the form of bicarbon ates contained in the recycle, thus eliminating or at least reducing the arnount of separately-added alkaline reagent, the problem of precise pH
control at the inlet of the anaerobic filter is greatly s1mplified. That is, if there still remains some net requirement of separately-added alkaline 10 reagent, the proportion of this reagent in the total alkali fed to the filter is reduced and there results a system which i~ more stable to rninor upsetY
in the rate of the alkali addi~ion.
It will be seen that in making allowanc~ for the alkaline buffer content of the recycle in adjusting pH conditions at the filter 1nlet, all that 15 is necessary is to trim the rate of alkali addition on the basis of pH readings of the mixed liquid entering the filter, employing either intermittent pH
measurements or else a continuous pH recorder or recorder-controller. In this connection it should be noted that it i8 preferable not to leave pH-measuring electrodes in the liquid entering the anaerobic ~ilter for an 20 indefinite period of t~me, since the microorganisms which are present tend to grow on the electrode surfaces and affect the accuracy of the pH r~eading adversely. It is recommended that the electrodes be removed periodica11y for thi~ rea~on.
~here are Qccasionally waste waters which are overly~alkaline and 25 oo require acidification, rather than the add1tion o~ alkaline reagent~, to achieve the desired pH range of a~out 6. 0 to 8. 0 before introduction Into the anaerobic filter or di~estor. While mineral acids can be employed for this purpose, the result is not only an expense due to the cost of the acid but, in ~ 14~

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~126 addition, an undesired increa~e in the amount of, for example, chlo`ride or sulfate salts in the filter effluent. The effluent recycle can be employed to advantage in these systems, just as those systems in which the waste water is insufficiently alkaline. That is, the effluent contains dissolved carbon dioxide along with the bicarbonate salts, and therefore serves to replace, with this dissolved carbon dioxide, the rnineral acid which would otherwise have to be added to the waste water entering the filter. Since the bicarbon-ate-carbon dioxide content of the recycle ~nakes it a buffered aystem, there i8 here again no upper limit, from the chemical standpoint, to the proportion of the filter effluent which can be recycled~ If the dissolved carbon dioxide content is in~ufficient in quantity to buffer all the alkali or alkaline earth ions contained in the waste water to the desired pE~ range, additional carbon dioxide can be introduced by contacting the waste water, for example in a conventional absorber, with the carbon dioxide-containing gases evolvlng from the anaerobic filter. Here again, 3ince the product resulting from the absorption operation can have a pH no lower than that characteristic of the bicarbonate-carbon dioxide buffer system, there is no adverse effect from incorporating into the liquid an excess of carbon dioxide over and above that which is nece~sary to convert all contained alkaline ions to the bicar-2 0 bona Le 8 alts .
If the alkalinity of the waste water should be 90 high that the effluen5 recycle and the carbon dioxide absorbed in the scrubber are still, taken together, insufficient to attain the desired pH adjustment, then further acidification is still needed employing, for example, A mineral acid or, - 25 preferably, an additional quantity of carbon dioxide from an extraneous source if one i3 available.
A8 explained ahove, pH and organic substrate loading (conveniently - rneasuxed by chemical oxygen demand as determined by standard methods) of the effluent-diluted waste water entering the anaerobic filter are the ;~
two most important parameters which must be controlled. An additional factor which is of substantial importance on some occasions, however, is the presence of heavy metals, e.g. copper, chromium, cobalt, and nickel in the waste water. These metals, as ~ell as zinc, mercury, cadmium, and antimony, frequently have a very adverse effect in aerobic treatment systems, and also in anaerobic systems which are operating without the effluent recycle with which the present invention is concerned. Employing ~ ~
the present effluent recycle, however, it becomes possible to greatly ~ `
increase the tolerance of the system to such heavy metals contamination.
While the exact mechanism is not entirely understood, it is believed that, in the anaerobic environment, the heavy metals are precipitatedl probably as the insoluble sulfides and are, for the most part, entrapped itl the bio-mass. The resulting ~ilter effluent is) then, comparatively free of such heavy metals and, upon recycle to the filter inlet, can be emplo~ed to dilute the original waste water stream sufficiently that the heavy metal content of the resulting mixture is below the level at which, regardless of the purifying effect of the biomass vis-a-vis heavy metals as just explain~
ed, it might nonetheless affect the microorganisms adversely. To facilitate ~ ;
conversion of heavy metals to insoluble sulfides, it is advantageous to in-corporate such bacterial nutrients as are required in the process in the form, at least in part, of sulates. ~or example, nitrogen nutrients can be employed in the form of a~monium sulfate. The sulfate is reduced in the filter to the sulfide, with ultimate precipitation of the heavy metals as sulfide salts. In many instances, of course, adventitious sul~ate ion will already be present. Alternatively, of course, it is possible to add a sulfide, such as hydrogen sulfideJ in small quantities. Alternatiuely,soluble sulfide salts of innocuous metals such as sodium can be added. In any case, the ' .. . . :

