Optimization and Controlling of Operational Cleaning of Sewer Networks

Optimization and Controlling of Operational Cleaning of

Sewer Networks

In most German cities and communities the drain systems are currently cleaned ac-cording to the strategy of the fire brigade or in fixed, inflexible intervals (cycle clean-ing). In this process existing deposit situations, constructional conditions and the lo-cal operational experience of the cleaning personnel are considered appropriately. The consequences of these methods are additional cost which have direct effect on the sewage fees. With regards to communal politics striving for a fee reduction, the available resources (budget, staff, vehicles, equipment, operating materials, etc.) have to be used in a optimized way. Turning away from cycle cleaning and towards optimized sewer cleaning based on demand-oriented cleaning strategy with comput-ing assistance can achieve a significant cost reduction and excessive pipe exposure can be avoided.

1 Introduction

Drain systems are for collection and discharge of sewage water in the form of foul and rain water. They cover the area “where the sewage water leaves the building / roof drainage or flows into a street drain, to the point where the sewage is led in to treatment system or discharge system. Waste water pipelines and sewers under-neath buildings are concluded at this, so long as they are not a component of the building drainage system“ according to DIN EN 752-1 [1]. The predominant number of drain systems is designed as a gravity system in the form of a mixed or separated system where “the drainage take places by gravity and where the pipelines are nor-mally operated with partial filling“ according to DIN EN 752-1 [1].

Up until the introduction of the ATV-A 110 in 1988 the gradients of the pipe networks were often measured according to Imhoff based on the empirical formula (Ic =

1000/DN for circle pipe with a half-filling or full-filling and τ = 2,5 N/m2). These net-works with nominal diameters larger than DN 800 are characterized which a too low invert incline and therefore often cause deposits in dependence on waste water con-sistency and drain variations. Furthermore networks with nominal sizes lower that DN 800 are often at great risk concerning deposits, this is due to low waste water quanti-ties, especially during nighttime with dry air discharge. The constructional condition of the sewers plays a major role in this context because a quite considerable part of particulate materials reaches the sewage water system via the groundwater which has infiltrated through leakages.

The status quo of our sewer systems will change for the worse in respect of deposit behavior, because the mixed and rain-water sewers are going to be predominantly operated with low partial filling levels and therefore also with low wall shear stress. Reasons are:

- still decreasing water consumption

- increased seepage of rainwater and

- efforts made to reduce the infiltration water emergence with rehabilitation measures.

Today’s sewer cleaning basically still takes place according to cleaning plans; the cleaning measures are always carried out with the same flushing parameters inde-pendent of fouling degree and constructional condition of the to be cleaned section. An improvement of economic efficiency of cleaning measures can be realized, ac-cording to the experiences of many communal operators, with the help of a demand-oriented cleaning with computing support based on network- and deposit-knowledge. During this procedure the necessary cleaning effort (cleaning cycle at optimal cost, correct cleaning equipment, required cleaning pressure and water, etc.) is deter-mined for each single cleaning section. The advantages for the operations are cost reduction via avoidance of too early or too late carried out cleaning measures, a bet-ter planning of the measures and therewith an more efficient usage of the existing resources and a reduction of an excessive pipe exposure. Futhermore the deter-mined cleaning priorities are coordinated chronologically and spatially with other maintenance measures in the drain network.

Within the scope of this contribution proposals are submitted concerning the optimi-sation of sewer cleaning.

2 State of the art of Cleaning methods 2.1 Introduction

The cleaning of sewer systems is an important part of maintenance. It is carried out [2]:

- For removing deposits within the scope of regular maintenance, in order to maintain free flow throughout the whole cross section and to prevent the ap-pearance of smells and gases caused by fouling processes and the creation of biogenic sulphuric acid corrosion,

- For the removal of blockage,

- As a preparatory measure for an inspection of the sewer.

Besides the above-mentioned applications within the scope of maintenance, cleaning also serves as a preparation for rehabilitation measures. This results in additional tasks such as intensive cleaning of the inner walls, removal of corrosion products, internally projecting laterals and pipes or other artificial flow obstacles [2]. The clean-ing processes mostly used can be divided into [2]:

- High pressure water jetting processes (HP water jetting processes);

- Flushing processes;

- Mechanical processes

- Other processes.

