use of the upflow sludge blanket (usb) reactor

7/30/2019 Use of the Upflow Sludge Blanket (USB) Reactor

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Use of the Upflow Sludge Blanket (USB) ReactorConcept for Biological Wastewater Treatment,Especially for Anaerobic Treatment

G . L E T T I N G A , A . F. M . V A N V E L S E N , S. W . H O B M A , W . DEZ E E U W , a nd A . K L A P W I JK , Depar tment of W ate r Pol lut ionControl , Agricultural Univers i ty , Wa geningen , 6703 BCWa g e n i nge n , l h e Ne t her l and s

SummaryIn recent years considerable effort has been made in the Netherlands toward thedevelopment of a more sophisticated anaerobic treatment process, suitable for treat-ing low strength wastes and for applications at liquid detention times of 3-4 hr. Theefforts have resulted in a new type of upflow anaerobic sludge blanket (UASB)process, which in recent 6 m3 pilot-plant experiments has shown to be capable ofhandling organic space loads of 15-40 kg chemical oxygen demand (COD).m-Ydayat 3-8 hr liquid detention times. In the first 200 m3 full-scale plant of the UASBconcept, organic space loadings of up to 16 kg COD.~n-~/dayould be treatedsatisfactorily at a detention time of 4 hr, using sugar beet waste as a feed. The mainresults obtained with the process in the laboratory as well as in 6 m3 pilot plant and

200 m3 full-scale experiments are presented and evaluated in this paper. Specialattention is given to the main operating characteristics of the UASB reactor concept.Moreover, some preliminary results are presented of laboratory experiments con-cerning the use of the USB reactor concept for denitrification as well as for the acidformation step in anaerobic treatment. For both purposes the process looks feasiblebecause very satisfactory results with respect to denitrification and acid formationcan be achieved at very high hydraulic loads (12 day-') and high organic loadingrates, i.e., 20 kg COD.~n-~/dayn the denitrification and 60-80 kg C O D ~ r ~ / d a ynthe acid formation experiments.

INTRODUCTIONIn recent years energy cons idera t ions and envi ronmenta l con-cerns have increased the interes t in direct anaerobic t reatment of

industrial w astes . Th e anaerobic method of waste t reatme nt offers ,under the present c i rcumstances, a nu mber of significant advantageswith l i t t le ser ious or insuperable drawbacks over other t reatmentmethods. Benefits and limitations of the process have been sum-marized in Table I . In spi te of i ts favorable prospects and i ts presentBiotechnology and Bioengineering, Vol. XXII, Pp. 699-734 (1980)@ 1980 John Wiley ti Sons, Inc. 0006-3592/80/0022-0699$0 .OO


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700 LETTINGA ET AL.TABLE IBenefits and Limitations of Anaerobic Treatment of Wastewaters

Benefits Limitations1)2)

Low production of waste biologicalsolidsWaste biological sludge is a highlystabilized product that as a rule canbe easily dewatered3) Low nutrient requirements

4) N o energy requirement for aeration5 ) Production of methane, which is a

useful end product6) Very high loading rates can be ap-plied under favorable conditions7) Active anaerobic sludge can be pre-served unfed for many months8.*o

1) Anaerobic digestion is a rather sen-sitive process, e.g., the presence ofspecific compounds, such as CHC13,CCI,, and CN-

2) Relatively long periods of time arerequired to start-up the process, asa result of the slow growth rate ofanaerobic bacteriaAnaerobic digestion is essentially apretreatment method; an adequatepost-treatment is usually requiredbefore the effluent can be dischargedinto receiving waters

4) Little practical experience has beengained with the application of theprocess to the direct treatment ofwastewater

3)

significance for the stabilization of sludge from municipal sewageworks, anaerobic digestion still is not a generally well-acceptedmethod of waste treatment, in particular not for the treatment oflow-strength wastes. The major obstacle to the application of theprocess for this specific purpose is perhaps the difficulty of extend-ing it to a stable and simple operational form. However, consider-able progress has been recently made in the Netherlands in thisrespect through the development of a more sophisticated form ofthe upflow sludge blanket ( U S B ) concept. Therefore an extensivefull-scale application of the process in the near future may now beexpected.

The main objective of this paper is to present and evaluate themost relevant results of recent pilot-plant and full-scale experimentswith the USB process for anaerobic treatment in the light of earlierresults obtained with this process in the laboratory for anaerobic

for the separate stage of acid formation, and for den-itrification. Detailed information on the pilot-plant and full-scaleexperiments has been presented in separate reports 5-8

ANAEROBIC WASTE TREATMENT METHODSThe loading rates permissible in an anaerobic waste treatment

process are primarily dictated by the sludge retention in the anaer-


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U SB REACTOR FOR WASTE TREATMENT 701obic reactor. The maintenance of a high sludge retention time (SRT)has been-at least until recently-the major problem in the practicalapplication of the process, especially for wastes with a chemicaloxygen demand (COD) below about 3000 mg/liter.

Obviously a waste treatment process for low-strength wastes isan economical one if large volumes of waste can be forced throughthe system in a relatively short time period. For this purpose pro-cesses are required in which the biomass retention time can becontrolled independently of the wastewater flow rate. Conventionalanaerobic treatment processes of the flow-through type are there-fore inadequate to .treat low-strength wastes. The solution for thebiomass retention problem resulted in the development of differentanaerobic treatment processes. These systems have been schemat-ically presented in Figure 1.

The essential feature of the anaerobic contact process is that

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702 LETTINGA ET AL.the w ashout of the act ive anaerobic bacter ia l mass f rom the reac toris controlled by a sludge separation-and-recycle sys tem . Th e majorproblem in the practical application of the contact process hasalway s been th e separat ion (and c oncentrat ion) of the s ludge fromthe eff luent solution. Fo r this purpo se several metho ds have beenused or were recommended for use , e .g. , pla in sedimenta-tion, 11-13,18--20 settl ing combined with chemical f locculation, withvacuum d e g a s s i f i c a t i ~ n , ~ ~ - ~ ~ ~ ~r eve n with f lota t ion,and centrifugation. 13,17 A basic idea underlying the co ntact processis that it is cons idered necessary to thoroughly mix the diges terco nte nts , e.g . , by ga s recirculation, s ludge recirculatio n, o r contin-uo us o r intermitten t mechanical agitation.

