CA1038511A - Removal of phosphorus from waste water - Google Patents

Abstract

Abstract of Disclosure Phosphate is removed from an aqueous medium by adding inorganic coagulant followed by a cationic polyelectrolyte which is a water-soluble salt or quaternary ammonium salt of a high molecule weight copolymer of acrylamide and an alkylaminoalkyl ester of acrylic or methacrylic acid. The process is of particular value in reducing the phosphate content of sewage effluent.

Description

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In the treatment of sewage and other aqueous wastes, (i.e. waste waters) generally two distinc-t areas of solid/
liquids separation are recognised.
The first of these concerns the removal of solid constituents from the bulk liquid effluent with the ob~ect of -producing a purified aqueous liquid effluent which may or may not require further purification before dlscharge or re-use. ;~
Examples of this are primary or secondary sedimenta-tion processes, filtration processes and flotation processes.
The solids removed in such processes usually are associa-ted with an appreciable quantity of water but because of their nature and consistency they are classified as sluclges.
The further dewatering of such sludges may also be by sedimen-tation, filtration, flota-tion or centrifuga-tion but constitu-tes a somewhat differen-t and often more difficult -teQhnology which is commonly referred to as sludge dewatering.
Whilst it is true -to say that there are certain common , features between the two, for instance,in relation to the site of operation, nevertheless in practice the overall technlques, ~ ;
handling methods, reagent dosages and reagent types are usually ~ `
quite different for the two areas.
There is often some confusion in the patent literature concerning these two areas and accordingly it is to be stated that the usage of the present invention is connected with technology of the first area, i.e., bulk effluent treatment and not at all with the second area i.e., sludge dewatering. This will be readily apparent to those skilled in the art.

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As a typical example of a conventional sewage treatment, the raw sewage generally undergoes a preliminary treatment for the removal of grit and coarse matter, followed by primary sedimenta-tion, where finer solids are settled ou-t by slow passage through large sedimentation tanks. This primary treatment may be followed by a secondary stage where ~;-~urther purification of the sewage is carried out biologically.
As the biological stage creates more insoluble solids, a secondary sedimentation step follows. The sludges ~rom each sedimentation step are then combined and de-watered while the effluent from the Pinal sedimentation step is discharged.
The presence of phosphorus in sewage e~fluents has been recognised as promoting the growth of algae and aquatic plants in receiving waters by providing a source of nutrition.
Excessive plant growth can cause clogging of water courses, and the growth and subsequent death of algae can be responsible for the eutrophication of lakes and other receiving waters.
Wh~rea~ it is recognised that oth~r elements, such as carbon and nitrogen, contribute towards the nu-tritive value o~ effluents, recent attention has focussed mainly on the remo~al of phosphorus as being -the simplest contributing element to remove.
Phosphorus can be found in sewage in a number of forms, for instance as both soluble and insoluble phosphates or complex phosphates. Thus a typical raw sewage enterlng a sewage treatment works migh-t contain two thirds of the i~

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phosphorus as soluble orth ~Q ~ ~ and polyphosphates and one third as insoluble phosphat~ Further, because part of the insoluble phosph~e~ may be present in a colloidally dispersed form) not all of the insoluble phospha~ can be removed from the sewage by a sedimentatio~ process.
The objec-t of the presen-t invention is to provide ways of reducing conveniently and satisfactori:Ly the phospha-te content of bulk effluent aqueous wastes. The invention is o~ particular value when utilised to reduce the phosphate content of the effluent from a sewage bulk effluent process, but is also applicable to the -treatment of a~y bulk effluent aqueous waste containing soluble phosphate, for example a chemical waste.
It is known to remove soluble phosphates from sewage by chemical treatment that causes precipitation o~ -the dissolved colloidal phosphates, followed by removal of the precipitated phosphates generally by settlement. Inorga~ic coagulants suitable for this purpose include certain soluble -~
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salts containing multivalent cations, such as aluminium sulphate, ferrous sulphate, ferric sulphate, ferric chloride, sodium aluminate and calcium hydroxide, the use of which result in the phosphates becoming precipitated as the ~ `
corresponding insoluble me-tal phosphates.
Unfortunately the settlement is much too slow and ineffective to be accomplished satisfactorily in a normal sewage sedimentation stage and it is already known to improve the settlement by adding polyelectrolyte flocculants.

