Local Limits Guidance
Pretreatment Related Links
Table of Contents
- DATA COLLECTION FOR POLLUTANTS OF CONCERN
- IDENTIFICATION OF APPLICABLE CRITERIA
- DEVELOPING MAXIMUM AVAILABLE HEADWORKS LOADINGS
- ALLOCATING POLLUTANT LOADINGS TO INDUSTRY
- SUBMITTING TECHNICAL JUSTIFICATION FOR LOCAL LIMITS
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY,
REGION 5 PRETREATMENT PROGRAM:
USEPA Region 5 Local Limits Guidance
77 WEST JACKSON BOULEVARD
CHICAGO, ILLINOIS 60604
RETREATMENT PROGRAM: LOCAL LIMITS GUIDANCE
Publicly-owned treatment works (POTWs) may need to develop new or revised local industrial user limitations intended to prevent pass through and interference. The purpose of this guidance is to assist POTWs in establishing a sound technical basis for local industrial user limitations and preparing justification for these limits for submittal to U.S. EPA for review and approval.
For further information refer to U.S. EPA's Guidance Manual on the Development and Implementation of Local Discharge Limitations Under the Pretreatment Program (December 1987) and U.S. EPA's Supplemental Manual on the Development and Implementation of Local Discharge Limitations Under the Pretreatment Program (May 1991).
This document is only additional guidance and not a replacement for the aforementioned U.S. EPA guidance manuals for local limits development. It consists predominately of Ohio EPA's Pretreatment Program: Local Limits Guidance (March 1991). Portions consist of excerpts from U.S. EPA, Region 8 local limits guidance, Michigan DNR local limits review checklist and U.S. EPA's 1987 Local Limits Guidance.
U.S. EPA, Region 5, which is presently the Approval Authority for pretreatment programs in Illinois and Indiana, must review and approve the technical justification for local limits before limits are incorporated into the sewer use ordinance or industrial user control documents. Technical justification for local limits should be submitted to:
U.S. Environmental Protection Agency
77 West Jackson Boulevard
Chicago, Illinois 60604-3590
Any questions regarding preparation or submittal of technical justification for local limits should be directed to U.S. EPA, Permits Section at (312) 886-6089.
1.1 POTWS REQUIRED TO DEVELOP APPROVED PRETREATMENT PROGRAMS
The General Pretreatment Regulations (40 CFR 403) require that each publicly owned treatment works (POTW), required to develop an approved pretreatment program, must develop and enforce local limits to protect against pass through and interference which may be caused by industrial discharges to the treatment facilities. Limits are developed originally by POTWs as a prerequisite to pretreatment program approval. However, a large percentage of the local limits developed in the early days of the pretreatment program were developed prior to publication of U.S. EPA's 1987 Local Limits Guidance Manual, and thus may not have been developed consistent with the procedures discussed in that guidance. In addition, conditions at POTWs change with time. Hence, each POTW will be required to reevaluate its local limits at least every five years to ensure a firm technical basis and address changing conditions.
1.2 POTWS WITHOUT APPROVED PRETREATMENT PROGRAMS
Some POTWs not having approved pretreatment programs may be required to develop local limits. 40 CFR 403.5(c)(2) gives U.S. EPA the authority to require any POTW to develop technically based local limits where the POTW receives pollutants contributed by industrial users which may cause or have caused pass through or interference at the POTW. In addition, some POTWs not required to develop local limits may wish to do so to assure compliance with their NPDES permit and the sludge disposal requirements.
1.3 COMPARISON OF LOCAL LIMITS TO CATEGORICAL STANDARDS
Federal categorical pretreatment standards, which are applicable to certain classes of industries, establish technology based minimum pretreatment standards. However, the categorical standards do not address POTW specific problems which may arise from discharges by categorically regulated industries. In addition, many types of industries which discharge significant quantities of pollutants are not regulated by the categorical standards. Hence, there is a need for many POTWs to establish site specific discharge limits in order to protect the treatment facilities, receiving water quality, worker health and safety, and to allow for beneficial use of sludge.
