PERMIT RATIONALE FOR REISSUANCE
Forest POTW
Scott County
Forest, Mississippi
Water NPDES No. MS0020362
April 2025

I.	CLASSIFICATION – NPDES Major Municipal

II.	DESCRIPTION OF WASTEWATER – Municipal Wastewater

III.	DESCRIPTION OF WASTEWATER TREATMENT – Municipal wastewater is collected and treated biologically by means of an activated sludge process. During periods of heavy rain excess municipal wastewater is routed through hydrographically controlled release (HCR) waste stabilization lagoons. The HCR lagoons discharges are included in the permit as Outfalls 002 and 003. Industrial wastewater is collected and treated through anaerobic lagoons followed by a sequential batch reactor system. The treated industrial wastewater is then combined with the treated municipal wastewater from the activated sludge process prior to chlorination and dechlorination.  The combined effluents are then discharged at Outfall 001. See Attachment No. 1 for a process flow diagram. No increase in flow is proposed within the application.

IV.	RECEIVING STREAM – Gordy Branch into Futchs Creek.  The receiving streams are located within the Pearl River Basin and are both classified as Fish and Wildlife.  See Attachment No. 2 for discharge location map.  At the point of discharge, neither of the receiving streams appear on the 2018 State of Mississippi Section 303(d) List of Impaired Water Bodies. The discharge is listed in a completed Total Maximum Daily Load (TMDL) for Organic Enrichment/Low Dissolved Oxygen for Tuscolameta, Tallabogue, and Shockaloo Creeks (June 2009). This facility has Biochemical Oxygen Demand, Total Nitrogen, and Total Phosphorous limitations in order to comply with the Organic Enrichment TMDL. The discharge is also listed in a completed Mercury TMDL (February 2004) and complies with the Mercury monitoring prescribed by the TMDL. The discharge from this facility should not further impair water quality.

The 7-day, 10-year minimum flow (7Q10) of the receiving stream is 0.0 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, 10 MGD or 15.48 cfs.
	QR = Receiving stream 7Q10, 0.0 cfs.

IWC = {(15.48 cfs)/(15.48 cfs + 0.0 cfs)}*100 = 100%

V.	APPLICABLE GUIDELINES – 11 Mississippi Admin Code pt. 6, Municipal Facility Guidelines (Technology), Wasteload Allocation (WLA, see Attachment No. 3), TMDL

VI.	CHANGES FROM PREVIOUS PERMIT –

Influent and effluent monitoring for Outfall 002 and Outfall 003 for Total Nitrogen and Total Phosphorous has been added in accordance with Major Municipal Facility Guidelines.

Phase II Ammonia Nitrogen limitations were added to the proposed draft permit, due to the updated ammonia toxicity standard.

The schedule of compliance states:

Phase II limitations shall apply as soon as the treatment system has demonstrated compliance with the Phase II limitations, but no longer than three (3) years after the issuance of the permit.

The permittee shall notify our office orally at least 24 hours prior to start-up and in writing no later than 48 hours after start-up.

Also, the permit requires the following:

The Permittee shall Submit engineering report: Due once within 6 months (180 days) after effective date. Said report shall outline the improvement activities needed to upgrade the wastewater treatment facility to comply with the site's Phase II limitations and shall include a proposed schedule for those improvement activities.

The Permittee shall Submit progress report: Due annually - every 1 year after effective date. Said report shall update the Permittee's progress in addressing needed upgrades to the wastewater treatment facility in order to meet the site's Phase II limitations. Said report should also include an updated schedule for completing these activities.

Based on the results of the WET testing, chronic bioassay requirements will be placed in the permit.

