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The main factors that affect the effect of organic matter treatment are:
Generally, the nutrient elements such as nitrogen and phosphorus in sewage can meet the needs of microorganisms, and there is a lot of excess. However, when the proportion of industrial wastewater is relatively large, attention should be paid to whether the ratio of carbon, nitrogen, and phosphorus meets 100:5:1. If the sewage is deficient in nitrogen, ammonium salt can usually be added. If there is a lack of phosphorus in wastewater, phosphoric acid or phosphate can usually be added.
The pH value of sewage is neutral, generally 6.5 to 7.5. The slight decrease in pH may be due to anaerobic fermentation in the sewage pipeline. Larger pH drops in the rainy season are often caused by urban acid rain, which is particularly prominent in a combined system. Sudden and large changes in pH, whether it is an increase or decrease, are usually caused by a large amount of industrial wastewater discharged. To adjust the pH value of sewage, sodium hydroxide or sulfuric acid is usually added, but this will greatly increase the cost of sewage treatment.
When the oil content in the sewage is high, the aeration efficiency of the aeration equipment will be reduced. If the aeration volume is not increased, the treatment efficiency will be reduced, but increasing the aeration volume will inevitably increase the sewage treatment cost. In addition, the higher oil content in the sewage will also reduce the settling performance of the activated sludge. In severe cases, it will become the cause of sludge expansion and cause the effluent SS to exceed the standard. For influent water with high oil content, it is necessary to add a degreasing device in the pretreatment section.
The influence of temperature on the activated sludge process is very extensive. First of all, temperature will affect the activity of microorganisms in activated sludge. When the temperature is low in winter, if no control measures are taken, the treatment effect will decrease. Secondly, temperature will affect the separation performance of the secondary settling tank. For example, temperature changes will cause density flow in the sedimentation tank, resulting in short flow; temperature reduction will reduce the settling performance of activated sludge due to increased viscosity; temperature changes will affect aeration The efficiency of the system, when the temperature rises in summer, it will be difficult to oxygenate due to the decrease of the saturated concentration of dissolved oxygen, resulting in a decrease in aeration efficiency and a decrease in air density. If the air supply is to be kept constant, it must Increase the air supply.
The removal of ammonia nitrogen in sewage is mainly based on the traditional activated sludge process by using a nitrification process, that is, using delayed aeration to reduce system load.
There are many reasons that cause the ammonia nitrogen in the effluent to exceed the standard, including:
Biological nitrification is a low-load process, and the F/M is generally 0.05-0.15kgBOD/kgMLVSS·d. The lower the load, the more fully the nitrification is carried out, and the higher the conversion efficiency of NH3-N to NO3--N. Corresponding to low load, the SRT of the biological nitrification system is generally longer, because the generation cycle of nitrifying bacteria is longer. If the sludge retention time of the biological system is too short, that is, the SRT is too short and the sludge concentration is low, the nitrifying bacteria will be cultivated. If you don't get it up, you won't get the nitrification effect. How much SRT is controlled depends on factors such as temperature. For biological systems whose main purpose is denitrification, SRT usually takes 11 to 23 days.
The reflux 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 already contains a large amount of nitrate. If the reflux ratio is too small, the activated sludge will stay in the secondary settling tank. The time is longer, and it is easy to produce denitrification and cause the sludge to float up. Usually the reflux ratio is controlled at 50-100%.
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 8h. This is mainly because the nitrification rate is much lower than the removal rate of organic pollutants, which requires a longer reaction time.
TKN refers to the sum of organic nitrogen and ammonia nitrogen in the water. BOD5/TKN in the 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; conversely, the smaller the BOD5/TKN, the higher the nitrification efficiency. The operation practice of many sewage treatment plants has found that the best range of BOD5/TKN value is about 2 to 3.
A 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. The typical value is 0.02gNH3-N/gMLVSS×d.
Nitrifying bacteria are obligate aerobic bacteria, which stop their 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. If sufficient oxygen is not maintained, nitrifying bacteria will "compete" and lose everything. Need oxygen. Therefore, it is necessary to keep the dissolved oxygen in the aerobic zone of the biological pond above 2 mg/L, and the dissolved oxygen content needs to be increased under special circumstances.
