List of Top 7 Sewer Appurtenances | Sewage | Sanitary Engineering

List of top seven sewer appurtenances:- 1. Manholes 2. Lamp-Holes 3. Drop Manholes 4. Street Inlets 5. Catch Basins or Pits 6. Sand, Grease and Oil Traps 7. Ventilating Shafts. 

Sewer Appurtenance # 1. Manholes:

A manhole is defined as the construction made to connect the ground level with the hole or opening made in the sewer line so that a man can easily, conveniently and safely enter through it and carry out the usual maintenance operation.

As far as possible sewers are laid straight. At every change of alignment, gradient or diameter of the sewer, manholes are constructed for giving access into the sewer for inspection, cleaning, repairs and maintenance. Manholes are masonry or R.C.C. chambers, which are constructed on the top of sewer. These are fitted with suitable cast iron covers at their top.

A manhole essential consists of:

(a) A working chamber;

(b) An access shaft; and

(c) A strong cover on the top flush with the road level.

The working chamber has such a size, so that necessary examination and cleaning can be done easily.

The minimum internal sizes of the chambers are as follows:

(i) For depth of 0.8 m or less – 0.75 × 0.75 m

(ii) For depth between 0.8 m and 2.1 m – 1.2 × 0.9m

(iii) For depth more than 2.1m – Circular chambers of 1.4 m diameter or rectangular chamber of 1.2 × 0.9 m

The access shaft provides an access to the working chamber. This shaft is formed by corbelling the working chamber on three sides at top as shown in Fig. 7.1, so that cover frame can be fitted in the opening. The minimum internal dimensions of the access shaft are 0.5 × 0.5m.

At the top of manhole, the manhole cover of cast iron or R.C.C. is provided to cover the opening. The cast iron cover is placed in the cast iron frame fixed at the top of access shaft. The cast iron covers may be light, medium or heavy weighing about 51, 153 or 255 kg respectively, which are used depending on the type of traffic on the road.

In case of R.C.C. covers a precast R.C.C. slab of suitable size is placed on the top of manhole opening. The, manhole covers are provided flush with the road level and have such a size that man can easily enter in the working chamber. The minimum size is 50 cm diameter.

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. The actual thickness is determined by design.

The circular shape is structurally more stable and stronger though it is difficult in construction. The manhole in circular shape should be atleast 180 cm high and 120 cm in diameter or it may be 90 × 120 cm in plan (the longer dimension should be along the flow).

For sewer larger than 130 cm in diameter, the manhole walls are made to spring from the sewer walls. The straight alignment of sewer line also requires manholes at regular intervals. The distance between two manholes on straight alignment depends mainly on the diameter of sewer line as below in Table 7.1.

If two or more sewers join at the same level in a manhole the branch channels should be given a smooth curve to meet the main channel. If the inlet and outlet pipes are of different diameters the crown of both the pipes should be placed at the same level and the necessary slope should be provided in the invert of the main channel.

Very often ignored element of sewer maintenance which requires careful attention and protective measures is the manhole work. The staff should be trained for removing a manhole cover, not only to avoid smashed toes and fingers, but also to prevent more serious back injuries. The most serious hazards of manhole work are, however, flammable as and oxygen deficiency.

The staff should be thoroughly trained to carry out simple tests on every manhole before entry for oxygen deficiency, combustible gases, carbon monoxide or hydrogen sulphide. If, however, an emergency demands to enter a gas tilled manhole or one where oxygen may be lacking, the worker should wear a self-contained air breathing mask and a safety harness with lifetime. Two other employees should be stationed at the manhole opening because one individual cannot lift an unconscious person out of a manhole.

Sewer Appurtenance # 2. Lamp-Holes:

In narrow lanes, change of gradient and slight curves where space is insufficient for the construction of manholes, a vertical shaft of20-30 cm diameter is connected to the sewer by a T-bend. These small size openings are covered by a cast iron or R.C.C. cover flush with the road level at the top. Fig. 7.3 shows a lamp hole mostly used in sewer lines.

While inspecting a lamp is lowered in the vertical shaft and is seen from the manholes on either sides to find that sewer is cleaned or obstructed. The lamp-holes are also provided at places when the regular manholes are placed at longer intervals.

Actually in practice during maintenance the lamp-holes are not used, therefore mostly local authorities do not recommend lamp-holes in the sewer lines.

Sewer Appurtenance # 3. Drop Manholes:

When it is uneconomical or impracticable to arrange the connection within 60 cm of the invert of the sewer and manhole, then a vertical shaft is constructed outside the manhole chamber through which the sewage of branch sewer is allowed to enter the manhole as shown in Fig. 7.2.

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 the 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 of sewage one 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 shaft, the vertical shaft is taken up to the ground level as shown in Fig. 7.2.

Sewer Appurtenance # 4. Street Inlets:

Street inlets or gullies are the openings in the street curb or gutter to collect the storm water and surface wash flowing along the street, and convey it to storm or combined sewer by means of stoneware pipes 25 to 30 cm in diameter. Inlets are placed at the road gutters generally at street junctions.

