Effectiveness of the Vertical Gas Ventilation Pipes for Promoting Waste Stabilization in Post-Closure Phase

To make inside of the municipal solid waste (MSW) landfill aerobic as much as possible is thought to be preferable for promoting waste stabilization, reducing pollutant's load in leachate, minimizing greenhouse gas emission and shortening post-closure-care period. In Japan, installation of semi-aerobic landfill structure has widely spread in order to promote waste stabilization in MSW landfill from 1980s. In semi-aerobic landfill structure, outlet of main leachate collection pipe is opened to atmosphere. Heat generated by aerobic degradation of waste causes natural convection and natural aeration arises from the outlet of leachate collection pipe to the gas vents. It is so-called stack effect. This air flow is thought to be effective for purifying leachate flowing through drainage layer and leachate collection pipes. And it is also thought to be contributing to expanding aerobic region in waste layer in landfill. Recently, measures attempting the promotion of waste stabilization are taken at several landfills at where stabilization of waste delays, in which many vertical gas vents are newly installed and close structure to semi-aerobic landfill is created. However, in many cases, these gas vents are not connected to leachate collection pipes. Many vertical gas vents are just installed without scientific proof regarding whether they can contribute for waste stabilization. In this study, how such installation of gas vents is effective for waste stabilization and aerobization of waste layer was discussed by numerical analysis. In numerical analysis, heat transfer, gas movement by pressure, gas diffusion, biological degradation of organic matter, and heat generation by biodegradation were taken into account. Simulations were carried out by using the general purpose simulator of finite element method. Three types of landfill structure were assumed. As the results, the following information were obtained. In dig-down type landfill, installation of gas vents has no effect for changing air flow. On the other hand, in pile-up type landfill, installation of vertical gas vents can accelerate air invasion and significantly promote waste stabilization, if it has high permeable horizontal layer.


Introduction
In Japan, installation of semi-aerobic landfill structure, which is generally called "FUKUOKA Method" (Hanashima, 1999, Matsufuji, 2004), has widely spread in order to promote waste stabilization in MSW landfill from 1980s because land reclamation is in particular important.In semi-aerobic landfill structure, outlet of main leachate collection pipe is opened to the atmosphere.This leachate collection pipe is designed to have relatively large diameter compared to ordinary system installed in anaerobic landfill and it is connected to vertical gas vents.
Almost a half of cross section of the leachate collection pipe is used for leachate to flow and the other half is used for air to flow.Air introduced from the outlet of leachate collection pipe flows towards the exits of each gas vents due to the heat generated by aerobic degradation of waste adjacent to the pipes.Driving force of this air flow is considered to be the natural convection caused by the difference between the temperature of the waste mass and the ambient temperature outside the landfill (Matsuto et al, 2015).It is so-called stack effect or buoyancy effect.This air flow is thought to be effective for purifying leachate flowing through drainage layer and leachate collection pipes.And it is also thought to be contributing to expand aerobic region in waste layer in landfill (Ishimori et al., 2011).Advantages of this system are low installation cost and low operational cost compared to the forced aeration (Hrad et al, 2013;Ritzkowski et al, 2006;Ritzkowski and Stegmann, 2012;Raga and Cossu, 2014) because it doesn't require additional energy for aeration.
Figure 1 shows typical semi-aerobic landfill structure.Semi-aerobic structure requires temperature rise generated by aerobic reaction for introducing air inside.It is known that temperature of waste mass rises up to 70-80°C under aerobic degradation.Therefore, if the waste mass is dry, landfill fire is likely to be caused.So, this structure may not be suitable for arid or semi-arid region.However, it is especially suitable for the areas in tropical climate and actually installation of this structure to several landfills in Southeast Asian country showed success (Chong et al., 2005;Matsufuji and Tachifji, 2007).In the survey conducted by Kim et al. (2010), they identified upward gas velocity in many vents.However, they also identified existence of oxygen and nitrogen in deep part of the vents.So, they concluded that "Inflow of air into the gas vents can occur at a wide range of depths, even 10-20 m below ground level.Air is induced not from the surface of the landfill, but horizontally along the waste layer" and "passive gas vents can help to aerate landfilled waste as well as collect and release LFG".In addition, from similar study, Yoshida (2010) reported that temperature of vents increased to 60°C even in winter and concluded that aerobic degradation of waste occurred around vents caused temperature increase and the aerobic condition was created by installation of vents.If just installing vertical gas vents simply can promote aeration, such methodology is regarded to be effective for landfill in which stabilization is delayed because the method is low-cost and low energy.But both studies mentioned above did not confirm practically two facts; from where did air enter?By what route did air flow?It is not clear whether simple installation of vertical gas vents contributes air induction or not.Furthermore, what air flow occur is not clear.So, in this study, how such installation of gas vents is effective for waste stabilization and aerobization of waste layer was discussed by numerical analysis.

