Microbial monitoring of the influence of the stabilization degree of sludge when applied to soil I.B. Estrada, E. Go´mez, A. Aller, A. Mora´n * Natural Resources Institute, University of Leon, IRENA-ESTIA, Chemical Engineering Department, Avda. Portugal # 41, 24071 Leo´ n, Spain Received 7 July 2004; received in revised form 3 May 2005; accepted 26 May 2005 Available online 26 July 2005 Abstract The aim of this study was to establish the degree of stabilization of sludge and its influence on the evolution of total aerobic micro-organisms and those of faecal origin when the sludge is applied to soil. Two different sludges were used, from a food industry (D) and from an urban treatment plant, (E). Both sludges were stabilized aerobically in laboratory conditions. The stabilization process was monitored by following the evolution of pathogenic micro-organisms and the content of total solids and volatile matter in suspension, while on the other hand the microbial evolution of the samples of soil plus sludge and control soil (S) were monitored over an incubation period of 80 days. After 20 or 40 days of incubation, in both sludge treatments the populations of pathogenic micro-organisms of faecal origin were below detectable limits, regardless of the degree of stabilization of the sludge. Soil without sludge treatment did not show pathogenic micro-organisms. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Sludge; Stabilization; Soil; Survival; Total aerobe micro-organisms; Faecal coliforms; Escherichia coli; Fungi and Salmonella spp. 1. Introduction The legislation of different countries establishes crite- ria for the agricultural use of sludge (in Spain, Real De- creto, 1310/1990, published in the Official State Gazette on the first November 1990; in general for the European Union, Council Directive 86/278/CCE of the 12th June, 1986; for the United States, USEPA, 1993, 40 CFR Part 503 Regulations), which establish a compulsory biolog- ical, chemical or thermal treatment, long-term storage or any suitable procedure to reduce significantly the fer- mentation power of sludge together with the sanitary drawbacks of its use. Therefore, the agricultural use of sludge inevitably requires its stabilization. The three main objectives of stabilization are the reduction of pathogens, the elimination of unpleasant smells and the prevention or reduction of potential rot- ting (Tchobanoglous and Burton, 1995). The criteria followed to determine the degree of sta- bility vary according to the definition of stability consid- ered (Krishnamoorthy and Loehr, 1988). Among other definitions, VesilindÕs (1979) states that a stable sludge can be used without damage to the environment or cre- ation of unpleasant conditions. However, HartensteinÕs (1981) definition states that it will only be possible to consider the sludge to be totally stable when the original solid wastes have been transformed into humic sub- stances, are no longer subject to putrefaction, are odour- less and their degradation is slight and very slow. Since Hartenstein, other authors have followed this definition based on the degree of humification (DeNobili et al., 1986; Herna´ndez et al., 1988; Inbar and Garcı ´a, 1990; Chen and Inbar, 1993). 0960-8524/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2005.05.024 * Corresponding author. Tel.: +34 987 291841; fax: +34 987 291839. E-mail address: dfqamp@unileon.es (A. Mora´n). Bioresource Technology 97 (2006) 1308–1315