PERMIT RATIONALE FOR REISSUANCE
Grenada POTW
Grenada County
Grenada, Mississippi
Water NPDES No. MS0020397
March 25, 2026

I.	CLASSIFICATION – Municipal NPDES Major

II.	DESCRIPTION OF WASTEWATER – Municipal/Domestic Wastewater

III.	DESCRIPTION OF WASTEWATER TREATMENT – Wastewater is collected and treated biologically by means of two sequential aerated lagoons, followed by a facultative lagoon. Before discharging, the facility uses chlorine disinfection, and sulfur dioxide dechlorination. The facility also utilizes a lagoon for flow equalization. No increase in flow is proposed within the application.  

IV.	RECEIVING STREAM – Yalobusha River.  The receiving stream is located within the Yazoo River Basin, and is classified as Fish & Wildlife.  See the application for discharge location map.  At the point of discharge the receiving stream not does appear on the current State of Mississippi 303(d) list of impaired waterbodies.  The permit is listed in a TMDL for total nitrogen and total phosphorus for selected large rivers in the Delta. All the limitations complies with the TMDL. 
	
	The 7-day, 10 year minimum flow, 7Q10, for the Yalobusha River at the point of discharge is 55.0 cfs.  Based upon the 7Q10 at the point of discharge the instream wastewater concentration (IWC) at the point of discharge is 8.73%.  Only one discharge to surface waters is described within the application with no increase in flow proposed.  The discharge from this facility should not further impair water quality.  

V.	APPLICABLE GUIDELINES – Mississippi Wastewater Regulations (11 MS admin code part 6), State of Mississippi Water Quality Criteria & “Wastewater Regulations for National Pollutant Discharge Elimination System Permits, Underground Injection Control Permits, State Permits, Water Quality Based Effluent Limitations , Water Quality Criteria (MSWQC), Water Quality Certification” applicable. Wasteload Allocation (WLA), Total Maximum Daily Load (TMDL)

VI.	SUMMARY OF LIMITATIONS –
	Parameter						Value			Basis
	Flow						3.4 MGD			Design
	BOD5						30 mg/l (Monthly Avg.)		WLA	
	BOD5 (Percent Removal)			65% Minimum			40 CFR
	TSS						90 mg/l (Monthly Avg.)		MSWQS
	TSS (Percent Removal)			65% Minimum			40 CFR*
	NH3-N						
•	May-Nov						14 mg/l (Monthly Avg.)		WLA
•	Dec-Apr						19 mg/l (Monthly Avg.) 		WLA
	Dissolved Oxygen					> 6.0 mg/l (Monthly Avg.)	WLA
	E. Coli						126 Col./100 ml			11 MS admin code part 6
	pH						6.0 – 9.0 SU			11 MS admin code part 6
	Chlorine, Total Residual			0.13/ 0.22 mg/l			IWC
	Total Nitrogen					326.3 lbs/day (Monthly Avg.)	TMDL**
	Total Phosphorus				147.5 lbs/day (Monthly Avg.)	TMDL**
Ceriodaphnia dubia	                        IC25 ≥ 8.73% 			MSWQS
Pimephales promelas				     IC25 ≥ 8.73% 			MSWQS
Copper, Total Recoverable			     Report mg/l			MSWQS

	See attached modeling data results attachment no. 1.

Wasteload Allocation (WLA), Whole Effluent Toxicity (WET), Instream Waste Concentration (IWC), Mississippi Water Quality Criteria (11 MS admin code part 6), Total Maximum Daily Load (TMDL)

VII.	CHANGES FROM PREVIOUS PERMIT – 

Because the failure occurred only after the 10X multiplier was applied, monitoring and reporting limitations for Copper, Total Recoverable will be included in the draft permit.

WHOLE EFFLEUNT TOXICITY AND CHEMICAL SPECIFIC EVALUATION

WHOLE EFFLEUNT TOXICITY EVALUATION

In accordance to State Regulation WPC-1 VI.B.2b., a whole effluent toxicity evaluation was performed 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 test were performed on Ceriodaphnia dubia (invertebrate) and Pimephale promelas (vertebrate).  The results of the wet tests are as follows:

March 2023
			Ceriodaphnia dubia		IC25    		>35%
			Pimephales promelas		IC25   		 >35%

			September 2023
			Ceriodaphnia dubia		IC25   		=30.16%
			Pimephales promelas                         IC25                   >35%
    
			November 2023

                                        Ceriodaphnia dubia		IC25    		>35%
			Primephales promelas 		IC25   		 >35%
                                        April 2024 
                                        Ceriodaphnia dubia		IC25    		=2.447%
			Pimephales promelas		IC25   		 >35%

Based on the results of the WET testing, inhibition concentrations were not greater that the predicted IWC of 8.73%.  Chronic bioassay requirements will be placed within the permit.

Chemical Specific Analysis

Chemical specific analysis has been performed in accordance with State Regulations WPC-1 V1.B1.  
wastewater.by analyzing for the toxic pollutant listed in Table III of appendix D of 40 CFR 122.  The results are tabulated in attachment No. 4.  The reported values are then analyzed and compared to water quality criteria (attachment No. 6) to determine any possible toxic effect to the receiving waters.  A synopsis of these calculations is listed in attachment No. 5.  A column by column description of the calculations in attachment No. 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 II of Appendix D of 40 CFR 122.  Metal analysis shall be for total recoverable metals.  Municipalities shall submit twelve influent and effluent samples collected.  Grenada POTW failed to submit the data as described above, submitting at this time only seven  (7) of the required twelve samples. Multiplying the average and maximum daily concentrations from Attachment 5 by 10 (due to a data set of less than 12 samples) determines the reasonable potential of the compound’s concentration to violate the Water Quality and Human Health Criteria. For Grenada POTW, the chemical specific data results submitted with the reapplication indicated a reasonable potential for violation of the criteria in Table III of Appendix D of CFR 122 for the following pollutants:

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 Volatile Organic Compounds, Acid-Extractable Compounds and Base-Neutral Compounds.

Attachments
Attachment 1	See Application
Attachment 2	See Application
Attachment 3	Waste Load Application 
Attachment 4	Tabulated Metals Results 
Attachment 5 	Synopsis of Metals Results 
Attachment 6 	MS State Water Quality Criteria 

Attachment 3

Attachment 4

Attachment5

Attachment 6