METHOD FOR CONTROLLING THE CONCENTRATION OF A SLURRY SUCKED UP BY AT LEAST ONE MOVEABLE SUCTION UNIT

Abstract: Method of controlling the sludge concentration of sludge sucked up by means of at least one first suction unit moving within a body of water, in accordance with a laid-down plan for collecting the sludge, which requires a predetermined minimum water content. Sludge having a water content higher than said predetermined content and sucked up by means of at least one movable or stationary second suction unit, or return water from a sludge deposit, is used for dilution of sludge sucked up by means of a movable first suction unit and having a water content lower than said predetermined content. Dilution is effected in such a proportion that the predetermined water content is imparted directly at the suction place to the sludge sucked up by means of said movable first suction unit. Control and coordination of the various suction units occurs on a pre-arranged plan, or a plan laid down while work is progressing, for utilisation or collection of the sludge and the water so that a minimum amount of water is recycled to the body of water.

Description: METHOD FOR CONTROLLING THE CONCENTRATION OF A SLURRY SUCKED UP BY AT LEAST ONE MOVEABLE SUCTION UNIT The present invention relates to a method of controlling the sludge concentration of sludge sucked up by means of at least one first suction unit moving within a body of water, in accordance with a laid-down plan for collecting the sludge, which requires a pre¬ determined minimum water content. The plan may be pre-arranged or laid down while work is progressing. By sludge is meant every type of material suspendible in water. The characteristic features of the method according to the invention will appear from the ap¬ pended claim 1, preferred embodiments being defined by the sub-claims. Man's activities have had an increasingly stronger influence on lakes and other watercourses, to the detriment of the ecosystems. The supply of nutrients increases the production- of settling particles, and the supply of erosion material from precipitation areas also causes accelerated silting-up. This influence results to a large extent in the rapid aging of the lakes. The introduction in Sweden of efficient purifica¬ tion plants for the treatment of municipal wastewater has to some extent normalised the external load on the waters. In a number of recipients, however, the external load for a long time has been so high that the lakes could not recover in spite of large invest¬ ments in wastewater relief and the construction of efficient purification plants. Direct intervention in the lakes is necessary to restore them to an environ- mentally acceptable level. Very often it is necessary to carry out suction dredging to remove the sludge layer (sediment) which has settled during the recipient period and which gives rise to a lasting internal load on the lake ecosystem. Because of its high density of efficient waste¬ water purification plants, Sweden's global position is unique. In most other countries, the pollution of lakes increases, simultaneously as water reservoirs and other watercourses are silted up at an increasing rate by wastewater sludge and erosion material deposits. In Sweden as well as internationally, there is a great need for efficient water conservation methods involving suction dredging and the return to land areas of sludge masses which have been deposited in water areas and frequently have a high concentration of plant nutrients. Projects to demonstrate how internal nutrient loads in overstrained silted-up lake recipients are to be relieved, have been presented in Sweden, the first time - in lake Trummen at Vaxjδ. However, the problem of the suction dredging technique so far available is on the one hand that large quantities of water have been pumped and, on the other hand, that the sludge/ water ratio has varied greatly during pumping. The large amounts of water "unnecessary" for sludge pump¬ ing have caused considerable difficulties in the deposi- tion of the sludge. Expensive settling basins had to be built for decantation of the excess water which, besides, had to be processed for nutrient salt reduction prior to its return to the subject of restoration. With this technique, restoration projects must be carried out as quickly as possible, which frequent¬ ly is unfavourably connected with concentrated costs which, from the budgetary point of view, cannot be distributed over any longer period of time. From the general viewpoint of environmental manage- ment, for financial reasons, and to make lake restoration by suction dredging possible within urbanised and other areas in which large settling basins cannot be built, a technique involving minimised water admixture must be applied, the admixture being adapted to the charac¬ ter of the