Mechanism of gas flow
First, as for natural convection in porous media caused by temperature increase, validity of theoretical formula was confirmed by conducting small scale experiment.Next, coupled model in which heat transfer, gas movement by pressure, gas diffusion, biological degradation of organic matter, and heat generation were taken into account was developed.Then, by using the model, how installation of vertical gas vents affect on the stabilization of waste in landfill was examined assuming three different landfill structures.So, temperature field with heat generation was created experimentally in order to confirm gas flow caused by natural convection.Then, numerical model that can simulate natural convection was developed and results of both experiment and numerical simulation were compared.

Confirmation of natural convection by small-scale lab test
Experimental apparatus is indicated in Figure 2. Size of the rectangular parallelepiped that is made of acrylic is 30 cm of height, 30 cm of width, and 5 cm of depth.At the side of the box, many thermocouples were equipped at intervals of 5 cm to measure temperature.
To exert the effect of natural convection more efficiently, steel shot and sand were used for filling material.In Table .1,specific weight, porosity, and intrinsic permeability of both materials are indicated.Steel shot was filled with 5 cm height at the bottom then sand was filled over the steel shot.
At the right side of the box, steel shot was again filled vertically to imitate gas ventilation pipe.Ribbon heater (10 cm long and 4 cm wide) was placed directly above bottom steel shot layer.After filling of material was completed, temperature of heater was raised to 110°C and it was maintained until steady state was achieved.
After temperature and gas flow reached at steady state, temperature of each thermocouple was measured and gas flux at the exit was measured.

Modelling of Natural Convection
Mass balance equation under advection of gas can be expressed by equation (1).
In above equation, density of gas (ρg) is calculated by equation of state.So, the equation ( 1) is depending on temperature.Then, heat balance equation is as follows.
Ceq can be calculated from equation (3).
) is specific heat of gas and CP (J•kg -1 •K -1 ) is that of porous media, ρP is true density of porous media (kg•m -3 ).Also, Keq is calculated by using thermal conductivity of gas (Kg) and porous media (KP).
Here, Mmix is molar weight of gas (g•mol -1 ) and Rg is gas constant (Pa•m -3 •K -1 •mol -1 ).By coupling equation ( 1), ( 2) and ( 5), change of temperature produces change of gas density and this change creates driving force of gas movement.By this way, natural convection was modelled.

Temperature Distribution and Flow Vector
Calculated temperature distribution and flow vector are showed in Figure 3.In Figure 3

Advection-diffusion equation
In this study, advection-diffusion equation by Maxwell-Stefan is used because multicomponent gas diffusion has to be considered.As for gas component, oxygen, carbon dioxide, and methane are taken into account.In addition, nitrogen was used as a balancing component.So, in total, four components are considered.For each gas component (i), the following Maxwell-Stefan equation can be formulated.

Formulation of biodegradation
Only two organic substrates were set such as easily-degradable organic matter and organic acid in the model.And four reactions are taken into account; aerobic biodegradation of easily-degradable organic matter (AeOm), aerobic degradation of organic acid that is formed under anaerobic condition (AeOa), acidogenesis of easilydegradable organic matter (AnOm), and methanogenesis of organic acid (AnMt).Mass balance equations of each organic substrate are as follows.To simplify the model, these organic substrates increase or decrease by only biodegradation and their transport by liquid was not incorporated.

 
).Then, the following three reaction rates are substituted for source-sink term (Ri) in Maxwell-Stefan equation.
In addition, by substituting sum of these three reaction rates for R in equation ( 1), coupling with advection is achieved.

