Size, Capacity of Landfill Sumps & Leachate Pumps

In a previous series of articles we covered various aspects of landfill design, including landfill layout, configuration and slope design; landfill liner systems; and leachate collection systems. In this article we are going to focus on the leachate drainage system and the landfill sumps and leachate pumps that are used within the collection system.

Leachate Drainage System

The primary role of the leachate drainage system is to collect and transport the leachate that accumulates within the liner. The type of pipes used, together with their dimensions, will be determined according to the weight of the waste, as well as the pressure exerted by the waste. As this network of pipes lies on the base of the cell, they have to endure a vast amount of pressure exerted by the weight of the waste above it. In order to support this weight and pressure, the pipes used in the drainage system can be either rigid or flexible; however it is recommended that the connecting joints are flexible. Alternatively, instead of laying the pipes under the waste at the bottom of the cell, conduits can be positioned in trenches. This network of collection pipes is responsible for collecting and transporting the leachate that filters through the drainage layer to a leachate well or collection sump, where it then pumped away for disposal or to be treated. In order to prevent leachate from accumulating in the drainage layer, the collection pipes can also act as drainage pipes within the drainage layer, where they are designed with perforations cut at an angle of 120 degrees to prevent solid materials and particles from entering the pipe.

Leachate Sumps

As liquid moves through a landfill cell, it passes through the filter and is collected in the leachate drainage network, which channels it to the sump. In order to maximize the efficiency of the landfill operation, careful planning in terms of the number, size and location of the sumps is essential. When designing leachate sumps, the volume of leachate and liquids anticipated needs to be taken into account. Landfills located in areas that receive higher than average rainfall tend to have larger leachate sumps installed to cater for the higher volumes of liquid.

Leachate Pumps

Another key factor to take into account when planning a leachate sump, is the capacity of the sump pump. If a pump with a low capacity is used then the volume of the leachate sump needs to be higher to compensate for this. It is vital that the sump is capable of storing the anticipated volume of leachate that is expected to accumulate in the sump between each pumping cycle. Sump pump cycles can be set automatically by presetting cycle times of leachate pumps. Alternatively, if flow rates are unpredictable, leachate pumps can be set to switch on automatically when the leachate reaches a predetermined level. Other factors to consider when planning a leachate drainage sump include sump maintenance and pump drawdown. Leachate is typically collected by the collection pipes, which drain into the sump/s by gravity. Once the leachate is collected in the sump, it is pumped out of the sump to a vehicle for removal offsite, or to an onsite storage or treatment facility.

Determining Leachate Sump Size

The size and dimensions of a sump are determined by the volume of leachate that needs to be stored, together with the capacity of the pump and the minimum pump drawdown. The sump needs to be of sufficient volume to store the maximum amount of leachate expected between each pump cycle, plus a further amount equivalent to the volume of the minimum pump drawdown. When determining sump size landfill operators should also take into account the space needed for conducting inspections and routine maintenance.

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Featured Image By SuSanA Secretariat [CC BY 2.0], via Wikimedia Commons