Note: Descriptions are shown in the official language in which they were submitted.
1(1~;~4Z~ l
; ~his invention relates to methods o~ treating waste
material by biological means and more particularly
to ~quch methods o~ treating water polluting waste
materials. '
In conventional methods oY treating water polluting wa~te 1~
materiala by anaerobic digestion, liquid carrying the ~ I
anaerobically digestible waate material is introduced
into a closed tank in which thewaste material is mixed'
with anaerobic bao'teria at a temperature enabling the 1 -;'
I0 ' bacteria to grow by feeding on the waste material, thereby ! :
; breaking down the waste material. Such anaerobic digestion
processes may be continuous processes or batch processes~ I
but in either case a treated liquid is removed ~rom the ~ ~ -
tank containing quantities of waste material and o~ 1 ;
bacteria. In general the liquid removed from the'tank ~
will contain substantially aimilar concentrations oY waste
material and oY bacteria as the bulk of the liquid
remaining in the tank. This haa led to the use of large
tanks and a long retention time oY liquid within the tanks
' 20 ~ in~order that the concentration o~ waste material in
' the treated liquid removed ~rom the tanks might be a~ low
as possible, and in order thst as large a proportion a~
pos~ible o~ the bacteria in the tank might be retained 1
within the tank. IY it is desired to use a very much
smaller tank to treat the same amount o~ waste material
or to treat a very much larger quantity o~ waste materlal '
., ~ . ~ .
in the same period oY time with the same size o~ ~ank
it has been found that the concentration o~ bacteria
~in the treated liquid removed Yrom the tank is sub tantially ' ~ I
'; .
~. : " , . ~ ~' ' ' ' ",' ,,'', '
~ C9579~Z~
increased, and it ha~ been necessary to perform
a separation step on the treated liquid in order to
reclaim bacteria for returning -to the tank, and
thereby to enable the liquid to be brought to a ~tate
of cleanliness in which it may be fed, for example,
in~o a river after a minimum of further treatment, for I I
` e~ample, by aerobic digestion.
,
There is described in the litera-ture the use o~ a gravity
separation process for separating sludge containing the
~l0 ~ bacteria from the liquid content of the ~reated liquid.
In practioe, it has been found that the degree of ii
eparation obtained using such a gravity separation
prooess is so low that a further separation would 1;
be necessary and recourse to more sophisticated separation !;,
` ~; techniques has therefore been necessary. However,
; 3eparator~ for carrying out the more sophisticated
;separation teohniques are expensive items of equipment.
The preaent Applicants have m vented a method of treating
waste material which is surprising in that it does not ~ ~
invo1ve the use of the more sophisticated sepa~ation ~ -
technique~ and provides a sati~actory degree o~ separation
in a gravity ~eparation proce~s
During the course of their researoh the Applicants have
found that the application of a thermal shook to the
treated liquid has the effect of temporarily causing i~
~ the gas produc~ng activity of the bacteria to be 1 -
i sub~tantially inhibited and that there 1S thereby oreated ,~
., !
:- :
.; 1..'.1 ~ .
' ; ~-:`
~oS~Z~29
a period of -time within which a gravity sepaLation may
take place without disturbance from the activity of
tne bacteria. Provided that the gravity separation is
effected wi-thin the period so crea-ted, sludge containing
the great majority of the bacteria from the treated
liquid may be removed from the liquid and returned for
mixing with further waste material, leaving a relatively
clear liquid as the other product of the gravity
separation.
: ~ :
However, if the gravity separation is allowed to
continue for too long a period of time before the sludge
is removed, the bacteria may recover their activity
to an extent sufficient for enough gas to be generated
to cause gas bubbles to carry sollds from the sludge
into the relatively clear liquid, thereby nullifying
such separation as has been achieved.
] :
.j .
According to the present invention, there is provided
a method of treating waste material comprising the
' ~ steps of supplying waste material to an anaerobic digestion : :
`i~ : tank in whlch anaerobic digestion is taking place,
maintaining the contents of the anaerobic digestion tank
at a substantially constant temperature, withdrawing
treated liquid from the tank, subjecting bacteria in the
treated liquid to a thermal shock by passing the treated
liquid through a cooler to effect a reduction in temperature
of the treated liquid of at least 8C in a period not
exceeding ten minutes, introducing the cooled treated
liquid from the cooler into a gravity separator, allowing
bacteria in the cooled treated liqùid whose gas-producing
Z activity is temporarily inhibited by the thermal shock to ~-
., j .
