CHAPTER XII

MAINTENANCE OF SEWERS

=198. Work Involved.=—The principal effort in maintaining sewers is to keep them clean and unobstructed. A sewerage system, although buried, cannot be forgotten as it will not care for itself, but becoming clogged will force itself on the attention of the community. Besides the cleaning and repairing of sewers and the making of inspections for determining the necessity for this work, ordinances should be prepared and enforced for the purpose of protecting the sewers from abuse. Inspections to determine the amount of the depreciation of sewers with a view towards possible renewal, or to determine the capacity of a sewer in relation to the load imposed upon it are sometimes necessary. The valuation of the sewerage system as an item in the inventory of city property may be assigned to the engineer in charge of sewer maintenance.

The work involved in the inspection and cleaning of sewers in New York City for the year ending May, 1914, included the removal of 22,687 cubic yards of material from catch-basins, and 14,826 catch-basin cleanings. This made an average of two and one-half cleanings per catch-basin per year, or 1½ cubic yards removed at each cleaning. The 6,432 catch-basins were inspected 71,890 times. There were 4,112 cubic yards of material removed from 517 miles of sewers, or about 8 cubic yards per mile. Inspection of 194 miles of brick sewers were made, 4.4 miles were flushed, and 27 miles were cleaned. Inspections of 198 miles of pipe sewers were made, 80 miles were examined more closely, 37 miles were flushed, and 91 miles were cleaned. The field organization for this work consisted of 17 foremen, 8 assistant foremen, 29 laborers, 71 cleaners, 13 mechanics, 7 inspectors of construction, 3 inspectors of sewer connections, 13 horses and wagons, and 28 horses and carts.[105]

=199. Causes of Troubles.=—The complaints most frequently received about sewers are caused by clogging, breakage of pipes, and bad odors. Sewers become clogged by the deposition of sand and other detritus which results in the formation of pools in which organic matter deposits, aggravating the clogged condition of the sewers and causing the odors complained of. Grease is a prolific cause of trouble. It is discharged into the sewer in hot wastes, and becoming cooled, deposits in thick layers which may effectively block the sewer if not removed. It can be prevented from entering the sewers by the installation of grease traps as described in Chapter VI. The periodic cleaning of these traps is as important as their installation.

Tree roots are troublesome, particularly in small pipe sewers in residential districts. Roots of the North Carolina poplar, silver leaf poplar, willow, elm, and other trees will enter the sewer through minute holes and may fill the sewer barrel completely if not cut away in time. Fungus growths occasionally cause trouble in sewers by forming a network of tendrils that catches floating objects and builds a barricade across the sewer. Difficulties from fungus growths are not common, but constant attention must be given to the removal of grit, grease, and roots. Tarry deposits from gas-manufacturing plants are occasionally a cause of trouble, as they cement the detritus already deposited into a tough and gummy mass that clings tenaciously to the sewer.

Broken sewers are caused by excessive superimposed loads, undermining, and progressive deterioration. The changing character of a district may result in a change of street grade, an increase in the weight of traffic, or in the construction of other structures causing loads upon the sewer for which it was not designed. The presence of corrosive acids or gases may cause the deterioration of the material of the sewer.

=200. Inspection.=—The maintenance of a sewerage system is usually placed under the direction of a sewer department. In the organization of the work of this department no regular routine of inspection of all sewers need be followed ordinarily. Attention should be given regularly to those sewers that are known to give trouble, whereas the less troublesome sewers need not be inspected more frequently than once a year, preferably during the winter when labor is easier to obtain.

The routine inspection of sewers too small to enter is made by an examination at the manhole. If the water is running as freely at one manhole as at the next manhole above, it is assumed that the sewer between the manholes is clean and no further inspection need be given unless there is some other reason to suspect clogging between manholes. If the sewage is backed up in a manhole it indicates that there is an obstruction in the sewer below. If the sewage in a manhole is flowing sluggishly and is covered with scum it is an indication of clogging, slow velocity and septic action in the sewer. Sludge banks on the sloping bottom of the manhole or signs of sewage high upon the walls indicate an occasional flooding of the sewer due to inadequate capacity or clogging.

If any of the signs observed indicate that the sewer is clogged, the manhole should be entered and the sewer more carefully inspected. Such inspection may be made with the aid of mirrors as shown in Fig. 140 or with a periscope device as shown in Fig. 141. Sunlight is more brilliant than the electric lamp shown in Fig. 141, but the mirror in the manhole directs the sunlight into the eyes of the observer, dazzling him and preventing a good view of the sides of the sewer. The observers’ eyes can be protected against the direct rays of the electric light, which can be projected against the sides of the pipe by proper shades and reflectors. It is possible with this device to locate house connection, stoppages, breaks of the pipe, and to determine fairly accurately the condition of the sewer without discomfort to the observers.