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anaerobic filter operating with the effluent recycle has been found to be surprisingly tolerant to heavy metal contaminants in the waste water and to be capable of abstracting them therefrom. Ultimately, of course, the accumulated heavy metal is removed from the anaerobic filter periodically 5 removing the biomass material in which it is entrapped.
In connection with the ability of the anaerobic filter or digestor to remove heavy metals from the waste water it is to be understood that this takes place even in once-through anaerobic systems if the initial metal concentration is not above the threshold level at which it inhibits or testroys 10 the biomass. Abo~te the threshold level, however, the non-recycle anaero,-bic systems cannot cope with the problem whereas it can be dealt with by employing the pre~ent method.
l~emperature and pressure of the anaerobic filter system are process parameters of relatively minor importance. Pressure is, of course, 15 typically about atrnospheric because there is little need for operatlng under pressure or vacuum. It will be understood, of coursei that operation under ~ome pressure has practis~al advantages if it is desired to collect the evolving gases for use as fuel, or if it is desired to pass the gas through acrubbing tower in order to incorporate the carbon dioxide contained there-20 in into the waste water for purposes of pH adjustment as discussed above.Regarding the f1lter temperature, temperatures in the range of approxi-- mately 30 to 50"C 3.re normally employecl in anaerobic dige3tions as generally known tQ the art, and~ such temperatures are applicable in the preqent process, Typically and preferably, temperatures of about 3S to 25 40 - C ars employed.
50ncernin~ bacterial nutrients which should be present in addition to the organic subatrate initially contained in the waste water, the pre~ent isnproved proces~ employs the ~ame nutrients, in the 3ame nutrient:organic ~17-~ubstrate ratio, aq already characteristic of the art. Typlcally, the- -diluted waste water entering the anaerobic filter (and here it should be noted that the present process provides a method for re-using such nutrients as have escaped the filter in the effluent) should contain any of 5 the common sources of nitrogen and phosphorll6 (e. g~, ammonium ion supplied as ammonium sulfate, and phosphate ion applied as phosphoric acid or ammonium phosphate) in a quantity such that the weight ratio of chemical oxygenden~and:nltrogen:phosphorus i9 approxirnate~rlO00:5:1.
It will be understood that this i~ an approximate figure which i8 subject 10 to wide variation as understood in the art, although this ratio i9 typical.
T~le following examples are given to illu~trate the invention further.
It will be recognized that many variations can be made thererom within the ~cope of the invention.
EXAMPLE I
An anaerobic filter was employed which compriaed a vertically-oriented section of glass pipe which was 6 inches in diameter and 4 feet tall, filled with 1-inch ceramic Raschig rings and having 15 liter~ of contained void space. The pipe was wrapped with electrical heating tape, connected to a variable power control whereby the interior of the filter 20 could be maintained at a constant temperature. Outside the heating tape, the pipe was insulated with glass wool, both to prevent heat loss and also, by excluding light, to prevent~growth~ of photosynthetic (hence oxygen-forming) organis~ms. ~ A thermqcouple w811 waa provided inaide the filter at approximately its midpoint. Means were provided for introducing a 25 waste water containing a mixture of organic contaminants into the bottom o~ the pipe at a controlled rate, along with auxiliary chernicala (bacterial nutrients, etc. ) as desired. Means were provided for measuring and .18-j sampling the liquid effluent, or outfall, discharged from the top of the filterand also for separating, measuring, and samplin~ the ga~es evolved from the top of the filter, A connection was providled from the~ effluent dis-charge at the top of the filter to the suction ~ e of the pump introducing 5 liquid into the bottom of the filter, whereby a~continuous recycle of effluent to filter inlet could be maintained at a controlled rate as de3ired. Ihe fresh filter "feed stockll (wa~te water),buffering reagents, nutrients, etc., as already discussed were introduced from a feedstock reservoir into this sffluent recycle on the ~uction side of the filter feet pump at a controlled 10 rate.
With the effluent recycle portion of the total liquid fed into the filter being varied from time to time as will be explained, the total throughput of fresh water feedstock plus recycled effluent waa continuously maintained at 150 liters per day. The void volume of the in1ulated pipe being 15 liters, 15 thexe were then 10 volume changes per day of the liquid contained in the insulated pipe anaerobic filter.
Ihroughout the operation, the filter was operated at atmoapheric prs~sure and at a temperatur~ of approximately 36-38C.
Prior to ~tarting up the filter, it wa~ initially charged with about 20 2 gallons of sludge which had been obtained from a municipal ;,ewage digestor and then passed through a 14-mesh screen to remove coaFse partlc1es. This initial inoculum was then conditioned to build up a popula-tion of methane-forming microorgani~s by continuously passing through the filter four hters per day of a synthetic sub~trate the composition of Z5 which is tabulated below, together with 146 litsrs per day of recycled filter effluent. Compo~ition of the synthetic conditioning substrate was as follows:

