Activated sludge process is an effective biological treatment method for urban sewage and organic industrial wastewater. And DO, the dissolved oxygen concentration of activated sludge mixture, is an important guarantee for fully utilizing various microbial life activities, and therefore it is also a key control point for the operation of activated sludge method and its transformation process in urban sewage treatment. In daily operation management, the DO value cannot be too high or too low. The currently recognized DO value in the industry should be controlled at around 2mg/l, and in actual operation, it should be determined according to the specific situation of each factory. However, for treatment plants aimed at biological nitrification and denitrification, their DO values are usually higher than those required for conventional treatment, because nitrifying bacteria are converted aerobic bacteria that stop their activity when there is no oxygen, and their oxygen uptake rate is much lower than that of bacteria that decompose organic matter. Therefore, the nitrification system needs to maintain a high concentration of DO. The manifestations of DO abnormality include two phenomena: excessive and insufficient DO. The phenomenon of low DO can be divided into two types: a sharp decrease in DO during a certain period of time and a gradual decrease in DO under the same blowing conditions. Analysis of the Reasons for DO Abnormality: The main reason for the sharp decline in DO is 1) a sudden change in incoming water quality, resulting in the inflow of high concentration organic wastewater (dissolved BOD). High concentration organic wastewater mainly refers to food processing wastewater, brewing wastewater, papermaking wastewater, etc. BOD is easily decomposed and removed by activated sludge, resulting in increased oxygen consumption and decreased DO. Discharge of high oxygen consuming wastewater. The inflow of sludge accumulated in sewage pipelines or sedimentation tanks, the large inflow of clear liquid from concentration tanks or digestion tanks, and the inflow of industrial wastewater such as high oxygen consuming oil wastewater, leather processing plant industrial wastewater, printing, fiber, and chemical synthesis wastewater can all lead to a sharp decrease in dissolved oxygen (DO). Affects the inflow of oxygen transfer wastewater. Surfactants (such as short chain fatty acids and ethanol), highly viscous substances, and oils in wastewater will accumulate at the gas-liquid interface, hindering the diffusion and transfer of oxygen molecules. Due to their increased resistance in the oxygen transfer process, the oxygen transfer coefficient decreases and the transfer efficiency decreases, resulting in a decrease in dissolved oxygen (DO). Inflow of high concentration FeO wastewater. High concentration FeO wastewater mainly comes from underground water or industrial and mining enterprises such as mines, ironworks, and cable plants. These wastewater contain a large amount of ferrous oxide, which is easily oxidized into Fe3+and consumes a large amount of oxygen, resulting in a decrease in dissolved oxygen (DO). 2) The formula for nitrification reaction in the aeration tank is: NH4+2O2 → NO3-+2H (+)+H2O. The nitrification reaction must meet the following conditions: suitable water temperature, pH, and DO, and SRT>1/Vn, where SRT refers to sludge age and Vn refers to the specific growth rate of nitrifying bacteria. The specific growth rate Vn of nitrifying bacteria in sewage treatment plants operating with the same SRT increases with temperature, or due to a sharp decrease in residual sludge discharge, when the conditions for nitrification reaction are met, nitrification reaction will suddenly occur. As can be seen from the above formula, nitrification will simultaneously consume oxygen, leading to a decrease in DO. The main reason for the gradual decrease in dissolved oxygen (DO) is that under the same blowing conditions, DO gradually decreases, mostly due to blockage of the aeration head or aging of the aeration membrane. Possible reasons for blockage include excessive dust in the air, inadequate filtration by the blower, cooling oil entering the pipeline, rust inside the aeration tube, and rust residue blocking the aeration head, leading to a decrease in dissolved oxygen (DO). The aging of the aeration membrane can cause the bubbles to become thicker and more dispersed. Larger bubbles reduce the contact area between the gas and liquid phases, shorten the contact time between the two, and thus reduce the efficiency of oxygen transfer. Under the same aeration conditions, DO will gradually decrease. The main reason for the sharp increase in dissolved oxygen (DO) is due to the discharge of a large amount of excess sludge, or the expansion of sludge in the secondary sedimentation tank, which causes the sludge to flow out with the effluent, or the high inlet load, all of which can lead to a decrease in the concentration of activated sludge in the aeration tank and a decrease in oxygen consumption, resulting in an increase in dissolved oxygen (DO). The inflow concentration is too low. For the drainage system of rainwater and sewage merging, due to long-term rainfall and the influx of large amounts of snowmelt water, the inflow load of the aeration tank will be too low, resulting in an increase in dissolved oxygen (DO). The inflow of toxic and harmful substances. Due to the inflow of industrial wastewater, toxic and harmful wastewater can enter, resulting in a decrease in the aerobic rate Sour of activated sludge and an increase in dissolved oxygen (DO). Excessive heavy metals are inhibitors and fungicides of bacteria, while bleach, liquid chlorine, and other substances have strong killing effects on bacteria, which can lead to a large number of bacterial deaths. A large amount of wastewater containing strong oxidants flows in. Strong oxidants such as potassium permanganate can oxidize the cellular material of bacteria, hindering their normal metabolism and even causing death. As a result, it inevitably leads to a decrease in microbial oxygen demand and an increase in dissolved oxygen. Nitrification reaction stops. When the nitrification reaction stops due to a decrease in water temperature or a shortened sludge age, oxygen consumption decreases and DO increases. In addition to the above factors, water temperature can also have an impact on DO. Within the temperature range where microbial enzyme systems are not affected by denaturation, an increase in water temperature will stimulate microbial activity and enhance reaction rate. Rising water temperature is beneficial for physical processes such as mixing, stirring, and precipitation, but not conducive to oxygen transfer. For biochemical processes, it is generally believed that the purification effect is good when the water temperature is between 20-30 ℃, and decreases when the temperature is above 35 ℃ and below 10 ℃. When the incoming water temperature suddenly increases, such as exceeding 40 ℃, it will cause protein degradation, oxygen loss of activity, and lead to deterioration of the treated water quality.
