1. What is biological wastewater treatment?

◆Biological treatment utilizes microorganisms to adsorb, decompose, and oxidize organic matter in wastewater, degrading unstable organic substances into stable, harmless substances, thereby purifying the wastewater. Modern biological treatment methods can be divided into two major categories according to the microorganisms involved: aerobic oxidation and anaerobic reduction. The former is widely used in the treatment of municipal wastewater and organic industrial wastewater. Aerobic oxidation is widely used and includes many types of processes and structures.

Biofilm methods (including biofilters, bio-rotating discs), biological contact oxidation, and other processes and structures. Activated sludge and biofilm methods are both artificial biological treatment methods. In addition, there are natural biological treatment methods such as farmland and ponds, namely irrigation fields and biological ponds. Biological treatment is inexpensive and therefore the most widely used wastewater treatment method currently.

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

◆Wastewater treatment capacity or total BOD5 removal: The total wastewater flow entering the wastewater treatment plant for treatment daily (in m3/d), which can serve as an indicator of the wastewater treatment plant's capacity. The total daily BOD5 removal can also serve as an indicator of the wastewater treatment plant's capacity. The total BOD5 removal is equal to the product of the treatment flow and the difference between the influent and effluent BOD5, in units of kg/d or t/d.

◆Treatment quality: Secondary wastewater treatment plants use the BOD5 and SS values of the effluent as treatment quality indicators. According to the newly formulated wastewater treatment plant effluent discharge standards, the effluent BOD5 and SS of secondary wastewater treatment plants are both less than 30 mg/L. Treatment quality can also be measured by removal rate. The removal rate is calculated by subtracting the effluent concentration from the influent concentration and dividing by the influent concentration. Ammonia nitrogen and TP effluent values or removal rates are also used as treatment quality indicators.

3. What is pH value and its significance?

◆pH indicates the acidity or alkalinity of wastewater. It is the negative logarithm of the hydrogen ion concentration in water, ranging from 0 to 14. A pH value of 7 indicates neutral water, less than 7 indicates acidity (the smaller the value, the stronger the acidity), and greater than 7 indicates alkalinity (the larger the value, the stronger the alkalinity). The pH value of wastewater has a certain impact on pipelines, pumps, valves, and wastewater treatment structures. The pH value of wastewater treatment plants mainly treating domestic wastewater is usually 7.2~7.8. Too high or too low pH values may indicate the presence of industrial wastewater. Too low a value can corrode pipes and pumps and may be hazardous. For example, sulfides in wastewater can form H2S gas under acidic conditions. High concentrations can cause headaches, runny noses, suffocation, and even death in operators. Therefore, when a decrease in pH is detected, monitoring must be strengthened, pollution sources must be identified, and countermeasures must be taken. At the same time, the allowable pH range for biological treatment is 6-10; values that are too high or too low can affect or damage biological treatment.

4. What is Total Solids (TS)?

◆Refers to the total amount of solids remaining after evaporation to dryness of a water sample at 100℃ in a water bath. It is the sum of dissolved and undissolved solids in wastewater. It reflects the total concentration of solids in wastewater. Analysis of the solids in the influent and effluent can reflect the effect of wastewater treatment structures on the removal of total solids.

5. What are suspended solids (SS)?

◆Refers to the amount of solid matter in wastewater that can be retained by a filter. Some suspended solids can settle under certain conditions. The determination of suspended solids is usually carried out using the asbestos filter layer filtration method. The main equipment is a Gooch 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. Due to the different filters used, there are often large differences in the determination of suspended solids.

◆This indicator is one of the most basic data for wastewater. Determining the suspended solids in the influent and effluent water can reflect the reduction of suspended solids after the wastewater passes through the primary sedimentation tank and secondary sedimentation tank, and it is the main basis for reflecting the sedimentation efficiency of the structure.

6. What is Chemical Oxygen Demand (COD)?

◆Chemical Oxygen Demand (COD) refers to the amount of oxygen required by the oxidant to chemically oxidize organic matter in wastewater. When potassium permanganate is used as the oxidant, the result is customarily called oxygen consumption, represented by OC. When potassium dichromate is used as the oxidant, the result is called chemical oxygen demand, represented by COD. The difference lies in the choice of oxidant. Using potassium permanganate as an oxidant can only oxidize straight-chain organic compounds in wastewater, while potassium dichromate is stronger and more complete in its action; in addition to straight-chain organic compounds, it can oxidize many structurally complex organic compounds that cannot be oxidized by potassium permanganate. Therefore, the COD value is much larger than the OC value for the same wastewater. In particular, when a large amount of industrial wastewater enters the wastewater treatment plant, the chemical oxygen demand of the potassium dichromate method should generally be measured. The COD value of municipal wastewater treatment plants is generally about 400-800 mg/L.

