Impact of sludge storage duration on its dewatering and drying ability Y.B Pambou, T. Salmon, L. Fraikin, M. Crine, and A. Léonard

Annual production of sewage sludge in Europe is estimated at more than twelve million tons of dry matter. Use in agriculture and incineration are the main ways of valorization. In this context, sludge drying appears as an essential step after mechanical dewatering. It reduces the costs of storage and transport, allows the stabilization and the hygienization of sludge while increasing its calorific value. However, sludge is a colloidal system in which particle form a stable suspension in water, making him difficult to be separated from water. The addition of polyelectrolytes chemical is necessary to help the sludge particles to agglomerate into large flocs that can be separate by mechanical dewatering. Before studying experimentally the influence of polymer’s type and dosage on dewatering performances and subsequent drying behaviour, it is necessary to assess the sludge variability during storage duration, because sludge is a highly fermentable material whose properties and composition can rapidly change.Résultats thèse de Doctora

IMPACT OF A HIGH LINEAR WEIGHT POLYMER CO-CONDITIONING WITH POLYALUMINIUM CHLORIDE ON DEWATERING AND CONVECTIVE DRYING OF URBAN RESIDUAL SLUDGE

This paper investigated the influence of Polyaluminium chloride (PAX) co-conditioning with a high linear weight polymer on the dewatering performance and the drying behavior of sludge. The CT linear polymer with a high molecular weight was used combined with PAX for sludge flocculation prior to mechanical dewatering and drying. It was found that sludge conditioned with the couple PAX/ CT led to better flocculation/dewatering process regarding size flocs and specific resistance to filtration. Concerning drying, it appeared that this chemicals combination led to improved drying rates with effect of a reduction of the drying time, when compared to polyelectrolyte used without PAX adding

Impact of sludge storage duration on its dewatering and drying ability

Annual production of sewage sludge in Europe is estimated at more than twelve million tons of dry matter. Use in agriculture and incineration are the main ways of valorization. In this context, sludge drying appears as an essential step after mechanical dewatering. It reduces the costs of storage and transport, allows the stabilization and the hygienization of sludge while increasing its calorific value. However, sludge is a colloidal system in which particle form a stable suspension in water, making him difficult to be separated from water. The addition of polyelectrolytes chemical is necessary to help the sludge particles to agglomerate into large flocs that can be separate by mechanical dewatering. Before studying experimentally the influence of polymer’s type and dosage on dewatering performances and subsequent drying behaviour, it is necessary to assess the sludge variability during storage duration, because sludge is a highly fermentable material whose properties and composition can rapidly change.Résultats thèse de Doctora

