IMPROVED METHOD FOR TREATING A LIQUID WASTE FRACTION FROM ANIMAL WASTE

Abstract

The present invention relates to an improved process for downstream processing of liquid waste fraction from mammalian, such as animal, waste. More specifically the present invention provides an improved and simplified method for downstream processing of liquid fractions of animal waste in which the nitrogen content is preserved and for inhibiting urease activity in said liquid.

Claims

1. A method for treating a liquid fraction of a waste matter from a mammal, such as an animal, or for preserving nitrogen in waste matter from a mammal, such as an animal, the method comprising providing a liquid fraction of a waste matter from a mammal, such as an animal, said fraction being urea rich; allowing the fraction to settle to provide a supernatant and a sediment; feeding the supernatant to an evaporating unit; and subjecting the supernatant to an evaporating step, wherein the evaporating step takes place at a pressure below atmospheric.

2. The method of claim 1, wherein the pressure below atmospheric is between 1 and 20 kPa.

3. The method of claim 2, wherein the evaporating step provides a urea fraction and a condensate fraction, substantially comprising water.

4. The method of claim 1, wherein the liquid fraction of a waste matter is provided by a method comprising the steps of: collecting waste matter from a mammal, such as an animal; inhibiting urease activity in the collected waste matter; and separating the urease-activity inhibited waste matter to provide the urea-rich liquid fraction of a waste matter from a mammal, such as an animal, and a urea-lean fraction.

5. The method of claim 4, wherein the collected waste matter is collected from a barn comprising a stable floor in which a separation means is installed underneath the stable floor having slats and in which the separation means comprises a conveyor belt or a pipe and scraper system, said conveyor belt conveying the waste matter for processing in a continuous manner.

6. The method of claim 5, wherein the conveyor belt is installed with an inclination, wherein the inclination of the conveyor belt is relative to a horizontal having an angle of greater than or equal to 2 degrees and less than or equal to 8 degrees.

7. The method of claim 6, wherein the waste matter is substantially liquid and is collected as liquid waste matter drained from the conveyor belt having the inclination.

8. The method of claim 3, wherein inhibiting urease activity in the collected waste matter is obtained by inhibiting the waste matter by one or more of a physical means, a chemical means, or both a physical means and a chemical means.

9. The method of claim 8, wherein the physical inhibition is a UV light, comprising at least two different wavelengths.

10. The method of claim 8, wherein the chemical inhibition is obtained by one or more of a reversible inhibition, or an irreversible inhibition.

11. The method of claim 10, wherein the reversible inhibition and/or the irreversible inhibition is obtained by adjusting the pH of the waste matter, and wherein the pH of the water is greater than or equal to 3.

12. The method of claim 10, wherein the irreversible inhibition is obtained by using one or more of: an N-(n-butyl) thiophosphoric triamide (NBPT), a salicylhydroxamic acid (SHAM), or a thymol.

13. The method of claim 12, wherein the irreversible inhibitor is the salicylhydroxamic acid and wherein the amount of acid added is greater than or equal to 45 mg/L and less than or equal to 55 mg/L.

14. A method for inhibiting urease in a liquid fraction of a waste matter from a mammal or for providing a urea rich liquid and/or preserving nitrogen in waste matter from a mammal, the method comprising: providing waste matter from a mammal, such as liquid waste or substantially liquid waste; and inhibiting urease present in the waste matter, wherein inhibiting urease present in the waste matter is obtained by adjusting the pH of the waste matter to a pH greater than or equal to 3 using an inhibitor comprising one or more of: an N-(n-butyl) thiophosphoric triamide (NBPT), a salicylhydroxamic acid (SHAM), or a thymol.

15. The method of claim 14 further comprising treating the urease inhibited waste matter.

16. The method of claim 1 wherein the pressure below atmospheric is between 2 to 10 kPa.

17. The method of claim 10, wherein the reversible inhibition and/or the irreversible inhibition is obtained by adjusting the pH of the waste matter, and wherein the pH of the water is greater than or equal to 12.

Description

[0041] The present invention will now be described in greater detail with reference to the appended drawings in which

[0042] FIG. 1 illustrates the process of the invention in its broadest sense.

[0043] FIG. 2 illustrates an embodiment of the invention in which upstream process steps for treating liquid waste are included.

[0044] FIG. 3 illustrates inhibition of urea versus urine with Agrotain. The dotted line is urea and the solid line is urine.

