Sewer Appurtanances
- 1. Sewer Appurtenances Dr. Akepati S. Reddy School of Energy and Environment Thapar University Patiala (PUNJAB) – 147 004
- 2. Ancillary structures in sewerage system • Ancillary structures – Manholes – Flushing tanks, flushing manholes and clean outs – Interceptor tanks – (Inverted) siphons – Pumping stations – Gutters, storm water inlets and catch basins – Culverts – Other appurtenances – Gully traps, and Oil & grease traps – Lamp holes and sewer ventilators • IS:4111 (Part I)-1967 Code of practice for ancillary structures in sewerage system: Part I Manholes • IS:4111 (Part II)-1967 Code of practice for ancillary structures in sewerage system: Part II Flushing tanks • IS: 4111 (Part III)-1967 Code of practice for ancillary structures in sewerage system: Part III Inverted siphons • IS : 4111 (Part 4)-1968 Code of practice for ancillary structures in sewerage systems: Part 4 Pumping stations and pumping mains (rising mains) • IS: 2470 (Part-1) – 1985 Code of practice for installation of septic tanks: Part-1 Design criteria and construction
- 3. Manholes
- 4. Manholes • Openings by which humans along with their machinery have access to sewers for inspection, cleaning, repair and other maintenance operations • Components of manholes – Manhole cover – Access shaft (passage from manhole cover to manhole chamber) – Manhole chamber (chambers constructed over the sewage channel within manholes) – Sewage channel with benching (sloping surface on either side) • Locations of manholes – At change of sewer diameter or slope or direction – At the upstream ends and at the sewer junctions – At regular intervals/distances along straight sewer stretches • Manholes for manually cleaned sewers and for the sewers cleaned mechanically • Pre-cast manholes and manholes constructed onsite
- 6. Spacing of Manholes In case of manually cleaned sewers • For smaller sewers which can not be entered for cleaning, the manhole spacing is ≤30 m • For sewers of >900 mm size, spacing of manholes is determined by – Distance through which silt and other obstructions can be conveyed manually – Distance by which materials for repair can be conveyed manually – Ventilation requirements For sewers of 900-1500 mm diameter on straight runs the spacing can be upto and above 90-150 m For sewers of 1.5 to 2.0 m diameter the spacing is 150-200 m For sewers of >2.0 m diameter the spacing can be above 300 m • General rule: spacing is 100 m x sewer diameter in meters In case of the sewers cleaned with mechanical devices, the spacing depends on the type of equipment used for cleaning
- 7. Types and sizes of manholes Rectangular, arch type and circular manholes • Rectangular manholes of 0.9 m x 0.8 m size are provided for sewers of <0.9 m depths, and 1.2 m x 0.9 m size manholes for sewer of 0.9 to 2.5 m depth • Arch type manholes with manhole chamber size 1.4 m x 0.9 m are used for sewers at >2.5 m depth – Width of the manhole is suitably increased from 0.9 m on bends and junctions, and for sewers of size >450 mm • Benching of >200 mm is ensured on either side • Circular manholes: stronger and better alternatives to rectangular and arch type manholes – These are straight down in the lower portion and slanting in the top portion – Can be provided for all depths starting from 0.9 m • 900 mm manhole is used for 0.9 - 1.65 m depth sewers, 1200 mm for 1.65 -2.3 m depth; 1500 mm for 2.3 – 9.0 m depth, and 1800 mm for 9 – 14 m depth
- 10. Manholes Sewage channels and benching • Channels and benching should be in cement concrete • If possible channels should be of the same material as the sewers • Benching is a sloping surfaces (1 in 12 slope) on either side of the sewer within the manhole • Meant to confine sewage flow to sewer/channel, to avoid accumulation of deposits, and to provide safe working platform • Channels should be semi-circular in the bottom half (diameter equal to that of sewer) and sides should be extended veritcally 50 mm above the sewer crown, and the channel top edge is rounded – Within the manhole the channel should be appropriately sloped – When a smaller sewer joins the main sewer, invert level of the smaller sewer should be located 2/3rd diameter above the main sewer invert – Branch sewers should deliver sewage in the manhole in the direction of flow in the main sewer – should not impede flow in the main sewer
- 11. Manholes Invert levels of incoming and leaving sewers • Maximum length of the building sewer should be 6 m • When sewer diameter is increasing, crown of the entering sewer is fixed at the same level as that of the leaving sewer Manhole rungs • Should be provided if the manhole depth is >0.8 m • Cast iron rungs of suitable dimensions are set in two staggering vertical runs 300 mm apart both vertically and horizontally • The rungs should project 100 mm beyond the finished wall • 1st rung should be 450 mm below the manhole cover and the last one 300 mm above the benching Manhole covers and frames • Clear opening of the manhole should be >560 mm for manhole depth is >0.9 m • Manhole frame should be firmly embedded in concrete alignment • Should be of heavy duty casting for manholes provided on heavy vehicular traffic roads, otherwise can be medium duty casting