.

. . .

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BL}~
Conc e nt r ation Compone nt ~=
Methanol 10. 0 Anhydrous sodi~lm aceta~l) 1 5 S Urea D. 4 85% Phosphoric acid 0.2 Epsom 9alt (MgS04 7H20~ ( ~ o. 08 Wate r _ _ remainde r (1) Buffering agent (2) Source of sulfate, to remove heavy metal contaminants as the s ulfide ~ .
With the anaerobic filter operating as described above, evolution of ga~ from a gas~liquid separator mounted at the head of the filter began to be apparent after a few days. The rate of gas evolution increased rapidly and lS then became iubstantially con3tant at about 7. 5 liters of gas (measured at about 24C and atmospheric pressure) for each liter of the synthetic ~sub-strate introduced into the system. At this point, and continuing over a period of two weeks, the proportion of synthetic substrate in the liquid being introduced into the filter was gradually reduced and replaced with an Z0 increasing proportion of an actual waste water from a petrochernical plant.
This plant waste water contained, as the major contaminants, acetic and~
formic acids along with minor amounts of acetaldehyde, acrylic acid, and -.
acrylate esters. It contained no nitrogen or phosphorus, and only traceamounts of alkaline ion~ ~i. e., Aodium). In order to provide the nutrients ZS and alkalimty that were lacking in thlY plant waste water, there were contin~ously incorporated into it, before it wa3 admixed into the liquid `
.

entering the anaerobic filter, urea and pho3phoric acid in such-amounts : that the plant waste water component of the anaerobic filter feed contained ~ .

