Efficiency improvement of biogas production from agricultural waste

Abstract

This research involves the in-situ methane enrichment for biogas upgrading process by desorption carbon dioxide from effluent and recirculating back to the system. Also, the investigation of microalgae growth as a mean to remove CO2 from biogas was conducted. This novel combined process can upgrade the biogas quality and simultaneously remove macro-nutrients from a biogas digester effluent. Laboratory-scale experiments were performed using 1,000 liters continuously channel digesters reactor operating with pre-mixed wastewater from chicken manure and tap water. The reactors were connected to a 30-liter external bubble column for selective desorption of CO2. This study consisted of two parts: the first part focused on the effects of gas bubble column parameters on the desorbed CO2 from the effluent and the impacts of effluent recirculation on the digester performance. The second part focused on the study of biological post treatment by microalgae for simultaneous CO2 and nutrient removal from effluent wastewater. In the first part, in-situ methane enrichment was studied. The biogas effluent was recirculated at the liquid to air flow (L/G ratio) from 0.20-0.70. The effluent flow rates through the column were varied from 1.75-3.30 liters/min and the air flow rates were in a range of 4.17-8.75 liters/min. It was found that the quantity of desorbed CO2 and the efficiency of CO2 desorption depended on the L/G Ratio in the desorber column, which observed a higher CO2 desorption at increasing L/G ratio. The desorbed CO2 amount was as high as 124.46±3.02 liters/day at L/G ratio of 0.70, indicating the efficiency of CO2 desorption of 72.68±4.07 %. The biogas effluent recirculation rate was varied from 230%- 430% of the digester volume at the L/G ratio of 0.40. Similarly, the CO2 desorption efficiency at the recirculation flow rate of 430% was found significantly higher than those of 230% and 330%, indicating more efficient CO2 removal. However, the high recirculation rate could cause higher methane loss. Under the investigated conditions, the organic removal efficiency and the CH4 yield of channel anaerobic digester were not significant affected by the gas desorption. In the second part, the biological post-treatment of digester effluent by microalgae was performed. It was found that Chlorella sp. was able to grow in high-COD wastewater as a growing medium. In the batch experiment, it was found that its nutrients uptake rate was high, especially in the condition with adequate lighting. Under mixotrophic cultivation, the growth rate of Chlorella sp. was higher than that in heterotrophic cultivation by approximately 2 times. While obtaining a moderate removal of COD, the culture could remove nutrients efficiently. In particular, NO2 and NO3 removal were exceptionally high. In a mixotrophic continuous experiment, the photobioreactors with the total volume of 98 liters each were used to perform the experiment. The combined system was operated at the L/G ratio of 0.60 and the algae growth was enhanced by feeding the off-gas which contained 1.91±0.12% of carbon dioxide. The effluent was diluted with distilled water to vary the COD loading of 1.89, 3.16, 4.93, and 6.30 kg COD/m3day, respectively. It was found that the growth curves of Chlorella sp. increased as the COD loading increased. At feed of 4.93 kg COD/m3day, the highest specific growth rate of Chlorella sp. (2.08±0.06 day-1) was observed. The CO2 uptake rate was high, especially in the condition with high feed livestock wastewater, resulted in 39% removal efficiency of CO2. The highest CO2 consumption rate was 1,664 mg/liter/day occurred at the feed of 4.96 kg COD/m3day. Also, Chlorella sp. showed high efficiencies in nutrient removal (44% COD, 30% TN and 54% TP). In addition, the amount of hydrogen sulfide was completely removed at all experiments. In conclusion, this study demonstrates that the novel biogas upgrading process by desorbing carbon from effluent and cultivating microalgae in the biogas digester effluent can be a potential technology for biogas upgrading. Not only the CO2 can be partially captured, but the wastewater can be simultaneously treated. The economic analysis showed that such technique is economically feasible, compared to conventional biogas upgrading processes.