Causes and Solutions of Activated Sludge Aging

1.   Summary of Activated Sludge Aging Phenomenon

The aging of activated sludge is widespread in most aerobic biochemical systems in operation at present, and the aging of activated sludge will not only lead to the increase of main pollution indexes of effluent, but also waste energy. Because the aging of activated sludge is usually related to overaeration and low load, and these operation problems will consume excessive energy.

2. Key Points for Judging Activated Sludge Aging  

(1) Performance of Activated Sludge Settlement Ratio Observe whether activated sludge is aging

1) Activated sludge settling rate. Generally, the settling rate of re-activated sludge is about 1.4 times faster than that of non-aged activated sludge. It is found in the experiment that aged activated sludge can complete the settling stage in a shorter time. Of course, the settling rate of other stages is also quite fast, which is usually about 1.4 times faster than that of non-aged activated sludge.

2) Size of activated sludge flocs. Aging activated sludge flocs are large, but loose, and their flocculation speed is also fast.

3) Activated sludge color. The color of aged activated sludge is very dark, gray-black and has no fresh luster.

4) Clear degree of supernatant. The aged activated sludge is easy to disintegrate, so there are more fine flocs free in the water body, but the interstitial water between flocs keeps good clarity.

5) Liquid level scum. The generation of scum is indeed related to the aging of activated sludge. Because aging activated sludge will lead to the death of some bacteria, the disintegrated bacteria will be scattered by aeration and adhere to bubbles, resulting in scum or foam.

(2) Observe whether the activated sludge is aged by microscope

Usually, the number of metazoans is dominant. On the surface, it seems that it has nothing to do with protozoan performance, but in fact it is obviously related. The main performance is that if metazoan is dominant, there will definitely be no obvious advantage of non-activated sludge protozoa, and a very small number of stragglers can be seen at most; On the contrary, the same is true. When non-activated sludge protozoa are dominant, metazoans are usually not seen. Therefore, the mass reproduction of metazoans can be used as an index of activated sludge aging.

(3) Confirmation of food/micro ratio

Under the condition that activated sludge aging usually occurs or may occur, the feed-to-micro ratio is at or at a low level for a long time, especially when the feed-to-micro ratio is lower than 0.05, the probability of activated sludge aging is very high.

3. Cause Analysis of Activated Sludge Aging

(1) The sludge discharge is not timely and the sludge age is too long

(2) The inflow water is in a low load state for a long time

(3) Activated sludge aging caused by overaeration

The direct result of overaeration is the disintegration and self-oxidation of activated sludge. The reason for disintegration is the disintegration of activated sludge caused by frequent shearing. The understanding of self-oxidation is that oxygen itself is an oxidant, and overaeration will naturally oxidize activated sludge.

(4) Activated sludge concentration control is too high. Activated sludge concentration control is too high, and there is not enough influent substrate concentration support, which will eventually lead to activated sludge aging.

4. Effective methods to inhibit the aging of activated sludge

(1) Requirements for Control of Activated Sludge Concentration in order to ensure that activated sludge will not age due to untimely sludge discharge during the operation of biochemical system, we should often confirm the relationship between current sludge discharge flow and activated sludge concentration, and indirectly guide the control of activated sludge discharge flow through confirmation of feed-to-micro ratio. At the same time, it is necessary to achieve the uniformity of sludge discharge flow, and avoid the sludge discharge mode with gap and excessive flow fluctuation.

(2) The uniformity of aeration and the prevention of overaeration require effective control of aeration volume to avoid overaeration. The DO concentration at the outlet of aeration tank can be controlled at about 2.5 mg/L. At the same time, it can also reduce the excessive consumption of electric energy by aeration, laying a foundation for reducing the treatment cost.

(3) Avoidance of low load operation state to avoid the occurrence of low load operation state, so as to avoid the occurrence of activated sludge aging. In addition to increasing the concentration and biodegradability of substrate in influent as much as possible, the concentration of activated sludge should be reduced as much as possible to ensure that the feed-micro ratio can be kept within a reasonable control value (about 0.15-0.25). When necessary, additional carbon sources can be added to ensure the normal operation and reproduction function of activated sludge, such as adding septic tank water and introducing domestic sewage.

5. Performance of each process control index during activated sludge aging

The relationship between each process index and activated sludge aging is quite close, and these relationships also help us to confirm whether activated sludge is aging and whether correcting aging is in place and accurate.

(1) The low control of feed-micro ratio is an important reason for the aging of activated sludge, which should be said to be relatively easy to adjust. There is a positive correlation between the aging degree and the low feed-micro ratio.

(2) The relationship with dissolved oxygen and the relationship with dissolved oxygen, in addition to the aging of activated sludge caused by excessive aeration and excessive control of dissolved oxygen, such problems will become more prominent under the condition of low feed/micro ratio. Aeration exceeding 4.0 mg/L should be classified as excessive waste aeration, and it is common for such aeration results to promote activated sludge aging.

(3) It is a reasonable range to keep the sludge age for 7-10 days in the relationship with the sludge age. Special attention should be paid to the phenomenon of sludge age exceeding 1 month. Such sludge age control is inevitable when activated sludge ages.