Systematic root-cause diagnostics for flocculator problems: pin floc, floc carry-over into downstream separation, excessive floc breakup, drive faults, and polymer overdose symptoms.
Flocculator performance is highly sensitive to G-value, retention time, coagulant dose, temperature, and raw water characteristics. Most field problems fall into four categories: insufficient floc growth, floc breakup, chemical dosing faults, and mechanical failures. Use the diagnostic cards below to identify and correct the root cause.
Root cause: G-value too high, coagulant overdose, or insufficient flocculant polymer. Primary particles are destabilised but the high shear prevents aggregate growth above 5โ20ย ฮผm. Often seen when raw water turbidity drops unexpectedly while mixer speed remains unchanged.
Reduce flocculator speed by 30โ50% and measure settled turbidity after 20 minutes. If improvement is seen, programme the VSD to a lower setpoint. Conduct a fresh jar test at current raw water quality to re-confirm optimum G and dose. Check coagulant feed rate โ overdosing alum or ferric restabilises charge and produces non-aggregating colloids.
Root cause: Floc is too buoyant (low-density, high-water-content aggregates) or G in the final flocculation chamber is too high, breaking floc back to a size the separation unit cannot capture. May also indicate insufficient flocculant polymer dose or the wrong polymer charge density.
Reduce G-value in the final chamber to 10โ20 sโปยน to allow floc consolidation. Check flocculant polymer dose โ increase by 10โ20% and re-evaluate carry-over at the downstream unit. For DAF carry-over, also check the air-to-solids ratio and recycle pressure. If flocs are forming correctly but settling poorly, the issue may be in the separation stage rather than the flocculator.
Root cause: Short-circuiting in the flocculation tank is bypassing water around the paddles or impellers without the full design contact time. Also caused by intermittent coagulant pump faults creating un-treated slugs of raw water.
Trace a dye test (food-grade dye or rhodamine) through the flocculator to confirm hydraulic residence time against theoretical HRT. If significant short-circuiting is confirmed, fit additional baffles between the inlet and outlet zones. Check coagulant pump operation and flow-pacing signal โ confirm dose proportional to flow is correct.
Root cause: G-value too high in the later stages. Fragile hydroxide flocs are broken by paddle or impeller shear at the point where they are most susceptible โ the 100โ500 ฮผm growth phase. Can also be caused by polymer overdose, which makes floc hydrophilic and weak.
Implement a tapered G-value sequence: reduce G in the second and third chambers to โค 25 sโปยน. If using a single chamber, reduce overall G by 30โ40% and extend HRT by 20โ30% to maintain the target GT product. Review polymer dose โ excessive cationic polymer can prevent charge bridging and weaken floc structure.
Root cause: Flocculant polymer overdose, or wrong polymer type (e.g., very-high-molecular-weight anionic polymer in a low-turbidity water). Stringy or filamentous floc indicates bridging is occurring without proper neutralisation of particle charge.
Reduce polymer dose by 25โ50% and conduct a zeta potential measurement on the post-coagulation sample (target: โ2 to +2 mV for charge neutralisation coagulation). If the polymer type is incorrect, trial an alternative charge density or molecular weight. Foam at the flocculator surface may also indicate excess polymer interaction with dissolved organics โ reduce dose and aerate the foam zone.
Root cause: Worn or misaligned bearing, bent shaft due to solids impact, or loose blade fasteners. In multi-chamber systems, cavitation from a submerged bearing operating dry can cause a high-frequency shriek.
Stop the unit and inspect submerged bearings for wear, correct lubrication, and alignment. Check shaft run-out with a dial gauge at mid-span (should be โค 0.5 mm for paddles up to 3 m span). Inspect blade fixings and re-torque to specification. If the bearing is submerged-oil type, verify the oil level in the bearing housing and replenish. If damage is confirmed, replace bearing and check shaft for permanent deflection.
Root cause: High-solids or high-viscosity feed (e.g., during cold weather when ฮผ rises), accumulated rags or stringy material around the paddle shaft, or VSD fault causing the motor to run at above-design speed.
Check current draw against the motor nameplate FLA and the process design basis. If within 20% of FLA, reduce VSD frequency setpoint by 10โ15% and monitor. If significantly over FLA, stop the unit and inspect for rags, debris, or seized bearings. In cold weather, increase the warm-up period before reaching design speed. Check VSD parameter settings for maximum frequency and torque limits.
Root cause: Scale or biological fouling of the mixing elements, or an increase in flow rate above the design basis. Particulate deposition within the element channels is common in high-alkalinity waters when coagulant injection pH is not controlled tightly.
Isolate the static mixer and flush with 2% citric acid or hydrochloric acid solution for 30 minutes to dissolve carbonate scale. Inspect elements for physical deformation or blockage. If fouling is recurring, lower the coagulant injection pH setpoint by 0.2โ0.3 pH units to reduce localised alkalinity precipitation within the mixer. Consider installing a bypass spool piece to allow online element cleaning without shutdown.
Slow-speed horizontal or vertical paddle wheels for gentle, sustained floc growth.
View PageAxial and radial turbine impellers for higher-intensity or multi-stage flocculation.
View PageChemical coagulant flash-mixing without moving parts using pipe-mounted static elements.
View PageCamp & Stein G-value calculations, GT product, tapered flocculation staging, and chamber sizing.
View PageDiagnose pin floc, carry-over, excessive breakup, and drive mechanical faults.
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