effects of intermittent aeration on carbon and nitrogen
EFFECTS OF INTERMITTENT AERATION ON CARBON AND NITROGEN TURNOVER IN LANDFILL SIMULATION REACTORS R. COSSU*, R. RAGA*, G. AGOSTINI*, E. PRIANTE, D. YUE**, A. PIVATO* * DII, Department of Industrial Engineering, University of Padua, via Marzolo 9, 35131 Padova, Italy ** School of Environment, Tsinghua University, Beijing 100084, China
SUMMARY: Landfill aeration is a method for enhancing degradation of organic matter and reducing ammonia present in the leachate. In the present research, six landfill simulation reactors (Ca, Cb, Ia, Ib, Aa, Ab) were filled with municipal solid waste and managed in order to compare an intermittent solution for the aeration of waste with continuous air injection. The experiment was divided in two main phases: landfill simulation (LS) and remediation simulation (RS). The LS phase was made by a pre-aeration, lasted 13 days, and a 53-days long anaerobic period. The RS phase lasted 142 days during which the six columns were managed in three different ways: Ca and Cb were continuously aerated; Ia and Ib were intermittently aerated using the same flux; Aa and Ab were maintained anaerobic. Intermittently aerated columns (Ia and Ib) were managed with three aerobic/anaerobic cycles that lasted respectively 18/20; 20/22 and 26/36 days. Therefore, the aeration time during RS phase of Ia and Ib was the 45% of the aeration time provided to Ca and Cb. Results obtained comparing performances of the different reactors during RS phase show a higher efficiency in organic carbon removal of continuous reactors Ca and Cb with respect to intermittent reactors Ia and Ib, while the ammonia removal efficiency was very similar. The efficiencies were also evaluated considering the oxygen consumed, highlighting better performances of intermittency both in terms of N-NH4+ and TOC removal, 1.65 times higher than continuous aeration.
1. INTRODUCTION Even though, since 1995, the share of waste disposal in EU-27 area more than halved (Eurostat, 2015), landfilling still represents the main method for the disposal of municipal solid waste (MSW) on a global scale (Brandstätter et al., 2015; OECD, 2017). With the development of an environmental awareness, in the last decades, due to the long lasting emissions (gas and leachate) produced by traditional landfills, the interest in sustainable biostabilization of MSW has increased (Hrad and Huber-Humer, 2016). With this aim, landfill aeration has been intensively studied with lab-scale and field-scale research projects (Hrad and Huber-Humer, 2016; Matsuto et al., 2015; Raga and Cossu, 2014; Ritzkowski et al., 2016; Ritzkowski and Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Stegmann, 2012). Results proved that in situ aeration could be a viable strategy for enhancing waste stabilization and shorten post-closure care (Raga and Cossu, 2014; Ritzkowski et al., 2006) but also a technique for reducing landfill emissions (Nag et al., 2015; Sun et al., 2017). Leachate emission produced inside traditional landfill bodies contain large amounts of organic carbon (TOC) and nitrogen, mainly in the form of ammonia (N-NH4+), that is considered to be one of the most long-lasting pollutants produced in sanitary landfills (Berge et al., 2006). In aerated environments, fast organic decomposition is promoted and, thanks to anaerobic pockets that are usually present inside the waste mass, due to the heterogeneity of waste, also nitrification and denitrification are achievable (Berge et al., 2005). So far, lab-scale experiment proved that continuous and intermittent aeration promote both TOC and N-NH4+ removal from recycled leachate (He and Shen, 2006; Nag et al., 2015; Sang et al., 2009; Shao et al., 2008). Nag et al. (2015) compared continuous aeration with two different intermittent aeration routines: continuous aeration had better results in terms of greenhouse gasses (GHGs) reduction and improvement of leachate quality. Short aerobic/anaerobic intermittence (6 h/day of aeration) exhibited improved leachate quality and more GHGs reduction than longer intermittence (3 days/week) and guaranteed a 75% reduction of the energy costs required by continuous aeration. The present research is aimed at studying the influence of intermittent aeration on improving leachate quality and to compare its efficiency with respect to continuous aeration, using longer aerobic/anaerobic periods (~20 days/month) and low flowrate. 2. MATERIALS AND METHODS 2.1 Waste sample The waste material used in this experiment was the residual fraction of MSW collected in a landfill located in Legnago, Northern Italy, before its disposal. Larger objects present in the collected material were removed and the entire mass of waste was grinded at 60 mm and thoroughly mixed to enhance homogeneity. Particle sizing was performed using 50 and 20 mm mash sieves and then a composition analysis was carried out (Figure 1). The water content (WC) was 36.1% and the waste material resulted rich in biodegradable substances with an initial respiration index (RI4) of 23.7 mgO2/g of dry matter (DM).
