Recovering phosphorus, nitrogen and potassium from urine as struvite, ammonium sulfate, and potash

What is it about?

Urine contains numerous plant nutrients, yet direct application of urine as a liquid fertilizer has disadvantages such as a predetermined NPK ratio that may be suitable for some plants but not others as well as labor and storage costs of collecting and transporting large volumes of urine. These disadvantages can be mitigated by producing separate N, P, and K products which will result in two main benefits: 1) the ability to customize the ratio of NPK in the fertilizer by recombining each individual product to fit the nutrient needs of any crop and 2) to concentrate nutrients in solid and liquid fertilizers that will have higher nutrient density than liquid urine. Therefore, the goal of this study was to investigate an integrated approach for nutrient recovery from urine. Three treatment processes, struvite precipitation for P recovery as struvite (MgNH4PO4·6H2O), ammonia stripping–acid absorption for N recovery as ammonium sulfate ((NH₄)₂SO₄), and evaporation for potassium (K) recovery as potash (soluble K) were integrated into a single treatment sequence. The impact of each operational parameter on downstream treatment processes was investigated to understand the complete picture of nutrient recovery from urine. Bench scale experiments were done with three urine solutions (i.e., real urine collected from individual’s ages 16-50, synthetic urine with organic metabolites, and synthetic urine with no organic metabolites) to investigate the influence of urine chemistry on treatment performance. Struvite (MgNH4PO4·6H2O) precipitation was chosen as the first treatment process because (1) it occurs naturally when urine is stored and (2) the concentration of NPK in urine is P<K<N, requiring minimal chemical input to recover a large portion of P and as a fertilizer. The effect of magnesium (Mg) sources (i.e., MgCl2·6H2O, MgO, and MgCO3) on P recovery via struvite precipitation and on downstream N and K recovery processes were investigated. MgCl2·6H2O was chosen because of its high solubility (52.9g/100mL water) and addition of chloride (Cl-). Theoretically, all the Mg added would dissolve and precipitate with P and N to form struvite and the addition of Cl- would increase the purity of the K product because it is a component of potash. MgCO3 and MgO were chosen because a pH increase of the solution was desired for subsequent N recovery. Results from a statistical analysis showed there was a statistically significant difference between P recovery and Mg sources where the amount of P recovered by each Mg source was MgCl2·6H2O > MgCO3 > MgO (see Table 1). To investigate the effect of pH and temperature on N recovery by ammonia stripping–acid absorption and K recovery by evaporation, five operational conditions were tested: (i) pH 9.6, 55°C (i.e., elevated pH and temperature); (ii) pH 10, 40°C (i.e., elevated pH and temperature); (iii) pH 10.5, 22°C (i.e., elevated pH) ; (iv) pH 9.2, 70°C (i.e., elevated temperature); (v) pH 9.2, 22°C (i.e., control). There was a statistically significant difference between N recovery and experimental stripping conditions where increasing both the pH and temperature recovered a higher percent of N compared to solely increasing the pH or temperature of urine (see Table 1). The purity of the N fertilizer product, i.e., (NH₄)₂SO₄, was dependent on the concentration and volume of sulfuric acid (H2SO4) used for N adsorption. To produce a product of equal or greater nutrient density to market fertilizer (i.e., >8-9% N w/w), approximately 28mL of 6M H2SO4 would be required to treat 1 L of urine. Experimental stripping conditions also had a significant impact on K recovery where conditions with elevated pH had the greatest impact on K fertilizer purity. No pH increase was observed after struvite precipitation; therefore, all samples required approximately similar amounts of base for experimental stripping conditions with elevated pH.

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Photo by Mahdi Kordi on Unsplash

Photo by Mahdi Kordi on Unsplash

Why is it important?

Our findings show that over 90% of NPK can be recovered from urine as separate fertilizer products however improved N and K recovery process must be investigated to produce fertilizer products similar to those on the market. This is important because the process of urine diversion is still new in the United States, and the economic gain of producing fertilizer products from a waste stream has not been accepted and recognized. Furthermore, it highlights the impact of each treatment process on the urine chemistry. Thus urine diversion