FiltrationFiltration

FiltrationFiltration

Single Media Rapid SandSingle Media Rapid Sand

Dual Media Rapid SandDual Media Rapid Sand

Filtration MechanismsFiltration Mechanisms

Surface Filtration - ScreeningSurface Filtration - Screening

Particles larger than pore openings between filter Particles larger than pore openings between filter Particles are removed on the surface of the filterParticles are removed on the surface of the filter

Rapid increase in head loss through filter – filter Rapid increase in head loss through filter – filter blindingblinding

Deep Bed FiltrationDeep Bed Filtration

Particles removed throughout depth of filter as the Particles removed throughout depth of filter as the collide with filter particles and stick. Small particles collide with filter particles and stick. Small particles may be removedmay be removed

Slower increase in head loss – longer filter runsSlower increase in head loss – longer filter runs

Filter CleaningFilter Cleaning

As filter removes more particles the area that will allow flow As filter removes more particles the area that will allow flow of water becomes smaller. A smaller cross sectional area means of water becomes smaller. A smaller cross sectional area means the velocity must increase to allow the same amount of water the velocity must increase to allow the same amount of water to pass. This results in higher head losses.to pass. This results in higher head losses.

As velocity increases, the likelihood of scouring particles off the As velocity increases, the likelihood of scouring particles off the filter media becomes higher. This results in an increase in filter media becomes higher. This results in an increase in particles in the effluent. Called filter beakthrough.particles in the effluent. Called filter beakthrough.

If the head loss gets too high or the particle count in the treated If the head loss gets too high or the particle count in the treated water gets too high (measured by turbidity or particle count), water gets too high (measured by turbidity or particle count), the filter must be cleaned. The cleaning process is called filter the filter must be cleaned. The cleaning process is called filter Backwash.Backwash.

BackwashBackwash

Open valve D, close Open valve D, close valves A and C, open valves A and C, open valve Bvalve B

Reverse direction of flow of water through the filter. Increase velocity until filter Reverse direction of flow of water through the filter. Increase velocity until filter media particles become fluidized (suspended in flow). Particles bump against media particles become fluidized (suspended in flow). Particles bump against each other knocking the “dirt” off of them.each other knocking the “dirt” off of them.

Design ParametersDesign Parameters

Filter Loading rateFilter Loading rate

Around 4 gal/(minAround 4 gal/(min..ftft22))

DisinfectionDisinfection

Chlorine CompoundsChlorine Compounds

Chlorine Dioxide (ClOChlorine Dioxide (ClO22))

Chlroamines (ClChlroamines (Cl22 and ammonia) and ammonia)

Hypochlorite ion (OCl-, bleach)Hypochlorite ion (OCl-, bleach)

OzoneOzone

Chlorine (ClChlorine (Cl22))

Disinfection by-products tri-halomethanesDisinfection by-products tri-halomethanes

C-TC-T

Concentration x timeConcentration x time

Effectiveness of disinfection a function of C-TEffectiveness of disinfection a function of C-T

FluoridationFluoridation