NZ260752A - Treatment of industrial effluent by ozonolysis following removal of coarse solids and also after removal of fine solids; apparatus therefor - Google Patents

Description

260752 NEW ZEALAND PATENTS ACT, 1953 No.: 260752 Date: 14 June 1994 N.Z. RATPMT OFF/CE 1 4 SEP 1995 P.^-CEIVlD COMPLETE SPECIFICATION SUBSTITUTION OF APPUOXT UNDtft SEC710U 24 'AN APPARATUS AND METHOD FOR TREATMENT OF A LIQUID -CONTAINING CONTAMINANTS INCLUDING SOLIDS IN SUSPENSION" VA oT> CrWeo WCyOfcONE INDUSTRIES LIMITED; a New Zealand ooniptHny, of 47 Kennedy Road, ■ Napier, New Zealand hereby declare the invention for which we pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- 26 0 751 The present invention relates to an apparatus and method for treating contaminated liquid. In the preferred embodiments the invention is designed for treatment of industrial effluents which contain various waste products. The invention is particularly useful for treating waste water from fellmongeries and tanneries, though it is not restricted to that use.

Many commercial and industrial processes result in production of unwanted and often toxic by-products. In the past such by-products were often disposed of by simply dumping or discharging them at or near the industrial site. Liquid by-products were often discharged in the form of waste water into the nearest water-way, with little or no thought as to possible environmentally damaging consequences.

In recent times such disposal has become unacceptable and often unlawful. Industries which produce unwanted by-products are now reliant on effective and cost-efficient means of treating waste, so that it can be disposed of without breaching the stringent regulations which now usually apply. This is particularly applicable to the fellmongering and tanning industries. These industries involve complex chemical treatments and require large volumes of water. Consequently a large amount of contaminated liquid is an end product of the fellmongering and tanning processes, both of which are often performed by a tannery.

To date, treatment of this effluent has been limited. Organic contaminants from treated hides and salts are often simply discharged in the waste water. One of the main by-products, sulphides, are oxidised. However, oxidation is generally only by exposure to air and relatively high concentrations of sulphides remain in the effluent. Another major contaminant consists of salts of the toxic heavy metal chromium and these salts are converted to hydroxides before being discharged.

As effluent discharge is now subject to far more stringent regulation, breach of which can occur heavy penalties, a highly efficient and cost-effective treatment producing high quality effluent is required. The present invention aims to provide this, or at least to provide the public with a useful choice. 26075? In a first aspect the present invention broadly consists in an apparatus for treatment of a liquid containing contaminants, including solids in suspension, comprising in sequence: means to remove coarser solids from the liquid; first means to ozonate the liquid, and thereby cause micro-flocculation of the finer solids remaining in suspension; means to remove the flocculated finer solids from the liquid; and second means to ozonate the liquid, so as to substantially neutralise any remaining contaminants.

Preferably the means to remove the coarser solids from the liquid comprises either one or, more preferably, both of a screening means, such as a rotary screen, and a settling tank. The preferred screening means removes solids having a particle size greater than about 3 mm.

The construction and arrangement of the preferred settling tank are such that the liquid flowing into the tank promotes sufficient turbulence within the tank to achieve a substantially homogenous mixture of the liquid and contaminants, including the finer solids remaining in suspension. The settling tank may have means to remove contaminants floating as scum or the like on the surface of the liquid in the tank, such means preferably being in the form of a scraper.

A further screening means is preferably located at an outlet of the settling tank, this screening means preferably removing solids having a particle size greater than about 0.75 mm.

Preferably the first means to ozonate the liquid comprises a first reaction tank which has an ozone input, preferably at or towards its bottom end. The apparatus preferably has means whereby a chemical flocculating agent, preferably a poly-electrolyte, can be added to the first reaction tank, and means to regulate the pH of the liquid in the first reaction tank, the purpose of each of these being to enhance the flocculation that occurs in the tank as a result of the presence of the ozone admitted to the tank. 260759 The preferred means to remove the flocculated finer solids from the liquid comprises a centrifuge.

