assessment of biogas use as an energy source from anaerobic digestion of brewery wastewater

ASSESSMENT OF BIOGAS USE AS AN ENERGY SOURCE FROMANAEROBIC DIGESTION OF BREWERY WASTEWATER

BAHAR KASAPGIL INCE1, ORHAN INCE2, G. KEN ANDERSON3

and SEMIHA ARAYICI41 Bosphorus University, Institute of Environmental Science, Istanbul, Turkey;2 Istanbul TechnicalUniversity, Department of Environmental Engineering, Istanbul, Turkey;3 Newcastle upon Tyne

University, Department of Civil Engineering, U.K.;4 Istanbul University, Department ofEnvironmental Engineering, Istanbul, Turkey

(Received 10 December 1998; accepted 27 March 2000)

Abstract. Energy recovery from a crossflow ultrafiltration (UF) membrane unit employed in orderto improve the performance of an anaerobic contact digester for the treatment of brewery wastewaterwas assessed. The performance of the pilot-scale anaerobic UF membrane system was studied forover 15 months. At steady-state conditions, an organic loading rate of 28.5 kg COD m−3 d−1, ahydraulic retention time of 4.2 days and overall COD and BOD removal efficiencies of 99% andalmost 100% were achieved, respectively. Percent methane in biogas was found to be in a rangeof 67–79% with the corresponding methane yield of 0.28–0.35 m3 CH4 kg−1 CODremoved. Thepotential energy recovery from the system treating brewery wastewater at an OLR of 28.5 kg CODm−3 d−1 was 87 MJ d−1 which would enable to maintain all energy requirements of the feed pump,mixing and heating of the reactor contents. In addition to this, 71% of the energy requirement forrecirculating the reactor content through the membranes would also be recovered.

Keywords: anaerobic contact digester, brewery wastewater, crossflow ultrafiltration, energy recovery

1. Introduction

The anaerobic treatment is an energy generating process rather than one whichdemands a regular high input of energy, as in an aerobic biological system. Theamount of energy resulting from production of biogas in any anaerobic treatmentsystem depends upon retention of adequate level of active biomass in the anaerobicreactor (Inceet al., 1994, 1995a). Various methods have, therefore, been developedfor a range of reactor configurations with the anaerobic contact process being theone of advanced anaerobic digestion technologies. However, the major difficultyencountered in the full-scale applications of these processes has been the settle-ment of biomass in the sedimentation tank. This has restricted its application in thetreatment of high strength industrial wastewaters for economic reasons.

The use of membranes as biomass separators in anaerobic digester systemstreating industrial effluents was pioneered in the early eighties by Epstein andKorchin (1981) and Choateet al. (1983) resulting in the development of ultrafiltra-tion membranes (UF) in combination with anaerobic digesters (by Dorr-Oliver)

Water, Air, and Soil Pollution126: 239–251, 2001.© 2001Kluwer Academic Publishers. Printed in the Netherlands.

240 BAHAR KASAPGIL INCE ET AL.

known as the MARS process, but this process has never been employed in full-scaleinstallations. Independent pilot-scale research into the use of locally manufac-tured UF membranes and modules (Strohwald, 1988) for solids-liquid separationin the anaerobic treatment of industrial effluents was begun in 1987 (Rosset al.,1988). Significant departures from overseas practice in the form of differencesin ultrafiltration membrane design, the use of unsupported tubular UF membrane(MEMTUR) modules (Strohwald, 1991) at low inlet pressures and integration withthe degister system led to the development of what has come to be known as theanaerobic digestion ultrafiltration process (ADUF) for the treatment of organicindustrial effluents. The design comprises two main unit processes; an anaerobicdigester and an external UF unit. In the ADUF process the permeate is the finaleffluent while the sludge concentrate containing the bacteria, is rapidly recycledback to the digester, enhancing its performance. This process has many advantagescompared to other systems, some of which are: (i) it enables the retention of ahigh concentration of active biomass thus minimizing the required reactor volume,(ii) it prevents biomass loss in the effluent resulting in greater stability under loadchanges and variations in influent characteristics, (iii) it provides an effluent almostfree of suspended solids, (iv) it eliminates the need for a sedimentation tank thusminimizing the biomass separation problems caused by system overloads which inturn are a major problem inherent in suspended growth processes, and (v) it enablespositive control of solid retention time (SRT) and hydraulic retention time (HRT).

