Use of organic waste materials in topcover
Henna Punkkinen, Markku Juvankoski, Tommi Kaartinen, Jutta Laine-Ylijoki, Elina Merta, Ulla-Maija Mroueh, Jarno Mäkinen, Emma Niemeläinen & Margareta Wahlström, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland.
Different types of organic materials can be used to cover sulphide containing wastes. The use of organic covers can offer affordable and effective solutions for the management of acid producing mine wastes (Peppas et al. 2000). When deposited, certain organic materials are capable to form an oxygen consuming cover in which a decomposition of organic material generates a significant biological oxygen demand (INAP 2009); thus reducing the ambient oxygen concentration at the waste/cover interphase (MEND 1994) and diminishing oxygen transport to the underlying waste (European Commission 2009). In addition, the use of organic material also promotes higher infiltration rates and maintains a high water table (INAP 2009), thus the saturated material on top of the waste physically inhibits the oxygen diffusion into the waste (European Commission 2009, Steffen Robertson and Kirsten Inc. 2001).
Various different organic waste materials, such as wood chips, forest industry wastes (e.g. bark, sawdust), sewage sludge, hay, straw, silage, pulp and paper mill residues, sanding dust, municipal solid waste compost, food industry wastes, carbonaceous matter, crumb rubber, and waste rock rich in organic matter are suggested as suitable materials for oxygen consuming covers (MEND 1994, Steffen Robertson and Kirsten Inc. 2001, INAP 2009, Lottermoser 2010); however, studies have shown some of these materials to be more suitable than others. In addition to these, the use of peat is discussed in this context due to its similar characteristics, although it is not a waste material. Also a wetland establisment where the decomposition of plants consumes oxygen can be considered as an oxygen consuming layer (European Commission 2009). The principles of wetlands are described in Passive treatment technologies section and only dry cover structures are discussed in this chapter.
The use of inorganic waste materials such as slags from the steel and metal industries, ashes from the energy production, and lime containing wastes, has also been studied in order to find out their suitability to cover mine wastes. Although inorganic materials do not have oxygen consuming properties, they can be used to prevent the infiltration of oxygen into the underlying waste. According to studies, many inorganic waste types such as cement kiln dusts and blast furnace slags have inhibiting properties and they are basically applicable to be used as impermeable layer material due to their small grain size, however, the lack of knowledge concerning their chemical and physical stabilities, as well as their availability and transportation costs, hinders their use. (Alakangas et al. 2014) These wastes are not further discussed here, however, more information on their properties and suitability can be found e.g. in the report by Alakangas et al. (2014).
Description of the methodology
The materials potentially suitable to act as an oxygen consuming layer need to be able to hold water in their pore spaces, and restrain the movement of oxygen into the waste when saturated. The ability of the material to retain pore water depends on the material type and its stage of compaction. Naturally, also precipitation and evaporation affect to these characteristics. The oxygen consumption takes place as a result of bacterial oxidation of organic carbon. (Steffen Robertson and Kirsten Inc. 2001) Both aerobic and anaerobic degradation processes take place in the biological decomposition of organic material. Decomposition of the organic material in aerobic conditions takes place in the upper part of the layer, while anaerobic conditions are prevailing in the lower parts of the layer. The different conditions generate an “oxygen trap” that minimizes the effects of oxidation of the underlying sulphidic waste layers. (Peppas et al. 2000)
The content of organic material in the cover structure needs to be high so that the efficient oxygen consumption and transportation can be achieved. (European Commission 2009) However, as the quick development of reducing conditions may accelerate the mobilization of iron and related trace elements, the amendments that consist of substantial amount of labile organic carbon should be soundly used (Lindsay et al. 2011). The organic layer is often a part of a dry cover structure, but organic material can also be used as an amendment for the creation of reactive, low permeability surface (Lottermoser 2010). Case studies demonstrate that different types of organic materials can be mixed together, compacted, and placed on the top of the waste material into one or more layers (European Commission 2009).
