Industrial wastewater treatment and zero liquid discharge

In short

Industrial wastewater treatment is the design and operation of unit processes that bring a factory's effluent — often strong, toxic, coloured, saline or shock-loaded — down to the discharge limits set under the Environment (Protection) Rules, 1986, or to a quality fit for reuse. Zero liquid discharge (ZLD) goes further: no liquid effluent leaves the plant boundary; water is recovered for reuse and dissolved salts leave as solids. It matters because Indian regulators (CPCB and the State Pollution Control Boards) now tie Consent to Operate for several highly polluting sectors to reuse and, in named sectors and states, to ZLD. It is examined in GATE-CE/ES environmental engineering, in university water-and-wastewater papers, and in interviews at Thermax, VA Tech Wabag, Ion Exchange, Praj, L&T Water, Aquatech, EPC consultancies, and in EHS roles at textile, tannery, pharma, distillery, dairy, steel and power companies.

Explained

Characterising industrial effluent

• Why characterisation comes first: the treatment train is chosen from the pollutant, not the flow. Measure flow pattern (24-h), pH, temperature, BOD₅, COD, TSS, TDS, oil and grease, colour, specific toxics (Cr, CN, phenol, F, NH₃-N, sulphide), and conductivity.
• BOD/COD ratio indicates biodegradability: > 0.5 readily biodegradable (dairy, sugar, food); 0.3–0.5 partly; < 0.3 largely non-biodegradable (textile dye, pharma intermediates, tannery liquor) and needs chemical or oxidative pre-treatment.
• Typical strengths (indicative ranges; vary widely by process): dairy COD 1,500–4,000 mg/L; textile dyeing COD 800–2,500 mg/L with TDS 3,000–8,000 mg/L; tannery COD 3,000–6,000 mg/L with chromium and sulphide; molasses distillery spentwash COD 80,000–130,000 mg/L; pulp and paper COD 1,000–3,000 mg/L with colour and lignin.
• Volumetric and organic loading vary hour to hour with batch processes, so equalization is almost always the first tank.
• Pollution prevention before treatment: segregation of streams (concentrated vs dilute, toxic vs biodegradable), counter-current rinsing, dry-sweeping before wash-down, condensate and cooling-water recovery, process substitution (chrome-free tanning, low-salt reactive dyes). Reducing volume and TDS at source is the cheapest ZLD step.

Primary and physico-chemical treatment

1. Screening and grit removal — rags, fibres, pulp knots, grit from ore or mineral handling.
2. Equalization — a mixed, often aerated tank (6–24 h, sometimes a full shift's or day's volume) that damps flow, pH and load swings and protects biology downstream.
3. Neutralization — lime, caustic or soda ash for acidic streams; sulphuric acid or CO₂ for alkaline streams; discharge pH must be 5.5–9.0. Self-neutralization by mixing acidic and alkaline process streams saves chemicals.
4. Oil and grease removal — API/TPI separator (gravity, free oil, refineries), tilted-plate interceptor, then dissolved air flotation (DAF) for emulsified oil and light solids; de-emulsification by acid cracking or polyelectrolyte where needed.
5. Coagulation–flocculation–clarification — alum, ferric chloride, ferrous sulphate, PAC with anionic/cationic polyelectrolyte; jar test fixes dose and pH. Removes colloidal COD, colour, phosphate and metal hydroxides.
6. Chemical precipitation of heavy metals — as hydroxides at the metal's minimum-solubility pH (Cr³⁺ ≈ 8–9, Zn ≈ 9–10, Ni ≈ 10–11, Cu ≈ 8–9), or as sulphides (lower residual solubility) or carbonates. Chelating agents (EDTA, citrate, ammonia) hold metals in solution and must be destroyed first.
7. Specific destruction reactions
   • Hexavalent chromium: reduce Cr⁶⁺ → Cr³⁺ at pH 2–3 with sodium metabisulphite, sulphur dioxide or ferrous sulphate, then precipitate as Cr(OH)₃ at pH 8–9.
   • Cyanide: alkaline chlorination at pH > 10 (CN⁻ → CNO⁻ → N₂ + CO₂) in two stages; never acidify cyanide waste (HCN gas).
   • Sulphide (tannery beamhouse): catalytic oxidation with air over manganese sulphate, in a segregated stream.
8. Chemical oxidation and AOPs — chlorine/hypochlorite, ozone, Fenton (Fe²⁺ + H₂O₂, pH 3–4), photo-Fenton, UV/H₂O₂, ozone/H₂O₂. They generate hydroxyl radicals that break rings and colour, raising BOD/COD so biology can finish the job. Costly, so used as a pre-treatment or polish, not as the main process.
9. Electrocoagulation / electro-oxidation — sacrificial Al or Fe anodes release coagulant in situ; compact, low sludge-handling, but electrode passivation and power cost limit scale-up.
10. Adsorption and ion exchange — granular activated carbon for residual COD, colour, phenol, pesticide; chelating resins for metal recovery; weak/strong acid-base resins for demineralisation of reuse water.

