It’s meltdown time, and we can all picture it: a young child stomping and howling when it’s time to leave the playground. Chances are, however, that you’ve never extended that imagery to nutrients such as phosphorus and nitrogen in wastewater: This “nice list” nutrient fraction calmly agrees to requests to leave and moves smoothly into the targeted phase, for example, pockets of polyphosphate embedded in cells in enhanced biological phosphorus removal (EBPR), or particulate nitrogen drifting to the bottom of a sedimentation basin.

But like the child on the playground, some nutrients just don’t want to leave. These “naughty” nutrients are more commonly referred to as non-reactive forms, or soluble non-reactive phosphorus and dissolved organic nitrogen (Figure 1).

Non-reactive nutrients are dissolved in the wastewater, so physical separation by gravity or filtration has no impact on them. Likewise, they are considered less chemically reactive and are not well removed by most conventional wastewater-treatment techniques, such as biological uptake or chemical adsorption/ion exchange.

Figure 1. Forms of phosphorus and nitrogen in water, highlighting the less readily removable, soluble, non-reactive nutrient fractions. The speciation hierarchy is modified from Venkiteshwaran et al. (2018)1 and Mallick et al. (2022)2, and the phospholipid layer image is from Biga et al. (2019).3

In their respective meta-analyses, Kaushik Venkiteshwaran et al. (2018)1 and Synthia Mallick et al. (2022)2 demonstrated wide variability in the relative abundance of non-reactive nutrients in different water matrices. More than 10 percent of wastewater, livestock manure, and environmental waters contained at least 10 percent non-reactive phosphorus (approximately three quarters of livestock manures contained that level of non-reactive phosphorus).1 In the case of nitrogen, approximately half of surface waters had 50 percent of nitrogen present as dissolved organic nitrogen, and approximately 90 percent of wastewaters had at least 10 percent of nitrogen in the less reactive dissolved organic form.2

These forgotten fractions tend to be less widely recognized and emphasized than their sibling fractions but can also have substantial impact. Although non-reactive nutrients are much less bioavailable than their reactive counterparts, they still have potential to contribute to eutrophication.

For example, when readily assimilable dissolved inorganic nitrogen is scarce, direct assimilation of dissolved organic nitrogen can occur, with algae incorporating 28 percent to 61 percent of the organic nitrogen fraction within two weeks.4 Moreover, an array of transformation processes can convert non-reactive species to reactive forms over time. Chao Qin et al. (2015)4 showed that the majority of the soluble organic phosphorus in treated wastewater effluent was transformed to phosphorus species available for algal growth within two weeks.

Given that non-reactive nutrients have the potential to contribute to eutrophication in environmental waters, it is important to address their removal in wastewater. Unfortunately, conventional nutrient-removal strategies do little to mitigate non-reactive fractions.

For example, less than 40 percent removal of soluble non-reactive phosphorus was observed during secondary treatment at a full-scale EBPR facility, whereas more than 93 percent of soluble reactive and particulate phosphorus was removed.5 At the same facility, tertiary treatment using ferric chloride coagulation offered poor removal of soluble organic phosphorus.5

No existing treatment processes intentionally target the removal of dissolved organic nitrogen, although several adsorption-based techniques, as well as reverse osmosis, have been observed to remove dissolved organic nitrogen to some extent.2 Its recalcitrance to wastewater-treatment processes can lead to much higher relative abundance of non-reactive nutrients in treated effluent compared to in influent waters. This concentrating effect is particularly relevant for operations with ultralow nutrient discharge permits.

For example, U.S. Environmental Protection Agency data showed that nine states had wastewater discharge permits of less than 100 micrograms per liter of phosphorus for at least one publicly operated treatment works (whereas the majority of permits are one milligram per liter).6 In such scenarios, even very low levels of non-reactive phosphorus, which is difficult to remove, can impede regulatory compliance.

Beyond removal, nutrient recovery as part of a more holistic water resource recovery operation is of increasing interest (Figure 2). In particular, recovery of phosphorus from wastewater is appealing, as it can help facilitate a circular phosphorus economy that both reduces phosphorus pollution to environmental waters and reduces reliance on the non-renewable phosphorus reserves currently mined to support global food production.7

Additionally, while the nitrogen used in fertilizers is readily accessible from the atmosphere, the Haber-Bosch process used to extract atmospheric nitrogen is highly energy intensive, accounting for approximately 1 percent of global energy consumption.8 The high energy input needed to secure nitrogen bolsters efforts toward removal and recovery from wastewater. Importantly, the first step in recovery of either type of nutrient is removal, which relies on the presence of reactive and/or particulate species.

