Errors involved in the estimation of Leachate Pollution Index of solid waste landfill in Bangladesh

ERRORS INVOLVED IN THE ESTIMATION OF LEACHATE POLLUTION INDEX OF SOLID WASTE LANDFILL IN BANGLADESH

Islam M. Rafizul1, Mahmud M. Minhaz^2*, Muhammed Alamgir1 and Nazmul Huda Chowdhury¹

¹ Department of Civil Engineering, Khulna University of Engineering & Technology (KUET), Khulna-9203, Bangladesh

² Research Associate, Shakkhor Envirorment & Education Development Society (SEEDS) , Bangladesh

ABSTRACT

An important part of maintaining a solid waste landfill is managing the leachate through proper treatment methods designed to prevent pollution into surrounding ground and surface waters . Any assessment of the potential impact of a landfill on groundwater quality requires consideration of the components of the leachate most likely to causze an envionental impact as well as the source of concentration of those components. Leachate pollution index (LPI)is an environmental index used to quantify and compare the leachate contamination potential of solid waste landfill. This index is based on the concentration of 18 pollutants of the leachate and their corresponding significance. That means, for calculating the LPI of a landfill, concentration of these 18 parameters are to be known. However, sometimes the data for all the 18 pollutants included in the LPI may not be available to calculate the LPI. In this study, the possible errors involved in calculating the LPI due to the nonavailability of data are reported by the author. The leachate characteristic data for solid waste landfill at Chittagong in Bangladesh have been used to estimate these errors. Based on this study, it can be concluded that the errors may be high if the data for the pollutants having significantly high or low concentration are not available. However, LPI can be reported with a marginal error if the concentrations of the nonavailable pollutants are not completely biased.

Keywords: Landfill, leachate, sub-index value, pollutant weight, error analysis, lechate pollution index

1. INTRODUCTION

Landfill leachate is liquid that moves through or drains from a solid waste landfill. Leachate is the main medium for the contaminants transportation from the landfill to groundwater and surface water (Rowe 1995). Landfill leachate is formed from the infiltration and passage of water through solid waste which results in a combination of physical, chemical and microbial processes that transfers pollutants from waste materials to the water (Jasper et al. 1985; Kjeldsen et al. 2002).

The most common source of landfill leachate is rainwater filtering down through the landfill and aiding bacteria in the process of decomposition (Kelley 1976). Modernlandfillsare often designed to prevent liquid from leaching out and entering the environment; however, if not properly managed, the leachate is at risk for mixing with groundwaternear the site, which can have terrible effects (Chian and DeWalle 1976).

Leachate can consist of many different organic and inorganic compounds that are typically either dissolved or suspended in the wastewater (Christensen and Kjeldsen 1995) .The leachate may be virtually harmless or dangerously toxic, depending upon what is in the landfill (Leckie et al. 1979; Kouzeli-Katsiri et al. 1999). Typically, landfill leachate has high concentrations chemical oxygen demand (COD) associated, BOD, nitrogen, phenols, pesticides, chloride, solvents and heavy metals are common in these systems (Lo 1996). As leachate liquid emerges from a landfill site, it is often black or yellow, with a strong acidic smell.

The potential long term environmental impact of a landfill on groundwater quality will depend on the leachate charecteristics, the mass of contaminant in the facility, the percolation of fluid through the waste, the nature of the leachate containment system and the site hydrology (Henry and Heinke 1996; Farquhar and Rovers 1973). Leachate characterstics may be expected to evolve over time, increasing from initial values to peak and then subsequently decreasing as the potential contaminants are either flushed out of the system, biodegraded or precipitaded (Jasper et al. 1985; Blight et al. 1999).

A large number of environmental indices have been eveloped in last four daades. Various indices are developed to quantify the pollution or quality of water and air. Usually, the indices are formulated based on studies conducted by the indices developers or on the Delphi technique (Kumar and Alappat 2009). In an effort to evelop a method for comparing the leachate pollution potential of various landfill sites in a given geographical area, an index known as Leachate Pollution Index (LPI) was formulated using Rand Corporation Delphi Technique (Kumar and Alappat 2003).

The LPI can be used to report leachate pollution changes in a particular landfill over time. The trend analysis so developed for the landfill can be used to assess the post closure monitoring periods. The leachate trend at a given landfill site can facilitate design of leachate treatment facilities for other landfills in the same region (Rafizul et al. 2011). The LPI can also be used to compare leachate contamination potential of different landfills in a given geographical area or around the world. To quantify and compare the leachate contamination potential of municipal landfills 18 characterstics to be known.The other potential applications of LPI include ranking of landfill sites based on leachate contamination potential, resource allocations for landfill remediation, enforcement of leachate standards, scientific research and public information (Kumar and Alappat 2003).

The intention of this study was to calculate the error invloved in estimation of LPI due to nonoavailability of data. In this study it is analyzed the possible error associated with estimation of LPI. The study area was Garbage Treatment Plant Landfill at Chittagong where daily waste disposal were 200 MT on average.

