GB2026462A

GB2026462A - Method and apparatus for the biological removal of nitrogen from waste water

Info

Publication number: GB2026462A
Application number: GB7917831A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-05-31
Filing date: 1979-05-22
Publication date: 1980-02-06
Also published as: JPS551895A; FR2427304A1; DE2823763A1

Abstract

In a method for purifying and clarifying waste water, the waste water is passed through a conventional mechanical-biological clarification plant (1, 9, 12) and thereafter ammonia contained in the waste water is oxidised to nitrate in a nitrification stage (14) and thereafter the nitrate is reduced to gaseous nitrogen in a denitrification stage (15), the stages (14 and 15) being separated from one another, the waste water passing successively through such stages directly or indirectly, and the nitrification stage and the denitrification stage being operated in accordance with the immersion percolating filter body method, in which the growth surfaces of the filter bodies are at least half immersed in the waste water and rotate. <IMAGE>

Description

SPECIFICATION Method and apparatus for the removal of nitrogen from waste waters The present invention relates to a method of purifying and clarifying waste water, preferably by using rotary immersion percolating filters in a biological purification plant to remove organic carbon, and an apparatus for carrying out this method.

Known multi-stage biological purification plants are comprised of immersion percolating filters having settling tanks in front of and after the biological stage and serve for the biological elimination of organic carbon.

It is known that the phosphorus and nitrogen contained in the discharge of a mechanical-biological settling plant leads to eutrophy and oxygen consumption in the waste waters. Whilst phosphorus can be removed to a great extent without difficulty by means of a chemical purification stage, a chemical precipitation of nitrogen compounds is not possible, since all nitrogen salts are water soluble. Therefore, nitrogen has to be removed from waste water by biological purification and clarification operations.

Nitrogen is contained in the discharge of mechanical-biological settling plants in the form of ammonia and minor amounts of organic nitrogen, nitrate or nitrite.

It is possible for the biological elimination of nitrogen to be carried out in a biological purification stage, known as a nitrification stage. Herein the conversion of the nitrogen to nitrate occurs by micro-organisms using large quantities of oxygen. The ammonium ion (NH4+) is oxidised by aerobic autotrophy bacteria (Nitrosomonas, Nitrobacter) in two stages via nitrite (NO2) to nitrate (NO3). As electron acceptor there is used molecular oxygen (02) dissolved in water. Hence the following course of biochemical reaction in a nitrification stage results: Nitrosomonas NH4+ + 1.502 (bacteria) NO2- + H2O + 2 H+ + 55 kcal.

2 H + 2 HCO3 2 H20 + 2 CO2 Nitrobacter NO2 + 0.502 (bacteria) 3 + 18 kcal.

In a denitrification stage, the oxygen molecule of the nitrate may be split from the nitrogen molecule, so that nitrogen escapes from the waste water in gaseous form.

The nitrate (NO3) can be reduced under anaerobic conditions by various kinds of bacteria to molecular nitrogen (N2).

The electron donors necessary for this would have to be added in the form of organic substances. The reaction then occurs via the following intermediate products: NO3- , NO2-NOo N2O , N2 The biological-chemical summation formular reads: NO3- + 5 H# + 5e##1i2 N2 + 2 H2O + OH- - 80 kcal.

The organic materials required for denitrification in the form of electron donors are comprised of: a) raw or mechanically purified sewage or waste water, b) methanol ethanol, sugar, etc.

The following disadvantages have been, until now, inherent in denitrification: a) supply of the electron donor If the supply occurs in the form of untreated or raw waste water or mechanically purified waste water, then the residual contamination is increased again in the discharge in view of the biological oxygen requirement (BSB).

b) The supply of "artificial" electron donors such as methanol is costly. Donors such as sugar, organic acids and the like may lead to an increase in the residual contamination.

c) Due to the escape of gaseous nitrogen, the settling operations in the settling tank connected in series with the denitrification stage (for the removal of the bacteria contained in the waste water for the denitrification stage) are impossible. If these bacteria - which represent organic materials - are not separated from the waste water, then there also occurs an increase in the residual contamination.

