FR2718732A1 - Process for improving water quality in lake - Google Patents

Abstract

Process for improving water quality in a lake comprises: (a) aerating the hypolimnion (deep cold water layer) with at least one aerator located in a deep zone below the thermocline, where the aerator entrains water upwards by means of a column of bubbles from a diffuser and returns the oxygenated water to a level just above the lake bed through horizontal outlets to avoid perturbation of the water; and (b) agitating and oxygenating the water in a (shallow) zone peripheral to the deep zone by generating a stream of bubbles which carries the water towards the surface. Also claimed is appts. for carrying out the above process.

Description

TREATMENT PROCESS FOR IMPROVING ORALITY
WATER CONTAINED IN A LAKE AND INSTALLATION FOR
IMPLEMENTATION OF SAID PROCESS.

The present invention relates to a treatment method for improving the quality of the water contained in a lake or in a eutrophic and deep reservoir.

It more specifically relates to a set of measures to act against the main causes of deterioration in the quality of lake waters, namely in particular - the deoxygenation of deep water during periods
which is the source of many sources of
nuisance, such as
. the release of phosphorus and heavy metals
toxic by sediment,
. the formation of ammonia,
. the establishment of an unfit environment
fish farming, - algal developments and, in particular,
cyanophyceae secreting toxins, - the heavy metals and phosphorus contributions by
tributaries of the lake.

We know that to combat these different causes, we have already proposed a multiplicity of solutions, each specific to a specific cause.

It turns out, in use, that these different solutions are often incompatible with each other, the use of a solution to fight against a source of nuisance often going against the action necessary to fight against a second source of nuisance.

Thus, for example, the reoxygenation of deep waters should preferably be carried out without disturbing the stratification of the body of water while the fight against algal developments may on the contrary involve delamination. The trapping of phosphorus and heavy metals discharged by the tributaries involves sedimentation according to a precipitation and decantation process which seem incompatible with mixing of the water.

A more particular object of the invention is therefore a method allowing the implementation of a set of measures which, up to now, seemed incompatible, and this, by combining their action so as to obtain a more complete and effective treatment.

To this end, it therefore proposes a treatment method involving simultaneously - a method of aerating deep layers using
at least one hypolimnic aerator placed inside
a deep sea zone in which the depth of the
lake is greater than a determined threshold, this aerator
generating a deep water suction
deoxygenated thanks to a column of bubbles generated
by a diffuser, then a return of the water as well
entrained, and which therefore underwent oxygenation, at
above the bottom, substantially horizontally so
to avoid any disturbance of the water layers
- a mixing of the water at its periphery, thanks to the
bubble curtain formation which ensures pumping
surface water device with mixing and
oxygenation of the entire water column
superior.

In this process, hypolimnical aeration allows the deep layers to be re-oxygenated and prevents the appearance of reducing conditions there. Because it does not stir the water column, it preserves the thermal stratification which may have taken place there, and this, with a good oxygen transfer efficiency. In addition, it does not require the energy necessary to break the thermal barrier between cold and deep water and warmer surface water.

Aeration with mixing, which takes place in an area located on the periphery of the deep area, only disturbs the stratification of the latter to a negligible extent.

Thanks to the oxygenation it produces, it helps to avoid the appearance of reducing conditions in the sediments. However, the effect which is most sought after results from the permanent mixing of the upper water column which it produces: this mixing locally maintains an "agitated" environment unfavorable to the development of cyanophyceae which - are at the origin of flowers d water, - secrete very violent toxins that cause
in bathers of dermatoses and problems
gastric, - can cause death of watering animals
in the areas where they accumulate.

In addition, these cyanophyceae are particularly harmful for the production of drinking water, because they can aggregate in colonies, thus causing clogging of the filters.

In addition, the toxins generated by these algae cause taste and odor problems which may require, in the drinking water systems, a combined treatment with activated carbon and ozone in high doses.

In addition to the two measures mentioned above, the method according to the invention may optionally include the reduction of the suspended matter in the waters delivered by the tributaries of the lake, this reduction being produced by means of a submerged threshold of flexible material stretched vertically between two banks at the mouth of the tributary, this threshold being maintained at its base by ballasts tackling it on the sediment and being supported in its upper part by floats so as to thus ensure the passage of a sheet of water of constant and adjustable thickness.

