Greening the City
Biodiversity, the city’s ally
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Earthworms to purify your water
In 2004, the town of Combaillaux in the Hérault department (France) equipped itself with a new kind of water treatment plant. What’s unusual about the plant is that it makes use of the ability of two species of earthworm, Eisenia andrei and E. fetida, to break down organic waste present in wastewater by ingesting it.
The big advantage of this biological treatment is that it doesn’t generate the foul-smelling sewage sludge that saturates incinerators and landfills, and that some farmers refuse to spread on their land. The technique also produces less final waste. And on top of that, the process uses less electricity and doesn’t need much room.
In practice, the worms act on two levels. Some of them are used for the treatment of solid waste with a size exceeding 2 mm, recovered after screening wastewater at the plant input. In less than three months, 80% of the waste is turned into vermicompost, which is humus that can be used as a natural fertilizer. The other worms operate on the residual liquid after an aeration stage needed to ensure the efficacy of the system. Around 2 500 000 Eisenia work together with bacteria in a tank roughly 100 m² in area containing pine bark, wood shavings and pebbles, and break down the remaining organic matter. In this way, every square meter of the ‘vermifilter’ can treat the wastewater produced by two to four people.
“The water recovered from the vermifilter complies with all the health standards in force, except with regard to phosphorus,” explains Patricio Soto, a research engineer at INRA (1) and the manager of LombriTek Eco-innovation, the company which performs the technical monitoring of the Combaillaux plant.“After chalking up this first success, we should soon be equipping two more towns in the Ain and Morbihan departments (France), but this time using second generation worm-based sewage plants.”The plants will be equipped with even finer screening facilities, a better-adapted tank, and a specific complementary treatment for phosphorus. All this without either the conventional bacteria bed or combined decanter-digester, which produce sludge. The only limitation is that such plants are only suitable for towns with no industrial activity likely to cause peaks in toxicity due to mercury, copper or arsenic. This is because the earthworms die if they ingest water contaminated with this type of pollutant.However, we haven’t heard the last of Eisenia. They are already being used by some individuals, companies and local authorities to turn organic waste into compost (2) on the spot. And in the future, they might even be used to treat water directly in our homes. With this in view, Soto’s company is taking part in the Napevomo project (3). The goal is to build a life-size modular positive-energy house equipped with a vermifilter.
1. French Institut National de la Recherche Agronomique
2. See www.lombritek.com
3. See www.napevomo.com