كليدواژه :
محل دفن پسماند , لاينر رسي , نفوذسنج , كهريزك , شيرابه
چكيده لاتين :
One of the most important factors of water pollution in municipal solid waste landfill is leachate which in the case that enters to ground or surface water; it can cause some noticeable health and environmental dangers. Impermeable liners act as barrier and donʹt allow leachate to permeate into deeper layers of landfills soil and subsequently into groundwater. Due to availability of proper materials in landfills (at most parts of Iran especially in Kahrizak) and also less expenditure of this liner in comparison with other liners it can be count as an acceptable choice. Different studies imply that one of the most important factors for soil liner selection is local conditions and also liner compatibility with generated leachate in studied site. Hydraulic behavior of soil liner, which can be condensed in liner permeability, in reaction with municipal solid waste leachate depends on properties of used leachate and linerʹs soil. This issue is very important, especially in the case of soil with high degree of salinity.
Because of this, changes of compacted soil liner permeability were measured by use of Kahrizak landfill soil subject to leachate in laboratory scale. Permeability test was done based on falling head method and by using of soil samples which was compacted in permeameter and real leachate which sampled from Kahrizak landfill. Although this method can be applied for measuring hydraulic conductivity of soil, but it has some defects that must be debugged. For example, by using of high hydraulic gradient, soil is saturated that it can lead to incorrect result of test. In addition in this method, soil must be compacted at first, and then it can be put into the permeameter. This issue causes that sample and permeameter body canʹt coincide with each other and as a result, possibility of piping phenomena formation by permeant liquid between sample and body increases. Also using of flexible body in permeameter is another fault of this method that as its result, obtained permeability value is less than real value. So to measure permeability more accurately, some improvements seemed imperative. For this purpose new permeameter was designed (Fig. 1). This apparatus has modified faults of this method while it has preserved generalities of EPA proposed method. In this method, soil isnʹt saturated before testing, permeameter has rigid body and leachate is used as permeant that these are the main modifications of this method in comparison with standard method. Ability of testing three samples simultaneously is another plus point of this method.
After setting and calibration of apparatus, leachate which has been brought from Kahrizak landfill was tested. pH, COD, BOD, TDS, EC, nitrate, phosphate, sulfate, chloride, carbonate and bicarbonate was parameters which measured each time (Table 1).
Table 1: Characteristics of generated leachate in Kahrizak landfill
Parameter First sample Second sample Third sample
COD (mg/lit) 67700 70350 55500
BOD5(mg/lit) 30362 33845 24855
Nitrate (mg/lit) 220 160 80
Phosphate (mg/lit) 177 275 275
Sulfate (mg/lit) 1700 2500 3500
Chloride (mg/lit) 4600 5320 8000
Bicarbonate (mg caco3/lit) 20600 15700 17600
pH 6.074 5.124 5.011
EC (ms/cm) 26.7 31.8 33.3
TDS (g/lit) 13.34 15.82 17.04
Before test, samples were passed through filter-paper No. 40. Obtained chloride and bicarbonate are results of two times titration average.
Physical characteristics of Kahrizak soil were tested. According to carried out tests, moisture content of soil in site and at the time of sampling was 11.06% and specific gravity (Gs) was 2.75. Also optimum moisture of Kahrizak soil obtained by standard proctor test is 15.16%. Fig. 2 that is related to permeability changes of each sample shows limit of soil permeability from the beginning until the time that soil reaches fixed state.
Figure shows that permeability of each sample increases by passing the time. Cause of this increase is interpreted in the way that water or any other liquid, for passing from one porous media chosen passage that requires less energy. Hence channels that are filled by water are chosen. Cavities which are occupied by air donʹt conduct liquid flowing and can be considered as soil solid particle. By passing the time that soil moves forward saturation point, trapped air in sample exits and leachate occupies its place. So more space for liquid movement is created and as result permeability increases. This increase continues until the time that soil is saturated and then it reaches fixed point. Relation between time and permeability and also correlation coefficient of the relations for three samples are given in Table 2.
Table 2: Relations between samples permeability and time
sample number saturation time (day) equation correlation coefficient
1 37 K = 2xl0"u Ln(t) + 3><10"u 0.9186
2 36 K = 2xlO"nLn(t) + 8xlO"12 0.8621
3 34 K = 4xl0-11 Ln(t)-2xl0"12 0.9005
In the equations the unit of K is m/sec and t is day.
Table show that the equation of permeability changes vs. time is one logarithmic equation. Three samples correlation coefficient approximately is close to one that it confirms this behavior. Results imply that however, liner permeability shows little increase by passing the time but this amount is one or two order of magnitude less than required permeability for liners (1*10~7 cm/sec). So this soil liner has needful competence for controlling of leachate permeation and it can be used as a material creating compacted soil liner according to related standards for Kahrizak landfill.
