Types of Biobeds
Depending on whether or not the bottom of the biobed is isolated from the environment, there are two types of biobeds: the lined respectively the unlined biobed.
This biobed resembles the original Swedish biobed but is lined by a synthetic impermeable layer (plastic, concrete, tarpaulin etc) that isolates it from the ground. This design allows the collection of drainage water in special wells that are built at the side of the biobed. Drainage layers (gravel, macadam or sand) are usually placed below the clay.
Illustration: Maria del Pilar Castillo/JTI
The unlined biobed has no impermeable synthetic layer that isolates it from the ground. The original Swedish-designed biobed belongs to this group. The bottom of this biobed is covered by a clay layer (10 cm). There is no collection of drainage water in this system. Most of the biobeds in use in Sweden are of the unlined type.