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Sewer Cleaning

The cleansing of a pipeline and particularly of a sewer means the effective cleaning and removal of deposited debris. Sewers, for instance are normally cleaned by one or more of the following methods:-
• flushing
• rodding
• winching
• jetting
• manual
Historically, this has been a popular technique to transport sediments in a sewer but is not so common these days. It is most effective in 100mm. - 375mm dia. pipes but can move debris in sewers up to 900mm. dia. The most common methods are:-
• Dam & Release - flow held by a stopper or hand operated gate to create an artificial storm when released.
• Automatic Gate - tipping bucket mechanism releases flow at regular intervals but is dependant on incoming flow.
• Automatic Syphon - most widespread relic of the flushing technique, water fills a chamber and discharges at regular intervals through syphonic action.
• Hydrant - water supply connection through 100mm. hose.
• Mobile Water Tank - filled from a hydrant and discharged directly into a manhole or purpose built flushing eye.
The advantages of this method include: no adverse effect on structure of sewer; little or no labour requirement; low machinery input and the disadvantages include: silt/debris not removed, merely moved along pipeline; little control over sudden rush of water.
Rodding is a cheap effective method of clearing blockages in small diameter sewers (up to 225 mm) for instance and when they are less than about 2 metres deep. It is a simple method using flexible jointed rods, made of cane, plastic, steel or more commonly now, continuous, semi-rigid lengths of coiled GRP, which are easier and quicker to use than jointed rods. The rods are simply pushed along the sewer to clear the blockage.
There are two methods of rodding - hand rodding and machine rodding, the latter being popular with drain cleaning contractors. These machine driven rods can accept a variety of tools for whichever purpose required. However, rodding is restricted to short lengths and small diameters for effective cleaning.
The main advantages are: cheap and effective if the blockage is cleared quickly; simple to use; does not require man entry into shallow manholes and the disadvantages include: distance able to rod effectively limited to 20/30m; can be time consuming and therefore expensive; can cause damage, particularly machine rodding; blockages can recur if material is not removed; size limitation.

