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A. DRAINAGE & SEWERAGE 601 General Reference may be made to the following for provisions not contained in this Chapter. In case of any variation, the latter shall prevail. (i) National Building Code of India 1983 - Part IX Plumbing Services, Drainage and Sanitation. (ii) Relevant BIS Codes as mentioned below :- IS 1172: 1983 Code of basic requirements for water supply drainage and sanitation. IS 1726: 1974 Specifications for cast iron manhole covers and frames. IS 1742: 1983 Code of practice for building drainage. IS 2470: 1985 Code of practice for installation of septic tanks. IS 4111: 1986 Code of practice for ancillary structures in sewerage system. IS 5329: 1983 Code of practice for sanitary pipeworks above ground for buildings. (iii) Manual on Sewerage and Sewage Treatment - Ministry of Urban Development, New Delhi. (iv) Regulations issued by Municipal Corporations or local Public Health Departments should also be referred. 602 Preparation of Schemes of Sewerage Definitions of some common terms are given below:- (i) Drain: A conduit/channel for the carriage of storm water or other used water (ii) Drainage: A system for removal of any unwanted liquid (iii) Sewer: A pipe/conduit used generally for carrying sewage or other water-borne wastes (iv) Sewerage: A system for the collection and conveying sewage to the point of disposal (v) Manhole: An opening by which a man may enter and leave a drain, a sewer or other closed structure for inspection, cleaning and other maintenance operations fitted with a suitable cover (vi) Soil Waste: The discharge from water closets, urinals, sinks, cattle sheds, gullies etc. (vii) Sewage: It includes all kinds of liquid wastes of a community or industry. It is classified as follows: - B. WATER-BORNE SEWERAGE 603 Adherence to Regulations In the preparation of under-ground sewerage schemes, the Drainage Regulations in regard to the installations of all sanitary equipments and construction of drains and sewers shall be followed. 604 Sewerage Schemes Reports of sewerage schemes should include- a) a historical retrospect leading to the justification of the project b) description of existing facilities, if any c) population studies, analysis and prediction based on a critical appraisal of available data d) location of the water supply headwork for the area and the neighbouring communities e) availability of piped water supply in the area, present and anticipated per capita water supply f) the nature of sewage, total sewage flow - present and prospective g) discussion of different possible disposal points and their comparative merits h) review of different possible methods of treatment i) engineering features and economical aspects of the sewer system, an index and a general layout plan with contours at intervals of 2m along with a schematic diagram. Sub-soil conditions including level of water table and location of other services - their positions, size etc. basis for computation of surface run-off, drainage area and silting of storm sewage and overflow, wherever necessary j) number and location of pumping stations with hydrostatic heads and frictional loss of heads on pumps and on mains k) a comparison of total costs (recurring and non-recurring) of the alternatives involved under major subheads l) the most satisfactory alternative scheme for the project, as recommended probable stages of construction, procurement of proprietary material for the project and any special problems relating thereto 605 Designing of Sewers Sanitary sewer system and storm water drainage system should be designed separately. The size of sanitary sewer depends on quantity of sanitary sewage, variation in peak and average flows, limiting velocities of flow, topography of the area, construction material etc. The size of storm water sewer depends on storm water run-off, Self Cleansing Velocity, construction material etc. Sanitary sewers are not expected to receive storm water. It should be ensured that proper design and construction of sewers and manholes are provided. 606 Quantity of Sewage The factors which influence the quantity of sewage are population and per capita sewage. Population estimates should cater for future requirements. a) Generally 80% of the water supplied may be considered to reach the sewers. b) The lean and peak flows of sewage will be about 50 and 150 percent of the average respectively. The sewer should be designed for the maximum flow rate, after allowing for seasonal fluctuations. For design purposes standard tables 607 Alignment of Sewer The alignment of the sewer is determined from a survey plan which should be prepared to a scale of 100 metres to a cm. The proposed sewer-lines are shown on a plan along with the ground levels and soil conditions, necessary for establishing grades and estimating the cost of the work. In the profiles, the invert and crown gradient lines should be shown. 