1.4 POLLUTANTS TO BE LIMITED
Each POTW with an approved pretreatment program must develop local limits for arsenic, cadmium, chromium, copper, cyanide, lead, mercury, nickel, silver and zinc or demonstrate that limits are not necessary for these pollutants. The POTW must also identify all other pollutants of concern and evaluate the need for limits for these pollutants. A pollutant of concern is defined as any pollutant limited in the POTW's NPDES permit or found in the collection system in sufficient quantity to have a reasonable potential to cause pass through or interference at the treatment plant, pose a threat to worker health and safety, or to cause other problems within the collection system or at the treatment plant, such as explosions or obstruction of wastewater flow. The priority pollutant scans performed periodically by POTWs with approved pretreatment programs are useful in identifying pollutants of concern. POTWs with multiple plants may wish to develop local limits for each plant or after calculating the limits for each plant choose the most stringent as uniform local limits across all plants.
Local limits are most often associated with the control of toxic pollutants. However, if a POTW has experienced violations of their NPDES permit effluent limits for conventional pollutants (such as BOD, TSS, ammonia, phosphorus, or oil and grease), or has experienced operational problems due to these pollutants, the POTW should investigate the need for local limits for conventional pollutants as well.
Many POTWs have surcharge programs for conventional pollutants. A surcharge program should not be confused with local limits for these pollutants. In addition to a surcharge program, which is generally triggered by specific pollutant concentrations, a POTW should set absolute upper limits for conventional pollutants in its sewer use ordinance (SUO) or industrial user (IU) permits, based on total plant capacity.
1.5 MAINTAINING AN ADEQUATE DATABASE
Since local limits must be revised periodically, an ongoing monitoring program is recommended to support review and revision of local limits and identification of pollutants of concern. An effective ongoing monitoring program should include periodic sampling of influent, effluent, sludge, domestic/commercial wastewaters, and industrial wastewater discharges.
1.6 PRELIM AND ROCK RIVER SUBMISSION
Many POTWs have found the PRELIM software to be useful in local limits development. PRELIM (which stands for pretreatment limits) is PC compatible and was developed by Science Applications International Corporation for U.S. EPA. Copies are available from U.S. EPA and State agencies.
The Rock River Water Reclamation District (RRWRD) designed a spreadsheet program using Lotus to calculate their local limits. U.S. EPA, Region 5 found this form of submission easy to use and understand so we borrowed RRWRD's program, modified it somewhat and made it ready for other POTWs to use. Copies of the spreadsheet may be downloaded from this web site.
If a POTW chooses to use the PRELIM or RRWRD program for local limits development, supporting documentation must be submitted along with the PRELIM and RRWRD reports. This will enhance our review.
1.7 STEPS IN LOCAL LIMITS DEVELOPMENT
In order to establish or revise local limits, the following steps must be performed:
- Data collection for pollutants of concern
- Identification of applicable criteria
- Development of maximum allowable headworks loadings
- Allocation of available pollutant loadings to industry
- Submittal of technical justification to U.S. EPA
Each of these steps of the local limits development process are described in the following pages.
2.0 DATA COLLECTION FOR POLLUTANTS OF CONCERN
The first step in the limits development process is to collect
and review monitoring data to be used in identifying pollutants
of concern and calculating the local limits. The POTW must determine
representative values for the following elements which influence
maximum allowable headworks loadings:
- Pollutant removal efficiencies
- Domestic/commercial pollutant contributions
- Industrial pollutant contributions
- Total plant flow
Conditions at each POTW are unique, therefore site specific sampling information is preferable to literature data. Conditions at the POTW also vary with time, therefore, the most recent representative sampling information available must be used. One year of sampling data is preferable for all the elements listed above, but where time is limiting, we suggest that the initial monitoring program for removal efficiencies and domestic/commercial contributions should include at least five consecutive days of sampling for both metals and toxic organics to adequately characterize the wastewater in a minimal time frame. Suggested guidelines for ongoing monitoring are for at least one day of sampling per month for metals and other inorganics, and one day of sampling per year for toxic organics, to asses long-term variations in wastewater composition. Sampling and analyses must be conducted using the methods of 40 CFR Part 136. Where it is not feasible for the POTW to obtain actual sampling information, substituting literature values for site specific sampling may be acceptable.