Biological treatment of industrial waste (where it differs from sewage)

• Nutrient balance: industrial waste is often nutrient-poor; dose to roughly BOD : N : P = 100 : 5 : 1 (urea and DAP in practice).
• Inhibition and shock: metals, sulphide, phenol, high salinity and surfactants inhibit biomass; acclimatised or halotolerant cultures, dilution and bioaugmentation are used. TDS above roughly 15,000–20,000 mg/L makes conventional activated sludge unreliable.
• Anaerobic first for strong waste: UASB or hybrid reactors treat high-COD streams (distillery spentwash, starch, brewery, pulp black liquor condensate), removing 60–80% COD at 8–15 kg COD/m³·d and yielding methane that offsets boiler fuel. Little sludge is produced.
• Aerobic polishing then follows (extended aeration, MBR, moving-bed media) to meet BOD and COD limits.
• Temperature: cool hot effluent (quench tanks, cooling towers) before biology; mesophilic optimum 30–38 °C.
• Detailed design of the aerobic reactors themselves belongs with activated sludge/SBR/MBBR; here the point is pre-treatment, segregation and conditioning that make biology survivable.

Zero liquid discharge: concept and the standard train

• Definition: no liquid effluent crosses the plant boundary; typically ≥ 95% of the water is recovered and reused, and the dissolved solids leave as a dry or semi-dry salt/sludge.
• Why it is pursued in India: scarce freshwater and high water tariffs; regulator directions for named sectors; river-basin programmes (grossly polluting industries in the Ganga basin); public litigation (the Tirupur textile dyeing cluster is the classic case, where dyeing units were restrained from discharging and moved to RO-plus-evaporation with salt recovery).
• Typical sequence
  1. Pre-treatment — equalization, neutralization, oil removal, chemical precipitation, biological COD removal.
  2. Tertiary polishing — clarification, pressure sand filter, activated carbon filter, ultrafiltration; softening (lime–soda, or weak acid cation resin) and antiscalant dosing to protect membranes from CaSO₄, CaCO₃, silica and BaSO₄ scaling.
  3. Primary RO at 70–80% recovery → permeate to reuse (cooling towers, process, dyeing after polishing by mixed-bed or EDI).
  4. Reject concentration — brine-concentrator RO / high-pressure or disc-tube RO, HERO (high-efficiency RO at high pH, which keeps silica soluble), electrodialysis reversal, or nanofiltration to split monovalent from divalent salts.
  5. Thermal concentration — multiple-effect evaporator (MEE), falling-film or forced-circulation, 3–6 effects, with thermal vapour recompression (TVR) or mechanical vapour recompression (MVR); condensate is high-quality reuse water.
  6. Crystallization / drying — forced-circulation crystallizer, or agitated thin-film dryer (ATFD) producing a 85–95% dry salt cake; filtered and dried salt is bagged.
  7. Salt management — mixed salt (NaCl + Na₂SO₄ + organics) is usually a hazardous waste requiring disposal at an authorised TSDF under the Hazardous and Other Wastes Rules, 2016. Selective nanofiltration plus fractional crystallization can recover saleable-grade Na₂SO₄ and NaCl (practised in textile clusters), which is what makes ZLD economically bearable.
• Energy and cost: membranes are far cheaper per m³ than evaporation, so every design pushes recovery upstream and minimises the thermal load. Indicative treatment cost rises from roughly ₹15–40/m³ for a conventional ETP to ₹60–150/m³ for an ETP with RO reuse, and ₹300–800 per m³ of reject sent to MEE-plus-ATFD; figures depend heavily on steam price, TDS and site, so quote them as ranges.
• Limitations: very high capex and opex, steam/power demand (and the carbon footprint that comes with it), scaling and fouling, corrosion requiring duplex/titanium/graphite surfaces, solid hazardous residue, skilled operation, and the fact that a stalled evaporator stops production because there is no outfall.

