Enhancement of the electrodewatering properties ofsynthetic and municipal sludges by the addition ofmetal cations.

Electrodewatering consists on the application of both pressure (mechanical effect) and electrical current (electrokinetic phenomena) to colloidal systems, such as municipal sludge, in order to achieve higher solid content (also designated by dryness). The influence of metal cations and its salts on the electrodewatering characteristics was systematically studied for municipal sludge and for two potential synthetic sludges, one composed of kaolin and the other of kaolin plus xanthan gum. The effect of the size, charge and concentration of the metal cations were studied in the three types of sludge. The effect of pH was also briefly addressed. The results were explained by the selective adsorption of metal cations to the surface of the dispersed particles. The adsorption was studied by ICP-AES (Inductively Coupled Plasma with Atomic Emission Spectrometer) metal analysis. This project was performed in collaboration with Ovivo. The first type of synthetic sludge was prepared by mixing kaolin powder with an electrolyte, where the nature of the salt and its concentration (from 0.0 to 0.2 M) were varied. In the presence of deionized water, the dryness of the sludge increases by 20%, from 62 to 74%. In general, the addition of salts of monovalent cations, such as MCl, where M=Li⁺, Na⁺, K⁺ and Rb⁺, causes an increase of the amount of water extracted from the sludge as long as the salt concentration is below 0.1 M. In the case of CsCl, the amount of water extracted decreases constantly with an increase in concentration. At the same concentration, the volume of water extracted is highest for cations with the largest hydrated radius: Li⁺> Na⁺ > K⁺ > Rb⁺ > Cs⁺. This series represents the opposite trend observed for the relative adsorption affinity of cations for the surface of the kaolin: Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺. Therefore, electrodewatering is favored in the presence of poorly adsorbed metal cations as a direct result of improvement of the electromigration. Moreover, for concentration in the range 0.0 to 0.2M, a charge increase on the cation of the chloride salts (NaCl, CaCl₂ et CeCl₃) results in a decrease in the volume of extracted water: Na⁺> Ca² ⁺> Ce³⁺, The adsorption of these cations to the kaolin surface is more significant for the cations with the highest charge: Na⁺ < Ca²⁺ < Ce³⁺. Once again, the electrodewatering is favored in presence of cations that do not adsorb to the kaolin surface. The electrodewatering of kaolin is also dependent on the pH that should be between 2 and 7. As a consequence, the addition of different sodium salts (NaCl, NaNO₃, NaClO₃, Na₂SO₄, NaHCO₃, NaOH, Na₂CO₃) causes a decrease of the extracted water if the pH of the electrolyte is higher than 8, which is the case of NaHCO₃, NaOH and Na₂CO₃.. In general, the extracted water decreases in the series: Cl⁻, NO₃⁻, ClO₃⁻> SO₄²⁻>> HCO₃⁻ >> OH⁻, CO₃²⁻. The energy spent in the electrodewatering process of kaolin mixed with deionized water is 0.9±0.1 kWh per kilogram of extracted water. However, 96% of the water is extracted in the first 10 minutes of electrodewatering with an energy consumption of 0.20±0.02 kWh/kg of extracted water, which corresponds roughly to 20% of the energy spent in the 60-minute experiments. Furthermore, the addition of the electrolyte solutions to kaolin causes an increase in the energy consumption. As an example, the addition of LiCl 0.2 M to kaolin leads to an increase of extracted water from 6.9 g without any salt to 8.0 g. At the same time the energy consumed is 1.1 and 2.1 kWh per kilogram of extracted water for the ten- and sixty-minute experiment, respectively. Therefore, the composition of sludge and the operation time are both crucial for the energy performance. The electrodewatering properties of kaolin are changed upon addition of a small amounts of xanthan gum (4.4 wt%). For example, a sludge composed of kaolin plus xanthan gum does not dewater by the application of pressure alone, which is the opposite of what happens with the kaolin sludge. The xanthan gum is helping to disperse the kaolin particles in solution, making the dewatering more difficult. Moreover, the total water extracted from kaolin plus xanthan gum sludge is 2.7 g, which is 2.6 times less water when compared to the water extracted from the kaolin sludge (7.2 g). All the salts of monovalent cations (MCl, with M=Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) have a similar effect on electrodewatering of kaolin plus xanthan gum sludge (Li⁺=Na⁺=K⁺=Rb⁺=Cs⁺) when compared at the same concentration. On the other hand, an increase of the charge of the metal cation results in an increase of extracted water: Na⁺ < Ca²⁺ < Ce³⁺. In this case, the amount of extracted water increases with an increase of the concentration of added salt (from 0 to 0.2 M). The final dryness is 74.2% when CeCl3 0.2 M is added, compared to 66.2% with deionized water. The presence of cations that adsorb strongly to the surface of the kaolin, such as Ce³⁺., facilitates the aggregation of the kaolin and xanthan gum particles, which results in an increase of electrodewatering. The replacement of chloride ion in NaCl was also studied. The amount of extracted water decreases in the series: Cl⁻ > NO₃⁻ > ClO₃⁻, SO₄²⁻ >> HCO₃⁻ > CO₃²⁻ > OH⁻. The electrodewatering of kaolin plus xanthan gum is favored by more acidic pH’s. The water lost by evaporation, as a result of the increase in temperature during the drying process, may represent more than 20% of the total extracted water. The energy consumed per kilogram of water extracted is 2.9±0.1 kWh in the presence of deionized water. The electrodewatering properties of municipal sludge from La Prairie, Quebec, were studied. This sludge has an initial dryness of 14.3%. Upon electrodewatering, the dryness of the sludge practically doubles (final dryness of 26±3%) and the initial volume is reduced by half. Twenty percent of the total extracted water is lost by evaporation. The energy consumed in the process is 0.29 kWh per kilogram of extracted water, which represents about four times less energy when compared to a thermal drying method. All monovalent cations have a similar effect on the electrodewatering of municipal sludge: Li⁺=Na⁺=K⁺=Rb⁺=Cs⁺. Furthermore, the +2 and +3 cations are more efficient in the dewatering of municipal sludge than +1 cations: Ce³⁺, Ca²⁺ > Na⁺. A bell shape curve is obtained for the dependency of extracted water with the amount of added metal cation. The highest dewatering is achieved at 4.4-7.5, 2.2-4.4, and 1.5-4.4 mmol of added, NaCl, CaCl₂ and CeCl₃, respectively. For example, the final dryness of the sludge reaches 42% upon addition of 4.4 mmol of NaCl. This represents a three times increase in dryness relative to the initial value. In these conditions, the volume of the sludge is reduced by 70%. For the optimal concentrations, the energy consumed per kilogram of extracted water is 0.32, 0.36-0.43 and 0.33-0.39 kWh for Na⁺, Ca²⁺ and Ce³⁺, respectively, values that are close to the one observed with deionized water. The energy consumption is still less than the case of thermal drying. Thus, electrodewatering of municipal sludge is an attractive method to treat municipal sludge residues and in fact the process may be successfully optimized by the addition of metal cations prior to treatment. Kaolin plus xanthan gum sludge and municipal sludge behave similarly in electrodewatering. The synthetic sludge is a useful tool that can be used in the understanding of the fundamental phenomena taking place in electrodewatering.

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