What Does Sand Do in Water Filtration?
Sand filtration is an essential part of water treatment. It is widely used in pools and spas, but also in industrial processes like car washes.
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Sand is an ideal filtration media because it strains out suspended solids and contains many natural bacteria that adsorb contaminants, making it suitable for both physical and chemical filtration of water.
There are many different kinds of sand filters, all of which work by removing suspended matter from source water. These can be categorized into two groups: gravity-fed and pressure-fed devices.
Gravity-fed devices are referred to as slow sand filters, while pressure sand bed filters are known as rapid sand bed filters.
Flow rates in both types of sand filters must be between 0.1 and 0.3 meters per hour for the filter to function properly. This is the rate that the sand must move through the system to allow for a continuous supply of nutrients and oxygen to the microorganisms within the filter.
It is important that the flow rate of water through the sand filter is high enough to allow for a microbial layer to form in the sand and this needs to be maintained, as it can take several weeks or even months to form (BRIKKE & BREDERO 2003). The top layer of sand must be scraped off every so often, washed, dried in the sun, and re-deposited back on the bed of sand.
A sand filter can be a single-pass or a multi-pass filter. A multi-pass filter has a larger surface area of sand than a single-pass filter and can handle much higher flows.
The sand in a filter can be of various grades, some are very fine and others are coarser. The sand in a filter can either be naturally silica filtration grade or synthetically formulated to provide the appropriate level of adsorption and adsorption stability for the specific application.
In addition to adsorption, the sand in a filter can contain colonies of beneficial bacteria that collect and convert harmful contaminants as they pass through the filter. This can be a good thing, as the beneficial bacteria will be present in the sand when it is cleaned, and this will help to reduce turbidity and disinfection levels in the final product water.
This adsorption can be combined with biological activity, such as biogas production, to help to increase the efficiency of a sand filter process and decrease the amount of rinse water that is required to maintain the sand bed clean.
There are some limitations to sand filtration, though. One is that sand is easily clogged by organic material or debris, so the sand must be periodically cleaned and added to the filter to ensure it does not become clogged over time.
Another limitation is that the sand must be treated to remove any chlorine or fluoride present in the water. This can be difficult to do, and chemicals may need to be added to the sand to improve its ability to adsorb these chemicals.