Impact of conditioning on sludge dewatering and drying abilities

Le traitement des eaux usées engendre de facto un sous-produit qui se retrouve sous la forme d’une suspension colloïdale, connu sous le terme générique de boue d’épuration. À l’heure actuelle, sa valorisation après une étape de traitement constitue un sujet environnemental de premier ordre. Ce travail porte sur l'étude de l’impact de la phase de conditionnement de la boue liquide, associant divers réactifs, c’est-à-dire un coagulant et un floculant, sur les étapes aval de déshydratation mécanique et de séchage thermique. Le coagulant utilisé (polychlorure d’aluminium) est chaque fois associé à un type de polymère cationique (floculant), allant de la forme linéaire aux structures plus branchées (ramifiée et réticulée). Deux des quatre floculants testés avec le coagulant sont de conformations linéaires se différenciant uniquement par la longueur de la chaîne polymérique. Concrètement, l’étude met en évidence le rôle joué par le polychlorure d’aluminium (PAX-14), comme agent co-activateur de la libération de l’eau intra floc durant la floculation, lorsqu’il est utilisé en complément avec le polymère cationique. Pour mieux comprendre le lien entre cette phase de floculation et les performances des procédés de déshydratation mécanique et de séchage, plusieurs protocoles de conditionnement ont été étudiés et la suspension floculée ainsi obtenue a été filtrée sous une différence de pression maintenue constante, dans une cellule de filtration normalisée. Les gâteaux de boue ainsi produits ont été caractérisés en termes de matières sèches, de propriétés texturales, d’isothermes de désorption, et enfin soumis à un post-traitement de séchage convectif, après une mise en forme cylindro-reproductible de l’échantillon à sécher. En déshydratation mécanique, le procédé de floculation par double conditionnement reste un excellent moyen permettant d’améliorer la siccité du produit déshydraté. La ‘‘complexation’’ de la suspension liquide avec du polychlorure d’aluminium et le floculant offrirait des rendements plus attrayants par rapport au polymère seul. Par ailleurs, l’activité des deux floculants linéaires testés en duo avec le PAX-14 produirait des performances en déshydratation plutôt similaires, tandis que le même coagulant couplé aux macromolécules ramifiées ou réticulées produirait des différences notables en termes d’augmentation du taux de siccité, différences pour lesquelles le polymère à plus haut degré de réticulation conférerait à la suspension une plus grande propension à éliminer l’eau contenue dans les flocs après filtration. Les isothermes de sorption, courbes traduisant l’hygroscopicité du produit, ont été réalisées sur les échantillons déshydratés en utilisant la méthode du DVS (Dynamic Vapor Sorption). Ces essais ont permis de confirmer d’une part, que les courbes obtenues pour la boue d’épuration présentent une allure sigmoïdale caractéristique de type II, et d’autres part, que l’influence du couple coagulant/floculant sur le caractère hydro-textural de la boue reste complexe à établir. Les paramètres statistiques mis à contribution pour évaluer le caractère significatif des différents ajustements montrent que la formulation de GAB apparaît comme l’équation décrivant au mieux l’isotherme de sorption de la boue des Grosses-Battes. En étudiant l’évolution de la texture de la boue pour les différentes teneurs en siccité, ce travail a permis de comparer les propriétés mécaniques issues des quatre types de conditionnement. Globalement, les résultats indiquent une fermeté plus prononcée du matériau déshydraté lors de l’utilisation du tandem PAX-14/polymère dans la phase de floculation, par rapport au floculant seul. Toutefois, la détermination des paramètres de texture montre qu’il semble difficile d’affirmer que la contribution du coagulant et des floculants réticulés est meilleure que celle associant ce sel d’aluminium polymérisé et les floculants linéaires. Enfin, il est à noter que les effets du sous/surdosage de réactifs peuvent mener à une déstructuration de la boue et conduire à une diminution de la fermeté du produit. Afin d’explorer l’effet du PAX-14/floculant sur le comportement de la boue au séchage, une comparaison des caractéristiques associées aux cinétiques a été réalisée. D’après l’analyse statistique y relative, les résultats les plus probants sont obtenus par l'association impliquant le PAX-14 et l’emploi des floculants de conformations branchées. Les polymères linéaires quant à eux, conduiraient à des performances presque équivalentes. Durant le processus de séchage, il se produit inévitablement un retrait du matériau. Par microtomographie à rayons X, ce phénomène a pu être caractérisé. Les analyses des courbes de retrait volumique laissent apparaître deux zones dont la transition semble être corrélée au caractère hydrique de la boue: la première, caractéristique d’un retrait idéal dont la réduction du volume de l’échantillon correspond à la quantité d’eau évaporée au cours du séchage et la seconde, attribuée aux propriétés rhéologiques de la boue. Le retrait final dépend vraisemblablement des caractéristiques intrinsèques à la boue, mais légèrement aussi de la nature/structure du type de conditionnement mis en œuvre, évalué à 70% pour les floculants linéaires et réticulés, combinés au sel d’aluminium polymérisé, alors qu’il est de 75% du volume initial de l’échantillon dans le cas des floculants ramifiés complexés au PAX-14. On note aussi une influence de ces réactifs sur la structuration de la porosité consécutive au séchage. Pour la floculation au polymère seul, il apparaît que celle-ci soit centrée sur le matériau solide tandis que pour le double conditionnement, une porosité en lamelles est identifiée. Cette différence de structure interne de la porosité créée, explique la différence de cinétique observée, notamment par la modification des surfaces d’échange accessibles et de la distance moyenne de transfert, induisant l’accélération ou le ralentissement de l’évacuation du solvant.Wastewater