Diagnosis, prediction and prevention of biofouling of NF and RO membranes

Biofouling phenomena were studied in 30 membrane elements (autopsy) for nanofiltration (NF) or reverse osmosis (RO) collected from 13 plants. Biomass parameters included adenosinetriphosphate (ATP), Total Direct Cell counts (TDC) and Heterotropic Plate Counts (HPC), respectively. The concentrations of biomass were related to the extent of variation of operational parameters as flux (normalized flux: MTC and/or pressure drops (normalized pressure drop: NPD), viz. highest biomass parameters were observed in plants with the highest changes of MTC and/or NPD. Monitoring of feed water from NF and RO plants was performed with biomass parameters (ATP and TDC), the easily assimilable organic carbon (AOC) test and the Biofilm Formation Rate (BFR) in a biofilmmonitor. Monitoring of the feed water showed that severe biofouling occurred in cases where the feed water had BFR-values - temporarily exceeding 120 pg ATP/cm(2).d and/or the AOC-value exceeded 80 mug Ac-C/l. BFR-values lower than 1 pg ATP/cm(2).d enabled stable operating for periods up to two year without cleaning. For BFR-values between 1 and 120pg ATP/cm(2).d as well as AOC-levels below 80 mug is still unknown whether biofouling occurs. Monitoring also showed that low concentrations of biodegradable compounds (mug/1) in the feed water could lead to biofouling. The results of monitoring of feed water agreed with the data from autopsies and extent of operational problems. Biofouling was observed in 12 of the 13 plants. Two plants suffered from biofouling caused by dosage of chemicals. Elucidation of the relationships between test parameters and extent of operational problems (NPD increase and MTC decrease) under different conditions requires more research. Based on such data more quantitative criteria for feed water quality can be defined enabling fin-ther prevention of biofouling. Also, further investigation is needed for the selection and preparation of effective cleaning chemicals and cleaning strategies.