GB2236103A

GB2236103A - Oxygenation of rivers

Info

Publication number: GB2236103A
Application number: GB9020346A
Authority: GB
Inventors: Martin Roy Whiteland; Richard Martin Bellis
Original assignee: Thames Water Ltd
Current assignee: Thames Water Ltd
Priority date: 1989-09-22
Filing date: 1990-09-18
Publication date: 1991-03-27
Also published as: CA2005513A1; GB9020346D0; GB8921454D0

Abstract

Apparatus for addition of oxygen to a body of water such as a river, comprises a vessel 1 in the form of a self-propelled boat or barge adapted to receive water extracted from the body of water 2, means 3 to pass oxygen into the extracted water, and means 4 to pass the oxygenated extracted water back into the main body of water 2. The extracted water may be filtered before the introduction of oxygen there into. The oxygen may be produced by adsorption of nitrogen from ambient air using a molecular sieve. The oxygenated water is returned to the main body of water through two series of nozzles 15 arranged on opposite sides of the vessel below the waterline. <IMAGE>

Description

OXYGENATION The invention relates to oxygenation, and particularly to apparatus and a system for addition of oxygen to a body of water.

Bodies of water such as rivers are becoming increasingly important from an environmental, ecological and leisure standpoint. In addition to providing pleasing venues which the population at large can enjoy, they also support ecosystems in the form of fish, and flora and fauna. From all these aspects it is important that any particular river be as pollution-free as possible. This, however, is not always achievable particularly in terms of heavy rainfall.

Taking the tidal Thames as but one example, increased polluting loads are discharged into it at times of heavy rainfall. This can lead to a rapid decline in dissolved oxygen concentrations which, on occasion, has resulted in fish mortality. The problem is caused by the cumulative effect of surface run-off, increased flows from sewage treatment works and storm sewage discharges from the combined drainage of Inner London.

The reduction in the pollution load discharged to the Thames estuary and the consequential improvement in river water quality that has taken place over the last 25 years or so is widely known.

These improvements led to the return of large numbers of fish to the estuary but also focussed attention on the short duration problems caused by heavy rainfall that had previously been masked by the overall inferior condition of the river.

During the summer months it is normal for freshwater flows to the head of the estuary of the Thames to fall to less than 10 cumecs.

The flow in the freshwater Thames would be much greater but large quantities of water are abstracted for use as potable supplies.

This means that there is very little dilution available in the upper reaches of the estuary and that the net seaward movement of water is only about 2km/day. This long retention period results in most of the oxygen demand of any input to the river system being satisfied within the estuary which strictly limits the polluting load that can be accepted.

Heavy rainfall in the London area rapidly produces very large quantities of run-off due to the highly impermeable catchment. Much of this run-off is contaminated by the very nature of the urbanised area and is bound to drain to the Tideway either by way of the non tidal watercourses, the sewage treatment works or as overflows from the combined drainage system.

It is estimated that total storm flows to the Tideway from all sources may be as high as 15 x 10 m3 compared to daily dry weather flows of 3 x 10 m3. In terms of pollution load discharged to the river this may represend a threefold increase in daily Effective Oxygen Loads (EOL).

If the rainfall is widespread throughout the Thames catchment then flows at the head of the estuary will also rapidly increase, retention times in the estuary will decrease and the storm loads will be accommodated without any disastrous effect on dissolved oxygen concentration. If, however, rainfall is confined to the London area (which often happens in the case of severe summer storms) the increase in flows will only be of short duration and will not significantly affect retention times. Under these conditions the very large polluting load will exert an oxygen demand of such magnitude that dissolved oxygen levels will fall rapidly and may result in widespread fish mortality. Water plants may also die, and the overall pollutant effect is increased, often locally, which can present a health hazard.

This storm problem was originally thought to be entirely due to the direct discharges of storm sewage. It is now recognised however that it is a cumulative problem due to extra polluting loads from the sources previously mentioned (i.e. watercourses, sewage treatment works, storm sewage overflows). The individual significance of any one component from all these discharges varies according to the rainfall pattern. The oxygen sag caused by the various storm discharges will oscillate with the tide and will move slowly seaward. The sag remains as a discrete body of water for many days before finally being attenuated by mixing and dispersion.

