AWWA MTC53939

The City of San Diego, California, is developing new water sources to serve itsarid region. In addition, water reclamation is being seriously evaluated by theCity to reduce the region's reliance on less dependable imported water. The Cityadopted the reverse osmosis (RO) process as the foundation of the advanced watertreatment of many of their reclamation projects because RO has been shown toaccomplish the best overall removal of organics, trace metals and total dissolvedsolids. In addition, RO has the potential for removal of all classes ofpathogens. In order to operate the RO membranes reliably and minimize membranefouling, a pretreatment step is required. A comparison of thin film composite(TFC) RO membrane operational performance from three different projects using thesame municipal wastewater source but different RO pretreatment processes wasperformed. The treatment scenarios that were evaluated included: microfiltration(MF); ultrafiltration (UF); membrane bioreactor (MBR); and, lime clarification.The lime clarified effluent wastewater proved a possible pretreatment to thin TFCRO membranes. However, the variable effluent water quality resulted in cleaningintervals between 600 and 1400 hours. The MBR and MF/UF pretreatment scenariosproduced a consistent effluent water quality. The MBR effluent did provide the ROmembranes with a run time as long as 3200 hours, however the MF/UF gave a runtime, as long as 10,000 hours with the MF effluent. There was no differenceobserved in quality of the MF or UF effluent as a pretreatment to RO. Althoughthe MBR did produce a similar effluent water quality as the MF/UF system, the runtime was significantly shorter. The exceptionally long run times experienced withthe MF/UF by far make it the best pretreatment to TFC RO membranes. Includes 6 references, figures.