RESUMEN
Controlling the residence time in paddle dryers and the drying efficiency imply the knowledge of rheological behaviour of highly concentrated and pasty sludge and its temperature dependency. However, because of perturbing effects such as evaporation, measurements are not fully representative of intrinsic sludge properties. Classical techniques usually considered in the literature for evaporation control are not efficient at high temperatures. This work gives a method to control the evaporation at high temperature that can be used with any commercial rheometer. The configuration concept is to prevent water loss by limiting the contact between the sheared sludge and the environment. This configuration allows preventing evaporation up to 80 °C at least during 2 h. Its efficiency is confirmed at different total solid (TS) contents ranging from 20 to 47 wt.%.
Asunto(s)
Desecación , Aguas del Alcantarillado , Eliminación de Residuos Líquidos/métodos , Calor , Reología , TemperaturaRESUMEN
Wastewater sludge are characterized by complex rheological properties, strongly dependent on solids concentration and temperature. These properties are required for process hydrodynamic modelling but their correct measurement is often challenging at high solids concentrations. This is especially true to model the hydrodynamic of dewatered sludge during drying process where solids content (TS) increases with residence time. Indeed, until now, the literature mostly focused on the rheological characterization of sludge at low and moderate TS (between 4 and 8%). Limited attention was paid to pasty and highly concentrated sludge mainly because of the difficulties to carry out the measurements. Results reproducibility appeared to be poor and thus may not be always fully representative of the effective material properties. This work demonstrates that reproducible results can be obtained by controlling cracks and fractures which always take place in classical rotational rheometry. In that purpose, a well-controlled experimental procedure has been developed, allowing the exact determination of the surface effectively sheared. This surface is calculated by scattering a classical stress sweep with measurements at a reference strain value. The implementation of this procedure allows the correct determination of solid-like characteristics from 20 to 45% TS but also shows that pasty and highly concentrated sludge highlight normal forces caused by dilatancy. Moreover the surface correction appears to be independent of TS in the studied range.