WO2002031273A1

WO2002031273A1 - Method for treatment of dredging soil

Info

Publication number: WO2002031273A1
Application number: PCT/FI2001/000883
Authority: WO
Inventors: Minna Katriina Aarnio
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-10-12
Filing date: 2001-10-11
Publication date: 2002-04-18
Anticipated expiration: 2003-04-12
Also published as: FI122359B; FI20002247L; AU2002210584A1; FI20002247A0

Abstract

The present invention relates to a method for reconditioning a eutrophied lake (1), where a waterlogged bottom sediment (3, 3a, 3b, 3c) of the lake (1) is removed to increase water depth. In the method, a dredged material (13) and an absorbent material (14, 20) is subjected to a mixing and compression process where at least an essential protion of the dredged material (13) is pressed by means of compression into a cell structure of said absorbent material (14, 20). The present invention further relates to a method for processing a waterlogged dredged sediment (13) containing organic material, whereby the material (14, 20) having an absorent cell structure and further subjected to a mutual mixing and a mechanical compression (16). The present invention further relates to teh use of a twin-screw press (15, 16) for compressing a dredged material (13) obtained from a bottom sediment (3, 3c, 9) of a lake (1) into a cell structure of an absorbent material (14, 20), such as peat.

Description

METHOD FOR TREATMENT OF DREDGING SOIL

The objective of the present invention is, in accordance with claim 1, a method for reconditioning especially a eutrophied lake or similar body of water, where waterlogged sediment is removed from the bed of the lake through dredging as known per se, in order to increase the depth of the lake. The objective of the present invention is, furthermore, a method developed for the processing of waterlogged sediment containing organic material dredged from the bed of the lake, and the specific use of a twin-screw press intended mainly for agricultural use.

The water of lakes and similar bodies of water created by the Ice Age hosts a continuous biological cycle of flora and fauna; for example, the aquatic flora of the lake stores solar energy and converts nutrients in the water into organic material. Over time, as the flora dies, this material descends to the bed of the lake, forming a sediment that could be best described as peat, consisting largely of decomposing remains of flora. The decomposition of organic material continues in this sediment, releasing nutrients into the water. In low-nutrient lakes, this cycle remains largely in equilibrium; the sediment on the bed of the lake increases in thickness very slowly if at all.

However, human action may cause nutrients to enter the system from outside the watercourse to such an extent that the natural low-nutrient cycle ceases and is replaced by a different cycle. This latter cycle is characterized by the presence of more nutrients than are required by the existing aquatic flora. In such a situation, the growth of the flora continues beyond the equilibrium described in the previous situation, increasing the amount of organic matter in the lake until a new level of equilibrium is attained. This equilibrium, known as eutrophication, is different from the low-nutrient equilibrium in that it is detrimental for the use of the watercourse; for example, the flora changes towards a majority of floating-leaf species. When this flora dies, its decomposition consumes oxygen so that even totally oxygen-free conditionsmay occur so that the fish die. Also, prolific anaerobic decomposition of flora causes unpleasant olfactory effects. A lake in such a state is essentially worthless for leisure use or any other useful purpose.

In addition to the above adverse effects, anaerobic conditions change the balance of the sediments on the bed of the lake, accelerating the dissolving of nutrients that under aerobic conditions would remain undissolved in the sediment. Thus, once eutrophication begins, it often becomes a self- accelerating process that escalates even after no further nutrients are introduced to the system from outside. In such a case, the sediment grows in thickness of its own accord, reducing the water depth. At the same time, the flora on the top of the sediment moves relatively closer to the sun, and the increased light and heat they receive further accelerate the process.

In all, lake eutrophication is a problem that can rapidly deteriorate the value of a body of water in terms of leisure use in particular. Since eutrophication is mostly due to human action, it is also typical that such a reduction of recreation value occur in locations where leisure use would be of the greatest benefit, i.e. near urban communities. It would thus be advantageous not only to cease feeding nutrients into such systems but also especially important to restore such lakes and ponds to leisure use instead of leaving them to become overgrown at an increasing rate.

