What is biological wastewater treatment method? Biological treatment is the use of microorganisms to adsorb, decompose, and oxidize organic matter in wastewater, degrading unstable organic matter into stable and harmless substances, thereby purifying wastewater. Modern biological treatment methods can be divided into two categories based on the different microorganisms involved: aerobic oxidation and anaerobic reduction. The former is widely used for treating urban sewage and organic industrial wastewater. Aerobic oxidation has a wide range of applications, including many techniques and structures.
Various processes and structures such as biofilm method (including biological filtration tank, biological turntable), biological contact oxidation, etc. Activated sludge process and biofilm process are both artificial biological treatment methods. In addition, there are natural biological treatment methods for farmland and ponds, namely irrigated fields and biological ponds. Biological treatment is currently the most widely used sewage treatment method due to its low cost.
What is the wastewater treatment capacity or BOD5 removal total amount and treatment quality? ◆ Sewage treatment capacity or total BOD5 removal: The total daily flow of sewage entering the sewage treatment plant (measured in m3/d) can be used as an indicator of the treatment capacity of the sewage treatment plant. The total amount of BOD5 removed daily can also be used as an indicator of the treatment capacity of sewage plants. The total amount of BOD5 removed is equal to the product of the treatment flow rate and the difference in BOD5 between the inlet and outlet water, measured in kg/d or t/d.
◆ Treatment quality: The secondary sewage treatment plant uses the BOD5 and SS values from the factory as treatment quality indicators. According to the newly formulated effluent discharge standards for sewage treatment plants, the BOD5 and SS of the effluent from secondary sewage treatment plants are both less than 30mg/L. The processing quality can also be measured by the removal rate. The removal rate is calculated by dividing the influent concentration by the effluent concentration. Ammonia nitrogen, TP effluent value or removal rate should also be used as treatment quality indicators. What is pH value and its indicative significance? PH represents the acidity or alkalinity of wastewater. It is the logarithmic value of the reciprocal of hydrogen ion concentration in water, ranging from 0 to 14. If the pH value is 7, water is neutral, and if it is less than 7, it is acidic. The smaller the value, the stronger the acidity, and if it is greater than 7, it is alkaline. The larger the value, the stronger the alkalinity. The pH value in sewage has a certain impact on pipelines, water pumps, gate valves, and sewage treatment structures. The pH value of sewage treatment plants that mainly treat domestic sewage is usually between 7.2 and 7.8. An excessively high or low pH value can indicate the entry of industrial wastewater. Low values can corrode pipelines, pump bodies, and may cause harm. For example, sulfides in wastewater can generate H2S gas under acidic conditions. High concentrations can cause headaches, runny nose, suffocation, and even death during operation. To address this, it is necessary to strengthen monitoring, identify sources of pollution, and take measures to prevent a decrease in pH. At the same time, the allowable pH range for biochemical treatment is 6-10, and too high or too low can affect or damage biological treatment.
What is Total Solids (TS)? ◆ It refers to the total amount of solids remaining after the water sample evaporates to dryness on a water bath at a temperature of 100 ℃. It is the sum of dissolved solids and insoluble solids in wastewater. It can reflect the total concentration of solids in wastewater. The analysis of inflow and outflow solids can reflect the effectiveness of sewage treatment structures in removing total solids.
What is suspended solids (SS)? ◆ refers to the amount of solid substances in sewage that can be intercepted by filters. A portion of suspended solids can precipitate under certain conditions. The determination of suspended solids is usually carried out using asbestos filter layer filtration method. The main equipment is the GuShi crucible. When the laboratory equipment conditions are not available, filter paper can also be used as a filter to determine the suspended solids content by subtracting the total solids from the dissolved solids. When measuring suspended solids, significant differences often occur due to different filters.
This indicator is one of the most basic data for sewage. The measurement of suspended solids in incoming and outgoing water can be used to reflect the reduction of suspended solids in wastewater after treatment in the primary and secondary sedimentation tanks. It is the main basis for reflecting the efficiency of sedimentation construction.
