Note: Descriptions are shown in the official language in which they were submitted.
2:L~ s3~rj
93-556/7
ACTIVATED SLUDGE PROCESS FOR SEWAGE PURIFICATION
The invention relates to an activated sludge process for ~ewage
purification in which the sewage is introduced into a first,
partially aerated activation stage, is then submitted to
intermediate clarification, is then introduced into a second,
partially aerated activation stage, i~ subjected to post-
clarification and is then removed, in which sludge is fed back from
the intermediate clarification into the first activation stage and
from the post-clarification into the second activation stage, and
in ~which excess sludge is removed from at least one activation
stage:is removed from thc ~ludge circuit.
..
The usual two-stage activated sludge process (Doctor W. Lindner,
"The Two-Stage Activation Process in Sewage Puri~ication" (Kempten
195~7)j: Thom~s-Verlag) is a known process. In this procefis,
sub~trate breathing: whcre the micro-organisms consume oxygen
through the oxidation of;organic compounds and where the biological
decomposition of carbon compounds is therefore most conspicllous
;takcs full effect in~the first activation stage with high sludge
load. The second activation stage in thi~ known proces~ i8
generally carried out with lower sludge load so that a
decomposition of the rcmaininy carbon compounds and the oxidation
of nitrate compounds (nitrification) t~kes place.
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Normally the known, two-stage activation proce6s is carried out
with a sludge proportion of 0.8 to 2.0 kg BORs(biochemical oxygen
requirement in 5 day~)/kg dry su~stances and per day in the fir~t
stage and from 0.15 to 0.5 kg BORs/kg dry substance and per day in
the second activation stage (Lehr- und Handbuch der Abwassertechnik
[Instructions and Nandbook for Sewage Technologyl, published by the
~Abwassertechnischen Vereinigung ~Sewage Technology Association)
e.V. in St. Augustin, Berlin Ernst Verlag, page 426, vol. IV, 3rd
edition, 1985). ID :the:~method which ifi typical for this proce6s,
the major part of carbon decomposition takes place as described in
the~first stage, and ~exteDsive nitrification in the second stage.
Denitrification is no longer possible in the fiecond stag~ due to
the~absence o~ easily decomposed carbon substrate. The withdrawal
of~exces~ s1udge from the overall plant takes place either during
pre-c1arification before~the first stage or, in the absence of pre-
clarification, from the sludge circuit of the first stase The
exce~s~sludge of~the second stage is withdrawn together with the
s1udge~of the first stage without giving rise to a controlled
rémovsl of nitrogen:because no anoxic conditions exist in the first
stage. ~
In a process known ~from AT-PS 318.503 for the elimination of
organically and inorganically:attached nitrogen from dome~tic and
industrial sewage,~ n~itrified sewage i~ taken from the ~edimentation
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basin of the second process stage and is fed into a conduit through
which sewage is conveyed from the aeration basin of the first stage
into the post-clarification basin of the first stage for the
denitrification of the ammonium nitrified in the second process
stage, and thereby for the elimination of nitric nitrogen from the
sewage to be purified. In the proces6 known from AT-PS 318.503
only denitrification takes place in the first proce~s stage
(aeration basin and'~sedimentation basin) and only nitrification
take~ place in the~second process stage.
: .
In~the process known from DE-OS 31 36 409, two plants, i.e. two
8tage8~ of~ equal rank~are parallel-connected for nitrification.
According~to the DE-OS 31 36 409 particulnrly favorable conditions
for~ the~ nitrification~;nre to be mnintained in the auxiliary
n~itrificntion~6tage~60~;thnt no interruption of pitrification may
occur in~cnse~of disturbances in the main nitrification stage but
thnt~biology held~in~ re~erve in the auxiliary nitrification stage
mny,~be introduced~ from~same into the main stage if necessary to
ensure thnt nitrificntion~can be continued therein substantially
without interruption~once;~a disturbance has occurred.
