Sewer Types: 9 Main Types of Sewers | Waste Management

The sewers may be of the following types: 1. Brick Sewers 2. Cement Concrete Sewers 3. Asbestos Cement Sewers 4. Stoneware or Vitrified Clay Sewers 5. Cast Iron Sewers 6. Steel Sewers 7. Plastic Sewers 8. Fibre Glass Reinforced Plastic (FRP) Sewers 9. Pitch Fibre Sewers.

Type # 1. Brick Sewers:

Bricks have been used for the construction of sewers, particularly for large diameters, since very early times. The advantage of brick sewers is that these could be constructed to any required shape and size. However, because of the comparatively higher cost, larger space requirement, slower progress of work and other factors, bricks are now used for sewer construction only in special cases.

For the construction of brick sewers purpose-made bricks are used instead of ordinary bricks. The purpose-made or wedge-shaped bricks make the mortar joints of uniform width. Further brick sewers have cement concrete or stone for invert and 12.5 mm thick cement plaster with neat finish for the remaining surface. Under special conditions protection against corrosion may be necessary, and for this sewers are provided with a lining of stoneware or ceramic blocks on the inner surface.

Such a lining makes the sewer smooth and hydraulically efficient, and also provides resistance against sulphide corrosion. The outer surface of the brick sewers is generally plastered to prevent groundwater infiltration and the entry of tree roots through the brick joints.

Type # 2. Cement Concrete Sewers:

Cement concrete sewers are very widely used. These may be constructed at the site or may be manufactured in the factories or casting yards and then transported to the site. The former are known as cast-in-situ and the later are known as precast.

Cast-in-situ reinforced cement concrete sewers are constructed where they are more economical or when non-standard sections are required, or when a special shape is required or when the headroom and working space are limited.

Precast cement concrete sewers consist of precast cement concrete pipes which may be of two types viz., non-pressure pipes and pressure pipes. The non-pressure pipes may be either plain or reinforced. The plain non-pressure pipes are manufactured with their internal diameters varying from 80 mm to 450 mm and wall thickness varying from 25 mm to 35 mm.

The reinforced non-pressure pipes are manufactured with their internal diameters varying from 80 mm to 2600 mm and wall thickness varying from 25 mm to 215 mm. The pressure pipes are invariably reinforced and these are manufactured with their internal diameters varying from 80 mm to 1200 mm and wall thickness varying from 25 mm to 120 mm. The lengths of precast cement concrete pipes vary from 2 m to 3 m.

The reinforcement in the reinforced cement concrete pipe consists of circumferential reinforcement in the form of circular rings or spiral and longitudinal reinforcement in the form of straight bars of hard drawn steel or mild steel . The circular rings or spiral and the longitudinal bars are tied together to form a cage. The pipes having wall thickness 75 mm or less are provided with a single reinforcement cage, and those having wall thickness more than 75 mm are provided with double reinforcement cage.

Plain cement concrete pipes are used in sewerage systems on a limited scale only and generally reinforced cement concrete pipes are used. Non-pressure pipes are used for gravity flow (i.e., for sewers running partially full), and pressure pipes are used for pumping mains, submerged outfalls, inverted siphons and for gravity sewers where absolute watertight joints are required.

The Indian Standard IS: 458-1988 provides the detailed specifications for the precast cement concrete pipes with and without reinforcement. As per IS: 458-1988 the non-pressure pipes have been classified as NP1, NP2, NP3, and NP4. The class NP1 represents the plain non-pressure pipes, and the classes NP2, NP3 and NP4 represent the reinforced non-pressure pipes. Similarly pressure pipes have been classified as P1, P2 and P3.

The reinforced cement concrete pipes are manufactured by the following two methods:

(i) In one of the methods the reinforcement cage consisting of circumferential and longitudinal reinforcement is introduced into the mould and the cement concrete is poured into the mould and tamped.

(ii) In another method the reinforcement cage is introduced into the mould which is rotated about its longitudinal axis as the concrete is poured into the mould. These pipes are known as spun concrete pipes.

The ordinary reinforced cement concrete pipes are not suitable for use in situations where pressures exceed 75 m (or 0.75 N/mm²). In situations where relatively high pressures are met with steel cylinder reinforced concrete pipes also known as hume steel pipes may be used. The reinforcement in these pipes consists of a steel cylinder of specified thickness surrounded by a reinforcement cage of mild steel bars.

