It is important to limit ammonia emissions due to their harmful impact on the environment. Excessive ammonium load 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 effects. Excessive nitrogen supply to areas in natural condition may change the balance of the vegetation, promote the evaporation of nitrous oxide known as a greenhouse 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 and further, for instance alkyl benzene sulphonates by a process using activated carbon. Generally, while the waste water already is very pure after these treatments, ammonium nitrogen is still a problem. It is difficult to remove ammonium nitrogen from waste water since the solubility thereof in water is high and it is a very stable compound in the form of ammonia.

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 chemical processes such as electrodialysis, reversed osmosis, stripping, chlorination, and ion exchange. Each of these processes has its assets and

drawbacks. Ion exchange was found out to be the most common process with several published variations and modifications.

As in known, ammonium nitrogen is adsorbed in ion exchangers to a zeolite material or e. g. to silica and the exhausted exchanger mass is regenerated chemically or biologically or with both ways. The details of the systems used vary according to the process. In US 3 929 600, regeneration is carried out using an alkaline eluate with alkali metal chlorides, the chlorine being further utilized to decompose ammonium nitrogen. In US 4 522 727 the regeneration is accomplished using gas containing oxygen, and in this case high temperatures are optionally necessary for the desired reaction. In general, potassium or sodium salt solutions containing for instance nitrates, carbonates and sulfates are used for the regeneration as described in US 4 695 386 and US 4 098 690.

From US 4 772 307 a multistep process for producing a fertilizer from waste water is known. In this process, waste water is passed via a first clarification tank to a mixing tank for the contact thereof with zeolite particles (preferably clinoptilolite) having a particle size of less than 200 jim, the waste water and the zeolite particles being further passed together via an adsorption zone and an aeration tank to a second clarification tank. Among others, the phosphates and part of the ammonium ions present in waste water are thus removed therefrom by the zeolite particles.

Excess sludge from the clarification steps containing for instance ammonium clinoptilolite formed is removed from the process and may be used in agricultural applications. Pretreated waste water leaving the second clarification tank is passed through a zeolite bed to bind the residual ammonium ions to the zeolite. While this publication teaches that the exhausted zeolite is preferably regenerated or deammoniated biologically, it also states that the zeolite bed may be regenerated with a regenerating solution containing a potassium salt. The solution from the regeneration containing ammonium may then be used directly as a plant nutrient, or it may be mixed with said excessive sludge to give high quality fertilizer. The publication also suggests the aeration of the exhausted potassium salt solution to form ammonia that may be absorbed to a phosphoric acid solution to give a liquid fertilizer. It is evident that the total process is complicated.

DE 41 19 869 discloses a ring made of zeolite soaked with a carboxylic acid before the adsorption of ammonium nitrogen or ammonia. The purpose of the carboxylic acid treatment is to desinfect the zeolite serving as a cultivation medium before use.

The ring is useful for biological removal of ammonium nitrogen or ammonia from

drinking water, service water or waste water by means of nitrite bacteria (nitrosomonas).

One of the problems associated with the ion exchange process is to find a suitable eluent solution for ammonium. Sodium sulphate, potassium nitrate and sodium chloride generally used may bring about precipitation problems or additional nitrogen emissions. Moreover, it is difficult to concentrate ammonium effectively enough from waste water.

As adsorbed ammonium was treated in the ion exchanger of the recovery process, it was surprisingly found that concentrated formate solution acts as a proper eluent.

Since most formates are readily dissolved in water, very concentrated solutions could be used without precipitation problems.

The object of the invention is to provide an effective and economic way to remove ammonium nitrogen from waste water and to recover it for reuse for instance as a fertilizer.

According to the invention, a process is provided for removal of ammonium nitrogen from waste water for a useful application using a chemical process based on ion exchange, comprising the following steps: i) as the first step, the adsorption of ammonium nitrogen contained in waste water to an adsorption mass in an ion exchanger, ii) as the second step, the desorption of ammonium nitrogen contained in the adsorption mass of the ion exchanger by means of an eluent solution containing a metal salt of a carboxylic acid, the adsorption mass being at the same time regenerated by the metal ions of said metal salt, iii) as the third step, the clarification of the solution from the second step containing ammonium nitrogen, iv) as the fourth step, the aeration of the clarified solution containing ammonium nitrogen thus liberating ammonium nitrogen as gaseous ammonia, and v) as the fifth step, the absorption of the gaseous ammonia from the fourth step for a useful application.

