GB2208164A - Purification of waters - Google Patents

Abstract

The purification of waste or raw waters or of sewage sludges or raw sewage by means of a basic ferric salt as a flocculating agent may be improved by control of the pH value of the water, as treated, by the addition of acid or base. Preferably the pH value is controlled within the range of greater than 5.0 to below 7.5 for example at from 5.5 to below 7.0.

Description

purification of water This invention relates to the purification of water.

Flocculating agents are currently in very large scale use in the purification waters such as raw waters, industrial waste waters, river waters, sewage sludge or even aqueous raw sewage suspensions. The classical flocculating agent for such use is aluminium sulphate.

Others, which have been developed more recently1 are basic aluminium sulphate, basic aluminium chloride and polyaluminium chloride. The word "basic" used with reference to salts means salts in which the anionic component includes hydroxyl ions.

Flocculants based on iron compounds are known. Such flocculants are useful additions to the range of water treatment chemicals and their use avoids a high residual aluminium content in waters where this may be undesirable.

While iron-based flocculants can be extremely effective under specific conditions they do not, in general, perform as well as aluminium-based flocculants in the treatment of neutral or nearly neutral waters. Ferric chloride is an example of a widely used iron-based flocculant which is relatively ineffective, in treating waters having pH values near to neutrality. Basic ferric chloride has been proposed as a flocculating agent for water treatment but, initially, proved to be unstable especially at higher basicities in manufacture and/or storage as a concentrated solution.

production of stable basic ferric chloride has been disclosed the said method comprising the manufacture of the basic ferric chloride in the presence of a phosphorus compound.

The Applicants now provide a new or improved process for the purification of waters by means of basic ferric salts such as basic ferric sulphate or, preferably, basic ferric chloride. The Applicants have found according to the present invention that in use in the flocculation of waste or impure natural waters for example those containing colloidal silica particles giving rise to turbidity and/or humic or fulvic acids giving rise to colour, an improvement in the reduction of turbidity and/or colour may be attained by the suitable control of the pH of the water being treated. The invention is also applicable to the treatment of sewage sludhes, raw sewage suspensions or any other aqueous system requiring flocculation.

According to one aspect thereof the present invention provides a process for the purIfIcation of water or of a sewage sludge by contacting the water or sludge with an effective quantity of a flocculating agent comprising a basic ferric salt wherein the flocculation is improved by adjusting the ps of the water or sludge by the addition to the water, in addition to the basic ferric salt, of an acidic or basic substance. There are a great many normally neutral or alkaline waters which alkaline waters may, for example, have a pH value in the range 7.5 to 9.0.

In purifying such waters according to the present invention an acidic substance is added. Preferably the acidic substance is added in a quantity such that, after the addition of the flocculating agent, the pH of the water is below 7.5 but not below 5.0. It is particularly preferred that the p. of the water is from 5.25 to below 7.0 a pH range of from 5.5 to 6.5 being the optimum for the treatment of at least some alkaline waters.

There are also acidic natural or industrial effluent waters which may be purified according to the present invention by the addition of an acidic or basic substance to bring the pH to the values taught above in relating to alkaline waters after the addition of theflocculating agent. Again, the effect of the addition of the basic ferric chloride flocculating agent.

The optimum pH may vary somewhat depending on the particular water being treated and it is envisaged that the benefit of the invention may be attained by process control based on the testing of the turbidity, colour or floc development of samples of water acidified to various extents and treated with the basic ferric salt flocculating agent. The optimum pH so determined need not necessarily, although It generally wi l, lie within the preferred or particularly preferred ranges set out above.

In relation to waters tested by the Applicant, however, the optimum pH has been found to be from 5.75 to 6.25.

The use of a basic ferric salt as a flocculating agent, of course, gives, In most cases treated water having a lower residual aluminium content than the use of an aluminium based flocculating agent. It has also been found that the treatment of water according to the present invention can result an extremely low content of residual iron. This content of residual iron can be considerably lower than that in the untreated water and can at least equal the content obtainable by the use of the preferred aluminium-based flocculating agent, that is, polyaluminium chloride.