The reinforcement cage consists of circular rings or spiral and straight longitudinal bars of mild steel. These pipes are manufactured with their internal diameters varying from 200 mm to 1800 mm and wall thickness varying from 40 mm to 70 mm. 

The cement concrete pipes are provided with either plain ends or spigot and socket ends. The pipes with plain ends are joined by cement collar joint and those with socket and spigot ends are joined by socket and spigot joint.

Advantages of Cement Concrete Sewers:

The various advantages of cement concrete sewers are as indicated below:

(i) These sewers are strong enough to withstand internal pressure when used for carrying sewage under pressure, as well as external loads. Thus these sewers can resist both tensile as well as compressive stresses.

(ii) These sewers can be constructed at the site or can be manufactured in the factory or casting yard and then transported to the site.

(iii) They are quite resistant to erosion and abrasion.

(iv) They are economical for medium and large sizes, and hence they are widely used for branch and main sewers.

(v) The precast cement concrete pipes may be easily used at places where due to the presence of groundwater it may not be possible to construct brick sewers or cast in situ cement concrete sewers.

(vi) These can be made of any desired strength by proper design of concrete mix, wall thickness and the percentage of reinforcement and shape of the reinforcement cage.

(vii) The cement concrete pipes which may be used as sewers are available in a wide range of sizes.

Disadvantages of Cement Concrete Sewers:

The main disadvantage of cement concrete sewers is that they get easily corroded and pitted by the corrosive action of the contents of the sewage.

Such a corrosion is known as crown corrosion which is caused as explained below:

Crown Corrosion:

The sewage or wastewater contains nitrogen, calcium, phosphorous, chloride, sulphate, etc., and hence these organic and inorganic matter are contained in the sewer. The insoluble inorganic matter as well as organic matter are gradually deposited at the bottom of sewer, which results in an increase in the resistance to flow and consequent decrease in the velocity of flow.

With the reduction in the velocity of flow the sewage in the lower layers becomes stale in which aerobic or anaerobic condition prevails, and by bacteria sulphates are chemically reduced to sulphides and to hydrogen sulphide gas (H₂S) as shown by the following equations.

The hydrogen sulphide gas (H₂S) is malodourous, colourless and inflammable having odour like rotten eggs. The hydrogen sulphide gas (H₂S) combines with iron (Fe) present in sewage and sludge and forms ferrous sulphide (Fe S) which causes blackening of sewage and sludge.

A variety of bacteria known as thiobacillus thioxidans convert hydrogen sulphide gas (H₂S) into sulphuric acid (H₂SO₄) as shown below.

The sulphuric acid (H₂SO₄) is deposited at the crown of sewer in the form of droplets. It acts with concrete and forms CaSO₄. As the droplets fall down the layer of CaSO₄ also falls down. Thus the crown of sewer becomes uneven and its thickness reduces as this process continues. This phenomenon is known as crown corrosion.

Besides crown corrosion the cement concrete sewers may also be subjected to erosion due to high velocity of sewage containing silt, grit, etc.

In order to protect cement concrete sewers against the erosion due to crown corrosion their inner surface is lined with vitrified clay blocks. These blocks are provided with projections on the back and fixed in cement concrete sewers. The joints between adjacent blocks are filled with rich cement mortar or with bituminous compounds. However, cement mortar joints are subject to attack, and bituminous joints are emulsified and dissolved by soaps, oil and grease. As such acid proof cement joints may be used which provide better protection but they are costly.

Another material used for lining of cement concrete sewers is plastic polyvinyl chloride sheet, having T-shaped projections on the back which key into the pipe wall at the time of manufacture.

Various measures which may be taken to avoid crown corrosion are:

(i) Ventilating the Sewers;

(ii) Making the sewers to run full;

(iii) Pretreating the sewage so as to reduce sulphate content;

(iv) Prohibiting the entry of sewage containing sulphides;

(v) Aerating and chlorinating sewage;

(vi) Neutralising sulphides by addition of chemicals.

