The CHT system accepts soil drains from water closets and urinals, and waste drains from showers, laundries,  and  galleys.  The  three  functional  elements  of sewage collection, holding, and transfer make up the CHT system. The collection element consists of soil and waste drains with diverter valves. Depending on the position of the diverter valves, the soil or waste can be diverted overboard or into the CHT tank The holding element consists of a holding tank. The transfer element includes sewage pumps, overboard and deck discharge piping, and deck discharge fittings. The CHT system can be used in any of three distinct modes of operation, depending on the situation. 1. When the ship passes through restricted zones, the  CHT  system  is  set  up  to  collect  and  hold  the discharges from the soil drains only. 2. During in-port periods, the CHT system will collect, hold, and transfer to a shore sewage facility all discharges  from  the  soil  and  waste  drains. 3. When the ship operates at sea outside restricted areas, the CHT system will be set up to divert discharges from soil and waste drains overboard. The Naval  Ships’  Technical  Manual,  chapter  593, has more information on the operation and maintenance of CHT systems. FUEL OIL STORAGE AND HANDLING Fuel oil systems aboard ship include fuel oil tanks, fuel oil piping, fuel oil pumps, and the equipment we use to strain, measure, and burn the fuel oil. FUEL OIL TANKS Navy ships use four kinds of tanks that are part of the ship’s system to receive, hold, and distribute fuel oil. They  are  (1)  storage  tanks,  (2)  overflow  tanks,  (3) service tanks, and (4) contaminated oil settling tanks. We’ll explain their functions in the following pages. Fuel oil tanks are vented to the atmosphere through pipes leading from the top of each tank to a location abovedecks.  The  vent  pipes  allow  air  to  leave  or  enter the tank as fuel is added or removed. Most fuel oil tanks are  equipped  with  manholes,  overflow  lines,  sounding tubes, liquid level indicators, and lines by which you may fill, empty, and cross-connect the tanks. Fuel Oil Storage Tanks The main fuel oil storage tanks are part of the ship’s structure. They may be located forward and aft of the machinery spaces or abreast of these spaces. They may be in double-bottom compartments as long as those compartments are not directly under boilers. Some tanks have connections that allow them to be filled with fuel oil or with seawater from the ballasting system. Fuel Oil Overflow Tanks Fuel oil overflow tanks receive the overflow from fuel oil storage tanks that are not fitted with independent overboard  overflows.  Overflow  tanks  also  serve  as ballast tanks because they can be filled with seawater from  the  ballasting  system. Fuel Oil Service Tanks Fuel oil is taken aboard by fueling trunks or special connections leading to the fuel oil storage tanks. The fuel oil is then pumped to the fuel oil service tanks. All fuel for immediate use is drawn from the service tanks. The fuel oil service tanks are considered part of the fuel oil service system described later in this section. Contaminated Oil Settling Tanks The contaminated oil settling tanks hold oil that is contaminated with water or other impurities. After the oil has settled, the unburnable material, such as water and  sludge,  is  pumped  out  through  low  suction connections. The burnable oil remaining in the tanks is transferred to a storage tank or a service tank. The contaminated oil tanks also can receive and store  oil,  or  oily  water,  until  it  can  be  discharged overboard  without  violation  of  environmental regulations. See OPNAVINST 5090.1 to learn when you may empty the contaminated oil settling tanks either  overboard  or  to  barges. FUEL OIL PIPING SYSTEM The fuel oil piping system includes the piping and pumps for three systems, each of which is connected at different levels of the storage or service tanks for (1) the fuel  oil  filling  and  transfer  system,  (2)  the  fuel  oil service  system,  and  (3)  the  fuel  oil  tank  stripping system. The pipes are connected to the storage and service tanks at different levels so the pumps can take suction from any of these levels. The service system is connected at the highest level, the filling and transfer 6-6