Removal of phenolic compounds and color from palm oil mill effluent by using grasses and their rhizosphere bacteria

Abstract

Palm oil mill effluent (POME) in stabilization ponds contains phenolic compounds and color higher than standard. Soil irrigation is usually used to reduce the wastewater volume; however the accumulation of phenolic compounds can be toxic to plants. This study aimed to reduce the concentration of phenolic compounds in soil by using rhizosphere bacteria of pasture grasses. Three grass cultivars namely Mulato or Brachiaria hybrid (B. ruziziensis x B. Decumbens x B. brizantha), Creeping signal (Brachiaria humidicola) and Guinea grasses (Panicum maximum Jacq) were compared in pot experiments under greenhouse conditions. Phenolic compounds and color significantly reduced in planted soil and highly effective in the 1 - 4 cycles of irrigation using POME from the last stabilization pond of a palm oil mill in Surat Thani province. Mulato, Creeping signal and guinea grasses removed 87-93%, 90-95% and 65-83% of phenolic compounds and 84-92%, 90-96% and 82-96% of color, respectively. At the same time, the control soil removed only 39-72% and 59-71% of phenolic compounds and color, respectively. Phenolic compound accumulation was minimal since their concentrations in grasses watered with POME and tap water were not significantly different. The number of phenol degrading bacteria in soil increased from 5.89 log CFU g-1 soil to 8.16, 7.52 and 7.27 log CFU g-1 soil after planted with Mulato, Creeping signal and Guinea grasses, respectively. On the other hand, the bacterial number in control soil was 6.14 log CFU g-1 soil. Thus, the degradation of phenolic compounds was corresponded to the growth of rhizosphere bacteria. The soil leachate from Mulato grass had the lowest toxicity to seed germination, thus Mulato grass was selected for further experiment. The addition of phenol-degrading bacteria, Acinetobacter sp. strain OPB to soil with Mulato grass slightly increased the efficiency of phenolic compounds removal. In the scale-up reactor, phenolic compounds in Mulato reactor were decreased around 77% after two irrigation cycles of POME from the last stabilization pond. The reduction was around 30% higher than the unplanted soil reactor. Thus, Mulato grass could be cultivated for reducing toxicity of phenolic compounds in soil.