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Generally sewers are laid in trenches by excavation in natural soil and then covered by refilling the trench to the original ground level. 'Characteristics of metallic, PSC and GRP pipes' and 'Characteristics of plastic, DI and GI pipes' can be referred to adjudge suitability of pipe material option for particular use.īedding shall be designed corresponding to laying condition of sewer in trench, embankment or tunnel as per CPHEEO manual, chapter 6. Characteristics of various pipes for gravity sewers makes comparison for different pipe options. The welded joints are leak proof and as such some people have started using these pipes.GRP pipes are widely used in other countries where corrosion resistant pipes are required at reasonable rates. The pipes are recently development in India but are costlier than RCC pipes and uPVC pipes. HDPE pipes are available up to 630 mm dia. These days these pipes are used for making connection from house to sewer but not prevalent in street sewers. uPVC pipes are smooth, light, easy to joint and leak proof joint. Cast iron, DI and steel pipes are not used due to high cost. AC pipes cannot stand high superimposed loads, subject to corrosion from acids in sewage and high sulphate soils, require special bedding and weak against erosion where high velocities are encountered as such use of AC pipe is not prevalent. However due to less length, more joints, difficulty in jointing, requirement of special bedding and less compressive strength of pipes manufactured in India use of these pipes is reducing in India. These pipes are good for corrosion resistance and erosion resistance. The length of these pipes is 60 cm, 75 cm and 90 cm. Salt Glazed Stoneware Pipes are mostly manufactured in sizes 80-1000 mm but sizes greater than 380 mm are generally not used due to economic considerations. Only high alumina cement concrete should be used when it is exposed to corrosive sewage or industrial wastes. However these pipes are subject to corrosion where acid discharges are carried or where velocities are not sufficient to prevent septic conditions or where the soil is highly acidic or contains excessive sulphates. Concrete pipes are commonly used now as these can be manufactured to any reasonable strength and laying is easy and jointing is leak proof.
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These slopes would ensure minimum velocity of 0.6 mps in the early years.īrickwork is used for large diameters as sewers can be constructed in any shape.
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As such for present peak flows up to 30 lps, the recommended slopes for minimum velocity are given in Table 3.7 of Manual.
Sewer network design software full#
Where : D = Full Depth of flow(internal dia), d Actual Depth of flow, V = Velocity at full depth, v = Velocity at depth ‘d’, n = Manning’s coefficient at full depth, nd = Manning’s coefficient at depth ‘d’, Q = Discharge at full depth, q = Discharge at depth ‘d’įor sewers running partially full, for a given flow and slope, velocity is little influenced by pipe diameter. Therefore the maximum flow depth should be 0.8 full at ultimate peak flow for all pipe diameters. The discharge at 0.8 full is 0.98 times the discharge at full flow. In case of circular sewers, table 3.6 of Manual(Hydraulic properties of Circular sections), the velocity is maximum at 0.8 full and is 1.14 times the velocity at full flow. Also from consideration of ventilation sewers should not be designed to run full. The closed sewers should not run full, otherwise the pressure will rise above or fall below the atmospheric pressure and condition of open channel flow will cease to exist. Velocity in a sewer is recommended not to exceed 3 mps. Erosion of sewers is caused by sand and other gritty material in the sewer and also by excessive velocity. Thus the sewers are designed on the assumption that although silting might occur at minimum flow, it would be flushed out during peak flows. Hence a minimum velocity of 0.6 mps, Para 3.4.3 Manual, for present peak flow and 0.8 mps at design peak flow is recommended for sanitary sewers. Provision of design capacities in the initial stages itself is economicalĪ velocity of 0.6 mps would be required to transport sand particles of 0.09 mm size with a specific gravity of 2.65. The construction shall be modular in phased manner as actual population less than design population and in Indian cities initially flows are much less due to connectivity problems. Sewer network (laterals, Trunk mains, Outfall et)c Land Acquisition for STP, SPS, sewers etc First phase of STP should be for about 10 years but land of STP should be taken for 30 years period. STP should be designed in phases, refer clarification in table 1.1.
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Table 1.1 CPHEEO Manual gives design period for different components of sewerage system and sewage treatment. The design period is different from planning horizon year.