Sanitary Engineering

Lecture 16

Storm water collecting Apparatuses( techniques)

Upper the Ground Surface RunoffThere are many ways and techniques to collecting the upper runoff, every way or technique has special characters and use. Some of these techniques are:

Roof top harvesting Roof top harvesting is an old technique and use since today it's practiced in many places throughout the world , the rain water can be diverted to the existing wells or tanks.

Streets as harvesting facilities

The simplest and most obvious facility for harvesting of site runoff is the use of streets and roads as channels. This technique is become the storm water harvesting system for most of the countries. There are two types of road drainage:

Roof type roads.Channel type roads

Open ChannelThis technique is more elaborate because they use does not involve flooding the streets also the channel can trap sediment

Micro basin It is a technique to collect storm water in localized basins served by small watersheds. Micro basins are appropriate for use on gently sloped or nearly flat land with low volumes of runoff water such as along sidewalks, in landscaped areas, in parking lot planters, etc. Micro basins can be used in areas with more concentrated runoff if they are arrayed in a series with spillways in between

French Drains French drains are rock-filled trenches that are designed to encourage rapid storm water infiltration through the sides, ends and bottom of the trench where soil and water meet. Because they are filled with rock, French drains hold only about one-third of the water that an open trench would hold. French drains can intercept low to moderate flows depending on their size. French drains are typically used on flat to moderate slopes. French drains should receive runoff water only from watershed areas that are relatively free of sediment like parking lots, rooftops, sidewalks, or areas stabilized with vegetation.

GabionsConstruct a semi permeable barrier or grade control structure to slow, but not stop, the flow of storm water in a small watercourse to help prevent or repair upstream erosion, trap rich detritus, and allow storm water to infiltrate into the channel sediments and adjacent soils.

ReservoirsThis technique is constructed to collect rain fall directly or by allow rain water to flow into the reservoirs. This may be ponds or pits out in open areas with impermeable base and walls such as concrete reservoirs

Lower the Ground Surface Runoff:

After collecting the storm water from the upper runoff, the water travel to its direction under the ground surface. The lower surface techniques are:

Pipes.ManholesChannels.Culverts.Inlets.Pumping station.

Sanitary Sewers Appurtenances

Manholes: Manholes are constructed in the following cases:

• when pipes change in diameter • change of direction• change of slope• intersection of pipes• at interval, ( 20-100 m)

Manufacture typesThere are two types of manholes manufacture:Precast concrete manholes Some company's facture standard manholes with typical dimensions, to be easy to use when need. Cast in situ concrete manholes Some dimensions are not a viable at precast or there need for special reinforce and other needs so the user prefer the cast in situ manholes.  Geometry typesThere are three geometry types for manholes:

Circular.Rectangular.Octagon.

The following table gives the allowable intervals of manholes relative to the diameter:

Pipe diameter ( inch ) Max. distance between manholes (m)

8 30

8-10 40

10-12 50

12-16 60

16-36 100

≥ 36 150

Note: The distance depends on the maintenance equipments available.

Manhole dimensionsThe diameter of the manhole or its side's dimensions depends on the depth of excavation.

The following table gives their relation. Depth of manhole Manhole dimensions

0.60 x 0.60 m [Square] Depth ≤ 0.90 m Φ 0.60 m [ Circular]

1.00x1.00 [Square]

Φ 1.00 m [ Circular] 1.50-2.00 m

0.80 x 1.20 m [Rectangular]

≥ 2.00 m Φ 1.50 m [ Circular]

The cover of the manhole should be strong enough to withstand the loads of traffic.

It is usually made of cast iron to carry a minimum concentrated load of 25 ton.

The manhole should be supplied with steps to allow for maintenance access.

The floor of the manhole should be lined with cement mortar which is called benching.

Brick Manhole

Has a cast iron frame and cover with 500 – 600 mm opening. The frame rests on brickwork which is corbelled as shown to form a cylinder from 1-1.25 m in diameter which extends downward to the lowest sewer. The walls are typically 200 mm thick for depths up 4 m and increase by 100 mm for each additional 2 m of depth. The interior of brick manholes is often plastered with Portland cement or mortar

The bottom of the manholes is normally concrete, sloping toward an open Channel which is an extension of lowest sewer. The open Channel is some-times lined with half-round or split sections of sewer pipe. The Channel should be sufficiently well-defined and deep enough to prevent sewage from spreading over the bottom of the manhole.

Concrete manhole with junction of branch sewer

Drop manhole are used when the

difference of elevation between the inlet pipe and the outlet pipe is ≥ 60cm. The drop manhole has a vertical pipe to prevent turbulence in the manhole and to allow the maintenance works to enter the manholes safely.

Grease and oil traps

Grease and oil traps: For the institutions, commercial units, restaurants and other places which produce oil and grease in there effluent, a grease and oil trap should be used to remove oil and grease before they enter the sewage pipes. Grease and oil affect the sewers and the treatment plant equipments that is why they should be removed. In case of the pipes, grease sticks to the walls and collects sand and other solids leading eventually to the decrease in the pipe diameter and some times to complete clogging.