Original Date: 03/17/1997
Revision Date: 01/18/2007
Best Practice : Cooling Tower Make-up Water Metering
Typically, sewer charges for industrial sites are based on the percentage of cubic feet of water metered to a company’s site regardless of its usage (e.g., drinking, cleaning, cooling). Water consumption charges are based on meter readings placed at an entry point to the site. Allowing for a small percentage for lawn watering, the Water Department calculates sewer charges from these same meter readings based on the assumption that water entering a site will exit the site through the sewer. In industrial sites such as Polaroid where film processing and machinery generate vast amounts of heat, large cooling towers are required to maintain stable temperatures and humidity levels. Although these cooling towers consume large amounts of water for operation, only about 10% of the water returns to the sewer system while the remaining 90% evaporates from the towers. As a result, Polaroid negotiated with the City of Waltham’s Water Department for an annual rebate of sewer charges for the water which evaporates from its cooling towers.
Until 1996, Polaroid paid full sewer charges for the evaporated water. Based on widely accepted engineering practices, on-site evaluations, and cooling tower blow-down cycles, Polaroid confirmed that an average of 90% of the water consumption volume for its 16 cooling towers evaporates, and the remaining 10% is discharged into the sanitation sewer system through the blow-down cycles. This breakdown equates to a 10:1 reduction of water consumed versus water entering the sewer system. Key to qualifying for the annual rebate was Polaroid’s presentation and demonstration to the city that the Water Department’s metering and sewer charging practices were inadequately reflecting the actual water discharged to the sewer system.
The city granted approval for the rebate, but required Polaroid to purchase and install new water meters at the intake of each of the water towers. The meters, which were compatible with the city’s present metering system, registered in cubic feet and allowed for remote readout from a touchpad using a smart gun.
By using the new method to estimate sewer discharge, Polaroid established a reliable accounting method for determining how much water evaporates at the cooling towers and how much enters the sewer system. Sewer charges are no longer based on the assumption that all water entering a site will be discharged through the sewer system. Polaroid’s annual sewer charge rebate for its 16 cooling towers is estimated at $150,000 to $200,000 with a hardware implementation cost of less than $4,000.
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