The MEND report 2.20 presents many material types that may be suitable to act as an organic oxygen consuming layer (MEND 1994). The key information presented in the report is summarized in Table 1. On the basis of the evaluation, wood wastes and paper mill sludges were chosen as the most potential candidates. These materials are widely distributed, cost effective and have potential biophysical characteristics to serve as effective oxygen consumers. The substantial availability and renewable nature of wood wastes are the most important benefits associated for their use. However, their physical and chemical variability, long term stability issues, difficulties related to cover design criteria, and the formation of undesirable leachates may hinder the feasibility of wood wastes. Although peat is a poor oxygen consumer when compared to wood waste, it is more uniform and may thus have potential as a combined oxygen consuming/moisture retention barrier. Also sewage sludge, compost, N-Viro soil, manure, waste paper, municipal refuse, and hay/straw/silage showed some potential characteristics and could have limited application for use in dry covers. (MEND 1994)
Table 1. Potentiality and characteristics of different organic materials to act as oxygen consuming covers (MEND1994, modified).
|Material Type||Form||Source and Quantity||Benefits / Concerns||Material Stability||Estimated Costs of Cover (in 1994), ($/m2)||Effective Cover Life, Guestimate||Care & Maintenance Needs, Guestimate||Potentiality|
|Wood Waste||Sawdust, bark, logs and other wood scraps generated by the lumber, pulp, and paper industries||Wastes from the forest products industry. Generally good availability in many mining districts.||+ Oxygen consumption can reduce oxygen flux significantly
– Material heterogeneity, high porosity, potential for combustion, poor quality control
|Unstable, material decomposes||<30||Approx. 10 years for 1 m cover – 50 years for 2-3 m cover.||Periodic reapplication may be needed||Potential: Although wood waste is not an ideal cover material, its usage offers potential cost savings if forest products industry operations are located near to the mine site|
|Crumb Rubber||Grinded used rubber tires||Produced in relatively small quantities||+ Good moisture retention capacity due to large particle size
– Biotoxicity, autocombustion
|Unstable||>30||Unknown||Unknown||Not potential: Material is costly, available in small quantities and can produce toxic leachates|
|Peat||May be found near mine sites in sufficient quantities||Primarily peat bog||+ Excellent water retention capacity, low permeability when compressed
– Long term performance, high costs, maintenance of perched water table within the peat
– Not very good oxygen consumer
|Unstable, material decomposes||<30||Not established||Not established||Low priority material: May have potential as a combined oxygen consuming/moisture retention barrier|
|Sewage Sludge||Dewatered sewage sludge||Applicable in large quantities, not usually located near to mines||+ Good oxygen consumer
– Limited quantities, transportation costs, quality of cover leachate, long term performance
|Unstable, material decomposes||<30||Not established||Frequent reapplication needed||Low priority material: Not a primary candidate, small scale applications may develop|
|N-Viro Soil||Processed dewatered sewage slugde and alkaline materials, such as cement kiln dust, fly ash and wood ash||Cities are the main source, small quantities may be crated at mine site||+ Provides a source of alkalinity and high organic oxygen demand
– Transport costs, limited quantities available
|Unstable, partly decomposes||<30||Not established||Not established||Low priority material: Available only in small quantities|
|Other Industrial Sludges||Sludges with a high biological oxygen demand. Residues of industrial processes/clean-up operations||Produced by various industries||+ High oxygen demand, available at little or no costs
– Limited quantities, diverse number of sources, varying characteristics
|Unstable, material decomposes||<30||Not established||Frequent reapplication may be needed||Not potential: Small quantities|
|Shredder Fluff||Mixture of non-ferrous materials, mostly polymers, produced as a by-product of the automobile shredding||Relatively small volumes available||+ Low costs
– Characteristics may change over time, material too porous, has insufficient organic content to be able to act effectively. Potential for low level PCB and metal leachate contamination
|Stability unknown||<30||Not applicable||Not applicable||Not potential: Contamination and technical concerns|
|Compost||Immature/mature compost, food and yard waste||Quantities within economic transport distance probably insufficient||+ High biological oxygen demand and moisture retention expected.
