The phenomenon of floating mud in the second sedimentation tank is common in urban sewage treatment plants and industrial wastewater treatment stations, especially after the temperature rises in summer, the floating mud situation is outrageous! It seriously affects the image of sewage treatment, resulting in an increase in effluent SS, which will seriously lead to excessive effluent standards.

1. Causes of floating mud in the second sinking tank

  1. Anaerobic rot of sludge

  The temperature is high in summer, and the bacterial metabolism is vigorous, and it is easy to produce an anaerobic environment in the secondary sedimentation tank, especially when the sludge discharge equipment is faulty or there are dead ends in the design! Main influencing factors:

  1) Water temperature

  In the temperature range suitable for bacteria to survive, the higher the temperature, the faster the metabolism and proliferation of bacteria, the higher the activity of sludge, the increase of oxygen consumption, and the risk of sludge decomposition.

  2) Water intake DO

  If the influent DO is too low, it will cause the secondary sedimentation tank to quickly fall into an anaerobic environment, and the sludge will undergo anaerobic decay and float up in an anaerobic environment!

  3) Sludge residence time

  The longer the sludge stays, the more thorough the anaerobic digestion is carried out, the more serious the floating sludge situation, and there are several reasons for the long residence time, the first sludge discharge cycle is too long; second, the failure of the mud discharge equipment; Third, there are dead ends in the second sinking pool!

  4) The concentration of sludge is too high

  If the sludge concentration is too high, the total amount of mud in the secondary sedimentation tank will be too much, and in the same sludge discharge cycle, more DO will be consumed, which is more likely to produce an anaerobic environment and lead to anaerobic digestion.

  2. The sludge is denitrified

  The phenomenon of denitrification floating sludge in the second sedimentation tank was relatively common in the past, and this situation may become less and less under higher and higher emission standards in the future! Denitrification floating mud is easy to understand, that is, denitrification occurs in the secondary sedimentation tank (mainly endogenous respiratory denitrification), and nitrogen floats up with mud when there is no way out! Main influencing factors:

  1) Temperature

  In the temperature range suitable for bacteria to survive, the higher the temperature, the faster the proliferation, the corresponding mud age also becomes shorter, and nitrification reactions may also occur in the process without denitrification, and nitrification is carried out more thoroughly in the denitrification process, resulting in an increase in nitrate nitrogen entering the secondary sedimentation tank. The higher the temperature in summer, the faster the endogenous respiration denitrification occurs in the secondary sedimentation tank, and the more N2 is produced by denitrification, the greater the risk of upscaling.

  2) Stay time

The longer the stay time of the secondary sedimentation tank, the easier it is to produce an anoxic environment, the more likely it is to occur endogenous respiratory denitrification, and the long-term stay of sludge will lead to the release of accumulated N2.

  3) Water intake DO

  In the presence of DO, bacteria give priority to aerobic metabolism and have an inhibitory effect on the denitrification process (O2's ability to accept electrons is much higher than that of NO2- and NO3-), so controlling a certain DO in the secondary sedimentation tank will delay the denitrification process and inhibit the production of nitrogen in the secondary sedimentation tank, which is generally required to enter the secondary sedimentation tank DO>2, and the DO out of the secondary sedimentation tank is greater than 0.5ppm.

  4) Influent nitrate nitrogen

  In many sewage treatment, because the nitrogen removal efficiency or TN requirements are not high, it will lead to too much nitrate nitrogen entering the secondary sedimentation tank, which provides the necessary conditions for the denitrification of the secondary sedimentation tank!

2. Measures to avoid denitrification of floating sludge

  1. Reduce the nitrate nitrogen content in the influent water

  The nitrate concentration entering the secondary sedimentation tank should be reduced as much as possible to avoid the occurrence of denitrification, and the removal of nitrate nitrogen can be achieved by improving the denitrification efficiency of the anoxic tank (adding carbon source, improving the internal reflux ratio, etc.) or through multi-stage denitrification.

  2. Increase the water intake DO

  Low DO is the common reason for sludge decomposition and denitrification, control the high content of dissolved oxygen in the influent water, and try to ensure that the effluent DO of the secondary sedimentation tank is greater than 0.5PPM, which can be achieved by opening the large tail aeration or adding the fan.

  3. Reduce the residence time of sludge

  The longer the sludge retention time, the greater the possibility of anaerobic or denitrification, which can be achieved by multi-discharge or shortening the sludge scraping cycle, and the system modification is required for the design dead space problem that cannot be eliminated.

  4. Reduce mud age (sludge concentration)

  In the non-denitrification process, too long the mud age will lead to an increase in the number of nitrifying bacteria, and nitrification reaction occurs under the condition of ammonia nitrogen.

  Too high sludge concentration also increases the risk of anaerobic spoilage, so reducing the sludge concentration, that is, reducing the age of the mud, kills two birds with one stone!

v5. Comprehensive means

  The main factors of floating sludge in the secondary sedimentation tank are: nitrate concentration in the influent water, influent water temperature, residence time, mud age, sludge concentration, etc. In the case of difficult judgment, the effect of eliminating the floating mud in the second sinking tank can be achieved by comprehensively eliminating these factors!