GB2119357A

GB2119357A - A process and apparatus for removing nitrogen from sewage

Info

Publication number: GB2119357A
Application number: GB08305896A
Authority: GB
Inventors: Volker Stengelin
Original assignee: Stengelin & Co KG GmbH
Current assignee: Stengelin & Co KG GmbH
Priority date: 1982-03-06
Filing date: 1983-03-03
Publication date: 1983-11-16
Also published as: DE3208173A1; ES520328A0; ES8400996A1; NL8204658A; IT8347863A0; FR2530610A1; GB8305896D0

Abstract

Apparatus for removing nitrogen from sewage characterised in that a two-stage immersion percolating filter plant (1) comprises a first stage (2) for carbon degradation, followed by an intermediate clarification tank (9) and a second stage (3) for the nitrification stage, which has an additional clarification tank (13), from which clarified sludge may be returned to the first stage (2) where denitrification occurs, the nitrates formed in the second stage now providing the necessary oxygen for carbon degradation. <IMAGE>

Description

SPECIFICATION A process and apparatus for removing nitrogen from sewage The present invention relates to a process and apparatus for removing nitrogen from sewage by means of so-called immersion percolating filter members or immersion trickling filters. In such a case, immersion percolating filter members which are preferably in the form of rotationally symmetrical, rotatable growth surfaces, and which are used for biological purification purposes are adapted, in such a manner, that they operate in the field of nitrification.

An object of the present invention is to construct and arrange such immersion percolating filter members in such a manner that denitrification (removal of nitrogen) can be achieved without enlarging the immersion percolating filter members used hitherto and without incurring additional expense, as well as enabling the degree of denitrification to be adjusted.

In order to achieve nitrification with immersion percolating filter members, the specific surface loading per unit surface, relative to the biological oxygen demand over 5 days (BOD5), needs to be less than8gO2/m2.d.

The construction of such immersion percolating filter members, operating within the nitrification range is known. A plurality of radially or axially traversed immersion percolating filter rollers may be disposed one behind the other and be designed for a mean surface loading (relative to the BODs) of less than 8 g O2/m2.d. To achieve positive nitrification, the immersion percolating filter plant may also be divided into two stages with an intermediate clarification step therebetween.

The first stage is designed for the degradation of organic carbon which operates within the range of approximately 18 to 30 g O2/m2. d - relative to the BOD5. The excess sludge produced in the first stage is removed in the intermediate clarification step. By utilising an intermediate clarification step, a biological green growth, which is optimum for nitrification, can be produced on the growth surfaces of the immersion percolating filter members in the second stage of the plant. The second stage of the immersion percolating filter plant (nitrification stage) operates within the range - relative to the BOD5 - of less than8gO2/m2.d.

An additional clarification means is connected downstream of the plant.

The present invention now seeks to remove the nitrate, which was produced in the nitrification stage, from the sewage without enlarging the im mersion percolating filter members, by means of controlled denitrification. This object is to be achieved by the process according to the invention, in that 2 can additionally be removed in the immersion percolating filter stage, which seeks to degrade carbon, from the air, on the one hand, and from the nitrate, on the other hand, after a transfer step which may be adjustable.

The biologically purified effluent from the additional clarification step contains the nitrogen which has been oxidised to form nitrate. According to the desired degree of denitrification, the effluent is returned to the first stage which now operates as a denitrification stage. In this denitrification stage, the oxygen of the nitrate is separated from the nitrogen.

This separation can only occur when there is no free, dissolved oxygen in the denitrification stage and the bacteria are compelled to use the oxygen of the nitrate for the 2 demand.

The immersion percolating filter stage, which had initially been useful for the degradation of organic carbon, is used as the denitrification stage. For the degradation of organic carbon, the mirco-organisms require the oxygen from the air, such oxygen being supplied, in the case of the immersion percolating filter member, because substantially half the growth surfaces extend into sewage and the other half of the growth surfaces extend into the air. As the immersion percolating filter rollers rotate, the microorganisms absorb oxygen when they rise into the air and absorb dirt substances when they are immersed in the sewage.

The micro-organisms of this immersion percolating filter stage for the degradation of organic carbon collect the oxygen from the air, therefore, since the rollers are only substantially half immersed in the sewage. By measuring the 2 content in the denitrification stage, the apparatus is controlled in such a manner that, once an 2 content of 0.2 mg/l is exceeded, the sewage is retained in this stage. By retaining the sewage until the immersion percolating filter rollers are totally immersed, the microorganisms on the immersion percolating filter rollers are compelled to collect the oxygen which is contained in the nitrate of the biologically purified sewage emerging from the additional clarification step.The guided circulation (feedback from the additional clarification step) has to be controlled so that the oxygen contained in the nitrate is sufficient to meet the oxygen demand of the micro-organisms which seeks to degrade organic carbon. Thus, the feedback of sewage may be regulated between an amount which is less than the supplied amount or which corresponds to the supplied amount, and an amount which is three times greater than the supplied amount.

If the nitrate oxygen of the feedback is inadequate, the excess immersion of the immersion percolating filter roller has to be reduced accordingly. A reduced immersion should eliminate any anaerobic conditions in the denitrification stage.

The present invention will be further illustrated, by way of example, with reference to the accompanying drawing, in which the drawing schematically illustrates a sewage purifying plant 1 having two biological stages 2 and 3 which are respectively provided with immersion percolating filter rollers 4 and 5 and 6,7and8.

