Microbe removal in secondary effluent by filtration

A study was carried out to assess the efficiency of filtration in reducing microbial contamination in municipal secondary effluent. After primary and secondary treatments, the wastewater underwent filtration through sand/hydroanthracite filters. A total of 20 samplings were made, each consisting of two instant samples (secondary effluent and filtered effluent). Each of the 40 samples was tested for: total and faecal coliforms, Escherichia coli, enterococci and somatic coliphages. The mean concentrations detected in the secondary effluent were in the order of 5 log for the total and faecal coliforms, 4 log for enterococci and Escherichia coli, and approx. 3 log for the coliphages. The filtration showed a higher efficacy in the reduction of faecal coliforms, Escherichia coli and in particular total coliforms. The results obtained for enterococci and coliphages were significantly lower. Filtration alone was not enough to reduce the bacterial indicators to within Italian legal limits, and showed a poor capacity to abate coliphages. However, by performing the filtration before disinfection, secondary effluents require a lower dose of disinfectant. This results in less pollution of surface waters from by-products and a consequent reduction in the chemical risk for the general population. Copyright © Source Interlink Media.