This article was downloaded by: [University of Sydney] On: 04 May 2015, At: 12:41 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Optimization of the treatment cycle of pressed-off leachate produced in a facility processing the organic fraction of municipal solid waste a
a
a
Luca d'Antonio , Massimiliano Fabbricino & Ludovico Pontoni a
Department of Civil, Architectural and Environmental Engineering, University of Naples Federico II, Via Claudio 21, Naples 80125, Italy Accepted author version posted online: 24 Nov 2014.Published online: 16 Dec 2014.
Click for updates To cite this article: Luca d'Antonio, Massimiliano Fabbricino & Ludovico Pontoni (2015) Optimization of the treatment cycle of pressed-off leachate produced in a facility processing the organic fraction of municipal solid waste, Environmental Technology, 36:11, 1367-1372, DOI: 10.1080/09593330.2014.990521 To link to this article: http://dx.doi.org/10.1080/09593330.2014.990521
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2015 Vol. 36, No. 11, 1367–1372, http://dx.doi.org/10.1080/09593330.2014.990521
Optimization of the treatment cycle of pressed-off leachate produced in a facility processing the organic fraction of municipal solid waste Luca d’Antonio, Massimiliano Fabbricino ∗ and Ludovico Pontoni Department of Civil, Architectural and Environmental Engineering, University of Naples Federico II, Via Claudio 21, Naples 80125, Italy
Downloaded by [University of Sydney] at 12:41 04 May 2015
(Received 26 August 2014; accepted 18 November 2014 ) The paper investigates, at a laboratory scale, the applicability of anaerobic digestion for the treatment of pressed-off leachate produced in a biomechanical treatment plant for municipal solid waste. Batch tests show that the anaerobic process proceeds smoothly and produces about 10,000 mL of methane per litre of treated leachate. The process is characterized by a lag phase lasting about 30 days, and is completed in about 2 months. Chemical oxygen demand (COD) and volatile fatty acids monitoring allows studying process kinetics that are modelled through a triple linear expression. Physical and biological treatments are also investigated to reduce the residual organic charge of the produced digestate. The best performances are obtained via aerobic degradation followed by assisted sedimentation. This cycle reduces the residual COD of about 85%, and allows the correct disposal of the final waste stream. Keywords: anaerobic digestion; biomechanical treatment; clariflocculation; digestate; pressed-off leachate
Introduction For many years sanitary landfill has been the most used, if not the unique system, for solid waste disposal, almost everywhere in the world. Although it is still widely applied, in the last three decades, because of the increasing attention to the safeguard and protection of the environmental quality, it has been more and more substituted by other options, which consider the waste as a potential source of energy and materials.[1–6] Sanitary landfill has therefore become just one of the elements of a so-called integrated scheme for municipal and industrial waste disposal, together with treatment plants aimed at resources reuse, recycle, recovery and convert (R4C).[7–9] The R4C situation in Italy is certainly not one of the best compared with other countries characterized by similar social and economic characteristics.[7] Yet many new facilities for solid waste valorization have been created during the last 10–15 years. Among them biomechanical treatment plants (BMTPs) for municipal solid waste play a non-secondary role.[10] According to the last available data,[11] about 9.2 million tons·year−1 , that is, 29% of the total produced amount of waste is processed in one of the 122 BMTPs existing in Italy. BMTPs receive the solid waste coming either from source-separated collection, or from commingled collection, and, according to the quality of the incoming waste, are aimed at producing refuse-derived fuel (RDF), stabilized organic matter (SOM) or materials to be reused. Together with about 1.7 million tons
*Corresponding author. Email: [email protected] © 2014 Taylor & Francis
of RDF, 1 million tons of SOM and 0.2 million tons of materials to be reused, the BMTPs produce, yearly, also about 4.5 million tons of solid residues, that are disposed in the existing sanitary landfills, and a consistent amount of wastewater, defined as pressed-off leachate, estimated to be around 71 × 103 tons.[11] Pressed-off leachate is generated by compaction operations aimed at reducing the water content required for SOM and RDF production, or is simply released by gravity from stockpiled wet waste, especially during rainy periods.[12,13] The characteristics of this pressed-off leachate are not dissimilar from those of the better known leachate produced in young sanitary landfills.[14–16] It has a very high organic load, mostly constituted by biodegradable compounds, and is generally poor in terms of phosphorous and nitrogen content, so that aerobic processes are difficult to apply. Although specific studies aimed at individuating the best option for pressedoff leachate treatment and disposal are very few,[12,17–19] it is generally suggested the recourse to anaerobic degradation, in order to supply part of the energy required by the BMT plant through biogas production. Unfortunately, the characteristics of the effluent of the anaerobic process do not meet the required standard for its safe disposal, and further treatment is therefore necessary. The present paper intends to investigate the whole treatment cycle of pressedoff leachate, evaluating its anaerobic biodegradability, calculating the process kinetics and individuating the most efficient process for the final treatment of the produced