, ~
. .
~1.0579LZ~ ~
settle in the cooled -treated liquid in the gravity
separator to leave an upper layer of relatively clear .
uid and a sludge at the bottom of the gravity
separator, removing relatively clear liquid from ~the
upper layer and removing sludge containing bacteria from
the bottom of the gravity separator at a rate which is
such that bacteria are removed from the gravity
separator before the bacteria can recover sufficient
gas-producing activity to cause substantial quantitie~
of solids in the sludge to rise towards the upper layer, ~ :
and reintroducing at least a portion of the sludge
oontaining bacteria into the anaerobic digesti.on tank.
Although a satisfa~ctory degree of separation may be
obtained
..
; ' ' / ~.: . ~
';1 / "~ ''' "
.~ / .',
:. / ':
- 5~
. .. , . -, . ... . - ... . ` . . ...... , . , . ~ .
... . . .. ~ . ... ... ... .. ..... . ..
- ~()S79L;~
in the gravity separator when no precautions are -taken to
prevent solids from the sludge being raised into the upper
layer other than removing the sludge from the bottom of
the gravity separator before the bacteria ln the
; sludge have regained sufficient gas producing activity
to raise substantial quan-tities o~ solids in the sludge
into the upper layer, it is preferred that solids
. should be preven-ted from floating into the upper layer
by separating the upper layer from that part of the cooled
treated liquid containing the sludge by a region I .
oontaining a packing material which is capable o~ causing
;~ : solids to be stripped from any gas bubbles passing
through the packing material and into the upper layer. I -
.- The packing material is arranged so that if any æolids
:: are raised by gas bubbles towards the sur~ace of the
`: oooled treated liquid, they encounter a turbulence
` ~ as.they float around the sharp edges of the packing
material, which is sufficie~t to cau~e the solids to be
dialodged from the. gas bubble~. ~he ga~ bubbles can then l.
oontinue to rise and escape through the surface of the
upper layer,.while the solid3 settle once more toward~
: the bottom of the gravity separator~
., i
. G The packing material can be any material which has the i.
desired hydraulic characteristics, but it has been
1 found particularly advantageous to use material having
: the configuration o~ Ra3chig rings made of a plastics
material having a density le3s than the density o~ ¦
water, and means such as a grid is provided to prevent
.the Raschig rings ~rom floating to the sur~aoe o~ the
6-
.
, .:. ~ . . . . .... . . ..
~0574z9 1
liquid in the gravity separa-tor. Ait~ an alternative
to a packing ma-terial it is possible to use sheets o~
perforatted metal or expanded metal, either as single -~
qheet~ or a sandwic~ constl~ction of several sheet3.
At least a portion o~ the sludge removed from the
gravity separator will be returned to the anaerobic digestion
tank and it is preferred for the gravity separator to
be arranged so that sludge may be returned under the action
of gravity to the anaerobic digestion tank rather than
~ having to be pumped.
!, ~ ~ . ~ , , .
In a preferred embodiment of the invention which will
; be described, the treated liquid is pumped vertically
upwards through the cooler. ~he preferred form of pump
for passing the treated liquid through the cooler is a ¦ ~
gas llft pump. ~ --
:i .
he period o~ time in which the reduction in temperature
must be effected will vary according to the;magnitude of
the reduction in temperature. ~he smaller reductions
; ~` in temperature are advantageously effected in periodq of
20 ~ ~ five minutes or leEtts, or even in periods of time as i
short as one minute or less. It has been found to be
particularly advantageous to effect a reduction in
. . .
-' ~ temperature o~ 10C in a period not greater than one
; ~ minute. In a preferred embodiment of the invention
which will be described the temperature of the treated
liquid is reduced from substantially 35C to substantially i~ -
25C in a period o~ time of the order of 30 seconds.
