Note: Descriptions are shown in the official language in which they were submitted.
~2~
., ,
Method and apparatus for biological treatment o~ waste
gases.
The lnvention relates to a me~hod of bio-
logically filtering gaces, in particular waste gases, by
passing them through a layer of carrier material whlch has
been provided with appropriate micro -organisms, and to an
apparatus for carrying out the me~hod.
For recent years biological ~iltrat~on $s
increasingly used for cleaning gas streams origi-
nating from various industrial activities like production
processes, waste water transport and waste water treatment
in order to reduce nuisance, trouble or damage to ~he envi-
ronment caused by malodorant and/or poisonous components.
In view of the material often used for bioLogical
filtration, one also speaks of compost 11tration. In
biological filters other materials like peat, wood chips or
fit-branches are used as the so called carrier materials.
In the biological method the removal of
har~ful components from the gas takes place by activity of
micro-organisms, malnly bacteria and fungi, which are present
on or in the carrier material. The Garrier materials for
that purpose~ are disposed in a container. The assembly of
container, carrier materials and micro-organisms usually
is called a biological fil~er. The gas to be treated is
introduced into the container by a fan, the gas passing
through the carrier material from abo~e to helow or vice
versa. The components to be~removed from the gas are adsor-
bed at the carrier material and converted by the micro-
; organisms present on or in;the~-carrier materlal into sub- -
~; stances whish are not harmful to ~he environment.~
A method and apparatus of the kind mentio-
ned in the preamble are disclosed in the Dutch patent appll-
cation 81.04g87 and in the German patent applicatlons
Z.445.315, 2.558.256 and ~.605.606.
The Dutch patent applicatlon Bl.04987
-~ discloses a biological filter for cleaning gases, wherein
;~ 35 rP1~tlvely large inactive particles of the ~tartlng com-
post are added to the active compo~t with ~he primary ob~ect
of reducing the pressure drop across ~he ~lter ~n~ hance
,~
54
the energy consumption.
According to the Weetgerman patent
application 2.445.315 the activity of the filter is in-
creased hy mixing the carrier material with bentonlte
earth~ Thereby hea~y metals which adversely affect the
biological activity of micro-organismC; arechemicallybound to
bentonlte so as to maintain the required biological activity.
It is noted, that the carrier material
in biological filtexs usually is oomposed of particles of
such size that besides a reasonable adsorbing surface also
an acceptable flow resistance is suaranteed. A too small
adsorbing surface leads to a too large and uneconomical
volume of the filter, whereas a too large filter resistance
is accompanied by a too large energy consumption in passing
the ga5 stream through the filter.
Furthermore lt is notad that for opti-
mal operation of the biological filter a water content of
40-60% in respect of the weigh~ of the carrier material
is necessary. In the known methods and apparatuses humidity
control, if any, is realized by spraying water across the
upper surface of the filter~
It has now appeared ~hat by ~praying
water onto or into the carrier material it is not or not
sufficiently possible torealize an adequate humidity control.
25~ In particular the degree of humidity of ~he most active
portion of the filter,~i.e. the lower side', when the flow
throuyh the filter is~from below to above, cannot suffi
ciently be ccntrolled by the known method. The known method,
e.g. by sprayin~ water externally or internally, is only
~capable of restorin~ to the desired water content of the
` carrier ma~terial, whlc~Ycalamities have dried too muchO
At too low~a mo~sture cvntent of the
carrier material the biolo~ical activity will ~trongly
dlminish or even totally disappear.
`:
Furthermore, shrlnk cracks develop in
the carrier material in consequence of drying up resultlng
in short circultlng the gas flow ln the bed so ~hat a
considerable portion of the waste gas 1~ withdrawn ~rom
the contemplated biologlcal treatment ShrlnX cralcks can
be avolded to a large extent by the choice of a ~ui~able
.
~ ~ 6
carrier material.
A too high moisture content in the car-
rier mar~rial results in the de~elopment of anaerobic zones.
Such zones have an increased flow re~istance precluding a
homogeneous flow of gas, which results in a considerable
decrease of the average residence time ofthe gas in the
filter bed.
Furt~.ermore the anaerobic zones pro-
duce volatile metabolic products leaving the bed with the
waste gas. Since theproduced metabolic products have malodo-
rant properties, they contribute so the undesired malodorant
nuisance.
