Note: Descriptions are shown in the official language in which they were submitted.
3~
The invention relates to a two-stage activated sludge process
for the purification of sewage,
Activated sludge processes, also called sludge activation processes,
constitute a qpecial form of biological purification of sewage. In the pa~t,
S sewage from housing estates and industrial plants was either allowed to
` seep into the subsoil, used for irrigating agricultural land or fed into
local watercourses either in a mechanically cleaned or uncleaned state,
It was only at a later date that the sewage treatment plant was extended
by a biological cltage, The function of sewage purification is to reduce
lû to a minimum the serious dangers for the health and life of hu~nans and
animals linked with inadequate purification. ~ sewage purification by
means of an activated sludge process the processes of biological self-
purification of a natural watercourse are compressed into the ~malle~t
possible space and their time scale i~ greatly reduced. As a result
of artificial aeration in special aeration tanks, and wlth the concomitant
action of the activated sludge (return sludge), initially the colloidally
~:
dissolved substances are partly agglomerated around the in part organic
and in part inorganic suspended matter, and optionally around any material
., .
present which has a capacity to settle, whereby flalces are formed,
However, in part under the influence of the aeration there i9 a coagulation
: .
and flocculation of the dissolved and semi-dissolved substances which form
,.
1~ ~ a good nutrient medium for bacteria and microorganisms. It is assumed
that through the vital activity of bacteria and microorganisms a type of
,
suction force is exerted on the dissolved and colloidal organic contaminants
~ ' .
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of the sewage and this can be called biogenic adsorption, Thus, the
biological purification action is obtained through the interaction of the
surface action of the flakes and the oxidation of the adsorbed substances
through said organisms with the aid of enzymes in the presence of oxygen,
The water which flows out of the aeration tank, together with the activated
sludge, is separated from the latter in a ~econdary settler and then flows
in regular manner as biologically purified water to a draining ditch, i. e,
a natural or artificial watercourse. Part of the sludge must be returned
to the aeration tank as return sludge where it completes the biological
decomposition of the putrefactive organic substance in the sewage, The
surplus sewage is either ~tabilised in the digestion tank, mixed with the
preclarification sludge or is fed to a sludge draining plant if it happens to
,: , .
be stabilised sludge, A constant rotation of the tank content is necessary
so that said flakes do not ~ink to the bottom of the aeration tank and die
due to lack of oxygen,
One- and two-staga activated sludge plants are known for carrying -` -
, ~ ~
;~ out the activated ~ludge proces~. Tha simplest form of an activated sludge
~; plant operates on the ba~is of a presettling of the sewage and a subsequent
single-stage highly-loadet aeration. However, this proces~ is extremely
sensitive to sewage surges and to the influences of industrial sewage
! .:
because all the sewage which has to be biologically purified is fed into the
I
aeration tank so that all the activated sludge can be destroyed, A greater
reliability relative to sewage ~urges and the influences of industrial ~ewage
can only be obtained in one-stage activation plants through a lower sludge
- 3 -
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loading and long residence times.
Efforts to make the activated sludge process less sen~itive but
nevertheless reduce the aeration time led to the development of processes
in which there is a two-stage aeration
On the basis of the latest findings, optimum results are obtained
with such a two-stage biological process by proceeding in the following
manner: the activated sludge of a first biological stage is used for
performing all the individual processes necessary for biological
decomposition such as adsorption of the dissolved organic substance
,
on the sludge ~lakes, their diffusion into the cell, substrate and optionally
'
endogenous breathing, and mixed with the partly puriffed sewage it is
supplied in the highly active state to the intermediate purification system. ~ -
After separating the sludge the partly purified sewage i8 introduced into
,
the second biological stage operated with a lower ~pace and ~ludge load.
lS Aided by the long re~idence time and large supply of oxygen, this ~tage
serve~ not only to~3ubstantially remove the remaining organic substance
' ~ .
but, dué to self-absorption, there iB alQo a significant reduction of the
biomaa~ content. The conventional proc0s~ mea~ures ~uch as secondary
settling and the like follow. The following are considered to be important
3 ~
proce~s parameters: a BOD5 ~ludge load between 0 5 and 1. 0 kg
"
BOD5/kg TS in the first biological ~tage and a low BODS space load of
below 0, 5 kgtm3/d with at the same time a low ~ludge load of approximately
, ~ ~ , ,,
O, 05 kg BOD5/kg TS, On implementing these parameter~ and utilising
, ,
the biological activity of the surplus sludge of the second stage by supplying
, .
