CA1098224A - Waste water treatment - Google Patents

Abstract

WASTE WATER TREATMENT
Abstract of the Disclosure In order to dissolve in a liquid amounts of gas giving a relatively high concentration of gas in the liquid, it is usually necessary to pass into the liquid considerably more gas than is theoretically needed. This is because some of the gas will pass out of the liquid without dissolving and thus is wasted. This wastage is reduced by the present invention wherein, prior to the gas dissolving step, the liquid in which the gas is to be dissolved is subjected to a reduced pressure to release therefrom gases therein.

Description

~ 2 ~t This inventi.on i~ concerned broadly with a me-thod of dissolvins gases in aqueous liquids, and has partic~l.ar utility (although it is not limited thereto) in the treatment of waste wa$ers such as sewage.
S In many fields of technology, it is neces~ary to dissolve gases in liquids. In general, this is ef~ected simply by passing the gas into the liquid. Where a relatively high concentration of dissolved gas is requlred ~i.e. a concentration approaching saturation), it is usually necessary to pass in-to the liquid considerably ~ore ga~
than is theoretically needed7 since some of the gas will pa.ss through and out of the liquid without dissolving.
Whilst it is sometimes possible to re-use soma o~ tha excess gas, this is not always practicable. In any event, the necessi-ty to inject excess ga~ incri!ases the overall cost of the process.
There are many processes for the treatment of waste waters (by which term we include surface and wa~e water~ ~ener-ally, sewage, sludges, and a~ueous effluents from indu~tr~ . :
such as fermentation liquors) in which it i~ de~irable to achieve a relatively high concen*ration o-.~ dissolved gas (usually oxygen)~ For example 9 in the treatment oP wa~te ~aters such as se~age to re~ove impuritieq therefrom, ~he sewage is aerated to promote the activity of micro-organi~ms Z5 therein which respire oxygen and it is desirable to maintain a relatively high dissolved oxygen (D~o.3 concentration in the sewager An oxygen-enriched gas (~or example commer-cial oxygen) i~ injected into the sewage and, in practiae~
in order to maintain a high DØ concentration, it is necessary to supply considerably more oxygen than is actually taken up by the micro-organisms.
In another example, it is known to use an oxygen-enriched gas to prevent the ~ormation of hydrogen sulphide in sewers, particularly rising main sewers (see Progress in Water Technology, Vol. 7 (1975) No. 2, pages 289-300). Again, in order to achieve the highest desirable D.O. concentration, more oxygen has to be supplied than is actually taken up and used by the micro~organisms in the sewage.
Apart from the wastage of gas referred to above~
the general technique of dissolving a gas in a liquid by injecting the gas therein is attractively simple and econo-mic in operation. It would, therefore, be highly advan-tageous if this general technique could be used without the necessity for considerable wastage of gas which, in the case for example of commercial oxygen of oxygen-enriched air, is expensive.
We have now fo~md that this wastage can be reduced if the aqueous fluid is first subjected to a preliminary treatment prior to injecting the desired gas, Furthermore, we have found that this preliminary treatment can be effect-ed relatively inexpensively and that, after the preliminary treatment, relatively high concentrations of dissolved gas can be obtained in the fluid more easily and efficiently.
In accordance with one aspect of this invention there is provided a method of dissolving a gas in an aqueous liquid, which comprises first subjecting the liquid to a reduced pressure by passing it through a siphon, whereby gases previously dissolved/ entrained or generated therein are released and removed from the top of the siphon, and then dissolving the said gas in the desorbed liquid.

-4~ 82~
The ~ethod of the invention is particularly, but not exclusively, useful in the treatment of waste waters, in which case the gas to be dissolved will usually by oxygen. Thus, in the case of oxygenation of sewage (for which the invention is particularly but not exclusively use-ful), the desorbtion of volatile materials (which is effect-ed prior to an oxygenation step) makes it possible to achieve high D~Oo concentrations in the sewage without using the large excesses of oxygen-enriched gas previously necessary.
Waste waters such as sewage naturally contain vola-tile materials, principally gases (for example nitrogen and carbon dioxide) which may be dissolved or entrained therein.
Some of these gases may have been generated in situ. The presence of these gases not only reduced the ease with whi~h oxygen can be dissolved in the sewage~ but also reducas the : maximum DØ concentration obtainable. In addition, the presence of dissolved carbon dioxide in sewage lowers its pH
value and this has the effect of suppressing the rate at which nitrification can be achieved in the activated sludye process, particularly where oxygen-enriched gas is used in place of air. Thus, the removal of these gases (preferably to as great an extent as possible) in accordance with the present invention, is particularly advantageous in the treat-ment of sewage.
In accordance wi-th another aspect of this invention there is provided a method of treating waste water to remove impurities therefrom by oxidative digestion, which comprises at one or more stages of the treatment, subjecting the waste water to a reduced pressure by passing it through a siphon whereby gases previously dissolved, entrained or generated "^ .

