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    Home > News

    Questions and Answers on 19 Special Terms in Wastewater Treatment

    Source: Author: Time: 2016-07-29 Views: Times

    Part 1: Sewage treatment-biological treatment method

    ◆ Biological treatment is the use of microorganisms to absorb, decompose, and oxidize organic matter in sewage, and degrade unstable organic matter into stable and harmless substances, so that sewage is purified. Modern biological treatment methods can be divided into aerobic oxidation and anaerobic reduction according to the different microorganisms. The former is widely used to treat urban sewage and organic industrial wastewater. Aerobic oxidation is widely used in many arts and crafts and structures. Biological membrane method (including biological filter, biological turntable), biological contact oxidation and other processes and structures. Activated sludge method and biofilm method are both artificial biological treatment methods. In addition, there are natural biological treatment methods for farmland and ponds, namely irrigated fields and biological ponds. The biological treatment cost is low, so it is currently the most widely used sewage treatment method.

    ◆ Activated sludge biological treatment method is usually physically treated before it. Therefore, activated sludge treatment belongs to the secondary treatment category. Sludge is produced after physical treatment and activated sludge treatment. The sludge in the secondary treatment sewage plant mainly includes primary sedimentation sludge and residual biological sludge. The general amount of sludge is about 5 ‰ ~ 7 ‰ (95% moisture content). Sludge is fertile, but contains bacteria and parasite eggs, and may also contain toxic heavy metals. In the utilization, it should be properly treated. The method used to treat sludge more often is that a large amount of digested gas (biogas) is generated during anaerobic digestion, which is a combustible useful gas. The moisture content of the digested sludge is still high and it is not easy to transport. Therefore, it is necessary to perform dehydration and drying.

    Part 2: What is the total amount of waste water treatment or BOD5 removal and treatment quality?

    ◆ Sewage treatment volume or total BOD5 removal: The total sewage flow (measured in m3 / d) that enters the sewage treatment plant daily can be used as an indicator of the sewage treatment plant's processing capacity. The total daily removal of BOD5 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 difference between the treated flow and the BOD5 of the inflow and outflow water, in kg / d or t / d.

    ◆ Processing quality

    The secondary wastewater treatment plant uses the factory BOD5 and SS values as the treatment quality index. According to the newly established effluent discharge standards for sewage treatment plants, the BOD5 and SS of the effluent from the secondary sewage treatment plant are less than 30mg / L. Treatment quality can also be measured by removal rate. Inlet water concentration minus outlet water concentration divided by inlet water concentration is the removal rate. Ammonia nitrogen, TP effluent value or removal rate should also be used for processing quality indicators.

    Part 3: What is the pH value and its significance:

    ◆ pH indicates the acid-base degree of sewage. It is the logarithmic value of the hydrogen ion concentration in water. Its range is 0 ~ 14, and the pH is equal to 7. The water is neutral and less than 7 is acidic. The larger the more alkaline it is. The pH value of sewage has a certain effect on pipelines, pumps, gate valves and sewage treatment structures. The pH value of sewage treatment plants, mainly domestic sewage, is usually 7.2 ~ 7.8. Too high or too low pH can indicate the entry of industrial wastewater. Too low values can corrode pipes and pump bodies and can be hazardous. For example, sulfide in sewage will generate H2S gas under acidic conditions. High concentrations can cause headaches, runny nose, suffocation and even death at work. To this end, it is necessary to strengthen the monitoring of pH reduction, find sources of pollution, and take countermeasures. At the same time, the allowable pH range of biochemical treatment is 6 ~ 10, too high or too low can affect or destroy biological treatment.

    Part 4: What is total solids (TS)?

    ◆ Refers to the total solids remaining after the water sample is evaporated to dryness in a water bath at 100 ℃. It is the sum of dissolved solids and insoluble solids in sewage. It reflects the total concentration of solids in the sewage. The analysis of solids in and out of water can reflect the effect of sewage treatment structures on the removal of total solids.

    Part 5: What is suspended solids (SS)?

    ◆ Refers to the amount of solid matter in the sewage that can be trapped by the filter. Part of the suspended solids can be precipitated under certain conditions. Determination of suspended solids is usually performed by asbestos filtration. The main equipment is the ancient crucible. When the conditions of the testing equipment are not available, filter paper can also be used as a filter to determine the amount of suspended solids 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. The determination of suspended solids in the incoming water and the factory water can be used to reflect the decrease 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.

    Part 6: What is chemical oxygen demand (COD):

    ◆ Chemical oxygen demand (referred to as COD) refers to the oxygen amount of oxidant required to chemically oxidize organic matter in sewage. Using potassium permanganate as the oxidant, the measured result is conventionally called oxygen consumption, which is expressed by OC. Using potassium dichromate as an oxidant, the measured result is called chemical oxygen demand expressed by 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, and using potassium dichromate as an oxidant has a stronger and more complete effect than the former. Except for linear organic compounds, it can oxidize permanganic acid. Potassium cannot oxidize many complex organic compounds. 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 COD of the potassium complexate 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 reference data for determining the five-day biochemical oxygen demand dilution factor in sewage plants.

