The purpose of this invention is to produce mineral fertilizer, like ammonium sulfate, ammonium phosphate and calcium phosphate from liquid waste, such as urine, reject water or any wastewater with an ammonium and a phosphorus content, particularly from a liquid waste having a high ammonium content. In particular, the present invention relates to production of mineral fertilizer, like ammonium sulfate, possibly together with phosphate salt, from human urine and reject water.

Background

There are only a few methods available to treat high ammonium containing liquid waste (urine, wastewater, reject water etc.) and these methods are high energy and carbon consuming. On the other hand, there are no simple methods to produce ammonium-based mineral fertilizers. The method disclosed herein offers one-step solution to harvest ammonia and phosphorus from any liquid waste, which contains high ammonium and phosphorus, particularly urine and reject water, and produce ammonium sulfate, ammonium phosphate and calcium phosphate fertilizer.

Municipal wastewater contains significant amount of plant nutrients; nitrogen (N), phosphorus (P) and potassium (K). Majority of these nutrients are contributed from human waste. In human waste, urine contains significant amount of NPK (Jonsson & al. 2004). If these nutrients could be harvested they can be used as a fertilizer.

Eco-toilet (urine collect separately from feces) is a good idea to collect urine and use as fertilizer. One person can excrete about 550 1 urine/year or about 4 kg of N, 0.4 kg of P and 0.9 kg K per year (Jonsson et al. 2004) and urine has already been successfully used in agriculture (Pradhan et al 2007). Use of urine as a fertilizer has not been successful at a large scale so far, however. There are several constraints which have made urine -based fertilizer unattractive at a large scale, for example: (1) it is expensive to transport large volumes of urine to farms, (2) the application of large amounts of urine is inconvenient, (3) urine has an unpleasant smell and (4) urine is not acceptable in several societies. Therefore, this invention provides a new method to harvest N and P from urine and the end product obtained is similar to mineral fertilizer. Biogas production is an environmentally friendly way to produce energy from different organic wastes and installation of biogas plants are increasing. However, biogas plants also produce significant amounts of reject water (reject water is the liquid fraction produced in the dewatering stage after anaerobic digestion of slurry). Reject water can contain 750-1500mg NH ₄ -N/L (Perez et al, 2007) and up to 130 mg P/L (Pitman, 1999) which contributes 10-30% of the N load and 10-80% of P load of total load in a waste water treatment plant (WWTP) (van Loosdrecht and Salem, 2006). Conventional WWTP equipped with nitrification-denitrification are not suitable to treat ammonium-rich reject water alone (Khin and Annachhatre, 2004). For example, Helsinki WWTP treats total influent of 130 million m3/year (i.e. 6500 tons of N) where reject water contributes 3.3 million m3/year (i.e. 975 tons of N). Reject water treatment is a serious issue in WWTP as it also reduces the efficiency of the wastewater treatment process.

The present state-of-the-art is disclosed for example in the patent document EP 1357087 A2, which discloses the sedimentation of phosphorus and recovery of ammonium in two separate stages. This two-stage process demands separate equipment for both steps being therefore less economical.

Ammonia ( H3) has been recovered using a gas permeable membrane from different sources (Vanotti et al. 2010, Rothrock et al. 2013). NH ₃ and other nutrients have also been extracted from human urine (Morales et al. 2013, Udert and Wachter, 2012).

WO 2006/005733 Al relates to a method for separating magnesium ammonium phosphate (struvite) from undiluted urine by adding a precipitant and by separating the precipitate formed. US 9005333 discloses a method for removing NH ₃ from an ammonia-containing liquid effluent and recovering the N as an ammonium salt, using hydrophobic gas-permeable membranes. In US 2014/019711 1 Al , pH of phosphorus and nitrogen containing wastewater is increased to above 8.5 with lime, the formed P precipitates are separated and the ammonia gas is absorbed and concentrated in an ammonia absorption tower as liquid material. However, a method used in this invention to harvest NH ₃ and P from liquid waste, such as urine has not been attempted previously. Our invention solves the problem in two ways; (1) this method removes the ammonium and P (phosphorus) from liquid waste such as urine or any wastewater, and (2) this method produces ammonium sulfate fertilizer and P fertilizer during the process by harvesting these nutrients. The harvesting process can be as short as 4 hours to harvest more than 90% of N and P and 8 hours to harvest 99%. This method provides a cost- effective alternative to produce ammonium sulfate fertilizer to the conventional ammonium sulfate producing process (Haber-Bosch process) which is high-energy consuming and expensive.