1368
L. d’Antonio et al.
digestate before its final disposal, investigating among those generally proposed in the available literature.[20–22]
Downloaded by [University of Sydney] at 12:41 04 May 2015
Material and methods Pressed-off leachate used for the experimental study was collected in a BMTP located in the province of Naples (South of Italy). After collection, the sample was analysed for a complete characterization (Table 1), and subject to anaerobic degradation in 200 mL glass reactors (Schott Duran, Germany). Tests were conducted in batch on triplicates to reduce experimental errors. Each reactor was filled with 1400 mL of fresh leachate and sealed with a 5-mm silicone disc, held tightly to the reactor head by a plastic screw cap punched in the middle (Schott Duran, Germany). The plastic cap was furthermore drilled and a polypropylene rigid tube was inserted and sealed through it in order to allow sampling inside the reactor. Reactors were immersed up to half of their height in a water bath kept at a constant temperature (308 K). During the degradation process, methane production was measured by the water displacement method, leaving the biogas bubbling in an inverted 1-L glass bottle containing a strongly basic solution (12% NaOH) in order to trap any CO2 present in the gas.[23] At fixed times 5 mL of leachate was withdrawn from the reactor, to monitor pH, chemical oxygen demand (COD) alkalinity and total volatile fatty acids (VFA) concentration. The process was ended when cumulative biogas production reached a plateau. At the end of the process, produced digestate was characterized (Table 1) in terms of total suspend solids (TSS), volatile suspended solids (VSS), COD, biochemical oxygen demand (BOD5 ), total nitrogen (TN), ammonia nitrogen (N-NH4 ) and total phosphorous (TP). After characterization, the digestate was divided into two equal fractions that were subject to different treatment sequences. The first fraction was simply clariflocculated, while the second one was oxidized biologically, and clariflocculated. Reagents used for Table 1. Pressed-off leachate and digestate characteristics. Fresh pressed-off leachate pH COD (mg L−1 ) BOD5 (mg L−1 ) TSS (mg L−1 ) VSS (mg L−1 ) −1 N-NH+ 4 (mg L ) −1 TN (mg L ) TP (mg L−1 ) VFA (mg L−1 ) Alkalinity (mg L−1 ) Pb (mg L−1 ) Cr (mg L−1 ) Ni (mg L−1 ) Cd (µg L−1 ) Cu (mg L−1 ) Zn (mg L−1 )
6.8 65,964 35,000 1750 880 111.4 1555 377 12,130 16,780 1.2 2.9 3.3 0.1 1.1 4.1
Digestate 8.6 6518 1000 1530 230 1112 1390 34 1395 2330 1.2 2.9 3.3 0.1 1.1 4.1
clariflocculation were poly-aluminum chloride (coagulant) ® and KLARAID PC4000 (flocculant). Both reagents were purchased from GE Infrastructure. Clariflocculation was conducted using a Jar Test apparatus. In all, 400 mL of the sample was introduced in each jar. The coagulant was added and rapidly mixed at 100 rpm for 3 min. The speed was then reduced to 30 rpm and the flocculant was introduced into the jar for an additional time of 20 min. The solution was finally left to settle for 1 h. Due to the high buffer capacity of the treated digestate, no attempt was made to correct the pH during the process. Tested concentrations of coagulant varied between 0.75 and 6.6 mg L−1 . Tested concentrations of flocculant varied, instead, between 100 and 900 mg L−1 . Biological oxidation was conducted at a bench scale using a batch reactor equipped with a magnetic stirrer and a mechanical pump to supply the required oxygen flow. The test was conducted in two successive steps. During the first step, the digestate was mixed with the aerobic sludge coming from a full-scale municipal wastewater treatment plant, ensuring a food/mass ratio equal to 0.5 BOD5 /VSS. The mixture was magnetically stirred and aerated for 24 h, to allow the acclimatization of the biomass. After 24 h the stirrer and the aeration system were turned off and the mixture was settled for 2 h. The supernatant was then discarded and the biomass was mixed with a fresh digestate, keeping the food/mass ratio equal to 0.3 BOD5 /VSS. The mixture was once more mixed and aerated for 24 h, and settled for 2 h. This time the supernatant was analysed and subject to clariflocculation, following the aforementioned procedure. Leachate, digestate and effluents from biological oxidation and clariflocculation were characterized following international standardized procedures. Alkalinity and total VFA concentrations were titrated according to Anderson and Yang.[24] COD was determined by the open reflux method, according to the APHA standard method 5220B. Heavy metals were determined by Atomic Absorption Spectroscopy, using a GBC AVANTA spectrophotometer. BOD5 was evaluated by a WTW-Oxi-TOP respirometric test. TSS and VSS were obtained by gravimetry. TN and N-NH4 were determined by the Kjeldahl method and steam distillation and titration (VELP, Italy). TP was obtained by the perchloric acid digestion and photometric vanadomolybdophosphoric acid method according to the APHA standard method 4500-P C. pH ( ± 0.05) was measured using a WTW ino-Lab Level 2 pH-meter. All reagents used for analytical determinations were ACS grade. Only ultrapure water was used for all dilutions. Results and discussion Leachate characteristics and anaerobic process development The first column of Table 1 summarizes the characteristics of the used pressed-off leachate. As it can be easily verified, it has a very high organic load, with COD being
1369
Downloaded by [University of Sydney] at 12:41 04 May 2015
Environmental Technology
Figure 1. pH and alkalinity trends during anaerobic digestion.
almost equal to 70 g L−1 . The organic load is, for a large part, biodegradable, as the ratio between BOD5 and COD is around 0.5. The amount of nitrogen and phosphorus compared with the available carbon content is sufficient for the anaerobic degradation, but not necessarily enough for the correct development of an aerobic process. The possibility of carrying out the anaerobic process is justified by the initial pH value and by the alkalinity concentration. The amount of trace metals, although not negligible, is not extremely high, and indicates the possibility of correct biological process development. Most probably, the anaerobic degradation is already started in the BMTP, as suggested by the presence of notable concentration of VFA. The characteristics listed in Table 1 are congruent with those obtained in previous research works.[12,25,26] Preliminarily, information derived by the analysis of the pressed-off leachate is confirmed by the trends of pH and alkalinity during anaerobic degradation, reported in Figure 1. These trends allow stating that the process development is regular and complete, and that no inhibitory factors affect it. What is more important, in terms of process convenience and efficiency, is the trend of cumulate methane production (Figure 2). After the initial lag phase, lasting about 30 days, the biogas production rate rapidly increases, reaching, in one month, an asymptotic value corresponding to 10,000 mL of methane. COD and VFA trends, summarized in Figure 3, are in agreement with methane production, and give some substantial information on the process kinetics. COD degradation and VFA concentration have a specular trend with respect to biogas production. Once again the lag phase lasts about one month and it is followed by a rapid decrease in the targeted concentration and by a final asymptotic value.
