Algal growth in primary settled sewage: The effects of five key variables

Photoperiod, at 3240 lx (300 ft candle) light intensity, was found to be the primary limiting factor for algal biomass growth in a bench scale series of experiments using primary settled sewage. At a retention time of seven days, a total effective photoperiod of greater than 6 h a day was required to produce algae at concentration above 500 mg 1 −1. Increasing dissolved CO 2 concentration promoted green algae production and resulted in a higher yield than that obtained from the blue green algae which dominated at low levels of dissolved CO 2. Unfortunately, most of the green algae were unicellular, discrete, small particles, and would be expensive to harvest compared with blue-green algae which often grow in colonies possessing a gelatinous sheath or are of filamentous construction. Low temperature favoured algal biomass production because of its effect upon the solubility of CO 2. The addition of bicarbonate also increased algal biomass yield but an excess of ammonium nitrate inhibited algal production. The removal of nitrogen and phosphorus by algae varied with the quantity of algal biomass produced. Under the experimental conditions, nitrogen removal corresponded to about 3 mg per 100 mg of biomass produced and phosphorus removal to about 0.5 mg per 100 mg biomass produced.