Sewers that are large enough to enter should be inspected by walking through them where possible. The inspection should be conducted by cleaning off the sewer surface in spots with a small broom, and examining the brick wall for loose bricks, loose cement or cement lost from the joints, open joints, broken bond, eroded invert, and such other items as may cause trouble. An inspection in storm sewers is sometimes of value in detecting the presence of forbidden house connections.

Certain precautions should be taken before entering sewers or manholes. If a distinct odor of gasoline is evident the sewer should be ventilated as well as possible by leaving a number of manhole covers open along the line until the odor of gasoline has disappeared. The strength of gasoline odor above which it is unsafe to enter a sewer is a matter of experience possessed by few. A slight odor of gasoline is evident in many sewers and indicates no special danger. A discussion of the amount of gasoline necessary to create explosive conditions is given in Art. 206. In making observations of the odor it should also be noted whether air is entering or leaving the manhole. The presence of gasoline cannot be detected at a manhole into which air is entering.

As soon as it is considered that the odors from a sewer indicate the absence of an explosive mixture, a lighted lantern or other open flame should be lowered into the manhole to test the presence of oxygen. Carbon monoxide or other asphyxiating gases may accumulate in the sewer, and if present will extinguish the flame. If the flame burns brilliantly the sewer is probably safe to enter, but if conditions are unknown or uncertain, the man entering should wear a life belt attached to a rope and tended by a man at the surface. Asphyxiating or explosive gases are sometimes run into without warning due to their lack of odor, or the presence of stronger odors in the sewer. Breathing masks and electric lamps are precautions against these dangers, the masks being ready for use only when actually needed. More deaths have occurred in sewers due to asphyxiating gases than by explosions, as the average sewer explosion is of insufficient violence to do great damage, although on occasion, extremely violent explosions have occurred. During inspections of sewers there should always be at least one man at the surface to call help in case of accident and the inspecting party should consist of at least two men.

It must not be felt that entering sewers is fraught with great danger, as it is perfectly safe to enter the average sewer. The air is not unpleasant and no discomfort is felt, but conditions are such that unexpected situations may arise for which the man in the sewer should be prepared. It is therefore wise to take certain precautions. These may indicate to the uninitiated, a greater danger than actually exists.

The inspection of sewers should include the inspection of the flush-tanks, control devices, grit chambers, and other appurtenances. A common difficulty found with flush-tanks is that the tank is “drooling,” that is to say the water is trickling out of the siphon as fast as it is entering the tank, and the intermittency of the discharge has ceased. If, when the tank is first inspected the water is about at the level of the top of the bell it is probable that the siphon is drooling. A mark should be made at the elevation of the water surface and the tank inspected again in the course of an hour or more. If the water level is unchanged the siphon is drooling. This may be caused by the clogging of the snift hole or by a rag or other obstacle hanging over the siphon which permits water to pass before the air has been exhausted, or a misplacement of the cap over the siphon, or other difficulty which may be recognized when the principle on which the siphon operates is understood. Occasionally it is discovered that an over zealous water department has shut off the service.

Control devices, such as leaping or overflow weirs, automatic valves, etc., may become clogged and cease to operate satisfactorily. They should be inspected frequently, dependent upon their importance and the frequency with which they have been found to be inoperative. An inspection will reveal the obstacle which should be removed. Floats should be examined for loss of buoyancy or leaks rendering them useless. Grit and screen chambers should be examined for sludge deposits.

Catch-basins on storm sewers are a frequent cause of trouble and need more or less frequent cleaning. Cleanings are more important than inspections for catch-basins for if they are operating properly they are usually in need of cleaning after every storm of any magnitude, and a regular schedule of cleaning should be maintained.

A record should be kept of all inspections made. It should include an account of the inspection, its date, the conditions found, by whom made and the remedies taken to effect repairs.

=201. Repairs.=—Common repairs to sewerage systems consist in replacing street inlets or catch-basin covers broken by traffic; raising or lowering catch-basin or manhole heads to compensate for the sinking of the manhole or the wear of the pavement; replacing of broken pipes, loosened bricks or mortar which has dropped out; and other miscellaneous repairs as the necessity may arise. Connections from private drains are a source of trouble because either the sewer or the drain has broken due to careless work or the settlement of the foundation or the backfill.

=202. Cleaning Sewers.=—Sewers too small to enter are cleaned by thrusting rods or by dragging through them some one of the various instruments available. The common sewer rod shown in Fig. 142 is a hickory stick, or light metal rod, 3 or 4 feet long, on the end of which is a coupling which cannot come undone in the sewer. Sections of the rod are joined in the manhole and pushed down the sewer until the obstruction is reached and dislodged. Occasionally pieces of pipe screwed together are used with success. The end section may be fitted with a special cutting shoe for dislodging obstructions. In extreme cases these rods may be pushed 400 to 500 feet, but are more effective at shorter distances. Obstructions may be dislodged by shoving a fire hose, which is discharging water under high pressure through a small nozzle, down the sewer toward the obstruction. The water pressure stiffens the hose, which, together with the support from the sides of the conduit, make it possible to push the hose in for effective work 100 feet or more from the manhole. A strip of flexible steel about ½ inch thick and 1½ to 2 inches wide is useful for “rodding” a short length of crooked sewer.