- 12. Manholes • Manholes should be built on a concrete bed designed to carry weight of walls, wheel loads, impacts of traffic and water pressure – Can be constructed in brick work or reinforced cement concrete – Sites with higher sub-soil water conditions can have RCC manholes • Still walls can be in brick masonry above the subsoil water level • A cement concrete collar should be provided over the sewer where it is passing through the manhole walls • On natural undeveloped ground the manholes are built to about 600 mm elevation from the ground level • Deeper manholes (>6 m) can have rest chambers at 6 m intervals • Manholes on sewers of >1.0 m diameter should have safety provisions, such as, – Safety chains across the sewer mouth – Galvanized pipe handrails provided on the benching edges, platforms, etc. • Manholes are not permitted in a building or passage
- 13. Types of Manholes Types • Straight through manholes • Junction manholes • Side entrance manholes • Drop manholes • Scraper type manholes Junction manholes (and junction chambers) • Precast or constructed onsite structures interconnecting two or more sewers • Soffit of a smaller sewer in the junction manhole should not be lower than that of the main sewer • Gradient of the smaller sewers if feasible may steepened from the previous manhole for reducing the invert level difference at junction manhole • Junction chambers: manholes constructed onsite – used when sewer diameters are large and precast manholes can not be used
- 14. Types of Manholes Drop manholes (external or internal) • Provided when – Water level in the main sewer during peak flow is >600 mm below the invert level of the branch sewer joining the main sewer – Difference in elevation between incoming and outgoing sewers is >0.6 m (flow of incoming sewer is dropped to the outgoing sewer elevation) • Drop is provided inside the manhole for smaller drops & smaller sewers – external drop is preferred for larger sewers & larger drops • Continuation of the sewer with half blank flange is built to facilitate access to the sewer for cleaning and repair and maintenance • External drop is usually encased in concrete and internal drops are supported on brackets • Internal drop sewer can be cast iron (if not, it is encased in 150 mm thick concrete encasement) • Diameter of the drop sewer should be ≥ to the sewer • Drop pipe terminates with a plain or duck foot band and water is discharged at 45 angle with the flow direction in the main sewer • 150 mm depth water cushion is provided under the drop sewer
- 16. Types of Manholes Side entrance manholes • Provided on larger sewers • Access shaft is constructed in the nearest convenient position and connected to the manhole chamber by lateral passage • Floor of the side entrance should be sloped 1 in 30 towards the manhole chamber Scraper (service) manholes • All sewers of >450 mm diameter should have the scraper manholes at 110-120 m intervals • The manhole should have a clear opening at the top of 1.2m x 0.9m size to facilitate lowering of buckets for clog material removal, etc., maintenance
- 17. Flushing tanks, Flushing manholes, and Cleanouts
- 18. Flushing Tanks • Devices that hold/store sewage/water and throw out or discharge into the sewer at intervals for the sewer flushing • Provided in the sewer sections where flow is never sufficient to generate self-cleansing velocity – At the heads of sewers • Quantity of sewage/water for the flushing is equal to half full 70 m long sewer (for sewers of <450 mm diameter) • Flushing tanks are operated either manually or they are automatic Sewer diameter Sewer length Flush water need 250 75 – 90 1.4 – 1.7 m3 350 75 – 90 1.7 – 2.7 m3 400 75 – 90 2.7 – 3.6 m3 450 75 – 90 3.6 – 4.5 m3
- 19. Flushing Tanks • Flushing without using an independent flushing tank is also possible – Using a flap valve or plug in the downstream manhole - opening the valve/plug at intervals to rush sewage through for flushing – Water is admitted into the sewer at controlled rate for achieving the desired level of backing – Fire hoses or water tankers may deliver the water for flushing • Flushing by manually operated flushing tank – Water supply connection to the flush tank and controlled flow of water into the flush tank – The flush tank is connected with the upstream manhole by a pipe of same size as the sewer – The flush tank drain has a slot with a wooden plank for manually regulating the flushing – The risk of sewage back up (in the event of flushing failure) is tackled by providing an overflow pipe connecting to the sewer
