Illinois Administrative Code › Title 35 - ENVIRONMENTAL PROTECTION › Part 375 - COMBINED SEWER OVERFLOW EXCEPTION CRITERIA AND FIRST FLUSH DETERMINATION › Subpart D - FIRST FLUSH DETERMINATION › Section 375.403 - Field Studies

Current through Register Vol. 48, No. 28, July 12, 2024
a)
				First flush volumes may be determined through direct field measurement at a
				point of overflow, modeling of the collection system or equally accurate
				techniques. Either approach is dependent on a field monitoring program as
				described in (c) below. Commonly, first flush for the design storm is
				determined by direct measurement of flush rate and volume for several storms of
				varying intensity and extrapolating to the design storm (1.2 in/hr.).
b) Detailed monitoring may be limited to less
				than the total number of overflow locations in the system when monitoring all
				locations would be impossible or would not be helpful in demonstrating the
				capabilities of the system. Sampling sites should be selected in order to
				monitor the largest discharges in terms of loading. Consideration shall also be
				given to the different land use patterns, population densities, and sewer
				system characteristics tributary to a given overflow site within the study
				area.
c) General guidance for field
				measurement includes:
1) A minimum of 3
				storms should be monitored for each sampling site.
2) There shall be a least one rain gauge
				located within the study area. For systems serving an area of over 2 squares
				miles, multiple gauges shall be used and located to achieve gauge spacing of 2
				miles or less throughout the study area. Rain gauges should be on the
				continuous recording type, otherwise manual readings should be recorded on
				intervals of 10 minutes or less. If only one gauge is used it should be located
				near the center of the study area; multiple gauges shall be located so as to
				identify variations in storm intensity across the study area.
3) Flow monitoring shall be initiated at the
				onset of a storm and continue through and beyond the sewer flushing period.
				Flow meters may be located in interceptor manholes, diversion structures,
				outfall sewers or other locations where dependable operation is assured.
				Caution should be taken to protect instrumentation from flooding, corrosion,
				excess humidity, vandalism or other factors which jeopardize either the
				equipment or reliability of the data.
4) Water samples should be collected at the
				onset of the storm and at 10 or 15 minute intervals thereafter. In most
				instances a moderate to heavy storm will fully flush the system within two
				hours. Most automatic discrete samplers have adequate capacity to contain the
				entire flush period with 10 minutes sampling frequency. Again, caution should
				be taken to protect against flooding, corrosion, or excess humidity that may
				cause malfunctioning or failure of the mechanical or electronic components of
				the sampler. Samples should be analyzed for BOD5 as
				defined in 35 Ill. Adm. Code
				304.120(a),
				total and volatile suspended solids with sample preservation, and analysis
				consistent with Standard Methods 15th Edition
				(1980).
5) Overflow points that are
				not subject to direct flow and quality monitoring should be inspected during
				the storm period to determine if the overflows are functioning and to estimate
				the rate and duration of overflow.