AT401048B - METHOD AND DEVICE FOR REMOVING AMMONIUM COMPOUNDS FROM WASTEWATER - Google Patents

Description

AT 401 048 B

The invention relates to a method and a device for removing ammonium compounds from waste water, in particular filtrate waste water from biological sludge.

It is known to post-treat the sedimentation sludge from digestion processes in sewage treatment plants in filter presses or centrifuges. While the solid obtained is then burned or deposited, the disposal of the ammonium-containing filtrate presents considerable difficulties. The concentration of ammonium compounds in such filtrates is usually between 0.7 and 1.6 g NH4-N / I.

A method for removing ammonium compounds from ammonium-containing wastewater with a concentration of approx. 8-20 g / l by so-called steam stripping is known from coking technology. In addition to ammonia, the coke oven wastewater also contains CO2, H2S and small amounts of HCN.

This method cannot be applied to the treatment of the filtrate waters mentioned, since they contain excess stoichiometric amounts of carbon dioxide, which binds all the ammonium in the form of volatile ammonium hydrogen carbonate (NH4.HCO3).

A discharge of the waste water into the sewage treatment plant is out of the question because this would lead to an ammonium cycle.

On the other hand, the legislator demands that the ammonium concentrations be limited to values less than 50 mg NH * -N / I.

In order to meet these legal requirements, it is known to add an excess of the stoichiometric amount of sodium hydroxide solution to the wastewater and then to subject the wastewater to air or steam stripping. By adding sodium hydroxide solution, the pH of the solution is increased to values of 10.5 -11. The balance of the reaction is shifted due to the shift in the pH in the direction of the free ammonia. CO2 is bound by adding sodium hydroxide solution.

Considerable amounts of sodium hydroxide solution are required to achieve the correspondingly high pH. For example, when using 30 percent sodium hydroxide solution, 20 liters of NaOH are required per m3 of wastewater. The result is severe salinization of the wastewater. Due to the considerable quantities of chemicals, the operating costs are also undesirably high.

Other chemical processes, such as the so-called MAP process (magnesium-ammonium-phosphate process) are also characterized by a very high consumption of chemicals.

In this respect, the invention is based on the object of offering a possibility for the treatment of ammonium-containing wastewater, with which removal of the ammonium compounds is made possible even without large additions of chemicals.

For this purpose, the invention offers a method with the features of claim 1 and an associated device with the features of claim 11. Advantageous embodiments of the method and the device are described in the subclaims.

In its most general embodiment, the process is determined by the following steps: The ammonium-containing waste water is fed to a packed column. Before being introduced into the packed column, it is heated (in an ambient atmosphere) to a temperature of at least 95 * Celsius. The wastewater should preferably be heated to boiling temperature. Heat exchangers or other indirect heating devices are used for this purpose. Due to the heating of the wastewater, there is a strong degassing. The separation of the gas phase from the liquid phase should take place in a separate facility. The gas phase consists mainly of ammonia (NH3) and carbon dioxide (CO2). Bound ammonium compounds (especially with organic constituents) remain in the liquid phase. As a result of this degassing, about 70-90% of the total amount of COi is broken down. - After this first separation stage, the liquid phase is fed to the top of the packed column. The packed column itself is subjected to superheated steam (inert gas) from below. The superheated steam serves as a carrier gas in order to maintain the previously set temperature (approx. 100 * Celsius) in the stripping column.

The packed column also has the purpose of stiffening the largest possible mass transfer area.

