Post on 16-Sep-2018




0 download

Embed Size (px)


<ul><li><p>57ime CONGRS CANADIEN DE GOTECHNIQUE 57TH CANADIAN GEOTECHNICAL CONFERENCE5ime CONGRS CONJOINT SCG/AIH-CNN 5TH JOINT CGS/IAH-CNC CONFERENCE</p><p>DEGRADATION OF MUNICIPAL SOLID WASTE UNDER AEROBIC CONDITIONS</p><p>M.A. Warith, Ryerson University, Toronto, Ontario, Canada L. Fernandes, University of Ottawa, Ottawa, Ontario, Canada S. Rendra, University of Ottawa, Ottawa, Ontario, Canada</p><p>ABSTRACT: The biodegradation of municipal solid waste (MSW) was investigated in simulated bioreactor landfills under aerobic conditions. The bioreactors were operated to determine the quantity of moisture and sludge that would optimize waste degradation. The leachate generated was recycled over 47 weeks, and leachate samples were collected on a weekly basis and analyzed for pH and organic concentration measured in terms of BOD and COD. Leachate recirculation and sludge addition at the rate of 855 and 85 mL/kg of waste/d in the aerobic bioreactors proved to be sufficient to enhance the MSW degradation within 27 weeks where COD concentration was about 1000 mg/L. This rate was the highest among the bioreactors. Reduction of leachate recirculation and sludge addition to 285 and 28 mL/kg of waste/d increased the stabilization period up to 45 weeks. These results revealed that the more moisture and sludge addition the more of rate biodegradation of MSW can be enhanced. Fitted empirical models indicated that COD concentration in the leachate was affected by leachate recirculation and sludge addition. However, the effect of leachate recirculation was much stronger than sludge addition. </p><p>RESUME: La dgradation biologique des dchets solides municipaux a t tudie en simulation dans des bioracteur en conditions arobiques. Les bioracteurs ont t utilis pour determiner le taux dhumidit et de boue qui optimiserait la degradation des dchets. Le lixiviat produit a t recycle pendant 47 semaines. On a pris des chantillons de lixiviat chaque semaine et on a fait les analyses de pH et de matires organiques mesures en terme de DBO et DCO. La recirculation de lixiviat et lajout de boue un taux variant entre 855 et 85 mL/Kg de dchets/jour dans les bioracteurs arobiques savre tre suffisante pour accrotre la dgradation des dchets solides en moins de 27 semaines et la concentration de DCO a t denviron 1000mg/L. Ce taux a t le plus lev obtenu parmi tous les bioracteurs. La rduction de la recirculation de lixiviat et lajout de boue 285 et 28 mL/Kg de dchets solides/jour a fait passer la priode de stabilisation de 27 45 semaines. Ces rsultats rvlent que plus on ajoute de lixiviat et de boue, plus le taux de dgradation biologique de dchets solides peut tre acclr. Le modle empirique qui ressort de cette exprimantation indique que la concentration de DCO dans le lixiviat a t influence par la recirculation de ce lixiviat et lajout de boue. Toutefois, limpact de la recirculation de lixiviat a t beaucoup plus fort que celui de lajout de boue. </p><p>1.0 INTRODUCTION </p><p>Several factors influence biodegradation of municipal solid waste (MSW). These factors can be manipulated to enhance biodegradation in a landfill environment. These manipulation techniques include moisture content management, the use of buffers, nutrient addition, reduction of waste particle size, and sludge addition. Moisture content can be most practically controlled through leachate recirculation, which is the most common enhancement mechanism used in the field. Numerous studies have been conducted to investigate the effects of leachate recirculation on biological waste stabilization, leachate quality, gas production, waste settlement, and other factors (Warith et al., 1999; Warith et al. 1998, Bae et al, 1998; Reinhart and Al-Yousfi, 1996; Townsend et al., 1995; Pohland and Al-Yousfi, 1994). The general conclusion of all these studies was that waste stabilization could be achieved by leachate recirculation. In most cases, leachate recirculation is practiced as a novel approach to managing leachate without regard to optimize the use of landfill as a bioreactor. </p><p>The aerobic degradation of MSW is mostly covered in the composting literature but not in aerobic landfill. The addition of water and sewage sludge is also commonly used in the composting system as amendments agents. However, the addition of water does not through leachate recirculation in which this technique is usually applied in landfilling practice. Presently, there is limited information in the literature regarding the use of aerobic landfill as a method of solid waste disposal. In the aerobic landfill, the addition of air, and in turn oxygen, promotes the aerobic stabilization of the landfilled waste. This is the same process that occurs in a waste composting system. However, aerobic composting is accomplished within the landfill rather than in windrows or other methods in which composting is normally practiced. The main products of aerobic degradation are carbon dioxide, water, and heat. In the aerobic landfill, methane energy is sacrificed by the addition of air. In addition, significant waste fractions such as lignin and ligneous materials, which are not degradable anaerobically are degraded aerobically (Stessel and Murphy, 1992). Based on the study of aerobic landfill conducted by Environmental Control System Inc., methane generation has decreased by more than 98% (Vitello, 2001). It is apparent that the aerobic bioreactor </p><p>Session 2DPage 1</p></li><li><p>landfill has some advantages over the anaerobicbioreactor landfill; however, there are very limitedexperimental data available to support this.