In this article we will discuss about the construction and maintenance of sewers.
Construction of Sewers:
The construction of sewers is started after completing the design work and preparation of all necessary plans.
Before starting the construction the specifications should be finalized, and all the changes and essential features should be noted actual construction consists of the following works:
1. Marking centre lines of sewers and locating the positions of sewer appurtenances.
2. Excavation of trenches including removal of road pavement and disposal of excavated materials.
3. Sheeting, bracing and dewatering of trenches.
4. Laying of pipe sewers and their jointing.
5. Testing of sewer lines.
6. Back filling of trenches.
7. Check for obstruction.
8. Removal of sheeting.
As all the construction works are to be carried out in the city area through streets, safety precautions must be taken for the public so that no accident may occur. Speed, limit boards, safety boards must be displayed at the proper places.
The buildings and other structure, whose foundations are undermined during the excavation work for laying the sewers, must be properly supported by the shores and other necessary arrangements must be made. Temporary railing should be provided all-round the trenches so that vehicle or public may not fall in them. The work should be completed as early as possible, because it will cause nuisance and inconvenience to the traffic.
1. Marking Centre Lines of Sewers and Positions of Sewer Appurtenances:
The centre lines of sewers are marked on the streets and roads from the plans starting from the lowest point or the outfall of the main proceeding upwards. The setting out of work is done by means of chain and theodolite or compass.
For checking the centre line during construction generally wooden pegs or steel spikes are driven at 10 metres intervals on a line parallel to the centre line at such a distance where while laying sewers, they will not disturb them.
For checking the levels of the sewers pipes and their alignment temporary bench marks are established at 200-400 metres intervals. The reduced level (R.L.) of these bench marks should be calculated with respect to a G. T.S. bench marks.
On the centre line of sewers the positions of the sewer appurtenances are also marked as per the plan which have been finalized.
2. Excavation of Trenches:
As in most of the towns the need of sewerage occurs after the development of the town i.e. when its roads, streets are constructed, laying the sewer line is usually done along the sides of the streets or in its middle. Therefore, after marking the layout of the sewer lines on i the ground, the first step is the removal of pavement.
The removal of the pavement is started from the lower end of the sewers and proceeds upwards. As the pavement also helps in supporting the trenches, it should be cut along the edges of the trenches. Pickaxes are used for removing softer pavements while spade or pneumatic drills can be used in case of removing concrete pavements.
After removing pavements, the excavation of trench is started. The excavation of trenches is done manually in India where as in some countries it is done by means of machinery. The width of trench depends on the diameter of the sewer and the depth of sewer-line below the ground level.
For large size sewer, the trench width should be 15 cm more than the external diameter of the sewer for easiness in lowering and adjusting the sewer pipe. The minimum trench width of 60 to 100 cm is necessary for conveniently laying and jointing of even very small size sewers.
Sometimes in case of small diameter sewer, the trench width is kept about 15 cm larger than the sewer dia but at both the end bigger trench is excavated for jointing the pipes as shown in Fig. 8.1.
Fig. 8.1 Excavation of trenches
If the trench has been excavated within one half of the diameter of the sewer pipe from the bottom and in case the soil material is firm, the remaining trench should be excavated in semicircular shape, to conform to the shape of the lower half of the outer side of the pipe. Fig. 8.2 shows the importance of the preparation of trenches for the proper bending of the pipes.
For accommodating the socket end of the pipe or collar end for facility of caulking, the excavation is done more near the joints. The excavation in hard rocks is done by means of pickaxes, crow-bars and by drilling holes side by side and splinting the rocks by driving plugs and feathers.
3. Bracing and Dewatering of Trenches:
In case of hard soils and rocks, the sides of the excavated trench will not collapse and will remain in cut position. But in case of made up soils and soft soils the trench sides require shoring and strutting to prevent their collapse till the sewers are laid and tested.
The following are the functions of the timbering or shoring:
(i) To prevent the collapse of the sides of the trenches.
(ii) To reduce the width of the trench at the top to the minimum possible.
(iii) To prevent the seepage of ground water into the trench.
