*August 16, 2005*

There are instances when a BaySaver Separation System may be used as a pretreatment device for a wet pond; when environmental regulations require it; or when a stand alone physical separator is not enough to meet an 80% removal efficiency using solely this product. There are cost benefits associated with using BaySaver products in conjunction with other Best Management Practices (BMPs). In the following example we will use the Modified Rational Method to determine the water quality treatment volume for a project site requiring a wet pond and the coinciding result of putting a BaySaver System in front of that pond. The results will be based on the removal criteria established and/or accepted by the State of New Jersey Department of Environmental Protection for wet ponds and for the appropriate sized BaySaver Systems.

A hypothetical site of 20 acres is being developed in New Jersey with an impervious surface area of 10 acres and a runoff coefficient of 1. New Jersey requires that a stormwater quality design storm of 1.25 inches/2 hour variable rate rainfall event be used. This means that .625 inches/hour of rainfall must be used in conjunction with the modified rational method to determine what volume of stormwater will need to be treated on any given site.

Q = CIA

Q = (1.0) (.625) (10.0)

Q = 6.25 cfs

This flow rate then needs to be converted into a volume:

V = runoff rate x storm duration

V = (6.25 cfs) (2 hours) (3,600 sec/hour)

V = 45,000 cf

**Wet Pond Only: **

Assume the developer decides to use a wet pond without extended detention to treat the runoff from this site. In order to obtain an 80% removal efficiency, the pond needs to have a permanent pool volume equal to three times the water quality treatment volume according to the New Jersey Stormwater Best Management Practices Manual. If the pond has an average depth of 6 feet then the area necessary to obtain an 80% removal efficiency would be calculated as follows:

Pond area = [(45,000 cf) (3)]/6

Pond area = 22,500 sf or .52 acres

**Wet Pond and BaySaver Separator: **

When a BaySaver and wet pond are used in series, the removal rate of each BMP necessary to obtain an 80% removal will be 50% and 60% respectively. This is calculated as follows:

Upstream TSS load = 1.0

TSS removed by BaySaver = 1.0 x 50% = .5

Remaining TSS load after BaySaver = 1.0 - .5 = .5

TSS removed by Wet Pond = .5 x 60% = .3

TSS load downstream of Wet Pond = .5 - .3 = .2

Total TSS removed = 1.0 - .2 = .8 or 80%

Therefore, if the developer were to use a BaySaver in addition to a Wet Pond then the area of the pond necessary to treat that same volume of water would be reduced dramatically. According to the New Jersey Stormwater Best Management Practices Manual, the volume of a Wet Pond necessary to obtain a 60% TSS removal rate is 1.5 times the water quality treatment volume. Assuming the same average depth of 6 feet as stated above, the area of the wet pond may be calculated as follows:

Pond area = [(45000 cf) (1.5)]/6

Pond area = 11,250 sf or .26 acres

Approximately a quarter acre of the developer's property has been saved from monetary oblivion. What is the value of a quarter

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