Wastewater Treatment Process Steps — A Technical Guide for Indian Industries

Industrial wastewater treatment is not a single process — it is a sequence of unit operations, each designed to remove specific contaminants, that must be matched to the effluent characteristics of your facility. A pharmaceutical plant's ETP looks nothing like a brewery's ETP, even if both are sized at 200 KLD. The process steps, the equipment, the retention times, and the treatment objectives are different because the contaminants are different.

This guide walks through the complete wastewater treatment process sequence — from screening at the inlet to ZLD at the discharge boundary — with the actual design parameters that matter: BOD/COD ranges, hydraulic retention times (HRT), suspended solids loadings, and the specific conditions under which each step is required or can be bypassed. It is written for plant heads, EHS managers, and engineering teams making real decisions about ETP design, upgrade, or troubleshooting.

Step 1: Preliminary Treatment — Screening and Grit Removal

Before any chemical or biological treatment can occur, coarse physical contaminants must be removed to protect downstream equipment from damage, plugging, and excessive wear. Preliminary treatment consists of two sub-steps:

Bar screens or rotary drum screens remove large solids — rags, plastic, fibrous material, packaging fragments. Screen opening size is typically 6–25 mm for coarse screens and 1–3 mm for fine screens. Flow-through velocity at the screen face should be 0.3–0.9 m/s to prevent solids from bypassing through the screen openings. For food processing plants with high fruit/vegetable solid loads, fine rotary drum screens (0.75–1.5 mm) are essential before any downstream unit.

Grit channels or vortex grit separators remove sand, grit, and dense inorganic particles (density >2.6 g/cm³) that would abrade pumps, centrifuges, and biological reactor media. Horizontal flow grit channels are sized for horizontal velocity of 0.3 m/s, which settles grit (>0.2 mm) while allowing organic matter to remain suspended for biological treatment. For small industrial plants, a simple inverted cone grit separator is often sufficient.

Step 2: Equalisation — Buffering Flow and Load Variation

This is the most undervalued step in industrial ETP design — and its omission is one of the most common root causes of ETP failure. Industrial processes generate wastewater in batch cycles, CIP (cleaning-in-place) events, shift changes, and seasonal production peaks. Raw flow rates and organic loads can vary 3–10x between peak and off-peak periods within a single day.

An equalisation tank with adequate volume — typically 6–24 hours HRT based on daily average flow — accepts this variable input and releases a steady, buffered flow to the downstream treatment units. This design choice:

• Prevents hydraulic overloading of biological reactors during peak production
• Prevents organic shock loading that crashes biological biomass
• Buffers pH swings from CIP acid/caustic discharges before they reach biological units
• Allows instrumentation to measure average inlet load for process control

Equalisation tank sizing: For factories with three-shift production, 8–12 hours HRT is typically sufficient. For batch-process plants (one or two batches per day), 12–24 hours HRT is needed. The tank should be equipped with coarse bubble aeration to keep solids in suspension and prevent septicity. A floating cover or covered design with ventilation is preferred for odour control.

Step 3: Primary Treatment — Physico-Chemical

Primary treatment removes contaminants that cannot be degraded by biological systems — or that would harm biological systems if not removed first. The specific primary treatment steps depend on what your effluent contains:

pH Adjustment: Most biological treatment systems operate optimally at pH 6.5–8.5. Many industrial effluents are outside this range: acid (pH 3–5) from pickling, CIP, or fermentation; alkaline (pH 10–12) from CIP caustic cleaning or saponification. Automatic pH dosing systems (sulphuric acid or caustic soda) with inline mixers maintain pH before biological reactors. Control pH to ±0.5 units of target set point.

Coagulation and Flocculation: For effluent with high suspended solids, colloidal matter, or colour, chemical coagulation (FeCl₃ at 50–200 mg/L, or alum at 50–200 mg/L) followed by flocculation with polyelectrolyte (1–5 mg/L) and settling in a lamella clarifier or tube settler. This step reduces TSS by 70–90% and can reduce COD by 30–50% if significant suspended organic matter is present. HRT in lamella clarifier: 1–2 hours; surface overflow rate: 1.0–2.5 m/h.

