Biological tertiary treatment of urban wastewaters with chitosan-immobilizedPhormidium

Summary Chitosan:Phormidium aggregates (chitosan: algae=1:2, dry weight basis) were used as a biological tertiary treatment to remove the nitrogen (NH ₄ ⁺ , NO ₂ ^- , NO ₃ ^- ) and phosphorus (PO ₄ ^3- ) from a secondary effluent. In a batch system, 71 and 92% of P–PO ₄ ^3- were removed after 6 and 24 h, respectively. The orthophosphate removal rate was identical for all three concentrations of algae-chitosan tested (3.3, 4.6, 5.9 g d. wt.·l^-1), and was 90 g±2 g P–PO ₄ ^3- ·l^-1·h^-1, for a 90% removal. Under control conditions (chitosan flakes only added to the effluent) 73 and 78% of PO ₄ ^3- were removed after 6 and 24 h respectively. A 95% removal of inorganic nitrogen (NH ₄ ⁺ , NO ₂ ^- , NO ₃ ^- ) was attained after 4–6 h withPhormidium immobilized on chitosan flakes, as compared to 30% with chitosan flakes alone (5 g d. wt.·l^-1). The system gave a similar performance when operated semi-continuously over 5 days at a daily retention time of 1.0. In the presence of chitosan-immobilized algae, medium P–PO ₄ ^3- levels were reduced by 87.3%±6.4% after 24 h (61.1 g±7.0 g P·l^-1·h^-1). The reduction of inorganic nitrogen in the medium was 98% after 24 h (370 g±50 g N·l^-1·h^-1). In the presence of chitosan alone, some 60% orthophosphate removal was recorded, whereas no reduction of nitrogen was observed. Disappearance of orthophosphate was attributed to its co-precipitation with calcium released from the chitosan by abrasion. The presence of the algae protected the chitosan from abrasion andPhormidium directly assimilated the orthophosphate and inorganic nitrogen, thus reducing their levels in the effluent.