Fifty-eight million tons of palm oil mill effluent (POME) is produced each year in Malaysian palm oil mills. This effluent can be used for biogas production, an especially viable renewable energy source in India, China, Malaysia, Thailand, Indonesia and the Philippines where palm residue is in high abundance (Renewable Cogen Asia). Typically, biogas is produced from industrial, municipal, and agricultural waste, such as cow manure. Raw palm oil contains high levels of fatty acids and oil, and, therefore, a high oxygen demand, which makes it necessary to treat in oxidation ponds where it can also be used to support bacterial growth (Alias and Tan 2005, Zakaria et al. 2008). This form of anaerobic digestion not only is a form of waste treatment, but it is also used for the production of biogas in the absence of oxygen (Santibanez 2011). Therefore, scientists expect that palm oil effluent can also be used as a beneficial additive in the treatment of cattle manure for biogas production (Nasir et al. 2012). —Shelby Long
Nasir, L.M., 2012. Palm oil mill effluent as an additive with cattle manure in biogas
production. Procedia Engineering 50, 904–912
Nasir et al. investigated the benefits of using palm oil effluent as an additive in cattle manure biogas production. Researchers created an anaerobic environment in a jacketed fermenter from which they sampled the products of the cattle manure and palm oil effluent digestion daily. They used a 10 L fermenter to conduct two treatments. In the first treatment they placed 500g of fresh cattle manure and 1.5L of POME in the fermenter. In the second treatment they only added 500g of cattle manure and added distilled water in order to achieve a solid content of 9%. For both treatments, they carried out a batch mode for the first ten days and a semi-continuous mode for the remainder of the experiment. In batch mode they added the contents to the fermenter and allowed it to digest for ten days. In semi-continuous mode they removed digested cattle manure daily and replaced it with the same volume of fresh cattle manure. For both treatments, they maintained a temperature of 53˚C in the digester, and they controlled the pH by adding 1 N HCl and 1 N NaOH as needed. The contents were stirred at a constant speed of about 150 rpm. In order to maintain the anaerobic environment in the sealed digester nitrogen gas was added to purge the oxygen. Researchers removed samples on a daily basis and analyzed the total solids (TS), volatile solids (VS), biochemical oxygen demand (BOD), chemical oxygen demand (COD), ammonia nitrogen content, and methane content. They used a gas chromatograph to analyze the methane content and a spectrophotometer to measure the methane content. In order to measure the biogas produced in the digester, researchers used the water displacement method.
Nasir et al. observed that biogas production in the first treatment, with cattle manure and POME, was continuous in the batch operation but declined after the sixth day. However, the production increased at a constant rate during the semi-continuous mode. The biogas production was more rapid for the first half of the digestion period and the methane content fluctuated between 52–55% through the course of the experiment. In the second treatment, with no POME, biogas production began on day 2 and peaked after four days. The biogas production only achieved a methane content of 20%. The level of biogas production in the first treatment (0.346 m3kg-1VS) was almost three times greater than the biogas production in the second treatment. Researchers determined that the increase in biogas potential of the cattle manure is most likely caused by microbial degradation of organic matter because of anaerobic bacteria in the POME additive (Zakaria 2007).
The ammonia nitrogen NH3-N content observed in the cattle manure and POME treatment seemed to stabilize throughout the experiment, only fluctuating between 400 and 600 mg/L, while the NH3-N content in the cattle manure treatment fluctuated in large amounts, between 400 and 800 mg/L. The stabilization of ammonia nitrogen content within the cattle manure and POME treatment suggests that the nitrogen ammonia did not inhibit the digestion process with POME present, which ultimately led to a higher biogas and methane production. Conversely, the high fluctuations in NH3-N in the cattle manure treatment suggest that the accumulation of ammonia nitrogen led to a lower amount of biogas production because of an inhibition on microorganisms. After three days, the volatile solids content in both treatments began to decrease more rapidly, which was most likely due to the ideal pH and temperature and adaptation of the microorganisms present (Dubrovskis et al. 2009). Overall, more VS were removed from both treatments during the semi-continuous period than in the batch period.
The chemical oxygen demand concentration decreased most rapidly for both treatments within the first ten days, which is most likely due to hydrolysis of the cattle waste. The fluctuations in the COD during the semi-continuous operation was likely due to the continuous adaptation of the microbial population to the changing environment within the digester (Muhammad 2011). The substrate concentrations sampled from both treatments throughout the experiment suggest that the reduction in TS, VS, and COD can be significantly improved by two-fold using POME additive during cattle manure biogas production. Nasir et al. recommend the use of POME as an effective additive in the biogas production from cattle manure through anaerobic digestion. They suggest that not only would the POME considerably improve the removal of TS, VS, COD, and ammonia nitrogen in cattle manure digestion, but the process would also create a use for the palm oil waste from mills.
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