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Breakpoint chlorination: chlorine dose, alkalinity and TDS for ammonia removal

Below a chlorine to ammonia mole ratio of one the residual is chloramine. Push past the breakpoint and ammonia leaves as nitrogen gas, at a cost in chlorine, alkalinity and dissolved solids.

Flowm³/d

Ammonia nitrogen inmg N/L

Ammonia nitrogen targetmg N/L

Design chlorine to nitrogen ratiomg Cl₂/mg NStoichiometric 7.6; practice 8 to 10.

Chlorine demand of other reduced speciesmg/LIron, manganese, sulfide, organic nitrogen.

Alkalinity availablemg/L as CaCO₃

Chlorine form and neutralisation

Design chlorine dose

36.0mg/L

Chlorine mass

136.8kg/d

Stoichiometric dose (nitrogen route)

30.4mg/L

Dose if ammonia went to nitrate

81.2mg/L

Alkalinity consumed

57.2mg/L as CaCO₃

TDS added

24.8mg/L

Dose at maximum chloramine residual

25.4mg/L

Below this the residual is combined chlorine, rising roughly one to one with dose.

How it works

Past the breakpoint ammonia is oxidised mainly to nitrogen gas, three moles of hypochlorous acid per two of ammonia:

2 {NH}_{4}^{+} + 3 HOCl \to N_{2} + 3 H_{2} O + 3 HCl + 2 H^{+}

(1)

\frac{{Cl}_{2}}{N} = \frac{1.5 \times 70.9}{14} = 7.6 \frac{mg}{mg} Alk = \frac{2 \times 100}{14} = 14.3 \frac{{mg CaCO}_{3}}{mg N}

(2)

where

${Cl}_{2} / N$: chlorine per unit ammonia nitrogen, mg per mg
$Alk$: alkalinity consumed, mg CaCO₃ per mg N

The nitrate route would take 20.3 mg per mg but is minor. The maximum chloramine residual sits at a mole ratio of one (5.07 mg per mg); between there and the breakpoint the residual falls as chloramines are destroyed. Organic nitrogen, iron, manganese and sulfide shift the whole curve to the right by their own demand. The reaction is fastest at pH 7 to 8 and needs rapid initial mixing.

Breakpoint chlorination: chlorine dose, alkalinity and TDS for ammonia removal

How it works

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