EP0048257A1

EP0048257A1 - Procedure for impriving the ecological state of a thermally and/or chemically stratified water basin

Info

Publication number: EP0048257A1
Application number: EP81900822A
Authority: EP
Inventors: Kalle Matti Lappalainen; Reijo Matti Oravainen; Ami Solin
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-03-28
Filing date: 1981-03-27
Publication date: 1982-03-31
Also published as: WO1981002731A1; FI59777B; FI59777C

Abstract

Processus permettant d'ameliorer les conditions ecologiques dans un lac stratifie (thermiquement ou chimiquement) ou tout autre bassin d'eau en ce qui concerne la teneur en oxygene des eaux a une grande profondeur. Le processus consiste a amener l'eau de surface du meme volume d'eau ou d'une autre source dans l'eau profonde a proximite du fond, ou elle est sensee exercer un effet favorable sur la teneur en oxygene de l'eau profonde et aussi sur la temperature de cette eau, a condition que la densite de cette eau permette de la maintenir a la profondeur desiree et, en outre, d'empecher par exemple toute participation non souhaitable dans l'economie de l'oxygene de l'eau capillaire presente dans les sediments. L'eau de surface est propulsee par exemple au travers d'un large tuyau au moyen d'energie produite par une source exterieure, ou transportee d'une hauteur plus elevee par effet de siphon, et des deflecteurs peuvent etre prevus au point de decharge pour faciliter la distribution desiree. A l'eau de surface ainsi transportee peuvent etre ajoutes plusieurs produits chimiques ou d'autres agents utiles.Process for improving ecological conditions in a stratified lake (thermally or chemically) or any other basin of water with regard to the oxygen content of waters at great depth. The process involves bringing surface water from the same volume of water or from another source into deep water near the bottom, where it is believed to have a favorable effect on the oxygen content of deep water. and also on the temperature of this water, provided that the density of this water makes it possible to maintain it at the desired depth and, moreover, to prevent, for example, any undesirable participation in the economy of the oxygen of the capillary water present in sediments. Surface water is propelled for example through a large pipe by means of energy produced by an external source, or transported from a higher height by siphon effect, and deflectors can be provided at the point of discharge to facilitate the desired distribution. To the surface water thus transported can be added several chemicals or other useful agents.

Description

Procedure for improving the ecological state of a thermally and/or chemically stratified water basin

The present invention concerns a procedure aiming to improve the ecological state of a thermally and/or chemically stratified water basin, for instance a lake with a low-oxygen depth zone.

Successful utilization in fish production of a water basin, such as for instance a marine region, lake, pond or equivalent, implies that an ecological state which is propitious with a view to the chances of fish to survive is maintained in the water body. One necessary prerequisite for fish production to take place in a given water body is an adequate plankton and bottom fauna production, that is, a given level of luxuriance, while on the other hand excessive luxuriance inhibits the fish life and thereby, utilization of this luxuriance. The level of luxuriance in a water basin can be con¬ sidered proper if its external loading, e.g. the effluents containing organic substances, and the internal loading, that is the oxygen consumption resulting from plankton disintegration, cause no oxygen depletion or harmful oxygen deficit in the water basin or in any part thereof.

Apart from being a habitat factor for fish and for the nutrient fauna of fish, the oxygen content has another significant linkage with luxuriance. To_. it, if the water strata adjacent to the bottom sediments in a lake become low in oxygen content, this results in a lowered reduction/oxidation potential of the sediment, and as a result of certain chemical reactions iron and manganese may dissolve in the water from the sediment, as well as phosphorus that has been bound to them previously. Gas bubbles, and mixing of the water produced by bubbling, may also occur. As a consequence of these events, the luxuriance of the lake rapidly increases. This is some- times referred to as wildfire or gallopping eutrophization.

Lack of oxygen is most clearly and most detrimentally evident speci¬ fically in the subwater of a lake. The lowest oxygen contents are found in immediate vicinity of the bottom.

In order to rectify the detriments described, that is the oxygen depletion and its consequent phenomena, endeavours have been made to increase the oxygen content of the water by aeration or oxygena- tion in that from a suitable source compressed air or oxygen is introduced under the water surface.

