PYROLYSIS OF SEWAGE SLUDGE FROM THE WASTEWATER TREATMENT PLANT IN BOGOTÁ (COLOMBIA)

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1 Proceedings of the 13 th International Conference on Environmental Science and Technology Athens, Greece, 5-7 September 2013 PYROLYSIS OF SEWAGE SLUDGE FROM THE WASTEWATER TREATMENT PLANT IN BOGOTÁ (COLOMBIA) S.L. RINCÓN 1, A.G. GÓMEZ 1 and M.P. ARAGONEZ 2 1 UniversidadNacional de Colombia - Sede Bogotá- Facultad de Ingeniería Departamento de Ingeniería Mecánica y Mecatrónica - Grupo de Investigación en Biomasa y Optimización Térmica de Procesos - Carrera 30 Calle 45-03, Bogotá, Código Postal Colombia 2 UniversidadNacional de Colombia - Sede Bogotá- Facultad de Ingeniería Departamento de Ingeniería Química y Ambiental - Grupo de Investigación en Biomasa y Optimización Térmica de Procesos- Carrera 30 Calle 45-03, Bogotá, Código Postal Colombia EXTENDED ABSTRACT The aim of this work is to study the Sewage Sludge pyrolysis. For this purpose the raw material was characterized and pyrolysis experiments were done in two different devices. The characterization of sewage sludge included proximate and elemental analysis (determination of carbon, hydrogen, nitrogen and sulfur), and the determination of the high calorific value. An analysis of the sewage sludge composition by the determination of iron, calcium, magnesium, sodium, potassium, cadmium, arsenic, nickel and mercury by Atomic Absorption Spectroscopy is also done. In addition the sewage sludge and its ashes were analyzed by X-Ray Diffraction for the determination of crystalline compounds, and finally a study of elements and oxides was studied by X-Ray Fluorescence. First, pyrolysis is done according to the International Standard ISO 647 Brown coals and lignites determination of the yields of tar, water, gas and coke residue by low temperature distillation. The product fractions were determined and characterized. The solid product is characterized by the same methods as the raw material. The bio oil was characterized by elemental analysis and determination of the calorific value. The second part of this study was the pyrolysis in a thermobalance. Essays were performed with variation of the particle diameter, the material quantity and the heating rate. In all experiments nitrogen was used as inert gas. The principal fraction of pyrolysis products was liquid (56%). And the other fractions were solid (26%) and gas (18%).The distribution of the calorific value for liquid, solid, and gas products were 53%, 12%, 35% respectively (all values are free of water and ash). It was found that the pyrolysis products could be used as an energy source. KEYWORDS: Sewage Sludge, Pyrolysis, Characterization. 1. INTRODUCTION Sewage treatment plants process contaminated water with the aim of producing usable water. The by-products of this process are gases, mainly composed of methane and carbon dioxide, and solids. The solid product called sewage sludge or biosolids contains organic material, mineral nutrients and heavy metals. Large quantities of biosolids are produced in the world. Bogota is the city with the largest biosolids production in Colombia, approximately 130 ton/day [1]. Currently this side product is used to restore damaged lands. A limitation of this final disposal is the possible lixiviation and the transport of heavy metals to water and plants. It is necessary to search a new way to take advantage of the properties of the Sewage Sludge by means of a clean process which does not affect the environment. The pyrolysis process is a thermal treatment under inert atmosphere until 550 C. The pyrolysis has demonstrated multiple benefits, such as the reduction of the volume of

