Thursday, October 16, 2014

Abandoned Mine Posts: Don’t Drown Your Passive Mine Drainage Treatment System

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:

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