In the process of sewage treatment, various sewage problems will be encountered. For example, indicators such as COD, ammonia nitrogen, and SS do not meet the standards, and sludge expansion, floating sludge, and death of active microorganisms occur. Since the principles of sewage treatment are the same, research on sewage treatment has basically been based on domestic sewage as the research blueprint from the beginning. Below, we will summarize the problems encountered in the operation process with domestic sewage as the target.
Inlet water quantity and quality
Inlet water volume
In China, the phenomenon of insufficient inflow of water into urban sewage treatment plants is common. The reasons for this lack of food are not only the commonly mentioned problem of lagging construction of sewage collection pipelines, but also the problem of advanced design capabilities. These two reasons have led to many sewage treatment plants in many places being built for several years but still unable to operate at full capacity. Some sewage treatment plants can only extract river water from the surrounding areas for treatment, making it more difficult to control the sewage treatment process and increasing the cost of engineering investment. This has resulted in idle and wasteful assets, unnecessarily consuming the already tight sewage treatment funds.
On the contrary, some sewage treatment plants have been operating at overload for a long time. For example, the first phase of a sewage treatment plant has a scale of 400000 m3/d, and the second phase has a scale of 240000 m3/d. However, due to a shortage of funds, the construction of the second phase has lagged behind. The actual treatment capacity of the first phase has reached 520000 m3/d, and the quality of the treated effluent has decreased. Therefore, a reasonable determination of the construction scale and stages of sewage treatment plants, efficient use of pollution control funds, and maximizing sewage collection rates are prerequisites for achieving sewage emission reduction.
Inlet water quality
The sewage collection pipeline network is not compatible, and the rainwater and sewage combined pipeline network is common. The management of the pipeline network is inadequate, resulting in a large proportion of rainwater, river water, and industrial wastewater entering the urban sewage treatment plant.
The following inlet water quality conditions are not conducive to the normal operation of the sewage treatment plant:
1. The BOD and COD contents in the influent are lower than the design values, while nitrogen, phosphorus and other indicators are equal to or higher than the design values, thereby increasing the difficulty of meeting the discharge standards for sewage denitrification and phosphorus removal treatment;
2. The carried oil or toxic substances in industrial wastewater have a huge impact on the biological system of urban sewage treatment plants. In extreme cases, these oil or toxic substances can paralyze the entire biological system, cause the death of microbial strains, and force the entire sewage treatment plant to retrain active sludge;
3. The incoming water quality is too high, and the specifications of the oxygen supply and sludge dewatering equipment cannot meet the requirements for sewage and sludge treatment. The impact of introducing leachate from garbage on the operation of urban sewage treatment plants needs to be given sufficient attention.
For the problem of the lack of coordination between sewage collection and sewage treatment capacity, relevant competent departments need to incorporate the construction of urban drainage networks and sewage treatment plants into the overall planning of urban construction in the short and long term, to ensure that the sewage collection system and sewage treatment plants are constructed synchronously or in advance. At the same time, conduct a survey of the sewage quality within the service area of the newly built sewage treatment plant to reasonably determine the design inlet water quality.
Effluent quality
The basic requirements for urban sewage treatment plants constructed in China in recent years are to meet the first level B standard in GB18918-2002, and in some areas, it is also required to meet the first level A standard. Even existing projects are gradually being upgraded and renovated to improve the effectiveness of wastewater reduction.
According to the prescribed sewage treatment discharge standards, each city's sewage treatment plant adopts sewage treatment technology suitable for local inflow water quality and other objective conditions, and strengthens operational management. However, in the actual operation and management process of sewage treatment plants, there may still be some problems from different aspects that result in the treated effluent quality not meeting the standards.
1. Excessive organic matter
The main function of traditional activated sludge process is to remove organic pollutants contained in urban sewage. A well-designed and well operated activated sludge process can achieve effluent BOD5 and SS levels of less than 20mg/L.
The main factors affecting the effectiveness of organic matter treatment are:
1. Nutrients
Nutrients such as nitrogen and phosphorus in general urban sewage can meet the needs of microorganisms, and there is a significant excess. 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 nitrogen is lacking in sewage, ammonium salts can usually be added. If there is a lack of phosphorus in the sewage, phosphoric acid or phosphate can usually be added.
2. pH
The pH value of urban sewage is neutral, generally ranging from 6.5 to 7.5. The slight decrease in pH value may be due to anaerobic fermentation in urban sewage pipelines. The significant decrease in pH during the rainy season is often caused by urban acid rain, which is particularly prominent in combined sewer systems. Sudden and significant changes in pH, whether it is an increase or a decrease, are usually caused by the massive discharge of industrial wastewater. Adjusting the pH value of wastewater usually involves adding sodium hydroxide or sulfuric acid, but this will greatly increase the cost of wastewater treatment.
