UV disinfection of wastewater coagulated with ferric chloride: Recalcitrance and fouling problems

Collimated beam and pilot tests using low pressure or medium pressure UV lamps were conducted to assess the disinfection performance of a municipal wastewater which had been pretreated by ferric chloride coagulation followed by sedimentation. The feed to the UV systems contained high levels of Fe (often above 3 mg/l); suspended solids and UV254 transmission values were also not ideal (mean approximately 30 mg/l and 32% respectively). The collimated beam tests indicated that the target level of 2,500 coliform forming units/100 ml often could not be reached, and the pilot tests revealed significant fouling after only a few hours. Thus physicochemical effluents of this nature likely require upstream filtration (or a relaxation of the target level) and a continual, automated cleaning system to ensure that performance levels can be maintained.