Regulations, Reporting, and Operator Math
The regulatory frame
Drinking water is governed by the Safe Drinking Water Act and its rules: MCLs, treatment techniques, MRDLs for disinfectants, monitoring and reporting, public notification, and the Consumer Confidence Report. Wastewater discharges operate under NPDES permits from the Clean Water Act, with effluent limits, monitoring, and Discharge Monitoring Reports. In South Carolina, DHEC administers primacy; sanitary sewer overflows carry strict reporting timelines, with 24-hour notification common for serious events.
Numbers worth memorizing
- lb/day = mg/L × MGD × 8.34 (converts mg/L in a million gallons to pounds).
- Dose = demand + residual.
- Detention time = volume ÷ flow; keep units consistent.
- Percent removal = (in − out) ÷ in × 100.
- 1 MGD = 694 gpm = 1.547 cfs; 1 psi = 2.31 ft of water.
- Feed rate in mL/min follows from solution strength and dose.
Reporting discipline
Read the permit before the exam. Know which parameters are daily, weekly, or monthly; the difference between a violation and an exceedance; how a monthly average is calculated; when resampling is allowed; and how a calibration is documented. Chain of custody and proper sampling technique protect the result.
Traps
The classic is plugging gpm into a formula that wants MGD, or using 8.34 when the answer wants pounds per year. Also remember MCLs apply to finished water, not raw.
Worked example
A 2.0 MGD plant doses 1.5 mg/L of chlorine: lb/day = 1.5 × 2.0 × 8.34 = 25 lb/day. If the solution is 12.5% available chlorine, you need 25 ÷ 0.125 = 200 lb/day of solution.