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Cellulose 4,57%
Composites 5,43% Glass 3,72% Inert 4,04% Metals 1,16% Plastic 8,82%
Undersieve 20 mm [PERCENTUA LE]
Putrascible 1,99% Reusables 0,06% Textiles 0,80%
Figure 1. Composition analysis on waste material used for the experiment. 2.2 Equipment The experiment was carried out using six Plexyglass® (polymethyl methacrylate) landfill simulation reactors (LSRs) with a height of 106 cm and a diameter of 24 cm (Figure 2). The bottom of the column was jointed to a wheeled-support by means of silicone while the top was closed through a lid. All the junctions were sealed with a rubber gasket to avoid leaching or gas leaks. The top lid had four valves for the collection of gas outflows, for the air injection, for the leachate recirculation and for the temperature monitoring. On the bottom of the reactors, a valve was present in order to allow leachate collection in a 5 L high-density polyethylene (HDPE) tank. The leachate recirculation was accomplished through polyvinyl chloride (PVC) pipes and the hydraulic head was ensured by a Heidolph PD 5001® peristaltic pump. A slotted, circular, PVC pipe allowed the homogeneous distribution of the leachate inside the column. The air injection into the waste body was provided using Prodac Air Professional pump 360® and Maxima · R® pumps. The air entered the reactor through a vertical, slotted PVC tube designed not to reach the saturated zone on the bottom of the reactor. The airflow was regulated through Sho-Rate GT1135 and Cole-Parmer® flow-meters. When collected, the output gas was stored into 25 L Tedlar® and Restek® bags and analyzed with a portable Eco-Control analyzer model LFG 20. Temperature monitoring was performed using a Thermo Systems TS100 temperature probe connected to a Endress-Hauser monitor. On the top and the bottom of the waste body in all the columns, two 10 cm drainage layers with small diameter gravel (Ø < 2 mm) were arranged. Reactors were maintained for the whole length of the experiment at a 36°C ± 2.8°C temperature using thermo-regulated insulation system covering the lateral surface.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Leachate distribution pipe Gas collection bag Temperature probe Thermo-regulated insulation system
Flow-meter
Waste
Air pump Gravel layer Output valve Leachate recirculation pump
Figure 2. Reactor design. 2.3 Methodology Each reactor was filled with 18 kg of waste and compacted to a density of 530 kg/m3; then, deionized water was added until the production of 2 L of leachate. Figure 3 shows how the experiment developed in two distinct phases: a landfill simulation phase (LS) and a remediation simulation phase (RS). During LS phase, all six LSRs were managed in the same way. This phase involved a 2 L/d (0.17 L/kgDM/d) leachate recirculation and two different aeration conditions. In the first step, named pre-aeration, an air injection with a 5 NL/h per 12 h/d flux (5.2 NL/kgDM/d) were performed. In the second step, the waste was maintained in anaerobic conditions for other 53 days. During the RS phase, the recirculation was increased in all reactors to 10 L/d (1.13 L/kgDM/d) and aeration regimes were operated in duplicates. Reactors Ca and Cb were managed with a 57.6 L/d (6.50 NL/kgDM/d) continuous aeration; reactors Ia and Ib with three 57.6 L/d aerobic/anaerobic cycles lasted 18/20, 20/22, 26/36 days; reactors Aa and Ab were maintained anaerobic until the end of the experiment.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 3. Experiment model. Vertical dotted lines (···) represents the beginning of anaerobic periods while vertical dashed lines (---) represents the beginning of aerations. 2.4 Sampling and analytical methods Samples of the waste mass were collected in three moments: at the beginning of the experiment, at the end of LS phase (day 66) and at the end of the experiment. The first sample was collected, in triplicate, before the filling of LSRs, after the waste was thoroughly mixed. DM, volatile solids (VS), WC, TOC and Total Kjeldahl Nitrogen (TKN) were measured in accordance with Italian standards and RI4 were analyzed by means of Sapromat (H + P Labortechnik, Germany) (method ANPA 3/2001 12.1.2.3). Another sampling, by each reactor, was performed at day 66 and DM, WC, RI4 and density of the solid matrix were analyzed; the average values are reported in Table 2. The last measurement, replicating the initial analysis, was carried out at the end of the whole experiment, collecting two samples by each column. Biogas composition (v/v % of oxygen (O2), carbon dioxide (CO2) and methane (CH4)) was analyzed by means of IR-analyzer model LFG20. In all aerated reactors, the gas was monitored directly from the gas output valve on the top of reactors and from gas-collection bags only during the anaerobic step of LS phase, since its production was high. During the whole experiment, 39 leachate samples with a volume from 100 to 500 mL were extracted from each reactors that were than backfilled to the original volume with deionized water. In total, roughly 6.5 L of leachate per reactor were extracted with the aim of analysis. Analysis were aimed at monitoring pH, nitrogen species (ammonia, nitrite, nitrate and TKN), organic compounds (BOD5, COD and TOC) and anions (sulfate and chloride) in accordance with Italian standards.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
3. RESULTS AND DISCUSSION 3.1 Solid sample Table 1 reports the evolution of parameters in solid sample during the experiment. In the first 66 days, LS phase produced a decrease of the dry matter (DM) and a reduction of the RI4 from 23.7 to 5.7 mgO2/gDM, indicating a stabilization of waste (Cossu et al., 2012). In addition, density increased in all reactors from 530 to 570 kg/m3. At the end of RS phase, RI4 further decreased in aerated reactors (with no significant difference between continuously or intermittently aerated reactors) to an average value of 3.6 mgO2/gDM while in anaerobic reactors the reduction was not quite so high. Density increased, especially in reactor Ca in which reached 698 kg/m3. Both TOC and TKN decreased along the whole experiment. Table 1 Solid waste parameters at the beginning of the experiment, of the RS phase and at the end of the experiment.