In a problem solution known in the art (US Patent No. 3,956,124; AT Patent No. 288.273), water that one wishes to treat is pumped from the depth to an oxygenating apparatus above the water surface and oxygen is added in this apparatus to the water which one desires to improve, whereupon the treated water is reconducted to the depth. It is further known, with a view to improving the operation of this type of oxygenator, to separate prior to treatment from_ι the water the gases which are dissolved therein. Procedure of this type have been disclosed in the Finnish Patents No. 49703 and 52661, the first teaching to separate the. ases by using pressure to atomize the water,^' while in the second mechanical vibrations directed on the in- take tube of the apparatus are employed.

Although procedures of prior art of the type described above are usable in themselves and have yielded positive results, the drawback still remains that the requisite apparatus is complex and expensive, as well as the drawback of high operating costs, energy costs in particular. The oxygenation capacity achieved by procedure of the kind described is usually on the order of 1 to 3 kg 0„ per kWh. In procedures of prior art the retention of the introduced oxygen in the subwater has also been incomplete in that large proportion of the oxygen contained in the bubbles escapes back into the atmosphere. The oxygen displacement in horizontal direction has likewise been insufficient as a rule.

The object of the present invention is to avoid the drawbacks hampering the procedures of prior art and to provide a simple procedure, and one which is low in operating and initial expenditure, by which it is possible efficiently to influence the ecological state of a water basin, such as a lake for instance, by controlling its oxygen status, its level of luxuriance or its temperature.

With a view to attaining the objects mentioned, and others which will become apparent later on, the invention is mainly characterized in that into the oxygen-deficient subwater is conducted through one or several flow passages, comparatively more oxygen-abundant and lighter topwater to the purpose of conveying this oxygen content, admixing it and displacing it horizontally among the subwater with the aid of mixing taking place by effect of convection tending to equalize differences in temperature and density, in such manner that the thermal and chemical stratification is preserved.

By the procedure of the invention the effect is achieved that all the oxygen-loaded water that is introduced remains in the subwater and there becomes uniformly and efficiently admixed. This highly advantageous effect is based on the fact that the temperature and density differences and equivalent are rapidly equalized due to convection and that the mixture that is produced rises to begin with, until it meets a depth where the densities of the surrounding water and of this mixture are the same, whereby the movement is deflected to be horizontal. Thus, the mixture is always heavier than the top¬ water and it will not rise to the surface. By the improvement achieved in the oxygen state, the unfavourable chemical bottom reactions are also prevented and the valuable bottom regions in the lake are released for fish production. In addition to the immediate effect elicited by the oxygen which the flow conducted into the sub¬ water carries along with it, the procedure of the invention reduces the water interchange between the capillary water in the sediment and the subwater because the lighter mixture of subwater and top¬ water floats better on top of the capillary water of the sediment. In wintertime, when the introduced topwater is cold, the procedure moreover cools the subwater, resulting in lower oxygen consumption and in a prolonged spring full cycle.

The economy of the procedure is partly based on the fact that the flow resistance of the flow passage remains quite minimal when a low flow rate and large diameter passage are used. The flow passage need only withstand the minimal pressure resulting from difference of density and from flow resistance, whereby the requisite apparatus can be made in a very light-weight construction. The economy of the procedure of the invention is partly based on the fact that in its application no air or gas bubbles are generated, which in other procedures require a large amount of energy at the bubble generating and conducting stages.

In a particularly favourable embodiment of the invention, the top- water to be conducted into the subwater is taken from the same water basin. This equalizing of the water body's oxygen resources is based on the fact that - excluding cases in which the water body is extremely heavily polluted - the topwaters of water bodies contain ^' water with adequate oxygen content in the wintertime too, because the greater part of the consumption takes place in the subwater and on the surface of the bottom sediment. Moreover, the volume of the subwater is only a fraction of that of the topwater.

The invention is described in the following in detail with the aid of an embodiment example, in association with the attached drawing, wherein:-

Fig. 1 presents the variation of water temperature, and density in a lake or equivalent basin at different depths during periods of stratification, and

Fig. 2 illustrates schematically an embodiment of the procedure of the invention.

