JP2007196141A - Sludge treatment method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a sludge treatment method which enables the easy execution of pH adjustment using a component contained in sludge to dispense with a pH controller or to reduce its consumption, facilitates the adjustment to target pH, and is capable of more enhancing a malodor preventing effect than when the pH controller is used. <P>SOLUTION: In the sludge treatment method for mixing primary sedimentation sludge 10 produced in a wastewater disposal plant or its concentrated sludge with excess sludge 9 or its concentrated sludge to subject them to dehydration treatment, the drawn primary sedimentation sludge from a first sedimentation basin is adjusted to an average SS concentration of 2 wt.% or above to be subjected to acid-putrefaction to be adjusted to a pH of 5.3 or below by a produced organic acid and a nitrite is added to the pH adjusted sludge to be mixed with excess sludge or its concentrated sludge or the nitrite is added to the mixed sludge. Then, the nitrite added sludge is subjected to dehydration treatment to prevent the occurrence of a malodor. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a sludge treatment method in which primary sludge generated in a treatment plant for organic wastewater or concentrated sludge thereof and concentrated excess sludge are mixed and dehydrated, and more specifically, nitrite is added to sludge. The present invention relates to a sludge treatment method that can effectively prevent the generation of odor.

  As a method for treating organic wastewater such as sewage, food industry wastewater, and pulp and paper wastewater, there is an aerobic treatment method such as an activated sludge treatment method. In such aerobic treatment, the wastewater to be treated is first solid-liquid separated in the settling basin, the separated liquid is subjected to aerobic biological treatment such as activated sludge treatment, and the aerobic treatment liquid is solid-liquid separated in the final sedimentation basin. The separated liquid is discharged as treated water. In the first settling basin, the separated sludge is discharged as the first settling sludge. In the final sedimentation basin, most of the separated sludge is returned to the aerobic treatment process as return sludge, and part is discharged as excess sludge. The initially settled sludge and excess sludge generated by such an aerobic treatment are usually mixed and dehydrated, but generally they are concentrated and mixed, and mechanically dehydrated by adding a dehydrating agent.

    In such sludge treatment, sulfur compounds such as hydrogen sulfide and methyl mercaptan, nitrogen compounds such as ammonia and trimethylamine, valeric acid, Malodorous substances such as lower fatty acids such as butyric acid and aldehydes are generated and the environment is deteriorated. For this reason, in the sludge treatment process, a sludge treatment method in which a deodorizer is added to prevent the generation of malodorous substances has been studied.

  In Patent Document 1 (Japanese Patent Laid-Open No. 2000-202494), nitrite, sulfite, or hydrogen sulfite is added to the mixed sludge of primary sedimentation sludge and excess sludge generated by aerobic treatment, and after 15 minutes or more, A method for dewatering sludge is shown. In addition, in Patent Document 2 (Japanese Patent Application Laid-Open No. 2002-28696), organic dibasic acid, ferric salt, etc. are added as a pH adjuster to the mixed sludge of primary settling sludge and excess sludge, and the pH is adjusted to 5.5 or less. A method of dehydrating sludge by allowing nitrite to be present as a bacteriostatic agent in the reduced adjusted sludge is shown.

These methods are sludge dewatering techniques that can effectively decompose the sludge slurry and dehydrated cake and prevent the generation of odor, and provide an effective deodorizing effect. However, in these methods, in order to increase the odor prevention effect of nitrite as a bacteriostatic agent, it is necessary to add a large amount of a pH adjuster such as an organic dibasic acid or a ferric salt, so that the processing cost is low. Get higher. Moreover, since the necessary amount of the pH adjusting agent for adjusting to a predetermined pH fluctuates with sludge change, the pH decrease is insufficient or the pH decrease is excessive, and the polymer flocculant during dehydration (polymer dehydrating agent) There is a problem that the agglomeration / dehydration effect may be reduced.
JP 2000-202494 A JP 2002-28696 A

  The problem of the present invention is that the pH contained in the sludge can be efficiently adjusted, so that a pH adjuster is unnecessary or the amount used is small, and the adjustment to the target pH is possible. It is easy to propose a sludge treatment method that can enhance the odor prevention effect as compared with the case of using a pH adjuster.

The present invention is the following sludge treatment method.
(1) A sludge treatment method in which a primary sludge generated in wastewater treatment or its concentrated sludge and surplus sludge or its concentrated sludge are mixed and dehydrated.
A sludge adjustment step of adjusting the average pH of the sludge to less than 5.3 by the organic acid produced by acidifying the first settling sludge or its concentrated sludge;
A sludge mixing step of mixing the adjusted sludge and excess sludge or its concentrated sludge;
A nitrite addition step of adding nitrite to the adjusted sludge, surplus sludge, its concentrated sludge or mixed sludge,
And a dehydration process for dehydrating nitrite-added sludge.
(2) The sludge adjustment step is a step of staying and concentrating so that the average SS concentration of the extracted initial sedimentation sludge in the first sedimentation basin is 2% by weight or more, acidifying the concentrated sludge, and lowering the pH of the concentrated sludge The method according to (1) above, comprising:
(3) The method according to (1) or (2) above, wherein the sludge adjusting step includes a step of storing the extracted initially settled sludge and further adjusting the pH to less than 5 by advancing acidic decay.
(4) The method according to any one of (1) to (3) above, wherein nitrite is added to the adjusted sludge, and the adjusted sludge to which nitrite is added and the excess sludge or the concentrated sludge thereof are mixed in the sludge mixing step.
(5) The method according to any one of (1) to (3) above, wherein after adjusting sludge and excess sludge or concentrated sludge thereof are mixed in the sludge mixing step, nitrite is added to the mixed sludge.
(6) The method according to any one of (1) to (5), further including a sludge pH lowering step of adding ferric salt or aluminum salt to the adjusted sludge, concentrated excess sludge, or mixed sludge.