WO 83/00856 describes n~ sludge treatment process in which only
cnrbon` compounds~nre~ to be decomposed and in which neither
; nit`rification nor~denitirification ta~es place. In this process
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treated sludge coming from a pre-thic~ener i~ fed to an aerobic
treatment stage for aerobic treatment. The discharge from the
aerobic treatment stage is conveyed to an anaerobic teatment stage
for a dige~tion proce~s in which additional decomposition processes
occur. Part of ;the ~ubstrate is fed back from the anaerobic
treatment stage into the aerobic treatment stage according to W0
83/00856. No mixed biocenoses are produced thereby, since the
bacteria which are active in the aerobic treatment stage sre not
vlable in the anaerobic treatment stage and vice versa.
A process known under by~the name ~AB Process~ from DE-PS 26-40~875
a~tw0-6tage 6ewage purification process in which the first
tage~, an adsorption 6t~ige, is heavily charged (~ludge proportion
2.0~ to~10.0 kg BORs/kg;dry substance and per day) and ~erves for
the decomposition;or~ adsorption of carbon compounds. The second
6tage operates with~a sludge proportion of 0.15 to 0.30 kg BORs/kg
dry ~sub6tance and~ per ~day and~serves for nitrification.
Denitrification ~is ~pos6ib1e only~ in the second stage in this
;process, and~then only in function~ of the integration of carbon
compounds not eliminated~;in the first stage, and this is difficult
to control in operation. In the process known from DE-PS 25 40 875
the biocenoses of the first and second stage must be kept strictly
6eparate ~rom each~ other in ~order ~o provide the advantage of
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removal of carbon compounds in the first stage with little energy
consumption and with adsorption. -~-
The purification of sewage through nitrification anddenitrification in activation plants can in carried out in
principle in single-stage and in two stage plants. In order to
achieve certain nitrification it i~ necessary to respect a given
sludge~age in the plant~in function of temperature of the sewage so
that the sIowly gxowing~nitrifying bacteria in the activated sludge
may indeed be present and are not washed out. The age of the
sludga~in an activation plant is however deci~ively influenced by
the proportion of carbon compounds in the sewage si~ce the
nitrifying bacteria ~represent only a small part of the entire
biomass ~less than~S~
Due to~these conditiao~ ehe si~e of the single-stage activation
plant~ for~sewage~purification through nitrification is~mainly
det~rmined~by the~proportion of decomposable carbon compounds.
n~two-stage processes~a~good basis for nitrification in the second
stage can be created through the exten6ive decomposition of the
carbon compounds~in the~first stage without nitrification and the
subsequent treatment~of;the pre-purified sewage in the second stage
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because it is possible to operate with comparatively small basin
volumes after removal of the carbon compounds of old-age sludge.
~ .
For the removal of nitrogen compounds from the sewa~e the reduction
of nitrate into gaseous nitrogen (denitrification) taking place in
: the absence of dissolved,;i.e. free oxygen and with the utilization
:
of the oxygen attached ~to the nitrate (anoxic conditions) is
however advantageouæ after thë oxidation of nitrogen compounds
(NH~) reduced to nitrate. In single-stage plants this can be
achie~ed with appropriate configuration of the basin and adaptation
of:the oxygen arri~al,~so that aerobic and anoxic conditions are
created.~ The inclusion of denitrification in the process requires
however~:additional enlar~ement of the required basin volume.
The~;in~cl:usion of~ de;nitrification in two-stage~ installation is
basical~ly~pos6ible,~but because of the spatial separation between
the~decomposition~of~the:carbon~compounds in the first sta~e and
the~;nitrification~in ;the~ second:~stage, the supply of carbon
compoùnds~requir~ed for the reduction~of the produced nitrate during
denitr:ification is~ now~very low. Effective denitrification in
conventional two-stage~processes is possible only through re-
airculation of nitrif~ied, essentially sludge-free discharge from
the post-clarification in the fls~t~stage and the establishment of
anoxic conditions ~in ;;same (see: also Wilhelm v.d. Emde
:
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93-5s6/7
"Betriebsweise von selebungsanlagen~ [Operation of activation
plants] in Wiener ~itteilungen Wasser - Abwasser - Gewass~r, Vol.
81, Vienna 1990, se~ond edition).
