1. Water Quality-Based Effluent Limits
(WQBELs)
Hi, my name is Catherine Grutsch.
I am an industrial wastewater permit writer with the Water Quality Division of the TCEQ, and I am going to talk about how water quality-based effluent limits are calculated.
Catherine Grutsch
May 2018

2. Permit limits apply to discharges
Permit limits apply to discharges. Water quality criteria apply to water bodies.
In other words, criteria in the Texas Surface Water Quality Standards do not apply directly to a discharge.
The TCEQ is responsible for maintaining and enhancing water quality in the state.
The Texas Surface Water Quality Standards are the legal standards for the quality of surface water in Texas.
The water quality criteria in the Texas Surface Water Quality Standards apply to WATER BODIES and do not directly apply to a wastewater discharge.

3. What factors influence water quality-based permit limits?
Numerical criteria (toxic pollutants)
Water body quality
Effluent fraction (mixing)
Bioavailable fraction
When it comes to calculating water quality-based effluent limits, the process is much like putting a puzzle together, and there are four main puzzle pieces:
- the numerical criteria for toxic pollutants = purple
- the water quality in the receiving water body = blue
- the effluent fraction associated with mixing = green
- the bioavailable fraction associated with aquatic life = red
Throughout the presentation, I will refer back to these associated puzzle pieces and colors.
This topic is pretty complex, so I help that this visual aid helps in understanding it.

4. Texas has numerical criteria for aquatic life and human health protection.
◊ Found in Texas Surface Water Quality Standards
(30 TAC Chapter 307 Section 6 – Toxic Materials)
The first puzzle piece that we will be discussing today is the purple one,
which represents NUMERICAL CRITERIA for toxic pollutants.
The state of Texas has established numerical criteria for toxic pollutants
so that aquatic life and human health are protected.
Again, remember that these criteria apply to the water body, not to the discharge, although the criteria do play into figuring out what limits apply to discharges.

5. Numerical criteria for toxic materials can change over time.
◊ Criteria revisited every three years
Pollutant
2010 Criteria
2014 Criteria
% Change
Aldrin Freshwater, acute
3.0 μg/L
No change
Hexachloroethane Human health, water & fish
27 μg/L
4.97 μg/L
-82 %
Benzo(a)anthracene Human health, fish only
0.33 μg/L
3.28 μg/L
994 %
Numerical criteria is revisited every three years.
The table in this slide shows that during our most recent revision, the criterion for Aldrin stayed the same, while the criterion for Hexachloroethane became more stringent, and the criterion for Benzo(a)anthracene became more relaxed.

6. Numerical criteria for aquatic life reflect an organism’s environment and exposure.
Table 1. Criteria in Water for Specific Toxic Materials
Aquatic Life Protection
Let’s discuss the numerical criteria for aquatic life.
Table 1 in the Texas Surface Water Quality Standards is divided up based on freshwater or saltwater environments and based on acute or chronic exposure.
When it comes to ACUTE toxicity, we are dealing with exposures of ≤ 4 days.
(Acute toxicity deals with lethality – things will die.)
When it comes to CHRONIC toxicity, we are dealing with exposures of ≥ 7 days.
(Chronic toxicity deals with growth and reproduction – life survives, but does it thrive?)

7. Not all of the numerical criteria are expressed in the same way.
◊ Most criteria are for total concentrations.
◊ Some metals criteria are for dissolved concentrations:
• aluminum
• arsenic
• cadmium
• chromium (tri and hex)
• copper
• lead
• nickel
• silver (free ion)
• zinc
But wait!
Permit limits are written for total concentrations.
While most criteria are in terms of total concentration,
all of the metals listed on this slide have criteria for the dissolved portion in water.
This is interesting because the limits that are placed in to permits are for total concentrations, not dissolved concentrations.
So your may be wondering how do we derive our limits?
Well, it requires that we incorporate some of those other four puzzle pieces.