3. Oil and Fat
When the oil content in sewage is high, it will reduce the aeration efficiency of the aeration equipment. If the aeration amount is not increased, the treatment efficiency will decrease. However, increasing the aeration amount will inevitably increase the cost of sewage treatment. In addition, the high oil content in sewage can also reduce the settling performance of activated sludge, and in severe cases, it can become a cause of sludge expansion, leading to excessive SS in the effluent. For influent with high oil content, it is necessary to add a degreasing device in the pretreatment section.
4. Temperature
The influence of temperature on the activated sludge process is extensive. Firstly, temperature can 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 can affect the separation performance of the secondary sedimentation tank, for example, temperature changes can cause the sedimentation tank to produce heavy flow, resulting in short flow; A decrease in temperature will reduce the settling performance of activated sludge due to an increase in viscosity; Temperature changes can affect the efficiency of the aeration system. In summer, when the temperature rises, it becomes difficult to oxygenate due to a decrease in the saturation concentration of dissolved oxygen, resulting in a decrease in aeration efficiency and a decrease in air density. To ensure a constant gas supply, it is necessary to increase the gas supply.
2. Excessive ammonia nitrogen
The removal of ammonia nitrogen in sewage mainly adopts nitrification process based on traditional activated sludge process, which uses delayed aeration to reduce system load.
The reasons for excessive ammonia nitrogen in effluent involve many aspects, mainly including:
1. Sludge load and sludge age
Biological nitrification is a low load process, with F/M generally ranging from 0.05 to 0.15 kg BOD/kg MLVSS • d. The lower the load, the more complete the nitrification process, and the higher the efficiency of NH3-N to NO3-- N conversion. Corresponding to low load, the SRT of biological nitrification systems is generally longer because the generation cycle of nitrifying bacteria is longer. If the sludge retention time of the biological system is too short, i.e. the SRT is too short, and the sludge concentration is low, nitrifying bacteria cannot be cultured and nitrification effect cannot be achieved. The SRT control depends on factors such as temperature. For biological systems with denitrification as the main objective, SRT can usually take 11-23 days.
2. Reflux ratio
The reflux ratio of biological nitrification system is generally higher than that of traditional activated sludge process, mainly because the mixed liquid of activated sludge in biological nitrification system already contains a large amount of nitrate. If the reflux ratio is too small, the residence time of activated sludge in the secondary sedimentation tank will be longer, which is prone to denitrification and causes sludge to float up. Usually, the reflux ratio is controlled between 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, thus requiring a longer reaction time.
4.BOD5/TKN
TKN refers to the sum of organic nitrogen and ammonia nitrogen in water, and BOD5/TKN in influent wastewater is an important factor affecting nitrification efficiency. The larger the BOD5/TKN ratio, the smaller the proportion of nitrifying bacteria in the activated sludge, the lower the nitrification rate, and the lower the nitrification efficiency under the same operating conditions; On the contrary, the smaller the BOD5/TKN, the higher the nitrification efficiency. The operation practice of many urban sewage treatment plants has found that the optimal range of BOD5/TKN value is around 2-3.
5. Nitrification rate
A specific process parameter of biological nitrification system is 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, with a typical value of 0.02gNH3-N/gMLVSS × d.
6. Dissolved oxygen
Nitrifying bacteria are specialized aerobic bacteria that stop their life activities when there is no oxygen, and their oxygen uptake rate is much lower than that of bacteria that decompose organic matter. If sufficient oxygen is not maintained, nitrifying bacteria will not be able to "compete" for the oxygen they need. Therefore, it is necessary to maintain the dissolved oxygen in the aerobic zone of the biological pool at 2mg/L or above, and in special circumstances, the dissolved oxygen content needs to be increased.
7. Temperature
Nitrifying bacteria are also sensitive to temperature changes. When the sewage temperature is below 15 ℃, the nitrification rate will significantly decrease. When the sewage temperature is below 5 ℃, their physiological activities will completely stop. Therefore, during winter, the phenomenon of excessive ammonia nitrogen in the effluent of sewage treatment plants, especially those in northern regions, is more pronounced.
8. pH
Nitrifying bacteria are highly sensitive to pH reactions, and their biological activity is strongest within the pH range of 8-9. 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 in the biological nitrification system should be controlled to be greater than 7.0 as much as possible.
3. Total nitrogen exceeds the standard
Wastewater denitrification is a process that adds biological denitrification technology to biological nitrification technology. Denitrification technology refers to the biochemical reaction process in which nitrate in wastewater is reduced to nitrogen by microorganisms under anaerobic conditions.