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the first activation stage a high BOD5 decomposition capacity i5
intended to be achieved in both biological stages, decomposition in
the first stage exceeding 85%, in order to obtain a discharge con-
centration of below 40 mg BOD5/1.
The known processes of the type described hereinbefore have
the disadvantage of a very considerable expenditure on energy~ The
purity levels obtained continue to be unsatisfactory because through
underloading the second stage and returning the surplus sludge of the
second stage to the first stage the deterioration of the activated -
sludge is very easily possible. It is also to be feared that the known
processes will not satisfy the future water conservation regulations
which are continually becoming more stringent. -
There~ore, the problem of the invention is to provide a two-
^ stage activated sludge process for the purification of sewage which
-~ 15 are able to achieve-much improved and much more stable purity levels
whilst significantly reducing energy costs, whereby the capital costs
must not reach thosé of a comparable conventional sludge activation
plant.
According to the invention, this problem is solved in that
~; 2Q all the sewage is fed to a first aerated activation stage which is
: ..
.,
operated as a maximum load stage with a space load BR of approximately
10 kg BOD5/m3/day and a sludge load BTS of at least 2 kg BOD5/kg of
dry substance/day, by intermediate settling of the sewage sludge
mixture removed from the first stage a strict separation of the
biocenoses of the two activation stages is ensured, whereby su~ficient
.,
surplus sludge is removéd from the intermediate settling stage that
the age of the sludge is kept low, the intermediately settled sewage
being fed to a second aerated activation stage which is operated as
,
a low load stage.
A further feature of the process according to the invention
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that compared with -the known two-stage activation processes, on
the crude sewage by-pass there is no need for additional feeding of -
the biocenose oE the second activation stage because in the process
according to the invention the polar easily decomposable organic
compounds are fed in adequate quantity to the second activation stage.
Within the scope of the present invention, the term sewage
is used in its widest sense. In general, it is an aqueous system in
which are dispersed organic substances, also in the presence of
dissolved or suspended inorganic substanoes. The particles of the
dispersed phase can be genuinely dissolved, emulsified, in colloidal
and/or suspended form. The degree of dispersion of the particles to
be separated is not in general of great importance. The performability
of the process according to the invention is also not dependent on
whether the substances will or will not settle. For example, sand
and mud will settle whereas the dissolved substances and colloids
which ma~e sewage turbid will not settle. The fact that sewage often
contains non-putrefactive as well as putrefactive substances is also
generally unimportant for the performability of the process.
Hereinafter a number of non-limitative examples of applica-
~20 tion of the inventive process are given. Thus, the process can beused for the purification of domestic sewage obtained in households
when washing, rinsing and bathing, as well as in toilets, for
purifying municipal sewage which also contains rainwater from roads,
roofs and courtyards, and for purifying purely industrial sewage. The
~ 25 process according to the present invention is particularly important
; in connection with the purification of industrial sewage. The process
i according to the invention is also suitable for use when combined
with other purification processés, adaptatio~ to the particular case
being necessary.
The two activation stages of the inventive process are
-6-
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_rated either by means oE compressed air or by breakiny up and
splashing the water surface by means of rapidly rotating rod rollers,
e.g. Kessener brushes, or by means of Simplex certrifuges, Simcar
fans or other known surface fans. Furthermore, if it is advantageous -
5 it is possible to combine different aerating means. -
On the basis of the teaching provided, it will also be
possible for the expert to select the most suitable aeration tanks
for the performance of the process of the invention. The following -~
enumeration of aeration tanks must only be considered in exemplified
manner and can be extended at random. Thus, for example, compressed
air tanks have proved advantageous. It is also advantageously
possible to use so-called Haworth channels which are aerated by means
of paddle wheels which simultaneously move the water forwards, whilst
it is also possible to use Kessener tanks, agitator tanks and tanks
which operate according to the Simplex process.
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The nature of the aeration and of the aeration tanks need not be the
same in the first and second activation or aeration stages according to the
process of the invention, Thus, for example, it is particularly advantageous
if the first activation stage (maximum load stage) involves aeration with
large bubbles, whereas ~ine bubbles are used in the aeration of the second
activation stage, Particularly good results can also be obtained if the
second activation stage is operated with a thoroughly mixed ditch system
which is independent of the construction of the aeration tank of the first
activation stage. Such a settling system represents a slight modification
o~ the Haworth channel, Air and/or oxygen are supplied by means of e. g,
centrifuges. A high level of efficiency is regularly obtained with such
plants such as e. g. low loaded oxidation ditches (roundabout plants),
A further advantage is that the construction costs of such plants are about
30% lower than known plants with a comparable decomposition capacity,
,
In general, preliminary settling of the sewage is not necessary.
