When soils filter wastewater


In Wallonia, one-tenth of domestic waste water doesn’t pass through a waste water treatment plant. Is it pollution? Not necessarily, because soils can filter and purify wastewater naturally under certain conditions.

Wherever people live, treating their domestic waste water must be addressed – the water exiting their bathrooms (toilets, sinks, showers, etc.), laundry rooms, dishwashers, kitchens, etc. In the end, all this waste water can return to groundwater, waterways or the sea. It’s not necessarily a bad thing. The question is whether and how it’s treated before being released back into nature.

Waste water purification in Belgium

In Belgium, waste water treatment is subject to European directives that the Regions are responsible for transposing into their legislation. In the Walloon Region, the two main waste water treatment frameworks are:

  • collective systems, based on a sewerage network that carries waste water to purification plants;

  • autonomous systems for isolated sites, which concern dwellings not connected to a sewerage network.

As Wallonia has more varied terrain and lower population density, autonomous systems account for 11% of dwellings, compared to 1 or 2% in Flanders.

Soil: a vast ‘living’ filter

Waste water that doesn’t pass through a waste water treatment plant is most often discharged into a waterway or infiltrates the soil. But contrary to what one might think, this isn’t necessarily an ecological disaster. This ‘pollution’ can even bring nutrients to the ground. ‘Soil isn’t an inert and passive material’, says Marnik Vanclooster, a professor at UCLouvain's Faculty of Bioengineering and a member of the Louvain4Water1 scientific committee. ‘On the contrary, it's a huge, living reactor with billions of microorganisms and minerals whose chemical composition can react to anything that percolates in it, including waste water. Many pollutants in waste water are degradable, at different rates. A significant proportion of waste water organic matter is rapidly biodegradable. The rest is also degradable, but more slowly.’2

How does soil purify water?

Soil can therefore filter and purify waste water, provided that systems are in place that perform this purification optimally. ‘At a technical level, waste water can be collected in a sealed septic tank and connected to dispersion drains, which percolate the water into the soil’, Prof. Vanclooster says. ‘This system can be individual, connected to a single dwelling, but it can also be collective. In the latter case, waste water from several households must be sent to a common dispersion site, such as individual purification plants or a soil bed planted with reeds.

But for this purification system to be optimal, several parameters must be taken into account. ‘First of all, we have to know the soils well, because not every soil has the same filtering capacity’, Prof. Vanclooster explains. ‘Then it’s necessary to envisage a sufficient dispersion area so as not to exceed the soil’s filtering capacity. Finally, we must choose dispersion sites. Because of odours and gases, these sites should neither be too close to homes nor, of course, installed near a drinking water catchment area, such as a spring or water table.

Pros and cons of purification by soil

Purification by natural soils compared to treatment plants requires less infrastructure and consumes (much) less energy. That said, the method has its limits or, rather, its challenges. ‘We have to respect environmental quality standards for the rejected water’, Prof. Vanclooster says. ‘In the treatment plant, measuring the different parameters and controlling the purification process is easy. In soil, it’s more difficult, but it’s feasible, for example by installing underground sensors to detect and quantify pollutants.’

Direct purification of waste water by soil is not yet authorised by the Walloon Region, but it’s possible to change the legislation. In 2018, a researcher at the UCLouvain Earth and Life Institute completed a mission and submitted a report on the subject,3 because this type of system is both ecological and economical, and not only in Belgium. ‘About two billion people dump their waste water without any treatment’, Prof. Vanclooster says. ‘For these populations, putting in place solutions based on the natural filtering capacities of soils would improve waste water treatment at lower costs, with certain benefits for the environment and public health.’

Candice Leblanc

(1) (1)	Louvain4Water is an interdisciplinary consortium that develops basic and applied research activities, advanced training programmes and services in the field of water.
(2) (2)	It should be noted that treatment plants also don’t address these tiny degradable organic materials.
(3) (3)	M. Phalempin, ‘Mission de guidance sur le traitement approprié et la capacité épuratoire des sols’, May 2018. 

A glance at Marnik Vanclooster's bio

Marnik Vanclooster is professor of water resources in the Faculty of Bioengineering at UCLouvain and a researcher at the Earth and Life Institute. He chairs the Belgian Commission of the UNESCO International Hydrological Programme. Prof. Vanclooster earned a bachelor’s degree in agricultural engineering in 1989 and a PhD in agricultural sciences and biological engineering in 1995, both at KULeuven. His research focuses on soil hydrology and integrated water management, particularly in agriculture.

Published on April 18, 2019