Aeration purposes; Fe/Mn removal; residual chlorine

Purposes of aeration

1. To oxidise soluble iron and manganese into insoluble precipitates.
2. To remove dissolved gases such as CO$_2$, H$_2$S and methane, which cause corrosion, taste and odour.
3. To remove taste and odour from volatile and algal substances.
4. To add oxygen to oxygen-deficient waters and make them fresh.
5. To reduce free CO$_2$ and so raise the pH and cut corrosion.

Removal mechanism of iron and manganese

Fe and Mn are present in ground water or reservoir bottom water as soluble ferrous and manganous bicarbonates. Aeration adds oxygen and drives off CO$_2$; the oxidation then makes them insoluble:

$$4Fe(HCO_3)_2 + O_2 + 2H_2O \rightarrow 4Fe(OH)_3\downarrow + 8CO_2$$ $$2Mn(HCO_3)_2 + O_2 \rightarrow 2MnO_2\downarrow + 4CO_2 + 2H_2O\ (\text{slow, pH}>9\text{ or by catalyst})$$

The precipitates are settled out in a tank and removed by filtration. Where aeration is not enough, chemical oxidants (chlorine, KMnO$_4$, ozone), manganese greensand filters, lime softening or ion exchange are used.

Significance of residual chlorine

Residual chlorine is the chlorine left in water after the demand has been satisfied and the required contact time has passed. A free residual of about 0.2 to 0.5 mg/l is usually maintained.

Significance

• It is a safety margin against re-contamination in service reservoirs and distribution pipes.
• Its presence proves that the chlorine demand has been met and the disinfection is complete.
• It is easy to measure (orthotolidine or DPD test) at any point, so it is a quick check of the plant performance.
• It continues to kill bacteria that enter through leaks or back-siphonage.
• Too high a residual gives a taste and may produce trihalomethanes, so it is limited (the WHO health-based guideline is 5 mg/l, and Nepal's NDWQS requires 0.1 to 0.2 mg/l in chlorinated supplies).