A som ewh at modified vers ion of the co ntact process is based onan upward movement of the l iquid waste through a den se blanketof anaerobic s ludge. This sys tem was f i rs t descr ibed by Cou lter e taLZ3n S ou th Africa , extens ive s tudies on the upflow con tact proc-ess have been made, and the process was successful ly applied bymeans of a reversed-flow modified DorrOliver Clarigester for thetreatment of glucose-s tarch,24~2 6ine distillery ~ a s t e , ~ ~ , ~ ~nd yeas twastewater .26The reactor was equipped to a l low an external recirculat ion ofsludg e, e.g., via the central com partm ent of the clarifier, in ord erto facili tate the return of settled sludge from the clarif ier back tothe diges ter com partmen t . Fo r the sam e reason a scraper has beeninstal led a t the bo ttom of the c larifier . Th e raw feed w as introdu cedvia adjus table inle ts s i tuated around the lower per imeter of thedigester compartment and via a rotating central-feed pipe with adis tr ibutor a t the lower par t of the d iges te r com par tmen t . The max-imum loading rate applied with wine distillery waste was 3 .2 kgC O D ~ n ; ~ /d a y a t a 6 .9 days de tent ion time and a t a tempera ture of33C. The CO D reduc t ion a tta ined under these c i rcumstances was97.3%. The primary limitation of the process with respect to theloading rate was shown to be the loss of s ludge in the clarif iedeff luent . Therefore the s ludge re tent ion was considered to be theprimary limitation of the process. The maximum loading rate per-missible without undue loss of sludge was 3.2 kg C O D * ~ n . - ~ / d a ytan operat ing temperature of 30C. An average total solids (TS)concen trat ion of about 25 g/liter could b e m aintained in th e reac torunder these c i rcumstances.A rather promising d evelopm ent is the anaerobic f il ter (AF ) proc-ess . T his sys tem s imply con sis ts of a vertical filter bed filled withan iner t suppor t m aterial such a s grave l , rocks , co ke , o r som e


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USB REACTOR FOR W ASTE TREATMENT 70 3plastic media. Laboratory and pilot-plant e x p e r i r n e n t ~ ~ ' - ~ ~ , ~ ~ , ~ ~aveshown that the AF process is suitable to treat various types ofchiefly dissolved wastes with a very satifactory treatment efficiencyat high hydraulic and organic loading rates. The SRT of the AFprocess is very satisfactory, which may be attributed to the gradualdevelopment of a highly settleable-more or less granular-sludge.This sludge is effectively entrapped in the packing.

In our laboratory we have made similar observations on experi-ments using methanol solutions,31 potato starch and so-lutions of volatile fatty acids (VFA). The process was found partic-ularly suitable for the treatment of wastewaters from potato starchplants, i.e., the process wastewater (composed of diluted potatosap) as well as the wash-and-tranport water. Small pilot-plant ex-periments with 150 cm high and 14 cm diam coke-packed columnsusing potato-sap solutions as feed indicated that the process wasfeasible to handle organic loads up to 10 kg COD.~n,-~/day t 30Cand could withstand hydraulic retention times as low as 10 hr.Moreover, it was found that shock loads up to 17 kg COD.~n,-~/daywere also fairly well accommodated.

Despite the good results obtained with the AF process, the furtherdevelopment of this concept has been abandoned in the Netherlandsin favor of the upflow anaerobic sludge blanket (UASB) process.

The remainder of this paper will be confined to a discussion ofthe features and prospects of the USB concept for the purpose ofanaerobic treatment in particular. Results of small (laboratoryphase) and large pilot-plant studies as well as of the first full-scaleexperiment will be considered.

USB CONCEPTThe USB process for anaerobic treatment resembles the USB

processes described in the l i t e r a t ~ r e , ~ ~ - ~ ~ , ~ ~xcept that: a) sludgerecirculation and/or mechanical agitation are kept at a minimum oreven completely omitted, and that-in particular-, b) the reactoris equipped in the upper part with a proper system for gas-solidsseparation. A schematic diagram of an UASB reactor is shown inFigure 1 .The basic ideas underlying the process are

a) The anaerobic sludge obtains and maintains superior settlingcharacteristics if chemical and physical conditions favorable tosludge flocculation and to the maintenance of a well flocculatedsludge are provided.


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704 LETTINGA ET A L .b) A sludge blanket (bed) may be considered as a separate-more

or less-fluid phase with its own specific characteristics. A well-established sludge blanket frequently forms a rather stable phase,capable of withstanding relatively high mixing forces. The redis-persion of the sludge in the liquid phase therefore may require asignificant amount of mixing energy.

c) The washout of discrete sludge particles (flocs) released fromthe sludge blanket can be minimized by creating a quiescent zonewithin the reactor, enabling the sludge particles to flocculate, tosettle, and/or to be entrapped in a secondary sludge blanket (presentin the settler compartment).

Laboratory ExperimentsThe USB concept has been extensively investigated in the labo-

ratory with reactors varying in volume from 1 to 60 liter and inheight from 0 .2 to 1 m. The potential feasibility of the USB concepthas been demonstrated for anaerobic treatment, for the separatestage of acid formation, as well as for denitrification.

Anaerobic treatmentThe USB concept has been investigated in our laboratory since

1971 for the one-step anaerobic treatment of a variety of indus-trial wastes, including wastes in which the acid fermentation wasalready complete or in a more or less progressed stage. The mostrelevant results of these laboratory-phase experiments, P4 are con-tained in Table 11. Part of these experiments has already beendescribed in detail previously. The maximum organic space loadingsthat could be achieved were between 10-14 kg COD-m-3/day cor-responding to maximum sludge loads in the range of 1 kg COD/kgVSS/day (VSS =volatile suspended solids).

Considerable emphasis has been given in the laboratory phase ofthe study to the start-up of the process with digested sewage sludgeas seed. Evidence was obtained that the first start-up of the processis predominantly important with respect to both the specific activityand the settleability of the sludge that develops in the reactor.Although the investigations are continuing in our laboratory,some important directions for the procedure to be followed in thefirst start-up should be mentioned here, viz.:1 ) The initial sludge load should be below 0.1-0.2 kg COD/kgtotal solids (TS)/day.


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706 LETTINGA ET A L.2) The loading rate of the reactor should not be increased, unless

all volatile acids present or formed are effectively decomposed.3) The environmental conditions for growth of the anaerobicbacteria should be favorable.In following these directions within 6- 12 weeks after the start-up,

sludge loads exceeding 0.5 kg COD/kg VSS/day can be handledsatisfactorily for most of the wastes investigated at temperatures ofabout 30C. Moreover, also the development of a well-settlingsludge will be promoted in such circumstances. However, in ignor-ing the directions mentioned above, a rather voluminous sludge mayresult that is also relatively low in specific activity. Such a sludgetype was formed especially in the experiments with the unsouredsugar-beet sap solutions. Together with the results of the experi-ments with soured sugar-beet sap solutions, these results indicatethat the two-phase concept for anaerobic treatment, as proposed byGosh et al.36 and Pohland and G h ~ s h , ~ ay be an attractive prop-osition. Significant higher loading rates could be applied with souredthan with unsoured sugar-beet sap solutions. Hence, even in thetreatment of relatively simple wastes of the type investigated, theintroduction of a separate acid-forming reactor may be justified.Evidently, this can be accomplished relatively easily by employinga holding period prior to treatment.Acid fermentation

The significance of the two-phase concept is reinforced by theevidence obtained from some preliminary souring experimentswith sugar-beet sap solutions, in which an U SB reactor has beenused for the acid fermentation step. From the results of these ex-periments-contained in Table 111-it appears that an almost com-plete souring is achieved at space loading rates up to 70 kgCOD-m-3/day and detention times as low as 3 hr. These loadingrates could be applied as the result of the high settleability (i.e.,SVI: 15-20 ml/g) and-presumably-of the high specific activity ofthe sludge formed. The main part of the sludge consisted of granulesof about 1-3 mm in size.