The following should additionally be considered in the choice of a suitable cleaning process or apparatus [2]:

- Possibility of access to the sewer;

- Coverage height;

- Cross sectional shape and sizes of the sewer;

- Cross sectional changes or displacements within a section of a sewer;

- Pipe material;

- Structural condition;

- Weather conditions (rain, snow, frost), especially for rainwater or combined wastewater sewers;

- Traffic conditions

- Filling level

The main focus of the following details and specifications is the High Pressure Water Jetting Process, which at present is being used mainly by sewer network operators.

2.2 Functionality of the HP water jetting process

The high pressure water jetting process (HP water jetting process) is the almost uni-versal process, used in about 90 % of all sewer cleaning processes for removing de-posits within the scope of regular maintenance as well as for cleaning as a prepara-tory measure for sewer inspection or rehabilitation. It can usually not be used when

hardened deposits are present or for removal of flow obstacles, e.g. internally pro-jecting laterals, artificial obstacles or roots or for achieving a very high degree of cleanliness on the inner pipe surface. In these cases additional use must be made of mechanical cleaning methods or apparatus, which can partly also be driven by the high pressure water jetting vehicles.

In the HP water jetting process, flushing water is pumped from a water tank by means of a high pressure pump through a hose with a cleaning nozzle at the end. The cleaning nozzle is equipped with holes into which are nozzle inserts, which beam the fast flowing water jets and direct them at the pipe walling. This causes a reaction force at the nozzle, which, in the first phase, force them and the hose in the sewer section, to move against the direction of the flow from the starting manhole to the tar-get manhole. After arrival of the nozzle at the tartar-get manhole, it is slowly pulled back in the second phase in the direction of flow. The jets of water leaving the nozzle in-crease the velocity of flow, loosen the deposits, whirl them up and convey them as a suspension towards the target manhole where they are usually vacuumed up by a hose. It is possible for many sections to be cleaned dependent on the section lengths and fouling degree, without having to open the intermediate manholes. The operating sequence of the HP water jetting process is shown in Figure 1.

Figure 1: Sequence of work in the high-pressure water jetting process (Source: Stein & Partner)

Phase 1: Tunneling of the cleaning nozzle Phase 2: Cleaning via retraction of the cleaning nozzle

Cleaning with the HP water jetting process is economically justified in drains and sewers up to approximately DN 2000. With the HP water jetting process it is very im-portant to have a reasonable combination a the single components, like high pres-sure pump, jetting hose and cleaning nozzle (Figure 2).

Figure 2: Significant components of the high pressure cleaning (Source: Stein & Partner)

High pressure pump Jetting hose Cleaning nozzle

The core piece of the high water jetting process is the cleaning nozzle. Special clean-ing nozzles are available for different types of foulclean-ing and sewer cross sections. Ac-cording to [2], they are divided into:

- Radial nozzles (water outlets distributed radially out of the jet circumference) - (Figure 3 a),

- Rotation nozzles (water outlets distributed radially on the jet circumference, nozzle rotates) - (Figure 3 b)

- Cleaning nozzles for removal of blockages (water sprays directed in forward and backward directions) - (Figure 3 c)

- Invert nozzles (water outlet directed at the invert) - (Figure 3 d).

Figure 3: Different cleaning nozzles [3]

a) Radial nozzles b) Rotation nozzles c) Cleaning nozzles for removal of blockages

d) Invert nozzles

The selection of the to be used nozzles depends on the type, quantity and consis-tency of the fouling, the nominal size, the quantity of the to be transported deposits and the cleaning target.

A new development is presented by the invert cleaner shown in (Figure 3 d) with in-tegrated, self-lighting TV camera. The transfer of the figure from the TV camera equipped with transmitter and the receiver is achieved by means of a wireless

con-nection [4]. Because of the continuous monitoring of the cleaning process, the clean-ing efficiency can be adapted to the degree of foulclean-ing, accordclean-ing to the manufac-turer’s information, so that a reduction of the water use as well as a minimising of time is achieved. Furthermore, a targeted inspection can be carried out when dam-age in the sewer is recognised and localised during the cleaning process [5].

