The role of Geonets in the management of landfill leachate

Every engineered landfill has to deal with leachate, and in a modern liner system the layer that carries it away is usually a geonet. Sustainable waste management still starts with Reduce, Reuse, and Recycle, but whatever is left has to go somewhere safe.

Government estimates from 2014 put India’s municipal solid waste (MSW) at about 62 million tonnes a year, projected to reach 165 million tonnes by 2030–31. More waste means more and larger landfills.

About half of India’s municipal waste is organic, so a large share of it rots and produces leachate and landfill gas. The World Bank’s What a Waste 2.0 report (2018) estimated that solid waste generated 1.6 billion tonnes of CO2-equivalent emissions worldwide in 2016.

What are landfills?

A landfill is a site engineered to isolate waste from the surrounding soil, groundwater and air. The waste may be municipal solid waste, industrial waste, hazardous waste, or organic/green waste.

A dump is an open pit or heap of trash. A sanitary landfill is a designed structure: a lined base, a leachate collection system, gas management and a final cover. India’s Solid Waste Management Rules, 2016 require sanitary landfills to have a liner and a leachate collection and treatment system.

What is landfill leachate?

Leachate is the liquid that percolates through landfill waste. It forms from the moisture in the waste as it decomposes, and from rainfall and runoff that enter the landfill.

Leachate carries dissolved organics, ammonia, salts and heavy metals. If it escapes, it moves into the subsoil and groundwater below the site. It has to be contained, collected, treated, and only then discharged, so the landfill design must collect and remove it efficiently from the base of every cell.

Geonets for leachate collection

Geosynthetic clay liners, geomembranes and geonets now make up most landfill liner systems. A geonet looks a little like a geogrid, but it does a different job: a geogrid reinforces soil, while a geonet carries liquid along its own plane. That makes it the usual drainage layer for leachate collection, placed directly on the HDPE geomembrane so leachate flows to the collection pipes and sump.

Geonets are often chosen over a traditional gravel drainage layer because they are a few millimetres thick instead of about 300 mm, so they save landfill airspace, are quicker to lay, and reduce the need for quarried aggregate.

A geonet is made of parallel sets of HDPE ribs extruded so they intersect and overlap a second set of ribs at an angle. On its own it is a bare net. When a geotextile is heat-bonded to one or both sides to stop soil or waste clogging the flow channels, the product is a drainage geocomposite. Geonets and geocomposites are used for leachate collection, leak detection between double liners, and gas venting and drainage in landfill caps.

Bi-Planar and Tri-Planar Geonets:

Geonets are either bi-planar or tri-planar. The choice depends on the flow rate and the load the drainage layer will carry.

Bi-planar geonets have two sets of intersecting HDPE strands forming diamond-shaped apertures. Liquids and gases pass through them in a fairly even pattern. They suit lower-load positions such as the drainage layer in a landfill cap.

Tri-planar geonets have a thick middle set of HDPE strands with diagonal strands above and below. The middle strands form wider, straighter flow channels that keep their transmissivity under high normal load, and the top strands limit geotextile intrusion into the channels. That makes them the better fit at the base of landfill cells, where the drainage layer sits under many metres of waste.

Choose your Geonets at Ocean Geosynthetics

Ocean Geosynthetics supplies geonets and drainage geocomposites for landfill leachate collection and capping. For an example, see the Ocean geonet geocomposite project in Vasco da Gama.