,, 1:
:~, , I ' :
-7~
. . ` '` ' ~'.
r. ; , , ~ , .. t
- ~057~29
The time which is available for gravity separation before
the bacteria in the sludge regain su~icient gas producing
activity to raise substantial quantities of ~olid~ in the
sludge into the relatively clear liquid may vary according
to the magnituAe of the reduction in temperature and the
rate o~ cooling -to whioh the treated liquid has been
subjected. However, in the preferred proces~ of the
:~ invention which will bs described, in which th~ temperature
of the treated liquid i.s reduced by 10C in a time of the
order of 30 seconds, the average time spent by the bacterla
in the cooled treated liguid before removal in the sludge is .
of the order o~ ten hours.
The present invention will be further understood ~rom the
following detailed description of preferred embodiments
- !
~ thereof, which ia made by way of example only and with
,~ ~ reference to the accompanying drawings in wbich: !
Figure 1 is a ''T910Ck diagram showing the basic step~
in a pre~erred effluent treatment prooess,
Figuré 2 i8 a cross-section through a preferred
20. ~ apparatus ~or the treatment of effluent, and I -
::, . I~s ~ Figure 3 is a cross-section of the apparatu~.. o~ !
. .~
~ igure 2 taken on the line I - I in that Figure~
~ ~ , .
Figure 1 is a block diagram showing the basic steps ~ :
involved in a preferred process in accordance with the
present inven~ion. Raw effl~lent is fed from a source of
.~ . 1 .,
~I raw e~fluent 1 through a heater 2 by which it is heated .
.. . ; ~efore it passes into an anaerobic digestion tank 3 in which
it i8 ~uba~ot~d to anaa~obio digestion. 1 -- ;
~ I :
.. . .
'' ',' , ~ '' . ~ ' ~ . '
~ )S79~29
Treated liquid is removed -from the anaerobic digestion tank
3 and is subjected to sudden cooling in a cooler 4. ~he
temperature drop to which -the treated liquid is subjected in
the cooler L~ is greater than 5C and is preferably o~ the
order of 10C, 90 that in a typical case in which the content~
of the anaerobic digestion tank 3 are at a temperature of the
` order of 35C the treated liquid which is fed to the cooler 4
has it~ temperature reduced to a temperature o~ the order of
25C. It is important that the drop in temperature should be
sudden in order to provide the thermal shock, and the time l- -
taken for the temperature drop is there~ore o~ the order of
minutes rather than hours. In a preferred application the I
temperature is reduced from 35C to 25C in a time of the
,
order of thirty seconds.
The cooled treated liquid is fed from the cooler ~ to a gravity
`~ ~separator 5. The bacteri~a in the cooled treated liquid enter-
ing the gravity separator 5 are present in the ~orm of a
dispersed suspension. ~he thermal shock has the effect of
~ temporarily reducing the gas producing activity o~ the bacteria ¦~~'f~ ; ' 20~ and after entering the gravity eparator 5 the bacteria tend to
flocculate into clumps which are relatively denqe and which
settle in the gravity separator 5 to form a sludge at the
bottom of the gravity separator 5. ~he sludge containing the , -
bacteria is then removed ~rom the bottom of the gravity i~
separator 5 and all or part of this sludge is returned to the
;. , .
i ~ ~ anaerobic digestion tank 3. A relatively clear liquid 1~ !~
removed ~rom the top of the gravity separator 5.
"
The e~fect of the thermal shock in reducing the activity
' . ,. I ~-
, _9_
. ~ ,
"; . , , ... - ,- . ,. .. : . : , :
~ [35~4Z~
of the bacteria is only temporary and, after retu~ling
to the anaerobic digestion tank 3, the bacteria regain
at least part of -their ~ormer aotivity. Even at the lower
temperature in the gravity separator 5, the bacteria m y
regain par-t of their former activity and the sludge i9
there~ore withdrawn ~rom the bottoJ~of the gravity
separator 5 before the bacteria have produced sufficient
gas to cause the sludge ~o rise to the surface of the
gravity separator 5. In the preferred proces~, in which
`the ælud~e is removed continuously ~rom the bottom of
the gravity separator, it has been found that the sludge ,~
is present in the gravity separator 5 for a period o~
the order of ten hours. '~
In some applicatio~s of the invention, the heater 2 and
the cooler 4 may be combined in a single unit ln which
the treated liquid from the digestion tank 3 is used to ,'
heat the raw ef~luent from the souroe of raw e`ffluent
l, and the raw effluent cools the s1udge from the anaerobic
digestion tank 3, thereby providing the thermal shock.
