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18 Specialized Terms for Sewage Treatment

1。 Sewage Treatment-Biological Treatment

_Biological treatment is to use microorganisms to absorb, decompose and oxidize organic matter in sewage, to degrade unstable organic matter into stable harmless substances, so that sewage can be purified. Modern biological treatment methods can be divided into aerobic oxidation and anaerobic reduction according to different microorganisms. The former is widely used to treat municipal wastewater and organic industrial wastewater. Aerobic oxidation is widely used in many arts and structures. Biofilm process (including biofilter, biological turntable), biological contact oxidation and other processes and structures. Activated sludge process and biofilm process are both artificial biological treatment methods. In addition, there are natural biological treatments of farmland and ponds, i.e. irrigated fields and ponds. Biological treatment is the most widely used wastewater treatment method because of its low cost.

2。 What is the total amount and quality of wastewater treatment or BOD5 removal?

_Total sewage treatment capacity or BOD5 removal capacity: The total sewage flow (measured in m3/d) that enters the sewage treatment plant every day can be used as an indicator of the treatment capacity of the sewage treatment plant. The total amount of BOD5 removal per day can also be used as an indicator of wastewater treatment capacity. The total removal of BOD5 is equal to the product of the difference between the treatment flow and the BOD5 of the effluent, in kg/d or t/d.

_Treatment quality: BOD5 and SS values of the secondary sewage treatment plant are taken as treatment quality indicators. According to the newly formulated effluent discharge standards of sewage treatment plants, the effluent BOD5 and SS of secondary sewage treatment plants are less than 30 mg/L. The treatment quality can also be measured by the removal rate. The removal rate is that the influent concentration is reduced by the effluent concentration and the influent concentration is divided by the influent concentration. Ammonia nitrogen, TP effluent value or removal rate should also be used for treatment quality indicators.

3. What is pH value and its indication?

_pH indicates the acidity and alkalinity of sewage. It is the logarithmic value of reciprocal hydrogen ion concentration in water. Its range is 0-14, and the pH value is equal to 7. The water is neutral and acidic when the value is less than 7. The smaller the value, the stronger the acidity and alkalinity when the value is greater than 7. The greater the value, the stronger the alkalinity. The pH value of sewage has a certain influence on pipelines, pumps, gate valves and sewage treatment structures. The pH value of sewage treatment plants mainly consisting of domestic sewage is usually 7.2-7.8. Excessive or low pH value can indicate the entry of industrial wastewater. Too low a value will corrode the pipeline, pump body and may cause harm. For example, sulfides in sewage can produce H2S gas under acidic conditions. High concentration can cause headache, runny nose, asphyxia and even death. For this reason, it is found that the monitoring must be strengthened to find out the source of pollution and take measures to reduce the pH. At the same time, the permissible range of pH of biochemical treatment is 6-10, too high or too low can affect or destroy biological treatment.

4. What is Total Solid (TS)?

_refers to the total amount of solids left by evaporation of water samples to dry in a water bath at 100 C. It is the sum of soluble and insoluble solids in sewage. It can reflect the total concentration of solids in sewage. The analysis of solid in and out water can reflect the effect of sewage treatment structure on removal of total solid.

5. What is suspended solids (SS)?

_refers to the amount of solid matter in sewage that can be intercepted by filters. A part of suspended solids can be precipitated under certain conditions. Asbestos filter is usually used to determine suspended solids. The main equipment is Gu's crucible. When the laboratory equipment is not available, filter paper can also be used as a filter to obtain suspended solids from the difference between total solids and dissolved solids. In the determination of suspended solids, great differences often occur due to different filters.

_This index is one of the most basic data of sewage. The determination of suspended solids in incoming and outgoing water can be used to reflect the reduction of suspended solids after the treatment of sewage through primary and secondary sedimentation tanks. It is the main basis for reflecting the sedimentation efficiency.

6。 What is Chemical Oxygen Demand (COD)?

