Physical property indicators of sewage

1. Temperature

It has a direct impact on the physical, chemical and biological properties of sewage and sludge. In the aeration tank of the activated sludge system, the treatment mainly relies on a large number of active microorganisms (bacterial flocs). The temperature they are more suitable for is generally around 20 to 30℃. Therefore, if a better organic matter treatment effect is to be ensured, the temperature should be controlled as much as possible at around 20 to 30℃.

Temperature monitoring is carried out on-site. Common methods include the water temperature meter method, deep water temperature meter method, inverted thermometer method and thermal sensitive thermometer method.

2. Chroma

The wastewater from urban sewage treatment plants is different from that from industrial wastewater. Its color intensity is not very obvious, but this does not mean that the monitoring of color intensity is unimportant. In fact, by observing the color of the sewage entering the sewage treatment plant, one can judge the freshness of the sewage. Generally, fresh urban sewage is gray. However, if it undergoes anaerobic spoilage during pipeline transportation with very little DO, the sewage will turn black and have a foul smell. In addition, in China, due to the drainage system that usually combines industrial wastewater with domestic sewage for discharge, the color intensity of urban sewage treatment plants sometimes varies significantly. Color intensity gives people an unpleasant feeling. In China's discharge standards for sewage treatment plants, there are discharge requirements for color intensity. Therefore, when the color intensity of the influent is relatively high, it should be given due attention in the monitoring indicators of the effluent.

3. Stench

The foul smell in water mainly comes from the decomposition of organic matter, which can also cause discomfort to people and even affect human physiology, such as breathing difficulties and vomiting. Therefore, odor is a relatively important physical indicator. However, at present, sewage treatment plants do not conduct dedicated monitoring of odor.

2. Chemical (including biochemical) property indicators of sewage

The chemical indicators of sewage quality include suspended solids, pH, alkalinity, heavy metal ions, sulfides, biochemical oxygen demand, chemical oxygen demand, total oxygen demand, total organic carbon, organic nitrogen, dissolved oxygen, etc.

Chemical Oxygen Demand (COD

Chemical Oxygen Demand (COD) is the amount of oxidant consumed when treating water samples under certain conditions with a certain strong oxidant. It is an indicator of the amount of reductive substances in water. The reducing substances in water include various organic compounds, nitrites, sulfides, ferrous salts, etc. But the main ones are organic substances. Therefore, chemical oxygen demand (COD) is often used as an indicator to measure the amount of organic matter in water. The greater the chemical oxygen demand, the more severely the water body is polluted by organic matter.

The determination of COD is a major daily monitoring item in sewage treatment plants. By measuring the COD of the influent and effluent of different structures, the operation status of the structures can be accurately grasped. Through the analysis of data over a certain period of time, the operation of the structures can be appropriately adjusted to ensure the treatment effect of sewage. In addition, for the effluent from sewage treatment plants, COD is a mandatory monitoring item, and the effluent should meet the corresponding national standards.

The determination of chemical oxygen Demand (COD) varies with the different reductive substances in the water sample and the determination method. At present, the most commonly used methods are the acidic potassium permanganate oxidation method and the potassium dichromate oxidation method. Potassium permanganate (KmnO4) has a relatively low oxidation rate but is relatively simple. It can be used when determining the relative comparative value of organic matter content in water samples. The potassium dichromate (K2CrO7) method features a high oxidation rate and good reproducibility, and is suitable for determining the total amount of organic matter in water samples.

2. Biochemical Oxygen Demand (BOD

Biochemical oxygen demand (BOD) refers to the amount of oxygen consumed when organic substances in water that can be decomposed are completely oxidized and decomposed under aerobic conditions due to the action of microorganisms. It is expressed as the reduction in dissolved oxygen (mg/L) of a water sample after being stored in a sealed container at a certain temperature (such as 20℃) for a certain period of time. When the temperature is 20℃, most organic substances can basically complete the oxidation and decomposition process in about 20 days, while it takes 100 days to fully complete this decomposition process. However, such a long period of time is lost for actual production control. Practical value. Therefore, it is currently stipulated that culturing for 5 days at 20℃ is the standard for determining biochemical oxygen demand. The biochemical oxygen demand measured at this time is called the five-day biochemical oxygen demand, denoted as BOD5. If the quantity and composition of organic matter in sewage are relatively stable, there may be a certain proportional relationship between the two, which can be calculated and determined mutually. The ratio of BOD to COD in domestic sewage is approximately 0.4 to 0.8. For a certain amount of wastewater, generally speaking, COD>BOD20>BOD5.

