JPS6349556B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improved method for anaerobic digestion of sewage sludge.

Over the past two to three years, with the boom in energy and oil conservation, the sewage treatment industry has begun to anaerobically digest the sludge generated at treatment plants and recover the gas produced as an energy source. . As a result, people were not satisfied with the conventional anaerobic digestion method, and various efforts were made to recover as much gas as possible.

However, when evaluating anaerobic digestion not only in terms of energy recovery, but also as a sludge treatment method, the solid-liquid separability of the digested sludge and the sludge must be taken into consideration. Must be. It has been written in books and other sources for some time that anaerobic digestion of sewage sludge improves solid-liquid separation, but in reality this is not the case; The solid-liquid separation property is not better than that of sludge (described as sludge). This is often expressed in numbers in foreign books.

In reality, in order to improve the solid-liquid separation of digested sludge, the digested sludge is washed with several times the amount of fresh water or treated water, and the digested sludge obtained by sedimentation and concentration is targeted for solid-liquid separation. However, sedimentation and concentration are poor here as well, and the separated liquid initially returns to the settling tank with a considerable amount of suspended solids in it. The suspended solids that initially returned to the settling tank then return to the anaerobic digestion tank, creating the most unfavorable situation known as sludge circulation within the site.
In addition, even if the digested sludge is not washed, in the current two-tank digestion system, the purpose of the second digestion tank is mainly to settle and concentrate the sludge that has exited the first digestion tank. Unfortunately, it also causes a large amount of sludge to be circulated within the site. Even if the suspended solids (hereinafter referred to as sludge) that have come out of the digester once return to the digester, they no longer have the ability to generate gas as an energy source, and are still thrown into the digester. It has to be heated to a certain temperature, which is a waste of energy.

The in-house circulation of these sludges is the biggest problem in achieving energy recovery in actual anaerobic digestion, and this is caused by the poor solid-liquid separation properties of digested sludge. Furthermore, poor solid-liquid separation greatly affects the dehydration process, increasing the amount of chemicals required in this process, and reducing the number of dehydrators required because the amount of solids processed per unit time by one dehydrator is small. or the operating time becomes longer,
This also increases the amount of power consumed.

In this way, the quality of solid-liquid separation has a large effect on the amount of chemicals and electricity consumed in the dehydration process, and even if the solid-liquid separation of anaerobic digested sludge is worse than that of mixed raw sludge, For example, when evaluating anaerobic digestion methods, it is necessary to comprehensively consider the advantage of energy recovery and the disadvantage of consuming a lot of energy in the solid-liquid separation process. However, the efforts that have been made to date for anaerobic digestion have been focused solely on energy recovery, without any consideration given to improving solid-liquid separation, and only on making the strengths even better. I can't deny what I've done.

In contrast, the present invention aims to provide an effective method that can improve the shortcomings in the solid-liquid separation process and thereby further improve the overall evaluation of anaerobic digestion. That is.

That is, the present invention employs a two-phase digestion method for the anaerobic digestion treatment process consisting of an acid production phase (first stage) and a gasification phase (second stage), and the sludge that has passed through the first stage of acid production is concentrated ( When the separated liquid obtained by sedimentation, flotation, or centrifugation is mixed with the digested sludge that can be discharged from the second stage, the pH of the separated liquid is
The method is characterized in that ferric chloride is added and mixed with digested sludge to give a concentration of 4.8 to 7.0, and the concentrated sludge obtained in the concentration step is transferred to the gasification phase for treatment. This is a sewage sludge treatment method that eliminates the circulation of digested sludge within a treatment plant and reduces the amount of chemicals required in the dewatering process.

One embodiment of the present invention will be described with reference to the drawings. Mixed raw sludge 1 is subjected to acid generation treatment in a first anaerobic digestion tank 2 and then treated in a sedimentation thickening tank 3. The thickened sludge 4 is gasified in a second anaerobic digestion tank 5,
Digested sludge 7 obtained flows into a mixing tank 8 together with separated water 6, where ferric chloride 9 is added and mixed, and then separated into separated water 12 and thickened sludge 11 by a flotation thickening tank 10, and the thickened sludge 11 is transferred to a belt. It is processed in a mechanical dehydration step 13 using a press type dehydrator or the like. In the figure, 14 is generated gas. Note that the sludge treated in the mixing tank 8 may be directly transferred to the mechanical dewatering step 13.

