JP2912960B2 - Pretreatment of sewage - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a pretreatment method for sewage which removes suspended substances (SS) which are time-consuming in the present treatment in advance and reduces a load in a subsequent step, and in particular, a biocoagulation method. The present invention relates to a pretreatment method for removing SS that is difficult to decompose.

[Conventional technology]

In the treatment of municipal sewage, the standard activated sludge method, that is, the solid matter that can be settled in the sedimentation basin is removed in advance, then the raw sewage is guided into the aeration tank, and the dissolved or suspended organic matter is aerated for a long time by microorganisms. Generally, a method of oxidatively decomposing, further leading to a final sedimentation basin to separate sludge, sterilizing the supernatant water and releasing the same is generally employed. This method has the advantage that the BOD removal rate is high and the amount of surplus sludge to be finally treated is small, but on the other hand, a considerable amount of land is required.

Looking at the recent treatment situation, the overcrowding in urban areas has become even more severe, and the amount of sewage to be treated has rapidly increased with this, and the current treatment facilities have fallen into a state where they cannot keep up with the increasing urban sewage. If the current treatment method would solve this problem, it would require a vast site to accommodate the increasing amount of sewage in the future, but it would be difficult to secure such a site in overcrowding.
There is a need for a treatment method that can sufficiently satisfy existing facilities and reduce facility costs and running costs.

Various proposals have been made for this processing method. Above all, JP-A-53-35263 discloses that the first stage of the two-stage activated sludge process has a high load, that is, a BOD volume load of about 10 kg BOD ₅ / m ^3.
It has been proposed that the sludge load be reduced to at least 2 kgBOD ₅ / MLSS, which is difficult to decompose in a day, by actively coagulating and sedimenting, and discharging excess sludge out of the system to reduce the subsequent load. In this example, the residence time of the first aeration tank is not specified, but when the concentration of the inflowing BOD is 200 mg /, the residence time is less than about 30 minutes.

In Japanese Patent Publication No. 1-18795, the first stage is an adsorption stage by the activated sludge method, the second stage is a filtration layer stage for biological treatment, and the first stage sludge load is 2 kgBOD ₅ / MLSS.・ BOD that is difficult to disassemble in the first stage by keeping the day and residence time as short as 45 minutes or less
Has been proposed to actively coagulate sedimentation and discharge excess sludge out of the system to make the facility compact.

[Problems to be solved by the invention]

However, in each case, it targets European sewage. In Europe, the proportion of cellulose in sewage is generally low due to differences in lifestyles, but in urban areas in Japan, suspended solids contain a large amount of fibrous material such as cellulose.
It accounts for 60-85% of the BOD (average about 200 mg /). When operated under such conditions, cellulose which is difficult to decompose cannot be completely treated in the preceding stage, and suspended substances mainly composed of cellulose (S
S) flows into the subsequent stage. In the former case, deterioration of water quality is expected, so a pretreatment facility for removing fiber is required, and the intended purpose cannot be fulfilled on the contrary.In the latter case, clogging of the filtration layer is expected. It becomes difficult, or even if the operation is maintained, the running cost may be high.

Accordingly, an object of the present invention is to provide a method for pretreating sewage in which the above-mentioned disadvantages are improved.

[Means for solving the problem]

In order to solve the above-mentioned problems, the present invention introduces raw sewage into a biological flocculation tank in which the oxidation-reduction potential is kept at -50 mV to 100 mV before subjecting the raw sewage to the main treatment, and the raw sewage is supplied for more than 45 minutes in the tank. The suspended solids suspended in the raw sewage that are retained for less than a minute are coagulated and adsorbed to the sludge, and then the raw sewage is guided from the bottom of the biological coagulation tank to the middle part of the adjacent settling tank, where the sludge is settled in the tank. Then, the supernatant water was sent to the next step of the main treatment, and excess sludge was drawn out of the system from the bottom of the sedimentation tank. A part of the sludge was returned to the biocoagulation tank, and the sludge concentration (MLSS) of the tank was 1.0 to 2.5 g. The method is characterized in that suspended solids (SS) that are difficult to decompose are removed in advance by keeping at /

