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5.1.1 REMOVAL EFFICIENCY CALCULATION METHODOLOGIES
This section explains the three removal efficiency calculation methodologies commonly used by POTWs.
They are the average daily removal efficiency, the mean removal efficiency, and the decile method.
Average Daily Removal Efficiency
The average daily removal efficiency (ADRE) calculation requires that an influent sample be paired
with a lagged effluent sample to reflect removal efficiency accurately. Samples are lagged by the
hydraulic residence time of wastewater within the
treatment plant. As shown in Equation 5.1, a series
of daily removal efficiencies based on paired
headworks influent (7J and POTW effluent data
(Epotw, n) is calculated first. This series of removal
efficiencies is then summed (symbolized in the
equation by the Greek letter £) and divided by the
total number of paired observations (N) to yield the
removal efficiency (RpotJ across the entire
wastewater treatment plant (from headworks to plant
effluent). To calculate the removal efficiency from
headworks to primary treatment effluent (Rprim), use
paired headworks influent (7J and primary treatment
effluent data (Eprim J. To calculate the removal
efficiency from headworks to secondary treatment
effluent (RseJ, use paired headworks influent (7J and
secondary treatment effluent data (EseC: J.
R „,. -
Mean Removal Efficiency
More flexible than the ADRE method, the mean
removal efficiency (MRE) can be used with paired
data lagged for retention time suitable for the ADRE
method and data that have not been lagged or paired.
As shown in Equation 5.2, instead of averaging
observed paired removal efficiencies, the MRE
calculation first averages (symbolized in the
equation by the overbars) all plant influent values
(Ir) and all plant effluent values (Epotw>t) separately
and then calculates removal efficiency across the
entire wastewater treatment plant from headworks to
plant effluent (RpotJ. The MRE calculation averages all headworks influent data (I) and all primary
treatment effluent data (Eprim J to calculate the removal efficiency from headworks to primary treatment
effluent (Rprim). The MRE calculation averages all headworks influent data (I) and all secondary
treatment effluent data (EseCiy) to calculate the removal efficiency from headworks to secondary treatment
effluent (RSJ.
Equation 5.1: Removal Efficiency
Calculated Using Average Daily Removal
Efficiency
N
N
N
Where:
Rpofw = Plant removal efficiency from headworks to
plant effluent, as decimal
Rprjm = Removal efficiency from headworks to
primary treatment effluent, as decimal
Removal efficiency from headworks to
secondary treatment effluent, as decimal
POTW influent pollutant concentration at
headworks , mg/L
POTW effluent pollutant concentration
Primary treatment effluent pollutant
concentration, mg/L
Secondary treatment effluent pollutant
concentration, mg/L
Paired observations, numbered 1 to N
5-3
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Equation 5.2: Removal Efficiency
Calculated Using Mean Removal
Efficiency
^potw,t
a - ' eec,y
"sec =
1,-E.
'pnm,x
Where:
D —
^ofw
Unpaired historical data from the same time period
(such as alternating months during the same year)
should not introduce bias. However, unpaired
historical data from different time periods, if used in
the MRE calculation, can introduce bias when
significant changes in the POTW's industrial base
(such as the opening or closing of an industry or the
installation of significantly more efficient
pretreatment equipment units or source control)
occurred between data collection times. Current
levels of POTW influent should be compared to
historical levels to determine if they are of the same
general magnitude. In addition, unpaired sampling
data representing some unusual one-time event should
not be included in the MRE calculation.
Decile Method
Mean removal efficiency does not indicate how often
the derived removal efficiency was achieved. The
decile method requires at least nine daily removal
efficiency values based on paired sets of influent and
effluent data. However, instead of averaging the daily
removal efficiency values, the decile method sorts
daily removal efficiency data from highest to lowest
and calculates the percentage of the daily removal
efficiency above or below a specified removal
efficiency. The methodology is similar to a data set
median. A median divides an ordered data set into
two equal parts: with half the data set above the
median and the other half below. The decile method
is similar except it divides the ordered data set into 10 ^^^^^^^^^^^^^^^^^^^^^^™
equal parts. Therefore, 10 percent of the data set is
below the first decile; 20 percent of the data set is below the second decile, etc. The fifth decile is
equivalent to the data set median.
The results of an applied decile
t =
r =
x =
y =
Plant removal efficiency from headworks to
plant effluent, as decimal
Removal efficiency from headworks to
primary treatment effluent, as decimal
Removal efficiency from headworks to
secondary treatment effluent, as decimal
POTW influent pollutant concentration at
headworks, mg/L
POTW effluent pollutant concentration, mg/L
Primary treatment effluent pollutant
concentration, mg/L
Secondary treatment effluent pollutant
concentration, mg/L
Plant effluent samples, numbered 1 to T
Plant influent samples, numbered 1 to R
Primary treatment effluent samples,
numbered 1 to X
Secondary treatment effluent samples,
numbered 1 to Y
method approach are shown in
Figure 5-2.
Figure 5-2 shows the decile values
(labeled "Deciles - Percent of Data
Set Less than Stated Efficiency") on
the Y-axis and the corresponding
removal efficiencies on the X-axis.
From this figure, a POTW can gain
an understanding of the likelihood of
certain removal efficiencies. As
illustrated at the fifth decile or
median, this hypothetical POTW has
an overall plant removal efficiency
(RpotJ of 64.5 percent less than half
TO
Q
+-(/)><
W _-
® *-
'o &
8 W
Q
TO 5
£ 'o
s ig
a) ni
Figure 5-2: Decile Results for Hypothetical
POTW
100% -,
90% -
80% -
70% -
60% -
50% -
40% -
30% -
20% -
10% -
0% -
0% 20% 40% 60% 80%
Removal Efficiency
100%
5-4
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of the time. As illustrated in the third decile, the POTW achieves a removal efficiency of below 36
percent less than 30 percent of the time. If concerned about recurring effluent limitation violations due to
plant operation variation, the POTW may decide, based on historical knowledge, to use the more
conservative third decile, instead of the median fifth decile, as the removal efficiency. However, POTWs
should be aware that a lower removal efficiency for those pollutants that accumulate in sludge would lead
to lower, more protective, effluent-based local limits but higher, less protective, sludge-based local limits.
Appendix P includes sample calculations of removal efficiencies using ADRE, MRE, and decile methods.
Conservative Pollutant Removal Efficiency Derived
For conservative pollutants, such as metals, the
portion removed during POTW processes ends up in
the sludge. Therefore, for conservative pollutants,
POTWs can also use sludge data to estimate removal
efficiency across the entire plant (RpotJ. Sludge data
should be used in place of effluent data when a
POTW has influent data above detection but does not
have adequate effluent data above detection and,
therefore, believes sludge data provide more
representative removal efficiencies. (In general,
accurate representative sampling results are more
difficult to attain in the sludge than in the POTW
effluent sampling.) As shown in Equations 5.3 and
5.4, ADRE and MRE can be used to calculate
removal efficiency across the entire plant (RpotJ by
comparing the sludge and headworks pollutant
loading. Sludge loading is calculated by multiplying
the sludge concentration (S) by the sludge flow rate
(Qsidg), specific gravity (Gsldg), and percentage solids
(PS). Influent pollutant loading is calculated by
multiplying the influent concentration (I) by the
average POTW flow rate (QpotJ. The influent
pollutant concentration (I) should be a monthly
average in order to be compared with sludge pollutant
concentration, which accounts for pollutants that have
accumulated for 20 to 30 days. The MRE method is
often more suitable technique than the ADRE in this
situation because:
from Sludge Data
Equation 5.3: Plant Removal Efficiency
Calculated Using ADRE and Sludge Data
P E(Sn* PS/100* Q^* Gsfda)/(/n* QpoJ
pot* N
Equation 5.4: Plant Removal Efficiency
Calculated Using MRE and Sludge Data
(S,*8.34* PS/100*
Where:
P —
"pofw ~
PS =
8.34 =
Sn, SB =
n =
u =
r =
(/r*8.34*Qpo(w)
Plant removal efficiency from headworks to
plant effluent, as decimal
POTW influent pollutant concentration at
headworks, mg/L
Percentage solids of sludge to disposal,
Total sludge flow rate to disposal, MGD
POTW average flow rate, MGD
Specific gravity of sludge, kg/L
Unit conversion factor
Sludge pollutant concentration, mg/kg
Paired observations, numbered 1 to N
Sludge samples, numbered 1 to U
Influent samples numbered 1 to R
1. Most POTWs will not have monthly average influent pollutant concentrations readily
available.
2. Sludge settling times are difficult to estimate when developing paired observations.
5.1.2 GUIDANCE ON USING DIFFERENT METHODOLOGIES
EPA offers the following guidance on implementing the three different methodologies:
• EPA recommends the MRE over the ADRE method if less than ten data pairs are available,
because it is generally less sensitive to variation in daily removal efficiencies.
5-5
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• Although requiring more data, the decile approach allows for a more comprehensive view of
removal rates than the ADRE and MRE methods because it provides a frequency distribution
and allows for explicit incorporation of daily removal efficiency.
• Although an overall depiction of the POTW removal efficiency frequency is gained in the
decile method, an individual decile estimate, depending on how conservative the POTW
wants to be in establishing removal efficiencies, can be less precise than the MRE and ADRE
estimates.
Appendix P of this manual provides additional guidance in the form of an example and an examination of
the different methodologies applied to one data set.
5.1.3 DATA QUALITY
This section reviews some issues related to data quality, quantity, and analytical method limits that often
cause problems during local limits calculations.
Outliers
The following two simple tests can be conducted to see if outliers exist in a given data set:
1. If the data are known to closely follow a "bell-shaped" normal distribution, then any data
point that lies more than two standard deviations from the mean is considered an outlier.
2. If the data values do not approximate a normal distribution, outliers can be determined based
on the interquartile range (IQR) of the data set. The IQR equals the values between the 1st
and 3rd quartile. Any data point that lies more than 1.5 times this IQR below Ql, or 1.5 times
this IQR above Q3 is considered an outlier.
Both of these methods are demonstrated in Appendix P with a sample data set.
Concentrations Below the Minimum Level of Quantitation (ML)
A POTW's sampling program will probably yield some sampling results that indicate a pollutant was
below the ML in the analyzed sample. The manner in which the POTW uses these data in the local limits
development process can significantly affect the MAHL calculation. Table 5-1 details the different
options available to POTW users.
Table 5-1: Options for Managing Sampling Results Below the ML in Removal Efficiency
Calculations
If only a few data values are below the ML:
Option 1 : Use surrogate value of % ML.
Option 2: Discard the few samples below the ML. (Influent
and effluent data should be discarded in pairs.)
If most data values are below the ML:
Option 1 : Re-evaluate the need for a local limit for the
pollutant. (However, if the pollutant is one of the 15 EPA
POCs an AHL should be developed.)
Option 2: Use removal rate data from other plants. (See
Section 5.1.4.)
In general, the surrogate value results in a greater bias when calculating the mean or standard deviation
and accuracy decreases as the proportion of non-detects increases.
5-6
-------
Other statistical methods—Regression order statistics (ROS), probability plotting, and maximum
likelihood estimations (MLE)—are detailed in Appendix Q. The probability plotting method provides
slightly more accurate results when non-detects represent 30 percent or more of the data set. The MLE
method works well when the data distribution is exactly normal or lognormal1 and when non-detects are
less than 30 percent of the data set. Other references for using statistics to analyze data sets containing
values below limits include:
• Appendix E in the Technical Support Document for Water Quality-based Toxics Control,
EPA/505/2-90-001, March 1991.
• Use of Statistical Methods in Industrial Water Pollution Control Regulations in the United
States, Journal of Environmental Monitoring and Assessment, Volume 12:129-148, 1989.
Although these methods can be applied by those without a background in statistics, EPA strongly
recommends that a statistician perform the necessary calculations.
Negative Removal Efficiency
Negative removal efficiencies, which reflect valuable operational data, should not be summarily
dismissed as outliers. Unless technical justification (such as poor sampling or analytical technique) to
remove them is discovered, negative removal rates should be retained in the data set. Described below
are methods to manage negative removal efficiencies. Appendix P provides sample calculations to
address negative removal efficiencies.
Use the MREMethod or Decile Approach. Negative removal efficiencies are attributable to the fact that
POTWs do not operate in a steady state. Deviations from steady state occur because of variability in
POTW influent, recycle streams and performance, accumulation of pollutants in POTW sludge, and
incidental generation of pollutants by POTW operations. This variability often leads to the ADRE
method of calculating removal efficiency, dependant on retention time lagged data, to yield negative
removal efficiencies. In these cases, the MRE method, less sensitive to data variability, should eliminate
negative removals efficiencies unless an underlying problem exists in the sampling, data analysis or plant
operations. The decile approach, which ranks instead of averages daily removal efficiencies, can be
applied to data sets with a few negative daily removal efficiencies because it determines efficiency based
on probability of occurrence and not averaging.
Manage data below the ML. In addition, negative removal rates often result from the influent and
effluent concentrations below the ML. Readings below the ML that can lead to negative removal
efficiencies should be examined as detailed above.
5.1.4 APPLYING REMOVAL EFFICIENCIES REPORTED BY OTHERS
Removal efficiencies are based largely on site-specific conditions such as climate, POTW design,
operation and maintenance, plant conditions, and sewage characteristics. Therefore, EPA strongly
suggests that site-specific data be used to calculate removal efficiencies. However, some POTWs still do
not have adequate data to calculate removals after conducting site-specific sampling and using analytical
Log-normal distributions are probability distributions that are closely related to normal distributions: if X is a
normally distributed random variable, then exp(X) has a log-normal distribution. In other words, the natural logarithm of a
log-normally distributed variable is normally distributed.
5-7
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methods that achieve the lowest detection levels possible. In these instances, POTWs may selectively use
removal efficiencies reported by other POTWs or by studies that have been published in professional
journals or by EPA. EPA urges POTWs to use performance data from plants employing the same
treatment technology and similar contributing sources. Appendix R provides a listing of removal
efficiency data for priority pollutants gathered from other POTWs. (These data are the same as those
presented in the 1987 Local Limit Guidance Manual.)
5.2 CALCULATION OF ALLOWABLE HEADWORKS LOADINGS
An AHL is the estimated maximum loading of a pollutant that can be received at a POTWs headworks
that should not cause a POTW to violate a particular treatment plant limit or environmental criterion. An
AHL is developed to prevent interference or pass through. An AHL is calculated for each applicable
criterion: pass through, sludge contamination, air quality standards, and the various forms of interference
(biological treatment inhibition, sludge digestion inhibition). The AHLs for each POC are calculated
based on the various suitable environmental criteria, plant flow rates, and plant removal efficiency. After
calculating a series of AHLs for each POC, the lowest AHL is chosen as the MAHL.
Local limits development uses a mass-balance approach to determine the AHLs for a POTW based on the
environmental and treatment plant criteria. With the mass-balance approach, the POTW calculates the
amount of loading received at the POTW headworks that will still meet the environmental or treatment
plant criteria that apply to each pollutant. Steady-state equations are used for conservative pollutants
because the amount of pollutant loading is "conserved" throughout the treatment plant. Conservative
pollutants can be removed from wastewater via chemical or physical separation or biological treatment
but always accumulate in the sludge or remain in wastewater. On the other hand, non-conservative
pollutants may be lost through degradation or volatilization in addition to accumulating in the sludge.
Because losses through degradation and volatilization do not contribute to pollutant loadings in sludge, it
is not valid to assume that all non-conservative pollutants removed during plant treatment are transferred
to sludge. Therefore, for non-conservative pollutants, different equations are used to calculate AHLs
based on sludge criteria.
Fate and transport software can estimate the effects of biodegradation, sorption onto solids, and
volatilization on substances entering a treatment plant. The most widely used model is EPA's Water9
model for wastewater collection and treatment systems available at:
http://www.epa.gov/ttn/chief/software/water/index.html.
5.2.1 DETERMINATION OF SUITABLE ENVIRONMENTAL CRITERIA
A properly functioning POTW will be in compliance simultaneously with air, effluent, and sludge
environmental criteria (see Figure 5-3). For each POC identified, the POTW should examine the
appropriate environmental criteria to guard against interference or pass through. From these
environmental criteria, along with flow rates and removal efficiencies, AHLs are calculated. These
environmental criteria should have all been evaluated as part of the POC development in Chapter 3.
Table 5-2 shows suggested criteria that should be evaluated for each POC. The next section provides
details regarding how to use these criteria in the AHL calculation.
5-
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Table 5-2: Suggested Criteria or Standards to be Considered
For Each POC in the Development of AHLs
Effluent Based
NPDES permit:
effluent limitations,
water quality-based
toxic pollutant limits,
Whole Effluent Toxicity
(WET)
[Source: POTWsown
NPDES permit]
State Water Quality
Criteria and Standards:
adoption of Federal
criteria or stricter
[Source: State
regulations]
National Recommended
Water Quality Criteria
for Priority Pollutants:
freshwater/saltwater
chronic and acute
criteria, human health
for consumption criteria
[Source: Appendix D or
National Recommended
Water Quality Criteria-
Correction. April 1999,
EPA822-Z-99-001]
Sludge-Based
State Sludge Quality
Criteria: adoption of
Federal criteria or
stricter
[Source: State
regulations]
Federal Sludge
Standards: land
application, surface
disposal, or
incineration
[Source: Appendix E
or Federal
regulations 40 CFR
Part 503]
Hazardous Waste
Criteria: Toxic
Characteristic
Leaching Procedure
(TCLP)
[Source: Appendix F
or Federal
regulations 40 CFR
Part 261 .24]
Inhibition-Based
POTW's own in-house
guidelines or criteria
for process inhibition
[Source: POTW
reports detailing
circumstances
surrounding last
inhibition]
Literature Inhibition
Values for activated
sludge, trickling filter,
and nitrification
processes
[Source: Appendix G]
Air Quality Based
Local regulatory
requirements to meet
National Ambient Air
Quality Standards
(NAAQS)
[Source: State
Implementation Plan
or local regulatory
requirements to meet
NAAQS ]
Resource Protection
Based
State and local
groundwater, aquifer,
and watershed
protection permits
[Source: State
regulations and local
codes]
5-9
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ra
(5
c
LU
TJ
C
ra
2
o
re
o
O
0.
CO
in
o
5- 10
-------
Where:
AHL
C,
• npdes~
'npdes ~
R_
nntw
5.2.2 EFFLUENT-QUALITY BASED AHLs
National Pollutant Discharge Elimination System (NPDES) Permit
One of the most effective means of restricting the
discharge of toxic substances into waters of the
United States is through a NPDES permit limit. As
illustrated in Equation 5.5, the AHL based on NPDES
permit limit (AHLnpdes) is the pollutant loading at the
NPDES permit limit (Cnpdes * QpoJ divided by the
fraction of the pollutant not removed by the plant (1-
RpotJ. The NPDES permit limit can appear in many
forms—specific technology-based effluent
limitations, water quality-based pollutant limits,
whole effluent toxicity—and is commonly expressed
as milligrams per liter and usually specified as a daily
maximum2 and/or a monthly average3 discharge limit.
POTWs should use actual average POTW flow rate
data for Qpotw and not use design flows (see Exhibit 5-
1).
Water Quality Standards or Criteria
In general, POTWs will not have NPDES permit limits
for all of the POCs established during the local limits
analysis. In such cases, EPA recommends a POTW
base its effluent-quality-based AHL on State Water
Quality Standards (WQS) or Federal Water Quality
Criteria (WQC).4 State environmental agencies have
developed WQS that set maximum allowable pollutant
levels for their water bodies, specific to the receiving
stream reach's designated uses. Designated uses are
identified by taking into consideration the use and
value of the water body for public water supply, for
protection offish, shellfish, and wildlife, and for
recreational, agricultural, industrial, and navigational
Equation 5.5: AHL Based on NPDES
Permit Limit
AHL,
_ (8.34)(Cnpdes)(QpoJ
'npdes
AHL based on NPDES permit limit, Ib/day
NPDES permit limit, mg/L
POTW average flow rate, MGD
Plant removal efficiency from headworks to
plant effluent, as decimal
Conversion factor
Exhibit 5-1: Be Conservative in
Selecting Criteria
A recurring theme in this guidance manual is to be
conservative in making your choices. For example,
a POTWs NPDES permit limit for a single pollutant
can sometimes be expressed in two forms - daily
maximum and monthly average. EPA recommends
that only the more conservative monthly average
should be used in calculating NPDES-based AHLs.
Specific policies regarding this issue should be
explored with your Approval Authorities. See Section
6.4.1 fora more detailed discussion of the duration
of local limits.
purposes. Even though the POTW's NPDES permit
may not contain a numeric effluent limit for a POC, the permit will probably contain narrative provisions
requiring compliance with State WQS and prohibiting the discharge of any toxic pollutants in toxic
amounts. A local limit based on a State WQS helps ensure that the POTW can comply with the narrative
permit requirement specifying "no discharge of toxics in toxic amounts." In the absence of State WQS,
local limits may be based on the EPA ambient WQC found in Appendix D. These criteria are EPA's
recommended maximum pollutant levels for protecting aquatic life. They offer a sound basis for
developing local limits for pollutants with the potential for causing toxicity problems in the receiving
Daily maximum is the maximum allowable discharge of a pollutant during a 24-hour period.
3 Monthly average is the arithmetic average value of all samples taken in a calendar month for an individual pollutant
parameter.
4POTWs should, if possible, use their State's methodology to convert a WQS to NPDES permit limits and then use
these calculated NPDES limits to develop the MAHL. Also see Section 3.2.2.
5- 11
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stream. A local limit based on WQC generally would fulfill the narrative permit requirement specifying
"no discharge of toxics in toxic amounts."
Equation 5.6: AHL Based on Water
Quality Criteria
AHL _
Vfw
Where:
"pofw
8.34 =
: AHL based on water quality criteria, Ib/day
Receiving stream background concentration,
mg/L
State WQS or EPA WQC, mg/L
Receiving stream (upstream) flow rate, MGD
POTW average flow rate, MGD
Plant removal efficiency from headworks to
plant effluent (as decimal)
Conversion factor
As illustrated in Equation 5.6, the AHL based on
water quality criteria (AHLvq) is calculated as the
hypothetical pollutant loading to the water body at the
water quality limit [Cvq(Qstr+QpotJ] adjusted for the
background loading of the water body (Cst*Qstr) and
divided by the fraction of the pollutant not removed
by the plant (1- RpotJ. The receiving stream
background concentration (Cstr) can be an average
background stream concentration. The receiving
stream (upstream) flow rate (QstrJ should be either
the 7Q10 or 1Q105 flow based on the particular
criteria used. The average POTW flow rate (Qpotw)
should be based on actual plant data and not on
design flows.6 Under most water quality based
analyses, Equation 5.6 is sufficient and, consequently,
is the only one presented here. Another method is the
five-step process based on the one described in EPA's
Technical Support Document For Water Quality-
based Toxics Control (EPA, 199la).
