How Cincinnati is Making Major Energy Efficiency Improvements in Wastewater Treatment Operations by Employing a Systems Analysis Approach

The Metropolitan Sewer District of Greater Cincinnati (MSDGC) serves a population of approximately 800,000 people in a service area that includes the city of Cincinnati and most of Hamilton County. The district is responsible for conveying and treating an average of 192 million gallons per day (MGD) of wastewater and it operates seven major treatment plants. The primary mission of MSDGC is to deliver responsive, customer-focused wastewater treatment services to its ratepayers. In recent years MSDGC has made significant strides towards the goal of reducing the energy required to treat wastewater within the district. The objectives and benefits of this ongoing effort include reduced economic burdens on the ratepayers, increased long-term sustainability of the metropolitan area, and a reduction of the overall carbon footprint of the district. This paper serves to highlight the experiences and successes that MSDGC has had in its efforts. The advances this municipality has made through plant optimizations, plant upgrades and innovative design solutions will most definitely provide useful information for other municipalities, both large and small.At several treatment plants within the district, large energy savings have already been realized through optimization of plant operations without any capital investment. For example, at the Mill Creek Wastewater Treatment Plant (110 MGD average flow), the electricity consumption for secondary process was reduced by over 30%% from 2006 to 2010. With a total of 16,173,000 kWh used last year, this decrease translates to a cost savings of 330,000 and a corresponding reduction in CO2 emissions equivalent to about 5,000 tons per year. This effort was accomplished by optimizing the treatment process through reducing solid retention time (SRT), sidestream reductions, and throughput maximization. Other efforts to save energy include solids handling optimization, improved pumping, and a broad range of other efforts. At the Little Miami Wastewater Treatment Plant (35 MGD average flow), a 30% reduction in incinerator fuel costs was attained by improving the feed sludge quality, stabilizing the sludge feed rate and optimizing incinerator operation on all shifts. By training and educating operations and maintenance (O&M) personnel about the goals and benefits of these optimization strategies and establishing performance measures and targets, MSDGC has been able to maintain these gains in efficiency over time and has had success in developing a culture in which employees actively seek to find ways to reduce power and fuel usage.In addition to the energy efficiency gains made by altering O&M strategies, MSDGC has realized other very significant efficiency gains after replacement or upgrade of treatment equipment assets through capital improvement projects (CIPs). Although CIPs are typically initiated to replace assets nearing the end of their useful life, or to upgrade treatment capabilities, or to do both, an energy efficiency component is often included. Energy efficiency is often a deciding factor when performing a life cycle analysis to optimize alternative selection in the CIP planning phase, and can affect the so-called triple bottom line that includes consideration of economic, social and environmental benefits. At the Mill Creek plant, air permit requirements drove the need to replace aging multiple hearth incinerators with cleaner burning fluidized bed incinerators. Project planning was initiated in 2005, and just last year the three new reactors were brought online. In addition to a roughly tenfold decrease in air emissions, the new incinerators will require significantly less fuel to operate because they are designed to burn autogenously. Together with other two CIP projects begun this year that include replacement of blowers, installation of automated dissolved oxygen (DO) control, and replacement of inefficient return activated sludge pumps, the Mill Creek plant is on track to reduce its annual carbon footprint for 2013 by approximately 13,000 tons of CO2 compared 2006 numbers. Taken together with the aforementioned operational changes, this translates to a 2,000,000 projected annual decrease in energy costs for this plant compared 2006.While the first two approaches mentioned deal with the optimization or replacement of existing plant processes, MSDGC also has significant opportunities at hand to construct new energy efficient, sustainable infrastructure as a part of the 3.25 billion combined sewer overflow reduction plan, known as Project Groundwork. One such opportunity within this project is the Lick Run sewer separation project that is currently in design. The goal of the first phase of Project Groundwork is to reduce combined sewer overflows by 2 billion gallons per year. While approximately 20% of this can be achieved by real time inline flow control, it was largely assumed that the remainder would have to be captured and treated by a new deep storage tunnel and high rate treatment facility. Capital and other O&M costs aside, the power used to pump and treat this volume amounts to approximately 2,500,000 kWh every year, equivalent to about 150,000 per year or about 1,900 tons carbon per year. If implemented as planned, this nontraditional project may very well eliminate the need for a deep tunnel solution and its energy intensive O&M costs, all while recreating a natural watercourse and revitalizing a blighted neighborhood.In this paper, the authors will expand upon all three of the approaches to increased sustainability listed in the paragraphs above. The projects mentioned will be discussed and analyzed in more detail and many other examples of completed and planned projects will be presented.