Characteristics of RO foulants in a brackish water desalination plant

This study identified the membrane fouling of a two-stage RO process of the Bai-sha brackish water desalination plant facing serious fouling problems. Two membrane modules, one from each stage, were autopsied and diagnosed for the cause for the failure of the RO system. Different morphologies were exhibited on the fouled membrane. Results suggested that the fouling on the first-stage membrane tended to be organic and biological, while that on the second-stage membrane appeared to be scaling. Higher than 85% of the foulants on the first stage membrane were organic substances in opposed to the 5 to 8% of those on the second stage membrane. However, the total organic matter on the second stage membrane was more than that on the first stage one. The thick layer of scalants on the surface of the second-stage membrane was identified to be calcium carbonate of which the crystal was clearly seen in the SEM images. The scaling was thickest at the outlet of the membrane. Huge amount of microbes were found on both membranes. The FTIR results also suggested the existence of extracellular polymeric substance (EPS) on the first stage membrane. This study recommended that the recovery rate, pH and the potential for bio-fouling should be defined at the real operation.

The foulings in the upstream and downstream of the RO train can be represented by the membrane analysis of the first and second-stage membrane. It is concluded that organic and biofouling occurred at the inlet end of the membrane in the RO system. And both of these two foulings were more serious in the beginning of the system, which might be due to the sedimentation of insoluble organic matter and microbes. The survived microorganisms then released EPS on the membrane. The brine water was concentrated down the stream, which lead to the precipitation of calcium carbonate. The population of the microbe was reduced by approximately an order although still very significant. Contrary to bio-fouling, the organic fouling was serious at the outlet end similar to the trend of scaling. This phenomenon suggested the co-precipitation of calcium salt and soluble organic substrates. In summary, the RO system has faced complex problem of organic fouling, bio-fouling and scaling. Consequently the recovery rate, pH and the bio-fouling potential should be well defined at the real time operation.