Trend in waste to landfills in Victoria indicates an increase from 2.2 MT in 1884/85 to 2.9 MT in 1991/92. Australians ranks second in the amount of waste generated in the world after the US, and generate 1.89 kg/capita/day of solid waste (Hoornweg and Thomas, 1999).
According to the waste management hierarchy, waste to landfill is the least preferred waste management option.
The Environment Protection (Resource Recovery) Act of 1992 objective was to reduce waste going to landfill such that only half the quantity of waste generated of 1991 was going to landfill by the year 2000. They hoped to achieve this target by the strategy of 3R’s which is reduce, reuse and recycle. However in a study conducted by Eco-Recycle Victoria in 2003, they found that the quantity of waste going to landfill was still constant which the overall waste volume had increased. This indicated that while the strategy of recycling waste was working. However increase in population also meant that there is more waste being generated.
While waste to landfill can be reduce, there will always be landfills. According to Waste and Recycling in Australia in 2006 it is projected that municipal waste will 10,984 MT by the year 2022-23 as compared to 6,045 MT in the year 2002-03.
Landfill sites if not properly designed and constructed can result in damage to the surrounding environment. Emission of methane gases to the atmosphere and seepage of leachae are some of the important issues that needs to be addressed with regard to landfill.
ASSIGNMENT – Part 1
With the Visual HELP model analysis, a best practice was done for all the three cities of Melbourne, Perth and Brisbane. These data has been taken from Best Practice Environmental Management: Siting, Design, Operations and Rehabilitation of Landfills (EPA, Victoria 2001). All setting have been taken from the default setting of the Visual HELP software except for:
– Base liner system “barrier soil” layer has been reduced saturated hydraulic conductivity from 1e-8 to 1e-9 m/s
- Site specified HDPE properties to 2 no pinholes per hectare for manufacturing defect, 8 no pinhole for installation defect density and use of poor liner quality placement.
The following table 1 represents the classification which of Layer in the best practice.
|Layer Classification||Layer Number||Top ( m)||Bottom ( m)||Thickness ( m)|
|Municipal Waste (312 kg/cub.m)||4||-0.9985||-15.9985||15.0000|
|High Density Polyethylene (HDPE)||6||-16.2975||-16.2985||0.0010|
Table 1: Layer Classification
Results & Discussion
The results for the accumulated precipitation, runoff, evapotranspiration, percolation in a 30 years period have been presented in Table 2.
|Melbourne Best Practice||Perth Best Practice||Brisbane Best practice|
|Lateral drainage collected from Layer 2 (m)||6.32E-02||6.82E-01||2.70E+00|
|Percolation or leakance through Layer 3 (m)||2.67E-01||1.22E+00||3.43E+00|
|Lateral drainage collected from Layer 5 (m)||2.67E-01||1.22E+00||3.43E+00|
|Percolation or leakance through Layer 7 (m)||2.51E-06||1.10E-05||2.97E-05|
Table 2: Total accumulated quantities of respective categories in 30 years in meter
A look at the total accumulated rainfall for 30 years for Melbourne, Perth and Brisbane in Figure 1 shows that Melbourne and Perth have about the same quantity of rainfall where as Brisbane received approximately double the rainfall quantity.
Figure 1: Total Precipitation for Melbourne, Perth and Brisbane in 30 years
Rainfall pattern for Melbourne, Perth & Brisbane over a 30 years period has been shown in Figure 2.
Figure: 2 Rainfall for Melbourne, Brisbane and Perth for thirty years
To study the performance of a landfill it is crucial to study the percolation through and out of the landfill. The less the percolation out of landfill means a better performance landfill.
An analysis of the results of the simulation shows that the percolation through the landfill shows the same trend for three layers 3, 5 & 7. There is maximum percolation in Brisbane landfill site followed by Perth landfill.
The trend in percolation is represented in Figure 3, 4 and 5. The most crucial is layer 7 presented in Figure 4 as this indicates leakage of leachae through base barrier soil and hence determines the performance of landfill.
Figure 3: Percolation or leakance in 30 years through layer 3 in meters
Figure 4: Percolation or leakance in 30 years through layer 5 in meters
Figure 5: Percolation or leakance in 30 years through layer 7 in meters
As the landfill best practice was designed for Victoria, we can see that least percolation through base barrier soil (layer 7) is observed in Melbourne and hence the best performance.
The main reason for the difference in percolation in different cities is the result of the hydraulic head. Hydraulic head is defined as ‘the pressure exerted by the weight of water above a given point’. Hence the more pressure at a given point, the more percolation would occur with ingress of water into the landfill cell through landfill cap soil barrier (layer 3).
