Bioresource Technology 99 (2008) 222–226

Short Communication

Development of an improved anaerobic Wlterfor municipal wastewater treatment

Sandeep Bodkhe ¤

National Environmental Engineering Research Institute, Nehru Marg, Nagpur-440 020, India

Received 5 September 2006; received in revised form 17 November 2006; accepted 20 November 2006Available online 5 January 2007

Abstract

Development of an improved reactor conWguration of anaerobic Wlter was carried out for the elimination of clogging of Wlter media.The experiments over diVerent hydraulic retention times (HRTs) indicated that the HRT of 12 h was the most appropriate one for the sys-tem studied while treating the municipal wastewater, which resulted 90% and 95% BOD and COD reduction, respectively. Reduction upto 95% in suspended solids concentration could be achieved without any pretreatment. The speciWc biogas yield obtained was 0.35 m3

CH4/kgCODr with 70% of CH4 content in the biogas generated from the system at the HRT of 12 h. Operational problems such as clog-ging of Wlter media were not observed throughout the period of study over 600 d.© 2006 Elsevier Ltd. All rights reserved.

Keywords: Anaerobic Wlter; Wastewater treatment; Suspended solids; Media clogging

1. Introduction

Fixed bed Wxed Wlm anaerobic reactors viz. anaerobicWlters (AF), use the porous medium packed in the reactorto support the biomass in the form of microbial Wlms.Media facilitates retention of biomass in the reactor forlonger duration achieving longer mean cell residence time(Henze and Harremoes, 1983; Ghangrekar and Kahalekar,2003; Bodkhe, 2003). This is important feature of anaerobicWlter (AF) for achieving better treatment eYciency. How-ever, AF faces clogging problem when wastewater passesthrough the media due to the presence of suspended solids(SS), leading to short circuiting of wastewater. This leads toincomplete treatment of wastewater impairing the reactoreYciency and yielding unacceptable eZuent quality (Bod-khe, 2003). Many researchers have also observed the clog-ging phenomenon and the associated problems in theoperation of the anaerobic Wlters due to the presence of SSin wastewater. They suggested removal of SS from waste-

* Tel.: +91 712 2249885 90x265; fax: +91 712 2249896.E-mail address: sy_bodkhe@neeri.res.in

0960-8524/$ - see front matter © 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.biortech.2006.11.026

water prior to its application to the anaerobic Wlter (AF).Jahren et al., 1999; Show and Tay, 1999; Elmitwalli et al.,1999, 2001; Bihan and Lessard, 2000; Harmand et al., 2002;Seghezzo et al., 2000; Foresti, 2002; Ghangrekar andKahalekar, 2003). Thus, for successful operation of AFfor longer duration, control of SS in the inXuent wastewateris essential.

In this study, an improved anaerobic Wlter was devel-oped in which a module of inclined tube settlers (ITS) wasintegrated with Wxed bed Wxed Wlm (FFFB) anaerobic reac-tor i.e. anaerobic Wlter (AF) as a solid–liquid separationsystem.

2. Methods

2.1. Reactor conWguration

A module of inclined tube settlers (ITS) was incorporatedin the reactor to control the input of SS to the anaerobicWlter to avoid media clogging. The module of ITS wasdesigned on the principles of high rate sedimentationproposed by Michael (1972). The module consisted of 11number of PVC tubes of 700 mm length and 40 mm diameter.

S. Bodkhe / Bioresource Technology 99 (2008) 222–226 223

Total empty bed volume of the reactor was 0.170 m3 witheVective volume of 0.120 m3. Locally available material(burnt brickbats) was used as a packing media. The averagediameter and speciWc surface area of media was 20mm and200 m2/m3 respectively. The total surface area available in thereactor for biomass attachment was 8.685 m2.

2.2. Wastewater Xow pattern

The municipal wastewater was collected in feed tank andwas pumped using a peristaltic pump. Inclined tube settler(ITS) module facilitated high rate sedimentation therebyarresting the suspended solids present in the wastewater.The partially treated wastewater with lower concentrationof suspended solids was further passed on to the middlechamber Wlled with media. Treated eZuent was withdrawnfrom top of media chamber by outlet pipe.

