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.

The NPDES permit requires that certain tests be made on the effluent on a regular schedule. Effluent testing may include, but is not limited to, flow measurement, temperature, BOD or COD, suspended solids, pH, DO, coliform count, and chlorine residual. Test results must be reported to the regulating agency. Along with the required tests, operators should check the receiving water, especially on small streams and lakes. Laboratory tests and visual checks may show that a problem exists in the receiving water and that something needs to be done. Plant operators cannot usually test large rivers, bays, lakes, and gulfs.

If an effluent containing a toxic substance is accidentally discharged to a receiving water that is used downstream as a drinking water supply, for recreation, or for livestock watering, operators must call the regulating agency and the downstream water users at once. Regulating agencies can then help curb the problem, and drinking water suppliers will have enough time to close their water intake lines until the problem is stopped. This will also warn people in recreation areas and give farmers and ranchers time to move livestock to a safe water supply.

Discharge for Recycling

In some areas where there is a shortage of water, wastewater effluent is recycled for industry, recreation, irrigation, and fire control use. Many industries can use treated wastewater for cooling and cleaning. Often this is cheaper for the industry than using potable (drinking) water. Lakes for fishing and boating have been maintained with recycled wastewater. Records show that these man-made lakes are often no more hazardous to the users than natural lakes. Recycled wastewater is seldom used as a drinking water supply.

Monitoring of effluent discharged for recycling is very important. Only by monitoring can the operator be sure that the effluent is good enough to be used. Recycling units may include extended settling and biological stabilization in holding ponds, sand filtering, and disinfection. Quality control is a must since the recycled water must be safe.

Discharge for Land Application (Irrigation)

Irrigation with wastewater effluent is frequently used in some areas. Before irrigating, it is necessary to consider the contour of the area for irrigation, soil type, ground water table, and potential damage to water supplies. The joint EPA/Army manual, EPA 625/1-77-008 Process Design Manual for Land Treatment of Municipal Wastewater, provides further guidance on this subject.

Hillsides and other areas with steep slopes are not often used for irrigation. Too much runoff may occur. Irrigation equipment is harder to move, control, and maintain. Each area to be used for irrigation should be surveyed by a qualified person. Often, areas with slopes on which normal farm machinery can be used can be irrigated by a sprinkler system or by a jet or spray gun. Terracing and contour furrowing help prevent runoff. Flooding, overland flow, and furrow irrigation may require special work done to the land. This may include leveling, grading, ditches, and dikes.

Soil type and structure affect the rate at which the wastewater can be applied and absorbed. Average loams and sandy loams absorb and filter well. Clay and other types of tight soil are not as good. Deep plowing and chiseling make these soils better for wastewater irrigation. Very sandy or gravelly soils have very high percolation or absorbing qualities. But when these soils are in contact with the ground water table, pollutants may get into underground water supplies before they can be filtered out. Tight, sandy, and gravelly soils can be improved for absorbing and filtering by plowing crops under.

Row crops may be watered by furrow, spray, and/or sprinkler irrigation. Spray irrigation is used where gravity flow is not practical in all parts of an effluent disposal plot or field. There should be gravity flow from one end of the row to the other. A lot of grading is needed to prepare a field for furrow irrigation. Long rows without enough slope will result in boggy parts of the field while other parts will not get enough water. Furrow irrigation on steep slopes may cause too much erosion. Operators in charge of this type of irrigation need special skill and experience to make sure a fairly even amount of water reaches all parts of the field.

Grass crops are often easy to irrigate. The grasslands may be pastures, meadows, parks, turf, or sodded areas of airfields and golf courses. The amount of water applied and how often it is applied are not as important as for row crops. Effluent can be applied to grassland by overland flow, sprinkler heads, or by jet or spray guns. The stems, leaves, and roots of the grasses make a good filter and help prevent rapid runoff. Grasses and some other plants purify and release to the air large amounts of moisture when they are growing. During times of sunny, hot, and dry weather with strong breezes, as much as 25 percent of the water applied to the land may evaporate, either straight from the plants or from the surface of the soil.

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