New technologies in nutrient removal and recovery are among major advances that can improve efficiency and save energy in wastewater treatment.
Clean-water plants and their operating teams face increasingly strict regulations and ever-greater expectations for efficiency and resource recovery. Emerging technologies can help them meet both.
Ralph “Rusty” Schroedel, P.E., BCEE, a wastewater engineering manager with AECOM’s engineering office in Milwaukee, Wisconsin, has observed a variety of advanced nutrient removal processes either being researched or applied commercially that can help treatment plants fulfill their obligations.
The processes he describes deal with both the liquid and solids sides of treatment. Among his basic premises is that, “The future of wastewater treatment will be based on carbon management.” That means treatment must optimize energy use and production, water use and reuse, and carbon utilization through nutrient removal, energy production and beneficial use of biosolids. In an interview with Treatment Plant Operator, Schroedel talked about the changes he sees coming for the wastewater treatment industry.
TPO: In the most basic sense, what has changed as it relates to nutrient removal in wastewater treatment facilities?
Schroedel: In the past, nutrient removal wasn’t much of an issue. In the early 1970s in the Great Lakes region, a reduction in phosphorus to 1 mg/L was implemented and was fairly readily achievable. The current trend is toward much lower limits of 0.5 mg/L and substantially lower. Some facilities are looking at even less than 0.1 mg/L.
Nitrogen removal is becoming stricter as well. In the past, for ammonia, the only requirement was for nitrification — conversion to nitrate. Now denitrification or total nitrogen removal is often being required, with limits on total effluent nitrogen sometimes less than 5 mg/L. These changes have impacts to treatment plants in requiring more energy, more chemical demand and more space at the plant.
TPO: How do emerging nitrogen reduction methods compare to traditional methods?
Schroedel: In the fundamental way things have been done and still are done commonly, plants go through full nitrification, converting ammonia to nitrite, then to nitrate, and then denitrification, converting nitrate to nitrogen gas, which is released and removes the nitrogen from the process. Now, work has been done on what is called nitritation/denitritation, where you skip the step of converting fully to nitrate. Instead, the process converts ammonia to nitrite and from there directly to nitrogen gas. That process can yield a 25 percent reduction in oxygen demand, a 40 percent reduction in carbon demand, and a 40 percent reduction in biomass production.
TPO: Is this the latest in nitrogen removal methods?
Schroedel: No, the newest approach is deammonification, which uses anaerobic ammonium oxidation (ANAMMOX) autotrophic bacteria. In this process, a portion of the ammonia is converted straight to nitrogen gas and the balance is first converted to nitrite and then nitrogen gas. This provides an even greater reduction in oxygen demand — up to 60 percent. You also get a 50 percent reduction in demand for alkalinity and eliminate the demand for supplemental carbon. That carbon can then be directed to energy recovery.
This two-step process first uses the conventional organisms to nitrify about half of the ammonia to nitrite, and then the ANAMMOX organism oxidizes the rest of the ammonia, using the nitrite as its electron donor. The result is that almost all the nitrogen is released as nitrogen gas directly, without using any of the organic carbon in the wastewater.
TPO: Where do the ANAMMOX bacteria come from, and how are they introduced to and sustained in a process?
Schroedel: That is one of the major challenges. The ANAMMOX bacteria is a granular material and is red in color, so you can see it when you have a concentrated amount of it. But it is extremely slow-growing, and it is extremely important to have a process than can retain the organisms and allow an adequate mass of them to perform the desired treatment.
TPO: How is the deammonification process deployed in actual applications?
Schroedel: It has been applied primarily on sidestreams. The Water Environment and Reuse Foundation has several projects ongoing to evaluate sidestream applications and potential mainstream implementation of the process.
TPO: What would be an example of applying this process to a sidestream?
Schroedel: If you have an anaerobic digester, the process breaking down sludge solids creates a highly concentrated ammonia stream. Then, depending on the type of dewatering or solids separation used, you have a readily treatable and much smaller-flow sidestream that is high in ammonia and is very suitable for application of the ANAMMOX bacteria. By removing the nitrogen in the sidestream before it is returned to the headworks, you significantly reduce the mainstream nitrogen loading.
TPO: Turning to phosphorus, what is the state of research on biological removal?
Schroedel: Biological phosphorus removal does require a carbon source. So that’s in this mix when we’re thinking about carbon management. There are a wide range of biological removal processes, and they are being implemented at hundreds of facilities. But there is continuous research and refinement of the processes. The process basics are fairly mature and relatively well understood, but there are more sophisticated approaches to design and control. There are also a variety of phosphorus-accumulating organisms, and work is being done to discover how to provide the environment to optimally grow the most efficient of those organisms.
TPO: What else is happening in the area of phosphorus management?
Schroedel: Another process that is being applied in several places is the recovery of phosphorus as struvite. Here again, the removal is typically from a sidestream after anaerobic digestion that has a high concentration of phosphorus. The interesting thing about recovering phosphorus as struvite is that it has multiple benefits. You make a product that can be used as a fertilizer. You minimize the potential for struvite precipitation on piping, pumps and other equipment. And you produce phosphorus at a time when the readily mineable supply of that nutrient is projected to be exhausted somewhere in the next 20 to 100 years.
TPO: Why do you say that carbon management is so important as a basis for wastewater treatment decisions?
Schroedel: The primary objectives going forward will be efficiency and cost. The optimum use of carbon is going to drive the economics and efficiency of the treatment process, whether that involves making energy through biosolids and methane gas production, or providing the carbon necessary for biological treatment or nutrient removal processes.
TPO: To what extent can these new processes be implemented in existing facilities, versus being deployed in new facilities or upgrades?
Schroedel: In some cases the processes can be retrofitted very readily. Many plants have some excess aeration tank capacity, and those tanks can be subdivided and reoriented for the new processes. The sidestream treatment processes should be readily implementable because those are much smaller flows and so require smaller tankage.
TPO: What do progressive utilities need to do in light of these new technologies?
Schroedel: The regulations are never going to get less stringent, and as they get more stringent, it’s important for operators, engineers and academics to keep abreast of what’s going on and be willing to implement new technologies as necessary to meet their permit requirements. Keep an eye on the firms and research organizations and the technologies being developed, and plan for them.