Disinfection and chlorination

CT valuechlorine demandchlorine residualbreakpoint chlorinationhypochlorite dose

CT value, dose–demand–residual, breakpoint ratio and hypochlorite product weight — getting the right chlorine on the right contact time.

CT Value for Disinfection Credit

\text{CT} = C \, t

Disinfection CT: residual concentration multiplied by contact time, returned in the regulatory unit of mg·min/L.

Chlorine Dose, Demand and Residual

D = C_{\text{demand}} + C_{\text{res}}

The fundamental chlorination balance: the dose applied equals the chlorine consumed by the water's demand plus the residual left for disinfection.

Breakpoint Chlorine-to-Ammonia Ratio

R = \frac{\mathrm{Cl_2}}{\mathrm{NH_3\text{-}N}}

The weight ratio of chlorine dose to ammonia nitrogen, which must reach about 7.6 to 1 to pass breakpoint and produce a free chlorine residual.

Hypochlorite Product Mass from Available Chlorine

m_{\text{prod}} = \frac{m_{\mathrm{Cl}} \times 100\%}{p}

Converts a required mass of available chlorine into the mass of hypochlorite product to weigh out, given the product's percent available chlorine.

Chlorine Dose from a Weight of Product

C = \frac{m \, p}{V}

Gives the free chlorine concentration produced by dissolving a known weight of hypochlorite product of known strength in a known volume of water.

How they fit together

Disinfection is not a dose, it is a dose held for a time: CT is residual concentration in mg/L multiplied by contact time in minutes, and the regulatory credit for inactivating Giardia or viruses is written against that product. Dose, demand and residual is the bookkeeping that gets you there — what you fed, what the water ate, what is left — and the breakpoint ratio explains the strange shape of the curve when ammonia is present.

Choose CT when you have to demonstrate compliance and dose–demand–residual when you are running the plant day to day. The traps are all in the fine print. CT credit uses T₁₀, the time in which 10% of the water has passed through — not the tank's hydraulic detention time — and in a poorly baffled clearwell T₁₀ can be a third of theoretical detention. CT is also strongly pH- and temperature-dependent: the same residual buys roughly half the credit at pH 8.5 that it does at pH 7, and cold water needs far more. On the breakpoint curve, adding chlorine between roughly 5:1 and 7.6:1 chlorine to ammonia destroys residual rather than building it, so an operator chasing a low residual by feeding harder can drive it to zero before it climbs — which is exactly the moment the free residual appears if you push past breakpoint.