Only when there are difficult constituents such as fibres and the like
may a preliminary sludge removal be necessary, and even then it is not
time-consuming (t C 0. 5 h), Intermediate and final settling are performed
by conventional processes,
It may be necessary to finish off the process according to the
invention by a filtration or flocculation filtration process in order to further
reduce the BOD$-value, This reduction can exceed 50%, I~ ~uch a
filtration is carried out the final ~ettling sy~tem can be m~de smallar
as an exception to the ATV-Guidelines (Working Repork of ATV-Committee
~ 8-
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5 "Settliny Processes"), because any separatiny sludye particles
are held back by the filtr~tion process. When account is taken of the
smaller final settling system, filtration does not lead to significant
additional construction costs. Compared with a conventional sewage
treatment plant (sTs = 0.15) the present process is still cheaper and
the efficiency is much higher.
The sludge from the intermediate and final settling stages
must in part be returned as return sludge to the particular activation
stages, whilst the remainder of the sludge is conveyed as surplus
10 -sludge to a sludge digestion tank, for example via a thickener. In
connection with such sludge returns, it must always be ensured that
there is a basic separation thereof. This leads not only to the
advantageous separation of the biocenoses of the two activation stages
but also ensures that unlike in known processes operating difficulties
do not occur after only a short time.
The expert wOula also not depart from the scope of the
~ invention by modifying the process tnereof by applying conventional
`~ measures of activated sludge technology which can be performed before-
hand, afterwards or intermediately. Thus, for example, adsorption in
the first activation stage can be improved by the addition of floccu-
lating agents, whilst a general stabilisation of the system can be
achieved by means of polyelectrolytes.
The addition of particular enzymes to the second active stage
may prove advan-ageous, whereby the enzymes should be matched to
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the sewage type,
The following statements serve to provide a better understanding
of the invention and relate to the basic technological interrelationships
of the invention.
S It can be fundamentally assumed that readily adsorbable substances
have a greater affinity to the adsorbent than solvents, in the present case
water. This means that the more difficultly water-soluble, i. e, higher
molecular weight, that is to say more difficultly decomposable ~ubstance~,
are held back in the first activation stage or maximum load stage, whereas
10 the readily water-soluble and consequently more easily decomposable
substances are supplied to the second activation stage and are biologically
decomposed therein. ~ .
The essential point for a successful and reliable operation of the
proces~ according to the invention is the knowledge that the sludge in the .....
. 15 maximum load stage must be constantly held in the so-called working-in
phase in which the substrate breathing starts. This sludge is of a fine-
~~ floccular structure with a very large specific surface area which is
important for the adsorbtive action and in general has a low sludge index, ~ .
The reactions taking place in the first activation stage are mainly of a
; ~ Z0 physical~chemical nature, whilst the biological processes which are in
the foreground o~ the known processes are in the background here,
espite the high purifying action (above 60%) the oxygen consumption and
consequently the energy requirements are relatively low,
However, when performing the process according to the invention,
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.
is necessary to ensure a yood intimate mixing and a high sludge
load accompanied by a low sludge age in the first activation stage,
whereby it has been found that a space load of BR oE approx. 10 kg
; BOD5/m /day and a sludge load BTS of at least 2 kg BOD5/kg TS/day
ensure the desired technical result.
The process of the invention differs from the prior art
purification processes in that without the addition of precipitation
or neutralisation agents in the first high-loaded activation stage,
tests have shown that in the second activation stage there is a
reduction of the high pH and conductivity surges to a harmless level
- for the biocenoses.
Compared with the known processes, the process of the
invention also differs to a certain extent by a reversal of the
puriflcation sequence. In the presently conventional biological
purification processes, the complete sludge load supplied to the
' biological processes~ is converted into biologically high grade
~: ,
' activated sludge with a large number of different types. The follow-
,~ ing purification sequence is maintained:
.!:
'~ ~ a) Addition (adsorption) of higher molecular weight compounds
on the flakes and decomposition of the lower molecular
. . ~ . .
weight (more easily decomposable) compounds,
b) Absorption of the higher molecular weight compounds after
, corresponding preparation through the enzyme activity in
the cell,
25 c) Oxidation of the contaminants for energy production and
conversion into new cell substance and separation of
products of metabolism.
