Contamination bactérienne des boues d'épuration utilisées à des fins agricoles en Tunisie

Bacterial contamination of sewage sludge used for agricultural purposes in Tunisia

¹ Laboratoire de microbiologie, Institut National de Recherches en Génie Rural Eaux et Forêts, BP. 10 Ariana 2080, Tunisie
² Laboratoire de contrôle des eaux et des denrées alimentaires. Institut Pasteur, BP.74, Tunis Belvédère 1002, Tunisie

Résumé

129 échantillons de boues résiduaires ont été analysés. Leur teneur en germes tests de contamination fécale varie selon le type de traitement subi en station d'épuration. Les salmonelles ont été retrouvées dans tous les types de boues étudiés. Dix sérotypes différents ont été isolés ; S. corvallis et S. entéritidis sont prédominants dans les boues. Les résultats obtenus ne confirment pas la corrélation entre le nombre d'indicateurs bactériens et la présence des salmonelles. En effet, 60 % des salmonelles isolées proviennent de boues qui contiennent moins de 10³ E. coli par gramme. Un stockage, même prolongé, des boues entraîne une baisse du nombre d'indicateurs bactériens mais n'assure pas l'élimination des salmonelles.

Abstract

Land application of municipal sewage sludge has been practised in Tunisia for many years. In 1995, more than 30 000 m³ of dry sludge were generated by 48 sewage treatment stations. About 50% of this volume were used as a soil fertiliser on food chain cropland.

It is well known that sewage sludge has beneficial plant nutrients; however, it may also contain bacteria and other micro-organisms that can cause disease. To evaluate heath risk associated to agricultural use of sewage sludge, microbial characteristics of this residue should be determined.

In this study, bacterial contamination of sewage sludge from 20 plants involving many treatment systems has been investigated. The number of faecal indicators varies between 6 and 1,1.10⁶ bacteria per gram depending of the treatment applied to the sludge. Investigation for Salmonella were performed in 98 samples of dry sludge. Results have shown that aerobic and anaerobic digestion had little effect on Salmonella which were found in 21,4% of the analysed sludge. Most of the strains of Salmonella (60%) were isolated from samples of sludge containing less than 10³ Escherichia coli per gram. The isolated strains of Salmonella belonged to 10 serotypes. S. corvallis and S. enteritidis were the most frequent. Storage of sludge leads to an important decrease of faecal indicators densities but doesn't permit a complete elimination of pathogenic bacteria : Samples of sludge stored for more than 3 years were positive for Salmonella.

Land application of sludge create a potential for human exposure to these pathogens through direct and indirect contact. To protect public health from theses pathogens, the use of sewage sludge for agricultural purpose should be regulated.