Dissolved Air Flotation (DAF): For oil-bearing effluent (food processing, dairy, tyre manufacturing) where fats, oils, and greases (FOG) need removal before biological treatment. DAF dissolves air under 4–6 bar pressure, releases it in the flotation tank as microbubbles (diameter 30–100 μm), which attach to oil droplets and suspended particles and float them to the surface. Skimmer removes the float layer. Achieves oil removal to <10–20 mg/L. HRT in DAF: 15–45 minutes. Surface loading rate: 3–10 m/h.

Heavy Metal Precipitation: Required for electroplating, paint, tannery, or metal finishing wastewater. Lime or caustic soda addition to pH 9.5–11.0 precipitates metal hydroxides. Followed by coagulation, flocculation, and clarification. The metal-bearing sludge is classified as Hazardous Waste and requires TSDF disposal.

Step 4: Secondary Biological Treatment

Secondary treatment is the heart of industrial wastewater treatment — where dissolved organic matter (BOD and COD) is metabolised by microorganisms and converted to biomass, CO₂, and water. The choice of biological process depends on the organic load (COD concentration), flow rate, and whether nitrification (ammonia removal) is required.

For high-strength effluent (COD >2,000–3,000 mg/L): Anaerobic pre-treatment first.

Anaerobic processes (UASB, AASP, AnMBR) degrade 70–80% of organic load without oxygen input, generating biogas (60–70% methane) as a useful by-product. UASB reactors operate at HRT 4–8 hours, upflow velocity 0.5–2.0 m/h, and temperature 30–37°C (mesophilic range) for maximum methane production. For every 1,000 kg COD removed anaerobically, approximately 350 m³ of biogas is generated — equivalent to ~200 m³ of natural gas for boiler use. This energy recovery significantly offsets ETP operating costs.

Aerobic biological treatment (following anaerobic pre-treatment, or as primary biological stage for COD <2,000 mg/L):

MBBR (Moving Bed Biofilm Reactor): Plastic carrier media (filling fraction 40–60% of reactor volume, specific surface area 300–500 m²/m³) provide substrate for biofilm growth. No sludge recycle required. Robust to load variation. HRT 4–12 hours. Achieved effluent BOD 15–30 mg/L, COD 80–150 mg/L from pre-settled feed.

Activated Sludge Process (extended aeration or conventional): Mixed liquor suspended solids (MLSS) maintained at 2,500–5,000 mg/L in the aeration tank. HRT 8–24 hours for high-strength industrial effluent. Dissolved oxygen (DO) maintained at 2.0–3.5 mg/L in the aeration zone. Sludge age (SRT) 10–20 days for stable nitrification. Secondary clarifier overflow rate 0.5–1.5 m/h for good sludge settling.

MBR (Membrane Bioreactor): Ultrafiltration membranes (pore size 0.04–0.4 μm) replace secondary clarifier. MLSS can be maintained at 8,000–12,000 mg/L — higher biomass in smaller footprint. Permeate TSS typically <1 mg/L, suitable for direct reuse in cooling towers or boiler makeup. Higher CAPEX than MBBR but smaller footprint and higher permeate quality.

Step 5: Tertiary Treatment and Polishing

Tertiary treatment addresses residual contaminants after biological treatment that would prevent compliance with CPCB discharge standards or make the treated water unsuitable for reuse. Not all plants need tertiary treatment — it depends on the discharge standard, treated water quality, and reuse requirements.

Sand/Multimedia Filtration: Removes residual suspended solids (<10 mg/L) and biological floc carry-over after secondary clarification. Essential before RO membranes (which require TSS <5 mg/L in feed). Filtration rate: 5–15 m/h. Backwash every 12–24 hours or when head loss exceeds 1.5 m.

Activated Carbon Filtration: GAC (Granular Activated Carbon) contactors remove trace organic compounds, residual COD, chlorine, colour, and odour. EBCT (Empty Bed Contact Time) 15–30 minutes for most applications. Carbon exhaustion at breakthrough — regeneration or replacement every 6–18 months depending on organic load. Used when tertiary COD target is <50 mg/L or when colour removal is required.

Disinfection: For treated wastewater reuse or STP effluent, chlorination (5–10 mg/L free residual), UV disinfection (dose 30–40 mJ/cm²), or ozonation achieves pathogen reduction to non-detectable levels. For industrial ETP discharge, disinfection is typically not required unless stipulated in consent conditions.

