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

Advantage of leachate recirculation on municipal

solid waste biodegradation: experimental and

field results

Mostafa A. Warith

Department of Civil Engineering, Ryerson Polytechnic University350 Victoria Street, Toronto, Ontario, Canada MSB 2K3E-mail: [email protected]

Abstract

The techniques that can be used to enhance biological degradation in landfillsinclude: leachate recirculation, addition of nutrients, shredding, sludge and buffersolutions addition, lift design, temperature and moisture content management.Manipulation of these variables promotes a more conducive environment formicrobial activity. This paper presents the results of a leachate recirculationexperiment into three solid waste cells. The leachate was recirculated over a periodof 65 weeks, and effluent samples were obtained on a weekly basis and analyzed forpH, BOD, and COD concentrations. The experimental results indicated that additionof supplemental materials to leachate during recirculation has a positive effect on therate of biological degradation of solid waste.

The paper also presents the results of leachate recirculation on solid wastebiodegradation in a full-scale landfill site, which is located in Ottawa, Ontario,Canada. The leachate was recirculated into the landfilled solid waste for 5 yearsperiod through infiltration lagoons. Similar results to the one obtained in thelaboratory scale experiments were noted. A decreasing trend of the organic load,measured as BOD and COD, was observed. Recovery of landfill air space was alsonoted due to the subsidence of solid waste.

1 Introduction

Major concerns regarding the impact of municipal landfills on the environment arerelated to leachate quantity and quality, gas generation and decomposition processes

Transactions on Ecology and the Environment vol 26, © 1999 WIT Press, www.witpress.com, ISSN 1743-3541

208 Water Pollution

occurring in the landfill. The main processes responsible for the degradation of solidwaste in the landfill are biological processes. It is desirable to minimize the timeperiod in which degradation occurs in order to reduce gas emissions after the landfillis closed, to ease the requirements of leachate treatment and to be successful inreclaiming the landfill site.

Several possible enhancement techniques can be implemented to increasebiological activity in the landfills (Pohland and Al Youssifi, 1994; Attal et al. 1992).These techniques include leachate-recycling, use of buffers and/or nutrients, sludgeaddition, reducing waste particle size, waste lift design and moisture contentmanagement.

Detailed leachate recycle investigations have been conducted in laboratory scaleexperiments (McCreanor et al. 1996) landfill lysimeters (Al-Youssifi and Pohland1993) and controlled landfill cells (Leuschner 1989). The general conclusion of allof these studies has been that the increased moisture content and leachaterecirculation had a positive effect on the waste stabilization process. Laboratoryexperiments investigating the effects of materials addition with leachate recycleconcluded that buffering the leachate and the addition of sludge, buffer and nutrientsboth have positive effects on leachate quality and methane production.

This paper summarizes the methodology and results of pilot scale solid wastebio-reactor cells to quantify the effect of moisture increase, nutrient addition, andsewage sludge addition on the rate of biological degradation of the solid waste. Inaddition, the paper briefly presents the results of a full-scale leachate recirculation atthe 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 Torontoin October 1996. Visual inspection of the refuse showed presence of a variety offood wastes, paper, cardboard, packaging materials, yard wastes and inorganicmaterials including glass containers and tin cans. The wet-weight moisture contentof the waste was calculated to be 9.8%, and the average specific weight of the wastecompacted in the waste cells was 278 kg/nf

Three landfill cells were used namely; Control Cell (C cell), Buffer and NutrientCell (B&N cell) and the Sludge Cell (S cell). Complete experimental cellconfiguration is shown in Fig. 1. The moisture content of the waste was quicklybrought to field capacity (45% by volume) by adding tap water to the waste cellsdaily, until the amount of leachate collected from the cell equaled the amount ofwater added the previous day. Leachate was recirculated three times a week for thefirst 6-month period, which corresponds to a volume of approximately 12 L/week, or15 % of the total volume of solid waste in each waste cell. Following the initiationperiod the leachate was recycled on a daily basis using a pumping system operatedand controlled by a computerized operation system. The leachate recirculation pumpinlet was attached to the tubing exiting the leachate collection tank. The outlet wasconnected to approximately 800 mm of tubing that extended vertically and was attached


Water Pollution 209

to a plastic hub that divided the flow into four 100-mm pieces of rubber tubing. Thefour equal streams were then inserted into the cover of the bioreactor (i.e. the solidwaste cell).

~̂̂ ~1;W##;#A

MUNICIPALSOLIDWASTE

— -^EACHATE X

n

**

p--̂*****

„*****.*,».

