Study of biogas production from Distillery Waste Water over Immobilised Biomass

Abstract

n a molasses distillery, a lot of waste water is generated as the bottoms product. These waters contain a lot of organics, are dark brown in colour and have low pH. Their free disposal is harmful to the environment. Anaerobic digestion has been the most preferred treatment technique of this water. Besides stabilizing the effluents, anaerobic treatment enable the recovery of energy in the form of biogas. However, there are problems associated with this method. Low pH, high salinity, recalcitrant compounds, inhibitory substances and oxygen permeation are the major problems that lower the degree of effluent stabilisation. New developments are being explored to suppress these effects. Currently, the focus is on cell biological immobilisation. This study investigated the immobilisation of methanogenic consortia using conditioned natural zeolite and calcium alginate polymers and benchmarked them with an unsupported system. The main focus was to enable the utilisation of high strength substrate besides increasing biogas yield and suppressing process instabilities due to oxygen permeation. The specific objectives considered included the determination of the sorption carrying capacity of the consortia, determination of the most suitable organic loading, biogas yield and degree of waste water stabilization. Also, the identification of applicable microbial growth models and evaluation of the rate controlling step of the process formed part of the specific objectives. The experimental process involved preparation and characterization of the support media and the waste water followed by immobilization of the consortia unto the support material. The most suitable operational parameters of pH and system temperature were then determined. Batch studies of biogas generation then followed where the kinetics of the processes were evaluated. Further studies were done on calcium alginate supported system on its operational behaviour with regards to mass transfer. The rate limiting step in the process was then identified. From the results obtained, it was determined that the sorption capacity of the consortia at 25C was 13.2 mg/g amino acid per zeolite support mass. The most suitable initial organic loading was obtained to be 25 g/l , in all the systems although the highest initial strength of 75 g/l was considered as the focus of making the comparisons. Activated natural zeolite supported system registered the highest effluent stabilization of 44% on organic loading of 75 g/l. The system supported by calcium alginate registered the highest biogas yield of on organic loading of 75 g/l. All the systems conformed to Modified Gompertz’s kinetics as opposed to Monod’s and Andrew’s kinetics. As regards the kinetic system performance, the reaction rate constants for the unsupported, activated natural zeolite and calcium alginate supported systems were obtained to be 0.1055, 1.2348 and 0.6750 day-1 respectively. The substrate kinetic behaviour of calcium alginate supported system, registered a Damkolhler number of 0.0643 with respect to substrate diffusive transfer and biochemical substrate degradation rates. It was therefore concluded that substrate degradation rate was the rate limiting step. The results obtained on the two support materials can be used to design a system that is able to utilize high strength waste water without dilution. This consequently reduces the hydraulic loading on the system. It was recommended that antagonistic agents like phosphates and sodium salts should be eliminated first before the experiment commences as they caused the dissolution of calcium alginate beads. Finally, identification of the specific inhibitors causing longer adaptation periods should be done so that preliminary treatment strategies could be formulated to avoid the longer fermentation periods shown by the natural zeolite supported system.