6. Modeling of pressure-driven membranes

Figure 5. Surface topography of PAm-ZTS as pH-responsive membrane, measured at 25C after treatment at pH 8.

10 Wastewater and Water Quality

The natural water resources contain solids in two forms, suspended and dissolved [16, 38, 39]. Suspended solid-state matters exist in insoluble particulates, debris, seawater microorganisms, silt, or colloids. Dissolved matters are present as ions, preferably as chloride, sodium, calcium, or magnesium. Principally, all desalination plants incorporate two-key treatment steps, sequentially designed to remove suspended and dissolved matters from their sources.

The first step of pretreatment removes the suspended solids from water resources or the naturally occurring soluble solids that may turn into a solid form and precipitates on the ROMs during separation processes. The second step of the RO system separates the dissolved solids from the pretreated saline source water, thereby producing fresh low-salinity water convenient for human utilization agricultural purposes and industrial implementations.

Subsequent pretreatment is designed for the left solids in the source stream; it includes the dissolved minerals. As long as the desalination system is operated in a manner that prevents these minerals from precipitating on the membrane surface, the ROMs could operate and produce freshwater of persistent nature at a high rate deprived of the need to clean these ROMs for long periods.

Notwithstanding pretreatment systems remove most but not all the insoluble solids contained in the saline source water and may not always effectively protect some of the soluble solids from precipitating on the membrane surface, the suspended solids, silt, and natural organic matter (NOM) that remained which may accumulate on ROM surface causing the loss of membrane productivity. In inclusion, saline water contains microorganisms as well as dissolved organics that could serve as food for these microorganisms. Consequently, a biofilm could form and grow on the ROM surface, causing loss of membrane productivity as well.

The protocol of reduction/loss of productivity of ROMs due to agglomeration of suspended solids and NOM, precipitation of dissolved solids, and/or establishment of biofilm on the ROMs surface is known as membrane fouling (MF). Excessive MF is undesirable since it has a negative impact on ROM productivity; it could also result in an increased consumption of energy for salt separation and in deterioration of product water quality.