Incorporation of ultrafiltration membranes resulted in improved digester loadingrates compared to conventional anaerobic digesters, organic loading rates (OLR) ofupto 6 kg COD m−3 d−1 (Inceet al., 1993). In other words, considering the samevolume of reactor, a significant increase in the amount of energy production canbe achieved in an anaerobic contact digester using a membrane technology. Undernormal circumstances, the yield of methane would, an average, be 0.33–0.35 m3

CH4 kg−1 CODremoved(at 35◦C and atmospheric pressure). The potential energyproduction from the utilization of 1000 kg COD by an anaerobic process is 350 m3

of methane production which is equivalent to net calorific value of 35 MJ m−3,energy value of 11.90×103 MJ, 280 L of heavy oil, 300 L of gas oil, and 0.39 tof coal (ETC Ltd., 1992). For example, a full-scale membrane anaerobic reactortreating 20470 kg COD d−1 produces 6140 m3 d−1 of methane, which is equivalentto about 4910 L of heavy oil, 5260 L of gas oil and 21× 104 MJ d−1 of energy(Li and Corrado, 1985). The energy from resulting methane may be used directlyfor reactor heating and mixing, while there may also be excess energy for use onsite at the industry. This paper will, therefore, assess the energy recovery from acrossflow ultrafiltration membrane anaerobic reactor system while treating brewerywastewater.

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241Figure 1.Schematic diagram of membrane anaerobic contact reactor system.


2. Materials and Methods

2.1. MEMBRANE ANAEROBIC REACTOR SYSTEM

Figure 1 shows a schematic drawing of the experimental system used in this study.It consisted of a crossflow ultrafiltration membrane unit and a 120 L completelymixed, suspended growth anaerobic reactor (CUMAR). The reactor pH and tem-perature were automatically maintained within the ranges of 6.9–7.2 and 36±1 ◦C.The reactor was fed at the same rate as the permeate rate by using a level controllerplaced within the reactor and controlling the reactor feed pump, hence the activevolume of the digester remained constant throughout the study. A variable speedpump was used to recirculate the reactor contents through the membranes whichwere themselves operated in parallel. The crossflow velocity and operating pressurewere controlled by adjusting the flow and pressure regulators. During the operation,a range of crossflow velocity of 2.4–3.2 m s−1 and pressure of 170–240 kN m−2

were used.The crossflow ultrafiltration membrane unit consisted of two independent but

identical cells with two cylindrical channels, each 12 mm diameter and 320 mmlong. Each cell held an UF membrane of 0.024 m2 total surface area. The UFmembrane used in this study (supplied by Paterson Candy International) was man-ufactured from fluoropolymer with a molecular weight cut-off of approximately200 000.

2.2. ANALYTICAL METHODS

Throughout the operation, routine analyses were carried out daily to check steady-state conditions with the monitoring schedule and analytical methods and instru-mentation used in this study being listed in Tables I and II.

2.3. MEMBRANE CLEANING

A chemical solution (2 g sodium hydroxide pellets + 5 mL sodium hypochloride toa litre) recommended by the UF membrane manufacturer (Peterson Candy Interna-tional) was used to clean the membranes. Residual chlorine levels were maintainedno higher than 300 mg L−1 in order to prevent any deterioration on the membranestructure. Membrane spares were stored horizontally in a storage area at a temper-ature of + 4◦C. The sealed plastic tube was kept closed until immediately prior touse. Cleaning with back washing was not carried out since it causes the membranetube collapse.