Figure 1: Experimental Set-up

Enhancement materials were added to the leachate collected from the designatedsolid waste cell prior to its recirculation once a week. In the Buffer and Nutrientcell (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 ofmunicipal sewage sludge added to the recirculated effluent equaled 5% of the totalleachate volume. The sludge was collected from a wastewater treatment plant inToronto, and provided a source of biomass, nitrogen and phosphorus as well as othernutrients and micronutrients to the solid waste during leachate recirculations. In theControl cell (C cell), only leachate was recirculated without any additionalenhancement. Leachate samples were collected weekly from leachate outlet portand pH values were measured. Leachate samples were then preserved and stored inat 4°C prior to being tested for biological oxygen demand (BOD) and chemicaloxygen demand (COD).

2.2 Full Scale Leachate Recirculation

Leachate recirculation was carried out at the Trail Road Landfill site-Stage III inOttawa, Canada to enhance organic waste biodegradation and to reduce the


210 Water Pollution

contaminant life span of the landfill site. In addition, leachate was recirculated todefer leachate treatment and handling for a period of time, until bettercharacterization of leachate was determined. The leachate was withdrawn from thelandfill cell and was pumped back into infiltration lagoons on the top of the waste.The locations of these infiltration lagoons were constantly changing to ensureuniform distribution of the leachate into the landfilled waste as well as toaccommodate the landfill operation and solid waste filling.

The organic load, measured as BOD and COD, was monitored for a period offive years to determine the effect of leachate recirculation on the decrease of theorganic load and landfilled solid waste subsidence.

3 Results and Discussion

3.1 Laboratory Results

1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 615.00

•r. -Control . - Sludge _ .*_ Buffer

Figure 2: pH variation in leachate

The variations in leachate pH during its recirculation in the control and the two othersolid waste cells are illustrated in Fig. 2. During the initial stage of leachaterecirculation, 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 organiccompounds into primarily volatile acids, alcohols, hydrogen and carbon dioxide andthe acetogenic bacteria convert the products generated by the fermenters to aceticacid, hydrogen and carbon dioxide. During the intermediate anaerobic degradationstage, methanogenic bacteria slowly start to appear. As the methane gas productionrate increases, hydrogen, carbon dioxide and volatile fatty acid concentrations


Water Pollution 211

decrease (Murphy et al. 1995). The conversion of fatty acids causes the pH withinthe waste cells to increase. This subsequently reduces the solubility of calcium, iron,manganese and heavy metals in the leachate solution, which are then precipitated assulfides. The leachate pH in the three solid waste cells (bioreactors) was stabilized atabout neutral (pH 7 to 8) after leachate recirculation for about 15 months. It isanticipated that this phase will last for an additional 6 to 12 months, and will befollowed by low methane production phase and constant level of pH in the leachateas well as low leachate strength.

Fig. 3 shows the BOD of the leachate from the control cell increased at theslowest rate, and generally remained below the values obtained from the two solidwaste experimental cells. After a lag period of about 4 weeks, the BODconcentration in the leachate from the waste cell to which sludge was addedincreased at a slightly higher rate. The concentration of BOD reached a peak valueof about 45,000 mg/1, while the BOD concentration in the leachate from the bufferand 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 analmost constant rate as shown in Fig. 3. The concentrations of BOD were decreasedto approximately 9,000 mg/1 and 11,000 mg/1 in both the sludge and the buffer andnutrients added solid waste cells, respectively, after a period of about 62 weeks.

1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61

(. . . Control . .Sludge _ .*— Buffer I

Figure 3: BOD variation in leachate

The experimental results indicated that the MSW stabilization was achieved in thesludge-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 valuewithin a reasonable time frame (80 to 100 weeks), while the control cell will


212 Water Pollution

continue eluting slightly elevated BOD concentration in the leachate for muchlonger period.

Similar trends were observed in the COD concentrations of the leachate fromeach of the three solid waste cells, as shown in Fig. 4. The COD of the leachate fromthe control cell remained below the concentrations from the other two experimentalcells throughout the investigation. A COD peak concentration of about 65,000 mg/1was 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 aCOD concentration of about 52,000 mg/1, a decrease of about 13,000 mg/1 from theformer cell. After a period of about 65 weeks, the COD concentrations in all threecells ranged from 17,000 to 25,000 mg/1 (Fig. 4), with the lowest CODconcentrations were obtained from the control cell and the highest CODconcentration were obtained from the buffer and nutrient added solid waste cell.

I

9 13 17 21 25 29 33 37 41 45 49 53 57 61

.Sludge __*__Bufferl

Figure 4 COD variation in leachate

BOD/COD ratio was computed for the three cells. It is documented that in thetransition stage the BOD/COD ratio lies in the range of 0.17 to 0.87, indicating theincreasing biodegradability of organics due to solubilization. A ratio of 0.4 to 0.8implies a highly biodegradable leachate.

