Tertiary treatment of wastewater by marine microalgae grown on a corrugated raceway

Two marine microalgal isolates from a sewage outfall site in St Andrews Bay, Scotland were cultured on corrugated raceways (2.5 m long, 0.2 m wide) to determine their ability to remove ammonium and orthophosphate from wastewater diluted with seawater. The isolates (SA91B33, preliminarily identified as Phaeodactylum tricornutum, and SA91CY1, Oscillatoria sp.) both have surface-adherent properties and were selected from 102 isolates for optimal nutrient removal and culture dominance in both batch and continuous culture on wastewater under controlled environmental conditions. Wastewater (primary sewage effluent) was diluted 1:1 with sterile seawater and continuously added to raceway algal cultures grown under ambient conditions. Nutrient concentrations in the diluted wastewater influent and in the effluent from the raceways were measured daily. Both isolates remained unialgal during the four month culture period and continuously removed 100% of ammonium and orthophosphate from the wastewater. Nitrite and nitrate levels in both influent and effluent were negligible. Measurement of influent and effluent nutrient concentrations over 24 h showed ammonium and orthophosphate removal remained unaltered during the diurnal cycle. These results indicate the potential for using microalgal species grown on raceway surfaces for wastewater treatment. (C) 1997 Elsevier Science Ltd.