It is important to limit emissions of nitrogen compounds due to their harmful impact on the environment. Excessive load of nitrogen compounds in different forms, for instance as nitrate nitrogen or ammonium nitrogen, is detrimental to plants, acidifies the soil, causes eutrophication of surface waters, or gives rise to unpleasant odor ef- fects. Excessive nitrogen supply to areas in natural condition may change the bal- ance of the vegetation, promote the evaporation of nitrous oxide known as a green- house gas, and pollute ground waters when absorbed in the soil. For instance, while industrial waters containing ammonium may be used in particular applications, they however corrode metals.

In the treatment of industrial, agricultural and municipal waste waters, organic and inorganic solids are removed by sedimentation, biochemical and chemical oxygen consuming substances by an activated sludge process. Generally, while the waste water already is very pure after these treatments, ammonium nitrogen is still a prob- lem. It is difficult to remove ammonium nitrogen from waste water since the solu- bility thereof in water is high and it is a very stable compound in the form of am- monia.

Various processes are developed for removal of ammonium nitrogen and ammonia from waste waters. Biological nitrogen removal processes are preferred in situations where the nitrogen content of waste water is relatively low and organic or inorganic matter is still present therein. On the other hand, an elevated nitrogen content of waste water and low processing temperatures are favourable for chemical nitrogen removal processes.

It is known to remove ammonium nitrogen from waste waters with various physical and chemical processes such as electrodialysis, reversed osmosis, stripping, chlori- nation, and ion exchange. Each of these processes has its benefits and drawbacks.

Ion exchange is the most common process with several published variations and modifications.

In ion exchangers ammonium nitrogen contained in the waste water is adsorbed on an ion-exchange substance, such as zeolite beds. The adsorption works well and the estimated life time of the zeolite beds is at least 5-7 years. The overall costs for the adsorption are very reasonable. However, the ion-exchange substance, such as zeo- lite beds need to be regenerated regularly and the overall (i. e. including invest- ments) regeneration costs are crucial for the competitiveness of the complete NH4 removal process. Several regeneration processes have been developed. The ex- hausted ion-exchanger material can be regenerated chemically or biologically. In the chemical regeneration the ammonium ions are eluted from the ion-exchange substance in an alkaline salt solution, such as an alkali metal chloride, f. ex. sodium chloride solution. The obtained solution containing ammonium can be subjected to stripping, thermal tretment or chemical treatment.

In US 3 929 600 the regeneration is carried out by passing an alkaline eluant liquid containing alkali metal chlorides through a column filled with an ion-exchange sub- stance, such as zeolite. The liquid eluate containing ammonium and chloride ions is electrolyzed to decompose the ammonium ion by the oxidizing effect of chlorine thereby evolved. The gas mixture generated in the electrolysis consists of hydrogen, oxygen, nitrogen, a small amount of chlorine, a trace of nitrogen oxides and chlor- amines. This gas mixture is brought into contact with an adjusting liquid containing alkali metal hydroxide to absorb chlorine and nitrogen oxides. An alkali metal hy- droxide is added to the electrolyzed liquid which is recirculated to the column filled with the ion-exchange substance.

US 6 132 627 discloses a process for removing nitrogen compounds, such as am- monium from waste water by adding an inorganic chloride to the waste water to a concentration which is a substoichiometric amount relative to the amount required to oxidize the nitrogen compounds, electrolyzing the waste water containing chlo- ride to form hypochlorite ions, and transferring the waste water from the electrolysis to a storage container where the nitrogen compounds are oxidized by the hypochlo- rite ions, and chloride ions are regenerated. The partially treated waste water may be circulated between the storage container and the electrolysis container. Subse- quently the waste water is contacted with a metal peroxide catalyst in a reaction column which peroxide catalyst further breaks down the nitrogen compounds and removes residual hypochlorite ions from the waste water.

In the two above discussed processes the waste water containing nitrogen com- pounds is subjected to direct electrolysis.

A drawback with the process of US 3 929 600 is that the control and optimization of the ammonium decomposition process in the electrolytic cell are difficult to imple- ment due to the fact that the flow rate through the electrolytic cell is dependent on the flow rate of the regeneration liquid through the zeolite bed. Also the pH and the retention time are difficult to control in the direct electrolysis. During the zeolite re- generation cycle there are great variations in the ammonium content of the filter bed effluent and in the direct electrolysis it is difficult to adjust the amount of generated hypochlorite to the amount needed for decomposing the ammonium. To avoid an excessive additional salt load in the wastewater plant effluent, the zeolite beds may have to be flushed with water after the regeneration with the salt solution. The used flushing water has to be taken to the regeneration loop. A direct electrolysis of such a water with low salt content is problematic.

