Biologic sulphate reduction in mine shaft lakesJari Väätäinen2018-11-13T13:45:01+02:00
Biologic sulphate reduction in mine shaft lakes
Jarno Mäkinen, VTT Technical Research Centre of Finland Ltd; P.O. Box 1000, FI-02044 VTT, Finland, e-mail: jarno.makinen(at)vtt.fi
Abandoned mine shafts are typically filled naturally with water when drainage pumping has been stopped. Especially in sulphide mines water becomes rich in sulphate and various metals causing well known environmental problems when water is discharging into the environment (Johnson 2003). In the technology presented here, mine shaft lakes are harnessed to a passive bioreactor-type of systems with the help of sulphate reducing bacteria (SRB) for sulphate and heavy metals removal.
Description of the technology
The mine shaft lakes, rich in sulphate and heavy metals, can be treated in-situ and passively with sulphate reducing bacteria (SRB). These bacteria require organic substrate and anoxic conditions to reduce sulphate to hydrogen sulphide, which precipitates, together with raised alkalinity and pH, various heavy metals from water as poorly soluble sulphides (Kaksonen et al. 2003, Liamleam & Annachhatre 2007):
3SO42− + 2CH3CHOHCOOH → 3H2S + 6HCO3−
H2S + M2+ → MS(s) + 2H+
Precipitates settle to the bottom of the mine shaft and are separated from water discharges (Lu 2004, Vestola & Mroueh 2008).
In Finland, biologic sulphate reduction in mine shaft lakes is used at least in four mines or their specific shafts: Ruostesuo, Hammaslahti, Kangasjärvi and Kotalahti (Vestola & Mroueh 2008). In Sweden, Rävlimyran mine shaft lake has been treated with lime and sewage sludge to enhance the biologic sulphate reduction and precipitation of heavy metals (Lu 2004). Performance of SRB water treatment of Hammaslahti mine site is presented in the case studies of Closedure pages: Biological sulphate redution of mine water – Hammaslahti mine site.
Biologic sulphate reduction can be utilised in mine shafts that have anoxic deep water layer throughout the year, i.e. rather stable stratification to prevent mixing of oxygen rich top water with anoxic bottom water. Proper stratification is developed due to sufficient depth or suitable shape of a shaft, but also due to high salinity of water. Moreover, seasonal variations in rainfall and temperature have an effect on stratification. In some cases, stratification is disturbed by oxygen rich seepage water flowing into the mine shaft lake through e.g. bedrock fissures. Situation may be improved by water management in the mine area, by constructing ditches that will lead the oxygen rich (non-polluted) water away from the shaft location (Lu 2004, Vestola & Mroueh 2008). The mine shaft depth has also an important role for water temperature, especially in northern regions. In Finnish case examples it is seen that in 50 meter depth the temperature is constant at 4- 6°C, which is rather low for sulphate reducing bacteria and therefore hampers the speed of sulphate reduction. However, if the mine shaft lake is clearly deeper, bottom water is usually warmer, favouring the biologic sulphate reduction. (Vestola & Mroueh 2008) Also case specific parameters, like pH and redox-potential have significant effect on biologic sulphate reduction. Main sulphate reducing bacteria require conditions of pH > 5.5 and redox < -100 mV to perform efficiently (García et al. 2001), but activity of some microbes is observed also in pH < 3.0 (Johnson 2003).
Advantages of the technology are related to its passive and in situ nature, providing rather economic and easily operated system. Practically, only anaerobic sludge or animal manure addition is required as they contain both the source of sulphate reducing bacteria and suitable organic substrate for them. Even the addition amount of these substrates is rather massive, and addition may have to be repeated several times to establish strong microbial colony, the price of the substrates is relatively low. When bacteria are thriving and there is enough substrate, it is possible that additions are not anymore needed and treatment can function without any actions. Only monitoring of the mine shaft lake and its discharge water quality is then needed (Lu 2004, Vestola & Mroueh 2008).
