JPH0692000B2 - Dewatering method of organic sludge - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a first settled sludge and an excess sludge produced in the treatment of waste liquid containing phosphorus and organic matter such as sewage, night soil, industrial wastewater or similar organic waste liquid. The present invention relates to a method for dehydrating anaerobic digested sludge that has been anaerobically digested, and to a method for improving dehydration and reducing phosphorus and nitrogen in the dehydrated liquid.

[Conventional technology]

In the treatment of organic wastewater such as sewage, not only removal of organic substances but also activated sludge treatment for removal of nutrient salts such as phosphorus and nitrogen is used. Then, in the activated sludge treatment, first sludge is produced from the first settling tank and surplus sludge is produced from the final settling tank. Anaerobic digestion is often used as a treatment method for this organic sludge in consideration of reduction of sludge and ease of disposal of dehydrated cake.

In this method of dehydrating anaerobic digested sludge, when a polymer coagulant is used in a dehydrator, the dehydrated liquid is returned to the water treatment system, but high concentration phosphorus and nitrogen are contained in this liquid. Since it is included, there are problems that the removal rate of phosphorus and nitrogen in the water treatment system (activated sludge treatment system) will decrease, and that the water content of the dehydrated cake will be higher than that of sludge that does not undergo anaerobic digestion. Was there.

When dehydrating digested sludge, magnesium hydroxide, magnesium oxide or flame retardant magnesium salt slurry and ferric chloride are used as dehydration aids, and the dehydrator is a vacuum dehydrator without using a polymer flocculant. A method using a pressure dehydrator (filter press) has also been known.

In this method, since the alkalinity component in digested sludge is high, the amount of magnesium hydroxide used is at least per SS.
It was necessary to add 10% or more, the amount of sludge generated during dewatering was large, and not only was the main purpose of anaerobic digestion not to reduce sludge, but the cost of the inorganic coagulant was enormous. Therefore, until now, it has been general practice to wash the digested sludge and employ the known dehydration method.
However, if the digested sludge is washed, not only phosphorus and ammonia contained in the digested sludge but also a large amount of fine sludge is returned to the water treatment system, and phosphorus and nitrogen removal in the biological reaction tank cannot be stabilized. There was a drawback.

Further, in the method of adding a magnesium compound to the digestion tank as in JP-A-56-150494 and JP-A-56-150500, long residence time in the digestion tank and sludge concentration in the digestion tank High temperature and weak agitation, the crystals of ammonium ammonium phosphate grow too large and are deposited in a large amount in the digestion tank, and the substantial capacity of the digestion tank is reduced.
It was difficult to maintain a stable anaerobic digestion process due to frequent occurrence of obstacles such as blockage of the digestion tank desorption tube.

[Problems to be Solved by the Invention]

As described above, in the conventional technology, the digested sludge dehydration technology still has various problems regardless of which method is adopted.

Therefore, the present invention, in the dehydration of digested sludge, reduction of phosphorus and nitrogen content to the water treatment system and improvement of the dehydration efficiency of digested sludge by a dehydrator such as a belt press (decrease in cake water content and treatment amount of sludge The purpose of the present invention is to provide a dehydration method capable of increasing (increasing).

[Means for Solving the Problems]

The present inventors, as a result of intensive studies to achieve the above object, found that by adding the polymer flocculant and the readily soluble magnesium compound as they are to the digested sludge, it is possible to suitably dehydrate, and the present invention was completed. I was able to do it.

That is, the present invention was obtained by anaerobically digesting the organic sludge containing phosphorus and nitrogen generated in the production and dephosphorization treatment consisting of an anaerobic tank and an aerobic tank containing organic wastewater containing phosphorus, nitrogen and organic matter. The polymer coagulant and the readily soluble magnesium compound were added without washing the digested sludge, and the amount of the easily soluble magnesium compound added was calculated as magnesium ions in a molar ratio (Mg) to the total phosphorus concentration in the digested sludge pore water. / T-
P) to 0.3 to 3, preferably 1.0 to 1.5, and then dehydrating the digested sludge.

Hereinafter, the present invention will be described in more detail with reference to the drawings.
FIG. 1 is a process drawing showing an embodiment of the present invention.

