Waste Management 31 (2011) 1232–1238
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Knowledge based ranking algorithm for comparative assessment of post-closure care needs of closed landfills Banu Sizirici a,⇑, Berrin Tansel b, Vivek Kumar b a b
Case Western Reserve University, Civil Engineering Department, 2104 Adelbert Road, Bingham Bld. Room: 216, Cleveland, OH 44106, United States Florida International University, Civil and Environmental Engineering Department, Miami, FL, United States
a r t i c l e
i n f o
Article history: Received 16 April 2010 Accepted 11 January 2011 Available online 15 February 2011
a b s t r a c t Post-closure care (PCC) activities at landfills include cap maintenance; water quality monitoring; maintenance and monitoring of the gas collection/control system, leachate collection system, groundwater monitoring wells, and surface water management system; and general site maintenance. The objective of this study was to develop an integrated data and knowledge based decision making tool for preliminary estimation of PCC needs at closed landfills. To develop the decision making tool, 11 categories of parameters were identified as critical areas which could affect future PCC needs. Each category was further analyzed by detailed questions which could be answered with limited data and knowledge about the site, its history, location, and site specific characteristics. Depending on the existing knowledge base, a score was assigned to each question (on a scale 1–10, as 1 being the best and 10 being the worst). Each category was also assigned a weight based on its relative importance on the site conditions and PCC needs. The overall landfill score was obtained from the total weighted sum attained. Based on the overall score, landfill conditions could be categorized as critical, acceptable, or good. Critical condition indicates that the landfill may be a threat to the human health and the environment and necessary steps should be taken. Acceptable condition indicates that the landfill is currently stable and the monitoring should be continued. Good condition indicates that the landfill is stable and the monitoring activities can be reduced in the future. The knowledge base algorithm was applied to two case study landfills for preliminary assessment of PCC performance. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The Resource Conservation and Recovery Act (RCRA)’s Subtitle D (40 CFR §258.61(b)) requires a post-closure care period (PCC) of 30 years for non-hazardous wastes in landfills. The length of the post-closure care period can be extended or shortened by the governing regulatory agency on a site-specific basis. A landfill is considered functionally stable when it no longer presents an unacceptable threat to human health and the environment (HHE). Currently, there are no specific technical criteria which can be used for systematic evaluation and comparison of relative stability of closed landfills which, at the same time, allow an objective determination of the adequacy of the 30-year PCC period on a case-by-case basis (Tansel et al., 2008; Sizirici, 2009; Sizirici and Tansel, 2010a; Butt et al., 2008; Butt and Oduyemi, 2003). This makes it difficult for regulators to decide whether to extend or reduce the PCC period. However, the decision to extend or shorten the PCC period should be based on whether the landfill is a threat to HHE (Sizirici and Tansel, 2010b). A closed landfill site can be reused for many purposes which serve the community needs such as a recreation park, ⇑ Corresponding author. Tel.: +1 216 368 2423. E-mail address:
[email protected] (B. Sizirici). 0956-053X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2011.01.014
wildlife refuge, golf course or a parking lot (Vesilind et al., 2001; Tansel, 1998; Morris et al., 2003; Morris, 2005; Anderson, 2004). Post-closure care needs for municipal solid waste (MSW) landfills include ground water (GW) and gas monitoring, leachate collection and treatment, gas recovery and management, final cover maintenance and risk assessment (Vesilind et al., 2001; Reinhart and Townsend, 1997; Tansel et al., 2008; Caldwell, 2004). Waste characteristics, climate, landfill design, and closure methods affect the rate and duration of leachate and gas production from a closed MSW landfill site. The landfill activity depends on factors that relate to operation and use of the landfill both before and after the closure (Tansel et al., 2007; Lee and Jones-Lee, 1992). Therefore, PCC decisions should be based on location specific factors, operational factors, design factors and most importantly the recipient (industrial or residential area around the landfill) factors (Golder Associates, 2002; Sizirici and Tansel, 2010a). The objective of this study was to develop an integrated data and knowledge based decision making tool which can be used for preliminary assessment of PCC needs of landfill sites. The knowledge based decision making tool would enable the decision makers to set priorities for monitoring and management of closed solid waste landfills and develop adequate PCC strategies. Guidelines were provided for assigning scores depending on the site conditions.
1233
B. Sizirici et al. / Waste Management 31 (2011) 1232–1238
Knowledge Base Categories
User Defined Weights
General
General
Liner
Liner
Cover
Cover
Design
Operation
End Use
End Use
Leachate Leachate
GW
GW
SW
SW
Soil
Gas
Climate
Climate
Gas
Monitoring
Soil Landfill score Fig. 1. Approach used to build the decision making tool.
