In reuse applications, the feedwater is often secondary effluent (biologically treated municipal wastewater).   This can be variable at times, with a high suspended solids and a high proportion of colloidal material, organics and bacteria.  Reverse osmosis (RO) membranes are well suited for removal of dissolved solids, but adversely affected by suspended solids.   Membrane fouling from suspended solids, colloidal material, bacteria or scale are major reasons for RO system failure.   The implications of membrane fouling are a decrease in flux and salt rejection, an increase in feed pressure and energy consumption and often irreversible membrane damage.   Appropriate pretreatment is the key to long term stable performance of RO membranes.

Microfiltration is used to remove suspended solids, colloidal particles, cysts and bacteria.  Unlike tighter membranes such as RO and NF that alter the chemical composition of the feedwater, microfiltration is a particle removal process, primarily used for clarification and disinfection and thus is an excellent pretreatment for RO and NF membranes.

Microfiltration (MF) has demonstrated the ability to significantly reduce membrane fouling and provide stable, predictable RO performance, which other conventional pretreatment technologies cannot match.   The pretreatment effectiveness of MF systems compared to conventional systems can be evaluated in terms of the:

• Quality and variability of the feedwater
• Capacity of the system and the space available
• Effect of feedwater quality on product water
• Amount of operator attendance required
• Amount of maintenance required
• Reliability, capital and operating costs of the RO or NF system

How does Microfiltration compare with Conventional Pretreatment Systems?
USFilter case study: Eraring Power Station