Referring now to Fig. 1, therein has been shown a typical example of the temperature and density stratification of the water in a lake or another equivalent basin, in the different seasons. The curves A represent the situation encountered in the winter and the curves B, the summertime conditions. As can be read from the graphs, the water temperature varies, in the different seasons, most strongly close to the surface. Close to the botton, the temperature varies rather

-^s ⁱEm - minimally around the value of 4 C, which corresponds to the maximum density of water. Naturally, the equivalent is true regarding density; that is, the heaviest water is always found at the greatest depth.

In Fig. 2 is schematically shown the cross section of a lake which is stratified as has been described. The zone under the water surface, or in the winter under the ice sheet 1, is the topwater 2, where the water has a relatively high oxygen content. In the underlying sub- water (indicated by shading in the figure) with low oxygen content or lacking oxygen, or in the depth zone 3, the water has a fairly uniform temperature and it has a density which makes it heavier than the topwater, and for these reasons it is nearly stationary. The top¬ water has as a rule a volume which is a multiple of that of the sub- water because the lake or equivalent water basin can be likened to a cone standing on its apex, as schematically indicated in the drawing. The depth of the subwater frequently represents one half of the total depth, but even in that case the subwater volume is only 10 to 20% of the total volume, depending on the morphology of the lake.

With a view to improving the oxygen state of the subwater, oxygen- loaded topwater 2 is conducted into the subwater 3 through a large diameter tube 4. The tube has to this purpose, on its top, intake ports 5 at an appropriate subsurface depth, through which the top- water flows into the tube and further to one or several exit ports 6 close to the bottom. It is well known that kinetic energy and friction increase proportionally as the square of the velocity. Therefore, in order to optimize the construction of the tube, the tube diameter is chosen large and the flow velocity low, whereby with a small amount of energy a relatively large flow rate is achieved. The flow is generated by the aid of a propeller 7 dis¬ posed within the tube 4 and driven over a shaft by a prime mover 8 mounted over the water surface. Suitable prime movers for the pro- peller 7 or equivalent means are, on the side of an electric motor or internal combustion engine, in view of the low energy requirement also alternative sources of energy, such as wind power, wave power or equivalent. Since the flow passage 4 is acted upon only by a minimal pressure due to different density and to the flow resistance, the apparatus can be constructed to be quite light, and the flow passage 4 may be formed of light, thin-walled tube material or, for instance, of plastic tubing. The apparatus may be mounted to be borne by a raft structure 11 or, in the winter, directly on the ice.

When the apparatus is in operation, the oxygen-carrying water entering the tube 4 through the intake ports 5 (which has a temperature typi- cally about 0.1 to 0.5 C in the winter and about 15 C in the summer) escapes close to the bottom through the lower end 6 of the tube and becomes mixed with the subwater, which has a 'temperature respectively of 2 to 4 C in the winter and e.g. 5 to 8 C in the summer. When top¬ water is conducted in among the subwater, there are immediately produced convection currents 9, with increasing efficiency for higher density and temperature differences between the topwater and subwater. The mixture thus formed is invariably heavier than the top¬ water and will not rise to the surface.

The intake flow rate and the depths for the intake and exit ports are selected so that the conducted topwater can be made as perfectly as possible to remain in the subwater. The intake port is placed in the oxygen-carrying water layer (usually 1 to 3 m below the water surface). The discharge port is located, as a principal rule, about 1 to 2 m above the bottom. The discharge may also be directed into another water stratum, as may be desired. For better mixing and for prevention of the fast upwardly directed flow at the initial stage, one may employ a disk-like flow guide 10 encircling the tube 4 some¬ thing like that shown in Fig. 2, by which the flow along the side of the tube is deflected outward from the tube.

The wintertime temperature distribution in the bottom sediment is such that the temperature in its top parts conforms to the variations of the water temperature, and at greater depth the temperature is usually 5 to 7 C. In case the water temperature is e.g. 3 C, then being heavier than water of e.g. 6 C it will tend to penetrate into the sediment and to displace capillary water therefrom, with the result that some of the oxygen consumption in the bottom is trans- placed to have its site in the water. Since i the procedure of the invention, in the winter, the temperature of the subwater is lowered, the water becomes lighter and will float on top of the heavier sedi- ment capillary water. Hereby the eminently greater part of convection will take place within the sediment, and less capillary water than before will be entrained with the water. In the summer, appropriate heating of the subwater has the effect that the subwater can better float upon the sediment, since the sediment is colder than the sub- water.