sludge to be found in the area concerned. The need for new technology enabling limnological- ly correct lake restoration by suction dredging of sludge (sediment) has long been emphasised. As early as about 20 years ago, S. Bjδrk was inquiring about technical activities to develop a suction dredging technique satisfying limnological requirements. Thus, it should be possible to pump sediment and other sludge by layers, without muddying, with a minimum amount of admixed water, and - regarding the individual sludge type - with as constant proportions between sludge and admixed water as possible. In recent years, these issues have been discussed on the basis of the inventions previously patented by A. Sandberg and relating to methods of steering and controlling devices and implements f . or sucking up lake sediment and other suspendible material, where- by the suction pump unit which satisfies limnological requirements for correct lake restoration by suction dredging was developed. The coordination of pump units utilised for syn¬ chronous pumping of sludge of different types has been analysed by M. Sandberg. Every restoration subject exhibits specific combi¬ nations of sludge types within the water area and their deposition possibilities on land. It should be emphasised that by sludge types are meant, in the context of this invention, all types of sludge, from densely packed sludges having a water content of but 70-80% to thin sludges which do not readily sediment and have very high water contents (99.9%). The densely packed sludges which readily sediment but are difficult to pump over longer distances in the above-mentioned concentration may find their specific use on land, for example for land elevation, while the thin sludge- after pumping may be used for, for example, irrigation. To be able to pump the thick sludge to its contemplated deposition area, the sludge must be diluted, and this has been done hitherto by direct admixture of water at the pumping location. This means that a large amount of "unnecessary" water is circulated and, as has been " mentioned above, causes considerable difficulties. Similarly, the pumping of thin sludges has caused considerable difficulties because large amounts of water are circulated. A consequence of the earlier want of concentration-controlled pumping technique was that one could not pump either thin or thick sludges in uniform concentrations, for which reason the sludges could not be used for the specific purposes for which they would eitherwise have been suitable. The difficulties encountered in the removal of sludge in exact and predetermined concentrations have been obviated by the inventions according to SE Patents 7410937-2, 7512265-5, 7703749-8 and 7903879-0 of ' which the present invention is a development. Lakes and other natural watercourses exhibit, like man-made water areas, individual characteristics which necessitate an individual design of the suction dredging equipment. In the individual case, moreover, the suction dredging equipment must be adapted not only to the conditions prevailing in the water area (the ecosystem), but naturally also to the conditions prevailing in the catchment area from which water is supplied naturally and in which sludge deposit sites must be established. The system according to the present invention, such as it has been defined in the appended claims, is characterised by maximum flexibility and is based on experience gained from the above-mentioned varying character of the subjects of restoration. To be able to select from the broad spectrum of method combinations according to the system of the present invention the environmentally correct subject-adapted alternative, one must have access to the result of limnological preliminary studies concerning the subject in question. When it is decided to restore a water area, the objectives of the intervention must be clearly de¬ fined, and usually all of the arguments in favour of the restoration are presented. Like the planning of the technical restorative intervention, the limno- logical preliminary study must be based upon the objec¬ tives formulated for the ecosystem to be established. Generally, the minimum time of the limnological preliminary study is a one-year cycle in both temperate and tropic zones. Sampling is carried out at intervals of at most one month. During periods with rapid and intense adjustment processes in the ecosystems, samples are taken more frequently. A normal study program comprises the following steps: I- Limnological analysis. 