Sewer management with qualitative and quantitative process control in arid areas by using the Cascade, Flush- and Discharge (CFD) Technology

Feb 26, 2010

1. Introduction In the densely populated cities of the world, drainage systems must be designed to carry away both the waste water from the population and the run-off from short and long rainfall events. The urban drainage method that has been mostly used is the so- called combined system, namely the simultaneous combination of waste-water and rainwater in a common pipe. Compared to the separate system this method has the economic advantage that urban drainage can be operated and managed using a single pipe system.

However, such mixed systems have to carry a wide range of flows and different concentrations of waste matter in the flow. As rainfall may be infrequent, there can be long periods of low flows without flushing of the sewers, resulting in the deposition of waste matter solids in the sewer invert. The operational problems listed below inevitably result in:

1.1 Deposits in the sewer invert cause the following offensive problems:
  • Odour problems from escaping fermentation gases in the form of hydrogen sulphide concentration (H2S). This is caused by fluctuations of the natural flushing waves on the sewage plant
  • Hydrogen sulphide corrosion in concrete pipes
  • Changing sewage load concentrations at the sewage plants during storm water flow

To reduce the aforementioned disadvantages or problems in combined systems, it is necessary to control the flows in the sewers at all times and to be able to adapt this control to a method of cleaning sewers of all diameters and profiles.

The most common cleaning methods of the sewer systems are:
  • Channel cleaning by hand using special collectors (skips)
  • High pressure flushing by means of sucking and flushing vehicles in diameters of DN 800 mm

1.2 Some remarks to the cleaning methods:
Channel cleaning in accordance with point 1.2.1: this is very personnel cost-intensive and for sewerage workers certainly not very healthy. High pressure flushing causes problems in pipes of the following materials:
  • Concrete,
  • Asbestos cement,
  • Synthetics

1.2.1 High pressures of up to 200 bar can lead to considerable damage and reduce the life span of the single pipe systems. New investment may be required sooner than expected.Since a combined sewer system has to carry flows from heavier rainfall events without causing flooding in the area, the extension of the system by correspondingly designed or calculated storm water tanks with overflow into the receiving water may have to be carried out. Flows in excess of the system capacity can be diverted into the tanks and stored or held for a time, which will retain the solid waste matter by settlement and protect the receiving water from pollution.

1.3 Storm water tanks can be added in the following system variants:
  • Storm water tanks in the form of a basin in a by-pass version
  • Storage capacity sewers with use of the accompanying volume in the pipe system

The usual carry-forward flow (2 Qt+Qf) [Two times dry weather flow + sewer infiltration flow] can lead particularly to sedimentation and offensive odour problems at flat pipe gradients in front of throttles. Owing to the siphon effect the danger exists, that suspended solids are diverted by passing underneath a solid scum board to the receiving water. This now completes the short introduction to the difficulties of the present combined sewer systems and my further explanations start with the question: "How can we optimize the combined sewer systems in their operation and if possible save investment and subsequent costs in the future?" If we want to proceed in this way, we only can participate with the knowledge of expert and experienced designers in this special field.

1.4 Opportunities for combined sewer systems
Although the extension of the sewage plants in Germany is nearly completed, we realize that technical deficiencies appear in the operation of sewage plants, because of changing influent solids loads from the natural flushing waves in the inlet area of the sewage plant. This can impair the sewage plant performance, operation and management considerably in rainy weather and worsens the treated effluent quality of the sewage plants. The consequences of storm water overflow still impair the receiving water and the ground water so that the experts now discuss how a synergy between the sewerage system, storm water overflow and sewage plant can be effected to achieve further necessary optimizations and improvements in the context of the complete water cycle. In my opinion it is necessary, that the complete complex of combined systems, storm water discharge and sewage plant has to be completely automated. Automation means complete sewer management with optimised available technical methods at the present time:
  • Use of the cascade volumes in combined sewer systems instead of constructing cost-intensive basins
  • Construction of sewer discharge devices for controlling all volumes from Qab = zero to Qmax
  • Putting into action of automated sewer cleaning devices for flushing in all diameters and distances to about kilometre
  • Reduction of offensive odour problems and prevention of hydrogen sulphide corrosion
  • Moderate comparing of sewage loads and accompanying inflow volumes on the sewage plant
  • Control of sewage volumes with different concentrations for a mixing process in parallel activated cascades to the sewage plant
  • Reduction of discharge of polluting load into the receiving water for protection of the ground water and aquatic life
  • Control of stormwater overflow under quality and quantity conditions
2. The idea of the CFD Cascade-, Flush- and Discharge technology