The severity of the sag depends on many factors such as rainfall intensity and pattern, upland flow, river temperature and tidal state at time of discharge.

It is therefore an object of the invention to seek to mitigate these effects in a body of water.

According to a first aspect, the invention provides apparatus for addition of oxygen to a body of water, comprising a vessel adapted to receive water extracted from the body of water, means to pass oxygen into the extracted water, and means to pass the oxygenated extracted water back into the main body of water.

According to a second aspect of the invention, there is provided apparatus for addition of oxygen to a body of water, comprising a vessel adapted to receive water extracted from the body of water, means to filter the water whereby to remove any foreign body entrained therein, means to pass oxygen into the extrated and filtered water, and means to pass the oxygenated extracted water back into the main body of water.

The vessel may be adapted to be placed in, on or adjacent the body of water. This provides for convenient siting of the vessel and quick response.

The vessel may be flotable on and manoeuvrable over the body of water. This avoids the use of in situ fixed installations, or ones which have to be transported by road.

There may be means to produce oxygen by adsorption of nitrogen from ambient air. This provides a relatively cheap and accessible source of oxygen.

The oxygen production means may comprise a molecular sieve which may be adapted preferentially to adsorb nitrogen from the atmos- phere. This "releases" the oxygen for use in the apparatus.

The means to pass oxygen into the water may comprise a venturi.

This construction ensures a thorough mixing and adsorption of the oxygen into the water.

There may be a plurality of venturis each connected to a respective pump for pumping the extracted water to the venturis for admixture (adsorption) of the oxygen.

The filter means may comprise a well or sink in which the water extracted from the body of water is received and in which any foreign body in the water is removed. This constructions provides a "flotsam" free source of water to be oxygenated.

The well or sink may comprise a screen through which water passes and which is adapted to retain a foreign body in the well or sink.

This constructions provides a relatively simple retention means.

The screen may comprise a part of a wall of the well or sink.

The pumps may be positioned on the downstream side of the screen.

This arrangement provides that additional filters are not required for the pumps, so reducing the cost of the apparatus.

The means to pass the oxygenated. extracted water back into the main body of water may comprise a plurality of nozzles projecting from the vessel.

There may be two series of nozzles spaced apart on opposite sides of a hull of the vessel below the waterline thereof. In this way the nozzles may be an integral part of the vessel.

The pumps may be arranged to provide substantially half the total flow of extracted water along each side of the vessel, whereby to obviate or hinder yawing of the vessel in use. This construction provides for consistency of operation and eveness of oxygen distribution into the body of water.

The pumps may be adapted to operate continuously.

The pumps may be adapted to pass about 2,000 litres per second of extracted water through the well or sink.

The apparatus may be self-propelled by a motor means as an electrically powered diesel generator means for driving the vessel forward and in reverse, and for turning the vessel.

According to a third aspect the invention provides a system for managing a body of water such as a river, comprising means to monitor the oxygen content of the water at a series of spaced locations, means to communicate from those locations to a control, and a vessel as hereinbefore defined for passing oxygenated extracted water back into the main body of water at a location where oxygen enhancement is required.

Using the invention, it is possible to "manage" a river such that an injection rate of 10 tonnes of oxygen per day can produce an increase of up to 2 mgl dissolved oxygen concentration; stated in another way, using the invention dissolved oxygen levels have been maintained at sufficiently high levels to prevent fish mortality.

Apparatus embodying the invention is hereinafter described, by way of example, with reference to the accompanying drawings.

Fig 1 is a side elevational view, partly in phantcm, of apparatus according to the invention in the form of a self-contained and self-prcpelled floating barge; Fig 2 is a plan view of the apparatus of Fig 1; Fig 3 is a plan view of a level of the barge of Figs 1 and 2 below the main deck thereof; Fig 4 is a schematic view of an oxygen adsorption system of the barge of Figs 1 to 3; and Fig 5 shows graphically results of oxygen enhancement using the apparatus of Figs 1-4.

