A POTW with adequate legal authority, a sound database, and adequate staffing and funding can proceed to develop effluent limitations for each industrial plant. At a minimum, all facilities are required to comply with federal prohibited discharge standards. The industries subject to federal categorical standards also must comply with the applicable categorical standards.

The POTW may also establish local limits more stringent than, or in addition to, the federal standards for some or all of its industrial users. To identify the need for and the nature of such limits, the POTW determines whether any public health or environmental problems related to the POTW will exist, even with full enforcement of the federal standards. This assessment addresses the following issues:

• Interference - Even with full implementation of federal standards, will the remaining pollutant loading interfere with the sewage treatment system? To answer this question, the POTW must analyze its treatment system's susceptibility to various problems and its history of breakdowns.
  
  
• Biosolids Contamination - Will any of the pollutants contaminate the biosolids resulting from the municipal treatment process? To answer this question, the POTW must determine the concentration of contaminants in its biosolids after full enforcement of federal standards and analyze the environmental residuals associated with each possible disposal method.
  
  
• NPDES Permit Violations - Will the pass-though of any pollutants cause an NPDES permit violation? To answer this question, the POTW must determine whether any of the pollutants that remain in the system after full enforcement of federal standards will pass through the treatment plant in quantities significant enough to cause a permit violation.
  
  
• Surface Water Impacts - Will any of the pollutants that pass through the treatment plant adversely affect the receiving water body? To answer this question, the POTW must determine whether any of the pollutants that remain in the system after full enforcement of federal standards will pass through the treatment plant in quantities significant enough to cause a permit violation. The POTW must examine the environmental condition of the receiving water body and determine whether the pass-though of any pollutants might have a substantial impact.
  
  
• Worker Safety - Will any of the pollutants create a safety hazard for municipal employees? To answer this question, the POTW must review the design and operation of its wastewater collection and treatment system and the chemical composition of its pollutant inflow to determine whether any of the pollutants individually, or in combination, will create a worker hazard.

If the answer to any of the above questions is "yes," the POTW will have to establish local limits to be incorporated into the discharge permits of some or all of the industries that it serves. To determine these limits, the POTW must establish the maximum concentration of each pollutant in the incoming wastewater that will not cause any of these problems. It can then calculate the maximum pollutant loading of each user that can be allowed without exceeding the maximum concentration of pollutants arriving at the treatment plant. These calculations must consider such factors as the level of pollutants already present in the water supply, the chemical decomposition of pollutants within the sewage system, and the need to accommodate future industrial growth. Based on these calculations, local limits for each pollutant are established for each industrial facility.