Circular Nutrient Recovery from Domestic Wastewater via Struvite Crystallization: Reactor-Scale Optimization and Fertilizer Valorization

Driven by the imbalance between nutrient discharge and availability of natural resources, there is a growing need for sustainable strategies for managing wastewater. Nutrient recovery from domestic wastewater through struvite crystallization presents one such efficient and circular technology for recovering nutrients and producing fertilizer. A blade-agitated reactor was designed and operated to investigate the effects of various magnesium sources and operating parameters on struvite crystallization. Results show that maintaining alkaline conditions and adding magnesium to wastewater enhance nutrient recovery. Struvite crystallization using magnesium carbonate was found to be most efficient in the removal of phosphorus and nitrogen. Additionally, wastewater treated through struvite crystallization exhibited a substantial reduction in nutrient concentrations. Struvite was found to have the same efficacy as chemical fertilizers in promoting the growth of crops when used on finger millet. This nutrient recovery and fertilizer production technology presents an advantage in the circular use of wastewater resources. Therefore, struvite crystallization is a promising technology that can be integrated into wastewater treatment plants as a means of recovering nutrients from domestic wastewater. Furthermore, struvite crystallization is a low-energy technology that is sustainable and has the potential to be used on a large scale in circular systems integrating water, energy, and food production. Under the optimum condition identified (magnesium carbonate at 5.0 g/L, 200 rpm, 20 min of reaction and 20-30 min of settling at the native wastewater pH of 8.9), removal efficiencies of 83.3% for phosphate, 81.8% for nitrogen, 88.4% for turbidity and 89.5% for biochemical oxygen demand were obtained, against phosphate removals of 50.0% and 33.3% for magnesium chloride and magnesium sulphate respectively. One-way analysis of variance on day-17 shoot height of finger millet confirmed a significant treatment effect (F(4,5) = 35.08, p = 0.00075, eta-squared = 0.97), and Tukey HSD grouping placed the magnesium-carbonate product in the same statistical group as urea and significantly above the unfertilised control, corresponding to a relative agronomic effectiveness of 76% of urea. A closing mass balance establishes that the recovered material is a struvite-bearing composite rather than a pure phase, and a preliminary techno-economic and energy screening identifies the magnesium dose, not the reactor, as the controlling barrier to full-scale adoption. The contribution of this work is therefore the coupling of reactor-scale magnesium-source optimisation with verified fertilizer valorisation and a transparent mass and cost balance for a single, unamended domestic sewage.