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Pure water and ultrapure water are essential to industry.

Each day industry in the US uses 25 billion gallons of source water for process water, boiler make-up water, condensate and potable water; these firms also generate an estimated 20 billion gallons of wastewater, which must be treated before discharge. Thermo-electric plants in the US and Canada withdraw 186 billion gallons of water, most of it used only once for cooling.

At the same time, drought, water shortages, increasing demand for water by urban populations and stricter standards for withdrawal and discharge have made water a major cost for industry.

All of these factors have created a compelling need for industry to recycle or reuse wastewaters.

Looking for new approaches
Analytical methodologies to measure impurities have evolved to a point where contaminants in the parts-per-trillion concentration are detected routinely and reveal previously undetected contaminants in drinking water supplies.

In the next three to five years, federal regulators are expected to require the drinking water industry to treat source waters to greater than 5-log removal of targeted microorganisms (e.g., protozoan cysts) and reduce levels of chemical contaminants. As a result, water industry designers, consultants, and managers are looking to new approaches such as membrane processes to meet these increasingly stringent requirements.

Membrane separation technologies, microfiltration (MF), ultrafiltration, nanofiltration and reverse osmosis have characteristics that make them attractive candidates to meet future regulatory requirements and for recycle and recovery processes (sidebar, page 50).

Hollow-fiber microfiltration
Municipalities are upgrading their facilities with low-pressure membrane technologies such as microfiltration and ultrafiltration.

Hollow-fiber microfiltration (MF) is very effective for treatment of all source waters. Microfiltration replaces existing conventional gravity sedimentation and multimedia filtration systems and minimizes chemical usage; operation and maintenance; and sludge dewatering, hauling and disposal costs. It does this while providing high-quality, non-variable drinking water.

Hollow-fiber MF systems, for example, process feedwaters to remove bacteria, protozoan cysts, viruses, iron, manganese and other solid particulates from surface water, ground water, secondary effluent, industrial wastewater, and, on occasion, municipal drinking water (note that potable water cannot be used for many industrial processes).

Microfiltration permeate is high-quality water, less than 0.05 ntu (nephelometric turbidity units), and free of microorganisms and particulate.

Integrated systems
Low-pressure MF systems are also used as effective pretreatment for spiral-wound nanofiltration and reverse osmosis systems, producing consistent water quality with a Silt Density Index of less than 2, usually 1 to 1.5.

Membrane materials such as polyvinylidene fluoride provide comprehensive oxidant capability, and allow integration of MF systems with emerging advanced oxidation technologies, aerobic and anaerobic biological systems, and nanofiltration and reverse osmosis systems coupled with electrodeionization.

These integrated systems effectively treat most industrial source waters to required levels of water quality. In drinking water applications, MF integrated with oxidation systems provides an effective treatment process to economically meet or exceed the requirements of current and future federal and state regulations.

Demands on industry
Public wastewater treatment facilities have a limited capacity to handle increased hydraulic and organic loading and still meet the US Environmental Protection Agency’s (EPA) guidelines on discharge limits. This is forcing them to demand that industry reduce hydraulic and organic loading to their plants.

California is leading the nation in requiring industries to reduce the concentration of total dissolved solids (TDS) in their effluent. Reverse osmosis systems coupled with microfiltration pretreatment allows industry in California to meet these strict TDS requirements.

In addition to the cost of buying municipal water, industry spends a great deal on water treatment chemicals.

Since municipal drinking water quality often does not meet industrial process water quality requirements, companies must then spend on the average up to eight times that cost to treat and dispose of used process water and residuals.

Conventional processes
Industrial process source water is currently treated using conventional coagulation sedimentation and filtration processes for turbidity removal (figure 1) and for precipitative softening (figure 2). These same processes are used as pretreatment for reverse osmosis (RO) membrane systems (figure 3).

Figure 1: Conventional Surface Treatment

Figure 2: Conventional Precipitative Softening

Figure 3: Conventional Pretreatment for Reverse Osmosis

Conventional gravity-based technologies are susceptible to upsets when influent water quality changes, negatively affecting all downstream treatment processes.

Industries that discharge their wastewaters to a sewer must often pay surcharges on the volume and strength of the wastewater. Those that discharge to a body of water must satisfy the requirements of their National Pollution Discharge Elimination System (NPDES) permit.

Reuse lowers industry costs
Industries that reuse all or a portion of their wastewaters lower their cost of water. In many cases they eliminate the requirement for an NPDES permit and the liability associated with not meeting effluent discharge requirements.

This approach also can reduce or eliminate discharge to sanitary sewers and associated sewer charges and surcharges, and reduce the volume of source water which must be purchased, pumped and treated.

Another cost associated with conventional wastewater technology is its use of inorganic and organic coagulants to enhance the solid-liquid separation of solids, microbes, and particulates in water. A byproduct of conventional treatment is a solid/liquid slurry, or sludge, which must be dewatered to reduce hauling and disposal costs. Sludge disposal involves additional cost.

Source water issues
One approach to effective industrial water reuse is to improve treated source water quality. When industry treats their source water with membranes, variability is removed and the downstream treatment process can be operated at less cost. All processes are operated with design parameters resulting in less chemical use and reduced maintenance.

For example, blowdown frequencies of cooling towers and biocide requirements are reduced when source waters are effectively treated.

Groundwater has, or is perceived to have, higher water quality, be less vulnerable to contamination, and require less treatment. But the geological composition of aquifers can significantly effect groundwater quality. Since most contaminants in groundwater are in dissolved form, they can be removed by oxidation or by adding coagulants, followed by microfiltation.

Secondary effluent, seawater
Secondary effluent can be treated to create high-quality water for industrial uses, such as boiler feed water and cooling tower water. Typical treatment of secondary effluent includes microfiltration followed by RO. Quality filtration systems transform secondary effluent into a high-quality resource and can reduce costs.

Similarly, the salt content of seawater can be reduced to less than 500mg/L by processing the seawater through reverse osmosis membranes. For every 100 gallons of seawater, 30 to 50 gallons of desalted water is produced. The remaining water is waste which is concentrated salts that may be discharged directly into the ocean, combined with other discharges (e.g. power plant cooling water or sewage treatment plant effluent) before ocean discharge, discharged into a sewer for sewage plant treatment, or dried out and disposed of in a landfill.