The acidogenic phase seems to be predominant during the first 10–20 days. In this period, the organic matter is transformed into volatile acids, but the latter are not yet converted into biogas. Correspondently, COD decreases, while VFA concentration increases (Figure 3), and no methane is produced (Figure 2). Successively, once the acidification is completed, it is possible to observe a decrease in both COD and VFA (Figure 3); the latter are transformed into biogas, and therefore, methane production drastically increases (Figure 2). According to methane, COD and VFA trends, it is possible to represent process kinetics using a triple linear expression, expressed by the following equations: PT1 = a1 (P) · t + b1 (P),
t ≤ 35 days,
(1a)
PT2 = a2 (P) · t + b2 (P),
35 < t ≤ 65 days,
(1b)
PT3 = a3 (P) · t + b3 (P),
65 days ≤ t,
(1c)
where P Ti is the value of the parameter (methane, VFA, COD) in the interval Ti, t is the time and ai (P) and bi (P) are values of the kinetic constants for the parameter P, in the interval Ti. The values of the kinetic constant are summarized in Table 2. From this table, it can be derived that the kinetic constants are related among them and can be verified by the following equivalences: ai (VFA) = 0.212 · ai (COD) − 9.64,
(2a)
ai (methane) = −0.337 · ai (COD) − 28.07,
(2b)
bi (VFA) = 0.189 · bi (COD) + 3355,
(2c)
bi (methane) = −0.351 · bi (COD) − 18, 968.
(2d)
Although other types of kinetic equations could have been applied, the triple linear expression has the advantage of being applicable for all the monitored parameters,
Downloaded by [University of Sydney] at 12:41 04 May 2015
1370
L. d’Antonio et al.
Figure 2. Cumulate biogas production.
Figure 3. COD and VFA trends. Table 2. Values of the kinetic constants. Parameter (P) COD a1 a2 a3 b1 b2 b3
− 150 − 1418 − 94 55.7 100.4 15.8
VFA − 15 − 312 − 55 12.9 22.8 6.9
Methane 16 450 10 30 − 16.5 13.1
and clearly gives an indication of the noted relationships among COD reduction, VFA consumption and methane production.
While Figure 2 gives a clear idea of the convenience of carrying out the anaerobic process to produce biogas, and Figure 3 indicates the possibility of reaching very good efficiency in terms of COD removal, the overall performances of the process can be even better assessed comparing the characteristics of the fresh pressed-off leachate and the characteristics of the digestate produced after 3 months, reported, respectively, in the first and in the last column of Table 1. From such a comparison, it can be noted that the digestate has a residual COD which is only 10% of the initial value, a BOD5 which is reduced of more than 95% and a TSS concentration which is reduced of 65%. The low concentration of VFA confirms that the process is completed and that no further degradation can be obtained from the anaerobic process. In terms of nitrogen and phosphorus,
1371
Environmental Technology Table 3. Characteristics of the digestate after: clariflocculation; biological oxidation and settling; biological oxidation and assisted settling. Digestate after clariflocculation
Downloaded by [University of Sydney] at 12:41 04 May 2015
pH COD (mg L−1 ) BOD5 (mg L−1 ) TSS (mg L−1 ) TVS (mg L−1 ) −1 N-NH+ 4 (mg L ) TN (mg L−1 ) TP (mg L−1 )
6.8 3405 700 206 72 506 1260 27.8
no important change can be noted so that the ratio C/N and C/P is drastically reduced, and therefore, an aerobic treatment of the digestate becomes more feasible. Despite the good removal efficiency, the digestate has still a residual organic charge that is not negligible and therefore needs a further treatment. Digestate treatment The first column of Table 3 summarizes the characteristics of the digestate after clariflocculation. The process was performed using 1.9 mg L−1 of poly-aluminum chlo® ride and 612 mg L−1 of KLARAID PC4000 . The amounts of used electrolyte and polyelectrolyte were optimized through jar tests, as indicated in Section 2. Overall process performances do not exceed 50% of COD removal, as it is typical of chemical–physical treatments. It is worth noting the substantial reduction of nitrogen compounds and suspended solids, which are likely to be present in the form of colloidal particles. It can be therefore deduced that most of the residual organic pollution is in the form of dissolved compounds. The second column of Table 3 lists instead the characteristics of the digestate after biological oxidation. This time it is possible to remove even a larger part of the organic load: COD removal attains 72%, BOD5 removal reaches 55% and TN decreases from 1390 mg L−1 to 440 mg L−1 indicating the development of nitrification processes, with nitrogen being almost completely in the form of ammonia, as a consequence of the anaerobic process carried out before the aerobic one. Moreover, using an assisted coagulation after the biological process it is possible to further increase the performances of the treatment (the last column of Table 3), with COD removal being 50% higher, and BOD5 removal 67% higher. In such a way, the final concentrations of all parameters are sufficiently low to permit the final disposal of the effluent. Conclusion The experimental study allowed evaluating the anaerobic biodegradability of pressed-off leachate produced
Digestate after oxidation and settling
Digestate after oxidation and assisted settling
8 1780 450 920 370 176 440 32.3
6.1 880 150 194 48 118 382 25.2
in BMTPS, to calculate the process kinetics, and to individuate the most efficient treatment process for the final treatment of the produced digestate. From the obtained results, it can be concluded that the pressed-off leachate can be certainly treated through anaerobic processes. The amount of produced biogas is high enough to partially cover the energy requirements of the solid waste treatment plant. Considering the heating value of methane, this corresponds to a potential power generation of 0.1 kWh per cubic meter of treated leachate. The process develops regularly, although an initial lag phase of about 30 days is to be expected. Process kinetics can be simulated using triple linear expressions. The equations to be used to model biogas production, COD reduction and VFA trend have the same form, and are characterized by parameters linearly correlated among them. Although the treatment of the digestate could be performed by clariflocculation, the obtained performances of the process do not allow the final disposal of the produced effluent. Digestate aerobic degradation followed by assisted sedimentation, instead, permits to reach extremely high removal rates, and turns out to be the preferred option to close the cycle of pressed-off leachate production, treatment and disposal.