Various methods of shoring and bracing of trenches have been shown in Fig. 8.3, 8.4 and 8.5.
Following thumb-rules will help in deciding the size and spacing of the timber sheeting and bracing for supporting trench walls:
(a) When the trench is 1.2 to 2.0 m deep timber piling (polling boards) about 5 cm thick are provided. If the depth is more than thickness of polling boards may be kept 7.5 cm
(b) For the trenches more than 2.5 m in width and more than 1.5 m deep, in hard soil, it may be bracked with walling of 5 x 15 cm size.
(c) Both the trench walls should be supported properly by means of walling and struts. When the width of the trench varies from 30 to 240 cm the strut size may vary from 10 x 75 cm to 15 x 15 cm. The struts should be provided at about 1.25 m. vertical spacing on the walling.
When sewer lines are to be laid below the ground water table, the ground water enters the trenches during excavation and causes much difficulties. Therefore, the dewatering of trenches is compulsory under such circumstances.
There are various methods for the removal of this water, but most common are:
(A) Gravity Method:
In this method, the excavation is started from the lowest level and is done upwards so that whatever water enters the trench, it automatically flows towards outlet due to gravity.
(B) Pumping Method:
In this method during excavation the amount of water entering the trench is pumped outside the trenches. In some cases the level of water table is depressed by driving driven wells along trenches and pumping the ground water.
At some places porous pipe is laid below the main sewer line to collect the ground water entering in the trench. This porous pipe carries the seepage water to the water courses. At the sewer construction work is started from the lower level, the water entering the trench must be carried by the sewer constructed. But even if it is not possible to convey the seepage water through the sewer under construction, it may be pumped out.
4. Laying of Sewers and their Jointing:
Trenches are excavated with proper grade so that the sewage may flow in sewers due to gravitational flow only.
Sewer pipes may have to be laid under the following conditions:
(a) Culvert Condition:
When the pipe is laid under embankment and it projects wholly or partly above the original surface or subgrade.
(b) Trench Condition:
When the pipe is laid in a trench excavated for the purpose.
(c) Negative Projecting Conditions:
When the pipe is laid in a relatively narrow and shallow trench in such a manner that the top of the pipes is at an elevation below the natural ground surface.
(d) Open Condition:
In this condition, the pipe is laid such that it projects wholly or partly above the ground surface.
When a sewer has to be laid in a soft underground strata or in a reclaimed land, the trenche shall be excavated deeper than what is ordinarily required. The trench bottom shall be stabilised by the addition of course gravel or rock; in case of very bad soil the trench bottom shall be filled in with cement concrete of appropriate grade.
In areas subject to subsidence, pipe sewer shall be laid on a timber platform or concrete cradle supported on piles.
In the case of case-in-situ sewers, and R.C.C. section with both transverse and longitudinal steel reinforcement shall be provided when intermittent variations in soil bearing capacity are encountered. In case of long stretches of very soft trench bottom, soil stabilization shall be done either by rubble, concrete or wooden crib.
The sewer pipes are not usually laid directly on the soil in the trenches. Before actual laying the bottom of the trench is prepared to receive the pipe such that the load is distributed uniformly. It is always preferred to provide concrete bedding in the trench below the sewer pipes.
Fig. 8.6 shows various types of pipe beddings usually provided under various conditions:
The centre line of sewers and their grades are transferred from the ground by means of sight rail and boning rod as shown in Fig. 8.7 by the following method:
1. Four stout stakes are driven into the ground or fixed over the pillars.
2. Horizontal boards called sight rail are fixed on the stakes spanning the trench.
3. The centre line of sewer is marked on the sight rail and small nails are fixed on the sight rails at the position of centre line.
4. The top of the nail or sight rail is fixed at some fixed distance from the invert level of the sewer at that spot. The line joining the top of nails fixed on the sight rails also conforms to the grade of the sewer.
5. Sight rails are usually fixed at 7.5 m centre to centre spacing and also at all junctions and change of gradient or alignment.
6. Now a strong cord or wire is stretched between the nails fixed on sight rails. This line is parallel to the grade of the sewer and also lies in the vertical plane passing through the centre line of the sewer.
