IE47896B1 - Method of removing mud from waters - Google Patents

Abstract

1. Method of removing mud from waters, characterized in that the mud is sucked up and the sucked-up water-mud suspension is filled into a floating vessel together with a flocculating agent, in that filling ist interrupted as soon as te vessel is filled, in that the supernatant water layer ist removed after the mud has settled, and in that filling with addition of flocculating agent, settling and removing the supernatant water layer is repeated until the vessel is filled with mud.

Description

The invention relates to a method of removing finely and very finely divided muds of all kinds from waters, for example from rivers, lakes, harbour basins, and the like.

The problem of keeping such waters clean after their state of pollution has passed acceptable limits is widely known.

As the pollution of water progresses, the fine and very fine particles of solid matter, which at first are suspended in the water, also increase and in a matter of weeks or months give rise to a continuous rise of the level of mud in the 1C waters. The fauna and flora of many lakes, including some situated in recreation areas near centres of population, already have their existence most seriously threatened through i bolting with mud. In river mouths situated in brackish water districts, where the silting of the water is further intensi15 fivd by the fact that microorganisms, which live only in fresh water, perish and sink on encountering seawater because of the excessive salt content, navigation is even hindered by the increasing mud or silt at certain points. It is attempted to master this silting in various ways. In par20 ttcularly affected port installations, for example ferry embarkation and disembarkation points, underwater ploughsknown as underwater rakes or harrows - are pulled through the masses of mud during the ebb-tide with the aid of tugs, in the hope that the mud swirled up under the water will be carried out towards the sea with the ebbing water.

This operation has very little effect because the high tide brings the mud back again. Dredging with the aid of grab dledgers or chain and bucket-conveyor excavators is far more effective. Grab dredgers can be used only for removing smaller amounts of mud. Suction dredgers and chain and bucket-conveyor io. avutors can be used only when anchored fast and while in use they constitute an obstacle to traffic In port Installations and water-ways. The floating suction dredger usually conveys the excavated material through a floating pipeline and a land line connected to the latter to an artificially constructed polder. Since the length of floating pipelines is limited in places where weather gives rise to heavy seas or where strong tidal currents occur, the method cannot be applied everywhere. the The combination of suction dredgers and barges,/so-called dumb barges, into which the excavated material is loaded and which are transported to the unloading site by means of tugs, would be an uneconomical solution to the problem, since in the case of suction dredging of mud the large amounts of water, which are always carried off at the same time because of the nature of the process, remain in the mud, which does not readily settle, and thus considerably increase the frequency of the barge journeys. A dumb barge cleaner (pump ship) is also necessary at the unloading station. It is more expedient to combine the use of barges, tugs, and barge cleaning ships with a chain and bucket-conveyor excavator, which continuously raises the mud, in practically its original concentration, from the seabed with the aid of its'buckets suspended on an endless chain, and empties it into barges lying alongside the excavator ship.

When a chain and bucket-conveyor excavator is used it is of course inevitable that the surface water will be polluted. Depending on the nature of the subsoil a chain and bucket-conveyor excavator, unlike n normal suction dredger, will generally be able to cut to plan a desired water profile modification, that is to say to lower the bed or widen the expanse of water. Since in most cases it is only desired to remove mud from the water and not to effect any particular modification of the profile, the use of a chain and bucket-conveyor excavator solely for mud removal is too complicated and too expensive. 7 8 9 6 The term hopper or hopper suction dredger is applied to a ship which combines all the functions necessary for dredging operations (the functions of a suction dredger, barge, tug, and barge cleaner) and which is much used for the removal of sand from waters. A hopper requires no .nchoring for the purpose of dredging and can, therefore, be removed at any time from the dredging site under its own power, so that it does not constitute an obstacle to shipping.

