JP2002361300A - Sludge treatment method - Google Patents

Abstract

(57) [Solution] A sludge treatment method capable of preventing sludge floating and odor generation in a concentration step, suppressing odor generation in a subsequent step, and improving sludge dewaterability in a final step. I will provide a. A method for treating sludge from a primary sedimentation basin containing excess sludge, wherein nitrite is added to the sludge from the sludge so that nitrite ions have a volume of 10 to 50 mg / L per sludge volume, and the pH is less than 6 In which the sludge is gravity-concentrated, or a method for treating sludge from the initial settling basin that does not contain excess sludge, wherein nitrite is added to the sludge and the gravity is concentrated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for treating sludge. More specifically, the present invention provides that the first settling tank drawn sludge containing excess sludge and the first settling tank drawn sludge containing no excess sludge are subjected to gravity concentration in the presence of nitrite,
The present invention relates to a sludge treatment method capable of preventing odor generated from a concentration step, suppressing odor generation in a subsequent step, and improving sludge dewatering property in a final step.

[0002]

2. Description of the Related Art Conventionally, sludge generated from a first settling pond in a sewage treatment process is usually subjected to a dewatering treatment through a gravity concentration process. However, in this gravity concentration tank, for example, (1) poor sludge concentration, (2) outflow of pollutants into overflow of the concentration tank due to sludge floating, and (3) sludge dewatering property by the above (1) and (2). Deterioration, (4) generation of foul odors typified by hydrogen sulfide and methyl mercaptan, and (5) corrosion of the enrichment tank itself and incidental equipment due to sulfuric acid generated by the biochemical oxidation of volatilized hydrogen sulfide. is there. Therefore, in order to deal with such a problem, for example, for the odor, measures such as covering the concentration tank and separately treating the odor have been taken. Studies are being conducted on the development of chemicals and on the efficiency of dehydrators. In addition, as a measure against solids flowing into the overflow of the thickening tank, the promotion of operation management such as preventing sludge from being initially stored in the sedimentation basin and keeping the sludge zone in the thickening tank low is being promoted. However, these measures have not been able to provide satisfactory results. By the way, as a means to eliminate the above-mentioned phenomenon itself in the gravity concentration tank of sludge or to reduce the chemical in the chemical treatment, oxidizing agents such as hydrogen peroxide, sodium chlorite, and sodium hypochlorite are concentrated by gravity. It is reported that it is effective in preventing sludge floating in the tank and odor. However, for example, since hydrogen peroxide floats with decomposed oxygen gas, it is necessary to perform a deaeration treatment by aeration after adding a chemical, and in this case, it is unavoidable that the apparatus becomes large-scale and control Problems such as difficulties arise. Also, with respect to sodium chlorite and sodium hypochlorite, a satisfactory effect commensurate with the addition cost cannot be obtained,
It has hardly been put to practical use. On the other hand, it is known that nitrite exerts the effect of suppressing the microbial activity in sludge to prevent the generation of hydrogen sulfide and the effect of decomposing hydrogen sulfide, which has already been generated, albeit slowly. However, Japanese Patent Application Laid-Open No. S56-38196 discloses a technique of adding nitrite to a sludge to float and concentrate, or "Control of bulking and biological foaming of activated sludge" (by Jiri Wanner, Kono According to Tetsuro et al., Gihodo Shuppan, p. 103), for nitrate, which causes biological denitrification as well as nitrite, the presence of 6-8 mg / L nitrate nitrogen in the final settling tank in activated sludge treatment It has been common knowledge that nitrite ions and nitrate ions are undesired substances, preferably substances that should be avoided in gravity sedimentation of sludge and gravity concentration of sludge. . In the sludge treatment, a technique using nitrite is disclosed in, for example, JP-A-200
0-202494, JP-A-2000-288592
JP-A-2000-351000 discloses that nitrite is added to a sludge concentration tank to deodorize a dehydrated cake. However, these techniques are all related to prevention of odor generation of the dehydrated cake,
There is no statement that the sludge is concentrated by gravity, and it also contains excess sludge.
There is no description to set the dose to 50 mg / L or less. Japanese Patent Application Laid-Open No. 2000-185290 describes that nitrite is added to a concentration tank or the like. However,
This technique uses hydrogen peroxide in combination and is different from the present invention.

