Borja, R. and Banks, C.J.
Comparison of an anaerobic filter and an anaerobic fluidized bed reactor treating palm oil mill effluent
Process Biochemistry, 30, (6), . (doi:10.1016/0032-9592(94)00035-2).
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A laborator-scale anaerobic filter (AF) and a fluidized-bed reactor (FBR) were compared for their ability to treat palm oil mill effluent (POME). The reactors were operated at mesophilic temperature (35°C). Reactor performance was assessed by measuring chemical oxygen demand (COD), volatile fatty acids (VFA), gas composition and gas production. Sand of 0·3–0·5 mm diameter was used as a biomass support medium in the FBR, and clay rings of 2–5 mm diameter were used in the AF. Start-up of the AF was achieved with 1·5–4·5-day residence times and FBR residence times were maintained at 6 h. After acclimatization, COD removals higher than 90% were reached in both reactors at 6 h residence time, equivalent to a loading of 10 g COD/litre/day. At higher loadings the FBR gave a better performance; even at 40 g COD/litre/day, with 6-h residence times, 78% COD was degraded. The AF could not be operated above 20 g COD/litre/day without clogging. The AF and FBR performed similar at reactor concentrations up to 1 g COD/litre, while above 2·2 g COD/litre the AF showed a maximum removal rate of 17·0 g COD/litre/day compared to 31·2 g COD/litre/day for the FBR. These differences were probably due to diffusion limitations and a less active biomass in the AF.
In both reactors the gas production rate and the levels of organic acids increased in response to a higher feed concentration in less than one day and this could be explained by substrate limitation. Other responses were slower with the microbial culture adapting over periods of 6–10 days; these were apparently growth related. A doubling in loading always resulted in a large increase in organic acids, especially acetic and propionic, as well as increasing the proportion of CO2 in the gas.
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