More stringent drinking water regulations have led to the need for many utilities to upgrade their treatment systems. A number of water treatment facilities throughout the world are considering the application of low-pressure membrane filtration to comply with new regulations.

Types of Low Pressure Membranes
Microfiltration (MF) and ultrafiltration (UF) membranes are low-pressure processes typically used for the removal of particulate and microbial contaminants in drinking water.

The majority of MF/UF membranes used in water treatment are hollow fibers manufactured from polymers and plastics.

Typical MF/UF Installations
MF/UF are typically used to treat surface water with slight to moderate turbidity and groundwater under the direct influence (GWUDI) of surface water. MF/UF membranes are usually installed as stand-alone treatment, a retrofit or replacement of conventional water treatment plants.

The following figure displays a process flow schematic for a typical pressurized membrane system. The major components of an MF or UF system usually include cartridge filters or strainers for debris removal, low pressure feed pumps, a compressed air system, membrane modules, high-pressure backwash pumps, a chemical cleaning system, a chlorine feed system, and a backwash waste handling and disposal system.

Size of Existing Membrane Market
In 2001, 110 full-scale MF/UF membrane installations were on-line (EPA, 2001). Approximately 75% of currently operating low-pressure membrane systems utilize MF technology. The average capacity for all installations is 1.71 mgd, but more than half of the MF/UF plants have a treatment capacity of less than 0.5 mgd. Currently, the largest operating facility is a 24 mgd UF plant, however, many membrane vendors have installation under contract for over 100 mgd.

The first low-pressure membrane system was installed in 1987 with a capacity of 0.06 mgd. From 1987 to 1996, 28 plants were installed, representing 25% of the 2001 market. From 1996 to 2001, an additional 83 facilities were placed on-line. Not only has the number of membrane facilities grown at an exponential rate in recent years, but facility size has increased as well. From 1996 to 2001, cumulative treatment capacity rose from 7 mgd for 28 facilities to almost 190 mgd for 110 facilities.

Membrane Costs
In recent years, low-pressure membrane filtration has become cost competitive compared to conventional treatment processes. Recent trends indicate that capital and operation and maintenance (O&M) costs per thousand gallons of treated water for low-pressure membranes are highest for small installations, and decrease linearly with increasing plant capacity,

Feed water quality and source (surface water vs. groundwater) typically does not impact low-pressure membrane equipment costs. However, poor water quality (turbidity greater than 50 NTU) will increase the cost of membrane systems due to greater pretreatment requirements. Membrane replacement contributes the majority of O&M costs for membrane systems. Replacement costs are a function of membrane life and membrane unit costs. Over a 20-year project life cycle, equipment costs will constitute the main cost for membrane systems. Excluding the membranes, treatment equipment usually accounts for approximately one-half of the life cycle cost. Initial membrane purchase and membrane replacement combined typically account for about one-third of life cycle cost. Energy and chemical costs usually account for around 10% of membrane life cycle costs.