A CrystalStream High Point

A CrystalStream High Point                                 TSS Removal     96%
Phosphorus       74%
Nitrogen              74%

A new independent study has shown once again that the basic CrystalStream water quality vault leads all competitors in performance.  In June of 2010 a CST Model 2056 was installed in the Town of Highlands, North Carolina, which is 4,118 feet above sea level.  While this may have been a high point in elevation above sea level, it also set a high point in performance for this type of manufactured BMP.  The study started out with lofty goals.  The intention was to remove 85% of the total suspended solids and 35% of the phosphorus.

Highlands is a beautiful community that sits atop the Eastern Continental Divide in the Blue Ridge Mountains. The town lies within one of only two rainforests in the United States with over 80 inches of annual rainfall, and enjoys an almost ideal average temperature of 78 degrees Fahrenheit in the summer “heat” of July.  The town has many shops, restaurants, and unique lodging opportunities, so it is no surprise that it is a favorite tourist destination and the home of many people who have come to appreciate the breathtaking scenery and temperate climate.  Of course, with development comes pressure on the environment, and that is what prompted the Town to look for ways to protect its pristine lakes and streams.

The plan was to install a CST hydrodynamic separator and an underground storage facility on a portion of a stream that had been piped through a part of the Town long ago.  The natural stream had been relocated into a small drainage ditch that ran between shops and a bed and breakfast adjacent to the project site. The area had been subject to frequent flooding in the past.  The goal was to clean up the water with the separator prior to it entering the storage facility, and to supplement the quality of the water coming out of the CST device by further treatment as it was slowed down passing through the storage.  The basin that drains through the system is 14.6 acres and could produce very high flows in excess of the 25 cubic feet per second (cfs) capacity of the CST separator, so an external weir bypass was incorporated to direct the first 25 cfs of the flow through both the separator and the storage facility.  Excess flows can bypass the entire system through an overflow pipe.  There was a very small standing flow in the basin, so a small weir in an upstream manhole diverted this low flow into the existing ditch, bypassing the entire project. This was all paid for by the Town through a Clean Water Management Trust Fund grant and matching local funds.

The system has worked very well at helping to control flooding with the added underground capacity and new pipe systems.  The system has far exceeded expectations in the water quality department.  Water entering the CST water quality vault had an average TSS concentration of 283.8 mg/L, while the water exiting the vault had an average TSS concentration of 11.9 mg/L.  This is an astonishing 96% removal rate!  Total phosphorus removal by the vault was at 74%, as was total nitrogen.  These are not composite estimates, but are grab samples taken live in the field during the 20 studied storm events.

Samples were taken above the CST vault and below the vault at a point just above the underground storage.  More samples were taken concurrently just below the underground storage system.  While the samples match quite nicely for the vault (at 5 cfs it has a 20 second resident time), the high capacity of the underground storage system makes matching sample times more problematic.  Nevertheless, the underground system did further improve the TSS removal as the concentration in was 11.9 mg/L and out was 5.5 mg/L.  This was a 54% removal rate for the underground storage itself. There was a slight rise in total phosphorus and nitrogen coming out of the underground storage system, and the TSS removal was not very good, but this can easily be explained in that the concentrations coming out of the CST device were already very low, and practically irreducible.  As a whole, the CST device and the underground storage removed 98% of the TSS from the stormwater runoff.   The effluent concentration of 5.5 mg/L was lower than the effluent TSS concentration for many wastewater treatment plants.

There is more interesting data in the study that is of interest to scientists and regulators.  Over 10,000 pounds of sediment was removed from the CST vault. It was analyzed for particle size distribution and found to be 57% sand, 35.5% silt, and 7.5% clay.  In addition, the trash screen collected over 1,000 pounds of organic debris.  When the sediments were analyzed for nutrients and metals, the results were impressive.  Twenty-seven pounds of total nitrogen and ten pounds of phosphorus were removed. There were 4.1 pounds of Zinc, 1.7 pounds of copper, 1.7 pounds of barium, and lesser amounts of mercury, arsenic, cadmium, chromium, lead, selenium and silver. Added to these totals was material trapped on the coconut fiber filter.  The filter targets dissolved pollutants.  The test plan called for swatches of fresh fiber to be tested, and for swatches of used fiber to be similarly tested.  The filters were changed four times during the test.  Each quarter the fiber filter collected 14.9 grams of total nitrogen, 3.4 grams of total phosphorus, 6 grams of zinc, 1.7 grams of copper and lesser amounts of other dissolved metals.  The overall data indicate that while most pollutants are attached to the sediments, there are significant levels of dissolved pollutants that can be removed by filter media.

Summary

This is a true third party study conducted and paid for by others.  It clearly demonstrates that the CST vault by itself exceeded the NCDENR (North Carolina Department of Environment and Natural Resources) requirements.  This was a bit surprising in that the basin is a mixture of natural steep slopes and highly developed “downtown” area, and is subject to a lot of rainfall and high energy storm flows.  Furthermore, the
analysis method used was the TSS method, which is not friendly to hydrodynamic systems.  That analysis method is highly small particle biased, so it always shows lower removal results than the more accurate and dependable SSC analysis method when used to evaluate these types of devices.  Finally, the device is in a large basin that can push it to its hydraulic capacity more often, as a weir diverts the first 25 cfs to the system.
The result is the system runs faster and longer than it would on a smaller basin.  The built in design of the CST devices to avoid re-suspension obviously works well when you assess the data from this test.

What does this mean?  Well, the numbers are nice and it is always good to get one more “passing grade” on a field test, but there is more here that just meeting an arbitrary removal rate standard.  This is significant in that the Town of Highlands set out to clean up a stream, and to protect a mountain lake.  They also wanted to protect their businesses from flooding and stormwater damage.  To that end, they hired an excellent  engineering team, McGill Associates (Ashville, NC, www.mcgillengineers.com) to come up with a good plan that fit into the existing drainage  system, and was compatible with the unique character of the region.  The Town went one step further, and was determined to get a system that could be cleaned and maintained on a regular basis.  They anticipated what might happen with an underground storage facility if it was unprotected, and took steps to make sure it would work well into the future.  Most importantly, they committed to the cost of cleaning and maintaining the system. No one has to guess whether or not the system is working.  Literally tons of material, bound for their lakes and streams has been removed and disposed of properly.  These are results you can see and touch.  A short walk downhill to the manhole at the end of the system that returns the flow to the natural stream bed tells the story.  The water is gin clear and the structure is clean and free of debris.  The contrast to the manholes above the system is immediately obvious.  This is as it should be.