Reasons for exceeding the effluent ammonia nitrogen standard

There are many reasons for exceeding the effluent ammonia nitrogen standard, mainly including:

　　(1) Sludge load and sludge age

　　Biological nitrification is a low-load process, F/M is generally 0.05 ~ 0.15kgBOD/kgMLVSS·d . The lower the load, the more complete the nitrification, and NH3-N to NO3–N the conversion efficiency is higher. Corresponding to the low load, the sludge retention time (SRT) of the biological nitrification system SRT is generally longer because the nitrifying bacteria have a longer generation cycle. If the sludge retention time of the biological system is too short, that is, SRT too short, and the sludge concentration is low, the nitrifying bacteria cannot be cultivated, and the nitrification effect cannot be obtained. SRT How much it is controlled depends on factors such as temperature. For biological systems where denitrification is the main purpose, usually SRT it is advisable 11 ~ 23d。

(2) Recirculation ratio

　　The recirculation ratio of the biological nitrification system is generally larger than that of the traditional activated sludge process, mainly because the activated sludge mixed liquor in the biological nitrification system already contains a large amount of nitrate. If the recirculation ratio is too small, the activated sludge will stay in the secondary sedimentation tank for a longer time, easily causing denitrification and sludge floating. Usually, the recirculation ratio is controlled at 50~ 100%。

　　(3) Hydraulic retention time

　　The hydraulic retention time of the biological nitrification aeration tank is also longer than that of the activated sludge process, and should be at least 8 hours or more. This is mainly because the nitrification rate is much lower than the removal rate of organic pollutants, so a longer reaction time is needed.

(3) Hydraulic retention time

　　The hydraulic retention time of the biological nitrification aeration tank is also longer than that of the activated sludge process, and should be at least 8 hours or more. This is mainly because the nitrification rate is much lower than the removal rate of organic pollutants, so a longer reaction time is needed.

　　(4)BOD5/TKN

　　TKN refers to the sum of organic nitrogen and ammonia nitrogen in water. In the influent wastewater, BOD5/TKN is an important factor affecting the nitrification effect. BOD5/TKN The larger it is, the smaller the proportion of nitrifying bacteria in the activated sludge, the smaller the nitrification rate, and the lower the nitrification efficiency under the same operating conditions. ; Conversely, BOD5/TKN the smaller it is, the higher the nitrification efficiency. Many wastewater treatment plants have found in practice that BOD5/TKN the optimal range is 2 ~ 3 around.

　　(5) Nitrification rate

　　A specific process parameter of the biological nitrification system is the nitrification rate, which refers to the amount of ammonia nitrogen converted per unit weight of activated sludge per day. The nitrification rate depends on the proportion of nitrifying bacteria in the activated sludge, temperature, and many other factors. The typical value is 0.02gNH3-N/gMLVSS×d.

　　(6) Dissolved oxygen

　　Nitrifying bacteria are obligate aerobic bacteria. In the absence of oxygen, their life activities cease, and the oxygen uptake rate of nitrifying bacteria is much lower than that of bacteria that decompose organic matter. If sufficient oxygen is not maintained, nitrifying bacteria will “compete” ” for the oxygen they need. Therefore, the dissolved oxygen in the aerobic zone of the biological pool needs to be maintained above 2mg/L In special cases, the dissolved oxygen content needs to be increased.

　　(7) Temperature

　　Nitrifying bacteria are also sensitive to temperature changes. When the wastewater temperature is below 15℃, the nitrification rate will decrease significantly. When the wastewater temperature is below 5℃ its physiological activity will completely stop. Therefore, the phenomenon of effluent ammonia nitrogen exceeding the standard is more obvious in winter, especially in wastewater treatment plants in northern regions.

　　(8)pH

　　Nitrifying bacteria are very sensitive to pH. In the range of pH of 8 ~ 9 their biological activity is strong. When pH<6.0 or >9.6 the biological activity of nitrifying bacteria will be inhibited and tend to stop. Therefore, the mixed liquor of the biological nitrification system should be controlled as much as possible pH greater than 7.0。