Palm Oil processing gives rise to highly polluting waste-water, known as Palm Oil Mill Effluent (POME), which is often discarded in disposal ponds, resulting in the leaching of contaminants that pollute the groundwater and soil, and in the release of methane gas into the atmosphere. POME is an oily wastewater generated by palm oil processing mills and consists of various suspended components. This liquid waste combined with the wastes from steriliser condensate and cooling water is called palm oil mill effluent.
On average, for each ton of FFB (fresh fruit bunches) processed, a standard palm oil mill generate about 1 tonne of liquid waste with biochemical oxygen demand 27 kg, chemical oxygen demand 62 kg, suspended solids (SS) 35 kg and oil and grease 6 kg. POME has a very high BOD and COD, which is 100 times more than the municipal sewage.
POME is a non-toxic waste, as no chemical is added during the oil extraction process, but will pose environmental issues due to large oxygen depleting capability in aquatic system due to organic and nutrient contents. The high organic matter is due to the presence of different sugars such as arabinose, xylose, glucose, galactose and manose. The suspended solids in the POME are mainly oil-bearing cellulosic materials from the fruits. Since the POME is non-toxic as no chemical is added in the oil extraction process, it is a good source of nutrients for microorganisms.
Biogas Potential of POME
POME is always regarded as a highly polluting wastewater generated from palm oil mills. However, reutilization of POME to generate renewable energies in commercial scale has great potential. Anaerobic digestion is widely adopted in the industry as a primary treatment for POME. Biogas is produced in the process in the amount of 20 m3 per ton FFB. This effluent could be used for biogas production through anaerobic digestion. At many palm oil mills this process is already in place to meet water quality standards for industrial effluent. The gas, however, is flared off.
Palm oil mills, being one of the largest industries in Malaysia and Indonesia, effluents from these mills can be anaerobically converted into biogas which in turn can be used to generate power through CHP systems such as gas turbines or gas-fired engines. A cost effective way to recover biogas from POME is to replace the existing ponding/lagoon system with a closed digester system which can be achieved by installing floating plastic membranes on the open ponds.
As per conservative estimates, potential POME produced from all Palm Oil Mills in Indonesia and Malaysia is more than 50 million m3 each year which is equivalent to power generation capacity of more than 800 GW.
Recovery of organic-based product is a new approach in managing POME which is aimed at getting by-products such as volatile fatty acid, biogas and poly-hydroxyalkanoates to promote sustainability of the palm oil industry. It is envisaged that POME can be sustainably reused as a fermentation substrate in production of various metabolites through biotechnological advances. In addition, POME consists of high organic acids and is suitable to be used as a carbon source.
POME has emerged as an alternative option as a chemical remediation to grow microalgae for biomass production and simultaneously act as part of wastewater treatment process. POME contains hemicelluloses and lignocelluloses material (complex carbohydrate polymers) which result in high COD value (15,000–100,000 mg/L).
Utilizing POME as nutrients source to culture microalgae is not a new scenario, especially in Malaysia. Most palm oil millers favor the culture of microalgae as a tertiary treatment before POME is discharged due to practically low cost and high efficiency. Therefore, most of the nutrients such as nitrate and ortho-phosphate that are not removed during anaerobic digestion will be further treated in a microalgae pond. Consequently, the cultured microalgae will be used as a diet supplement for live feed culture.
In recent years, POME is also gaining prominence as a feedstock for biodiesel production, especially in the European Union. The use of POME as a feedstock in biodiesel plants requires that the plant has an esterification unit in the back-end to prepare the feedstock and to breakdown the FFA. In recent years, biomethane production from POME is also getting traction in Indonesia and Malaysia.