Landfilling of Waste

8. Leachate Collection

8.1 Definition and strategy
8.2 Components of a leachate collection system
8.3 General design criteria
8.4 Design intensity
8.5 Design of individual components
8.5.1 Drainage layer
8.5.2 Side drains
8.5.3 Main drains
8.5.4 Intake system for collection well
8.5.5 Collection wells
8.6 Control systems
8.7 Quality assurance

8.1 Definition and Strategy

A leachate collection system is a system designed to remove efficiently and quickly any leachate from the liner surface of a landfill, so that at no time there will be a hydraulic pressure over the liner surface exceeding 30cm. If this is fulfilled, no significant hydraulic pressure will occur on the liner surface. The risk of release of leachate through leaks, if any, in the liner is therefore substantially reduced.

Furthermore, the leachate collection system must guarantee that leachate is discharged for treatment before final discharge to the recipient.

The leachate collection system must be operational throughout the active phase of the landfill. For that reason the leachate collection system must be simple, safe and durable. The system should not include components in inaccessible places that may require repairs. Inaccessible components must be designed so that their useful life covers at least the active phase of the landfill.

Components of the leachate collection system that may require replacement or repair must be easily accessible.

8.2 Components of a Leachate Collection System

A leachate collection system of a landfill is a system of construction elements (components) that collect and remove leachate from the landfill. The main components of a leachate collection system are:

	Collection unit (drainage gravel layer, side drains and, to some extent, the main drain);
	transport unit (main drain and, to some extent, side drains);
	intake system for the collection well (load distribution plate, etc.);
	collection well, inspection wells, sampling well and, as the case may be, pumping well;
	discharge pipe (gravitation and pressure pipes).

8.3 General Design Criteria

The main concept behind the design of the leachate collection system is:

	a simple, safe and durable design; and
	to avoid any use of inaccessible components that may fail.

Generally speaking, the following rules should be observed:

	The design of the drainage system must be chosen on the assumption that the variations of the leachate in terms of quantity, chemistry and biology cannot be regulated.
	The flow rate in the selected components must be kept as low as possible and without sudden variations in places without realistic prospects of repair.
	Only justified flow-disturbing components such as drain pipes and wells should be used. Geotextiles should be avoided.
	Components that require maintenance or have a useful life which cannot meet the demands for active environmental protection systems should be used only as an exception and then only as strictly necessary.
	In connection with any choice of functions it should be considered if the functions could be carried out in a simpler manner and with more resistant materials. If so, it should be considered to change the design.

8.4 Design Intensity

The design basis for the hydraulic capacity of a drainage system must be determined on the basis of, among others:

	The precipitation pattern at the relevant site.
	Recirculation and/or watering.

Where no recirculation/watering is carried out at a landfill, a leachate collection system should be dimensioned to a capacity corresponding to the actual precipitation. As a starting point, the maximum monthly precipitation for an average year (data from the most recent 30 years) for the relevant site should be used. This capacity will be necessary until the liner surface of the landfill is covered by at least one layer of waste.

Where recirculation/watering is planned for the landfill, this must be included in the design intensity. Please refer to DS/R 466.

8.5 Design of Individual Components

All components incorporated into the leachate collection system should be dimensioned with a safety factor of at least 2. This implies that the leachate collection system must be capable of draining the design intensity if the capacity of the drainage system is reduced by half due to clogging.

8.5.1 Drainage and protective layer

The hydraulic pressure level of the leachate over the liner surface may nowhere exceed the thickness of the drainage and protective layers. In the design, the hydraulic pressure should be set as not more than 30cm.

The permeability and grain size of the drainage layer should be adapted to the nature of the leachate likely to be generated from the waste - so that efficient drainage is ensured and no clogging of the drainage layer occurs.

Otherwise, any design of the drainage and protective layer should adhere to the rules in DS/R 466.

8.5.2 Side drains

A network of side drains with drainage direction towards the main drain should be installed in the drainage gravel layer, cf. para. 8.5.3.

The design of side drains should be based on the instructions in DS/R 466.

Side drains should be built as stone fascines made of mutually stable filter materials (shingle, pea gravel, perlite, filter gravel, etc.). Geotextiles should not be used as a solution to meet the filter criteria. If, none the less, it is necessary to use geotextiles, the permeability and risk of clogging of the geotextiles should be carefully considered.

The distance between two side drains should not exceed 0.5 x the theoretical distance and be at a maximum of 20 metres.

The width of the side drain fascine should be determined as at least 2 x the theoretical width of the fascine based on the necessary capacity.

The length of the side drain must be adapted to the "catchment area" of the main drain, cf. para. 8.5.3.

8.5.3 Main drains

Main drains should be placed in the drainage gravel at the lower level of the liner surface with direct drainage towards the intake system and collection well.The dimension of the main drain is determined by the design intensity and on the basis of the catchment area of the main drain (the liner surface with run-off to the main drain). If a drainage pipe is installed in connection with the stone fascine, its capacity should not be taken into consideration.

