Sewer appurtenances are those structures and devices of a sewerage system which are constructed at suitable intervals along a sewer line to assist in the efficient operation and maintenance of the system.
Following are the important sewer appurtenances: 1. Inlets 2. Catch Basins or Catch Pits 3. Clean-Outs 4. Manholes 5. Drop Manholes 6. Lamp-Holes 7. Flushing Devices 8. Grease and Oil Traps 9. Inverted Siphons 10. Storm Water Regulators.
1. Inlets:
Inlets are the devices meant to admit storm water (or rain water) and surface wash flowing along the roads/streets and convey the same to storm water sewers or combined sewers. An inlet is a small box like chamber made of brickwork or concrete, having an opening at the top in vertical or horizontal direction for the entry of storm water (or rain water) and surface wash. The water from this chamber leaves through an outlet provided at its bottom and carried by a pipeline to a nearby manhole.
Inlets are of three types viz.:
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(i) Curb Inlets
(ii) Gutter Inlets and
(iii) Combination Inlets
(i) Curb Inlets:
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Curb inlets (also called vertical inlets) have vertical openings in the road curbs through which storm water flows. The openings are provided with gratings of closely placed bars. These inlets are preferred where heavy traffic is anticipated. The curb inlets are termed as deflector inlets when equipped with diagonal notches cast into the gutter along the curb opening to form a series of ridges or deflectors. The deflector inlet also does not interfere with the flow or traffic as the top level of the deflectors lie in the plane of the pavement.
(ii) Gutter Inlets:
Gutter inlets (also called horizontal inlets) have horizontal openings in the gutter which are covered by gratings through which storm water flows. The clear opening between the bars of the gratings should not be more than 25 mm and the gratings should be capable of sustaining heavy traffic loads.
(iii) Combination Inlets:
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Combination inlets are composed of a curb and gutter inlet acting as a single unit. Normally, the gutter inlet is placed right in front of the curb inlet but it may be displaced in an overlapping or end-to-end position.
Each of the above noted three types of inlets may be either undepressed (or flush) or depressed depending upon their elevation with reference to the pavement surface. The depressed inlets may, however, result in some interference with traffic.
The inlets are usually located by the side of the roads. Maximum spacing of inlets depends on conditions of road surface, size and type of inlet and rainfall. A maximum spacing of 30 m may be adopted for locating the inlets.
Further inlets are also located at intersections of roads, and are so placed that cross walks are not flooded. However, road corners are avoided for this purpose.
2. Catch Basins or Catch Pits:
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A catch basin or catch pit is a device meant for the retention of heavy debris in storm water which otherwise would be carried into the sewer. It is an inlet with its outlet being placed well above its bottom so that heavy debris such as grit, sand, etc., flowing along with storm water is allowed to settle down and thus prevented from entering the sewer.
The outlet from the basin is provided with a hood or it is trapped to prevent escape of foul gases from the sewer and to retain floating matter. At the bottom of the basin space is provided for the accumulation of the settled matter.
Catch basins need periodical cleaning, as otherwise the settled organic matter may decompose, producing foul odours, and may also become a breeding place for mosquitoes.
Catch basins are considered indispensable parts of a combined sewerage system. However, at present these are not considered very essential, as it is more usual to have well-paved roads/streets with little grit, sand or debris to be carried into the sewers by the storm water (or rain water) and to provide such gradients for the sewers that self-cleansing velocities are achieved.
3. Clean-Outs:
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Clean-outs are the devices meant for cleaning the sewers. These are generally provided at the upper ends of lateral sewers in place of manholes. A clean-out consists of an inclined pipe, one end of which is connected to the underground sewer and the other end brought up to ground level. A cover is provided at the top end of the clean-out pipe at the ground level.
For cleaning the sewer the cover of the clean-out pipe is removed and water is forced through clean-out pipe to the sewer to remove obstacles in the sewer. If obstructions are large enough, flexible rod may be inserted through the clean-out pipe and pushed backward and forward to remove such obstacles.