2.4. WASTEWATER CHARACTERISTICS ANDSEED SLUDGE

The wastewater used throughout the study was collected from a local brewery andthe characteristics are given in Table III. Although the wastewater had a high COD

ASSESSMENT OF BIOGAS USE AS AN ENERGY SOURCE 243

TABLE I

Monitoring schedule

Parameter Frequency Sampling location

Influent rate Daily Pump setting

COD: Influent 3×/week Feed line

Effluent 3×/week Permeate

Reactor 1×/week (unsteady-state) Sampling point

3×/week (steady state)

Gas: Production Daily Wet gas meter

Composition Daily Gas line

Solids: Feed SS 1×/week Feed line

Effluent SS/VSS 1×/week Permeate

Digester SS/VSS 1×/week (unsteady state) Sampling point

3×/week (steady state)

Temperature Continuous Temperature probe port

TKN 1×/week Permeate/feed

NH3-N 1×/week Permeate

PO4-P 1×/week Permeate/feed

Alkalinity 3×/week Permeate/feed

Volatile Fatty Acids Daily Permeate/feed

Turbidity 3×/week Permeate/feed

Color 3×/week Permeate/feed

Particle size 1×/2 weeks Permeate

pH Daily pH probe port

concentration, glucose was added to increase strength of the feed after an OLR of20 kg COD m−3 d−1 had been reached since the HRT of the CUMAR system waslargely determined by the flux rate of the membrane. The raw wastewater had aCOD:N:P ratio of 400:0.7:0.4. Throughout the operation of the CUMAR system,the COD:N:P ratio was maintained in the influent at a ratio of 400:5:1 by addingurea and KH2PO4 in order to supplement nitrogen and phosphorus. The pH of thefeed was adjusted by adding NaHCO3 to a level close to neutral during the start-upperiod of the CUMAR system while alkalinity was maintained in the range 1000–3000 mg L−1 as CaCO3. During the start-up period, the VFAs were in a rangeof 20–50 mg L−1 in the feed and 50–1600 mg L−1 in the effluent. After that, theamount of NaHCO3 added to the feed was gradually decreased to a point afterwhich there was no need to add alkalinity (after an OLR of 7 kg COD m−3 d−1

had been reached) due to the quantity of alkalinity produced in the digester. Seedsludge from a local municipal wastewater treatment plant was used for inoculating


TABLE II

Analytical methods and instrumentation

Parameter Method Instrument/reference

Influent rate Feed pump setting Peristaltic Watson Marlow (S170)

COD Dichromate closed reflux Standard Methods (1985)

Gas: Production Gas meter Wet gas meter

Composition Gas chromatography Pye Unicam 304

Suspended Solids Gravimetric Standard Methods (1985)

Volatile Suspended Solids Gravimetric Standard Methods (1985)

Temperature Probe/Indicator RS components

Heater controller Churchill Thermo circulator

Cooler Grand FC15 cooler

TKN Distillation and titration Standard Methods (1985)

NH3-N Distillation and titration Standard Methods (1985)

PO4-P Ascorbic acid Standard Methods (1985)

Alkalinity Titration Standard Methods (1985)

Volatile Fatty Acids Gas-liquid chromatography Becker 403 with Pye Unicam

autojector and integrator

pH pH meter Kent EIL 9143

Particle size Counter Coulter Counter Electronics

Crossflow velocity Pump setting and flow Mono Merlin pump

meter CAB12H1R4/H1

Turbidity Turbidity meter HACH model 2100 A

Color Lovibond discs BDH, Lovibond Nesslerisep

the contact digester. The concentration of volatile suspended solids in the sludgewas found to be 11 000 mg L−1 which was 75% of the total suspended solids.A specific methanogenic activity (SMA) test was immediately carried out beforeseeding the anaerobic reactor and a maximum of 1.5 mL CH4 g−1 VSS d−1 wasmeasured (Inceet al., 1995a).

