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Energy Efficiency in Wastewater Treatment Plants
By Robert Farmer
The pressures to use our energy resources efficiently have never been greater. Energy engineers in all sectors of the economy are striving to keep electricity costs, and therefore net purchases, to a minimum by increasing operating efficiencies and reducing overall energy consumption in their installations. But, at the end of it all, we’re still growing. More people, more businesses, more energy demands, the world over.
Nowhere are these demand-side pressures felt more than in the utility industries. To meet growing demand, electric utilities have focused their efforts not only on supply-side efficiencies but increasingly on demand-side solutions. By having customers operate more efficiently, existing but under-utilized capacity has been freed up to meet new demand. In the short term this allows the utilities to maintain competitive prices for their product by deferring the addition of expensive new capacity.
This theme of optimizing existing infrastructure by improving energy dynamics and system efficiencies to handle increasing demand has a common application in another type of utility.
Take, for example, some of the members of the Water Environment Federation, which just wrapped up its 68th annual conference and exposition in Miami Beach last week. WEFTEC is the “Super Bowl” for the water and wastewater industry; the conference had over 16,000 attendees and 600 exhibits.
Municipal wastewater treatment plant (WWTP) processes are energy intensive. Electricity represents 20% to 30% of operating expenses in the conventional activated sludge process, the most common method of municipal wastewater treatment in North America.
Typically, electricity is recorded at a single point at the WWTP reflecting the total used by the entire facility. Unfortunately this reveals nothing about individual processes, and those such as aeration and major pumping operations can be very energy intensive. The data required to assess energy savings which could be achieved with modifications to individual unit processes is hidden within the total plant measurement.
In the mid 1980’s one of Canada’s major environmental research laboratories, the Wastewater Technology Centre (WTC), developed an in-depth wastewater treatment plant performance assessment methodology to tackle this problem.
Referred to as the process audit, this involves temporarily installing sophisticated on-line monitoring instruments to collect real-time data to assess process operating conditions. Oxygen transfer limitations, biomass imbalances, and hydraulic disturbances are all part of this highly specialized process audit. Together with historical data and off-line sampling results, the information is interpreted to provide specific details about plant operation, design and capacity limitations, and identify optimization and energy conservation initiatives. To date, the procedure has been used at numerous treatment facilities ranging in size from 4 MGD to 250 MGD.
WTC’s chief operating officer John Neate said that application of audit technology to the municipal sector has been necessitated by pressure to expand and upgrade existing plants in North America to handle hydraulic growth and to meet increasingly stringent environmental regulations concerning wastestreams.
The continually escalating costs of these capital works make it imperative for municipalities to maximize the operation of existing facilities. Often, optimization can eliminate or reduce the cost of expansion through the incorporation of relatively low-cost modifications, the use of on-line instrumentation and control techniques, and the elimination of process and hydraulic bottlenecks.
The WTC has demonstrated that not only can operating costs be significantly reduced, but increased reliability in the control of the process can lead to better overall performance and maintenance of effluent quality. As an example, Neate said that aeration energy savings in the order of 15% to 50% are achievable with automated versus manual control.
And there you have it — EMS for WWTP. •
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