Comprehensive studies on the acid fermentation step are beingpresently undertaken by Zoetemeyer and Cohen at the Universityof Amsterdam.Denitrification

The USB concept has been investigated in a number of experi-ments in our laboratory for its feasibility for denitrification; acetate


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708 LETTINGA ET A L .solut ions a s well a s an a lcoholic w aste have b een used a s the carbonsou rce in these exper im ents . The m ost re levant resul ts obtained inthe exper iments wil l be mentioned here , because they are a lsoappropr iate to the use of the U SB process for anaerobic t rea tment ,such a s to the ques t ion wh ether mechanical agi ta t ion is required o rnot a t high hydraulic load s and/or a t low organic loads . T he ex per-iments have been conducted in 24 and 41 l i ter reactors both ap-proximately 1 m in height . The s ludge bed compartment of thereactor was equipped with a centra l axis , carrying three pi tched-blade turbines in th e 24 l iter re acto r and th ree anchor- type impellersin the 41 l i ter reactor. The main results of these experiments arecontained in T able IV. Th e performance of the proc ess with respectto n i t ra te and/or C O D reduc t ion was very sa t is factory , even a t theextremely high organic and hydraulic loading rates applied. Againthis should mainly b e attr ibute d to the form ation of a highly settle-able ( i .e . , SV I values approach ing 20 ml/g) and active sludge. Withthe alcoholic waste a gran ular s ludge (pellets of 1 to 3 mm in diam)develop ed in the cou rse of 6-8 weeks .In order to ensure suff ic ient contact between s ludge and feedsolution-and he nc e a satisfactory op eration with respec t to deni-tr if ication an d/o r COD-reduction-it app eared nec essary to exerc isea mo derate-intermittent o r continuous-mech anical agitation inthe sludge bed, particularly at high hydraulic loading rates and atlow gas production ra tes . In this way the formation of cracks andcanals in the sludge bed could be effectively combatted.A s table , a lthough exp and ed, s ludge bed could be maintained a tsuperficial velocities as high as 4 d h r . M o r e o ve r, in s p it e of t hemechanical mixing, the sludge retained its high settleability.In the anaerobic expe r iments mechanical s t ir ring was a lw ays keptat a minimum, viz. never exceeding 1 min at 10-30 rpm every 10min. Because of the rather fragile character of anaerobic s ludgef locs , as com pared to denitri fying s ludge, a more intens ive mechan-ical agitation w as con sidered to b e detrimental for the sludge reten-tion in the reactor. Moreover, in view of the treatment efficiencyobtained and the loading ra tes applied a t that t ime, there was noser ious need in these par t icular exper iments for a more intens iveagita tion than th at brought a bo ut by the gas production.

Pilot-Plant and F ull-scale Stu dies with the U A S B Pro c e s sT he firs t pilot-plant ex perim ents hav e been carrie d out with sugar-beet wastes (see Table V for the main character is t ics) in c losecooperation with the CSM sugar-beet company. Initially a 6 m3


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USB REACTOR FOR WASTE TREATMENT 71 1reactor was used. Figure 2 shows a schematic diagram of the re-actor . In the seco nd s tage a 30 m3 reacto r twice a s high a s the 6 m3reac tor (6 instead of 3 m) was tested, in order to obtain additionalda ta for the design of a full-scale plant. Based o n th e informationthus obta ined, a 200 m3 full-scale plant (4.5 m high) was designedand built. T he full-scale plant (F ig. 3) was put in t o opera t ion a t theend of Septem ber 1977.On ce the exper im ents with sugar-beet waste had been com pleted,th e 6 m3 pilot plant was employed to investigate the anaerobictreatment fo r po tato processing waste (see Table V ). These exper -im en ts h av e b ee n re ce nt ly ~ o m p l e t e d . ~In the Nether lands a number of pilot-plant s tudies are runningpresently or will be started in the near future, e.g. , with distil leryw astes , vegetable canning w astes , and sewage. S om e of these s tud-ies (as was the case for the exper iments with sugar-beet wastes)w ere mad e possible through grants of the M inistry of Env ironm entalProtection. In the next sections the most relevant results obtainedwith suga r-beet and potato-processing wastes will be discussed an deva luated in the light of the exp erienc e gained during the lab orato ryph ase of th e investigations.Start-up

T h e 6 m3 pilot-plant exp erim ents with both sugar-beet w aste andpotato-processing wa stes we re started up with sludge obta ined froma municipal digester . In view of our laboratory experience muchattent ion was paid to the s tar t-up procedu re, viz . ca re was taken tofollow a s s tr ictly a s possible th e directions given ab ov e. In accordwith the laboratory experience the results indicated that with bothtypes of waste a well-adapted sludge could be obtained within aperiod of 8-12 weeks . T hen sp ace loadings up to 10 kg C O D . I ~ - ~ /day could b e handled a t 30C and hydraulic lo ads up t o 3 m3-m-3/day .T h e 30 and 200 m3 reacto rs were see ded with well-adapted sludgefostered during the former experiments, and no difficulties wereencou ntered to s tar t the process . Conformably the 200 m3 reactor-seeded with approximately 1800 kg sludge TS (84% VS)-couldalready diges t a COD load of 8.5 k g C O D * ~ n - ~ / d a yfter only 14day s of operat ion. Th e s tar t-up af ter a feed interruption of even afew weeks never gave any dif f icul ty; the gas production a lwaysremained a t the desired level as measu red a few hou rs af ter feedingwas resumed.


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U SB RE A CT O R FO R WA ST E T RE A T M E N T 713

Fig. 3. First full-scale UASB plant (200 m3), installed and tested in 1978 at theCSM sugar-beet factory in Halfweg.

Loading rates and treatment efficiencyA s app ears from the resul ts presented in Table V I, exceptional lyhigh organic and hydraulic loading rates were possible in combi-nation with satisfactory treatment efficiencies. Moreover and per-hap s the more interesting, this ability was ob served for both so ured

and a lmos t unsoured was tes . Th e reason for the modera te trea tmentefficiency c orresp ond ing to the sugar-beet campaign w astew aterprobably was the presence of a not unsignificant quantity of finelydispe rsed and poorly biodegradable m atter in this w aste , viz. rang-ing in the or de r of 10-20% of the COD. Only 20-25% of this matterwas retained in the reactor.The process accommodates fair ly well to hydraulic and organicshock loads , temperature f luctuat ions , and low inf luent pH values ,provided the d iges ter pH remains well abov e pH 6.0 and that thesludge load applied is below the maximum specific COD removalrate of the sludge at the tem peratu re prevailing in the d igester .From the resul ts presented in Figure 4,which were obtained withthe liquid sugar waste, i t appears that a low influent pH value didnot ha ve an y de tectab le harmful effect. Th e results in this f igure doreveal the ad ve rse effect of a nutrient deficiency, i.e. , phosphate in


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17/36

USB RE A CT O R FO R WA ST E T RE A T M E N T 715

I n

260 284 308 332 d o y mFig. 4. Results of the 6 m3 pilot-plant experiments with liquid sug ar wa ste a s feed(influent CO D: 2500-5500 mgfliter). N utrients were no t supp lied until day 271, butbeyond day 271 an amount of 6-7 mg P0,3--P/liter wa s supplied to the influentsolution. Low influent pH values @Hi) uring various periods of the experiment

resulted from in terrupt ions in the supply of NaHC O, (Na2 C0,) to the feed solutions.

this case. The response of the process upon resuming the supply ofphosphorus was immediate.

