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Application of pressure-driven membrane techniques to biological treatment of landfill leachate
Process Biochemistry 36 (2001) 641– 646 www.elsevier.com/locate/procbio
Application of pressure-driven membrane techniques to biological treatment of landfill leachate Jolanta Bohdziewicz a,*, Michal* Bodzek a, Joanna Go´rska b a
Institute of Water and Wastewater Engineering; Technical Uni6ersity of Silesia, Faculty of En6ironmental and Energy Engineering, ul. Konarskiego 18, 44 -100, Gliwice, Poland b En6ironmental Biotechnology Department, Technical Uni6ersity of Silesia, Faculty of En6ironmental and Energy Engineering, ul. Konarskiego 18, 44 -100, Gliwice, Poland Received 29 March 2000; received in revised form 31 March 2000; accepted 29 October 2000
Abstract The treatment strategy of landfill leachate is not easy to define in general terms due to differences, involving its composition, dumping place, and age of wastes. Since it is of specific type, characterized by high loading caused by refractive compounds, organic and inorganic substances, they are much more difficult to treat in comparison with municipal sewage. For this reason, the application of hybrid processes is advantageous, because, they enable the treatment of all contaminants present in the leachate. Preliminary tests of sanitary landfill leachate were carried out, using the following hybrid systems: activated sludge-chemical oxidation, activated sludge-ultrafiltration-chemical oxidation and activated sludge-ultrafiltration-reverse osmosis. © 2001 Published by Elsevier Science Ltd. All rights reserved. Keywords: Treatment of landfill leachate; Ultrafiltration; Reverse osmosis; Hybrid systems
1. Introduction One of the major problems connected with the management of waste dumps involves the collection and treatment of landfill leachate generated there. It permeates into ground and surface waters, contributing to their pollution, and hence it poses considerable hazards to the natural environment. Since the landfill leachate contains significant amount of toxic organic and inorganic compounds, it cannot be directly introduced to a sewage system. High loading of landfill leachate, divergent composition and different volume of leachate in particular seasons of the year make the treatment of such wastewater much more difficult as compared for example with the treatment of municipal sewage [1 – 3]. Very frequently, the reduction of contaminants present in the waste water is impossible with the application of a single classical treatment system, or necessitates high * Corresponding author. Tel.: +48-32-2371698; fax: + 48-322371047. E-mail address: [email protected] (J. Bohdziewicz).
financial expenditures. Therefore, to ensure the required level of purity, and minimization of financial expenditures connected with it, very often the hybrid systems are used which consist of combining a few single operations [4–7]. Due to limited biodegradability, the treatment of landfill leachate, apart from biological methods, necessitates the application of physicochemical methods which complement and support the major process. Therefore, the combination of biological processes with pressure-driven techniques seems to be a very promising solution [4,8 –12]. This investigation was undertaken to define the efficiency of treatment of landfill leachate from a municipal waste dump in the systems combining the following processes: biological (activated sludge method), chemical oxidation (ferrous sulphate) and pressure-driven membrane techniques (ultrafiltration, reverse osmosis). To reach these objectives, three separate systems constituting the following integrated methods were subjected to analysis: activated sludge and chemical oxidation;
0032-9592/01/$ - see front matter © 2001 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 2 - 9 5 9 2 ( 0 0 ) 0 0 2 5 9 - 4
J. Bohdziewicz et al. / Process Biochemistry 36 (2001) 641–646
642
activated sludge, ultrafiltration and chemical oxidation; activated sludge, ultrafiltration and reverse osmosis.
2. Experimental
nitrogen forms was inconsiderable. The control of the composition of landfill leachate and the efficiency of its purification involved standard designations such as reaction, COD, BOD5, suspended matter, dry residue and conductivity [13]. Basic parameters of the investigated waste water is presented in Table 1.
2.1. Research substrate (leachate characteristics) The treatment involved landfill leachate generated around the waste site ‘Lipo´wka’ for dumping of municipal wastes from Da˛browa Go´rnicza (province of Katowice, Poland). The composition of the leachate was unknown. The landfill leachate proved to be of high stability, both with respect to organoleptic properties and chemical composition. The ratio BOD5/COD for all investigated samples had a very low value and oscillated between 0.1 and 0.4, which means that the leachate was resistant to biodegradation, and the contaminants can not easily be neutralized using the activated sludge method. In addition, the relation between the amount of ammonium nitrogen and total nitrogen was constant. Almost all total nitrogen constituted ammonium nitrogen, and the contribution of other Table 1 Characteristics of landfill leachate from the municipal waste site ‘‘Lipo´wka’’ Loading index of landfill leachate
Unit
pH BOD5 COD Ammonia nitrogen
8.0 mgO2/dm3 331 mgO2/dm3 1183 mg 743 3 NH+ 4 –N/dm 3 mg NO− Trace values 2 –N/dm 3 mg NO− 0.8 3 –N/dm mg/dm3 240 mg/dm3 21
Nitrite nitrogen Nitrate nitrogen Total solids Suspended solids
Mean value
Fig. 1. Diagram of the system for biological treatment of landfill leachate (1. tank with raw landfill leachate, 2. peristaltic pump, 3. phase reactor of SBR type, 4. tank for cleaned landfill leachate, 5. air pump).