In FIG. 1, the organic waste liquid 11 containing phosphorus and nitrogen, such as domestic sewage, is first solid-liquid separated into the liquid to be treated 11 ′ and the first settling sludge 14 in the settling tank 1, and then the treated liquid 11 ′. Is introduced into the biological reaction tank 2 and biologically treated with the returned sludge 13 and separated into treated water 12 and sludge in the final settling tank 3.

For the biological reaction tank 2, a treatment method can be selected depending on the purpose of treatment. That is, when the main purpose is to remove BOD, aerobic treatment is performed, and when the purpose is to remove BOD, phosphorus and nitrogen, anaerobic treatment (stirring without blowing air) and aerobic treatment (air blowing treatment). ) Is a rational process.

Most of the sludge from the final settling tank 3 is returned to the biological reaction tank 2 as return sludge 13, but part of it is transferred to the sludge treatment process (concentration, anaerobic digestion, dehydration) as excess sludge 15.

The first settling tank sludge 14 is concentrated by the first settling sludge concentrator 4, and the excess sludge 15 is individually concentrated by the surplus sludge concentrator 5 and transferred to the anaerobic digestion tank 6.

When the first settling tank sludge 14 and surplus sludge 15 are sufficiently concentrated in the settling tank (when the TS concentration is about 1.5% or more)
Is transferred to the anaerobic digestion tank 6 as it is, omitting the concentration step depending on the sludge concentration.

Further, it is also possible to first mix the sludge 14 in the settling tank and the excess sludge 15 to concentrate them and transfer them to the anaerobic digestion tank 6.

The digested sludge 18 that has been subjected to the anaerobic digestion treatment is transferred to the mixing tank 7, where the polymer flocculant 31 and the easily soluble magnesium compound 32 are added and mixed, and the digested sludge 18 is transferred to the dehydrator 8 as the chemical-feeding sludge 19 and dehydrated. It The order of addition of the polymer coagulant and the readily soluble magnesium compound can be set arbitrarily, and the readily soluble magnesium compound can be added at the same time as the addition of the polymer coagulant or before or after the addition.

As the polymer flocculant that can be used in the present invention, strong cation-based, weak cation-based, nonionic, anion-based ones are used.
A single species or multiple species are used.

As the readily soluble magnesium compound that can be used in the present invention, those having a solubility (mass of the anhydrous compound contained in 100 g of a saturated solution and wt%) of 5% or more at a water temperature of 20 ° C. can be used, for example, MgCl ₂ ( Magnesium chloride), MgS
O ₄ · 7H ₂ O (magnesium sulfate), Mg (NO ₃₎ a ₂ (magnesium nitrate), or the like. On the other hand, magnesium compounds such as Mg (OH) ₂ and MgO have a solubility of less than 5% and can be distinguished as a sparingly soluble magnesium salt. This point is based on the Chemical Handbook edited by the Chemical Society of Japan, Basic Edition (revised 2nd edition), published on June 20, 1975, 77.
Clarification will be made by referring to the matters on pages 7 and 784.

In the present invention, the above-mentioned easily soluble magnesium compounds may be used alone or in combination of two types depending on the properties of the digested sludge.

Further, as the dehydrator 8, a belt press dehydrator, a centrifugal dehydrator or the like which is used by adding a conventional polymer coagulant can be used.

The method for installing the mixing tank 7 will be described. When a single type of polymer flocculant and a readily soluble magnesium salt are added at the same time, one mixing tank 7 may be installed. When adding a readily soluble magnesium compound before and after addition and mixing of a single flocculant, or when adding and mixing a readily soluble magnesium compound using a plurality of types of polymer flocculants, a plurality of mixing tanks 7 are provided, Polymeric flocculant is added and mixed in the first half tank, and easily soluble magnesium compound is added to the latter half tank, or conversely, easily soluble magnesium compound is added to the first half tank and polymeric flocculant is added to the latter half tank. By doing so, the purpose can be achieved.

The digested sludge 18 contains ammonia nitrogen, phosphorus, alkalinity components, colloidal substances and the like in high concentration due to the decomposition of the initial sludge and excess sludge.