2.3. Analytical assessment

The physico-chemical characteristics of municipalwastewater are given in Table 1. The analysis of variousparameters of inXuent and eZuent was done as per Stan-dard Methods (APHA, 1995). Biogas generation and itscharacterization were carried out by using wet gas Xowmeter and gas chromatography respectively.

Municipal wastewater was collected from the municipalwastewater collection and treatment works of Nagpur City(India) and the inoculum was brought from the biogasplant of the same facility. To start with, the reactor wasWlled with municipal wastewater and the anaerobic sludgein 50:50 proportion. The mixture was allowed to stay oneweek in the reactor under anaerobic conditions; thereafterthe wastewater was fed to the reactor at a very low rate.The organic loading rate (OLR) and hydraulic retentiontime (HRT) were maintained at 0.0333 kg COD/m3/d and12 d respectively. Longer HRT and low OLR are prerequi-sites to the successful start-up of anaerobic reactor as itallows biomass to acclimatize to the new environment and

Table 1Physico-chemical characteristics of municipal wastewater

Parameter Concentration

Temperature (°C) Winter 10–30Summer 20–42

pH 7.5–8.2Alkalinity (mg/l as CaCO3) 230–300SS (mg/l) 300–450VSS (mg/l) 240–382BOD (mg/l) 200–300COD (mg/l) 350–450COD:N:P ratio 100:10:1.3Total Kjeldhal nitrogen (mg/l) 30–45Total phosphorus (mg/l) 5–6Sulphate as S (mg/l) 60–100Total sulphide (mg/l) 2–6Oil and grease (mg/l) 15–20Phenol (�g/l) 30–40

help to increase their population. After 71 d of operation atthe same OLR and other operational conditions, reductioneYciencies of COD, BOD and SS were found to be consis-tent. Rate of biogas production and its methane contentwere also found to be consistent during this period. Thissteady state of operation indicated complete acclimatiza-tion of the biomass and hence the completion of the phaseof reactor start-up.

3. Results and discussion

The performance of reactor was studied at varioushydraulic retention times (HRTs). Once the steady state ofoperation was achieved at that particular HRT, the HRTwas further shifted to next lower value and so on byincreasing the volumetric Xow rate of inXuent wastewater.Steady state of operation was deWned on the basis of thesteady observations of the reduction in the parameters suchas BOD, COD, SS and biogas generation and its methanecontent. Performance of the reactor studied for 600 d atdiVerent HRTs is shown in various Wgures and discussed infollowing paragraphs. Data indicated that the HRTs higherthan 12 h had a marginal improvement in the performanceof the reactor system in terms of reduction in organic pollu-tants and generation of biogas whereas, the HRTs lowerthan 12 h resulted in deteriorated quality of eZuent. Hence,HRT of 12 h was considered to be appropriate for theeYcient operation of the reactor system.

3.1. pH, alkalinity and total volatile acid

The municipal wastewater was slightly alkaline in naturedue to presence of detergents, soaps etc. During the studyperiod the pH values of the inXuent were observed to be inthe range of 6.8–7.6. The pH values of the eZuent wereobserved to be in the range of 6.4–7.2 at various HRTs. Itwas observed that during the shift from a particular HRTto a next lower HRT, the reactor had developed acidic con-ditions causing a drop in pH values of the reactor contentand the treated eZuent. However, this happened for a veryshort period; therefore there was no need to supply extraalkalinity. The inXuent alkalinity (as CaCO3) was observedto be in the range of 230–300 mg/l whereas, the eZuentalkalinity varied between 178 and 400 mg/l. At the HRT of12 h and at higher HRTs, the eZuent alkalinity was noticedto be 25–33% more than the inXuent alkalinity due toformation of carbonates and bicarbonates in the reactor.Manariotis and Grigoropoulos (2006) observed 19–21%higher alkalinity in the Wlter eZuents compared to thecorresponding inXuent level, reXecting the formation ofammonium bicarbonate (NH4HCO3). The ratio of totalvolatile acids to alkalinity i.e. TVA/ALK represents a usefuland sensitive tool for monitoring of anaerobic biodegrada-tion. From HRT of 12 d to the HRT of 12 h, the value ofTVA/ALK ratio for the eZuent was lower than unity asrecommended by Amatya (1996), which prevented reactorinstability.

