Resource recovery is a major trend in wastewater treatment — to the task of cleaning water, facility teams are adding the capture of energy and nutrients.

One increasingly popular type of technology is the capture of phosphorus as struvite, which has market value as a fertilizer additive. One entry in that sector is Schwing Bioset, which offers a phosphate and nitrogen recovery technology under license from Belgium-based NuReSys.

The offering is unique in that it is not necessarily designed to extract revenue from nutrient capture, although that option is available. The process is designed to be tailored to each facility’s objectives in dealing with phosphate-related issues.

Besides nutrient recovery, the process has the benefits of preventing buildup of struvite scale in treatment equipment and improving dewaterability in biosolids. Wim Moerman, chief technology officer for NuReSys, and Chuck Wanstrom, director of new business development with Schwing Bioset, talked about the technology in an interview with Treatment Plant Operator.

TPO: What led to the creation of this technology?

Moerman: We received an inquiry from an industrial client who was having issues with struvite clogging up his facility’s pipework. This condition is common in waters that are high in phosphate after anaerobic treatment. We saw an opportunity to control the process so as to prevent the clogging and get a valuable side product that could be used as a fertilizer.

TPO: How does the NuReSys process differ from other phosphorus recovery technologies on the market?

Moerman: Our clear distinction is that our business model is not necessarily focused on capturing and selling a product. For us, the focus is first of all to control phosphate-related issues. Even if struvite had no fertilizer properties at all, we would still offer this technology, because other features related to struvite control are much bigger economic drivers.

TPO: What would some of those other economic drivers be?

Moerman: One is controlling the clogging of pipes, pumps and centrifuges. Another is improving the process of biological phosphorus removal, or bio-P. Facilities that operate with bio-P do a very good job of absorbing phosphate into the sludge, but a significant part of that is released again during digestion. The return of biosolids filtrate or centrate to the headworks creates an internal phosphorus loop that can diminish the capacity of the plant. But if you put a struvite unit in between, you can break that loop and get better performance on bio-P without having to expand the treatment plant.

TPO: Can a customer who installs a NuReSys process without nutrient recovery capability add that process later?

Moerman: Yes. Our system is quite modular, so if at any time they want to start nutrient recovery, they can do that with the simple add-on of a cyclone and grit washer.

TPO: How does this NuReSys process work?

Moerman: We have a two-stage process. First digestate or centrate is pumped to an air stripper where carbon dioxide is removed to raise the pH to the desired level. Overflow from that stage proceeds to a crystallizer where a reagent, commonly magnesium chloride, is added according to the phosphate content of the wastewater stream. In this continuously stirred tank reactor, the magnesium reacts with orthophosphate ions and nitrogen, allowing struvite crystals to form and grow. Struvite prills are removed from the crystallizer bottom.

TPO: Are there specific advantages to a two-stage rather than single-stage process?

Wanstrom: The advantage of using two tanks is first of all that we can have more stable process control because we reduce the number of variables to monitor in each tank. Second, the equipment size is smaller. In the smaller first tank where we control pH, we have lower air and pressure requirements, so we can reduce the power demand from the blowers.

TPO: At what points in the wastewater treatment process can this technology be placed?

Moerman: We can apply our process on water as centrate or filtrate, or on digested sludge. Because we have a stirred crystallizer tank, we can simply adapt the tank size and stirring capacity to work on the water phase or a stickier digestate. On the digestate side, we also offer a hybrid solution. By locating the air stripper in the digestate line, we can manipulate the pH to form microcrystals of struvite with the magnesium that is naturally present. This controlled precipitation prevents accumulation in areas that would cause downtime and maintenance costs.

TPO: What impact does the NuReSys process have on biosolids?

Moerman: In digestate applications, there is potential to increase the dry solids content of the biosolids cake, which can be a major financial advantage. Our experience is that if you see, for example, a 3% gain in dry solids content, half is from the improved dewaterability of the material and half is because you’ve converted soluble compounds into solid struvite. 

TPO: In nutrient recovery applications, how do you assess the value of the struvite?

Moerman: Although struvite has excellent fertilizer properties, it isn’t a very good fertilizer by itself, in that it contains equal amounts of nitrogen, phosphorus and magnesium. These are all essential plant nutrients, but they need to be in the right proportion to ensure good growth of crops. We believe struvite should be blended with other fertilizers to get the optimum product mix.

TPO: What types of treatment plants are the best candidates for this technology?

Moerman: If you have a municipal plant operating in bio-P mode, that is where struvite technology is practically a no-brainer in terms of the benefits you can get from it. If the phosphate concentration is high enough, it’s beneficial to transform it into struvite, which has an added value.

TPO: How many NuReSys systems are in operation now?

Moerman: In Europe we have eight larger systems, and we are finalizing a ninth large one in Germany.

Wanstrom: We have our first order in the U.S. at the Tres Rios Water Reclamation Facility in Tucson, Arizona, and we’re now going through engineering submittals.

TPO: What is your general approach when responding to inquiries from prospects?

Wanstrom: Let’s look at each individual plant. What problems are they having? How can we solve them? With the flexibility of our design, we can configure a system in a way that resolves the phosphate or struvite issue. We sit down and define a problem they want to solve and then adapt the technology to do that.