2.1 Sewage sedimentation occurs particularly under low flow speed conditions and where there is a minimal gradient. In such cases larger pipe diameters, such as those used for sewers with storage capacity and overflow, cause problems. These are aggravated by the fact that traditional throttle facilities have only a limited discharge capacity, which again increases sedimentation with the tractive force approaching the zero mark. The only possible solution to these problems is provided by use of stationary flushing systems. Sewer management with the CFD Cascades-, Flood and Discharge technology can make possible the aforementioned optimizations for combined systems. It is important to know, that it is possible to control the sewer cleaning of flushing waves for several kilometres by computer analysis.

2.2 The diagram Pic. 1 shows the effect of four waves running over a sewer length of 1600 m in diameters from 800 to 1200 mm with their polluting load concentration of each of the waves. In general, pipe monitoring technology must be seen as inadequate as the continuous flow of varyingly strong waste material movements can be determined, especially in larger pipe cross-section sewer stretches where there is a sedimentation hazard; this is even more evident in heavy rainfall after long dry periods. Odour pollution mostly occurs in the case of sewer lines with only slight gradients particularly with storage sewers; this may be an indication of hydrogen sulphide corrosion on the pipe material.

2.3 Cleaning large diameter pipe sewers with conventional high-pressure flushing equipment is only partially feasible and continues to be extremely costly. Possible damage as a result of using high-pressure flushing equipment cannot be excluded and fresh water required for the flushing must subsequently also be treated in the sewage treatment plant. As a general rule the sewer network should be cleaned at least once a year, although as most communities continually discharge waste water, this largely prevents optimal cleaning, especially in the base area. This particularly applies to checking socket sealings and possible material defects in the vicinity of the permanently wet area. Interruption of the sewage flow for monitoring, maintenance and rehabilitation purposes is therefore essential, particularly in large diameter pipe systems > 1.2 m. It is also generally known that installation of valves as cut-off instruments in combined sewer systems where there is a sedimentation hazard also entails considerable problems.
3. Flush Wave Runtime Process (FWR)

3.1 General data on the method
Flushing waves in cascade-controlled combined sewer systems are not only used to minimize sedimentation deposits in the sewerage systems, but in conjunction with the FWR Flush Wave running process described below they can also serve the purpose of determining, inter alia, volumes, speed and sewage loads involved. This however, requires the allocation of at least two water level measuring devices (WM1 and WM2) at a specially defined measuring point spacing (ML) provided with on-line connection to a computer. Balancing of the CPU time can be effected via a radio clock card and/or computer link. The measurement data to be evaluated for the determination of the flushing wave volume and speed are explained in detail according to Pic. 2. The individual phases until providing evidence of "No deposits found in the area of the flushing line” will be shown below:
  • Scanning the flushing waves in the areas of WM 1 and WM 2
  • Calculation of flushing wave volumes
  • Measuring flushing wave runtime
  • Calculation of flushing wave speeds
  • Taking wastewater samples in the areas of WM 1 and WM 2
  • Determination of Concentration Difference CD
  • Calculation of Sewage Load Accumulation SLA
  • ML = Scanning points stationing difference between (sewer profile 1 and sewer profile 2) measured in sewer axis
  • TL = Absolute-time difference of wave peaks centre of gravity WS1 and WS2
This provides the additional possibility of improving the previous sewage load theory significantly by means of the innovative FWR process approaches in determining the CD concentration difference for the most varied parameters in the measuring points [1] and [2] arranged in series. This is achieved through measurement of the SLC Sewage Load Concentration in the ongoing wave. A sewerage system can be considered to be free of sedimentation when e.g. the following equation is fulfilled for the sewage load parameter CSB:

CD = SLC {2} (CSB) - SLC {1} (CSB) = 0 [mg/l]
If CD ≥ = 0 then we can calculate the Sewage Load Accumulation SLA
3.1.1 Method description
The FWR process is certainly also analytically transferrable to all current section shapes without any major problems involved, ranging from a rectangular section to a circular one and any other special types, whereby after each cyclical measurement of the wave height in the section shape (KP) the pertaining areas of partial fill are calculated and taken into account in the given SWL process formulae in lieu of the wave heights. A corresponding evaluation of the test in the main sewer of the City of Mengen/Danube is given in Pic. 3.
In this example the wave was generated by means of a cascade storage height of 2.95 m, whereby in section shape WM 1 the wave reached a water level of ca 0.78 m, in section shape 2 WM2 the water level was ca 0.30 m, and the flush wage volume determined based on the process approaches varied between 172.17 m³ for KP 2 and 172.60 m³ for KP 1. It is therefore feasible to watch the cleansing of a sewerage system on-line on a screen by means of a possible data remote supervision.The flush wave heights (SH) resulting from a given upstream storage and the pertaining measured peak durations (SL) in the individual cascade measuring points (KM) are always reproducible where a "clean" sewerage system is involved. In the event that the stored measurement values SH and/or SL are not nearly reached in a flushing process, this means that disturbances resulting from other restraining influences must exist somewhere along the sewer line which should then be subjected to an extensive visual inspection.
4. Storage structures

4.1 Centrifugal jets are mostly used to prevent sedimentation in storage structures. The operating costs incurred e.g. depending on the time spent, power, repair and wear and tear are exceptionally high and the energy losses which are indicative of this concept are certainly no longer justifiable from an economic aspect.

4.2 The use of potable water for cleaning storage sewers and retention tanks is a costly evil. Intensive network cleaning using expensive potable water is not justifiable in the long run, as waste water charges will increase according to the use and throughput of the treatment plant.

4.3 Additional building land procurement in densely populated areas can also lead to problems as essential hydraulic requirements can be decisive for the optimal location.

4.4 Odour pollution occurring in storage tanks can only be prevented by the provision of a cover with an integrated tank ventilation system. When site conditions are restricted, it may be necessary to build stacked retention tanks which in turn have to be emptied with the use of pumps. Construction and on-going costs for such installations are naturally considerable.

4.5 With the present throttle devices, constant emptying of the tanks is only partially possible. The danger of blockages is also a further hindrance to optimal utilization.

4.6 Fixed scum boards are used to hold back floating matters in the combined sewer system overflow process. In the past such fixed scum boards in steel or reinforced steel construction used to be installed before the receiving water and/or retention tank overflows. For hydraulic reasons, the lower edges of the fixed scum boards must be high enough to prevent the floating materials which have accumulated in front of the scum board from being dragged along, depending on the flow speed. Fixed scum boards create siphon-like overflow conditions which in their detrimental effect on the receiving waters can no longer be disregarded, as they inevitably lead to overflows from already settled waste water areas.

4.7 Floating scum boards according to the German patent DE 3503407 offer the advantage that floating materials throughout the entire area of varying water levels can be retained by combined sewer system overflow installations. Tipping of the floating scum boards is excluded as they travel in corresponding channel section grooves, ball bearing-equipped.

4.8 High discharge sills usually entail long overflow structures. Should the discharge sill height lie within the high water level variation range, backflow in the sewer network cannot be prevented without special technical installations.

4.9 Utilization of the sewer storage capacity is only possible to top edge of sill and, in general, must be seen as insufficient due to the lack of retention volumes, so that critical overflows can occur where there is contamination.

4.10 The inflow speed of the waste water is not taken into consideration when measuring water overflow quantities and this leads to increased inaccuracy. Quality-related "on line" overflow monitoring has not been undertaken to date.

4.11 Throttle facilities usually limit to a maximum outflow (2* Qt-Qf) and cannot be adapted to possible waste water control criteria (extension of the intake shaft). The discharge times of such scum boards have been found to be extraordinarily high, therefore favouring sedimentation and the resultant odour pollution. As a result of inconstant inflow water levels, the quantity to be passed on varies, and with motor controlled throttles there is a danger of discharge fluctuation by displacement of the run-off cross section during level control.

