Chlorine Demand Spike Before Discharge Permit Limit
A 2.5 MGD secondary plant disinfects with sodium hypochlorite in a contact chamber designed for 30 minutes at peak flow. The discharge permit requires fecal coliform below 200 CFU/100 mL (geometric mean) and total residual chlorine below 0.1 mg/L at the outfall. On Tuesday the effluent is unusually turbid (25 NTU, normally 5 NTU) because of a clarifier upset, and the chlorine dose is raised from 3.0 to 5.0 mg/L to hold the coliform count. The contact chamber effluent shows 0.6 mg/L total residual chlorine, and the dechlorination sulfite pump is running at its normal setting. The afternoon coliform sample comes back at 150 CFU/100 mL, just under the limit. The operator has one day before the next sample and must decide how to proceed.
1. Explain why raising the chlorine dose did not produce a comfortable coliform margin and what the residual reading indicates.
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Turbidity shields organisms and consumes chlorine, so a large fraction of the added dose is spent on particles and reduced compounds rather than on disinfection. That is why 5.0 mg/L produced only a marginal coliform result: the chlorine demand of the turbid effluent is high, and the contact time available to the surviving organisms is effectively reduced.
The 0.6 mg/L total residual at the chamber outlet shows that chlorine is present, but total residual includes combined chlorine (chloramines), which is a much weaker disinfectant than free chlorine. The key diagnostic is the free versus total residual split: if free chlorine is low while total is 0.6 mg/L, the plant is mostly forming chloramines and getting little disinfection benefit. The right response is to fix the clarifier upset and reduce turbidity first, then verify free residual and contact time rather than continuing to raise dose.
2. The dechlorination pump is at its normal setting but outfall residual must stay below 0.1 mg/L. What should the operator do before the next sample, and what records or notifications are involved?
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The operator should not simply keep the high dose and hope the sulfite pump keeps up. Steps: (1) confirm the sulfite feed is actually reaching the effluent and check for pump or injection problems; (2) measure free and total chlorine at the chamber outlet and at the outfall to see where the residual is breaking through; (3) increase dechlorination feed in a controlled way while monitoring outfall residual, because an overfeed can drive residual to zero but also indicates the disinfection step is being defeated; (4) address the clarifier upset to bring turbidity down, since that is the root cause.
If the outfall residual exceeds the permit limit, that is a reportable excursion under most permits and should be documented and reported per the permit's notification requirements. Records should include chlorine dose, residuals, turbidity, sulfite feed rate, and the corrective actions taken. The coliform result of 150 CFU/100 mL should be flagged as a marginal result even though it is under the limit, because the geometric mean is what matters and one high value can pull it up.