The study of hydraulic head on cap barrier soil (layer 3) reveals that the hydraulic head for Perth and Brisbane is higher than Melbourne as shown in Figure 6. There is more fluid percolating through cap barrier soil (layer 3) as there is more hydraulic head on it.
This hydraulic head also has the same percolation effect on layer 6 and is shown in Figure 7.
Figure 6: Hydraulic head above layer 3
Figure 7: Hydraulic head above layer 6
Hence we can conclude that the effectiveness of the landfill depends largely on the having the lowest hydraulic head.
Therefore the Victorian ‘best practice’ cannot be directly applied to other states. However with modification it will be possible to apply the best practices. The following recommendations will give some of the modifications that can be done.
There are many options for the modification of landfill designs to suit the landfills locations and situations. Issues to consider would site geology, weather patterns, rainfall patterns, water table, use of ground water, level of ground water, groundwater gradient, quality of ground water etc.
However after considering all the above issues issues, there are particular issues that can be directly addressed in the landfill designs that can help ensure that the landfill performance is optimum for the given location.
Recommendations changes that can be made in the cap liner and the base liner to improve the performance of landfill are listed:
- Increasing the slope of the cap liner will ensure more runoff quickly thereby decreasing the time for water to percolate into the soil. EPA recommends not exceeding 20% slope as this would result in erosion.
- Installation of drainage pipes would quickly remove water. This will ensure that the hydraulic head is reduced. However EPA guidelines recommend that the design of drainage in cap layers should be properly done to ensure that the surface is not dried out too quickly. The effect of drying out the surface quickly would result in the vegetation dying as well as prevents the continuous hydration of the cap low permeability barrier soil. The consequence of not hydrating the barrier soil continuously would result in dry cracked clay which would result in more percolation of water during rainfall.
- Adding geomembrane (HDPE) and geotextile in the above the cap barrier layer would reduce percolation into the landfill cell.
- Increasing the thickness of low permeability barrier soil (greater than 0.6 meters as per EPA landfill cap design requirements)
- Use of lower permeability barrier soil and to ensure that they are compacted properly during construction
- Ensure that vegetation roots are shallow as deep roots could damage the cap barrier layer
The most important aspect of the landfill base is minimizing percolation of leachae through base liner. Base liner performance can be improved by increasing the slope of drainage or reducing the pipe spacing so that leachae is drained out more quickly. This will result in decreased hydraulic head which would result in less percolation through layer 7 (base liner).
The EPA guidelines recommend that the clay layer of 1 meter. Increasing the thickness and with proper compaction during construction would reduce the percolation rate.
ASSIGNMENT – Part 2
A sensitivity analysis was done comparing Melbourne best practice and with changing the following parameters:
|Case 1||Changing the saturated hydraulic conductivity of ‘barrier soil’ layer using HELP default setting of 1x10e8 m/s|
|Case 2||Changing the Leaf Area index to 5 (instead of 2). This also meant that we changed the vegetation class to excellent strand of grass instead of poor strand of grass|
|Case 3||Changing the slope gradient for the leachae collection to 5% instead of 2% in the base liner|
Table 2: Different case studies
The following were the result were obtained with the change in parameters in the Melbourne best practice with different saturated hydraulic conductivity of barrier soil, change in leaf index to 5 and change of slope to 5%.
|Melbourne Best Practice||k (1e-8m/s)||leaf index 5||slope = 5%|
|Lateral drainage collected from Layer 2 (m)||6.32E-02||6.39E-02||8.67E-02||6.39E-02|
|Percolation or leakance through Layer 3 (m)||2.63E-01||2.63E-01||2.95E-01||2.63E-01|
|Lateral drainage collected from Layer 5 (m)||2.63E-01||2.63E-01||2.95E-01||2.63E-01|
|Percolation or leakance through Layer 7 (m)||2.51E-06||1.27E-05||2.74E-06||8.09E-08|
Table 3: Comparison of change in saturated hydraulic conductivity, change in leaf index, and slope in Melbourne Best practice
- In the case of change of saturated hydraulic conductivity of barrier soil all parameters are same till layer 7. However as the saturated hydraulic conductivity of barrier soil has been changed from 1x10e-9 to 1x10e-8 for the barrier soil in layer 7 we can see that there is increase in percolation.
Figure 8: Comparison of different saturated hydraulic conductivity
Hence we can see that saturate hydraulic conductivity is very important in the design of a landfill. Using lower permeability (k = 1e-9 instead of k = 1e-8 m/s) means that the barrier soil is more effective in mitigating the flow of leachae. This result in decrease in leachae flow through the layer and hence the landfill performance is better with saturated hydraulic conductivity of k= 1 e-9 m/s.