.. ~ ~ j~:
~ 20 ~ ~ Re~er~ing to ~igures 2 and 3 of the drawings, there is l~
.; ~ .
shown an apparatus which includes the anaerobic digestion
tank 3, the cooler ~ and the gravity separator 5 o~
~ Figure l. The apparatus includes a mild steel tank 6
-~ ~ which is rectangular in shape and which has a base 7,
. . , .
end walls 8 and side walls 9. The internal surfaces of
the tank 6 are coated with an epoxy resin pla3tics
material as a protection against corrosion.
The tank 6 i~ provided with a roo~ 10 whlch ma~ also be o~ ~ ~
-10- , ,, ~,,
' ' ' ' '":
~`.:, , :, . .
S7429
~: mild steel and coated on the inside with an epoxy re~in
plastics material. ~he upper part of the space defined by
the tank 6 and roof 10 i~ used as a ga~ ~torage compartment
. 11. ~he lower par-t o~ the space de~ined by the tank 6 and
the roo~ 10 is divided into an anaerobic digestion tank 3
and two gravity separators 5 by a pair o~ sloping walls 12.
~he sloping wallq 12 are of sandwich construction ha~ing. ,~ .
outer surfaoes of glass fibre reinforced plastic~ material
, qeparated by a foamed plas-tics material. This sandwich ,
construction has been found to provide desired qualities
of rigidity, durability and thermal in~ulation. ~ ¦
The ends 13 o~ the ~loping wall~ 12 fit flush with the end i~
walls 8 of the tank 6. The lower edges 14 of the 810ping ~:
walls 12 approach closely the side walls 9 of the tank 6. : ~ ;
However, a ~mall gap 15 is left between the lower edges 14 ' : -~
and the side walls 9 in order to allow ~or the~return of ,
sludge containing bacteria from the bottom of the gravity '
eparators 5 into the anaerobic digestion tank 3. . ~
Pre-heated raw ef~luent which ha~ been de-aerated is :, ,.' :
', .: 20 ~ ~ pumped into the anaerobic dige tion tank 3 through ' ''~'~'"'~''.'
a waqte inlet 16 in order to maintain thevo,lume of the
liquid cont.ent~ o~ the anaerobic digestion tank 3
i substantially constant at a le~el indicated by the line 17.
. ..
When the proce~s.is set up, the e~luent in the anaerobi¢~
digestion ~ank 3 i~ ~eeded with anaerobic bscteria. . ',:
~J ' "~
.` ' '' ~, '
~LC3 5742~ ;
In general, the anaerobic digeqtion process can be operated
~atis~actorily at temperatures within the range o~
~rom 30C to 40C. However, it is pre~erred for the proces~
to be maintained at a temperature of the order of 35C
for optimum e~iciency.
In order to maintain the contents o~ the anaerobic
digestion tank 3 at a constant temperature it i9 `
necessary to heat the contents of the anaerobic digestion
tank 3 and to ensure that the contents are thoroughly
mixed. Heat is supplied to the contents o~ the anaerobic
digestion tank 3 through heat exchange pipes 18 which i
are supplied with heated fluid by means of inlet and
outlet pipes 19 and 20~ Mixing of the contents of . ':
the anaerobic digestion tank 3 is achieved by passing : 1.
gas ~rom a per~orated tube 21 into the content~ o~ the
anaerobic digestion tank 3. Gas is pumped from the gas
storage compartment ll through an inlet tube 22 to the
perforated tube 21. Gas emitted from the perforations
in the per~orated tube 21 causes the contents of the i~
t~nk to pass vertically upwards over the sur~aces of the
heat e~change pipes 18 by a gas lift e~ect, and a
thorough mixing o~ the c~ntents o~ the anaerobic :~
digestion tank 3 is achieved as shown by the arrows 23.
, .
~he ga~ ~rom the per~orated tube 21 and gaseous produots
o~ the anaerobic digestion pass from the anaerobic i ~;
digestion tank 3 into the gas storage compartment ll. . I :
Gas is removed from the ga~ storage oompartment ll through
a gas outlet 24~ A pressure regulator (not shown) may
. , ~'. '
. -12-
.