Summary of the invention

It is an object of the invention to provide a novel method for producing mineral fertilizers from ammonium and phosphorus containing liquid waste.

The invention is mainly based on application of Ca(OH) ₂ (calcium hydroxide) or a Ca related substance in liquid waste that contains ammonium and phosphorus, to increase the pH to convert ammonium into NH ₃ gas (dissolved) and to sediment the P present in the liquid waste. The dissolved NH ₃ gas is stripped using aeration and trapped into an acid, such as diluted H2SO4 (sulfuric acid) or H ₃ PC>4 (phosphoric acid). Alternatively, the dissolved NH ₃ gas is trapped using an acid, such as diluted H2SO4 or H ₃ PC>4 running through a GPHM (gas permeable hydrophobic membrane). It is also possible to use a combination of aeration and membrane (GPHM) techniques.

Accordingly, the present invention provides a method for producing mineral fertilizers from ammonium and phosphorus containing liquid waste, wherein the method comprises the following steps:

- application of Ca(OH) ₂ (calcium hydroxide) or a Ca related substance in the liquid waste to increase the pH and to convert ammonium into dissolved NH ₃ gas and to sediment P;

trapping the dissolved NH ₃ gas into an acid or a mixture of acids to form ammonium salts; recovering the sediment of P and the saturated ammonium salts by dewatering and drying.

A further object of this invention is to harvest NH ₃ and P from human urine and to produce a fertilizer containing ammonium sulfate, ammonium phosphate and calcium phosphate.

A still further object of the invention is the use of the method according to the invention for preparing fertilizer from liquid waste, for example from urine or reject water, particularly from human urine.

Brief description of the drawings

Embodiments of the invention will be further described in the following examples and figures of which:

Fig 1 presents the experimental setup for the aeration method.

Fig 2 presents the experimental setup for the membrane method.

Fig. 3 presents the experimental setup for the combination of aeration and membrane methods. Fig 4 presents the amount of harvested NH ₃ from urine using aeration method.

Figure 5 presents the amount of harvested NH ₃ from urine through GPHM (gas permeable hydrophobic membrane).

Figure 6 shows the amount of harvested NH3 from reject water (RW) through GPHM (gas permeable hydrophobic membrane).

Detailed description of the invention

In the present context, the term "liquid waste" comprises any wastewaters having an ammonium content, such as municipal wastewater, reject water, urine, compost leachate, landfill leachate or any other wastewater of industrial, commercial or agricultural origin, which contains also phosphorus in addition to ammonium. The method of the invention is particularly applicable to liquid waste having a high ammonium content, such as 500 mg/1 of ammonium or above, or 5000 mg/L of ammonium or above, but the method can work even with higher ammonium containing liquid waste. Further, the method of the invention is also applicable to liquid waste having an ammonium content of above 50 mg/1 and a phosphorus content of for example <10 mg/1 P to very high concentration of P. Concentrations of for example 50mg PO4-P to 350 mg P/l have been particularly tested.

In an embodiment of the invention, liquid waste comprises urine, either human or animal urine. Human urine can be collected for example from urine separating toilets and used in the method of the invention after a storage time which can be for example a week when mixed with old urine, several months or even a year. It is also possible to mix urines stored for different time periods. Urea present in fresh urine dissociates rather rapidly to ammonium ions and carbon dioxide and therefore pH of urine increases.

The starting pH of the liquid waste to be treated with a method of the invention may vary. For a human urine, the starting pH is usually about pH 9, while pH of reject water is about pH 8.

First, the pH of the liquid waste is increased to pH at or above 12 by adding a Ca containing base, preferably calcium hydroxide Ca(OH) ₂ or other Ca related substance and by mixing the Ca containing base properly with the liquid waste. For example in the case of urine, increasing the pH of the liquid waste to above 12 converts more than 99.9% of the ammonium nitrogen in the liquid waste into ammonia gas (dissolved) and at the same time sediments more than 99% of phosphorus.

