JP2000093990A - Treatment of sewage by activated sludge - Google Patents

Abstract

PROBLEM TO BE SOLVED: To minimize the amount of a flocculant used and to prevent the clogging of a sand filter bed by controlling the addition amt. of the flocculant by an automatic control mechanism measuring the turbidity of waste water and regulating the addition amt. of the flocculant to activated sludge on the basis of the measured result. SOLUTION: The sludge of an excessive sludge tank 5 is drawn out by a pump to be mixed with the flocculant sent from a flocculant tank 9 by a pump and the resulting mixture is mechanically dehydrated by a dehydrator 10. The formed waste water is allowed to flow in a waste water sump 11 and sent out to a supernatant water tank 6 by a pump. A turbidity meter 12 is arranged on the way from the dehydrator 10 to the waste water sump 11 to automatically measure the turbidity of the waste water. The measured result is sent to the control panel of the motor of the pump sending out the flocculant from the flocculant tank 9 and the number of rotations of the motor is regulated to control the addition amt. of the flocculant. The turbidity can be measured continuously and intermittently and the control of the addition amt. of the flocculant based on the measured result can be also performed by an arbitrary method.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method for treating wastewater with activated sludge. More specifically, the present invention relates to a method for reducing the amount of a coagulant used when dehydrating excess activated sludge.

[0002]

2. Description of the Related Art The treatment of wastewater with activated sludge is widely practiced. In one of the typical methods, sewage is continuously introduced into an aeration tank, and sewage is aerated in the presence of activated sludge to decompose organic matter in the sewage. The sewage that has undergone aeration treatment flows into a settling tank together with the activated sludge to settle the activated sludge. The activated sludge is extracted from the settling tank and circulated to the aeration tank. The supernatant water flowing out of the settling tank is passed through a sand filtration layer to remove suspended matters, and then discharged into public waters. If the supernatant water is colored, it is passed through a sand filter layer and subsequently through an activated carbon layer, decolorized, and discharged into public waters.

In this method, since excess activated sludge is generated, a part of the activated sludge circulated from the settling tank to the aeration tank must be discharged out of the system. Activated sludge has an extremely high water content, so that a flocculant is usually added to the activated sludge and then dewatered by a dehydrator such as a pressure filter or a centrifugal separator and discharged to the outside of the system. The wastewater squeezed out of the activated sludge is passed through a sand filtration layer together with the supernatant water from the settling tank to remove suspended matters.

[0004]

In this method, it is important to prevent clogging of the sand filtration layer. As is well known, the properties of activated sludge are liable to change, so when dewatering excess activated sludge, the content of suspended matter in the squeezed wastewater is considerably large even when the addition ratio of the coagulant to sludge is constant. Changes and, in some cases, suspended matter can cause clogging of the sand filtration layer. If the sand filtration layer is clogged, it must be backwashed to eliminate the clogging, and normal operation is significantly impaired. Therefore, the coagulant is usually added excessively for safety of operation so that the suspended solids concentration of the wastewater does not rise above the upper limit even if the properties of the activated sludge change. However, it is economically disadvantageous to add a coagulant more than necessary. Accordingly, an object of the present invention is to provide a method for preventing clogging of a sand filtration layer by adding a minimum amount of a flocculant.

[0005]

According to the present invention, sewage is aerated in an aeration tank in the presence of activated sludge, and treated water flowing out therefrom is allowed to flow into a settling tank to settle activated sludge.
Activated sludge and supernatant water are extracted from the sedimentation tank. After adding a flocculant to the excess of the activated sludge circulated to the dewatering unit, dewatering is performed by using a dewatering device.The wastewater is passed through a sand filtration tank together with the supernatant water of the sedimentation tank, and the turbidity of the wastewater is reduced. The amount of flocculant added is controlled by an automatic control mechanism that measures the amount of flocculant added to the activated sludge based on the measurement results and minimizes the amount of flocculant used. Clogging can be prevented.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, in the treatment of sewage using activated sludge, when a coagulant is added to excess activated sludge and dewatering is performed, the turbidity of the wastewater squeezed out by dehydration is measured, and the measurement result is obtained. The addition of the coagulant is controlled by an automatic control mechanism that adjusts the addition ratio of the coagulant to the activated sludge based on the above. As a result, the amount of the coagulant used can be minimized, and the subsequent sand filtration layer can be prevented from being clogged.