A loading tank (hopper tank) occupying almost the entire ship serves to receive the sand brought up. The sand mixed with water is sucked up continuously through a vertically movable nose-like suction pipe mounted on the outside of the hopper dredger, and is passed into a connecting pipe system which ends in two hopper tank inlet pipes extending on the port and far-board sides above the hopper tank and provided with numerous closable flaps. The mixture of water and sand flows •-nt ol the inlet pipes over the entire length into the hopper tank, in which the sand, having a higher specific gravity, settles immediately, while the separated water flows off continuously into the sea by way of overflow hatches which are provided, directly under the top edge of the deck, in the ships's side and are vertically adjustable. In this way a mixture of water and sand is continuously sucked up from the bed and passed into the hopper tank until the latter is filled with sand.

After the hopper tank has been filled and the suction pipe nose has been raised and stowed away, the hopper dredger sails under its own power to an unloading site, which may be at a distance of several kilometres, and is there unloaded either by means of a discharge flap provided in the bottom of the hopper or by a flushing operation- with the aid of its own pump and of bottom valves (slide valves). Because of the nature of its operation the hopper dredger works economically.

For sand dredging a hopper tank having a capacity of e.g. 500 to 5000 cubic metres can be effortlessly filled with sand by the overflow process within 1 to 3 hours. Attempts to use hopper dredgers for removing mud from waters have failed because even dredging for days on end does not enable the hopper tank to be filled with a worthwhile amount of solid material. The suspensions of mud and water pumped into the hopper tank show no tendency to settle because difference between the density of the solid material'and the is very small writer/, but are washed out of the overflow hatch with the same concentration of solid material, when the overflow process characteristic of a hopper dredger is applied, as when they flow into the hopper tank. The concentration of solid material in the suspension of mud and water pumped up is far too low to permit economic operation.

The object of the present invention therefore is to provide a simple, economical method of removing mud from waters.

The method of the present invention for the removal water- suspension of mud from waters is characterised in that the/mud/is sucked up and the suspension of mud and water sucked up is filled together with a flocculating agent into a floating vessel, the filling is interrupted as soon as the vessel is full, and after the mud has settled the supernatant layer of water is removed, and the filling with the addition of flocculating agent, allowing to settle, and removal of supernatant water are repeated until the vessel is filled with mud.

In order to ensure that the mud from the suspension of mud and water will settle to form a supernatant layer of clear water, it is necessary to add a flocculating agent.

Flocculating agents are known. They are usually high polymeric products, such as for example rubber phosphonium .salts, starch products, proteins, polyacrylates, bone glue, polyethylene oxides, polyacrylamides, polyethyleneimines, and -;o on.

Flocculating agents are divided into non-ionic, anionic, and cationic agents. In order to be able to select the most suitable flocculating agent for any particular mud, sedimentation tests are carried out in the laboratory with a sample of suspension of mud and water which has been pumped up, and the optimum amount of flocculating agent for-total i1oceulation is determined. Good results are obtained in practice with flocculating agents based on e.g. polyacrylamide, partially saponified polyacrylamide, non-ionic polyethylene oxide, cationic polyethyleneimine, cationic polyethyleneamine, cationic poly-dimethylaminoethyl methacrylate, and/or poly-dimethylaminoethyl acrylate, cationic copolymerisates of dimethylaminoethyl methacrylate and acrylate, and/or acrylamide.

The flocculating agent is normally added in the form of an aqueous solution direct to the suspension of mud and water pumped up, expediently immediately downstream of the pump, in the optimum amount determined by previous tests.

J'he aqueous solution of flocculating agent may have a concentration of 0.0001% by weight or higher, particularly from 0.01 to 0.5% by weight. If aqueous solutions having a concentration higher than 0.5% by weight can still be metered, more concentrated solutions can also be used. It may be expedient to measure the concentration of the mud in the suspension of mud and water continuously during the pumping operation, for example by rndiometry and to meter the flocculating agent in accordance with the mud concentration measured. From 10 to 100 ppm of flocculating agent (referred to 100% flocculating agent) are normally added per cubic metre of mud and water suspension pumped up. Mud ox· silt deposited on a bed usually contains from 40 to 50% by weight of solids. During the pumping operation it is normally diluted with water in the ratio 1 : (1 to 20).

