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%.