[0003]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems in the conventional gravity sludge tank in sludge treatment, prevents odor generated from the concentration step, and reduces odor generation in the subsequent steps. It is an object of the present invention to provide a sludge treatment method capable of suppressing and improving the sludge dewatering property in the final step.

[0004]

Means for Solving the Problems In order to achieve the above object, the present inventors have focused on nitrite which has been applied as a sludge deodorant, and have conducted detailed studies on its use conditions and the like. By adding nitrite so that the amount of nitrite ions is within a certain range to the sludge initially drawn from the sedimentation basin and concentrating the sludge by gravity, the floating of the sludge is not unexpectedly promoted (adverse effect). Not found), the ascent time is much longer than that of no treatment, that is, nitrite can exert a remarkable anti-floating effect, contrary to the conventional wisdom, and can achieve the above object. The present invention has been completed based on this finding. That is,
The present invention relates to (1) a method for treating sludge drawn from a first sedimentation basin containing excess sludge, wherein nitrite is added to the drawn sludge;
A method for treating sludge, characterized by adding nitrite ions in an amount of 10 to 50 mg / L with respect to sludge volume, and subjecting the sludge to gravity concentration at a pH of less than 6, (2) withdrawing a first sedimentation basin containing no excess sludge A method for treating sludge, wherein nitrite is added to the extracted sludge and the sludge is concentrated by gravity. (3) A method for treating nitrite, wherein nitrite is contained in an amount of 10 to 70 mg / L relative to nitrite. The sludge treatment method according to Item 2, wherein the sludge treatment method is added so as to have a sludge capacity; (4) The sludge treatment method according to Item 1, 2, or 3 for diluting drawn sludge; (5)
The sludge treatment method according to any one of items 1 to 4, wherein an acidic substance is added to the drawn sludge, and (6) a method in which nitrite is added to a sludge storage tank for storing sludge after gravity concentration.
A method for treating sludge according to any one of Items 5 to 5.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION The sludge treatment method of the present invention comprises (1)
There are two modes, a method of treating the first settling tank drawn sludge containing excess sludge, and a method of treating (2) the first settling tank drawn sludge not containing excess sludge. In the sludge treatment method of the present invention, first settling tank drawn sludge containing excess sludge (hereinafter, may be abbreviated as first settled sludge).
Alternatively, nitrite is added to the initial settled sludge containing no excess sludge, and the sludge is gravity-concentrated. If the amount of the nitrite is too large, when the decomposition of nitrous acid occurs promptly, nitrogen gas is rapidly generated, and as a result, sludge floats and concentration becomes difficult. In particular, this tendency becomes remarkable in the initial settling sludge containing excess sludge. This is because excess sludge contains a large amount of microorganisms (such as nitrite-reducing bacteria) that consume nitrite ions. Therefore, the amount of nitrite to be added to the initial settled sludge containing surplus sludge must be 50 mg / L as nitrite ion or less per sludge volume. Nitrite ion content is 50mg /
When the volume exceeds L, the sludge floats and the concentration becomes difficult, and the object of the present invention cannot be achieved. The lower limit of the amount of nitrite added is 10 mg / L as nitrite ion per sludge volume. If the amount of nitrite ions is less than 10 mg / L per sludge volume, the effects of preventing odor generation, floating prevention by suppressing carbon dioxide gas generation, and preventing rot are not sufficiently exhibited, and the object of the present invention cannot be achieved. The preferred amount of nitrite added is selected in the range of 15-45 mg / L as nitrite ion per sludge volume. On the other hand, in the case of the initial settling sludge that does not contain excess sludge, the consumption rate of nitrite ions is slow, so that compared to the case where the excess sludge is mixed,
Even if a large amount of nitrite is added, nitrogen gas is not rapidly generated, and sludge hardly floats. The upper limit of the amount of nitrite that does not cause sludge floating by nitrogen gas varies depending on the treatment plant. Therefore, the amount of the nitrite added is not particularly limited, but is generally 10 to 70 mg / L as nitrate ion in consideration of economy, safety, effects, and the like.
It is the sludge capacity, and is preferably selected in the range of 15 to 45 mg / L. When the sludge concentration is high, the required amount of nitrite added for preventing odor generation, suppressing carbon dioxide gas generation, and preventing rot increases. However, as described above, on the other hand, when the amount of addition of nitrite increases, the amount of generated nitrogen gas increases, and as a result, the risk of sludge floating increases.
Therefore, when the sludge concentration is high, it is preferable to dilute the sludge. The concentration of the extracted sludge before concentration is 200
It is advantageous to adjust it to less than 00 mg / L, in particular in the range from 8000 to 15000 mg / L. If the concentration of the drawn sludge is too low, the water area load is excessively large with respect to the capacity of the concentration tank, which is not preferable. For this dilution, inflow sewage or first settling pond overflow water can be used.

