Methane production from anaerobic co-digestion of Chlorella vulgaris and wastewater sludge

Includes bibliographical references (p. 42-48).Increasing demand for energy coupled with concerns over limited fossil fuel reserves and apprehensions over their contributions to greenhouse gas emissions have made the search for low carbon energy sources a high priority. Algal biomass could serve as an alternative source of renewable biofuels. Research efforts to date have primarily focused on the production of algal biofuels through lipid extraction, which involves high temperature and high pressure, resulting in an energy intensive process. In this research, the use of algal biomass as a supplementary feedstock to anaerobic digesters for the production of methane gas is evaluated. To test the potential of algal biomass as a supplementary feedstock, labscale anaerobic digesters are set-up. The methane gas production of various combinations of thickened waste activated sludge (TWAS) and algal biomass is investigated. Chlorella vulgaris (C. vulgaris) is used as representative microalgae. In addition, the effects of operational parameters, such as biomass loading, temperature and alkalinity, on biogas production are investigated. The results show that the biogas production for all biomass loading combinations of C. vulgaris and TWAS ranged from 0.47-0.57 mL per mg volatile solids (VS) digested. On average, VS and chemical oxygen demand (COD) were reduced 48 and 38%, respectively, at 35??C. Average total coliform (TC) and fecal coliform (FC) concentrations of 6.3x10??? and 1.0x10??? CFU per gram of total solids (TS), respectively, were measured in the digested waste at 35??C. Thus, the residual meets the USEPA requirements for pathogen reduction (FC < 2x10??? CFU per g TS) and vector attraction reduction (> 38% reduction in VS) for land application. The total nitrogen and phosphorus content of the residual was determined to be in the range of 9-17% as N and 3-7% as P (7-16% as P???O???), respectively, revealing its potential value as a fertilizer. It was also observed that decreased digestion temperatures resulted in lower biogas yields, while initial alkalinity in digesters did not appear to affect biogas production. From the results of the research, it can be inferred that algae can be co-digested with wastewater sludge, or by itself, to produce methane gas at wastewater treatment plants (WWTPs). This suggests that algae can be utilized as an energy source through anaerobic co-digestion with wastewater sludge. This is significant because algae can be grown with the nutrient and CO??? contained in waste streams at WWTPS, thereby minimizing the release of nutrients and effluent water to the environment. This reduced nutrient load results in treatment cost savings, while the reduction in effluent discharge decreases environmental pollution