The design of main drains should otherwise be based on the instructions in DS/R 466.

Main drains should be built as stone fascines made of mutually stable filter materials (shingle, pea gravel, perlite, filter gravel, or similar) with the addition, as the case may be, of drainage pipes of polyethylene, etc. Geotextiles must not be used as a solution to meet the filter criteria.

The distance between main drains should be adapted to the capacity of the main drains and their catchment area.

The width of the main drain fascine should be determined as at least 2 x the theoretical width of the fascine fixed on the basis of the necessary capacity.

The catchment area of a main drain should not exceed what corresponds to a damming up of 30cm in the main drain at the intake system to the collection well. The catchment area of the main drain can therefore be calculated as follows:

where:

O_H = Maximum catchment area (m²) of the main drain.
b_H = Width of the main drain (metres)
K_H = Capacity of the main drain (m/sec.)
I = Leachate design intensity (m/sec.)

In most cases the length of a main drain should not exceed 75 metres.

8.5.4 Intake system for collection well

An intake system for a collection well is intended to ensure that the leachate has direct access to the collection well. This system is of vital importance and must be fully operational throughout the active phase of the landfill.

An intake system should consist of a high-permeable layer of gravel in which drainage pipes have been laid out with intake to the collection well. The intake system should be protected by a load distribution plate installed on top of it.

The dimensions of the intake system must be based on the design intensity, cf. sect. 8.4, and the liner surface having run-off to the collection well.

As a minimum, the dimensioning basis for the intake system should be as follows:

	The intake capacity is determined as at least 2 x the theoretical capacity of the intake system, with not less than two intake strings to the collection well.
	The intake system should be able to resist a load corresponding to at least 2 x the theoretical pressure from the overlying waste and soil.
	The intake system should not receive direct infiltration of leachate generated from overlying waste (to avoid direct precipitation of particles in the intake system).
	As a minimum, the intake system should cover an area around the collection well which corresponds to a distance from the collection well of at least 5 metres.

The intake system should be built as stone fascines made corresponding to the most permeable material used in the main drain.

The intake strings to the collection well should be made of drainage pipes of materials resistant to leachate.

8.5.5 Collection wells

The collection of leachate from a landfill unit should occur via a collection well. Thereby it is possible to draw samples of leachate and to gain access as required to the drainage pipes of the intake system. Leachate is removed from the collection well by pumping or by a gravitation outlet under the liner to a common pumping station for the landfill.

The dimensions of the collection well are determined by the design intensity, cf. sect. 8.4, and the catchment area having run-off to the collection well. The design of the capacity of the collection well and the necessary pumping output is otherwise subject to the general practice for drainage systems.

In order to avoid excessive heights of the collection wells they should be placed where the filling height of waste is lowest. At the same time this provides improved access to the collection well.

A collection well should be able, as a minimum, to resist a load corresponding to at least 2 x the theoretical pressure from waste and soil.

The collection well should be designed in a manner which allows for the design criteria in sect. 8.3, and which makes it possible to draw samples of leachate for analysis from all intakes.

Furthermore, the collection well should be designed in such a manner that most of the activities that are to be carried out in the collection well can be completed without any persons being present in the collection well. If it is expected that there will be a need to send persons down into the collection well, it must be designed so that this can occur only subject to special safety precautions.

To the extent that discharge is made to sewer system and rainwater discharge systems, Statutory Order of the Directorate of National Labour Inspection No. 473 of October 7, 1983, on Sewage Work, etc., shall apply. Where this is not the case, the design requirements in the Statutory Order on Sewage Work etc. shall form the basis for a specific assessment.

If concrete is used for the construction of collection wells, the concrete must be treated - inside as well as outside - with leachate-resistant coating agents.

Suitable materials for collection wells are, in particular, glass-fibre reinforced polyester (GRP) and - where the height of the collection well is moderate - polyethylene high density (PEH). These materials, too, must be coated on the inside with leachate-resistant coating agents.

8.6 Control Systems

To be able to check the effectiveness of the leachate collection system, small vertical monitoring pipes should be installed with hydraulic contact to the drainage gravel layer. The control pipes are placed so that they adequately reflect the leachate level over the liner.

The monitoring pipes can be led through the waste gradually as it is filled. When the pipes are installed, measures should already be in place to prevent puncture of the underlying liner.

Another feasible method to check the effectiveness of the leachate collection system is to assess the collected quantity of leachate in relation to the quantities of precipitation and leakage control in connection with groundwater monitoring.

8.7 Quality Assurance

When designing all the components of a leachate collection system, documentation should be prepared, including the choice and durability of the materials. The design basis should be enclosed with the application for environmental permit.