When conditions are such that a great amount of grease necessarily enters the kitchen-sink waste, it is necessary to use a grease trap, a form of which is to be seen in Fig. 7, this form representing the best type of such traps.

As already stated, contact with cold surfaces causes the grease in sewage to separate from the liquid, a fact which is made use of in the operation of this or any other grease trap. The body of the trap is surrounded entirely by a water jacket or chamber, with the exception of the top. In addition, the partition in the center of the trap, which is designed to aid in breaking up the sewage and deflecting the grease upward, is also formed into a water chamber.

The water pipe supplying the kitchen sink is connected at the inlet and outlet ends of this water jacket, cold water thus flowing through the jacket constantly and changing whenever water is drawn at the sink. If the jacket were simply filled with water and not changed there would be no cooling effect, but the method described keeps the surfaces against which the waste comes in contact always cool, resulting effectively in the separation of the grease, which rises to the top and is taken out through the removable cover. The trap outlet is made at the bottom of the trap, instead of at the top, to aid in preventing the escape of the grease.

The partition through the center of the trap also helps to prevent grease entering the trap from being carried over into the outlet.

While the water jacket surrounding the trap does effective work, a large part of the results obtained is due to the presence of the hollow partition or deflector. This trap is of cast iron and made in several sizes.

Less expensive and less satisfactory grease traps are made on the same general lines as the trap just described, but not provided with a water jacket. Many of them do very good work, but it is not to be expected that they can hold back as large a part of the grease as the trap does which is cooled continuously by the water supply. There is one point in the use of the grease trap which does not always receive consideration - the amount of money to be derived from the sale of grease coming from the grease trap. In large establishments this is often a very respectable sum of money. Traps similar in design to the one described are also made of wrought steel. Cast iron, however, would seem to be less in danger of deterioration than wrought steel, which is more easily acted upon by acids. The grease trap, on a larger scale, in the form of a catch basin, is sometimes located outside the building, underground, and into this receptacle all the kitchen waste from kitchen sinks, pantry sinks, dishwashing sinks, etc., is discharged. The great advantage gained in the use of such a catch basin is that it is constantly cooled by the moisture of the ground in which it is located. It should always be set low enough in the ground to be out of danger of freezing. If it is impossible to so install it, the catch basin should never be used. A serious disadvantage in the use of the underground catch basin is that generally its use necessitates a long line of horizontal waste pipe from the kitchen to the catch basin, and in this pipe and its connections grease has abundant opportunity to collect before reaching the catch basin, resulting in the ultimate stoppage of such pipes.

These pipes generally run in cool cellars and for a distance underground, which favors the collection of more or less of the grease on their surfaces. The better plan would seem to be the use of grease traps under the fixtures in the kitchen, with systematic attention given to the removal of grease that accumulates.

In the case of a line of kitchen sinks or of dishwashing sinks, one grease trap of proper size may be used for the accommodation of the entire number of fixtures. Catch basins for kitchen waste may be of brick or cast iron, and should never be less than 30 in. in internal diameter, tapering toward the top, if desired, to about 22 to 24 in., and provided with a cast-iron cover. If of brick, they should be made water-tight. The drain from the kitchen catch basin to the sewer may be of glazed tile, and should be not less than 5 in. in diameter and provided with a trap having a deep seal.