[0006] In addition, even in a new treatment plant or an existing treatment plant,
When improving the sludge treatment flow, the initial settled sludge and the excess sludge may be mixed after being concentrated respectively. And in this case, since the concentration of the initial settled sludge is increased, the required amount of nitrite ion per sludge volume is increased. Therefore, as described above, diluting the initial settled sludge includes, in particular, excess sludge. Not effective against sludge. In the sludge treatment of the present invention, when excess sludge is contained, the sludge is concentrated by gravity at a pH of less than 6,
Even when excess sludge is not contained, it is preferable to carry out gravity concentration at a pH lower than 6, but the pH to which the present invention can be applied differs depending on the sludge properties. It is particularly preferable to adjust the pH to a range of 4.5 to 5.5 regardless of the presence or absence of excess sludge. By adjusting the pH in this manner, the rate of decomposition and disappearance of nitrous acid is reduced, and the effect of nitrous acid is maintained in a small amount.At the same time, nitrogen gas is not generated suddenly, so that sludge floating by nitrogen gas hardly occurs. Become. If this pH is too low, the pH-dependent effect will be reduced, and the corrosion of the device and the saturated concentration of carbon dioxide gas will be reduced,
Carbon dioxide gas is easily generated. For this pH adjustment, for example, at least one acidic substance selected from mineral acids such as sulfuric acid and hydrochloric acid, ferrous sulfate, ferric chloride, and ferrous chloride can be used. Those which do not contain a sulfate group that causes generation of hydrogen sulfide in the process are preferable. Also,
Iron salts are preferred because they fix phosphorus, reduce the concentration of decomposed water returned to the water treatment, and ultimately reduce the phosphorus concentration in the treated water. Therefore, it is particularly preferable to use a ferric salt. In conventional facilities, surplus sludge is often returned to the sedimentation basin or the preceding stage, and the initial settled sludge is concentrated together with the surplus sludge. In such a case, although the sludge concentration is low, it contains a large amount of microorganisms (such as nitrite-reducing bacteria) that consume nitrite ions. Therefore, the pH adjustment by the addition of the acidic substance is particularly effective for drawn sludge containing excess sludge. Furthermore, in the present invention, the addition of nitrite to a sludge storage tank for storing sludge after gravity concentration can effectively prevent odor generation in the entire sludge treatment process.
That is, by adding nitrite to the initial settled sludge and performing gravity concentration, odor generation in the gravity concentration tank can be prevented, but odor generation in the storage tank, the sludge dewatering step, and the dewatered cake is completely prevented. It is difficult to prevent. Therefore, by adding nitrite to the storage tank, it becomes possible to prevent odor generation in the entire sludge treatment process. The nitrite to be added in the sludge treatment method of the present invention is not particularly limited as long as it is a water-soluble one, but it is preferable to use sodium nitrite or calcium nitrite from the viewpoint of economy and the like. Can be. Zinc nitrite is not preferred in terms of toxicity and the like. The nitrites may be used alone or in combination of two or more. Also, in the present invention, JP-A-2000-28
In JP-A-8592 and JP-A-2000-351000, a metal salt such as nitrate used in combination with nitrite, an organic fungicide sorbic acid, and the like can be used in combination with nitrite. FIG. 1 and FIG. 2 each show a method of treating the first settling tank drawn sludge containing excess sludge and the first settling tank drawn sludge not containing excess sludge in the present invention.
It is a process flow chart showing an example.

