“This project represents a model energy efficient system not yet widely adopted in the State of Illinois.” Nicholas J. Menninga, General Manager, Downers Grove Sanitary District

CASE STUDY:
Downers Grove Sanitary District, IL:
Aeration System Retrofit Improves Energy Efficiency, Cuts Energy Bills

Background

A leader in energy efficiency in municipal wastewater treatment in Illinois, Downers Grove Sanitary District commissioned a study in 2005 to identify energy efficiency improvements to the activated sludge aeration system at the District’s 11 MGD Wastewater Treatment Center. The District decided to implement the following improvements:

• replacement of medium-bubble diffusers and turbine aeration equipment with fine-bubble diffusers
• addition of a single-stage centrifugal blower (first of its kind in Illinois)
• replacement of several shallow aeration tanks with a deeper tank converted from sludge processing use
• addition of automatic controls for dissolved oxygen in the mixed liquor

Solution

Baxter & Woodman, Inc. completed the design of the improvements, and the construction contract was bid in mid-2007. The District decided to separate the dissolved oxygen control system and its integration into the plant’s SCADA system from the general construction contract, relying instead on a separate contract with Baxter & Woodman’s technology subsidiary B&W Control Systems Integration (BWCSI).

Technology Integration

BWCSI has provided technology design and SCADA service for the District for years and was able to quickly integrate the dissolved oxygen (DO) controls with the District’s fully redundant GE Proficy iFIX SCADA software and Allen Bradley hardware.  District staff is now able to change the required DO, airflow, and even the process control (PID) parameters from virtually anywhere that has SCADA access, including remotely. According to the District, of all the aspects of the project gradually coming on line, it was the newly acquired ability to match the air delivery to process demand that brought the “real moment of change” in the energy efficiency of the plant.

Results

Construction was completed in July 2008, and energy consumption and power audit data available since then allow the practical outcome of this project to be quantified and its energy performance to be used as a benchmark for other facilities.

The comparison of data before and after the improvements indicates that the average power usage by the entire plant decreased by one third, from 1,833 kWh/MG to 1,225 kWh/MG. With energy at $0.08/kWh and $1,400,000 in total capital cost partially offset by a $250,000 grant from Illinois Clean Energy Community Foundation, this means that the District can expect its investment in the project to pay for itself in less than 7 years of reduced energy bills energy bills.

The District’s example demonstrates that the advancing aeration technology offers relevant and surprisingly cost-effective energy efficiency retrofits even for mature and complex wastewater treatment facilities.