Decommissioning of a landfill for industrial or municipal waste
Discontinuation of waste disposal
The decommissioning of a landfill begins once the surface area reserved for the landfill site and site capacity are reached. Environmental permits for landfills contain provisions on the surface area reserved for the landfill as well as its total capacity and final height. The permit also contains provisions on the construction of capping structures, including their materials and layer thicknesses, as laid down in the Government Decree on Landfills (Decree 331/2013, section 7 and Annex 1, paragraph 2). Separate requirements apply to the structures of landfills for conventional and hazardous waste.
Different legislation has been applied in different periods, which means that the decommissioning methods for current and old landfills are not the same. Consequently, the permit provisions and submitted plans may differ somewhat. In addition to containing provisions on final decommissioning, the permit usually also requires that the open surface area of the landfill should be minimised. This is why a landfill may contain sections in different operational stages. At least an intermediate cover is laid on areas which have reached the design height before the entire site is closed down. At the start of the decommissioning contract, it is often necessary to shape the disposed waste to obtain sufficient but safe slopes in the berm, and it may also be necessary to compact the waste. This may make it necessary to redo the intermediate cover. Con-sideration must also be given to overlapping any finished layers of the capping and joining them with new structures if the work is carried out in stages.
Approval for construction work
Before constructing the capping layers, the operator commissions plans for the decommissioning contract. The plans should include an exhaustive description of the materials to be used and their properties as well as quality control measures. The plans also include layout drawings and detailed drawings of the layers to be built, the installation of gas collection pipes, and possible penetrations and their sealing. The plans are usually first approved by an independent quality controller who issues a statement on the matter. The plans and the statement are then submitted to the authority for approval, and the authority issues its opinion on the plans. If the plans are first approved by the authority, an independent quality controller must be designated in connection with the approval. After the approval round, construction work can begin.
During the construction period, meetings are held regularly at the landfill site between the contractor, operator, independent quality controller and the authority. A test structure is constructed for each structural layer before it is built in full. The work stages are documented with photographs and as-built measurements. The independent quality controller inspects all construction stages, takes the necessary quality control samples and checks, among other things, the water permeability results of the materials used and the analysis results of wastes to be reused and their suitability for the structures. A final report is produced on the contract, which contains information on the construction work and any deviations. Attached to the final report is a statement issued by the independent controller verifying that the construction work was carried out in compliance with the plan and environmental permit. The supervising authority issues a statement on the final report. If significant deviations from the original plan turn out to be necessary during the construction period, the new plan should be approved by an independent quality controller and the authority.
Layers to be built and waste reuse
The following layers are built in the landfill on top of the compacted waste layer (from top to bottom):
- Top soil/growth layer ≥ 1 m
- Geotextile filter
- Drainage layer ≥ 0.5 m
- Impermeable mineral layer > = 0.5 m
- Geotextile liner if artificial sealing is used
- Artificial sealing, such as an HDPE membrane, is not mandatory in an ordinary landfill but it is re-quired in a hazardous waste landfill
- Geotextile filter
- Gas drainage layer; at least 0.3 m is recommended, in a hazardous waste landfill if necessary
- Geotextile filter
- Intermediate capping layer
Photo: Closed landfill in Elimäki, game field after use
The layers of the structures can also be thinner than this. In this case, a geosynthetic drainage medium is used as the drainage layer and a bentonite mat as the impermeable mineral layer. When using thinner layers, it must be ensured that their properties, including water permeability, correspond to those of thicker layers.
In addition to new materials, waste materials can be used in the surface layer structures. However, in this case it should be ensured that the waste is classified according to the category of the decommissioned landfill and that its properties, including water permeability and granularity, meet the requirements set for the layer. Environmental permits contain provisions on the reuse of waste in landfill structures. If the permit contains no provisions on these matters, approval of the independent quality controller and the relevant authority must be obtained.
Clean surface waters should be led to a drain system that is separate from landfill leachate. Clean surface waters can be discharged into the environment without treatment. Landfill leachate is often directed to the municipal sewer network through a detention basin. Some landfills have on-site pretreatment equipment for leachate, such as a biological treatment plant or a soil filtration system, from which the water is led to a sewer. Waters from industrial landfills are usually treated in the industrial plant’s sewage treatment system, to which they are pumped or transported by a tanker if necessary. In old decommissioned municipal waste landfills, not only clean surface waters but also leachate have been directed to a drain after the detention basin (at old landfills, this may also refer to as a sedimentation basin) and from there into the water system. The Guide to landfill decommissioning gives limit values for the need and methods of water treatment (Guidelines of the environmental administration 1/2008).
Photo: Leachate basin of Kääpäkä closed landfill
The methods of gas collection and treatment depend on the volume of gas the waste generates, or is expected to generate. As placing organic waste in landfills has been prohibited, the volume of gas-releasing
waste taken to landfills has been reduced, and there usually is no need for gas collection at landfills for hazardous industrial waste, for example. If necessary, the collection system can be equipped with a gas burner, which means that the gas can also be used to generate energy. If less gas is produced, biological gas purification is sufficient. At small sites, the gas has been released through permeable stone layers, or no collection pipeline has been built. Estimates and gas measurements are used to determine the volume of gas and the required treatment technique. See the Guide to landfill decommissioning for instructions regarding the required treatment, which depends on the gas volume and concentrations (Guidelines of the environmental administration 1/2008).