The denitrification in the two-stage AB process (DE-PS 26 40 875)
can only be achieved through limitation of the carbon removal in
the first high-load stage, e.g. by reducing air supply, and ~hereby
by shifting the carbon renloval into the second ~tage, also while
maintaining anoxic conditions. Recirculation before completion of
the second stage into the first stage which i8 a highest-load stage
would serve little purpose because of the short sojourn time. On
`::
the other hand however, the ~dvantage with respect to mai~taining
optimal condition~ in nitrification is lo~t because of the
limitation of carbon removal in the irst stage.
To~sum up it can therefore be said for the ~peration methods used
at thi~ time in acti~ation processes that in the single-stage
process the pr~duction~ of a mixed biocenosi~ for carbon
decomposition, nitrification and denitrification creates good
conditions for the obtention of good results, but the volume
requirements for activation basins for this are relatively high,
e.g. 200 1/EGW (liter/population equivalence). In the two-stage
proces~es the conditions for nitrification are generally better in
~:::
the second stage because of the decomposition of the carbon
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compounds in the first stage. For denitrification however,
considerable streams of volume must be fed back, and this ~leads to
a heavy load for the sedimentation basin following the activation
ba~in of the first or of the second stage. A reduction of the
decomposition effect of the first stage (in the A~ process) on the
other hand represents a partial elimination of the advantages with
~; respect to the nitrification in the second stage and can be
achieved only at high operating costs with a reduction of the
oxygen supply and the danger of odor emissions.
: ~
It is the object of the invention to propose a process of the type
mentioned initially in which the advantages of the single-a~age and
of~the~ two-stage processes are combined.
According to the inve~ntion this object is attained in a process of
the~type;mentioned~initially in that at least part of the activated
sludge~con6tituted~in~thé~first activation stage, in particular the
e~cess~sludge, is;~tra~nsferred into the ~econd activation stage and
at least~part of~the~activated sludge constituted in the second
;activation stage,~in~ part~icular ~the~excess sludge is transferred
into the first activation stage and in that in addition to the
decomposition of carbon compounds, a decomposition of nitrogen
compounds is carried out~through nitrification and denitrification
in the first activation~stage as well as in the second activation
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21~ !~03~
93-556/7
stage. Advantageous and prefe~red variants of the proces~ are
indicated in the sub-claims. In the invention, controlled mixing of
the biocenoses in a two-stage plant makes it possible to avoid the
disadvantage of difficulties in denitrification which results from
the separation of the biocenoses carried out in the second stage
for the purpose of carbon decomposition in the first stage and
nitrification in the second stage. The controlled transfer of
activated sludge ~from the first stage, with high oxygen
consumption, into;the second stage makes it possible to bring
denitrifying biomass into the second stage. This controlled
transfer of biomass from the first stage into the second stage
resul~s in a mixing with the nitrifying biomass of the ~econd
stage, so that it becomes;~possible in this second stage, by means
of a controlled formation of a mixed biocenosifi, to achieve
denitrification in;add~ition to nitrification. It is advantageous
in~this case for the transfer of biomass from the first into the
second stage to be~controlled in such manner that the age of the
sludge required for e~xtensive nitrification can be respected.
By contrast to the~method of operation in a single-staqe plant
where the denitrification is produced by the pollution present in
~, .
the sewage ~the extent~of pollution is reflected substantially by
the BOR5 value of the sewage), denitrification in the process
:~ i
according to the invention is achieved in that the denitrifying
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biomass from the first stage is transferred in form of an activated
bio-sludge into the second stage and there, together w~th the
nitrifyîng biomass of the second stage, constitutes a mixed
biocenosis which is capable of nitrification as well a6 of
denitrification. The carbon compounds absorbed in the first stage
due to the biomass are brought into the second stage through the
transfer of the activated sludge. They are used therein as an
additional carbon source for denitrification in the 6econd 6tage.
The ratio between nitrificatlon and denitrification capability of
the formed mixed biocenoses depends in the invention on the
quantity of sludge transferred from the first stage into the second
tage. ~ Since only small volumes are involved in the transfer of
sludge from one stage into another stage by comparison to the
quantity of sewage~ ~ratio~ approx. 1:20), no additional hydraulic
load is imposed upon~the~plant.
