Passive treatment system

The type of system selected is dependent upon the chemistry of the acid mine drainage and the flow of the discharge, as well as relevant regulations.

Passive treatment systems are a relatively recent technology that involves using sulfate-reducing bacteria or limestone or both to neutralize acidity and precipitate metals.

[1] Aerobic wetlands are shallow (1–3 foot deep) ponds; they may be lined or unlined and some are nearly filled with soil or limestone gravel.

[1] Anaerobic wetlands may be lined or unlined shallow ponds filled with organic matter, such as compost, and underlain by limestone gravel.

If the system is improperly sized, if flow dries up, or if extended low temperatures are encountered, the microorganisms will die and the performance will be decreased.

[1] Another passive treatment system uses lime dispensing technology to neutralize acidity and precipitate metals in a settling pond.

The disadvantages of passive treatment systems are that they may require complex discharge permits unless taking a Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) action; may not meet stringent water-quality-based effluent standards; may fail because of poor design or severe winter conditions; and, are a relatively new technology and an area of active research.

Nonetheless, regular monitoring, inspection, and maintenance are required, although to a much lesser extent than with active water treatment systems.