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Arizona city manages to protect its expanding wastewater needs
In 1937, a one square mile parcel of land on U.S. Route 60 in Maricopa County near Surprise, Ariz., was little more than a gas station and a few small houses. That was until State Legislator and Founder Homer Charles Ludden subdivided the rural square mile parcel into low-cost homes sites for the area's agricultural workers.
Looking at this piece of rural property, Ludden remarked that if this ever became a city, it would be a surprise. No surprise, but it took a while--63 years.
The city was incorporated in 1960 with a population of 1,000. A perfect barometer for the growing popularity of the Valley of the Sun, Surprise expanded to 7,122 in 1990 and to 10,737 in 1995. Five years later, the city's population tripled to 30,848. In 2003, the population was expected to top 65,000. With this explosive growth came the need for expanded water and wastewater facilities. The original wastewater treatment plant had to be expanded, not once but twice.
Planning for 2020
Home to the brand new $48 million Surprise Stadium--a training facility for Major League Baseball's Texas Rangers and Kansas City Royals--the city of Surprise today owns and operates a 3.2-mgd water reclamation facility (WRF).
The existing WRF consists of Plants I and II, with treatment capacities of 0.8 mgd and 2.4 mgd, respectively. In order to meet the growing wastewater treatment needs, the city found it necessary to build a new 4.0-mgd treatment expansion facility in 2002. The Plant III expansion facility design was completed in Dec. 2001 by a local consulting engineering firm, RT Engineers.
One of the key tasks of the design phase was to master plan the entire 187-acre site for the ultimate plant build-out capacity of 24 mgd, according to the city's Water Service Director Richard Williams, Sr.
"Our planners warned us of the colossal growth yet to come," said Williams. "They were projecting 124,534 by the end of 2010, and a nearly doubling again--213,951--by the year 2020. We knew that today's capacity of 7.2 mgd would have to triple by then."
New 4.0-mgd plant
The new Plant III 4.0-mgd liquid stream facilities consist of influent pumping, rotary drum screening, grit removal, bio-denitro oxidation ditch biological treatment, secondary clarification, tertiary filtration, effluent pumping and artificial recharge basins.
The solid stream treatment included RAS/WAS pumping, rotary drum thickening, auto-thermal thermophillic aerobic digestion and centrifuge dewatering. The polymer is added to enhance the dewaterability of the digested sludge. The 25-30% dewatered sludge is hauled offsite and is used as a soil conditioner. The Plant III design included a state-of-the-art Operation and Maintenance Building where the plant's central SCADA system is located.
Williams knew he had to provide corrosion protection for a considerable amount of new concrete substrate. He was hoping for a 15+ year coating life span. To achieve this goal, Williams needed a high-performance coating system because of the increased life expectancy, unusually aggressive H2S environment, long detention times and higher sludge temperatures brought on by the intense desert heat.
Drawing on his past experience with the high-performance coatings system for Plants I and II, Williams turned to the Tnemec Co. and Joe Keilbey, a coatings consultant, for corrosion protection counsel.
For the Plant III expansion, the wastewater bearing structures included the influent pumping wet well, influent channels, grit chamber and RAS/WAS pumping wet well.
For concrete surfaces exposed to hydrogen sulfide gas, Keilbey specified a three-step coating system of Tnemec Series 218 Mortar-Clan (concrete patch), Series 434 Perma-Shield at 1/8-in. DFT, and a topcoat of Series 435 Perma-Glaze at 15 mils DFT. This epoxy system would assure Williams long-term performance and design life.
Keilbey selected the 434/435 Perma-Shield H2S system because it was specifically designed and tested to resist microbiologically-induced corrosion, hydrogen sulfide gas permeation, abrasion, carbonic acid, and the contaminants always found in industrial waste streams--major contributors to corrosion and concrete deterioration.
"In addition for the extensive areas of submerged metals, we recommended two coats of Series N69 Hi-Build Epoxoline II, a polyamidoamine advanced generation epoxy," Keilbey said. "For the above grade non-immersible metal surfaces, we used a two coat epoxy/polyurethane coating system that included Series 1074 Endura-Shield II, an aliphatic acrylic polyurethane coating that is highly resistant to abrasion, wet conditions, corrosive fumes and exterior weathering. Both systems are used extensively and have a long, successful history in waste and water treatment facilities in the southwest."
The system was approved and applied without problem in the 1,000 square ft Sewer Line Junction Structure in June 2002 during 110° weather. The Plant III Influent Structure (3,500 square ft), the Plant III Influent Water Pump Station Wet Well and Baffle Well (4,300 square ft), the Sewer Interceptor Junction Structure (3,100 square ft), and other unscheduled miscellaneous structures were coated over the next two months.
Accordingly, all of these areas have been in service for over a year without problems with the coating applications.
Williams was impressed with the three-step application process and solution that was incorporated.
"These areas have been in service and performing well," he said. "Reliable performance is vital because of the expansion plants we know we will have to build in the future to increase capacity. It is important to me that all of our wastewater equipment and concrete substrates are well protected for the next 15 plus years--that's one less thing for me to worry about."