Biogas (LFG) collection efficiency of traditional landfill systems

The objective of the entire landfill biogas collection system must be to prevent the dispersion of biogas into the atmosphere by keeping the pressure inside the waste storage within values just below atmospheric ones and conveying the biogas from the landfill to the treatment and energy recovery equipment.

Since the topic lends itself to considerable and extensive technical discussions, in order to be both simple and concise, as well as to give only an idea of the plant engineering situation, we will limit ourselves to setting out some indications regarding the main traditional collection systems used so far for the extraction of biogas from landfills.

It goes without saying how important each element of the entire extraction system is in ensuring its efficiency. Many times, the problems encountered in a landfill (e.g. overpressures and migration through preferential biogas routes, atmospheric emissions, ‘bad smells’, fire or explosion phenomena, poor productivity of energy recovery plants etc.) refer to the efficiency of its collection system.

Collection systems, therefore, take on fundamental importance from both an environmental and economic point of view and, in a landfill, they are mainly composed of the following elements:

1. the INTAKE UNIT which provides a pressure gradient for biogas extraction;
2. the MAIN LINES for transporting the biogas, connecting the adjustment stations to the intake unit;
3. the ADJUSTMENT STATIONS, collector plants for the biogas coming from the secondary transport lines that make it possible to adjust biogas collection manually;
4. the SECONDARY BIOGAS TRANSPORT LINES connecting the collection well to the adjustment station;
5. the collection systems present in the landfill body are made up for the most part of STATIC ELEMENTS POSITIONED INSIDE THE LANDFILL BODY, including:

• collection wells in progress,
• drilled vertical collection wells,
• drainage in progress,
• mechanical bank protection drains,
• under-sheet drains,
• horizontal drainage inside the waste storage,
• perimeter trenches for biogas collection,
• cortical drains during the closure of the landfill

which, on the whole, are made up of ‘passive’ elements with the exception of the central intake unit, the only ‘active’ collection element.

Considering this context, we cannot forget that landfill biogas can have a dual environmental effect: if collected and exploited, it can be a potential alternative energy source thanks to its high methane content; on the other hand, it can have a significant environmental impact when released into the atmosphere, since methane is the second most important greenhouse gas after carbon dioxide, with a GWP 28 times higher than that of CO₂ (https://www.ghgprotocol.org/sites/default/files/ghgp/Global-Warming-Potential-Values%20%28Feb%2016%202016%29_1.pdf).

This is why it is so important to predict what the total production of biogas (LFG) will be in a landfill, and to prepare an adequate and efficient collection system for it right from the design stage.

From a 2002 study by the University of Strathclyde conducted on 2364 landfills in the United States and 55 landfills in Europe, the average state of the art in terms of biogas collection efficiency was obtained. Standard calculation models (CAA, AP-42, EPA) were applied in order to assess production and the results were compared with collection performance levels to check efficiency. Some of the assessments are shown below (https://www.epc.it/contenuti/magnano_biogas_sito.pdf):

Ultimately, the international state of the art identifies an average efficiency of about 50%, with maximum efficiency performance levels of around 70%.

There are currently very few regulatory methodological references to assess the extractive efficiency of landfill biogas, at national, EU or international level.

To the best of our knowledge, the only and most recent technical legislation – which in any case does not directly deal with assessing the collection efficiency of the intake systems present in the landfill, but deals with the assessment of fugitive emissions from the landfill biogas – is that issued by the UK ENVIRONMENT AGENCY (EA) called “Guidance for Monitoring Landfill Gas Surface Emissions”, which could find support and feedback regarding the values returned in a contextual thermographic survey of site emissions.