Consistent with the laboratory experiments it has been found thatsubstrate degradation occurred for 80-90% in the lower part of thereactor. Similar observations have been made with the anaerobicfilter.27,29*32*33n important reason for this phenomenon is the pres-ence of a high concentration of active anaerobic sludge in the lowerpart of the reactor. A second main cause is the effective mixing(due to the upward escape of the produced gas) of the incomingwaste flow with the partially purified liquor present in the upperpart of the reactor. The occurrence of colloidal particles and othersuspended solids in specific wastes, and the precipitation, sedimen-


18/36

71 6 L E T T IN G A E T A L .ta t ion, and/or entrap m ent of such undissolved matter are factors ofimpo rtance to an effic ient subs tra te removal in the bottom par t ofthe reacto r . These kinds of physical processes hav e been o bservedin our laboratory and elsewhere with var ious wastes , i.e., pota tos tarch wa stes , dairy w astes , sewage, e tc . To what exten t absorpt ionof dissolved com poun ds may con tr ibute to the C O D removal isinsufficiently known as yet.Sludge characteristics

One of the principal features of the USB process certainly is i tsquali ty to en han ce t he de v e l opme n t of a sludge with a high speci f icactivity and superior set t ling pro pert ies . Th e feasibility of the U SBreac tor concept for anaerobic t rea tment , as demons t ra ted in thelaboratory experiments, has not only been confirmed by the pilot-plant and full-scale experiments, but in fact appreciable better re-sul ts were obtained dur ing this s tage of the s tud y. So i t was foundthat s ludge loads could b e applied ranging from 0.5-0.8 kg COD/kgVSS/day for sug ar-beet wastes to I .O- I .4 kg COD/kg VSS/day forpotato-process ing wastes . M oreover , it was sho wn in the exper i-ments with potato-processing waste that such loading rates werepossible with almost unsoured wastes, viz. with 10% of the totalCO D conve rted to VFA-COD a t the maximum.Conforming to the results o btained in the laboratory U SB exper-iments with denitr if ication and acid ferm entatio n (acido gen esis) , i twas now found tha t in due co urse a s ignif icant par t of the anaerobics ludge occu rred in a granular form a s well . This appeared to be thecase with both the sugar-beet and potato-processing wastes. Thesettleability of the gran ular sludge was som ew hat better in com par-ison to the "flocculated" sludg e, viz. SVI,, , , = 10-20 mlig andSVIf l , , = 20-40 mlig. Accordingly the granular s ludge occurredmainly in the low er regions of the sludge bed. So m e scanning elec-tron micrographs (SE M s) of granular sludge are sho w n in Figure 5 .From exper iments car r ied out and/or underway in the Nether -lands with a var ie ty of wastes , we o btained evidence that anaerobicsludge as a rule ex hibits fairly satisfa ctory flocculation chara cter-is t ics, provided th e f i rs t s tar t-up has been careful ly made and thatthere is no n utrien t deficiency. M oreo ver, the f locculation ability ofthe s ludge was shown to depend on the occurrence of divalentcation s (e.g . , C a2+ ) and of f inely dispe rsed, poo rly f locculating,matter in the waste. Calcium ions have an evident positive effecton the f locculation ability of anaerobic s ludge, presumably mainlybecause they improve the mechanical s trength of the f locs . The


19/36

USB REACTOR FOR WASTE TREATMENT 717

Fig. 5 . S E M s of the surface of an an aerobic sludge granule from the 6 m3 U A SBexperiments with potato processing waste (3000 X ) (TFDL, electron microscopy,Wageningen).

effect of calcium is clearly illustrated in Figure 6, which showssome relevant results of the ex per imen ts with po tato waste. 'T he results in Figure 6 show th at the sys tem respon ded within afew day s af ter NaH CO , was subs t i tuted by Ca(OH), (7- 12 mequivlliter) as supplied buffer to the wastewater, i .e . , the SV I values ofthe s ludge decreased and at the same t ime a comparably sharpincrease of the s ludge concen trat ion occurred in the lower regionsof the reac tor . An even m ore pronou nced effect of calc ium has beenfound recently in exper im ents with a distillery w aste , i.e. , co nsistingof an alcoholic solution in almost salt-free water; here a similarsubstitution resulted in an in crease of the sludge SVI from 100-150to 30-40 ml/g within a few weeks.39


20/36

718 LETTINGA ET AL .

S V I (rnl/g)OD-load (kg/rn:/doy) x x50-40

20

10-0 . I

,-- +space load+ -

25

20

15

10

I1

0100 150 200 250 300 350 400 450d a y no

I

Fig. 6. Results of experiments in the 6 m3 UASB pilot plant with potato pro-cessing waste.' COD reduction vaned from 70-90% on the basis of raw effluentsamples, from 85-98% on the basis of centrifuged effluent samples, and from 85-95% on the basis of settled (3 0 min) effluent sam ples (compare Table VI) .

Some additional settl ing tests with digested sewage sludge havebeen conducted in order to il lustrate the effect of the presence ofdifferent con centra tions of Mg2+,Ca2 +, nd Ba2+salts in the sludge.A s an index for the f locculation ability of the sludge w e hav e usedthe a mo unt of s ludge remaining in the sup ernata nt . The resul ts inTable VII clearly indicate the positive effect of these salts. As theexper iments were conducted a t a pH of about 6.0, the effect shou ldbe attr ibu ted mainly to the ionic forms of these e lemen ts and not toa clarification brought about by the precipitation of CaCO, andBaCO,. Lik ew ise, the im prov em ent of the sludge settleability in theU A S B exper iments as a resul t of the presence of calcium in th ewaste certainly should not be exclusively attr ibuted to a possibleen t r apment of precipi ta ted CaCO, in the s ludge; the as h con tent ofwell-settling anaerobic sludge is striking low!


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USB REACTOR FOR W ASTE TREATMENT 719Finely dispersed, poorly f locculating, matter exerts an adverseeffect on th e f locculation of the sludge, and therefo re on the sludge

retentio n of the reacto r. This has been particularly ob served in thelaboratory experiments with a s imulated closed sugar-beet waste-water c ircui t .2 Th e susp ended matter accumulating in the c ircui t ,which pro bab ly m ainly co nsisted of acid-forming orga nism s, greatlyham pered th e reflocculation of disintegrated sludge flocs especiallyat concentrat ions exceeding approximately 1000 mg/liter. Similarindications h ave b een o btained in the 6 m3 pilot-plant ex pe rim en tswith su gar-b eet campaign wa ste. H ere the dispersed organic ma tterreached concent ra tions up to 1500 mg/liter at the end of the cam-paign. N either in the la bora tory nor in th e pilot-plant o r full-scaleexp erim ents was this organic matter retained in the rea cto r for morethan 20-25%. As a rule sugar-beet campaign waste a lso containsvarying quantities of dispersed inorganic matter (mainly clay); oc-casionally con centra tions exceeding I g/liter may o ccu r. This matte ris retained t o som e exten t in the reacto r. T he reten tion of inorganicsolids is clearly reflected in the trend of the volatile solids contentcu rve of the sludge as il lustrated in Figure 7 .A s shown in Figure 7 a sha rp decre ase in the frac tion of volatilematter of the s ludge occurs beyond day 80. This was caused by theexceptionally high quantity of inorganic sludge solids (SS) in the

1974 campaign wastewater , viz . 0.5-1.5 g/liter a t the a verag e withpeaks during some days occasionally reaching values up to 30 giliter.In the 1975 campaign wastewater (period 436-524) the quant i ty

TA BLE VIIResults of Settling Tests with an Anaerobic Sludge Conductedin the Presence of Increasing Concentrations of DivalentCationsa

SS in supernatant solutionCation Conc. 10 20 40 100added cation added (mgiliter)

800- 000

Mgz+ 750 680 580 580CaZ+ 480 365 140 120BaZ+ 550 350 160 30a The numerical values represent the sludge concentration (in

mg SS /liter) rem aining in the sup ernata nt after 45 min of settling(sludge TS: 50-60 giliter).