At the lower end of the packed column, the discharge, which is practically free of ammonium-containing compounds, is drawn off and fed, for example, to a sewage treatment plant. - The steam generated in the top of the packed column is then fed to a condenser. In order to achieve a gas (vapor) phase which is as homogeneous as possible, the gas drawn off from the degassing container can be fed into the top of the packed column at this point, so that the gas phase as a whole is fed to the downstream condenser unit. - The main part of the hot steam used for stripping and the steam supplied from the degassing condensate in the condenser, which can be followed by a heat exchanger. 2nd

AT 401 048 B

Both the condenser and the heat exchanger are preferably cooled with cooling water or air. - The condensate obtained in the condenser or the downstream heat exchanger and essentially contains ammonium hydrogen carbonate is then returned to the packed column (in the top). A base, preferably sodium hydroxide solution, is metered into the condensate beforehand. The amount of sodium hydroxide solution which is necessary to achieve a further separation of ammonia corresponds here at most to the stoichiometric amount which is necessary for the corresponding conversion of the residual ammonium hydrogen carbonate. It is therefore significantly less than the amount of sodium hydroxide solution required in the prior art.

The returned condensate is treated again in the packed column in the manner described above. - Crystalline ammonium hydrogen carbonate, which has not yet been dissolved, continues to form in the form of a mist (hereinafter referred to as aerosol) in the downstream heat exchanger. This is withdrawn from the condenser and washed out in a downstream absorber using the circulating liquid. The ammonium hydrogen carbonate removed from the absorber is then disposed of.

If this is desired for further digestion, a portion of a base, preferably a sodium hydroxide solution, can also be added to the liquid phase before introduction into the packed column. In this way the pH of the liquid phase is raised and the ammonia still present is driven off. This creates sodium carbonate and ammonia.

In order to prevent the carbonate formed in the condenser or heat exchanger from adhering to the walls, it is advantageous to rinse them with a liquid. For this purpose, an advantageous embodiment suggests subtracting a partial flow (approx. 3-10%) from the original wastewater and using this partial flow for flushing the surfaces of the condenser or heat exchanger. The return flow described from the condenser into the packed column ensures that the ammonium compounds still present here are also broken down.

The absorber is preferably charged with the aerosol from below. Due to the overpressure set in the packed column, the aerosol flows through the absorber from bottom to top. Desorbed carbon dioxide can be drawn off at the top of the absorber and released into the atmosphere. Since the desorbed CO2 is often still contaminated with odorous substances, an advantageous embodiment suggests sending the carbon dioxide through a filter, preferably a biofilter, before being introduced into the atmosphere. For example, this filter can be filled with compost.

Otherwise, the absorber is preferably operated in a circuit, that is to say the liquid is removed from the lower end of the absorber and redistributed at the top of the absorber, so that the ammonium hydrogen carbonate solution is concentrated, which is finally drawn off. In order to keep the amount of circulating water constant, fresh water is metered in along the return line.

According to an alternative embodiment, a packed column is arranged upstream of the packed column. The liquid phase is to be fed into the lower part of the column and superheated steam - as described - is applied in countercurrent, while the gaseous phase is conducted into the upper part and is contacted there with a cooling liquid. Released carbon dioxide is drawn off at the top of the column and - if necessary via a biofilter - is fed into the atmosphere.

The premature removal of the carbon dioxide makes it possible to obtain so-called strong water (15-20 percent NH * OH solution) or ammonia as the end product after the condenser.

At the same time, a further thermal decomposition of ammonium hydrogen carbonate takes place in the lower part of the upstream column described by the steam supply.

The feed for the packed column, which in turn is preferably preheated to boiling temperature, is mixed with the return from the condenser or the downstream heat exchanger. Here, too, the addition of sodium hydroxide solution serves to raise the pH and bind CO2 in the solution as sodium carbonate.

The above description of the processing method already shows the essential components of the associated device. For the rest, reference is made to the following description of the figures, which in this respect also contains general features.

Show, each highly schematic:

Figure 1: A system according to the invention in a first embodiment.

Figure 2: An alternative embodiment of the system of Figure 1.

In Figure 1, the reference line 10 for a filtrate waste water of the type mentioned is shown. The waste water is conveyed into a pipeline 14 via a pump 12, along 3

AT 401 048 B, a heat exchanger 16 is arranged to heat the waste water to a temperature of at least 80 * Celsius, preferably 100 * Celsius.