Type # 3. Asbestos Cement Sewers:

Asbestos cement sewers consist of asbestos cement pipes which are manufactured from a mixture of asbestos fibres and cement combined under pressure into a dense homogeneous structure in which a strong bond is effected between the cement and the asbestos fibre.

These pipes are manufactured with their internal diameters varying from 50 mm to 900 mm. The lengths of asbestos cement pipes vary from 1.5 m to 4 m. The Indian Standard IS: 1592-1989 provides the detailed specifications for the asbestos cement pressure pipes.

These pipes are provided with plain ends and the pipes are joined by means of a special type of joint or coupling called slip type sleeve joint. The commonly used slip type sleeve joint with different trade names are Simplex joint or Ring-tite coupling.

Advantages of Asbestos Cement Sewers:

The asbestos cement sewers possess the following advantages:

(i) The asbestos cement sewers are corrosion resistant in most natural soils and they offer enough resistance to salts, acids and other corrosive materials.

(ii) The inside surface of these sewers is exceptionally smooth and hence the least resistance is offered to the flow of sewage.

(iii) The sewers of this material are light in weight and hence they are easy to handle.

(iv) These sewers can be easily cut, drilled and threaded.

(v) These sewers can be easily joined without skilled labour.

(vi) The mechanical joints provided for these sewers are highly flexible, which may permit a deflection upto 12° when laid around curves.

(vii) These sewers are relatively cheap. Disadvantages of asbestos cement sewers.

The disadvantages of asbestos cement sewers are as indicated below:

(i) These sewers cannot resist heavy external loads and may be broken easily.

(ii) These sewers are brittle and cannot stand impact forces during handling operations.

(iii) They are subject to corrosion by acids, highly septic sewage and by highly acidic or high sulphate soils.

(iv) These sewers may be eroded due to high velocity of sewage containing grit, sand, etc.

Type # 4. Stoneware or Vitrified Clay Sewers:

The stoneware or vitrified clay sewers consist of stoneware or vitrified clay pipes which are manufactured from clays and shales of special qualities.

For manufacturing these pipes following procedure is adopted:

(i) The ingredients are taken in required proportion, pulverised and mixed together with water.

(ii) From the mixture pipes of standard lengths and diameters are formed in a pipe press at a pressure of about 0.85 N/mm² (or 8.5 kg(f)/cm²).

(iii) The pipes so formed are allowed to dry in warm air.

(iv) The dried pipes are then burnt in a kiln under regulated temperatures. The temperature in the kiln is maintained at 150°C in the beginning for several hours and it is then raised to about 650°C to 750°C. Finally the temperature is raised to about 1200°C at which vitrification takes place.

(v) At high temperature, a small quantity of salt is added to the kiln. The high temperature vaporises the salt and a glass like glaze is formed on the surface of the pipes. As such these pipes are also known as salt glazed stoneware pipes.

(vi) The exterior surface of spigot end and the interior surface of socket end are not glazed so as to make a watertight joint.

The stoneware pipes are manufactured with their internal diameters varying from 80 mm to 1000 mm but pipes of internal diameters greater than 380 mm are not generally used because of economic considerations. The lengths of stoneware pipes are 600 mm, 750 mm and 900 mm. These pipes are provided with spigot and socket ends and hence these pipes are joined by socket and spigot joints. The Indian Standard IS: 651-1992 provides the detailed specifications for the stoneware pipes.

The stoneware sewers are generally use for house drainage and for laterals.

Advantages of Stoneware Sewers:

The various advantages of stoneware sewers are as indicated below:

(i) These sewers are highly resistant to corrosion from most acids and to erosion due to grit and high velocities of flow.

(ii) The inner surface of these sewers is smooth and hence they are hydraulically efficient.

(iii) They are highly impervious.

(iv) These sewers are capable of withstanding hydraulic pressure up to 0.15 N/mm² (or 1.5 kg(f)/cm²), and bearing the load of soil of about 4.5 m depth.

(v) These sewers are cheap and easily available. Disadvantages of stoneware sewers.

The various disadvantages of stoneware sewers are as indicated below:

(i) These sewers are brittle in nature, and are therefore likely to be damaged during transport or handling operations.

(ii) They are weak in tension and hence these cannot be used where sewage is to be carried under pressure.

(iii) These sewers are bulky and heavy and hence it is difficult to handle, transport and lay them.