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As compared with this in the maximum load/low load process
according to the invention, a large proportion of the higher molecular
weight compounds are not decomposed, Instead, following adsorption,
in the maximum load stage they are removed after a short time with the
low-age sludge and consequently do not further load the purification
process for the lower molecular weight and, in particular, polar compound~
in the second activation stage, This results in a considerable energy
saving, This is proved by a comparison of the oxygen consumption of
the process according to the invention and the prior art processes, : -
!: 10 For this purpose it is possible to use the oxygen requirement formula
(see ATV-Handbook)
OVR = ~ 5 ~,. BR ~ 0.1 , TSR [g 2/
in which the individual quantities have the following meaning: ~
~, OVR oxygen requirement, . ~ -
: 15 7, purification capacity or proportion of decomposed :
: : substance,
BR organic space load/m3/d,
TSR % of solids/rn3,
Taking account of sub~trate and soil breathing, as well as the
i: zn oxygen requirement for nitrification and denitrification processes, the
¦ ~ following is obtained:
i:
-12 -
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.. ... .. . .. . .. . . . . . . . . .
~- 10~4~;3~3
Stabi- Nitri- Residual Residual
lised fication BOD5 BOD5
20 mg/l 30 mg/l
BTS - 05 0, 15 0, 30 0, 60
Soil and substrate
breathing 0, 42 0. 56 0, 78 1, 18 OVR
.
Nitrification and
denitrification 0~ 05 0. 23 0, 34 0, 26
OVR 0, 47 0. 79 1, 12 1, 44 R R
Increase of soil
and substrate
~, ~ breathing due to 112% 141% 144% 122'1u m
3~ nitrification and
1: denitrification
: ~ ~ * Den, = denitrification,
The power consumption K is functionally related to the oxygen
I i ,
requirement OVR in accordance with the following equation:
m, OV
~: K =
k
~: ` in which k is the amount of oxygen supplied per kWh, which in the case
: :
of aeration with medium-sized bubble~ is 1800 g of 2~
The actual total power consumption KG - KN ~ Kae . including
ancillary equipn~ent, can be gathered from the following aft~r applying the
,: ~
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ATV-Guidelines (see ATV-Han(lbook):
_ _
BTS~ 05 0,15 0. 300. 60 l, 0
:,
TS 5 3.33.3 3.3 3-3 g/l
K 21 15 11 9 8 kWh/I a
I = inhabitant
In this the power consumption is given in kilowatt hour~ per
, inhabitant and year,
'~ ~ The total power consumption which is related to the OVR- value
can be gathered from the following formula:
j~ KG - KN + 0.1014 . m . r1, . a ~ 0. 0203 . m . ,~3 BTS
,::
..
The new symbols in this formula have the following meanings:
KN specific power requirements for ancillary equipment,
aer, plant BTS = 0, 05 - KN = 2, S kWh/I- a,
aer. plant BTS 0. 10 - KN = 3. kWhtI- a;
m additional consumption for nitrification and denitrification;
; ~, proportion of biological purification level;
purification of organic substance and
, a ~pecific organic decomposable load
g 13OD5/I- d~
.
,;
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10~63~
For the various dctivation proce~ses the factor~ can, for example,
be gathered from the following:
BTS 0.05 0.15 0,30 0,60 1,00Dim,
_
m 1, 12 1,41 1,44 1,22 1, 10 -
_ ~,.' '
~rl 96 94 91 86 81 %
a 57 48 45 42 42 g/I d
_ _
,~ O, 50 0. 60 0, 60 0, 70 0, 70 -
_
S(dec, ) 10 15 20 30 40 mg/l
.
`
On the basis of these calculations the following i~ obtained for
the two-~tage activated sludge proces~ (without ~ilter) according to the
, ~ ~ 5 invention:
KN ~ 3, 00 kWh/I~ a
KH = (1, 10 ~ 0, 09) = 1, 19 kWh/I. a
KS ~ (2,27 ~ 2,16) = 4,43 kWh/I. a
KN ~tand~ for the power consumption of the ancillary equipment.
