Bioresource Technology xxx (2014) xxx–xxx

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Short Communication

Anaerobic digestion of municipal wastewater sludges using anaerobic fluidized bed bioreactor Nizar Mustafa a, Elsayed Elbeshbishy b, George Nakhla a,c,⇑, Jesse Zhu c a

Department of Civil and Environmental Engineering, University of Western Ontario, London, Ontario N6A 5B9, Canada Department of Civil and Environmental Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada c Department of Chemical and Biochemical Engineering, University of Western Ontario, London, Ontario N6A 5B9, Canada b

h i g h l i g h t s  Performance of AnFBR for treating primary and waste activated sludges was evaluated. 3

 Different organic loading rates (OLRs) ranged from 4.2 to 93 kg/m -d were evaluated. 3

 70% solids destruction of primary sludge was achieved at OLR of 19 kg/m -d. 3

 56% solids destruction of waste activated sludge was achieved at OLR of 8 kg/m -d.  First-order coefficient for primary sludge was higher than waste activated sludge.

a r t i c l e

i n f o

Article history: Received 7 July 2014 Received in revised form 16 September 2014 Accepted 17 September 2014 Available online xxxx Keywords: Anaerobic digestion Fluidized bed bioreactor Primary sludge TWAS

a b s t r a c t The anaerobic digestion of primary sludge (PS) and thickened waste activated sludge (TWAS) using an anaerobic fluidized bed bioreactor (AnFBR) employing zeolite particles as the carrier media was investigated at different organic loading rates (OLRs). PS was tested at OLRs from 4.2 to 39 kg COD/m3-d corresponding to hydraulic retention times (HRTs) from 1.0 to 8.9 days. The highest COD removal and VSS destruction efficiencies for primary sludge of 85% and 88%, respectively, were achieved at an HRT of 8.9 days and OLR of 4.2 kg COD/m3-d. For TWAS, VSS destruction efficiencies varied from 42% at an HRT of 2.6 days and OLR of 13.1 kg COD/m3-d to 69% at an HRT of 8.8 days and an OLR of 4.2 kg COD/ m3-d. The first-order COD biodegradation rates in the AnFBR for PS and TWAS were 0.4 d1 and 0.1 d1, respectively, almost double the rates in conventional high-rate digesters. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction The primary challenge associated with the anaerobic treatment of biosolids in conventional systems is the slow rate of biodegradation not only requiring very long retention times and large reactor volume but also achieving low solids destruction efficiencies (Bunrith, 2008). The common low rate digestion system is continuously stirred tank reactor (CSTR), and high rate systems are upflow anaerobic sludge bed (UASB), expanded granular sludge bed (EGSB), fluidized bed bioreactor (FBR), anaerobic membrane bioreactor (AnMBR) (De Mes et al., 2003). Although high rate anaerobic digesters such as UASB, AnMBR, EGSB, and FBR are not suitable for high solids or thickened wastes, high rate systems can be used as a part of multi-stage system for treating high solids ⇑ Corresponding author at: Department of Civil and Environmental Engineering, University of Western Ontario, London, Ontario N6A 5B9, Canada. E-mail address: [email protected] (G. Nakhla).

wastes (Angelidaki et al., 2003). Typically conventional completely mixed anaerobic digesters (CAD) treating municipal wastewater sludge have solids retention times (SRT) of 15 days or more and are characterized by relatively low organic solids destruction efficiencies of less than 50%. Grady et al. (1999) observed that a lower SRT limit of 10 days at a temperature of 35 °C is sufficient to ensure an adequate safety factor against washout of the methanogenic population, and incremental changes in volatile solids destruction are relatively small for SRT values above 15 days. Fluidized bed reactors have been used as a substitute for the conventional bioprocesses in various biotechnological applications e.g. fermentation, production of enzymes, and bioconversions (Grady et al., 1999). The structure and physical properties of natural zeolite such as channel and pore cavities, high specific surface area, low bulk density, high exchange (CEC) and adsorption capacities make it ideal for use in biological purification wastewater processes (Christidis, 1998; Wong and Yeung, 2007; Carretero and Pozo, 2009; Marty et al., 2010; Park et al., 2010). Consequently, the use

http://dx.doi.org/10.1016/j.biortech.2014.09.081 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Mustafa, N., et al. Anaerobic digestion of municipal wastewater sludges using anaerobic fluidized bed bioreactor. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.09.081

