International Journal of Environment Science and Technology Center for Environment and Energy Research and Studies (CEERS) ISSN: 1735-1472 EISSN: 1735-2630 Vol. 3, Num. 1, 2006, pp. 69-77

High rate anaerobic treatment of Sago wastewater using HUASB with PUF as carrier

^1*J. Rajesh Banu, ²S. Kaliappan and ³D. Beck

¹Department of Biotechnology, Jeppiaar Engineering College Chennai, India
²Centre for Environmental Studies (CES), Anna University, Chennai- 600025 India
³ Senior Advisor, Indo-German Project, CES

*Corresponding Author, E-mail: rajeshces@gmail.com.

Received 4 November 2005;
revised 18 February 2006;
accepted 3 March 2006
available online 18 April 2006

Code Number: st06009

ABSTRACT

Sago industry is one of the major small-scale sectors in India and over 800 units are located in the southern State of Tamilnadu. Processing of sago generates enormous quantities of high strength wastewater requiring systematic treatment prior to disposal. The present study is an attempt to treat the sago wastewater using Hybrid Upflow Anaerobic Sludge Blanket (HUASB) reactor, which offers the advantages of both fixed film and up flow anaerobic sludge blanket treatment. HUASB reactor with a volume of 5.6 L was operated at Organic Loading Rates varying from 10.7 to 24.7 kg COD/m³.day. After 130 days of startup, the reactor produced appreciable decrease in COD of wastewater and removed solids efficiently. The COD removal varied from 91-87%. While the removal of Total Solids was in the range of 61-57%, that of volatile solids varied from 70-67%. The ideal OLR for the reactor was 23.5 kg COD/m³.day. The findings of the study open up newer possibilities of design low cost and compact onsite treatment systems with very short retention periods.

Key words: Sago effluent, HUASB, treatment efficiency, biogas

INTRODUCTION

Sago, the common edible starch in the form of globules is processed from the tubers of tapioca (Mannihot utillisema). Processing of tapioca requires 20,000 to 30,000 L of water per ton of Sago; besides it produces equal quantity of wastewater, which is highly organic, foul smelling and acidic (Murthy and Patel, 1961; Sastry and Mohan, 1963). Various anaerobic technologies including conventional anaerobic treatment (Sastry et al., 1964; Tongkasane, 1970; Saroja and Sastry, 1972; Pescod and Thanh, 1977), high rate anaerobic treatment such as Anaerobic Filter (AF- Khageshan and Govindan, 1998) and Fluidized Bed (FB -Saravanane et al., 2001) have been attempted to treat Sago wastewater. The conventional treatment options are known to have low treatment efficiency owing to high concentrations of solids present in the wastewater. Published works indicate that most of the negative aspects of high rate anaerobic digestion can be overcome by restricting the supporting material to the top 25 to 30% of the reactor volume (Guiot and Van den berg, 1984; 1985). This would further help realize the advantages of both fixed film and up flow sludge blanket treatment. This kind of reactor often calledHybridUp flowAnaerobic Sludge Blanket (HUASB) reactor and is considered more stable for the treatment of a series of soluble or partially soluble wastewaters (Tilche and Vieira, 1991). Over the years, HUASBs have been used to treat wastewaters from sugar industry (Coates and Colleran, 1990), pharmaceutical units (Hentry et al., 1996), distilleries (Shivayogimath and Ramanujam, 1999) and domestic sectors (Elmitwalli et al., 2002 a,b). In the present study, an attempt has been made to use the HUASB reactor to treat Sago wastewater.

The Schematic of HUASB reactor is illustrated in Fig. 1.Thelaboratoryscale reactor wasfabricated using PVC tube with an internal diameter of 110 cm and an overall height of 87cm. Total volume of the empty reactor was 6.7 L. A gas headspace equivalent to 1.5 L was maintained above the effluent line. A screen was placed at a height of 59 cm to arrest the floating packing material. A peristaltic pump (Make: Miclins PP20) was used for feeding the wastewater into the reactor. The effluent pipeline intern was connected to a water seal to prevent the escape of gas. The gas outlet was connected to a wet gas meter (Make: Ritter. Model No: TG1)

Carrier material

Polyurethane foam (PUF) cubes 150 cubes each (measuring 2 cm x 2 cm x 2 cm) was used as carrier material. Polyurethane is known to offer excellent colonization matrix in high rate reactors (Hysman et al., 1985) and entrap suspended solids in wastewater (Elmitwalli et al., 2002a) leading to an increase in the overall treatment efficiency.

Seed and inoculation

Digested slurry from an active biogas plant located at MurugappaChettiarResearch Institute, Chennai was used as seed. To accelerate the start up, 40 % (v/v) slurry was mixed with the feed as recommended by Shapiro and Switzenbanum (1984) and Hickleyet al., (1991).

Sago effluent

Synthetic wastewater was prepared by following Khageshan and Govindan (1995) and the physico chemical characteristics of the synthetic Sago wastewater was listed in Table 1.

