Precipitation to remove calcium ions from stabilized human urine as a pre-treatment for reverse osmosis

Concentration of Ca(OH)₂ stabilized urine by reverse osmosis (RO) has the potential to cause CaCO₃ scaling on the membranes. The aim of this research was to determine whether the addition of carbonate salts could be used to precipitate CaCO₃ prior to RO concentration and how to accurately dose the salts. Dosing of NaHCO₃ or Na₂CO₃ reduced the calcium concentration to <0.18 mmol L^-1, whilst maintaining a pH > 11. This is the pH threshold for enzymatic urea hydrolysis in urine, but above the operating pH range of most membranes. However, the pH could be decreased by adding an acid. Measuring conductivity as a proxy for the calcium concentration was found to be an effective method to determine the dose of salt required. Simulations with other carbonate-producing salts (KHCO₃, MgCO_3, and NH₄HCO₃) were also shown to be effective. However, NH₄HCO₃ ($0.53 m^-3 urine) was the only other salt comparable in cost to NaHCO₃ ($0.49 m^-3 urine) and resulted in a final pH within the normal operating range of membranes. The addition of NH₄HCO₃ would add extra N to the urine rather than sodium ions when dosing NaHCO₃. The choice of salt will ultimately depend on what liquid fertilizer composition is desired.