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N. Mustafa et al. / Bioresource Technology xxx (2014) xxx–xxx

of natural zeolite in different wastewater biological treatment processes has increased significantly over the past few years (Montalvo et al., 2012). An Anaerobic fluidized bed reactor (AnFBR) with zeolite as carrier media (425–610 lm) developed by Nakhla and coworkers of the University of Western Ontario (Andalib et al., 2012a), was recently tested for the treatment of stillage with TCOD of 120 g/L and TSS of 60 g/L (Andalib et al., 2012b). The AnFBR showed up to 88% TCOD and 78% TSS removal at organic and solids loading rates (OLR and SLR) of 29 kg COD/m3 d and 10.5 kg TSS/m3 d respectively and hydraulic retention time (HRT) of 3.5 days (Andalib et al., 2012b). The main objectives of this study were to test the performance of the AnFBR with more challenging municipal waste e.g., primary sludge (PS) and thickened waste activated sludge (TWAS) and bench-mark it against CAD at the lab-scale. In this study, two anaerobic fluidized bed bioreactors (AnFBR) with zeolite particles as carrier media (610–825 lm) were used to digest municipal wastewater sludge under high OLRs. To the best of the authors’ knowledge, this is the first study which evaluated the capability of treating municipal wastewater biosolids in an anaerobic fluidized bed. 2. Methods The schematic diagram of the mesophilic AnFBR is presented in Fig. 1. The details of the set-up and operation of the lab-scale AnFBR can be found elsewhere (Andalib et al., 2012b). Anaerobic digested sludge (TSS and VSS concentrations of 17,500 and 13,400 mg/L) from the secondary digester was collected from the St. Mary wastewater treatment plant (Ontario, Canada) and used as the seed sludge for the acclimatization of the AnFBRs. The reactors were started by feeding synthetic solution containing 10,000 mg/L acetate and 5000 mg/L glucose, at a flow rate of 3.4 L/d corresponding to a volumetric OLR of 3.2 kg/m3-d based on the 16 L AnFBR liquid volume. Details of composition of the synthetic feed are presented elsewhere (Andalib et al., 2012a). After the acclimatization period, the PS and TWAS were fed to the AnFBR. The operating conditions for the AnFBR over the course of the study are presented in Tables 2 and 3 for the PS and TWAS, respectively. Samples were analyzed for different water quality parameters such as TSS, VSS, TCOD, SCOD, VFA, and alkalinity and the gas production as well as the gas composition was monitored daily. The analytical techniques of the aforementioned parameters are presented elsewhere (Andalib et al., 2012b). Attached biomass concentrations (biosolids) were measured using APHA Standard Method No. 2540G (APHA, 1998). Biomass detachment rate constant was calculated using the following equation:

Biomass detachment rate constantðbÞ ¼

Q effluent  VSSeffluent Mmedia  Bd

and 70% were achieved at OLRs of 4.2, 9.5 and 19 kg COD/m3-d, respectively. Increasing the OLR to 39 kg/m3-d in Phase IV resulted in sharp deterioration in performance as evidenced by COD removal efficiency of 30% and VSS destruction efficiency of 31% (Table 3) due to sudden doubling of OLRs. Reducing the OLR to 28 kg COD/m3-d in phase V resulted in slight improvement in the system performance, with both COD and VSS removal efficiencies increased to 42%. The VFA-to-alkalinity ratio (a) is a widely accepted measure of anaerobic digestion stability (Chen et al., 2007). The steady-state VFA (as acetate)-to-alkalinity ratio (a) were consistently below 0.2 in phases I–IV, although during the transition from one phase to another a increased due to sudden doubling of OLRs. However, in phases IV and V, an increase in a value to 0.5 was observed due to the accumulation of the VFA. The widely variation of a value between 0.29 and 0.41 clearly suggesting that digestion stability at OLR of 28–39 kg/m3-d was compromised. 3.2. Performance of the AnFBR treating TWAS The steady-state performance data of the AnFBR treating TWAS is presented in Table 3. The operation of the AnFBR2 was over four phases corresponding to various OLR. In phase I, at an OLR of 4.2 kg COD/m3-d, COD removal and VSS destruction efficiencies were 68% and 69%, respectively (Table 3). The VSS destruction efficiencies decreased to 56% at OLR of 8.1 kg COD/m3-d. Increasing the OLR to 13.1–18.2 kg/m3-d resulted in a sharp a drop in performance as evidenced by COD removal efficiency of 34–42% and VSS destruction efficiency of 33–42% (Table 3). Methane yields ranged from 0.5 to 0.54 m3/kg VSS destroyed. VFA-to alkalinity ratios (a) were consistently below 0.3 in Phases I and II. However, in Phases III and IV, a values varied more widely between 0.35 and 0.45 clearly indicating that digestion stability at OLR of 13.1–18.2 kg/ m3-d (Phases III and IV) is compromised. 3.3. Volatile solids destruction modeling In general, volatile solids destruction efficiencies range from 40% to 55% depending on the characteristics of the sludge and the operating conditions such as the HRT, SRT and OLR. The following empirical equation (Liptak, 1974) is applied for high-rate digestion systems, with SRT of 15–20 days.

