PNNL

Abstract

Wastewater algal treatment systems show improved economic viability and enhanced energy return on investment when the algal biomass is converted to biofuel. One biofuel option is to anaerobically digest the algae to generate bio-methane. This method is appropriate for filamentous algae that are typical of turf scrubbers®, because they have relatively low lipid content. But unbalanced carbon to nitrogen (C/N) ratio and the resistance of algal biomass to biodegradation can limit it conversion into bio-methane. To evaluate options to improve algal derived bio-methane production, an indigenous assembly of macro-algae was established and cultivated in CO2-infused secondary wastewater effluent, then harvested and either anaerobically digested using pretreatments or co-digested with sewage sludge. Results were used to develop methane production kinetic models and perform an AD system energy balance analysis in order to evaluate the feasibility of pretreatment and co-digestion for a scaled process. Dominated by Ulothrix and Oedogonium periphyton biomass production rates averaged 3.7±0.4 g VS m-2 d-1 (±1SD) and increased by 62 % in the second half of the 14 day cultivation experiment. Ultimate methane yield from harvested biomass (306±13 mL gVS-1) was improved through thermal pretreatment by 15%, dilute acid by 5%, dilute alkali by 17%, acid- and alkali-assisted thermochemical pretreatments by 23 and 27%, respectively. However, all pretreatment methods had negative impact on energy balance parameters except alkali-assisted thermochemical pretreatment, which provided only a minor positive effect. In contrast, co-digestion of algal biomass with sewage sludge enhanced methane yield and reached maximum value of 401±3 mL gVS-1 at algae to sludge ratio of 20% to 80%. Most importantly, co-digestion strongly improved AD system energy balance. Net Energy Ratio and Net Energy Efficiency increased from 2.8 and 73% for algae as sole substrate to 4.3 and 81%, respectively, for an algae to sludge ratio of 20%:80%. Moreover, the Net Energy Recovery during co-digestion reached 39% compared to only 26% and 33% when algae or sewage sludge were processed as single-substrates. Thus, co-digestion of algae with sewage sludge serves as an attractive option for extracting bioenergy from biomass harvested from filamentous algal polyculture treatment systems.