Water costs get you coming and going. A facility pays for using city water and then for discharging it to the city sewer. Combined fees for water and sewer use have risen between 100 and 400 percent around the country from 1986 to 1996, to between $4 and $9 per thousand gallons.¹ As water is regarded as a precious and ever more costly resource, companies are taking a closer look at reusing wastewater. One such process is ion exchange water treatment.

Ion exchange is a common unit application used for recycling rinse waters from cleaning operations. Technically defined, ion exchange is a process whereby "ions held by electrostatic forces to charged functional groups on the surface of a solid are exchanged for ions of similar charge in a solution in which the solid is immersed."²

The solid referred to is the ion exchange resin. Resins can be naturally occurring or synthetic polymeric materials containing charged functional groups that electrostatically attract ions of opposite charge. For example, a cation exchange resin has a negatively charged functional group (e.g., sulfonic acid) to retain cations while anion resin has a positively charged functional group (e.g., quaternary amine) to retain anions.

A wastewater recovery ion exchange system at its most basic level consists of one or more resins that remove cations and anions from wastewater. In the loading cycle, cations such as sodium (Na+) are exchanged for hydrogen ions (H+) and anions such as chloride (Cl-) are exchanged for hydroxide ions (OH-). The hydrogen and hydroxide ions then combine to form water. When the resin is saturated, it is regenerated and returned for another loading cycle.

Regeneration of ion exchange resins involves use of an acid and an alkali and can be performed either on or off-site.

All ion exchange systems are configured with some common components as shown in Figure 1. Pre-filters (using cartridge filters) are used to remove suspended solids from the wastewater, typically to 5 microns or less. Activated carbon is used to remove absorbable organic compounds and oxidizing substances (such as free chlorine, which can damage the ion exchange resin) and also to provide water clarification. The carbon and ion exchange resins are contained in vessels (canisters) usually constructed of fiberglass reinforced plastic (FRP), steel (lined with thermoplastic), or stainless steel and sized based on the flow rate through them.

In wastewater applications, the design flow rate through the system is usually 2.0 to 2.5 gallons per minute per cubic foot of resin (gpm/ft³). Post-filters are used following deionization. The quality of the recovered water is usually monitored in-line with a resistivity indicator, either a "quality light" or an analog or digital meter.

In recovering rinse water from precision cleaning operations, the key factors affecting ion exchange performance include the following:

Cleaning solution chemistry can be as benign as hot water or can be a mixture of water and cleaning chemicals. Aqueous cleaning chemicals have the advantage of allowing a number of formulations to fit various applications. Cleaning chemicals may contain alkaline materials (sodium hydroxide, sodium metasilicate), complexing agents (citrates, gluconates, glucoheptanates, EDTA), surface-active agents (surfactants), and other organic compounds (ethanolamines, glycol ethers).

The levels and types of inorganic cleaning compounds directly affect the use of resin in a recovery system. Complexing agents (designed to retain metals in solution) can render any treatment system ineffective for removing metals. Surfactants and other organic compounds may be removed by the activated carbon prior to ion exchange. However, many of these types of materials are not amenable to removal by carbon and/or ion exchange and must be controlled by bleeding a portion of rinse water from the system on a continuous or intermittent basis.

Minimizing drag-out volume is important to maximize resin usage and minimize treatment costs. As contaminants are introduced to the rinse water, the ion exchange resin is depleted. Drag-out can be minimized many ways, including:

o Efficient racking of parts

o Maximizing the drain time of the rack or basket upon withdrawal from the cleaning bath

o Using less concentrated cleaning solutions

o Using a stagnant rinse following the cleaner and prior to the flowing rinses

o Installing fog rinses over the cleaning bath

The rinse water ionic background refers to all of the ionic materials present in the rinse water, which must be removed by the ion exchange system in order to have high-purity water. Cleaning applications that use hard and/or soft water for bath make-up and rinsing use more resin in recovering higher-purity rinse water than those that use deionized water.

