Alexandra Everhart
18 March 2015
Professor White
English 202C
Bio Retention Filter Process Description & Definition
A major problem in today’s world especially in urbanized areas is the management of storm
water runoff. Rain or snow hits the sidewalks and streets and gathers all the chemicals and
pollutants on the pavement and then flows into our rivers and streams. To prevent the pollutants
from going into our water systems Best Management Practices (BMPs) have been installed.
These devices are put in to catch the runoff water before it goes into the water. Some different
types are sand filters, riparian buffers, tree box filters, and bio-retention filters. Described below
is how a bio-retention filter functions to slowly cleanse the contaminated water throughout a
series of filtration levels. As stated above most BMPs can be found in urbanized areas, but they
can also be found agricultural production areas. This may seem contradictory, however,
agricultural farms have large amounts of soil and water runoff that contains farming nutrients
like phosphorous and nitrogen. Once these chemicals come into our water eutrophication can
occur. Eutrophication can cause plant life to increase to such an extent that it shortens the life of
a water body and raises the amount of sedimentation in the water. BMP devices can prevent this
from happening.
Above Ground Level
Figure 1 – Bio Retention Filter Above Ground View
Inlets
A bio-retention filter has usually two sometimes three inlets where water can flow from. One is
a gentle sloping two to three feet grass filter strip making a ring around the filter. This helps to
decrease the velocity of the water flowing in and possibly catch any suspended solids before the
water reaches the mulch bed. An inlet can also be from a street of parking lot up gradient from
the filter that has been piped down to the bio-retention area. You can see these two inlets in
Figure 2 below.
Trees &Plants
In a bio-retention filter there is a significant amount of small trees, plants, and shrubs in a very
small area. The purpose of these plants is to help purify the water through evapotranspiration
and nutrient cycling. By drawing up the water with their roots the plants lessen the amount of
water that needs to be filtrated by the retention area simultaneously.
Evapotranspiration – is the sum of transpiration and evaporation. Transpiration is the water that
plants emit after the water has cycled through their system and evaporation is when water
because of temperature vaporizes into the atmosphere. Water being transpired is in a sense
filtrated by the plant is also helpful because it decreases the amount that flows intot the poding
area which will be described later.
Nutrient Cycling – is the use, transformation, movement and reuse of nutrients in an eco system.
This is necessary for the filtration because it is one more way excess nutrients are removed from
the water.
Mulch & Ponding Area
Mulch serves many purposes. It is an organic later that stimulates microbial activity and these
microbes break down pollutants, especially petroleum-based pollutants. This is why you hear of
microbes being used to help clean up oil spills. Also, the mulch protects the layer of soil
underneath from erosion. Erosion is not usually a significant issue with the bio-retention areas
because tend to be cut as a very shallow and small circular or rectangular valley. In the center,
there is a slight indentation. This is called the ponding area and it is so if there is a surplus of
storm water it can be held there until it can be infiltrated down through the mulch and soil.
Figure 2 – Bio Retention Filter Cross Sectional View
Under Ground Level
Soil Layer
Soil is one of the most important factors in a bio-retention filter that contributes to the removal of
contaminants. Clay is known for its ability to collect hydrocarbons, metals, and nutrients. Clay
is able to do this because it has the highest cation exchange capacity out of sand, silt, and clay.
cation exchange capacity is the soils ability to hold onto essential nutrients. Clay tends to be
negatively charged so it can “exchange” some of those anions to absorb positively charged
metals and hydrocarbons. Therefor, the soil is specifically engineered to have slightly more clay
than sand and silt because it can collect more positively charged pollutants. The soil layer is
about 4 feet deep at its minimum depth and is vital in supporting all of the plants and trees above
ground which help in removing excess nutrients from the runoff water through nutrient uptake
and nutrient cycling.
Figure 3 – Bio Retention Filter Side View
Sand Bed
In-between the sand bed and engineered soil layer there is a very thing 3 to 5 inch pea stone
layer. This is installed so that the filter layers under the ground do not mix to an extent where
they no longer serve their purpose. The sand bed is about one foot and is just another layer the
storm water must flow through to remove more contaminants. It has slightly more open pore
space than clay soil so this is where the water starts to flow more quickly.
Drainage Layer
After the water comes from the sand bed it will either slowly keep working it’s way downward
to ground water storage or an aquifer or it will be piped to a water body. These pipes help to
distribute the water evenly again through our water system. Because through the inlet structures
a lot of water is brought into the filter that would not have naturally flowed to the filtration area.
The pipes underneath the layers are important because as mentioned above they redistribute the
water. There are usually one to three pipes underlying the system. The sections of the pipes
right under the filter are made from a porous material so that water is able to flow into the pipe
from the entire filter. From there the pipes can lead to bigger piping systems which will
eventually take the water to a lake or river, or they can be smaller and just route the water to the
local stream. Figure 1 above does not show the piping system but in figure 2 below one can see
a pipe underlying the filtration bed.
References
1) http://www.lenntech.com/eutrophication-water-bodies/eutrophication-effects.htm
2) http://www.lakesuperiorstreams.org/stormwater/toolkit/bioretention.html
3) http://www.stormwatercenter.net/Assorted%20Fact%20Sheets/Tool6_Stormwater_Practi
ces/Filtering%20Practice/Bioretention.htm