ENHANCEMENT OF METHANE OXIDATION IN COVER SOIL OF TROPICAL LANDFILL BY VEGETATION AND LEACHATE IRRIGATION

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1 ENHANCEMENT OF METHANE OXIDATION IN COVER SOIL OF TROPICAL LANDFILL BY VEGETATION AND LEACHATE IRRIGATION Chart Chiemchaisri, Wilai Chiemchaisri, Kittipon Chittanukul, Thipsuree Kornboonraksa Department of Environmental Engineering, Kasetsart University 5 Phaholyothin Road, Chatuchak, Bangkok 19, Thailand Sayan Tudsri Department of Agronomy, Faculty of Agriculture, Kasetsart University 5 Phaholyothin Road, Chatuchak, Bangkok 19, Thailand ABSTRACT The effect of vegetation and leachate irrigation on methanotrophic activity in cover soil of landfill was investigated. Leachate was applied to maintain moisture content for microbial activity while being treated when irrigated on topsoil. Laboratory-scale experiment was conducted to examine the effect of leachate loading on methane oxidation in non-vegetated and vegetated cover soil (with S. virginicus, a tropical grass). Methane oxidation rate in the soil column has been monitored for more than 3 days in which the concentration of applied leachate was increased from 1,88 mgcod/l to to 3,7 and 9,4 mg/l. Subsequent experiment was conducted to examine the pattern of leachate application on methane oxidation activity. It was found that S. virginicus could be successfully grown in typical condition of landfill (exposed with landfill gas and rainwater). The results also showed that the growth of plants on final cover layer of landfill could be promoted when optimal supplement nutrients from leachate were provided. The vegetation helped promoting methane oxidation in soil while leachate application helped increasing methane oxidation rate in non-vegetated cover soil. At higher organic loading, adverse effect of organic loading on methane oxidation rate and plant growth was observed. The inhibition of plant growth and methane oxidation at high organic loading caused by the shortage of oxygen in the root zone where active methane oxidation was taken place. The pattern of leachate application also affected methane oxidation rate in cover soil. Intermittent application of leachate (once every 4 days) improved methane oxidation activity as compared to daily application. This study suggests that employment of vegetation and appropriate leachate irrigation enhance and sustain methane oxidation in landfill cover soil. KEYWORDS Methane oxidation; final cover soil; landfill vegetation; leachate irrigation INTRODUCTION Landfill is one of the most common disposal methods for municipal solid wastes in developing countries. However, significant environmental impacts such as leachate and gas production could be created if the landfills are not designed and operated properly. For example, the gas produced from biodegradation of solid waste in landfill that contains various compounds including ammonia (NH 3 ), nitrogen (N 2 ), carbon dioxide (CO 2 ), hydrogen (H 2 ), hydrogen sulfide (H 2 S), methane (CH 4 ) and volatile organic compounds (VOCs) are normally released directly to the atmosphere without any treatment. Methane and carbon dioxide are the major principal gases generated which can cause the rising of earth temperature so called greenhouse effect. Proper management of landfill gas is therefore one of the most important components in landfill operation. The produced gas from landfill could be utilized as source of energy in large landfills but it should be eliminated when the utilization is not feasible especially in small landfills. For the purpose of controlling gas emission from landfill, cover soil plays an important role in reducing the emission of methane gas. Final cover may consist of multi-components, including surface layer (vegetative support), protection layer, drainage layer, hydraulic barrier layer, foundation layer, and gas collection (control) layer. However, it is not always possible to have of all these components especially in small landfills of developing countries. In such case, single layer using clay material is common and thus allow substantial gas emission from landfill through final layer when soil moisture content decreased to the shrinkage limit and cracking of soil layer took place.

