PERMIT RATIONALE FOR REISSUANCE
South Gate Sewer District
Lowndes County
Columbus, Mississippi
Water NPDES No. MS0052728
February 21, 2023

I.	CLASSIFICATION –Municipal NPDES Minor 

II.	DESCRIPTION OF WASTEWATER – Municipal Wastewater

III.	DESCRIPTION OF WASTEWATER TREATMENT – Domestic wastewater is collected and treated through a 3-cell lagoon.  

IV.	RECEIVING WATERS – Tombigbee River.  The Tombigbee River does appear on the current 303(d) List of Impaired Waterbodies. The receiving stream for the treatment facility has  a TMDL placed on it for sediments. This waterbody is in the Tombigbee River drainage basin.  Tombigbee River is classified as Fish and Wildlife.  At the point of discharge the 7Q10 for the receiving stream is 98.5 cfs.  All the limitations of this permit complies with the TMDL. The instream wastewater concentration at the point of discharge can be determined by the following calculation:

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

	Where, Qw = Design flow of the wastewater treatment facility, 0.16 MGD or 0.248 cfs
		 Qr = Receiving stream 7Q10, 150.0 cfs.

IWC = {(0.248 cfs)/(0.248 cfs + 98.5 cfs)}*100 = 0.25 %

V.	APPLICABLE GUIDELINES – 11 MS admin code part 6

VI.	SUMMARY OF LIMITATIONS –
Parameter			Flow				Basis
Flow				0.16 MGD			Design
BOD5				30 mg/l 				Modeled
BOD (Percent Removal)	65% Minimum			Technology
Minimum DO			6 mg/l				Modeled
TSS				90 mg/l				Technology
E. Coli				126 Col./100 ml			Modeled
Chlorine, Total Residual	<1.0 mg/l			MSWQS & IWC, Modeled
pH				6.0 – 9.0 SU			MSWQS
Total Nitrogen			Report (lbs/day)			WLA
Total Phosphorus		Report (lbs/day)			WLA