Mass (kg) DM (%) VS (%DM) WC (%) RI4 (mgO2/gDM) Density (kg/m3) TKN (mgN/kgDM) TOC (gC/kgDM)
End of the experiment
Initial
RS phase
Ca
Cb
Ia
Ib
Aa
Ab
18.0 63.9 44.2 36.1 23.7 530 6693 217
18.0 49.2 50.8 5.7 570 -
18.0 37.5 13.8 62.6 3.8 698 3685 189
18.0 39.4 15.3 60.6 3.4 585 2499 176
18.0 41.6 13.8 58.4 3.5 612 3276 185
18.0 37.4 14.0 62.6 3.7 612 3156 198
18.0 43.5 13.6 56.5 4.8 612 3402 162
18.0 43.5 13.6 56.5 5.1 663 3999 180
3.2 Gas In Figure 4 it is shown the characterization of the biogas during the whole experiment. After the 13 days of pre-aeration, which is proved to enhance methanogenesis (Cossu et al., 2016; Morello et al., 2017), a high biogas production occurred. During the anaerobic step of LS phase, 464, 274, 355, 375, 181 and 359 NL of biogas were produced in Ca, Cb, Ia, Ib, Aa and Ab consisting, on average, of 57% CH4 and 39% CO2. During RS phase, in aerated LSRs, CH4 production dropped almost to 0% with only a low production during the first anaerobic period. CO2 production decreased to values lower than 10% in aerated reactors with some production during all the anaerobic phases in Ia and Ib. At the end of the experiment, the amount of O2 provided was 1685 NL (146.5 NL/kgDM) and 759 NL (66 NL/kgDM) respectively in continuously and intermittently aerated reactors. Of these, 43% was used for reactions in reactor Ca, 71% in Cb, 66% in Ia and 72% in Ib. The low percentage of utilization of reactor Ca may be associated with the higher compaction that occurred in this reactor and some preferential pathways that the higher density could have created.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
(a)
(b)
Figure 4. Evolution of biogas composition in continuous (a) and intermittent (b) reactors. 3.3 Leachate Table 2 shows the evolution of leachate parameters. LS phase (from day 1 to 66) showed a high reduction of carbon content and biodegradable substrate with the TOC and BOD5 that reduced respectively by ~81 and ~93% in all reactors. In the same phase, only a slight reduction (~18% on average) of N-NH4+ occurred, mainly attributable to pre aeration and some dilution. Since reactors behaved similarly, the RS phase started with leachates characterized by similar initial conditions (TOC ~3700 mg/L and ~1600 mg N-NH4+/L). Table 3 shows removal performances and efficiencies of all LSRs from the beginning ofhe RS phase. From the beginning to the end of RS phase, N-NH4+ removal efficiency reached almost 99% in all aerated LSRs. In Aa and Ab the N-NH4+ reduction efficiency was lower, as usual in anaerobic reactors (Sun et al., 2014). Figure 5a and 5b highlights how this reduction is largely attributable to dilution. N-NH4+ evolution during the experiment is reported in Figure 6a. TOC evolution during time is reported in Figure 6b. TOC removal efficiency was found higher in continuously aerated reactors, while anaerobic reactors exploited a way lower reduction. The increase in the
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Table 2 Leachate parameters at the beginning of the experiment, of the RS phase and at the end of the experiment. Parameter
Ca
Cb
Ia
Ib
Aa
Ab
Initial (day 1)
pH TKN (mgN/L) N-NH4+ (mgN/L) TOC (mgC/L) COD (mgO2/L) BOD5 (mgO2/L)
6.2 2442 1910 16600 52353 26623
6.2 3075 2094 21150 61153 39858
6.2 2950 1899 19500 62796 39845
6.3 2933 2047 18550 61564 37178
6.2 3549 1940 20450 60262 38431
6.1 2908 2027 19050 55982 30197
Beginning of RS phase (day 67)
pH TKN (mgN/L) N-NH4+ (mgN/L) TOC (mgC/L) COD (mgO2/L) BOD5 (mgO2/L)
7.7 2210 1451 3485 14867 2528
7.8 2324 1721 4078 14383 1544
7.9 2090 1486 3378 13304 1966
7.9 2272 1736 3578 16555 2519
7.9 2474 1778 3925 15739 1821
7.8 2324 1583 3765 14694 3102
End of experiment (day 208)
pH TKN (mgN/L) N-NH4+ (mgN/L) TOC (mgC/L) COD (mgO2/L) BOD5 (mgO2/L)
7.1 139 57 546 2168 < 10
7.1 150 14 370 1240 < 10
7.4 112 23 1138 1816 < 10
7.3 54 27 942 2296 < 10
7.9 1247 1183 2600 8700 36
7.7 1200 1058 1750 6848 71
(a)
(b)
Figure 5 N-NH4+ removal, dilution curve and N-NH4+/Cl ratio in reactor Aa (a) and Ab (b).
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
(b)
(a)
Figure 6. Evolution of N-NH4 (a) and TOC (b) parameters during the whole experiment. Vertical dotted lines (···) represents the beginning of anaerobic periods while vertical dashed lines (---) represents the beginning of aerations.
