terial action destroys a portion of the organic matter, changing a portion of the raw sludge to a non-offensive digested sludge. During this action a large quantity of gas, high in methane, is produced. This gas is collected and used about the plant for heating purposes, the excess being burned in a specially constructed burner outside.= The contents of these digesters tend to separate into three distinct layers (Figure 3). These consist of lighter solids that are carried up with the gas, tend to stay on top, and form a scum layer. Just below this is a layer of material mostly liquid, and from this to the bottom the amount of solids increase. This bottom layer is called sludge. As the bacterial action continues, the sludge becomes more digested, and its specific gravity increases, the more completely digested sludge settling to the bottom. In the primary digester, these layers are not as pronounced as in the secondary because of the large amount of gas that is given off, keeping the contents fairly well mixed. The partially digested sludge is transferred from the primary to the secondary digester, the transfer being made ordinarily once each day. Raw sludge is pumped into the primary digester, usually three times per day. In the model these tanks are fourteen inches in diameter and six and one-half inches high. They were mounted on blocks of wood to bring them to the proper elevation. The base was made of a thick piece of wood to which was nailed and glued thin strips for the sides. The top was made of a piece of five-ply veneer to which was fastened a piece of galvanized iron in the shape of a cone with a diameter of fourteen inches and altitude of three inches. Two sampling wells were made of one-eighth-inch copper tubing which was soldered in place. A manhole was made by soldering a portion of a bouillon cube can to the cover, while a small tin can was soldered in place to represent the trap in which the gas is collected. A gas-pipe housing, permitting the cover to move up and down over the gas outlet pipe was made from an old Bunsen burner barrel, plugged on one end with putty. Inside the digesters the beating coils were made of heavy copper wire. A piece of one-eighth-inch cast % A tpresent there are about 40,000 cubic feet of gas being produced per month.
iron pipe was installed as the digested sludge drawoff pipe. In order that these parts inside could be seen a section of the roof was cut away. The digester control house containing valves and pipes for controlling the flow of material from one digester to the other is included in the model although not many of the pipes and valves were shown. One set, however, was made of one-fourth inch doweling. These represented a set of pipes and valves from which samples of sludge could be drawn a t various levels from the digesters, the purpose of these samples being to determine a t which level sludge should be taken from the digesters. The digested sludge is carried from the secondary digester to the basement of the main control house through a pipe in a tunnel. The tunnel was included in the model, galvanized iron wire being used for the pipe. This tunnel also contains pipes for carrying the raw sludge to the digesters, the hot water with cold water return, the gas from the digesters, and the supernatant liquid from the digesters. A portion of the top of the tunnel was cut away so that the inside could be seen. From the sludge well, the sludge is lifted by bucket elevators to the vacuum sludge filter. This filter consists of a pair of drums covered with canton flannel. A vacuum inside causes the liquids to be forced in, leaving the dry sludge on the outside. Before the digested sludge is placed on the filter, ferric chloride and hydrated lime are added to coagulate the finely divided solids, making the filtration more complete. This sludge cake is sold, being used as top dressing for lawns. The sludge well and filter were not included in the model. Labels were placed a t various points throughout the model describing the size and function of that part of the actual plant. In addition to the model a description of the entire sewage system of the city has been prepared and is used in the chemistry course. To accompany this description a number of photographs have been taken, showing different parts of the plant. Some of these show similarities between model and corresponding parts of the actual plant while others show views of the plant not included in the model. This has served as a very effective means of presenting this phase of the course.
INCREASE REPORTED IN JUNIOR COLLEGE ENROLMENT Enrolment in junior colleges in the United States has doubled in the last seven years, according to the 1940 Junior College Directory, just issued by the American Association of Junior Colleges. Enrolment has increased from 155,588 to 196,510 in the last
year. This 40.922 increase. which is 26.4 oer cent.. is the ereat;st ever reported, according to Walter C. 'Eells, secretary of the Association. There are now five hundred seventy4ve junior colleges, as compared with five hundred fifty-six reported a year ago.
