Phosphorus recovery from aqueous solution via a microbial electrolysis phosphorus-recovery cell.

The coming global phosphorus (P) crisis makes P recovery from wastewater become an inevitable choice. Hydroxyapatite (HAP) crystallization is an important approach for P recovery, but its requirements for high alkali and acid are unaffordable. Thus, a microbial electrolysis phosphorus-recovery cell (MEPRC) was developed to cut down the alkali cost via raising the wastewater pH (over 11) in the cathode chamber, and the acid cost via producing acid in the acid-production chamber. HAP was confirmed to be the final recovered products, and P recovery efficiency over 80% was achieved at 24-h operation. To optimize the P recovery performance of this system, the effects of the key factors including applied voltage, P initial concentration and Ca/P ration were investigated. High voltage could promote the rate of P recovery but had slight effect on the eventual recovery efficiency (elevated from 88.5 to 91.1%). High P initial concentration (15.0 mM) could slow down the pH elevation, contributing to the low P recovery efficiency (50.1%) within 24 h. However, prolonging the operation could break the buffering and obtain a satisfactory P recovery efficiency (87.2%) at 36 h. Besides, sufficient calcium ions were favorable to the P recovery. In addition, P recovery cost analyses of the MEPRC indicated that it might be a low-cost technology for P recovery. Moreover, the simultaneously produced acid could be used to neutralize the effluent after P recovery with high pH value. These results demonstrate the feasibility of MEPRC for cost-effective P recovery from wastewater.

Bioelectrochemical acidolysis of magnesia to induce struvite crystallization for recovering phosphorus from aqueous solution.

A novel struvite crystallization method induced by bioelectrochemical acidolysis of magnesia (MgO) was investigated to recover phosphorus (P) from aqueous solution using a dual-chamber microbial electrolysis cell (DMEC). Magnesium ion (Mg2+) in the anolyte was firstly confirmed to automatically migrate from the anode chamber to the cathode chamber, and then react with ammonium (NH4+) and phosphate (PO43-) in the catholyte to form struvite. Recovery efficiency of 17.8%-60.2% was obtained with the various N/P ratios in the catholyte. When MgO (low solubility under alkali conditions) was added into the anolyte, the bioelectrochemical acidolysis of MgO naturally took place and the released Mg2+ induced struvite crystallization in the cathode chamber for P recovery likewise. Besides, there was a strong linear positive correlation between the recovery efficiency and the MgO dosage (R2 = 0.935), applied voltage (R2 = 0.969) and N/P ratio (R2 = 0.905). Increasing the applied voltage was found to enhance the P recovery via promoting the MgO acidolysis and the released Mg2+ migration, while increasing the N/P ratio in the catholyte enhanced the P recovery via promoting the struvite crystallization. Moreover, the electrochemical performance of the system was promoted due to more stable anolyte pH and lower pH gradient between the two chambers. Current density was promoted by 10%, while the COD removal efficiency was improved from 78.2% to 91.8% in the anode chamber.

Electrochemical acidolysis of magnesite to induce struvite crystallization for recovering phosphorus from aqueous solution.

A novel struvite crystallization method induced by electrochemical acidolysis of cheap magnesite was investigated to recover phosphorus from aqueous solution. Magnesite was confirmed to continuously dissolve in the anolyte whose pH stabilized at about 2. Driven by the electrical field force, over 90% of the released Mg2+ migrated to the cathode chamber via passing through the cation exchange membrane. The pH of the phosphate-containing aqueous solution in the cathode chamber was elevated to the appropriate pH fit for struvite crystallization. The products were identified as struvite crystals by scanning electron microscopy and X-ray diffraction. Increasing the magnesite dosage from 0.83 to 3.33 g L-1 promoted the phosphorus recovery efficiency from 2.2% to 78.3% at 3 d, which was attributed to sufficient Mg2+ supply. Increasing the applied voltage from 3 to 6 V improved the recovery efficiency from 43.6% to 76.4% at 1 d, since the enhanced current density of the electrochemical system markedly accelerated both the magnesite acidolysis and the catholyte pH elevation. The initial catholyte pH between 3 and 5 was found to benefit the phosphorus recovery due to the final catholyte pH fit for the struvite crystallization.