PERMIT RATIONALE FOR REISSUANCE
Harrison County Utility Authority, Delisle Wastewater Treatment Facility
Harrison County
Pass Christian, Mississippi
Water NPDES No. MS0052574
July 2021

I.	CLASSIFICATION – NPDES Municipal Minor

II.	DESCRIPTION OF WASTEWATER – Domestic/Municipal Wastewater

III.	DESCRIPTION OF WASTEWATER TREATMENT – Wastewater is biologically treated by means of an activated sludge system followed by ultraviolet (UV) disinfection prior to discharge. Only one discharge to surface waters is described within the application with no increase in treated effluent flow proposed. 

IV.	RECEIVING WATERS – Unnamed Tributary of Delisle Bayou. The receiving waters are located in the Coastal Streams drainage basin and are classified as Fish and Wildlife. At the point of discharge, the receiving stream does not appear on the current State of Mississippi 303(d) List of Impaired Waterbodies. The discharge does appear on a completed Total Maximum Daily Load (TMDL) for Nutrients and Organic Enrichment/Low Dissolved Oxygen in listed Tributaries to St. Louis Bay (June 2009). The TMDL does not prescribe any new or reduced limitations for the discharge. The discharge also appears on a completed TMDL for Fecal Coliform in St. Louis Bay (July 2001). The facility utilizes UV disinfection, which complies with the conditions of the TMDL. 

At the point of discharge, the 7Q10 for the receiving stream is 0 cfs. The instream wastewater concentration of discharge can be determined by the following calculation:

IWC = {(QW)/(QW + Qr)}*100

Where, Qw = Permitted flow of the wastewater treatment facility, 0.8 MGD or 1.24 cfs
		 Qr = Receiving stream 7Q10, 0 cfs.

IWC = {(1.24 cfs)/(1.24 cfs + 0 cfs)}*100 = 100%

V.	APPLICABLE GUIDELINES – 11 Mississippi Admin Code pt. 6, Municipal Facility Guidelines (Technology), State of Mississippi Water Quality Criteria (MSWQC), Wasteload Allocation (WLA)

VI.	CHANGES FROM PREVIOUS PERMIT – 

E. coli has replaced fecal coliform as the bacterial indicator for all freshwaters in Mississippi designated as Recreation, Fish and Wildlife Support, and/or Public Water supply. 

Total Nitrogen and Total Phosphorus reporting requirements have been added to the draft permit based on the Wasteload Allocation for the discharge dated April 21, 2021.

Total residual chlorine limitations have been included in the draft permit in the event chlorine is utilized as a backup to the UV disinfection system. See Condition L-1 of the draft permit for applicability. 

VII.	SUMMARY OF DISCHARGE LIMITATIONS – 

Parameter			Value		Basis
Flow				0.8 MGD		Design
BOD5				10 mg/l 		WLA
BOD5 (Percent Removal)	85% Minimum		Technology
TSS				30 mg/l 		Technology
TSS (Percent Removal)	85% Minimum		Technology
Ammonia Nitrogen		2 mg/l 		WLA
Dissolved Oxygen		6.0 mg/l Minimum		WLA
E. coli			 	126 Col./100mL		MSWQC
pH				6.0 – 9.0 SU		MSWQC
Total Nitrogen			Report lbs/day		WLA
Total Phosphorus		Report lbs/day		WLA
Total Residual Chlorine	0.011 mg/l		MSWQC, IWC

Minimum and maximum values for influent pH, dissolved oxygen in the aeration basin, and 30-miunte sludge settleability in the aeration basin must also be reported.