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NPDES Permitting

NPDES Permit Program Load Allocation

  1. Introduction
  2. Effluent Limited Stream Segments
  3. Water Quality Limited Stream Segments:
  4. Reserving Stream Assimilative Capacity

I. Introduction

The Bureau of Water (Bureau) determines the total maximum point source load in pounds of ultimate oxygen demand that can be discharged to a surface water body segment without causing a water quality violation. A computerized mathematical model of a receiving stream is normally used in this process. For each discharger to a surface water body, the Bureau starts with the maximum loading allowed by the appropriate technology based limitations (secondary effluent limits for domestic discharges and effluent limitation guidelines or best professional judgment for industrial discharges) as the initial inputs into the model. The model is then run to determine whether or not discharges can take place at technology based allowed loadings without causing a predicted water quality violation.

II. Effluent Limited Stream Segments

If the model shows that each discharger can discharge its technology based allowed loadings without causing water quality violations and the antidegradation rules of SC Regulation 61-68, Water Classifications and Standards, are properly addressed, then each discharger is issued a permit with technology based effluent limitations. This situation (whether there are one or more dischargers) is commonly called an effluent limited situation or technology based situation.

III. Water Quality Limited Stream Segments

Overview. When the model predicts that the technology based discharge effluent limitations will cause water quality violations, more stringent effluent limits must be established. This means that the water quality standards will determine the allowed loading to the stream and not the technology based effluent limits. This situation is commonly called a water quality limited situation or water quality based effluent limitation situation. When this situation occurs, loading must be established so that the model predicts water quality standards are met. Where multiple dischargers are involved, this process can be very complicated as there are a number of factors that must be considered when dividing the assimilative capacity among several dischargers. In the Designated 208 Planning Areas of the State, the appropriate Council of Government is responsible for establishing a process for how this will be accomplished. In the Non-Designated 208 Areas of the State, the Bureau of Water is responsible for this process. The antidegradation rules of the water quality standards regulation must also be properly addressed in water quality limited situations.

Basic Principles. In making decisions on permit reductions that may be required because of a new discharger, an expansion of an existing discharger, water quality violations, or as a result of the use of a new or updated water quality model, the Bureau follows the basic principles below:

  1. The process should be reasonable and fair to all parties;
  2. Preferably existing and proposed dischargers should cooperatively determine how the loading to the stream will be reallocated in these situations. Existing and new or expanding dischargers will be given the opportunity to determine how the maximum allowed loading will be allocated among themselves. When the dischargers cannot agree, the Bureau must make the final decisions on the allocation;
  3. The Bureau will normally reduce affected permits by the same percentage from their technology based limitations;
  4. Once a loading has been divided between two or more dischargers, it is expected that most expansions will not require further division of the allowable loading. Increased flow will be allowed while holding the poundage constant, thus requiring better treatment on the part of the expanding discharger. In instances where an expansion is of such magnitude that a serious inequity in treatment costs would result or the expanding discharger would be required to treat beyond the limits of technology, reallocation would be considered.
  5. With respect to new dischargers, whenever possible, reductions in an existing discharger's permitted loadings will be made so that adverse impact to existing dischargers will be minimized. For example, any existing permits with excess capacity will normally be reduced first so that whenever possible no actual costs are incurred by an existing discharger;
  6. New or expanded dischargers will normally have reductions in their allowed technology based loadings at least equal to the largest percent reduction of any existing discharger;
  7. Reductions in permitted loadings will be limited to the limits of treatment technology;
  8. A permit for a new or expanded discharger will not be issued until after all the existing dischargers' permits that must be reduced to allow for the new or expanded discharger are issued and effective (no appeals pending); and
  9. No permit issued by the Department shall be interpreted as creating any vested right in any person.
The sections below explain the process the Bureau uses to accomplish the reduction in the total permitted loading to a stream when necessary. The process cannot possibly cover every situation that can occur. For situations not covered in the following, the Bureau will review them on a case-by-case basis using the basic principles listed above.

Single New or Expanding Discharger. The simplest water quality limited situation is when there is only one discharger in a stream segment. In these situations, the one discharger's permitted loading must be equal to or less than the total maximum point source load that the model indicates can be discharged without causing a water quality violation. Obviously, in this situation, there is no allocation process needed for the loading since there is only one discharger whose loading must established so as to maintain water quality standards.

One or More Existing Dischargers With Either a Proposed New or Expanded Discharger: The Allocation Process. With more than one discharger in a stream segment, the situation is much more complicated since there must be an allocation of the allowed stream loading among the existing dischargers and the proposed discharger. Depending on how this is accomplished, the proposed discharger and some or all of the existing dischargers will not be allowed to discharge the maximum loading allowed by their appropriate technology based limitations. Normally any existing discharger should not receive a larger percent reduction from its technology based limitations than a proposed discharger would receive.

