Comparison of different thickening methods for active

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Comparison of different thickening methods for active biomass recycle for anaerobic digestion of wastewater sludge A. Ya. Vanyushina, A. M. Agarev, S. I. Moyzhes, Yu. A. Nikolaev, M. V. Kevbrina and M. N. Kozlov

ABSTRACT The effect of returning solids to the digester, after one of three thickening processes, on volatile solids reduction (VSR) and gas production was investigated. Three different thickening methods were compared: centrifugation, flotation and gravitational sedimentation. The amount and activity of retained biomass in thickened recycled sludge affected the efficiency of digestion. Semi-continuous laboratory digesters were used to study the influence of thickening processes on thermophilic sludge digestion efficiency. Centrifugation was the most effective method used and caused an increase of VSR from 43% (control) up to 70% and gas generation from 0.40 to 0.44 L g1 VS. Flotation and

A. Ya. Vanyushina (corresponding author) A. M. Agarev S. I. Moyzhes Yu. A. Nikolaev M. V. Kevbrina M. N. Kozlov MGUP ‘Mosvodokanal’, 1-st Kuryanovsky pr., 15, Moscow, Russia E-mail: [email protected]

gravitational sedimentation ways of thickening appeared to be less effective if compared with centrifugation. These methods increased VSR only by up to 65 and 51%, respectively and showed no significant increase of gas production. The dewatering capacity of digested sludge, as measured by its specific resistance to filtration, was essentially better for the sludge digested in the reactors with centrifugated and settled recycle. The VS concentration of recycle (g L1), as reflecting the amount of retained biomass, appeared to be one of the most important factors influencing the efficiency of sludge digestion in the recycling technology. Key words

| anaerobic digestion, municipal sludge, sludge recycle, sludge thickening, SRT and HRT decoupling

INTRODUCTION As wastewater treatment improves, the volume of waste activated sludge also increases. Proper treatment of excess sludge as well as primary sludge demands the expansion of sludge treatment facilities, both for meeting sanitary requirements and as well as reducing volumes of disposable sludge. In megalopolises, even a small expansion of wastewater treatment plant (WWTP) facilities is problematic and expensive. At Moscow wastewater treatment plants (MWWTP), the whole volume of excess and primary sludge is stabilized by anaerobic thermophilic digestion. High load of digesters results in a low rate of gas generation and volatile solids (VS) reduction. The volatile solids reduction (VSR), on average, is about 41% and biogas production about 0.33 L g1 VS. Construction of new industrial digesters requires considerable capital investment and, hence, it is necessary to find an efficient way of doi: 10.2166/wst.2012.405

increasing the solids retention time (SRT) in digesters without increasing hydraulic retention time (HRT). Decoupling of SRT and HRT by retention or returning of solids to the digester should enhance the VSR and biogas production. Research in this area has developed into two branches: the first, thickening with the application of membrane filtration, and the second, by other methods of solids-liquid separation. The usefulness of membrane for sludge anaerobic reactors is currently disputed by technologists. One of the first membrane applications in a coupled membrane/digester system helped to increase volumetric throughput from 70 to 130 L d1 while SRT was maintained at 26 days (Pillay et al. ). Positive results were also reported by Jackivicz et al. () at the pilot-scale level. Nour et al. () successfully demonstrated this process using an ultrafiltration membrane with a 50 L digester. Xu et al. () revealed

