Industrial Water Reuse Makes Cents

Stricter standards and the increasing demand for water is raising water treatment costs and making wastewater recycling more attractive to industry

• By Anthony M. Wachinski
• Oct 01, 2004

Until just recently, water was viewed as a low-cost commodity. This perception has changed as communities across the United States face water supply limitations and plant managers look for ways to cut their process water treatment cost. Awareness of the concept that water is not a low-cost commodity, but in fact a major cost for industry in order to do business has been heightened by:

• Widespread drought -- drought conditions that have afflicted certain geographical regions in the last few years have made water reuse and recycling critical.
• Increasingly strict standards for withdrawal and discharge of water.
• Increasing demands of urban populations.

Water shortages in the United States and stringent environmental regulations have created a compelling need for industry to recycle or reuse wastewaters. Recent drought conditions in some geographical areas have made industrial water reuse and recycling crucial since water supplies are unlikely to increase in the future. Each day U.S. industry uses 25 billion gallons of source water for process water, boiler make-up water, condensate, and potable water.

These same industries generate an estimated 20 billion gallons of wastewater, which must be treated before discharge to the receiving bodies of water. Every day approximately 2,000 thermoelectric plants in the United States and Canada withdraw 186 billion gallons of water each day, with most of this water being used only once for cooling.

Analytical methodologies developed to measure impurities have evolved to a point where contaminants in the part-per-trillion concentration are detected routinely and reveal previously undetected contaminants in drinking water supplies. Federal rules and regulations, which will probably be promulgated in the next three to five years, are expected to require the drinking water industry to treat source waters (Table 1 and Table 2) to zero levels of microorganisms and to reduce the use of chemicals.

Options in Membrane Technologies
To meet these projected stricter regulations, municipalities are upgrading their facilities with low-pressure membrane technologies, such as microfiltration (MF) and ultrafiltration (UF). Hollow-fiber microfiltration is extremely effective for treatment of all source waters (as shown in Table 1).

Microfiltration replaces existing conventional gravity sedimentation and multimedia filtration systems and minimizes chemical usage, operation and maintenance, sludge dewatering, and hauling and disposal costs, as well as providing high-quality, non-variable drinking water 24 hours each day, seven days each week, and 365 days each year.

An example of a microfiltration system is Pall's hollow-fiber MF system, which processes feedwaters to remove bacteria, protozoan cysts, viruses, iron, manganese, and other solid particulate from surface water, groundwater, secondary effluent, industrial wastewater, and on occasion municipal drinking water (note: potable water cannot be used for many industrial process water purposes). With this system, the microfiltration permeate is high quality water, less than 0.05 nephelometric turbidity units (ntu) and free of microorganisms and particulate.

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

Integrating MF systmes with other types of technologies can be problematic; however, innovative technologies are overcoming these challenges. For example, Pall's fluorocarbon membrane material, polyvinylidene fluoride, provides comprehensive oxidant capability from chlorine to ozone. In addition, it allows integration of MF systems with emerging advanced oxidation technologies, aerobic and anaerobic biological systems, nanofiltration, and reverse-osmosis systems coupled with electrodeionization.

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

Wastewater includes all spent and used -- often once -- industrial process water. Industrial wastewater can be discharged to a sanitary sewer, treated and discharged to a body of water such as a river, or reused. The limited capacity of publicly owned treatment works (POTWs) to handle increased hydraulic and organic loading, and still meet the U.S. Environmental Protection Agency's (EPA) guidelines on wastewater discharge limits, is forcing the POTWs to demand industries to develop wastewater recycling, recovery, and reuse options to reduce hydraulic and organic loading to their plants. Furthermore, the state of California is leading the nation in requiring industries to reduce the concentration of total dissolved solids (TDS) in their effluent. Source water and in-process treatment technology selection is affected. Reverse-osmosis systems coupled with microfiltration pretreatment allow industry in California to meet these strict TDS requirements.

The Growing Cost of Water and Wastewater Treatment
In their article "Making Water Work Harder" that appeared in Environmental Protection's November 2001 issue, Tom Wingfield and Jim Schaefer reported that industries now spend tens of thousands of dollars per year on water-treatment chemicals. This is in addition to the cost of buying municipal water.

The average cost of water in the United States is about $2.06 per 1,000 gallons, but the costs in areas west of the Mississippi River are above that average -- e.g., the cost in Los Angeles is 12.3 percent higher at $2.31 per 1,000 gallons and in Fort Smith, Arizona, it is $2.83 per 1,000 gallons. And, since municipal drinking water quality often does not meet industrial process water quality requirements, it requires further treatment at additional cost. Industry must then spend an average of up to eight times the cost of treating municipal drinking water to treat and dispose of used process water and residuals.

