Prokaryotic communities and potential pathogens in sewage sludge: Response to wastewaster origin, loading rate and treatment technology.

Sewage sludge features high nitrogen and phosphorous contents encouraging its use as a biosolid in agriculture, but it bears potential chemical and microbiological risks. To tease apart the relative contribution of main factors determining the sludge chemical and microbial features, we analysed 28 treatment plants differing in the wastewater origin (municipal residues, agro-food or chemical industries), organic loading rate and treatment technology (extended aeration, activated sludge or activated sludge followed by anaerobic digestion). We found that the treatment technology and the organic loading rate are main determinants of the sludge chemical properties, including its organic load, nutrient and metal contents, and override the effect of the wastewater origin. Sludge bacterial and archaeal community structure and diversity, characterized through massive sequencing of the 16S rRNA gene, were also mostly determined by the treatment technology partly through shifts in the sludge nutrient load. The same factor conditioned the relative abundance of sequenced bacteria most closely related to potential pathogens, but not that of cultivable Escherichia coli or Salmonella spp. We did not find an effect of the geographic location of the plant on any of the variables at the regional scale of our study. Operational parameters appear as major determinants of the sludge chemical and microbial properties, irrespective of the source of wastewaters, thus leaving a broad management window for improving the agronomic value of sewage sludge.