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

Abstract

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 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, MgCO3, 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. HIGHLIGHTS NaHCO3 addition can reduce calcium concentrations in stabilized urine to 0.18 mmol L−1.; NaHCO3 dosing is preferred to Na2CO3 as additional Na+ ions and costs are minimized.; pH is not a good parameter to control NaHCO3 dosing, but conductivity is.; Acid addition or dosing NH4HCO3 can achieve desired pH values.; Additional pre-treatment would increase cost of fertilizer product by 0.13%.;

Keywords

• calcium carbonate
• calcium hydroxide
• fertilizer
• membranes
• scaling