[0045] FIG. 4 illustrates the effect of inhibition at various pH values according to the invention and comparative, respectively.

[0046] Referring now to FIG. 1 the process of the invention is illustrated schematically relating to waste matter obtained from animals. Liquid waste matter rich in urea (such as by being urease inhibited or free of urease) is provided by upstream processing as will be explained in greater detail below. The liquid feed is provided to the settling tank, 1, where the liquid is allowed to sediment for a suitable period of time. In the context of the present invention a suitable period of time is determined by the number of animals, such as pigs, particle size of suspended solids and tank size. Typically, but not limited to, 1 hour to 5 days.

[0047] After sedimentation the liquid supernatant is fed e.g. by a liquid pump, 2, to a low pressure evaporator, 3. From the low pressure evaporator, 3, the evaporated liquid is condensed in condensate tank, 4, and the urea rich fraction (liquid) is stored in a urea tank, 5. According to the present invention the evaporation should be performed at low pressure, i.e. below atmospheric. Suitable evaporators for the present invention should be capable of large scale evaporation; commercially available evaporators can be obtained, for example, from the firm Veolia under the trade name Evaled. Other commercially available low pressure evaporators are suitable and known to the skilled person. The evaporation concentrates the liquid. The evaporation takes place at a pressure in the range of 1 to 90 kPa, preferred 2 to 20 kPa, more preferred 5 to 10 kPa. It is preferred that the liquid is concentrated to a urea concentration of at least 15% w/V, more preferred at least 20% g N (in urea)/L, such as 15% to 50% g N (in urea)/L, 20 to 30% g N (in urea)/L. According to the invention as much as 70-80% w/V of the nitrogen (in urea) in the original waste matter is recovered in the urea fraction, in preferred embodiments almost 100% w/V urea is recovered. This is contrary to the prior art methods that denitrify the manure and recover less than 20% w/V after concentration. Consequently, with these new downstream processing steps, a complete process for recovery of waste matter can be provided for which it has proven possible to retain as much nitrogen as possible also when the volume of the matter is reduced to a manageable volume. Consequently, the need for supplemental nitrogen for use, e.g., as fertilizer is reduced and a simple solution is provided for farms or plants that wish to have a state of the art environmental program in which waste is turned into a valuable product in an efficient manner and without the need for supplemental nutrients. Furthermore, the process is simple to retrofit in existing farms and plants and the operation of the process requires very little input of power.

[0048] Referring now to FIG. 2 the invention is illustrated with a further inclusion of upstream processing and preparation of the liquid waste matter in addition to the features illustrated in FIG. 1. Prior to being fed to the settling tank, the liquid waste is provided by collection in a barn, 6. The collection is either by the use of conveyor belts continuously transporting the waste matter from the barn or by regular scraping of the stable floor and collecting the waste matter in an appropriate manner. In the preferred embodiment the waste matter is removed from the barn in a continuous manner by a conveyor belt and more preferred where the conveyor belt is slightly skewed allowing the liquid waste to drain from the belt. It has been found that when the belt skews specifically with an angle to the horizontal of 2 to 8, preferably, 3 to 5, more preferred 4, it will substantially only be the liquid waste that drains off the belt. If the angle is higher than 8, solids will drain too, in particular when the solids are semisolid. If the angle is lower than 2 no separation will take place. After the initial separation in liquid and solid, the liquid fraction is provided to a sump, 7, for inhibition of urease activity. In the embodiment shown, both physical and chemical inhibition takes place but it is also contemplated that the inhibition may be either physical or chemical. In the sump, 7, irradiation with UV light using an UV lamp, 8, and addition of chemical inhibitor fed from an inhibitor tank, 9, through a liquid pump, 10, takes place. From the sump, 7, inhibited liquid waste matter is transported, e.g. by use of a liquid pump, 10, or by gravity as appropriate to the settling tank, 2. It is also contemplated that the inhibition takes place before separation into solid and liquid. If a sediment is formed in the sump, 7, it may be returned to the barn, 6, through a recirculation pump, 11, for reuse in a solid treatment process.

[0049] In the following, the invention will be described in more details with reference to all embodiments of the invention.

[0050] Within the context of the present invention it is intended that the expression waste matter designates matter discharged from the body of a mammal, such as from an animal or human, in particular a farm animal such as pig, cow, sheep, etc. Waste matter comprises liquid excrete, e.g. urine and further waste matter includes solid excrete, e.g. faeces. In a preferred embodiment the animal is a pig.