In the process flow diagram of FIG. 1, raw water such as sewage is first supplied to the sedimentation basin 1 together with excess sludge from the final sedimentation basin 3 to separate sludge. The sludge initially drawn from the sedimentation basin 1 is added with nitrite A or B, concentrated in the gravity concentration tank 4, and then stored in the sludge storage tank 5 as concentrated sludge. If necessary, nitrite C is added to the sludge storage tank 5, and the concentrated sludge is dewatered by the dehydrator 6 and discharged as a dewatered cake. On the other hand, the supernatant liquid from the first settling basin 1 is first passed through the activated sludge tank 2 to the final settling basin 3, where the sludge is separated. Part of the separated sludge is returned to the activated sludge tank 2 as return sludge, and the remainder is first transferred to the sedimentation basin 1 as excess sludge.
The supernatant liquid of the final sedimentation basin 3 is discharged as it is or after being subjected to necessary treatment. In the process flow diagram of FIG. 2, raw water such as sewage is first supplied to the sedimentation basin 1 and sludge is separated. First, the sludge extracted from the sedimentation basin 1
The nitrite A or B is added and concentrated in the gravity concentration tank 4. First, the supernatant liquid from the sedimentation basin 1 is led to the final sedimentation basin 3 through the activated sludge tank 2 where sludge is separated.
A part of the separated sludge is returned to the activated sludge tank 2 as a part of returned sludge, and the remainder is concentrated by a centrifugal concentrator 7 or the like. It is stored in the sludge storage tank 5 together with the concentrated sludge. If necessary, nitrite C is added to the sludge storage tank 5, and the mixed concentrated sludge is dewatered by the dehydrator 6 and discharged as a dewatered cake. The supernatant of the final settling basin 3 is
It is discharged as it is or after necessary processing has been performed. Examples of the dehydrator 6 include a centrifugal dehydrator, a belt press dehydrator, a screw press dehydrator, a filter press dehydrator, and a vacuum dehydrator.

[0008]

Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples. In Examples and Comparative Examples, the analysis of hydrogen sulfide was performed using a gas detector tube [Gastec 4M,
4L or 4LL], and analysis of methyl mercaptan was performed using a gas detector tube [Gastec 71H or 71]. The lower detection limit concentration is 1 ppm for both hydrogen sulfide and methyl mercaptan. Examples 1 to 3 and Comparative Examples 1 to 3 The following tests were carried out on sludge from the initial settling basin containing excess sludge. The properties of the drawn sludge used are as shown in Table 1.

[0009]

[Table 1]

(1) Settling test of 2 liter graduated cylinder With respect to the test sludge, no treatment or addition of various amounts of nitrite ions was carried out.
A sedimentation test at ℃ was conducted to examine the relationship between sedimentation time and sedimentation interface. The results are shown in Table 2 and FIG.

[0011]

[Table 2]

(Note) In the table, the mark * indicates that the sludge is floating or that a water layer is observed in the sludge layer. (2) Odor measurement test of supernatant liquid and settled sludge In the settling test of the above (1), the supernatant liquid after elapse of 10 hours and the sludge after elapse of 20 hours were concentrated to 1.5 times the original concentration. An odor measurement test was performed. The floating sludge after elapse of 20 hours was concentrated by removing the supernatant water. Table 2 shows the results. The concentrations of hydrogen sulfide and methyl mercaptan gas in the table are the concentrations of each gas present in the gas phase after 50 mL of a sample is placed in a container having a void volume of 530 mL and shaken for 2 minutes.

[0013]

[Table 3]

(3) Odor measurement test, sludge property analysis and dehydration test after storage of concentrated sludge In the sedimentation test of the above (1), the sludge after 20 hours has been concentrated to 1.5 times the original concentration. 25 more sludges
After storage at 5 ° C. for 5 hours or 10 hours, an odor measurement test was performed, and a sludge analysis after storage for 10 hours and a dehydration test were performed according to the following method. In addition, Comparative Example 1-2 and Example 1-
2. Examples 2-2 and 3-2 are examples in which the amount of nitrite ions shown in Tables 2 and 3 is further added to the sludge 1.5 times concentrated. NO ₂ in said surface ^- the total amount of a value converted to a 1.5-fold concentration prior to the sludge. <Dehydration test> 100 mL of sludge was collected in a 200 mL beaker, and a commercially available cationic polymer flocculant [Kurita Industry Co., Ltd.
Registered trademark name "Crifix CP604"] 150mg
/ L was added, and the mixture was subjected to coagulation and stirring, followed by gravity filtration for 1 minute, and the filtrate was subjected to a dehydration treatment at a compression pressure of 0.10 MPa for 1 minute to obtain a dehydrated cake. The moisture content of the cake obtained by the dehydration test in this way was measured, and the cake solid content was 1%.
The cake generation weight per t (t) and the cake loss rate relative to the untreated cake generation weight were calculated. The results are shown in Tables 2 and 3.

