and some other plants purify and release to the air large amounts of moisture when they are grow- ing. 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 pre- vent 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  pro- cessed. 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 open- ings  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  sur- rounding  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. 10-29