_Chemical oxygen demand (COD) refers to the amount of oxidant oxygen required by chemical oxidation of organic substances in sewage。 Using potassium permanganate as oxidant, the measured results are customarily called oxygen consumption, which is expressed by OC。 Using potassium dichromate as oxidant, the measured result is called chemical oxygen demand expressed by COD。 The difference between them lies in the choice of oxidant。 Potassium permanganate as oxidant can only oxidize straight-chain organic compounds in sewage, but potassium dichromate as oxidant has stronger and more complete effect than the former。 Besides straight-chain organic compounds, it can oxidize many complex organic compounds which potassium permanganate can not oxidize。 Therefore, COD value of the same sewage is much larger than OC value。 Especially when a large amount of industrial wastewater enters the wastewater treatment plant, the chemical oxygen demand of potassium dicarboxylate method should be generally measured。 The COD value of municipal wastewater treatment plants is generally about 400-800 mg/L。

_The consumption value of potassium permanganate method is often used as a reference data to determine the dilution multiple of biochemical oxygen demand for five days in sewage treatment plants.

7. 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 decomposition of organic matter usually has two stages。 The first stage is mainly the oxidation of carbon-containing organic matter, called carbonization stage, which takes about 20 days to complete。 The second stage is mainly the oxidation of nitrogen-containing organic matter, called nitrification stage, which takes about 100 days to complete。 Under accepted circumstances, the general standard practice is to incubate for 5 days at 20 C。 The measured data are called biochemical oxygen demand for 5 days。 BOD5 is abbreviated as BOD5, so BOD5 represents the oxygen demand of decomposition of some carbon-containing organic matter, and the BOD5 of domestic sewage should be about 70%。

_The determination of biochemical oxygen demand for five days is to take raw water samples or water samples diluted appropriately and make them contain enough dissolved oxygen to meet the requirements of biochemical oxygen demand for five days。 The water samples are divided into two parts。 One part is to measure the dissolved oxygen content of the day, and the other part is to be placed in the incubator at 20 C。 The dissolved oxygen content is determined after incubation for five days。 The difference between them multiplied by the dilution factor is BOD5。

In the determination of BOD5, it is very important to select the dilution factor correctly。 It is generally believed that the dilution factor should be chosen so that the dissolved oxygen of diluted water can be reduced by 20%~80% after 5 days'incubation in a constant temperature chamber at 20 C。 However, sometimes the inappropriate control of the dilution multiple of BOD5 results in numerical errors, and even the data of BOD5 can not be obtained because the dilution multiple is too small。

8。 Determining the use of BOD?

_BOD can reflect the degree of organic pollution in sewage. The more organic matter in sewage, the more oxygen consumption, the higher BOD value, and vice versa. Therefore, it is the most important indicator of sewage quality. Although it takes a long time to determine BOD and the data are not timely, the BOD indicators are comprehensive, reflecting the total amount of organic matter synthetically, simulating and imitating the self-purification of water body. Therefore, it is difficult to replace other indicators.

For sewage treatment plants, the purpose of this indicator is:

A. Reflect the concentration of organic matter in sewage. For example, the concentration of organic matter in the effluent from the factory and the concentration of organic matter in the effluent from the factory. The influent BOD5 of municipal sewage treatment plants can generally reach 150-350 mg/L.

B. Used to indicate the treatment effect of sewage treatment plant. The BOD5 removal rate of the plant is the difference between the inlet and outlet BOD5 and the influent BOD5, which is an important index.

C。 Total removal capacity and effluent BOD5 of wastewater treatment plants indicate the total treatment capacity of wastewater treatment plants and the impact on water environment。

D. Used to calculate operation parameters of treatment structures, such as sludge load BOD5kg (MISS) of aeration tank or volume load BOD5kg/(m3/d).

E。 Technical and economic data reflecting the operation of sewage treatment plants, such as the removal of power consumption per kg BOD and the removal of air demand per kg BOD 5。

F. Measuring the biodegradability of sewage, when BOD5/COD is greater than 0.3, it shows that sewage can be treated biochemically. When it is less than 0.3, it is difficult to biochemical treatment. When the ratio is between 0.5 and 0.6, the biochemical process can be easily carried out.

