2.8 What are the advantages and disadvantages of planted gravel filters?

Effluent Management 2 Overview of Post-Treatment Techniques: The Planted Gravel Filter 2.8 What are the advantages and disadvantages of planted gravel filters?

The table below presents the advantages and disadvantages of horizontal planted gravel filters.

Advantages of Horizontal Planted Gravel Filters	Disadvantages of Horizontal Planted Gravel Filters
The well-sized horizontal systems efficiently reduce the concentration of organic matter (BOD, COD), TSS, N, and P	It requires relatively large and open land areas (20 m² per m³ effluent/day or app. 3 m² per PE¹ in warm regions)
The pathogens concentrations are also strongly reduced (by a factor of 1,000 or 10,000)	Clogging occurs after 10 to 25 years or more (depending on the quality of effluent to be treated)
The flow in a horizontal filter is a “Subsurface flow” under the surface of gravels, and there is no contact with animals or humans and no odour	Requires (small) slopes for gravitational flows
It is natural and produces reeds, grasses, fodders, woody plants, and compostable plants for the population and the field workers	Loss of nutrients and water (evaporation) reduces the potentiality of the post-treated effluent reuses in agriculture and water recharge
The system can be continuously fed

Advantages and Disadvantages of Horizontal Planted Gravel Filters

⁽¹ Person Equivalent producing 150 L wastewater per day and 60 g biodegradable organic matter (BOD5) per day. A displaced person/refugee may produce only half of the given amounts. This is considered by assuming 0.5 PE instead.

The second table below presents the advantages and disadvantages of vertical planted gravel filters.

Advantages of Vertical Planted Gravel Filters	Disadvantages of Vertical Planted Gravel Filters
Vertical filters have been mainly developed since the 90s and are now well-experienced	Vertical filters are aerobic and are fed by batches (to allow aeration during the rest period); electric pumps or manual opening valves are commonly needed to feed multiple filters alternately
The vertical systems efficiently reduce the concentration of organic matter (BOD, COD) and TSS. The pathogens concentrations are also strongly reduced (by a factor probably lower than the horizontal system because of shorter retention time)	The effluent is spread on the surface of the gravel/sand layers: contact with animals or humans and odour is possible
It is natural, producing reeds and compostable plants for the population and the field workers	It requires relatively large areas (<10 m² per 1 m³ effluent/day)^[1]
The vertical filters require a lower surface than the horizontal filters	Clogging occurs regularly (for example 1 year), and periodic cleaning is required
The sludge accumulates on the surface of the filter and is theoretically more straightforward to remove than the sludge in the gravel of a horizontal filter	Requires slopes for gravitational flows (up to 1 m)
	Low reduction of nitrogen and phosphorus

Advantages and Disadvantages of Vertical Planted Gravel Filters

[1] In industrialized countries, 1 to 2 m² is mentioned per person equivalent to producing 150 L wastewater/d and 60 g biodegradable organic matter (BOD₅) per day!

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