3. Results

3.1. START-UP PROCEDURE AND INITIAL LOADING CONDITIONS

One hundred litres of digesting sludge, taken from a municipal wastewater treat-ment plant, was first sieved through a mesh with a diameter of 0.1 mm in orderto remove waste materials which could cause pump failure. Secondly, the sludgewas left at room temperature for 24 hr so that the biomass settled, after which 80 L


TABLE III

Characteristics of brewery wastewater

Parameter Concentration (mg L−1)

COD 80000–90000

BOD5 65000–80000

TKN 110–210

PO4-P 90–100

Suspended Solids 100–150

pH (units) 3.5–4.5

of settled sludge was then drawn from the bottom of the tank and introduced intothe reactor through the overflow line. The rest of the reactor was filled with tapwater to a level of 120 L. Following this, the reactor contents were flushed withnitrogen for 30 min so that anaerobic conditions could be established. Finally thetemperature of the reactor content was gradually increased from room temperatureto 36◦C over a period of 48 hr without feeding.

The anaerobic contact reactor was initially fed with brewery wastewater (TableIII) at a strength of 2.5 g L−1 to give an OLR of approximately 1 kg COD m−3

d−1 with a HRT of 2.5 days. The VFA concentration in the digester immediatelyincreased to about 1600 mg L−1 in the first week of the operation which resulted ina high VFA/alkalinity ratio of 0.7 and a COD removal efficiency of 14%. The OLRwas, therefore, reduced to 0.7 kg COD m−3 d−1 for the following 3 weeks duringwhich the COD removal efficiency improved to about 80%.

Acclimatization of the digester sludge was completed after 40 days operationfollowed by exponential increases in OLR while the MLVSS concentration in thedigester increased from approximately 8500 mg L−1 to over 10 000 mg L−1 whichin turn resulted in an increase in the MLVSS/MLSS ratio of 5% (Inceet al., 1993).

3.2. PERFORMANCE OFCUMAR SYSTEM

The overall performance of the system are shown in Figures 2–3. As seen fromFigure 2, a maximum OLR of 28.5 kg COD m−3 d−1 with a HRT of 4.2 days,at a F/M ratio of 0.55 kg COD kg−1 VSS d−1 was achieved towards the end ofthe study at which point the system performed well, 99% COD and almost 100%BOD removal efficiencies. As seen from Figure 3, the methane content of thebiogas produced decreased from 79 to 67% which resulted in a methane yieldof 0.28 L CH4 g−1 CODremoved. This was probably due to the high OLRs beingapplied to the digester causing a change in dominant methanogenic species (Inceet al., 1995b). During the operating period, the MLVSS increased from 5 to 50 gL−1 which contained highly active methanogenic bacteria (Inceet al., 1994).


Figure 2.COD and BOD removal efficiencies and HRT of CUMAR system against OLR.

Figure 3.Methane yield and methane percentage against OLR.

The limiting MLSS concentrations are given in Table IV from which it maybe seen that the digester could be operated at any biomass concentration less thanthose given in Table IV without a separation problem, provided that the sludge canbe recirculated.


TABLE IV

Limiting MLSS concentrations in contact digester

MLSS Average pressure Crossflow velocity

(g L−1) (kN m−2) (m s−1)

212 240 2.4

176 171 2.4

240 206 2.9

279 206 3.2

The true color of the wastewater fed to the digester was in the range 2400–3500◦ Hazen, approximately 50–70% color removal was achieved through themembrane (Inceet al., 1993). The turbidity of the permeate was found to be 0.2–0.6 NTU while the SS and VSS were in the range of 0.5–2.0 and 0.5–1.0 mg L−1,respectively, indicating almost 100% biomass separation by the membrane unit.In addition to this, particle sizes which passed through the UF membrane weremeasured by Coulter Counter with no particles with a diameter greater than 0.4micron being found in the permeate (Inceet al., 1993). Throughout the operationof the system, SMA tests, most probable number (MPN) and microscopic countswere carried out and the results obtained from SMA tests showed that almost nomethanogenic activity was found in the permeate. This was also confirmed by theresults of MPN, microscopic count and plate count (Inceet al., 1993).

