Water Treatment
Delaware's Water Treatment domain tests whether you can run a treatment train from raw water to finished water: coagulation, flocculation, sedimentation, filtration, disinfection, corrosion control, and the calculations and records that keep it compliant. Most Delaware public supply is groundwater, where iron, manganese, hardness, and 4-log virus treatment dominate; surface plants on the Brandywine, White Clay, and Red Clay add filtration and DBP control.
Core rules
- Conventional filtration: combined filter effluent turbidity ≤0.30 NTU in at least 95% of monthly measurements and never >1 NTU. Slow sand and diatomaceous earth: ≤1 NTU in 95% of measurements, never >5 NTU.
- Disinfection is judged by CT, where C is the residual disinfectant concentration (mg/L) and T is T10 contact time (minutes). T10 = theoretical detention time × baffle factor. Use peak hourly flow, not average flow.
- Groundwater Rule: provide 4-log virus inactivation, or state-approved monitoring and corrective action.
- Stage 2 DBPR: TTHM LRAA ≤80 µg/L and HAA5 LRAA ≤60 µg/L at each monitoring location.
- Lead and Copper Rule: 90th-percentile lead action level 0.015 mg/L and copper 1.3 mg/L; maintain optimal corrosion control.
- Secondary limits: iron 0.3 mg/L and manganese 0.05 mg/L.
Worked example
A plant treats 1.0 MGD at peak hourly flow. Contact basin volume is 12,000 gal, baffle factor 0.5, and free chlorine residual is 0.8 mg/L. Detention = 12,000 ÷ 1,000,000 gal/day = 0.012 day = 17.3 min. T10 = 17.3 × 0.5 = 8.6 min. CT = 0.8 × 8.6 = 6.9 mg·min/L. If the required CT for the target inactivation is 6.0, the process passes, but only with a small margin; using dose instead of residual would overstate CT.
Traps
Using applied chlorine dose rather than measured residual; using average flow instead of peak hourly flow; forgetting the baffle factor; applying free-chlorine CT tables to chloramines; quoting individual-filter turbidity when the rule is combined-filter effluent; treating 1 NTU as always allowed.