Oil Palm Tree Wastes 8: The uses of the palm oil mill effluent

Published Date: 23rd November 2020

Introduction

The application of novel treatment technologies to the palm oil mill effluent (POME) management has led to its re-classification from waste material to a by-product or useful resource by an increasing number of palm oil mills. This re-classification has been driven mostly by an international campaign on pollution prevention through cleaner production based on the so-called 5R policy of reduction, replacement, reuse, recovery, and recycling of wastes. Environmentally sound biotechnologies that change the status of waste materials to valuable resources are therefore gaining ground. Such green technologies have been shown to minimize the costs of waste management, while also ensuring compliance with global environmental standards.

The high concentrations of carbohydrate, protein, nitrogenous compounds, lipids, and minerals in the POME makes it specifically a good raw material for bioconversion through numerous biotechnological processes. For example, the available bio-resources in the POME could be concentrated, and recovered with the aid of ultrafiltration processes for use as a fermentation medium, fertilizers, and animal feeds. Again, through anaerobic digestion processes that yield methane as biogas, POME has been converted to energy which is used to run gas turbines for the generation of electricity or heating steam boilers. Other processes have been employed to convert POME into value-added products such as carotenoids, vitamins A and E, citric acid, ethanol, and enzymes. POME as a resource material, therefore, can be transformed into beneficial products, while at the same time promoting pollution prevention through cleaner production. Of these approaches, however, the combined objective of simultaneous effluent treatment, and the production of renewable bioenergy in the palm oil industry has proven to be more environmentally sound, and has also resulted in much cleaner production, and greater sustainability.

POME as an energy source

Although different studies have established that POME is a source of greenhouse gas emissions in the form of carbon dioxide, and methane, this potential could be a reliable source of renewable energy in the form of bio-methane and bio-hydrogen. Both the aerobic and anaerobic digestion of POME can be employed to produce biogas, with the aerobic digestion exhibiting a higher microbial growth rate, and lower retention time compared to anaerobic digestion. The anaerobic method has however been proven to be more effective in terms of cost, and conversion into useful products. Biogas production from POME involves multi-stage microbiological processes of hydrolysis, acidogenesis, acetogenesis, and methanogenesis to convert carbohydrates, fatty acid, and proteins in the POME into CH4 and CO2. The processes are affected by the microbial population, and environmental factors, such as pH, temperature, nutrient content, mixing condition, chemical equilibrium, pressure, hydraulic retention time, and the presence of inhibitory materials. About 20 to 28 m3 of biogas can be produced per m3 of POME, while the biogas contains 65 percent CH4, 35 percent CO2, and traces of hydrogen sulphide.

Researchers at the Universiti Malaysia Sarawak, Malaysia, used an anaerobic reactor configured for hydrolysis at the first stage, and acetogenesis and methanogenesis at the second stage to produce biogas of 3.8 L/day at optimum pH of 6.9, carbon-to-nitrogen (C/N) ratio of 30, and organic loading rate of 6 VSSg/L.d. The flow chat of biogas production from POME is shown in figure 1.  It has been estimated that 1 m3 biogas produced from POME will generate about 1.8 kW of energy per hour.

Several approaches such as pretreatment, bioreactor modification, and co-digestion have been used to improve the biogas production from POME. The pretreatment methods include deoiling, sedimentation, pre-hydrolysis, and addition of biological or inorganic additives. These are used essentially to overcome the high lipid content of POME that might cause a problem during the anaerobic digestion. While ozonation as a pre-hydrolysis method has been used to increase the methane yield of POME, inorganic additives such as red mud-iron, calcium oxide-cement Kiln dust, and chitosan have been used to improve biomass retention during the digestion process. Similarly, consortia of microorganisms have been used to enhance the anaerobic digestion of POME in terms of sufficient acclimatization, startup period, and better removal of chemical oxygen demand. Again, co-digestion of POME and lignocellulosic material such as oil palm empty fruit bunches, sewage sludge, cow manure, and microalgae has been used to improve the anaerobic digestion efficiency, and biogas yield, probably through improved process stability, carbon/nitrogen ratio, and micronutrient contents, resulting from synergistic activities of the microorganisms.

Bio-hydrogen has also been produced from POME using the less energy-intensive dark-and photo-fermentation techniques. POME has been converted by bacterial strains such as Clostridium butyricum, and Rhodopseudomonas palustris into hydrogen-rich gas through anaerobic processes. In a study by Chong and coworkers, using C. butyricum, the maximum hydrogen yield was 914 ml H2/h at optimum pH, temperature, and chemical oxygen demand of 6.52, 410C, and 60 g COD/l respectively. Supplementation of the substrate with nitrogen, phosphorus, and iron sources was found to significantly increase the hydrogen yield, and rate of production.

