Why Are Solids Are Floating on My Secondary Clarifier?Sludge Bulking

One of the common problems in sewage treatment using biochemical methods is that the microorganisms in the activated sludge will be affected by various internal and external factors, causing the specific gravity of the activated sludge to decrease and float away. Sludge bulking refers to the phenomenon that the sludge structure is extremely loose, increases in volume, floats, and is difficult to settle and separate, affecting the quality of the effluent. Basically, sludge expansion will occur in all types of activated sludge processes at present. Sludge bulking not only has a high incidence rate and is common, but also difficult to control once it occurs, and it usually takes a long time to adjust. Generally speaking, when the SVI value of activated sludge is around 100, its settling performance is the best. When the SVI value exceeds 150, it indicates that the activated sludge is about to expand or is already in a state of expansion, and should be paid attention to immediately.This not only increases the amount of suspended solids in the effluent, but also increases the amount of suspended solids in the effluent. And it will greatly reduce the activity and quantity of activated sludge in the biological reaction system. This has caused certain difficulties in the operation, operation and control of the sewage treatment plant, seriously affecting the quality of the effluent.

1、Reasons for activated sludge floating

The causes of activated sludge floating can be roughly divided into two categories. One type is caused by changes in incoming water quality, and the other type is caused by process operation control.In addition, when the oil content in the incoming water is too high, after aeration and mixing, the oil will agglomerate on the surface of the bacterial jelly mass, causing the bacteria to die due to lack of oxygen, causing the specific gravity to decrease and float.

1）. Incoming water quality causes activated sludge to float

(1) Excessive surface active substances and grease compounds

When normal sewage enters the aeration tank for operation, specific surfactants partially degrade organic matter to form foam, which causes the foam to grow rapidly. These foams are generally white and light and disappear when the activated sludge reaches maturity. When there are excess surfactants in sewage, these substances can affect the stability and permeability of the cell plasma membrane, causing the loss of certain essential components of the cells, leading to stagnant growth and death of microorganisms. A large amount of foam (bubbles) is generated during aeration, and these bubbles can easily agglomerate on the bacterial jelly, causing the specific gravity of activated sludge to decrease and float. In addition, when the oil content in the incoming water is too high, after aeration and mixing, the oil will agglomerate on the surface of the bacterial jelly mass, causing the bacteria to die due to lack of oxygen, causing the specific gravity to decrease and float.

(2) pH value shock

A pH value that is too high or too low will affect the catalytic effect of activated sludge microbial extracellular enzymes and enzymes present in the cytoplasm and cell wall, as well as the absorption of nutrients by microorganisms. When the pH in the continuous flow aeration reaction tank is <4.0 or pH>11.0, in most cases the microbial activity in the activated sludge is inhibited, or loses activity, or even dies, resulting in sludge floating.

(3) Influence of water temperature and salt content

The suitable temperature range for the microorganisms that make up activated sludge is generally 15-35°C. When it exceeds 45°C, most of the microorganisms in the activated sludge will die and float (except for long-term domesticated or special microorganisms).

Adjusting the pH value of the incoming water cannot eliminate the impact of alkalinity on activated sludge. Adjusting the pH value of alkaline feed water will neutralize alkaline substances, but produce salt. Different salt solution concentrations have different osmotic pressures. Osmotic pressure is one of the important factors affecting the survival of microorganisms. If the osmotic pressure of the solution in which the microorganism is located changes, it will lead to cell death.

(4) Toxic substrates

Substrates that are toxic to aerobic activated sludge microorganisms mainly include: excessive COD, organic matter (phenols and their derivatives, alcohols, aldehydes and certain organic acids, etc.), sulfides, heavy metals and halides. High substrate concentration can form stable compounds with the enzyme activity center of cells, making the matrix inaccessible, unable to be degraded, and even causing cell poisoning and death. After heavy metal ions enter human cells, they mainly combine with -SH groups on enzymes or proteins to inactivate or denature them. Trace amounts of heavy metal ions can also accumulate in cells and eventually cause poisoning (micro-action) to microorganisms. The most common halides are iodine and chlorine. Iodine irreversibly combines with the tyrosine of bacterial proteins (or enzymes) to generate diiodotyrosine, which inactivates the bacterial cells. Chlorine and water form hypochlorous acid, which decomposes to produce a strong oxidizing agent. Moreover, the mutation of organic matter in wastewater reduces or eliminates the microorganisms that were originally domesticated and can degrade organic poisons.

2）. Activated sludge floating caused by process operation

(1) Excessive aeration

Microorganisms are in a state of starvation, causing their own oxidation to enter the senescence stage, and the dissolved oxygen concentration (DO) in the pool rises; or due to poor sludge activity, the linear speed of the aeration impeller is too high and the oxygen supply is too much. In short, when DO rises, the sludge activity may be very good in the short term because the metabolism is fast and the organic matter is decomposed quickly. However, over time, the sludge will be beaten into light and broken pieces (but without bubbles), and it will float around like mist. On the surface of the sedimentation tank, it flows with the water. This kind of sludge is light in color, has poor activity, decreases oxygen consumption rate, increases sludge volume and sludge index, and significantly reduces the treatment effect.

