Nutrient recovery is a growing component of clean-water plant operations. Most often the target is phosphorus.

The various motivations to isolate phosphorus include combating struvite accumulation in piping and process equipment, meeting stringent effluent phosphorus permit limits, generating revenue from sale of a marketable fertilizer and simply making operations more sustainable.

CalPrex technology, which Centrisys/CNP has licensed from Nutrient Recovery & Upcycling (or NRU), is the newest arrival among phosphorus recovery processes. The technology captures phosphorus in the form of brushite (dicalcium phosphate dehydrate), which is then dried and pelletized.

The company says the process can recover 45% or more of the total phosphorus from sludge going to the digesters. It removes soluble phosphorus by adding calcium hydroxide to centrate from waste activated and primary sludges in a special reactor before anaerobic digestion. Menachem Tabanpour, vice president of business development, talked about the process in an interview with Treatment Plant Operator.

TPO: What is the history of this technology’s development?

Tabanpour: I started working on a struvite project in 2002 as a high school student during a summer internship at the University of Wisconsin-Madison in Professor Phillip Barak’s lab. The project evolved and I continued while pursuing my undergraduate degree. We began working with the Nine Springs Wastewater Treatment Plant in Madison and sampled different locations in the process to identify the most feasible place to remove phosphorus. We found that in an acid digester tank, basically a 1.5-day solids retention time digester where the pH goes down, the phosphorus becomes very soluble and extractable. So we abandoned the struvite research and launched in a new direction with brushite recovery in 2007.

TPO: How did you move this idea toward a commercially viable technology?

Tabanpour: We launched NRU in 2011 with the aim to commercialize a calcium phosphate predigester phosphorus recovery process. We built and ran our first pilot in 2014-15 in Woodridge, Illinois. We later licensed the technology to CNP exclusively, and they picked up the development work. A second pilot was conducted in Woodridge in 2017. In 2018, we worked with the Water Research Foundation to demonstrate CalPrex as a fully scalable technology. That successful project was conducted at Nine Springs with support from the Milwaukee Metropolitan Sewerage District, Metro Wastewater Reclamation District (Denver) and Massachusetts Water Resources Authority.

TPO: Fundamentally, how does CalPrex differ from other phosphorus-recovery processes?

Tabanpour: One key difference is that instead of producing struvite, we produce brushite, a calcium phosphate mineral versus a magnesium-ammonia phosphate mineral. Also, our technology can typically recover two to three times as much phosphorus as other technologies that have been commercialized. Usually the pinch point in phosphorus-removal efficiency is how much phosphorus you can make soluble. Capturing soluble phosphorus is the easy part. The problem is how much of the total phosphorus in the sludge can get into soluble form.

TPO: How does the process maximize the amount of soluble phosphorus?

Tabanpour: Typically phosphorus that comes in wastewater gets sequestered in the sludge. About 10% to 20% ends up in the primary solids, and most of the balance is in the waste activated sludge. Maybe 10% gets sent out in the effluent. The sludge, once thickened, is basically a phosphorus sink: It contains mainly cellular or particulate-bound phosphorus. To do recovery, you need that phosphorus to become soluble. That’s where a fermentation tank comes in. In that tank at lower pH, more phosphorus gets released and stays in a soluble form.

After centrifuge dewatering, you have a centrate that has high-soluble phosphorus, on the order of 500 parts per million, and low TSS. That’s the perfect solution for doing the reaction.

TPO: What happens in the CalPrex reactor vessel?

Tabanpour: We add calcium hydroxide (hydrated lime), which raises the pH typically from 5.5 to 6.5. That creates conditions where the calcium phosphate mineral forms. We settle it out in a lamella clarifier and then dewater it with a centrifuge and dry it into fertilizer. The centrate, which now has low phosphorus, is sent back into the treatment facility.

TPO: Are there any ancillary benefits to the process?

Tabanpour: It helps prevent struvite buildup in plant piping and digesters. In addition, the centrate from the fermentation tank has around 5,000 ppm volatile fatty acids. That could serve as a carbon source for facilities that are carbon-limited and are buying methanol or sugar to keep their biological process functioning.

TPO: How would you quantify the phosphorus removal from the process?

Tabanpour: We typically have 65% to 75% phosphorus release from fermentation. We divert 80% toward the centrate, react 90% of that and settle 95% of the brushite mineral particulate. Then we capture 95% of the particulate in dewatering.

TPO: What is the basic nature of the brushite mineral?

Tabanpour: It is similar to struvite in that it is a very good slow-release fertilizer. In its pure form it’s 18% phosphorus by weight versus 12.6% for struvite, so it’s a higher-analysis mineral that is a good replacement for other phosphorus fertilizers on the market. While struvite forms crystals that grow on themselves and can create large crystals, or pearls, brushite does not. It forms 150- to 300-micron particles. When we dewater, it makes a paste that looks like thick concrete. When dry, it is an off-white or gray powder, from which we make pellets.

TPO: What kinds of treatment plants would be the best served by this process?

Tabanpour: There are four major types. For a facility using some type of predigestion hydrolysis process, it couples really well. Another category includes facilities that need most of the phosphorus removed before the digester, either because they have a really bad struvite problem or because they are regulated for phosphorus in their biosolids. Another category is facilities with post-aerobic digesters, because the PAD destroys alkalinity and drives down the pH, making the phosphorus soluble and available to recover. Another category includes plants that want to make more fertilizer product.

TPO: Once the brushite material is produced, how is it taken to market?

Tabanpour: NRU will handle that. We launched a fertilizer brand called Steady State in fall 2019. The fertilizer being marketed will be struvite from our AirPrex installations and brushite from CalPrex installations, as well as various blends of nitrogen, phosphorus and potassium, according to customer needs.