CPCB ETP Design Guidelines — Coagulation and Flocculation — Explained

Role of Coagulation-Flocculation in Industrial ETPs

Coagulation-flocculation is the primary physicochemical treatment stage in most industrial ETPs. It removes:

• Colloidal suspended solids (particles 0.001–1 µm) that cannot settle by gravity alone
• Emulsified oils and fats (from food, dairy, and edible oil ETPs)
• Phosphates (when using alum or ferric chloride as coagulants)
• Colour from textile, paper, and chemical ETPs (colloidal and some dissolved colour)
• Turbidity from wash water and process effluent

It does NOT remove dissolved organic compounds (BOD, dissolved COD) — these require biological treatment downstream. Coagulation-flocculation is therefore a pre-treatment or primary treatment step that prepares the effluent for biological treatment, not a standalone solution for CPCB compliance.

Coagulation Chemistry and Coagulant Types

Coagulation works by neutralising the negative electrical charge (zeta potential) on colloidal particles — which prevents them from aggregating naturally. The primary coagulants used in Indian industrial ETPs:

• Alum (Al₂(SO₄)₃·18H₂O): Dose 50–200 mg/L; optimal pH 6.5–7.5; produces large fluffy flocs; generates relatively large sludge volumes; lowest cost. Most widely used in food, dairy, and paper ETPs.
• Ferric chloride (FeCl₃): Dose 20–100 mg/L; effective pH range 4–9; denser sludge than alum; excellent phosphate precipitant; stains equipment rust-red. Preferred for pharmaceutical, chemical, and municipal applications.
• Polyaluminium chloride (PAC): Dose 10–80 mg/L; effective at lower doses than alum; suitable for slightly acidic or high-turbidity effluent; more expensive than alum but lower sludge volume. Used in textile, pharmaceutical, and food ETPs where sludge disposal cost is high.
• Lime (Ca(OH)₂): Used for high-pH coagulation in combination with alum/ferric chloride; also removes phosphate as calcium phosphate; essential for heavy metal precipitation (Pb, Cd, Zn).

Flocculants (polyelectrolytes) are always used after coagulation to grow micro-flocs into larger, faster-settling macro-flocs. Anionic polyelectrolyte (0.5–2 mg/L) is most common; cationic polyelectrolyte is used when the floc is negatively charged after coagulation.

Jar Test — The Essential Design Tool

The jar test is the fundamental tool for designing a coagulation-flocculation system. Procedure:

• Step 1: Fill 6 × 1-litre jars with fresh effluent sample.
• Step 2: Add increasing doses of coagulant (e.g., alum at 50, 100, 150, 200, 250, 300 mg/L) to each jar simultaneously.
• Step 3: Flash mix at 150–200 RPM for 60 seconds (simulates flash mixer).
• Step 4: Reduce speed to 30–40 RPM for 20 minutes (simulates flocculator).
• Step 5: Stop mixing, allow to settle for 30 minutes, and measure supernatant turbidity, TSS, BOD, COD, and pH.
• Step 6: Plot turbidity vs. dose to identify the optimal coagulant dose and pH.
• Step 7: Repeat with polyelectrolyte at the optimal coagulant dose to find the optimal flocculant dose.

Jar tests must be run at the actual effluent temperature and pH — results from one pH are not transferable. CPCB ETP design guidelines require that the chemical dosing design be backed by jar test data.

Flash Mixer Design

The flash mixer (rapid mix tank) ensures instantaneous, uniform coagulant distribution throughout the effluent:

• Hydraulic retention time (HRT): 10–30 seconds — rapid and short.
• Velocity gradient (G): 300–1,000 s⁻¹ — high turbulence to disperse coagulant before it hydrolyses.
• Coagulant injection: Added as close to the point of maximum turbulence as possible — at a pipe restriction, inline static mixer, or centrifugal pump suction.
• Tank geometry: For tank-based flash mixers — square tank with vertical turbine impeller, depth equal to width. Alternatively, inline static mixers or hydraulic jump channels (for flow-through designs) are preferred for continuous-flow ETPs.

Flocculator Design

The flocculator grows micro-flocs into large, fast-settling macro-flocs under gentle agitation:

• HRT: 15–30 minutes for industrial ETPs; 30–60 minutes for difficult-to-flocculate effluent (high colour, low TSS).
• G value: 10–75 s⁻¹ — much lower than flash mixer to avoid breaking fragile flocs.
• Tapered flocculation: Best practice — three compartments with decreasing G (75 → 40 → 15 s⁻¹) as flocs grow larger and more fragile.
• Flocculant injection: Polyelectrolyte (0.5–2 mg/L) added at the inlet of the flocculator — after coagulant has destabilised particles.
• Tank geometry: Rectangular tanks with horizontal shaft paddle flocculators or vertical turbine; baffled channels are used in smaller ETPs.

Clarifier Sizing After Flocculation

After flocculation, the floc-containing effluent flows to a settling tank (clarifier):

• Surface overflow rate: 1.0–2.0 m/h for conventional floc (food, dairy ETP); 0.5–1.0 m/h for light, slowly-settling floc (textile ETP with high colour).
• HRT in clarifier: 2–4 hours.
• Lamella clarifiers: Used in compact ETPs — inclined plates increase effective settling area; allow surface overflow rate 3–5× higher than conventional clarifiers in the same footprint.
• Sludge collection: Sloped hopper bottom with sludge scraper; sludge withdrawn to sludge thickener continuously or on a timed cycle.

Sludge Generation and Handling

Coagulation-flocculation generates significant sludge volumes that must be managed:

• Alum sludge: 15–40 L sludge per m³ treated effluent (at 1% solids); very compressible — dewaters to 15–25% solids on a filter press.
• Ferric sludge: 10–25 L per m³ (denser and less voluminous than alum sludge); dewaters to 20–30% solids.
• Sludge from food/dairy ETPs is often non-hazardous and can be composted or land-applied with SPCB approval.
• Sludge from chemical, pharmaceutical, or electroplating ETPs may contain heavy metals or toxic organics — characterisation and hazardous waste authorisation required before disposal.

Integration into the ETP Treatment Train

Coagulation-flocculation in the overall ETP treatment train:

• Typically positioned after screening and equalisation — influent is equalised to smooth out composition variations before coagulation.
• For high-strength organic effluent (food, dairy, paper): coagulation-flocculation → primary clarifier → anaerobic pre-treatment (UASB) → aerobic biological treatment (ASP/MBBR) → secondary clarifier → discharge.
• For emulsified oil-bearing effluent: coagulation with alum/PAC → DAF (instead of gravity clarifier) — DAF combined with coagulation achieves 90–98% oil removal.
• For colour-bearing textile effluent: biological treatment → coagulation-flocculation → clarifier — the biological stage reduces BOD, and coagulation removes residual colour in the effluent polishing step.

Frequently Asked Questions