US Patent Application Publication 2004/0007533 A discloses a method of treating a nitrogen compound-containing (e. g. ammonia) water. The nitrogen compound is ad- sorbed to an adsorbent (e. g. zeolite) and decomposed by treating the adsorbent with an oxidizer-containing liquid. The oxidizer can be hypochlorous acid or hypobro- mous acid produced by electrolysis. In a typical embodiment 19 (Fig. 23) hy- pochlorous acid produced by electrolysis from a sodium chloride solution is sup- plied to an adsorbing tower filled with zeolite. The hypochlorous acid decomposes the ammonia adsorbed to the zeolite to nitrogen gas. A drawback with this embodi- ment is that the hypochlorous acid may deteriorate the zeolite. US 2004/0007533 A suggests an other embodiment 9 wherein ammonia adsorbed to zeolite is desorbed in a sodium chloride solution and the obtained solution is introduced into a denitri- fication tank in which the ammonia is oxidized with hypobromous acid to nitrogen gas. This embodiment prevents the contact between hypobromous acid an zeolite.

This Application Publication generally suggests keeping the pH of the hypobro- mous acid neutral to acid in order to inhibit the formation of harmful bromic acid.

An object of the present invention is to provide a process which avoids the above drawbacks.

According to the invention there is provided a process for removal of ammonium from an ammonium-containing aqueous solution obtained by regenerating an am- monium-loaded ion exchange material with an eluant liquid comprising a chloride- containing salt, said process comprising the step of feeding said ammonium- containing aqueous solution or an ammonia-containing condensate obtained by stripping said ammonium-containing aqueous solution and removing ammonia from the stripping gas by condensation, and a separate aqueous solution of an alkali metal

hypochlorite into a reactor for converting the ammonium/ammonia to nitrogen gas and for producing an essentially ammonium-free effluent stream, a portion of said effluent stream from the reactor being returned as the eluant liquid to the regenera- tion of the ammonium-loaded ion exchange material.

Thus, in a first embodiment of the invention, the process comprises the step of feed- ing said ammonium-containing aqueous solution and said separate aqueous solution of an alkali metal hypochlorite into the reactor for converting the ammonium to ni- trogen gas.

In a second embodiment of the invention, the process comprises the step of feeding said ammonia-containing condensate obtained by stripping said ammonium- containing aqueous solution and removing ammonia from the stripping gas by con- densation, and said separate aqueous solution of an alkali metal hypochlorite into the reactor for converting the ammonia to nitrogen gas. At least a portion of the liq- uid from the stripping can be returned as the eluant liquid to the regeneration of the ammonium-loaded ion exchange material.

In a preferred embodiment of the invention the pH of the aqueous solution in the re- actor is kept at a level of at least 7, preferably at least 8 and more preferably at least 8.5. The pH of the aqueous solution can be adjusted to the desired level with a al- kali, such as sodium hydroxide. The benefit of the alkaline pH is that the formation of undesired by-products is prevented.

The aqueous solution of alkali metal hypochlorite can be produced on-site by direct electrolysis of an aqueous solution of alkali metal chloride. In that case a portion of the effluent stream from the reactor can be returned as the aqueous solution of alkali metal chloride to the electrolysis.

According to the present invention it is also possible to take the aqueous solution of alkali metal hypochlorite from a storage container. In that case a portion of the ef- fluent stream from the reactor is returned as the eluant liquid to the regeneration of the ammonium-loaded ion exchange material.

A preferred ion exchange material comprises zeolite. The zeolite can be a commer- cially available zeolite including American, Greek, Cuban or Australian clinoptilo- lite, zabonite, phillipsite or synthetic zeolite.

The chloride-containing salt is preferably sodium chloride, the alkali metal hy- pochlorite is preferably sodium hypochlorite, and the alkali metal chloride is pref- erably sodium chloride.

The eluant liquid can contain from 20 g to 75 g NaCl/l, preferably from 50 g to 60 g NaCl/l, or corresponding amounts of another alkali metal chloride.

Said ammonium-containing aqueous solution can contain 0.2 g to 4 g ammonium nitrogen/1, preferably 0.4 g to 2 g ammonium nitrogen/l.

Said reactor can be a pipe reactor, the retention time being preferably between 5 and 60 seconds, more preferably between 10 and 30 seconds. The benefit of the short re- tention time is that a small equipment can be used. The pipe reactor can be equipped with static mixers.

