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Slow sand filtration relies on both physical and biological activity in controlling plant pathogens. In a slow sand filter, the filter bed is constructed of a medium with high surface area which can be colonized by suppressive micro-organisms. Clean drinking water This fine media also presents a phy

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Slow sand filtration use physical & biological to make clean drinking

Slow sand filtration relies on both physical and biological activity in controlling plant pathogens. In a slow sand filter, the filter bed is constructed of a medium with high surface area which can be colonized by suppressive micro-organisms. Clean drinking water

This fine media also presents a physical barrier to the passage of spores of plant pathogens. Bacteria, such as representatives of the genus Pseudomonas and Trichoderma have been demonstrated as biological control agents effectively controlling plant pathogens in hydroponics systems. In a SSF, plant pathogens recirculating in the irrigation water are captured in the filter media, and at slow rates of water filtration (100-200 l/hr/m2 surface area of filter), are acted upon by the antagonistic micro organisms that colonized the filter bed.

The efficiency of SSF depends on the particle size distribution of the sand, the ratio of surface area of the filter to depth and the flow rate of water through the filter. The finest grade sand fractions and granulated rockwool have been shown to be most efficient in controlling diseases such as Phytophthora, Pythium and Fusarium oxysporum, the most widespread nursery diseases. 

Advantages of SSF
There are several advantages of slow sand filtration over other methods of water disinfestation:

• It is a low energy consuming process
• It has great adaptability in components and applications maintenance is minimal
• Systems can be built and installed by laymen
• Costs of building and running significantly lower than other disinfestation methods

In Australia, chlorination/bromination processes are most widely used for water disinfestation in nurseries. The effectiveness of such chemical treatments in controlling plant pathogen depends on correct dosages and treatment times, control of suspended particulate matter in the recycled water, and foolproof monitoring systems. Chemical treatments have proven effective when used properly, however they are relatively expensive and present safety issues to the handlers and the environment.

Other water treatment processes such as UV and Ozone are being adopted by some nursery crop producers. The perceived expense, difficulty and/or questionable effectiveness of some of these water treatment methods has not encouraged wide use.

Establishment of SSF in a nursery is similar to the installation of holding tanks and pumps to allow for batch treatments with chemical disinfectants. SSF requires no purchase of chemicals, there is no technical dosing equipment to service or replace and there is no chance of crop damage if dosing equipment or the operator miscalculates.

SSF used as a replacement to other forms of chemical, ozone or UV treatments would lead to significant savings in equipment upkeep and purchase of chemicals and avoid potential crop phytotoxicity.

Construction of a Slow Sand Filter
A slow sand filter consists basically of the following components:

Housing
Water layer
Filter bed
Drainage system
Flow control