A MODEL of a MODERN SEWAGE TREATMENT PLANT as a PROJECT in HIGH-SCHOOL CHEMISTRY WILLIAM MAYROSE Senior High Schaol. Muskegon, Michigan
A display model of a sewage treatment plant was construded by a group of eight high-school boys. The greater N r t was made of wood, on a scale such that onefourth of an inch on the model equaled twelve inches on the actual plant. The blueprints used in constructing
the plant were used for details on the model. All important parts were represented i n their relative positions, a number of the smaller details also being shmun. The entire model was labeled, the- course of the sewage through the pluntbeing indicated by numbers.
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volunteered then selected parts that they desired to construct. One boy had a workshop in the basement of his home where he built the main control house and the grit chamber. The other work was done in the chemistry rooms which were equipped with a work bench and a few tools. One boy was in a course in cabinet-making and had access to the machinery and tools of that department. The main control house is-a small hrick building containing the office, chlorination apparatus, sludge filters, chemical laboratory, pumps, and so forth. I n the model this was constructed of one-fourth inch white pine. The partitions were installed in such manner that the arrangement of the rooms and corridors were accurately shown. Windows were made of clear isinglass. There was only a single story and basement, these being made in separate parts so that they could he taken apart and cleaned. The floor was made of a piece of ordinary window glass so that the labels and descriptions placed on placards in the various rooms could he read. A piece of window glass was placed over the top of this huilding and fastened in place by a strip of molding. This building is nine and one-half inches by twelve inches by six inches.
HIS project was undertaken partly because a
model along similar lines of the water filtration plant of this city had previdusly been built1 and because of a desire on the part of the writer to have the two models for use in teaching that part of the course in chemistry on water purification and sanitation. A set of plans used in the construction of the treatment plant were obtained from the superintendent of the plant. These were used in building the model, the scale being reduced such that one-fourth inch on the model was equal to one foot on the actual plant. A trip was made to the treatment plant, by the students who had volunteered to build the model, where the details of construction and operation were explained by the superintendent. The students then met to study the blueprints and to formulate plans for building the model. The work very conveniently fell into three distinct parts; the main control house and the grit chamber, the clarifiers with their troughs and scraping mechanisms, and the digesters with their control house (Figure 1). The students who had MAYROSE, "A model of a modern water filtration plant as a project in high-school chemistry," J. &EM. Eouc., 16, 239 (1939).
All sewage entering the treatment plant flows into the grit chamber, a tank twenty feet square and about four feet deep. Sewage passes through this tank a t a rate of about one foot per second. At this rate the very heavy solids settle to the bottom and are removed by a scraping mechanism. This is a large three-blade scraper that is turned by an electric motor, such that the settled'solids are scraped to one side of the tank and removed by being scraped up an
separate from the liquids. The heavier solids settle to the bottom and are called raw sludge, while the lighter solids such as grease, oil, matches, and so forth, rise to the surface and are called scum. To collect these solids each clarifier has three sets of collectors; two designated as longitudinal and one as a cross collector. The longitudinal collector consists of two endless chains running nearly the complete length of the clarifiers. To these are attached as
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FIGURE 1.-GENERAL LAYOUT 03 THE TREATMENTPLANT incline and discharged into a pit a t the side of the tank. This scraper mechanism was made of wood, the three blades heing made of a piece of white pine onto which were glued small pieces of balsam sloping toward the floor. These blades resemble somewhat the backbone of a fish. This mechanism is guided around the grit chamber by a railroad track on top of the wall of the chamber. A piece of one-eighth inch track was obtained from a supply house furnishing materials for .models. The wheel that runs on the track was carved out of a piece of lead. The sewage from the grit chamber containing the more finely divided solids in suspension passes into the clarifiers. There are two clarifiers, each one hundred thirty-six feet long, thirty feet wide, and ten feet deep. The velocity of the sewage in these tanks is retarded sufficiently to allow the suspended matter to
scrapers pieces of redwood two inches by six inches. The whole mechanism resembles a huge ladder, the cross pieces being the scrapers and being spaced about six feet apart, the sides heing the two chains. This scraper mechanism moves about three feet per minute over sprockets that are driven by electric motors. It is guided along the sides of the clarifiers by steel guide rails. The clarikrs, contact chamber, and diversion chamber were constructed as a single box, twentyone inches wide, thirty-four inches long, and two and one-half inches deep, with partitions setting off each of these sections. Ordinary furnace chain was used to represent the chain in the collectors, various links heing spread apart and pieces of soft iron, one-eighth inch square and three inches long to represent the scrapers were slipped into these links which were then