– Contaminant, pathogens, high costs, long term performance
|Unstable, material decomposes||<30||Not established||Frequent reapplication may be needed||Low priority material: Availability issues|
|Municipal Refuse||Municipal refuse excluding recycled material||Produced in abundant quantities nationally but not necessarily regionally||+ May offer significant cost savings, if disposal costs of refuse were applied to cover construction
– Leachate management, complexity of permitting requirements
|Unstable, material decomposes||<30||Indefinite if landfill constructed at site||Landfill monitoring||Low priority material: Concerns with leachate management and controlling|
|Manure||Animal and plant manure||Usually available only in small quantities near to mine sites, depending on case||+ Use as a soil conditioner, high oxygen demand, moisture retaining capacity
– Long term effectiveness
|Unstable, material decomposes||<30||Not determined (possibly <5 years)||Frequent reapplication may be needed||Low priority material: High collection and transportation costs|
|Hay/Straw/Silage||Farm products||Available in large quantities, often in near distance||+ Much better uniformity if compared to wood wastes, use as soil conditioner, high oxygen demand and moisture retention capacities
– Relatively high costs, porosity, poor compressibility
|Unstable, material decomposes||<30||Not determined (possibly <5 years)||Frequent reapplication may be needed||Low priority material: Widespread use is unlikely|
|Paper Mill Sludge||Sludges from primary and secondary wastewater treatment||Available in moderate quantities in places where paper mills exist||+ High biological oxygen demand and moisture retention is expected, low costs
– Long term effectiveness
|Unstable, material decomposes||<30||Not determined||Not determined||Potential: May act as diffusion barrier and oxygen consuming barrier. The massive use is not likely|
East Sullivan mine site in north-western Québec, Canada represents a well-known example of a case where an oxygen consuming cover has been constructed. The mine was closed in 1966, and tailings impoundment was covered with organic cover consisting of various forestry wastes in 1984. Link 1, Link 2 (INAP 2009, Steffen Robertson and Kirsten Inc. 2001)
Oxygen consuming covers are also constructed for example in Galgberget (Link 1, Link 2) and Garbenberg mine sites located in Central Sweden. In Galgberget, the deposition of tailings was ceased in 1981. The construction of the cover started in 1989 and was completed in 1997. The cover consists of mixed paper mill sludge and fly ash (35% sludge and 65% fly ash), and is placed under a protective layer containing wood waste or till (Hallberg et al. 2005).
The oxygen consuming cover structure is applicable for acid-forming tailings, but the suitability on covering waste rocks is uncertain and more testing work would be justified. However, it is assessed that some difficulties met when covering tailings, such as keeping the organic layer moist and stable in right place, could be even more troubled if waste rocks were covered with organic layer. In addition, the coarse grain size and the steep side slopes of the waste rock piles would allow oxygen flow into the waste, and hinder the oxygen consumption in a fairly thin cover structure. (Steffen Robertson and Kirsten Inc. 2001) However, it is estimated that an organic layer could be incorporated to an multi-layer cover system when covering waste rocks; for example, a placement of protective overburden layer on top of the organic layer would protect the layer from erosion, decrease water loss via evaporation, and compact the layer. It may be also possible to construct a coarse capillary break layer between the organic and waste layers to prevent water drainage from the organic structure. (Pierce 1992, cited by Steffen Robertson and Kirsten Inc. 2001)
Table 2 presents the most important advantages and disadvantages linked to the use of oxygen consuming covers in general, whereas some material specific benefits and concerns are presented in Table 1.
Table 2. Advantages and disadvantages related to the use of oxygen consuming covers (MEND 1994, Peppas et al. 2000, Steffen Robertson and Kirsten Inc. 2001, INAP 2009, Lottermoser 2010).
|Cover is able to minimize the amount of oxygen penetration into the tailings due to low hydraulic permeability, high cation exchange capacity, and high alkalinity of the organic materials||Poor availability of suitable materials in some locations, high travelling costs|
|The establishment of organic cover does not disturb the natural environment||Long term performance is uncertain, may require regular reapplication. The expected cover life is strongly dependent on the rate of decomposition|
|Low costs, if the material is readily available||Humid climates may be required to maintain sufficient conditions|
|Offers a recycling option for organic wastes||Pre-treatment of some material streams (such as municipal sewage sludge) is needed prior to discharge|
|The organic nature of the material allows the development of vegetation on the surface of the cover||Concerns related to quantity and quality of effluent, and its impacts on the surrounding environment|
|Limited amount of demonstration and experiences|
|In some cases it is possible that layer may cause leaching of acids. However, this is yet uncertain and needs to be studied more|
|Difficulties in keeping the organic layer in place, stable and moist|
As the organic layer has to remain moist to ensure the proper function of the cover, it is suggested that organic covers are applicable only in areas where the amount of rainfall constantly stays from moderate to high throughout the year (Steffen Robertson and Kirsten Inc. 2001).