In addition, the process of the invention may comprise before the first step a step of saturating the adsorption mass of the ion exchanger with said metal ions to be exchanged by using a solution of a carboxylic acid metal salt.

Preferably, said carboxylic acid metal salt is an alkali metal salt of a carboxylic acid having 1 to 3 carbon atoms. Carboxylic acids having 1 to 3 carbon atoms include the formic, acetic, and propionic acids. Alkali metal formates, especially sodium or potassium formate are particularly preferable.

The concentration of the carboxylic acid metal salt solution, being preferably an alkali metal formate solution, used for saturating the adsorption mass of the ion exchanger may be below 6 moles/l, preferably 1 to 4 moles/1.

The concentration of the carboxylic acid metal salt solution, being preferably an alkali metal formate solution, used for eluting ammonium ions may be 0.2 to 6, preferably 4 to 6 moles of the metal per liter.

Adsorption yield of ammonium nitrogen after the first and the second steps is preferably at least 90%, particularly preferably at least 95%.

Waste suitable for a useful application is removed from the clarification step.

In the fourth step, the pH of the clarified ammonium nitrogen solution is preferably adjusted with a base such as an alkali metal hydroxide to a value between 10 and 12. It is preferable that the cation of the base corresponds to the ion to be exchanged in the ion exchanger, preferably a Na or K ion. Aeration is preferably carried out with carbon dioxide free air to liberate pure gaseous ammonia to said aeration air. In the fourth step, pure ammonia is removed, the yield thereof being preferably in the range from 70 to 90%.

In the fourth step, the pH of the aerated solution containing a metal salt of a carboxylic acid, preferably an alkali metal formate, is preferably adjusted to a value between 5 and 9 and recycled to the second step to be used therein as the eluent solution. The pH adjustment may be carried out with a carboxylic acid, preferably formic acid. Also other acids such as hydrochloric acid or nitric acid may be used.

In the fifth step, the gaseous ammonia may be absorbed to a fixed bed impregnated with an acid, or to an acidic or neutral washer containing liquid.

Preferably, said fixed bed consists of porous natural material such as clay material, preferably perlite or damoline. The acid used to impregnate the fixed bed is

preferably sulphuric acid, phosphoric acid or a mixture of these acids. The acid concentration of the fixed bed may be between 60 and 70%.

The useful waste leaving the fifth step may be used either as such as a nitrogen fertilizer or a preservative, or as the starting material therefor.

According to the invention, the first and the second steps may also be carried out using two parallel ion exchangers in different phases, one of them being in the adsorption phase and the other being in the desorption phase, or vice versa.

The invention is also directed to a fertilizer product, suitable as a nitrogen fertilizer, obtained according to the process of the invention.

The invention is now described with reference to the appended figure 1 showing schematically a preferable process of the invention.

In the following description, while sodium formate solution is used for elution, it is clear that also other carboxylic acid metal salt solutions are useful in the present invention.

As shown by the Figure 1, waste water is first passed in the first step to an ion exchanger a containig a suitable adsorption material such as commercially available zeolite including Greek, Cuban or Australian clinoptilolite, phillipsite or synthetic zeolite, preferably Australian clinoptilolite being mechanically sufficiently strong to resist erosion during ion exchange, sufficiently regenerable, having no memory effect, and causing moderate operation cost. Adsorption material is loaded with sodium ions by saturating it for instance with concentrated sodium formate solution having a concentration below 6 moles/1, preferably 1 to 4 moles/1, and after the saturation, by removing the liquid sodium formate solution as completely as possible with suction, for instance by means of a vacuum, and thereafter, waste water is passed through said adsorption material with a flow rate ensuring a sufficient residence time for the adsorption of ammonium ions, the flow rate being not more than five times the volume of the ion exchanger used per hour, preferably not more than three times the volume of the ion exchanger used per hour. Passing waste water through the ion exchanger is continued until 70% of the adsorption capacity is exhausted. In this first step, pure water is obtained from the ion exchanger as the effluent.