2.2. Biological and chemical treatment of landfill leachate 2.2.1. Apparatus The process was carried out in a phase reactor of SBR type fed with landfill leachate cyclically once very 24 h. The apparatus used in the investigation studies is presented in Fig. 1. Thirty dm3 per day of landfill leachate was pumped with a peristaltic pumps (Elpon, type 372C) from the tank with raw landfill leachate into a reactor of capacity 45 dm3, and then, after purification and sedimentation, into the tank for purified landfill leachate. It was subjected to aeration with an air pump (Hiblow, type SPP-30-GJ-2). 2.2.2. Methodology The method of activated sludge was the first step of leachate purification. This process enabled partial oxidation of organic compounds and removal of organic and ammonium. The biological reactor was fed with activated sludge collected at a domestic sewage treatment plant. First, the activated sludge was adapted to the contaminants present in the leachate, and then the main tests were started. The adaptation process lasted for 2 months, which corresponded to the period between three and five cycles of the sludge. The reactor worked in one cycle per day, both during sludge adaptation and the main tests. The duration of particular phases of the cycle was as follows: inflow of the leachate into the reactor, 0.75 h; aeration, 21.0 h; sedimentation, 1.0 h; drainage of purified leachate from the reactor, 1.0 h; idle time, 0.25 h. In order to assess the actual physiological state of the activated sludge, its respiratory activity during the tests was controlled, applying the following procedure; 125 cm3 samples of activated sludge were taken from the reactor, transferred into a measuring chamber equipped with an oxygen probe and the decrease in oxygen concentration over time was measured. The rate of oxygen consumption by activated sludge microorganisms, expressed in mgO2 per dm3 × h, was calculated on the basis of the rectilinear segment of the decrease in oxygen concentration.
J. Bohdziewicz et al. / Process Biochemistry 36 (2001) 641–646
Fig. 2. Diagram of cross-flow membrane system (1. tank with feeding liquid, 2. pump, 3. heat exchanger, 4. manometer, 5. membrane module, 6. pressure control valve).
Since the sewage treated was characterized by high biodegradation resistance (high COD value after its biological purification), it was subjected to chemical oxidation in the subsequent phase of the purification. The oxidation consisted of the introduction of ferrous sulfate (concentration of 15 mg/dm3) into the biologically pretreated sewage and subsequently its aeration for 1 h. The process was preceded by ultrafiltration which enabled the removal of activated sludge and macromolecular refractive compounds from the treated sewage.
2.3. Treatment of landfill leachate in the processes of ultrafiltration and re6erse osmosis 2.3.1. Apparatus Studies were carried out using the apparatus illustrated in Fig. 2. It consisted of a tubular or plate-frame membrane module, pressure feed pump, fittings and tank with landfill leachate subjected to treatment. 2.3.2. Membranes Three types of membranes were applied: ultrafiltration membranes prepared from polyvinyl chloride (PVC); tubular membranes prepared from polysulfone (PSF); flat osmotic membrane prepared from cellulose acetate (SS). The membranes from PVC were produced using the phase separation method. The casting thickness of the polymer film was 0.2× 10 − 3 m and the active surface of the membranes was 0.0155 m2. By changing the concentration of the polymer, membranes of different compactness, were obtained and hence membranes of different transport-separation properties. Three types of membranes of 9% weight, 10% weight and 11% weight polymer content were tested. The membranes were designated, respectively, with the following symbols; PVC-9 (cut-off 20 000), PVC-10 (cut-off 30 000), and PVC-11 (cut-off 55 000).
643
Tubular ultrafiltration membranes from polysulphone (PSF) were also prepared by means of the phase separation method from the casting solution of polysulphone in dimethyloformamide. Their casting thickness was 0.25× 10-3 m, and the active surface 0.025 m2. The polymer concentration in the casting solution was changed within the range 15% weight –16% weight. The following membranes were used in the tests; PSf-15 and PSf-16. They were typical microfiltration membranes which was indicated by very low retention coefficients of the dextran whose molar mass was 300 000 (B 20%). The osmotic membrane from cellulose acetate (OCSS) was a flat membrane of the surface area 0.0155 m2 (Osmonics).
2.3.3. Methodology By testing the ultrafiltration membranes with landfill leachate obtained after its biological pretreatment, it was possible to select a membrane which retained all activated sludge. The second evaluation criterion in view of their applicability potentials was the rate of ultrafiltration process expressed in terms of the value of volume permeate flux. The ultrafiltration process was carried out using the following process parameters: trans-membrane pressure 0.3 Mpa, cross-flow velocity 2.5 m/s, and temperature 298 K. The efficiency of reverse osmosis was assessed based on the retention coefficients of low-molecular refractive compounds and inorganic salts remaining in the leachate after its pretreatment in the biological and chemical module, as well as, basing on the value of volume permeate flux. Reverse osmosis was carried out applying the pressure 2.76 Mpa, cross-flow velocity 1.5 m/s and temperature 298 K.
