Isabelle Seyssiecq

State-of-the-art: rheological characterisation of wastewater treatment sludge

Since wastewater treatment has been a subject of interest, non-Newtonian flows, such as sludge flows, have been widely studied in the literature, most of the time from a rheological point of view. It is well known that the hydrodynamic behaviour of sludge flows is of prime importance to optimise process parameters of wastewater plants on one hand, as well as those of excess sludge retreating processes on the other hand. The present study is devoted to a synthesis of articles dealing with the rheological characterisation of biological wastewater treatment plant sludges (activated or concentrated ones) and some other concentrated suspensions such as microorganism ones. Attention is notably given to the rheological methods used and to rheological equations and behaviours observed in the different articles. Correlation proposed by some authors linking rheological properties to physico-chemical parameters of suspensions, or to some operating parameters of a process involving the use of sewage sludges are also given in this literature review.

From the conventional biological wastewater treatment to hybrid processes, the evaluation of organic micropollutant removal: A review

Because of the recalcitrance of some micropollutants to conventional wastewater treatment systems, the occurrence of organic micropollutants in water has become a worldwide issue, and an increasing environmental concern. Their biodegradation during wastewater treatments could be an interesting and low cost alternative to conventional physical and chemical processes. This paper provides a review of the organic micropollutants removal efficiency from wastewaters. It analyses different biological processes, from conventional ones, to new hybrid ones. Micropollutant removals appear to be compound- and process- dependent, for all investigated processes. The influence of the main physico-chemical parameters is discussed, as well as the removal efficiency of different microorganisms such as bacteria or white rot fungi, and the role of their specific enzymes. Even though some hybrid processes show promising micropollutant removals, further studies are needed to optimize these water treatment processes, in particular in terms of technical and economical competitiveness.

Agglomeration of gibbsite Al(OH)3 crystals in Bayer liquors. Influence of the process parameters

Agglomeration of gibbsite (Al(OH)3) crystals is an important stage of the Bayer process. In the present study, using a crystallizer working at constant supersaturation with time, we determine the agglomeration kinetics of gibbsite as a function of different experimental parameters. The effect of the crystallization temperature, supersaturation, seed mass, stirring rate and seed size is investigated. An interpretation of the results is proposed, they are also compared with previous results drawn from the literature.

The influence of additives on the crystal habit of gibbsite

Abstract Crystallization of gibbsite (Al(OH) 3 ) is an important stage of the Bayer process, production of alumina from bauxite ores. In both pure or industrial supersaturated sodium aluminate solutions, gibbsite crystals are always agglomerated. In the present paper, we present results of a study concerning the influence of different polycarboxylic acids as crystal habit modifier for gibbsite. In pure solution, agglomerated hexagonal plates are observed. Whereas acicular and tabular morphologies are found in the presence of different additives. These results are discussed referring to the crystallographic structure of gibbsite. It is found that only oxygen atoms are present on gibbsite surface. This observation leads us to propose an additive way of acting by formation of a molecular complex between the growth unit and the carboxylic groups of the additive.

The influence of organic additives on the crystallization and agglomeration of gibbsite

Abstract During the crystallization of gibbsite Al(OH)3, which leads to the synthesis of alumina in the Bayer process, crystals go through a step of agglomeration. In this work, we present a study concerning the influence of different organic compounds, polycarboxylic acids and polyalcohols on the crystallization and agglomeration of gibbsite crystals. It has been determined that they act as crystal habit modifiers and inhibitors of the agglomeration occurring during the formation of alumina crystallites. Simply by following the kinetics of the reaction, it has been observed that polyalcohols are stronger inhibitors than polycarboxylic acids, which can be linked to the structure of gibbsite crystals. The influence of the characteristics of the additive, such as the number of carboxylic groups and the distance between them for polyacids, the stereochemistry and the length of the carbon chain in the case of polyalcohols, is demonstrated to have a significant effect.