22/36

720 LETTINGA ET A L .type of wastet" 'b" I ~';;, ' ' I i i i ! ,~~~iII J' I C mm

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-x- -x-rs ludgenreac tmY skidge in effluent-%< -x & L x /

coops'4 0o o O - 1 "0xo

of ash content of the s ludge remains a t about the same leve l. T heresul ts obtained thus far in the var ious exper iments indicate thatthe accu m ulatio n of clay in the sludge will be limited, as long as th esludge mainly co nsists of gran ular material and th e q uan tity of clayin the wastewater is below 0.25 g/liter.Dynamic behavior of the sludge blanket (bed)

The presence of the different types of s ludge is reflected in TSprof i les determined over the height of the reactor . Some prof i lesare sh ow n in Figure 8 . The specific shape of a profile, which is nomore than an ins tantaneous shot , depends s trongly on the pa t te rnof hydraulic and organic loading during a longer or shorter periodpreceding th e sampling mom ent. Many ho urs o r eve n days maypass before a stea dy profile is reestablished on ce the loading ratesand/or othe r factors affecting the s ta te of the s ludge bed hav e beenaltered. Although th e s ludge bed may be co mp osed of tw o o r moredis t inct par ts , the blanket as a whole should be considered as acoherent dynam ic unity , even if i t extends within the set t ler com-par tment .At low loading ra tes the s ludge appe ars to be a lready markedlyexpanded, mainly as a resul t of occ luded gas . As w as observed ,


23/36

U SB R E A C TO R F O R W A S T E T R E A T M E N T 72 1particularly in the small pilot-plant experiments conducted in thelaboratory, the occluded gas occasionally causes smaller or largerparts of the bed to move upwards, inducing a more or less pulse-like liberation of the entrapped gas and a local turnover of thesludge bed (vertical mixing of the sludge). At increasing gas pro-duction the gas eruptions succeed each other more rapidly, givingthe surface of the sludge blanket somewhat of the appearance of aboiling fluid, ejecting gas bubbles and sludge particleshlocs. Thehalf-freed particles eventually become liberated from the blanketthrough the tearing forces of fast upward moving separate gas bub-bles. They are kept dispersed in the liquid bulk above the sludgeblanket as a result of the agitation caused by the gas bubbles. Whenthe concentration increases here the reflocculation to larger-moreeasily settling-flocs may gradually improve, ultimately leading toa balance between sludge ejection from and entrapment in the sludgeblanket. The gas-solids separator has an important function in thereflocculation of the sludge. This point will be discussed below.Sludge retention in an UASB reactor

The sludge retention of an UASB reactor primarily depends onthe sludge settleability. In turn, the sludge settleability is closely

he ight (rn)&y 411 d ay 424

HRT = 61.0

0Mt=128

0 2 0 4 0 6 0 0 2 0 4 0 6 0day 430

2.0 HRT = 3.91.0

0 Mt 149 Mt=2050 x f 4 0 60 0 20 40 60sludge T.SA kg/rn3)

Fig. 8. So me T S p r o fd es as measured in the 6 m3 pilot-plant expe riments withpotato-processing waste^.^ (0)rganic spac e load applied (kg COD.m-Yday); H R T= hydraulic retention tim e (hr).


24/36

122 LETTINGA ET AL.connected to the waste characteristics (i.e., Ca2+ concentration,nature of pollutants, and presence of dispersed poorly flocculatingmatter), the start-up procedure followed, and the time past sincestart-up. For a given situation (reactor, waste, and sludge) thesludge retention is essentially determined by operational factorssuch as the hydraulic and organic loading rates.

The effect of the hydraulic loading rate on the sludge retentionis closely related to the agitation intensity in the sludge bed. In theabsence of mechanical mixing or mixing through gas evolution, thewastewater may find its way through the bed predominantly viacanals and cracks formed in the bed, leaving the thickened characterof the main part of the sludge unaffected. This has been demon-strated in the laboratory U SB reactor for denitrification as well asin adsorption experiments in a fluidized bed of flocculated activatedcarbon.40 n both cases mechanical agitation was required to ensurea sufficient liquid-solids contact. If mechanical agitation is applied,the sludge bed expansion will increase appreciably with an increas-ing hydraulic load, and consequently the quantity of sludge that canbe retained in the reactor will decrease.

In view of what has been discussed in the preceding paragraphsthe organic loading rate (gas production) is a rather crucial factorfor the sludge retention of the reactor. Already at low loading ratesa marked expansion of the sludge bed always occurs. There is littledoubt that at increasing organic load (and gas production), a con-traction of the bed will soon take place, although it is uncertain atpresent to what extent and at what load. The improved agitationbrought about by the increased gas production will result in a de-crease of the hold-up of entrapped gas and-even more important-will improve the thickening of the sludge. Moreover, also certainlythe formation as well as the erosion of granular sludge will beaffected by the gas production rate. At present the effect of the gasproduction on either of these factors is insufficiently known. Theresults obtained up to now-especially those obtained with potatowastewater-do not indicate that the sludge retention is adverselyaffected by the gas production. A s a matter of fact the contrary hasbeen found. This is illustrated by the results of the potato waste-water experiments, as summarized in Table VIII. In all situationsconsidered in Table VIII, the sludge bed was expanded (compareFig. 8) over almost the entire reactor height. Despite the exception-ally high loading rates applied, the data in Table VIII indicate thatthere is an evident increase in the amount of sludge retained in thereactor. Attempts to incorporate the effect of the hydraulic and


25/36

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26/36

724 LETTINGA ET ALorganic loading rates in a dynamic model are being made at theDelft Technical University by Heertjes and van der M e e ~ . ~ lSludge washout

Sludge washout is closely connected to the amount of finelydispersed sludge present in the reactor, and therefore to the factorsthat are important in this respect.

Some relevant data concerning the washout of sludge from the200 m5 reactor in the treatment of sugar-beet campaign wastewaterhave been summarized in Table IX.