In general, WQS and WQC are classified into three
groups: freshwater aquatic life protection, saltwater
aquatic life protection, and human health protection.
Freshwater and saltwater aquatic life criteria include chronic and acute toxicity criteria. Chronic toxicity
criteria are designed to protect aquatic organisms from long-term effects over the organisms' lifetime and
across generations of organisms, while acute toxicity criteria generally are designed to protect organisms
against short-term lethality. EPA offers the following guidance on the use of WQS and WQC:
• Hardness, pH, and Temperature Dependence. WQS and WQC for some metals depend on
the hardness of the receiving water. If the State has not factored this in, then the POTW
should obtain from the State the appropriate hardness value for its receiving stream and use
this value to determine the applicable WQS or WQC. Formulas for the common pollutants
that are affected by hardness can be found in footnote E to Appendix D. In addition, WQS or
WQC for some inorganic pollutants (e.g., ammonia) are pH- and/or temperature-dependent
and should be treated similarly. If the State has not established site-specific values, the
POTW should contact the State permitting authority to obtain appropriate temperature and
pH values for its receiving stream. These values should then be used to calculate WQS or
WQC for AHL determinations.
1Q10 refers to the lowest average flow for a one-day period that is expected to occur once every ten years. 7Q10
refers to the lowest average flow for a seven-day period that is expected to occur once every ten years. Both values are available
in the background documentation for the POTW's NPDES permit issuance and also can be obtained from the local district office
of the US Geological Survey (http://water.usgs.gov/local_offices.html).
Some States develop WQS to take into account dilution from the receiving stream and therefore the AHL calculation
in Equation 5.6 would not need to be adjusted for the background loading of the water body, Cstr*Qstr. POTWs should consult
with their State water quality control agencies.
5- 12
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Exhibit 5-2: How to Convert
Dissolved Metals Criteria to Total
Metals Criteria
NPDES permit writers often use metals
translators to convert dissolved water quality
standards or criteria to total recoverable
equivalents. Translators are specific to each
metal and may be 1) the theoretical
partitioning coefficients; 2) experimentally
determined through site-specific translator
studies; or 3) the EPA conversion factors
used to convert dissolved metals criteria to
total metals criteria. For establishing an
AHL, EPA recommends the theoretical
partitioning coefficient to calculate metal
translators detailed in The Metals Translator:
Guidance For Calculating A Total
Recoverable Permit Limit From A Dissolved
Criterion (EPA/823-B-96-007).
• Converting Dissolved Metals to Total Metals.
WQS and WQC for some metals may be
expressed in the dissolved form. Most metals
measurements, however, are reported in the total
or total recoverable form. Total and total
recoverable metals concentrations are always at
least as high as dissolved metals concentrations
because a fraction of the metal has sorbed to
particulate matter in the water. If dissolved
metals WQS or WQC are used to develop local
limits that are expressed as total metals, local
limits will be more stringent than if total metals
concentrations are used for the WQS. Therefore,
POTWs should convert dissolved metals WQS or
WQC into the total metals form before using
them to calculate water quality-based AHLs (see
Exhibit 5-2).
• Chronic and Acute Criteria Guidance. Chronic
and acute criteria should be used in the
calculation of AHLs to protect receiving water
quality. POTWs should not develop a monthly
average limit based solely on chronic criteria or a daily maximum limit based exclusively on
acute criteria. AHLs should be calculated based on chronic and acute criteria and the more
stringent criterion used for comparison with other AHLs.
• Stream Flow Guidance. To calculate limits based on chronic WQS, the receiving stream
flow rate should be consistent with State recommendations for chronic criteria, such as 7Q10
flows. To calculate limits based on acute criteria, the POTW should also use the State-
recommended receiving stream flow (e.g., 1Q10). POTWs should consult with their State
water quality agencies to confirm the correct flow values.
Resource Protection
Many State water quality protection laws that are the basis for POTW permits protect all waters of the
State including groundwater. Some POTWs have discharges that have the potential to impact
groundwater resources such as water reclamation projects to recharge groundwater, saline intrusion
barriers (to minimize the intrusion of saline groundwater into fresh groundwater) or disposal of treated
effluent via underground injection control (UIC) wells. Potential groundwater impacts can also be of
concern in effluent dominated streams in arid regions of the country. Therefore, groundwater protection
may need to be considered during local limits development. Some examples of groundwater protection
requirements that might need to be considered in local limits development include the following:
• Aquifer Protection Permits and Water Reuse Permits. Arizona issues aquifer protection
permits and water reuse permits to POTWs that discharge to effluent-dominated streams or
reuse the water for irrigation or other uses. The effluent limits in these permits are designed
to protect diminishing groundwater resources and to assure adequate effluent quality for the
reuse activity.7
Communication with John E. Watson, City of Phoenix Water Services Division, February 12, 2003.
5- 13
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• State NPDESPermits. New York State law specifies groundwater effluent discharge
limitations to protect groundwater quality. When an effluent may have an impact on
groundwater, State Pollutant Discharge Elimination System permits include effluent limits to
protect groundwater.8
• Under ground Injection Control (UlC)Program Permits. The Miami-Bade County POTW
system disposes effluent into underground injection wells. The POTW is required to comply
with UIC permits as well as its NPDES permits. The most stringent standards are being used
in local limits calculations.9
UIC, groundwater, or aquifer protection criteria can be used in place of NPDES permit limit (Cnpdes) in
Equation 5.5 to calculate AHLs based on resource protection.
5.2.3 SLUDGE-QUALITY BASED AHLs
In February 1993, EPA issued the Part 503 Biosolids regulations governing the use or disposal of sewage
sludge. Pollutant levels were established for three disposal alternatives: land application to condition the
soil or fertilize crops grown in the soil, surface disposal for final disposal, and incineration. The pollutant
levels, however, are different for each alternative. In addition to the Federal standards, States may have
sludge standards that are more stringent or that regulate more pollutants. Therefore, POTWs should
check with their State environmental agencies to confirm the applicable standards. Regardless of how a
POTW disposes of sludge, POTWs may wish to consider using land application "clean sludge" values
from 40 CFR 503.13 in their calculation of AHLs. Use of these criteria can improve a POTW's beneficial
use options for disposal of sludge. The further achievement of these standards is consistent with the
objectives of the National Pretreatment Program, which are listed at 40 CFR 403.2. Moreover, the land
application standards have a more extensive list of pollutants than either surface disposal or incineration
and they help control discharges of toxic pollutants that the other disposal alternatives do not address.
The Part 503 Biosolids Regulations also indicate that biosolids placed in a municipal solid waste landfill,
a fairly common practice, must meet only the Federal provisions of Part 258 RCRA Subtitle D landfill
regulations or delegated States' regulations. These provisions generally include a hazardous waste
evaluation, which is detailed in the last part of this section discussing municipal solid waste landfills.
Land Application
Federal sludge use or disposal regulations, found at 40 CFR Part 503, establish limitations for nine
common metals (arsenic, cadmium, copper, lead, mercury, molybdenum, nickel, selenium, and zinc) that
are primarily controlled by the Pretreatment Program. As shown in Appendix E, four types of land
application limitations were established and are known by the table number in which they appear:
• Table 1: Ceiling Concentrations [milligrams per kilogram (mg/kg)] establish the maximum
concentration that can be in sludge when it is land applied.
• Table 2: Cumulative Pollutant Loading Rates [pounds per acre (lb/acre)] establish the limits
that cannot be exceeded over the lifetime of the disposal site.
See Title 6 of the Official Compilation of Codes, Rules and Regulations of the State of New York (6 NYCRR)
Chapter 10, Part 703.6.
9 Memo from M. Mallard Greene, US EPA Region IV dated January 14, 2003 with a copy of the UIC Permit and
NPDES permit.
5- 14
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• Table 3: Pollutant Concentrations (mg/kg) set levels considered "clean" sludge and are
subject to less restrictive reporting requirements.
• Table 4: Annual Pollutant Loading Rates (Ib/acre/year) establishes maximum loadings that
can be applied in any given year.
As illustrated in Table 5-3, sludge standards are applied based on biosolid end use. For all land
application of biosolids, POTWs must comply with Table 1 ceiling concentrations. If its biosolids are
applied to agricultural land, a forest, a public-contact site, or a reclamation site, a POTW must comply
with either the cumulative loading rates in Table 2 or the monthly average pollutant concentrations in
Table 3. If its biosolids are applied to a lawn or home garden, the sludge pollutant concentration may not
exceed the monthly average pollutant concentrations in Table 3. If its biosolids are sold or given away in
a bag or other container for land application, the POTW must comply with monthly average pollutant
concentrations in Table 3 or the annual pollutant loading rates in Table 4.
Table 5-3: Land Application Requirements
Biosolids End Use
Applied to agricultural land, forest,
public contact site, reclamation
site
Applied to lawn or garden
Sold or given away in bag or
container
Table 1
Ceiling
limits
(mg/kg)
X
X
X
and
Table 2
Cumulative
limits
(Ib/acre)
X
or
and
and
Table 3
"Clean
Sludge"
Pol. Cone.
(mg/kg)
X
X
X
or
Table 4
Annual limits
(Ib/acre/
year)
X
To calculate AHLs based on sludge land application criteria, a POTW should:
• Determine which land application criteria apply to its biosolids by using Table 5-3.
Determine the applicable Table 1, 2, 3, or 4 criteria in Appendix E for each POC.
Convert the applicable Table 2 cumulative loading rates (CCUJ and applicable Table 4 annual
pollutant loading rates (Cann) to equivalent sludge standards (Cslgstd) using Equation 5.7 and
Equation 5.8, respectively. The values for site life (SL) and site area (SA) are determined by a
POTW's sludge management plan. The POTW determines how long the sites will be used
and how much land or acreage is needed for disposal of the total annual volume of sludge
generated. Generally, the amount of land needed is determined by dividing the total annual
sludge production by the agronomic application rate for nitrogen based on the crop grown.
• Determine the lowest sludge concentration standard (Cslgstd) derived from Equation 5.7,
Equation 5.8, Table 1 Ceiling Concentrations, Table 3 Monthly Average Pollutant
Concentrations, and suitable State sludge standards.
5- 15
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• Use Equation 5.9 with the lowest sludge
concentration standard (Cslgstd) to
determine the sludge land-application-
based AHL for conservative pollutants.
As shown in Equation 5.9, the AHL for
land application (AHLsldg) is the pollutant
loading of sludge at the sludge standard
[(CslgJ * (PS/100) * (Qslds) * (GsMg)J,
divided by the overall plant removal
efficiency (RpoJ.
EPA offers the following guidance in performing the
calculations in Equations 5.7 through 5.9:
• Values greater than the Table 1 ceiling
concentrations can not be used for Cslgstd,
because the regulations governing use or
disposal of sewage sludge (40 CFR Part
503) expressly prohibit any form of land
application if the sludge exceeds these
concentration levels for any regulated
component. In addition, EPA
recommends that the POTW consider
using the more conservative pollutant
concentration levels for "clean sludge"
specified in Table 3 because these levels
are more protective of the environment,
promote greater flexibility in the
beneficial use of sludge, and are subject
to less restrictive reporting and
management requirements. This grade of
sludge would meet the criteria for
"exceptional quality"or "low pollutant
concentration" sludge.10
Equation 5.7: Converting Table 2
Cumulative Loading Rates to Dry Sludge
Concentrations
(Ccum)(S/\)
3046(SL)(QWa)(PS/100)(GsB9)
Equation 5.8 : Converting Table 4 Annual
Loading Rates to Dry Sludge
Concentrations
(cann)(s/o
3046(Q,a
Where:
PS =
Q,a =
SA =
SL =
3046 =
Equivalent sludge standard, mg/kg dry
sludge
Federal or State land application cumulative
pollutant loading rate, Ib/acre over the site
life
Federal or State land application annual
pollutant loading rate, Ib/acre/yr
Specific gravity of sludge, kg/L
Percent solids of sludge to disposal
Sludge flow rate to bulk land application
(agricultural, forest, public contact, or
reclamation site), MGD
Sludge flow rate to non-bulk land application,
MGD
Site area, acres
Site life, years
Unit conversion factor
Generally, POTWs can assume the specific gravity of sludge (Gsldg) equals that of water (1
kg/L). For atypical wet sludge containing about 5 percent solids (PS) the specific gravity of
the sludge does not differ significantly from that of water. However, drier sludges such as
dewatered sludges with 30 percent solids may have a specific gravity of 1.1 kg/L or greater.
In these circumstances, if the specific gravity is not considered, AHLs will be understated and
any local limits based on these AHLs may be unnecessarily conservative. Therefore, the
POTW can measure the specific gravity of its sludge to correct for the error introduced as the
percent solids rises. If the POTW does not have data on the specific gravity of its sludge, it
should assume conservatively that the specific gravity is 1 kg/L. Guidelines for determining
the specific gravity of sludge are provided in Appendix S.
10,
'See Chapter 2 in A Plain English Guide to the EPA Part 503 Biosolids Rule, EPA/832/R-93/003, September 1994
5- 16
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If the POTW's data for sludge flow rate to disposal are expressed in dry metric tons per day
(or can be converted to dry metric tons per day), a specific gravity factor is not needed. An
equation for calculating an AHL using dry metric tons per day is provided in Appendix T.
Table 1 sludge ceiling concentrations are instantaneous maximum concentrations, while the
"clean sludge" criteria in Table 3 are monthly average concentrations. See Section 6.4.1 for a
discussion of how the types of criteria - monthly average, instantaneous maximum - affect
the type of local limit developed.
Surface Disposal
Sludge surface disposal occurs at dedicated disposal
sites, surface impoundments, waste piles, monofills,
or dedicated beneficial use sites. The difference
between surface disposal and land application is that
land application is performed at rates that do not
exceed the agronomic rates of the fertilizer value of
the sludge. For a more extensive discussion of
surface disposal, see the sludge regulations at 40 CFR
503.20. Surface disposal regulates only three metals
(arsenic, chromium, and nickel) at levels near the
"clean sludge" levels for land application. The
standards apply to sludge disposed at facilities
without a liner or a leachate collection system. AHLs
based on sludge surface disposal quality should be
calculated in the following manner:
Table 1 (40 CFR 503.23) sludge surface
disposal criteria should be used directly
as the sludge standard (Cslgstd) in Equation
5.9 for conservative pollutants.
Equation 5.9: AHLs Based on Sludge
Land Application and
Surface Disposal Criteria
(for conservative pollutants)
AHL.
_ (8.34)(Cs)gsM)(PS/1 OOKQ^KG^)
r\__|...
Cslgstd
PS =
Where:
AHLsldg = AHL based on sludge, Ib/day
Sludge standard, mg/kg dry sludge
Percent solids of sludge to disposal,
Qsldg = Total sludge flow rate to disposal, MGD
Rpotw = Plant removal efficiency from headworks to
plant effluent, as decimal
GsBg = Specific gravity of sludge, kg/L
8.34 = Unit conversion factor
• If the sewage sludge unit is less than 150 meters from the property line, Table 2 (40 CFR
503.23) sludge disposal criteria, based upon distance from the property line, should be used
directly as the sludge standard (Cslgstd) in Equation 5.9 for conservative pollutants. See
Appendix E for a list of Table 1 and Table 2 surface disposal options.
In addition, POTWs should be aware that surface disposal regulations allow for site-specific limits. Site
owners or operators may have requested surface disposal criteria from the permitting authority in place of
the Table 1 or Table 2 sludge surface disposal criteria. Therefore, the POTW should check with the
disposal site owner/operator to determine standards that apply. If the State has developed more stringent
sludge disposal standards for surface disposal, the POTW needs to use those standards in its calculation of
AHLs when using Equation 5.9.
Incineration
Incineration, the third method of sludge disposal, typically regulates arsenic, cadmium, beryllium,
chromium, lead, mercury, and nickel. Limits are site-specific and based on feed rate, stack height
(dispersion factor), incinerator type, and control efficiency. EPA offers the following guidance on
incineration-based AHLs:
5- 17
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• POTWs that dispose of their sludge through incineration should determine AHLs based on
the calculated sludge standards that apply to the sludge feed to the incinerator. These
standards may have been calculated by the owner/operator of the incinerator (and listed in a
sludge disposal agreement), the State, or EPA from the equations provided in 40 CFR Part
503, and should be expressed in mg/kg dry sludge. These standards should be used directly
as the sludge standard (CslgsJ in Equation 5.9 to determine the AHL.
• If no sludge standards have been calculated for the sludge feed to the incinerator, POTWs
should use the 40 CFR Part 503 equations (provided in Appendix T) to determine the
maximum pollutant concentrations for the incinerator feed. These standards should be used
directly as the sludge standard (Cslgstd) in Equation 5.9 to determine the AHL. As a general
rule, an AHL for incineration will be an order of magnitude or greater than an AHL based on
land application.
Municipal Solid Waste Landfill's Hazardous Waste Requirements
According to 40 CFR 503.4, "any person who prepares sewage sludge that is disposed in a municipal
solid waste landfill unit shall ensure that the sewage sludge meets the requirements of 40 CFR Part
258. . . ." Part 258 does not allow municipal solid waste landfill units to accept hazardous waste.
Whether a POTW's sewage sludge is hazardous waste may be determined by using EPA's TCLP test. If
determined to be hazardous waste, sludge must be disposed of according to RCRA requirements. POTWs
cannot dispose of sludge determined to be hazardous waste in solid waste landfills designated for non-
hazardous waste. In general, POTWs will not generate sludge that exceeds TCLP limits.
However, because the costs and liabilities associated with the management and disposal of hazardous
sludge are high, POTWs may find it advantageous to periodically run the TCLP test on their sludge to
identify any trends of increasing pollutant concentrations that may lead the sludge to be considered
hazardous waste. The POTW should compare the quality of its sludge with the limits in the TCLP and, as
necessary, set local limits to help ensure that the pollutant levels in its sludge do not exceed TCLP levels.
If TCLP test results are close to or exceed the TCLP limit, the POTW needs to develop AHLs based on
TCLP criteria. To develop TCLP-based AHLs, the POTW should:
• Determine the dry weight metals and toxic organics concentrations (in mg/kg dry sludge) that
would be protective against sludge being classified as hazardous based on the TCLP test from
sampling data. The POTW can collect site-specific data for both total pollutant
concentrations in the sludge and TCLP concentrations (10-12 data pairs) and use these data to
correlate TCLP concentrations with total concentrations in the sludge.
• Use these dry-weight, correlation-based concentrations directly as the sludge standard (Cslgstd)
in Equation 5.9 to determine the AHL.
5- 18
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Exhibit 5-3: The Challenge in
Determining
Plant Inhibition Values
Determining site-specific inhibition values is
difficult because the exact point at which
pollutant concentration inhibition takes place
is difficult to identify. For instance, an
activated sludge system's mixed liquor may
run at about 1 mg/L zinc. An industrial
discharge causes the plant to violate its
NPDES permit by upsetting the plant and
raising the mixed liquor concentration to 100
mg/L zinc. How can one determine at which
concentration the inhibition took place? The
concentration lies somewhere between 1 and
100 mg/L. An inhibition value set at 100
mg/L would be incorrect because a lower
value could have caused the inhibition.
Some POTWs have attempted to estimate
site-specific inhibition values by simply using
the highest observed pollutant concentration
in the biological process that did not cause
interference.
5.2.4 INHIBITION-BASED AHLs
Secondary and Tertiary Treatment Unit Inhibition
Pollutant levels in a POTW's wastewater or sludge may
cause operational problems for biological treatment
processes involving secondary and tertiary treatment.
Disruption of a POTW's biological processes is referred to as
inhibition and can interfere with a POTW's ability to remove
biochemical oxygen demand (BOD) and other pollutants. A
POTW should assess any past or present operational
problems related to inhibition and follow the protocol
outlined below.
• No Past Inhibition Problems at POTW.
POTWs may not need to calculate AHLs to
protect against inhibition because current
loadings are acceptable to the treatment work's
biological processes. However, a POTW may
still choose to calculate AHLs based on
biological process inhibition criteria to prevent
future loadings that may cause inhibition and
should follow the steps outlined below for
POTWs with past inhibition problems.
• Past Inhibition Problems at POTW. POTWs
should calculate AHLs based on inhibition
criteria. If site-specific data are needed (see
Exhibits 5-3 and 5-4), the POTW may choose to
substitute pollutant concentrations that either
have occurred in the applicable biological
process or are currently in its influent and have
not caused inhibition, in place of process
inhibition values that have been reported in
studies published by EPA or in professional
journals. Inhibition criteria for select secondary
treatment units (such as activated sludge and
trickling filters) and one tertiary treatment unit
(nitrification) are presented in Appendix G.
Site-specific inhibition data are preferred to literature data
because they more accurately measure pollutant
concentrations that cause inhibition in actual biological
treatment environments. Inhibition of biological treatment
processes could be a function of toxic compounds (not a
single toxic compound), synergism, antagonism, pH, temperature, hardness, stressed conditions,
microorganism acclimation, and the number and variety of microorganisms present. Sometimes based on
laboratory studies using pure cultures, literature values can indicate inhibition at much lower
concentrations than in actual biological treatment environments for the following four main reasons: 1)
organic chemicals combine with the metals and reduce metal availability to the microbes; 2) activated
sludge environments generally have a variety of organisms present that may not be as sensitive to metal
Exhibit 5-4: Inhibition Value Study
by Chesterfield County (VA)
Chesterfield County's Pretreatment Program
conducted a site-specific evaluation of
inhibition values for several heavy metals as
part of its recent recalculation of local limits. A
pilot system was fed with primary effluent from
the full-scale facility and was loaded with
varying levels of several heavy metals to
determine the loading rate that caused
measurable deterioration in process
performance. The measured inhibition values
for this plant were typically found to be much
higher than those given in Appendix G. In this
case, the controlling factor became the
inhibition potential of the anaerobic digesters,
and it was possible to substantially increase
the local limits as a result of the data
generated from pilot testing. [Contact Abha
Sharma of Chesterfield County (VA)
Pretreatment program.]