3 Optimisation of the operational cleaning planning

The cleaning plan is a concept for the organisation of cleaning operations. The plan should be based on a demand-oriented cleaning strategy, where only the fouled net areas and sections are cleaned. Furthermore a detailed analysis of the net data (master data, conditional data, operational data and hydraulic data) should be carried out. The concept for the demand-oriented cleaning strategy can be classified as fol-lows [6]:

- Strategic planning

- Operational planning

- Constructive planning of preventive measures

In the context of the strategic planning the present drain and cleaning processes (equipment and personnel application, selection of operational parameters, logistic, failure documentations, complaints, etc) have to be analysed – and consequently identify economical optimisation- and improvement-potentials. These can also be carried out by questioning and interviewing the staff concerning the daily assign-ments or by escorting and documenting the current cleaning measure. By working on a daily basis in sewer systems you have exact knowledge of the cleaning require-ments of single sections.

The regulation of realistic targets and cost reduction potentials is carried out based on the evaluation results. These are defined during discussions with the network op-erator. Cost-saving can e.g. be defined to be approx. 30 % in the first year. The re-sults of strategic planning are cleaning demand-plans. For this the individual sections have to be recorded concerning the quantity and composition and subsequently clas-sified. The classification can take place according to ATV-M 149 [7], technical docu-mentations sewage [8], or individually (e.g. according to quantity and composition). Based on this cleaning intervals for the treated section depending on the identified deposit behaviour is determined. In this process the following has to be closely con-sidered: very short cleaning intervals can lead to very high costs due to the high fre-quency; very long cleaning intervals also cause very high cost, because the cleaning process tales longer. The target has to be to find the optimum between a very low cleaning effort and a low level of operational problems for the respective sewage

network. Considering economical aspects a optimal cleaning cycle for a single sec-tion results, as shown at the minimum of the displayed graph in Figure 4.

Figure 4: Qualitative Development of the cleaning costs for a section (Source: Stein & Partner) according to [9]

Our gained experiences concerning this matter, in the context of a research project [10], have shown that a demand-oriented cleaning plan (Figure 5) can be prepared in the best way using a GIS (geographic information system). The GIS consists of a graphic surface and a database (e.g. Access, Oracle, etc.), in which all section pa-rameters, like nominal size, substances, section lengths, inclines, condition pictures and so on, are deposited. The deposit data was also integrated, administered, ana-lysed and visualised here. Using all the above mentioned information it was possible to clearly identify the to be cleaned sections and define the optimal cleaning cycle for all sections.

The saving of cost based on this newly developed demand-oriented sewer cleaning amounts to approx. 77.000 € [10] for a sewer network length of 13,1 km and a time period of 15 years – this is compared to the used preventive strategies until now.

Cleaning costs

Figure 5: Demand-oriented computer-aided cleaning plan considering as example the community Möhnesee [10]

The realisation of the cleaning operation takes place based on the operative clean-ing planning in the form of action and flushing plans. They include a priority se-quence of the cleaning measures stating the to be preferred cleaning methods (high pressure cleaning, Surge Flushing, mechanical cleaning, etc.) and the appropriate cleaning tools (nozzle types, flap types, etc.). Furthermore the cleaning parameters (pump pressure, entrance- and retraction-tensile-speed, surge frequency, etc.) are determined in consideration of the deposit behaviour and the constructional condition in the sections.

The realisation / 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 used devices (e.g. bore diameter of the nozzle application) and their operational parameters (nozzle pressure, delivery flow) should be checked by a cleaning measure. Furthermore a hydraulic coordination of the pump capacity, the hose and the nozzle is needed to achieve an optimal performance of the high pressure system. If this does not take place then high pressure losses can occur and the efficiency of the high pressure system is reduced. During daily cleaning opera-tions the following has to be considered: The cleaning parameters have to be ad-justed to the constructional condition of the single section. By doing this a damage increase in already damaged sewers (e.g. abrasion, corrosion, cracks, fragment for-mation, bursting of pipes) or even the structural collapse of caused by cleaning can be avoided [2]. Specific potentials dangers result from the knocking of the cleaning nozzle, raising sediment or stones as well as the working cleaning nozzle remaining at one place.

During the cleaning process the accruing deposits have to be checked permanently (Figure 6). Larger parts of mineral materials and fragments of the pipes are signs for immense damages, e.g. for fragment formation, pipe bursting or even collapse.

Figure 6: Watching the deposits in the manhole (Source: Stein & Partner)

In such cases the cleaning process has to be stopped immediately and security measures have to be initiated; for the continuation of the cleaning process a more sparing method should be used if required.

