BURNING SEWAGE SLUDGE FROM A MUNICIPAL WASTE WATER TREATMENT PLANT THE MIGRATION OF METALS INTRODUCTION

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1 bubbling fluidised bed combustor heavy metals, sewage sludge Witold ŻUKOWSKI * Jerzy BARON * Sylwia CHRUPEK ** Małgorzata PILAWSKA ** * Institute of Inorganic Chemistry and Technology, Cracow University of Technology, Cracow, Poland ** Institute of Thermal Engineering and Air Protection, Cracow University of Technology, Cracow, Poland BURNING SEWAGE SLUDGE FROM A MUNICIPAL WASTE WATER TREATMENT PLANT THE MIGRATION OF METALS The migration of selected heavy metals has been analysed when sewage sludge from a municipal waste water treatment plant was burned in a bubbling fluidised bed. It has been shown that manganese, lead, arsenic, mercury and cadmium, as opposed to chromium, nickel and copper, leave the reactor mainly via the gas phase. To reduce the emissions of heavy metals to the atmosphere associated with the thermal utilization of sewage sludge requires more effective flue gases cleaning than is possible with filters. INTRODUCTION Progress, increasing urbanization and growing awareness of the need to protect the environment create the need of building more waste water treatment plants. This leads to increased amounts of sewage sludge. In the year 2000 Poland produced about tons (dry mass) of sewage sludge [1]. It has been estimated that by 2014 the rate of production of the sludge will rise to tons p.a. Because of the character of the wastes treated, sewage sludge from municipal waste water treatment plants differs from that from industrial ones and also from those of mixed character. The sludge produced in mains water treatment plants differs from all of these. The source or sources from which waste waters are derived determine both their quantity and character and hence also the mass and composition of the sludge. The data collected in Table 1 indicate that increased concentrations of metals are characteristic of sewage sludge from in industrialised areas (Stalowa Wola, Częstochowa). The extent of urbanisation appears to be less important (Kołobrzeg). Waste water treatment, independently of the method used, leads to high concentrations of pollutants in the sewage sludge. This is particularly important if biological utilization of the sludge is envisaged [2]. In spite of controls, in areas where sewage sludge has been employed as a fertiliser, the concentrations of heavy metals in the soil have increased. Burning the sewage sludge, possibly with energy recovery, would help to prevent further soil contamination. Most of the heavy metals present in waste waters are derived from industrial effluents. Particularly noxious waste streams come from the production of batteries 781

2 Units Unieście (raw sludge) [1] Kołobrzeg [1] Wysokie Mazowieckie [1] Częstochowa [1] Świlcza [3] Jasienica Rosielna [3] Stalowa Wola [3] and dry batteries, metal etching, electroplating, inorganic pigments, artificial fibers and leather tanning. Tab. 1. Physico-chemical characteristics of sewage sludge from areas differing in the degree of urbanisation and industrialisation ph moisture % organic matter % total N % * 6.6 * 3.27 * total P % * 4.66 * 5.77 * Pb mg/kg * < Cu mg/kg * none Ni mg/kg * < Cr mg/kg * Zn mg/kg * Hg mg/kg * Cd mg/kg * sampling, year * in dry mass From the economic point of view, the elimination of metals from industrial effluents would be justified at the point of origin, when the streams to be treated are much smaller than those of waste waters at a municipal plant. Metals can be removed, or partly removed, using various physico-chemical processes (precipitation, ionic exchange, inverse osmosis etc.) as well as bio-technological ones. Although such methods are in use, the concentrations of metals in the waste waters reaching treatment plants are often elevated. As a rule, the sewage sludge produced during waste water treatment comprises about 34% of the cadmium discharged, 57% of the chromium, 66% of copper, 61% of lead, 35% of nickel and 56% of zinc [ 4]. During the thermal utilisation of sewage sludge by burning, the final products consist of clean flue gases, liquid wastes generated in wet scrubbing of the flue gases and mineralised solids (ash, fly ash) containing most of the metallic elements from the sludge. 782

