Water Quality and Wastewater Management
Water Quality and Wastewater Management
This domain tests the barriers that keep drinking water safe, the treatment that makes wastewater safe to discharge, and the regulatory numbers attached to both. Expect treatment-train reasoning, disinfection calculations and pollutant limits.
Drinking water
Source protection → treatment → distribution. Conventional surface water treatment: coagulation/flocculation, sedimentation, filtration, disinfection. Groundwater may need only disinfection, but requires sanitary surveys and wellhead protection. Multiple barriers are the organising principle.
Key metrics:
- Turbidity is a pathogen surrogate: conventional filtration ≤0.3 NTU in at least 95% of monthly samples and never above 1 NTU.
- Disinfection uses CT (residual disinfectant concentration × contact time, mg·min/L) compared with tables for log inactivation of Giardia and viruses; Cryptosporidium is far more chlorine-resistant and is controlled by UV or ozone.
- Disinfection by-products: TTHM 80 µg/L and HAA5 60 µg/L as locational running annual averages.
- Lead action level 15 µg/L and copper 1.3 mg/L at the 90th percentile - action levels, not MCLs.
- Nitrate MCL 10 mg/L as nitrogen; arsenic 10 µg/L; fluoride MCL 4 mg/L with a 2 mg/L secondary standard.
Wastewater
Primary (physical), secondary (biological, activated sludge or trickling filters), tertiary (nutrient removal, filtration). Regulated by BOD5, TSS, ammonia and pathogen indicators. Discharge runs through NPDES permits, with TMDLs where the water body is impaired. Onsite systems need siting, soil evaluation and maintenance; combined sewer overflows and cross-connections are recurring contamination routes.
Worked example
A plant must achieve 3-log Giardia inactivation at 10 °C, pH 7. Measured free chlorine residual is 1.2 mg/L with a contact time of 25 minutes at peak hourly flow: CT = 30 mg·min/L. The table requires about 78 mg·min/L. Shortfall. Options: raise the residual (78/25 = 3.1 mg/L, worsening DBP formation), extend contact time to about 65 minutes, earn additional credits from improved filtration, or switch primary disinfection to ozone or UV, which also targets Cryptosporidium. Reasoning: CT credit uses the residual at the end of the contactor and the peak flow, not the average.
Traps that cost marks
- Treating the lead action level as an MCL.
- Forgetting that CT depends on temperature and pH.
- Confusing primary and secondary standards.
- Using average rather than peak flow in the CT calculation.
- Reporting nitrate as NO3 when the limit is expressed as N (10 mg/L as N is about 45 mg/L as NO3).