2.1 POLLUTANT REMOVAL EFFICIENCIES
Representative removal efficiencies must be determined for each pollutant of concern. Removal efficiencies should be determined across the entire POTW and for the individual primary and secondary treatment operations. Hydraulic retention time should be accounted for during sampling where feasible. Influent samples should be obtained upstream from recirculating flows.
For additional information on calculating removal efficiencies please refer to the U.S. EPA guidance manuals discussed in the introduction. These guidance manuals discuss the three ways to calculate removal efficiencies (the decile approach, mean removal efficiency, and average daily removal efficiency) as well as how to handle common problems encountered in calculations such as pollutant levels below analytical detection and negative removal efficiencies. Also, there is guidance from Region 3 that includes a discussion on how to handle monitoring results that are below analytical detection. The Region 3 guidance is available on this web site.
If removal efficiencies vary greatly from sampling to sampling, the decile approach may be preferred and negative removal rates excluded. Note, that the decile approach requires a minimum of 10 data points. If removal efficiencies are fairly consistent from sampling to sampling, either the mean or daily average removal efficiency approaches may be preferred and negative removal rates included. Of these two, the mean removal efficiency is less sensitive to extreme removal efficiencies.
There is also literature data available for removal efficiencies. Literature removal efficiency data should only be used when site-specific removal efficiencies obtained from POTW in-plant monitoring programs cannot be obtained. However, site-specific data is always preferable to the literature values.
2.2 DOMESTIC/COMMERCIAL CONTRIBUTIONS
Sampling must also be performed to determine pollutants contributed by domestic and other uncontrollable sources (also referred to as domestic/commercial or background contributions.) The sampling locations and number of samples taken should ensure that the data is representative of domestic and uncontrollable sources in the POTWs system. We recommend sampling at a minimum of three separate locations within the service area where only domestic or uncontrollable wastewater flow is present. Since infiltration and inflow are components of background wastewaters, the POTW may wish to schedule background sampling to account for the influence of infiltration and inflow. Literature data is also available for the domestic/commercial contribution.
2.3 INDUSTRIAL POLLUTANT CONTRIBUTIONS
At a minimum, a value for total industrial wastewater flow must be determined. In addition, POTWs which do not use the uniform concentration limit method for allocating available industrial pollutant loadings must establish representative flows and pollutant contributions for each industrial user.
Most often local limits are developed for a POTW's industrial users. However, commercial users such as dry cleaners, service stations, photo processors, hospitals and dental offices can be significant sources of pollutants. A POTW may wish to calculate local limits for certain commercial users as well, especially when domestic/commercial pollutant loadings use up nearly all of the allowable loading. If commercial users are to be regulated, please discuss the control mechanism to be used (e.g. permits, ordinance requirements) and how compliance will be monitored.
2.4 TOTAL POTW TREATMENT PLANT FLOW
A representative value for total plant flow is another important parameter in the local limits development calculation. Actual flows should always be considered rather than plant design flow.
3.0 IDENTIFICATION OF APPLICABLE CRITERIA
The second step in the limits development process is to identify applicable criteria in order to identify which is the limiting criteria. All applicable criteria must be examined to identify which is the limiting criteria. Applicable criteria may include:
- NPDES permit limits
- Biological process inhibition
- Sludge disposal
- Water quality standards
- Collection system problems
3.1 NPDES PERMIT LIMITS
The NPDES permit for each treatment plant may contain effluent limitations for some or all of the pollutants for which local limits are to be established. NPDES permits may contain daily maximum limits only, or both daily maximum and monthly average limits. Where the NPDES permit contains both daily maximum and monthly average limits, the POTW has the option of deriving one set of local limits based on the monthly average NPDES permit limits, or developing both daily maximum and monthly average local limits.