- 21. Flushing tanks Automatic flushing tank • A masonry/concrete/RCC flushing tank has an Adam’s siphon (automatic siphon) at the bottom – Adam’s siphon is in cast iron and comes in different diameters (65, 80, 100 mm diameter) – The jointing should be leak proof • The tank has a water supply connection – A physical break (of 30 mm) between the water supply connection and maximum water level in flushing tank is ensured • When the tank is full, the siphon goes into operation and water is quickly discharged into the sewer – The flushing frequency should be once a day
- 23. Cleanouts • An inclined pipe connected from above ground to the sewer • Provided at the upper ends of the lateral sewers in place of manholes • When needed, cover of the cleanout is removed and water is forced through for cleaning the sewer • If needed a flexible rod is inserted for removing the obstructions
- 24. Inverted Siphons
- 25. Inverted Siphon • It is the section of a sewer at a lower elevation than the adjacent sewer • It is actually a depressed sewer or a sag pipe used to carry sewage below obstructions (ground depressions, streams, rivers, railway lines, etc.) • These pipes flow full under the pressure greater than the atmospheric pressure • Inverted siphon usually includes two or more pipes of ≥200 mm diameter running parallel – 1st pipe carries dry weather minimum flow – 2nd pipe carries the difference of dry weather minimum flow and dry weather maximum flow – 3rd carries the storm water flow – Large fluctuations in storm water flows demand >3 pipes • Minimum dry weather flow, maximum dry weather flow, and maximum wet weather flow for the design period are required for the design of inverted siphons
- 26. Inverted Siphon • Flow velocity in the siphons is maintained at ≥1.2 m/sec. for self cleansing – Velocity produced in the siphon pipe is function of the liquid level difference between the siphon’s inlet and the outlet • Deposits are prone to form at siphon pipe bends – Sharp bends are avoided and only easy bends are allowed – Hatch boxes of adequate size in manholes are provided at bends for access to pipes for rodding – Hatch boxes are often omitted and only water tight manholes are provided – but floating matter can accumulate here • Provisions to isolate any of the siphon pipes for cleaning – Suitable penstocks or stop planks are provided at both the inlet and outlet ends for this purpose • Draw off valve on siphon pipe at lowest point for draining – Provision of wash out into a manhole and pump out from there – Portable pumps with suction end connected to the draw off of the siphon pipe and discharge end connected to the draw off of the another siphon pipe
- 27. Inverted Siphon • Siphon inlet (fore bay) is designed for bringing the siphon pipes successively into action – The fore bay or inlet chamber with the same number of channels as the number of siphon pipes is used – The incoming sewer extends as a middle channel and feeds the siphon pipe carrying the minimum dry weather flow – On one side the middle channel has a overflow weir at specified elevation for diverting the flow in excess of the minimum dry weather flow into the 2nd channel – On the other side the middle channel has another overflow weir at another specified elevation for diverting the flow in excess of the maximum dry weather flow into the 3rd channel • Outlet of the inverted siphon is arranged in such a way that the inverts of all the siphon pipes merge into a single channel – Outlet of the 2nd pipe is maintained higher to the 1st pipe and so on to avoid eddies and accumulation of solids – Groves for stop planks are provided in the outlet chambers for facilitating closure of siphon pipes for cleaning
- 29. Inverted Siphon • The inlet and outlet chambers of the siphon should allow sufficient room for the access, entry and maintenance – Ramps or if not feasible vertical access shafts are provided • Bypass arrangements are made to the inlet chambers • Siphon pipes under river beds are – Surrounded by RCC of appropriate thickness to prevent floating when empty – Protected against the water currents and sifting bottoms or channels • Outlet chamber is designed to prevent backflow of sewage • Inverted siphons are constructed of cast iron pipes or reinforced pressure pipes
- 31. Septic Tanks and Interceptor Tanks
- 32. Septic tank • Septic tank is an on-site sewage treatment unit • Clarification (separation of both settlable solids and floating materials) of raw sewage occurs • TSS, BOD and oil & grease are removed by 60-80%, 50- 60% and <80% efficiencies respectively • Digestion or stabilization and storage of the settled sludge till pumped out once in 3 to 5 years for cleaning • Removal of nutrients, pathogens and metals also occur • Conditioning of sewage for further treatment and equalization also occurs • Located in open to sky areas and should be accessible for cleaning • Should not be located in swamp areas and not in areas prone to flooding
