PERMIT RATIONALE FOR REISSUANCE
West Rankin Utility Authority Wastewater Treatment Facility
Rankin County
Richland, Mississippi
Water NPDES No. MS0061743
January 2025

I.	CLASSIFICATION – NPDES Major Municipal

II.	DESCRIPTION OF WASTEWATER – Municipal Wastewater

III.	DESCRIPTION OF WASTEWATER TREATMENT – Wastewater is collected and treated by means of an activated sludge mechanical treatment plant followed by either chlorine or UV disinfection. There were no increase to effluent flow on the application.

IV.	RECEIVING STREAM – Pearl River.  The receiving stream is classified as Fish and Wildlife, and is in the Pearl River Basin. Pearl River has completed Total Maximum Daily Loads (TMDLs) for pH (December 2000), Pathogens (January 2012), and Nutrients (June 2009). The permit complies with the conditions of pH TMDL because the effluent is required to comply with water quality criteria before discharging. Total Nitrogen and Total Phosphorus limitations are included in the permit to comply with the nutrient TMDL. The facility will disinfect before discharging in order to comply with the pathogen TMDL.

The 7-day, 10-year minimum flow (7Q10) of the receiving stream is 277 cfs. The Instream Wastewater Concentration (IWC) at the point of the discharge is determined by the following calculation:

IWC = {(QW)/(QW + QR)}*100
	Where:
	QW = Design flow of the wastewater treatment facility, 20 MGD or 30.96 cfs.
	QR = Receiving stream 7Q10, 277 cfs.

IWC = {(30.96 cfs)/(30.96 cfs + 277 cfs)}*100 = 10.05%

V.	APPLICABLE GUIDELINES – 11 Mississippi Admin Code pt. 6, Municipal Facility Guidelines (Technology), Wasteload Allocation (WLA, see Attachment No. 1), TMDL

VI.	CHANGES FROM PREVIOUS PERMIT – Since facility does not utilize chlorine disinfection at this time, it may be used as back up disinfection. The following clause has been added to the proposed draft as L-1:

“Chlorination may be utilized by facility in the event the ultraviolet (UV) system is not available for disinfection. When chlorine is used as the method for disinfection, the facility must comply with the Chlorine, Total Residual limitations provided in the permit.  If chlorination is not being utilized, the permittee shall report NODI_CODE 9 (Conditional Monitoring - Not Required This Period) in the monthly Discharge Monitoring Report (DMR).”

VII.	SUMMARY OF LIMITATIONS –

Parameter			Value					Basis
Flow				20 MGD				Design
CBOD5			12 mg/l (Nov-Apr)		WLA 
CBOD5			6 mg/l (May-Oct)		WLA 
CBOD5 (Percent Removal)	85% Minimum		Technology
TSS				30.0 mg/l 			Technology
TSS (Percent Removal)		85% Minimum		Technology
NH3-N				2.0 mg/l 	(Nov-Apr)		WLA
NH3-N				1.7 mg/l 	(May-Oct)		WLA
Dissolved Oxygen		> 6.0 mg/l (Minimum)		WLA
E. coli				126 Col./100 ml 		WLA
Nitrogen, Total		1918 lbs/day		WLA
Phosphorus, Total		434 lbs/day	 		TMDL
pH				6.0 – 9.0 SU			WLA

The minimum and maximum values for Dissolved Oxygen in aeration unit and 30-Minute Sludge Settleability in aeration unit must be reported

See attached modeling data results Attachment No. 1.

BOD5, NH3-N and DO limits are based on the Wasteload Allocation and STREAM modeled, which accounts for oxygen depletion in the receiving stream. The toxicity limit for NH3-N was determined. For NH3-N the more stringent of the modeled limit and the toxicity limit is what is given in the Wasteload Allocation, to meet Water Quality Based Effluent Limitations (WQBELs).

BIOASSAY AND CHECMICAL SPECIFIC EVALUATIONS

BIOASSAY

The permit includes limitations requiring the facility to perform the testing required in order to perform the Whole Effluent Toxicity Testing, which will meet the requirements of the Form 2A application.   The applicant shall test in accordance with Short – Term Methods for estimating the Chronic toxicity of Effluents and Receiving Water to Freshwater organisms.  Short term chronic WET tests are to be performed on Ceriodaphnia dubia (invertebrates) and Pimephales promelas (vertebrates).  Monitoring shall be required once per quarter for a period of one year.  Monitoring shall begin within 90 days after the facility has been in operation for a period of one year.  The chronic toxicity value is equivalent to the facilities instream waste concentration (IWC) which is 10.05%.  Should any of the analysis demonstrate amounts that violate water quality standards a reopener clause has been included in permit. The facility submitted four (4) quarterly bioessays with the following results:

February 2024									
Ceriodaphnia dubia									IC25>40%							
Pimephales promelas									IC25>40%

June 2024
Ceriodaphnia Dubia									IC25>40%
Pimephales promelas									IC25>40%

August 2024
Ceriodaphnia Dubia									IC25>40%
Pimephales promelas									IC25>40%

November 2024
Ceriodaphnia Dubia									IC25>40%
Pimpephales promelas									IC25>40%

The results were greater than IWC; therefore, WET monitoring and limitations were not added to the proposed draft permit.

CHEMICAL SPECIFIC ANALYSIS

The permit includes limitations requiring the facility to perform the testing required in order to perform the chemical specific analysis screening, which will meet the requirements of the Form 2A application.  Monitoring shall be conducted twice per month for a six month period for total recoverable metals, free cyanide, phenol, pentachlorophenol, and hardness.  Sampling is to be conducted on the influent to and effluent form the treatment works. Additionally, monitoring shall be required for pollutants listed in Part D under the headings of volatile organic compounds, acid extractable compounds, and base neutral compounds.  Sampling for these pollutants shall be performed once per month for a three month period.  Sampling is to be conducted on the effluent only.  Monitoring shall begin within 90 days after the facility has been in operation for a period of one year.  Should any of the analysis demonstrate amounts that violate water quality standards a reopener clause has been included in permit.

VOLATILES, ACID-BASE NEUTRALS, AND ACID EXTRACTIBLES

The facility submitted three (3) samples of each compound of the Volitiles, Acid-Base Neutrals, and Acid Extractables. The lab sheets indicated no detects of the compounds.
ATTACHMENTS

Attachment No. 1 – Wasteload Allocation
Attachment No. 2 – MS State Water Quality Standards
Attachment No. 3 -Chemical Specific Analysis
Attachment No. 4- Toxicity Screen

-Attachment No. 1 – Wasteload Allocation

Attachment No. 2 - MS State Water Quality Standards

Attachment No. 3 Chemical Specific Analysis

Attachment No. 4 Toxicity Screen