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Current IssueSeptember 04, 2015
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Ask The Expert: Spot-Free Rinse Water for Bus Washer -- 3 Answers
Question: Can you provide any insight as to the best process to use to produce 3-to-5 gpm of spot-free rinse water for our bus washer application? The use of potable drinking water has resulted in etched glass or polycarbonate panels on the side windows of our buses (not hard water spots). The washing takes place at night and window blow-drying is not feasible. The conservation of water and electrical power is of some concern, as industrial sewer discharge fees rise.

Stephen Watt
Long Beach, Calif.

Answer: Three members of WC&P's Technical Review Committee volunteered responses to this query. We highlight their answers below:

No. 1: The typical solution is reverse osmosis, which provides a cost-effective rinse. It's hard to believe the use of city water causes etching of the windows. A more likely cause is the detergent used in the wash cycle. Consider changing the formulation or quantity. By the time you rinse the damage has already been done.

Jim Hunt, DWC Division, Duff Co. (now with DI Water Group, Boston)

No. 2: The most practical technology for spot-free rinse water generation is reverse osmosis. In general, the water quality produced by this process will not leave a spot when it evaporates. Conservation of water can be a problem, as anywhere from 10-to-50 percent of the feed water to reverse osmosis systems is discharged as concentrate. It is possible to reuse the concentrate in other concentration, perhaps as makeup water for the washing solution. Any of the water conditioning dealers that handle commercial and industrial applications should be able to help you with this. Good Luck.

Peter S. Cartwright, P.E., CWS-VI, Cartwright Consulting Co., Minneapolis

No. 3: Recently we conducted an evaluation for a car wash operation that wanted to get rid of white pinhead spots from calcium carbonate deposits on vehicles washed using city water. The customer asked us to evaluate a device supposed to cure this problem. The manufacturer claimed conversion of the depositing crystalline calcium carbonate to a form that would slide off of the washed vehicles and not have white deposits. Our study could not validate the manufacturer's claims. The take home lesson is: "Buyer Beware." I am sure you will get offers to fix your problem.

Henry Nowicki, Ph.D., PACS Inc., Pittsburgh

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