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Detailed explanation of sludge settling ratio (SV)


In the process of sewage treatment, sludge sedimentation ratio (SV) is a very key index. It refers to the percentage volume of sludge after the mixture of aeration tank settles in the measuring cylinder for 30min.

Sludge settling ratio (SV) test, not only need a data result, but also need to understand the whole process of sludge settling. Through detailed observation and analysis, we can get a comprehensive and correct conclusion to guide production control. Therefore, how to carry out the daily sludge settling ratio (SV) test is a very important problem.

Misunderstandings in sludge settling ratio (SV)

In fact, sludge settling ratio test should include three parts, one is test data; the other is observation and record of settling process; the third is comprehensive analysis of results and records.

In fact, in the actual operation and management, SV measurement is convenient, fast and irreplaceable. Through the test, we can understand the structure and settling performance of sludge, and in the absence of other abnormalities, it can be used as a reference for excess sludge discharge.

At the same time, some abnormal phenomena of sludge can also be reflected by sedimentation test, that is to say, if the operator only knows the sedimentation ratio after 30 minutes, but does not carefully observe and analyze some situations in the process of sludge sedimentation measurement, then when the operation is abnormal, the fault information that can prompt us in the process of sludge sedimentation measurement is difficult to obtain And we may not be able to obtain these information from other channels in a timely and accurate manner.

Therefore, in the process of sludge settling test, we should not only observe the settling ratio, but also observe other characteristics of sludge, such as appearance, settling rate, clarity of mud water interface, turbidity of upper liquid, and whether there is suspended solids.

Settlement rate and settlement performance

In the determination of SV, the condition of removing the upper liquid can be divided into fast and slow sludge only in terms of sedimentation rate. The sludge with fast sedimentation rate is not necessarily good, and the sludge with slow sedimentation rate is not necessarily bad. Of course, the so-called “fast” and “slow” are relative.

Sedimentation performance sedimentation performance is a comprehensive assessment index, and sedimentation rate is only one of the most basic contents to correctly observe the sedimentation performance of sludge.
To further understand the reasons for the different sedimentation rate of sludge, through a large number of actual operation data comparative analysis, according to the process characteristics and operation of the unit to measure whether it is within the normal range.

When doing the sedimentation test, the operator should also pay attention to observe the sedimentation at the initial stage of sedimentation and the amount of sludge sedimentation per unit time.
The purpose of sludge settling ratio (SV) test is to judge the settling performance of sludge by test, rather than simply need a data.

Therefore, according to the actual situation, different sludge sedimentation ratio test plans can be made, and the sedimentation ratio in different time can be used respectively.
The test results can be compared with the sludge appearance and sedimentation process records to find out the most suitable test time for the actual sedimentation of sludge in this unit.

Problems in daily test

  1. Test time

    For example, if there is sludge on the surface of the sedimentation tank and whether the bubble rises or not, it is caused by denitrification in the sedimentation tank. It is possible to extend the settlement test time to confirm and confirm it, because in the case of nitrate nitrogen, the sludge that will sink for thirty minutes after the test is finished, then continue to let it rest for a period of time, then the sludge will float along with nitrogen foam when it is anoxic.
    However, in a high load activated sludge system, if the temperature is high and the residence time of sludge in the sedimentation tank is too long and acidification occurs, there will be bubble bubbles and random sludge floating. These bubbles are usually caused by ammonia generated during acidification.
    In addition, when the amount of aeration is too large, and the air can not be fully released after the mixture enters the sedimentation tank, it will also cause the phenomenon of sedimentation tank sludge. So we need to make a comprehensive analysis according to other situations.

  2. Test instrument

    Generally, 1000 ml measuring cylinder (or measuring cup) is used for SV measurement, and 100 ml measuring cylinder is used for some units, which will cause errors. Because the diameter of small measuring cylinder is small, it has a certain blocking effect on sludge sedimentation, and the measured value is likely to be higher, and the error will be greater when the sludge structure is loose.
    The sludge with different settling performance was tested with 1000 ml and 100 ml measuring cylinders respectively. The results showed that for the sludge with good settling performance, the difference between the two measuring results was not significant (5-10% higher for the small measuring cylinder), while the difference between the measuring values of the expanded sludge was more, the maximum error was 40%. That is to say, when the sludge expanded, the SV measured by the small measuring cylinder was much higher than that measured by the large measuring cylinder.

Relationship between sludge settling ratio (SV) and settling performance

The SV value of activated sludge can only roughly reflect the sludge settling performance. The tightness and settling performance of sludge structure are measured by sludge index (SVI), which is calculated according to the sludge concentration and sludge settling ratio. The measurement error of sludge settling will cause the calculation error of sludge index, which is easy to cause misleading.

The SV test is different from the actual sedimentation effect of sludge in the sedimentation tank. Generally speaking, the sludge with low sedimentation ratio has better separation effect in sedimentation tank, otherwise, the effect is poor. However, there are some inconsistencies in the actual operation. The main reasons are as follows:

  1. Different states: SV is measured under static state, while sedimentation tank is dynamic.
  2. Different settling time: the settling time of settling tank is much longer than that of settling test.

  3. Different influencing factors: the operation condition of the sedimentation tank is affected by many factors, including the disturbance of the sludge layer caused by the influent water and the control of the sludge layer required by the production process.

    Therefore, the production and operation management is very important, which has a great impact on the sedimentation of sludge. The settling performance of sludge not only depends on the settling ratio, but also on the working system, effluent quality and sludge utilization rate of the settling tank. And then determine the production cost and product quality.

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