When the nitrificat~ion is~reduced ~it is possible, in the process
according to the~invention~, to achieve~an improvement ~ery quickly
by~rèducing the volume~of~deni~rification bioma~s coming from the
first~;stage. ~Depending~to~ the degree ~to which the two biocenoses
are mixed, the conditions~of a single-stage plant (with extensive
transfer of the sludge~from the first into the ~econd stage) and
conditions of a ~two-stage plant (with interruption of ~ludge
transfer) can be established in a plant with mixed biocenoses.
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Similarly a mixing of the biocenosis of the second stage with that
of the first stage takes place since activated sludge (e.g!. excess
sludge) is transferred from the second stage into the first stage.
Thanks to this measure nitrifying bacteria are transferred from the
second fitage into the first stage where they cannot be held
automatically because of the prevailing conditions ~the actual age
of the sludge is not sufficient for stable colonization by
nitrifying bacteria in; the~first stage). Because of the constant
arrival of nitrifying~bacteria in the first fitage these are however
present in the mixed biocenosis of the first ~tage to the extent
that activated sludge of the second stage is fed back. Because of
the high concentration of ammonia nitrogen prevailing in this ~tage
the growth rates of~tbe nitrifying ~bacteria are however not
rest;ricted and can operated at nearly maximum speed. By contrast,
a~lowe}ing of the~growth rates always occurs in the second stage
because of ~the defiired~;~low concentration of ammonia nitrogen in
said~second stage~based~on the Monod relationship: V = V~XS/(K~S)
where'~V~= the current turnover ~(growth) rate, V~x = maximum
turnoveF~(growth)~`rate, S = substrate concentration and K =
substrate~co~ncentration;~with growth rate at half of maximum.
Due to this fact it~is possi~le~to nitrify in the aerobic portion
of the first stage ~simultaneously with the decomposition of the
carbon compounds, where~y the ~ormed nitrate is denitrified very
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rapidly because of the high breathing activity of the activated
sludge in this stage in the anoxic ranges. In this mannerJ through
the controlled mixing of the biocenoses of the second and of the
first sta~e, the aerating capacity exiting in the first stage is
also used optimally to remove nitrogen compounds through
nitrification and denitrification in addition to the removaL of
carbo~ compounds~ with the extent of growth depending on the
composition of the mixed biocenoses. Here too, the variation of a
purely two-stage operation suffices as back-feeding of biomass from
the second into the first stage is interrupted until a nearly
sinqle-stage operation i8 obtained in case of compl~te sludge
circuits.
In the process according to the invention, activated sludge can be
takén out directly ~from the basin in whic,h the respective
activation stage~(first or second) is being carried out. It is
furthermore possib,le~ to~,take activated sludge from the first or
second~activation ~stage~and to transfer it to the other activation
stage. Finally, activated sludge can be taken from the intermediate
~, ~ :, : , . ~
clarification basin~`downstream of the first activation stage and be
~ transerred into the second activation stage. Activated sludge can
;~ logically be taken from the post-clarification basin downstream of
the second activation stage and be transferred to the first
activation stage.
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93-5S6/7
Overall the process according to the invention is a~ follows, and
can be carried out for instance in plants such as shown as
: schematic examples: in Figs. l and 2.
.
Fi~. l shows a two-~tage activation process with mixed biocenosis
for:nitrification and denitrification and Fig. 2 shows a variant of
the process with biologi:cal removal of phosphor.