Disclosure statement No potential conflict of interest was reported by the authors. References [1] Allen A. Containment landfills: the myth of sustainability. Eng Geol. 2001;60:3–19. [2] Fabbricino M. An integrated programme for municipal solid waste management. Waste Manage Res. 2001;19:368–379. [3] D’Antonio G, Fabbricino M, Pirozzi F. Decisional model for integrated management of municipal solid waste – a case study. J Solid Waste Technol Manage. 2002;28:28–43. [4] Renou S, Givaudan JG, Poulain S, Dirassouyan F, Moulin P. Landfill leachate treatment: review and opportunity. J Hazard Mater. 2008;150:468–493.
Downloaded by [University of Sydney] at 12:41 04 May 2015
1372
L. d’Antonio et al.
[5] Rada EC, Istrate IA, Ragazzi M. Trends in the management of residual municipal solid waste. Environ Technol. 2009;7:651–661. [6] Di Maria F. Upgrading of a mechanical biological treatment plant with a solid anaerobic digestion batch: a real case study. Waste Manage Res. 2012;30:1089–1094. [7] Achard PO, Fabbricino M, d’Antonio L. Global resources, recovery, reuse, recycling and conversion in Italy. J Solid Waste Technol Manage. 2014;39:260–274. [8] Davis G. Global resources recovery, reuse, recycling and conversion: an overview from Queensland, Australia. J Solid Waste Technol Manage. 2014;39:225–233. [9] Su M, Shih K. Projects resource recovery, reuse, recycling and conversion (PR4C). J Solid Waste Technol Manage. 2014;39:234–259. [10] Rada EC, Ragazzi M, Panaitescu V, Apostol T. Some research perspectives on emissions from bio-mechanical treatments of municipal solid waste in Europe. Environ Technol. 2005;26:1297–1302. [11] ISPRA (Italy) Municipal Solid Waste Report: Ed 2013; 2014. [12] Nayono S, Winter J, Gallert C. Co-digestion of press water and food waste in a biowaste digester for improvement of biogas production. Bioresource Technol. 2010;101:6987– 6993. [13] Salati S, Scaglia B, Di Gregorio A, Carrera A, Adani F. The use of the dynamic respiration index to predict the potential MSW-leachate impacts after short term mechanical biological treatment. Bioresource Technol. 2013;128:351–358. [14] Ehrig HJ. Quality and quantity of sanitary landfill leachate. Waste Manage Res. 1983;1:53–68. [15] Yildiz ED, Ünlü K, Rowe KR. Modelling leachate quality and quantity in municipal solid waste landfills. Waste Manage Res. 2002;22:78–92. [16] Kulikowska D, Klimiuk E. The effect of landfill age on municipal leachate composition. Bioresource Technol. 2008;99:5981–5985.
[17] Mokhtarani N, Bayatfard A, Mokhtarani B. Full scale performance of compost’s leachate treatment by biological anaerobic reactors. Waste Manage Res. 2012;30: 524–529. [18] Di Maria F, Micale C, Sordi N, Cirulli G. Leachate purification of mechanically sorted organic waste in a simulated bioreactor landfill. Waste Manage Res. 2013;31: 1069–1074. [19] Tran HN, Munnich K, Fricke K, Harborth P. Removal of nitrogen from MBT residues by leachate recirculation in combination with intermittent aeration. Waste Manage Res. 2014;32:56–63. [20] Borzacconi L, Lopez I, Ohanian M, Vinas M. Anaerobic– aerobic treatment of municipal solid waste leachate. Environ Technol. 1999;20:211–217. [21] Kochany J, Lugowski A. Multi stage treatment of high strength leachate. Environ Technol. 2000;21:623–629. [22] Fang F, Abbas AA, Chen Y-P, Liu Z-P, Gao X, Guo J-S. Anaerobic/aerobic/coagulation treatment of leachate from a municipal solid waste incineration plant. Environ Technol. 2012;33:927–935. [23] Liotta F, d’Antonio G, Esposito G, Fabbricino M, Frunzo L, van Hullebusch ED, Lens PNL, Pirozzi F. Effect of moisture on disintegration kinetics during anaerobic digestion of complex organic substrates. Waste Manage Res. 2014;32:40–48. [24] Anderson GK, Yang G. Determination of bicarbonate and total volatile acid concentration in anaerobic digesters using a simple titration. Water Environ Res. 1992;64: 53–59. [25] Kjeldsen P, Barlaz MA, Rooker AP, Baun A, Ledin A, Christensen H. Present and long-term composition of MSW landfill leachate: a review. Critical Rev Environ Sci Technol. 2002;32:297–336. [26] Nayono S, Winter J, Gallert C. Anaerobic digestion of pressed off leachate from the organic fraction of municipal solid waste. Waste Manag. 2010;30:1828–1833.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Optimization of the treatment cycle of pressed-off leachate produced in a facility processing the organic fraction of municipal solid waste a
a
a
Luca d'Antonio , Massimiliano Fabbricino & Ludovico Pontoni a
Department of Civil, Architectural and Environmental Engineering, University of Naples Federico II, Via Claudio 21, Naples 80125, Italy Accepted author version posted online: 24 Nov 2014.Published online: 16 Dec 2014.
