Numerical Modeling of Surface Waters For TPDES Permits

Mark A. Rudolph, P.E.TCEQ Water Quality Division

What Purpose Does Modeling Serve in the TPDES Permitting Process?

• Develop permit limitations for the protection of surface water dissolved oxygen levels

• Develop dilution estimates for discharges using diffusers

Models: What are Their Strong Points?

• Results are repeatable and consistent with scientific principles

• Environmental conditions resulting from discharges can be evaluated quickly

• Can be set up to run conservative (protective) scenarios without the need to collect a large amount of site-specific data

Natural systems are complex. Even the most sophisticated models are simplifications.

Models: What are Their Limitations?

Models can be difficult to learn and use properly; Specialized training required.

Models: What are Their Limitations?

This Photo by Unknown Author is licensed under CC BY-SA

Results are sensitive to assumptions and data quality (garbage in = garbage out).

Models: What are Their Limitations?

No one model is appropriate for all situations.

Models: What are Their Limitations?

This Photo by Unknown Author is licensed under CC BY-SA-NC

Regulatory use requires the use of guidance documents to ensure consistency.

Models: What are Their Limitations?

Models Routinely Used by the Water Quality Division

• CORMIX– Diffusers

• QUAL-TX– Streams, rivers, estuaries, other linear water bodies

• Continuously Stirred Tank Reactor (CSTR)– Ponds, small lakes/reservoirs, coves of larger lakes

• Water Quality Analysis Simulation Program (WASP)– Lakes, reservoirs, estuaries, can also be used for rivers

Dissolved Oxygen

Why Model Dissolved Oxygen?

• It’s a good general measure of water quality• It’s essential for the survival of aquatic life• The science is reasonably well understood• Oxygen demanding substances are commonly

found in wastewaters• There’s a long regulatory history of dissolved

oxygen modeling and numerical WQ criteria exist

Dissolved Oxygen Models: What are They Really?

A collection of mathematical equations meant to describe interrelated chemical and physical processes controlling DO in natural waters.

Or, more simply

A dissolved oxygen accounting program that considers the major “deposits” and “withdrawals” of DOto/from a surface water system.

Simple Model Equation(Streeter-Phelps)

D is the saturation deficit, (D = DOsat - DO)k1 is the deoxygenation ratek2 is the reaeration rateLa is the initial oxygen demand of organic matter in the water, also called the ultimate BOD (BOD at time t=infinity). Da is the initial oxygen t is the elapsed time

Stream/River Dissolved Oxygen Response to a Waste Load Input

0

1

2

3

4

5

6

7

8

9

10

024681012

Diss

olve

d O

xyge

n

Upstream

10/3/4*

10/12/4*

20/12/4*

Saturation

DO Criterion

Downstream

Waste Load Input

*Permit limits for CBOD5/Ammonia-N/Dissolved Oxygen

FFLUENT LIMITATIONS AND MONITORING REQUIREMENTS Outfall Number 001

During the period beginning upon the date of issuance and lasting through the date of expiration, the permittee is authorized to discharge subject to the following effluent limitations: The annual average flow of effluent shall not exceed 2.5 million gallons per day (MGD); nor shall the average discharge during any two-hour period (2-hour peak) exceed 4,403 gallons per minute (gpm).

Effluent Characteristic Discharge Limitations Min. Self-Monitoring Requirements Daily Avg. 7-day Avg. Daily Max. Single Grab Report Daily Avg. & Daily Max. mg/l (lbs/day) mg/l mg/l mg/l Measurement

Frequency Sample Type

Flow, MGD Report N/A Report N/A Continuous Totalizing Meter Carbonaceous Biochemical Oxygen Demand (5-day)

10 (208) 15 25 35 Two/week Composite

Total Suspended Solids 15 (313) 25 40 60 Two/week Composite Ammonia Nitrogen 3 (63) 6 10 15 Two/week Composite Total Nitrogen Report (Report) N/A Report N/A One/month Composite Total Phosphorus Report (Report) N/A Report N/A One/month Composite E. coli, CFU or MPN/100 ml 126 N/A 399 N/A One/week Grab

. The effluent shall contain a chlorine residual of at least 1.0 mg/l after a detention time of at least 20 minutes

(based on peak flow) and shall be monitored daily by grab sample. The permittee shall dechlorinate the chlorinated effluent to less than 0.1 mg/l chlorine residual and shall monitor chlorine residual daily by grab sample after the dechlorination process. An equivalent method of disinfection may be substituted only with prior approval of the Executive Director.

