A novel nutrient removal/waste heat utilization process was simulated using semicontinuous cultures of the thermophilic cyanobacterium Fischerella. Dissolved inorganic carbon (DIC)-enriched cultures, maintained with 10 mg l(-1) daily productivity, diurnally varying temperature (from 55 degrees C to 26-28 degrees C), a 12:12 light cycle (200 mu E sec(-1) m(-2) and 50% biomass recycling into heated effluent at the beginning of each light period, removed > 95% of NO3- + NO2--N, 71% of NH3-N, 82% of PO43- -P, and 70% of total P from effluent water samples containing approximately 400 mu g l(-1) combined N and 60 mu g l(-1) P. Nutrient removal was not severely impaired by an altered temperature gradient, doubled light intensity, or DIC limitation. Recycling 75% of the biomass at the end of each light period resulted in unimpaired NO3- + NO2-removal, 38-45% P removal and no net NH3 removal. Diurnally varying P removal, averaging 50-60%, and nearly constant > 80% N removal are therefore projected for a full-scale process with continuous biomass recycling.