- 33. Septic tank • Usually rectangular tanks with 2 to 4 aspect ratio • Minimum width is 750 mm, minimum depth is 1000 mm and minimum capacity is 1000 L • HRT is 24 to 48 hours for average daily flow • Septic tank is compartmentalized (has two or more compartments) when size is >2000L • 1st chamber is twice the size of the second chamber • The two chambers are connected by 100-150 mm size openings at 300 mm below the top water level • Duplicate septic tanks are used when the serving population is >100 (to facilitate desludging) • Septic tank may have a provision for the decanting of the top water during cleaning
- 34. Septic tank • Inlet introduces raw sewage into the septic tank without disturbing the settled sludge and the floating scum • Outlet withdraws sub-surface clarified sewage from septic tank without disturbing the settled sludge and the floating scum, while maintaining constant water level within • Contains a bottom settled sludge zone, a middle clear wastewater zone and a top floating scum zone • Has manholes one per chamber of 455x600 mm size rectangular manholes or 500 mm size circular manholes • Vent pipe of 50 mm size and >2 m height
- 35. Septic tank: inlets • Inlet introduces raw sewage into the septic tank without disturbing the settled sludge and the floating scum • One or more T shaped dip pipes of diameter larger than the incoming sewer with top limb rising above scum layer and bottom limb extending about 300 mm below the water level are used as the inlets • In case of larger septic tanks (>1200 mm width), submerged bends are used as inlets • A small benched chamber with invert level 58 mm above the top water level receives sewage and delivers into the submerged bends at the bottom • The submerged bend opens into the tank at >75mm below the water level – invert level of the bend is 300-325 mm below the water level • A baffle extending 150 mm below the invert level of the inlet and 150 mm above the water level is provided at 150 mm distance from the inlet end
- 36. Septic tank: outlets • Outlet withdraws the sub-surface clarified sewage from the septic tank without disturbing the settled sludge and the floating scum and while maintaining constant water level within the tank • A T shaped dip pipe with top limb rising above scum layer and bottom limb extending to 1/3rd of the liquid depth below the liquid level is used as the outlet • Invert level of the outlet is 50 mm below that of the inlet • In case of larger septic tanks (>1200 mm width), a weir outlet extending full width is used as the outlet • Outlet is protected by a scum board extending by 150 mm above the weir and by 1/3rd liquid depth to the surface water level • A deflector protruding by 150 mm is provided at 150 mm below the scum board base submerged bends are used as inlets
- 37. Septic tank - Design • On-site septic tanks are provided for serving upto 300 users • Area provided for sedimentation is 0.828 m2/9LPM (peak flow) – 9 LPM peak flow per fixture unit is assumed – Upto 20 users 1 fixture unit equivalent is assumed per household of 5 members and 70% of the fixtures are assumed to operate simultaneously – For residential colonies of 10 to 60 households, 2 fixture units per household are assumed and 60% of the fixture units are assumed to be in simultaneous operation – For hostels or boarding schools, 1 fixture unit is assumed per every 4 students and simultanous operation of 70% of the fixtures assumed • Depth provided for sedimentation is 250-300 mm • Septic tank volume for the sludge digestion and for the digested sludge storage – sludge digestion: 0.003 m3/person/day – digested sludge storage: 0.00021 m3/person/day – Per capita suspended solids load assumed is 70 g
- 41. Septic tank
- 43. Interceptor Tanks • Interceptor tanks on house connections are designed to settle solids and allow solids-free sewage to flow into sewers – Interceptor tanks reduce peak flows, and hence sewers can be smaller (typical dia. of 50mm or 75mm, min. dia. of 38mm – Because the sewage do not have solids, sewers can be laid on flatter gradients than conventional sewers – Interceptor tanks reduce oxygen demand by about 30% • Interceptor tanks are rectangular in plan, designed with two compartments and have T pipes on the outlet – Existing septic tanks can be used as interceptor tanks • Interceptor tanks tend to concentrate maintenance requirements at the interceptor tanks – Periodic de-sludging of the interceptor tanks is required – Institutional arrangements for fecal sludge management will have to be strong and effective • Smaller interceptor tanks operate as traps for large objects
- 45. Gutters, Storm water inlets and Catch basins
- 46. Street Gutters • Gutter: A channel alongside the road for conveying storm water flows to sewer inlets • Gutters have – Both depth and width (gutter spread) – Both cross slope and longitudinal slope • Flow in gutters is estimated by modified Manning’s equation • Flow spread allowed in the gutter or depth of flow at the curb is used as the criterion for locating sewer inlets • Rational method is used for quantifying the runoff – Area contributing to the runoff, Design rainfall intensity and Runoff coefficient are required 5.03 8 67.25.087.1 1376.0 376.0 Sd Sn Q TSS n Q x x Q is gutter capacity ‘n’ is manning’s roughness coefficient T is gutter spread (width of flow) Sx is gutter cross slope S is longitudinal slope of gutter ‘d’ is depth of flow at the curb