Preferably a second reaction tank is provided after the means to remove the flocculated finer solids. This can be used for pH control and preferably this second reaction tank also has an ozone input at or towards its bottom end.

The second means to ozonate the liquid so as to substantially neutralise any remaining contaminants is preferably a diffuser system by which ozone is diffused through the liquid passing through, or is a venturi contactor system. In either case the preferred system is in the form of a multi-stage system comprising a series of connected tanks through which the liquid flows, with ozone being brought into contact with the liquid in each tank by the diffusers or by the venturi contactors.

Preferably the apparatus is lined with an ozone-inert material, such as Teflon (trade mark) or an epoxy type compound.

In a second aspect the present invention broadly consists in a method for treatment of liquid containing contaminants including solids in suspension, comprising the steps of: removing coarser solids from the liquid; ozonating the liquid to cause micro-flocculation of the finer solids remaining in suspension; removing the flocculated finer solids from the liquid; and ozonating the liquid so as to substantially neutralise any remaining contaminants.

Preferably after the coarser solids are removed the finer solids are flocculated using a poly-electrolyte, preferably Nalco Alfloc 627 (trade mark).

The micro-flocculation with ozone and, optionally, the poly-electrolyte, preferably takes place in alkaline conditions, most preferably at a pH between about 7.8-10. Sodium hydroxide can be used to achieve this pH level. 26075? After the removal of flocculated finer solids the pH level is preferably lowered, preferably to a pH of less than about 7, most preferably to a pH between about 4.5-6.5. Sulphuric acid can be used to lower the pH.

The ozone is preferably delivered as an ozone/oxygen mixture and preferably a ratio of about 0.5-5% wt/wt ozone/oxygen is used in the ozonation steps.

The above gives a broad definition of the present invention some preferred forms of which will now be described with reference to the accompanying drawing in which: Figure 1 shows an example of the apparatus of the present invention.

The apparatus provides for three main treatment stages identified in the drawing as primary, secondary, and tertiary treatment stages. First there is a means for removing coarse solids from the contaminated liquid which comprises the raw influent. If the contaminated liquid is tannery effluent it will contain organic material (for example hair/wool, fats and greases), chromium salts, sulphides, and acids and alkalis amongst other contaminants. The means for removing coarse solids can comprise a screening means 10 and/or a settling tank 11. Preferably both are used. In a preferred form, the screening means consists of a rotary screen and the preferred rotary screen removes coarse solids having an average particle size greater than about 3 mm. Such coarse solids may be in the form of wool or other organic particles from the treatment of animal hides. Wool recovered at this stage can be washed and is then reusable.

The preferred construction of the settling tank 11 is such that the influx of liquid through inlet 12 creates turbulence in the tank. This turbulence should be sufficient to achieve a substantially homogenous mixture of the liquid and contaminants, including the finer solids, remaining in suspension. The liquid should not be sufficiently turbulent to prevent settling of denser solids at the bottom 13 of the tank 11, however.

The settling tank 11 can optionally have a scraper 14 to remove any contaminants that float on the surface of the liquid in the form of a scum or the like.

The preferred construction of the settling tank 11 also incorporates a screening means 15 at its outlet. This screen preferably removes solids having average particle size greater than about 0.75 mm. 260 752! In the secondary treatment stage, finer solids are removed from the liquid. For this purpose the apparatus has means to ozonate the liquid to cause micro-flocculation of the finer solids remaining in the suspension. Preferably this means consists of a first reaction tank 16 which has an ozone diffuser inlet 17 which is preferably located at or towards the bottom of the tank. This diffuser allows the ozone gas to bubble through the liquid creating turbulence and enhancing the flocculating process. As an alternative to a diffuser, a venturi contactor may be used. In a preferred treatment, the flocculation is enhanced by use of a poly-electrolyte added to the first reaction tank 16. Nalco Alfloc 627 (trade mark) is an example of a suitable poly-electrolyte. It is an anionic latex polymer.