1. Bureau Issues

In the allocation process, the Bureau's primary concern is that water quality standards must be maintained by ensuring that the total loading to the stream from all existing and proposed dischargers does not cause water quality violations as predicted by the model. Whether this is accomplished by one discharger or more than one discharger lowering their maximum allowed technology based loadings does not really matter to the Bureau provided the water quality model predicts water quality standards will be maintained. What does matter to the Bureau is that the total loading to the stream is somehow established so that water quality standards are maintained under the critical conditions evaluated.

2. Selecting Dischargers for Reductions and the Water Quality Evaluation Process

If dischargers cannot agree how the maximum allowed loading will be allocated among themselves, a simple way for the Bureau to reallocate the allowed stream loading would be to lower each discharger's allowed technology based loadings by the same percentage until the model predicts no water quality violations. However, this may require some dischargers to spend money unnecessarily as other dischargers may be able to take the reduction without any adverse impact to their operation. The Bureau must evaluate the situation to determine how the permitted loadings should be allocated in a manner that is reasonable and cost-effective.

In the allocation process among dischargers, the Bureau determines if any of the existing dischargers can reduce their present permit limitations without adversely affecting their operations. If all existing dischargers have technology based limits and none of them can take a reduction without adversely affecting their operations, the Bureau will reduce all existing and proposed dischargers by the same percentage until the model predicts no water quality violations.

If the Bureau determines one or more dischargers can take a reduction without adverse impact on their operations then those dischargers are reduced first. For example, if an existing discharger is not discharging the maximum loading as given in its permit and the discharger's facility is operating at its maximum production or wastewater flow rate, then the discharger has excess capacity that may be available in the reallocation process. Further, if any existing discharger can take a reduction without incurring any substantial costs associated with meeting the reduced permit limit, this discharger is also a candidate for a reduction. The Bureau makes these evaluations by comparing a discharger's existing effluent data (DMR & CMR data) to its permit effluent limitations. Also, the Bureau may make a judgment on whether a discharger can reduce its actual loading by improving operation and maintenance at its wastewater treatment plant or by using in-plant controls that are not costly. If these reductions are sufficient to allow the proposed new or expanded discharger at the same percent reduction, the selected permits are modified. When the "selected" permits are modified with no appeals pending, the new or expanded discharger will be permitted.

If the reductions for the selected dischargers are not sufficient to allow the new or expanded discharger and in situations where the Bureau is not able to determine if there are any facilities that can take a reduction without adversely affecting their operations, the Bureau will determine if any dischargers have permitted loadings that are already reduced below their technology based limitations. If there are dischargers with permit limits that are already lower than their maximum allowed technology based limits and there is at least one discharger with no reductions below its technology based limitations, the Bureau will normally reduce the permit limitations of each discharger that has technology based effluent limitations by the same percentage and reevaluate the situation using the water quality model. This percent reduction evaluation is then repeated using only the selected dischargers until either the model predicts water quality standards will be maintained or the percent reduction used for determining the loading inputs to the model equals the percent reduction for a discharger with a permit which already has a reduction below technology based limits, whichever occurs first.

If the water quality model predicts water quality standards will be maintained before the percent reduction equals the existing percent reduction that other dischargers have, the selected dischargers will have their permit loadings reduced by permit modification. If the model, using the loadings from the proposed percent reduction evaluation, shows that water quality standards are not being maintained and the percent reduction used in the evaluation reaches an existing non-selected discharger's actual percent reduction, that discharger is added to the selected dischargers that must further reduce their loadings and the model is run again. This process is repeated until the water quality model predicts water quality standards will be maintained.

In this reduction evaluation process, no discharger's permit loadings will be reduced below their limits of treatment technology (can't treat any better regardless of what they do) as determined by the Bureau. When a discharger's proposed permit reductions reach their limits of treatment technology, their permit loadings are not reduced any further in the reduction evaluation process. When the evaluation is completed, the Bureau modifies the "selected" permits. When the "selected" permits are modified with no appeals pending, the new or expanded permit will be issued.

3. Limits of Treatment Technology

In situations where one or more existing dischargers are at their limits of treatment technology, the Bureau will reduce the permitted loadings of the dischargers that are not at the limits of treatment technology by the same percentage until the model predicts no water quality violations will occur. The new or expanded discharger will have the same percent reduction from their technology based limits. In this evaluation when an existing discharger reaches their limits of treatment technology, their loading will not be reduced any further and the modeling will be repeated using further reductions for the dischargers that have not reached their limits of treatment technology. If the evaluation reaches a point where all existing and proposed dischargers' loadings have been reduced to their limits of treatment technology and the model still predicts water quality violations will occur, the new or expanded discharger cannot be allowed as proposed. The new or expanded discharger may be allowed on a smaller scale than was originally proposed such that the total loading from all dischargers will meet water quality standards.