Temperatures of cleaning rinse waters vary from ambient to 150°F. The ion exchange resins themselves are effective in removing ions at these temperatures. In fact, the kinetics of the ion exchange mechanism are enhanced by elevated temperatures. However, it has been shown that at operating temperatures above 120°F, the functional groups on the strong base anion resin are degraded,^3,4 which results in a loss of strong base capacity. Once these functional groups are gone, the resin is no longer effective in producing high-quality water.

There are many possible configurations for ion exchange systems used in wastewater recovery. Systems are configured based on the recovered water quality desired. The most common types of systems are briefly described below.

An example of an ion exchange configuration to produce "medium" quality recovered water (Figure 2) utilizes a separate bed system for deionization. Maximum achievable water quality with this configuration ranges from 1 to 2 MW -cm. The configuration shown is commonly referred to as a primary-secondary arrangement. Two canisters on line in series allows any leakage from the first to be picked up in the second, thereby utilizing the carbon and resin to their fullest capacities.

An example of an ion exchange configuration to produce "high" quality recovered water (Figure 3) uses a mixed cation-anion bed. The mixed bed system can be used alone for deionization or in conjunction with a separate bed system as shown in Figure 4. The mixed bed system would be used in applications where the contaminant level is fairly low in the rinse water. Typical achievable water quality is a minimum of 8 MW -cm.

Ion exchange has been used for many years to deionize water before being used in a process. Performance can be predicted from a good water analysis. Wastewater reuse applications, on the other hand, range from fairly straightforward to very complex. If a company is considering recovering rinse water, they should take the following steps before purchasing and installing a system:

High-purity water has its price. The higher the quality required, the higher the cost. It is important to realistically define the water quality needed for your process. It makes no sense to continuously generate (and pay for) 15 MW -cm water when 1 MW -cm quality will be sufficient. Your "customer" sets the water quality needed, either internal (such as the next process following cleaning) or external (the end user).

Look at the cleaning chemicals in use and also at any chemicals added to the rinse tanks (such as wetting agents or other "rinse aids"). Will the chemistries of these components interfere with the ion exchange process? If so, determine if alternate chemicals (or none at all) could be used. Use the experience of your ion exchange vendor or service provider for input.

Review the operating conditions of the cleaning process such as required flow rates, temperatures, and throughput. Compare these conditions with those necessary for optimum ion exchange performance and make any necessary adjustments possible.

Determine the quality of the water used to feed the process. What type of treatment (if any) is presently being done? You need to look at the impact of using untreated make-up water on your process and the ion exchange system. It is typically a case of "pay me now or pay me more later" when comparing the costs of water pretreatment and wastewater treatment.

A properly designed ion exchange system operates relatively trouble-free. Operational activities include monitoring flows, pressures, and canister performance. One of the most neglected areas of ion exchange operation is replacement of the carbon. Breakthrough detection on carbon canisters is unreliable, so it is best to change carbon on a predetermined time schedule.

Maintenance activities include changing cartridge filters and regenerating or replacing spent resin. Disposing of regenerant and/or resin must be performed according to applicable local, state, and federal regulations. Even resin used in a recycle application may be classified as a hazardous waste; do not assume otherwise.

Aqueous processes have become the mainstay of cleaning. As the costs of water use and disposal rise, recovering rinse water from cleaning processes has become a necessity for many companies. Ion exchange, when properly applied, can recover cleaning rinse waters for reuse. The keys are properly defining process requirements and knowing how the components of the cleaning process affect ion exchange. Optimizing the process results in the right quality water at the lowest possible cost.

1. The Gallery, Water Technology, April 1997, 12.

2. Weber, W.J., Physicochemical Processes for Water Quality Control, John Wiley & Sons (1972) 261.

3. Kunin, R., "Helpful Hints in Ion Exchange Technology," Amber Hi Lites, Number 173, November 1973.