WO1991007354A1

WO1991007354A1 - Water treatment method

Info

Publication number: WO1991007354A1
Application number: PCT/GB1989/001351
Authority: WO
Inventors: Alban Timmons
Original assignee: Irving Ainsworth (Holdings) Limited
Priority date: 1989-11-15
Filing date: 1989-11-15
Publication date: 1991-05-30
Also published as: ES2021489A6; CA2006512A1

Abstract

The invention provides a method for the purification of contaminated water such as leachate from a land fill site. The treatment involves the application to the water of anionic and cationic coagulents at different stages, and between the application of the coagulents precipitation agents are added to precipitate the contaminants which typically are metal ions in solution. The coagulants are polyelectrolytes which in the case of the first added coagulant cause formation of a floc and the addition of the second coagulant causes heavy deposition of contaminants. Separation follows and the separated solids are passed to a sludge thickening tank whilst the separated liquid which is of high pH is stripped of ammonia and other nitrogenous compounds. The pH is subsequently reduced to zero and subsequently the carbon dioxide is removed. The remaining liquid can either be subjected to a bio-oxidation treatment or filtered by reverse osmosis in the latter case to provide clean water which can be returned to a stream or river. The invention also provides a novel form of stripping of ammonia and carbon dioxide and a vessel for use in the same.

Description

Water Treatment Method

The present invention relates to an improved process for the treatment of contaminated water such as household or industrial sewage or leachate from a waste material dump or land fill site.

It is well known that metal ions dissolved in water can be removed by the addition of various treatment agents which react with the metal ions and precipitate them as insoluble salts. This is of use in the treatment of contaminated water when for example the addition of calcium hydroxide (lime) to hard water will cause the precipitation of metal carbonates and so remove the metal and bicarbonate ions from the water. This precipitation process can be further improved by the addition to the water of a polyelectrolyte which promotes the flocculation of the solid particles and a weighting agent such as calcium carbonate which increases the specific gravity of the flocculated matter and therefore increases the rate of sedimentation and thus the rate of clarification of the liquor.

This process can be used for the removal of most metal ions from contaminated water. In addition to calcium hydroxide, other chemical treatment agents such as sodium hydroxide and sodium carbonate may also be added to the solution. Whilst calcium hydroxide reacts effectively with bicarbonate ions, sodium carbonate reacts with for example metal sulphates to precipitate metal carbonates.

British Patent No 2095226 describes a composition which is of use in the purification of water and which contains an alkaline earth metal hydroxide and an anionic oligomeric polyelectrolyte and may additionally contain a weighting agent and a cationic polyelectrolyte. However, although it stabilises the composition, the anionic polyelectrolyte is not particularly effective as a flocculant and when a cationic polyelectrolyte is also used, the anionic polyelectrolyte may counteract its effect. The amount of calcium hydroxide or sodium carbonate added to the water will depend on the concentration of bicarbonate or sulphate ions in the water.

In British Patent No 2157278 there is described a method of treating water using a composition containing calcium sulphate as a weighting agent, an electrolyte having a multivalent cation such as iron (III) or aluminium and a cationic or anionic polyelectrolyte. This composition is primarily of use in treating soft water or water with a low concentration of bicarbonate ions and in this treatment, the metal cation will react to form an insoluble hydroxide which absorbs suspended solids. In all the examples described in this patent the polyelectrolyte used is cationic and it is stated that cationic polyelectrolytes stabilise the compositions. However, the polyelectrolytes do not necessarily give the best flocculation effects under the conditions described.

The approach to treatment of contaminated water and effluent has in the past comprised the precipitation of the metal salts as described above followed by their separation and then the removal of unwanted gases or vapours from the liquor by storing the liquor in a shallow lagoon where it is aerated. This process is not particularly efficient because the shallow depth of liquor in the lagoon does not allow the oxygen supplied to the liquor to be under sufficient pressure to saturate the liquor.

It is an object of at least a preferred aspect of the present invention to provide a method of purification of contaminated water which is simple to operate, provides effective flocculation of the solid salts precipitated when chemical treatment agents are added to the solution and effective aeration and removal of unwanted vapours or gases.