In some cases, small eutrophied ponds have been reconditioned by removing a significant portion of the bottom sediment. For technical reasons, this almost always requires that the pond be temporarily drained. The extracted bottom sediment is spread out to dry nearby, causing serious aesthetic, functional and olfactory adverse effects to the environment due to the decomposition of organic material. In other cases, the dredged sediment is composted. In both methods, the volume of sediment to be processed is considerable in projects of any significant extent, requiring either a large area of land on the shore of the body of water for processing or lengthy transport. The size of the land area required is alone enough to demonstrate that such a method is not feasible for reconditioning lakes of any significant size.

Another reconditioning method includes artificial oxygenation of water in order to forcibly prevent anaerobic conditions from evolving in the bottom sediment . Such a method requires constant expenditure of energy and does not solve the original problem, i.e. shallowness and surplus of nutrients. As a result, the situation today is that the problem of eutrophication is acknowledged but there are no practical measures available to solve it.

The objective of this invention is to create a method for efficiently, economically and ecologically removing large volumes of bottom sediment from lakes and similar bodies of water.

Furthermore, the objective of the invention is to provide procedures that can be used near human habitation.

An additional objective is to create a method where the surplus nutrients, which are useful as such and would in fact become available near their potential end users, can be put to use as a substrate for plants. Alternatively, the dredged organic material can be used to generate energy, as shown in an alternative embodiment of the present invention. The above objectives can be achieved in the manner outlined in the enclosed claims and in accordance with the characteristics defined therein. The reconditioning method according to the present invention is characterized by: combining the dredged material with an absorbent material, expecially with peat and mixing and compressing the resultant slurry so that the water in the dredged material is trapped in the cell structure of the absorbent material. The dredged sediment processing method according to the present invention is characterized by: bringing the sediment material into contact with an absorbent material that has an absorbent cell structure, favorably with peat, bringing said sediment material and said peat into a mutual agitation and subjecting the resultant slurry to mechanical compression. One embodiment of this invention includes the use of a twin-screw press intended mainly for agricultural use for the compression of the dredged material from the bottom sediment into the cell structure of the absorbent material, such as peat or the like. Other benefits and characteristics of the present invention are disclosed in the enclosed dependent patent claims.

The following is a more detailed description of the present invention, referring to a beneficial embodiment of same as well as to the enclosed drawing, which in principle discloses in cross-section a body of water and its bottom sediment.

Referring to the Figure a body of water 1 such as a lake, pond or similar body of water contains water, whose surface is referred to as 2. A bottom sediment 3 contains organic material derived from the biotope of the lake, such as flora, fish and micro-organisms . The strata of the sediment are in various stages of decomposition: the deepest strata 3a are in an advanced state of mineralization, while the topmost strata 3b and 3c contain more recent organic material. Especially in lakes in an advanced state of eutrophication, the topmost strata 3c of the bottom sediment 3 contain a large proportion of water, sometimes so much so that any boundary 4 between open water and bottom sediment is rather difficult to identify; the proportion of solid matter increases towards the bed 5 of the lake, as the darkening tone in the Figure depicts .