Referring to the drawings, there is shown apparatus for addition of oxygen to a body of water such as a river, comprising a vessel 1 in the form of a self-prcpelled boat or barge adapted to receive water extracted fron the body of water 2, means 3 to pass oxygen into the extracted water, and means 4 to pass the oxygenated extracted water back into the main body of water 2.

The vessel or barge 1 has a hull 5 of overall dimension of 50.5 metres length and 10 metres beam, and an on-board motor means for supply of power to the oxygen supply means, water extraction means and return means, and propulsion, in the form of three diesel alternator sets 6 (two "duty" and one back-up or stand-by) having a total capacity of over 3,000 Watts. The vessel 1 is electrically propelled by separate bow and stern 280 kW thruster units 7 each capable of turning through 3600 arc, providing an attainable speed of 8 knots and the facility to turn the vessel 1 in its own length.

The vessel thus has a high degree of manoeuvrability.

Oxygen is produced on board the vessel at the rate of 30 tonnes per day from ambient air at atmOspheric pressure by oxygen production means 8 in the form of a PSA (Pressure Swing Adsorption) plant.

The vessel 1 is adapted by a large aperture 9 in the bows 10 to receive water extracted from the river whence it passes to a well or sink 11 from which it is extracted by means in the form of four submersible pumps 12 in the water in the well 11 to means in the form of four venturis 13 where the extracted oxygen is injected from the PSA plant 8 into a high pressure stream of the river water from the pumps 12. The oxygen/water mixture, the oxygen being adsorbed into the water, is then passed via pipe-lines 14 to a series of injectors in the form of two spaced rows of nozzles 15 (corrprising the means 4) along either side of the hull 5.The nozzles 15 are integral with the hull, are below the water line and are angled downwards (as viewed in Fig 1) at 206. The nozzles 15 have a tapered outlet which injects the extracted, oxygenated water back into the river 2 so that bubbles of oxygen are shattered into a myriad of microbubbles which rapidly dissolve in the body of the river water before they reach the surface of the river. Thus there is no "leak" or waste to ambient atmosphere and all the injected oxygen is injected into the river, so oxygenating it, raising its oxygen content, and providing life support for fish and aquatic plants.

The well or sink 11, as shown in Fig 2, is a through one (the inboard) wall of which is defined by a screen 16. The screen 16 comprises a plurality of substantially vertical metal bars at a spacing of about 12 rrrn. This constructions provides a means to prevent coarse debris or flotsam from entering the well, and thus fram entering the venture system 13 or nozzle injection system 15.

The screen 16 also ensures that river water does not create acid imbalance in distribution systems of the venture and PSA systems, which can give rise to adverse handling characteristics from adverse pump performance.

The oxygen adsorption means 8 utilises the principle of specialised adsorbents to adsorb one component of a gas mixture preferentially.

A zeolite molecular sieve is used which has a higher adsorbent capacity for nitrogen than oxygen and is contained in three pressure vessels 17 or adsorber beds housed in a hold of the barge. Air is drawn from the atmosphere by the process air carpressor, then passed through a river water cooled heat exchanger to reduce the temperature of the air charge. Water droplets are removed in a mist eliminator before the pressurised air is fed to the adsorber beds where preferential adsorbtion of nitrogen occurs, giving a product gas better than 93% oxygen. When a bed becomes saturated with nitrogen a high vacuum is applied which draws off the nitrogen which is vented to the atmosphere. The three beds operate on a cyclic system to give a continuous supply of oxygen to the venturis via a buffer tank.

In order to use the apparatus 1 herebefore described, a plurality of monitoring stations are set up at a body of water to be managed, say along a river. The stations include continuously recording oxygen probes, and readings are monitored and transmitted by a suitable communication system such as by radio, telephone, satellite or the like to a control point. When the dissolved oxygen content of the river is perceived to be falling to a dangerously low level at a particular site, the control point either manually or automatically notifies the crew of the vessel 1 which then proceeds to the site, fine positioning of the vessel at the site being achieved by determination of readings from on-board dissolved oxygen meters.