</p><p>The purpose of this paper is to evaluate the use ofaerobic processes for the rapid stabilization of MSW insimulated bioreactor landfills. In addition, the optimumfactors of leachate recirculation and sludge addition thatenhance the degradation of MSW are examined.Another aspect of this research is to evaluate the use of aerobic bioreactor landfills technology in term of thebiodegradation using empirical model.</p><p>2.0 MATERIAL AND METHODS</p><p>2.1 Simulated bioreactors setupThe experimental setup consisted of six simulatedbioreactor landfill models made from Plexiglas withdimensions of 150-mm in diameter and 550-mm inheight. Bioreactors were operated with leachaterecirculation and equipped with an outlet port at thebottom to collect leachate from the bioreactors, and aninlet port at the top of the bioreactors to distributesimulated rainfall and the recirculated leachate. Theleachate from the bioreactors were collected and storedin tanks before being recycled back at the top of eachreactor. Also all bioreactors were sealed usingconstruction-type sealant in order to avoid leachatefrom leaks. A perforated PVC pipe with diameter of12.7-mm used as an air distribution systems. This pipe was wrapped in a plastic mesh prior to inserting into thewaste matrix and was connected to the laboratory airsupply system. The configurations of the bioreactorsare shown in Figure 1. </p><p>Inlet port Air inlet</p><p> Pump </p><p> Outlet port</p><p> Leachate reservoir</p><p>Figure 1 Configuration of Simulated Bioreactor.</p><p>The MSW used in this experiment was collected fromcurbside of Ottawa City in order to achieve representative sample of municipal solid waste that arenormally disposed in a landfill. As is presented in Table1, the major components of this waste were paper(36.6%), food (36.2%) and yard trimmings (27.2%). Thecollected waste was shredded manually to about 50 to 100-mm in diameter and was mixed before being loaded into the bioreactors. A 100-mm layer ofshredded solid waste was filled and compacted to adensity of 350 kg/m3. This technique was repeated untilthe thickness of solid waste in each reactor reached aheight of 400-mm. A supporting media, consisting ofmarbles (15-mm in diameter), with thickness of about50-mm was placed at the bottom of each reactor and used as a leachate collection system. In addition, themarbles beds were used to prevent clogging of theleachate outlet at the bottom of the bioreactors. Aperforated plate was placed at the top of the wastematrix to provide equal distribution of water andleachate recirculation. Approximately 2 liters of tap water was added per day to the bioreactors in order to</p><p>attain field capacity. After day 4, when leachate hadbeen generated, simulated rainfall was reduced to 2 L/wk. Leachate, then, was recycled daily at the top ofthe bioreactors.</p><p>In all bioreactors, the pH was controlled by addingsodium bicarbonate (NaHCO3). Addition of bufferdepended on the pH of the leachate in each bioreactor.Anaerobically digested municipal sewage sludge wasadded to the bioreactors through leachate recirculationsystem. The sludge was collected from ROPEC Wastewater Treatment Plant in Ottawa. Air wassupplied to the aerobic bioreactors to maintain theaerobic conditions. The flowrate supplied of air mustsatisfy the oxygen demand exerted by the organicwaste decomposition (stoichiometric demand). In thisresearch, the assumption of solid waste composition isC64H104O37N (Haug, 1993). The Consequently, theoxygen demand calculated was 1.53 g O2 /g waste(dry). This is a total quantity of oxygen that must besupplied to the aerobic bioreactors to meet thestoichiometric demand. However, in this research a</p><p>100 mm</p><p>400 mm Thermocouple</p><p>50 mm</p><p>Session 2DPage 2</p></li><li><p>safety factor of 2 was applied to avoid anaerobicpockets. A total flow rate of air was 185 L air/day.</p><p>In order to record the progress of landfill stabilizationwithin the bioreactors, a combination of parametersconcerning solid waste and leachate were monitoredthroughout this experiment. Leachate samples collectedfrom the leachate outlet port were stored in arefrigerator at 40C prior to measurement for chemical,physical and biochemical parameters. Of particularinterest in evaluating the effect of leachate recirculation,with sludge addition, on solid waste biodegradationwere biochemical oxygen demand (BOD), chemicaloxygen demand (COD); these were measured on abiweekly basis. BOD and COD were analyzed byprocedures outlined in Standard Methods (5210B and5220C, AWWA-APHA 1998).