Chapter seven Operation and Maintenance Drainage systems may be, corrode, erode, clog, collapse, and ultimately may reach to the point of failure. Maintenance is needed to maintain the operational function of the system and to extend its working life.

Maintenance strategies:

There are several reasons for the maintenance of a drainage system.

1-Public healthThe system itself should not cause a health hazard to either its users or its operators.

2-Asset managementAll systems were costly to construct and would be even more costly to replace. High tension must, therefore, be given to maintaining the physical integrity of the assets.

3-Maintain hydraulic capacityA primary function of maintenance is to preserve the as-built hydraulic capacity of the system. This will minimize the possibility of wastewater. This can be done by cleaning and ensuring, as far as is practicable, that the system is watertight.

4- Minimize pollutionAll combined and storm sewer systems have discharge points to the environment that come into operation periodically. Maintenance has a role in reducing the frequency of operation as far as possible, and in avoiding conditions in the system that cause build-up of pollutants.

Planned maintenance:In planned maintenance, problems are dealt with prior to failure.

Reactive maintenance: In reactive maintenance, problems are dealt with on a corrective basis as they arise (i.e. after failure).

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Operational functions:

The major 0peration and maintenance functions are: 1- Location and inspection. 2- Cleaning and blockage clearance. 3- Chemical dosing.

4 -Repair, or replacement.

Drain maintenance steps:

1- Periodic cleaning.2- Blockage removal.3- Repair.4- Replacement.

Sewer Location and I nspection :

The first steps in a maintenance are to check on the accuracy and completeness of existing records of the system. Before an inspection, it is necessary to locate the manholes and thereby determine the route of all the sewers in the system. The position and level cover, of each manhole can be determined using standard land surveying techniques. This procedure can now be substantially speeded up using GPS (global positioning satellite) technology, allowing positional data to be logged on-site in seconds. Visual inspection of flow directions in manholes is sometimes sufficient. Electronic tracing is also becoming common (see Figure). A probe or ‘sonde’ which emits radio signals is pushed, jetted or floated along the sewer and its progress tracked from the surface using a hand-held receiver. Using this approach, sewers up to 15 m deep can be traced to accuracy of 10%. Interference from signals generated by other buried metallic assets can, however, cause problems.

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Figure: Electronic tracing of sewers

Closed-Circuit Television (CCTV): A small TV camera incorporating a light source is propelled through the sewer and the images are relayed to the surface for viewing and recording (see Figure).

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Figure: CCTV image of a sewer in poor condition

Figure: sewer CCTV camera

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CCTV is a popular choice for sewer inspection, because internal investigation of sewer systems can be carried out quickly, and with minimal disruption, avoiding lengthy shutdowns and unnecessary excavation. This method is particularly useful in environments that are too small or hazardous for people to enter. CCTV can also be used to locate and define the cause of a known condition or defect, and to help establish a plan of maintenance. Progress is relatively rapid with typical rates of 400–800 m/day.The method is commonly used in pipes from 100–1500 mm in diameter.Generally, for sewers of <150 mm diameter the camera must be winched between manholes. For sewers of diameter greater than 150 mm, it is usual for the camera to be mounted on a self-propelled remotely- controlled tractor, with speeds ranging from 0.1 to 0.2 m/s.

Sewer cleaning techniques:

- Objectives

Sewer cleaning is carried out:

• to remove sediment in order to restore hydraulic capacity and limit pollutant accumulation.• to deal with blockages or offensive odors.• to permit sewer inspection.

•to aid sewer repair.

-Problems

1- Blockages:

These are defined as full or partial restrictions within the sewer and are most commonly found in smaller diameter pipes. A blockage is normally associated with a system defect (e.g., displaced joint, severe change of direction). The effect of a blockage ranges from partial loss of capacity to complete stoppage.

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2- Sedimentation:

Sediment is defined as any settle able particulate material that may, under certain conditions, form bed deposits in sewers. Sedimentation rarely completely chokes the pipe, but can still have a significant impact on capacity.

3-Tree roots:

Sewers are susceptible التأثر of tree) (تطفلto intrusion) (سرعةroots, which seek out moist conditions.The roots themselves are a nuisance( مضايقة), both in retarding the flow and also in initiating further blockage with larger solids.

Cleaning techniques:

A number of cleaning techniques and methods are in use, dependingparticularly on location and severity( قوة), including: Roding( .jetting, flushing and hand excavation ,(سحب )winching , (دفع A combination of more than one method may well be used in anyparticular locality.

Roding or Boring:

Boring is primarily procedure, in which short flexible rods are screwed together and then inserted into the blocked sewer. The principal action is the physical contact of the tools, although the compression of air by plungers( غطاس) and dense brushesفرشاة) ( can contribute.

Winching or Dragging:

Dragging is a technique involving the use of purpose-shapedbuckets ( دلو)that are dragged through the sewer collecting sediment, which is emptied out at a manhole. Although the drag can be manually operated, power driven devices are normally used.The procedure is capable of removing most materials, even in large pipes, but is most effective in sewers up to 900 mm diameter, up to 50% silted.

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Care has to be taken that damage is not caused to the sewer fabric.

Jetting:

Jetting is a widely used technique that relies on the ability of an applied high-pressure (100–350 bar) stream of water to dislodge material from sewer inverts and walls and transport it down the sewer for subsequent removal. Water under these high pressures is fed through a hose to anozzle containing a rosette وردة علىشكل of jets sited in ) (شيءsuch a way that the majority of flow is ejected in the opposite direction to the flow in the hose. The jets propel the hose through the sewer, eroding the settled deposits in the process. A range of nozzles is available to cope with specific situations.Modern combination units incorporate vacuum or air-displacement lifting equipment to remove the material, as well as to dislodge it without the need for man entry.Jetting is a versatile الجوانب and efficient procedure for ) (متعددremoving a wide rangeof materials and is widely used in practice. Concern over the possibility of damaging pipes during the jetting process has resulted in the publication ofthe Sewer Jetting Code of Practice.

Flushing:

Flushing is a technique in which short duration waves of liquid are introduced or created so as to scour the sediment into suspension and, hence, transport it downstream.

Waves may be induced by:

• hydrant and hose• mobile water tanker..

Hand excavation:

Historically, large diameter sewers were cleared by manual digging-out of deposited material. Laborers entered the sewer and shoveled sediment into skips that were transported to the surface for

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emptying. The method is limited in application to larger size pipes (>900 mm) and has significant health and safety implications .

Comparison of methods

No one method of cleaning is superior to the others on all occasions; each has its advantages and disadvantages.These are summarized in table.

Table: Relative performance of sewer cleaning techniques.

Topic Roding Winching Jetting Flushing

Sewer size (mm):<375 Good Fair Good Good

450–900 Poor Good Fair Fair

Max. cleansing distance (m) 25 100 100 50

No. of manholes required 1 2 1 1

Dislodging materials:

Invert Fair Good Good Good

Walls Fair Fair Good Poor

Joints Fair Fair Good Poor

Materials encountered:

Silt Fair Fair Good Good

Sand/gravel Poor Good Good Good

Rocks Poor Good Fair Poor

Grease Fair Fair Fair Poor

Roots Good Good Fair Poor

Material removed? No Yes Yes No

Damage potential Low Medium High Low

Flooding potential? No No No Yes

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