In a typical mid-sized city, 30 to 40 percent of the electric bill for municipal purposes goes to water and wastewater treatment. And in a wastewater treatment plant, energy is typically the second largest expense, after salaries and benefits.
It’s no wonder that treatment plants are getting serious about saving energy — even if they already generate some of their own power and heat with digester gas.
In a wastewater treatment plant, there are many ways to cut energy costs. The only questions are where to look and where to place priorities. Well, looking is easier if you have a copy of the Water and Wastewater Energy Best Practice Guidebook, funded by Wisconsin’s Focus on Energy (a part of the state Department of Administration). I’m glad to see my home state do something this positive for the clean-water profession.
On the hunt
Clearly, energy conservation is on the radar screens of plant managers and operators. A number of state and regional WEF Member Associations have energy committees. Every month, Treatment Plant Operator publishes a Greening the Plant feature, and most of those stories are about saving energy.
But where exactly do you look for saving opportunities? Some are obvious. Aeration is the biggest power consumer in most treatment plants, and operators are cutting back there with innovations like putting variable-speed drives on blowers and using automated blower controls to optimize oxygen concentrations.
After that, what? This is where the guidebook can provide lots of clues. But first, what exactly constitutes good energy performance in a treatment plant? The Focus on Energy team answered that question by conducting on-site surveys at 85 treatment plants in the state and establishing baseline energy use and benchmark performance levels for three types of plants: activated sludge, oxidation ditch, and aerated lagoon.
The benchmark levels represent the performance of treatment plants already applying best practices. Tables in the booklet compare those best-practice plants against the average current energy consumption they observed. Other tables illustrate the potential savings plants can achieve if they apply best practices.
The book then outlines a six-step plan for undertaking energy improvement projects, and a five-step plan for ongoing energy management.
Save how much?
But how much can you actually save by putting best practices in place? “Many energy efficiency projects provide a high return on investment (as much as 100 percent or more) and are low risk,” the guidebook states. “When compared to other investment opportunities, these projects can be very attractive. Typically you can achieve 10 percent to 30 percent energy cost savings in the first year by implementing a systematic energy management program.”
Longer-term, energy savings generally range from 20 to 40 percent, but some aggressive facilities have cut consumption by nearly 75 percent, the book reports. How do they do it? Here are some of the best practices the guidebook outlines:
• Assess variations in facility flows and apply control systems to address minimum, average and peak design flows.
• Investigate renewable energy — not just biogas but wind for power generation and solar thermal energy for water and spacing heating.
• Cover basins and tanks for heat retention: grit removal, comminution, clarification, aeration, gravity thickening, aerobic digestion, disinfection.
• Consider fine-bubble aeration for activated sludge treatment, and consider combining that with dissolved oxygen monitoring and control, and a variable-capacity blower.
• Evaluate mixing options for aerobic and anaerobic digesters and choose the most effective and energy-efficient methods.
• Increase efficiency in UV disinfection by reducing the number of lights, bulb orientation, bulb type (pressure and intensity), turn-down ratio (bank size and lamp output variability) and dose-pacing control (system output automatically controls to the disinfection requirement).
• Reuse final effluent to replace potable water use for washdown of tanks and process-related applications.
• Install an accurate, real-time energy monitoring system that allows collection and analysis of 15-minute energy data for each treatment process and pump installation.
• Educate all treatment system personnel in the relationship between energy efficiency and facility operations.
• Develop strategies to manage on-peak electricity demand, such as shutting down non-essential equipment during on-peak hours.
• Survey existing motors for possible replacement with new, high-efficiency motors and specify the most energy-efficient motors on all new equipment. Include an emergency motor replacement program that specifies energy-efficient motors.
Cases in point
The book includes case studies on successful energy management programs and testimonial statements from plant managers about the benefits of putting best practices to work. If your plant is in Wisconsin, you may want to look up Wisconsin Focus on Energy at www.focusonenergy.com. I also found a digital copy of the guidebook on the Water Environment Research Foundation Web site at www.werf.org.
Here’s wishing you abundant energy savings. And by the way, if you want to offer us a story about what your plant is doing to save energy or create a greener, more sustainable world, drop me a note to editor@tpomag.com.
