~ Pergamon

PU: 80273-1223(99)00397-2

War Sci. Tech. Vol. 40, No.1, pp. 289-295, 1999C 1999IAWQ

Published by Elsevier SCIence LtdPnnted in Great Britain. All rights reserved

0273-1223/99 520.00+ 0.00

TWO-PHASE ANAEROBIC TREATMENTOF CHEESE WHEY

Giiliim Yilmazer and Orhan Yenigiin

Institute ofEnvironmental Sciences. Bogazi~i University, Bebek 80815. Istanbul.Turkey

ABSTRACT

Performance of two-phase anaerobic digestion of cheese whey was investigated in a system consisting of acontinuous stirred tank reactor (CSTR) as the acidogenic reactor and an upflow anaerobic filter (UFAF) asthe methanogenic reactor. The acidogenic reactor was operated at various hydraulic retention times (HRTs)between 18 hours and 4 days. The results showed that an optimum HRT for the acidogenic reactor with thesame organic loading rate (OLR) between 0.5-2 g CODIMLSS day was 24 hours. At this retention time theacidification rate increased up to a maximum of50%. Volatile fatty acids (VFAs) produced in the acidogenicreactor operating at an HRT of 24 hours were 52% acetic acid, with 14% propionic, 27% butyric and 7%isovaleric acids. Operating the acidogenic reactor at this HRT, the effluent was fed to the upflow anaerobicfilter. Here HRT was varied between 3-6 days for the best COD removal efficiency and biogas production.At an HRT of 4 days a 90% soluble effluent COD removal efficiency was obtained with an outmost biogasyield of0.55 m3/kg COD removed. 0 1999 IAWQ Published by Elsevier Science Ltd. All rights reserved.

KEYWORDS

Acidogenesis; anaerobic treatment; cheese whey; methanogenesis; upflow anaerobic filter; volatile fattyacids.

INTRODUCTION

A considerable effort has been made to develop anaerobic digestion processes specifically for treatingindustrial wastewaters. With the present state of process technology, anaerobic digestion represents anattractive alternative to aerobic processes which do not only have high operating costs but also generatelarge quantities of excess sludge which is costly to dispose of. Anaerobic processes offer the potential forenergy savings and the ability to operate at organic loadings far in excess of equivalent types of aerobicreactors, which ultimately results in more economic industrial wastewater treatment.

The anaerobic digestion process is identified as occurring through six distinct conversion processes (Henzeand Harrernoes, 1983). These can generally be grouped into three steps; hydrolysis, acid production andmethane production (Gujer and Zehnder, 1983). Complex organic substrates are first metabolised by non­methanogenic organisms to produce volatile acids and carbon dioxide. These intermediates are thendegraded by the methanogenic organisms to methane (McCarty, 1964). The growth requirements of the twogroups of organisms are rather different. In order to create optimum conditions for the process, Pohland andGhosh (1971) first proposed for separating the process into two phases.

289

290 G. YILMAZER and O. YENIGON

Operating a system with these two different groups of micro-organisms in two separate reactors provid ingfavourable environmental conditions may be expected to give improved process efficiency and the system isreferred to as a two-phase anaerobic digestion system. As the time growth rate of acidogenic organisms isrelatively faster than that of methanogic organisms, this work used a completely mixed reactor for acidformers, also referred to as an acid-phase reactor or acid tank. To provide a greater solids retention time forslowly growing methanogenic organisms, also referred to as methane formers, an upflow anaerobic filterwas used.

Operating variables applicable for the selection and enrichment of microbial populations in phased digestorsinclude digestor loading, hydraulic retention time (HRn, pH, temperature, reactor design and operatingmode, for instance, the optimum pH range for acidogens is 5.5-5.9 whereas the best pH level formethanogens is given as 6.6-7.6 (McCarty and Mosey, 1991). Phase separation can be achieved by keepingthe HRT in the acidogenic reactor short enough to wash out methanogenic organisms (Ghosh and Pohland,1974; Ghosh et al., 1975). Although the importance of the separation of the acidogenic and methanogenicphases is well known, only a few studies were carried out for the investigation of the two-phase anaerobicdigest ion mechanism as well as the mutual relationship between microb ial populations and their productdistributions (Zhang and Noike, 1991).

