SCRAPER BRIDGES for SEDIMENTATION TANKS

Sedimentation is the process by which the solid substances suspended in water settle on the tank bottom, thanks to the force of gravity only.

The sedimented material is scraped, collected and conveyed out of the sedimentation tank. The sedimentation of waste water, before the biological treatment, is defined as Primary Sedimentation (Primary Clarifiers), while the sedimentation of water after the biological treatment is called Final or Secondary Sedimentation (Final/Secondary Clarifiers).

The Scrapers used for primary sedimentation are equipped with bottom scrapers and a system to remove the floating material.

The Scrapers used for secondary sedimentation are equipped with a scraping or suction system, in the second system the sludge is continuously sucked from the tank bottom.

Our range of production includes the following types of Scrapers for Sedimentation Tanks:

Scraping system

Central drive scraper bridge mod. EM16
Peripheral drive (simple - double – triple) scraper bridge mod. EM17 Scraper bridge with torsion trestle mod. EM18
Scraper for rectangular tank with travelling bridge mod. EM50
Scraper for rectangular tank with dredging chain mod. EM53

Suction system

Peripheral drive (simple - double – triple) suction type scraper bridge mod. EM29
Suction type scraper bridge with torsion trestle mod. EM30
Suction type scraper for rectangular tank with travelling bridge mod. EM51

ACCESSORIES

Overflow weirs
Scum baffles
Quick loading scum boxes

Outlines of the Sedimentation Process

A good efficiency of the sedimentation process gives as a result a considerable reduction of BOD, COD, heavy metals, azote, phosphorus, etc. in the treated water. As the suspended substances in the waste water become larger, their sedimentation becomes easier.

Stokes’ law emphasizes the importance of the floc size for the speed of sedimentation.

Stokes’ law:

V = speed of sedimentation in m/s
ρ = density of solids in kg/m³
ρ₀ = densità dell’acqua kg/m³
g = 9,81 m/s²
μ = viscosity of water in in Pa x s (at 15° C the viscosity of water is 1,06 x 10^-3 Pa x s)
D = diametre of the particle in m.

A good way to obtain large flocs in the clarifier feeding is the installation of baffles using the kinetic energy of water to agglomerate the flocs to each other

In the secondary clarifiers, the type of scraping blades and the sizing of the central drain well are very important.

Actually, given the enormous volume of “sludge” compared to the feed flow rate (50%), we can hypothize that the sludge characteristics are just a little different from the ones of the clarified water (under static conditions the clarified water and the sludge stratify with a horizontal separation surface) and that, consequently, due to the slight bottom slope (∼ 4%), it flows naturally towards the central well. However, we should not forget that the particles which touch the bottom, and the ones which stratify, adhere and loose the properties of “fluid”: so they must be actively moved and “pushed” towards the drain well before the layer becomes too thick and, above all, before the anoxic condition causes a strong de-nitrification with the production of floating plates.
Therefore all of the sedimented sludge particles conveyed by the bottom scrapers to the central well from the tank periphery, must reach it quickly. This is achieved by giving a continuous profile with logarythmic shape to the blade; the relevant equation in polar coordinates is:

where r and α are the polar coordinates having the tank centre as a pole and r_o is the radius of the drain well. Under these conditions, the scraper moves the sludge, makes it more “fluid” (by eliminating the adhesion) and conveys it by a perpendicular force to its surface (Pascal); this force has a radial centripetal component, not null and constant.
The angle γ is determined by:

CENTRAL DRIVE SCRAPER BRIDGE FOR CIRCULAR TANK
Mod. EM16

Principles of sizing

Hydraulic sizing (process)
The sizing of the diffusion drum is based on the following parametres:
• influent flow rate
• piping diametre
The water coming out of the diffusion drum to the sedimentation zone must have a very low kinetic energy in order to guarantee a good separation and settling of the solids. Moreover, the bottom scrapers must have a continuous and logarythmic profile in order to successfully convey the sludge into the central well in less than a complete rotation of the shaft, 270° (¾).