An inspection and documentation of the accomplished cleaning result as done after completion of each cleaning measure. An acceptance record is called for in the form of a report, which must include the following information among other things [11]:

- Location, date and time of the operations

- Name of the contractor and the person/company realising the operations

- Condition of the sewer before/after the cleaning

- Used cleaning method

- Type and extent of the deposits.

Concerning the cleaning quality it is recommended for the contractor and ordering party to select the performance indicator so that it is possible to definitely identify if the required service was performed. A possible performance indicator for the inspec-tion and checking of the cleaning quality could possibly be the measurement and in-spection of the deposit heights in the sections which have already be cleaned. This can be done with mirroring the sections, manhole checks or sewer-TV-cameras. Fur-ther performance indicators for the evaluation of the cleaning work are included in [11]. This information is a part of the report and form the base for future, demand-oriented and quality assured cleaning planning. In the context of the cleaning plan-ning not only sewage water pipelines and sewers have to be considered but also other drain system constructions, such as manhole, road drains, culverts, pumping stations and rainwater basins.

A further important aspect of optimisation of operational sewer cleaning is the pro-phylactic prevention of particulate materials entries with constructive preventive measures in sewage systems. Significant entry points of particulate materials are the surface drain above road drains and damaged or leaky sewers.

Rehabilitation strategies for drain systems should aim at preventing or minimising these particulate materials entries. This can also be achieved through the construc-tional rehabilitation of the leakages and the application of optimised road drains with improved particulate material retention [15,17].

The particulate materials are inadequately retained with road drains according to DIN 4052 [12] which have been used until now. Especially concerning road drains with floor drains the particulate materials flow through the connecting sewer into to the main sewer with a rate of 100 %, after passing the trench opening. Road drains with an alluvium space have a better particulate material retention, even though a great part is flushed with the rainfall flow via mobilisation in to the sewer.

In consideration of this fact a new road drain (Separation-Road-Drain SSA) was de-veloped in the context of a research project [13] as a preventive cleaning measure

with the objective of elimination of the above mentioned weak spots of the conven-tional systems according to DIN 4052 [12]. The new road drain has a better particu-late material retention and prevents the mobilisation of sediment materials in the allu-vium space with the help of constructive measures. Therefore this new system makes a important contribution to the prevention of deposits in the sewer system [14, 15]. The analysis of the laboratory test with a mineral alloy (0,125 mm - 8 mm) re-sulted in a magnification of the particulate material retention of the newly developed road drain up to 20,4 % above the conventional road drain with alluvium space. No retention of particulate materials takes place in road drains with floor drains and buckets, this is due to the relation of the trench opening (8 mm × 60 mm) to the maximum grain size. Even in situ tests confirmed this result in respect of the road drains with silts space (23 %). Compared to the road drains with floor drains and buckets the retention was twice as high.

Which saving potential can be realised is made very clear by the results from Great Britain and the tests done there [16]. These have shown that the cost for the removal of deposits in sewage treatment plants concerning the cleaning of road drains, road surfaces and sewers are at a ratio of 1 : 2 : 5 : 10.

4 Future Prospects

Particulate materials are fed in to the sewer system from different sources. Espe-cially mineral materials are significantly involved concerning the formation of deposits in sewers. If these deposits are not removed regularly then they can harden in the course of time dependent on their composition and residence time. These deposits cause a reduction of the hydraulic capability and in an extreme case lead to a total closure of the drainage cross section. Furthermore this can also result in a biogenous sulphuric acid corrosion (BSC) in partially filled concrete sewage water systems. Therefore the cleaning of sewer systems is an important part of the lawfully required maintenance of drainage systems. An efficient and sustainable cleaning plan re-quires knowledge about specific net data. This includes master data, condition data and hydraulic data and also operating data and especially information about deposit levels. This requires the writing of a comprehensive, transparent and always avail-able documentation in consideration of electronic and graphic data processing. A Geo-Information-System (GIS) is especially qualified for this purpose. It offers the possibility of the implementation, administration, analysis and visualisation of the de-posit data, which are recorded per section, so that cleaning plans can be prepared with regard to sections and legs.

During the operational realisation of the cleaning planning the selection of cleaning parameters for the HP water jetting process (water pressure, volume flow, type of nozzle) and especially the constructional condition of the sewers should be consid-ered. Regular and significant checks of the cleaning results with regard to the corre-lation of tendering and the work performed are necessary for the recording of a cleaning planning. Further important factors, like origin, amount and composition of the particulate materials are crucial factors for the determination of causes concern-ing the particulate materials entries. They are also very important for the formulation of a prophylactic cleaning strategy.