. The pH shall not be less than 6.0 standard units nor greater than 9.0 standard units and shall be monitored once per week by grab sample.

. There shall be no discharge of floating solids or visible foam in other than trace amounts and no discharge of visible oil.

. Effluent monitoring samples shall be taken at the following location(s): Following the final treatment unit.

. The effluent shall contain a minimum dissolved oxygen of 5.0 mg/l and shall be monitored twice per week by grab sample.

. The annual average flow and maximum 2-hour peak flow shall be reported monthly.

Example DO Modeling AnalysisProblem Statement

TPDES Application Details:• 1.0 MGD domestic discharge for the City of Friendly, TX• Discharge proposed into Lake Slough thence into the

San Antonio River• Applicant proposing secondary treatment; 20 mg/L

BOD5, 20 mg/L TSS, 2 mg/L DO (mechanical plant)

Example DO Modeling AnalysisOther Key Information

• Receiving water dissolved oxygen criteria; Lake Slough 3 mg/L, San Antonio River 5 mg/L

• Regulatory base flow of 0.0 cfs for Lake Slough and 132 cfs for the San Antonio River

• Other discharges to consider in the analysis; none• Site-specific hydraulic information available for these water

bodies; none

Example DO Modeling AnalysisModel Choice and Rationale

Water bodies receiving this discharge are streams/rivers, no calibrated models are available for this system, no site-specific stream data provided in the application use a default QUAL-TX model with statewide hydraulic equations in the analysis.

Example DO Modeling AnalysisResults

Treatment Level (mg/L)(BOD5/Ammonia-N/DO)

DO Criteria (mg/L)(Lake Slough/San Antonio River)

Minimum Predicted DO (mg/L)(Lake Slough/San Antonio River)

20/12/2 3 / 5 1.67 / 5.95

20/12/6 3 / 5 1.70 / 5.95

10/12/6 3 / 5 1.80 / 5.95

10/3/4 3 / 5 3.73 / 5.98

Diffuser Modeling

What are Diffusers?

Primary Purpose: Enhance mixing of effluent with receiving waters

Why Model Diffusers?

• Most water quality standards/criteria apply at the edge of mixing zones, so effluent fraction estimates at these distances are important.

• Processes controlling mixing are complex. • Enhanced mixing characteristics of diffusers can

be predicted quantitatively with models.• Yields better site-specific predictions of

necessary permit limits for Toxics.

ZID

Aquatic Life Mixing Zone

(MZ)

Human HealthMixing Zone

(HH)

50 ft.

Point of Discharge

60 ft.20 ft.

Z I D 300 ft.100 ft.

Mixing Zone & Human Health MZ

Point of Discharge

Direction of Flow

Diffuser Modeling: What Factors are Considered?

Factors affecting mixing:Diffuser geometrySingle/multi-portPosition/orientation in the water column

Boundary interactionsShore/bottom hugging plumes

Mass/Volume fluxFlow RatePollutant concentration

Mixing ProcessesJet mixingBuoyant spreadingDensity CurrentsDiffusionAdvection

1. During the period beginning upon the date of issuance and lasting through the completion of the installation of the 25-port diffuser and the permittee=s request 1 to advance to the Tier I phase, or the date of expiration, whichever occurs first, the permittee is authorized to discharge treated industrial and domestic wastewaters 2 subject to the following effluent limitations:

The daily average flow of effluent shall not exceed 30.0 million gallons per day (MGD). The daily maximum flow shall not exceed 77.0 MGD.

