PERMIT RATIONALE FOR REISSUANCE
Lexington POTW
Holmes County
Lexington, Mississippi
Water NPDES No. MS0024601
October 9, 2025

I.	CLASSIFICATION –Municipal NPDES Minor 

II.	DESCRIPTION OF WASTEWATER – Domestic / Municipal Wastewater

III.	DESCRIPTION OF WASTEWATER TREATMENT – Wastewater is collected and treated by means of a conventional lagoon followed by disinfection.  No increase in flow is proposed within the application.  

IV.	RECEIVING WATERS – Black Creek.  The receiving water is located within the Yazoo River Basin and is classified as Fish and Wildlife.  Black Creek does not appear on the updated State of Mississippi 303(d) list impaired waterbodies. The facility is listed in a completed TMDL for pathogens. The effluent is disinfected to meet water quality criteria before discharging into the receiving waters, which complies with the conditions of the TMDL. The facility is also listed in a completed TMDL for nutrients. The facility is required to report nitrogen and phosphorus as required by the TMDL. At the point of discharge the 7Q10 for the receiving stream is 8.71 cfs.  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, 0.5 MGD or 0.775 cfs
		 Qr = Receiving stream 7Q10, 8.71 cfs.

IWC = {(0.775 cfs)/(0.775 cfs + 8.71 cfs)}*100 = 8.16%

V.	APPLICABLE GUIDELINES – Mississippi Admin Code pt. 6, Municipal Facility Guidelines (Technology), Wasteload Allocation (WLA), Federal Regulations (40 CFR)

VI.	CHANGES FROM PREVIOUS PERMIT – Chlorine limitations were reduced to reflect the change in the instream waste concentration (IWC). The IWC was changed to reflect a lower 7Q10 base flow in the receiving stream than previous permits were based. 

VII.	SUMMARY OF LIMITATIONS –
Parameter			Flow					Basis
Flow				0.5 MGD				Design
BOD5				30 mg/l (Max Monthly Average)	WLA
BOD (Percent Removal)	65% Minimum			40 CFR
Minimum DO			6 mg/l					WLA
NH3-N				Report					Previous Permit
TSS				90 mg/l (Max Monthly Average)	40CFR
TSS (Percent Removal)	65% Minimum			40 CFR
E. coli				126 Col./100 ml			MS Admin Code
Chlorine, Total Residual	0.135					MS Admin Code & IWC
pH				6.0 – 9.0 SU				MS Admin Code
Total Nitrogen			Report					TMDL
Total Phosphorus		Report					TMDL