18 professional terms for sewage treatment

(Summary description)18 professional terms for sewage treatment

18 professional terms for sewage treatment

(Summary description)18 professional terms for sewage treatment


1. Sewage treatment - biological treatment method

◆ Biological treatment is to use microorganisms to absorb, decompose and oxidize organic matter in sewage, and degrade unstable organic matter into stable and harmless substances, so as to purify 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 in the treatment of urban sewage and organic industrial wastewater. Aerobic oxidation has a wide range of applications, including many techniques and structures. Biofilm method (including biological filtration 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 of farmland and ponds, ie irrigated fields and biological ponds. Biological treatment has low cost, so it is the most widely used sewage treatment method at present.

2. What is wastewater treatment volume or BOD5 removal total and treatment quality?

◆Sewage treatment capacity or total removal of BOD5: the daily total sewage flow (in m3/d) entering the sewage treatment plant can be used as an indicator of the treatment capacity of the sewage treatment plant. The total daily removal of BOD5 can also be used as an indicator of the treatment capacity of the sewage treatment plant. The total amount of BOD5 removed is equal to the product of the treatment flow rate and the difference between the BOD5 value of the incoming and outgoing water, in kg/d or t/d.

◆Treatment quality: The secondary sewage treatment plant takes the ex-factory BOD5 and SS values ​​as the treatment quality indicators. According to the newly formulated sewage treatment plant effluent discharge standard, the BOD5 and SS of the secondary sewage treatment plant effluent are all less than 30mg/L. Treatment quality can also be measured by removal rate. The influent concentration minus the effluent concentration divided by the influent concentration is the removal rate. Ammonia nitrogen, TP effluent value or removal rate are also used as treatment quality indicators.

3. What is pH value and its indication meaning?

◆pH indicates the acidity and alkalinity of sewage. It is the logarithmic value of the reciprocal of the hydrogen ion concentration in water. The larger it is, the stronger its alkalinity is. The pH value in sewage has a certain influence on pipelines, pumps, gate valves and sewage treatment structures. The pH value of sewage treatment plants mainly composed of domestic sewage is usually 7.2~7.8. Too high or too low pH value can indicate the entry of industrial wastewater. Values ​​that are too low can corrode piping, pump bodies and may be hazardous. For example, sulfide in sewage will generate H2S gas under acidic conditions. At high concentration, it can cause headache, runny nose, suffocation and even death. Therefore, it is necessary to strengthen monitoring, find pollution sources, and take countermeasures when pH is found to decrease. At the same time, the allowable range of pH for biochemical treatment is 6~10, too high or too low can affect or destroy biological treatment.

4. What is total solids (TS)?

◆ refers to the total solids remaining in the water sample evaporated to dryness on a water bath at a temperature of 100°C. It is the sum of dissolved and undissolved solids in sewage. It reflects the total concentration of solids in wastewater. The analysis of influent and effluent solids can reflect the effect of sewage treatment structures on the removal of total solids.

5. What is suspended solids (SS)?

◆ refers to the amount of solid matter in the sewage that can be retained by the filter. A portion of the suspended solids can settle out under certain conditions. Determination of suspended solids is usually carried out using asbestos filter layer filtration. The main equipment is Gooch crucible. When the 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 of sewage. Determination of suspended solids in influent 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.

6. What is chemical oxygen demand (COD)?

◆Chemical oxygen demand (COD for short) refers to the oxygen content of the oxidant required for chemical oxidation of organic matter in sewage. Using potassium permanganate as an oxidant, the measured result is customarily called oxygen consumption, expressed in OC. Using potassium dichromate as an oxidant, the measured result is called chemical oxygen demand and expressed in 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, its effect is stronger and more complete than the former. In addition to linear organic compounds, it can oxidize permanganic acid. Many structurally complex organic compounds that potassium cannot oxidize. Therefore, the COD value of the same sewage is much larger 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 potassium permanganate method is often used as the reference data to determine the dilution ratio of BOD in five days in sewage treatment plants.

