(From Water Technology, Volume 18, Number 7, July 1995)

LAUNDRY USES REDOX ALLOY TO TAME CHLORINE

Linens saved from the effects of municipal chlorination.

By John C. Dalton

Water quality may be more important to commercial laundries than to almost any other commercial customer you can find, but to avoid being washed up by the challenge of treating the volumes of water these prospects require, equipment selection and system design are critical.

Take, for example, the case of one major East Coast commercial laundry that was faced with chlorine levels in incoming city water that ranged between 3 and 3.5 parts per million (ppm). Iron content varied between 1 and 5 ppm, manganese was 0.3 to 0.5 ppm, and hardness was 5 to 7 grains. The result was that clothes weren't coming out clean, and customers were upset.

To help control the way excessive chlorine was ravaging washed linens, the laundry had been chemically treating incoming water to reduce chlorine concentration to 2 ppm or less. This was expensive and wasn't always effective because of drastic changes in the incoming chlorine concentration. Even after chemical treatment, excessive chlorine degraded downstream water softening resins, rendering them ineffective at reducing hardness.

CHALLENGES DEFINED:

Diamond Water Systems, Holyoke, MA, initiated a pilot study to confirm the efficiencies of several different treatment technologies. The pilot system flow rate was 10 gallons per minute (gpm) which was discharged into a 500 gallon holding tank. Water from the holding tank was pumped to one of the laundry washing machines through a 5-micron automatic backwashable filter that removed suspended solids.

Additional filtration was installed to remove dissolved iron and manganese, an ion exchange water softener controlled hardness, and an activated carbon filter was installed to control chlorine levels.

The pilot plant demonstrated two important considerations that influenced the choice of treatment technology and system design: flow rate and filter replacement costs.

Flow rate was indeed a challenge. The laundry operated 14 to 18 hours per day during the week, and 8 to 16 hours per day on weekends. Water demand could range from 150 to 500 gpm. The wide variation required the treatment system to be vastly oversized to meet peak demand without jeopardizing water quality.

The water flow problem was solved by installing a 50,000-gallon holding tank for treated water. The treatment system could then be sized for a flow of 180 to 200 gpm and the reservoir of treated water could handle peak demand during periods of heavy usage.

The other problem revealed by the pilot study was the replacement and disposal costs for the activated carbon filters used for dechlorination. Although activated carbon is an effective dechlorination medium, it rapidly reached its saturation point on this job, even at a flow of 180 gpm. Because the laundry didn't wish to continue chemical treatment for chlorine control, redox alloy media(RAM) was chosen as an alternative.

RAMMING CHLORINE:

RAM is a patented, high-purity, copper-zinc alloy that uses oxidation/reduction (redox) chemistry to remove contaminants. The redox reaction essentially exchanges electrons to convert contaminants into innocuous components. In the case of chlorine, the zinc loses two electrons (oxidation) and the chlorine gains two electrons (reduction), converting chlorine to chloride ions.

What makes RAM so attractive for chlorine removal is that it has a longer bed life than granular activated carbon (GAC). While the adsorbed contaminants in a GAC filter eventually obstruct (occlude) catalytic sites and exhaust the filter's capacity, the kinetic action of RAM remains constant for approximately 15 years. It's then replaced, and the high-purity metal content is reclaimed.

Although the initial cost of the chlorine removal system using RAM was higher than that for a system based on activated carbon, the reduced long-term capital and operating costs of the RAM system represented a major saving for the laundry.

SPACE CONSIDERATIONS:

RAM systems also occupy far less space than those using activated carbon filtration. The dechlorination system cited previously uses two 36-inch diameter vessels containing 20 cubic feet of RAM. The vessels are shorter than conventional vessels, standing only 72 inches high. An automatic control package monitors chlorine input and throughput levels.

A further consideration in sizing the water treatment system was the 50,000-gallon outdoor holding tank. Water from the tank is brought back inside the building for use in laundry processes and for domestic purposes such as supplying employee drinking water and restroom water.

Concern about the growth of bacteria in stored water was addressed by purposefully undersizing the dechlorination system to allow for chlorine levels of from 1 to 1.5 ppm to remain in the storage tank for disinfection. RAM also inhibits bacterial growth.

The complete water treatment plant has been on-line continuously since it was installed a year ago. In addition to the RAM dechlorination system, the plant includes a proprietary quartz filtration system that removes suspended solids down to a nominal 5 microns, a dissolved iron and manganese removal system, and an ion exchange water softening system. All systems include fully automatic control packages.

John C. Dalton is an engineering consultant with Diamond Water Systems, Inc, Holyoke, MA. His areas of expertise include low-micron suspended solids filtration for process, potable, cooling and wastewater applications.