Combined Sewer System Long Term Control Plan - DC Water

Recommended Control Plan
13.2.1 System Wide Controls: Low Impact Development - Retrofit
The objective of low impact development (LID) is to mimic predevelopment site hydrology by using
site design techniques that store, infiltrate, evaporate and detain runoff from rainfall events.
Examples of such facilities include biofilters, tree planting, rain gardens, sand filters, porous
pavement, storm water detention, rooftop greening and others. LID has the potential to reduce CSOs
by diverting runoff into LID facilities and thereby preventing that runoff from reaching surface
waters. Also, certain LID facilities (e.g. biofilters) can provide some treatment to runoff and reduce
the pollutant load in discharges to surface waters.
Low Impact Development Retrofit (LID-R) is the application of LID to the modification of an
existing site. Since most of the District is developed, LID-R appears to offer the practicable
approach for implementation of LID in the District. Public comments indicated a strong preference
for LID-R, and this section describes the LID-R measures that have been included in the LTCP.
Benefits of LID-R
The analyses conducted as part of the LTCP indicate that LID-R can reduce the magnitude and
frequency of CSOs. Generally, CSO reduction benefits of LID-R are in proportion to the quantity of
storm water that would be diverted from the receiving waters (e.g. Anacostia River) by the LID-R
measures. In order to achieve a high degree of CSO control, a large application rate for LID-R is
required.
In order to meet the requirements of the CSO Policy, the degree of CSO control proposed in the
LTCP is extremely high in that the controls are sized for large and intense storms. As a result, the
analyses indicate that application of LID-R by itself cannot be expected to provide the degree of CSO
control proposed in the LTCP. However, LID-R can be coupled with structural controls to reduce
CSOs or to reduce the size of capital facilities required for the degree of control proposed in the
LTCP.
In addition to CSO control, LID-R can provide other ancillary benefits. These include beautification
and aesthetic improvements, reduced cooling costs and reduction in pollutants from the separate
storm water system. Because LID diverts storm water from the collection system, it also has the
potential to reduce the total amount of flow treated by the BPWWTP. This would potentially
provide capacity to handle additional dry weather flow or increase treatment performance without
constructing additional facilities at the plant.
Challenges to Implementation
There are several challenges associated with the implementation of LID-R. These have been divided
into technical, institutional and regulatory issues below:
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