ETP (ETP)

An Effluent Treatment Plant (ETP) is a process facility that treats industrial wastewater — distinct from sewage — to bring pollutant levels (BOD, COD, TSS, TDS, pH, heavy metals, oils & grease, specific organics) within SPCB discharge standards under the Environment (Protection) Rules 1986, Schedule VI, before discharge to inland water, sewer or land. Every industrial unit holding a Consent to Operate that generates wastewater must operate and maintain an ETP scaled to its hydraulic load and pollutant profile.

A generic ETP runs in four stages. Preliminary: bar screens, grit chamber, equalisation tank — removes coarse solids and buffers variable flow. Primary: pH correction (HCl/H2SO4 for caustic streams, lime/NaOH for acidic), coagulation-flocculation (alum, ferric chloride, polyelectrolyte), primary clarifier — drops 50-70% of TSS, 30-40% of COD. Secondary: biological aeration (activated sludge, MBBR, SBR, or UASB for high-BOD anaerobic load) followed by secondary clarifier — removes 80-95% of remaining BOD/COD. Tertiary: sand filter, activated carbon adsorption, disinfection (chlorine, UV, ozone), and where TDS is the binding constraint, reverse osmosis followed by evaporation and crystallisation for Zero Liquid Discharge. The choice and sizing of stages depends entirely on raw effluent characterisation.

Sizing is hydraulically straightforward — design flow with 20% margin — but pollutant-load economics drive the bill of materials. A 50 m³/day PET wash-line ETP costs Rs 35-65 lakh capex with opex of Rs 8-18 per m³. A 100 m³/day e-waste hydrometallurgy ETP with heavy-metal precipitation, sulphide polishing and ZLD runs Rs 4-8 crore capex with opex of Rs 200-450 per m³. CBG digestate handling adds anaerobic digestion of the slurry itself, plus aerobic polishing on the bleed.

For recycling plants, the recurring failure modes are: undersized equalisation — surge flow during shift change shocks the biology; insufficient nutrient dosing — biological tanks need N:P ratios of about BOD:N:P = 100:5:1 which several recycling effluents lack; sludge disposal cost neglect — primary chemical sludge and biological excess sludge add Rs 8-25/kg to opex via cement-kiln co-processing; and untreated high-TDS bleed — biological treatment removes none of the TDS, so high-TDS streams need RO + evaporation that doubles ETP capex.