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that the implemented ultrasound did not have a negative effect on anaerobic microorganisms but effectively controlled membrane fouling. VSR of 51.3% was achieved at the loading rate of 2.7 g VS L1 d1. Furthermore, Pierkiel & Lanting () discovered that both the vibrating and tubular membrane configurations proved to be effective for membrane-coupled anaerobic digestion of sewage sludge whilst digester operation was stable achieving VS reduction of 59%. The application of membranes makes possible the operation of anaerobic sludge digesters at a hydraulic retention time of 1–3 days depending on solids concentration of the waste sludge, while maintaining the solids retention time in the range of 8–12 days. The latest positive example was a full-scale anaerobic membrane bioreactor (AnMBR) incorporating anaerobic digestion and membrane filtration in one process for treating high-strength industrial wastewater (Christian et al. ). The first AnMBR installation in North America was built at Ken’s Foods in Massachusetts, USA. The AnMBR system consistently produced a high quality effluent with non-detectable total suspended solids (TSS) concentrations and with average chemical oxygen demand (COD) and biochemical oxygen demand (BOD) removals of 99.4 and 99.9%, respectively. The AnMBR system provides superior performance and a very low rate of membrane fouling with the aid of biogas scour across the membrane surface. At the same time, other researchers revealed some difficulties and disadvantages of membrane usage for digested sludge recycling. In , Ghyoot & Verstraete tested a polymer ultrafiltration and ceramic microfiltration membrane for filtration of anaerobic sludge. They discovered that it was not effective to increase volumetric loading rates. Shear stress disrupted the interaction between the different species in the anaerobic consortia. Padmasiri et al. (), in accordance with Ghyoot’s previous work, indicated that an increase in cross-flow velocity resulted in poor anaerobic digestion performance. Jeison et al. (a, b) studied submerged membrane in anaerobic bioreactors for the treatment of wastewater containing suspended solids and revealed that the reactor operation was unstable, with sudden increases in filtration resistance, due to excessive cake layer formation. Membranes themselves represent a significant cost for the full-scale application of AnMBRs and their usefulness on anaerobic biomass is still under question. Other thickening technologies were applied to increase the SRT by returning solids to the digester. The technology was developed between 1991 and 1994, based on the recycle of anaerobic biomass after flotation by anoxic gas (biogas), and it came to be known as anoxic gas flotation (AGF)

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(Burke , , ). Three pilot and one full-scale plant treating wastewater sludge and food waste were launched on the basis of this technology. Applied AGF technology with a hydraulic retention time of 6.1 days achieved a 60% VS reduction in the experimental reactor, as opposed to 55% in the conventional reactor with a hydraulic retention time of 19 days. Li et al. () tested two parallel anaerobic reactors, fed with waste activated sludge, with an SRT of 53 days and an HRT of 25 days. They revealed an increase of VS destruction efficiency (from 45 to 65%) and cumulative biogas production. However, it remains unclear what method was used to thicken digested sludge. Nagao et al. () fulfilled a continuous anaerobic digestion experiment for food waste. Discharged sludge was centrifugated, and solids including undegraded substrate and microorganisms were returned to the reactor. The high carbon conversion rate of food waste to biogas was observed and reached steady state at around 80%. Comparison of the processes of conventional digestion and recycled by centrifugation, as demonstrated in laboratory reactors (Vanyushina et al. ), also revealed an increase of VS reduction from 34 to 68% with 65% of daily discharged sludge being thickened and returned to the digester. VSR increased from 41 to 52% with 47% recycled effluent. All mentioned studies approve the increase of VSR due to retaining or returning solids into the digester. However, the choice of thickening method for accelerated digestion remains very important as well as inconclusive and thus demands further studies before it can be recommended for full-scale application. Filterability of digested sludge is also of interest as it influences dewatering costs significantly. The objective of this research has been to test different methods of recycling solids separation, apart from expensive membranes, and to evaluate their effect on digestion efficiency and filterability.

METHODS Experiments for studying the process of thermophilic anaerobic digestion with recycle were conducted in two identical thermostatic, completely stirred reactors; one of them worked as a control under conventional conditions with HRT equal to SRT. The experimental reactor was operated with recycle of thickened digested sludge. The HRT of both reactors was 6 days. The SRT of the experimental reactor was changed by the use of three thickening methods coupled to digestion: centrifugation, flotation and gravitational sedimentation. Duration of each experiment was 36–41 days


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Table 1

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Operational parameters of experimental reactors according to different thickening procedures

