Sand filtering consists of the physical retention of dirt in holes formed between grains used in the entire filter layer. It is depth filtration, and this characteristic makes it the most effective of all systems. It can be used with highly contaminated water.
Sand is able to remove small-sized organic and inorganic solids, due to the effect of electrical forces caused by friction as the water passes through the filter bed.
Water must be made to flow at low speed; if not, canals are formed in the sand and no filtration occurs.
The system produces a minimum load loss due to the use of collector arms or nozzles at the filtered water outflow; these arms cover the entire surface of the bed and ensure that water circulates uniformly.
In cases where there is a considerable amount of colloidal substances or saline precipitates, flocculation is used for the purpose of bringing together particles, which facilitates their retention in the layer of sand grains. This prevents small sized particles from getting through the filter bed. Flocculation is carried out using chemical substances known as polyelectrolytes.
Filtration occurs in a downward direction. As dirt accumulates, there is an increase in the pressure difference between the area “upstream” of the filter bed and the area “downstream” of the bed. Once a pre-established pressure differential is reached, the filter is cleaned. Two versions of the Lama Globe valve – metal and plastic – are used for this cleaning process.
Carbon steel components are treated externally and internally using baths and the application of a base primer. They are then painted by electrostatic spraying, with epoxy polyester powder paint.