2. METHODOLOGY ADOPTED

2.1 Leachate Pollution Index (LPI)

The formulation process and complete description on the development of the LPI has been discussed elsewhere (Kumar and Alappat 2003). The LPI represents the level of leachate contamination potential of a given landfill. It is a single number ranging from 5 to 100 (like a grade) that expresses the overall leachate contamination potential of a landfill based on several leachate pollution parameters at a given time.

2.2 LPI Variables and Their Weight

The 18 parameters chosen and their corresponding weights are as follows: chromium (Cr): 0.064; lead (Pb): 0.063; chemical oxygen demand (COD): 0.062; mercury (Hg): 0.062; biochemical oxygen demand (BOD5): 0.061; arsenic (As): 0.061; cyanides (Cn): 0.058; phenolic compounds: 0.057; zinc (Zn): 0.056; pH: 0.055; total kjeldhal nitrogen (TKN):0.053; nickel (Ni): 0.052; total coliform bacteria (TCB): 0.052; ammonia nitrogen (NH4-N): 0.051; total dissolved solids (TDS): 0.050; copper (Cu): 0.050; chlorides (Cl-): 0.048; and total iron (Fe): 0.044. The weight factor indicates the importance of each pollutant variable to the overall leachate pollution. The sum of the weights of all 18 parameters is one.

2.3 Variable Curves

The averaged sub-index curves for all the pollutant variables have been reported by Kumar and Alappat (2003).

2.4 Variable Aggregation

illustration not visible in this excerpt

(1)

The weighted sum linear aggregation function was found to be the most suitable one for the calculation of LPI (Kumar and Alappat 2004) and is as follows:

illustration not visible in this excerpt

Where, where LPI is weighted additive leachate pollution index; wi = the weight for the ith pollutant variable;

pi = the sub-index value of the i th leachate pollutant variable, number of leachate pollutant parameters; n =18 and ∑ wi =1.

However, when the data for all the leachate pollutant variables included in LPI are not available, the LPI can be calculated using the data set of the available leachate pollutants by the equation:

illustration not visible in this excerpt

Where m=number of leachate pollutant for which data are available, but in that case, m<18 and ∑ wi <1.

2.5 Errors Involved in Calculating LPI Due to Nonavailability of Data

To assess the errors involved in calculating LPI due to nonavail ability of data, a case study is taken up. Leachate samples from Chittagong Garbage Treatment Plant Landfill were collected and analyzed in the laboratory is provided in Table 1 to evaluate the error invlolved in calculation LPI due to nonavailability of data.

Table 1: Leachate characterstics of Chittagong Garbage Treatment Plant Landfill

illustration not visible in this excerpt

All values in mg/L except pH and total coliform unit (cfu/100ml). * Average of 4 samples taken between 1st August and 30th September 2011.

3. CASE STUDY

To estimate the possible errors involved in calculating LPI, due to the nonavailability of leachate data, two approaches have been made as

- Ignoring pollutant data based on weight factor and
- Ignoring pollutant data based on sub-index value.

The sub-index values of all the pollutant parameters in lechate based on their concentrations are reported in Table 1. The subindex values have been derived from the subindex curves for all the parameters reported by Kumar and Alappat (2003). The LPI value based on these sub-index values has been calculated using Equation (1) and provided in the fifth column, Table 2. The LPI calculated based on these 18 parameters is considered to be the true LPI value of the landfill.

3.1 Errors Introduced by Ignoring Pollutant Data Based on Weight Factor

In this approach, two options are discussed. In the first option, the data of the pollutants having low weight factors is ignored and in the second option, the data of the pollutants with high weight factors are assumed to be not available.

3.1.1 Removing Pollutants with Low Weight Factors

1. In the first step, the concentration of the total iron, the parameter having lowest weight, is presumed to be unknown. Hence, by deleting the subindex value of total iron, the LPI value is derived by using Eq. (2). The derived LPI value is reported in the sixth column, Table 2.
2. In the next step, the concentration of chlorides, the parameter having second lowest weight, is also presumed to be unknown in addition to the concentration of total iron. Again using Eq. (2), the LPI of the data set with 16 parameters is calculated and reported in the seventh column, Table 2.
3. In a similar fashion, it is presumed that the concentrations of copper, total dissolved solids, ammonia nitrogen, total coliform bacteria, nickel, total kjeldhal nitrogen, pH, and zinc are also not known one by one in addition to the earlier unknown concentrations of the parameters. The derived LPI values considering concentration of 15, 14, 13, 12, 11, 10, 9,8,7,6,5 and 4 parameters are calculated and reported in columns 8, 9,10, 11, 12, 13, 14,15,16,17,18 and 19 of Table 2, respectively.
4. The percentage error introduced calculating LPI values, with respect to the LPI value when data are available for all 18 leachate pollutants, is also reported in the last row of respective columns of Table 2.
5. The variation in LPI values with respect to the number of parameters considered in calculating LPI is provided in Figure 1. It also gives the percentage error introduced in calculating LPI values with respect to the number of parameters considered.

[...]