The University of Texas at Austin power plants provide heating and air conditioning for two hundred buildings totaling over one-hundred million cubic feet of air space in offices, lecture halls, classrooms, laboratories, kitchens, cafeterias, and dormitories. The air conditioning is derived from six miles of underground chilled water loops.
Texas has been under moderate to severe drought conditions. The University of Texas at Austin's cooling water use for power generation and chilled water production was around 350 million gallons per year. The chilling station water use was 245 million gallons annually. They buy water from the City of Austin which costs $13.45/1,000 gallons for water and sewer—$5.22 for fresh water and $8.23 to discharge it to the sewer. As an alternative, the city offered reclaimed wastewater at a cost of $1.50/1,000 gallons.
The university elected to bring reclaim water to the campus, selecting Chilling Station 5 as the trial site. They replaced roughly 80 million gallons of treated potable water with City reclaim wastewater at a savings of almost $300,000 a year.
Identification of Objectives for Systems with Reclaim Water
Before committing to a reclaimed water program, the university required a study to justify the expense of bringing the water to the campus. U.S. Water evaluated the facilities and proposed a combination of service, equipment, and chemistry that would:
- Decrease the corrosion rates in distribution lines, heat transfer stations, and refrigeration units
- Eliminate fouling that impeded desired temperature control at all locations
- Minimize water losses from rust-damaged lines
- Minimize corrosion from microbiological sources
- Decrease the energy cost to create the chilled water
- Extend the life of seals, pumps, and heat exchangers
The water treatment challenges presented by the reclaimed wastewater (Figure 3) included high concentrations of organic carbon which had the potential to increase energy robbing biofilm growth, high total phosphate and ortho-phosphate with attendant deposit potential, and high chloride and sulfate residuals contributing to increased potential for localized corrosion. Like many distributive cooling systems, the university equipment cycles on and off according to need. Off-line condensers are subject to solids precipitation and subsequent fouling or corrosion.
U.S. Water implemented a water treatment program to minimize issues in off-line condensers by activating pumps at full flow for two to four hours, three days per week. This action reduced the settling of solids while introducing microbicides and fresh inhibitors. They formulated a custom polymer blend to address the stresses of using reclaimed water with high hardness and phosphate in the chilled water loops. Condenser water boxes and tube sheets were epoxy coated to reduce corrosion potential, and acid pH control was established to minimize insulating deposits on heat transfer surfaces. U.S. Water established a microbiological and biofilm control program, which included continuously feeding chlorine based on real time control using ORP (Oxidation – Reduction Potential) technology, chlorine dioxide fed in timed additions several times a week for additional control and lastly, a non-oxidizing microbicide fed weekly.
Installation of up-to-date feed and control equipment made it possible to constantly measure and control all treatment parameters. Information was outputted directly to the U.S. Water web-based data management program providing access to real time conditions for both university and U.S. Water personnel.
Corrosion rates were continuously monitored using LPR style corrosion sensors. A DATS deposition monitor continuously measured heat transfer efficiency. Heat exchanger surface temperatures were set at 10°F over the operating condensers to provide early warning of any possible fouling issues. Microbiological activity was monitored by plating bacteria cultures and with ATP (Adenosine TriPhosphate) testing. Regular field tests measured conductivity, pH, ATP, iron, copper, phosphate, and corrosion inhibitor concentrations. Additionally, once per quarter, samples were sent to the U.S. Water analytical laboratory, in order to verify field results with more extensive testing.
The results pointed to a program that provided solid corrosion, deposition and microbiological control. There have been no deposition or corrosion related issues after almost two years in Chiller Station 5. Biomonitoring showed less than 1000 cfu/ml total aerobic bacteria counts with less than 10 cfu/ml of sulfate reducing bacterial (SRB) present in the system. Based on this multi-year study, the University of Texas at Austin is building a new 15,000 ton chilling station which will use reclaim water for cooling tower make-up.
Municipal reclaim water can successfully replace potable water in district energy/power applications. Actions to take include analyzing the reclaim water over extended time frames to capture the diurnal and seasonal variability of the reclaimed water quality, identifying the critical parameters that are unique to reclaim water such as chlorides, phosphate, ammonia, total organic carbon, and total suspended solids and selecting options to address or mitigate the problems that each may cause. These options may include changes in system design, materials of construction, chemical treatment, or system operation and control.
The University of Texas at Austin employed all of these in their solution producing excellent results.