Coagulant and Flocculant Selection for Industrial ETPs — A Practical Guide

You can install a perfectly designed DAF unit and get 60% FOG removal. Or you can install the same DAF unit with proper coagulant selection and jar test optimization and get 92% FOG removal. The capital cost is identical. The operating cost difference is maybe ₹8,000-15,000 per month in chemical costs. The performance difference is whether your biological system downstream is being fed a manageable 200 mg/L FOG or an overwhelming 500 mg/L FOG every day.

Coagulation chemistry is the part of ETP design that is most often handed off to the chemical supplier and then ignored. The supplier recommends their product, sets an arbitrary dose, and leaves. The dose never gets optimized. The coagulant-to-flocculant ratio never gets adjusted for seasonal change. This guide gives plant chemists and ETP operators the knowledge to select, optimize, and maintain coagulation systems properly.

What Coagulation Actually Removes

Coagulation-flocculation-flotation or settling removes colloidal and suspended material from the effluent. What it removes well: suspended solids (TSS), fats, oils, and greases (FOG), colloidal BOD (particulate organic matter), phosphate (with ferric or aluminium coagulants), colloidal colour (reactive dyes from textiles, melanoidins from sugar/dairy).

What coagulation does not remove well: dissolved BOD (soluble organics like sugars, short-chain fatty acids, amino acids), dissolved COD from non-coagulable organics, ammonia nitrogen, dissolved TDS. If your ETP report shows 90% suspended BOD removal from the DAF but only 30% total BOD removal, this is expected — the dissolved fraction passes through.

This distinction matters because coagulation is upstream of biological treatment. Removing colloidal and particulate BOD in the DAF reduces biological loading by 30-50% in a typical food ETP, which is significant. But dissolved BOD removal requires biological treatment regardless of how good your coagulation is.

Alum vs. PAC vs. Ferric Chloride

The three standard inorganic coagulants for industrial ETPs each have different cost, performance, and operational characteristics:

Alum (Aluminium Sulfate, Al₂(SO₄)₃·18H₂O):

• Cost: ₹12-18 per kg (commercial grade)
• Effective pH range: 6.5-7.5
• Typical dose: 150-350 mg/L for industrial effluent
• Advantages: cheap, widely available, well-understood
• Disadvantages: pH-sensitive (performance drops sharply outside 6.5-7.5), poor performance in cold water (<15°C), floc can be fine and slow to settle without good polymer addition

PAC (Polyaluminium Chloride, [Al₂(OH)ₙCl₆₋ₙ]ₘ):

• Cost: ₹20-30 per kg (18% Al₂O₃ content)
• Effective pH range: 5.5-8.5
• Typical dose: 80-200 mg/L (30-40% lower than alum for same removal)
• Advantages: wider pH range, better cold-water performance, faster coagulation, less sludge generation than alum
• Disadvantages: 20-50% higher purchase cost than alum (partly offset by lower dose)

Ferric Chloride (FeCl₃):

• Cost: ₹15-25 per kg (40% solution)
• Effective pH range: 4.5-6.5
• Typical dose: 100-300 mg/L
• Advantages: excellent phosphate removal, effective for colored wastewater (textiles, dairy melanoidins), works at lower pH where aluminium coagulants are less effective
• Disadvantages: produces more sludge than PAC, corrosive (requires FRP or stainless steel dosing equipment), can impart a yellow-brown tint to effluent at high doses

For most Indian food, dairy, and packaged goods ETPs, PAC is the best choice. For textile and tannery ETPs with colored wastewater, ferric chloride or a combination of ferric and PAC gives better colour removal. For plants where phosphate in the outlet is a compliance requirement, ferric chloride is preferred.

Polymer Selection and Dose Optimization

Coagulants destabilize colloids by neutralizing surface charge. Polymers (polyelectrolytes or flocculants) bridge the destabilized particles into larger, faster-settling or -floating flocs. Both are required for good performance in most industrial ETPs — coagulant alone produces small pinpoint flocs that are hard to separate.

Polymer charge type matters: anionic polymers work with metal coagulants (alum, PAC, ferric) because the metal hydroxide floc is positively charged; the anionic polymer bridges the positively charged flocs. Cationic polymers are used as standalone coagulants for some organic sludge conditioning and paper mill effluents.

Typical polymer doses for industrial ETPs: 2-8 mg/L of anionic polyacrylamide (PAM) with medium to high molecular weight (10-15 million Daltons). Higher molecular weight gives larger flocs. Start at 3 mg/L and optimize via jar test. Overdosing polymer is wasteful and can cause re-stabilization of the floc at doses above 10-15 mg/L — the polymer bridges so many particles that it re-stabilizes the suspension rather than flocculating it.