2 In order to minimize the emission of methane gas, several reports suggested the existence of microbial activity of methanotrophic bacteria in the final cover soil which help converting them to carbon dioxide (Whalen et.al.,199; Boeckx and Van Cleemput, 199; Bogner et.al.,1997). Application methane biodegradation via methanotrophic activity in topsoil is preferable to control of methane emission from landfill with low cost association. Investigation on the optimum design and operation of landfill final cover layer in order to promote the methanotrophic activity is essential if this technique is to be implemented in actual practice. This technology will be attractive for small landfills in developing countries, as they require minimum operating and maintenance cost in postclosure period. Several reports of methane oxidation found in a final cover soil in different regions may associate with many factors such as type of soil, soil depths, soil temperature, soil water content or age of landfill etc. Moreover, vegetation grown on the landfill surface might also influent this biochemical reaction. In this study, methane oxidation in top cover soil with two species of tropical grass is studied. The effect of leachate irrigation on methane oxidation and plant growth is also investigated. MATERIALS AND METHODS Six soil columns made of acrylic having 15-cm diameter and 1-cm height were used in laboratory scale experiment. The schematic of experimental unit is shown in Figure 1. The columns were purged with synthetic landfill gas containing % methane and 4% carbon dioxide at a flow rate of 3 ml/min. Sandy loam soil consists of 5% sand and 5% organic material (wt/wt) was used as cover soil. Their characteristics are shown in Table 1. Organic material was prepared by sieving organic compost through 35- mm opening screen to remove straw, coconut husk and large particles. Then, organic materials and sand were mixed until mixture became homogenous. The plants used in the study are local grass found in tropical climate, S. virginicus and C. plectostachyus. They were grown for about two weeks in the nursery pots before being planted into soil columns. A Light bulb for plant growing purpose is used to supply light for plant during daytime (light intensity 3, lux). Rainwater/synthetic leachate (Table 1) was irrigated to the soil column to control soil moisture content at 1-15%. Leachate was diluted with rainwater and final concentration of 1,88 mgcod/l was obtained for the 1 st experiment (day -154). Applied concentration was then increased to 3,7 mg/l and 9,4 mg/l at day 155 and 225 respectively. Two soil columns without plant are prepared as the control experiment. One of them was irrigated with rainwater and the other with leachate. Two plant species, i.e. S. virginicus and C. plectostachyus, were grown others four columns, two columns each. They were operated in the same manner as control columns. The effect of leachate irrigation on vegetation was studied by determining the following parameters; plant growth rate (height, number of leaves, root length and weight of dry mass) and plant damage (abnormal appearance). In addition, soil organic content and electrical conductivity were monitored to determine the degree of organic matter and salt accumulations in soil. Methane Oxidation Rate (MOR) was used for the determination of microbial activity. It can be calculated from following equation: MOR (CH 4 mol/m 3.d) = Q [(CH 4 ) In (CH 4 ) out]/v where Q = Gas flow rate (ml/day) (CH 4 ) In = Inflow methane conc. (moles/ml) (CH 4 ) out = Outflow methane conc. (moles/ml) V = Volume of soil (m 3 ) RESULTS AND DISCUSSION The effects of leachate strengths on plant growth had been initially investigated in pot study where two species of plants (S. virginicus and C. plectostachyus) were irrigated daily with 5 ml of rainwater and leachate at different concentrations. It was found that C. plectostachyus grew well when it was applied with rainwater or diluted leachate of 1,88 mgcod/l. At higher leachate concentrations of 3,7 and 9,4 mgcod/l, the plant growth was obviously retarded after 13 th and the 7 th weeks respectively. When comparing the growth rates of grass between irrigated with rainwater and diluted leachate, it was found that leachate application could promote the growth of C. plectostachyus. Similar result was found in case of S. virginicus. The growth of S. virginicus was promoted at higher concentration of leachate (3,7 mgcod/l) and it was found to be the most tolerant grass specie to leachate irrigation in this study. Nevertheless, all three species was severely damaged when being irrigated with higher concentration of leachate (9,4 mgcod/l). The growths of C. plectostachyus and S. virginicus in soil column were observed for almost 3 days. The result showed that both plants could be grown in typical condition of landfill (exposed with landfill gas and rainwater) when irrigated with rainwater and diluted leachate. Significant accumulations of salt (EC) and organic carbon (OC) contents in soils from leachate irrigation were not observed. Generally, the level of soluble salts (any soluble ions, EC content) in