3 COMBUSTION OF SEWAGE SLUDGE Sewage sludge Flue gas Pilot flame Flue gas analyzer Temp. registration Fly ash Heat loss control and cooling blower Thermocouple Distributor Wind box Gas fuel Air Fig. 1. Scheme of the fluidised bed reactor with the associated control and measuring systems Experimental tests on the combustion of sewage sludge from a municipal water treatment plant were carried out using a laboratory size 5 kw bubbling bed fluidised bed combustor. The system is fully described in a number of publications [5, 6] and a schematic of the reactor is shown in Fig. 1. It consists of a cylindrical section of a quartz tube, 400 mm long and 98 mm in diameter, resting on a flat distributor plate, made of Cr-Ni steel, with regularly spaced holes occupying 1.8% of the total area. Solid fuels are added from the top, from a disc feeder. A small pilot flame directed downwards is used for ignition. 783

4 The combustor was started on natural gas, with a bed of sand. After the desired temperature was reached, the sludge feeder was started and the combustible gas supply reduced, until fuel replacement was complete. The combustor was then run on sludge alone, with the air excess coefficient between The bed temperature was measured using a thermocouple placed 50 mm above the distributor. Under laboratory conditions de-ashing consisted of using a solids probe to withdraw bed material at suitable intervals, so as to keep the bed height constant. Before the flue gases were exhausted to a flue duct, they passed through an ash trap and a cyclone where the elutriated solids were captured. Tab. 2. Summary of measurements and analytical methods used 1. Analysis flue gases a. ECOM SG Plus analyser Method electrochemical sensors Concentrations determined O 2, CO, NO, NO 2, SO 2 Accuracy O 2, 0.1 % Accuracy CO, NO, NO 2, SO 2 1 ppm b. AWE S analyser Method FID Concentrations determined total VOCs Accuracy 0.1 mg/m 3 c. Infralyt 4 analyser Method NDIR Concentration determined CO 2 Accuracy 0.1 % 2. Temperature Method Measuring system thermoelement NiCr-Ni analogue-numerical converter ECOM 3. Chemical analysis, metals Method ICP/MS Error limits 5-10 % The flue gases were sampled in the freeboard space and were analysed on-line for O 2, NO, NO 2, CO, CO 2, SO 2 and total volatile organic compounds, VOCs. All of the analytical methods used are summarised in Table 2. The metals determined were 784

5 those for which the maximum permissible concentrations are specified in environmental regulations. The sewage sludge burned was air dry and came from three areas, differing in the degree of industrialization and was collected at different times. The proportions of combustibles present in the samples were very similar, but there were considerable differences in the concentrations of metals (Table 3). The sludge collected at the earliest date, from Myślenice, contained Cr at concentrations over an order of magnitude higher than in the other two sludges. This is because the Myślenice area is well known for a large number of very small tanneries, and it has always been difficult to enforce environmental standards and to monitoring the effluents produced. The prominent presence of lead is a result of the use of lead tetraethyl as an additive to petrol. This additive is now being phased out the other samples of sewage sludge had been collected when most of the petrol sold was already Pb-free. In the sludge from Lublin nickel and cadmium are high considerable quantities of these metals were used in the motor industry and in electroplating. The concentrations of metals, except for copper, are lowest in the sludge from Niepołomice, a small town near Kraków, with relatively little industrialisation. Tab. 3. The concentrations of selected heavy metals in dry sewage sludge, (mg/kg) Metal Sewage sludge Myślenice 1988 Sewage sludge Lublin 2002 Sewage sludge Niepołomice, 2003 Mn Cu Pb Cr Ni As Hg Cd DISCUSSION Burning of air-dry sewage sludge in a bubbling fluidised bed combustor is broadly similar to that of coal. Other independent measurements have shown that, on the average, the sludges contained 30.35% (mass) of non-combustible components, while in the coal used for the comparison the proportion was under 20%.. Figure 2 785

6 t, oc gases. Fig. 5ThefluidizedbedtemperatureandtheconcentrationofCO, NOx, SO2, VOCintheflue 1000 Sewage sludge Niepołomice 200 kg/m 2 h CO, mg/m CO time, s NO x, mg/m 3 SO 2 SO 2, VOC, mg/m VOC time, s Fig. 2. Bed temperature and the concentrations of selected components of the flue gases, normalised to 11%vol. O 2, during the combustion of sewage sludge from Niepołomice 786