  In this invention, the sludge used as the object of a process is the primary sedimentation sludge and excess sludge which generate | occur | produce in a wastewater treatment plant. Such sludge is first separated in a sedimentation basin in an aerobic treatment method such as an activated sludge treatment method generally used as a treatment method for organic wastewater such as sewage, human waste, food industry wastewater, and pulp and paper wastewater. First sludge and surplus sludge that is separated and discharged as surplus sludge. These sludges are dehydrated as mixed sludge of concentrated sludge, but the concentration may be before or after mixing.

  In the present invention, in the sludge adjusting step, among these sludges, the initial settling sludge or its concentrated sludge is intentionally spoiled and adjusted to a pH of less than 5.3. Here, the pH decreases due to the production of organic acid due to the decay of sludge, but decay and the accompanying pH decrease are likely to occur in primary sludge containing a large amount of organic substances that produce organic acids such as carbohydrates and fats. is there. The surplus sludge is mainly composed of microorganisms, and the organic substances that generate organic acids and the organic acids themselves are already decomposed, so that the pH is neutral and the pH is not easily lowered. For this reason, in the present invention, the initial settling sludge or its concentrated sludge is spoiled and adjusted to a pH of less than 5.3 with the generated organic acid.

  In the present invention, the initial sedimentation sludge is sludge separated in the first sedimentation basin of the biological treatment process, but about 30% or less of the excess sludge generated from the final sedimentation basin is returned to the first sedimentation basin, together with the raw water. In the processing method to isolate | separate, the sludge isolate | separated by a sedimentation basin first is also included. The method of the present invention can be applied even in the case of the conventional treatment in which the entire amount of excess sludge is returned to the initial sedimentation. However, since there is a lot of excess sludge, the sludge in the first sedimentation basin is compared with the case where the excess sludge is not included. Concentration is difficult to proceed and the pH reduction rate is slow, so the odor prevention effect is reduced or the required amount of nitrous acid is increased. Moreover, since the concentration of concentrated sludge is low, the sludge slurry capacity to be dewatered increases.

  After mixing primary sludge or its concentrated sludge and surplus sludge or its concentrated sludge, the mixed sludge is spoiled. It is difficult to adjust to the target pH. Further, in this case, odor generation is severe in the process of decay, which is contrary to the original purpose of preventing odor in the sludge treatment process. In addition to this, the dewaterability of sludge deteriorates due to the progress of sludge decay, the moisture content of the dehydrated cake increases, and the amount of cake disposal and disposal costs increase. Thus, in order to intentionally rot the sludge and lower the pH, the first settling sludge that is susceptible to acidic rot is suitable. For this reason, in the present invention, the excess sludge before acid mixing with the concentrated concentrated sludge is acidified and the average pH of the sludge is reduced to less than pH 5.3, preferably less than 5.0 by the organic acid produced. adjust.

  In ordinary organic wastewater treatment plants such as sewage, organic solids in raw water are first precipitated in a settling basin, and the separated liquid is sent to an aerobic reaction tank (aeration tank) for biological treatment. The sludge drawn from the first sedimentation basin (primary sedimentation sludge) is sent to the gravity concentration tank for concentration. The concentration of the first sedimentation sludge drawn into the gravity concentration tank is generally 1.0% by weight or less. The pH is slightly lower than the inflowing sewage, specifically, pH 6.0 to 6.5. If the concentration of the drawn initial sludge is increased, concentration inhibition such as generation of carbon dioxide gas occurs, and the gravity concentration function is lowered, which is not preferable. The points to be noted above are described in pages 308 and 309 of the sewerage maintenance management guideline 2003 edition.

  In the present invention, in order to acidify the first settled sludge or its concentrated sludge and adjust the pH to less than 5.3, preferably less than 5.0, the first treatment sludge is contrary to the treatment recommended in the guidelines for sewerage maintenance. It is preferable to draw out the initial sedimentation sludge by staying and concentrating so that the SS concentration of the extracted sludge in the sedimentation basin is 2% by weight or more, preferably 2.5% by weight or more. As a result, in the sediment sludge phase in the first sedimentation basin, the sludge is concentrated to a high concentration, and mainly the carbohydrate-based organic matter is spoiled to produce an organic acid, which remains in the sludge phase and accumulates to lower the pH. . Depending on the substrate of the incoming sewage organic matter, most of the extracted sludge is adjusted to a pH of less than 5.3 by this operation.