In a first aspect of the present invention there is provided a method of treating water comprising the steps of adding to the water agents to cause precipitation of impurities and separating out said impurities, characterised in that the agents for causing precipitation are added sequentially and comprise but not necessarily in this order, a batch of an anionic or cationic coagulant; agent(s) which react with dissolved metal ions to form insoluble salts; and a batch of coagulant of opposite charge to the first batch.

The anionic coagulant may be selected from anionic polyacids and salts therof and may be for example an alkali metal salt of a simple or complex oligomer of acrylic or methacrylic acid, low-viscosity sodium carboxymethylcellulose or an oligomeric sulphonate.

The cationic coagulant may be a polyamide.

Preferably, the one coagulant (preferably the anionic coagulant) is added to the incoming contaminated water followed by the said agents, and the liquor is then passed to a preflocculation tank where it is thoroughly mixed. After the preflocculation stage, the other coagulant is added to the liquor to cause precipitation of the insoluble salts which are then separated out. The coagulants and agents are preferably added to the contaminated water via vortex valves to ensure thorough mixing, the vortex valve at which the said other batch of coagulant is added being positioned between the preflocculation tank and the separation vessel. The method of adding the respective coagulants before and after the other agents produces a much denser precipitate which is much more easily separated from the liquor. This is thought to be because the addition of the first coagulant induces a charge on the particles suspended in the liquor and so when a coagulant of the opposite charge is added, precipitation and coagulation of the charged particles is faster and more efficient.

The treatment agents used may be an alkaline earth metal hydroxide and calcium carbonate as a weighting agent. Preferably the alkaline earth metal hydroxide is calcium hydroxide. Other treatment agents such as sodium carbonate or sodium hydroxide may also be used.

The treatment agents described above are especially useful in the treatment of water which has a high concentration of bicarbonate ions.

After the addition of the treatment agents mentioned above, the pH of the liquor will preferably be in the range of 10.5 to 13.

After precipitation and removal of the suspended solids the remaining liquor may be aerated and stripped of ammonia and other nitrogenous compounds, the pH may then be adjusted to between 6.5 and 7.5 by the addition of acid and the liquor may then be aerated and stripped of carbon dioxide.

After the addition of the acid to neutralise the liquor, instead of aerating and removing the carbon dioxide the solution may be bio-oxidised by the addition of activated sewage sludge to the liquor to consume residual organic matter and the aeration of the liquor. The humus resulting from this process can then be separated from the water. Other treatment agents may also be used in the water treatment process of the invention and these may comprise an electrolyte having a multivalent cation and calcium sulphate as weighting agent.

The multivalent metal cation may comprise iron (III) or aluminium and the electrolyte may be iron (III) sulphate, iron (III) chloride or aluminium sulphate. These reagents are particularly suitable for the treatment of contaminated water which has a low concentration of bicarbonate ions.

After the separation of the suspended solid impurities, the liquor may be bio-oxidised by the addition of activated sewage sludge to consume residual organic matter and the aeration of the liquor. The humus formed by this process may then be separated from the purified water.

In a second aspect of the invention there is provided a method for treating contaminated water comprising adding to said water treatment agents, raising the pH of the liquor to between 10.5 and 13, separating out the solids suspended in the liquor and stripping off ammonia and other nitrogenous compounds contained in said liquor.

In a third aspect of the invention there is provided a method for treating contaminated water comprising adding to said water treatment agents, separating out the solids suspended in the liquor and stripping off the carbon dioxide dissolved in the liquor.

The purified water obtained from any of the methods described may be further treated by passing it through a carbon filter and/or carrying out reverse osmosis.

Once purified water is obtained, it may be discharged to a watercourse or used for other purposes such as agriculture or horticulture.

In a fourth aspect of the invention there is provided a method for use in removing ammonia and other nitrogenous compounds, or removing carbon dioxide or carrying out a bio- oxidation process during the treatment of water, characterised in that the method is carried out in a vessel having an aspect ratio of at least 2:1 and supplying water to the top of the vessel and removing water from the bottom of said vessel and at the same time aerating the water.

The vessel may be provided with means for regulating the height of the head of water contained within the vessel and the water may be sprayed into the top of the vessel to provide additional aeration.

