Urban wastewater disinfection by filtration technologies

The microbiological quality of effluents from different macrofiltration systems (pressure sand filter and disc filter) used as pre-treatment and membrane technologies (microfiltration and ultrafiltration) was evaluated in order to determine their possible application as alternatives to disinfection of urban wastewater prior to reutilization. Microbiological quality was determined by reference to nematode egg content, fecal coliforms, E.coli and somatic coliphages. Pathogenic nematode eggs were efficiently retained by the macrofiltration systems. However, since other types of nematode eggs were present in the effluents treated by both systems, the possibility of such infective agents appearing after this type of treatment cannot be discounted. The membrane technologies proved highly efficient at retaining micro-organisms, achieving effluents of excellent microbiological quality. However, the effluents could not be classified as sterile, since contamination of permeation zones gave rise to the presence of micro-organisms. This result casts doubt on the validity of using the fecal coliform indicator to assess microbiological quality of effluents from these systems. Differences between the two membrane technologies were noted with regard to viral particle retention capacity, with only the ultrafiltration module achieving effluents with total absence of fecal contamination indicators. The macrofiltration systems may present problems when used as pre-treatments to standard disinfection systems (UV radiation, reactive oxidant disinfection). Such problems do not arise with the membrane technologies, which offer a valid alternative for the disinfection of urban wastewater prior to reutilization.