Site Energy Management Plan Helping Wastewater Plant
Reduce Carbon Footprint

The LaGrange wastewater treatment facility, located in LaGrange, Indiana, is a 50-year old extended air activated sludge plant, designed to treat an average daily flow of 1.8 mgd (6,815 m³/day) and an average design peak flow of 3.6 mgd (13,630 m³/day). Upgrades to the plant in 2002 included a new headworks, oxidation ditch, secondary clarifiers, digesters and sludge dewatering equipment. In a recent review of the plant's performance, all indications were that the operating plan for the plant, which has been managed via public-private partnership by Severn Trent Services since 1997, was resulting in compliant, efficient and cost effective operations. The staff was operating all processes as intended by the engineer's design and fulfilling the facility's intended purpose.

But in an era when the global "green" movement is focusing increased attention on conserving resources, compliance is not enough. In 2008, under the direction of Seven Trent, the LaGrange plant underwent a process to identify ways to reduce its carbon footprint.

Sewer collection system maintenance program reduces unwanted flow
Over the 13 years of the partnership, LaGrange town officials and Severn Trent have worked to eliminate discharges from numerous combined sewer overflows (CSOs) and sanitary sewer overflows (SSOs), as well as excessive flows at the facility during rain events due to severe collection system inflow and infiltration. The CSOs were found and have since been disconnected and, although four SSOs remain listed in the National Pollutant Discharge Elimination System (NPDES) permit, there have been no discharges from an SSO since 2007.

The effort to eliminate these discharges began with a large-scale storm sewer separation project that was completed in 1999. The town continued to experience wet weather discharges from what were now SSOs, albeit less than with the previous CSOs, and very high flows to the treatment plant during rain events. Severn Trent continued to work with the town and took action to identify the remaining problem areas. LaGrange was the first site to implement Severn Trent's sewer collection system maintenance program, in which the company and other contractors performed the following tasks:

• utilized portable flow meters to monitor manholes for increased flows during rain events
• established a record of flows throughout the collection system during rain events and used the data to locate additional problem areas
• smoke tested the collection system to find illegal connections and pipe failures
• conducted inflow and infiltration studies by monitoring lift stations during dry weather versus wet weather
• videotaped sections of the collection system and found numerous points of inflow and infiltration
• conducted door-to-door inspections where necessary to find illegal clear water connections, an ongoing effort

A variety of conditions including illegal residential connections, poor pipe joints, failed pipe and previously unknown combined sewer pipe connections have been identified during the past 11 years. Once discovered, the town was able to take the necessary action through capital planning and construction projects to repair the causes of unwanted flow. This included repairing old manholes, replacing failed collection pipes and slip-lining old leaking pipes.

Site Energy Management Plan: identifying ways to reduce the plant's carbon footprint
Implementation of the sewer collection system maintenance program is indicative of the successful partnership between Severn Trent and the town of LaGrange. Defining problems and working together to devise and implement solutions is an ongoing, proactive effort. So, when Severn Trent made a formal, company-wide commitment to monitor and reduce its carbon footprint in 2008, company personnel implemented a Site Energy Management Plan (SEMP) audit to evaluate and create action plans to improve plant energy efficiency. The ratio of kWh used per million gallons treated is the unit of measure used to evaluate a treatment plant's carbon footprint and performance results against any set objectives. Long viewed as a financial burden, power consumption at water and wastewater treatment plants is now considered a major environmental burden as well. Energy costs are the second largest expense for treatment facilities – 28 percent of total operating expenses for wastewater facilities. Reducing power consumption, resulting in reduced operating costs and maximized process efficiencies, is one way utilities are generating savings and minimizing their carbon footprint.

Before Severn Trent recommends any action items resulting from a SEMP, all permit limits and client contractual requirements must be met. Unless all effluent permit and client commitments and obligations are met, no amount of kWh reductions can be justified.

The formal SEMP:

• evaluates site energy consumption by collecting a minimum of one year's history of kWh and plant influent flow and loading data; collecting information on equipment size and horsepower; collecting all data on chemical usage; performing a process analysis to determine a variety of optimum plant operation parameters; analyzing current operations to determine the optimum electric power consumption; calculating sludge hauling energy based upon round trip distances to sludge disposal sites and miles-per-gallon data; and calculating sludge incineration energy using latent heat of evaporation figures as applicable
• challenges current practices
• identifies opportunities for energy reduction
• produces priority actions for increasing energy efficiency

Evaluating current facility operations
The Severn Trent SEMP Process Model was used to evaluate the current operations of the LaGrange facility. The following table details the treatment facility's base model annual average operating characteristics:

While the process model proved the effectiveness of the LaGrange plant's current operation, it also helped Severn Trent personnel identify possible areas of energy reduction, including:

1. discontinuing the use of the larger of two oxidation ditches (ditch #1), which was constructed during the 2002 plant upgrade, and operating the plant with the smaller ditch (ditch #2) that was then out of service, but could be rehabilitated and modified with a smaller, more efficient aerator
2. discontinuing the use of oxidation ditch #1 and operating with a refurbished but unmodified ditch #2
3. discontinuing aerobic digestion and converting digesters to non-aerated gravity settling tanks
4. operating the grit pump on demand instead of continuously, and continue operation of only one of two available ultraviolet gravity flow open channel disinfection systems – as long as the use of just one ultraviolet system proves to be adequately effective
5. continue operating with one of two available ultraviolet banks and with no changes to the grit pump
6. operating the grit pump on demand but with no reduction in the number of ultraviolet banks utilized

These six strategies, determined by Severn Trent to be the most viable methods of reducing kWh demand at the plant, would represent estimated savings ranging from about $800 per year to more than $13,000 per year. In evaluating the feasibility of each of the approaches, Severn Trent officials have recommended the fourth option (above). This approach would operate the grit pump on demand, rather than continuously, and with one ultraviolet bank seven months of the year to meet seasonal disinfection requirements. The grit pumps and classifier are currently operated continuously because there is a lack of confidence in the pump priming system and the system freezes in the winter if not run. The issues with the priming system will have to be addressed prior to operating the pump in automatic mode. The estimated annual savings from this approach is 59,220 kWh, a kWh/million gallons savings of 10 percent compared to current performance.

According to Clint Houseworth, process specialist for Severn Trent Services' Technical Services Group, "The results of the SEMP audit show that plant management and staff are putting forth a strong effort to optimize operations. Severn Trent is committed to continuously looking for opportunities to save energy through creative thinking that will be incorporated into long range comprehensive planning goals. While the LaGrange plant is currently operating efficiently and effectively, creating a SEMP to guide our operating decisions will ensure even greater process and operational efficiencies in the years to come through reduced energy usage."

Terry Helmer, president of the LaGrange Town Council, said, "Prior to hiring Severn Trent we were out of compliance in many areas and in a lot of trouble with state and federal regulators. We hired Severn Trent Services and they immediately began working with the regulators and initiated a plan to get us back into compliance at the wastewater treatment plant and in the collection system. They have since made all of our compliance problems go away. Severn Trent's efforts have gone above and beyond contractual requirements and their services have been a gift to the community."

For more information, e-mail info@severntrentservices.com.

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