Water balance comparison between a dry and a wet landfill — a full-scale experiment

Abstract

This paper describes a water balance study conducted in a full-scale experimental municipal solid waste landfill in Melbourne, Australia. The investigation identified the significance of various hydrological components of a ‘dry’ landfill (represented by half of the experimental cell as a control section) and a ‘wet’ landfill (represented by other half of the cell allowing leachate recirculation and working as a bioreactor). The information obtained is important and useful in terms of leachate management for both dry and wet cell operations, especially for landfills located in a similar climate region. The study also determined the in situ field capacity of the waste and compared it to published data. The implication of using this field capacity value in water balance study is discussed.

Introduction

The current design of municipal solid waste (MSW) landfills is primarily based on a permanent storage and containment or ‘dry cell’ concept. The idea is to minimise the amount of water entering the waste to restrict formation of leachate and gas to reduce environmental impacts. However, experience reveals that as the containment system ages it may become ineffective and introduce a long-term risk of uncontrolled leachate and gas leaks. In recent years, with a better understanding of landfill decomposition processes and behaviour, there has been a strong trend to shift the philosophy of landfill design from the permanent storage concept towards a bioreactor or ‘wet cell’ approach (e.g. Maurer, 1994, Krol et al., 1994). This concept, in contrast to the permanent storage approach, focuses on enhancing biodegradation to stabilise waste and aims to bring forward the inert state of a landfill in a relatively short time. Moisture is commonly introduced to a wet cell through leachate recirculation (LR) (Yuen et al., 1995). A better understanding of the hydrological aspects of landfill design thus becomes more crucial as the decomposition of waste, in situ treatment of leachate, and production of gas are all closely related to moisture level. Water is not only essential for the first step of the biodegradation process, i.e. hydrolysis, a high moisture content is also important in terms of facilitating the redistribution of nutrients and microorganisms within a wet cell (Christensen and Kjeldsen, 1989). Even for the dry cell approach, a reasonable prediction of various water components is also necessary for efficient leachate management in terms of in situ storage, treatment and disposal. Recent hydrological research has thus expanded to focus on the presence and mobility of water within a landfill.

Several water balance models have been reported in the literature to quantify various landfill hydrological components. Some examples are the WBM (Fenn et al., 1975), HSSWDS (Perrier and Gibson, 1981), LSM (Meeks et al., 1989), and the popular HELP model (Schroeder et al.,1994, Peyton and Schroeder, 1988). They share a common approach in water balance estimation — regard a landfill as a spatially lumped system and calculate leachate volume using the continuity equation. However, there appears to be limited field data to calibrate and validate these models, and particularly, to test their suitability for a certain climate. Examples of such attempts include Blight et al., 1992, Bengtsson et al., 1994, Bendz et al., 1997. For wet cell water balance, a scheme has been proposed by Baetz and Onysko (1993) to specifically address the storage volume sizing for LR management.

In order to quantify the significance of various hydrological components that contribute to the water balance of a bioreactor landfill, an investigation was conducted in a full-scale experimental cell which forms part of a larger bioreactor study (Yuen, 1999). The study also aimed to quantify the waste stabilisation process and to evaluate the performance of the LR system and moisture flow mechanism. This paper presents a summary of the methodology and results of the water balance investigation. A complete report is available in Yuen (1999).

The water balance has been separated into two distinct phases: the pre-capping phase related to the open cell between December 1993 and December 1995, and the post-capping phase relevant to the closed cell, covering the period from January 1996 up to October 1997. It is assumed that water produced or consumed by chemical and biological activities and the moisture loss through vapour in gas are negligible as justified by Bengtsson et al. (1994).