7. Now with the help of Boning rods using plumb bob the line and grade to the sewer line is given as shown in Fig. 8.7.
Smaller size pipes can be laid by the pipe-layers directly by hand only, but heavier and larger size pipes are lowered in the trenches by passing ropes around them and supporting through a hook.
It is the common practice to lay the pipes with their socket end upgrade for easiness in jointing. After lowering the pipes; these are brought near the spigot end of one pipe is placed in the socketed end of the other. After properly placing and arranging the pipes they are suitably joined. The joints are carefully cured for sufficient time.
Large size sewers are constructed at the site itself by R.C.C. or brick masonry. The common material used for the construction is R.C.C. The most common sewer sections are circular, egg-shaped, horse-shoe, catenary and Basket handled etc.
When large size sewers are to be constructed at greater depth more than 9.0 m., it is more economical to do their construction by tunnelling than open excavation. The various methods of tunnelling have been dealt in ‘Roads, Railway, Bridge and Tunnel Engineering’ book by Ahuja-Birdi.
Jointing of Sewers:
a. Stoneware pipes:
All the pipe joints shall be caulked with tarred gasket in one length for each joint and sufficiently long to entirely surround the spigot end of the pipe. This gasket shall be caulked lightly home but not so as to occupy more than a quarter of the socket depth.
The gasket shall then be filled with a mixture of one part of cement and one part of clean fine sand mixed with just sufficient quantity of water to have a consistency of semi-dry condition and a fillet shall be formed round the joint with a trowel forming an angle of 45° with the barrel of the pipe (IS: 4217) Rubber gasket may also be used for jointing.
b. Concrete pipes:
The collars shall be placed symmetrically over the end of two pipes and the annual space between the inside of the collar and the outside of the pipe shall be filled with hempyarn soaked in tar or cement slurry tamped with just-sufficient quantity of water to have a consistency of semi-dry condition, well packed and thoroughly rammed with caulking tools and then filled with cement mortar 1:2.
The joints shall be finished off with a fillet sloping at 45° to the surface of the pipe. The finished joints shall be protected and cured for at least 24 hours. Any plastic solution or cement mortar that may have squeezed in the pipe shall be removed to leave the inside of the pipe perfectly clean.
c. C.I. pipes:
The C.I. pipes shall be examined for line and level and the space left in the socket shall be filled in by pouring molten pig lead. This shall be done by using proper leading ring. One or two air vents shall be provided around the lower end of the joint. The lead used shall be soft and of best quality conforming to IS: 782. The quantity and depth of lead to be used per joint as well as general procedure for joining shall be as per IS: 3114.
In special cases rubber or plastic rings may be used.
5. Hydraulic Testing of Pipes Sewers:
Following two tests are done for testing the pipe sewers:
(a) Water Test:
Each section of the sewer is tested for water- tightness preferably between manholes. To prevent change in alignment and disturbance after the pipes have been laid, it is desirable to backfill the pipes up to the top, keeping at least 90 cm length of the pipe open at the joints. But in case of shorter pipe lengths of stoneware and R.C.C. pipes, it is not possible. With concrete encasement or concrete cradle, partial covering of the pipe is not necessary.
In the case of the concrete and stone-ware pipes with cement mortar joints, the testing shall be done after making the joints. It is necessary that the pipelines are filled with water for about a week before commencing the application of pressure to allow for the absorption by the wall of the pipe.
The testing of the sewers is done by plugging the upper end with a provision for an air outlet pipe with stopcock. The water is filled through a funnel connected at the lower end provided with a plug. After expelling the air through the air outlet, the stopcock is closed and water level in the funnel is raised to 2 to above the invert at the upper end. Water level is noted after 30 minutes in the funnel and the quantity of water required to restore the original water level in the funnel is determined.
The pipe line under pressure is then inspected while the funnel is still in position. There should not be any leaks in the pipe or the joints except small sweating on the pipe surface which is allowed. Leakage in 30 minutes determined by measuring the replenished water in the funnel should not exceed 15 ml in the smaller dia. and 60 ml in the larger dia. per cm dia. of pipe for 100 m length. Any sewer or part that does not meet the test shall be emptied and repaired or re-laid as per requirements and tested again.