What is Chemical Oxygen Demand (COD)? Chemical Oxygen Demand (COD) refers to the amount of oxygen required by oxidants to oxidize organic matter in wastewater using chemical methods. The result obtained by using potassium permanganate as an oxidant is commonly referred to as oxygen consumption, expressed as OC. The result obtained by using potassium dichromate as an oxidant is called chemical oxygen demand (COD), and the difference between the two lies in the choice of oxidant. Using potassium permanganate as an oxidant can only oxidize linear organic compounds in wastewater, while using potassium dichromate as an oxidant has a stronger and more complete effect than the former. In addition to linear organic compounds, it can oxidize many structurally complex organic compounds that potassium permanganate cannot oxidize. Therefore, the COD value of the same wastewater is much larger than the OC value. Especially when a large amount of industrial wastewater enters the sewage treatment plant, the chemical oxygen demand of the potassium dichromate method should generally be measured. The COD value of urban sewage treatment plants is generally around 400-800mg/L.
The consumption value of potassium permanganate method is often used as a reference data to determine the dilution factor of five-day biochemical oxygen demand in sewage treatment plants.
What is Biochemical Oxygen Demand (BOD)? Biochemical Oxygen Demand (BOD) refers to the amount of oxygen required by microorganisms in water to decompose organic matter under aerobic conditions. It is an indirect indicator of the degree of organic pollution. The biochemical oxidation and decomposition of organic matter usually have two stages. The first stage is mainly the oxidation of carbon containing organic matter, called the carbonization stage, which takes about 20 days to complete. The second stage is mainly the oxidation of nitrogen-containing organic compounds, known as the nitrification stage, which takes about 100 days to complete. Under recognized circumstances, the general standard practice is to cultivate for 5 days at a temperature of 20 ℃ and measure the data, which is called five-day biochemical oxygen demand. Abbreviated as BOD5, therefore BOD5 represents the oxygen demand for partial decomposition of carbon containing organic matter, and the BOD5 of domestic sewage should be around 70%.
The determination of five-day biochemical oxygen demand is to take the original water sample or a water sample that has been appropriately diluted to contain sufficient dissolved oxygen to meet the requirements of five-day biochemical oxygen demand. The water sample is divided into two parts, one part measures the dissolved oxygen content of the day, and the other part is placed in a 20 ℃ incubator for 5 days of cultivation before measuring its dissolved content. The difference between the two is multiplied by the dilution factor to obtain BOD5.
During the BOD5 measurement process, it is crucial to correctly select the dilution factor. It is generally believed that the selected dilution ratio should be such that the dissolved oxygen of the diluted water sample decreases by 20% to 80% after being cultured in a 20 ℃ constant temperature incubator for 5 days. However, sometimes numerical errors are caused by improper control of the dilution ratio of BOD5, and even if the dilution ratio is too small, BOD5 data cannot be obtained.
What is the purpose of measuring BOD? BOD can reflect the degree of organic pollution in sewage. The more organic matter is contained in sewage, the more oxygen is consumed, and the higher the BOD value, and vice versa. Therefore, it is the most important indicator of sewage quality. Although the determination of BOD takes a long time and the data is not timely, the BOD index has comprehensiveness - it comprehensively reflects the total amount of organic matter and simulates the self purification of water bodies. Therefore, it is difficult to replace it with other indicators.
For sewage treatment plants, the purpose of this indicator is to reflect the concentration of organic matter in sewage. Such as the concentration of organic matter in the incoming wastewater and the concentration of organic matter in the outgoing wastewater. The influent BOD5 of urban sewage treatment plants can generally reach 150-350mg/L.
b. Used to indicate the treatment effect of sewage treatment plants. The difference in BOD5 between the inlet and outlet water is divided by the inlet BOD5 to obtain the BOD5 removal rate of the plant, which is an important indicator.
c. The total removal capacity and effluent BOD5 of a sewage treatment plant represent the total treatment capacity and impact on the water environment.
d. Used to calculate the operating parameters of the treatment structure, such as the sludge load BOD5kg (MISS) or volumetric load BOD5kg/(m3/d) of the aeration tank.
e. Reflecting the technical and economic data of the operation of sewage treatment plants, such as the amount of electricity consumed per kilogram of BOD (kWh) and the amount of air required to remove every kilogram of BOD5.
f. To measure the biodegradability of wastewater, when BOD5/COD is greater than 0.3, it indicates that the wastewater can undergo biochemical treatment. When it is less than 0.3, it is difficult to carry out biochemical treatment. When the ratio is between 0.5 and 0.6, biochemical processes are easily carried out.
