ENVIRONMENTAL ENGINEERING

441

Lecture 5:

Water Treatment (3)

Sedimentation & filtration

Philadelphia UniversityFaculty of Engineering

Department of Civil EngineeringFirst Semester, 2013/2014

RAPID MIXING - FLOCCULATION

The process whereby the chemicals are quickly & uniformly dispensed in the water

During coagulation and softening, the chemical reaction that take place in rapid mixing for precipitation – in less 10-13 second

For flocculation, enough mixing to bring the flocs from settling in the basin. To much will shear the flocs particles so that the flocs is small & finely dispersed.

SEDIMENTATION- OVERVIEW

Process of heavier solid particles in suspension, settle to the body of the tank by gravity

Designed to remove settled water from the basin without carrying away any of the floc particles.

Sedimentary Basin/ clarifier are divided into four zones: Inlet, settling, outlet and sludge storage

SEDIMENTATION- OVERVIEW

SEDIMENTATION-CONCEPT Velocity of flowing water (Over

flow rate/surface loading rate (is proportional to the flow rate divided by the surface area

Where,

Vo = water velocity (m/s)

Q = water flow (m3/s)

Ac= cross surface area (m2)

o

c

SEDIMENTATION-CONCEPT Particle removal is dependent on the overflow

rate and independent of the depth of sedimentary tank

If the particle to be removed from the bottom of the clarifier and not go out in the settled water, then the particle settling velocity must be greater than the over flow rate (Vs>Vo);

If (Vs>Vo); 100% removal of particle

If (Vo>Vs); Zero% removal of particle Often Vo is set as 50 to 70%

For designing a sedimentary tank, first we have to determine the Vs before set the over flow rate Vo . Vs is dependent on the type of particles

Surface Area=A

Vo

Vs

FILTRATIONFiltration is a process for separating suspended

or colloidal impurities from water by passing through a porous medium, usually a bed of sand or other medium.

Settled water (Sedimentation effluent) turbidity range 1-10 TU – due to residue of flocs particles. So turbidity need to be reduce to less than 0.3

Common materials for granular bed filters: Sand (slow, rapid or high) Anthracite coal Dual media (Coal plus sand) Mixed media (coal, sand & garnet)

FILTRATION- FILTER MEDIA

Loading rate

Where, Vo = water velocity (m/s) Q = water flow (m3/s)Ac= cross surface area (m2)

FILTRATION Slow sand filters Low filtration rate with the use of smaller sand Filter sand is less uniform Particles are removed on the surface of the filter

(forming a mat of materials , called schmultzdecke) Schmultzdecke forms a complex of biological

community that degrade some organic compounds. Pretreatment is not important Require large are of land and are operator intensiveRapid sand filters ( most common) Graded (layered) within the bed to optimize the

passage of water while minimizing the passage of particulate mater

Cleaned in place by backwashing process Pretreatment to destabilize particles is essential

Dual-media Filters• Constructed of silica sand and anthracite coal. • Depth of sand is about 0.3m and coal 0.45 m. Size

and uniformity is selected to produce a distinct separation after backwashing

• Disadvantage is that filtered materials are held loosely in the anthracite layer and can dislodge with sudden changes in hydraulic loading. The material can then bind to the sand layer

Mixed Media Filters• The perfect filter is composed of a grading of

large media at the top to small at the base. This is best achieved by the use of 3 or more media with ranging size, density and uniformity coefficient

• Typical installation – Overall bed depth 0.75m; 60% anthracite; 30% silica sand; 10% garnet sand

• Size range from 1.0mm anthracite to 0.15 garnet sand

• Filtration rates range from 10 to 20m/hr