A plumbing system serves two purposes: to supply water for human use, and to get rid of human wastes. It consists of a store of water that is delivered to various outlets via a distribution system.
All Indian plumbing systems have an underground tank. This collects water from the municipal supply line, which is normally delivered at low pressure, and therefore cannot push water to the height of an overhead tank. The most basic decision in a plumbing system is whether to provide an overhead tank or not. Traditionally, the underground tank is connected to an overhead tank via a pump and a supply line; the water flows from there to any tap or outlet by gravity. The height of the water column above the outlet, known as head, supplies the required pressure in the outlet. The overhead tank can be eliminated, however, by the use of a pump that pumps the water directly from the underground tank to all outlets. Such a pump should be able to continuously supply the line with pressure, so that whenever one opens a tap, water gushes out.
Such pumps are available these days; they are called hydro-pneumatic systems. They consist of a smallish steel tank, divided into two compartments by a rubber membrane. One contains water, the other air. A pump periodically switches on and pumps water into the wet side; this causes the membrane to expand, and compresses the trapped air on the other side. The water on the wet side is thus under pressure; it is connected to the water supply line, which in turn gets pressurized. Whenever someone opens a tap, water flows out, and the pressure in the hydro-pneumatic tank drops, which causes the pump to be switched on again, thereby maintaining pressure in the line while supplying the required quantity of water to the outlets.
Such systems have several advantages. One, they eliminate the need for heavy water tanks on the tops of buildings. Two, one can design them to supply water at any pressure one desires, unlike a traditional system, where the pressure is determined only by the height difference (or head) between the tank and the outlet, and where people on the top floors get low pressure and people on the ground very high pressure. Three, in theory, they consume less energy than a traditional system, because if one has an overhead tank, even the water that goes to the person on the ground or first floor has to first be pumped up say ten stories high before coming back down. However a hydro-pneumatic system has a disadvantage: no power, no water.
Therefore, if the power supply is good and backup power is available, use a hydro-pneumatic system, else provide overhead tanks.
Pumps may be of submersible or open type. Submersible pumps are placed inside the water tank (at the bottom) and require little maintenance. Either type of pump may be used in traditional or hydro-pneumatic systems.
There are a range of choices for the distribution network of pipes; GI, CPVC (chlorinated PVC), HDPE (high-density polyethylene), and copper. These days plastic (CPVC, HDPE) pipes are preferred to others because they do not rust, are light and easy to install, and inexpensive.
In a very tall building, floors should be divided into zones of perhaps 15-20 floors. Each of these will have its own pumping system. This serves to eliminate the very high pressures that result from high water heads, and also reduces pipe and pump sizes. Remember that centralized hot water boilers are placed at the bottom as hot water rises by itself.
The other aspect of plumbing is drainage, which is of two types. Waste water is from showers, basins, kitchen sinks, washing machines, and the like. This is also called greywater. Soil water or sewage is from WCs and urinals. This is also called blackwater. Normally a minimum of 100mm dia pipes are used for soil water and 75mm for waste water. When run horizontally, soil water pipes should be run at a steeper slope, such as 1:40, as they have solids. These can be of cast iron, or, of late, of PVC.
A grease trap should be used when draining waste from large kitchens or massage parlours; grease should not be allowed to enter the normal drainage system. A grease trap is nothing but a small inspection chamber. The grease floats, and should be removed manually on a daily basis. The inlets and outlets into this chamber should be designed in a way that minimises disturbance of the floating grease layer.
Stone ware (ceramic) pipes are used when soil and waste water is to be transported in external soil. An inspection chamber is used to clean blockages in the line and change direction. A manhole is a larger version of an inspection chamber, used in the main drainage line or in the street.
If municipal drainage is not available on a small project, provide aseptic tank and a soak pit. A septic tank is a rectangular underground tank with compartments. It is always full of sewage. Solids are allowed to sink to the bottom, where they break down by the process of anaerobic decomposition, and eventually form a sludge that should be removed manually after several years. The less water put into a septic tank, the better it will function. This process is termed primary treatment. The effluent that flows out of this, which is about 70% purified, is then put into a soak pit. A soak pit is a cylindrical tank with porous brick walls surrounded by a layer of gravel. The effluent will therefore enter this and soak into the soil via the holes in the walls and the gravel, which acts both as a filter and as a device for bacterial decomposition. A soak pit should not be placed near any occupied structure, water body, or water supply pipe. It also cannot be used where the water table is high, as groundwater will then enter and flood the pit through the porous walls.
A sewage treatment plant is recommended for the disposal of large amounts of sewage. This is a plant that will process sewage and produce sludge and (relatively) clean water from it. This water may then be used for landscaping, or even perhaps for HVAC cooling towers – not for drinking or washing. If one has to dispose of quantities of water containing chemical contaminants, an effluent treatment plant is the answer.
When you design a plumbing system, remember the following:
Underground tanks: as mentioned earlier, water must flow from the municipal supply first to the firefighting tanks and then to the domestic water tanks to prevent stagnation. The firefighting tanks should remain full at all times. If one does not have overhead tanks, the domestic water stored underground should be segregated into two tanks, so that when it is cleaned (every 6 months) the water in the other tank can be used. However the two compartments should be connected by a valve at the bottom. This valve should be closed when one tank is cleaned.
Underground tanks should be designed for two days’ storage if possible.
On-ground tanks may be used instead of underground if the soil is good. This reduces construction costs as well as the pumping head.
There are essentially two different types of pumps one may use:submersible pumps, which are designed to be placed inside the water in the tank, and conventional centrifugal pumps, which should be placed in the pump room.
If using conventional pumps, the outlet from each tank to the pumps should be placed in a sump; if no sump is provided and the outlet is placed say 150mm above the tank bottom, then a 150mm high pool of water will remain in the tank. Whenever the tanks are to be cleaned, a de-watering pump will have to be brought in to remove this water.
If using submersible pumps, provide a deep sump (at least 600mm), as these pumps are normally installed vertically, and the water inlet is in the middle of the pump and not at the bottom. The pump should be supported only by the delivery pipe, which is bolted to a flange attached to a sleeve in the roof of the tank. If the pump is to be removed for maintenance, this arrangement allows the bolts to be undone, allowing one to easily lift the pump out of the tank without draining the water. For this purpose a manhole must be placed next to the delivery pipe. Such pumps must not be mounted on the floor of the tank because the fixing bolts will puncture the waterproofing layer of the tank.
Overhead tanks: some municipalities, such as the BMC, require that flushing water be segregated from the other water in overhead tanks. There is no technical reason for this.
These days automatic sensors that start and stop the pumps when the overhead tanks are full or empty are cheap and reliable.
All pump rooms should without fail have an arrangement for floor drainage; pumps always leak. The best way to do this is to slope the floor towards a sump, and install a de-watering pump if the water cannot flow out by gravity.
When you attach any water pipes to the side of a building, make sure to keep a gap of at least 50mm between the pipe and the face of the building, so that any leakages do not affect or seep into the plaster. Also use non-corroding screws (and clamps) like brass or SS to attach the pipes to the building.
It is possible to have a common tank for firefighting and domestic water. In this case the outlets for the domestic water must be placed at a sufficiently high level as to leave the correct volume of water for firefighting. Pump rooms for firefighting and plumbing may be combined in any case.
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