V d ¼ 13:7LnðSRTÞ þ 18:9

ð2Þ

where Vd is the volatile solids destruction efficiency (%) and SRT is the SRT of the system in days. Based on the AnFBR performance, Eqs. (3) and (4) describe the kinetics of VSS destruction efficiency as a function of SRT for PS and TWAS, respectively.

ð1Þ

V d ¼ 20:7LnðSRTd Þ þ 35:4

ð3Þ

where b is the first-order detachment rate constant (d1), Qeffluent is the effluent flow rate (L/d); Mmedia is the mass of zeolite particles (g); and Bd is the attached biofilm concentration (mg/g).

V d ¼ 17:8LnðSRTd Þ þ 19:7

ð4Þ

3. Results and discussion 3.1. Performance of AnFBR treating adelaide WWTP primary sludge Table 2 summarizes the steady-state performance of the AnFBR treating PS over the five testing phases. PS feeding to the AnFBR was started at an OLR of 4.2 kg COD/m3-d and increased gradually to 19 kg COD/m3-d to 39 kg COD/m3-d in Phase IV, prior to reduction to 28 kg COD/m3-d in phase V. During the first three phases, both COD and VSS removal efficiencies decreased gradually with increasing the OLRs. The VSS removal efficiencies of 88%, 79%

Fig. 2 shows the volatile solids destruction as a function of time for both AnFBRs using Liptak equation and the measured volatile solids destruction at each corresponding SRT. It is evident that the Liptak model significantly underestimated the AnFBR performance at all OLRs. Discrepancies between the model and the measured VSS destruction efficiencies varied from 55% of the model to 110% of the model for primary sludge while for TWAS deviations from the model ranged from 3% to 27%. As expected, the first-order VSS destruction coefficient for PS was higher than for TWAS. 3.4. First-order COD degradation rates Since in a fixed-film process like the AnFBR, determination of SRT is complicated, the COD and correspondingly VSS degradation

Please cite this article in press as: Mustafa, N., et al. Anaerobic digestion of municipal wastewater sludges using anaerobic fluidized bed bioreactor. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.09.081

N. Mustafa et al. / Bioresource Technology xxx (2014) xxx–xxx

3

Fig. 1. Schematic of anaerobic fluidized bed bioreactor (AnFBR).

rates are more conveniently correlated with HRT. The first-order degradation rate constants for both AnFBRs were calculated by plotting Ln(Ceff/Cin) vs HRT (Fig. 3a and b). The first-order degradation rate constants for PS and TWAS are 0.396 d1 and 0.098 d1, respectively. By comparison, the first-order COD degradation coefficient in the completely mixed anaerobic digestion treating TWAS was 0.045 d1 at an SRT of 15 days (data are not shown). Furthermore, considering the 20% nonbiodegradable VSS in TWAS, the maximum COD biodegradation efficiency of 80% can be achieved in the AnFBR at a HRT of about 12 days, as compared to 60–90 days in conventional anaerobic digestion. Based on the digestion kinetics, in order to achieve the typical 50% VSS destruction efficiencies, the AnFBR should be sized for an HRT of only 2 days when treating primary sludge and 5.2 days when treating TWAS.

