Last reviewed 16 Sept 2026 · 8 min read
Characteristics of industrial wastewater
Industrial wastewaters differ widely in quantity and quality, and generally differ from domestic sewage:
- Very high or variable organic strength (BOD/COD), sometimes non-biodegradable.
- Extreme pH (strong acids or alkalis).
- Toxic substances — heavy metals (chromium, lead, mercury, cadmium, nickel), cyanides, phenols, pesticides.
- High dissolved solids (salts), colour, oil and grease, high temperature.
- Deficiency of nutrients (N, P) needed for biological treatment.
- Intermittent or batch discharges and seasonal operation.
Effects of untreated industrial wastes
| On | Effects |
|---|---|
| Sewers | Corrosion by acids and sulphides; clogging by fibres, grease and solids; explosive or toxic gases; high temperatures damage pipes |
| Sewage treatment plants | Toxic substances kill microorganisms; shock loads upset biological processes; excess sludge; grease interferes with settling and filters |
| Receiving waters | Oxygen depletion, fish kills, colour and taste, toxicity, bioaccumulation of heavy metals (e.g. Minamata disease from mercury), eutrophication, thermal pollution |
| Land and groundwater | Soil salinity, heavy-metal contamination, groundwater pollution |
General approach
Waste minimisation (cleaner production)
- Volume reduction — classify and segregate wastes, conserve water, reuse process water, change production methods, control leaks and spills.
- Strength reduction — process changes, chemical substitution, equipment modification, segregation of concentrated wastes, equalisation of flows, by-product recovery (e.g. chromium recovery in tanneries, lignin/chemical recovery in paper mills, yeast and potash from distillery spent wash).
- Recycling and reuse of treated water — towards zero liquid discharge (ZLD).
Treatment options
- Treatment within the industry and discharge to a water body or land (meeting standards).
- Pre-treatment and discharge into municipal sewers (meeting sewer discharge standards).
- Common Effluent Treatment Plants (CETPs) — joint treatment for clusters of small and medium industries (e.g. tanneries, textile dyeing, electroplating).
Pre-treatment and treatment units
| Unit / process | Purpose |
|---|---|
| Screening | Remove fibres, rags, large solids |
| Equalisation tank | Dampens variations in flow and concentration; prevents shock loads; allows controlled feed to treatment |
| Neutralisation | Adjusts pH: acidic wastes — lime, caustic soda, soda ash, limestone beds; alkaline wastes — sulphuric acid, CO₂ (flue gas); mixing acidic and alkaline wastes of the same plant |
| Oil–water separators (API separators) and skimmers | Remove free oil and grease |
| Dissolved air flotation (DAF) | Removes emulsified oil, fats and light suspended solids with fine bubbles |
| Chemical coagulation and precipitation | Removes suspended solids, colour, phosphorus and heavy metals (as hydroxides or sulphides at controlled pH) |
| Oxidation–reduction | Chromium reduction — hexavalent Cr(VI) reduced to trivalent Cr(III) at low pH (using SO₂, sodium bisulphite or ferrous sulphate), then precipitated as Cr(OH)₃; cyanide destruction by alkaline chlorination; oxidation of sulphides and phenols |
| Biological treatment | Activated sludge, extended aeration, trickling filters, aerated lagoons for biodegradable organics; anaerobic processes (UASB, anaerobic lagoons, digesters) for high-strength wastes with biogas recovery |
| Adsorption | Activated carbon for colour, phenols and refractory organics |
| Ion exchange | Heavy metals, recovery of valuable metals |
| Membrane processes | Ultrafiltration, reverse osmosis for reuse and ZLD |
| Evaporation and crystallisation | Concentrate reject brines to solid salts (ZLD) |
| Advanced oxidation | Ozone, Fenton's reagent, UV/H₂O₂ for recalcitrant organics and colour |
Nutrients (N and P) may be added to biological reactors when the waste is deficient (a BOD : N : P ratio of about 100 : 5 : 1 is a common guide for aerobic treatment).
Characteristics and treatment of important industrial wastes
| Industry | Main characteristics | Typical treatment |
|---|---|---|
| Textile (dyeing, printing) | Strong colour, high pH, high COD, TDS, starch (sizing), detergents | Equalisation, neutralisation, coagulation, biological treatment, colour removal (adsorption, ozonation), RO for reuse |
| Tannery | Chromium (chrome tanning), sulphides (lime–sulphide unhairing), high TDS and salinity, high BOD, suspended solids and hair | Segregation of chrome and lime liquors, chrome recovery, sulphide oxidation, coagulation, biological treatment; CETPs in tannery clusters |
| Dairy | High BOD (milk, whey), fats, casein; biodegradable; turns acidic quickly | Grease removal, equalisation, aerobic or anaerobic biological treatment |
| Sugar mills | High BOD (sugars), seasonal, suspended solids, oil | Equalisation, anaerobic lagoons/UASB, aerobic treatment |
| Distillery (molasses-based) | Spent wash — extremely high BOD and COD, dark brown colour (melanoidins), low pH, high potassium and TDS | Biomethanation (anaerobic digesters) followed by aerobic treatment, concentration and incineration, composting with press mud; ZLD increasingly required |
| Pulp and paper | Lignin, colour, high COD, suspended fibres, black liquor, toxic chlorinated organics (from chlorine bleaching) | Chemical recovery of black liquor, fibre recovery, clarification, biological treatment; elemental-chlorine-free bleaching |
| Electroplating and metal finishing | Heavy metals (Cr, Ni, Cu, Zn, Cd), cyanides, acids and alkalis | Segregation, cyanide oxidation, chromium reduction, hydroxide precipitation, ion exchange, metal recovery |
| Iron and steel | Coke oven wastes with phenols, cyanides, ammonia; oil and scale from rolling mills; suspended solids | Ammonia stripping, biological phenol removal, settling, oil separation, recycling of cooling water |
| Fertiliser (nitrogenous) | Ammonia, urea, nitrates; fluorides (phosphatic plants) | Ammonia stripping/recovery, hydrolysis of urea, lime treatment for fluoride |
| Petroleum refinery | Oil, phenols, sulphides, ammonia | API separators, DAF, sour water stripping, biological treatment |
| Slaughterhouse and meat processing | Blood (very high BOD), fats, manure, suspended solids, pathogens | Screening, blood recovery, grease removal, anaerobic and aerobic treatment |
| Thermal power plants | Thermal pollution (cooling water), fly ash slurry, boiler blowdown | Cooling towers/ponds, ash ponds or dry ash handling, recirculation |
| Pharmaceuticals and chemicals | Solvents, antibiotics, high COD, toxic organics | Solvent recovery, physico-chemical plus biological treatment, advanced oxidation, incineration |