Drain and sewer systems serve to collect and discharge sewage in the form of wastewater and/or stormwater. According to DIN EN 752-1 , they cover the area "where the sewage leaves the building or roof drainage system, or enters a road gully, to the point where it is discharged into a treatment works or receiving water. Drains and sewers below buildings are included provided that they do not form part of the drainage system of the building."
The vast majority of drain and sewer systems is designed as a gravity system that is either configured as a combined or a separate system (Figure 1). According to DIN EN 752-1 , the gravity system is a "drain or sewer system where flow is caused by the force of gravity and where the pipeline is designed normally to operate partially full."
Up until the introduction of the standard ATV-A 110  in 1988, the gradients of the pipe networks were often measured according to Imhoff based on the empirical for-mula (Ic = 1000/DN for circular pipes with a half filling or full filling and t = 2.5 N/m2
). These networks with nominal sizes larger than DN 800 are characterized by an invert incline that is too low and therefore frequently cause deposits depending on the wastewater composition and the drainage deviations. Furthermore, it is also networks with nominal sizes lower than DN 800 that are often susceptible to deposits if the wastewater quantities are low, especially during nighttime with dry weather. Likewise, the structural condition of the sewers plays a major role in this context: Apart from damages that directly impede the free outflow (positional deviations, root ingress, pipe fractures etc.), it is also leakages that cause deposit problems in that solid parti-cles reach the sewer system via the infiltrating groundwater. Vice versa, the accidental escape of wastewater leads to a loss of "transport water" and thus to increasing deposits.
One has to assume that the time and effort for cleaning of sewer systems will increase even further as the combined sewers and stormwater sewers are predominantly operated with lower partial filling levels and therefore with lower wall shear stresses. Reasons are:
- decreasing water consumption by the use of water-saving technologies
- increased seepage of stormwater
- the use of stormwater as service water
- efforts made to reduce the emergence of extraneous water by rehabilitation measures
- the increased convertion from combined systems to modified combined or separate systems.
Today's cleaning of sewer systems in most German cities and communities still takes place according to the (rotation) strategy of the fire brigade. However, in many cases, this strategy does not sufficiently consider the present situation of deposits, the structural conditions and the local operation experience of the cleaning staff. In addition, the sewers are often overloaded and partially damaged by the application of a flushing pressure that is too strong and the incorrect use of the jets. For economic reasons, the necessary quality control of the cleaning results is often spared.
The consequences of this procedure may be additional costs, i.e. increasing wastewater taxes. As, for political and economic reasons, taxes are to be reduced, it will be necessary in the future to use the available resources (budget, staff, vehicles, machines, operating materials etc.) in an optimised way.
According to many communal network operators, the economic efficiency of cleaning measures can be improved by a computer-aided sewer cleaning strategy that takes the demand and condition of sewer systems into account. During this procedure, the necessary cleaning effort (cleaning cycle at optimal costs, suitable cleaning equipment, required flushing pressure and amount of flushing water, etc.) is determined for each cleaning section. The acquired cleaning priorities are coordinated with other maintenance measures in the sewer network with respect to time and place. There-fore, for example, a planned cleaning process can be omitted if rehabilitation meas-ures are scheduled for the same sewer section. In the course of such rehabilitation procedures, it is necessary anyway to carry out sewer cleanings - sometimes even several times. By turning away from rotation strategies to demand- and condition-oriented sewer cleanings, essential reductions in costs can be achieved and pipe overloads may be prevented.
Within the scope of this contribution, different suggestions will be presented concerning the development of demand- and condition-oriented cleaning strategies for sewer systems.
2. State of the art of cleaning processes
The cleaning of sewer systems is an essential part of maintenance. It is generally carried out :
- to remove deposits within the scope of regular maintenance in order to main-tain free flow throughout the whole cross section of the discharge,
- to prevent the formation of smells and gases caused by fouling processes and the creation of biogenic sulphuric acid corrosion in partially filled sewers made of cement bound materials
- to remove blockages
- as a preparatory measure for a sewer inspection.
Besides the above-mentioned applications within the scope of maintenance, cleaning also serves as a preparation for rehabilitation measures. In this context, additional tasks arise such as the intensive cleaning of the inner walls, the removal of corrosion products, internally projecting laterals and pipes or other artificial flow obstacles. The cleaning processes used in sewer systems can be divided into the categories illustrated in Figure 2 according to .