608 Velocity of Flow - The limiting velocities for existing conditions should be determined and gradients decided accordingly. Normally, a minimum velocity of 1 metre per second and a maximum of 2 metres per second, are allowed. On steep slopes, one or more drop manholes may be provided. a) Self Cleansing Velocity - It is necessary to- maintain a minimum velocity or self-cleansing velocity in a sewer to ensure that suspended solids do not deposit and cause blockage. Self cleansing velocity is determined by considering the particle size and specific weight of the suspended solids in sewage. A minimum velocity of 0.8 mps at design peak flow in the sanitary sewers is recommended subject to a minimum velocity of 0.6 mps for present peak flow. b) Velocity at Minimum Flow-To avoid steeper gradients which will require deeper excavations, it is the practice to design sewers for the self cleansing velocity at ultimate peak flows. This is done on the assumption that although silting might occur at minimum flow, the silt would be flushed out during the peak flows. However, the problem of silting may have to be faced in the early years, particularly for smaller sewers which are designed to flow half- full, as the actual depth of flow then is only a fraction of the full depth. Transport of suspended solids should be achieved in such a manner that deposition and odour nuisance therefrom are kept to a minimum. Sewers are normally designed for flows with free water surface and self-cleansing velocities. Pressure sewers, including siphons, should be avoided as far as practicable. c) Maximum Velocity - Erosion of sewers is caused by sand and other gritty material in the sewer at excessive velocity. Velocity of flow in a sewer is recommended not to exceed 2.0 mps. 609 Ventilation in Sullage From consideration of ventilation in wastewater flow, sewer should not be designed to run full. Sewers upto 400 mm dia may be designed to run at half depth, 400-900 mm diameter at two third depths and larger sewers at three fourth depth at ultimate peak flows. 610 Size of Sewer Minimum diameter of sewer shall be 200 mm except for hilly areas where steep slopes are available. In those areas minimum size may be 100 mm. 611 Storm Water Quantity of storm run-off is dependent on intensity and duration of rainfall, characteristics of catchment area and time required for such flow to reach the sewer. Storm water flow for this purpose may be determined by using rational methods, hydrograph methods or empirical formulae. 612 Manholes - A manhole is an opening constructed on the alignment of a sewer to facilitate a person to access the sewer for the purpose of inspection, testing, cleaning and removal of obstructions from the sewer line. According to IS:4111 - spacing of manholes is designed with reference to sewer diameter as given below: Sewer dia in mm Upto 300:301 to 500:501-900 Spacing in metres 45 ' 75 90 When silt and grit loads are heavy, catchpits may be provided at suitable intervals depending on local conditions and as approved by the Divisional Engineer. 613 Choice of Material for Sewers a) Salt glazed stoneware pipes - For all sewers and drains in all soils except where supports are required, glazed stoneware pipes shall be used. They are particularly suitable where acid effluents or acid sub-soil conditions are likely to be encountered. b) Cement Concrete Pipes - These can be used as an alternative to glazed stoneware pipes of over 150 mm diameter. These shall not be used to carry acid effluents or sewage under conditions favourable for the production of hydrogen sulphide and shall not be laid in those sub-soils which may affect the strength of concrete. c) Cast Iron Pipes - These shall be used in unstable or made-up grounds or where sewer is laid at insufficient depth. These are particularly useful where pipes are suspended in basements or similar situations or for crossings of water courses. These pipes are suitable where the velocity is more than 2.4 m/s. d) Asbestos Cement Pipes - These pipes are commonly used for house drainage systems and are not recommended for underground situations. They are not suitable to carry acid effluents or sewage under conditions favourable for the production of hydrogen sulphide. e) PVC Pipes - Unplasticised PVC pipes may be used for drainage purpose. 614 Laying of Sewer Pipes a) Excavation - The safety precautions required as per construction practices shall be ensured. (i) Excavation shall proceed to within about 7.5 cm of the finished formation level. This final 7.5 cm is to be trimmed and removed as a separate operation immediately prior to the laying of the pipes or their foundations. (ii) Unless specified otherwise by the Authority, the width at bottom of trenches for pipes of different diameters laid at different depths shall be as given below: 1) For all diameters, upto an average depth of 120 cm, width of trench in cm = diameter of pipe+30 cm 2) For all diameters for depths above 120 cm, width of trench in cm = diameter of pipe+40 cm. 3) Notwithstanding (1) and (2), the total width of trench at top should not be less than 75 cm for depth exceeding 90 cm. iii) All pipes, ducts, cables, mains or other services exposed in the trench shall be effectively supported by timber and/or chain or rope-slings. iv) All drainage sumps shall be sunk clear of the work outside the trench or at the sides of manholes. After the completion of the work, any pipes or drains leading to such sumps or temporary sub-soil drains under permanent work shall be filled in properly with sand and consolidated. b) Laying of Pipes - Laying of pipes shall be done in accordance with good practice i.e. unloading of pipes should be by using mechanical means to avoid damages, adjusting to proper levels, minimum thickness of