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Pyrolusite® treatment beds

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, we’ve achieved a high rate of treatment at one site, with minimal maintenance, while we’ve 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 bed’s 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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