8. 𝑪𝒅 𝑪𝑻 = 1 1+(𝐾𝑝 × 𝑻𝑺𝑺 × 10−6) 𝐾𝑝=10𝑏 × 𝑻𝑺𝑺 𝑚
Metals criteria may be expressed as a dissolved concentration because local water quality affects toxicity.
Metals criteria may be expressed as a dissolved concentration because local water quality affects toxicity.
◊ Conversion from dissolved criteria
to total limits uses ambient total
suspended solids (TSS) of the nearest downstream classified segment.
◊ Hint:
Dissolved fraction = bioavailable fraction.
𝑪𝒅 𝑪𝑻 = 1 1+(𝐾𝑝 × 𝑻𝑺𝑺 × 10−6) 𝐾𝑝=10𝑏 × 𝑻𝑺𝑺 𝑚
There is a definable ratio between the total concentration of a metal and how much of it is dissolved. That ratio can be calculated and measured using the ambient total suspended solids concentration of the receiving water body. Remember the blue piece represents water body quality, in this case levels of TSS.
The dissolved concentration of a metal is also known as the bioavailable fraction. Having a high concentration of dissolved metals can be harmful to the environment and human health, and this is indicated through toxicity.
We will discuss this more later on, but we are starting to see how the different puzzle pieces are connected and build on each other.

9. Criteria for pentachlorophenol are affected by pH.
𝐴𝑐𝑢𝑡𝑒 𝑐𝑟𝑖𝑡𝑒𝑟𝑖𝑜𝑛= 𝑒 (1.005 𝒑𝑯 −4.869)
𝐶ℎ𝑟𝑜𝑛𝑖𝑐 𝑐𝑟𝑖𝑡𝑒𝑟𝑖𝑜𝑛= 𝑒 (1.005 𝒑𝑯 −5.134)
◊ Pentachlorophenol is more toxic at lower pH values.
Pentachlorophenol provides a good example of how water body quality affects numerical criteria.
The toxicity of pentachlorophenol is affected by pH.
And pentachlorophenol is more toxic in water that has a low pH (or water that is more acidic).
Therefore, permit limits for pentachlorophenol are more stringent
for facilities whose receiving waters have low pH.
For pH of 3: For pH of 6:
Acute criterion = Acute criterion = 3.19
Chronic criterion = Chronic criterion = 2.45

10. Some freshwater criteria depend on the hardness of the receiving water.
◊ These include:
• cadmium
• chromium (trivalent)
• copper
• lead
• nickel
• zinc
Another example of water body quality impacting numerical criteria
can be seen in case of total hardness.
In general, most metals are more toxic in water that has low hardness values
(soft water).
Therefore, water quality criteria are more stringent for receiving waters
that have low hardness values.
Copper is an example of a toxic metal that is affected by hardness.
Example: copper
𝐴𝑐𝑢𝑡𝑒 𝑐𝑟𝑖𝑡𝑒𝑟𝑖𝑜𝑛= 𝑚𝑒 ( ln ℎ𝑎𝑟𝑑𝑛𝑒𝑠𝑠 −1.6448)
𝐶ℎ𝑟𝑜𝑛𝑖𝑐 𝑐𝑟𝑖𝑡𝑒𝑟𝑖𝑜𝑛= 𝑚𝑒 ( ln ℎ𝑎𝑟𝑑𝑛𝑒𝑠𝑠 −1.6463)

11. Metals affected by hardness are more toxic in soft water.
◊ Freshwater criteria are
lower at smaller hardness values.
Example: copper
Segment Number
Water Body Name
Hardness
(mg/L of CaCO3)
Acute Criterion (µg/L)
Chronic Criterion (µg/L)
0505
Sabine River Above Toledo Bend Reservoir 42
6.27
4.51
1412
Colorado River Below Lake J. B. Thomas
310
41.2
24.8
Again, looking at Copper, which is an example of a metal that is affected by hardness.
Copper is more toxic in receiving waters that have low hardness values.
Let’s compare Segment No to Segment No. 1412…
0505: Lower hardness value leads to Higher toxicity which results in Lower criteria values
1412: Higher hardness value lead to Lower toxicity which results in Higher criteria values

