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CHEMICAL PHOSPHORUS REMOVAL. Phosphorus removal (chemical precipitation) Al 3+ + PO 4 3-  AlPO 4 = converting of dissolved P compounds to a low solubility metal phosphate (through use of a metal salt) Precipitants: Aluminium salts Iron salts Lime.

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  • Phosphorus removal (chemical precipitation)
  • Al3+ + PO43-AlPO4
  • = converting of dissolved P compounds to a low solubility metal
  • phosphate (through use of a metal salt)
  • Precipitants:
    • Aluminium salts
    • Iron salts
    • Lime


Precipitation chemicals precipitate the dissolved inorganic phosphates as insoluble compounds (to be more exact: compounds with small solubility)

At the same time metal-hydroxides are formed

 jelly-like flocs which bind the precipitated metal phosphates and any other suspended substances in the water (coagulation-flocculation)

This also removes organically combined P, as the amount of suspended matter is greatly reduced by chemical precipitation



    • Phosphorus removal (chemical precipitation)
  • Al3+ + PO43-AlPO4
    • Removal of organic matter (coagulation-flocculation)
        • Al3+aluminium-hydroxide
      • Good coagulant: contacts suspended matters (mainly organics) of wastewater rapidly and strongly
      • Organics are originally mainly in colloidal form – do not settle well – settling characteristics can be improved due to coagulation-flocculation


    • destabilization of the colloidal particles
  • Flocculation:
  • increase the size of flocs


  • as the only treatment process
    • primary (direct) precipitation
  • or in combination with biological treatment processes
    • pre-precipitation
    • simultaneous precipitation
    • post-precipitation

significant part of the organic pollutants is connected to suspended solids

 increasing of their removal efficiency in the primary settling tank results low organic pollutant load in the activated sludge processes



  • Addition of calcium
    • Usually in the form of lime (Ca(OH)2)
    • Reacts with the natural bicarbonate alkalinity to precipitate CaCO3
    • As pH increases beyond 10, excess Ca ions react with the phosphate to precipitate hydroxylapatite
  • 10 Ca2+ + 6 PO43- + 2 OH- Ca10(PO4)6(OH)2
    • pH has to be adjusted back before biological treatment
    • No simultaneous P removal can be applied


  • Addition of aluminium or iron
    • Al3+ + HnPO43-n AlPO4 + nH+
    • Fe3+ + HnPO43-n FePO4 + nH+
    • 1 mole aluminium or iron ion will precipitate 1 mole of phosphate
    • Many competing reactions (the above ratio never occurs)
    • We can not estimate the required dosage based on stoichiometry
    • Dosages established based on bench-scale tests
    • Solubility of AlPO4 is the smallest around pH = 6
    • Solubility of FePO4 is the smallest around pH = 5
pre precipitation

metal salt



activated sludge basin


grit chamber




BOD removal  90%

TP removal > 90%

pre precipitation1

Direct precipitation followed by a biological treatment stage

Introduced to biological treatment plants to reduce the loading to the biological stage

Reduction in energy consumption and in hydraulic retention time

simultaneous precipitation

metal salt



grit chamber



activated sludge basin

BOD removal: 90%

TP removal: 75-90%

simultaneous precipitation1

Phosphorus is chemically precipitated at the same time as biological treatment in an activated sludge process

The biological stage also serves as a flocculation tank, with both the biological and chemical sludge being separated in a subsequent stage

Results 1 mg/L TP

post precipitation

metal salt

20 min

10 min

coagulation tank and flocculator

activated sludge basin



grit chamber



BOD removal  90%

TP removal > 95%

post precipitation1

Phosphorus is separated from biologically treated water in a separate post-treatment stage

