This study describes a method to formulate struvite fine powder into pellets that are easy to spread on agricultural land. To evaluate the quality of produced pellets, some chemical and physical properties commonly measured for fertilizers were tested. The findings indicated that the salt index and heavy metal content ofstruvite pellets were significantly lower than those of commercial NPK fertilizers. In addition, the percentage of nutrient released from struvite pellets after 105 days was in the range of 9.6-23.2, 8.4-26.7 and 11.3-32.6% for nitrogen, phosphorous and magnesium, respectively, which is considerably lower than that of commercial NPK fertilizer. Among different formulations between struvite crystals and binders, starch and bentonite were the most efficient in agglomerating struvite powder, leading to an increase in the crush strength to over the recommended limit of >2.5 kgf for fertilizer hardness.

Recovering nutrients from urine as solid minerals such as struvite can eliminate many hurdles related to the use of liquid urine as fertilizer in the current urine separation systems. The focus of this study has been to increase nitrogen and phosphorus removal efficiencies by developing a struvite formation and recycling process. Struvite crystallization was carried out in urine samples by addition of MgO where 92% of phosphorus as orthophosphate was recovered. In order to optimize the use of calcined struvite in the struvite recycling step, four calcination treatments were tested and calcination at 200°C for 3 h was chosen for further experiments. To remove the remaining ammonia present in urine a struvite recycling step was used subsequently. Through sequential struvite formation and struvite recycling process up to 90% of ammonium nitrogen was successfully removed.

One of the major setbacks of struvite recovery processes is the difficulty in harvesting struvite crystals. This study evaluates the use of different coagulants to improve precipitation of struvite (MgNH4PO4.6H20) crystals. Chitosan and poly(diallyldimethyl ammonium chloride) (Poly-DADMAC) as a coagulant-flocculent and alginate and bentonite as a coagulant aid have been examined in jar tests. Also, a continuous three-phase process, i.e., struvite crystallization, coagulation/flocculation and precipitation process, was set up for real wastewater. Addition of chitosan as the coagulant and bentonite as the coagulant aid was significantly more efficient in forming struvite flocs in comparison to Poly-DADMAC alone or with coagulant aid, which did not show any positive effect. The calculated average settling velocity of struvite with chitosan-bentonite addition in synthetic and in real wastewater increased by approximately 5.3 and 2.8 folds, respectively, compared with that of no coagulant/flocculent addition. Phosphorus recovery of over 70% was achieved by the continuous process. Findings in this study clearly confirmed the possibility of using chitosan and bentonite as an efficient coagulant-flocculent to enhance the recovery of struvite crystals.