Steady-state design of vertical wells for liquids addition at bioreactor landfills

Abstract

The rate at which liquids can be added to a vertical well, the lateral zone of impact of the well, and the liquids volume needed to wet the waste within the zone of impact of the well are the key inputs needed to design a vertical well system. This paper presents design charts that can be used to estimate these inputs as a function of municipal solid waste properties (porosity, hydraulic conductivity, and anisotropy ratio), well dimensions (radius and screen length), and injection pressure. SEEP/W modeling was conducted to estimate the key design inputs for a range of conditions practically encountered for a vertical well installed in landfilled waste. The flow rate, lateral zone of impact of a well, liquids volume added, and injection pressure were normalized with the waste properties and well dimensions to formulate dimensionless variables. A series of design charts were created to present dimensionless steady-state flow rate, lateral zone of impact, and the dimensionless liquid volume needed to reach a steady-state condition, as a function of dimensionless input variables. By using dimensionless variables formulated for this work, these charts permit the user to estimate the steady-state design variables described above for a wide range of configurations and conditions beyond those simulated without the need for further modeling. The results of the study suggest that the lateral extent of the well can be estimated using Darcy’s equation and assuming saturated unit-gradient vertical flow regime below the well bottom. An example problem is presented to illustrate the use of the design charts. The scenario described in the example problem was also modeled with SEEP/W, and the results were compared with those obtained from the design charts to demonstrate the validity of design charts for scenarios other than those used for the development of the design charts. The methodology presented in this paper should be thought of as a means to provide a set of bounds that an engineer can use along with their judgment in the design of a system for a specific site.

Introduction

Bioreactor landfills are designed and operated to optimize the waste stabilization process rather than to simply contain the wastes, as is prescribed by most regulations (Reinhart and Townsend, 1997, Reinhart et al., 2002). Bioreactor landfill operation most commonly involves adding liquids (leachate and/or other liquids) to the waste to increase its moisture content. An effective liquids-addition system, (also known as leachate recirculation system or moisture addition system) therefore, is a prerequisite to the bioreactor operation of a municipal solid waste (MSW) landfill. Vertical wells and horizontal trenches are the most commonly used systems for adding liquids to bioreactor landfills (Reinhart and Townsend, 1997). Some of the important inputs required for the design of liquids addition systems include the range of flow rates achievable through a device (vertical well, horizontal trench) at a given liquid pressure, the lateral extent of the zone of impact of a device, and the liquids volume that should be added to a device to wet the waste within its zone of impact. As of yet, however, adequate field experience and data regarding the operation and design of these liquids addition systems at landfills have not been reported. Although operational experience is paramount, mathematical modeling of liquids flow through these devices in porous media can provide insight into the impact of waste properties (e.g., media permeability, anisotropy) and operating conditions (e.g., injection pressure) on the design inputs and, thus, be of benefit to designers of liquids addition systems at landfills.

Fluid flow through landfilled waste has been previously simulated using principles governing fluid flow in unsaturated soil (Straub and Lynch, 1982, Korfiatis et al., 1984, Schroeder et al., 1984, Khire and Mukherjee, 2007, McDougall, 2007). Most of these investigations focused on estimating the leachate generation rate from the landfill as a function of climatic conditions, landfill geometry, and waste properties.

Flow through liquids-addition devices such as wells, trenches, and galleries has been modeled previously. McCreanor and Reinhart (1996) used the saturated–unsaturated flow and transport model (SUTRA) to simulate the saturation profiles that would occur around a vertical well in a homogenous and isotropic waste in response to 44 days of leachate recirculation at several constant flow rates (0.2, 0.4, and 0.8 m³ day⁻¹). The pressures required to achieve the flow rates used for modeling were not reported in the study. Khire and Mukherjee (2007) simulated the impact of the leachate injection rate on the steady-state injection pressure, lateral extent of liquids movement, and head on the bottom liner for isotropic waste. The impact of well radius, well depth, and screen length were also investigated. The researchers assumed that the waste was isotropic. Modern MSW landfills, however, because of the manner in which waste is deposited, compacted, and covered, are typically considered anisotropic with respect to permeability (Townsend, 1995, McCreanor, 1998, Landva et al., 1998, Hudson et al., 1999).

When designing a vertical well liquid-addition system for a landfill, the design engineer can utilize numerical fluid flow models such as those used in the studies presented above, but this process requires specialized expertise and software that may not always be available. In the present study, previous contributions in modeling liquids addition at landfills are built upon by incorporating anisotropy to develop a series of design charts for estimating key variables needed to design and operate vertical well systems for liquids addition at bioreactor landfills.