Build a reliable particle-removal train

Three processes, three jobs

Fine particles can remain suspended because their size, surface chemistry and interactions make settling slow or aggregation unfavourable. Coagulation changes those interactions using suitable chemicals and mixing. Flocculation then promotes collisions and growth into larger aggregates. Clarification separates the resulting solids from the water.

Combining the names into one treatment box hides distinct failure modes. Poor chemical conditions cannot be repaired merely by increasing settling volume. Excessive shear can break flocs after they have formed. An inadequately operated sludge-removal system can compromise a clarifier whose nominal overflow rate looks acceptable.

Chemistry and mixing must be designed together

Aluminium and iron salts, pre-hydrolysed products and polymers can act through mechanisms such as charge neutralisation, precipitation and bridging. Their behaviour depends on pH, alkalinity, temperature, natural organic matter and particle characteristics. The “best” dose is therefore a condition-dependent operating choice, not a fixed property of the chemical.

Rapid mixing distributes the dose before local concentration differences produce inconsistent treatment. Flocculation uses gentler mixing to create collisions without excessive breakup. Actual performance depends on the energy distribution and hydraulics, not only the motor rating or nominal basin volume.

Use G and Gt correctly

G: average velocity gradient, s⁻¹; P: power transferred to the water, W; μ: dynamic viscosity, Pa·s; V: mixed water volume, m³; t: seconds.

For an illustrative basin with P = 100 W transferred to the water, μ = 0.001 Pa·s and V = 100 m³, G = √1,000 = 31.6 s⁻¹. For 20 minutes, Gt ≈ 37,900. A 100 W motor would not necessarily deliver 100 W to the water, and temperature changes the viscosity. State those assumptions.

Gt is a useful comparison parameter, but it cannot make different water qualities or hydraulic arrangements equivalent. A short high-shear period and a longer gentle period can produce different floc behaviour despite the same product. Bench and pilot evidence should support the intended operating envelope.

A jar-test programme that answers the right question

1. Collect representative raw water and record temperature, pH, alkalinity, turbidity and relevant organic indicators. Include difficult seasonal or event conditions.
2. Test dose and pH systematically, holding other conditions controlled enough to interpret the result. Use chemicals and stock concentrations that match the intended basis of reporting.
3. Apply a mixing and settling sequence that reasonably represents the process. Document the limits of the scale-up.
4. Assess more than visible floc size. Compare settled turbidity, filtered quality, organic removal, pH, residual metals where relevant, and residuals handling.
5. Repeat promising conditions and check sensitivity. Select an operating range and response rules rather than a single optimum from one sample.

If a commercial product dose is reported in mg/L of solution, do not compare it directly with a dose reported as active metal or dry chemical. Clearly distinguish product strength, density and dosing-pump delivery.

Clarification: area, depth and hydraulic behaviour

Use consistent units, for example Q in m³/day, A in m² and V in m³.

An ideal discrete-settling model compares particle settling velocity with Q/A. It explains why plan area is important. It does not establish that clarifier depth is irrelevant in practice. Inlet energy, short-circuiting, floc growth, density currents, sludge storage and removal all affect performance.

For a teaching example, Q = 12,000 m³/day and surface area A = 500 m² give Q/A = 24 m/day, or 1.0 m/hour. At an active volume of 1,500 m³, nominal detention is three hours. These calculations describe loading, not a demonstrated treatment outcome. Validate the appropriate rate and hydraulic design for the water and process.

Why Stokes’ law is a starting point

This expression assumes an isolated spherical particle settling in the creeping-flow regime; check its applicability.

The squared dependence on diameter helps explain the benefit of aggregation. But a floc is usually porous, irregular and capable of changing size while it settles. Its effective density differs from that of a solid mineral grain. Concentrated suspensions also exhibit hindered settling or a sludge blanket, so an isolated-particle equation is not a full clarifier design method.

Lamella systems increase effective settling area within a compact footprint, but still require suitable flow distribution and solids removal. Direct filtration removes the clarification step only where the water quality and filter loading support that process choice. It is not a universal shortcut.

Capacity, residuals and practical troubleshooting

Clarification transfers contaminants into a residuals stream; it does not make their mass disappear. Include sludge production, thickening or dewatering, washwater, chemical deliveries and waste disposal in the options assessment. Dependable plant output may be governed by an auxiliary system or maintenance outage rather than the main basin’s design flow.

What to retain in the design record

Keep raw-water scenarios, treatability results, dose definitions, hydraulic assumptions, power basis, solids estimates and operational response limits together. Link the process assessment to the plant water balance and the storage required during reduced production. Continue to filtration design and integrity and disinfection and effective contact time.

Sources & further reading

• Australian Drinking Water Guidelines ↗NHMRC and NRMMC · Living guidelines; publication and draft status checked 25 September 2026
  External source · Checked 25 September 2026
• Best Practice Modelling Guidelines ↗eWater · 2011
  External source · Checked 24 September 2026

Source findings are distinguished from editorial interpretation. Apply current local criteria and project evidence when making engineering decisions.