It can be seen that the determination of BOD5 is very useful, and it is the most important measurement item in sewage treatment plant。 However, it takes a long time to determine and the data can not be obtained in time。 COD test reflects the amount of oxygen required for the oxidation of organic matter by oxidant in sewage, and its data value is close to the total amount of oxygen demand of organic matter。 Therefore, it is also of great use, and the COD determination is short。 Generally, COD > BOD in municipal wastewater treatment plants。 If there are less changes in organic species in wastewater, COD and BOD have a certain relationship。 Therefore, BOD5 can be predicted by COD of the same day。

According to the operation data of municipal wastewater treatment plants, SS and BOD5 are generally similar or slightly higher in value。 For example, the SS of each sewage treatment plant in Shanghai is about 50 mg/L higher than that of BOD5 on average。

If BOD5 and SS are found to increase exponentially in the effluent, there may be a high concentration of organic wastewater flowing into the plant or a large amount of feces entering the plant. This will increase the processing load. In order to reduce the treatment efficiency and even block the pipeline, it is necessary to trace the causes and take measures.

9. Indicative Significance of Total Nitrogen, Ammonia Nitrogen, Nitrite Nitrogen, Nitrate Nitrogen (N, NH4+, NO2-NO-3)?

There are a lot of carbon-containing organic matter and nitrogen-containing organic matter in sewage. The former is based on carbon, hydrogen and oxygen. The latter is based on nitrogen, sulfur and phosphorus. In the process of aerobic decomposition, 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, the determination of these three indicators can reflect the process of sewage decomposition and the degree of inorganization after treatment. When only a small amount of nitrite nitrogen appears in the secondary sewage treatment plant, the treated effluent is not stable. When the oxygen content is insufficient, most of the organic nitrogen in the sewage is converted into inorganic substances, and the effluent is relatively stable after it enters the water body. In general, the ammonia nitrogen value of the sewage entering the factory is about 30-70 mg/L. Nitrite and nitrate are generally not contained in the water entering the plant. Secondary sewage treatment plants generally can not remove large amounts of nitrogen fertilizer. When the treatment degree is high, part of ammonia nitrogen can be converted into nitrate nitrogen.

10. Significance of phosphorus and nitrogen (P, N) indicators?

The content of phosphorus and potassium in sewage affects the growth of microorganisms。 The proportion of BOD5:N:P in activated sludge sewage treatment should be maintained at 100:5:1, which can be generally achieved in urban sewage treatment plants。 Some industrial wastewater can not reach this ratio, so nutrients must be added to the wastewater。

10。 Significance of phosphorus and nitrogen (P, N) indicators?

The content of phosphorus and potassium in sewage affects the growth of microorganisms. The proportion of BOD5:N:P in activated sludge sewage treatment should be maintained at 100:5:1, which can be generally achieved in urban sewage treatment plants. Some industrial wastewater can not reach this ratio, so nutrients must be added to the wastewater.

11. What is dissolved oxygen and what is the purpose of determination?

Dissolved oxygen refers to the amount of oxygen dissolved in water, which is closely related to temperature, pressure and biochemical action of microorganisms. At a certain temperature, only a certain amount of oxygen can be dissolved in water at most. For example, at 20 C, the saturation value of dissolved oxygen in distilled water is 9.17 mg/L.

Dissolution value in effluent and aeration tank is often measured in sewage treatment. Air supply is adjusted according to its size. Oxygen consumption in aeration tank is known to judge oxygen consumption rate of aeration tank under various water temperature conditions. In the operation process, dissolved oxygen in the aeration tank is required to be more than 1 mg/L. The low dissolved oxygen value indicates that the aeration tank is anoxic. Excessive dissolved oxygen not only wastes energy consumption, but also may cause sludge crushing and aging.

Dissolved oxygen in effluent of sewage treatment plant is beneficial to water environment. Under possible conditions, some dissolved oxygen should be allowed in effluent.