224 S. Bodkhe / Bioresource Technology 99 (2008) 222–226

3.2. Suspended solids removal

EVect of the lowering of HRT on removal eYciency ofsuspended solids (SS) and volatile suspended solids (VSS)seemed to be insigniWcant. At HRT of 12 h, surface over-Xow rate and retention time available in individual tubesettler were 18.25 m3/m2/d and 0.1 h, respectively. It wasobserved that inXuent SS concentration varied in the rangeof 300–450 mg/l whereas, the eZuent SS concentration wasless than 25 mg/l most of the times during study period asindicated in Fig. 1. The SS reduction eYciency was 95% atHRT of 12 h and was around 90% at nearly all the lowerHRTs considered in this study. The ability of reactor toretain solids even at high upXow velocities (Vup) caused due

to lower HRTs may be attributed to the incorporation ofa module of tube settlers. During the study period of 600 dthe problem of clogging of anaerobic Wlter was notobserved. Such long term operation without cleaning(Manariotis and Grigoropoulos, 2006) saves cost of opera-tion and maintenance.

3.3. Removal of BOD and COD

Variation in HRT and its eVect on performance of sys-tem in terms of eZuent BOD and COD concentrations andreduction eYciencies have been indicated in Figs. 2 and 3,respectively. As the HRT was increased, BOD and CODreduction eYciencies of the system were also found to be

Fig. 1. Reduction in SS concentration at various HRTs.

0

50

100

150

200

250

300

350

400

450

0.125 0.25 0.33 0.5 0.75 1 1.5 3 4 6 8 12

HRT (d)

SS C

once

ntra

tion

(mg/

l)

80

82

84

86

88

90

92

94

96

98

Red

ucti

on E

ffic

ienc

y (%

)

Influent SS Concentration (mg/l)Effluent SS Concentration (mg/l)% Reduction Efficiency (%)

Fig. 2. Variation in BOD reduction eYciency at various HRTs.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.125 0.25 0.33 0.5 0.75 1 1.5 3 4 6 8 12HRT (d)

Org

anic

Loa

ding

Rat

e (K

gBO

D/c

um/d

)

50

55

60

65

70

75

80

85

90

95

100B

OD

Red

ucti

on E

ffic

ienc

y (%

)

Organic Loading Rate (KgBOD/cum/d)

BOD Reduction Efficiency (%)

S. Bodkhe / Bioresource Technology 99 (2008) 222–226 225

increased and vice-versa. At HRT of 12 d, the BOD andCOD reduction eYciencies were 98% and 97%, which werereduced to 91% and 89%, respectively at the HRT of 12 h.EZuent BOD concentration was observed to be below100 mg/l at the lowest HRT of 3 h and it was 28 mg/l at 12 hHRT. The eZuent COD concentration was 41 mg/l at 12 hHRT which was found to be increasing with HRT and was160 mg/l at 3 h HRT.

At lower HRTs, higher upXow velocities (Vup) throughthe reactor caused shorter contact period between organ-isms and organics resulting in decreased mass transfer rate.The reduced mass transfer rate has led to reduced perfor-mance eYciency. Considering hydrolysis as a rate limitingstep in anaerobic degradation process, the alternativesalready studied include, a previous suspended solids separa-tion step (Elmitwalli et al., 2001). Although the inXuentwastewater characteristics such as concentration of sus-pended solids, BOD and COD were changing diurnally, theeZuent quality was observed to be consistent at a particu-lar HRT.

3.4. Biogas generation

SpeciWc biogas yield (calculated as methane generatedper kg of COD removed) at the HRT of 12 h was found tobe 0.35 m3 CH4/kgCODr. However, the values of speciWcbiogas yield were observed to be lower at lower HRTs. Thebiogas composition was 60–70% methane, 30–35% carbondioxide with nitrogen and hydrogen sulphide in traces. Atsame HRT diVerent biogas generation rates were observedduring diVerent operation periods resulting in delayed sta-bilization of accumulated organic matter. This was moreprominent when the unit was operated at a higher HRTthan in the preceding period as also noted by Manariotisand Grigoropoulos (2006).