POME as a fertilizer

POME contains significant amounts of plant nutrients in the form of nitrogen, phosphorus, potassium, calcium, and magnesium. The application of raw POME to the soil is however harmful to crops because of its very high biochemical oxygen demand and, the temperature at the time of discharge. Several treatment approaches such as anaerobic stabilization ponds, coagulation, and flocculation among others have been used to render the POME more amenable to agricultural soil application. While the ponding approach may be cost-effective, and the most effective biological treatment method, it requires substantial land area for building the ponds, and extended retention time to conclude the process. In the ponding system, the organic nitrogen contained in the POME is changed to ammonia, which is then converted to nitrate, by the process of nitrification over a relatively long hydraulic retention time, which is thus, a major constraint for most factories. To overcome this constraint, researchers have investigated the bioaugmentation of POME with a mixed culture of nitrifiers (ammonia, and nitrite oxidizers), and found them very effective both in speeding up the nitrification process, and also for improving the quality of POME as a liquid fertilizer. Indigenous organisms isolated from agricultural soils, and genetically modified organisms have been effectively used as nitrifiers in the bioaugmentation of POME.

The coagulation, and flocculation of the POME, which can be achieved with several agents such as aluminum sulphate (alum), polyaluminium chloride (PAC), ferric sulphate, and organic-based biocoagulants like chitosan require much shorter time and limited land area. They also yield significant amounts of sludge, which subsequently is used as fertilizer. Researchers at the University Malaysia Pahang, Malaysia, evaluated the effectiveness of chitosan in the removal of biochemical oxygen demand, chemical oxygen demand, ammonium nitrogen, phosphate, and potassium, in addition to the fertility of the sludge obtained from the treatment process. The study showed that chitosan was effective in reducing the nitrogen and phosphorus content, but not the potassium content of the POME, while the fertility of the sludge on a common fast-growing vegetable was found to be superior to that of commercial fertilizer.

Estimates reported by Wood and coworkers show that the application of properly treated POME at 4.5 x 106 1iter per hectare represents a fertilizer application level of about 30 kg ammonium sulphate, 7 kg rock phosphate, 52 kg potash, and 18 kg kieserite per year in a palm plantation. Improvements in soil productivity, root health, and crop yield have been associated with the use of treated POME in the fertilization of agricultural soils.

Composting has also been used as a suitable method of converting POME into manure that can be applied as organic fertilizer. The co-composting of partially treated POME with empty fruit bunches or sawdust has been investigated. The co-composting of partially treated POME obtained from an anaerobic pond, and shredded empty fruit bunches for 60 days yielded manure containing considerable amounts of calcium, magnesium, phosphorus, potassium, and other micro-minerals. The C/N ratio decreased from 45 to 12, while the moisture content changed from the initial values of 65 percent to 75 percent, and pH from 7.75 to 8.10. Co-composting of POME with sawdust however required augmentation with sand or other ideal substrates before good fertilizer value could be achieved. The composting of POME could therefore be a good valorization strategy for reducing its volume for more efficient land application. Such POME based manure has been used to replace up to 66 percent of chemical fertilizers in palm plantations.

Animals and aquaculture feeds

Palm oil sludge produced from artisanal, and small-scale oil palm mills is routinely used by smallholder farmers in the formulation of the pig, and small ruminant diets. A Nigerian study of the nutrient composition of such oil mill effluent samples reported that on a dry matter basis, the crude protein content ranged from 8.62 - 9.89 percent, crude fiber (0.14 - 0.15 percent), fat (69.39 - 80.03 percent), and ash (8.21- 10.49 percent). The mineral values were given as potassium (495 - 835 mg/100g), phosphorus (655 - 1042 mg/100g), calcium (0.8 - 1.14 mg/100g), copper (35 - 45 mg/100g), and zinc (30 – 50 mg/100g). POME from industrial-scale palm oil mills using wet milling process on the other hand contains mostly water (90-95%), but on drying could contain more than 90 percent dry matter. The amount of residual oil in the dried product could vary from 5 to 70 percent or more, while other constituents such as crude protein, ash, and neutral detergent fiber have been shown to range from 10 - 15 percent, 9 - 30 percent, and 15 - 60 percent respectively.

Feeding value trials with sheep have shown that up to 40 percent of POME can be used either alone in molasses urea-based diets or combined in equal proportions with palm press fibre. Raw or concentrated POME has been mixed with cassava meal, dehydrated grass, or palm kernel cake, and fed to buffaloes, and cattle with good results. In a laying chicken trial, 10 percent inclusion of POME was found to be the optimum dietary level, with the average percent egg production, total egg mass, and feed/gain ratio being 76.4 percent, 8.9 kg, and 2.77:1 respectively. Similarly, 10 – 15 percent levels of inclusion have been shown to give good results in the broiler, Pekin ducks, and rabbit trails.

