The present invention relates to a method for improving the ecological state of a water basin, such a lake, and its bottom sediment .

In recent years it has been evidenced ever more undeniably that the ecological state of a lake or similar water basin badly eutrophicated or contaminated due to waste loading does not change significantly unless a reduction of so¬ called internal phosphorus loading and other organic matter loading caused by reentry of such loading matter from the bottom sediment of the water basin is achieved. As commonly known, the phosphorus content of bottom sediment in healthy and low-eutrophication lakes is very high in all cases, and in general, even higher than in unhealthy lakes. From this it can be inferred that the main goal in the improvement of sediment status is to slow down the back-dissolution pro- cesses of phosphorus bound in the bottom sediment rather than to achieve reduction of phosphorus concentration or overall quantity in the sediment. Namely, this is because the phosphorus reserve of bottom sediment in lakes tends to increase continually, which is a natural result therefrom that, independently from the condition of the lake, phos¬ phorus inflow into lakes is annually larger than its removal .

An essential factor contributing to the occurrence and per- sistence of the high internal phosphorus loading problem is the change of the disintegrating microbial activity in the superficial layers of the sediment into an predominantly anaerobic one, which releases disintegration products that are toxic to aerobic microorganisms such as hydrogen sul- fide and also gases such as methane and nitrogen that float the particulate matter of the bottom sediment upward into the lake water, whereby phosphorus is dissolved and

released with the particulate matter into water. As also known, anaerobic disintegration occurs at a lower rate than that of aerobic activity, so the slowed disintegration rate acts as an additional factor that promotes accumulation of excess organic matter in the bottom sediment thus support¬ ing the vicious circle of phosphorus recirculation.

In excessively eutrophicated water basins, the reserves of active organic matter and gases in the bottom sediment have accumulated a detrimentally high proportion of the exo¬ genous or endogenous photosynthetic energy which in healthy water basins is chiefly circulated in the food chains of the fauna such as fish living in the top and littoral/ sublittoral water layers. Accordingly, the excess energy of the bottom sediment is the primus motor of internal loading that must be eliminated in order to improve the ecological state of a water basin and its bottom sediment.

To the end of improving the ecological state of lakes, one of the attempts has been excavation of the decaying and organic-matter-rich bottom layer and dumping thereof outside the lake. This approach, however, is expensive. An alternative approach has been to cover the bottom sediment with external inert material, which also is expensive and quite often ineffective. Furthermore, different types of lake water oxidation methods have been developed with the goal to shift the disintegration processes of excess or¬ ganic matter in the aerobic direction, whereby an end pro¬ duct of the process is carbon dioxide. All conventional oxidation methods of lake water are characterized in that the transport of oxygen into the sediment is by diffusion only, which makes oxygen transport rather slow and re¬ stricts the healing rate of the lake status to the limiting value of oxygen transport. On the other hand, the end or intermediate products of microbial disintegration such as the nitrite, ammonium ion and ammoniacal forms of nitrogen, nitrous oxide, hydrogen sulfide and methane as well as the

harmless carbon dioxide cannot escape sufficiently rapidly into water by diffusion only, whereby compounds harmful to aerobic microorganisms may enrich in the sediment in detri¬ mental quantities. If the sediment is oxidized directly by invasive means, it will become scattered and mixed into the entire water volume. Also apparatuses of the injection type have been employed to take chemicals directly into the bottom sediment.

It is a broad object of the present invention to provide a method capable of improving the quality of the bottom sedi¬ ment and the entire ecological state of a water basin. It is a particular object of the invention to offer a method suited for removing gases and excess organic matter from the bottom sediment and for lifting matter from the deeper layers of the bottom sediment to its superficial layers.

The goal of the invention is attained by means of a method characterized by what is stated in the annexed claims.

In the method according to the invention, the bottom sedi¬ ment of a water basin is worked mechanically in order to intermix its different layers. When organic-matter-rich superficial sediment of inferior quality is mixed according to the invention with the deeper-lying oxygen-depleted layer of higher density and mineral content, the fines-rich layer of the superficial sediment is blended with the deeper-lying matter resulting in higher activity of the anaerobic microorganisms. With certain provisions, this results in abundant methane and carbon dioxide production with minor production of secondary microbial biomass. In turn, this causes rapid release of gases (mostly methane and nitrogen) as bubbles and partially via diffusion into the atmosphere, and additionally, release of carbon dioxide via diffusion into the atmosphere, which provides an aux¬ iliary route to the removal of organic carbon and the energy bound therein from the sediment. Resultingly, the

bottom sediment is mineralized and compacted, the microbial activity is quantitatively reduced and the redox potential elevated, all of which factors contribute to improved oxy¬ gen content of the littoral/sublittoral water layers thus gradually improving the general status of the lake. By the removal of the excess energy from the sediment layers, the phosphorus binding capacity of the sediment is improved and its phosphorus content increased, resulting in overall reduction of phosphorus content in the lake water and lower eutrophication of the lake, and in the human standpoint, finally rendering the lake improved biological health and higher attraction to recreational use. The process may also be enhanced by oxidation and chemical treatment in conven¬ tional manners.

