Optimization of pre-treated palm oil mill effluent digestion in an up-flow anaerobic sludge fixed film bioreactor - A comparative study

An up-flow anaerobic sludge fixed film (UASFF) bioreactor was used to treat physically and chemically pre-treated palm oil mill effluent (POME) under different operating conditions. In physical pre-treatment, POME was pre-settled for 2 h and the supernatant was fed to the reactor. In chemical pre-treatment, optimum dosages of a cationic and an anionic polymer were used. Experiments of pre-treated POME digestion were conducted based on a central composite face-centered design (CCFD) with two independent operating variables, feed flow rate (QF) and up-flow velocity (Vup). The operating variables were varied to cover a wide range of organic loading rates (OLR) from 3.8 to 29 g COD/(l d). Six dependent parameters were either directly measured or calculated as response. These parameters were total COD (TCOD) removal, effluent pH, effluent total volatile fatty acid (TVFA), effluent bicarbonate alkalinity (BA), methane yield (YM), and solids retention time (SRT). The performance of the reactor was compared for the pre-settled and chemically pre-treated POME. The chemical pre-treatment approach was shown to be more predictable, reliable and practical as the sludge produced was very compressible and was easy to separate. At a comparable range of QF and Vup,
the pre-settled POME yielded slightly better reactor performance in terms of COD removal (%), bicarbonate alkalinity and methane yield. At an OLR of about 16.5 g COD/(l d), higher COD removal efficiency (90–94%) was achieved compared to that of the chemically pre-treated POME (82–88%) despite about 33% of organics in the pre-settled POME was contributed by suspended solids. The optimum conditions for digestion of the pre-settled and chemically pre-treated POME were determined as QF of 1.65 l/d, Vup of 0.6 and QF of 2.45 l/d, Vup of 0.75, respectively. The experimental findings were in close agreement with the model prediction.