TP below 0.5 mg/L


1-litre glass cylinders with Kemira's flocculator device


  • to compare the efficiency of different coagulants
  • to determine optimal dosage
colloidal systems
Colloidal systems
  • There is no thermodynamic equilibrium in colloidal systems
  • Proteins, foam (egg white), jelly (gelatinous, gelation)
  • Main group of colloidal system:
    • Colloid dispersions
    • Associated colloids
    • Colloidal macromolecules
colloid dispersions
Colloid dispersions
  • Suspension (solid particles in liquid phase e.g. surface waters – suspended matter in water )
  • Emulsions (liquid particles in liquid phase e.g. oil in water)
  • Gels
    • Hydrophilic matters in water – special structure, gelatinous appearance
  • Sols
    • Hydrophilic matters in water – special structure, gelatinous appearance
    • theoretical diameter < 1m (real diameter is in nm range)
    • Can aggregate into gels
stability of colloid systems
Stability of colloid systems
  • Dispersions
    • Suspensions
      • Distributive stability
        • Colloid (<0.5 m) and quasi colloid (0.5-50 m) particles have high specific surface
        • Cutting a cube (1 cm side length) into 1012 pieces (1 cm = 10 mm = 104 m)  A = 6 m2 1012 = 6104 cm2

A = 6 cm2

A = 6104 cm2

Same volume and mass!!

1 m

1 cm

stability of colloid systems1
Stability of colloid systems
  • Dispersions
    • Suspensions
      • Distributive stability
        • Smaller particle size – higher surface/mass ratio
        • With higher specific surface – special forces develop between the particles and the water, which will decrease the effects of gravity
      • Aggregative stability
        • Electrical charges on the surface of particles (negative, except powdered glass)
        • Repulse each oher (same electrical charge)
        • Result: no aggregation, high stability
    • Emulsions
      • Distributive stability
      • Aggregative stability
stability of colloid systems2
Stability of colloid systems
  • Sols
    • Aggregative stability
  • Gels
    • Distributive stability
    • Structural stability (dewatering)

Truly in solution: <10-9 m (<1 nm)

Colloidal solution: 10-9 – 10-7 m (1 nm – 0.1 m)

Suspension: >10-7 m (>0.1 m)

Settleable: >10-4 m (>100 m)

  • Other classifications
    • dissolved <0.01 m
    • colloidal 0.01-1.0 m (<0.5m)
    • suspended 1.0-100 m
    • settleable suspended solids >100 m
Small particles
    • difficult to separate
    • more strongly affected by surface chemistry forces
    • surface forces prevent particles from clumping together
    • Brownian motion
Hydrophobic colloidal particles:
    • Insoluble in water
    • Clay, fats
    • Maintain themselves in suspension
  • Hydrophilic colloidal particles:
    • Protein, starch, carbohydrate, humic acid
    • Prefer to bind water molecules (rather than bind with each other)
    • Stabile through hydratisation
    • Stability can only be effected by changing the solubility of the molecules on the surfaces of particles (temperature change or adding salt)
    • Surface is electrically charged (no aggregation because of the same electrical charge – usually negative for suspended solids in the water)

Electric charges in the phase boundary surface between colloidal particles and water

  • These charges attract ions of the opposite charge (establishing neutral charge for the colloidal particle and its immediate surface)
  • Stern’s layer: positive charge (strongly bound)
  • Diffusion layer: fixed and mobile layer
  • Charge potential relative to the water (changing with the distance from the particles)
  • Z-potential: potential difference at the boundary of the fixed layer
reduction of z potential
Reduction of Z potential
  • Compression of the thickness of the double layer as a result of the effect of simple counter-ions
    • Neutralising the negative charge we need to add positive ions, but we can not buy ions – we always add negative ions as well
    • Charge neutralisation can be effuicient if we add multivalent + charged ions
    • Coagulation effect of trivalent, bivalent and monovalent cations: Al3+: 11 times larger than Ca2+, 730 times larger than Na+
reduction of z potential1
Reduction of Z potential
  • Specific adsorption of counter-ions on the particle surface
    • Hydrolysis products of Al and Fe depends on pH
    • Low dosages of the hydrolysis compounds will neutralize the surface charges
    • Hydrophobic particles can be covered by hydrophobic material

In order to be able to remove small stable particles by sedimentation, or flotation, it is first necessary to coagulate them

