The slow sand filter
Water is purified by a mechanical and a simultaneous microbial filter effect. Due to the small particle size of the sand, a mechanical filter effect is initially achieved by retention due to small pore sizes. In addition, at a slow flow rate of less than 0.25 m/h, microbial growth develops due to the enlarged surface on the sand particles. Depending on the water and the quality of the sand, this takes several weeks, which is why the final filter performance is only achieved after some time.
The microorganisms metabolise organic material in the water and other dissolved inorganic substances, such as salts and nitrates into gaseous substances such as carbon dioxide. In order to prevent the microorganisms from dying, the sand filter must therefore be regularly flowed through with fresh water and must not run dry, otherwise the microorganisms will no longer be sufficiently supplied with nutrients.
The sand filter is based on three layers of sand, a diffuser, an overflow and a rising pipe. The lowest layer consists of gravel with a diameter between 5 and 8 mm, above it lies a layer of coarse sand with a particle diameter of about 2-4 mm. The two coarse layers are each about 10 cm thick and ensure the fine sand layer above isn’t washed out. The fine sand layer has a height of about 40 cm to ensure a sufficient separating effect.
In addition, the outlet is protected by using very coarse gravel (approx. 1-2 cm in diameter) to protect it from clogging with fine sand or finer gravel. To prevent the filter from running dry, the outlet is equipped with a rising pipe that ends about 4 cm above the sand dumping height. At the same time, an overflow protection (about 6 cm above the sand layer) ensures that the bacteria do not die due to excessive static pressure on the sand layer. This overflow leads the excess water back into the cistern. For an additional oxygen input, an even distribution of the water over the entire cross-section and as little disturbance of the microorganisms as possible by whirling up the sand, a diffuser is installed above the sand layer.
In order to ensure permanent effectiveness of the filter, it must be cleaned about once a year. Indicators of necessary maintenance are a drop-in filter performance or regular overflow of the sand filter. For cleaning, the filter is opened, and the topmost centimetres of the sand layer are removed from the filter, then cleaned several times with clear water and refilled into the filter. This method represents the smallest intervention in the microorganism layer and therefore requires the shortest time until the filter has reached its full efficiency again. In cases of heavy pollution, this layer is replaced by new sand. In the worst case, the entire sand of the filter must be replaced. The caretaker on site is trained in the various methods and is provided with illustrated instructions to ensure professional cleaning. There are also multiple sand filters running in parallel to allow maintenance and sufficient recovery time of the microbial layer without loss of quality.