3 Rainwater/Leachate Flowmeter Synthetic gas FIGURE 1 SCHEMATIC OF EXPERIMENTAL SYSTEM TABLE 1 CHARACTERISTICS OF SOIL AND LEACHATE USED IN THE EXPERIMENT Soil Characteristics Leachate Characteristics ph (Soil: H 2 O = 1:1) 4.4 ph 5.7- EC (Soil: H 2 O = 1:5).159 BOD, mg.l -1 13,-1,5 Moisture content, % 4 COD, mg.l -1 1,-19, Organic matters, % 1.91 TKN, mg.l Available P, mg.kg 1 9 NH 3 -N, mg.l Available K, mg.kg NO 3 -N, mg.l NH 3 -N, mg.kg Orthophosphate, mg.l -1-8 NO 3 -N, mg.kg Chloride, mg.l ,5 CEC, cmol c.kg EC, ds.m -1 8

4 soil can have significant impact on plant growth by causing nutrient imbalance. Nevertheless, soil irrigation with diluted leachate had little influence on EC and OC content in this study. The oxygen uptake rate (OUR) in soil was found increasing with an increase of organic loading. Therefore the main reason for plant growth inhibition when high strength leachate was applied could be the shortage of oxygen in soil. Proper control of leachate loading could, however, benefit the growth of plant by providing supplement nutrients. Methane oxidation rate (MOR) was determined in soil column operated under different condition. In case of non-vegetated cover soil irrigated with rainwater, MOR was found to be 1- mol/m 3.d during the first 5 days and gradually declined to 4 mol/m 3.d after 2 days of operation (Figure 2). In case of soil column with leachate irrigation, MOR could be maintained above 8 mol/m 3.d when low concentrations of leachate (1% and 2%) were used. However, it was reduced significantly to the same level as rainwater application case of 1-2 mol/m 3.d when high strength leachate of 5% concentration was applied. In case of C. plectostachyus, MOR was initially maintained at the same level as non-vegetated soil but rapidly dropped down to less than 2 mol/m 3.d after only 8 days of operation. Leachate application could not help improving MOR in this case. For soil column with S. virginicus, MOR was gradually reduced from 1- mol/m 3.d to 8 and 5 mol/m 3.d after 15 days for rainwater and leachate application cases. Nevertheless, leachate irrigation was not significantly affected MOR in this vegetated soil except at higher concentration of 5% where MOR was reduced to only 3 mol/m 3.d. From the experimental results, methane oxidation was successfully sustained for over 2 days in nonvegetated soil with leachate irrigation and cover soil with S. virginicus. Adverse effect of high organic loading on MOR and plant growth was observed in all cases. At 2% of leachate concentration, MOR of soil column without plant was about 8 mol/m 3.d while that of column with S. virginicus maintained in a range between 7-9 mol/m 3.d. As leachate concentration was increased to 5% after day 225, MOR in all columns dropped down to 1-4 mol/ m 3.d. Most of methane oxidation took place at 5-15 cm. depth from soil surface in nonvegetated case but the active zone was deeper (15-3 cm.) in vegetated cover soil. All plant could not survive at this high organic loading condition however small MOR in cover soil still existed. Subsequent experiment was conducted by irrigating 2% leachate concentration on the cover soil with S. virginicus. Different amount of leachate was applied at corresponding time interval in order to maintain constant overall hydraulic loading, i.e. 5 ml/d, 1 ml/2days and 2 ml/4days respectively. Figure 3 shows the variation of MOR along the experimental period of 15 days. During the experiment, MOR was gradually increased from about 4 to 1 mol/m 3.d within the first 5 days. There was not much different in MOR among different leachate application pattern in this period. As the experiment continued beyond 5 days, MOR in cover soil with 2 ml/4days of leachate application case kept gradually increasing and finally reached a plateau of mol/m 3.d until the end of experimental period. In cases of 5 ml/d and 1 ml at every 2 days application, MOR reached a maximum value of about 11 mol/m 3.d and finally declined to 9 mol/m 3.d. From the experimental results obtained, it was found that intermittent application of leachate was preferred to continuous application for methane oxidation. This could be due to the fact that oxygen for methane oxidation could be easily transferred into the soil during the resting period. The determination of oxygen content in soil gas also confirmed that higher oxygen content was available for methane oxidation in case of intermittent leachate application. When higher leachate concentration of 4%, 8% and 1% was applied while maintaining constant hydraulic and organic loading as the previous experiment, it was found that lower MOR was achieved in all cases. As shown in Figure 3, MOR was increased from 4 mol/m 3.d to a maximum of 8 mol/m 3.d within the first 5 days and gradually declined afterwards. In the case of 4% leachate concentration, MOR was maintained slightly higher than the other two cases. These results suggested that the application of low strength leachate was preferred to higher concentration cases. This could possibly due to rapid consumption of oxygen in soil as higher strength leachate was applied affected the activity of methanotrophic bacteria and thus methane oxidation. Comparing the MOR obtained from this experimental cases with the former cases employing 2% concentration leachate, it was found that MOR in this experiment using higher leachate concentration were much lower, being only 5% of the previous experiment. Therefore, the concentration of applied leachate on vegetated cover soil should be limited to 2% or 3,7 mgcod/l in order to achieve high MOR.

