Soil, sewage sludge and digestate samples contamination with Gram-negative bacteria from Enterobacteriaceae

Abstract

Background and aim. In Poland, organic fertilizers may be used in agriculture, if no Salmonella are found in 100 g of the sample and the number of Enterobacteriaceae is lower than 1000 cfu/g. However, there are no standards of microbiological contamination methods dedicated directly to sewage sludges or digestate. The aim of the study was to assess the contamination levels of Enterobacteriaceae in sewage sludge, digestate and arable soil samples. Methods. The microbiological tests included 82 soil samples, 9 sewage sludge and 9 digestate samples. Due to the lack of applicable standards in the field of microbiological testing of sewage sludge and digestate for the presence of pathogenic bacteria, the tests were based on the procedures used to test soil, feed, and food. Detection of Salmonella, was performed according to the standard PN-Z-19000-1/2001, Escherichia coli - PN-EN ISO 16649-2:2004, the total number of bacteria - PN-EN ISO 4833-2:2013-12/AC, and Enterobacteriaceae - PN-EN ISO 21528-2:2017-08. Results. The total number of mesophilic bacteria ranged from 1,1 x105 to 1,0 x106 cfu/g, from 0,1 x108 to 9,8 x108 cfu/g, and from 1,6 x103 to 5,7 x108 cfu/g for soil, digestate and sewage sludge samples, respectively. For the soil samples, contamination with Enterobacteriaceae, was estimated at an average level of 1,1 x104 cfu/g, while for sewage sludge and digestate samples these values were 9,4 x105 cfu/g and 5,6 x106 cfu/g, respectively. The presence of Escherichia coli was confirmed in 2 soil, 6 digestate and 4 sewage sludge samples. None of the samples were contaminated with Salmonella, excluding one positive of digestate sample. Conclusions. The development of standards for the use of sewage sludge and digestate as fertilizers should take into account microbiological contamination with Enterobacteriaceae. Keywords: Soil, sewage sludge, digestate, Enterobacteriaceae. Funding: The research was financed by NCBiR (Poland); Project No.: GOSPOSTRATEG-III/0061/2020-00.Article