The retention of particulate materials at source through the constructional rehabilitation of the leakage and also through road drain with a improved particulate material -retention contributes to a remarkable cost reduction concerning cleaning of drainage system. This circumstance is exemplified by the cost relation of road drain cleaning compared to the cleaning of sewers, which are at a ratio of 1 : 5.

The introduction of a demand-oriented cleaning strategy has great advantages com-pared to the conventional preventive strategy and the strategy of the fire brigade, especially concerning economic and ecological aspects, e.g.:

ƒ Optimisation of the cleaning effort and reduction of cost

ƒ Deposit-free operation

ƒ Preventive BSC-Avoidance

ƒ Preservation of the net substance

ƒ Preservation of the hydraulic capability

ƒ Reduction of the abrasions of pumps and screw-conveyors

5 Literature:

[1] DIN EN 752: Entwässerungssysteme außerhalb von Gebäuden. Teil 1: Allge-meines und Definitionen (01.1996)

[2] Stein, D.: Instandhaltung von Kanalisationen, 3. Auflage, Verlag Ernst & Sohn, Berlin, 1998.

[3] Firmeninformation KEG mbH, Burgstädt/Herrenhaide.

[4] R. Stein: Trends und Entwicklungen der Zustandserfassung; BI Umwelt Bau Juni 2004.

[5] Stein, D., Körkemeyer, K., Cakmak, H.: Expertise zur Analyse und Beurteilung des von der KEG mbH entwickelten Sohlenreinigungsverfahren „Sehende Dü-se“ für Abwasserleitungen und -kanäle. Bochum, November 2002.

[6] Stein, R., Cakmak, H.: Erstellung bedarfsorientierter Reinigungspläne für Ent-wässerungssysteme auf Grundlage der Europäischen Norm prEN 14654-1, Schriftenreihe aus dem Institut für Rohrleitungsbau Oldenburg – Rohrleitungen – für eine sich wandelnde Gesellschaft, Iro Band 30, Vulkan-Verlag Essen, 2006.

[7] ATV-M 149: Zustandsklassifizierung und -bewertung von Entwässerungsnet-zen außerhalb von Gebäuden (1999).

[8] Bundesministerium für Verkehr, Bau- und Wohnungswesen / Bundesministeri-um der Verteidigung; „Arbeitshilfen Abwasser - Planung, Bau und Betrieb von abwassertechnischen Anlagen in Liegenschaften des Bundes“; Berlin / Bonn, 2004.

[9] Krämer, St.; Scholz, K.: Bedarfsorientierte Reinigung von Abwasserkanälen, Korrespondenz Abwasser 49 (2002), Nr. 4, S. 453-462.

[10] Freitag, St.: Erstellung eines Spülplanes auf Grundlage einer bedarfsorientier-ten Kanalreinigung für die Gemeinde Möhnesee. Diplomarbeit an der Arbeits-gruppe Leitungsbau und Leitungsinstandhaltung (AGLL) der Ruhr-Universität Bochum, Januar 2005.

[11] DIN EN 14654-1: Management und Überwachung von Reinigungsmaßnah-men in Abwasserkanälen und –leitungen – Teil 1: Reinigung von Kanälen, De-zember 2005.

[12] DIN 4052: Bauteile und Eimer für Straßenabläufe. Teil 1 bis 4 (Entwurf). [13] Stein, R., Cakmak, H., Dettmar, J.: Untersuchungen von bestehenden

Stra-ßenabläufen bezüglich ihrer Leistungsfähigkeit und Realisierung von techni-schen Möglichkeiten zur Verbesserung des Feststoffrückhaltevermögens. For-schungsbericht im Auftrag des MUNLV NRW, Mai 2005.

[14] Stein, R.: Präventive Reinigungsmaßnahmen am Beispiel des Feststoffrück-haltes in Straßenabläufen, Vortragsreihe Niederschlagsbehandlung der TU-München, Garchingen, 2005.

[15] Stein, R.: Entwicklungen bei der Straßenentwässerung im innerstädtischen Betrieb, ATV-Vortragsreihe, Trier, 2004.

[16] 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.