PERMIT RATIONALE FOR REISSUANCE
Isola POTW
Humphreys County
Isola, Mississippi
Water NPDES No. MS0020109
June 2024

I.	CLASSIFICATION – Municipal Minor

II.	DESCRIPTION OF WASTEWATER – Domestic Wastewater

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

IV.	RECEIVING WATERS – Directly to Lake Dawson.  At the point of discharge the receiving waters are located within the Yazoo River Basin and are classified as Fish and Wildlife.  At the point of discharge the receiving stream does not appear on the State of Mississippi 303 (d) list for impaired waterbodies.  The facility is denoted on the TMDL for Organic Enrichment, Nutrients and Sediment for the Big Sunflower River.  The permit has been updated to include reporting for Nitrogen and Phosphorus to comply with this TMDL. Only one discharge to surface water is described within the application with no increase in flow. 

At the point of discharge the 7Q10 for the receiving stream is 0.0 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 0.22 MGD or 0.341 cfs
	  Qr = Receiving stream 7Q10, 0.0 cfs.

IWC = {(0.341 cfs) / (0.341 cfs + 0.0 cfs)}*100 = 100%

V.	APPLICABLE GUIDELINES – 11 MS Admin Code pt. 6, Total Maximum Daily Load (TMDL), 2020 Wasteload Allocation (WLA), Previous Permit, Municipal Guidelines (Technology)

VI.	SUMMARY OF LIMITATIONS –

Parameter			   Value		Basis
Flow				   0.22 MGD		Design
BOD5					30 mg/l		Previous Permit
BOD5 % Removal		   65 % Minimum		Technology
Dissolved Oxygen			6.0 mg/l		Admin Code
TSS				   90 mg/l		Technology
TSS %  Removal		   65 % Minimum		Technology
Ammonia Nitrogen 			6  mg/l		WLA
E. Coli			 	  126 Col./100 ml		Admin Code
Chlorine, Total Residual	  0.011 mg/l		WLA
pH				  6.0 – 9.0 SU		Admin Code
Total Nitrogen			  Report		TMDL
Total Phosphorus		  Report		TMDL