Biological Treatment Process Fundamentals
What you must know
Biological treatment relies on microorganisms converting dissolved and colloidal organics into biomass, CO2, and water. Aerobic processes require dissolved oxygen (typically 1.5–3 mg/L in activated sludge); anoxic zones enable denitrification; anaerobic processes (UASB, anaerobic digesters) handle high-strength streams and produce biogas. Key kinetics: substrate utilization and biomass growth (Monod), yield coefficient, decay, and solids retention time (SRT).
Process selection depends on loading, biodegradability, and toxicity. Suspended-growth (activated sludge) vs attached-growth (trickling filters, MBBR) systems differ in biomass management. Nutrient requirements follow roughly BOD:N:P = 100:5:1 for balanced growth. Temperature affects rates (Arrhenius); pH near 6.5–8.5 supports bacteria. Toxicity and shock loads can inhibit biomass; acclimation matters for industrial streams.
Traps in questions
- Assuming more aeration always means better treatment - oxygen transfer and mixing limits exist.
- Forgetting nutrient balance (N and P) for industrial streams lacking nutrients.
- Confusing SRT with hydraulic retention time (HRT).
- Overlooking that anaerobic systems are sensitive to temperature and pH.
Worked example / rule of thumb
For influent BOD 1000 kg/d, nutrient need ≈ N 50 kg/d and P 10 kg/d using 100:5:1. Rule of thumb: SRT controls biomass inventory and settling; longer SRT favors nitrification but increases aeration demand.