As mentioned earlier, urban sewage, mainly in residential areas in Japan, contains a large amount of suspended solids (SS) mainly composed of cellulose.
(Average number 200 mg /) often accounts for about 60 to 85%. In such municipal sewage, removing as much SS as possible without biodegradability as it is before applying biological treatment as it is will reduce the burden of treatment and reduce energy consumption. Volumetric efficiency can also be increased. The present invention has been made for such a purpose, and unlike European and American countries, in order to effectively remove SS containing a large amount of fiber in a short time, a mutual correlation is established based on standard sewage in Japan. It is necessary to determine the MLSS concentration of the biocoagulation tank and the oxidation-reduction potential of the tank, and the residence time appropriately.

Therefore, the present inventors tentatively set the residence time in the biocoagulation tank to 50 minutes, set the surface area load of the sedimentation tank to 5 m / hr, and set the sludge concentration suitable for SS removal using the average wastewater shown below. Upon examination, the results shown in FIG. 1 were obtained.

Raw sewage water quality BOD: 100-220mg / SS: 200-220mg / Soluble BOD: 40-60mg / According to the results of this experiment, the suitable sludge concentration for removing SS is 1.0-2.5g /. When the sludge concentration was less than 1.0 g /, the coagulation effect was reduced, and when it exceeded 2.5 g /, there was a tendency that sedimentation in the sedimentation tank became poor.

The redox potential suitable for the activity of aerobic microorganisms is
It is generally known that it is between −200 mV and +400 mV. Then, when the present inventors conducted an experiment using the same sewage as above, the results shown in FIG. 2 were obtained.

Operating conditions Residence time in biological flocculation tank: 50 minutes Sedimentation tank surface area load: 5 m / hr In this experiment, as the oxidation-reduction potential became higher than -200 mV, SS in the treated water gradually decreased, and
At 50mV, it suddenly decreased and then became asymptotically constant. From this result, it was found that the redox potential of the biocoagulation tank should be set to -50 mV or more in order to increase the SS removal efficiency. In addition, the upper limit is preferably set to +100 mV in consideration of economic efficiency because the processing amount may increase in the future and the cost may increase.

An experiment was also conducted on the residence time.
The result as shown in the figure was obtained. From this result,
The longer the residence time, that is, the longer the aeration time, the higher the SS removal rate.However, there is a limit at about 45 minutes, and the removal rate may not increase significantly even if it is longer. understood. At the time exceeding 45 minutes, a removal rate of 70% or more can already be secured, so that at least the residence time is desirably a length exceeding 45 minutes. Therefore, SS cannot be effectively removed if the residence time is kept at 45 minutes or less as in the conventional case. The upper limit of the residence time is desirably 70 minutes in consideration of efficient operation of the site area and running costs.

When operating under such conditions, it is desirable to set the surface load of the precipitation tank to 4 to 6 m / hr.

In the treatment, raw sewage flows from the bottom of the biological flocculation tank to the middle of the sedimentation tank, so that it is difficult for light flocs to flow, and the water quality can be improved.

[Action]

When applied to existing facilities, the present invention may be used instead of the sedimentation basin at first. In this way, even if the processing amount increases, it is not necessary to increase the area of the separate site, and the load on the subsequent stage can be reduced, so that the processing capacity can be further improved. Further, in the present invention, various combinations are possible, for example, in addition to activated sludge aeration tank-final sedimentation tank, contact oxidation tank-sand filtration tank, aerobic filter bed tank, or anaerobic fluidized bed tank, etc. Can be combined. In any case, the SS that is difficult to disassemble is removed in the first stage, so that the load in the second stage can be increased.

〔Example〕

 Hereinafter, description will be made with reference to FIG.

First, the outline of the plant will be described, and then the operation method will be described. The main part of the plant is constituted by a biological flocculation tank 1 and a sedimentation tank 2. And one biological coagulation tank 1
Is formed in such a size that a residence time of more than 45 minutes and not more than 70 minutes can be secured. Of course, the size of the biocoagulation tank 1 will naturally vary depending on the treatment amount, but the effective height is 4.5 m. Raw sewage drained from each household is received in the raw water tank 3 as in the conventional case, and is quantitatively sent to the biological flocculation tank 1 via the raw water pump 4 from here. An air blowing nozzle 5 is provided at the bottom. Also,
In order to perform a continuous treatment, the bottom of the biocoagulation tank 1 is communicated with the center of the adjacent settling tank 2.