Installed at the outlet of tributaries loaded with nutrients, this threshold ensures, under optimal operating conditions, a theoretical reduction in incoming phosphorus, which can reach 70 to 90 t in summer.

The load of nutrients flowing into the water body is reduced and, ultimately, algal development is reduced accordingly.

The basin which is isolated upstream of the threshold can be advantageously aerated, periodically, so as to obtain optimal operation of the "phosphorus trap" thus produced.

An embodiment of an installation for implementing the method according to the invention will be described below, by way of nonlimiting example, with reference to the appended drawing in which
The single figure is a representation
schematic showing in section a lake and
the implantation, in this lake, of the different
parts of the installation.

In this example, the lake L has a bowl-shaped profile whose flared edges B gradually rise to the surface S of the water. This lake is fed by a river R whose mouth E is visible on the left of the figure.

In this drawing, the horizontal line T corresponds to the thermocline which theoretically separates at a depth of ten meters, the lower layer of water poor in oxygen (hypolimnion) from the upper layer of water, rich in oxygen (epilimnion).

As we know, in summer, the phytoplankton, present in the epilimnion, produces organic substances which, in the long run, are deposited in the hypolimnion. In it, organic substances are broken down into inorganic substances by bacteria, this process taking place thanks to the oxygen contained in water.

Under normal conditions, the natural renewal of oxygen in the hypolimnion is enough to fuel this process and there is thus created a balance.

If, for whatever reason, the quantity of nutritive matter becomes too large (for example a too high phosphorus level), the quantity of organic substances which is deposited in the hypolimnion becomes itself too large. The oxygen present in the hypolimnion then becomes insufficient for the decomposition of all of the organic matter to occur.

In winter, the nutrient salts are distributed throughout the body of water (release of phosphorus) and thus become available for new production of organic matter.

This process therefore leads to a continuous increase in the production of organic matter and, at the same time, a continuous depletion of the oxygen present in the hypolimnion.

The object of the invention is therefore more particularly - to limit the supply of nutrients, and
particular phosphorus from the R river
as well as the release of sediments, - to avoid the deoxygenation of the hypolimnion so
to avoid the phenomena of release of phosphorus and
other heavy metals by sediment as well as the
formation of substances unfit for fish farming.

In addition, it is clear that the presence of nutritious matters in great quantity and, in particular of phosphorus, favors algal developments and in particular of cyanophycées secreting toxins.

The invention therefore also aims to limit the development of these algae.

To achieve these results, the invention combines different measures, a priori not very compatible, so as to produce a mixing and a selective and simultaneous oxygenation of the epilimnion and the hypolimnion.

The first measurement consists of hypolimnical aeration carried out using a device which can include, as shown - a cylindrical bell 1, open in its part
lower and closed in its upper part by a
bottom 2 at the top of which opens a conduit
air exhaust 3, - a tubular channel 4 which extends inside the
bell 1, substantially coaxial thereto, - an air injection nozzle 5 connected to a compressor
(not shown) via a conduit
flexible, this nozzle 5 being arranged in the part
bottom of channel 4, and - water discharge orifices 6 provided in the part
bottom of the side wall of the bell 1.

The assembly comprising the bell 1, the channel 4 and the injection nozzle 5 is kept immersed in the hypolimnion, in the vicinity of the bottom F, while the air exhaust duct 3 is held on the surface by a float 7 so as to lead to the open air.

The operation of this device is then as follows
The air coming from the compressor, which is injected through the nozzle 5, generates in the channel 4 a column of bubbles which drives a current of water by oxygenating it.

This stream of water rises to the upper part of the bell 1 then descends to escape through the water discharge orifices 6 provided in the bell 1.

At the same time, the excess air is evacuated through the exhaust duct 3.

It is clear that thanks to this device, oxygenation of the hypolimnion is obtained without disturbing the thermal stratification of the water in the lake.

As a result, the rate of decomposition of organic substances into inorganic substances can reach a sufficient level and the phenomenon of nutrient release is thus avoided. The harmful self-deoxygenation process of the hypolimnion is therefore stopped.

Furthermore, this device has the advantage of having reduced operating costs because it does not require the energy necessary to break the thermal barrier between cold and deep water and warmer surface water.

