The reason for the poor treatment effect of COD, ammonia nitrogen, total phosphorus and total nitrogen!

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The COD treatment effect is poor

The main factors affecting the effect of COD treatment are:

1. Nutrients: Nutrients such as nitrogen and phosphorus in general sewage can meet the needs of microorganisms, and there is a lot of surplus. However, when the proportion of industrial wastewater is large, attention should be paid to whether the ratio of carbon, nitrogen and phosphorus meets 100:5:1. If there is a lack of nitrogen in the effluent, ammonium salt can usually be added. If there is a phosphorus deficiency in the effluent, phosphoric acid or phosphate can usually be added.

The pH value of 2.pH sewage is neutral, generally 6.5~7.5. The slight decrease in pH may be due to anaerobic fermentation in the sewage pipes. Large pH drops during the rainy season are often caused by urban acid rain, especially in combined systems. Sudden, large changes in pH, whether up or down, are usually caused by large amounts of industrial wastewater. Adjusting the pH of the effluent, usually by adding sodium hydroxide or sulfuric acid, will greatly increase the cost of wastewater treatment.

3. When the content of oil substances in the sewage is high, the aeration efficiency of the aeration equipment will be reduced, and the treatment efficiency will be reduced if the aeration volume is not increased, but the increase in aeration volume will inevitably increase the cost of sewage treatment. In addition, the higher grease content in the sewage will also reduce the sedimentation performance of the activated sludge, and in severe cases, it will become the cause of sludge expansion, resulting in excessive SS in the effluent. For influent water with high oil content, it is necessary to add an oil removal device in the pretreatment section.

4. The influence of temperature on the activated sludge process is very extensive. First of all, the temperature will affect the activity of microorganisms in the activated sludge, and the treatment effect will be reduced if no control measures are taken in the winter when the temperature is low. Secondly, the temperature will affect the separation performance of the secondary sedimentation tank, for example, the temperature change will cause the sedimentation tank to produce abnormal gravity flow, resulting in short flow; the temperature decrease will reduce the sedimentation performance of the activated sludge due to the increase of viscosity; the temperature change will affect the efficiency of the aeration system, when the temperature rises in summer, it will make it difficult to fill the oxygen due to the decrease of the saturation concentration of dissolved oxygen, resulting in the decrease of the aeration efficiency, and will reduce the air density, if the air supply is to ensure that the air supply is unchanged, the air supply must be increased.

The effect of ammonia nitrogen treatment is poor

The removal of ammonia nitrogen in sewage is mainly based on the traditional activated sludge process using nitrification process, that is, delayed aeration is used to reduce the load of the system.

There are many reasons that affect the effect of ammonia nitrogen treatment, mainly including:

1. Sludge load and sludge age biological nitrification is a low-load process, and F/M is generally 0.05~0.15kgBOD/kgMLVSS·d. The lower the load, the more fully nitrification is carried out, and the higher the efficiency of the conversion of NH3-N to NO3--N. Corresponding to the low load, the SRT of the biological nitrification system is generally longer, because the generation cycle of nitrifying bacteria is longer, if the sludge residence time of the biological system is too short, that is, the SRT is too short, and the sludge concentration is low, the nitrifying bacteria cannot be cultivated, and the nitrification effect will not be obtained. The amount of SRT control depends on factors such as temperature. For biological systems with denitrification as the main purpose, SRT can usually be taken for 11~23 days.

2. The reflux ratio of the biological nitrification system is generally larger than that of the traditional activated sludge process, mainly because the activated sludge mixture of the biological nitrification system has contained a large amount of nitrate, if the reflux ratio is too small, the residence time of the activated sludge in the secondary sedimentation tank is longer, and it is easy to produce denitrification, resulting in sludge floating. Usually the reflux ratio is controlled at 50~100%.

3. Hydraulic retention timeThe hydraulic retention time of the biological nitrification aeration tank is also longer than that of the activated sludge process, at least more than 8h. This is mainly due to the fact that the nitrification rate is much lower than the removal rate of organic contaminants, which requires a longer reaction time.

4. BOD5/TKNTKN refers to the sum of organic nitrogen and ammonia nitrogen in water, and BOD5/TKN in influent sewage is an important factor affecting the nitrification effect. The larger the BOD5/TKN, 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; The operation practice of many sewage treatment plants has found that the optimal range of BOD5/TKN value is about 2~3.

