PASSIVE TREATMENT
The term passive treatment refers to treatment technologies that can function with little or no operation or maintenance over long periods of time. Of course, all systems will eventually require some O&M, but passive systems can function for weeks to years, even decades, with little interference from humans. The current standard is to design systems to operate for 25 year with no major O&M. The opposite of passive treatment is active treatment, which requires regular maintenance and/or additional of treatment chemicals. As you would expect, active treatment is often prohibitively expensive.Examples of passive treatment system components include:
Ponds- Wetlands
- Anoxic Limestone Drains (ALDs): a buried bed of limestone that the water runs through in a controlled manner
- Vertical Flow Ponds (VFPs): a pond that drains through the bottom, which is constructed of organic material and limestone
- Open Limestone Channels
Usually, passive treatment systems contain several of the above elements in series. Each component and each passive treatment system is suited for a particular purpose in a particular type of water. Although there are a few general rules about passive treatment, all sites and and situations are different.
In order to design successful passive treatment systems, the following information is necessary:
- High, low, and average flow rates
- Iron content and form (Ferrous or Ferric)
- Aluminum content
- Total acidity
- Total alkalinity
- pH
- Dissolved oxygen content
- Site layout (elevations and area available to construct treatment)
If AMD is net alkaline, usually only ponds and/or wetlands are necessary to allow the metals to precipitate.
If AMD is net acidic but does not contain aluminum, dissolved oxygen, or ferric iron, an ALD may be used to add alkalinity to the water. The ALD is usually followed by ponds and/or wetlands to precipitate the metals, but metals are not retained within the limestone. ALDs are very reliable when designed correctly, and some ALDs have been operative successfully for over 10 years. However, ALDs have are limited in the amount of alkalinity that they can generate and are not suitable for use with many types of AMD.
If AMD is net acidic and contains aluminum, dissolved oxygen, or ferric iron, ALDs cannot be used. In this case, VFPs are usually used. Water flows through the organic matter and the limestone, leaving behind metals and generating alkalinity. Unlike ALDs, VFPs are expected to clog and are equipped with special plumbing that allows them to be flushed. VFPs can be used in series with one another to generate additional alkalinity if necessary.
Click HERE to read about one of our passive treatment projects.
Click HERE to view "The Creation of a Vertical Flow Pond".
Click HERE to view "The Passive Treatment System Slideshow".
