Water Conservation for Electroplaters: Rinse Tank Design

Are your pouring money down the drain? You are if your firm does not have a water conservation program. Water is expensive to buy, treat, and dispose of. A water conservation program can cut water consumption by up to 50%, or even more, and the cost savings can pay for the required materials in a very short period of time. With average water and sewage rates about $2/1000 gallons, reducing the flow to a rinse tank by just one gpm can save $250 per year. Multiply this by the 20 or more rinse tanks in an average shop and it can add up to thousands of dollars per year in savings. Add to this the reduced capital costs of a waste treatment system and the savings can be even greater.

There are numerous methods which have been developed to conserve water in electroplating shops. Three of the major techniques, rinse tank design, counter current rinsing and rinse water reuse are the subjects of this series.

Rinse Tank Design

Proper rinse tank design is the cornerstone of any water conservation program. The most important factor in rinse tank design is to ensure that the rinse water is completely mixed. This will eliminate short circuiting of the feed water and effectively use all of the tank's volume. As shown below, a rinse tank must include a feed water distribution line, air agitation, and a flow control valve.

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Proper Rinse Tank Design

A water distribution line placed on the bottom of the far end of the tank will evenly distribute the fresh water throughout the tank's volume. The incoming water creates a rolling action which will help to scour the work piece clean as well as dilute the drag-out throughout the tank's volume. The distribution line can easily be constructed by drilling holes at 3-inch intervals into a or 1 inch PVC pipe and capping-off one end.

Air agitation will help remove any plating solution clinging to the surface of the work piece as well as mix the contents of the tank. Air can be introduced into the tank by an aerator placed diagonally across the bottom of the tank. This can be a commercially available aerator or one can be constructed by drilling 1/8 inch holes at 1 to 5 inch intervals in a 1 inch PVC pipe and capping-off one end. The total area of the holes must be somewhat less than the cross-sectional area of the pipe, but not less than 2/3 as this would put too much back pressure on the blower. The air must be supplied by a blower, as an air compressor can contaminate the solution with oil.

A flow control valve is used to restrict the fresh water feed rate to an optimum level. The simplest and least expensive type is a mechanical flow control valve. This type of valve will maintain a specific flow rate independent of the water faucet setting. These flow controllers are usually rated in gpm intervals from 1 gpm, and are available in plastic or plated brass.

An alternative is the use of a conductivity control system. This uses a conductivity probe to measure the level of dissolved solids in the rinse tank. When the level exceeds a predetermined maximum value, the controller will open the fresh water feed valve. When the level reaches a preset minimum value the water flow is stopped. This type of system is best used when the amount and type of work varies greatly from day to day. The major draw backs to this type of system are the relatively high system cost, difficulty in establishing the concentration limits and the need for regular system maintenance.

Capital and Operating Costs

These water conservation techniques are very inexpensive to install and operate. The typical costs of these techniques are detailed below:

The total cost for installing a water distribution line, an aerator and a mechanical flow control valve would be about $100 for a typical rinse tank.

There are no direct operational costs associated with these techniques. Though, they should be included in the facility's routine inspection and maintenance program.

Pay Back

The pay back period for installing or retrofitting these water conservation techniques into a rinse tank will depend on the quantity of water saved, as well as the water and sewer costs. For example, if water flow is reduced from 6 gpm to 3 gpm, and the water and sewer charges are $2/1000 gallons, the pay back period will be about 7 weeks for a typical rinse tank. After this initial pay back period the water conservation system will reduce operating costs by $750 per year, through water savings.

In addition to these direct cost savings, a water conservation program can reduce the capital cost of any required end-of-pipe wastewater treatment system. A rough "rule of thumb" is that a precipitation system costs about $2,000 per gallon of flow per minute. Thus a substantial reduction in water flow can produce corresponding savings in treatment system costs. However, small or token reductions will yield very little, if any, savings.

Selected References

  1. JRB Associates. Three Case Studies to Improve Industrial Waste Management in Baltimore-Area Metal Finishing Plants. 83-4, Maryland Hazardous Waste Facilities Siting Board, Annapolis, Maryland. 1984.
  2. Kushner, J. B. and A. S. Kushner. Water and Waste Control For the Plating Shop. Second Edition. Gardner Publications. Cincinnati, Ohio. 1981.
  3. Roy, C. H. In-Plant Conservation. American Electroplaters Society. Winter Park, Florida. 1980.

The North Carolina Division of Pollution Prevention and Environmental Assistance provides free, non-regulatory technical assistance and education on methods to eliminate, reduce, or recycle wastes before they become pollutants or require disposal. Call DPPEA at 919-715-6500 or 800-763-0136 or e-mail us for assistance with issues in this Fact Sheet or any of your waste reduction concerns.