Sewer appurtenances

Sewer appurtenances are those structures and devices of a sewerage system which are constructed at suitable intervals along a sewer line to assist in the efficient operation and maintenance of the system.

Following are the important sewer appurtenances:

 1. Inlets 

2. Catch Basins or Catch Pits

3.Manholes 

4.Lamp-Holes

5. Flushing Devices

6.Grease and Oil Traps

1. Inlets:

Inlets are the devices meant to admit storm water (or rain water) and surface wash flowing along the roads/streets and convey the same to storm water sewers or combined sewers.

 An inlet is a small box like chamber made of brickwork or concrete, having an opening at the top in vertical or horizontal direction for the entry of storm water (or rain water) and surface wash. 

The water from this chamber leaves through an outlet provided at its bottom and carried by a pipeline to a nearby manhole.

Inlets are of three types 

(i) Curb Inlets

(ii) Gutter Inlets and

(iii) Combination Inlets

(i) Curb Inlets:

Curb inlets (also called vertical inlets) have vertical openings in the road curbs through which storm water flows. 

The openings are provided with gratings of closely placed bars.

 These inlets are preferred where heavy traffic is anticipated.

(ii) Gutter Inlets:

Gutter inlets (also called horizontal inlets) have horizontal openings in the gutter which are covered by gratings through which storm water flows.

 The clear opening between the bars of the gratings should not be more than 25 mm and the gratings should be capable of sustaining heavy traffic loads.

(iii) Combination Inlets:

Combination inlets are composed of a curb and gutter inlet acting as a single unit. Normally, the gutter inlet is placed right in front of the curb inlet but it may be displaced in an overlapping or end-to-end position.

2. Catch Basins or Catch Pits:

A catch basin or catch pit is a device meant for the retention of heavy debris in storm water which otherwise would be carried into the sewer.

 It is an inlet with its outlet being placed well above its bottom so that heavy debris such as grit, sand, etc., flowing along with storm water is allowed to settle down and thus prevented from entering the sewer.

The outlet from the basin is provided with a hood or it is trapped to prevent escape of foul gases from the sewer and to retain floating matter. At the bottom of the basin space is provided for the accumulation of the settled matter.

Catch basins need periodical cleaning, as otherwise the settled organic matter may decompose, producing foul odours, and may also become a breeding place for mosquitoes.

3. Manholes:

Manholes are the masonry or R.C.C. chambers constructed at suitable intervals along the alignment of sewers to provide access to the sewers for the purpose of inspection, testing, cleaning and removal of obstructions from the sewer lines.

They also help in joining sewer lines and in changing the direction or alignment as well as gradient of sewer lines.

 Manholes may receive sewage from sewers coming from various directions and also from sewers of various sizes.

 Manholes are usually constructed directly over the centre line of the sewer.

 They are circular, rectangular or square in shape.

Location of Manholes:

Manholes are provided at every change in alignment of sewers, at every change in gradient of sewers.

Alternatively the values of spacing of manholes on straight reaches of sewers recommended in the Manual on Sewerage and Sewage Treatment prepared by Central Public Health and Environmental Engineering Organisation are as indicated below:

(i) For sewers of diameter 0.9 to 1.5 m, spacing of manholes above 90 to 150 m may be allowed.

(ii) For sewers of diameter 1.5 to 2.0 m spacing of manholes at 150 to 200 m may be allowed, which may further be increased up to 300 m for sewers of over 2 m diameter.

(ii) In case of very large sewers a spacing allowance of 100 m per 1 m diameter of sewer may be considered as a general rule.

 It is provided at suitable place so as to keep the sewer line below ground level and to lay it at a limiting gradient.

4. Lamp-Holes:

Lamp-holes are small openings provided on sewer lines mainly to permit the insertion of a lamp into the sewer for the purpose of inspection of sewer lines and detecting the presence of any obstructions inside the sewers.

A lamp-hole consists of a vertical stone ware or concrete or cast iron pipe 225 to 300 mm diameter, connected to the sewer line through a ‘T’ junction.

The pipe is surrounded by concrete to make it stable. 

At the ground level the lamp-hole is provided with a manhole cover with frame strong enough to take up the load of traffic.

Flushing Devices:

When sewers are to be laid in a flat country, it is not possible to obtain a self-cleansing velocity even once a day due to flatness of gradient especially at the top ends of branch sewers which receive very little flow.

Thus flushing devices help to prevent clogging of sewers and permit the adoption of flatter gradients than those required to maintain self-cleansing velocity.

Grease and Oil Traps:

Grease and oil traps are the chambers provided on the sewer line to exclude grease and oil from sewage before it enters the sewer line.

 These are located near the sources contributing grease and oil to sewage, such as automobile repair workshops, garages, kitchens of hotels, grease and oil producing industries, etc.

It is essential to exclude grease and oil from sewage due to following reasons:

(i) If grease and oil are allowed to enter the sewer, they will stick to the inner surface of the sewer and will become hard, thus cause obstruction to flow and reduce the sewer capacity.

(ii) The suspended matter which would have otherwise flown along with sewage will stick to the inner surface of the sewer due to sticky nature of grease and oil, thus further reduce the sewer capacity.

(iii) The presence of grease and oil in sewage makes the sewage treatment difficult as they adversely affect the bio-chemical reactions.

(iv) The presence of a layer of grease and oil on the surface of sewage does not allow oxygen to penetrate due to which aerobic bacteria will not survive and hence organic matter will not be decomposed. This will give rise to bad odours.

(v) The presence of grease and oil in sewage increases the possibility of explosion in the sewer line.

The principle on which grease and oil traps work is very simple. 

The grease and oil being light in weight float on the surface of sewage.

 Hence, if outlet draws the sewage from lower level, grease and oil are excluded. 

The wastewater obtained from garages, particularly from floor drains and wash racks, contains grease, oil, sand and mud. 

To trap all these a combined sand, grease and oil trap is provided.