[0051] Generally, according to the invention, the method comprises collecting waste matter from the animals whereby the waste matter to be treated is provided for processing.

[0052] Collection of waste matter from animals can be carried out in any suitable way known in the art. Preferably, waste matter is collected when the animals are concentrated in locations for their tending, e.g. for feeding, for drinking, and/or for yielding milk, or when they are concentrated in locations for transportation, or for retention before slaughtering. Other locations include locations where the conditions for producing waste matter are good, or locations where such waste-matter producing conditions could be made to stimulate the animals to urinate or defecate.

[0053] Preferably collection of waste matter takes place in stables where the animals are stocked. However, collection of waste matter can take place in the free as well, e.g. at locations where free going animals are stocked for their transportation to slaughterhouses.

[0054] Known waste matter collections facilities comprise stables wherein the animals are located and tended. Below the stables are stable basements wherein waste matter, e.g. liquid manure, faeces, litter, and other matter disposed from the animals are collected. Stable floors through which said waste matter is guided for collection and storage, detach the stables and stable basements.

[0055] According to the invention, the method also comprises inhibiting urease activity in said collected waste matter whereby it is obtained that urease-catalyzed hydrolysis of urea to ammonia is inhibited, either reversibly or irreversibly, and that loss of nitrogen and/or production of ammonia from the waste matter is substantially reduced or avoided. Consequently, the unpleasant odour and unhealthy condition due to ammonia in the stables can be reduced or avoided.

[0056] Generally, it is known to inhibit the catalytic activity of urease on hydrolysis of urea, either by removing water so that hydrolysis cannot take place, or by inhibiting the active site of the enzyme urease as such. Known methods comprise addition of inhibitors as cited e.g. in the referenced prior art, such as U.S. Pat. No. 3,565,599, heat treatment, and irradiation with ionizing radiation. Generally, the method of inhibiting urease activity in said collected waste matter depends on the intended use of the urease-inhibited waste matter.

[0057] Thus, for example for urease-inhibited waste matter used in the preparation of a fertilizer to be used for disposing waste matter through the agricultural system, the urease-inhibited waste matter should be compatible with components of the prepared fertilizer. Also, urease-inhibited waste matter should not adversely affect the environment where the fertilizer is applied.

[0058] Consequently, the selection of method of inhibition of urease activity generally depends on the application.

[0059] According to prior art methods as described in WO2005/009925, said inhibition may comprise: reversibly inhibiting urease activity, irreversibly inhibiting urease activity, and/or a combination thereof whereby it is obtained that the urease inhibition can either be applied for a period wherein reversible inhibition conditions apply, or for a longer period wherein irreversible inhibition conditions apply.

[0060] Reversible inhibition conditions as described in WO2005/009925 include conditions of temporarily different pH, e.g. buffering about isoelectric point of urease about pH 5.5, temperature, or pressure, or presence of a reversible inhibitor component. After a reversible inhibition period, the inhibition condition can be returned to its previous state of no or substantially no inhibition of the urease catalytic activity. Reversible inhibition can be applied to both the urea-rich and the urea-lean fractions. That is, for example to the total waste matter before liquids and solids are separated, or, if separated, to the liquid fraction comprising small amounts of faeces (solid matter).

[0061] Hence, the method of the invention provides a method for concentrating the preserved nitrogen.

[0062] According to a particular preferred embodiment according to the present invention, the inhibition provides a complete reversible inhibition which is unexpectedly obtained by adjusting the pH of the waste matter to about pH 3 or pH 12 in step ii).

[0063] It is contemplated that any acid or base normally used for adjusting pH can be used. Such compounds are generally known to the skilled person and may for example include, but is not limited to: sulfuric acid, hydrochloric acid, HNO.sub.3, sodium hydroxide, potassium hydroxide or KH.sub.2PO.sub.4 or a combination.

[0064] Irreversible inhibition conditions include conditions of permanent or essentially permanent inhibition of the urease catalytic activity. Also, a combination of reversible and irreversible inhibition of urease activity can be applied. This is particularly advantageous for applications wherein the urea-lean fraction (the substantially solid fraction) does not require addition of an irreversible inhibitor. Then the liquid waste fraction rich in urea can be treated to irreversibly inhibit the urease-catalytic activity whereas the solid fraction need not. For many applications of the urea-lean fraction, the amount of urea and the amount of water for the hydrolysis of urea are so low that only insignificant hydrolysis takes place.

