Influence of the airflow rate on heat and mass transfers during sewage sludge and bulking agent composting.

The aim of this study was to characterize the gas flow during the composting, at a pilot scale, of a mixture of sludge and bulking agent, in order to model heat and mass transfers involved in the process. Thus, a closed 300-litre cylindrical pilot was fed with a mixture of wastewater treatment sludge and pine bark. Aeration was supplied from the bottom via an air blower and gases were collected at the top. Three experiments were led with constant gas flow rates and one with varying aeration rate. Temperatures within the pilot reactor were monitored all along the trials and their evolutions were discussed in term of heat transfers and parameters influencing the heat balance. Concurrently, Retention Time Distribution curves were obtained by injecting a pulse of methane in the entering airflow and by analysing the methane concentration in the exhaust gas, every two or three days during composting. The gas flow, within the composting medium, was characterized by a dispersion model, which is a deviation of the plug flow model. The dispersive effect of the flow was correlated to the evolution of the experimental temperature, and a convective dispersion model was used to describe the heat and mass transfers through the gas flow. These equations will be, in future work, coupled with heat production and mass degradation terms in order to model the global mass and heat balances of this composting process. Finally, axial dispersion coefficients of gases were determined and correlated with the airflow rate.

A respirometric method for characterising the organic composition and biodegradation kinetics and the temperature influence on the biodegradation kinetics, for a mixture of sludge and bulking agent to be co-composted.

A respirometric method was set up to study kinetics of biological reactions involved in the treatment of organic wastes-sludge mixed with pine barks--by composting. Oxygen consumption rates of this type of mixture were monitored during 10-20 days, using a 10 l respirometric cell kept at constant temperature and moisture. Oxygen consumption kinetics were modelled and organic matter composition was characterised as biomass, easily-biodegradable, slowly-biodegradable and non-biodegradable organic matter. The influence of temperature on kinetics was tested. Results show that this respirometric method is a useful tool for the characterisation of solid organic matter biodegradability and for the modelling of the biological kinetics of the composting process.

[Manganese in the water supply of the town of Beaucaire: origins and solutions]. / Manganese dans l'eau d'alimentation de la ville de beaucaire: origines et solutions.

Changes in domestic water supply appearance and quality linked with increasing manganese amount and iron at a lesser degree, having been found by the inhabitants of Beaucaire (Rhône Valley), a study was made by chemists, hydrogeologists and biologists to detect the origin. The problem is not specific to this town and can frequently be found in the Saône and Rhône valleys. The results show that we have to deal with an aquifer with semi-captive groundwater whose supply is double: the Rhône and calcareous mountain masses. The low-oxygen waters are enriched, by dissolution from gravel, with manganese and iron which are preferentially fixed on porous and cracked surface elements. The conditions for manganese liberation are connected with the presence of organic matters, bacteria (capable to reduce the manganic compounds) and anaerobiosis. The supply does not seem to be the only cause in the low-oxygen contents; the action of heterotrophic microorganism using organic matter and consuming oxygen, at the groundwater periphery, must be taken into account. Thus the central part of the groundwater system is a reducing environment which allows the modification of the bacterial metabolism towards a ferric and manganic compounds reduction process. The result is the transformation of insoluble manganic and ferric compounds into manganous and ferrous compounds which are liberated into the groundwater.