1. Hydrological balance calculation. In the absence of data regarding inflow and runoff, stored water volumes etc., the requisite measurements are made. 2. Status study regarding environmental factors. A. Water. Nutrient salt contents (primarily phosphorus and nitrogen) and concentrations of dominant ions are analysed in inflows and runoffs as well as in vertical profiles within the deep areas of the subject. The magnitude and limnological complexity of the subject are decisive of the sampling frequency. Temperature, light, turbi- dity and visual depth conditions are recorded. A nutrient salt budget is drawn up. The stratifi¬ cation during the year for pH and dissolved gases is illustrated. The supply and self-production of organic matter are estimated. B. Sediment. The horizontal extension and vertical thickness of the sediments are mapped. The stratigraphy (arrange¬ ment of strata) of the sediments is established in respect of the soil character, the physical and chemical character (oxygen consumption, solubi¬ lity of int.al. nutritive and toxic substances etc.). The integrated significance of the sediments for the character of the ecosystem is established by experiments involving the exchange of substances between sediment and water, sediments from different stratigraphical levels being analysed. Sediment beha- viour upon drying is investigated, and the useful¬ ness of the sediments on land for different applica¬ tions is analysed. 3. Organism status studies . A. Analysis of plankton (the microorganism population of the free water) and periphyton (attached microorga¬ nism populations) and bacteriological conditions. B. Analysis of large-plant communities and mapping of vegetation. C. Analysis of bottom fauna. D. Analysis of fish population. E. Analysis of production conditions in respect, of plankton and fish. F. Analysis of ornithological conditions and, if desired, mapping of organisms especially 1 valuable to the subject. 4. Studies of the natural and civilisation-related development of the water area are based on analyses of the sediment character and contents of subfossil (plant and animal remains). Sediment layers are dated. II. Limnological synthesis. The present limnological state of the water area is described, i.e. the interconnection of environ¬ mental factors and organism populations, the relationship between environmental factors and organism populations, the relationships between and within organism species, between production and destruction of organic matter, including sediment growth, range of vegetation etc. Descrip- tion of the development of the water area up to the present day, and prognosis of the area's future development, in case the restoration is not carried through. III. Limnological restoration plan. 1. Description of the restorative measures that must be taken in order to reach the objectives, such as the removal of vegetation (mowing and root rot destruction with amphibious and/or pontoon- borne machines) and pumping by suction dredging of specific sediment layers within defined areas. The requirements regarding the vegetation treat¬ ment and the suction dredging in respect of areas, levels, times etc. are laid down. As for the suction dredging, especial importance is attached to minimising the admixture of water and the keeping apart of sediment types from different layers and areas. 2. Description of the procedure to be followed in depositing and processing plant material and sediments for utilisation or other forms of col¬ lection on land, including directions for avoiding excess water, or including the processing of such water. Recommended programs for testing sediments as culture substrates. 3. Cost budget for the restoration to be drawn up in consultation with representatives of the tech¬ nical branch of the project. 4. Prognosis for the limnological character and development of the water area and its useful¬ ness for various applications after the resto¬ ration has been carried through. The control method according to the present inven¬ tion makes it possible to carry through efficiently a laid-down restoration plan which may be a long-term plan extending over several years and controlled, with regard to its rate of progress, by external fac- tors, such as the need for soil improving agents and/or purification capacity. Furthermore, the invention improves the restoration possibilities by the special conditioning by means of additives. In addition, the invention provides an advantageous method for .