The reasons for excessive total nitrogen in effluent involve many aspects, mainly including:
1. Sludge load and sludge age
Due to the fact that biological nitrification is a prerequisite for biological denitrification, only good nitrification can achieve efficient and stable denitrification. Therefore, the denitrification system must also adopt low load or ultra-low load, and use high sludge age.
2. Internal and external reflux ratio
The external reflux of biological denitrification system is smaller than that of simple biological nitrification system, mainly because the majority of nitrogen in the inflow sewage has been removed, and the concentration of NO3-- N in the secondary sedimentation tank is not high. Relatively speaking, the risk of sludge floating up in the secondary sedimentation tank due to denitrification is already very low. On the other hand, the sedimentation rate of sludge in denitrification systems is relatively fast. While ensuring the required concentration of returning sludge, the reflux ratio can be reduced to facilitate the extension of the residence time of sewage in the aeration tank.
A well functioning sewage treatment plant can control the external reflux ratio below 50%. The internal reflux ratio is generally controlled between 300-500%.
3. Denitrification rate
Denitrification rate refers to the daily amount of nitrate that is denitrified per unit of activated sludge. The denitrification rate is related to factors such as temperature, with a typical value of 0.06-0.07gNO3-- N/gMLVSS × d.
4. Dissolved oxygen in hypoxic areas
For denitrification, it is desirable to have DO as low as possible, preferably zero, so that denitrifying bacteria can "fully" carry out denitrification and improve denitrification efficiency. However, from the actual operation of the sewage treatment plant, it is still difficult to control the DO in the anoxic zone below 0.5mg/L, which affects the process of biological denitrification and thus affects the total nitrogen index of the effluent.
5. BOD5/TKN
Because denitrifying bacteria carry out denitrification and denitrification during 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 lagging construction of supporting pipeline networks in many sewage treatment plants, the BOD5 entering the plant is lower than the design value, while nitrogen, phosphorus and other indicators are equivalent to or higher than the design value, which makes the incoming carbon source unable to meet the carbon source requirements of denitrification, and also leads to the occurrence of excessive total nitrogen in the effluent from time to time.
6. pH
Denitrifying bacteria are not as sensitive to pH changes as nitrifying bacteria, and can perform normal physiological metabolism within the pH range of 6-9. However, the optimal pH range for biological denitrification is 6.5-8.0.
7. Temperature
Although denitrifying bacteria are not as sensitive to temperature changes as nitrifying bacteria, their denitrification efficiency also varies with temperature changes. The higher the temperature, the higher the denitrification rate, and the denitrification rate reaches its maximum at 30-35 ℃. When the temperature is below 15 ℃, the denitrification rate will significantly decrease, and by 5 ℃, denitrification will tend to stop. Therefore, in order to ensure denitrification efficiency in winter, it is necessary to increase SRT, increase sludge concentration, or increase the number of operational tanks.
4. Total phosphorus exceeds the standard
The phosphorus removal in urban sewage treatment plants mainly relies on biological phosphorus removal, that is, adding an anaerobic section before the aerobic section, allowing polyphosphate accumulating bacteria to alternate between anaerobic and aerobic states, achieving the release and absorption of phosphate, and achieving phosphorus removal by discharging excess sludge. On the other hand, under conditions where biological phosphorus removal is difficult to meet standards, chemical agents can also be considered to assist in phosphorus removal. Chemical phosphorus removal is mainly achieved through methods such as coagulation, precipitation, and filtration to make phosphorus an insoluble solid precipitate, which is separated from wastewater.
The reasons for the excessive total phosphorus in biological phosphorus removal effluent involve many aspects, mainly including:
1. Sludge load and sludge age
The anaerobic aerobic biological phosphorus removal process is a high F/M low SRT system. When the F/M ratio is high and the SRT is low, the remaining sludge discharge is also higher. Therefore, under the condition of a certain phosphorus content in sludge, the more phosphorus is removed, the better the phosphorus removal effect.
For biological systems with phosphorus removal as the main objective, the F/M ratio is usually 0.4-0.7kgBOD5/kgMLSS × d, and the SRT is 3.5-7d. However, SRT cannot be too low and must be based on ensuring effective removal of BOD5.