In the second step, ammonium ions are desorbed from the adsorption material of the ion exchanger a by circulating therethrough for few hours an amount of neutral sodium formate solution corresponding to volume of the material, the pH of the solution being between 5 and 9, preferably 7. The concentration of the sodium formate solution to be circulated may vary between 5 and 400 grams of Na per liter (corresponding to 0.2 to 6 moles of Na per liter). No desorption takes place with more diluted solutions. Ammonium nitrogen stays in the eluent solution and at the same time, the adsorption material is regenerated with Na ions. Eluent solution now contains ammonium nitrogen and sodium formate. The yield of the adsorbed ammonium ions is at least 90%, preferably 95%.

In the third step, the formate solution containing ammonium nitrogen is passed to a clarifier b shown in Figure 1 to remove any precipitate passed through the ion exchanger a containing Ca and Mg ions less soluble than Na ions, for instance dead microbial waste such as algae or bacteria, for optional useful applications.

According to the type of the waste water used, the precipitate formed may consist of material that may be recycled by composting or reused as nutrient for microbes.

In the fourth step, the pH of the clarified solution containing ammonium nitrogen is adjusted e. g. with NaOH to a value between 10 and 12 at the temperature of 25 °C.

A higher pH value is needed if the temperature is lowered. Thereafter the formate solution with the controlled pH value is aerated in the aeration apparatus c shown in Figure 1 where the solution is purged with carbon dioxide free air from a compressor via an air flow manifold. High pH value is favourable for the aeration and evaporation of ammonia, and reduces the formation of precipitates. Aeration time is for instance 10 to 12 hours at 30 °C. According to the aeration time, the ammonia yield may vary between 70 to 90%. Ammonium level remaining in the eluent solution is preferably below 100 ppm. Foaming during aeration may be controlled and prevented by using antifoam agent containing silicate. In the aeration step, pure gaseous ammonia is obtained as the product.

In the fifth step, gaseous ammonia leaving the aeration unit c is absorbed into an absorption apparatus d as shown in Figure 1, being for instance a fixed bed impregnated with an acid. Alternatively, ammonia may be absorbed into an acidic or neutral gas washer. The fixed bed consists of a porous natural material such as commercially available clay materials, including perlite, vermiculite or damoline, and the mineral acid used for impregnation of the bed, such as sulphuric acid, phosphoric acid or the mixture thereof. The acid concentration of the fixed bed may

be 60 to 70% by weight. The fixed bed consists preferably of damoline or perlite soaked with phosphoric acid. If a mixture of sulphuric acid and phosphoric acid is used, the amount of phosphoric acid relative to sulphuric acid is preferably 25 to 75%. The gas washer used alternatively may for instance be a countercurrent washer with circulating phosphoric acid, nitric acid, sulphuric acid or formic acid.

The absorption apparatus d used in the process of the invention is capable of absorbing 80 to 90% of the ammonium relative to the theoretical value (14%). A recycled product suitable for reuse is thus obtained as a liquid solution, slurry or solid matter according to the recovery method. Depending on the chemicals and absorption methods used, this fixed bed produces a solid product containing ammonium phosphate useful as such as a fertilizer, and the gas washer produces ammonium salts useful as starting materials for fertilizers or formates useful as preservatives.

In one embodiment of the invention, the step a shown in Figure 1 may be carried out by using two or more parallel ion exchangers operating in different phases relative to one another. In this case, during the regeneration of the one adsorption mass, waste water may passed to another adsorption mass used in parallel, if necessary. The exhausted adsorption mass is washed with pure water, the washing water is removed with suction and an amount of neutral or slightly basic sodium formate solution corresponding to one volume of the mass is circulated therethrough for few hours to release adsorbed ammonium ions. With this procedure, during the adsorption phase of the other mass, the mass is also loaded with Na ions. So the masses in different phases may be used alternately thus providing a continuous process for the recovery of ammonium nitrogen.

