Phosphorus Removal from Dairy and Swine Manure

Definition

Manure phosphorus (P) removal systems reduce the amount of P applied to cropland by removing soluble P from the manure stream.

Purpose

Dairy and swine manure contain appreciable P concentrations with a low nitrogen (N) to P ratio that can lead to soil P accumulation. Reducing the amount of P in manure can result in a concentrated solid with a more favorable N:P ratio for crops, helping to reduce soil P accumulation where manure is repeatedly applied.

How Does This Practice Work?

Manure P removal systems vary from simple solid-liquid separation to more elaborate methods combining liquid-solid separation with chemical additions to precipitate P. Solid-liquid separation systems typically use screening separators (e.g. sloped screen or rotary drum), auger or screw-press type separators, and gravity-based settling structures. Solid separation can remove substantial total solids (from 8 to > 90%), but total P removed through this process is often much lower, ranging from 3 to 50% (Varma et al., 2021; Cortus et al, 2026), because the majority of P is associated with smaller manure solids. Separated solids can be used as animal bedding or soil amendments.

This fact sheet covers treatment systems proven effective for P removal based on simple chemical addition along with mechanical systems combining physical and chemical processes. Except for the Manure Phosphorus Extraction System, chemicals containing sulfur were not included here due to potential hazards of hydrogen sulfide production when sulfur compounds are added to liquid manures (Moore, 2023). Chemical-based P removal is more effective when implemented after a preliminary solids-separation step. This preliminary step reduces the buffer capacity of the manure slurry thereby reducing amount of chemicals added. Post chemical dosing, a separation step is required to extract the P-rich phase, either by passive settling in a tank or using a mechanical or size separation step.

Additionally, we have provided information on the effectiveness, cost, and some operation and maintenance information for each system.

Aluminum Chloride Treatment

Liquid dairy and swine manure can be treated with aluminum chloride (AlCl₃) at a 1:1 molar ratio to effectively remove soluble P and is summarized in another SERA17 fact sheet (Moore, 2023). This patented method can achieve total P removal rates of 60% (Timby et al., 2004), is easy to implement and inexpensive compared to other methods ($420/ton of AlCl₃). Another benefit of AlCl₃ is enhanced flocculation and formation of Al-P particles that can be recovered (Vanotti et al, 2020). However, recovered solids have low P bioavailability and are therefore not a good candidate for P fertilizer (Moore, 2023). One advantage of this method is the minimal mechanical processing required compared to more automated methods using liquid-solid separation with chemical treatment.

Iron Chloride Treatment

Like AlCl₃, iron chloride (FeCl₃) also encourages flocculation and P removal from liquid manures, with total P removal rates from 67 to 88% (Chastain, 2013). Compared to Al compounds, Fe is more effective across a broader range of manure pH (Vanotti et al, 2019). Optimum molar Fe: P ratios for P removal are not clear from the literature, however FeCl₃ concentrations reviewed generally range from 13.9 to 40%. Treatment occurs with direct addition of FeCl₃ to waste storage facilities during agitation and requires separation via settling or a screening phase. The cost of FeCl₃ can vary widely depending on grade, concentration and supplier, however industrial grade FeCl₃ is in the range of $200 to 260/ton.

Quick-Wash System

The Quick-Wash system for P removal is a more sophisticated process designed to treat poultry and swine manure solids (Figure 1) but can be adapted to dairy manure systems with liquid-solid separation (Szögi et al., 2020). The Quick-Wash system combines acid pre-treatment, hydrated lime addition, and a polyelectrolyte to extract and recover P (Vanotti et al., 2019). From 80 to 90% of total P is extracted in the first step with a 10 mM citric acid solution at pH 3 to 5 (Szögi et al., 2014). Liquid is then pumped to a treatment tank where a 2% hydrated lime solution is added to increase pH to the range of 9 to 11, precipitating out Ca-P compounds. The P-rich solids are further dewatered and liquid is recycled within the system or can be applied to cropland.

Figure 1. Schematic of the Quick-Wash system for P removal from manure solids. From Vanotti et al (2020).

While this system is more complex, it offers some advantages. Recovered solids are mainly comprised of Ca-P compounds and considered a slowly available P source to crops. In addition, the process conserves more carbon and nitrogen in manure solids compared to other methods.

As with other technologies, the cost for implementing the Quick-Wash system depends on farm-specific factors, including equipment needs and existing manure management infrastructure. Szögi et al. (2020) estimated a cost of $0.13/cow based on a manure volume of 1.93 m3/day for 300 lactating Jersey cows.

Manure Phosphorus Extraction System (MAPHEX)

The Manure P Extraction System (MAPHEX) is a multistep process combining solid-liquid separation, centrifugation and chemical treatment to remove and recover P-rich solids (Figure 2). It was designed to treat liquid dairy manure but could also be applicable to swine manure. It works as a standalone system or can be integrated into existing solid-liquid separation systems.

The first step is separating bulk solids using a screw type or auger press (Figure 2) which can be used for animal bedding, mixed with other solids for composting, or applied to cropland. The liquid fraction is then pumped to a decanter/centrifuge tank to remove medium-size solids (~ 25 µm) before pumping to a chemical treatment tank. The chemical tank uses a dosing rate of 3 g l^-1 of Fe₂(SO₄)₃ to sorb P in solution. In a final step the liquid is pumped through diatomaceous Earth to further reduce P concentrations. Phosphorus removal rates for the system can be as high as 90% (Church et al., 2018). Estimated costs for a fully mobile unit including a screw press separator was $291,000 in 2016, while annual cost was estimated at $750 cow^-1 yr (~$38 kg^-1 of P removed).