5- 19
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concentrations; 3) metals can chelate toxic organics, reducing their toxicity to nitrifiers; 4) acclimated
biological treatment populations can accept higher concentrations of metal and organic toxins than
laboratory cultures. In addition to the technical drawbacks, literature values, if eventually the limiting
basis of a local limit, will most likely engender more regulatory scrutiny.
Equation 5.10 is used to calculate inhibition-based
AHLs for secondary treatment processes such as
aerated lagoons, stabilization ponds, activated
sludge, rotating biological contactors, and trickling
filters. Equation 5.11 is used to calculate inhibition-
based AHLs for tertiary treatment for various
processes to remove nitrogen, phosphorus,
suspended solids, organics, metals, and dissolved
solids (see Figure 5-3). As shown in Equation 5.10,
the AHL based on secondary treatment unit
inhibition (AHLsec) is calculated by dividing the
pollutant loading to the secondary treatment unit at
the inhibition criterion (Cinhib2 * QpotJ by the fraction
of the pollutant not removed after primary treatment
(1 - RpriJ. As shown in Equation 5.11, the AHL
based on tertiary treatment unit inhibition (AHLter) is
calculated by dividing the pollutant loading to the
tertiary treatment unit at the inhibition criterion
(Cmhibs * Qpo J by the fraction of the pollutant not
removed after secondary treatment (1 - Rsec). The
POTW flow rate (Qpotw) should be calculated using
actual average flow data and not design flow.
Appendix U shows where to sample in various plants
to calculate inhibition-based loading. (Note that in
many POTWs nutrient removal is often more like an
advanced secondary process that occurs in the same
basin as an activated sludge process. In these cases,
the same primary removal efficiency (Rprim), would
be used in both Equations 5.10 and 5.11.)
Equation 5.10: AHLs Based On
Secondary Treatment Inhibition
,,,, 8.34(Cfn/,j62)(QpoJ
Equation 5.11: AHLs Based On Tertiary
Treatment Inhibition
Where:
AHLter =
Rprim =
8.34 =
AHL
AHL based on secondary treatment
inhibition, Ib/day
AHL based on tertiary treatment inhibition,
Ib/day
Inhibition criterion for secondary treatment,
mg/L
Inhibition criterion for tertiary treatment, mg/L
POTW average flow rate, MGD
Removal efficiency from headworks to
primary treatment effluent, as decimal
Removal efficiency from headworks to
secondary treatment effluent, as decimal
Unit conversion factor
Sludge Digester Inhibition
Sludge digestion is also a biological process that can
be upset if pollutants are allowed to accumulate to toxic levels. Plant-specific sludge digestion inhibition
thresholds, like inhibition of secondary treatment, are difficult to know. Literature data on sludge digester
inhibition criteria are listed in Appendix G. The preponderance of sludge digestion inhibition data are for
anaerobic digesters. There is no publicly available data about the effect of metals on aerobic digestion of
sludge.
5-20
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Equation 5.12: AHLs Based On Sludge
Digestion Inhibition (Conservative
Pollutants)
AHL,
Equation 5.13: AHLs Based On Sludge
Digestion Inhibition (Non-conservative
Pollutants)
•'dgsinhlb
Cdgstr
Where:
AHLdgstr= AHL based on sludge digestion inhibition,
Ib/day
LM = POTW influent loading, Ib/day
Sludge digester inhibition criterion, mg/L
Existing pollutant level in sludge, mg/L
Qdgsfr = Sludge flow rate to digester, MGD
Rpotw = Plant removal efficiency from headworks to
plant effluent, as decimal
8.34 = Unit conversion factor
modeling to predict existing air emissions (Cair)
wastewater collection and treatment systems, is available at:
http://www.epa.gov/ttn/chief/software/water/index.html.
POTWs can determine pollutant removal efficiency by
volatilization (Rvol) by examining sampling data of
influent, effluent, sludge, and air and determining the
portions of the total removal efficiency associated with
adsorption to the sludge, biodegradation, and
volatilization. In addition, POTWs can model the
removal process to predict pollutant removal efficiency
by volatilization.
5.3 AHLs FOR CONVENTIONAL AND NON-
CONVENTIONAL POLLUTANTS
This section provides guidance on the development of
AHLs for three conventional pollutants [BOD, Total
Suspended Solids (TSS), oil and grease] and one non-
conventional pollutant (ammonia), whose unique
circumstances allow for special mechanisms for their
AHL development.
Using the steady-state mass balance approach across
the influent to the digester, Equation 5.12 calculates
the AHL based on sludge digestion inhibition
(AHLdgstr) for conservative pollutants such as metals.
AHLdgstr is calculated by dividing the pollutant loading
at the inhibition criterion to the digester (Cdgstinhib *
Qdgstr) by the removal efficiency across the entire
POTW (RpotJ. As shown in Equation 5.13, for non-
conservative pollutants (AHLdgstr) is found by
multiplying the POTW influent loading (Linfl) by the
ratio of the sludge digester inhibition criterion
(Cdgstinhib) and the level of the POC in the sludge
(Cdgstr) •
5.2.5 AIR-QUALITY BASED AHLs
In rare circumstances, POTWs that have been
regulated as air pollution sources and have air
emissions standards for specific toxics may need to
consider calculating AHLs for those toxics (see
Section 3.2.4). AHLs based on air emissions
standards can be calculated using either Equation
5.14, which uses the air standard and removal
efficiency by volatization, or Equation 5.15, which
uses air standards and existing air emissions. The
POTW can conduct air emissions sampling or conduct
The most widely used model, EPA's Water9 model for
Equation 5.14: AHLs Based On Air
Criteria and Volatization Rates
_ 0.0022(Ca/reftld)
Equation 5.15: AHLs Based On Air
Criteria and Existing Emissions
C_,
Where:
AHLa!r = AHL based air emission standards, Ib/day
Lin,i= POTW influent loading, Ib/day
Ca»5h7
-------
5.3.1 BOD/TSS
One of the most commonly documented industry-related causes of POTW effluent violations is the
discharge of excessive conventional pollutants, particularly BOD and TSS (see Exhibit 5-5). As stated
earlier in the chapter on POC development, POTWs should develop MAHLs for all NPDES-permitted
conventional pollutants and understand the degree to which the plant is loaded. In fact, some EPA
regions require any wastewater treatment plant that operates at 80 percent of any NPDES permitted
conventional pollutant MAHL for three months of the calendar
year to calculate a MAIL and establish local limits for those
pollutants. To establish MAHLs for BOD and TSS, EPA
recommends the following:
The POTW's rated average design capacity, along
with any improvements subsequent to construction
that have increased plant capacity, should be used
as a "monthly average"- based MAHL. The
treatment works is designed to have the capacity to
consistently treat a specified amount of
conventional pollutants to acceptable levels for
discharge. A copy of the approved design capacity
may be available from the State as part of the . . _ .„„,. „,„„„„„ „
, •: . , „ ~^ increased from 100:5:1 to 100:11:2.
design or operating manual for the POTW.
Exhibit 5-5: Less BOD, More
Ammonia and Phosphorous
In the late 1980s, the City of Trenton
Wastewater Treatment Plant (WWTP)
violated NPDES permits due to excessive
BOD5 loading. Today, BOD5 loading has
been cut in half after two industries that
accounted for half of the BOD5 loading
upgraded their existing treatment facilities by
including nutrient addition and longer
retention times. However, the industries'
nutrient addition led to problems with high
amounts of Ammonia-N and Phosphorous
discharged to the WWTP. The ratio of BOD
to Ammonia-N to Phosphorous has
The POTW's peak loading capacity should be
used as the "daily maximum"- based MAHL. Based on a peaking factor, peak loading
capacity reflects the plant's ability to handle diurnal, wet weather, or seasonal peaks.
EPA recognizes that sometimes average design capacity and the corresponding peak loading factor may
be too conservative when considering the industrial allocation of conventional pollutants. Therefore, the
POTW can provide a technically defensible argument for establishing a MAHL for the plant. These
arguments could include the following:
• Performing mass balance calculations on the entire plant for the current condition, and scale
up the plant loading until loading rates for individual processes exceed design guidelines,
including solids handling facilities.
• Verifying capacity of hydraulic structures.
• Performing detailed modeling of biological process capacity under current loading conditions
using software (e.g., BioWin by Envirosim). Calibrate the model to current conditions and
then increase loading rates to estimate failure.
• Determining maximum biological process loading compared to typical design guidelines -
including aeration equipment capacity, basin sizing, mixing energy, secondary clarifier
sizing, return activated sludge/waste activated sludge capacity, nutrient removal capacity,
winter and peak operation.
Evaluating current operating conditions. For example, a plant with three activated sludge
trains is operating reliably at 2/3 of its design loading with only one train in service.
5-22
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• Stress testing of individual processes. Increase loading through a single process train until
failure is recognized.
Benchmarking against similar plants and processes.
Pilot or bench-scale testing of unit operations that have been determined to possibly be a
bottleneck for plant capacity.
Smaller plants should incorporate a safety factor in developing the BOD/TSS MAHL for the plant using
these methods.
5.3.2 AMMONIA
Typical concentrations of ammonia in untreated domestic wastewater range from 10 to 50 mg/L.
Therefore, significant non-domestic industrial sources of ammonia will be unusual and the result of
industry-specific activities. If the POTW was designed to remove ammonia through specific processes
such as nitrification and denitrification, breakpoint chlorination, or ammonia stripping, the engineering
specifications that establish design loading rates should be used as the MAHL. However, for most
conventional activated sludge and trickling filter plants, ammonia removal is incidental, and a study of the
plant will have to be conducted to determine its removal efficiency. The AHL for ammonia can then be
determined using Equation 5.5. When the AHL is determined using site-specific removal efficiencies and
Equation 5.5, a safety factor of at least 20 percent should be applied. NPDES ammonia limits are often
seasonal, with more stringent limits in place during warmer weather. This needs to be taken into
consideration in the development of local limits. A seasonal limit for ammonia might be developed for
Ills as well.
5.3.3 OIL AND GREASE
The term fats, oil, and grease (FOG) includes materials of vegetable, animal, and mineral origin. Mineral
oils include petroleum, hydrocarbon, and or non-polar fats, oils, and grease. Petroleum-based oil and
grease (non-polar concentrations) occur at businesses using oil and grease; and can usually be identified
and regulated by municipalities through local limits and associated pretreatment permit conditions.
Animal-based and vegetable-based oil and grease (polar concentrations) are more difficult to regulate
when the major source is a large number of restaurants and fast-food outlets in the collection system.
Collection system issues related to animal-based and vegetable-based oil and grease are addressed in
Section 8.3 dealing with flow obstructions.
The pretreatment regulations 40 CFR403.5(b)(6) prohibit the discharge of "petroleum oil, non-
biodegradable cutting oil, or products of mineral oil origin in amounts that will cause interference or pass
through." Most POTWs have adopted 100 mg/L as their local limit for petroleum-based oil and grease
because of its history of being protective of the treatment plant and receiving stream. Additionally, the
5-23
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limit of 100 mg/L is achievable with the application
of best management practices (BMPs) or generally
available pretreatment. The basis of the 100 mg/L
limit is an April 1975 EPA document titled
Treatability of Oil and Grease Discharged to
Publicly Owned Treatment Works. This study
found a dilution of at least two occurs in collection
systems and that influent to biological treatment
systems should contain less than 75 mg/L and
preferably less than 50 mg/L oil and grease of
mineral or petroleum origin to prevent interference.
The 100 mg/L was recommended as the value that
prevents interference based on the dilution.
However, the basis for the 100 mg/L FOG limit is
not site specific. The limit should be justified with
additional information in order to be considered a
technically based limit. See Exhibit 5-6 for a
description of how the City of Richland,
Washington addressed this limit.
Developing a technically based local limit for FOG
requires an understanding of the unique manner in
which oil and grease can cause interference or pass
through. EPA recommends two different methods:
• With FOG limits often included in
NPDES permits, POTWs could
determine FOG removal efficiency
using Equation 5.5 to develop an AHL
based on the plant's numeric NPDES
permit limits.
Although animal- and vegetable-based FOG at reasonable concentrations are easily broken
down, petroleum-based, non-polar FOG can interfere with both aerobic and anaerobic
treatment. Petroleum-based oils can coat the organisms responsible for biological treatment
and result in less effective oxygen transfer rates. In anaerobic processes, excessive
concentrations of solid grease in digesters can reduce the effectiveness of the process, lead to
structural damage to pipes and supports as a result of the weight of scum and grease, and
present accumulation problems when supernatant is recycled. When digesters are well mixed
and heated to minimize scum loads, reasonable FOG concentrations can be anaerobically
digested. If these types of process inhibition are occurring, POTWs could calculate FOG
primary and secondary removal efficiencies, determine FOG inhibition criteria, and use
Equations 5.10 and 5.11 to determine AHLs based on inhibition. See Exhibit 5-7 for a
description of how the City of Portland established an inhibition-based local limit for non-
polar FOG.
Exhibit 5-6: City of Richland, Washington,
POTW Evaluates FOG Removal Efficiency
The Richland POTW and the Washington Department of
Ecology ("WDOE") sought to address a laundry's inability
to meet its local limits permit limit of 100 mg/L FOG.
During 1995, the laundry discharged to the POTW at an
average of 200 mg/L FOG.
Monitoring of the POTW indicated average influent levels
for FOG of 25 mg/L and effluent levels averaging less
than 1 mg/L — a FOG removal efficiency of 96 percent.
Respirometer tests on samples of the laundry's
wastewater indicated that the wastestream was a
biodegradable food source and easily metabolized by the
POTWs microorganisms.
Despite the relatively high concentration of FOG (200
mg/L) in the laundry's effluent, based on the results of this
evaluation, the city eliminated the laundry's FOG effluent
limit but continued a sampling schedule. Furthermore,
the results support previous EPA findings that petroleum
based oil and grease compounds "can be degraded to
various degrees especially if the microorganisms are
acclimated to use the compounds as a substrate", and
that "[i]f oil and grease are biodegradable and in a
physical state [i.e., emulsified] that does not cause
clogging or undue maintenance problems in the
wastewater facilities, the discharge of these substances
can be accepted in a wastewater treatment system."
(EPA, Treatability of Oil and Grease Discharged to
Publicly Owned Treatment Works (April 1975), p. 11)
City of Richland POTW, Richland Laundry & Dry
Cleaning, Inc. Wastewater Discharge Permit CR-IU003
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Exhibit 5-7: City of Portland, Oregon Uses
Current Influent Loading to Develop Non-
Polar FOG Local Limit
The City of Portland wanted to develop a local limit for
non-polar FOG to avoid any potential for inhibition at its
POTW. However, as often is the case in developing
inhibition- based local limits, the MAHL was difficult to
define (see Exhibit 5-3) because the plant had never
experienced inhibition. The City determined that "the
POTW had not experienced process inhibition from non-
polar-FOG under these conditions. Therefore the
development of a local limit based upon current loading
will be protective against process inhibition." The current
loading of 8.6 mg/L of non-polar FOG was used as the
inhibition-based MAHL.
Although using current loading to establish an inhibition
based MAHL is conservative, the methodology provides a
scientific basis for the development of local oil and
grease limits. Based on this MAHL, the City established
a non-polar FOG local limit of 110 mg/L.
See Industrial Source Control Division, Bureau of
Environmental Affairs, City of Portland, Final Report -
Update of Local Discharge Standards (April 1996).
5.4 DETERMINATION OF THE MAXIMUM
ALLOWABLE HEADWORKS LOADING
After calculating AHLs for each POC for a variety
of environmental criteria, MAHL determination is
simple. The lowest (i.e., most stringent) of the AHLs
for each POC is selected as the MAHL for that
pollutant. Influent loadings below the MAHL will
lead to compliance with the AHLs based on all
environmental and treatment plant criteria. The
MAHL will be used for all further steps of local
limits development and evaluation.
5.5 SAMPLE MAHL CALCULATION
A POTW is attempting to determine the MAHL for
copper. From its local limits sampling plan, the
POTW has determined the following plant data:
• Plant removal efficiency from
headworks to plant effluent, Rpotw = 0.85
• Removal efficiency from headworks to
primary treatment effluent, Rprim= 0.65
• Average plant flow rate, Qpotv = 10 MOD
• Percent solids in the sludge, PS = 5 percent
• Specific gravity of sludge, Gsludge = 1 kg/L
• Average sludge flow rate, Qsiudge = 0.05 MOD
For copper, the POTW determines that the suitable environmental criteria are the following:
• The POTW has a specific copper limit in its NPDES permit, Cnpdes = 1 mg/L copper.
• With biosolids being used ultimately for lawn application, Federal Sludge Land Application
Table 3 "Clean Sludge" Limits, Cslgstd= 1,500 mg/kg copper, are applicable.
• Although inhibition has never taken place at the plant's activated sludge secondary treatment
unit, the POTW wants to develop an AHL based on activated sludge inhibition. Based upon
the highest observed copper concentration in the secondary treatment unit that did not cause
inhibition, the POTW sets the inhibition criterion for secondary treatment, Cinhib2 = 1 mg/L
copper.
The following equations for AHLs based on NPDES limits (Equation 5.5), sludge standards (Equation
5.9), and secondary treatment inhibition (Equation 5.10) are used.
AHL
npdes
(8.34)(lmg/L)(10MGD) = 556
(1 - 0.85)
AHL,
_ (8.34)(Cs)gsM)(PS/100)(Q^>)(Grtfc) (8.34)(1,500mg/frg)(5/100)(0.05M3D)(1 kg/L)) _
R,
potw
0.85
37/ft/day
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AHL _ .jnM,2po 8.34(1mg/L)(10MGD)_238
-
From these three AHLs, the most stringent (lowest) AHL based on the sludge standard (AHLsldg) was
chosen as the MAHL for copper at 37 Ib/day.
5.6 SUMMARY
After reviewing Chapter 5, POTWs should be able to:
• Calculate POTW removal efficiencies for each POC
• Calculate AHLs for each environmental criteria
• Determine MAHL as the most stringent AHL for each POC
Chapter 6 describes how to assess the need for local limits, allocate the maximum allowable industrial
loadings, and develop and implement final local limits and BMPs.
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CHAPTER 6 -
DESIGNATING AND IMPLEMENTING LOCAL LIMITS
Chapter 6 provides guidance on how to:
• Determine the need for new local limits after establishing Maximum Allowable
Headworks Loadings (MAHLs).
• Calculate Maximum Allowable Industrial Loadings (MAILs).
• Compare MAIL allocation and implementation methods.
• Allocate MAILs to controlled dischargers.
• Perform a common sense assessment of local limits.
• Use best management practices.
• Provide public participation.
• Gain Approval Authority approval.
• Conduct public outreach.
• Select the appropriate control mechanism to apply local limits.
6.1 DETERMINATION OF THE NEED FOR NEW LOCAL LIMITS
Once a POTW has calculated MAHLs for all of its pollutants of concern (POCs), it can determine for
which pollutants it will require local limits. In making this pollutant-by-pollutant evaluation, the POTW
will also want to consider historical issues and the degree to which current influent loadings approach
calculated MAHLs. For example, the concentration of some pollutants in the POTW influent may be far
below the calculated MAHLs. These pollutants are unlikely to cause problems for the POTW, so the
treatment works may conclude that local limits for them are unnecessary. EPA recommends that the
POTW document such decisions and discuss them with its Approval Authority, as needed.
Some Approval Authorities require that local limits be established for a specific set of pollutants
regardless of the outcome of the headworks loading analysis. For example, some Approval Authorities
specify that local limits be developed for arsenic, cadmium, chromium, copper, cyanide, lead, mercury,
molybdenum, nickel, selenium, silver, and zinc regardless of whether they are in the POTW's influent. If
such specific guidance is not available, EPA recommends that the POTW conduct evaluations for each
POC.
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No single approach applies for all pollutants at all POTWs. The approaches presented below are
intended to determine which POCs deserve to be covered by new local limits. In EPA's view, a POTW
should not use the approaches below in deciding whether to continue to control a particular
pollutant by a local limit because the enforcement of the local limit may be the reason that the
pollutant loading has been reduced or is no longer causing problems. If the local limit were
removed, industrial users (IDs) may discontinue their use of wastewater pretreatment and POTW
loadings may increase above the threshold in the criteria. Re-evaluation of existing local limits is
discussed in Chapter 7.
6.1.1 ACTUAL LOADINGS vs. MAHL
Equation 6.1 compares actual POTW loadings to the calculated MAHLs for individual POCs. A POTW
would use this equation to calculate the percentage of MAHL being received at the POTW. The average
and highest daily influent loading should be calculated. EPA recommends that local limits are needed
when:
Average influent loading of a toxic pollutant
exceeds 60 percent of the MAHL.
Maximum daily influent loading of a toxic
pollutant exceeds 80 percent of the MAHL any
time in the 12-month period preceding the
analysis.
Monthly average influent loading reaches 80
percent of average design capacity for BOD,
TSS, and ammonia during any one month in the
12-month period preceding the analysis.
Equation 6.1: Actual Loading vs.
MAHL Calculation
"-INFL
MAHL
x100
Percentage of the MAHL
Current influent loading (average or
highest daily), Ib/day
MAHL = Calculated MAHL Ib/day
EPA recognizes that these percentages to trigger local
limits development are default assumptions that can vary from plant to plant. The approach used for
toxic pollutants is more conservative because most POTWs are not designed to treat toxic pollutants.
6.1.2 NONCOMPLIANCE DUE TO PASS THROUGH OR INTERFERENCE
The basic purpose of the pretreatment program is to prevent pass through and interference, and the
General Pretreatment Regulations require that local limits be established to prevent them. EPA
recommends that in the absence of strong evidence that the cause of pass through or interference has
been eliminated, a POTW retain local limits for the pollutants causing historic violations. By reviewing
past NPDES permit violations, sludge disposal restrictions, or inhibition incidents, the POTW can
identify the pollutants for which it should set or maintain local limits.
6.1.3 ESTABLISHING LOCAL LIMITS FOR CONVENTIONAL POLLUTANTS
Conventional pollutants such as BOD, TSS and ammonia require additional evaluation before decisions
are made to set a MAIL and put in place a local limit. Controlling conventional pollutants from Ills must
be evaluated in a broader context, because the POTW was designed to treat conventional pollutants. A
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comprehensive evaluation of the POTW may be needed (see Section 5.3) and many alternatives in lieu of
or in addition to local limits may be considered.