2 sewage sludge, the maintenance of the nutrients in the char and the inactivation of the heavy metals. In addition solid, gas and liquid fuel products are obtained. In the present work the physicochemical characterization of biosolids produced in a sewage treatment plant in Bogotá and its behavior during pyrolysis in laboratory mass scale is performed. Moreover a physicochemical characterization of the pyrolysis products is done, so that the influence of the pyrolysis process in the obtained products is determined. Decomposition profiles of the sewage sludge were obtained by thermogravimetric analysis (TGA).This information plays an important role in the determination of the pyrolysis kinetics, the design of a pyrolysis reactor and its optimization. 2. CHARACTERIZATION 2.1. Wastewater sludge materials Digested wastewater sludge samples of Bogotá-Colombia were analysed. All the samples were collected from an urban wastewater treatment plant where the sewage comes mainly from domestic sources. The sewage sludge has high initial moisture. In a previous study, a similar sample had moisture of 67.7 % [2]. It was necessary to dry the raw material for a better handling; the drying of 7kg of material was done at 105 C for three hours in a rotary kiln. Then the raw material was milled to a particle size less than 1 mm Proximate analysis The proximate analysis included the determination of moisture, ash, and volatile matter contents. All experiments were done according to DIN standards. All measurements were performed in the Engineer Faculty of the National University of Colombia. The results are shown in Table 1. The moisture content of sewage sludge (after the dry in the rotary kiln) was determined according to DIN CEN/TS :2004. This standard proposes a method for determination of moisture of solid biofuel. Samples of less than 1g were dried at 105 C until constant weight using an electric furnace type 1300 Thermolyne. The determination of the ash content was done according to the standards DIN CEN/TS 14775:2004 for solid biofuel and DIN for solid mineral fuel. The difference between the standards is the temperature of the ash synthesis. The experiments were done using a furnace LINDBERG Type The ash content for the first standard (at 550 C) was 52.11% and the ash content for ash obtained at 850 C according with the second standard was 49.4% both values are free of water. Finally the determination of volatile matter in biosolids was done according to DIN for solid fuels Elemental analysis The elemental analysis included the determination of carbon, hydrogen and nitrogen using an Elemental Analyzer. The sulfur was determined by Atomic Absorption Spectroscopy and oxygen was calculated by difference. The results in dry basis are shown in Table Determination of gross calorific value The gross calorific value at constant volume in dry basis was determined according to DIN The measurements were obtained using a bomb calorimeter. Also, the gross calorific value was calculated using the correlation of Boie [3].

3 Table 1. Results of the Sewage Sludge characterization Elemental analysis /% Proximate analysis /% Calorific Value / MJ/kg c dab h dab n dab o dab s dab w VM db A db HHV,db LHV,db W, moisture; VM, volatile matter; A, ash content at 550 C; dab, free of water and ash; db, dry basis; HHV, higher heating value; LHV, lower heating value Chemical composition Determination of elements in biosolids by Atomic Absorption Spectroscopy Taking into account the different composition of sewage sludge depending on the origin, it was pertinent to determinate the content of heavy metals and nutrients for the soil relevant when a research with biosolids is done. The quantification of those elements was done by Atomic Absorption Spectroscopy. Table 2 shows the obtained contents of Fe, Si, Ca, Na, S, Al, P, K, Zn, Mg, Mn, Cu, Cr, Ni As, Hg, Cd and Pb. Table 2. Results of the analysis by Atomic Absorption Spectroscopy Elemen Fe Si Ca Na S Al P K Zn t Ω db / % Elemen t Ω db / % Mg Mn Cu Cr Ni As Hg Cd Pb Ω, mass fraction; db, free of water; N.D., not detected The element found in greatest proportion is iron, followed by silicon, calcium, sodium, aluminum and sulfur. Cadmium and lead were below the detection limits (Cd 0.01ppm and Pb 0.1 ppm) Composition of biosolids and its ashes Due to the high ash content in sewage sludge and the different ash contents at 550 C and 815 C. It was necessary to realize a special study about the ashes and the biosolids. Ashes were obtained at different temperatures and subsequently the ashes and the sewage sludge were analyzed by X-Ray Fluorescence and X-Ray Diffraction Ashes at different temperatures First, the ash content at different temperatures was determined according to the general condition of the standard DIN EN 14775:2004 but changing the final temperature to 550 C, 700 C, 815 C, 900 C and 1000 C. Table 3 shows the obtained results. Table 3. Ash content at different temperatures T / C A db / % A, ash content; db, free of water An increase in the temperature at which the ashes were determined causes a decrease in the percentage of ash of biosolids. This is possibly due to reaction between the ash components at experimental conditions. ND ND

