Making Water Work Harder
Water needs to work harder. On the surface, it seems a rather odd statement to make, but when you look at factors and trends in water use worldwide, it seems intuitive. The human population of earth continues to grow rapidly. The supply of fresh water is finite. Communities and industry vie for this limited resource. Yesterday's large, central treatment facilities are reaching their design capacities and have nowhere to expand.
Regulations governing water treatment tighten yearly. At the same time, new generations of more sensitive water monitoring and measurement devices reveal that previously unseen contaminants persist in drinking water supply systems. Conventional gravity-dependent technologies, such as clarifiers and loose media filters, do not reliably remove many of the target pathogens, which can affect public health. The performance of the older technologies is also heavily dependent upon the appropriate application of water treatment chemicals.
Industries dependent upon water spend tens of thousands of dollars per year in water chemical treatments. It is easily one of the highest costs within many treatment systems. Although water treatment chemicals have helped to clean up water supplies, they have also complicated its production. Some sectors, such as paper plants, maintain on site "live-in" chemical supplier personnel to solve complex water chemistry problems. Today, physical separation of water contaminants is rapidly becoming the preferred solution over one more chemical addition. In the public arena, awareness of the potential physical and neurological side effects of water treatment chemicals also grows more acute. Both community and industry have their own motives for finding ways to cut chemical use. Cash-strapped communities and large industrial water users are looking for a cost efficient means to provide rational growth planning and regain control over their water quality.
Although water treatment chemicals have helped to clean up water supplies, they have also complicated its production. |
Water reuse represents a practical and reliable means of extending water supplies in areas with water shortages. Typically, highly treated wastewater has been reused by industry for high quality process water, which could justify the higher cost of treatment for reuse. In the past, multiple processes were employed using conventional technology. As "per square foot" membrane capital costs and operating costs have decreased, their use has spread across market segments. Ten years ago, membranes were largely used for the biotech industry. Today, membrane technology has found wide acceptance by both municipalities and industry and is seeing steady growth in the range of five to 10 percent annually.
New modular membrane treatment units are reducing costs to make water reuse practical for irrigation of parks and landscaping, and to protect watersheds that supply drinking water from treated wastewater discharges. A line of easy-to-operate, modular microfiltration systems are being successfully applied to a wide range of reuse applications. Although pleated, backwashable microfilters are a growing competitor to hollow fiber systems, this article includes only examples with the hollow fiber technology platform.
The Bedford Hills Correctional Facility and Toppan Electronics use small flow microfiltration units to meet tough water quality standards at low cost. The large installations for Fountain Hills Sanitary District and Sonoma County Water Agency represent new applications for water reuse, designed to meet tough standards, provide flexibility and meet increasing water demands.
Water Reuse and Wine in Sonoma County
Sonoma County, in the heart of California's wine country, is reusing water to assure adequate supplies for drinking water, commercial and agricultural uses. The 3 million gallon per day (mgd) water reuse facility treats secondary effluent from a partially aerated lagoon treatment plant near the county airport in Santa Rosa. The reused water will irrigate fields near the airport and some of the many nearby vineyards. In the future, reused water may also replenish water in local geysers, which have recently been less active because of low water levels. Microfiltration was chosen because it provides high quality filtered water with low suspended solids, which is required for vineyard irrigation. A detailed evaluation of capital and operating and maintenance (O&M) costs over a 20-year period showed that microfiltration was the most cost-effective alternative.
Aquifer Storage and Recovery in Arizona
The Fountain Hills Sanitary District's 2 mgd microfiltration installation is part of a system for conjunctive use of ground water. During the fall, winter and spring, the microfilter treats an average of 1.7 mgd of tertiary wastewater effluent, which is used to recharge the District's three aquifer storage and recovery (ASR) wells. During the summer, tertiary effluent is used directly for golf course irrigation, and water is pumped from the ASR wells to meet peak water demands. This ASR system is one of only three similar installations in the United States.
Awareness of the potential physical and neurological side effects of water treatment chemicals also grows more acute. |
This microfiltration system was chosen because of its excellent turbidity removals, its ability to accept high chlorine residuals and the ability of the existing system to accept ultrafilter modules, if virus must be disinfected without using chlorine. Feed turbidity ranges between 0.5 and 1.5 nephelometric turbidity unit (ntu), while the filtered turbidity is consistently 0.03 to 0.04 ntu. The extremely low turbidity improves ASR well longevity by minimizing the amount of solids pumped into the aquifer by the recharged water. A four train, 2 mgd system was designed such that expansion to 3 mgd involved only the addition of a fifth module rack and modules.
