2. Routine monitoring project of mixed liquid in aeration tank
2.1 How to control the MLSS or MLVSS value of the aeration tank?
2.2 What is the sludge settling ratio (SV) of the aeration tank mixture? What is the function?
2.3 What abnormal phenomena are prone to occur when measuring the SV value? Why?
2.4 What is Sludge Volume Index (SVI)?
2.5 What is the reason for the increase in the SVI value of the aeration tank mixture?
2.1 How to control the MLSS or MLVSS value of the aeration tank?
The aeration tank mixture must maintain a relatively fixed sludge concentration MLSS in order to maintain a good treatment effect and stable operation of the treatment system. Each aerobic activated sludge treatment process has its optimum MLSS of aeration tank. For example, the optimum value of MLSS of activated sludge in ordinary air aeration tank is about 2g/L, while the MLSS of section A of AB process is the most The optimal value is about 5g/L, and there is a big gap between the two.
Generally speaking, the treatment effect is best when the MLSS in the aeration tank is close to its optimum value. When the MLSS is too low, the expected processing effect is often not achieved.
When the MLSS is too high, the sludge age is prolonged, and the dissolved oxygen required to maintain the normal activity of microorganisms in these sludges will increase a lot, resulting in an increased requirement for the capacity of the oxygenation system. At the same time, the density of the mixed liquid in the aeration tank will increase, and the resistance will increase, which will also increase the power consumption of mechanical aeration or blast aeration.
That is to say, although the MLSS is high, the resistance of the aeration tank to changes in the influent water quality and impact load can be improved, but it is often uneconomical in operation. And sometimes it will lead to excessive aging of sludge, decreased activity, and finally even affect the water quality of the treatment.
In actual operation, it is sometimes necessary to forcefully reduce the MLSS value of the aeration tank by increasing the discharge of excess sludge, stimulate the growth and reproduction of microorganisms in the mixed solution of the aeration tank, and improve the activity of activated sludge to decompose and oxidize organic matter.
2.2 What is the sludge settling ratio (SV) of the aeration tank mixture? What is the function?
Sludge Settling Velocity (SV) in English is Settling Velocity, also known as 30min settling rate, which is the ratio of the volume of sedimented sludge formed after the mixed solution of the aeration tank is allowed to stand in the graduated cylinder for 30min to the volume of the original mixed solution, expressed in % .
Generally, take 1000ml of mixed liquid sample and measure it with a 1000ml measuring cylinder with full scale. After standing for 30min, the height of the mud surface is exactly the value of SV. Because the determination of SV value is simple and fast, it is a common method for evaluating the concentration and quality of activated sludge.
The SV value can reflect the amount of sludge and the cohesion and settling performance of the sludge during normal operation of the aeration tank. It can be used to control the discharge of excess sludge. The normal value of SV is generally between 15% and 30%. If the value is lower than this value, it means that the sludge has good sedimentation performance, but it may also be due to poor sludge activity.
It can reduce or not discharge sludge or increase the amount of aeration. Above this value range, it means that sludge discharge operation is required, or measures should be taken to increase the amount of aeration, or it may be due to the action of filamentous bacteria that the sludge expands, and it is necessary to increase the amount of sludge or reduce the amount of aeration.
2.3 What abnormal phenomena are prone to occur when measuring the SV value? Why?
2.3.1 After 30-60 minutes of sedimentation, the sludge floats up in layers and the water quality is relatively clear. It shows that the activated sludge has a strong reaction function, produces a nitrification reaction, and forms more nitrates, which stay in the aeration tank for a long time, and then enter the secondary sedimentation tank to denitrify and generate gaseous nitrogen; some sludge The flocs float up. It can be solved by reducing the amount of aeration or reducing the residence time of sludge in the secondary sedimentation tank.
2.3.2 The supernatant in the graduated cylinder contains a large number of suspended micro flocs, and the transparency is poor and turbid. The explanation is that the sludge is disintegrated, and the reasons are excessive aeration and too low load, resulting in excessive oxidation of the activated sludge itself and the entry of harmful substances. It can be solved by reducing the amount of aeration or increasing the amount of mud.
