(7) RBCOD (easily degradable COD)
Studies have shown that when acetic acid, propionic acid and formic acid and other easily degradable carbon sources are used as the phosphorus release substrate, the release rate of phosphorus is relatively large, and its release rate has nothing to do with the concentration of the substrate, only the concentration of activated sludge and the composition of microorganisms. , The release of phosphorus caused by this type of matrix can be expressed by the zero-order reaction equation. For other types of organic matter to be used by phosphorus-accumulating bacteria, they must be converted into such small molecules as easily degradable carbon sources before phosphorus-accumulating bacteria can use their metabolism.
Glycogen is a branched macromolecular polysaccharide composed of multiple glucoses, which is the storage form of intracellular sugars. As shown in the figure above, glycogen in the phosphorous accumulating bacteria is formed in an aerobic environment, and the stored energy is metabolized in an anaerobic environment to form NADH, which is the raw material for PHAs synthesis, and provides energy for the metabolism of the phosphorous accumulating bacteria. Therefore, in the case of delayed aeration or peroxidation, the phosphorus removal effect will be poor, because excessive aeration will consume part of the glycogen in the phosphorus-accumulating bacteria in an aerobic environment, resulting in insufficient NADH, which is the raw material for PHAs during anaerobic conditions. .
For a well-functioning municipal sewage biological nitrogen and phosphorus removal system, it generally takes 1.5-2.5 hours and 2.0-3.0 hours for phosphorus release and phosphorus absorption, respectively. Overall, it seems that the phosphorus release process is more important. Therefore, we pay more attention to the residence time of sewage in the anaerobic section. The HRT of the anaerobic section is too short to ensure the effective release of phosphorus. Facultative acidifying bacteria cannot fully decompose the macromolecular organic matter in the sewage into lower fatty acids that can be taken by the phosphorus-accumulating bacteria, and will also affect the release of phosphorus; HRT is too long and unnecessary, which not only increases infrastructure investment and operating costs, but also There may be some side effects. In short, phosphorus release and phosphorus uptake are two interrelated processes. Phosphorus accumulating bacteria can only absorb phosphorus better in the aerobic section after sufficient anaerobic release of phosphorus, and only phosphorus accumulating bacteria with good phosphorus absorption can be anaerobic. Excessive release of phosphorus in the oxygen stage and proper regulation will form a virtuous circle. The data obtained by our factory in actual operation is: the HRT of the anaerobic section is 1 hour and 15 minutes to 1 hour 45 minutes, and the HRT of the aerobic section is 2 hours to 3 hours and 10 minutes.
(10) Reflux ratio (R)
A/O process is extremely important to ensure the dephosphorization effect, which is to make the system sludge “carry” enough dissolved oxygen into the secondary settling tank in the aeration tank, and its purpose is to prevent the sludge from becoming annoying Oxygen releases phosphorus, but if the sludge cannot be discharged quickly, the sludge layer in the secondary settling tank is too thick, no matter how high the DO, the sludge will not release anaerobic phosphorus. Therefore, the reflux ratio of the A/O system should not be too high.
If it is low, a sufficient reflux ratio should be maintained, and the sludge in the secondary settling tank should be discharged as soon as possible. However, a too high reflux ratio will increase the energy consumption of the reflux system and the aeration system, and will shorten the actual residence time of the sludge in the aeration tank, and affect the removal of BOD5 and P. How to reduce the reflux ratio as much as possible on the premise of ensuring rapid sludge discharge requires repeated exploration in actual operation. It is generally believed that R is in the range of 50 to 70%.
5. Excessive suspended matter
Whether the suspended solids in the effluent meets the standard depends mainly on whether the quality of the sludge in the biological system is good, the sedimentation effect of the secondary settling tank, and whether the process control of the sewage treatment plant is appropriate.
The reasons for the excessive suspended solids in the effluent of the secondary settling tank are as follows:
(1) Selection of process parameters of secondary sedimentation tank
Whether the design parameters of the secondary settling tank are properly selected is an important factor for whether the effluent suspended solids index will exceed the standard. At the beginning of the design of many sewage treatment plants, in order to save construction costs, the hydraulic retention time was greatly shortened, and the hydraulic surface load was increased as much as possible, resulting in frequent mud turning in the secondary sedimentation tank during operation, resulting in excessive suspended solids in the effluent. In addition, some sewage treatment plants need to control the sludge concentration of the biological tank at a higher level due to the actual process adjustment requirements, which will also cause the solid surface load of the secondary settling tank to be too large, which will affect the effluent water quality. Therefore, it is generally believed that there is more room for the setting of these process parameters in the secondary settling tank to facilitate the control and adjustment of the process of the sewage treatment plant.
