Advantage of leachate recirculation on municipal solid waste biodegradation: experimental and field results

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1 Advantage of leachate recirculation on municipal solid waste biodegradation: experimental and field results Mostafa A. Warith Department of Civil Engineering, Ryerson Polytechnic University 350 Victoria Street, Toronto, Ontario, Canada MSB 2K3 Abstract The techniques that can be used to enhance biological degradation in landfills include: leachate recirculation, addition of nutrients, shredding, sludge and buffer solutions addition, lift design, temperature and moisture content management. Manipulation of these variables promotes a more conducive environment for microbial activity. This paper presents the results of a leachate recirculation experiment into three solid waste cells. The leachate was recirculated over a period of 65 weeks, and effluent samples were obtained on a weekly basis and analyzed for ph, BOD, and COD concentrations. The experimental results indicated that addition of supplemental materials to leachate during recirculation has a positive effect on the rate of biological degradation of solid waste. The paper also presents the results of leachate recirculation on solid waste biodegradation in a full-scale landfill site, which is located in Ottawa, Ontario, Canada. The leachate was recirculated into the landfilled solid waste for 5 years period through infiltration lagoons. Similar results to the one obtained in the laboratory scale experiments were noted. A decreasing trend of the organic load, measured as BOD and COD, was observed. Recovery of landfill air space was also noted due to the subsidence of solid waste. 1 Introduction Major concerns regarding the impact of municipal landfills on the environment are related to leachate quantity and quality, gas generation and decomposition processes

208 Water Pollution occurring in the landfill. The main processes responsible for the degradation of solid waste in the landfill are biological processes.

2 208 Water Pollution occurring in the landfill. The main processes responsible for the degradation of solid waste in the landfill are biological processes. It is desirable to minimize the time period in which degradation occurs in order to reduce gas emissions after the landfill is closed, to ease the requirements of leachate treatment and to be successful in reclaiming the landfill site. Several possible enhancement techniques can be implemented to increase biological activity in the landfills (Pohland and Al Youssifi, 1994; Attal et al. 1992). These techniques include leachate-recycling, use of buffers and/or nutrients, sludge addition, reducing waste particle size, waste lift design and moisture content management. Detailed leachate recycle investigations have been conducted in laboratory scale experiments (McCreanor et al. 1996) landfill lysimeters (Al-Youssifi and Pohland 1993) and controlled landfill cells (Leuschner 1989). The general conclusion of all of these studies has been that the increased moisture content and leachate recirculation had a positive effect on the waste stabilization process. Laboratory experiments investigating the effects of materials addition with leachate recycle concluded that buffering the leachate and the addition of sludge, buffer and nutrients both have positive effects on leachate quality and methane production. This paper summarizes the methodology and results of pilot scale solid waste bio-reactor cells to quantify the effect of moisture increase, nutrient addition, and sewage sludge addition on the rate of biological degradation of the solid waste. In addition, the paper briefly presents the results of a full-scale leachate recirculation at the Trail Road landfill site located in Ottawa, Ontario, Canada. 2 Materials and Methods 2.1 Experimental Set-Up The solid waste used in this experiment was collected from the curbsides of Toronto in October Visual inspection of the refuse showed presence of a variety of food wastes, paper, cardboard, packaging materials, yard wastes and inorganic materials including glass containers and tin cans. The wet-weight moisture content of the waste was calculated to be 9.8%, and the average specific weight of the waste compacted in the waste cells was 278 kg/nf Three landfill cells were used namely; Control Cell (C cell), Buffer and Nutrient Cell (B&N cell) and the Sludge Cell (S cell). Complete experimental cell configuration is shown in Fig. 1. The moisture content of the waste was quickly brought to field capacity (45% by volume) by adding tap water to the waste cells daily, until the amount of leachate collected from the cell equaled the amount of water added the previous day. Leachate was recirculated three times a week for the first 6-month period, which corresponds to a volume of approximately 12 L/week, or 15 % of the total volume of solid waste in each waste cell. Following the initiation period the leachate was recycled on a daily basis using a pumping system operated and controlled by a computerized operation system. The leachate recirculation pump inlet was attached to the tubing exiting the leachate collection tank. The outlet was connected to approximately 800 mm of tubing that extended vertically and was attached

3 Water Pollution 209 to a plastic hub that divided the flow into four 100-mm pieces of rubber tubing. The four equal streams were then inserted into the cover of the bioreactor (i.e. the solid waste cell). ^~^~1 ;W##;#A n p--^ ***** MUNICIPAL SOLID WASTE EACHATE X -^ Figure 1: Experimental Set-up ** *****.*,». Enhancement materials were added to the leachate collected from the designated solid waste cell prior to its recirculation once a week. In the Buffer and Nutrient cell (B&N cell), the buffer used for the adjustment of ph of the leachate was NaOH, and the nutrients were added in the form of plant food with nitrogen content of 20%, and phosphorus content of 20%. While in the Sludge cell (S cell), the amount of municipal sewage sludge added to the recirculated effluent equaled 5% of the total leachate volume. The sludge was collected from a wastewater treatment plant in Toronto, and provided a source of biomass, nitrogen and phosphorus as well as other nutrients and micronutrients to the solid waste during leachate recirculations. In the Control cell (C cell), only leachate was recirculated without any additional enhancement. Leachate samples were collected weekly from leachate outlet port and ph values were measured. Leachate samples were then preserved and stored in at 4 C prior to being tested for biological oxygen demand (BOD) and chemical oxygen demand (COD). 2.2 Full Scale Leachate Recirculation Leachate recirculation was carried out at the Trail Road Landfill site-stage III in Ottawa, Canada to enhance organic waste biodegradation and to reduce the