Figure 2. Illustration of the circular nutrient economy featuring removal of phosphorus and nitrogen from wastewater, recovery of a reusable nutrient fertilizer product, and return of the nutrients to the agricultural system followed by human use.

Just like children (and many adults), nutrients can rapidly swing from the naughty list to the nice list, as speciation in both engineered and environmental systems is dynamic. As such, processes that transform non-reactive phosphorus or nitrogen to the reactive forms facilitate improved nutrient removal. For example, chemical or biological (i.e., enzymatic) transformations hold promise for improving removal. Additional research is needed to further establish non-reactive nutrient transformation mechanisms and efficiency relative to process inputs such as energy and chemicals.1,2,9

References

  1. Venkiteshwaran, Kaushik, Patrick J. McNamara, and Brooke K. Mayer. “Meta-Analysis of Non-Reactive Phosphorus in Water, Wastewater, and Sludge and Strategies to Convert It for Enhanced Phosphorus Removal and Recovery,” Science of the Total Environment 644 (December 2018): 661-74. https://doi.org/10.1016/j.scitotenv.2018.06.369.
  2. Mallick, Synthia P., Zayed Mallick, and Brooke K. Mayer. “Meta-Analysis of the Prevalence of Dissolved Organic Nitrogen (DON) in Water and Wastewater and Review of DON Removal and Recovery Strategies,” Science of the Total Environment 828 (July 2022): 154476. https://doi.org/10.1016/j.scitotenv.2022.154476.
  3. Biga, Lindsay M., Staci Bronson, Sierra Dawson, Amy Harwell, Robin Hopkins, Joel Kaufmann, Mike LeMaster et al. “The Cell Membrane,” in Anatomy and Physiology. https://open.oregonstate.education/aandp/chapter/3-1-the-cell-membrane/ (accessed 2023-10-23).
  4. Qin, Chao, Haizhou Liu, Lei Liu, Scott Smith, David L. Sedlak, and April Z. Gu. “Bioavailability and Characterization of Dissolved Organic Nitrogen and Dissolved Organic Phosphorus in Wastewater Effluents,” Science of the Total Environment 511, (April 2015): 47-53. https://doi.org/10.1016/j.scitotenv.2014.11.005.
  5. Gu, A.Z., L. Liu, J.B. Neethling, H.D. Stensel, and S. Murthy. “Treatability and Fate of Various Phosphorus Fractions in Different Wastewater Treatment Processes,” Water Science & Technology 63, no. 4 (February 2011): 804-810. https://doi.org/10.2166/wst.2011.312.
  6. Hutchison, Justin M., Faten B. Hussein, and Brooke K. Mayer. “Evaluating Sustainable Development Pathways for Protein- and Peptide-Based Bioadsorbents for Phosphorus Recovery from Wastewater,” Environmental Science & Technology 57, no. 43 (October 2023): 16317-16326 https://doi.org/10.1021/acs.est.3c04016.
  7. Mayer, Brooke K., Lawrence A. Baker, Treavor H. Boyer, Pay Drechsel, Mac Gifford, Munir A. Hanjra, Prathap Parameswaran, et al. “Total Value of Phosphorus Recovery,” Environmental Science & Technology 50, no. 13 (July 2016): 6606-20. https://doi.org/10.1021/acs.est.6b01239.
  8. Marcal Capdevila-Cortada. “Electrifying the Haber-Bosch,” Nature Catalysis 2 (December 2019): 1055. https://doi.org/10.1038/s41929-019-0414-4.
  9. Brooke K. Mayer, Daniel Gerrity, Bruce E. Rittman, Daniel Reisinger, and Sherry Brandt-Williams. “Innovative Strategies to Achieve Low Total Phosphorus Concentrations in High Water Flows,” Critical Reviews in Environmental Science and Technology 43, no. 4 (February 2013): 409-41. https://doi.org/10.1080/10643389.2011.604262.

About the author

Dr. Brooke K. Mayer is an associate professor in the Department of Civil, Construction and Environmental Engineering as part of the Opus College of Engineering at Marquette University. She holds bachelors, masters and doctorate degrees in civil engineering with an emphasis in environmental engineering from Arizona State University. She is a registered Professional Engineer in the state of Arizona.

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