CT Shortfall on the Night Shift
A groundwater-under-influence plant chlorinates with free chlorine ahead of a 12,000-gallon clearwell. The state requires 3-log virus inactivation and 2-log Giardia inactivation. Design flow is 1.5 MGD. On the day shift, CT calculations routinely show compliance with margin. On the night shift, the operator notices that the clearwell level drops to 40 percent because of high distribution demand, and the chlorine residual entering the clearwell falls from 1.2 mg/L to 0.6 mg/L while the pH rises from 7.6 to 7.9. The plant has no online chlorine analyzer at the clearwell outlet, only a grab sample taken every four hours. The most recent grab sample at 02:00 showed 0.55 mg/L free chlorine. Disinfection is the only pathogen barrier; there is no filtration.
1. Explain why CT compliance may fail at night even though the day shift passes, and identify the variables the operator must check.
Read the worked answer
CT is concentration times contact time. At night, both terms can degrade: lower clearwell level reduces effective contact time, and lower residual reduces C. The pH rise also matters because free chlorine speciation shifts toward the less effective hypochlorite ion as pH rises, so the required CT for a given log credit increases. The operator must check actual clearwell volume at the low level, actual flow, outlet residual, pH, and water temperature, then compute CT against the required values for the target logs. The 02:00 grab of 0.55 mg/L is close to or below what many states require as a minimum residual, so the operator should treat the plant as potentially out of compliance until the calculation is done.
2. What immediate operational actions should be taken, and what should be done about the monitoring gap?
Read the worked answer
Immediate actions: raise the clearwell level if demand allows, increase chlorine feed to restore residual, and verify pH control. If CT cannot be met, the operator should notify the primacy agency per the plant's protocol and consider reducing flow to extend contact time. The four-hour grab interval is too coarse for a variable that can swing with demand; the operator should push for an online chlorine analyzer at the clearwell outlet with alarm setpoints, and in the interim increase grab frequency during high-demand periods. Document every reading and action.
3. A supervisor suggests simply increasing the chlorine dose to fix the problem. Evaluate that advice.
Read the worked answer
Increasing dose raises C and can restore CT, but it is incomplete. If the clearwell level is low, contact time is short and the dose may need to be impractically high, risking disinfection byproduct formation and taste and odor complaints. The pH rise reduces chlorine effectiveness, so dose alone does not address speciation. The better approach is to restore clearwell level and pH first, then adjust dose as needed, and verify with a CT calculation rather than assuming the residual is sufficient. The supervisor's advice is a partial fix that could create a new compliance problem.