The following aspects should additionally be considered for the selection of a suitable cleaning process or device :
- type and extent of deposits
- accessibility to the sewer
- depth of cover
- cross-sectional shape and dimensions of the sewer
- cross-sectional changes or displacements within a sewer section
- pipe material
- structural condition
- weather conditions (rain, snow, frost), especially for stormwater sewers or combined sewers)
- traffic conditions
- filling level.
The high-pressure (HP) water jetting process is the almost universal process that is used in about 90% of all sewer cleaning processes for removing deposits within the scope of regular maintenance as well as for cleaning as a preparatory measure for sewer inspection or rehabilitation. Usually, it can neither be used for the removal of hardened deposits or flow obstacles, e.g. projecting laterals, artificial obstacles or roots nor for the achievement of a very high degree of cleanliness of the inner pipe surface. In these cases, additional use must be made of mechanical cleaning proc-esses or devices (e.g. chain rotation devices, milling robots etc.), some of which can also be driven directly by high-pressure water jetting vehicles .
In the high-pressure water jetting process, flushing water is pumped from a water tank by means of a high-pressure pump through a hose that is equipped with a cleaning nozzle at the end. The cleaning nozzle has boreholes with nozzle inserts, which bundle the water jets that flow at high speed and direct them against the pipe walls. This causes a reaction force in the nozzle, which, in the first phase, conveys the jets and the hose in the sewer section from the starting manhole to the target manhole against the direction of the flow. After the cleaning nozzle has reached the target manhole, it is, in a second phase, slowly pulled back at the water jetting hose in the direction of the flow. The jets of water leaving the nozzle increase the flow velocity, loosen the deposits, whirl them up and convey them as a suspension towards the target manhole where they are usually extracted by means of a vacuum via a hose. It is also possible to clean several sections depending on the length of the section and the degree of pollution without having to open the intermediate manholes. The operating procedure of the high-pressure water jetting process is shown in Figure 3.
The universal high-pressure water jetting process is currently used to clean drains and sewers up to a nominal size of approximately DN 2000. If the high-pressure water jetting process is applied, it is very important to have a well-balanced combination of the single components, such as a high-pressure pump, a water jetting hose and a cleaning nozzle (Figure 4).
The core piece of the high-pressure water jetting process is the cleaning nozzle. Specific cleaning nozzles (Figure 5) are available for different types of pollution, dif-ferent shapes of sewer cross sections and different cleaning purposes. According to , they are divided into:
- Radial nozzles (water outlet distributed radially on the nozzle circumference) – (Figure 5a),
- Rotation nozzles (water outlet distributed radially on the nozzle circumference, nozzle rotates) – (Figure 5b),
- Cleaning nozzles for the removal of blockages (water jets situated to the front and back) – (Figure 5c)
- Invert nozzles (water outlet directed towards the invert) – (Figure 5d).
The selection of the nozzles to be used in each case depends on the type, quantity and consistency of the pollution, the nominal size, the quantity of the deposits to be transported and the cleaning purpose.
3. Demand-and-condition-oriented planning for cleaning processes
The cleaning plan is a concept in order to organise the cleaning works and must be set for the following purposes according to :
- to reduce the number of reactive cleaning measures down to a minimum and
- to find out the ideal cycle of anticipatory cleaning measures.
It should be based on a demand-and-condition-oriented cleaning concept which specifically aims at the cleaning of the polluted sections considering the structural condition of the sewer network. This concept serves to reduce the usual cleaning costs and to prevent excessive pipe loads. The introduction of this concept can be divided into the four steps illustrated in Figure 6.
Within the scope of condition acquisition in step 1, the present methods and efforts for cleaning have to be analysed first in order to find out economic potentials for optimisation or improvement, respectively. If documentations of former cleaning measures are available, they should be analysed in advance. In addition, important information about the actual situation can be gained by questioning and interviewing the staff about their daily assignments or by supervising the current cleaning measures. By working in sewer systems on a daily basis, these members of staff have got the most accurate knowledge of the cleaning requirements of single sections.