    Part 7: What is BOD:

    ◆ Biochemical oxygen demand: (referred to as 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 pollution of organic matter. There are usually two stages of biochemical oxidation and decomposition of organic matter. 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. In a generally accepted case, the general standard practice is to perform a measurement at 20 C for 5 days, and the measured data is called 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%.

    ◆ The determination of the five-day biochemical oxygen demand is to take the original water sample or the appropriately diluted water sample to make it contain enough dissolved oxygen to meet the five-day biochemical oxygen demand. This water sample is divided into two, one One part measures the dissolved oxygen content of the day, and the other part is placed in a 20 C incubator. After 5 days of incubation, the dissolved content is measured. The difference between the two is multiplied by the dilution factor to obtain BOD5.

    ◆ During the determination of BOD5, it is important to choose the correct dilution factor. Generally, it is considered that the dilution factor should be selected so that the diluted water sample is more suitable when the dissolved oxygen is reduced by 20% to 80% after being cultured in a 20 C incubator for 5 days. However, sometimes the value of BOD5 is incorrect due to improper grasp of the dilution factor of BOD5, and even the dilution factor is too small to obtain BOD5 data.

    Part 8: 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 consumption, the higher the BOD value, and vice versa. Therefore, it is one of the most important water quality indicators. Although the determination of BOD takes a long time and the data is not timely, the BOD indicator is comprehensive-comprehensively reflects the total amount of organic matter, and simulation-mimics the self-purification of water bodies. It is therefore difficult to substitute other indicators.

    For sewage treatment plants, the indicator uses are:

    a. Reflect the concentration of organic matter in sewage. Such as the concentration of organic matter in the sewage entering the factory, and the concentration of organic matter in the sewage leaving the factory. The BOD5 of urban sewage treatment plant can reach 150 ~ 350mg / L.

    b. Used to indicate the treatment effect of the sewage treatment plant. The difference between the inlet and outlet water BOD5 divided by the inlet water BOD5 is the BOD5 removal rate of the plant, which is an important indicator.

    c. The total removal capacity and effluent BOD5 of the sewage treatment plant indicate the total treatment capacity in the sewage treatment plant and the amount of impact on the water environment.

    d. Used to calculate the operating parameters of the structure, such as the sludge load of the aeration tank BOD5kg (MISS) • d or volume load BOD5kg / (m3 • d)

    e. Reflect the technical and economic data of the operation of the sewage treatment plant, such as removing the power consumption (degrees) per kg of BOD, and removing the amount of air required per kg of BOD5.

    f. Measure the biodegradability 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 becomes difficult. When the ratio is 0.5 ~ 0.6, the biochemical process is easy to carry out.

    It can be seen that the determination of BOD5 is very useful. It is the most important measurement item of a sewage treatment plant. However, the measurement takes a long time, and the data cannot be produced in time. The COD test reflects the amount of oxygen required to oxidize the organic matter in the wastewater by the oxidant. 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, urban sewage plants have COD ﹥ BOD. If there is less change in the type of organic matter in the sewage, then there is a certain correlation between COD and BOD. .

    ◆ According to the operating data of each city's sewage treatment plant, SS and BOD5 are generally similar in value or slightly higher. For example, the SS of each sewage plant in Shanghai is about 50mg / L higher than BOD5 on average.

    ◆ If BOD5 and SS are found to increase exponentially in the sewage entering the plant, there may be a high concentration of organic wastewater flowing into the plant or a large amount of manure entering the plant. This will increase the processing load. Reduce the processing efficiency, and even block the pipeline, you must track down the cause and take measures.

    Part 9: Total nitrogen, ammonia nitrogen, nitrite nitrogen, nitrate nitrogen (N, NH4 +, NO2-NO-3) indicating meaning:

    ◆ There are a large number of carbon-containing organic matter and nitrogen-containing organic matter in the sewage, the former taking carbon, hydrogen and oxygen as basic elements. The latter is based on nitrogen, sulfur, and phosphorus. During the aerobic decomposition of nitrogen-containing organic matter, it will eventually be converted into inorganic matter such as ammonia nitrogen fertilizer, nitrite nitrogen fertilizer, nitrate nitrogen, water and carbon dioxide. Therefore, the measurement of the above three indicators can reflect the sewage decomposition process and the degree of inorganicization after treatment. When only a small amount of nitrogen nitrite appears in the secondary sewage treatment plant, the treated effluent is not yet stable. When the amount of oxygen is insufficient, most of the organic nitrogen in the sewage is converted into inorganic matter. After the effluent flows into the water body, it is relatively stable. 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. When the treatment degree is high, it can convert part of the ammonia nitrogen into nitrate nitrogen.

