Cleaner discharge
Palm oil milling is far from benign, but the industry is frantically conducting research to realise low-emission technologies. The zero-discharge plant is one such bold concept.
THE extraction of palm oil involves a number of steps: fruit bunch sterilisation, stripping, digestion, pressing, classification, purification, and vacuum drying. When all these step are added together, a significant amount of water is needed for a palm oil mill’s operations. On average, about 1.5cu.m of water is needed to process one tonne of fresh fruit bunch, and half of this liquid ends up as palm oil mill effluent (POME).
POME consists of water soluble components of palm fruits as well as suspended materials like palm fibre and oil residues. Despite its biodegradability, POME cannot be discharged without first being treated because it is acidic and contains residual oil that cannot be easily separated using conventional gravity-based systems. Essentially, this oily mix needs a lot of oxygen before it can decompose completely, and this phenomenon is called having a high biochemical oxygen demand (BOD), and raw POME can sometimes have a BOD of up to 100 times higher than that of domestic sewage.
In the past, palm oil mills usually discharged either partially treated or raw POME into nearby rivers as this was the easiest and cheapest method for disposal. However, excessive quantities of untreated POME deplete a waterbody of its oxygen and suffocate aquatic life, and many small rivers have been devastated by such discharges.
The Department of Environment (DOE) is well aware of the impact of POME discharges, and under the Environmental Quality Act (1974), had set a limit of 100 parts per million (ppm) for BOD levels before a mill is allowed to discharge is effluent into the surrounding environment.
Tightening standards
The good news for the past decade or so is that the majority of mills are able to comply with DOE’S requirement. However, DOE has recently proposed a tightening of the standard to 20ppm, especially for Sabah, Sarawak and other environmentally-sensitive areas. “And in general, the industry has a problem in complying with this,” said Dr Loh Soh Kheang, head of the energy and environment unit of the Malaysian Palm Oil Board’s (MPOB) engineering and processing research division.
DOE is not unfairly targetting the industry, as even Indah Water Konsortium, the national sewerage company, also needs to meet the 20ppm requirement if the discharge point is upstream of a water treatment plant intake point, and 50ppm if the intake point is downstream. “Due to this new requirement, the industry is under pressure to investigate a wide-range of approaches towards the treatment of POME,” said Loh.
According to Tang Meng Kon, head of plantation sustainability for Sime Darby Plantation Sdn Bhd, dozens of methods have been developed to control POME pollution, including decanting and drying, evaporation, coagulation, flotation, ultrafiltration, and various aerobic and anaerobic biodegradation technologies. As POME contains a significant amount of nutrients like nitrogen, phosphorus, potassium and magnesium, another way of dealing with POME is to spread them on agricultural land as fertiliser. “There has yet to be a one-sizefits-all solution that is perfect, and research is ongoing,” said Tan, who added that SDP is carrying out various experiments in-house in the quest for more efficient treatment methods.
Beyond obvious water pollution problems, the appropriate treatment of POME is a very critical factor in curbing global warming. Mills that choose to treat POME using the anaerobic process will generate a lot of methane, a greenhouse gas 22 times more potent than carbon dioxide in trapping heat. Already, palm oil mills are fingered by climate change authorities as being the second largest source of methane generator in the country (38%), next to landfills (53%).
In this context, research on some possible zero-discharge treatment technologies for POME is an area that holds great promise. “To my knowledge, there has yet to be any mill in the world which is operating on a zero-discharge basis,” said Tan.
The majority of POME treatment methods today are based on anaerobic, facultative or aerobic pond system, otherwise known as the open lagoon system, which is adopted by nearly 85% of the mills here. This method requires a sizable land area, a problem made worse by the fact that the wastewater needs to sit for a very long time in order for microbes to digest the sludge, usually for more than three weeks. Other options to speed up the digestion process would mean building costly digesters. “The two most common means for the industry to move beyond the open lagoon system is turning POME and empty fruit bunches into compost, or capturing the methane from the ponds for either flaring or power generation,” Tan said.
Zero discharge
MPOB itself is currently evaluating a solution proposed by the Shanghai Jiaotong University as well as Malaysia’s own Ronser Bio-tech Sdn Bhd. A specialist in high organic content industrial wastewater treatment, Ronser offered two proprietary systems called ANAEG and BIOAX for evaluation at a pilot plan at Labu, Negri Sembilan.
Ronser claims to have the world’s most advanced wastewater treatment technology, a feat credited to Professor Zhang Zhenjia of the Shanghai Jiao Tong University’s School of Environmental Science and Engineering.
ANAEG is touted as being capable of tackling a wide range of wastewater and is particularly efficient for problematic wastewater with very high organic content, while BIOAX, also designed and patented by Dr Zhang, is an advanced aerobic system which is touted as faster and more efficient than the conventional activated sludge process.
The five-tonne per hour pilot plant at Labu is intended to demonstrate that it is possible to achieve a zero emission of POME into the environment. In theory, the system should be able to generate biogas (mainly methane) at a rate of 150cu.m per hour, produce sludge that can be used as fertiliser, and produce wastewater that is good enough for boiler use. The first stage of the plant will see the recovery of waste oil from POME, before the sludge is treated by the ANAEG or BIOAX processes. The biogas that is produced will be captured for power generation by burning them in gas engines, while some solid high-potash fertiliser is also recovered. The final bits involve membrance separation, followed by ultrafiltration, and reverse osmosis, to produce clean water,
MPOB is far away from giving the system its endorsement. “There are various treatment options out there, and each treatment method still has its strengths and weaknesses. For example, if the system is optimised to produce more clean water, then this would lead to a reduction in biogas production. More work still need to be done, like subjecting the plant to more rounds of continuous 24-hour operations and loading from heavy downpours,” said Loh.