VII.	SUMMARY OF LIMITATIONS –

Outfall 001
Parameters	Value		Basis	
Flow	4.9 MGD	Design	
BOD5	8 mg/l (Monthly Avg)	TMDL	
TSS				30 mg/l (Monthly Avg)	Technology Based	
Ammonia Nitrogen (Phase I)	2.0 mg/l (Monthly Avg)	WLA	
Ammonia Nitrogen (Phase II)	1.37 mg/l (Monthly Avg)	WLA	
E. coli		126 Col./100 ml	MS WQS	
Dissolved Oxygen	6.0 mg/l	WLA	
pH	6.0 – 9.0 SU	MS WQS	
Residual Chlorine		0.011 mg/l – 0.019 mg/l	MS WQS	
Total Nitrogen	539.21 lbs/day (Monthly Avg)	TMDL	
Total Phosphorus	175.84 lbs/day (Monthly Avg)	TMDL	
Ceriodaphnia dubia	IC25 >100%	Admin Code	
Pimephales promelas		IC25 >100%	Admin Code	
Silver	0.00098 mg/l	Chem Screen, MS WQS	
Copper, Total Recoverable	0.011 mg/l (Monthly Avg)
0.016 mg/l (Weekly Max)	Chemical Translator & Hardness Study	
Total Hardness	Report	Chemical Translator & Hardness Study	

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

After reviewing two years of DMR data, Total Recoverable Copper demonstrated a reasonable potential to violate toxicity in the receiving stream. Total Recoverable Copper limits will remain in the permit based on the calculations in Attachment No. 7. The hardness was reduced to 50 mg/l and was adjusted to apply a chemical translator using total suspended solids.

Outfall 002
Parameters			Value					Basis
Flow				Report *				Design *
BOD5				30 mg/l (Monthly Average)		WLA
TSS				90 mg/l (Monthly Average)		Technology Based
Ammonia Nitrogen		5.0 mg/l (Monthly Average)		WLA
Dissolved Oxygen		2.0 mg/l				WLA
pH				6.0 – 9.0 SU				MS WQS
	Total Nitrogen				Report lbs/day 			WLA
	Total Phosphorus			Report lbs/day 			WLA

*Discharge flow must comply with the following equations
	
1.	At a stream flow greater than or equal to 10 cfs but less than 52 cfs
(10 cfs <= stream flow < 52 cfs)
Allowed discharge (MGD) = 0.35 x Stream Flow (cfs) – 2.3

2.	At a stream flow greater than or equal to 52 cfs but less than 675 cfs
 (52 cfs <= stream flow < 675 cfs)
Allowed discharge (MGD) = 0.536 x Stream Flow (cfs) – 12

3.   At a stream flow greater than or equal to 675 cfs 
Stream flow >= 675 cfs)
Allowed discharge (MGD) = 0.536 x Stream Flow (cfs) 

Outfall 003
Parameters			Value					Basis
Flow				Report *				Design *
BOD5				30 mg/l (Monthly Average)		WLA
TSS				90 mg/l (Monthly Average)		Technology Based
Ammonia Nitrogen		5.0 mg/l (Monthly Average)		WLA
Dissolved Oxygen		2.0 mg/l				WLA
pH				6.0 – 9.0 SU				MS WQS
	Total Nitrogen				Report lbs/day 			WLA
	Total Phosphorus			Report lbs/day 			WLA

*Discharge flow must comply with the following equations
	
1.	At a stream flow greater than or equal to 10 cfs but less than 20 cfs
(10 cfs <= stream flow < 20 cfs)
Allowed discharge (MGD) = 0.50 x Stream Flow (cfs) – 0.9

2.   At a stream flow greater than or equal to 20 cfs 
Stream flow >= 20 cfs)
 Allowed discharge (MGD) = 0.460 x Stream Flow (cfs) 

BIOASSAY AND CHECMICAL SPECIFIC EVALUATIONS

BIOASSAY

In accordance to State Regulation 11 Mississippi Admin Code pt. 6, VI.B.2b., Forest POTW 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:

February 2023
		Ceriodaphnia dubia		IC25 		=7.775%
		Pimephales promelas		IC25		>100%