Calcium hydroxide is a preferred chemical for the method of the invention as it is more effective, less aggressive and comparatively cheaper but also other Ca containing substances, such as calcium carbonate and calcium oxide can be used. In the method of the invention, calcium is recovered as calcium phosphate which can be used as a fertilizer. Phosphorus as calcium phosphate is more bioavailable compared to commonly recovered P as struvite. The dissolved ammonia gas is a) stripped using aeration and trapping into an acid; or b) trapped by using an acid that runs through a gas permeable hydrophobic membrane (GPHM); or c) it is stripped by using a combination of the techniques a) andb). The acid may be any acid that can form a salt of with the ammonium ion, such as sulfuric acid, phosphoric acid or a mixture of these. A preferred acid for the method of the invention is sulfuric acid or a mixture of sulfuric acid and phosphoric acid. The ammonium sulfate formed in the reaction of sulfuric acid and ammonium ions is readily usable as a fertilizer as such or mixed with other fertilizer ingredients. Also the ammonium phosphate formed in the method of invention where phosphoric acid is used, is ready to be used as a fertilizer.

Aeration

In one embodiment of the invention, the ammonia gas is stripped using aeration. The rate of aeration depends on the system applied and can be for example 1200 ml/min/L. The higher the aeration, the faster the N harvesting will occur. The stripped gas is passed into an acid, preferably sulfuric acid, where ammonia reacts with the acid and forms an ammonium salt. The process can be continued until the formation of the ammonium salt is saturated. At the same time, P has been precipitated with the calcium related substance, such as calcium hydroxide.

The aeration technique is more effective at 30 to 40 °C. Operation of this process in less than 30 °C is not very efficient.

Membrane technique

In one embodiment of the invention, the ammonia gas is passed through a gas permeable hydrophobic membrane (GPHM) and contacts an acid, which is preferably circulating inside a GPHM tube. In the GPHM tube, ammonia reacts with the acid and forms ammonium salt. The process can be continued until the formation of ammonium salt is saturated. It is also possible to use a flat GPHM through which the ammonia gas passes to react with an acid or a mixture of acids on the other side of the GPHM.

Commercially available GPHM made up of PTFE materials having a wall thickness of for example 0.01 -0.045 inch (0.02-0.12 cm) are suitable for the purposes of the invention. However, also other GPHMs are applicable as long as ammonia gas is able to pass through the membrane.

At the same time, P is precipitated with a calcium related substance, such as calcium hydroxide. The membrane technique is more efficient at 30°C but this process can harvest significant amount of N and P even at 8°C.

Combined membrane technique and aeration (continuous pilot plant) An example of a continuous method that combines membrane technique and aeration is illustrated in Figure 3. The illustrated pilot plant comprises a pretreatment unit, a GPHM unit, an aeration unit and a sedimentation unit.

Pretreatment unit

In an embodiment of the invention which combines aeration and membrane techniques, the pH of the liquid waste is preferably adjusted in a pre -treatment unit. Calcium hydroxide or a Ca related substance is added and mixed well (for example 30 to 45 minutes) to increase pH above 12. Adjusting and keeping pH above 12 turns more than 99% of ammonium present in the liquid waste, for example urine and reject water, into dissolved ammonia gas. At the same time, pH above 12 sediments phosphorus present in the liquid waste.

GPHM unit

In the next step, the upper layer of the liquid waste from the pretreatment unit is passed into a GPHM column. In the column an acid or a mixture of acids is circulating inside the GPHM tube. Preferred acids include for example sulfuric acid and phosphoric acid. The ammonia gas passes through the GPHM tube and contacts the acids which are circulating inside the tube. Ammonium salts, such as ammonium sulfate and ammonium phosphate, are formed.

The process will be continued until the ammonium salts are saturated or the pH of the acid reaches up to pH 4-6. The saturated liquid ammonium salts are dewatered, dried and recovered as crystal ammonium sulphate containing about 20% N. Aeration unit

After a retention time in GPHM unit, the liquid waste is passed into an aeration column. The retention time may be a couple of hours, for example 4 hours. In the aeration unit, the liquid waste is aerated to harvest any residual ammonia. The aeration also decreases the pH of the effluent.

The stripped ammonia (air) is passed into an acid or a mixture of acids, where the ammonia reacts with the acid(s) and forms ammonium salts. Preferred acids are sulfuric acid and phosphoric acid, which lead to formation of ammonium sulfate and/or ammonium phosphate.

The stripping will be continued until the acid solution is saturated. The exhausted air from the acid bottle can be passed to the environment. The saturated liquid ammonium salts are dewatered, dried and recovered as crystal ammonium salts containing about 20% N.

