35 Ill. Adm. Code 355.209
Calculation of Preliminary Effluent Limitation
Section 355
Section 355.209 Calculation
of Preliminary Effluent Limitation
a) The preliminary effluent limitation (PEL) is calculated in a
mass balance approach reflecting allowed dilution as referenced in Section
355.207:
WQS = [(Q
e
)(PEL)
+ (Q
d
)(C
d
)]/(Q
e
+ Q
d
)
or
PEL = [WQS(Q
e
+ Q
d
) - (Q
d
)(C
d
)]/Q
e
where:
WQS
=
applicable total ammonia
nitrogen water quality standard pursuant to Section 355.203
Q
e
=
effluent flow rate
Q
d
=
allowed mixing flow rate as
determined in accordance with the provisions of 35 Ill. Adm. Code 302.212(c)
C
d
=
background ammonia nitrogen
(as N) concentration in mixing water
Effluent flow
rate shall be selected to coincide with the critical stream flow condition used
to quantify allowed dilution. Typically this will be estimated to be the
average of the lowest three months average flow rate during the previous year
for domestic wastewater sources. For industrial and other wastewater sources
where flow rates are not directly correlated to climatic patterns, Q
e
will be estimated as the average of the highest three monthly average flow
rates. With either approach, Q
e
shall be modified when future flows
are expected to vary significantly from historical data.
b) The reasonable potential analysis shall be completed
separately for the winter and summer periods and for acute, chronic and
subchronic water quality standards. The Agency may subdivide summer or winter
periods into quarterly or monthly segments with analysis of reasonable
potential corresponding to those smaller time segments in individual permit
applications. WQBELs based on the acute water quality standard shall be
expressed as a daily maximum. WQBELs based on the chronic water quality
standard shall be expressed as a monthly average. WQBELs based on the
subchronic WQS shall be expressed as a weekly average.