This~is:a two-stage act~ivated sludge procefi~ (stageæ ll and 12) for
the purification of sewage in which sewage 9, introduced into the
fi}st activation basin 3~having at leafit one aerated and at least
'one non~-aerated zone and being set up and operated with,a sludge
age~:of at leaæt:1~to S~days, is then subjected to intermediate
c~larif~ication in~:an ~intermediate clarificat~on basin 4, and is
thereupon~introduced~;into a second clarification basin S with at
least~one aerate:d~and;~at::least one:non-aerated zone, set up and
operated:~with~a s~1udge~age of 3 to 1~5 days, is then Eubjected to
post-clarification~ in~ a ~post clarification ba~in 6 and is then
re'moved,~ whereby~ s;ludge~ i8 ~ withdrawn from the intermediate
c~Iarif~ication in:~a~,~first~;sludge circuit l, is fed back in part in
; ,form of feed-back 81udge~13 into the first activation stage and is
in~part introduced,as:an~active biomass into the second activation
stage ~'(s1udge circuit 1) and in part withdrawn from the first
s1udge circuit in~:form of exces6 sludge 14. From the post-
13
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93-55Ç/7
clarification in the post-clarification basin 6 of the second stage
12, the sludge is withdrawn in a second sludge circuit 2, î~ fed
back in part in form of feed-back sludge 25 into the second
activation basin 5, is in part fed back as active biomass 23 into
the first stage ll and is in part withdrawn as excess sludge 24
from the sludge circuit 2.
Thereby nitrification and denitrification takes place in the fir~t
stage ll as well as in the~second stage 12, respectively compri6ing
at least one activation basin and at least one sedimentation ba6in.
The biomass constituted in the second stage 12 i~ introduced into
the~first stage Il and the first stage is thereby inoculated with
nitrifiers. Thus~ nitrlfication, normally impossible with an
aerobic s1udge age of e~.g. two days at a sewage temperature of
10C,~a160~takes place~in the first stage.
Ammonium~nitrogen,~as~;a~rule in a concentration of over lO mg/l, is
contained in the fir~st- stage ll, and for this reason nitrifiers
grow~at nearly maximum~speed. At the same time a sufficient amount
of easiIy decomposed~; carbon compounds and highly active
~; heterotrophic bacteria is present here too, and therefore the
nitrified nitrogen can be denitrified in the same stage. At the
; same time over 75~ of ~the carbon compound~ are removed from the
sewage in the first stage ll. Part of the biomass 15 constituted
~ ~ 14
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in the ~irst stage is introduced into the s~cond stage via ~ludge
circuit 1. Organic hydrocarbon compounds are adsorbe~ extensively
on this bioma~s. The heterot.rophic bacteria composing the m~jor
part of this biomass use nitrate as the electron acceptor and
organic compounds as electron donors in the anoxic zones of the
second stage 12. As a result denitrification ~ecomes possible,
this being normally impossible in a conventional two-stage
activation plant because of the lack of carbon compounds and active
eterotrophic bacter~ia. ~
Sufficient aerobic 61udge age is a pr~-conditîon for certain
nitrification in;an active sludge system. The r~itrificat~on in the
second staqe 12 is;~barely~affected by organic compounds because as
a~ru;le;more than 75~o~the carbon compounds are already removed in
the~first stage ~ Poss~ible fluctuations in nitrogen content in
the~arrivi~ng sewage~are~compensated for in the first stage 11, with
app~oximately 50~ o~the~nitrogen being~removed as a rule in this
st~agé. For this~reason an aerobic sludge àge o~ 10 day~ and a
sewa~e temperature~of~10~C is `sufficient for the second ~tage 12 in
;order to keep the~;~ammonium nitrogen lower than 2 mg/l in the
dischar~e.
:: : ~ : : :
; Three basic conditions must be met for biological phosphor
elimination which~can be carried out according to Fig. 2 in a plant
211 ~ 3 ~i
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in combination with the process according to the invention:
Anaerobic and aerobic conditions in the sludge circuit, volatile
fatty acid and a suitable sludge age. In the first stage of this
process easily decomposed, volatile fatty acids are as a rule
present in sufficient quantity. In an additionally constituted
;anaerobic basin 7, the polyphosphate-accumulating bacteria are able
to~use the energy stored in the polyphosphate to maintain their
metabolism by decomposing the easily decomposed volatile fatty
~acids~and by releasing phosph~ate into the solution. In the aerobic
zone the bacteria again store decomposed polyphosphate in the cell,
with~;~phosphate being~ accepted in greater quantity from the
80~1ution.~ In~order to keep phosphate at less than 1 mg/l in the
discharge, a chemical precipitation can be used by adding a dosage
of thè~precipitant~ in the $irst stage and/or in the second stage.
