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Sludge production : F SP = X e × (Q-Q e )+ X R × Q W – (X 0 × Q 0 )

Q , S 0 , X 0. ( Q -Q w ), S, X e. V , X , S. Q w , S, X R. Q r , S , X R. Sludge production : F SP = X e × (Q-Q e )+ X R × Q W – (X 0 × Q 0 ) In mass balance the source has to be included (bacteria are growing with the use of substrate) (in + source – sink = out)

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Sludge production : F SP = X e × (Q-Q e )+ X R × Q W – (X 0 × Q 0 )

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  1. Q, S0, X0 (Q-Qw), S, Xe V,X, S Qw, S, XR Qr, S, XR • Sludge production: • FSP = Xe×(Q-Qe)+ XR×QW– (X0×Q0) • In mass balance the source has to be included (bacteria are growing with the use of substrate) (in + source – sink = out) • Excess sludge production: • QW×XR 0

  2. Q, S0, X0 (Q-Qw), S, Xe V,X, S Qw, S, XR Qr, S, XR • Sludge age (solid retention time): • average time during which the sludge has remained in the system • SRT = X = (V× X)/ (Xe×(Q-Qw) + XR×QW) d • kg sludge present in the areation tank • sludge leaving the system kg/d • SRT < 2 days  high load • SRT = 2-6 days  normal load • SRT > 6 days  low load

  3. 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

  4. 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

  5. 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

  6. 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

  7. Stability of colloid systems • Sols • Aggregative stability • Gels • Distributive stability • Structural stability (dewatering)

  8. Particles 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

  9. Small particles • difficult to separate • more strongly affected by surface chemistry forces • surface forces prevent particles from clumping together • Brownian motion

  10. 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)

  11. 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

  12. 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+

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

  21. 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 oranics - 15-30%

  22. Main processes of coagulation • Coagulant feeding • Coagulant mixing • Connection to the suspended solids and/or dissolved organics (parallel processes)

  23. 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???

  24. Definitions • 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)

  25. 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

  26. 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

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