J. Agric. Engng Res. (1999) 73, 59}63

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Article No. jaer.1998.0391, available online at http://www.idealibrary.com on

Treatment of Dairy Wastewater Using an Up#ow Anaerobic Sludge BlanketReactor

H. N. Gavala; H. Kopsinis; I. V. Skiadas; K. Stamatelatou; G. Lyberatos

Department of Chemical Engineering, University of Patras and Institute of Chemical Engineering and High Temperature Chemical ProcessesGR-26500 Patras, Greece; e-mail: gavala@chemeng.upatras.gr

(Received 16 March 1998; accepted in revised form 5 November 1998)

Wastewater coming from cheese-producing industries in Greece is high in organic matter (about 40}60 g/lChemical Oxygen Demand, COD) since it generally contains discarded cheese-whey as well. This wastewater isrich in easily biodegradable carbohydrates and has a relatively low content in suspended solids (1}5 g/l).Because of the high organic content of dairy wastewater, anaerobic digestion is essentially the only viabletreatment method.

An Up#ow Anaerobic Sludge Blanket (UASB) reactor is a high rate treatment system, especially feasiblefor treating soluble (containing low solids) wastewaters. A well-performing UASB reactor is characterized byhighly #occulated, well-settling, compact methanogenic sludge granules, resulting in a very high biomasscontent.

The aim of this work is the study of a UASB reactor treating dairy wastewater. A UASB reactor of 10 luseful volume was constructed and inoculated with anaerobic mixed liquor from dairy wastewater and glucosefed digesters. The digester e$ciency of treating dairy wastewater at various organic loading rates was studiedand its performance was assessed by monitoring pH, dissolved chemical oxygen demand (COD), biogasproduction and composition. Operation at an organic loading rate of 6)2 g COD/l d was found to be safeand could be increased to a maximum of 7)5 g COD/l d . A draw and "ll digester fed with non-dilutedwastewater was also studied in parallel for comparison purposes. The advantages of using a UASBreactor versus a conventional digester become questionable for the high COD ('42 g/l), non-diluted dairywastewater case.

( 1999 Silsoe Research Institute

1. Introduction

Dairy wastewater comes either from milk or cheese-producing industries; in the "rst case the wastewater hasa low chemical oxygen demand (COD) of 3000 mg/l,whereas in case of cheese-producing industries thegenerated wastewater generally contains discardedcheese whey as well, with a COD value of 50 000 mg/l.1

Aerobic processes are most suitable for the treatmentof milk-producing industries wastewater, althoughthere are several studies on anaerobic methods fortreating this kind of wastes as well.2,3 On the otherhand, anaerobic digestion is essentially the only viablemethod for treating high organic content wastewatercoming from cheese-producing plants. The dilutionof cheese whey by mixing with other wastewater is

0021-8634/99/050059#05 $30.00/0 59

a method for reducing the instability and low e$ciencyproblems caused by its high organic content, espe-cially for high-rate anaerobic systems, such as Up#owAnaerobic Sludge Blanket (UASB) reactors2 or Up#owAnaerobic Filters.4 The two-stage anaerobic digestionof cheese whey has been studied also by Cohen et al.5

using two pilot scale UASB reactors. The combinationof two treatment methods, biological and chemical, isanother possible option for treating cheese whey anddairy wastewater.6,7 Co-digestion of cheese whey withanimal wastes8,9 or domestic wastewater10 has also beenstudied.

The aim of this work is to study the behaviour ofa high rate system, a UASB reactor, when treating dairy(including cheese whey) wastewater and to compare itwith the use of a conventional digester.

( 1999 Silsoe Research Institute

H. N. GAVALA E¹ A¸ .60

2. Materials and methods

Determinations of dissolved (after "ltration) chem-ical oxygen demand (COD), total Kjeldahl nitrogen(TKN) and total suspended solids (TSS) were carried outaccording to standard methods.11 For total P determina-tion the persulphate digestion method and the ascorbicacid method (standard methods11) have been employed.