~t)6~L'2~
at all times five p~rts of nitro~en and one p~rt of pho~phorus, by weight, per 1000 parts of ~hernical oxygen demaDd. These ccncentrations of nitrogen and phosphorua ære lowar by a factor of 10 than the concentra-tion~ normally employed in ~erobic treatment. Sodium hydroxide was 5 incorporated into the waste water in a concentration of 50Q mg per liter, and also' Epsom salts (MgS04 7H2O), in a concentration of 78 mg/l to provide 10 mg/l of S, this latter intended for control of heavy metals.
l~he actual composition of the plant waste water being introduced into the filter as descrlbed abol,-e changed over a period of time, and as 10 the chemical oxygen demand fluctuated the nitrogen and phosphorus incorporation rate3 were controlled to maintain the ratios ju3t set forth above. Compositions of individual sequential batches of the plant waste water passed through the anAerobic filter were as tabulated below, each batch representing ~pproximately 3 weeks of anaeroblc filter operation:
~ABLE II
Batch A B C D E F G
p~ 2.4 2.7 3.6 3.42.5 3.0 2.8 TOC (1), mg/l12300 6800 2300 22007400 5600 6100 COD (Z), mg/l15000 7800 830017800 11600 15500 _odium, m~62 _ 13 60 0. 50. 5 22 150 (1) ~otal organic carbon content.
.
(2) Chemic~l o~ygen demand, rng oxygen per liter of ~liquid.
A~ explained z~bo~e~ the proportion of plant waste water in the filter feed was gradually increased, while the proportion of synthetic substrate 25 was decreased, until, aEter two weeks, the liquid being introduced into the filter con~isted entirely Of recycled filter effluent and plant waste water (admixed with nuer~ents and aodium hydroxide as explained above). After thi3 point was reached, the rate of introduction of the pla~t waste water was .

gradually increased until~ after an additional three weeks, i~ had reached 15 liters per day. The effectiveness of organic substrate removal during ~he treatment process was judged by comparing the total organic carbon content of the filter effluent or outfall with that of the plant waste water ~ ~
before treatment. It was found that, although the total organic carbon ~`
content of the waste water varied widely as tabulated above, the percent removal of organic carbon in the digestion process was relatively constant, ranging from 75% to 95% and normally being within the range of 85% to ;
90%. Although the more concentrated waste waters were characterized by relatively higher residual organic carbon content in the outfall, the percent removal in the case of these higher-concentration feedstocks was better than with those which were relatively more dilute.
Guring the above-described operation of the anaerobic filter, t~e p~l of the outfall was always in the relatively narrow range of 6.7 to 7.2 despite the fact that the waste water belng treated (even after incorporation of sodium hydroxide into it was explained above) was distinctly acidic, having a pH of about 3.5 to 4.5, which is below the pH level required by ~`
the methane-forming bacteria. It will be seen, of course, that the pH-adjusting effect of the effluent recycle was responsible for the satisfactory results which, from the standpoint of pH, could not have be0n obtained on a once-through operating basis.
With the exception of brief periods o modified operation to test the effects of temporary upsets in treatment conditions, the anaerobic fil-ter was operated continuously on the above-described basis for six months, during which time the outfall p~I and total organic carbon removal efficiencies described above were maintained. Among the changed operating conditions which Nere tested during this period of ti~e were included abrupt changes in waste water substrate concentration between , ;,~ : : :

~i0~ 6 about 2.5 and about 7.5 grams per liter of total organic carbon; the anaerobic filter took ~hese changes in stride without evidence of any operating problems, with the ratio of offgas to waste water feed input immediately reflecting in each case the change in organic carbon throughput. Likewise, the addition of known amounts of speciIic sub-strates, e.g. sodium acetate, in~o the feed was also reflected within a few hours in a corresponding increase in gas:waste water ratio.
Resistance of the microorganism in the recycling anaerobic filter to poisoning by heavy metals was tested by incorporating into the waste water 20 mg per liter of each of several heavy metals along with magnesium sulfate in a proportion of at least 1 equivalent of sulfur .
per equivalent of heavy metal ion. Specifically, filter performance was completely unaffected by such incorporation into the waste water feed-stock of the ions o iron, cobalt, copper, nickel, and chromium3 for a period of one week of operation with each of these ions. In each case none of the heavy metals could be detected in tne filter effluent by ;-analytical methods which were sensitive to concentrations of }ess than 1 milligram per liter. Analysis of dried biomass sludge taken from the `- -filter at the end of this period of operation with the heavy metal con- ;~
tamination showed it to contain 5.6% chromium, 2.9% nickel, 1.2~ copper, j ~`