lead at the face of socket not to reduce less than 6mm, alignment to be repeatedly checked and refilling of trenches etc. c) Jointing - All soil pipes, waste pipes, ventilating pipes and other such pipes above ground, shall be gas-tight. All sewers and drains laid below the ground shall be water tight. Jointing shall be done in accordance with specification prescribed for particular type of pipes. d) Connection to Existing Sewers - (i) The connection to an existing sewer shall, as far as possible, be done at the manholes. The work of connection with the existing sewerage system shall be carried out by the Authority owning the system or under its supervision. ii) Connection with the sewer shall be affected by the cautious enlargement of a small hole and every precaution shall be taken to prevent any material from entering the sewer. No connection shall be formed in such a way as to constitute a projection into the sewer or to cause any diminution in its effective size. e) Back-Filling - (i) Filling of the trench shall not be commenced until the length of pipes therein has been tested and passed (ii) All timber may be withdrawn safely as filling proceeds. (iii) Where the pipes are unprotected by concrete haunching, the first operation in filling shall be carefully done to hand-pack and tamp selected fine material around the lower half of the pipes so as to buttress them to the sides of the trench. (iv) The filling shall then be continued to 15 cm over the top of the pipe using selected fine hand-packed material, watered and rammed on both sides of the pipe with a wooden hammer. On no account shall material be tipped into the trench until the first 15-cm of filling has been completed. The process of filling and tamping shall proceed evenly so as to maintain an equal pressure on both sides of the pipeline. (v) Filling shall be continued in layers not exceeding 15cm in thickness, each layer being watered and well rammed. (vi) In roads, surface materials previously excavated shall be replaced as the top layer of the filling, consolidated and maintained satisfactorily till the permanent reinstatement of the surface is made. (vii) In gardens, the top soil and turf, if any, shall be carefully replaced. f) Pipes should preferably be laid with the socketted ends against the direction of flow and in straight lines. g) In manholes, no pipes should project. Pipes should be laid flush with the inside face of the manhole walls. Where a pipe passes through the manhole wall, an arch should be built round the pipe to carry the weight of the masonry above it. 615 Sewage Disposal Where water-borne sewerage is not possible, septic tank system of disposal of sewage is normally adopted. 616 Septic Tanks Modern septic tank system is an on a site disposal method, which uses standard flushing. The septic tank acts as sedimentation-cum-digestion tank. Anaerobic digestion of the settled sludge occurs in its bottom zone and the supernatant liquid has to undergo treatment in a soak pit/filter bed. Use of septic tank without follow-up treatment is not permitted, as the effluent from the septic tank is hazardous from the point of view of health and pollution. Since, it is usually not possible to provide soak pit/filter bed in built up urban areas, septic tank system would not be appropriate in such areas. The capacity of the septic tank should be such as to take care of the variations, in the flow. A detention period of 24 to 48 hrs. for tanks serving less than 50 persons and a period of 10 to 18 hrs. for tanks serving more than 50 persons are generally adopted. The size and shape of the septic tank shall be generally in agreement with the minimum dimensions given in the table below:- Table 6.1
Septic tanks should be cleaned when a large quantity of sludge has collected in the bottom of the tank. The interval of cleaning should not normally exceed 12 months. After cleaning 3 or 4 shovelful of surface earth containing grass roots and decaying vegetable matter should provide a good start. No disinfectants should be used in latrines attached to septic tanks as they kill the organisms, which digest sewage. For providing any soakpit/filter bed with septic tanks the water table must be well below their depth and the rain water from surface also should not interfere with their functioning. Hence, these are not usually suitable where water tables are high or where ground slopes are flat. 617 Aqua Privy Latrines The latrine will usually consist of an under ground masonry tank on top of which are fixed the latrine pans. The outlet of the pans are dipped 7.5 to 10 cm below the flow line in the tank. The pans may be fixed with or without traps to the outlet pipe. The excreta will get digested in the masonry tank below and will be cleared at intervals through manhole left for this purpose. The effluent may be then absorbed by sub-surface irrigation process or by other alternative means. 618 Bio-Latrines These are improved aqua privies used for sewage ranging from very small quantities to comparatively larger quantities. They need 1.5 to 2 litres/capita of water for flushing. Standard drawings are available from RDSO. The following points should be taken care of in bio-latrines. 1. No water tap should be provided in the water closet to prevent flooding of digesters, which will prevent efficient anaerobic action. 