NL9401806A - Method for purifying carbon-, phosphorus- and nitrogen- containing wastewater - Google Patents

Abstract

A method is described for removing compounds of carbon, phosphorus and/or nitrogen from wastewater, wherein the wastewater is successively brought into contact, in an anaerobic zone, a first aerobic zone, an anoxic zone and a second aerobic zone, with activated sludge containing microorganisms of the genus Acinetobacter, after which the phosphate-enriched sludge is separated from the effluent and is subjected, at least partially, to dephosphatation, and part of the dephosphated sludge is recycled to the first aerobic zone and part of it is recycled to the anoxic zone.

Description

Method for purifying waste water containing carbon, phosphorus and nitrogen

The invention relates to a method for removing carbon, phosphorus and / or nitrogen compounds from waste water, wherein the waste water is successively brought into contact in an anaerobic zone, a first aerobic zone, an anoxic zone and a second aerobic zone. sludge containing micro-organisms, after which the phosphate-enriched sludge is separated from the effluent and at least partially dephosphated and the dephosphated sludge is returned at least partially to the first aerobic zone

Such a method is known from the publication of Rensink et al. In Wat. Sci. Techn. 23t 651-657 (1991) · Bacteria of the genus Acineto-baater play a role in the phosphate removal according to this method. The bacteria can store significant amounts of phosphate in the cell under aerobic conditions. After the treatment in the four mentioned zones of the bioreactor, the separated, phosphate-rich sludge is partially dephosphated, by adding lower fatty acids, for example acetate, under anaerobic conditions. According to the known method, the dephosphated sludge - apart from the excess amount withdrawn and removed from the process - is returned in its entirety to the first aerobic zone. The supernatant obtained during dephosphatation is treated with sand in a fluidized bed, whereby calcium phosphate is deposited on the sand, after which the residual liquid can be returned to the bioreactor (anaerobic zone). Part of it separated in the reactor after treatment. phosphate-rich sludge is returned directly to the anaerobic zone of the bioreactor without dephosphating.

In the above-mentioned known method - which is referred to as "Renpho" process - removal of nitrogen compounds is effected by means of the anoxic zone, in which nitrate is present in the waste water, as well as nitrate which in the previous aerobic zone from ammonium compounds present in the waste water. and organic nitrogen compounds are formed (nitrification), it is denitrified, mainly to molecular nitrogen. For an acceptable degree of denitrification, according to the known method it is necessary that at least 25% of the waste water is introduced directly into the anoxic zone.

While the known method permits a stable process operation with which an effective phosphate removal can be achieved, the result with regard to the removal of nitrogen compounds is in most cases insufficient.

It has now been found that improved results, especially in nitrogen compound removal, can be achieved by a change in the known method.

The invention accordingly relates to a method as described in the preamble, which is characterized in that part of the dephosphated sludge is returned to the anoxic zone.

More effective denitrification is achieved as a result of the direct introduction of dephosphated sludge, including dephosphated bacteria of the genus Acinetobacter, into the anoxic zone. Namely, it has been found that Acinetobacter has the ability to denitrify and simultaneously absorb phosphate.

The volume ratio between the dephosphated sludge supplied to the first aerobic zone and the dephosphated sludge fed to the anoxic zone is mainly determined by the nitrate content of the waste water: at a relatively high nitrate content of the waste water, a larger proportion is directed to the anoxic zone. Preferably, 20-80% by volume of the dephosphated sludge is returned to the anoxic zone, in particular from 0% to 60% by volume.

In the method according to the invention it is undesirable that part of the supplied waste water is led directly to the anoxic zone, i.e. by skipping the anaerobic zone and the first aerobic zone, as in the known Renpho process. Therefore, the waste water is preferably passed entirely into the anaerobic zone of the bio-reactor.

The method according to the invention can be carried out in an installation as schematically shown in the accompanying figure.

The four reaction zones anaerobic (1) - aerobic (2) - anoxic (3) -aerobic (4) can be accommodated in a bioreactor (5) which consists of several consecutive rooms, possibly more than one room per zone. Before the waste water, which is supplied via line (6), is fed into bioreactor (5), it can be passed through a pre-settler (7), whereby the majority of the solid matter from the supplied waste water (6) becomes as primary sludge. captured. A clarifier (8) is located behind the bioreactor (5). The treated wastewater is discharged via (9). A portion, for example 50-95¾, of the phosphate-rich sludge separated in settler (8) can be returned via line (10) directly to the beginning of the bioreactor. The remaining part of the separated sludge is led via line (11) to stripper (12), where acetate or another fatty acid is added. A thickener (13) is located after the stripper (12). where the dephosphated sludge is largely separated from the phosphate-rich stripping water. The sludge is then distributed via lines (14) and (15) to the first aerobic zone (2) and the anoxic zone (3) respectively. The excess sludge is discharged via (16). The phosphate-rich stripping water is dephosphated in reactor (17). This reactor can for instance be a fluidized bed reactor with sand grains. Lime is added if necessary. The dephosphated water is then returned to the bioreactor via line (18), and optionally via selector (19) in which flocculation can be promoted.

Example I

Domestic wastewater with an average composition as treated in a treatment plant according to the accompanying figure. The capacity of the bioreactor was 1000 l. The bioreactor was partitioned into 10 spaces of 100 l, namely two anaerobic, three aerobic, four anoxic and 1 aerobic, the connection between the spaces being formed by not in each other extended horizontal openings in the walls with a diameter of about 3 cm. The aerobic spaces were aerated, the others were stirred alone. The process conditions were as shown in Table A. The calcium consumption of the grain reactor was low. The purification results are summarized in Table B.

Compare example

The process of Example I was carried out in the same manner, except that 252 of the waste water was led directly to the anoxic zone, and the dephosphated sludge was passed in its entirety to the first aerobic zone, as in the known Renpho process. The results are shown in Table B for comparison in the column "Renpho".

Claims (5)

A method for removing carbon, phosphorus and / or nitrogen compounds from waste water, in which the waste water is successively brought into contact with sludge-containing sludge in an anaerobic zone, a first aerobic zone, an anoxic zone and a second aerobic zone, after which the phosphate-enriched sludge is separated from the effluent and at least partially dephosphated and the dephosphated sludge is returned at least partially to the first aerobic zone, characterized in that part of the dephosphated sludge is returned to the anoxic zone. The method of claim 1, wherein the microorganisms contain bacteria of the genus Acinetóbacter. A method according to claim 1 or 2, wherein the waste water is brought in its entirety into the anaerobic zone. The method of any one of claims 1 to 3, wherein 20-80 vol. # Of the dephosphated sludge is returned to the first aerobic zone and 80-20 vol. # Of the dephosphated sludge is returned to the anoxic zone. A method according to any one of claims 1-4, wherein part of the separated phosphate-enriched sludge is returned to the anaerobic zone.