Wastewater Treatment Processes and Unit Operations
What the candidate must know
Wastewater treatment typically includes preliminary (screening, grit removal), primary (sedimentation), secondary (biological treatment), and solids handling. Know the purpose of each stage and key parameters such as BOD, TSS, and the solids retention time.
Concepts to master
- Preliminary treatment: bar screens, comminutors, grit chambers.
- Primary clarifiers: removal of settleable solids; sludge and scum handling.
- Secondary treatment: activated sludge (aeration basin + clarifier) and the roles of mixed liquor suspended solids (MLSS), return activated sludge (RAS), waste activated sludge (WAS), and food-to-microorganism ratio.
- Trickling filters and lagoons as alternatives.
- Nitrification/denitrification basics and the effect of temperature and dissolved oxygen.
- Sludge thickening, digestion, and dewatering.
Traps in questions
A classic trap is confusing RAS (returned to maintain biomass) with WAS (wasted to control solids). Another is assuming more aeration is always better - over-aeration wastes energy and can affect denitrification. Questions may also test that low DO impairs nitrification.
Worked example / rule of thumb
Rule of thumb: control biomass with wasting; if MLSS climbs too high, clarifier solids loading rises and effluent TSS degrades. For nitrification, maintain adequate DO and solids retention time; cold water slows nitrifiers.