Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention is concerned with a method
for the elimination or prevention of blockages in bottom
aerators in water treatment and sewage treatment under
operating conditions.
Bottom aerators, i.e. aerators that are placed
near the bottom of the aerator basin, have a lower
energy requirement and a better air introduction capacity
than aerators placed laterally in the aerator basin.
During the operation of aeration systems in water
or sewage treatment plants, depositions are formed in
the pores of the aerators in the course of a shorter
or longer time. As a rule, those deposits mainly consist
o~ calcium carbonate andlor iron phosphate and/or organic
substances. How sQon these deposits result in blockage
of the aerat~rs, does not depend only on the material
of which the aerators are made (e.y., various plastics,
such as polyethylene, polypropylene or polystyrene, or
ceramic sinter materials) and on the air transmission
capacit~, but in the first place, on the composition of the
water to be treated. The deposits deteriorate the oxygen
introduction capacity of the aerators and cause an in-
crease in the ener~y consumption. De~osits occur both
in so-called disc aerators (filter domes~ and in candle
aerators.
In a method of cleaning bottom aerators the
aerator basin is emptied, the aerators are disassembled
soaked and washed ~in an acid bath mos~ly consisting of
hydrochloric acid) and assembled again, and the basin
is again taken into operation. Since the aerator basins
could include up to 1,000 or even more disc aerators,
this procedure is highly laborious and time-consuming.
Moreover, thereby the functioning of tha clarifier
,~
~%033æs
plants is deteriorated/ or the sewage water must even
be allowed to by-pass the clarifier plants. In certain
plants, such a cleaning must take place several times
per year.
There are also pivotab,e aerator systems, which
can be pivoted out of the aerator basin for the
purpose of cleaning, but such systems are expensive
owing to the necessary articulated joints.
There are also methods, wherein the cleaning is
effected with the aerators in place by using e.g. chlorine
or hydrogen chlorine, see for instance R.B. Jackson,
"Maintaining Open Diffuser Plates With chlorine",
Water Works & Sewerage, September 1942, pages 380-382,
W~M~ Franklin, "Purging Diffuser Plates With Chlorine",
Water Works & Sewerage, JunP 1939, pages 232-233,
"Manual of Practice No 5", Federation of Sewage and
Industrial Wastes Associations, Champaign, Illinois,
1952, pag~s 60-61, U.S. Patent No. 2,686,138 and
EP application No. 49154. These methods are characteriz-
ed hy the use of strong acids and sxidizing agents,
which in practice caus~ several problems, e.g.
- corrosion of pipes and diffusers,
- the handling of the very dangerous gases
stored in pressure vessels, is riskful and disturban~.e
in the~use of dosage devices may be hazardous,
- the risk, that the biological purification of
the water will be disturbed sr that the bacterial
culture will be destroyed,
- due to corrosion and viewpoints of safety the
arrangement of apparatuses is expensive.
In the method of the present invention the dis-
advantages mentioned above have been eliminated.
The present invention is concerned with a method
for the elimination or prevention of blockages in
bottom aerators in water treatment and sewage treatment
under operating conditions. The method is characterized
~2~3a~
in that formic acid is introduced into the gas fed into
the aerators without taking the aerators out of opera-
tion.
The gas fed into the aerators is, as a rule, air
or oxygen, e.g. compressed air. Formic acid is supplied
preferably in the form of gas or vapour, but it may
also be introduced in the form of a fine spray.
Compared to the mineral acids mentioned above
~he organic acids are weaker. However, even small
amounts of formic acid is strong enough to dissolve
normal blockages caused for instance by Ca and Fe
deposits and to prevent sliming under operating condi-
tions. Formic acid is also volatile enough to be sprayed
into the pipe distributing the gas to be fed into
the aeratoxs.
An essential requirement is that the cleaning
chemical does not cause corrosion of the air distribu-
tion network or aerators. The air distribution network
above the basin is usually made of Cr, Ni alloyed steel.
In certain cases galvanized pipes a:re used~ In the
following table the effects of formLc acid and hydro-
chloric acid on steel containing 18 ~ Cr and 9 ~ Ni
have been compared (data taken from "Korrosionstabeller
for rostfria stal, Jernkontoret, Stockholm, D226,
Stockholm 1979, Carlson press offsetstryckeri AB,
pages 20, 45, 46 and 58)
Corrosion effect
Cont. Temp. HCOOH HCl
% C
0.1 20-50 - 1, P
0.5 70 0 1, P
0.5 50 - 2
1 ~0-50 0 2
5- 20 0 2
0 2
0 2
0 2
3}3~2~
0 Corrosion rate < 0.1 mm/year the material is
corrosion-resistant
1 Corrosion rate 0.1 - 1.0 mm/year the material is not
corrosion-resistant, but it can be used in certain
cases.