It is an object of the present invention to at least minimize these difficulties and to solve the problems arising thereby in a practicable manner with simple means, which have been proved in a different manner in purifying processes and to make special arrangements therefor.

According to the present invention there is provided a method of purifying and clarifying waste water in which the waste water, in order to remove organic carbon therefrom, is passed through a mechanicalbiological clarification plant, the waste water thereafter, for oxidation of the ammonia contained therein to nitrate, being passed through a biological nitrification stage and then, for reduction of the nitrate to gaseous nitrogen, is passed through a biological denitrification stage, the nitrification and denitrification stages being separated from one another, the waste water passing successsively through such stages directly or indirectly, in which the nitrification stage and the denitrification stage operate in accordance with the immersion percolating filter body method, and in which the growth surfaces of said filter bodies are at least half immersed in the waste water and rotate.

Also according to the present invention there is provided an apparatus for carrying out the method of the invention comprising a mechanical-biological clarification plant having a preclarification tank with a series connected digestion tank, a gas storage tank and a reclarification tank, and utilising immersion percolating filters of known kind, to which is connected an immersion percolating filter body tank for the nitrification stage and, after a subsequent clarification filter, an immersion percolating filter body tank for the denitrification stage is arranged, behind which a reclarification filter is disposed.

In a conventional mechanical-biological settling plant, the accruing fresh sludge is digested in a digesting chamber. During the digestion process large quantities of sewer gas occur. Sewer gas is comprised of about 70% methane (CH4) and about 30% carbon dioxide (CO2).

The nitrification stage and the denitrification stage utilise immersion percolating filter bodies.

Immersion percolating filter bodies present every possible type of rotating growth surfaces for bacteria.

The nitrification stage is comprised of single or multi-stage immersion percolating filter bodies, which are arranged, e.g. according to the guidelines of the Land Baden-Wuerttemberg or Bayern for the complete nitrification of ammonia (about 6 - 8 g) BSBS/m2. d). Immersion percolating filter bodies are used since it is known that Festbeet-reactors are highly suitable for biological nitrification.

The very light biological surplus sludge is separated in a filter plant and partly returned again into the nitrification stage or removed from the waste water in the preliminary sedimentation and conveyed into the sewage chamber to increase the sewer gas quantity. If the immersion percolating body stage for the nitrification is connected in series to an existing settling plant, then about 5 - 10% of the purified waste water has to be supplied for the optimal function of the immersion percolating filter body nitrification stage.

In the immersion percolating filter body nitrification stage, the ammonia is converted into nitrate. The discharge of high nitrate content in the immersion percolating filter denitrification stage is conducted via an after-settling filter or an after-settling tank. the rotating growth surfaces of the immersion percolating filter denitrification stage are hermetically sealed. In this stage, sewer gas from the sewer chamber which acts as an electron donar is conducter to the mechanical-biological settling plant.

H+ + NO, + CH4 denitrification agents 1/2 N2 + CO2 + H2O The high oxygen requirement of the nitrification stage and the complete absence of oxygen during denitrification, make it compulsory for the two biological stages to be separated. A reclarification filter is used for this purpose.

Sewer gas is produced in sufficient quantity during sludge putrefaction in every mechanical-biological settling plant and therefore is available in sufficient quantity for the immersion percolating filter body denitrification stage. In the immersion percolating filter method, the addition of sewer gas is particularly simple. The space above the water surface has to be filled with sewer gas instead of the usual atmospheric air. For screening against atmospheric air only a minor excess pressure of max. 500 mm WS (water column) is required.

The immersion percolating filter denitrification device is built up in such a manner that the immersion percolating body rollers can be installed underground and the drive located outside the enclosure.

Moreover, the water level in the immersion percolating filter body troughs must be adjustable for optional heights up to complete immersion of the immersion percolating rollers in the waste water.

Sewer gas is supplied to the immersion percolating filter denitrification stage via a low-pressure gas storage tank (max. pressure = 500 mm WS) with a gravel filter and water separator connected at the front.

