Phosphorus Recovery and Passivation Controls using Sacrificial Magnesium Electrodes in Anaerobic Digestor Centrate
Abstract
Batch experiments were conducted to identify the impact of pH, Mg:P ratios, total solids, and dissolved ions on phosphorus recovery efficiency from anaerobically digested sludge centrate using electrochemically dosed magnesium. The mechanisms for phosphorus recovery, such as coagulation by charge neutralization and struvite precipitation were explored. The recovery mechanism was determined to be struvite precipitation.
The magnesium dosage rate was proportional to the current which was observed to decrease over time in a process called electrode passivation. Batch experiments were conducted to identify the impact of operating conditions such as voltage, alternating pulsed current interval duration, and scouring airflow rate on electrode passivation rate in anaerobically digested sludge centrate. Two electrode materials were tested, aluminium and AZ31B magnesium alloy. The optimal operating conditions for each electrode material were found with the Box-Behnken design of experiments with three factors and three levels, which generated multiple response surfaces. The three factors were the operating conditions specified above, and the levels were 7, 10.5, and 14 volts for voltage; 2, 6, and 10 L/min/electrode for airflow rate; and 5, 10, and 15 minutes for polarity reversal interval duration. Each point in the Box-Behnken design represented two batch tests operated for 90 minutes. After the optimal conditions to minimize passivation was found, magnesium electrodes were operated with the optimal conditions in a long-term continuous reactor. The long-term current was best described with a power function, and a 94.76% phosphorus recovery efficiency was sustained at an HRT of 1 hr until the electrodes totally dissolved after 4 days.