Wastewater Treatment and Collection Operations
Wastewater Treatment and Collection Operations
This domain covers collection-system integrity and the biological treatment used to meet SPDES limits. New York plants typically must meet secondary treatment: 30-day average effluent CBOD5 and TSS of 30 mg/L or less, with at least 85% removal, plus pH 6–9 and permit-specific ammonia, phosphorus, or disinfection limits.
Core rules and concepts
- Secondary treatment is a concentration and percent-removal standard. Calculate removal as (influent − effluent) ÷ influent × 100.
- Activated sludge control uses MLSS, sludge age (MCRT), F/M ratio, dissolved oxygen, and sludge volume index (SVI). Rising SVI with poor settling suggests bulking.
- Collection systems must control infiltration and inflow (I/I). Sanitary sewer overflows (SSOs) are reportable; combined sewer overflows (CSOs) are permitted, but both need prompt response and documentation.
- Lift stations need backup power, alarms, and wet-well level control; grease and grit cause most preventable pump failures.
Worked example
Influent BOD5 is 220 mg/L and effluent BOD5 is 18 mg/L. Removal = (220 − 18) ÷ 220 × 100 = 91.8%. Influent TSS is 240 mg/L and effluent TSS is 22 mg/L, giving (240 − 22) ÷ 240 × 100 = 90.8%. Both concentrations are below 30 mg/L and both removals exceed 85%, so the monthly average is compliant. A single grab sample above 30 mg/L is not automatically a monthly violation, but repeated highs can push the average over the limit.
Traps
Computing percent removal from mass loading when the permit specifies concentration, or mixing BOD5 with CBOD5, changes the result. Do not treat a CSO as an unpermitted SSO; the permit and reporting path differ. A low effluent concentration does not excuse a missed 85% removal if influent is weak.