Microbial Removal from Secondary Treated Wastewater Using a Hybrid System of Ultrafiltration and Reverse Osmosis

Abstract

The efficiency of advanced membranes towards removal of general and specific microbes from wastewater was investigated. The treatment included a subsequent system of activated sludge, ultrafiltration (hollow fibre membranes with 100 kDa cut-off, and spiral wound membranes with 20 kDa cut-off), and RO (reverse osmosis). The removal evaluation of screened microbes present in treated wastewater showed that hollow fibre membrane rejected only 1 log (90% rejection) of the TPC (total microbial count), TC (total coliforms), and FC (faecal coliforms). A higher effectiveness was observed with spiral wound, removing 2-3 logs (99%-99.9%) of TPC and complete rejection of TC and FC. The RO system was successful in total rejection of all received bacteria. The removal evaluation of inoculated specific types of bacteria showed that the hollow membranes removed 2 logs (99%) of inoculated E. coli (107-108 cfu/mL inoculum), 2-3 logs (99%-99.9%) of Enterococus spp. (107-1010 cfu/mL inoculum), 1-2 logs (90%-99%) of Salmonella (108-1010 cfu/mL inoculum) and 1-2 logs (90%-99%) of Shigella (105-106 cfu/mL inoculum). The spiral wound was significantly efficient in rejecting further 3 logs of E. coli, 5 logs of Enterococus spp., 4 logs of Salmonella, and a complete rejection of all received bacteria was accomplished by RO membrane. The results indicate that Gram positive bacteria were removed much more efficiently compared to the Gram negative ones, the rationale behind such behaviour is based on cell walls elasticity.