Landfilling of Waste

Annex C: "Bioreactor landfill"

C.1 Strategy

In regions of Denmark where, due to special circumstances, it is inappropriate or impossible to treat waste suitable for incineration in an incineration plant, the waste may be landfilled in a "bioreactor landfill".

The aim is that a "bioreactor landfill" will optimise the decomposition of landfilled organic waste in a manner that will reduce the organic impact of the leachate and promote the generation of gas as quickly as possible.

Generally, a "bioreactor landfill" must be designed as any other landfill unit with active environmental protection systems. In addition, the "bioreactor landfill" must be designed so that it is possible to optimise the extraction of gas from the landfill waste. The extracted gas will be recovered for energy generation.

Admittedly, the overall landfill strategy described in Chapter 2 of this Guideline cannot be fulfilled for a "bioreactor landfill".

C.2 Acceptance Criteria for Waste

Waste for landfilling in a "bioreactor landfill" may be accepted on the basis of the following, restrictive positive list:

	Domestic waste.
	Bulky waste.
	Sewage sludge which has been stabilised in accordance with Statutory Order of the Ministry of the Environment No. 736 of October 26, 1989, and dewatered down to ³ 30% dry matter.
	Waste from the positive list for Category III - defined in sect. 4.3 of this Guideline.

In addition, the acceptance criteria for landfill units for mixed wastes, Category III, must be observed.

Liner and leachate collection

A "bioreactor landfill" must be designed with a composite liner system of the type described in Chapter 7 and with a leachate collection system as described in Chapter 8. Furthermore, a "bioreactor landfill" must be designed so that recirculation of leachate is possible at all times.

If possible, there must be access to recover the gas from a "bioreactor landfill", and the design must make allowance for that.

To determine how the gas is to be managed, a number of estimates should be made of the quantity of gas generated.

Such estimates must be based on experience from other sites where waste of a similar nature is landfilled. The most important elements of such an estimate are the content of organic substances in the waste and the half-life of these substances, as well as the conditions under which it was landfilled in the actual "bioreactor landfill", i.e., when anaerobic and methanogene conditions are likely to have been achieved in the landfill unit.

Extraction of gas

Gas may be extracted from the "bioreactor landfill" either by drilling gas wells in the completed landfill unit or by installing gas drains continuously during filling. From these the gas may be drawn with a view to being burnt in a combustion chamber.

When the gas contains 5-15% methane and is in contact with atmospheric air, there is a risk of explosion. The management of gas in a "bioreactor landfill" must therefore be arranged in a manner so as to prevent gas explosions within the landfill itself - but also to prevent gas from escaping into the surroundings where an explosion risk may arise.

In connection with the design of leachate collection wells it is necessary to take special precautions to prevent gas explosions.

C.4 Operation

Compactors should be used for the filling of domestic waste and other mixed wastes, which may be highly inhomogeneous. The waste must be broken open (plastic bags, etc., must be opened) and homogenised (in practice: mixed) with a compactor before being filled. Furthermore, there may be certain types of waste that require shredding prior to filling.

The waste should be filled in thin layers (not exceeding a thickness of 30cm) and compacted by being run over repeatedly by a compactor.

The first layer should consist entirely of domestic waste. The domestic waste should have been broken open with a compactor away from the tip face and laid out in layers of about 1 metre each.

The purpose of daily covering is to prevent escape of waste and to reduce the risk of odour and vermins.

If the waste is thoroughly compacted in accordance with the above, daily covering can normally be dispensed with or be reduced to a minimum. The first layer, however, should be covered with permeable soil.

Where daily covering becomes necessary, it should be laid out in layers that are no thicker than necessary to obtain the desired effect.

Material used for daily covering must be permeable so as to ensure that precipitation is evenly distributed in the waste and, thereby, ensure that the leaching behaviour for the waste is comparatively even.

The waste should be unloaded at the top of the tip face. This enables the driver to assess the composition of the waste upon unloading. During filling it is also possible for the driver to observe any unwanted fractions in the unloaded waste. Unwanted fractions should be removed for other treatment.

Because of its high content of water, sludge may present operating problems. The quantity of sludge landfilled should therefore be confined to a minimum. Sludge should be spread in thin layers and immediately covered with other waste. Where the quantity of sludge becomes excessive, special landfill techniques should be applied.

During wet periods, and when the content of leaves is high, road sweepings can present problems for driving on the waste. It is therefore recommended that road sweepings should be placed in thin layers and mixed with other waste at the tip face.

C.5 Control and Inspection

The procedures for control and inspection will be as described in Chapter 11 of this Guideline. In connection with the analysis parameters for leachate control, the analysis programme for mixed wastes should be used.

C.6 Restoration

When restoring a "bioreactor landfill" it must be ensured that infiltration to the waste is adequate to ensure optimum decomposition of the landfilled waste. Also, measures should be taken to prevent damage from gas to the vegetation in the top layer.