12. Water Body Quality Water Body Quality
Critical values for water quality parameters for each classified segment are found in Procedures to Implement the Texas Surface Water Quality Standards (IPs), June 2010, Appendix D.
◊ Total suspended solids (TSS)
◊ pH
◊ Total hardness
◊ Total dissolved solids (TDS)
◊ Chloride
◊ Sulfate
Now let’s discuss the water body quality puzzle piece in more detail
The Procedures to Implement the Texas Surface Water Quality Standards, or what we refer to as the IPs, explain the procedures that the TCEQ uses when applying the Texas Surface Water Quality Standards to permits that are issued under the TPDES (Texas Pollution Discharge Elimination System) program.
Critical values for water quality parameters can be found in Appendix D of the IPs, and these ambient values are considered to be critical conditions.
Critical conditions of a water body are those combinations of environmental conditions and wastewater inputs that may result in the most stringent permit limits. These are conditions such as lowest stream flow or low dissolved oxygen availability.

13. Effluent fractions help convert numerical criteria into limits.
Numerical criteria apply at the edge of each zone:
So now that we have discussed how the local water quality provides us with critical conditions,
let’s discuss how the other puzzle pieces are used to derive effluent limits.
In particular, let’s discuss the use of effluent fractions.
An effluent fraction is the amount of wastewater relative to the amount of receiving water.
And effluent fractions are important for converting numerical criteria into actual permit limits.
There are three zones associated with wastewater discharges:
– Zone of Initial Dilution
– Aquatic Life Mixing Zone
– Human Health Mixing Zone
Numerical criteria apply at the edge of each zone:
– Acute Aquatic Life criteria – apply at the edge of the Zone of Initial Dilution (ZID)
– Chronic Aquatic Life criteria – apply at the edge of the Aquatic Life Mixing Zone (MZ)
– Chronic Human Health criteria – apply at the edge of the Human Health Mixing Zone (HHMZ)
(The distances from the point of discharges depend on the type of receiving water body that the discharge is going into…. Diagram in slide is of a wide tidal river)

14. Texas assumes critical low flow or low mixing conditions.
Expressed as:
◊ Critical effluent percentages
(lakes, bays, estuaries, wide tidal rivers) or
◊ Critical flows
(streams, rivers, narrow tidal rivers)
Our permits are written to account for critical low flow conditions or critical low mixing conditions.
The critical-condition concept asserts that if a discharge is controlled so that it does not cause water quality criteria to be exceeded in the receiving water during critical flow conditions, then the discharge will likely be protective of those same water quality criteria at all flows.
Critical effluent percentages or critical flows are assigned depending on the type of receiving water body.
For example, intermittent streams use the simplest model. With no dilution available in the stream, discharges are evaluated at 100% effluent (i.e. no zone of initial dilution or mixing zone).

15. Resulting effluent fractions depend on the type of water body.
Zone of Initial Dilution
(Acute)
Aquatic Life Mixing Zone
(Chronic)
Human Health Mixing Zone
Stream
Least simple
𝑄 𝐸 𝑄 𝐸 +0.25(7𝑄2)
𝑄 𝐸 𝑄 𝐸 +7𝑄2
𝑄 𝐸 𝑄 𝐸 +𝐻𝑀
Lake
60 % effluent
15 %
effluent
8 %
Wide tidal
30 % effluent
4 %
Intermittent
Most simple
100 % effluent
Here, we can see that the effluent fraction is determined by the type of water body and the type of mixing zone.
Qe = effluent flow
7Q2 = seven-day, two-year low-flow value
HM = harmonic mean flow value
Intermittent streams use the simplest model. With no dilution available in the stream, discharges are evaluated at 100% effluent (i.e. no zone of initial dilution or mixing zone).
For streams, a mass-balance between the effluent and the receiving water is used to determine the size of the mixing zones for the Zone of Initial Dilution, Aquatic Life Mixing Zone, and the Human Health Mixing Zone.
The information in the chart can be found in the Implementation Procedure (IPs).
Lake mixing is based on a Zone of Initial Dilution of 25 feet (60% effluent), Aquatic Life Mixing Zone of 100 feet (15%), and Human Health Mixing Zone of 200 feet (8%).
Wide Tidal River mixing is based on a Zone of Initial Dilution of 50 feet (30% effluent), an Aquatic Life Mixing Zone of 200 feet (8%), and a Human Health Mixing Zone of 400 feet (4%).

