WO2011026758A1

WO2011026758A1 - Method for purifying water flowing in a river or canal

Info

Publication number: WO2011026758A1
Application number: PCT/EP2010/062288
Authority: WO
Inventors: Regina Kawai
Original assignee: Evonik Degussa Gmbh
Priority date: 2009-09-02
Filing date: 2010-08-24
Publication date: 2011-03-10
Also published as: AR078243A1; BRPI0903291A2

Abstract

Water flowing in a river or canal is purified by a step of adding hydrogen peroxide continuously or intermittently to the flowing water in an amount of at least 0.1 mg/l through an installation providing a multitude of addition points across the width of the river or canal and a step of floatation carried out downstream of the step of adding hydrogen peroxide after at least 70 % of the added hydrogen peroxide has been consumed, the step of floatation comprising adding a metal salt of Al³⁺ or Fe³⁺ as a coagulant, adding a polymeric flocculant, injecting air bubbles for floatation and removing the sludge floating on the water surface.

Description

Method for purifying water flowing in a river or canal

Field of the invention

The invention relates to a method for purifying water flowing in a river or canal to make it suitable for

transferring it to a reservoir for potable water.

Background of the invention

Water reservoirs are often used for several purposes at the same time, such as obtaining potable water, obtaining water for irrigation purposes, providing recreational areas and providing hydroelectric power. Such multiple use leads on the one hand to the requirement of high water quality for recreational use and for obtaining potable water and on the other hand to the requirement of a large water throughput for irrigation and for providing hydroelectric power.

One practical way of increasing water throughput of a reservoir is to transfer water from a river or canal to the reservoir. Such a transfer also allows use of the reservoir as a device for storing hydroelectric power by pumping water from a river or canal using surplus hydroelectric power at times of low power demand and generating

additional hydroelectric power from the reservoir at times of high power demand. However, the amount of water that can be transferred from a river or canal without deteriorating the water quality in the reservoir is often limited by pollution of the water in the river or canal.

Purifying water before transferring it to a reservoir allows an increase in water throughput of a reservoir without deteriorating the water quality in the reservoir. Therefore, there is a need for methods for purifying water flowing in a river or canal to make it suitable for

transferring it to a reservoir for potable water.

BR 9702430 discloses a process for purifying water flowing in a river or canal by a continuous step of floatation, which comprises adding a coagulant to the flowing water, followed by adding a flocculant, injecting air bubbles for floatation and removing the sludge floating on the water surface. This process has found application for purifying water flowing in the Pinheiros river at Sao Paulo before transferring it to the Billings reservoir. However, the degree of purification achieved by this process is not satisfactory and still limits the amount of water that can be transferred to the reservoir. Therefore, there is still a need for improving the efficiency of this process. Hydrogen peroxide has been used to combat or prevent starvation of dissolved oxygen in polluted ponds, lakes and rivers, which can lead to the killing of fish and in the extreme to anaerobic conditions, where toxic and intensely smelling hydrogen sulphide is formed by anaerobic bacteria. For this purpose, hydrogen peroxide is added batch-wise to provide dissolved oxygen by the slow decomposition of hydrogen peroxide. Such an addition of hydrogen peroxide is more efficient than aeration of the water, because the slow decomposition can provide amounts of dissolved oxygen in excess of oxygen solubility. CN 101 088 927 discloses such a process for purifying contaminated river or lake water, where hydrogen peroxide is stirred into the water in an amount of from 50 to 5000 ppm from a motor vessel that is driven across the river or lake. CN 101 139 129 discloses a method for killing and

decomposing blue algae in a pond, a lake or a river stretch by spraying hydrogen peroxide in an amount of 1000 to

35000 ppm onto the water. The document also teaches to flocculate the decomposition products formed from the algae by spraying an aluminium sulphate solution onto the same area in an amount of from 5 to 2000 ppm after all the hydrogen peroxide has reacted. The problem of excessive blue algae growth occurs only in stagnant waters and not in rivers having a substantial water flow. The method of CN 101 139 129 using the aluminium sulphate solution is also adapted to treating stagnant water and is not useful for treating a river with flowing water.

