CLARIFIER PRINCIPLE OF OPERATION

PRINCIPLE OF OPERATION

The Solids Contact Clarifier works on two basic principles of Coagulation / Flocculation and Hydraulic Separation.

Coagulation and flocculation occur in the flocculation zone when the feed stream comes in intimate contact with chemical mixtures and suspended sludge particles from previously treated water.  This contact also promotes floc growth as smaller particles agglomerate into larger heavier particles.

The hydraulic separation principle uses an upflow design to move water into the settling zone for clarification.

WORKING

Raw water enters the central draft tube above the re-circulator impeller where it is mixed with treatment chemicals and re-circulated precipitates.  Precipitate re-circulation is accomplished by a variable speed impeller which acts as an air lift pump.  The mixture of raw water and sludge rises through the central draft tube and is discharged into the flocculation compartment.

This mixture undergoes gentle hydraulic turbulence as it passes through this zone.

Part of the water (equivalent to the instantaneous influent flow rate) enters the clarified water zone and rises toward the effluent collector.

Settled precipitates (sludge) are moved continuously along the floor toward the centre of the unit by means of a slowly rotating scraper, which covers the entire floor area.  The accumulated sludge is transferred to the sludge pit at the centre of the unit, where sludge-thickening pickets concentrate the sludge, reducing the total amount of blowdown.

Excess sludge is removed by a blowdown system.  Backflushing with water under pressure clears the blow-off line.  Opening of the blow-off valve permits the sludge to flow to waste.  These operations are either manual or automatic.  Most of the water and suspended precipitates enter the lower end of the draft tube to be re-circulated, providing positive solid contact regardless of sludge inventory level.  Clear water rises and is uniformly collected.

PLATE TYPE CLARIFIER

The  Plate Type Clarifier is a new development in filtration technology for effective removal of High Suspended Solids present in the water.

Clarity of liquid overflow and desnity of underflow discharge are the two fundamental process requirements of all gravity settling equipment. 

In many applications the area needed to provide the desired overflow clarity exceeds that required for thickening of the settled solids.  This means that, in a cylindrical settling tank, the lower section including the rakes and drive mechanism is overdimensioned.

Pre-treatment is given to water to make it suitable for subsequent treatment, which makes it suitable for use in a particular process.  Actually what we term as pre-treatment plant is a misnomer.  The pre-treatment plant generally consists of Coagulations, Clarification and Filtration.  If one examines the worldwide statistics of water treatment plants - 85% of the plants will fall in this category.

COAGULATION :

The suspended solids in water acquire a negative charge which prohibits these particles incoming together to form a large mass for easier settleability.  Coagulants such as Alum, Ferrous sulphate, Ferric chloride - which have positive charged ions like aluminum, Iron are then dosed into the water to hasten the settling of the suspended particles.  Suitable dosing system comprising tanks, pumps etc. are employed for this purpose.  Coagulated particles are known as “Flocs”.

CLARIFICATIONS :

The purpose of clarification is to bring the flocs together to form a larger mass.  The larger the mass of flocs, the heavier the particles and easier settleability.

Inclined plate clarifier with solid contract re-circulation achieves solid - liquid separation by directing the liquid between a series of inclined plates.  The settling surface of each plate is equivalent to its horizontal projection.  These plates are normally spaced approx 50 mm apart, with the result that large settling surfaces are concentrated within a relatively small floor area.

EQUIPMENTS :

FLOCCULATION CUM FLASH MIXING TANK :

This tank is constructed with MS plates and having a circular cross section with a diameter of 2200 mm and a conical bottom.  There are TWO compartments in this tank.  In flash mixing chamber a motorized agitator with 0.5 H.P. motor is mounted.  In flocculator chamber a geared motor       mixer is mounted.  This tank is internally painted with coal tar epoxy after sand blasting.

CLARIFIER :

Constructed from MS plates, rectangular in cross section with a conical     hopper bottom.  FRP plated are fitted inside the clarifier with an equidistant of 50 mm apart.

OPERATION :

Raw water is pumped to a chamber outside the flash-mixer tank, Coagulant (Alum) is dosed into this chamber through a dosing pump.  Water then enters the flash-mixing chamber.  Mixing of coagulant with water takes place with the help of motorized mixer and then it enters the flocculation chamber.  Slow speed agitator mounted on this chamber rotates at gentle speed, which coalesces the floc.  The overflow of flocculator tank passes into the clarifier chamber.  The clarifier has a launder at its inlet.  Water flows through this launder to the bottom portion and slowly rises up in between the plates.   The particles settle down on the plates and slide down to the sludge-collecting zone.  Clear water moves up and is collected in the outlet launder through pipes having small orifices.  These small orifices regulate the laminar flow of water to the outlet launder.   A sample to be drawn from the bottom portion of clarifier where ½” tapping is provided to check for sludge concentration.  Whenever sludge concentration is found more, open the valves provided at the drain nozzles and drain the sludge till clear water comes out.

Cross sectional drawing showing the operation is enclosed for reference.

MAINTENANCE :

The advantage of this type of clarifier is fact that not only it saves area but very easy to operate and maintain.  the only rotary equipments are two small motors, a small gearbox, which can be repaired / replaced quickly if needed.

The FRP plates can be lifted one by one by hand (Weight approx 6 kg) from the platform.  They can be cleaned and put back again with out affecting treated water quality.   This exercise is to be monitored bimonthly or monthly.

ADVANTAGES  :

1.                  Heavy Duty rigid construction of tank, sludge hopper and plate packs.

2.                  White spacing of plates to handle high density feed pulps and coarse solid particles.

3.                  Lower capital costs

4.                  Lower installation costs.

-          One piece or pre-fabricated delivery

-          Smaller foundations

-          Less building space

-          Rectangular plan suits conventional building structures

5.                  Prevents short circuiting and reduces turbulences from surface wind or feed temperature variations.

6.                  Simple flat sheet and standard section construction

-          Special materials and protective coatings easily incorporated.

-          Repair and replacement of parts is simplified.