If the streets are very long more than 200 m, inlets are also provided at the intermediate points at 100, 130 m spacing. Fig. 7.4 shows most useful location of street inlets at some common places. At the street junction inlets should be placed in such a way that storm water may not flow across any of the streets or flood the cross-walks causing interference with the traffic.

The inlets are of the three types:

(i) Curb Inlets:

In which an opening is provided in the road curb for the entrance of storm water. Fig. 7.5 shows such inlet. In the gutter openings bars can be fixed to prevent the passage of dry-leaves, papers, etc. in the sewer line.

These inlets are more suitable than gutter inlets, because less quantity of floating solids enter in the catch pits. As the grating is fixed with the road curb, it cannot be easily stolen. Secondly the grating is cheap, as it does not has to bear the traffic load, as in case of gutter inlet.

(ii) Gutter Inlet:

These are placed directly below the road gutter and storm water directly enters them from the top. Such inlets catch very large volume of water and are most suitable in roads having steep slopes, because in such cases curb inlets may fail to catch all the storm water. These inlets are provided with cast iron gratings at their top to prevent the floating matters entering the sewer. The top grating should be sufficiently strong the bear the traffic loads. (Fig. 7.7).

The main difficulty with such inlets is that of the heavy cost. Grating placed at the top of the inlets to collect the water, is very costly and has good scrap value. Therefore, these are mostly stolen and the pit remains uncovered and becomes the source of accident. Such trouble does not arise in case of curb inlets, because their grating is fixed with the curve. Recently to avoid thefts of gutter inlet gratings, use of R.C.C. gratings has been started instead of C.I.

(iii) Combined Gutter and Curb Inlet:

Fig. 7.7 shows such an inlet in which the storm water enters from both the gutter and curb.

Curb inlets are preferred than gutter inlets and are mostly used in practice.

Sewer Appurtenance # 5. Catch Basins or Pits:

A catch basin is a structure in the form of a chamber which is provided along the sewer line to admit clear rain water free from silt, grit, debris, etc. into the combined sewer.

These are small masonry chambers (75 to 90 cm in diameter and 75 to 90 cm deep) which are constructed below the street inlets to prevent the flow of grit, sand or debris in the sewer lines. When storm water enters these basins, the grit, sand etc. settle in the bed and the storm water free from all these enters the sewers.

The outlet pipe of catch basin is fixed about 60 cm above the bottom as shown in Figs. 7.5, 7.6 and 7.7. The outlet pipe is provided with a trap to prevent the escape of odours from the sewers to the catch basins.

Catch-pits should be provided in the following sections:

(i) When the sewers are laid at very small gradient and the velocity of flow is less than self-cleansing velocity. At such situations as the solids present in the sewage will not move and will go on accumulating and will require continuous cleaning and maintenance of the sewer, which can be reduced by providing catch-pits.

(ii) When the drains are passing along the water bound macadam road or in sandy area, and the market (where lot of refuse is dropped in the drains). Because in such areas if all the solids moving in the sewage are allowed to enter the sewer, they will choke the sewer line.

The catch basins collect the solids from the storm water. These solids are to be removed at frequent intervals for the proper functioning of the catch basins, otherwise they will block the passage of storm water to the sewers, resulting in the flooding of the streets creating nuisance.

Previously when roads were not good, catch pits were mostly used, but now a days if the roads are good and streets are well-paved it is not necessary to provide catch-basins and the street inlets may be directly connected with the sewers.

Sewer Appurtenance # 6. Sand, Grease and Oil Traps:

The sewage from hotels, restaurants, kitchen and industries contains grease, oils and fats, which if not removed before it enters the sewers, will stick to the interior surface of the sewer conduit and will become hard and cause obstruction in the movement of the sewage. To check them, grease traps are required, which are placed in the pipe connecting the kitchen with sewer line. Fig. 7.10 shows the section through a grease trap.

Sewage from garages and service stations contains sand, mud, oils and grease which should also be removed before the sewage enters sewer line. Fig. 7.10 shows the section through a combined silt and oil trap which is used at such places. Such traps also prevent gasoline from entering the sewer and causing explosion hazard. These traps should be regularly cleaned for their proper functioning.

Sewer Appurtenance # 7. Ventilating Shafts:

Various gases are produced in sewers due to decomposition of organic materials of sewage. These gases are very foul in nature, cause harm to human health and corrode the sewers reducing their life.

The gases so produced are highly explosive and in high concentration may cause fatal accidents to the maintenance people on duty due to their explosive and poisonous character. Due to the above difficulties, ventilation is provided to the sewer lines at every 80-100 meters which will provide fresh air to the workers working in the manholes.

Fig. 7.15 shows a ventilating shaft commonly used in practice. It may be of R.C.C. or cast iron 15 to 23 cm in diameter with a cowl provided at the top. The height of the ventilating shaft should be more than the roof of the tallest building in its neighbourhood.

The ventilating shaft is generally connected to the manholes by a 15 cm diameter pipe as shown in the figure. In open areas, the manhole covers may be provided with vent pipes, but in crowded areas, they should be air-tight and connected with ventilating shafts.