Method of recycle thickening

Experiment duration, days

HRT

SRT

Average organic loading rate, kg VS m3

SRT/ HRT

Polymer dose, g kg1 TS

Volume ratio of thickened recycled solids to the digested daily effluent

Centrifugation

36

6.0

17.0

2.8

3.4 ± 0.6

–

0.32

Flotation

41

6.0

11.3

1.9

3.5 ± 0.6

6

0.53

Sedimentation

36

6.0

8.5

1.4

3.8 ± 0.6

–

0.46

(Table 1). The working volume of each completely stirred reactor was 1.7 L. The digestion process was conducted at 54 С. The primary sludge (PS) and thickened excess activated sludge (ES) were mixed in the ratio of 3:2 on the basis of VS. The PS and ES were both taken from Kuryanovskoe WWTP (KWWTP), Moscow. Loading and unloading of both reactors was daily. Organic loading rate depended on sludge properties in order to conduct the experiments under real conditions and to have the full-scale control comparison. Gas volume for both reactors was measured daily by displacement of NaCl saturated solution by biogas in 1 L measuring cylinders. CH4 and CO2 concentrations in biogas of both reactors were measured three times during each experiment by gas chromatograph (Crystal 2000M, Chromatech software, catarometer detector, coloumn packed with Parapack Q, He carrier).

Filterability properties

W

Thickening procedures All thickening procedures were carried out for digested sludge of the experimental reactor. Centrifugation of effluent digested sludge was conducted daily at laboratory centrifuge at the low acceleration rate of 109 g for 10 minutes without polymer. In spite of gentle centrifugation to avoid anaerobic flocs disruption, the degree of compaction (0.32) was the most intensive in comparison with flotation and sedimentation (Table 1). Flotation was carried out by adding of water-air mixture saturated at 6 bars through a bottom sleeve in a column with flocculated sludge. The volume of ‘working’ liquid was 200% of the sludge volume, time of flotation was 30 min. Gravitational sedimentation was performed in two identical columns with the HRT of 3 days without polymer addition. Solids were returned to experimental digester replacing an equivalent volume of digested sludge. Thickened solids, liquid supernatant or decanted water and digested sludge were analyzed for total solids content (TS), volatile solids (VS), pH, COD, volatile fatty acids (VFA) ammonium nitrogen and phosphates. Properties of decanted liquid after thickening allowed us to evaluate the influence of digestion upon the properties of water flux returning to the head of WWTP.

Specific resistance to filtration under 0.5 bar vacuum (SRF) of sludge from both digesters was periodically evaluated for comparison of their dewaterability. Test was conducted every 7 days for a pair of control and experimental sludge from centrifugation and sedimentation trials (Water Treatment Handbook ). Parameters of digestion efficiency VSR was evaluated by mass-balance equation (EPA ): FYF ¼ SYS þ DYD þ loss. VRS ¼ loss/FYF ¼ (FYF SYS DYD)/FYF, where F, S, D are volumetric flow rate of feed (F), digested sludge (S) and decanted (supernatant) water (D), respectively, L day1; Y is volatile solids concentration of these flows, g L1. In the case of conventional digestion, there was no supernatant water withdrawal. Gas production (RG) was calculated as RG ¼ Vsp ρ 100, where Vsp is specific gas generation, i.e. the volume of gas evolved per unit of weight of influent VS, L g1, ρ is density of gas, kg m3.

RESULTS AND DISCUSSION General parameters of digestion process were normal for a mixture of PS and ES and didn’t vary significantly between variants of experiments and in time. So, pH was 7.7–7.8, VFA 7–12 mmol L1, NH4-N500–600 mg L1. Total and volatile solids concentrations in the experimental reactor with recycle increased to different extents depending on the method of recycle thickening (Table 2). The most intensive increase of TS concentration by 10 g L1, as compared with the control, was observed for sedimentation trial (Figure 1). VS content in total solids had the opposite trend – solids in reactor coupled to centrifugation were less mineralized in contrast to the sedimentation variant. TS and VS parameters for digestion with flotation thickening of recycle were closer to those of centrifugation variant.


