Environmental Engg. 3(2+1)

Lesson-14 Introduction to sewers

Important points to be considered before selecting sewer material

The following points are to be considered before selecting a material for constructing sewer:

Strength and durability

The sewer should have sufficient strength to withstand all the forces that are likely to fall on them. Sewers are subjected to considerable external loads of backfill material and traffic load, if any. They are not subjected to internal pressure of water. To withstand external load safely without failure, sufficient wall thickness of pipe or reinforcement is essential. In addition, the material selected should be durable and should have sufficient resistance against natural weathering action to provide longer life to the pipe.

Resistance to abrasion

Sewage mostly contains grit.  These particles moving at high velocity can cause wear and tear of sewer material. This abrasion can reduce thickness of pipe and reduces hydraulic efficiency of the sewer by making the interior surface rough. Therefore, the material of the sewer should be in a position to offer enough resistance to abrasion.

Resistance to corrosion

Sewer carries wastewater that releases gases such as H₂S. This gas in contact with moisture can be converted in to sulphuric acid. The formation of acids can lead to the corrosion of sewer pipe. Hence, selection of corrosion resistance material is needed for long life of pipe.

Weight

To facilitate easy handling and transportation, the sewers should have less specific weight.

Imperviousness

This property is also important in selecting a sewer material.  To eliminate chances of sewage exfiltration and infiltration, the material selected for pipe should be impervious.

Cost

Sewer should be less costly to make the sewerage scheme economical.

Hydraulically efficient

The sewer shall have smooth interior surface to have less frictional coefficient

Materials for sewer

Asbestos Cement Sewers

These types of sewers are manufactured from a mixture of asbestos fibre, silica and cement. These pipes are available in size 10 to 100 cm internal diameter and length up to 4.0 m. These kind of sewers are normally used for house drainage.  They are not recommended for underground situations.  These pipes are used for vertical transport of water.  For example, transport of rainwater from roofs in multistoried buildings, for transport of sewage to grounds, and for transport of less foul sullage i.e., wastewater from kitchen and bathroom.

Advantages

• These pipes are light in weight and hence, easy to carry and transport.

• Easy to cut and assemble without skilled labour.

• Interior is smooth (Manning’s roughness coefficient = 0.011) hence, can make excellent hydraulically efficient sewer, offering least resistance to flow.

• They are durable against corrosion.

Disadvantages

• These pipes are not very strong and hence should not be subjected to heavy super-imposed loads.

• These are susceptible to corrosion by sulphuric acid. When bacteria produces H₂S, in presence of water, H₂SO₄ can be formed.

Plain Cement Concrete or Reinforced Cement Concrete

Plain cement concrete (1: 1.5: 3) pipes are available up to 0.45 m diameter and reinforcement cement pipes are available up to 1.8 m diameter. Large sized cement concrete sewers are generally reinforced.  These pipes can be cast in situ or precast pipes. Precast pipes are better in quality than the cast in situ pipes.  The reinforcement in these pipes can be different such as single cage reinforced pipes, used for internal pressure less than 0.8 m;  double cage reinforced pipes used for both internal and external pressure greater than 0.8 m;  elliptical cage reinforced pipes used for larger diameter sewers subjected to external pressure; and hume pipes with steel shells coated with concrete from inside and outside.  Cement concrete pipes may be used for surface water drains in all diameters.

Advantages

• Economical for medium and large sizes.

• It is possible to effect saving in the cost of jointing, owing to the longer lengths in which these pipes are generally available.

• They can be made of any desired strength.

• Strong in tension as well as compression.

• These pipes are available in wide range of size and the trench can be opened and backfilled rapidly during maintenance of sewers.

Disadvantages

• These pipes can get corroded and pitted by the action of H₂SO₄.

• The carrying capacity of the pipe reduces with time because of corrosion.

• The pipes are susceptible to erosion by sewage containing silt and grit.

When cement concrete pipes are subjected to chemical and corrosive actions, one method of protecting them from such actions is the lining of the sewer by vitrified clay blocks.  These blocks have projections, projecting downwards and are fixed in cement concrete of the sewers.  The joints between adjacent blocks are filled with cement mortar or with bituminous compounds.