2. Methodology Landfill is a complex system where environmental conditions and location specific factors affect the rate of stabilization of wastes as well as the potential risks to human health due to proximity to sensitive receptors. The adequacy of PCC activities at a landfill site depends on both the conditions at the landfill and the environmental factors. The factors which affect the PCC efforts include: (1) General landfill characteristics (e.g., size, fill material, fill volume). (2) Climate (e.g., temperature, precipitation, evapotranspiration, wind speed). (3) Cover system (e.g., type, thickness). (4) Soil characteristics (e.g., type, permeability, contamination). (5) Groundwater characteristics (e.g., velocity, direction, depth to aquifer, contamination). (6) Liner characteristics (e.g., presence, type, thickness). (7) Leachate characteristics (e.g., quantity, quantity and quality trends). (8) Surface water management (e.g., runoff, erosion, flooding control, quality). (9) Gas characteristics (e.g., quantity, quantity and quality trends). (10) End use (e.g., type, regrading). (11) Sensitive receptors near the site (e.g., type, distance). The approach to build a decision making tool involves both the use of monitoring data from the site and knowledge base in an integrated manner. Some landfills sites have extensive data collected for monitoring leachate, groundwater, soil, surface water and landfill gas. However, there are also landfill sites with limited data, but with significant amount of knowledge about the site history, landfill development, and the site characteristics. For such cases, the available knowledge base can help for evaluation of landfill components and for assessment of PCC needs. The knowledge base approach uses a relative rating methodology where the user can assign scores to design, operation, and monitoring quality categories. Based on the significance and the anticipated effect on PCC, a weight is assigned to each category as presented in Fig. 1.
Post-closure care activities at a solid waste landfill ensure that the solid waste facility will be managed after its closure so that it does not pose any threat to HHE. Hence, the PCC process ensures that the landfill does not affect or hinder the well being of neighboring communities and sensitive receptors. Fig. 2 presents how the various landfill parameters are linked through different pathways (i.e., surface water, groundwater, soils, and gas) which may affect human health and environmental quality. To develop the decision making tool, eleven categories of parameters were identified as critical areas which could affect the PCC needs. Each category was defined by a set of questions which can be answered based on the existing knowledge about the site. Relative importance of each question can be defined by the user depending on the site location and sensitive receptors. Based on the relative importance of the questions, weighted sums were calculated for each category as shown in Fig. 3. Rating guidelines were provided to aid the users with the scoring of each category. The knowledge base landfill ranking approach was developed based on evaluation of each component of the landfill. Each factor were scored from 1 to 10 (i.e., 1 is the best and 10 is the worst). The weights for each category can be assigned by the user depending on the importance of that category for the PCC needs and characteristics of the sensitive receptors. The weighted sums of the responses to the questions were used to calculate the overall landfill score. Table 1 presents the categories, descriptive questions to be considered in each category, and the score ranges for preliminary PCC assessment. Detailed score ranges for age with material, layer type and receptors with distance questions listed in Table 1 were presented in Tables 2-4, respectively. 2.1. Ranking and scores Weighted sum method (US EPA, 1992) was used for assessment of the site parameters. This is a quantitative method for screening and ranking options and involves the following three steps: (1) Identification of important factors: eleven categories of parameters related with design, operation and geographic factors were identified as critical areas which affect the PCC needs. (2) Detailed analysis of each category: parameters in each category were further analyzed by detailed questions which could be answered by decision makers. Depending on the
1234
B. Sizirici et al. / Waste Management 31 (2011) 1232–1238
Fig. 2. Various landfill parameters linked through different pathways.
Fig. 3. General schematic of the decision making tool.
knowledge base, a score was assigned to each question (based on 1–10, as 1 being the best, 2 being better, 3 being good, 4 being right, 5 being reasonable, 6 being fair, 7 being poor, 8 being bad, 9 being worse and 10 being the worst case). If the question was a Yes/No question, the score was assigned 1 to 10 based on the importance of the question for PCC needs. If the question had multiple choices, suitable score range was provided according to the importance of the parameters. For example, for the climate categories, the most favorable scenario would be arid climate type. In arid climate, there is not significant precipitation; hence it is favorable for leachate quantity or surface water quality. However, arid climate would affect the landfill gas quantity
negatively since there would not be sufficient humidity. For this reason, a score of 3 was assigned for this question. For the depth to groundwater question, if the depth to GW was deeper than 4.5 m, a value of 1 was assigned as the most favorable score. Since, the higher the distance to GW, the lower the risk due to leachate migration with GW. Age with fill material question was evaluated based on the material placed in the landfill. If the landfill only received construction and demolition debris, there would not be significant degradation or significant amounts of contaminants which would leach. However, these score should be assigned based on the users’ collective opinion and knowledge about the landfill. Some construction and demolition landfills may
1235
B. Sizirici et al. / Waste Management 31 (2011) 1232–1238 Table 1 Summary of the categories, questions and score range. Categories General landfill characteristics
Questions
Scores a
Age with fill material Fill area (area where waste is placed)
Fill volume (volume of waste placed)
Climate
Local climate
Precipitation rate (average rainfall/year)
Evapotranspiration rate
Hurricane activity Soil
Subsoil type under landfill
Soil contamination Soil contamination type
Liner
Liner presence Liner extent
What is the liner type
Barrier layer
Drainage layer Leachate
Is there a collection system? Generated quantity
Quantity trend
Leachate constituent trend
Projected quantity
Ground water
Depth to GW
GW flow direction GW velocity
GW constituents
Min score
Max score
1 1
10 10
1
10
60 cm 15 10–15 5–10