  • The stable state needs to be destroyed so that the particles are attracted to each other and can be bound together by mass attraction forces
  • Highly charged metal ions, their hydroxide complexes or polymeric compounds will be adsorbed on the surfaces of the particles
The reaction occurs in less than 0.1 s
  • Intensive mixing is necessary for coagulation
  • The surface of the particle has been altered so that it is no longer soluble in water – it will combine with other particles through the action of the hydroxide radical of the metal
Al3+, Fe3+ ions are never by themself in water (surrounded by water molecules)
  • In an octaeder structure: [Al(H2O)6]3+
  • Among special circumstances hydrolysis occurs
hydrolysis of metal ions
Hydrolysis of metal ions

[Al(H2O)6]3+ + H2O [Al(H2O)5OH]2+ +H3O+

[Al(H2O)5OH]2+ + H2O  [Al(H2O)4(OH)2]+ +H3O+

[Al(H2O)4(OH)2]+ + H2O  Al(OH)33H2O+H3O+

  • Insoluble aluminium or iron hydroxide
  • Change in pH
necessary circumstances
Necessary circumstances
  • Reaction can only occur if H3O+ is taken away from the vicinity of hydroxides (if not – reverse reaction)
  • In waters with high buffering capacity:

HCO3- + H3O+ = H2CO3 + H2O

reaction can occur + pH decrease is moderate

  • Sulphate (chloride) ions have to be taken away
  • Fast mixing
    • Even concentration distribution
    • Preventing the aggregation of hydroxides with each-other (H-bond) – more efficient colloid destabilisation and floc formation
factors influencing destabilization
Factors influencing destabilization
  • Chemical factors:
    • Certain types of pollutants have a greater tendency to react with iron-based coagulants (sulphide), others prefer aluminium-based coagulants
    • Dissolved pollutants – complex forming substances, tensides, humus substances, biopolymers, phosphates compete with the flocculant
  • pH
    • Solubility of the molecules that make the particle stable is pH-dependent
    • Wastewater has no isoelectric point (there is no pH wherethe compounds agglomerate spontaneously)
    • Performance of coagulant depends on pH
possible pathways of colloid destabilization
Possible pathways of colloid destabilization
  • Destabilization by aluminium or iron(III) ions
  • Destabilization by water soluble aluminium or iron(III) hydroxide polymers
  • Destabilization by weakly water soluble aluminium or iron(III) hydroxide sols
  • Destabilization by aluminium or iron(III) hydroxide flocs
tasks of coagulation flocculation sedimentation processes drinking water treatment
Tasks of coagulation, flocculation, sedimentation processes (drinking water treatment)
  • Suspended solids removal

- requirement – less than 10 mg/L suspended solids

- efficiency - 97-99% (after filtration)

  • Dissolved organic matters removal

- at Lake Balaton - 15-20%

- others - 40-60%

  • Remaining organics - 15-30%
main processes of coagulation
Main processes of coagulation
  • Coagulant feeding
  • Coagulant mixing
  • Connection to the suspended solids and/or dissolved organics (parallel processes)
Coagulant transformation (parallel processes)
  • Water soluble Al-hydroxides formation (parallel processes)
  • Connection to the suspended solids and/or dissolved organics (parallel processes)
  • Al-hydroxide sols formation (parallel processes)
  • Sol-suspended solids, sol-dissolved organics connection (parallel processes)
  • Sol-sol aggregation (parallel processes)
  • Floc formation – flocculation
  • Flocs-suspended solids, flocs-dissolved organics connection???
  • Coagulation

Decreasing and elimination the stability against the aggregation of solid particles by aluminium and iron(III)-salts, or their hydrolysis products

  • Flocculation

Aggregation of destabilized particles

  • Sedimentation (removal of pollutants)
floc formation
Floc formation
  • The already destabilized particles can combine to form large, densely packed flocs
  • Floc formation is encouraged by high turbulence, but large flocs are easily broken up by it
    • Intensive stirring for rapid floc formation – break-up is not significant as flocs are small
    • Slow stirring to allow floc growth

Combination of metallic salts with polymers can result in better treatment performance

  • Polymers can destabilise colloids by charge neutralisation (same way as metal salts)or by bridge building mechanisms - polymer is adsorbed onto particles
  • Size of the flocs are larger – sedimentation is more efficient


dissolved CODCr:

227-405 mg/l

50-85% of the total organic matter