    Part 10: Significance of phosphorus and nitrogen (P, N) indicators:

    ◆ The content of phosphorus and potassium in sewage affects the growth of microorganisms. The activated sludge treatment of sewage should maintain a BOD5: N: P ratio of 100: 5: 1 or more. In urban sewage plants, this ratio can generally be achieved. Some industrial wastewater does not reach this ratio, and it is necessary to add nutrients to the wastewater.

    Part 11: What is dissolved oxygen and what is the purpose of the measurement?

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

    ◆ In sewage treatment, the dissolved values in water and aeration tank are often measured, the air supply is adjusted according to its size, and 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 must be above 1 mg / L. Too low dissolved oxygen value indicates lack of oxygen in the aeration tank. Excessively high dissolved oxygen not only wastes energy consumption, but also may cause sludge loosening, Ageing.

    ◆ Dissolved oxygen in the effluent of the sewage treatment plant is beneficial to the environment of the water body. When possible, the effluent zone should be allowed to have some dissolved oxygen.

    ◆ Dissolved oxygen is an important parameter in the process of self-purification of water, it can reflect the equilibrium relationship between oxygen consumption and dissolved oxygen in water.

    Part 12: The relationship between water temperature and operation:

    ◆ Water temperature has a lot to do with the aeration tank. The water temperature of a sewage treatment plant gradually changes slowly with the season, and there is almost no change in the day. If it is found that the change is large within a day, check it to see if industrial cooling has entered. 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 less than 80%.

    Part 13: What is the sludge load? How to adjust it?

    a. Sludge load = BOD5 quantity (flow concentration) entering the aeration tank / total MLSS in the aeration tank (MLSS pool product)

    b. Because the amount of BOD5 in the effluent of the primary sink is determined by the water quality of the plant, it is generally difficult to adjust. To adjust the sludge load and reduce MLSS, increase the sludge load and increase or decrease MLSS by increasing or decreasing sludge.

    ◆ The sludge load has a great influence on the treatment effect, sludge growth and oxygen demand, so we must pay attention to it. Generally, the sludge load is 0.2 ~ 0.5kg (BOD5) / (kg.d), and it is controlled at about 0.3kg (BOD5) / "kg (MLSS) .d".

    Part 14: Volumetric load of aeration tank:

    ◆ The BOD5 load per unit volume of the aeration tank is called volume load kg (BOD5) / (m3.d). The volume load indicates the economics of constructing the aeration tank. The volume load has the following relationship with the concentration of the mixed liquid and the sludge load:

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

    Part 15: Meaning of sludge age:

    ◆ Sludge sludge age = number of MLSS in the aeration tank (MLSS pool volume) / solid content in the remaining sludge (discharge amount drainage concentration)

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

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

    ◆ The sludge age and sludge load have the opposite relationship. The sludge age is long and the load is low, and vice versa, but it does not form an absolute inverse proportional relationship.

    Part 16: Suspended Solids Concentration (MLSS):

    ◆ The concentration of suspended solids in the mixed liquid is the amount of suspended solids in the mixed liquid after the sewage and activated sludge are mixed in the aeration tank, and the unit (mg / L) is an indicator for measuring the amount of activated sludge in the aerated tank. It is often used as an indicator to roughly measure the microbial biomass of activated sludge. The MLSS is generally 1000 ~ 4000mg / L in the push flow aeration. In the complete mixed aeration tank constructed, the MLSS root of the air aeration rarely exceeds 8000mg / L. This is because MLSS is too high. It prevents oxygenation and makes it difficult to settle in the secondary sink.

    Part 17: Mixed liquid volatile suspended solids concentration (MLVSS):

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

    Part 18: Sludge Index (SVI):

    ◆ The sludge index refers to the volume (in ml) occupied by the corresponding 1g of dry sludge after 30 minutes of static aeration in the aeration tank.

    SVI = sludge sediment (ml) / sludge dry weight (g)

    The SVI value can well reflect the looseness of the activated sludge and the cohesive sedimentation performance. Good activated sludge SVI is usually between 50 and 300. SVI concentration is too high, the SVI value measured at the same concentration is only valuable. In addition, because the size of the measurement container has a certain effect on the number of measurements, the measurement containers must be unified.

    Part 19: Method for measuring sludge volume index (SVI):

    a. Take samples for SS measurement.

    b. Take 100ml sample, put it into a 100ml graduated cylinder washed with tap water, leave it to stand for 30min, and take the volume (ml)

    Calculation: SVI (mg / L) = SVn 1000 / MLSS 100

    Among them: SVn?——100ml mixed solution, the volume occupied by the sludge layer after settling for 30min (ml)

    MLSS——Concentration of suspended solids in mixed solution (g / L)


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