Project

Biogeochemical properties of organic matter in landfill waste

In the Netherlands current landfill management isolates the waste from the surrounding environment, resulting in waste and contaminant conservation and an indefinite aftercare period. Potentially more sustainable management techniques introduce air and/or water into landfills to increase decomposition of the waste and reduce contaminant emissions. We investigate the effect of these new techniques on waste properties, with specific focus on the properties and partitioning of organic matter and their influence on contaminant emissions.

Background

Landfills are the terminus of unrecyclable waste materials. Despite progressive developments in recycling, nearly 2.5 million tons of waste is landfilled annually in the Netherlands. Landfill waste poses a two-fold threat to human health and the environment (HHE):

  • Landfills are the second largest source of anthropogenic methane emissions in Europe;
  • - high concentrations of contaminants, most crucially ammonium, heavy metals and organic micropollutants, in the leachate are a potential source for soil and groundwater pollution.

Traditionally landfill waste is isolated from the environment by encapsulating it with engineered barriers. These barriers are essential for the protection of HHE as they limit rainwater infiltration and leachate percolation to soil and groundwater and thereby limit contaminant emissions. Yet, these barriers are unsustainable as they limit decomposition and stabilisation of the waste and thereby preserve its contaminant emission potential. Additionally, the lifespan of the barriers is estimated to be 50 to 75 years. This results in an indefinite after-care period, requiring replacement of these barriers and treatment of contaminant-rich leachate for the unforeseeable future.

Description

The main objective of our experimental research is to characterise the development of the amount and properties of organic matter (OM), the contaminant binding capacity, and the resulting contaminant (im)mobilisation in the solid landfill waste, as influenced by aeration and leachate recirculation. This objective is divided into three main aims:

  1. Describe the development of OM characteristics affected by aeration and leachate recirculation.
  2. Evaluate the effect of OM characteristics on contaminant (im)mobilization. Contaminants of primary concern are NH4+ and an array of heavy metals.
  3. Identify best-practice treatment conditions to steer waste decomposition and stabilization processes towards enhanced removal and/or immobilization of contaminants.

To do this landfill simulation reactors (LSRs) are designed with a volume of 60L. The LSRs enable us to study the fundamental processes in actively treated landfill waste under controlled laboratory conditions.

In view of the relevant organic matter properties, there is no all-purpose analytical technique that can comprehensively characterise OM. In our research we will therefore use a selection of complementary techniques to characterise the changes in organic matter properties during

In view of the relevant organic matter properties, there is no all-purpose analytical technique that can comprehensively characterise OM. In our research we will therefore use a selection of complementary techniques to characterise the changes in organic matter properties during its decomposition, particularly in relation to the partitioning of OM between the solid waste and leachate, and the resulting (im)mobilisation of contaminants. Examples of these complementary techniques are fractionation and characterisation of solid and dissolved humic substances, Rock-Eval analyses and physical fractionation in particulate and mineral-associated OM.

Results

The landfill simulation reactors are filled with approximately 30kg DM of landfill waste from three distinctly different landfills, the LSR design is shown in Figure 1. Six distinct operating conditions are defined to simulate and investigate the effect of the active treatments on the solid waste and leachate properties, 1) anaerobic, 2) anaerobic with leachate recirculation, 3) anaerobic with N-reduced leachate recirculation, 4) aerobic, 5) alternated aerobic and anaerobic conditions with leachate recirculation, and 6) treatment (5) with N-reduced leachate. In total 16 LSRs are running simultaneously.

Figure 1: Design of the landfill simulation reactor, with a redox lance with sensors at four different depths; Rhizons for pore water sampling; Sprinkler for leachate recirculation; A gas entry ring for introducing air or N2 into the system; A heating-mat to control temperature of the system.
Figure 1: Design of the landfill simulation reactor, with a redox lance with sensors at four different depths; Rhizons for pore water sampling; Sprinkler for leachate recirculation; A gas entry ring for introducing air or N2 into the system; A heating-mat to control temperature of the system.

Fractionation of dissolved organic matter (DOM) and quantification of the contribution of humic substances allows us to better understand the importance of DOM for contaminant leaching and transport. Intensive monitoring of humic substances in the pore water of a landfill subject to leachate recirculation as active treatment, showed little variation in time but high heterogeneity with depth (Figure 2). Fulvic acids are found to be the dominant fraction of in-situ DOM, characteristics and concentrations of this fraction are thus likely to control the concentration of contaminants with a high organic matter affinity. These findings will be used as essential input for subsequent geochemical modelling of contaminant leaching as a function of the different waste treatment conditions.

Figure 2: Humic substances concentration with depth in a landfill subject to leachate recirculation as active treatment.
Figure 2: Humic substances concentration with depth in a landfill subject to leachate recirculation as active treatment.