Treating Biosolids with Water Treatment Residuals to Reduce Phosphorus Runoff

Treating Biosolids with Water Treatment Residuals to Reduce Phosphorus Runoff


Water treatment residuals (WTRs) are added to biosolids to reduce phosphorus runoff.


Biosolids, formerly known as sewage sludges, are waste products from wastewater treatment plants that have tremendous potential for agricultural use due to their high nutrient content. However, biosolids can have high concentrations of water soluble phosphorus which can cause non-point source phosphorus pollution. Research has shown that the amount of phosphorus runoff from biosolids is directly related to their soluble phosphorus content. Adding water treatment residuals to biosolids greatly reduces the soluble phosphorus levels, which results in much less phosphorus runoff and leaching.

How Does This Practice Work?

Water treatment residuals should be blended with biosolids at a rate equivalent to 10-30% by weight (WTR/biosolids). Biosolids are typically processed into what is referred to as “cake” at municipal wastewater treatment plants. The cake is the material remaining after water removal through centrifugation or belt pressing and usually contains around 80% moisture. Water treatment residuals are the waste products from drinking water treatment plants following the addition of metal salts, such as aluminum sulfate or ferric sulfate, to flocculate impurities, like organic matter and sediment, present in surface waters. After this flocculation process, the WTRs will be primarily composed of aluminum or iron hydroxides, which have a very high capacity for phosphorus adsorption.

Biosolids cake being collected at a wastewater treatment plant.

When WTRs are mixed with biosolids the soluble phosphorus levels in biosolids are reduced, resulting in less phosphorus runoff or leaching following land application. The blended mixture can be applied right after mixing or incubated for several weeks prior to application. The aluminum and iron oxides and hydroxides present in the WTRs adsorb soluble phosphorus and reduce its release during runoff events. The incubation period allows for greater interaction and adsorption to take place, increasing the effectiveness of the overall blend.

Where This Practice Applies and Its Limitations

This practice applies to areas of pasture or row crop production where biosolids and WTRs applications are allowed, where both are reasonably accessible, and where water treatment plants are using aluminum or iron salts to create their WTRs. There are no known limitations of this practice at these blending ratios.


The use of aluminum-flocculated WTRs to reduce soluble phosphorus is a highly effective way to reduce soluble phosphorus in runoff when biosolids are land-applied. When blended and applied the same day, a 20% blend reduced soluble phosphorus loads from runoff on small plots by 48% as compared with runoff from untreated biosolids. When mixed and allowed to incubate for 3 weeks prior to application, a 15% blend reduced soluble phosphorus loads by as much as 78%.

Blend of 15% WTR/biosolids prior to application.

Cost of Implementing the Practice

The costs associated with using these blends include those associated with transporting and blending the two substrates.

Operation and Maintenance

Biosolids and water treatment residuals can be applied anytime that rainfall is unlikely to occur.

Land application of a mixture of water treatment residuals and biosolids.


Agyin-Birikorang, S. G.A. O’Connor, L.W. Jacobs, K.C. Makris, and S.R. Brinton. 2007. Long-term phosphorus immobilization by a drinking water treatment residual. J. Environ. Qual. 36:316-323.

Bayley, R.M., J.A. Ippolito, M.E. Stromberger, K.A. Barbarick, and M.W. Paschke. 2008. Water treatment residuals and biosolids co-application affect semiarid rangeland phosphorus cycling Soil Sci. Soc. Am. J. 72:711-719.

C.G. Cogger, A.I. Bary, S.C. Fransen, and D.M. Sullivan. 2001. Seven years of biosolids versus inorganic nitrogen applications to tall fescue. J. Environ. Qual. 30:2188-2194.

Dayton, E.A. and N.T. Basta. 2005. A method for determining the phosphorus sorption capacity and amorphous aluminum of aluminum-based drinking water treatment residuals. J. Environ. Qual. 34:1112-1118.

de Koff, J.P., P.A. Moore, Jr., R.D. Williams, R. Young, and P.J.A. Kleinman. 2013. Utilizing water treatment residuals to reduce phosphorus runoff from biosolids. J. Environ. Sci. Eng. 2:405-417.

Elliott, H.A., G.A. O’Connor, P. Lu, and S. Brinton. 2002. Influence of water treatment residuals on phosphorus solubility and leaching. J. Environ. Qual. 31:1362-1369.

Elliott, H.A., G.A. O’Connor, and S. Brinton. 2002. Phosphorus leaching from biosolids-amended sandy soils. J. Environ. Qual. 31:681-689.

Ippolito, J.A., K.A. Barbarick, M.E. Stromberger, M.W. Paschke, and R.B. Brobst. 2009. Water treatment residuals and biosolids long-term co-applications effects to semi-arid grassland soils and vegetation. Soil Sci. Soc. Am. J. 73:1880-1889.

Madison, R.K., L.R. McDowell, G.A. O’Connor, N.S. Wilkinson, P.A. Davis, A.T. Adesogan, T.L. Felix, and M. Brennan. 2009. Effects of aluminum from water-treatment-residual applications to pastures on mineral status of grazing cattle and mineral concentrations of forages.

Makris, K. C., W. G. Harris, G. A. O’Connor, and T. A. Obreza. 2005. Long-Term Phosphorus Effects on Evolving Physicochemical Properties of Iron and Aluminum Hydroxides. J. Colloid Interface Sci. 287:552–560.

O’Connor, G.A., H.A. Elliott, N.T. Basta, R.K. Bastian, G.M. Pierzynski, R.C. Sims, and J.E. Smith, Jr. 2005. Sustainable land application: An overview. J. Environ. Qual. 34:7-17.

U.S. EPA. 2023. Biosolids. Accessed Jan. 19, 2023.

U.S. EPA. 2011. Drinking Water Treatment Plant Residuals Management Technical Report. Summary of residuals generation, treatment, and disposal at large community water systems. US EPA (U.S. Environmental Protection Agency)

EPA 820-R-11-003. Accessed Jan. 19, 2023.

For Further Information

Contact your local soil and water conservation district, USDA-NRCS or Cooperative Extension Service office. To find your local USDA Service Center, visit

Current Author
Philip A. Moore, Jr.
Editing and Design
Deanna Osmond
NC State University
Forbes Walker
University of Tennessee

Moore, P.A., Jr. 2023 Treating Biosolids with Water Treatment Residuals (WTRs) for Surface-applications in Pastures. SERA17 Phosphorus Conservation Practices Fact Sheets.

Funding for layout provided by USDA-NRCS Grant 69-3A75-17-45
Published: Feb 25, 2023