.: . . . .
l~S74Z9
,
be connected to the gas outlet 24 to control the pressure
of gas in the gas storage compartment 11. The pres~ure
within the ga~ storage compartment 11 is advantageously
maintained at a relatively low level, for example,
a pre~sure of six inches on a water manometer. ~he pressure
regulator i9 advantageou~ly provided with an automatic
conden~ation removal facility and a flame trap. Such
pressure regulators are known items of equipment and the
pressure regula-tor i~ therefore not de3cribed nor i9 it j ;~
illustrated. Some of the gas from the pressure regulator
is passed to a compressor pump for supply to the inlet j~
pipe 20, and ~ome of the ga~ from the pressure regulator
is removed ei~ther for storage or for use, for example, in
a gas-~ired boiler system or a suitably adapted internal
combustion engine.
~Treated liquid is removed from the anaerobic digestion ~ ~
tank 3 through a discharge outlet 25 by means of a pump ~ -
26 which pumps the treated liquid through the cooler 4 !~
` to a weir box 27. The pump 26 may be any known type of !`
;20 pump which i8 suitable for pumping sludge-containing liquid.
However, it is preferred for the pump 26 to be a ga~ t
pump which is supplied by the compressor pump with gas
from the gas storage compartment 11. !
~he cooler 4 is provided with cooling fluid through a
cooler inlet 28. ~he cooling fluid pa ses vertically !~
upwards through ~he cooler ~ to a cooler outlet 29. ~he ¦`
cooling ~luid may be, for example, water from a river or
lake, or water in a closed circuit system which i~ 1
. . I .
-13- :
1: .
.
~ `,
... . . .. .. . . . . . . . . . . .
7~Z9
circulated between the cooler 4 and a cooling tower.
Alternatively, raw effluent may be used as the cooling
fluid, in which case the cooler 4 will act as a pre-heater
for the raw effluent which will be passed directly from the
eooler outlet 29 -to the waste inlet 16.
,
The temperature of the cooling ~luid as it enters the
cooler inlet 28 and the rate of flow of fluid through the
cooler 4 are ~uch that the treated liquid from the anaerobic ',
digestion tank 3 i9 subjected to a temperature drop of the
order of 10C, i.e. in the optimum process the temperature
is reduced from a temperature of the order of 35C to a
temperature of the order of 25C. The fall in the temperature
of the treated liquid has to be suf~iciently rapid for the
gas producing activity of anaerobic bacteria in the treated
; . I . .
I liquid to be substantially inhibited. In one embodiment of
the apparatus, it is estimated that the temperature of the
treated liquid is reduced from 35C to 25C in a time of 1 1
. . . ~ the order o~ 30 seconds.
~ The weir box 27 is provided with a plurality of
gravity flow weirs 30. ~he weirs 30 are connected
to a plurality of ~eed pipes 31 which ensure that the
flow o~ fluid from the weir box 27 into the gravity
, . . .
~ separators 5 is substantially even at different points l~ ~
~ .
j along the length of eaoh of the gravity SeparatorJ 5. t
~ . ``'~
A baffle 32 is provided in each of the gravity separators
5. The ba~fles 32 extend along the whole length of the
., ,, ~ :
., .
'i , , j;:
.
.
. . . ~ , . . ... .. ., , . :
~57429
gravity separators 5 between the end walls 8 of the tank 6
and extend from above the sur~ace of the liquid in the
gravity separators 5 to a point below the surface 33
of the liquid. The feed pipes 31 deliver cooled treated
liquid into the regions between the baffles 32 and the side
walls 9 of the tank.
: s
The flow of cooled treated liquid into the separators 5
is so arranged that the surfaces 33 in the separators 5
are maintained at a higher level than the level
indicated by the line 17 in the anaerobic digestion tank 3.
The bacteria in the cooled treated liquid entering the i
~gravity separator 5 are present in the form of a dispersed
suspension. Since the thermal shock imparted to the
bacteria by the cooler ~ temporarily reduces the gas
producing activity of the bacteria, after entering
the gravity separators 5 the bacteria tend to flocculate
into clumps which are relatively dense and which settle j~
in the gravity separators 5 to form a sludge 34 at the
bottom of the gravity separators 5. The slight pressure
difrerence between the gravity separators 5 and the
anaerobic digestion tank 3 due to the surfaces 33 being
higher than the line 17 causes sludge to pass from the
gravity separators 5 back into the anaerobic digestion
tank 3 through the small gaps 15. Some of the sludge
34 may be removed from the gravity separators 5 for 1 `-
:
disposal through sludge outlets 35 provided in the side
walls 9 of the tank 6.
