What’s the Difference between a Clogged Biofilter and a Failed Biofilter?
One of the great challenges that Green Infrastructure faces is perception vs. reality. When someone drives up to a site and sees porous concrete that is raveling, permeable pavers that are silted in and shedding water or a biofiltration system that is holding water, we automatically want to believe it’s a product failure, when in fact so many other things could be at play.
The photo above is from a project that was completed in 2014. The weeds are out of control, there is water ponding around the overflow inlet. The first thing that comes to mind is that the system failed and isn’t performing like it was intended to. If you dig down deeper (literally), you will find a sandy soil beneath a layer of mulch clogged with silt and sediment. This sandy soil is engineered media designed to infiltrate and treat stormwater at 100″/hr, but it can’t do it’s job if there is an impermeable layer of silt and sediment on top of it.
During construction the energy dissipation component (rip rap), designed to slow down the flow of stormwater from the head wall, was never installed. When there is an outfall pipe 15 feet in front of any type of biofiltration or bioretention system, erosion issues will increase the risk of clogging.
Luckily this project isn’t a failure, just a clogged layer of mulch. The good news is that the clogging is on the surface, and not in the under drain. It’s much easier to restore the performance of a clogged biofilter when the mulch layer is full of sediment. If the clogging occurred subsurface, the entire system might have to be removed and replaced.
If the top layer of sediment clogging the mulch is removed, the underlying engineered media can be tested to determine the infiltration rate. When the biofiltration system was tested, the infiltration results met the specification of 100″/hr. The mulch layer trapped the sediment and prevented it from clogging the engineered soil below. The takeaway from this project is that if water quality is your goal, maintenance must be your number one priority.
Mistakes happen, especially with new technologies that most designers and contractors haven’t used. We have to consider all the reasons why something might fail (or in this case simply clog). These systems, are resilient, but like all stormwater quality systems, are filters. If you overwhelm the filter, just like the one in your car or your house, it will stop filtering.