Sewers are seldom so clogged that no channel whatever remains. As a sewer becomes more and more clogged, the passage becomes smaller, thereby increasing the velocity of flow of the sewage around the obstruction and maintaining a passageway by erosion. This phenomenon has been taken advantage of in the cleaning of sewers by “pills.” These consist of a series of light hollow balls varying in size. One of the smaller balls is put into the sewer at a manhole. When the ball strikes an obstruction it is caught and jammed against the roof of the sewer. The sewage is backed up and seeks an outlet around the ball, thus clearing a channel and washing the ball along with it. The ball is caught at the next manhole below. A net should be placed for catching the ball and a small dam to prevent the dislodged detritus from passing down into the next length of pipe. The feeding of the balls into the sewer is continued, using larger and larger sizes, until the sewer is clean. This method is particularly useful for the removal of sludge deposits, but it is not effective against roots and grease. The balls should be sufficiently light to float. Hollow metal balls are better than heavier wooden ones.

Plows and other scraping instruments are dragged through pipe sewers to loosen banks of sludge and detritus and to cut roots or dislodge obstructions. One form of plow consists of a scoop[106] similar to a grocer’s sugar scoop, which is pushed or dragged up a sewer against the direction of flow. As fast as the scoop is filled it is drawn back and emptied. The method of dragging this through a sewer is indicated in Fig. 143. At Atlantic City the crew operating the scoop comprises five men, two are at work in each manhole and one on the surface to warn traffic and wait on the men in the manholes. The outfit of tools is contained in a hand-drawn tool box and includes sewer rods, metal scoops for all sizes of sewers, picks, shovels, hatchets, chisels, lanterns, grease and root cutters, etc., and two winches with from 400 to 600 feet of ⅜-inch wire cable.

Another form of plow or drag consists of a set of hooks or teeth hinged to a central bar as shown in Fig. 144. A root cutter and grease scraper in the form of a spiral spring with sharpened edges, and other tools for cleaning sewers are shown in Fig. 145. A turbine sewer cleaner shown in Fig. 146 consists of a set of cutting blades which are revolved by a hydraulic motor of about 3 horse-power under an operating pressure of about 60 pounds per square inch. The turbine is attached to a standard fire hose and is pushed through the sewer by utilizing the stiffness of the hose, or by rods attached to a pushing jack as shown in the figure. This machine was invented and patented by W. A. Stevenson in 1914. Its performance is excellent. The blades revolve at about 600 R.P.M., cutting roots and grease. The revolving blades and the escaping water also serve to loosen and stir up the deposits and the forward helical motion imparted to the water is useful in pushing the material ahead of the machine and in scrubbing the walls of the sewer. In Milwaukee four men with the machine cleaned 319 feet of 12–inch sewer in 16 hours, and in Kansas City 7,801 feet of sewers were cleaned in 14 days.

Sewers large enough to enter may be cleaned by hand. The materials to be removed are shoveled into buckets which are carried or floated to manholes, raised to the surface and dumped. In very large sewers temporary tracks have been laid and small cars pushed to the manhole for the removal of the material. Hydraulic sand ejectors may also be used for the removal of deposits, similar to the steam ejector pump shown in Fig. 97. The water enters the apparatus at high velocity, under a pressure of about 60 pounds per square inch, leaps a gap in the machine from a nozzle to a funnel-shaped guide leading to the discharge pipe. The suction pipe of the machine leads to the chamber in which the leap is made. In leaping this gap the water creates a vacuum that is sufficient to remove the uncemented detritus large enough to pass through the machine, and will lift small stones to a height of 10 to 12 feet. Occasionally barricades of logs, tree branches, rope, leaves, and other obstructions which have piled up against some inward projecting portion of the sewer, must be removed by hand either by cutting with an axe or by pulling them out. Projections from the sides of sewers are objectionable because of their tendency to catch obstacles and form barricades.

Little authentic information on the cost of cleaning sewers is available. A permanent sewer organization is maintained by many cities. The division of their time between repairs, cleaning, and other duties is seldom made a matter of record. From data published in Public Works[107] it is probable that the cost varies from $3 to $15 per cubic yard of material removed. From the information in Vol. II of “American Sewerage Practice” by Metcalf and Eddy the combined cost of cleaning and flushing will vary between $10 and $40 per mile; the expense of either flushing or cleaning alone being about one-half of this.

=203. Flushing Sewers.=—Sewers can sometimes be cleaned or kept clean by flushing. Flushing may be automatic and frequent, or hand flushing may be resorted to at intervals to remove accumulated deposits. Automatic flush-tanks, flushing manholes, a fire hose, a connection to a water main, a temporary fixed dam, a moving dam, and other methods are used in flushing sewers. The design, operation, and results obtained from the use of automatic flush-tanks and flushing manholes are discussed in