GB2256154A

GB2256154A - Method and apparatus for the removal of ammonium compounds from waste waters

Info

Publication number: GB2256154A
Application number: GB9210956A
Authority: GB
Inventors: Jerzy Mackowiak; Stanislaw Filip; Antony Kosiol
Original assignee: ENVICON ENG GmbH
Current assignee: ENVICON ENG GmbH
Priority date: 1991-05-25
Filing date: 1992-05-22
Publication date: 1992-12-02
Anticipated expiration: 2012-05-22
Also published as: ATA91892A; NL9200888A; GB2256154B; CH684090A5; BE1006843A3; GB9210956D0; FR2676728A1; AT401048B; FR2676728B1; DE4117171C1

Description

22-)6154 Method andApparatus for the Removal of Ammonium Compounds from

Waste Waters The invention relates to a method andapparatus for removal of ammonium compounds from waste waters, in particular filtrate waste waters from biological slurries.

A method is known of subsequently treating the sedimentation slurry from rotting processes of clarification plants in filter presses or centrifuges. While the solid matter obtained thereby is subsequently burned or deposited, the disposal of the ammoniumcontaining filtrate presents considerable difficulties.

The concentration of ammonium compounds in filtrates of this kind usually lies between 0.7 and 1.6 g NH4-NI1.

From coal carbonizing practice a method is known for the removal of ammonium compounds from ammonium- containing waste waters with a concentration of about 8 - 20 g/1 by means of socalled stripping by means of steam. In addition to ammonia, the coke- oven plant effluent also contains C02, H2S and small quantities of HCN.

This method cannot be used for the treatment of the named filtrate waters, since these contain quantities of carbon dioxide which are leaner than stoichiometric, binding the entire ammonium in the form of volatile ammonium hydrogen carbonate (NH4HCO3)' An introduction of the waste waters into the clarification plant is out of the question because this would lead to a circulation of ammonium.

On the other hand, the law demands a restriction of the ammonium concentrations to values smaller than mg NH4-N/1.

In order to fulfil these legal requirements a method is known of adding to the waste water a quantity of sodium hydroxide solution which is leaner than stoichiometric and of subsequently subjecting the waste water to an air or steam stripping. Through the addition of sodium hydroxide solution the pH value of the solution is raised to values around 10.5 - 11. In the process, the balance of the reaction is displaced in the direction of the free ammonia due to the displacement of the pH value. Through the addition of sodium hydroxide solution C02 is bound.

To attain the appropriately high pH value considerable quantities of sodium hydroxide solution are required. For example, with the use of 30% sodium hydroxide solution 20 litres of NaOH per m3 waste water are required. The consequence is a great salting of the waste water. Through the considerable quantities of chemicals, moreover, the operational costs are undesirably high.

Other chemical processes, like the so-called MAP process (magnesiumammonium-phosphate process), are also distinguished by a very high consumption of chemicals.

In this respect, the object of the invention is to offer a possibility of the preparation of ammonium- containing waste waters, with which a removal of the ammonium compounds is also made possible without greater additions of chemicals.

Accordingly, in one aspect the present invention provides a method for the removal of ammonium compounds from waste waters, in particular filtrate waste waters from biological slurries, comprising:

heating the waste water to at least 95' centigrade and subsequently separating a gas phase, comprising C02 and NH3. from the liquid phase, comprising additionally ammonium compounds; supplying the liquid phase to the head of a packed column and loading said column from below with a hot gas, suitably steam; conducting steam produced in the head of the packed column, optionally together with the gas phase previously separated from the waste water, to a condenser, to produce a condensate comprising ammonium hydrogen carbonate; removing from the condenser, and disposing of, any ammonium hydrogen carbonate which is released therein as a mist; adding a base to the condensate and thereafter returning the condensate into the head of the packed column; and drawing off treated waters from the packed column, suitably at the base thereof.

In a second aspect the present invention provides an apparatus for the removal of ammonium compounds from waste waters, in particular filtrate waste waters from biological slurries, comprising: heating means for heating the water to at least 95' centigrade; means for separating a gas phase from a liquid phase of the water after said heating; a packed column and means for supplying the liquid phase to the head of the packed column and for loading the column from below with a hot gas; a condenser and means for conducting thereto steam produced in the head of the packed column, optionally together with the gas phase previously separated from the waste water; a return line connecting the condenser to the packed column for returning condensate to the packed column and means for adding a base to the condensate prior to said return thereof to the packed column; means for removing from the condenser any ammonium hydrogen carbonate which is released as a mist therein; and means for drawing off treated waters from the packed column, suitably at the base thereof.

