Effects on greenhouse gases of the application of MBT output, bio-solids and compost on land

By Anna-Maria Skartsila: September 2010

Supervisors: Dr Nick Voulvoulis and Dr Martin Head

Introduction

This project aimed to investigate the effect on greenhouse gases (GHG) emitted by soil, following the application of bio-solid wastes to the soil’s surface. The four main project objectives were:

Testing soil pH

  1. Gain primary data of the GHG concentrations emitted after the application of MBT (Mechanical Biological Treatment) output, quality compost and bio-solids on land
  2. Study the effects on greenhouse gas concentrations, of the application of compost, bio-solids and MBT output to soil
  3. Research the way that different soil pH may affect these concentrations
  4. Research the way that different soil moisture contents may affect these concentrations

Atmospheric concentrations of the three main greenhouse gases: carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) have increased since the industrial revolution due to anthropogenic activities. Population growth and an observed change in lifestyle during the last decades have led to an increasing rate of waste generation. 70% of the generated Municipal Solid Waste goes to landfill, the most dominant method of waste disposal, which contributes 25% of total CH4 emissions.

MBT is a method that processes a wide variety of waste and can leave a large proportion of residue with uncertain biodegradability potential, which may be applied to land as a soil conditioner. It is preferred by many local authorities as it holds a good public perception and has potential energy and materials recovery. While the use of MBT plants may have beneficial effects on meeting the Landfill Directive targets, soil sequestration of atmospheric CO2 through land application of organic residues may also have beneficial effects as a strategy to offset the increase in the concentration of GHGs in the atmosphere.

Method

Gas capture cell

A literature review was carried out to illustrate the drivers for this thesis and to show the necessity of the specific study, while reviewing what is already known about applications of organic treated waste. The experimental part of this study, took place at Imperial College’s Silwood Park campus, and research was made prior to the experiment in order to define the three most suitable sites that would be used for the experiment, in terms of differences in soil pH.

Compost, bio-solids and MBT output samples were obtained from different facilities of organic waste treatment, and were described in terms of pH, moisture content, treatment methods and feedstock material used for their generation. These samples were then applied to soil; in dedicated closed chambers that were placed in the three sites with the different soil pHs that were selected in the campus. Flux measurements by gas sample collection and analysis for CO2, CH4, O2, N, H2, Ethylene, Ethane and Acetylene through gas chromatography took place.

After the concentrations of these gases were known, an effort to interpret the factors that were responsible for different concentrations was made. Soil pH and moisture, as well as the treatment method used for the production of each of the treated organic samples were the parameters that were studied during the analysis.

Results and discussion

After analysing the results, the following trends were observed:

  • CO2 was the only gas emitted through soil after the application of MBT output, bio-solids and compost
  • Outputs from different facilities have a different behaviour in terms of gas emissions after application on soil
  • Soil with pH 5.67 was observed to be the soil that enhances CO2 emissions through soil respiration
  • Soil with pH 6.7 seemed to be the soil with less emissions after the application of MBT output, bio-solids and compost on soil
  • Bio-solids was the sample that when applied on soil had the most significant emissions compared with the other two samples
  • MBT output was the sample that when applied on soil had the most optimum behaviour in terms of gas emissions through soil.


However, the data obtained should be used keeping in mind the implications related to this study. Firstly, there are numerous parameters that may interact during the application of such samples onto soil. A major problem of researching the application of treated organic material on soil is that the experimental conditions and the tested organic samples are so different that it is difficult to concentrate results into general conclusions.

Chemical and physical properties of the applied samples include several factors that may affect the emitted gases after their application on soil. Chemical and physical properties of the soil, as well as the fact that micro-populations of the soil may be disturbed by this application, made the analysis of the results extremely challenging. External factors such as seasonal effect and atmospheric pressure, along with soil management techniques, also have an effect on gas production and emission. Other factors affecting gas generation and emission from soil are the available C and N content.

Another point to consider in terms of implications related to this study is the limited scientific data available on this subject for comparison to the data obtained. Indeed, there is a lack of scientific knowledge in terms of effects on the environment of the application of CLOs (Compost-like Outputs) on land and this is mainly due to the fact that long-term field experiments would be required for this study that should involve a major number of variables.

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