Alternative Cover Systems Research Articles

Biogeochemical versus Conventional Landfill Soil Covers: Analysis of Gas Flow Profiles, Microbial Communities, and Mineralogy

In this study, a novel biogeochemical cover system comprising biochar-amended soil and basic oxygen furnace (BOF) steel slag was explored as a sustainable alternative cover system to mitigate methane (CH4), carbon dioxide (CO2), and hydrogen sulfide (H2S) simultaneously from landfill gas (LFG). Long-term column studies of a simulated biogeochemical cover (BGCC) profile investigated CH4, CO2, and H2S removal potential. The performance of the BGCC system was compared with a conventional soil cover (SC) profile. The CH4 oxidation rates of biochar-amended soil were significantly higher, ranging from 185 to 407 µg CH4/g-day in comparison with the barrier soil in the SC system (6–7.5 µg CH4/g-day), based on the batch incubation of column-exhumed samples. In addition, the biochar-amended soil showed higher relative abundance of methanotrophic bacterial communities (20%–51%) in comparison with soil cover (10%–27%). In both columns, complete attenuation of H2S occurred near the inlet (75 cm bgs) and sulfur oxidizing bacteria (e.g., Thiobacillus) and methanotrophs were both detected. The sulfur content was elevated (0.68%) at the base of both columns and H2S may have imparted an inhibitory effect on CH4 oxidation rate in the SC system. The BOF slag showed a CO2 removal potential of 67 g CO2/kg BOF slag. Overall, the BGCC system outperformed the SC system, effectively mitigating CH4, CO2, and H2S simultaneously.

Cover systems with synthetic water‐repellent soils

AbstractA cover system is a crucial component of engineered landfills, to minimize water percolation into the underlying waste. Capillary barriers are an alternative cover system, which has been widely used in the arid and semiarid regions as no cohesive, low‐permeability materials are used. However, the performance of capillary barriers in tropical climate has been unsatisfactory (breakthrough observed). In recent years, synthetic water‐repellent granular materials have drawn increasing attention due to their distinctive hydraulic behavior (inhibited water infiltration and high water entry pressure), suggesting they may also be used to improve the performance of cover systems. In this study, flume tests were conducted with inclined model slopes under artificial rainfall. By monitoring the surface runoff, lateral diversion, and basal percolation and conducting water balance analysis, the performance of monolithic cover, conventional capillary barrier, and water‐repellent cover systems were evaluated. The study revealed that (a) the barrier effect and diversion capacity were significantly strengthened by induced water repellency, providing a promising solution to extend the application of capillary barrier covers; and (b) cover systems can be formed using one raw material to decrease the construction cost, by using synthetic water‐repellent soil as the underlying layer.

Modeling and Performance Assessment of Alternative Cover Systems on a Waste Rock Storage Area

The selection and design of an appropriate mine waste cover system for the local climatic conditions, unsaturated and saturated material properties, and available cover materials is important to mine waste management. We investigated the performance of various cover configurations for minimizing the ingress of water and oxygen into the northern waste rock storage area of the Kisladag gold mine, in Usak, Western Turkey. SEEP/W and VADOSE/W software were used to model the flow in unsaturated and saturated zones and to assess the performance of various cover systems. The accuracy of input data was checked during calibration for steady-state conditions with SEEP/W. Subsequently, bedrock, waste rock, and three different cover alternatives were modeled under transient conditions for 20 years using daily climatic data. All three alternatives (enhanced store-and-release, and double and single capillary barriers) were effective in limiting infiltration. However, capillary barrier covers were more effective in limiting oxygen ingress than enhanced store-and-release covers.

Quantifying capture efficiency of gas collection wells with gas tracers

A new in situ method for directly measuring the gas collection efficiency in the region around a gas extraction well was developed. Thirteen tests were conducted by injecting a small volume of gas tracer sequentially at different locations in the landfill cell, and the gas tracer mass collected from each test was used to assess the collection efficiency at each injection point. For 11 tests the gas collection was excellent, always exceeding 70% with seven tests showing a collection efficiency exceeding 90%. For one test the gas collection efficiency was 8±6%. Here, the poor efficiency was associated with a water-laden refuse or remnant daily cover soil located between the point of tracer injection and the extraction well. The utility of in situ gas tracer tests for quantifying landfill gas capture at particular locations within a landfill cell was demonstrated. While there are certainly limitations to this technology, this method may be a valuable tool to help answer questions related to landfill gas collection efficiency and gas flow within landfills. Quantitative data from tracer tests may help assess the utility and cost-effectiveness of alternative cover systems, well designs and landfill gas collection management practices.