5. Nitrification rateA special 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, and the typical value is 0.02 gNH3-N/gMLVSS×d. 6. Dissolved oxygen nitrifying bacteria are obligate aerobic bacteria, which stop life activities when there is no oxygen, and the oxygen uptake rate of nitrifying bacteria is much lower than that of bacteria that decompose organic matter. Therefore, it is necessary to keep the dissolved oxygen in the aerobic area of the biotank above 2mg/L, and the dissolved oxygen content needs to be increased under special circumstances.

6. Temperature nitrifying bacteria are also sensitive to temperature changes, when the sewage temperature is lower than 15 °C, the nitrification rate will drop significantly, and when the sewage temperature is lower than 5 °C, its physiological activities will stop completely. Therefore, the phenomenon of excessive ammonia nitrogen in the effluent of sewage treatment plants, especially in northern China, is more obvious in winter.

8.pH nitrifying bacteria are very sensitive to pH reaction, in the range of pH 8~9, their biological activity is the strongest, when the pH < 6.0 or > 9.6, the biological activity of nitrifying bacteria will be inhibited and tend to stop. Therefore, the pH of the mixture of the biological nitrification system should be controlled as much as possible to be greater than 7.0.

The total nitrogen treatment effect is poor

Sewage denitrification is based on the biological nitrification process, adding the biological denitrification process, where the denitrification process refers to the biochemical reaction process in which nitrate in sewage is reduced to nitrogen by microorganisms under anoxic conditions.

There are many reasons that affect the effect of total nitrogen treatment, mainly including:

1. Sludge load and sludge ageSince biological nitrification is the premise of biological denitrification, only good nitrification can obtain efficient and stable denitrification. As a result, denitrification systems must also be low- or ultra-low-load and use a high sludge age.

2. The internal and external reflux is smaller than that of the biological denitrification system, which is mainly due to the fact that most of the nitrogen in the inflow sewage has been removed, and the concentration of NO3--N in the secondary sedimentation tank is not high. Comparatively speaking, the danger of sludge floating due to denitrification in the secondary sedimentation tank is very small. On the other hand, the sludge sedimentation rate of the denitrification system is fast, and the reflux ratio can be reduced under the premise of ensuring the required reflux sludge concentration, so as to prolong the residence time of the sewage in the aeration tank. A well-run sewage treatment plant can control the external return ratio below 50%. The internal reflux ratio is generally controlled between 300~500%.

3. Denitrification rate: The denitrification rate refers to the amount of nitrate denitrified per unit of activated sludge per day. The denitrification rate is related to temperature and other factors, and the typical value is 0.06~0.07gNO3--N/gMLVSS×d. 4. For denitrification of dissolved oxygen in the anoxic zone, it is hoped that the DO should be as low as possible, preferably zero, so that the denitrifying bacteria can carry out denitrification with "full strength" and improve the denitrification efficiency. However, from the actual operation of the sewage treatment plant, it is still difficult to control the DO in the anoxic area below 0.5mg/L, so it also affects the process of biological denitrification, and then affects the total nitrogen index of effluent. 5. BOD5/TKN Because denitrifying bacteria carry out denitrification and denitrification in the process of decomposing organic matter, there must be sufficient organic matter in the sewage entering the anoxic area to ensure the smooth progress of denitrification. Due to the lag in the construction of supporting pipe networks in many sewage treatment plants, the BOD5 of the incoming plant is lower than the design value, while the indicators of nitrogen and phosphorus are equal to or higher than the design value, so that the carbon source of the influent water cannot meet the demand for carbon source for denitrification, and the total nitrogen of the effluent exceeds the standard from time to time. 6.pH denitrifying bacteria are not as sensitive to pH changes as nitrifying bacteria, and can carry out normal physiological metabolism in the range of pH 6~9, but the optimal pH range of biological denitrification is 6.5~8.0.

7. Although temperature denitrifying bacteria are not as sensitive to temperature changes as nitrifying bacteria, the denitrification effect will also change with temperature changes. The higher the temperature, the higher the denitrification rate, and the denitrification rate increases to the maximum at 30~35 °C. Below 15°C, the denitrification rate decreases significantly, and at 5°C, denitrification tends to stop. Therefore, in order to ensure the denitrification effect in winter, it is necessary to increase the SRT, increase the sludge concentration or increase the number of tanks put into operation.