In its most general embodiment the method is determined by the following steps:

The ammoniumcontaining waste water is conducted to a packed column. Before introduction into the packed column it is heated (at ambient atmosphere) to a temperature of at least 950 centigrade. The waste water should preferably be heated to boiling temperature. This purpose is served, for example, by heat exchangers or other indirect heating devices. Due to the heating of the waste water there results a great degasification. In this respect, the separation of the gas phase from the liquid phase should take place in a separate device. In this respect, the gas phase predominantly consists of ammonia (NH3) and carbon dioxide (C02) In the liquid phase bound ammonium compounds still remain (in particular with organic constituents). As a result of this degasification, about 70-90% of the entire quantity of C02 is decomposed.

After this first separator stage the liquid phase is supplied to the head of the packed column. The packed column itself is loaded from below with hot steam (inert gas). In this respect, the hot steam serves as carrier gas, in order to maintain the temperature previously set (about 1000 centigrade) also in the stripping column.

Moreover, the packed column has the purpose of making available as large a mass-transfer area as possible.

At the lower end of the packed column the discharge which is practically free of ammoniumcontaining compounds is drawn off and, for example, supplied to a clarification plant.

The steam produced in the head of the packed column is subsequently conducted to a condenser. In order to achieve a gas (steam) phase which is as homogeneous as possible, for this purpose zhe gas drawn off from the degasification container can be supplied at this point into the head of the packed column, so that the gas phase as a whole is conducted to the subsequently connected condenser unit.

The main portion of the hot steam used f or the stripping and the steam supplied from the degasification container condense in the. condenser, to which there can be subsequently connected a heat exchanger. In this respect, both the condenser and the heat exchanger are preferably cooled with cooling water or air.

The condensate produced in the condenser or in the -6 subsequently connected heat exchanger and containing substantially ammonium hydrogen carbonate is subsequently returned into the packed column (into its head). In this respect, a base, preferably sodium hydroxide solution, is previously added to the condensate. The quantity of the sodium hydroxide solution which is necessary in order to achieve a further separation of ammonia therefore corresponds here maximally to the stoichiometric quantity which is necessary for the appropriate conversion of the residual ammonium hydrogen carbonate. It is therefore clearly less than the necessary quantity of sodium hydroxide solution to be added in the prior art.

The ret-urned condensate is treated anew in the packed column in the manner previously described.

Furthermore, crystalline ammonium hydrogen carbonate which has not yet dissolved is produced in the subsequently connected heat exchanger in the form of a mist (hereinafter called aerosol). This is drawn off from the condenser and is washed out in a subsequently connected absorber with the aid of the circulating liquid. The ammonium hydrogen carbonate removed from the absorber is thereafter disposed of.

So far as it is desired for further decomposition, a partial quantity of a base, preferably a sodium hydroxide solution, can likewise be added to the liquid phase before introduction into the packed column. In this way the pH value of the liquid phase is raised and the ammonia still present is expelled. In the process, sodium carbonate and ammonia arise.

- 7 In order to prevent adhesion of the carbonate, formed in the condenser or heat exchanger, to the walls, it advantageous to rinse them with a liquid. For this purpose, an advantageous embodiment proposes that a partial flow (about 3 - 10) be drawn off from the original waste water and that this partial flow be used for wetting the surfaces of the condenser or heat exchanger. By way of the described return from the condenser into the packed column it is ensured that the ammonium compounds still present herein are also decomposed.

is The absorber is preferably loaded by the aerosol from below. Due to the excess pressure set in the packed column the aerosol flows through the absorber from below upwards. Desorbed carbon dioxide can be drawn off at the upper end of the absorber and can be conducted into the atmosphere. Since the desorbed C02 is frequently still loaded with smelling substances, an advantageous embodiment suggests that the carbon dioxide be sent through a filter, preferably a biofilter, before introduction into the atmosphere. This filter can be filled with compost, for example.

Moreover, the absorber is preferably operated in circulation, that is to say, the liquid is removed at the lower end of the absorber and is distributed again at the head of the absorber, so that there results a concentration of the ammonium-hydrogen- carbonate solution, which is finally drawn off. In order to keep constant the quantity of the circulating water, fresh water is added along the return line.

According to an alternative embodiment it is provided to connect a twopart column in front of the packed column. In this respect, the liquid phase should be supplied into the lower part of the column and loaded with hot steam - as described - in the counterflow, while the gaseous phase is conducted into the upper - 8 part and is contacted there by a cooling liquid. In the process, released carbon dioxide is drawn off at the upper part of the column and - suitably by way of a biofilter - is released to the atmosphere.

By means of the premature separation of the carbon dioxide it is possible to obtain so-called heavy water (15 - 20% NH4-OH solution) or ammonia as end product after the condenser.

At the same time, in the lower part of the column described which is connected in from there takes place by means of the supply of steam a further thermal decomposition of ammonium hydrogen carbonate.

