Biodiesel wastewater contained residual of alkali catalyst, soap and glycerol which causing in high chemical oxygen demand (COD). The optimum conditions of aluminium sulphate (alum), poly-aluminium chloride (PAC) and laterite soil (silifloc) in the coagulation of biodiesel wastewater for COD reduction were explored through a sequence of studies including the effects of pH and dosage of coagulants. Coagulation process were conducted in the fixed conditions of mixing rate, mixing time, settling time and anionic polymer aid as flocculant. Silifloc could treat acidified biodiesel wastewater with 32.35 % at pH 2 and dosage of 4000ppm, the highest among these three coagulants. This was due to the silifloc rich in silica component had a higher rate law at sufficient dosage and had an effective range of at around pH 2. Whereas, PAC had a highest efficiency with 61.58% at pH 6 and dosage of 1100ppm in coagulation of raw biodiesel wastewater due to its solubility and elements of higher ions content. On other hand, alum had a moderate performance in acidified and raw wastewater with 26.42 % at pH 6 and 57.63 % at pH 4, respectively.