		May 2023
		Ceriodaphnia dubia		IC25 		>100%
		Pimephales promelas		IC25		>100%

August 2023
		Ceriodaphnia dubia		IC25    	 	>100%
		Pimephales promelas		IC25		>100%

		November 2023
		Ceriodaphnia dubia		IC25 		>100%
		Pimephales promelas		IC25		>100%

The above submitted bioassays demonstrate a reasonable potential to violate Whole Effluent Toxicity based on the IC25 shown above being less than the IWC of the discharge (100%). Based on the results of the WET testing, chronic bioassay requirements will be placed in the 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 No. 4.  The reported values are then analyzed and compared to water quality criteria (see Attachment No. 5) to determine any possible toxic effect to the receiving waters.  A synopsis of these calculations is listed in Attachment No. 6.  A column by column description of the calculations in Attachment No. 6 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 Forest POTW, the chemical specific data results submitted with the reapplication indicated the reasonable potential for violation of the appropriate criteria (Copper) in Table III of Appendix D of CFR 122. See Attachment No. 6 for Chemical Specific Screening sheet and results.

Chemical specific analysis for Volatile Organic Compounds, Acid-Extractable Compounds, and Base-Neutral Compounds and Results

The following Volatile Organic Compounds were detected and evaluated for the potential to violate Human Health Criteria: 

Bromodichloromethane
Avg. Daily Concentration = 4.3 μg/L 

According to EPA’s National Recommended Water Quality Criteria, the Human Health Criteria (consumption of organism only) for the following compound(s) are shown below: 

Bromodichloromethane
Recommended Water Quality Criteria = 27 μg/L
 
Multiplying the average daily concentration above by 10 (due to a data set of less than 12 samples) then dividing by the mean annual flow of 5.02 cfs determines the reasonable potential of the compound’s concentration to violate the Human Health Criteria. 
[4.3 ug/L * 10 ÷ 5.02] = 8.57 μg/L
		
8.57 μg/L < 27 μg/L 

The above results demonstrate that the average concentration for bromodichloromethane does not present a reasonable potential to violate Human Health Criteria. 

Chloroform
Avg. Daily Concentration = 16.8 μg/L 

According to EPA’s National Recommended Water Quality Criteria, the Human Health Criteria (consumption of organism only) for the following compound(s) are shown below: 

Chloroform
Recommended Water Quality Criteria = 2000 μg/L
 
Multiplying the average daily concentration above by 10 (due to a data set of less than 12 samples) determines the reasonable potential of the compound’s concentration to violate the Human Health Criteria. 
[16.8 ug/L * 10] = 168 μg/L 
168 μg/L < 2000 μg/L 

The above results demonstrate that the average concentration for Chloroform does not present a reasonable potential to violate Human Health Criteria.

The chemical specific data results submitted had no detections for Base-Neutral Compounds or Acid Extractable Compounds.

ATTACHMENTS

Attachment No. 1 – Flow Diagram
Attachment No. 2 – Discharge Location Map
Attachment No. 3.a – Wasteload Allocation (Outfall 001)
Attachment No. 3.b – Wasteload Allocation (Outfall 002)
Attachment No. 3.c – Wasteload Allocation (Outfall 003)
Attachment No. 4 – Tabulated Metals Testing Data
Attachment No. 5 – MS State Water Quality Standards
Attachment No. 6 – Synopsis of Metals Results
Attachment No. 7 – Copper and Hardness Calculations

Attachment No. 1 – Flow Diagram

Attachment No. 2 – Discharge Location Map

Attachment No. 3.a – Wasteload Allocation (Outfall 001)

Attachment No. 3.b – Wasteload Allocation (Outfall 002)

Attachment No. 3.c – Wasteload Allocation (Outfall 003)

Attachment No. 4 - Tabulated Metals Testing Data

Attachment No. 5 - MS State Water Quality Standards

Attachment No. 6 - Synopsis of Metals Results

Attachment No. 7 – Copper and Hardness Calculations