◆The oxygen consumption value of the potassium permanganate method is often used in wastewater treatment plants as a reference data for determining the dilution ratio of five-day biochemical oxygen demand.

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 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 generally accepted conditions, the standard procedure is to measure it after culturing at 20℃ for 5 days, and the measured data is called the five-day biochemical oxygen demand, abbreviated as BOD5. Therefore, BOD5 represents the oxygen demand for the decomposition of some carbon-containing organic matter, and the BOD5 of domestic sewage should be about 70%.

◆To determine the five-day biochemical oxygen demand, a raw water sample or a properly diluted water sample is taken, ensuring it contains sufficient dissolved oxygen to meet the requirements of the five-day biochemical oxygen demand. This water sample is divided into two portions. The dissolved oxygen content of one portion is measured on the day, while the other portion is placed in a 20℃ incubator and cultured for 5 days before its dissolved oxygen content is measured. The difference between the two multiplied by the dilution factor is the BOD5.

◆In BOD5 determination, the correct dilution ratio is crucial. It is generally believed that the selected dilution ratio should allow the dissolved oxygen of the diluted water sample to decrease by 20%~80% after being cultured in a 20℃ incubator for 5 days. However, sometimes the improper control of the BOD5 dilution ratio may cause numerical errors, or even if the dilution ratio is too small, no BOD5 data can be obtained.

8. What is the purpose of determining BOD?

◆BOD can reflect the degree of organic pollution in wastewater. The more organic matter in the wastewater, the more oxygen is consumed, and the higher the BOD value, and vice versa. Therefore, it is the most important indicator of wastewater water quality. Although the determination of BOD takes a long time and the data is not timely, the BOD indicator has comprehensiveness—it comprehensively reflects the total amount of organic matter, and simulation—it simulates water self-purification. Therefore, it is difficult to replace it with other indicators.

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

a. Reflect the concentration of organic matter in wastewater. Such as the concentration of organic matter in influent wastewater and effluent wastewater. The influent BOD5 of urban wastewater treatment plants generally reaches 150~350mg/L.

b. Used to indicate the treatment effect of the wastewater treatment plant. The difference between the influent and effluent BOD5 divided by the influent BOD5 is the BOD5 removal rate of the plant, which is an important indicator.

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

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

e. Reflect the technical and economic data of the operation of the wastewater treatment plant, such as the electricity consumption (degrees) per kg of BOD removed and the amount of air required to remove per kg of BOD5.

f. Measure the biodegradability of wastewater. When BOD5/COD is greater than 0.3, it indicates that the wastewater can be biochemically treated. When it is less than 0.3, it is difficult to biochemically treat. When the ratio is between 0.5 and 0.6, the biochemical process is easy to carry out.

◆It can be seen that the use of BOD5 determination is very large, and it is the most important determination item in wastewater treatment plants. However, the determination takes a long time and cannot provide data in a timely manner. The COD test reflects the amount of oxygen required for the oxidation of organic matter in wastewater by an oxidizing agent, and its data value is close to the oxygen demand of all organic matter. Therefore, it is also very useful, and the COD determination is short. In general urban wastewater plants, COD﹥BOD. If the types of organic matter in wastewater change less, then COD and BOD have a certain mutual relationship, so the BOD5 value can be predicted using the COD of the day.

◆According to the operation data of wastewater treatment plants in various cities, SS and BOD5 are generally similar in value or slightly higher. For example, the SS of wastewater treatment plants in Shanghai is on average about 50mg/L higher than BOD5 in value.

◆If it is found that BOD5 and SS increase exponentially in the influent wastewater, there may be a high concentration of organic wastewater inflow or a large amount of feces entering the plant. This will increase the treatment load, reduce the treatment efficiency, and may even block the pipeline. The cause must be investigated and measures must be taken.