Figure 7 BOD5/COD ratio showing an increase of waste stability. Vertical dotted lines (···) represents the beginning of anaerobic periods while vertical dashed lines (---) represents the beginning of aerations. Table 3 N-NH4+ and TOC removal and O2 efficiency consumption in RS phase. Ca
Cb
Ia
Ib
Aa
Ab
N-NH4
Initial N-NH4+ (gN) Final N-NH4+ (gN) Removed N-NH4+ (gN) Removal (%) Consumed O2 (kgO2) Efficiency (gN/kgO2/kgDM)
20.86 0.67 20.19 97 727 3.13
22.77 0.17 22.60 99 1203 2.12
19.47 0.26 19.21 99 504 4.30
20.93 0.34 20.59 98 543 4.28
23.34 13.81 9.53 41 -
20.04 12.35 7.69 38 -
TOC
Initial TOC (gC) Final TOC (gC) Removed TOC (gC) Removal (%) Consumed O2 (kgO2) Efficiency (gC/kgO2/kgDM)
44.25 8.41 35.85 81 727 5.56
48.37 5.52 42.85 89 1203 4.02
43.11 7.36 35.75 83 504 8.00
47.58 9.89 37.69 79 543 7.84
53.89 30.36 23.53 44 -
53.10 20.43 32.67 62 -
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
3.4 Carbon and Nitrogen balance Table 4 shows the carbon balance built considering the initial and final TOC content in solid samples and the total biogas (CH4 and CO2) and leachate emission. In nitrogen balance, it was considered the initial and final TKN in solid samples and the emission as leachate. High unaccounted percentages can be attributed to initial and final solid characterization that, due to the heterogeneity of waste, can be subject to high margin of error and to the impossibility to take into account the N2 produced by denitrification processes. Table 4 Carbon mass balance considering initial and final TOC on solid sample and leachate and gas emissions. Nitrogen mass balance considering initial and final TKN on solid sample and leachate emission. Ca
Cb
Ia
Ib
Aa
Ab
217 (100) 108 (50) 8 (4) 20 (9) 2 (1) 79 (36)
217 (100) 121 (56) 9 (4) 15 (7) 2 (1) 71 (33)
217 (100) 116 (53) 10 (5) 15 (7) 2 (1) 74 (34)
217 (100) 110 (51) 5 (2) 5 (2) 2 (1) 95 (44)
217 (100) 122 (56) 9 (4) 8 (4) 2 (1) 75 (35)
6.7 (100) 2.1 (32) 0.8 (12) 3.7 (55)
6.7 (100) 1.9 (28) 1.0 (15) 3.8 (56)
6.7 (100) 2.5 (37) 1.1 (17) 3.1 (46)
6.7 (100) 2.9 (43) 1.1 (17) 2.7 (40)
Carbon balance (gC/kgDM and (%)) Initial content Final content CH4 emission CO2 emission Leachate emission Unaccounted
217 (100) 111 (51) 12 (5) 19 (9) 2 (1) 74 (34)
Nitrogen balance (gN/kgDM and (%)) Initial content Final content Leachate emission Unaccounted
6.7 (100) 2.2 (32) 0.8 (12) 3.7 (55)
6.7 (100) 1.6 (23) 0.7 (11) 4.38 (65)
4. CONCLUSIONS Aeration using 6.5 NL/kgDM/d air flux showed, on average, a 70% efficiency in term of O2 consumed per O2 provided that was similar in all aerated reactors, with the exception of reactor Ca in which the higher compaction may have reduced the efficiency (43%). Intermittently and continuously aerated reactors showed also similar carbon and nitrogen removal efficiencies. NNH4 removal was high in all reactors, reaching efficiencies higher than 97%. TOC removal was slightly higher in Ca and Cb (continuously aerated reactors), in which reached 81 and 89% efficiency compared to 83 and 79% efficiency obtained in reactor Ia and Ib (intermittently aerated reactors). Intermittent aeration caused the consumption of 759 NL of oxygen (45% of the consumption in continuously aerated reactors), proved higher C and N removal per kg of O2 consumed with an efficiency in reactor Ia and Ib 1.65 times higher than in reactor Ca and Cb. These results prove the effectiveness of intermittent aeration in carbon and ammonia removal and suggest it as a viable and cost-saving alternative to continuous aeration. REFERENCES Berge, N.D., Reinhart, D.R., Dietz, J., Townsend, T., 2006. In situ ammonia removal in
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bioreactor landfill leachate. Waste Manag. 26, 334–343. doi:10.1016/j.wasman.2005.11.003 Berge, N.D., Reinhart, D.R., Townsend, T.G., 2005. The Fate of Nitrogen in Bioreactor Landfills. Crit. Rev. Environ. Sci. Technol. 35, 365–399. doi:10.1080/10643380590945003 Brandstätter, C., Laner, D., Fellner, J., 2015. Nitrogen pools and flows during lab-scale degradation of old landfilled waste under different oxygen and water regimes. Biodegradation 26, 399–414. doi:10.1007/s10532-015-9742-5 Cossu, R., Lai, T., Sandon, A., 2012. Standardization of BOD 5/COD ratio as a biological stability index for MSW. Waste Manag. 32, 1503–1508. doi:10.1016/j.wasman.2012.04.001 Cossu, R., Morello, L., Raga, R., Cerminara, G., 2016. Biogas production enhancement using semi-aerobic pre-aeration in a hybrid bioreactor landfill. Waste Manag. 55, 83–92. doi:10.1016/j.wasman.2015.10.025 He, R., Shen, D. sheng, 2006. Nitrogen removal in the bioreactor landfill system with intermittent aeration at the top of landfilled waste. J. Hazard. Mater. 136, 784–790. doi:10.1016/j.jhazmat.2006.01.008 Hrad, M., Huber-Humer, M., 2016. Performance and completion assessment of an in-situ aerated municipal solid waste landfill - Final scientific documentation of an Austrian case study. Waste Manag. 63, 397–409. doi:10.1016/j.wasman.2016.07.043 Matsuto, T., Zhang, X., Matsuo, T., Yamada, S., 2015. Onsite survey on the mechanism of passive aeration and air flow path in a semi-aerobic landfill. Waste Manag. 36, 204–212. doi:10.1016/j.wasman.2014.11.007 Morello, L., Raga, R., Lavagnolo, M.C., Pivato, A., Ali, M., Yue, D., Cossu, R., 2017. The S.An.A.® concept: Semi-aerobic, Anaerobic, Aerated bioreactor landfill. Waste Manag. doi:10.1016/j.wasman.2017.05.006 Nag, M., Shimaoka, T., Komiya, T., 2015. Impact of intermittent aerations on leachate quality and greenhouse gas reduction in the aerobic-anaerobic landfill method. Waste Manag. 55, 71– 82. doi:10.1016/j.wasman.2015.10.018 OECD, 2017. Waste: Waste Management [WWW Document]. OECD Environ. Stat. URL http://dx.doi.org/10.1787/data-00601-en (accessed 7.5.17). Raga, R., Cossu, R., 2014. Landfill aeration in the framework of a reclamation project in Northern Italy. Waste Manag. 34, 683–691. doi:10.1016/j.wasman.2013.12.011 Ritzkowski, M., Heyer, K.U., Stegmann, R., 2006. Fundamental processes and implications during in situ aeration of old landfills. Waste Manag. 26, 356–372. doi:10.1016/j.wasman.2005.11.009 Ritzkowski, M., Stegmann, R., 2012. Landfill aeration worldwide: Concepts, indications and findings. Waste Manag. 32, 1411–1419. doi:10.1016/j.wasman.2012.02.020 Ritzkowski, M., Walker, B., Kuchta, K., Raga, R., Stegmann, R., 2016. Aeration of the teuftal landfill: Field scale concept and lab scale simulation. Waste Manag. 55, 99–107. doi:10.1016/j.wasman.2016.06.004