A POTW that is approaching its design capacity for BOD/TSS should begin planning to avoid future
violations. NPDES permits sometimes include a reporting requirement when the POTW begins to
operate at 80-90 percent of its original design capacity for 90-180 consecutive days. EPA recommends
using a similar threshold as a basis for investigating alternatives for reducing or responding to future
conventional loadings. If the rate of increase in influent conventional pollutants loadings suggests that
the full capacity of the plant will be utilized within five to seven years, then planning may need to begin
immediately. The planning need not automatically assume that local limits would be set for conventional
pollutants. Several alternatives should be investigated in addition to local limits. These include:
• Minimizing growth of the community by controlling sewer connections.
• Initiating POTW modifications to optimize performance (through chemical additions,
filtration, membrane filtration, and other methods).
• Modifying operation or flow configurations.
• Expanding POTW capacity via facilities planning.
• Reducing industrial sources of conventional pollutants through incentives and
disincentives.
Each POTW has a unique, historical background of successful operation with respect to conventional
pollutants, and whether each POTW can operate successfully at a given (elevated) loading will vary from
plant to plant. Some of these concepts are reviewed in Section 5.3.
POTW expansions can take up to 5 years. Therefore, it is vitally important to monitor loadings to the
plant against the POTW design capacity. Failure to plan in a timely manner can result in NPDES
violations. With respect to nitrogen management, it is useful to note that nitrogen removal at the POTW
typically requires four times the biological treatment volume needs of BOD, hence the need to quantify
significant industrial sources of nitrogen to optimize control and treatment.
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6.2 CALCULATION OF MAXIMUM ALLOWABLE INDUSTRIAL LOADING
MAHLs estimate the maximum combined loadings that can be received at the POTW's headworks from
all sources. MAILs developed by the POTW represent the amount of pollutant loadings the POTW can
receive from controlled sources (i.e.,
industrial users, some commercial
sources1, and some hauled waste) that
the POTW chooses to control through
local limits. As shown in Equation 6.2,
the MAIL is calculated by subtracting
estimates of:
Loadings from
uncontrolled sources
\Lunc)
Hauled waste not
regulated through local
limits (HW)
Growth allowance (GA)
Equation 6.2: MAIL Calculation
MAIL= MAHL(\ - SF)- (LUNC+ HW+ GA)
Where:
MAIL =
MAHL =
SF =
HW =
GA =
Maximum allowable industrial loading, Ib/day
Maximum allowable headworks loading, Ib/day
Safety factor, if desired
Loadings from uncontrolled sources (uncontrolled sources=
domestic + some commercial + l&l)
Loadings from hauled waste, if not regulated through the local
limits
Growth allowance.
from a MAHL adjusted with a safety factor (SF). These four elements of the MAIL calculation-
loadings from uncontrolled sources, hauled waste, growth allowance, and safety factor - are further
explained in the next four subsections. Table 6-1 provides a summary on the information needed to
calculate the MAIL.
Table 6-1: Data for Implementation of MAHLs
Parameter
IU and significant
industrial user (SIU)
flow
Uncontrolled Source
Pollutant
Concentrations
Uncontrolled Source
Flow
Hauled Waste Loadings
Safety Factor
Growth Allowance
Comments
Total flow from all SlUs and lUs, plus any commercial dischargers
that the POTW intends to control
Levels of POCs in domestic and commercial discharges that the
POTW does not intend to control with local limits
Flow from all uncontrolled sources, either in total or divided by
type of facility (domestic, commercial, l&l, storm water)
Based on volume and pollutant concentration data
Varies depending on quality and amount of data
Varies based on the projected growth for the area
Source of Data
POTW local use sampling
program, periodic reports from
SI Us
POTW local use sampling
program
POTW local use sampling
program
POTW sampling of waste
hauler loads
POTW choice based on data
analysis
POTW choice based on data
analysis
For example, a POTW may choose to regulate or limit the discharges from some or all of its commercial dischargers
(e.g., dental offices, hospitals, and restaurants), in which case they would be considered controllable sources.
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6.2.1 UNCONTROLLED SOURCES
As noted above, some sources of pollutant loadings to the POTW are considered uncontrolled. They
include domestic users, inflow and infiltration (I&I), treatment chemicals added to sewers, storm water,
and some or all of a POTW's commercial dischargers. Because the POTW does not control the loadings
that these users discharge [except through the general and specific prohibitions in the POTW's sewer use
ordinance (SUO)], the POTW needs to subtract these loadings from its MAHLs before it can determine
the MAIL (see Equation 6.2). EPA recommends the following approach for calculating the contribution
to the MAHL from these uncontrolled loadings: First, the POTW conducts site-specific monitoring of
the uncontrolled discharges at sewer trunk lines that receive wastewater from only these sources (see
Section 4.1.2). This activity will enable the POTW to develop data on average pollutant concentration
levels. The POTW then multiplies the concentration loadings for each pollutant obtained from these
locations (Cmc) by the POTW's total uncontrolled flow rate (Qmc) to determine total loadings to the
POTW for that specific pollutant from all uncontrolled sources (see Equation 6.3).
EPA strongly encourages POTWs to use site-specific data for uncontrolled loadings whenever possible.
Appendix V includes data on pollutant concentrations found in typical domestic wastewater discharges,
which can be used if site-specific data are not available. Because domestic wastewater values may not be
representative of the uncontrolled discharges in their systems, POTWs should use care with these data.
A POTW may find that the total uncontrolled loadings of a particular pollutant approach or exceed the
MAHL. In these cases, little or no pollutant loading is available for Ills. This situation may arise in part
because some of the facilities considered uncontrolled are commercial facilities such as gas stations,
radiator repair shops, car washes, or hospitals, which may discharge high levels of pollutants. These
facilities may be grouped initially with uncontrolled sources because they are small or have low
discharge flows. The POTW may need to carefully evaluate the sources it considers uncontrolled to see
if some of them would be better classified as controlled sources with reducible pollutant loadings. Refer
to the Supplemental Manual on the Development and Implementation of Local Discharge Limitations
under the Pretreatment Program (EPA-W21-4002,
May 1991) for typical pollutant loadings for selected
commercial industries. This is recommended for
POTWs whose allocations to uncontrolled sources
consume most or all of its MAHLs for some
pollutants. In addition, see Section 9.5 for additional
guidance addressing this issue.
Equation 6.3: Uncontrolled Loading
Calculation
Where:
'-u/vc =
8.34 =
Uncontrolled loading, Ib/day
Uncontrolled pollutant concentration, mg/L
Uncontrolled flow rate, MGD
Unit conversion factor.
If a POTW has considerable loadings from I&I and
storm water (from combined sewer systems), it
should try to estimate their loadings and include them
in the uncontrolled loadings estimate. The POTW
may be able to select sampling locations that include
these flows, or it may be able to estimate them by
analyzing the variations in flow between periods of
wet and dry weather. In some cases, the POTW may be able to decrease the flows and loads from I&I
and storm water through sewer system rehabilitation and pollution prevention programs so that loads
from these sources do not consume a substantial portion of the POTW's MAHLs.
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The POTW may be able to estimate loadings from uncontrolled sources by subtracting loadings of
controlled sources from total influent loadings. This method may be useful when most or all of a
POTW's data for uncontrolled sources are below detection levels for a pollutant. When the data are
mostly below detection levels, the POTW should carefully evaluate how to handle these data because
these decisions can greatly affect the loadings available for Ills. Additional guidance on setting local
limits when uncontrolled source loading exceeds the MAHL has been developed by EPA Region 5 and
can be found at: http://epa.gov/r5water/npdestek/npdprtg3.htm.
6.2.2 HAULED WASTE
As previously noted, POTWs that do not regulate
waste haulers through local limits will want to
determine the loads they receive from hauled waste
and subtract these loads from their MAHLs before
determining their MAILs. EPA recommends that
POTWs base the allocations for hauled waste on
actual data - pollutant concentrations and flows from
waste haulers collected by sampling hauled waste
brought to the treatment works. EPA further
recommends that POTWs regularly sample these
loads to ensure that they are not hazardous waste, do
not contain toxic pollutants in amounts greater than
expected or greater than local limits, and will not
pose risks to the treatment plant or its workers. In
addition, EPA reminds POTWs that hauled waste
subject to categorical limitations must meet those
limits when accepted at the POTW and that
pretreatment standards apply to wastes hauled from
Ills. Additional information on the acceptance and
characterization of hauled wastes at POTWs is
available in Guidance Manual for the Control of
Waste Hauled to Publicly Owned Treatment Works
(EPA/833-B98-003). The guidance discusses
collection of information on waste haulers,
characterization of hauled waste received, evaluation
implementation of controls.
6.2.3 SAFETY FACTOR
Exhibit 6-1: Safety Factor Example
If a POTW's data for cadmium were all below detection
and the POTW used literature data for cadmium
removal efficiencies, the treatment works should
consider using a safety factor for cadmium. At the
same time, if the POTW's zinc data were mostly above
detection and the daily removal efficiencies were all
between 60 and 80 percent, the POTW may not need
to use a safety factor for zinc. The decision to use a
safety factor for zinc removal on pass through would
depend on the quality of the data used to calculate the
removal efficiency. In this example, assume that the
removal efficiency is based on 12 months of paired
influent and effluent samples that range from 60 and
80 percent and collected as hydraulically lagged pairs.
Because this data set is of high quality, the POTW
might not use a safety factor. If an ADRE is
calculated, it will lie in the 60 to 80 percent range. If
the ADRE is 72 percent, the POTW will want to
consider the degree of safety that would exist should
the actual removal efficiency be lower. This, along with
the potential to violate water quality standards or
NPDES effluent limits, also needs to be considered.
Note that the ADRE for pass through is the same value
used for sludge quality protection calculations. The
POTW should also examine the data set to determine
the potential for removals to be higher than the ADRE
leading to violations of sludge disposal quality criteria.
of potential impacts and development and
Determining safety factors is an imprecise process, which has the potential to affect significantly the final
local limits. A safety factor is site specific and depends on local conditions. The main purpose of a
safety factor is to address data "uncertainties" that can affect the ability of the POTW to calculate
accurate local limits. Some Approval Authorities may have mandatory safety factors. At a minimum,
EPA generally recommends a 10 percent safety factor. The determination of whether a safety factor is
needed and, if it is, how large the safety factor should be depends on the following elements:
The variability of the POTW's data.
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• The amount of data the POTW used to develop its MAHLs.
The quality of the POTW's data.
• The amount of literature data the POTW used.
• The history of compliance with the parameter.
• The potential for IU slug loadings (e.g., as a result of chemical spills).
• The number and size of each IU with respect to the POTW's total flow rate.
The POTW may use different safety factors for different pollutants. The above elements may vary from
pollutant to pollutant, making it appropriate for a POTW to use different safety factors (see Exhibit 6-1).
6.2.4 EXPANSION/GROWTH ALLOWANCE
A POTW that anticipates a significant amount of growth in the future can consider holding in reserve a
portion of its MAHLs for this growth. This expansion/growth allowance is separate from the safety
factor. Anticipated growth should be projected for known, planned expansions such as Ills moving into
the POTW's service area or existing Ills expanding their operations, the development of a shopping mall
or the opening of other commercial businesses in a new office park, or the construction of a new housing
development. The expansion and growth allowance is most commonly justified for BOD, TSS, and other
pollutants the POTW was designed to remove. By holding in reserve some of the MAHL, the POTW has
a portion to allocate to the new discharges and may not need to revise its existing IU permits or SUO.
6.3 COMPARISON OF MAIL ALLOCATION AND IMPLEMENTATION METHODS
Uniform-concentration local discharge limitations have become synonymous in the Pretreatment Program
with the term "local limits." However, local limits can take many forms based on how MAILs are
allocated to lUs. The designation and implementation of these MAILs, including the allocation of
loadings to lUs, are left to each POTW, as long as the implementation procedures do not allow the
calculated MAHL to be exceeded and provide a reasonable method for making allocations to the lUs.
This section describes some of the implementation decisions facing POTWs. The selection of an
appropriate implementation approach is an integral aspect of a POTW's local limits process.
A POTW may select any allocation and implementation method that results in enforceable local limits to
prevent pass through and interference and to comply with the prohibitions in the Federal regulations.
The POTW should choose the allocation approach that best fits its own situation. It may choose one
approach for some pollutants and another approach for other pollutants, depending on the amount of
loading available to lUs and the number of lUs discharging a given pollutant. For example, if only three
of a POTW's ten lUs discharge silver, the POTW may prefer to allocate its allowable industrial silver
loading among the three lUs that discharge silver so that these lUs receive more achievable limits. At the
same time, if all of the users discharge copper, the POTW may choose to allocate the MAIL for copper to
all of the users on a uniform basis. All regulated lUs should receive at least a background allocation for
copper and all other POCs.
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Table 6-2 on the next page lists issues that POTWs will want to consider when determining how to
allocate and implement its local limits. Ultimately, the POTW will want to allocate pollutant loadings in
a fair and sensible way that does not favor any one industry or group of industries, considers the
economic impacts, maintains compliance with the NPDES permit, and otherwise achieves the
environmental goals of the program. The allocation method selected may be subject to State and local
public participation requirements in order for the resulting local limits to become legally enforceable.
6.4 ALLOCATION OF MAILs AMONG CONTROLLED SOURCES
A POTW can apply to its controllable sources concentration-based limits (typically in mg/L), or mass-
based limits (typically in Ib/day), or both. The type of limit depends in part on the method chosen by the
POTW to allocate its MAILs among the controlled dischargers. For example, a POTW that uses the
uniform concentration method based on total IU flow typically implements a pollutant limit as a single
concentration (generally in its SUO) applicable to all controlled users. If the POTW allocates its MAILs
on a case-by-case basis depending on an Ill's need for a certain loading allocation, the POTW may find it
easier to apply mass-based limits (in individual permits) that allow for the needed loading at the IU. The
POTW needs to consider the ability to determine and enforce compliance. EPA recommends that the
POTW consider the lU's sampling capabilities when determining the type of limits to apply to an IU. An
IU may not have flow meters or sampling points necessary to determine mass-based limits. In these
cases, the POTW may instead put concentration-based limits in the IU permits or, potentially, both types
of limits in the permit. Thus, the POTW may first allocate its MAILs based on loadings, but then apply
the allocations to lUs as concentration-based limits based on flow. EPA recommends that POTWs use
mass-based limits only for users that have the capability (or are required to develop the capability) to
accurately measure their flows at the designated sampling points. Mass-based limits have the added
benefit of allowing lUs to reduce their water consumption through conservation or recycling without
affecting their ability to meet local limits.
6.4.1 LIMIT DURATION
When applying its local limits, a POTW needs to determine the appropriate limit duration. The POTW
may establish limits that are daily maximums, monthly averages, or instantaneous maximums. In
general, a POTW should base the limit duration on the type of criteria - long-term or short-term - used to
develop the local limit. However, most local limits will be implemented as daily maximums based upon
two main factors: 1) the short-term nature of the event that the local limit is protecting against; and
2) the infrequency of IU sampling. Scenarios illustrating this are presented below.
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Table 6-2: Options for Allocating and Implementing Local Limits
Method
Pros
Cons
Allocate MAILs uniformly
among all Ills and place
uniform concentration limits in
the local SUO
-Limits are clear to I Us
-Requires little time to calculate limits
-Easy to determine compliance
-Need to update SUO when limits change
-Inflexible
-Limits may be overly stringent because some
I Us may get an allocation but do not discharge a
pollutant
Place general language about
complying with local limits in
the local SUO and announce
the actual uniform limits outside
the SUO
-Do not have to revise the SUO every
time local limits change
-Easy to monitor for compliance
-Relatively easy to calculate limits
-lUs may not be clear on the limits with which
they must comply
-Action may be overlooked by the general public
and interested parties
Place general language about
complying with local limits in
the local SUO and place
individual limits in IU permits
-Do not have to revise the SUO every
time local limits change
-Provides flexibility
-Requires issuing a permit to all lUs to which the
POTW wants limits to apply
-Action may be overlooked by the general public
and interested parties
Put MAILs in SUO, allocate
loadings on an IU contributory
flow or mass proportion basis,
and place limits in IU permits
-Only I Us that discharge a pollutant are
given a full allocation so limits are more
efficiently allocated
-Helps avoid setting excessively
stringent or unattainable limits
-Requires knowing more about IU discharges
(need to know their pollutant content)
-Requires updating the SUO when MAILs change
-Requires issuing permits to all lUs with specific
limits
-May penalize I Us that are currently pretreating if
others are not
Put MAILs in SUO, allocate
loadings on a case-by-case
basis to those I Us that need an
allocation for a specific
pollutant, and place limits in IU
permits
-Only I Us that discharge a pollutant are
given a full allocation so limits are more
efficiently allocated
-Helps avoid setting excessively
stringent or unattainable limits
-Provides flexibility
-Requires knowing more about IU discharges
(need to know their pollutant content) and
applicable pretreatment systems
-More time-consuming to determine allocation
-Can lead to an inequitable allocation among I Us
-Requires updating SUO when MAILs change
-Requires issuing permits to all lUs with individual
limits
EPA recommends use of a daily maximum in the following circumstances:
• A local limit based upon short-term criteria should be a daily maximum. For
example, local limits based upon NPDES permit limits expressed as daily maximums
should be considered daily maximums.
• A local limit based upon long-term criteria, BUT protecting against a short-term
event, should be a daily maximum. For example, a local limit based on chronic water
quality criteria would appear to warrant assigning a long-term limit duration such as
monthly average. However, the local limit should be considered a daily maximum
because the MAHL calculation using water quality criteria is based on either the
receiving stream's 1Q10 or 7Q10 flows, both of which are short-term phenomena (see
Equation 5.6). Another short-term condition that leads to a daily maximum limit is
biological inhibition for both secondary and tertiary treatment, both of which have short
residence times.
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A local limit based upon long-term criteria and protecting against a long-term
event, BUT the sampling cannot generate a true monthly average, should be a daily
maximum. For example, monthly average "clean sludge" criteria, can be the basis of a
local limit. Residence times in sludge digesters and storage facilities are commonly 20
to 30 days or more. Consequently, to change the concentration to any appreciable
degree, any excessive loading would have to be maintained for three to four weeks - a
long-term event. These two factors favor a monthly average type local limit. However,
an IU will rarely sample for the metals that end up in the POTW sludge more than once a
month. Therefore, local limits for sludge disposal, although based upon a long-term
criteria and protecting against a long-term event, should be considered a daily maximum
limit.
This means of assigning local limit duration is protective in that it leads to enforcing local limits based on
monthly average criteria as daily maximums.
In terms of other duration types, EPA recommends that local limits should be monthly averages when
the environmental criteria that they are based upon is long term, the protected event is long term, and
frequent IU sampling can generate a true monthly average. EPA recommends that instantaneous limits
be developed for pollutants that cannot be composited. A limit derived from a MAHL based on one-hour
acute toxicity water quality criteria may not be protective if it is implemented as a daily maximum
instead of as an instantaneous limit. However, if the instantaneous limit is converted to a daily maximum
limit using a statistical procedure that
accounts for the variation in
concentrations over a 24-hour period, the
daily maximum limit should be adequately
protective. The EPA Technical Support
Document (TSD) approach, described in
the Technical Support Document for Water
Quality Based Toxics Control (EPA,
199 la), accounts for these variations.
Instantaneous limits may also be
appropriate where Approval Authorities
require lUs to accumulate all wastewater
flows in batch tanks. Grab samples can
then be collected to evaluate an
instantaneous limit.
6.4.2 ALLOCATION APPROACHES
A POTW can use several basic approaches
to assign limits to its controlled
dischargers. As noted above, the POTW
can select different allocation methods for
different pollutants. Several common
approaches for allocating MAILs for
conservative pollutants are described in
this section. A POTW may choose to use
Exhibit 6-2: Background Allocation
When using the IU Contributory Flow Method or Mass Proportion
method, any user that discharges at or below the background level is
given a background allocation (unless a different allocation can be
justified based on actual sample data). Please note that:
• Background loading can be calculated for each pollutant using the
uncontrolled concentration for that pollutant and the flow of that
pollutant from the "non-contributing" industries. (Background flow
from non-contributing industries may be different for each
pollutant.)
• These background "limits" are then applied to non-contributing
industries.
Similar to how estimated uncontrolled source loading can actually
exceed the MAHL (see Section 6.2.1), estimated loadings from
non-contributing I Us discharging the pollutant at background
levels can result in an over-allocation of the MAIL. In other words,
the estimated loading from lUs discharging at pollutant
background levels plus the loading from I Us discharging the
pollutant at local limit levels is greater than the MAIL. Generally,
this occurs because background levels are set too high. POTWs
should make sure that their determination of background levels is
sound and check their allocation method. For instance, a uniform
concentration specified in a Sewer Use Ordinance for a
background concentration can lead to an over-allocation error
(see Equation 6.7 on the next page).
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another method, such as a statistical method, as long as it results in local limits that are enforceable and
adequately protective.
Limits Based on IU Contributions of a
Pollutant
Two allocation methods divide the MAILs
among only the controlled dischargers that
discharge a particular pollutant. These
methods develop Ill-specific discharge
limits. Any user that discharges at or below
the background level is given a background
allocation unless a different allocation can
be justified based on actual sample data (see
Exhibit 6-2 on the previous page).
The IU Contributory Flow method is similar
to the uniform method described below,
except that the portion of MAILs above
background (MAIL - LBACK) is divided by the
flow rate from controlled sources (Qcomo)
discharging the pollutant above background.
The concentration-based limits (CLIM) apply
only to those users (see Equation 6.4).
The Mass Proportion method allocates
MAILs to each controlled discharger in
proportion to the discharger's loading of
that pollutant. To calculate the allowable
loading for a user (LALLx) the portion of the
MAIL above background (MAIL - LBACK) is
multiplied by the ratio of the current loading
from user x (LCURRJ to the current total
loading of a pollutant from controlled
sources (LCURRt). The mass-based loading
calculated using the mass proportion
method can be converted to a concentration-
based limit (see Equations 6.5 and 6.6).