More specifically, the present invention will be described with reference to FIG. 1 which shows an example of a flow sheet when the present invention is carried out. In the aeration tank 1, sewage is continuously supplied through a conduit 2, and air is continuously supplied through a conduit 3. The organic matter in the sewage is decomposed by activated sludge.
The treated water flowing out of the aeration tank 1 is allowed to flow into the settling tank 4, where it is allowed to stand still to settle the activated sludge. The settled activated sludge is extracted by a pump, and most of the sludge is circulated to the aeration tank 1, and the excess is discharged to the excess sludge tank 5. The supernatant water in the settling tank 4 is the supernatant water tank 6
, And is drawn out by a pump, sequentially passed through a sand filtration layer 7 and an activated carbon layer 8 to remove suspended matters and coloring components, and then discharged into public waters. Usually, a plurality of sand filtration layers and activated carbon layers are provided and periodically regenerated one after another so that water can be passed almost continuously as a whole. On the other hand, the sludge in the surplus sludge tank 5 is extracted by a pump, mixed with a flocculant sent from the flocculant tank 9 by a pump, and then mechanically dewatered by a dewatering device 10. The generated wastewater is allowed to flow into the drainage reservoir 11 and sent out to the supernatant water tank 6 by a pump. In many cases, a plurality of dehydrators are installed as shown in the figure. A turbidity meter 12 is installed on the way from the dehydrating device 10 to the drainage reservoir 11,
The turbidity of the wastewater is automatically measured. The measurement result is sent to the control panel of the motor of the pump that sends out the flocculant from the flocculant tank 9, and the addition amount of the flocculant is controlled by adjusting the rotation speed of the motor. The turbidity can be measured continuously or intermittently, and the amount of the flocculant added can be controlled based on the measurement result by any method. Preferably, a centrifuge is used as the dewatering device, and the coagulant is added to the sludge in a supply conduit for supplying the sludge to the centrifuge. As soon as possible. In addition, the amount of coagulant to be added is measured by measuring the turbidity of the wastewater and comparing the measurement result with a predetermined reference turbidity range. If so, it is preferable to repeat the turbidity measurement-addition amount adjustment for a short time, for example, every 30 seconds to 1 minute so as to induce the turbidity of the wastewater into the reference turbidity range. In a preferred embodiment of the present invention, a control time period in which such adjustment of the amount of the coagulant is performed for a predetermined time and a non-control time period in which the coagulant is added for a predetermined time at an addition rate selected at the end of this time period Is repeated. The control time period of 5 to 15 minutes is sufficient, and the non-control time period of 30 minutes or less is unlikely to hinder the passage of water through the subsequent sand filtration layer due to the increase of suspended solids in the wastewater. Absent. In this way, the turbidity of wastewater from a plurality of dehydrators can be measured by one turbidity meter by sequentially switching the target wastewater.

When the present invention is applied to the treatment of wastewater generated in a coke manufacturing plant, it is compared with a case where the operator checks the turbidity of the wastewater every few hours and manually adjusts the amount of the coagulant added. As a result, the use amount of the flocculant could be reduced by about 60%. In a coke manufacturing plant, the coke oven gas generated contains water adhering to the raw material coal and water generated by the decomposition of the coal, and the coke oven gas is cooled and cooled.
During the purification process, colored BOD-rich wastewater is discharged. Sewage is also discharged during the process of collecting coal tar in coke oven gas. Therefore, in a coke manufacturing plant, these wastewaters are discharged into public waters through activated sludge treatment, sand filtration, and activated carbon adsorption, and it is very significant to reduce the cost and labor required for this treatment. is there.

[Brief description of the drawings]

FIG. 1 is an example of a flow sheet when implementing the present invention.

[Explanation of symbols]

 Reference Signs List 1 aeration tank 2 sewage supply pipe 3 air supply pipe 4 settling tank 5 excess sludge tank 6 supernatant water tank 7 sand filtration layer 8 activated carbon layer 9 coagulant tank 10 dehydrator 11 drainage tank 12 turbidity meter

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Motoyoshi Shimomura 1st Bancho-cho, Sakaide City, Kagawa Prefecture Mitsubishi Chemical Corporation Sakaide Office (72) Inventor Kishi 1st Bancho-cho, Sakaide City, Kagawa Prefecture Mitsubishi Chemical Corporation 4D028 AA01 AB05 AC01 AC09 BE02 BE08 CA00 CA12 CB01 CB03 CC00 CD05

Claims (4)

[Claims] 1. Aeration of sewage in an aeration tank in the presence of activated sludge, treated water flowing out therefrom is allowed to flow into a settling tank to settle activated sludge, and activated sludge and supernatant water are extracted from the settling tank. The former is circulated to an aeration tank, and the latter is a method of treating wastewater that is at least passed through a sand filtration layer to remove suspended matter and then discharged. After the addition of water, the water is dehydrated with a dehydrator, and the wastewater is passed through a sand filtration layer together with the supernatant water of the sedimentation tank, and the turbidity of the wastewater is measured. A method characterized in that the amount of coagulant added is controlled by an automatic control mechanism for adjusting the amount of coagulant added to the activated sludge to be provided. 2. The method according to claim 1, wherein a centrifugal separator is used as the dewatering device, and the flocculant is added to the activated sludge in a supply conduit for supplying the activated sludge to the centrifuge. 3. The method of adding a flocculant to activated sludge by measuring the turbidity of wastewater and comparing the measurement result with a predetermined reference turbidity range to increase or decrease the rate of addition of the flocculant. A cycle consisting of a control period during which the turbidity of the wastewater is guided to the reference turbidity range by repeating the above, and a non-control period during which the flocculant is added at the addition rate selected at the end of the control period. 3. The method according to claim 2, wherein the method is performed by: 4. The method according to claim 1, wherein the sewage supplied to the aeration tank is mainly colored sewage generated in a coke plant.