WATER PLANT OPERATOR TRAINING PROGRAM


Coagulation and Flocculation 2

Coagulation:
  • “Addition of coagulant to help collect the finely divided suspended particles in the raw water into larger clumps in a short time”
  • High speed mixing

    Flocculation:
  • “Growth of floc through constant collisions”
  • Slow mixing

    Coagulation and Flocculation
  • Particulate - Impurities in water result from:
  • Land erosion
  • Pick up of minerals
  • Decay of plant material
  • Additional: airborne contamination, industrial discharges, animal wastes

    Surface water sources polluted by man and nature likely to contain:
  • Suspended
  • Dissolved
  • Organic and inorganic material

    Biological forms
  • Bacteria
  • Plankton

    Particulates - suspended solids (size varies)
  • Larger: sand & silts – settleable solids
  • Smaller: bacteria, fine clays, non-settleable solids or colloidal matter

    Purpose of Coag, Floc: -- To remove particulate impurities especially non-settleable solids and colour from water being treated

    Conventional Treatment
  • Coagulation
  • Flocculation
  • Sedimentation
  • Filtration
  • Disinfection
  • Distribution

    Direct Filtration
  • Coagulation
  • Flocculation
  • Filtration
  • Disinfection
  • Distribution

    In the coagulation process, chemicals added to cause particles to become destabilized and clump together. These particles gather together to form larger particles in the flocculation process

    Coagulants: Chemicals used to thicken finely divided suspended solids into larger clumps for east removal.

    Coagulation: Destabilization and initial aggregation of colloidal and finely divided Suspended matter by the addition of a floc. Forming chemical (coagulant).

    Floc: Small jelly-like masses formed in a liquid by adding a coagulant

    Flocculation: The collection of coagulated suspended solids into a mass by gentle Stirring.

    Coagulants: In practice, chem. coagulants referred to as primary coagulants or as coagulant aids
  • cause particles to become destabilized
  • adds density to slow settling flocs
  • adds toughness
  • also called floc or sedimentation aids

    Basic Coagulant Theory
    • Coagulant is a physical and chemical reaction between the alkalinity of the water and the coagulant added --- insoluble flocs.
    • For a specific coagulant, the ph of the water determines which hydrolysis chemicals pre-dominate
    • Lower ph values favor the compounds which are desirable for reacting with –ve colloids and particulates.
    • Optimum ph for coagulant: ph 5-7 Residual alkalinity serves to: buffer ph in system, aids in complete precip. of chemicals
    • Lime or soda ash used to control ph and/or alkalinity
    • As OH- ions assumed, PH - Alum Complex + Suspended Matter = Neutralized Particle
    • Electrical charges become neutralized
    • Suspended particles become stuck in alum complex
    • Takes place in a few seconds
    • Its important that alum complex ions are instantly dispersed. High speed mixers are needed
    • This is called “flash” mixing.
    • This is the coagulation process.
    Methods Of Flash Mixing
    • Hydraulic: need sufficient H20 velocity
    • Mechanical: versatile and reliable, uses greatest amount of electrical energy
    • Diffusers And Grid Systems: has performed tubes and nozzles, can provide uniform distribution of coag. Over entire basin, sensitive to flow changes
    • Pumped Blender: coag. Added to H20 thru, a diffuser in a pipe, provides rapid dispersion, no significant head loss in the system, elec. use < mechanical mixer
    • Flocculation: slow stirring, small coag. Suspended particles grow into larger settleable solids. This process provides contact between particles to form larger particles called floc. For ease of removal by: sedimentation and filtration.
    • Floc formation controlled by: rate at which collisions occur, effectiveness of collisions. The purpose is to create a floc of good size (0.1 mm --- 3 mm), density, toughness.
    • Floc Formation:
      a) Alum Complex + Suspended Matter = Neutralized Particle
      Coag: Initial floc formed
      b) floc: Floc grows through const. Collisions
      As floc grows collisions with suspended matter and algae take place.
      Lge #’s algae ----- ‘stringy appearance’ floc ~ 96% H20 delicate
    • Efficient Flocculation Involves:
      Selection of right stirring time (detention time)*
      Proper stirring intensity
      Properly shaped basin for uniform mixing
      Mechanical equipment or other means of stirring
    • 5-20 min. for direct filtration, 30 min. for conventional filtration


    Flocculation Aids:
    1. Adsorbants – Weighting Agents, e.g. Bentonite clays, used with waters of - high colour, low turbidity
    2. Activated Silica
      NA20.SI02 + activator = SI02 + byprod.
      Activator: Sodium Bicarbonate, Chlorine, Alum, Ammonium sulphate
    3. Polyelectrolytes:
      Long, linear chains of carbon atoms
      Colloidal in nature & action
      Cationic Anionic Non-Ionic
    Uses For Coag-Aids
  • Small or slow forming flocs.
  • Low raw water temp.
  • Reduced Chemical Dosages: Primary Coagulant, Disinfectant, Coloured flocs.

    Advantages Of Coag-Aids
    • Improved quality of filtered water
    • Broader pH range of treatment
    • Good flocculation of widely varied water
    • Increased compaction of sludge
    • Long filter runs, use of higher filter rates
    • Reduced chemical cost
    • Increase of plant capacity
    • Helps to reduce the residual content
    • Therefore: less chance for after-floc problems.
    Importance Of Proper Chemical Dosage
    • Good turbidity removal
    • Good colour removal
    • Good algae removal
    • Long filter runs
    • Low filter runs
    • Low residual aluminum
    • Good $ value