[0015]

[Table 4]

[0016]

[Table 5]

Examples 4 to 7 and Comparative Example 4 The following test was carried out on sludge from the first settling basin without excess sludge, which was first diluted at a predetermined ratio using settling water. The properties of the drawn sludge used are as shown in Table 3.

[0018]

[Table 6]

(Note) The test sludge is a mixture of 3 parts by volume of sludge withdrawn from the first settling basin and 2 parts by volume of first settling water. (1) Settling test of 2 liter graduated cylinder For test sludge, leave untreated or add various amounts of nitrite ions,
A sedimentation test at ℃ was conducted to examine the relationship between sedimentation time and sedimentation interface. The results are shown in Table 4-1 and FIG.

[0020]

[Table 7]

(Note) In the table, the * mark indicates the floating of sludge or the presence of a water layer in the sludge layer. (2) Odor measurement test of supernatant liquid and settled sludge In the settling test of the above (1), the sludge obtained by concentrating the supernatant liquid after elapse of 10 hours and the sludge after elapse of 20 hours to 1.8 times the original concentration. An odor measurement test was performed. The floating sludge after elapse of 20 hours was concentrated by removing the supernatant water. The results are shown in Table 4-2.

[0022]

[Table 8]

(3) Odor measurement test, sludge property analysis and dewatering test after storage of mixed sludge of concentrated raw sludge and surplus concentrated sludge First-stage sedimentation tank with sludge properties shown in Table 3 The sludge concentration (SS21700mg / SS21) calculated by diluting 5/3 times and further condensing 1.8 times
L) to obtain the same concentration as the excess concentrated sludge (pH 6.4).
6, TS31200mg / L, VTS86.8% / TS,
SS 30400 mg / L, VSS 87.2% / SS, fiber content 1.2% / SS) were diluted with the final settling water, and the excess adjusted sludge (pH 6.54, TS22400 mg / L, VTS8)
6.2% / TS, SS21800mg / L, VSS87.3
% / SS, fiber content 1.1% / SS). Next, in the sedimentation test of the above (1), 3 volumes of sludge obtained by concentrating the sludge after 20 hours to 1.8 times the original concentration and 2 volumes of the surplus adjusted sludge were mixed to obtain test sludge. Was. After storing the test sludge at 25 ° C. for 5 hours or 10 hours, an odor measurement test was performed, and a sludge property analysis after storage for 10 hours and a dehydration test were performed according to the method described above. Note that Comparative Example 4-2,
Example 4-2, Example 5-2, and Example 6-2 are examples in which nitrite ions in the amounts shown in Table 4-3 and Table 4-4 were further added to the test sludge. NO ₂ in said surface ^- the total amount of, excluding excess adjustment sludge portion mixed, a value obtained by converting the former without the excess sludge sludge (1.8 fold concentrate before sludge). These results are shown in Table 4-3 and Table 4
It is shown in FIG.

[0024]

[Table 9]

[0025]

[Table 10]

Examples 8 to 11 Sludge drawn from the first settling basin without excess sludge was first diluted with a settling water to a predetermined dilution ratio to obtain sludges having different concentrations. A predetermined amount of nitrite ion was added to the sludge, and a sedimentation test at 25 ° C. was performed in a 2-liter measuring cylinder to examine the relationship between the sedimentation time and the sedimentation interface.
The results are shown in Table 6 and FIG. The properties of the first settling basin sludge that does not contain the used excess sludge are as shown in Table 5.

