Three Reasons for Sludge Floating in Secondary Clarifier
The sludge floating in secondary clarifier is one of the common problems in wastewater treatment. The direct causes are three: first, the sludge has died; second is sludge aging or disintegrating; the third is the generation of bubbles. All the fundamental reasons for the sludge floating in secondary clarifier can be attributed to these three aspects.
I The sludge has died
First answer a question: Why does the sludge float on the water surface when it has died?
After the death of organism, the cells lose their activity and the cytoplasm undergoes diffusion, resulting in a decrease in density. When the density is lower than that of water, it will float. For example, dead fish, and the same principle applies to bacterial colonies containing a large number of microorganisms.
Common causes of dead sludge：
1. Excessive oil content
When the oil content in the wastewater is too high, when mixed with activated sludge, the oil will agglomerate on the surface of the bacterial flocs, causing aerobic bacteria to die of hypoxia and the sludge to float.
2. PH value too high or too low
Microorganisms require a pH range of 6-9 for their living environment, which means the acidity and alkalinity should be moderate. When pH<4.0 or pH>11.0, the activity of microorganisms will be inhibited or even die, causing sludge to float.
3. The impact of salt content
When the salt content in wastewater is too high, it will increase the osmotic pressure of the wastewater, and microorganisms will die due to water shortage. The mechanism of microbial death is the same as the principle of using salt to kill bacteria in human life.
Excessive salt content, microbial death, and sludge floating.
4. Excessive temperature
All living organisms can only survive within an appropriate temperature range. The temperature range suitable for most microorganisms that make up activated sludge is generally 20-40 ° C. When it exceeds 45 ° C, most microorganisms in the activated sludge will die and the sludge will float.
5. Sludge poisoning
Just like human poisoning can lead to death, microbial poisoning can also lead to death, and in situations where wastewater is specifically treated, it is highly susceptible to poisoning. Toxic substances that can kill microorganisms include heavy metals and their compounds (Hg, Ag, Cu, etc.), organic compounds (phenols and their derivatives, alcohols, aldehydes, and certain organic acids, etc.), halogenated elements and their compounds (iodine, chlorine, and their compounds), oxidants (potassium permanganate, etc.), surfactants, dyes, high levels of COD, etc.
Microbial poisoning leads to death and sludge floats up.
II Sludge aging or disintegrating
First answer a question: Why does sludge aging cause sludge to float?
The aging of sludge itself will accelerate the flocculation speed and shorten the sedimentation time, but at the same time, the bacterial flocs will become loose or even disintegrate, producing small activated sludge particles and cause floating scum.
Common causes of sludge aging：
1. Low sludge load
Long term low load operation, microbial lack of nutrition, sludge aging, and flocs disintegrating.
2. Nutritional imbalance
Imbalance of microbial nutrition, high ammonia nitrogen in the influent, and low C/N lead to the disintegration of the sludge colloid mechanism.
3. Excessive sludge age
The sludge age is too long, the sludge structure is dispersed, and the sludge disintegrates and floats.
4. Excessive aeration
Excessive aeration causes the sludge to be stirred too vigorously, causing it to be crushed and generating a large number of small bubbles that aggregate on the flocs, causing the sludge to float.
III Generation of bubbles
First answer a question: Why do bubbles cause sludge to float?
This can be likened to a phenomenon in daily life, where in the summer, when playing in the water at an amusement park, people will use the buoyancy of airbags such as swimming rings to float on the water surface. The principle of bubbles in the secondary sedimentation tank to float sludge is the same. The bubbles generated in the tank are equivalent to swimming rings, and the bacterial colonies are equivalent to people carrying on the swimming rings. When a large number of bubbles are generated in the tank, the bubbles will attach to the bacterial flocs and carrying sludge to float up.
The reason for the generation of bubbles in the secondary sedimentation tank：
1. Denitrification occurs in the secondary sedimentation tank
We know that biochemical processes often have designs for nitrification/denitrification. When there is a significant nitrification reaction in the aerobic tank and the secondary sedimentation tank accumulates sludge or stays for too long, denitrification will occur in the secondary sedimentation tank to produce N2 bubbles, which attach to the sludge particles and cause the sludge to float.
2. Anaerobic digestion of accumulated sludge at the bottom of the secondary sedimentation tank
If the bottom sludge of the secondary sedimentation tank is fermented, the generated CO2 and H2 will also agglomerate on the activated sludge, causing the sludge to float up. In this case, the floating sludge appears black.
3. Water contains a large amount of surfactants
When there is an excess of surface active substances in water, the stirring flow process will produce bubbles, which are easily attached to the bacterial flocs, reducing the specific gravity of the activated sludge and causing it to float.
4. The bubbles in the aeration tank are carried to the sedimentation tank
The bubbles in the aeration tank float up to the sedimentation tank, and this sludge is granular in shape with no change in color. The direction of upward flipping is turbulent flipping from the wall of the diversion zone to the wall of the sedimentation zone.