2.1. =astewater characterization

Characterization of a cheese-producing industrywastewater gave a pH of 4)7, TSS of 2)5 g/l, dissolvedCOD of 60 g/l, TKN of 830 mg/l and total P of 280 mg/l.Di-ammonium phosphate (NH

4)2HPO

4was added to

make up for N and P de"ciencies; NaOH was used forpH adjustment of the feed.

2.2. Reactor design and operation

A 10 l (useful volume) UASB reactor was constructedof Plexiglas (total height of 113 cm with inner diameter of11 cm), inoculated with anaerobic mixed liquor fromdairy wastewater and glucose fed digesters and fed withdairy wastewater. The objective was to determine themaximum possible organic loading rate (OLR) as well asthe maximum in#uent COD concentration that allowssatisfactory COD removal. The digester was initially fedwith diluted dairy wastewater (2500 mg/l COD); sub-sequently the in#uent COD concentration was increasedgradually (by reducing the dilution) while at the sametime the hydraulic retention time (HRT) of the system

Fig. 1. Inyuent and e{uent chemical oxygen demand (COD) concenfor an Upyow Anaerobic Sludge Blank

was changed so as to allow a satisfactory COD removal.A recirculation rate of approximately 52 ml/min wasused throughout. Although long-term population shiftscould be observed upon changing the HRT or the OLR,changes were e!ected whenever it was felt that a quasi-steady state was reached (typically two residence times).

For comparison purposes, an 8 l (useful volume) drawand "ll digester was inoculated with anaerobic mixedliquor from dairy wastewater and glucose fed digestersand was fed with non-diluted dairy wastewater (COD of60 g/l) at progressively lower hydraulic retention time.

The two reactors were operated at the mesophilictemperature range (353C) throughout the period of theexperiments. Wastewater was continuously fed into theUASB reactor using a variable-speed peristaltic pump,whereas the draw and "ll digester was fed manually ona daily basis.

3. Results and discussion

3.1. ;p-ow anaerobic sludge blanket experimentwith dairy wastewater

Figure 1 shows the digester in#uent and e%uent CODconcentration as well as the system e$ciency (based on%COD removal) versus time while Fig. 2 shows thehydraulic retention time (HRT) changes, the organicloading rate (OLR) changes and pH values over time.

The maximum digester COD removal e$ciency of98% was reached at an HRT of 6 d with an in#uent CODconcentration of 37 g/l (OLR"6)2 g COD/l d). Whenincreasing the in#uent COD concentration to 42 g/l(OLR"7)5 g COD/l d), the COD removal e$ciency

tration and system ezciency (based on COD removal) versus timeet reactor treating dairy wastewater

Fig. 2. (a) For an Upyow Anaerobic Sludge Blanket reactor treating dairy wastewater: hydraulic retention time (HRT) and organicloading rate (OLR) changes versus time; (b) biogas production and organic loading rate (OLR) changes versus time; (c) pH values

versus time

61TREATMENT OF DAIRY WASTEWATER

was reduced to 85}90% with a mean COD e%uentconcentration of 5 g/l. After this point, the increase ofin#uent COD resulted in even lower e$ciencies accom-panied by a sharp decrease in biogas production and pHvalues as well. Consequently, the total experiment maybe divided into three periods (Fig. 1): period I, with al-most complete COD removal; period II, with constantin#uent COD concentration but with a progressive in-crease in HRT from 6 d to approximately 20 d in order tomaintain satisfactory COD removal (80}90%); andperiod III during which the in#uent COD concentrationwas increased to the undiluted value. In this period, theHRT had to increase above 30 d in order to sustainreasonable reactor performance.

Thus, an organic loading rate for a UASB reactortreating dairy wastewater of 6)2 g COD/l d (diluted to37 g COD/l, with an HRT to 6 d) may be safely used andcould be increased up to 7)5 g COD/l d. Above thatOLR, reduced performance is observed; while for non-diluted dairy wastewater, an HRT in excess of 30 d isrequired.

