Laguna Watershed Knowledgebase

Constructed wetlands can be used to lower levels of residual contaminants and nutrients in water, while biogasification of vegetation harvested from the wetlands can generate useable energy. Two gravity-flow 400 ft^2 Channelized Aquatic Scrubbers (CAS) were constructed at the City of Santa Rosa Laguna Treatment Plant, each composed of three channels ranging in depth from 5 to 20 inches and stocked with native floating aquatic plants and algae common in the Laguna de Santa Rosa. The CAS displayed a high efficiency of nitrate removal (1.0 ± 0.5 g N/m^2/d from July 2008 to June 2009; mean ± SD), primarily owing to denitrification. Additionally, results from juvenile trout bioassays demonstrated that the CAS substantially decreased levels of estrogen-mimicking compounds in the water. Net productivity of the CAS, estimated from regular harvests, averaged from 1.3 to 13.7 g dry weight/m^2/d. CAS could potentially be applied in the Laguna watershed for scrubbing nutrient-rich tributaries.
Energy generation from harvested biomass would enhance the cost effectiveness of larger scale applications of CAS. Currently, the most technically feasible procedure for extracting usable energy from the biomass is anaerobic digestion, which produces methane-rich biogas that can offset fossil fuel consumption. Anaerobic digestion of harvested biomass at 35 °C in the laboratory yielded 145 ± 22 ml biogas/g dry weight (mean ± SE); an approximate 40% synergistic enhancement of biogas production from the biomass was attained by co-digesting with a mix of winery and dairy waste. Two 1500 gallon digesters, recently built on the treatment plant grounds, will be fed with varying proportions of CAS-harvested vegetation, Ludwigia from the Laguna, and agricultural wastes. The digested material will be utilized as a soil amendment for an on-site garden. Deployment of this type of integrated nutrient removal/bioenergy system would recycle carbon and nitrogen, support local food production, and reduce the demand for fossil fuels in our community.