  Generally, the drawn initial settling sludge is gravity concentrated in a gravity concentration tank. In the present invention, the concentration of the initial settling sludge concentrated so that the SS concentration becomes 2% by weight or more in the gravity concentration tank is described above. This is difficult as described in the guidelines for sewerage maintenance. Therefore, in the present invention, the conventional gravity concentration tank can be used as a relay and storage tank for concentrated primary sedimentation sludge. Even when the pH of the initial settling sludge is insufficient, the storage time can be adjusted in a storage tank using a gravity concentration tank, and the sludge can be adjusted to a pH of less than 5.3.

  The sludge mixing step is a step of mixing the adjusted sludge adjusted to a pH of less than 5.3 and the excess sludge or its concentrated sludge. This sludge mixing may be performed before or after the next nitrite addition step. When sludge mixing is performed before the nitrite addition step, nitrite is added to the mixed sludge. When sludge mixing is performed after the nitrite addition step, nitrite is added to the adjusted sludge, excess sludge, or its concentrated sludge, and then the nitrite addition adjusted sludge and the excess sludge or its concentrated sludge are mixed.

  The nitrite addition step is a step of adding nitrite to the adjusted sludge, excess sludge, its concentrated sludge, or mixed sludge. When adding nitrite before the sludge mixing process, nitrite can be added to the adjusted sludge, excess sludge, or its concentrated sludge, and when adding after the sludge mixing process, nitrite is added to the mixed sludge. can do. Among these, it is preferable to add nitrite to adjusted sludge or mixed sludge. Examples of the nitrite include sodium nitrite and potassium nitrite. The amount of nitrite added is 20 to 200 mg / L, preferably 40 to 150 mg / L, as nitrite ions with respect to the total capacity of the primary sedimentation sludge or its concentrated sludge and concentrated excess sludge.

  When adding nitrite to the adjusted sludge whose pH of the initially extracted sludge is adjusted to less than 5.3, the drawn sludge and the storage tank to which the sludge is guided, or the gravity concentration tank converted to a relay / storage tank, etc. In addition, nitrite can be added. When further reducing the pH of the adjusted sludge in a storage tank or a gravity concentration tank diverted to a relay / storage tank, mixing with these tanks, withdrawn sludge from these tanks, or concentrated excess sludge as the destination Nitrite can be added to sludge storage tanks. When adding nitrite to mixed sludge of adjusted sludge and concentrated excess sludge, nitrite can be added to a mixed sludge storage tank or the like.

  The time from the addition of nitrite (when adding a pH lowering regulator to both) to the dehydration treatment can be 0.3 to 48 hours, preferably 3 to 12 hours. . In general, the average sludge retention time of the mixed sludge storage tank of primary sludge and excess sludge is 1 hour or more, but in the case of dehydration operation only during the daytime, the added nitrite may be consumed and decomposed at night. In this case, however, nitrite can be additionally added all at once about 1 hour before the start of dehydration.

  When the adjusted sludge having a pH of about 5 and less than 5.3 is mixed with the concentrated excess sludge, the pH of one of the concentrated excess sludge is about 6, so that the mixed sludge is less than pH 5 to about pH 5.5. Even if the pH of the mixed sludge is about 5.5, the sludge slurry by nitrous acid and the deodorizing effect of the dehydrated cake are exhibited. In order to enhance the deodorizing effect of the dehydrated cake, the mixed sludge is adjusted to less than pH 5. It is preferable to do. In order to adjust the mixed sludge to less than pH 5, the septic time in the sludge adjusting step is lengthened and the adjusted sludge is adjusted to about pH 4.7. In addition, the mixed sludge can be adjusted to a pH of less than 5 by adding a ferric salt or an aluminum salt as a pH lowering adjusting agent to the adjusted sludge, concentrated excess sludge or mixed sludge.

  In this pH lowering regulator, ferric chloride, specifically ferric chloride, polyferric sulfate, and aluminum salt, specifically, aluminum chloride, aluminum sulfate and the like can be used. . These ferric salts and aluminum salts not only enhance the deodorization effect by nitrite due to the pH lowering action of the mixed sludge, but also fix the soluble phosphorus in the excess sludge as a metal salt, so water treatment as sludge treatment return water Phosphorus returning to the process can be reduced, and as a result, the amount of phosphorus contamination in the discharged water can be reduced. The addition amount of a ferric salt and an aluminum salt is 1000-10000 mg / L with respect to a concentrated excess sludge as a liquid product.

  The addition site of the pH lowering regulator may be any of the concentrated surplus sludge tank, the transfer line of the concentrated surplus sludge to the sludge mixing tank, and the sludge mixing tank. However, when both nitrite and ferric salt or aluminum salt are added to the sludge mixing tank, it is preferable to pay attention to the place of addition so that both chemicals do not contact directly or at a high concentration.

