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

CHEMICAL PRECIPITATION OF PHOSPHORUS 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

CHEMICAL PHOSPHORUS REMOVAL 1.Phosphorus removal (chemical precipitation) Al 3+ + PO 4 3-  AlPO 4 2.Removal of organic matter (coagulation-flocculation) Al 3+  aluminium-hydroxide Good coagulant: contacts suspended matters (mainly organics) of wastewater rapidly and stronglyGood 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-flocculationOrganics are originally mainly in colloidal form – do not settle well – settling characteristics can be improved due to coagulation-flocculation

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

CHEMICAL TREATMENT ·as the only treatment process primary (direct) precipitationprimary (direct) precipitation ·or in combination with biological treatment processes pre-precipitationpre-precipitation simultaneous precipitationsimultaneous precipitation post-precipitationpost-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

CHEMICAL PHOSPHORUS REMOVAL Addition of calcium Usually in the form of lime (Ca(OH) 2 )Usually in the form of lime (Ca(OH) 2 ) Reacts with the natural bicarbonate alkalinity to precipitate CaCO 3Reacts with the natural bicarbonate alkalinity to precipitate CaCO 3 As pH increases beyond 10, excess Ca ions react with the phosphate to precipitate hydroxylapatiteAs pH increases beyond 10, excess Ca ions react with the phosphate to precipitate hydroxylapatite 10 Ca PO OH -  Ca 10 (PO 4 ) 6 (OH) 2 pH has to be adjusted back before biological treatmentpH has to be adjusted back before biological treatment No simultaneous P removal can be appliedNo simultaneous P removal can be applied

CHEMICAL PHOSPHORUS REMOVAL Addition of aluminium or iron Al 3+ + H n PO 4 3-n  AlPO 4 + nH + Fe 3+ + H n PO 4 3-n  FePO 4 + nH + 1 mole aluminium or iron ion will precipitate 1 mole of phosphate1 mole aluminium or iron ion will precipitate 1 mole of phosphate Many competing reactions (the above ratio never occurs)Many competing reactions (the above ratio never occurs) We can not estimate the required dosage based on stoichiometryWe can not estimate the required dosage based on stoichiometry Dosages established based on bench-scale testsDosages established based on bench-scale tests Solubility of AlPO 4 is the smallest around pH = 6Solubility of AlPO 4 is the smallest around pH = 6 Solubility of FePO 4 is the smallest around pH = 5Solubility of FePO 4 is the smallest around pH = 5

PRECIPITANT ADDITION

PRE-PRECIPITATION screengrit chamber sedimentation activated sludge basin sedimentation flocculator metal salt min BOD removal  90% TP removal > 90%

PRE-PRECIPITATION 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 screen grit chamber sedimentationactivated sludge basin sedimentation metal salt min BOD removal: 90% TP removal: 75-90%

SIMULTANEOUS PRECIPITATION 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 screen grit chamber sedimentation activated sludge basin sedimentation metal salt 20 min sedimentation 10 min coagulation tank and flocculator BOD removal  90% TP removal > 95%

POST-PRECIPITATION Phosphorus is separated from biologically treated water in a separate post-treatment stage TP below 0.5 mg/L

COAGULATION-FLOCCULATION LABORATORY JAR TESTS to compare the efficiency of different coagulantsto compare the efficiency of different coagulants to determine optimal dosageto determine optimal dosage 1-litre glass cylinders with Kemira's flocculator device

PO 4 -P precipitation

TP REMOVAL

Colloidal systems There is no thermodynamic equilibrium in colloidal systemsThere is no thermodynamic equilibrium in colloidal systems Proteins, foam (egg white), jelly (gelatinous, gelation)Proteins, foam (egg white), jelly (gelatinous, gelation) Main group of colloidal system:Main group of colloidal system: –Colloid dispersions –Associated colloids –Colloidal macromolecules

Colloid dispersions Suspension (solid particles in liquid phase e.g. surface waters – suspended matter in water )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)Emulsions (liquid particles in liquid phase e.g. oil in water) GelsGels –Hydrophilic matters in water – special structure, gelatinous appearance SolsSols –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 DispersionsDispersions –Suspensions Distributive stabilityDistributive stability –Colloid (<0.5  m) and quasi colloid (  m) particles have high specific surface –Cutting a cube (1 cm side length) into pieces (1 cm = 10 mm = 10 4  m)  A = 6  m 2  = 6  10 4 cm 2 1 cm A = 6 cm 2 1  m A = 6  10 4 cm 2 Same volume and mass!!