Studies on anaerobic digestion of cheese whey are generally with single-phase digestion systems (Barford etal., 1986; Van et al., 1989; Schroder and Dehaast, 1989). More recently Patel et al. (1995) investigatedanaerobic digestion of high strength cheese whey with COD of 70,000 mg/l, using an upflow fixed filmreactor with various support media, obtaining a maximum COD removal of 81%. A reasonably high CODremoval of98 % was reported in Malaspina et al. (1996) who used a downflow-upflow hybrid reactor.

The aim of this study was to determine the best combination of hydraulic retention time and organic loading'rate at preset values of pH and temperature in both acidogenic and methanogenic reactors set up for the two­phase anaerobic treatment of cheese whey. For this purpose the acidogenic conversion of higher organics tovolatile fatty acids (VFAs) and the distribution of major VFAs produced during acidogenesis wereexamined. After having determined the HRT for highest acid ification, optimum operation of themethanogenic upflow biofilter was investigated through monitoring COD conversion and biogas yield.

MATERIALS AND METHODS

The experimental set-up used in this study is schematically shown in Figure I. It consisted of a continuouslystirred cylindrical container as the acidogenic reactor and an upflow anaerobic filter as the methanogenicreactor.

The experimental work consisted of acclimation and optimization stages . The seed sludges for both reactorsare obta ined from a full-scale two-phase anaerobic wastewater treatment plant. The substrate used for thestudy was cheese whey powder. Appropriate amounts ofthe powder were dissolved in distilled water. Thechemical composition of cheese whey with a COD concentration of 20 000 mg/l can be seen in Table 1. Itwas necessary to add some micro and macro nutrients to the cheese whey feed solution. Ammoniumbicarbonate and dipotassium hydrogen phosphate were added to adjust the COOl NI P ratio to 250:5:1. Theconcentrations of the micro and macro nutrients added to the cheese whey feed solution are given in Table 2.Stock solutions were prepared and consumed daily. No acidification was observed in the feed bottle duringexperiments. The acclimation of acidogenic seed sludge to the cheese whey substrate was stepwise. Thetarget concentration of 20 000 mg/l cheese whey as COD organic loading was achieved by increasing theconcentration from 5000 mg/l to 10 000 mg/l, 15 000 mg/l and 20 000 mg/l in the final step. All this wascompleted in 114 days.

Two-phase anaerobic treatmentofcheese whey 291

Gas collectionPump1

Gas

EffluentcontainerUpflow anaerobic

filter

pH adjustmentCompletely mixedacidification reactor

Feed bottle

Figure I. Schematicdiagram of laboratory-scaletwo-phase anaerobic digestion system.

Table 1. Chemical characteristics of substrate used

ParameterCODBODsTKNNH4-NSuspended solidsVolatile Suspended SolidsP0 4-PCalciumMagnesiumPotassiumSodiumIronpH (units)Food Energy

Concentration (mgll)20000120009571571750152020511140847.0826 Kcal

Table 2. Nutrient solution composition

Macronutrient Concentration Micronutrient Concentration

448 mg/l2256 mg/l

FeCh.6H20NiS04.6H20

CoCh.6H20<NH4)~07024.4

H20

4.830mg/l0.444 mg/l0.402 mg/l0.036mg/l

In the optimization stage, for the acidogenic phase, glass and plexiglass bottles plugged and sealed withrubber stoppers with 1.5 litres and 4.5 litres of working volumes were utilised. Acidogenic reactors werekept in a temperature controlled water bath at 35 ± 1.0 DC. Mixing was provided by magnetic stirrers placedunderneath the water bath. The reactors were gassed with nitrogen gas at the beginning of the experiments topromote anaerobic conditions. For the methanogenic phase; the upflow anaerobic filter was constructedusing a plexiglass cylinder of 14.8 cm diameter and filled with filter media of plastics pall rings. The filter

292 O. YILMAZERand O. YENIGON

had a volume of 5.36litres with a void ratio of90 percent. The details of the reactors and the characteristicsof the filter media can be seen in Tables 3 and 4, respectively.

