Tertiary domestic waste water treatment by Scenedesmus sp. pilot-scale culture
Two experimental pilot-scale culture systems have been operated at Valcartier (domestic effluent) and Vaudreuil (semi-industrial urban effluent) in order to test the feasibility of a tertiary treatment using microalgae (Scenedesmus sp.). Tanks with a capacity of 15 000 L were used for batch cultures conducted outdoors; CO2 being provided through atmospheric air bubbling. Physical (temperature, relative insolation, solar radiation), chemical (pH, NH4+, NO2-, NO3-, PO4-3, heavy metal, pesticides) and biological parameters (biomass dry weight, cell counts) were mesured. Results show that cultures were limited for CO2 (high final pH) and for nitrogen flow N/P ratio with value of 3-7). During the summer period, nutrient removal from the Valcartier effluent was ~ 95 % for N and ~ 60 % for P; corresponding figures at Vaudreuil were 92 % and 98 %. Biomass productivity was lower (0,39 mg/L-h) at Valcartier compared to that obtained at Vaudreuil (1,02 mg/L-h). Among the differences that could explain the results at both sites, one may suggest a better CO2 input, a more favorable N/P ratio, slightly higher average temperature and light conditions, less ammonia stripping and larger inocula in Vaudreuil. The quality of biomass obtained was satisfactory : heavy metals were present at acceptable levels and no contamination by organochlorinated compounds occurred. According to our results, it appears that biological tertiary treatment of urban effluent and algal biomass production are technically possible under Québec summer conditions.