3. Results and discussion
3.1. Treatment of landfill leachate using the biological method of acti6ated sludge Changes of COD concentration of raw landfill leachate in the process of its treatment with activated sludge are illustrated in Fig. 3. As a result of the tests, carried out, it was found that when the COD of raw landfill leachate was low and did not exceed 2000 mgO2 per dm3 (raw landfill leachate 1 and 2), it contained a large number of compounds which were difficult to neutralize using biological methods. Numerous attempts to grow activated sludge able to neutralize these refractive compounds yielded each time only a slight removal of COD (B 10%). In the course of the tests, a slow drop of respiratory activity of the microorganisms of the activated sludge and an increase in the concentration of dissolved oxygen in the
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J. Bohdziewicz et al. / Process Biochemistry 36 (2001) 641–646
reactor was observed while the volumetric index of the activated sludge did not undergo any substantial changes. The situation changed when COD of landfill leachate was characterized by high values (raw landfill leachate 3 – 6548 mgO2 per dm3). In which case, the aeration of landfill leachate with activated sludge contributed to the rapid decomposition of the organic compounds. As early as after 5 h of the process, the concentration of COD obtained was lower by 75% and equalled 1657 mgO2 per dm3. However, in each successive attempt, the biological decomposition of organic compounds present in landfill leachate characterized by an increased COD took place only to a certain definite level, typical of raw landfill leachate of low COD, i.e. up to 1500 – 2000 mgO2 per dm3. After that time period, all attempts to lower this
index on a permanent basis were unsuccessful. The results of the tests carried out indicate low biodegradability of the investigated sewage.
3.2. Selection of ultrafiltration membrane Figs. 4 and 5 illustrate the permeate fluxes obtained in the ultrafiltration process of landfill leachate following its pretreatment using the biological method of activated sludge. Considerable differences in the transport velocity of landfill leachate were observed. The flux obtained for the membrane PSf-15 was the highest, and this membrane retained all suspended matter. With respect to the flat membranes produced from PVC, the removal of suspended matter was ensured by PVC-10 and PVC-11 membranes. However, the permeate flux obtained in the filtration process of landfill leachate in the case of the more open membrane, i.e. PVC-10, was almost four times lower compared with membrane PSf-15. Therefore, the value of volume, permeate flux determined the selection of an ultrafiltration membrane for further tests, which membrane PSf-15 turned out to be the most advantageous.
3.3. Application of a hybrid processes to the treatment of landfill leachate
Fig. 3. Dependence of volume permeate flux on the volume reduction factor for flat membranes from PVC (DP= 0.3 Mpa, u= 2.5 m/s).
Fig. 4. Dependence of volume permeate flux on the volume reduction factor for tubular membranes from PSf (DP= 0.3 Mpa, u= 2.5 m/s).
Fig. 5. Dependence of COD concentration of raw landfill leachate on the time of its biological treatment.
3.3.1. Integrated method of acti6ated sludge with chemical oxidation method Since the biological treatment of landfill leachate was not successful due to the high COD of the treated landfill leachate, it was subjected to an addition chemical treatment which consisted of oxidation of the remaining contamination with ferrous sulphate of the concentration 15 mg/dm3. The treatment process of landfill leachate was carried out as follows; The system for landfill leachate aeration (aeration intensity –800 dm3/h) consisted of a reactor of capacity 10 dm3 with a fine-bubble diffuser installed on the bottom. The efficiency of landfill leachate treatment in this system is presented in Table 2. In the process of biological treatment of landfill leachate having a low content of organic substances, the COD stayed almost unchanged, whereas as a result of the additional treatment of chemical oxidation that index decreased by 66%. The cleaned sewage was, however, characterized by a considerably higher conductivity, a higher quantity of dry matter and the settling of the suspension was much more difficult. Setting after a 30-min sedimentation was 0.98 cm/cm, and the volumetric index of the suspension was at the level of around 813 cm3/g. The landfill leachate had a very low pH of 3.07. In the initial stage of the aeration process a large volume of foam was formed, which was a few
J. Bohdziewicz et al. / Process Biochemistry 36 (2001) 641–646
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Table 2 Composition of landfill leachate subjected to chemical treatment following its pretreatment by means of the biological method of activated sludge Loading index of landfill leachate
Unit
Raw landfill leachate
Landfill leachate after biological treatment
COD Suspension Dry matter pH Conductivity
mgO2/dm3 mgO2/dm3 mgO2/dm3
1600 26 164 8.0 9.5
1680 225 395 8.7 11.2
mS/m
Landfill leachate after chemical treatment 546 345 547 3.1 16.1
Table 3 Composition of landfill leachate subjected to treatment in the system combining the processes of: biological treatment, ultrafiltration and chemical oxidation Loading index of landfill leachate pH COD Suspension Dry matter Conductivity
Unit
mg/dm3 mg/dm3 mg/dm3 mS/m
Raw landfill leachate
Landfill leachate after biological treatment
Landfill leachate after ultrafiltration process
8.0 1780 56 264 9.8
8.6 1660 295 495 11.2
8.8 846 0 459 9.00
times larger than the volume of the sewage. The foam did not disappear even after a sedimentation process lasted for several hours.