In situations where the Bureau determines that all existing dischargers are already reduced to their limits of treatment technology, the existing total loading to the stream cannot be reduced through better treatment. In this situation, the new or expanded discharger cannot be permitted to surface waters unless reductions are made in other ways. The Bureau will normally encourage existing dischargers to reduce their loadings by other means such as source reduction, recycling, land application of effluent, water conservation, alternate manufacturing processes, consolidation of facilities through regional planning, etc. In situations where the Bureau determines that the existing loading exceeds the allowed stream loading, the Bureau may require the actual loading to the stream to be reduced by the existing dischargers utilizing the above methods even when there is not a proposed new or expanding discharger.

4. Actual Water Quality Violations Exist

a. TMDL Is Required.

For water bodies that are impaired (not meeting one or more water quality standards), the Bureau must develop a Total Maximum Daily Load (TMDL) for each pollutant that causes the impairment. The TMDL must:

  • include the load allocation (nonpoint sources)
  • include the wasteload allocation (point sources)
  • include a margin of safety;
  • address seasonal variations; and
  • have an allowance for any foreseeable reasonable growth.

In the development of the TMDL, the total allowed loading is divided among the point source dischargers (Wasteload Allocation) and the nonpoint source discharges (Load Allocation). Once established and approved by EPA, the TMDL is implemented.

To implement the TMDL, the NPDES permits for the point source discharges to the impaired water body are either modified or reisssued, as appropriate, to comply with the wasteload allocation (WLA) portion of the TMDL. Each discharger's permitted loading of the pollutant(s) that caused the impairment will be modified to comply with the TMDL. Actual loading may be used as interim permit limits. These permits will also contain schedules of compliance, as appropriate, and new final effluent limits based on the TMDL for the pollutants of concern. Also, the load allocation portion of the TMDL is divided between the nonpoint sources for implementation. After implementation of the TMDL, the water quality of the water body should meet standards. This allocation process will take place even if there is not a proposed new or expanding discharger.

b. New or expanded Discharge Is Proposed Before the TMDL Is Established.

When a TMDL has not been established for an impaired waterbody, a new or expanded discharge that contains the pollutant(s) which caused the impairment will not be allowed unless there is a plan in place that is acceptable to the Bureau that addresses no net increase in the loading of the pollutant into the impaired water body from all dischargers. In this situation, the appropriate existing discharges to the impaired water body will have their permits modified to include the limits necessary to insure no net increase in loading of the pollutant of concern. These permits will also contain a reopener clause that says the permit may be modified to the comply with the TMDL when it is established. Further, these permits must include appropriate schedules of compliance for the work that must be completed to meet the reduced loadings. Upon establishment of the TMDL, the permits will be modified, as necessary, to comply with the TMDL.

c. New or expanded Discharge Is Proposed After the TMDL Is Established

If there is a proposed new or expanding discharger, the portion of the WLA reserved for reasonable foreseeable growth (if it exists) will be used and/or the wasteload allocation portion of the TMDL must be reallocated among the existing and proposed discharges. The margin of safety must be maintained in this process. Therefore, in this situation, new final effluent limits for each discharger may be established depending on whether or not the reserved WLA by itself is sufficient to allow the new point source load. Any reduced limits will be based upon the percent reduction necessary to allow the new or expanded discharge at the same percent reduction as the existing discharges such that the Wasteload Allocation portion of the TMDL is not exceeded.

All affected permits will be modified to include their allocated portion of the Waste Load Allocation portion ofthe TMDL. These new final TMDL limits will go in effect when the new or expanding discharge occurs or on a date that the Bureau determines to be appropriate based on the situation.

5. New or Expanded Permit Issuance

When permit modifications are necessary to reallocate the maximum allowed stream loading so that the new or expanded discharger can be permitted, the new or expanded permit is not issued until the necessary permit modifications have been made with no outstanding appeals. Any reductions to an existing discharger's permit limitations will go into effect when the new or expanded discharge actually occurs.

IV. Reserving Stream Assimilative Capacity When TMDLs Are Not Applicable

The Bureau is asked on numerous occasions why it does not reserve assimilative capacity in streams for use by new or expanding dischargers. The argument is made that by reserving assimilative capacity the permitting of new dischargers and expansions would be easier and reallocation with permit modifications would not be necessary. This may be true up to a point. However, the Bureau does not think reserving stream assimilative capacity is an appropriate practice. This practice can lead to situations where existing dischargers are required to meet more stringent effluent limitations than necessary to meet water quality standards to allow for future development that may or may not ever occur. The exception to this is the Total Maximum Daily Load (TMDL) Program where there must be reserve capacity for any foreseeable reasonable growth. In other situations, if assimilative capacity is reserved, existing dischargers may have higher capital and operation and maintenance costs for their wastewater treatment facilities to allow for future development. Further, if future development which uses the reserve capacity does occur, the Department must still reallocate the allowed stream loading among all the existing dischargers and the proposed discharger so that reserve capacity is always provided.

While reserving capacity will allow new or expanding dischargers to be permitted before existing dischargers' permits are reduced, it does not eliminate the reallocation process and it can result in unnecessary costs for existing dischargers. Therefore, the Bureau does not reserve assimilative capacity for future development except as part of a TMDL.


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