Design Approach to Protect Water Resources from MSW Disposal

This innovative design provides the guideline for Engineers to locate a suitable site for MSW disposal. The design facilitates for a natural or compacted clay barrier with or without a leachate collection system. The governing equation for one-dimensional contaminant transport model includes the effects of advection, dispersion, adsorption and decay of the contaminants in saturated soils. The model was solved by a hybrid numerical technique (Green Element Method) using Hermitian interpolation functions and validated for two case studies. Subsequently, the analysis was conducted taking a finite mass of the contaminant as top boundary condition, while natural or flux condition was at the bottom. The maximum relative concentration of the contaminant is determined as it percolates. Deducing from the results, design charts (Monographs) were developed to determine the minimum thickness of the barrier. The principal design parameters are "Equivalent Leachate Heights" and "Darcy Velocities". The flow direction can be either downward or upward. The Darcy velocity represents the hydro-geological condition while; the equivalent leachate height represents the density of the waste in a landfill and the prerequisite of leachate collection system. Conservative contaminants are used for determining the minimum barrier thickness. However, the thickness is necessary to check for reactive contaminants.


Introduction
The streams at their birth have already been polluted due to Municipal Solid Waste (MSW) dumping in Urban Centres at high altitudes.
The MSW disposals do not conform to the modern practices of sanitary land filling and the waste is largely dumped at convenient locations.As a pioneering effort, the Nuwara-Eliya Municipal Council has commissioned a Sanitary Landfill thus preventing further pollution of "Bomburella" stream.(Upper Catchments of Uma Oya) In the Western Province, disposal is carried out in low-lying areas prone to flooding and often susceptible to contamination of water.The Groundwater pollution, which has not been properly assessed is another threat posed by the dumping of wastes.Therefore, the contamination of precious ground or surface water is unavoidable unless Sri Lanka too resorts to the modern practices of sanitary land filling, at least in urban centres, as an integrated approach to environmental protection.

The forecasts of the population distribution of Sri
Lanka in year 2025 shows that the urban population will reach nearly 60% of the predicted overall population of 23 million.About 10 million of this urban population will crowd the urban centres in the Western Province, already inhabited by more than half of the urban population.Waste disposal facility design must involve some form of a barrier that separates the waste from the groundwater system located below it.The barrier is intended to minimise the migration of contaminant from the waste facility.Natural clay deposits, decomposed rock formations, abandoned quarries and compacted clay liners can be used as landfill barriers, thus preventing pollution of water resources.

The residential per capita waste generation in
The leakage through the barrier system and its environmental impact on the ground water quality depends on the nature of the site, the climate, the type of waste, the local hydrogeology and the presence of dominant flow path.

(Jayawardena & Mathur 2000a)
The design of a landfill barrier system is generally based on a prescriptive standard type.The regulatory organisations often specify some guidelines to determine the minimum thickness of the compacted clay liner for satisfactory performance.The bottom-lining system of the prescriptive standard for municipal waste landfills is varied from either one country to another or is practically non-existent.
There is however no consensus on what an appropriate minimum thickness ought to be due to difficulties encountered while analysing soil liners.
The disadvantage of the perspective standard is that it could lead to gross over design of barrier system where either the hydro-geologic environment is favourable or the landfill is small with low toxicity waste.
The present state of knowledge enables to promote the performance standard design for barrier system, ensuring the environment safety in an efficient manner.(Jayawardena & Mathur 2000b) In this paper, design curves (monographs) developed in the study are presented to determine the minimum barrier thickness required in an "Engineering Landfill" or "Naturally Attenuated Landfill" with a view for protecting the environment.The above minimum thickness should be checked against other reactive contaminants such as heavy metals, dissolved organic compounds and biodegradable substances.
Moreover, this innovative design approach promotes performance standard design instead of transposing perspective standard practices of other countries, which is expensive for developing countries such as Sri Lanka.

Methodology:
The retardation factor R^can be expressed as It is assumed that the sorption processes are linear and can be represented in terms of partition or distribution coefficient K p .The detailed descriptions of the special boundary conditions used in this study are described below, since the reader should understand the assumptions made in developing the monographs and its limitations in applying it to the complex situation.

Finite Mass Boundary Condition
The finite mass boundary condition may be used to represent contaminant sources in landfills.When the mass of contaminant is finite, the concentration of contaminant at the source will decline as contaminant mass is transported into the layers below or is removed by a leachate collection system.The boundary condition is defined by

Thus / r (c.T) -nJJfiriMl-n L D k -n,{U,C T (fi,l)-0.0(0,0}
Also, Hence the numerical scheme the finite mass boundary condition can be defined as If the landfill contaminant source is assumed as finite mass; the waste density, infiltration through the cover, and percentage of mass are to be specified for the contaminant.