4. Manholes:
Manholes are the masonry or R.C.C. chambers constructed at suitable intervals along the alignment of sewers to provide access to the sewers for the purpose of inspection, testing, cleaning and removal of obstructions from the sewer lines.
They also help in joining sewer lines and in changing the direction or alignment as well as gradient of sewer lines. Manholes may receive sewage from sewers coming from various directions and also from sewers of various sizes. Manholes are usually constructed directly over the centre line of the sewer. They are circular, rectangular or square in shape.
Manholes are provided at every change in alignment of sewers, at every change in gradient of sewers, at every junction of two or more sewers, at head of all sewers or branches, and wherever there is a change in size of sewer. On straight reaches of sewers manholes are provided at regular spacing which depends on the size of the sewers.
The larger the diameter of the sewer the greater may be the spacing between two manholes. Further the spacing between manholes also depends on the type of equipment to be used for cleaning sewers. Table 6.1 gives the spacing of manholes as recommended by the Indian Standard IS: 4111 (Part 1)-1991.
Alternatively the values of spacing of manholes on straight reaches of sewers recommended in the Manual on Sewerage and Sewage Treatment prepared by Central Public Health and Environmental Engineering Organisation are as indicated below:
(i) For sewers of diameter 0.9 to 1.5 m, spacing of manholes above 90 to 150 m may be allowed.
(ii) For sewers of diameter 1.5 to 2.0 m spacing of manholes at 150 to 200 m may be allowed, which may further be increased up to 300 m for sewers of over 2 m diameter.
(ii) In case of very large sewers a spacing allowance of 100 m per 1 m diameter of sewer may be considered as a general rule.
5. Drop Manholes:
A drop manhole is a type of manhole which is constructed on a sewer line where a sewer at a high level is to be connected to another sewer at a lower level.
Such situations may arise as indicated below:
(i) Branch sewers are generally situated at lower depths below the ground level whereas main sewers are laid at greater depths below the ground level. When a branch sewer located at a higher level is to be connected to a main sewer located at a lower level then if ordinary manhole is provided the sewage from the branch sewer will fall from above into the manhole which is not desirable and is to be avoided.
Thus at the junction of branch sewer and main sewer when the difference between the invert level of branch sewer and peak flow level of main sewer is more than 600 mm a drop manhole is provided. The construction of drop manhole permits the sewage from branch sewer to be discharged at the bottom of the manhole without necessitating steep gradient for branch sewer and thus reduces the quantity of earth work.
(ii) In case of ground having steep slope it is not possible to lay the sewer line at a uniform gradient that will not produce scouring velocity. In such cases when a drop of more than 600 mm is required to be given in the same sewer line a drop manhole is provided at suitable place so as to keep the sewer line below ground level and to lay it at a limiting gradient.
As shown in Fig. 6.9 the high-level branch sewer is connected to a low-level main sewer in a drop manhole by forming a ramp by increasing the grade of the last length of the branch sewer to about 45° to the horizontal. This is accomplished by providing a ‘Y’ piece at C as shown in Fig. 6.9. This arrangement is suitable only for small drops. In case of large drops a large amount of cement concrete is required to support the horizontal length of sewer and thus the whole construction becomes costly.
Now-a-days the common practice is to provide a vertical drop pipe (instead of inclined drop pipe) with a bend at the bottom and a ‘T’ piece at the top as shown in Fig. 6.10. The sewage coming from the branch sewer trickles down the vertical pipe and it is discharged at the bottom of the manhole through a horizontal pipe.
When the sewage falls through the vertical pipe it clinges to the sides and thus velocity of fall gets reduced, requiring no water-cushion arrangement at the bottom. The bend at the bottom also minimises the scouring action. However, the bottom of the manhole should be so arranged that the high velocity sewage flow entering the manhole does not splash over the benching.
