SAPS & Vertical Flow Reactors
A variety of prolix terms—including Vertical Flow Reactors (VFR), Successive Alkalinity-Producing Systems (SAPS), Vertical Flow Ponds, Vertical Flow Wetlands, and Reducing and Alkalinity-Producing Systems (RAPS)—exists to describe passive treatment systems that incorporate the benefits of anoxic limestone drains and anaerobic wetlands.
A vertical flow reactor consists of a layer of compost organic compost on top of a layer of limestone. AMD flows vertically downward through the system (compare this to horizontal flow in an anaerobic wetland). In the top layer, bacteria in the compost remove oxygen from the AMD. Oxygen is necessary for iron oxide to precipitate, so this removal of oxygen reduces the amount of iron oxide that will "armor" (clog) the limestone layer. Acidic AMD flows through the limestone, dissolving it, and thereby adding alkalinity and increasing pH. At the bottom of these layers is a system of pipes, which passes the treated water on to a settling pond, where metals can finally precipitate. Highly acidic AMD can be treated by running the water through a series of such reactors.
Operations & Maintenance Considerations
Even though the compost reduces armoring within the limestone, some metal precipitation does occur, and a vertical flow reactor needs to be cleaned ("flushed") regularly. This is accomplished by simply adjusting the pipes to increase water pressure as a result of gravity. Water flows through the system at a higher rate, forcing accumulated metals and sediment out through the pipes. The frequency and duration of flushing depend on the particular system, but it should generally occur at least four times per year.
It may be necessary to replace the compost layer because compost may become compacted or clogged, reducing its permeability. Also, flows of water may create preferential paths through the compost over time. This results in short-circuiting and decreased retention time. Compost replacement will probably take place once during the life of a system.
Of course, like most passive treatment systems, basic maintenance including mowing grass around a system and removing leaves that may clog conduits is necessary. Also, animals may damage a system, especially muskrats, whose tunnels must be blocked with dirt and rocks.