[0065] The irreversible inhibition according to the invention may be selected from the group most of which are also described in WO2005/009925 comprising: urea compounds such as hydroxyurea, selenourea, phenylurea, thiourea; hydroxamates such as amino acid hydroxamates, acetohydroxamate; benzoates such as p-substituted mercuribenzoate, p-chloromercuribenzoate, p-hydroxymercuribenzoate, iodosobenzoate; sulfonates such as p-chloromercuribenzene sulfonate; imides such as N-ethylmaleimide; phosphor compounds such as phosphoramidate, phosphate; monovalent ions such as F.sup., Na.sup.+, and K.sup.+; divalent metal ions such as Hg.sup.2+, Cu.sup.2+, Fe.sup.2+, Co.sup.2+, Zn.sup.2+, Ni.sup.2+, Mn.sup.2+, Cd.sup.2+, Ag.sup.+, Mg.sup.2+ (weak), Ba.sup.2+, preferably Cu.sup.2+, Ag.sup.+, or Pb.sup.2+, or a combination thereof in form of at least one water-soluble salt, and/or at least one electrochemically-released ion; or as an oxide, preferably ferrate; trivalent ions such as As.sup.3+; and at least one nickel-complexing agent, preferably dimethylglyoxime, ethylenediamine, EDTA, or a combination thereof, and other compounds such as beta-mercaptoethanol, iodine, suramin, phenylsulfinate, KH.sub.2PO.sub.4 and furacin; and/or physical inhibiting means, such as UV light at suitable wavelenghts, such as in the range of 240-300 nm, with concentration at 270 nm, whereby it is obtained that hydrolysis of urea is not catalyzed by inhibited urease.

[0066] In a presently preferred embodiment, according to the invention irreversible inhibition is obtained by use of one or more among N-(n-butyl) thiophosphoric triamide (NBPT) available under the tradename Agrotain, salicylhydroxamic acid (SHAM), and thymol.

[0067] It is also contemplated that the waste is treated by a combination of reversible and irreversible inhibition.

[0068] Consequently, the irreversibly urease-activity inhibited urea-rich fraction can be stored or processed without urease-catalysed conversion of urea whereby loss of nitrogen and/or generation of ammonia can be avoided.

[0069] This is particularly advantageous for long storage periods of the urea-rich fraction before it is subjected to a subsequent treatment or is stored. Another advantage is that once the urease activity is irreversibly inhibited; the urease cannot function to catalyse the hydrolysis of urea to ammonia. Hence, urease in the urea tank will be maintained under inhibition so that ammonia is not developed during storage or use.

[0070] As mentioned previously the total waste matter can be separated in solids and liquid by a number of means. If periodically scraped from the stable floor the separation may be accomplished by sedimentation of solids whereby said solids make up the solid fraction and the supernatant liquid makes up the liquid waste matter fraction.

[0071] The liquid fraction is removed from the solid by any suitable method, e.g. by surface layer pumping or by decantation. Particularly it is preferred that the sedimentation of solid waste from liquid waste of the total waste matter is carried out so that substantially the entire liquid fraction has become separated from the solid fraction.

[0072] It is contemplated that the liquid fraction and the solid fraction of the waste matter may be separated before and/or after inhibition of the urease activity. When the waste matter is removed by continuous mode using a conveyor belt, which is skewed, the majority of the liquids and solids have been separated before inhibition. In this embodiment the liquid fraction may be sedimented once more if necessary before, during or after inhibition and/or before being settled prior to evaporation.

[0073] An entire process for processing total waste matter is also contemplated wherein concurrently to the liquid treatment the solids are treated as disclosed in WO2008/1014182, hereby incorporated by reference. More specifically the solids are treated by a method comprising the steps of

[0074] a. solid waste matter from mammals is wholly or partially dissolved using an oxidizing acid,

[0075] b. solid acid-insoluble components are separated, if necessary, and

[0076] c. the liquid component is neutralized with a base

[0077] The solid acid-insoluble components are typically non faecal related components e.g. dirt, gravel etc.