The Figure shows that large floating-leaf plants 6 such as yellow water-lilies grow with their leaves 7 very close to the surface 2, whereas their roots 8 are in the bottom sediment 3 at a level advantageous to the plant . The Figure also shows minor aquatic plants 6a, whose roots form a sometimes very tough top layer to the bottom sediment 3. A depth of two metres is often considered critical for the state of a lake in the sense that aquatic plants 6 usually cannot reach the surface from a depth of more than two metres, and thus cannot be under influence of the most effective solar radiation. However, at depths of less than two metres, growing conditions in terms of both light and temperature are favourable for strong growth of aquatic plants, which in turn leads to an increase in biomass if nutrients are available. Cross-hatching in the Figure shows such a depth zone 9 in the bottom sediment 3 that would need to be removed for successful reconditioning of the lake as a whole. If said zone 9 is, for example, one metre thick, a reconditioning produces 10,000 cubic metres of sediment per hectare of water area. This volume of material would then need to be placed somewhere where a decomposition of organic material would not be detrimental to the environment. In this context it should be observed that although a detritus of aquatic plants 6, 6a, i.e. a sediment material comparable to peat, has a very high water content as such, the water it contains is very rich in nutrients. It is thus quite important to ensure that a sediment layer 9 is removed in such a way that the water it contains does not flow back into the body of water 1.

According to the present invention it is appropriate that the sediment zone to be removed 9 be dredged for instance using a suction dredger so as to remove the entire sediment stratum 3c. The drawing shows, only schematically, an existing dredging system which may include, e.g., a pontoon 10 supporting an appropriately cutting suction head 11, whereby waterlogged dredged material 13 is removed from the body of water 1 by means of an arrangement known per se, e.g., a piping 12, for further processing.

Processing of the dredged material according to the present invention includes that the dredged material 13 is brought essentially as it is to be mixed with a porous absorbent material 14, especially with peat, so that the water and fine solids in the dredged material 13 are pressed into the cell structure of the absorbent material 14. Favorably, this phase is implemented using an existing screw press generelly referred to by reference 15. Said screw press 15 has a cooperating twin screw arrangement 16, which in connection with a mutua intermixing of said materials 13, 14 also subject the resultant slurry to a mutual compression. Through the compression, a considerable proportion of the dredged material 13 is forced into the cell structure of the absorbent material 14.

In the case of a typical slightly eutrophied lake, the dredged material 13, especially that from near the surface, usually contains less than 20 % of fine solids, in most cases <2 to 15 % and typically 6 to 10 %. These solids consist mainly of plant detritus, i.e. material akin to peat and as such similar to fully composted plant peat. Initially, the chemical composition of the sediment 3, 3c includes organic carbon compounds as well as ordinary substances classified as fertilizers and trace elements such as phosphorus, nitrogen, potassium, sodium, calcium, magnesium, boron, copper, iron, etc. These substances are beneficial as such, and contribute to the fact that according to one embodiment of the present invention the end product is a material excellently suited for use as a growth substrate for plants or as a raw material for such a substrate. This type of material is in short supply in urban communities. However, the material may also contain, due to external contamination, heavy metals and other harmful substances. The presence of such substances is often local and well-defined, as disclosed in the Figure by reference 17, and thus it is appropriate to use sediment analysis to determine the zones and depths where the dredged material 13 or the mix material 18 produced through the compression process should be appropriately brought to further special treatment.

The greater the dredging depth, the higher the proportion of solids in the dredged organic material; accordingly, in one beneficial embodiment of the present invention, the compression process is regulated, at least in part, by adjusting the suction depth of the suction head 11 of the dredging apparatus, as shown with a dotted line in the drawing. The water content of the sediment stratum 3, which may be 80 to >98 % of the total volume, contains a significant proportion of nutrients that need to be extracted from the body of water 1. Thus, it is appropriate to absorb most of the water into the absorbent material 14 in accordance with the present invention. Peat has proven a particularly feasible absorbent material 14, because its vacant cell structure can receive water and fine solids to an amount which almost corresponds to the initial peat volume.