Generally speaking, the reaction mechanism of anaerobic digestion of sewage sludge consists of two stages: acid generation and gasification of the generated volatile organic acids, and the two-phase digestion method has separate processes for each function. The digested sludge obtained from the gasification phase (second stage) is no different in properties from the digested sludge obtained from a normal anaerobic digestion method that does not use a two-phase digestion method. It's not something you have.
Therefore, the digested sludge discharged from the gasification phase (second stage) has a high alkalinity, and acid is added to it to adjust the pH.
When the sludge is lowered, carbon dioxide gas is generated, and as a result, the digested sludge produces separated water during flotation concentration without using pressurized water as in normal pressure flotation concentration. As mentioned above, in reality, in order to improve the solid-liquid separation of digested sludge, the digested sludge is washed with several times the volume of fresh water or treated water, and the digested sludge obtained by sedimentation and concentration is converted into solid-liquid. However, due to poor sedimentation separation, the separated liquid returns to the settling tank with a considerable amount of suspended solids, creating a situation called sludge circulation within the site.

In contrast, in the present invention, digested sludge is washed with a solution obtained by sedimentation or flotation separation of sludge that has gone through the stage of acid production, with ferric chloride added to it. Because of this, carbon dioxide gas is generated. As a result, the digested sludge is floated and concentrated to produce separated water, so sludge can be effectively concentrated by flotation and concentration or by sedimentation and concentration after deaeration. The purpose of adding ferric chloride is not only to improve the concentration of digested sludge, but also to increase the acidity of the washed water, and the suspended solids concentration in the separated water after washing is Sludge recovery rate is 99% without exceeding 100mg/
Because of the above, an unfavorable situation such as internal circulation of sludge does not occur. Moreover, the amount of washing water does not require more than several times the amount of digested sludge.

In the present invention, the above-mentioned washing water refers to a separated liquid obtained by sedimentation or flotation separation of sludge that has gone through the first stage of acid production. It may be less than the amount. Naturally, since the amount of washing water is larger than the amount of digested sludge, the more sedimentation and separation area is required for the cleaning effect, taking this into consideration, it is desirable to increase the cleaning effect by reducing the amount of washing water as much as possible. Then, washed concentrated digested sludge with a much higher concentration can be obtained by allowing the sludge to stand for about the same amount of time as washing with washing water several times the amount of digested sludge and then sedimentation and concentration.

In other words, in the present invention, a sufficient cleaning effect can be achieved by adding a small amount of ferric chloride to a very small amount of cleaning water compared to the conventional method, and the method can achieve the same level of sedimentation concentration or flotation concentration time as the conventional method. Washed digested sludge with a higher concentration can be obtained, and the concentration of suspended solids in the separated water is also much lower than in conventional methods, making it possible to break the vicious cycle of internal circulation of sludge. If the concentration method after washing is flotation concentration, for example, the time required for flotation is longer than that of pressure flotation concentration, but no pressurized water is required, and therefore no electricity is required to create pressurized water. I don't. In addition, even if you perform sedimentation and concentration after deaeration, deaeration is very easy.
Sedimentation and concentration properties are also significantly improved compared to conventional methods.

In the present invention, the separated liquid obtained by sedimentation or flotation separation of sludge that has undergone an acid generation period, that is, the washing water, has a pH of 4.8 to 5.5, and about the same amount of digested sludge is mixed with this washing water. PH after is 4.8
It is important to add ferric chloride so that the ratio is 7.0 to 7.0, preferably 5.5 to 6.5, and the amount of ferric chloride added is in the range of 40 to 70 g per 1 kg of solids after mixing.

If the sludge is sedimented and concentrated in the second digestion tank without washing the sludge in the first digestion tank, a much larger amount of sludge will be circulated within the site than when cleaning with the conventional method, and it will take a longer time for sedimentation and concentration. However, the concentration of the concentrated digested sludge obtained is also low, so the present invention can be said to be a much more advantageous method. As described above, the present invention has superior advantages over conventional methods in terms of washing and concentrating digested sludge.