How to handle DO exceptions? Dissolved oxygen is an important indicator for the operation control of aeration tanks in activated sludge processes. The activity of activated sludge can be determined by the consumption of dissolved oxygen. Good activated sludge has a high oxygen demand, and the dissolved oxygen (DO) in the mixed solution quickly disappears after sampling. Even if it is saturated with oxygen for a few minutes, it will be consumed, while the deactivated sludge will not be consumed after a few minutes. Due to the different sizes of activated sludge flocs, the minimum required dissolved oxygen concentration also varies. The smaller the floc, the larger the contact area with sewage, and the more suitable it is for sample uptake, resulting in a lower required dissolved oxygen concentration; On the contrary, the larger the floc, the higher the required dissolved oxygen concentration. Dissolved oxygen should not be too low, as it cannot meet the oxygen demand of microorganisms in the aeration tank, leading to a decrease in microbial population and hindering normal metabolic processes, growth of filamentous bacteria, decreased sludge purification function, incomplete decomposition of organic pollutants, and affecting water discharge efficiency. If the DO in the effluent section is too low for a long time, it can also cause denitrification in the secondary sedimentation tank and cause sludge to float up. Dissolved oxygen should not be too high, as excessive dissolved oxygen means consuming too much energy and causing an excessive increase in actinomycetes that prefer high dissolved oxygen, which affects the treatment effect. In addition, excessive aeration can cause some sludge to not settle and become floating sludge, and may also lead to sludge disintegration or peroxidation, disrupting the biological nutrient balance of activated sludge, reducing microbial biomass and losing activity, decreasing adsorption capacity, shrinking flocs, and decreasing sludge volume index (SVI); Excessive aeration will also cause abnormal phenomena such as the increase of foam in the aeration tank. Therefore, the higher the dissolved oxygen in the aeration tank, the better. For traditional activated sludge process and its transformation technology, the DO value should be minimized as much as possible without affecting the effluent. For traditional activated sludge process, the maximum oxygen demand occurs in the first section of the aeration tank where sewage and sludge start to come into contact and mix, namely Zone I. The editor believes that for activated sludge processes that do not require denitrification, controlling the dissolved oxygen in zone I (inlet zone) between 0.8 and 1.2 mg/l, zone II (middle zone) between 1.0 and 1.5 mg/l, and zone III (outlet zone) around 2 mg/l can meet the treatment needs. The slightly higher dissolved oxygen in the effluent area is for the full absorption of phosphorus and to prevent sludge from anaerobically floating up in the secondary sedimentation tank. Abnormal DO also indirectly reflects the abnormality of incoming water quality or process control, and different measures should be taken based on the causes of its occurrence. If there is a problem with the quality of the incoming water, it is necessary to strengthen communication with the environmental protection department, identify the source of water quality, strengthen source management, or timely avoid peak periods and reduce the amount of incoming water in different time periods. If DO abnormality occurs due to process control, it should be adjusted according to the reasons for the above phenomenon. In addition, due to high water temperature in summer, the aeration rate should be appropriately increased, while the opposite is true in winter. If the dissolved oxygen decreases due to blockage of the aeration system, a comprehensive inspection of the aeration tank should be carried out, including cleaning or replacing the aeration membrane, clearing the blockage inside the aeration tube, and allowing air to enter the aeration tank smoothly, providing normal dissolved oxygen for microorganisms. In summary, dissolved oxygen (DO) is an extremely important process control measure in activated sludge treatment, and its value can affect a series of indicators. When there is an abnormality in DO, it is necessary to analyze the cause carefully, prescribe the right medicine, adjust it in a timely manner, and try to control the abnormality within the minimum range to ensure that the sewage is discharged in compliance with standards.