Watershed and Marine Protection in New York
For many years, New York City has conducted a program to evaluate a number of different treatment technologies to reduce contamination of its drinking water reservoirs by communities and industries in the region. Sub-micron microfiltration of secondary effluent was determined to provide the level of water quality necessary for watershed protection. The first of many of these plants is being installed at the Bedford Hills Correctional Facility in upstate New York.
All along the East Coast from Maine to Chesapeake Bay, contaminants in wastewater effluents, especially bacteria and nutrients such as nitrogen and phosphorous, damage marine fishing and shellfish industries. A microfiltration treatment system, which provides controlled separation of contaminants, supplies extra measures of protection for people and the environment. Not only does this treatment serve to protect the public health, ecosystems and habitat, it is also preserving jobs and a way of life. In the heavily populated eastern seaboard, this high quality effluent is a valuable resource for industrial make-up water and irrigational purposes.
Drought-Proofing a Macroelectronics Industry in San Diego
Secondary wastewater effluent is a readily available source of water in water-short southern California. In San Diego, Calif., treated water replaces potable water purchased from the San Diego water system for Toppan Electronics Inc. An advanced microfiltration system treats 200,000 gallon per day (gpd) of secondary wastewater effluent for reuse as process water. The new microfiltration equipment is a small flow system providing consistently high removals of solids, pathogenic cysts and bacteria from the secondary effluent and producing excellent feed water to Toppan's existing reverse osmosis (RO) system.
New modular membrane treatment units reducing costs to make water reuse practical for irrigation of parks and landscaping. |
A growing body of data at this and other water reuse facilities demonstrates that microfiltration significantly improves RO system operation. Allowable fluxes increase by 20 percent, operating costs are reduced by 40 percent and the time interval between chemical cleanings for the RO system is extended by four times compared to the conventional means of lime settling and filtration of secondary effluent.
Water Reuse and Economic Development in Chandler, Arizona
The City of Chandler, Arizona, has managed to attract several large semiconductor manufacturers to the Phoenix area by providing the infrastructure necessary for their growth. A large part of that is the Chandler Industrial Process Water Treatment Facility (IPWTF), where approximately 80 percent of the high quality reclaimed water processed is used to recharge the local aquifer. Another portion of that water is used for landscape irrigation and for make-up water to cooling towers.
The manufacturer achieves a net water use of 1 mgd when contrasted with a residential development occupying the same land area that would use approximately 2 mgd. An added benefit is that this conservation reduces the net load on city services.
The IPWTF employs membrane filtration to achieve its goals. After initial pH adjustment of the raw wastewater from the manufacturing facility, the water flows through a microfilter, then to a cartridge filter and finally to a spiral wound RO unit.
Prior to the introduction of microfiltration into the process train, replacement cartridge filters, replacement RO elements and cleaning chemicals cost almost $400,000 per year. A pre-RO microfiltration step resulted in a 40 percent improvement in operating effectiveness due to drastically reduced frequencies of change-outs, extended intervals between RO cleanings and longer RO element life.
Oil and Water Pay Dividends at European Ethylene Plant
Dispersed oil or hydrocarbons can be separated from water using liquid/liquid coalescers. Coalescers are specialized membrane filters that intercept small droplets of the dispersed liquid on the filter medium. Oil and water flow along the surface modified medium allowing smaller droplets to aggregate with other droplets to form progressively larger drops. A European ethylene plant installed a liquid/liquid coalescer in late 1997 to remove hydrocarbon contaminants from quench water, which, if left untreated, can cause fouling of the steam generators. In reducing hydrocarbon concentrations to the five parts per million (ppm) range, the coalescer saved $300,000 per year in the operation of the steam system, as well as lowering maintenance requirements on heat exchangers and other equipment. The hydrocarbons were recovered and blended back into gasoline products and water treatment costs were significantly reduced.
A growing body of data demonstrates that microfiltration significantly improves reverse osmosis system operation. |
A Day's Work
Water is a valuable resource; its use is subject to competition between industrial and municipal consumers. Wastewater is a form of water that can be transformed by membrane filtration into a reusable resource. Membranes are "clean" separation technologies that provide industrial and municipal end users with a greater degree of control over their water quality. They are gaining wide acceptance for a range of applications including drinking water, process water and water from wastewater. Membrane separation provides recovery of high value products and transforms wastewater into a renewable water resource. Excellent performance, reliability and competitive economics make membrane systems a good solution for integrated water management applications.
Water reuse is no longer a practice relegated to arid and semi-arid regions of the country. Today, advanced treatment and reuse of wastewater can be found wherever competition for water use is high, such as the mid Atlantic and northeastern states.
This article appeared in the November 2001 issue of Environmental Protection, Vol. 12, No. 11, on page 30.
This article originally appeared in the 11/01/2001 issue of Environmental Protection.