Again, 10 – 30 percent of fresh or dried POME have been used to partially replace maize in pig diets. A Nigerian study specifically reported that weaner pigs fed up to 25 percent fresh POME recorded a good daily gain, feed intake, and backfat thickness values, although feed efficiency, and diet digestibility decreased. Similarly, researchers at the Central Agricultural University, Selesih, India, studied the performance of growing Large White Yorkshire pigs fed a diet containing 0, 15, and 25 percent inclusion levels of POME sludge as a replacement for maize during 42 days of feeding trial. They reported no significant differences in the average feed intake, average daily gain, and feed conversion ratio among the different treatment groups, although the final body weight decreased slightly with increasing inclusion level of POME.

Conversion of POME into other value-added products

Palm oil mill effluent contains both insoluble, and soluble carbohydrates such as starch, cellulose, hemicellulose, and sucrose among others that could be utilized in microbial fermentation to produce several value-added products. The amount of total soluble carbohydrates in the POME is however much lower than the insoluble ones. Therefore, to aid or speed up the microbial fermentation process the POME is usually hydrolyzed to release the fermentable sugars, which are subsequently converted to the desired products. Products, such as carotene, acetone, butanol, and ethanol, as well as citric acid, biohydrogen, enzymes, and bioplastic have been successfully synthesized from POME.

Acetone, butanol, and ethanol (ABE) production: POME is a good substrate for the production of acetone, butanol, and ethanol through the activities of several saccharolytic clostridia. Researchers at the University of Kebangsaan, Malaysia, studied the direct use of POME as a fermentation medium for acetone-butanol-ethanol production using Clostridium acetobutyicum and C. saccharoperbutylacetonicum as cultural organisms. They reported that at 90 percent POME sedimentation, and an initial pH of 5.8 the C. acetobutyicum produced a higher total acetone-butanol-ethanol of 4 g/L, although butanol production was maximized at an initial pH of 6.0. The use of POME in bioethanol production particularly offers an alternative treatment route for the oil mill waste, which can also ease the competition in the utilization of food-based raw materials such as grains, and tubers in the production of bioethanol. Under controlled laboratory conditions, POME has been used as the main substrate for Saccharomyces cerevisiae fermentation to produce up to 16 percent bioethanol. Other cultures that have been used in the direct bioconversion of POME into bioethanol include Trichoderma harzianum, Phanerochaete chrysosporium, and Mucor hiemalis,

Bioplastic formation: Numerous microorganisms invade, and grow in POME mostly during the ponding period to break down complex molecules into simpler ones. Some of these microorganisms, especially bacterial species have shown the potential for polyhydroxyalkanoates (PHAs) production. PHAs are biodegradable, and biocompatible, thermoplastic compounds produced mainly from renewable resources, and having mechanical properties similar to those of polypropylene. A two-stage process for the production of PHA from POME aided by the phototrophic bacterium, Rhodobacter sphaeroides, and involving an initial production of organic acids (acetic and propionic acids) by anaerobic fermentation of the POME, followed by conversion of the organic acids into PHA has been demonstrated. This relatively inexpensive production of PHAs is believed to have significant economic advantages, since more than 40 percent of the cost of PHAs production has been attributed to the carbon source such as POME.

Citric acid production: The fungal fermentation of cheap substrates such as glucose, sucrose, inulin, date fruit syrup, and sugarcane molasses among others with Aspergillus niger is traditionally employed in the production of citric acid. Seven days of POME fermentation with A. niger has however been shown to record a much higher citric acid yield of 5.2 g/l than these conventional substrates, indicating the potential of this waste material as a major source of the large quantities of citric acid used as an acidulant, stabilizer, flavor enhancer, preservative, antioxidant, emulsifier, and a chelating agent in various industries.
Enzyme production: The POME has been utilized as a medium for industrial enzyme (lignin peroxidase) production by microorganisms such as the white-rot fungus, Phanerochaete chrysosporium. Pre-filtered POME has also been used as a substrate for protease production by Aspergillus terreus. Other fungal organisms that have been employed in the production of enzymes from POME fermentation include Rhizopus oryzae, and R. rhizopodiformis, Candida cylindracea, and C. cylindracea for lipase production, and A. niger and T. reesei for cellulase production.

Conclusion

POME is a good raw material for bioconversion through different biotechnological processes, because of its high concentrations of carbohydrate, protein, nitrogenous compounds, lipids, and minerals. Numerous attempts have therefore been made to produce value-added products from the readily available POME in order to help solve the challenges usually associated with its disposal. There is a need to optimize these bioconversion opportunities in order to create additional revenues from the processing of palm oil, especially in Africa.

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