squeezed together holding the iron in place. The guide rails were made by tacking thin strips of wood to the walls of the clarifiers. A number of different types of sprockets were considered. One boy suggested alarm clock wheels; another thought they might he made by cutting lead washers. Finally three-fourth-inch brass washers were used, the teeth being cut on them a t the school machine shop. The shafts on which the sprockets are fastened were represented by three-eighth-inch doweling, these being nailed and glued to pillars supporting the cross walks on top of the clati6ers. The scrapers while on top of the clarifiers are about half submerged in the sewage and in this position are enabled to carry the scum to the scum trough. This trough extends over the entire width of the two clarifiers and in the model was made of a piece of grooved
the longer collector except, of course, smaller. The sprockets and shafts were made in the same manner. On top of the partition separating the clarifiers and contact chamber are concrete walks, represented in the model by strips of wood painted light gray to resemble the concrete. In the actual plant, that section of the clarifiers beyond the scum trough has a series of troughs, covered with iron grating. In the model these troughs were covered with strips of wood. cut away so that an observer might see below. Sewage from the clarifiers flows through weirs into these troughs and then to the contact chamber. This
is a tank about one hundred thirty-six feet long, sixteen feet wide, and ten feet deep. Its purpose is to allow the chlorine, when added, to remain in contact with the sew.age long enough to kill the bacteria present in the clarified sewage. On the outer wall of this chamber are two conduits, one above the other. Each is three feet wide, the wood. The scum flows from this trough to the scum lower one being two and one-half feet deep, while the well, represented in the model .by a small box fastened upper one is five feet deep. Sewage from the clarifiers to the side of the clarifier. From here the scum is flows through this lower conduit to the contact chamber. Chlorine as chlorine water is added about midway pumped to the digesters. Q This scraping mechanism passes under the scum in this conduit. A set of baffles directs the flow into trough and to the far end of the clarifier several feet the contact chamber. The sewage leaves the contact under the sewage. Here they pass over a pair of chamber flowing through side weirs, into the upper sprockets that guide them to the bottom of the tank. conduit, and out to the river through the diversion On the bottom they scrape the raw sludge to the op- chamber. These conduits were installed and a portion of the posite end of the clarifier where it is discharged into a cross collector sump. This occupies about seven feet outside wall cut away so that the construction details of one end of the clarifiers and has a floor about three could be seen. The side weirs in the contact chamber and one-half feet lower than the other portion of these can be easily seen in Figure 2. The sludge from the clarifiers is pumped to the tanks. It has a cross collection mechanism, similar to, hut smaller than, the transverse collector and primary digester. There are two digesters, the other operates a t right angles to the direction of flow of being called the secondary. The primary digester sewage in the clarifier. This cross collection mechanism has twice as many hot-water heating coils as the other, scrapes the raw sludge into a smaller sump from where since the raw sludge that is added is cold and must be it is removed by suction and transferred to the di- heated in this tank to a temperature where bacterial action will take place. These digesters are concrete gesters. On the model these sumps were made separate from tanks, fifty-five feet in diameter, and twenty-five the other portion of the tanks by cutting a piece of and oue-half feet deep. Each has a steel cover which two by four, then installing i t in the proper place. The floats on the contents, rising and falling as additions cross collectiou mechanism was made the same as and withdrawals are made. In these digesters hac-
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 slndge 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 oue-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 iq place to represent the trap in which the gas is collected. A gas-pipe housing, permitting the cover to move up 8nd down over the gas outlet pipe was made from an old Bunsen burner barrel, plugged on one end with putty. Inside the digesters the heating coils were made of heavy copper wire. A piece of one-eighth-inch cast %t present there are about 40,000 cubic feet of pas 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 camed 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 slndge 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 slndge on the outside. Before the digested slndge 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. I n 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 diierent 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.
