Chemical Coagulation and Flocculation

Chemical coagulation destabilizes colloidal particles and precipitates dissolved pollutants enabling removal via sedimentation or flotation, achieving 70-90% color removal, 60-80% COD removal, and 85-95% TSS removal in textile effluent. Coagulation mechanisms include charge neutralization (cationic coagulants neutralizing negatively charged dye molecules, colloids causing destabilization and aggregation), sweep flocculation (metal hydroxide precipitates enmeshing particles, dyes, organics in settling flocs), and complexation (coagulants forming insoluble complexes with dyes, heavy metals precipitating). Coagulants include aluminum-based (alum aluminum sulfate Al2(SO4)3 50-500 mg/L dosage, effective pH 5.5-7.5, generating Al(OH)3 flocs, economical $200-400/tonne, common for color and TSS removal, residual aluminum concerns limiting use in potable reuse), polyaluminum chloride PAC (pre-hydrolyzed aluminum, 30-300 mg/L dosage, wider pH range 5-9, faster flocculation, lower sludge generation 30-40% vs. alum, cost $400-800/tonne, preferred for textile applications), iron-based (ferric chloride FeCl3, ferric sulfate Fe2(SO4)3, ferrous sulfate FeSO4 100-600 mg/L dosage, effective pH 4-11 depending on salt, generating Fe(OH)3 flocs, good for reactive dyes, cost $150-400/tonne, corrosion concerns requiring plastic or coated equipment), and organic polymers (cationic polyelectrolytes—polyDADMAC, polyamines 5-50 mg/L dosage, charge neutralization mechanism, expensive $2,000-5,000/tonne but low dosage, effective for color removal, combining with metal coagulants for synergistic effects reducing total chemical cost 20-40%). pH optimization critical as coagulation pH-dependent: alum optimal pH 5.5-7.5 (lower pH requiring more coagulant, higher pH preventing floc formation), PAC wider tolerance pH 5-9, ferric salts pH 4-11 (acidic pH for anionic dyes, alkaline for cationic dyes, direct dyes). Jar testing determines optimal dosage and pH via laboratory-scale simulation: preparing samples at various coagulant doses (25, 50, 100, 200, 400 mg/L), adjusting pH (4, 5, 6, 7, 8, 9), rapid mixing (100-150 rpm, 1-2 min dispersing coagulant), slow mixing (20-40 rpm, 15-30 min promoting floc growth), settling (20-30 min), measuring supernatant color, turbidity, COD, identifying optimal conditions (lowest residual color, turbidity, COD at minimum coagulant dose balancing performance and cost). Coagulation-flocculation process comprises rapid mix tank (1-3 min retention, high-intensity mixing 100-200 rpm, G value 300-1000 s⁻¹ dispersing coagulant uniformly, initiating particle destabilization), flocculation tank (15-30 min retention, gentle mixing 20-40 rpm, G value 20-80 s⁻¹ promoting collisions, floc growth without shear breakup, multiple stages with decreasing intensity), polymer addition (0.5-5 mg/L cationic or anionic polyelectrolyte in later flocculation stage bridging microflocs into larger settleable flocs 5-20 mm diameter), sedimentation (2-4 hours retention, settling flocs, clear effluent overflow, sludge removal 10-50 kg dry solids/1000 m³ treated), or dissolved air flotation DAF (air dissolved under pressure 4-6 bar, released at atmospheric pressure forming micro-bubbles 10-100 μm, attaching to flocs, floating to surface, skimmed off, faster than sedimentation 20-30 min, suitable for low-density flocs from textile dyes). Performance includes color removal 70-90% (reactive dyes 60-80%, direct dyes 75-90%, disperse dyes 80-95%, acid dyes 70-85% depending on structure, charge, coagulant type), COD removal 60-80% (particulate and colloidal COD removed, soluble low-molecular-weight organics passing through), TSS removal 85-95%, heavy metal removal 80-95% (chromium, copper precipitating as hydroxides), sulfide oxidation (ferric salts oxidizing sulfide preventing odor, toxicity), and partial dye degradation (ferric flocs exhibiting Fenton-like oxidation of certain dyes 10-30% enhancing color removal). Sludge generation significant (10-50 kg dry solids per 1000 m³ treated, 2-5% solids concentration requiring thickening to 10-15%, dewatering to 20-40% solids for disposal, chemical sludge non-biodegradable limiting reuse, disposal via landfill $50-150/tonne or incineration $150-400/tonne if hazardous). pH adjustment post-coagulation may be required (alum, PAC generating acidity via hydrolysis, pH drop 1-2 units requiring neutralization with lime or caustic before discharge or biological treatment). Advantages include high color removal (superior to biological treatment), rapid treatment (30-60 min total vs. 6-24 hours biological), compact footprint (0.01-0.03 m²per m³/day), and flexibility (adjusting dose and pH for different effluent characteristics, batch campaigns). Limitations include high chemical costs ($0.20-0.60/m³ for coagulant plus pH adjustment), sludge disposal costs ($0.10-0.30/m³ for handling, disposal), residual metals in effluent (aluminum, iron 1-5 mg/L potentially requiring polishing), not addressing dissolved organics (soluble low-MW dyes, auxiliaries requiring advanced oxidation or adsorption), and sensitivity to effluent variability (requiring frequent jar testing, dose optimization 2-3 times per week for varying influent). Applications include primary treatment (before biological, removing color, reducing COD load improving biodegradability), tertiary treatment (after biological polishing effluent, removing residual color, TSS, preparing for reuse), or standalone treatment (small operations, low-BOD effluent dominated by color where biological not justified). Coagulation often combined with other processes: coagulation + biological (pre-treatment removing toxic dyes enabling biological, or post-treatment polishing), coagulation + advanced oxidation (coagulation removing bulk color and COD 60-80%, ozone or Fenton oxidizing remaining 20-40% achieving >95% removal at lower oxidant dose), and coagulation + membrane filtration (coagulation pretreatment reducing membrane fouling, extending life 2-5× in MF, UF, RO systems).