3.3. ENERGY BALANCE OF THE CUMAR SYSTEM

Equivalent energy values of the methane produced from the CUMAR system at anOLR of 28.5 kg COD m−3 d−1 are given in Table V while energy balance for thetreatment of the brewery wastewater is given in Table VI. As seen from Table VI,the overall energy production from the CUMAR system would cover 75% of theenergy requirements. Additional 1.3 MJ hr−1 energy is needed to maintain thesystem self-sufficient from the energy point of view. The operation was stoppeddue to the limitation of increasing cross flow velocity and transmembrane pressureand the surface area of membrane used further studies should, therefore, be carriedout in order to determine the maximum energy production capacity of the system.

4. Discussion

OLRs can be applied efficiently up to 6 kg COD m−3 d−1 in full-scale applica-tions of conventional anaerobic contact digesters. Table VII shows a comparison ofOLRs of a range of anaerobic bioreactors. Further increases in OLR are restricted


TABLE V

Energy equivalent values of methane produced from the CUMAR system

Flow rate Methane production Heavy oil Gas oil Coal

(L d−1) (L d−1) (L d−1) (L d−1) (kg d−1)

30 940 0.75 0.80 1.1

TABLE VI

Energy balance for treatment of brewery wastewater

Average flow (L d−1) 30

Methane production (L d−1 at OLR of 28.5 kg COD m−3 d−1) 940

Energy of methane produced in anaerobic contact digester (MJ d−1) 32

Energy resulting from high-rate recirculation (MJ d−1) 55

Total energy production (MJ d−1) 87

Energy requirements of the CUMAR system (MJ d−1)

Mechanical mixer (contact digester) 6.5

Feed pump 0.3

Recirculation pump 108

Heating (anaerobic contact digester, PVC) 3.0

Total energy requirement (MJ d−1) 118

Approximate energy recovery (%)∗ 75

a Energy recovery (%) = (Energy of methane produced/total energy requirement)× 100.

by an inadequate amount of biomass retained in the anaerobic contact digesters.In this study, therefore, a method of operating a completely mixed digester usinga crossflow UF membrane technique for the retention of active biomass and forthe determination of the extent of any other advantageous that can be gained overother reactor configurations (Table VII) was studied. Retaining an adequate amountof active biomass in the digester determined by MLVSS measurement and SMAtests using a membrane separation technique resulted in excellent organic matterremoval throughout the study (Inceet al., 1994).

Although it has been reported (Stronachet al., 1986) that methanogenic bacteriahave relatively long generation times of 0.5–2 days and that 4–8 months may berequired for the attainment of microbial steady-state in suspended biomass systems,the acclimatization period of both the acetogenic and methanogenic populationin the CUMAR system was completed after 40 days operation. This could be


TABLE VII

Comparison of reactor loading rates of anaerobic bioreactors

Anaerobic reactor type Operational Loading rates Units COD

temperature removal◦C (kg units m−3 d−1) (%)

Conventional

(a) Standard rate Ambient 0.32–0.8 VS N.R.

(b) High rate 35 3.2 VS N.R.

Anaerobic contact reactor 35 1–6 COD 80–95

Upflow anaerobic sludge 35 5–30 COD 80–95

blanket reactor

Two-stage system 25–50a 10–60 COD –

30–40b 8–25 COD –

Anaerobic filter 35 2–16 COD 80–95

Expended bed reactor 35 1–20 COD 80–85

Fludised bed reactor 35 1–20 COD 80–87

Source: Donnelly (1984), Anderson and Saw (1986) and Speece (1983).N.R. = Not reported.a Acidogenic reactor.b Methanogenic reactor.

explained by the retention of a sufficient quantity of active biomass using themembrane process in this study (Inceet al., 1995a).

