Causes of poor biochemical removal rate of pollutants and solutions

Nutrients such as nitrogen and phosphorus in general sewage are able to meet the needs of microorganisms, and there is a lot of excess. However, when the proportion of industrial wastewater is large, attention should be paid to accounting for the proportion of carbon, nitrogen and phosphorus to meet the 100:5:1. If there is a lack of nitrogen in the sewage, ammonium salts can usually be added. If the lack of phosphorus in the sewage, usually can be added phosphate or phosphate.

The pH of the effluent is neutral, usually 6.5 to 7.5. Small decreases in pH may be due to anaerobic fermentation in the effluent conveyance pipeline. Larger decreases in pH during the rainy season are often the result of urban acid rain, which is especially prominent in combined flow systems. sudden large changes in pH, either increases or decreases, are usually caused by large discharges of industrial wastewater. Adjusting the pH of the effluent is usually done by adding sodium hydroxide or sulfuric acid, but this will greatly increase the cost of sewage treatment.

When the higher content of oil substances in sewage, it will reduce the aeration efficiency of the aeration equipment, such as not increasing the amount of aeration will make the treatment efficiency, but increasing the amount of aeration is bound to increase the cost of sewage treatment. In addition, the higher content of oil and grease in the sewage will also reduce the settling performance of activated sludge, and when serious, it will become the reason for sludge expansion, resulting in the SS effluent exceeds the standard. The higher content of oil substances in the influent, the need to increase the oil removal device in the pretreatment section.

The influence of temperature on activated sludge process is very wide. Firstly, temperature affects the activity of microorganisms in activated sludge, and in winter when the temperature is low, the treatment effect will decrease if no regulation measures are taken. Secondly, the temperature will affect the separation performance of the secondary sedimentation tank, for example, the temperature change will make the sedimentation tank produce heterogeneous heavy flow, resulting in short flow; the lower temperature will make the activated sludge due to the viscosity increase and reduce the settling performance; the temperature change will affect the efficiency of the aeration system, the temperature will be increased in summer, due to the reduction of the saturated concentration of dissolved oxygen, and oxygenation is difficult, resulting in the reduction of the aeration efficiency, and it will reduce the density of the air, and if you want to ensure that the air supply remains unchanged, it is necessary to increase the air supply.

The reasons affecting the effect of ammonia nitrogen treatment involve many aspects, mainly:

Biological nitrification is a low load process, F/M is generally 0.05 ~ 0.15kgBOD/kgMLVSS - d. The lower the load, the fuller the nitrification, the more efficient the conversion of NH3 - N to NO3 - N. The lower the load, the fuller the nitrification, NH3 - N to NO3 - N conversion efficiency is higher. Corresponding to the low load, the SRT of biological nitrification system is generally longer, because the nitrifying bacteria have a long generation cycle, if the sludge retention time of the biological system is too short, i.e., the SRT is too short, and the concentration of the sludge is low, the nitrifying bacteria can not be cultivated, and the effect of nitrification can not be obtained.The amount of the SRT is controlled by the temperature and other factors. For biological system with nitrogen removal as the main purpose, usually SRT can be taken as 11-23d.

The reflux ratio of biological nitrification system is generally larger than that of traditional activated sludge process, mainly because the activated sludge mixture of biological nitrification system contains a large amount of nitrate, if the reflux ratio is too small, the activated sludge in the second sedimentation tank will have a longer residence time, which is prone to denitrification, resulting in sludge floating. Usually nitrogen removal process reflux ratio is controlled at 50-100%.

Biological nitrification aeration tank hydraulic residence time is also longer than the activated sludge process, at least should be more than 8h. This is mainly because the nitrification rate is much lower than the removal rate of organic pollutants, thus requiring a longer reaction time.

The reasons affecting the effect of total nitrogen treatment involve many aspects, mainly:

Since biological nitrification is a prerequisite for biological denitrification, only good nitrification can obtain efficient and stable denitrification. Thus, the denitrification system must also use low load or ultra-low load, and use high sludge age.

In denitrification, the size of reflux ratio determines the efficiency of denitrification, too low reflux ratio will lead to a decrease in denitrification efficiency, effluent TN exceeds the standard, but too high reflux, on the one hand, will carry more DO, consume the carbon source and destroy the anoxic environment, higher than a certain ratio, the efficiency of denitrification will not be improved a lot, the general denitrification process external reflux ratio can be controlled at 50%~100%, the internal reflux ratio is generally controlled between 200-400%, and the external reflux ratio is controlled at 200-400%. ～The general denitrification process external reflux ratio can be controlled at 50%~100%, internal reflux ratio is generally controlled at 200~400%.

Denitrification rate refers to the amount of nitrate denitrified per unit of activated sludge per day. Denitrification rate is related to temperature and other factors, the typical value is 0.06～0.07gNO3--N/gMLVSS×d.

For denitrification, it is hoped that the DO is as low as possible, preferably zero, so that the denitrifying bacteria can be “full” denitrification to improve the efficiency of nitrogen removal. However, from the point of view of the actual operation of the sewage treatment plant, to control the DO in the anoxic zone at 0.5mg / L or less, there are still difficulties, and therefore also affects the biological denitrification process, which in turn affects the total nitrogen indicators.

Because denitrifying bacteria are in the process of decomposition of organic matter denitrification denitrification, so the sewage into the anoxic zone must have sufficient organic matter to ensure the smooth progress of denitrification. As many sewage treatment plants supporting pipeline network construction lagging behind, into the plant BOD5 lower than the design value, while nitrogen, phosphorus and other indicators are equivalent to or higher than the design value, so that the influent carbon source can not meet the denitrification of carbon demand, but also led to the effluent of total nitrogen exceeds the standard from time to time. The general CN ratio is controlled at 4~6.

Denitrifying bacteria are not as sensitive to pH changes as nitrifying bacteria, and can carry out normal physiological metabolism within the range of pH 6~9, but the best pH range for biological denitrification is 6.5~8.0.

Although denitrifying bacteria are not as sensitive to temperature changes as nitrifying bacteria, the effect of denitrification also changes with temperature. The higher the temperature, the higher the denitrification rate, in 30-35 ℃, denitrification rate increased to the maximum. When lower than 15 ℃, denitrification rate will be significantly reduced to 5 ℃, denitrification will tend to stop. Therefore, to ensure the effect of nitrogen removal in winter, it is necessary to increase the SRT, increase the sludge concentration or increase the number of pools in operation.