1999-01-26
HEIJNEN JOSEPH JOHANNES (NL); VAN LOOSDRECHT MARINUS CORNELI (NL)
Applicant: GRONTMIJ ADVIES & TECHNIEK
BV (NL)
Also published as: EP0826639 / EP0826639 / NL1003866C (C2)
Abstract: A method is described for the biological treatment
of ammonium-rich wastewater in at least one reactor which has a
temperature
of at least 25 DEG C., which involves the wastewater being passed
through
the said reactor(s) with a population, obtained by natural selection in
the absence of sludge retention, in the suspended state of nitrifying
and
denitrifying bacteria to form, in a first stage with the infeed of
oxygen,
a nitrite-rich wastewater and by the nitrite-rich wastewater thus
obtained
being subjected, if required, in a second stage without the infeed of
oxygen,
to denitrification in the presence of a carbon source such as methanol,
in such a way that the contact time between the ammonium-rich
wastewater
and the nitrifying bacteria is at most about two days, and the pH of
the
medium is controlled between 6.5 and 8.5 by the infeed of the said
carbon
source, and the excess, formed by growth, of nitrifying and
denitrifying
bacteria and the effluent formed by the denitrification are extracted,
the demand for the said carbon source during the treatment being
controlled
as a function of the amount of heat produced in the reactor. In
addition
the growth rate of the nitrifying and denitrifying bacteria is
expediently
controlled by means of the retention time, in the reactor, of the
wastewater
to be treated which is fed in.
The invention relates to a method for the biological treatment of
ammonium-rich wastewater in at least one reactor which has a
temperature of at least 25.degree. C., by the wastewater being passed
through the said reactor(s) with a population, obtained by natural
selection in the absence of sludge retention, in the suspended state of
nitrifying and denitrifying bacteria to form, in a first stage with the
infeed of oxygen, a nitrite-rich wastewater and by the nitrite-rich
wastewater thus obtained being subjected, if required, in a second
stage without the infeed of oxygen, to denitrification in the presence
of a carbon source such as methanol, in such a way that the retention
time of the ammonium-rich wastewater is at most about three days, and
the pH of the medium is controlled between 6.5 and 8.5 by the infeed of
the said carbon source, and the excess, formed by growth, of nitrifying
and denitrifying bacteria and the effluent formed by the
denitrification are extracted.
Such a method is known from a publication in Delft Outlook, 95.2, pp.
14-17. However, the research reported in this publication was carried
out on a laboratory scale and does not provide any suggestion
whatsoever on the measures required for using such a process in
practice to achieve adequate cleaning of the wastewater in question.
As a result of discharge standards having become more stringent, in
particular for nitrogen, there is a need for efficient, cost-effective
purification systems for the treatment of wastewater. Examples of these
concentrated industrial wastewater streams are, wastewater streams like
those released with off-gas treatment etc. Another example of the
concentrated nitrogen-rich wastewater stream is the so-called rejection
water. This rejection water stream is formed after dewatering of fully
digested sewage sludge and has not only a high ammonium concentration
(about 1000 mg of NH.sub.4 -N per litre) but also a high temperature
(usually about 30.degree. C.). The ammonium in the rejection water may
account for as much as 15% of the total nitrogen loading of a
wastewater treatment installation, while the volume flow of the
rejection water is only less than 1% of the wastewater volume flow to
be processed. This rejection water therefore makes a considerable
contribution to the nitrogen loading of the treatment installation.
The biological treatment of such wastewater streams normally makes use
of treatment processes in which the high sludge concentrations required
are obtained by employing a form of sludge retention such as settling,
membrane filtration, attachment to filter media, etc. In that context
it is worth drawing attention to the STOWA report 95-08, which relates
to the treatment of nitrogen-rich return streams in sewage plants, and
to the Proc. 18th IAWQ Biennial, Water Quality International '96, 23-28
June 1996, Singapore, pp. 321-328.
An, as it happens, frequently used treatment process is known as the
activated-sludge system. Such a system is characterized on the one hand
by employing sludge retention by sludge settling and, on the other
hand, by the bacteria mainly being present in so-called
activated-sludge flocculae. Such flocculae usually have a size of 0.1-2
mm.
SUMMARY OF THE INVENTION
It should be noted that the present process of biological nitrogen
removal preferably proceeds in two successive stages, an aerobic and an
anoxic stage. Both stages can, in the present invention, take place in
one reactor, separated in time, or in separate reactors which may or
may not involve a return stream to the first stage. In the first stage
the nitrogen present as ammonium is largely converted into nitrite,
with the aid of oxygen and nitrifying bacteria. The second stage
comprises the conversion of nitrite into molecular nitrogen, said
conversion being anoxic and taking place with the aid of denitrifying
bacteria. These denitrifying bacteria require a carbon source such as
methanol, to carry out the said conversion.
We have now found, surprisingly, that the method as set forth in the
preamble can be carried out on an industrial scale, with an ammonium
removal efficiency of more than 90% being achieved, by controlling the
demand of the denitrifying bacteria for a carbon source, in this case
methanol.
