Target Zero

Wastewater contains more energy per gallon than is needed to treat it. The question is how to put more of it to work in the treatment plant.

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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.

TPO: Typically, the energy plants generate from digester methane falls pretty far short of the total plant energy demand. So how do we get to net zero?

Roehl: I talk about this as the technology gap. The size of the gap varies depending on the treatment plant, what its effluent treatment objectives are, and the type of processes used. Plants today generally can expect to cover maybe 25 to 30 percent of their electric power needs through the installation of an on-site cogeneration system. So there’s a big gap from there to meeting all the plant’s power needs.

Our company has been working on that problem for the past two years. We have a research and development facility in Singapore where we’re working on technologies for all facets of the water and wastewater industry, and one project we’re working on is focused on closing that technology gap, so we can get very close to or actually achieve the target of net zero energy.

TPO: Does closing of the gap primarily involve achieving greater efficiency in existing operations?

Roehl: It involves a few different things. The U.S. EPA has gathered data to identify the biggest power consumers in a treatment plant. Typically about 50 to 60 percent of a treatment plant’s energy demand is for aeration. Next biggest are pumping and the heating of anaerobic digesters.

Clearly, we can have the biggest impact by reducing the amount of energy used for aeration. Our research is focused on reconfiguring the process so we can rely much less on aerobic treatment and have most of the treatment occur anaerobically. When you do that, you shift the load from the energy-intensive process of aeration to the biogas-generating process of anaerobic digestion.

So you convert a much greater percentage of the organic matter in wastewater to methane rather than carbon dioxide, which is what you produce in the aeration basins. You essentially scale back the amount of aeration and increase production of methane, so that in the end you have a new balance where you come pretty close to producing enough biogas to power aeration and all the other processes on the site.

TPO: Does this involve changing the nature of the entire treatment process?

Roehl: Yes and no. We’re looking at using much of the existing tankage and most of the processes already on site. The process wouldn’t look that different. We’re seeking ways to concentrate the waste stream so more of it can be sent to the anaerobic digesters.

TPO: What are some of the ways of doing that?

Roehl: That’s really the heart of the research project, and it would be premature to talk about the details, but it’s essentially a waste concentration challenge. We don’t have an actual solution yet, but we have pretty good evidence that we’re on the right track. It’s an ongoing project, and we expect to have an answer in 12 to 18 months.

TPO: What would be the impact of this change on a typical activated sludge wastewater treatment plant?

Roehl: You would end up with much smaller aeration basins and perhaps slightly larger anaerobic digesters. One of our challenges is to see whether we can get by with the same-size anaerobic digesters. Essentially we would just have a more concentrated stream going into the digesters than we have today.

TPO: In general terms, what is involved in concentrating the waste stream? Is it some sort of mechanical or filtration process?

Roehl: We’re trying to do it biologically — basically manipulating the microorganisms so that they do it for us. If you can get the bugs to help you concentrate the waste stream without actually treating it, you can look at moving that concentrated waste stream over to the anaerobic system. In broad terms, that’s what we’re trying to do.

TPO: Would this have any impact on plant throughput? Can you get the same gallons per day through the plant as before?

Roehl: We don’t see this affecting plant capacity. We should be able to get the same flow out of the plant.

TPO: Would you characterize this as somewhat futuristic — not something someone would be able to do in one or two years, but maybe longer down the road?

Roehl: It’s definitely in the future but it’s more near-term. I’d say realistically two years out we’d be able to bring this to market. It really comes down to some pretty basic ideas of improving energy recovery, reducing reliance on aerobic processes, and improving some process efficiencies throughout the plant, so we’re not wasting energy.

TPO: Turning to process efficiency, what are some of the ways to make substantial improvements?

Roehl: One technology that really helps reduce energy requirements is aerated-anoxic treatment. It involves providing a certain amount of treatment under aeration but without a positive dissolved oxygen (DO) presence.

That allows you to have simultaneous nitrification/denitrification. It also gives you a little better driving force for getting oxygen into the system, since you don’t have any residual DO present. You use less energy because you’re not putting excess oxygen into the system. This technology has been in use for the past 20 or 25 years.

Another improvement in aeration is smart biological nutrient removal control. It basically consists of tying your aeration system to DO and oxidation-reduction potential (ORP) sensors. In that way you’re tuning the process to adjust itself with varying flows and loads so that, for example, you’re not burning excess energy at night creating high DO levels when you don’t need to. Again, this is nothing new, but a lot of plants could benefit from it.

TPO: What about efficiencies in other areas of the treatment train?

Roehl: Depending on the treatment plant, things can be done with enhanced primary treatment, essentially doing chemical additions in the primary clarifiers to help reduce TSS and BOD going into the aeration basins. That allows you to move more material over to the digesters, and so use less power for aeration. The most common ways of doing this are through addition of alum or ferric chloride to precipitate more constituents out of the waste stream.

TPO: What about making plant equipment itself more efficient?

Roehl: Blowers available today in many cases can improve efficiency by 20 percent or more. If your plant is using coarse-bubble diffusers, you can certainly improve efficiency with state-of-the-art fine-bubble diffusers.

Then, looking around the plant, you can install variable-frequency drives on pumps and motors, so you can avoid using more energy than is needed at any given time. And finally it can be worth considering premium-efficiency motors, especially for the high-horsepower motors on site. Depending on the plant, you might see a 5 to 10 percent reduction in total plant power usage just by deploying variable-frequency drives and premium-efficiency motors.

When you combine all those things, the total impact varies, because some plants have better practices already in place than others. But for a fairly average treatment plant, you might be able to reduce energy usage by 20 to 30 percent just by making those kinds of process im-provements and upgrading equipment. And you might save even more.

TPO: What role do renewable energy sources other than biogas play in treatment plant energy efficiency and your net-zero-energy concept?

Roehl: I’m starting to hear about more treatment plants putting up wind turbines on site or doing solar installations. I’ve been exposed to several such projects around the country. But that’s a different scenario because those things really go beyond the scope of the treatment plant itself. Wind turbines or solar panels aren’t required to sustain treatment or to meet regulations. That’s more of a community initiative — but treatment plant sites can be good places to do those things because they have space available.

TPO: How would you characterize the state of development on the net-zero-energy treatment scheme?

Roehl: We started by doing some modeling — if we did certain things with the process, would net zero be achievable? The answer was yes. So we did bench-scale testing with an activated sludge process to confirm our initial theories. That looked promising, and so we moved to a pilot-scale test.

Now we’re doing a demonstration-scale project to evaluate the concept thoroughly. That system is to begin operation in the first half of 2011.



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