San Jose State University Uses Control System to Reduce Wastewater and Improve the Use of Recycled Water: A Cooling Tower Case Study

SAN JOSE STATE UNIVERSITY (SJSU) Central Plant (Plant) provides steam heating, chilled water cooling, and electrical distribution for approximately five million square feet of urban university building space. The facility uses a forced draft cooling tower for the production of chilled water. Since 1999 SJSU has enjoyed the cost benefits and the challenges of using recycled water for cooling tower make up.

Additionally the campus community strives to be a good steward of the earth. By conserving water, using recycled water for non-potable uses such as cooling, and lowering sewer discharge, they were able to attain this more nebulous goal.

Problem

The existing cooling tower treatment control was setup conservatively to ensure that fluctuations in recycled water chemistry would not disturb operation of the system. Conductivity was controlled at a fixed rate, and adjusted to accommodate wet chemistry results, chlorine was fed to keep a sufficient residual to handle biological growth, and corrosion inhibiter was controlled by using a trace dye in the product. Since the control method was not precise, the chemical usage was programmed within a range, and it was never exact. More chemicals were being used than needed, thus providing the Plant with more frequent discharge to sewer because they were using the ideal water chemistry for the maximum cycles in the tower.

Goals

SJSU had a dual goal with this project:

  • Reduce the Plant's flow to the sanitary sewer.
  • Improve the efficiency of recycled water use in the cooling system.

Solution

SJSU has access to an on campus well or recycled water. They evaluated which was more cost effective. A computer based modeling program determined the program control limits for the cooling tower using both recycled water and well water. The modeling program determined that the Plant could run the cooling water system at a maximum of six cycles with the new control using either recycled water or well water; keeping the sewer flow the same regardless water source.

When operational costs are compared, there is a cost benefit to having better automation regardless of water source. Because recycled water has a lower unit cost, the plant is better off using recycled water than using well water, as shown in the figure below:

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SJSU purchased a controller to improve their ability to monitor the Plant. It accurately traces the active polymer used in the process for scaling control. This allows the Plant to measure the proportional amount of product fed to the system, and the amount of active polymer remaining in the water. With the new technology, they no longer need to carry artificially high residual polymers to compensate for changes in tower loading. Conductivity is controlled and run at higher levels, and significant changes in make up water quality are automatically monitored. Bio-control is improved by measuring active biological growth in the tower basin with a color sensitive dye.

The new controller increased the number of cycles of concentration in the cooling tower from between two and three to up to between five and six, drastically reducing the campus’ industrial waste water discharge to the sanitary sewer.

Results

Since installing the new control technology in October 2006, the plant has reduced blowdown and sewer flow by an average of 60%, (14,700 gallons per day). They are meeting their goal of reducing the daily sewer flow and have realized a significant cost savings. Additionally, operators have more information about system performance which helps to improve the reliability of the cooling water system and minimize maintenance costs.

The new system benefits exceed performance and cost benefit expectations in some unanticipated ways. Previously the makeup water contained more chlorine than was necessary to maintain system performance. By actively controlling the chlorine feed base on biological growth, chlorine use dramatically decreased. With reduced chlorine residual, other treatment chemicals are less likely to be consumed by the oxidizer. This coupled with the active polymer control has significantly reduced the cost of water treatment at the Plant.

Return on Investment Analysis

Modification cost (includes purchase of equipment, installation and annual maintenance package) $11,545
Chemical Savings Realized $ 1,345/month
Recycled Water Cost Savings Realized $ 78/month
Sewer Cost Savings Realized $ 1,185/month
Total Operational Cost Savings $2,608/month

Time to Payout: 135 days - 0.37 years


Thanks to Chris Nordby of SJSU and Chris Buchholz of Nalco for their assistance in preparing this case study.

 

 
 
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