Coagulant Dosing Calculator

Coagulation in Wastewater Treatment: How It Works

Destabilisation of colloidal particles

Industrial wastewater contains colloidal particles — typically 1 nm to 1 µm in size — that carry a negative surface charge (zeta potential). This charge prevents particles from coming together and settling by gravity. Coagulation works by introducing positively charged metal ions (Al³⁺ or Fe³⁺) that neutralise the surface charge, allowing particles to collide and aggregate into larger flocs. This process is known as charge neutralisation. A second mechanism — sweep flocculation — occurs when the metal hydroxide precipitate (Al(OH)₃ or Fe(OH)₃) forms a voluminous floc that physically entraps and sweeps down colloidal particles as it settles.

Coagulation is almost always followed by flocculation — gentle, prolonged mixing (G value 10–50 s⁻¹) that allows the destabilised microflocs to collide and grow into settleable macroflocs. In a well-designed effluent treatment plant (ETP), rapid mixing (flash mix, G ~ 300–1000 s⁻¹, 30–60 s contact time) is used for coagulant dispersion, followed by a flocculation stage (15–30 minutes) before clarification or DAF.

Alum vs. PAC vs. ferric chloride: mechanisms and trade-offs

Alum (Al₂(SO₄)₃·18H₂O) is the most widely used coagulant in Indian ETPs due to low cost and easy availability. It works well in the pH range 6.5–7.5. Below pH 6 or above pH 8, aluminium hydroxide becomes more soluble and coagulation efficiency drops. Alum hydrolysis consumes alkalinity and lowers pH — approximately 0.5 mg alkalinity consumed per mg of alum dosed.

PAC (poly-aluminium chloride) is a pre-polymerised coagulant with higher charge density and larger polynuclear aluminium species. It works across a wider pH range (5–8), produces less sludge than alum for equivalent turbidity removal, requires lower doses (30–50% less than alum on a product basis), and performs better at low temperatures. The higher unit cost of PAC is often offset by lower dose and reduced sludge disposal costs.

Ferric chloride (FeCl₃) is preferred for high-colour wastewater (textile, tannery), phosphorus removal, and acidic wastewater where the low optimal pH (4.5–6.5) is compatible with the inlet quality. Fe(OH)₃ floc is denser and settles faster than Al(OH)₃, giving higher clarification efficiency in DAF and lamella settlers. FeCl₃ is highly corrosive — material selection is critical.

pH sensitivity and optimal coagulation pH

Optimal coagulation pH varies by coagulant: alum works best at pH 6.5–7.5; PAC at pH 5.0–8.0; FeCl₃ at pH 4.5–6.5; ferric sulphate at pH 4.0–7.0. Outside these ranges, coagulation efficiency drops significantly. Always measure and control inlet pH before coagulant addition. For low-alkalinity wastewater, add lime or soda ash upstream to buffer pH during hydrolysis.

How to Determine Coagulant Dose: Jar Test Procedure

Step-by-step jar test

1. 1. Collect a representative 10-litre sample of your wastewater immediately before the coagulation point. Measure baseline turbidity (NTU), colour (Pt-Co), TSS (mg/L), phosphorus (mg/L), and pH.
2. 2. Prepare a 1% w/v stock solution of the coagulant (10 g/L in distilled water). This gives 1 mg of coagulant per 0.1 mL of stock.
3. 3. Fill 6 × 1-litre beakers with wastewater. Add doses of 5, 10, 20, 30, 50, and 75 mg/L coagulant to each beaker using a graduated pipette.
4. 4. Rapid mix at 100–150 rpm for 1 minute (flash mixing), immediately followed by slow mixing at 30–40 rpm for 15–20 minutes (flocculation).
5. 5. Allow 20–30 minutes quiescent settling. Collect the supernatant without disturbing settled floc and measure turbidity, colour, pH, and TSS.
6. 6. Plot turbidity (or colour) vs. coagulant dose. The minimum dose achieving your target quality is the optimum dose. Add 10–20% safety factor for full-scale design variability.