The supply, preferably preheated in turn to boiling temperature, for the packed column, is in this respect mixed with the return from the condenser or the subsequently connected heat e2changer. Here also the addition of sodium hydroxide solution serves again to raise the pH value and to bind C02 in the solution as sodium carbonate.

For ease of understanding the invention, embodiments will now be described by way of example and without limitation, with reference to the accompanying drawings, in which:

Figure 1 shows schematically a waste water treatment plant; and Figure 2 shows schematically an alternative waste water treatment plant.

In Figure 1 the supply line for a filtrate waste water of the type named at the beginning is represented by the reference number 10. The waste water is conveyed by way of a pump 12 into a pipeline 14, along which there is arranged a heat exchanger 16, in order to heat the waste water to a temperature of at least 801 centigrade, preferably 100' centigrade.

The heated waste water thereafter arrives into a degasification station 18. The mixture of steam and liquid is separated here, whereby the gases, which predominantly consist of C02 and NH3, are drawn off at the upper end by way of the line 20 and the liquid (about 30% of the original quantity) is drawn off at the lower end by way of the line 22. Both lines open into the head 24 of a packed column 26. Along the line 22 is located another dosing device 28, by way of which sodium hydroxide solution is admixed, as well as a further heat exchanger 30 for maintaining the desired increased temperature (about 1000 centigrade).

The packed column 26 is loaded from below with hot steam by way of a line 32. The hot steam flows through the column 26 filled with conventional packing material from below upwards.

By means of the preheating of the main flow of the waste water, the degasification station 18 and the further heating at 30, a most extensive decomposition of ammonium hydrogen carbonate has already taken place when the liquid phase enters into the packed column 26 A further intensive mass transfer takes place in the -10 column, so that the discharge 34 of the packed column 26 is practically free of ammonium and ammonia and, for example, can be supplied to a clarification plant.

The hot steam and the quantity of gas supplied from the degasification station 18 are subsequently conducted by way of a line 36 into a condenser 38, to which a heat exchanger 40 is subsequently connected.

Moreover into the heat exchanger 40 there opens a line 42, by way of which a partial flow of the waste water separated at 44 is introduced into the heat exchanger, in order to rinse off the ammonium hydrogen carbonate which has formed on the cold walls of the cooler 40.

is This is drawn off from the heat exchanger 40 by way of a line 46 and is returned into the head 24 of the packed column 26. On this path, sodium hydroxide solution (by way of the line 48) is again admixed to the returned liquid, in order to achieve an even better separation. In this respect, it is to be preferred that this partial flow also contains a portion of the untreated water.

By way of a connection line 50 the carbon dioxide desorbed in the heat exchanger 40 and in the column 26 leaves the condenser unit 38, 40. In this respect, a mist is drawn off, which contains above all ammonium hydrogen carbonate which has not yet dissolved either.

This is subsequently conducted into an absorber 52. The aerosol flows through the absorber 52 from below upwards, where an ammonium-hydrogencarbonate absorption in the liquid conducted in the cycle takes place. A heat exchanger 54 serves to cool the liquid.

By means of the circulatory conduction a concentration of ammonium hydrogen carbonate is achieved. In this respect, the solution is concentrated to near the limit of saturation. The concentrate is removed by way of a line 56 from the absorber 52 and the appropriate liquid equalization is equalized by way of a supply of fresh water (line 58) along the recirculation line 60. A pump 62 ensures the circulation of the liquid.

In a further stage (for example in a reactor or a further packed column) the discharge (by way of the line 56) can be treated further at increased temperatures. Through additions of aqueous solutions of strong bases (for example sodium hydroxide solution) there can then be obtained as products, for example, calcium carbonate or sodium carbonate and a heavy water with a, for example, 15 - 20 % NH4-OH solution.

if high-grade heavy water or ammonia is required as end product, then by a small change of the process control compared with the embodiment represented in Figure 1 gaseous carbon dioxide can be separated in a column 70 connected in front (Figure 2). The column is divided in two. In the upper part 72 the gas drawn off from the degasification station 18 is supplied. A cooling is achieved in that a partial flow of the waste water is supplied by way of the line 42 in this region.

However, the gas supplied from the degasification station 18 after expansion and containing, among other things, excess carbon dioxide is not absorbed here because of its low solubility, but is discharged in a gaseous form (line 74) and is subsequently cleaned by way of a biological filter 76 before it is conducted into the atmosphere (Figure 1).

A similar biological filter 76 can also be arranged along a line 68, by way of which carbon dioxide drawn off from the absorber 52 is removed.

In the lower part of the column 70 (Figure 2) there takes place in turn the supply of hot steam (line 78), leading to a further thermal decomposition of ammonium hydrogen carbonate.

The supply, preheated to boiling temperature, into the packed column 26 is mixed there with the return of the condenser unit 38, 40 (analogous to the exemplary embodiment according to Figure 1).