[0027]

[Table 11]

[0028]

[Table 12]

[0029] (Note) NO ₂ ^- amount of 1% Correction: Sludge Concentration 1
This is a conversion value of the addition amount based on 0000 mg / L. Maximum calculated concentration: [sludge concentration / minimum settling interface (%)] x 1
00 *: Floating sludge or a water layer is observed in the sludge layer. Examples 12 to 14 and Comparative Examples 5 to 7 The following test was carried out on sludge from the initial settling basin containing excess sludge. The properties of the drawn sludge used are as shown in Table 7.

[0030]

[Table 13]

(1) 2 liter graduated cylinder settling test First settling basin sludge containing excess sludge was used as it was.
Alternatively, a predetermined amount of an acidic substance is added, and without adding nitrite ion or 30 mg / L, a sedimentation test is performed in a 2-liter measuring cylinder at 25 ° C., and the relation between the sedimentation time and the sedimentation interface is determined. Examined. The results are shown in Table 8-1 and FIG.
Shown in

[0032]

[Table 14]

(Note) PFS: Ferric polysulfate (Liquid containing 11% Fe) FC: Ferric chloride (Liquid containing 13% Fe) *: Floating sludge or water layer in sludge layer. (2) Odor measurement test of supernatant liquid and settled sludge In the settling test of the above (1), the supernatant liquid after elapse of 10 hours and the sludge after elapse of 20 hours were concentrated to 1.5 times the original concentration. An odor measurement test was performed. The floating sludge after elapse of 20 hours was concentrated by removing the supernatant water. The results are shown in Table 8-2.

[0034]

[Table 15]

[0035]

According to the sludge treatment method of the present invention, the first settling tank drawn sludge containing surplus sludge and the first settling tank drawn sludge not containing surplus sludge are gravity-concentrated in the presence of nitrite, and are subjected to gravity concentration. In addition to preventing the floating of sludge and the generation of odor, the generation of odor in the subsequent steps can be suppressed, and the dewatering property of the sludge in the final step can be improved.

[Brief description of the drawings]

FIG. 1 is a process flow chart showing one example of a method for treating sludge drawn from a first sedimentation basin containing excess sludge in the present invention.

FIG. 2 is a process flow chart showing one example of a method for treating sludge from a first sedimentation basin without excess sludge according to the present invention.

FIG. 3 is a graph showing a relationship between a settling time and a settling interface in a settling test in Examples 1 to 3 and Comparative Examples 1 to 3.

FIG. 4 shows the results obtained in Examples 4 to 7 and Comparative Example 4.
It is a graph which shows the relationship between the sedimentation time and the sedimentation interface in a sedimentation test.

FIG. 5 is a graph showing a relationship between a settling time and a settling interface in a settling test in Examples 8 to 11.

FIG. 6 is a graph showing a relationship between a settling time and a settling interface in a settling test in Examples 12 to 14 and Comparative Examples 5 to 7.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 First settling tank 2 Activated sludge tank 3 Final settling tank 4 Gravity thickening tank 5 Sludge storage tank 6 Dehydrator 7 Centrifugal thickener A, B, C Nitrite

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4D059 AA04 AA05 BD19 BE08 BE16 BE26 BE31 BE38 BE53 BE57 BF12 BK01 CA28 CA29 DA23 DA24 DA32 DA33 DA70 DB11 EB05 EB11

Claims (6)

[Claims] 1. A method for treating sludge from a primary sedimentation basin containing excess sludge, wherein nitrite is added to the sludge so as to have a nitrite ion content of 10 to 50 mg / L with respect to the sludge volume. A sludge treatment method, wherein the sludge is concentrated by gravity. 2. A method for treating sludge drawn from an initial sedimentation basin that does not contain excess sludge, wherein nitrite is added to the drawn sludge and the sludge is gravity-concentrated. 3. A nitrite comprising 10 to 70 mg of nitrite ions.
3. The method for treating sludge according to claim 2, wherein the sludge is added so as to be / L to sludge capacity. 4. The method of claim 1 wherein the sludge is diluted.
The sludge treatment method described in the above. 5. The method according to claim 1, wherein an acidic substance is added to the drawn sludge.
A sludge treatment method according to any one of claims 1 to 4. 6. The sludge treatment method according to claim 1, wherein nitrite is added to a sludge storage tank for storing sludge after gravity concentration.