Documents about concentrated efforts of the local fire brigade to dry up flooded basements are also helpful even though they may not always indicate deposit problems of a sewer system. Other useful information may be acquired by bills of hourly paid workers coping with strong deposits in the preliminary stage of large TV inspections (e.g. for the first condition acquisition according to the German SüwVKan-NRW) or with spontaneous emergency efforts.
Another important basis for the determination and evaluation of the actual state of drain and sewer systems is to be provided by all available data of the network (master data, data on condition, operation and hydraulics). However, the experiences gathered within the scope of a research project  have revealed that the attempt to identify sections that are susceptible to deposits only on the basis of their hydraulical parameters (invert slope, partial filling level, flow velocity, dragg stress) and calculations would represent reality only insufficiently. In the concrete case, a cleaning process would be considered necessary for too many sections. For this reason, they must absolutely be verified by a visual inspection of the sewer network.
The deposits within a network can be determined by visual manhole controls (Figure 7a), by mirrorings of the section (Figure 7b) or by the application of a TV manhole camera (Figure 7c). The visual manhole controls have proved to be the most efficient process in order to obtain an overview of the degree of pollution within the sewer system in no time and at low costs. A manhole control can be carried out within the scope of a rotational cleaning procedure of dirt traps or during a manhole inspection, which is required every two years according to DWA-M 174 . A simple classification into three categories (no deposits; slight, flow-retarding deposits; flow-impeding deposits) are often sufficient for the time being. In particular cases, TV manhole cameras and sewer mirrors may support this simple procedure. Despite the doubtlessly excellent picture quality and range of sight of TV manhole cameras, they have not yet been designed to the full practical satisfaction as regards handling and required set-up times.
A deposit control by inspecting the section with the aid of a sewer TV camera is generally not considered as it requires too much time and great expenses.
After their determination and classification, the deposits are evaluated in order to derive suitable cleaning intervals. The result of this step is the first estimation of the cleaning effort that is actually required.
On the basis of the evaluation of the actual state, the aims and the saving potentials that are closely connected to the available resources (budget, personnel, vehicles and machines) are determined in step 2 of the strategic cleaning planning.
The results of this second step of strategic cleaning planning are inspection plans used to establish sustainable demand- and condition-oriented cleaning plans.
The experiences gathered in this regard within the scope of a research project  have revealed that a demand- and condition-oriented cleaning plan (Figure 8) can best be established by the use of a GIS (geographical information system).
The GIS consists of a graphic user interface and a related database (e.g. Access, Oracle, etc.) in which all section parameters like nominal size, material, section length, inclines, condition pictures etc. are deposited. The network and deposit data have additionally been integrated, administered, analysed and visualised here, e.g. by a coloured marking of the different deposit classes. Using all the above-mentioned information it was possible to clearly identify the sections that need to be cleaned and to define the optimal cleaning cycle for all sections. The spectrum of the cycles ranged between the sections that need to be cleaned once a year and those network areas that do not need to be cleaned before the preliminary stage of the next TV in-spection (according to SÜwVKan-NRW after 15 years).
Next to the findings on the actual need for cleaning, further considerations must be included. It must be clarified in every particular case whether sections which are free of deposits but which are situated between sections that have more severe deposits should also be cleaned in order to optimise operation processes (operation schedule, moving times).
The strategic cleaning plan created along the above-mentioned working steps for the partial drainage area “Völlinghausen” in the community of Möhnesee has been subjected to a profitability analysis according to the German LAWA-regulations for cost comparison methods [8, 10]. This analysis has revealed that considerable cost savings can be achieved by the use of a demand-oriented sewer cleaning incl. regular follow-up inspections in contrast to the rotational cleaning procedures. The costs of a completely inspected sewer length of the area of Völlinghausen could be reduced by about 50% compared to the preventive strategy used until then (annual rotational cleaning) .
The cleaning process is carried out on the basis of the operative cleaning planning in the form of action and flushing plans. They include a priority sequence of the cleaning measures and can be supplemented by further information on preferred cleaning methods (high-pressure cleaning, surge flushing, mechanical cleaning, etc.) and the appropriate cleaning tools (nozzle types, flap types, etc.). Furthermore, the performance parameters (pump pressure, inlet and retraction speed, surge frequency, etc.) are determined in consideration of the deposit behaviour and the structural condition in the sections. Any information on structural damages and/or rehabilitation devices (e.g. short liners) are particularly important for the cleaning staff in order to prevent damages due to cleaning pressures that are too high or similar reasons. In addition, logistic aspects must also be considered like e.g. the minimisation of driving times between the sections to be cleaned by appropriate routeing.