Nitrifying bacteria are also very sensitive to temperature changes. When the sewage temperature is lower than 15°C, the nitrification rate will obviously decrease, and when the sewage temperature is lower than 5°C, its physiological activities will stop completely. Therefore, in winter, the effluent ammonia nitrogen of sewage treatment plants, especially in northern areas, is more obvious.
Nitrifying bacteria are very sensitive to pH response. In the range of pH 8-9, their biological activity is the strongest. When pH<6.0 or >9.6, the biological activity of nitrifying bacteria will be inhibited and tend to stop. Therefore, the pH of the mixed solution of the biological nitrification system should be controlled to be greater than 7.0.
Sewage denitrification is based on the biological nitrification process by adding a biological denitrification process. The denitrification process refers to the biochemical reaction process in which nitrate in the sewage is reduced to nitrogen by microorganisms under anoxic conditions.
There are many reasons that cause the total nitrogen in the effluent to exceed the standard, mainly including:
Since biological nitrification is the prerequisite of biological denitrification, only good nitrification can obtain efficient and stable denitrification. Therefore, the denitrification system must also use low load or ultra-low load, and use high sludge age.
The external backflow of the biological denitrification system is smaller than that of the pure biological nitrification system. This is mainly because most of the nitrogen in the influent sewage has been removed, and the NO3--N concentration in the secondary sedimentation tank is not high. Relatively speaking, the risk of sludge floating in the secondary settling tank due to denitrification is very small. On the other hand, the sludge sedimentation rate of the denitrification system is relatively fast. Under the premise of ensuring the required return sludge concentration, the return ratio can be reduced to extend the residence time of the sewage in the aeration tank.
For a well-functioning sewage treatment plant, the external reflux ratio can be controlled below 50%. The internal reflux ratio is generally controlled between 300 and 500%.
The denitrification rate refers to the amount of nitrate denitrified per unit of activated sludge per day. The denitrification rate is related to factors such as temperature, and 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 denitrify with "full force" and improve the efficiency of denitrification. However, judging from the actual operation of the sewage treatment plant, it is still difficult to control the DO in the hypoxic zone below 0.5 mg/L, which affects the process of biological denitrification, which in turn affects the total nitrogen index of the effluent.
Because denitrifying bacteria perform denitrification and denitrification in the process of decomposing organic matter, there must be sufficient organic matter in the sewage entering the anoxic zone to ensure the smooth progress of denitrification. Due to the lag in the construction of supporting pipe networks in many sewage treatment plants, the incoming BOD5 is lower than the design value, and indicators such as nitrogen and phosphorus are equal to or higher than the design value, making the influent carbon source unable to meet the carbon source demand for denitrification. It has also led to occasions where the total nitrogen in the effluent exceeds the standard.
Denitrifying bacteria are not as sensitive to pH changes as nitrifying bacteria. They can perform normal physiological metabolism within the range of pH 6-9, but the optimal pH range for biological denitrification is 6.5-8.0.
Although 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. When it is lower than 15°C, the denitrification rate will obviously decrease, and when it reaches 5°C, denitrification will tend 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 operational ponds.
In biological phosphorus removal, phosphorus accumulating bacteria release phosphorus in an anaerobic state, and excessive intake of phosphorus in an aerobic state. After the phosphorus-rich surplus sludge is discharged to remove phosphorus, there are many reasons for the effluent TP to exceed the standard. The main reasons are:
The effect of temperature on the effect of phosphorus removal is not as obvious as that on the process of biological denitrification. Within a certain temperature range, when the temperature change is not very large, biological phosphorus removal can run successfully. Experiments have shown that the temperature of biological phosphorus removal should be greater than 10°C, because the growth rate of phosphorus accumulating bacteria will slow down at low temperatures.
