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Zhucheng Blue Green Environmental Equipment Co., Ltd. explains to you the professional terminology of sewage treatment

Zhucheng Blue Green Environmental Equipment Co., Ltd. is a professional sewage treatment equipment manufacturer with many years of experience in the production of environmental pollution prevention and control equipment. Today our experts from Blue Green Environmental Protection are here to explain to you some issues in sewage treatment terminology.

1. What is biological sewage treatment method?
Biological treatment uses microorganisms to absorb, decompose, and oxidize organic matter in sewage, and degrade unstable organic matter into stable and harmless substances, thereby purifying sewage. Modern biological treatment methods can be divided into two categories: aerobic oxidation and anaerobic reduction according to different microorganisms. The former is widely used to treat urban sewage and organic industrial wastewater. Aerobic oxidation is widely used and includes many types of technologies and structures. Biofilm method (including biological filter tank, biological turntable), biological contact oxidation and other processes and structures. Activated sludge method and biofilm method are both artificial biological treatment methods. There are also natural biological treatments for farmland and ponds, namely irrigated fields and bioponds. Biological treatment is the most widely used wastewater treatment method because of its low cost.
2. What is the wastewater treatment volume or the total BOD5 removal volume and treatment quality?
Sewage treatment volume or total BOD5 removal volume: the total sewage flow (in m3/d) entering the sewage plant for treatment every day, which can be used as an indicator of the sewage plant's treatment capacity. The total amount of BOD5 removed every day can also be used as an indicator of the treatment capacity of the sewage plant. 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, expressed in kg/d or t/d.
Treatment quality: The secondary sewage treatment plant uses the factory BOD5 and SS values ​​as treatment quality indicators. According to the newly formulated sewage treatment plant effluent discharge standards, the BOD5 and SS of the secondary sewage treatment plant effluent are both less than 30mg/L. Treatment quality can also be measured by removal rate. The removal rate is the inlet water concentration minus the outlet water concentration divided by the inlet water concentration. Ammonia nitrogen, TP effluent value or removal rate are also used as treatment quality indicators.
3. What is pH value and its meaning?
pH represents the acidity and alkalinity of sewage. It is the logarithmic value of the reciprocal of the hydrogen ion concentration in water. Its range is 0~14. If the pH value is equal to 7, the water is neutral. If the pH value is less than 7, the water is acidic. The smaller the value, the stronger the acidity. 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 pipes, water pumps, gate valves and sewage treatment structures. The pH value of sewage treatment plants, which are mainly domestic sewage, is usually 7.2~7.8. A pH value that is too high or too low can indicate the entry of industrial wastewater. Too low a value will corrode pipes and pump bodies and may cause harm. For example, sulfide in sewage will generate H2S gas under acidic conditions. High concentrations can cause headaches, runny eyes, suffocation and even death during operation. For this reason, it is necessary to strengthen monitoring to find the source of pollution and take countermeasures if the pH is found to be reduced. At the same time, the allowable pH range of biochemical treatment is 6 to 10. If it is too high or too low, it can affect or destroy the biological treatment.
4. What is total solids (TS)?
It refers to the total solid amount remaining after the water sample is evaporated to dryness in a water bath at a temperature of 100°C. It is the sum of dissolved solids and undissolved solids in wastewater. It reflects the total concentration of solids in wastewater. The analysis of solids in and out of the water can reflect the effectiveness of the sewage treatment structure in removing total solids.
5. What is suspended solids (SS)?
It refers to the amount of solid matter in sewage that can be retained by the filter. Part of the suspended solids can settle under certain conditions. The determination of suspended solids is usually carried out using asbestos filter filtration. The main equipment is Gu's crucible. When laboratory equipment conditions are not available, filter paper can also be used as a filter, and the amount of suspended solids can be obtained from the difference between total solids and dissolved solids. When measuring suspended solids, large differences often occur due to different filters.