If an experienced gang is used, winching can be an effective method for silt or sand removal, albeit slow progress can be expected. Hand or power winching techniques are used, the latter requires particular attention to safety and a "sense" for the job by the operators, e.g. stuck buckets, cable break, etc.
The method is most effective in the larger sizes non man-entry sewers, i.e. 450 mm - 900 mm dia. A small bucket, size dependent on the problem, say from 150 mm dia. is pulled through a sewer length. The bucket size is increased to a maximum of 450 mm or 600 mm - it is obviously difficult to pull anything larger, even with a power winch. Holes are located in the base of the bucket to allow water to pass through. Pulleys should be fixed in manholes to assist the "pull", without which the winch cable may cut into the manhole roof slab, often getting stuck.
The collected material is usually deposited in skips for removal from site by a skip wagon.
The main advantages of the technique are: effective removal of debris; can be used in urban areas without major traffic disruption; can be used on large diameter sewers however the disadvantages have probably made this an out dated method: must have access at both ends and clear route between manholes, e.g. bends - wire cuts into side of pipe, intruding connections, displaced or large open pipe joints, loose bricks, pipe diameter change, etc.; can cause structural damage; slow progress, therefore expensive. Close monitoring and control is essential, particularly when power winching is used.
This is the most popular and important method of cleansing, particularly with the increasing use of recycling jet/vac machines. To be effective a short length affected by silt and debris, say 10m, must be pulled back to a manhole, say before moving onto the next 10 m section. The purpose of jetting is twofold - to loosen sediments, debris and material coating a sewer and to then move this material along the pipe, usually in a downstream direction, to a location from where it can be removed. Removal of the material is normally carried out using a vacuum or air displacement device, which, these days, is usually combined with the jetting equipment and mounted together on one vehicle. Jetting machines for sewer cleaning are usually of the low pressure - high volume type, with operating pressures of between 140 and 200 bar and flow rates of 200 up to 750 litres per minute.
Water is fed at pressure through small diameter jetting hoses (40mm) to a jetting nozzle which can be equipped with both perpendicular jets and backward inclined jets. The purpose of the jets is to provide the forward propulsive force and to remove material on the sewer walls and invert. A variety of nozzles and jets allow various cleansing activities to take place ranging from blockage clearing to large diameter sewer desilting projects. Jetting is most effective in smaller pipe sizes 150mm - 375mm diameter but can be used successfully in removing debris from the inverts of pipe diameters 450mm - 900mm. Jetting machines using very high flow rates suffer from limitations on the length of time they can be operated, due to the fact that the water storage capacity in the vehicles themselves is limited to a maximum of around 8000 litres. This can be overcome to some extent by using slave tankers to feed the main machine but the ability to recycle the used water and thus operate continuously is generally the favoured method for large diameter sewer cleaning – see 'Recycling Machines' below.
Jetting at higher pressures of 800 bar up to 2000 bar can be used (with limited success) for cutting tree roots and intruding laterals – it is more common to use specialist high pressure jetting systems or robotic mechanical grinding methods to remove these – see Robotic Repair Systems in the Localised Repair Techniques section.
There are five types or classes of jetting unit:-
Super combination: jetting unit with high suction lift
Combination suction jetting: lower pressure jetting unit with limited suction
HGV jetting unit: high pressure jetting unit only
Non HGV jetting unit: trailer or van mounted
Small drain units: drain cleaning contractors
The main advantages include: flexibility - can be used from one manhole, usually downstream; removal of debris can be carried out without entry into manhole by operator, i.e. Class 1 and 2 jetting units; efficient transportation of debris; effective for final cleaning prior to rehabilitation procedure.
Disadvantages are: maintaining water supply; plant maintenance; HGV drivers required for Classes 1, 2 and 3.
Jetting Nozzles
Jetting nozzle technology has become a science in its own right in recent years, particularly as the desire to avoid man-entry methods is encouraged and the use of large recycling machines to remove vast deposits of silt and debris becomes the 'standard' method of work.
Further details of this topic will be provided in a forthcoming review of the available technology.
Recycling Machines
Much of the recycling technology advances have come from European sources, particularly (but not exclusive) from German companies. The ability to remove the larger deposits in sewage waste and recycle the less dirty water to use in the jetting mode has clear advantages. A recycling machine can frequently work all day without the need to refill the water tanks with more water – the unit becomes more cost effective than a similar approach with a basic jetting unit, it is more environmentally friendly as it reduces the volume of clean water used and the need to travel to fill up (or receive water from a 'slave' tanker'. Consequently, the technology helps to reduce the carbon footprint of a cleansing project.
These machines can be supplied with single or two tank for the water recycling and mineral & organic material separation. The 'solid' waste can be removed by auger into another waste vessel such as a skip to allow continuous cleaning without the main machine leaving site or tipped at the end of a day to an approved disposal site. Depending on the size of vehicle and tank configuration and sewer conditions, silt and debris up to 10-15 tonnes per day can be removed without leaving the site, consequently this method is particularly good for the removal of large volumes of deposited materials.

Manual Removal of Debris

The earliest form of sewer cleaning was hand excavation whereby labourers loaded sediment into barrows which were moved down the sewer and then lifted out at manholes by bucket. The work is not only dirty, unpleasant and dangerous, it is also generally very costly. For these reasons it is now only considered when all other methods are not possible for one reason or another.
In extreme situations, silt and debris may have to be removed by open excavation to the sewer and removal of a section of the pipe or brick to gain access to the interior of the sewer.
Removal and Disposal of Sediment
Sediment removed from sewers is an objectionable mixture of inorganic, inert material, rags, grease and rotting organic matter. The ultimate disposal of this material should be to approved tips or to a sewage treatment works where permitted. The location of the disposal site should be known to the sewerage undertaker.
As already identified, modern jetting units are often equipped with suction devices for removing detritus from the sewer. In order to overcome the problem of lifting from deep sewers, the original vacuum or suction lifting units have now often been replaced by air displacement lifting equipment, which is capable of operating at depths of up to 30 metres. These machines tend to be very large. Manual lifting of detritus and sediment has been superseded by these more modern methods, although manual methods do have the advantage that a minimum amount of water is removed with the silt. Water removed with the sediment when using air lift and suction devices can be decanted or separated by other methods before the silt is transported to the disposal site.

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