As of summer 2000, two Pyrolusite® beds have been
constructed and two are in construction. The systems consist of limestone
filled beds, where the limestone is inoculated with aerobic microorganisms
provided under a contract with Allegheny Mineral Abatement (AMA). The
beds are preceded by small aerobic wetlands to provide nutrients for
the microorganisms and to provide some initial treatment. To date, weve
achieved a high rate of treatment at one site, with minimal maintenance,
while weve experienced a number of difficulties at the second
site. The first site, known as Laurel Run, is in the upper reaches of
the Loyalhanna Creek watershed in Westmoreland County. A 30 gpm
discharge with low iron and aluminum concentrations and moderate acidity
and manganese levels is being successfully treated in a bed measuring
40 ft. by 120 ft. by 5 ft. deep. Effluent
water quality has been consistently outstanding. The only problem experienced
has been some plugging and ponding that occurred at the head of the
system shortly after the initiation of treatment. This was addressed
by removing and replacing limestone at the head of the system with larger
diameter stone (AASHTO No. 57, 1 _" to 3/32", was replaced
with ASHTO No. 3, 2 _" to _"). Also, a shallow trench was
dug at the head of the system to aid in flow distribution. This project
was completed in the early fall of 1997, and the above described work
was completed in the spring of 1998. Since then, the system has not
required additional maintenance, although a recent inspection found
some ponding that will need to be addressed. Recently completed biological
surveys have shown that this system, and another AMD treatment systems
constructed under the 319 program, are having a beneficial impact to
macroinvertebrate life in Laurel Run.
Unfortunately, the second site has not been nearly as successful. While
the bed has produced effluent of very good water quality, plugging at
the head of the system has been a problem from the beginning of operation.
This system has much higher flows (up to 100 gpm (6.3 L/s)) and much
higher iron concentrations and acidity than the Laurel Run system. The
system, known as the Squatter Falls site, located on Glenwhite Run in
Blair County, was completed in the early summer of 1999. The bed measures
80 ft. by 212 ft. by 6 ft. deep (24.3 m by 64.6 m by 2 m) and is preceded
by a small aerobic wetland. Based on our experience at Laurel Run, this
system was constructed with larger diameter limestone.
When plugging with iron precipitate occurred at the bed entrance, a
number of actions were taken to correct the problem. First, a trench
was constructed across the width of the bed and inoculated with microorganisms
by AMA. When the partially treated AMD entering the bed from the wetland
was characterized by depressed dissolved oxygen levels during the summer,
there was a concern that the aerobic
organisms in the bed would be killed. In order to add oxygen, an 8 inch
(20.3 cm) pipe was cut in half, notched, laid along the previously constructed
trench at the bed entrance, and connected to the pipe leaving the wetland,
allowing the water to cascade over the notches and become aerated. While
this action and cooler temperatures increased the oxygen levels, it
also seemed to increase plugging. Consequently, a subsurface piping
system was installed that directed the discharge into the bed below
the standing water level, thus eliminating oxidation caused by the cascading
action. The system continued to plug, raising the water level, and at
one point, causing water to overflow the beds embankment. It was
learned that a short-term fix was to simply use an excavator bucket
to stir up the stone at the head of the trench. This initially
had the effect of causing the water levels to drop almost instantly,
as water started to pass through the bed. Over time, however, even this
became less successful.
AMA was consulted prior to any of the above actions being taken and
was involved in the decision-making process throughout these attempts.
Finally, in early 2000, a decision was made to take more drastic action
before the area of plugging became more widespread (holes excavated
in areas of the bed away from the head of the system found clean limestone
with no evidence of plugging). A small vertical flow alkalinity system
and settling pond are being constructed in front of the wetland, with
the idea of pre-treating the discharge and removing a larger portion
of iron prior to the AMD entering the bed. The inoculated limestone
bed will then remove manganese and any remaining iron, raise the pH
and add alkalinity prior to the final discharge. This system, which
is nearing completion, will have a somewhat unique design. The vertical
flow alkalinity system is being constructed without using compost. This
was done to test a theory being discussed among BAMR staff, that if
a vertical flow system has adequate flushing capability, any iron entering
as ferric iron can be flushed from the system. This will be discussed
further under vertical flow systems.
Interestingly, with the number of problems this site has had, beneficial
impacts have been observed in the receiving stream and at a downstream
water supply reservoir. Treated AMD leaving the bed has had consistently
good water quality. The Blair County Conservation District has completed
one other site on Glenwhite Run, and several more are under construction
by BAMR and Natural Resource Conservation Service (NRCS).
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