5 MOR (mol/m 3.day) % leachate 2 % leachate 5 % leachate rainwater, no veget leachate, no veget MOR (mol/m 3.d) % leachate 2 % leachate 5 % leachate rainwater,c. plectostachyus leachate,c. plectostachyus 15 1 % leachate 2 % leachate 5 % leachate MOR (mol/m 3.d) rainwater,s. virginicus leachate, S. virginicus FIGURE 2 VARIATION OF METHANE OXIDATION RATE IN COVER SOILS WITH/WITHOUT PLANTS

6 methane oxidation rate (mol/m 3.day) leachate 2 % 5 ml/day leachate 2 % 1 ml/2day leachate 2 % 2 ml/4day methane oxidation rate (mol/m 3.day) leachate 4 % 5 ml/2day rainwater 5 ml/day leachate 8 % 5 ml/4day rainwater 5 ml/day leachate 1 % 15 ml/day rainwater 27 ml/day FIGURE 3 VARIATION OF METHANE OXIDATION RATE UNDER DIFFERENT LEACHATE APPLICATION PATTERN

7 CONCLUSION From the experimental results obtained, the following conclusion can be drawn. 1) In laboratory scale soil column study, S. virginicus, but not C. plectostachyus promoted methane oxidation. Irrigation of 1% and 2% leachate concentration (equivalent to 1,88 and 3,7 mgcod/l) could also promote methane oxidation in non-vegetated cover soil. The study suggests that selection of the appropriate plant species and optimum concentration of leachate were important to achieve sustainable gas and leachate control by cover soil. 2) When the cover soil was irrigated with higher leachate concentration (e.g. 9,4 mgcod/l), the grown vegetation was severely damaged and methane oxidation rate in soil dropped significantly. It was possibly due to the shortage of oxygen for methane oxidation. 3) Leachate application pattern also affect methane oxidation rate in vegetated cover soil. Intermittent application of diluted leachate (3,7 mgcod/l) promoted methane oxidation by allowing oxygen transfer into the soil during resting period. The application of higher leachate concentration, though maintaining constant hydraulic and organic loading, gave lower methane oxidation rate ACKNOWLEDGEMENT This research project is financially supported by Swedish International Development Cooperation Agency (SIDA) in Asian Regional Research Program on Environmental Technology (ARRPET). REFERENCES Boeckx P. and Van Cleemput O.(199), Methane Oxidation in a Natural Landfill Cover Soil: Influence of Moisture Content, Temperature and Nitrogen Turnover, Journal of Environmental Quality, 25, Bogner J.E., Spokas K.A. and Burton E.(1997), Kinetic of Methane Oxidation in a Landfill Cover Soil: Temporal Variation, a Whole-Landfill Oxidation Experiment and Modeling of Net CH 4 Emissions, Environmental Science and Technology, 31, Whalen S.C., Reeburg W.S. and Sandbeck K.A. (199), Rapid Methane Oxidation Rate in Landfill Cover Soil, Applied and Environmental Microbiology, 5,