Indian standards, Acts and regulatory practice

• Water (Prevention and Control of Pollution) Act, 1974 (amended 2024) — Consent to Establish and Consent to Operate from the SPCB; CPCB coordinates and sets standards. The Water Cess Act, 1977 is a former law, repealed with effect from 1 July 2017.
• Environment (Protection) Act, 1986 and the Environment (Protection) Rules, 1986 — Schedule VI general discharge standards and Schedule I industry-specific standards (distillery, tannery, textile, pulp and paper, pharma, thermal power, etc.). Industry-specific limits override the general ones.
• General Schedule VI limits (mg/L except pH): inland surface water — pH 5.5–9.0, BOD (3 days at 27 °C) 30, COD 250, TSS 100, oil and grease 10, temperature not more than 5 °C above ambient; public sewer — BOD 350, TSS 600; land for irrigation — BOD 100, TSS 200. Toxics (inland): Cr⁶⁺ 0.1, total Cr 2.0, Pb 0.1, Cd 2.0, Ni 3.0, Cu 3.0, Zn 5.0, Hg 0.01, As 0.2, cyanide 0.2, phenolics 1.0, fluoride 2.0, sulphide 2.0, ammoniacal-N 50, TKN 100.
• Note on TDS: the general standards do not set a TDS limit for inland discharge; TDS and chloride/sulphate limits appear in sector or state conditions, which is why saline streams are driven to reuse or ZLD rather than dilution.
• Hazardous and Other Wastes (Management and Transboundary Movement) Rules, 2016 — ETP sludge and evaporator salts from listed processes, their storage, manifest and TSDF/co-processing disposal.
• CPCB practice: red/orange/green/white categorisation of industries (2016 list) with the 17 highly polluting categories; the CREP charter commitments; online continuous effluent and emission monitoring (OCEMS) with data to CPCB/SPCB servers for the 17 categories and CETPs; environmental compensation levied for exceedances; NGT orders drive many site-level ZLD directions. ZLD obligations are sector- and state-specific (for example molasses-based distilleries and several textile dyeing clusters), not a single nationwide rule — check the applicable consent conditions.
• CETP (common effluent treatment plant): cluster-level treatment for SSI units in notified industrial estates, with member-level primary treatment and pre-treatment norms for inlet quality; CETPs have their own notified standards.
• Codes and references: IS 3025 (several parts) for sampling and test methods for water and wastewater; CPHEEO Manual on Sewerage and Sewage Treatment (2013) for common unit-process design data; the old IS 2490 tolerance limits for industrial effluents has been withdrawn — the legal limits are those notified under the Environment (Protection) Rules.

