Filtration

Introduction

Removal of TSS by filter media.

For fine particles not removed by coagulation

Filters can also capture Giardia cysts, viruses, and asbestos fibers

Water flows downward through a bed of sand and gravel

Particles are captured on and between sand grains

Filtered water is collected in the underdrain, sent to disinfection

Some filters also contain activated carbon, to remove organics

Sand Filtration

Driving Force

Filtration Rate

Slow sand filter

Design criteria for slow sand filters

*Parameter*	Recommended level
Design life	10 - 15 yr
Period of operation	24 hr/day
Filtration rate	0.1 - 0.2 m/h
Filter bed area	5 - 200 m2/filter
No. of filters	2 minimum
Initial bed height	0.8 - 0.9 m
Minimum bed height	0.5 - 0.6 m
Sand effective size	0.15 - 0.30 mm
Uniformity coefficient	< 3.0
Underdrain height (including gravel layer)	0.3 - 0.5 m
Supernatant water height	1 m