To~inar~ease the overall capacity in nitrogen removal according to
the~iDventi~on, part of`the~;nitrate can be denitrified in the first
aativàti~on basin 3~ through the introduction of part of the
di~c~har~e~of the post clarification 4 back into the anoxic zone of
the~;first stage (see conduit in Figs. 1 and 2 indicated by broken
lines).
The process according to the invention may have the following
characteristics: ~
16
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93-556/7
Operation of a two-stage activation plant with transfer of part of
the sludge (excess sludge) constituted in the fir6t stage 11 into
the second stage 12 (sludge circuit 1,15) and transfer of part of
the sludge (excess sludge) constituted in the second stage 12 into
the first stage 11 (sludge circuit 2,23) so that mixed biocenoses
are formed in both stages and denitrifying biomass is on the one
hand transferred through sludge circuit 1 into the second stage and
:on the:other hand nitrifying biomass is transferred through the
sludge circuit 2 into the~ first stage whereby nitrogen compounds
can be removed through nitrification and denitrification in the
fist stage as well~ as in the second stage, in addition to the
decomposition of carbon~compounds.
The~desired sludge age~;in~the first ~tage 11 is two days at a
sewage~temperature~of~10C~and with a basin share of 50% and can be
var:ied:b:etween l:and::~5~days depending on:the aerobic contents of
the activation ba~in 3 and the temperature of the sewage.
The~ sludge~loads~ in~ the first stage 11 re~ulting from this are
typically~0.4 kg~BORJkg~dry substance and per day with a range of
fluctuation from 0.2~to~0~.8, 50 that a spacial load of 2 kg BOR~/m3
,; . ; . ~
and per day with a fluctuation range of 0.8 - 4 re~ults with a
typical sludge content o~f 5 g/l (range 2 - 10 g/l).
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93-556/7
The desired sludge age in the second stage 12 with a sewage
temperature of 10C and an aerobic basin share of 50~ is 10 days
and can also be varied between 3 and 15 days depending on the
temperature and the aerobic share of the activation basin 5.
the sludge loads resulting from this in the second stage 12 are
typically 0.08 kg BOR5/kg dry substance and per day with a
variation range of~ 0.~03-; - 0.15, so that with a typical sludge
content of S gJl ~range~2~- lO:~g/l) a spacial load of 0.4 kg BOR5/m3
and per day with a fluctuation range of 0.1 - 0.75 results.
The share~of non-aerated basin volume in the overall volume of the
activation basin 3,5;of~he~s:i~ngle s~ages 11 and 12 is typically 30
to~50%~,~ it~also being~possib~le to operate within a range of 10 to
70~:~to~optimize spe~c~ial operating condition (sewage concentration,
tempe~ature, etc.)~
The~ ~ludge circuit~ caA also be implemented in whole or in part
through the:suspended~particles which~are~pre~ent in the discharge
of~the~first stage with~suitably~heavy loading of the intermediate
clarification ba~in~4.
The~sludge circuit 2~ can also be completed in part through a back-
:feeding:of discharge:of discharge ~rom the post-clarification, with
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nitrate present in the discharge being denitrified in the first
stage.
The removal of phosphor compounds is not ~f~ected by the process
according to the invention and can be achieved either through
chemical precipitation, with - precipitation chemicals being
preferabIy added mostly in the first stage. However it is also
possible to carry out biological removal of pho~phor through the
process according to the invention by providing (see Fig. 2~ a non-
aera~ed anaerobic bas;n 7 before the activation basin 3 of step 11
into which the arriving and the fed back sludge of ~he first stage
,
,
: i8 introduced. In the anaerobic section phosphate is ~i~solved
back and in the subsequent aerobic section an increased acceptance
:and thereby biological removal of phosphate takes place.
When:increased phosphor removal is required (P less than 0.5 mg/l)
a~purification stage with flocculation filtration can be added.
~ , :
Excess ~sludge is~taken~ from the system preferably at the fist
~ stage, and if biological P-removal is used care must be taken that
: no P is fed back from the sludge treatment together with cloud~
water. If necessary, excess sludge can also be taken from the
second stage.