3.2 BIOLOGICAL PROCESS INHIBITION
Pollutants may cause toxicity of biological processes, such as activated sludge, trickling filters, nitrification, or sludge digestion. The POTW must evaluate the potential for pollutants to disrupt biological processes when local limits are developed. Unless site specific information on biological process inhibition is available, we recommend the minimum reported inhibition threshold in the literature be used for process inhibition values in the local limits calculations. This means that most often, POTWs must consult the literature for biological process inhibition values. In some cases, biological processes at the POTW may be exposed to concentrations of pollutants in excess of the minimum reported threshold in the literature without any disruption in plant performance. In these cases, it may be acceptable for the POTW to substitute pollutant concentrations which have occurred in the applicable biological process without causing any disruption in plant performance for the process inhibition value.
3.3 SLUDGE DISPOSAL
The POTW must consider establishing local limits to prevent any restriction of the POTW's sludge disposal options. A discussion on developing local limits using the land application regulations follows. For a discussion on toxicity based limitations for sludge disposal or incinerator emission requirements please see U.S. EPA's 1987 Local Limits Guidance Manual, section 3.2.2. Please note that the EP toxicity test, discussed in section 3.2.2, has been replaced with the TCLP test.
If a POTW land applies their sludge they should evaluate all applicable regulations including 40 CFR Part 503 regulations and any State sludge regulations that might be more stringent for the development of local limits. The land application criteria in 40 CFR Part 503 is contained in four tables which are based on cumulative loading rate limits, annual loading rate limits, ceiling concentration limits and high quality limits. The applicability of the limits in each table depends on the facilities sludge disposal practices. Table 1 sludge criteria are a minimum set of limits that all land applied sludge must meet. Table 3 criteria are considered high quality criteria and if they are met then no other criteria need be considered. If sludge is sold in bags or containers it must meet Table 1 and Table 4 criteria. If sludge is applied in bulk to agricultural land, forest, a public contact site, or a reclamation site then it must meet Table 1 and Table 2 criteria. If sludge is applied in bulk to home gardens or lawns then it must meet Table 3 criteria.
The sludge criteria in Table 2 and Table 4 are not in the units of mg/kg as required by the formula to calculate maximum allowable headworks loadings. They must be converted using the cumulative application rate or annual application rate formulas which require the site life and/or the site area of the land application site.
3.4 WATER QUALITY STANDARDS
Where there are no NPDES limits for a pollutant or pollutants for which local limits are being developed, the POTW should use State water quality standards as a basis for local limits. Illinois' water quality standards are at 35 Ill. Ad. Code 302.208 and Indiana's water quality standards are at 327 IAC 2-1-6. Other State water quality standards may be found in their respective State Administrative codes.
3.5 COLLECTION SYSTEM PROBLEMS
The POTW may wish to consider potential problems in the collection system as a basis for establishing local limits. Collection system problems may include fire or explosion hazards, corrosion, flow obstruction, worker health and safety, or other site specific issues.
Any pollutant concentration in the wastewater treatment plant collection system which exceeds the screening levels listed in Table B-1 of the Guidance to Protect POTW Workers from Toxic and Reactive Gases and Vapors (June 1992) has the potential to cause adverse effects on worker health and safety. Therefore, pollutant concentrations in the collection system that are near or above the screening levels should be controlled. One simple approach to control pollutants of concern based on worker health and safety is to adopt the screening levels, identified in Table B-1 of the EPA guidance document identified above, as local limits. Table B-1 is included in the appendix.
All areas of the collection system that are suspected of receiving discharges of organic pollutants that may approach the screening levels should be evaluated on the basis of protecting worker health and safety. If there are no suspected areas in the collection system then the pollutant concentration in the total influent may be compared to the screening criteria.
4.0 DEVELOPING MAXIMUM ALLOWABLE HEADWORKS LOADINGS
The third step in the limits development process is to calculate maximum allowable headworks loadings based on each limiting criterion. Maximum allowable headworks loadings based on all applicable criteria for each pollutant are then compared and the lowest allowable loading values are selected. Next, a safety factor must be applied to the entire maximum allowable headworks loadings and then the domestic/commercial contributions subtracted to establish the pollutant loadings available for industry. Equations for each of the limiting criteria are presented below.
