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A laboratory-scale hybrid UASB reactor, which combined an UASB in the lower part and a ®lter in the upper part, was used for the treatment of distillery spentwash. The reactor was operated under ambient conditions for 380 days. Using anaerobically digested sewage sludge as a seed, the start-up of the reactor and the cultivation of active granular sludge was completed within three months period. Scanning electron microscopic (SEM) observation of the granules showed the presence of Mehtanonthrix-like bacteria as the dominant species. Following the start-up the organic loading rate (OLR) was increased, stepwise, to 36 kg COD/m 3 á d at a constant hydraulic retention time (HRT) of 6 h. COD removal ef®ciency was 80% even at a high OLR of 36 kg COD/m 3 á d. Biogas rich in methane content (80%), with a maximum speci®c biogas yield of 0.40 m 3 CH 4 /kg á COD was produced. Polypropylene pall rings ®lter medium in the upper-third of the reactor was very effective as a gas-liquid-solid (GLS) separator, and retained the biomass in addition. The study indicated that hybrid UASB is a very feasible alternative for the treatment of high-strength wastewaters like distillery spentwash. IntroductionAn aerobic wastewater treatment (AWT) is becoming an accepted technology, not only for the treatment of variety of wastewaters but also for the environmental protection and recovery of renewable resources, particularly after the development of high-rate anaerobic reactors [1]. These reactors achieve the separation between hydraulic retention time (HRT) and solids retention time (SRT), thereby making an application of signi®cantly high volumetric loading rates possible. Various types of high-rate anaerobic reactors such as anaerobic ®lter (AF), down¯ow stationary ®xed ®lm (DSFF) reactor, up¯ow anaerobic sludge blanket (UASB) reactor,¯uidized bed (FB) and expanded bed (EB) reactors have been developed. Hybrid UASB [2] is among the newer designs which combines the advantages of UASB and AF concepts. In literature, hybrid UASB is also called as sludge blanket ®lter (SBF), up¯ow sludge blanket ®lter (UBF), hybrid up¯ow anaerobic sludge blanket (HUASB) reactor and the up¯ow anaerobic¯oat-ing bed (UAFB) or more simply anaerobic hybrid reactor.In India, there are over 200 distilleries [3] producing annually approximately 10,000 million litres of spentwash or vinasse. The distillery waste in the form of spentwash is amongst the worst pollutants produced by any industry both in magnitude and strength (BOD 5 ³ 30000 mg/l). It has been a cause of concern in India for the past few decades. In developing and tropical countries such as India, anaerobic digestion of high-strength wastewaters like spentwash becomes more attractive. Various anaerobic systems such as conventional digester, open lagoon, AF and two-staged ®xed ®lm reactor have been attempted for the treatment of distillery spentwash with a limited success. Applicability of hybrid UASB, which is considered most suitable for series of soluble and partially soluble wastewaters [4], appears to b...

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