Integrated multi-objective membrane systems for surface water treatment: pretreatment of reverse osmosis by conventional treatment and ultrafiltration

Within the scope of this project funded by AWWARF and USEPA three very promising IMS's were identified for surface water treatment. This paper will cover some highlights of the research carried out by the Water Supply Company of North Holland and Kiwa on the combination ultrafiltration/ultra low pressure reverse osmosis. Productivity of the ultrafiltration proved to be stable for the first 6000 h. Then, at the end of summer, the backwash regime was not able to prevent accumulation of biofouling. This biofouling coincides with an increase in bacterial counts in the ultrafiltrate. Also the ultra low pressure reverse osmosis unit showed no significant fouling up to 6000 hours. Then a rapid decrease of the second stage MTC started caused by BaSO4 precipitation, coinciding with an increase in feed-concentrate pressure drop caused by a severe biofouling. Ultrafiltration reduced the indicator organisms E. coli and spores of sulphite reducing Clostridia to under the detection limit. To determine the elimination capacity the feed of UF and ULPRO were spiked with MS-2 phages. Generally UF showed a complete (>5 log) removal. Once only a 2.7 log removal was found caused by defects in UF-membranes. ULPRO showed a high (4.7-4.9 log) although no complete removal. The results show that integrity measurement and monitoring of both membrane steps are essential. UF-integrity is measured by a pressure hold test and monitored by particle counting. integrity of ULPRO is measured by vacuum testing and monitored by in situ conductivity measurement and particle counting. ULPRO gives a very high precursor removal. Potential measurements (24 h, 20 degrees C, 1-2 mg/l Cl-2-residual) showed >95% removal of THMFP and HAAFP. IMS with UF and ULPRO are exceptional tools for control of microbials and DBP-precursors. Crucial for application may be control of biofouling in the system.