A suitable machine for the mixing and compression process is, e.g., a screw press used as such in agricultural manure processing 15. This machine has parallel transporting screws 14 that mix material placed on the screws, in this case the dredged material 13 and the absorbent peat 14. The dredged material 13 which is conveyed in appropriate manner by means of a pipe 12 is sprayed on top of a layer 20 of absorbent peat blown or otherwise conveyed from a peat silo 19. In one embodiment of the invention, it is appropriate to moisten the peat 14 slightly to avoid airborne dust. Accordingly, in some embodiments, additives to improve a particular desired feature of the end product 18 are added to the peat 14 and/or the dredged material 13 at this point in the process. Such additives may, e.g., include bacteria that contribute to a further composting of the end product 18, possible additional nutrients to improve the nutrient balance of the end product, tensides, buffering materials such as lime, and/or some other appropriate additive 21.

As the screws 16 rotate, the absorbent peat 14, 20, the dredged material 13 added to it and any additives 21 are mixed with one another, and at the same time the dredged material 13 with a high water content and any additives 21 are compressed into the cell structure of the peat 14. The proportions of the materials being mixed depends on the properties of the dredged material 13; a typical mixture involves 1 part dredged material to 1 to 1.5 parts peat. The mixture ratio and compression pressure are kept within such limits that essentially all of the dredged material 13, including the water it contains, is absorbed into the peat 14; thus, the nutrient-rich water is contained as a beneficial additive in the peat instead of flowing back into the body of water. The parameters may be suitably adjusted by controlling the dredging power, the rotational speed of the screws 16 and/or the rate of adding peat to the process. Correspondingly, the water level 2 of the body of water 1 will also be regulated during the dredging with regard to the water removed from it with the dredged material 13, for instance by damming, as shown in the drawing at reference 22. Said dam 22 also prevents the nutrient-rich water, which may be temporarily clouded by the dredging, from progressing further in the watercourse.

Due to the properties of the absorbent peat 14 in particular, the volume of the end product 18 is significantly smaller than the combined volume of the mixed materials 13, 14, since under pressure the cell structure of the peat 14 contains the water in the dredged material 13 very efficiently. Typically, 1 part of dredging material 13 mixed with 1.5 parts of peat 14 yields only 1.5 parts of end product 18 after mixing and compression. Unlike the original nutrient-poor peat 14, which is unsuitable as a substrate for growing plants, the end product 18 produced by means of the method according to the present invention contains all the nutrients required in a good substrate. Thus, if a dark, already composted peat is used as said absorbent peat 14, the end product 18 is usable as a substrate with no further processing at all. Again, if light raw' peat having a higher absorption capacity is used, the end product 18 is primarily suitable for energy production. In both cases, the absorbent peat 14 prevents also the unpleasant smells released by the decomposing dredging material 13 and renders the end product 18 hygienic, solid and easy to process and transport.

The present invention provides a number of significant benefits due to which the reconditioning even of large lakes becomes a feasible proposition. Firstly, the need for land areas to spread the dredging material to dry, and to smell in the process of decomposition, often for years, is eliminated, because the dredged material does not need to be dumped. Secondly, instead of the dredging producing mainly a problem waste, the dredged material is converted into a useful material of which there is in many cases a shortage especially in the immediate vicinity of eutrophied lakes. The process also eliminates the unpleasant olfactory effects of dredged material, and the end product is commercially viable. Furthermore, the end product is easy to handle, transport and process further. As such, the method according to the present invention is usable in the field, easy to implement and reliable. It recovers the greatest proportion of nutrients possible; even the nutrient-rich water involved is contained in th eproduct and will not be released back into the watercourse. The technology involved in the method is existing, tested and easily available.

A further benefit to be noted is that even the dredging material 13 extracted from even very problematic locations 17 can easily be controlled, enabling harmful materials to be removed from the watercourse and preventing their spreading into the environment. The method according to the present invention enables even lakes in an advanced state of eutrophication to be reconditioned into their natural state, and in any case increasing their water depth safely and without causing environmental problems. With the increase in depth and the removal of flora, the leisure use value of the lakes will increase.

