October/November 2025 - Sally Brown Research Library & Commentary

Provided for consideration to MABA members by
Sally Brown, PhD., University of Washington

Phosphorus +/-

So, why don’t we recover more P? Because the cost of the mineral form is much less than it would cost for us to recover it. Sales of struvite cover about a third of the cost of making the struvite. But the dollar value isn’t a true reflection of what you can get when you recover P. The article offers a review of P in wastewater including ways to maximize P removal in association with anaerobic digestion. It also places P recovery into the context of resource recovery (water, energy, nutrients and metals), something the authors see as essential to the mission of treating wastewater. But just because we can doesn’t mean that we currently are:

Technological advances must be integrated with improved business models, systems level understanding, policy support, and increased public awareness and acceptance. Implementation of P recycling and reuse requires an approach that involves total value recovery at local, national, and international scales. 

In this, the US falls way short of what is possible, reusing less than 10% of the wastewater and importing a lot of mineral P. So, how do we get the needle to move? The authors talk about the operational cost savings of struvite, the use of Life Cycle Assessment to get a better handle on true costs/ benefits and the need for regulatory factors to drive change. 

Article #3 Phosphorus Recovery in Municipal Wastewater and Socioeconomic Impacts in Canada and the United States takes this discussion and puts it into a Canadian/ US context. They start by noting enhanced P recovery rules to take effect in Germany and Switzerland with reduced effluent concentrations (0.5 mg L) set in Europe as a whole. The authors note that P recovery can be costly and that the cost will be higher for smaller plants. Here is how it breaks down:

They also consider how much it costs to release P- here considering the environmental impact using a value from the state of WI ($74.5 per kg P released). Recovering P from aqueous digestate costs between $1-35.2 per kg P recovered. If you spread that cost across the country rather than putting it directly to the ratepayers for a utility it comes to $0.14-$4.8 per kg P recovered per yr. It is much more expensive to recover P from solids ($4.7-$20.8). This is much less than the environmental cost of excess P release. The article separates recovery costs by region of each country based on size of plants and availability of anaerobic digestion and the household affordability index (HAI).
Are you convinced that P recovery is a good thing yet? 
 
The last two articles in the library are new publications on this general topic. Article #4 Recycled Phosphorus Bioamendments from Wastewater Impact Rhizomicrobiome and Benefit Crop Growth: Sustainability Implications at Water Food Nexus focuses on plant and microbial response to recycled P. It is a bit ironic here the authors are talking about response to biosolids from enhanced P removal from manure lagoons. Biological P removal is a wastewater technology developed in S Africa that makes readily plant available P. Note that the first article in the library was talking about the horrors of excess P in biosolids. Put a cow in front of it instead of a person and it becomes something to rave about. Here the authors note increased populations of polyphosphate accumulating organisms (PAOs) in the rhizospheres of plants and the associated benefits for P cycling and plant growth. You can see the change in population of PAOs in the fertilizer, biosolids with extra P and manure compost below: 

Article #5 BioWin Modeling Comparisons of Municipal Wastewater Phosphorus Recovery between Brushite and Struvite - talks about integrating a new process either with an existing struvite recovery system or on its own. The CalPrex reactor takes an acidogenic digest and treats it with lime to precipitate brushite (a form of calcium phosphate). The work described here was done at the Madison, WI treatment plant that features enhanced biological P removal (like I said about article #4). It also has mesophilic digesters that treat primary and waste activated sludge. The plant has a history of nuisance struvite precipitation within and past the digester. It has a Ostara Pearl reactor in place that treats liquid from the WAS, clarifiers and the biosolids dewatering. Here is a version of the treatment train that includes the flows of P:

If the results described are reflective of what could happen, they would reflect an operator’s and environmentalist's dream come true. Higher P capture via precipitates that can store and be transported easily, lower biosolids volume with reduced P concentrations making it easier to dewater and less of a concern to land apply for those that worry about excess P. Did I mention less nuisance struvite precipitation in the digester?

It would appear that there are ways to capture the P and save the world. I say that that would be OK by me. 

Sally Brown is a Research Associate Professor at the University of Washington, and she is also a columnist and editorial board member for BioCycle magazine.  

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