Effluent disinfection: chlorine contact tank

Description

Chlorine contact tank for a treated effluent, with sodium hypochlorite dosing. Sized for the contact time at the peak flow, as a serpentine channel long and narrow enough to behave as plug flow:

$V = Q_{max} \, t_c \qquad b = \sqrt{\frac{A}{L/b}} \qquad L = \sqrt{A \cdot L/b}$

A length-to-width ratio of 40 or more keeps short-circuiting small; the effective contact time is the theoretical one times the baffle factor, $t_{10} = BF \, t_c$. The channel velocity at average flow is checked against the minimum that keeps the remaining solids from settling.

Chlorine dose as the demand plus the residual to keep at the outlet, unless the dose is given, and the hypochlorite it takes:

$D = D_{cons} + C_R \qquad V_{hip} = \frac{D \, Q}{C_{hip} \, \rho_{hip}}$

When the influent coliforms are given, the kill at the residual and effective time follows Collins-Selleck, and a target gives the residual it needs:

$\frac{N}{N_0} = \left( \frac{C_R \, t_{10}}{b} \right)^{-n} \quad (C_R t_{10} > b)$

$b = 4.0$ mg.min/L and $n = 2.8$ are typical values for coliforms; site data is better. Hypochlorite loses strength in storage, so the storage time is kept short.

References

METCALF & EDDY. Wastewater Engineering: Treatment and Resource Recovery. 5th ed. McGraw-Hill, 2014. Chapter 12 (disinfection).

U.S. EPA. Design Manual: Municipal Wastewater Disinfection. EPA/625/1-86/021, 1986.