When the mid-1950s wastewater treatment plant became too small to meet demand, the community of Washington, Iowa, decided to build new and look for as many operational efficiencies as possible to help hold down operating costs. The new $15 million plant went online in August 2012 to serve the 7,200 residents.

A 6.4 mgd sequencing batch reactor (SBR) facility with UV disinfection and 17 million gallons of flow equalization replaced the old 2 mgd single-stage trickling filter plant. The modern plant performs better while saving energy and money.

All automated

“Everything is run off the SCADA system,” says Fred Doggett, plant superintendent. Besides providing process controls, it monitors equipment’s energy use, and that helps preventive maintenance. If a piece of equipment suddenly starts using more energy, that may indicate a developing problem, which operators can then correct before a failure happens.

“We have variable-frequency drives [VFDs] and a geothermal system with in-floor radiant heat,” says Doggett. “An energy recovery ventilator for the lab building, maintenance shop and administration building recovers heat from air as it is discharged to heat fresh air that is coming in. All the buildings have occupancy sensors to control energy efficient T-8 fluorescent lighting.”

A gas-fueled makeup air unit in the headworks building has variable-speed fans for efficient performance. Due to the high rate of air exchanges needed to meet the NFPA 820 code, the geothermal system would not keep up with building heating needs.

The plant also uses insulation, low-E windows and window film to increase efficiency. The project, designed by FOX Engineering Associates, Shive-Hattery and Riesberg Engineering, earned a 2013 Excellence in Energy Efficiency honor from Alliant Energy, the local electric utility, for demonstrating leadership and for an estimated savings of 4.7 million kWh annually. “We traveled to several plants to look at equipment during the planning process,” says Doggett.

Ready for growth

Rob Baker, project manager with Fox Engineering, adds, “The city wanted to accommodate future industrial growth. The plant can take quite a bit of variable industrial loading in terms of organics, nitrogen and ammonia. About 45 percent of the total maximum monthly design loading is set aside for industrial loading.”

The plant has 17 million gallons of flow equalization, versus 2 million gallons at the old plant. It also has new screening, grit removal and raw wastewater pumping to the four SBR basins. Each basin is 150 feet long and 38 feet wide and up to 20 feet deep and holds up to 855,000 gallons.

“The 150 hp main blowers for the SBR are the largest motors in the plant and big energy users,” notes Baker. “The VFDs can be turned down when the plant isn’t receiving the full design loading. Having more than one SBR, we can shut down one for a while if there isn’t enough flow and loading.”

That happened shortly after the plant went online. “It just wasn’t getting the loading to grow the bacteria,” says Baker. “Two reactors were shut down and the other two operated at lower speeds to be able to achieve a mixed liquor suspended solids level that we were targeting.” When flows increased in spring, sludge from the two reactors that had been running seeded the two previously idle reactors.

DO control

Dissolved oxygen sensors in the SBRs reduce blower use. “They maintain a minimum oxygen level so the blowers can be shut off when not needed during the react cycle,” says Baker. “Aeration is done just before the settle and decant phases to elevate and maintain the DO.” All blowers in the new plant are from Aerzen USA, while aeration and decanting equipment is from Sanitaire, a Xylem Brand.

After the effluent is decanted from the SBRs, it passes through a vertical ultraviolet disinfection system (Ozonia). “Most of the time only one bank is operating,” says Baker. “The UV system uses more power as the flow increases, and then a second unit comes online if needed. That way it doesn’t use any more power than it has to.”

Helping to reduce energy needs are 18 Allen-Bradley VFDs on the raw wastewater pumps, equalization basin pumps, SBR blowers, SBR decanters, and digester blowers. Three covered digesters with medium-bubble diffusers normally operate in series but can operate in parallel. Just as in the SBRs, DO in the digesters is monitored so that aeration blowers only run as needed.

Much to learn

The biosolids are dewatered to about 2 percent solids and pumped to a storage tank for eventual land application to area farms in spring and fall. “We may go to spreading just once per year, since we have a 1-million-gallon biosolids storage tank,” adds Doggett.

A new plant with different technology presented a steep learning curve for Doggett and operators Jason Whisler and Danny Martin. “We had nothing like this,” says Doggett. “We had no SCADA, just a few electrical boxes. For pumping biosolids, we had to manually start the pump and control the valves.”

New plant construction took about 15 months. Training lasted for about two months, and all three operators have upgraded their state certifications. Doggett was struck by the complexity: “I’ve never been around this kind of construction; the amount of concrete, rebar, wiring and all the automation that goes into a new plant.”