An intermediate clarification tank 9 is disposed between the first and second immersion percolating filter stages 2 and 3 and excess sludge can be removed from the intermediate clarification tank 9 via the conduits 11 and 12. An additional clarification tank 13 is disposed downstream of the second immersion percolating filter stage 3, and, on the one hand, clarified sludge is removed from, the additional clarification tank 13 via the conduit section 14; whilst on the other hand, biologically clarified sewage is returned to the inlet 16 from the additional clarification tank 13 via the return conduit 15 and is therefore supplied to the first immersion percolating filter stage 2.

However, the immersion percolating filter trough 17 of the first immersion percolating filter stage 2 has to be constructed so that its upper edge 18 always, in each case, protrudes beyond the culmination point of the immersion percolating filter mem bers in the trough 17.

The outlet 19 of the immersion percolating filter trough 17 is vertically adjustable, so that each level of the water surfaces can be set between the lowermost position 20 and the highest position 21.

An oxygen measuring device 22 is installed in the immersion percolating filter trough 17 to measure the oxygen content therein, and the device 22 seeks to set the 02 content in this stage to a range of from 0.1 to 0.2 mg/l . d.

The measuring device 22 causes the outlet 19 to be raised when the limit of 0.2 mg/l 02 has been exceeded in the immersion percolating filter trough 17. However, the feedback of sewage in the conduit 15 is discontinued if, despite the maximum position 21 being reached, the 02 content still does not drop in this trough 17.

So that the plant 1 according to the invention can operate in the described manner, the pre-requisite is for the plant to be operating in the second biological nitrification stage 3 within the nitrification range, while carbon has previously been degraded in the first stage 2. Thus, the plant has first to be used before dentrification can occur in the first stage 2, which then becomes the denitrification stage.

This occurs in the following manner: To remove the nitrogen, the immersion percolating filter plant 1 has to operate within the nitrification range, i.e. the BOD5 surface loading has to be less than 8 g O2/m2. d. After use of the plant and after nitrogen oxidization has produced nitrate, the feedback from the additional clarification step (guided circulation) is intensified in accordance with the nitrate content and the BODs loading, whilst the water level is simultaneously raised in the first stage 2 which is itself designed for the degradation of organic carbon. The accumulation level is also dependent on the 02 demand of the microorganisms for the degradation of organic carbon and on the 02 demand of the nitrate from the oxygen which is available from the feedback of the additional clarification step.It is therefore necessary for 1) thefeedbackto be capable of being regulated within the range of between 100 % and 300 % of the in-fiowing sewage; and for 2) the water level in the immersion percolating filter rollers 4 and 5, which have to effect denitrification, to be capable of being regulated likewise steplessly until the rollers are fully immersed.

As a result of these considerations, the immersion percolating filter plant 1 itself does not need to be enlarged to achieve denitrification. The oxygen of the nitrate is utilised to meet the oxygen demand of the micro-organisms during degradation of the organic carbon in the sewage. It is thereby simultaneously possible to make substantial savings in the energy requirements of the immersion percolating filter rollers. By utilising the nitrate oxygen, there is no need for the micro-organisms to obtain oxygen from the air in an energy-consuming manner.

If the nitrate oxygen is utilised for the 02 supply of the micro-organisms for the degradation of organic carbon, the speed of the immersion percolating filter rollers can be reduced to 0 revs/min. This enables an energy saving of approximately 50% to be achieved with the same purifying output as with known plants.

Finally, the control means also has to be provided so that the accumulation around the immersion percolating filter rollers 4 and 5 is reduced if there is an inadequate amount of nitrate oxygen in the return conduit 15.

Claims

1. A process for removing nitrogen from sewage by means of biologically operating immersion percolating filter members which, in addition to the carbon degradation stage, have a nitrification stage provided with an additional clarification tank, excess sludge being returnable from the additional clarification tank, characterised in that 02 can also be removed in the first immersion percolating filter stage, which seeks to degrade carbon, from the air, on the one hand, and from the nitrate, on the other hand, after a transfer step which may be adjustable.

2. An apparatus for carrying out the process as claimed in claim 1, characterised in that a two-stage immersion percolating filter plant comprises a first stage for carbon degradation, followed by an intermediate clarification tank having sludge feedback, and a second stage for the nitrification stage, which has an additional clarification tank, from which clarified sludge is removable via a conduit section and sewage is returnable via a return conduit and the two-stage immersion percolating filter plant has a vertically adjustable outlet in the first stage, the sewage level in this stage being continuously variable by means of an outlet, from the centres of the immersion percolating filter rollers to the upper edges thereof.

3. An apparatus as claimed in claim 2, in which the sewage feedback in the return conduit is capable of being regulated between an amountwhich is less than the supplied amount or corresponds to the supplied amount, and three times the supplied amount.

4. A means for controlling the apparatus as claimed in claims 2 and 3, in which 02 measuring devices are arranged in the denitrification stage and set the dissolved 02 content in this stage to a range of from 0.1 to 0.2 mg/l, in which the 02 measuring devices cause the outlet of this denitrification stage to be raised when this limit is exceeded, the feedback of sewage in the return conduit being discontinued thereafter if the maximum setting is reached and the 02 content does not drop.

5. A means as claimed in claim 4, in which the rotational speed of the immersion percolating filter members in the denitrification stage is capable of dropping to 0 when the predetermined denitrification value is reached.

6. A means as claimed in claim 4 or 5, which is so constructed that the accumulation around the immersion percolating filter rollers is reduced if there is an inadequate amount of nitrate oxygen in the return conduit.

7. Apparatus for removing nitrogen from sewage substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.