[0078] The reaction time for the dissolution has to be sufficient to obtain the desired solubilization of the solids. The type and quantity of reactants is usually balanced so that the reaction time is less than 10 hours to ensure sufficient process economy. Normally, the reaction time cannot be less than 30 minutes as this would require a relatively large proportion of the oxidizing acid. The reaction time may, for example, be from approx. 1 to approx. 10 hours, preferably 3 to 6 hours. The reaction should preferably be carried out at a slight underpressure, for example approx. 800 mbar, such as 500 mbar to 900 mbar or at least below 1 bar (10 kPa). The underpressure may be provided by means of a vacuum pump serving at the same time to vent off gases from the reaction. The pressure may suitably be measured in a pipe venting the gas from the reaction container, which thereby provides a measure of the pressure in the container.

[0079] The reaction between the oxidizing acid and the faecal matter will lead to the development of heat, and therefore measurement of the temperature in the reaction container may be relevant. Accordingly, a thermometer or a temperature electrode may be arranged in connection with the reaction mixture. The heat developed may possibly be conducted away from the reaction mixture, thereby causing a simultaneous cooling of the reaction mixture. If necessary, further heat may also be supplied to advance the reaction. Both the supply to and conducting away of heat from the reaction mixture may be carried out by passing a suitable fluid, such as water or steam, at a suitable temperature through a pipe or conduit encapsulating the reaction container. The flow rate of the liquid may be adapted according to need, for example relative to the amount of heat to be conducted away or supplied.

[0080] Accordingly, the invention also discloses various embodiments of a plant for processing animal waste in which solid waste matter produced by animals as illustrated in the appended drawings. The plants, processes and sections illustrating the invention include trivial equipment such as pipes, liquid pumps etc. which are well known to the skilled person.

[0081] Embodiments of the invention will now be illustrated by means of the following non limiting examples.

EXAMPLES

Example 1

[0082] An initial study was made to investigate the effect of testing the various inhibition in vivo versus in vitro before further tests were made.

[0083] Hence, the TAN value (total ammonical nitrogen) was measured in a sample where the same amount of Agrotain was added to a sample of urea and urine respectively. The results were obtained as described below in example 2 and also illustrated in FIG. 3.

[0084] From the figure it is apparent that both samples follow the same path and therefore it was concluded that the in vitro testing of urea would be representative for the effect on urine as well.

Example 2

[0085] To assess the effectiveness of selected urease inhibitors at preserving urea in swine urine over a six-week period, a set of experiments were established. The experiments were run in triplicate and average results are rendered in the below Table 1.

[0086] The inhibition selected for the experiment were inhibitors Agrotain ((N-(n-butyl) thiophosphoric triamide (NBPT)), SHAM (salicylhydroxamic acid), thymol, and at pH 3, and finally at pH 12.

[0087] The levels of each inhibitor used in the experiment were as follows:

[0088] Agrotain, 50 mg/L;

[0089] SHAM, 50.5 mg/L and 100.9 mg/L;

[0090] thymol, 1.5 mg/L.

[0091] A set of controls was included to test urease activity over the first eight hours of the experiment and then after six weeks. At each time interval samples were analyzed by standard methods for total Kjeldhal nitrogen (TKN) and total ammonical nitrogen (TAN) while urea was determined as the difference between the two (i.e. TKNTAN).

[0092] In an effort to reduce variability and increase confidence in results, the experiment used a 1.0 molar urea solution rather than swine urine, although in one experimental series Agrotain (see also example 1) was added to swine urine collected from sows at the NCSU Lake Wheeler Road Field Laboratory Swine Education Unit for verifying the validity of the in vitro results in vivo.

[0093] Jack Bean urease was added to all experimental units to ensure that urease was present in sufficient quantity for the breakdown of the urea.

[0094] Urea solution was prepared as a 1 molar solution by dissolving 60.06 g urea per litre of water.

[0095] Agrotain was diluted according to manufacturer's recommendation of 2 quarts per ton of manure.

[0096] Thymol was dissolved in ethanol at the ratio of 1 mg per mL.

[0097] Salicylhydroxamic acid (SHAM) was prepared at a 1:1 concentration of SHAM in 0.1 molar Tris-HCL buffer at pH 7.5.

[0098] The pH adjustments were made with 1.0 M sulfuric acid or 1.0 M sodium hydroxide, respectively. It is however contemplated that any acid or base can be used.