A vessel as described above with an aspect ratio (height:largest cross sectional dimension) of at least 2:1 provides more efficient aeration of the liquid inside it than does a lagoon of the type previously used. This is because the depth of liquid within the vessel has a head of pressure and so the oxygen supplied to the bottom of the vessel will be pressurised and so it will be easier to saturate the liquid with oxygen and to force out any unwanted gases.

In a further aspect of the invention there is provided a method of aerating water, removing unwanted dissolved gases and bio-oxidising water to be treated comprising supplying the water to a vessel as described above, and supplying air to the water via the aeration means such that air bubbles rise through the solution and assist in the removal of dissolved gases.

This method of scrubbing ammonia or carbon dioxide or bio- oxidising water to be treated may be used in any of the water treatment processes described above.

The methods of water treatment described may be used for the removal from water of such contaminants as bicarbonate, sulphate, carbonate or other anions, calcium, magnesium, beryllium, iron, mercury, molybdenum, silver, tin, zinc, manganese, barium, cadmium, lead or other metals, inorganic substances such as arsenic or selenium or soluble or insoluble organic substances.

The invention will now be further described with reference to the accompanying diagrammatic drawings wherein:-

Fig. 1 is a circuit diagram of a water processing system;

Fig. 2 is a plan view of part of the systemof Fig. 4;

Fig. 3 is a front view of the part of the system of Fig. 2; and

Fig. 4 is a circuit diagram of an alternative water processing system.

Firstly, with reference to the Figs. 1, 2 and 3, contaminated water such as household or industrial sewage or leachate from a landfill site enters the system via a pipe 10 having a stop valve 11. At a vortex valve 12, a coagulant which may be either anionic or cationic is added to the incoming liquor and at a second vortex valve 14 a mixture of calcium carbonate and calcium hydroxide is also added to the liquor. The function of the coagulant is to induce a charge in particles suspended in the liquor and the function of the calcium hydroxide is to react with metal irons dissolved in the liquor and precipitate them. The calcium carbonate is a weighting agent. The liquor is then caused to flow tangentially into a preflocculation tank 16 where it is mixed and where a floe forms. The liquor then passes via a line 17 from the base of tank 16 and tangentially into a separator 18 which is preferably of the type described in British Patent Application No. 2205512 via a vortex valve 20 through which a second coagulent is added. This second coagulent is of opposite charge to the first coagulent and therefore is extremely effective in precipitating the particles in which a charge has been induced by the first coagulent. The solid particles dispersed in the solution are removed using the separator 18 and the sludge leaving the bottom of the separator is discharged via line 19 to a sludge thickener 22 from which sludge can be extracted via outlet 22A.

The pH of the water has been adjusted purposely to make it alkaline (about 12) by the addition of the treatment agents, calcium hydroxide and calcium carbonate.

Clear water may be taken from the top of the separator 18 and the sludge thickener 22 via line 21 and 23 and passed to an ammonia stripping vessel 24. Each of vessel 16 and 18 has a float trap 7. The water is sprayed into the top of the vessel 24 over a splash plate 24A in order to aerate it as much as possible and also, air is passed into the bottom of the tank at 26. The tank should have an aspect ratio (height:maximum - cross sectional dimension e.g. diameter) of at least 2:1 in order to ensure that the water at the bottom of the vessel and the air as it enters the vessel are supplied under pressure. This enables air to move up through the liquid in the vessel slowly so that the liquid becomes saturated with oxygen and the ammonia and other nitrogenous compounds are forced out of the liquid and are vented to the atmosphere. The water therefore becomes progressively cleaner as it moves down the vessel. The water taken from the bottom of the vessel at 28 still has a pH of about 12 and therefore a mineral acid such as hydrochloric or sulphuric acid is added at a vortex valve 30 (Fig. 1 only) to neutralise the liquor. The neutralised liquor may then be passed via line 25 to a second stripping vessel 32 which is of the same design as vessel 24 and either may be aerated as indicated at 27 so as to remove the carbon dioxide dissolved in the water or the water may be bio-oxidised by the addition of activated sewage sludge to the water accompanied by the aeration so that the organisms in the sewage sludge consume the remaining organic matter in the water and die once they have completed their life cycle to leave humus. The humus liquor can then be removed via line 29 to a separating vessel 34 in which the humus is removed from the water. Separating vessel 34 may be of the same design as vessel 18.