Polymer must be prepared correctly. Dissolve as a 0.1-0.5% solution in clean water, not in effluent. Allow 30-60 minutes hydration time after mixing before use. Undissolved polymer granules pass through the system without effect. Many poor coagulation results in Indian ETPs are simply due to poorly prepared polymer solution.

Jar Test Protocol: Step by Step

The jar test is the standard method for optimizing coagulant dose. Run it on fresh representative effluent — composite sample from the equalization tank outlet, taken during the production period you want to represent.

1. Fill six 1,000 mL beakers with 900 mL of raw effluent each. Measure and record pH and turbidity of each beaker.
2. Prepare coagulant stock solution at 1% concentration (10 g/L). Prepare polymer solution at 0.1% (1 g/L).
3. Add coagulant at increasing doses to each beaker: 50, 100, 150, 200, 250, 300 mg/L (equivalent to 4.5, 9, 13.5, 18, 22.5, 27 mL of 1% solution per 900 mL beaker).
4. Flash mix immediately at 150-200 rpm for 60 seconds using gang stirrer.
5. Reduce speed to 30-40 rpm. Add polymer at 3 mg/L (equivalent to 2.7 mL of 0.1% solution) to all jars. Slow mix for 20 minutes.
6. Stop stirring. Allow 30 minutes settling (or if testing for DAF, observe floc floating in first 10-15 minutes).
7. Collect supernatant from 10 cm below the surface. Measure turbidity, TSS, and COD. Observe floc size and character.
8. The optimal coagulant dose is the lowest dose that achieves target turbidity with good, fast-settling flocs and minimum chemical addition.
9. Repeat the test varying polymer dose at the optimal coagulant dose to find the minimum effective polymer dose.

Also test pH correction. If the optimal coagulant dose drops pH below 6.5 (for PAC) or 6.8 (for alum), add lime or sodium carbonate to maintain pH and observe the effect on flocculation. Some effluents with low alkalinity (<150 mg/L as CaCO₃) need alkalinity addition with the coagulant.

DAF Coagulation vs. Gravity Settling Coagulation

Coagulation chemistry is slightly different for DAF versus gravity settling. For DAF, you want smaller, less dense flocs that float readily with attached air bubbles. For gravity settling, you want large, heavy, fast-settling flocs.

For DAF: use slightly lower polymer doses (2-4 mg/L) and prefer medium molecular weight polymer. High-MW polymer produces flocs that are too large and fragile — they break up in the recycle water injection zone and settle instead of floating. Flash mix time should be shorter (30-45 seconds) and slow mix time can also be shorter (10-15 minutes) because DAF relies on bubble attachment, not purely on floc mass.

For gravity settling: use higher polymer doses (4-8 mg/L) and high molecular weight polymer. Longer slow mix time (20-30 minutes) to build larger flocs. The target is the largest, most rapidly settling floc possible.

Seasonal Variation in Coagulant Demand

Indian industries see 20-40% higher coagulant demand in winter (November-February) compared to summer. The reasons: lower water temperature slows coagulation kinetics, and some process changes in winter (different raw material batches, slower CIP, higher fat content in dairy in winter flush season) change effluent composition.

A coagulant dose optimized in March (water at 28°C) may give noticeably worse DAF performance in December (water at 14°C). Run a new jar test at the start of each winter season. Typical adjustment: increase PAC dose by 25-30% in winter. PAC is more cold-resistant than alum — another reason PAC is preferred for plants in northern India where water temperatures drop below 15°C.

Also check whether your chemical supplier is delivering consistent quality. PAC quality varies significantly between suppliers — nominal 18% Al₂O₃ product can be 14-15% from some suppliers. If your coagulant demand suddenly increases without apparent change in effluent, test the Al₂O₃ content of the coagulant stock.

Chemical Cost at 100 KLD Scale

For a 100 KLD industrial ETP with typical food processing effluent, monthly chemical costs for primary treatment:

• Alum at 250 mg/L: 250 g/m³ × 100 m³/day × 30 days = 750 kg/month × ₹15/kg = ₹11,250/month
• PAC at 150 mg/L: 150 g/m³ × 100 m³/day × 30 days = 450 kg/month × ₹25/kg = ₹11,250/month
• Ferric chloride at 200 mg/L: 200 g/m³ × 100 m³/day × 30 days = 600 kg/month × ₹20/kg = ₹12,000/month
• Anionic polymer at 4 mg/L: 4 g/m³ × 100 m³/day × 30 days = 12 kg/month × ₹180/kg = ₹2,160/month

Total monthly chemical cost for primary treatment at 100 KLD: approximately ₹13,000-14,000. This seems small — and it is small relative to the biological treatment operating cost. But the quality of primary treatment directly determines the stability and performance of the biological system, which is where the real operating cost is. A ₹3,000 monthly saving on coagulant by using a lower dose is not worth the recurring cost of biological upsets from higher FOG loading.