Removal of phosphorus and residual aluminium with the simultaneous use of chitosan and alum on the effluent of an MBBR biological system during start–up

Physico–chemical treatment with aluminium salts is a common practice to remove total phosphorus (TP) from wastewaters. However, the use of alum can increase the residual aluminium concentration both in the effluent and biosolids. Chitosan, an alternative coagulant, does not allow for the removal of TP below the requirement level when lower than the soluble phosphorus fraction of the water. Hence, simultaneous dosage of alum and chitosan solutions was evaluated on the effluent of a newly installed MBBR (moving bed biofilm reactor) system for residual TP and aluminium removal. At alum optimal dosage, the most effective chitosan solution generated: 1) an optimal dosage zone for which residual TP and aluminium concentrations were minimal; 2) maximal abatements for TP and aluminium that reached 35 and 85%, respectively, the concentrations observed with alum only. Also, the fraction of residual aluminium in the biosolids was increased, particularly from 84 to 98% at optimal chitosan dosage.