This indicator is one of the most basic data on sewage. Measuring the suspended solids in the incoming and outgoing water can be used to reflect the reduction of suspended solids after the sewage passes through the primary sedimentation tank and the secondary sedimentation tank. It is the main basis for reflecting the sedimentation efficiency of the structure.
6. What is chemical oxygen demand (COD)?
Chemical oxygen demand (COD for short) refers to the amount of oxygen required by the oxidant to chemically oxidize organic matter in sewage. Potassium permanganate is used as the oxidant, and the measured result is customarily called oxygen consumption, expressed in OC. Potassium dichromate is used as the oxidant, and the measured result is called chemical oxygen demand and expressed as COD. The difference between the two lies in the choice of oxidant. Using potassium permanganate as an oxidant can only oxidize linear organic compounds in sewage, 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 organic compounds with complex structures that potassium permanganate cannot oxidize. Therefore, the COD value of the same sewage is much greater than the OC value. Especially when a large amount of industrial wastewater enters the sewage plant, the chemical oxygen demand of the potassium dichromate method should generally be measured. The COD value of urban sewage plants is generally about 400~800mg/L.
The consumption value of the potassium permanganate method is often used in sewage plants as reference data to determine the dilution factor of the five-day biochemical oxygen demand.
7. What is biochemical oxygen demand (BOD)?
Biochemical Oxygen Demand: (BOD for short) refers to the amount of oxygen required by microorganisms in water to decompose organic matter under aerobic conditions. It is an indicator that indirectly indicates the degree of pollution of organic matter. The biochemical oxidative decomposition of organic matter usually has two stages. The first stage is mainly the oxidation of carbon-containing organic matter, which is called the carbonization stage. It takes about 20 days to complete. The second stage is mainly the oxidation of nitrogen-containing organic matter, called the nitrification stage, which takes about 100 days to complete. Under generally accepted circumstances, the general standard practice is to incubate for 5 days at a temperature of 20°C and then measure it. The measured data is called the five-day biochemical oxygen demand. It is referred to as BOD5, so BOD5 represents the oxygen demand for the decomposition of some carbon-containing organic matter. The BOD5 of domestic sewage should be about 70%.
To measure the five-day biochemical oxygen demand, take the original water sample or a properly diluted water sample to make it contain enough dissolved oxygen to meet the five-day biochemical oxygen demand requirements. Divide the water sample into two parts. One part is measured for the dissolved oxygen content on that day, and the other part is placed in a 20°C incubator. After incubation for 5 days, the dissolved content is measured. The difference between the two times the dilution factor is BOD5.
During the BOD5 determination process, it is crucial to correctly select the dilution factor. It is generally believed that the dilution factor should be selected so that after the diluted water sample is incubated in a 20°C incubator for 5 days, its dissolved oxygen will be reduced by 20% to 80%, which is more appropriate. However, numerical errors are often caused by improper control of the dilution factor of BOD5, or even the dilution factor is too small to obtain BOD5 data.
8. What is the purpose of measuring BOD?
BOD can reflect the degree of pollution of sewage by organic matter. The more organic matter 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 water quality. Although it takes a long time to measure BOD and the data is not timely, the BOD index is comprehensive - comprehensively reflects the total amount of organic matter, and simulated - imitates 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:
a. Reflects the concentration of organic matter in sewage. Such as the concentration of organic matter in the incoming sewage and the concentration of organic matter in the effluent leaving the factory. The BOD5 of incoming water from urban sewage treatment plants can generally reach 150~350mg/L.
b. Used to indicate the treatment effect of the sewage treatment plant. The difference between the inlet and outlet BOD5 divided by the inlet BOD5 is the BOD5 removal rate of the plant, which is an important indicator.
c. The total removal volume and effluent BOD5 of the sewage treatment plant represent the total treatment capacity of the sewage treatment plant and the 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. Reflect the technical and economic data of the operation of the sewage treatment plant, such as removing the electricity consumption (kWh) per kg BOD and the amount of air required to remove per kg BOD5.
f. Measure the biochemical degree of sewage. When BOD5/COD is greater than 0.3, it means that sewage can be biochemically treated. When it is less than 0.3, it will be difficult to biochemically treat. When the ratio is between 0.5 and 0.6, the biochemical process proceeds easily.