The amount of the easily soluble magnesium compound added is 0.3 to a molar ratio (Mg / TP) with respect to the total phosphorus concentration in the pore water of the digested sludge.
By setting it to 3, preferably 1.0 to 1.5, it is possible to reduce the concentration of phosphorus and nitrogen in the dehydrated liquid, and to improve the dehydratability.

The pH of the digested sludge upon addition of the easily soluble magnesium compound is 6.5 to 9.5, preferably 7.0 to 8.5. When the pH range is less than 6.5, the solubility of magnesium ammonium phosphate is high, the removal of phosphorus and nitrogen is insufficient, and when the pH is 9.5 or more, the sludge cohesiveness is insufficient and the phosphorus content is low. The water content of the dehydrated cake, on the contrary, was found to be high even if magnesium acid ammonium was produced.

When the pH in the digested sludge is low, it can be solved not only by adding an alkaline agent, but also by aeration of the digested sludge to strip dissolved carbon dioxide gas. When the pH is high, it is effective to add an acid such as sulfuric acid and strip the dissolved carbon dioxide gas.

The phosphorus concentration and nitrogen concentration of the dehydrated liquid 22 from the dehydrator 8 become much lower than the concentration of pore water in the digested sludge 18, and are first returned to the sedimentation tank 1 or the production reaction tank 2.
Further, the dehydrated cake 21 is dried and incinerated, or disposed of by returning to farmland.

〔Example〕

The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to these examples.

Example-1 Municipal sewage was treated as a liquid to be treated by the method of the first illustrated example. An anaerobic tank (front end of the tank) and an aerobic tank (rear end of the tank) are provided in the biological reaction tank of FIG. In the anaerobic tank, the liquid to be treated and the returned sludge were mixed with an underwater stirrer.

The specifications of each device are as follows.

First settling tank: 500m ³ Biological reaction tank Anaerobic tank: 200m ³ Aerobic tank: 600m ³ Final settling tank: 500m ³ Primary sludge thickener: 50m ³ Excess sludge thickener: 50m ³ Anaerobic digester: 1000m ³ belt Press dewatering machine: Filter cloth width 1m, processing capacity 120kg / m / h Using such a facility, the amount of liquid to be treated is 5000m ³ / d, and returned sludge is 1500m ³ / d. A treatment liquid was obtained.

Generation of primary sedimentation sludge per liquid to be treated 1 m ³ is 100 g / m
³ , the amount of excess sludge generated was 105 g / m ³ . Anaerobic digestion was carried out with a sludge concentrator of 4% in the sedimentation tank sludge concentration and a sludge concentration of 3% in the surplus sludge concentrator.

The pH of digested sludge is 7.5, SS concentration is 1.5%, total phosphorus concentration in interstitial water and ammonia nitrogen concentration are 315 mg / l and 9 respectively.
It was 80 mg / l.

To this digested sludge, 2% of a cationic polymer was added per SS, and at the same time, a solution of MgSO ₄ was added so that the Mg / TP molar ratio was 1.0.

The water content of the cake was 82% when the treatment speed of the dehydrator was 80 kg / mh, the total phosphorus concentration in the dehydrated liquid was 5 mg / l, and the ammoniacal nitrogen was 840 mg / l. Further, the results were the same when the addition of MgSO ₄ was added before the mixing of the cationic polymer and when it was added after the cationic polymer.

Comparative Example-1 The same cationic polymer was added to the digested sludge of Example-1 as SS.
2% was added and dehydrated.

When the treating speed of the dehydrator was set to 80 kg / mh as in Example 1, the water content of the cake was 86%, the total phosphorus concentration in the dehydrated liquid was 330 mg / l, and the concentration of ammoniacal nitrogen was 980 mg / l. It was.

When the dehydrated liquid was first returned to the sedimentation tank in this state, the total phosphorus concentration in the treated liquid increased to 3 mg / l and the total nitrogen increased to 24 mg / l.

Example-2 After adjusting the pH of the digested sludge of Example-1 using sulfuric acid or caustic soda, the dehydration test was conducted with the same amounts of the cationic polymer and the readily soluble magnesium as in Example-1.