It is noted that the micro~organisms
responsible for decomposing the mainly organic components
which are present in the gas to be treated, require various
inorganic nutrients for their metabolism. As basic material
various kinds of compost axe satisfactory for this purpose.
These materials, however, have the common disadvantage of
being strongly subject to aging. Some kindsofcompost have
a tendency of lumping accompanied by a strong decrease of
the specific surface area ~surface area per unit of
volume) aging phenomena of the carrier material can be
caused by local loss of moisture by a too low relative
h~midity of the air and~or the occurrence of t`amperature
gradients in the carrier material.
The occurrence of temperature gradients
in the carrier material is inherent to the microbiological
I
activity.
This activity may be locally different.
Where this activity is high, the temperature will be
~ slightly higher than elsewhere as a result of the realised
;; energy of ~xidation. ~ ~ ;
a result of these temperature gra-
dients will develop alao gradients in the maximum
water vapour pressure. This lmplies that biologically acti-
ve zones have a tend~ncy of dry:ing up whlle l~ss active
zones take up thls excess water by condensatlan.
As a result active zones will expe-
rience a loss of activity and wet, mostly lnactive zones
will take up still more water.
~L26~45~
Aging phenomena in the carrier material mostly
manifest themselYes by the deYelopment of shrink cracks in
the carrler material in which drying occurs, and by the deve-
lopment of wet, of~en anaerobic zones. Such aging phenomena
5 are mostly irreve.rsIble, which means, that a shrink crack
once developed will not disappear automatically.
It has been known that aging phenomena of this
kind can be prevented by continuously moving the carrier
material throughout the bed and mixing it up outside of the bed
lO as disclosed by the German patent application 2.445.315.
Another problem presented by the known methods
and apparatuses is that in the microbiological conversion of
certain organic components in the waste gases acid components,
: a.o. acetates, are produced as intermediate products. Such
l5 acid compo~ents contribute to acidiflcation of the carrier
material and as a result the microbiologtcal activity will
drastically decrease or even totally dlsappearO
Furthermore the decomposition of other organic
compoments by acid-intolerant organisms can be considerably
20 hampered. This, for example, is the case with the decomposi-
tion of toluene.
In the use of biological filters for cleaning
waste gas strongly varying loadings often occur due to dis
continuous processes.
~he active micro-organism~ in the filter con-
sequently are subject to loadings fluctuating in time. In
such cases the concentrations of polluting components in
the effluent gas may strongly vary unless one proceeds to
the use of a strongly oversi~ed filterbed.
~ It can be generally stated that these fluc-
tuations may adver:sely affect maintaining an optimal mlcro-
biological aotivity. In discontinuous gas discharges no
loading at all takes place at the filter during certain
periods of the day. Though the nIc~organ1sms may well survive
35 during perIods of shut-down, also of lon~er duration,for
example, weeks, it still means, that the decomposlng capa-
city of the mlcro-organi~m~ is not fully exploited.
The lnvention now has the object of providing
a method and apparatus for blologically flltering gase~
wherein the above mentioned disadvantages are removed ln an
~.264at~
efficient ~anner.
For this purpose the method of the
lnven~ion i5 characteriz~d ln that the gases prior to
being passed through the carrie~ material are brought into
intimate contact with water in such amannerthat the gases
contain the quantity of watPr required for the optimum
activity of ~he micro-organisms , the gases are brou~ht to
thQ temperature required for that purpos~, and any water
soluble substances present in the gases dissolve at }east
partially.
The gases to be treated are brought
into intimate contact wi~h water by spraying.
This brings the gas streams to the right humidity
and to the required temperature as is necessary for the
micro-organisms o~ the carrier material in the filter to
function optimally.
Furthermore as a result of spraying the
gases to be treated in a socall~d pretreatment chamber the
water soluble components of the gas stream will at least
partially dissolve in the water so as to be removed from
the gas stream. Thereby the components which are toxic
for the micro-organisms can be removed from the gas stream
during this pretreatment so as not to affect the activ~ty
of the micro-organisms.
In case of non-toxic but water soluble
componen~s decomposable by micro-organisms a considerable
portion thereof can be removed from the gas stream by
dissolving~ For decomposiIIg the so dissolved components ln
accordance with the lnvention now a specific biological
population, for example, ac~iYesludge, canadvantageously be
introduced into the spray water.
