Determination of operation parameters for magnesium-air fuel cell to recover nitrogen and phosphorus from wastewater.

Recovering phosphorus (P) and nitrogen (N) from wastewater not only contributes to environmental protection but also aligns with sustainable development goals. This study employed a magnesium-air fuel cell (Mg-O2-FC) to extract P and N from wastewater in the form of struvite (MgNH4·6H2O), based on the removal efficiency of ammonia and phosphate, electricity generation capacity and struvite purity to determine the optimal operation parameters. These parameters included hydraulic retention time (HRT), service life of magnesium sheet, and precipitation discharge frequency. The results showed that the removal efficiency of ammonia from 0 to 4h was 55.99%, and that from 4 to 12h was only 15.74%. The phosphate removal efficiency in the initial cycle was 97.68% but decreased to 63.25% after 24h. The phosphate removal rate in 2 min increased by 145% when the precipitation discharge frequency increased from 4 h/time to 24 h/time. Consequently, the HRT, service life of the magnesium sheet, and precipitation discharge frequency were selected as 4 h, 24 h, and 24 h/time. These optimized conditions provide valuable insights for the practical implementation of Mg-O2-FC in recovering N and P from wastewater.

Condition optimization of iron-air fuel cell to treat phosphate-containing wastewater regarding sustainable development.

Increasing use of phosphorus products and excessive exploitation of phosphorus resources become two major problems in perspective of phosphorus sustainable development. Phosphorus recovery is the shortcut to solve this dilemma. Combining electrochemistry, an iron-air fuel cell was adopted to recover phosphate and electricity from phosphate-containing wastewater in our previous studies. The present study focused on investigating the effects of catholyte/anolyte conductivity, external resistance, and anolyte pH on the performance of iron-air fuel cell, and obtaining the optimized conditions. Furthermore, the electrochemical methods of phosphate recovery were compared and assessed, and it is concluded that iron-air fuel cell has great potential for energy recovery. The phosphate removal efficiencies and vivianite yield roughly positively correlated with the catholyte conductivity and the anolyte pH, but negatively correlated with the external resistance and the anolyte conductivity. The electricity generation roughly positively correlated with the catholyte conductivity and anolyte conductivity, but showed limitations in the test range of anolyte pH and external resistance. To pursue high phosphate removal efficiencies and vivianite yield, the catholyte conductivity, external resistance, anolyte pH and anolyte conductivity were suggested to be 35 g-NaCl/L, 10 Ω, 8 and 0 g-NaCl/L. While if electricity generation was the primary goal, these parameters should be 35 g-NaCl/L, 220 Ω, 5 and 70 g-NaCl/L. The optimized conditions will help to improve the phosphate removal efficiency, vivianite yield and electricity generation, and to promote the development of iron-air fuel cell technology.

Recovering phosphate and energy from anaerobic sludge digested wastewater with iron-air fuel cells: Two-chamber cell versus one-chamber cell.

Anaerobic sludge digested (ASD) wastewater is widespread in wastewater treatment plants. Recovering phosphate from ASD wastewater not only removes pollutants but also solves the phosphorus deficiency problem. Iron-air fuel cells were chosen to recover phosphate and generate electricity from ASD wastewater. To optimize cell configuration, a two-chamber and a one-chamber iron-air fuel cell were set up. The phosphate removal efficiency, the vivianite yield and the electricity generation efficiency of the two fuel cells were evaluated. It turned out that the volumetric removal rate (VRR) of phosphate of the two-chamber cell was 11.60 mg P·L-1·h-1, which was about five times of that in the one-chamber cell. The phosphate recovery product vivianite was detected on the surface of the iron anodes and the calculated purities of the two-chamber fuel cell and one-chamber fuel cell were 90.6% and 58.7%, respectively. Considering the content and purity, the iron anode surface in the two-chamber fuel cell was the best point to recover phosphate. The proton exchange membrane (PEM) in the two-chamber fuel cell provided low pH conditions suitable for vivianite formation. Moreover, under the low pH condition, metal ions of Fe2+, Ca2+, Al3+ and so on were kept soluble, leading to a high conductivity. The high conductivity caused low internal resistance, which benefited the electricity generation. The total output electric power of the two-chamber fuel cell was 2.4 times that of the one-chamber fuel cell when treating 25 mL ASD wastewater (0.62 vs. 0.26 mW·h). Overall, the two-chamber fuel cell was the better choice for phosphate recovery and electricity generation from ASD wastewater. Further studies on the long-term operation of two-chamber fuel cells should be carried out.