As shown in Figs. 6.9 and 6.10 the branch sewer is extended to intersect the wall of the manhole. A plug is provided at the end of the extended branch sewer. The length of branch sewer between the vertical pipe and the wall of the manhole is known as inspection arm and after opening the plug it can be used for cleaning and inspecting the branch sewer.
6. Lamp-Holes:
Lamp-holes are small openings provided on sewer lines mainly to permit the insertion of a lamp into the sewer for the purpose of inspection of sewer lines and detecting the presence of any obstructions inside the sewers.
A lamp-hole consists of a vertical stone ware or concrete or cast iron pipe 225 to 300 mm diameter, connected to the sewer line through a ‘T’ junction as shown in Fig. 6.11. The pipe is surrounded by concrete to make it stable. At the ground level the lamp-hole is provided with a manhole cover with frame strong enough to take up the load of traffic.
The manhole cover with frame is carried on a concrete-surround which is separated from the lamp-hole by 15 cm vertical clay joint and thus permits the concrete-surround to bear the traffic load passing on the cover without causing the pipe-column to be subjected to heavy loads resulting in its possible collapse.
A lamp-hole may serve the following purposes:
(i) Inspection:
For inspection of sewer line an electric lamp is inserted in the lamp-hole and the light of the lamp is observed from the two manholes one upstream and the other downstream of the lamp-hole. If the sewer length is unobstructed, the light of the lamp will be seen.
(ii) Flushing:
Under certain circumstances when no other flushing devices are available, lamp-holes may be used for flushing the sewers.
(iii) Ventilation:
If the cover at the top of the lamp-hole is perforated the ventilation of the sewer is affected. Such a lamp-hole is also known as fresh-air inlet.
Lamp-holes are found suitable for use under the following situations:
(1) A lamp-hole may be provided when there is change in the alignment or gradient of the sewer line between two manholes that are a short distance apart.
(2) When the sewer is straight for a considerable distance beyond the usual spacing of manholes a lamp-hole may be provided.
(3) When the construction of a manhole is difficult, a lamp-hole may be provided in the place of manhole.
It may, however, be noted that the lamp-holes have become more or less obsolete and hence at present times these are rarely provided.
7. Flushing Devices:
When sewers are to be laid in a flat country, it is not possible to obtain a self-cleansing velocity even once a day due to flatness of gradient especially at the top ends of branch sewers which receive very little flow.
Similarly near the dead ends of the sewer lines, the self-cleansing velocities cannot be achieved because the discharge coming at the starting point happens to be small. In both these cases the discharge is required to be increased and this is done by adding a certain quantity of water by means of flushing devices. Thus flushing devices help to prevent clogging of sewers and permit the adoption of flatter gradients than those required to maintain self-cleansing velocity.
The various flushing devices may be broadly classified under the following two categories:
(1) Hand operated flushing devices; and
(2) Flushing tanks.
8. Grease and Oil Traps:
Grease and oil traps are the chambers provided on the sewer line to exclude grease and oil from sewage before it enters the sewer line. These are located near the sources contributing grease and oil to sewage, such as automobile repair workshops, garages, kitchens of hotels, grease and oil producing industries, etc.
It is essential to exclude grease and oil from sewage due to following reasons:
(i) If grease and oil are allowed to enter the sewer, they will stick to the inner surface of the sewer and will become hard, thus cause obstruction to flow and reduce the sewer capacity.
(ii) The suspended matter which would have otherwise flown along with sewage will stick to the inner surface of the sewer due to sticky nature of grease and oil, thus further reduce the sewer capacity.
(iii) The presence of grease and oil in sewage makes the sewage treatment difficult as they adversely affect the bio-chemical reactions.
(iv) The presence of a layer of grease and oil on the surface of sewage does not allow oxygen to penetrate due to which aerobic bacteria will not survive and hence organic matter will not be decomposed. This will give rise to bad odours.