US 6,596,176 discloses a process for treating water to be made potable by adding hydrogen peroxide to untreated water and shortly thereafter adding a metallic coagulant

comprising between 1 to 10 parts

Polydimethyldiallylammionium chloride and between 1 to 5 parts metal salt. The metal salt can be a salt of Al³⁺ or Fe³⁺. US 6,596,176 teaches a synergistic effect between hydrogen peroxide and the metallic coagulant due to a rapid reaction between both components occurring within 30 to 60 s. The process of US 6,596,176 therefore requires mixing the water comprising hydrogen peroxide with the metallic coagulant before a substantial amount of the hydrogen peroxide is consumed. Furthermore, the rapid reaction of hydrogen peroxide and metallic coagulant requires a rapid mixing of the water comprising hydrogen peroxide with the metallic coagulant. In practice, this can be achieved only by adding hydrogen peroxide and the metallic coagulant to a pipeline though which the water flows as shown in figure 1 of US 6,596,176, but not in a river or canal. This makes the process of US 6,596,176 unsuitable for treating water flowing in a river or canal.

It has now been found that the efficiency of the process known from BR 9702430 can be surprisingly improved by adding hydrogen peroxide in a suitable manner to the flowing water and carrying out a step of coagulation, flocculation and floatation downstream of the step of adding hydrogen peroxide after at least 70 % of the added hydrogen peroxide has been consumed. Summary of the invention

The present invention relates to a method for purifying water flowing in a river or canal, comprising a step of adding hydrogen peroxide continuously or intermittently to the flowing water in an amount of at least 0.1 mg/1 through an installation providing a multitude of addition points across the width of the river or canal and a step of floatation carried out downstream of the step of adding hydrogen peroxide after at least 70 % of the added hydrogen peroxide has been consumed, the step of floatation

comprising adding a metal salt of Al³⁺ or Fe³⁺ as a

coagulant, adding a polymeric flocculant, injecting air bubbles for floatation and removing the sludge floating on the water surface.

Detailed description of the invention

The method of the present invention comprises a step of adding hydrogen peroxide continuously or intermittently to the flowing water.

Preferably, hydrogen peroxide is added continuously. If hydrogen peroxide is added intermittently, the product of the time interval interrupting hydrogen peroxide addition in seconds and of the average water flow in m/s is

preferably less than 200 m, more preferably less than

100 m. Selecting such short time intervals for interrupting hydrogen peroxide addition ensures that essentially all of the water has been treated with hydrogen peroxide before it reaches the location where the step of floatation is carried out downstream of the step of adding hydrogen peroxide . In the method of the present invention hydrogen peroxide is added in an amount of at least 0.1 mg/1, preferably in an amount of from 1 to 50 mg/1. At least 70 % of the hydrogen peroxide added to the flowing water will be consumed in the water, either by decomposition with formation of oxygen dissolved in the water or by reaction with oxidisable compounds present in the water. The decomposition of hydrogen peroxide occurs by catalytic action of substances naturally present in the water. The amount of hydrogen peroxide that is added is preferably selected to achieve a level of dissolved oxygen at the location where the step of floatation is carried out downstream of the step of adding hydrogen peroxide of at least 2 mg/1, more preferably at least 3 mg/1. In the method of the present invention hydrogen peroxide is added through an installation that provides a multitude of addition points across the width of the river or canal. The installation preferably provides from 2 to 30 addition points across the width of the river or canal and the distance between addition points across the river or canal is preferably from 1 to 20 m. The installation may be submerged in the water with the hydrogen peroxide feed to the addition points through a conduit. Preferably the conduit is running at the bottom of the river or canal and has flexible hoses carrying floats connected to the conduit to provide the addition points for the hydrogen peroxide near the water surface, preferably less than 1 m below the water surface. Alternatively, the installation may be floating on the river or canal with the installation anchored at the bottom or the shore of the river or canal to keep it in place. In a third alternative, the

installation may be arranged as a bridge or as part of a bridge across the river or canal with conduits for hydrogen peroxide leading from the bridge into the river or canal at the addition points. In a preferred embodiment of this third alternative, the conduits for hydrogen peroxide comprise flexible hoses reaching down into the river or canal to avoid damaging of the structure by objects

floating down the river or canal. The use of a multitude of addition points across the width of the river or canal provides for better initial distribution of the added hydrogen peroxide in the flowing water and leads to an improved efficiency of the step of floatation carried out downstream. At the same time, harmful effects of high hydrogen peroxide concentrations on organisms living in the water are avoided.

The method of the present invention further comprises a step of floatation carried out downstream of the step of adding hydrogen peroxide. The step of floatation comprises adding a metal salt of Al³⁺ or Fe³⁺ as a coagulant, adding a polymeric flocculant, injecting air for floatation and removing the sludge floating on the water surface.

The addition of a metal salt of Al³⁺ or Fe³⁺ to the water leads the formation of finely dispersed hydroxides or hydrated oxides of aluminium or iron, which act as

coagulants for colloidal particles. Preferably, a water soluble metal salt of Al³⁺ or Fe³⁺ is used. A suitable metal salt of Al³⁺ is aluminium sulphate. Preferably, a metal salt of Fe³⁺ is added as a coagulant. A suitable metal salt of Fe³⁺ is iron (III) chloride. The metal salt is preferably added in an amount of from 5 to 200 mg/1, more preferably from 10 to 130 mg/1, of metal. The metal salt is preferably added to the flowing water.