4.1 NPDES PERMIT LIMITS
Lhw = 8.34 * Ccrit * Qpotw 1 - Rpotw Where: Lhw = Maximum allowable headworks loading (lbs/d) Ccrit = NPDES permit limit (mg/l) Qpotw = POTW flow (mgd) Rpotw = Removal efficiency across POTW, as a decimal 8.34 = Unit conversion factor
4.2 BIOLOGICAL PROCESS INHIBITION
For Secondary Treatment (e.g. activated sludge): Lhw = 8.34 * Ccrit * Qpotw 1 - Rprim For Tertiary Treatment (e.g. nitrification): Lhw = 8.34 * Ccrit * Qpotw 1-Rsec Where: Lhw = Maximum allowable headworks loading (lbs/d) Ccrit = Threshold inhibition concentration (mg/l) Qpotw = POTW flow (mgd) Rprim = Removal efficiency across primary treatment, as a decimal Rsec = Removal efficiency across primary treatment and secondary treatment, as a decimal 8.34 = Unit conversion factor For Anaerobic Digesters - Conservative Pollutants only (Metals): Lhw = 8.34 * Ccrit * Qdig Rpotw For Anaerobic Digester - Non-Conservative Pollutants (Organics/Cyanide): Lhw = Linf * Ccrit Cdig Where: Lhw = Maximum allowable headworks loading (lbs/d) Ccrit = Threshold inhibition level (mg/l) Qdig = Sludge flow to digester (mgd) Rpotw = Removal efficiency across POTW, as a decimal Linf = POTW influent pollutant loading (lbs/d) Cdig = Pollutant level in sludge to digester (mg/l) 8.34 = Unit conversion factor
4.3 SLUDGE DISPOSAL
To calculate maximum allowable headworks loadings based on sludge disposal considerations, first all applicable sludge disposal criteria must be expressed as a unit of pollutant per unit of sludge (mg/kg dry sludge). Sludge criteria based on annual (Table 4) and cumulative (Table 2) application rate limits may be converted to mg/kg dry sludge using the following equations. Please see the tables in the appendix for a summary of the sludge criteria contained in 40 CFR Part 503 regulations.
Clim(a) = AAR * SA Qsldg * (PS/100) * 3046 Clim(c) = CAR * SA SL * Qsldg * (PS/100) * 3046 Where: Clim(a) = Sludge disposal criteria based on annual application rate limits (mg/kg dry sludge) Clim(c) = Sludge disposal criteria based on cumulative application rate limits (mg/kg dry sludge) AAR = Annual application rate limit (lbs/acre/year) CAR = Cumulative application rate limit (lbs/acre over site life) SA = Site area (acres) SL = Site life (years) Qsldg = Sludge flow to disposal (mgd) PS = Percent solids in sludge to disposal, as a percent not as a decimal 3046 = Unit conversion factor
Once all applicable sludge criteria are in units of mg/kg dry sludge then the most stringent of the applicable disposal criteria must be used to calculate the maximum allowable headworks loading. The most stringent sludge disposal criteria may then be substituted for Cslcrit in the following equation to calculate the maximum allowable headworks loadings.
For Conservative Pollutants (Metals):
Lhw = 8.34 * Cslcrit * (PS/100) * Qsldg Rpotw For Non-Conservative Pollutants (Organics/Cyanide): Lhw = Linf * Cslcrit Csldg Where: Lhw = Maximum allowable headworks loadings (lbs/d) Cslcrit = Sludge disposal criterion (mg/kg dry sludge) PS = Percent solids of sludge to disposal Qsldg = Sludge flow to disposal (mgd) Rpotw = Removal efficiency across POTW, as a decimal 8.34 = Unit conversion factor Linf = POTW influent pollutant loading (lbs/d) Csldg = Pollutant level in sludge to disposal (mg/kg dry sludge)
4.4 WATER QUALITY STANDARDS
Lhw = 8.34 * [Ccrit * (Qstr + Qpotw) - (Cstr * Qstr)] 1 - Rpotw Where: Lhw = Maximum allowable headworks loading (lbs/d) Ccrit = Water quality standard (mg/l) Qstr = Receiving stream (upstream) flow (mgd) Qpotw = POTW flow (mgd) Cstr = Receiving stream background level (mg/l) Rpotw = Removal efficiency across POTW, as a decimal 8.34 = Unit conversion factor
The POTW should calculate the maximum allowable headworks loadings based on both the chronic and acute water quality standards. When calculating the maximum allowable headworks loading based on the chronic water quality standards the receiving stream flow (Qstr) should be the 7Q10. When the calculation is based on the acute water quality standard the receiving stream flow should be the 1Q10. The most stringent loading should then be used to derive the local limits. Please note that it is not correct to develop monthly average limits based on the chronic criteria or to develop daily maximum limits based on the acute criteria. Please refer to Section 188.8.131.52 of U.S. EPA's 1987 guidance manual on local limits development for further discussion on the use of chronic and acute criteria.