4 X-Ray Fluorescence analysis (FRX) The results of the analysis by X-Ray Fluorescence presented in Table 4 indicate that the major elements in biosolids are iron, silicon, calcium, sulfur, aluminum and potassium. In a lower proportion titanium, potassium, zinc, magnesium, chloride, sodium and barium are present. Additionally traces of copper, manganese, zirconium, strontium, chromium, cerium, lead, rubidium and yttrium were detected. Table 4. Results of the sewage sludge analysis by X-Ray Fluorescence. Element Fe Si Ca S Al P Ti K Zn Mg Cl Ω db / % Element Na Ba Cu Mn Zr Sr Cr Ce Pb Rb Y Ω db / % Ω, mass fraction; db, free of water; N.D., not detected The proportion of the elements determined by X-Ray Fluorescence is similar to the determined by Atomic Absorption Spectroscopy. The differences between the specific values are due to the fact that the X-Ray Fluorescence is a semiquantitative technique, and it shows a proportion between components, not exact concentrations. The high presence of iron in the sewage sludge is possibly due to the contribution of iron from mineral material as sediments, soil, sand and stones. Silicon basically comes from mineral material, specifically sand and rocks. Other possible source of iron is the addition of ferric chloride as coagulant agent in the treatment of wastewater treatment plant. When ferric chloride (FeCl 3) is added to the residual water is dissociated into ions, Fe 3 + ion reacts with the OH - ions of the water, and precipitated as iron hydroxide (III) [4]. Silicon basically comes from mineral material, specifically sand and rocks. The same elements detected in the biosolids by X-Ray Fluorescence were detected in the ashes, but the proportion of that depends on the process temperature. Figure 1 shows the comparison between the composition of sewage sludge and ashes in percentage Sewage Sludge Ash 550 C Ash 700 C Ash 815 C Ash 900 C Ash 1000 C 15 db / % Fe Si Ca S Al P Element

5 Figure 1. Percentage of major elements in sewage sludge and its ashes Under the experimental conditions of this study, when the process temperature increases the concentration of iron, silicon, calcium, aluminum and potassium in the ash increases too. However this does not occur for sulfur. Sulfur can be released to the atmosphere by decomposition of sulfates in form of oxides of the respective metal. Also, sulfides can produce sulfur oxides, for example with temperature increase of 440 C to 666 C the iron disulfide in the presence of oxygen produces iron sulfide and sulfur oxides [5] X-Ray Diffraction analysis (DRX) The sewage sludge and the ashes were analyzed through X-ray diffraction. The diffractograms of biosolids and ash are shown in Figure 2. Ash 1000 C Ash 900 C Ash 815 C Ash 700 C Ash 550 C Sewage Sludge Conventions Ca(Al 2 Si 2 O 8 ) Intensity / CPS SiO 2 Fe 3 O 4 Fe 3 S 4 MgO Si P 2 O 7 Fe 2 O 3 Al 2 O 3 CaO Ca 3 (PO 4 ) 2 AlPO / Figure 2. Diffractograms of biosolids and ash The results of the DRX Analysis are according to the results of FRX. Among the crystalline compounds identified the quartz was detected with a greater intensity. It is possibly due to the existence of that compound in sand or other minerals that come from the Bogota River and were not removed in the sewage treatment plant. In the literature it was found that it is possible to find mineral quartz in sand samples [6] [7]. Also iron, calcium, aluminum, phosphorus and magnesium oxides were identified. According to the analysis of the diffractograms and the literature, the possible presence of greinite (F 3S 4), silicon pyrophosphate, calcium phosphate and aluminum phosphate were found. In addition a mineral called anorthite (CaAl 2Si 2O 8) was identified.