In some favourable cases a certain separation already occurs between the mud and the water during the filling of the floating vessel with the suspension of mud and water mixed with flocculating agent, so that a layer of clear water first reaches the level boundary in the floating vessel. In such cases it is expedient for the layer of clear water to be removed during the actual filling operation, which can most simply be done by allowing to clear water to overflow over the edge of the vessel or through overflow hatches provided in the wall of the vessel. However, even in favourable cases a layer of clear water can be removed in this way only for the space of a few minutes, whereupon the suspension of mud and water reaches the level boundary of the vessel. The filling operation is then interrupted. After the mud has settled, the supernatant layer of clear water is removed, and the filling, allowing to settle, and the removal of the layer of clear water are repeated until an economic amount of mud has collected in the vessel, whereupon the latter is transported to the unloading site and the mud is there emptied out.

Known suction devices are used for pumping up the mud deposited on the bed, in the form of a suspension of mud and water, and for filling it into the floating vessel. The layer oi clear water above the mud deposited in the floating vessel can be removed in various ways, for example, a vertically adjustable slide valve can be provided in the wall of the vessel, this slide valve closing an outlet opening, for example a discharge hatch, when in its highest position. After 4789® the suspension has separated into a layer of mud and a layer of clear water thi' outlet opening, for example the outlet hatch, is gradually opened, so that the layer of clear water can flow off into the sea. A separate receiving chamber, into which the supernatant layer of clear water can flow off through a closable opening, may also be provided in the floating vessel, to clear water being removed from this chamber for example with the aid of a pump.

Furthermore, the layer of clear water can be removed through an outlet pipe or outlet hose, through which the water flows freely into the sea or is pumped off.

Various possible ways of removing the layer of clear voter may also be combined. In all cases the removal of the layer of clear water should be effected in such a manner that . ·; little swirling as possible is produced in the layer of mud. i'nis car, for example be achieved by arranging for the overflow, aifaiission, or sucking-off of the water always to take place just below the surface of the clear water. The overflow edges, inlet and outlet openings must for this purpose be adapted to tin? falling surface of clear water, that is to say must be i ontiuuously lowered as the level of clear water falls. It is useful for the inlet openings of outlet pipes, suction hoses, or the like to be so disposed that the clear water can flow from above downwards into the outlet pipes, suction hoses, or the like. The clear water flowing into the inlet openings thus comes mainly from the surface of the water. By this means it is ensured that none of the collected mudiwill be entrained, during the drawing-off of the clear water, through the action of suction, such as may occur with unfavourably constructed or disposed inlet openings.

In addition, it is expedient for the inlet opening of an outlet or suction pipe to be made funnel-shaped, because the overflow rate of clear water flowing from the surface will thereby be reduced. It is also advantageous for overflow edges and inlet openings for the clear water to be serrated.

It is useful for the suspension of mud and water pumped up to be introduced into the vessel, together with the flocculating agent, in such a manner that during the filling operation also the least possible turbulence is caused. In the case of elongated vessels, such as are generally provided, for example, in boats and ships, it is expedient for the suspension of mud and water together with the flocculating agent to be introduced only at one longitudinal end. It is thereby ensured that the heaviest turbulence occurs during the filling operation only at one longitudinal end, while the deposition of the mud is already started during the filling operation. The apparatus for removing the layer of clear water is advantageously disposed at the other longitudinal end of the vessel. It is also advantageous for the suspension of mud and water to be filled into the vessel below the level of 1iquid present during the filling operation. The suspension is thereby prevented from impinging on the surface of the liquid in the vessel, with the consequent destruction of the flakes of mud already formed, and in addition with the disturbing formation of foam.

