The principle of purifying water using sand is an age-old method of water treatment that remains important to this day. When water flows slowly through a layer of sand, suspended solids and microorganisms are initially trapped mechanically. Additionally, a natural biofilm forms on the sand grains that biologically breaks down organic substances. Some dissolved substances can also be bound to surfaces or converted into solid compounds. Undesirable compounds in drinking water also adhere to these compounds and are thus removed.
Limitations of sand filtration
However, sand filtration has its limitations when it comes to certain groups of substances. For example, dissolved salts such as nitrates and chlorides remain in the water, as do many modern trace substances such as medicine and pesticide residues. Sand alone cannot reliably ensure complete disinfection either.
Historical water treatment
Historically, slow sand filtration played a central role in water treatment, as seen at the Berlin-Friedrichshagen plant. In this process, water was passed slowly and over a large surface area through sand beds – an effective method, but one that required a great deal of space and time. This was important because contact with oxygen can lead to the ‘oxidation’ of water and the formation of iron and manganese compounds. Iron and manganese are dissolved in oxygen-free groundwater, but they must be removed from drinking water. This was achieved in sand filters.
Modern drinking water treatment
Sand still plays a role today. For example, in a process called bank filtration, water from rivers or lakes seeps through layers of sandy soil into groundwater, where it is naturally purified over long distances. Initially, two-thirds of Berlin’s drinking water undergoes this process.
Sand is also used in modern drinking water treatment, usually in conjunction with technical processes. Activated carbon filters, ozone or UV treatment and membrane processes are now employed to remove trace substances and disinfect the water. This creates an efficient hybrid system in which sand serves as a robust pre-treatment stage while modern technologies ensure precise fine purification and the hygienic safety of the drinking water.