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;,, . ;. : . . . . ... .. .; ,.. ,.,. , ;.. ; ;, Thus U.S. Patent ~,506,570 teaches the use of trivalent aluminium ions and anionic polyelectrolyte flocculants, being high molecular weigh copolymers of from 80 to 50 weight percent acrylamide or methacrylamide and from about 20 to 50 weight percent acrylic or methacrylic acid or water-soluble salts thereof.
Likewise, U.S. Patent 3,617,569 discloses that the separation of precipitated metal phosphates is facilitated by the use of a water-soluble organic polyelectrolyte flocculating agent such as a partially hydrolysed poly-acrylamide. The use is described7 in U.S. Patent 3,171,802, of metal salts and anionic or nonionic polyelectrolyte flocculants followed by filtration -through coal, sand and activated carbon, for sewage -treatment. U.S. Patent 3,655,552 teaches the removal of phosph~s by the synergistic admixture of a water-soluble, high molecular weight nonionic polymer, preferably polyacrylamide, and a water-soluble salt containing ferric ions, preferably ferric chloride. Also, U-50 Patent, 3,607,738 describes thë use of lime and ca-tionic polyamines for phosphate removal during tertiary treatment of sewage, and U.S. Patent 3,453,207 describes the use of alum and a homogeneous latex emulsion comprising water, polybutadiene and a cationic emulsifying agent. ~ ~

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Also of course it is well known to use a variety of flocculants~ either by themselves or in combination with other materials, to promote de-watering o~ sewage sludge.
For example one such process is described in U.S0 Patent No. 3,472,767 in which specified cationic copolymers are added to sewage sludge in the presence of polyvalent metal ions to facilitate de-watering of the sludge. This however,is not relevant to the problem of reducing the phosphate content of the effluent from a sewage sedimentation process.
A disadvantage of the processes described abo~e for the reduction of phosphate in bulk e~fluent is that all the described processes tend to be rather slow. For example UOS. Specification No. 3,506,570 suggests that a time of at least two minutes and preferably five minutes should elapse between the addition of coagulent and flocculant while U.S. Patent No. 3,453,207 suggests periods of five -to thirty ~ '~
minutes with agitation as being required. It would be i~
desirable to be able to obtain good results by-adding the flocculant much more quickly after the coagulant and quickly thereafter passing the mixture to the sedimentation stage. -;
Thus in many sewage works now there is a relatively high speed of flow such that the time be-tween the raw sewage entering the works and reaching the primary sedimentation tank may be less than five minutes. Similarly after biological treatment of the effluent from the first step insufficient time may be available before the secondary sedimentation step to allow adequate precipitation by known processes. ~;`
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According to our invention we remove phosphate from an aaueous waste bulk effluent con-taining phosphate by precipitating soluble and colloidal phosphates in the medium by adding -to the , .
medium an inorganic coagulant, and therea~ter we add to the medium certain wa-ter soluble high molecular weight cationic polyelectrolyte ~locculants, and then we subject the waste to a liquids-solids separation process.
The high molecular weight cationic polyelectrolyte flocculants used in the invention are water soluble salts or quaternary ammonium salts of copolymers containing (a) units of alkylaminoalkyl esters of methacrylic or, pre~erably, acrylic acid and (b) acrylamide units. ~;
The addition of the cationic polyelectrolyte appears to result in assisting and accelerating flocculation and sedimentation of the precipitated phosphates. Whatever the mechanism, by the invention it is possible to achieve phosphate removal ~rom sewage very quickly and to a greater degree than by previous methods. The method has the advantage that a wide variety of metal salts can be used ~or precip-itation o~ the phosphorus compound and it generally requires no elaborate agitation means. This method usually also results in good reductions in other pollution parame-ters, such as suspended solids content and Biochemical Oxygen Demand and generally can be applied at any par-t of a sewage puri~ication process immediately prior to a sedimentation `
stage As the inorganic coagulant, metal salts such as those mentioned above can be used. In general the salt is a water soluble compound, usually an acid salt, containing a multivalent ` `
metal cation and in particular is usually one of the

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compo~mds that is of-ten used in was-te water treatment, examples being compounds of A13 , Fe2~, Fe3 and Ca~
e.g. aluminium sulphate, sodium aluminate, ferrous sulphate, ferric sulpha-te, ferric chloride and calcium hydroxide. ` ;~
Preferred copolymers contain the following groups~
(a) R3 (b) ,, : ~ .
-CH2- R and -CH2-~H~

¦ R4 ~
00- ~ NH2 2 ;~
in which Rl and R2 may be the same or different and are hydrogen or alkyl or, together with the nitrogen a-tom to which they are attached, form a heterocyclic ring, R4 is alkylene containing 1 to 8, usually 2 to 4, carbon atoms and R is methyl or, preferably, hydrogen. If Rl and R2 form a heterocyclic radical they preferably represent an alkylene chain of 4 to ~ carbon atoms. Alkyl groups represented by Rl and R2 usually contain no more than 8 carbon atoms, preferably 1 to 4 carbon atoms, preferably methyl or ethyl. R4 preferably contains 2 carbon atoms (e-thylene).
Salts can be formed with an acid capable of forming salts with the amino groups present in the copolymers, ~ ~-2~ for example hydrogen bromide and hydrogen chloride.

