Wastewater polishing by a channelized macrophyte-dominated wetland and anaerobic digestion of the harvested phytomass

Abstract

Constructed wetlands (CW) offer a mechanism to meet increasingly stringent regulatory standards for wastewater treatment while minimizing energy inputs. Additionally, harvested wetland phytomass subjected to anaerobic digestion can serve as a source of biogas methane. To investigate CW wastewater polishing activities and potential energy yield we constructed a pair of secondary wastewater-fed channelized CW modules designed to retain easily harvestable floating aquatic vegetation and maximize exposure of water to roots and sediment. Modules that were regularly harvested averaged a nitrate removal rate of 1.1 g N m⁻² d⁻¹; harvesting, sedimentation and gasification were responsible for 30.5%, 8.0% and 61.5% of the N losses, respectively. Selective harvesting of a module to maintain dominance of filamentous algae had no effect on nitrate removal rate but lowered productivity by one-half. The average monthly productivity for unselectively harvested modules was 9.3 ± 1.7 g dry wt. m⁻² d⁻¹ (±SE). Cessation of harvesting in one module resulted in a significant increase in nitrate removal rate and decrease in phosphate removal rate. Compared to the influent, the effluent of the harvested module had significantly lower levels of estrogenic activity, as determined by a quantitative PCR-based juvenile trout bioassay, and significantly lower densities of E. coli. In mixed vertical-flow reactors anaerobic co-digestion of equal dry weight proportions of harvested aquatic vegetation, wine yeast lees and dairy manure was greatly improved when the manure was replaced with the crude glycerol by-product of biodiesel production. Remaining solids were vermicomposted for use as a soil amendment. Our results indicate that incorporation of constructed wetlands into an integrated treatment system can simultaneously enhance the economic and energetic feasibility of wastewater and organic waste treatment processes.

Keywords:

• Anaerobic co-digestion
• aquatic weeds
• biofuel
• biogas
• constructed wetland
• denitrification
• endocrine disruptors
• methanogenesis
• nutrient removal.

Acknowledgments

Technical assistance was provided by many City of Santa Rosa staff and Sonoma State University students and staff, including A. Agostini, J. Collins, M. Cupp, S. Horne, A. Simpson, M. Vieira, and N. Warden. We greatly appreciate the guidance given by K. Nielsen and T. Lundquist. Funding was provided by the City of Santa Rosa (D. Tredinnick and N. Dorotinsky, project managers) and grants from the California Energy Commission Energy Innovations Small Grant Program, the Bay Area Air Quality Management District and the California State University Program for Education and Research in Biotechnology.