Industrial process source water is currently treated using conventional coagulation sedimentation and filtration processes for turbidity removal, and for precipitative softening, i.e., removal of calcium and magnesium hardness. These same processes are used as pretreatment for reverse-osmosis (RO) membrane systems. Conventional gravity-based technologies are susceptible to upsets when influent water quality changes. These upsets negatively affect all downstream treatment processes.

Many conventional plants have exceeded their design life and require extensive refurbishment or replacement. In addition, these plants require high levels of maintenance employee-hours and resources.

Industries that discharge wastewater streams to a sewer often must pay surcharges on the volume and strength of the wastewater. Volume or flow, suspended solids, dissolved solids, and biochemical oxygen demand or chemical oxygen demand are the wastewater characteristics most often monitored. California is the first state to limit industry's discharge of total dissolved solids. This issue will have widespread consequences for industry using ion exchange to treat source and wastewaters due to the high TDS of the regenerants.

Industries that discharge wastewater to a body of water must satisfy the requirements of their National Pollution Discharge Elimination System (NPDES) permit. Any violation of this permit not only carries heavy fines, but also provides a non-green image to the local community.

Industries that reuse all or a portion of their wastewater lower their cost of water. In many cases, they eliminate the requirement for a NPDES permit and the liability associated with not meeting effluent discharge requirements, reduce or eliminate discharge to sanitary sewers and associated sewer charges and surcharges, and reduce the volume of municipal water purchased, which is source water that must be pumped and treated.

Conventional wastewater technology uses inorganic and organic coagulants to enhance the solid/liquid separation of solids, microbes, and particulates in water. A byproduct of conventional wastewater treatment is a residue stream, which is the solid/liquid slurry referred to as sludge. Sludges range in concentration from 0.5 percent to 3 percent in most plants. Before disposal, these solids must be dewatered to reduce hauling and disposal costs. Sludge disposal of residues is accomplished on site by using the industrial facility's real estate for non-revenue-producing activities, or it can be hauled away at the expense of the industrial facility to commercial or private landfills for final disposal. Sludge disposal is not included in the United States' $8.00/1,000 gallon for wastewater treatment cost.

Improving the Quality of Source Water
One approach to effective industrial water reuse is to improve treated source-water quality. When industry treats its source water with membranes, the variability currently experienced is removed and all downstream treatment processes can be operated at less cost. All processes are operated with design parameters, which results in less chemical use and reduced maintenance. Blowdown frequencies of cooling towers, such as biocide requirements, are reduced when source waters are effectively treated.

Groundwater is, or is perceived to be, higher-quality water than raw water taken from surface water bodies, less vulnerable to contamination, and requires less treatment, thereby making it a preferred source for drinking water. But the composition of an aquifer's geological formation can have a significant effect on groundwater quality. For example, groundwater from limestone aquifers is usually rich in calcium carbonate, while groundwater formed in dolomitic rock contains a high fraction of magnesium. Since most contaminants in groundwater are in dissolved form, they can be removed by oxidation or by adding coagulants, followed by using the appropriate treatment technology.

Most soft drink facilities use either groundwater or municipally treated water as a source for their product and production processes. Municipal-supplied water can contain treatment chemicals, particles, and microorganisms, as well as salts and metals that may affect the quality of the product or the industrial processes involved in its manufacture. Groundwater taken from a well generally contains a lesser amount of particles and microorganisms than raw water taken from lakes and rivers, but it may suffer from the influx of metals and salts drawn from the geology of the aquifer or the accompanying strata. In either case, the geological setting and geographical characteristics of a plant's location will determine the type of water that is fed to it.

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. The two-stage membrane process provides high-quality water suitable for process uses (such as high-pressure boilers), conserves water for domestic uses, and significantly lowers costs -- both for purchasing water and additional water treatment.

Seawater is characterized by high salt content compared to brackish or other natural waters. Average ocean water has a salinity of 35 i.e., 1,000 grams (g) of average seawater contains 965 g of water and 35 g of salts. Seawater with a salinity of 35 is considered as "standard seawater" as the composition is nearly the same all over the world, consisting of about 55 percent chloride, 31 percent sodium, 8 percent sulfate, 4 percent magnesium, 1 percent calcium, and 1 percent potassium. Typical seawater pH is 8.1 to 8.3.

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

Reference
Wingfield, T. and Schaefer, J., "Making Water Work Harder," Environmental Protection, November 2001 (available at no charge on this Web site under Archives).

• Surface Waters -- rivers, lakes, reservoirs
• Groundwater
• Seawater
• Secondary Effluent -- municipal/industrial wastewater treated biologically
• Treated/Untreated Industrial Wastewater -- e.g., cooling-tower blowdown
• Municipal Drinking Water -- potable water quality purchased from city

This article originally appeared in the 10/01/2004 issue of Environmental Protection.