Notes to accompany 1014 drainageDocument Transcript
Notes to accompany 1014 “Drainage”Slide1-3 Good public drainage is unglamorous but one of the great achievements of the industrial revolution. Clean water supplies and efficient drainage and sewage treatment are more important to human health than all medical advances ever achieved. Slides 4-5 Drains are simply a well laid network of connected, ventilated pipes, which carry away waste water by gravitational flow. Pumping is sometimes unavoidable, but wherever possible, simple sloping pipes provide all the technology needed to move the water. Waste pipes can be broken into two main classes, surface water (rain water essentially) and foul water (used water from buildings.) Grey water is used to describe waste water that has just been used for washing. This is much less dangerous than kitchen and lavatory waste, which is always considered a health hazard. Grey water is increasingly used for flushing lavatories, to save on the use of clean potable water for this process. Slides 6-8 Surface water is non-hazardous and non-noxious (it doesn’t smell). The problem is simply to collect it (gutters and downpipes) and then direct it to a disposal point. Because it is non- hazardous, it can be disposed of directly into a nearby water course, if one is available. Otherwise on permeable sub-soils it can be taken into an underground soak away, where it disperses into the sub soil. IN urban areas, there is likely to be a dedicated network of surface water sewers, which collect all of the surface water and convey it to a river. If possible surface water should not flow into foul sewers, as it could overwhelm them in volume during periods of heavy rainfall. In old, pre 1960 systems, foul and surface were often combined, which rsults in the release of sewage into rivers if there is a heavy rain storm. The sewage farms simply can’t cope with the volume of water. Slides 9-11 Foul water is just that, foul, dangerous a smelly. It is collected in a series of sealed pipes which are trapped at all appliances, to ensure that smells do not come back up into the building. The pipework must be ventilated at its highest point to ensure there is no significant pressure variation in the system as water flows through it. Above ground these pipes are always made of plastic in modern buildings. In early 20th century buildings they are likely to be cast iron. Slides 12-17 Underground drains are essentially a near horizontal network of large diameter (100-150mm) pipes. They are never completely horizontal. They must always slope in the direction of the
ultimate disposal point. There must be inspection chambers, rodding points and manholes onthe system to give access for repairs and maintenance.In most system, the final disposal point can be considered to be the public sewer. Before this thedrains must comply with the Building Regulations. Once the foul drain has connected to thesewer, the disposal of the waste is the responsibility of the local sewerage company. They areusually the local water supply company as well. The sewer will run to a sewage treatment plant,where cleaned water is discharged into a river, and the separated waste is spread on the land afertiliser. It is now illegal to dump sewage sludge at sea, which had been done for centuries.Slides 18-20If local sewers are either non-existent, or higher than a newly constructed property, it may benecessary to deal with sewage on site, rather than via the public sewerage system. Sewage canbe stored in a cess pit and removed by tanker, but this is risky and should be avoided. It is muchbetter to treat the sewage fully on site and disperse the effluent.Slides 21-26Sewage treatment is a biological process carried out in traditional septic tanks or modern bio-disk units. Both work in the same way. Waste is stored in anaerobic conditions. Bacteria breakdown the effluent into carbon dioxide, water and a small amount of sludge. The bacteria arethen killed by aerating the liquid effluent, which can then be dispersed into the soil ordischarged into a water course. The small volume of sludge needs to be removed by tanker atintervals, often just once a year. Bio-disks speed up this process, but require an electrical supplyto turn the disks.Slides 27-28If sanitary appliances or whole houses are below the local drains, the effluent can be pumped upto the drains. Foe a WC this is just a small macerator behind the WC, which grinds and pumpseach time the lavatory is flushed. It works, but everyone can hear what you are doing…For a whole house at low level, the waste goes into a large underground tank, essentially a cesspit. When this is partly full, a switch is tripped which starts a submersed pump which pumps outthe entire tank to the high level sewer through a high pressure run of pipes. The pump can beextracted at intervals for servicing. In practice, a company will pull out the old pump, drop in areplacement, and take the old one for servicing. It is turn will pass on to the next customer. Allthe householder has to do is pay the bills of the service company.