Wastewater Treatment: From Preliminary to Disinfection
What You Must Understand
Wastewater treatment removes solids, organics, nutrients, and pathogens. Know the train: preliminary (screening, grit removal) → primary (settling to remove settleable solids, producing primary sludge) → secondary (biological treatment, typically activated sludge or trickling filters) → secondary clarification → disinfection → discharge or reuse. Some plants add tertiary nutrient removal or filtration.
Key concepts: BOD (biochemical oxygen demand) measures biodegradable organic strength; TSS measures suspended solids; both drive permit limits. Activated sludge relies on a microbial community; operators control MLSS, SRT (sludge age), F/M ratio, and dissolved oxygen. Too low DO or too high F/M causes poor settling (bulking). Nitrification converts ammonia to nitrate and consumes oxygen and alkalinity; denitrification converts nitrate to nitrogen gas. Sludge handling includes thickening, digestion (aerobic or anaerobic), and dewatering.
Traps to Watch
- BOD vs. COD: COD includes non-biodegradable organics and is measured faster; COD is generally higher than BOD.
- F/M direction: high F/M = young sludge, poor settling; low F/M = old sludge, possible pin floc.
- Nitrification consumes alkalinity-questions often omit this link to pH crashes.
- Return vs. waste sludge: return keeps biomass in the aeration tank; wasting controls SRT. Confusing the two is common.
Rule of Thumb
For settling problems, check the sludge volume index (SVI): roughly 80–150 mL/g indicates good settling; values well above that suggest bulking. Confirm with DO, F/M, and microscopic exam before changing wasting rates.
Study Focus
Trace the treatment train, link operational controls to effluent quality, and know BOD/TSS fundamentals.