The aqueous fraction o~ the treated liquid pasced into the
-15-
:
: ...... .. . . . . . .
. . ~ , . . , - . .
. , . . . ., . . : .
.. . . . . . . . . ..
~ ~ ~ 7 4 Z 9
gravity separators 5 through the feed pipes 31 circulates
round to t~e opposite side of the baffles 32 from the
feed pipes 31, as indicated by the arrows 36, This
aqueous fraction passes upwards through a bed of packing
material 37 which is disposed between the baffle 32 and
the sloping wall 12 of each gravity separator 5.
.
It has been found that material having the configuration
of Raschig rings is particularly suitable for use as the
packing material 36. Raschig rings are hollow open
cylinders, having a diameter of similar magnitude
to the length. It is preferred to use Raschig rings I ^
; having a dia~eter of the order of three inches to four
:. inches and made of a plastics material which is less
.~. , I .
dense than water, so that the packing material 37
tends to float towards the surface 33 of the liquid in ,~ :
the gravity separators 50 A grid 38 is provided in each l~ -
~: ~
of the gravity separators 5 between the baffle 32 and the .
sloping wall 12 in order to prevent the packing material
37 from floating to the surface 33.
Any gas which may be produced by the bacteria in the
sludge 34 will tend to carry solid material towards
the surface of the liquid in the gravity separators 50
The packing material is such that as gas bubbles float l~
towards the surface of the liquid, they encounter a
turbulence as they float around the sharp edges of the
. . ~
packing material 37, and this turbulence is sufficient
to cause the solids to be dislodged from the gas bubbles.
The gas bubbles can then continue to rise and escape
through the surface 37 of the liquid, while the solids
I -16-
I
. ~ . .
!
':,' , ' .' .''' ' . ,'.' ' ` ,'. ~, .. . . . .
':' ~ ' ., ~ .
~57~zg
settle once more towards the bottom o~ the gravity
~eparator 5. The liquid in the volumes which are de~ined ~ :
by the sur~aces of the liquid 33, the grids 38, the
baffles 32 and the 910ping walls 12 contain a relatively
¢lear liquid which flows over the ~ide~ o~ open-topped
channels 39 and passes out from the tank 6 through liquid
outlet pipes 40. The liquid'outlet pipes 40 are connected ~i
to conventional gas trap~ ~not shown). :
~he sloping side walls 12 Porm a thermally insuIating
barrier between the gravity separators 5 and the anaerobic
digestion tank 3, but there will inevitably be a slow 1 : :
transPer of heat from the anaerobic digestion tank 3 to
the gravity separators 5. Even at the temperatures
to which the treated liquid is cooled in the cooler 4, l `~
however, the inhibition of the gas producing activity j
of the anaerobic bacteria in the liquid may be only
temporary and the bacteria may 310wly regaln part of ~ ~ :
their gas producing activity, although no substantial ,~
regaining of ga~ producing.activity ha~ been observed 1-.
in the gravity separators 5 under the oonditions present
in the pre~erred proce~s. .
.. . .
.~ , 1 , ~ .
., .
It ha~ been Pound that iP a ~low rate through the
gravity separator~ 5 i~ maintained such that oooled :
, treated liquid is present in the region~ between the baPPle~
:'l 32 and the ~ide wall~ 9 for a period oP the order of ten
to ~iPteen minute~ and the ~ludge 34 is present at
..,
` I
~ -17- ~
. . ,
:,,
, ,
, ~
, ' , , .
'''-' '' '' ,, '
. .
D574z9
the bottomof the gravity separators ~ ~or a period
o~ the order of ten hours, the liquid passing out
~rom the tank 6 through the liquid outlet pipes 40
is ~ound to be relatively clear, the B.O.D. removal
effioiency being commonly of the order of 85% to 95~/o.
~he li~uid from the outlet pipes ~0 may be subjected
to further anaerobic dige~tion treatment and/or to
aerobic polishing to provide a discharge liquid which is
su~iciently pure to meet the most stringent
anti-pollu-tion requirements, and to be fed, ~or example,
; into a river. 1~-
', 1 ~,
Metallic parts in the interior o~ the apparatus
illustrated in ~igures 2 and 3 o~ the drawings are
pre~erably either o~ stainless steel or provided with a
~ coating o~ epoxy resin plastics material.
