Wastewater treatment plants are a relatively untapped opportunity in the world of energy efficiency. Although these energy costs represent the largest controllable expense for municipalities that provide water or wastewater services, efficiency measures have not been adopted as quickly as they have been in other industries. However, the fact is that wastewater efficiency projects can have a significant impact on municipal governments given that these plants account for 30 to 40 percent of a city’s total energy consumption.
At the same time, there is much need for updated infrastructure at these plants. Many plants are more than twenty years old and use technology that is outdated and inefficient.
Faced with competing pressures to cut costs, modernize and meet growing regulatory requirements, municipalities often struggle to make all the improvements needed given today’s budget restraints. But the City of Riverbank, California found a cost-effective way to improve plant efficiencies and reduce site utility bills, while also providing much needed upgrades to aging equipment. This article takes a closer look at how the City achieved all its goals simultaneously.
The City of Riverbank is a small city with just over 23,000 residents. It operates its own wastewater treatment plant, which has an average flow of 1.67 million gallons per day (MGD). The wastewater treatment plant has primary treatment only through aerated lagoons and uses percolation ponds rather than discharging the effluent. The primary treatment is accomplished in four treatment ponds through the use of surface aerators to provide oxygen for the biologic process. Once the sewage is adequately treated, it is transferred to the percolation ponds through the opening of sluice gates or weir gates.
The plant was primarily operated manually with limited dissolved oxygen control in the treatment ponds and intermittent use of the transfer pumps. Lights were controlled through photocells and only ran at night. Electricity is the only utility on-site, and data was collected from Modesto Irrigation District for the plant electric meter, which had an annual utility spend of $367,137 for 3,458,190 kWh. In addition, the existing treatment ponds used constant speed surface aerators to provide oxygen for the biologic process. Typically, only two of the four treatment ponds were used at a time and not all of the surface aerators would be in operation at any given time. These aerators had low oxygen transfer efficiency, and thus required excessive horsepower to deliver the appropriate amount of oxygen to the biologic process.
The City of Riverbank hired Schneider Electric to evaluate its wastewater treatment plant for energy savings opportunities. This project consisted of replacing the surface aerators with submersible fine bubble diffusers and blowers with variable frequency drives (VFDs). It also included the installation of a SCADA control system to provide better control and visibility into the plant processes, particularly controlling the dissolved oxygen level in the treatment ponds.
The solution replaced twelve surface aerators with Parkson’s Biolac Treatment System, which uses moving aeration chains with suspended fine bubble diffusers, motorized and controlled air valves, blowers and an automated control system. Four 60 horsepower blowers with VFDs were installed to provide air to this system and are controlled to maintain a dissolved oxygen set point in the treatment ponds.
While this solution was chosen to maximize energy savings, there are also several maintenance benefits from changing system types. For example, subsurface aeration reduces the build-up of sludge in the treatment ponds. Currently, when the sludge build-up reaches a certain depth, the treatment ponds need to be taken out of service, dredged and then have the sludge hauled away to a dump site. This happens every twelve to fifteen years, but is very costly for the city. Additionally, the new system is modular and upgradeable, so if plant flows increase, the system can be added on to. Or, if new permit requirements are enforced, a tertiary treatment system can be added to the existing system to increase the levels of treatment.
In order to determine the energy and utility savings that would be seen from this project, a five-step process was used including:

  1. Benchmarking
  2. Baseline Utility Analysis
  3. Baseline Energy Analysis
  4. Post-Retrofit Energy Analysis
  5. Post-Retrofit Utility Analysis

In next month’s conclusion, we’ll illustrate how the partnership between the City of Riverbank and Schneider Electric successfully upgraded wastewater treatment plant and created a significantly reduced electric utility bill. ◆
About the Author:
Ben Johnson is energy engineering manager for Schneider Electric Energy and Sustainability Services. Schneider Electric offers integrated solutions across multiple market segments, including leadership positions in utilities and infrastructures, industries and machine manufacturers, non-residential buildings, data centers and networks, and the residential sector. For more information, visit www.schneider-electric.com.
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