Does your landfill suffer from gas pains?

With landfill operators under pressure to increase revenue streams, increasing landfill gas production rates makes financial sense. Anne Udaloy explains how multiple-phase flow designs can help achieve this

When landfill gas extraction rates are low, operators often assume it's because of an intractable problem such as perched leachate blocking wells, low permeability or low methane generation rates. Other concerns may include high condensate production, leachate wells producing gas but little leachate, or leachate levels that fail to fall despite constant leachate extraction.

Multiple-phase extraction wells can address these issues and they are also highly productive - wells penetrating 30m of municipal solid waste commonly produce 200m3/hour. And there are additional benefits - multiple-phase designs demonstrably reduce leachate levels even at problematic sites and can also enhance the overall stability of the waste mass, an important issue if slope stability is of concern.

Multiple-phase flow designs are highly efficient because they rely upon energy already present in the waste to drive flow. These designs are site specific and are developed to allow the landfill operator to manipulate total head and phase production in order to maximise production of the desired phase - leachate, landfill gas, or both. Careful planning and construction management not only minimises construction time and costs but can also improve overall system reliability.

An attack of the vapours
Landfill gas extraction wells must manage water vapour both as vapour phase and as condensate. Unfortunately, typical landfill gas extraction system designs consider only the vapour phase and maximise condensate production at the worst possible location - within and near the well. This reduces the efficiency of the extraction system because landfill gas cannot pass through condensate-filled pores without first displacing the condensate.

System designs that drain condensate into gas extraction wells, or that force condensate formation near wells, are self-defeating. To the degree that the wells extract gas, they plug their screens with condensate and limit gas production. From a functional standpoint, these systems are better designed to manufacture condensate than to extract gas.

At many facilities, leachate contains significant quantities of dissolved gas, while waste that is not saturated with leachate is saturated with gas. Leachate extraction systems must consider how landfill gas will affect extraction system operation. Standard leachate extraction wells minimise leachate production and favour gas production. Leachate wells are pumped so that leachate levels in the wells are 2m above the base of waste or lower.

Bubble trouble
As leachate levels are lowered, leachate in the area degasses and bubbles or foam form. Pores that had previously been filled with leachate fill with gas and as leachate-filled pores are replaced by gas-filled pores, the continuous network of leachate-saturated pores is broken and replaced by a continuous network of gas-filled pores. This decreases the flow of leachate from the well and increases the flow of gas.

Leachate discharge to the well is also significantly reduced as gas bubbles in waste can prevent leachate flow to a well. The leachate must first either absorb or physically displace the gas before it can flow through the vapour-locked pore. Worse still, some pores that are newly filled with gas will collapse because landfill gas is more compressible than leachate. When leachate is extracted from gas-saturated landfills using a standard dewatering approach, the results are predictable - as the leachate level is lowered, the leachate extraction well becomes a gas production well.

Multiple-phase extraction systems can be integrated into new cell construction or retrofitted to existing cells. Multiple-phase extraction designs are applicable to blanket collectors (granular or geogrid-types), toe collectors/trenches/horizontal collectors, and vertical extraction wells. Specific well or collector designs consider natural boundaries, such as underlying clays, and existing and planned engineered systems, such as liners. For many sites, a prototype system is designed, installed and tested before a final design is developed for extension throughout the target area.

Client specific
Designs are developed per client specification to emphasise either short-term or long-term performance. Designs that emphasise short-term performance minimise initial capital costs under the assumption that the well or collector will be redundant within a few years. Designs that emphasise long-term performance minimise long-term operations and maintenance costs under the assumption that the wells or collectors will be required to function for more than a decade.

System construction is carefully supervised to ensure that the system is constructed to meet the intended design criteria, with trained and experienced professionals overseeing the operation. These field managers remain in constant contact with the design team so that changes can be integrated as needed to address actual field conditions.

In summary, multiple-phase extraction schemes can maximise landfill gas production, enhance leachate management, improve condensate management, reduce operations and maintenance costs and improve extraction system reliability and slope stability. These schemes are particularly applicable to sites where putrescible waste has been placed into cells with geomembrane or clay liner and cover systems. They can be implemented retrospectively and can significantly increase gas extraction rates - making any investment required more than self-financing.

Anne Udaloy is a principle at SLR Consulting

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