A further advantage of the invention is that the method lifts dormant spores, differentiated cells, cysts and other latent-phase cells of microorganisms along with the parti¬ culate matter of the bottom layer sediment into the super- ficial sediment layers and the overlying water. In this manner, the species ratios of the microorganisms occurring in the superficial layers of the bottom sediments are changed closer to those of the prevailing periods, simulta¬ neously improving the viability of the originally prevailed microorganisms.

Additional processes involved are as follows: The propor¬ tion of methane oxidation in water is reduced, since the bubbles transport methane rapidly into the air, whereas diffusion in turn is a slow process and permitting release of methane only at the lake surface subjects the entire lake water volume to the action of methane-oxidizing aerobic bacteria. Such an oxidation of methane causes a heavy oxygen demand and large production of secondary bacterial biomass, which tends to detain methane in the lake. Flotation of bottom sediment particulate matter and internal loading sustained by microbubbles are reduced,

because the specific surface and flotation power of large bubbles are smaller than those of microbubbles and since particulate matter suspended in water contains abundantly the already mineralized particles of the 10-30 cm thick sediment layer. This is contrary to that occurring in normal flotation and resuspension. Mineralization of the bottom sediment and rise of the redox potential improve the lake bottom conditions for the microorganisms of the lake bottom, which further hastens the mineralization of the lake bottom and means more nutrition to fish, for instance. Also the plankton species can avail of the improved condi¬ tion of the bottom sediment and the sublittoral zone water layer, which means faster mineralization already in sub¬ littoral water and reduction of organic matter settling as a sediment on the lake bottom. The prerequisites for the predominance of blue algae, which in a certain stage of their lifecycle utilize the lake bottom, may be jeopard¬ ized. The present concept is based on the idea that the mixing of the bottom sediment to a depth of 10-50 cm is an entirely novel and incompatible challenge to blue algae, which are the oldest species and most successful survivors on the earth.

In an embodiment of the invention, the superficial layer (having a depth of 0-5 cm) and the underlying sediment are intermixed. According to a preferred embodiment of the invention, the superficial layer (having a depth of 0-5 cm) and the underlying intermediate sediment (to a depth of 5-20 cm) are intermixed. In both cases, the biomass matter of the loose superficial layer is brought deeper in the sediment, where it is rapidly brought in the anaerobic state and thuε can enter the methane-releasing fermentation process. While the temperature of the unmixed bottom sedi¬ ment is on its top surface equilibrated with the overlying water, deeper in the sediment a much lower temperature will be encountered during summer. When the mixing is made during summer, the mixed sediment is warmed, whereby the

process is speeded through the entire depth of the mixed sediment, which means a strong methane production. Further¬ more, the top layer of the mixed sediment becomes in regard to organic matter a diluted substrate for microbial growth.

In a preferred embodiment of the invention, the superficial layer of the bottom sediment is mixed with the underlying sediment by transferring underlying sediment on top of the superficial layer. This operation is chiefly done locally at each point of the bottom. According to the invention, the bottom sediment is advantageously overturned with the help of a bucket or similar implement so that a suitable amount of bottom sediment is lifted with, e.g., a bucket, overturned and placed back to the same point wherefrom it was excavated. In comparison with, e.g., bubbling aeration of the sediment, this method offers the benefit that the mixing of the sediment into the total volume of the water basin remains much lower-.

In a further embodiment of the invention, ferrous salts are mixed with the bottom sediment to enhance methane fermenta¬ tion. Hydrogen sulfide contained in the bottom sediment is known to disturb methane fermentation. Therefore, hydrogen sulfide is bonded with ferrous salts, whereby ferrous sulfide, FeS, is formed that due to its low solubility stays in solid form and thus cannot disturb the methane fermentation process. Obviously, also other chemicals may be added into the bottom sediment as noted above.

The bottom sediment is mixed with a controllable, mechani¬ cally operating apparatus. Instead of using a bucket or similar implement, the mixing operation according to the invention may be done by means of a plurality of alterna¬ tive manners, e.g., by mixing the sediment by means of a single- or multi-propeller agitator or a cultivator; a paddle-wheel-type roll equipped with fixed or adjustable blades and possibly also with suction and discharge slots;

a harrow of the bucket rotor, plain disc or spike-tooth type; a subsoil-plow-type underdrainage excavator apparatus performing deep cultivation and distribution of the sedi¬ ment; or a bucket excavator apparatus performing grabbing and scattering of the sediment in a stepwise manner.

Apparatuses operating in the above-described principles may be supported by a vessel floating on the lake, cables spanned over selected areas of the lake, or alternatively the apparatus can be implemented as a robotic unit moving remote-controlled or according to a preset program over the bottom of the water basin. For automatic determination of its position, the apparatus may utilize satellite position¬ ing. The apparatus may be moved by means of a separate propeller- or paddle-wheel-type of power source exerting a pulling or propulsing force in a dedicated vessel. When driven by a cable actuated by a winch or similar actuator, the apparatus may be designed for manual or automatic control equipped with limiters and direction changers. The apparatus may be powered with an electric motor, combustion engine or other power source of sufficient output. Electric energy can be generated by a combustion-engine-driven power aggregate, wind power generator and/or solar power genera¬ tor. Obviously, electric energy may be taken by a cable from a mains service line.

Without being limited by the above-described exemplifying embodiments, the invention may be varied within the scope and inventive spirit of the annexed claims.