Concerning the washout of sludge three situations should be dis-tinguished:a) The top of the sludge blanket remains well below the effluent

weir of the reactor. Under such circumstances sludge washout isconsiderably less than sludge accretion from growth. This is evenmore true because the sludge lost with the effluent generally willnot consist completely of bioactive matter. A considerable part ofthe effluent suspended solids may originate from suspended solidssupplied with the influent solution; this has been particularly ob-served for sugar-beet campaign waste. Moreover, strong indicationshave been obtained that a majority of the bioactive sludge can berecovered from the effluent by plain sedimentation.

b) The sludge blanket reaches the effluent weir under steadyloading conditions. In this situation the washout of sludge and theaccretion of sludge by growth will range over a similar order ofmagnitude per unit of time.

c) An excessive expansion of the sludge blanket may occur as aresult of a shock loading or due to suddenly deteriorating conditions(i.e., nutrient deficiency, high concentrations of finely dispersedpoorly flocculating matter, etc.). Under such circumstances a tem-porary drastic washout of the sludge may occur and last until a newsteady bed has been established.Gas-solids separator

Apart from some technical imperfections the performance of thegas-solids separator originally employed in the 6 m3 reactor (seeFig. 2 ) was fairly satisfactory. Nevertheless, considerable effortwas made to develop a more sophisticated device. However, as, infact, none of the alternative means proved to be more effective norwere simpler in construction, they will not be discussed here. Itshould be emphasized that no additional measures are required to


27/36

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28/36

726 LETTINGA ET AL.enhance the re turn of the s ludge f rom the set t ler back into thediges te r compar tment , provided tha t a number of condi t ions a refulfilled, viz.:a) The incl ined wall of the set t ler should be a t an angle of ap-proximately 50".b) The su rface load of the s et t ler should b e kept below ab out 0.7m/hr , and the average f low through the aper ture between the gascollectors below abo ut 2 m/hr.c) The development and maintenance of a well se t t l ing s ludgeshould b e enhanc ed by adequ a te measures.d) T he sludge presen t a t the liquid-gas interface in the gas col-lector should be kep t well imm ersed.e) Ex cess ive foaming in th e gas col lector should be prevented.Th e f irs t two condit ions can be readily met by selecting a prop erdepth-area ra t io for the reactor . Fo r low-s trength w astes this factoris primarily d ictated by th e restr iction s placed o n the surface load.For medium- and high-strength wastes the volume of gas releasedper unit of cross-sect ional are a may becom e the threshold cr iter ionat extreme high loading ra tes . However , in view of the resul tsobtained until now, li t t le if any difficulties are expected providedtha t loading rates exceed ing 20 k g . ~ n - ~ / d a yre avoided and tha t theheight of the reactor remains below about 10 m. In order to meetthe third c ond ition an app ropria te insight into the fa cto rs controllingthe settleability of the sludge is required. A s will be discussed inmo re detai l below , the bes t gu arantee for the form ation and main-tenance of a good sett ling s ludge is to kee p the co ndit ions forf locculat ion a s favorable as poss ible. T he pe rformance of the gas-sol ids separator is closely connected to the condit ions for f loccu-lation prevailing within the system, because its operation is basedon a com bine d proc ess of sedim entatio n, f loccu lation , and-if pres-ent-sludge bl an ke t filtration. A s a matter of fact the f locculat ion,sedimentat ion, and f i l t ra t ion processes a lready s tar t in or evenbelow the aper tu re between the gas col lector and rea ctor wall .In order to prevent any difficulty with buoying sludge the gas-liquid interfa ce in the g as collecto r should be k ept well s tirred. T hesurface area of the interface should be dimensioned so that en-trapp ed ga s can readily escap e. If not, buoying sludge will be p ushe dthrough the aper ture into the set t l ing compartment , which mayresult in a poor sedimen tat ion performan ce of this de vice and l it tlere turn of s ludge. I t mu st be em phasized that an effective separat ionof entrapped o r a t tached gas f rom the s ludge f locs , before the s ludgeenters the settl ing compartment, is a primary condition for good


29/36

USB REACTOR FOR WASTE TREATMENT 727operation of the gas-solids separator. At high organic loading ratesthe agitation caused by gas evolution will usually be sufficient toprevent an excessive buoying of sludge. However, at low organicloading rates-depending upon the design of the gas-solids sepa-rator (i.e., liquid surface area) and the type of waste treated-someform of mechanical agitation of the gas-liquid interface in the gascollector may be required. This applies in particular to wastes thatcan readily form a scum layer, i.e., dairy wastes' and raw sewage.

Excessive foaming has only been observed in the event of rela-tively poor treatment efficiencies-due to overloading or a nutrientdeficiency-and at very high gas production rates. However, withboth the sugar-beet and potato-processing wastes foaming couldalways be effectively repressed by adding a small quantity of anti-foaming agent to the feed solution. According to our laboratoryresults more difficulties may be expected in this respect if the wastestreated are relatively rich in proteins, such as potato starch waste-water.

DISCUSSIONThe results obtained so far clearly demonstrate the feasibility of

the USB reactor concept for anaerobic treatment. This certainlyalso applies to acid fermentation and denitrification. The UASBprocess matches its simplicity in construction and operation withits ability to tolerate extremely high organic and hydraulic loadingrates, viz. exceeding 30 kg COD-m-3/day and 8 m3/m3/day, espec-tively. Obviously for practical reasons such excessive loading ratesshould not be pursued. From an economic viewpoint there is hardlyany argument to justify such excessive claims. Moreover, to acertain extent, overcapacity is always desirable for the sake ofprocess stability but also with a view to a possible future expandingproduction.

In terms of construction the UASB process is very simple indeed.As was shown in the experiments with sugar-beet and potato-pro-cessing wastes the reactor requires no specific mechanical parts.However, in other situations provisions for mechanical agitation atthe gas-liquid interface or in the digester compartment may beuseful or even necessary. This is certainly the case if wastes com-posed of mainly undissolved solids must be treated. Either an in-termittent or a continuous agitation mode should be applied, inorder to prevent an accumulation of biodegradable waste solids inthe lower part of the reactor and/or to ensure a good contact be-


30/36

728 LETTINGA ET AL.tween bacteria and substrate (i.e., at low gas production rates orhigh hydraulic loading rates). At present no decisive answer can begiven on the best way of mixing; tentatively, gas recirculation seemsto be the most suitable method.

However, provided that the reactor is equipped with any appro-priate system to cause the degree of turbulence required, the UASBprocess proves to be a suitable method to treat wastes with a highconcentration of undissolved solids.42Obviously in this respect thebehavior of the AF is different, although it appears, according toMcCarty and to personal observations of the authors, that a smallamount of biodegradable suspended solids can be accepted by anAF. Colloidal suspensions can be handled' by the AF if coagulationremains insignificant. Taking into consideration the lack of an ef-fective mixing of the solids in the AF process, it is obvious that thisprocess is more sensitive to low influent pH values, to the presenceof toxic compounds, and to shock loadings than the UASB process.Moreover, as a result of the less efficient contact between activesludge and substrate (i.e., blocking of canals, etc.), loading rates ashigh as in the UASB process are impossible with the AF process,even if the loss of actual volume by filter media is accounted for.As has been already mentioned no additional equipment (i.e., theinstallation of a scraper system) is necessary to ensure the returnof settled sludge from the settler to the digester compartment. Infact, the upward liquid flow through the aperture between the gascollectors fluctuates considerably, depending on the local gas erup-tions in the sludge bed. Occasionally (everywhere in the passage)a temporary downward flow may even occur, thus facilitating (ini-tiating) the backsliding of agglomerate sludge particles into thedigester compartment. Simple solutions for the construction of alarge-scale UASB plant are shown in Figure 9.