Stability of colloid systems DispersionsDispersions –Suspensions Distributive stabilityDistributive 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 stabilityAggregative 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 stabilityDistributive stability Aggregative stabilityAggregative stability

Stability of colloid systems –Sols Aggregative stabilityAggregative stability –Gels Distributive stabilityDistributive stability Structural stability (dewatering)Structural stability (dewatering)

Particles Truly in solution: <10 -9 m (<1 nm) Colloidal solution: – m (1 nm – 0.1  m) Suspension: >10 -7 m (>0.1  m) Settleable: >10 -4 m (>100  m) Other classificationsOther classifications –dissolved <0.01  m –colloidal  m (<0.5  m) –suspended  m –settleable suspended solids >100  m

Small particlesSmall particles –difficult to separate –more strongly affected by surface chemistry forces –surface forces prevent particles from clumping together –Brownian motion

Hydrophobic colloidal particles:Hydrophobic colloidal particles: –Insoluble in water –Clay, fats –Maintain themselves in suspension Hydrophilic colloidal particles: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 waterElectric 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)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)Stern’s layer: positive charge (strongly bound) Diffusion layer: fixed and mobile layerDiffusion layer: fixed and mobile layer Charge potential relative to the water (changing with the distance from the particles)Charge potential relative to the water (changing with the distance from the particles) Z-potential: potential difference at the boundary of the fixed layerZ-potential: potential difference at the boundary of the fixed layer

Reduction of Z potential Compression of the thickness of the double layer as a result of the effect of simple counter-ionsCompression 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: Al 3+ : 11 times larger than Ca 2+, 730 times larger than Na +

Reduction of Z potential Specific adsorption of counter-ions on the particle surfaceSpecific 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 themIn 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 forcesThe 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 particlesHighly 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 sThe reaction occurs in less than 0.1 s Intensive mixing is necessary for coagulationIntensive 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 metalThe 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

Al 3+, Fe 3+ ions are never by themself in water (surrounded by water molecules)Al 3+, Fe 3+ ions are never by themself in water (surrounded by water molecules) In an octaeder structure: [Al(H 2 O) 6 ] 3+In an octaeder structure: [Al(H 2 O) 6 ] 3+ Among special circumstances hydrolysis occursAmong special circumstances hydrolysis occurs

Hydrolysis of metal ions [Al(H 2 O) 6 ] 3+ + H 2 O  [Al(H 2 O) 5 OH] 2+ + H 3 O + [Al(H 2 O) 5 OH] 2+ + H 2 O  [Al(H 2 O) 4 (OH) 2 ] + + H 3 O + [Al(H 2 O) 4 (OH) 2 ] + + H 2 O  Al(OH) 3  3H 2 O + H 3 O + Insoluble aluminium or iron hydroxideInsoluble aluminium or iron hydroxide Change in pHChange in pH

Necessary circumstances Reaction can only occur if H 3 O + is taken away from the vicinity of hydroxides (if not – reverse reaction)Reaction can only occur if H 3 O + is taken away from the vicinity of hydroxides (if not – reverse reaction) In waters with high buffering capacity:In waters with high buffering capacity: HCO H 3 O + = H 2 CO 3 + H 2 O HCO H 3 O + = H 2 CO 3 + H 2 O reaction can occur + pH decrease is moderate reaction can occur + pH decrease is moderate Sulphate (chloride) ions have to be taken awaySulphate (chloride) ions have to be taken away Fast mixingFast mixing –Even concentration distribution –Preventing the aggregation of hydroxides with each- other (H-bond) – more efficient colloid destabilisation and floc formation

Factors influencing destabilization Chemical factors: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 pHpH –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 Destabilization by aluminium or iron(III) ionsDestabilization by aluminium or iron(III) ions Destabilization by water soluble aluminium or iron(III) hydroxide polymersDestabilization by water soluble aluminium or iron(III) hydroxide polymers Destabilization by weakly water soluble aluminium or iron(III) hydroxide solsDestabilization by weakly water soluble aluminium or iron(III) hydroxide sols Destabilization by aluminium or iron(III) hydroxide flocsDestabilization by aluminium or iron(III) hydroxide flocs

Tasks of coagulation, flocculation, sedimentation processes (drinking water treatment) Suspended solids removalSuspended solids removal - requirement – less than 10 mg/L suspended solids - efficiency % (after filtration) Dissolved organic matters removalDissolved organic matters removal - at Lake Balaton % - others % Remaining organics %Remaining organics %

Main processes of coagulation Coagulant feedingCoagulant feeding Coagulant mixingCoagulant mixing Connection to the suspended solids and/or dissolved organics (parallel processes)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???

Definitions CoagulationCoagulation Decreasing and elimination the stability against the aggregation of solid particles by aluminium and iron(III)-salts, or their hydrolysis products FlocculationFlocculation Aggregation of destabilized particles Sedimentation (removal of pollutants)Sedimentation (removal of pollutants)

Floc formation The already destabilized particles can combine to form large, densely packed flocsThe 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 itFloc formation is encouraged by high turbulence, but large flocs are easily broken up by it 1.Intensive stirring for rapid floc formation – break- up is not significant as flocs are small 2.Slow stirring to allow floc growth

Combination of metallic salts with polymers can result in better treatment performanceCombination 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 particlesPolymers 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 efficientSize of the flocs are larger – sedimentation is more efficient

RESIDUAL COD CR CONCENTRATIONS VS. COAGULANT DOSAGE dissolved COD Cr : mg/l 50-85% of the total organic matter