The addition of carbonate salts to urine stabilized with Ca(OH)2 was shown to be a simple and effective method to reduce the concentration of calcium ions and the scaling potential of RO membranes. Both NaHCO3 and Na2CO3 addition were shown experimentally to reduce the calcium concentration in Ca(OH)2 stabilizedurine to at least 0.18 mmol L-1. NaHCO3 is preferred over Na2CO3 as it is cheaper, adds half the amount of Na+ ions, and results in a change in the measured conductivity which can then possibly be used as a proxy for the calcium concentration. Dosing either of these salts would also effectively prevent enzymatic urea hydrolysis from occurring because a high operating pH (>11) would be maintained. It was found that the pH buffering capacity of urine varies with composition and pH
0 5 10 15 20 25 30 35 40 45
0 10 20 30 40 50 60
Calcium remaining (mmol -1L)
Dose (mmol/L)
NaHCO3 KHCO3 NH4HCO3 MgCO3
9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0
0 10 20 30 40 50 60
pH (-)
Dose (mmol/L)
15 17 19 21 23 25 27
0 10 20 30 40 50 60
Conductivity (mS cm-1)
Dose (mmol L-1)
NaHCO3 KHCO3 NH4HCO3 MgCO3
NaHCO3
KHCO3
NH4HCO3
MgCO3
A B
C D
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
NaHCO3 NaHCO3 pH10.5
KHCO3 NH4HCO3 MgCO3 Cost ($ m-3urine)
Carbonate salt Reactant ($/kg) Dose (kg/m3) Cost ($/m3)
NaHCO3 NaHCO3 KHCO3 NH4HCO3 MgCO3
pH10.5
Chemical cost ($ kg-1) Dose (kg m-3) Total cost ($ m-3) NaHCO3
KHCO3
NH4HCO3
MgCO3
there was a correlation between calcium concertation and conductivity. It was shown experimentally that conductivity could be used as a proxy for the calcium concentration and would only require 3.2 to 5.9% additional NaHCO3.
It is advised that either complexometric titration, to determine the initial calcium concentration, or using conductivity as a proxy for calcium concentration should be used to determine the exact dose of NaHCO3 required. This will minimize chemical costs and the addition of undesirable Na+ ions. Should the operating pH of the membrane process be lower than 11, then H2SO4 should be dosed to correct the pH. Whilst simply dosing additional NaHCO3 to a fixed pH is simpler, the addition of H2SO4 would minimize the number of additional ions added.
Modelling of the experiments was shown to be accurate and used to simulate the addition of other carbonate salts. Excess calcium can also be removed by dosing other carbonate salts such as KHCO3, NH4HCO3, and MgCO3. Whilst K+ ions from KHCO3 are more desirable than Na+ ions and MgCO3
results in the lowest TDS, these two options were significantly more expensive and therefore not favored. Ammonium bicarbonate ($0.53 m-3 urine) was comparable in cost to NaHCO3 ($0.49 m-3 urine) and would result in a solution pH within the operating range of most RO or NF membranes and extra N would be added to the stream instead of Na+ ions. It is advised that both NaHCO3 and NH4HCO3
addition be assessed as a pre-treatment method for an RO system to determine the preferred dosing chemical. The concentration of Ca(OH)2 stabilized urine using reverse osmosis without pre-treatment is likely to cause significant membrane scaling that would render the process economically unfeasible.
However, the addition of a bicarbonate salt (NaHCO3 or NH4HCO3) has been proven to be a quick, simple, and cost-effective method to reduce the scaling potential of Ca(OH)2 stabilized urine.
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