The sewer gas is supplied to the individual immersion percolating filter rollers via a central sewer gas pipe provided with tap pipes for the individual supply of the immersion percolator filter body denitrification rollers. The central sewer gas pipe is provided with cut-out and throttle valves, such valves are also incorporated in the tap pipes.

The CH4 concentration in the atmosphere of the immersion percolatorfilter-denitrification stage is measured over the individual immersion percolator body rollers. If the CH4 concentration drops below the value required for the denitrification process, then an impulse is generated to the individual metering valves to add a suitable quantity of CH4.

If the metering valves are opened, then the air discharge valve must also be opened.

The waste water is supplied to and discharged from the immersion percolator filter denitrification stage via a siphon, to permit a maximum excess pressure of 500 mm WS to be retained in the immersion percolator body stage.

The bacteria compositions (denitrificants) necessaryforthe reduction of nitrate to gaseous nitrogen grow on the growth surfaces of the immersion percolator filter body. These bacteria have to be separated from the waste water during resettling. A precipitation by sedimentation is not possible since the escaping gaseous nitrogen does not allow sedimentation. The sludge from the denitrification stage is therefore precipitated via a rotating filter. The rotating filter drum at the same time accelerates the nitrogen degasification of the waste water. The sludge precipitated on the filter blanket is partly recycled as excess sludge into the preliminary sedimentation tank and partly supplied again to the immersion percolating filter denitrification stage.

This recycling increases the concentration of the denitrificants in the immersion percolator filter denitrification stage. The higher the concentration the more nitrate can be reduced to nitrogen gas. The excess sludge may be precipitated from the denitrification stage in the preliminary sedimentation and thereby increases the quantity of fresh sludge. Increase of the fresh sludge quantity also increases the sewer gas quantity.

The carbon of the CH4 is converted in the immersion percolating filter denitrification stage into bacteria sludge and CO2. From this bacteria sludge CH4 is recovered again in the digestion tank. The recycling of the excess sludge from the denitrification stage (via the preliminary sedimentation tank) in the digestion tank enables a considerable proportion of the CH4 consumption in the denitrification stage to be compensated.

The addition of CH4 as electron donor does not impair the residual contamination of the waste water in any way. This impairment would occur if untreated waste water were added to the denitrification stage as electron donor.

The denitrification process proposed operates without any addition of foreign substances.

Recycling the sludge from the denitrification stage into the digestion tank substantially balances the CH4 consumption of the denitrification stage.

The present invention will be further illustrated, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a known mechanical-biological clarifying plant comprised of a preliminary clarifying tank with a digestion tank connected thereto, a biological clarifying tank provided with immersion percolating filters and a reclarifying tank to which reclarifying tank is connected, in accordance with the invention, a nitrification tank and a denitrification tank each provided with immersion percolating filters.

Figure 2 is a similar plant in accordance with the invention in which the nitrification tank is included in the biological clarifying tank of the mechanical-biological clarifying plant.

Figure 3 is a schematic view of the sewer gas supply from the sewer tank into the immersion percolating filter denitrification tank, together with associated parts.

In the purifying and clarifying plant shown in Figure 1, untreated waste water is supplied to a preliminary clarifying tank 1 through an inlet 2. In conventional manner, raw sludge is extracted from the preliminary clarifying tank 1 via a sludge pipe 3 and supplied to a digestion tank 4. Activated sludge is removed from this digestion tank 4 via a pipe 5. The sewer gas forming in this tank is supplied via a sewer gas pipe 6 to a gas storage tank 7. The mechanically preclarified waste water from the preliminary clarifying tank 1 is supplied through pipe 8 to a biological clarifying tank 9 provided with immersion percolating filter bodies. The usual reclarifying tank 12 adjoins the immersion percolator filter tank via a pipe 11.

