Filtration Single Media Rapid Sand Dual Media Rapid Sand
Filtration Mechanisms Surface Filtration - Screening Particles larger than pore openings between filter Particles are removed on the surface of the filter Rapid increase in head loss through filter – filter blinding Deep Bed Filtration Particles removed throughout depth of filter as the collide with filter particles and stick. Small particles may be removed Slower increase in head loss – longer filter runs
Filter Cleaning As filter removes more particles the area that will allow flow of water becomes smaller. A smaller cross sectional area means the velocity must increase to allow the same amount of water to pass. This results in higher head losses. As velocity increases, the likelihood of scouring particles off the filter media becomes higher. This results in an increase in particles in the effluent. Called filter beakthrough. If the head loss gets too high or the particle count in the treated water gets too high (measured by turbidity or particle count), the filter must be cleaned. The cleaning process is called filter Backwash.
Backwash Open valve D, close valves A and C, open valve B Reverse direction of flow of water through the filter. Increase velocity until filter media particles become fluidized (suspended in flow). Particles bump against each other knocking the “dirt” off of them.
Design Parameters Filter Loading rate Around 4 gal/(min.ft2)
Disinfection Chlorine Compounds Chlorine (Cl2) Disinfection by-products tri-halomethanes Chlorine Dioxide (ClO2) Chlroamines (Cl2 and ammonia) Hypochlorite ion (OCl-, bleach) Ozone
C-T Concentration x time Effectiveness of disinfection a function of C-T