PERMIT RATIONALE FOR MODIFICATION
Calhoun City POTW
Calhoun County
Calhoun City, Mississippi
Water NPDES No. MS0028134
December 2020

I.	CLASSIFICATION – NPDES Minor Municipal

II.	DESCRIPTION OF WASTEWATER – Municipal Wastewater

III.	DESCRIPTION OF WASTEWATER TREATMENT – Municipal wastewater is collected and treated by means of two conventional lagoons followed by a polishing cell thence a storage cell. The system operates as a hydrographically controlled release (HCR). Also, the discharge shall be automatically controlled between 0 and 0.32 MGD in order to maintain the river’s dissolved oxygen concentration of 5.0 mg/l. No increase in flow is proposed within the application. 

IV.	PURPOSE OF MODIFICATION – The facility’s discharge is listed on the Total Maximum Daily Load For Nutrients and Organic Enrichment / Low Dissolved Oxygen For the Yalobusha River. The TMDL applies a 2 mg/l monthly average limitation for Ammonia Nitrogen (NH3-N), which is applicable to the previously modeled minimum flow condition of >3.32 cfs and dilution ratio of 6.7:1. The HCR was remodeled under minimum flow condition of >5 cfs and no greater than 10:1 dilution ratio (Attachment 1).

This modification changes the minimum flow condition from >3.32 cfs to >5cfs for Yalobusha River and the dilution ratio from 6.7:1 to no greater than 10:1. The model under these flow conditions allows for a 14 mg/l Ammonia-Nitrogen (NH3-N), which will replace the 2 mg/l limitation in the existing permit.

V.	RECEIVING WATERS – Yalobusha River. The receiving water does not appear on the updated State of Mississippi 303(d) List of Impaired Waterbodies. The facility appears in a completed TMDL for Nutrients and Organic Enrichment/ Low DO. The limits proposed in this draft permit reflect the limits suggested in the TMDL. The receiving water is located in the Yazoo River drainage basin, and is classified as Fish & Wildlife. 

VI.	APPLICABLE GUIDELINES – 11 MS Admin Code pt. 6, 2020 Wasteload Allocation (Attachment 1), and Total Maximum Daily Load (TMDL)

VII.	SUMMARY OF DISCHARGE LIMITATIONS – 
Parameter			Value			Basis
Flow				0.32 MGD			Design
BOD5
-	Effluent		30 mg/l (Monthly Average)		WLA
-	Mass			80.0 lbs/day 
BOD5 (Percent Removal)	65% (Minimum)			Technology
TSS
-	Effluent		90 mg/l (Monthly Average)		WLA
-	Mass			240.0 lbs/day			
TSS (Percent Removal)	65% (Minimum)			Technology
pH				6.0 – 9.0 SU			MSWQS
Ammonia Nitrogen 		14 mg/l (Monthly Average) 		WLA
Dissolved Oxygen		6.0 mg/l (Minimum)			WLA
Total Nitrogen			30.7 (Annual Avg)			TMDL
Total Phosphorus		13.9 (Annual Avg.)			TMDL 

Permittee shall Submit a report: Due annually by the 28th of January detailing date, time, and stream stage when the discharge began; date, time, and stream stage when the discharge ceased; and the volume of wastewater released the previous year.  

VIII.	CHANGES TO PREVIOUS PERMIT – 

The minimum flow for discharge has been updated to >5 cfs from >3.32 cfs (2.14 MGD). The previous ratio of minimum river flow to discharge flow has been changed from 6.7:1 to 10:1.

The Ammonia-Nitrogen (NH3-N) limitation has been changed from 2 mg/l to 14 mg/l.

Attachment 1: 2020 Wasteload Allocation