When the pH is between 6.5 and 8.0, the phosphorus content and phosphorus absorption rate of the phosphorus-accumulating microorganisms remain stable. When the pH value is lower than 6.5, the phosphorus absorption rate drops sharply. When the pH value suddenly decreases, the phosphorus concentration rises sharply in both aerobic and anaerobic areas. The greater the pH decrease, the greater the release, which indicates that the phosphorus release caused by the decrease in pH is not caused by the change in pH by the phosphorus accumulating bacteria itself. Physiological and biochemical reactions are purely chemical "acid solubilization" effects, and the greater the anaerobic release caused by the pH drop, the lower the aerobic phosphorus absorption capacity, which shows that the release caused by the pH drop is destructive. Invalid. A slight absorption of phosphorus occurs when the pH increases.
Each milligram of oxygen can consume 1.14mg of easily biodegradable COD, which inhibits the growth of phosphorus-accumulating organisms, making it difficult to achieve the expected phosphorus removal effect. The anaerobic zone should maintain a low dissolved oxygen value to facilitate the fermentation of anaerobic bacteria to produce acid, so that the phosphorus-accumulating bacteria can release phosphorus better. In addition, less dissolved oxygen is more beneficial to reduce the consumption of easily degradable organic matter. In turn, the phosphorous accumulating bacteria can synthesize more PHB.
In the aerobic zone, more dissolved oxygen is needed, so that the phosphorous accumulating bacteria can decompose and store PHB substances to obtain energy to absorb the dissolved phosphate in the sewage to synthesize cellular polyphosphate. 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 release and aerobic phosphorus absorption.
The presence of nitrate nitrogen in the anaerobic zone consumes organic substrates and inhibits the release of phosphorus from PAO, thereby affecting the phosphorus absorption of phosphorus accumulating bacteria under aerobic conditions. On the other hand, the presence of nitrate nitrogen will be used by Aeromonas spp. as an electron acceptor for denitrification, thereby affecting its use of fermentation intermediates as electron acceptors to produce acid through fermentation, thereby inhibiting the release and uptake of PAO. Ability and PHB synthesis ability. Each milligram of nitrate nitrogen can consume 2.86 mg of easily biodegradable COD, which will inhibit the anaerobic release of phosphorus, which is generally controlled below 1.5 mg/l.
Since the biological phosphorus removal system mainly achieves phosphorus removal by discharging the remaining sludge, the amount of the remaining sludge determines the phosphorus removal effect of the system, and the length of the sludge has a direct effect on the discharge of the remaining sludge and the uptake of phosphorus by the sludge. Impact. The younger the sludge age, the better the phosphorus removal effect. This is because reducing the age of sludge can increase the discharge of surplus sludge and the amount of phosphorus removal in the system, thereby reducing the phosphorus content in the effluent of the secondary settling tank. However, for the biological treatment process for simultaneous phosphorus and nitrogen removal, in order to meet the growth requirements of nitrifying and denitrifying bacteria, the sludge age is often controlled to be relatively large, which is the reason why the phosphorus removal effect is unsatisfactory. Generally, the sludge age of the biological treatment system for the purpose of phosphorus removal is controlled within 3.5-7 days.
In the sewage biological phosphorus removal process, the type and content of organic substrates in the anaerobic stage and the ratio of nutrients required by microorganisms to phosphorus in the sewage are important factors that affect the effect of phosphorus removal. When different organic substances are used as substrates, the effects of anaerobic release and aerobic uptake of phosphorus are different. Small molecular weight easily degradable organics (such as volatile fatty acids, etc.) are easily used by phosphorus-accumulating bacteria, decomposing the polyphosphate stored in the body to release phosphorus, which has a strong ability to induce phosphorus release, while the polymer is difficult to degrade organic matter. Phosphorus-accumulating bacteria have poor phosphorus release ability. The fuller the release of phosphorus in the anaerobic phase, the greater the uptake of phosphorus in the aerobic phase. In addition, the energy produced by phosphorus-accumulating bacteria releasing phosphorus in the anaerobic stage is mainly used for its absorption of low-molecular organic substrates as the basis for survival under anaerobic conditions. Therefore, whether the influent water contains enough organic matter is an important factor related to whether the phosphorus accumulating bacteria can survive smoothly under anaerobic conditions. It is generally believed that the COD/TP of the influent water must be greater than 15 to ensure that the phosphorus accumulating bacteria have enough substrate to obtain the ideal phosphorus removal effect.
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