Figure 2. Design schematic for the manure phosphorus extraction system (MAPHEX) designed to remove P and dry matter from liquid dairy manures. From Church et al (2018).

Magnesium Phosphate Precipitation (Struvite)

Recovering manure P via controlled struvite (MgNH₄PO₄) precipitation has received considerable attention over the last two decades. While originally envisioned for swine and poultry manure (Westerman et al, 2010), the technology is also applicable to dairy manure systems using solid-liquid separation (Zhao et al., 2010; Varma et al, 2021). Struvite formation requires ammonium (NH₄⁺), magnesium (Mg₂⁺) and phosphate (HPO₄^2-) at proper molar concentrations and a pH between 7 to 7.5 (Vanotti et al, 2019). One benefit is that recovered solids can be used as a slow-release P fertilizer for cropland.

Different reactor vessels and systems are used for struvite crystallization of manure P, including gas agitated fluidized bed reactors, mechanical stirring, and water-agitated bed reactors. Phosphorus removal efficiency varies depending on system-specific factors
(manure P concentrations, pH, calcium concentration, temperature and presence of chelating agents). Successfully designed struvite recovery systems report removal efficiencies from 62 to 95% (Varma et al, 2021).

After bulk solids separation, manure liquids are pumped to a treatment vessel (often a fluidized bed reactor). Since dairy manure is high in calcium concentration, additional steps must be taken to bind calcium to favor struvite formation. Zhao et al (2010) showed that EDTA was an effective chelating agent and that a concentration of 40 mmol EDTA resulted in 80% P recovered as struvite (without EDTA, only 15% P recovery resulted due to preferential formation of Ca-P). Calcium to phosphate ratios of < 0.5 are recommended to reduce Ca-P formation.

Dairy and swine manure both have sufficient NH₄⁺, however Mg₂⁺ must be added to achieve proper stochiometric ratios for struvite precipitation. Zhao et al (2010) showed the greatest P recovery when Mg₂⁺ concentration was 94.5 mmol along with 40 mmol EDTA. One drawback is the high cost of EDTA ($1500 to 2500/ton), however other chelating agents are being explored. One alternative is the use of oxalic acid to complex calcium cations. Oxalic acid is considerably cheaper ($400 to $800/ton) than EDTA, however additional research is required to test its efficacy.

References

Chastain, J.P. 2013. Solid-liquid separation alternatives for manure handling and treatment. USDA-NRCS. Washington, D.C.

Church, C.D, A.N. Hristov, P.J. Kleinman, S.K. Fishel, M.R. Reiner, R.B. Bryant. 2018. Versatility of the MAnure PHosphorus EXtraction (MAPHEX) system in removing phosphorus, odor, microbes, and alkalinity from dairy manures: a four-farm case study. Appl. Eng. Agric., 34: 567–572.

Cortus, E. L., M.Sharara, N.C. Soriano, K. Jones, D.S. Andersen, & B.C. Ramirez 2026. Nutrient and carbon partitioning and performance for swine manure management technologies. J. Environ. Qual., 55(4), e70223.

Krumpelman, B. W., Daniel, T. C., Edwards, F. G., McNew, R. W., and Miller, D. M. 2005. Optimum coagulant and flocculant concentrations for solids and phosphorus removal from pre-screened flushed dairy manure. Appl. Eng. Agric. 21:127–135.

Moore, P.A., Jr. 2023. Treating Liquid Swine and Dairy Manure with Aluminum Chloride. SERA17 Phosphorus Conservation Practices Fact Sheets. https://sera17.wordpress.ncsu.edu/treating-liquid-swine-and-dairy-manure-with-aluminum-chloride/

Szögi, A.A., M.B. Vanotti, & P.G. Hunt. 2014. Process for Removing and Recovering Phosphorus from Animal Waste. U.S. Patent 8,673,046. U.S. Patent and Trademark Office, Washington, DC.

Szögi, A.A., V.H. Takata., & P.D. Shumaker. 2020. Chemical extraction of phosphorus from dairy manure and utilization of recovered manure solids. Agronomy 10:1725.

Vanotti M.B., M.C. Garcia-Gonzáles, A.A. Szögi, J.H. Harrison, W.B. Smith, and R. Moral. 2020. Removing and recovering nitrogen and phosphorus from animal manure. Anim Manure 67:275–321.

Varma, V.S., R. Parajuli, E. Scott, T. Canter, T.T. Lim, J. Popp, & G. Thoma. 2021. Dairy and swine manure management–challenges and perspectives for sustainable treatment technology.Sci. Total Environ., 146319.

Westerman, P. W., K.E. Bowers, & K.D. Zering. 2010. Phosphorus recovery from covered digester effluent with a continuous-flow struvite crystallizer. Applied Engineering in Agriculture, 26:153–161.

Zhao, Q., Zhang, T., Frear, C., Bowers, K., Harrison, J., & Chen, S., 2010. Phosphorous Recovery Technology in Conjunction with Dairy Anaerobic Digestion. CSANR Research Report.

For Further Information

Contact your local USDA-NRCS office or soil and water district for further information.

Current Authors

Eric Young
USDA-ARS
[email protected]

Steph Kulesza
NC State University
[email protected]

Mahmoud Sharara
NC State University
[email protected]

Editing and Design

Deanna Osmond
NC State University

Citation:
Young, E., M. Sharara, and S. Kulesza. 2026. Phosphorus Removal from Dairy and
Swine Manure. SERA17 Phosphorus Conservation Practices Fact Sheets. https://sera17.wordpress.ncsu.edu/phosphorus-removal-from-dairy-and-swine-manure/