The October 17 issue of Abandoned Mine Posts featured this article on designing passive mine drainage treatment systems by Joe Schueck, formerly with DEP’s Bureau of Abandoned Mine Reclamation. The article was edited by Anne Daymut, Western PA Coalition For Abandoned Mine Reclamation Watershed Coordinator.
The text of the article follows—
Experts of abandoned mine drainage passive treatment are often called upon to triage under-performing systems. Frequently, their diagnosis is: The system was well designed, but just too small for the flows encountered.
In a passive treatment system, adequate retention time is mandatory for the needed chemical reactions to take place in order to neutralize acid mine drainage. When the flow rate is higher than the system was designed for, the system can't function as designed and decreased performance or failure results.
Flows greater than the design flow rate happen for a number of reasons. Perhaps the pre-design monitoring dates failed to coincide with peak flow occurrences or additional flows are opened up during construction.
Whatever the reason, these higher flows need to be dealt with but it is often not possible to redesign or construct systems due to funding and space limitations. Failure of passive treatment systems due to overloading is very expensive and the number of failures on record is giving passive treatment a bad reputation.
Unfortunately, the vast majority of passive treatment systems direct all of the flow through the ponds, regardless of what the ponds were designed to handle. Bypassing excess flow is an option but is often tedious and inaccurate because flow adjustments have to be made manually on a daily basis. One watershed group was able to find a solution.
The Shamokin Creek Restoration Alliance and the Northumberland County Conservation District installed a passive mine drainage treatment system for the Corbin Mine drift, a deep mine discharge with a highly variable flow, in the Shamokin Creek watershed at Scarlift Site 15 in the summer of 2006. Space was limited so an attempt was made to bypass flows in excess of the design flow.
Initially the group made flow adjustments reaching the treatment system manually. Then, a flow distribution box, fabricated by Agri Drain, was inserted between the discharge and the ponds to simplify yet make this process more precise.
The box is constructed using 1" PVC for the base and 1/2" PVC for the sides and ends and comes complete with a lockable lid. The inlet and the bypass are made of 12" PVC pipe. The interior of the box consists of three chambers separated by Agri Drain's stop logs. An 8" Valterra valve is mounted in the center chamber. This valve is adjusted to control the volume of discharge going to the treatment ponds.
The operation of the distribution box is rather simple. To begin with, the 8" Valterra valve is closed with all of the flow going out the bypass. The stop log located between the Valterra valve and the bypass port serves as a broad crested weir. The depth of flow over top of the weir is measured and a flow-rating chart is consulted to determine to total discharge rate.
The flow that is to be directed into the treatment ponds is subtracted from the total discharge rate, leaving the flow that should be going through the bypass once all adjustments are made. The rating chart is once again consulted to determine the depth of flow that will give the proper bypass discharge rate. The Valterra valve is slowly opened until that depth of flow is obtained. A setscrew on the Valterra valve stem is tightened, the lid replaced and everything is done.
If flow rates of the discharge increase dramatically, there will only be a slight increase in the flow to the ponds because the head will only increase slightly. If the discharge rate falls below the design rate then all of the discharge will be directed through the ponds. When maintenance needs to be done to the treatment system, the valve is simply closed, bypassing all of the discharge until the necessary maintenance work is completed.
This box can be easily adapted to discharge to multiple ponds by lengthening the flow distribution box and adding additional Valterra valves and stop logs. Placement of stop logs between the Valterra valves allows the treatment ponds to receive different flow rates.
Treatment system design and installation is a long and arduous process and unfortunately, it's not always flawless. Construction costs can run into the millions of dollars, space can be limited, and discharge characteristics can change or be different than initially observed.
Luckily, there are many innovative minds out there working towards solutions to mine drainage problems. The flow distribution box used at Corbin Mine drift is a tool that can be adapted to many situations, is insignificant to the overall cost of a treatment system, has the potential to reduce the amount of manual adjusting, will increase the precision of flow control, and may serve as an insurance policy to a treatment system.
Joe Schueck retired from the Bureau of Abandoned Mine Reclamation in 2006 after serving 36 years with the Commonwealth. He is currently a Manufacturer's Representative for Agri Drain. He can be reached by sending email to: firstname.lastname@example.org.