Sedimentation unit

The treated liquid waste passes into the sedimentation unit where residual calcium phosphate, any magnesium phosphate, calcium carbonate and other P compounds are sedimented and the effluent is disposed. The sediment is dewatered, dried and pulverized to produce a powder, which contains about 1.5% P as phosphates. Separation of fertilizer

In the final steps of the method, the saturated liquid ammonia sulfate obtained via any of the above alternatives is dewatered, dried and thus a crystal ammonium sulfate product is formed. The ammonium sulfate produced contains about 20% N.

The precipitate of P which has been sedimented with Ca(OH) ₂ or other Ca based substance is also dewatered and dried and optionally pulverized to produce a powder, which contains about 1.5% P mainly as Ca-P04 and Mg-P04 compound.

The method of the invention is preferably performed at room temperature, but can be accomplished within a temperature range of from 8 °C to 30 °C when membrane technique (GPHM) is used. When aeration technique is applied, operating temperature is preferably from 30 °C to 40 °C.

The method of the invention is able to harvest N and P within a few hours, for example in 8 hours, but also longer times are possible and within the scope of the invention. When membrane technique (GPHM) is used, 99% of N and P can be harvested within 8 hours at 30 °C and within 24 hours at 8 °C. When aeration technique is used, 90-99% of N and P can be harvested within 24 hours at 40 °C and within 36 hours at 30 °C.

The present invention thus provides a method, which enables to turn more than 99% of ammonium present in liquid waste, such as urine or reject water, into ammonia gas and to recover 99% of nitrogen present in said ammonia gas. At the same time more than 99% of the phosphorus of the liquid waste, such as urine, is recovered. The present method has also successfully harvested N and P from reject water and produced ( H4) ₂ S04, ammonium phosphate and calcium phosphorus compound sediment.

A further advantage of the present invention is that sedimentation of phosphorus and recovery of ammonia take place in the same step, which makes the method uncomplicated and economically attractive. It is also possible to concentrate the harvested ammonium salts, such as ammonium sulfate and ammonium phosphate, by using a commercial reverse osmosis membrane. Another possibility for further treatment of the concentrated ammonium sulfate and ammonium phosphate is drying in a hot air oven, thus producing a crystal fertilizer.

It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

Example 1 (Aeration method) Materials and Methods Urine was collected from a urinal placed in Helsinki festival in summer 2014.

Experiment 700 ml of urine was taken into 1 L glass bottle and 14 g of Ca(OH) ₂ was added. The solution was stirred for 30 minutes to reach the pH about 12±0.5. The urine was placed into water bath at 40°C. 150 ml of 1M H ₂ SC>4 was taken in 250 ml bottle. Now, the urine bottle was aerated with 1000-1200 ml/min air using a diffuser for ammonia stripping. The stripped air from the urine bottle was passed into the 150 ml sulfuric acid (Figure 1). In this experiment pH of the urine was kept above pH 1 1 for the entire experiment. After 12 hours of experiment, 2.5 g of Ca(OH) ₂ was added again into urine to maintain the pH >11. The experiment was carried out for 28 hours at 40 °C. similar experiment was repeated for 36 hours at 30 °C. Results

After 28 hours of stripping the result showed that 98% of ammonium-N stripped, and among them 98% of ammonium was recovered in H ₂ SC>4 (Figure 3). In fact we found that 1 1 of urine could produce about 20 g (dry weight) of ammonium sulfate compound which could further be used as a commercial fertilizer. Besides this, phosphorus is recovered as sediment with used Ca(OH) ₂ . The result showed that 99% of the P in urine is harvested in sediment i.e. about 15 g P/kg sediment.

Economic consideration, In general, 1 m ³ of urine contains 4.5 kg of NH ₄ -N and 0.35 kg of total-P ^virhe - ^{viitteen iahdetta ei 18ytynyt} \ Among them, 4.2 kg of NH ₄ -N and 0.35 kg of total-P can be harvested using 20.9 kg of H ₂ S0 ₄ (98%) and 22 kg of Ca(OH) ₂ . The cost of the used H ₂ S0 ₄ will be€6 (€290/ton), and the cost of the Ca(OH) ₂ will be€2 (€93/ton). The energy needed to aerate 1 m ³ of urine is about 10 cents (calculated as 0.025 kWh/nm ³ and€0.072/kWh in Finland). In the revenue generation segment; the price of the harvested 25.2 kg of ( H4) ₂ S04 (13% N) will be€7.3 (€461/ton of ( H ₄ ) ₂ S0 ₄ of 21% N, Cemagro Finland), the price of PO ₄ -P will be€0.4 (€1363/ton of phosphate, USDA 2013) and the price of 30 kg of CaC0 ₃ will be€4.05 (€135/ton of 36% Ca, Nordkalk). Thus, the total production/treatment cost is€8.1 (€6 +€2 +€0.1) and the total revenue generated is€1 1.75 (€7.3 +€0.4 +€4.05). Based on the calculation the harvesting of N and P from 1 m ³ of urine can make a profit of€3.65. The price of ammonium sulfate is calculated as the price of N in ammonium sulfate and the price of Ca(OH) ₂ is calculated as the price of Ca. Investment and labor costs are not included in the calculation.

