Water Online

SEP 2016

Water Innovations gives Water and Wastewater Engineers and end-users a venue to find project solutions and source valuable product information. We aim to educate the engineering and operations community on important issues and trends.

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By Ben Johnson W astewater 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 Many plants are more than 20 years old and use technology that is outdated and inefficient.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, CA, 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. Energy Efficiency Overhaul Riverbank is a small city with just over 23,000 residents. It operates its own WWTP, which has an average flow of 1.67 MGD, and conducts primary treatment only through aerated lagoons, using 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 ran only at night. Electricity is the only utility on-site, and data was collected from Modesto Irrigation District for the plant's electric meter, which showed 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 hired Schneider Electric to evaluate its WWTP for energy savings opportunities. The 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 (supervisory control and data acquisition) control system to provide better control and visibility into the plant's processes, particularly controlling the dissolved oxygen level in the treatment ponds. The solution replaced 12 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-hp 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 buildup of sludge in the treatment ponds. Currently, when the sludge 26 wateronline.com n Water Innovations ESCO Analysis: 5 Steps To WWTP Energy Efficiency How do energy savings performance contracts work? An energy services company (ESCO) breaks down the process and shares recent results from a participating wastewater treatment plant (WWTP). Subsurface aeration system with suspended fine bubble diffusers

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