Undoubtedly one of the main features of the USB process is thedevelopment of a highly settleable sludge, more especially the for-mation of the granular sludge. The results obtained so far with theUASB process indicate that the pellet formation (size enlargement)proceeds fairly slowly with anaerobic sludge; viz. in the order ofmonths. As has been shown in the laboratory this process proceedsconsiderably faster for denitrification and acid fermentation. Themechanism underlying the size enlargement process is not yet suf-ficiently understood and will therefore be subjected to further studyin our laboratory. However, a number of factors have been recog-nized to play an important role in this process, namely:

1) The presence of sufficient nutrients and other requirements for


31/36

USB REACTOR FOR WASTE TREATMENT 72 9

Fig. 9. Schematic diagramsof a large-scaleUASB plant. (a) Rectangular reactor;(b) cylindrical reactor. (1 ) Sludge bed; (2) bulk of liquid with suspended sludge; (3 )gas bowl (collector);(4) gas seal; (5) feed inlet; (6) settler compartment;(7) launderer;(8) gas collector with gas outlet pipe to (3); (9) water seal.

bacterial growth and for the formation of the bonding agents re-sponsible for the solid links between the bacterial cells (comparethe SEMs of granular sludge as shown in Fig. 5 ) .2) The continuous removal (washout) of non- or poorly floccu-lating finely dispersed matter from the system, present in or formedfrom the feed.

3) The creation and maintenance of conditions favorable to theflocculation process in order to improve the development of gran-ular sludge, i.e., presence of Ca2+ ons, and absence of high con-centrations of finely dispersed poorly flocculating matter.

4) The homogeneous and gentle agitation brought about by thegas production. Being mixed in this way rather effectively in thevertical direction, the sludge is exposed to varying forces of gravitycompression (thickening).

Except through the action of bacterial growth in and on the sludgeflocs, the formation of granular sludge is presumably mainly drivenby gravity compression forces. Sludge thickening is stimulated byincreasing the height of the sludge blanket and-within certain lim-


32/36

130 LETTINGA ET A L .its-also by increasing the stirring intensity. These two factors mayat least partly explain the significant improvement in sludge reten-tion observed upon scaling up the reactor (from 0.3- 1 .O m labora-tory scale to 3 m pilot-plant scale), and upon increasing gas pro-duction (see Fig. 6). Contrary to earlier pre~urnptions,~,~hepresence of clay or other inorganic particles seems to be harmful tothe formation of granular sludge. As was shown in the experimentswith liquid sugar and potato wastes, granular sludge is fairly wellformed in the absence of dispersed inorganic matter. The granularsludge then has a very high volatile solids content; SEMs indicatethat probably a significant part of the organic matter consists ofbacteria. This observation is supported by the fact that the nitrogencontent of the volatile moiety of the granular sludge is in the rangeof 11 - 12.5%, whereas it was about 5-7% in the seed sludge.

It should be recognized that besides the formation of sludgegranules, erosion also takes place in the sludge bed under the influ-ence of friction forces to which the sludge flocs are exposed; inparticular, at high mixing intensities. As was already mentioned, abetter insight into the factors determining the mechanical strengthof the sludge particles should be gained. Once more emphasis islaid on the significance of the first start-up to the quality of thesludge obtained. Sludge of a superior quality develops if the processis started up cautiously according to the directions outlined in theprevious section.

What remains to be briefly discussed are the prospects of thetwo-phase concept for the anaerobic treatment of more complexwastewaters, viz. consisting of almost unsoured dissolved and/orundissolved solids. The results obtained in the pilot-plant experi-ments with either liquid-sugar or potato-processing wastes are cer-tainly disclosing and rather unexpected as far as the utility of thetwo-phase concept is concerned. In spite of the underlying philos-ophy, the two-phase ~ o n c e p t ~ ~ * ~ 'nd more or less contrary to theexperience obtained from the laboratory experiments with sugarsap solution^,^^^ the necessity of separating the stages of acid andmethane fermentation has been rejected by the pilot-plant experi-ments for both types of wastes considered. Apparently in a well-adapted system with almost unsoured wastes acidogenesis andmethanogenesis can proceed simultaneously fairly well. In the ex-periments with potato-processing waste (see Table V ) it even ap-peared that the rapid growth of the unsoured fraction at the sharplyincreased organic loads did not affect to any noticeable extent theperformance efficiency of the process.


33/36

USB REACTOR FOR WASTE TREATMENT 731How far these findings may apply to other types of waste is

uncertain at present, but undoubtedly there are wastes for which aphase-separation may be very useful. We recently obtained ev-idence that such might be the case for methanolic waste^.^' Thesame may hold for wastes with a high fraction of undissolved matter.However, more research is required before decisive judgments canbe made. Special attention should be given in this respect to thesignificance of optimum start-up and to the development of a gran-ular type of sludge.

CONCLUSIONS1 ) The U S B reactor concept has been found to be very promising

for anaerobic treatment of low-strength wastes. Full-scale and pilot-plant experiments using sugar-beet and potato-processing wastesrevealed that exceptionally high organic and hydraulic loading rates(up to 25 kg COD.m-3/day and 5 m3.m-3/day at 30C, respectively)can be satisfactorily handled in an U S B reactor, once the sludgehas been adapted to the waste.Results of small pilot-plant-scale experiments conducted in thelaboratory indicate that the U S B reactor concept is also feasible fordenitrification and acid fermentation.2) The use of an U S B reactor promotes the development of asludge of superior quality with respect to settling characteristicsand specific activity. Presumably this is the result of a combinationof factors, i.e.:

The sludge is exposed to varying forces of gravity compression(dependent on the height of the sludge bed, sludge concentration,and place of the sludge in the reactor).

Growth will take place in and on the sludge flocs/particles present.Disintegration (erosion) of sludge flocs/particles is kept a t a min-

imum by allowing only a gentle and homogeneous agitation mode,e.g., as resulting from the escape of produced gas.

An important additional factor in the development of sludge ofdesired quality is the creation and maintenance of favorable con-ditions for flocculation within the system, i.e., the presence of Ca2+ions, adequate mixing, and the absence of a high concentration ofpoorly flocculating suspended matter in the wastewater. Moreover,sufficient nutrients should be present and available to ensure bac-terial growth.

3) In terms of construction the U S B process is very simple.


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732 L E T T IN G A E T A L .Except if applied at low organic loads (2-4 kg COD.m-3/day), athigh hydraulic loads (3-4 m3.md3/day)and in treating wastes con-taining a significant amount of undissolved solids, there will be noneed for any form of mechanical mixing.

Concerning the construction of the gas-solids separator, it shouldbe looked after a ready liberation of adherent/entrapped gas inbouying sludge (i.e., a well-mixed liquid-gas interface in the gascollector of sufficient area). Moreover, the inclined walls of thesettler should be at an angle of approximately 50, in order tofacilitate the return of the collected sludge back into the digestercompartment.4)The results obtained with unsoured or slightly soured sugar-beet and potato-processing wastes indicate that there does not existany real need to separate the acid and methane fermentation phases,provided the sludge is well adapted.