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Table 2

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Properties of influent and digested sludge during digestion experiments with

Table 3

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The main efficiency parameters of digestion with different recycle thickening

different methods of recycle thickening (average data)

Centrifugation

Experiment duration, days

36

Flotation

41

Sedimentation

36

TS of influent, g L1

29.2 ± 4.8

33.0 ± 4.1

34.2 ± 4.1

TS of digested sludge (control), g L1

18.9 ± 3.2

21.6 ± 3.0

20.2 ± 3.0

TS of digested sludge (recycle), g L1

20.8 ± 3.3

22.8 ± 4.9

30.5 ± 3.0

VS of influent, g L

20.4 ± 3.3

20.8 ± 2.9

22.6 ± 2.9

VS of digested sludge (control), g L1

11.6 ± 1.8

12.2 ± 2.7

11.3 ± 1.7

VS of digested sludge (recycle), g L1

12.6 ± 1.9

12.5 ± 4.5

16.1 ± 1.7

VS/TS of influent, %

70.7 ± 4.1

63.4 ± 2.2

66.2 ± 1.0

VS/TS of digested sludge (control), %

61.3 ± 4.0

57.2 ± 2.8

56.2 ± 1.6

VS/TS of digested sludge (recycle), %

60.9 ± 2.6

55.5 ± 2.5

52.7 ± 1.8

COD of influent, g L1

29.1 ± 4.5

31.3 ± 5.5

31.8 ± 5.0

COD of digested sludge (control), g L1

15.6 ± 3.2

15.5 ± 3.0

16.2 ± 3.0

COD of digested sludge (recycle), g L1

16.2 ± 2.7

13.2 ± 3.0

21.5 ± 1.2

1

All three recycle thickening methods demonstrated significant VSR increase. The highest VS reduction was achieved by centrifugation coupled to digestion (70%) in contrast to 43% in the control (Table 3). Flotation thickening caused the VSR to increase by up to 65% and the sedimentation variant by up to 51%. Regarding gas production, the only variant that provided an enhancement of gas yield was centrifugation. It gave 10% of a raise up to 0.44 L g1 VS in contrast to 0.4 L g1 VS in the control reactor. For two other methods of recycle thickening, an increase of gas production was not fixed. Methane

Figure 1

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Volatile solids reduction, %

Biogas production, L/g VS

recycle

Control

Reactor with

Control

Reactor

thickening

reactor

recycle

reactor

with recycle

Centrifugation

43 ± 6.0

70 ± 4.5

0.40 ± 0.07

0.44 ± 0.07

Flotation

42 ± 7.0

65 ± 6.0

0.42 ± 0.06

0.42 ± 0.06

Sedimentation

42 ± 5.0

51 ± 5.0

0.45 ± 0.06

0.45 ± 0.06

Method of

concentration in biogas samples from both control and recycle reactors was 65 ± 0.3% and did not change through all three experiments. The disparity between efficiency rates of gas yield and VS reduction can be explained by the less specific gas yield of organic substances decomposing in recycle. The organic matter which was additionally digested in recycle experiments was depleted with easily degradable compounds like lipids, with a specific gas yield of about 1.25 L/g and enriched with more resistant substances such as polymer carbohydrates (e.g. cellulose) with specific gas yield less than 0.79 L/g (Angelidaki & Sanders ). The decay of these substances contributes more to mass reduction than to gas generation. Similar difference of effects on gas yield and VSR was previously stated for sludge recycled digestion (Li et al. ). It was previously discovered (Vanyushina et al. ) that sludge digestion coupled to centrifugation could provide even twice VSR and 17% of gas yield. During six months of the process exploration TS concentration in the experimental reactor remained only 5 g L1 higher than in the control. Low acceleration rate of laboratory centrifugation does not produce an adverse effect on anaerobic flocs. However, the influence of higher acceleration rate of industrial decanters on anaerobic biomass activity of recycled sludge should be proven. Flotation method of recycle thickening was supposed to avoid sand deposition in the reactor as it separated the most light-weighted fraction of sludge. It was confirmed that VS fraction in TS of

Dynamics of total solids concentrations in digested and thickened sludge in periods before and after the start of the digestion with settled recycle.