This reclarifying tank 12 is now followed, in the actual inventive part of the plant, with an immersion percolating filter nitrification plant 14 and an immersion percolating filter denitrification plant 15 connected by a pipe 13, each of the tanks 14 and 15 being provided with immersion percolating filter bodies 16 and 17, respectively. When these two tanks, in accordance with the invention, viz. the nitrification tank 14 and the denitrification tank 15, are connected to a conventional mechanical-biological plant, it is necessary for the optimal utilisation of the plant for from 5 to 10% of the mechanically preclarified waste water from preliminary clarifying tank 1 to be supplied to the nitrification tank 14. This is effected by means of a by-pass pipe 20 which connects the outlet pipe 8 of the preliminary clarifying tank directly with the pipe 13.

Whilst the nitrification tank 14 operates with a considerable amount of oxygen, same is not to be allowed to have any access whatsoever to the denitrification tank 15. Accordingly, it is necessary that both tanks 14 and 15 are hermetically separated from each other. For such purpose, a reclarification filter 18 is used, which is incorporated in the connecting pipe 19 between the immersion percolating filter body nitrification tank 14 and the immersion percolating filter body denitrification tank 15.

A further reclarification filter 21 is provided in conjunction with the immersion percolation filter denitrification tank 15.

In order to keep the oxygen utilised in the nitrification tank 14 completely remote from the denitrification tank 15, as shown in Figure 3, a hermetically sealed housing 22 is provided to which the waste water is supplied via a siphon 23 and discharged therefrom via a siphon 23'. To prevent the supply of oxygen from the environmental air, it is sufficient to keep the excess pressure in the housing 22 at a maximum of 500 mm WS. This excess pressure is produced by the low pressure gas storage tank 7 from which a sewer gas main pipe 30 emanates. This main pipe 30 branches into several tap pipes 24 having metering valves 28. The number of tap pipes corresponds to the number of immersion percolating filters 17. Each immersion percolating filter 17 has a measuring sensor 25 associated therewith which serves for the measurement of the CH4 concentration. Measuring leads 26 conduct the sensor results to an evaluating device 27. If the CH4 concentration drops below a value necessary for the denitrification process, the evaluating device 27 emits an impulse which causes the metering valves 28 to permit a suitable quantity of CH4 to pass through into the associated immersion percolator body space.

Figure 3 shows that, depending upon requirement, the liquid level is adjustable to between a maximum and a minimum. Thus, both levels are dependent upon the mounting and size of the immersion percolating filters 17. With the lowermost liquid level, the immersion percolating filters 17 are only half immersed in the waste water and with the maximum level they are completely immersed in the waste water.

The light biological excess sludge produced in the nitrification tank 14 is precipitated in the reclarification filter 18 and partly returned, via a sludge return pipe 29, into the nitrification tank 14. A proportion of the sludge accruing in the reclarification filter 21 is supplied, via a sludge return pipe 31, to the associated biological tank, in this case the immersion percolating filter denitrification tank 15. The same applies also to the sludge from the reclarifying tank 12. From here also a part of the sludge is returned via a sludge return pipe 32 in front of the associated biological tank 9. The other parts of the excess sludge accruing in the tank 12, filter 18 and filter 21 are returned via an excess sludge return pipe 33 to the inlet 2 for the oncoming untreated waste water.

Figure 2 shows an embodiment in which a biological tank 9a both process steps, viz. the removal of organic carbon and the nitrification of the nitrogen, are combined. The remaining parts of the plant are the same as in the embodiment shown in Figure 3. They bear the same designations and have the same reference numerals and consequently have the same function.

In both embodiments, clarified clear water may be discharged from the reclarification filter 21 through an outlet pipe 34. Moreover, a mixture of nitrogen N2, carbon dioxide CO2 and methane CH4 issues from the waste gas pipe 35 when a valve 3 (Figure 3) in this pipe is opened simultaneously with the metering valves 28.

A gravel filter having a water precipitator 37 is arranged in the gas storage tank 7.

Claims

1. A method of purifying and clarifying waste water in which the waste water, in order to remove organic carbon therefrom, is passed through a mechanical-biological clarification plant, the waste water thereafter, for oxidation of the ammonia contained therein to nitrate, being passed through a biological nitrification stage and then, for reduction of the nitrate to gaseous nitrogen, is passed through a biological denitrification stage, the nitrification and denitrification stages being separated from one another, the waste water passing successively through such stages directly or indirectly, in which the nitrification stage and the denitrification stage operate in accordance with the immersion percolating filter body method, and in which the growth surfaces of said filter bodies are at least half immersed in the waste water and rotate.

