hypochlorite should be used to disinfect the water in the tank trailer. The procedure for preparing and adding the solution to the tank trailer is as follows: 1. Fill the tank trailer half full of water. 2.   Prepare a small amount of HTH concentrate by dissolving the required amount of HTH in a canteen cup or other container. In this case, place the 10 ounces of HTH in a l- or 2-gallon bucket of water. 3.   Stir the mixture thoroughly. All of the granules will not dissolve. Allow undissolved granules to settle to the bottom of the bucket. 4.   Add only the clear concentrate liquid to the tank trailer. By pouring the supematant into the tank trailer slowly, you can see that the settled granules will remain in the bucket. 5.   Fill the remainder of the tank with the water to be treated 6.   The water now must be agitated to distribute the HTH. In this example, you could simply attach the tank trailer to a vehicle and go for a short drive. 7. The final step is to take a FAC reading 30 minutes after adding the HTH. The reading must be at or above the required ppm. In this example, the FAC must read 100 ppm or higher. If the ppm is not high enough,  add  more  HTH  until  the  desired  ppm  is maintained. NOTE Remember  that  during  superchlorination the FAC may not fall below 50 ppm wihin 4 hours or the whole procedure must be repeated. After the 4-hour contact time and the tank trailer ppm has stayed above 50 ppm, the trailer is now disinfected.  Rinse  the  tank  thoroughly  with  potable water and then refill it for usage. An occasion may occur   when   you   must   use   the   water   that   you superchlorinated. If the chlorine ppm is too high, you may use sodium thiosulfate or sodium bisulfate to dechlorinate the water. Table 2-3 shows water sources and the required chlorine  residual. For  further  information,  see  chapter  9  of  the Manual for Preventive Medicine, NAVMEDP-50109. Table 2-3.—Chlorine Residual Chart WATER SOURCE REQUIRED CHLORINE RESIDUAL Public  water  supply 5.0  ppm  FAC  after  30- system  of  questionable minute  contact  time  and quality maintain at a minimum of 2.0 ppm FAC throughout the  distribution  system Engineering   water 5.0   ppm   FAC   at   the points standpipe or fill hose Water tankers, trailers, Maintain between 5.0 ppm bladders,  and  cans and  2.0  ppm  FAC  when filling from an approved water point. Maintain 5.0 ppm FAC when used as a source   of   distribution (piping system) Distribution   (piping) system Maintain  5.0  ppm  at  the source and 2.0 ppm FAC at the spigot Lyster bags and canteens Maintain  2.0  ppm  FAC when   filling   from   an approved   water   source. Chlorinate to 5.0 ppm FAC initially  and  maintain  at  2.0 ppm FAC when filling from an unapproved or raw water source Diatomite Water Purification Unit (3000-D) The 3000-D Water Purification System is portable and completely self-contained, as shown in figure 2-25. The unit purifies turbid and bacteria-polluted water. Particular attention was paid to design and packaging to increase efficiency, mobility, and cost effectiveness. It also provides a trouble-free method of producing potable water at the rate of 3,000 gallons per hour. The  system  is  constructed  in  separate  modules, interconnected, and mounted in a common chassis. The chassis is not required for operation, but it greatly eases the transportability of the system. The system contains all of the functional apparatus and supplies necessary to process  approximately  20,000  gallons  of  potable water.  The  user  must  have  a  water  source  and  a 2-30