- 47. Street Gutter
- 48. Storm water inlets • May be located either on a continuous grade or in a sag location – Part of the gutter flow can bypass the inlet in case of the Inlets on continuous grade (carry over, bypass or runby flow) • Storm water inlet types: – Gutter (grate) inlets – Curb opening inlets – Slotted drain inlet – Combination inlets • A concrete box with grating or opening in the vertical or horizontal direction – Vertical or curb inlet – Horizontal inlet or gutter • Grates of the inlets can be reticular, rectangular or parallel bars
- 49. Storm water inlets • Storm water inlets act as weirs for shallow flows and as orifices for deeper depth flows • Located on road sides at 30 to 60 m distance at low points and at cross slope reversal points – Inlets in sag are located at points where runoff from a given area ultimately accumulates • Open into catch basins and connected to nearby manholes of the sewerage system • Have clear opening of ≤25 mm size
- 50. Curb Inlet • Recommended length is 1.2 m for 0.08 m3/sec. flow/discharge • Curb opening height is <150 mm • Flow through a curb inlet (Qi) 2 1 5.1 0 5.1 2 267.0 8.127.1 27.1 h dgAQ dWLQ LdQ i i i ---- for a gutter without depression ---- for a gutter with depression ---- for a gutter with d ≥1.4 h Qi is discharge through curb inlet L is length of the curb inlet ‘d’ is depth of flow ‘h’ is height of the curb opening A is area of the curb opening W0 is width of the depression
- 52. Catch Basins • Masonry chambers of 75-90 cm dia. & 75-90 cm depth provided along the storm sewer (or combined sewer) line for collecting and clarifying storm water and allowing into the storm sewer • Storm water inlets open into these catch basins and outlets of these basins provided 60 cm above the basin bottom convey storm water into the sewer • Frequent maintenance (degritting and cleaning) of the catch basins is required
- 54. Sewage Pumping Stations Pumping stations have wet well (and dry well!), pumps and drives, electrical power control panels, header and other piping with necessary fittings Has a sub-structure of 2 underground compartments and a super structure Sub-structure typically has two underground compartments: a dry well housing the pumps, pipe work and control valves, and a wet well containing sewage – both separated by a common wall The pumping station should be designed to resist floating due external water pressure Frequently located in low lying areas that may possibly be flooded Highest recorded flood levels should be known and pumping station should be located above the highest recorded flood level Ground water level of the site should be recorded Soil of the site should be free from the danger of subsidence Should be located as far away as possible from the residential properties Noise and odours can be problems
- 55. Wet Well • Capacity depends of maximum and minimum flows of sewage to be handled – With any combination of inflow and pumping, each of the pumps operating cycle should be >5 minutes – Maximum HRT of the wet well is 30 minutes • Settlement of solids and formation of scum and dead pockets within the wetwell should be the minimum – Suction pipes of all the pumps should be suitably placed apart • Bottom of the well is sloped towards the suction end of the pump suction pipe – Can not be flatter than 1 in 10 slope • Suitable overflow arrangement for the wet-well • The site should have arrangement for diverting the overflows into near by watercourse (avoid the site flooding and the watercourse pollution)
- 56. Dry well • Walls and floor should be rendered water proof to external water pressure • Each of the pumps should have a separate foundation block – Insulation of the foundation from the building to prevent transmission of vibrations • Adequate lighting and ventilation within the dry well • Bottom should be sloped to a small sump with adequate dewatering arrangement – Flooding of pumps and accessories should be avoided • Sufficient floor space at the motor and pump levels for facilitating dismantling and overhauling • Traveling gantries (overhead cranes) for handling the pumps and motors (for larger stations!) and chain controls for crane movement
- 57. Sewage pumping station types 4 types • Pumping stations with only wet well and no dry well, and using submersible (non-clog) pumps – Submersible grinder pumps may replace non-clog pumps (solid matter is grounded to facilitate sewage pumping) • Pumping stations with only wet well and no dry well and self- priming pumps or pumps primed by smaller vacuum pumps pumps located at ground level are used • Pumping stations with both wet well and factory built dry well – Cylindrical steel chamber cylindrical access tube installed underground next to the wet well – Non-clog pumps with drives mounted on the pump top are used • Pumping stations with both wet well and onsite built dry well – Good for larger capacity installations – Pumps are installed at the bottom of the dry well