3.5. Comparison with other digestion technologies COD removal and VSS destruction efficiency in PS using AnFBR was about 70% at an OLR of 19 kg COD/m3-d, HRT and SRT of 1.9 and 2.9 days, respectively. On the other hand, conventional anaerobic digestion has a HRT and SRT of 15 days or more and are characterized by relatively low organic solids destruction efficiencies of less than 50% (Lee et al., 2011). Moreover, the AnFBR at SRT of 2.9 days achieved successful digester performance and stability which confirmed by a values less than 0.15. COD removal and VSS destruction efficiency in TWAS using the AnFBR was 56% at an OLR of 8.1 kg COD/m3-d, HRT and SRT of 4 and 7.2 days, respectively, as compared with the typical 30–48% achieved in conventional anaerobic digesters (Gossett and Belser, 1982).

Please cite this article in press as: Mustafa, N., et al. Anaerobic digestion of municipal wastewater sludges using anaerobic fluidized bed bioreactor. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.09.081

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N. Mustafa et al. / Bioresource Technology xxx (2014) xxx–xxx

Table 1 Comparison of various anaerobic treatment processes. Type of reactor

Type of wastewater

Influent COD (mg/L)

Influent TSS (mg/L)

OLR (kg COD/ m3-d)

COD removal (%)

HRT (day)

References

AnFBR AnFBR UASB

Primary Sludge TWAS Municipal WW

37,488 34,414 369–958

38,989 34,824 120–443

19 8.3

68 56 69–84

1.9 4.0 4h

UASB CSTR

Municipal WW Primary sludge

341 32,400

190 42,400

2.6 0.6–0.79

34 35

3.2 h 20

CSTR CSTR CSTR

Primary sludge Primary sludge Primary sludge

50,000–65,000

45,000–50,000 40,000 27,000–38,000

2.1–2.9 0.83 1–2

33–47 30–40

10–15 40 13–32

AnMBR

Waste activated sludge

19,800

16,200

2.4–2.6

48

This study This study Von Sperling et al. (2001) La Motta et al. (2007) Ghyoot and Verstaete (1997) Han and Dague (1977) Gomez et al. (2006) Bayr and Rintala (2012) Dagnew et al. (2012)

Semi Continuously Digester CSTR ASBR UASB

Mixed primary sludge and activated sludge Waste activated sludge Thin stillage Food waste Leachates

69,300– 120,800

28,900–40,300

1.55–2.7

97,000 5400–20,000

26,000–33,000 42,000

*

1 9.5 4.3–16

7–15, 30a 20

24 90 58–93

20–40 10 1.25

Bouallagui et al. (2010) Bolzonella et al. (2005) Agler et al. (2008) Agdag and Sponza (2005)

* kg VS/m3 d, CSTR: continuous stirred tank reactor, UASB: upflow anaerobic sludge blanket, AnFBR: anaerobic fluidized bed bioreactor, TWAS: thickened waste activated sludge, AnMBR: anaerobic membrane bioreactor, ASBR: anaerobic sequencing batch reactor. a SRT.

Table 2 Operating conditions and steady-state performance data of AnFBR fed primary sludge. Parameter

Phase I

Phase II

Phase III

Phase IV

Phase V

Operating conditions Time of operation (d) Feed flow rate (L/d) OLR based on anaerobic reactor (kg COD/m3 d) Anaerobic HRT(d) SRT (d) SRT/HRT

12–42 1.8 4.2 8.9 17.2 1.93

43–76 4 9.5 4.0 6.9 1.72

77–118 8.3 19 1.9 2.9 1.53

119–148 16 39 1.0 1.1 1.1

149–180 11 28 1.5 1.7 1.1

Primary sludge characteristics TCOD (mg/L) sCOD (mg/L) TSS (mg/L) VSS (mg/L)

38,900 ± 2880 2490 ± 300 39,860 ± 3410 30,400 ± 2510

37,100 ± 3010 2500 ± 360 39,800 ± 3000 30,500 ± 2960

37,500 ± 2900 2230 ± 250 39,000 ± 2870 31,100 ± 2770

36,000 ± 3200 1940 ± 470 39,300 ± 2800 30,750 ± 3060

38,700 ± 2910 2610 ± 470 40,000 ± 2880 31,300 ± 1980

Digested primary sludge characteristics TCOD (mg/L) sCOD (mg/L) TSS (mg/L) VSS (mg/L)