Start up phase

During the start up, the reactor operation was initiated using wastewater with a COD of 2000 mg/L. The initial retention time was 56h. The HRT was gradually decreased to 56 h., which is equal to the volume of the reactor. This was achieved by increasing the flow rate from 100 mL/h to 1000 mL/h over a period of 130 days.

Treatment phase

After the start up, the reactor was operated by varying the influent Chemical Oxygen Demand (COD) at a constant HRT 5.6 h. The efficiency of the treatment was evaluated intermsof removal ofTotal Solids (TS),Volatile Solids (VS) and COD and generation of biogas.

Chemical analysis

COD, Volatile Fatty Acids as acetate (VFA), Total Alkalinity, TS, VS and Total Kjeldhal Nitrogen (TKN) ofthe raw and treated wastewater wereanalysed following Standard Methods(1998). Anionssuchas phosphate(PO₄³) sulphate(SO₄^2-) and chloride (Cl^-) wereanalysed employing ion exchangechromatography (Make: Dionex,Model:DX120) after filtering the samples through a 0.45 μ filter. The eluent was a combinationof3.5 mMbicarbonateand 1 mM carbonate; theflowratewas 1.2 mL/minutewith an injection volume of 25 μL. Methane content in the biogas was measured byGas Chromatography (Make:Chemito, Model: GC1000) equippedwith FlameIonization Detector(FID). The columnused was Proapak Q.

The loading regime and biogas production of the reactor during the start up phase are presented in the Fig. 2. The initial OLRappliedduringstartup was0.85 kg COD/m³.day ata HRT of 56hours. This HRT was preferred to prevent the washout of inoculated biomass (Hickely et al., 1991). When the OLR was increased in a stepped manner to8.5 kg COD/m³.day over a periodof 130 days, the biogas production also increased gradually reaching a maximumof 9.4 L/day at an OLR of 8.5 kg COD/m³.day. Fig. 3 presents COD removal and VFA accumulation patternduringstartup period.The CODremoval increased with time; this is in conformity with the findings of Saravanane et al., (2001) during the treatment of Sago wastewater. The VFA concentration in the effluent atthe initial OLR0.85kg COD/m³.daywasin the rangeof 780-720 mg/L from thatit fell down to548 mg/L at OLR2.5 kg COD/ m³.day. Higherlevels ofVFA inthe wastewaters duringthe initial phases of operation indicate the prevalence of acid fermentation(VanHanndel and Lettinga, 1994).Subsequently, theVFA in the wastewaterdecreasedand was inthe rangeof 525 to 447mg/L indicating healthyanaerobic environment and satisfactory methanogenic activity. The overall performance of the reactor during the start-up was more than satisfactory. It is known that selection of seed material plays a crucial rolein minimizingtime required forinitial bioflim establishment (Bull et al., 1983; Salkinoja – Salonen et al., 1983). Itis likely that theslurrycollected from anactive biogas plant and used as a seed had sufficient numbers of physiologically activemicroorganisms.

Treatment phase

Fig. 4 illustrates the loading pattern and biogas production during the treatment. The initial OLR applied during this phase (Day-131) was 10.7 kg COD/m³day. It was increased in a stepped manner to 24.7 kg COD/ m³.day over a period of 245 days. The increment between successive OLR was about 2.1 kg COD/m³.day. The gas production increased as the OLR increased, reaching a maximum28.2 m³/dayat an OLRof 23.5 kg COD/m³.day. Beyond this loading, the gas production decreased with increase in OLR (24.7 kg COD/m³.day). Table 2 presents data on Normalized Methane Production (NMP), and methane content of biogas at various phases. It is evident from the Table that at an OLR of 10.7 kg COD/m³.day the NMP and methane content of the reactor were 0.11 m³CH₄/Kg COD.day and 65 ±3, respectively.Asthe OLRincreased, the NMP exhibited a gradual increase; at an OLR of 23.5 kg COD/ m³.day, the NMP reached a maximum of 0.13 m³CH₄/Kg COD.day. The NMP and methane content of biogas touched an all time lowof 0.06 m³CH₄/Kg COD.day and 55±2, respectivelywhen the OLR wasincreased to24.7 kg COD/m³.day.