In case of concrete, R.C.C. and A. C. pipes of more than 600 mm dia the quantity of water in flow can be increased by 10% for each additional 100 mm of pipe dia. For brick sewers, regardless of their dia. the permissible leakage of water shall not exceed 10 m3/24 hrs. per km. pipe length.
(b) Air Testing:
This testing is done in large dia. pipes when the required quantity of water is not available. It is done by subjecting the stretch of pipe to an air pressure of 100 mm of water by means of a hand pump. If the pressure is maintained at 75 mm, the joints shall be assumed to be watertight.
In case the drop is more than 25 mm the leaking joints shall be traced and suitably treated to ensure water-tightness. The exact point of leakage can be detected by applying soap solution to all the joints in the line and looking for the air bubbles.
6. Back-Filling of Trenches:
After testing and removing defects of pipe line, the trenches are back-filled with earth. Generally the excavated soil of trench is used for back-filling but before using it, the pebbles, stone-pieces and lumps must be removed from it. The back-filling is not done at a time. First the back filling is done by ramming the soils in layers, using water for proper consolidation.
When the height of the back-filled rammed soil reaches 60 cm above the crown of the pipe, the back-tilling is stopped for at least one week for weathering. After a week, again the backfilling is started in layers and the trench is filled 15 cm above the ground level. During the course of time back-filled soil gets compacted and the filled soil comes to the ground level.
The back-filling is not done immediately after construction of the sewer lines. It is done after 7 days for precast pipes and after 14 days in case of cast-in-situ after casting the sewer. Tamping should be done carefully when doing it near the crown of the sewer.
Reinstatement of the pavement is carried out after about two months after the proper consolidation of the back-fill material and there is no danger on risk of crank or settlement in the pavement.
7. Check for Obstruction:
Immediately after laying and testing the sewer, a double disc or solid or closed cylinder, 75 mm less in dimension than the internal dimension of the sewer, is run through the stretch of the sewer to ensure that it is free from obstruction.
8. Removal of Sheeting:
Sheeting driven below the spring line of a sewer shall be withdrawn a little at a time as the back-filling progresses. Some of the back-filled earth is forced into the void created by withdrawing the sheeting by means of a water jet.
To avoid any damage to buildings, cables, gas mains, sewers, water mains, telephone cables etc. near the excavation or to avoid disturbance to the sewer already laid portions of the sheeting may be left in the trenches and buried.
Maintenance of Sewers:
Maintenance of sewers consists mainly of the removal or prevention of stoppages, cleaning of sewers and other sewer appurtenances, and repair works. Maintenance of sewers becomes costly only when they are laid on flat gradients and tree roots find easy entrance in sewers through defective joints. The maximum expenditure in maintenance comes on the cleaning of sewers, which have been clogged due to deposition of silt, grease and oily materials.
For good maintenance up-to-date plans of sewers system showing location of manholes, and other appurtenances, direction of flow, house sewers, grades of sewer lines etc. are necessary. Before actual cleaning and repair work the inspection is done. In some cities inspections are made and maintenance is done only when difficulties arise, whereas in other cities periodical inspection is done.
The period of inspection is generally as follows:
Sewers on flat grades – 3 months
Sewers troubled by roots – 3 months
Sewers having no trouble – 6 to 12 months
Intercepting sewers – 7 to 30 days
Flushing tanks – 1 month
Inverted siphons – 7 to 30 days
Storm water overflows – during rains.
During inspection the clogging of sewers, breakage of pipes etc. are noted and then the cleaning and repair works are done later on. In few cities trained gangs are given the duties of inspection and maintenance.
These gangs follow on inspection routine and cleaning and repairs according to the prearranged schedule. During inspection an explosion-proof lamp is lowered in the manhole and is seen from the manhole on either side to see if the sewer line is clean or not.
Causes of Damage to Sewers:
Following are the main causes of damage to the sewers:
(i) Bad workmanship and use of low specification material.
