PERMIT RATIONALE FOR REISSUANCE
Harrison County Utility Authority, Riverhills Wastewater Treatment Facility
Harrison County
Gulfport, Mississippi
Water NPDES No. MS0061191
March 2024

I.	CLASSIFICATION – NPDES Minor Municipal

II.	DESCRIPTION OF WASTEWATER – Domestic Wastewater 

III.	DESCRIPTION OF WASTEWATER TREATMENT – Domestic wastewater is collected and treated biologically by means of an activated sludge/oxidation ditch process. The proposed wastewater treatment scheme includes screening, grit removal, biological treatment by oxidation ditch, followed by chlorination/dechlorination, and post aeration prior to discharge. The treatment capacity for the Riverhills Wastewater Treatment Facility is 0.5 MGD. 

IV.	RECEIVING WATERS – Little Biloxi River. The receiving stream is located within the Pascagoula River drainage basin and is classified as Fish & Wildlife. At the point of discharge, the receiving stream does not appear on the current State of Mississippi 303(d) List of Impaired Waterbodies nor does the discharge appear on a completed Total Maximum daily Load (TMDL). 

The 7Q10 for the Little Biloxi River at the point of discharge is 2.6 cfs. The instream wastewater concentration at the point of discharge can be determined using the following calculation:

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

Where:
Qw = Design flow of the wastewater treatment facility, 0.5 MGD or 0.77 cfs.
Qr = Receiving stream 7Q10, 2.6 cfs

IWC = {(0.77 cfs)/(0.77 cfs + 2.6 cfs)} * 100  =  23 %

V.	APPLICABLE GUIDELINES – 11 Mississippi Admin Code pt. 6, State of Mississippi Water Quality Criteria (MSWQS), Municipal Facility Guidelines (Technology), 2024 Wasteload Allocation (WLA)

VI.	SUMMARY OF DISCHARGE LIMITATIONS – 
Parameter			Value		Basis
Flow 				0.5 MGD		Design
BOD5				10 mg/l 		WLA
TSS				30 mg/l 		Technology
Ammonia Nitrogen		2 mg/l 		WLA
Dissolved Oxygen		≥ 6.0 mg/l		WLA
Fecal Coliform		200 Col./100ml		MSWQS
Chlorine, Total Residual	0.05 mg/l		WLA
pH				6.0 - 9.0 SU		MSWQS
Total Nitrogen			Report lbs/day 		WLA
Total Phosphorus		Report lbs/day		WLA

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

If practical, the permittee shall discharge treated effluent from an appropriately designed diffuser for Outfall 001 to provide appropriate dispersion of the effluent to the receiving waters.  In low flowing receiving streams where diffusers would not be practical, discharge energy dissipation shall be provided.