From this, it can be seen that the determination of BOD5 is very useful and it is the most important measurement item in sewage treatment plants. But the measurement takes a long time and data cannot be obtained in a timely manner. The COD assay reflects the oxygen required for the oxidation of organic matter in wastewater by oxidants, and its data value is close to the oxygen demand of all organic matter. Therefore, it is also of great use, and COD measurement is brief. Generally, COD>BOD in urban sewage plants. If there is little change in the types of organic matter in the sewage, there is a certain correlation between COD and BOD. Therefore, the COD of the day can be used to predict the BOD5 value.
According to the operational data of sewage treatment plants in various cities, SS and BOD5 are usually roughly similar or slightly higher in numerical values. On average, the SS of various sewage treatment plants in Shanghai is about 50mg/L higher than BOD5 in terms of numerical value.
If BOD5 and SS are found to increase exponentially in the incoming wastewater, there may be high concentrations of organic wastewater flowing into the plant or a large amount of feces entering the plant. This will increase the processing load. Reducing processing efficiency and even blocking pipelines requires investigating the cause and taking measures.
What are the indicative meanings of total nitrogen, ammonia nitrogen, nitrite nitrogen, and nitrate nitrogen (N, NH4+, NO2-NO-3)? There are a large number of carbon containing organic compounds and nitrogen-containing organic compounds in sewage, with carbon, hydrogen, and oxygen as the basic elements. The latter is based on nitrogen, sulfur, and phosphorus as basic elements. During aerobic decomposition, nitrogen-containing organic compounds will ultimately be converted into inorganic compounds such as ammonia nitrogen fertilizer, nitrite nitrogen fertilizer, nitrate nitrogen, water, and carbon dioxide. Therefore, measuring the above three indicators can reflect the degree of sewage decomposition process and inorganic transformation after treatment. When only a small amount of nitrite nitrogen appears in the secondary sewage treatment plant, the treated effluent cannot be stable. When the oxygen level is insufficient, most of the organic nitrogen in the sewage is converted into inorganic substances, and the effluent is relatively stable after flowing into the water body. The ammonia nitrogen value of the general incoming wastewater is about 30-70mg/L. The incoming water generally does not contain nitrite and nitrate. Secondary sewage treatment plants generally cannot remove a large amount of nitrogen fertilizer. When the treatment level is high, they can convert some ammonia nitrogen into nitrate nitrogen.
What is the significance of phosphorus and nitrogen (P, N) indicators? The content of phosphorus and potassium in sewage affects the growth of microorganisms. The ratio of BOD5: N: P in activated sludge sewage treatment should be maintained at 100:5:1 or above. In urban sewage treatment plants, this ratio can generally be achieved. Some industrial wastewater cannot reach this ratio, so it is necessary to add nutrients to the wastewater.
What is dissolved oxygen and what is the purpose of measurement? Dissolved oxygen refers to the amount of oxygen dissolved in water, which is closely related to temperature, pressure, and the biochemical processes of microorganisms. At a certain temperature, water can only dissolve a certain amount of oxygen at most. For example, at 20 ℃, the saturation value of dissolved oxygen in distilled water is 9.17 mg/L.
In sewage treatment, it is common to measure the dissolved values in the effluent and aeration tank, adjust the air supply based on their size, and understand the oxygen consumption in the aeration tank to determine the oxygen consumption rate under various water temperature conditions. During operation, it is required that the dissolved oxygen in the aeration tank be above 1 mg/L. A low dissolved oxygen value indicates oxygen deficiency in the aeration tank, while a high dissolved oxygen value not only wastes energy but may also cause sludge to loosen and age.
The presence of dissolved oxygen in the effluent of sewage treatment plants is beneficial to the water environment, and where possible, some dissolved oxygen should be allowed to enter the effluent.