Next, regarding the dehydration process, which is the main part of solid-liquid separation operation, when dewatering is performed using a polymer flocculant, the amount required in the present invention is considerably smaller than that for conventional washed and digested sludge, and is only 50 to 60%. Adequacy can be cited as an advantage. Since the ferric chloride added during the cleaning process has the effect of acting as a coagulation aid, the amount of polymer flocculant required is gradually reduced. Increasing the amount will not necessarily reduce the amount of polymer flocculant required during the dehydration process; if too much is added, the polymer will not flocculate even if a large amount of polymer flocculant is added during the dehydration process, making dehydration very difficult. becomes.

From this point of view, there are limits to the amount of ferric chloride that can be added to bring about an effect in the dehydration process, and the amount must be added to the separated liquid obtained by sedimentation or flotation separation of sludge that has passed through the acid generation phase. Afterwards, when this is mixed with the digested sludge flowing out from the gasification phase, its PH
The amount should be such that the ratio is 4.8 to 7.0, preferably 5.5 to 6.5.

Of course, the unit price of polymer flocculant is quite expensive, and the amount required is much lower than that for conventional washed and digested sludge.
A reduction of 50 to 60% means that the cost of chemicals required for the dehydration process can be reduced accordingly. In the case of the present invention, since ferric chloride is added in the cleaning process, the cost must also be taken into account, but even if this cost is included, the total chemical cost is the same as the polymer flocculation required for conventional cleaning and digestion sludge. It is cheaper than the drug price.

As described above, in the present invention, when the separated liquid obtained by concentrating sludge that has passed through the acid generation phase by sedimentation separation, flotation separation, etc. is mixed with the digested sludge that can flow out from the gasification phase, the pH of the sludge increases. After ferric chloride is added to give a concentration of 4.8 to 7.0 and mixed with digested sludge, the mixed sludge is separated into solid and liquid. It is possible to obtain highly concentrated washed and concentrated digested sludge, and the sludge recovery rate reaches over 99%.
Not only can the vicious cycle of internal sludge circulation be broken, but the amount of polymer flocculant required in the solid-liquid separation process can be reduced by nearly half, and the cost of all chemicals, including ferric chloride, can also be reduced. This method offers many benefits, such as being cheaper than the cost of just the polymer flocculant required in the conventional method.

Examples based on the present invention are shown below.

Example 1 Solids concentration (hereinafter abbreviated as TS) 3.5%, volatile solids concentration (hereinafter abbreviated as VS) 2.7%, loss on ignition (hereinafter abbreviated as VS/TS) 77.1%, pH 5.7. The mixed raw sludge was put into an anaerobic digestion tank (corresponding to the acid production phase of the first stage) whose internal temperature was controlled at 35±1°C at a rate of 3 days per day, and the tank was stirred for 3 days. When retained, TS3.0%, VS2.2%, VS/TS72.9%, PH
Acid-generated sludge of 5.1 was obtained. When this is sedimented and concentrated in a sedimentation and thickening tank for 4 days, the volume occupied by the sedimented and thickened sludge becomes 54%, with a TS of 5.3% and a VS of 3.8%.
It was 1.62/day. Sedimentation separated water has TS0.23%,
VS was 0.17%, 1.38/day. When this thickened sludge with a TS of 5.3% is fed at this flow rate, the temperature inside the tank is 35±1.
When it is put into an anaerobic digestion tank (corresponding to the second stage methane production phase) maintained at a temperature of 21.2°C and allowed to stay there for 10 days while stirring, gas is generated at 21.2°C.
Digested sludge with TS 3.9%, VS 2.5%, VS/TS 63.0%, and PH 7.75 was obtained at 1.62/day.

This and the previous sedimentation separated water (TS0.23%, VS0.17%)
1.38/day TS1g after mixing
50mg of ferric chloride (1105mg/mixture)
) was added. As a result, TS2.2%, VS1.4%,
VS/TS was 63.8% and PH was 6.4. When this was transferred from the mixing tank to another tank and left for 2 days, the capacity of the flotation concentration section was 54.5% (i.e. 1.64/day) and the TS was 4.0%.
It became. Also, the TS of separated water by flotation concentration is 83
mg/day, 1.37/day. When this floating concentrated sludge was dehydrated using a belt press type pressure dehydrator, 0.56% of polymer flocculant was required per TS, and the water content of the dehydrated cake was 82.7%. The cost of the consumed chemicals was 4.25 yen for ferric chloride and 10.36 yen for polymer flocculant per 1 kg of TS, for a total of 14.61 yen, and the TS contained in the water separated by flotation concentration was 0.12 g/day.