2 Corrosion rate > 1.0 mm/year
P Spot corrosion is possible
It is known that hydrocloric acid strongly
dissolves zink. In order to determine the effect o~
formic acid the action on a galvanized sheet (160 g/m2)
was studied. Xn the test the sheets were subjected to
saturated formic acid vapour. Corrosion right through
the zink was observed after 400 hours. According to
the general heat galvanization standards the thickness
of the zink layer of a 1-10 mm pipe is 420 g/m2.
According to the test such a layer would have been
corroded right through after about 1000 hours.
In periodic cleaning ~see examples) the yearly
cleaning time is less than 10 hours and the conditions
are not as severe as the test conditions. Thus formic
acid is also harmless to galvanized pipes.
Compared to the risks associated with the storing
and the dosage of chèmicals-in the known methods and
to the complicated apparatuses used therein th~e use of
formic acid is easy and simple. The dosage device may
be a conventional dosage pump for chemicals provided
with a nozzle by which the formic acid is sprayed into
the air. Formic acid can be sucked directly from a
transport container, e.g. a plastic can, whereby the
handling of the chemical is reduced to a minimum. When
an adjustable dosage pump is used a flow indicator is
unnecessary.
3~
As a pump a high-pressure paint sprayer can be
used. A pump with a feeder pipe may also be concerned;
in this way, readily volatile or gaseous chemicals can
be introduced. However, the chemical is preferably dosed
by means of a pump and a carburator, e.g. an overflow,
float or injection carburator, into the gas. Thereby
it is guaranteéd that no non-gasified formic acid
vapours are separated in the gas distribution network,
but the formic acid vapours are distributed uniformly
to the individual aerator apparatuses. Combinations
of such dosage devices may also be used.
Formic acid may be introduced into the gas
supply pipe, preferably a pipe of compressed air,
before or after the compressor. For example, the chemi-
cal may be fed into the principal gas distribution
pipe or into the supply pipe system of the individual
aerator sectors.
Formic acid is highly suitab]e, because it is
biologically decomposable and non-toxic for the
biocenosis in the organic sediments in the sewage treat-
ment plants. Formic acid diluted in a great amount of
water acts as a substrate for aerobic bacteria. The
formic acid used is normally 50 to 85-% formic acid.
Preferable is ~5-% technical formic acid because it is
a cheap commercial product and because of its low water
content.
By means of the method in accordance with the
present invention, e~tensive blockages of the aerators
may already be dissolved after a treatment time of few
hours, so that the pressure loss at the aerators is,
as a rule, reduced almost to the value of new aerators.
The chemicals may, however, also be dosed continually
as smaller quantities, whereby blockages of the pores
are prevented efficiently. Appropriate dosage quantities
~2~æ~
can be found out readily by means o~ experiments.
Since in weakly buffered water, e.g. so~t
water, the pH-value of the water to be treated might
become excessively high on addition of larger quanti-
ties of acids, in such cases, practically a buffer agent,
such as sodium bicarbonate, is added to the aerator
basin. Under normal conditions this is, however, un-
necessary.
The invention will be described more closely in
the following with referellce to the attached drawings.
~ ig. 1 shows an arrangement of trials for testing
a continuously clogfree plant in a large-scale installa-
tion.
Fig. 2 shows the pressure trend at the manometer
according to Fig. 1 using aeration with three different
blower stages, measured in bar during one year.
Exam~le 1
Experiments were carried out in a sewage treat-
ment plant having an approx. 70,000 population e~uiva-
lent. The waste water accumulation comes mainly from a
major slaughterhouse and a factory producing ~nimal
feed; in addition approx. 5000 inhabitan~s are linked
to the municipal sewage network.
The aeration tanks studied wer`e likewise equipped
with Nokia disk aerators, type HKL-215 with the follo-
wing numerical distribution in both stages:
1568 ~erators per aeration tank, 1st stage,
~28 aerators per aeration tank, 2nd stage.
After a run for about two years all the aerators
already had to be replaced ~wice. Quite precise analyses
have revealed that the disks were caked up not onl~ on
the surface exposed to the water but also in their pores.
The grime was composed primarily of calcium
carbonate. Clogging in the first stage was generally
heavier than in the second stage.
The order of the tests is presented in Fig. 1.
The order of the tests is presented in Fig. 1.