References1 . G. Let t inga and A. F. M.van Velsen, H20 , , 281 (1974).2. G. Let t inga, J . van der Ben, and J . van der Sar , H@, 9, 38 (1976).3 . P. J . Schel lekens, Feasibi l ity of anaerobic t reatment for was tes of the foodindustry, Agricultural University of Wageningen, Internal report no. 765, 1976 [inDutch].4. G . Let t inga, A. G. N. Jansen , and P. Terps t r a , H@, 8, 530 (1975).5. G. Let t inga, K. Ch. Pet te , R. e W et ter , and E . W ind , HZO,10, 526 (1977).6. G. Lettinga, K. Ch . Pet te , R. e We t ter , and E . Wind , Anaerob ic t r ea tmentof sugar beet w aste wa ter at pilot plant scale, f inal report, Se pte m ber , 1977 [inDutch] .7. A. J . Verspr il le , A naero bic t reatmen t of potato processing wastes , repor t

216, 1978 I BV L , Bo x 18, 6700 AA Wageningen [ in Dutch].8. K. Ch. P et te , A naerobic t reatment of sugar beet wa ste in 200 m3 ull scaleplant, rep ort in preparation [ in Dutch].9. G. L et t inga and J . Stel lema, H@, 7, 129 (1974).

10. G. Lettinga, Feasibili ty of anaerobic digestion for the purification of indus-tr ial waste water , in 4th European Sewa ge and Refuse Sym posium (EAS, M unich ,1978).

1 I . R. R. Dague, R. E. McKinney , and Y. T. Pfeffer, J . Wa ter Pollut. ControlF e d . , 38, 221 (1966); 42, R29 (1970).12. D. New ton , H . Keinath, and L. S. Hillis, Pilot plant studies for the evaluationof methods of treating brewery wastes, in Proceedings of the 16th Industrial Wa steConference (Purdue U. P. , Lafayet te , IN, 1962), p. 332.13 . M. B . Rands and D. E. Cooper , Development and operat ion of a low costanaerobic plant for meat waste, in Proceedings of th e 2 l s t Industrial Waste Con-ference (Purdue U. ., Lafayet te , IN,1966), p. 613.14. A. J . Steffen and M. Be dker, O peration of full-scale anaerobic con tac t


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US B R EAC TOR F OR WAS TE TR EATM ENT 733treatm ent plant for meat packing wastes, in Proceedings of the 16th IndustrialWa ste Conference (Purdue U. P., Lafayette, IN, 1962) p. 423.

15. J. T . Ling, Pilot investigation of starch-glutek wa ste treatment, in Pro-ceedings of the 16th Industrial W aste Conference (Purdue U. p., Lafayette , IN,1966), p. 217.16. C. A. Cow ie, Industrial waste treatm ent and disposal in Ne w Zealand, in

15th Industrial W as te Confe rence (Purdue U. P. , Lafayette , IN , 1965). p. 1% .17. G . J . Schroepfer and N. R. Ziemke, Sewage Ind. Wastes , 31, 164,697 (1959).18. R. Kohler, Wasser Lufl Betr . , 17, 10, 342 (1973);Branntweinwirtschaft, 12,

264 (1974).19. L . van den Berg and C. P. Lentz , Food processing waste treatment byanaerobic digestion, in Procee dings of the 32nd Industrial W as te Conference (Pur-

due U. P., Lafayette, IN, 1977).20. S . Schlegel, Wa sser Lufr Be tr. , 19, 447 (1975).21. M. G. Black, J . M. Brown, and E . Kaye, Water Pollut. Control Fed ., 73(5),

532 (1974).22. A. V. Babayants, Ya. A. Khanukaere, A. V . Korchinskii, L. N. Kondova,

and A. A. Petrenko, A p p l . Biochem. Microbiol., 7 ( l) , 35 (1973).23. J. B. Coulter and M. B. Ettinger, Sew. Ind. W as tes , 29, 468 (1957);J . Water

Pollut. Control Fed ., 33, 1045 (1961).24. J. Hemens, P. G. J . Meir ing, and G. J. Stander , Water W aste Treatment , 9,

16 (1962).25. G. J . Stan der, G. C. Cillie, W. R. Ro ss , and R. D. Baillie, Tr eatm ent of wine

distillery wa stes by gnaerobic digestion, in Proceedings of the 22nd IndustrialWa ste Conference (Purdue U. P. , L afaye t te , IN, 1%7), p. 892.26. G. C. Cillie, M. R. H enz en, G. J. Sta nde r, and R. D. Baillie, W a t er R es . , 3,

623 (1969).27. J. C. Young and P. L. McCarty, The anaerobic filter for waste treatment,in Ref. 25, J. Wa ter Pollut. Control Fed., 41, R 160 (1969).28. C. R. Lovan and E. G . Force, The anaerobic filter for the treatment ofbrewery press liquor waste, in Procee dings o f the 26th Industrial W as te Conference(Purdue U. P. , Lafay ette , IN, 1971), p. 1074.29. G. Lettinga, P. G. Fo hr , and G. G. W. Janssen, H a (22), 510 (1972).30. A. T. E l -Shafe and D. E. Bloodgood, J. Wa ter Pollut. Control Fed., 45, 2345(1973).31. G. Lett inga, A. Th. van de r Geest, J . van der Laan, and S. Hobm a, Anaer-obic treatment of methanolic wastes, Water Res., 13, 725 (1979).32. Th . van Bellegem a nd G. Lettinga, An aerobic treatment of wastes of fenpeat-colonial industries, repo rt 1976, Proefstation van A ardappelverwerking, Groningen[in Dutch].33. A. H . Plummer and J . F. Malina, Stabilization of a low solids carbohy drate

wa ste by an anaerobic submerged fdter, in Proceedings of the 23rd IndustrialWa ste Conference (Purdu e U. P. , Lafayette, IN , 1968), p . 462.34. W. A. Pretoruis, W a t er R es . , 5 , 681 (1971).35 . D . E. Simpson, Water Re s . , 5 , 523 (1971).36. S. Ghosh , J . R. Co nrad, and D. L . Klass, J. Wa ter Pollut. Control F ed ., 47,37. F. G. Pohland and S. Ghosh , Environ. Le t t . , 1(4), 255 (1971).38. A. C ohen, R . J . Zoetemeyer , and J . G. v.Andel, H a , 10, 2% (1977).

30 (1975).


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734 LETTINGA ET AL.39. A . J . van der Vlugt, Zuidnederlandse spiritusfabriek, Bergen op Zoom , private40. G . Lettinga, W . A. Beverloo, and W . C. van Lier, P ro g . W a f e rZechnol., 10,41. P. M. Heertjes and R. R. van der Meer, Biofechnol .Bioeng. , 20, 1577 (1978).42. A . F. M . van Velsen, E. van het Oeve r, and G. Lettinga, H @ , 12, 59 (1979).

communication.(1/2), 537 (1978).

Accepted for Publication May 31, 1979

use of the upflow sludge blanket (usb) reactor

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