Uniform Limits For All Controlled
Dischargers
As illustrated in Equation 6.8 (on the
following page), the uniform limits method
of allocating MAILs for conservative
pollutants yields one limit per pollutant
(CUM) that applies to every controlled
discharger. It requires that the MAIL for
each pollutant be divided by the total flow
rate from all controlled dischargers (QcoNr),
Equation 6.4: ID Contributory Flow Calculation
Equation 6.5: Mass Proportion Method fora
Mass-Based Local Limit
LCURR,
Equation 6.6: Mass Proportion Method fora
Concentration-Based Limit
<-ALLx
(Qx)(8.34)
Equation 6.7: Uniform Allocation of
Background Loading
Where:
Q/M =
r* —
^BACK ~
MAIL =
Qco/vro =
1-CURRx ''
•—C.IIRRt ~
(CWX8-34)
Concentration-based limit for all users discharging a
pollutant, mg/L
Concentration-based limit for all users discharging
pollutant at or below background, mg/L
Maximum allowable industrial loading, Ib/day
Total background loading allocation for all users for
which no contributory flow limit is being established for
that pollutant, Ib/day
Flow rate from all industrial and other controlled
sources discharging the pollutant, MGD
Flow rate from all industrial and other controlled
sources not discharging the pollutant at or below
background, MGD
Allowable loading allocated to userx, Ib/day
Current loading from userx, Ib/day
Total current loading to POTWfrom controlled sources,
Ib/day
Discharge limit for user x, mg/L
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even those that do not discharge the pollutant. This method can be overly stringent because some Ills
that do not discharge the pollutant will be given an allocation of the MAIL that they may not need. Other
Ills that do discharge that same pollutant may have to pretreat to comply with the local limit.
Basis of IU Needs for Discharge Loading/Case-by-Case Basis
A POTWmay set lU-specific limits case by case. This type of allocation relies on the POTW's judgment
to determine the amount of the MAIL to allocate to each controlled discharger. The limits can be based
on the discharger's current loading, its need for a continued loading allocation, its ability to apply
pretreatment to achieve certain discharge pollutant levels (i.e., treatability), or any other factor that the
POTW determines is relevant. The POTW needs to ensure that the sum of the allocated loadings does
not exceed the MAIL and that it provides for at least a background allocation for each pollutant for each
user, unless a lower allocation can be justified by sampling data. To ensure that it does not allocate more
than the MAIL, the POTW should develop a mechanism to track the loading allocated to each IU and
compare the allocated total to the MAIL.
Creative Allocation Methods
In general, once the MAIL is calculated, the
POTW has substantial flexibility in
allocating the pollutant load among its Ills
as long as a margin of safety is maintained,
the POTW has carefully accounted for all
allocations, and public notice of the
allocation is properly issued and allocation
is adopted. For example, the Hampton
Roads Sanitation District (HRSD) has
developed flow-based local limits.
Industries are placed in one of the following
flow categories:
Equation 6.8: Uniform Concentration Limit
Calculation
MAIL
Where:
CUM =
MAIL =
Qco/vr =
8.34 =
Uniform concentration limit, mg/L
Maximum allowable industrial loading, Ib/day
Total flow rate from industrial and other controlled
sources, MGD
Unit conversion factor
• 0 to 9,999 gallons per day (gpd)
10,000 to 19,999 gpd
20,000 to 29,999 gpd
30,000 to 39,999 gpd
40,000 to 199,999 gpd
200,000 to 399,999 gpd
Greater than 400,000 gpd
Uniform limits are applied to each industry within the same flow category. The local limits become
progressively more stringent as the industry's discharge flow increases. lUs that discharge above 400,000
gpd are assigned specifically calculated local limits based on domestic loadings and the industrial
processes from the specific facility. As an illustration, lUs with a flow rate of 0 to 10,000 gpd would
have a nickel limit of 10.0 mg/L, while those with a rate of 200,000 to 400,000 gpd would have daily
maximum nickel limit of 1.0 mg/L. HRSD uses this scheme for its local limits for the following
parameters: arsenic, cadmium, chromium, copper, cyanide, lead, mercury, nickel, phenolic compounds,
silver, zinc, and non-saponifiable oil and grease.
Another creative form of MAIL allocation that POTWs may consider is pretreatment trading or effluent
trading. These programs allow one source to meet its regulatory obligations by using pollutant
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reductions created by another source that has lower pollution control costs. Trading capitalizes on
economies of scale and the control cost differentials among and between sources. Trading policy is
applicable to local limits, only. The policy does not apply to categorical standards. EPA supports a
municipality or regional sewerage authority developing and implementing trading programs among
industrial users that are consistent with the pretreatment regulatory requirements at 40 CFR Part 403 and
the municipality's or authority's NPDES permit. See Final Water Quality Trading Policy, EPA, Office of
Water, Water Quality Trading Policy, January 13, 2003. Available at:
http://www.epa.gov/owow/watershed/trading/finalpolicy2003.html.
6.5 COMMON SENSE ASSESSMENT
After developing and allocating local limits, POTWs should determine whether their local limits pass a
"common sense test." An effective public participation process can help with this assessment. Some of
the questions a POTW should ask to determine if its limits pass the "common sense" test are:
• Are the limits technologically achievable? Are Ills and other controlled dischargers
likely to meet these limits with currently available forms of pretreatment and pollution
prevention (e.g., process modifications)? Local limits are meant to protect the POTW
and the environment and therefore are not specifically based on technological
achievability.
• Can the POTW and dischargers determine compliance with the local limits? Are
the limits above sampling method detection levels? If the limits are below the detection
level of the most sensitive analytical method, neither the POTW nor the Ills will be able
to definitively determine compliance.
• Are the limits sensible in light of actual conditions at the treatment plant and past
compliance experience? For example, if the POTW is currently violating its NPDES
limit for copper but the local limits analysis indicates that the POTW can accept its
current influent loading and maintain compliance with that limit, the calculations and the
past experience are in conflict. In this situation, the POTW should determine the
reason(s) for the inconsistency.
If a POTW's calculated limits do not pass the "common sense test," the POTW may need to reassess its
limits development process or investigate other options for reducing pollutant loads (e.g., source
reduction measures). Besides the environmental criteria used in the calculations, the two pieces of data
that can have the greatest impact on the local limits calculations are the removal rates and the
uncontrolled pollutant concentrations. A reassessment of the limits development process may show that
several of the limits are affected by a lack of data and the use of literature values. By conducting
additional sampling (possibly using lower detection limits), a POTW may obtain better data and, thus, be
able to calculate more appropriate limits.
Despite the POTW's efforts to obtain the best data available for the calculations, the local limit
calculated for a specific pollutant may at times be unreasonable and warrant other actions to establish
valid limits. Other options for reducing pollutant loads to the POTW include the following:
• Adding other commercial facilities to the set of controlled sources and requiring those
facilities to reduce the pollutant load in their discharges. For example, a POTW's
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MAHL for silver could be less than the uncontrolled loading resulting in a negative local
limit. By adding other silver dischargers (e.g., photoprocessors) to the group of
controlled Ills, the uncontrolled loading may be reduced significantly enough to
calculate a reasonable limit.
• Instituting a public education program to reduce problem discharges from domestic and
other non-industrial (e.g., dental offices) sources. Some POTWs have worked with area
dental associations to help educate dentists about proper disposal practices for mercury
amalgam. Other POTWs have held hazardous waste disposal days to reduce the amount
of household hazardous wastes discharged into sewers. See more on working with
industry on Best Management Practices (BMPs) in Section 6.6.
• Limiting acceptance of hauled waste to fewer loads, smaller loads, or lower pollutant
levels. If hauled wastes contribute significantly to uncontrolled loadings, the POTW
may need to stop accepting some hauled waste.
• Conducting an I&I reduction program. Although I&I will generally contain lower
concentrations of most pollutants than typical domestic sewage, it may contribute
loadings that can increase problems with limits calculations.
• Encouraging the replacement of piping that contributes significant loads of copper and
lead.
• Examining impurities, such as mercury, in chemicals used by industry, POTWs and
water suppliers. Additionally, POTWs should be aware that the chemicals used in
potable water treatment, such as fluoride (hydrofluorosilicic acid additive to prevent
tooth decay) and zinc (zinc orthophosphate for corrosion control), can contribute to
POTW pollutant loads.
A POTW that cannot develop reasonable local limits may need to consider changing sludge disposal
methods (if sludge is the limiting factor) or, in the long term, expanding the capacity of its treatment
plant (especially for pollutants such as BOD, TSS, or ammonia). In any event, a POTW that is
experiencing difficulty developing reasonable limits should contact its Approval Authority to discuss
possible solutions.
6.6 BEST MANAGEMENT PRACTICES
The General Pretreatment Regulations do not specifically address the use of BMPs. The regulations at
40 CFR 403.5(c) require the POTW only to develop "specific limits" for prohibited discharges. The
current regulatory language is ambiguous as to whether BMPs may serve in lieu of numeric limits.
However, the proposed Pretreatment Streamlining Rule (40 CFR Part 403, Streamlining the General
Pretreatment Regulations for Existing and New Sources of Pollution, July 22, 1999) states that BMPs
may be enforceable as local limits as an alternative to numerical limits or may supplement local limits.
BMPs would need to be included in the technical evaluation of local limits. BMPs are defined in the
NPDES regulations (40 CFR 122.2) as scheduled activities, prohibitions of practices, maintenance
procedures and other management practices to prevent or reduce pollution. Some recently developed
Effluent Limitation Guidelines, such as those for Pulp, Paper and Paperboard (40 CFR 430),
Transportation Equipment Cleaning (40 CFR 442) and Pesticide Formulating, Packaging and
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Repackaging (40 CFR 455), allow for use of BMPs in meeting prescribed limits. BMPs also include
treatment requirements, operating procedures, sludge or waste disposal, or drainage from raw material
storage, and practices to control plant site runoff, spillage or leaks.
Some commercial establishments may discharge pollutants in quantities that can be controlled either by
local limits or by BMPs. A photofmisher that discharges to a POTW that is critically loaded with silver
is one example. The POTW might elect to require silver BMPs in lieu of a permit and account for this
allocation and anticipated reduction in silver in coordination with more traditional permits issued to Ills
with mass-based or concentration-based local limits. However, to the extent that BMPs are used as an
alternative or supplement to technically based local limits, the technical evaluation will need to assign an
allocation to the pollutants and users covered by the BMP. A series of BMP mini-case studies is
presented in Appendix W.
EPA suggests the following resources in POTW development of BMPs:
• Pollution Prevention Information Clearinghouse Resource List: This comprehensive Web
site has sector-specific guidelines on pollution prevention.
http://www.epa.gov/opptintr/library/ppicdist.htm
• Guides to Pollution Prevention: Municipal Pretreatment Program, (EPA 625/R-93/006
October 1993)
• Guidance Manual for Developing Best Management Practices, (EPA 833/B-93/004
October 1993)
• Pollution Prevention (P2) Guidance Manual for the Pesticide Formulating, Packaging,
and Repackaging Industry: Implementing the P2 Alternative, (EPA 821-B-98-017 June
1998)
• The Massachusetts Water Resources Authority (MWRA) currently prohibits the
discharge of mercury by industrial facilities to its sewer system. Additionally, MWRA
imposes an effective discharge limitation for mercury of 1.0 part per billion (ppb) from
its regulated sources, including hospitals and institutions. To address this complex issue,
the MWRA established a Mercury Products Work Group to examine the problem and
develop strategies to reduce the amount of mercury being discharged. Read about this
effort at: http://www.masco.org/mercury/index.htm.
6.7 APPROVAL AUTHORITY AND ADOPTION PROCESS
A Control Authority's legal authority to impose local limits on industrial and commercial users derives
from State law. Therefore, State law must confer the minimum Federal legal authority on a Control
Authority. Where deficient, State law must be modified to grant the minimum requirements. In order to
apply regulatory authority provided by State law, the Control Authority generally must establish local
regulations to legally implement and enforce pretreatment requirements. If the Control Authority is a
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municipality, legal authority is detailed in a Sewer Use Ordinance (SUO),2 which is usually part of a city
or county code. Regional Control Authorities frequently adopt similar provisions in the form of "rules
and regulations." Likewise, State agencies implementing a State-wide program under 40 CFR 403.10(e)
set out pretreatment requirements as State regulations, rather than as a SUO. However, local regulations
cannot give the Control Authority greater authority than that provided by State law.
Establishing or revising local limits is considered to be a modification of the POTW's pretreatment
program. Therefore, the new or changed local limits must be submitted to the Approval Authority for its
review and approval. The POTW must submit a notice to the Approval Authority that states the basis for
the modification and must provide a modified program description and other documentation requested by
the Approval Authority. After a modification is approved by the Approval Authority, it will be
incorporated into the POTW's NPDES permit [40 CFR 403.18(e) and 40 CFR122.62].
In most instances, the initial adoption of a MAIL or BMP will be a substantial modification where it
replaces a different form of local limits. Unless the mass-based limit or BMP is specifically tied to an
existing concentration limit, the switch to mass-based limits or to BMPs will likely result in less stringent
local limits for at least some group of industrial users. As specified at 40 CFR 403.18(b)(2), making a
local limit less stringent is considered a substantial modification of a POTW's pretreatment program.
Not only is the relaxation of a uniform concentration limit considered a substantial modification, but if a
POTW calculates a less stringent concentration limit, the MAHL or MAIL also becomes less stringent.
If this is the case, the Approval Authority may be required to process any new local limits as a substantial
modification as well. For substantial modifications, the Approval Authority must issue a public notice of
the request for approval and must provide an opportunity for interested parties to comment or request a
public hearing. After deciding whether to approve the modification, the Approval Authority must issue a
public notice of approval or disapproval, unless certain conditions are met [40 CFR 403.18(c)(3)].
Non-substantial modifications may be implemented after 45 days, unless the Approval Authority notifies
the POTW that a modification is disapproved or determines that the modification is substantial (e.g.,
would result in an increase in pollutant loadings at the POTW) [40 CFR 403.18(d)]. To be approved by
the Approval Authority, local limits must first be made legally enforceable by the POTW. This is
generally done by incorporating them in the local SUO by following local public noticing procedures.
The SUO need not contain local limits already allocated to industries. However, at a minimum, the SUO
should authorize the POTW to establish individual limits through the permits based on the MAIL.
The activities described above are regulatory requirements that must be met by all Approval Authorities
and POTWs. Approval Authorities may have different procedures for implementing these requirements,
and POTWs should check with their Approval Authority for details. In general, however, the approval
and adoption process includes the following steps:
Consult Model Pretreatment Ordinance, (EPA 833-B-92-003, June 1992) for recommended formats for a Sewer Use
Ordinance.
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(1) The POTW develops or recalculates draft local limits.
(2) The POTW submits the draft new or revised local limits and supporting documentation
to the Approval Authority for review,3 makes the proposed new or revised limits
available to the public for comment, and provides individual notice to the affected
parties.
(3) The Approval Authority notifies the POTW of the adequacy of its submission. The
submission may be:
• Not accepted. The Approval Authority provides comments to the POTW, the POTW
addresses the issues raised in the comments and repeats Step 2.
• Accepted. The Approval Authority notifies the POTW that its proposed limits have
been accepted.
(4) Once accepted by the Approval Authority, the POTW adopts the new or revised limits,
which also are adopted by all the contributing jurisdictions (i.e., all municipalities in the
service area). Note that the public must be given the opportunity to review and comment
according State and local law (see Section 6.8 for a discussion on public participation).
(5) Once approved and adopted by the control authority (and thereby enforceable), the
proposed changes to local limits become a formal pretreatment program modification and
need to be publicly noticed and approved (as noted in the above discussion of regulatory
requirements) by the Approval Authority. (The specific procedures for review and final
approval may vary among Approval Authorities. POTWs should check with their
Approval Authority.)
6.8 PUBLIC PARTICIPATION
Section 101(e) of the CWA establishes public participation as one of the goals in the development,
revision, and enforcement of any regulation, standard, effluent limitation, plan, or program established by
EPA or any State. The General Pretreatment Regulations encourage public participation by requiring
public notices or hearings for program approval, removal credits, program modifications, local limits
development and modifications, and Ills in significant non-compliance.
POTW pretreatment program approval requests require the Approval Authority (a State or EPA) to
publish a notice (including a notice for a public hearing) in a newspaper of general circulation within the
jurisdiction served by the POTW. All comments regarding the request as well as any request for a public
hearing must be filed with the Approval Authority within the specified comment period, which generally
lasts 30 days. The Approval Authority is required to account for all comments received when deciding to
3 Although not required, POTWs are encouraged to submit draft local limits to their Approval Authority for review
prior to formal submission. This step can be helpful in identifying revisions necessary to make limits approvable and can save
the POTW (and any contributing jurisdictions) from having to re-adopt revised limits after addressing Approval Authority
comments.
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approve or deny the submission. The decision is then provided to the POTW and other interested parties,
and published in the newspaper. All comments received are made available to the public for inspection
and copying.
Once a local pretreatment program is approved, the Control Authority (usually the POTW) must
implement that program as approved. Before there is a significant change in the operation of a POTW
pretreatment program, a program modification must be initiated. For a substantial program modification,
such as the development of new or less stringent local limits, the Control Authority is required to notify
the Approval Authority of the desire to modify its program and the basis for the change. Approval
Authorities (or POTWs) also are required to issue public notice of the request for a modification, but are
not required to issue public notice of the decision if no comments are received and the request is
approved without changes. These changes become effective upon approval by the Approval Authority.
Federal regulations also require POTWs to notify
affected persons and groups and give them an
opportunity to respond before final promulgation
of a local limit [40 CFR 403.5(c)(3)]. While the
regulations do not specify the exact public notice
process that a POTW should follow, EPA
recommends that the POTW conduct public
participation in the local limits process as openly
as possible. This process would include
notifying affected users and other parties that the
POTW knows are interested that the POTW is
beginning a detailed reevaluation of its local
limits. When new limits are drafted, EPA
recommends notifying the Ills and other
interested parties, individually, of the proposed
limits and announce a public comment period in
the local newspaper. This public comment
period can be open while the proposed limits are
submitted to the Approval Authority for initial
review, or the POTW can wait until it receives
comments from the Approval Authority. In EPA's view, POTWs should allow sufficient time in their
limits development process to provide for public participation. A POTW that plans to establish
individual limits through the permits issued to users also should provide for public comments in the
permit issuance process. During the comment period, the public may present technical challenges to the
rationale for a particular local limit. To be adequately prepared to address such challenges, the POTW
needs to thoroughly document its local limits development process. Similar issues need to be addressed
during the re-evaluation process as well (see Exhibit 6-3).
6.9 CONTROL MECHANISMS
POTWs have discretion in selecting the control mechanism through which they will apply local limits to
Ills and thereby making them enforceable. Examples of control mechanisms may include a SUO,
individual permits, and orders. A POTW's choice of control mechanism may depend on the type of user
(SIU or non-SIU) and on the method the POTW uses to allocate its MAHLs among its lUs. A POTW
should consider the following:
Exhibit 6-3: Local Limits Documentation
Among the items a POTW should keep to document its
local limits development process are:
• All data used for determining pollutants of concern and
performing calculations.
• Rationale for choosing pollutants of concern.
• Record of calculations (formulas used) and related
assumptions.
• Printouts from any spreadsheets or computer
programs used.
• Rationale for choosing local limits (comparison of
maximum allowable headworks loadings for all
applicable criteria, allocation methods and
calculations).
• Reasons for not setting limits for particular pollutants or
deleting any existing limits.
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• An SUO alone may not be adequate with any allocation method other than the uniform
concentration method.
• The POTW does not need to allocate its local limits in an SUO. It may instead include
MAILs in the SUO, then allocate the loadings in individual control mechanisms. Again,
care must be taken to ensure that the sum of each pollutant allocation does not exceed the
MAIL.
• Limits based on the contributory flow method may result in over-allocation of the MAIL
when uniform concentration values are specified in the SUO for "background
concentrations" for SIUs that do not discharge the pollutant. POTWs should ensure that
the implementation of the allocation scheme into a control mechanism does not result in
an over-allocation of the MAIL.
• An individual control mechanism (such as a permit) is necessary for most POTW-IU
relationships. Even if one uniform set of local limits were applicable for all lUs, an
individual control mechanism may be desirable to specify monitoring locations and
frequency, special conditions such as solvent management or spill prevention plans,
applicable categorical standards, and reporting requirements, and to provide clear
notification to lUs (as required by 40 CFR 403.8). Note that 40 CFR 403.8(f)(l)(iii)
requires a POTW to control the contribution of SIUs through individual control
mechanisms (e.g., permits). The development of IU permits is discussed in detail in
EPA's Industrial User Permitting Guidance Manual (EPA, 1989a).
6.10 SUMMARY
After reviewing Chapter 6, POTWs should understand how to:
• Determine the need for new local limits after establishing MAHLs.
• Calculate MAILs.
• Compare MAIL allocation and implementation methods.
• Allocate MAILs to controlled dischargers.
• Perform a common sense assessment of local limits.
• Use best management practices.
• Provide public participation.
• Gain Approval Authority approval.
• Select the appropriate control mechanism to apply local limits.
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CHAPTER 7-
LOCAL LIMITS REVIEWS AND DETAILED RE-
EVALUATIONS
According to 40 CFR 122.44(j)(2)(ii), POTWs must "provide a written technical evaluation of the need
to revise local limits under 40 CFR 403.5(c)(l), following permit issuance or reissuance." EPA
recommends that a periodic evaluation of local limits be tied to the permit cycle and that more detailed
evaluations be conducted on an "as needed" basis. Chapter 7 provides guidance on two means to meet
this requirement - local limits reviews and detailed re-evaluations -depending on the conditions at the
POTW. Reviews compare current headworks loadings with the maximum allowable headworks loading
(MAHL) and examine any recent violations. When plant conditions have changed, EPA suggests a
detailed re-evaluation be conducted that includes an in-depth look at all the data, criteria, and
assumptions on which local limits are based to determine whether any changes affecting the local limits
have occurred.
7.1 REVIEWS
For POTWs with past performance problems (pass through, interferences, or collection system issues),
EPA suggests performing reviews annually as part of its preparation of the Annual Pretreatment Report.
Reviews are intended as a quick check for any obvious signs that local limits may not be adequately
protective of its treatment works, its workers, and the environment. This review will help ensure that any
changes made during the previous year have not weakened the local limits' effectiveness in protecting the
POTW from pass through and interference. Presented below is a suggested methodology for performing
reviews.
7.1.1 COMPARISON OF CURRENT LOADINGS WITH MAHLs
During a local limits review, EPA recommends that a POTW identify its maximum daily and maximum
monthly average headworks loadings during the previous year for each pollutant of concern (POC) for
which it calculated a MAHL—regardless of whether a local limit for each POC was adopted. Similar to
the calculations made to determine the need for local limits in Section 6.1, comparisons of the MAHL to
the headworks loadings will determine if local limits need to be recalculated, or established for additional
POCs. The comparisons also may indicate if there is a need for an investigation into the cause of
increased loadings, possibly due to noncompliant industrial users (Ills).
