Passive treatment technologies
Kaisa Turunen (GTK), Geological Survey of Finland, P.O. Box 1237, Kuopio FI-70211 FINLAND, kaisa.turunen(at)gtk.fi
The passive water treatment methods are based on natural chemical and biological reactions without or little nutrient and energy addition. They use mainly naturally available energy sources such as microbial metabolic energy, photosynthesis and chemical energy. In addition, the nutrients needed in passive treatment are commonly available in the nature and only compost and/or limestone addition may be needed. Since passive treatments are self-sustaining processes, they require usually only infrequent maintenance and function well without or little human interference when established. Therefore passive treatment is the most applied method to closure and post-closure phases of mine.
The environmental conditions in the different passive treatment systems will dictate the metal removal mechanisms and due to uniqueness of each mine site and their water chemistry, a comprehensive understanding of mine water chemistry is required. In addition, passive water treatment methods can be mainly applied to low flow rates and the treated water quality is often poorer and more variable than in active or in-situ treatments. Passive treatment systems are best suited to mine water with low acidity (<800 mg CaCO3/L), low flow rates (<50 L/s) and therefore low acidity loads (<100-150 kg CaCO3/day). However, passive water treatment has longer term liability and due to its self sustaining character the operational and maintenance costs for passive treatment are fairly low. Yet due to the large amount of required land, the investment costs may be high compared to the active treatment methods. Since several treatment methods are based on biochemical and microbial reactions, cold climate and/or winter conditions may hinder the performance of passive water treatment in which the reaction rates decrease as temperatures drop. Moreover, passive systems are more sensitive to low temperatures in the beginning than during operation and should be started during warmer months to give the sulphate reducing bacteria (SRB) and associated microorganisms a chance to build up population density before they encounter cold temperature conditions. In most cases, the SRB hosting organic substrate is covered with soil layer to protect it from freezing. In addition, the prediction of the lifetime and performance of passive treatment methods is difficult and always site specific (Younger et al. 2002, Wolkersdorfer 2008, Gusek & Figueroa 2009, Zipper et al. 2011).
Passive water treatment methods rely mainly on the following mechanisms (modified from Younger et al. 2002 and Gusek & Figueroa 2009):
- Precipitation of metal hydroxides or oxides
- Microbial sulphate reduction forming metal sulphides
- Microbial reduction of dissolved Fe3+ and Fe hydroxides
- Complexation and adsorption onto organic matter
- Adsorption of metals by iron hydroxides
- Ion exchange with other cations on negatively charged sites
- Direct uptake by plants
- Neutralization by carbonates
- Attachment to substrate materials
- Adsorption and exchange of metals onto algal mats
- Filtering suspended and colloidal material from water
Table 1. Advantages and disadvantages of passive water treatment (modified from Younger et al. 2002, Heikkinen et al. 2008, Wolkersdorfer 2008)
|No or low external energy and chemical requirements after construction and commissioning||Large physical area|
|Self-sustaining processes, no on-going maintenance need, thus suitability also for remote locations without access to utilities||Fairly low contaminant remediation|
|Longevity and low operational costs||Prone to seasonal and other variations in performance due to reliance on biological processes|
|Can often be integrated directly to surrounding environment||Need for monitoring, due to lack of day-to-day intervention in treating processes|
|Use of non-hazardous materials||Sensitivity of biological systems to high concentrations of contaminants, extreme pH conditions, salinity, and/or significant changes in hydrologic regime|
|Landscape, ecological and fauna rehabilitation||Relatively high capital costs for construction|
Figure 1. Flow chart for passive water treatment selection. (Modified from Younger et al. 2002, PIRAMID consortium 2003, Wolkersdorfer 2008).
More detailed descriptions and evaluations of selected passive water treatment technologies are presented in the following pages:
- Aerobic constructed wetlands
- Anaerobic constructed compost wetlands
- Open limestone channel (OLC)
- Limestone / Slag Leach Beds
- Anoxic limestone drains (ALD)
- Reducing and alkalinity producing systems (RAPS/SAPS)
- Natural wetlands (natural attenuation)
Table 2. The average effluent range for different passive water treatment technologies (Modified from MEND 1996).
|Treatment system||Average acidity range ( mg CaCO3/l)||Average acidity load (kg CaCo3/day)||Average flow rate (l/s)||Dissolved oxygen (mg/L)||Average pH range||Max attainable pH|
|ALD||< 500||< 150||< 20||< 1||> 2||6–8|
|OLC||< 500||< 150||< 20||Ambient||> 2||6-8|
|Aerobic wetlands||< 500||≤ 1||Residence time 1-5 days||Ambient||> 6||n/a|
|Anaerobic wetlands||< 500||1||Residence time 1-5 days||Ambient near surface,<1 mg/l subsurface||> 2.5||6-8|
|RAPS||< 300||< 100||< 15||< 1-3||> 2.5||6-8|
Gusek, J. & Figueroa, L. 2009. Mitigation of Metal Mining Influenced Water. Part 2. SME. 164 p.
Heikkinen, P.M. (ed.), Noras, P. (ed.), Salminen, R. (ed.), Mroueh, U.-M., Vahanne, P., Wahlström, M., Kaartinen, T., Juvankoski, M., Vestola, E., Mäkelä, E., Leino, T., Kosonen, M., Hatakka, T., Jarva, J., Kauppila, T., Leveinen, J., Lintinen, P., Suomela, P., Pöyry, H., Vallius, P., Nevalainen, J., Tolla, P. & Komppa, V. 2008. Mine closure handbook. 169 p.
MEND (Mine Environmental Neutral Drainage). 1996. Review of Passive Systems for Treatment of Acid Mine Drainage. MEND Report 3.14.1 Canada: Mine Environmental Neutral Drainage.
PIRAMID consortium, 2003. Engineering Guidelines for the Passive Remediation of Acidic and/or Metalliferous Mine Drainage and similar Wastewaters. 166 p.
Wolkersdorfer, C. 2008. Water Management at Abandoned Flooded Underground Mines. Fundamentals, Tracer Tests, Modelling, Water Treatment. Springer. 465 p.
Younger, P.L., Banwart, S.A., Hedin, R.S. 2002. Mine Water – Hydrology, Pollution, Remediation. Environmental Pollution. vol. 5. Kluwer Academic Publisher. 442 p.
Zipper, C., Skousen, J. & Jage, C. 2011. Passive Treatment of Acid-Mine Drainage. Publication 460-133 of The Powell River Project. Virginia Polytechnic Institute and State University.