The flocculation is preferably carried out in alkaline conditions, ideally at about pH 7.8-9 or even up to a pH of about 10. An alkali such as sodium hydroxide is used, if necessary, to achieve a desired pH level in this range.

One of the properties of ozone is that it is a powerful micro-flocculating agent When used in this process, preferably in conjunction with a poly-electrolyte, substantially all the finer particles coagulate into an amphorous gel that tends to settle to the bottom of the first reaction tank 16. The pH control enhances the flocculation of chromium during this process. Chromium, as with all metals, has a minimum solubility point at a particular pH. When these conditions are achieved, precipitation of the chromium into the amorphous gel that settles to the bottom of the reaction tank takes place. Trials show that pH values of up to 10 enhance this precipitation process. The control systems for pH control may be interlocked so that the optimum pH cannot be exceeded. The formation of insoluble chromium hydroxide and the settling of the components which have bound chromium are the dominant removal mechanisms. The formation of chromium hydroxide may be represented by: 2Cr3+ + 3HzO + 03- 2Cr(OH)3 26 0 75? Following flocculation, the coagulant and the liquid flow out of the first reaction tank 16 via the outlet 18.

A means for removing the flocculated (coagulated) finer solids from the liquid is provided. Preferably this is in the form of a centrifuge 19. The centrifuge is used to separate the solid material from the liquid which then contains mostly non-solid contaminants. At this stage of the treatment most of the originally suspended solids, including fats and other organic solids and also chromium salts have either been screened or flocculated out of the liquid. There is also some oxidation of contaminants by ozone in the first reaction tank, though this is secondaiy to the flocculating process which takes place there.

Preferably the liquid is then transferred to a second reaction tank 20, through inlet 21. In this tank the pH is lowered to a level of at least about pH 7 and preferably to a pH of 4.5-6.5. Sulphuric acid may be used to achieve this pH level. In a preferred construction, the second reaction tank 20 has an ozone inlet 22, which may be a diffuser or a venturi contactor, preferably located at or towards the bottom of the tank. Liquid can be ozonated at this stage to oxidise some of the remaining contaminants.

The third treatment stage of the apparatus comprises means for ozonating the liquid to neutralise any remaining contaminants. By "neutralize" is meant to render substantially non-toxic and substantially inert to the environment to which the liquid will be released at the end of the treatment process. The liquid is transferred from the second reaction tank 20 through outlet 23 to the ozonating means. The ozonating means is preferably in the form of a diffuser system 24, or a venturi contactor system.

The preferred diffuser system 24 is a multi-stage system comprising a series of connected tanks 25 through which the liquid flows. As shown in the drawing each of these tanks 25 has one or more ozone diffuser inlets 26 which are ideally located at the bottom of the tank. Ozone from an ozone-generator 27 is pumped through the inlets 26 into the diffuser system (and optionally also through inlets 17 and 22 of the first and second reaction tanks 16 and 20). 2 6 0 7 5 ?J At this stage of the treatment, the powerful oxidising properties of ozone are used to neutralise the remaining contaminants in the liquid. As shown in the following examples, ammonia and inorganic forms of sulphur are oxidised into less toxic end products: Examples Ammonia removal: 403 + NH3 N03" + 402 + H30+ Sulphide removal: S2" + 303 -»• S032' + 30z S032 +03-S042 +02 Total: S2" + 403 - S042" + 402 Maintenance of pH levels, at levels preferably lower than about pH 10, more preferably lower than about pH 7, and most preferably in the range 4.5-6.5, and increased contact/reaction time in the diffuser system enhance the reactions.

Micro-pollutants in the form of any remaining suspended solids are also significantly reduced by the action of ozone. Colour, which is a by-product found in tannery effluent cannot be filtered or settled out but is substantially "bleached out" with a strong oxidant like ozone.