Restoration and post-closure use of the site
After decommissioning, the landfill site is restored. The restoration plan is usually drawn up as a separate project. Suitable vegetation on a landfill site consists of grasses or low shrubs. If trees are planted at the site or allowed to grow there naturally, there is a risk of the structural layers being damaged. Closed landfill sites are occasionally used for new activities. Examples of such activities in Southeast Finland include a golf centre and a mountain biking track. In post-closure use, particular attention should be paid to maintaining the integrity of the landfill structures and environmental safety for those using the area. The necessary agreements on using the site should be concluded between the municipality and the operator or association.
Aftercare and monitoring
The obligations associated with a closed landfill do not end with the completion of the decommissioning contract. The waters from landfills must be monitored for at least 30 years after the site’s decommissioning (section 21 of the Waste Act 646/2011). Water treatment should usually also be continued if this is re-quired under the environmental permit. The sampling frequency, substances to be tested for and sampling locations are specified in the environmental permit or a monitoring programme approved by the authority on a case-by-case basis, taking into account the waste materials treated at or tipped into the site in question. The sampling frequency usually is twice a year, at least initially. Sampling locations should be set up as follows:
- Gases (at all vents)
- Surface waters (upstream and downstream from the landfill)
- Water inside the landfill (at least one point)
- Leachate (each point where water is led outside the site)
- Groundwater (one upstream and two downstream)
Not only sample-taking but also regular monitoring of surface structures and vegetation are an important part of the aftercare. Heavy rain can cause erosion and degrade surface layers, increasing the potential environmental load. If trees or other vegetation that may damage the structures have started growing in the area, or if invasive alien species or litter are detected, they must be removed. Measuring settlement is also an essential part of monitoring both municipal and industrial waste landfills. The functioning of gas collection pipes, subsurface drains and any water treatment structures should be checked regularly. Drain systems carrying water away from the landfill should also be checked. It is also a good idea to take care of the condition of sampling points and access to the site, ensuring that representative samples can be collected safely.
Substances to be tested for
Below is a list of common parameters that should be tested for in landfills. Case-by-case discretion should be used regarding the analyses, taking into account the types of waste contained in the landfill. The list of samples is different for municipal and industrial waste landfills, as well as for non-hazardous and hazardous waste landfills. In addition to taking water samples, the water level inside the landfill site, the level and flow direction of groundwater, and the flow of surface waters should be monitored.
Parameters typically analysed in leachate and waters inside the landfill: pH, different forms of nitrogen, phosphorus, biological or chemical oxygen demand and chloride or electrical conductivity. Once, or every few years: heavy metals, petroleum hydrocarbons and volatile organic compounds.
Parameters typically analysed in surface water: pH, different forms of nitrogen, phosphorus, biological or chemical oxygen demand, chloride or electrical conductivity. Once, or every few years: heavy metals, petroleum hydrocarbons and volatile organic compounds.
Typical parameters analysed in groundwater: pH, electrical conductivity, ammoniac nitrogen and chloride. In addition, it is often necessary to monitor the leaching of arsenic, chromium, copper, lead, zinc or nickel.
The results should be gathered into an annual report describing the current and previous years’ results and averages. The reports should also contain a calculation of the surface water load originating from the land-fill by comparing an upstream and downstream point. Nitrogen and phosphorus loading has often been described as population equivalents (only municipal waste landfills), making it possible to assess any need for water treatment. When waters are led to a sewer, the local sewage treatment plant sets limit values for harmful substances in industrial sewage treatment contracts.
When to stop or reduce the frequency of sampling
The sampling frequency may be reduced when the values of the analysed parameters have decreased and remained unchanged for a long time and no impacts have been observed in natural waters. This is decided on a case-by-case basis. If the permit does not contain limit values for emissions into water, the results obtained are compared to monitoring programme reference points and, for example, to general results on natural waters and quality standards laid down in legislation. The limit values and sampling frequency of waters led to the sewer have generally been agreed with the local wastewater treatment plant. If deviations are observed in the results after reduced sampling frequency, the frequency must be increased. Any reductions in frequency and additional sample-taking are approved and ordered by the authority. If the waters from the landfill are treated on site, the possibility of discontinuing treatment can be examined as described above. The monitoring of waters led to the sewer should be discussed with the local wastewater treatment plant. A reasoned proposal should be made on discontinuing water treatment. Depending on the permit provisions, the proposal is approved by the supervising authority or permit authority. Before monitoring ends, it should be ensured that the landfill no longer poses a risk to health or the environment. Gas monitoring and treatment by burning can be discontinued when the gas volume decreases or its quality changes to the point that burning becomes unprofitable or impossible.
Indicative values for concentrations in discharge waters when making decisions on reducing the frequency of or discontinuing monitoring (Guidelines of the environmental administration 1/2008).
• BOD7(ATU) 30 mgO2 /l
• CODCr 125 gO2 /l
• solids 35 mg/l
• P-tot 2 mg/l
• N-tot 15 mg/l.
Oversight by authorities
Oversight by authorities takes place annually on the basis of sampling reports. In addition to sampling results, the reports should include descriptors of long-term results, making it easier to monitor the trend. The report should contain a comparison of results from downstream and upstream sampling points, description of the effectiveness of any water treatment equipment, and conclusions on the possible impacts of the landfill. On-site inspections are conducted as set out in the monitoring programme. Decommissioned land-fills are usually placed in control category 4, which means that inspections are carried out around every 10 years. If necessary, the authority may inspect the site as a result of deviating results or a report made by a member of the public. The inspection pays attention to the condition of surface structures and drain net-works, general tidiness, and the implementation of appropriate monitoring.