SEWER APPURTENANCES
Sewer system requires various types of appurtenances for their proper functioning and maintenances. If sewerage system will not be maintained properly salt, ashes, Fats, oils and greasy matters etc. will choke the sewer line. Therefore for the proper operation and maintenance of sewerage system, various devices like manholes, lamp holes, drop manholes, street inlets, flushing tanks, catch basins, ventilating shafts and storm role of works etc are essential, which should be inaccessible to the public.
1. Manholes
Location: Manholes are provided at every change of alignment, gradient or diameter of the sewer. Function: Manholes are provided for inspection, cleaning, repairs and maintenance of the sewer. Construction: A Manhole consists of
(a) Working chamber,
(b) An access shaft, and
(c) A strong cover on the top flush with the road level
(a) Working chamber: The working chamber has such a size, so that necessary examination and cleaning can be done easily, the minimum internal size of the chamber are as follows:
(i) For depth of 0.8 m or less - 0.75 m × 0.75 m
(ii) For depth between 0.8 m and 2.1 m -1.2 m x 0.9 m
(iii) For depth more than 2.1 m -1.2 m × 0.9 m or 1.4 dia circular chamber.
(b) Access shaft
The access shaft provides an access to the working chamber. The shaft is formed, by corbelling the working chamber on three as shown in Fig So that the cover frame can be fitted in the opening; the minimum internal dimensions of the access shaft are 0.5 x 0.5 m.
(c) Cover:
At the top of manhole the manhole cover of cast iron or R.C.C is provided to cover the opening depending upon the type of traffic on the road. The manhole covers are provided flush with the road level. The bottom of the manhole is usually made of concrete slightly sloped at the top towards the open channels, which are in continuation of the sewer line. The channels are sometimes lined with half-round sewer pipe section. The top surface of the concrete is called benching and the man stands on its top during cleaning and inspection of the sewer lines over the cement concrete walls not less than 20 cm thickness are constructed. Circular shape is structurally more stable and stronger though it is difficult in construction. The maximum distance between two manholes should be 30 m and the distance between the manhole and gully chambers should not exceed 6 m.
2. Drop Manhole
If the difference in level between the branch sewer and main sewer is within 60 cm and there is sufficient roof within the working chamber, the connecting pipe may be directly brought through the manhole wall by providing a ramp in benching. Such manholes which drop the level of invert of the incoming sewer, by providing a vertical shaft are called drop manholes. The main purpose being to avoid the splashing in sewage on the man working and on the masonry work. The branch sewer line is connected to the manhole in such a way that it can be cleaned and rodded when necessary. For inspection of the incoming sewage and cleaning of vertical shall the vertical shaft is taken up to the ground level as shown in Fig
1. 3. Catch Basins - These are small masonry chambers (75. to 90 cm in diameter and 75 to 90 cm deep) which are constructed below the street inlet to prevent the flow of grit, sand or debris in the sewer lines. The outlet pipe of catch basin is fixed about 60 cm above bottom as shown in Fig. The outlet pipe is provided with a trap to prevent the escape of odors from the sewer to the catch basins. Catch basins are provided in the following sections[1]
l. When the sewers are laid at very small gradient and velocity of flow is less than self-cleaning velocity.
2. When the drains are passing along the water bound macadam road or in sandy area and market. Catch basins collect the solids from the storm water. These solids are to be removed at frequent l intervals for the proper functioning of the catch basin otherwise they will block the passage of storm water in the sewers resulting in the flooding of the streets creating nuisance. Nowadays catch basins are not providing and street inlet may be directly connected with the sewers.
2. 4. Flushing Tanks: These are masonry or concrete chambers to flush the sewers when the sewers gradients are flat and velocity of sewage is very low. These are usually provided at the beginning point of the sewers and may be either are automatic or worked by hand. In automatic flushing tank, the water is automatically released from the tank at required intervals which can be adjusted by supply tap and flushes the sewer as shown in Fig. it consists of U-tube with bell cap at its one end connects the chamber with sewer. With the water level reaches certain level in the chamber, siphonic action takes place and the whole water of the chamber rushes to the sewer pipe and flushes t. The capacity of these tanks is usually 9 to 14 litres and may be adjusted in such a way as to work twice or thrice a day depending upon the quantity of deposits in the sewer and size of sewer.
3. 5. Inverted Siphon: Inverted siphon is a sewer section which is constructed lower than the adjacent sewer section and which runs full under gravity with pressure greater than atmosphere as shown in Fig.
Purpose: The main purpose of inverted siphon is to carry the sewer line below obstructions such as ground depressions, streams, rivers, railway etc. Siphon is so designed that a self-cleaning velocity of about 90 cm/sec during achieved the period of minimum discharge. For this purpose, the siphon is usually made of three pipe sections-one for carrying minimum discharge, the other for maximum discharge and the third for combined flow in mnonsoons. The inlet chamber contains three channels One for each pipe section. When channel no. 1 overflows, the sewage enters in channel no. 2 and pipe no. 2 comes into commission. Similarly, when channel no. 2 also overflows the sewage enters channel no. 3 and pine no. 3 comes into commission as shown in Fig. The inlet chamber should be provided with screens to remove silt, grit etc. from | sewage before enters the siphon.
Disadvantages
I. As the down gradient is not continuous in inverted siphon: the silting takes place.
2. It is not possible to give side connections to inverted siphons.
3. If inlet chamber is not properly designed, the floating matter contained in sewage will separate out and it will accumulate in the inlet chamber results in the inefficient functioning of the inverted siphon.
