Illinois Administrative Code › Title 35 - ENVIRONMENTAL PROTECTION › Part 378 - EFFLUENT DISINFECTION EXEMPTIONS › Subpart C - FECAL COLIFORM DIE-OFF MODEL › Appendix B - Application of the Die-off Equation

Current through Register Vol. 48, No. 28, July 12, 2024
a) Sketch the receiving stream and the progression of higher
				  order streams it flows into, up to and including the major river basin. Major
				  river basins are listed in Appendix C. Also identify on your sketch:
1) Smaller streams which are tributary to the
				  receiving water below the point of discharge.
2) All point source dischargers.
3) All public and food processing water supply
				  intakes.
4) Water body reaches wherein primary contact
				  activities are feasible or known to be engaged.
b) Sources which discharge directly to receiving waters which
				  are obviously suitable for primary contact use and therefore applying for a
				  seasonal exemption only, do not need to assess downstream primary contact
				  potential or use. Sources which are applying for a year-round exemption must
				  carefully assess such potential or use for the entire affected reach of the
				  undisinfected discharge.
c) Subdivide downstream waters into segments where discharge
				  and stream cross-sectional area are relatively uniform. Segments will typically
				  begin at confluences with other streams. Number the segments and identify each
				  on the above sketch. Where available note stream mile numbers established by
				  U.S. Geological Survey (see Appendix C) for the receiving stream.
d) Establish discharge rates for each segment. If no
				  discharge data is available, the equations developed by the Illinois State
				  Water Survey (Appendix C) may be used. For waters protected to 200 fecal
				  coliform per 100 ml, the median discharge (50% recurrence frequency) shall be
				  utilized. For waters protected to 2000 fecal coliform per 100 ml, calculate
				  discharges for the 10%, 30%, 50%, 70%, and 90% recurrence frequencies.
e) Derive average velocities for all necessary recurrence
				  frequencies in each segment. In the absence of field measurements, velocity is
				  best estimated through the use of the Manning equation (Appendix D). For some
				  situations, equations developed by the Illinois State Water Survey (Appendix C)
				  may suffice; however, these equations tend to over-estimate velocity, so it may
				  be beneficial for a discharger to go to a more detailed analysis.
f) Assess the average concentration of fecal coliform
				  directly upstream of the source (Nu) and for all
				  significant tributaries listed in Part A. Data from Agency ambient monitoring
				  stations may be useful in some instances.
g) Assess the concentration of fecal coliforms in the
				  effluent prior to disinfection (No). An average over at
				  least 3 months is preferable, but a minimum of 4 samples in 30 days will be
				  acceptable. A conservative value of 400,000 fecal coliform per 100 ml may be
				  utilized when effluent specific data is not available.
h) Determine the appropriate die-off rate constant (k).
				  Available literature values for k range from 0.01 /hour to greater than 1.00
				  /hour. In the absence of stream-specific data, the following values may be
				  used: 0.06 /hour for the months May thru October, and 0.03 /hour for the months
				  November thru April. Stream assessments are preferred and may be necessary to
				  demonstrate compliance. (See Appendix E).
i) Calculate fecal coliform levels at intervals downstream
				  using the design average flow for the source, for all necessary recurrence
				  frequencies and values of k. Incorporate the contributions of additional
				  downstream sources as necessary. Compare the results to the required levels of
				  protection. (These levels are 200/100 ml for primary contact and 2000/100 ml
				  for water supplies).
j) In cases where the predicted level approximates the
				  required level of protection, the Agency will require additional
				  stream-specific information. Such information may include, but is not limited
				  to:
1) Die-off studies to determine k.
2) Dye tracer studies to determine V.
3) Stream surveying to determine Q.