-status studies of water and sediments. The invention.will be described in more detail below, reference being had to a number of possible applications and the accompanying drawings. Fig. 1 illustrates in principle an example of units that may be included in the system according to the inven¬ tion. Figs. 2-8 illustrate different possibilities of combining the units shown in Fig. 1. The units that may be used with the system accord¬ ing to the invention are shown in Fig. 1 and comprise a number of suction units 1-5, a straining unit 6, a sludge lagoon 7, irrigation units 8 and 9, a sort¬ ing unit 10, a brushing and/or flushing unit 11, a return water pumping unit 12, and an additive unit 13. Example 1; Integrated sludge suction and irrigation. In a part of a lake lies a sludge deposit having a water content of about 95% and an average thickness of 1 m. The sludge has been analysed and it has been found that it may be used for irrigation of a given land area during a given time of the year. The fluid volume withdrawn is calculated at 1 m /min and is deemed to contain 1% sludge, which implies a water content of 99%. The suction unit 1 is rigged up, in accordance with Fig. 2, over the area concerned and is caused to travel back and forth at a suitable constant velo¬ city. A sludge content meter and a flow meter are mounted in the pump conduit of the suction unit. Round the suspension lines of the suction nozzle a line angle transducer is mounted which, when the nozzle strikes the 95% sludge or firm bottom, indicates the angle alpha. The sludge-contai ing water is strained so as to not clog the spreader. Control 1. The flow meter controls the pump speed, such that the flow is kept at a constant rate of 1 m /min, and to counteract clogging tendencies. 2. The sludge content meter controls raising and lowering of the suction nozzle, such that the water content of the pumped mix¬ ture will be 99%. 3. As the suction nozzle travels through the 95% sludge, a given line angle alpha is obtained. This line angle will increase when the suction nozzle starts to drag on firmer bottom. When the lag has reached a preset maximum value alpha, this func- tion takes over the raising and lowering of the suction nozzle. If there is obtained, simultaneously with a large lag, a sludge having a water content exceeding e.g. 99.5%, the bottom of the lake is free from sludge. In this manner, a sludge having a high water content can be pumped up from a firm bottom without necessitat- ing the removal of parts of the firmer bottom material. Example 2: Integration between several section units and sludge deposition. In a part of a lake lies a sludge deposit having a water content of about 95%. In another part of the same lake lies another sludge deposit having a water content of about 80%. Both sludge deposits are to be removed and used for soil improving purposes on adjacent fields. Geographically, the sludge supplies are close to each other. For pumping away these sludges, the present inven¬ tion utilises two suction units simultaneously, as shown in Fig. 3. Sludge having a water content of about 80% is pumped up to the surface by the suction unit 4. At the surface, the sludge is diluted with sludge from the suction unit 3 to form a mixture that can be pumped to the place of deposition. Example 3: Integrated sludge suction and recycling of coarse material . A part of a lake that has previously been a spawn¬ ing ground for fish, has silted up. On the original bottom which consists of coarse sand, a sludge layer having a water content of above 98% has formed. The sludge may be used for the dilution of thicker sludge, in accordance with Example 2. According to Fig. 4, the 98% sludge is pumped together with a thinner layer of the solid bottom material. By some type of separation, such as strain¬ ing, centrifugation or sedimentation, on the suction unit, the coarsest bottom material is separated and recycled to the bottom. The remaining sludge water is pumped to the suction unit 4 as water of dilution. Example 4 : Integrated irrigation and pumping of eutrophic water without sludge. The bottom in a part of a lake consists of a eutrophic sludge having a thickness of several metres. An irrigation enterprise wishes to take out 2 m /min of water for irrigation of crops. During a specific part of the year, irrigation with sludge admixture cannot be permitted. The eutrophic sludge is rich in phosphorus and nitrogen which diffuse out of the sludge, implying that the clear water phase adjacent the sludge contains large amounts of nutrients. The suction unit 1 is rigged up, in accordance with Fig. 5, over the area concerned and is caused to travel back and forth at a velocity controlled by a conductivity meter in the pump conduit of the suction unit. A flow meter and a light-sensitive tur¬ bidity meter are also provided in the same pump conduit. The flow meter controls the pump speed, and the tur¬ bidity meter controls the raising and lowering of the suction nozzle. Example 5: Integration of sludge withdrawal and return water from sludge deposition. In a lake lies a sludge having a water content of about 80%. The highly eutrophic sludge is to be used for soil improvement. On land, a sludge lagoon - has been arranged from which the sludge is to be taken by loaders. The sludge has a high settling rate and is pumpable at a water content of about 88%. According to Fig. 6, the suction unit 4 pumps the 80% sludge up to immediately above the surface of the water where it is diluted with return water