Wastewater Treatment
Wastewater Treatment
This domain covers influent characterization, preliminary and primary treatment, secondary biological treatment, disinfection, solids handling, effluent limits, and process math. North Dakota plants range from activated sludge and trickling filters to lagoons; cold weather slows biological activity and can freeze lines and clarifiers, so temperature effects are fair game.
Core rules
- BOD and TSS measure organic load and solids; effluent limits are permit-specific.
- Activated sludge control uses MLSS, DO, F/M, and sludge age (SRT); wasting controls SRT.
- Nitrification consumes oxygen and alkalinity and is temperature-sensitive.
- Lagoons rely on detention time, light, and settling; short-circuiting reduces treatment.
- Disinfection must meet CT and residual limits while controlling byproducts.
Worked example
An aeration tank holds 0.15 MG and influent flow is 0.75 MGD. Hydraulic detention time = 0.15 MG ÷ 0.75 MGD = 0.20 day. Convert to hours: 0.20 × 24 = 4.8 h. If flow increases to 1.0 MGD, detention time = 0.15 ÷ 1.0 = 0.15 day = 3.6 h. The reasoning: keep units consistent (MG and MGD), then multiply days by 24.
Traps
Mixing gpm and MGD; treating SRT as only aeration-tank solids while ignoring clarifier solids; over-wasting and losing nitrifiers; letting DO fall too low; ignoring cold-temperature effects on nitrification and settling.