9. Significance of total nitrogen, ammonia nitrogen, nitrite nitrogen, and nitrate nitrogen (N, NH4+, NO2−N O−3)?

◆There are a large number of carbon-containing organic matter and nitrogen-containing organic matter in wastewater. The former is based on carbon, hydrogen, and oxygen as basic elements. The latter is based on nitrogen, sulfur, and phosphorus as basic elements. In the aerobic decomposition process, nitrogen-containing organic matter will eventually be converted into inorganic substances such as ammonia nitrogen fertilizer, nitrite nitrogen fertilizer, nitrate nitrogen, water, and carbon dioxide. Therefore, determining the above three indicators can reflect the degree of wastewater decomposition and mineralization after treatment. When only a small amount of nitrite nitrogen appears in the secondary wastewater treatment plant, the treated effluent is not yet stable. When the oxygen is insufficient, most of the organic nitrogen in the wastewater is converted into inorganic matter, and the effluent is relatively stable after entering the water body. The ammonia nitrogen value of influent wastewater is generally about 30~70mg/L. The influent water generally does not contain nitrite and nitrate. Secondary wastewater treatment plants generally cannot remove a large amount of nitrogen fertilizer. When the treatment level is high, part of the ammonia nitrogen can be converted into nitrate nitrogen.

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

◆The content of phosphorus and potassium in wastewater affects the growth of microorganisms. The activated sludge wastewater treatment needs to maintain the ratio of BOD5:N:P above 100:5:1. In urban wastewater treatment plants, this ratio is generally achieved. Some industrial wastewater does not reach this ratio, so nutrients must be added to the wastewater.

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

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

◆In wastewater treatment, the dissolved oxygen value in the effluent and aeration tank is often measured. According to its size, the air supply is adjusted, and the oxygen consumption in the aeration tank is understood to judge the oxygen consumption rate in 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 indicates that the aeration tank is hypoxic. Too high dissolved oxygen not only wastes energy but may also cause sludge to loosen and age.

◆The dissolved oxygen in the effluent of the wastewater treatment plant is beneficial to the water environment. Under possible conditions, the effluent should contain 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. The relationship between water temperature and operation?

◆Water temperature has a great relationship with the operation of the aeration tank. The water temperature of a wastewater treatment plant changes slowly with the seasons, and there is almost no change within a day. If a large change is found within a day, it should be checked to see if industrial cooling water has entered. Within the range of 8~30℃ throughout the year, when the aeration tank operates at a water temperature below 8℃, the treatment efficiency decreases, and the BOD5 removal rate is often below 80%.

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

a. Sludge loading = BOD5 entering the aeration tank (flow × concentration) / Total MLSS in the aeration tank (MLSS × tank volume).

b. Since the amount of BOD5 in the effluent from the primary sedimentation tank is determined by the influent water quality and is generally difficult to adjust, adjusting the sludge loading and reducing MLSS increases the sludge loading. Increasing or decreasing MLSS is generally achieved by increasing or decreasing sludge discharge.

◆ Sludge loading has a great impact on treatment efficiency, sludge growth, and oxygen demand, and must be carefully controlled. Generally, the sludge loading is 0.2~0.5kg(BOD5)/(kg.d, and is controlled at around 0.3kg(BOD5)/kg(MLSS).d.

14. Aeration tank volume loading?

◆ The amount of BOD5 that an aeration tank bears per unit volume per day is called the volume loading kg(BOD5)/(m3.d). The volume loading indicates the economic efficiency of building the aeration tank. The volume loading, mixed liquor concentration, and sludge loading have the following relationship: BV = x.B5, where (x is MLSS).

15. Meaning of sludge age?

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

◆ Sludge age is the ratio of the total amount of activated sludge working in the aeration tank to the amount of waste sludge discharged daily, with units of d. During stable operation, it can be understood as the average residence time of activated sludge in aeration.

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

◆ Sludge age and sludge loading have an inverse relationship. A long sludge age means a low load, and vice versa, but it is not an absolute inverse proportional function relationship.

16. Mixed liquor suspended solids concentration (MLSS)?

◆ The mixed liquor suspended solids concentration is the amount of suspended solids in the mixture of wastewater and activated sludge in the aeration tank, with units of (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 a rough indicator for measuring the amount of activated sludge microorganisms. In push-flow aeration, MLSS is generally 1000~4000mg/L. In a fully mixed aeration tank, the MLSS of air aeration rarely exceeds 8000mg/L. This is because excessive MLSS hinders oxygenation and makes it difficult to settle in the secondary sedimentation tank.

17. Mixed liquor volatile suspended solids concentration (MLVSS)?

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

18. Sludge volume index (SVI)?

◆ The sludge volume index refers to the volume (in ml) occupied by 1g of dry sludge after 30min of sedimentation of the mixed liquor in the aeration tank, that is:

SVI = Sludge sedimentation volume after 30min sedimentation (ml) / Dry sludge weight (g)

The SVI value can better reflect the looseness and coagulation sedimentation performance of activated sludge. The SVI of good activated sludge is often between 50 and 300. The SVI value measured under the same concentration is meaningful only for sludge with high SVI. In addition, since the size of the measuring container has a certain impact on the measured quantity, the measuring container must be unified.