Basic ferric chloride or sulphate, for the avoidance of doubt, is understood to be a range of compounds having the general formula Fe(OH)mCl3#m, or Fe2(OH)m(SO4)3#m the % basicity being the % of Fe valency occupied by hydroxyl ions. The % basicity, for utility according to the invention, is preferably from 20% to 90% and particularly preferably from 25% to 85%. Such basic iron salts may be produced by basifying a solution of the ferric salt. The concentration of the ferric salt is preferably initIally from 2 to 20% by weight calculated as Fe203 and the pH thereof is preferably from 2.0 to 0.0. Ferric chloride or sulphate liquors having a suitable concentration and containing the corresponding acid in a quantity to give a suitable pH are available commercially.

The base utilised to produce the basic salt may be, for example, an alkali metal, preferably sodium, hydroxide carbonate or bicarbonate. Alternatively, the use of the corresponding ammonium compounds, particularly preferably ammonium bicarbonate, gives rise to particularly suitable basic ferric salts. The quantity of base may be determined by reference to the stoichiometric quantity required for reaction with the required proportion of the amnion of the ferric salt making allowance for the presence of free acid and any other substances which may react with base. The base is preferably added to the solution of the ferric salt slowly and with agItation at ambient temperature and the reaction mixture so formed is preferably aged for at least 15 minutes.If any precipitate is formed it may be removed, the residual solution of-the basic salt constituting a utilisable product. Preferably the reaction to produce the basic ferric salt is conducted in the presence of a phosphorus compound, particularly preferably a phosphate, a polyphosphate or phosphoric acid as more fully described in the aforementioned Japanese Patent Publication No. 59116127. Preferably the phosphorus compound is present in a quantity, calculated as PO4 of from 0.01 to 0.3 moles, particularly preferably from 0.025 to 0.25 moles, per atom of Fe. The effect of the present invention is particularly noticable in relation to basic ferric chloride flocculating agents containing such quantities of phosphorus compound.

In use according to the invention the basic ferric salt is preferably added to water to be treated to give a concentration of the basic salt in the water of from 1 to 50 mg.l#1 particularly preferably from 2 to 20 mg.l -l a particularly useful treatment concentration being from 3 to 15 mg.' i calculated as Foe203.

The acidic substance used to control the pH of the water to be treated according to the present invention may be any suitable acid, preferably used at an initial concentration, for example, of from 5% to 25% by weight.

Hydrochloric acid may be used although due to its tendency to fume if spilled it is not preferred on a practical usage basis. Sulphuric acid is preferred environmentally, and on a handling basis and tends to be more effective.

The basic substance used according to the present invention may very suitably be, for example, a form of lime, or caustic soda, in a similar range of preferred concentrations. It is understood however that more concentrated acid or base may be used if desire. The addition of the acidic or basic substance is preferably conducted prior to or concurrent with the addition of the basic ferric salt to the water to be treated although the benefit of the Invention may be attained, although possibly to a lesser extent, by a later or a progressive addition of the acidifying agent.

The invention will now be more specifically described by reference to the following Tests of particular embodiments thereof and to Figure 1.

Figure 1 is a three part plot of the pH of water being treated against (A) Turbidity, (B) Hazen Colour and (C) Residual Fe in the treated water. Four different flocculating agents, two according to the invention and two not according to the invention and present to provide a basis for comparison, as identified hereafter were used.

In the Tests a number of parameters are investigated.

Certain of these, for example, pH and dosage rates, and residual aluminium and iron contents require no further definition. The Tests for Colour, Turbidity and Floc size are defined below.

Colour Measured in "Hazen" units by the technique described in "Analysis of Raw Potable and Waste Waters" published by HMSO 1972 page 26. The technique involves the measurement of optical density at a wavelength of 400 nm using a spectrophotometer (SP600) and calibration against standard solutions.

Turbidity Measured in FTU (Formazin turbidity units) by the technique described in the above reference on page 29.

The technique involves measurement of optical density at a wavelength of 580 nm before and after filtration through a membrane filter and calibration against formazin standards.

Floc growth Measured on a scale of A to G, after standard addition of flocculant followed by stirring at 60 rpm, then at 20 rpm and finally allowing to settle for 30 minutes. A denotes a multiplicity of 0.3 to 0.5 mm flocs, C denotes a very few 3.0 to 4.5 mm flocs and B to F denote intermediate conditions.

In all the Tests the waters treated were input waters to existing water treatment plants, not artificially produced systems, e.g. suspensions of clays. The Examples are therefore directly relevant to operational conditions.

These following Tests illustrate the criticality of the pH at which water treatment according to the invention is carried out.