Click for updates To cite this article: Luca d'Antonio, Massimiliano Fabbricino & Ludovico Pontoni (2015) Optimization of the treatment cycle of pressed-off leachate produced in a facility processing the organic fraction of municipal solid waste, Environmental Technology, 36:11, 1367-1372, DOI: 10.1080/09593330.2014.990521 To link to this article: http://dx.doi.org/10.1080/09593330.2014.990521
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2015 Vol. 36, No. 11, 1367–1372, http://dx.doi.org/10.1080/09593330.2014.990521
Optimization of the treatment cycle of pressed-off leachate produced in a facility processing the organic fraction of municipal solid waste Luca d’Antonio, Massimiliano Fabbricino ∗ and Ludovico Pontoni Department of Civil, Architectural and Environmental Engineering, University of Naples Federico II, Via Claudio 21, Naples 80125, Italy
Downloaded by [University of Sydney] at 12:41 04 May 2015
(Received 26 August 2014; accepted 18 November 2014 ) The paper investigates, at a laboratory scale, the applicability of anaerobic digestion for the treatment of pressed-off leachate produced in a biomechanical treatment plant for municipal solid waste. Batch tests show that the anaerobic process proceeds smoothly and produces about 10,000 mL of methane per litre of treated leachate. The process is characterized by a lag phase lasting about 30 days, and is completed in about 2 months. Chemical oxygen demand (COD) and volatile fatty acids monitoring allows studying process kinetics that are modelled through a triple linear expression. Physical and biological treatments are also investigated to reduce the residual organic charge of the produced digestate. The best performances are obtained via aerobic degradation followed by assisted sedimentation. This cycle reduces the residual COD of about 85%, and allows the correct disposal of the final waste stream. Keywords: anaerobic digestion; biomechanical treatment; clariflocculation; digestate; pressed-off leachate
Introduction For many years sanitary landfill has been the most used, if not the unique system, for solid waste disposal, almost everywhere in the world. Although it is still widely applied, in the last three decades, because of the increasing attention to the safeguard and protection of the environmental quality, it has been more and more substituted by other options, which consider the waste as a potential source of energy and materials.[1–6] Sanitary landfill has therefore become just one of the elements of a so-called integrated scheme for municipal and industrial waste disposal, together with treatment plants aimed at resources reuse, recycle, recovery and convert (R4C).[7–9] The R4C situation in Italy is certainly not one of the best compared with other countries characterized by similar social and economic characteristics.[7] Yet many new facilities for solid waste valorization have been created during the last 10–15 years. Among them biomechanical treatment plants (BMTPs) for municipal solid waste play a non-secondary role.[10] According to the last available data,[11] about 9.2 million tons·year−1 , that is, 29% of the total produced amount of waste is processed in one of the 122 BMTPs existing in Italy. BMTPs receive the solid waste coming either from source-separated collection, or from commingled collection, and, according to the quality of the incoming waste, are aimed at producing refuse-derived fuel (RDF), stabilized organic matter (SOM) or materials to be reused. Together with about 1.7 million tons
*Corresponding author. Email: [email protected] © 2014 Taylor & Francis
of RDF, 1 million tons of SOM and 0.2 million tons of materials to be reused, the BMTPs produce, yearly, also about 4.5 million tons of solid residues, that are disposed in the existing sanitary landfills, and a consistent amount of wastewater, defined as pressed-off leachate, estimated to be around 71 × 103 tons.[11] Pressed-off leachate is generated by compaction operations aimed at reducing the water content required for SOM and RDF production, or is simply released by gravity from stockpiled wet waste, especially during rainy periods.[12,13] The characteristics of this pressed-off leachate are not dissimilar from those of the better known leachate produced in young sanitary landfills.[14–16] It has a very high organic load, mostly constituted by biodegradable compounds, and is generally poor in terms of phosphorous and nitrogen content, so that aerobic processes are difficult to apply. Although specific studies aimed at individuating the best option for pressedoff leachate treatment and disposal are very few,[12,17–19] it is generally suggested the recourse to anaerobic degradation, in order to supply part of the energy required by the BMT plant through biogas production. Unfortunately, the characteristics of the effluent of the anaerobic process do not meet the required standard for its safe disposal, and further treatment is therefore necessary. The present paper intends to investigate the whole treatment cycle of pressedoff leachate, evaluating its anaerobic biodegradability, calculating the process kinetics and individuating the most efficient process for the final treatment of the produced
