Ask the Expert Question-and-Answer Archive (Wastewater Treatment)

by Mike McGinness, EcoShield Environmental Systems, Inc.
March, 2002

Water Use In Metal Finishing Operations

Q. We had spoken a couple of months ago on the phone. I had a couple of question about water use. Which of the metal finishing processes use most water for rinsing and how much? Also what are the standard tank sizes for rinsing purposes used by engineers when making an estimate for market penetration?

A. Rinse water volume seems to vary more by facility that by process in my opinion partly due to better control and management in some facilities.

If you assume a single stage rinse with a very low drag-in concentration limit (3 ppm) for the rinse water, as in precision parts cleaning applications, and a high plating bath concentration of say nickel chloride (20,000 ppm), and assume a part shape that does not drain well then you would have a worst case scenario. Redesigning the part or rack for better drainage, and adding multiple counter flow rinses could reduce the waste water volume for the above situation by as much as 99.99 %, or from 1,000,000 gallons per day to 100 gallons per day!

In general the ultraclean precision cleaning areas such as electronics (hard drives, silicon chips for example) will use more rinse water than the low tech cleaning applications.

There are no standard rinse tank sizes that I know of. Tank sizes, including rinse dip tanks, vary in size more as a function of part size or rack or basket size. Tank size for spray rinses will need to be large enough to supply the spray rinse pump with out letting it draw air which can make it cavitate. This is a function of pumping rate and drain back time. For a pump rate of 200 GPM a 400 to 600 gallon tank would probably work fine for a spray rinse.

Finally rinse tanks were size optimized (larger) in the past to allow multiple part or rack rinsing before rinse was dumped and replaced, but methods and strategies have changed and now rinse tank size is minimized and additional rinse tanks are added in stages with a counterflow to minimize total waste water volume.