The POTWs must take a number of steps to implement the effluent limits established in their programs. First, the industrial plants must be notified of the effluent limitations that apply to them. These limitations might be based on categorical pretreatment standards, prohibited discharge standards, or local limits. The effluent limits are then incorporated in a permit issued by the POTW to the industrial facility.

POTWs must then ensure that the industrial facilities comply with the effluent limits in their pretreatment permits. They require industrial plants to submit self-monitoring reports in which they report the total volume and pollutant concentrations of their wastewater discharges. Federal regulations require that these reports be submitted semi-annually, at a minimum. The industrial facility's pretreatment permit might also require the submittal of additional information such as descriptions of any accidental discharges into the sewage system.

The POTW cannot rely exclusively on the information supplied by industry in self-monitoring reports. It must, conduct its own inspection and monitoring activities. POTWs identify locations within the industrial facility for collecting samples of wastewater for chemical analysis. Sampling locations might be at the end of the industrial process or at the point of connection to the public sewer. The effluent limits considered acceptable at each sampling location are specified in the facility's pretreatment permit.

Municipal personnel periodically visit each industrial site to collect wastewater samples at the designated sampling locations within the facility. Some of these inspections are held on a regularly scheduled basis. There are also unannounced monitoring visits. Monitoring also may occur in response to a suspected violation of a pretreatment permit, a public complaint, the suspected presence of explosive or corrosive materials, operating difficulties in the sewage treatment plant, or violation of the POTW's NPDES permit. Monitoring is generally undertaken immediately following the onset of a serious problem.

The frequency and extent of monitoring and inspection of an industry by the POTW depends on the industries' potential impact on the sewage system and the environment. In general, major industrial facilities such as those covered by categorical standards are subject to at least one monitoring visit per year; more if resources allow. The volume of wastewater discharges, the toxicity of the discharge, or the variability of monitoring results are used to determine the frequency of monitoring visits.

When an industrial plant violates its permit, the POTW takes enforcement action. Before taking this step, however, the POTW verifies the violation. In most cases, verification involves sampling and laboratory analysis of the industries' effluent to confirm that a violation has occurred.

In emergency situations, the POTW may take immediate action to halt all discharges from a facility. In less serious cases, however, the POTW may immediately inform the violator verbally of the violation, then later will do so in writing. The facility is required to retrun to compliance within a specified period of time. Monitoring of the facility's discharge is then instituted to ensure that the compliance deadlines are met.

When compliance deadlines are not met, civil and/or criminal proceedings may be initiated against the violator. In some cases, violations can be handled without litigation. However, when a facility persists in violations that endanger public health and the environment, the POTW may take strong enforcement action. It may levy fines and/or seek injunctions to force the violating facility to come into compliance.