Following advantages are obtainable by the process of the present invention.

The flow rate through the electrolysis device is independent of the flow rate of the regeneration liquid through the ion exchange material and due to that the control and the optimization of the ammonium decomposition process conditions are easy to perform. Due to the optimal process conditions and easier process control, there are less formation of such unwanted by-products which most probably need to be removed from the regeneration liquid before the liquid can be reused in the regen- eration of the ion exchange material.

In the separate reactor of the present invention the pH and retention time are easy to control. The pH and retention time are both important parameters in the optimiza- tion of the process conditions.

According to the present invention it is possible to arrange a hypochlorite tank be- tween the electrolysis and the ammonium decomposition reactor. Such a tank serves as both a hydrogen removal vessel and a buffer tank that at any moment gives ac- cess to the right amount of hypochlorite needed for the optimal process conditions in the separate ammonium decomposition reactor. The hypochlorite tank also makes it possible to use a smaller-sized electrolysis device and optimal process conditions in the electrolysis.

From an environmental and/or economical point of view recycling of at least a por- tion of the essentially ammonium-free effluent stream as the eluant liquid to the re- generation of the ammonium-loaded ion exchange material is essential, because a

local disposal of the essentially ammonium-free effluent stream is causing consider- able additional salt load in nearby recipients like lakes and rivers, and because transportation of the used salt solution for treatment and/or disposal elsewhere means considerable additional costs compared to recycling and on-site reuse.

Recycling of a portion of the essentially ammonium-free effluent stream from the reactor to on-site production of the alkali metal hypochlorite by electrolysis is mini- mizing the need of make-up alkali metal chloride.

In the following the present invention will be described in more detail with refer- ence to the enclosed drawing wherein Fig. 1 is a flow diagram illustrating one preferred embodiment of the present inven- tion.

Ammonium is removed from wastewater by feeding wastewater through a zeolite bed 1 wherein ammonium is adsorbed on granular zeolite. The ammonium-loaded zeolite bed 1 has to be regenerated regularly. The ammonium-loaded zeolite bed 1 is regenerated by feeding a sodium chloride regeneration solution 2 through the zeo- lite bed. The pH of the regeneration solution 2 is about 9 and the concentration of the sodium chloride is about 20 g to 75 g per litre. The eluate solution 3 leaving the zeolite bed contains ammonium and has a reduced concentration of sodium chlo- ride. Said ammonium-containing eluate 3 contains about 0.2 g to 4 g ammonium nitrogen/1. The eluate 3 is then fed to a breakpoint chlorination reactor 4. A sodium hypochlorite solution 5 generated in an electrolytic cell 6 equipped with a buffer tank is also fed to the reactor 4. In the reactor 4 the sodium hypochlorite reacts with the ammonium to form nitrogen gas 7 and sodium chloride. The essentially ammo- nium-free sodium chloride solution 8 leaving the reactor 4 can be fed to a storage container 9 wherefrom a portion 10 can be recirculated as regeneration solution to the zeolite bed 1. Another portion 11 can be recirculated to the electolytic cell 6. If needed make-up sodium chloride is introduced into the process.

In the following the invention will be illustrated by following examples Example 1 In an autoclave having a capacity of 2. 0 1 and equipped with a magnetic stirring unit there were placed 1000 ppm of NH4+ ion, 2.2 g of NaOH and 1800 g of water. The autoclave was at atmospheric pressure. The reactor was heated to 25°C and NaClO

(7 per cent by weight) was fed into reactor at rate 1.2 g/min. Within a period of 4 hour the amount of NH4+ was dropped to 1 ppm.

Example 2 In a similar manner 1100 ppm of NH4+ ion, 144 g of NaCl and 1800 g of water were placed in reactor. Within a period of 1.25 hour NaClO (7 per cent by weight) was fed into reactor at rate 4.5 g/min as well as NaOH (5 per cent by weight) was fed at rate 1.25 g/min. The amount of NH4+ was dropped to 4 ppm.

Example 3 Effect of pH on hypo-reaction - Experiments were made in a fiberglass reactor (1. 2m3) with mixing - NH4-containing eluent liquid was pumped into reactor - in some of the experiments pH was adjusted to 8.5-9 with NaOH in the beginning of experiments and in some of the experiment pH was as it was after regeneration (6.5-7. 5) -after that both NaOH and NaClO were fed into reactor with such a speed that reac- tion happened with wanted speed. pH was kept over 8.5 with NaOH, preferably pH was let to rise up to around 11 - in some of the experiment pH was let to drop down to see if unwanted by-products were formed The results are compiled into Table 1.

