The traditional pretreatment for RO has been conventional treatment with deep bed granular media filters. These are made up of layers of graded sands, gravel and anthracite. Suspended solids are removed from the feedwater as it flows downwards through the media Filter media have erratic performance because of the way they remove particles, which includes straining, sedimentation, interception, adhesion and adsorption, generally created by a filter aid or flocculant dosed into the feedwater to the filter. Filter media can achieve removal down to particle sizes of 10 – 20 micron or, with the addition of upstream coagulant dosing, removal down to 1 micron.
A microfilter removes particles as the feedwater flows through the microfilter membrane. Microfilters remove particles down to 0.1 micron in size – 10 to100 times finer than media filters. Microfiltration is a purely physical process in which particles are captured on the surface on the membrane. Any particle larger than the pore size of the membrane cannot squeeze through.
Conventional pretreatment has limitations in its effectiveness as pretreatment to RO, related to the variable quality of the treated water produced. Microfiltration is ideal as RO pretreatment because it produces filtrate of a consistent quality irrespective of variations in the feedwater. The Silt Density Index (SDI), an empirical parameter, is used by RO membrane manufacturers as an indicator of the propensity of a feedwater to foul membranes. For most manufacturers, an SDI of 5 is the maximum recommended. The conventional pretreatment system often achieves an SDI of approximately 5, while MF pretreatment typically achieves an SDI less than 3.
In the past, cellulose acetate (CA) RO membranes were used in water reuse applications, because of the ability of CA membrane to withstand high chlorine dosages (as biocide) that reduced its fouling tendency compared to polyamide composite membranes. The disadvantage to using CA membranes is that they require significantly higher pressures to achieve the same production rate as polyamide composite membranes. Polyamide membranes, although able to operate at lower pressures and produce higher quality product water, were subject to rapid fouling and possible irreversible bio-fouling due to their incompatibility with oxidants. The adoption of MF as the pretreatment for RO systems reduced this fouling potential, enabled the reliable use of polyamide composite membranes on wastewater and allowed for a 20% increase in flux, resulting in a significant reduction in capital and operating costs.
USFilter case study: Eraring Power Station
Membranes for Reuse: Industrial Applications using Microfiltration as RO Pretreatment
RO Membrane Fouling is the #1 Problem in most Reuse Applications