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Sang, N.N., Soda, S., Inoue, D., Sei, K., Ike, M., 2009. Effects of intermittent and continuous aeration on accelerative stabilization and microbial population dynamics in landfill bioreactors. J. Biosci. Bioeng. 108, 336–343. doi:10.1016/j.jbiosc.2009.04.019 Shao, L.M., He, P.J., Li, G.J., 2008. In situ nitrogen removal from leachate by bioreactor landfill with limited aeration. Waste Manag. 28, 1000–1007. doi:10.1016/j.wasman.2007.02.028 Sun, F., Sun, B., Li, Q., Deng, X., Hu, J., Wu, W., 2014. Pilot-scale nitrogen removal from leachate by ex situ nitrification and in situ denitrification in a landfill bioreactor. Chemosphere 101, 77–85. doi:10.1016/j.chemosphere.2013.12.030 Sun, X., Zhang, H., Cheng, Z., Wang, S., 2017. Effect of low aeration rate on simultaneous nitrification and denitrification in an intermittent aeration aged refuse bioreactor treating leachate. Waste Manag. 63, 410–416. doi:10.1016/j.wasman.2016.12.042
SUMMARY: Landfill aeration is a method for enhancing degradation of organic matter and reducing ammonia present in the leachate. In the present research, six landfill simulation reactors (Ca, Cb, Ia, Ib, Aa, Ab) were filled with municipal solid waste and managed in order to compare an intermittent solution for the aeration of waste with continuous air injection. The experiment was divided in two main phases: landfill simulation (LS) and remediation simulation (RS). The LS phase was made by a pre-aeration, lasted 13 days, and a 53-days long anaerobic period. The RS phase lasted 142 days during which the six columns were managed in three different ways: Ca and Cb were continuously aerated; Ia and Ib were intermittently aerated using the same flux; Aa and Ab were maintained anaerobic. Intermittently aerated columns (Ia and Ib) were managed with three aerobic/anaerobic cycles that lasted respectively 18/20; 20/22 and 26/36 days. Therefore, the aeration time during RS phase of Ia and Ib was the 45% of the aeration time provided to Ca and Cb. Results obtained comparing performances of the different reactors during RS phase show a higher efficiency in organic carbon removal of continuous reactors Ca and Cb with respect to intermittent reactors Ia and Ib, while the ammonia removal efficiency was very similar. The efficiencies were also evaluated considering the oxygen consumed, highlighting better performances of intermittency both in terms of N-NH4+ and TOC removal, 1.65 times higher than continuous aeration.
1. INTRODUCTION Even though, since 1995, the share of waste disposal in EU-27 area more than halved (Eurostat, 2015), landfilling still represents the main method for the disposal of municipal solid waste (MSW) on a global scale (Brandstätter et al., 2015; OECD, 2017). With the development of an environmental awareness, in the last decades, due to the long lasting emissions (gas and leachate) produced by traditional landfills, the interest in sustainable biostabilization of MSW has increased (Hrad and Huber-Humer, 2016). With this aim, landfill aeration has been intensively studied with lab-scale and field-scale research projects (Hrad and Huber-Humer, 2016; Matsuto et al., 2015; Raga and Cossu, 2014; Ritzkowski et al., 2016; Ritzkowski and Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Stegmann, 2012). Results proved that in situ aeration could be a viable strategy for enhancing waste stabilization and shorten post-closure care (Raga and Cossu, 2014; Ritzkowski et al., 2006) but also a technique for reducing landfill emissions (Nag et al., 2015; Sun et al., 2017). Leachate emission produced inside traditional landfill bodies contain large amounts of organic carbon (TOC) and nitrogen, mainly in the form of ammonia (N-NH4+), that is considered to be one of the most long-lasting pollutants produced in sanitary landfills (Berge et al., 2006). In aerated environments, fast organic decomposition is promoted and, thanks to anaerobic pockets that are usually present inside the waste mass, due to the heterogeneity of waste, also nitrification and denitrification are achievable (Berge et al., 2005). So far, lab-scale experiment proved that continuous and intermittent aeration promote both TOC and N-NH4+ removal from recycled leachate (He and Shen, 2006; Nag et al., 2015; Sang et al., 2009; Shao et al., 2008). Nag et al. (2015) compared continuous aeration with two different intermittent aeration routines: continuous aeration had better results in terms of greenhouse gasses (GHGs) reduction and improvement of leachate quality. Short aerobic/anaerobic intermittence (6 h/day of aeration) exhibited improved leachate quality and more GHGs reduction than longer intermittence (3 days/week) and guaranteed a 75% reduction of the energy costs required by continuous aeration. The present research is aimed at studying the influence of intermittent aeration on improving leachate quality and to compare its efficiency with respect to continuous aeration, using longer aerobic/anaerobic periods (~20 days/month) and low flowrate. 2. MATERIALS AND METHODS 2.1 Waste sample The waste material used in this experiment was the residual fraction of MSW collected in a landfill located in Legnago, Northern Italy, before its disposal. Larger objects present in the collected material were removed and the entire mass of waste was grinded at 60 mm and thoroughly mixed to enhance homogeneity. Particle sizing was performed using 50 and 20 mm mash sieves and then a composition analysis was carried out (Figure 1). The water content (WC) was 36.1% and the waste material resulted rich in biodegradable substances with an initial respiration index (RI4) of 23.7 mgO2/g of dry matter (DM).
