PERMIT RATIONALE FOR REISSUANCE
DeSoto County Regional Utility Authority, Johnson Creek Wastewater Treatment Facility
Desoto County
Walls, Mississippi
Water NPDES No. MS0061271
February 2025

I.	CLASSIFICATION – NPDES Major Municipal 

II.	DESCRIPTION OF WASTEWATER – Domestic / Municipal Wastewater

III.	DESCRIPTION OF WASTEWATER TREATMENT – Currently wastewater is collected and treated biologically using screening, grit removal, biological treatment via activated sludge/oxidation ditch, UV disinfection, and post aeration prior to discharge. See Attachment 1 for a flow diagram. The current design capacity of the facility is 2.0 MGD. The facility proposes to expand the facility to a design capacity of 16 MGD by constructing two additional oxidation ditch treatment trains.  

IV.	RECEIVING WATERS 

Outfall 001

Johnson Creek. The receiving stream is located in the Yazoo River drainage basin and is classified as Fish & Wildlife. At the point of discharge, the receiving stream does not appear on the current 303(d) List of Impaired Waterbodies. The discharge does appear on a completed Total Maximum Daily Load (TMDL) for Biological Impairment due to Nutrients and Organic Enrichment/Low Dissolved Oxygen in Johnson Creek (June 2008). The draft permit contains limits in accordance with the allocation developed by the TMDL.

The 7-day, 10-year minimum flow (7Q10) of the receiving stream is 0.77 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, 2.0 MGD or 3.09 cfs 
QR = Receiving stream 7Q10, 0.77 cfs

IWC = {(3.09 cfs)/(3.09 cfs + 0.77 cfs)}*100 = 80.08 %

Outfall 002

Mississippi River. The receiving stream Mississippi River drainage basin and is classified as Fish & Wildlife. At the point of discharge, the receiving stream does not appear on the current State of Mississippi 303(d) List of Impaired Waterbodies. Also, there are no completed Total Maximum Daily Loads (TMDLs) that establish limitations on the discharge. The draft permit contains limits in accordance with the allocation developed by the TMDL.

The 7-day, 10-year minimum flow (7Q10) of the receiving stream is 107,000 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, 16 MGD or 24.8 cfs 
QR = Receiving stream 7Q10, 107,000 cfs

IWC = {(24.8 cfs)/(24.8 cfs + 107,000 cfs)}*100 = 0.02 %

V.	APPLICABLE GUIDELINES – 11 Mississippi Admin Code pt. 6 Chapter 1, 11 Mississippi Admin Code pt. 6 Chapter 2 (MSWQS), Wasteload Allocation (WLA, Attachment 3 and 7), Municipal Facility Guidelines (Technology), TMDL

VI.	CHANGES DONE DURING PERMIT REISSUANCE – 

No change from the previously modified permit. Outfall 001 will be removed from permit on next reissuance after facility decommissioned Outfall 001. As of now, the proposed draft will show both outfalls. Condition L-2 added to proposed draft permit.

Condition L-3 is added to the proposed draft permit and outlines the facility’s use of back up disinfection.

Condition L-1 is added to the proposed draft permit and outlines a requirement of submitting two (2) additional sample sets.

VII.	SUMMARY OF LIMITATIONS – 

Outfall 001 Johnson Creek

Parameter				Value				Basis
Flow					2.0 MGD			Design
	BOD5					10 mg/l 			Previous Permit
	TSS					30 mg/l 			Technology
	Ammonia Nitrogen			2 mg/l 				Previous Permit
	Dissolved Oxygen			> 6.0 mg/l 			Previous Permit
	Chlorine, Total Residual		0.014 mg/l 			MSWQS, IWC
E. coli					126 Col. /100 ml		MSWQS
	pH					6.0 – 9.0 SU			MSWQS
	Nitrogen (Total)			135.4 lbs/day			Johnson Creek TMDL 
	Phosphorus (Total)			13.8 lbs/day			Johnson Creek TMDL

Outfall 002 Mississippi River

Parameter				Value				Basis
Flow					16 MGD			Design
	BOD5					30 mg/l 			WLA
	TSS					30 mg/l 			Technology
	Ammonia Nitrogen			Report 				WLA
	Dissolved Oxygen			> 6.0 mg/l 			Previous Permit
	Chlorine, Total Residual		Report	 			MSWQS, IWC
E. coli					126 Col. /100 ml		MSWQS
	pH					6.0 – 9.0 SU			MSWQS
	Nitrogen (Total)			Report				WLA 
	Phosphorus (Total)			Report				WLA

BIOASSAY AND CHEMICAL SPECIFIC EVALUATION

BIOASSAY

In accordance with State Regulation 11 Mississippi Admin Code pt. 6, VI.B.2b., DCRUA, Johnson Creek WWTF is required to perform a Whole Effluent Toxicity (WET) evaluation as part of the renewal application. The applicant was to have tested in accordance with Short – Term Methods for estimating the Chronic Toxicity of Effluents and Receiving Water to Freshwater Organisms.  Short – term chronic WET tests were performed on Ceriodaphnia dubia (invertebrate) and Pimephale promelas (vertebrate).  The results of the wet tests are as follows:

NOEC = No Observable Effect Concentration

February 2024
		Ceriodaphnia dubia		NOEC		>80.08 %
		Pimephale promelas		NOEC		>80.08 %

April 2024
		Ceriodaphnia dubia		NOEC	 	>80.08 %
		Pimephale promelas		NOEC		>80.08 %

October 2024
		Ceriodaphnia dubia		NOEC		>80.08 %
		Pimephale promelas		NOEC		>80.08%

July 2018
		Ceriodaphnia dubia		NOEC		>80.08 %
		Pimephale promelas		NOEC		>80.08 %

All of the inhibition concentrations of the above submitted bioassays were greater than the largest predicted IWC of 80.08%. Based upon the results of the WET Testing, chronic bioassay requirements will not be placed in the draft permit.  

CHEMICAL SPECIFIC ANALYSIS

	Chemical specific analysis has been performed in accordance with State Regulations 11 Mississippi Admin Code pt. 6, V1.B1.  Municipalities shall determine the toxic characteristics of their wastewater by analyzing for the toxic pollutant listed in Table III of appendix D of 40 CFR 122.  The results are tabulated in Attachment 4.  The reported values are then analyzed and compared to water quality criteria (see Attachment 6) to determine any possible toxic effect to the receiving waters.  A synopsis of these calculations is listed in Attachment 5.  A column by column description of the calculations in Attachment 5 is hereby provided:

Column No. 1 – Maximum concentration.  The highest effluent reading of the parameter

Column No. 2 – The maximum concentration mixed with the receiving water IWC.  This is 			calculated by the following equation:

		{Maximum Concentration (Column 1) * IWC %} / 100

Column No. 3 – Acute allowable – Chemical Specific State Water Quality Criteria                                   
	Column No. 4 – Pass or Fail. If column 2 > column 3 water quality criteria is exceeded                                		and failure occurs.

Column No. 5 – Long term average (LTA) of all tests.  The summation of the 12 samples divided                                         by 12.

Column No. 6 – The long term average concentration mixed with the receiving water                                		 	IWC.  This is calculated by the following equation:

		{LTA Concentration (Column 5) * IWC%} / 100

Column No. 7 – Chronic allowable – Chemical Specific State Water Quality Criteria           

Column No. 8 – Pass or Fail. If column 6 > column 7 water quality criteria is exceeded                               			and failure occurs.

Column No. 9 – Human health determination.  Long term average (Column 5) is mixed                  		with the mean annual flow IWCMA .  This is calculated by the following equation:

{LTA Concentration (Column 5) * IWCMA %} / 100

Column No. 10 – Human health allowable – State Water Quality Criteria   Note: Organisms only                              column is used if receiving waters are not drinking water supply. 

	Column No. 11 – Pass or Fail.  If column 9 > column 10 water quality criteria is exceeded                       	         	 and failure occurs.
	
	Chemical Specific Analysis for Metals and Results

Major municipalities shall determine the toxic characteristics of their wastewater by analyzing for toxic pollutants listed in Table III of Appendix D of 40 CFR 122.  Metal analysis shall be for total recoverable metals.  Municipalities shall submit twelve influent and effluent samples collected.  Chemical specific limits shall be placed in a permittee’s permit if any of the parameters evaluated in the toxic screening procedure indicate the reasonable potential for violation of the appropriate criteria (acute, chronic, and human health). 

For DCRUA, Johnson Creek WWTF, the chemical specific data results of the ten (10) sample sets submitted with the reapplication did indicate no reasonable potential..

See Attachment 5 for Chemical Specific Screening sheet and results.

Chemical Specific Analysis for Volatile Organic Compounds, Acid – Extractable Compounds, and Base – Neutral Compounds and Results

The chemical specific data results submitted had no detects for all Volatile Organic Compounds, Base-Neutral Compounds, and Acid-Extractable Compounds.

ATTACHMENTS:

Attachment 1 – Flow Diagram
Attachment 2 – Location Map
Attachment 3 – Wasteload Allocation Johnson Creek
Attachment 4 – Chemical Specific Tabulated Test Results
Attachment 5 – Chemical Specific Screening
Attachment 6 – MS Water Quality Criteria
Attachment 7 – Wasteload Allocation Mississippi River

Attachment 1 – Flow Diagram

Attachment 2 – Location Map

Outfall 001

Plant Location

Attachment 3 – Wasteload Allocation Johnson Creek

Attachment 4 – Chemical Specific Tabulated Results

Attachment 5 – Chemical Specific Screen

Attachment 6 – MS Water Quality Criteria

Attachment 7 – Wasteload Allocation Mississippi River