A.I. García

Dye adsorption by sewage sludge-based activated carbons in batch and fixed-bed systems.

The present research work deals with the production of activated carbons by chemical activation and pyrolysis of sewage sludges. The adsorbent properties of these sewage sludges based activated carbons were studied by liquid-phase adsorption tests. Dyes removal from colored wastewater being a possible application for sludge based adsorbents, methylene blue and saphranine removing from solution was studied. Pure and binary adsorption assays were performed in batch and fixed bed systems. In all cases studied, the adsorbents produced from sewage sludges were able to adsorb both the compounds considered here. Nevertheless, time required for reaching equilibrium, adsorptive capacity and fixed bed characteristic parameters were different for these two compounds. Methylene blue adsorption occurred faster than that of saphranine, and it was preferably adsorbed when treating binary solutions. It could be concluded that the sewage sludge-based activated carbons may be promising for dyes removal from aqueous streams.

Kinetic and equilibrium modelling of the methylene blue removal from solution by adsorbent materials produced from sewage sludges

Abstract Sewage sludge utilisation must be the preferred management solution for a residue which does not show the desired improvement on wastewater treatment. This research work’s aim was to study the dye binding capacity of adsorbents produced from sewage sludges. The quality of sewage sludges as starting materials has been investigated by using sludges both from an urban and from an agrofood industry wastewater treatment plant. Dried sewage sludges, pyrolysed sewage sludges and both chemically activated and pyrolysed sewage sludges have been used as adsorbent materials in single batch liquid-phase adsorption tests. The adsorption equilibriums of methylene blue by these materials have been described in terms of both Langmuir and Freundlich equations. In order to investigate the mechanisms of adsorption, the first- and second-order kinetic models have been used. All the sludge-derived adsorbents produced have been able to uptake methylene blue from solution, the second-order rate expression being preferred to the first-order one. Nevertheless, the time needed for reaching the equilibrium and adsorptive capacity have differed from one to another adsorbent. Equilibrium and kinetic results have showed that dried urban sewage sludges are the most efficient materials for removing the methylene blue from the solution.

Elimination of organic water pollutants using adsorbents obtained from sewage sludge

Sewage sludge valorisation must be the preferred management solution for a residue which minimising does not keep with a desirable improvement on wastewater treatment. This research work deals with the potential application of adsorbents produced from sewage sludge in organic pollutants removal. After chemical activation and pyrolysis treatment, sewage sludge provides materials of great porosity and high surface area. The properties of this type of material was studied by liquid-phase adsorption using crystal violet, indigo carmine and phenol as adsorbates, experiments being performed for two different adsorbent particle sizes. Firstly, single adsorption batch experiments were carried out to obtain kinetic and equilibrium data. The adsorbates preferential adsorption was then investigated by studying adsorption from mixed solutions. The sludge-derived activated carbon produced could adsorb the three adsorbates considered with the time required to reach equilibrium and full adsorptive capacity varying between the adsorbates. Crystal violet adsorption has been higher and faster than indigo carmine or phenol. It is proposed that activated carbons made from sewage sludge show promise for the removal of organic pollutants from aqueous streams.

Adsorption of heavy metals onto sewage sludge-derived materials.

Two materials were produced from sewage sludge by: (1) pyrolysis of dried sewage sludge (PS); (2) chemical activation of dried sewage sludge with ZnCl(2) followed by pyrolysis (AS). The aim was to study the application of these materials for metal purification from water and to determine the efficiency of each material. Although AS displayed higher capacity, both PS and AS were able to adsorb these metals and the preferential order was equal: Hg(II)>Pb(II)>Cu(II)>Cr(III). For each metal-adsorbent pair, metal adsorption was highly pH dependent. In all cases the equilibrium was well described both by the Langmuir and the Freundlich isotherms. At the corresponding optimum pH, AS showed the following adsorption capacities: 175.4, 64.1, 30.7 and 15.4 mg/g of Hg(II), Pb(II), Cu(II) and Cr(III), respectively. These results indicate the potential application of these sewage sludge based adsorbents for the treatment of metal polluted effluents.

Analysis of the co-combustion of sewage sludge and coal by TG-MS

Combustion of sewage sludge may be a viable solution for its management in some cases and so is its co-combustion with coal. The aim of the present paper is to study the behavior of three different sludges during combustion and the modifications of combustion parameters that take place when sludges are mixed with coal, all measurements being done by thermogravimetry. The combustion of pyrolyzed sewage sludges has also been studied, related to the possibility of combining combustion with pyrolysis when trying to valorize sludges by producing active carbon. From the burning profile data it has been possible to observe that a rapid devolatilization occurs during sludge combustion compared to coal and previously pyrolyzed sludge. The exact temperatures of devolatilization and of maximum speed of weight loss have also been ascertained together with differential thermal analysis and measurements of gas emissions during combustion. The behavior of the sludges tested is qualitatively similar but quantitatively different, so tests have to be carried out before they are put in the furnace.