Mechanical Sizing
The sizing of the bottom scraper is based on the type of sludge to be evacuated from the tank bottom. Normally the central shaft and the scrapers are designed to bear a stress of 20 Kg. per linear metre. The torque to transmit is calculated by:
T [kgm] = r² x K
r is the tank radius
K is the load on the scrapers per linear metre (20 kg for biological clarifiers)
For a reliability purpose, the drive unit shall be capable of transmitting a 1.8 higher torque than the design value.

Torque limiting device (dynamometric cell)
The dynamometric cell measures the torque transmitted by the drive unit.
This value is evident directly on the instrument and allows to evaluate the stresses on the central shaft during its rotation.
The device is equipped with limit switches set at two different torque values, one for alarm and one for the machine shutdown.

Selection of the electric motor.
The electric motor size is selected so that, in case of shutdown, its static torque is lower than the one which can be borne by each downstream unit. For this reason the electric motor is usually very small. Exuberant motors may cause, in case of shutdown, serious damages to the structures if the torque limiting device fails to intervene or if it is not properly connected.

Example of calculation:
Tank diametre 10 m.
K = 20
Scrapers peripheral speed 1,5 m/min
Slow shaft rpm 0,047
Torque r² x K = 5² x 20 = 500 kgm
so the drive unit shall be designed to transmit a 1.8 higher torque.

Electric Motor:

To transmit a torque of 500 kgm at 0.047 rpm, 25 watt are necessary.
If we hypothize a drive unit output of 60%, the electric motor shall have a power of 40 watt.
Normally it is difficult to find electric motors which are smaller than 0,18 kw - 180 watt.
If the selected motor has got a power of 180 watt, in case of shutdown it might give a 2.6 times higher power for a few seconds, i.e. 468 watt.
The torque transmitted by the motor to the slow shaft, with a drive unit output of 60%, would be equal to 5700 kgm.
In this case it should be evaluated whether to install a shear pin or a dynamometric cell to limit the torque.

PERIPHERAL DRIVE SCRAPER BRIDGE FOR CIRCULAR TANK
Mod. EM17

Principles of sizing

Hydraulic sizing (process)
The sizing of the diffusion drum is based on the following parametres:
• influent flow rate
• piping diametre
The water coming out of the diffusion drum to the sedimentation zone must have a very low kinetic energy in order to guarantee a good separation and settling of the solids.
Moreover, the bottom scrapers must have a continuous and logarythmic profile in order to successfully convey the sludge into the central well in less than a complete rotation of the shaft, 270° (¾).

Mechanical Sizing
The sizing of the bottom scraper is based on the type of sludge to be evacuated from the tank bottom.
The central column, the trolleys and the scrapers are designed to bear a stress of 20 Kg. per linear metre.
The torque to transmit is calculated by:
T [kgm] = r² x K
r is the tank radius
K is the load on the scrapers per linear metre (20 kg for biological clarifiers) The trolleys drive unit shall be capable of transmitting a 1.8 higher torque than the design value.

Torque limiting device (dynamometric cell)
The dynamometric cell measures the torque transmited by the drive unit.
This value is evident directly on the instrument and allows to evaluate the stresses on the gearbox during rotation.
The device is equipped with limit switches set at two different torque values, one for alarm and one for the machine shutdown.

Selection of the electric motor.
The electric motor size is selected so that, in case of shutdown, its static torque is lower than the one which can be borne by each downstream unit.
For this reason the electric motor is usually very small.
Exuberant motors may cause, in case of shutdown, serious damages to the structures if the torque limiting device fails to intervene or if it is not properly connected.
Example of calculation:
Tank diametre 20 m.
K = 20
Scrapers peripheral speed 1,5 m/min
Slow shaft rpm 0,047
Torque r² x K = 10² x 20 = 2000 kgm
(in case of radial scrapers, the torque is halved) so the drive unit shall be designed to transmit a 1.8 higher torque

