Washington, IA

New Sequencing Batch Reactor Plant

  • Client City of Washington
  • Type Wastewater
  • Services Design, Construction Administration
  • Construction Cost $14.8 million
  • Project Manager Steve J. Troyer, P.E., BCEE


The city (pop. 7326) has treated wastewater at this site since 1924.  Facilities included preliminary treatment, primary clarification, single-stage trickling filter, final clarification, and anaerobic digestion.  Frequent violations associated with sanitary overflows during storm events mandated action to gain control of the collection system and treatment facilities.  The challenge was compounded with increasing loads, new regulations, and deteriorating equipment.  The city forged partnerships with important local industrial stakeholders so all could invest in the municipal plant upgrade.

The city was under an IDNR administrative consent order to eliminate bypassing of wastewater to the receiving stream and replace their out-dated trickling filter plant.  To obtain necessary IDNR approval and move the project forward, FOX was hired to revise a facility plan prepared by another consulting engineering firm.  Our facility evaluation and report recommended construction of a new sequencing batch reactor plant.  FOX was retained to design the new facility.

1. Grit Vortex

Grit Vortex

SBR in React

SBR in React


FOX began design in June 2009. The project included new 15 million gallon earthen flow equalization basin with return pump station and reuse of an existing 2.3 MG equalization pond.  The new operations building houses automatic screening and grit removal, automatic raw wastewater sampling and flow metering, a pump station with self-cleaning trench-style wet well, positive displacement blowers for sequencing batch reactors (SBR), and an outdoor backup power generator. New secondary treatment consists of four continuous inflow SBRs with fine bubble membrane diffusers, automatic measurement of dissolved oxygen and liquid level, a post-SBR flow equalization basin, a new ultraviolet (UV) disinfection process, and automatic sampling, TSS measurement, and flow metering of final effluent. The new solids treatment building contains three covered aerobic digesters with medium bubble aeration; a new sludge storage tank with floating surface mixers; sludge transfer and loadout pumps; and indoor storage for UV modules and electrical equipment. Finally, a new building provides a laboratory, office, and garage/maintenance.

The new treatment plant was a large financial commitment and with this comes the expectation that it will be efficient in its operation.  The design provided operational economy; with peak wet weather flows exceeding 30 MGD, only 6.4 MGD of this flow is pumped, and only once – the remainder flows by gravity to locations were facilities will not be damaged; power use is automatically reduced to what is necessary when flows are low. The largest electric motors are on the SBR blowers, which are operated at lower speeds and run intermittently to provide minimum oxygen to meet discharge limits. At high loadings, three aerobic digesters can be used in series to meet treatment goals; at lower loadings, the digesters can be operated in parallel, or even with one out of service to meet the same goals. Sludge transfer pumps are provided but when static tank levels are favorable, gravity driven transfer is possible when filling tanks which saves energy.  The UV disinfection system is equipped with turndown capability during flows lower than the design flow.  Geothermal heat transfer for the lab building heating and cooling system, coupled with direct digital controls, optimize power usage.

Sludge Storage

Sludge Storage


Design was finalized in January 2011. Construction occurred from 2011 to 2013, and the project reached final completion on September 12, 2013.  The project was funded with a $1.9 million I-Jobs forgivable loan (grant) and Clean Water State Revolving Fund (CWSRF) loan.  The final treatment plant project cost was $14.77 million.


SBR, looking SW from North side stair tower

SBR, looking SW from North side stair tower


FOX Engineering Associates, Inc. and the city of Washington are recipients of the American Council of Engineering Companies (ACEC) Engineering Achievement Award for Engineering Excellence in the Water and Wastewater Category for the 2013 Washington Wastewater Treatment Plant.  The project also received the Award for Energy Efficiency from Alliant Energy.

UV Module

UV Disinfection Module