Switching a water plant from chlorine to chlorine dioxide under a THM limit
A water utility treats 20,000 m³ day⁻¹ and must reduce trihalomethanes (THMs). The current chlorine gas dose is 1.5 mg L⁻¹ as Cl2 equivalent. The proposed replacement is chlorine dioxide, generated on site by
2NaClO2 + Cl2 → 2ClO2 + 2NaCl.
The target dose remains 1.5 mg L⁻¹ expressed as Cl2 equivalent. ClO2 is reduced to Cl⁻ in the disinfection process, a five-electron change per ClO2, and its available chlorine is therefore about 2.63 g Cl2 equivalent per g ClO2. The plant can store sodium chlorite solution but not chlorine dioxide, which must be generated continuously. The distribution system has a chlorite guideline of 0.7 mg L⁻¹, and the operators are worried about both chemical cost and chlorite formation.
1. Calculate the daily mass of ClO2 required, the daily mass of NaClO2 consumed, and the daily mass of Cl2 needed for generation. Use M(ClO2) = 67.45 g mol⁻¹, M(NaClO2) = 90.44 g mol⁻¹ and M(Cl2) = 70.90 g mol⁻¹.
Read the worked answer
Flow = 20,000 m³ day⁻¹ = 2.00 × 10⁷ L day⁻¹.
Target dose = 1.5 mg L⁻¹ as Cl2 equivalent, so the required Cl2-equivalent mass is:
2.00 × 10⁷ L × 1.5 mg L⁻¹ = 3.00 × 10⁷ mg = 30.0 kg Cl2-eq day⁻¹
ClO2 needed = 30.0/2.63 = 11.4 kg day⁻¹
Moles of ClO2 = 11,400 g/67.45 g mol⁻¹ = 169 mol day⁻¹
From the generation equation, 2 mol NaClO2 gives 2 mol ClO2, so NaClO2 required = 169 mol day⁻¹:
169 × 90.44 = 15.3 kg day⁻¹ NaClO2
Chlorine required = 169/2 = 84.5 mol day⁻¹:
84.5 × 70.90 = 5.99 kg day⁻¹ Cl2
So the plant needs about 11.4 kg ClO2 per day, made from about 15.3 kg sodium chlorite and 6.0 kg chlorine per day. The ClO2 mass is much smaller than the 30 kg Cl2-equivalent figure because each ClO2 molecule accepts five electrons, whereas Cl2 accepts two.
2. The oxidation state of chlorine in ClO2 is +4. Explain why chlorite can appear in treated water and why it is a concern. Give one chemical method to control chlorite and write a balanced equation for it.
Read the worked answer
In ClO2, each oxygen is −2, so chlorine is +4. That intermediate oxidation state can disproportionate, especially in alkaline solution:
2ClO2 + 2OH⁻ → ClO2⁻ + ClO3⁻ + H2O
Chlorite (ClO2⁻, Cl +3) is also formed when ClO2 oxidizes natural organic matter or other reductants by a one-electron step, and when ClO2 is used in excess or the generator is inefficient. Chlorite is a concern because it is a regulated disinfection by-product, it can cause haemolysis at high doses, and it indicates that oxidant has been wasted rather than delivered to the distribution system.
One control method is to reduce chlorite to chloride with sulfite:
ClO2⁻ + 2SO3²⁻ → Cl⁻ + 2SO4²⁻
Alternatively, ferrous ion can be used:
ClO2⁻ + 4Fe²⁺ + 4H⁺ → Cl⁻ + 4Fe³⁺ + 2H2O
Both reactions convert chlorite to chloride before the water leaves the plant. The choice depends on the allowable sulfate or iron residual and on sludge handling.
3. A cheaper option is sodium hypochlorite bleach. Compare its available chlorine with that of ClO2 and decide whether the switch to ClO2 is justified. Include the effect on THM formation, chloride load and chlorite control in the decision.
Read the worked answer
For NaOCl, the active chlorine is reduced from +1 to −1, a two-electron change. With M(NaOCl) = 74.44 g mol⁻¹:
available chlorine = 2 × 35.45/74.44 = 0.952 g Cl2-eq per g NaOCl
To deliver 30.0 kg Cl2-eq day⁻¹ would therefore need about 30.0/0.952 = 31.5 kg day⁻¹ of pure NaOCl, far more mass than 11.4 kg ClO2. Bleach is usually supplied as a dilute solution, so the actual delivered volume and chloride load are larger still. It also adds sodium and chloride and can raise conductivity.
The key reason to switch is THM control. Hypochlorite reacts with natural organic matter to form trihalomethanes and other chlorinated by-products. ClO2 is a one-electron oxidant and largely avoids chlorine substitution, so THM formation drops sharply. That advantage is the justification for the higher chemical complexity of on-site generation.
The decision is not automatic. ClO2 must be generated continuously, and chlorite can exceed the 0.7 mg L⁻¹ guideline if dosing or generation is poor. The plant must therefore control the ClO2 dose, monitor chlorite, and have a chlorite-reduction step. If chlorite control is unreliable, a lower bleach dose or a different primary disinfectant may be safer despite the THM pressure. On balance, ClO2 is justified only if chlorite can be kept below the guideline and the generator efficiency is verified.