The action or flushing plan describes exactly when, where and how (with which method and parameters) the cleaning should take place. To achieve an optimal cleaning performance, the local and temporal use of cleaning personnel and cleaning vehicles of the drainage companies or the service providers must be regulated accordingly. The places of the water take-up must also be determined in advance.
The previously planned cleaning process is now carried out in step 3. The vehicle is placed as close as possible at the starting manhole. The place of operation is safe-guarded and the restriction of the traffic flow is reduced to the lowest possible level. Once the hose and nozzle have been inserted, the cleaning process begins. During the cleaning process, the above-mentioned cleaning parameters must also be adjusted to the structural condition of the respective section in order to prevent a damage increase of predamaged sewers (e.g. abrasion, corrosion, cracks, fragmentation, pipe fracture) or a complete collapse (Figure 9a). Special moments of risk arise if the cleaning nozzle is slammed, sediments or stones are raised or the cleaning nozzle is maintained at one place (Figure 9b).
During the cleaning process, the excavation masses must be constantly controlled. Larger amounts of mineral solid materials and pipe fragments are indications of stronger damages, e.g. fragmentation, pipe fracture or collapse.
In these cases, the cleaning process must be interrupted and security measures must be taken; before the cleaning process is continued, a more careful process must be selected or a rehabilitation measure must first be carried out.
After the cleaning works have been finished, a quality control and a final documentation of the cleaning performance and results is provided in step 4. This measurement serves to determine insufficient cleaning works so that the provider can be called on to rectify the cleaning process. A quality control of the cleaning works shall serve to constantly expand the network and operation expertise and is therefore incorporated into the planning process. Consequently, the cleaning control can only be carried out by the use of sewer mirrors and TV manhole cameras as they perfectly recognise the cleaning level inside the section. It should be done directly after the cleaning and not several days later. Although this may cause a considerable effort of the network operator or authorising company, random checks should at least be performed. Quality controls have revealed, among other things, that if the retraction speed of the nozzle is too high, the nozzle can slide over the accummulated deposits without pushing them into the target manhole. The section is consequently considered as cleaned, although there are still in fact flow-impeding deposits.
Within the scope of cleaning planning, it is not only drains and sewers that need to be considered but also every other structural part of the drainage system like e.g. manholes, road gullies, grit traps, culverts, pumping stations and stormwater retention basins.
Another important aspect in the preparation of demand- and condition-oriented cleaning plans is to prevent the entry of solid material into the sewer system by taking constructive preventive measures. The most frequent entry points of solid materials are surface drains above road gullies and damaged or leaky sewers.
Rehabilitation strategies for drain and sewer systems should aim at preventing or minimising these particulate material entries. This can also be achieved by the constructional rehabilitation of the leakages and the application of optimised road drains with improved particulate material retention .
The road drains according to DIN 4052 , which have been used so far, retain the particulate materials only insufficiently. Especially road drains with floor drainage cause the particulate materials to reach the main sewer via the connecting sewer with a rate of 100 % once they have passed the trench opening. Road drains with a sludge compartment have a slightly better particulate material retention, even though a great part is flushed into the sewer along with the precipitation discharge.
Due to this fact, a new road drain (separation road drain SSA) has been developed in the context of a research project  as a preventive cleaning measure in order to eliminate the above-mentioned weak spots of the conventional systems according to DIN 4052 . It is characterised by a better particulate material retention and pre-vents the mobilisation of sediment materials in the sludge compartment by means of constructive measures. Therefore, this new system makes an important contribution to the reduction and prevention of deposits in sewer systems [13, 14, 15].
The analysis of the laboratory tests by means of a mineral alloy (0.125 mm - 8 mm) resulted in an increase of the particulate material retention of the new road drain (SSA) by up to 20.4 % above the conventional road drain with a sludge department. No retention of particulate materials takes place in road drains with floor drains and buckets due to the relation of the trench opening (8 mm ´ 60 mm) to the maximum grain size. Even in-situ tests have confirmed this result with regard to the road drains with sludge compartments (30 %). Compared to road drains with floor drains and buckets, the retention was three times as high.
