Sedimentation with CoagulationChemicals are added to neutralize the forces preventing agglomeration, or help in increased floc formation, and settled.Factors affecting Types of coagulant, dose of coagulant, characteristics of water, mixing.

Theory of CoagulationDouble Layer surface chargesAdsorptionHydrolysisPolymerizationCharge neutralization

Common CoagulantsAlumChlorinated CopperasFerrous sulphate and limeSodium aluminatePolyelectrolytes

Aluminium SulphateAl2 (SO4)3 18H2OAl2 (SO4)3 18H2O + 3 Ca(HCO3)2 = 2 Al (OH)3 + 3CaSO4 + 18 H2O + 6 CO2If Lime is added (in the absence of sufficient Alkalinity)Al2 (SO4)3 18H2O + 3 Ca(OH)2 = 2 Al (OH)3 + 3CaSO4 + 18 H2O When Soda Ash is addedAl2 (SO4)3 18H2O + 3 Na2CO3 = 2 Al (OH)2 + Na2SO4 + 18 H2O + 3 CO2Alum is effective when pH between 6.5 to 8.5. Dosage for water 10 30 mg/l. Generally not used for sewage treatment. If used dosage 40 60 mg/l

Ferrous SulphateFeSO4. 7H2OReacts with natural alkalinity, but slow.FeSO4. 7H2O + Ca(OH)2 = Fe (OH)2 + CaSO4 + 7 H2O4 Fe(OH)2 + O2 + 2H2O = 4 Fe(OH)3 Effective in pH ranges 4 to 7 and above 8.5.Dosage 5-10 mg/l for water; 35 40 mg/l for sewage.

Ferric chlorideFe Cl32Fe Cl3 + 3Ca(OH)2 = Fe (OH)3 + CaCl2 Effective in pH range 5.5 7.0Forms dense, heavy floc which settles rapidly. Used extensively for waste water treatment.Dosage

Chlorinated CopperasChlorine introduced along with Copperas (Hydrated Ferric sulphate, FeSO4.7 H2O)Oxidises to Ferric sulphate{(Fe2 (SO4)3} and Ferric Chloride ( Fe Cl3)FeSO4.7 H2O + 3 Cl2 = 2Fe2 (SO4)3 + FeCl3 + 7 H2OFe2 (SO4)3 + 3 Ca(OH)2 = 3 CaSO4 + 2 Fe(OH)3Effective over pH range 3.5 to above 9.0, useful in waste water treatment; dosage 30 80 mg/l

Sodium AluminateNa2Al2O4 + Ca(HCO3)2 = Ca Al2O4 + Na2CO3 + CO2 + H2ONa2Al2O4 + CaCl2 = Ca Al2O4 + 2 NaClNa2Al2O4 + CaSO4 = Ca Al2O4 + Na2SO4Effective in removing hardness; effective pH ranges of 6 to 8.5

PolyelectrolytesHigh molecular weight water soluble polymersAnionic, cationic and non-ionic.Trade names such as Floccal, Magnifloc, Mogul etc.Broadens the range of pHUsually less than 1 mg/l dosage.

Coagulant AidsOxidizing agentsWeighting agentsActivated SilicaPoly electrolytes

MixingUsing Hydraulic JumpsTurbinesRapid and slow

Feeding of ChemicalsDry FeedCheap, difficult to controlChemical may clog, useful for non uniform materialAlum etc.Wet FeedEasy to control, but costlyNon-clogging type, Suitable for even very small dosages, uniform feedingCopperas, lime etc. which absorb moisture and tend to clog.

Design Criteria for FlocculatorsDetention time 1.5 3 hours ( mixing unit 45 60 sec)Paddle Area10 25 % of C.S.AreaPaddle speed 4-5 RPMInlet velocity15 25 cm/secPeripheral velocity of paddle blades = 0.2 to 0.6 m/sec. (0.75 paddle velocity. Geberally the ratio of fluid velocity to paddle velocity is 25 %)Velocity gradient ( G)10 75 per sec(for mixing>300 per sec)Gt factor10,000 to 100,000Power Consumption10 36 KW/m3/dayOutlet velocity0.15 to 0.25 m/dayWeir loading 300 m3/m/daySludge scraper tip velocity0.5 cm/sec at 1 RPM ( 0.75 KW/m)