EXAMPLE 1

The procedure of the invention was tried out in a lake suffering from subwater oxygen depletion during periods of stratification. The lake by the name Teernijarvi has 76 hectares surface and a total depth about 14 m. An apparatus as in the embodiment example above pre¬ sented was installed in the winter, about one month after the lake froze over.

The propeller was rotated with a 0.55 kW electric motor running at 127.5 r.p.m. The water was taken into the tube from 1.5 m depth under the ice (temperature 0.6 C) and conducted to the bottom through a plastic tube 0.32 m in diameter. The lower end of the tube was located at about 1 m height over the sediment surface (total depth 13.3 m) . The water quantity flowing through the tube was esti¬ mated to be 20 to 30 litres per second.

The results obtained are seen in Table A reproduced below, showing the temperature and oxygen content in the lake at different depths at the time when the apparatus was started operating and one month thereafter, together with the figures from two reference years. TABLE A

Temperature, C Reference years

Depth, m At starting time One month after Year 1 Year 2

Jan. 11, 1980 starting Feb. 22, Feb. 1,

Feb. 11, 1980 1971 1972

1 0.5 0.6 1.2 0.7

3 1.2 1.6 3.2 2.1

5 1.9 2.6 4.1 3.2

7 2.2 2.8 4.3 3.5

9 2.6 2.8 4.7

10 2.7 2.8 3.9

11 2.9 2.8

12 3.0 2.8 4.0

13 3.2 3.0

14 3.4 3.0 4.9 4.3

Oxygen content, mg 0„ per litre Reference years

Depth, m At starting time One month after Year 1 Year 2 Jan. 11, 1980 starting Feb. 22, Feb. 1

Feb. 11, 1980 1971 1972

1 11.5 10.4 9.4 9.5

3 10.9 9.3 7.0 8.7

5 9.4 5.8 3.1 5.6

7 8.0 3.4 1.8 3.0

9 6.3

10 5.2 3.6 0.6 0.5

11 4.0

12 1.6 3.3

13 0 2.6 0.2

14 0 2.5 As can be seen from Table A, the effects are very clearly observable in the values determined one month after start-up. The subwater had attained almost uniform temperature, and the normally seen continuous increase of subwater temperature had stopped. Water devoid of oxygen was no longer found in the depth region. In the corresponding period of the reference years, the subwater below the 10-metre depth level had been virtually devoid of oxygen, while the values measured one month after starting up the apparatus revealed in the said water layer an oxygen content of 2.5 to 3.5 mg 0„ per litre. The oxygen state of the topwater was not inferior to that of the reference years. The sub¬ water temperature had also clearly gone down and it was 2.8 to 3.0 C after the test. In the reference years, the subwater temperature exceeded 4.0 C. In a temperature range as low as this, the change is significant with a view to the intensity of decomposition activity, in favour of the test apparatus.

It could be concluded from the test results that the apparatus operated as expected.

EXAMPLE 2

Trials concerning the procedure were carried on with a prototype apparatus in the summer of 1980. The apparatus was installed in the Teernijarvi lake on May 21st, 1980 in the same location which had been used in the winter. The lower end of the tube was at the same depth (about 13.3 m) .

The results of the trial and reference years are stated in Table B. These results reveal that already on the day of installation the water was slightly stratified and that there had already been oxygen consumption.

Comparison of the temperatures stated in Table B reveals that the invention has homogenized and heated the subwater as is consistent with its principle of operation. It should be particularly noted that application of the procedure did not disrupt the temperature stratification, whereby it is evident that the mixing convection took place in the subwater. The ascending movement of the mixture composed of conducted topwater and of the subwater that has united therewith has been forced to turn horizontal and to flow in the upper part of the subwater radially outwards from the tube. This implies that the conducted topwater and the oxygen which arrived along with it have stayed in the subwater and have spread out there with great effi¬ ciency.