The results of the investigations carried out in Great Britain clearly represent the respective saving potential  as they have revealed that the costs for the removal particulate materials (sand) in sewage treatment plants as well as for the cleaning of road drains, road surfaces and sewers are at a ratio of 1 : 2 : 5 : 10.
An efficient and sustainable planning of cleaning processes presupposes the knowledgee of specific network data. These are not only master data, condition data or hydraulic data but also operation data and especially information about the amount and composition of deposits in the different section. This requires the preparation of a comprehensive and transparent documentation that is accessible at any time taking the electronic and graphical data processing into consideration. A specific geographical information system (GIS) is especially suitable for this purpose. It offers the possibility of implementing, administering, analysing and visualising the deposit data recorded for each section in order to establish demand- and condition-oriented cleaning plans for each section or pipe string.
For the operative implementation of the cleaning planning, the pipe material and especially the structural condition of the sewers should be taken into consideration when it comes to the selection of the cleaning parameters for high-pressure water jetting processes (water pressure, volume flow, type of nozzle). Regular and explanatory tests of the cleaning results for conformity between the tendering and the work performed are indespendable to updating cleaning strategies.
Further influencing factors like e.g. origin, amount and composition of the entered solid materials play an important role in the finding of the cause of the solid matter entry and therefore in the elaboration of a prophylactic cleaning strategy.
The retention of solid materials at their origin by the structural rehabilitation of leak-ages as well as by road drains with optimised solid matter retention contributes to a considerable reduction in costs for cleaning processes in sewer systems. This fact is exemplified by the cost ratio of the cleaning of road drains to the cleaning of sewers, which is 1 : 5 according to . A highly productive road drain in this regard is the separation road drain (SSA) .
The concept of demand- and condition-oriented cleaning planning presented in this study has successfully been applied in several communities and is generally transferable to other sewer networks. The introduction of this cleaning strategy has significant advantages over the conventional preventive strategy and the strategy of the fire brigage especially from an economic and ecological point of view, e.g.:
- Optimisation of the cleaning effort and reduction in costs
- Operation without deposits and thus higher operation safety
- Prevention of odour nuisances
- Verification of the compliance with legal and technical requirements
- Preservation of the substance of the network by reducing the pipe load
- Minimisation of substantial water pollution.
 DIN EN 752: Drain and sewer systems outside buildings. Part 1: Generalities and definitions (01.1996).
 ATV-A 110: Hydraulische Dimensionierung und Leistungsnachweis von Abwasserkanälen und –leitungen, 09/2001.
 Stein, D.: Instandhaltung von Kanalisationen. 3. Auflage, Verlag Ernst & Sohn, Berlin, 1998.
 Firmeninformation KEG mbH, Burgstädt/Herrenhaide.
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 DWA - M 174: Betriebsaufwand für die Kanalisation – Hinweise zum Personal-, Fahrzeug- und Gerätebedarf (10.05).
 Länderarbeitsgemeinschaft Wasser (LAWA): Leitlinien zur Durchführung Dy-namischer Kostenvergleichsrechnungen (KVR-Leitlinien). 6.Auflage, Berlin 1998.
 DIN 4052: Bauteile und Eimer für Straßenabläufe. Teil 1 bis 4 (Entwurf).
 Stein, R., Cakmak, H., Dettmar, J.: Untersuchungen von bestehenden Straßenabläufen bezüglich ihrer Leistungsfähigkeit und Realisierung von technischen Möglichkeiten zur Verbesserung des Feststoffrückhaltevermögens. Forschungsbericht im Auftrag des MUNLV NRW, Mai 2005.
 Stein, R.: Präventive Reinigungsmaßnahmen am Beispiel des Feststoffrückhaltes in Straßenabläufen. Vortragsreihe Niederschlagsbehandlung der TU-München, Garchingen, 2005.
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 Butler, D., Thedchanamorothy, S., Payne, J. A.: Aspects of surface sediment characteristics on an urban catchment in London. Water Science Technology. Vol. 25, No. 8.