4 210 Water Pollution contaminant life span of the landfill site. In addition, leachate was recirculated to defer leachate treatment and handling for a period of time, until better characterization of leachate was determined. The leachate was withdrawn from the landfill cell and was pumped back into infiltration lagoons on the top of the waste. The locations of these infiltration lagoons were constantly changing to ensure uniform distribution of the leachate into the landfilled waste as well as to accommodate the landfill operation and solid waste filling. The organic load, measured as BOD and COD, was monitored for a period of five years to determine the effect of leachate recirculation on the decrease of the organic load and landfilled solid waste subsidence. 3 Results and Discussion 3.1 Laboratory Results Figure 2: ph variation in leachate r. -Control. - Sludge _.*_ Buffer The variations in leachate ph during its recirculation in the control and the two other solid waste cells are illustrated in Fig. 2. During the initial stage of leachate recirculation, the ph of the leachate varied from 6.0 to 6.5. During this initial stage, the fermenter bacteria hydrolyze and ferment solid and complex dissolved organic compounds into primarily volatile acids, alcohols, hydrogen and carbon dioxide and the acetogenic bacteria convert the products generated by the fermenters to acetic acid, hydrogen and carbon dioxide. During the intermediate anaerobic degradation stage, methanogenic bacteria slowly start to appear. As the methane gas production rate increases, hydrogen, carbon dioxide and volatile fatty acid concentrations

5 Water Pollution 211 decrease (Murphy et al. 1995). The conversion of fatty acids causes the ph within the waste cells to increase. This subsequently reduces the solubility of calcium, iron, manganese and heavy metals in the leachate solution, which are then precipitated as sulfides. The leachate ph in the three solid waste cells (bioreactors) was stabilized at about neutral (ph 7 to 8) after leachate recirculation for about 15 months. It is anticipated that this phase will last for an additional 6 to 12 months, and will be followed by low methane production phase and constant level of ph in the leachate as well as low leachate strength. Fig. 3 shows the BOD of the leachate from the control cell increased at the slowest rate, and generally remained below the values obtained from the two solid waste experimental cells. After a lag period of about 4 weeks, the BOD concentration in the leachate from the waste cell to which sludge was added increased at a slightly higher rate. The concentration of BOD reached a peak value of about 45,000 mg/1, while the BOD concentration in the leachate from the buffer and nutrient added solid waste cell reached a concentration of about 43,000 mg/1. These peak periods were followed by a decrease in the BOD concentration at an almost constant rate as shown in Fig. 3. The concentrations of BOD were decreased to approximately 9,000 mg/1 and 11,000 mg/1 in both the sludge and the buffer and nutrients added solid waste cells, respectively, after a period of about 62 weeks (... Control..Sludge _.* Buffer I Figure 3: BOD variation in leachate The experimental results indicated that the MSW stabilization was achieved in the sludge-added cells at a higher rate than that of the buffer and nutrient added cell. The results also indicate that, assuming the total BDOF in all three cells is the same, the concentration of BOD in the sludge added cell will reach a minimum value within a reasonable time frame (80 to 100 weeks), while the control cell will

6 212 Water Pollution continue eluting slightly elevated BOD concentration in the leachate for much longer period. Similar trends were observed in the COD concentrations of the leachate from each of the three solid waste cells, as shown in Fig. 4. The COD of the leachate from the control cell remained below the concentrations from the other two experimental cells throughout the investigation. A COD peak concentration of about 65,000 mg/1 was detected in the leachate samples with sludge addition after about 30 weeks, while the leachate from the buffer and nutrients added solid waste cell exhibited a COD concentration of about 52,000 mg/1, a decrease of about 13,000 mg/1 from the former cell. After a period of about 65 weeks, the COD concentrations in all three cells ranged from 17,000 to 25,000 mg/1 (Fig. 4), with the lowest COD concentrations were obtained from the control cell and the highest COD concentration were obtained from the buffer and nutrient added solid waste cell. I Figure 4 COD variation in leachate Sludge * Bufferl BOD/COD ratio was computed for the three cells. It is documented that in the transition stage the BOD/COD ratio lies in the range of 0.17 to 0.87, indicating the increasing biodegradability of organics due to solubilization. A ratio of 0.4 to 0.8 implies a highly biodegradable leachate. 3.2 Field Experience Figure 5 illustrates the relation between the average monthly precipitation, the volume of leachate generated and the volume of the leachate pumped from Stage III

Water Pollution 213 of the Trail Road landfill site. As noted the leachate generation rate was approximately 25 to 30% of the total precipitation.

