Fouling water quality guidelines

Table 3.4 lists generally-accepted water quality guidelines to minimize fouling of RO membranes/... 

Suspended solids are typically measured using turbidity. Turbidity measures the light-scattering ability of particles in water. The water quality guidelines call for an influent turbidity of less than 1 Nephelometric Turbidity Units (NTU), which also happens to be a warranty requirement of membrane manufacturers. Exceed 1 NTU and the membrane warranty is voided. The lower the turbidity, the less likely the membranes are to foul with suspended solids. RO best practices call for feed water turbidity less than 0.5 NTU. 

Membrane performance tends to naturally deteriorate over time due to a combination of material wear-and-tear and irreversible fouling of the membrane elements. Typically, membrane elements have to be replaced every 5-7 years to maintain their performance in terms of water quality and power demand for salt separation. Improvements of membrane element polymer chemistry and production process have made the membranes more durable and have extended their useful life. Use of elaborate granular media pretreatment technologies and ultra and microfiltration (UF and MF) membrane pretreatment systems prior to RO desalination is expected to allow extending the membrane useful life to 7 years and beyond, thereby reducing the costs for their replacement and the overall cost of water. Detailed guidelines for designing SWRO plants are provided elsewhere (AWWA, 2007). 

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