Energy efficiency has become a big issue in wastewater treatment. There are two basic ways to deal with energy in a treatment plant: Use less and make more.

Using less means deploying more energy-efficient equipment and processes. Making more primarily means increasing production of digester methane. Today, researchers are pursuing the ideal of the net-zero-energy wastewater treatment facility.

Is that possible? In theory, yes — because the energy content of wastewater (about 8 kWh for 1,000 gallons) far exceeds the energy required for treatment (0.8 to 1.6 kWh for 1,000 gallons). But is it also feasible in practice? Some experts think so.

Among them is Marc Roehl, P.E., product manager, biosolids technology, with Siemens Water Technologies. Roehl and colleagues are working on technologies that, along with current best practices in energy management, could make net-zero-energy treatment a commercial reality within a few years.

Roehl holds a master’s degree in environmental engineering from the University of Iowa and has been in the wastewater industry for 18 years. He spent the first half of that career with MSA Professional Services, consulting with municipal treatment plants in and around Wisconsin. In nine years with Siemens, he has dealt with anaerobic digestion, solids minimization, biological treatment and clarification.

Roehl talked about energy efficiency and his company’s research and development project on net zero energy in an interview with Treatment Plant Operator.

TPO: What is your impression of the state of the industry today in terms of how energy efficient treatment plants are?

Roehl: There’s an incredible opportunity to improve efficiency and reduce power usage in treatment plants. That’s partly because many plants are older and due for upgrade. Until recently, we haven’t had really good ways to measure efficiency, and it didn’t become a focal point in plant design or operation until the past five or ten years.

TPO: Do you see energy efficiency becoming a major priority?

Roehl: Most definitely. We’re seeing that in the technologies plant designers and operators are evaluating when looking at upgrades, and in the technologies being installed in retrofits. There’s a general mindset today that if we’re going to improve the plant, let’s make sure we do it with the best practices available.

TPO: What’s mainly driving this trend? Cost savings? Sustainability? Concerns about climate change?

Roehl: Cost is the primary driver, but I think the others are also of interest, depending on the community. We’re seeing more and more communities say that in the interest of good citizenship, they’re going to choose investments that are as sustainable or as green as possible. But at the end of the day, there’s money to be saved, and that’s really the biggest driver.

TPO: What exactly is a net-zero-energy facility?

Roehl: Basically it’s a facility that can produce as much energy on site as is needed for plant operations. That’s not to say the facility wouldn’t need a utility connection, but the general concept is that the utility bill should be close to zero.

TPO: Why are treatment plants good candidates for net zero energy?

Roehl: They have a waste stream that can be readily converted into biogas that can be burned to generate electricity and heat. That’s nothing new — it’s been done for decades through anaerobic digestion. Still, the fact that you have this waste stream and you have a digester on site gives you potential to set up a renewable energy system on site to handle the plant’s power demands.

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