This calculator uses the jar test dose as the input — it provides the consumption and cost estimates, not the dose selection. The dose must always come from experimental work on your actual wastewater, as coagulant requirements are highly site-specific.

Dosing Pump Selection for Coagulant Dosing Systems

Metering pump types: peristaltic vs. diaphragm

Peristaltic pumps are ideal for small ETPs (under 200 KLD) and liquid coagulant dosing — no wetted metal parts, self-priming, easy to clean, and capable of handling viscous or slurry-type coagulants. The tubing (Hytrel, PTFE, or Tygon) is the only item requiring periodic replacement. Flow range is typically 0.001–100 L/hr for metering-grade peristaltic pumps.

Diaphragm metering pumps (PTFE-lined, SS 316L or PVC valve bodies) are used for larger plants and more precise flow control. They accept 4–20 mA control signals for automatic dosing proportional to flow or inline measurement (turbidity, phosphorus). Rated accuracy ±1–2% at full stroke.

Pump sizing from dosing rate

This calculator outputs the dosing pump flow rate in both L/hr and mL/min. Size the pump so that the calculated flow rate falls at 40–80% of the pump's rated maximum — this allows headroom for peak demand and turndown capability. Pump turndown is typically 10:1 (minimum 10% of maximum flow). A pump rated for 10 L/hr can typically be turned down to 1 L/hr — use this range for full-scale to pilot load variation. See the Chemical Dosing Calculator for full pump sizing across multiple chemical streams.

Material compatibility: FeCl₃ is highly corrosive

Ferric chloride attacks carbon steel, mild steel, galvanised steel, copper, brass, and aluminium. All wetted components — pump heads, valves, piping, tanks — must be SS 316L or HDPE. FRP tanks are acceptable for storage. Avoid standard 304-grade stainless steel for concentrated FeCl₃ solutions above 30% — 316L is required. For dosing lines, use HDPE or PTFE tubing. Inspect all FeCl₃ dosing equipment at quarterly intervals for pitting corrosion.

Solution day tank sizing

The day tank should hold at least 1.5–2 days of liquid coagulant requirement to absorb supply variability. The calculator's daily solution volume output directly gives the daily draw-down from the tank. For powder coagulants, prepare 10% make-up solutions in HDPE tanks — size the tank for 1 day's prepared solution. FeCl₃ day tanks must be HDPE or FRP with a polypropylene (PP) lid and SS 316L dip tubes.

Coagulant Chemical Costs in Indian ETPs

Typical market prices (2024–2025)

Indicative coagulant prices in India vary by region, quantity, and supplier: alum powder ₹15–25/kg; PAC liquid (10% Al₂O₃) ₹18–35/kg; PAC powder (28–30% Al₂O₃) ₹25–45/kg; ferric chloride liquid (40%) ₹18–28/L; ferric sulphate powder ₹18–28/kg. Bulk purchases (tanker quantities) attract 10–20% discounts. Prices are subject to seasonal variation tied to bauxite and iron ore feedstock costs. Enter your current local price in the calculator for accurate cost estimates.

PAC vs. alum: trade-off analysis for Indian ETPs

While PAC costs more per kg than alum, the lower dose requirement (30–50% less on a product basis) often makes total chemical costs comparable. The additional advantages — wider pH range, less sludge, better low-temperature performance — can reduce sludge handling costs and improve downstream settling performance. For a 500 m³/day ETP treating textile wastewater with a colour problem, the difference in annual chemical cost between alum at 200 mg/L and PAC at 100 mg/L (at equivalent colour removal efficiency) may be modest, while the sludge disposal savings from PAC can be significant.

For detailed annual cost comparison between coagulant options, use the ETP Cost Calculator. For coagulation system design, procurement, and commissioning, contact Spans Envirotech.

Frequently Asked Questions