1368
L. d’Antonio et al.
digestate before its final disposal, investigating among those generally proposed in the available literature.[20–22]
Downloaded by [University of Sydney] at 12:41 04 May 2015
Material and methods Pressed-off leachate used for the experimental study was collected in a BMTP located in the province of Naples (South of Italy). After collection, the sample was analysed for a complete characterization (Table 1), and subject to anaerobic degradation in 200 mL glass reactors (Schott Duran, Germany). Tests were conducted in batch on triplicates to reduce experimental errors. Each reactor was filled with 1400 mL of fresh leachate and sealed with a 5-mm silicone disc, held tightly to the reactor head by a plastic screw cap punched in the middle (Schott Duran, Germany). The plastic cap was furthermore drilled and a polypropylene rigid tube was inserted and sealed through it in order to allow sampling inside the reactor. Reactors were immersed up to half of their height in a water bath kept at a constant temperature (308 K). During the degradation process, methane production was measured by the water displacement method, leaving the biogas bubbling in an inverted 1-L glass bottle containing a strongly basic solution (12% NaOH) in order to trap any CO2 present in the gas.[23] At fixed times 5 mL of leachate was withdrawn from the reactor, to monitor pH, chemical oxygen demand (COD) alkalinity and total volatile fatty acids (VFA) concentration. The process was ended when cumulative biogas production reached a plateau. At the end of the process, produced digestate was characterized (Table 1) in terms of total suspend solids (TSS), volatile suspended solids (VSS), COD, biochemical oxygen demand (BOD5 ), total nitrogen (TN), ammonia nitrogen (N-NH4 ) and total phosphorous (TP). After characterization, the digestate was divided into two equal fractions that were subject to different treatment sequences. The first fraction was simply clariflocculated, while the second one was oxidized biologically, and clariflocculated. Reagents used for Table 1. Pressed-off leachate and digestate characteristics. Fresh pressed-off leachate pH COD (mg L−1 ) BOD5 (mg L−1 ) TSS (mg L−1 ) VSS (mg L−1 ) −1 N-NH+ 4 (mg L ) −1 TN (mg L ) TP (mg L−1 ) VFA (mg L−1 ) Alkalinity (mg L−1 ) Pb (mg L−1 ) Cr (mg L−1 ) Ni (mg L−1 ) Cd (µg L−1 ) Cu (mg L−1 ) Zn (mg L−1 )
6.8 65,964 35,000 1750 880 111.4 1555 377 12,130 16,780 1.2 2.9 3.3 0.1 1.1 4.1
Digestate 8.6 6518 1000 1530 230 1112 1390 34 1395 2330 1.2 2.9 3.3 0.1 1.1 4.1
clariflocculation were poly-aluminum chloride (coagulant) ® and KLARAID PC4000 (flocculant). Both reagents were purchased from GE Infrastructure. Clariflocculation was conducted using a Jar Test apparatus. In all, 400 mL of the sample was introduced in each jar. The coagulant was added and rapidly mixed at 100 rpm for 3 min. The speed was then reduced to 30 rpm and the flocculant was introduced into the jar for an additional time of 20 min. The solution was finally left to settle for 1 h. Due to the high buffer capacity of the treated digestate, no attempt was made to correct the pH during the process. Tested concentrations of coagulant varied between 0.75 and 6.6 mg L−1 . Tested concentrations of flocculant varied, instead, between 100 and 900 mg L−1 . Biological oxidation was conducted at a bench scale using a batch reactor equipped with a magnetic stirrer and a mechanical pump to supply the required oxygen flow. The test was conducted in two successive steps. During the first step, the digestate was mixed with the aerobic sludge coming from a full-scale municipal wastewater treatment plant, ensuring a food/mass ratio equal to 0.5 BOD5 /VSS. The mixture was magnetically stirred and aerated for 24 h, to allow the acclimatization of the biomass. After 24 h the stirrer and the aeration system were turned off and the mixture was settled for 2 h. The supernatant was then discarded and the biomass was mixed with a fresh digestate, keeping the food/mass ratio equal to 0.3 BOD5 /VSS. The mixture was once more mixed and aerated for 24 h, and settled for 2 h. This time the supernatant was analysed and subject to clariflocculation, following the aforementioned procedure. Leachate, digestate and effluents from biological oxidation and clariflocculation were characterized following international standardized procedures. Alkalinity and total VFA concentrations were titrated according to Anderson and Yang.[24] COD was determined by the open reflux method, according to the APHA standard method 5220B. Heavy metals were determined by Atomic Absorption Spectroscopy, using a GBC AVANTA spectrophotometer. BOD5 was evaluated by a WTW-Oxi-TOP respirometric test. TSS and VSS were obtained by gravimetry. TN and N-NH4 were determined by the Kjeldahl method and steam distillation and titration (VELP, Italy). TP was obtained by the perchloric acid digestion and photometric vanadomolybdophosphoric acid method according to the APHA standard method 4500-P C. pH ( ± 0.05) was measured using a WTW ino-Lab Level 2 pH-meter. All reagents used for analytical determinations were ACS grade. Only ultrapure water was used for all dilutions. Results and discussion Leachate characteristics and anaerobic process development The first column of Table 1 summarizes the characteristics of the used pressed-off leachate. As it can be easily verified, it has a very high organic load, with COD being
1369
Downloaded by [University of Sydney] at 12:41 04 May 2015
Environmental Technology
Figure 1. pH and alkalinity trends during anaerobic digestion.
almost equal to 70 g L−1 . The organic load is, for a large part, biodegradable, as the ratio between BOD5 and COD is around 0.5. The amount of nitrogen and phosphorus compared with the available carbon content is sufficient for the anaerobic degradation, but not necessarily enough for the correct development of an aerobic process. The possibility of carrying out the anaerobic process is justified by the initial pH value and by the alkalinity concentration. The amount of trace metals, although not negligible, is not extremely high, and indicates the possibility of correct biological process development. Most probably, the anaerobic degradation is already started in the BMTP, as suggested by the presence of notable concentration of VFA. The characteristics listed in Table 1 are congruent with those obtained in previous research works.[12,25,26] Preliminarily, information derived by the analysis of the pressed-off leachate is confirmed by the trends of pH and alkalinity during anaerobic degradation, reported in Figure 1. These trends allow stating that the process development is regular and complete, and that no inhibitory factors affect it. What is more important, in terms of process convenience and efficiency, is the trend of cumulate methane production (Figure 2). After the initial lag phase, lasting about 30 days, the biogas production rate rapidly increases, reaching, in one month, an asymptotic value corresponding to 10,000 mL of methane. COD and VFA trends, summarized in Figure 3, are in agreement with methane production, and give some substantial information on the process kinetics. COD degradation and VFA concentration have a specular trend with respect to biogas production. Once again the lag phase lasts about one month and it is followed by a rapid decrease in the targeted concentration and by a final asymptotic value.
