Phosphorus harvesting from fresh human urine: A strategy of precisely recovering high-purity calcium phosphate and insights into the precipitation conversion mechanism

Factors influencing K-struvite purity via phosphorus coprecipitation in synthetic urine: Verification, quantification, and modelling

Due to the plethora of nitrogen (N), phosphorus (P), potassium (K) in urine, it is bound to trigger phosphorus coprecipitation, thereby adversely affecting K-struvite purity in the coprecipitates. To obtain high pure K-struvite, the present study was to innovatively explore the effect of residual NH4+ concentration, pH and initial Mg2+ concentration on phosphorus coprecipitation in synthetic urine. Importantly, a Back-Propagation Artificial Neural Network (BPANN) model was innovatively proposed to simulate and predict crystal purities in the coprecipitates. It was revealed that K-struvite, struvite, hydroxyapatite and cattiite dominated the coprecipitates. Comparatively, the content of calcium in synthetic urine is far lower than that of phosphorus and potassium, resulting in low hydroxyapatite purity in the coprecipitates. Notably, cattiite purity is highly dependent of Mg2+ concentration, because it was low at the Mg2+ concentration of <10 mmol/L, but increased up to above 50% at the Mg2+ concentration of 50 mmol/L. At 10 mmol/L Mg2+ and pH 10, K-struvite purity in the coprecipitates decreased from 64.6% to 43.3% following the increase of NH4+ concentration from 0 to 300 mg/L. The BPANN model well simulated and predicted the purities of the crystals in the coprecipitates from synthetic urine. At 10 mmol/L Mg2+ and 100 mg/L NH4+, an increase in pH from 8.5 to 10 can facilitate K-struvite crystallization in synthetic urine. The adjustment of pH and initial Mg2+ concentration can significantly mitigate the inhibitory effect of residual NH4+ on K-struvite crystallization. The BPANN model herein can effectively obtain optimized operational parameters for the full-scale implementation of slow-release NPK fertilizers from urine, which can also provide an effective reference for nutrient recovery from various waste streams.

Self-driven electrochemical system for struvite and energy recovery from digested wastewater: Device optimization strategy and long-term operation

A self-driven electrochemical system (SDES) was utilized to treat anaerobic digestate wastewater, aiming to achieve wastewater resource utilization and energy generation. The efficiencies of pollutant removal, resource recovery, and energy production were enhanced by adjusting device parameters (anode area, external resistance, and electrode spacing). The high pollutant removal rates and struvite purity were achieved with the magnesium anode area of 15 cm2, external resistance of 10 Ω, and electrode spacing of 10 cm. The appropriate anode area (3.0 cm2), external resistance (50 Ω), and electrode spacing (7.5 cm) were prone to achieve high electric energy output. For one cycle, the removal rates of PO43--P and NH4+-N were 95.37% and 39.10%, respectively, with an average output power of 50.98W/m³, and 0.0275g of struvite was recovered(50 ml digested wastewater). For the long-term operation (20 cycles), the average PO43--P and NH4+-N removal rates were 89.3% and 23.4%, the CV (Coefficient of Variation)for PO43--P and NH4+-N were 0.1998 and 0.0504, and the average output power was 8.90 W/m3. The SDES showed satisfactory performance without replacing the magnesium anode. Based on the comprehensive efficiency of pollutant removal, resource recovery, and energy production, a replacement cycle of 20 cycles for magnesium anode was determined. In summary, the SDES for treating the anaerobic digested wastewater was demonstrated with stability and efficiency.

Benchmarks for urine volume generation and phosphorus mass recovery in commercial and institutional buildings

Phosphorus (P) is a finite resource and necessary nutrient for agriculture. Urine contains a higher concentration of P than domestic wastewater, which can be recovered by source separation and treatment (hereafter urine diversion). Commercial and institutional (CI) buildings are a logical location for urine diversion since restrooms account for a substantial fraction of water use and wastewater generation. This study estimated the potential for P recovery from human urine and water savings from reduced flushing in CI buildings, and proposed an approach to identify building types and community layouts that are amenable to implementing urine diversion. The results showed that urine diversion is most advantageous in CI buildings with either high daily occupancy counts or times, such as hospitals, schools, office buildings, and airports. Per occupant P recovery benchmarks were estimated to be between 0.04-0.68 g/cap·d. Per building P recovery rates were estimated to be between 0.002-5.1 kg/d, and per building water savings were estimated to be between 3 and 23 % by volume. Recovered P in the form of phosphate fertilizer and potable water savings could accrue profits and cost reductions that could offset the capital costs of new urine diversion systems within 5 y of operation. Finally, urine diversion systems can be implemented at different levels of decentralization based on community layout and organizational structure, which will require socioeconomic and policy acceptance for wider adoption.

Solar distillation of human urine to recover non-potable water and metal phosphate mineral

Human urine is a readily available nutrient source that can complement commercial fertilizer production, which relies on finite mineral resources and global supply chains. This study evaluated the effectiveness of a simplified solar distillation process for urine to recover phosphorus (P) and nitrogen for agricultural use and water for non-potable purposes. Synthetic fresh, synthetic hydrolyzed, real fresh, and real hydrolyzed urine were exposed to direct sunlight for 6 h in a simple distillation apparatus, which produced distillation bottoms and distillate. Metal phosphate precipitation in the distillation bottoms was evaluated to recover P. The non-potable water was recovered as distillate. Hydrolyzed urine recovered more metal phosphate solid in the distillation bottoms and had a higher conductivity in the distillate than fresh urine. Hydrolyzed urine also achieved greater distillate volume recovery than fresh urine. Hydrolyzed urine had a greater presence of UV-absorbing organics in the distillate than fresh urine and therefore produced a lower-quality product water. There was no significant correlation between the daily high air temperature and the volume of distillate recovered. This study provides a comprehensive data set on simplified solar distillation of human urine considering the fate of nutrients and water for different types of urine.

Phosphorus recovery from wastewater via calcium phosphate precipitation: A critical review of methods, progress, and insights

Phosphorus (P) is one of the important elements for human, animal, and plant life. Due to the development of the circular economy in recent years, the recovery of P from wastewater has received more attention. Recovery of P from domestic, industrial, and agricultural wastewater in the form of calcium phosphate (CaP) by precipitation/crystallization process presents a low-cost and effective method. Recovered CaP could be used as P fertilizer and for other industrial applications. This review summarizes the effects of supersaturation, pH, seed materials, calcium (Ca) source, and wastewater composition, on the precipitation/crystallization process. The recovery efficiency and value proposition of recovered CaP were assessed. This in-depth analysis of the literature reports identified the process parameters that are worth further optimization. The review also provides perspectives on future research needs on expanding the application field of recovered CaP and finding other more economical and environmentally friendly Ca sources.