(ii) Faulty design.
(iii) Excessive superimposed load.
(iv) Settlement due to loose foundation or made up ground.
(v) Too small cover at the top of sewer, which is insufficient to protect it from the impact and other loads, which sewer has to bear.
(vi) Undermining of sewer due to any reasons.
(vii) Explosion inside the sewer due to improper ventilation of the explosive gases inside the sewer.
(viii) Deterioration of the sewer due to corrosive gases.
(ix) Abrasion of sewer due to frictional movement of the solids present in the sewage.
(x) Old age of the sewer.
Problems in Sewer Maintenance:
Following are the main problems which are faced in maintenance of sewers:
(a) Clogging in Sewers:
Following factors are mainly responsible for the clogging of sewers:
(i) deposition of grit or other detritus which causes stagnation, resulting in the putrefaction of organic matter giving rise to odours and poisonous gases; deposition of grease from hot liquid wastes from kitchen finding entry into sewers, getting cooled and deposited on the sides which in course of time clog the sewer;
(ii) Penetration of roots from nearby trees through the joints or cracks in the sewer which choke up the sewers;
(iii) Deposition of tarry materials assisting in binding the detritus and leading to their growth;
(iv) Growth of fungi forming a network of tendrils, which starts floating and obstructing the free flow of sewage;
(v) Stagnation of sewage in the sewers due to improper working of pumping units leading to settlement of grit and other materials and dumping of solid wastes in the manhole indiscriminately.
The persons engaged in the operation and maintenance of sewerage systems are exposed to different types of occupational hazards like physical injuries, injuries caused by chemicals and radioactive wastes, infections caused by pathogenic organism present in the sewage and dangers inherent with explosive or noxious vapours and oxygen deficiency.
The health and safety of personnel can be safeguarded to a great extent by taking the likely hazards into consideration at the time of designing the sewers, sewers appurtenances and pumping stations. During maintenance the still possible hazards can be reduced by using safety equipment and precautions appropriate for each hazardous condition. Even then the maintenance work should be supervised by the competent personnel for preventing accidents.
‘Sewer gas’ is a mixture of gases in sewers and manholes containing abnormally high percentage of carbon-dioxide, varying amount of methane, hydrogen, hydrogen sulphide and low percentage of oxygen caused by septic action through the accumulation of organic matter inside the sewer.
The main hazard is due to the presence of high levels of methane forming an explosive mixture or the oxygen deficiency or hydrogen sulphide in excess of permissible levels. Some trade wastes also contribute to other gases like chlorine, ammonia, sulphur dioxide etc.
A noxious gas or vapour which is directly or indirectly injurious or harmful to the health or life. It can be a simple asphyxiant, chemical asphyxiant, irritant, volatile solvent or the combustible gas.
Simple asphyxiants are the physiologically inert gases like nitrogen, methane and hydrogen which when breathed in high concentration act mechanically by excluding oxygen. Chemical asphyxiants are substances like carbon monoxide which by combining with the hemoglobin of the blood or with some constituents of the tissues either prevent oxygen from reaching the tissues or prevent the tissues from using it.
Irritants are the substances like chlorine which injure the air passage and lungs and induce inflammation in the surface of the respiratory tract. Volatile solvents and drug like substances exert little or no effect on the lungs but affect the nervous system including anaesthesia.
Combustible vapours burn as long as they are in contact with a flame, spark or a heated material having a temperature equal to or greater than ignition temperature of the vapour, provided there is sufficient oxygen present for combustion.
When the sewer or a manhole has to be entered for cleaning or clearing an obstruction, it is advisable to go in for efficient ventilation either natural or forced and get assured that the atmosphere inside the manhole or sewer is free from oxygen deficiency and noxious gases or vapours.
When clearance of the inside atmosphere is not possible, or time-consuming, following precautions should be taken before entering the sewer:
(i) Traffic warning signs should be placed on road.
(ii) No smoking or open flames should be allowed and sparks should be guarded.
(iii) Only safety, explosion-proof electric lighting equipment or mirrors for reflection of light should be used.