3.2 Field Experience

Figure 5 illustrates the relation between the average monthly precipitation, thevolume 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 wasapproximately 25 to 30% of the total precipitation. The generated leachate waspumped into infiltration lagoons, which were constructed using on-site stockpiledclay for containment dykes. The infiltration lagoons were relocated periodically toensure even distribution of the moisture and to accommodated the landfilling of thesolid waste. In addition Figure 5 illustrates the effect of short-circuiting within thewaste which restricts the landfilled waste from reaching 100% of its field capacity.

1

Time (Month)

- Leachate Generated - Leachate Pumped - PrecipitationFigure 5 Average monthly precipitation, leachate generated and leachate pumped

A decreasing trend of BOD and COD was noted over a period of eight years ofleachate recirculation. Figure 6 displays the relationship of BOD, COD and the ratiobetween BOD/COD over time. The ratio of the BOD/COD was decreased fromabout 0.9 to 0.4 over a period of eight years, which illustrates the reduction in thebiodegradable organic compounds and the increase of the microbial activities due tothe increase in the solid waste moisture.

Leachate management at the Trail road Landfill site through lagoonedrecirculation has not been without its challenges. What it has successfullyaccomplished, was the effective management, on an interim basis, of the leachateand the deferral of the leachate treatment until such time, the quantity and quality ofthe leachate is determined. Leachate recirculation reduces the contaminant life span


214 Water Pollution

of the landfill and effectively utilizes the engineering components of thecontainment system during the landfill operation.

100000

90000

80000

^ 70000

? 60000c•ji 50000

§ 40000

O 30000

20000

10000

CODBODBOD/COD Ratio

May-90

1.8

1.6

1.4

1.2

0.8 O

I0.6 5

0.4

0.2

Sep-91 Mar-97 Jul-98

Figure 6. Changes in BOD, COD and BOD/COD ratio over time

Leachate recirculation at the Trail Road Landfill site enhanced the settlement ofthe solid waste in Stage III and resulted in the recovery of 40% of landfill air spacewhich was utilized for land-filling more solid waste. On the other side, leachaterecirculation increases the rate of landfill gas generation and pronounces the odorproblem, which results from the landfilling operation.

4 Conclusion

Addition of supplemental materials to the leachate during recirculation was found tohave positive effect on the rate of biological degradation. The addition of primarysludge and supplemental nutrients enhanced conditions such that there was a rapidincrease 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 theleachate will be achieved within a reasonable time frame.

The results of this investigation indicated that the primary sludge is an excellentsource of microbial inoculum. The addition of supplemental nutrients (nitrogen andphosphorus) with buffer also increase the concentration of the BOD and COD in theeffluent samples proving that a balance of pH and an increase in the availablenutrients increase biological activities in the solid waste cells compared to thecontrol cell.

Enhancing and recycling leachate proves to be an effective tool in landfillmanagement. It helps lessen the distinctive biological phase, which in return allowsfor the landfill to reach a state of stabilization at a quicker rate. This acceleration not


Water Pollution 215

only increases the life of the landfill, but also reduces overall monitoring costsincurred with post-closure.

Acknowledgment

The National Research Council of Canada provided support for this research. Theauthor wish to acknowledge the work carried out by Ryerson Polytechnic Universityundergraduate students. The author also wishes to acknowledge the ColderAssociates and the Regional Municipality of Ottawa Carleton for providing the fieldresults.

References

Al-Youssifi A and Pohland F G. 1993. Modeling of leachate and gas generationduring accelerated biodegradation at controlled landfills. Pore. 31st Annual SolidWaste Exposition of the Solid Waste Association of North America. San Jose, CA.

Attal A, Akin J, Yamato P, Salmon P and Paris I. 1992. Anaerobic degradation ofmunicipal wastes in landfill. Water Science and Technology, 25(7):243-253.

Leuschner AP. 1989. Enhancement of Degradation: Laboratory Scale Experiments.Sanitary Landfilling: Process, Technology and Environmental Impact, Edited byChristensen, R. Cossu and R. Stegmann, Academic Press Ltd. pp. 83-98.

McCreanor, PT, and Reinhart DR. 1996. Hydrodynamic modeling of leachaterecirculating landfills. Water Science and Technology, 34(7-8):463-470.

Murphy RJ, Jones DE and Stessel RI. 1995. Relationship of microbial mass andactivity in biodegradation of solid waste. Waste Management and Research, 13:485-497.

Pohland FG and Al-Yousifi B. 1994. Design and operation of landfills for optimumstabilization and biogass production. Wat. Sci. Tech. 30:117-124.


Documents

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