BY

AAKANSHA DANTRE

TUBE SETTLER IN SEDIMENTATION

INTRODUCTION

Pure water can never be available in nature as when flows over the surface of earth, picks up/ dissolves particles.

The impurities dissolved can be useful or harmful making water HARMFUL & UNFIT for human consumption.

Hence it is necessary to treat raw available water to safe and permissible limits.

Methods Involved

In conventional water treatment technology, following various methods which may be adopted for purifying the public water supplies are-:

Screening Plain Sedimentation Sedimentation aided with coagulation Filtration Disinfection Aeration Softening Miscellaneous treatment

Raw

sewage

P P C P P CTo consumers

sedimentation

screening Coagulation and Flocculation

sedimentation

filtration

Disinfection

Note: P- Physical Process C- Chemical Process

SCHEMATIC DIAGRAM OF CONVENTIONAL WATER TREATMENT PROCESS

SEDIMENTATION

Suspended impurities present in water have specific gravity greater than that of water.

In normal raw supplies, these particles remain in suspension because of turbulence.

As the turbulence is retarded, impurities settle down and this is PRINCIPLE behind sedimentation.

Theory of sedimentation

The settlement of a particle in water is brought to rest, is

opposed by the following factors-: The velocity of flow which carries the particle

horizontally. The viscosity of water in which particle is travelling. The size, shape and specific gravity of particle.

The settling velocity of a spherical particle is given by

Stoke’s law which takes the above three factors into

account.

DESIGN CONCEPT

V

DESIGN CONCEPT

Consider a particle settled which was at surface and settled through depth.

V/Vs = L/ H Vs= (V.H )/L Vs = (QH)/ LBH Vs = Q/A Thus DEPTH OF THE BASIN IS NOT A

FACTOR IN PARTICLE REMOVAL

TYPES OF SEDIMENTATION TANK

Typical sedimentation tanks: (a) rectangular horizontal flow tank; (b) circular, radial-flow tank; (c) hopper-bottomed, upward flow tank

(a) INLET ZONE – At the central well, which has a round baffle plate, the flow is established in a uniform radial direction so that short-circuiting does not take place.

(b) SETTLING ZONE – Where settling is assumed to occur as the water flows towards the outlet.

(c) OUTLET ZONE – In which the flow converges up and over the decanting weirs.

(d) SLUDGE ZONE – Where settled material collects and is pumped out.

IMPORTANT PARTS OF A SEDIMENTATION TANKS

ZONES IN SEDIMENTATION TANK

TUBE SETTLERS

Tube settlers increase the settling capacity of circular clarifiers and/or rectangular sedimentation basins by reducing the vertical distance a floc particle must settle before agglomerating to form larger particles.

The inclination provides for a particle settling depth that is significantly less than the settling depth of a conventional clarifier, reducing settling times.

TUBE SETTLERS

DESIGN CONSIDERATION

There must be laminar (or viscous) flow conditions within the tubes at the maximum flow rate required.

The residence time within each tube must be ample so that a floc particle entering at the extreme upper edge of the tube will have sufficient time to settle to the collecting surface a vertical distance below.

The velocity of flow through the tubes must not exceed a critical maximum that would cause the settled sludge to lose stability and be swept out of the tube in the direction of normal flow.

The performance of tube settler is normally estimated by a parameter, S

S= vs /vo(sin+Lcos)Where vs = settling velocity of the particlevo = velocity of flow along the tube settlers = angle of inclination of the tube settler with the horizontalL = relative effective length of the settlerl = length of the tube settlerd = diameter of tube settler

Size, Shape and Configuration The vertical settling height within a tube should

be kept as short as possible, within the restraints of cost and plugging potential, to minimize the settling distance for the particles. 

Careful consideration should also be given to the vertical height of the modules. 

Tube settlers manufactured with the tubes aligned in the same direction avoid the formation of crossing points that cross flow tubes promote. 

Tube module design should

incorporate features that

would prevent gaps along

the installed modules. Uni-directional tubes are easier

to clean and have less chance for

plugging.

Material of Construction

Evenly-spaced sheets of PVC which are solvent-welded to form a durable bond between sheets and channels. 

PVC modules have a specific gravity considerably greater than water and will not float.

PVC is denser, has greater tensile strength, higher impact strength, and is much less flammable then ABS.

TUBE SETTLERS V/s PLATE SETTLERS

Plate settlers are more expensive than tube settlers.

These tend to be hydraulically unstable. Tube settlers eliminate crossflows and eddy

currents. Tube settlers allow for the use of corrosion-

resistant, lighter-weight PVC, resulting in a 50% cost savings.

Tube settlers are a common, economically-viable alternative/solution to parallel plates (plate settlers).

APPLICATIONS FOR TUBE SETTLERS

Potable Water Clarification Ballasted Flocculation Membrane Pre-Treatment Desalinization Pre-Treatment Backwash Reclamation Resin Recovery Secondary/Tertiary Wastewater Clarification Phosphorous Removal

ADVANTAGES

Clarifiers/basins equipped with tube settlers can operate at 2 to 4 times the normal rate of clarifiers/basins without tube settlers.

It is possible to cut coagulant dosage by up to half while maintaining a lower influent turbidity to the treatment plant filters.

Less filter backwashing equates to significant operating cost savings for both water and electricity.

New installations using tube settlers can be designed smaller because of increased flow capability.

Flow of existing water treatment plants can be increased through the addition of tube settlers.

Tube settlers increase allowable flow capacity by expanding settling capacity and increasing the solids removal rate in settling tanks.

Why tube settlers?

Tube settlers offer an inexpensive method of upgrading existing water treatment plant clarifiers and sedimentation basins to improve performance.

They can also reduce the tankage/footprint required in new installations or improve the performance of existing settling basins by

reducing the solids loading on downstream filters. Made of PVC, tube settlers can be easily supported with minimal structures that often incorporate the effluent trough supports. 

REFERENCES

http://en.wikipedia.org/wiki/Sedimentation_(water_treatment)

http://en.wikipedia.org/wiki/Sedimentation_(water_treatment)#Potable_Water_Treatment

http://www.brentwood-ind.com/water/tubesettlersystems.html

http://www.corix.com/corix-companies/water-systems/water-treatment/technology/tube-settler.aspx