~' ~ 19
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If necessary the process according to the invention may also
contain a mechanical pre-clarification stage in addition to raking
and sand catching.
: :
When a quantity of mixed water occurs during rain, up to a multiple
of the quantity during dry weather can be processed according to
: : :
the required specifications.
The~invention is expla~ined through the plant shown schematically in
' ~ig. 1:
:: :
The ~process according~to the invention was tested in a two-stage
clarifi~cation plant. Th~ essential technical data of the plant are
: ~ ,
indicated below~
Activation basin~3~ 200 m'
Intermediate~basin 4~ 72 m' ~170 m~)
Activation basin~5 ~ 330 m'
Post-clarificatian~ basin 6 194 m' (650 m')
In addition, raking and sand catching i8 provided.
~: ~
~ During the test the~two~activation basins 3,S were operated 80 that
: ~ :
~ 50% was aerated (aerobic);and the remaining 50% non-aerated (anoxic
, ~
~ 20
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211~5
93-556J7
- oxygen supplied through nitrate or anaerobic). At the time of
the test the plant was loaded with approximately 6000
EGW (population equivalence). With an arrival of 290 mg/l BOR5 and
45 mg/1 TKN (= total Kjeldahl nitrogen) as well as with a 6upply
amount of 1250 m'/d representing a volume load of 1.8 kg BOR5/m'
and~per day and with a sludge content in the activation ba~in of
:: : ~
5.1 g/l representing a sludge load of 0.35 ~g BO~/kg dry substance
and per day the following discharge values were obtained in the
f~ist stage 11 at a temperature of 10C and with a filudge age of
approximately 2 days in the first stage 11:
,
BOR5~di~æolved) 25 mg/l
TKN ~ ~ 27;mg/1
In~the activation basi~n~5 of the second stage 12 the sludge content
amounted~to~5~.0 g/~ and;the volume load resulting from the arrîval
0~ 81udge ~from the;~ first stage and the discharge of the
intermediate clarif ication amounted to approximately 0.4 kg BOR5/m'
and~per day~; the ~appertaiAing sludge load was calculated to ~e
0.08~ kg BOR5/kg dry~ substance and per~day and the sludge age was
approximately 12 days.~ The results~obtained in the discharge 10
were:
BO~ 7~mg/1
TKN 2 mg/l
21
, :~
:
211~3~
. .
93-55~/7
NO3-N 6 mg/l
Based on this, the efficiency calculated for the entire plant i~
over 97% for the BORs and 8Z% for the nitrogen.
The sludge circuits for the obtention of mixed biocenoses in th~
two activation stages were adjusted in thi~ case so that
approximately 30~ of the excess sludge production of the first
stage was transferred into the second stage (approximately 150
kg/d) and the total excess sludge production of the second stage
(approximately 140 kg/d) was transferred back into the first st~ge.
In the case described here, the excess sludge of the entire plant
was taken only from the first stage 11. At higher sewage
temperatures an greater quan~ity of sludge can be tran~ferred from
the sludge circuit 1:into:the second stage without adverse efects
:
on ni;trification, ~o that nitrate value in the discharge can be
lowered to n~arly~0 while the efficiency of nitrogen removal can be
: increa~ed to 95~
: ~ ,
In practical application of the process according to the invention
it is pofisible to operate with a comparatively low ~pecific
: activation basin volume o~ approximately 90 l/EGW (liter/population
equivalence) for the purification of communal sewage. A typical
22
21 ~ 5 ~
- 93-556/7
value for single-stage activation plant~ functioning according to
; the state of the art and with comparable purification capacity
(total nitrogen removal over 80 %) is approximately 200 l/EGW
(liter/population equivalence). This considerable elimination of
~: ~ nitrogen is not achieved by the known two-stage processes.
,
Another advantage of the process according to the invention is the
fact that as existing ~plants for nitrogen removal are being
expanded, the exis~ti~ng basin6 can be incorporated extensively into
:the new process, and the existing activation stage can be used as
fi~rs~t or;as second stage, depending on basin volume and sewage
c~nditionfi. ; : ~ ,
~i: . : :
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