(2) Sludge floating caused by anoxia and denitrification

When the content of organic ammonia compounds or ammonia nitrogen in the wastewater is high, it can be oxidized to NO3- by nitrate bacteria and nitrite bacteria under suitable conditions. If the secondary sedimentation tank accumulates mud or the residence time is too long, the N2 produced by the reduction of NO3- will be The activated sludge flocs are adsorbed, causing the activated sludge to float. Because denitrification occurs in the secondary sedimentation tank or in the dead space caused by insufficient aeration, tiny nitrogen bubbles are released, thereby reducing the density of the sludge, which is conducive to the floating of activated sludge. This phenomenon is obvious in the secondary sedimentation tank, and the suspended foam produced is usually unstable.

(3) Floating sludge caused by excessive return flow

A sudden increase in the return flow will result in incomplete separation of gas and water, and the bubbles in the aeration tank will be brought to the sedimentation area to float. This sludge will be granular and the color will not change.

(4) Floating sludge caused by mud accumulation at the bottom of the secondary sedimentation tank

If the bottom sludge of the secondary sedimentation tank is fermented, the CO2 and H2 produced will also agglomerate on the activated sludge, causing the sludge specific gravity to decrease and float. Sludge decays to produce CH4 and H2S and then floats. First, small bubbles escape from the water surface, and then black sludge floats up.

(5) Sludge floating caused by excessive growth of activated sludge filamentous bacteria

When the incoming water N and P are insufficient, the PH is too low, the dissolved oxygen of the mixed solution is too low, and the incoming water fluctuates too much, it will cause abnormal growth of microorganisms such as filamentous bacteria and actinomycetes, and the ratio of filamentous bacteria to The growth rate is higher than that of colloid bacteria, and because the specific surface area of filamentous bacteria is larger, filamentous bacteria are much more advantageous than colloid bacteria in obtaining BOD5 substances in sewage and the oxygen required to oxidize BOD5 substances. , As a result, the filamentous bacteria in the aeration tank became the dominant species and increased their value in large quantities, resulting in the production of biological foam. In addition, most of these microorganisms are filamentous or branch-shaped and can easily form nets, which can catch particles and bubbles and float to the surface of the water. The bubbles surrounded by the wire mesh increase the surface tension, making it difficult for the bubbles to break and cause pieces of sludge to float.

2. Control of activated sludge floating

According to the activated sludge floating mechanism and its influencing factors, physical, chemical and biological methods can be used to control sludge floating.

1）. Physical and chemical methods to control foam

(1) spray water

By utilizing the final water return, the sprayed water flow can break up the bubbles and floating sludge floating on the surface of the secondary sedimentation tank to reduce the floating of activated sludge. However, it cannot completely eliminate the phenomenon of sludge floating. It is the most commonly used and simple physical method.

(2) Adjust the pH value of sewage

The entrance of the aeration tank is equipped with a neutralization tank and an automatic pH adjustment system consisting of an alkali tank, an acid tank, a pH detector, a pH automatic adjustment valve, etc., so that the pH value of the water entering the aeration tank is controlled within the required range.

(3) Reasonable addition of nutrients

Due to the imbalance of nutrients in industrial wastewater, carbon sources are often sufficient but nutrients such as nitrogen and phosphorus are insufficient, so additional nutrients must be added when treating industrial wastewater. Generally, urea and phosphate are used as nitrogen and phosphorus sources, but the dosage should not be excessive.

(4) set up the regulating pool

When the incoming water contains excessive surfactants, grease compounds and substrates that are toxic to aerobic activated sludge microorganisms, a final water return adjustment tank should be set up in front of the aeration tank to dilute and adjust the aeration tank. The concentration of organic matter in the incoming water must be stabilized within a certain range. The prerequisite for final water return is that the treatment capacity of the sewage treatment plant must be greater than the actual incoming water volume.

2）. Biological methods to control activated sludge floating

(1) Add a creature selector

There are two types of biological selectors: aerobic selectors and anoxic selectors. Their purpose is to fully mix the sewage entering the aeration tank before the return sludge. By adjusting factors such as F/M and DO, the selectivity can be developed. Flocculant-forming bacteria inhibit excessive proliferation of filamentous bacteria, etc. When designing the selector, the selector needs to be set in grids, generally 4 to 6 grids are used; try to increase the F/M value of the first grid of the selector to form an F/M gradient; the hydraulic retention time of the selector must also be controlled. Usually 15 to 30 minutes.

(2) Optimization of process selection

The microbial composition of activated sludge mainly depends on wastewater composition, flow pattern, operating conditions and appropriate design. Since it is almost difficult to control wastewater composition during actual treatment, optimal selection of operating conditions to control activated sludge flotation is crucial.

(3) Increase or adjust the aeration volume

Sludge floating due to lack of oxygen (including dead corners caused by uneven aeration) or sludge poisoning can increase the aeration volume, reduce water inflow and remove dead sludge.

(4) Reduce sludge age

Generally, the residence time of sludge in the aeration tank is reduced to inhibit the growth of actinomycetes with a long growth period. Practice has proved that when the sludge residence time is 5 to 6 days, the problem of sludge floating caused by it can be avoided.