~ ` ~\

therein are released and removed from the top oE the siphon, and then dissolving oxygen in the waste water (for example by passing oxygen-enriched air or commercial oxygen into the waste water). It will be appreciated that, in the overall 5 treatment of sewage to render it safe and disposable, the raw sewage may be processed into one or more sludyes and liquors, and that the preliminary desorption step of the invention is applicable at any one or more stages in the process prior to an oxygenation step.
In accordance with another aspect of this invention there is provided a method of treating waste water while it - is being held in, or is flowing through, a sewer which com-prises subjecting the waste water to a reduced pressure by passing it ~hrough a siphon whereby gases previously dlssol-15 ved, entrained or generated there:in are released and removed from the top of the siphon, and then dissolving oxygen in the waste water. This aspect of the invention is an improve-ment in the process described and claimed in U. K. patent No. 1,452,961.
A further ad~antage of ~he step of the present invention is that, after removal of a major proportion of volatile materials, the oxygen may dissolve almost completely in the waste water so that ~in contrast to prior Xnown pro-cedures) there is little, if any, undissolved gas in the 25 waste water. This minimises pressure increases which would otherwise occur because of the presence of entrained gases, and a]so increases the maximum D.O. concentration obtainable.
It will be appreciated that the step of ~emoving volatile materials ~rom sewage will effect removal not only ,'. ;;.
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., ~,. ' ~

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of "inert" gases such as nitrogen, but also of noxious and n~alodorous gases such as hydrogen sulphide. The removal of`
such gases under controlled condi-tions is, in itself~ advan-tageous since it reduces the risk of dang~r and inconvenience to personnel Crom the presence of those gases in the sewage and ~urrounding environment~
The volatile materials remo~ed from waste waters by the method of the in~ention can themselves provide useful information abou-t the waste water. Thus, the pre~ence and/or amount of one or more gases present in the desorbed materials, for example, can pro~ide an indication as to the chemical constitution of the waste waters which can be important in the case of, for example, sewage whose constitution con-tinuously changes as a result of the activit~ of the micro-organisms thereinO Analysis of the volatile materials remoYed can gi~e a more accurate guide thaII the con~ntional pseudo head space analysis techniques, palrticularly when a large proportion of the volatile ma-terials inthe waste ~a~er is removed.
The sol-uble organic content of settled sewage is u~ually removed by one of two processes, i.e. either ~h~
suspended-growth activated-sludg~e process or the atta~hed-growth percolating-filter system. Both of these proce~ses require relatively large areas of land. Recentl~? a process has been demonstrated in which the concentration o~ biomass per Imit volume iB greatly increased giving an enormous reduction in the size of equipment to achieve a givan degree of -treatment. This proc0ss employs expanded or ~luidized beds of sand or other small media (typically 0.2 - 2.0 mm size) ha~ing a very large surface area on which the micro--7~
organisms which effect the treat.ment grow. It is possible to operate an expanded or fluidized attached-growth system with the equivalent of 50 - 100 g biomass/l that can be held in the reac-tor compared with the normal 3 - 5 biomass/l 5 possible in conventional acti~ated-sludge systems. This allows the retention time required for treatment to be reduced from the 6 - 10 hours commonly used in activated-sludge plants to the order of 0.5 - l hour in a fluidized attached-growth reactor. This process has been proposed for 10 aerobic treatment of settled sewage, and for anoxic treatment of settled nitrified effluent to give effluent denitrifica-tion using an exte.rnal carbon source such as methanol.
The preliminar~ desorption step of the present in~ention can.with advantage be utilised in such a process 15 immediatel.y prior to the, or each, o~genation of step.
Accordingiy, in a preferred aspect the invention provides a method of treating waste water to reduce the con-centration of carbonaceous and nitrogenous substances therein, which comprises: (a) passing the waste water through a first 20 expanded or fluidized bed comprising particles having attach-ed thereto facultative heterotrophic bacteria, under anoxic conditions, to con~ert nitrate in the waste water to nitro- ;
gen gas, (b) subjecting the treated waste water produced in step (aj to a reduced pressure by passing it through a 25 siphon whereby gases previously dissolved, entrained or gene-rated therein are released and rsmoved from the top of the siphon; (c) dissol~ing oxygen in the treated waste water from step (b); and (d) passing the treated waste water from step ~c) through a second expanded or fluidised bed, comprising ., particles having micro-organisms attached thereto, to oxidise carbonaceous and nitro~enous substances in the waste water.
A highly preferred feature of ~his method is to subject part of the effluent from step (d) to a reduction in pressuxe to remove, inter alia, oxygen therefrom and then to recycle this efEluent to stage (a). In this way, -~he necessity for providing an external carbon source, such as methanol, for step ~a) can be avoided. If it is desired to substantially completely remove nitrate from the effluent from stage (d), the effluent (apart from any which is re-cycled to stage (a))- can be mixed with a source of carbon and then passed through a third fluidised bed, comprising particles having micro-organisms attached thereto, to remove nitrate from the said effluent.
In the method of the invention, the manner in which the~aqueous fluid is subjected to a reduced pressure is not critical. Those skilled in the art will appreciate that there are many possible techniques. With small quanti-ties of aqu~ous fluid, for example, the fluid can be placed in a vessel which is then evacuated to reduce the pressure above the fluid surface~ After sufficient time has elapsed for the desired quantity of volatile materials to be removedr the vessel is returned to atmospheric pressure ~internally) and the fluid released. The desorbed volatile materials may be collected for analysis.

Whe.re larser quantities of fluid, e~gO wast~
water, are ~oncerrled7 however, and where an inexpensive continuous operation is required~ we prefer to pa~s th~
waste water through a siphon, at the head of which a reduced pressure is maintained to desorb volatile materials from the waste water passing through the siphonO
In order that the invention may be more fully understood, various preferred embodiments thereof ~ill now be described, by way of example only, with reference to the accompanying drawings, in which:
FIGI~E 1 is a sche~atic vertical se~tional ~iew - of one form of siphon arrangement;
FIGURE 2 is a schematic vertical se~tion~l vi~w of another form of siphon arrangement;
FIGURE 3 is a sche~atic representation o~ a waste water treatment process embodying the inven-tion;
FIGURE 4 is a diagrammatic vertical elevation of one form of siphon and oxyg~n injection devic~ used in Figure 3;
FIGURE 5 is a schematic repre~entation of another water treatment process ernbodying -the invention; and FIGURE 6 shows an alternative arrang~ment for carrying out essentially the process of Figure 5.
Referring to the drawings~ in Figure 1 wa te water is ~ed to a reservoir tank 1 from ~hence it pa~ses up conduit 2 into vessel 3. From-vessel 3, it exits vla conduit 4 which terminates at a level below tank 1. Tank 3 includes a gas exi-t line 5 connected to a vacuum pump 6 having a vent 7.
In operation~ conduits 2 and 4 and tank 1 operate _ g _ 3L~9~3224 ~-s a siphorl for the waste water, and once -the siphon has been initiated, the waste wa-ter flows -through th~ conduits 2 and 4, ancl tar~ 3, as shown~ In the head ~pace o in tank

3 the vaouum pump 6 maintains the reduced pressure produced by the siphon and the gases desorbed from the waste water exit via line 5 and vent 7 (and may be analysed)~ The height of the water in the siphon can be up to the maximum attainable under prevailing at~ospheric conditions and the vacllum pu~p can be operated a* any pressure down to the minimum pressure attain-able. In prac-tice, the operating conditions will depend on (i) the flow rate of the waste water, which i~ depend-dent on its viscosity and on *he dimensions of the conduits and the height of *he sipho~;
(ii) the amount of gRS removed which depznds inter alia on the height of the siphon, the gas solubility and the vacuum applied; and ~iii) the vapour pressure of the waste water at th~ pre-vailing temperature.
The arrangement in Figure 2 is slightly differ~nt from that in Figure 1, in that conduits 2 and 4 and tank 3 are replaced by a vertically mounted cylinder 10, closed at one end except for connecti.on to a vacuum pump 11, and having a partition 12 located diametrically across the tube over a major part of the length of the tube. An inle-t ~3 ~or waste water is pro~ided at the i`oot of the -tube 10 on one 3ide of the ~artition, and an outlet 14 is provided on the other side of the partition at a level below inlet 13 (*o provide a siphon). The apparatus is operated in essentially the same way as that of Figure 1~
In the method of the invention as applied to the oxygenation of waste water in a sewer (principally to pre-vent or recluce hydrQgen sulph;de formation), a siphon can be provided i~ or adjacen-t the sewer, suitably at a 3ump or wet ~ell, a~d the sewage passed there-through (with de-sorption of gases) immediately prior to oxygenation.
Figure 3 of the accompanying drawings show~
schsmatically and hy way o-f ill~stration only, a wa~te water treatment process embodying the in~ention as applied to municipal sewage. Figure 4 is a diagrammatic vertical elevation of the siphon and oxygen i~jection devlce 50 in Figure 3.
Referring to Figure 3, crude sewage is fir~t sub-jected to con~entional treatment involving screens ~nd grit removal (40). The sewage i5 then subjected to a desorp~ion step according to the invention, followed b~ oxygenation, in a device 50 (to be described in connection with ~igure l~)o In device 50, volatile materials are removed from the ~ewage which is then immediately oxygenated to a DØ concentration o~
40 to 50 mg~litre~ The oxygenated sewage passes to a con~entional primary sedimentation step 60 in which a primary sludge is separated from a liquorO The liquor from this step (DØ
may be zero) is then passed through a desorption/oxygenation devicc 70 (same as device 50) to raise the DØ concentration to 40 to 100 mg/litre~ Biological oxidation takes place in tank 80 (under agitation wi-th stirrer 81) followed hy a con~en-tional secondary settle~e~t ~tep 90 and the ~inal ef~luent emerges from pipe 91~ Se-ttled qludge is recirculated via a desorption/oxygenation device 100 (same as device 50) to the tank 80.
Figure 4 ~hows a siphon 110 with an oxygen ~i.e.
oxygen-containing gas) injection device 120 immediately d~wnstre~m thereof~ This arrangement o~ Figure ~ ~in which the heigh* of the wa-ter in the siphon is -the maximum attainable under normal atmospheric pressure) is the devlce 50, 70 and 100 referred to in Figure 3. Gas is removed from the siphon head 111~
By operating as descri~ed in Figure 3, the efficiency overall of the process is improved over con-ventional procedures, in that the DØ levels r~equired are more easily and efficiently achieved~
The removal of gases (or reduction in the amount of gases) in waste waters according to the in~ention is also useful in the treatment of waste ~ters to remove ammoniacal nitrogen. At present, ammoniacal nitrogen i~
removed by increasing the pH of the waste water and stripp-ing out the ammonia b~ p~ssing the wa~te water through a scrubber with a counter-current flow of air. There are problems in this process including the formatio~ o~ lime scale in the scrubbing tower. Accordins to an aspect o~
the present invention9 in a waste water treatment method o~ the in~ention, ammoniacal nitrogen can be removed by subJecting -the alkaline waste water to reduced pressure to desorb the ammonia~ For this purpose, it may be advantageous to use a series of siphon arrangements a~ described abovec It will be appreciated that an important are~ o~
use of the invention is in the pre-treatment o~ waste waters prior to an aeration or oxygenation step. Example~ of this have bcen given above. A further example is in the re-aeration of surface waters such as river waters~ Thu~, the river water is first subjected to reduced pre~sure to desorb the gases therein, and is then oxygenated or aerated.

Z2~

~igures 5 and 6 illustrate fluidised (or e~panded) bed attached-g.rowth ~aste water treatment methods embodying the invention~ In ~igure 5, settled sewage is deli~ered via lines 200 and 201 to the first fluidized bed (A) 7 with a racycle stream from 1.ine 202, containing nitrate ions. The recyc:le stream may have pre~iously had the dissol~ed oxygen removed from it. Under the anoxic conditions that prevail in this bed, the f`aculative heterotrophic bacteria modify their metabolism to use nitrate ions as a source of ox~gcn in the absence of dissolved oxygen and in doing so break down the nitrate via ni*rite to nitrogen gas w~ich bubbles off (arrow 203) from the surface. The bacteria use the carbonaceous material in the 3ettled sewage as the necessary energy source~ The uses of (i) nitrified effluent recycling ~5 and (ii) settled sewage as the carbon source in modified activated-sludge plants have enabled the efflue~t nitrate concentration to be reduced by between 70 and oOYo, leading to concentra-tions of 5 - 10 mg N/l in the effluellt. Thi~
concentration is below the World Health Orga~isatio~ recommended drinking-wate~ standard of 11.4 mg N/l. The ~luidiæed-bed process described here can be so operated as to be equally eff-ective in terms of overall remo~al of nitrate1 and can achieve denitrification in a shorter period of retention because of the much grea-ter we:ight of biomass present. The bacteria are contained in the slime layer that develops on the ~and particles. The superficial retentiQn time necessary will be in the range 3 - 10 minutes depending upon temperature.
The ammonia present in the settled sewage will pass through the anoxic f'luidized (or expanded~ bed unchanged~ In ~ewage, ammonia is present in two forms, either as ammonium ions (r~H~) in true solutiorl or dissolved gas (NH3). At the normal operat:ing temperatures of` 10 - 20 C and at the pH value encouIltered at this stAge in the system (7.5 - o,5) gre~ter than 90% Or the ammonia will be present as ammonium ions in true solution. At this point the liquid is pas~sed along line 20ll to a siphon degasser ~B)~ Some of the liquid in line 204 may be recycled ~ia line 205 to line 201. Gase~ are removed from the head of degasser (B) via vacuum line 206. Di~solved carbon dioxide and nitrogen tog0ther ~lth a small amount iO o~ ammonia will bestripped out in degasser (B~, Stripping of these waste gases then allows greater concantratiGns of oxygen to be dissolved in the liquor at the next stage of treatmentO
The degassed liquor from (B) passes ~ia l~e 2~0 to (C~ where o~ygen is dissolved in the liquor prior tG itS
passage via line 211 to the second fluidized bed o~ ~and particles (D). The superficial retention time ~eces~ary is of the order of 0.5 - 1.0 hour.
Oxygen is dissol~ed in so3ution by a de~ice such as a Venturi dissolver or more effecti~ely by the EZ-GAS
system. Sufficient oxygen is dissol~ed in the li~uid ~o supply the oxidation requirements of the carbonaceou~ and nitrogenous conten-t of the liquid~ The oxygen m~y be supplied as (i) commercial oxygen7 (ii~ oxygen-er~iched air, liii) air, (i~) hydrogen peroxide, or any other suitable sourceO The effluent line 2/3 will contain some di~001ved oxygen but the input of oxygen at ~C) is controlled ~o t,hat this is the miniml~n which will still allow nitrifyins con~
ditians *o be mai~tained in (D~ since some 70 to 850/D of the effluent is now recycled via lines 214 and 202 to the first fLuidized bed which is kept anoxic. This effluent may be recycl,ed to the first (anoxic~ flu:idized bed (A) via line 214 and another siphon degasser (E). This siphon de,gasser ser~es to remove di~sol~ed oxygen (220) which would be detrimental to the operation of the first fluidi~ed bed since it would consu~e some of the carbonaceous material present in tha settled sewage which is nece~sary ~or the ~enitrification reaction. The remaining ~5 _30% of the liquid flowing from the sec~nd fluidized bed is discharged as a completely treated effluent, possibly after settlement, or some further polishing process.
If complete remo~al of nitrate is ~eeded~ a further small fluidized or expanded bed may be added to treat the effluent. It would be necessary to use an ex~er-nal sour,ce of carbon; methanol has been commonly used ~or this purpose. According to the present in~ention, the meth-anol requirement is reduced by between 70 and 800/Q, co~p~rod wi*h existing denitrifying reactors, as a result of the efficient use of the carbonaceous material in the ~ettl*d sewage as described abo~eO
The control of growth of the biomass in the colwmn may be effected by pumping the coated sand from the twc fluid~
i~ed beds via devices which shear the biomas~ from the sandO The mixtures of sand and biomass are ~hen ~eparated by equip~ent such as hydrocyclones, elutriation column~, centrifuges, or ~ibrating sieves. The cleaned sand media is recycled to the fluidi~ed beds while the biological sludge may be thickened before disposal. If vibrating sie~as or centrifuges are used the sludge produced may be concentrated ~0 enough to dispose of without further treatment, ha~ing a dry-- ~5 -2~

solicls content of 5 - 20% w/wO
The amount of s~lspended solids passin~ fro~n the fluidi~,ed beds will be relatively low and in some cases secondary clarification o-f this effluent may not be necessary - treatment of the effluent by sand filtration will probably be the most effecti~e way to achie~e a very high quality effluent, containing less than 10 mg SS/l.
The system described can be operated with a diuranally ~aryinS flow r~te. It is essential that the sand bed is not displaced fro~ the reactor by an increa~ed up~low ~elocity. There are several ways of ensuring that this does not happen.
i) The reactors ma~ be built in a conical o;r pyramidal ~orm allowîng the volume of the sand bed to be varied o~er a wide range of upflow velocities.
ii~ The amount of liquid recyc:led via lines ~05 ~nd 212 arolmd each fluidi~ed-bed reac-tor can be varied to maintain a constant upflow velocity through the bed~
:iii) The syste~ can be so designed that at the lowcst flow rates the sand bed is no-t fluidiz~d but acts as an expanded bed occupying about half the total ~olume of the reactor, but as the flow rates (and hence upflow velocity) increase the bed expands and becomes fully fluidized, but it is s-till r~tain~d within the reactor.
Amon$ the ad~antages which can be obtained by use o~ a system in ~ich degassers are used in conJuncklo~ ~ith two or more fluidized beds, for example, as shown in Figure 5, are:

~ 16 -i) Cvnlpl.o~e ~oevrl~ary ~:reatmerlt Or settled ~wage i.n a re~erl~iorl time ol` about l~ ours, witl~
:removal of about 75% of the nitrat~.
ii) ~hc llig}~ concentration of biomass allows.the size of plant to be very small, which can reduce the land area needed by about 8CP/o compared with th~t of cunventional plant.
iii) Removal of nitrate to less than th~ W~O drinking-water standard without use of an addition~l carbon ~ource.
The need for ~econdary c3.arification i9 elimin-ated or greatly reduced.
~r) A capital C09t reduetion o~ about 3C)% may b~ possible because o~ the greatly ~educed reactor voltun~s needed~
vi~ Existing overload~d plants can be easll~ uprated without using more land becau~e the plant m~y be gradually replaced by a much smaller plant btlilt on the exi~ting ~ite~
~ii ) Thick sludses are produced.
~0 viii ) Diurnal vari.ation in feed flo~ rate can be accomtnociated.
i~) The si~holl degas~er en~ures maximum utili~ation of oxygen since none is tu~ed in displ~ing dls-solved gases~
'~5 An alternative arrangement of using the siphon degasser is tv incorporate one or more flu;disCd beds in the suction side ~hus avoiding the use of separate columns. The method of operation and functions of the ~pparatus would be similar to th~t desoribed for the configuration shown in Fig~-ure 5.
.

- ~7 -Orls ~ulch ~4rran~3emerlt i~ 3ho~ in FLfi5llrç9 6., Th~
corlic~lly~ha~p~d unit A 1~ the he~d o~ th~ ~3ipha,~
fr s~m which ga~es Are with~awn throu~h llne~ 302.
Settled ~ew~ge aff'lu~nt ~n~er0 throu~h llne 300 in 5 ad~ with ~ ps~rtion s3~ tlr0at~d ~ltrifled a~flu~nt in li~e 308 ~nd is pumped int~ A (~la plL~Dp 301~. llh~

l~ws3r p~rt of` Yassel A ee~prise~ ~ bi.olo~;lc~l fluidi~ad bed of aand whi~h operates und0r ~r~o:c:Lc corldition3 to denitrif'y the ~eed. The upp~r p~rt O~e th~3 ~a~ l la 10 mainltained at a low pr~3a~ure ~nd acts~ th~ he~d s~ th~ ~
degas~er. The clegae~ed ~luid p~ B ~Ut 0~ ~ down l~n~
303 in which oxygen is i~aJeeted ~via line 304. PW~P 305 p~sse~ th~ .mixtur~ to 9. ~acon~ ~luldiJ~d b~d D o~ nd particl~0 Th~ ~and i~ recy~led Yi~ llne 310, p~p 311, 15 ~3~nd ela~nillg d~Yice 312 ~adl r0tlsrn l~ne 3t3. I;n bed D
th~ liquid :L8 ~ub~ect~d to oxidlaltio~ o:~ lt~ csrbonAceou~
~nd nitrO~eJaOU~ compon~lt~. Ihe tre~ted liquidl p~e~q Yi~l' line 306 to ~ third f`luidl~ed b~d ~F l.n ~hich the ~mmoniated cl)ntent i8 ~xidl~d to rsitrat~. P~rt Dr th~

