PERMIT RATIONALE FOR REISSUANCE
Richton POTW
Perry County
Richton, Mississippi
Water NPDES No. MS0024686
May 2022

I.	FACILITY INFORMATION

Facility Name:     Richton POTW
Facility Address:  206 Dogwood Ave East 
 Richton, MS  39476    
Permit No.:  	      MS0024686
SIC:  4952
Permit Writer:	      Dmitriy Asanov 
EPD Branch:         Municipal and Private Facilities

II.	CLASSIFICATION – Municipal NPDES Minor

III.	DESCRIPTION OF WASTEWATER – Municipal Wastewater

IV.	DESCRIPTION OF WASTEWATER TREATMENT – Municipal wastewater is collected and treated biologically by means of a conventional lagoon, which consisted of one main cell and two wetland cells, followed by chlorination/dechlorination and post-aeration.  

V.	RECEIVING WATERS – Thompson Creek.  The receiving stream is located within the Pascagoula River Basin and is classified as Fish and Wildlife.  At the point of discharge the receiving stream does not appear on the most recent State of Mississippi 303(d) list of evaluated or impaired waterbodies. Nor does the facility appear on an established TMDL.
		At the point of discharge, the 7Q10 of the receiving stream is 4.6 cfs.  The 	instream wastewater concentration (IWC) 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, 0.33 MGD or 0.51 cfs
		QR = Receiving stream 7Q10, 4.6 cfs

	IWC calculation for the Richton POTW is as follows:

IWC = {(0.51 cfs)/(0.51 cfs + 3.6 cfs)} * 100 = 12.43 %

VI.	APPLICABLE GUIDELINES – 11 Miss. Admin Code pt. 6, 2022 Wasteload Allocation (WLA) , Municipal Guidelines (Technology)

VII.	SUMMARY OF LIMITATIONS –
	Parameter					Value			Basis
	Flow						0.33 MGD			Design
	BOD5						
		(May – Oct)				15 mg/l (Quarterly Avg.)		WLA
		(Nov. – April)			30 mg/l (Quarterly Avg.)		WLA
	TSS						30 mg/l (Quarterly Avg.)		Technology
	Dissolved Oxygen				> 6.0 mg/l (Quarterly Avg.)		WLA
	E. Coli						126/410 #col/100ml			Admin Code
	Chlorine, Total Residual		0.089 mg/L			See Calculation
	pH						6.0 – 9.0 SU			Admin Code
	NH3-N						10 mg/L			WLA
	Total Nitrogen					Report mg/L			WLA
	Total Phosphorus				Report mg/L			WLA
	Chromium Hexavalent	   0.011 mg/L			See Calculation
	Zinc, Total Recoverable 		0.65 mg/L			See Calculation
	Ceriodaphnia dubia				12.43% Minimum			Application, IWC
	Pimephales promelas			12.43% Minimum			Application, IWC

BOD5, NH3-N and DO limits are based on the wasteload allocation and modeled for oxygen depletion in the receiving stream.  The toxicity limit for NH3-N was determined for the parameters that are potentially acute and chronic toxicity.  The more stringent of the modeled limits and the toxicity limit (for NH3-N) is what is given in the wasteload allocation, to meet water quality based effluent limitations (WQBELs).  

	Total Residual Chlorine Calculations

Limitation Calculation = (Numeric Criteria)/(IWC)

	IWC = 12.43 %
	
	Chlorine Acute Limitation Calculation

	Chlorine Acute Freshwater Stream Standard = 0.19 mg/l

(0.019 mg/l)/(0.1241) = 0.15 mg/l

	Chlorine Chronic Limitation Calculation

	Chlorine Chronic Freshwater Stream Standard = 0.011 mg/l

(0.011 mg/l)/(0.1241) = 0.089 mg/l

VIII.	WHOLE EFFLUENT TOXICITY AND CHEMICAL SPECIFIC EVALUATION

BIOASSAY

In accordance to 11 Mississippi Admin Code, pt. 6, VI.B.2b, Richton POTW is required to perform Whole Effluent Toxicity (WET) tests as part of the renewal application.  The applicant should 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 were performed on Ceriodaphnia dubia (invertebrate) and Pimephales promelas (vertebrate).
The results of the wet tests are as follows:

December 2020
Ceriodaphnia dubia 	IC25 > 49.64 %
Pimephales promelas 	IC25 > 49.64%

March 2021
Ceriodaphnia dubia 	IC25 > 49.64%
Pimephales promelas 	IC25 > 5.47 %

June 2021
Ceriodaphnia dubia	 IC25 > 49.64%
Pimephales promelas 	IC25 >  49.64 %

September 2021
Ceriodaphnia dubia 	IC25 > 49.64 %
Pimephales promelas 	IC25 > 49.64 %

Based on the results of the WET testing, the inhibition concentration was below the predicted IWC in March of 2021. As a result, bioassay requirements will be placed in the permit.

CHEMICAL SPECIFIC ANALYSIS

Chemical specific analysis has been performed in accordance with State Regulation 11 MS Admin Code, pt. 6, VI.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 of this analysis are tabulated as Attachment #4.  The tabulated results are then analyzed and compared to water quality criteria to determine any possible toxic effect to the receiving waters.  A synopsis of these calculations is listed in Attachment #5. 

Chemical Specific Analysis for Metals, Cyanide and Total Phenols

The chemical specific data results submitted with the reapplication indicated a reasonable potential for violation of the appropriate criteria (shown below) found in Table III of Appendix D of CFR 122.
	-Total Recoverable Zinc—acute
	-Chromium Hexavalent—acute, chronic 

Based on the results of the Chemical Screening Evaluation, limitations for Zinc (Total Recoverable) and Chromium Hexavalent have been placed in the draft permit.

The following limitations will be added to the permit for Total Recoverable Zinc:
	Maximum Monthly Average:	0.65 mg/L
	Maximum Weekly Average:	0.65 mg/L
The following limitations will be added to the permit for Chromium Hexavalent:
	Maximum Monthly Average:	0.011 mg/L
	Maximum Weekly Average:	0.016 mg/L

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

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

A column by column description of the calculations in attachment no. 4 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.

Attachment #1—Wasteload Allocation
Attachment #2—Discharge Point
Attachment #3—Process Flow Diagram
Attachment #4—Chemical Specific Screening Calculations
Attachment #5—Toxicity Screening
Attachment #6—MS Water Quality Criteria

Attachment #1

Attachment #2

Attachment #3

Attachment #4

Attachment #5

Attachment #6