Natural or Flux Boundary Condition
The

Model Validation:
The numerical model was simplified sufficiently so that it could be compared with the analytical solution to conduct a sensitivity analysis and discuss the importance of the Courant and Peclet numbers that result into oscillations and numerical dispersion in the numerical solution.(Jayawardena 2002).(Eq.12)

Rowe & Booker (1985) introduced the concept of equivalent height of leachate H f defined in equation (Eq 4) and found that in the absence of a first order decay in a landfill, the one dimensional (vertical) spatial and temporal variation of solute concentration below the landfill is given by
The notations were defined similar to the governing equation (Eq 1).
As The development of a monograph is also explained graphically in Figure 4 to Figure 6.(c) By removing the leachate from the landfill for treatment and disposal, the mass of contaminant available for transport into the hydro-geological system will be reduced.
(d) Avoid damage to the liner system.
Therefore, the introduction of leachate collection system reduces "Effective Leachate Height" by one tenth, since 90% of the leachate generated is taken out by the leachate collection system.For natural attenuation (without a leachate system) the designer should assume that entire leachate generated due to infiltration is available for migration.(Bagchi, 1994).
However, when a leachate collection system is provided at a landfill site, the value of the equivalent leachate height is small.For a case when the leachate height is equal to lm (implying that 90% of the leachate is collected in the leachate collection system located at the bottom of the landfill), the amount of leachate that infiltrates the barrier is about 10% only.
In the illustration in the Appendix B, when equivalent leachate height is lm and 10m respectively, which corresponds to the situation whether or not a primary leachate collection system is provided at the landfill site, the minimum (safe) barrier thickness required can be compared.The comparison is given in the Appendix B.
It has been reported that contamination migration can be reduced if the landfill is located in the discharging zone (Rowe, 1988).In such a case, the advective transport is through the barrier and into the landfill (flow in upward direction), and this in turn tends to reduce the downward movement of the contaminants on account of the concentration gradient.Such a system is also called a hydraulic trap where the flow is from the subsurface zone to the landfill through the barrier (inward flow).This situation is desired at times since it prevents any significant migration of the contaminant out of the barrier, however it does not necessarily imply that the migration stops completely when the flow direction is reversed.
A study was also conducted for a conservative substance to determine the effect of the advective transport in the upward direction.
It can be seen from the Monographs that as the velocity of flow in the upward direction increases, diffusion becomes a more predominant transport mechanism.However, when the flow is in the downward direction at such high velocities, dispersion is more important.Also, at an upward flow velocity of 0.003m/yr (Figure 12) and a leachate height of lm, there was hardly any variation in the two cases of upward and downward flow and there was a very marginal change when the leachate height was 10m.This is because since the Darcy velocity is very small, the effect of the dispersion is less compared to diffusion in the expression for hydrodynamic dispersion.Whether the flow is in the Upward or downward direction the values of the maximum relative concentration at various depths for the two leachate heights is more or less the same implying that diffusion is the likely predominant transport mechanism.
Thus, when the advective flow is reversed (upward direction), the minimum barrier thickness can reduce drastically, but one has to resort to pumping out the groundwater that needs to flow in the upward direction through the barrier.
When the upward velocity is increased to a very high value (0.1 m/yr), the advection in the upward direction is so high that the impact of diffusion in the downward direction is negligible.Therefore the barrier thickness required is very small, however, this is not a practical situation since a large volume of water at a very high rate would have to be pumped out of the system.Moreover, in cases of engineered hydraulic traps, the base contours are sufficiently lowered thereby reducing the thickness of the natural barrier that separates the underlying aquifer and the waste.Figure 21 shows that the variation of maximum relative concentration with depth has no effect with the thin aquifer boundary conditions located at various depths and the zero concentration boundary condition at a depth of 30.0m.Therefore introducing zero concentration at deeper depth and the thin aquifer boundary at lesser depths has no effect on the end results of the design charts.

Conclusions:
The Colombo is about 0.54 kg/day (Gamage & Costa, 2001).In India the amount ranges from about Eng.(Dr.) L P. Jayawardena B.Sc. Eng.(Hons).MEng.. PhD.. CEng.. MIE(SU) Deputy Resident Engineer.National Water Supply & Drainage Board.Kalu Ganga Treatment Plant Site Office.Kandana.Horana A Former Research Scholar.Indian Institute of Technology Delhi.Prof. Shashi Mathur.B.Tech(Hons).M.Tech.PhD.Professor.Dept. of Civil Engineering, Indian Institute of Technology.Delhi 0.1 kg/ day in small towns to 0.5 kg/day in large towns.(Pachauri & Sridharan, 1998).This figure varies from 1.0 to 2.2 kg/person/day in industrialised nations (Hanashima, 2000).Therefore, the Municipal Engineers of Sri Lanka should be prepared to solve the problem of disposing about 7,450 Metric Tonnes per day in year 2025.Please note that at the moment, Colombo Municipal Council (C.M.C.) is handling approximately 500 Metric Tonnes per day.The need of a National Programme is highlighted here on infrastructure development of MSW disposal with minimum damage to the environment.