5780 ± 490 490 ± 30 10,400 ± 800 3700 ± 380

7600 ± 300 380 ± 30 12,000 ± 770 6330 ± 440

12,200 ± 270 250 ± 20 15,900 ± 1080 9400 ± 680

25,300 ± 1900 180 ± 20 28,800 ± 2450 21,300 ± 2010

22,600 ± 1860 180 ± 10 27,200 ± 2300 18,100 ± 1300

Removal efficiencies COD removal eff. (%) VSS removal eff. (%)

85 88

80 79

68 70

30 31

42 42

Methane yields Methane yield (L CH4/g VSSremoved) Methane yield (L CH4/g VSSadded)

0.47 0.41

0.41 0.32

0.41 0.29

0.42 0.13

0.43 0.18

Table 1 compares different anaerobic systems and their performances with different high strength wastewaters. As shown in the Table, HRTs in range of 13–40 days were used in CSTR for treating primary sludge at OLR of 0.6–2.9 kg COD/m3-d. The maximum OLR of 2.9 kg VS/m3-d was achieved with COD removal efficiency of 33%. The maximum VS destruction efficiency of the primary sludge of 47% was achieved at OLR of 2.1 kg COD/m3-d (Han and Dague, 1977). As shown in Table 1, mostly CSTRs have been used to treat the primary sludge due it its higher solids content, 27,000–65,000 mg TS/L and to the authors’ best knowledge, there is no single study in the literature that addressed the anaerobic digestibility of primary sludge in a fluidized bed reactor. Furthermore, Agler et al. (2008) reported 90% COD removal in thin stillage with 42 g TSS/L in an anaerobic sequencing batch reactor (ASBR) at an OLR of 9.5 kg COD/m3-d, and an HRT of 10 days.

Agdag and Sponza (2005) have achieved 58%-79% COD removal efficiency for food waste leachates in an UASB at an OLR of 4.3– 16 kg COD/m3-d while Zhang et al. (2008) achieved 93% COD removal from palm oil mill effluent with 35 g COD/L and 12 g TSS/L in an EGSB at an OLR of 10 kg COD/m3-d. It must be asserted that although high-rate anaerobic technologies such as the ASBR, UASB, EGSB have been successfully applied to highstrength industrial wastes, however, applying those technologies to PS and TWAS digestion are more challenging due to the high solids content of the two aforementioned wastes and relatively lower biodegradability. Although a detailed comprehensive economic analysis between the AnFBR and conventional digestion is beyond the scope of this study, the superficial liquid up flow velocity required for fluidization, which constitutes 94% of the operational cost of FBR is

Please cite this article in press as: Mustafa, N., et al. Anaerobic digestion of municipal wastewater sludges using anaerobic fluidized bed bioreactor. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.09.081

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N. Mustafa et al. / Bioresource Technology xxx (2014) xxx–xxx Table 3 Operating conditions and steady-state performance data of AnFBR fed TWAS. Parameter

Phase I

Phase II

Phase III

Phase IV

Operating conditions Time of operation (d) Feed flow rate (L/d) OLR based on anaerobic reactor (kg COD/m3 d) Anaerobic HRT(d) SRT (d) SRT/HRT

23–62 1.8 4.2 8.8 16.7 1.89

63–103 4 8.3 4.0 7.2 1.8

104–141 8.3 18.2 1.9 2.7 1.4

142–172 6 13.1 2.6 2.8 1.07

TWAS characteristics TCOD (mg/L) sCOD (mg/L) TSS (mg/L) VSS (mg/L)

34,900 ± 3230 290 ± 40 32,900 ± 2050 30,200 ± 2120

34,400 ± 2690 500 ± 80 34,800 ± 3860 30,500 ± 2280

35,000 ± 3810 510 ± 90 36,000 ± 3700 30,100 ± 3050

35,200 ± 1910 540 ± 60 36,300 ± 3100 30,800 ± 2100

Digested TWAS sludge characteristics TCOD (mg/L) sCOD (mg/L) TSS (mg/L) VSS (mg/L)