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Table 4

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Total and volatile solids concentrations of recycling sludge during the digestion with different recycle thickening

Method of recycle thickening

VSR in reactor with recycle, %

TS of thickened recycled sludge, g/L

SRT

VS of thickened recycled sludge, g/g TS

VS of thickened recycled sludge, g/L

Recycled VS to digested VS ratio, g/g

Centrifugation

70 ± 4.5

61.2 ± 4.0

17.0

0.51 ± 0.3

31.2 ± 3.0

2.47

Flotation

65 ± 6.0

36.8 ± 3.5

11.3

0.58 ± 0.3

21.3 ± 2.8

1.63

Sedimentation

51 ± 5.0

60.9 ± 4.0

8.5

0.48 ± 0.2

28.7 ± 3.0

1.79

flotation recycle was the highest (0.58 g VS g1 TS) as regards to that of centrifugation recycle (0.51 g VS g1 TS) and sedimentation recycle (0.48 g VS g1 TS) (Table 4). However, laboratory flotation set did not allow getting sufficient TS concentration of recycle. In contrast to 61 g TS L1 of centrifugation and sedimentation recycle, flotation method provided thickening of TS to only 36.7 g TS L1. This caused low circulation rate of VS in the system, on the one hand, and did not elevate SRT as was found for centrifugation case (11.3 days for flotation and 17 days for centrifugation). SRT appeared to be the main factor influencing the efficiency of VS reduction. Thus, the degree of recycle thickening and its VS concentration play an important role in the digestion effectiveness. Gravitation sedimentation of recycle was tested as the cheapest and simplest method of sludge thickening. This technology is based on heavy fraction separation and settled recycle caused the increase of TS in the experimental reactor immediately after recycle start (Figure 1, Table 5). VS fractions in TS (%) of all recycled system components (such as digested sludge, decanted water, and thickened recycling sludge) were lower than those for control digester and thickener. Digested sludge, forwarded to dewatering, contained 10 g TS L1 more than conventional digested sludge. However, decanted water from recycling thickener also was more polluted by TS solids than in the conventional system. Levels of ammonium nitrogen and phosphates were

equal for both variants of decanted waters (control and experiment). In terms of digestion efficiency, sedimentation appeared to be the least effective method of recycle thickening as its VSR improvement by up to 51% was the smallest and no gas yield promotion was observed. In general, the thickening method should provide safe treatment of digested sludge for recycle. It should be fast so as not to drop thermophilic methanogenic bacteria activity, it should be gentle enough not to disrupt bacterial flocs, and sludge should not be exposed to oxygen as the latter inhibits methanogenic activity as well. Considering all these requirements, centrifugation thickening looks the most suitable method for recycle separation. Sludge flotation by air could poison methanogens but it might be useful to test anoxic gas (biogas) for the purpose of recycle separation as was proposed by Burke (, , ). Another drawback of flotation is flocculant usage as it increases expenditure. Specific resistance to filtration of sludge digested with recycle was lower than that of the control in both centrifugation and sedimentation experiments (Table 6). Better filterability of sludge with recycle might be due to deeper decomposition of organic colloids during more profound digestion. On the basis of accumulated data it could be presumed that the SRF decrease of 0.5 × 1014 m kg1 for recycled digested sludge means polymer dosage reduction for thickening by 0.5–1 kg t1 TS and allows costs to be reduced significantly. Technical and economic benefits

Table 5

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Total and volatile solids concentrations of sludge and decanted water during the digestion with settled recycle