The heated wastewater then passes into a degassing station 18. The vapor-liquid mixture is separated here, the gases, which consist mainly of CO2 and NH3 at the upper end via line 20 and the liquid (approx. 30% of the original amount) at the lower end withdrawn via line 22. Both lines open into the head 24 of a packed column 26. Along the line 22 there is also a metering device 28 via which sodium hydroxide solution is mixed in, as well as a further heat exchanger 30 for maintaining the desired elevated temperature (approx. 100 * Celsius).

The packed column 26 is charged with superheated steam via a line 32 from below. The superheated steam flows through the column 26 filled with conventional packing elements from bottom to top.

Due to the preheating of the main flow of the waste water, the degassing station 18 and the further heating at 30, ammonium hydrogen carbonate has already been largely decomposed when the liquid phase enters the packed column 26.

A further intensive mass transfer takes place in the column, so that the outlet 34 of the packed column 26 is practically free of ammonium and ammonia and can be fed to a sewage treatment plant, for example.

The superheated steam and the amount of gas supplied from the degassing station 18 are then passed via a line 36 into a condenser 38 which is followed by a heat exchanger 40.

A line 42 also opens into the heat exchanger 40, via which a partial flow of the wastewater separated at 44 is introduced into the heat exchanger in order to rinse off the ammonium hydrogen carbonate formed on the cold walls of the cooler 40.

This is withdrawn from the heat exchanger 40 via a line 46 and returned to the top 24 of the packed column 26. In this way, sodium hydroxide solution (via line 48) is added to the recirculated liquid in order to achieve an even better NH3 separation. It should be taken into account that this partial flow also contains part of the raw water. The carbon dioxide desorbed in the heat exchanger 40 and in the column 26 leaves the condenser unit 38, 40 via a connecting line 50. A mist is drawn off, which above all also contains undissolved ammonium hydrogen carbonate. This is then passed into an absorber 52. The aerosol flows through the absorber 52 from bottom to top, where ammonium bicarbonate absorption takes place in the circulated liquid. A heat exchanger 54 is used to cool the liquid. A concentration of ammonium hydrogen carbonate is achieved by the circulation. The solution is concentrated close to the degree of saturation. The concentrate is led away from the absorber 52 via a line 56 and the corresponding liquid compensation is compensated for via a fresh water supply (line 58) along the recirculation line 60. A pump 62 ensures that the liquid is circulated.

In a further stage (for example in a reactor or another packed column), the outflow (via line 56) can be further treated at elevated temperatures. By adding aqueous solutions of strong bases (for example sodium hydroxide solution), calcium or sodium carbonate and a strong water with a for example 15-20 percent NH + -OH solution can then be obtained as products.

If the high-quality high-pressure water or ammonia is required as the end product, gaseous carbon dioxide can be separated off in an upstream column 70 due to a slight change in the process control compared to the embodiment shown in FIG. 1 (FIG. 2). The column is divided into two. The gas drawn off from the degassing station 18 is supplied in the upper part 72. Cooling is achieved in that a partial stream of the waste water is fed in via line 42 in this area.

However, the gas that is supplied from the degassing station 18 after expansion and contains, inter alia, excess carbon dioxide is not absorbed here because of its low solubility, but is discharged in gaseous form (line 74) and subsequently cleaned via a biological filter 76 before it is passed into the atmosphere (FIG ).

A similar biological filter 76 can also be arranged along a line 68, via which carbon dioxide drawn off from the absorber 52 is led away.

In the lower part of column 70 (FIG. 2), hot steam (line 78) is again supplied, which leads to a further thermal decomposition of ammonium hydrogen carbonate.