The methane content in the biogas at different phases varied from 55±2 to 61±2 %, which is comparable to 52 to 63%, reported during the treatment of Sago wastewater in anaerobic filter by Khageshan and Govindan (1995). Influence of OLR on COD removal and VFA concentration in the medium during the treatment of Sago wastewater is illustrated in Fig. 5 and Table 2. It is evident from the Fig. that at an OLR of 10.7 kg COD/m³.day theCOD removal was 91%.As the OLR increased, the COD removal exhibited a gradual decrease; at an OLR of 23.5 kg COD/m³.day the COD removal was 87% and as mentioned elsewhere in the discussion the gas production was maximum at this OLR. The COD removal touched an all time low of 70% when the OLR was increased to 24.7 kg COD/m³.day. During the stable operational phase (OLR: 10.7 to 23.5 kg COD/m³.day) of the reactor, the VFA (as acetate) levels in the medium varied from 420 to 640 mg/L. VFA started building up in the wastewater as the digestion proceeded and the maximum concentration of 2650 mg/ Lwas recorded at an OLR of 24.7 kg COD/m³.day. VFA has been recognized as one of the important intermediates during the anaerobic digestion (Wang et al., 1999; Ahring Angelidaki 1997) and is considered a central parameter for anaerobic treatment (Ahring and Angelidaki, 1995; Pind et al., 1999; 2002). The impact of VFA accumulation was reflected in the marked decrease of COD removal from 86 to 68% when the OLR was increased to 24.7 kg COD/m³.day. Working on synthetic dairy wastewater using UASB, Fang et al., (1994) have reported a VFA concentration of over 2500 mg/L responsible for souring of the anaerobic reactor. Fig. 6 presents variations in pH during the treatment. The pH of the treated wastewater was in the range of 7.5 – 7.9 up to an OLR of 23.5 kg COD/m³.day which is indicative of the satisfactory condition of the reactor. It is known that pH less than 6.8 and greater than 8.3 would cause souring of the reactor during anaerobic digestion (Wheatly 1991). The pH of the wastewater dropped to 6.2 when the OLR was increased to 24.7 kg COD/m³.day from 23.5 kg COD/m³.day. Alkalinity of the effluent increased from 1510 mg/L at an OLRof 10.7kg COD/m³.day to 3120 mg/Lat an OLR of 23.5 kg COD/m³.day (Fig. 7). Increase in OLR beyond this level caused a decrease in alkalinity in the wastewater. Alkalinity isknown to be a critical buffering factor for neutralizing VFA during methanogenesis (Pohland 1969). Decreasein alkalinity obviouslyaffects the buffering capacity and leads to dropping down of pH. Fig. 8 and Table 2 presents the influence of OLR on the removal of TS and VS from the Sago wastewater during the treatment phase. The determination of VS is useful in control of wastewater treatment plant operation as it offers rough approximation of the amount of organic matter present in the solid fraction of wastewater (Standard Methods, 1988). Under stable operation conditions (up to an OLR of 23.5 kg COD/ m³.day) the removal of TS and VS in the wastewater was in range of 61-57% and 70-67%, respectively. Like all other parameters the removal efficiency of both TS and VS decreased drastically when the OLR was increased from 23.5 to 2.4.7 kg COD/m³.day; the respective values for TS and VS being 50 and 61%. Table 3 presents the utilization pattern of nutrients (organic nitrogen and phosphorous) and the removal of sulphate during the treatment of Sago wastewater. From the Figs. it is evident that the utilization of organic nitrogen was markedly higher than that of phosphorus. Higher utilization of nitrogen in comparison with phosphorous during the anaerobic treatment of Sago wastewater has been reported by Subramanian and Sastry (1989) also. Utilization of organic nitrogen fell from 39 to 36% when the OLR was increased from 10.7 to 23.5 kg COD/m³.day. Similarly, the phosphorous utilization fell from 22% at an OLR of 10.7 kg COD/ m³.day to 16% at an OLR of 23.5 kg COD/m³.day. The decrease in nutrient utilization at higher OLRs can be attributed to higher flow rate of the wastewater and the consequent reduction in contact time between nutrients and microbes as well as the nutrient washout. The removal of sulphate in the wastewater was in the range of 84-81%. The chloride concentration in the effluent remained unaffected during the treatment.

The higher loading rate achievable (23.5 kg COD/ m³.day observed) in the present study can be attributed to the structural modifications effected to UASB and also the filter material used. For instance, working on the treatment of Sago wastewater using AF, with granite stones as filter medium Subrahmanyam and Sastry (1989) have reported that the gas production decreased at an OLR of 16 kg COD/m³.day. Khageshan and Govindan (1995) have reported that beyond an OLR of 11.6 kg COD/m³.day, gas production decreased during the treatment of synthetic Sago wastewater using anaerobic filter with basalt stone chips as filter medium. In contrast to the present observations, Saravanane et al., (2001) have reported a very high OLR of 60.5 kg COD/m³.day during the start up phase of treatment of synthetic Sago wastewater using a fluidized bed reactor. Interestingly, the efficiency of this reactor in terms of COD removal (82%) is lower than the value recorded during the present study (92%) during the start up. Further comparison between these two studies is not possible in the absence of data on HRT maintained by Saravanane et al., (2001). The anaerobic treatment of Sago wastewater using the Hybrid Upflow Anaerobic Sludge Blanket indicated promising results. The reactor could be operated at a considerably higher OLR for 23.5 kg COD/m³.day, which is twice the loading rate suggested for the treatment in Anaerobic Filter. Further, it was possible to achieve higher COD removal (86%) and considerable generation of biogas (28.7 m³/day) at a HRT as low as 5.6 h. The results are significant, especially in the context of wastewater treatment in tropical developing countries, where reactor design with low HRT and high OLR would be a technologically viable and economically affordable option.

ACKNOWLEDGEMENT

Authors are very much thankful to Indo-German project at Centre for Environmental Studies for providing facilities under their project.

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