Palm oil mill effluent (POME) contains high COD and biochemical oxygen demand (BOD). POME consists of a wide range of biological substances from complex biopolymers such as proteins, starches and hemicelluloses to simple sugars and amino acids. POME may also contain dissolved oil and fatty acids, glycerin, crude oil solids and short fibers as well as soluble materials that are harmful to the environment. Since POME is discharged at high temperatures (80-90oC), both mesophilic and thermophilic temperatures have been widely applied for POME treatment by anaerobic digestion.

2.2 Effect of chitosan on UASB treating POME

It has been reported that thermophilic operation of anaerobic reactors provides some advantages over mesophilic operation in areas such as higher rates of substrate degradation and biogas production. However, mesophilic reactors can be preferable because of greater process stability (Mustapha et al., 2003; Poh & Chong, 2009). Operating temperature is a major factor that greatly influences digester performance (Choorit & Wisarnwan, 2007; Poh & Chong, 2009; Yu et al., 2002).

The effects of chitosan as a sludge granulation accelerator during the transition from mesophilic (37oC) to thermophilic condition (57oC) has been investigated by Khemkhao et al. (2011). They used two UASB reactors, with a working volume 5.3 L, both of which they inoculated with mesophilic anaerobic sludge. The sludge was then acclimatized to a thermophilic condition with a stepwise temperature increase of 5oC from 37 to 57oC. The OLR ranged from approximately 2 to 9.5 g COD/L-d. One of the reactors was then injected with a chitosan dosage of 2 mg chitosan g/VSS on the first day of operation and the second reactor was used as a control.

At all times during the operation of the two reactors, the UASB with chitosan addition was found to have 5% higher COD removal efficiency and 16 L/d higher biogas production rate (7.82 L/g VSS removed-d) than that of the control. The methane contents of both reactors were found to be similar, with approximately 78% methane content for UASB with chitosan addition and 76% for the control. The effluent VSS in both reactors was found to increase with increase of OLR. The UASB with chitosan addition was found to have 6 to 23% lower effluent VSS than that of the control. Khemkhao et al. (2011) concluded that the UASB with chitosan addition had consistently better performance than the control.

2.3 Effect of chitosan on microbial diversity in UASB treating POME

The mechanism of anaerobic digestion in methane production consists of a series of complex metabolic interactions between various types of microorganisms in the absence of oxygen. Anaerobic digestion is mediated through processes of hydrolysis, acidogenesis, acetogenesis and methanogenesis. Khemkhao et al. (2011) used 16S rRNA targeted denaturing gradient gel electrophoresis (DGGE) fingerprints to study the microbial communities during anaerobic digestion. They found that bacteria and methanogens could both be detected in the UASB reactors operating both with and without chitosan addition.