Key points

• Characterisation first — the train follows the pollutant: pH, COD, BOD/COD, TDS, toxics and flow pattern.
• BOD/COD ratio — above ~0.5 biology alone works; below ~0.3 you need chemical, oxidative or anaerobic pre-treatment.
• Equalization — damps flow, pH and shock load; the single most protective tank in an ETP.
• Segregation at source — keeping concentrated/toxic streams out of the dilute stream shrinks every downstream unit.
• Minimum-solubility pH — each metal hydroxide has its own; a single pH cannot optimally precipitate all metals.
• Chrome rule — reduce Cr⁶⁺ at pH 2–3, then precipitate Cr³⁺ at pH 8–9.
• Cyanide rule — oxidise at pH > 10; never acidify (HCN release).
• AOPs — hydroxyl radicals break refractory COD and colour; used as pre-treatment to make waste biodegradable.
• Nutrient dosing — BOD:N:P ≈ 100:5:1 for industrial biology.
• Anaerobic for strong waste — UASB on spentwash/starch gives COD removal plus methane and low sludge yield.
• ZLD definition — no liquid leaves the boundary; water recovered, salts solidified.
• Membrane-before-thermal — maximise RO recovery because evaporation costs an order of magnitude more per m³.
• MEE economy — steam economy rises with the number of effects; MVR/TVR cuts live-steam demand further.
• ATFD/crystallizer — the final step converting concentrate into a dry salt cake.
• Salt value — mixed salt is hazardous waste; selective NF and fractional crystallization make recovered Na₂SO₄/NaCl saleable.
• Schedule VI — inland: BOD 30, COD 250, TSS 100, pH 5.5–9.0, oil and grease 10; sector standards in Schedule I override.
• Consent and compliance — CTE/CTO under the Water Act, 1974; OCEMS telemetry and environmental compensation for the 17 highly polluting categories.
• Failure mode of ZLD — scaling, fouling and corrosion in the concentration stages; no outfall means an evaporator trip halts production.

Formulas and examples

Example 1 — lime demand for neutralization. A pickling line discharges 150 m³/d with free acidity of 0.02 eq/L; neutralize with 90%-pure hydrated lime.

• Equivalents/day = 150,000 L × 0.02 = 3,000 eq/d.
• Pure Ca(OH)₂ = 3,000 × 37 g = 111,000 g = 111 kg/d; commercial = 111/0.90 = 123.3 kg/d.
• At ₹8/kg → 123.3 × 8 = ₹986/day ≈ ₹3.6 lakh/year.

Example 2 — RO mass balance for a reuse scheme. Feed 1,000 m³/d at TDS 3,000 mg/L; recovery 75%; salt rejection 98%.

• Permeate = 750 m³/d, reject = 250 m³/d. Feed salt = 1,000 × 3,000/1,000 = 3,000 kg/d.
• Permeate salt = 2% of 3,000 = 60 kg/d → C_p = 60 × 1,000/750 = 80 mg/L (fit for boiler make-up after polishing).
• Reject salt = 2,940 kg/d → C_c = 2,940 × 1,000/250 = 11,760 mg/L; CF = 11,760/3,000 = 3.92.
• Check by formula: CF = (1 − r·(1−R))/(1 − r) = (1 − 0.75 × 0.02)/(1 − 0.75) = 0.985/0.25 = 3.94 ✔ (small difference from rounding).

Example 3 — MEE and ATFD load on that reject. Concentrate the 250 m³/d of 11,760 mg/L reject to 25% TDS (250,000 mg/L), then dry.

