SBR and supporting technologies give life to new Missouri WWTP
Federal economic stimulus funds soon will flow into modernizing the nation’s wastewater treatment infrastructure. The funding should help resurrect projects stalled in recent years by capital shortfalls and launch many new facilities with improved treatment systems. With stricter effluent regulations on the near horizon, advanced technologies will become the building blocks of many projects.
The new Kimmswick Wastewater Treatment Plant (WWTP) developed by Rock Creek Public Sewer District (RCPSD), south of St. Louis, offers a model integration of state-of-the-art technologies that satisfied both initial and long-term considerations. This exceptional facility continues to exceed operational expectations in operator manpower requirements and in delivering biochemical oxygen demand (BOD), total suspended solids (TSS) and ammonia levels far less than those of state requirements.
Its advanced automation enables one operator to run the new plant, versus four for the decentralized network of plants previously operated. Without the need for the three additional experienced personnel—$93,000 per year in labor costs—for daily operation, their time is now better utilized in other assignments within the district, according to a district official.
Time to Consolidate
In the late 1990s, the short distance from Jefferson County to St. Louis employment centers had spurred development that eventually began overloading existing wastewater plant capacities of two private sewer companies. These operations owned and operated 11 stand-alone package plants—rated 4,500 gal per day (gpd) to 1.2 million gpd—and 16 lift stations in service areas adjacent to the RCPSD. The pollution in receiving streams for the private plants heightened to the point that county government suspended issuing new building permits unless developers pledged to truck any additional wastewater to an adequate treatment facility.
These issues and forthcoming stricter effluent standards led the Missouri Department of Natural Resources to convince the Rock Creek public utility to acquire both private utilities and undertake significant upgrades that included a new, consolidated treatment plant to serve the consolidated area.
The scope of improvements following the acquisition included a new 4.8-million-gal-per-day (mgd) treatment plant at Kimmswick fed by 91,000 ln ft of 8- to 36-in. interceptor lines and an 11,600-gal-per-minute (gpm) pumping station. All but one of the former plants were either removed or infilled, and the transmission system was reduced from 16 lift stations to 12 lift stations, yielding significant maintenance savings.
The improvements should meet the projected growth within the consolidated service area through 2020, although the master plan presently is being updated to prepare for the state’s upcoming changes in disinfection and nutrient removal standards. Unlike the limited financing resources of the private-sector utilities, Rock Creek District was eligible for a $22-million loan from a state revolving fund. The district eliminated the 10 stand-alone plants and converted the original Kimmswick plant to the district’s efficient sludge storage and processing program.
“The acquisition immediately expanded the district’s service area to 32 sq miles and brought a tenfold increase in our customer base, from the original 800 to over 8,200 connections,” said Don Daniel, district operations manager. “The small plants had employed a mix of inadequate treatment processes, ranging from lagoons to extended aeration, contact stabilization and oxidation ditch facilities that presented different operational issues along with I&I [infiltration and inflow] problems from leaking manholes and failed lines. At the outset, we faced high maintenance.”
Horner & Shifrin, Inc., a consulting engineering firm based in St. Louis, was selected to design the new Kimmswick WWTP and began by evaluating sites and alternative processes. These included oxidation ditch, membrane biological reactor (MBR) and enhanced sequencing batch reactor (SBR) technologies for a five-acre surplus site within the district’s original Kimmswick WWTP. Following comparative study by the engineers and district management, the planning team ruled out an oxidation ditch plant because of site constraints, and high cost eliminated the MBR option. An advanced SBR system was the favored solution.
Sanitaire, a business unit of Xylem Water & Wastewater, supplied the new Kimmswick WWTP with a four-basin SBR that incorporates a unique intermittent cycle extended aeration system (ICEAS) and an automated sludge age control system called solids inventory management system (SIMS). The ICEAS differs from conventional SBR design by incorporating unique features that eliminate the need to manually divert flow during critical settling and decanting phases. Under low-load conditions, the new Kimmswick WWTP can handle the flow with just two basins. This essentially reduces the projected operating costs by half, according to the district’s operations manager.