Dissolved oxygen is an important parameter in the process of water self-purification。 It can reflect the balance between oxygen consumption and dissolved oxygen in water。

12. The relationship between water temperature and operation?

Water temperature and water temperature have a great relationship with the work of aeration tank。 The water temperature of a sewage treatment plant changes slowly with the seasons, and hardly changes in a day。 If it is found that there is a great change in a day, it is necessary to check whether there is industrial cooling in。 The treatment efficiency of the aeration tank decreases when the water temperature is below 8 and the removal rate of BOD5 is usually less than 80% in the range of 8 30 during the whole year。

13。 What is the sludge load and how to regulate it?

Sludge load = the amount of BOD5 entering the aeration tank (flow x concentration)/the total amount of MLSS in the aeration tank (MLSS x pond volume).

B。 Because the quantity of BOD5 in the effluent of the primary clarifier depends on the quality of the water entering the plant, it is generally difficult to adjust the sludge load and reduce MLSS, then the sludge load will be increased, and the increase or decrease of MLSS will generally be achieved by increasing or reducing the discharge of sludge。

_Sludge load has a great influence on treatment effect, sludge growth and oxygen demand。 We must pay attention to it。 Generally speaking, the sludge load is 0。2-0。5 kg (BOD5)/(kg。d), which is about 0。3 kg (BOD5)/"kg (MLSS)。 d"。

14. Volume load of aeration tank?

_The daily BOD5 load per unit volume of aeration tank is called volume load kg (BOD5)/(m3.d). Volume load indicates the economy of building the aeration tank. Volume load is related to mixture concentration and sludge load as follows:

BV = X. B5, in formula (x is MLSS).

15。 Meaning of sludge age?

_Sludge age = MLSS amount in aeration tank (MLSS * pond volume) / solid amount in excess sludge (discharge * sludge concentration)。

_Sludge age is the ratio of the total activated sludge in the aeration tank to the excess sludge discharged every day. The unit of sludge age is d. When running smoothly, it can be understood as the average residence time of activated sludge in aeration.

_Sludge age of general aeration tank system is about 5-6 days. When the nitrification stage is to be reached, the sludge age should be 8-12 days or higher.

_Sludge age and sludge load have the opposite relationship。 Sludge age is long and sludge load is low, and vice versa, but it is not an absolute inverse proportional function。

16. Suspended Solid Concentration of Mixture (MLSS)?

_The suspended solid concentration of mixed liquor is the amount of suspended solid in the mixed liquor of sewage and activated sludge in aeration tank (mg/L). It is an index to measure the amount of activated sludge in aeration tank. Because of its simplicity, it is often used as a rough indicator to measure the microbial biomass of activated sludge. MLSS in propellant flow aeration is generally 1000-4000 mg/L. In the fully mixed aeration tank, there are fewer MLSS roots aerated by air than 8000 mg/L. This is because MLSS is too high. It hinders the aeration and makes it difficult to settle in the secondary sedimentation tank.

17. Volatile Suspended Solid Concentration (MLVSS) of Mixture?

Volatile suspended solids concentration of mixed liquor refers to the weight of organic matter in suspended solids of mixed liquor (usually measured by burning reduction at 600 C). Therefore, some people think that it can more accurately represent the number of microorganisms in activated sludge than MLSS. However, MLVSS also includes inactive and non-degradable organic matter, which is not the best indicator for measuring MLSS. For domestic sewage, it is usually around 0.75.

18。 Sludge Index (SVI)?

_Sludge index refers to the volume (in ml) of 1 g dry sludge after 30 minutes of static settling of the mixed liquor in the aeration tank.

SVI = sludge deposition (ml) / sludge dry weight (g) after 30 minutes of static settling of mixed liquor

SVI value can better reflect the loose degree of activated sludge and the coagulation sedimentation performance. Good activated sludge SVI usually ranges from 50 to 300. Excessive SVI sludge concentration is of value only when the SVI value is measured at the same concentration. In addition, because the size of the container has a certain impact on the quantity determined, it is necessary to unify the determination of the container.

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