Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
PROCESS FOR PURIFYING EXHAUST AIR
This invendon relates to a process for purifying exhaust air contaminated with
substantially water-insoluble, biologically degradable pollutants which are absorbed
by a washing liquid containing activated sludge and at least one high-boiling solvent
for ~he pollutants and which are degraded by the activated sludge, the solvent and the
activated sludge being separated from at least part of the remaining washing liquid
which is discharged
One such process is known from DE 36 41 442 Al The exhaust air contamin-
a~ed with the pollutants is passed through an absorp~ion tower in which the pollutants
are absorbed by the washing liquid containing activated sludge and solvent The
washing liquid then flous in~o an activated sludge tank where the pollutants are bio-
logically degraded by the microorganisms of the activated sludge In order to prevent
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the conccnlralion of salts and Ihe possiblc accumulation of biological growth inhibilors
in lhe washing liquid, part of thc washing liquid is continuously removed from the
washing liquid circuit by a pump and delivercd to a flotation lank. In this tank, the
high-boiling solvent is removed with the activated sludge from the remaining washing
s liquid by flotation and mtllrned to the acliva~ed sludge tank. The clear water is dis-
charged into the main drainage system. Dialkyl phthalates, particularly dioctyl phthal-
ate and dinonyl phthalate, are used as solvents in the known process.
The disadvantage of the known process is that the clear water separated by flO-
tation has a relatively high contcnt of solvent and, optionally, activa~ed sludge, so that
.o an additional working-up step has to be subsequently carried out or the washing liquid
separated off has to be disposed of as waste water at considerable expense.
l he problem addressed by the present invention was further to develop the pro-
cess mentioned at the beginning to the extent that the contenl of ecologically and
physiologically harmful substances in the washing liquid to be separated off is consid-
.s erably reduced without any expensive additional measures having to be taken.
According to the invention, the solution to this problem is characterized in thatthe part of the remaining washing liquid is separated off by membrane filtration using
inorganic and hydrophilicized organic membranes.
On the one hand, the use of silicone oil provides for excellent absorption of
20 the pollutants of the exhaust air in the washing liquid. On the other hand, silicone oil
is physiologically inert and, hence, largely safe and, according to the invention, may
be separated very effectively together with the activated sludge from the remaining
washing liquid by membrane filtration. Using the process according to the invention,
the residual silicone oil content in the washing liquid separated off can be reduced to
2s one twentieth or less by comparison with the known process. It has surprisingly been
found that, even in the event of prolonged operation, separation of the three-compon-
enl mixlure of silicone oil. activated sludge and washing liguid can readily be carried
out withou~ blockage of the membrane, provided thal inorganic or hydrophilicized or-
ganic membranes are used. Solvent contents far below the corresponding legal limits
30 are achieved by the further developmen~ of the process men~ioned at the beginning in
accordance with the present invenlion.
In one embodiment, the process according to the invention is carried oul con-
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tinuously by recycling thc rclcntatc con~aining solvcnl and activa~ed sludge and replac-
ing the quanti~y of pcrmca~e rcmoved by fresh washing liquid (water). A procedure
such as this is adapted lo the continuous operation of a waste-air purifying plant. By
virtue of the low contents of ccologically safe substances, the permeate removed may
s readily be dischargcd into the main drainage system.
In some cases, i~ may be necessary to remove the activated sludge circulated
in the process according to the invcntion from the circuit, for example in order to re-
place it. One particular advantage of the process according to the invention lies in the
possibility of thickening the mixture of activaled sludge, solvent and residual washing
o liquid to such an extent that an almost punctureproof mass is obtained. Accordingly,
it is proposed that the process be carried out discontinuously for concentrating and,
in particular, thickening the solvent-containing activated sludge. In this case, too, the
contents of ecologically unsafe substances in the permeate remain small. It has been
found to be particularly suitable to use a surface-hydrophilicized fluoropolymer as a
s membrane material in the process according to the invention. There is thus no danger
of the membrane nnaterial losing any of its hydrophilic properties under certain influ-
ences which would impair the efficiency of the process.
Microfiltration, ultrafiltration or nanofiltration membranes may be used as the
membranes. Crossflow membrane filtration is applied with particular advantage in the
process according to the invention.
The process according to the invention may readily be carried out continuously.
In order, however, to provide a particular safety margin for certain cases, it is propos-
ed that the membrane be periodically washed with a cleaning liquid. This can be done
by fixedly installing a container accommodating the cleaning liquid in the plant and
periodically interrupting the flow of washing liquid through or over the membrane
and, instead, passing the cleaning liquid through the membrane.
Examples
Embodiments of the invention are described in detail in the following with ref-
erence to the accompanying drawing which is a schematic flow chart of the process.
Referring to the drawing (Fig. 1), the exhaust air contaminated with the sub-
stantially water-insoluble, biologically degradable pollulants en~ers an absorption col-
umn I at its lower end, is drawn through the column I by a fan 2 arranged at the head
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of thc column and discharged as purified exhaus~ air at lhc hcad of the column. A
washing liquid containing activaled sludge and solvent, i.e. silicone oil, is pumped
~hrough the column in countercurrent to the upwardly llowing exhaust air, i.e. from
the head to the boltom of the column, by means of a first circulation pump 4. The
s washing liquid then flOws back to the activatcd sludge tank 3. In order to avoid the
above-mentioned concentration of salts and accumulation of biological growth inhibit-
ors, part of the liquid in the activated sludge tank is removed by a second circulation
pump 5 and re~urned to ~he activated sludge tank 3 through a membrane module 6.
Permeate collecting in the membrane module 6 is discharged as wastewater. To re-,o place the wastewater removed, a corresponding quantity of fresh water is delivered to
the activated sludge tank 3.
In tests, it was shown that the concentration of the high-boiling solvent, sili-cone oil, could be reduced to levels of 50 mg/l to 60 mg/l by the use of suitable mem-
branes. By contrast, the wastewater from the flotation tank contains more than 1000
~s mg/l to 1500 mg/l of the high-boiling solvent.
Further Example:
An activated sludge dispersion containing 20 % silicone oil and a biomass con-
tent of 20 g/l was used as starting material for the membrane stage. A DDSS Hekla
20 A ultrafiltration membrane was used. The pressure on the concentrate side was at
most 3 bar, the membrane crossflow rate was 3 m/s. The temperature was between
19 and 29 C.
In this case, the membrane stage was operated in batches, i.e. the concentrate
was returned to the supply tank. In this way, there was a continuous increase in the
viscosity and a corresponding reduction in the water content of the starting material.
x Accordingly, the permeate flows decreased from, initially, 70 Vm2 h to levels
of 20 Vm2~. The concentrate was concentraled virtually to the individual star~ing
componcnts - silicone oil and biomass. Mixed samples of the permeate obtained had
a content of 4 mgA to 6 mgA of silicone oil, determined as substances soluble in pe-
troleum ether.
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List of reference numerals
I Absorption column
2 Fan
3 Activated sludge tank
4 First circulation pump
5 Second circulation pump
6 Membranc module