3.2. Suspended growth experiment

For comparison purposes, an 8 l (useful volume) drawand "ll digester was fed with non-diluted dairy waste-water (COD of 60 g/l) at progressively lower hydraulicretention times as shown in Fig. 3. The highest possibleOLR was found to be 2)3 g COD/l d at a hydraulicretention time of 26 d.

Table 1 presents typical values for the performanceof each reactor during the di!erent phases of theexperiments.

In general, it is di$cult and risky to compare systemsoperated in di!erent laboratories, meaning that the an-aerobic sludge history and characteristics might be quitedi!erent. However, in Yan et al.12 the removal e$ciencyof a UASB reactor has been reported to be about81}86% at HRT 5}6 days with an in#uent COD concen-tration of 41 g/l (OLR"7)9}8)2 g COD/l d) and 97}99%at an HRT of 5 d with an in#uent COD concentration of5}29 g/l (OLR"0)91}6 g COD/l d). In Yan et al.,13 itwas stated also that the UASB reactor could treat cheese

Fig. 3. E{uent chemical oxygen demand (COD) concentration, percentage COD removal and hydraulic retention time (HRT) versustime for a draw and xll digester treating dairy wastewater

H. N. GAVALA E¹ A¸ .62

whey of strength up to 28)8 g COD/l. From this study, itwas concluded that the maximum digester COD removale$ciency of 98% was reached at an HRT of 6 d withan in#uent COD concentration of 37 g/l (OLR"

6)2 g COD/l d). When increasing the in#uent CODconcentration to 42 g/l (OLR"7)5 g COD/l d), theCOD removal e$ciency was reduced to 85}90%. Addi-tionally, the removal e$ciency of a continuous stirredtank reactor was reported12 to be about 18}58% forHRT values of 14}70 d with an in#uent COD concentra-tion of 69 g/l. The present experiments showed that theremoval e$ciency of the draw and "ll digester lay be-tween 83 and 92% for HRT values of 26}40 d with anin#uent COD concentration of 60 g/l. In each case, thetreatment e$ciencies were similar and/or better thanthose reported by Yan et al.12,13 The organic loadingswere reported12 to be much higher (up to 27 g COD/l d)than those in this paper only with systems other than

TablTypical values for the performance of each reacto

Duration,d

pH MeanCOD

UASB experiment1st period 76 7)0}7)3 122nd period 111 6)6}7)2 453rd period 77 6)6 57

Draw & Fill experiment1st period 41 7)0 602nd period 12 7)0 603rd period 12 (6)5 60

COD, chemical oxygen demand.

UASB or suspended growth reactors (mostly with sys-tems based on attached microorganism growth). Indeed,this fact is not surprising. The present paper focused onthe comparison of a UASB and a draw and "ll reactorthat were inoculated with similar anaerobic sludge andtreated the same wastewater.

4. Conclusions

An Up#ow Anaerobic Sludge Blanket (UASB) re-actor was used successfully for treating dairy waste-water containing large amounts of cheese whey, whilea draw and "ll digester fed with non-diluted waste-water was also studied in parallel for comparison pur-poses. Operation of the UASB reactor at an organicloading rate of 6)2 g COD/l d was found to be safeand could be increased to a maximum of 7)5 g COD/l d.

e 1r during the di4erent phases of the experiments

inyuent, g/l

Hydraulicretentiontime, d

Organicloading rate,g COD/l d

Removal, %(mean vlaue)

}44 6 2)0}7)3 85}9910}20 2)3}4)5 79}91

}60 30}40 1)5}1)9 81

35}40 1)5 9429}32 2)1}1)9 9426 2)3 (83

63TREATMENT OF DAIRY WASTEWATER

The high retention times required for non-diluted waste-water, however, do not justify the use of a UASB reactor,as a less expensive conventional reactor, such as a Con-tinuous Stirred Tank reactor could equally well be em-ployed for such a high COD wastewater ('40 g/l).

Acknowledgement

The authors wish to thank the Commission of theEuropean Communities for the "nancial support of thiswork under grant No BRE2-CT92-0355.

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