Dissolved oxygen is an important parameter in the process of water self purification, which can reflect the balance between oxygen consumption and dissolved oxygen in the water.
What is the relationship between water temperature and operation? Water temperature has a significant impact on the operation of the aeration tank. The water temperature of a sewage treatment plant gradually changes slowly with the seasons, and there is almost no significant change within a day. If significant changes are found within a day, an inspection should be conducted to check for any industrial cooling entering. When the aeration tank operates within the range of 8-30 ℃ throughout the year and the water temperature is below 8 ℃, the treatment efficiency decreases, and the BOD5 removal rate is often below 80%.
What is sludge load? How to adjust? a. Sludge load=number of BOD5 entering the aeration tank (flow rate x concentration)/total MLSS in the aeration tank (MLSS x tank volume).
b. Due to the fact that the amount of BOD5 in the effluent of the primary sedimentation tank is determined by the incoming water quality and is generally difficult to regulate, adjusting the sludge load and reducing MLSS can increase the sludge load. Increasing or decreasing MLSS is usually achieved by increasing or decreasing the sludge discharge.
The sludge load has a significant impact on treatment efficiency, sludge growth, and oxygen demand, and must be carefully controlled. Generally speaking, the sludge load ranges from 0.2 to 0.5 kg (BOD5)/(kg. d), and is controlled at around 0.3 kg (BOD5)/"kg (MLSS). d".
Capacity load of aeration tank? The daily burden of BOD5 per unit volume of the aeration tank is called the volumetric load kg (BOD5)/(m3. d). The volumetric load represents the economic feasibility of constructing the aeration tank. The relationship between volumetric load, mixed liquid concentration, and sludge load is as follows:
BV=x.B5， In the equation (x is MLSS).
What is the meaning of sludge age? ◆ Sludge age=MLSS quantity in the aeration tank (MLSS x tank volume)/solid content in the remaining sludge (discharge volume x sludge concentration).
The sludge age is the ratio of the total amount of activated sludge working in the aeration tank to the remaining sludge discharged daily, measured in days. When running smoothly, it can be understood as the average residence time of activated sludge in aeration.
The sludge age of a typical aeration tank system is about 5-6 days. When reaching the nitrification stage, the sludge age needs to be 8-12 days or higher.
There is an inverse relationship between sludge age and sludge load, with longer sludge age and lower load, and vice versa, but it does not form an absolute inverse proportional function relationship.
What is the concentration of suspended solids (MLSS) in a mixed solution? The concentration of suspended solids in the mixed solution is the amount of suspended solids in the mixed solution of sewage and activated sludge in the aeration tank, measured in mg/L. It is an indicator for measuring the amount of activated sludge in the aeration tank, and due to its simplicity, it is often used as a rough measure of the microbial biomass of activated sludge. In promoting flow aeration, MLSS is generally 1000-4000mg/L. In fully mixed aeration tanks built together, MLSS roots for air aeration rarely exceed 8000mg/L. This is because MLSS is too high. Obstructing oxygenation also makes it difficult for it to settle in the secondary sedimentation tank.
What is the concentration of volatile suspended solids (MLVSS) in a mixed solution? The concentration of volatile suspended solids in a mixed solution refers to the weight of organic matter in the suspended solids of the mixed solution (usually measured by the loss on ignition at 600 ℃), so some people believe that MLSS can more accurately represent the number of activated sludge microorganisms. However, MLVSS also includes non reactive and non degradable organic matter, which is not the most ideal indicator for measuring MLSS. For domestic sewage, it is often around 0.75.
Sludge Index (SVI)? The sludge index refers to the volume (in ml) occupied by 1g of dry sludge after 30 minutes of static settling of the mixed liquid in the aeration tank, that is, SVI=sludge sedimentation (ml)/sludge dry weight (g) after 30 minutes of static settling of the mixed liquid. The SVI value can better reflect the looseness and coagulation settling performance of activated sludge. A good activated sludge SVI is often between 50 and 300. A sludge concentration with excessively high SVI is only valuable when measured at the same concentration. Additionally, since the size of the measuring container has a certain impact on the quantity being measured, it is necessary to measure the container uniformly.
