Setting out to find information on stormwater inlet protection devices, which by the way is very boring and tedious, led me to a very interesting article.  The article caught my attention because of the title of the article, “Who built a beach on a stormwater pond?”

The article discussed a method of effectively removing some of the phosphorus out of the stormwater before it enters into a lake.  Phosphorus, in high amounts, is a nutrient when entering a body of water can cause an unsightly algae blooms and is detrimental to aquatic life because algae blooms  deplete the oxygen in the water column.

Phosphorus sources are fertilizers that are applied in excess in both agricultural and urban settings, animal waste, soil erosion and certain soaps and detergents.  Even wastewater when sewers or septic systems are not operating properly can add to the nutrient load.

The method is to have the stormwater filter through a beach like structure which contains a designed mixture of iron and sand.  The iron chemically attaches to the dissolved phosphorus.  The iron enriched sand filter was developed by the University of Minnesota’s St. Anthony Falls Laboratory.

The method is part of a treatment train.  When there is a storm event, stormwater crosses lawns and paved surfaces and picks up various pollutants and then enters into the storm drains, from the storm drains the water is then conveyed to the stormwater pond.  As the pond rises, solids such as soil will settle out, the stormwater then over flows into the iron enriched sand filter.  The water filters down through the mixture, where the phosphorus is removed, then enters into perforated pipe which then conveys the water to a ditch which enters the lake.

I found the article very compelling because Des Moines has such a big problem with algae blooms.  I wanted more information so I contacted Mitch Haustein the author of the article.  Here are some of the questions I asked and Mitch’s responses.

1)            How is iron mixed into the sand? By the contractor or done commercially?

The mixing procedure is critical to the success of the iron enhanced sand filter (IESF).  If the iron is poorly mixed in the sand, it will clump and reduce the available number of binding sites.  Poorly mixed iron can also result in a crust that will prevent water from passing through the IESF.  The iron-sand media was mixed commercially using a soil blending unit paired with a trommel screen, half of which was covered with a membrane to only provide mixing rather than traditional separation.  We collected multiple samples of the final mix and used magnets to remove and weigh the iron to ensure the percentage of iron was within the specification tolerance (i.e. 5-8% with a target of 6.5% by weight).  It is important to note that we found stockyard sand typically has a moisture content of 3-5% by weight.

2)            What gradation of iron did you use?  Is it expensive?

                The iron filings were specified to be Type ETI-CC-1004 (-8+50) as supplied by Connelly-GPM, Inc. (Chicago, IL) or an approved equal.  For the project highlighted in the article, the raw iron filings were approximately $1,500 per ton.  It is important to note that the original research conducted at the St. Anthony Falls Laboratory of the University of Minnesota used the iron filing mix from Connelly.  Also, please note for this project the Connelly filing mix was specified to be 6.5% by weight of the iron-sand media even though the Connelly mix is actually 80% iron.  It was verified with St. Anthony Falls Laboratory researchers that this is how the original experiments were conducted.

3)            Can iron itself become a pollutant?

I’ve been in close contact with Andy Erickson from St. Anthony Falls Laboratory at the University of Minnesota, who conducted much of the initial research on IESFs.  Based on the monitoring they have conducted, there is an initial pulse of fine iron dust that washes out of the filter shortly after installation, but the phenomenon is short-lived.  Actually, the IESFs they monitor have shown decreases in dissolved iron relative to the inflowing water after the initial pulse immediately following installation.

4)            What gradation of sand?

                The sand was specified to be washed Fine Filter Aggregate (Mn/DOT 3149.J.2).

5)            Is the filter system expensive?  What is the life expectancy?  Are you planning on doing more in your district or is this one your beta project and waiting for more results?

                When phosphorus is the target pollutant, our modeled reductions and cost estimates suggest the IESFs can be highly cost effective.  The project was originally identified in a stormwater retrofit analysis completed for the Golden Lake subwatershed.  The Anoka Conservation District has been completing these analyses in watersheds of impaired waterbodies to identify cost effective retrofit options to improve water quality.  Essentially, we’re striving to ensure the best project is installed in the best place for the best price.

Based on the research conducted at the St. Anthony Falls Laboratory, we assigned a 30-year lifespan to the filter.  The microcosm studies in the lab suggested sufficient binding sites for 30 years of phosphorus loading.  Monitoring results will of course guide management decisions in the field.  That being said, removing and replenishing the iron-sand media would not be an extraordinarily expensive maintenance activity.

We will have another IESF installed in the City of Fridley this year.  That project is similar, in that an existing pond is being retrofit with an IESF (the pond itself is also being enlarged because of the contributing drainage area).  As I mentioned, IESFs appear to be cost effective projects when phosphorus is the target pollutant, so we plan to continue pursuing funding for this innovative BMP until more cost effective options arise or if we discover the IESFs do not perform as expected.

Other elements that Mitch bought up that I did not ask about were:

• It is critical that the IESF dries between storm events to avoid anoxic conditions that would result in phosphorus release from the iron. Therefore, the IESF should include an EPDM liner to ensure no water seeps in due to high groundwater or from the pond itself.  An understanding of the downstream hydrology is also important to ensure backwater effects during extremely wet periods will not be an issue.
• This project specified marine-grade, treated plywood to be installed vertically along the pond-side edge of the filter to achieve a uniform, specified elevation across the entire length of the IESF.  This ensures that water enters the IESF evenly across the entire interface with the pond surface, rather than through low spots.
• Corrugated polyethylene pipe with slits and no sock was installed along the bottom of the trench and covered with washed pea rock.  The IESF media was then installed to the top elevation of the plywood.

Mitch was a great help for me in writing this first Blog.  You can read the article that Mitch wrote at the following link.

http://abcnewspapers.com/2016/01/09/column-who-built-a-beach-on-a-stormwater-pond/