treatment plants generate a byproduct which is found in the form of a colloidal suspension, known under the generic term of sludge. At present time, its valorization after a treatment step is a topical environmental issue. This work focuses on the study of the impact of the conditioning step, i.e., the addition of chemicals such as coagulants and flocculants, on the mechanical dewatering and thermal drying steps. The coagulant used (polyaluminum chloride) is combined with a cationic polymer type (flocculant), ranging from linear to complex structures (branched and cross-linked), two of the four flocculants tested with the coagulant are linear structures only differing by their molecular weight. Specifically, the study highlights the role of polyaluminum chloride (PAX-14) as a co-activating agent of the water release during the flocculation stage, when it is combined with the polymer. To better understand the link between this flocculation stage and mechanical dewatering and drying performances, many conditioning ways have been studied and the flocculated sludge obtained was dewatered under a constant pressure using a normalized filtration-expression cell. The dewatered cake obtained after the filtration stage was then characterized in terms of dry solids content, textural properties, desorption isotherms, and finally submitted to a convective drying post-treatment, after the sample was extruded through a circular diameter of 14 mm and cut at a height of a same length, yielding cylindrical samples with a mass of around 2.5 g. Results show that, dual conditioning by applying PAX-14 as a coagulant is an excellent way to improve the cake dryness after the mechanical dewatering step. The interaction between the liquid suspension with polyaluminum chloride and the flocculant offers more attractive yields compared with the use of single conditioning to flocculate the bio-material. Moreover, the efficiency of the two linear flocculants including the use of PAX-14, were shown to produce rather similar performances, while for the same coagulant coupled with trendy or branched macromolecules produce significant differences in terms of increase in dry solids content of the dewatered cake, differences for which the polymer having the highest degree of cross-linking appears to be more beneficial to remove more water contained in the floc after filtration. Sorption isotherms, curves reflecting the hygroscopicity of the product were carried out on the dewatered samples using the DVS equipment (Dynamic Vapor Sorption). Firstly, these experiments confirmed that, curves obtained for wastewater sludge exhibit a sigmoidal shape characteristic of type II, and secondly, an impact of dual PAX-14/polymer conditioning on hydro-textural properties remains difficult to assess. Based on the statistical parameters, GAB equation appears the best formulation to model the sludge sorption isotherms of the Grosses-battes wastewater treatment plant. By studying the texture evolution of sludge for different levels of cake dryness, this work has compared the sludge mechanical properties from the four conditioning types. Globally, results show a more pronounced firmness of the dewatered cake when using the couple PAX-14/polymer in the flocculation stage compared to the single conditioning one. However, from the determined sludge texture parameters it seems difficult to argue that the contribution of both coagulant and cross-linked polymers is better than the association involving aluminum polymerized salt and linear flocculants. Finally, it can be noted that the effects of under or overdosing of chemical can lead to the destruction of material and then, a decrease of sludge cohesion drying rate can be observed. In order to access at the effect of the dual PAX-14/polymer conditioners on drying behavior of sludge samples, some characteristics associated with drying kinetics were compared. According to the statistical analysis, the most significant results were obtained by the association involving PAX-14 and the use of cross-linked flocculants, for which, the polymer which has the highest degree of cross-linking provides a better propensity to remove water contained into flocs after filtration. Linear polymers were shown to produce the same performance. During drying, the texture of the material evolves, including shrinkage and voids development. By X-ray microtomography, this phenomenon has been characterized. The curves of shrinkage volume exhibited a linear decrease with two straight line periods, which the transition seems to be correlated to the sludge water behavior. On the first period, the shrinkage volume can be correlated to the quantity of evaporated water. The texture of the solid material does not hamper the shrinkage process. However, during the second period, the structure of solid stiffens and then influences the shrinkage phenomenon: water is replaced by air and porosity is created. This period corresponds to a phase of decreasing deformation rate (ε) marking the gradual rigidifying of the matrix until no further deformation was observed, the solid was totally rigidified. After drying, the final volume reduction was found to be close to 70% of the initial sample volume for linear and cross-linked flocculants combined with PAX-14, while it was around 75% of the sample initial volume in the case of branched flocculant co-conditioning with the sample coagulant. According to this result, shrinkage depends on both the nature of the sludge and the polyelectrolyte types or dosage. An impact was also found of chemical conditioners on the evolution of porosity. For sludge flocculated by single conditioning, it appears that the porosity was centered on the solid material while for dual conditioning a porosity structured in slats, was identified. This difference in internal structure of the porosity explains the difference in drying kinetics, including the modification of the exchange surfaces available and the average distance of transfer, allowing acceleration or a slow-down of the water removal