4.4% cobalt, and 14.8% iron by weight.
Calcium, added to the waste water as calcium carbonate, was tested as a buffering agent for a period of several weeks. Performance of ` ~
the anaerobic filter was as satisfactory as when using sodiu~, but it would ~ ;
be expected that over a long period of time the coninued use of calcium in this manner would result in calcium carbonate deposition within the filter so that soluble bu~fers (e.g., alkali metals or ammonia) would be prefer~
able.
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When, at the conclusion of the above-described six-month period of operation~ the anaerobic filter was emptied and its interior examined, it was found that approximately 85% of the original void volume was still available (the remaining 15% baing occupied, by this time, by the biomass, entrapped heavy metals, calcium carbonate, etc.)~ A simple water wash of the packing of the anaerobic filter removed substantially the entirety of the contained biomass. -EXAMPLE II
When operating the anaerobic filter as described in Example I ``~
. ~
but with an increasing effluent recycle while maintaining the net through-put of waste water at the same rate as in Example I, there is no adverse efect from increasing the recycle ~and thereoro the linear velocity of liquid through the filter) until a recycle rate is reached at which, with the packing employed in Example I, the linear velocity of liquld through the filter reaches a level of about 0.1 foot per minute. Up to this level the biomass is not adversely affected, but the retention time in the filter be-comes low enough that, with this particular feedstock, the degree of diges-tion begins to be adversely affected and a rise in chemical oxygen demand begins to be noticed in the effluent. If it is desired to continue opera~
tion with this relatively high recycle rate and still obtain the maximum degree of chemical oxygen demand removal from the waste water, it is helpful, after withdrawing the recyele as before, to pass the net filter outfall ti.e.~ that portion which is being discharged rom the system as distinguish-ed from that portion which is being recycled) through a finishing treatment, -`
which can be either a once-through second anaerobic stage (which will in- ~
herently have a suitably bufered inlet liquid) or else through a stage of `~`
conventional aerobic treatment as generally employed in the aerobic art.
At even highar anaerobic filter liquid velocities, e.g., at linear - 2~ -~ ' :
- .

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liquid velocities thr.ough the filter in exçess of about one foot per minute detachment of biomass from the surface of the packing may become ~
noticeable, indicating that the liquid throughput rate i8 too high to allow retention of an optimal bioma~s and that the lilquid velocity should be

5 reduced. It i9 to be underqtood that the ma~imum liquid velocity which the filter can tolerate before hydraulic erosion of the biomass occurs will vary somewhat with the type of packing. ~ -EXAMPLE III
An anaerobic filter as described in Example I was acclimate:d to~
10 digest the synthetic substrate solution described in Table I above. When the digestor wa8 properly acclimated as shown by it~ operation at avirtually constant pH value in the outfall and with a sub~tantially cc>nstant ratio of off-ga~ volume to sub~trate throughput, the initial synthetic sub~trate feedstock was gradually changed, over a period of approximately two week~, to 15 another synthetic substrate o known composition as set forth in Table III
: .
below:
TABLE III

Components Concentration, Grams per Liter Acrolein 1. 56 Acrylic acid 2. 00 Ethyl acrylate 1. 67 Maleic acid 2~34 Glyce rine 2 . 5 6 Urea 4. 80 ~ ~

Ammonium aulfate(a) ~ ~ - 1. 60 ~ ~ -P~hoqphoric acid,~ 85% ~ 2. aa : :

Sodium bicarbonate 4 Water remainder ~a) Intended to provide both N as nutrient and S for heavy rnetal~

control.