- 59. Pumping stations for water and wastewater Traditional pumping stations had both wet well and dry well – Smaller foot print, availability of submersible non- clog pumps, reduced health and safety concerns discourage their use
- 61. Pumping stations for water and wastewater Designed to handle both normal and peak flows – Redundancy is built into the system Capacity of the wet well is designed such that – pump starts and stops are minimized – sewage in the sump will not become septic Have level switches in the sump to turn on or turn off the pumps – Level switches may also be provided in the receiving reservoir/overhead tank There can be a high water alarm to alert pump failures and not pumping fast enough to keep up with inflow Settling of solids and accumulation of grit in the wet well is avoided by design – Screens and grit channels/chambers are provided up stream
- 62. Pumping stations for water and wastewater Sewage can enter the dry well from failures and leaks of pumps and seepages from wet well – Electrical motors mounted above the top sewage level in the wet well with extended vertical shafts are often used – Separate pump of appropriate capacity and head are often provided to take care of the seepage Usually designed to allow removal of pumps and other equipment from outside the wet well – Poisonous gases can accumulate in the wet well and human entry into wet well may require correct confined space entry method DG sets are often used for power backup
- 63. Sewage Pumps • Pumping stations should deal with • Both daily and seasonal flow variations • Both dry weather and wet weather flows – whether the sewage is sanitary, storm or combined matters • Number of and size of pumps should be chosen on the basis of • Overall economy of pumping
- 66. Sewer and Storm Sewer Outfalls
- 67. Storm water Regulators • Storm water regulators are used to divert part of the sewage from combined sewer into a natural stream or river • The regulators include leaping weir, overflow weir and siphon spillway • Leaping weir – A gap or opening is provided in the invert of the combined sewer – And intercepting sewer running at right angles to the combined sewer the diverted sewage – When flow is quite high additional water will flow forward in the combined sewer • Overflow weir – Excess sewage is allowed to overflow into the channel made in the manhole
- 68. Float activated gates and valves • automatic mechanical regulators actuated by water level in the sump • These regulators involve moving parts and require periodic maintenance • Flap gates and flood gates are installed at or near sewer outlets to prevent backflow of water
- 70. Storm Sewer Outfall • For erosion control velocity is reduced and energy dissipators provided • Stilling basin, stone rip-rap, erosion control mat, Sod and Gabion • Stone rip-rap – Type and size of stone – Thickness of stone lining – Length and width of apron 3 4 50 02.0 oD Q TW D D50 is median stone size TW is tail water depth in feet Do is maximum pipe/colvert width Q is design discharge
- 72. Lampholes and Sewer Ventilators Lampholes • Openings/holes constructed in sewer for lowering lamp - often used also as a flushing device • Located at change of gradient or direction, specially if providing a manhole is not possible, and on straight sewer stretches between maintenance manholes • Has a vertical pipe, encased in concrete and connected to the sewer line through a T-junction • Lamphole is covered at ground level by manhole cover with a frame – If the cover is perforated, the lamphole can also ventilate the sewer Sewer ventilators • 15-23 cm diameter cast iron or RCC pipe with cowl at the top • Ventilating shaft is connected to a manhole by 15 cm diameter pipe or it may be provided directly on the manhole cover • Provided on sewers at every 80-100 m • Ventilators are needed if intercepting traps are provided in house connections
- 74. Traps Gully traps • It is a 60-70 mm water seal in stoneware provided inside a masonry chamber with cast iron grating on its top • Located near the external face of wall, and these are starting points of horizontal flow of sewage • Leads sewage to sewer, inspection chamber or manhole • Prevents entry of gases from sewer line to household drainage Oil & grease traps • It is a trap chamber with outlet near bottom and inlet near top • If enough space is provided at the bottom sand and grit can also be excluded from sewage • Located near the source of oil & grease (Automobile garages, oil & grease producing industries, kitchens of hotels, etc.) • Excludes oil & grease from sewage prior to entry into the sewer line • If not excluded SS become sticky and stick to sewer walls • Oil & grease trap requires regular inspection and cleaning