Enhanced sludge dewatering and drying comparison of two linear polyelectrolytes co-conditioning with Polyaluminium chloride

Annual production of sewage sludge in Europe is estimated at more than eleven million tons of dry matter content [1]. Use in agriculture and incineration are the main ways of valorization. In this context, drying of residual sludge appears as an essential step after mechanical dewatering. It reduces the costs of storage, transport and allows the sludge stabilization. However, this process is highly energy consuming and still needs to be optimized as it constitutes an important economic and environmental issue [2]. Polymers are usually employed in the conditioning step in order to promote particle aggregation, making the dewatering easier. The past decades, the application of pre-hydrolized polyaluminium chlorides (PACls) as coagulant has increased, particularly in China, Japan, Russia and Western Europe [3]. As a consequence, PACls are extensively investigated for their coagulation performance, characterization and speciation [3]. In this work, the influence of Polyaluminium chloride (PAX-14) co-conditioning with linear polyelectrolytes on sludge dewatering and drying performances was investigated. Experiments were conducted on activated sludge samples collected after thickening from the wastewater treatment plant of the Grosses- Battes (Belgium). Two cationic polymers were tested, each of them in combination with PAX for sludge flocculation prior to mechanical dewatering and their effects on sludge convective drying. The one referenced as 640 LH was a linear polymer with a low molecular weight, whereas the 640 CT was a linear one with a high molecular weight. After conditioning, the dewatering step was realized by using a normalized filtration-expression cell (AFNOR 1979) under 5 bar of pressure. Then, the specific resistance to filtration was determined from the follow-up of filtrate mass with time, using the Carman-Kozeny equation [4]. Figure 1 shows the experimental design. For convective drying experiments, the cake recovered after filtration was extruded through a circular die of 14 mm diameter and cut at a height of 14 mm, yielding cylindrical samples with mass of approximately 2.5 g, as used in several industrial belt dryers. Individual extrudates have been dried in a specially designed convective micro-dryer. Results reported in this paper were obtained with the following operating conditions: air temperature of 130 °C, at ambient humidity (absolute humidity ~ 0.005 kgwater/ kgdry air) and a superficial velocity of 1 m/ s. Results show that, the dry solids content of the dewatering cake increases with increasing Polyaluminium chloride dosage for both series of experiments. It supposes that PAX contributed to improve solids capture, depicted on Fig. 2. Concerning drying, it appeared that the samples treated by the dual PAX/ polymers combination showed higher drying rates than samples conditioned by polymer without PAX addition, allowing a reduction of the drying time (see Fig. 3). Table 1 shows the drying characteristics of the samples. The drying time decreases and the average drying rate increases with the dual conditioners addition, while the initial water content decreases as well as the total amount water to be removed

Impact of sludge storage on dewatering and convective drying

Annual production of sewage sludge in Europe is estimated at more than ten million tons of dry matter. Use in agriculture and incineration are the main ways of valorization. In this context, sludge drying appears as an essential step after mechanical dewatering. It reduces the costs of storage and transport, allows the stabilization and the hygienization of sludge while increasing its calorific value. However, this process is highly energy consuming and still needs to be optimized as it constitutes an important economic and environmental issue. This implies the improvement of knowledge about sludge drying, including the impact of the mechanical dewatering step. Before studying experimentally the influence of polymers type and dosage use for dewatering on the rheological properties and the drying process, it is necessary to assess the stability of the sludge during of the storage. Because sludge is a living material that can rapidly change, we use the method of experimental design in order to get maximum information with minimal testing, regarding sludge stability during storage.Thèse de Doctora

Impact du conditionnement sur l'évolution de la structure interne des boues durant le séchage

This paper investigated the influence of dual conditioning of waste water treatment sludge on the evolution of sludge internal structure during convective drying. Results showed clearly an impact of chemical conditioners on the evolution of the created porosity. For sludge flocculated by single conditioning, a centered porosity delimiting by external crust was shown, while for dual conditioning, a porosity structured in lamellae, was identified

Enhancement of dewatering and drying behavior of activated sludge when using PAX coagulant for conditioning

This paper investigated the influence of dual conditioning of waste water treatment sludge on the evolution of sludge internal structure during convective drying. Results showed clearly an impact of chemical conditioners on the evolution of the created porosity. For sludge flocculated by single conditioning, a centred porosity delimiting by external crust was shown, while for dual conditioning, a porosity structured in lamellae, was identified

Evolution de la structure interne de la boue durant le séchage : effet du conditionnement

This paper investigated the influence of dual conditioning of waste water treatment sludge on the evolution of sludge internal structure during convective drying. Results showed clearly an impact of chemical conditioners on the evolution of the created porosity. For sludge flocculated by single conditioning, a centered porosity delimited by an external crust was shown, while for dual conditioning, a porosity structured in lamellae, was identified