3.3.2. Integrated method of acti6ated sludge with an ultrafiltration method and with chemical oxidation At the next stage of these tests, the process of biological treatment of landfill leachate was separated from chemical treatment by ultrafiltration to remove from the leachate the remaining, poorly sedimenting, nitrificating activated sludge and macromolecular refractive substances left after the biological process. It was expected that the removal of the suspension at this stage would allow a decrease in the volumetric index of the activated sludge resulting from the chemical oxidation of the leachate. The investigations were carried out as follows: The landfill leachate pretreated by the activated sludge method was then subjected to ultrafiltration in the concentrating system. The permeate devoid of suspension was then subjected to chemical treatment. Table 3 presents the results obtained. In the process of biological treatment of landfill leachate the COD decreased slightly (6.7%) and, as in the previous, system, the quantity of suspension, dissolved matter and conductivity increased. By the introduction of membrane filtration, the suspension was totally removed from the landfill leachate, and the values of other indexes of its loading decreased. The ultrafiltration process was carried out in the concentrating system, with the volume reduction factor being five times higher. The dependence of the values of
Landfill leachate after chemical treatment 2.3 482 323 765 12.7
permeate flux on the volume reduction factor is illustrated in Fig. 6. By reducing the initial volume of the landfill leachate five times, the volume permeate flux decreased almost three times. The decrease in filtration velocity, notably in the first phase of leachate concentration, probably resulted from the occurrence of fouling indicated by the initial sudden decline in filtration velocity with time. As a result of the application of the next stage of sewage treatment after the ultrafiltration process i.e. chemical oxidation, the COD decreased by another 43% (482 mgO2 per dm3), but again the content of the suspension and the conductivity increased. The landfill leachate was characterized by very low pH. Better sedimentation properties where exhibited by the suspension formed in this process, whose volumetric index was smaller several times and equaled 148 cm3/g.
Fig. 6. Dependence of permeate flux on the volume reduction factor in the ultrafiltration process of landfill leachate cleaned by means of biological method (DP=0.2 Mpa, u = 2.5 m/s).
J. Bohdziewicz et al. / Process Biochemistry 36 (2001) 641–646
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Table 4 Composition of landfill leachate subjected to treatment in the system combining the processes of: biological treatment, ultrafiltration and reverse osmosis Loading index of landfill leachate pH COD Suspension Dry matter Conductivity
Unit
mgO2/dm3 mg/dm3 mg/dm3 ms/m
Raw landfill leachate
Landfill leachate after biological treatment
Landfill leachate after ultrafiltration process
8.0 1780 56 264 9.8
8.6 1660 295 495 11.2
8.8 846 0 459 9.00
3.3.3. Integrated method of acti6ated sludge with pressure-dri6en membrane techniques: ultrafiltration and re6erse osmosis Since the treatment of landfill leachate in the hybrid system combining the biological process of activated sludge with ultrafiltration yielded a comparatively high level of its purification, and the introduction of the next stage, chemical oxidation, again brought about a deterioration of its quality, the least stage of the process was initiated. In the final phase, the sewage having been treated by ultrafiltration was subjected to final purification with the use of reverse osmosis. The whole process is presented as follows: The process of reverse osmosis was carried out with the trans-membrane pressure recommended by the manufacturer of the membranes, i.e. 2.76 Mpa. The pH of the landfill leachate was adjusted to 6.5 before it was passed to the osmotic module. The results of landfill leachate treatment obtained in the last of the investigated hybrid systems were most advantageous (Table 4). In the ultrafiltration process, the suspension was removed from the landfill leachate flux after the biological treatment to protect osmotic membranes against fouling. Refractive organic compounds and inorganic salts left in the sewage were almost totally removed by the process of reverse osmosis. The leachate thus purified may be discharged into a natural water reservoir.
4. Conclusions Landfill leachate generated during the operation of solid wastes dumping grounds pose a considerable hazard to the aquatic environment due to high loading with both inorganic compounds and organic substances of poor biodegradation. Its treatment is extremely difficult. The above-mentioned problem is more and more frequently solved by the application of hybrid systems which ensures neutralization of all components present in landfill leachate. From the systems investigated the most advantageous was the system combining the process of biologi.
Landfill leachate after reverse osmosis 7.4 56 0 110 1.2
cal treatment with pressure-driven membrane techniques; ultrafiltration and reverse osmosis. The applied technological solution ensured such a level of treatment that the purified landfill leachate could be introduced into a natural water reservoir.