As previously explained, dividing the headworks loading of all POCs by their respective MAHL will
yield a "percentage of MAHL" represented by the POC headworks loading (see Equation 6.1). If a POC
headworks loading is a high percentage of the MAHL, the POTW may choose to revise the local limit for
that pollutant or develop a local limit for it if none exists. For example, a POTW may decide to develop
a local limit for any pollutant whose headworks loading is above a "threshold value" of 50 percent of the
MAHL. EPA recommends maximum threshold values of 60 percent for metals and toxic organics and 80
percent for non-toxic organics, and conventional pollutants. However, in most circumstances, a POTW
will use threshold values that are consistent with the criteria it used to determine if a local limit was
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needed for a POC. EPA offers the following guidance on this comparison between MAHLs and POCs
for which local limits were not established:
1. If the current POC headworks loading exceeds the MAHL, EPA recommends that
the POTW establish a local limit for the pollutant, investigate the cause of elevated
loading, increase its IU monitoring, identify any noncomplying industries, and consider
undertaking pollution prevention efforts.
2. If the current POC headworks loading exceeds the established threshold value for
the first time (i.e., the loading was below the threshold value during the year
before), EPA recommends the POTW increase monitoring for the POC, or establish a
local limit for it.
3. If the current POC headworks loading exceeds the established threshold value for
the second time, EPA recommends establishing a local limit and increasing POC
monitoring.
4. If the current loading is below the established threshold, EPA recommends that the
POTW review the pollutant's loading as part of its preparation of next year's annual
report.
Similarly, EPA recommends that the POTW prepare to address situations involving POCs for which local
limits have already been established in the follow circumstances:
• If the current POC loading exceeds the MAHL, EPA recommends revising the local
limit (unless an investigation reveals that the elevated loading is due to an unusual, one-
time event), investigating the cause of the high loading, identifying any noncomplying
industries, increasing monitoring of Ills, and considering adopting pollution prevention
efforts.
• If the current POC loading has increased significantly from the previous year (e.g.,
from 55 percent to 75 percent of the MAHL), EPA recommends that the POTW
investigate the cause of the increased loading, increase its monitoring for the POC, or
revise the local limit.
• If the current POC loading is below the established threshold, EPA recommends that
the POTW review the POC's loading when it prepares next year's report.
As part of its investigation into the cause of an elevated loading, the POTW will investigate whether the
loading is an aberration. If the high loading resulted from an unusual, or one-time, occurrence, the
POTW may not need to establish or recalculate the local limit for the POC. For example, if the POC
load increased as a result of an IU oil spill, the POTW may better address the situation by ensuring that
the IU properly implements a spill control plan, rather than by setting or revising a local limit. In
addition, the POTW should also look at whether any sampling or analytical problems caused the
aberration.
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When the current loading of one or more POCs approaches the MAHL, the POTW can respond in several
ways. It can compare current IU loadings with the MAILs. If the comparison shows that the increased
loadings come from domestic or commercial sources, the POTW can educate these sources about
pollution prevention, or it can impose local limits on the commercial sources rather than change the IU
local limits. If the IU loadings exceed the MAILs, one or more IU may be violating local limits. Such
violations should be found during the POTW's regular review of IU monitoring data. Another response
is to review the data used to set the local limits in the first place. If changing conditions have affected the
removal efficiencies, flow rate, or other criteria on which the MAHLs were based, the POTW should
recalculate the MAHLs.
7.1.2 REVIEW OF COMPLIANCE HISTORY
If a review is performed, the POTW will also want to consider its compliance record over the previous
year to determine whether the local limits it has set provide sufficient protection from pass through and
interference. If the POTW has violated its NPDES permit or sludge disposal standards, has caused or
contributed to violations of water quality standards in its receiving waters, or has experienced
interference of its treatment processes, the POTW's local limits may not be adequately protective.
Unless it has identified as the cause of the violation a specific, unusual incident that is unlikely to recur,
the POTW is required to investigate the violation's cause and take appropriate enforcement action
against any noncomplying lUs. Alternatively, the POTW may revise the local limit, or establish a local
limit if none exists for the pollutants that caused the violations.
7.1.3 NEXT STEPS
POTWs that find further action is necessary after conducting reviews outlined above can turn to the
earlier chapters of this document for guidance on ensuring that local limits remain protective. Chapter 4
has information about sampling issues; Chapter 5 covers the calculation or recalculation of MAHLs; and
Chapter 6 discusses the reallocation of existing MAHLs and other implementation issues, such as control
mechanisms and revisions to the POTW's sewer use ordinance.
7.2 DETAILED LOCAL LIMITS RE-EVALUATION
Periodically, POTWs need to re-evaluate their
local limits to ensure that they remain protective,
or to determine whether they should be revised,
reallocated, or developed for additional
pollutants (see Exhibit 7-1). As discussed above,
POTWs may wish to review their local limits
when preparing their annual Pretreatment
Program Reports. However, the annual review
may not have addressed conditions that can pass rou9
Exhibit 7-1: Why Local Limits Should Be
Re-evaluated
Conditions change overtime, and these changes may
make it necessary to revise some or all of a POTW's local
limits. Periodic re-evaluation of local limits will help the
POTW ensure that the limits are effective in protecting the
treatment works, its workers, the local collection system,
and the environment from the effects of interference and
change over time and undermine the ^^^^^^^^^^^^^^^^^^^^^^^^^^™
effectiveness of local limits. When a POTW
needs to address changes in its operating conditions or environmental criteria, the data or assumptions
used to establish local limits in the first place may no longer be appropriate (see Exhibit 7-2).
As these and other changes occur, the POTW will need periodically to undertake a more detailed re-
evaluation of its local limits. In addition, if a POTW violates its NPDES permit or sludge requirements,
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but all of its regulated sources have been maintaining compliance, the POTW will need to evaluate the
adequacy of its local limits to protect the treatment works, its workers, and the environment.
POTWs can avoid having to re-evaluate local limits for some of the events described in Exhibit 7-2 if
adequate growth allowances (covered in Section 6.2.4) were used during local limits development. In
addition, if Ill's have stopped discharging a pollutant, or reduced their discharge of a pollutant, POTWs
should place the load formerly contributed by those Ills into a reserve account to accommodate future
growth. If local limits are developed with flexibility, POTWs can respond to changes in IU loadings
without a complete recalculation and approval of their local limits.
The detailed re-evaluation of local limits
is a four-step process:
1. Assess current conditions
to determine whether
existing MAHLs should
be recalculated or
reallocated, or additional
local limits should be
developed. Also
determine which
pollutants need to be
further evaluated and for
which criteria. (If only
re-allocation of existing
MAHLs is needed, skip to
step 4.)
2. Based on the pollutants
and criteria identified in
step 1, determine whether
existing data are
sufficient. If not, develop
and implement a local
limits sampling plan, then
analyze the data
collected.
Exhibit 7-2: When to Recalculate or Develop
Local Limits
A POTW that answers "yes" to any of these questions should re-
evaluate its local limits:
• Has the treatment plant been modified, or has a new
treatment plant been brought on line?
• Have the treatment plant processes or operation changed
in a way that affected the removal efficiencies?
• Has the flow to the treatment plant changed significantly?
• Is the POTW subject to new or revised NPDES limits?
• Have the State water quality standards changed for the
receiving water?
• Has the POTW changed, or intend to change, its sludge
disposal method? If yes, will this change affect the sludge
quality standards that the POTW must meet?
• Have loadings been affected by new I Us discharging to the
POTW?
• Have loadings been affected by I Us that have stopped
discharging to the POTW?
• Have loadings been affected by changes in discharges
from current I Us?
• Are new data available about the POTW or the I Us that
invalidate assumptions made during the last local limits
development effort?
3. Recalculate the MAHLs
of pollutants for which local limits have been developed, and determine MAHLs for new
pollutants.
4. Implement the local limits. This step may include the reallocation of existing MAILs, if
required.
The following sections describe these four steps in more detail.
7.2.1 STEP 1: ASSESS CURRENT CONDITIONS
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To determine whether MAHLs should be recalculated, MAILs reallocated, or additional local limits
developed, the POTW first will need to compare its current conditions and requirements with those that
existed when the local limits were last developed. In this process, EPA suggests that the POTW also
evaluate whether a new MAHL is required for a POC, or if the previously determined MAHL remains
valid, but needs to be reallocated. To determine which response is appropriate, the POTW will want to
consider the change that led it to re-evaluate its local limits in detail.
Usually, a POTW will undertake a detailed re-evaluation of its local limits in response to one or more
significant changes at the treatment works or in the discharges it receives. Recalculating existing
MAHLs or determining MAHLs for new POCs is generally an appropriate response to changes in:
• Removal efficiencies
Total POTW or IU loading
• Limiting criteria (NPDES permits, water quality standards, sludge criteria)
• Sludge characteristics or method of disposal (e.g., percent solids, disposal site life)
• Background concentrations of pollutants in receiving water
Simply reallocating existing MAHLs may be appropriate when:
• Some Ills need a larger loading allocation and other Ills are not using all of their
allocations.
• Total POTW flow is unchanged, but the amount of uncontrollable loading relative to the
IU loading has changed.
• Total POTW flow has not changed but new lUs have come on line while existing lUs
have stopped discharging.
In these cases the current MAHLs are usually still appropriate, and the POTW can skip to step 4.
Some Approval Authorities have worksheets that POTWs can use to determine whether existing local
limits need to be recalculated. The worksheets help POTWs compare existing local limits and the data
on which they are based with current conditions and applicable environmental and treatment plant
criteria. They consider such parameters as POTW and SIU flows; sludge disposal method and associated
disposal criteria; occurrence of violations, upsets, and interference; current influent and effluent
loadings; water quality criteria; and NPDES permits. A copy of one of these worksheets and instructions
for its use can be found in Appendix X.
On occasion, a relaxation of local limits may be appropriate. However, in EPA's view, the POTW first
should demonstrate that the revised local limits will satisfy all of the minimum Federal and State
requirements and will adequately protect in-stream water quality and sludge quality. If its analysis shows
that local limits can be relaxed, the POTW would next determine whether their relaxation will result in
new or increased IU discharges that will affect the volume or character of POTW influent or effluent.
Relaxation of local limits would likely result in a major modification that must be approved by the
Approval Authority in accordance with 40 CFR 403.18(b)(2).
7.2.2 STEP 2: COLLECT AND ANALYZE DATA
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Properly re-evaluating local limits requires representative sampling data. If sufficient data are not
available, the POTW obviously will want to develop and implement a sampling plan to provide
additional data on relevant POCs. The availability of accurate site-specific data is critical to the
development of sound, technically based local limits. Local limits developed using data from the
literature are often conservative.
The data necessary to calculate a MAHL for a new POC may not be available if that pollutant was not
part of the POTW's local limits monitoring. Similarly, data collected to support development of a
current MAHL may not be valid for recalculating the MAHL if the data were collected before any
changes occurred. For example, upgrading a treatment unit may increase removal efficiencies beyond the
levels when the POTW conducted most of the sampling for local limits. Consequently, the POTW may
need to collect new samples to obtain sufficient data that represent current conditions in order to support
the MAHL's recalculation. Chapter 4 covers the data needed to develop local limits.
7.2.3 STEP 3: RECALCULATE EXISTING, OR DETERMINE NEW, MAHLs
If the results of the analyses conducted in Steps 1 and 2 warrant, the POTW will next recalculate existing
MAHLs or determine MAHLs for new POCs. Chapter 5 of this guidance covers MAHL calculations.
The POTW will want to ensure that current data are used for all the variables in the equations for
calculating MAHLs.
7.2.4 STEP 4: IMPLEMENT THE LOCAL LIMITS
The evaluation conducted in Step 1 may indicate that the MAHL for a POC need not be recalculated, but
rather should be reallocated among the sources of pollutant loadings (IDs, domestic and commercial
sources, hauled waste, and any reserve for future growth). In such cases, the POTW will go directly from
step 1 to this step.
Implementing local limits may involve:
• Allocating or reallocating MAHLs (between the group of Ills and uncontrollable
sources, as well as to individual non-domestic sources).
• Public participation.
• Approval of revised local limits considered either a "non-substantial" or "substantial"
modification as defined in 40 CFR 403.18(b).
• Adoption of local limits and revision of the SUO.
• Revisions of control mechanisms or IU permits.
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Implementing new and revised local limits is
covered in Chapter 6 of this guidance.
Although most of the information presented in
Chapter 6 applies to both new and revised local
limits, the POTW may have to take additional
considerations into account when implementing
revised local limits. For example, the POTW
may want to use the same allocation method it
used previously but may have a different
number of Ills to consider. Or the POTW may
want to use a new allocation method (see
Exhibit 7-3). In addition, the POTW does not
have to use the same allocation method for
every POC, but it should document which
method is used for which pollutant and why. If
a POTW wants to change its allocation method,
it should consider how the change may affect its
existing users. If some Ills become subject to more
equipment to remain in compliance with local limits
7.3 SUMMARY
Exhibit 7-3: An Example of Changing the
Method for Allocating Local Limits
Using the uniform allocation method, a POTW gave all of its
Ills the same local limit for cadmium through its sewer use
ordinance. Since then, an IU changed its operating process
and now generates a significant amount of cadmium. If the
POTW reallocates cadmium using the same method, the IU
may be subject to a local limit that will be difficult for it to
meet.
The POTW can change its local limits implementation method
by including the MAILs for cadmium in its SUO and allocating
cadmium loadings to lUs through individual permits. The new
allocations would be based on how much loading each IU
discharger needs. In this way, the POTW can provide the IU
that changed its operating process with a cadmium allocation
sufficient for its needs. This would be considered a
"substantial" modification as defined in 40 CFR 403.18(b).
stringent limits, they may need to install pretreatment
Chapter 7 provides the tools for POTWs to evaluate the circumstances that would lead it to conduct a
review or re-evaluation of the local limits program.
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CHAPTER 8-
LOCAL LIMITS TO ADDRESS CONCERNS ABOUT
COLLECTION SYSTEMS
POTWs may need to develop local limits to address concerns about their collection systems and meet the
requirements found at 40 CFR 403.5(b), which include protecting the health and safety of workers at the
POTW. Chapter 8 describes methods to address the following collection system concerns:
Fires and explosions [40 CFR 403.5(b)(l)]
Corrosion [40 CFR403.5(b)(2)]
Flow obstructions [40 CFR 403.5(b)(3)]
Temperature [40 CFR403.5(b)(5)]
• Toxic gases, vapors, or fumes [40 CFR 403.5(b)(7)]
POTWs should address each of these potential problems through their local limits development and re-
evaluation processes.
8.1 FIRES AND EXPLOSIONS
The General Pretreatment Regulations prohibit the discharge of pollutants that will create a fire or
explosion hazard in the POTW. This prohibition includes wastestreams shown to have a closed cup
flashpoint of less than 140 degrees Fahrenheit (60 degrees Celsius) using the test methods specified at 40
CFR 261.21. This provision is intended to protect POTW workers and the POTW collection system. To
comply, a POTW can establish a local limit equal to the flashpoint provision, or opt to develop other
protection methods. The flashpoint provision and three common alternatives are described below.
8.1.1 FLASHPOINT LIMIT
The flashpoint is the lowest temperature at which vapor combustion will propagate away from its source
of ignition. At temperatures below the flashpoint, vapor combustion immediately above the liquid either
will not occur, or will occur only at the exact point of ignition. Temperatures above the flashpoint are
required for combustion to spread. If a POTW prohibits discharges, typically volatile organic
compounds, that have a closed cup flashpoint of less than 140°F, it will protect against fires and
explosions. (A flashpoint limit applies to the entire wastestream, not to a specific pollutant.)
A flashpoint limit ensures that discharges to a POTW will not combust. It is important to note that a
flashpoint prohibition does not necessarily account for the flammability of mixtures from more than one
discharger. Dilution effects in sewer systems, however, generally prevent the creation of explosive
conditions.
The closed cup is used because this test simulates the confinement of vapors in a sewer. EPA requires a
flashpoint of less than 140°F [see 40 CFR 403.5(b)]for several reasons:
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• Ambient temperatures in a sewer are not expected to exceed 140°F.
• Typical industrial discharges of wastewater are cooler than 140°F.
• The specified flashpoint is consistent with hazardous waste regulations, which will help
ensure that POTWs do not face increased hazardous waste liabilities.
Regulations require that the flashpoint be determined by a Pensky-Martens Closed-Cup Tester, using the
test method specified in ASTM Standard D-93-79 or D-93-80, or by a Setaflash Closed-Cup Tester, using
the test method specified in ASTM Standard D-3278-78, or as determined by an equivalent test method
approved by the EPA Administrator under specified procedures. Appendix H lists closed cup flashpoints
for select organic compounds.
8.1.2 LOWER EXPLOSIVE LIMIT MONITORING
Another way to protect POTW workers is to monitor the collection system for combustible gases. A
combustible gas detector measures the concentration of these gases and vapors in the air as a percentage
of the lower explosive limit (LEL). The LEL is the minimum concentration in air at which a gas or vapor
will explode or burn in the presence of an ignition source.
LEL monitoring measures pollutant concentrations in the headspace above the wastewater, rather than in
the wastewater itself. This method makes setting local limits difficult. Consequently, POTWs often use
LEL monitoring to identify potentially problematic discharges, rather than as a numerical limitation to
implement and enforce against Ills. LEL monitoring is also an important way to protect POTW workers
who enter the collection system.
One approach to monitoring explosion potential is to measure LEL levels at key locations in the
collection system. Continuous monitoring at pump stations or key manholes can provide a constant
source of data on the potential for an explosion. Many POTWs establish a percentage of the LEL,
often 10 to 30 percent, as the level of concern. This ensures that discharges are safely below an
explosive level. The entire LEL should not be used to establish the level of concern.
8.1.3 SAMPLE HEADSPACE MONITORING
Sample headspace monitoring is a discharge screening technique to detect the presence of explosive
compounds and toxic gases and vapors. Initial screening using this method can identify discharges that
warrant detailed chemical-specific screening.
Sample headspace monitoring involves collecting a wastewater sample using proper volatile organic
sampling techniques (i.e., zero headspace), withdrawing a set percentage of the sample, injecting nitrogen
gas into the sample container (to maintain a total pressure of one atmosphere), and performing a gas
chromatography analysis of the sample headspace gas.
Volatile organic concentrations of the sample headspace gas are converted to an equivalent
concentration of hexane and compared to a set hexane limit (usually 300 parts per million of
hexane). Concentrations below the limit are usually deemed sufficient to protect the collection system
from fires and explosions and to provide minimal protection from toxic gases and vapors. Details of this
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method are available in Guidance to Protect POTW Workers from Toxic and Reactive Gases and Vapors
(EPA/812-B-92-001).
8.1.4 FLAMMABILITY AND EXPLOSIVITY DISCHARGE SCREENING LEVELS
Discharge screening levels can be used to set local limits on the discharge of pollutants that can create
flammable or explosive conditions in sewers. This approach requires converting the LELs of individual
compounds into corresponding IU discharge screening levels. These levels are then compared with
actual IU discharge concentrations. Appendix I contains a table of discharge screening levels based on
explosivity. A variety of screening levels have been developed for limiting flammable and explosive
discharges, including the four-step approach summarized here:
1. Identify the LEL for each POC.
2. Use the following equation to convert the compound's LEL concentration to a vapor
phase concentration (CVAP) expressed as moles per cubic meter (mol/m3). (Ten percent
of the LEL often is used in this equation, instead of the full LEL.)
CVAP = LEL x P/RT x 1000 = LEL x 40.87 (at 1 atm and 25°C)
Where:
P = total pressure, 1 atmosphere (assumed)
R = ideal gas constant, 0.08206 atm L/mol °K
T = absolute temperature, 298.15°K (equal to 25°C) (assumed)
3. Determine the Henry's Law Constant (H) for the POC. This constant converts LEL air
phase values to corresponding water phase discharge levels. Note that H is presented in
a variety of units [e.g., (atm m3)/(mol), (mol/m3)/(mg/L), and (mg/m3)/(mg/L)] and may
require converting H into the appropriate units of (mol/m3)/(mg/L). Appendix I contains
a listing of Henry's Law constants in various units and provides the appropriate
conversions.
4. Calculate the IU discharge screening level (CLVL) using the Henry's Law expression:
CLVL = CVAP/H
Where CLVL is the discharge screening level in mg/L.
Screening levels derived by this method should be compared directly with the actual IU discharge
concentrations. Some of the assumptions made using this method are:
• Although temperature dependent, H typically is reported at 25 °C (77°F), which is a
reasonable estimated temperature of discharges to POTWs. Warmer wastewaters will
exhibit higher concentrations in the vapor phase, while cooler wastewaters will exhibit
more of the pollutant in the liquid phase.
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• The pollutant instantly volatizes to the sewer atmosphere. Although this is a
conservative assumption, the more turbulence in the sewer, the closer the assumption is
to actual conditions. In addition, air flow through the sewers prevents the reaching of
equilibrium, thereby acting to reduce concentrations below threshold levels in the vapor
phase.
• The equation does not take into account the solubility effects that result from organic
contaminants in the wastewater, thereby limiting volatilization into the atmosphere.
For details of this method, see Guidance to Protect POTW Workers from Toxic and Reactive Gases and
Vapors (EPA 812-B-92-001).
8.2 CORROSION
The General Pretreatment Regulations prohibit discharges of pollutants that will cause corrosive
structural damage to a POTW. The regulations also prohibit discharges with a pH lower than 5.0 unless
the POTW is specifically designed to accommodate such discharges.
8.2.1 PH
Besides the low-end pH limit specified in the General Pretreatment Regulations, EPA recommends
POTWs evaluate the need to set upper pH limits or more stringent low-end pH limits. A POTW should
set an upper pH limit if corrosion damage attributable to high-pH discharges is identified. An upper limit
pH of up to 12.5 may be an appropriate upper limit in lieu of any identified high pH corrosion concerns.
However, because wastewater of pH 12.5 or higher is considered a hazardous waste (exhibiting the
characteristic of corrosivity) under 40 CFR 261.22(a)(l), additional reporting and liability results when
hazardous waste is discharged to a sanitary sewer. The POTW needs to set an upper pH limit that is
protective of the POTW, but also allows for some margin of safety to avoid characterization as hazardous
waste.
