Water Treatment · Solve a Problem

Clarification Troubleshooting — Cloudy Water and Poor Floc

Start from what the plant is actually doing, not from the chemical. Most clarification failures are a pH, alkalinity, mixing or polymer-preparation problem before they are a coagulant-choice problem — and changing the chemical first hides the real variable. Pick your symptom and work the checks in order.

What is the plant actually doing?

Choose the symptom you can see. One symptom at a time — chasing two at once is how the real variable gets missed.

Symptom

Water remains cloudy

Settled or clarified water turbidity is above target although a coagulant is being dosed.

Likely causes

Coagulation pH is outside the range in which the coagulant hydrolyses effectively — commonly because the coagulant itself consumed the available alkalinity.
Dose is below the point of charge neutralisation for the current raw-water solids and organic loading.

Possible causes

Overdose has restabilised the colloid — the turbidity curve turns back upward past the optimum.
Raw water changed (turbidity, colour, organics, temperature) and the fixed dose no longer matches it.
Rapid mix is not distributing the coagulant before hydrolysis completes.

What to check

CHECK coagulated-water pH and raw-water alkalinity, together, at the same time.
CHECK whether raw turbidity, colour or TOC has moved since the dose was set.
CHECK the actual chemical feed rate against the intended one — pump calibration, suction, dilution water.
CHECK rapid-mix energy and whether the mixer is running at all.

Possible direction

POSSIBLE: if alkalinity is being consumed to the point of pH depression, an alkalinity source alongside the coagulant is a recognised response.
POSSIBLE: where alkalinity is genuinely limiting, a pre-hydrolysed coagulant consumes less of it per unit metal.
VALIDATE any change by jar test before altering the plant.

When to jar test: Any change of dose, coagulant or pH target must be validated by jar test on the current water before it is applied to the plant.

The variables behind every one of these symptoms

Work these in order. Changing the chemical before ruling them out hides the real cause.

pH and alkalinity: Metal-salt coagulants hydrolyse and consume alkalinity, depressing pH. On low-alkalinity water this moves coagulation pH out of the effective range faster than operators expect.
Temperature: Hydrolysis and floc-formation kinetics slow in cold water; floc is smaller and slower to settle. A jar test run at room temperature will not predict a winter plant.
Coagulant type: Pre-hydrolysed, metal-sulfate and organic coagulants differ in alkalinity demand, temperature sensitivity, floc density and sludge production on the same water.
Dose: Both underdose and overdose leave turbidity high. Past the optimum, restabilisation turns the turbidity curve back upward.
Rapid mix: Coagulant must be distributed before hydrolysis is complete. Rapid-mix energy and dispersion at the injection point are part of the chemistry, not separate from it.
Flocculation conditions: Aggregation needs energy and time, tapered towards the outlet. Too much energy breaks floc; too little never builds it.
Polymer selection and preparation: A flocculant bridges floc a coagulant has already destabilised; dosing it first wastes it. Make-down concentration, mixing and ageing govern whether the chain length is available at all.
Raw water variability: Turbidity, colour, organics, alkalinity and temperature all move. A fixed dose against a moving load is inconsistency by design.
Settling time and hydraulics: Surface loading, flow split, weir levelling and blanket control decide whether floc that formed correctly is actually removed.

Running a jar test that predicts the plant

· Run at plant temperature, not bench temperature. Cold-water kinetics are the most commonly ignored variable in clarification troubleshooting.
· Match rapid-mix energy and duration, then the flocculation regime, as closely as the apparatus allows. A jar test with the wrong mixing tests the wrong thing.
· Record coagulated pH as well as residual turbidity. A dose that hits target turbidity while pushing pH out of range is not a stable operating point.
· Measure settled sludge volume alongside turbidity when sludge handling is a constraint.
· Bracket the winning dose either side to see how flat the optimum is. A sharp optimum means the plant will need tighter dose control.

Everything a jar test produces is a candidate for the plant, not a setting for it. Confirm at full scale, in a controlled step, with the ability to revert.

Use this when

Turbidity out of the clarifier has risen and nothing obvious changed in the raw water.
Floc is forming but stays small, or forms and then breaks before it settles.
The same dose gives a different result on different shifts or in different seasons.

How this decision is structured

Each symptom maps to documented causes: alkalinity consumption and pH drift, coagulation temperature kinetics, rapid-mix and flocculation energy, overdose restabilisation, polymer make-down, and raw-water variability.
Output is separated into LIKELY CAUSES · WHAT TO CHECK · POSSIBLE DIRECTION · WHEN TO JAR TEST.
Language is graded. LIKELY and POSSIBLE describe candidates; CHECK and VALIDATE describe what you must do before acting.

What this page does not determine

It does not diagnose your plant. It narrows what to measure, in what order.
It never issues a dose, a pH setpoint, a mixing energy or a polymer concentration. Those are site-specific and validated on your water.
It does not guarantee that following any direction here will restore treated-water quality.
It makes no potable-water, discharge or regulatory compliance statement.

Relevant Allzone products

These are catalogue identities relevant to this decision. A listing here is not a statement that the material is suitable for your system, and it is not a certification, performance or drinking-water claim.

Ask for a quotation or a specification

Where this comes from, and what it cannot tell you

Water-treatment engineering references on coagulation and flocculation (AWWA Water Treatment Plant Design; MWH's Water Treatment: Principles and Design)
Supports: That coagulation performance is governed by charge neutralisation and sweep-floc mechanisms, that metal-salt coagulants consume alkalinity and depress pH, and that rapid-mix and flocculation energy and time control floc formation and strength.
Limitation: Design references describe mechanisms and typical ranges. They do not give a dose for a specific raw water; that comes from jar testing on the actual water.
Engineering literature — general practice · Established water-treatment engineering.
Standard jar-test method literature (ASTM D2035 Standard Practice for Coagulation-Flocculation Jar Test of Water)
Supports: That the validated dose for a coagulant or polymer is determined experimentally on the actual water, at representative temperature and mixing energy, and expressed in mg/L.
Limitation: A jar test predicts direction and approximate optimum. Full-scale mixing, retention time and raw-water variability shift the result; confirmation on plant is required.
Standard practice — North America · Current standard practice; confirm the edition in force.
Peer-reviewed and utility literature on cold-water coagulation (temperature effects on hydrolysis kinetics and floc formation)
Supports: That coagulation kinetics slow at low temperature, that floc formed in cold water is typically smaller and slower-settling, and that pre-hydrolysed coagulants are less temperature-sensitive than metal sulfates.
Limitation: The magnitude of the effect is water-specific. Nothing here establishes that a given coagulant will perform adequately at your winter temperature.
Scientific and utility literature — cold-climate relevance · Established consensus.
Polyacrylamide manufacturer technical literature on make-down, ageing and dosing of dry and emulsion polymers
Supports: That polymer solutions require controlled dilution and an ageing period to develop chain extension, that over-shearing degrades performance, and that emulsion and dry products are quoted on different active-polymer bases.
Limitation: Make-down times, concentrations and shear limits are grade-specific. Always work from the datasheet of the exact grade purchased.
Manufacturer documentation · Grade-specific; verify against the current TDS.