4.5 OTHER CRITERIA
For a discussion on EP toxicity limitations or reduction of incinerator emissions please refer to sections 184.108.40.206 and 220.127.116.11 of U.S. EPA's Guidance Manual on the Development and Implementation of Local Discharge Limitations Under the Pretreatment Program (December 1987).
4.6 COLLECTION SYSTEM PROBLEMS
Refer to Chapter 4 of U.S. EPA's Guidance Manual on the Development and Implementation of Local Discharge Limitations Under the Pretreatment Program (December 1987).
5.0 ALLOCATING POLLUTANT LOADINGS TO INDUSTRY
Once maximum allowable headworks loadings have been calculated, the POTW may then determine the pollutant loadings available for industrial users. First a safety factor must be applied to the entire maximum allowable headworks loadings. Next, the domestic/commercial pollutant loadings are subtracted, leaving the available pollutant loadings for industry.
5.1 SAFETY FACTOR
A safety factor must be applied to the maximum allowable headworks loading to account for future industrial growth or any uncertainty in measured input values and to protect the POTW during slug loadings or periods of industrial noncompliance with local limits. A minimum 10% safety factor is required, however, a 20% safety factor is recommended in most cases. Safety factors greater than 30% are not recommended. POTWs should reevaluate local limits whenever there is a substantial change in the industrial loading to the POTW, rather than applying a large safety factor to account for changes in wastewater contributions. Use of a large safety factor does not constitute an appropriate substitute for periodic review and revision of local limits.
5.2 DOMESTIC/COMMERCIAL CONTRIBUTIONS
Domestic/commercial flow is estimated by subtracting the total industrial flow from the total plant flow (Qdom = Qpotw - Qind). The domestic/commercial loading may then be estimated for each pollutant as follows:
Ldom = Cdom * Qdom * 8.34 Where: Ldom = Domestic/commercial loading (lbs/d) Cdom = Domestic/commercial pollutant concentration (mg/l) Qdom = Domestic/commercial flow (mgd) (may vary for each pollutant if uniform concentration limits using only industrial contributory flow are employed) 8.34 = Unit conversion factor
In some cases the domestic/commercial loading may exceed the maximum allowable headworks loading. This will result in a negative allowable industrial loading. A negative limit is impractical but this situation indicates that loadings to the POTW may need to be reduced. U.S. EPA, Region 3 developed Guidance for Setting Local Limits (PDF) (25K, 10pp) for a Pollutant Where the Domestic Loading Exceeds the Maximum Allowable Headworks Loadings which describes alternative approaches that POTWs can use when negative allowable industrial loadings result.
5.3 CALCULATING AVAILABLE INDUSTRIAL LOADINGS
The pollutant loadings available for industrial users may then be calculated as follows:
Lind = Lhw * (1 - SF) - Ldom Where: Lind = Available loading for industry (lbs/d) SF = Safety factor expressed as a decimal Lhw = Maximum allowable headworks loading (lbs/d) Ldom = Domestic/commercial loading (lbs/d)
The next step is to allocate the available pollutant loadings to the industrial users. Any of the following accepted methods for allocating available industrial loads may be used by the POTW:
- Uniform concentration limits - Uniform concentration limits using only industrial contributory flows - Mass proportion allocation - Selected industrial reduction - Individual loading allocation
Uniform concentration limits and uniform concentration limits using only industrial contributory flow, the two most commonly used methods, are described below. The individual loading allocation method is also described below since it is not discussed in U.S. EPA's 1987 Local Limits Guidance Manual. If you are interested in the mass proportion allocation or the selected industrial reduction methods please see Section 3.3.3 of U.S. EPA's 1987 Local Limits Guidance Manual.