EPA acknowledges that there are advantages to accepting high pH industrial wastewater. These include:
• Reducing odor emissions from the collection system and plant processes due to a
reduction in the amount of aqueous hydrogen sulfide.
• Aiding the nitrification process (which often requires an external source of alkalinity).
• Improving precipitation and removal of toxic heavy metals by primary clarification.
• Limiting IU use of acids to neutralize high pH effluent and thus minimizing chloride and
sulfate ions detrimental to POTW operation.
8.2.2 CORROSIVE POLLUTANTS
In addition to discharges whose pH is high or low, the following pollutants can contribute to the
corrosive properties of wastewater:
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• Sulfide and sulfate. Much of the sulfide in collection systems is present as hydrogen
sulfide due to the anaerobic degradation of sulfate. This degradation occurs where
oxygen is absent and organic matter is present. Collection systems are particularly
conducive to this reaction if wastewater is allowed to stagnate. The formation of
hydrogen sulfide is primarily a function of the collection system's design, however, and
not a function of the characteristics of industrial discharges. Hydrogen sulfide corrodes
metals such as iron, copper, lead, and zinc. It is also a precursor to sulfuric acid, which
corrodes concrete and metals. Sulfate causes corrosion by reacting with the calcium in
concrete to form calcium sulfate, which can cause concrete to crack. For more
information, see Detection, Control, and Correction of Hydrogen Sulfide Corrosion in
Existing Wastewater System, (EPA-832-R92-001, September 1992).
• Chloride. This pollutant can adversely affect inorganic films and precipitates that form
on sewer wall and provide a physical barrier that protects from chemical corrosion. Not
only can chloride decay and penetrate these coatings, it can also prevent them from
developing by forming more soluble metal chloride instead.
• Chlorine. By reacting to form hydrochloric (HC1) and hypochlorous (HOC1) acids that
decrease the pH of wastewater, chlorine can increase the rate at which iron and steel
corrode.
• Nitrate and nitrite. They can contribute to iron and steel corrosion.
• Dissolved salts. The electrolytic action of dissolved salts on the base material can
corrode concrete, asbestos-cement, and cement mortar.
• Suspended solids. The abrasive and erosive contact of suspended solids with sewer
pipes and pumps can cause corrosion, particularly at joints, elbows, bends, and other
non-uniform areas.
• Organic compounds. If present in excessive concentrations, organic compounds such
as solvents will promote the dissolution of gaskets and rubber and plastic linings.
8.3 FLOW OBSTRUCTIONS
The discharge of solid or viscous pollutants in amounts that will obstruct flows to POTWs and result in
interference is prohibited by the General Pretreatment Regulations. The greatest threat of obstruction in
POTWs comes from polar fats, oils, and greases (FOG) of animal and vegetable origin. Typical sources
include restaurants, residences, food processors, and food-based industries. Certain polar FOGs, such as
non-ionic surfactants, do not contribute to flow obstruction. Additional discussions on the potential for
interference and pass through due to FOG are provided in Section 5.3.3.
Although more compatible with wastewater treatment operations than non-polar mineral oil or
petroleum-based oil and grease, polar FOG can accumulate and congeal in collection systems, pumping
stations, and treatment plants. By obstructing influent flows, polar FOG reduces the capacity of pipes
and pumps, interferes with POTW instruments (such as flow meters and probes), reduces treatment
efficiency, and increases POTW operation and maintenance costs. Polar FOG can interfere with the
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POTW's collection system through blockages when the wastewater cools sufficiently to allow the
suspended fat, oil, or grease to congeal. This condition is a function primarily of interceptor size, length,
and slope; ambient temperature; wastewater temperature; and concentration of FOG. These factors vary
throughout the collection system. To develop a technically based FOG limit for protecting the collection
system, empirical data (observations and measurements) are needed to document problems and
contributing factors. The empirical data along with generally available pretreatment and control
measures for FOG become the technical basis for the proposed local limit.
To collect data, the POTW first identifies collection system sections that have a critical low slope (i.e.,
relatively flat) profile and may be subject to low temperatures. Data are collected that identify FOG
levels corresponding to deposition rates of solidified oil and grease. The level of oil and grease at which
deposition is negligible would be the basis for the collection system MAHL.
Local limits on FOG may require POTWs to investigate and monitor the activities of non-SIUs that are
the sources of FOG. The use of controls other than numerical limitations may be a more appropriate way
to address the problem of FOG from non-SIUs. These controls can include:
• Requirements to install and maintain grease traps
• Pretreatment requirements
• Best management practices
• Prohibitions of specific materials, such as free-floating FOG
• Prohibitions of FOG that are in a solid or semisolid form
• Surcharge programs
• Cost recovery efforts to defray the expenses associated with cleaning sewers
• Pollution prevention measures
Many POTWs have oil and grease control programs. The Oregon Association of Clean Water Agencies
has authored Fats, Oil, and Grease Best Management Practices Manual: Information, Pollution
Prevention, and Compliance Information for Publicly Owned Treatment Plants. The manual provides
municipal pretreatment staff, along with restaurant and fast food business managers and owners, with
information about animal and vegetable-based oil and grease pollution prevention techniques focused on
their businesses. The techniques are effective in both reducing maintenance costs for business owners,
and preventing oil and grease discharges to the sewer system. Go to:
http://www.oracwa.org/Pages/intro.htm to review the manual.
8.4 TEMPERATURE
The General Pretreatment Regulations prohibit heat discharges that will inhibit biological activity in a
POTW and result in interference. And in no case can discharges increase the temperature at the
POTW headworks above 40°C (104°F) unless the Approval Authority, upon request of the POTW,
approves alternative temperature limits.
The dilution of heated industrial wastewaters in the collection system typically ensures compliance with
this prohibition. Temperature is generally more of a hazard to workers who must enter the sewer system
than it is to POTW treatment operations. A POTW that encounters IU discharges hot enough to prevent
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or restrict sewer entry should require the IU to reduce the temperature of its discharge. The installation
of heat exchangers on high-temperature discharges may help the IU save on heating costs for its facility
or its process streams.
8.5 Toxic GASES, VAPORS, AND FUMES
The General Pretreatment Regulations prohibit the discharge of pollutants that lead to the accumulation
of toxic gases, vapors, or fumes in the POTW in sufficient quantity to cause acute worker health and
safety problems.
Discharge screening levels can be developed to identify IU discharges that have the potential to generate
toxic gases or vapors in the sewer. A common approach is to convert gas and vapor toxicity criteria for
individual compounds into corresponding IU discharge screening levels using Henry's Law Constants.
These constants relate the concentration of a constituent in the air to the corresponding equilibrium
concentration in the water. The screening levels should be compared to the actual pollutant
concentrations in the IU discharge. Calculating these wastewater screening levels is a three-step process:
• Identify the toxicity criteria, also known as the threshold concentration (CVAP, in mg/m3),
for the POC. Typical threshold values are available from the National Institute for
Occupational Safety and Health's (NIOSH's) Recommended Exposure Limits (RELs),
the Occupational Safety and Health Administration's (OSHA's) Permissible Exposure
Limits (PELs), and the American Conference of Governmental Industrial Hygienists'
(ACGIH) Threshold Limit Values (TLVs). Each organization can provide chronic and
acute exposure thresholds that can be used to develop screening levels. See Appendix J
for a listing of some of these threshold concentrations. Consistent with the specific
prohibitions for toxic gases, vapors, and fumes, screening levels may be based most
appropriately on acute worker health and safety levels (i.e., short-term exposure levels or
ceiling concentrations).
• Identify the Henry's Law Constant (H) for the POC and convert the constant to the
appropriate units of (mg/m3)/(mg/L). Appendix I contains a listing of Henry's Law
constants in various units and the appropriate conversions.
• Calculate the IU discharge screening level (CLVL) from the Henry's Law expression:
CLVL = CV
Where:
CLVL = IU discharge screening level (in mg/L)
CVAP = Threshold concentration (in mg/m3)
As with the flammability and explosivity screening level, this screening method assumes instantaneous
volatilization of the pollutants to the atmosphere and does not consider the dilution of IU wastewater in
the collection system. Therefore, these screening levels will in many cases be more conservative than
necessary to protect POTW workers.
These screening levels address only the toxicities of individual compounds, but mixtures of toxic
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compounds can be evaluated against an adjusted threshold value of the mixture of all the toxic
compounds. Appendix I contains a table of discharge screening levels based on fume toxicity. Details
on the specifics of using the discharge screening level method, including evaluating mixtures of toxic
gases, vapors, or fumes, is available in EPA's Guidance to Protect POTW Workers from Toxic and
Reactive Gases and Vapors (EPA 812-B-92-001).
8.6 SUMMARY
After reviewing Chapter 8, POTWs should be able to address collection system concerns: fire and
explosions, corrosion, flow obstructions, temperature, and toxic gas, vapors and fumes.
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CHAPTER 9 -
QUESTIONS AND ANSWERS
This chapter presents EPA's responses to many commonly asked questions about local limits
development and implementation. The questions and answers are grouped by topic for ease of finding
subjects of interest.
9.1 GENERAL
Q: Once I establish a local limit, will I ever be able to drop it?
A: As emphasized throughout this guidance, development of local limits is a continuing, dynamic
process. EPA recommends a re-evaluation of specific local limits whenever there are significant
changes in the overall program as a step that every prudent Control Authority should do on a
regular basis. If changes in IU discharge conditions or installed treatment technologies at the
POTW dictate that some pollutants of concern (POCs) are no longer present or are present only
in concentrations that will not cause pass through, interference, or degradation of sludge
quality, then the local limits for those pollutants may be dropped after appropriate procedures
are taken. However, POTWs should be cautioned that dropping a particular local limit
completely may motivate lUs to discontinue a treatment process designed to remove or recycle
that particular pollutant. POTWs should have a complete understanding of the makeup of
untreated IU waste streams before dropping a local limit completely. The regulations at 40 CFR
403.18(c) specify that eliminating or changing a local limit to make it less stringent requires
notification of the Approval Authority and appropriate public notice because such actions are
considered substantial program modifications.
Q: How do multi-jurisdictional systems affect local limit requirements?
A: For multi-jurisdictional systems in which one Control Authority accepts industrial wastes from
one or more other, independent municipalities, EPA strongly recommends that all contributing
jurisdictions adopt a set of local limits that are at least as stringent as those of the Control
Authority that maintains the collection system and operates the receiving POTW. If this policy is
impractical, then the contributing jurisdictions should agree to a maximum total mass loading of
pollutants that would be discharged to the primary collection system and POTW. As an
alternative, the contributing jurisdiction may adopt two sets of local limits and apply to each IU
the limit appropriate to the treatment works to which the user discharges. Consult EPA 's
Multijurisdictional Pretreatment Programs Guidance Manual (EPA 833-B-94-005, June 1994) for
additional information.
Q: Do a minimum number of parameters need to be evaluated?
A: There is no minimum number of parameters required by regulation. EPA recommends that the
need for local limits be evaluated for a list of specific pollutants. EPA recommends that
technical evaluations for POCs by every POTW should include a determination of the need for
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limits for arsenic, cadmium, chromium, copper, lead, mercury, nickel, silver, zinc, and cyanide.
This Guidance adds total suspended solids, 5-day biochemical oxygen demand, ammonia,
molybdenum and selenium to the list of recommended minimum pollutants to be considered as
POCs.
Q: Do local limits have to be developed individually for multiple treatment works? Is it necessary
that identical numeric local limits be established?
A: There is no regulatory requirement that a Control Authority develop local limits that are specific
to a single treatment works. However, EPA recommends that the Control Authority perform a
separate evaluation for each works to determine if each plant is being protected and not subject
to pass through or interference problems. After completing these independent evaluations, the
Control Authority can determine whether individual local limits should be provided to the lUs
that discharge into the parts of the system served by a particular treatment works. The only
regulatory requirement is that there be local limits developed that prevent pass through and
interference and are enforceable on a technical basis. The preferred method is to establish
MAILs individually for the treatment plants, but if that is politically infeasible, then set a single,
conservative local limit (i.e., the lowest limit developed in the assessment for the individual
treatment works) for a POC. The limit should then apply to all lUs that discharge to the POTW,
without regard as to which works actually treats the wastewater discharged by a particular IU.
Q: Can best management practices (BMPs) and best professional judgment (BPJ) limits be applied
in lieu of the traditionally derived numeric local limits?
A: The General Pretreatment Regulations do not specifically address the use of BMPs and BPJ as
local limits. The regulations at 40 CFR 403.5 (c) require the POTW only to develop "specific
limits " for prohibited discharges. The current regulatory language is ambiguous as to whether
BMPs could serve in lieu of numeric limits. BMPs may reduce the amount of the POC at the
headworks thus leaving more pollutant loading to be distributed as numerical limits to facilities
that cannot control their discharge through BMPs. If adopted, the proposed Pretreatment
Streamlining Rule would specify that BMPs could be considered as local limits and also fulfill
the statutory requirements of Section 307 (d) of the Clean Water Act. As with BMPs, using BPJ
to develop local limits is not specifically prohibited. If adopted following the process in 40 CFR
403.5, BPJs are enforceable.
Q: Can local limits evaluation and development be contracted out?
A: In EPA 's view, the optimum process is for the Control Authority to evaluate and develop the
appropriate local limits because it provides the Control Authority with a better understanding of
limit development and the importance of compliance. However, recognizing the fact that some
Control Authorities may be severely constrained by an overextended workforce, or require
access to technical expertise that is not internally available, the Control Authority may secure
the necessary manpower and expertise through an outside consultant or engineering firm.
However, the Control Authority should be aware that any mistakes or improper determinations
would be its legal responsibility if the Approval Authority, an IU, or any outside party challenges
the POTW on the assignment of the limits.
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9.2 POTENTIAL POLLUTANTS OF CONCERN
Q: If a pollutant is below the detection level in influent, effluent, and sludge, may a POTW exclude
it as a POC (and not develop a MAHL), even if it is one of EPA's 15 pollutants?
A: Yes, it may. If a POC is not detected in the influent, effluent, or sludge during the POTW 's
assessment of the need for local limits, an accurate calculation of the MAHL for that particular
pollutant is not possible. The goal of setting stringent local limits is to protect the POTW and
avoid violations ofNPDES permit. However, if no MAHL is established for a "potential" POC,
there is always the possibility that a new industrial user (or users) of the system will discharge
wastes that are in excess of the POTW's ability or capacity to treat such wastes. Therefore, EPA
recommends that MAHLs be developed for all 15 EPA-designatedPOCs even if local limits are
not adopted. Of course, POTWs should assess a new user's impact on local limits before
granting authorization to discharge.
Q: Should local limits be developed as dissolved metals, total metals, or both? How does
hexavalent chromium relate to total chromium, and which should be used for local limits
development?
A: While it may be desirable to develop local limits for both dissolved and total metals, in reality it
may be impractical because of cost. POTW data are developed almost exclusively in terms of
"total" because of NPDES requirements and the fact that Categorical Pretreatment Standards
are always expressed as total. Because the POTW should be able to apply the more stringent of
either the local limit or the Categorical Standard, it makes sense to develop the local limits as
"total" values. Although the dissolved form of metals is usually more toxic, POTWs need to
control the total metal entering the treatment works because paniculate metal or metal
compounds may exert some toxicity or may later be resolubilized. A large percentage of the
toxic metals present in aeration basins at some treatment works has come from recycled solids
handling sidestreams. These contributions can continue to exert a toxic effect long after the
source has been controlled. Although most heavy metals "passing through " a treatment works
are discharged into receiving waters in the dissolved form, significant concentrations of heavy
metals may accumulate as fine particulates in the sludge produced at the POTW. By
implementing local limits to control total metal concentrations, a POTW will reduce the chances
for pass through and ensure that the quality of the sludge is not degraded. Local limits should
be developed for total chromium. Hexavalent chromium is the more toxic of the two forms of the
metal, but it can be converted to a total chromium value by using proper mathematical
equations. If a POTW has contributions of hexavalent chromium, EPA recommends it develop
local limits for both hexavalent chromium and total chromium. The basis of the limits will likely
be different because the allowable holding time for hexavalent chromium samples is less than 24
hours.
9.3 SAMPLING AND ANALYSIS
Q: What analytical requirements and quality assurance/quality control procedures apply to local
limits evaluation sampling?
A: There are no different or "special" quality assurance/quality control procedures that apply
strictly to local limits sampling. EPA recommends that all wastewater sampling for POCs follow
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prescribed protocols found in 40 CFR Part 136 (Guidelines for Establishing Test Procedures for
the Analysis of Pollutants^ and information provided in EPA-issued technical guidance. When
sampling sludge for metals and total solids, however, the requirements in the sludge regulations
at 40 CFR Part 503 apply. Therefore, EPA recommends that the analysis of sludge for the
presence of metals be performed according to EPA test method SW-846 and for total solids
according to Part 2540 G of the Standard Methods for the Examination of Water and
Wastewater, 18th Edition.
Q: Are there minimum analytical detection levels that should be achieved when analyzing samples
for local limits?
A: As discussed in Chapter 3, a POTW's NPDES permit conditions, sludge disposal practices, and
State and local requirements need to be addressed through local limits. Therefore, the analytical
techniques for detecting POCs need to be able to identify and quantify concentration levels that
are at least as stringent as the prescribed maximum concentrations for conventional and non-
conventional pollutant effluent limitations, water quality-based toxic pollutant limitations, whole
effluent toxicity (WET) requirements, and any numeric criteria for sludge use and disposal
practices. In addition, POTWs will want to specify the lowest reasonable detection limit for a
local limit monitoring to minimize the possibility of a POC being reported as "non-detectable. "
Q: Is it necessary to account for hydraulic detention time through the treatment works when
conducting sampling?
A: Treatment works sampling should account for hydraulic detention times within the plant
whenever possible. Developing relevant removal efficiencies depends in part on accounting for
hydraulic detention times. For some systems, such as lagoon systems, hydraulic detention times
may be lengthy (e.g., 21 days). If it is not feasible to account for detention times, local limits can
still be developed, but the options for determining removal rates will be reduced. Various
methods for determining removal efficiencies are reviewed in Chapter 5.
Q: Do I have to outline a sampling plan for the local limits evaluation?
A: Outlining a sampling plan for local limits evaluation is not required by 403 regulations,
although some Approval Authorities may require submission of such a plan. However, EPA
highly recommends that a POTW develop a sampling program to ensure that it has adequate
data for developing local limits that have sound technical bases. A sampling program can also
enable a POTW to use fewer resources for evaluating local limits by providing the data
necessary to determine and justify that local limits are not necessary for some pollutants and by
enabling the POTW to manage its data and ensure that unnecessary sampling is not performed.
Information regarding local limits data collection is reviewed in Chapter 4.
Q: Is sampling and analysis of the receiving stream necessary?
A: Receiving stream data (flow and ambient background concentrations of pollutants) provide key
input parameters for allowable headworks loading (AHL) calculations when NPDES permit
limits do not exist and the POTW needs to evaluate for pass through based on water quality
standards. These data may already be available from sources such as the U.S. Geological
Survey, State environmental agencies, and the POTW's NPDES permit. Therefore, a POTW may
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not need to conduct sampling and analysis of the receiving stream to gather these values.
However, if these data are not available, the POTWwill want to consider sampling the receiving
water so that AHLs can be calculated based on applicable values. The Approval Authority may
require this information on a case-by-case basis for individual lUs. Other dischargers to the
same portion of the receiving stream may already have performed sampling and may be willing
to share the data or the costs of new monitoring.
9.4 DETERMINING MAHLs
Q: Water quality standards have been established for our treatment works' receiving waters, but no
water quality-based effluent limitations are included in our permit. Is it necessary to include the
analysis for an allowable headworks loading (AHL) based on water quality standards in this
case?
A: Yes, it is. If a POC loading measured at the headworks exceeds a MAHL that was set by the
AHL for a water quality standard, there may be pass through of the pollutant, thereby causing a
violation of the water quality standard and (consequently) of the Clean Water Act. In general,
POTWs will not have NPDES permit limits for all of the POCs established during the local limits
analysis. In such cases, a POTWmay base its effluent-quality-based AHL on State Water
Quality Standards (WQS) or Federal Water Quality Criteria (WQC). State environmental
agencies have developed WQS that set maximum allowable pollutant levels for their water
bodies, specific to the receiving stream reach 's designated uses. Even though the POTWs
NPDES permit may not contain a numeric effluent limit for a POC, the permit probably will
contain narrative provisions requiring compliance with State WQS and prohibiting the discharge
of any toxic pollutants in toxic amounts. A local limit based on a State WQS fulfills the narrative
permit requirement specifying "no discharge of toxics in toxic amounts. " See Section 3.2.2 and
the associated footnotes for additional information.
Q: How much literature data are acceptable in deriving MAHLs? How much site-specific data are
sufficient? How recent must data be for deriving MAHLs?
A: The answers to these questions will vary significantly from facility to facility. Depending on the
POC and on the type and accuracy of the data available, a considerable range of techniques are
acceptable for deriving the MAHL. EPA recommends that the Control Authority make a case-
by-case determination about type and age of data that are sufficient to calculate accurate,
technically defensible MAHLs. For example, data collected prior to major construction should
not be used. However, the most accurate and technically defensible limits are the result of using
site-specific data, rather than "generic " removal efficiency data derived from average, national-
level treatment works "literature " data.
Q: We do not have NPDES or sludge limits for all of the POCs required to be evaluated; further,
there are no State WQS for these pollutants. What criteria are we supposed to use in our
evaluation?
A: Sludge, NPDES, or water quality criteria may not exist for all POCs. In these instances, the
POTWmay want to develop MAHLs based on system design criteria, air quality standards,
inhibition criteria, or worker health and safety standards. In addition, the POTWwill want to
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determine the original purpose for adding a POC (e.g., WET test failure) and establish criteria
through researching other applicable standards and guidelines.
Q: How does a POTW develop local limits based on a NPDES WET limit?
A: Nothing in the pretreatment regulations prohibits using Whole Effluent Toxicity (WET) test data
as the basis for developing a local limit. WET tests are primarily designed to protect the
receiving waters from the aggregate toxic effect of a mixture of pollutants in the effluent. The
WET approach is most useful for complex effluents where it may be infeasible to identify and
regulate all toxic pollutants in the discharge, or where chemical-specific pollutants are set, but
synergistic effects are a problem. However, unless you can identify each compound in the
effluent that produces measurable acute or chronic toxicity concentrations, WET testing cannot
be used to set local limits for a particular POC. If the toxic pollutant or pollutant parameter
cannot be identified, then a POTW will want to evaluate all of the possible POCs present in the
mixture. In this situation, WET test data may not be a cost-effective methodology for identifying
POCs for evaluation in the local limits development process. The guidance Toxicity Reduction
Evaluation Guidance for Municipal Wastewater Treatment Plants (EPA/833-B-99-002, August
1999) provides further information on conducting a Toxicity Identification Evaluation.
