Challenge

Planned expansion and asset renewal of an operating company’s water reclamation facility (WRF) in the Chicago suburbs required the need to comply with new stringent nutrient discharge limits. Located in a residential/commercial area, the 1.5 million gallon per day (MGD) facility currently serves a population of about 7,500. The selected biological nutrient removal (BNR) system would have to handle double that population over the next few decades and remain odor-free.

The operating company’s engineering firm, Strand Associates Inc. of Madison, Wis., suggested installing an Orbal® multichannel oxidation ditch from Siemens Water Technologies. One of many such installations throughout the Midwest, the suburban WRF is the first expandable BNR Orbal solution that Strand and Siemens designed.

Solution

Aerated anoxic nutrient removal technology is at the heart of the Orbal solution, where oxygen is added to a zone that remains anoxic with the majority of the zone at zero dissolved oxygen (DO) level. This is done in a unique two-channel design, with an anaerobic zone built in the outline of the future third channel. Disc aeration, with its high oxygen transfer and superior mixing efficiency, forms the mechanical backbone of the process.

The Oak Valley WRF design includes Sim-Pre® recycle for enhanced denitrification as well as an internal recycle for nitrogen removal.  The anaerobic zone design allows the new system to also meet phosphorus removal requirements by promoting enhanced biological uptake rate. This zone can eventually be easily incorporated into the future third channel, once it is built, by knocking down the side walls.

A SmartBNR™ control system is incorporated into the design, to provide control operation and variable frequency drive (VFD) speed of the aerators, based on continuous readings from oxygen reduction potential (ORP) and DO probes and process objectives. The system also has the capability of notifying operators of high influent flow during storm events, and suggesting changing inflow location based on influent peak flow factor, if needed.

Results

Since the Orbal system became operational in December 2008, the Midwest water reclamation facility has consistently achieved below 3.0 mg/L total nitrogen (TN) and 1.0 mg/L total phosphorus (TP). This equates to more than 90% TN removal and 75% TP removal, without tertiary filtration or chemical addition. The system has significantly decreased odorous conditions. Improved monitoring and control options also have added to the facility's operation and maintenance (O&M) savings.

“And it will be relatively easy to expand to meet the 2.7 MGD Phase 2 design flow in the future,” said Matthew Smith, PE, at Strand Associates.