The design/build/operate (DBO) alternative to traditionally fragmented procurement and operation continues to gain favor with utilities needing new wastewater treatment plants for often fast-growing service areas. DBO plants reduce initial plant investment, accelerate schedules to startup and can improve life-cycle operating costs and performance when the contract includes contracted operations.
Pacific Environmental Resources Corporation (PERC) is among the new generation of firms that operates as a single-source DBO contractor specializing in wastewater treatment facilities. The company’s modular plants, typically in the 1- to 10-mgd range, apply a hybrid sequential batch reactor process (PERC ASP). The PERC ASP delivers the benefits of the batch process by adding a conventional anoxic pre-reactor.
“The advanced design and process, supported by proven, standardized equipment, are combined to a plant’s specific scale,” explained Bob Nespeca, PERC vice president of operations. “The relatively small footprint of our facilities clearly distinguishes them from other plants readily available in the market. PERC has 10 to 12 plants in operation, four under construction and still others in various stages of development in Arizona, California and New Mexico, where shortfalls in water resources mandate reclamation.”
Meeting criteria
The 4.1-mgd Palm Valley Water Reclamation Facility, serving the Goodyear, Ariz. area stands out for the prototypical concepts applied to the DBO contract awarded to PERC. Built adjacent to a golf course, the plant presents an admirable setting fronted by a minipark with a cascading water feature fed by the plant’s high-quality effluent, a contemporary sculpture and liberal landscaping. However, beyond the aesthetics, this showcase facility had to meet the most stringent performance criteria.
“The Litchfield Park Service Co. (LPSCo.) contracted for the facility to support a residential golf community within 14,000 ac held by SunCor Development,” Nespeca said in explaining the project’s background. “A DBO contract enabled the private utility and developer to bring the plant on-line much sooner and at a lower cost than participating in an expansion of the city of Goodyear wastewater treatment facility.”
City of Goodyear specifications imposed on the project required treated effluent to meet not only Arizona Department of Environmental Quality (ADEQ) Class A+ criteria but also EPA Class A Biosolids. PERC guaranteed the plant to produce ADEQ Class A+ values of 10 mgl BOD, 10 TSS and 10 nitrogen, but delivered 2, 2 and 2, respectively, during the period the company operated the facility.
The high-quality effluent can be used to irrigate golf courses, parks and residential common areas, and can also be diverted into an infiltration basin to recharge the aquifer.
A Phase II expansion, provided for in the original land acquisition and design, will double the present capacity within the existing property footprint. The entire project required 181/2 months, 11 months of which involved actual construction. PERC initiated a public education effort, including personal contact with nearby residents.
“The design and aesthetic features of the plant are what overcame any NIMBY objections,” Nespeca said. “The plant stands within 50 yd of neighboring property but requires only 6 ac; has less visible infrastructure; and eliminates the objectionable noise and odor so often associated with traditional wastewater treatment facilities.”
The PERC ASP process avoids the objectionable traits of traditional plants because the process occurs below grade. The majority of the plant lies beneath one-story buildings that could easily pass for a cluster of office structures.
Plant design
Operational since 2002, the unique plant was designed to achieve biological nutrient removal with a two-stage activated sludge process. The links in the process chain consist of two, 1.6-mg SBRs, disc filtration, UV disinfection and an Auto Thermophilic Aerobic Digester (ATAD). At the time it entered service only 25 ATAD units operated in the nation.
“The design relies upon a distributed control system to regulate the breakdown of concentrated sludge as pumps transfer it through the process,” Nespeca noted. “The microbial activity in the process depends on closely controlled time and high temperature.”
Equalization, biological treatment and secondary clarification occur sequentially within the SBR tanks, versus the simultaneous stages in separate units that characterize a conventional activated sludge system.
Five operating modes—fill, react, settle, draw and idle—occur over a controlled period in SBRs to achieve the specified quality of effluent. The level of automation enabled PERC to operate the 4.1-mgd Palm Valley facility for the duration of its contract with only three operators compared to 10 operators required at a 3.0-mgd municipal plant designed around the traditional activated sludge process.
The transfer of influent and concentrated sludge required advanced fluid-handling pumps that could perform the mission without clogging. ITT Flygt Model N3300 pumps were specified universally throughout the plant, including the SBRs, the anoxic tank, ATAD and aerobic post digester. The plant required 20, 45-hp submersible pumps to compensate for the plant’s low hydrological flow. Using identically sized units instilled maximum redundancy within the plant and operational flexibility.
The N-model pumps were ideally suited for the thicker sludge of the PERC hybrid SBR process. The semi-open, self-cleaning impeller on an N pump represents a patented innovation in wastewater-handling pumps. The design keeps the leading edges of the impeller vanes unobstructed where fouling often sets the stage for clogging. The leading edges of the impeller vanes pass across a stationary relief groove that clears any snared fibrous solids, grease or sludge, creating a self-cleaning flow path through the pump.
Unique findings
The Palm Valley facility remains unique among recent PERC plants because an ATAD was needed to produce the initially specified EPA Class A Biosolids. The Flygt pumps provided the low net positive suction needed to handle 4–6% solids from the main treatment process aeration. The ATAD performed the secondary treatment at 55 to 60?C, typically associated with the traditional activated sludge process.
An exception to the series of Flygt pumps are two, 10-hp, 150-gpm Allweiler progressing cavity pumps that transfer the thick solids off the digesters into the centrifuge for the dewatering. These progressing cavity pumps create a series of cavities that progress from the suction point to the discharge. Their positive displacement was well-suited for this application because the flow is directly proportionate to the speed, independent of head, whereas submersible pumps would have a variable flow rate that could cause the flow to change with the head.
Production of EPA Class A biosolids was eventually waived in favor of Class B due to a lack of any market for the soil enhancing end product. The Palm Valley plant was so efficient that four, 500,000 gpd cycles yielded only 71/2 cu yd per day. Therefore, it became more practical to adjust the process and produce Class B sludge collected by a regional waste management company for land application on eligible crops. This change to Class B biosolids now enables the plant to operate with a solids concentration of 2.03%.
“Many wastewater utility executives have visited the plant to see the ATAD process during the two years that PERC operated the facility under the DBO contract, although it is an unlikely candidate for duplication,” Nespeca said. “PERC now markets other variations that can produce Class A biosolids.” In the period after entering service, LPSCo. was acquired by a Canadian utility holding company that directly operates the facility with LPSCo. personnel. Meanwhile, opportunities for DBO—and advanced processes—continue to increase as utilities adopt a more progressive approach toward procurement.