In this manner a lager quantityocomponents
can be removed per unit of time from the gases to be
treated so as to increase the efficiency of the b$ologlcal
3~ filter which subsequently is contacted by the gas.
Research at the Eindhoven University of
Technology on which the invention 1~ based, has shown that
adding certaln additional materials to ~he carrler materlal
yields very favourable results.
-6- ~ ~ 6~ ~S~
The additional materials of the i~Yention can
be sub-di~ided into materials pre~enting the development of
shri~k cracks in the carrier material and reducing the fl~w
resistance of the biological filter on the one hand, and
additional materials counteractiny -aeidificatlon of the
filtermaterial on the other hand.
In particular it has been shown in accor-
dance with the inventlon that modifying the structure of the
carrier material by adding additional material of the first
type may prevent the very disadvantageous aging phenomena.
As a result of adding the additional material, which current-
ly is inert material to the carrier material, ~or example,
compost, and by mixing it therewith the development of
shrink cracks can be counteracted.
As additional material of the first-mentio-
ned type, which mainly is inert, and has a diametrical par-
ticle size of 3-10 mm, the following substances having a more
or less rigid structure can successfully used in accordance
with the invention: organic materials, like polyethylene,
~ 2Q: polystyrene, particles of ground automobile tyres, as well as
,~f' inorganic materials like fired clay particIes, ground lava
bits, ground coalcin~er particles and pelletized flue gas
particIes, perlite and active coal.
The proportion of mixing the additional
material with the carrier material is be~ween 30-70% and
70-30% on the basis of volume.
The additional material should pxeviously
be thoroughly mixed with the carrier material, for ex~mple
compost, so as to obt in a loose structure having a large
specific surface.
~ This mixing can take place simultaneously
; with other additions if necessary or preferable. In this
respect, for example, mlcro-organisms which by nature are not-
present in or on the carrier materlal can be applied to
the carrier material by inoculatlon~
~;~ Furthermore, the research at the Elndhoven
UniversityofTechnology has ~hown that by thelchoice of 2
sultable additional material also the flow resl~tance can be
consider~bly decreased. So, fox example, the 1OW resi~tance
at a surface loading of 200 m3/m2/hour is about 8 mm head
: ,~
5~
.
of water per meter of bed height when using a mixture of 50%-60%
polyethylene particl~s (low density nr. 1500, particle diameter
about 4 mm) and 50%-40% compost.
In contrast therewith there is at a same value of sur-
face loading a pressure drop of about 120 ~n head of water per
meter of bed height when no polyethylene particles are added.
Porous additional matsrials having a high internal
porosity and hydrophilic properties are advantageous, since such
materials may function as a buffer for excess moisture in the
carrier material, which excess moisture in case of excessive loss
of molsture can be yielded by such materials. This effects a
certain degree of humidity control in t.he carrler material.
The disadvantage of acidification of the carrier ma~e-
rial can be advantageously removed by the use of additional mate-
rials of the second typa counteracting the acidification of the
filter material. For this purpose a pH-reducing alkaline sub-
stance is added to the carrier material and intimately mixedtherewith. ~dvantageously~ for example, marl, limestone and cal-
cium ca~bo~ate in a weight proportion of 2-40% in respect of the
carrier material can be considered for preventing acidification
of thé carrier material. It has appeared that ln particular
cases (for example, th~ micro-biologlcal decomposition of methy-
lene chloride) considerably higher weight proportions should b~
applied, which may range up to a value of 40~ ln respect of the
total filling quantity.
The research at the Eindhoven Un1versity of T GhnOlOgy
besides the previously mentioned two types of additional material
has shown active carkon to be very suitable as additionaI mate-
rial.
Active carhon added to the carrier material in a quan-
~ity of 1-50 kg/m3 appears to be very favourable in case the pre-
-- 7 --
.
...
- :
~ 5~
sent biological filters of the invention are sub~ected to various
loads as in various and/or discontinuous processes. The active
carbon in such case is or is not mixed with the carrier material.
These carbon particles have adsorbing properties for organic com-
5 ponents which are present in the waste gas to be treated.
.
