The palm tree from which oil is harvested is perhaps one of the most versatile crops known to man and an important element for the implementation of the circular economy (Figure 1). As the diagram below indicates, the list of products obtainable from palm oil is very wide and ranges from animal feed and beauty products through to nutritional supplements and even explosives. Also, once the fruit has been removed, the fibres and husks can be used as a fuel. Wastewater from the process was, until relatively recently, a considerable problem for mill owners but now is considered to be a resource and an active mechanism for reducing the carbon footprint of the industry.
Invariably, any plan for ensuring the sustainability of industrial activities centres around two pivotal points: the financial and the environmental. If there is little or no profit to be made, the industry becomes financially inviable and, if there is no consideration of the environmental aspects, the local economy as well as the environment will suffer, and consumers will be disincentivised to purchase the products. An integral aspect of whether a palm oil operation is sustainable is the way it interacts with its immediate environment. Anaerobic digestion of palm oil mill effluent (POME) is an effective way to fulfil many of the criteria necessary to ensure both financial viability and environmental sustainability. This paper highlights the advances that have been made in the transition of POME from being thought of as a liability to a significant resource.
Palm oil mills generate large amounts of both solid and liquid wastes, but it is the POME that has been singled out by the operators as the most expensive and difficult to manage. A palm oil operation relies heavily on the use of water, with about 0.5-0.75 tonnes of POME being generated for every tonne of fresh fruit bunch (FFB) processed. Until relatively recently this highly contaminated water has been the most expensive and difficult waste product to manage by mill operators simply because the processing of the FFB produces a lot of POME. If released directly to the environment, raw POME depletes water bodies of oxygen and kills aquatic life. Added to this, in Malaysia alone, palm oil mills have been cited as being the second-largest source of methane next to landfills and, if this is extrapolated across the increasingly important global palm oil industry, they are therefore major potential contributors to the balance of global warming gas emission.
Palm Oil Mill Effluent (POME) is a by-product of the production of palm oil that, because of the large production, can result in significant quantities of wastewater that has to be treated. It is a thick brown mix of water, oil and fine suspended solids that is emitted from the oil production process and is released to the settling lagoon at a temperature of 80 – 90ºC. A mixture of condensate, sludge and cyclone wastewater, about 2.5-3.0 tonnes of POME per tonne of crude palm oil produced is generated in the extraction processes. It has a very high BOD, normally more than 25.000 (Table 1) and has a pH of around 4.5 and this is the major factor that makes it highly contaminating if released untreated to the environment. As it is a physical process that depends on many factors for its production, the composition of the liquor produced can be highly variable between batches, days, factories or even the age of the fruit used.
|Oil and grease||4,000||–|
|Biochemical oxygen demand (BOD)||25,000||10,250-43,750|
|Chemical oxygen demand (COD)||51,000||15,000-100,000|
|Suspended solids||18,000||5,000- 54,000|
|Total volatile solids||34,000||9,000- 72,000|
|Ammoniacal nitrogen (NH3–N)||35||4- 80|
|Total nitrogen (T.N.)||750||180-1,400|
|Table 1: Characteristics of palm oil mill effluent (POME)|
POME is rich in organic material, a parameter that can be exploited by using recent advances in anaerobic digestion technology. It can now therefore be considered as a valuable resource, the capitalisation of which significantly reduces not only the locally induced environmental damage caused by a palm oil operation but also its public perception. Apart from the obvious use of extracting the energetic value from POME, the potential for using POME as a cheap organic fertilizer also offers an alternative to the application of chemical fertilizers as well as contributing to an improvement in soil fertility. As a fertilizer applied to the palm oil plantation, it has also been shown to contribute towards increased productivity.
Because of their low cost and simple operation, open lagoon systems for stabilising POME have been used extensively for many years. With the introduction of the closed anaerobic lagoon, it is converted into a highly efficient mechanism not only for stabilising POME to meet discharge limits but, depending on the method used, can result in significant quantities of biogas with a high concentration of methane being collected. It has been estimated that for each tonne of FFB processed, 0.7 tonnes of POME will be produced, and each tonne of POME can generate up to 28m3 of biogas with a concentration of 60% or more of methane. This means that from a typical mill rated at 40 tonnes per hour FFB between 1 and 2 Mega Watt (MW) of electricity can be generated from the biogas collected in the anaerobic digester.
Anaerobic digestion involves the breakdown of organic waste by bacteria in an oxygen-free environment. It is commonly used as a waste treatment process but also produces a methane rich biogas which can be used to generate heat and/or electricity.
In the production of biogas from POME, anaerobic digestion equipment consists, in simple terms, of an anaerobic reactor volume, a gas holder to store the biogas, and a gas-burning engine/generator set, if electricity is to be produced.
The organic waste is broken down in the reactor with up to 60% of this waste being converted into biogas; the rate of breakdown depends on the nature of the waste, the reactor design, and the operating temperature.
The process of anaerobic digestion (AD) consists of three steps: The first step is the decomposition (hydrolysis) of plant or animal matter. This step breaks down the organic material to usable-sized molecules such as sugar. The second step is the conversion of decomposed matter to organic acids. Finally, the acids are converted to methane gas. Process temperature affects the rate of digestion and should be maintained in the mesophillic range (30ºC to 35ºC). It is possible to operate in the thermophillic range (approx. 55ºC) but the digestion process at this temperature is subject to relatively easy upset if not closely monitored.
There is little doubt that the economic well-being of millions of people depends on the palm oil crop and it is incumbent on those who have historically been the major producers of global pollution to constructively engage in the development of sustainable practices that ensure environmental protection. By ensuring ecological practices, from certified sourcing of sustainable palm oil to construction of a truly circular economic model which focuses on the comprehensive reuse of waste products, there is little reason why palm oil cannot be perceived as being a benefit to humanity rather than a liability.