2. A method as claimed in claim 1, in which the mechanical-biological clarification plant comprises a preliminary clarifying tank, a biological clarifying tank and a reclarifying tank.

3. A method as claimed in claim 1 or 2, in which the immersion percolating filter nitrification stage operates with atmospheric air and the immersion percolating filter bodies are in the form of rollers which rotate over about one half in the waste water and about one half in the air and operate at a load margin below 8 g BSB5lm2. d.

4. A method as claimed in claim 2 or 3, in which from 5 - 10% of the waste water is supplied from the preliminary clarifying tank via a by-pass pipe directly to the immersion percolating filter nitrification stage.

5. A method as claimed in any preceding claim, in which the immersion percolating filter denitrification stage operates with sewer gas, comprised of about 70% CH4 and 30% CO2, and the filter bodies are in the form of immersion percolating filter rollers which immerse from at least one half to completely in the waste water, and in which the immersion percolating filter rollers are partitioned off against the outer atmosphere.

6. A method as claimed in any preceding claim, having a reclarification filter which completely separates the nitrification and denitrification stages from each other and which prevents the aerobic sludge (Nitrosomonas and Nitrobacter) from reaching the denitrification stage from the nitrification stage.

7. A method as claimed in claim 2, in which the biological clarification and purification tank for removal of organic carbon accommodates the nitrification stage.

8. A method as claimed in any preceding claim, in which a reclarification filter having a rotatable sieve drum is provided for reclarification of the denitrification stage for separating and/or precipitating the sludge from the denitrification stage whereby the rotation of the sieve drum at the same time obtains an accelerated nitrogen degassing.

9. A method as claimed in any preceding claim, in which excess sludge from the reclarification filters of the nitrification andlor denitrification stages is returned via a pipe into a digestion tankforthe increased production of sewer gas and to compensate for the CH4 consumption of the denitrification stage.

10. A method as claimed in claims 5 and 8, in which a proportion of the sludge precipitated from the reclarifying filter of the denitrification stage is returned via a pipe into the denitrification stage to increase the sludge concentration thereof.

11. A method of purifying and clarifying waste water as claimed in any one of claims 1 to 10, substantially as hereinbefore described and illustrated.

12. Apparatus for carrying out the method according to any one of claims 1 to 11 comprising a mechanical-biological clarification plant having a preclarification tank with a series connected digested tank, a gas storage tank and a reclarification tank, and utilising immersion percolating filters of known kind to which is connected an immersion percolating filter body tank for the nitrification stage and after a subsequent clarification filter an immersion percolating filter body tank for the denitrification stage is arranged, behind which a reclarification filter is disposed.

13. Apparatus as claimed in claim 12, in which the gas storage tank which accommodates sewer gas from the digestion tank of the mechanical-biological clarification plant, is so formed that it is able to emit sewer gas with a pressure of a maximum 500 mm WS to the immersion percolating filter denitrification stage.

14. Apparatus as claimed in claim 2 or 3, in which tap pipes from a main supply pipe for sewer gas leading from the gas storage tank to the immersion percolating filter denitrification stage lead to the individual immersion percolating body filter denitrification rollers which lead to a position in front of the immersion percolating filter rollers.

15. Apparatus as claimed in claims 2, 13 or 14, in which measuring devices are provided above the individual immersion percolating filter denitrification rollers, measuring the CH4 concentration, which devices when the CH4 concentration drops, open metering valves and a discharge air valve.

16. Apparatus as claimed in any one of claim 12 to 15, in which siphons are provided in the supply and discharge pipes of immersion percolating filter denitrification stage to prevent the escape of sewer gas from the immersion percolating filter denitrification stage, which siphons, with an increase of gas pressure in the immersion percolating filter denitrification stage beyond 500 mm WS, cause degasification.

17. Apparatus for purifying and clarifying waste water, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

18. Waste water whenever treased by a method as claimed in any one of claim 1 to 11.