</p><p>2.2 Factorial Design</p><p>An empirical mathematical model was used to describethe effects of operating variables on the CODconcentration of the leachate (process response, E(Y)).In this model, at a certain time, the data was fitted to the first order linear model using a factorial design. Themodel is expressed,</p><p>E(Y) = 0 + 1 x1 + 2 x2 +12 x1 x2 Eq. 1 </p><p>where E(Y) is the expected value of the observedresponse, i is a model parameter and xi is a coded</p><p>operating variable. The model parameters wereestimated using data collected according to a 22</p><p>factorial experimental design including two center pointreplicates from which reproducibility and lack of fit wasevaluated.</p><p>To assess the effects of leachate recirculation andsludge addition on the biodegradation of solid waste,three scenarios providing different rates of leachaterecirculation and sludge addition had been decided. . These scenarios are coded as a factorial experimentalTo simplify notation and subsequent calculation x1 andx2 in equation 1 would be dimensionless codedvariables defined as follows:</p><p>x1 = (rate of leachate recirculation-10)/5x2 = (sludge addition-1)/0.5</p><p>Each operating variable was tested at only twoconditions. Therefore, four possible combinations ofoperating conditions using the coding x1 and x2 wasderived from Table 2. The following design strategy offour combinations of operating conditions and twocenter point replicates, as outlined in Table 3, was putforward for this (Box et al., 1978) and according to a 22 factorial experimental design, the results of 22 factorial experimental design are outlined in Table 2. </p><p>Table 1 Composition of MSW Used in Simulated Bioreactors</p><p>Component Total weight of MSW (wet basis)</p><p>(kg) (%)</p><p>Corrugated and Kraft 0.319 12.77</p><p>Newspaper 0.174 6.97</p><p>Computer and high grade paper 0.099 3.96</p><p>Others 0.323 12.91</p><p>Fruits 0.120 4.78</p><p>Vegetables 0.239 9.56</p><p>Pasta, Rice 0.085 3.42</p><p>Bread 0.085 3.42</p><p>Meat/Fish 0.085 5.47</p><p>Diary product 0.051 2.05</p><p>Coffee/Tea 0.034 1.36</p><p>Others 0.154 6.15</p><p>Grass 0.256 10.25</p><p>Leaves 0.075 3.01</p><p>Trimmings 0.348 13.93Total 2.50 100.00</p><p>Session 2DPage 3</p></li><li><p> Table 2 Different scenarios at which operating variables were investigated in the factorial design.Levels</p><p>Type of operating variableLow Center High</p><p>Rate of leachate recirculation (L/week) 5 10 15Rate of sludge addition (L/wk) 0.5 1 1.5Coded -1 0 +1</p><p> Table 3 A 22 factorial design in coded variables with two center point replicates</p><p>Bioreactor Name x1 x2 x1 x2</p><p>AER1 -1 -1 +1</p><p>AER2 +1 -1 -1</p><p>AER3 -1 +1 -1</p><p>AER4 +1 +1 +1</p><p>AER5 0 0 0</p><p>AER6 0 0 0</p><p>3.0 RESULTS AND DISCUSSION </p><p>The variations of pH in leachate during theexperimental period are presented in Figure 2. Duringthe initial phase, the pH in the leachate varied from 5.6-6.7 from all bioreactors in the period starting from week1 to 5 with the exception of the AER2 and AER4 inwhich the pH was stabilized at about neutral (pH 7-7.5)after 3 weeks of operation. While the pH in the leachatefrom the AER1, AER3, AER5 and AER 6 reachedbetween 7 to 7.5 after 10 weeks. The data show that</p><p>the pH in the AER2 and AER4 reached at about neutralcondition faster than in anaerobic bioreactors. Thisindicated that an acclimation period or initial lag time ofthe AER2 and AER4, where highest leachaterecirculations were applied, faster than the rest ofbioreactors. The leachate pH, then, was controlled byadding sodium bicarbonate (NaHCO3). Therefore, thepH in the leachate samples was consistently at therange of neutral condition until the end of experimentalstudy.</p><p>5</p><p>5.5</p><p>6</p><p>6.5</p><p>7</p><p>7.5</p><p>8</p><p>1 5 9 13 17 21 25 29 33 37 41 45 49</p><p>Time, weeks</p><p>pH, u</p><p>nits</p><p>AER1 AER2</p><p>AER3 AER4</p><p>AER5 AER6</p><p> Figure 2 pH in Leachate Collected from Aerobic Bioreactors</p><p>The two parameters used to evaluate the effect of leachate recirculation and sludge addition on waste</p><p>degradation were COD and BOD. Figure 3 depicts theconcentration of COD in the leachate from the </p><p>Session 2DPage 4</p></li><li><p>bioreactors. At the initial stage, the COD concentrationin the leachate from AER1, AER3, AER5 and AER 6increased slowly for a period of 9 weeks and generallyremained below 10,000 mg/L. After this stage, the CODconcentration increased exponentially and reached atpeak value of about 40,000 mg/L for AER5 and AER6,45,000 mg/L for AER3, and 51,000 mg/L for AER1 until17 and 19 weeks. While, after 3 weeks of operation, theCOD concentration increased exponentially andreached at peak value of about 38,000 mg/L for AER1,and 40,000 mg/L for AER3 until 13 weeks and werefollowed by a decrease in the COD concentration atalmost constant rate until 25 weeks. It can bee seenthat all curves had a similar trends, with rising part,peak concentration, and declining part. The CODconcentration in the leachate from AER4 was about1000 mg/L after 25 weeks of the operation. While, at the same period the COD concentration from the rest of bioreactors were still above 8000 mg/L. Theconcentration of COD trends showed that the MSWstabilization was achieved in AER4 and AER2, where...</p></li></ul>