Calcium removal from stabilized human urine by air and CO₂ bubbling

Stabilization of urine with calcium hydroxide prevents enzymatic urea hydrolysis, thus allowing for maximum nitrogen recovery. The process also produces a calcium phosphate bi-product which has value as a fertilizer. However, the treated solution is saturated with calcium that could ultimately result in calcium carbonate scaling of reverse osmosis membranes during urine concentration. This would result in a decrease in maximum water removal and an increase in operational costs. This study therefore investigated if bubbling air and carbon dioxide through stabilized urine could remove calcium ions as calcium carbonate. The process was modelled to better understand the mechanisms controlling the reactions in the process. The model was then used to determine the most cost and time efficient operating conditions. Calcium removal of between 85-98% was achieved at air flow rates of 1.5 to 9 L min^-1. Increasing the CO₂ concentration from 0.04% (air) to 1% decreased the reaction time from 20.5 h to 2.5 h but the cost of CO₂ outweighed the shorter operating time. Air bubbling was the more cost-efficient option. It was estimated that 95% of the calcium could be removed in 7.6 h at an air flow rate of 4 L min^-1 L^-1 of urine and at a cost of $0.65 m^-3. It was also determined that even if the pH decreased to below 11, the urine remained stabilized and no enzymatic urea hydrolysis occurred.