Slow sand filter
The slow sand filter is one of three types of sand filters that are widely used in water purification around the world, namely:
- Quick Sand Filters
- Flowing Sand Filters Up
- Slow Sand Filters
First used in the United States in 1872, slow sand filters are the oldest type of municipal water purification. Today, slow sand filters are the preferred water purification technology in many developing countries due to their low power requirements and strong performance. It is also used to treat water in some developed countries, such as the United Kingdom, where it is used to treat water supplied to London.
Slow Sand Filter Characteristics
Slow sand filters work using a complex biological membrane called schmutzdecke that grows naturally on the surface of the sand. This biological membrane is fairly effective in removing coliform microorganisms such as Giardia, Cryptosporidium, Salmonella, Escherichia coli, Total and Faecal Coliforms, Streptococcus faecalis, and bacteria. The sand itself does not perform any filtering function but simply acts as a substrate, unlike sand used in UV rays and pressure treatments.
A properly designed slow sand filter consists of a tank, a layer of fine sand, a layer of gravel to support the sand, a system of drains to collect the filtered water, and a flow regulator to control the filtration rate. No chemicals are added to aid the filtration process.
A typical slow sand filter has a depth of 1 to 2 meters, can be rectangular or cylindrical in cross-section, and is primarily used for surface water treatment. The SSF has a slow flow rate (0.1-0.2 m3/h).
The filter head is only 10-15 cm long, but it increases as impurities and contaminants are attached to it. The filter must be cleaned as necessary when the filter head reaches the maximum allowable limit of 75 cm. The sand must be changed every 5-8 years.
What are the plankton and pollutants that a slow sand filter can eliminate?
Reverse osmosis filters are best for removing a large proportion of water contaminants, including dangerous waterborne bacteria. But slow sand filters have consistently proven effective in removing suspended particles with flowing turbidity of less than 1.0 NTU, achieving 90 to 99 percent reductions in bacteria and viruses.
However, slow sand filtration is generally not effective for the majority of chemicals that are a secondary concern in water supplies subject to severe microbial contamination as declared by the World Health Organization in 1996.
What are the advantages and disadvantages of a slow sand filter?
The slow sand filter has many advantages and disadvantages, including:
- Simplicity of its design and operation.
- Low maintenance cost.
- Low overall cost.
- Its ability to remove small-sized dirt (20-40 microns) and debris.
- Requires no pre-treatment or extensive factory control.
- It does not require a lot of energy to run.
However, the slow sand filter also suffers from some drawbacks, which are:
Incurring additional costs due to the necessity of treating the water that is being disposed of.
- Not very effective against all viruses and pathogens.
- Exposure to clogged filters.
- Difficulty removing fine dust in water.
- Difficulty transporting filters due to their high weight.
- Continuous backwashing can lead to a decrease in the chemical equilibrium.
- It requires a large amount of land.
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