2. Excessive water should not be used in the water closet. 3. The pan in the bio-latrines should have a slope not less than 40 degrees with horizontal. 4. No foreign matter e.g. garbage, soap water etc. should be thrown into the water closet pan. 5. The digesters should be linked directly to the toilet with least possible inter distance. 6. The flow from the toilet to digesters is to be by gravity. 7. When combined digesters are used, the sewage will first get collected in a chamber attached to each water closet and then flow to a combined digester. The slope of the pipe lines from the collection chamber to the digester should not be flatter than 1 in 20 so as to allow for easy flow. 8. A water seal of 50mm above the crown level of the inlet pipe at the inner face of the digestion tank wall, shall always be maintained in the digestion tank. 9. The plinth level of the latrines should be decided working backwards from the invert level of the outlet pipe of the digestion tank. 10. The sludge settled at the bottom of the digestion tank may be cleared once in two years. 11. Satisfactory working of the anaerobic digesters in the tank, is indicated i) if the effluent is odourless ii) if the effluent does not attract flies and mosquitoes; and 12. Outlet drains should have proper slope and rounded corner. 13. Adequate ventilation should be provided in bio-latrines. 14. In case of public or community type of toilets, users are likely to throw rags, stones etc. into the toilets. In such cases an inspection chamber with 25-50mm spaced grating should be provided just before the entry of water into the digesters to arrest any such material. 15. A minimum retention of 30 days is required for anaerobic digestion, while a septic tank provides retention only for 2-7 days and hence the effluent from the septic tank must have its main treatment in a soakpit/filter while effluent from a digester can be discharged directly in a drain system. 619 Conservation of Water There is a general shortage of water supply in India and in some parts even the drinking water is scarce. Therefore, it has to be seen that water is conserved to the maximum extent possible and that it is not polluted and the waste disposal systems should be designed with these ends in view. This applies to both ground water and surface water. In case of non-residential toilets at stations etc., and community toilets provided with 'standard flushing' arrangements, instead of flushing cisterns of 13.5 litres capacity, the use of 'coach' handles should be preferred so that only the required amount of water is used. In case of urinals, instead of automatic flushing cisterns, dripping ferrules may be used to conserve water. 620 Recommended Toilet systems Based on the above rationale, directions for selecting appropriate toilets system are summarised below:- (i) Where an underground sewerage system can be connected, use a 'flush' type toilet as under-
(iii) In case of urinals, automatic flushing cisterns should not be used and only dripping ferrules may be used. All toilets on the Railways must be sanitised on top priority program basis. Thus only flush toilets with underground sewerage, flush toilets with septic tank systems (with soakpit/filter-bed for the septic tank effluent) and bio-latrines (also acquaprivy) are the approved sanitised toilet systems for use on the Railways. Hence - - all 'dry' (Service type) toilets must be sanitised. - all septic tanks, if not provided with prescribed soakpit/filterbed, must be provided with these or if possible, converted into digesters by minimising the flushing so that the same tank can provide clear minimum 30 days retention (in addition to the space for digestion at the bottom). C. MAINTENANCE OF SEWERAGE AND DRAINAGE 621 General Due consideration shall be given to maintenance requirements at the time of designing sewerage system. Provision of openings, staircases and due provisions for eliminating gas hazards, should be ensured. 622 Sewer Lines a) Maintenance of sewers involves removal of stoppages and clearing out the deposits. Clogging may be due to grit deposition, penetration of roots from adjoining trees, growth of fungi or stagnation of sewage. Sewers may be flushed by firehose from the nearest fire hydrant if conveniently located if the stoppage is too stubborn to respond to flushing, it may be removed by rodding from the next manhole or it may be necessary to dig down to the point where the trouble exists. (b) Records of the location and depth of all sewers and house connections should be kept in sufficient detail and accuracy for locating any sewer in case it becomes necessary to dig it up. Connections of house drainage to the sewer should be permitted to be done only by competent and experienced labour. (c) Sewer line gangs, if under the Engineering Department, should attend to the cleaning and flushing of all sewers at least twice every month. Manholes should be left open for few hours for gases to escape, before any man enters into it. 623 Open Sullage Drains Open sullage drains should be maintained in a thoroughly satisfactory condition; cleaning and flushing should be done at least twice a month or more often, as considered necessary. Soak-pit should be dug up and cleaned at least once a month or as required and fresh broken stones, broken bricks or cinder filled in. 624 Storm Water Drains Special examination should be made of storm water drains, particularly before the commencement of monsoon, to ensure correct grades. Out falls should be particularly examined with a view to ensure satisfactory disposal of the storm water. 