- 7a -
~.1
.,., .. ,,.,. ,; :~ ;~
' ` ~ ' '', ~ '
-8- ~26~4
Besides acti~e carbon other substances having
adsorbing properties may be used, for ex~nple polyethylene
glycol.
Besides their good adsorbing properties such
substances should also have good desorbing propertie~.
Thereby it is possible that during a per1od of incr~ased
supply the adsorbens temporarily adsorbs the concentration
excess anddesorbsit at lower loadson b,ehalfofthe micro-biological
decomposition. This permits in principle a considerable
reduction in the filter volume requirled at discontinuously
discharging plants.
When Yery concentrated waste gas streams are
discharged during a short period, in order to reduce the
volume of the filter a quantity of adsorben~ (active
carbon) is required which will be in excess o 50 kg/m3
and will be at maximum 250 kg/m3.
At such high concentrations of carbon these
particles can also take overthe function of inertcarrier material.
It i5 noted that in accordance with the present
method the temperature of the spray water is chosen so that
the temperature of the gases to be treated is between lO and
40C.
In accordance with the invention ~he relative
humidity of the gases after pretreatment with water ~s
usually 95-100%.
The invention also relates to an apparatus for
carrying out the method of the invention for biologically
flltering gases, in particular waste gases~`
Such apparatus c~mprlses a container lncluding
a biological filter unit, a supply conduit for the gases
to be treated at the lowex side and a discharge conduit for
the gases to be treated at the uppPr side.
The apparatus of the invention is characterized
~n that at least one f~lter unit ls arranged in the contai-
ner and a pretreatment chamber for the gases to be treatedi9 di~posed underor at the lower sideo~the lowermost filterunit.
Usually the contain2r ~s cylindrlcal, though
it may be rectangular or be differently shaped.
In the apparatus of the lnvention the pretreat-
ment chamber and the lowermost filter unit are separated
-9- ~26~45~
from each other by a support layer permeable for the gas
to be treated. The support layer us~ally is a perfcrated
metal or plastics plate~ The pretreatment chamber is proYi-
ded withaspraying water facility for intimately contactinS
5 the waste gases with water.
In a favourable emhodiment the pretreatment
chamber is provided with a contact bed arranged above the
supply conduit for the gases to be treated and resting on
the gas-permeable support layer, a discharge conduit for
10 spraywater under the support layer and the spraylng Eacility
for the water above the contact bed. The contact bed enhances
the intimate contact between the yases to be treated and the
spray w~ter. The contact bed advantageously can be a gravel
bed, ~hough other suitable materials also can be used.
To reduce the quantity of water used, it
is advantageous, when the spraying facilitiesareconnected to a
dlscharge conduit for spraywater via a cixculation conduit
including a circulation pump for circulating the spraywater.
For adjusting the temperature of the spray-
20 water it is favourable when the circulation conduit includesa heat exchanger. Adjacent to the pretreatment chamber is
a filter unit, which, in a simple embodiment, consists of a
gas-permeable support plate and a biologically active zone.
Currently, the gas-permeable plate ls a perforated metal or
25 plastics plate.
In order to obtain a more homogeneous dis
tribution of the gases to be treated across the biologically
active zone use can be made of a contact material on which
the biologically active zone rests. As a contact material
30 advantageously lava bits or graveI are used, though other
~uitable materials al o can be considered for this purpose.
According to the invention the biologically
a~tive zone can be composed of a suitabl~ carrier material,
for example compost, peat, wood chips, etc. Preferably, the
35 bIological active zone is composed of such carrier material
and an appropriate additlonal material as di cussed pr~-
viou~ly.
Advantageously, spraying acilities~re dispa~-
sed in the upper portion of the filker unit ln order to
40 prevent ~he biologlcally active zone from clrying up ~n case
1 264~5~
of calamities.
There can be circumstances requirin~ that
the apparatus has a plurality of filter units disposed
above each othPr and separated from each other by a gas-
5 permeable suppoxt plate being a perforated metal or plasticsplate as mentioned previously.
A favourable embodiment of the apparatus is
illustrated in the single drawing.
The container 1 which in thi5 embodiment is
10 cylindrical is composed of a pretreatment chamber 2 for the
waste gases and a filter unit 3. In the pretreatment chamber
is provided the supply conduit for the gases to be treated
4 and the support layer 9 which in this embodiment is a
perforated metal plate, The pretreatment chamber may be
15 arranged separately from the container.