(iv) The atmosphere should be tested for the presence of noxious gases and oxygen deficiency.
(v) When atmosphere is normal, the worker may enter in the manhole or sewer, with safety harness attached with two men available at the top.
(vi) In case oxygen deficiency or noxious gas is detected, forced ventilation should be resorted to using a portable blower.
(vii) Frequent tests for oxygen deficiency and noxious gas should be done, even if the initial tests are satisfactory, because conditions may change during the working period inside the manhole and sewers.
(viii) If forced ventilation is not possible or not satisfactory and men have to enter urgently, such as in case of saving the life of fallen persons, a gas mask should be worn and extreme care should be taken to avoid all sources of ignition if inflammable gas is present. Only permissible safety lights, rubbers or non-sparking shoes and non-sparking tools should be used.
(ix) Only experienced personnel having experience of working under such conditions and fully equipped with the proper protective safety equipment should be allowed to enter the sewers.
Following precautions should be taken for the personnel working in sewerage maintenance system which are prone to infections:
(i) Emergency first aid kit should be provided to take care of all minor injuries, cuts and burns, to the gang.
(ii) In case of emergency the services of a doctor should be available.
(iii) All the concerned workers should be educated about the hazards of water-borne diseases through sewage and tetanus through cuts and wounds.
(iv) The importance of personal hygiene should be emphasized and the workers should be instructed to keep finger nails short and well-trimmed, not to brush fingers when they are sore, wash hands with soap and hot water before taking food, keep the fingers away from nose, mouth and eye.
(v) Rubber gloves should be used so that sewage may not come in direct contact of the hands.
(vi) Workers should be provided with complete change of work clothes to be worn during working. Workers should also use gum-boots.
Following are the common safety equipment’s, which are used by the workers connected with sewer maintenance works:
(i) Gas Masks:
In general purpose gas masks are used for respiratory protection from low and moderately high concentrations of all types of toxic gases and vapours present in the atmosphere in which there is sufficient oxygen to support life.
Masks afford necessary respiratory protection under many circumstances, but the various limitations of these gas masks should be known to the workers using them. In the general purpose gas masks provide protection against organic vapours, acid gases, carbon monoxide upto 2% concentration, toxic dusts, fumes and smoke.
The gas mask mainly consists of a face piece, a canister containing purifying chemicals, a timer for showing duration of service and a harness for support. Protection against specific contamination can be achieved by the selection of the appropriate canisters. Gas masks cannot be used in oxygen deficient atmosphere or in unventilated location or areas.
(ii) Oxygen Breathing Apparatus:
This apparatus is used for respiratory protection from atmosphere containing high percentage of toxic gases and vapour which are deficient in oxygen. This apparatus fully protects the workers against all gases, vapours, dusts, fumes, smokes and oxygen deficiencies in atmosphere including petroleum vapours. This is a dependable device. The apparatus may be of air hose respirator type or a pure oxygen respirator.
(iii) Portable Lighting Equipment:
Portable electric hand lamps of permissible type, electric cap lamps and explosion proof flash lights are used while doing work inside sewer lines.
(iv) Portable Air Blowers:
Forced ventilation of manholes, pits and tanks can be provided by portable air blowers powered by enclosed explosion proof electric motors. Special precautions are required to check that blowers do not become the source of ignition for inflammable gases. Trailer mounted blower having a capacity of 210 m2/min can easily ventilate many metres of medium sized sewer.
(v) Non-Sparking Tools:
Non-sparking tools to be used during sewage maintenance works are made of an alloy (containing at least 80% of copper) that will not spark, when struck against other objects and metals and yet have the required strength and resistance to wear and tear.
Approved inhalators employing a mixture of oxygen and carbon dioxide are used for resuscitating victims of collapse, drowning or electric shock. Artificial respiration should be started at once on the patient and an inhalator face piece attached to the victim’s mouth as soon as the equipment can be made ready.
The carbon dioxide used in small percentages stimulates deep breathing so that more oxygen may be inhaled. Pure oxygen should be used only when irritant gases such as hydrogen sulphide or chlorine have caused the victim’s collapse.