The ozone is supplied mixed with a carrier gas, either air or preferably oxygen. This carrier gas can also play an important role in floating the small quantities of other suspended contaminants still remaining in the effluent to the surface of the effluent in the diffuser (or venturi contactor) system 24 where this solid material is preferably caught and removed, for example by the side solids receivers 28 shown in the drawing. These receivers are attached to the sides 29 dividing the tanks 25, being angled upwardly to about the surface of the liquid towards the centre of the respective tank 25. As effluent flows either upwardly or downwardly past the side receivers they tend to catch fine solids material on their upper surfaces. Solids accumulated by the side receivers can be discharged for recovery. These solids can comprise fats, greases, proteins, and residual hair/wool, but are not restricted to these. 26 0 75 ? BOD (Biological Oxygen Demand) and COD (Chemical Oxygen Demand) are measures of the contamination of waste. Trials have shown that ozone destroys much of the organic material which contributes to the BOD and COD and thus minimises the impact of discharge of the treated waste on receiving waters. Ozone is particularly effective when used in conjunction with screening means and flocculating treatments as in the earlier stages of this invention.

Trials have shown that a preferred ozone concentration of about 0.5-5% wt/wt ozone/oxygen produces optimum results for ozonation. Ozone/oxygen mixes are preferred for best oxidation, but ozone/air mixes can be used instead.

Trial results illustrate the effectiveness of ozone use in treatment of tannery waste water: The average of numerous trials. % remnval/filiminatinn Chrome Sulphides Suspended Solids Fats and Greases B.O.D. (Biological Oxygen Demand) 94 90 91 73 91 AfTTTAT, T .AST TRTAT.

Before Treatment Chrome (mg/L) ppm Sulphides (mg/L) ppm Suspended Solids (mg/L) ppm Fats and Greases (mg/L) ppm B.O.D. (mg/L) ppm 1,520 3,400 11,322 1,388 8,400 810 318 520 139 2 One of the advantages of the process is that the gaseous by-products, any excess ozone and oxygen, are relatively harmless and can be simply discharged from the 26 0 7 5 diffuser system 24 through an outlet 30. Due to the powerful oxidising nature of ozone, in a preferred construction all elements of the apparatus exposed to ozone are lined with an ozone-inert material, for example Teflon (trade mark) or an epoxy type compound.

Trial results show that ozone is a very effective agent for treatment of waste water, particularly as a decontaminant of tannery effluent When used in a staged process, as in the preferred embodiment of this invention, ozone used as both as a micro-flocculent and an oxidising agent removes a high percentage of contaminants and suspended solids from waste water. Ozone, being such a powerful oxidizing agent, will kill viruses, and bacteria in a much shorter time, at a lower concentration than chlorine. Further, it attacks double bonds, and destroys phenols and tannins by a type of chain reaction, and it removes amino acids and detergents. The high quality effluent produced is capable of being recycled through the tannery system.

Because ozone provides oxidation and sterilization at faster rates than do oxygen and chlorine together, this means that a treatment plant using the apparatus and method of the present invention can be of a significantly smaller size and thereby occupy a significantly smaller site than can a conventional plant dealing with the same type and quantities of effluent The above describes some preferred forms of the invention and indicates a number of possible modifications but it will be appreciated by those skilled in the art to which the invention relates that various other modifications can be made without departing from the scope of the invention as has been broadly described. 26 0 75?

Claims (56)

WHAT WE CLAIM IS:

1. An apparatus for treatment of a liquid containing contaminants, including solids in suspension, comprising in sequence: means to remove coarser solids from the liquid; first means to ozonate the liquid, and thereby cause micro-flocculation of the finer solids remaining in suspension; means to remove the flocculated finer solids from the liquid; and second means to ozonate the liquid, so as to substantially neutralise any remaining contaminants.

2. An apparatus according to claim 1 wherein the means to remove the coarser solids from the liquid comprises a screening means and/or a settling tank.

