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.

Introduction

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).

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).

Appropriate applications

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).

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)

Technological maturity

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)

References

Alakangas, L., Maurice, C., Macsik, J., Nyström, E., Sandström, N., Andersson-Wikström, A. & Hällström, L. 2014. Kartläggning av restproducter för efterbehandling och inhibering an gruvavfall –Funktion, tillgång och logistic. Lulea Tekniska Universitet. 2/28/2014. (In Swedish)

European Commission 2009. Reference Document on Best Available Techniques for Management of Tailings and Waste Rock in Mining Activities, January 2009.

Germain, D., Tassé, N. & Cyr, J. 2010. The East Sullivan Mine Site: Merging Prevention and Treatment of Acid Mine Drainage –update. Presentation in Proceedings of the 16th Annual BC-MEND ML/ARD workshop, Vancouver, BC., December 2-3, 2009, W.A. Price and K. Bellefontaine (Eds.).

Hall, G. & Sudbury, M.P. 1997. Evaluation of the Use of Covers for Reducing Acid Generation from Strathcona Tailings. MEND Project 2.25.3

Hallberg, R.O., Granhagen, J.R. & Liljemark, A. 2005. A fly ash/biosludge dry cover for the mitigation of AMD at the Falun mine. Chemie der Erde Geochemistry 65 S1, 43–63.

INAP 2009. The GARD Guide. The Global Acid Rock Drainage Guide. The International Network for Acid Prevention (INAP). http://www.gardguide.com/

Lindsay, M.B.J., Blowes, D.W., Condon, P.D., & Ptacek, C.J. 2011. Organic carbon amendments for passive in situ treatment of mine drainage: Field experiments. Applied Geochemistry 26, 1169-1183.

Lottermoser, B. 2010. Mine Wastes: Characterization, Treatment and Environmental Impacts. Springer, ISBN: 978-3-642-12418-1.

Peppas, A., Komnitsas, K. & Halikia, I. 2000. Use of Organic Covers for Acid Mine Drainage Control. Minerals Engineering 13, No. 5, 563-574.

MEND 1994. Evaluation of Alternate Dry Covers for the Inhibition of Acid Mine Drainage from Tailings. MEND Project 2.20.1. March 1994. Prepared by: SENES Consultants Limited. Mine Environment Neutral Drainage Program (MEND).

Steffen Robertson and Kirsten Inc. 2001. Methods for Delaying the Onset of Acidic Drainage – A Case Study Review, Final Report. Mend Project 2.37.2.