  In the dehydration process, the nitrite-added sludge is dehydrated. The nitrite-added sludge to be dehydrated here is a mixed sludge in which nitrite is added to the adjusted sludge in the nitrite addition step, and the adjusted sludge to which nitrite is added and the excess sludge or its concentrated sludge are mixed in the sludge mixing step, Mixing sludge and excess sludge or its concentrated sludge in the sludge mixing process, mixing the sludge to add nitrite in the nitrite addition process, and ferric salt or aluminum salt in these sludges in the salt addition process Mixed sludge to which is added.

  A well-known method can be adopted as a method for dehydrating nitrite-added sludge. For example, the sludge slurry can be dehydrated using a centrifugal dehydrator, a belt press dehydrator, a screw press dehydrator, a filter press dehydrator, a vacuum dehydrator, or the like. In order to improve the dewaterability of the sludge slurry, the anionic polymer flocculant, the cationic polymer flocculant, the amphoteric polymer flocculant, etc. It can be dehydrated by adding slaked lime.

  In the above treatment, nitrite is added to the sewage sludge or its sludge that has been spoiled and adjusted sludge adjusted to a pH of less than 5.3 with the generated organic acid, or surplus sludge and mixed sludge with its concentrated sludge. Odor generation from sludge after nitrous acid addition to dehydration process and storage of the dehydrated cake can be prevented. In this case, since the pH is adjusted by using the components contained in the sludge, the pH can be adjusted efficiently, so that a pH adjuster is unnecessary or the amount of use can be reduced. And adjustment to target pH is easy, and also the odor prevention effect can be heightened rather than the case where a pH adjuster is used.

  As described above, according to the present invention, the initial settling sludge or its concentrated sludge is spoiled, adjusted to a pH of less than 5.3 with the generated organic acid, and nitrite is added to the adjusted sludge, excess sludge, its concentrated sludge or mixed sludge In addition, since the dehydration treatment is performed while preventing the generation of odor, the pH can be adjusted efficiently by using the components contained in the sludge, and therefore a pH adjuster is unnecessary or the amount used. Therefore, the adjustment to the target pH is easy, and the effect of preventing odor can be enhanced as compared with the case of using a pH adjuster.

  Embodiments of the present invention will be described below with reference to the drawings. In addition,% is weight%. FIG. 1 is a flowchart showing a sludge treatment method according to an embodiment of the present invention, which is configured to use a conventional sewage treatment apparatus as it is. In FIG. 1, 1 is an initial settling pond, 2 is an aerobic reaction tank (aeration tank), and 3 is a final settling pond, and these constitute an aerobic biological treatment apparatus. In the aerobic biological treatment, the raw water 4 is firstly solid-liquid separated in the precipitation basin 1, the separation liquid 5 is aerobically biologically treated in the aerobic reaction tank 2, and the reaction liquid 6 is solid-liquid separated in the final precipitation basin 3 and separated. The liquid is discharged as treated water 7, a part of the separated sludge is returned as return sludge 8, and the remainder is taken out as excess sludge 9. The separated sludge from the first settling basin 1 is taken out as the first settling sludge 10.

  Reference numeral 11 denotes a gravity concentration tank. Here, the first settling sludge 10 is concentrated by gravity and sent to the gravity concentration sludge storage tank 12 for storage. These are used as a relay / reservoir for primary sedimentation sludge, where the pH of primary sedimentation sludge is reduced. 13 is a surplus sludge storage tank, and the surplus sludge 9 is stored here. 14 is a surplus sludge concentrator, and a centrifugal concentrator or the like is used, where the surplus sludge in the surplus sludge storage tank 13 is concentrated and sent to the concentrated surplus sludge storage tank 15 for storage. Reference numeral 16 denotes a sludge mixed storage tank, in which the gravity concentrated sludge in the gravity concentrated sludge storage tank 12 and the concentrated excess sludge in the concentrated excess sludge storage tank 15 are mixed and stored. 17 is a sludge dewatering device, which uses a centrifugal dewaterer, a belt press dewaterer, etc., where the mixed sludge in the sludge mixed storage tank 16 is dehydrated, and the dehydrated cake is sent to the dehydrated cake storage tank 18 for storage. The dehydrated cake in the dehydrated cake storage tank 18 is incinerated in the incinerator 19 or is carried out for other processing.

  The above flow is configured to use a conventional sewage treatment apparatus as it is, but in the first sedimentation basin 1, the initial sludge 10 is stored and concentrated, and the amount of drawn sludge is reduced. As a result, the drawn sludge concentration is concentrated from 2% to 3%, and the pH is less than 5.3. The gravity concentration tank 11 is used as a relay tank or a storage and adjustment tank for further lowering the initial sedimentation sludge pH. When the initial settling sludge 10 is stabilized at a pH of less than 5.3 at the time of drawing, nitrite can be added in the gravity concentration tank 11. Gravity-enriched sludge storage tank 12 is used as a relay tank, where gravity-enriched sludge can be stored, acidified, and adjusted by reducing pH. If the initial settling sludge 10 is stable at a pH of less than 5.3 at the extraction stage, the gravity concentrated sludge storage tank 12 can be omitted, but if this is present, add nitrite here. You can also. Appropriate operations can be determined for these depending on the equipment status of the treatment facility and the degree of pH decrease due to the storage and decay of the first sludge.

