Coagulation / Flocculation Tanks

Coagulation/Flocculation Tanks

  • Coagulation neutralizes repelling colloidal solution
  • Allowing polluting matter to form floccules
  • Coagulant added into fully mixed tanks with short retention time and high turbulence mixing
  • Coagulants can be chitosan (natural), poly-aluminium chloride, aluminium sulfate
  • Flocculation is considered more gentle mixing process
  • Addition of flocculants aid small floccules to gather for easier separation
  • Modified polyacrylamides are manufactured and sold by the flocculant producing business

It is well known that the presence of dissolved and suspended particles in groundwater are mostly arise from land erosion, minerals dissolution and vegetation decay and also domestic and industrial waste discharges. These particles may be dissolved organic and/or inorganic matter, including various biological organisms, for instance bacteria, algae or viruses. Removal of these suspended solids are essential to avoid deterioration of water quality by reducing the clarity, more importantly these might eventually carrying pathogenic organisms or toxic compounds, adsorbed on their surfaces.

To get rid of the dissolved and suspended particles from groundwater or wastewater, processes like coagulation and flocculation are used. Both coagulation and flocculation are simple and economical, given that chemicals needed are widely available and dosage is adapted to the water composition. Nevertheless, coagulation-flocculation is usually applied, either as pre-treatment or as post-treatment step after sedimentation.

Suspended solids mostly possess negative charge, hence repulsion occurs and prevents them from agglomerating, causing them to remain suspended. Coagulation destabilises the particles’ charges. Coagulants with charges opposite to those of the suspended solids are added to the water to neutralise the negative charges on dispersed non-settable solids such as clay and organic substances. Once neutralised, the small-suspended particles are capable of sticking together. The slightly larger particles formed through this process are called microflocs and are still too small to be visible to the naked eye. A high-energy, rapid-mix to properly disperse the coagulant and promote particle collisions is needed to achieve good coagulation and formation of the microflocs. Over-mixing does not affect coagulation, but insufficient mixing will leave this step incomplete. Proper contact time in the rapid-mix chamber is typically 1 to 3 minutes.

Following coagulation, flocculation, a gentle mixing stage, increases the particle size from sub microscopic microflocs to visible suspended particles. Slow mixing cause collisions of the microflocs, leading them to bond to produce larger, visible flocs. Macroflocs are formed. High molecular weight polymers, called coagulant aids, may be added during this step to help bridge, bind, and strengthen the floc, add weight, and increase settling rate. Once the floc has reached its optimum size and strength, the water is ready for the separation process, could be through sedimentation, floatation or filtration. Design contact times for flocculation range from 15 or 20 minutes to an hour or more.