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Spreading sludge: recovery route or danger for the environment?

Following the articles published in l’Est Républicain 1, 2, where the sludge of spraying are accused of producing an odor nuisance, Grégorio Crini, environmental chemist at the Chrono-environnement laboratory, returns to the subject

I was surprised to read in the regional press these claims that sludge pollution represents all popular misconceptions and that other channels are anti-environmental.
The agricultural spreading of sludge is not a sustainable recovery solution but a danger for our ecosystems and therefore for humans. The societal acceptability of spraying has fallen sharply, more and more residents and tourists are complaining about odor nuisance. The smell of rotten egg is characteristic, but that’s just the tip of the iceberg. You have to look at the submerged part, that is, all the so-called emerging chemical and biological substances that you are not looking for. Even a part of the agricultural world is wary of these "biosolids" because these sludges are concentrators of substances. A highly sensitive subject confirmed by numerous scientific studies around the world.

What is the role of a wastewater treatment plant?
The main objective of a wastewater treatment plant, urban (WWTP) or municipal (STEP), is to treat our wastewater in order to make it comply with regulations before returning it to the environment. The principle is to use biological purification: it is bacteria that eat our pollution, and it works from a regulatory point of view! However, any sewage treatment plant also produces sludge which must be disposed of.

Odor nuisance is a reality nonetheless, so where does the problem come from?
A so-called compliant WWTP sludge should not generate odor pollution. If there are odors, there is a malfunction in the process, a problem with stabilization, aeration, fermentation, etc. A WWTP that does not function properly can also produce odor nuisances, such as certain agricultural practices or specific effluents (pigsties, cheese dairies). If the nuisance is significant and regular, the prefect can ask the inspection of classified installations to ensure that the operator complies with the requirements imposed in the spreading plans. There are no European regulations on the problem of smelly molecules, and yet this is the subjective criterion expressed by residents who complain about this inconvenience.

What are the substances treated by a wastewater treatment plant?
WWTPs only treat what is called “CHONPS”: bacteria eliminate biodegradable organic matter (CHO, carbon, hydrogen, oxygen), forms of nitrogen (N), phosphorus (P) and odors (S, sulfur derivatives). WWTPs are not there to remove chemicals, that is, refractory organic filler; they were not designed to deal with the many substances that we dump in our waste: drugs, hormones, cosmetics, shampoos, detergents, disinfectants, metals, etc. Not to mention plasticizers, perfluorinated compounds, microplastics or nanoparticles that WWTPs cannot even filter!

Why does WWTP sludge have a bad reputation? Are they beneficial or toxic?
In addition to odor nuisance, it is the question of toxicity that questions citizens. For the water police, sludge is waste and not fertilizer, it must meet qualitative regulatory criteria on only 12 chemical substances (metals, PAHs, PCBs) and a few microbiological parameters. Knowing that the Chemical Abstract Service lists more than 80 million chemical substances and 100,000 are listed in the European inventory, one can legitimately wonder where all the substances used in our daily lives end up. In chemistry, nothing is lost, everything is transformed. Sludge has and will continue to have a bad reputation!
What happens to this sludge?
France has chosen the practice of agricultural spreading. Sludge has an agronomic and energy value because it contains nutrients for plants (carbon, nitrogen, phosphorus), which is why they are considered as "natural" fertilizers to fertilize and / or amend the soil. Stabilized and limed, they play a role in soil pH and avoid the problem of odors. Spreading is also a practice recommended by state public agencies for practical, ecological and above all financial reasons. It is a sector under high surveillance with very strict regulations in terms of compliance, self-monitoring, traceability, etc. However, we must stop saying that it is a sector of the future and a natural alternative to chemical fertilizers. Switzerland, for example, has banned the spreading of sewage sludge in agriculture since 2006, as a precautionary principle against the risk of accumulation of substances in the soil. It turned to pretreatments (thickening, stabilization, hygienation), recovery and recovery treatments, and specific incineration with suitable furnaces.

What’s the important question?
Spreading on agricultural soils poses the problem of the fate of substances in the soil, and their transfer to organisms, and therefore to the food chain. What is worrying are indeed the environmental impacts that the use of this sludge could cause. The composition of the spreading sludge has changed significantly with less metals but with other substances that we do not seek! Regulations on substances (endocrine disruptors, cosmetics, plant protection products, etc.) should evolve and become more restrictive. What is the risk for the environment and for humans? What is the temporal and spatial, chemical and biological composition of the sludge? What is their toxicity? How to assess the real danger of spraying? What are the substances accumulating organisms? What are the other methods of sludge recovery? So many transdisciplinary questions on which Chrono-environnement members work on a daily basis in their research projects.

Contact: Gregorio Crini

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