Performance and design requirements
The performance of organic cover depends on the capacity of the material to consume oxygen, the moisture content, and the degree of compaction (MEND 1994). According to the experiments performed by Germain et al. (2010) organic covers may also have a role in water treatment. Organic cover can act as an alkaline reducing barrier if the collected waters are recirculated through the organic cover. This treatment neutralizes acidic waters and removes soluble sulphide oxidation products (iron and other metals). (Germain et al. 2010)
The long term efficiency of the oxygen consumption barrier is the single most important thing to consider in the cover design process. Organic material that is placed at the surface of a cover structure is constantly in contact with atmosphere and thus decomposing over time. Such circumstances eventually cause the exhaustion of oxygen consumption ability of materials. If the addition of organic waste is not done at regular intervals, a construction of organic cover is only a short term solution to a long term problem (Lottermoser 2010), as the expected life time of the cover is relatively short. For example it is estimated that a wood waste cover of 1 m depth will meet a lifespan of approximately 10 years, whereas a cover of 2-3 m can stay operational around 50 years. (MEND 1994)
The other important thing to assess is the selection of organic material, as the suitability of the organic material to act as an oxygen consuming material varies between different material types. It is yet notable, that although the material characteristic properties may seem similar than in some other material, it is not an evidence the materials will function similarly (Hall & Sudbury 1997), and careful feasibility studies should be performed prior to material selection. Not only the characteristics of the material should be evaluated, but also the availability of the material is an essential factor as it differs between sites. It is necessary for method applicability that enough suitable organic material is available. (European Commission 2009)
Organic oxygen consuming covers are classified as potentially inexpensive cover options. For example, although the total reclamation costs for the East Sullivan mine site were approximately $7.5 to 8 million, the construction of organic cover was basically free to the mine company. (Steffen Robertson and Kirsten Inc. 2001) However, there is only a limited amount of information available on the overall costs related to organic covers. It should be noted that the cost inspections should also include maintenance and the reapplication costs of the cover structure. (MEND 1994) As the organic structure needs regular reapplication the costs can increase significantly, although the materials as such are considered to be quite affordable. Also pre-treatment of some organic materials (for example municipal sewage sludge) may be necessary prior to their usage in cover structure so that the potential environmental and health risks related to their use can be minimized (Peppas et al. 2000), and has also its effects to the overall costs. The economic issues can also make some options more attractive than others, for example many municipalities are obligated to pay for the municipal waste disposal. The costs are also strongly dependent on the distance between source material and mine site (Steffen Robertson and Kirsten Inc. 2001). Long distance transports can be expensive, and make the estimated covering option economically unfeasible.
Unfortunately, no general instructions for organic cover design are available. Potential organic materials often have varying physical and chemical properties, and due to this cover designing is usually based on trial and error. Numbers of different variables are affecting to the cover design, and even at best the estimates of optimal depth of the cover, particle size, and whether to use one-time vs. periodic application are only hypothetical. (MEND 1994)
When assessing the performance of oxygen consuming layer, the monitoring of oxygen, carbon dioxide, and methane concentrations, as well as the moisture content of the cover are needed. Also the seasonal effects such as wetting and drying of the cover are important aspects to consider. (Steffen Robertson and Kirsten Inc. 2001)
The demonstration results of the method applicability in varying climatic conditions are limited (INAP 2009). It is safe to consider that the method should be used only in humid areas to ensure sufficient moisture content of the cover. Organic covers have been proven to work effectively for tailings, but the suitability for waste rock covering needs more testing. (Steffen Robertson and Kirsten Inc. 2001) More studies connected to life time expectancy of the organic covers, and the behaviour of the cover permeability after the organic material is decomposed should also be performed (Hallberg et al. 2005). The cover’s life time expectancy is linked to the rate of decomposition, however, the relation between long term performance of the cover and degradation rate of the organic material needs more investigations. (Steffen Robertson and Kirsten Inc. 2001)
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