from the sludge lagoon to an about 88% sludge, where¬ upon it is pumped up to the sludge lagoon. Example 6: Integration of flow and sludge volumes. In a part of a lake lies a contaminated sludge having a thickness of 0,1 m. The sludge has a water content of about 84% and must be diluted to become pumpable over longer distances. Suitable pumping con¬ centration about 90%. A suction unit is rigged up, in accordance with Fig. 7, over the area concerned. The working width of the suction nozzle is established, thereby to de¬ termine the relationship between the pumped sludge volume and the nozzle movement. Analysis has shown that the 84% sludge should be diluted on the spot to a 90% sludge. Calculation: Volume of 84% sludge 0.1 m 3/m2. Volume of 90% sludge 0.16 m 3 /m 2 . Calculated pump volume 1.6 m /min cor¬ responds to a surface area of 10 m-Vmin. A working width of 2 m will thus give a travelling speed of 5 m/min. Example 7: Integration of pH and sludge/surface unit. In an acidified lake lies a sludge contaminated by heavy metals. The acidification has caused the heavy metals to precipitate. The contaminated sludge has a pH of 5.5, a water content of 95%, and a thickness of 0.15 m. The underlying sludge has a pH of 6.8. Suppose now that analysis has shown that mixing a pH 5.5 sludge with a pH 6.8 sludge in specific pro- • portions will give pH 6.0. This pH value may be used for controlling the vertical movement of the suction nozzle in accordance with Fig. 8. Example 8 ; Irrigation ponds. An irrigation pond receives its water from a river. The pond is filled with water during the rainy season when the discharge from the river is high. During this period, the river water contains large amounts of suspendible material which is maintained suspended in the river water by turbulence. The sus- pendible material will settle on the bottom of the pond, a fact which is used as a basis for an irriga¬ tion scheme using both the water and the bottom sludge. By mixing water and sludge in .proportions from prac¬ tically pure water to water containing several percent of sludge, it should be possible to execute the said irrigation scheme according to a plan based upon the times of the year, the type of cultivation and soil improvement (compare Examples 2 and 5). Example 9; Integration of lake restoration and munici¬ pal wastewater purification plant A lake having an area of about 2 k .2 lies within a metropolitan district. The lake has previously been the recipient of various discharges from neighbouring built-up areas. On the bottom of the lake lies a eutro¬ phic sediment having an average thickness of 1 m and a water content of 92%. Investigations have shown that the municipal purification plant in a normal day is capable of receiving between 2 and 8 m-- * /min of water having a sludge content of about 1%. The daily average value is calculated at 4 m^/min. Depending upon the city sewer system, two or more suction units are put into operation. Each suc- tion unit has been set to pump 1% sludge concentration. The purification plant supplies an information signal which indicates the load on the plant and the possibi¬ lities of receiving 1% lake sludge. The time taken in transit between the respective suction unit and the plant are included in the calculations). Calculation: Sludge volume in lake: 2 million m 8% sludge = 160,000 tons DS (= dry substance). 4 m 3 /min 1% sludge = 40 kg DS/min = 57.6 tons DS/day. This would correspond to a pumping time of 2,777 days (approximately 8 years). Suppose now that the sludge is dried in the plant to 25% DS, which would correspond to a sludge volume of about 230 m-Vday. Example 10: Integration of discharge/turbine effect and restoration of power station basin. Sedimentation occurs in a power station basin. The thickness of the sediment grows, on an average, 2 by 0.6 m/year. The basin area is 1 km . The sludge water content is about 80%. River discharge varies with the time of the year. Investigations have shown that the sludge can be pumped past the power station during 10 months of the year and then proportionally to the discharge. During the remaining two months, no sludge must -be pumped. The pumpability of the sludge is analysed, and it is found that, with current energy prices, the best economy is obtained by pumping the sludge past the power station with a water content of about 90%. Calculation: Volume of 10% sludge is about 1,200,000 m to be pumped past the power station during a 9-month period. Continuous pumping dur- ing this time gives a pump capacity of, on an average, 3 m /min. The normal river discharge is set at 100%, and variations will amount to a maximum of 150% and a minimum of 50% of the normal discharge, the variations between high and low being about the same. This means that sludge pumping varies from 1.5 to 4.5 m /min during the period. Example 