11,300 ± 870 520 ± 50 9960 ± 820 9390 ± 680

15,400 ± 1100 600 ± 50 18,000 ± 900 13,300 ± 690

23,200 ± 2010 420 ± 50 26,500 ± 1700 20,400 ± 1860

20,400 ± 2000 310 ± 30 24,800 ± 1800 17,800 ± 1200

Removal efficiencies COD removal eff. (%) VSS removal eff. (%)

68 69

55 56

34 33

42 42

Methane yields Methane yield (L CH4/g VSSremoved) Methane yield (L CH4/g VSSadded)

0.51 0.35

0.5 0.28

0.54 0.18

0.5 0.21

VSS Destruction based on Liptak Eqn for PS VSS destruction measured for PS VSS destruction based on Liptak Eqn for TWAS VSS destruction measured for TWAS Linear ( VSS destruction measured for PS ) Linear ( VSS destruction measured for TWAS)

VSS Destruction Efficiency

100 y = 20.705x + 35.416 R² = 0.8894

80 60

y = 17.767x + 19.756 R² = 0.9526

40 20

A

0 0

1

2

3

4

0.35 cm/s (Eldyasti et al., 2012). For a conventional 10 MGD plant, the anaerobic digester capital cost needed to handle about 11,000 kg TSS/d (70% volatile) is about $10 million dollar or $6/gallon of digester volume. Accordingly using a conservation design COD loading rate of 12 kg COD/m3-d for combined (50% PS and 50% TWAS) would result in a capital cost of $1.4 million dollar excluding the zeolite media. Thus, using a typical fill factor of 22%, a bulk density of 944 kg/m3, and a cost of $1.0/lb to include delivery, shipping, and installation in the digester, the total capital cost of the AnFBR would be less than $2 million dollar. The fluidization energy, based on a liquid up-flow velocity of 0.35 cm/s (Andalib et al., 2012b), an AnFBR height of 10 m. and a unit electricity cost of $0.1/KWH would cost about $54,000/year, or 0.675% of the capital cost savings, not to mention the value of the additional biogas due to the improved volatile solids reduction.

Ln (SRT) Fig. 2. Comparison of Liptak model and observed VSS destruction efficiencies at various SRTs.

0

1

2

3

HRT (d) 4 5

6

7

8

9

10

Ln (CODeff/CODin)

0.00 -0.20

TWAS

-0.40

PS

-0.60

The AnFBR successfully treated the primary sludge at OLR of 19 kg/m3-d, achieving COD removal efficiency of 68% and VSS destruction efficiency of 70%, with VSS destruction efficiency dropping to 42% and 31% at OLR of 28 and 39 kg/m3-d, respectively. The AnFBR successfully treated the TWAS at OLR of 8 kg/m3-d, achieving COD removal efficiency of 55% and VSS destruction efficiency of 56%, with VSS destruction efficiency decreasing to 42% and 33% at OLR of 13 and 18 kg/m3-d. Acknowledgements

-0.80

y = -0.0984x - 0.2978 R² = 0.9373

-1.00 -1.20 -1.40

4. Conclusions

y = -0.3961x - 0.0688 R² = 0.868

This work was jointly funded by the Tomlinson Environmental Services and the Applied Research and Commercialization program of the Federal Economic Development Agency for Southern Ontario. References

-1.60 -1.80 Fig. 3. First-order COD degradation in the AnFBR for primary sludge and TWAS.

Agdag, O.N., Sponza, D.T., 2005. Anaerobic/aerobic treatment of municipal landfill leachate in sequential two-stage up-flow anaerobic sludge blanket reactor (UASB)/completely stirred tank reactor (CSTR) systems. Process Biochem. 40, 895–902.

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Please cite this article in press as: Mustafa, N., et al. Anaerobic digestion of municipal wastewater sludges using anaerobic fluidized bed bioreactor. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.09.081

Anaerobic digestion of municipal wastewater sludges using anaerobic fluidized bed bioreactor.

The anaerobic digestion of primary sludge (PS) and thickened waste activated sludge (TWAS) using an anaerobic fluidized bed bioreactor (AnFBR) employi...
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