References [1] Peters T, Stanford P. L’osmose inverse et al disc-tube module´ dans le traitement des lixiviats. Tribune de l’eau 1993;556/6:67– 72. [2] Keramchemie GmbH, Multi-stage leachate treatment plant for a landfill site in Germany, Waste Management and Recycling International 1995: 73-74. [3] Welander U, Henrysson T, Welander T. Nitrification of landfill leachate using suspended-carrier biofilm technology. Wat Res 1997;31(9):2351– 5. [4] Staab KF, Eichberger M, Zwickl S. Deponiesickerwasser-Behandlung auf der Sonderabfalldeponie Billigheim. Entsorgungs Praxis 1999;5:38– 41. [5] Steensen M. Chemical oxidation for the treatment of leachate – process comparison and results from full-scale plants. Wat Sci Tech 1997;35(4):249– 56. [6] Robinson H. Exporting waste expertise. Water 1999;21:35–6. [7] Leitzke O. Chemische Oxidation mit Ozon und UV-Licht unter einem Druck von 5 bar absolut und Temperaturen zwischen 10 und 60°C. Wasser-Abwasser gwf 1993;134(4):202– 7. [8] Weber B, Holz F. Landfill leachate treatment by reverse osmosis. In: Turner MK, editor. Effective Industrial Membrane Processes: Benefits and Opportunities. London, New York: Elsevier Applied Science, 1991:143– 54. [9] Linde K., Jonnson A.N., Wimmerstedt R., ‘‘Treatment of three different types of landfill leachate with reverse osmosis, w: Preprints of 7th International Symposium: Synthetic Membranes in Science and Industry’’, Tubingen, Germany, 1994, pp. 488491. [10] Mulder M. The use of membrane processes in environmental problems. An introduction, w. In: Membrane Processes in Separation and Purification. Dordrecht-Boston London: Kluwer Academic Publishers, 1994:229– 62. [11] Slater C, Ahlert R, Uchrin C. Treatment of landfill leachates by reverse osmosis. Environ Prog 1983;2:251– 6. [12] Syzdek AC, Ahlert RC. Separation of landfill leachate with polymeric ultrafiltration membranes. J Hazard Mater 1984;9:209– 20. [13] Hermanowicz W., Doz; an´ska W., Dojlido J., Koziorowski B., Physical and chemical examination of water and wastewater, Arkady, Warszawa, 1976.
Application of pressure-driven membrane techniques to biological treatment of landfill leachate Jolanta Bohdziewicz a,*, Michal* Bodzek a, Joanna Go´rska b a
Institute of Water and Wastewater Engineering; Technical Uni6ersity of Silesia, Faculty of En6ironmental and Energy Engineering, ul. Konarskiego 18, 44 -100, Gliwice, Poland b En6ironmental Biotechnology Department, Technical Uni6ersity of Silesia, Faculty of En6ironmental and Energy Engineering, ul. Konarskiego 18, 44 -100, Gliwice, Poland Received 29 March 2000; received in revised form 31 March 2000; accepted 29 October 2000
Abstract The treatment strategy of landfill leachate is not easy to define in general terms due to differences, involving its composition, dumping place, and age of wastes. Since it is of specific type, characterized by high loading caused by refractive compounds, organic and inorganic substances, they are much more difficult to treat in comparison with municipal sewage. For this reason, the application of hybrid processes is advantageous, because, they enable the treatment of all contaminants present in the leachate. Preliminary tests of sanitary landfill leachate were carried out, using the following hybrid systems: activated sludge-chemical oxidation, activated sludge-ultrafiltration-chemical oxidation and activated sludge-ultrafiltration-reverse osmosis. © 2001 Published by Elsevier Science Ltd. All rights reserved. Keywords: Treatment of landfill leachate; Ultrafiltration; Reverse osmosis; Hybrid systems
1. Introduction One of the major problems connected with the management of waste dumps involves the collection and treatment of landfill leachate generated there. It permeates into ground and surface waters, contributing to their pollution, and hence it poses considerable hazards to the natural environment. Since the landfill leachate contains significant amount of toxic organic and inorganic compounds, it cannot be directly introduced to a sewage system. High loading of landfill leachate, divergent composition and different volume of leachate in particular seasons of the year make the treatment of such wastewater much more difficult as compared for example with the treatment of municipal sewage [1 – 3]. Very frequently, the reduction of contaminants present in the waste water is impossible with the application of a single classical treatment system, or necessitates high * Corresponding author. Tel.: +48-32-2371698; fax: + 48-322371047. E-mail address: [email protected] (J. Bohdziewicz).
financial expenditures. Therefore, to ensure the required level of purity, and minimization of financial expenditures connected with it, very often the hybrid systems are used which consist of combining a few single operations [4–7]. Due to limited biodegradability, the treatment of landfill leachate, apart from biological methods, necessitates the application of physicochemical methods which complement and support the major process. Therefore, the combination of biological processes with pressure-driven techniques seems to be a very promising solution [4,8 –12]. This investigation was undertaken to define the efficiency of treatment of landfill leachate from a municipal waste dump in the systems combining the following processes: biological (activated sludge method), chemical oxidation (ferrous sulphate) and pressure-driven membrane techniques (ultrafiltration, reverse osmosis). To reach these objectives, three separate systems constituting the following integrated methods were subjected to analysis: activated sludge and chemical oxidation;
0032-9592/01/$ - see front matter © 2001 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 2 - 9 5 9 2 ( 0 0 ) 0 0 2 5 9 - 4
J. Bohdziewicz et al. / Process Biochemistry 36 (2001) 641–646
642
activated sludge, ultrafiltration and chemical oxidation; activated sludge, ultrafiltration and reverse osmosis.
2. Experimental
nitrogen forms was inconsiderable. The control of the composition of landfill leachate and the efficiency of its purification involved standard designations such as reaction, COD, BOD5, suspended matter, dry residue and conductivity [13]. Basic parameters of the investigated waste water is presented in Table 1.
