This portion of the construction and installation process
for the P removal structure is NOT necessary for the typical-everyday use. What is described here is for very detailed
monitoring of performance for the purpose of research. It is completely unnecessary for the applied
purpose of the P removal structure. We
are doing this so that we can demonstrate the effectiveness of the unit and
also produce more data for model verification.
This section is mostly relevant to researchers.
That being said, we will not put the PSM (treated steel slag
in this case) into the structure until all the monitoring equipment is
installed. For monitoring, our purpose
it to measure the amount (volume) of runoff treated by the structure during
each runoff event and also measure the P concentrations flowing in (i.e. before
treatment) and after (i.e. after treatment).
Knowing this, we can then estimate how much (mass) P has been trapped by
the P removal structure.
On the “inflow” side of the structure where water will
enter, we will use an ISCO automatic sampler that will be triggered to collect
samples based on the detection of flow at the “outflow” side of the structure
where the treated water will be located.
Below you can see some photos of the construction of the approach for
the 3 foot flume that will be used to monitor flow rate.
Monitoring flow rate is critical if one is serious about
getting a real estimate on the P removal performance of a P removal
structure. Many researchers have
utilized only water sampling in their monitoring regime without the use of flow
monitoring. Simply put, P concentration testing
of the treated water compared to untreated water alone is not sufficient for
assessing the capacity of a P removal structure; knowledge of the flow rates
and therefore the volumes of water treated is absolutely critical. Any assessment without the use of detailed
and thorough flow monitoring should not be taken too seriously. Why? The short answer is “P load”. If I measure a 100% P concentration reduction
and there is only 1000 gallons of water treated, it is not correct to compare
that concentration reduction to another scenario where there is only 50% P
concentration reduction and 1 million gallons of water treated. Simply put, the most important factor is the
P load (i.e. mass) reduction. Instead,
it is more correct to compare the P load reduction (i.e. mass of P removed).
Too often, we focus on the final concentration of runoff
water and treated water. This makes little
sense, especially when surface water quality thresholds (i.e. critical P
concentrations for streams and lakes) are applied to the context of runoff
water concentrations. For example,
runoff concentrations from a certain field (call it field 1) may test at 1 mg
P/L, while field 2 may produce 0.2 mg P/L.
That does not mean that field 2 is somehow more “safe” than field
1. Consider a scenario where field 1
produces 2 million L runoff for an event, while field 1 only produces 1000
L. The P load transported off site for
field 1 and 2 would be 1 g vs 400 g, respectively. Concentrations alone would be deceiving in
that case. What matters is the mass or
load of P that reaches the lake or stream; concentrations (mass/volume) change
with dilution, evaporation, etc., but it is the mass of P that does not
change. This is why the USEPA has moved
to a “total maximum daily loading”, or TMDL system for point and non-point
source pollution concerning nutrients.
For the same reasons, it is important to assess the
performance of a P removal structure based on P load reduction, not P
concentration reduction.
Below you can see the flume installation process.
This flume can handle much more water than our projected 2
yr storm (~16 cfs). This flume is being
used courtesy of Dr. Sherry Hunt, Kem Kadavy, and Ron Tejral, located at the
USDA-ARS Hydraulics Engineering Research Unit.
All of the treated water plus any water that might overflow
the structure will flow through the flume for measurement. There will not be any water overflowing the
structure if none of the storms exceed a 2 yr return period for that
location. In addition, if that does
happen, there will be a flow sensor (actually a depth sensor) placed on top of
the structure that will also monitor exactly how much water overflows the
structure and remains untreated. Below is
the view looking from the structure downhill toward the flume:
This area between the outflow drainage of the structure and the flume will have an impermeable liner placed on the ground and “bordered” with stabilized railroad ties to force all water from the structure to flow through the flume where flow rate will be monitored and also where samples will be taken by the ISCO sampler.
The photos below show the installation of the small building that will house the ISCO sampling equipment:
