Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~ 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
"^ .
~ ` ~\
--5--
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
,'. ;;.
,,~ "
., ~,. ' ~
2;~
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-
-- 10 --
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~)
- 13 -
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 .
18
