Decrease of enteric micro-organisms from rural sewage sludge during their composting in straw mixture.

AIMS: To study the decrease of enteric micro-organisms including viable nematode eggs, enteroviruses, faecal indicators (Escherichia coli and enterococci) and pathogenic bacteria (Listeria monocytogenes, Salmonella sp. and Clostridium perfringens) of a rural sewage sludge when it is composted for 7 months in mixture with straw. METHODS AND RESULTS: Numbers of the test organisms and the physico-chemical parameters were measured on a monthly basis on the mixture, on the compost after being turned, and on the pile in three positions representing the part by which air is incoming, the bottom of the pile and the part through which air is outgoing. The lowest temperature in the pile was observed at the bottom, where it did not exceed 50 degrees C against 66 degrees C in the two other areas. There were no significant differences between the three areas in terms of micro-organism survival. Infectious enteroviruses were inactivated rapidly and were not found after the first turning whereas some genomes were detected until after the third turning. Escherichia coli and enterococci presented a similar survival rate and their number decreased by 4 log(10) whereas Salmonella decayed at a greater rate than L. monocytogenes. The numbers of C. perfringens decreased gradually to reach a final concentration in the mature compost of about 10(2) CFU g(-1) dry matter (d.m.), which was similar to that of the faecal indicators. CONCLUSIONS: The hygienic effect of sludge composting in mixture with straw results in a significant reduction of enteric micro-organisms, the concentration of the faecal indicators in the final product being < 64 most probable number g(-1) d.m. The concentrations of Salmonella, enteroviruses and viable nematode eggs in the final product were not detectable which is in accordance with the French legislation. SIGNIFICANCE AND IMPACT OF THE STUDY: The results which pointed out the different behaviour of the test micro-organisms reflect the difficulty to propose a relevant indicator of hygienization. Otherwise, they show that composting is an efficient means for hygienization of sludge of rural wastewater treatment, where the straw is available close to their place of production.

Enumeration and characterization of cellulolytic bacteria from refuse of a landfill.

Enumeration and phenotypic characterization of aerobic cellulolytic bacteria were performed on fresh, 1 year old and 5 years old refuse samples of a French landfill site. Numbers of cellulolytic bacteria ranged from 1.1x10(6) to 2.3x10(8) c.f.u. (g dry wt.)(-1) and were lower in 5 years old refuse samples. A numerical analysis of phenotypic data based on 80 biochemical tests and performed on 321 Gram-positive isolates from refuse, revealed a high phenotypic diversity of cellulolytic bacteria which were distributed into 21 clusters. Based on the phenotypic analysis and the sequencing of 16S rDNA of five representative strains of major clusters, the predominant cellulolytic groups could be assigned to the family of Bacillaceae and to the genera Cellulomonas, Microbacterium and Lactobacillus. Furthermore, chemical parameters such as pH, carbohydrates and volatile solid contents influenced the composition of the cellulolytic bacterial groups which were reduced essentially to the family of Bacillaceae in the oldest refuse samples.

Detection of a whitening fluorescent agent as an indicator of white paper biodegradation: a new approach to study the kinetics of cellulose hydrolysis by mixed cultures.

A simple and reliable method to estimate paper degradation by cellulolytic bacteria is described. This method is based on the detection in the culture medium of a fluorescent whitening agent (FWA) added to white paper during the manufacturing process. Preliminary results using a Cellulomonas strain cultivated in a liquid medium containing FWA, indicated that this component is non-toxic at a final concentration of 0.01 per thousand (v/v) and that the fluorescence decreased during the first 24 h of incubation, i.e. during exponential growth phase, suggesting an adsorption of FWA on bacterial cells. Consequently, all experiments have been performed with a liquid medium containing FWA (0.01 per thousand v/v) and white paper (8.0 g/l) as cellulose source. Mixed bacterial populations (MBPs) were prepared from refuse samples. These MBPs, which mainly consisted of bacterial rod cells, were used as inocula and fluorescence was measured after 30 h of incubation, i.e. after the stationary phase was reached. A high linear correlation (R(2) = 0.979) was found between the percentages of degraded paper (%P) deduced from residual paper weight and the fluorescence values (F) of the culture medium and the following equation between %P and F was determined: %P = 8.71x10(-5) x F. An additional experiment using a second MBP showed a strong correlation (R(2) = 0.990) between the measured %P and the %P estimated from F values, confirming the reproducibility of the method. Moreover, the time course of paper degradation by five replicate flasks from a unique MBP was set up. Paper degradation was detected 3 to 5 days after the beginning of the stationary phase. The average degradation rate between the 7th and the 11th day of incubation was 11.4% per day. Rates of paper degradation ranged from 31 to 60% after 10 days and from 77 to 88% after 3 weeks of incubation, depending on the inoculum.