The water from vessel 32 may be taken via line 31 to a reverse osmosis treatment 33 from whence the treated water is clean enough to be returned to a stream or river.

The method described above is extremely effective for the purification of water contaminated with household or industrial sewage or with metals, inorganic or organic impurities.

Significant purification of the liquor has been achieved by the time it leaves the separator 18 and table 1 gives a list of contaminants and their concentration in the water on entering the system and their concentration in the water on leaving the separator 18. TABLE 1

Analysis Units Start Concentration

Concentration after separation

BODr mg/1 433 105

Total suspended solids mg/1 400

Ammonia as nitrogen mg/1 110 78

Total dissolved solids mg/1 2308 5344

True colour colour 500 35 units

It can be seen therefore that the water is considerably purified by the time it reaches the ammonia stripping vessel. Soluble metal salts etc can be removed from the water by reverse osmosis in a final purification stage.

Fig. 4 shows a system for the treatment of water which is similar to the first system in that water enters the system at pipe 10, a first coagulant and various reagents are added at vortex valves 12 and 14 and the mixture passes through a preflocculation tank 16 and via a vortex valve 20 where the second coagulent is added to a separator 18. However, the pH of the water treated in this system is adjusted so as not to be higher than pH 9 and therefore the water from the separator which is considerably purified as shown in table 1 can proceed directly to bio-oxidation in a tank 36 to which activated sewage sludge may be added and which may be aerated at 38. After bio-oxidation the humus can be removed from the water as before in a separator.

The first and second coagulants preferably are polyelectrolytes as used in the method described in relation to Fig. 1, but the agents added at vortex valve 14 may be for example lime or lime carbonate as the water to be treated will probably be softer than the hard water which is treated in the Fig. 1 process. However, any of the features of the Fig. 1 process may be embodied in the process described with reference to Fig. 4. Thus, the liquor from the treatment tank 36 may have its acidity adjusted to a neutral level i.e. of the order of pH 7, and the treated liquor may then be passed to another treatment tank as shown by reference 40 in Fig. 4 for example for the removal of carbon dioxide, as described in relation to Fig. 1 or for further bio-oxidation processes. Again, air may be bubbled through the tank 40 and the resulting liquor may be passed to reversed osmosis treatment to provide ultra clean water which can be returned to a river or stream.

In the processes according to the invention, the quantity of coagulant and/or agents which is or are used will depend upon the composition of the original contaminated water to be treated. It will be appreciated that the nature of the contaminants in the water to be treated will vary depending upon the site from which the water eminate. Obviously industrial and domestic sewage liquids will vary in contaminants and these in turn will vary from land fill leachates. Also the quantity of contaminants in the respective waters to be treated will demand that in some cases more or less of the polyelectrolyte and treatment agents will be used, and indeed the particular ones of the polyelectrolytes and agents which are used will to some extent depend upon the nature of the contaminants in the water. Before any particular plant is commissioned, the contaminated water to be treated will preferably be analysed in order to identify the nature and quantity of the contaminants in the water.

Claims

1. A method of treating contaminated water comprising the steps of adding to the water agents to cause precipitation of. impurities and separating out said impurities, characterised in that the agents for causing precipitation are added sequentially and comprise, but not necessarily in this order, a batch of an anionic or cationic coagulant, at least one agent which reacts with dissolved metal ions to form insoluble salts and a batch of coagulant of opposite charge to the first batch.

2. A method according to Claim 1 wherein the anionic coagulant is selected from anionic polyacids and salts thereof.

3. A method according to Claim 2 wherein the anionic coagulant is selected from the alkali metal salts of simple oligomers and complex oligomers of acrylic or methacrylic acid, low-viscosity sodium carboxymethyl cellulose and oligomeric sulphonates.

4. A method according to Claims 1 to 3 wherein the cationic coagulant comprises a polyacrylamide.

5. A method according to Claims 1 to 4 wherein one coagulant and the other agents are added to the contaminated water and the liquor produced is passed to a pre flocculation tank before the other coagulant is added.

6. A method according to claim 5 wherein the said one coagulant and other agents are added to the contaminated water via a vortex valve.

7. A method according to Claims 5 and 6 wherein the preflocculated liquor is passed to a separator via a vortex valve to which the other coagulant is applied.