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

Here too, the main stripping process takes place in the packed column 26, in which the steam flows in countercurrent to the liquid flowing downward from the top, which steam then flows in the fourth

Claims (14)

AT 401 048 B condenser 38 or the heat exchanger 40 is deposited. FIG. 2 describes a further deviation from FIG. 1 in the area of the outlet 34 from the packed column 26. The largely ammonium-free outlet is supplied to the heat exchanger 16 via a pump 80 in order to use the heat content of the outlet to preheat the waste water. Otherwise, the device according to FIG. 2 and the treatment process carried out with it essentially correspond to those according to FIG. 1. Claims 1. Process for removing ammonium compounds from waste water, in particular filtrate waste water from biological sludges, characterized by the following steps: 1.1 The waste water is on its Way to a packed column heated to at least 95 * Celsius and then passes through a device for separating a gas phase consisting predominantly of CO2 and NH3 from the liquid phase which still contains ammonium compounds, 1.2.1 while the liquid phase is fed to the top of the packed column and from below with superheated steam is charged, 1.2.2 the steam accumulating in the top of the packed column, if appropriate together with the gas phase previously separated from the waste water, are fed to a condenser, 1.3 the Ko which is obtained in the condenser and essentially contains ammonium hydrogen carbonate ndensat is returned to the top of the packed column after addition of a base, 1.4 the ammonium hydrogen carbonate drawn off from the condenser as a mist is then washed out in an absorber in a liquid circuit, 1.5 the ammonium hydrogen carbonate removed from the absorber is then disposed of and 1.6 the taken from the packed column, discharge from a sewage treatment plant or the like essentially free of ammonium compounds. 2. The method according to claim 1, characterized in that a base is added to the liquid phase before introduction into the packed column. 3. The method according to claim 1 or 2, characterized in that sodium hydroxide solution is used as the base. 4. The method according to any one of claims 1 to 3, characterized in that the base is added in an amount which leads to the stoichiometric conversion of the ammonium compounds into sodium carbonate and ammonia. 5. The method according to any one of claims 1 to 4, characterized in that a partial flow of the waste water is separated before heating and passed directly into the condenser. 6. The method according to any one of claims 1 to 5, characterized in that the waste water and / or the liquid phase are heated to boiling temperature before being introduced into the packed column. 7. The method according to any one of claims 1 to 6, characterized in that in the absorber desorbed-tes carbon dioxide is cleaned in a filter before introduction into the atmosphere. 8. The method according to any one of claims 1 to 7, characterized in that the ammonium bicarbonate solution in the absorber is concentrated by a circuit up to the vicinity of the degree of saturation and the loss of liquid is compensated for by removing the concentrate with a corresponding fresh water supply. 9. The method according to any one of claims 1 to 8, characterized in that the packed column is preceded by a two-part column, the liquid phase being supplied in one part of the column and subjected to superheated steam in countercurrent, while the gaseous phase is conducted in the other part and there is contacted with a cooling liquid, the released carbon dioxide being drawn off and the liquid phase subsequently being fed to the packed column. 5 AT 401 048 B 10. The method according to claim 9, characterized in that a partial flow of the raw waste water is used as the cooling liquid. 11. Device for performing the method according to one of claims 1 to 10, characterized by 11.1 at least one packed column (26.70), the 11.2 at least one heat exchanger (16.30) and 11.3 a device (18) for separating the gas from upstream of the liquid phase, and 11.4 a condenser (38, 40) connected downstream of the packed column (26,70), to which an absorber (52) is connected 11.5, and from which 11.6 a return line (46) to the packed column (26) 11.7 the components are interconnected by pipes (14, 20, 28, 36, 42, 50). 12. The apparatus according to claim 11, characterized in that the condenser (28) is followed by a heat exchanger (40). 13. The apparatus of claim 11 or 12, characterized in that the absorber is formed with a return line (60) for at least partial recirculation of the liquid passed through the absorber (52). 14. The apparatus according to claim 13, characterized in that a fresh water supply line (58) opens into the return line (60). Including 2 sheets of drawings 6 1