To determine the soil-pollutant interaction, the available leachate is not sufficient to perform the laboratory experiment. The municipal solid waste was therefore collected from the dumping sites in order to prepare the leachate, and uncontaminated natural soil is also collected in an airtight polythene bag.
2.1 Municipal Solid Waste
The collected MSW was carried to the laboratory and placed under ordinary laboratory temperature in the air-tight container until it was actually used. Municipal solid waste involves all kinds of waste such as cooking, paper, cardboard, clothing, plastic and glassware etc.
2.2 Natural clay soil
The uncontaminated natural soil was collected from the dumping site and stored until its real use in the air-tight polythene bag. Air is dried and crushed into the wooden mallet before undertaking the laboratory testing of the soil sample and then testing was performed as per IS 2720. The geotechnical properties of soil sample as shown in Table 1.
Table 1
Geotechnical characteristics of Natural soil
Sl.no
|
Parameters
|
Values
|
1
|
Clay content (%)
|
38.50
|
2
|
Silt Content (%)
|
61.50
|
3
|
Liquid Limit (LL) (%)
|
58.00
|
4
|
Plastic Limit (PL) (%)
|
28.66
|
5
|
Plasticity Index (%)
|
29.34
|
6
|
Shrinkage Limit (%)
|
11.52
|
7
|
Max. dry density (g/cc)
|
1.77
|
8
|
Optimum moisture content (%)
|
17.05
|
9
|
Unconfined compressive strength (UCS) (kPa)
|
167.80
|
10
|
Hydraulic conductivity (cm/s)
|
8.63x10− 5
|
2.3 Methodology
Laboratory studies were conducted using two experiment set-ups to determine the impact of soil-pollutant interaction;
First experiment set-up: Extraction of leachate from municipal solid waste
Second experiment set-up: One-Dimensional soil column method has been used to determine the effect of soil on the municipal solid waste leachate.
2.3.1 Leachate preparation experimental set-up
The extraction of leachate was done by taking five barrels with 68cm height and 35cm diameter. In the bottom of each container a metal wire mesh has been provided. Above the wire mesh 10cm thickness gravel layer was placed to prevent the blocking of the waste which enabled to flow the leachate continuously. The weight of the MSW filled in each barrel has been calculated depending on the real density of the municipal solid waste, which is approximately 700 kg/m3 in the container (without compaction) and 1,200 kg/m3 in the landfill, the average is 950 kg/m3. The weight of the municipal solid waste introduced to each barrel has been calculated by the following equation (1)
Weight = Density X V olume …… (1)
Where,
W = Weight of the MSW, kg
D = Density of the real MSW, kg/m3
V = Volume of the container, m3
From the equation the weight of each sample was calculated as 57 kg and was placed into the barrel without compaction.
2.3.2 Amount of water needed to extract leachate from MSW
The amount of water needed to extract leachate from MSW has been calculated by the annual precipitation that is taken in the Puducherry region by using the following Eq. 2.
L = P (1-C) – E ………… (2)
Where,
L = Depth of the leachate water, mm
P = Precipitation, mm
C = Runoff-Coefficient
E = Evaporation, mm
The leachate coefficient (C) in this study was taken as 0.15 depending on the average daily precipitation in Puducherry (Indian meteorological department).
Depending on this data the quantity of water required for each container has been calculated by using the Eq. 3
W = L X A ……………. (3)
Where,
W = amount of the water to be added, mL
L = Depth of the leachate water, mm
A = Area of the container or Cylinder, m2.
The quantity of water was distributed within 60 days in each container. Since the average rainy days take place in Puducherry region is between 45 days − 60 days (Department of Agricultural and farmer welfare). The rainfall and evaporation take place in winter season and the laboratory experiments were performed in the month of February and March as shown in Table 2.
Table 2
Month
|
Precipitation (mm)
|
Evaporation (mm)
|
winter seasons
|
September
|
114
|
144
|
October
|
229
|
118
|
November
|
285
|
83
|
December
|
144
|
65
|
Total
|
772
|
410
|
Experimental season
|
February
|
09
|
48
|
March
|
0 8
|
85
|
Total
|
17
|
133
|
L1 = 772(1–0.15) – 410 = 246.2 mm in rainy season.
2.4 MSW Leachate effect on soil using a one-dimensional technique of soil columns
The experimental set-up comprises of seven PVC pipe soil columns. Figure 1 indicates the seven-soil columns schematic perspective. Each column was made by unification of 90 mm outer diameter and 80 mm height with an anti-corrosive coated hopper portion with a wire mesh placed between the soil-column and hopper portion. An outlet control valve was installed in the hopper portion to regulate the outlet flow, which is connected to a drain outlet (8 mm dia).
The leachate extracted from the MSW was percolated into the soil column by feeding through the feeding tank (140 mm dia x 450 mm height) fitted with the inlet control valve at the bottom and the inlet feed tube (8 mm diameter tube) connecting the soil column with the feed tank.
2.4.1. Preparation of soil-column and loading conditions
Soil samples (weighing 2.7 kg) were mixed with leachate corresponding to their optimum moisture content and loaded in the seven columns of the soil. The first columns stated as OT and the remaining six columns as stated as ST. Leachate outflow samples were collected from the drain outlet at a 48-hour interval. The chemical analysis was carried out for the outlet leachate collected from the soil column marked as OT. The soil sample was gathered for soil testing at a periodic interval of every 15 days at the remaining six columns as called ST. For each of the soil columns, about 1.2 L of extracted leachate was packed in the feed tank.
2.5 Analysis of geotechnical characteristics of soil
The soil sample was collected at an interval of 15 days for soil testing. The testing parameter are Liquid limit (LL), Plastic Limit (PL), Shrinkage Limit (SL), and Grain Size Distribution by Hydrometer Analysis, Unconfined Compressive Strength (UCS), Proctor Compaction, Hydraulic Conductivity and Scanning Electron Microscopy (SEM) are conducted as per standard IS and ASTM soil testing methods.