8. A method according to Claims 1 to 7 wherein the treatment agents comprise an alkaline earth metal hydroxide and calcium carbonate as weighting agent.

9. A method according to Claim 8 wherein the alkaline earth metal hydroxide is calcium hydroxide.

10. A method according to Claims 8 or 9 wherein one or both of sodium hydroxide or sodium carbonate are used as additional treatment agents.

11. A method according to Claims 8 to 10 wherein the pH of the water once the impurities have been removed is between 11.5 and 12.5.

12. A method according to Claim 11 further comprising the steps of aerating the water and removing nitrogenous impurities, adjusting the pH of the water to between 6.5 and 7.5 by the addition of acid and aerating and removing carbon dioxide from the water.

13. A method according to Claim 11 further comprising the steps of aerating the water and removing nitrogenous impurities, adjusting the pH of the water to between 6.5 and 7.5 by the addition of acid, adding activated sewage sludge to the water to remove residual organic matter, aerating the mixture and finally separating the resulting humus from the water.

14. A method according to Claims 8 to 13 for the purification of contaminated water having a high concentration of bicarbonate ions.

15. A method according to Claims 1 to 7 wherein the treatment agents comprise an electrolyte having a multivalent metal cation and calcium sulphate as a weighting agent.

16. A method according to Claim 15 wherein the metal cation comprises iron (III) or aluminium.

17. A method according to Claim 16 wherein the electrolyte comprises iron (III) sulphate, iron (III) chloride or aluminium sulphate.

18. A method according to Claims 15 to 17 further comprising the steps of adding activated sewage sludge to the water after the removal of the solid impurities, aerating the resulting mixture and removing the humus thus formed in a separator.

19. A method according to any one of Claims 15 to 18 for the treatment of contaminated water having a low concentration of bicarbonate ions.

20. A method according to any one of Claims 15 to 18 wherein the contaminated water is treated with calcium hydroxide before the addition of the first electrolyte.

21. A method of treating contaminated water comprising adding to said water treatment agents, raising the pH of the liquor formed to between 10.5 and 13, separating out the solids suspended in the liquor and stripping off ammonia and other nitrogenous compounds contained in the liquor.

22. A method of treating contaminated water comprising adding to said water treatment agents, separating out the solids suspended in the liquor thus formed and stripping off the carbon dioxide dissolved in the liquor.

23. A method according to any one of the preceding claims further comprising passing the water through a carbon filter.

24. A method according to any one of the preceding claims further comprising treating the water by reverse osmosis.

25. A method according to any one of the preceding claims wherein after treatment the purified water is discharged to a water course.

26. A vessel for use in: a) removing ammonia and other nitrogenous compounds; b) removing carbon dioxide or c) carrying out a bio-oxidation process during the treatment of contaminated water, characterised in that said vessel has an aspect ratio of at least 2:1 and that there is provided means for supplying water to the top of said vessel and means for removing water in a purer state from the bottom of said vessel and aeration means positioned at or near the bottom of said vessel.

27. Apparatus according to Claim 26 additionally being provided with means for regulating the height of the head of water within the vessel.

28. Apparatus according to Claims 26 and 27 wherein the water is sprayed into the top of the vessel.

29. A method of aerating water and removing unwanted dissolved gases and bio-oxidising water to be treated comprising supplying the water to a vessel according to Claims 26 to 28 and supplying air to said water via said aeration means such that air bubbles rise through the water and assist in the removal of said unwanted dissolved gases.

30. A method according to Claims 12, 13, 21 or 22 wherein the nitrogenous impurities and carbon dioxide are removed and bio-oxidation is carried out using apparatus of Claims 26 to 28.

31. A method according to Claims 12, 13, 21 or 22 wherein the nitrogenous impurities and carbon dioxide are removed and bio-oxidation is carried out using the method of Claim 29.

32. A method according to any of Claims 1 to 25, 30 or 31 for the treatment of contaminated water containing one or more of bicarbonate, sulphate, carbonate or other anions, calcium, magnesium, beryllium, iron, mercury, molybdenum, silver, tin, zinc, manganese, barium, cadmium, lead or other metals, inorganic substances such as arsenic or selenium or soluble or insoluble organic substances.