(v) The presence of grease and oil in sewage increases the possibility of explosion in the sewer line.
The principle on which grease and oil traps work is very simple. The grease and oil being light in weight float on the surface of sewage. Hence, if outlet draws the sewage from lower level, grease and oil are excluded. Thus grease and oil trap is a chamber with outlet provided at a lower level near the bottom of the chamber and inlet provided at a higher level near the top of the chamber. However, in addition to grease and oil if it is desired to exclude sand, space should be kept at the bottom of the chamber for sand to be deposited.
Fig. 6.14 shows a typical grease and oil trap. It consists of two chambers interconnected through a pipe. The inlet with grating is provided near the top of one of the chambers while the outlet is provided in the other chamber. The end of the outlet is located at a height of about 0.6 m above the bottom of the chamber and it is held submerged.
The wastewater obtained from garages, particularly from floor drains and wash racks, contains grease, oil, sand and mud. To trap all these a combined sand, grease and oil trap is provided which is shown in Fig. 6.15.
These traps should be cleaned at regular intervals for their proper functioning. If this precaution is not taken there will not be free flow of sewage.
9. Inverted Siphons:
An inverted siphon is a section of sewer which is constructed lower than the adjacent sections to pass beneath a valley, river, stream, road, railway and such other obstructions. It runs full at greater than atmospheric pressure because the crown is depressed below the hydraulic grade line.
An inverted siphon differs from a true siphon in this respect that whereas a true siphon flows full with the flow-line above the hydraulic grade line and, therefore, under pressure less than atmospheric, an inverted siphon runs full with the flow-line depressed below the hydraulic grade line and, therefore, under pressure above atmospheric. For this reason inverted siphons are frequently referred to as depressed sewers.
Disadvantages of Inverted Siphons:
The inverted siphon should be adopted only under unavoidable circumstances.
It possesses the following disadvantages:
(1) The down gradient in the siphon pipes is not continuous, and they are likely to get silted if not properly designed to maintain self-cleansing velocity at all flows. When once silted, it is very difficult to remove the silt. Sometimes, a chain extending from inlet chamber to outlet chamber is provided to stir up the deposited silt in the siphon. This will help to keep the silt in suspension.
(2) If the inlet chamber is not properly designed, the floating matter contained in sewage will separate out and it will accumulate in the inlet chamber. The working of inlet, chamber will then be seriously affected and it will result in the inefficient functioning of the inverted siphon.
(3) It is not possible to give side connections to the inverted siphons.
(4) When the inverted siphons are built on or under river beds, it should be seen that they possess sufficient weight or anchorage to prevent their floatation when empty.
10. Storm Water Overflow Devices or Storm Water Regulators:
In the case of combined system of sewerage a large quantity of storm water (or rain water) flows through the sewer along with domestic sewage. Usually it is neither advisable nor practicable, to pump and/or treat such a large quantity of sewage. Moreover, the percentage of domestic sewage in a combined system is very much less, and hence the domestic sewage gets diluted by storm water to a considerable extent.
It is, therefore, possible to divert a portion of this combined sewage and discharge it into a watercourse such as stream, nalla, river, etc., without any trouble. This is achieved by providing storm water overflow devices or storm water regulators in an intercepting sewer so that the outfall sewer will carry only a portion of the combined sewage to the sewage treatment plant.
For example – if the combined sewage is 6 times dry weather flow, 3 times dry weather flow will be taken for treatment as in the case of a separate system of sewerage and the remaining 3 times dry weather flow will be discharged directly into a water course.
Thus storm water overflow devices or storm water regulators are the devices which are provided in a combined system of sewerage to permit the diversion of excess sewage through relief sewer to a watercourse and thus prevent overloading of sewers, pumping stations, treatment plants or of disposal arrangements.
Following are the three types of storm water overflow devices or storm water regulators:
(1) Side flow weirs or overflow weirs
(2) Leaping weirs or jumping weirs
(3) Siphon spillways.