A new frontier
WHAT ARE MEMBRANES AND WHAT DO THEY DO?
A simple definition of a membrane is a thin, semi-permeable barrier that selectively allows some types of materials to pass through while blocking the passage of others. A classic example from nature is a cell wall. The membranes used in water and wastewater treatment are man-made, generally of plastic or ceramic. For modern wastewater and water treatment systems, membranes are formed into hollow fibers, which are slightly thinner than spaghetti, or into flat sheets. The membranes are arranged in parallel rows and mounted in pressure vessels or open tanks, depending on the manufacturer or the project requirements.
WHAT ARE THE APPLICATIONS FOR MEMBRANES?
The main use of membranes in wastewater treatment, the membrane bioreactor (MBR), is relatively new, with large-scale municipal installations dating back only about 15 years. MBRs employ microfiltration or ultrafiltration (MF/UF) membranes that have pores so small — about 0.2 micron for MF and a tenth of that for UF — that they reject the passage of protozoa, cysts, bacteria and most of the particles that constitute the turbidity, or haziness, in water. An MBR consists of an MF/UF system integrated with a biological activated sludge treatment process. The biological process breaks down the pollutants in the waste, and then the membranes filter out microscopic debris, producing very clear, low-turbidity water. Concurrently, the membranes produce a concentrated biological solution that is returned to the activated sludge basin for further treatment. An MBR occupies a smaller footprint than a conventional activated sludge plant because it operates with a higher mixed-liquor concentration and eliminates the need for secondary clarification and tertiary filtration.
MBRs’ high-quality product water and compact footprint have captured the interest of municipal wastewater treatment plants that need cleaner effluent and/or need to expand capacity at a land-locked site. The membranes produce water with turbidity less than 0.1 NTU (Nephelometric Turbidity Unit, a measurement of turbidity). By augmenting MBRs with coagulant addition, utilities can further reduce concentrations of other constituents, such as total organic carbon (TOC), total nitrogen and total phosphorus. Increasingly, utilities that have MBRs are selling their higher-quality effluent to industrial users.
A very high quality, almost distilled water, is produced by further treating MBR filtrate with reverse osmosis (RO), removing almost all of the dissolved organic and ionic substances, such as sodium, chloride, hardness and nitrate. Advanced oxidation, such as ultraviolet (UV) light and hydrogen peroxide (H2O2), can be applied to further remove trace organic materials. Combined in sequence, the processes yield reclaimed water that meets a host of drinking water standards. With high-quality reclaimed water, water utilities can offer industrial customers a valuable drought-proof source of supply, or the utilities can store reclaimed water in an aquifer.
- Cloudcroft, N.M. – It is no secret where this small town’s water comes from
- Orange County, Calif. – With reduced supply, county reuses and recycles
- Oxnard, Calif. – Cooperation turns the tide of city’s water supply
Scott Freeman is the membrane technology leader, and Scott Levesque is a wastewater process specialist for Kansas City, Mo.-based Black & Veatch .