Wastewater treatment plants produce sludges which are likely to contain microbial pathogens, metallic trace elements and organic micropollutants. The aim of this study was to assess factors influencing the quality of raw sludge, i.e. freshly-produced and non-treated sludge. The survey of raw sludge quality was conducted each season over a year with controlled factors such as sludge type (primary or biological; rural or urban area origin) and seasonal evolution. Quality of raw sludge was characterized by the determination of microbiological (Salmonella spp., Escherichia coli, enterococci, spores of Clostridium perfringens) and chemical parameters (metallic trace elements, organic index). The organic index is a new parameter based on a gas chromatography method and developed in this study in order to estimate global organic semi-volatile load in sludge. Results showed significant differences in raw sludge quality depending on controlled factors. Thus, E. coli, and enterococci concentrations were higher in primary and biological urban raw sludge compared to biological rural sludge. Concentrations of Hg and organic semi-volatile compounds, estimated by organic index, were higher in primary urban raw sludge than biological sludges of rural or urban origin. As, Ni and Co loads were higher in biological rural raw sludge compared to primary and biological urban sludge. Spores of C. perfringens concentration in raw sludge was lower in autumn. Organic index in raw sludge was lower in spring and in summer. In conclusion, results showed that sludge quality varies significantly, depending of sludge type and seasonal evolution.