Table 3. Details ofUFAF

CharacteristicsInternal diameterOverall heightPacking heightVoidageEffluent port (from top)Influent port (from base)

Plexiglass UFAF14.8 em42.0cm25.0cm90%5.00cm5.00cm

Table 4. Characteristics of filter medium

PVC Pall Rings322 mZ/mJ

1.59 em1.8cm112 kglm3

CharactericticsSpecific surface areaOutside diameterLengthApproximate weight

Throughout this study, pH, DO, alkalinity, gas production, COD removal rates, mixed liquor suspended andvolatile suspended solids (MLSS, MLVSS) and volatile fatty acids were monitored daily. DO measurementswere made by a Hach dissolved oxygen apparatus. Volatile fatty acids were analysed by a HP 5890 Series IIgas chromatograph equipped with a flame ionization detector. An HP-Innowax capillary column (15 m x0.25 mm x 0.15 um) was used. Oven, detector and injection port temperatures were 120°C, 300°C and250°C, respectively. Before injection adequate amounts of samples were centrifuged at 1500 g for 15minutes and supernatants were Milipore filtered (0.45J.1.lll). All other analyses were carried out according toStandard Methods (APHA 1992).

RESULTS AND DISCUSSION

Determination of operating criteria for the acidification phase of anaerobic digestion of cheese whey wasstudied in a completely mixed anaerobic digester initially using feed and draw type operation-mode due tothe difficulties associated with feeding at very low flow rates. The acidogenic reactor was operated at HRTsof 4 and 2 days. Later, the HRT of the reactor was decreased subsequently to 24 hours and 18 hours. Atthese smaller HRTs the reactor was fed semi-continuously from the feed bottle via a peristaltic pump. At 18hours HRT, settling of acidogenic sludge became difficult so smaller retention times were not tested. Theoperating pH range was 5.7-5.8. MLVSS / MLSS ratio varied between 0.80-0.86 in all the runs. DO levelswere also continuously monitored and were found to vary between 0.2-0.6 mg/l. The aim of this part of thestudy was to obtain maximum acidification rate, which points out to the optimum conversion of highmolecular weight organics to VFAs in the acidogenic reactor. The method used here for the assessment ofacidification rate using COD equivalent of each VFA was adopted from Hajipakkos (1987). The CODequivalents of the five VFAs are presented in Table 5. The use of the following formula allows an easierassessment of the overall acidification process:

COD ofVFA(mg/L)Percentage of Acidified COD (%)= ()x 100

SolubleCOD mg/L

Two-phase anaerobic treatment ofcheese whey

Table 5. Conversion factors for volatile fatty acids

293

VFAAcetic acidPropionic acidButyric acidValerie acidCaproic acid

COD Equivalent1.0661.5121.8162.0362.204

__Acidification% atHRT=4 days)

........Acidification% atHRTs2days

-Ir-Acidlfication % atHRTs 24 hrs

21.954.91 9.65 20.97

OlR'(kg/m3day)

60.,.-----------------,

50~6 40

'"lJ 30

'":5! 20~ l*.......-

10 ,....~J::t~~ _+o-t:-:.....:.-+--_l__--+----l---+---I----'1.31

Figure 2. Acidification % vs OLR at different HRTs.

In the acidogenic reactor operating at an HRT of 4 days, the optimum acidification rate was observed atorganic loading rate (OLR) of4.5 kg/nr'day. When HRT was decreased to 2 days there was a slight increasein the acidification rates. However at an HRT of 24 hours a measurable increase in acidification rates wasobserved. This is probably because at this HRT the activity of acidogenic microorganisms reaches a peak.The concentration of individual volatile fatty acids exhibited variations with respect to the HRT. Thecompositions ofVFAs at different HRTs are given in Table 6. At an HRT of 4 days with the organic loadingrate between 0.5-2 COD / MLSS day, acetic and propionic acids comprised 39% and 21% of the totalvolatile fatty acids respectively. At an HRT of2 days and with the same organic loading rate composition ofacetic and propionic acids changed to 31% and 40% respectively. At an HRT of 24 hours acetic acidcomprised 52% of the total volatile acids produced while propionic, butyric acids were 14% and 27%respectively. Although at HRTs of 4 and 2 days comparable amounts of valerie acid was recorded, at HRTof 24 hours there was no valerie acid produced but instead presence of isovaleric acid at about 7% wasobserved.

Table 6. Composition ofVFAs at different HRTs

HRT(days)

42I

Acetic acid(mgll) %269 39405 311625 52

Propionic acid(mgll) %lSI 21517 40462 14

Butyric acid(mg/l) %147 20226 17845 27

Valerie acid(mg/l) %145 20149 17

IsovaI. Acid(mgll) %

235 7

The experimental results showed that the best reactor performance was obtained at an HRT of 24 hours.