An interesting relationship is given in Darwin’s Law (Formula 1). As we increase the saturated hydraulic conductivity there will be an increased flow rate through the linear. This is important to consider during landfill design. Lowering the permeability of liner means a lower flow rate.
Flow rate Q = K i A (Formula 1)
Where K = permeability of liner
i = hydraulic gradient = H / L
A = area perpendicular to flow
If the flow rate is decreased and there is a good drainage system then more leachae could be collected and removed from the lateral drainage, thereby improving the performance of the landfill.
- Changing the Vegetation type results in a change in percolation through all layers of the landfill starting from the cap.
As can be seen in Figure 9, increase in leaf index from 2 to 5 which is fair to excellent vegetation, there is an increase in percolation through the cap barrier soil (layer 3).
Figure 9: Percolation through landfill cap barrier soil (layer 3) with leaf index of 2 and 5
To determine the effect of vegetation a number of scenarios were modeled. Result of the scenarios effects thorough the cap barrier soil (layer 3) in shown in Figure 10. Bare vegetation has the maximum percolation whereas poor vegetation has lowest percolation. From fair to excellent vegetation there is an increase in percolation. Other attributes that also effect percolation is runoff and evapotranspration.
Figure 10: Percolation through cap barrier (layer 3)
Vegetation cover has a direct impact on the surface runoff and the evapotranspration. With the bare vegetation, there is maximum run off. As the vegetation increases there is decrease in run off as seen in Figure 11.
Figure 11: Run off affected by change in leaf index
Vegetation cover also has a direct impact on evapotranspiration. Fair type of vegetation has the about the same affect as excellent vegetation as can be seen in Figure 12.
Figure 12: Evapotranspiration affected by different leaf index
However while having no vegetation has maximum runoff, it is crucial to have at least fair to good vegetation on the top soil. This is so that the vegetation would retain water in it roots which would ensure that the clay barrier soil does not dry up as clay desiccation will reduces barrier effectiveness.
With good vegetation clay moisture to layers below the top soil would remain optimum and hence clay barrier soil would act best to reduce percolation into the landfill. As per the EPA guidelines it states that the fundamental objective of landfill cap is to “store water during periods of elevated precipitation and low evapotranspiration for subsequent release during drier periods”.
In addition it is recommended that while good vegetation is recommended we should ensure that the top soil layer should have vegetation with shallow roots to ensure that they do not go deep thereby damaging the cap barrier layer.
- Increasing the slope of drainage has an immense impact on the amount of percolation through base liner (layer 7) as can be seen from the figure 13.
Figure 13: Comparison of percolation through layer 7 of Melbourne best practice (slope of 2%) and slope of 5%.
Increasing the drainage slope has a very big impact on the landfill performance. As can be seen from the figure, with increase in slope there is a vast reduction in leachae percolation through the base barrier soil (layer 7).
This is because as the slope is increased from 2 to 5%, the flow of leachae is faster to the drainage giving it less time for it to seep through the HDPE liner. Further, with a steeper slope, the drainage pipe would technically get submerged quicker and hence drainage of leachae would be faster.
ASSIGNMENT – Part 3
A typical landfill site situated in south-eastern part of Melbourne has the following conditions such as: existing water quality TDS of 9,500 mg/l, current ground water use limited to stock watering, ground water is relatively shallow, site is not within any ground water recharge area, site geology comprises a sequence of Tertiary age sand and clays of 15 to 35m depth underlain by granite rock, and ground water gradient is very flat which results in a slow groundwater flow rate in the region.
The objective is to determine if it is justifiable to construct landfill with best practice which would cost 40% more than constructing landfill with current practice.
The main difference between the best practice landfill and the current practice landfill is the absence of the drainage layer in the cap and the absence of the HDPE membrane in the base liner.
A simulation was done for both the best practice and the current practice using the Visual-HELP software. Given that the most crucial criteria for the landfill performance is to the eventual outflow of leachae from the base liner, we have taken and compared data at year 30 which is the expected landfill life period.