- 7 -Preferred copolymers are quaternary ammonium salts, and especially such salts where Rl and R2 contain 1 or 2 carbon atoms each. Preferred quaternising reagents are those that introduce the alkyl group, for example Cl 4 alkyl (especially methyl or e-thyl), onto the nitrogen atom. Thus typical quaternising reagents are dimethyl sulphate and methyl chloride.
The preferred copolymers have R4 represen-ting C2H4, Rl and R2 being the same of different and representing methyl or ethyl and which are quaternised with dimethyl sulphate.
The copolymers must contain from 80 to 97 mole per cent (b), (i.e. a molar ratio of (a) : (b) of approximately of l:L~ to 1:33) and -the most preferred copolymers generally contain from 85 to 97 mole per cent, especially 85 to 93 mole per cent, b (i.e. molar ratios of approximately 1:6 to 1:33 or 1:6 to 1:12). All mole percentages are based on a - plus b.
Generally the copolymer consists solely of the two specified types of groups bu-t of course smaller amounts of other vinyl groups may be included in the copolymers.
As these other vinyl groups, there may be used any vinyl monomers that can copolymerise with the other vinyl monomers-that are specified and that do not interfere deleteriously with the properties of the copolymer. Such vinyl monomers may be selected from all the conventional vinyl monomers.
The amount is usually below 20 mole per :: . .. . :: ~ : . : : :: : .: ..................... -: ," ' ' ~ , ' ' . ' .. .. ,~ : , . ,~ ;' . ' ; . .

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cent preferably below lO mole per cent.
We prefer to specify the molecular weight o~ the polyelec-troly-tes we use in this in~ention in terms o~
the viscosity of their solutions. The polymers are pre~erably such that at 25C the viscosity in centipoise of their aqueous solutions a-t pH 6.0~ containing 1% by weight of -the polymer, and in the absence of added salts, using a Brookfield Model RVT viscometer with spindle No. 3 and at 20 rpm is greater than 2000. A ~iscosity greater than 3000 is especially pre~erred.
Suitable copolymers are well known and are commercially available. They can be made in~any convenient manner, for example by well known vinyl type polymerisation methods including, ~or example, free radical initia-ted solution polymerisation.
The aqueous medium -to which the additions are made may be, ~or example, a chemical e~luent but pre~erably ~ -it is raw sewage or the e~luent from a primary or secondary sedimentation stage. The sludge resulting ~rom the process when the aqueous medium is raw sewage or a sedimentation e~luent may be subjected to de-watering by any suitable method.
The process is conveniently carried out by adding the metal salt or other inorganic coagulant in an amount such that the metal cation : phosphate molar ratio is at least 1:1, preferably about 2:1. Naturally it is necessary that the metal cation becomes mixed rapidly through the aqueous medium but this is conveniently achieved by adding the metal cation to a ~lowing stream of the medium. The pH of - _ 9 _ . .. .. ,, ~.. .. . . - ..
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the stream is preferably such as to promote precipi-tation of insoluble phosphate. It is important that the polyelectrolyte is added a~-ter -the coagulant but it is desirable that it should not be added too long after the coagulant. Thus pre~erably it is added within five minutes of adding the coagulan-t and generally wi-thin two minutes. Mos-t preferably it is added less than a minute after adding the coagulant~ There must be a de~inite -time interval between adding the coagulant and the flocculan-t, for example, at least five and preferably at least ten seconds. A preferred interval is a quarter of a minute -to one minute. Naturally it is necessary that the flocculant shall mix rapidly through the aqueous medium: conveniently this is achieved by adding it to a flowing stream of the medium. The amount of polyelectroly-te added is generally from 0.1 to 2 or perhaps 3 mg/l, preferably about 0.5 -to 1 mg/l. `
It is generally introduced in the form of a dilute a~ueous solution having a concen-tra-tion of from 0.01 to 0.05 per cent by weight. `
The invention is illustrated by the following examplesD ~
E~{amPle 1 ':
~, To 200 ml aliquots of a raw sewage, obtained before primary sedimentation, aluminium sulphate was added in various amounts as a 5% weight by volume solution of A12(S04)3. 16H20, with stirring at 160 rpm.
After 30 seconds, various polyelectrolyte flocculants were added at a doseo`f 1 mg/l, with further stirring at 160 rpm for 30 seconds, folIowed by stirring at 40 rpm for 1 minute and stirring at 5 rpm for 3 minutes~

