Biological treatment of wastewater

Details Biological treatment of wastewater Biological treatment of wastewater

1999-01-25

2001-02-06

Upton, Christopher (Department: 1724)

Liquid purification or separation

Processes

Treatment by living organism

C210S610000, C210S614000, C210S630000, C210S903000

Reexamination Certificate

active

06183642

ABSTRACT:

FIELD OF THE INVENTION
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° 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, in a second stage without the infeed of oxygen, to denitrification in the presence of an electron donor, 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 6.0 and 8.5 and the excess, formed by growth, of nitrifying and denitrifying bacteria and the effluent formed by the denitrification are extracted.
DESCRIPTION OF RELATED ART
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.
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
4
—N per liter) but also a high temperature (usually about 30° 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, Jun. 23-28 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.
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 using electron donors of inorganic nature. Preferably said electron donor of inorganic nature is selected from the group consisting of hydrogen gas, sulfide, sulfite and iron (III) ions, while said electron donor of organic nature is selected from the group consisting of glucose and organic acids, aldehydes and alcohols, having 1-18 carbon atoms.
More in particular we have found that the electron donor 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.
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. 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. Preferably, glucose or hydrogen gas are added 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 electron donor as explained above. The conversion of nitrite into molecular nitrogen requires on its own, however, about 40% less electron donor 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° C. The biological conversion such as the nitrification will cause the temperature to rise by about 15° C. per gram of nitrogen per liter removed. Increasing the process control temperature beyond 40° 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. Monitoring of the pH is obviously possible by the pH of the medium being measured directly. 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
NH
4
+
+1.5O
2
→NO
2
−
+H
2
O+2H
+
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 du

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