Example 2 (GPHM method)

Materials and Method- Membrane method Urine was collected from a urinal placed in Helsinki festival in summer 2014.

About 700 ml of urine was taken in a 1 1 glass bottle and 14 g of Ca(OH) ₂ was added to achieve pH 12±0.5. 100 ml of 1 M H ₂ SO ₄ was taken in a 250 ml bottle. Gas permeable hydrophobic membrane (GPHM) was coiled and connected with PVC pipe (figure 1). The coiled GPHM was submerged into the urine and the PVC pipe was passed through a pump, each end of the PVC pipe was submerged into the H ₂ SO ₄ . The acid was circulated at 30 ml/min and the urine was stirred at 250 rpm. During the circulation process the gas ammonia passes through GPHM, reacts with circulated acid and forms ammonium sulfate. The acid and urine samples were analyzed every 4 hours. The experiment was conducted at 8 °C for 24 hours, at 20 °C 16 hours and at 30 °C for only 8 hours. Results

The result showed that 99% of ammonium-N was harvested using membrane at different temperature (figure 5). In fact we found that 1 1 of pure urine can produce about 20 g of ammonium sulfate/ammonium sulfate like compound (dry weight) which can be used as a commercial fertilizer. Besides this phosphorus is recovered as sediment with used Ca(OH) ₂ . The result showed that 99 % of the P in urine is harvested in sediment i.e. about 15 g P kg sediment.

Basic economic consideration; 1 m ³ of urine contains 4.5 kg of NH ₄ -N and 0.35 kg of total-P (Jonsson et al 2004). Among these N and P, 4.4 kg NH ₄ -N and 0.35 kg of total-P can be harvested using 20 kg of H ₂ S0 ₄ (98 %) and 18.6 kg of Ca(OH) ₂ . The cost of H ₂ S0 ₄ and Ca(OH) ₂ will be€ 5.8 (€ 290/tons, Rothrock et al. 2013) and€ 1.7 (€ 93/tons, Miller 2012), respectively. The energy needed for pumping acid for 16 hours is about€0.04 and drying the acid is€ 1 (calculated as 0.4 kWh and€ 0.072/kWh in Finland). The acid needed to neutralize the pH of effluent is€ 0.4. The price of the harvested (NH ₄ ) ₂ S0 ₄ (i.e. 23.3 kg) will be€ 9.72 (€ 461/ton of ( H ₄ ) ₂ S0 ₄ ), price of P0 ₄ -P will be€ 0.5 (€ 1363/ton phosphate) (USDA 2013) and price of CaC0 ₃ will be 3.24. In quick calculation harvesting of ammonium and phosphorus from 1 m ³ of urine can make profit of€ 4.5. The investment cost is not included in the calculation.

In the future the plant will be optimized so that the treatment cost will be decreased and efficiency of recovery of ammonia will be increased which means the profit will also be increased. In conclusion, ammonium harvesting from urine using both methods (aeration and GPHM) has been successful and the method is profitable.

References

Patent documents

EP 1357087 A2

WO 2006/005733 Al

US 9005333

US 2014/019711 1 Al

Other references

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Khin, T., Annachhatre, A.P., (2004). Biotechnol Advances. 22, 519-532.

Morales, N., Boehler, M.A., Buettner, S., Liebi, C, Siegrist, H. 2013. Water, 5, 1262-1278. Pradhan, S.K., Nerg, A.M., Sjoblom, A., Holopainen, J.K., Heinonen-Tanski. H., 2007. Journal of agricultural and food chemistry 55, 8657-8663.

Perez, R., Gali, A., Dosta, J., Mata-Alvarez, J. (2007), water. Ind. Eng. Chem. Res. 46, 6646- 6649.

Pitman A.R. (1999). Water Res., 33, 1 141-1 146.

Rothrock Jr. M.J., Szogi A.A., Vanotti M.B. 2013. Waste Management 33, 1531-1538.

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