Table 1 pH N03 Chlorinated am-Chlorinated hydro-Ammonia Chloride monia copounds* carbons pH < 5 from hundreds not observed tens of ppms not observed couple of ppms to thousands of ppms 5 < pH < 7 tens of ppms not observed couple of ppms not observed not observed 7 < pH < 8.5 not observed not observed not observed not observed not observed 8. 5 < pH not observed not observed not observed couple of ppms not observed * this is not measured value, but according to literature chlorinated ammonia com- pounds are formed when pH is lower than 8.5 and the lower the pH the more these compounds are formed

Benefits of high pH: not unwanted by-product formation, the higher the pH the less byproducts are formed.

Suitable pH: over 7, preferably over 8.5 Example 3a (run 11, initial pH as it was after regeneration) 550 1 of ammonium-containing eluent liquid was pumped into the reactor. NaOH and NaClO feeding (constant speed) was started at the moment t=0 min. Reactor was well mixed. In Table 2 below are presented the amounts of chemicals pumped into reactor, ammonium content in the reactor, and pH of the liquid at different moments.

Table 2 pH time Ammonium 100% 100% NaClO NaOH min mg/1 g 7.38 0 712 0 0 9.83 30 483 1114 241 11.74 60 234 2123 482 11.16 190 8 3114 1709

No formation of unwanted by-products.

Example 3b (run 16, pH was adjusted before the start of the experiment) 870 1 of ammonium-containing eluent liquid was pumped into the reactor. At first 920 g of 100% NaOH was added to the reactor to adjust pH. At the moment t=0 the feeding of NaClO was started (constant speed). NaOH was added when needed to keep pH high enough. Reactor was well mixed. Results of the experiment are pre- sented Table 3.

Table 3 100% 100% pH time ammonium sodium hypochlorite NaOH min m/1 8.89 0 934 0 920 10.33 30 767 1513 1175 11.83 60 486 3024 1278 11.45 90 125 4488 1278 9. 48 110 2.4 5438 1443 No harmful by-products were formed.

Example 3c (run 22, low pH) 590 1 of ammonium-containing eluent was pumped into the reactor. Within 20 first minutes 81 g of 100% NaOH was added to the reactor. NaClO addition to the reac- tor was started at t=0 (constant speed). Reactor was well mixed. NaOH was added more when pH had dropped dramatically. Results of the experiment are presented in Table 4.

Table 4 pH time ammonium 100% 100% min mg/l NaClO NaOH 7.26 0 1061 0 0 9.27 20 731 994 80.8 10.19 40 569 1762 80.8 10.95 60 493 2646 80.8 7.06 80 313 3511 80. 8 2.69 100 103 4345 84. 8 7. 44 120 55310 276 Lot of by-products was formed between 80-100 minutes.

- 80-100 minutes: 60% of ammonium nitrogen was transformed into N03 Example 4 Retention time - ammonium-containing eluent liquid was pumped through a pipe reactor having static mixers - start sample was taken before chemical additions - NaOH and NaClO were added to the flow after sampling, before pipe reactor - order NaOH first, NaClO after that is important to be able to control the pH - retention time in pipe reactor was 15 seconds - sample was taken immediately after pipe reactor and ammonium was analysed Benefits of short retention time: -short retention leads to a smaller equipment and the investment costs are smaller

Example 4a (run pipe reactor 1) Ammonium-containing eluent liquid was pumped through the pipe reactor which was equipped with static mixers. Initial ammonium concentration was measured be- fore chemical additions. NaOH was added first to be able to control pH and after that NaClO was added. In the next phase liquid was fed into pipe reactor. Retention time was 15 seconds. Immediately after pipe reactor sample was taken from which ammonium was analysed. Results of the experiment are presented in Table 5 below.

Table 5 Time Ammonium ammonium eluent liquid 100% 100% NaOH initial after reactor flow NaClO addition addition Min mg/l mg/l l/min g/min g/min 0 2573 647 4. 8 51. 86 21.60 30 2046 606 4. 8 56. 18 19.28 60 1888 276 4 52. 08 19.44 90 1823 464 4 45. 00 15.20 120 1667 702 4 49. 90 12.99 150 1500 3. 2 4 30. 83 9.57 180 1391 296 4 31. 59 4.80 210 1105 2. 8 4 23. 70 6.72