2. BOD5/TP
To ensure phosphorus removal efficiency, the BOD5/TP ratio in the wastewater entering the anaerobic zone should be controlled to be greater than 20. Due to the fact that polyphosphate bacteria belong to the genus Acinetobacter, their physiological activity is weak and they can only take up the easily decomposable parts of organic matter. Therefore, the BOD5 content should be ensured in the influent to ensure the normal physiological metabolism of polyphosphate bacteria. However, many urban sewage treatment plants actually have low carbon sources and high concentrations of nitrogen, phosphorus, and other pollutants in their influent, resulting in BOD5/TP values that cannot meet the needs of biological phosphorus removal, which affects the effectiveness of biological phosphorus removal.
3. Dissolved oxygen
The anaerobic zone should maintain a strict anaerobic state, with dissolved oxygen below 0.2mg/L, at which point polyphosphate accumulating bacteria can effectively release phosphorus to ensure subsequent treatment effectiveness. The dissolved oxygen in the aerobic zone needs to be maintained above 2.0mg/L for polyphosphate accumulating bacteria to effectively absorb phosphorus. Therefore, improper control of dissolved oxygen in anaerobic and aerobic zones will greatly affect the effectiveness of biological phosphorus removal. In addition, some sewage treatment plants feed river water with high dissolved oxygen content. If it directly enters the anaerobic zone, it is not conducive to controlling the anaerobic state and affects the phosphorus release efficiency of polyphosphate accumulating bacteria.
4. Reflux ratio
The reflux ratio of anaerobic aerobic phosphorus removal system should not be too low, and sufficient reflux ratio should be maintained to discharge the sludge in the secondary sedimentation tank as soon as possible to prevent phosphorus accumulating bacteria from releasing phosphorus in the anaerobic environment in the secondary sedimentation tank. On the premise of ensuring rapid sludge discharge, the reflux ratio should be minimized as much as possible to avoid shortening the actual residence time of sludge in the anaerobic zone and affecting the release of phosphorus.
In anaerobic aerobic phosphorus removal systems, if sludge.
The suspended solids in the effluent of the secondary sedimentation tank exceed the standard
Selection of process parameters for secondary sedimentation tank
The selection of appropriate design parameters for the secondary sedimentation tank is an important factor in determining whether the suspended solids index in the effluent will exceed the standard. At the beginning of the design of many urban sewage treatment plants, in order to save construction costs, the hydraulic retention time is greatly shortened and the hydraulic surface load is maximized, resulting in frequent mud overturning in the secondary sedimentation tank during operation, leading to excessive suspended solids in the effluent.
In addition, some sewage treatment plants need to control the sludge concentration in the biological tank at a high level due to actual process adjustments, which can also cause excessive surface load on the solid surface of the secondary sedimentation tank and affect the effluent quality. Therefore, it is generally believed that there is room for adjustment in the setting of these process parameters for the secondary sedimentation tank, in order to facilitate the control and adjustment of the sewage treatment plant process.
Generally speaking, the main process parameters that affect the sedimentation effect of sedimentation tanks are hydraulic retention time, hydraulic surface load, and sludge flux.
Hydraulic retention time of secondary sedimentation tank
The hydraulic retention time of sewage in the secondary sedimentation tank is an important parameter for its operation. Only sufficient residence time can ensure good flocculation effect and achieve high sedimentation efficiency. Therefore, it is recommended to set the hydraulic retention time of the secondary sedimentation tank at around 3-4 hours.
Hydraulic surface load of secondary sedimentation tank
For a sedimentation tank, the size of particles it can remove is also constant when the inlet water volume is constant. Among the particles that can be removed, the settling velocity of the smallest particle is exactly equal to the hydraulic surface load of this sedimentation tank. Therefore, the smaller the hydraulic surface load, the more particles can be removed, the higher the sedimentation efficiency, and the lower the index of suspended solids in the effluent. Designing a secondary sedimentation tank with a smaller hydraulic surface load is beneficial for the effective sedimentation of suspended solids such as sludge. It is generally recommended to control the hydraulic surface load of the secondary sedimentation tank at 0.6-1.2m3/m2 × h.
Solid surface load of secondary sedimentation tank
The size of the solid surface load on the secondary sedimentation tank is also an important factor affecting the sedimentation effect of the secondary sedimentation tank. The smaller the solid surface load of the secondary sedimentation tank, the better the concentration effect of sludge in the secondary sedimentation tank. On the contrary, the concentration effect of sludge in the secondary sedimentation tank is worse. Excessive solid surface load can cause the mud surface of the secondary sedimentation tank to be too high, and many sludge flocs will flow out with the sewage before settling, affecting the suspended solids index of the effluent. The maximum solid surface load of a general secondary sedimentation tank should not exceed 150kg MLSS/m2 × d.
Quality of activated sludge
The quality of activated sludge is an important factor affecting whether the suspended solids in the effluent exceed the standard. High quality activated sludge is mainly reflected in four aspects: good adsorption performance, high biological activity, good settling performance, and good concentration performance.