4. Conclusion

Although the inXuent wastewater characteristics such asconcentration of suspended solids, BOD and COD werechanging diurnally, the eZuent quality was observed to beconsistent at a particular HRT. The reactor exhibited con-sistent performance and high treatment eYciency at lowerHRTs. The problem of clogging of media was not observedduring study period of 600 d, indicating that the pretreat-ment of municipal wastewater was not necessary whileusing this improved reactor conWguration.

References

Amatya, P.L., 1996. Anaerobic treatment of tapioca starch industry waste-water by bench scale upXow anaerobic sludge blanket (UASB) reactor.M. Eng. Thesis. AIT. Bangkok. Thailand.

APHA (American Public Health Association), 1995. Standard Methodsfor the Examination of Water & Wastewater, 19th ed. Water Environ-ment Federation, Alexandria, VA.

Bihan, L., Lessard, P., 2000. Monitoring bioWlter clogging: biochemicalcharacteristics of the biomass. Water Res. 34 (17), 4284–4294.

Bodkhe, S.Y., 2003. Concept and application of a novel anaerobic reactorsystem for decentralised sewage treatment. In: The Proceedings of 5thGVC-Abwasser Kongress, Bremen (FRG), during Sept. 22–24, 2003,Vol. 2, pp. 557–563.

Elmitwalli, T.A., Zandvoort, M., Zeeman, G., Lettinga, G., 1999. Low tem-perature treatment of domestic sewage in upXow anaerobic sludgeblanket and anaerobic hybrid reactors. Water Sci. Technol. 39 (5), 177–185.

Elmitwalli, T.A., Soellner, J., Keizer, A.D., Bruning, H., Zeeman, G., Lett-inga, G., 2001. Biodegradability and change of physical characteristicsof particles during anaerobic digestion of domestic sewage. Water Res.33 (5), 1311–1317.

Foresti, E., 2002. Anaerobic treatment of domestic sewage: establishedtechnologies and perspectives. Water Sci. Technol. 45 (10), 181–186.

Ghangrekar, M.M., Kahalekar, U.J., 2003. Performance and cost eYcacyof two-stage anaerobic sewage treatment. J. Environ. Eng., Institutionof Engineers (India). 84 (September), 16–22.

Fig. 3. Variation in COD reduction eYciency at various HRTs.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.125 0.25 0.33 0.5 0.75 1 1.5 3 4 6 8 12

HRT (d)

CO

D R

educ

tion

Eff

icie

ncy

(%)

40

45

50

55

60

65

70

75

80

85

90

95

100

Org

anic

Loa

ding

Rat

e (k

gCO

D/c

um/d

)

Organic Loading Rate (kgCOD/cum/d)

COD Reduction Efficiency (%)

226 S. Bodkhe / Bioresource Technology 99 (2008) 222–226

Harmand, J., Miens, F., Conte, T., Gras, P., BuYère, P., Steyer, J.P., 2002.Model based prediction of the clogging of an anaerobic Wxed bed reac-tor. Water Sci. Technol. 45 (4–5), 255–262.

Henze, M., Harremoes, P., 1983. Anaerobic treatment of wastewaters in WxedWlm reactors: a literature review. Water Sci. Technol. 15 (8–9), 1–101.

Jahren, S.J., Rintala, J.A., Ødegaard, H., 1999. Anaerobic thermophilic(55 °C) treatment of tmp whitewater in reactors based on biomassattachment and entrapment. Water Sci. Technol. 40 (11–12), 67–76.

Manariotis, I.D., Grigoropoulos, S.G., 2006. Municipal wastewater treat-ment using upXow anaerobic Wlters. Water Res. 78 (3), 233–242.

Michael, M.C., 1972. Sedimentation in inclined tubes and its applicationsfor the design of high rate sedimentation devices. J. Hydraul. Res. 10(1), 1338–1341.

Seghezzo, L., Castaneda, M.L., Trupiano, A.P., Gonzalez, S.M., Guerra,R.G., Torrea, A. Cuevas, C.M., Zeeman, G., Lettinga, G., 2000. Anaero-bic treatment of presettled sewage in UASB reactors in tropical regions(Salta-Argentina). In: Preprints of the VI Latin-American Workshopand Seminar on Anaerobic Digestion, Vol. 1, pp. 7–13.

Show, K.U., Tay, J.H., 1999. InXuence of support media on biomassgrowth and retention in anaerobic Wlters. Water Res. 33 (6), 1471–1481.