[0099] Urease was prepared by dissolving 100 mg of the Jack Bean enzyme (40,318 units per gram) in 125 mL phosphate buffer at pH 7.0. Application to the treatments was at the rate of 5.0 mL per 100 mL or 161 units per mL urea solution or urine.

[0100] The various treatments were added to 100 mL of the urea solution (and urine in the case of Agrotain), and the urease enzyme was added in random order. The obtained solutions were stirred to ensure distribution, sealed and stored under ambient conditions.

TABLE-US-00001 TABLE 1 TKN, TAN, Urea, Urea/ Treatment Time mg/L mg/L mg/L TKN % Control Urea 8 h 13122 75 13047 99 Urea week 6 19027 16400 2623 14 Urine 8 h 7115 338 6777 95 Urine week 6 5681 4688 993 17 Thymol in Week 1 no data 4751 no data no data urea, Week 2 26694 4928 21766 82 1.5 mg/L Week 3 26109 4756 21353 82 Week 4 20045 4240 15805 79 Week 5 22503 4036 18467 82 Week 6 24778 4927 19851 80 Agrotain in Week 1 no data 1723 no data no data urea, Week 2 10686 2248 8438 79 50 mg/L Week 3 10622 2763 7859 74 Week 4* 10718 309 10409 97 Week 5 26834 4040 22794 85 Week 6 24638 3040 21598 88 Agrotain in Week 1 no data 1087 no data no data urine, Week 2 6302 2310 3992 63 50 mg/L Week 3 7725 2617 5108 66 Week 4 4721 1958 2763 59 Week 5 5858 2768 3090 53 Week 6 5645 2797 2848 50 SHAM in Week 1 18450 2527 15923 86 urea, Week 2 23091 3783 19307 84 50.5 mg/L Week 3* 23366 385 22981 98 Week 4 25625 4864 20761 81 Week 5 26378 4579 21798 83 Week 6 26098 5012 21085 81 SHAM in Week 1 17320 4627 12693 73 urea, Week 2 21956 6342 15615 71 100.9 mg/L Week 3* 19856 695 19162 97 Week 4 22455 8141 14314 64 Week 5 25466 7780 17687 69 Week 6 22443 8570 13873 62 pH = 3.0 Week 1 24895 12 24883 ~100 Week 2 27341 18 27323 ~100 Week 3 27316 22 27294 ~100 Week 4 29225 26 28199 ~100 Week 5 27948 29 27919 ~100 Week 6 27753 37 27716 ~100 pH = 12.0 Week 1 no data 142 no data no data Week 2** 535 75 460 86 Week 3** 106 52 54 51 Week 4** 536 62 474 88 Week 5 29431 65 29366 ~100 Week 6 22066 11 22055 ~100 *the results in week 3 for SHAM and week 4 for Agrotain in urea deviate from the remaining results and have been disregarded **without the wish to be bound by any theory it is currently believed that the first results from the high pH N-determinations are not representative of the real values due to sensitivity issues with the method. Over time (and also after longer storage, results no shown) the TAN/TKN values are in line with the results of weeks 5 and 6 illustrated above and these are believed to be the true values.

[0101] As appears from the results, a considerable inhibition of urea breakdown was attained with all the tested treatments. In particular the pH treatments were surprisingly effective. The effect of pH was therefore further investigated.

Example 3

[0102] Example 3 is a comparative example showing the effect of pH outside the preferred range. pH was adjusted as appears from the first column of table 2 the result of the pH 3 and 12 treatments of example 2 is included as well. The tests were run similar to that of example 2 and nitrogen content was measured after two weeks. The results appear from table 2 below and FIG. 4. For comparison the result for the pH 12 measurement is the one from week 5 since the measurement method is compromised at high pH and no meaningful calculation could be made for week 2.

TABLE-US-00002 TABLE 2 pH adjusted to TKN, mg/L TAN, mg/L 3 27341 18 4 28012 1795 5 27266 2305 6 26074 3884 7 27703 5206 8 27428 4432 9 27503 4131 10 26355 4105 11 24981 3640 12 29431 65 Initial urea, 28729 2 pH 7.05

[0103] As can be seen from the results the effect of adjusting initial pH is significant when pH is decreased from 4 to 3 such that the amount of ammonia goes from 1795 mg/L to 18 mg/L. Similarly, when pH is increased from 11 to 12 a significant decrease in ammonia formation occurs. The relation between TAN and pH is illustrated in FIG. 4.