6 3. PYROLYSIS 3.1. Low temperature pyrolysis by the standard ISO 647 First, pyrolysis was done according to the International Standard ISO g of sewage sludge were processed by low temperature distillation. The principal fraction of pyrolysis products was liquid (56%). And the other fractions were solid (26%) and gas (18%). All percentages are in a free of water and ash basis. The calorific value for each product was determined and the distribution of the energy content of the products was calculated. The calorific value of the products is shown in Table 5 in free of water and ash basis. The gas has a higher calorific value than the solid. The major percentage of the distribution of the calorific value corresponds to the liquid (53%), follow by de gas (35%) and finally the solid (12%) all value in free of water and ash basis. Table 5. Calorific value of products Liquid db /MJ/kg Solid db /MJ/kg Gas db /MJ/kg Db,Dry basis 3.2. Thermogravimetry A thermogravimetry analysis was done using nitrogen as carrier gas. The experimental conditions were selected taking into account the minimization of effects of mass transfer, heat transfer and secondary reactions. 1.7 l/min of Nitrogen and 950 C of final temperature were used in all experiments. The results of the variation of the material quantity and particle diameter show that it exist a bigger influence of the grain size than of the amount of that material in the process Variation of the heating rate. Considering the conditions previously cited and using 0.5g of sewage sludge with a particle diameter between 0.15 and 0.18 mm, experiments at 3K/min, 5K/min and 10K/min were done. The results of the experiments are presented in Figures 3 and 4. The heating rate has a considerable effect on the sewage sludge pyrolysis. The final fraction of non-decomposed material with variation of the rate is less than the fraction obtained with variation of the particle diameter. When the heating rate decreases the curve is displaced to the left. Possibly this is due to the higher time of reaction. Mass loss dab / 1 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0, Temperature / ºC 3K/min 5K/min 10K/min d dab/ dt / 1/K 0,0010 0,0005 0,0000-0,0005-0,0010-0,0015-0,0020-0,0025-0,0030-0,0035-0, Temperature / ºC 3K/min 5K/min 10K/min Figure 3. Mass loss of biosolids during the pyrolysis. Figure 4. Variation of mass loss with temperatu

7 4. CONCLUSIONS Considering the characterization of sewage sludge it was found that the major elements present in the sewage sludge are iron, silicon, calcium, sodium and sulfur. All these elements except the sulfur exist in its ashes. The sulfur is a volatile element that is released to the atmosphere under the conditions of the experiments. Sewage sludge of Bogotá has traces of heavy metals, copper, chromium, nickel, arsenic, mercury and lead. Between the regulated heavy metals only selenium was not identified, however the sewage sludge analised has zinc, copper and chromium in concentrations above the fixed limits by the European Union for the use of biosolids as fertilizer in agriculture. The remaining elements (As,Cd,Hg,Pb,Zn) are present in concentrations less than the limits. The high presence of iron and silicon in the sewage sludge is possibly due to the presence of traces of mineral materials as sediments, soil, sand and stones in the sewage sludge. The results of DRX are according with de results of FRX. Minerals as quartz, greinite and anorthite were detected by R-XDifraction. The higher fraction of the pyrolysis products corresponded to the liquid ones (56%), followed by the solid one (26%). The liquid has a calorific value of 34 MJ/kg. The gas (17.5 MJ/kg) has a higher calorific value than the solid one (11MJ/kg). All values in basis free of water and ash. A variation of the particle diameter and of the heating rate during pyrolysis have a bigger effect on the pyrolysis results than the variation of the material quantity. REFERENCES 1. Dáguer, G.P. GESTIÓN DE BIOSÓLIDOS EN COLOMBIA. In Congreso internacional ACODAL, Mendoza, L.E.G., Pirólisis de biosólidos y degradación de sus alquitranes, in Facultad de Ingeniería. Universidad Nacional de Colombia: Bogotá D.C., 2010 p Boie, W., Vom Brennstoff zum Rauchgas: Feuerungstechnisches Rechnen mit Brennstoffkenngrässen und seine Vereinfachung mit Mitteln der Statistik Ghaly, A.S., Treatment of grease filter washwater by chemical coagulation. American Journal of Environmental Sciences, 2007: p Pelovski, Y. and V. Petkova, Investigation on Thermal Decomposition of FeS2 and BaO2 Mixtures Part II. Journal of Thermal Analysis and Calorimetry, (1): p Almanza Montero, E.E.C.T., R. Cogollo Pitalúa, Caracterización De Arena Para Su Posible Uso Como Dosímetros De Radiación por EPR. Revista Colombiana de Física, p Padmakumar, G.P., et al., Characterization of aeolian sands from Indian desert. Engineering Geology,