Wastewater Treatment and Collection
Wastewater Treatment and Collection
This domain tests how Nevada collection systems and wastewater treatment plants protect public health and receiving waters. Expect collection-system hydraulics, lift stations, infiltration/inflow, secondary treatment, disinfection, solids handling, NPDES permit limits, and operator math.
Core rules
- Collection: gravity sewers need adequate slope and velocity (often about 2 ft/s minimum for self-cleansing); lift stations use wet wells, pumps, force mains, and air or vacuum relief. Control grease, roots, and H2S corrosion; confined spaces require entry permits and gas testing.
- Treatment: preliminary screening and grit, primary settling, secondary biological treatment (activated sludge, trickling filters, lagoons), disinfection, and solids dewatering. Nevada plants commonly must meet BOD5, TSS, pH, fecal coliform, and sometimes ammonia or phosphorus limits.
- Activated sludge control uses mixed liquor suspended solids (MLSS), sludge age (SRT), F/M ratio, and return and waste sludge rates. Lagoons rely on detention time and seasonal biology.
- Biosolids and effluent reuse must meet permit and state requirements; Southern Nevada's recycled water returns to the Las Vegas Wash and Lake Mead.
Worked example
An aeration tank holds 0.50 MG. Influent flow is 1.5 MGD. Detention time = volume ÷ flow = 0.50 MG ÷ 1.5 MGD = 0.333 day × 24 h/day = 8.0 hours. If the same plant receives BOD5 at 220 mg/L, BOD load = 1.5 MGD × 220 mg/L × 8.34 = 2,752 lb/day. Use the load, not the concentration alone, to judge whether the biomass can handle it.
Traps
- Forgetting 8.34 in the pounds formula, or mixing gpd with MGD.
- Confusing detention time with sludge age (SRT); SRT uses solids inventory, not tank volume alone.
- Reporting BOD when the permit limit is CBOD5 or TSS.
- Missing infiltration/inflow: stormwater and groundwater entering sewers can overwhelm the plant.
- Sampling before disinfection when the permit limit is for fecal coliform after disinfection.