Chlorine Dosing and Contact Tank Sizing: 18-Env-B9 Environmental Chemistry and Microbiology, May 2013, Q2

Question 2: Chlorine Dosing and Contact Tank Sizing (5 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. Population served $P=100{,}000$; chlorine demand (dose) $=1\ \text{mg/L}$. No per-capita demand or contact time is stated, so both are assumed and flagged below.

Find. (2.1) chlorine mass required per day, kg/d; (2.2) required volume of the chlorine contact tank, m³.

Check: assumes an average-day per-capita water demand of 400 L/(person·d), a typical Canadian municipal design value (Davis & Cornwell, Table 4-3 range 380–600 L/(person·d)), and a chlorine contact time of 30 minutes, the standard minimum used to size a plug-flow contact basin/clearwell ahead of the CT (concentration × time) disinfection requirement.

Approach. Compute the plant's average daily flow from population × per-capita demand, apply the chlorine dose as a simple mass balance to get the daily chlorine mass, then size the contact tank as flow × assumed detention time.

Average daily flow. $$Q = P \times q = 100{,}000\ \text{person} \times 400\ \tfrac{\text{L}}{\text{person}\cdot\text{d}} = 4.0\times10^{7}\ \text{L/d} = 40{,}000\ \text{m}^3/\text{d}$$
Chlorine mass required. The chlorine demand is applied as a dose over the full flow: $$m_{\text{Cl}_2} = C \times Q = 1\ \tfrac{\text{mg}}{\text{L}} \times 4.0\times10^{7}\ \text{L/d} = 4.0\times10^{7}\ \text{mg/d}$$ Converting to kilograms, $$\boxed{m_{\text{Cl}_2} = 40.0\ \text{kg/d}}$$
Contact tank volume. Sizing the tank on the assumed 30-minute hydraulic detention time, $$V = Q \times t = 40{,}000\ \tfrac{\text{m}^3}{\text{d}} \times \frac{30\ \text{min}}{1440\ \text{min/d}} = \boxed{833\ \text{m}^3}$$ (equivalently 0.5787 m³/min × 1440 min).

Chlorine dosing and contact tank — final results
Quantity	Value
Average daily flow, $Q$	40,000 m³/d
Chlorine required	40.0 kg/d
Chlorine contact tank volume (30-min HRT)	833 m³

Concept & Theory

Topic. Chlorine mass-balance dosing and contact-tank hydraulic sizing.

Key relations. $m=C\cdot Q$ (dose × flow = mass rate); $V=Q\cdot t$ (flow × detention time = tank volume); CT concept, $C\times t \ge$ the required value for a target log-inactivation (Chick–Watson).

Why this works. Chlorine demand is defined as the mg of chlorine consumed per litre to satisfy the immediate oxidant demand of the water before a measurable residual remains; multiplying by the daily volume gives the daily chlorine mass the plant must feed, and the contact tank must hold that flow long enough (CT) for disinfection to occur before the water leaves the basin.

Common pitfall. Confusing chlorine demand (mg/L consumed) with the chlorine dose (mg/L applied, which must exceed demand to leave a measurable free residual) — and sizing the contact tank on peak rather than average flow, which under-sizes CT during high-demand periods.

Source. MWH's Water Treatment: Principles and Design, ch. 17 (disinfection, chlorine chemistry, CT concept); Davis & Cornwell, ch. 8.

Question 2 of 25: Chlorine Dosing and Contact Tank Sizing

Section 1: Chemistry (11 questions, 50 marks)

Question 2: Chlorine Dosing and Contact Tank Sizing (5 marks)