There  are  two  styles  of  service  sinks  (slop sinks):  the  trap-to-wall  and  the  trap-to-floor.  They are used for disposal of wash water, filling swab buckets, and washing out swabs. The trap-to-wall type  requires  a  2-inch  or  3-inch  waste  pipe;  the trap-to-floor, a 3-inch waste pipe. For both types, if  copper  tubing  is  used,  a  one  size  reduction  is allowed. Scullery sinks are large sheet metal sinks used for washing large pots and pans and for general scouring  purposes.  The  large  amount  of  grease that  usually  passes  through  a  scullery  sink  makes a  2-inch  waste  pipe  necessary. Drinking   fountains   carry   only   clear   water wastes and a 1 1/4-inch waste pipe is suitable. An indirect drain (covered later in this chapter) should be used. Sizing Sanitary Collecting Sewers The design and sizing of collecting sewers, the subtrunks, and the main trunk lines are provided by  engineers.  However,  the  UT  should  understand the factors that contribute to the design and the requirements that must be met. While  the  unit  system  is  used  to  size  the building sanitary piping and the building drain, the  sewage  quantities  used  in  sewer  design normally  are  computed  on  a  contributing population   basis.  The  population  to  be  used  in design  depends  upon  the  type  of  area  that  the sewer is to serve. If the area is strictly residential, the design population is based on full occupancy of  all  quarters  served.  If  the  area  is  industrial, the  design  population  is  the  greatest  number employed in the area at any time. There are ex- ceptions to the general rule that sewers must be designed on a population basis. Among these ex- ceptions are laundry sewers and industrial-waste sewers.  The  per  capita  contribution  for  sewer design varies. Typical values are 100 gallons per person  per  day  for  permanent  residents  and  30 gallons per person in the industrial area per 8-hour period. The sizing of the sewer includes the average rate and the extreme (peak) rate of flow (which occurs  occasionally).  The  ratio  of  the  peak  rate of flow to the average rate of flow may vary with the area served, because the larger the area or the greater  the  number  of  persons  served,  the  greater the  tendency  for  flow  to  average  out.  Typical  peak flows  might  range  from  6  for  small  areas  down to  1.5  for  larger  areas. An  allowance  for  infiltration  of  subsurface water  is  added  to  the  peak  flow  to  obtain  the design  flow.  A  typical  infiltration  allowance  is  500 gallons  per  inch  of  pipe  diameter,  per  mile  of sewer per day. Additional capacity to provide for population increase is usually included for areas that are likely to   continue   to   develop.   Provision   of   approxi- mately  25  percent  additional  capacity  over  the initial requirements is advisable. Each length of pipe from one manhole to the next is sized to carry the design flow. However, to   help   prevent   clogging   and   to   facilitate maintenance, a minimum size is usually specified which may be larger than is necessary to carry the design  flow  at  the  upper  ends  of  the  system. Typical minimum sizes are 6-inch pipe for house and industrial-waste sewers and 8-inch pipe for all other sewers. It is sometimes the practice to select a pipe size that will carry the design flow when the pipe is half full, thus allowing for expansion. More often, however,  sufficient  safety  factors  in  the  future population estimate and the peak flow factor are included so the pipe may be designed to carry the design  flow  when  flowing  full. The formulas or tables used in sizing the pipe are based on experiments and experience. One of the  factors  taken  into  account  is  the  roughness of  the  pipe.  Asbestos-cement  pipe,  for  example, is smoother than concrete pipe. Because there is less friction on the inside of the asbestos-cement pipe, it will carry a greater flow than concrete pipe of the same size. Another factor is the slope at which the pipe will be laid. The slope will generally be determined by the fall available on the natural ground area through  which  the  sewer  runs.  The  plans  for collecting sewer systems generally show slope (or grade) in terms of fall per hundred feet. Slope is sometimes expressed as a percent rather than in inches  per  foot.  A  1  percent  slope  means  1  foot of frill in a 100-foot length of pipe, or about 1/8 inch per foot. A 0.5 percent slope (6 inches in 100 feet  is  about  1/16  inch  per  foot. Table 7-6 gives the minimum slope for some of the most commonly used pipe sizes. The slope should  remain  constant  in  the  section  between each  manhole.  Each  section  between  successive manholes  should  be  analyzed  and  the  slope  for that particular section determined. If the fall is relatively steep, the velocity of the flow is faster and a smaller pipe size may be used. If the slope is relatively flat, the velocity is slower and a larger pipe size may be used. In the larger pipe, the depth of  flow  may  decrease  to  such  extent  that  the velocity might be no greater than a smaller pipe 7-8