Generally speaking, the main process parameters that affect the sedimentation effect of the sedimentation tank are hydraulic residence time, hydraulic surface load and sludge flux.
Hydraulic retention time of secondary sedimentation tank
The length of hydraulic retention time of sewage in the secondary sedimentation tank is an important parameter for the operation of the secondary sedimentation tank. Only enough residence time can ensure good flocculation effect and obtain higher precipitation efficiency. Therefore, it is recommended that the hydraulic retention time of the secondary settling tank be set at about 3 to 4 hours.
Hydraulic surface load of secondary sedimentation tank
For a sedimentation tank, when the water inflow is constant, the size of the particles that it can remove is also constant. Among the particles that can be removed, the sedimentation speed of the smallest particle is exactly equal to the hydraulic surface load of the sedimentation tank. Therefore, the smaller the hydraulic surface load, the more particles can be removed, the higher the sedimentation efficiency, and the lower the index of suspended solids in the water. The design of the secondary settling tank with a small hydraulic surface load is conducive to the effective precipitation of suspended solids such as sludge. It is generally recommended that the hydraulic surface load of the secondary settling tank be controlled within 0.6～1.2m3/m2×h.
Solid surface load of secondary sedimentation tank
The solid surface load of the secondary settling tank is also an important factor affecting the sedimentation effect of the secondary settling tank. The smaller the solid surface load of the secondary sedimentation tank, the better the thickening effect of the sludge in the secondary sedimentation tank. On the contrary, the thickening effect of the sludge in the secondary settling tank is worse. Excessive solid surface load will cause the mud surface of the secondary settling tank to be too high, and many sludge flocs will flow out with the sewage before it can settle, which will affect the effluent suspended solids index. Generally, the maximum solid surface load of the secondary settling tank should not exceed 150kgMLSS/m2×d.
(2) Activated sludge quality
The quality of activated sludge is an important factor that affects whether the suspended solids in the effluent exceed the standard. High-quality activated sludge is mainly reflected in four aspects: good adsorption performance, high biological activity, good sedimentation performance and good concentration performance.
Pollutants in colloidal state must first be adsorbed on the activated sludge flocs, and then further adsorbed near the surface of the bacteria to be decomposed and metabolized. Therefore, activated sludge with poor adsorption performance has poor ability to remove colloidal pollutants. The biological activity of activated sludge refers to the ability of microorganisms in the sludge flocs to decompose and metabolize organic pollutants. Activated sludge with poor biological activity must be slower to remove organic pollutants. Only activated sludge with good settling performance can be effectively separated from the sludge and water in the secondary settling tank. Conversely, if the sludge sedimentation performance deteriorates, the separation effect will inevitably decrease, resulting in turbid effluent from the secondary sedimentation tank, excessive SS, and in severe cases, it may cause a large amount of loss of activated sludge, resulting in insufficient biomass in the system, which in turn affects organic pollutants The catabolic effect. Only when the activated sludge has good concentration performance can a higher concentration of sludge be obtained in the secondary settling tank. Conversely, if the concentration performance is poor and the sludge concentration is reduced, it is necessary to ensure sufficient return sludge volume and increase the return ratio. However, increasing the reflux ratio will shorten the actual residence time of the sewage in the aeration tank, resulting in insufficient aeration time and affecting the treatment effect.
(3) Water inlet SS/BOD5
The ratio of MLVSS in the activated sludge of biological system has a great relationship with influent SS/BOD5. When the ratio of SS/BOD5 in influent is high, the ratio of MLVSS in activated sludge of biological system is low, and vice versa. According to operating experience, when SS/BOD is below 1, the MLVSS ratio can be maintained above 50%, and when SS/BOD5 is above 5, the VSS ratio will drop to 20-30%. When the proportion of MLVSS in the activated sludge is low, in order to ensure the nitrification effect, the system must maintain a higher sludge age. The sludge aging is more obvious, causing the effluent SS to exceed the standard.
(4) Toxic substances
The influent sewage contains toxic substances such as strong acid, strong alkali or heavy metals, which will poison the activated sludge and lose its treatment effect. In severe cases, the sludge may even disintegrate, causing the sludge to fail to settle and the effluent suspended matter to exceed the standard. The fundamental solution to the problem of activated sludge poisoning is to strengthen the management of upstream pollution sources.
The influence of temperature on the activated sludge process is very extensive. First of all, temperature will affect the activity of microorganisms in activated sludge. When the temperature is low in winter, the treatment effect will decrease if no control measures are taken. Secondly, the temperature will affect the separation function of the secondary settling tank. For example, the change of temperature will cause density flow in the secondary settling tank, leading to short-flow phenomenon; when the temperature is reduced, the activated sludge will decrease the settling performance due to the increase in viscosity.