WT0968 Aberystwyth University
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i Treatment of non-coal mine water – establishing new pilot trials using alternative technologies. Final Report - WT0968 Aberystwyth University Aberystwyth University Biochar - DEFRA – non-coal mine drainage ii WT0968 Treatment of non-coal mine water - establishing new pilot trials using alternative technologies Executive summary: Water, draining from abandoned metal mines in the United Kingdom causes pollution of the local streams and rivers. In many cases this pollution is invisible because the contamination is dissolved in the mine water. This problem is responsible for around 10% of UK rivers failing to meet water quality standards dictated by the EU Water Framework Directive. It has been estimated that over 170 tonnes of the metal zinc (Zn) is being released into UK rivers each year from these old metal mines. These mines are distributed throughout the UK in the old metal mining regions such as southwest England, central and north Wales, Shropshire, the Peak District, the Pennines and north Yorkshire. The UK has been successful in treating the water coming from abandoned coal mines through the work of the Coal Authority but to date there has been very little work done on the treatment of water coming from abandoned metal mines. Metal mining in the UK was at its peak during the 18th and 19th centuries but many of the mines became uneconomic when faced with cheaper imports from overseas. The result was that the mines were simply abandoned long before any environmental legislation was enacted. The mines were left with open tunnels (adits) which allow water to enter the mine and interact with the remaining ore materials underground. When this water returns to the surface, through the tunnels which drain the lowest point of the mine, it is contaminated with a number of potentially toxic elements such as lead, zinc, cadmium and arsenic. Many of these abandoned mines are in remote locations with only limited infrastructure. This project was funded by Defra to investigate the potential of biochar to treat contaminated metal mine water. Biochar is produced from organic material by heating it in an atmosphere without oxygen. The process is known as pyrolysis and it generates a product which typically retains the original organic structure in a carbon-rich material called biochar. There has been a considerable volume of research on the use of biochar to amend soils to improve their fertility and even to remove contamination from the soil but very little work has been done on the possible use of biochar as an absorber of metal pollution from water. Whilst the term absorber is used here the Aberystwyth University Biochar - DEFRA – non-coal mine drainage iii true physical process taking place should, more correctly, be described as adsorption and thus the material is an adsorber. The project was set up to study metal-rich mine waters which are not strongly acidic or alkaline. These waters are, therefore, known as circum-neutral mine waters. These mine waters also typically have low iron content which makes the application of coal mine treatment technologies difficult or impossible because in coal mine drainage the main contaminant is usually iron. The low iron content also explains the fact that the waters do not look ‘contaminated’ because the potentially harmful elements and compounds are fully dissolved in the water. The mine water also carries elements in solution which are naturally present in the environment and which do not pose a toxicity threat. One of the aims of this project was to test the application of biochar to remove the toxic elements but not non-toxic elements in the water. Thus an element of selectivity is highly desirable in any absorber of metals. The main element of concern at abandoned metal mines is zinc because of its toxicity to aquatic organisms. This project developed in stages from laboratory tests of different biochar materials to the deployment of different scales of treatment systems at two metal mine locations in mid-Wales. The project aimed to determine which types of plant-based materials were the best sources for biochar production; with the emphasis on the use of low cost, by-products from agricultural practices or the use of plant materials which are subject to cutting in grazing management schemes. The project also considered wood-based materials from softwood production to hardwood by-product and material produced through the management of ash dye-back disease. In the laboratory tests there were several factors which needed to be considered. Not only did different starting materials need to be tested but the best charring conditions for each material had to be tested. A review of the literature on biochar production showed that there is a wide range of temperatures used to produce biochar. Generally, as the temperature of the charring process is increased the resulting material becomes more carbon-rich and has less of the other major components such as hydrogen, oxygen and nitrogen attached to the char. It is suggested that the other major components form compounds attached to the char particles that are responsible for the chemical interaction with metals in water and that these compounds may help to remove the metals from the water. Therefore it is likely that the temperature of heating the biological material will be important in controlling the properties of the biochar produced. This had not been fully investigated before this project took place so one of the early tasks of this project was to test the effect of varying the temperature at which char is produced and testing the resulting material with mine water from one of the test sites in mid-Wales. Aberystwyth University Biochar - DEFRA – non-coal mine drainage iv The results of the early laboratory tests showed that producing biochar at relatively low temperatures (200 – 350 °C) produced the best material to adsorb metals. The tests were carried out using a small scale furnace in which it was possible to tightly constrain the temperature and the rate of heating. The furnace also has the facility to introduce different inert gasses during the charring process. The tests also showed that grass-type materials were the best feedstock for the charring process. Here grass-type refers to ryegrass together with oil seed rape residue, wheat straw, the common rush plant (Juncus) and the common reed (Phragmites). Some of the plants now grown for bio-energy bio-fuel production were also tested. These included willow (Salix) and the large sterile hydrid Miscanthus. The tests carried out in the laboratory considered the capacity of the char to remove metals and were presented as metal adsorbed per gram of biochar. Further testing was required to show how quickly the biochar was able to remove the metals once it was placed in contact with the contaminated water. It is important to know how rapid the process is so that the size and scale of any treatment system can be properly designed. The results showed very good removal of metals from mine water within 15 minutes of contact time. When the laboratory tests had demonstrated the best conditions for producing biochar it was possible to scale up the production using a larger facility at Aberystwyth University. This facility has the capacity to treat around one cubic metre of starting material but still offers the operator control over the temperature of charring and the duration of the heating process. The gasses which are produced during the charring process are used to fuel the heating of the feedstock and this reduces the running costs and the impact of biochar production at this facility. Producing biochar on a larger scale was necessary for the trials which were to take place at the mine sites. The field scale trials developed in several stages. The first set of trials used small volumes of char (around 1 litre) packed into vertical columns produced from plastic drainpipe. The trials were conducted at the Bwlch mine site near Aberystwyth. This mine produces a flow of water all year round and it is particularly zinc-rich water which also has a range of other metal contaminants present which makes it an ideal test site. The mine water was delivered to the field trial columns and the flow of water was controlled to give sufficient contact time between the water and the selected adsorber. During these early trials the biochars produced during this project were compared to the performance of commercially available activated carbon and ion exchange materials. The advantage of the small scale system is the flexibility to quickly change the type of material in the column and thus to test a wide range of different adsorbers. The second stage of the field trials was to test the effect of passing the mine water through a ‘bed’ of biochar in which the water flowed in a horizontal and not a vertical fashion. This trial was conducted because there may be mine sites at which it would be appropriate to collect the Aberystwyth University Biochar - DEFRA – non-coal mine drainage v contaminated mine water and pass it through a ditch or channel filled with a low-cost adsorber such as biochar. The test illustrated a problem with the packing of the biochar into the tank because some of the material floated to the top and some sank leaving a region in the centre which the mine water was able to flow through and not interact fully with the char. This problem was overcome by careful re-packing of the bed of biochar. The first and second field trials showed that the biochar was able to remove all of the zinc, lead, cadmium and copper from the mine water during the early period of operation but as time went on, and the volume of water which had passed through the adsorber increased, the performance of the biochar deteriorated so that some zinc was able to pass through the bed and not be removed.