4.12 The overflow rate in combined sewer systems decisively influences possible waste material discharge occurring at structures with overflow. Irrespective of the storage structure arrangement, centrifugal jets are usually installed to prevent sedimentation. These centrifugal jets are switched on at low water level depending on the respective tank fill. Post-rain effects however, cause problems as they often lead to the overflow volumes "slopping over" several times, which in turn cause increased waste material discharge owing to the fact that the premature stirring up of waste material cannot be prevented at such short time intervals. Although these problems are generally known, they are mostly not investigated in more detail as no better alternative appears to be available. In areas with stationary weir systems, structures with overflow are equipped with mesh screen systems in order to retain floating matter in the combined sewer system mixing water overflow, similar to the floating scum boards, and to reduce the CSB and BSB receiving water loads. The major disadvantage involved is the mesh screen systems' need of permanent mechanical cleaning. In addition their design only allows floating matter retention with particle sizes > 4 mm.
5. Sewer control equipment in the CFD Technik

5.1 Cascade Weirs
Cascade weirs can be lifted with a speed up to 7 m/min by hydraulic cylinders, fitted on both sides and restrained by guide ways. The weirs have a plane-parallel weir plate (thickness = 200 mm) formed from a box-like outer skin with welded airtight stainless steel sheets. (thickness = 2 mm) The weir plate is filled with concrete, reinforced and when the concrete is set, a vacuum is applied to keep the stainless steel flat against the concrete core. By a special manufacturing method the weir plate can be produced with a width of 1.20 m to 3.70 m and a lift height of 6.00 m (Pic. 4).

5.1.1 Essential functions for weir and slide systems are:
  • Activation of the cascade volume
  • Flushing of sewage networks for automated sewer cleaning
  • Ventilating of sewage networks by shifting of the flood wave volumes
  • Comparing moderation of the sewage load in the intake of the sewage plant 

 At the present time sewer cleaning distances are managed up to 8.2 km in the project "D-Eisenach"
5.2 Slide Systems
FT slides are inclined against the direction of flow at less than 60°. The slide plate consists of stainless steel and is ground on the underside. It slides by means of the lift of hydraulic cylinders with a speed up to 7 m/min into up and down positions. SD slides can be manufactured up to a diameter of 1.10 m. The essential functions are the same as the functions of the cascade weirs. The effect of sewer cleaning could be tested approximately over 1.6 km in the project of D-Spaichingen.

Skimmer Slides are similar in their construction to the FT slides and provide a skimming effect in order to control the discharge of scum or other floating materials. Under fast lowering conditions, they also can contribute further flushing management in the sewer network. Especially for controlling a constant effluent into a sewage plant a combination of an FT Slide Pic. 5 and as Skimmer Slide Pic. 6 is very helpful.

Usual measurements are widths up to 1.10 m with variable lifts up to 2.00 m.
5.3 Discharge Weirs
Discharge weirs have the task of activating cascade volumes up to a predefined storage level and to discharge surplus stormwater into the receiving water by lowering the weir edge (Pic. 7). The construction features of discharge weirs correspond to those of the cascade weirs in which the lift velocity is between 10 to 30 cm/min. The measurements of discharge weirs are: width = 1.20 m to 7.70 m at a maximum lift of approximately 3.00 m.
6. Floating Scum Board

In all cases of installing discharge weirs, the reduction of floating solids can be provided with scum boards (Pic. 8). The floating scum board supports the retention of swimming substances from lowest to highest water-level situation. An essential advantage is the fact that the floating scum board is fitted in parallel guide-ways where swimming is provided with out any blockage, because the diameter of a rolling circle smaller than the light distance of the guide slots to be fastened in the building. Floating scum boards are conveniently installed in front of discharge weirs to reduce scum or other floating substances at discharge structures. Scum boards are manufactured as floating bodies and consist of stainless steel and cannot bend or twist under floating conditions. Usual production lengths are 1.20 m to approximately 8.00 m.
7. Special Structures for sewer management

7.1 Pump-Flush-Cylinder PFC Structure
The discharge of pumped waste water in gravity sewers mostly causes odour problems and deposits in the inlet of the sewer system. By erecting of a PF Pump-Flush-Cylinder Structure Pic. 9 in the outlet of a pressure pipe, it is possible to reach a time based flushing wave by activating of the integrated Flush-Slide inside the PFC Structure in a long distance of the sewer system. The achievable flush length arises by the activation of the predefined storage level in the FC-Structure and an automatic sewer cleaning can be solved over several kilometres.