Q: Influent and effluent pollutant concentrations are below quantifiable levels yet the pollutant is
detected in the sludge. What removal rate should I use?
A: EPA recommends that a POTW first evaluate those levels below the minimum level of
quantitation (ML) as outlinedin Section 5.1.3. If the methodologies outlined in Section 5.1.3 do
not allow the calculation of a removal rate, a POTW then may selectively use removal
efficiencies reported by other POTWs or by studies that have been published in professional
journals or by EPA. Appendix R provides a list of removal efficiency data for priority pollutants
gathered from other POTWs.
Q: Why should POTWs use the Table 3 Land Application Part 503 sludge standards when the
POTW's sludge is disposed in a landfill?
A: POTWs are encouraged to use the Table 3 standards because the Pretreatment Regulations list
recycling of sludge as one of the goals of the program. Land application standards help meet
this goal and also allow for more sludge disposal options, because the Table 3 standards are the
most stringent. EPA recommends that POTWs consider the attainment of EPA "clean sludge "
standards, that are delineated in Table 3 of 40 CFR 503.13, and provide the broadest choice of
beneficial use options for sludge disposal. Further achievement of these standards is consistent
with the objectives of the National Pretreatment Program, which are listed at 40 CFR 403.2.
Additionally, until a sludge landfill is properly closed and abandoned there is always a potential
for the leachate to affect groundwater. See Appendix Kfor landfill leachate loadings. In some
cases, collected leachate can be trucked (as hauled waste) to a POTW and treated to non-toxic
concentration levels. For this option to be viable, the metals content of the sludge should be
limited to concentrations that will not cause potential pass through or interference problems for
the POTW. Table 3 sludge standards for land application cover all nine toxic metals, while the
surface disposal sludge standards specify limits only for arsenic, chromium and nickel.
Imposing land application standards on sludge increases the probability that the leachate can be
successfully treated in the future at a POTW. Nevertheless, if a POTW has a choice of disposal
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options, EPA recommends that it use land application disposal techniques because they are
generally more controllable and have less potential for serious environmental degradation of
surface water and groundwater.
9.5 ESTABLISHING LOCAL LIMITS
Q: All of my influent, effluent, and sludge concentration data for a specific pollutant are below the
method detection limit. Can the pollutant still be considered a POC and local limits established?
A: Yes. The Control Authority (generally, the POTW) has the authority to consider any chemical
compound or pollutant as a potential POC and establish a local limit for that pollutant. If your
POTW serves a high-growth municipality or incorporated area where the number and type of
non-domestic users change frequently, it may be prudent to establish aMAHL limit in your
ordinances for any pollutant that could potentially cause interference, pass through, or degrade
your sludge quality—even if the concentration of that pollutant is currently below detection
levels. Several statistical approaches to evaluating "below detection level" or below
quantitation level data are discussed in Section 5.1.3 and Appendix Q.
Q: If a POTW's local limits evaluation indicates that its sludge disposal method (e.g., land
application) is the most limiting factor, may the POTW pursue a less stringent sludge disposal
method (e.g., landfill)?
A: The determination of the manner in which the sewage sludge is used or disposed of is a local
determination. As long as a POTW adheres to all of the regulatory requirements specified in 40
CFR Part 503, it may select the optimum method of sludge disposal. EPA recommends that
POTWs consider the attainment of EPA "clean sludge " standards, that are delineated in Table 3
of 40 CFR 503.13, and provide the broadest choice of beneficial use options for sludge disposal.
Further, achievement of these standards is consistent with the objectives of the National
Pretreatment Program, which are listed at 40 CFR 403.2.
Q: What do I do when my total domestic/background loading of a pollutant is equal to or greater
than my MAHL, so I have no allowable loading for Ills?
A: The POTW may wish to consider a program that involves short-term, intermediate, and long-
term measures. Short-term measures include evaluating the data and calculations used to
develop the local limits to assess the validity of results. Intermediate measures include
establishing interim local limits, looking into other possible sources of pollutants (including
expansion of your list oflUs), and determining how to manage these sources. Long-term
measures involve evaluating controls for users not already covered by your pretreatment
program. If the short-term measures do not take care of the problem and provide loadings to
allocate to lUs, the POTW would proceed to intermediate measures, and then, if necessary, to
long-term measures. Examples of activities for each of the steps are listed below:
Short-term
• Ensure that all significant industrial and commercial dischargers of the pollutants have
been identified.
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Evaluate all sampling sites that have been used to estimate background concentration to
ensure that commercial facilities were not missed and are not contributing pollutants of
concern to the sampling location.
Use actual sewer trunk line monitoring data in place of any literature data used in
determining total domestic pollutant loadings to the POTW.
Use removal efficiencies based on in-plant monitoring in place of any literature removal
efficiencies used in determining MAHLs.
Verify the applicability of criteria (e.g., sludge disposal standards, and water quality
criteria) used as the basis for AHL calculations.
Verify that appropriate sampling locations have been used, and that samples are
representative (i.e., do not reflect peak loading periods only).
Check the accuracy of all calculations made and the reliability of data used.
Evaluate the method for handling non-detect monitoring results (e.g., equal to the
detection level was used) and consider using other conventions (e.g., half the detection
level).
If the MAHL is based on inhibition criteria, current headworks loadings are greater than
the inhibition criteria and the POTW has not experienced inhibition, the current
loadings may be a more appropriate basis for inhibition values.
Intermediate
• Verify the sampling frequency through statistical methods.
• Collect additional sampling data to refine values used (e.g., for removal efficiencies) or
replace literature values.
• If hauled waste is being accepted, consider discontinuing this practice or instituting a
program to determine individual wastewater components versus those contained in the
septage.
• If chemicals are added in the plant or sewer system (e.g., to control root growth),
consider alternatives that do not introduce POCs.
• Calculate a mass balance for the collection system (i.e., check if the sum of industrial
plus domestic/commercial plus any hauled waste loadings are between 80 percent and
120 percent of the total influent loading). If not, one or more sources may not be
accounted for or data may be invalid.
• Establish interim local limits such as a local limit equal to the POTW's NPDESpermit
limit, to the NPDES limit adjusted for the POTW removal efficiency for a particular
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pollutant, or to the lowest achievable method detection level (so that IU compliance with
the limit can be determined). If the POTWis not experiencing pass through or
interference for a given pollutant (e.g., no NPDES limit or sludge disposal criterion
violations, no collection system problems), consider substituting the current influent
loading for the MAHL and recalculate the allowable industrial loading. The interim
limits should be replaced as long-term measures take effect.
Long-term
• Require industries to perform pollutant minimization/prevention evaluations.
• Consider implementing measures to address or regulate elevated loadings from non-
industrial sources. These non-industrial sources include nonpoint sources (e.g., runoff)
discharging to combined sewers, elevated pollutant levels in water supplies, household
disposal of chemicals into sanitary sewers, and toxic pollutant discharges from
commercial sources (e.g., photo labs or dry cleaners).
Pollution prevention/minimization programs can address each of these sources. Nonpoint
sources of pollutants may be addressed through combined sewer overflow control programs and
urban and agricultural chemical management programs. The POTWmay be able to reduce
elevated pollutant levels in water supplies by working with the local water department. For
example, elevated levels of metals in water supplies often arise from corrosion in water
distribution pipes. The local water department may be able to reduce corrosion by adjusting the
pH of the water supply. The POTWmay be able to assist the water company in developing a
program to optimize the use of chemical additives in lieu of making simple adjustments to the pH
by using acidic or caustic chemical agents. The POTWcan make efforts to educate the public on
proper disposal of household chemicals and to provide chemical and used-oil recovery facilities.
Each of these efforts is not directly part of the local limits process.
Reducing toxic pollutant discharges from commercial facilities is generally most effectively
addressed through local limits. Commercial sources of pollutants, such as radiator shops, car
washes, hospitals, laundries and photo processors, are often not considered significant sources
of toxics because they typically have relatively low flows or are assumed to have insignificant
pollutant levels in their discharges. However, these commercial sources may discharge at
surprisingly high pollutant loading levels and are potential lUs that should be considered for
control during local limits development. In some cases, the POTWmay best address these
sources through pollution prevention/minimization efforts, such as providing guidance to small
commercial dischargers (e.g., informing dentists about how they can reduce mercury discharges
to sewers).
Q: How useful are priority pollutant data in determining the need for and in setting local limits?
A: The "best case scenario " is that a POTW knows everything about each of its lUs, including the
manufacturing processes involved and the types and amounts of pollutants discharged into the
collection system by a particular facility. However, despite the requirements to notify the POTW
of any changed discharges, some facilities might install new process technology, change to the
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production of new chemical compounds, or use new or substitute chemicals in their processes.
In these cases, new POCs might be introduced into the POTW. Use of priority pollutant scan
data would provide added insurance that none of the 126 priority pollutants are being
introduced (inadvertently or otherwise) into a POTW before problems with pass through,
interference or sludge quality are detected by other analytical means.
Q: Do local limits apply to all Ills? Do they have to be included in all permits issued by the
POTW?
A: The assignment of local limits depends on how the MAIL calculations were performed and how
the sewer use ordinance requires the local limits to be implemented. There is no regulatory
requirement that "all limits " be included in every permit. However, the regulations at 40 CFR
403.8(f)(l) require that the contribution to the POTW by each Industrial User be 'controlled'
through permit, order, or similar means, to ensure compliance with applicable Pretreatment
Standards and Requirements. The regulations also specify that permits issued to Significant
Industrial Users (SIUs) must contain certain minimum conditions, which include: "Effluent
limits based on applicable general pretreatment standards in part 403 of this chapter,
categorical pretreatment standards, local limits, and State and local law. " [40 CFR
The applicability issue is determined by the local limit allocation method (i.e., uniform
concentration, mass proportion, industrial contributory) that the POTW chooses when
developing the local limits and how the POTW expressly states the applicability of the local
limits within its sewer use ordinance (SUO). The Control Authority may elect to codify local
limits in the local SUO or place general enabling authority language about local limits in the
SUO and announce the actual limits by another mechanism (e.g., as a technical directive, etc.).
Including the limits in the SIU permit provides individual notice to a permittee of the pollutant
limits that are applicable to that particular SIU.
Q: My local limits re-evaluation indicates that a less stringent local limit than the one currently in
the ordinance can be applied. Is this allowed in light of EPA' s anti -backsliding policy?
A: First, you need to consider the full meaning of the "anti-backsliding" policy. The "anti-
backsliding " concept associated with NPDES permit limits does not apply to local limits. Local
limits apply to a particular IU and can be raised or lowered based on the periodic re-evaluation
of the need for those limits. Second, a POTW may need to modify its SUO before it may impose a
less stringent limit. Otherwise, the permit may conflict with the POTW' s authority. Third, in the
case of a Categorical Industrial User discharge regulated by a categorical effluent standard, the
more stringent limit (either the local limit or the categorical standard) must be
applied — regardless of the local limit established for that pollutant. Though rare, some
categorical standards may be made less stringent as a result of removal credits (40 CFR 403. 7).
Also, because any less stringent change in prescribed local limits would be a significant
program modification, you must notify and seek the approval of the Approval Authority prior to
making such a change.
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Q: Is effluent trading of local limits allowed?
A: Yes. A POTWmay decide to negotiate with its lUs in allocating its calculated allow able
industrial loadings. However, the POTWneeds to ensure that no more than the total MAHL is
allocated among domestic/background sources, lUs, commercial sources not considered lUs by
the POTW, and other sources of loadings such as hauled waste. Effluent trading, which must be
authorized in the POTW's sewer use ordinance, may result in a program modification, as defined
in 40 CFR 403.18 and results of the trades should be incorporated into any control mechanisms
(see Section 6.4.2).
Q: If a calculated local limit is excessive (i.e., a large number), should the POTW implement this
limit?
A: The POTW should consider the potential IU discharge for the particular pollutant and the
possibility that a high limit might encourage increased discharges to the system. Of course, the
POTW must receive Approval Authority concurrence on the local limit.
Q: How do I develop local limits for other pollutants (e.g., BTEX compounds) that may be specific
to certain users?
A: For BTEX, some options to consider for determining if pass through or interference will occur
include:
• Fume toxicity criteria.
• Aquatic life protection criteria.
• Worker safety and health criteria. Consult the Guidance to Protect POTW Workers
from Toxic and Reactive Gases and Vapors (EPA, 1992).
Once the most stringent criteria are determined, POTWs may want to compare the proposed
local limit with BTEX treatment technology. The Model NPDES Permit for Discharges Resulting
from the Cleanup of Gasoline Released from Underground Storage Tanks (EPA, 1989) contains
two sets of effluent limits: 1) BTEX of 100 /ug/L and benzene of 5 /ug/L (assumes approximately
15 mg/L of dissolved product is treated to a removal efficiency of 99.5 percent, which can be
achieved with a commercially available stripper unit), and 2) BTEX of 750 /ug/L and benzene of
50 jUg/L (assumes approximately 15 mg/L of dissolved product is treated to a removal efficiency
of 95 percent, using equipment that a small business is more likely to purchase).
Q: How should lU-specific limits be developed for "atypicaP'dischargers (i.e., groundwater
cleanups, hauled waste, landfill leachate, and underground storage tank cleanups) containing
pollutants for which no local limits or MAHLs are established and which cannot be measured at
the headworks?
A: First, EPA recommends you ensure that your local ordinance gives you the authority to impose
limits for pollutants that are not specifically listed in your ordinance limits or other document
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pertaining to local limits adoption policy. Second, EPA suggests that you review the
Supplemental Manual on the Development and Implementation of Local Discharge Limitations
under the Pretreatment Program (EPA-W21-4002, May 1991) and relevant RCRA site
remediation guidelines (for underground storage tanks and groundwater contamination) to
determine what types and concentrations of pollutants are typically discharged by these
wastewater sources. The POTWnext may determine (on a site-specific basis) which of these
sources are likely to be a problem and establish a sampling program for the sewer trunk lines
into which the wastewater is discharged. If this sampling program identifies the potential for an
adverse impact on the POTW, then specific local limits can be developed and incorporated into
the discharge permit of the IU(s) that are problematic. The Guidance to Protect POTW Workers
from Toxic and Reactive Gases and Vapors (EPA, 1992) provides additional data relating to
health and safety concerns.
9.6 OVERSIGHT AND PUBLIC PARTICIPATION
Q: What kind of public participation should I expect during the local limits development process?
A: Although the public does not usually become actively involved in the development process, the
CWA established public participation as an integral part of developing any regulatory program,
including standards and effluent limitations associated with the pretreatmentprogram.
Obviously, "public " participation includes all affected entities. The lUs are critically important
participants in the whole local limits development process. The General Pretreatment
Regulations encourage public participation by requiring public notices or hearings on local
limits development. Federal regulations require POTWs to notify affected persons and groups
and give them an opportunity to respond before final promulgation of a local limit [40 CFR
403.5(c)(3)]. Any subsequent modifications that are deemed significant modifications (as
defined in 40 CFR 403.18 (b) ) must be publicly noticed. Minor modifications, such as the
adoption of a more stringent local limit for a POC, do not require public notice. However, the
POTW must ensure that it has the authority to impose more stringent limits. Modifications to
local limits for pH and reallocation of the MAIL are considered to be minor program
modifications and do not require public notice (see Sections 6.7-6.9).
Q: Do I need Approval Authority approval to implement and enforce local limits?
A: No, you do not unless you are making changes to your legal authority or amending your local
limits to make them less stringent than those currently incorporated in your approved
pretreatment program. In accordance with 40 CFR 403.18, changes to legal authority or
making local limits less stringent is considered a significant modification to the approved
pretreatment program and must therefore be approved by the Approval Authority. However,
modifications to local limits for pH and reallocation of the MAIL are considered to be minor
program modifications and do not require Approval Authority approval or public noticing. As
prescribed in 40 CFR Part 403, the authority to develop and enforce local limits needs to be
incorporated into a POTWs pretreatment program at the time of program approval (see
Sections 6.7-6.9).
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9.7 IMPLEMENTATION AND ENFORCEMENT OF LOCAL LIMITS
Q: Are local limits enforceable if not contained in a sewer use ordinance (SUO)?
A: Local limits are enforceable if included in a valid user permit or similar enforceable control
mechanism. From a notification standpoint, local limits may be more difficult to enforce if the
SUO does not specifically reference them so that lUs know what is expected of them. Even if the
limits are not in the SUO, the Control Authority must ensure that it has the legal authority to
enforce limits or procedures in documents other than the SUO and that all required public
participation procedures are conducted. The Control Authority will need to evaluate the
availability of resources and the respective burden of enforcing local limits before deciding
whether to use general language about complying with local limits versus putting specific MAIL
values in its SUO (see Sections 6.7 - 6.9).
Q: Can my State or EPA take enforcement action against Ills in my jurisdiction for violations of
local limits?
A: All local limits developed in accordance with the provisions stated in 40 CFR 403.5(c) are
deemed to be Pretreatment Standards for the purposes of Section 307 (d) of the Clean Water Act.
Consequently, EPA or the State Approval Authority may take enforcement action against any
industrial user for a violation of a local limit. The CWA also provides that affected third parties
may bring "citizen suits " against users for violations of these local limits.
Q: How can a POTW justify imposing stringent local limits on Ills when the POTW is not subject
to an NPDES permit limit or sludge standards for the same pollutant?
A: If a POTW believes that one or more POCs may cause or have the potential to cause damage to
the system infrastructure (i.e., corrosion, erosion, disruption of plant treatment efficiencies),
affect worker safety and health, or negatively impact water quality, it must impose a local limit
for these POCs. The use of site-specific data (rather than less precise "literature " data) for local
limits calculations will always produce better, more technically defensible limits. In addition,
POTWs have the ability to establish land application of its sludge as the goal ofitspretreatment
program and to use sludge land application criteria (as opposed to sludge surface disposal
criteria) in the development of the limits.
Q: Can a POTW allocate local limits to non-categorical SIUs only and require CIUs to comply with
the categorical standards only?
A: This is an allocation method issue. As long as the appropriate categorical standards are
imposed on the CIUs and the sum of the loadings allocated to alllUs does not exceed the total
MAIL, the POTW may assign MAILs as it sees fit (i.e., each IU'need not be given the identical
limit for a particular POC). Note that if the POTW establishes a MAIL for a pollutant, then EPA
recommends that CIUs receive an allocation for that pollutant even if the categorical standard
does not regulate that pollutant. Also, note that local limits based on the general prohibitions
(e.g., corrosion, flammability, etc.) would still need to be applied to categorical industries.
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9.8 POTW OPERATIONS
Q: Our POTW consists of multiple treatment plants. Wastewater flow and sludges can be diverted
between them. How does this affect local limits evaluation and development?
A: To ensure that all treatment plants are protected from pass through, interference, and sludge
degradation, each treatment plant should calculate allowable headworks loadings. The MAHL
can then be selected from the most stringent AHL. This practice will effectively impose a safety
factor on all of the treatment plants in the POTW and avoid any disruption of the plant treatment
process or violation of the POTW's NPDES discharge permit.
Q: Is expansion of my POTW's service area cause for me to re-evaluate local limits?
A: EPA recommends that a POTW evaluate the characteristics of its "new " service area to
determine how the POTW's current local limits requirements would be affected. Although not an
absolute requirement (due to presumed safety factors built into a POTW's local limits
determination), it is always prudent to re-evaluate the local limits calculations if the expansion
will add a number ofSIUs to the POTW's collection system. The decision about what triggers
the need for a re-evaluation is left to the POTW. However, as has been previously noted, EPA
recommends that local limits be re-evaluated periodically whenever there are significant
changes in the mix oflUs or in the total daily flow through the system (see Exhibit 7-2).
Q: How do contract operations or privatization affect local limits evaluations and development?
A: A POTW's type of management should have no impact on the evaluation and development of
local limits. Local limits are designed to protect the POTW from pass through, interference, or
degradation of sewage sludge. As long as the public has some fiduciary interest in the POTW
the need for local limits should be assessed on a routine basis. If the POTW is sold to a private
entity, then the 403 regulations regarding local limits would no longer apply upon reissuance of
the permit. The new owner of the treatment plant is not required to develop or implement local
limits unless it is made a management practice requirement in its new NPDES permit.
Q: Is it possible to develop local limits for a wastewater treatment lagoon where sludge is dredged
only every 20 years?
A: The POTW can always develop local limits based on water quality. A lagoon system would not
be significantly different than any other type of system in that respect. For sludge, the POTW
should ensure that the sludge, when dredged, will meet the standards for its chosen sludge
disposal option by establishing local limits protective of that option.
9.9 INDUSTRIAL USERS
Q: If a new significant industrial user/categorical industrial user (SIU/CIU) commences its process
discharge, or if an existing SIU/CIU ceases its process discharge, is a local limits re-evaluation
necessary?
A: It depends. If the SIU/CIU contributes a "significant percentage " (as determined by the POTW
based on total design flow or number oflUs contributing a particular POC) of the total loading
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for a particular pollutant or pollutants, then EPA recommends that the POTW recalculate the
local limits. However, if the SlU/CIUin question does not have the capability of adversely
affecting the entire POTW, then (depending upon the allocation method, SUO language, or
applicable categorical standards) the local limits can be specified in the lU's discharge permit.
Q: If I have CIUs with specific, numeric categorical pretreatment standards, is it necessary for me to
apply local limits to these CIUs for these pollutants?
A: No, it is not necessary unless the numeric categorical standards for a specific POC covered by
local limits are less stringent than the values specified in the local limits. In this case, the more
stringent local limits must prevail (see Section 1.5).
Q: Does promulgation of new categorical pretreatment standards affect local limits evaluation?
A: The promulgation of a new categorical standard should have no effect on local limits
requirements. All industrial users subject to the categorical standard(s) will have to meet that
discharge standard. However, if the categorical standard for a particular POC is less stringent
than the local limit set for that pollutant, the more stringent local limit must be met by the lUs
subject to the categorical pretreatment standard. In addition, if the new categorical standard is
more stringent than the local limit, the "freed up " loading could be reallocated to the other lUs.
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