TP treatment is poor

In biological phosphorus removal, phosphorus is released in an anaerobic state by phosphorus-gathering bacteria, and excessive phosphorus is taken in an aerobic state. Phosphorus is removed by discharging phosphorus-rich residual sludge!

There are many reasons that affect the effect of total phosphorus treatment, mainly including:

1. The influence of temperature on the phosphorus removal effect is not as obvious as the influence on the biological denitrification process, and the biological phosphorus removal can be successfully operated in a certain temperature range when the temperature change is not very large. The test showed that the temperature of biological phosphorus removal should be greater than 10°C, because the growth rate of polyphosphate bacteria will be slowed down at low temperatures.

When the 2.pH pH value was 6.5-8.0, the phosphorus content and phosphorus absorption rate of phosphorus-polyphosphate microorganisms remained stable, and when the pH value was lower than 6.5, the phosphorus absorption rate decreased sharply. When the pH value is suddenly reduced, the concentration of phosphorus in both the aerobic and anaerobic areas rises sharply, and the greater the pH decrease, the greater the release, which shows that the phosphorus release caused by the pH reduction is not the physiological and biochemical reaction of the polyphosphate bacteria themselves to the pH change, but a pure chemical "acid solubility" effect, and the greater the anaerobic release caused by the pH drop, the lower the aerobic phosphorus absorption capacity, which indicates that the release caused by the pH drop is destructive and ineffective. When the pH rises, there is a slight absorption of phosphorus.

3. Dissolved oxygen can consume 1.14mg of easily biodegradable COD per milligram of molecular oxygen, resulting in the inhibition of the growth of polyphosphorus organisms, and it is difficult to achieve the expected phosphorus removal effect. In addition, less dissolved oxygen is more conducive to reducing the consumption of easily degradable organic matter, so that polyphosphate bacteria can synthesize more PHB. In the aerobic zone, more dissolved oxygen is required, which is more conducive to the decomposition of phosphorous substances and the energy obtained by the polyphosphate bacteria to absorb the dissolved phosphate in the sewage to synthesize the cellular polyphosphorus. The DO in the anaerobic zone is controlled below 0.3mg/l, and the DO in the aerobic zone is controlled above 2mg/l to ensure the smooth progress of anaerobic phosphorus release and aerobic phosphorus absorption.

4. The nitrate nitrogen in the anaerobic zone of the anaerobic pond consumes the organic substrate and inhibits the release of phosphorus from PAO, thereby affecting the phosphorus absorption by phosphorus bacteria under aerobic conditions. On the other hand, the presence of nitrate nitrogen will be used by Aeromonas as an electron acceptor for denitrification, which will affect the fermentation intermediate product as an electron acceptor for acid production, thereby inhibiting the phosphorus release and phosphorus uptake of PAO and the synthesis of PHB. Each milligram of nitrate nitrogen can consume 2.86mg of COD that is easily biodegradable, resulting in the inhibition of anaerobic phosphorus release, which is generally controlled below 1.5mg/l.

5. Sludge ageSince the biological phosphorus removal system mainly achieves phosphorus removal through the discharge of the remaining sludge, the amount of remaining sludge determines the phosphorus removal effect of the system, and the length of the sludge has a direct impact on the discharge of the remaining sludge and the uptake of phosphorus by sludge. The younger the sludge age, the better the phosphorus removal effect. This is because reducing the age of the sludge increases the discharge of the remaining sludge and the amount of phosphorus removed from the system, thereby reducing the phosphorus content in the effluent of the secondary sedimentation tank. However, for the biological treatment process of simultaneous phosphorus and nitrogen removal, in order to meet the growth requirements of nitrification and denitrifying bacteria, the sludge age is often controlled large, which is the reason why the phosphorus removal effect is unsatisfactory. Generally, the mud age of the biological treatment system for the purpose of phosphorus removal is controlled at 3.5~7d.