Further advantages can be summarized in the following notes:
  • Time based activation of the sewer flush technology with PF Pump-Flush-Cylinders, by sewage water or salty ground water, which causes no problems in the in the biological clearing process of the sewage treatment plant.
  • Short time flushing waves permit oxygen transfer capacity under sewage process conditions
  • Avoiding sewer sedimentation
  • Avoiding H2S-Corrosion
  • Avoiding odour problems
  • Rats drown in the flushing wave, because the aeration reduces the specific weight
  • Equalization of inlet flow to the sewage plant according quality and quantity
8. Round Cyclone

8.1 Round cyclone with integrated screen function
The round cyclone is an innovative new development and the task is to destroy the high-energy flushing by the forced radial flow in the round cyclone in front of the sewage treatment plant or a following pumping station. Together with an upstream rotating material screen it can achieve a significant removal of sand grit and waste matter solids. A great benefit is that downstream connected wastewater pumps cannot be damaged by sediments carried forward. The 3D sketch in Pic. 10 shows the special equipment in the structure of a round cyclone, which has to be installed for an optimized activation and treatment of flushing waves. The necessary diameter for the round cyclone must be calculated with more than four times the width of the inlet main sewer. The injector pump is necessary to provide the radial flow in all cases of influent volumes and to avoid sludge sedimentation in the cone of the round cyclone. The round cyclones can advantageously be produced in prefabricated parts.
8.2 Round cyclone with integrated quality discharge function
The target is to use the round cyclone for different functions. A new version for the quality management of rain water discharge shows Pic. 11. as an overlaid picture.
After flushing of parts of retention volumes in the cascaded sewer system the rainwater retains in the sedimentation channel. The sedimentation process of the suspended solids begins and the clear water level lowers down. By permanently measuring the SAC with the vertically adjustable sensor in layers of the water body, the discharge weir can lift down in order to the qualitative measurement for example using several sensor systems. The clear water discharge to closed percolation tanks can be used to increase the level of the sweet water body in the arid areas.
9. Control concept for sewer management equipment

The sewage control plants are controlled locally and all regular process data like top water-level, sub-water-level, weir and slide level, status reports of the hydraulic system etc. are recorded (Pic. 12). The respective plants can exchange the necessary global control functions over fibre optic or ISDN lines and are supervised and remotely controlled.
10. Conclusion

The CFD Cascade-, Flush- and Discharge Technique [1; 2] with their components Weir-structures and Flush-Throttle-Slides, as well as the accompanying process control bases on a volume oriented flow control. Target is the optimal usage of the available sewer retention volumes, simultaneous controlling of throttle flow and sewer cleaning for kilometres by flushing wave impulses. Discharge to receiving water, using weirs, are executed only after fulfilling of the required storage volume. Parallel to the optimization of controlling the sewer retention volume, the online measurement of quality discharge in rainwater and combined water systems is very useful. First tests with continuous measuring of the pollution load of combined sewage, are carried out at the moment, by measuring the parameter SAC Sequenced Adsorption Coefficient. This idea, to control the quality discharge in a combined sewer system is also a solution for discharge of rain water systems for optimising the groundwater body in arid areas. The only differences are that every structure has to be designed in a new version.

The CFD technology will be extended with a sewage load orientated process control using the flushing wave run time method. Only lowering weir gates are a suitable control element due to their high flexibility especially to respond to the variable demands of future flow control. The installation of a round cyclone with a post-connected circulation and rotating wastewater screen in the intake of a sewage treatment plant optimizes the flushing management of the sewer system allowing a simultaneous withdrawal of grit, sand and wastewater screen material. Otherwise the installation of a circulation wastewater screen in the outlet can avoid problems during discharge of combined sewer overflow.


[1] Weikopf, M.:
Das Mischsystem mit seinen bisherigen Unzulänglichkeiten und Trendwende zur MSR-Kaskaden- und Entlastungstechnik
Korrespondenz Abwasser 7/95
(The combined system with its previous shortcomings and trend reversal towards the MSR cascade and overflow technique "Korrespondenz Abwasser" 7/1995 page 1155).

[2] Weikopf, M.; Mordelt, T.:
Sewerage System Management based on the Cascade-, Flush- and Discharge Technique Fundamentals of CFD Technique and Process Control
VDI/GMA Convention; D-Langen on 22/23/11.1999

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