Control

With recycle

Digested sludge

TS, g L1 VS, g L1 VS/TS, %

20.2 ± 3.0 11.3 ± 1.7 56.2 ± 1.6

30.5 ± 3.0 16.1 ± 1.7 52.7 ± 1.8

Decanted water


3.0 ± 0.3 2.0 ± 0.2 65.8 ± 3.0

4.8 ± 1.0 2.4 ± 0.5 52.2 ± 3.2

Thickened sludge


49.2 ± 4.2 26.0 ± 2.2 53.0 ± 1.8

59.6 ± 4.0 28.7 ± 1.8 49.5 ± 2.1

In order to determine the technical and economic benefits for the digestion technology with recycle, the parameters of KWWTP industrial digesters and the data from the Table 6

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Specific resistance to filtration of digested sludge from control and experimental reactors during centrifugation and sedimentation coupled digestion, r0.5 * 1014, m kg1 (average data of five experiments)

Centrifugation

Sedimentation

Control digested sludge

2.82 ± 0.4

2.92 ± 0.4

Digested sludge with recycle

2.35 ± 0.4

2.38 ± 0.4


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Table 7

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Profits and losses of the recycling digestion sludge technology

Digestion with biomass recycling

Conventional digestion

Operational cost reduction Influent initial flow of PS and ES

m3/day

VS reduction

%

70

43

VS of digested sludge

t/day

102

194

VS reduction in digested sludge in comparison with conventional digestion

t/day

92

0

16,800

16,800

Expected profit m3/kg VS

Output of biogas per 1 kg of influent VS

3

0.44

0.40

Output of biogas per day

m /day

150,000

136,400

Additional biogas volume in relation to conventional digestion

m3/day

13,600

0

kWh/day

13,000

Additional expenses Energy consumption for thickening of recycle (100% of daily volume input)

centrifugation experiment, as the most effective, were taken as a basis for calculations (Table 7). The major advantage of the technology with recycling resulted in a 67% increase of VSR in comparison with conventional digestion. This improvement leads to reduced expenses for sludge transportation, dewatering and landfilling. Sludge landfill contributes up to 40% of total wastewater treatment costs, especially for megalopolises. An increase of VS reduction from 43 to 70% during anaerobic digestion for a wastewater treatment plant with a 3 million m3/day capacity would result in a sludge reduction of 92 tons of TS per day. An additional benefit is an increase of 13,600 m3 of gas per day as a result of digestion with recycling biomass for a plant of the same capacity. However, the benefit of the extra gas production would be compensated by energy consumption and operational expenses for thickening of sludge recycle according to Moscow region energy and gas prices. The only remaining but pretty significant benefit is the cost reduction for sludge dewatering and landfilling. Monetary benefits are determined by landfill charges, as well as polymer and energy costs for each region. In the current economic situation in the Moscow region this project is supposed to be very profitable with payback in half a year.

CONCLUSIONS

•

The technology of anaerobic digestion with sludge recycle thickened by three tested technologies has proven to be

•

• •

highly profitable as it enables intensification of digestion without the physical expansion of digesters capacity or significant reconstruction of existing facilities. Centrifugation at low acceleration rate was shown to be the most effective method for recycle separation compared with air flotation and sedimentation, and caused the increase of VSR from 43% in control up to 70% and gas generation from 0.40 to 0.44 L g1 VS. The VS concentration of recycle appeared to be one of the most important factors influencing the efficiency of sludge digestion in the recycling technology. Significant reduction of digested sludge solids in comparison with conventional technology provides reduction of expenses for dewatering, transportation and utilization.

REFERENCES Angelidaki, I. & Sanders, W.  Assesment of the anaerobic biodegradability of macropollutants. Reviews in Environmental Science and Biotechnology 3, 117–129. Burke, D. A.  Sewage sludge digestion utilizing the AGF process. In: Proceedings the WEF Specialty Conference Series: The Future Direction of Municipal Sludge (Biosolids) Management; Where We Are and Where We’re Going. 26–30 July Portland, Oregon, USA. Burke, D. A.  Improved Biosolids Digestion Utilizing Anoxic Gas Flotation. In: WEFTEC ’96; Proceedings of the Water Environment Federation 69th Conference & Exposition. 5–9 October, Dallas, Texas, USA.