2. 1 .
Governing Equation: In one dimension, the mass transport of a decaying solute through a porous medium can be expressed by following equation, which constitutes the governing equation.It is based on the theory discussed in Spitz and Moreno (1996).The retardation due to adsorption discussed by Kim et al., (2001) is incorporated in this equation to yield P^C of parabolic and hyperbolic characteristics depending on the relative values of the parameters included in the equation.When hydrodynamic dispersion is dominant, the equation behaves as a parabolic equation, and when advection is large it behaves as a hyperbolic equation.The relative importance of these two transport processes is indicated by the value of the dimensionless parameter known as the Peclet number (Taigbenu 1998a, 1998b, 1999a).
The rate of leachate available for migration at the top of the barrier.9 0 =The rate of leachate generated due to infiltration from the cover, and f T (c/t) , is the surface flux (mass per unit area per unit time) entering the soil.For the case of advective-diffusive transport the mass flux f, is given by, / -nVfi -nDJfi (Rowe et al., 1995).
second type of boundary condition is referred to as the Neumann (Natural or flux condition), and it specifies the flux (fluid flux) across an area.This condition is expressed as and transport equation this boundary condition is used when there is an impermeable layer at the bottom of the domain.Similarly in most of the analytical models the boundary condition at end of the infinite domain .isalso 3C modelled by taking.-=0 The same condition dx preavails when the contaminant moves out of soil with moving soil-water.However dispersion and diffusion do not contribute to this movement.(Mass flow without hydrodynamic dispersion).

Figure ( 4 )* 1 Figure 4 :Figure 5 :The relative maximum concentration is the ratio of the maximum concentration at a particular depth and the initial source concentration defined at the top boundary. The maximum relative concentration attained for a time period of 200 years and the depth from the contaminant source were plotted forFigure 6 :Figure 7 :Figure 8 :Figure 9 :Figure 10 :Figure 11 :
Figure (4) shows the temporal variation on concentration of conservative solute for various depths up to 300mm (50mm intervals) for Darcy velocity of 0.003m/yr and leachate height of 1.0m.The solid points represent the maximum concentration attained at a particular depth.The values plotted up to a period of 30 yrs were shown

Figure 12 :Figure 13 :Figure 14 :Figure 15 :Figure 16 :Figure 17 :Figure 18 :Figure 20 :
Figure 12: Variation of Maximum Relative Concentration with Depth (For a Hydraulic Trap -More Pumping & Leachate Treatment) Upward Darcy Velocity = 0.003 m/yr (Conservative Solute) a non-conservative solute is discussed in this section of the study for two cases of Darcy velocity and a leachate height of 10m.At a low Darcy velocity (0.003m/yr) and for a product of partition coefficient and dry density ranging from 0 to 50 for most of the solutes found in municipal landfills (Rowe et al., 1995), the computed retardation factor assumes a value ranging between 1.0 and 126.0.It is found that corresponding barrier thickness obtained from conservative solutes are safe for the entire range of retardation factors for a metal like zinc, where the maximum relative concentration is about 0.075 (assuming maximum zinc concentration in a typical landfill is 67 mg/1 and the permissible value of zinc is 5 mg/1 (Rowe etal.,1995).One can determine the depth when the solute attained the required permissible concentration in the barrier and the respective depth is illustrated in the Appendix B. An exercise was also conducted for a leachate height of lm to study the impact of attenuation by decay and attenuation by adsorption for Darcy velocity of 0.003m/yr.Figures (17) and (18) illustrates the minimum thickness of the barrier for the non-conservative solute.Similarly, first order decay of a non-conservative substance is considered at an equivalent leachate height of 10.0m for various values of half life ranging from 1 year to 50 years for two values of Darcy velocity (Figures 19 and 20).From these design curves the relevant depth of attenuation of a non-conservative solute can be determined.From these figures it appears that there is very little change in the curves for cases where the halflife is 1,5 and 10 yrs when the velocity of flow is 0.003m/yr and O.lm/yr (low and large velocities respectively).However, as the half-life increases to about 50 yrs, the difference in the relevant depths of attenuation becomes quite obvious.The lower boundary condition was taken as the zero concentration.The effects of the lower boundary conditions on design charts were investigated subsequently by changing the domain length and a thin aquifer boundary was introduced at various depths such as 10m, 5m and 3 m.The parameters of the thin aquifer were taken as porosity 0.30; the Darcy velocity of the thin aquifer lm/yr and the thickness is 2m.The length of the Landfill along the aquifer flow direction was taken as 200m.

Figure 21 :
Figure 21: Effect of the Bottom Boundary Condition on Variation of Maximum relative Concentration (Darcy Velocity = 0.03 m/yr Conservative Solute)

The above assumption is valid at relatively low concentrations found in MSW disposal sites. Both Langmuir's and Freundlich's isopleth reduces to linear representation at very low concentrations
(Rowe et al., 1995).