On the support layer 9 a gravel bed is pro-
vided for enhancing the contact between the upward gas-
stream and the downwardly sprayed liquid.
Up in the pretreatmbnt chamber 2 spraying
20 means 5 are provided. Down in the pretreatment chamber 2
the spray water may be tapped through the discharge conduit
20 and the valve 21 or it may be circulated through the
circulation conduit 19 including the circulation pump 6 and
the heat ~xchanger 7. In the heat exchanger 7 the spray
25 water can be brought to the desired temperature, i.e~ when
a higher temperature is desired, the spray water can be
heated in the heat exchanger or when a lower temperature is
desired, the spray water can be cooled.
Above the pr~trea~lent chamber 2 is prov~ded
30 the filter unit 3 separated by the gas permeable support
:: plate 10 on which rests the biologically actlve zone 11.
The biologically active ~zone 11 can be composéd of~the car-
rier material alone but~preferahly of a mixture of carrier
mat~riaL and~the previously dlscussed additional materials.
Above the blologically active zone 11 are
provided sprayers 12 which in case of emergency, for example,
when the spraying means 5 in the pretreatment chamber ~
should fall, can ecure the moistening o~ the biolo~lcally
active zone.
On the filter unlt 3 is fastened the top
section 14 through the fastenin~ means 13, for example a
screw means.
Th~ biologically filtered gas can now be dis-
charged to the atmosphere through the discharge 15 and
the valve 16 or it can be conducted through the conduit 17
and the valve 18 to a measuring and sampling apparatus.
On the filter unit 3, if desired, a plurality
of filter units can be arranged which then can be fastened
~ogether by a similar fastening means as 13, for example.
For convenience, such fas~ening means can be
a screw mean~.
The use of more than one filter unit can be ne-
cessary in cases wherein the waste gases to be treated con-
tain components requiring for their decomposition different
lS conditions possibly including dlfferent miorQ-organisms or
when the waste gases to be treated include one certain com-
ponent in such a high concentration that the capacity of
one filter unit is inadequate for sufficiently decomposing
it.
By a uniform construction of the filter units
the apparatus of the invention, if desired, can be adapted
to biologically cleaning various waste gases of differant
compositions by simple fastening the said filter units on
each other.
~; 25 The principle of ~ultiplé filter units can also
; be applied by dividing a gas stream to be treated into two
of more equal streams and by conducting the separate streams
to separate filter units~isposed above each other in one
column.
: 3a ~hen the method is effected in such an appara-
U5, a considerable further reduction of the pressure drop
of ~he gas stream through the fiLter can be reached. This
decrease of the pressure drop for two units arranged inpa-
rallel theore~ically amounts to a fa~tor 4 in comparison
with the case of two units ln series: namely a ~ac~or 2 ~or
~hs reduction of the gas loadlng per m~ o~ filterarea
traversed and a ~ac~or 2 for the reduction of the filter
helght.
The method of the lnvention will now be ex-
plalned by way o~ the followlng examples.
-12- ~6~
Exam~le I.
An apparatus accordiny to the in~ention and ln-
cluding a pretreatment chamber and five filter units arran-
ged ln series was continuously fed with a synthetic wast~
gas from a paint spraying shop. The diameter of a filter
unit was 15 cm, the filling height was 60 cm, the ~olume o
carrier material thus being about 11 1 per filter unit.
The composition of the filter material was 29% by weight of
peat compost,59%by weight ofpol~ethylelle particles tlowdensitY
nr.1500,average particle diameter about4mm),2~by weight ofmarl.
The filter material was inoculated with an active sludy~
suspension and a toluene decomposing organism which was iso-
lated from a ground sample of a petrol filling station.
The synthetic waste gas contained a mixture of the
components aethyl-acetate, butyl-acetate, butanol and toluene.
The gas output practised was 67 l/min corresponding to a gas
load of 220 m3/m2/hour, the temperature was about 20C.
The measured total pressure drop across the filter units at
the said gas output was 26 mm head of water.
