DISPOSING OF AND MONITORING SEWAGE EFFLUENTS

The wastewater treatment process includes taking the solids out of the wastewater, getting rid of the solids, and getting rid of the treated wastewater or effluent in a way the federal and state regulating agencies approve. Sludge handling and disposal are covered in chapter 13. This chapter describes many ways to dispose of plant effluent.

All plants that discharge an effluent must have NPDES permits issued by the EPA or by a state agency for the EPA. Before these permits are given to the plant, officials make a careful survey of the water use nearby that might be hurt by the effluent of the treatment plant. The permit may list top, bottom, or average limits for some kinds of pollutions. It may also state in what way the plant can dispose of its effluent. If the plant does not meet the limits on the permit, the operator should contact the regulating agency at once. The permit can be changed or revoked by the agency. Sometimes the plant may be allowed to discharge more than the limit on the permit, but that is up to the regulating agency. The purpose of the permit is to protect human health and natural resources. All operators should know the permit limits and make every effort to ensure that the treatment plant complies with them.

EFFLUENT DISCHARGE METHODS

The two major methods of discharging effluent are continuous discharge and intermittent discharge.

Most treatment plants discharge an effluent to a receiving water all the time. The effluent may go to an ocean, gulf, bay, lake, or stream. The point of discharge may be above or below the surface of the receiving water. Continuous discharge is often cheaper than other methods because it takes less manpower, equipment, and storage to operate. However, a very good monitoring program must be used to make sure toxic waste is not discharged. After a toxic waste is discharged, there is no practical way to stop or isolate the toxic substance.

Intermittent discharge means that the effluent is not discharged all the time, but only from time to time. This type of discharge requires a place to store the effluent. It is not often used at large plants. But it does work well at lagoons and small treatment plants that have holding or "polishing" ponds.

Intermittent discharge lets the operator choose the time and rate of discharge. A controlled amount of effluent can usually be discharged without hurting the quality of the receiving water if the operator picks the right time for all discharges. In some cases, the receiving water has even been improved. Intermittent discharge may cost more to build, but it does not require as costly a monitoring program. When there is no discharge, there is no effluent to be tested.

A special type of intermittent discharge is seasonal discharge. This type of discharge is often used to protect high-quality streams, especially during the season when the stream is used a great deal for recreation. More storage is needed for seasonal discharge because there are usually only two discharges, one in spring and one in autumn. The effluent is discharged under controlled conditions approved by the regulating agencies.

METHODS OF DISPOSING AND MONITORING SEWAGE EFFLUENTS

Several methods of disposing of sewage effluents are used today. All methods must conform to the NPDES permit requirements and must be closely monitored. This section discusses these methods as well as troubleshooting problems with sewage effluent quality.

Direct Discharge to Receiving Water Most treatment plants discharge effluent right into the receiving water. The abilities of the receiving water to dilute and accept the effluent is shown in the NPDES permit limits. The NPDES permit also considers the use of the receiving water. The effluent may come from a final clarifier, a disinfection contact basin, a lagoon, a polishing pond, or a storage pond. However, it must pass through some type of outfall sewer to the point of discharge.

The outfall sewer may be an enclosed pipe or an open channel or some of both. It is used to transport the effluent from the final treatment or storage unit to the point of discharge. The outfall sewer may be built to include cascades or stairsteps, channels, mechanical aerators, or a filter bed of coarse rock. The purpose of these aerators is to increase the DO content of the effluent.