PNNL

Abstract

In the current study the feasibility of cultivating a lipid-containing algal-bacterial polyculture in municipal primary wastewater and enhancing biomethanation of lipid-extracted algal residues (LEA) through hydrothermal pretreatment and co-digestion with sewage sludge (SS) was investigated. In high-rate algal ponds, the polyculture of native algal and bacteria species demonstrated a monthly average net and gross biomass productivity of 30±3 and 36±3 gAFDW/m2/day (summer season). The polyculture metabolic activities resulted in average reductions of wastewater volatile suspended solids (VSS), carbonaceous biochemical oxygen demand (cBOD5) and total nitrogen (Ntotal) of 63±18%, 98±1% and 76±21%, respectively. Harvested biomass contained nearly 23% lipid content and extracted blend of fatty acid methyl esters satisfied the ASTM D6751 standard for biodiesel. The methane production from LEA was hampered by limited biodegradability and by toxicity of the residual solvent (hexane). Hydrothermal pretreatment improved ultimate methane yield and production rate by 15-30% but did not mitigate solvent toxicity effects completely. While, the prospected AD system energy output increased by 5-20% compared to untreated LEA the Net Energy Ratio did not exceed 1. In contrast, co-digestion of LEA with sewage sludge (10% to 90% ratio) was found to be a feasible solution for elimination methane inhibition and for maximizing the energy output from processed biomass, which resulted in enhancing energy output and NER about 3-4 fold compared to LEA as a single substrate.