MIL-HDBK-1005/16
to each classification. These standards are then used to develop
the effluent limits for the discharger.
The inclusion of water quality-based effluent limits in
the permit is based on a review of the effluent characterization
presented in the discharger's permit application (EPA Form 2C).
This review, conducted by the permit writer, assesses the
presence of compounds that could violate the water quality
standards. For these compounds, permit limits will be identified
wherever possible.
2.3.2.1
Waste Load Allocation. Most NPDES permits include
limits on oxygen-demanding substances (such as carbonaceous
biochemical oxygen demand [CBOD] and ammonia). Development of
these limits is typically based on a waste load allocation for
the receiving stream. Stream modeling is used to assess the
assimilative capacity of the stream based on the applicable
dissolved oxygen (DO) standard. This capacity is then allocated
among all the dischargers in the area. Generally, some portion
of the stream's capacity is reserved for future dischargers.
Waste load allocation modeling typically consists of a
desktop effort for small discharges and a calibrated and verified
model based on field measurements for larger discharges.
Modeling can be performed by the discharger or by the state
agency. Regardless of who performs the modeling, the results
receive a detailed review by both the state and the EPA.
Typically, these results are put out for public comment. In many
cases, the public comment period is concurrent with the public
notice for the NPDES permit.
2.3.2.2
Chemical-Specific Criteria. Water quality-based
effluent limits can be based on chemical-specific criteria from
the water quality standards (such as for metals or toxics) or on
general narrative criteria. Specific criteria are used to
develop effluent limits, and in many cases an allowance for
dilution in the receiving stream is provided. Typically, some
portion of the 7Q10 low-flow (the seven-day low stream flow
projected to recur every ten years) for the receiving stream is
used for dilution purposes. Background concentrations in the
receiving stream must also be considered in these calculations.
Where the 7Q10 low-flow is zero, the criteria will apply at the
point of discharge, prior to any dilution. In these cases it may
be more economical to go to zero-discharge systems, reuse, or
alternate discharge points.
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