Type # 5. Cast Iron Sewers:

Cast iron sewers consist of cast iron pipes. Cast iron pipes possess high strength. They are structurally strong to withstand tensile, compressive as well as bending stresses. The three types of cast iron pipes which are commonly manufactured are centrifugally cast iron pipes, vertically cast iron pipes (or pit-cast iron pipes) and centrifugally cast ductile iron pipes.

Centrifugally cast iron pipes are manufactured with their internal diameters varying from 80 mm to 1050 mm, and lengths varying from 3 m to 5.5 m. Vertically cast iron pipes are manufactured with their internal diameters varying from 80 mm to 1500 mm, and lengths varying from 2 m to 5.5 m. Centrifugally cast ductile iron pipes are manufactured with their internal diameters varying from 80 mm to 2000 mm, and lengths varying from 4 m to 6 m.

The Indian Standard IS: 1536-1989 provides the detailed specifications for the centrifugally cast iron pipes. The Indian Standard IS: 1537-1976 provides the detailed specifications for the vertically cast iron pipes. The Indian Standard IS: 8329-1994 provides the detailed specifications for the centrifugally cast ductile iron pipes. The cast iron pipes are joined either by socket and spigot joints or by flanged joints. For the socket and spigot joints each pipe is provided with socket and spigot ends.

The advantages of cast iron pipes are:

(i) Longer lengths and watertight joints;

(ii) When properly designed can withstand relatively high internal pressure and external loads; and

(iii) Corrosion resistant in most natural soils.

However, the cast iron pipes are subject to corrosion by acids or highly septic sewage and acidic soils. Moreover, the cast iron pipes are relatively costlier.

As such cast iron sewers are not very commonly adopted but these are adopted only under special circumstances as indicated below:

(i) Heavy External Load:

The cast iron sewers are more suitable at places where external loads on sewers may be heavy as in case of sewers to be laid under railway line, foundation walls, high overburden of soil, etc.

(ii) High Internal Pressure:

When sewage is to be conveyed under pressure, as in case of pumping of sewage, it is desirable to adopt cast iron sewers. Thus for conveying sewage through pumping stations and treatment works, cast iron sewers may be adopted.

(iii) Crossing Low Level Areas:

When sewers are to be carried above ground level on trestles while crossing low level areas, cast iron sewers may be used.

(iv) Under Expensive Road Surface:

If road surface is expensive such as cement concrete, it becomes a costly affair to disturb such a surface for carrying out repair operations for a sewer laid under such a surface. As such under such circumstances cast iron sewers are preferred because they do not require frequent repairs.

(v) Projection against Contamination:

The cast iron sewers are adopted where there is danger of contamination of groundwater due to leakage of sewage from sewers. Cast iron pipes and their joints are completely leak proof. Further they are also preferred when sewers are to be laid under or over the water supply pipelines.

(vi) Temperature Variation:

Cast iron sewers are highly suitable at places where there is large variation in temperature during a day or during the year.

(vii) Vibrations:

Cast iron sewers are suitable where ground is likely to be subjected to heavy movements and vibrations.

(viii) Wet Grounds:

It is desirable to provide cast iron sewers through wet grounds. This results in considerable reduction in the quantity of infiltration into the sewers.

Type # 6. Steel Sewers:

Steel sewers consist of steel pipes which are fabricated by rolling the mild steel plates to proper diameter and either welding or riveting the edges. Welded pipes are smoother and stronger than riveted pipes and hence welded pipes are more commonly used.

Welded steel pipes are made with their internal diameters varying from 150 mm to 900 mm and lengths up to 12 m. However, welded steel pipes of internal diameter as large as 2400 mm are also made. The Indian Standard IS: 3589-1991 provides the detailed specifications for electrically welded steel pipes.

Steel pipes can withstand internal pressure, impact load and vibrations much better than cast iron pipes. Further they are more ductile and can withstand higher water hammer pressures.

The disadvantages of steel pipe is that it cannot withstand high external load. Further it is likely to collapse when it is subjected to negative pressure.

Steel pipes are susceptible to various types of corrosion. Therefore steel pipes should not be used for partially full sewers. A thorough soil survey is necessary all along the alignment where steel pipes are proposed to be laid. Steel pipes should be protected against both internal and external corrosion.