11: Integration of sludge withdrawal, return water and sludge processing. Outside a closed-down cellulose plant lies a fibre bank contaminated by mercury. The fibre material of the bank has a water content of about 90%. The fibre sludge is to be dewatered in a temporarily erect- ed dewatering plant. The sludge is estimated to be dewatered to a water content of 75%, i.e. 25% DS. The dewatering plant lies about 1 km from the sludge bank and has a capacity of 2 tons DS/hour. The fibre sludge is pumped up to the surface in about 10% DS where it is diluted with return water from the sludge dewatering to about 2% DS. The 2% sludge is pumped ashore to a settling basin where the sludge concentration is adjusted to suit the sludge dewatering plant. The reject from the sludge dewater- ing plant is recycled to the settling plant. Some of the water from the settling basin goes unpurified back to the suction unit according to the above, and the remainder is purified by, for example, chemical precipitation and is recycled to the recipient. This procedure gives the following water balance: 3 From the fibre bank 1 ton DS 9 m water 3 After dewatering 1 ton DS 3 m water To the recipient 0 ton DS 6 m water Normal procedure: The sludge is taken up uncontrollably in concen¬ trations between 1 and 3% DS , on an average 1.5% DS, which should give the following water balance: From the fibre bank 1 ton DS 66 m water After dewatering 1 ton DS 3 m water 3 To the recipient 0 ton DS 63 m water This implies that the water volume to be purified is about 10 times larger. As will appear from the above Examples, the system described provides, according to one aspect of the present invention, a method of controlling one or more jointly movable and stationary suction devices for sucking suspendible material or a specific layer in a stratified body of water according to a pre-arranged plan or a plan laid down while work is progressing, for using or collecting the sludge and the water, respectively. For optimal utilisation of a water volume required for carrying a project into effect, use is made of sludge from areas having a high water content, such as sludge from temporarily erected settling spaces or sludge obtained upon flushing/brushing of stony areas, and/or return water from a provisional sludge deposit on land or in water, which deposit may be for instance in the process of being filled or has previously been filled, or has been in the process of settling, for dilution of sludge which originates from low water content areas and which would not normal¬ ly be practically pumpable over longer distances. Preferably, the sludge concentration is controlled directly at the place of suction, for example on the suction side of a pump 14 (see Figs. 3, 4 and 6), such that the sludge may be used for' a specific purpose, for example land elevation or thin-layer deposition without the need for decantation on arable and pasture land. By using sensing devices of various types, the concentration and composition of the sucked sludge may be controlled according to need. To control the integrated system, use may thus be made of signals from one or more solids concentration meters, such as viscosity or density meters, which are mounted immediately adjacent the respective suction place, or in the conduit from the respective suction place. Similarly, flow meters may be mounted in the conduit from the respective suction place to provide control signals for the suction system. pH meters, turbidity meters, temperature gauges or conductivity meters mounted immediately adjacent the respective suction place, or in the conduit from the respective suction place, may provide further or alternative control signals for the integrated suction system. Further¬ more, use may be made of ion-selective electrodes to provide control signals for the suction system. The integrated suction system according, to the present invention may thus be controlled in response of one or more of the above-mentioned parameters, although control may also be effected in response to other parameters, such as the pumping effect and the pump speed of the respective suction unit. Con- trol may also occur empirically on the basis of para¬ meters which are bottom and sludge responsive, such as the line angle alpha, the skewing angle beta of the suction unit, and the line tension, and by means of implements located in or at the nozzle and adapted to loosen the sludge, such as water flushing means and cutting devices. The- control primarily aims at providing a sludge having a composition which is desired for a specific use, but the signals used for controlling the integrated system may also or alternatively be used as monitoring signals for connection and discon¬ nection of suction units. According to a second aspect of the present inven¬ tion, there is provided a deposited sludge