Fecal Coliform Excursion at Peak Flow in an Undersized Contact Chamber
A 0.5 MGD plant disinfects with 12.5% sodium hypochlorite (about 1.24 lb available chlorine per gallon) fed to a chlorine contact chamber with 8,500 gallons of liquid volume and a baffling factor of 0.5. The permit requires a 1.0 mg/L residual after 20 minutes of contact and a fecal coliform limit of 200/100 mL monthly geometric mean. At design flow, a feed rate of 10 gal/day holds 1.0 mg/L against a demand of about 2.0 mg/L. After a weekend storm, flow peaked at 1.1 MGD, the outlet residual fell to 0.3 mg/L, and Monday's fecal coliform result was 900/100 mL. The bench chlorine demand of the storm-time effluent was 1.6 mg/L. The night operator wants to run the feed up to 30 gal/day during storms to push the residual to about 2.5 mg/L.
1. Calculate the actual and effective contact time at design and peak flow, and the feed rate needed to hold 1.0 mg/L at the peak. What do the numbers tell you?
Read the worked answer
Detention time (V = 0.0085 MG)
- At 0.5 MGD: 0.0085 ÷ 0.5 = 0.017 day = 24.5 min
- At 1.1 MGD: 0.0085 ÷ 1.1 = 0.00773 day = 11.1 min
Effective contact time (× baffling factor 0.5)
- Design: 24.5 × 0.5 = 12.2 min
- Peak: 11.1 × 0.5 = 5.6 min
Chlorine dose at peak flow with the current 10 gal/day
- Feed = 10 gal × 1.24 lb/gal = 12.4 lb/day
- Dose = 12.4 ÷ (1.1 × 8.34) = 1.35 mg/L
- Demand is 1.6 mg/L, so the theoretical residual is negative - the measured 0.3 mg/L is consistent with a plant that has essentially no free chlorine left at the outlet.
Feed rate to hold 1.0 mg/L at peak
- Dose needed = 1.6 demand + 1.0 residual = 2.6 mg/L
- lb/day = 1.1 × 2.6 × 8.34 = 23.9 lb/day
- Gallons = 23.9 ÷ 1.24 = 19.3 gal/day
What it means: two failures happened at once - the dose per gallon was diluted by more than half, and the contact time was cut from 12 minutes to under 6. Fixing only the dose leaves the kill incomplete, because coliform inactivation depends on concentration × time, not concentration alone.
2. What is the maximum flow that preserves the design contact time, and how would you actually achieve it?
Read the worked answer
Solve for flow at 10 minutes effective contact
- t_eff = (V × 1440 × BF) ÷ Q
- 10 min = (0.0085 × 1440 × 0.5) ÷ Q
- Q = 6.12 ÷ 10 = 0.61 MGD
Any flow above about 0.6 MGD puts the chamber below its design basis. At the observed 1.1 MGD the plant cannot meet the contact component of the permit no matter how much hypochlorite it feeds - a useful thing to say out loud, because it reframes the problem from "chemistry" to "hydraulics."
Practical ways to get there
- Equalize ahead of disinfection: throttle the flow to chlorination and store the excess upstream (in the aeration basin freeboard, a storm tank, or the unused holding tank if one exists). This is the only option that genuinely restores contact time.
- Reduce infiltration/inflow: smoke testing and rehabilitation of the collection system is the long-term answer to a peak that is 2.2× design.
- Improve the chamber: add baffling, verify there is no short-circuiting or dead space, and confirm the 8,500 gallons is real liquid volume rather than nominal tank volume.
- Upsize the chamber or switch to UV if peaks are a recurring permit threat - but note UV is also flow- and transmittance-sensitive, so it does not remove the need to control peak flow.
What not to do: do not sign the DMR while assuming the higher residual compensates. If the contact time is short, the disinfection result is unreliable regardless of the residual number.
3. Critique the night operator's plan to run 30 gal/day to reach 2.5 mg/L residual during storms.
Read the worked answer
It is the wrong fix for four separate reasons.
- It does not restore contact time. At 5.6 minutes effective contact, a higher residual improves kill somewhat but cannot substitute for CT. Compliance would become a gamble on the day's coliform density rather than a design margin.
- Chlorine is consumed before it disinfects. At 30 gal/day, dose = 30 × 1.24 ÷ (1.1 × 8.34) = 4.06 mg/L, against 1.6 mg/L demand - so the outlet residual rises, but a large part of the extra chlorine simply oxidizes whatever else is in the water (ammonia, organics, reduced sulfur) rather than inactivating organisms.
- DBP and residual-limit exposure. Trihalomethanes and other disinfection byproducts form faster at higher dose, and many permits cap effluent total residual chlorine (commonly around 0.5–1.0 mg/L) or require dechlorination. Pushing 2.5 mg/L may trade a coliform exceedance for a chlorine residual exceedance and a receiving-water toxicity issue.
- Safety and cost. Hypochlorite at high feed rates means more storage, more off-gassing and splash exposure, more frequent deliveries, and accelerated scaling of the metering pump and injection quill (which itself reduces reliability when you need it most).
What the operator should be doing during a storm: verify the metering pump is actually delivering what it is set to (drawdown check, not just the dial), confirm the injector is not fouled, check for short-circuiting through the chamber, take residual readings at multiple points along the chamber, collect the coliform sample downstream of the full contact path, and document the flow and residual curve so the pattern is visible before the next storm. An elevated dose during a storm is a reasonable temporary mitigation only when combined with flow control and with the residual and DBP limits in view.