The acidogenic phase seems to be predominant during the first 10–20 days. In this period, the organic matter is transformed into volatile acids, but the latter are not yet converted into biogas. Correspondently, COD decreases, while VFA concentration increases (Figure 3), and no methane is produced (Figure 2). Successively, once the acidification is completed, it is possible to observe a decrease in both COD and VFA (Figure 3); the latter are transformed into biogas, and therefore, methane production drastically increases (Figure 2). According to methane, COD and VFA trends, it is possible to represent process kinetics using a triple linear expression, expressed by the following equations: PT1 = a1 (P) · t + b1 (P),
t ≤ 35 days,
(1a)
PT2 = a2 (P) · t + b2 (P),
35 < t ≤ 65 days,
(1b)
PT3 = a3 (P) · t + b3 (P),
65 days ≤ t,
(1c)
where P Ti is the value of the parameter (methane, VFA, COD) in the interval Ti, t is the time and ai (P) and bi (P) are values of the kinetic constants for the parameter P, in the interval Ti. The values of the kinetic constant are summarized in Table 2. From this table, it can be derived that the kinetic constants are related among them and can be verified by the following equivalences: ai (VFA) = 0.212 · ai (COD) − 9.64,
(2a)
ai (methane) = −0.337 · ai (COD) − 28.07,
(2b)
bi (VFA) = 0.189 · bi (COD) + 3355,
(2c)
bi (methane) = −0.351 · bi (COD) − 18, 968.
(2d)
Although other types of kinetic equations could have been applied, the triple linear expression has the advantage of being applicable for all the monitored parameters,
Downloaded by [University of Sydney] at 12:41 04 May 2015
1370
L. d’Antonio et al.
Figure 2. Cumulate biogas production.
Figure 3. COD and VFA trends. Table 2. Values of the kinetic constants. Parameter (P) COD a1 a2 a3 b1 b2 b3
− 150 − 1418 − 94 55.7 100.4 15.8
VFA − 15 − 312 − 55 12.9 22.8 6.9
Methane 16 450 10 30 − 16.5 13.1
and clearly gives an indication of the noted relationships among COD reduction, VFA consumption and methane production.
While Figure 2 gives a clear idea of the convenience of carrying out the anaerobic process to produce biogas, and Figure 3 indicates the possibility of reaching very good efficiency in terms of COD removal, the overall performances of the process can be even better assessed comparing the characteristics of the fresh pressed-off leachate and the characteristics of the digestate produced after 3 months, reported, respectively, in the first and in the last column of Table 1. From such a comparison, it can be noted that the digestate has a residual COD which is only 10% of the initial value, a BOD5 which is reduced of more than 95% and a TSS concentration which is reduced of 65%. The low concentration of VFA confirms that the process is completed and that no further degradation can be obtained from the anaerobic process. In terms of nitrogen and phosphorus,
1371
Environmental Technology Table 3. Characteristics of the digestate after: clariflocculation; biological oxidation and settling; biological oxidation and assisted settling. Digestate after clariflocculation
Downloaded by [University of Sydney] at 12:41 04 May 2015
pH COD (mg L−1 ) BOD5 (mg L−1 ) TSS (mg L−1 ) TVS (mg L−1 ) −1 N-NH+ 4 (mg L ) TN (mg L−1 ) TP (mg L−1 )
6.8 3405 700 206 72 506 1260 27.8
no important change can be noted so that the ratio C/N and C/P is drastically reduced, and therefore, an aerobic treatment of the digestate becomes more feasible. Despite the good removal efficiency, the digestate has still a residual organic charge that is not negligible and therefore needs a further treatment. Digestate treatment The first column of Table 3 summarizes the characteristics of the digestate after clariflocculation. The process was performed using 1.9 mg L−1 of poly-aluminum chlo® ride and 612 mg L−1 of KLARAID PC4000 . The amounts of used electrolyte and polyelectrolyte were optimized through jar tests, as indicated in Section 2. Overall process performances do not exceed 50% of COD removal, as it is typical of chemical–physical treatments. It is worth noting the substantial reduction of nitrogen compounds and suspended solids, which are likely to be present in the form of colloidal particles. It can be therefore deduced that most of the residual organic pollution is in the form of dissolved compounds. The second column of Table 3 lists instead the characteristics of the digestate after biological oxidation. This time it is possible to remove even a larger part of the organic load: COD removal attains 72%, BOD5 removal reaches 55% and TN decreases from 1390 mg L−1 to 440 mg L−1 indicating the development of nitrification processes, with nitrogen being almost completely in the form of ammonia, as a consequence of the anaerobic process carried out before the aerobic one. Moreover, using an assisted coagulation after the biological process it is possible to further increase the performances of the treatment (the last column of Table 3), with COD removal being 50% higher, and BOD5 removal 67% higher. In such a way, the final concentrations of all parameters are sufficiently low to permit the final disposal of the effluent. Conclusion The experimental study allowed evaluating the anaerobic biodegradability of pressed-off leachate produced
Digestate after oxidation and settling
Digestate after oxidation and assisted settling
8 1780 450 920 370 176 440 32.3
6.1 880 150 194 48 118 382 25.2
in BMTPS, to calculate the process kinetics, and to individuate the most efficient treatment process for the final treatment of the produced digestate. From the obtained results, it can be concluded that the pressed-off leachate can be certainly treated through anaerobic processes. The amount of produced biogas is high enough to partially cover the energy requirements of the solid waste treatment plant. Considering the heating value of methane, this corresponds to a potential power generation of 0.1 kWh per cubic meter of treated leachate. The process develops regularly, although an initial lag phase of about 30 days is to be expected. Process kinetics can be simulated using triple linear expressions. The equations to be used to model biogas production, COD reduction and VFA trend have the same form, and are characterized by parameters linearly correlated among them. Although the treatment of the digestate could be performed by clariflocculation, the obtained performances of the process do not allow the final disposal of the produced effluent. Digestate aerobic degradation followed by assisted sedimentation, instead, permits to reach extremely high removal rates, and turns out to be the preferred option to close the cycle of pressed-off leachate production, treatment and disposal.
