Enhancement of the electrodewatering properties of
\nsynthetic and municipal sludges by the addition of
\nmetal cations.

Electrodewatering consists on the application of both pressure (mechanical effect) and \nelectrical current (electrokinetic phenomena) to colloidal systems, such as municipal sludge, in \norder to achieve higher solid content (also designated by dryness). The influence of metal cations \nand its salts on the electrodewatering characteristics was systematically studied for municipal \nsludge and for two potential synthetic sludges, one composed of kaolin and the other of kaolin \nplus xanthan gum. The effect of the size, charge and concentration of the metal cations were \nstudied in the three types of sludge. The effect of pH was also briefly addressed. The results were \nexplained by the selective adsorption of metal cations to the surface of the dispersed particles. \nThe adsorption was studied by ICP-AES (Inductively Coupled Plasma with Atomic Emission \nSpectrometer) metal analysis. This project was performed in collaboration with Ovivo. \nThe first type of synthetic sludge was prepared by mixing kaolin powder with an \nelectrolyte, where the nature of the salt and its concentration (from 0.0 to 0.2 M) were varied. In \nthe presence of deionized water, the dryness of the sludge increases by 20%, from 62 to 74%. In \ngeneral, the addition of salts of monovalent cations, such as MCl, where M=Li⁺, Na⁺, K⁺ and \nRb⁺, causes an increase of the amount of water extracted from the sludge as long as the salt \nconcentration is below 0.1 M. In the case of CsCl, the amount of water extracted decreases \nconstantly with an increase in concentration. At the same concentration, the volume of water \nextracted is highest for cations with the largest hydrated radius: Li⁺> Na⁺ > K⁺ > Rb⁺ > Cs⁺. \nThis series represents the opposite trend observed for the relative adsorption affinity of cations \nfor the surface of the kaolin: Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺. Therefore, electrodewatering is \nfavored in the presence of poorly adsorbed metal cations as a direct result of improvement of the \nelectromigration. Moreover, for concentration in the range 0.0 to 0.2M, a charge increase on the \ncation of the chloride salts (NaCl, CaCl₂ et CeCl₃) results in a decrease in the volume of \nextracted water: Na⁺> Ca² ⁺> Ce³⁺, The adsorption of these cations to the kaolin surface is more \nsignificant for the cations with the highest charge: Na⁺ < Ca²⁺ < Ce³⁺. Once again, the \nelectrodewatering 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 \nand 7. As a consequence, the addition of different sodium salts (NaCl, NaNO₃, NaClO₃, Na₂SO₄, \nNaHCO₃, NaOH, Na₂CO₃) causes a decrease of the extracted water if the pH of the electrolyte is \nhigher than 8, which is the case of NaHCO₃, NaOH and Na₂CO₃.. In general, the extracted water \ndecreases in the series: Cl⁻, NO₃⁻, ClO₃⁻> SO₄²⁻>> HCO₃⁻ >> OH⁻, CO₃²⁻. \nThe energy spent in the electrodewatering process of kaolin mixed with deionized water \nis 0.9±0.1 kWh per kilogram of extracted water. However, 96% of the water is extracted in the \nfirst 10 minutes of electrodewatering with an energy consumption of 0.20±0.02 kWh/kg of \nextracted water, which corresponds roughly to 20% of the energy spent in the 60-minute \nexperiments. Furthermore, the addition of the electrolyte solutions to kaolin causes an increase in \nthe energy consumption. As an example, the addition of LiCl 0.2 M to kaolin leads to an increase \nof extracted water from 6.9 g without any salt to 8.0 g. At the same time the energy consumed is \n1.1 and 2.1 kWh per kilogram of extracted water for the ten- and sixty-minute experiment, \nrespectively. Therefore, the composition of sludge and the operation time are both crucial for the \nenergy performance. \nThe electrodewatering properties of kaolin are changed upon addition of a small amounts \nof xanthan gum (4.4 wt%). For example, a sludge composed of kaolin plus xanthan gum does not \ndewater by the application of pressure alone, which is the opposite of what happens with the \nkaolin sludge. The xanthan gum is helping to disperse the kaolin particles in solution, making the \ndewatering more difficult. Moreover, the total water extracted from kaolin plus xanthan gum \nsludge is 2.7 g, which is 2.6 times less water when compared to the water extracted from the \nkaolin sludge (7.2 g). \nAll the salts of monovalent cations (MCl, with M=Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) have a \nsimilar effect on electrodewatering of kaolin plus xanthan gum sludge (Li⁺=Na⁺=K⁺=Rb⁺=Cs⁺) \nwhen compared at the same concentration. On the other hand, an increase of the charge of the \nmetal cation results in an increase of extracted water: Na⁺ < Ca²⁺ < Ce³⁺. In this case, the amount \nof extracted water increases with an increase of the concentration of added salt (from 0 to 0.2 \nM). The final dryness is 74.2% when CeCl3 0.2 M is added, compared to 66.2% with deionized \nwater. 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 \nof electrodewatering. \nThe replacement of chloride ion in NaCl was also studied. The amount of extracted water \ndecreases in the series: Cl⁻ > NO₃⁻ > ClO₃⁻, SO₄²⁻ >> HCO₃⁻ > CO₃²⁻ > OH⁻. The \nelectrodewatering of kaolin plus xanthan gum is favored by more acidic pH’s. The water lost by \nevaporation, as a result of the increase in temperature during the drying process, may represent \nmore than 20% of the total extracted water. The energy consumed per kilogram of water \nextracted is 2.9±0.1 kWh in the presence of deionized water. \nThe electrodewatering properties of municipal sludge from La Prairie, Quebec, were \nstudied. This sludge has an initial dryness of 14.3%. Upon electrodewatering, the dryness of the \nsludge practically doubles (final dryness of 26±3%) and the initial volume is reduced by half. \nTwenty percent of the total extracted water is lost by evaporation. The energy consumed in the \nprocess is 0.29 kWh per kilogram of extracted water, which represents about four times less \nenergy when compared to a thermal drying method. \nAll monovalent cations have a similar effect on the electrodewatering of municipal \nsludge: Li⁺=Na⁺=K⁺=Rb⁺=Cs⁺. Furthermore, the +2 and +3 cations are more efficient in the \ndewatering of municipal sludge than +1 cations: Ce³⁺, Ca²⁺ > Na⁺. A bell shape curve is obtained \nfor the dependency of extracted water with the amount of added metal cation. The highest \ndewatering is achieved at 4.4-7.5, 2.2-4.4, and 1.5-4.4 mmol of added, NaCl, CaCl₂ and CeCl₃, \nrespectively. For example, the final dryness of the sludge reaches 42% upon addition of 4.4 \nmmol of NaCl. This represents a three times increase in dryness relative to the initial value. In \nthese conditions, the volume of the sludge is reduced by 70%. For the optimal concentrations, the \nenergy consumed per kilogram of extracted water is 0.32, 0.36-0.43 and 0.33-0.39 kWh for Na⁺, \nCa²⁺ and Ce³⁺, respectively, values that are close to the one observed with deionized water. The \nenergy consumption is still less than the case of thermal drying. Thus, electrodewatering of \nmunicipal sludge is an attractive method to treat municipal sludge residues and in fact the \nprocess may be successfully optimized by the addition of metal cations prior to treatment. \nKaolin plus xanthan gum sludge and municipal sludge behave similarly in \nelectrodewatering. The synthetic sludge is a useful tool that can be used in the understanding of \nthe fundamental phenomena taking place in electrodewatering.

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