The properties of the dehydrated liquid and the water content of the dehydrated cake are shown in Table-2 according to each pH.

When the pH is less than 6.5 and above 9.5, the water content of sludge increases and the phosphorus concentration and ammonia concentration in the liquid increase.

When the pH became 9.5 or more, ammonia in the liquid diffused into the atmosphere, and the working environment deteriorated significantly. From this result, the optimum pH of the present invention is in the range of 6.5 to 9.5, preferably 7.0 to 8.5.

〔The invention's effect〕

As described above, the following effects can be obtained by adding the easily soluble magnesium compound according to the method of the present invention.

The first effect is that phosphate ions in digested sludge pore water (supernatant liquid obtained with a filter paper opening of 4 μm or less) are converted to magnesium ammonium phosphate (MgNH ₄ PO) according to formula (1) in the presence of magnesium ions. _When the compound of ₄ ) is formed and dehydrated, it is fixed in the dehydrated cake. Therefore, the concentration of phosphorus and the concentration of ammonia nitrogen in the dehydrated liquid decrease.

Mg ²⁺ ＋ NH ₄ ⁺ ＋ PO ₄ ^3- → MgNH ₄ PO ₄ ↓ (1) In general, digestive sludge has higher ammoniacal nitrogen than phosphorus in terms of molar concentration. Therefore, the phosphorus is completely removed by adding the easily soluble magnesium compound.

The second effect is that, when magnesium ammonium phosphate is present in the chemical sludge 19 due to the addition and mixing of the easily soluble magnesium compound, the compound of magnesium ammonium phosphate plays a role of a coagulation aid, and therefore, during dehydration, The processing speed of the dehydrator increases by 20% to 50%, and at the same processing speed, the water content (true water content) of the dehydrated cake decreases.

Therefore, by dehydrating the digested sludge by the method of the present invention, the throughput of the dehydrator is increased and the water content of the dehydrated cake is lowered, and phosphorus and nitrogen in the dehydrated filtrate are removed, and the biological reaction tank It was possible to always stabilize the removal of phosphorus and nitrogen.

[Brief description of drawings]

FIG. 1 is a process drawing showing an embodiment of the present invention. 1 ... First settling tank, 2 ... Biological reaction tank, 3 ... Final settling tank, 4
… First settling sludge concentrator, 5… Excess sludge concentrator, 6… Anaerobic digestion tank, 7… Mixing tank, 8… Dehydrator, 11… Organic waste liquid, 11 ′
… Biologically treated liquid, 12… Treatment liquid, 13… Returned sludge, 14… First sludge, 15… Excess sludge, 16… Concentrated initial sludge, 17… Concentrated excess sludge, 18… Digestion sludge, 19… Medication sludge , 21 ... dehydrated cake,
22 ... Dewatered liquid, 23 ... Primary sludge concentrated separation water, 24 ... Excess sludge concentrated separation water, 31 ... Polymer flocculant, 32 ... Easy soluble magnesium compound

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shoichi Goda 4-2-1 Honfujisawa, Fujisawa City, Kanagawa Prefecture Ebara Research Institute Ltd. (56) Reference JP 59-92100 (JP, A) Special Features Kai 62-289300 (JP, A) JP-A-53-15277 (JP, A)

Claims (2)

[Claims] 1. A digestion product obtained by anaerobically digesting an organic sludge containing phosphorus and nitrogen generated in a biological dephosphorization process comprising an anaerobic tank and an aerobic tank containing organic wastewater containing phosphorus, nitrogen and organic matter. Without washing sludge, a polymer flocculant and a readily soluble magnesium compound were added, and the amount of the readily soluble magnesium compound was added as magnesium ions to the total phosphorus concentration in the digested sludge pore water in a molar ratio (Mg / 0.3 for T-P)
3. The method for dehydrating organic sludge, which comprises dehydrating the digested sludge after setting 3. 2. The pH of the digested sludge when the polymer flocculant or the easily soluble magnesium compound is added to the digested sludge.
The method for dehydration according to claim 1, wherein the water content is 6.5 to 9.5.