Modelling gibbsite agglomeration in a constant supersaturation crystallizer

An important stage of the Bayer process leads to the formation of gibbsite particles (Al(OH)3) mostly grown by agglomeration (aggregation and crystal growth). In this work, a population balance model is developed to account for experimental data generated in a constant supersaturation crystallizer. The objective is to quantify the effect of the seed concentration on the agglomeration kinetics. The population balance equation is discretized and solved with the method of classes. Several classical agglomeration and fragmentation kernels are tested. The size-independent agglomeration kernel taken alone results in the most suitable description of the experiments. The kinetic constant must then strongly decrease with the seed mass. The hypothesis of an agglomeration kernel independent of the seed mass combined with a fragmentation kernel increasing with the seed mass is also tested. Results are compared and discussed with the previous ones drawn from the literature.

A non-immersed induction conductivity system for controlling supersaturation in corrosive media: the case of gibbsite crystals agglomeration in Bayer liquors

Agglomeration of gibbsite Al(OH)3 crystallites is an important stage of the Bayer process, aiming at increasing the initial size of the particles. In the present work, a semi-continuous crystallizer working at constant and imposed supersaturation, and equipped with an automatic withdrawal system was developed to study the agglomeration of gibbsite crystals in supersaturated Bayer liquors. The liquor conductivity was measured using an induction conductivity system placed around the crystallizer, the conductivity regulation being used to work at constant supersaturation. Using this system allowed one to work with both a small crystallizer and a highly corrosive and abrasive suspension of gibbsite in a five molar caustic soda solution at 70°C. Analyses of the withdrawals were carried out with an Elzone particle counter, in order to draw N(t)N(0) = f(t) plots, representing the decrease of crystal number with time, due to agglomeration.

Effect of exo-polysaccharide concentration in the rheological properties and settling ability of activated sludge

The non-Newtonian properties of activated sludge (AS) suspension lead to transfer limitations (oxygen, substrate…) and operation difficulties in Waste Water Treatment Plants (WWTP). The current approach involves assuming the sludge behaves like water on a rheological point of view, and then oversizing pumping and aeration devices, which represent over 60% of the operating cost in WWTP. The objective of this work is to understand the effect of bioflocculation on the rheological properties and the settling ability of AS suspensions, by means of variations in concentration of exo-cellular polysaccharides. Experiments have been conducted in a 20 L laboratory scale bioreactor at a constant retention time of 20 days and with a total suspended solid concentration between 15 and 20 g L−1. The bioreactor was fed with a synthetic substrate at a constant mass loading rate of d−1. Our results show that increasing the exo-polysaccharide (EPS) concentration from 10 to leads to an increase in shear-thinning properties of AS. An improvement of the settling ability is also obtained, at least when the EPSs increase from 10 to . Above of adsorbed polysaccharides, the settling ability seems to decrease again.

Thermo-physical properties of synthetic mucus for the study of airway clearance

In this article, dynamic viscosity, surface tension, density, heat capacity and thermal conductivity, of a bronchial mucus simulant proposed by Zahm et al., Eur Respir J 1991; 4: 311-315 were experiementally determined. This simulant is mainly composed of a galactomannan gum and a scleroglucan. It was shown that thermophysical properties of synthetic mucus are dependant of scleroglucan concentrations. More importantly and for some scleroglucan concentrations, the syntetic mucus, exhibits, somehow, comparable thermophysical properties to real bronchial mucus. An insight on the microstructure of this simulant is proposed and the different properties enounced previously have been measured for various scleroglucan concentrations and over a certain range of operating temperatures. This synthetic mucus is found to mimic well the rheological behavior and the surface tension of real mucus for different pathologies. Density and thermal properties have been measured for the first time.

Rheological characterization of macromolecular colloidal gels as simulant of bronchial mucus

Mucus is mainly composed of water (90-95%) and mucins (2-5%), these mucins being high molecular weight macromolecules forming a 3D cross-linked matrix. It makes it a complex non-Newtonian fluid, displaying viscoplasticity, viscoelasticity, shear-thinning and thixotropy. These properties were qualitatively and separately tested by different authors. Due to the difficulties to collect samples and the extreme sensibility of the tested materials, the results are widely variable. A complete and intrinsically consistent characterization remains to be done to develop a reliable rheological model for future numerical simulations of mucus displacements in airways, in the case of pathologies such as cystic fibrosis. For this purpose, samples of mucus simulants were tested using a controlled stress rheometer. The material consists of macromolecular colloidal gels at different concentrations in macromolecules to mimic the variability in mucin production depending on the disease state and environmental factors. The rh...