Anaerobic treatment of palm oil mill effluent in a two-stage up-flow anaerobic sludge blanket (UASB) system

Abstract

A high rate anaerobic treatment of palm oil mill effluent (POME) was achieved in a two-stage up-flow anaerobic sludge blanket (UASB) reactor. The acidogenic reactor acclimated rapidly to the wastewater and was tolerant to a suspended solids (SS) concentration of 5.4 g l −1 in the influent wastewater. Loading was gradually increased over a period of 100 days resulting in a satisfactory hydrolysis and acidification giving a maximum rate of acid production of 4.1 g l −1 d −1 acetic acid at a loading rate of 16.6 g l −1 d −1 COD at a hydraulic retention time of 0.9 days. An increase in alkalinity throughout the acclimatization maintained the effluent from the reactor at around pH 5.8. The methanogenic reactor was initially fed on dilutions of the effluent from the first stage reactor after pH adjustment. The loading was gradually increased, and then stepwise, to 60 g l −1 d −1 COD at which point COD removal efficiency had declined significantly and an accumulation of long-chain volatile fatty acids was observed. It was concluded that the reactor could work efficiently up to loadings of 30 g 1 −1 d −1 COD, whilst producing a good methane yield and a COD reduction of greater than 90%. Effluent recirculation alleviated the need for alkali additions to the feed of the methanogenic reactor and a direct coupling of the two reactors was achieved towards the end of the experimental run of 175 days. Both reactors showed granule formation with distinct morphological characteristics; these were observed to be formed after 80 days in the acidogenic reactor and after 110 days in the methanogenic reactor.