Brick Sewers

Brick sewers are generally made at site.  They are used for construction of large size sewers or particularly for storm water drains. The pipes are plastered from outside to avoid entry of tree roots and ground water through brick joints. These are lined from inside with stone ware or ceramic block to make them smooth and hydraulically efficient. Lining also make the pipe resistant to corrosion.  If brick sewers are to be used on a large scale, the purpose made bricks should be used instead of ordinary bricks.  The advantage of brick sewers is that these could be constructed to any required shape and size.

Galvanized iron sewers

Corrugated iron sewers are used for storm sewers.  They are made from ingot iron or rust resisting alloys, with galvanizing for additional protection.  Bituminous coatings are also used inside and outside, and the invert is sometimes, covered with abrasion resistant bituminous material.  It is made in varying metal thickness and in diameters up to 4.5 m.

Cast Iron

Cast iron pipes may be used in the form of sewers, where they have to withstand high internal pressures and external loads.  However, these are costly.  Cast iron pipes are used for outfall sewers, rising mains of pumping stations, and inverted siphons, where pipes are running under pressure. These are also suitable for sewers under heavy traffic load, such as sewers below railways and highways.  Light cast iron pipes are used for house drainage works, whereas, heavy ones are adopted for city sewers. 

Steel pipes

These types of sewers are used where lightness, imperviousness and resistance to high pressure are of paramount importance.  These sewers are flexible and can absorb vibrations and shocks efficiently.  They are more ductile and can withstand water hammer pressure better.  They are susceptible to corrosion and are not generally used for partially flowing sewers. They are protected internally and externally against the action of corrosion.  These sewers may be protected from corrosion by galvanizing or by applying a bituminous coating or by using special corrosion-resistant steel. Their use may be made in outfall or trunk sewers.

Stoneware sewers

The stoneware sewers are also known as vitrified clay sewers.  This type of sewer is made of clay or shale which has been ground, mixed with water, moulded into shape, dried and finally burnt in the kiln at a very high temperature.  At the end of the burning period, some common salt is put in the kiln.  The intensive heat cause the salt to vaporize and there is a reaction between salt vapours and the clay to form a hard water proof glaze.  The heat also causes a fusion or vitrification of the clay which makes it very dense and hard.  The temperature in the kiln is maintained at 150°C for several hours in the beginning, but it is raised later to about 650°C to 750°C.  Finally, the temperature is raised to 1200°C where vitrification takes place. 

These sewers are normally favoured for house drainage connections and laterals.  The interior surface of these sewers is smooth and impervious.  If properly laid, they are strong enough to take the load of back-filling and traffic.  They are cheap, easy to lay and join, resistant to corrosion and extremely durable.  But they are brittle, heavy and get damaged while handling and transportation.

Plastic sewers

Plastic is recent material used for sewer pipes. These are used for internal drainage works in house. These are available in sizes 75 to 315 mm external diameter and used in drainage works. They offer smooth internal surface. The additional advantages they offer are resistant to corrosion, light weight of pipe, economical in laying, jointing and maintenance, the pipe is tough and rigid, and ease in fabrication and transportion of these pipes.

High Density Polythylene (HDPE) Pipes

Usage of HDPE sewers is in its nascent stage of development. They are not brittle like asbestos cement pipes and other pipes and hence does not cause damage to pipes during loading, unloading and handling. They can be joined by welding or can be jointed with detachable joints up to 630 mm diameter.  These are commonly used for conveyance of industrial wastewater. They offer all the advantages offered by PVC pipes.

Glass fibre reinforced plastic pipes

This material is widely used where corrosion resistant pipes are required. GRP or FRP can be used as a lining material for conventional pipes to protect from internal or external corrosion. It is made from the composite matrix of glass fiber, polyester resin and fillers. These pipes have better strength, durability, high tensile strength, low density and high corrosion resistance. These are manufactured up to 2.4 m diameter and up to 18 m length.

Pitch Fibre Pipes

The pitch impregnated fibre pipes are of light weight and have shown their durability in service. The pipes can be easily jointed in any weather condition as internally tapered couplings join the pipes without the use of jointing compound. They 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. They can be cut to required length on the site. Because of the larger lengths, cost of jointing, handling and laying is reduced. These are generally recommended for 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 India.