The ICEAS system eliminated the significant cost of building primary and secondary clarifiers, pumping and larger piping. Horner & Shifrin designed the plant around a battery of four 2.2-million-gal basins to handle a normal flow of 4.8 million gal, but with a wet-weather peak capacity rated at a formidable 16.7 mgd that can prevent mixed liquor washouts. Jeremy Lay, project manager with Horner & Shifrin, now is calling upon the system’s flexibility while updating the master plan to schedule the addition of disinfection and adding submersible mixers to enhance nutrient removal.
“The decision for the ICEAS system was driven by the immediate economics, ability to handle high peak flow conditions and to accept future nutrient removal features,” Lay said. “The Sanitaire system is also a great example of the benefits derived from automation and was actually selected over a lower-cost alternative SBR technology quoted for the project. The variable frequency drives on the decanters enable the plant to efficiency handle a wide range of flows without sacrificing a quality effluent.”
The ICEAS technology operates in three time-based phases:
1. The React Phase originates with raw wastewater entering the system and reacting with mixed liquor suspended solids. It undergoes aeration and anoxic mixing sequences, setting the stage for biological oxidation and denitrification of the wastewater.
2. The Settle Phase suspends the agitation to allow the solids to reach the basin bottom as clear water builds up atop the basin.
3. In the Decant Phase, the variable frequency drive-controlled decanter slowly rotates downward to extract the clear treated water for discharge into the effluent line, simply by gravity. The positively controlled decanter uses proprietary software from Sanitaire to ensure high-quality effluent. Sludge typically is wasted from the process during this step in the sequence, automatically controlled by SIMS.
Sanitaire also provided a SCADA system accommodating the entire treatment plant. The fully integrated equipment at the facility consists of an influent pump station, a headworks building that includes flow measurement, fine screening and aerated grit removal, and a solids handling system. In addition to the ICEAS process, the plant has dissolved oxygen (DO) sensor-activated control over three centrifugal blowers, a Sanitaire fine-bubble aeration system and the SIMS solids retention time control. Sanitaire coordinated with Flygt, a sister brand within Xylem Water & Wastewater, to supply the plant’s high-performance submersible pumps.
After biological treatment in the four ICEAS reactors, the plant is equipped with flow metering and effluent pumping brought into play when required by the level of the Mississippi River, which receives the plant discharge. The process equipment also includes three aerobic digesters with Sanitaire fine-bubble diffusers in each aerobic digester, a waste activated sludge storage tank, sludge pump station, gravity belt thickener and thickened sludge storage tank.
Contract haulers remove the thickened sludge two to three times a year for land application after it is thickened from 2% to 5.5%. The SIMS lessens the frequency of hauls, according to the district’s operations manager.
“The district has experienced twice the anticipated amount of ammonia, yet the results have actually been better than predicted,” Lay said. “They target a sludge age value, and in conjunction with the SIMS system, the operators consistently maintain it every day.”
The ICEAS system, in tandem with the SIMS feature, produces excellent effluent quality at RCPSD. Sludge retention time is set at 19 days and is maintained by the SIMS (its set point is checked and adjusted once daily). The 19-day sludge age consistently results in reliable settling characteristics and, in turn, consistently high effluent quality. Most importantly, the district consistently achieves BOD and TSS far below the state’s 30 mg/L limit, despite a 1 million gal increase in raw influent now under the district’s responsibility.
“I credit Sanitaire service with optimizing our favorable experience because they remote monitor this plant’s operation in real time,” Daniel said.
The DO blower control automation features of the system also have impacted the effluent quality while reducing operational costs. By maintaining the DO levels in an optimal range, the system is capable of achieving anoxic conditions. As a result, the system acts as a “selector” that enables high-performance settling organisms to flourish, Lay said.
“The automation delivers DO control that saves an estimated $14,000 per year,” Daniel said.
As concepts moved to realistic applications, advanced systems such as those at the new Kimmswick WWTP are rewriting the expectations for wastewater treatment facilities likely to enter service in the foreseeable future.