2.1. Research substrate (leachate characteristics) The treatment involved landfill leachate generated around the waste site ‘Lipo´wka’ for dumping of municipal wastes from Da˛browa Go´rnicza (province of Katowice, Poland). The composition of the leachate was unknown. The landfill leachate proved to be of high stability, both with respect to organoleptic properties and chemical composition. The ratio BOD5/COD for all investigated samples had a very low value and oscillated between 0.1 and 0.4, which means that the leachate was resistant to biodegradation, and the contaminants can not easily be neutralized using the activated sludge method. In addition, the relation between the amount of ammonium nitrogen and total nitrogen was constant. Almost all total nitrogen constituted ammonium nitrogen, and the contribution of other Table 1 Characteristics of landfill leachate from the municipal waste site ‘‘Lipo´wka’’ Loading index of landfill leachate
Unit
pH BOD5 COD Ammonia nitrogen
8.0 mgO2/dm3 331 mgO2/dm3 1183 mg 743 3 NH+ 4 –N/dm 3 mg NO− Trace values 2 –N/dm 3 mg NO− 0.8 3 –N/dm mg/dm3 240 mg/dm3 21
Nitrite nitrogen Nitrate nitrogen Total solids Suspended solids
Mean value
Fig. 1. Diagram of the system for biological treatment of landfill leachate (1. tank with raw landfill leachate, 2. peristaltic pump, 3. phase reactor of SBR type, 4. tank for cleaned landfill leachate, 5. air pump).
2.2. Biological and chemical treatment of landfill leachate 2.2.1. Apparatus The process was carried out in a phase reactor of SBR type fed with landfill leachate cyclically once very 24 h. The apparatus used in the investigation studies is presented in Fig. 1. Thirty dm3 per day of landfill leachate was pumped with a peristaltic pumps (Elpon, type 372C) from the tank with raw landfill leachate into a reactor of capacity 45 dm3, and then, after purification and sedimentation, into the tank for purified landfill leachate. It was subjected to aeration with an air pump (Hiblow, type SPP-30-GJ-2). 2.2.2. Methodology The method of activated sludge was the first step of leachate purification. This process enabled partial oxidation of organic compounds and removal of organic and ammonium. The biological reactor was fed with activated sludge collected at a domestic sewage treatment plant. First, the activated sludge was adapted to the contaminants present in the leachate, and then the main tests were started. The adaptation process lasted for 2 months, which corresponded to the period between three and five cycles of the sludge. The reactor worked in one cycle per day, both during sludge adaptation and the main tests. The duration of particular phases of the cycle was as follows: inflow of the leachate into the reactor, 0.75 h; aeration, 21.0 h; sedimentation, 1.0 h; drainage of purified leachate from the reactor, 1.0 h; idle time, 0.25 h. In order to assess the actual physiological state of the activated sludge, its respiratory activity during the tests was controlled, applying the following procedure; 125 cm3 samples of activated sludge were taken from the reactor, transferred into a measuring chamber equipped with an oxygen probe and the decrease in oxygen concentration over time was measured. The rate of oxygen consumption by activated sludge microorganisms, expressed in mgO2 per dm3 × h, was calculated on the basis of the rectilinear segment of the decrease in oxygen concentration.
J. Bohdziewicz et al. / Process Biochemistry 36 (2001) 641–646
Fig. 2. Diagram of cross-flow membrane system (1. tank with feeding liquid, 2. pump, 3. heat exchanger, 4. manometer, 5. membrane module, 6. pressure control valve).
Since the sewage treated was characterized by high biodegradation resistance (high COD value after its biological purification), it was subjected to chemical oxidation in the subsequent phase of the purification. The oxidation consisted of the introduction of ferrous sulfate (concentration of 15 mg/dm3) into the biologically pretreated sewage and subsequently its aeration for 1 h. The process was preceded by ultrafiltration which enabled the removal of activated sludge and macromolecular refractive compounds from the treated sewage.
2.3. Treatment of landfill leachate in the processes of ultrafiltration and re6erse osmosis 2.3.1. Apparatus Studies were carried out using the apparatus illustrated in Fig. 2. It consisted of a tubular or plate-frame membrane module, pressure feed pump, fittings and tank with landfill leachate subjected to treatment. 2.3.2. Membranes Three types of membranes were applied: ultrafiltration membranes prepared from polyvinyl chloride (PVC); tubular membranes prepared from polysulfone (PSF); flat osmotic membrane prepared from cellulose acetate (SS). The membranes from PVC were produced using the phase separation method. The casting thickness of the polymer film was 0.2× 10 − 3 m and the active surface of the membranes was 0.0155 m2. By changing the concentration of the polymer, membranes of different compactness, were obtained and hence membranes of different transport-separation properties. Three types of membranes of 9% weight, 10% weight and 11% weight polymer content were tested. The membranes were designated, respectively, with the following symbols; PVC-9 (cut-off 20 000), PVC-10 (cut-off 30 000), and PVC-11 (cut-off 55 000).
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Tubular ultrafiltration membranes from polysulphone (PSF) were also prepared by means of the phase separation method from the casting solution of polysulphone in dimethyloformamide. Their casting thickness was 0.25× 10-3 m, and the active surface 0.025 m2. The polymer concentration in the casting solution was changed within the range 15% weight –16% weight. The following membranes were used in the tests; PSf-15 and PSf-16. They were typical microfiltration membranes which was indicated by very low retention coefficients of the dextran whose molar mass was 300 000 (B 20%). The osmotic membrane from cellulose acetate (OCSS) was a flat membrane of the surface area 0.0155 m2 (Osmonics).
