Assessment of survival of Listeria monocytogenes, Salmonella Infantis and Enterococcus faecalis artificially inoculated into experimental waste or compost.

AIMS: To evaluate survival of pathogenic strains, Listeria monocytogenes and Salmonella Infantis and a sanitation indicator Enterococcus faecalis in composts at different stages of the composting process and during storage. METHODS AND RESULTS: The studied pathogenic and indicator strains, originally isolated from compost, were inoculated into compost samples from the various stages of the composting process. During incubation, indigenous microflora diversity was monitored with DGGE analysis. After 90 days of incubation, strain survival was observed in compost sampled before the beginning of the cooling phase, and DGGE analysis demonstrated an increase of microbial diversity up to the cooling phase. However, inoculated strains were not detected in composts after 30, 60 or 90 days of incubation in compost sampled after the start of the cooling phase. Microbial diversity also became stable, and DGGE profiles reached a maximum number of bands at this stage. CONCLUSIONS: Strain survival was not observed in stabilized composts. The cooling phase seems to be the turning point for pathogen survival and at this stage the indigenous microflora appeared to play a significant role in suppression. SIGNIFICANCE AND IMPACT OF THE STUDY: The importance of indigenous microflora in the survival of pathogens in four different composts was demonstrated. Stabilized composts were recommended for spreading on land.

Predicting the biochemical methane potential of wide range of organic substrates by near infrared spectroscopy.

The use of near infrared spectroscopy (NIRS) as an alternative method to predict the biochemical methane potential (BMP) of a broad range of organic substrates was investigated. A total of 296 samples including most of the substrates treated by anaerobic co-digestion were used for NIRS calibration and validation. The NIRS predictions of the BMP values were satisfactory (Root Mean Square Error = 40 ml CH(4) g(-1) VS(fed); r(2) = 0.85). The integration of the entire substrate diversity in the model remained nevertheless difficult due to the specific organic matter properties of stabilised substrates and the high level of uncertainty of the BMP values. The elaboration of a model restricted to "fresh" substrates allows the practical use of the NIR technique to design and operate anaerobic co-digestion plants. The addition of more samples in the dataset in order to perform local calibrations would probably make the elaboration of a global NIR-model possible.

Composting in small laboratory pilots: performance and reproducibility.

Small-scale reactors (<10 l) have been employed in composting research, but few attempts have assessed the performance of composting considering the transformations of organic matter. Moreover, composting at small scales is often performed by imposing a fixed temperature, thus creating artificial conditions, and the reproducibility of composting has rarely been reported. The objectives of this study are to design an innovative small-scale composting device safeguarding self-heating to drive the composting process and to assess the performance and reproducibility of composting in small-scale pilots. The experimental setup included six 4-l reactors used for composting a mixture of sewage sludge and green wastes. The performance of the process was assessed by monitoring the temperature, O(2) consumption and CO(2) emissions, and characterising the biochemical evolution of organic matter. A good reproducibility was found for the six replicates with coefficients of variation for all parameters generally lower than 19%. An intense self-heating ensured the existence of a spontaneous thermophilic phase in all reactors. The average loss of total organic matter (TOM) was 46% of the initial content. Compared to the initial mixture, the hot water soluble fraction decreased by 62%, the hemicellulose-like fraction by 68%, the cellulose-like fraction by 50% and the lignin-like fractions by 12% in the final compost. The TOM losses, compost stabilisation and evolution of the biochemical fractions were similar to observed in large reactors or on-site experiments, excluding the lignin degradation, which was less important than in full-scale systems. The reproducibility of the process and the quality of the final compost make it possible to propose the use of this experimental device for research requiring a mass reduction of the initial composted waste mixtures.

Influence of bulking agents on organic matter evolution during sewage sludge composting; consequences on compost organic matter stability and N availability.

The influence of bulking agents on organic matter (OM) stability and nitrogen (N) availability in sludge composts was investigated. Seven mixtures were composted over a 12-week period in 170 L-reactors using the same sludge with different bulking agent mixtures. The OM evolution was characterised by carbon (C) and N mass balances in biochemical fractions. The evolution of OM stability and N potential availability were measured during soil incubations. The type of bulking agent had little influence on the intensity of OM stabilisation and the N availability in final composts. However, they influenced the time to reach similar OM stability and the biochemical evolution of OM. Depending on their ligno-cellulosic characteristics and a careful control of aeration, initial mixtures with high C:N ratio and rather biodegradable carbonaceous materials could favour the organisation of initially present mineral N or easily mineralisable sludge N and therefore limit NH3 volatilisation responsible for the low compost N availability measured.

Distribution of C and N mineralization of a sludge compost within particle-size fractions.

The contribution of particle-size fractions to carbon (C) and nitrogen (N) mineralization of sludge compost was investigated. Particle-size fractionation was performed using "dry" (sieving of total dry compost) and "wet" (dispersion of compost in water, followed by sieving) fractionation methods, then C and N mineralization of the separated fractions were measured during incubation in soil. The "dry" fractionation did not allow the actual particle-size distribution of compost to be estimated accurately. Out of all the "wet" fractions, the [0-50 microm] fraction was the most significant fraction in compost mass and contributed the most to the N mineralization of sludge compost in soil. Its low degradability, positive N mineralization and similarities with sludge OM suggest that the most humified sludge organic matter was located in this fraction, which would probably contribute to C storage and N availability after compost application in soil. Other fractions (>200 microm) were more readily biodegradable and induced N immobilization.