Biological treatment of livestock slurry
using bacteria and fungi

By Blandine Cailliez

Euralis Environnement slurry treatment system and filter beds (pilot unit in a hog farm located in the southwest of France).

French livestock specialists are using bacteria and, more recently, filamentous fungi to treat pig and poultry slurry. This technology offers several advantages: a large reduction in odour, an improvement in the fertilizing value of the slurry, and a reduced risk of pollution.

Until recent years, livestock slurry in France was stored on farms without any particular treatment, then used as fertilizer on arable land. The concentration of livestock, over-fertilization in certain regions and changes in regulations led hog and poultry producers to become more concerned with the environment. They began to look for ways of reducing the smell of the slurry and to limit the harmful effects on water quality. Various biological processes were explored and French laboratories have now succeeded in developing natural processes to treat slurry.

Reducing odours from slurry

The Hygéfac laboratories in Paris succeeded between 1994 and 1996 in isolating aerobic bacteria which, when incorporated in hog, poultry, or cattle slurry, reduce odour while improving the material's fertilizing value. The bacteria consisted of almost eighty different strains that live in symbiosis, and are now marketed under the product name Azofac. "Aerobic bacteria that are introduced into the slurry combat the anaerobic bacteria that usually develop in slurry pits," explains Colette Senan, researcher and co-director of Hygéfac. "However, it is anaerobic bacteria that are, in general, responsible for the emission of gases having unpleasant odours, in particular, ammonia (NH₃) and hydrogen sulphide (H₂S)."

The potential of this technology has been confirmed by France's National Test Laboratory (LNE), in accordance with techniques from the American NIOSH institute. Analyses performed by the LNE have shown evidence of reductions in ammonia of 80% and hydrogen sulphide of 90%. These results earned recognition for the technique from the French Ministry of the Environment, with the result that the minimum distance from dwellings necessary for spreading treated slurry was reduced from 339 feet to 165 feet when this process was utilized.

The treatment of the slurry with Azofac aerobic bacteria also makes it possible, by reducing ammonia emissions, to maintain the concentration of nitrogen absorbed by the slurry, thus improving its fertilizing value. A trial conducted by AGPM (General Association of Corn Producers) with duck breeders was very conclusive: treatment of the slurry made it possible to increase the yield of corn by 19%, saving 50% of the mineral fertilizer usually used in cultivation.

Aerobic phase of the Euralis slurry treatment system. Photo - Euralis

Animal welfare also improved

"The use of this process to treat slurry has advantages not only for the environment and the quality of people's lives, but also for animal welfare," adds Julien Castaing of AGPM, which also carried out tests on pig slurry in the pits beneath the animals. "By reducing the emission of gases, in particular ammonia, the use of bacteria led to an improvement in breeding performance (Mean Daily Gain and Index of Consumption) by 1.5%."

Pollution-removing fungi

While French biologists have been working on pollution control using bacteria for almost 15 years, the idea of combining fungi to achieve the same results is much more recent. Those behind it are Euralis, a cooperative of farmers and breeders in the Pau region of southwest France, and the team of Professor Jean Villard, director of the Plant Biology Laboratory at the Faculty of Pharmacy at Paris V University.

After two years of research, Professor Villard and his team succeeded in isolating four species of natural filamentous fungi (not genetically modified) for their ability to "digest" slurry: two lower fungi (Mucor and Rhizopus) and two higher fungi (Aspergillus and Fusarium). These four species act in synergy. "The advantage of fungi is their high richness in enzymes," explains Villard. "After treatment in the biodigester, the residual biomass is much smaller than with bacteria."

Pilot tests conducted since 1998 on farms have shown that the manure after treatment was completely odourless and had lost 80% of its pollutants. Its installation costs around (US) $4.50 for 35 cubic feet processed, whereas more complex systems may reach three times that amount. The process has just obtained European Eureka approval.

The system of slurry treatment developed by Euralis consists of two phases: slurry is first treated with anaerobic bacteria and then with fungi in an aerobic environment. In the first biodigestion tank, the bacteria convert the nitrogenated material in the slurry into nitrites (NO₂^-). In the second biodigester, the fungi absorb these nitrites, which they expel into the air in the form of atmospheric nitrogen (N₂). Sulphur, the second source of odour in slurry, is complexed in the residual biomass. The fungi also consume a large part of the phosphorus and potash contained in the waste and trap heavy metals, particularly lead, mercury, and cadmium. The reduction in Chemical Oxygen Demand (COD), nitrogen, phosphorus and potassium makes it possible to use the effluent after treatment as irrigation water.

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