Scrutiny of the oxygen content figures reveals that the oxygen state of the subwater has been superior to that of the reference years, though not fully satisfactory. This is by no means any indication of a poorly operating procedure, because the apparatus had not been dimensioned to have a power rating adequate for the Teernijarvi lake. The low oxygen content cannot either be taken as an indication of accelerated oxygen consumption resulting from the heating of the sub¬ water. It is easy to demonstrate that the oxygenating capacity of the procedure outweights the effect of temperature elevation. Moreover, heating of the subwater promotes the floating of the water on the cooler capillary water of the sediment and thereby reduces the con- sumption of oxygen by the deep sedimentary layers from the water thereabove.

On the strength of the test results reported, the invention could be observed to operate in expected manner also during summertime stratification.

C'*?I - TABLE B

Temperature, C

Depth, m Start-up About one At sum- Reference years month later mer's enc 1971 1973 1974 May 21 June 26 Sept. 10 June 29 June 25 Sept. 11

1 9.9 17.1 14.7 19.0 19.4 16.2

3 - 17.1 - 18.5 18.1 -

5 9.7 12.4 14.7 16.8 12.5 16.0

7 8.1 10.1 13.7 12.0 7.9 15.1

10 7.6 9.6 12.3 9.5 7.2 8.7

12 6.8 9.4 12.1 7.5 - -

13 - - - 6.5 6.5 7.6

14 6.1 9.3 11.8 - — -

Oxygen content, mg 0_ per litre

Depth, m Start-up About one At sum¬ Reference years month later mer 's end 1971 1973 1974

May 21 June 26 Sept. 10 June 29 June 25 Sept. 11

1 11.9 8.7 8.4 8.6 9.2 9.2

3 - 8.7 - 9.7 9.2 7.5

5 12.0 5.3 8.3 7.7 4.1 2.9

7 9.1 2.6 3.5 4.3 1.6 0

10 8.5 2.3 0.5 0.4 0.6 0

12 6.4 2.0 0.2 0.1 - 0

13 - - - 0.1 0.4 0

14 3.4 1.5 0 0 0 0 Although in the foregoing the invention has been disclosed in asso¬ ciation with en embodiment example, it is obvious that the invention may be modified in many different ways within the scope of the claims following hereinbelow. It is for instance possible to conduct into the subwater in the basin to be treated, also oxygen-carrying water derived from another suitable source and which is lighter than the subwater in hand. It is in some instances possible to use water conveyed by the siphon principle from a water body, or another source, at higher altitude, whereby external energy is only needed to put the flow under way. It is possible in connection with the invention, also to add to the flow introduced into the subwater, chemicals known in themselves in the art, to the purpose of admixing them with the subwater.

Claims

1. Procedure for improving the.ecological state of a thermally and/or chemically stratified water basin^* for instance of a lake with a low- oxygen depth zone, characterized in that into the low-oxygen or oxygen-depleted subwater (3) is conducted through one or several flow passages (4) topwater (2) having a relatively higher oxygen content and lighter weight in order to convey, mix and horizontally displace its oxygen contents into the subwater by the aid of mixing taking place through the effect of convection tending to equalize temperature and density differences, in such manner that the temperature and/or chemical stratification is preserved.

2. Procedure according to claim 1, characterized in that topwater is conducted into the subwater within the framework of the intensity implied by the state of the object to be rehabilitated and by the mixing events, e.g. in the range from 10 to 2000 litres per second.

3. Procedure according to one of claims 1-2, characterized in that the topwater (2) to be conducted into the subwater (3) is taken from the same water basin.

4. Procedure according to one of claims 1-3, characterized in that the topwater (2) to be conducted into the subwater is taken from a separate water basin.

5. Procedure according to claim 4, characterized in that the flow to be conducted into the subwater (3) is taken from another water basin located at higher altitude, by the siphon principle.

6. Procedure according to any one of claims 1-5, characterized in that the augmentation of the subwater's oxygen content, the reduction of other substances' excessive contents or another desired alteration of the state or contents of the subwater is boosted or the alteration is accomplished by the aid of gases, chemicals and/or equivalent active agent additions added to the water that is conducted.