5.4 UNIFORM CONCENTRATION LIMITS
Using this method, the POTW develops industrial user limits which are applied equally to all industrial users. Its advantages are relative ease of calculation and application and that it is not necessary for the POTW to know effluent characteristics for each individual industrial user. Its disadvantages are that all industries are granted the same allocation for each pollutant regardless of the level of pollutants in each industry's discharge. This may result in relatively stringent limits when compared to other methods of allocation. The following equation is employed:
Clim = Lind 8.34 * Qind Where: Clim = Uniform concentration limit (mg/l) Lind = Available loading for industry (lbs/d) Qind = Total industrial flow (mgd)
5.5 UNIFORM CONCENTRATION LIMITS USING ONLY INDUSTRIAL CONTRIBUTORY FLOWS
Using this method, the POTW establishes a common discharge limit for all industrial users identified as having pollutant concentrations in process wastewaters at concentrations greater than domestic/commercial wastewaters. The flow from a particular industrial user is considered either as part of the domestic/commercial flow or as part of the industrial contributory flow in calculating limits for each pollutant, depending on the concentration of the pollutant present in that industrial user's raw wastewater before pretreatment. Industries not considered to have any contributory flow for a given pollutant must be restricted in individual control documents to discharging that pollutant at or below domestic/commercial levels. One advantage of this method is that a more prudent use of the available industrial loading is achieved, because the available pollutant loadings are granted only to those industries that have pollutants in the raw wastewater at levels above domestic/commercial. A disadvantage of this method is that the POTW must have detailed knowledge of each industrial user's raw wastewater characteristics. The following equation is employed:
Clim = Lind 8.34 * Qcont Where: Clim = Uniform concentration limit for contributing industries (mg/l) Lind = Available loading for industry (lbs/d) Qcont = Industrial contributory flow (mgd) (may be different for each pollutant)
5.6 INDIVIDUAL LOADING ALLOCATION
Using this method, the POTW allocates available pollutant loadings to industrial users on an individual basis. The sum of allocations for each pollutant must not exceed the total available loading for industry.
6.0 SUBMITTING TECHNICAL JUSTIFICATION FOR LOCAL LIMITS
Technical justification for new or revised local limits must be submitted to the approval authority for review and approval prior to incorporation into the sewer use ordinance and/or control documents. Technical justification for retaining existing local limits must also be submitted to the approval authority for review. The POTW must submit the following information in order to demonstrate an adequate technical basis for local limits:
- POTW treatment plant process schematic or description.
- All data used in the calculations including data such as: domestic/commercial pollutant contributions; process inhibition thresholds; POTW pollutant removal efficiencies; sludge disposal criteria including site area and site life if used in developing applicable criteria; POTW, industrial, and domestic/commercial flows; and where applicable, effluent quality measured or assumed for industrial users.
- Supporting assumptions, methodologies, and calculations used in establishing the items required in number 2 above.
- A comparison of maximum allowable headworks loadings based on all applicable limiting criteria in tabular form.
- The proposed local limits. Where industry specific local limits are proposed, include the limits for each industry or, if there are too many individual industrial limits, describe the allocation method used.
- Justification for decision to not consider a pollutant referred to on pages 3-4 of this guidance as a pollutant of concern.
- Support for deleting any local limits.
- Comparison of existing local limits to proposed if revising local limits.
- If your POTW has an approved pretreatment program and revisions to local limits are proposed, technical justification for local limits must be submitted as a pretreatment program modification request. Please refer to 40 CFR 403.18 for federal requirements on modifications of POTW pretreatment programs.