2.3.3. Methodology By testing the ultrafiltration membranes with landfill leachate obtained after its biological pretreatment, it was possible to select a membrane which retained all activated sludge. The second evaluation criterion in view of their applicability potentials was the rate of ultrafiltration process expressed in terms of the value of volume permeate flux. The ultrafiltration process was carried out using the following process parameters: trans-membrane pressure 0.3 Mpa, cross-flow velocity 2.5 m/s, and temperature 298 K. The efficiency of reverse osmosis was assessed based on the retention coefficients of low-molecular refractive compounds and inorganic salts remaining in the leachate after its pretreatment in the biological and chemical module, as well as, basing on the value of volume permeate flux. Reverse osmosis was carried out applying the pressure 2.76 Mpa, cross-flow velocity 1.5 m/s and temperature 298 K.
3. Results and discussion
3.1. Treatment of landfill leachate using the biological method of acti6ated sludge Changes of COD concentration of raw landfill leachate in the process of its treatment with activated sludge are illustrated in Fig. 3. As a result of the tests, carried out, it was found that when the COD of raw landfill leachate was low and did not exceed 2000 mgO2 per dm3 (raw landfill leachate 1 and 2), it contained a large number of compounds which were difficult to neutralize using biological methods. Numerous attempts to grow activated sludge able to neutralize these refractive compounds yielded each time only a slight removal of COD (B 10%). In the course of the tests, a slow drop of respiratory activity of the microorganisms of the activated sludge and an increase in the concentration of dissolved oxygen in the
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reactor was observed while the volumetric index of the activated sludge did not undergo any substantial changes. The situation changed when COD of landfill leachate was characterized by high values (raw landfill leachate 3 – 6548 mgO2 per dm3). In which case, the aeration of landfill leachate with activated sludge contributed to the rapid decomposition of the organic compounds. As early as after 5 h of the process, the concentration of COD obtained was lower by 75% and equalled 1657 mgO2 per dm3. However, in each successive attempt, the biological decomposition of organic compounds present in landfill leachate characterized by an increased COD took place only to a certain definite level, typical of raw landfill leachate of low COD, i.e. up to 1500 – 2000 mgO2 per dm3. After that time period, all attempts to lower this
index on a permanent basis were unsuccessful. The results of the tests carried out indicate low biodegradability of the investigated sewage.
3.2. Selection of ultrafiltration membrane Figs. 4 and 5 illustrate the permeate fluxes obtained in the ultrafiltration process of landfill leachate following its pretreatment using the biological method of activated sludge. Considerable differences in the transport velocity of landfill leachate were observed. The flux obtained for the membrane PSf-15 was the highest, and this membrane retained all suspended matter. With respect to the flat membranes produced from PVC, the removal of suspended matter was ensured by PVC-10 and PVC-11 membranes. However, the permeate flux obtained in the filtration process of landfill leachate in the case of the more open membrane, i.e. PVC-10, was almost four times lower compared with membrane PSf-15. Therefore, the value of volume, permeate flux determined the selection of an ultrafiltration membrane for further tests, which membrane PSf-15 turned out to be the most advantageous.
3.3. Application of a hybrid processes to the treatment of landfill leachate
Fig. 3. Dependence of volume permeate flux on the volume reduction factor for flat membranes from PVC (DP= 0.3 Mpa, u= 2.5 m/s).
Fig. 4. Dependence of volume permeate flux on the volume reduction factor for tubular membranes from PSf (DP= 0.3 Mpa, u= 2.5 m/s).
Fig. 5. Dependence of COD concentration of raw landfill leachate on the time of its biological treatment.
3.3.1. Integrated method of acti6ated sludge with chemical oxidation method Since the biological treatment of landfill leachate was not successful due to the high COD of the treated landfill leachate, it was subjected to an addition chemical treatment which consisted of oxidation of the remaining contamination with ferrous sulphate of the concentration 15 mg/dm3. The treatment process of landfill leachate was carried out as follows; The system for landfill leachate aeration (aeration intensity –800 dm3/h) consisted of a reactor of capacity 10 dm3 with a fine-bubble diffuser installed on the bottom. The efficiency of landfill leachate treatment in this system is presented in Table 2. In the process of biological treatment of landfill leachate having a low content of organic substances, the COD stayed almost unchanged, whereas as a result of the additional treatment of chemical oxidation that index decreased by 66%. The cleaned sewage was, however, characterized by a considerably higher conductivity, a higher quantity of dry matter and the settling of the suspension was much more difficult. Setting after a 30-min sedimentation was 0.98 cm/cm, and the volumetric index of the suspension was at the level of around 813 cm3/g. The landfill leachate had a very low pH of 3.07. In the initial stage of the aeration process a large volume of foam was formed, which was a few
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Table 2 Composition of landfill leachate subjected to chemical treatment following its pretreatment by means of the biological method of activated sludge Loading index of landfill leachate
Unit
Raw landfill leachate
Landfill leachate after biological treatment
COD Suspension Dry matter pH Conductivity
mgO2/dm3 mgO2/dm3 mgO2/dm3
1600 26 164 8.0 9.5
1680 225 395 8.7 11.2
mS/m
Landfill leachate after chemical treatment 546 345 547 3.1 16.1
Table 3 Composition of landfill leachate subjected to treatment in the system combining the processes of: biological treatment, ultrafiltration and chemical oxidation Loading index of landfill leachate pH COD Suspension Dry matter Conductivity
Unit
mg/dm3 mg/dm3 mg/dm3 mS/m
Raw landfill leachate
Landfill leachate after biological treatment
Landfill leachate after ultrafiltration process
8.0 1780 56 264 9.8
8.6 1660 295 495 11.2
8.8 846 0 459 9.00
times larger than the volume of the sewage. The foam did not disappear even after a sedimentation process lasted for several hours.
