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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.

Physical, chemical and microbiological characterization of three tropical acidic soils of Rwanda under natural fallows and productivity constraints. Three acidic soils of highland (HL), middle land (ML) and lowland (LL) from Rwanda were sampled respectively in the experimental stations of Gakuta, Tonga and Cyabayaga for a physical, chemical and microbiological characterization in order to determine their productivity constraints. Soils are strongly acidic in the HL and ML (pH 4.1 and 4.7) and moderately acidic in LL (pH 5.5). The rate of total organic carbon (Corg) is acceptable in Gakuta (4.5%), but decreases with altitude. The total nitrogen (Ntot), the phosphorus and the effective cation exchange capacity follow the same trend, their levels remaining nevertheless weak. Base saturation and Bray2-P content are weak in the HL and ML, but also they are negatively correlated with Corg content. The Al3+ ion occupies 32 and 18% of the exchange complex at Gakuta and Tonga, and is non-existent in Cyabayaga soils. Carbon microbial biomass (Cmic : Corg) and nitrogen microbial biomass (Nmic : Ntot) ratios indicate a weak mineralization/immobilization rate in the highly acidic soils, and are positively correlated with soil pH. Cmic : Nmic ratio values obtained indicate a soil microbial population dominated by fungi, which predicts a poor organic matter (OM) quality. The small proportion of the assimilate carbon fraction at Gakuta (5% of Corg) testifies also to the OM poor quality. Basal respiration rate is very low (0.30 - 0.37 μg CO2 .h-1.g-1). The addition of glucose for the substrate inducedrespiration has considerably stimulated the metabolic activity, which results in low values of respiratory activation quotient(QR = 0.4-0.5). Dehydrogenase and acidic phosphatase activities are very low but positively correlated with Corg. The soil acidity remains the principal constraint to land productivity in the HL and ML regions while low soil OM and N contents would constitute a limitation in LL zone if appropriate land management practices are not used.