.. , , ~.:
It will be appreciated that numerous safety features
can be incorporated in the apparatus descrlbed with
re~erence to ~igures 2 and 3 o~ the drawings. ~or !~
~ ~ example, the roof 10 and the gas outlet 24 may be
'~ 20 provided with pressure switches which are connected to shut
o~ the compressor which supplies gas to the gas li~t ~
pumps and connected to isolate the gas storage vessel ~ -
or apparatus which is connected to use gas supplied
through the gas outlet 24 from the gas ~torage compartment 11,
in the event that pressure in the gas storage compartment 11
drops below a pre-determined ~alue which may be, ~or j~
example, ~our inches on a water manometer. The pressure
'
.
-
1~S79~Z9
~ :.
switches may also be arranged to cause an alarm to sound.
The gas storage compartment 11 may be e~uipped with oxygen ',
, detectors in order to detect a pre-set concentration of
oxygen in -the event o~ an air leak into the tank. The oxygen
,~ detectors are advantageously coupled to an automatic carbon
;i . . ~ .
,I dioxide pur~ing system arranged to flood -the entire gas : '~
. , .
.1 storage compartment 11 and pipes leading from the gas outlet 2~
with ¢arbon dioxide whenever the pre-set concentration of j~ ,'~ ,.. ,;
~j oxygen is exceeded. 1,' :'~` :.
""i, 10 The roof 10 may also be provided with pressure relie~ valves
which relieve pressure through flame traps to prevent excessive
,~: !` :
"i build up o~ pressure within the gas storage compartment 11. '
Explosion relief doors may be fitted in the roo~ 10, Such
explosion relie~ doors are sealed by their own weight, and
: l . .: .
chained to ths roof 10 in order to reduce the risk o~ ~njury to 1.
psrsonnel from ~lying debris i~ an explosion should occur. 1 `~
Instead of using a packing material in the gravity separator
to inhibit solids in the sludge from being carried lnto the 1'~ ', ;
upper layer as a result o~ the bacteria regaining gas , I'
~ producing activity, a rotatable rake may be employed.
Advantageously the gravity separator and the anaerobic
~ digestion tank are then both ~ormed as circular tanks, with the
.1 gravity separator located above the anaerobic digestion tank. ¦'-
,~ As the rake is rotated slowly in the gravity separator, the
:~ movement o~ the tines causes gas bubbles on the sludge to be ,
stripped from the solid materials and provides an upward path
, through the settling sludge ~or the ga~ bubbles and the clear
.;, ~'- :
., :
... . I .
-1 9
., . ~
.i ~ .,.
. . .. . . . . ` .
- `. . . ., . . . . .
S'79~2~9 ~
.
~: liquid which is being displaced upwards by the settling of the ~.
solids. i ~
~.
~he proce~ses of the present invention which have been
described with reference to the accompanying drawings have ~`
partioularly advantageous application in the treatment of
effluen-t from faotories operating the wheat starch/gluten/ ` .
dextrose process. They are also particularly suitable for the
treatment of effluents from abattoirs, confectionery faotories, I
creameries9maize staroh/gluten/dextrose factories, single cell ~.
protein production processes and winemaking.
Other examples of effluents which are suitable for treatment
by processes in accordance with the present invention include
domestic sewage and organic effluents from animal feed
. ~
production processes,breweries, distilleries, fermentation, ,:
food processing, fruit juice production processes, mal-tings,
.
meat processing, milk or dairy production processes, paper j.
making and recycling~ pharmaceutical manufactures, soft dri~
: production processes, starch processing, suga.r refining, textile i`
processing, vegetable processing and yeast production processes.
"
When the process of the present invention i9 used to treat
effluent from processes such as the whea-t starch/gluten/
: : dextrose proce~s, the waste material in the effluent may be
- almost entirely (90a/1a to 95~io for example) soluble. ~he solids
,:, :`
:. in the sludge obtained following gravity separation can then
~ : consist essentially of bacteria,
; i-
.. .
: -~ ::
.
-20- ~
:. '
.. , . ,, : . . . :