Disclosure statement No potential conflict of interest was reported by the authors. References [1] Allen A. Containment landfills: the myth of sustainability. Eng Geol. 2001;60:3–19. [2] Fabbricino M. An integrated programme for municipal solid waste management. Waste Manage Res. 2001;19:368–379. [3] D’Antonio G, Fabbricino M, Pirozzi F. Decisional model for integrated management of municipal solid waste – a case study. J Solid Waste Technol Manage. 2002;28:28–43. [4] Renou S, Givaudan JG, Poulain S, Dirassouyan F, Moulin P. Landfill leachate treatment: review and opportunity. J Hazard Mater. 2008;150:468–493.
Downloaded by [University of Sydney] at 12:41 04 May 2015
1372
L. d’Antonio et al.
[5] Rada EC, Istrate IA, Ragazzi M. Trends in the management of residual municipal solid waste. Environ Technol. 2009;7:651–661. [6] Di Maria F. Upgrading of a mechanical biological treatment plant with a solid anaerobic digestion batch: a real case study. Waste Manage Res. 2012;30:1089–1094. [7] Achard PO, Fabbricino M, d’Antonio L. Global resources, recovery, reuse, recycling and conversion in Italy. J Solid Waste Technol Manage. 2014;39:260–274. [8] Davis G. Global resources recovery, reuse, recycling and conversion: an overview from Queensland, Australia. J Solid Waste Technol Manage. 2014;39:225–233. [9] Su M, Shih K. Projects resource recovery, reuse, recycling and conversion (PR4C). J Solid Waste Technol Manage. 2014;39:234–259. [10] Rada EC, Ragazzi M, Panaitescu V, Apostol T. Some research perspectives on emissions from bio-mechanical treatments of municipal solid waste in Europe. Environ Technol. 2005;26:1297–1302. [11] ISPRA (Italy) Municipal Solid Waste Report: Ed 2013; 2014. [12] Nayono S, Winter J, Gallert C. Co-digestion of press water and food waste in a biowaste digester for improvement of biogas production. Bioresource Technol. 2010;101:6987– 6993. [13] Salati S, Scaglia B, Di Gregorio A, Carrera A, Adani F. The use of the dynamic respiration index to predict the potential MSW-leachate impacts after short term mechanical biological treatment. Bioresource Technol. 2013;128:351–358. [14] Ehrig HJ. Quality and quantity of sanitary landfill leachate. Waste Manage Res. 1983;1:53–68. [15] Yildiz ED, Ünlü K, Rowe KR. Modelling leachate quality and quantity in municipal solid waste landfills. Waste Manage Res. 2002;22:78–92. [16] Kulikowska D, Klimiuk E. The effect of landfill age on municipal leachate composition. Bioresource Technol. 2008;99:5981–5985.
[17] Mokhtarani N, Bayatfard A, Mokhtarani B. Full scale performance of compost’s leachate treatment by biological anaerobic reactors. Waste Manage Res. 2012;30: 524–529. [18] Di Maria F, Micale C, Sordi N, Cirulli G. Leachate purification of mechanically sorted organic waste in a simulated bioreactor landfill. Waste Manage Res. 2013;31: 1069–1074. [19] Tran HN, Munnich K, Fricke K, Harborth P. Removal of nitrogen from MBT residues by leachate recirculation in combination with intermittent aeration. Waste Manage Res. 2014;32:56–63. [20] Borzacconi L, Lopez I, Ohanian M, Vinas M. Anaerobic– aerobic treatment of municipal solid waste leachate. Environ Technol. 1999;20:211–217. [21] Kochany J, Lugowski A. Multi stage treatment of high strength leachate. Environ Technol. 2000;21:623–629. [22] Fang F, Abbas AA, Chen Y-P, Liu Z-P, Gao X, Guo J-S. Anaerobic/aerobic/coagulation treatment of leachate from a municipal solid waste incineration plant. Environ Technol. 2012;33:927–935. [23] Liotta F, d’Antonio G, Esposito G, Fabbricino M, Frunzo L, van Hullebusch ED, Lens PNL, Pirozzi F. Effect of moisture on disintegration kinetics during anaerobic digestion of complex organic substrates. Waste Manage Res. 2014;32:40–48. [24] Anderson GK, Yang G. Determination of bicarbonate and total volatile acid concentration in anaerobic digesters using a simple titration. Water Environ Res. 1992;64: 53–59. [25] Kjeldsen P, Barlaz MA, Rooker AP, Baun A, Ledin A, Christensen H. Present and long-term composition of MSW landfill leachate: a review. Critical Rev Environ Sci Technol. 2002;32:297–336. [26] Nayono S, Winter J, Gallert C. Anaerobic digestion of pressed off leachate from the organic fraction of municipal solid waste. Waste Manag. 2010;30:1828–1833.