which, in every respect, has the desired composition regard¬ ing, for example, grain fractions, nutrients, pH etc., by continuously adding, while sludge suction is pro- gressing, the additives required for obtaining the desired composition. The addition may be carried out proportionally to the pumped flow volume and/or the volume of pumped solids and/or the volume of pumped water and/or the pH of the pumped flow. According to a third aspect of the invention, the suction units comprised by the system may be uti¬ lised as movable measuring stations. If the suction units are used for mapping only, the pumped medium preferably is collected or recycled in such a manner that the measuring result is not affected. The measur¬ ing station or measuring units may, however, also be utilised during the actual sludge suction phase and may then operate as a source of information for controlling the sludge composition adjustment of the system. The information supplied by the measuring station may also be utilised for controlling the supply of additives in order to obtain the desired composition of the deposited sludge. In this case, the measuring unit may be operating somewhat ahead of the suction unit and thus supply the requisite information for steering and controlling the suction unit. Alterna¬ tively, the measuring unit may be operating beside the suction unit and record a pumping program which is used for the suction unit when this is travelling across the measured area during its return trip. Utilising one or more movable suction units as measuring stations thus implies that the sludge to be sucked can be continuously analysed by utilising the same type of sensing devices as may be used for controlling the sludge concentration and composition, and that the dosing of additives can be carried out in response to the result of the analysis made by means of the measuring unit or units. The additives may be supplied simultaneously with the measuring operation or separately. Measuring may also be ef¬ fected after the additives have been supplied. Control may preferably be effected on the basis of empirically established relations between the infor¬ mation provided by the measuring unit and the parameter to be controlled. According to a fourth aspect of the present inven- tion, one or more movable suction units may also be used for depositing additives on the bottom of the lake for conditioning purposes. Thus, the limnological mapping may show that a stabilisation of existing sludge strata is required before suction is carried out, or that some other conversion, preservation or adjustment should be effected to be able to conduct a restoration in the desired manner. At or immediately adjacent the respective suction unit, substances for nutrient salt precipitation, sludge stabilisation, pH adjustment, oxidation, etc. may thus be added. Such sub¬ stances are iron chloride, calcium nitrate, lime and ben- tonite. The addition of these substances may be con¬ trolled in response to such parameters as the water depth and/or the conductivity, measured on the pumped medium, either from the suction unit or from a measur¬ ing unit, and/or the pH value, measured on the pumped medium, either from the suction unit or from a measur- ing unit, and/or the sludge depth and/or the gas dis<- charge. Furthermore, adding may be effected separate¬ ly or in connection with measuring, sludge suction or the withdrawal of pure water. The conditioning according to the invention thus provides for, for example, temporary preservation of sludge, binding sludge to the bottom, oxidation of sludge and binding of heavy metals to prevent sludge escape and/or in preparation of a subsequent sludge suction and, possibly, also in order to improve the usefulness of the sludge. As will appear from the above, the present inven¬ tion provides an integrated system for the restoration of lakes and other watercourses, said system making efficient use of the sludge and water layers so that an economic long-term restoration becomes possible, a restoration which also pays regard to the deposition and purification possibilities- as well as to weather changes. ' Finally, it is pointed out that the invention proposes the utilisation of return water as a carrier of sedimentation material . The return water need not be purified and merely should have a lower sludge concentration. The invention also offers an advan- tageous alternative to prior art sludge concentration control technique which merely utilised suction unit movement control. In addition, the invention makes it possible, by admixing sludge to water taken from a body of water, to maintain or reduce the sludge volume in said body of water, which is highly important in connection with power station basins. The invention thus makes it possible to maintain sludge balance in different watercourses.