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Cellulose 4,57%
Composites 5,43% Glass 3,72% Inert 4,04% Metals 1,16% Plastic 8,82%
Undersieve 20 mm [PERCENTUA LE]
Putrascible 1,99% Reusables 0,06% Textiles 0,80%
Figure 1. Composition analysis on waste material used for the experiment. 2.2 Equipment The experiment was carried out using six Plexyglass® (polymethyl methacrylate) landfill simulation reactors (LSRs) with a height of 106 cm and a diameter of 24 cm (Figure 2). The bottom of the column was jointed to a wheeled-support by means of silicone while the top was closed through a lid. All the junctions were sealed with a rubber gasket to avoid leaching or gas leaks. The top lid had four valves for the collection of gas outflows, for the air injection, for the leachate recirculation and for the temperature monitoring. On the bottom of the reactors, a valve was present in order to allow leachate collection in a 5 L high-density polyethylene (HDPE) tank. The leachate recirculation was accomplished through polyvinyl chloride (PVC) pipes and the hydraulic head was ensured by a Heidolph PD 5001® peristaltic pump. A slotted, circular, PVC pipe allowed the homogeneous distribution of the leachate inside the column. The air injection into the waste body was provided using Prodac Air Professional pump 360® and Maxima · R® pumps. The air entered the reactor through a vertical, slotted PVC tube designed not to reach the saturated zone on the bottom of the reactor. The airflow was regulated through Sho-Rate GT1135 and Cole-Parmer® flow-meters. When collected, the output gas was stored into 25 L Tedlar® and Restek® bags and analyzed with a portable Eco-Control analyzer model LFG 20. Temperature monitoring was performed using a Thermo Systems TS100 temperature probe connected to a Endress-Hauser monitor. On the top and the bottom of the waste body in all the columns, two 10 cm drainage layers with small diameter gravel (Ø < 2 mm) were arranged. Reactors were maintained for the whole length of the experiment at a 36°C ± 2.8°C temperature using thermo-regulated insulation system covering the lateral surface.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Leachate distribution pipe Gas collection bag Temperature probe Thermo-regulated insulation system
Flow-meter
Waste
Air pump Gravel layer Output valve Leachate recirculation pump
Figure 2. Reactor design. 2.3 Methodology Each reactor was filled with 18 kg of waste and compacted to a density of 530 kg/m3; then, deionized water was added until the production of 2 L of leachate. Figure 3 shows how the experiment developed in two distinct phases: a landfill simulation phase (LS) and a remediation simulation phase (RS). During LS phase, all six LSRs were managed in the same way. This phase involved a 2 L/d (0.17 L/kgDM/d) leachate recirculation and two different aeration conditions. In the first step, named pre-aeration, an air injection with a 5 NL/h per 12 h/d flux (5.2 NL/kgDM/d) were performed. In the second step, the waste was maintained in anaerobic conditions for other 53 days. During the RS phase, the recirculation was increased in all reactors to 10 L/d (1.13 L/kgDM/d) and aeration regimes were operated in duplicates. Reactors Ca and Cb were managed with a 57.6 L/d (6.50 NL/kgDM/d) continuous aeration; reactors Ia and Ib with three 57.6 L/d aerobic/anaerobic cycles lasted 18/20, 20/22, 26/36 days; reactors Aa and Ab were maintained anaerobic until the end of the experiment.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 3. Experiment model. Vertical dotted lines (···) represents the beginning of anaerobic periods while vertical dashed lines (---) represents the beginning of aerations. 2.4 Sampling and analytical methods Samples of the waste mass were collected in three moments: at the beginning of the experiment, at the end of LS phase (day 66) and at the end of the experiment. The first sample was collected, in triplicate, before the filling of LSRs, after the waste was thoroughly mixed. DM, volatile solids (VS), WC, TOC and Total Kjeldahl Nitrogen (TKN) were measured in accordance with Italian standards and RI4 were analyzed by means of Sapromat (H + P Labortechnik, Germany) (method ANPA 3/2001 12.1.2.3). Another sampling, by each reactor, was performed at day 66 and DM, WC, RI4 and density of the solid matrix were analyzed; the average values are reported in Table 2. The last measurement, replicating the initial analysis, was carried out at the end of the whole experiment, collecting two samples by each column. Biogas composition (v/v % of oxygen (O2), carbon dioxide (CO2) and methane (CH4)) was analyzed by means of IR-analyzer model LFG20. In all aerated reactors, the gas was monitored directly from the gas output valve on the top of reactors and from gas-collection bags only during the anaerobic step of LS phase, since its production was high. During the whole experiment, 39 leachate samples with a volume from 100 to 500 mL were extracted from each reactors that were than backfilled to the original volume with deionized water. In total, roughly 6.5 L of leachate per reactor were extracted with the aim of analysis. Analysis were aimed at monitoring pH, nitrogen species (ammonia, nitrite, nitrate and TKN), organic compounds (BOD5, COD and TOC) and anions (sulfate and chloride) in accordance with Italian standards.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
3. RESULTS AND DISCUSSION 3.1 Solid sample Table 1 reports the evolution of parameters in solid sample during the experiment. In the first 66 days, LS phase produced a decrease of the dry matter (DM) and a reduction of the RI4 from 23.7 to 5.7 mgO2/gDM, indicating a stabilization of waste (Cossu et al., 2012). In addition, density increased in all reactors from 530 to 570 kg/m3. At the end of RS phase, RI4 further decreased in aerated reactors (with no significant difference between continuously or intermittently aerated reactors) to an average value of 3.6 mgO2/gDM while in anaerobic reactors the reduction was not quite so high. Density increased, especially in reactor Ca in which reached 698 kg/m3. Both TOC and TKN decreased along the whole experiment. Table 1 Solid waste parameters at the beginning of the experiment, of the RS phase and at the end of the experiment.
