Primary Treatment - Screening and Equalization

Primary treatment removes gross solids, equalizes flow and load variations, and neutralizes pH preparing effluent for secondary biological treatment. Screening removes large solids (fibers, lint, threads, packaging materials) via bar screens (25-50 mm openings removing coarse debris), fine screens (3-10 mm removing lint, short fibers preventing pump clogging, biofilm growth), and micro-screens (0.5-2 mm cartridge or drum screens for final solids removal before downstream treatment), manual cleaning for small operations (daily maintenance) or automatic self-cleaning (backwash or mechanical scrapers for continuous operation), captured solids (1-5 kg/1000 m³ treated) disposed via landfill or incineration. Equalization balances flow surges and concentration peaks via large tanks (6-24 hours retention, 500-5,000 m³ capacity depending on plant size) with mixing (mechanical aerators, diffused air, recirculation pumps preventing settling, maintaining homogeneity), accommodating batch discharges from dyeing machines (concentrated slugs of high-COD, high-color effluent), continuous processes generating steady lower-strength streams, and non-production periods (nights, weekends with no discharge but treatment continuing), benefits include dampening concentration fluctuations ±50% to ±10%, enabling consistent biological treatment (shock loads causing biomass washout, process upset, poor removal efficiency), optimizing chemical dosing (coagulation, neutralization based on average characteristics not peak), and managing peak flows (preventing hydraulic overload of downstream units, surface overflow, poor settling). pH adjustment (neutralization) corrects acidic (pH 2-5 from carbonizing, acid dyeing) or alkaline (pH 10-13 from scouring, mercerization, reactive dyeing) effluent to neutral range pH 6-9 optimal for biological treatment and discharge, using acids (sulfuric H2SO4 most economical $50-150/tonne, hydrochloric HCl, nitric HNO3) or alkalis (lime Ca(OH)2 economical $100-200/tonne but generating sludge, sodium hydroxide NaOH more expensive $400-600/tonne but no sludge, sodium carbonate Na2CO3), dosing via automated pH control systems (online pH probes triggering chemical pumps, feedback control maintaining setpoint ±0.2 pH units), two-stage neutralization (coarse adjustment to pH 5-8 with lime or soda ash, fine tuning to pH 6.5-7.5 with NaOH or H2SO4 preventing overshoot, reducing chemical consumption 10-20%), reaction tanks (10-30 min retention allowing complete reaction, mixing), and safety considerations (exothermic reactions, corrosive chemicals requiring proper materials HDPE, FRP, stainless steel). Sedimentation (primary clarifiers) removes settleable suspended solids via gravity separation in circular or rectangular tanks (2-4 hours retention, surface overflow rate 20-40 m³/m²/day, depth 3-5 m), mechanisms include discrete settling (individual particles settling independently, Stokes' Law velocity proportional to particle size squared and density difference), flocculent settling (particles aggregating during descent, increasing effective size and settling velocity), and zone settling (concentrated suspensions settling as mass, compression zone at bottom), typical removal efficiency 50-70% TSS, 20-40% BOD, 10-20% COD (suspended organics removed, dissolved organics passing through), sludge collection via scrapers (rotating arms in circular clarifiers, chain-and-flight in rectangular), pumping to sludge handling (5-20 kg dry solids per 1000 m³ treated, 2-5% solids concentration requiring thickening, dewatering before disposal), and scum removal (floating oils, grease, fibers skimmed from surface 0.5-2 kg/1000 m³). Design considerations include hydraulic loading (flow rate per unit area balancing throughput and settling efficiency, overloading causing carryover of solids), solids loading (kg solids per m² per day, excessive loading overwhelming sedimentation capacity), short-circuiting prevention (baffles, proper inlet/outlet design ensuring plug flow, avoiding dead zones, preferential flow paths), temperature effects (cold temperature increasing viscosity slowing settling, warm temperature decreasing requiring larger tanks), and wind effects (outdoor tanks susceptible to wind-driven currents disrupting settling, covers or windbreaks recommended). Primary treatment costs relatively low (capital $50,000-500,000 depending on capacity, operating $0.05-0.20/m³ for power, chemicals, maintenance) but limited pollutant removal (not addressing color, most COD, dissolved organics requiring secondary biological or tertiary advanced treatment), essential for protecting downstream processes (screening preventing clogging, equalization preventing shock loads, neutralization protecting biomass, sedimentation reducing organic load by 20-40% easing biological treatment burden).