Sand Filter System
A distinction can be made between continuous and discontinuous filters. In continuous filters (often upward-flowing filters), the polluted sand is removed, rinsed and re-used continuously, without interrupting the filtration process. Discontinuous filters (often downward-flowing filters) are stopped, and a rinse takes place in the opposite direction. Air bubbles are blown into the sand bed to make it swirl around. Filtered water then flows through the filter bed in the opposite direction. The polluted matter is released and flows away along with the rinse water. The filtration process can then resume. The yield of a sand filter is determined by two sand filter functions, namely surface filtration and depth filtration.
Surface filtration involves collecting the particles above the filter bed. These particles jointly form a macro-porous lump, which is able to collect new particles in a very effective manner. Depth filtration generally involves smaller particles that are more difficult to collect, and which are bound to the sand particles by adsorption. Dirt from surface filtration is easier to remove during reverse rinsing compared to dirt from depth filtration.