6. In the biological phosphorus removal process of COD/TP sewage, the type and content of organic substrate in the anaerobic section and the ratio of nutrients required by microorganisms to phosphorus in sewage are important factors affecting the phosphorus removal effect. The anaerobic release and aerobic uptake of phosphorus are different when different organic matter is used as a substrate. Easily degradable organic compounds with small molecular weight (such as volatile fatty acids, etc.) are easily used by polyphosphate bacteria to decompose and release phosphorus from polyphosphate stored in their bodies, which has a strong ability to induce phosphorus release, while polymer refractory organic matter has a poor ability to induce phosphorus release from polyphosphate bacteria. The more abundant the release of phosphorus in the anaerobic phase, the greater the uptake of phosphorus in the aerobic phase. In addition, the energy generated by phosphorus release in the anaerobic stage is mainly used for its absorption of low-molecular-weight organic substrates as the basis for survival under anaerobic conditions. Therefore, whether there is enough organic matter in the influent water is an important factor related to whether the polyphosphate bacteria can survive smoothly under anaerobic conditions. It is generally believed that the COD/TP in the influent water should be greater than 15 in order to ensure that the phosphorus-gathering bacteria have enough substrate to obtain the ideal phosphorus removal effect.

7. RBCOD (easily degradable COD) studies show that when easily degradable carbon sources such as acetic acid, propionic acid and formic acid are used as phosphorus release substrates, the release rate of phosphorus is larger, and its release rate is not related to the concentration of the matrix, but only related to the concentration of activated sludge and the composition of microorganisms, and the release of phosphorus caused by this type of substrate can be expressed by the zero-order reaction equation. In order for other organic compounds to be utilized by polyphosphate bacteria, they must be converted into such small molecules of easily degradable carbon sources before polyphosphate bacteria can use their metabolism.

8. Glycogen glycogen is a branched macromolecular polysaccharide composed of multiple glucoses, which is the storage form of intracellular sugar. As shown in the figure above, glycogen is formed in an aerobic environment, and the stored energy is metabolized in an anaerobic environment to form NADH, a raw material for the synthesis of PHAs, and provides energy for the metabolism of polyphosphate bacteria. Therefore, in the case of delayed aeration or peroxidation, the phosphorus removal effect will be very poor, because excessive aeration will consume a part of the glycogen in the polyphosphate bacteria in the aerobic environment, resulting in the deficiency of NADH, the raw material for the formation of PHAs during anaerobic.

9. HRT: For a well-run urban sewage biological denitrification 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. In general, 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 in the anaerobic section is too short, will not be able to ensure the effective release of phosphorus, and the facultative acidifying bacteria in the sludge can not fully decompose the macromolecular organic matter in the sewage into low-grade fatty acids that can be ingested by the polyphosphate bacteria, which will also affect the release of phosphorus; HRT is too long, it is not necessary, it will not only increase the infrastructure investment and operating costs, but also may produce some side effects. In short, phosphorus release and phosphorus absorption are two interrelated processes, and only after sufficient anaerobic phosphorus release can the polyphosphate bacteria better absorb phosphorus in the aerobic section, and only the polyphosphate bacteria with good phosphorus absorption will release excessive phosphorus in the anaerobic section, and a virtuous circle will be formed if the regulation is properly controlled. The data obtained by our factory in the actual operation are: the HRT of the anaerobic section is 1 hour 15 minutes ~ 1 hour 45 minutes, and the HRT of the aerobic section is 2 hours ~ 3 hours 10 minutes is more appropriate. 10. The reflux ratio (R) A/O process ensures the extremely important point of phosphorus removal effect, that is, the sludge of the system "carries" enough dissolved oxygen into the secondary sedimentation tank in the aeration tank, the purpose of which is to prevent the sludge from releasing phosphorus due to anaerobic in the secondary sedimentation tank, but if the sludge cannot be discharged quickly, the mud layer in the secondary sedimentation tank is too thick, and no matter how high the DO is, it cannot ensure that the sludge does not release phosphorus anaerobically, therefore, the reflux ratio of the A/O system should not be too low, and the reflux ratio should be maintained sufficient, and the sludge in the secondary sedimentation tank should be discharged as soon as possible. However, an excessively high reflux ratio will increase the energy consumption of the reflux system and the aeration system, and shorten the actual residence time of sludge in the aeration tank, which will affect the removal effect of BOD5 and P. How to reduce the reflux ratio as much as possible on the premise of ensuring rapid sludge discharge needs to be repeatedly explored in actual operation. It is generally believed that R is in the range of 50~70%.