According to Figure 1, the solution containing formate may be recycled from the aeration step c to the ion exchanger a of the second step following the adjustment of the pH thereof to a sufficiently low operation value, being between 5 and 9, for instance with formic acid. This recycling makes the process more profitable.

According to the invention, carbon dioxide free air acting as the vehicle for gaseous ammonia leaving the absorption apparatus d in the fifth step may be recycled to the aeration apparatus c of the fourth step for reuse, as shown in Figure 1.

In the process of the invention for the recovery and reuse of ammonium nitrogen, the waste water feed may be wastewater having solid matter removed for instance by sedimentation in a municipal waste water treatment plant, and having an

ammonium nitrogen content below 500 mg/1. The waste water used is preferably municipal waste water.

In contrast to biological treatment processes, the use of the formate solution according to the invention in the chemical process for recovery of ammonium nitrogen allows the recovery to be carried out even at low temperatures, such as at- 20 °C since the use of concentrated salt solutions is still possible under such conditions. In addition, the inorganic or microbial precipitate obtained in the ion exchange as a by-product may be recovered and reused. As is known, formate inhibits bacterial and microbial growth in the process apparatus.

Formates being highly soluble in water, less disturbing precipitates are formed in the process relative to the use of other salts in eluent solutions. Moreover, by using concentrated formate solution, ammonium nitrogen may be removed efficiently and economically. An advantage of formates is also the fact that they are biodegrable and they control bacterial growth, thus preventing the adsorption material from becoming mucous in continuous operation.

The invention will now be illustrated with examples without limiting the invention thereto.

Example 1 Waste water from a municipal waste water treatment plant was fed to a process for recovery of ammonium nitrogen following the removal of solid matter from the waste water by sedimentation and chemical purification thereof, that is, the chemical precipitation of e. g. phosphorus. In the process, a zeolite mineral column was used for ion exchange, the zeolite being 0.2 m3 Australian clinoptilolite. The zeolite column was loaded with sodium ions by percolating therethrough sodium formate solution having a concentration of 1 mole/1 (Perstorp) for 2 hours.

Waste water containing 35 mg of ammonium nitrogen per liter was passed through the zeolite column with a flow rate of two times the volume of the column per hour.

The amount of the waste water to be passed through the column was estimated on the basis of the ammonium nitrogen contained therein. Waste water was passed through the zeolite column for a period sufficient to exhaust 70% of the ammonium nitrogen adsorption capacity of the zeolite (7 kg NH4 per 1 tonne of zeolite).

The column was then washed with pure water, the washing water being drained off as completely as possible. The zeolite column was regenerated with sodium formate solution containing sodium 25 g/1 and having a pH of 7, the volume of the solution corresponding to that of the column. The regeneration was carried out by pumping the solution through the column for one hour, while recycling it. Finally, the regenerating solution was pumped via a clarification step and a step for pH adjustment to aeration. Following removal of the regenerating solution, waste water flow through the regenerated zeolite was restarted.

The pH of the regenerating solution containing ammonium eluted from the column was adjusted to a value of 12 with sodium hydroxide (Na 25 g/1) and the solution was passed to an aeration tank. In the aeration tank, the regenerating solution was purged with air for 20 hours, the volume of air being 2500 times the volume of the solution per hour. The air was depleted of carbon dioxide and other impurities by activated carbon filtration. The ammonia contained in the regenerating solution, for instance 1 g/1, was entrained with the air. Following neutralization, the aerated sodium formate solution was suitable for recycling to the zeolite mineral column as the regenerating/eluting solution.

Ammonia entrained with the air was washed in the countercurrent washer using 1M nitric acid to give ammonium nitrate solution that was further processed by crystallization and filtering to obtain a raw material for nitrogen fertilizer.

In case of a plant treating waste water produced by 100 000 people, 70 to 90% of the ammonium contained in waste water was removed with the process described and used for producing 3500 tonnes raw material per year for a fertilizer.