(vii) Safety Belt:
This belt consists of a body belt with a buckle and a shoulder harness. The life line is of high grade spliced manila rope or a steel cable anchored with rings, on each side of the belt and provided with safety straps for anchoring or securing to a stable support.
The life line should be 15 m in length and the overall assembly should be capable of withstanding a tensile load of 2000 kg. The safety belt and life line should be daily tested before use.
The clogging of the sewers can be prevented by periodic cleaning and removal of silt accumulations from sewer lines, including manhole repairing, during the functioning of the sewerage system.
Each sewer maintenance gang should be under the supervision of a competent person well trained in the use of all the necessary tools, machines, safety equipment’s and qualified to give first-aid-service also.
The covers of all manholes immediately upstream and downstream of the particular manholes into which men are to enter should be removed and kept open for at least half an hour, for natural ventilation. Necessary protective barricades and boards should be provided on the road to prevent entry of the traffic in the area.
When the foul atmosphere is cleared from the sewers, the workers may enter the sewer. If the natural ventilation is not sufficient and the foul atmosphere is not cleared, forced ventilation should be provided. Ordinarily the flexible sewer rod with manila rope is used for sewer cleaning.
The composite flexible rod composed of rope tied together with bamboo strips is lowered inside the manhole by a person on top, while another worker inside the manhole thrusts the same into the sewer in the direction of flow.
The worker inside the next downstream manhole receives the rod and pushes it out of the manhole. As soon as the end of the 60 m rod is thrust into the sewer, it is connected to a thick manila rope which is dragged and kept coiled on the surface near the downstream manhole.
When the rope is dragged through the sewer, the silt is drawn out into the downstream manhole from where it is taken out by buckets. Now this process is repeated for the next section of the sewer. This process is continued till the sewer line is cleaned. With the cleaning of the sewer other repairs, if any, to the inside of the manhole, the footsteps, cover etc. are also done simultaneously.
Depending upon the local requirements the periodical repair and cleaning programme for all the sewers in the system is prepared and followed. The frequency of cleaning depends on the nature of sewage flowing in the particular section. Flushing of sewers is also carried out periodically to clear laterals and sewers laid within sufficient slope for maintaining a velocity so as to remove the settled material.
Cleaning of Catch Basins:
All the catch basins should be regularly cleaned by removing the silt and mud from their basins. The responsibility of this cleaning generally lies with the street sweepers.
During inspection the portions of the sewers which are to be repaired are noted and the repairs are done to prevent the further deterioration and damage of the sewers.
Mostly the following repairs are required:
(i) Large brick-sewers will require pointing up of joints and replacement of fallen out arch bricks.
(ii) Plastering and pointing of manholes.
(iii) Replacement of manhole frames and covers that have broken or worn out and have become noisy.
(iv) Replacement of cracked and crushed portion of the pipe.
(v) Lowering or raising the manhole heads so that they may remain flush with the road level.
(vi) Raising or lowering of manhole heads with change in the surface of the roads and streets.
(vii) Tightening of loose manhole covers that give noise under movement of the traffic.
(viii) Replacement of the broken pipes.
(ix) Reconstruction of damaged house connections.
The new connections are also made to the sewer lines from the newly constructed buildings.
Explosions in Sewers:
Explosions in sewer lines may occur and the manhole covers may be blown into the air if the sewers are not properly ventilated. Combustible gases are produced inside the sewers due to the presence of gasoline, grease solvents, illuminating gas, methane gas or naptha. Gasoline comes from the discharges of garages, filling station and chemical industries. Miscellaneous chemicals such as calcium carbide may also cause explosions inside the sewers.
The following precautions should be taken to prevent the explosions in sewers:
(i) The workmen should open the manhole covers at least one hour earlier before they enter in it.
(ii) Workmen before entering in the manhole first lower a burning candle inside the manhole to check the presence of explosive gases.
(iii) Only dry-cell torches or electric lamps should be used inside the sewer line for light purpose.
(iv) The presence of hydrogen sulphide gas is detected by lowering a wet lead acetate paper inside the manhole.