  The surplus sludge 9 passes through the surplus sludge storage tank 13, is concentrated by a centrifugal, levitation, and gravity-type surplus sludge concentrator 14, and is sent to the concentrated surplus sludge storage tank 15 for storage. In the sludge mixing storage tank 16, the initially settled sludge in the gravity concentrated sludge storage tank 12 or its concentrated sludge and the concentrated excess sludge in the concentrated excess sludge storage tank 15 are mixed and stored. When nitrite is not added by this step, nitrite is added to the sludge mixed storage tank 16.

  When ferric salt or aluminum salt is added to the concentrated adjusted sludge to which nitrite is added, it is added to either the concentrated surplus sludge storage tank 15, the sludge being added to the sludge mixed storage tank 16, or the sludge mixed storage tank 16. Can be mixed.

  The place where odor generation is particularly large in the normal sludge treatment process is a treatment process in which the first settling sludge of the gravity concentration tank 11, the gravity concentration sludge storage tank 12, the sludge mixing storage tank 16, and the sludge dewatering device 17 is present, and the excess sludge treatment system. In particular, before the surplus sludge concentrator 14, there are few odor problems. The gravity concentration tank 11, the gravity concentration sludge storage tank 12, the sludge mixed storage tank 16 and the like can be deodorized by having a sealed structure, but the sludge dewatering device 17 is difficult to have a sealed structure, and the odor appear. On the other hand, in the present invention, deodorization is possible in most of the sludge treatment processes having a large odor generation problem by adjusting pH and adding nitrite.

The deodorizing action of nitrite includes (a) the decomposition action of sulfur compounds that are the main cause of odor of sewage sludge such as hydrogen sulfide, methyl mercaptan, etc., and (b) stop the activity of spoilage bacteria It is considered to be bacteriostatic.
It is thought that the odor generation prevention of the sludge slurry and the odor generation prevention of the dewatered cake are due to the action (b). For this reason, in order to prevent odor generation of the dehydrated cake, it is important to maintain a nitrite ion concentration of about 20 mg / L or more in the sludge slurry before dehydration for at least 1 hour, preferably 3 hours.

  On the other hand, since nitrite ions are biochemically decomposed by microorganisms in the sludge, their concentrations are consumed, decreased, and disappear over time. The consumption rate of nitrous acid depends on pH, and is fast at neutral pH, decreases with decreasing pH, and records a minimum value at about pH 4.5-4.7. When the pH is 4.5 or less, the rate starts to increase again. This is not a biochemical decomposition but a chemical decomposition caused by destabilization of nitrous acid. The pH decrease due to the decay of the first settling sludge and the generation of the organic acid proceeds rapidly to about pH 4.7, there is almost no subsequent pH decrease, and the minimum pH is about 4.4. Therefore, the conditions for optimizing the effect of nitrite can be obtained by utilizing the properties of the primary sedimentation sludge itself in the primary sedimentation sludge treatment method of the present invention.

In the sludge whose pH is less than 5.3 by septicing the initial settling sludge, the former is more odorous by adding nitrite to the sludge which is less than pH 5.3 with acid or ferric salt, even if the pH is the same. Excellent prevention effect. The following can be considered as a reason for this odor prevention effect improvement.
1) Even if pH is the same, there are many equivalents of acid, and the biodegradation rate of nitrous acid is further reduced accordingly.
2) At the stage of spoilage and acidification, changes in the preferred species of sludge microflora occur, and this is also an artificial acidity in the composition of the microflora after receiving bacteriostatic action such as rot of nitrite. As a result, the effect of preventing odor generation of the dehydrated cake was further improved.

  The sludge treatment method of the present invention was carried out using the first settling sludge and excess sludge from the A sewage treatment plant. In the flow shown in FIG. 1, the surplus sludge concentrating device 14 is a centrifugal concentrator, the sludge dewatering device 17 is a belt press dewatering machine, and the dewatered cake is incinerated.

  In the above treatment process, the initial settling sludge in normal operation is extracted by intermittently operating the extraction pump with a timer. The pump runs every 5 hours for 5 minutes. This first settling sludge is extracted with high concentration sludge (hereinafter referred to as high concentration first settling sludge (A1)) from 0.5 minutes to 1.0 minute immediately after pump operation, and the pump operation 3.5. Both drawn sludges with almost no solid content (hereinafter referred to as low-concentration primary sedimentation sludge (A2)) after 5 minutes were collected. The pH of the high concentration initial sedimentation sludge was 5.34, the water temperature was 23 ° C., and the analysis SS concentration during the test was 3.03%. The pH of the low concentration initial sedimentation sludge was 6.61, the water temperature was 22 ° C., and the analysis SS concentration during the test was 0.14%. The operating conditions at the time of collecting low-concentration initial sedimentation sludge, in which the sludge solids in the latter half of the pump run out, are generally recommended conditions for managing the initial sedimentation basin and the gravity concentration tank into which this sludge is charged. The average concentration (arithmetic average value) of the pump for 5 minutes was 0.93% and the pH was 6.13.