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 oxidation and decomposition of organic matter usually has 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 matter, called the nitrification stage, which takes about 100 days to complete. Under the recognized circumstances, the general standard practice is to incubate at 20°C for 5 days, and then measure, and the measured data is called 5-day biochemical oxygen demand. BOD5 for short, so BOD5 represents the oxygen demand for the decomposition of some carbon-containing organic matter, and the BOD5 of domestic sewage should be about 70%.

◆The determination of the five-day BOD is to take the original water sample or the properly diluted water sample to make it contain enough dissolved oxygen to meet the requirements of the five-day BOD. This water sample is divided into two parts, one The dissolved oxygen content of the day was measured in one part, and the other part was placed in a 20 °C incubator, and the dissolved content was measured after culturing for 5 days.

◆ In the process of BOD5 determination, it is very important to choose the dilution factor correctly. It is generally believed that the selected dilution factor should be such that after the diluted water sample is incubated in a 20°C incubator for 5 days, its dissolved oxygen reduction is more appropriate when it is 20% to 80%. However, sometimes numerical errors are often caused by improper control of the dilution factor of BOD5, and 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 sewage pollution by organic matter. The more organic matter contained in the sewage, the more oxygen consumption will be, and the higher the BOD value will be, and vice versa. Therefore, it is one of the most important indicators of sewage water quality. Although it takes a long time to measure the BOD and the data is not timely, the BOD index is comprehensive - reflecting the total amount of organic matter comprehensively, and simulation - imitating 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. Reflect the concentration of organic matter in sewage. Such as the concentration of organic matter in incoming sewage, and the concentration of organic matter in outgoing sewage. The BOD5 of the influent water of the urban sewage treatment plant can generally reach 150~350mg/L.

b. It is used to represent the treatment effect of the sewage treatment plant. The reduction of BOD5 in the influent and effluent divided by the BOD5 in the influent is the BOD5 removal rate of the plant, which is an important indicator.

c. The total removal amount 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. It is used to calculate the operating parameters of the treatment structures, such as the sludge load BOD5kg (MISS) or the volume 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 per kg BOD (kWh) and removing the air volume required 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, biochemical treatment is difficult. When the ratio is between 0.5 and 0.6, the biochemical process is easy to carry out.

◆It can be seen that measuring BOD5 is very useful, and it is the most important measuring 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 oxygen required for the oxidation of organic matter in the sewage by the oxidant, 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, the urban sewage plant COD﹥BOD, if the type of organic matter in the sewage changes less, then the COD and BOD have a certain relationship, so the COD of the day can be used to predict the BOD5 value .

◆According to the operation data of various urban sewage 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 BOD5 in value.

◆If BOD5 and SS are found to increase exponentially in the wastewater entering the plant, 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. The processing efficiency is reduced, and the pipeline is even blocked. The cause must be traced and measures must be taken.

9. What is the indication meaning 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 takes carbon, hydrogen and oxygen as basic elements. The latter uses nitrogen, sulfur and phosphorus as the basic elements. In the process of aerobic decomposition, nitrogen-containing organic matter will eventually be transformed into inorganic substances such as ammonia nitrogen fertilizer, nitrite nitrogen fertilizer, nitrate nitrogen, water and carbon dioxide. Therefore, the determination of 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 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 flowing into the water body. Generally, the ammonia nitrogen value of the sewage entering the plant is about 30~70mg/L. The incoming water generally does not contain nitrite and nitrate. The secondary sewage treatment plant generally cannot remove a large amount of nitrogen fertilizer, and can convert part of the ammonia nitrogen into nitrate nitrogen when the treatment degree is high.