7 Water Pollution 213 of the Trail Road landfill site. As noted the leachate generation rate was approximately 25 to 30% of the total precipitation. The generated leachate was pumped into infiltration lagoons, which were constructed using on-site stockpiled clay for containment dykes. The infiltration lagoons were relocated periodically to ensure even distribution of the moisture and to accommodated the landfilling of the solid waste. In addition Figure 5 illustrates the effect of short-circuiting within the waste which restricts the landfilled waste from reaching 100% of its field capacity. 1 Time (Month) - Leachate Generated - Leachate Pumped - Precipitation Figure 5 Average monthly precipitation, leachate generated and leachate pumped A decreasing trend of BOD and COD was noted over a period of eight years of leachate recirculation. Figure 6 displays the relationship of BOD, COD and the ratio between BOD/COD over time. The ratio of the BOD/COD was decreased from about 0.9 to 0.4 over a period of eight years, which illustrates the reduction in the biodegradable organic compounds and the increase of the microbial activities due to the increase in the solid waste moisture. Leachate management at the Trail road Landfill site through lagooned recirculation has not been without its challenges. What it has successfully accomplished, was the effective management, on an interim basis, of the leachate and the deferral of the leachate treatment until such time, the quantity and quality of the leachate is determined. Leachate recirculation reduces the contaminant life span

8 214 Water Pollution of the landfill and effectively utilizes the engineering components of the containment system during the landfill operation ^ 70000? c ji O COD BOD BOD/COD Ratio O I May-90 Figure 6. Sep-91 Mar-97 Jul-98 Changes in BOD, COD and BOD/COD ratio over time Leachate recirculation at the Trail Road Landfill site enhanced the settlement of the solid waste in Stage III and resulted in the recovery of 40% of landfill air space which was utilized for land-filling more solid waste. On the other side, leachate recirculation increases the rate of landfill gas generation and pronounces the odor problem, which results from the landfilling operation. 4 Conclusion Addition of supplemental materials to the leachate during recirculation was found to have positive effect on the rate of biological degradation. The addition of primary sludge and supplemental nutrients enhanced conditions such that there was a rapid increase in BOD and COD concentrations in the effluent samples. This rapid increase in BOD and COD concentrations suggest that, following a lag phase prior to the methanogenesis phase, a rapid decrease in the organic load in the leachate will be achieved within a reasonable time frame. The results of this investigation indicated that the primary sludge is an excellent source of microbial inoculum. The addition of supplemental nutrients (nitrogen and phosphorus) with buffer also increase the concentration of the BOD and COD in the effluent samples proving that a balance of ph and an increase in the available nutrients increase biological activities in the solid waste cells compared to the control cell. Enhancing and recycling leachate proves to be an effective tool in landfill management. It helps lessen the distinctive biological phase, which in return allows for the landfill to reach a state of stabilization at a quicker rate. This acceleration not

9 Water Pollution 215 only increases the life of the landfill, but also reduces overall monitoring costs incurred with post-closure. Acknowledgment The National Research Council of Canada provided support for this research. The author wish to acknowledge the work carried out by Ryerson Polytechnic University undergraduate students. The author also wishes to acknowledge the Colder Associates and the Regional Municipality of Ottawa Carleton for providing the field results. References Al-Youssifi A and Pohland F G Modeling of leachate and gas generation during accelerated biodegradation at controlled landfills. Pore. 31st Annual Solid Waste Exposition of the Solid Waste Association of North America. San Jose, CA. Attal A, Akin J, Yamato P, Salmon P and Paris I Anaerobic degradation of municipal wastes in landfill. Water Science and Technology, 25(7): Leuschner AP Enhancement of Degradation: Laboratory Scale Experiments. Sanitary Landfilling: Process, Technology and Environmental Impact, Edited by Christensen, R. Cossu and R. Stegmann, Academic Press Ltd. pp McCreanor, PT, and Reinhart DR Hydrodynamic modeling of leachate recirculating landfills. Water Science and Technology, 34(7-8): Murphy RJ, Jones DE and Stessel RI Relationship of microbial mass and activity in biodegradation of solid waste. Waste Management and Research, 13: Pohland FG and Al-Yousifi B Design and operation of landfills for optimum stabilization and biogass production. Wat. Sci. Tech. 30:

EFFECT OF LEACHATE RECIRCULATION ON ORGANIC WASTE AND LEACHATE STABILIZATION IN ANAEROBIC BIOREACTOR

EFFECT OF LEACHATE RECIRCULATION ON ORGANIC WASTE AND LEACHATE STABILIZATION IN ANAEROBIC BIOREACTOR International Journal of Civil Engineering and Technology (IJCIET), ISSN 976 638(Print) ISSN 976 6316(Online) Volume 1 Number 1, May - June (21), pp. 87-11 IAEME, http://www.iaeme.com/ijciet.html International