2.1 What are planted gravel filters?

Planted Gravel Filters (PGFs), also known as constructed wetlands or reed beds, are artificial wastewater treatment systems consisting of shallow ponds or channels planted with aquatic plants and use the natural functions of vegetation, soil, and organisms.

There are various designs and configurations of planted gravel filters. Amongst others, the classification can be according to the life form of the dominating macrophytes, the flow pattern in the system and the type of configurations of the cells. The following schema shows the classification according to the flow pattern.

The free surface flow PGF, characterised by wastewater flowing on the surface and the need for extensive land areas, has low pathogen reduction and is challenged by issues with insects and odours as well as human and animal contact. Therefore, it is not suitable for refugee camps or other densely populated areas and will not be further discussed in this module.

The issues are avoided with the subsurface flow PGF, making effluent treatment possible in emergency settings. Subsurface flow PGFs realise the treatment process under a layer of gravel with a vertically or horizontally directed flow. Different treatment results can be achieved depending on the specific flow pattern and design. The combination of horizontal and vertical PGF, also described as hybrid systems, can further enhance nutrient removal efficiency compared to individually constructed PGF.

Subsurface planted gravel filters are operated as a secondary or tertiary treatment step downstream of a septic tank, biogas digester, anaerobic baffled reactor (ABR) / anaerobic filter or trickling filter. Following the PGF, the post-treatment depends on the reuse purpose.

For further information on planted gravel filters, please click on the Materials tab at the top of the page.