. .
, ~0~2~
The above-described suhstrate contains one gram per liter~of carbon from each of the named organic compounds comprising the mixtur~.
AB can be seen, this mixture contains acrylic a~id and ethyl acrylate, ~which have appreciable toxicity toward microorgani~ms, a~ well a~ acrole~ln, 5 which i9 very toxic and is in fact sometimes ennployed as~a bioclde. It waa found that the anaerobic filter, operating with this rather refractory feed-stock but with the effluent recycle as described in Example I produced,~ wi~h a four-day retention time within the filter, an outfall which averaged approx-imateiy 600 mg per liter of total organic carbon, which i9 an amount~of or~
10 ganic carbon which is les~ than that contributed by any of the individual compounds in the feed mixture. Gas chromatographlc analysis for volatllè~
components in the outfall showed that acrolein, acrylic acid, and e~hyl acrylate (all o which normally affect microorganisms adversely~ were each present in the filter effluent or outfdll al; less than 0. 01 weig~ht percent 15 ~ concentration. This indicates an efficiency of removal of these compounds of greater than 95~ during the digestion process.
When, with the anaerobic filter operating on the above-de~scribed feedstock and according to the method set forth in Example I, the effluent recycle was discontinued with resulting conversion of the digestion process~
20 to the once-through mode of operation characteristic of the prior art, off gas production decreased almost immediately and was virtually nil after twenty-four hours. Re-starting of the dig~stion by simply resuming the~
effluent recycle mode of operation was not possible, indicating that the ~ -biomass in the~filter had been de~troyed. Resumption Qf normal operation . .
: .
- 25 required remov`al of the overly-concentrated aqueous liquid from the filter, ~ollowed by re-innoculation with a live biomass and resumption of the~

~ ~ ~ recycle type of operation. With wa~ts waters which are, without dilution, ; ~ toxic-to the anaerobic b~omasd it is thus indicated that it is essential to ~ - -Z6-.
' ~, ~

- ~ 3 either maintain continuous dilution at the inlet of the fiiter or digestor by ~, employing either the effluent recycle or else an external sourc~ of diluent water (in which latter case buffering chemicals and/or bacterial nutrlents ~;
mU9t al90 be supplied externally). Otherwi~e, it ia neces~ary to 3top the . ~: : J.
S flow of the waqte water into the filter or digestor until the dilution can~be resumed. It will be understood, of cour~e, thZLt de3tructioD o~ the ~ aD~
by undiluted toxic waste water i9 not nece~sarily instàntaneous; very bri`ef ~ . .
contact, e. g., contact of up to a few hours, may not always destroy the~

biomass beyond the point of revivabillty.

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Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a process for the anaerobic biological treatment of waste.
water containing organic pollutants which are of such a nature as to be amenable to anaerobic digestion, said process comprising passing said waste water through an anaerobic digestor or filter containing a biomass consisting essentially of microorganisms capable of digesting said pollutants while converting said waste water to a purified effluent having a reduced content of said pollutants, the improvement which comprises recycling to the inlet of said anaerobic digestor filter, and admixing into said waste water entering said digestor filter, a portion of said effluent in such an amount that the content of said pollutants and any biostatic or biocidal species which are present in the resulting mixture of recycled effluent and waste water is below that level above which there results inhibition of the growth processes of said microorganisms.

2. The improvement of claim 1 wherein said anaerobic digestor or filter is an anaerobic filter comprising a tank or vessel packed with solid support means upon which said biomass is permitted to grow, and wherein the rate of recycle of said effluent: is controlled below that at which the velocity of diluted liquid passing through said filter is high enough to sweep said biomass away from the surfaces of said support means.

3. The improvement of claim 1 wherein said process includes.
the step of adding a pH-buffering reagent to the waste water entering said digestor or filter to adjust the pH at the filter inlet to about 6.0 to about 8.0 and wherein, after admixing said recycled effluent liquid into said waste water, the rate of addition of said buffering reagent is reduced by an amount stoichiometrically corresponding approximately to the amount of buffering moiety contained in said recycled effluent.

4. The improvement of claim 3 wherein said pH-buffering reagent is acidic and wherein, in addition to admixing said recycled effluent into said waste water, carbon dioxide is also absorbed therein to provide acidic pH-buffering moiety.

5. The improvement of claim 4 wherein said carbon dioxide comprises carbon dioxide from the gases evolved from said anaerobic digestor or filter.