The second measurement used in the method according to the invention consists of aeration with mixing, carried out in an area which extends linearly around the lake, preferably at the place where the depth of the lake corresponds to that of the thermocline T which separates the hypolimnion from the epilimnion.

This ventilation with mixing involves at least one pipe 8, installed at the periphery of the water body, supplied with compressed air by means of a compressor (not shown) installed on the bank and arranged in a completely soundproof enclosure. This pipe 8 comprises a multiplicity of air outlet orifices so as to deliver a curtain of air bubbles which generates a circulation of water by "air-lift" effect up to the surface.

This treatment, which ensures additional oxygenation of the water as well as a homogenization of the water, is mainly used for its mechanical action: it maintains an "agitated" environment unfavorable to the development of cyanophyceae, the drawbacks of which have been previously mentioned. The mixing it produces is not contrary to the effect sought by the first measurement, (hypolimnetic aeration without destratificaion, because it is carried out on the periphery of the deep areas and not inside these areas.

The third measure mainly concerns the outlet areas of the tributaries of the lake, that is to say in areas crossed by a stream of water loaded with nutrients.

It involves an immersed threshold 10 made of flexible material stretched between two banks and held vertically on the bottom by means of weights 11 and floats 12, its height being determined so as to allow the passage of a sheet of water from constant and adjustable thickness h. This threshold 10 isolates upstream a part of the lake which functions as a settling tank, thus ensuring the reduction of suspended matter and, therefore, of particulate phosphorus and of phosphorus absorbed on ferric hydroxides. The load of nutrients flowing into the water body is reduced.

Advantageously, the settling tank delimited by the threshold 10 can be aerated periodically, thanks to a network of pipes 13, so as to promote the bacterial action and thus to obtain optimal operation of the phosphorus trap.

Claims (6)

Claims  1. Treatment method for improving the quality of the water contained in a lake (L) or the like, characterized in that it involves, in combination - aeration of the deep layers by means of at least one  hypolimnic aerator (1 to 7) placed inside a  deep sea area in which the depth of the lake  (L) is greater than a determined threshold (T), this  aerator generating deep water suction  deoxygenated to the surface thanks to a column of  bubbles generated by a diffuser (4, 5), then a  return of the water thus entrained, and which has therefore undergone a  oxygenation, above the bottom (F), substantially  horizontally so as to avoid any disturbance  of water, - mixing of the water in a peripheral area to  the deep sea area, thanks to the formation of a curtain  of bubbles which ensures peripheral pumping of water  deep to the surface with mixing and  oxygenation of the entire water column  peripheral.  2. Treatment method according to claim 1, characterized in that it further comprises the reduction of the suspended matter in the waters delivered by the tributaries (R) of the lake (L), this reduction being produced thanks to a threshold submerged (10) in flexible material, stretched vertically between two banks at the mouth of the tributary (R), this threshold (10) being held at its base by ballast weights (11) pressing it against the sediments and being supported in its upper part by floats (12) so as to thus ensure the passage of a blade of water of constant thickness (h) and adjustable.  3. Method according to one of claims 1 and 2, characterized in that the aforementioned stirring takes place in a peripheral zone in which the depth corresponds substantially to the thermocline (T) which separates the hypolimnion from the epilimnion.  4. Method according to one of the preceding claims, characterized in that it further comprises periodic aeration of the settling tank delimited by the submerged threshold made of flexible material (10).  5. Installation for implementing the method according to one of the preceding claims, characterized in that it comprises at least - a hypolimnetic aeration device comprising a  cylindrical bell (1), open in its part  lower and closed in its upper part by  a bottom (2) at the top of which opens a conduit  air exhaust (3), a tubular channel (4) which  extends vertically inside the bell (1),  an air injection nozzle (5) arranged in the part  bottom of the channel (4) and the discharge orifices  water (6) provided in the lower part of the wall  of the bell (1), and - a ventilation and stirring device comprising at  minus one pipe (8) installed on the bottom of the lake  and supplied with compressed air by a compressor,  this pipe (8) comprising a multiplicity  air outlet openings capable of delivering a curtain of  air bubbles which cause water circulation from  the bottom to the surface.  6. Installation according to claim 5, characterized in that it comprises a settling tank delimited by an immersed threshold (10) of flexible material stretched between two banks and held vertically on the bottom by means of ballasts (11) and floats (12), this settling tank preferably being equipped with means capable of performing periodic aeration of the water.