Mass (kg) DM (%) VS (%DM) WC (%) RI4 (mgO2/gDM) Density (kg/m3) TKN (mgN/kgDM) TOC (gC/kgDM)
End of the experiment
Initial
RS phase
Ca
Cb
Ia
Ib
Aa
Ab
18.0 63.9 44.2 36.1 23.7 530 6693 217
18.0 49.2 50.8 5.7 570 -
18.0 37.5 13.8 62.6 3.8 698 3685 189
18.0 39.4 15.3 60.6 3.4 585 2499 176
18.0 41.6 13.8 58.4 3.5 612 3276 185
18.0 37.4 14.0 62.6 3.7 612 3156 198
18.0 43.5 13.6 56.5 4.8 612 3402 162
18.0 43.5 13.6 56.5 5.1 663 3999 180
3.2 Gas In Figure 4 it is shown the characterization of the biogas during the whole experiment. After the 13 days of pre-aeration, which is proved to enhance methanogenesis (Cossu et al., 2016; Morello et al., 2017), a high biogas production occurred. During the anaerobic step of LS phase, 464, 274, 355, 375, 181 and 359 NL of biogas were produced in Ca, Cb, Ia, Ib, Aa and Ab consisting, on average, of 57% CH4 and 39% CO2. During RS phase, in aerated LSRs, CH4 production dropped almost to 0% with only a low production during the first anaerobic period. CO2 production decreased to values lower than 10% in aerated reactors with some production during all the anaerobic phases in Ia and Ib. At the end of the experiment, the amount of O2 provided was 1685 NL (146.5 NL/kgDM) and 759 NL (66 NL/kgDM) respectively in continuously and intermittently aerated reactors. Of these, 43% was used for reactions in reactor Ca, 71% in Cb, 66% in Ia and 72% in Ib. The low percentage of utilization of reactor Ca may be associated with the higher compaction that occurred in this reactor and some preferential pathways that the higher density could have created.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
(a)
(b)
Figure 4. Evolution of biogas composition in continuous (a) and intermittent (b) reactors. 3.3 Leachate Table 2 shows the evolution of leachate parameters. LS phase (from day 1 to 66) showed a high reduction of carbon content and biodegradable substrate with the TOC and BOD5 that reduced respectively by ~81 and ~93% in all reactors. In the same phase, only a slight reduction (~18% on average) of N-NH4+ occurred, mainly attributable to pre aeration and some dilution. Since reactors behaved similarly, the RS phase started with leachates characterized by similar initial conditions (TOC ~3700 mg/L and ~1600 mg N-NH4+/L). Table 3 shows removal performances and efficiencies of all LSRs from the beginning ofhe RS phase. From the beginning to the end of RS phase, N-NH4+ removal efficiency reached almost 99% in all aerated LSRs. In Aa and Ab the N-NH4+ reduction efficiency was lower, as usual in anaerobic reactors (Sun et al., 2014). Figure 5a and 5b highlights how this reduction is largely attributable to dilution. N-NH4+ evolution during the experiment is reported in Figure 6a. TOC evolution during time is reported in Figure 6b. TOC removal efficiency was found higher in continuously aerated reactors, while anaerobic reactors exploited a way lower reduction. The increase in the
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Table 2 Leachate parameters at the beginning of the experiment, of the RS phase and at the end of the experiment. Parameter
Ca
Cb
Ia
Ib
Aa
Ab
Initial (day 1)
pH TKN (mgN/L) N-NH4+ (mgN/L) TOC (mgC/L) COD (mgO2/L) BOD5 (mgO2/L)
6.2 2442 1910 16600 52353 26623
6.2 3075 2094 21150 61153 39858
6.2 2950 1899 19500 62796 39845
6.3 2933 2047 18550 61564 37178
6.2 3549 1940 20450 60262 38431
6.1 2908 2027 19050 55982 30197
Beginning of RS phase (day 67)
pH TKN (mgN/L) N-NH4+ (mgN/L) TOC (mgC/L) COD (mgO2/L) BOD5 (mgO2/L)
7.7 2210 1451 3485 14867 2528
7.8 2324 1721 4078 14383 1544
7.9 2090 1486 3378 13304 1966
7.9 2272 1736 3578 16555 2519
7.9 2474 1778 3925 15739 1821
7.8 2324 1583 3765 14694 3102
End of experiment (day 208)
pH TKN (mgN/L) N-NH4+ (mgN/L) TOC (mgC/L) COD (mgO2/L) BOD5 (mgO2/L)
7.1 139 57 546 2168 < 10
7.1 150 14 370 1240 < 10
7.4 112 23 1138 1816 < 10
7.3 54 27 942 2296 < 10
7.9 1247 1183 2600 8700 36
7.7 1200 1058 1750 6848 71
(a)
(b)
Figure 5 N-NH4+ removal, dilution curve and N-NH4+/Cl ratio in reactor Aa (a) and Ab (b).