The main disadvantage of the technology is significant lag phase – development of proper bacterial colony and positive changes in mine shaft water take usually 2 – 3 years. Other parameters that can slow down the start-up and sulphate/metals reduction from waters is the low temperature and pH of the bottom water, as well as high concentration of sulphate, metals, As, and some anions like F and Cl. (Tsukamoto et al. 2004, Vestola and Mroueh 2008) As inhibition levels in different studies seem to vary, importance of laboratory testing is essential before starting the large-scale projects. One particular concern is also the release of nutrients (N, P) and pathogens to the surrounding environment (Vestola & Mroueh 2008).
Successful case example of Vehkankuilu (Kotalahti, Finland) has shown that biologic sulphate reduction in mine shaft lakes can be very efficient method for removing sulphate and heavy metals from water. In the Vehkankuilu a ten year monitoring campaign has revealed that initial concentrations of sulphate (960 mg/l), iron (3.0 mg/l), nickel (3.0 mg/l) and manganese (4.6 mg/l) have decreased by 88%, 76%, 97% and 93%, respectively. Simultanously, the pH has increased from 6.6 to 7.5. The reason for successful treatment is probably the depth of Vehkankuilu (hundreds of meters), only slightly acidic conditions, moderate concentrations of sulphate and heavy metals, and finally, multiple additions of substrate and microbial inoculum during 1996 – 1997. However, in the case examples of Kangasjärvi and Hammaslahti “N” mine shaft lakes, both in Finland, results are not so encouraging, even though some sulphate and iron reduction has been observed. Common for these two mine shaft lakes is the depth of approximately 50 meters and therefore very low bottom water temperature (Vestola & Mroueh 2008). Results of the performance of the Hammaslahti case study are discussed in a more detail in the article: Biological sulphate redution of mine water – Hammaslahti mine site.
The costs of biologic sulphate reduction consist of characterization of the site, and then purchasing, transportation and pumping of the substrate and bacterial inoculation. After the establishment of the treatment, costs consist of monitoring of mine shaft and discharge water. If proper sulphate reduction is observed, monitoring can be done in longer intervals, but if not, more substrate must be added, or other actions, like liming, executed. (Lu 2004, Vestola & Mroueh 2008)
Design requirements include the understanding of the mine shaft geochemical and limnological properties. These include mine shaft depth, shape, water quality and stratification in all seasons as well as water flow volume, quality and seasonal variations into the shaft. These parameters identify how successful the passive and in situ biologic treatment would be. The most important factor is the stratification throughout the year to maintain bottom water anoxic as biologic sulphate reduction is not occurring when oxygen is present. Therefore, also leakages of oxygen rich water into the mine shaft bottom must be prevented. If the water is strongly acidic prior lime treatment is suggested to lift the pH ≥ 6. This also decreases concentrations of metals, which may be initially high and therefore cause inhibition to bacteria (Lu 2004, Vestola & Mroueh 2008). The mine shaft depth and resulted bottom temperature are parameters that cannot be affected on and therefore it must only be understood that ≤ 6 °C temperatures significantly slow down the process (Tsukamoto et al. 2004, Vestola & Mroueh 2008).
Proper selection and dosing of substrate and microbial inoculum is very important. The easiest way is to utilise anaerobic sludge or animal manure to provide microbes, carbon, nitrogen and phosphorus all in the same feed. Theoretically, sulphate reducing bacteria require 0.25 tons of total organic carbon (TOC) to remove 1 ton of sulphate, which serves as a basis for dosing calculations. However, in the nature utilisation ratio is never 100% and also rivalry is observed between sulphate reducers and other microbes. Therefore, the needed dosing is more or less higher than theoretical value (Vestola & Mroueh 2008). Optimal performance of sulphate reducing bacteria is observed with C:N:P ratios of 100:7:1 (Cocos et al. 2002), while anaerobic sludges or manures contain clearly higher shares of nitrogen and/or phosphorus, compared to carbon. Therefore, these nutrients are not completely used by bacteria and they may be discharged into the surrounding environment. The C:N:P ratio of a feed can be improved by addition of e.g. ethanol into the mine shaft lake (Vestola & Mroueh 2008). Laboratory-scale testing is essential when selecting and determining adequate dosage of the organic substrate, and determining the amount of excess nutrients in the system to avoid discharges. The more detailed review of organic substrates is presented by Liamleam & Annachhatre (2007).
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