The pressure air pipework from the blower to the aera
tion tank consists of zinc-plated steel, in the aera-
tion tank itself of V4A steel. The air distributing
grids on the bottom of the tank are made of PVC plastic,
diameter 120 mm.
The formic acid is fed into the V4A pipeline with
two dosing pumps Qmax = 18 l/h. In order to prevent
backing up of the acid into the blower, the acid in the
vertical pipe section leading down to the tank bottom
is sprayed into the air flow through a jet.
Fig. 2 shows the pressure trend, expressed in
bars over a measured period of time, of blowers 1, 1
2 and 1 + 2 + 3. To facilitate interpretation three
phases were distin~uished: Phase I presents the condi-
tions shortly before the last changing of the aerators.
Blower 3 could not, however, any longer be switched on
since its air would have been dissipated through the
relie~ valve. On 2 December, the beginning of
Phase II, the installation was again put into operation
with the cleaned replacement disks. Withln three months
of operation without the addition of formic acid a
clear pressure build-up was already to be noted again.
From 2 March on, 85% formic acid was introduced
into the pressure air with the successful result that
during Phase III the pressure almost sank to the
readings of new-ranking aerators.
When workin~ out the acid dosage, a distinction
must be made between the first and the second stage.
After a number of preliminary tests in which by and
large an excessively large amount of acid was introduced
into the pressure air, the following standard ratings
can be recommended:
q = 0.8 ml/~disk.d),
q = 0.9 ml/~disk.d),
where ~ corresponds to the daily dose of acid per disk.
The acid dosing should preferably be carried out once
to twice weekly since owing to the long-term effect one
can expect the next days to be without the occurrence
of caking or crusty formations. A quater hour sufices
to carry out -the dosing. In the event that duriny the
subsequent two to three months no pressure build-up
was noted with dosage in this amount, the speci~ic acid
quantity q can be reduced by approx. 30 %.
In the sewage treatment plant studied, the annual
requirement for technical formic acid (85%) was appro~.
580 litres, which translates into chemical costs of
about sFR 900. In comparison with the otherwise high
costs of current, the large-scale cleaning works and
the shutdown and return to operation of the aeration
tank, this expense is so small as to be neglibile, It
has been established that the method has no influence
on the biological process, does not attack the pipework
or aerator and exerts a tolerable en~ironmental load.
E~ample 2
The sewage treatment plant of the city o Turku,
Finland, comprises five aerator bas:Lns in which the
total number of disc aerators is 5250.
Ferrosulphate was added to the activated sludge
process to remove the phosphor. After a run for 2.5
years the back pressure of the aerators had increased
0.06 bar d-le to the ferrosulphate a~ld power failure.
The aerators of one basin at the time were cleaned ky
feeding formic acid through a nozzle connected by a
sleeve ~oint with the pipe directed downwards. A high-
pressure paint sprayer was used as a pump. The dosage
were 0.4 kg/aerator, i.e. about 2~100 l 85% technical
formic acid during two hours. As a result of the
cleaning the back pressure of the aerators decreased
0.06 bar, i.e. to the initial value. Because of the
fall of the kack pressure the energy consumption of
the plant was reduced from 74S0 kwh/d to 6550 kwh/d.
~he cleaning with formic acid did not affect the
biological unction in any way, which is proved by the
results o~ the analysis of an average sample on a
cleaning ~ay:
1 0
pH COD BOD7 Susp. solids Total -P
Influent
to plant 7.5 76 196 345 9.3
Influent
to aeration7.5 46 103 132 3.4
Outlet from
secondary
sedimentation 7.5 12 19(ATV8) 9 0.3
These data correspond to the normal operation
data of the sewage treatment plant.
Example 3
The effect of formic acid on sliming was tested
at the Toronto Lakeview waste water treatment plant.
Two Nokia Nopol HKP-600 diffusers were mounted on pipes
of an indiviclual length of 3/4" and installed in the
aeration tank. The air supply to each diffuser was moni-
tored and controlled at weekly interwalls. The diffusers
were pulled up above the mixed liquid lewel and inspect-
ed. One of the pipe diffusers acted as a control. 30 g
HCOOH/week was fed into the othex pipe diffuser. The
essential results were as follows:
- The control diffuser fouled quickly;
a slime coating of a thickness of more than
1 cm grew on this diffuser within a period of
3-4 weeks.
- The test diffuser also fouled at the beginning
of the test period. However, dosages of 30 g
formic acid/week injected into the diffuser
air stream over a period of 0.5 h reduced
substantially the biofouling on this diffuser.