More in particular we have found that the methanol demand during the
treatment can be controlled as a function of the amount of heat
produced in the reactor. These parameters proved to be directly
proportional to one another. As will be explained hereinafter, the pH
of the medium is controlled at the same time by means of the methanol
being metered.
It should be noted that during the nitrification two moles of protons
are produced per oxidized mole of ammonium. The pH drops as a result.
The pH is usually controlled by feeding alkali and/or acid into the
reactor. During denitrification, on the other hand, protons are
consumed. Denitrification furthermore takes place under anoxic
conditions, nitrite being used as an electron acceptor. For
denitrification to be possible, the presence of not only an electron
acceptor, but also of an electron donor is required. Methanol, for
example, is indeed added at the same time in the present process as an
electron donor.
In addition, the following may be noted with respect to the present
process. For the purpose of nitrogen removal, the ammonium present in
the wastewater is not nitrified to nitrate but only to nitrite. Indeed,
the term of nitritifying bacteria is sometimes used, to indicate more
clearly that what takes place predominantly is the formation of
nitrite. The denitrifying bacteria which are capable of anoxic
conversion of both the nitrate and the nitrite into molecular nitrogen,
consume a carbon source such as methanol, as explained above. The
conversion of nitrite into molecular nitrogen requires on its own,
however, about 40% less methanol than the conversion of nitrate into
nitrogen. Moreover, the oxidation of nitrite to nitrate costs oxygen.
Indeed, direct conversion of nitrite into nitrogen provides another
(approximately) 25% savings on the oxygen account. The conversion via
nitrite instead of nitrate is therefore very advantageous in economic
terms.
If, under certain circumstances, the conversion via nitrate is more
attractive, however, than the conversion via nitrite, this can
obviously be achieved by extending the retention time, of the
wastewater to be treated, in the present process.
In an expedient variation of the present process in addition the growth
rate of the nitrifying and denitrifying bacteria is controlled by means
of the retention time, in the reactor, of the wastewater to be treated
which is fed in. This retention time is an important parameter, since
the stability of the nitrifying process may be put at risk as a result
of the maximum growth rate of the biomass decreasing as the temperature
decreases. This therefore requires a higher temperature than with
known, more conventional processes. In practical trials the influent of
the reactor was found to have a temperature of 30.degree. C. The
biological conversion such as the nitrification will cause the
temperature to rise by about 15.degree. C. per gram of nitrogen per
litre removed. Increasing the process control temperature beyond
40.degree. C., however, is not advantageous to the stability of the
present process. By controlling the amount to be fed in of wastewater
to be treated it is therefore possible to control the growth rate of
the biomass; the temperature in the system and consequently the heat
production therein then reflects the conversion in the system.
It was found that a retention time of the amount of wastewater to be
fed in of 0.5-2.5 days, preferably of 1.3-2.0 days, affords optimum
results, i.e. an overall removal efficiency of more than 90%.
Expediently, the retention time in the aerobic phase is from 0.5 to 2
days and in the anoxic phase from 0.4 to 1 day. A reduction in the
retention time in the aerobic phase may lead to an improvement in the
ammonium conversion ratio. This is caused by a longer retention time
for the denitrifying bacteria then being achieved with an identical
cycle time of the aerobic and anoxic period. This produces a higher
average pH, as a result of which the ammonium conversion rate is
increased. If the retention time in the aerobic phase is extended at
the expense of the retention time in the anoxic phase, the pH is not
sufficiently stabilized by the denitrifying bacteria and the conversion
ratio drops again. However, if the retention time in the aerobic phase
is reduced too far, the nitrifying bacteria will be flushed out and as
a result the conversion ratio again drops.
Although control of the pH of the process according to the invention is
effected by methanol being metered as a function of the amount of heat
produced by the biological treatment, monitoring of the pH is obviously
possible by the pH of the medium being measured directly. As explained
above, protons (or acid ions) are produced during the nitrification
process, as a result of which the pH of the medium drops in accordance
with the equation
The nitrification rate is therefore pH-dependent, so that conversely
the pH can be regarded as a relevant process parameter. It was found,
incidentally, that during the nitrification buffering may take place by
bicarbonate (HCO.sub.3.sup.-) which is present in the rejection water
fed into the reactor or is added, in accordance with the equation
For an optimum effect it is important, in this context, that the carbon
dioxide is transported (stripped) from the liquid phase to the gas
phase. With respect to dimensioning the reactor to be used in the
method according to the invention it was indeed found, in this context,
that in the case of a ratio of volume to bottom area of the reactor in
the range of 2-10 very beneficial results are achieved in terms of the
nitrification-denitrification process according to the invention.
The characteristic feature of the invention is that the process takes
place without sludge retention being employed, i.e. the sludge
retention time is equal to the liquid retention time. To achieve this,
both the mixing and the discharge of the treated water need to be
effective. Good mixing can be obtained by employing, for example,
aeration in the aerobic phase, and in the anoxic phase, for example, by
employing mechanical agitators, liquid injection, introduction of
low-oxygen or oxygen-free gases etc. As a result of these measures a
very active bacterial population is obtained, which is mainly present
in the liquid phase as free cells and/or very small clusters of a
limited number of cells, rather than activated-sludge flocculae.
