An efficient cover on a MSW landfill’s body and a good biogas collection system are not enough to guarantee the absence of uncontrolled gas flux from the surface, both during the operative and in the post-closure phases.
The monitoring of diffuse emissions from a landfill body is a crucial process that can provide information about:
- environmental impact: methane generates a greenhouse effect 21 times higher than CO2.Both VOC and H2S can cause odorproblems
- Landfill management: functioning of the biogas collection system, identification of areas with higher soil biogas emission, and presence of fractures on the cover and consequent leakage
The diffuse biogas emission from the landfill body is measured by means of the accumulation chamber technique, which allows a direct, timely and inexpensive quantification of the gas flux.
Such procedures involve geo-referenced flux measurements, on a regular grid, of the entire landfill body. A further processing of the acquired data by geo-statistical methods allows the creation of isoflux maps in relation to the various measured contaminants and the quantification of the total biogas emission from the landfill body. These maps, showing the spatial distribution of soil biogas emissions, play a fundamental role in the planning of every management intervention such as:
- Biogas collection
- coverage of the landfill body
- identification of areas with anomalous degassing activity
- efficiency of the anaerobic digester
- calibration of the production model.
The instrument used to perform the measurements on site is a flux-meter, designed and produced by West Systems, based on the static non-stationary technique. The flux-meter performs real-timemeasurements of the gas concentrations inside the accumulation chamber allowing an immediate assessment of growth rates. The obtained results are reliable regardless of the knowledge of soil composition and flux regime.
The reliability of global emission estimates is given by the comparison between these results and the measurements carried out by the landfill management.
Theoretical global biogas production = Captured quantity + diffused quantity
In almost all cases, the global production estimate is significantly higher than the amount of captured biogas, due to the atmospheric dispersion of uncontrolled emissions.
This methodology provides different results:
- Estimation of the global amount of each gas species in the investigated area
- Highlighting the spatial variations of methane and carbon dioxide emissions in terms of isoflux maps
Such instrumentation allows investigation on various gas emissions such as methane, carbon dioxide, hydrogen sulfide and VOC by means of the analytical techniques reported in the following table:
|CH4||IR Spectrometry based on TLD Tunable Laser Diode with multipass cell||0.001||750|
|VOC||PID - Photo Ionization detector||0.01||0.5|
- B.Raco,R.Cioni, M.Guidi et al., “Monitoraggio del flusso di biogas dal suolo da discariche RSU: il caso di Legoli, Peccioli (PI)”; RS Rifiuti solidi vol.XX n.2 marzo-aprile 2006
- R.Battaglini, M.Guidi, G.Virgili, J.Salazar, “Una nuova metodologia per la valutazione del flusso diffuso di biogas all'interfaccia aria-suolo emesso dalle discariche RSU”; Geologia Tecnica
- R.Cioni, M.Guidi, B.Raco et al., “CO2 Flux from soil: a methodology to estimate the diffuse biogas” Swemp