Under stationary conditions the following con-
centrations were measured: 3
Gasconcentration in mg/m
Component toluene butylace- aethylace- butanol
tate tate
supply pretreatment 958 218 205 127
25 chamber
discharge pretreat- 941 214 200 0
ment chamber
discharge firstfilter 936 89 95 0
unit
d'schar~e secondfilte~ 928 17 38 0
discharge thirdfilter 908 0 0 0
unlt ;
dlscharge fourth filter 872 0 0 0
unit
discharge fif h filter 810 0 0 0
., . ~ _ ~ _
,, . ~ ~ , ,. , ~ ,
-13-
~6~5~
Example II.
A heaYily loaded apparatus includlng a pre~
treatment chamber and a one-stage filter unit was fed wlth
a synthetic waste gas which containedaethyl-acetate as a
polluting cQmponent. The filter unit had a diameter of 15 cm
and a filling height of 100 cm, thus a filling volume of
about 18 1. The filling material of the filter unit consis-
ted of 7920 g of peat compost' 108 g of polystyrene parti-
cles ~diameter about 3 mm) and 216 g of marl.
The said filling material was inoculated with
micro-organisms from an active sludg~ suspension taken from a
waste water treatment plant.
The filter unit was daily intermittently
loaded: during 8 hours per 24 hrs a waste gas at a volume
output of 100 l/min and therefore a gas load of 340 m3/m2j
hour, and having an aethyl-acetate inle~ concentratlon of
825 mg/m3 was fed. During the remaining 16 hours aeratlon
of the filter with clean air (air output 10 l/min) took
place.
Eight days after start-up of the filter the
dynamic behaviour was studied. For that purpose the concen-
tration of aethy~acetate in the waste gas from the filter
unit was measured after application of the load.
This concentration appeared to increase sub-
stantially immediately from zero value until after about
30 min a stationary waste gas concentration of about 320 mg/
m3 was reached. Thls concentration was maintained during
tha remaining period of 7~ hours for which the filter was
loaded.
:
To the fi}ling material in ~he apparatus des-
cribe~ ln Example lI 549 of active carbon was ~dded to be
mlxed therewith.
Also in this case the filter was dally inter~
mittently loaded in accordance with ~he data in Example II,
lt being under~tood that at the s~me tlme the lnle~ concen-
tration ofaethyl-ac~tate ln the ~ynthetic waste ga~ to be
cleaned had been further lncreased to about 1610 mg/m3.
A~ter the filter in this way had been loaded
for about 14 days, the dynamic behaYiour wasagain studied.
After application of the load the concentration of aethyl-
acetate in the waste gas during the first 30 mln appeared
to ~e substantiallyzero to slowly increase subsequently.
This increas~ continued during the remaining
time of the loaded period. At the end of this periodS after
8 hours, the ethylene-acetate concentration in the waste
gas was about 700 mg/m3.
Example_IV,
An apparatus according to the invention incl1~-
ding a pretreatment chamber and three filter units arranged
in series was discontinuously ~ed with a waste gas from
a pharmaceutical plant. The diame~er of a filter unit was
150 cm, the filling height was 100 cm, the volume of carrier
material thus being 1,77 m3 per filter unit. The composition
o~ the filling material in the filter unit was 21~ by
weight of peat compost, 39~ by weight of marl and 40~ by
weight of active carbon.
Once a week a waste gas was treated by the fil-
ter, said gas a.o. containing a methylene chloride concen-
tration of ~00 mg/m3 after the pretreatment chamber. The gas
output practi ed was 350 m3/hour corresponding to a gas load
o~ 200 m3/m2/hour. The discharge time of the waste gas was
S~hours. In these S hours thè methylene chloride was quan-
titavely captured in the upper filter unit by adsorption
at the active carbon present in the filling material.
~After the dlscharge of the waste gas the methylene chloride
was desorbed~from the active carbon by passing an output o~
50 m3/hour of clean ~oom air through the filter. The desor~ed
; methylenechloridewas adsorked by the biologically active frac-
.
tion of the filling material and subsequently biologically
decompo~ed.
The biological decomposing capacity of the fil-
llng ma~erial for^recalcltrant component methylene chlorlde
was obtained in that be~ore the start-up a suspension of
methylene chloride decompQslng micro-organisms, which had
been especially prepared for this purpose, was ~ixed through
the fillina material of the upper filter section.
15 ~2~4~
The biological decQmposin~ capacity of
methylene chloride at 20C was 30 g/~3/hour.
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