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The polyelectrolytes added were~
1. An anionic polyelec-trolyte, being a copolymer : ;
of about 60% by weight acrylamide and about 40/0 by weight sodium acrylate, that iSJ about 66 mole percen-t acrylamide and about 54 mole percent sodium acrylate.
2. An anionic polyelectroly-te as No. 1, but with about 10% sodium acrylate, -that is, about 92 mole percent acrylamide iand about 8 mole percent sodium acrylate.

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3. A cationic polyelectroly-te, being a copolymer of about 90% by weight acrylamide and about 10% by weight of dimethyl sulphate quaternised diethylamino ethyl acrylate, that is about 97 mole percent acrylamide and about 3 mole percent quaternised diethylaminoethylacrylate.
4. A cationic polyelectrolyte as Nro. 3, but wi-th about 70% acrylamide 9 that is, about 90 mole percent acrylamide and about 10 mole percent quaternised diethyl~
aminoethylacrylate.
5. A nonionic polyelectrolyte, being a homopolymer of acrylamide.
Polyelectrolytes Nos. 3 and 4 had viscosities as previously described o~ 2140 cp and 3370 cp respectively.
Analyses were made to determine the total phosphorus contents of the superna~nk liquors produced after treatment of the sewage with aluminium sulphate and the above polymers, resulting in the following data. Results are given as mg/l phosphorus and the initial total phosphorus content of the ~;
sewage was 14 mg/l. Aluminium sulphate doses are given as mg/l A12(S0~)3 16H20. ~;

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Supernatant total phosphorus content (mg/l) ~or aluminium sulphate doses of Polymer _ _ ~ . ~

lO0 mg/l 200 mg/l 300 mg/l 400 mg/l ~
. . .. _ .~
None 9.8 7.6 5.9 2.2 No. l 7.2 4.5 203 l.l No. 2 7.5 4.9 2.6 1.3 ~ ;
No. 3 6.0 2.4 0.6 0.5 ;
No. 4 5.2 2.0 0.4 0.3 No. 5 9.8 7.2 4.9 _ , The results show that whileincrea9ed phosphate removal results ~rom the use of the anionic polyelec-trolytes compared with the aluminium sulphate alone, much better resul-ts are possible with the cationic polymers, particularly No. 4. The nonionic polyelectrolyte, No. 5, gave almost no improvement over aluminium sulphate alone.
Similarly, good results can also be obtained i~ other quatenary or other salts of polymer 4 are used or i~ polymers similar to poly~er 4 are used but in which either or both R
and R2 are replaced by methyl.
Example II ~;
A similar series o~ tests was carried out with a sample of sewage obtained just before secondary settlement, that is, a~ter the raw sewage had undergone primary sedimentation and biological puri~ication in bacteria filters, but before settlement for removal of solids produced by the biological purification.

1038Sll A 10% slurry of calcium hydroxide was added at :;~
various doses ~or the precipitation o~ the phosphates and the polyelectrolytes of Example 1 were added at a dose of 1 mg/l. ~ :
The total phosphorus content of the sample was ini-tially 7.4 mg/l and the ~ollowing results were obtained. ~-..... ~ ~
Supernatan-t to-tal phosphorus content (mg/l) :~:
Polymer ~or calcium hydroxide dose of ,,_ , . _ ._ .... .. . _ . 100 mg/l 200 mg/l300 mg/l :
._ .~ ~ I ' ' ~:
None 5 5 1.8 0.4 No. 1 4.1 0.9 0.3 No. 2 3.6 0~8 0.3 No. 3 2.4 0.6 0~1 No. 4 1.9 0.3 0.07 _ _ __ _~ 4-6 _ _ _ -1`

As with Example 1, -the results show that, whlle the nonionic polymer, No. 5, gives only slight improvement and the anionic polymers Nos. 1 and 2 give fairly good improvements ;~
in phosphate removal, very superior results are obtained with the cationic polymers, Nos. 3 and 4, again particularly No. 4.