On the other hand, the sedimentation tank 2 is formed so that the surface area load is 5 m / hr, and the effective height is 4.5 m as in the biocoagulation tank 1. The bottom of the sedimentation tank 2 is formed in a conical shape so that sludge is easily deposited, and a condenser 6 formed in a substantially similar shape is installed in the inside thereof. The condenser 6 is rotated at a speed of three revolutions per minute by a motor (not shown) installed above the settling tank 2. A discharge pipe 7 for discharging surplus sludge is connected to the bottom. The discharge pipe 7 is connected with an excess sludge pump 8 for discharging the excess sludge out of the system and a return pump 9 for returning a part of the sludge to the biological flocculation tank 1.

 Next, the operation method will be described.

First, raw sewage drained from each household is received in a raw water tank 3, where the water quality is adjusted, and then supplied to the biological flocculation tank 1 continuously by a raw water pump 4. Next, air is blown from the air blowing nozzle 5 so that the oxidation-reduction potential in the tank becomes −50 to +100 mV. The raw sewage is supplied so that the residence time is 70 minutes. After a few days, the SS that has passed through the bottom of the biocoagulation tank 1
The concentrated sludge is drawn out by the return pump 9 and returned to the biological flocculation tank 1. The return is performed so that the MLSS concentration in the biocoagulation tank 1 becomes 1.0 to 2.5 g /. This operation may be performed intermittently. It takes several days for this system to stabilize. When the system is stabilized, excess sludge accumulates in the sedimentation tank 2, so that a certain amount of the sludge is drawn out by the excess sludge pump 8 and discharged out of the system.

Table 1 shows the results of the processing performed in this manner. This data is the average of four consecutive weeks,
2,4,6,8 were conducted for 3 hours (from 7:00 am to 10:00 am) in the morning when the load fluctuation was high even during the day. Experiment numbers 1, 2, 3, 4, 5, 6, and 7, 8 were performed during the same period, respectively.

〔The invention's effect〕

According to the present invention, it is possible to use the sedimentation basin in the first place, so that it is not necessary to expand the step area according to the amount of treatment, and it is also possible to reduce the load on the subsequent step, so that the processing capacity is further improved. be able to.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the MLSS concentration in the biocoagulation tank and the SS removal rate. FIG. 2 is a graph showing the relationship between the oxidation-reduction potential in the biocoagulation tank and the SS concentration of the treated water. The figure is a graph showing the relationship between the residence time in the biocoagulation tank and the SS removal rate,
FIG. 4 is a schematic diagram showing an outline of a plant used for carrying out the present invention. 1 ... biological coagulation tank, 2 ... sedimentation tank, 3 ... raw water tank, 4 ...
... air blowing nozzle, 6 ... condenser, 7 ... discharge pipe,
8 Excess sludge pump, 9 Return pump

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-53-35263 (JP, A) JP-A-62-38296 (JP, A) JP-A-58-89992 (JP, A) 47455 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) C02F 3/12

Claims (1)

(57) [Claims] (1) Before subjecting raw sewage to the present treatment, raw sewage is introduced into a biological flocculation tank in which the oxidation-reduction potential is kept at -50 mV to +100 mV, and retained in the tank for more than 45 minutes and up to 70 minutes. Aggregate and adsorb fibrous suspended solids such as cellulose that are difficult to decompose suspended in sewage, and then guide raw sewage from the bottom of the biological coagulation tank to the middle of the adjacent settling tank, where the sludge is settled. Then, the supernatant water is passed to the next step of the main treatment, and excess sludge is drawn out of the system from the bottom of the sedimentation tank. A part of the sludge is returned to the biological flocculation tank, and the sludge concentration (MLSS) of the tank is adjusted to 1.0 to 2.5. A method for pretreatment of sewage, characterized in that said suspended matter that is difficult to decompose is removed in advance while maintaining g / liter.