Electric motor:
To transmit a torque of 1000 kgm, the thrust of each trolley must be 1000 : 5 = 200 kg. If the wheel diameter is 300 mm., then the torque on the shaft is 200 x 0.15 (wheel radius) = 30 kgm. Therefore, at a speed of about 1.5 rpm, 62 watt are necessary.
If we hypothize a drive unit output of 60%, the electric motor shall have a power of 113 watt.
Normally it is difficult to find electric motors which are smaller than 0,18 kw - 180 watt.
If the selected motor has got a power of 180 watt, in case of shutdown it might give a 2.6 times higher power for a few seconds, i.e. 468 watt. The torque transmitted by the motor to the wheel slow shaft, with a drive unit (on the trolleys) output of 60%, would be equal to 48 kgm., the thrust of the trolley would be 320 kg. and the torque 3200 kgm. for each trolley (total 3200 kgm.)
Therefore the central column must be designed to bear a torsion stress (in case of shutdown) of 3200 kgm.

SCRAPER BRIDGE WITH TORSION TRESTLE FOR CIRCULAR TANK
Mod. EM18

Principles of sizing

Hydraulic sizing (process)
The sizing of the diffusion drum is based on the following parametres: • influent flow rate • piping diametre The water coming out of the diffusion drum to the sedimentation zone must have a very low kinetic energy in order to guarantee a good separation and settling of the solids.
Moreover, the bottom scrapers must have a continuous and logarythmic profile in order to successfully convey the sludge into the central well in less than a complete rotation of the shaft, 270° (¾).

Mechanical Sizing
The sizing of the bottom scraper is based on the type of sludge to be evacuated from the tank bottom. Normally the central shaft and the scrapers are designed to bear a stress of 20 Kg. per linear metre. The torque to transmit is calculated by
T [kgm] = r² x K
r è is the tank radius
K is the load on the scrapers per linear metre (20 kg for biological clarifiers)
For a reliability purpose, the drive unit shall be capable of transmitting a 1.8 higher torque than the design value.

Torque limiting device (dynamometric cell)
The dynamometric cell measures the torque transmitted by the drive unit.
This value is evident directly on the instrument and allows to evaluate the stresses on the trestle during the rotation.
The device is equipped with limit switches set at two different torque values, one for alarm and one for the machine shutdown.

Selection of the electric motor.
The electric motor size is selected so that, in case of shutdown, its static torque is lower than the one which can be borne by each downstream unit. For this reason the electric motor is usually very small. Exuberant motors may cause, in case of shutdown, serious damages to the structures if the torque limiting device fails to intervene or if it is not properly connected.
Example of calculation:
Tank diametre 20 m.
K = 20
Scrapers peripheral speed 1,5 m/min
Slow shaft rpm 0,023
Torque r² x K = 10² x 20 = 2000 kgm
so the drive unit shall be designed to transmit a 1.8 higher torque.

Electric Motor:

To transmit a torque of 2000 kgm at 0.023 rpm, 64 watt are necessary. If we hypothize a drive unit output of 60%, the electric motor shall have a power of 107 watt.
Normally it is difficult to find electric motors which are smaller than 0,18 kw - 180 watt.
If the selected motor has got a power of 180 watt, in case of shutdown it might give a 2.6 times higher power for a few seconds, i.e. 468 watt.
The torque transmitted by the motor to the torsion trestle, with a drive unit output of 60%, would be equal to 5700 kgm.
In this case it should be evaluated whether to install a shear pin or a dynamometric cell to limit the torque.

SCRAPER WITH TRAVELLING BRIDGE FOR RECTANGULAR TANK
Mod. EM50

Principles of sizing

Hydraulic sizing (process)
The sizing of the feeding zone is based on the following parametres: • influent flow rate • dimensions of the pipings The water flowing to the sedimentation zone must have a very low kinetic energy in order to guarantee a good separation and settling of the solids.