Furthermore, it is expedient to effect the filling of the vessel with suspension of mud and water at one end of the elongated vessel, and foi- this part to be separated, in the region of higher liquid levels, from the remainder of the vessel. With higher liquid levels a pacification zone is thereby formed and the rate of sedimentation is increased. 7 8 9 6 Before the next removal of the layer of clear water, however, the division between the filling region and the remainer of the vessel must be removed. Normally after a few cycles, for example 2-4 cycles, of the process of the invention - which can be carried out within a few hours, for example 1 to 3 hours a container of e.g. 500 to 5000 cubic metres capacity is already filled with so large an amount of highly concentrated mud that it is economic for it to be transported away. At the unloading station the mud is removed from the vessel, for example by way .:1 flaps in the bottom of the vessel. The mud in the vessel . an also be diluted with water at the removal point to a pumpable state and then pumped off.

The floating vessels used for applying the process of (lie invention may for example be suitable boats and ships, for example dumb barges, but are preferably hoppers or hopper dredgers. When a hopper dredger is used the finely and very finely divided mud suspension entering the hopper tank during the dredging operation, which may be a suspension of any kind, is mixed with a flocculating agent and the hopper tank is filled to a point just below the overflow. Dredging is then suspended for a short time. The layer of clear water forming above the layer of mud is drawn off with the aid of a suitable apparatus, and dredging is then resumed with the addition of a flocculating agent to the incoming mud suspension, until the hopper tank is once again filled to a point just below the overflow. The new layer of clear water formed is drawn off, and filling with the addition of flocculating agent, allowing to settle, and removal of the layer of clear water lying above the solid material are repeated until the hopper tank is filled with an economic amount of flocculated mud. Unlike the practice of continuous filling of the hopper which was customary up to the present time, the process of the invention is carried out 7 896 discontinuously.

In order to apply the method of the invention an ordinary hopper dredger must practically be simply provided with an apparatus for removing the clear water, for which only slight conversion is usually required.

In view of numerous unsuccessful attempts to use hoppers for removing mud from water in an economic manner, the method of the invention must be regarded as surprising. It was also not to bo expected that by working in accordance with the invention it would be possible for a vessel to be substantially filled with concentrated mud within a relatively short time, particularly as it had to be feared that because the hopper tank is unfavourably shaped for rapid separation of mud, and also because of the movements of the ship caused by heavy seas, it would not be possible to achieve reasonable settling speeds or high concentrations of the mud.

The invention will be further explained with the aid of tile accompanying drawings. The drawings illustrate diagrammatically and by way of example a hopper dredger suitable for carrying out the method of the invention.

Figure 1 shows diagrammatically a cross-sectional side view of a hopper dredger, the ship’s wall being partly broken away.

Figure 2 is a top plan view of the hopper dredger shown in Figure 1. 'flic drawings show only those devices which are of importance for the carrying-out of the method of the invention. Apparatus for the propulsion of the ship, for navigation, and for accommodation of the crew, and so on, is not shown in the drawings.

Tlie hopper dredger is designated 1 and floats in the water 2. The actual hopper tank is designated 3. The 7 8 9 6 vertically movable suction pipe 4 is fastened to the ship by means of the joint 5. The pump for sucking up the suspension of mud and water is designated 7. The two pipes 6 and 37, in which valves 8 and 9 are disposed, lead into the suction side of the pump 7. From the delivery side of the pump 7 the two pipes 10 and 11, in which the valves 12 'and 13 are provided, lead off. Through the suction pipe 4 and the pipe 6 connected to it the pump 7 pumps up a suspension of mud and water when the valve 8 is open and the valves 9 and 13 are ij closed, this suspension being pumped through the open valve 12 into the pipe 10 and then into the two outlet pipes 14.

The two outlet pipes 14 are mounted one on each side of the hopper tank 3. The outlet pipes 14 have outlet openings 15 and 16, through which the suspension of mud and water is pumped into the hopper tank 3. The outlet pipes 14 are arranged with their outlet openings 15 and 16 in such a position that the suspension of mud and water flows into the hopper tank only at the stern end. A solution of flocculating is supplied in metered amounts from the reservoir 17 containing an aqueous solution of flocculating agent by way of the pipe IS bj’ the metering pump 19 to the pumped-up suspension of .mud and water, downstream of the pump 7.

