Disinfection CT Versus Rising Turbidity
A groundwater-under-the-influence plant uses free chlorine for primary disinfection before a clearwell. Design flow is 1.8 MGD; the clearwell has a baffling factor of 0.5 and a volume of 90,000 gallons. The operator calculates CT each shift. Today raw turbidity rose to 8 NTU after a nearby river stage increase, and filtered turbidity is 0.35 NTU. Chlorine residual after the clearwell is 1.1 mg/L free, pH 7.6, water temperature 12°C. The required CT for the target log inactivation at these conditions is 8 mg/L·min. The operator's worksheet shows a calculated CT of 9.5 mg/L·min and concludes the plant is in compliance, so no changes are made. A grab sample later shows total coliform-positive results in the distribution system.
1. Check the CT calculation and identify any errors or assumptions that could make the reported 9.5 mg/L·min unreliable.
Read the worked answer
CT = residual concentration × contact time. Contact time must use the baffling factor: effective volume = 90,000 gal × 0.5 = 45,000 gal. At 1.8 MGD, detention time = 45,000 gal ÷ (1.8 × 10^6 gal/day) × 1,440 min/day ≈ 36 minutes. With 1.1 mg/L residual, CT ≈ 1.1 × 36 ≈ 39.6 mg/L·min, far above 8 - so either the operator used a much lower contact time, a different residual point, or the worksheet has a unit error. The reported 9.5 suggests a contact time near 8.6 minutes, which would imply a much smaller effective volume or a higher actual flow.
Key checks: was the residual measured at the clearwell outlet (correct) or at the plant tap (wrong)? Was the actual flow higher than design during the event? Was the baffling factor applied? Was pH and temperature used to select the correct CT table value? The 0.35 NTU filtered turbidity is a separate concern: many CT tables require turbidity ≤0.3 NTU for the credited inactivation, so the credit may not be valid at 0.35 NTU regardless of the arithmetic.
2. Given the coliform results and the turbidity excursion, what immediate actions and confirmations do you take?
Read the worked answer
Immediate actions:
- Confirm the positive results with repeat samples at the same and adjacent sites, and check for any confirmed fecal indicator.
- Verify disinfectant residual throughout the distribution system; boost if residual is low.
- Recheck the CT worksheet against actual flow, residual, pH, temperature, and the turbidity condition required for credit; if the credit is invalid at 0.35 NTU, treat the plant as not meeting primary disinfection and act accordingly.
- Increase coagulation/filtration performance to bring filtered turbidity back under 0.3 NTU and document.
- Notify the primacy agency per the coliform rule timeline if repeat samples confirm, and follow the public notification requirements.
- Investigate the source of contamination: cross-connection, storage, or treatment breakthrough. The 0.35 NTU turbidity is the likely root cause of lost disinfection credit; the CT arithmetic alone does not prove compliance.
3. What monitoring and documentation changes would you make to prevent a repeat?
Read the worked answer
- Require continuous filtered turbidity and a documented turbidity condition check before claiming CT credit each shift.
- Add a second-person verification of the CT worksheet, with actual flow and residual recorded at the same time as the calculation.
- Trend raw turbidity and river stage so the operator anticipates events and adjusts coagulation proactively.
- Increase distribution residual and coliform monitoring during and after turbidity events.
- Keep a written record of any CT credit disallowance and the corrective action, so the compliance file is defensible.
- Train staff on the link between turbidity and disinfection credit; the error here was treating CT as arithmetic only.