It can be seen that measuring BOD5 is of great use. It is the most important measurement item in sewage treatment plants. However, the measurement takes a long time and the data cannot be obtained in time. The COD test reflects the amount of oxygen required for the oxidation of organic matter in sewage by oxidants, and its data value is close to the oxygen demand of all organic matter. Therefore, it is also of great use, and the COD measurement is short. Generally, COD﹥BOD in urban sewage plants. If there are few changes in the types of organic matter in the sewage, there is a certain relationship between COD and BOD. Therefore, the COD of the day can be used to predict the BOD5 value.
According to the operation data of sewage treatment plants in various cities, SS and BOD5 are usually roughly similar in value or slightly higher. For example, the SS in Shanghai's sewage treatment plants is on average about 50 mg/L higher than BOD5.
If BOD5 and SS are found to increase exponentially in the sewage entering the factory, there may be high-concentration organic wastewater flowing in or a large amount of feces entering the factory. This will increase the processing load. It will reduce the processing efficiency and even block the pipeline. The cause must be traced and measures must be taken.
9. What are the indications of total nitrogen, ammonia nitrogen, nitrite nitrogen, and nitrate nitrogen (N, NH4+, NO2-NO-3)?
There are a large amount of carbon-containing organic matter and nitrogen-containing organic matter in sewage. The former uses carbon, hydrogen and oxygen as basic elements. The latter uses nitrogen, sulfur and phosphorus as basic elements. During the aerobic decomposition process, nitrogen-containing organic matter will eventually be converted into inorganic matter such as ammonia nitrogen fertilizer, nitrite nitrogen fertilizer, nitrate nitrogen, water and carbon dioxide. Therefore, measuring the above three indicators can reflect the decomposition process of sewage and the degree of inorganicization after treatment. When only a small amount of nitrite nitrogen appears in the secondary sewage treatment plant, the effluent from the treatment is not stable. When the amount of oxygen is insufficient, most of the organic nitrogen in the sewage is converted into inorganic matter, and the effluent is relatively stable after flowing into the water body. Generally, the ammonia nitrogen value of the wastewater entering the factory is about 30~70mg/L. The water entering the factory generally does not contain nitrite and nitrate. Secondary sewage treatment plants generally cannot remove large amounts of nitrogen fertilizer. When the treatment level is high, part of the ammonia nitrogen can be converted into nitrate nitrogen.
10. What is the significance of phosphorus and nitrogen (P, N) indicators?
The phosphorus and potassium content in sewage affects the growth of microorganisms. The ratio of BOD5:N:P in activated sludge sewage treatment must be maintained above 100:5:1. This ratio can generally be achieved in urban sewage plants. Some industrial wastewater cannot reach this ratio, so nutrients must be added to the wastewater.
11. 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 effects of microorganisms. At a certain temperature, water can only dissolve up to a certain amount of oxygen. For example, at 20°C, the dissolved oxygen saturation value of distilled water is 9.17 mg/L.
In sewage treatment, the dissolved value in the effluent and the aeration tank is often measured, and the air supply is adjusted according to its size. The oxygen consumption in the aeration tank is understood to determine the oxygen consumption rate of the aeration tank under various water temperature conditions. During operation, the dissolved oxygen in the aeration tank is required to be above 1 mg/L. A too low dissolved oxygen value indicates a lack of oxygen in the aeration tank. An excessively high dissolved oxygen not only wastes energy, but may also cause the sludge to become loose and aged.
The presence of dissolved oxygen in the effluent of sewage treatment plants is beneficial to the water environment. Where possible, the effluent should be allowed to carry some dissolved oxygen.
Dissolved oxygen is an important parameter in the self-purification process of water bodies. It can reflect the balance between oxygen consumption and dissolved oxygen in the water body.