Coupled Simulation Model
The model is created based on the following assumption; installation of vertical gas vent creates air flow and the induced air flow changes anaerobic condition in landfill to aerobic then aerobic degradation occurs.As the result, temperature distribution changes by reaction heat then air flow is further promoted.In order to simulate this scheme, equations for heat transfer, gas movement by pressure, gas diffusion, biological degradation of organic matter, and heat generation were coupled.Their interrelations are indicated in Figure 5.
In the model, output of each equation is utilized by others mutually.Calculation was done by using The COMSOL Multiphysics engineering simulation software environment (COMSOL, Inc.) which is multi-purpose finite element model solver.These structures are indicated in Figure 6.The depth and width of landfill were set to 20 m and 40 m respectively.For each structure, two cases were considered; namely with or without installation of vent.In the case with gas vent, vent whose diameter is 30 cm was assumed to be installed at the left side of landfill.Since the simulated landfill structure is designed to be symmetrical about an axis of left edge, the vent is in actual located at the center of the landfill.As for Type-3 landfill, horizontal layer was set to have high permeability that was two orders of magnitude higher than waste layer.

Simulated Landfill Structure
Changes of gas flow and temperature and degradation of organic matter in landfill for 50 years after installation of vertical gas vent were simulated by using the developed coupled model.Simulated landfill was assumed to be a closed landfill in which much organic matter still remained.Three structures were assumed; dig-down landfill (Type-1), pile-up landfill (Type-2), and pileup landfill with horizontal layer (Type-3).Figure 7 indicates transition of organic carbon in Type-1 landfill after 10, 30 and 50 years.Initial content of organic carbon was set to 7.8 kg-C•m -3 .Left column shows landfill without gas vents and right column (9) (10) (8) (7) shows that with vents.Any significant difference cannot be found.Air gets in from only top surface, so organic carbon disappears gradually from top towards bottom in both cases.However, organic carbon still exists even after 50 years.Hence, even if vertical gas vent is installed, no specific flow change arises in the digdown type landfill and it can be concluded that installation of gas vent does not have effect for acceleration of waste stabilization.
Figure 8 shows temperature distribution at 10, 30, 50 years in Type-2 landfill.Initially, temperature of entire landfill was set to 15°C.In pile-up landfill, air can get in from both top and side slope.Induced air creates aerobic zone and the zone expands gradually into inside.So, temperature rise by aerobic reaction occur from side slope.Temperature reaches to approximately 50°C at area in which aerobic reaction is most active.Effect of air invasion is more prominent in the case with vent.And its high temperature area spreads wider than the case without vent.At 50 years, temperature of most of landfill body becomes steady (15°C) but its distribution is not so different in both cases with or without vent.In Type-3 which has high permeable horizontal layer, organic carbon disappears from not only top and side but also along with the horizontal layer.Installation of gas vent leads to more drastic promotion of air induction if horizontal layer exists.This is because stack effect is produced more effectively with horizontal layer.The difference of organic carbon content is evident at 30 years when only a little organic carbon remains at the bottom central region in Type-3.And it is vanished completely at 50 years.From these results, it can be concluded that installation of gas vent becomes particularly effective when there is high permeable horizontal layer.So, creating such horizontal layer with coarse material before installation of gas vents is recommended when remediation countermeasure to promote stabilization of waste in landfill of high organic matter content is taken.

Conclusion
In this paper, waste stabilization in landfill construction was discussed.Heat transfer, gas movement by pressure, gas diffusion, biological degradation of organic matter, and heat generation by biodegradation were calculated by using coupled model for three landfill structures.The effect of gas vent was various depending on landfill structure.In dig-down landfill, installation of vertical vent has no effect.On the other hand, the effect was obvious in pile-up landfill.Especially, in pile-up landfill with high permeable horizontal layer, installation of gas vent changes gas flow drastically and induction of air is promoted.The induced air makes circumstance of landfill aerobic from anaerobic and aerobic degradation of waste readily stabilizes organic carbon remaining in landfill.Therefore, it can be concluded that creating high permeable horizontal layer with coarse material before installation of vertical gas vents is recommended when remediation countermeasure to promote stabilization of waste in landfill of high organic matter content is taken.This measure is thought to be especially effective for landfills/open dumps under tropical climate at which remediation is required because rainy climates is favorable to maintain water content and to control excessive rise in heat.Landfill Symposium, 1, 313-326. Hrad, M., Gamperling, O., Huber-Humer, M. (2013) Comparison between lab-and full-scale applications of in situ aeration of an old landfill and assessment of long-term emission development after

Figure 1 .
Figure 1.Schematic diagram of semi-aerobic landfill structure

Table 1 .
Basic characteristics of filling

20 30 40 50 60 70 80 90
And gas flux at right upper edge was 0.62 L/hour in experiment.Meanwhile, the flux calculated from the model was 0.79 L/hour.They were slightly different but we evaluated that developed model is able to express air flow caused by natural convection.