The Results of the simulation is shown in Table 3 & 4.
|Melbourne Best Practice||(meters)||(in cum)|
|Lateral drainage collected from Layer 2||6.32E-02||8.53E+02|
|Percolation or leakance through Layer 3||2.67E-01||3.61E+03|
|Lateral drainage collected from Layer 5||2.67E-01||3.61E+03|
|Percolation or leakance through Layer 7||2.51E-06||3.39E-02|
Table 3: Results of landfill performance in 30 years for Best Practice
|Melbourne Current Practice||(meters)||(cum)|
|Percolation or leakance through Layer 2||3.12E-01||4.21E+03|
|Lateral drainage collected from Layer 4||2.75E-01||3.72E+03|
|Percolation or leakance through Layer 5||3.62E-02||4.89E+02|
Table 4: Results of landfill performance in 30 years for Current Practice
|Percolation through Base liner for Best & Current Practice||(cum) in 30 years||(cum) per year||(liters per hectare per day)|
|Percolation or leakance through Base liner in Best practice||3.39E-02||1.13E-03||2.29E-03|
|Percolation or leakance through Base liner in Current practice||489.00||16.30||33.08|
Table 5: Percolation through Base liner for Best and Current Practice
- The landfill that is constructed is a Municipal landfill site
- Municipal waste is classified as type 2 and waste consist of Putrescible waste, solid inert waste and fill material
According to Type 2 waste, the EPA Victoria guidelines stipulate the performance of landfill and that the best available technology be used to ensure that seepage through the liner does not exceed 10 liters/ha/day (EPA, Siting, Designing, Operation, and Rehabilitation of Landfill 2001, pg.21,Table 4).
From table 5 we can see that with the best practice leachae percolation through base liner (layer 7) is 2.29E-03 liters/ha/day which is within the EPA standard requirement.
However for the current practice the leachae percolation through the base liner (layer 7) is 33.1 liters/ha/day which exceeds the EPA requirements of 10 liters/ha/day.
While the existing quality of ground water around the landfill site has TDS of 9,500 mg/l. As per the SEPP, Ground waters of Victoria, this is classified as segment C which is used as stock watering. This stock watering category falls within the TDS range of 3,501 to 13,000 mg/l. The 33 l/ha/day of leachae from a current landfill would mix and this could result in increase in the overall TDS of ground water. While the TDS as such may still fall within the range for stock watering the combination the increase could result in reduction in beneficial use.
We have assume that the while the water table is shallow it has a depth more than the landfill (greater than say 20 meters) as per the assumption in the HELP modeling. This depth of water table would not reach the affect the landfill through the landfill life as we assume shift of water table greater than 20 meters depth. This would mean that there is no inward pressure on the sides of the landfill at any point. With the current practice as the hydraulic head on base liner (layer 7) increase there is more percolation through. However with the best practice, while the same might be the case with the hydraulic head, there is a HDPE liner which has a very low permeability. Hence there is less percolation though this layer and therefore minimal leachae into the ground water.
The location of landfill site is not within any ground water recharge area. This would mean that the mixing of contaminated leachae and ground water would not result in dilution, but over the period of 30 years become more toxic. This would mean that over a 30 year period with the current practice there would be a huge quantity of 489.00 m3 of leachae (refer Table 5) mixing with the ground water. This would have a major impact on the TDS of ground water and could negatively affect the use of the ground water as stock watering. In contrast with the use of best practice, only 3.39E-02 m3 of leachae percolated through base liner (layer 7).
The water gradient in the area is very flat would mean that the toxic mix would remain in the area. Hence cattle consuming stock water would end up consuming the mix of polluted ground water and leachae. With good practice landfill, this would not be an issue as the base liner and HDPE would prevent percolation.
The site geology comprises of a sequence of tertiary age sand and clays of 15 to 35 m depth underlain by granite rock. We assumed that the clay in site geology is of high permeability material than our base barrier soil (layer 7). Hence there would be seepage of combined lechae mix and ground water mix through this medium easily. In addition, sand has high permeability so together they would have low barriers effect. With best practice, we do not have to be concerned about the effect of percolation through the base liner.
It is determined through the HELP computer simulation that the current practice landfill the leachae through the base liner would be 33 liter/hectare/day. This is above the EPA recommended limit of 10 liters/hectre/day for type 2 landfills.
In addition, as the water is used for stock (cattle) watering it is important to study the TDS of the mix of leachae with ground water. Over the period of 30 years, 489.00 m3 of leachae would percolate through the base liner (layer 7) of the current landfill practice. Given that there is a low ground water gradient and that the ground water is relatively shallow, the toxic combination of leachae and ground water would remain in the area getting more toxic each year. An increase in TDS would have adverse effect on the beneficial use of the ground water for stock watering.
Over a period of time, the land in the nearby areas may not be suitable for rearing stock and the community would protest. The result would be that the landfill be required to do a massive clean up programme. This would not only be very costly but also very difficulty and would cost much more than the 40% cost difference between the current and best practicelandfill.
With the best practice, the landfill management would be able to operate in an environmentally friendly way. In addition, the relationship between the landfill management and the community can be much better and hence there would be less risk of protest from local community disruption the operation of landfill.
Based on all the above consideration, we recommend the best practice landfill type to be constructed in the south-eastern part of Melbourne, Australia.
Prepared by: Jigme T Tsering