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ExamPle III ~03~
This example illus-trates processes in which the coagulant and flocculant are added to the effluent going to a tertiary sedimentation process involving settlement or fil-tration. ~
Such processes are desirable where it is necessary to remove ~ `
residual suspended solid in the effluent from a secondary ~;~
sedimentation stage or where it is considered advan-tageous to remove nutrients after the conventional sedimentation process is complete.
For this example, a series of jar tests was carried out by the procedure described above for Examples I and II on effluent following a secondary sedimentation process.
Aluminium sulphate was added a-t two doses for the precipitation of the phosphate a~d the polyelectrolytes of Example I were added subsequently a-t 0.5 mg/1. , l . . . - ' -----'I : ~' ' Supernatant total phosphorous content (mg/1) for Aluminium Sulphate Polymer Dosage ¦~
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120 mg/l 150 mg/l _ -None 1.02 0.61 No. 1 0.64 0.55 No~ 2 0.64 0.54 ;~
No. 3 0.52 0.35 No. 4 0.40 0.20 No. 5 0.71 ~ O.70 ~' , --- - 15 - ~

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These results show that while all polyelectrolyte treatments give better results than aluminium sulphate ~ ~
alone the best results are obtained in accor~ance with ~ :
the invention, using copolymers 3 and 4, especially 4.

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Exam~le IV ~0 A series of tests was carried out by a similar procedure to tha-t of Examples I and II, to investigate the e~fect of varying the time between acldi-tion of the metal salt and addi-tion of the polyelectrolyte, which we call the intervening mixing time. A sample of raw sewage was used, with a total phosphorus content of 12.0 mg/l.
The metal sal-ts used was aluminium sulphate, and a constant ;
amount of 150 mg/l A12(S04)3. 16H20 was added in each case~
The polyelectrolyte flocculants, No. 1 and No. 4 ;~
from Example I were added at a dose of 0.5 mg/l, producing ~;~
the following results. , __ . ................................. ~ _ _ ~.,, Supernatant phosphorus con-tent Intervening Mixing Time (minutes) (mg/l) , ~ -~ , _ ,, :
Flocculant Flocculant `
No.l No.4 ~;~
. ~ . . .
0 10.4 6.
0.25 9.0 0.43 ;
0.5 4.1 0.43 1 0.96 0.45 2 0.74 3 0.75 4 0.83 0059 1.0 0.73 ; ~
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The resul-ts illustrate an important bene~it of our invention, namely that a very short intervening mixing time, o~ten a minute or even half a minute or lesæ, is su~ficient and that in fact the degree of suspended solids increases if mixing is conducted for as long as has often been necessary in the past.
This benefit means that the process of our inven-tion can be readily applied to existing sewage works where longer intervening mixing times are not possible without substantial alterations involving high capital outlay. Such a situation would occur where phosphate removal was -to be carried out be~ore the primary sedimentation stage and, due to the speed of flow of the sewage, only a short time would elapse between entry of the raw sewage to the works and its entry to the primary sedimentation tanks. Also, where a new works is planned to incorporate phosphate removal by chemical treatment, this ~eature o~ our invention means that extra land need not be taken up in order to ensure long periods o~ ~low of sewage between addition of metal salt and polyelectrolyte.
ExamPle V
The data presented in this example are obtained from results on the plant scale at a sewage treatment works. At this works, the raw sewage stream is divided into -two parts which then pass separately through dif~erent primary sedimentation tanks.
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A trial was carried ou-t to assess the process of our invention by trea-tin~ one part of the flow and making a comparison with the other~ untrea-ted part.
To one part of the ~low, ferric sulphate was added as a 62.5% weight by volume solution7 foLlowed by addition of a high molecular weight cationic polyelectrolyte of type No. ~ of the previous examples. The time taken for the sewage to flow between the two addition points was approximately 20 seconds. Mixing of the chemicals with the sewage was simply effected by placing a wooden baffle across the channel at each addition point to provide the necessary turbulence. ~ ;
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Samples were taken of the sewage at various s-tages and ` ;
-the total phosphorous content was measured. In -the table ;
below Sample A denotes raw sewage en-tering the plant. Sample B
denotes the sewage effluent leaving the sedimenta-tion tanks when no coagulant or flocculant is added. Sample C denotes -the sewage effluent leaving the sedimentation tanks when ferric sulphate and polyelectrolyte were added to the sewage leading ;
to the sedimen-tation tanks in the manner described abo~e.
Addition o~ ferric sulphate and polyelec-troiyte was~;
carried out only during the period of maximum flow, which was for about 8 hours per day, but analyses were made on composite -~
samples obtained over 24 hour periods. As these composite samples will therefore con-~ain some sewage which had not been chemically treated and as the performance of -the sedimentation tanks is subject to adverse effects from the build-up of sludge, a further series of samples was taken. These samples, D, were obtained from the chemically treated sewage before -~ - 19-... : ,~- ... ~ . ,. . , , : , ....................... . : .- , . .:: .