7 illustrates the changes in bed temperature and the composition of the flue gases (normalised to 11% vol. O 2 ) while the sludge from the municipal waste water treatment plant at Niepołomice was burned. During this run the stream of sludge fed in (per unit distributor area) was 200 kg/m 2 h. Changes in the bed temperature within the range o C did not appear to cause any major changes in the flue gases concentrations of CO, NO x, SO 2 and VOCs. Detailed analysis shows [7] that the quality of combustion is more strongly (but not to a degree that is statistically significant) influenced by uncontrollable fluctuations in the air excess coefficient, due to the polydisperse and inhomogeneous character of the sludge fuel. Table 4 gives the mean bed temperature, the air excess coefficient and the normalised concentrations of the components of the flue gases registered during the test. These are compared with those recorded during the combustion of coal dust. The concentrations of the oxides of sulphur and of nitrogen are higher with the sludge because both S and N are present in the sludge in chemically combined form, in various substances of biological origin (such as proteins). The solids analysed for selected metals came from the bed after the reactor had cooled, fly ash was collected from the cyclone and elutriated particles were captured in the ash trap. The bed material was also sampled during operation, directly from the combustor, via a solids probe (working at reduced pressure). The results obtained for the sludge from Niepołomice are set out in Table 5. Tab. 4. The experimental conditions used and the mean, normalised (to 11% O 2 ) composition of the flue gases. Comparison of the combustion of sewage sludge from Niepołomice and coal dust, coal class 23/200/08 from the mine Wujek Sewage sludge Coal dust Mean Standard deviation Mean Standard deviation Bed temperature, o C Air excess coefficient Actual conc. of O 2, % Normalised conc. of CO 2, g/m CO, mg/m NO, mg/m NO 2, mg/m SO 2, mg/m VOC, mg/m

8 Tab. 5. The concentrations of selected heavy metals (mg/kg, dry mass) in the sewage sludge from the waste water treatment plant Niepołomice and in the bed material from the combustor, ash from the trap and fly ash from the cyclone Sewage Metal Bed Ash trap Fly ash sludge Mn Cu Pb Cr Ni As Hg Cd The concentrations expected in the ash were calculated from the analysis results, taking into account the ash content of the dry sludge and assuming all the metals stay in the ash. In addition, it was assumed that metals are evenly distributed between the ash streams (bed, trap and cyclone). The data of Table 5, columns 3, 4 and 5, support this assumption. The concentrations of lead, arsenic and cadmium in the three types of ash samples differ more than those of the other elements, suggesting some preferential retention of these elements in the bed ash. Although the differences are rather large for experimental errors, these results may not be statistically significant. Table 6 summarises the calculated metal concentrations and the measured concentrations, as % of the respective expected levels. In the subsequent argument the figures for the bed material are ignored because of its rather inhomogeneous character. Table 6 gives the degree of metal detection as the mean of the figures for the ash trap (column 4) and the cyclone (column 5). This is then taken as the degree of retention of the metals in the solids and any missing metal is assumed to leave the system with the gases. The metal can, in principle, be present in the gaseous phase, in the form of vapours of the element or its volatile compounds. Alternatively, the metal can be associated with the smallest particles, which cannot be captured in the cyclone. The estimated proportions of the metals retained in ash and leaving the system with the gases are given in columns 7 and 8 of Table