Mechanical Sizing
The sizing of the bottom scraper is based on the type of sludge to be evacuated from the tank bottom.
Normally the trolleys and the scrapers are designed to bear a stress of 20 Kg. per linear metre.

Checks
The following checks regarding the movement of the machine components are very important for this kind of machine: • bridge: forward-backward movement • bottom scraper: lifting-lowering • surface scraper: lifting-lowering • electrical supply Our travelling bridges are always equipped with two limit switches for each movement (1 for operation and 1 for emergency) Buffer groups are also installed to limit the run of the moving units in case of failure the electrical limit switches and they are sized to bear the maximum stress.

Motorization
We use a double motorization for bridges exceeding 6 m. of width and moving over a runway of more than 12 m. This solution allows to achieve a good alignment of the bridge, even if the two motorized wheels diametres are not exactly equal.
In this case, a simple motorization through a central shaft may cause the side-slip of one of the two motorized wheels.

Characteristics
Both the motorized hweels and the idle wheels must be perfectly parallel and at the same distance; for this purpose, after the installation of the bridge, the diagonals between the wheels must be carefully measured.

Contrast wheels and buffers
The contrast wheels and the buffers must be designed and sized so that the bridge cannot skid laterally out of the runways, under no circumstances.

SCRAPER WITH DREDGING CHAIN FOR RECTANGULAR TANK
Mod. EM53

Principles of sizing

Hydraulic sizing (process)
The sizing of the feeding zone is based on the following parametres: • influent flow rate • dimensions of the pipings The water flowing to the sedimentation zone must have a very low kinetic energy in order to guarantee a good separation and settling of the solids.

Mechanical Sizing
The sizing of the scrapers is based on the type of sludge to be evacuated from the tank bottom. Normally the scrapers, shafts and chains are designed to bear a stress of 20 Kg. per linear metre.

Torque limiting device (dynamometric cell)
The dynamometric cell measures the torque transmitted by the drive unit. This value is evident directly on the instrument and allows to evaluate the stresses on the moving members during the rotation. The device is equipped with limit switches set at two different torque values, one for alarm and one for the machine shutdown.

Checks
The movement of the chains must be carefully checked because it is very important for this type of machine.
For this purpose, we use to apply some sensors to check the idle shafts rotation.

PERIPHERAL DRIVE – SUCTION TYPE SCRAPER BRIDGE FOR CIRCULAR TANK
Mod. EM29

CENTRAL DRIVE – SUCTION TYPE SCRAPER BRIDGE WITH TORSION TRESTLE, FOR CIRCULAR TANK
Mod. EM30

Principles of sizing

Sludge suction unit sizing.
For the project of the sludge suction unit we have to make sure that the sludge is quickly removed from the tank bottom because if it tends to settle or get too thick for staying in the tank for a too long time, it may happen that: • a part of the recycled sludge returns to the oxidation tank when it is no more fresh and this will affect the treatment efficiency • a shortage of dissolved oxygen may cause a leakage of sludge in the effluent

Mechanical Sizing
Same principles as described for the Central drive scraper bridge mod. EM16.

Operation Description

The bottom of the suction scraper bridge tank is flat and the suction pipes, sized according to sludge recycle flow rate, are placed at a distance which does not exceed 3 m. The sludge is conveyed towards the suction pipe by V-shaped scrapers which make the flowing of the sludge towards the piping much easier. Each suction pipe is equipped with a telescopic valve to regulate the flow into the hopper. The quality and quantity of sludge sucked by each pipe can be checked from the bridge. The sludge flows into the hopper according to the principle of the communicating vessels. The sludge flow rate depends on the difference of level between the water surface and the sludge level in the hopper. The sludge sucked by all of the suction pipes is conveyed out of the tank by means of a self-regulating siphon. The triggering of the siphon can be made during the commissioning phase by means of a pump or ejector.

Technical and dimensional data necessary to make a technical proposal and quotation

SUCTION TYPE SCRAPER WITH TRAVELLING BRIDGE FOR RECTANGULAR TANK
Mod. EM51