Embrace the Solids – Rethinking How Pretreatment is Used with MF/UF Membranes

Conventional wisdom would dictate that as raw water Total Suspended Solids and Turbidity rises, the head-loss across a filter or trans-membrane pressure (TMP) of a membrane filtration system should also rise. This prompts designers and operators to reduce filtration time, reduce filtration flux, and increase filter cleaning (backwashing or otherwise) when elevated levels of turbidity enter the filtration system. Therefore, many of our guiding principles and rules of thumb for filter and membrane operation dictate that influent raw water turbidity must be tempered with clarification before a filtration step. This may be true for a conventional filtration system, but a growing number of pilot studies and operating plants are demonstrating that this methodology does not apply to an ultrafiltration (UF) or microfiltration (MF) membrane system. Rather, a rise in feed turbidity in a UF/MF membrane system correlates to an improvement in system performance, indicated by a decline or stabilizing of TMP. In this way, a ìsolids toleranceî of a UF/MF membrane may be more accurately described as a ìsolids preferenceî of that membrane. Recent pilot studies conducted by SUEZ Water Technologies & Solutions (SWTS) in partnership with our municipal customers in the United States and Canada have indicated that selecting a UF/MF membrane with the appropriate solids tolerance specification for direct filtration of a source water will outperform a UF/MF membrane in a flow sheet combined with conventional and high-rate clarification as pretreatment.