9 Tab. 6. The expected concentrations of metals in the ash and the experimental concentrations with respect to the expected level Expected metal conc. in ash Proportion of metal detected bed ash trap fly ash ash trap & fly ash average in solid phase in gas phase mg/kg dry mass % % % % % % Mn % 91% 86% 88.6% 90% 10% Cu % 92% 108% 100.1% 100% 0% Pb % 81% 87% 83.9% 80% 20% Cr % 113% 115% 114.1% 100% 0% Ni % 109% 119% 113.6% 100% 0% As % 66% 77% 71.5% 70% 30% Hg % 21% 24% 22.4% 20% 80% Cd % 58% 72% 65.3% 70% 30% No attempt has been made to identify the compounds of the metals investigated that could be present in the ash. It can be supposed that these compounds must be thermally stable and were either already present in the sewage sludge fuel or were Tab. 7. Melting and boiling temperatures ( o C) of the metals, their oxides and chlorides [8] Oxidation Proportion with Metal Oxide Chloride state solids gases T melt. T boil. T melt. T boil. T melt. T boil. Mn II 90% 10% Cu II 100% 0% Pb II 80% 20% Cr III 100% 0% Ni II 100% 0% As III 70% 30% Hg II 20% 80% Cd II 70% 30% at a pressure of 28 atm 2 sublimation 789

10 formed in the combustor. Since the conditions are uniformly oxidizing, the metals should be present in their most stable oxidation states. As a general guide, selected physical properties of the metals, their oxides and chlorides are collected in Table 7. As could be expected, the metals which are relatively volatile and/or have volatile oxides or chlorides, i.e. mercury, cadmium, arsenic and lead, tend not to be wholly retained in the ash. The others are retained practically completely. With respect to the chlorides, chlorine is ubiquitous and it is known that HgCl 2 is important in the transport of mercury within combustion systems [9], and it has long been known that in combustion systems which contain high concentrations of water vapour many metals form volatile hydroxides, the thermal stability of which tends to be similar to that of chlorides. It must be also borne in mind that the high temperature chemistry of metallic elements is highly complex and with many metals polyatomic molecules can be formed [10,11]. It is also relevant that during the combustion of coal (in conventional PF furnaces at temperatures much higher that those used in FBC) most of the trace elements present pass through the gas phase, free or combined, and as the flue gases cool, are adsorbed or condense on submicron particles of fly ash [12-15]. The possibility that in combustion systems metals can pass into many different compounds has been confirmed by e.g. Wu i Biswas [16]. They found that the 6 metallic elements selected by them and present in coal, after combustion were distributed between 91 chemical compounds. Depending on the conditions under which a solid fuel containing Cr is burned, different proportions of the Cr can pass into compounds that include: CrO 2, CrO 3, CrO 2 Cl, CrO 2 (OH), CrO 2 (OH) 2 [17,18]. To assess the extent of possible migration of metals during FBC combustion not only data such as those of Table 7 have to be considered, but also the thermal stabilities, chemical equilibria and kinetics of formation the various species that could be formed. It would be easier to carry out detailed experimental element balance for all the material streams entering and leaving the combustor than to perform a theoretical analysis of this type. From the practical point of view, the combustibles in the sewage sludge, together with air, are the source of all of the components of the flue gases. The composition of the gases discharged to the atmosphere depends on the elemental composition of the sludge, combustion conditions and the effectiveness of the flue gases cleaning system, including the removal of particulates. In Poland, the emission of pollutants from technological processes and technical operations is regulated [19]. An incineration plant burning between 1 Mg and 3 Mg of waste per hour cannot discharge flue gases with dust loading above 100 mg/nm 3. The regulations also specify the maximum permissible emissions of heavy metals. Assuming O 2 concentration of 11 % vol. the quantity of wet flue gases produced on burning 1 Mg of the sludge from Niepołomice has been calculated. Supposing that the particulate loading is at the maximum permissible level and assuming that the metal concentration is as given in Table 6 (columns 7 and 8) the quantities of metals in the particulates were obtained. This made it possible to estimate the total metal loading 790