BOD5 is also one of the important daily monitoring items in sewage treatment plants. The specific significance of BOD5 monitoring is basically the same as that of COD.

However, due to the drainage system of rivers in our country, the wastewater from urban sewage treatment plants contains a certain amount of industrial wastewater. Compared with domestic sewage, the quality of industrial wastewater varies greatly and is more difficult to degrade. By monitoring the BOD and COD in the influent of sewage treatment plants, the biodegradability of the wastewater can be roughly determined.
The classic method for determining biochemical oxygen demand is the dilution inoculation method.

3. Dissolved oxygen (DO)

Molecular oxygen dissolved in water is called dissolved oxygen. The dissolved oxygen content in natural water depends on the balance of oxygen between the water body and the atmosphere. The saturated content of dissolved oxygen is closely related to the partial pressure of oxygen in the air, atmospheric pressure and water temperature. The solubility of clean surface water is generally close to saturation. Due to the growth of algae, dissolved oxygen may become supersaturated. When water bodies are polluted by organic or inorganic reducing substances, dissolved oxygen decreases. When the oxygen in the atmosphere cannot be replenished in time, the dissolved oxygen in water gradually decreases until it approaches zero. At this point, anaerobic bacteria multiply, water quality deteriorates, and fish and shrimp die.

The content of dissolved oxygen in wastewater depends on the treatment process before the sewage is discharged. Generally, the content is relatively low and varies greatly. Most fish deaths are caused by the large amount of sewage being absorbed, which increases the oxygen-consuming substances in the water body and leads to a very low dissolved oxygen level, resulting in the suffocation and death of fish. Therefore, dissolved oxygen is one of the important indicators for evaluating water quality.
Throughout the entire operation of the sewage treatment plant, great importance is attached to the determination of dissolved oxygen in water.

Both at home and abroad, the main approach to urban sewage treatment is the biological secondary treatment system, which is mostly aerobic. As the name suggests, it is to utilize the metabolic process of aerobic microorganisms to decompose and remove organic matter in water. From this, it can also be seen that the control of DO oxygen is very important. First of all, it is necessary to ensure that there is sufficient dissolved oxygen in the water so that aerobic microorganisms can work normally. This is a prerequisite for achieving better operational results. However, if too much oxygen is supplied, it will cause waste and lead to an increase in operating costs. Therefore, the DO in the aeration tank is generally controlled between 2 and 4mg/L.

When the dissolved oxygen is insufficient due to equipment problems or other reasons, the treatment system will malfunction. For instance, insufficient DO in the aeration tank often leads to the bulking of filamentous bacteria in the activated sludge. The reason is that bacteria and filamentous bacteria compete for the insufficient DO. However, under the condition of insufficient DO, the competitiveness of filamentous bacteria is far greater than that of bacteria. Therefore, bacteria obtain less DO, and their growth is inhibited. On the contrary, filamentous bacteria have the opportunity to multiply in large numbers. The ultimate result is the expansion of filamentous bacteria.

In certain nitrogen and phosphorus removal processes such as A/O and A2/O, the control of DO is also very important. To achieve the desired removal rates of N and P, it is necessary to ensure an appropriate DO value.

It can be seen that in the daily operation monitoring of sewage treatment plants, the monitoring of DO is of great significance. The methods adopted for general singing include iodometric method and its correction method, membrane electrode method and on-site rapid dissolved oxygen meter method.

4. Total Oxygen Demand (TOD

Total Oxygen Demand (TOD) Organic substances contain elements such as C, H, N, and S. When all the organic substances are oxidized, these elements are respectively oxidized to CO2, H2O, NO2, and SO2. The oxygen demand at this time is called total oxygen demand (TOD).

