Sludge liming : quality of the blend and impact on the mechanical properties | Chaulage des boues : qualité du mélange et impact sur les propriétés mécaniques

[Departement_IRSTEA]EA [TR1_IRSTEA]TESAD / TECHPROPRE

anglais. Nowadays, sewage sludge are either incinerated or valorised, mainly through agricultural spreading. However, they are fermentescible and the metabolism of micro-organisms induces phenomena such as unpleasant odours and consistency evolution. Liming is one of the most economic solution that was found to overcome those drawbacks. Its effects are: -an hygienisation of the material -a decrease of the odours -higher possibilities of handling -the neutralisation of metallic trace elements -an increase of the agronomical value. Finally, lime should appear like a perfect solution. However, to reach those purposes, the blend of sludge and quicklime must be as intimate as possible. Practically, limed sludge currently often presents heterogeneities: agglomerates, lime particles, non limed sludge pellets. In order to maintain criteria imposed by legislation, such as a pH higher than 12, a temperature higher than 55°C during more than 75 minutes, overdoses are common. With the purpose of decrease reactive consumption, we are working on a new blending process which allows us to respect the legislation while using less lime. The results presented here concern the rheological and textural characterization of samples of limed sludge both taken in a waste water treatment plant and produced into our lab thanks to the new process. Different parameters were measured. Creep tests, relaxation tests, determination of the fracturability and the adhesion force were performed. It appears that not only the lime content but also the blending process influence the properties of the samples. Finally, we show that our new process allows to minimize lime quantity for a better efficiency of the mixture. In parallel, the evolution of the temperature revealed that high temperature were reached and that the decrease of temperature evolves very slowly: the hygienisation process is then respected. Moreover, pH measurements show that the lime content quickly induces basic values of pH. The evolution of the pH will be modified with the time as a function of the quality of the blend and of the lime content.