Two endemic strains of the marine microalgal species Phaeodactylum tricornutum (designated B2 and B4), previously isolated from a sewage outfall site in St Andrews Bay, Scotland, were cultured in 20-litre mini-ponds to determine their ability to remove ammonium and orthophosphate from wastewater diluted with seawater. These strains had been selected from 102 species 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 algal cultures grown in an open greenhouse under ambient conditions. Nutrient concentrations in the diluted wastewater and in outflow from the cultures were measured daily. Both strains remained unialgal with little change in biomass during the 14-day culture period and continuously removed >80% of ammonium from the wastewater. However, while strain B2 removed >80% of orthophosphate, there was a gradual accumulation of orthophosphate in the culture of strain B4. Measurement of nutrient concentrations in diluted wastewater and outflow from the continuous culture of strain B2 over 24 hours showed that at night nutrient removal dropped to a minimum of >70% for both ammonium and orthophosphate. These results indicate the potential value of strain B2 for use in scaled-up treatment ponds.

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