16. Most metals are not entirely bioavailable. Conversion is required.
For most metals, numerical criteria for aquatic life are dissolved concentrations, but…
Effluent limits are expressed as total concentrations.
The bioavailable fraction, which is a function of TSS, is used to make this translation.
Back in the beginning of the presentation we talked about how the numerical criteria for most metals are expressed as dissolved concentrations and how TSS is used to calculate the ratio of the dissolved concentration to the total concentration.
The fraction of the pollutant that is available organisms for use is the BIOAVAILABLE FRACTION, and I would like to discuss it more in detail.
The bioavailable fraction is the fraction of the pollutant that is available to organisms, and TSS concentrations have an impact on this value.

17. Bioavailable Fraction
The bioavailable fraction equals:
where:
Cd = dissolved concentration
CT = total concentration
This fraction depends on TSS :
𝐶 𝑑 𝐶 𝑇
The bioavailable fraction is the portion of the pollutant that is biologically available to organisms to affect them, and it is equal to the dissolved concentration
divided by the total concentration.
It depends on the ambient total suspended solids concentration
of the receiving water body.
𝐶 𝑑 𝐶 𝑇 = 1 1+( 𝐾 𝑝 ×𝑇𝑆𝑆 × 10 −6 )

18. Bioavailable Fraction Cont.
𝐶 𝑑 𝐶 𝑇 = 1 1+( 𝐾 𝑝 ×𝑇𝑆𝑆 × 10 −6 )
The term KP, the partition coefficient, also depends on TSS:
where “b” and “m” are values found in Table 6 in the 2010 IP (p. 160).
𝐾 𝑝 = 10 𝑏 × 𝑇𝑆𝑆 𝑚
Because effluent limits are expressed as total concentrations, TCEQ staff has to use partition coefficients in order to determine instream compliance with the numerical standards for dissolved concentrations.
The use of partition coefficients determines how much metal TCEQ considers to be dissolved in the receiving water.

19. Putting All the Pieces Together
Water Body Quality
Numerical Criteria
SO NOW LET’S PUT ALL OF THE PIECES TOGETHER…
Effluent Fraction
Bioavailable Fraction

20. Four easy steps to calculate WQBELs for aquatic life and human health!
Putting All the Pieces Together
Four easy steps to calculate WQBELs for aquatic life and human health!
◊ Calculate waste load allocation – WLA
◊ Calculate long-term average – LTA
◊ Calculate effluent limits:
• daily average (DLY AVG)
• daily maximum (DLY MAX)
◊ Final permit limits are determined by comparison
There are FOUR steps in calculating water quality-based effluent limits for both aquatic life and human health:
The waste load allocations are calculated
Waste load allocations (WLAs) equal the effluent concentrations
that will not cause the overall instream criteria to be exceeded.
2. The long-term average concentrations are calculated using WLAs
The second step is to calculate the Long Term Average of the treatment system performance that is necessary to meet the respective WLA with a given probability.
3. The daily average and daily maximum effluent limits are calculated using LTAs
4. Final potential water quality-based permit limits are determined after comparing the effluent limits calculated for aquatic life with the effluent limits calculated for human health,
TEXTOX, a set of spreadsheets, does these calculations for us.
I say “potential” because these calculated limits are then compared to any technology-based limits, and the more stringent limit or limits of the two become the final limits in the permit.

21. Call your permit writer!
Help!
Help!
My draft permit includes a new or more stringent WQBEL – what can I do?
Call your permit writer!
Now, what happens if you receive a draft permit with a new or more stringent water quality-based effluent limit included?
What should you do??
YOU SHOULD CALL YOUR PERMIT WRITER

22. Water Quality-Based Effluent Limits
QUESTIONS
Water Quality-Based Effluent Limits
Are there any questions???
Catherine Grutsch
May 2018