  From the above test results, it is assumed that the residence time in the first sedimentation basin is set as a setting condition for appropriately retaining and concentrating in the first sedimentation basin and lowering the sludge pH. Stored as adjusted sludge. Those stored for 3 hours (hereinafter referred to as high concentration initial precipitation 3 (A3)), those stored for 8 hours (hereinafter referred to as high concentration initial precipitation 8 (A4)), those stored for 16 hours (hereinafter referred to as high concentration) First sedimentation 16 (A5)) and 24 hours storage (hereinafter referred to as high concentration first sedimentation 24 (A6)) were used as the adjusted sludge.

Sludge preparation:
Mix 1 volume of high-concentration initial sedimentation sludge (A1) and 2 volumes of low-concentration primary sedimentation sludge (A2) and concentrate for 12 hours in a test gravity concentration tank of 300 mmΦ and 1.5 m height. Gravity-enriched sludge (B) is obtained. The mixed sludge had a pH before gravity concentration of 6.01 and an SS concentration of 0.96%, and the sludge after gravity concentration had a pH of 5.64 and an SS concentration of 3.01%. Moreover, the gravity concentration tank extraction sludge (hereinafter referred to as the actual gravity concentration sludge (C)) of the A treatment plant was collected. At the time of collection, the pH was 5.57, the water temperature was 24 ° C., and the SS concentration was 3.17%.

  On the other hand, the concentrated excess sludge (D) is collected from the concentrated excess sludge storage tank 15, and the high concentration primary sedimentation sludge (A1), high concentration primary sedimentation (A3) to (A6), indoor gravity concentrated sludge (B), or actual It mixed with equipment gravity concentration sludge (C), and it was set as mixed sludge. At the time of collecting the concentrated excess sludge (D), the pH was 6.14, the water temperature was 24 ° C., and the SS concentration was 3.32%. Mixing of concentrated excess sludge (D) with the high-concentration primary sedimentation sludge (A1), high-concentration primary sedimentation (A3) to (A6), indoor gravity concentrated sludge (B), or actual equipment gravity concentrated sludge (C) The conditions were 35% concentrated excess sludge (D) by volume ratio and 65% of the first seven types of primary sedimentation sludge in accordance with the actual equipment performance.

  The above mixed sludge was subjected to a dehydration test. The time from the sludge collection to the start of the dehydration test was such that the pH drop occurred in a short time in the high concentration first precipitated sludge, so the test was started as soon as possible after collection. The time from collection to the start of the test was 3 hours.

Testing and evaluation methods:
1) Storage conditions for test sludge and chemical sludge;
The sludge was collected and sealed in a gas impermeable storage bag (product name: mighty pack) with a capacity of 0.85 L, and gently stirred with a shaker. The room temperature was set at 23 ° C.

2) Sludge slurry odor test;
About 60 mL of sludge in the Mighty pack is taken out, 50 mL of it is collected with a syringe, taken into a 500 mL polyethylene bottle (total volume 630 mL), covered, shaken vigorously for 2 minutes with a shaker, hydrogen sulfide and methyl mercaptan detector tube Then, the gas concentration in the container was measured.

3) Dehydrated cake adjustment conditions;
Collect 200 mL of sludge after the prescribed chemical injection and storage, form aggregated flocs with the cationic polymer flocculant CLIFIX CP111 (manufactured by Kurita Kogyo Co., Ltd.), perform gravity filtration, and filter the total amount. The dehydrated cake was obtained by pressing and dewatering for 120 seconds at a pressure of 0.04 Mpa through a cloth. In Comparative Examples 10 and 11, the aggregating power of the cationic aggregating agent was decreased, so that the aggregating operation was performed using the amphoteric polymer aggregating agent Crivest P359 (manufactured by Kurita Kogyo Co., Ltd.).

4) Storage conditions for dehydrated cake;
Put the whole amount of dehydrated cake (range 26g to 28g) by cutting one end of a gas-impermeable gas sampling bag (trade name: Tedlar bag), plug the cut end with a heat sealer, and put 200mL of air and 500mL in the bag. Nitrogen gas was sealed. The sealing port was sealed, and this bag was stored in a 30 ° C. thermostat.
5) Measurement of dehydrated cake odor;
Hydrogen sulfide and methyl mercaptan in the bag of the cake stored for a predetermined time were measured with a detector tube.

6) Contents of the additive drug;
As the nitrite, a 38% sodium nitrite solution was used. When added to the primary sedimentation sludge, 100 mg / L was added as nitrite ions, and when added to the mixed sludge, 65 mg / L was added. The addition amount 100 mg / L to the primary sedimentation sludge is 65 mg / L for the mixed sludge.
Liquid products were all used for ferric chloride (description symbol FC), polyferric sulfate (PFS), and aluminum chloride (LAC). FC is a product containing 13% iron, PFS 11% iron, and LAC 10% alumina (A1203). The addition was performed on the concentrated surplus sludge, and the sludge pH was set to be substantially the same for Examples 4, 5, and 6.

7) Analysis of nitrite ion;
The sludge remaining in the slurry odor test of 2) above is aggregated with the cationic polymer flocculant CLIFIX CP111 to obtain clear water, and a nitrite ion measurement kit (Merck Co., Ltd., trade name RQ Flex; sold in Japan) (Measured by Kanto Chemical Co., Inc.)