The above highlights some beneficial embodiments of the present invention, but to a professional in the field it is immediately apparent that the invention can be used in a number of other ways within the scope of the enclosed claims. For instance, it is feasible in some cases to use other methods to complement the reconditioning method included in this invention. Aeration and/or chemical treatment of water may be necessary to increase oxygen content and for instance to precipitate the nutrients and solids stirred up in the water by the dredging. It is also clear that although the absorbent material is above consistently referred to as peat' , this should be taken to refer to any organic plant detritus with an absorbent cell structure. Furthermore, in some cases any other material with approximately the same absorbent properties might be usable.

Also, the additives 21 discussed above may include a binding agent to render the end product 18 solid and malleable and thus usable for gardening and landscaping, so that the binding agent and/or the end product 18 itself also in a hardened state decomposes over time and/or forms a substrate by itself. The binding agent 21 may also be such that the end product 18 in a shaped and more hardened form as discussed above is usable for other purposes, for instance as an essentially solid construction material or the like.

Claims

1. A method for reconditioning a eutrophied lake (1) or similar body of water, where a waterlogged bottom sediment (3, 3a, 3b, 3c) of the lake (1) is dredged using a dredging method known per se to increase the water depth, which method is c h a r a c t e r i z e d in subjecting a dredged material (13) and an absorbent material (14, 20) to be mixed therewith to a mixing and compression stage so that at least an essential portion of said dredged material (13) is pressed into the cell structure of said absorbent material (14, 20) .

2. A method as defined in claim 1, c h a r a c t e r i z e d in that before dredging, a sediment stratum to be dredged (3c, 9) is cleared of at least a major portion of aquatic plants (6, 6a), their roots (8) and/or other larger elements.

3. A method as defined in claim 1 or 2, c h a r a c t e r ! z e d in that said dredged material (13) to be absorbed includes both the solid material and at least an essential portion of the water contained in a dredged stratum (9), and that the water level (2) of the body of water (1) is controlled by means of a dam (22) , so that the material dredged is replaced by water running into the watercourse.

4. A method as defined in any one of claims 1 to 3, c h a r a c t e r i z e d in that said mixing and compression stage is implemented using an existing agricultural waste-processing unit (15) , favorably an existing twin-screw press (16) .

5. A method as defined in any one of claims 1 to 4, c h a r a c t e r i z e d in that an end product (18) produced by said compression is further composted to produce a product suitable for use as a plant substrate.

6. A method as defined in any one of claims 1 to 5, c h a r a c t e r i z e d in that the lake (1) is subjected, after dredging, to a chemical treatment known per se in order to clarify the water and/or to aerate the water to raise its oxygen content .

7. A method as defined in any one of claims 1 to 6, c h a r a c t e r i z e d in that the solid material content of the dredging material (13) fed into the mixing and compression process is controlled by controlling the suction depth of a suction head (11) .

8. A method for processing waterlogged dredged sediment containing organic material (13), c h a r a c t e r i z e d in bringing said dredged material (13) into connection with an absorbent material (14, 20) having an absorbent cell structure, subjecting said dredged material (13) and said absorbent material (14, 20) to a mutual mixing and further subjecting the resultant mixture to a mechanical compression (16).

9. A method as defined in claim 8, c h a r a c t e r i z e d in that said method is implemented as one suitably continuous working process within a twin-screw press (15, 16) .

10. A method as defined in claim 8 or 9, c h a r a c t e r i z e d in using a dark, at least partly composted absorbing peat as said absorbent material (14, 20) in order to produce an end product (18) suitable for use as a plant substrate, or alternatively using a light raw peat in order to produce an end product (18) suitable for energy production, suitably so that additives (21) are added to the materials to be mixed (13, 14, 20) prior to the mixing compression, especially in order to improve the properties and/or a further processing of the end product (18) .

11. A use of a twin-screw press (15, 16), used mainly in agriculture, for compressing a dredged material (13) obtained from the bottom sediment (3, 3c, 9) of a lake (1) into the cell structure of an absorbent material (14, 20), such as peat.