Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article


Master of Engineering Science


Civil and Environmental Engineering


Dagnew, Martha


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.

Summary for Lay Audience

Phosphorus is an essential nutrient for plants, and phosphorus fertilizers are widely applied in agriculture. Phosphate rock is the most abundant form of unprocessed phosphorus in the environment. Around 90% of phosphate rock production is used to produce fertilizers. However, the world’s phosphate rock reserves are expected to be depleted within the century. This will affect the price and availability of food. This study extracted phosphorus from water to produce phosphorus fertilizer. This process creates a short phosphorus cycle and allows phosphorus reuse. Phosphorus in water can be converted into fertilizer by adding magnesium and ammonia ions to precipitate struvite, a phosphorus fertilizer that can be recovered from the water. This reaction is favoured at high pH and therefore usually requires the addition of sodium hydroxide to increase pH. Using a novel process to dose magnesium electrochemically, the magnesium concentration and pH can be increased simultaneously without the addition of sodium hydroxide. This process will reduce the pH adjustment cost. This study recovered phosphorus from side-stream wastewater, which is already high in phosphorus and ammonia concentrations and requires only the addition of magnesium and a pH increase. Optimization of the process was conducted to increase phosphorus recovery efficiency, resulting in 96.87% recovery when optimized compared to 17.56% recovery when unoptimized. The product was experimentally confirmed to be struvite fertilizer. In conclusion, phosphorus can be recovered from water as struvite using electrochemically dosed magnesium. This technology can serve as an alternative source of phosphorus when phosphate rock reserves are depleted.

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