Water is then extracted from the river into the well 11, passed by the pumps to the venturis 13 where it is mixed with the oxygen produced by the on board system extraction means 8, and is then passed back into the river through the nozzles 15, of which there are 160 in the embodiment. The river is enriched with oxygen at the site and when the oxygen level has reached an acceptable level the vessel moves to another site, or back to base, as required.

Fig 5 shows a graph the plots A, B and C being plots of dissolved oxygen (as a percentage of saturation level) against distance in km from a particular point, plot A shows a "normal" reading (acceptable) before a storm, plot B shows the reading after a storm, and after proposed use of the vessel, and plot C shows the level without use of the vessel. It can be seen that graph C shows a dangerously low oxygen level, whilst plot B shows that the oxygen level can be maintained to an acceptable level using a vessel 1 according to the invention.

Claims

1. Apparatus for addition of oxygen to a body of water, comprising a vessel adapted to receive water extracted from the body of water, means to pass oxygen into the extrated water, and means to pass the oxygenated extracted water back into the main body of water.

2. Apparatus for addition of oxygen to a body of water, comprising a vessel adapted to receive water extracted from the body of water, means to filter the water whereby to remove any foreign body entrained therein, means to pass oxygen into the extracted and filtered water, and means to pass the oxygenated extracted water back into the main body of water.

3. Apparatus according to Claim 1 or Claim 2, the vessel being adapted to be placed in, on or adjacent the body of water.

4. Apparatus according to Claim 3, the vessel being flotable on and manoeuvrable over the body of water.

5. Apparatus according to Claim 4, comprising means to produce oxygen by adsorption of nitrogen from ambient air.

6. Apparatus according to Claim 5, the oxygen production means comprising a molecular sieve which is adapted preferentially to adsorb nitrogen from the atmosphere.

7. Apparatus according to any of Claims 5 to 6, the means to pass oxygen into the water comprising a venturi.

8. Apparatus according to Claim 7, there being a plurality of venturis each connected to a respective pump for pumping the extracted water to the venturis for admixture of the oxygen.

9. Apparatus according to Claim 8, when appendant on any one of Claims 2 to 7, the filter means comprising a well or sink in which the water extracted from the body of water is received and in which any foreign body in the water is removed.

10. Apparatus according to Claim 9, the well or sink comprising a screen through which water passes and which is adapted to retain a foreign body in the well or sink.

11 Apparatus according to Claim 10, the screen comprising a part of a wall of the well or sink.

12. Apparatus according to Claim 10 or Claim 11, the pumps being positioned on the downstream side of the screen.

13. Apparatus according to any of Claims to 12, the means to pass the oxygenated extracted water back into the main body of water comprising a plurality of nozzles projecting from the vessel.

14. Apparatus according to Claim 13, there being two series of nozzles spaced apart on opposite sides of a hull of the vessel below the waterline thereof.

15. Apparatus according to any of Claims 12 to 14, the pumps being arranged to provide substantially half the total flow of extracted water along each side of the vessel, whereby to obviate or hinder yawing of the vessel in use.

16. Apparatus according to any of Claims 12 to 15, the pumps being adapted to operate continuously.

17. Apparatus according to Claim 16, the pumps being adapted to pass about 2,000 litres per second of extracted water through the well or sink.

18. Apparatus according to any preceding Claim, which is self -prcpelled by a rrotor means.

19. Apparatus according to Claim 18, the motor means comprising electrically powered diesel generator means for driving the vessel forward and in reverse, and for turning the vessel.

20. Apparatus for addition of oxygen to a body of water, substantially as hereinbefore described with reference to the accoupanying drawings.

21. A system for managing a body of water such as a river, comprising means to monitor the oxygen content of the water at a series of spaced locations, means to communicate from those locations to a control, and a vessel according to any preceding Claim for passing oxygenated extracted water back into the main body of water at a location where oxygen enhancement is required.