Laurent Vachoud

A contribution to the understanding of micro-pollutant sorption mechanisms in wastewater biological processes: case of the tributyltin

Micro-pollutant fluxes distribution throughout the physical separation and biological units of wastewater treatment plants (WWTPs) are very dependent of sorption phenomena. The understanding and the control of the sorption stage is thus essential for the optimization of micro-pollutant removal in WWTPs, and particularly in biological treatments where these mechanisms influence the bioavailability towards micro-organisms. If the influence of the micro-pollutant physicochemical characteristics (e.g. K ow, pKa) on their ability to sorb on biological media (i.e. sludge) has been demonstrated, it appears that some other parameters, like the biosorbent characteristics, have to been taken into account. The aim of this study is thus to correlate the capacities of sorption of an environmentally relevant substance (tributyltin), with a thorough characterization of different types of sludge. The characterization of three biological media (raw, sonicated and flocculated activated sludges) is proposed according to various characterization parameters related to biochemical composition, aggregate size, rheological behaviour etc. The results show first that, whatever the sludge characteristics may be, the sorption mechanisms are very rapid and that an equilibrium state is reached after a few minutes. The influence of the sludge characteristics, notably the floc size and the chemical oxygen demand partition between solid and colloidal fraction, on sorption efficiency is demonstrated. A Langmuir modelling allows giving the maximum sorption capacity, as well as the binding energy for the three studied sludges, according to their physicochemical characteristics.

Preparation of Composite Hydrogels for Medical Applications: Experimental Study and Modeling of Mass Transfers

The aim of this work is to prepare wound dressing hydrogels permitting (i) to cure chronicle pathologies; (ii) to remove unsuitable odors coming from healing. Thus, chitosan, a natural polymer with biological properties, and activated carbon (AC), a well-known adsorbent, are mixed within a composite hydrogel. To get a suitable porous structure for healing, the vapor induced phase separation process (VIPS) is used. It permits to slow down mass transfers kinetics compared to other elaboration processes such as the wet process. Hence, a better control of the final porous matrix is reached throughout the elaboration process.In this work, the mass transfers occurring through the elaboration of the composites were studied thanks to gravimetric measurements carried out during the VIPS process. Results showed there were two main steps in the gelation process. A weight intake, due to an ammonia flux allowing the gelation to happen, was followed by a weight loss, mainly controlled by a water outtake. This second part could be compared to a slow drying in smooth conditions. Gelation with a color indicator was also performed, which provided complementary experimental information on mass transfers barrier.A modeling of mass transfers mechanisms occurring during the gel elaboration is presented to get an overview of limiting phenomena leading to the final composite structures. It involves a coupling between chemical reactions, external mass transfers and internal diffusion within the polymeric matrix. Results show that internal diffusion is the main barrier to gelation.To qualify the gel structure, drying kinetics were compared on the elaborated gels. Whatever the gel, the drying kinetics were globally the same and could be divided into three steps. Considering the process used and this result, it was believed that the matrixes prepared were homogeneous and rather symmetric. They could constitute the first layer of wound dressings.

How the nature of the compounds present in solid and liquid compartments of activated sludge impact its rheological characteristics

ABSTRACT Although the role of the solids concentration on the rheological characteristics of sludge is greatly documented in the literature, few studies focused on the impact of the nature of these solids. How the nature of solutes can modify the solid–liquid interactions and thus the rheological properties of the sludge are also slightly explored. Thus, the objective of this study is to investigate the rheological characteristics of activated sludge in relation with the nature of the compounds present in the solid and liquid phases. Rheological measurements were carried out on raw sludge and on sludge modified by mechanical actions and/or addition of solids or solutes. The rheological properties of raw and modified sludges were measured according to flow and dynamic measurements. Results demonstrated that if suspended solid concentration affected sludge rheological parameters, the nature of the solids was quite of importance. The key role of nature and molecular weight of solutes was also highlighted. The results contribute to a better knowledge of the relationship between sludge composition and its rheological properties, which is useful for the optimization of sludge mixing, pumping or aeration and also for the improvement of sludge dewatering, notably by a relevant choice of adjuvant.

An environmental application of functionalized chitosan: enhancement of the separation of the solid and liquid fractions of digestate from anaerobic digestion

Abstract A high molecular weight chitosan was chemically modified to quantitatively incorporate quaternary ammonium groups. Its efficiency was evaluated in the liquid-solid separation for various liquors, and compared with the one of a polyelectrolyte usually used for this utilization. The performance of the liquid-solid separation was estimated through the determination of two parameters measured after the screening- settling of the mixture liquor/flocculating agent: the separation efficiency (EV) and the TS removal efficiency (ETS). Apart for liquor 6, TS removal was always better after an addition of functionalized chitosan. Furthermore, whatever the type of liquor, the distribution was modified by an increased presence of high-size particles when functionalized chitosan was added. Moreover, chitosan addition tended to homogenize the size of the particles, which could facilitate the choice of the liquid-solid separation process. This homogenization was particularly observed for the liquor initially highly dispersed in size, i.e. liquors 1, 4, 6 and 8.