It should further be noted that the denitrification in the reactor is
carried out under essentially oxygen-free conditions. Such conditions
can be formed spontaneously as a result of denitrifying bacteria
consuming the oxygen present, the environment consequently
automatically becoming anoxic. Expediently, and to accelerate the
process if required, the denitrification is carried out, however, with
recycling of the nitrogen already formed previously by denitrification.
An additional advantage of this is that the nitrogen stream through the
reactor at the same time strips the carbon dioxide from the reactor.
As indicated above, the excess, formed by growth, of nitrifying and
denitrifying bacteria is extracted. In practice this involves these
bacteria being entrained by the effluent from the reactor and being
added to the main stream of the wastewater treatment process, after
which the further removal of residual ammonium is carried out.
It should be noted that the effluent from the reactor is preferably
withdrawn therefrom at a point below the liquid level prevailing in the
reactor, expediently with local intensive mixing. While at the moment
this cannot be stated with certainty, this measure may be essential for
a process without sludge retention.
According to an attractive variation of the method according to the
invention, the nitrite-rich, acidic effluent formed by nitrification is
used, at least in part, for the neutralization of ammonia. This ammonia
can be present both in the rejection water to be treated and,
alternatively, in a process stream of whatever origin. The treatment
can be carried out, for example, in a gas scrubbing installation known
per se, whereas the effluent obtained after treatment can be recycled,
for further treatment, to the nitrification reactor according to the
invention.
SURVEY OF THE DRAWINGS
The accompanying
FIGS. 1 and 2
schematically show the progress of the nitrification/denitrification
process according to the invention. More in particular, FIG. 1 provides
a sketch of the pH profile in the reactor, the pH being controlled
between 8 and 7 with the addition of methanol.
FIG. 2 also represents the change in time, produced by methanol being
fed in, of the nitrite concentration and ammonium concentration for one
cycle.
It should be noted that if it is undesirable or less desirable for the
bacteria present in the effluent to reach the main stream of the
wastewater treatment process when the effluent is recycled to said main
stream, said effluent, according to a very expedient embodiment of the
method according to the invention, is first subjected to a treatment
with protozoa. Such a variation is of interest, in particular, if the
influent of the reactor is a COD-containing wastewater. The term COD
refers, as usual, to chemical oxygen demand; the component relevant
thereto in solution is primarily formed by carbon bound in organic
compounds. This material acts as a nutrient for the bacteria present in
the reactor. By subjecting the effluent of the biological treatment to
a treatment with protozoa it proved possible to largely remove the
bacteria suspended in the effluent and entrained from the reactor.
It should also be noted that the principle, employed in the method
according to the invention, of the absence of sludge retention can also
expediently be employed in the treatment of COD-containing wastewater.
More in particular this then means replacing the present nitrite route
by the COD route, in which case an overall removal efficiency of more
than 50% was obtained.
The invention is explained in more detail with reference to an
exemplary embodiment.
EXAMPLE
In this example a continuous flow reactor without sludge retention was
employed. Such a reactor makes it possible for the bacterial population
having the lowest maximum growth rate to be flushed from the system
selectively.
The reason for this is that a retention time can be used which is lower
than the maximum reciprocal growth rate of the one bacterial population
(in this case the nitrite oxidizers which oxidize the nitrite present
to nitrate), but is higher than the maximum reciprocal growth rate of
the other bacterial population (in this case the ammonium oxidizers).
Flushing out the nitrite oxidizers therefore leads to a build-up of
nitrite in the reactor.
The reactor used had a diameter of about 20 m and a height of 6 m and
therefore an effective volume of about 1150 m.sup.3.
The influent for the reactor, the so-called rejection water, had a
temperature of about 30.degree. C. and an ammonia concentration of
about 1000 mg of N/l, while the total amount of rejection water fed in
was about 760 m.sup.3 per day.
For the purpose of converting ammonia into nitrite, followed by the
conversion into nitrogen, the reactor contained about 120 kg of biomass.
The treatment of the rejection water took place in the reactor with a
cycle configuration as shown in FIG. 2, i.e. a cycle time of about 2
hours consisting of an aeration period of .+-.80 min, followed by a
period involving recycling of the nitrogen gas formed in a first period
of bout 40 min.
In the steady state of the process the amount of rejection water fed to
the reactor was such that the retention time was about 1.5 days. The
infeed of methanol was about 1 kg per kg of nitrogen bound as ammonium
and was effected in such a way, while the temperature difference
between the input and the output of the reactor was being measured,
that the pH of the medium could be kept between about 7.2 and 7.7.
The rejection water thus treated had a nitrogen concentration of as
little as about 80 mg of total N.multidot.l.sup.-1, which could be
recycled for treatment to the main stream of the treatment
installation. The result of the treatment of this rejection water was
therefore a purification efficiency of about 90%.
rexresearch.com
