Slow Sand Filter

 Slow sand filters are used for purification of raw water on a large scale

It makes use of a complex biological film that grows on the surface of sand known as the Schmutzdecke layer which provides effective purification

Schmutzdecke layer consists of bacteria, fungi and protozoa and aquatic insect larvae which are formed in the sand layer after 10-20 days of operation  

Slow sand filter loses efficacy when biofilm thickens and hence should be refurbished

Advantages of slow sand filters are its simple design and little need of sophisticated technology for running the equipment

They need no mechanical power and only little operator training

Disadvantage is that they are time consuming