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Rinsing Manual

Estimating and Realizing Potential Savings

Tools and information for evaluating rinse improvement options Once you have collected data and characterized your rinse systems, you are ready to evaluate options for improvement. Appendices A, B, and C of this manual describe well-proven options for good rinsing:
  • Appendix A covers methods that reduce the formation and carryover of dragout
  • Appendix B describes methods of improving rinse efficiency
  • Appendix C covers methods of water use control

The STERC website has two on-line calculators that will help you to evaluate rinsing options:

If you are performing a cost/benefit analysis (determining which option returns the optimal ratio of costs to savings) and/or calculating the payback period (the length of time required to recover the cost of an investment), you will want to identify both the costs of making rinse system changes and the potential savings for various options. Costs will vary significantly from shop to shop, depending on many site-specific factors and are therefore not discussed here. Various sources of cost information are available, including suppliers and online catalogs. But whatever the relative costs, the opportunities for savings generally apply to a wide variety shops, as indicated below. Table 4 identifies the key benefits that can be derived from making rinse system improvements. The table shows the range of savings that can be expected. If you know your actual costs, you can narrow the range to apply to your shop.

Table 4. Potential Savings from Rinse System Improvements
Potential Savings
What to Focus On
Water and Sewer Nearly all municipalities charge separate fees for water and sewer, which show up on a combined monthly or quarterly bill. Typically, more than 90% of the water used at metal finishing shops ends up being discharged to the wastewater treatment system. The remainder is evaporated or discharged to the sanitary sewer. Water and sewer costs vary widely across the US, so be sure to use your actual costs when determining potential savings. Median water/sewer cost for commercial entities across major US cities is $8/1,000 gal ($67/1,000 cubic feet). However, you should take into account future increases. Water/sewer costs have doubled over a recent 12 year period and continue to rise three times faster than the Consumer Price Index (CPI). Source: Black & Veatch Report Potential reductions can be accurately estimated. For evaluating multiple rinse tank options use STERC calculators or equations. Reduction of water use can be accomplished in numerous ways, including reducing dragout (increasing drain time, adding drain boards), improving rinsing efficiency (e.g., adding air agitation or additional rinse tanks) and improving water use control (e.g., conductivity controllers, flow restrictors)
Wastewater Treatment (WWT) Chemicals Various chemicals are used for treating wastewater from metal finishing operations. Examples include sodium hydroxide, sulfuric acid, sodium hypochlorite, sodium bisulfite and polymer. Chemical costs for wastewater treatment are typically in the range of $8 to $12/1,000 gal of wastewater treated. The quantity of WWT chemicals used depends both on the volume of wastewater treated and on its composition. The presence of hexavalent chromium, cyanide or complexed metals increases costs. Therefore, reducing/eliminating these constituents and the flow rate will lower WWT chemical use.
Sludge Disposal Most wastewater treatment sludge generated by metal finishing facilities is disposed of off-site as hazardous waste (RCRA waste code F006). The average disposal cost for F006 sludge is $1 to $2 per pound. Disposal for non-hazardous sludge is $0.10 to $0.20 per pound. Sludge mostly consists of precipitated metals and water. Reducing dragout will proportionally reduce sludge generation. Focus on use of dragout tanks, increasing drain time, installing drip trays, etc.
Process Chemistry Process chemicals, especially plating solutions, are expensive to purchase. Some of this chemistry is consumed by the metal finishing process. However, a portion of the chemistry ends up in the wastewater. Process chemical costs vary widely. Check with your purchasing department for prices. Reducing dragout will reduce process chemical purchases. Focus on use of dragout tanks, increasing drain time, installing drip trays, etc.
Work Quality/Re-work Parts rejected due to improper finish are either stripped/reworked or discarded. Rework typically more than doubles the cost of finishing a given part. Significant potential savings come from reductions in labor, process chemistry, water/sewer, WWT chemicals and sludge disposal. Rework often occurs when rinsing is inadequate. By improving rinsing efficiency you can get cleaner parts using less water.

Implementing changes, measuring results, and calculating savings Once you have analyzed and selected options, establish an implementation schedule. Unfortunately, for making most changes to rinse systems, the plating line will need to be idle. Large jobs such as adding rinse tasks will take days to accomplish, whereas smaller jobs, such as adding a spray bar or air agitation can be performed over weekends or other downtimes. A schedule will help you to keep on track.

After implementation is complete, measure the effect of your changes using the same methods used to characterize the old systems (i.e., dragout, water use, rise efficiency). Also, collect current data for water/sewer costs, WWT reagent use, sludge generation, process chemistry use and work quality. You will likely need several months of data in order to get an accurate picture of change.

Compare the baseline and new data to determine savings. The comparison should be based on how much product was run during the time periods being compared. To compare apples to apples, the total water use and sludge data should be "normalized" by dividing the totals by a normalizing factor such as pounds of product plated, square footage plated or dollars of sales. For example, the volume of water used on a barrel line can be expressed as gallons of water used per pound of parts plated.

Instituting a program of continuous monitoring and recordkeeping Continuous monitoring and recordkeeping will allow you to quickly identify glitches within the rinse systems and make corrections before substantial losses occur. It will also allow you to accurately track long-term savings. Further, keep in mind the saying: data is knowledge and knowledge is power. Documented success with a rinse system project will help generate management's support for your next pollution prevention project.



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