Colloidal pollutants must first be adsorbed onto activated sludge flocs and further adsorbed near bacterial surfaces before they can be decomposed and metabolized. Therefore, activated sludge with poor adsorption performance also has poor ability to remove colloidal pollutants. The biological activity of activated sludge refers to the ability of microorganisms in the sludge flocs to decompose and metabolize organic pollutants. Activated sludge with poor biological activity will inevitably have a slower removal rate of organic pollutants.
Only activated sludge with good settling performance can effectively separate sludge and water in the secondary sedimentation tank. On the contrary, if the settling performance of sludge deteriorates, the separation effect will inevitably decrease, resulting in turbid effluent from the secondary sedimentation tank, excessive SS, and in severe cases, a large amount of activated sludge may be lost, leading to insufficient biomass in the system, which in turn affects the decomposition and metabolism of organic pollutants.
Only activated sludge with good concentration performance can achieve a high sludge concentration in the secondary sedimentation tank. On the contrary, if the concentration performance is poor and the sludge concentration decreases, it is necessary to ensure sufficient reflux sludge volume and increase the reflux ratio. However, increasing the reflux ratio will shorten the actual residence time of sewage in the aeration tank, resulting in insufficient aeration time and affecting the treatment effect.
Inlet SS/BOD5
The proportion of MLVSS in activated sludge of biological systems is closely related to the influent SS/BOD5. When the influent SS/BOD5 is high, the proportion of MLVSS in activated sludge of biological systems is low, and vice versa. Based on operational experience, when SS/BOD is below 1, the MLVSS ratio can be maintained at over 50%. However, when SS/BOD is above 5, the VSS ratio will decrease to 20-30%. When the proportion of MLVSS in activated sludge is low, in order to ensure the nitrification effect, the system must maintain a high sludge age. The aging of sludge is more obvious, resulting in excessive SS in the effluent.
Toxic Substances
The inflow of sewage contains toxic substances such as strong acids, strong alkalis, or heavy metals, which can cause activated sludge to be poisoned, lose its treatment effectiveness, and even lead to sludge disintegration, resulting in the inability of sludge to settle and excessive suspended solids in the effluent. The fundamental solution to the problem of activated sludge poisoning is to strengthen the management of upstream pollution sources.
Temperature
The influence of temperature on the activated sludge process is extensive. Firstly, temperature can 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 function of the secondary sedimentation tank. If the temperature changes, it will cause the secondary sedimentation tank to produce density currents, resulting in short flow phenomena; When the temperature decreases, the sedimentation performance of activated sludge will decrease due to the increase in viscosity.
Moisture content of mud cake
At present, the main indicator for assessing sludge from urban sewage treatment plants is the moisture content of the sludge cake.
In China, urban sewage treatment plants that have been put into use or are under construction generally use activated sludge method for sewage treatment. The sludge age design of activated sludge is relatively short, and there are basically no sludge concentration and digestion facilities in the design, resulting in a large amount of residual sludge, high organic components in sludge, and difficulty in dewatering. Therefore, in order to control the moisture content of the mud cake below 80%, it is necessary to increase the dosage of PAM, thereby increasing the cost of sewage treatment.
In order to ensure the concentration and dewatering effect of sludge, the concentration of flocculants in the preparation of sludge dewatering flocculants should be controlled within the range of 0.1% to 0.5%. If the concentration is too low, the amount of solution added will be large and the frequency of dispensing will increase; Excessive concentration can easily cause high viscosity of the medication, which may result in uneven mixing, increased resistance when the screw pump transports the medication, and accelerated equipment loss and pipeline blockage. In addition, the specific gravity of coagulants varies greatly among different batches and models. It is necessary to regularly or irregularly calibrate the concentration of the prepared agents according to the actual situation, adjust the dosage of the agents in a timely manner, ensure the dewatering effect of sludge, and reduce the waste of agents. At the same time, attention should be paid to moisture prevention and failure prevention during the storage and use of dry powder agents.
Nowadays, the demand for sludge moisture content is increasing, and sludge drying is gradually put on the agenda. Sludge drying generally includes methods such as air drying, high-temperature drying, and adding lime as an additive. However, many places now prohibit the addition of lime to reduce moisture content. Air drying requires a large space and odor cannot be prevented. High temperature drying consumes too much energy, so it can only be solved through chemicals, dehydrators, and processes. Currently, advanced methods include high-pressure diaphragm mud presses, low-temperature drying, etc. In the future, anaerobic digestion of sludge from large-scale sewage treatment plants to produce biogas will be a trend.
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