Effluent Characteristics 3 Discharge Limitations Minimum Self-Monitoring Requirements Daily Average Daily Maximum Single Grab Report Daily Average and Daily Maximum lbs/day mg/l lbs/day mg/l mg/l Measurement Frequency Sample Type Flow (MGD) (Report) (Report) N/A Continuous Totalizing Meter Biochemical Oxygen Demand, 5-day (BOD5)

Report N/A Report N/A 160 1/day 24-hr Composite

Total Suspended Solids (TSS) Report N/A Report N/A 149 1/day 24-hr Composite Oil and Grease Report N/A Report N/A 51.0 3/week Grab Total Organic Carbon (TOC) Report N/A Report N/A 262 3/week 24-hr Composite Ammonia -Nitrogen (NH3-N)1 Report N/A Report N/A 26.0 3/week 24-hr Composite Temperature (Degrees Fahrenheit, _F) (105) (115) 4 N/A Continuous In-Situ Residual Chlorine, Total N/A N/A Report N/A 0.0175 1/week Grab Enterococci (CFU/100 ml) (168) 5 (500) 6 N/A 3/week Grab Fluoride N/A 4.20 N/A 6.10 12.2 3/week 24-hr Composite Phenol Report N/A Report N/A 0.150 1/month Grab Phenolics (i.e., phenols) Report N/A Report N/A 0.679 1/week Grab Sulfide Report N/A Report N/A 0.631 3/week Grab Chromium, Hexavalent Report N/A Report N/A 0.0397 1/month 24-hr Composite Chromium, Total 125 N/A 250 N/A 2.00 3/week 24-hr Composite Copper, Total 8.43 N/A 17.8 N/A 0.142 3/week 24-hr Composite Cyanide, amenable to chlorination 7 2.90 13

2.20 14 N/A N/A

6.10 13 3.63 14

N/A N/A

0.0490 13 0.0290 14

3/week 3/week

24-hr Composite 24-hr Composite

Lead, Total 37.1 N/A 78.5 N/A 0.627 1/week 24-hr Composite Mercury, Total 0.673 13

0.0572 14 N/A N/A

1.79 13 0.120 14

N/A N/A

0.0114 13 0.0010 14

1/week 1/week

24-hr Composite 24-hr Composite

Nickel, Total Report N/A Report N/A 0.218 3/week 24-hr Composite

Example Diffuser Modeling AnalysisProblem Statement

TPDES Application Details:• New 1.0 MGD industrial discharge into a Ship

Channel (SC)• Single 6 inch port diffuser• Oriented horizontally and toward the center of the

SC

Example Diffuser Modeling AnalysisOther Key Information

• Port located 2 meters off the bottom, 50 meters from shore • SC is 12 meters deep and 300 meters wide at the diffuser location• SC current velocity is 0.05 m/s• Effluent density is a constant 1010.00 kg/cubic m• Ambient density cases calculated from long term monitoring

station(s)• Radial mixing zones of 50’, 200’, and 400’

Example Diffuser Modeling AnalysisCases Evaluated

CORMIX Cases Evaluated

Case

Ambient Effluent ΔDensity(kg/m3)

Case DescriptionDensity (kg/m3)

Density(kg/m3)

Flow (MGD)

A 1015.14 1010.00 1.0 5.14 Proposed flow, (T5,S5 ambient)

B 1027.56 1010.00 1.0 17.56 Proposed flow, (T5,S95 ambient)

C 1010.76 1010.00 1.0 0.76 Proposed flow, (T95,S5 ambient)

D 1022.61 1010.00 1.0 12.61 Proposed flow, (T95,S95 ambient)

Example Diffuser Modeling AnalysisResults

CORMIX Case Results

Case

Percent Effluent (%)Case DescriptionZID

(50’)AL MZ (200’)

HH MZ (400’)

A 1.8 1.0 0.9 Proposed flow, (T5,S5 ambient)

B 1.8 1.5 1.4 Proposed flow, (T5,S95 ambient)

C 4.7 0.4 0.4 Proposed flow, (T95,S5 ambient)

D 1.7 1.4 1.3 Proposed flow, (T95,S95 ambient)

Role of Modeling Regulatory Guidance Documents

• Ensures technical legitimacy• Consistency – level playing field• Helps with remembering all steps in a

complex process• Public transparency

Conclusions

• Two types of modeling performed by the Water Quality Division: dissolved oxygen and mixing from diffusers.

• Domestic limits from dissolved oxygen modeling include 5-day Carbonaceous Biochemical Oxygen Demand, Ammonia Nitrogen, and minimum dissolved oxygen.

More Conclusions

• Mixing estimates from diffuser modeling are used to develop limits for toxics in certain industrial permits.

• Models are valuable tools for developing protective permit limits but require proper use to provide meaningful results.

• Contact information: Mark A. Rudolph, P.E.

512-239-4534 Mark.rudolph@tceq.texas.gov

• Questions?