The principle and process of total oxygen demand determination is to inject a certain amount of water sample into the oxygen content and send it into a combustion tube with platinum steel as the catalyst, where it is burned at a high temperature of 900℃. The organic matter in the water sample consumes the oxygen in the carrier gas due to combustion. The remaining oxygen is measured by an electrode and recorded by an automatic recorder. The total oxygen demand is obtained by subtracting the remaining oxygen after the combustion of the water sample from the original oxygen content of the carrier gas.

The determination of this indicator is faster and simpler compared with the determination of BOD and COD, and its result is also closer to the theoretical oxygen demand than COD.

5. Total Organic Carbon (TOC

Total organic carbon (abbreviated as TOC in English) It represents the total carbon content of all organic pollutants in water and is a comprehensive parameter for evaluating organic pollutants in water. It is a comprehensive determination index that reflects the total amount of organic matter in water by measuring the total organic carbon content in water samples through combustion. The determination result is expressed in terms of C content, with the unit being mg/L.

Its determination principle and process are: Acid is added to the water sample, and the inorganic carbonates in it are dehydrated by compressed air to eliminate interference. Then, the water sample is quantitatively injected into a combustion tube with platinum steel as the catalyst. In a gas flow with sufficient and certain oxygen content, it is burned at a high temperature of 900℃. During the combustion process, carbon dioxide is produced, which is measured by an infrared gas analyzer and recorded by an automatic recorder. Then convert the carbon content within it.

The determination of TOC uses the combustion method, which can oxidize all organic matter. It can more directly represent the total amount of organic matter than BOD5 or COD, and is therefore often used to evaluate the degree of organic pollution in water bodies.

In recent years, various types of TOC analyzers have been developed both at home and abroad. According to different working principles, it can be classified into combustion oxidation - non-dispersive infrared absorption method, conductivity method, gas chromatography method, wet oxidation - non-dispersive infrared absorption method, etc. Among them, the combustion oxidation - non-dispersive infrared absorption method only requires one-time conversion, has a simple process, good reproducibility and high sensitivity. Therefore, this type of TOC analyzer is widely adopted at home and abroad.

6. Nitrogen (organic nitrogen, ammonia nitrogen, total nitrogen)

Organic nitrogen is a water quality indicator that reflects the total amount of nitrogen-containing organic compounds such as proteins, amino acids, and urea in water.

If organic nitrogen is subjected to biological oxidation under aerobic conditions, it can gradually decompose into forms such as NH3, NH4+, NO2-, and NO3-. NH3 and NH4+ are called ammonia nitrogen, NO2- is called nitrite nitrogen, and NO3- is called nitrate nitrogen. The content of these forms can all be used as water quality indicators, representing different stages of the conversion of organic nitrogen into inorganic substances.

Total nitrogen (abbreviated as TN in English) is a water quality indicator that encompasses all contents from organic nitrogen to nitrate nitrogen.
Ammonia nitrogen (NH3-N) is an important monitoring indicator for the effluent of sewage treatment plants. The sources of ammonia nitrogen in water are mainly the decomposition products of nitrogen-containing organic matter in domestic sewage under the action of microorganisms, certain industrial wastewater, such as coking wastewater and wastewater from synthetic ammonia fertilizer plants, as well as agricultural drainage. In addition, in an anaerobic environment, the nitrite present in water can also be reduced to ammonia by the action of microorganisms. In an aerobic environment, ammonia in water can also be transformed into nitrite and may even continue to be converted into nitrate.

Measuring various forms of nitrogen compounds in water is helpful for evaluating the pollution and "self-purification" status of water bodies. Fish are relatively sensitive to ammonia nitrogen in water. When the ammonia nitrogen content is high, it can lead to the death of fish.

It exists in water in the form of free ammonia (NH3) or ammonium salts (NH4-), and the composition ratio of the two depends on the pH value and water temperature of the water. When the pH value is relatively high, the proportion of free ammonia is also higher. Conversely, the proportion of ammonium salts is high, while the water temperature is the opposite. Therefore, sufficient attention should be paid to pH and water temperature during monitoring.

The determination methods of ammonia nitrogen usually include Nessler's colorimetric method, gas-phase molecular absorption method, phenol-hypochlorite (or salicylic acid-hypochlorite) colorimetric method and electrode method, etc.