Numerical parametric study of an alternative three-layer capillary barrier cover system

A common practice for disposing of municipal waste is to dump it in landfills, where it is then protected by a cover system. The main purpose of this cover system is to minimise the amount of water percolating into the waste and, hence, to reduce the excessive formation of leachate. To achieve this objective, conventional landfill covers employ low permeability materials, such as composite liners, to meet the permeability design criterion. However, their long-term hydraulic behaviour, cost and shear resistance are not entirely satisfactory. Increasingly, covers with capillary barrier effects (CCBEs) have been considered as an alternative cover system in semi-arid and arid regions. It is questionable, however, whether CCBEs can be successfully applied in humid climate conditions where the annual rainfall often exceeds 2000 mm. In this paper, an alternative three-layer capillary barrier cover system for use in humid climates is proposed, and its feasibility is investigated by a numerical parametric study. This alternative system consists of a fine-grained soil layer overlaying a coarse-grained layer, which in turn overlies a fine-grained soil, such as clay, to minimise water percolation in humid climates. This bottom clay layer is protected by the upper two coarser soil layers. The factors considered in the numerical parametric finite element analyses include the thickness of this additional clay layer, rainfall conditions and degrees of saturation of the municipal waste. It was verified that the middle sand layer serves as a capillary break when the rainfall intensity is light or the duration is short. After the upper two layers are permeated, the bottom clay layer serves as an impeding layer, whereas the sand layer shifts to serve as a lateral drainage layer. It was also found that the amount of percolation increases with an increase in rainfall duration but decreases with saturation of the waste. Based on the six simulated durations of rainfall, the most severe rainfall duration is 1 day, irrespective of the return period.

Accelerated methane oxidation cover system to reduce greenhouse gas emissions from MSW landfills in cold, semi-arid regions

Many regional landfills for municipal solid waste (MSW) and industrial, commercial, institutional (ICI) wastes in cold, dry regions do not produce enough gas to support conventional gas extraction, treatment, and utilization or flaring. Yet, some solution is required to reduce emissions of methane and trace constituents to the atmosphere for the protection of the public and of the global climate. Methane oxidation, as a natural biochemical process, offers an opportunity to reduce methane emissions with a simple, passive alternative cover system. The goal of this article is to develop an effective design of Methane Oxidation Covers to achieve superior methane management performance while still producing equivalent closure conditions to conventional covers in semi-arid, cold climates. specifically, the goal is to reduce methane surface emissions by 50% to 80%, with no significant increase in leachate production compared with conventional covers of clay and topsoil.

Evaluation of alternative cover systems using GIS

Abstract Geographic Information Systems (GIS) can be used to determine and display the depths of native soil covers required for waste containment sites. Readily available data for the State of New Mexico are used in a GIS environment to determine the required depth of two separate cover systems. The depth of an evapotranspirative (ET) soil cover is determined using the available water capacity of the soil and the amount of dormant precipitation. The thickness of a capillary barrier cover system is determined using unsaturated hydraulic parameters, the amount of dormant precipitation, and an iterative numerical model. A difference map showing the savings using a capillary barrier instead of an ET cover is then created. The use of a capillary barrier in areas with fine-grained soils can decrease the necessary soil cover thickness by 90 cm of soil. The computed cover thickness compare well with those determined using Hydrologic Evaluation of Landfill Performance (HELP) (Schroeder et al., 1994), a model commonly used in New Mexico to determine if alternative cover systems can meet the required performance criteria.

On the performance of capillary barriers as landfill cover

Abstract. Landfills and waste heaps require an engineered surface cover upon closure. The capping system can vary from a simple soil cover to multiple layers of earth and geosynthetic materials. Conventional design features a compacted soil layer, which suffers from drying out and cracking, as well as root and animal intrusion. Capillary barriers consisting of inclined fine-over-coarse soil layers are investigated as an alternative cover system. Under unsaturated conditions, the textural contrast delays vertical drainage by capillary forces. The moisture that builds up above the contact will flow downdip along the interface of the layers. Theoretical studies of capillary barriers have identified the hydraulic properties of the layers, the inclination angle, the length of the field and the infiltration rate as the fundamental characteristics of the system. However, it is unclear how these findings can lead to design criteria for capillary barriers. To assess the uncertainty involved in such approaches, experiments have been carried out in a 8 m long flume and on large scale test sites (40 m x 15 m). In addition, the ability of a numerical model to represent the relevant flow processes in capillary barriers has been examined.