For this steel pipes may be galvanized (i.e., inside and outside surfaces of the pipes are coated with zinc) or inside and outside surfaces of the pipes may be coated with bituminous material or may be lined with cement mortar. Steel pipes may also be protected by cementing plasticized polyvinyl chloride sheets to the pipes.

The steel pipes are joined either by welded joints or by mechanical coupling or mechanical joint.

The cost of steel pipes is also quite high and hence these are not commonly used as sewers. However, some of the situations where steel pipes may be preferred are pressure sewer mains, under water river crossings, bridge crossings, necessary connections for pumping stations, self-supporting spans and railway crossing.

Besides smooth-walled steel pipes corrugated steel pipes (or corrugated iron pipes) are also used as sewers. The corrugated iron sewers are mainly used for carrying storm water (or rain water) only. The corrugations of the sewer reduce the velocity of flow and consequently, for given diameter and slope, the carrying capacity of sewer is also reduced. These sewers also needs to be protected from the effects of corrosion by galvanization or by bituminous coatings.

Type # 7. Plastic Sewers:

Plastic sewers are not yet very common in our country. These sewers consist of plastic pipes. The two types of plastic pipes which may be used as sewers are unplasticized Poly Vinyle Chloride pipes (PVC pipes) and High Density Polyethylene pipes (HDPE pipes). However, PVC pipes are not very suitable for sewerage works and hence mostly HDPE pipes are used as sewers.

The various advantages of plastic pipes are:

(i) They possess excellent flow characteristics which permit flatter gradients;

(ii) They are corrosion resistant;

(iii) They are available in long lengths (usually 6 m or so); and

(iv) They are not brittle like asbestos cement and other pipes and hence a hard fall at the time of loading, unloading, handling, etc., cannot do any harm to them.

The PVC pipes are usually joined by socket and spigot joint and HDPE pipes are joined by welding.

The detailed specifications for these types of plastic pipes which may be used as sewers are provided by the following Indian Standards:

Type of Plastic Pipe Indian Standard Indian Standard

(i) High Density Polyethylene pipes (HDPE pipes) – IS: 4984-1987

(ii) Poly Vinyle Chloride pipes (PVC pipes) – IS: 4985-1988

Type # 8. Fibre Glass Reinforced Plastic Sewers (FRP Sewers):

Fire glass reinforced plastic sewers consist of fibre glass reinforced plastic pipes. These pipes are made up of fibre glass, polyester resin and fillers. These pipes possess better strength, durability, high tensile strength, low density and highly corrosion resistant. Fibre glass reinforced plastic pipes are manufactured with diameters up to 2400 mm and lengths upto 18 m.

These pipes are widely used in other countries where corrosion resistant pipes are required at reasonable costs. In our country also these pipes are now being manufactured. The Indian Standard IS: 12709-1989 provides the detailed specifications for the fibre glass reinforced plastic pipes.

Fibre glass reinforced plastic pipes are joined by using double bell coupling.

Fibre glass reinforced plastic can also be used as a lining material for conventional pipes which are subject to corrosion. Fibre glass coating can resist external and internal corrosion whether the corrosion mechanism is galvanic or chemical in nature.

Type # 9. Pitch Fibre Sewers:

Pitch fibre sewers consist of pitch impregnated fibre pipes. These pipes are of light weight and are quite durable. These pipes are flexible, resistant to heat, freezing and thawing and earth currents which set up electrolytic action. They are also unaffected by acids and other chemicals, water softeners, sewer gases, oils and greases and laundry detergents.

These pipes can be easily jointed in any weather condition as internally tapered couplings join the pipes without the use of jointing compound. These can be cut to required length on the site. Because of larger lengths, cost of jointing, handling and laying is reduced. These are generally recommended for all drainage uses such as house connection to sewers and septic tanks, farm drainage, down pipes, storm drains, industrial waste drainage, etc.

These have recently been manufactured in our country. These are manufactured with their internal diameters varying from 50 mm to 225 mm and lengths varying from 1.5 m to 3.5 m. These pipes are joined by taper coupling joints or rubber ring joints. The Indian Standards IS: 11925-1987 provides the detailed specifications for the pitch fibre pipes.