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:. - ::: . . , . . .. . . . : -entry to the sedimentation tanks to assess the performance of the invention under optimum condi-tions. The samples were allowed to settle for 5 minu-tes, and analysis was made o~ the total phosphorus conten-t o~ the supernatan-t liquid.
The average results and the range of results obtained over a period of 6 months are shown below.
A~erage ferric sulpha-te dose = 23~ mg/l Fe2 (S04)3. -~
Average polyelectrolyte dose = 0.5-mg/1.

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Average Average Total Phosphorus 10Sample PhosphorusRange Removal Range content (mg/l) (mg/l) (%) (%) A 5.2 3.7 - 7.9 , _ _ B 3.8 2.5 - 5.1 29.8 17.2 - 51.5 C 0.57 o.oL~_ 2.0 90.0 72.1 - 99.0 ¦ D ¦ 0 09 ¦ ~ 00- I 26 L 99 ~ 9Y~9 ¦

, Comparison of the results o~ samples B and C illustrates the e~fectiveness of the process o~ our invention on a plant ;`
scale. Although -the composite samples C contain some sewage to which the chemical additions had not been made, a high average removal o~ phosphates resulted. The results of samples D show that, under optimum conditions, virtually complete removal of `; ~
~ phosphates is possible. - -.
Further analysis of the above samples showed that compared with the untreatad part o~ the sewage after sedimentation, ... .. , ., ~ . , ., , , ~ . " . . . . . . . .. . . .. . . .. .... .
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i.e. samples B, the treated par-t of -the sewa~e a~ter ~;
sedimentation, samples C, had a suspended solids content lower by 66% and a Biochemical Oxygen Demand lower by 55%. :
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Claims (11)

1. A process for removing phosphate from a bulk effluent aqueous waste containing phosphate comprising the steps of, in order, (1) precipitating soluble and colloidal phosphate in the waste by adding an inorganic coagulant to the waste, (2) adding to the waste a cationic polyelectrolyte which is a water soluble salt or quaternary ammonium salt of a high molecular weight copolymer containing (a) units of alkylaminoalkyl esters of acrylic or methacrylic acid and (b), 80-97 mole %
acrylamide units, based on (a) plus (b), and (3) subjecting the waste to a liquids-solids separation process.

2. A process according to claim 1 in which the copolymer contains units of:
(a) (b) and in which R1 and R2 may be the same or different and are hydrogen or alkyl. R4 is alkylene containing 1 to 3 carbon atoms and R3 is methyl or hydrogen.

3. A process according to claim 2 in which R3 is hydrogen and R1 and R2 are methyl or ethyl and R4 is ethylene.

4. A process according to claim 2 or claim 3 in which the copolymer is present as a quaternary ammonium salt.

5. A process according to claim 3 in which the copolymer is present as a quaternary ammonium salt with dimethyl sulphate or methyl chloride.

6. A process according to claim 1, 2 or 3 in which the copolymer contains from 85 to 97 mole percent (b) based on (a) plus (b).

7. A process according to claim 1, 2 or 3 in which the copolymer has a molecular weight such that at 25°C the viscosity in centipoise of an aqueous solution having a pH of 6.0, containing 1% by weight of the polymer and in the absence of added salts using a Brookfield Model RVT viscometer with spindle No.3 at 20 rpm is greater than 2000.

8. A process according to claim 1, 2 or 3 in which the inorganic coagulant is a compound containing aluminium, ferrous or ferric or calcium ions.

9. A process according to claim 1, 2 or 3 in which the polyelectrolyte is added 15 seconds to 4 minutes after the coagulant.

10. A process according to claim 1, 2 or 3 in which the polyelectrolyte is added fifteen seconds to one minute after the coagulant.

11. A process according to claim 1, 2 or 3 in which the aqueous medium is raw sewage or effluent from a primary or secondary sedimentation process or the effluent from a biological sewage treatment process.