11 of the flue gases, in the gas phase (including submicron particles) and with the particulates. The results are given in Table 8. Tab. 8. Actual concentration levels of metals compared with the legal maximum for the flue gases stream Mass of metal in 1 m Total with the of flue gases Metal Total metal gases introduced 1) with with total in solids gases gases experim. limit [19] mg/m 3 mg mg mg % mg/m 3 mg/m Mn % Cu % Pb % Cr % Ni % As % Hg % Cd % ) Total mass of metal introduced into the reactor, calculated with respect to 1 m 3 of the flue gases produced Of the metals considered, copper, nickel and chromium leave the combustor with the particulates. Therefore, with a more efficient particulate removal system would eliminate their emissions. The emissions of the remaining metals, manganese, lead, arsenic, mercury and cadmium cannot be reduced by dry methods of flue gases cleaning. CONCLUSIONS The regulations divide the metals listed in Tables 5 8 into 3 groups [19]. For the combustion of municipal wastes, the maximum permissible emissions are specified for each group, Table 8, column 8. Comparison of these figures with the experimental results in column 7 shows that the limit is exceeded by the metals in the Mn, Cu, Cr and Pb group. The concentrations of these metals are also high in the original sewage sludge. The fact that the limit is exceeded can be ascribed mainly to emission of lead and manganese or their compounds. In the group containing nickel and arsenic the measured emissions amounted to about 30%, due mostly to As. For the third group, containing two of the most volatile metals, mercury and cadmium, the measured emissions were close to the limit. The experiments and calculations carried out have thus demonstrated that when wastes containing metals are burned in a atmospheric, bubbling fluidised bed, the metals are 791

12 largely retained in the ash. Emissions of metals to the environment are with the flue gases and flue gases cleaning should take this fact into account. ACKNOWLEDGEMENTS This work has been carried out within the project NNE5/2001/468 Sludge for Heat financed by the European Union and the Polish State Committee for Scientific Research. The authors would also like to thank Prof. Stanisław Kandefer for the part he played in devising the experiments and for help in interpreting the results. REFERENCES [1] NOWE SPOJRZENIE NA OSADY ŚCIEKOWE - ODNAWIALNE ŹRÓDŁA ENERGII, (red. G. Malina), Wyd. Politechniki Częstochowskiej, Częstochowa 2003 [2] Rozporządzenie Ministra Środowiska z dnia 1 sierpnia 2002 r. w sprawie komunalnych osadów ściekowych. (Dz. U. nr 134/2002, poz. 1140) [3] Bień B. J., Inżynieria i ochrona środowiska, XI Konferencja Naukowo Techniczna, Osady ściekowe Technologie Wspomaganie decyzji, Tom 3, nr 1-2, Wyd. Politechniki Częstochowskiej, Częstochowa 2000 [4] Łebkowska M., Karwowska E., Usuwanie metali ciężkich ze ścieków przemysłowych i z osadów ściekowych, Polskie Zrzeszenie Inżynierów i Techników Sanitarnych, Warszawa 2003 [5] Baron J., Bulewicz E.M., Żukowski W., Kandefer S., Pilawska M.: Combust. Flame, 128: , 2002 [6] Baron J., Żukowski W., Kandefer S.: Chem. Inż. Ekol., 8: , 2001 [7] Project NNE5/2001/468 SFH, Cracow University of Technology, January-June 2003, (unpublished Interim Technical Report) [8] [9] Hall B., Schager P., Lindqvist O.: Water, Air and Soil Pollution, 96:3-14, 1991 [10] Hastie J.W.: HIGH TEMPERATURE VAPORS, Academic Press, New York - San Francisco London, 1975 [11] Alkemade C. Th., Hollander Tj., Snelleman W., Zeegers P. J. Th.: METAL VAPOURS IN FLAMES, Pergamon Press, 1982 [12] Linak W.P., Wendt J.O.L.: Prog. Energy Comb. Sci., 19: , 1993 [13] Linak W.P., Wendt J.O.L.: Fuel Proc. Technol., 39: , 1994 [14] Bool L.E., Heble J.J.: Energy Fuels, 9: , 1995 [15] Zeng T., Sarofim A.F., Senior C.L.: Combust. Flame 126: , 2001 [16] Wu C.Y., Biswas P.: Combust. Flame 93:31-40, 1993 [17] Bulewicz E.M, Padley P.J.: Proc. Roy. Soc. Lond. A., 323: , 1971 [18] Kashireninov O.E., Fontijn A.: Combust. Flame, 113: , 1998 [19] Rozporządzenie Ministra Środowiska z dnia 30 lipca 2001 r. w sprawie wprowadzania do powietrza substancji zanieczyszczających z procesów technologicznych i operacji technicznych. (Dz. U. nr 87/2001, poz. 957) 792