Perfect mixing of the suspension of mud and water with the flocculating agent is thereby effected by the time the mixture passes out of the openings-15 and 16, and substantial I Iocculation of the mud is thereby achieved. In the irontpart that is to say the bow end of the hopper tank 3 an outlet pip 20 adapted to be raised and lowered is disposed. The outlet pipe 20 is fastened by means of a flexible, incompressible 30 rubber sleeve 21 and of suitable flange elements on the box 22, which covers tiie outlet opening 23 provided in the bottom 7896 of the hopper tank 3. The apparatus for raising and lowering the outlet pipe 20 is not shown in the drawings. A suitable mounted pulley block arrangement can for example be used for raising and lowering the outlet pipe 20. In the hopper tank additional outlet openings 24 to 29 are provided, which can be separately opened and closed by the valves 30 to 36 and which lead into the common pipe 37. In the rear, stern part of the hopper tank 3 a plato 38 is disposed which extends over the entire width of the hopper tank and which is mounted for rotation about the horizontal axis 39. The plate 38 in its vertical arrangement extends downwards about 70 - 150- cm from the highest possible filling level of the hopper tank, and thus forms a pacification zone in the rear, stern part of the hopper tank.

For the purpose of filling the hopper the outlet pipe 20 is raised above the highest filling level of the hopper tank and the valves 9, 13, and 30 to 30 are closed; the plate 38 is brought into the position in which it points vertically downwards. By means of the suction pipe 4 and of the pipe 6, with the valve 8 open, a suspension of mud and water is sucked up by the pump 7, and at the same time flocculating agent solution is metered through the pipe 18 downstream of the pump 7. The suspension of mud and water accompanied by flocculating agent is delivered through the pipe 10 into the two outlet pipes 14 and pumped by way of the outlet openings 15 and 16 into the rear part of the hopper tank. The filling operation is interrupted when the maximum possible level of liquid has been reached in the hopper tank. As soon as the flocculated mud has settled, which is usually the case after 10 to 25 minutes, the plate 38 is swung up into its horizontal position, which is shown in phantom outline, and the suction pipe 20 is lowered below the level of the clear water layer. At the same time the valve 30 is opened and the water flowing off through the pipe 20, the box 22, and the opening 23 into the pipe 37 is pumped back into the sea by the pump 7, with the valves 9 and 13 open and the valves 8 and 12 closed, by way of the pipe 12 and the connection 40 provided in the ship's wall. The outl.et pipe 20 is lowered in accordance with the falling level of clear water, until the opening of the suction pipe 20 reaches the surface of the deposited mud layer. The suction pipe 20 is then again raised above the maximum filling height of the hopper tank 3, the valves 23, 9, and 13 are closed, and the valves 8 and 12 are opened, while the plate 38 is. brought back into the vertical position. The filling operation is then repeated. After the layer of mud has again been allowed to settle, the supernatant layer of clear water is removed ι., the manner already described. After repeating the process tips described only three times the hopper tank is normally filled to tho extent of more than 70% with mud whose con.-entration is normally practically identical with that of the mud or ;;ilt deposited on the bed of the sea or other expanse of water. The suction pipe 4 of the hopper dredger is then raised the hopper moves under its own power to the discharge point, '.lirre the mud is emptied out.

