As the world's population continues to grow, the demand for food will increase by about one third by 2050 (Fukase et al. 2020). Most commercial fertilizers containing ammonia bicarbonate, calcium phosphate, potassium chloride or potassium sulfate have been successfully prepared to improve soil fertility and promote crop growth (Pogorzelski et al. 2020, Lodi et al. 2021). Since the raw materials of traditional fertilizers are derived from ore and are non-renewable, the development of new technologies to recover these nutrients from the environment as alternative fertilizers has attracted widespread attention (Flores et al. 2017). Urine is rich in nutrients and is the source of more than 50% of phosphorus and 80% of potassium in municipal wastewater, even though it makes up only 1% of total wastewater volume (Yan et al. 2021). Therefore, extracting urine elements before urine reaches municipal wastewater would both lower the load of sewage plants and produce valuable fertilizers (Badeti et al. 2021). Although urine storing for more than 6 months can be used directly in agricultural production, this technique is limited by the need for storage space, odor treatment during storage, and high transportation costs (Udert et al. 2012), calling for the development of new technologies.
Actually, struvite precipitation (Rodrigues et al. 2019), ion exchange (Guan et al. 2020), adsorption (Liu et al. 2021a) have enabled the recovery of nutrient elements from urine. All methods have advantages and disadvantages in terms of water quality, chemical use, site and cost (Patel et al. 2020). Struvite precipitation is considered to be one of the promising directions, because it can recover nitrogen and phosphorus in urine at the same time and struvite is an excellent alternative fertilizer (Kumari et al. 2020). Traditionally, struvite is obtained by adding substances that provide magnesium and alkalinity to the urine, resulting in magnesium and alkalinity sources accounting for more than 70% of the total process cost (Hoevelmann et al. 2016). Researchers have investigated the use of magnesium-containing materials such as magnesium oxide (Luo et al. 2018), magnesite (Krähenbühl et al. 2016), and seawater (Merino-Jimenez et al. 2017) to reduce the cost, although the source of magnesium and alkali still remains a major challenge.
Most studies have focused on the recovery of nitrogen and phosphorus, whereas there is little knowledge on the recovery of potassium. Potassium and phosphorus can be recovered as magnesium potassium phosphate hexahydrate (Xu et al. 2011). Huang et al. recovered all phosphorus and 70.0% potassium from urine as K-struvite using low-grade MgO and phosphoric acid at a MgO:K:P molar ratio of 4:1:1.6 (Huang et al. 2019). Since the kinetics and thermodynamics of K-struvite precipitation are different from those of conventional struvite, some of the experiences of struvite precipitation studies are not fully applicable (Gao et al. 2018).
Electrochemical treatment of wastewater for resource recovery is also of increasing interest due to its high efficiency, flexibility and absence of chemical addition (Crini et al. 2019, Liu et al. 2021b). For instance, electrochemical recovery of phosphorus using iron as a sacrificial anode is about four times cheaper than the chemical crystallization method (Martin et al. 2020). Li et al. achieved 86.6–93.3% phosphorus recovery in the form of struvite by electrochemical decomposition of natural magnesite, providing both magnesium and alkali, with reduced costs by 60.0% compared to conventional chemical processes (Li et al. 2021). The practical application of electrochemical methods for simultaneous recovery of phosphorus and potassium from urine has not been reported yet. Therefore we present here an electrochemical system for the simultaneous recovery of phosphorus and potassium from urine, in which magnesium metal is used as a sacrificial anode. The effect of current density on the effectiveness of recovering phosphorus and potassium at low (P/K = 0.25) and high (P/K = 0.6) phosphate levels, respectively, was investigated. Next the composition of the products and their morphology were determined. Finally, the crystal seed feeding technique was applied to the K-struvite precipitation process.