How the Green Wall works

The intervention and its objectives

The main objective of the project is to reduce water consumption, in anticipation of a lower availability of water resources also due to climate change.

The plumbing systems of buildings are today designed to use potable water for all uses, even those for which non-potable water could be used, such as toilet flushing. Ferla’s school was no exception: both the washbasins and the toilets were supplied with potable water. The average water consumption due to the presence of 190 pupils amounted to about 8,600 litres per day.

The pilot project implemented in the framework of NAWAMED intervened on the existing bathrooms, separating the water coming from the washbasins (greywater) from that coming from the toilets (blackwater). Greywater – much less polluted than blackwater – is treated with natural techniques and used for toilet flushing, thus replacing drinking water.

The intervention provides for a saving of almost 3000 litres of water per day, equal to over 30% of daily consumption.

How it works

The water from the washbasins is sent to a small tank located outside the building, at the base of the wall which houses the natural treatment system: in practice a ” green wall ” made up of a series of small pots filled with filter medium in which appropriate plants are planted.

Many types of plants can be used as long as they are “hydrophilic”, therefore with root systems suitable for living in humid or even submerged soils. In Ferla, among others, aquatic and cervine mint, sedges, calamus, nasturtium, Anemopsis californica, but also more common plants such as ivy and Potus were used.

From the tank the water is pumped to the top of the filtering system and from here the water percolates by gravity from a pot to the one below until, once treated, it is collected in a tank from which it is pumped into the toilets to feed the toilet cisterns. In summer, a small part of the treated water is used to irrigate the vines that complete the green wall of the school.

The pollutants contained in greywater – mainly the organic substance that makes up soaps and detergents – are treated through a combination of physical-chemical and biological processes which decompose the organic molecules, transforming them into water, carbon dioxide and small quantities of minerals (such as nitrogen, phosphorus and potassium) which are used by plants as nutrients.

Greywater can also be treated with technological systems – which use the same decomposition mechanisms – but treatment with natural systems, in addition to reducing management costs (including energy consumption), offers other advantages. The green wall in fact improves the aesthetics in the building and contributes to summer cooling. Furthermore, the green wall constitutes a small ecosystem that hosts a real ecological community, contributing in a small way to supporting biodiversity.

The energy consumption of a green wall can be estimated at around 900 kWh/year, approximately equal to the annual consumption of two large refrigerators.