Wastewater Treatment Facility Improvements

Evansville has started construction of a major upgrade to our wastewater treatment facility (WWTF).  Construction began late summer 2009 and is scheduled to be completed with the upgraded WWTF operating in late 2010.  The cost is estimated at $7.25 million.

Current Facility.  The current WWTF was built in 1982, designed to last for 20 years with a capacity of 600,000 gallons per day. At its most basic level, our WWTF is a series of open ponds. In the first ponds, blowers pump air into the wastewater, this oxidation contributes to the growth of micro-organisms, or bugs, that eat the nutrients in the wastewater. From the aeration ponds, the wastewater is transferred to a settling pond to settle out the solids. Eventually, the clean water is discharged into seepage ponds that allow the water to seep into the ground.  Every few years, we pump stabilized sludge from the bottom of the ponds to apply to farm fields.

The Need.  Analysis of the wastewater is performed at various steps in the process to ensure that the discharged water meets the standards of the Wisconsin Department of Natural Resources (DNR).

In the early 1990’s the DNR established a new standard for nitrates in the groundwater.  Nitrogen, of which nitrates are one form, occurs naturally in residential waste, so it is nothing special or unique.  But our WWTF was not designed to remove it from the wastewater.

A condition of our current (April 2006) wastewater discharge permit was to address our ability to meet the groundwater limit of 10 milligrams per liter for nitrates and report a plan by December 2008. In the fall of 2006, we installed several monitoring wells both above and below the WWTF to evaluate the amount of nitrate in the groundwater due to the discharge at the WWTF. The resulting analysis is that we are not consistently meeting the limits for nitrates or chlorides.

The requirement to change our wastewater treatment to reduce nitrates is the reason for some significant upgrades for our WWTF.  Wastewater treatment alternatives were evaluated for sufficient treatment capacity to meet future flow and loading conditions, performance, and other factors.  Cost effectiveness was based on life-cycle costs, including future operation and maintenance expenses.  Based on these evaluations, the recommended WWTF plan, a mechanical plant, was developed and submitted to the DNR in November 2008.

Upgraded WWTF.  Several features have been central to the design of the upgraded WWTF.  Obviously, the treatment process would have to meet the DNR’s groundwater standards for nitrates.  The mechanical treatment would also allow the city the ability to accommodate a “wet” industry, something we currently are unable to do, so there is also an economic development component to the improvements. 

Even though we are several years beyond the existing WWTF’s designed lifespan, the plant currently runs at about 75 to 85 percent of its designed capacity.  Much of our ability to stay within the designed capacity despite the city’s population growth is due to improvements we’ve made over the years to the collection system.  But we expect Evansville to continue growing at a moderate rate, and it makes sense to increase the capacity to meet the city’s needs for the next 20 years while undertaking the treatment improvements.  Just as importantly, the mechanical system’s small footprint also allows room for future expansion when needed.

The WWTF will also have lower operating costs without the need for additional chemical treatment and lower electricity needs.  In addition to energy efficiency with the mechanical plant, it will have other “green” elements to reduce its operating costs in the long run.

Vertical Loop Reactor.  The mechanical system will include a number of other new elements.  A headworks facility with mechanical screening, grit removal, and screenings and grit washing will remove sand and debris not needing to go through the treatment process.  The vertical loop reactor (VLR) will remove nitrates and other nutrients.  A pair of clarifiers will separate cleaned wastewater from activated sludge.  Aerobic digesters will stabilize the waste sludge.  A  sludge drying bed will  dry the stabilized sludge.  The existing infiltration basins will continue to be used to discharge the clean wastewater effluent.

The central feature of the new mechanical plant is the VLR.  The VLR is essentially an oxidation ditch turned on end, think of it as a large, concrete oval or doughnut buried 20 feet in the ground.  As the wastewater circulates through the oval, bubble diffusers and mechanical spindles will mix oxygen into the wastewater to encourage the growth of the micro-organisms necessary for the biological treatment and process of nutrients.  One time through the system can result in an 80% reduction in nitrates; by recycling nitrates back to the start of the system, the denitrification efficiency is projected to meet the permit limits.     