Advanced Oxidation (Fenton, Ozonation): For recalcitrant COD — synthetic dyes, pharmaceutical active compounds, resin-derived organics — that resists biological degradation. Fenton oxidation (H₂O₂ + Fe²⁺ at pH 3.0–4.0) generates hydroxyl radicals that non-selectively oxidise recalcitrant organics, reducing COD by 40–70% and improving biodegradability. Ozonation achieves similar effect. These are expensive processes and are used selectively for specific problem contaminants.

Step 6: Sludge Treatment and Disposal

Sludge — the concentrated solids separated in primary and secondary treatment — is often the overlooked cost driver in ETP operation. For a 200 KLD food processing ETP, sludge dewatering and disposal can account for 25–40% of total operating cost. Getting sludge treatment right matters.

Sludge thickening: Gravity thickeners or dissolved air flotation thickeners (DAFT) increase sludge solids from 0.5–1.0% (as-wasted) to 3–6% before dewatering. This reduces dewatering equipment size and energy requirement.

Sludge digestion (optional): Aerobic or anaerobic digestion of biological sludge reduces sludge volume by 30–50% and stabilises it for disposal. Anaerobic digestion of sludge produces additional biogas. Typically applied for larger plants (>500 KLD ETP) where the economics justify the additional unit.

Sludge dewatering: Volute screw press (25–30 kW, MLSS input 3–6%, dry cake output 18–22% DS) or belt filter press (higher throughput, 20–25% DS cake) or centrifuge (higher DS, higher energy). Dewatered cake is transported to authorised disposal — composting for non-hazardous bio-sludge, TSDF for hazardous chemical sludge. Sludge from chemical precipitation (heavy metals, DAF float) must be characterised and disposed of per Hazardous Waste Management Rules.

Step 7: ZLD — Membrane and Thermal Concentration

Zero Liquid Discharge is the extension of the treatment process beyond discharge-grade quality to near-complete water recovery. It is mandatory for certain industries and locations under CPCB directives. ZLD adds two concentration stages after tertiary treatment:

Reverse Osmosis (RO): High-pressure membranes (15–70 bar operating pressure depending on feed TDS) separate water (permeate, 65–75% of feed) from dissolved salts (reject/concentrate brine, 25–35% of feed). RO permeate TDS is typically 50–200 mg/L — suitable for cooling tower or boiler makeup reuse. RO requires feed SDI <3, TSS <1 mg/L, oil <0.1 mg/L — extensive pre-treatment is therefore essential. Spiral wound polyamide membranes are standard; flux 15–30 LMH at 25°C; membrane replacement every 3–5 years.

Thermal Evaporation (MEE/MVR/ATFD): The RO reject — typically 25–35% of feed at TDS 50,000–120,000 mg/L — is further concentrated by evaporation to recover the remaining water as clean condensate. MEE (3-effect) or MVR is used for bulk concentration up to 200,000–250,000 mg/L. Final stage (ATFD or crystalliser) produces dry solid for disposal. Total system water recovery: 92–98%.

The capital cost of ZLD is significant — ₹2–50 crore depending on capacity and complexity — but the operating cost per KLD recovered water (₹80–250/KL) is often lower than the cost of purchasing fresh water in water-stressed regions. See the ZLD cost calculator for site-specific estimates.

How to Choose the Right Process Sequence for Your Plant

The single most important input to process design is accurate effluent characterisation — not a brief sample but a systematic 4-week monitoring programme that captures weekday/weekend variation, shift-to-shift variation, and seasonal fluctuation. From this characterisation, the process design engineer can determine:

• Whether anaerobic pre-treatment is needed (COD >2,000–3,000 mg/L → yes; COD <1,000 mg/L → typically no)
• Which primary treatment steps are required (oil → DAF; heavy metals → chemical precipitation; high TSS → coagulation-flocculation)
• Biological process selection (MBBR vs. activated sludge vs. MBR based on flow, COD, and reuse requirements)
• Whether tertiary treatment is needed (for colour, recalcitrant COD, or reuse quality targets)
• Whether ZLD is required (regulatory mandate, or water cost/scarcity economics)

Skipping the characterisation step and designing from assumed parameters is the most expensive mistake in ETP engineering — as documented in Spans Envirotech's analysis of why most ETPs fail in India. Design for the effluent you actually have, not the one you assume you have.

If you are designing a new ETP or auditing an existing one, the ETP cost calculator provides capacity-based cost benchmarks. For a detailed techno-commercial proposal specific to your effluent and regulatory requirements, contact Spans Envirotech.