N in water can lead to eutrophication of water bodies. The N in the effluent of sewage treatment plants should be treated in accordance with the corresponding requirements of the state and local governments before being discharged up to standard. Therefore, the monitoring of N in the effluent is one of the important items in the water quality monitoring of sewage treatment plants.

In addition, for urban sewage treatment plants that widely adopt secondary treatment as the main method, in order to ensure the normal operation of the sewage treatment plant, it is necessary to guarantee the nutrient requirements of microorganisms in the biochemical tank. The aerobic method is generally controlled at: The ratio of BOD:N:P is 100:5:1. Therefore, monitoring the influent N of sewage treatment plants is beneficial for controlling the nutrition of microorganisms. When the phosphorus content in the sewage is relatively low, it is necessary to supplement it artificially to ensure the nutritional requirements of microorganisms and thus guarantee the normal operation of the sewage treatment system.

7. Phosphorus (total phosphorus, dissolved phosphate and total dissolved phosphorus)

In natural water and wastewater, phosphorus almost always exists in the form of various phosphates, which are classified as orthophosphates, condensed phosphates (pyrophosphates, metaphosphates and polyphosphates), and organically bound phosphorus (such as phospholipids, etc.). They are present in solutions, humus particles or aquatic organisms.

The phosphate content in general natural water is not high. Industrial wastewater and domestic sewage from industries such as fertilizer production, smelting, and synthetic detergents often contain a relatively large amount of phosphorus. Phosphorus is one of the essential elements for the growth of living organisms. However, if the phosphorus content in water bodies is too high (such as exceeding 0.2mg/L), it can cause excessive reproduction of algae until the quantity reaches a harmful level (referred to as eutrophication), resulting in reduced transparency of lakes and rivers and deterioration of water quality. Phosphorus is an important indicator for evaluating water quality.

To further prevent eutrophication of water bodies caused by P in water, the P in the effluent of sewage treatment plants should be treated in accordance with the corresponding requirements of the state and local governments before being discharged up to standard. Therefore, the monitoring of P in the effluent is one of the important items in the water quality monitoring of sewage treatment plants.

In addition, for urban sewage treatment plants that widely adopt secondary treatment as the main method, in order to ensure the normal operation of the sewage treatment plant, it is necessary to guarantee the nutrient requirements of microorganisms in the biochemical tank. The aerobic method is generally controlled at: The ratio of BOD:N:P is 100:5:1. Therefore, monitoring the P in the influent of a sewage treatment plant is beneficial for controlling the nutrition of microorganisms. When the phosphorus content in the sewage is relatively low, it is necessary to supplement it artificially to ensure the nutritional requirements of microorganisms and thus guarantee the normal operation of the sewage treatment system.

8. pH value

pH value is an important indicator of the acidity or alkalinity of water and numerically equals the negative logarithm of the hydrogen ion concentration. The determination of pH value usually adopts the glass electrode method based on electrochemical principles, and the colorimetric method can also be used.

The pH value can represent the most fundamental properties of water and has an impact on changes in water quality and the effectiveness of water treatment. The measurement and control of pH value are of great practical significance for maintaining the normal operation of sewage treatment facilities, preventing corrosion of sewage treatment and transportation equipment, protecting the growth of aquatic organisms and the self-purification function of water bodies.

If the pH value of sewage is too high or too low, it will affect biochemical treatment, because the pH range suitable for the survival of organisms is often very narrow and also very sensitive. For instance, in the aeration tank of an activated sludge process system, if the pH changes, such as from the normal range of 6.5 to 8.5 to 5.5, it is highly likely that filamentous bulking of the activated sludge will occur in the system. This will directly affect the effluent quality and lead to deterioration of the effluent. The main reason is that in activated sludge, bacteria should be dominant. The optimal pH range they prefer is 6.5 to 8.5. When the pH value is normal, bacteria are dominant and the number of filamentous bacteria is limited. However, when the pH changes to 5.5, it is highly suitable for the growth of filamentous bacteria, which inhibits their growth. This will lead to the dominance of filamentous bacteria in the activated sludge, causing sludge bulking.

In addition, when anaerobic digestion treatment is carried out on sludge or high-concentration wastewater, special attention should also be paid to the control of pH value. Because, in the anaerobic digestion process, it is mainly methanogenic bacteria