3. An apparatus according to claim 2 wherein the screening means is a rotary screen.

4. An apparatus according to claim 2 or 3 wherein the screening means removes solids having an average particle size greater than 3 mm.

5. An apparatus according to any one of claims 2-4 wherein the construction and arrangement of the settling tank are such that the liquid flowing into the tank promotes sufficient turbulence within the tank to achieve a substantially homogenous mixture of the liquid and contaminants, including the finer solids remaining in suspension.

6. An apparatus according to claim 5 wherein the settling tank has means to remove contaminants floating as scum on the surface of the liquid in the tank. 26075? -12-

7. An apparatus according to claim 6 wherein the means to remove said contaminants floating on the surface comprises a scraper.

8. An apparatus according to any one of claims 2-7 wherein a screening means is located at an outlet of the settling tank.

9. An apparatus according to claim 8 wherein the screening means at the outlet of the settling tank removes solids having an average particle size greater than 0.75 mm.

10. An apparatus according to any one of the preceding claims wherein the first means to ozonate the liquid comprises a reaction tank which has ozone input means.

11. An apparatus according to claim 10 wherein the ozone input means is located at or towards a bottom end of the reaction tank.

12. An apparatus according to claim 10 or 11 wherein the ozone input means comprises a diffuser.

13. An apparatus according to claim 10 or 11 wherein the ozone input means comprises a venturi contactor.

14. An apparatus according to any one of claims 10-13 wherein the apparatus has means to admit a chemical flocculating agent to the reaction tank to enhance the flocculation occurring in that tank.

15. An apparatus according to any one of claims 10-14 wherein the apparatus has means to regulate the pH of the liquid in the reaction tank to enhance the flocculation occurring in that tank. -13- 260752

16. An apparatus according to claim 15 wherein the means to regulate the pH allows the pH to be maintained in the range from 7.8 to 10.

17. An apparatus according to any one of claims 10-16 wherein a second reaction tank is provided after the means to remove the flocculated finer solids and before the second means to ozonate the liquid.

18. An apparatus according to claim 17 wherein the second reaction tank has ozone input means.

19. An apparatus according to claim 18 wherein the ozone input means of the second reaction tank is located at or towards a bottom end of the second reaction tank.

20. An apparatus according to claim 18 or 19 wherein the ozone input means of the second reaction tank comprises a diffuser.

21. An apparatus according to claim 18 or 19 wherein the ozone input means of the second reaction tank comprises a venturi contactor.

22. An apparatus according to any one of claims 18-21 wherein the apparatus has means to regulate the pH of the liquid in the second reaction tank.

23. An apparatus according to claim 22 wherein the means to regulate the pH of the liquid in the second reaction tank allows the pH to be maintained in the range from 4.5 to 7.

24. An apparatus according to claim 23 wherein the means to regulate the pH of the liquid in the second reaction tank allows the pH to be maintained in the range from 4.5 to 6.5. -14- 2 6 0 75 2.

25. An apparatus according to any one of the preceding claims wherein the means to remove the flocculated finer solids from the liquid comprises a centrifuge.

26. An apparatus according to any one of the preceding claims wherein the second means to ozonate the liquid is a diffuser system by which ozone is diffused through the liquid passing through.

27. An apparatus according to any one of claims 1-25 wherein the second means to ozonate the liquid comprises a venturi contactor system.

28. An apparatus according to claim 26 or 27 wherein the second means to ozonate the liquid is in the form of a multi-stage system comprising a series of connected tanks through which the liquid flows, with ozone being released into the liquid in each tank.

29. An apparatus according to claim 28 wherein the liquid follows a serpentine path in flowing through the series of tanks, the liquid flowing countercurrently with respect to the ozone in some of the tanks and flowing cocurrently with respect to the ozone in alternate tanks.

30. An apparatus according to claim 28 or 29 wherein the tanks of the second means to ozonate the liquid have solids collecting means to collect fine solids still remaining in the liquid.