3.3.2. Integrated method of acti6ated sludge with an ultrafiltration method and with chemical oxidation At the next stage of these tests, the process of biological treatment of landfill leachate was separated from chemical treatment by ultrafiltration to remove from the leachate the remaining, poorly sedimenting, nitrificating activated sludge and macromolecular refractive substances left after the biological process. It was expected that the removal of the suspension at this stage would allow a decrease in the volumetric index of the activated sludge resulting from the chemical oxidation of the leachate. The investigations were carried out as follows: The landfill leachate pretreated by the activated sludge method was then subjected to ultrafiltration in the concentrating system. The permeate devoid of suspension was then subjected to chemical treatment. Table 3 presents the results obtained. In the process of biological treatment of landfill leachate the COD decreased slightly (6.7%) and, as in the previous, system, the quantity of suspension, dissolved matter and conductivity increased. By the introduction of membrane filtration, the suspension was totally removed from the landfill leachate, and the values of other indexes of its loading decreased. The ultrafiltration process was carried out in the concentrating system, with the volume reduction factor being five times higher. The dependence of the values of
Landfill leachate after chemical treatment 2.3 482 323 765 12.7
permeate flux on the volume reduction factor is illustrated in Fig. 6. By reducing the initial volume of the landfill leachate five times, the volume permeate flux decreased almost three times. The decrease in filtration velocity, notably in the first phase of leachate concentration, probably resulted from the occurrence of fouling indicated by the initial sudden decline in filtration velocity with time. As a result of the application of the next stage of sewage treatment after the ultrafiltration process i.e. chemical oxidation, the COD decreased by another 43% (482 mgO2 per dm3), but again the content of the suspension and the conductivity increased. The landfill leachate was characterized by very low pH. Better sedimentation properties where exhibited by the suspension formed in this process, whose volumetric index was smaller several times and equaled 148 cm3/g.
Fig. 6. Dependence of permeate flux on the volume reduction factor in the ultrafiltration process of landfill leachate cleaned by means of biological method (DP=0.2 Mpa, u = 2.5 m/s).
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Table 4 Composition of landfill leachate subjected to treatment in the system combining the processes of: biological treatment, ultrafiltration and reverse osmosis Loading index of landfill leachate pH COD Suspension Dry matter Conductivity
Unit
mgO2/dm3 mg/dm3 mg/dm3 ms/m
Raw landfill leachate
Landfill leachate after biological treatment
Landfill leachate after ultrafiltration process
8.0 1780 56 264 9.8
8.6 1660 295 495 11.2
8.8 846 0 459 9.00
3.3.3. Integrated method of acti6ated sludge with pressure-dri6en membrane techniques: ultrafiltration and re6erse osmosis Since the treatment of landfill leachate in the hybrid system combining the biological process of activated sludge with ultrafiltration yielded a comparatively high level of its purification, and the introduction of the next stage, chemical oxidation, again brought about a deterioration of its quality, the least stage of the process was initiated. In the final phase, the sewage having been treated by ultrafiltration was subjected to final purification with the use of reverse osmosis. The whole process is presented as follows: The process of reverse osmosis was carried out with the trans-membrane pressure recommended by the manufacturer of the membranes, i.e. 2.76 Mpa. The pH of the landfill leachate was adjusted to 6.5 before it was passed to the osmotic module. The results of landfill leachate treatment obtained in the last of the investigated hybrid systems were most advantageous (Table 4). In the ultrafiltration process, the suspension was removed from the landfill leachate flux after the biological treatment to protect osmotic membranes against fouling. Refractive organic compounds and inorganic salts left in the sewage were almost totally removed by the process of reverse osmosis. The leachate thus purified may be discharged into a natural water reservoir.
4. Conclusions Landfill leachate generated during the operation of solid wastes dumping grounds pose a considerable hazard to the aquatic environment due to high loading with both inorganic compounds and organic substances of poor biodegradation. Its treatment is extremely difficult. The above-mentioned problem is more and more frequently solved by the application of hybrid systems which ensures neutralization of all components present in landfill leachate. From the systems investigated the most advantageous was the system combining the process of biologi.
Landfill leachate after reverse osmosis 7.4 56 0 110 1.2
cal treatment with pressure-driven membrane techniques; ultrafiltration and reverse osmosis. The applied technological solution ensured such a level of treatment that the purified landfill leachate could be introduced into a natural water reservoir.
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