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
(b)
(a)
Figure 6. Evolution of N-NH4 (a) and TOC (b) parameters during the whole experiment. Vertical dotted lines (···) represents the beginning of anaerobic periods while vertical dashed lines (---) represents the beginning of aerations.
Figure 7 BOD5/COD ratio showing an increase of waste stability. Vertical dotted lines (···) represents the beginning of anaerobic periods while vertical dashed lines (---) represents the beginning of aerations. Table 3 N-NH4+ and TOC removal and O2 efficiency consumption in RS phase. Ca
Cb
Ia
Ib
Aa
Ab
N-NH4
Initial N-NH4+ (gN) Final N-NH4+ (gN) Removed N-NH4+ (gN) Removal (%) Consumed O2 (kgO2) Efficiency (gN/kgO2/kgDM)
20.86 0.67 20.19 97 727 3.13
22.77 0.17 22.60 99 1203 2.12
19.47 0.26 19.21 99 504 4.30
20.93 0.34 20.59 98 543 4.28
23.34 13.81 9.53 41 -
20.04 12.35 7.69 38 -
TOC
Initial TOC (gC) Final TOC (gC) Removed TOC (gC) Removal (%) Consumed O2 (kgO2) Efficiency (gC/kgO2/kgDM)
44.25 8.41 35.85 81 727 5.56
48.37 5.52 42.85 89 1203 4.02
43.11 7.36 35.75 83 504 8.00
47.58 9.89 37.69 79 543 7.84
53.89 30.36 23.53 44 -
53.10 20.43 32.67 62 -
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
3.4 Carbon and Nitrogen balance Table 4 shows the carbon balance built considering the initial and final TOC content in solid samples and the total biogas (CH4 and CO2) and leachate emission. In nitrogen balance, it was considered the initial and final TKN in solid samples and the emission as leachate. High unaccounted percentages can be attributed to initial and final solid characterization that, due to the heterogeneity of waste, can be subject to high margin of error and to the impossibility to take into account the N2 produced by denitrification processes. Table 4 Carbon mass balance considering initial and final TOC on solid sample and leachate and gas emissions. Nitrogen mass balance considering initial and final TKN on solid sample and leachate emission. Ca
Cb
Ia
Ib
Aa
Ab
217 (100) 108 (50) 8 (4) 20 (9) 2 (1) 79 (36)
217 (100) 121 (56) 9 (4) 15 (7) 2 (1) 71 (33)
217 (100) 116 (53) 10 (5) 15 (7) 2 (1) 74 (34)
217 (100) 110 (51) 5 (2) 5 (2) 2 (1) 95 (44)
217 (100) 122 (56) 9 (4) 8 (4) 2 (1) 75 (35)
6.7 (100) 2.1 (32) 0.8 (12) 3.7 (55)
6.7 (100) 1.9 (28) 1.0 (15) 3.8 (56)
6.7 (100) 2.5 (37) 1.1 (17) 3.1 (46)
6.7 (100) 2.9 (43) 1.1 (17) 2.7 (40)
Carbon balance (gC/kgDM and (%)) Initial content Final content CH4 emission CO2 emission Leachate emission Unaccounted
217 (100) 111 (51) 12 (5) 19 (9) 2 (1) 74 (34)
Nitrogen balance (gN/kgDM and (%)) Initial content Final content Leachate emission Unaccounted
6.7 (100) 2.2 (32) 0.8 (12) 3.7 (55)
6.7 (100) 1.6 (23) 0.7 (11) 4.38 (65)
4. CONCLUSIONS Aeration using 6.5 NL/kgDM/d air flux showed, on average, a 70% efficiency in term of O2 consumed per O2 provided that was similar in all aerated reactors, with the exception of reactor Ca in which the higher compaction may have reduced the efficiency (43%). Intermittently and continuously aerated reactors showed also similar carbon and nitrogen removal efficiencies. NNH4 removal was high in all reactors, reaching efficiencies higher than 97%. TOC removal was slightly higher in Ca and Cb (continuously aerated reactors), in which reached 81 and 89% efficiency compared to 83 and 79% efficiency obtained in reactor Ia and Ib (intermittently aerated reactors). Intermittent aeration caused the consumption of 759 NL of oxygen (45% of the consumption in continuously aerated reactors), proved higher C and N removal per kg of O2 consumed with an efficiency in reactor Ia and Ib 1.65 times higher than in reactor Ca and Cb. These results prove the effectiveness of intermittent aeration in carbon and ammonia removal and suggest it as a viable and cost-saving alternative to continuous aeration. REFERENCES Berge, N.D., Reinhart, D.R., Dietz, J., Townsend, T., 2006. In situ ammonia removal in
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