Co-combustion of different sewage sludge and coal: A non-isothermal thermogravimetric kinetic analysis

The kinetics of the combustion of coal, two different sewage sludge and their blends (containing different dried weight percentages of sewage sludge) was studied by simultaneous thermogravimetric analysis. Once the weight percentage of sludge in the blend was 10%, the effects on the combustion of coal were hardly noticeable in terms of weight loss. The Arrhenius activation energy corresponding to the co-combustion of the blends was evaluated by non-isothermal kinetic analysis. This showed that, though differences between coal and sewage sludge, the combustion of their blends kept kinetically alike to that of the coal. This work illustrates how thermogravimetric analysis may be used as an easy rapid tool to asses, not only mass loss, but also kinetics of the co-combustion of sewage sludge and coal blends.

Thermogravimetry as a technique for establishing the stabilization progress of sludge from wastewater treatment plants

Stabilization processes are key to the reliable performance of any wastewater treatment plant. However, a simple, yet reliable and widely acceptable analytical tool to evaluate the degree of stability reached during sewage sludge treatment remains to be found. Mesophilic aerobic digestion has been carried out on laboratory scale in aerated reactors using non-stabilized sludges from three different wastewater treatment plants. Temperature programmed combustion tests were carried out to investigate the stabilization degree of samples throughout the lab treatment. Differential thermal analysis (DTA) has shown that energy release during sludge combustion shifts to higher temperatures as stabilization advances. The integration of the DTA curve gives an area which is proportional to the energy released so it has been possible to quantify this shift by representing the percentage of the cumulative area at each temperature along each sample combustion. Although the initial composition of the sludge should be considered in each particular case, this preliminary study shows that thermogravimetry may offer a means of ascertaining whether an acceptable degree of stability has been reached by aerobic digestion.

Upgrading sewage sludges for adsorbent preparation by different treatments

Addressing the adequate management of sludges produced at sewage plants is becoming a fundamental need as a consequence of the high production volumes, both current and forecasted, of this byproduct. European waste-treatment policies consider reuse of sludges as one of the preferred actions; along those lines this study proposes using sewage sludges as adsorbents for pollutants contained in wastewaters. As potential adsorbents, sludges dried at 105 degrees C, dried and pyrolyzed, or dried and chemically activated were tried. As adsorbate, methylene blue was used in order to characterize the adsorption capacity of the different materials. Although surface areas corresponding to pyrolyzed and chemically activated sludges were around 80 and 390 m2/g, respectively, both these materials exhibited poor levels of methylene blue adsorption which may have been due to their high proportions of micropores. Sludges only dried, on the contrary, showed significant methylene blue adsorption capacities.

Heating process characteristics and kinetics of rice straw in different atmospheres

Rice growing engenders large quantities of straw, which is usually burnt in the open field, a practice implying a public health risk. There are, however, several ways of making use of this material, including its conversion into a valuable fuel. The aims of this research are to describe the thermal characteristics of the heating processes of rice straw in different atmospheres of air and nitrogen and to fit the heating profiles to two mathematical models proposed. This research was followed by thermogravimetric analysis, visual observation of the heating profiles revealing three stages in the rice straw heating process: drying, devolatilization and burning. The models described were found to adequately describe the weight loss of rice straw. The approximate integral method (AIM) is less complex than the direct method (DM), but the latter shows the order of reaction. A better fit is obtained for the heating atmospheres with a lesser proportion of oxygen. The presence of oxygen increases the activation energy and decreases the final temperature of each stage considered.

Gasification of rice straw in a fluidized-bed gasifier for syngas application in close-coupled boiler-gasifier systems.

The feasibility and operation performance of the gasification of rice straw in an atmospheric fluidized-bed gasifier was studied. The gasification was carried out between 700 and 850 °C. The stoichiometric air-fuel ratio (A/F) for rice straw was 4.28 and air supplied was 7-25% of that necessary for stoichiometric combustion. Mass and power balances, tar concentration, produced gas composition, gas phase ammonia, chloride and potassium concentrations, agglomeration tendencies and gas efficiencies were assessed. Agglomeration was avoided by replacing the normal alumina-silicate bed by a mixture of alumina-silicate sand and MgO. It was shown that it is possible to produce high quality syngas from the gasification of rice straw. Under the experimental conditions used, the higher heating value (HHV) of the produced gas reached 5.1 MJ Nm(-3), the hot gas efficiency 61% and the cold gas efficiency 52%. The obtained results prove that rice straw may be used as fuel for close-coupled boiler-gasifier systems.