The water treated had the following properties.

pH 7.8 Turbidity 9 FTU Colour 17 Hazen Residual Al 0.05 mg A1/l-1 Residual Fe 0.21 mg Foe/1 1 The flocculating agents used were polyaluminium chloride (PAC) and ferric chloride, for comparative purposes and two basic Iron chlorides according to the invention, one of 40% Basicity and the other of 80% Basicity. The dosage rate was 5 mgl 1 as M203 except for the PAC which was used at 3mug/1~1 as Awl203. Samples of the water having pH values of 5.0, 5.5, 6.0, 6.5 and 7.0 by addition of sulphuric acid (10% w/w) were dosed over a period of 3 minutes with stirring at 60 rpm and were subsequently stirred at 15 rpm for 20 minutes.The samples were then allowed to stand for 30 minutes before examination and analysis.

The Turbidity, Colour and Residual Fe of the treated samples are indicated in Figure I, parts A, B and C respectively the pH related performance of the basic iron chloride samples being highlighted by the superimpos tion of a curve.

It is remarked, generally, that since the interact ions involved are on the molecular level the effectiveness of dosage rates requires comparison on a molar basis between the iron-based and the aluminium-based flocculating agents which implies a weighting factor of 0.64 to be applied to the basic iron salts. Thus the dosage rates of these salts and of the PAC are equivalent.

The following Tests illustrate the effect of various flocculating agents at a standard pH of 6.0 but at varying dosage rates.

The water to be treated had the following properties.

pH 7.3 Colour 38 Hazen Residual Fe 0.92 mg.l 1 ResidualAl 0.05 mg.l#1 The treating technique was the same as that described in the previous Tests.

The results are summarised in the following Table.

Test No. Flocculant Dose Floc Colour Residual mg M2O3.l-l Size Hazen Fe mg.l 1 1 Aluminium 8 D 6 0. 38 2 Sulphate 9 D-E 9 0.40 3 10 E 8 0. 41 4 PAC 8 E 8 0.22 5 Basic Fe 8 D 6 0.20 5 Chloride 40% 9 D 5 0.18 7 10 D-E 4 0.14 8 Basic Fe 8 D-E 7 0.17 9 Chloride 9 E 7 0.16 10 80% 10 E-F 6 0.12 In the instances where the basic iron chlorides had been used the residual aluminium content of the water remained unchanged as a result of treatment. In the instances where aluminium containing flocculating agents had been used the residual aluminium content of the treated water was found to have increased as follows : Dose mg.l#1 Residual Al mg.l- Aluminium Sulphate 8 + 0.15 9 + 0.20 10 + 0.05 PAC 8 + 0.10

Claims (13)

1. CLAIMS 1. A process for the purification of water by contactIng the water with an effective quantity of a flocculating agent comprising a basic ferric salt wherein the flocculation is improved by adjusting the pH of the water or sludge by the addition to the water, in addition to the basic ferric salt, of an acidic or basic substance.
2. 2. A process as claimed in claim 1 wherein the flocculating agent is a 20% to 90% basic ferric chloride or sulphate.
3. 3. A process as claimed in claim 1 or 2 wherein the flocculating agent is present in the water at a concentration of from 1 mg/l to 50 mg/l.
4. 4. A process as claimed in any preceding claim wherein the flocculation is improved by adjusting the pH of the water to a point in the range of 5 to less than

   7.5.
5. 5. A process as claimed in claim 4 wherein the pH is adjusted to a point in the range of 5.25 to 6.75.
6. 6. A process as claimed in claim 4 wherein alkaline water is to be purified and there is added thereto in addition to the basic ferric salt a quantity of an acidic substance sufficient to adjust the pH to within the said range.
7. 7. A process as claimed in claim 6 wherein the acidic substance is selected from hydrochloric and sulphuric acids.
8. 8. A process as claimed In claim 6 wherein the water has a pE of from 7.5 to
9. 9.C 9. A process as claimed In claim 4 wherein acidic water is to be purified and there is added to the water, in addition to the basic ferric salt, a quantity of a basic substance sufficient to adjust the pH to within the said range.
10. 10. A process as claimed in claim 9 wherein thebasic substance is selected from lime or caustic soda or chemIcal equivalents thereof.
11. 11. Water or other aqueous liquid whenever purified by a process as claimed in any receding claim.
12. 12. A process as claimed in claim 1 and substantially as described herein.
13. 13. A process as claimed in claim 12 with particular reference to any one of Tests 5 to 7 described herein or to Figure 1.