2.3.3 The mud-water interface in the graduated cylinder is unclear, and the water quality is turbid. The reason may be that high-concentration organic wastewater flows into the water. It can be solved by increasing the amount of aeration or prolonging the residence time of sewage in the aeration tank.
2.4 What is Sludge Volume Index (SVI)?
Sludge Volume Index (SVI) in English is Sludge Volume Index, which refers to the volume of sedimented sludge formed by each gram of dry sludge after the mixture at the outlet of the aeration tank is allowed to settle for 30 minutes. Units are in ml/g.
The relationship between SVI and SV value:
SVI value excludes the effect of sludge concentration on sludge sedimentation volume, so it can more accurately evaluate and reflect the coagulation and sedimentation performance of activated sludge than SV value. Generally speaking, if the SVI value is too low, the sludge particles are small, the inorganic matter content is high, and the activity is lacking. The SVI value of urban sewage treatment plants is generally between 70 and 100.
The SVI value is related to the sludge load. If the sludge load is too high or too low, the metabolic performance of the activated sludge will deteriorate, and the SVI value will also become high, and there is the possibility of sludge bulking.
2.5 What is the reason for the increase in the SVI value of the aeration tank mixture?
2.5.1 The sudden drop of water temperature reduces the activity of microorganisms and the function of decomposing organic matter.
2.5.2 The inflow of acid-containing wastewater keeps the pH value of the mixed solution in the aeration tank under acidic conditions for a long time, and the acidophilic filamentous microorganisms multiply in large numbers. In addition, the periodic shedding of the filamentous microbial films grown in the pipelines that discharge the acid wastewater will also lead to mixing. Proliferation of filamentous microorganisms in liquid.
2.5.3 The ratio of nitrogen and phosphorus nutrients in the influent water is low, while the filamentous bacteria can multiply in the case of serious shortage of nitrogen and phosphorus and other nutrients, and dominate the mixed liquid, thereby causing sludge bulking.
2.5.4 The high organic load of the aeration tank leads to the deterioration of the coagulation performance and sedimentation performance of the activated sludge, and the increase of the SVI value.
2.5.5 The content of low-molecular-weight organic matter in the influent water is large, and low-molecular-weight organic matter is the most easily absorbed and utilized component of filamentous bacteria, so that the filamentous microorganisms multiply and the sedimentation performance of the aeration tank mixture is reduced.
2.5.6 Insufficient dissolved oxygen in the mixed solution of the aeration tank inhibits the growth of flocs. On the other hand, filamentous bacteria can multiply in the condition below 0.1mg/L, which leads to the expansion of activated sludge and the increase of SVI value.
2.5.7 The increase of toxic and harmful substances in the influent water, such as the sudden increase of the content of phenol, aldehyde, sulfide and other substances, reduces the cohesion performance of microbial bacteria micelles, and a large number of flocculation, while the filamentous bacteria can proliferate, and the SVI increases .
2.5.8 The high-concentration organic wastewater enters the aeration tank after hypoxia and corruption, which contains a large amount of low-molecular-weight organic matter and sulfide, etc., so that the filamentous bacteria multiply and the SVI value increases.
2.5.9 The supernatant of the digester enters the aeration tank for a short time. Among them, the high concentration of organic matter increases the organic load of the aeration tank, and the filamentous bacteria multiply. The influent of
2.5.10 has a low SS and a large proportion of dissolved organic matter, which reduces the bulk density of the sludge and makes it difficult to separate the solid and liquid, which increases the SVI value.
2.5.11 If the sludge stays in the secondary sedimentation tank for a long time, the dissolved oxygen content will decrease, the sludge will be corrupted and deteriorated, and the filamentous bacteria in the return sludge will multiply, causing the activated sludge in the aeration tank to expand, SVI increase.