In order to investigate the performance of the methanogenic phase of the two-phase digestion of cheesewhey, effluent from the acidogenic reactor operating at an HRT of24 hours was fed to the upflow anaerobicfilter. The influent COD to the anaerobic filter was around II 000 mg/l. To be able to assess the best reactorperformance with respect to the HRT of the upflow anaerobic filter, the rate of feeding was varied. HRTsfrom 3 to 6 days were considered. For each HRT, biogas yield and total soluble COD removal weremeasured. The effect ofHRT on methanogenesis in terms of COD removal % is presented in Figure 3.

294 G. YILMAZER and O. YENIGON

100

z 90

~ 80

'ii> 700

i 60ll:C0 50(J

401 12 23 35 42 50

Time (Weeks)

__HRTz4 days

-.-HRT=5days

-+--HRT=6 days__HRT"3 days

Figure3. CODremoval rate vs HRT in methanogenesis at steadystate.

As can be seen in Figure 3 the methanogenic reactor was first operated at an HRT of 6 days with a CODremoval rate of about 67%. The performance of the methanogenic upflow filter showed improvement as theHRT of the reactor was decreased to 4 days. At an HRT of4 days the upflow anaerobic filter reached a CODremoval rate of about 95% but when the HRT was further decreased to 3 days, COD removal rate droppeddown to 63%. This sudden decrease in COD removal rate showed that there was considerable washout ofmethanogenic microorganisms. In all these runs pH of reactor contents varied in the range 7.4-7.7. Figure 4shows the biogas production rates at HRTs ofbetween 3 and 6.

The upflow anaerobic filter operated at an HRT of 4 days yielded an average COD removal rate of over 90%and an outmost biogas yield of 0.55 m3/kg COD removed. The results of this study demonstrated that for thetwo-phase anaerobic digestion of cheese whey a system comprising a completely stirred acidogenic reactoroperated at an HRT of 24 hours and an upflow anaerobic filter operated at an HRT of 4 days yields highlysatisfactory results in terms ofCOD removal rate and biogas production.

7000.,-------------------,

"',.. 6000.afte 5000..'i 4000se3000ll.:: 2000

! 1000 .......--..............- ..........._ ......~H....>_e---~O+- I---ll---l_+-+-+-+_+___-+--+--+--+-I

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Time (days)

__HRTz4 days

-+-HRT"5 days

-+--HRTz6 days

Figure4. Biogasproduction rate vs HRTat steadystate.

CONCLUSIONS

The following conclusions were drawn from this study:

e Maximum acidification rate of 50% was achieved in the completely mixed acidification reactor at a HRTof24 hours

Two-phase anaerobic treatment ofcheese whey 295

• the VFA composition in the well mixed acidogenic reactor working with a 24 hour HRT was 52% acetic,14% propionic, 27% butyric and 7% isovaleric acids

• in the upflow anaerobic filter, biogas production (0.55 m3 per kg COD removed) and COD removal(95%) was maximum at an HRT of4 days

• the two-phase anaerobic treatment system ofa CSTR as the acidogenic and a UFAF as the methanogenicreactors is demonstrated to be an efficient combination in the treatment of cheese whey.

ACKNOWLEDGEMENTS

This study is a part of Project No 96Y0012, financially supported by the Bogazici University ResearchFund. This support is gratefully acknowledged. The authors are also thankful to PAKMAYA factory whoprovided the innoculum sludges.

REFERENCES

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Ghosh, S. and Pohland, F.G. (1974). Kinetics of substrate assimilation and product formation in anaerobic digestion. J. WaterPoll. Control Fed., 46, 748-759.

Ghosh, S., Conrad, J.R. and Klass, D.L. (1975). Anaerobic acidogenesis of wastewater sludge. J. Water Poll. Control Fed., 47,30-45.

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Ferment. Bioeng., 79(4), 398-399.Pohland, F.G. and Ghosh, S. (1971). Developments in Anaerobic Treatment Process, BiotechnoI. Bioeng. Symp., No.2, 85-106.Schroder, E.W., Dehaast, J. (1989). Anaerobic digestion of deproteinated cheese whey in an upflow sludge blanket reactor. J.

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