• Salt to be handled = 250 × 11.76 = 2,940 kg/d.
• Concentrate volume = 2,940/250 = 11.8 m³/d to the ATFD.
• Water evaporated W = 250 − 11.8 = 238.2 m³/d = 238,200 kg/d.
• With a 4-effect MEE at steam economy 3.2: steam = 238,200/3.2 = 74,440 kg/d ≈ 74.4 t/d.
• At ₹2,200/t of steam → ₹163,770/day, i.e. 163,770/250 = ₹655 per m³ of reject, or ₹164/m³ spread over the 1,000 m³/d feed — the number that decides whether ZLD or source-TDS reduction is the right answer.
• A single-effect unit (SE ≈ 0.9) would need 238,200/0.9 ≈ 265 t/d of steam — showing why multiple effects or MVR are mandatory in practice.

Example 4 — chrome reduction and sludge. Tannery chrome liquor: 20 m³/d at Cr⁶⁺ = 500 mg/L, reduced with sodium metabisulphite (Na₂S₂O₅, M = 190).

• Cr⁶⁺ load = 20,000 L × 0.5 g/L = 10 kg/d.
• Electron balance: Cr⁶⁺ + 3e⁻ → Cr³⁺; each Na₂S₂O₅ gives 2 S(IV) → S(VI) = 4 e⁻. Moles SMBS per mole Cr = 3/4.
• Mass ratio = 0.75 × 190/52 = 2.74 kg SMBS per kg Cr; theoretical dose = 27.4 kg/d; at 1.3× practical excess ≈ 35.6 kg/d.
• Precipitated Cr(OH)₃ (M = 103) = 10 × 103/52 = 19.8 kg/d dry solids; at 25% cake solids ≈ 79 kg/d of cake, a hazardous waste.

Example 5 — anaerobic energy recovery from distillery spentwash. 400 m³/d at COD 100,000 mg/L; UASB removes 60%.

• COD in = 400 × 100,000/1,000 = 40,000 kg/d; removed = 24,000 kg/d.
• CH₄ = 24,000 × 0.35 = 8,400 m³/d; biogas at 65% CH₄ = 8,400/0.65 ≈ 12,900 m³/d.
• Energy = 8,400 × 35.8 MJ/m³ = 300,700 MJ/d ≈ 83,500 kWh thermal/day, replacing roughly 90–110 t/d of steam from a boiler at ~80% efficiency — which is how distilleries fund their downstream ZLD.

Where it is used

• Textile dyeing and processing clusters — Tirupur, Pali, Balotra, Jetpur, Panipat: RO reuse with MEE/ATFD and recovery of Na₂SO₄ and NaCl for re-use in dyebaths.
• Molasses-based distilleries and sugar complexes — biomethanation of spentwash, then concentration and incineration/composting with ZLD conditions in the consent.
• Tanneries and CETPs — Ambur, Ranipet, Jajmau, Kanpur: chrome recovery and reuse, sulphide oxidation, common RO and evaporation.
• Pharma, bulk drug and specialty chemicals — stripper columns, AOPs, solvent recovery, segregated high-TDS streams to MEE; Hyderabad, Ankleshwar, Vapi, Vizag estates.
• Thermal power, steel and refineries — cooling-tower blowdown recovery, DM-plant regenerant neutralization, oily-water treatment; ZLD is standard in new coal-based stations.
• Job roles — process design engineer (ETP/STP/ZLD), commissioning and O&M engineer, EHS/environment manager handling consents and OCEMS data, water-treatment sales and proposals engineer, SPCB/CPCB and consultant roles preparing EIA and compliance reports.

