Rapid Sand Filter
Rapid sand filters require a smaller land footprint compared to slow sand filters. They are typically designed as part of multi-stage treatment systems used by large
municipalities, and use relatively coarse sand and other granular media to remove particles and impurities that have been trapped in a floc. Water and floc flow
through the filter medium under gravity or under pumped pressure and the flocculated material is trapped in the sand matrix. Filters will generally be backwashed with
water, have a sequential air scour stage or a simultaneous air/water backwash for maximum effect.
Much higher flow rate than a slow sand filter; about 150 to 200 million gallons of water per acre per day.
Requires relatively small land area.
Less sensitive to changes in raw water quality.
All Rapid Sand Filter function in a similar way in that raw water is passed through a bed of media where any
impurity is retained on the media; after a designated period the media must be cleaned to prevent it becoming compacted or clogged. The rate at which raw water can be
applied, the type of media, the depth of the media and the frequency, rate and method of backwashing are all variables that must be considered based upon the volume of
water to be treated, the quality to be processed and the quality required.
Slow Sand Filters
This traditional design of filter typically has a bed 1 to 2 metres deep, and can be rectangular or cylindrical in cross section; it is used primarily to treat surface
water. Slow sand filters have a number of unique qualities:
Use biological processes and are non-pressurized systems.
Do not require chemicals or electricity to operate.
Cleaning is traditionally by mechanical scraper; however, some operators use "wet harrowing", where the Rapid Sand
is scraped while still under water, and the water used for cleaning is drained to waste.
Maximum required throughput of water is achievable with one or more beds out of service.
Require relatively low feed turbidity levels.
Produce water at a slow, constant flow rate and are usually used in conjunction with a storage tank for peak usage.
In the base of each bed is a series of drains covered with a layer of pebbles, which in turn is covered with coarse gravel. Further layers of sand are placed on top
followed by a thick layer of fine sand. Slow sand filters work through the formation of a biofilm in the top few millimetres of the fine sand layer; the underlying
sand is the support medium for the biological treatment layer.
Minimal operator training.
Only periodic maintenance.
Appropriate technology for poor and isolated areas.
Dual Media Filters
Dual media filters are used where raw water contains suspended particles; turbidity and iron provide highly efficient particle removal at a high filtration rate.
Layers of anthracite and garnet particles are the filter medium. Dual media filters feature automatic backwashing, which employs a combination of air and water to
maintain unit efficiency. Air scouring loosens suspended solids and dirt, sand removes suspended particles and anthracite removes odor and color, etc., so as to make
the water fit for different applications.
Automatic self backwashing.
High filtration efficiencies.
Able to operate at very high flux rates.
Custom designed packages.