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Burke, D. A.  Producing exceptional quality biosolids through digestion, pasteurization, and redigestion. Biosolids 2001: Building Public Support Conference. In: Proceedings WEF/ AWWA/CWEA Joint Residuals and Biosolids Management Conference 45–53. Christian, S., Grant, S., McCarthy, P., Wilson, D. & Mills, D.  The first two years of full-scale anaerobic membrane bioreactor (AnMBR) operation treating high-strength industrial wastewater. In: Proceedings 12th IWA World Congress on Anaerobic Digestion, 31st October – 4th November. Guadarajara, Mexico. EPA l625/R-921013  Environmental Regulations and Technology. Control of Pathogens and Vector Attraction in Sewage Sludge. Ghyoot, W. R. & Verstraete, W. H.  Coupling membrane filtration to anaerobic primary sludge digestion. Environmental Technology 18 (6), 569–580. Jackivicz, L., Lanting, J. & Kirschbaum, J.  Using membranes for recuperative thickening of anaerobically digesting sludge. In: Proceedings of the Water Environment Federation. WEFTEC Session 71 through Session 83, pp. 183–192(10). Jeison, D., Díaz, I. J. B. & van Lier a Anaerobic membrane bioreactors: Are membranes really necessary? Electronic Journal of Biotechnology [online] 11 (4). Iss. Oct. 15. Jeison, D., van Betuwa, W. & van Lier, J. B. b Feasibility of anaerobic membrane bioreactors for the treatment of wastewaters with particulate organic matter. Separation Science and Technology 43 (13), 3417–3431. Li, T., Camacho, P., Mouilleron, I., Martin, S. & Dauthuille, P.  Improvement of municipal sludge anaerobic digestion yield by dissociating solid retention time and hydraulic retention time. In: Proceedings 12th IWA World Congress on


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Anaerobic Digestion, 31st October – 4th November, Guadarajara, Mexico. Nagao, N., Tajima, N., Niwa, C. & Toda, T.  Continuous anaerobic digestion process of food waste in the sludge circulation process. In: Proceedings 12th IWA World Congress on Anaerobic Digestion, 31st October – 4th November, Guadarajara, Mexico. Nour, A. H., Nour, A. H., Vissaliny, A. P. S. & Rajaletchumy, A. P. V.  Kinetics study of sewage sludge treatment by an aerobic digestion. Journal of Applied Sciences 10, 226–230. Padmasiri, S. I., Zhang, J., Fitch, M., Norddahl, B., Morgenroth, E. & Raskin, L.  Methanogenic population dynamics and performance of an anaerobic membrane bioreactor (AnMBR) treating swine manure under high shear conditions. Water Research 41 (1), 134–144. Pierkiel, A. & Lanting, J.  Membrane-coupled anaerobic digestion of municipal sewage sludge. Water Science and Technology 52 (1–2), 253–258. Pillay, V. L., Townsend, B. & Buckley, C. A.  Improving the performance of anaerobic digesters at wastewater treatment works: the coupled cross-flow microfiltration/digester process. Water Science and Technology 30, 329–337. Vanyushina, A. Ya., Nikolaev, Yu. A. & Kharkina, O. V.  Anaerobic digestion of sewage sludge with recycled active biomass. In: Proceedings 12th IWA World Congress on Anaerobic Digestion, 31st October – 4th November Guadarajara, Mexico. Water Treatment Handbook  10th Russian edn., Degrémont. V. 1, St-Petersburg. Xu, M., Wen, X., Yu, Z., Li, Y. & Huang, X.  A hybrid anaerobic membrane bioreactor coupled with online ultrasonic equipment for digestion of waste activated sludge. Bioresource Technology 102 (10), 5617–5625.

First received 13 February 2012; accepted in revised form 28 May 2012