The percentage of COD removal after the start-up period was generally over99% in the permeate with no sudden increases in VFA concentrations with in-creasing OLRs, confirming that the digester was performing very well. The resultsobtained from this study were most encouraging with respect to biomass reten-tion and the treatment efficiency of the anaerobic system. Clearly, the CUMARsystem was capable of higher OLRs and had not reached its maximum treatmentcapacity. During the operation period, the maximum OLR applied was 28.5 kgCOD m−3 d−1. at which point COD removal efficiencies of approximately 99%in the permeate and over 97% in the digester were achieved. The excellent CODremoval efficiencies of the digester throughout the operation can be explained bythe separation process which was employed in this study.

The methane content of the biogas produced in the digester ranged from 79 to67% and the methane yield ranged from 0.35 to 0.28 m3 CH4 kg−1 CODremoved

over the experimental period. The methane yield and methane percentage werereasonably constant up to an OLR of 9 kg COD m−3 d−1 after which a decreasein both methane percentage and methane yield was observed, corresponding tothe applied OLRs. This might be explained by the high OLRs which favor anincrease in growth rate for acidogenic bacteria over methanogenic bacteria. Even


under these conditions, the digester performed well which indicated that a newequilibrium might have been established between the bacterial populations in thedigester, resulting in the process continuing with no sign of an impending failure(Inceet al., 1995b).

The OLR was increased, after 20 kg COD m−3 d−1, by adding glucose, howeverthe OLR could also be increased either by increasing the flux rates, employing ahigher membrane area or by increasing the crossflow velocity or increasing dra-matically the deliberate sludge wastage rate which would reduce sludge retentiontime significantly. The latter was not applied since it might have had a negativeaffect on the performance of the digester. On the other hand it would have beenvery expensive to buy a membrane module simply to increase the surface areaand flux rate. Therefore, only the crossflow velocity and transmembrane pressurewere increased in order to increase the flux. A crossflow velocity of 3.2 m s−1 wasapplied towards the end of study. A crossflow velocity greater than this was notused since it caused continuous sludge leaking from the recirculation pump whichwas not easy to control.

The results showed that flux rate is partly dependent upon the concentrationof biomass. During wastage of sludge under steady-state conditions the flux ratesremained reasonably constant whereas no biomass was wasted at unsteady-stateconditions, during which period the flux rates decreased as the biomass concen-tration increased at all applied crossflow velocities and pressures. The maximumreduction in flux rate was experienced at a crossflow velocity of 2.4 m s−1 andan average pressure of 171 kN m−2 within the range 17–22 g L−1 MLSS whereasthe minimum reduction in flux rate was observed at a crossflow velocity of 3.2 ms−1 and at an average pressure of 206 kN m−2 within a range of 49–64 g L−1

MLSS. This implies that the flux rate is not only dependent on the concentration ofbiomass, but is also a function of crossflow velocity and applied pressure (Inceetal., 1993).

It is worth noting that the membranes were not cleaned during each set of flowvelocities and pressures. This did not cause biomass separation problems and aconsistent flux rate was maintained.

The results demonstrated that the UF membrane had an ability to remove color,turbidity and retain almost 100% biomass in the digester. This can be seen from theresults of SS, VSS, particle size measurements, total counts, microscopic counts,plate counts, SMA and MPN tests (Inceet al., 1993).

5. Conclusions

Results obtained from this study showed that a maximum OLR of 28.5 kg CODm−3 d−1, a HRT of 4.2 days and overall COD and BOD removal efficiencies of 99%and almost 100% were achieved respectively. The potential energy recovery treat-ing brewery wastewater at an OLR of 28.5 kg COD m−3 d−1 was 87 MJ d−1 which


would enable to maintain approximately 75% of the total energy requirements ofthe CUMAR system, the feed pump, mixing and heating of the anaerobic contactreactor contents and recirculation through the membranes. From the above results,it can be concluded that the treatment of brewery wastewater by the CUMARsystem would be an efficient solution.

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assessment of biogas use as an energy source from anaerobic digestion of brewery wastewater

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