10. What is the meaning 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 should be maintained above 100:5:1 in activated sludge sewage treatment sewage. In urban sewage plants, this ratio can generally be achieved. Some industrial wastewater cannot reach this ratio, and 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 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 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, the air supply is adjusted according to its size, and the oxygen consumption in the aeration tank is known to judge 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. Too low dissolved oxygen value indicates lack of oxygen in the aeration tank. Too high dissolved oxygen not only wastes energy consumption, but may also cause sludge to become loose and broken. Ageing.

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

◆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 water.

12. What is the relationship between water temperature and operation?

◆Water temperature, water temperature has a great relationship with the work of aeration tank. The water temperature of a sewage plant varies gradually and slowly with the seasons, and hardly changes within a day. If you notice a lot of variation within a day, check to see if industrial cooling is coming in. In the range of 8~30°C throughout the year, when the aeration tank is operated below 8°C, the treatment efficiency decreases, and the BOD5 removal rate is often lower than 80%.

13. What is the sludge load? How to adjust it?

a. Sludge load = the number of BOD5 entering the aeration tank (flow × concentration) / the total amount of MLSS in the aeration tank (MLSS × pool volume).

b. Since the amount of BOD5 in the effluent of the primary sedimentation tank is determined by the water quality of the incoming plant, it is generally difficult to adjust. Adjusting the sludge load and reducing MLSS will increase the sludge load. Increasing or reducing MLSS is generally achieved by increasing or reducing sludge discharge.

◆The sludge load has a great influence on the treatment effect, sludge growth and oxygen demand, and must be mastered. Generally speaking, the sludge load is 0.2~0.5kg(BOD5)/(kg.d, and it is controlled at about 0.3kg(BOD5)/“kg(MLSS).d”.

14. Volume load of aeration tank?

◆The amount of BOD5 burdened by the unit volume of the aeration tank per day is called volume load kg (BOD5)/(m3.d). The volumetric load represents the economics of constructing the aeration tank. The relationship between volume load, mixed liquor concentration and sludge load is as follows:

BV=x.B5, where (x is MLSS).

15. What is the meaning of sludge age?

◆Sludge age = the amount of MLSS in the aeration tank (MLSS × pool volume) / the amount of solids in the remaining sludge (discharge amount × sludge concentration).

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

◆The sludge age of general aeration tank system is about 5~6d. When the nitrification stage is to be reached, the sludge age should be 8~12d or higher.

◆There is an opposite relationship between the sludge age and the sludge load. The longer the sludge age is, the lower the load is, and vice versa, but it is not an absolute inverse proportional function relationship.

16. Mixed liquid suspended solids concentration (MLSS)?

◆The concentration of suspended solids in the mixed solution is the amount of suspended solids in the mixed solution after the sewage and activated sludge in the aeration tank are mixed, in units (mg/L). It is an indicator for measuring the amount of activated sludge in the aeration tank. It is often used as a rough measure of the microbial biomass of activated sludge. In push flow aeration, the MLSS is generally 1000~4000mg/L. In the fully mixed aeration tank built together, the MLSS root of 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 concentration of volatile suspended solids in the mixture (MLVSS)?

◆The concentration of volatile suspended solids in the mixed solution refers to the weight of the organic matter in the suspended solids of the mixed solution (usually measured by the calcination loss at 600℃), so some people think that it can more accurately represent the number of microorganisms in activated sludge than MLSS. However, MLVSS also includes inactive non-degradable organic matter, and it is not the most ideal indicator for measuring MLSS. For domestic sewage, it is often around 0.75.

18. Sludge Index (SVI)?

◆Sludge index refers to the volume (in ml) occupied by the corresponding 1g of dry sludge after the aeration tank mixture settles for 30min, namely:

SVI=Sludge deposition after 30min static sedimentation of the mixed solution (ml)/sludge dry weight (g)

The SVI value can better reflect the loosening degree and coagulation and sedimentation performance of activated sludge. A good activated sludge SVI is often between 50 and 300, and the SVI is too high.

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