Underflocculation

Underflocculation is a term used in coagulation and flocculation processes in water and wastewater treatment to describe insufficient formation of flocs. In underflocculation, primary particles and colloids do not aggregate into sufficiently large or cohesive flocs, resulting in small, weak, and easily disrupted flocs that fail to settle, form sludge blankets, or be removed by filtration.

Causes of underflocculation include inadequate coagulant dose or incorrect dosing regime, suboptimal mixing intensity or duration, rapid or uneven mixing, and poor coagulation conditions such as unfavorable pH or low alkalinity. The presence of certain natural organic matter, high particle loading, temperature effects, or competing ions can also hinder floc formation. Poor pretreatment and improper polymer dosages can contribute when polymers are added but flocs remain fragile.

Consequences of underflocculation are reduced solids removal, higher residual turbidity, increased need for filtration or backwash, and greater downstream treatment energy use. Flocs that are too small or loosely bound are more susceptible to shear and breakup, leading to poor hydrocyclone or clarifier performance and potential membrane fouling in subsequent filtration steps.

Detection methods include jar tests and settling tests to observe floc growth and settling velocity, as well as turbidity measurements, particle size analysis, and zeta potential assessments. Process indicators are slow or incomplete clarification, persistent turbidity after coagulation, and fragile flocs that break under shear.

Mitigation strategies focus on optimizing dosing (coagulant and polymer), adjusting mixing intensity and time, tuning pH and alkalinity, staged dosing, and improving pretreatment to reduce interfering substances. In some cases, modifying the process to promote controlled growth of flocs or extending the maturation time can restore effective flocculation.