PERMIT RATIONALE FOR REISSUANCE
Columbia POTW, South
Marion County
Columbia, Mississippi
Water NPDES No. MS0044164
July 1, 2024

I.	CLASSIFICATION – Municipal Major

II.	DESCRIPTION OF WASTEWATER – Municipal Domestic

III.	DESCRIPTION OF WASTEWATER TREATMENT - Wastewater is biologically treated by means of a conventional lagoon.  Disinfection is provided by chlorination.  A process schematic is attached to the facility’s application.  The current design capacity of the facility is 1.47 MGD.

RECEIVING WATERS – Pearl River.  The receiving water, at the point of discharge, is classified as Recreational, and is in the Pearl River Basin. This discharge is included in a TMDL for pathogens and pH.  The permit as drafted will comply with the conditions of both the pH and pathogen TMDL. The facility is also listed in a draft TMDL for nutrients. The permit includes a Total Phosphorus limitation in order to comply with the conditions of the nutrient TMDL. A map of the discharge location is attached to the facility’s application.

The 7-day, 10-year minimum flow (7Q10) of the stream is 765 cfs. At the point of discharge, the Instream Wastewater Concentration (IWC) is determined by the following calculation:

Qw = Design flow of the wastewater treatment facility = 1.47 MGD or 2.28 cfs
Qr =  Receiving stream 7Q10 = 765 cfs

IWC = (Qw/ (Qw + Qr)) * 100 =  (2.28/ 2.28+ 765) =  0.30 %

IV.	APPLICABLE GUIDELINES –11 MS Admin Code pt. 6, Municipal Guidelines (Technology), Wasteload Allocation (WLA, see Attachment No. 3), Total Maximum Daily Load (TMDL)

V.	SUMMARY OF LIMITATIONS –

	Parameter						Value				Basis
	Flow						1.47 MGD				Design
	BOD5						45 mg/l (Monthly Avg.)			WLA
	BOD5 (Percent Removal)			65% Minimum				Technology	
	Ammonia-Nitrogen					Report				WLA
	Minimum D.O.						6.0 mg/L				Admin Code
	TSS						90 mg/l (Monthly Avg.)			Technology
	TSS (Percent Removal)				65% Minimum				Technology
	Total Phosphorus					33.81 lbs/day (Monthly Avg.)		TMDL
Total Nitrogen				     141.08 lbs/day (Monthly Avg.)	TMDL
	E. Coli						126 Col./100 ml (Monthly Avg.)		Admin Code
	pH						6.0 – 9.0 SU				Admin Code
Chlorine, Total Residual		     1.0 mg/l (Maximum Weekly Average)	Admin Code
Ceriodaphnia dubia			IC25 	> 0.30%	See Bioassay Evaluation
Pimephales promelas			IC25	> 0.30%	See Bioassay Evaluation

The monitoring and reporting requirements of the draft permit are consistent with State Municipal Guidelines, Section B (All Lagoons).

BIOASSAY AND CHEMICAL SPECIFIC EVALUATION

BIOASSAY

In accordance to State Regulation 11 MS Admin Code pt. 6 – 1 VI.B.2b, Columbia POTW, South 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 Acute Toxicity of Effluents and Receiving Water to Freshwater Organisms.  Short-term Acute WET tests were performed on Ceriodaphnia dubia (invertebrate) and Pimephales promelas (vertebrate).   

The facility did not submit four sets of WET tests as required of the application. A reasonable potential analysis cannot be conducted, therefore Columbia POTW, South will be limited for WET testing in the permit. The facility will be required to begin the WET testing monitoring within 10 days of the date of permit issuance. Columbia POTW, South should use a dilution ratio of 0.30 % when performing the Whole Effluent Toxicity testing.  Regulations require that those facilities with an IWC of less than 1% performs an acute whole effluent toxicity evaluation.  This test should be performed in accordance with Short-Term Methods for Estimating the Acute Toxicity of Effluents and Receiving Water to Freshwater Organisms (EPA/600/4-91/002) or most recent addition.  The acute WET tests should be performed on Ceriodaphnia dubia (invertebrate) and Pimephales promelas (vertebrate).

The permit limit can be reevaluated once the applicant provides a minimum one test per quarter of a year.

CHEMICAL SPECIFIC ANALYSIS

Categorical Guideline Limitations Calculations

The chemical specific analysis has been performed in accordance with State Regulations 11 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 2. A toxicity screening is performed by analyzing and comparing the results to water quality criteria (Attachment 3) to determine any possible toxic effect to the receiving waters.  See Attachment 4 for the toxicity screening results. A column by column description of the toxicity screening calculations 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 the toxic pollutant listed in Table III of appendix D of 40 CFR 122.  Chemical specific 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).

The chemical specific data results submitted did not indicate reasonable potential for violation of the appropriate criteria in Table III of Appendix D of CFR 122. 

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

For Volatile Organic Compounds, the permittee had an average daily concentration for the following compounds:
	
	Chloroform: Average Daily Concentration: 2.0 µg/L 
	Toluene: Average Daily Concentration: 2.13 µg/L

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

	Chloroform: Average Daily Concentration: 2000 µg/L 
	Toluene: Average Daily Concentration: 520 µg/L

Multiplying the average daily concentrations 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.

	Chloroform: [2.0 µg/L*10] = 20.0 µg/L < 2000 µg/L 
	   Toluene: [2.13 µg/L*10] = 21.3 µg/L < 520 µg/L

The above results demonstrate that the average concentrations for Chloroform, and Toluene do not present a reasonable potential to violate Human Health Criteria.
	
The chemical specific data results provided no-detects for Acid-Extractable Compounds or Base-Neutral Compounds.

Attachments

Attachment No. 1 – Wastewater treatment plant flow diagram – See Application
Attachment No. 2 – Discharge location map – See Application
Attachment No. 3 – Wasteload Allocation (WLA)
Attachment No. 4 – Chemical Specific Analysis Results
Attachment No. 5 – Toxicity Screening
Attachment No. 6 – MS State Water Quality Standards

Attachment 3

Attachment 4

Attachment 5

Attachment 6

MS State Water Quality Standards