Flocculation and Coagulation Mustafa Nasser 2012

Flocculation and Coagulation Water Treatment Agglomeration of solids prior to sedimentation and rapid sand filtration Wastewater Treatment Physico-chemical treatment of primary and secondary effluents and also raw wastewater Treatment of individual wastewater where it is necessary to coalesce solids when they are present in appreciable quantities Mustafa Nasser 2012

Theory of Aggregation of Colloidal Particles Basic properties <10 m Almost non-settling Dispersions sols or emulsions Large surface area Electrostatic charge Hydrophobic (tendency to repel water Hydrophilic (affinity for water) Mustafa Nasser 2012

Coagulation of Colloids (Destabilisation) Forces acting on colloidal particles (<10 m) Some cause in attraction between particles and cause repulsion electrostatic van der Waals Hydrodynamic lubrication force Length scale of action is up to say 100 nm Mustafa Nasser 2012

Ways of reducing the repulsion between particles reduce the zeta potential ( ), charge neutralisation induce aggregation by inter-particle bridging using polyelectrolytes Mustafa Nasser 2012

Theory of Inter-particle Interaction DLVO Theory(after Derjaguin, Landau, Verwey and Overbeek) The total energy of interaction, , is considered to be the sum where = energy of repulsion = energy of attraction Mustafa Nasser 2012

DLVO Theory(after Derjaguin, Landau, Verwey and Overbeek) Mustafa Nasser 2012

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Electrical Double Layers When colloidal particles are dispersed in aqueous electrolyte, they acquire an electrical charge this leads to the electrostatic repulsion between particles. The most water based system most natural particles have a negative charge around neutral pH. The charged particles are surrounded by ions. Ions having the same charge are called co-ions Ions having the opposite charge are called counter-ions There is electrostatic repulsion between the particles and co-ions and attraction between the particles and counter-ions. The result is that each particle is surrounded by a diffuse electrical double layer. Mustafa Nasser 2012

Electrical Double Layers Depending on the valency (charge number) of the counter-ions some will be adsorbed onto the particle surface. These form the innermost layer of ions, which is known as the fixed layer or Stern layer. Adsorbed ions cause charge neutralisation. Outside the fixed layer is the diffuse layer or Gouy layer of mobile co- and counter-ions. Overall the electrical neutrality of the bulk electrolyte is maintained as there are equal numbers of positive and negative charges. Mustafa Nasser 2012

Electrical Double Layers The Diffuse Double Layer The diffuse double layer consists of an outer part where there are mainly co-ions and an inner part where the are mainly counter ions.. The layers do not consist entirely of either co-ions or counter-ions as there is always some Brownian diffusion of ions through the double layer, although on average the double layer is thought to be in the steady state. The diffuse double layer gives rise to an electrokinetic property known as the zeta potential. Mustafa Nasser 2012

Electrical Double Layers The Zeta Potential If a colloidal particle has an electrical surface charge it will have a corresponding electrical surface potential. Typically the magnitude of this surface potential is between 20 – 100 mV. The potential decays with distance from the particle surface until it is zero in the bulk electrolyte. The rate of decay depends on the concentration of the electrolyte. (higher concentration more rapid decay) Mustafa Nasser 2012

Electrical Double Layers The zeta potential is related to the stability of colloids. STABLE – No tendency to coagulate UNSTABLE – Coagulation can take place Mustafa Nasser 2012

Coagulation and Flocculation These terms describe the addition of a floc-forming chemical to water containing suspended solids to induce aggregation of the colloidal material that is present. Note that if excess chemical is used then a precipitate is formed and this can enmesh or combine with both the non-settling and slow settling solids to produce a rapid-settling floc. Mustafa Nasser 2012

Coagulation and Flocculation Coagulation refers to the addition and rapid mixing of a coagulant, the resultant destabilisation of the colloid and fine suspended solids and the initial aggregation of the destabilised particles. Flocculation refers to the slow stirring or gentle agitation of the treated suspension together with the subsequent grow in aggregate size to from fast settling flocs. Mustafa Nasser 2012

Mechanisms Addition of inorganic chemicals (Aluminium and Iron salts) - coagulant salt dissolves in water and dissolved ions are formed - metal ions undergo hydrolysis and polymerise forming positively charged hydroxometallic complexes e.g. Al2(SO4)3   Al6(OH)135+ and forms Al7(OH)174+ etc. Mustafa Nasser 2012

Mechanisms These complexes are: polyvalent highly positively charged adsorbed onto the surfaces of negatively charged colloids The effect is to reduce the zeta potential by neutralising the surface charge and consequently removing the electrostatic repulsion from between the particles – the particles are destabilised. Mustafa Nasser 2012

Mechanisms If more coagulant is added then the polymerisation is stopped and the metal hydroxide is precipitated. This can enhance the process as the precipitate can enmesh the suspended particles. In some wastewater treatments a significant excess is used. In both water and wastewater treatment the optimum dose of chemical should be sought. This is usually achieved by jar testing. Mustafa Nasser 2012

Inter-particle Bridging Mechanisms Addition of organic polymers (polyelectrolytes, e.g. CTAB, SDS, Magna Floc) Inter-particle Bridging Charge neutralisation Mustafa Nasser 2012

Mechanisms Inter-particle bridging Polymers – very high molecular weight chain molecules Polymers have ionisable groups ( e.g. amino, sulphonic etc.) which find reactive sites on the surface of the colloidal particles Several particles may be bound by a single polymer molecule Optimum conditions for bridging occur when the colloid is half covered by segments of polymer Excess polymer should be avoided as this will restabilise the colloidal particles (i.e. flocculation is destroyed) Jar testing is required to determine the optimum dose Mustafa Nasser 2012

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Initial adsorption at the optimum polymer dose BRIDGING MECHANISM + Polymer Particle Destabilised Particle Initial adsorption at the optimum polymer dose Mustafa Nasser 2012

Destabilsed particles BRIDGING MECHANISM Floc Formed Destabilsed particles Floc formation Mustafa Nasser 2012

Destabilised Particle Restabilised Particle BRIDGING MECHANISM Destabilised Particle Restabilised Particle Secondary adsorption of polymer No contact with vacant sites on another particle Mustafa Nasser 2012

Excess polymer dose uses up all available adsorption sites BRIDGING MECHANISM Stable particle Excess polymer dose uses up all available adsorption sites Mustafa Nasser 2012

Aggregate, large flocculated particle BRIDGING MECHANISM Floc fragments Aggregate, large flocculated particle Prolonged or over-intense agitation leads to floc rupture Mustafa Nasser 2012

BRIDGING MECHANISM Secondary adsorption of polymer following floc rupture leads to restabilisation. Mustafa Nasser 2012

END Mustafa Nasser 2012