Biological Treatment - Activated Sludge

Biological treatment uses microorganisms (bacteria, protozoa, fungi) degrading dissolved organic matter reducing BOD 85-95% and COD 60-80% in textile effluent via aerobic processes requiring oxygen or anaerobic processes without oxygen. Activated sludge process most common aerobic treatment comprising aeration tank (6-24 hours retention, 2,000-10,000 mg/L MLSS mixed liquor suspended solids concentration of active biomass, mechanical aerators or diffused air supplying 1.5-2.5 kg O2 per kg BOD removed maintaining dissolved oxygen 2-4 mg/L), biological reactions (heterotrophic bacteria oxidizing organic matter—BOD, biodegradable COD—to CO2, H2O, new cells via aerobic respiration: Organic matter + O2 → CO2 + H2O + Energy + New cells, autotrophic bacteria nitrifying ammonia to nitrate: NH4+ → NO2- → NO3- consuming 4.6 kg O2 per kg NH4-N oxidized), secondary clarifier (2-4 hours retention, 20-30 m³/m²/day surface overflow rate settling activated sludge, clear treated effluent overflows, concentrated sludge returns to aeration tank maintaining biomass or wastes as excess sludge), return activated sludge RAS (50-100% of influent flow rate recycling settled biomass maintaining MLSS 2,000-10,000 mg/L in aeration), and waste activated sludge WAS (5-10% of RAS removing excess biomass growth maintaining steady-state MLSS, sludge age or solids retention time SRT 5-15 days for conventional systems, 20-30 days for nitrification). Performance parameters include BOD removal 85-95% (influent 400-2,000 mg/L reducing to 20-50 mg/L effluent), COD removal 60-80% (lower than BOD due to non-biodegradable recalcitrant organics from dyes, auxiliaries passing through, influent 1,000-5,000 mg/L reducing to 200-500 mg/L), TSS removal 90-95% (biomass settling in clarifier, effluent TSS 10-30 mg/L meeting discharge limits), nitrogen removal (partial via nitrification-denitrification, 40-70% ammonia removal, complete removal requiring modified systems with anoxic zones), phosphorus removal (limited 10-30% via biomass uptake, enhanced removal requiring chemical precipitation), and color removal (poor 10-30%, dyes largely passing through biological treatment, advanced oxidation or adsorption required for decolorization). Process variants include conventional plug-flow (influent at one end, effluent at other, concentration gradient along tank length, simple, susceptible to shock loads), complete-mix (influent distributed throughout tank, uniform conditions, shock load dampening, common for industrial wastewater), extended aeration (long SRT 20-30 days, low F/M ratio 0.05-0.15 kg BOD/kg MLSS/day achieving higher removals, endogenous respiration reducing sludge production 30-50%, suitable for small plants, variable loads), sequencing batch reactors SBR (fill-react-settle-draw cycles in single tank, flexible operation, no clarifier required, 4-6 cycles per day, popular for small-medium installations 100-5,000 m³/day), moving bed biofilm reactor MBBR (biomass growing on plastic carriers providing 200-500 m²/m³ specific surface area, suspended in aeration tank via air, compact design 50% smaller footprint than conventional, higher shock load tolerance), and membrane bioreactor MBR (ultrafiltration membranes replacing clarifier achieving excellent solids removal TSS <5 mg/L, compact, high MLSS 8,000-15,000 mg/L, footprint reduction 50-70%, higher capital and operating cost, used for water reuse applications). Limitations for textile effluent include recalcitrant organics (synthetic dyes, certain auxiliaries resisting biodegradation, COD removal limited to 60-80% vs. 85-95% for municipal wastewater), color persistence (90% of color remaining post-biological treatment requiring tertiary decolorization), toxicity issues (some dyes, heavy metals, surfactants inhibiting biomass activity at high concentrations, requiring dilution or pretreatment), high salinity (TDS 3,000-10,000 mg/L from dyeing salts inhibiting biomass above 10,000-15,000 mg/L, requiring salt-tolerant biomass acclimation or dilution with low-TDS streams), pH fluctuations (requiring strict pH control 6.5-8.5 via equalization, neutralization), temperature variations (optimal 20-35°C, cold weather slowing kinetics requiring larger tanks or longer retention, hot weather accelerating but reducing oxygen solubility), and nutrient requirements (textile effluent deficient in nitrogen and phosphorus, BOD:N:P ratio 100:5:1 required for optimal growth, supplementation with urea, ammonia, phosphoric acid 5-20 kg/1000 m³ treated). Operational monitoring includes daily dissolved oxygen (2-4 mg/L target via aeration rate adjustment), MLSS (maintaining 2,000-10,000 mg/L via RAS and WAS control), SVI sludge volume index (100-150 mL/g good settling, >200 mL/g bulking sludge requiring corrective action—selector zones, antifoam, reducing organic load), pH (6.5-8.5 range), temperature, microscopy (weekly assessment of floc structure, filamentous bacteria, protozoa indicating process health), and effluent quality testing (daily TSS, BOD, COD verifying performance). Costs include capital ($500,000-5,000,000 for 1,000-10,000 m³/day capacity depending on process complexity), operating ($0.30-0.80/m³ dominated by energy for aeration 0.5-1.5 kWh/m³, nutrients, sludge disposal $50-200/tonne dry solids), and footprint (0.05-0.15 m²per m³/day for conventional, 0.02-0.08 m² for compact systems like MBBR, MBR). Biological treatment essential for reducing biodegradable organics economically but insufficient for complete textile effluent treatment requiring combination with chemical coagulation for TSS and advanced oxidation or adsorption for color and recalcitrant COD.