PERMIT RATIONALE FOR REISSUANCE
Carthage POTW
Leake County
Carthage, Mississippi
Water NPDES No. MS0020061
May 2013

I.	CLASSIFICATION – Municipal NPDES Minor

II.	DESCRIPTION OF WASTEWATER – Domestic Wastewater

III.	DESCRIPTION OF WASTEWATER TREATMENT – Domestic wastewater is collected and treated by a conventional facultative lagoon.  The effluent of the lagoon is then discharged into a series of artificial wetlands for further treatment.  Disinfection is provided by means of chlorination.  Chlorine is then removed by use of sulfur dioxide.  No increase in flow is proposed within the application.  

IV.	RECEIVING WATERS – Pearl River.  The receiving stream is located within the Pearl River Basin and is classified as Fish and Wildlife.  The receiving at the point of discharge does not appear on the 2012 State of Mississippi 303(d) list of evaluated or impaired waterbodies.  At the point of discharge the receiving stream’s 7Q10 is 44.0 cfs.  The instream wastewater concentration at the point of discharge is determined by the following calculation:

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

	Where:
	Qw = Design flow of the wastewater treatment facility, 0.95 MGD or 1.47 cfs.
	QR = Receiving stream 7Q10, 44.0 cfs.

IWC = {(1.47 cfs)/(1.47 cfs + 44.0 cfs)} * 100 = 3.23 %

	Only one discharge to surface waters is described within the application with no increase in flow proposed.  The discharge is an existing source with no proposed increase in discharge flow, therefore the facility should not further impair water quality.  

V. 	APPLICABLE GUIDELINES – State of Mississippi Wastewater Regulations, WPC – 1 

VI.	SUMMARY OF LIMITATIONS –
	Parameter				Value		Basis
	Flow				0.95 MGD		Design
	BOD5				40 mg/l 		Previous Permit
	TSS				90 mg/l 		Technology Based
	Ammonia Nitrogen			12 mg/l 		MSWQS
	Dissolved Oxygen			> 6.0 mg/l 		MSWQS

	Parameter				Value		Basis
	Fecal Coliform
	May – October			200 Col./100 ml		MSWQS
	Nov. – April 				2000 Col./100 ml		MSWQS
	Chlorine, Total Residual 	0.34 mg/l		MSWQS
	pH				6.0 – 9.0 SU		MSWQS
	Nitrogen (total)			Report		WLA
	Phosphorous (total)			Report		WLA

BOD5, NH3-N and DO limits are based on the Wasteload Allocation and modeled for oxygen depletion in the receiving stream, and the toxicity limit for NH3-N and Cl2 were determined for the parameters that are potentially acutely and chronically toxic.  The more stringent of the modeled limit and the toxicity limit (for NH3-N) is what is given in the Wasteload Allocation, to meet Water Quality Based Effluent Limitations (WQBELs). 

	Chlorine Acute Limitation Calculation

Freshwater Acute Chlorine Numeric Criteria = 0.019 mg/l

Limitation Calculation = (Numeric Criteria) / (IWC)

0.019 mg/l / 0.0323 = 0.59 mg/l

Chlorine Chronic Limitation Calculation

Freshwater Chronic Chlorine Numeric Criteria = 0.011 mg/l

Limitation Calculation = (Numeric Criteria) / (IWC)

0.011 mg/l / 0.0323 = 0.34 mg/l