Investigations on energy conversion of waste coconut water through an Up-flow Anaerobic Hybrid Bioreactor

Abstract

Many Agro-industries discharges considerable amount of wastewater to water bodies. Anaerobic digestion of organic effluents from agro-industries has a great importance in pollution abatement as well for renewable energy production. Waste coconut water (WCW) is a medium strength waste water for which high rate anaerobic treatment is an affordable technology. This technology offers simultaneous production of energy in the form of biogas along with pollution control. Conventional biogas plants are slow in operation with long Hydraulic Retention Times (HRTs) in the order of 35 to 55 days, necessitating very large digester volumes. Hence, anaerobic treatment of WCW is technically and economically feasible only through high rate bioreactors, where we can reduce the HRTs in the range of 6 to 8 days. Hence, an investigation was taken up to study the performance of a high rate bioreactor viz. Up-flow Anaerobic Hybrid Bioreactor (UAHBR) for biomethanation of WCW. It was revealed that the WCW had a low pH along with high Bio-chemical Oxygen Demand (BOD) and Total Solids (TS). The semi-continuous digestion WCW was carried out in a lab scale floating gas holder digester. The digester was operated at different HRTs of 35, 30 and 25 day and performance evaluated. During all HRT there was a profound effect of pH over the working of the digester. The maximum daily biogas production and biogas productivity were 21.9 L and 3.5 L.L -1 during 30-day HRT. The TS reduction had the maximum value of 51.94 at 35-day HRT. The performance of the digester deteriorated at 25 day HRT and the minimum reduction was only 1.38 %. The system showed signs of failure. Existing full scale UAHBR was operated at different HRTs of 16.67 and 15 day and performance evaluated. The reactor was stable in operation throughout the period of operation and revealed high organic reduction with biogas production. The maximum specific biogas production and biogas productivity were 354.31 Lkg 1 TS added and 13.50 L.L -1 during 15-day HRT. The TS reduction was in the range of 79.35 and 81.40 during the period of 15-day HRT. Experimental UAHBR was fabricated and performance evaluated at different HRTs of 15, 12, 10, 8 and 6 day. Reactor was stable in operation during 15, 12, 10, 8 and 6 day HRTs and exhibited high process efficiency characterised by good organic reduction and biogas production. The performance was slightly deteriorated with 8 and 6-day HRT. The maximum daily biogas production and volumetric biogas production were 114 L and 877 L.m -3 for 6-day HRT. The maximum specific biogas production and biogas productivity were 225.73 L.kg -1 TS added and 8.7 L.L -1 during 15-day HRT.