The coagulated particles, which are still finely dispersed, are then flocculated by the addition of a polymeric

flocculant. Preferably, a cationic polymer is used as the flocculant. Suitable flocculating polymers are known to a person skilled in the art from the fields of sewage

treatment and potable water purification. The polymeric flocculant is preferably added in an amount of from 0.1 to 5 mg/1. After the addition of the polymeric flocculant, air bubbles are injected for floatation and the sludge floating on the water surface is removed. The air bubbles may be injected by direct injection of compressed air through a device which forms small air bubbles, preferably of less than 100 ym. In a preferred embodiment, however, air is

dissolved in water under pressure and the air saturated water is injected through nozzles submerged in the water to form air microbubbles by depressurization . Suitable

equipment for providing and injecting air saturated

pressurized water is known from US 5,989,437. The air bubbles attach to the flocculated particles, which then rise to the water surface and form a sludge floating on the water surface. The sludge floating on the water surface is then removed by any suitable means, such as mechanical skimmers, rotating blades or nozzles located at the water surface for pumping off the sludge. Preferably, a barrier for collecting the sludge is arranged across the width of the river or canal to facilitate removal of the sludge. In a preferred embodiment, the step of flotation is carried out in the flowing water by adding the metal salt, adding the polymeric flocculant and injecting air bubbles at consecutive positions along the river or canal.

The step of flotation carried out downstream of the step of adding hydrogen peroxide is carried out after at least 70 % of the added hydrogen peroxide has been consumed.

Preferably, at least 80 % and more preferably at least 90 % of the hydrogen peroxide has been consumed before the step of flotation is carried out. Preferably, the step of adding hydrogen peroxide and the step of flotation carried out downstream are carried out at a distance along the river which is larger than the product of the average water flow in m/s and of a time of 1000 s. More preferably, the distance along the river is larger than the product of the average water flow in m/s and of a time of 5000 s. Most preferably, the distance along the river is larger than the product of the average water flow in m/s and of a time of 30000 s.

The process of the invention leads to an increase in the efficiency as compared to the process known from

BR 9702430, in that a higher fraction of the organic matter present in the flowing water is removed. Surprisingly, the fraction of organic matter that is removed by the step of flotation is increased by the addition of hydrogen peroxide and contrary to what could be expected from the teachings of US 6,596,176, this increase is higher the more of the hydrogen peroxide is consumed before the step of flotation is carried out. Without wishing to be bound by theory, it is believed that the enhanced removal of organic matter in the floatation step is not due to a direct oxidation of organic matter by hydrogen peroxide, but to a microbial action on the organic matter effected by the dissolved oxygen that was supplied through the addition of hydrogen peroxide . In a preferred embodiment, the water comprises Fe²⁺ prior to the addition of hydrogen peroxide. The amount of Fe²⁺ is preferably from 1 to 40 mg/1, preferably from 2 to 30 mg/1, of Fe²⁺. The Fe²⁺ may be naturally present in the water, it may be present as a result of a water contamination or it may be added on purpose to the water upstream of the step of adding hydrogen peroxide. Preferably, the Fe²⁺ is naturally present in the water. The presence of Fe²⁺ prior to the addition of hydrogen peroxide further improves on the efficiency of removing organic matter in the floatation step and allows to reduce the amount of metal salt used in the step of floatation carried out downstream without losing efficiency in the water purification.

In a preferred embodiment, the method of the present invention further comprises an additional step of adding a metal salt of Al³⁺ or Fe³⁺ as a coagulant, adding a polymeric flocculant, injecting air bubbles for floatation and removing the sludge floating on the water surface upstream of adding hydrogen peroxide. This additional step may be carried out in the same manner as the step carried out downstream of the step of adding hydrogen peroxide. Preferably, the same metal salt is used upstream and downstream of the step of adding hydrogen peroxide. The additional floatation step carried out upstream of the step of adding hydrogen peroxide reduces the amount of hydrogen peroxide that is necessary to carry out the invention and provides higher levels of dissolved oxygen in the purified water using less hydrogen peroxide. It also has the effect of providing purified water having a lower content of organic matter. In a preferred embodiment of the method of the present invention, the purified water is pumped to a reservoir for potable water. Purifying water by the method of the

invention before pumping it to the reservoir reduces algae growth in the reservoir and reduces the effort necessary to obtain potable water from the reservoir.

