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.

Wooded areas and some wastelands have been used for effluent disposal. In these areas the disposal of the treated wastewater is most often the only reason for applying the effluent since crops are not grown there. These areas may absorb and filter the wastewater very well. The amount of wastewater applied to these lands is not as important as it is with crop irrigation.

The use of wastewater irrigation for vegetable and fruit crops that can be eaten uncooked may be restricted to protect public health. The health department or local regulating agency should be contacted before wastewater irrigation is applied to fruit and vegetable crops.

Wastewater effluent is often held in storage ponds or basins before it is used for irrigation. All discharge permit requirements must be met before the effluent can be used. Wastewater used on parks, golf courses, and other recreation areas should be disinfected just before it is applied. A chlorine contact chamber installed just upstream of the irrigation should be enough, but disinfection must meet the rules of the regulating agencies.

Testing of surface and subsurface water supplies in the immediate area of irrigation is important and must not be forgotten. Samples should be taken from all surface waters exposed to drainage and seepage of the irrigation water. Ground water should be tested using samples from existing wells or from test wells dug for that purpose. In areas where the water table is only a few feet below ground level, tests should be run very often. It is not as hard or as costly to prevent pollution as it is to clean up polluted water. Qualified laboratory technicians should check the soil to see if it is being hurt by buildups of toxics or by too many plant nutrients. Grazing and/or harvesting crops may help control soil conditions.

There must be enough storage capacity to hold the wastewater effluent until it is time to irrigate. The weather, type of soil, and type of crop are important in finding out how much to apply and how often to apply it. The wastewater should be disinfected before it is used for recreation. A power unit and a pump must be used unless gravity flow can be used to transport and distribute the effluent. A lightweight pipe like an aluminum alloy or plastic is often used to carry the wastewater. Quick-coupling joints are needed so the pipe can be put together and taken apart easily. Sections of flexible hose are needed for mobile spray equipment and can also be used for bends in pipelines carrying the effluent. Valves are needed to control the amount of flow. Pressure release or bypass devices are needed to control pressure. Spray or sprinkler nozzles, heads, and guns must be able to adapt to the volume of water to be applied. They must also be designed to work in the range of water pressure maintained.

Evaporation and Percolation Basins

Evaporation and percolation basins are used to dispose of wastewater effluent by letting it evaporate and by letting it percolate or seep into the soil. The correct use of these ponds depends on the area of the basin compared to the amount and kind of the wastewater effluent to be processed. The larger the surface and bottom of the pond, the faster the wastewater evaporates and percolates. The climate and kind of soil are important in finding out whether this type of disposal can be used in a given area. This kind of system can be a good and cheap way to dispose of wastewater effluent.

It is often better to build two basins or one basin with a dike that separates it into two parts. After a time, suspended solids will change to settleable solids and build up in the pores or openings of the soil. Percolation will slow down and sooner or later will stop. To get the basin back in working order, it must be drained, dried, and cleaned. Scars must be cut in the bottom. With two basins, one can be kept in service while the other is being restored. The bottoms of the ponds should be sloped for quick and complete draining.

The berms or dikes must be checked often for erosion and rodent damage. The dikes and surrounding areas should be mowed often to keep vegetation at a maximum height of 6 to 10 inches (15 to 25 centimeters). This will help keep rodents out of the area. The area should be fenced to keep out larger animals.

Signs should be built to show that the ponds are wastewater treatment plants and are dangerous. As with wastewater lagoons, trees should not be allowed to grow within about 500 feet (150 meters) of the pond. There must be enough surface drainage around the edge of the pond to keep surface water from entering the unit.