20 e~f~luent 1 eaving F by lin~ 307 .i0 t~ken up l ine 308 to th~3 t~p of the siphon ~f3 pr~io~a~ly dY,~ ilbed.
In thi~ {~rrang~ent, A, F and 1~ ~r~ ~rr~nged in ve~tic~l ~tack, the over~all heiLgh~ o~ which equ~l~ th~l:
~ th~3 ~3iphon n~ce~ry to ~chievo the reguir~dl dega~ing.
25 In e~eet, unit~ F ~nd 1) ~re in- the u~s~.r Aer li~b o~ the ~ipholl .

Claims (14)

WHAT IS CLAIMED IS:

1. A method of dissolving a gas in an aqueous liquid, which comprises first subjecting the liquid to a reduced pressure by passing it through a siphon, whereby gases previously dissolved, entrained or generated therein are released and removed from the top of the siphon, and then dissolving the said gas is the desorbed liquid.

2. A method according to claim 1 wherein the aqueous liquid is a waste water and the gas to be dissolved therein is oxygen.

3. A method of treating waste water to remove impurities therefrom by oxidative digestion, which com-prises at one or more stages of the treatment, subjecting the waste water to a reduced pressure by passing it through a siphon whereby gases previously dissolved, entrained or generated therein are released and removed from the top of the siphon, and then dissolving oxygen in the waste water.

4. A method according to claim 3 wherein sewage is treated to render it safe and disposable and wherein, after subjecting the sewage at one or more stages during its treatment to a reduced pressure, air or another oxygen-containing gas is passed into the sewage to dissolve oxygen therein.

5. A method of treating waste water while it is being held in, or is flowing through, a sewer which com-prises subjecting the waste water to a reduced pressure by passing it through a siphon whereby gases previously dissolved, entrained or generated therein are released and removed from the top of the siphon, and then dissolving oxygen in the waste water.

6. A method according to claim 5 wherein the waste water is sewage and wherein air or another oxygen-con-taining gas is injected into the sewage to dissolve oxygen therein.

7. A method of treating waster water to reduce the concentration of carbonaceous and nitrogenous sub-stances therein, which comprises:
(a) passing the waste water through a first expanded or fluidised bed comprising particles having attached thereto facultative heterotrophic bacteria; under anoxic conditions, to convert nitrate in the waste water to nitrogen gas;
(b) subjecting the treated waste water produced in step (a) to a reduced pressure by passing it through a siphon whereby gases previously dissolved, entrained or generated therein are released and removed from the top of the siphon;
(c) dissolving oxygen in the treated waste water from step (b); and (d) passing the treated waste water from step (c) through a second expanded or fluidised bed, comprising particles having micro-organisms attached thereto, to oxidise carbonaceous and nitrogenous substances in the waste water.

8. A method according to claim 7 wherein part of the effluent from step (d) is subjected to a reduced pressure by passing it through a siphon whereby gases dissolved, entrained or generated therein are released and removed from the top of the siphon, and then recycling the effluent to the first fluidised bed.

9. A method according to claim 7 or 8 wherein effluent from step (d) is mixed with a source of carbon and then passed through a third fliudised bed, comprising particles having micro-organisms attached thereto, to remove nitrate from the said effluent.

10. A method according to claim 7 or 8 wherein the waste water is sewage and wherein oxygen is dissolved in the treated waste water by injecting therein commercial oxygen, oxygen-enriched air, or hydrogen peroxide.

11. A method according to claim 1 wherein the height of the liquid in the siphon is, or is close to, the maximum attainable under prevailing atmospheric conditions.

12. A method according to claim 11 wherein oxygen injected into the liquid after it has passed the top of the siphon but before it leaves the siphon.

13. A method according to claim 11 wherein oxygen is injected into the waste water immediately after it leaves the siphon.

14. A method according to claim 7 or 8 wherein effluent from step (d) is mixed with a source of carbon and then passed through a third fluidised bed, comprising particles having micro-organisms attached thereto, to remove nitrate from the said effluent, the waste water being sewage, and wherein oxygen is dissolved in the treat-ed waste water by injecting therein commercial oxygen, oxygen-enriched air, or hydrogen peroxide.