Example Silt was removed from a brackish water region of the ',,'cser with a hopper dredger converted in accordance with Figure 1. The capacity of the hopper tank amounted to 1200 cubic metres. The overflow hatches provided in the stern part of the hopper tank were closed watertightly. The optimum quality and quantity of the flocculation agent to be used were previous iy determined in the laboratory by means of known tests for determining the rate of sedimentation. For the silt which v.i;. to be removed a commercially available partly saponified Jv’ po]ynerylamido flocculation agent (Ilydropur of Cassella, Frankfurt am Main-Fvchenheim, Germany was found to be suitable, this agent being used in a 0.5% by weight solution and in an amount of 19 ppm (referred to 100% flocculating agent) and addl’d to the suspension of silt and water pumped up. As diagrammatical] y indicated in Figure 1, the filling zone was separated by a rotatably mounted pacification wall, having a height of aiiaut 120 cm and extending vertically, from the forward part of the hopper tank. The outlet pipe 20 in the front part ol the hopper tank had a diameter of 40 cm. The first filling operation, comprising about 1200 cubic metres, lasted twenty uinutes. At that time solid flocculated particles threatened to be (‘ui rained through the front overflow hatches togetherwith the clear water flowing overboards. 15 minutes after termination of the filling operation, removal of the layer of vli-nr water which had formed in the meantime was commenced.

Sti-mova 1 of the clear water with the aid of the outlet pipe in the manner described took 15 minutes, The hopper tank then contained about -led cubic metres of concentrated silt with a concentration of more than 40% by weight. For the next filling operation therefore a loading space of 1200 - 460 « approximately 740 cubic metres was available. The outlet pipe 20 was raised and the hopper tank refilled in the manner previously described. The second filling operation was suspended after 12 minutes; after 15 minutes more the removal of the new layexof clear water foimed by the method already described was started. The removal of the second layer of clear water- took 10 minutes. About 745 cubic metres of concentrated mud were now contained in the hopper tank. The hopper tank was then filled fox- the third time, the filling taking 7 1/2 minutes.

Aftvi- 15 minutes more the removal of the clear water was begun. 7 8 9 6 I’h i s 1 a.·. ΙυιΙ about ϋ minutes. About 92(1 cubic metros of conccntrat ι-l nun! were now contained tn the hopper tank. The entire operation lasted in all .scarcely two hours. bivers1tests ascertained that the concentration of the iilt deposits on the riverbed was identical with the concentraiion of the silt in the hopper tank, containing about 40% by if.iit of solid matter. The hopper dredger then moved to the .'Iii·.;· unloading site and unloaded the silt-with the aid of the ut.u ,1 flushing process.

Claims (10)

1. Method of removing mud from waters, characterised in water- suspension that the/mud/is sucked up and the sucked-up water-mud suspension is filled into a floating vessel together with a flocculating agent, the filling is interrupted as soon as the vessel is filled, the supernatant water layer is removed after the mud has settled, and the filling with addition of flocculating agent, settling and removing the supernatant water layer are repeated until the vessel is filled with mud.

2. Method according to claim 1, characterised in that the floating vessel is a suction hopper.

3. Method according to claim 1 or 2, characterised in that the filling of the vessel is discontinued only when there is a risk that mud flakes will overflow the edge of the vessel.

4. Method according to claims 1 to 3, characterised in that, viewed in the direction of flow, the flocculating agent is added downstream of the pump employed for sucking up the water-mud suspension.

5. Method according to claims 1 to 4, characterised in that the flocculating agent is metered into the sucked-up suspension of mud and water in amounts dependent on the concentration of mud.

6. Method according to claims 1 to 5, characterised in that a flocculating agent based on polyacrylamide is added. 478S6

7. Method according to claims 1 to 5, characterised in that the flocculation agent added is a non-ionic flocculating agent based on polyethylene oxide

8. Method according to claims 1 to 5, characterised in that 5 the flocculation agent added is a cationic flocculating agent based on polyethyleneimine.

9. Method according to claims 1 to 5, characterised in that the flocculating agent added is a polymer based on dimethylaminoethyl methacrylate and/or dimethylaminoethyl acrylate, 10. If appropriate, copolymerised with acrylamide.

10. A method of removing mud from a water-mud suspension according to claim 1 / substantially as hereinbefore described and exemplified.