Preferably, at least a part of the water pumped to the reservoir is used for hydroelectric energy generation.

Purifying water by the method of the invention before pumping it to the reservoir allows to pump a larger amount of river water to the reservoir without deteriorating water quality in the reservoir and thereby increases the

electrical power during storage of hydroelectric energy.

The following examples illustrate the invention, but do not limit the scope of the invention.

Examples Example 1

A water sample was taken from the Pinheiros river at Sao Paulo upstream of the water treatment installations installed in the river. The water had a content of

dissolved oxygen of 0.3 mg/1, a content of Fe²⁺ of

6.6 mg/1, a turbidity of 15.7 NTU and a biological oxygen demand of 17 mg/1. 50 % aqueous hydrogen peroxide was diluted with distilled water to a concentration of 0.05 % by weight and added to the river water sample in an amount of 4.7 mg/1 hydrogen peroxide. 155 min after the addition of hydrogen peroxide, the water sample had a content of hydrogen peroxide of 0.3 mg/1, a content of dissolved oxygen of 3.3 mg/1, a content of Fe²⁺ of 1.7 mg/1, a turbidity of 11.8 NTU and a biological oxygen demand of 16 mg/1.

Example 2

Example 1 was repeated with water taken from the Pinheiros river at Sao Paulo downstream of a water treatment

installation where floatation is carried out by adding iron (III) chloride as a coagulant, adding a cationic polymeric flocculant, injecting pressurized air saturated water for floatation and removing the sludge floating on the water surface. The water had a content of dissolved oxygen of 0.2 mg/1, a content of Fe²⁺ of 6.4 mg/1, a turbidity of 9.5 NTU and a biological oxygen demand of 23 mg/1. 5.0 mg/1 hydrogen peroxide was added to the water sample. 151 min after the addition of hydrogen peroxide, the water sample had a content of hydrogen peroxide of 0.5 mg/1, a content of dissolved oxygen of 3.4 mg/1, a content of Fe²⁺ of 0.7 mg/1, a turbidity of 9.0 NTU and a biological oxygen demand of 17 mg/1.

Claims

Claims :

1. A method for purifying water flowing in a river or canal, comprising a) a step of adding hydrogen peroxide continuously or intermittently to the flowing water in an amount of at least 0.1 mg/1 through an installation providing a multitude of addition points across the width of the river or canal and b) a step of floatation carried out downstream of the step of adding hydrogen peroxide after at least 70 % of the added hydrogen peroxide has been consumed, comprising adding a metal salt of Al³⁺ or Fe³⁺ as a coagulant, adding a polymeric flocculant, injecting air bubbles for floatation and removing the sludge floating on the water surface .

2. The method of claim 1, wherein hydrogen peroxide is added continuosly.

3. The method of claim 1, wherein hydrogen peroxide is added intermittently and the product of the time interval interrupting hydrogen peroxide addition in seconds and of the average water flow in m/s is less than 200 m, preferably less than 100 m.

4. The method of any one of claims 1 to 3, wherein the step of adding hydrogen peroxide and the step of flotation carried out downstream are carried out at a distance along the river which is larger than the product of the average water flow in m/s and of a time of 1000 s.

5. The method of any one of claims 1 to 4, wherein

hydrogen peroxide is added in an amount of from 1 to 50 mg/1.

6. The method of any one of claims 1 to 5, further comprising an additional step upstream of adding hydrogen peroxide, said step comprising adding a metal salt of Al³⁺ or Fe³⁺ as a coagulant, adding a polymeric flocculant, injecting air bubbles for floatation and removing the sludge floating on the water surface.

7. The method of any one of claims 1 to 6, wherein a

metal salt of Fe³⁺ is added as a coagulant.

8. The method of any one of claims 1 to 7, wherein the metal salt is added in an amount of from 5 to

200 mg/1, preferably from 10 to 130 mg/1, of metal.

9. The method of any one of claims 1 to 8, wherein the polymeric flocculant is a cationic polymer.

10. The method of any one of claims 1 to 9, wherein the polymeric flocculant is added in an amount of from 0.1 to 5 mg/1.

11. The method of any one of claims 1 to 10, wherein air bubbles are injected by injecting water saturated with air under pressure through nozzles submerged in the water.

12. The method of any one of claims 1 to 11, wherein the water prior to addition of hydrogen peroxide comprises from 1 to 40 mg/1, preferably from 2 to

30 mg/1, of Fe²⁺.

13. The method of any one of claims 1 to 12, wherein the purified water is pumped to a reservoir for potable water.

14. The method of claim 13, wherein at least a part of the water pumped to the reservoir is used for hydroelectric energy generation.