Claims: CLAIMS 1. A method of controlling the sludge concentration of sludge sucked up by means of at least one first suc¬ tion unit moving within a body of water, in accordance with a laid-down plan for collecting the sludge, which requires a predetermined minimum water content, c h a ¬ r a c t e r i s e d in that sludge having a water content higher than said predetermined water content and sucked up by means of at least one movable or stationary second suction unit, or return water from a sludge deposit, is used for dilution of sludge sucked up by means of a movable first suction unit and having a water content lower than said pi ^determined water content, in proportions such that the said predeter¬ mined water content is imparted, directly at the suc- tion place, to the sludge sucked up by means of said movable first suction unit. 2. A method as claimed in claim 1, c h a r a c ¬ t e r i s e d in that different suction units are jointly connected or disconnected on a pre-arranged plan for optimal utilisation of time responsive con¬ ditions prevailing in the body of water. 3. A method as claimed in claim 1 or 2, c h a ¬ r a c t e r i s e d in that, for dilution control, use is made of signals from solids concentration meters, flow meters, pH meters, conductivity meters, turbidity meters, gas discharge meters or temperature gauges, or ion-selective electrodes, said meters or electrodes being mounted adjacent the suction place or in a pipe conduit from the respective suction unit. 4. A method as claimed in claim 3, c h a r a c ¬ t e r i s e d in that dilution is controlled by the length and diameter of the pipe conduit. 5. A method as claimed in any one of claims 1-4, c h a r a c t e r i s e d in that the capacity and speed- of a pump associated with the respective suction unit are used as variables for dilution control. 6. A method as claimed in claim 3, c h a r a c ¬ t e r i s e d in that the suction place or places of the movable suction unit or units are moved hori¬ zontally or vertically, separately or jointly in re¬ sponse to said signals. 7. A method as claimed in any one of claims 1-6, c h a r a c t e r i s e d in that solid material from the bottom of the body of water is separated and recycled at the respective suction unit. 8. A method as claimed in any one of claims 1-7, c h a r a c t e r i s e d in that the dilution con¬ trol signals also, or alternatively, are used as moni- toring signals for the connection and disconnection of suction units . 9. A method as claimed in claim 1, c h a r a c ¬ t e r i s e d in that one or more substances are added proportionally to the volume of the pumped flow, or proportionally to the volume of the pumped solids, or proportionally to the volume of the pumped water, or proportionally to the pH of the pumped flow, there¬ by to obtain a composition which, in respect of grain fractions, nutrients, pH etc. of the deposited sludge, is suitable for the contemplated use. 10. A method as claimed in claim 1, c h a r a c ¬ t e r i s e d in that one or more suction units are used as a mobile measuring unit, and that the pumped medium preferably is collected or recycled in such a manner that it does not affect the measuring result. 11. A method as claimed in claim 10, c h a r a c ¬ t e r i s e d in that the measuring unit operates together with the suction unit, but records its own functions independently of said suction unit. 12. A method as claimed in claim 10 or 11, c h a ¬ r a c t e r i s e d in that said measuring unit operates somewhat ahead of or behind the suction unit and supplies information thereto. 13. A method as claimed in claim 10 or 11, c h a ¬ r a c t e r i s e d in that said measuring unit operates beside said suction unit and records a program which is used by said suction unit during its return movement across the measured area. 14. A method as claimed in any one of claims 10-13, c h a r a c t e r i s e d in that the medium pumped by said measuring unit is pumped into the pump conduit of the suction unit after it has passed the respective measuring unit. 15. A method as claimed in an -one of claims 1-14, c h a r a c t e r i s e d in that substances for nutrient salt precipitation, sludge stabilisation, pH adjustment, oxidation etc., such as iron chloride, calcium nitrate, lime and bentonite, are added to the body of water at or immediately adjacent the re¬ spective suction unit. 16. A method as claimed in claim 15, c h a r a c ¬ t e r i s e d in that the water depth or the conducti¬ vity of the medium pumped from the suction unit or from a measuring unit, or the pH of the medium pumped from the suction unit or from a measuring unit, or the sludge depth or the turbidity is used for controlling the amount of additives. 17. A method as claimed in claim 15 or 16, c h a ¬ r a c t e r i s e d that addition is effected separate¬ ly or in combination with measuring, sludge suction or the withdrawal of pure water.

Inventors: BJOERK SVEN, SANDBERG ARNE MIKAEL, SANDBERG NILS ARNE, BJOERK, SVEN, SANDBERG, ARNE, MIKAEL, SANDBERG, NILS, ARNE
Applicants: INDUSTRIKONSTRUKTIONER AB
Application year: 1986
Main classification: E02F5
IPC codes: E02F3