  Hereinafter, although the dehydration test of Reference Examples 1-2, Comparative Examples 1-2, and Examples 1-7 is described, the test conditions and results are shown in Tables 1-2.

[Reference Examples 1-2]:
Reference Example 1 is a mixed sludge of concentrated excess sludge (D) and actual equipment gravity concentrated sludge (C), Reference Example 2 is a mixed sludge of concentrated excess sludge (D) and indoor gravity concentrated sludge (B), This is an example of dehydration without adding nitrite ions. When no odor generation prevention measures are taken, 200-400 ppm of hydrogen sulfide and 10-20 ppm of methyl mercaptan are generated from the gravity-concentrated sludge of the first settling sludge by a shaking method. A dehydrated cake generates 100 ppm or more of methyl mercaptan within one day, resulting in an unbearable odor. The concentrations at which the odor intensity is 3 (a odor that can be easily detected) are 0.063 ppm of hydrogen sulfide and 0.0041 ppm of methyl mercaptan, and methyl mercaptan has a foul odor effect 15 times that of hydrogen sulfide.

[Comparative Examples 1-2]:
Comparative Example 1 is the actual equipment gravity concentrated sludge (C), and Comparative Example 2 is the indoor gravity concentrated sludge (B). After storage and storage, the concentrated excess sludge (D) was mixed, stored and stored for 5 hours, and dehydrated.
In Comparative Examples 1 and 2, nitrite ions disappear after 5 hours from the addition of 100 mg / L of nitrite ions, and odor is generated from the sludge slurry after mixing the concentrated excess sludge. Although the deodorized cake is reduced in odor compared to untreated Reference Examples 1 and 2 after 1 day, there is no significant difference from untreated after 2nd day, and intense odor generation is observed.

[Examples 1, 2, 3, 4]:
Example 1 is high concentration initial precipitation 3 (A3), Example 2 is high concentration initial precipitation 8 (A4), Example 3 is high concentration initial precipitation 16 (A5), and Example 4 is high concentration initial precipitation 24 (A6). ) Add 100mg / L of nitrite ion to the initial sedimentation sludge with a concentration of about 3%, store and store for 5 hours, mix concentrated excess sludge (D), store and store for 5 hours, and perform dehydration treatment went.
In Example 1, the pH of the initially precipitated sludge falls to 5.28, and nitrite ions remain after 5 hours, and the effects of preventing odor generation and dehydrated cake odor after mixing the concentrated sludge are obtained to some extent.
In Examples 2 to 4 in which the pH of the initial settling sludge was less than 5, 62 mg / L or more of nitrite ions remained after 5 hours as compared with Comparative Examples 1 and 2, and the sludge slurry after mixing the concentrated excess sludge Completely prevents odor generation. And the generation of dehydrated cake odor is also prevented for more than one day, and the effect is greater as the pH of the precipitated sludge is initially lowered.
It should be noted that both hydrogen sulfide and methyl mercaptan are evaluated as being capable of satisfactorily preventing odors at 10 ppm or less, while those having methyl mercaptan as 100 ppm or more are evaluated as having a bad odor.

[Examples 5, 6, and 7]:
In Examples 5, 6, and 7, in Example 2, the pH of the concentrated excess sludge was lowered with ferric chloride (FC), polyferric sulfate (PFS), and aluminum chloride (LAC), respectively. In this example, the pH after mixing the sludge is less than 5. In this case, the odor generation preventing effect of the dehydrated cake is improved, and the odor generation preventing period is 5 days or longer.

  Hereinafter, although the dehydration test of Comparative Examples 3-8 and Examples 8-14 is described, the test conditions and results are shown in Tables 3 and 4.

[Comparative Examples 3 and 4]:
Comparative Example 3 is the actual equipment gravity concentrated sludge (C), and Comparative Example 4 is the indoor gravity concentrated sludge (B), after mixing the concentrated excess sludge (D) with the primary sedimentation sludge having a concentration of about 3%. Nitrite ions 65 mg / L were added to the mixed sludge, stored and stored for 5 hours, and dehydrated.
Compared to the case where nitrite is added to the first precipitated sludge of Comparative Examples 1 and 2 and then the concentrated excess sludge is mixed, it is slightly improved, but the mixed sludge slurry is stored for 5 hours and dehydrated cake is stored for 1 day. The effect is unsatisfactory.

[Examples 8, 9, 10, 11]:
Example 8 is high concentration initial precipitation 3 (A3), Example 9 is high concentration initial precipitation 8 (A4), Example 10 is high concentration initial precipitation 16 (A5), and Example 11 is high concentration initial precipitation 24 (A6). In each case, after mixing concentrated excess sludge (D) with primary sedimentation sludge with a concentration of about 3%, nitrite ions 65 mg / L are added to the mixed sludge, stored and stored for 5 hours, and dehydrated. Went.
In Examples 9, 10, and 11 where the initial sludge pH was less than 5, the cake odor generation prevention period was 2 days or longer as in Examples 3 and 4.
In Example 8 where the primary sediment concentration sludge has a pH of 5.18, mixed sludge slurry odor is prevented for 5 hours or more and dehydrated cake odor generation is also prevented for up to 1 day, but in Examples 9, 10 and 11 where the pH is less than 5, The cake odor generation prevention period is 2 days or more, and the effect is greatly improved.