625 Disposal Works All sullage disposal works should be examined once a quarter and their sanitary condition ensured. When an appreciable e) extent of silting has taken place, septic tanks should be cleaned in co-ordination with the Medical Department whose responsibility lies in either doing it on contract or departmentally. 626 House Connections Surface drains should be examined f) frequently and there should be no stagnation of sullage near the premises. Gully traps with gratings should be examined installed and prompt action taken for renewal and replacements necessary. 627 Conservancy and Sanitary Arrangements Conservancy arrangements will be in the charge of either the Medical, or Operating or Engineering Department. Any irregularity noticed in their working should be promptly reported to the officials concerned. 628 Maintenance of Sewerage System While assisting sewer maintenance crews entering a deep manhole or sewer where dangerous gas or oxygen deficiencies may be present, the following precautions should be taken: a) allow no smoking or open flames and guard against sparks b) erect warning signs c) use only safe gas-proof electric lighting equipment d) test for noxious gases and oxygen deficiencies (presence of hydrogen sulphide is detected using lead acetate paper and that of oxygen by safety lamps) e) If the atmosphere is normal, workmen may enter with a safety belt attached and with two men available at the top. For extended jobs, the gas tests shall be repeated at frequent intervals depending on circumstances f) If oxygen deficiency or noxious gas is found, the structure shall be ventilated with pure air by keeping open at least one manhole cover each on upstream and downstream side for quick exit of toxic gases or by artificial means. The gas tests shall be repeated and the atmosphere cleared before entering. Adequate ventilation shall be maintained during the work and the test repeated frequently g) If the gas or oxygen deficiency is present and it is not practicable to ventilate adequately before workers enter a manhole, mask shall be worn and extreme care shall be taken to avoid all sources of ignition. Workers should be taught how to use the oxygen hose equipment. In these cases, they shall always use permissible safety lights (not ordinary flashlights), rubber boots or non-sparking shoes and non-sparking tools. h) Workmen descending a manhole shaft to inspect or clean sewers, shall try each . ladder step or rung carefully before putting the full weight on it to guard against insecure fastening due to corrosion of the rung at the manhole wall. When work is going on in deep sewers, at least two men shall be available for lifting workers from the manhole in the event of serious injury i) Portable air blowers, for ventilating sewer manhole are recommended for all tank, pit or manhole work where there is a question of the presence of noxious gases, vapours or oxygen deficiency. The motors for these shall be of weather proof and flame-proof types; compression ignition-diesel type (without sparking plug). When used, these shall 629 Periodical Cleaning of Drainage System - The following operations shall be carried out during periodical cleaning of a drainage system:- a) The covers of inspection chambers and manholes shall be removed and the side benchings and channels scrubbed b) The interceptive trap, if fitted, shall be adequately cleaned and flushed with clean water. Care shall be taken to see that the stopper in the rodding arm is securely replaced. c) All lengths of main and branch drains shall be rodded by means of drain rods and a suitable rubber or leather plunger. After rodding, the drains shall be thoroughly flushed with clean water. Any obstruction found shall be removed with suitable drain cleaning tools and the system thereafter shall be flushed with clean water. d) The covers of access plates to all gullies, shall be removed and the traps plunged and flushed out thoroughly with clean water. Care shall be taken not to flush the gully deposit into the system. e) Any defects revealed, as a result of inspection or tests shall be made good. f) The covers or inspection chambers and gullies shall be replaced, bedding them in suitable grease or other materials; and g) Painting of ladders/rings in deep manholes and external painting of manhole covers shall be done with approved paints. 630 Covered Stormwater Drains-All stormwater drains shall be periodically rodded by means of drain rods and a suitable rubber or leather plunger. After rodding, they shall be thoroughly flushed with clean water. Any obstruction found shall be removed with suitable drain cleaning tools. 631 Subsoil Drains .-All subsoil drains shall be periodically examined for obstruction at the open joints due to the roots of plants or other growth. 632 Responsibilities of the Assistant Engineer and Staff -The Assistant Engineer and the Section Engineer (Works) should inspect all drainage systems in their charge and ensure their maintenance in an efficient condition. Complaints from residents should be promptly attended to. |
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