Wastewater Treatment - Ohio Operator Study Guide
Wastewater Treatment covers the biological, physical, and chemical processes that meet Ohio NPDES permit limits. Exam questions target secondary treatment standards, activated sludge control, nitrification/denitrification, phosphorus removal, solids handling, disinfection, and permit reporting.
Core rules
- Secondary treatment: monthly average 30 mg/L BOD5 and 30 mg/L TSS, with at least 85% removal of each; pH 6–9. Ohio permits may be stricter and may use CBOD5, ammonia, or phosphorus limits.
- Activated sludge is controlled by F/M, mean cell residence time (SRT/MCRT), MLSS, dissolved oxygen, and sludge volume index. Return sludge maintains MLSS; wasting sets SRT.
- Nitrifiers are slow growers and are washed out at low SRT or low temperature. Denitrification needs an anoxic zone and a carbon source; alkalinity is consumed by nitrification.
- Phosphorus removal can be chemical (alum or ferric) or biological; Ohio's Lake Erie basin permits often drive tighter phosphorus limits.
- Solids handling: thickening, digestion, dewatering; track percent solids and mass, not just volume.
- Disinfection must meet CT or UV dose and coliform limits; dechlorinate where required.
Worked example
Aeration volume 1.0 MG, MLSS 2,500 mg/L. Waste flow 0.1 MGD at 8,000 mg/L; effluent TSS 10 mg/L at 1.0 MGD. Solids in aeration = 1.0 × 2,500 × 8.34 = 20,850 lb. Wasted solids = 0.1 × 8,000 × 8.34 = 6,672 lb/day. Effluent solids = 1.0 × 10 × 8.34 = 83.4 lb/day. SRT = 20,850 ÷ (6,672 + 83.4) = 3.1 days. The reasoning: include effluent solids, and use 8.34 to convert mg/L × MGD to lb/day.
Traps
Omitting the 8.34 factor; ignoring effluent TSS in SRT; confusing BOD5 with CBOD5; assuming 30/30 is the only limit; and wasting too much sludge, which drops SRT and kills nitrification.