12. What is the relationship between water temperature and operation?
Water temperature has a great relationship with the work of the aeration tank. The water temperature in a sewage treatment plant changes gradually and slowly with the seasons, and there is almost no change within a day. If it is found that there is a big change within a day, it is necessary to check whether there is industrial cooling entering. In the range of 8~30℃ throughout the year, when the aeration tank is operated below 8℃, the treatment efficiency decreases, and the BOD5 removal rate is often lower than 80%.
13. What is the sludge load? How to adjust?
a. Sludge load = the amount of BOD5 entering the aeration tank (flow rate × concentration) / the total amount of MLSS in the aeration tank (MLSS × pool volume).
b. Since the amount of BOD5 in the primary sedimentation tank effluent is determined by the quality of the incoming water, it is generally difficult to adjust. To adjust the sludge load and reduce MLSS, the sludge load will be increased. Increasing or reducing MLSS is generally achieved by increasing or decreasing sludge discharge.
The sludge load has a great influence on the treatment effect, sludge growth and oxygen demand, and must be carefully controlled. Generally speaking, the sludge load is between 0.2~0.5kg(BOD5)/(kg.d) and should be controlled around 0.3kg(BOD5)/kg(MLSS).d.
14. What is the volumetric load of the aeration tank?
The amount of BOD5 that the unit volume of the aeration tank bears every day is called the volumetric load kg (BOD5)/(m3.d). The volumetric load indicates the economics of constructing the aeration tank. The volumetric load has the following relationship with the concentration of the mixed liquid and the sludge load:
BV=x.B5, where (x is MLSS).
15. What is the meaning of sludge age?
Sludge age = the number of MLSS in the aeration tank (MLSS × pool volume) / the amount of solids in the remaining sludge (discharge volume × sludge concentration).
Sludge age is the ratio of the total amount of activated sludge working in the aeration tank to the remaining sludge discharged every day, and the unit is d. When the operation is stable, it can be understood as the average residence time of activated sludge in aeration.
Generally, the sludge age of the aeration tank system is about 5 to 6 days. When reaching the nitrification stage, the sludge age needs to be 8 to 12 days or higher.
There is an opposite relationship between sludge age and sludge load. Longer sludge age means lower load, and vice versa, but it does not form an absolute inverse proportional functional relationship.
16. Mixed liquor suspended solids concentration (MLSS)?
The concentration of suspended solids in the mixed liquid is the amount of suspended solids in the mixed liquid after mixing sewage and activated sludge in the aeration tank, unit (mg/L). It is an indicator for measuring the amount of activated sludge in the aeration tank. Because it is easy to measure, it is often used as an indicator to roughly measure the microbial biomass of activated sludge. In push flow aeration, the MLSS is generally 1000~4000mg/L. In the jointly constructed complete mixing aeration tank, the MLSS in air aeration rarely exceeds 8000mg/L. This is because the MLSS is too high. It hinders oxygenation and makes it difficult to settle in the secondary sedimentation tank.
17. What is the mixed liquid volatile suspended solids concentration (MLVSS)?
The concentration of volatile suspended solids in the mixed liquor refers to the weight of organic matter in the suspended solids in the mixed liquor (usually measured by the loss on ignition at 600°C). Therefore, some people think that it can represent the number of activated sludge microorganisms more accurately than MLSS. However, MLVSS also includes inactive organic matter that cannot be degraded, and it is not the most ideal indicator for measuring MLSS. For domestic sewage, it is usually around 0.75.
18. Sludge Index (SVI)?
The sludge index refers to the volume (in ml) occupied by the corresponding 1g of dry sludge after the mixed liquid in the aeration tank has settled for 30 minutes, that is:
SVI=Sludge deposition after 30 minutes of static settling of the mixed solution (ml)/sludge dry weight (g)
The SVI value can better reflect the looseness and coagulation settling performance of activated sludge. The SVI of good activated sludge is usually between 50 and 300. If the SVI is too high, the SVI value measured at the same concentration is valuable. In addition, because the size of the measurement container has a certain impact on the number of measurements, the measurement containers must be unified.