K~ ~tand~ for the power con~umption of the maximum load stage reali~ed
according to the invention and KS or the low load stage so that the total
power con~umption i8 8, 6 kWh/I. a for the two activation ~tage~,
, ~
If it i8 a~umed that both the ~ingle ~tage comparative
proce~ with BTS ~ 0~1S and the proce~ according to the invention
.~
-15-
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10~q~63~
operate in the final stage with a BTs-value of 0, 15, then an energy
saving of
I = (15. 0 - 8, 6=) 6. 4 kWh/I~ a (without filter)
is obtained,
In a conventional process which finally operates with a BTs-value
of 0, 15, a B~)D5-residual value of 15 mg/l is on average obtained, The
' same result is obtained with the process of the invention, If a filter
`~, station is used with the process of the invention, the BOD5-value drops
~ to 8 to 10 mg/l, Thus, the following values are obtained for a connected ùp
: 10 figure of 300, 000 I and IE (number of inhabitants/inhabitant equivalents):
~ ~ _ _ _ .
~: ~ Process Actual space occupied Residence time
1 ~ Ideal space occupied* t in h,
-~ ~ _ ___ _ _ _
Conventional 77, 000 m
, 14
~: BTS = 0. 15 80, 000 m3
.1 , _ _ - : '
Acc, to the invention 60, 300 m 9, 1 ~;
Without filter station 68, 300 m ,
_ _
, Acc, to the invention 56, 000 m3 7, 6 :
With filter station 74, 500 m ~ Filter
,, . ~
~: -
* The ideal space occupied was determined by relatin~ the useful
volume of each treatment unit to a unit price of DM 300. --/m,
Thus, the ideal volumes also constitute an estimated cost comparison
. . .
'~ of the main units of the sewage treatment plant,
.; : .
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-16-
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~, ' , ' ' .' :'' ' ':
.
10~ 39
The conventional process operates with a low load stage,
followed by final settling, According to the invention, there is a maximum
load stage, a low load stage, intermediate settling and final settling,
whereby the process may if desired be terminated by a filtration stage,
As can be gathered from the above calculations, a considerable
advance in the art is provided by the process of the invention, particularly
with reference to the power consumption and space occupied, whilst the
purification capacity is as good o~ better than the prior art processes.
Thus, the advantages obtainable by means of the invention are
that the process thereof can be used in plants which can be operated
with a much lower power consumption than plants operated by conventional
processes, whilst there is no disadvantageous effect on the construction
costs. In fact, a plant operated in accordance with the process of the
invention has a much smaller space requirement than the prior art plants.
, 15 Finally, the process according to the invention in conjunctian with the
filtration stage brings about extremely high purification capacities
(BOD5 c 10 mg/l), Plants operated in accordance with the process of
the invention are much less sen~itive to sewage surges, Another
¦~ important advantage is the suitability of the invention for extending older
or overloaded sewage treatment plants. By connecting in beforehand a
maximum load plant which would have to be constructed and by converting
the preliminary settling aystem into an intermediate settling system, the
load on the existing activation stage can be significantly reduced, In
addition, itwill also be necessary to enlarge the final settling system.
;: ' ,i " .
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. .
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.
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~0~63~3
However, it should be noted that when using the inventive concept the
old parts of the sewage treatment plant can continue to be used,
Hereinafter an embodiment of the process of the invention is
described relative to the drawing in order to further illustrate the
invention.
: Sewage enters via an intake 1 and is fed by means of a pump 2
via a pipe ll to a preliminary sludge removal apparatus 3, After removing
the difficult constituents, such as fibres and the like, the sewage passes
via pipe 12 into the first aeration tank or the first activation stage 4
which is operated as a maximum load stage. The aerated medium then
passes via pipe 13 into an intermediate settling apparatus 5, The
clarified phase is passed via pipe 14 to the second aeration tank or the
second activation stage 6 which is operated as a low load stage, A sludge
is removed from apparatus 5 for intermediate settling and is supplied via
a pipe Z2 and a pump 23 to pipeq 24 and 25. Pipe 24 serves to return
return sludge into the first stage system, whilst pipe. 25 serves to drain
off surplus sludge from the system, e, g, by means of a thickener into a . .
,~ sludge digestion tank, The same happens to the initial sludge removed . .
- via pipe Z7, At the end of biological decomposition in apparatus 6 the
: 20 aqueou~ phase paqses into a inal settler 7, from which sludge is removed
via a pipe 18 and a pump 19, By means of pipe Z0 this sludge is either
~: returned to the second stage sy~tem as return sludge or it is removed from
the sy~tem as surplus sludge by means of pipe 21, By means of a pipe 16,
. ~ ~ a pump 8 and a pipe Z6 the clarified aqueous phase i~ fed to a high speed
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filter 9, from where the clarified water i8 supplied via a drainpipe
10 to a watercourse, By means of pipes 28 and 27 flushing water can
be fed from high speed filter 9 to the second stage system.
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