31. An apparatus according to claim 30 wherein the solids collecting means comprise solids receiving plates attached to sides of the tanks of the second means to ozonate the liquid, the plates being angled upwardly to about the surface of the liquid towards the centre of the respective tank whereby solids in liquid flowing either upwardly or downwardly past the plates tends to settle on upper surfaces of the plates. -15- 26 0 ^

32. An apparatus according to any one of the preceding claims wherein those parts of the apparatus which are exposed to ozone in use are lined with an ozone-inert material.

33. An apparatus according to any one of the preceding claims wherein the apparatus includes a source of ozone and means which supplies the ozone in a mixture with air or oxygen for ozonation of the liquid.

34. An apparatus according to claim 33 wherein the ozone is supplied in a 0.5-5% wt/wt ozone/oxygen mixture.

35. An apparatus substantially as herein described with reference to the accompanying drawings.

36. A method for treatment of liquid containing contaminants, including solids in suspension, comprising in sequence the steps of: (a) removing coarser solids from the liquid; (b) ozonating the liquid *o cause micro-flocculation of the finer solids remaining in suspension;;(c) removing the flocculated finer solids from the liquid; and;(d) ozonating the liquid so as to substantially neutralise any remaining contaminants.;

37. A method according to claim 36 wherein the coarser solids are removed by passing the liquid through one or more screening means and/or a settling tank.;

38. A method according to claim 36 or 37 wherein solids having a particle size greater than 3 mm are removed from the liquid prior to step (b).;- 16-;26 0 7 5 2;

39. A method according to claim 38 wherein solids having a particle size greater than 0.75 mm are removed from the liquid prior to step (b).;

40. A method according to any one of claims 36-39 including the step of removing any contaminants floating as scum from the surface of the liquid prior to step (b).;

41. A method according to any one of claims 36-40, characterised by including with step (b) the adding of a chemical flocculating agent to the liquid to enhance the micro-flocculation of the finer solids.;

42. A method according to claim 41 wherein the chemical flocculating agent is a poly-electrolyte.;

43. A method according to any one of the preceding claims characterised by including with step (b) regulation of the pH of the liquid at a pH in the range from 7.8-10 to enhance the micro-flocculation of the finer solids.;

44. A method according to claim 43 wherein sodium hydroxide is used to regulate the pH of the liquid.;

45. A method according to any one of claims 36-44 wherein the finer flocculated solids are removed from the liquid by centrifugation.;

46. A method according to any one of claims 36-45 wherein between steps (c) and (d) the pH of the liquid is lowered to less than 7.;

47. A method according to claim 46 wherein the pH is lowered=t?f^pH;m the range of 4.5 to 6.5. ^ ;:x;11;i f h;O ft;'"V*c c ^ 26 0 75 2 - 17-

48. A method according to claim 46 or 47 wherein sulphuric acid is used to lower the pH of ihe liquid.

49. A method according to any one of claims 36-48 wherein there is a further ozonation of the liquid between steps (c) and (d).

50. A method according to any one of claims 36-49 wherein step (d) is effected by passing the liquid through a multi-stage system comprising a series of connected tanks with ozonation of the liquid occurring in each tank.

51. A method according to claim 50 wherein the liquid follows a serpentine path in flowing through the series of tanks, the liquid flowing countercurrently with respect to the ozone in some of the tanks and flowing cocurrently with respect to the ozone in alternate tanks.

52. A method according to any one of claims 36-51 wherein step (d) includes collecting any further fine solids remaining in the liquid.

53. A method according to any one of claims 36-52 wherein the ozone is supplied in a mixture with air or oxygen.

54. A method according to claim 53 wherein the ozone is supplied in a 0.5-5% wt/wt ozone/oxygen mixture.

55. A method according to any one of claims 36-54 performed using the apparatus of any one of claims 1-35.

56. A method according taelaim 36 and substantially as herein described with reference to any embodiment ^closed. " ^ t | Had] ffiac/dC Qbocpa......