Development of a High- rate Anaerobic Bioreactor for Energy Production from Rice-mill Effluent

Abstract

Anaerobic digestion of agricultural, industrial and municipal wastes has a great relevance in the global renewable energy scenario, since it combines waste stablisation with net fuel production. RME is a low strength, high volume waste for which anaerobic treatment can be economically and technologically made feasible by adopting high rate processes. Hence, an investigation was taken up to develop an anaerobic high rate reactor for biomethanation of RME. It was revealed that the RME had a low pH along with high BOD and COD. The batch digestion studies proved that it is amenable to anaerobic digestion. The semi-continuous studies to test media compatibility could reveal that the reactor could be feed with RME without prior neutralisation. The study established the compatibility and suitability of rubber seed outer shells as packing media in high rate reactors and hence this was selected to be used in Up-flow Anaerobic Hybrid Reactors (UAHRs). Eight lab scale UAHRs were designed and fabricated, with two different media for immobilization viz. polyurethane rings and rubber seed outer shell. The daily feeding in the reactors were started from the 25 th day after initial charging and operated for 31 days, with a startup HRT of 10 day. The UAHRs were then operated at HRTs of 10, 5, 4 ,3 , 2, 1 and 0.8 day and the performance evaluated. All reactors were stable in operation and exhibited high process efficiency characterised by good organic reduction and biogas production. This was due to the high degree of cell immobilisation obtained in the hybrid design. The performance deteriorated with reduction in HRT. The methane content of the biogas remained fairly high (60-65 per cent) during the above period with a near neutral effluent pH (7.7 t0 7.8). The reactor performance models showed a high degree of fit within the ranges of loading rates investigated. The major parameter which affected reactor performances was HLR, which is a function of HRT. The maximum loading rate and volumetric gas production (at 0.8 day HRT) were 2.2 kg/m 3 .d and 855 l/m 3 (Reactor 1). The maximum specific gas production was 858.2 l/kg TS observed in Reactor 2 at 10 day HRT. The BOD reduction had the maximumvalue of 82.9 per cent at 10 day HRT in R 2 and the minimum reduction was on the 0.8 day HRT during which 77.1 per cent reduction was obtained for all reactors. The UAHR was found to be appropriate in energy conversion of RME and 20 MJ/m3 of energy could be produced as biogas by operating the bioreactor at 2 day HRT, simultaneously reducing the pollution load of RME considerably (81 per cent BOD reduction). A HRT of 2 day was found optimum for moderate biogas production. An aerobic polishing treatment would be required to meet the effluent standards prescribed by the pollution control board. The overall performance of the reactor with rubber seed outer shell media was found to be significantly better than the polyurethane media reactor, possibly due to the enhanced microbial attachment on the more favorable surface.