ELECTRODEWATERING OF DIFFERENT SEWAGE SLUDGES UNDER CONSTANT ELECTRIC CURRENT: IMPACT OF PHYSICO-CHEMICAL PROPERTIES ON THE DEWATERABILITY AND THE SEPARATION KINETICS
Résumé
A sewage sludge is the by-product of a wastewater treatment plant (WWTP), and is mainly composed of water and the rest of its mass is the dry solids (DS). Prior to the transport and valorisation of these sludges, a liquid-solid separation is necessary. Various processes allow this aim, but their performance is limited and can be enhanced to avoid the use of processes like thermal drying which consume high amounts of energy, that needs to be limited. Electro-dewatering (EDW) is a hybrid process in which a conventional filtration-compression process is combined with the application of an electric field in order to enhance sludge dewatering (with lower energy consumption compared to a conventional drying process). Previous studies have proposed an empirical model that reveals a relationship between the evolution of DS over time. This mathematical model includes a kinetics coefficient that is proportional to the sludge's physicochemical properties. This model was developed for EDW tests carried out under a constant electric current (I-EDW), unlike most of the studies presented in the literature where a constant voltage (U-EDW) is commonly used. Although less studied in the literature, the I-EDW mode remains an interesting operating mode since it limits the raise of temperature which can be detrimental for the electrodes. The objectives of this study are to evaluate the influence of the physicochemical properties on the response of the process, notably the kinetics of the liquid-solid separation and the DS evolution. Then, a mathematical relationship between the kinetics coefficient and the influencing parameters is proposed. First step was to conduct I-EDW tests on different sludge types in a lab-scale equipment running with a constant current of I=0.32A, a constant pressure of P=36kPa and a constant sludge thickness (H) of 5mm height. The duration of tests was fixed at 15min. For every sludge, several physicochemical properties were measured (pH, conductivity, hydrophobicity, volatile suspended solids (VSS), surface charge and extracellular polymeric substances (EPS)). Then simple correlation models were used among the studied parameters. Initial DS and soluble EPS seems to be the most influencing parameters on the kinetics. VSS and conductivity happens to have an important influence as well. Later, a mathematical relationship between the kinetics coefficient and the physicochemical properties was established using a multiple linear correlation. The experimental and the modelled kinetics coefficient appears to be comparable, we could then assume that this model is viable for our sludge types. These results will help improving the existing empirical model, by integrating the influencing parameters on the kinetics. The model will help predict the evolution of DS over time depending on the sludge properties, the applied current, the sludge loading rate and the initial DS.
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