The main stripping process also takes place here in the packed column 26, in which process, in the counterflow to the liquid flowing from above downwards the steam flows upwards which is then condensed in the condenser 38 or the heat exchanger 40.

Figure 2 describes a further difference compared with Figure 1 in the region of the discharge 34 from the packed column 26- The most extensively ammonium-free discharge is supplied by way of a pump 80 to the heat exchanger 16, in order to use in this way the heat content of the discharge for preheating the waste water.

Moreover, the device according to Figure 2 and the preparation method exercised therewith correspond substantially to those of Figure 1.

i 13 -

Claims

1. A method for the removal of ammonium compounds from waste waters, comprising: heating the waste water to at least 950 centigrade and subsequently separating a gas phase, comprising C02 and NH3.. f rom the liquid phase, comprising additionally ammonium compounds; supplying the liquid phase to the head of a packed column and loading said column from below with a hot gas, suitably steam; conducting steam produced in the head of the packed column, optionally together with the gas phase previously separated from the waste water, to a condenser, to produce a condensate comprising ammonium hydrogen carbonate; removing from the condenser, and disposing of, any ammonium hydrogen carbonate which is released therein as a mist; adding a base to the condensate and thereafter returning the condensate into the head of the packed column; and drawing off treated waters from the packed column, suitably at the base thereof.

2. A method according to claim 1, in which the ammonium hydrogen carbonate mist is removed by washing through an absorber in a liquid circulation.

3. A method according to claim 2, in which any carbon dioxide desorbed in the absorber is subsequently cleaned, suitably by a filter, before introduction into the atmosphere.

4. A method according to claim 2 or 3, in which the ammonium hydrogen carbonate solution in the absorber is concentrated through a circulation to near the limit of saturation and the liquid loss is compensated for by the removal of the concentrate with an appropriate supply of fresh water.

5. A method according to any preceding claim, in which a base is added to the liquid phase before introduction into the packed column.

6. A method according to any preceding claim, in which sodium hydroxide solution is used as base.

7. A method according to any preceding claim, in which the base is added in a quantity which leads to the stoichiometric conversion of the ammonium compounds into a carbonate and ammonia.

8. A method according to any preceding claim, in which a partial flow of the waste water is separated before the heating and is conducted directly into the condenser.

9. A method according to any preceding claim, in which the waste water and/or the liquid phase are heated to boiling temperature before introduction into the packed column.

10. A method according to any preceding claim, in which a two-part column is connected in front of the packed column, whereby the said liquid phase is supplied to one part of the two-part column and is loaded with a hot gas, suitably steam, under counterflow, while the said gas phase is conducted into the other part of the two-part column and is contacted there by a cooling liquid, whereby released carbon dioxide is drawn off and the liquid phase is subsequently supplied to the packed column.

11. A method according to claim 10, in which a partial flow of the untreated waste water is used as cooling liquid.

12. An apparatus for the removal of ammonium compounds from waste waters, comprising: heating means for heating the water to at least 950 centigrade; means for separating a gas phase from a liquid phase of the water after said heating; a packed column and means for supplying the liquid phase to the head of the packed column and for loading the column from below with a hot gas; a condenser and means for conducting thereto steam produced in the head of the packed column, optionally together with the gas phase previously separated from the waste water; a return line connecting the condenser to the packed column for returning condensate to the packed column and means for adding a base to the condensate prior to said return thereof to the packed column; means for removing from the condenser any ammonium hydrogen carbonate which is released as a mist therein; and means for drawing off treated waters from the packed column, suitably at the base thereof.

13. An apparatus according to claim 12, in which an absorber is provided whereby the ammonium hydrogen carbonate is removed by washing through the absorber in a liquid circulation.

14. An apparatus according to claim 13, in which the absorber is provided with a return line for the at least partial recirculation of the liquid conducted through the absorber.

15. An apparatus according to claim 14, in which a fresh-water supply line is provided, opening into the return line.

16. An apparatus according to any one of claims 12 to 15, in which a heat exchanger is present as the means for heating the waste water to at least 950 centigrade and/or a further heat exchanger is provided after the condenser.

- 16

17. An apparatus according to any one of claims 12 to 16, in which a two- part column is provided before the packed column, means being also provided for supplying the said liquid phase to one part of the two-part column and for loading the same with a hot gas under counterflow, and for conducting the said gas phase into the other part of the two-part column to be contacted there by a cooling liquid, whereby released carbon dioxide is drawn off and the liquid phase is subsequently supplied to the packed column.

18. A method for the removal of ammonium compounds from waste waters, substantially as herein described with reference to the accompanying drawings.

19. An apparatus for the removal of ammonium compounds from waste waters, substantially as herein described with reference to the accompanying drawings.