Interview questions

• Q: What is zero liquid discharge, and is 100% really achieved? A: No liquid effluent leaves the boundary — water is recovered for reuse and salts leave as solids. In practice 95–99% water recovery with a small moisture content in the salt cake is called ZLD; the solids still leave the plant as waste.
• Q: Why do designers push RO recovery as high as possible before the evaporator? A: Membrane separation costs a few kWh/m³ while evaporation costs the equivalent of hundreds of kWh/m³; every cubic metre kept out of the MEE is the biggest single saving in a ZLD plant.
• Q: How do you treat hexavalent chromium? A: Reduce it to trivalent chromium with metabisulphite, SO₂ or ferrous sulphate at pH 2–3, then raise pH to 8–9 to precipitate Cr(OH)₃ and settle or filter it.
• Q: Why is BOD/COD ratio asked for before choosing a train? A: It measures biodegradability — a high ratio justifies biological treatment, while a low ratio means refractory organics that need coagulation, adsorption, AOP or anaerobic pre-treatment first.
• Q: Which step is almost always first in an industrial ETP and why? A: Equalization, because batch industrial discharges swing in flow, pH and load, and biology or membranes downstream cannot absorb shocks.
• Q: What limits conventional activated sludge on a saline effluent? A: Above roughly 15,000–20,000 mg/L TDS, osmotic stress deflocculates the biomass and settling fails; options are halotolerant acclimatised cultures, dilution, or separating the saline stream for RO/evaporation.
• Q: What is steam economy and how is it improved? A: Kilograms of water evaporated per kilogram of live steam; it is improved by adding effects, by thermal vapour recompression, or by replacing live steam with mechanical vapour recompression.
• Q: Why is the salt from an evaporator usually a problem? A: Mixed NaCl–Na₂SO₄ with organics has no market and must go to an authorised TSDF as hazardous waste, so plants add nanofiltration and fractional crystallization to recover saleable single salts.
• Q: What are the general inland surface-water discharge limits for BOD, COD and TSS? A: Under Schedule VI of the Environment (Protection) Rules, 1986: BOD (3 days, 27 °C) 30 mg/L, COD 250 mg/L, TSS 100 mg/L, pH 5.5–9.0 — unless an industry-specific Schedule I standard applies.
• Q: Is there a TDS limit for discharge to inland surface water? A: The general standards do not prescribe one; TDS and chloride/sulphate constraints come through industry-specific standards, state norms or consent conditions, which is a main driver for reuse and ZLD.
• Q: A tannery CETP's RO membranes foul within weeks. What do you check? A: Upstream failures — inadequate biological COD removal, no softening or antiscalant for calcium sulphate and silica, poor UF/cartridge filtration, surfactant carry-over, and whether the CF has pushed a sparingly soluble salt past saturation.
• Q: What is the difference between an ETP and a CETP, and who is responsible for compliance? A: An ETP serves one industry; a CETP serves a cluster, with members doing primary treatment to meet inlet norms. Both the member unit and the CETP operator carry obligations under their respective consents.
• Q: Why can a ZLD plant shut down production? A: There is no outfall — if the evaporator or crystallizer trips, there is nowhere for the effluent to go once the holding tanks fill, so reliability and spare capacity are design conditions, not options.

Common mistakes

• Quoting the withdrawn IS 2490 tolerance limits instead of the standards notified under the Environment (Protection) Rules, 1986, or using general Schedule VI limits where a sector-specific Schedule I standard applies.
• Treating ZLD as a bolt-on RO plus evaporator without first reducing flow and TDS at source and segregating streams — the resulting evaporator load is unaffordable.
• Precipitating all metals at one pH, ignoring each hydroxide's minimum-solubility pH and the effect of chelating agents.
• Acidifying a cyanide-bearing stream, or mixing chrome and sulphide streams — both release toxic gases.
• Dropping the 1/1000 factor in mass loading, or mixing mg/L with kg/m³ and m³/d with m³/h, so the computed dose or evaporator duty is off by an order of magnitude.
• Forgetting nutrient dosing and temperature control, then blaming the aeration system when an industrial biological reactor fails to settle.
• Reporting evaporator "steam consumption" without stating the number of effects or whether MVR/TVR is used — the figure is meaningless alone.
• Assuming recovered salt is a revenue item by default; without selective separation it is a hazardous waste with a disposal cost.

Related topics

Membrane separations · Biological wastewater treatment: activated sludge, SBR and MBBR · Sludge treatment and disposal · Wastewater engineering · Water treatment plant design · Groundwater contamination and remediation