The VLR is an extremely energy efficient and state of the art system.  The VLR is a high-rate treatment process, a drop of wastewater entering the system is treated and discharged in about 30 hours; the same drop of wastewater would take up to 6 weeks to be treated in our current lagoon system.

Energy Efficiency.  Evansville’s Energy Independence Team has recently completed an energy usage baseline to evaluate the municipality’s services.  It includes all of the city’s energy use: electricity, natural gas, gasoline, and diesel.  Measured in BTU’s, water treatment accounted for 46% of all of the city’s energy usage.  This particular measurement does include energy used in the pumping of clean, drinking water, but it does give perspective on the large energy demands in our water treatment system.

The current WWTF has 60 to 100 horsepower blowers that diffuse oxygen into the aeration ponds, aiding in the wastewater treatment.  In the upgraded WWTF, the VLR will have 30 to 40 horsepower blowers.  The motors in the VLR system will also use variable frequency drives and will be hooked up to a dissolved oxygen sensor tied into an automated computer system.  This on-line monitoring will allow precision use of the blowers based on the actual needs of the wastewater being treated.

A major electricity user in the wastewater system is actually upstream of the WWTF at the lift stations.  Currently, the main lift station periodically switches on and off, pumping wastewater to the WWTF in regular surges.  And the fluctuation of high and low flows of wastewater is not well-suited to the precision blowers in the VLR.  Two of the lift station pumps will have their motors replaced with ones using variable frequency drives.  This will enable the lift station to maintain a steady flow, better suited to the VLR system and reducing energy demand.  And, like the others, these motors are much more efficient than those in current use.

The buildings, including retrofitting the current building, will include a geothermal system for heating and cooling, further reducing the operating costs of the WWTF over the long run.  With the variety of energy efficiency and conservation attributes in the new WWTF, we expect it to qualify for Leadership in Energy and Environmental Design (LEED) certification.  Assuming we decide to apply for the certification, this would be the first WWTF in the state recognized by the LEED program.

The other notable life-cycle feature is the wind turbine.  A wind turbine in the 90 to 100 kilowatt (kW) range is a size that will  help meet the WWTF’s typical daily needs.  The turbine is being bid separately from the WWTF project and its final decision will depend on grant funds from Focus on Energy and WPPI Energy which are expected to cover about 70% of the cost.

Union Township’s recent Windtower Siting Ordinance does not have jurisdiction in the City of Evansville.  However, with all the attention Union had; there will inevitably be questions about how the turbine for the WWTF compares.  EcoEnergy had proposed three 1.5 megawatt (MW) towers (4.5 MW total) in Union; at 90 kW (or 0.09 MW) the WWTF wind turbine is significantly less.

Although the exact make and model may differ; the WWTF turbine would have a hub height of 135 feet, a rotor diameter of 56 feet, and a total height of 163 feet.  These are much less than the hub height of 262 feet, rotor diameter of 269 feet, and total height of 397 feet that were proposed in Union.  Actually, the size considered for the WWTF is small enough that it would be exempt from the requirements of Union’s ordinance.

Construction.  The city advertised construction of the WWTF in the spring of 2009 and opened bids on June 11.  CD Smith Construction of Fond du Lac was the lowest, qualified bidder. 

Because of the project's eligibility for American Relief and Recovery Act (stimulus) funding, we delayed construction to apply.  The project made the "draft" list of approved projects (we expect to sign contracts in late fall 2009).  The Clean Water Fund will finance half of the $7.25 million cost through a low-interest loan; the other half will be a forgivable loan through the ARRA award. 

Construction started August 31, 2009.  The project will be substantially complete around September 2010 and will be fully operational in December 2010.