A controlled stream mesocosm for tertiary treatment of sewage

Freshwater stream ecosystems are well known for their capabilities for ''self-purification'' of sewage and other wastewaters. Unfortunately, the efficiencies of treatment are low and concentrations and volumes now discharged cannot be treated by self-purification alone. This paper describes an experiment with a stream mesocosm, in central California, USA, using controlled ecosystem methodologies in the format of an algal turf scrubber (ATS(TM)). This system was used to drive primary production and export in the mesocosm to bring secondary sewage to tertiary levels. The mesocosm consisted of a natural, mixed assemblage of attached periphyton, microalgae and bacteria which colonized an inclined floway 152 m long and 6.7 m wide, over which wastewater flowed in a series of pulses. The capacity of the wastewater flow varied between 436 and 889 m(3) per day and various operational parameters were tested. Biomass was mechanically harvested from the floway at 1- or 2-week intervals depending upon the season. This paper presents the results for nitrogen and phosphorus removal as well as that of other contaminants and productivity of the algal turf. Nitrogen and phosphorus removal from the secondary wastewater was measured twice a week during four, 8-week quarters corresponding to the solar seasons. Nitrogen and phosphorus content of the harvested solids was also measured during these periods. Based on the percentage of nutrients in the harvested solids (3.1% N and 2.1% P) and the operational productivity of 35 g dry solids m(-2) day(-1), the yearly mean removal of nitrogen and phosphorus was 1.11 +/- 0.48 gNm(-2) day(-1) and 0.73 +/- 0.28 gP m(-2) day(-1), respectively. Results indicate the strong potential of controlled stream mesocosms for the removal of nutrients and other contaminants from wastewaters to achieve tertiary levels.