[Examples 12, 13, and 14]:
In Examples 12, 13, and 14, in Example 9, the pH of the mixed sludge was lowered with ferric chloride (FC), polyferric sulfate (PFS), and aluminum chloride (LAC), respectively. In this example, the pH after mixing is less than 5. That is, when the pH of the concentrated excess sludge is 4.7 with FC, PFS, and LAC, mixed with the first precipitated sludge with a pH of less than 5, and nitrite is added with the mixed sludge with a pH of less than 5, the generation of dehydrated cake odor occurs for more than 5 days. Prevented.

[Comparative Examples 5, 6, 7, 8]:
In Comparative Examples 5, 6, 7, and 8, each of the indoor gravity concentrated sludges (B) was mixed with the primary settled sludge having a concentration of about 3% and the concentrated excess sludge (D), respectively, and then the concentrated excess sludge was FC or This is a comparative example in which LAC was added to lower the mixed sludge pH. In Comparative Examples 6, 7, and 8 where the mixed sludge has a pH of less than 5, the effect of preventing the odor generation of the dehydrated cake is up to 2 days. On the other hand, in Examples 10 and 11, the pH after mixing of the concentrated excess sludge was 5.05 and 4.97, which is slightly higher than or equal to Comparative Examples 6, 7, and 8, but 3 days and 5 days. There is an effect of preventing the above odor generation.

[Solubility Phosphorus Fixation Effect of Examples 5, 6, and 7]
Solubilized phosphorus (PO ₄ -P) in Reference Example 2, Comparative Example 2 and Examples 2 to 7 The measurement results are shown in Tables 5 and 6, and the fixed phosphorus (PO ₄ -P) in Examples 5 to 7 is shown. Table 6 shows the measurement results.
In Examples 5, 6, and 7, FC, PFS, and LAC added to make the mixed sludge pH less than 5 fix not only the pH lowering effect but also soluble phosphorus (PO ₄ -P) in the sludge. It has been shown to have an effect.
As a result, for the purpose of reducing the soluble phosphorus in the sludge dehydrated filtrate, the removal efficiency when using the inorganic flocculant (number of moles of phosphorus that can be fixed and reduced per mole of added metal) is approximately 0.65. It can be seen that the present invention has a phosphorus reduction effect equivalent to or higher than that.

  A sludge treatment method that mixes dewatered primary sludge generated in an organic wastewater treatment plant or concentrated sludge with concentrated excess sludge, especially by adding nitrite to the sludge, thereby effectively generating odor. It can be used in sludge treatment methods that can be prevented.

It is a flowchart which shows the sludge processing method of embodiment.

Explanation of symbols

1 First sedimentation basin, 2 Aerobic reaction tank, 3 Final sedimentation tank, 4 Raw water, 5 Separation liquid, 6 Reaction liquid, 7 Treated water, 8 Return sludge, 9 Surplus sludge, 10 Initial sedimentation sludge, 11 Gravity concentration tank, 12 Gravity-concentrated sludge storage tank, 13 surplus sludge storage tank, 14 surplus sludge concentrator, 15 concentrated surplus sludge storage tank, 16 sludge mixing storage tank, 17 sludge dewatering device, 18 dehydrated cake storage tank, 19 incinerator

Claims (6)

A sludge treatment method in which a primary sludge generated by wastewater treatment or its concentrated sludge and excess sludge or its concentrated sludge are mixed and dehydrated,
A sludge adjustment step of adjusting the average pH of the sludge to less than 5.3 by the organic acid produced by acidifying the first settling sludge or its concentrated sludge;
A sludge mixing step of mixing the adjusted sludge and excess sludge or its concentrated sludge;
A nitrite addition step of adding nitrite to the adjusted sludge, surplus sludge, its concentrated sludge or mixed sludge,
And a dehydration process for dehydrating nitrite-added sludge. The sludge adjustment step includes a step of staying and concentrating so that the average SS concentration of the initially-settled initial sludge in the first sedimentation basin is 2% by weight or more, acidifying the concentrated sludge, and lowering the pH of the concentrated sludge. Item 2. The method according to Item 1. The method according to claim 1 or 2, wherein the sludge adjusting step includes a step of storing the extracted initially settled sludge and further adjusting the pH to less than 5 by advancing acidic decay. The method according to any one of claims 1 to 3, wherein nitrite is added to the adjusted sludge, and the adjusted sludge to which nitrite is added and the excess sludge or the concentrated sludge are mixed in the sludge mixing step. The method according to any one of claims 1 to 3, wherein nitrite is added to the mixed sludge after the adjusted sludge and the excess sludge or the concentrated sludge thereof are mixed in the sludge mixing step. The method according to any one of claims 1 to 5, further comprising a sludge pH lowering step of adding ferric salt or aluminum salt to the adjusted sludge, concentrated excess sludge or mixed sludge.

Images