Example 2 The same sludge as in Example 1 was used until the first stage of acid production phase and sedimentation concentration, and the tank temperature was 35 ± 1°C.
When it is put into an anaerobic digestion tank (corresponding to the second stage methane generation phase) maintained and managed in the anaerobic digestion tank and left to stay for 8 days while stirring inside the tank, 20.0 gas is generated per day, TS 4.0%, VS2.6%, VS/TS64.2%, PH7.72
of digested sludge was obtained at a rate of 1.62/day.

This and the previous sedimentation separated water (TS0.23%, VS0.17%)
1.38/day TS1g after mixing
60mg of ferric chloride (1356mg/mixture)
) was added. As a result, TS2.3%, VS1.5%,
VS/TS was 64.6% and PH was 5.8. When this was transferred from the mixing tank to another tank and left for 2 days, the capacity of the flotation concentration section was 57.0% (i.e. 1.71/day), TS 4.0%.
It became. Also, the TS of the separated water is 38mg/
, it was 1.29/day. When this floated concentrated sludge was dehydrated using a belt press pressure dehydrator, 0.5% of polymer flocculant was required per TS, and the moisture content of the dehydrated cake was 83.1%. The cost of consumed medicine is 1Kg.
per TS of ferric chloride 5.1 yen, polymer flocculant 9.25 yen
The total cost was 14.35 yen, and the TS contained in the water separated by flotation concentration was 0.05 g/day.

Next, a comparative example will be shown in which only the solid-liquid separation section was performed using the conventional method.

Comparative Example The same sludge as in Example 2 was used up to the outlet of the anaerobic digestion tank corresponding to the second methanogenic phase. In other words, when treated sewage water was mixed with 1.62/day of digested sludge with a TS of 4.0% and sedimentation was concentrated for 2 days, the sedimentation/concentration part was 25.5% and the separated liquid part was 74.5%.
It became. The sedimentation thickened sludge had a TS of 3.3%, 1.69/day, and the separated liquid had a TS of 0.18%, 4.93/day. When this thickened sludge was dehydrated using the same dehydrator as in Examples 1 and 2, 0.82% of polymer flocculant was required per TS, and the water content of the dehydrated cake was 83.3%. The cost of the consumed chemicals was 15.17 yen per 1 kg of TS, and the TS contained in the separated sediment water was 8.93 g/day.

[Brief explanation of drawings]

The drawings are system explanatory diagrams showing embodiments of the present invention. 1... Mixed raw sludge, 2... First anaerobic digestion tank, 3...
Sedimentation concentration tank, 4, 11... Thickened sludge, 5... Second anaerobic digestion tank, 6, 12... Separated water, 7... Digested sludge, 8
...mixing tank, 9...ferric chloride, 10...floating concentration tank,
13...Mechanical dehydration process.

Claims (1)

[Scope of Claims] 1. In a method of subjecting sewage sludge to anaerobic digestion and then solid-liquid separation, the anaerobic digestion step is a two-phase digestion consisting of an acid production phase (first stage) and a gasification phase (second stage). In this method, sewage sludge is acid-generated in the acid generation phase of the first stage, the sludge that has passed through the first stage is separated into thickened sludge and separated liquid in the concentration process, and the thickened sludge is gasified in the second stage. The separated liquid is mixed with the digested sludge flowing out from the second stage, and ferric chloride is added so that the pH thereof becomes 4.8 to 7.0, and then solid-liquid separation is performed. How to treat sewage sludge. 2 The amount of ferric chloride added is adjusted so that the pH after addition is
The method according to claim 1, wherein the ratio is set to 5.5 to 6.5. 3. The method according to claim 1 or 2, wherein the solid-liquid separation treatment comprises a flotation concentration treatment in the first stage and a mechanical dehydration treatment performed by adding a polymer flocculant in the latter stage.