The process of filtration of wastewater has been used widely with different types of filtering media. In this article I take a look at unconventional designs of wastewater sand filters.

Dual media wastewater filters or multimedia wastewater filters contain two or more different types of filtering media. A multimedia wastewater filter has densely packed finer particles as a bottom layer in the filter while a coarser medium, made of much larger grains and packed less densely, lies at the top. A common way of constructing the filtration medium is to start with garnet as the bottom layer, covered with sand forming the middle layer, and topped with anthracite coal. During normal usage, wastewater percolates down the layers in the multimedia filters due to the force of gravity. However, sometimes Biflow, Upflow and Horizontal types of filters are used in wastewater treatment.

To operate efficiently, such multimedia filters require regular cleaning through forced back flows. The wastewater filter supposedly reaches a limiting value or a limiting point when the filtering process is severely affected due to clogging by filtered and trapped suspended solids from the wastewater. This reduces the flow of wastewater through the filter and it then has to be cleaned by reversing the direction of flow of water in a process commonly called backwashing. When such back flow is used, the trapped solids are freed by the flowing water. Air can also be forced in through the filter bed to dislodge trapped solids; this helps in the cleaning process. This water containing the flushed out particles is then fed back into the incoming wastewater.

Various granular media filtration processes are used as a means of tertiary treatment of wastewater in municipal wastewater treatment plants. When chemical treatment processes are used along with this type of filtration, the suspended solids that might remain in the outflow can be separated by passing it through the filter. Land biological treatment processes, wherever used, can also be considered as a type of granular media filter process. Wastewater sand filters are being used more and more to treat effluent flowing out of municipalities as an on-site treatment mode.