Temperature
Temperature plays a vital role in the biochemical culture process. The measurement and analysis of temperature in the sewage biochemical reaction system and each operation stage plays a guiding role in the biochemical sludge acclimatization and culture process. It can provide a basis for the interpretation of various phenomena in the biochemical culture process, and help managers and operators make correct and timely judgments on the system operation and management.
Temperature greatly affects the activity of microorganisms in activated sludge (including anaerobic, facultative and aerobic), and affects dissolved oxygen, aeration volume, etc., and also affects the biochemical reaction rate.
Different types of microorganisms grow in different temperature ranges, about 5℃~80℃. Within this temperature range, it can be divided into the lowest growth temperature, the highest growth temperature and the most suitable growth temperature. According to the temperature range to which microorganisms adapt, microorganisms can be divided into three categories: mesophilic, thermophilic and thermophilic.
The growth temperature range of mesophilic microorganisms is 20℃~45℃, the growth temperature of thermophilic microorganisms is below 20℃, and the growth temperature of thermophilic microorganisms is above 45℃. The biochemical aerobic biological treatment of wastewater is mainly based on mesophilic bacteria, and the optimum temperature for their growth and reproduction is 20℃~37℃. When the temperature exceeds the maximum biological growth temperature, the protein of the microorganism will be rapidly denatured and the enzyme system will be destroyed and lose its activity. Low temperature will reduce the metabolic activity of microorganisms, and then they will stop growing and reproducing, but they will still preserve their vitality. The optimum temperature range of mesophilic methanogens in anaerobic biological treatment is between 20℃~40℃, and the temperature range of mesophilic methanogens is 50℃~60℃. The temperature of anaerobic biological treatment is usually 33℃~38℃ and 50℃~57℃.
PH
Different microorganisms have different pH adaptation ranges. For example, the pH adaptation range of bacteria, actinomycetes, algae and protozoa is between 4 and 10. Most bacteria are suitable for neutral and alkaline environments (pH 6.5-7.5); Bacillus sulfoxidans prefers acidic environments, with an optimal pH of 3, and can also live in an environment with a pH of 1.5; yeast and mold require living in an acidic or slightly acidic environment, with an optimal pH of 3.0-6.0, and an adaptable pH range of 1.5-10. It is very important to maintain the optimal pH range in the wastewater biological treatment process.
For example, when the activated sludge method is used to treat wastewater, when the pH value of the aeration tank mixed liquor reaches 9.0, the protozoa will turn from active to sluggish, the sticky substances of the bacterial flocs will disintegrate, the activated sludge structure will be destroyed, and the treatment efficiency will be significantly reduced. If the influent pH value suddenly decreases, the aeration tank mixed liquor will become acidic, the activated sludge structure will also change, and a large amount of floating sludge will appear in the secondary sedimentation tank. Well-cultivated and mature biological systems have a strong ability to withstand shock loads, but if the pH value changes within a wide range, it will affect the efficiency of the reactor and even cause toxicity to the microorganisms, rendering the reactor ineffective, because changes in pH may cause changes in cell charge, thereby affecting the microbial absorption of nutrients and the activity of enzymes in microbial metabolism.
Chemical Oxygen Demand (COD)
The chemical oxygen demand is the amount of oxidant consumed when chemical oxidants are used to oxidize organic pollutants in water, expressed in terms of oxygen (mg/L). The higher the chemical oxygen demand, the more organic pollutants there are in the water. Commonly used oxidants are mainly potassium dichromate and potassium permanganate. When potassium permanganate is used as an oxidant, the measured value is called CODMn or OC for short. When potassium dichromate is used as an oxidant, the measured value is called CODCr, or COD for short. If the composition of organic matter in wastewater is relatively stable, there is a proportional relationship between chemical oxygen demand and biochemical oxygen demand. Generally speaking, the difference between the chemical oxygen demand of potassium dichromate and the biochemical oxygen demand of the first stage can be roughly expressed as organic matter that cannot be decomposed by aerobic microorganisms.
The testing and analysis of COD is an important part of the commissioning and operation of wastewater treatment. On the one hand, it is necessary to understand the inlet and outlet water conditions of each treatment unit in the process flow to ensure the stability of the inlet water and avoid large fluctuations and impact on the system; on the other hand, the treatment effect and efficiency of the treatment unit can be understood through the changes in COD of the inlet and outlet water before and after each treatment unit.
Biophase of activated sludge
The observation of the biophase of activated sludge plays an extremely important role in the biochemical treatment of wastewater. It not only reflects the degree of microbial cultivation and sludge domestication, but also directly reflects the treatment of wastewater. Activated sludge is a mixed culture composed of various microbial groups such as bacteria, fungi, protozoa and metazoa. Bacteria have a high proliferation rate and a strong function of decomposing organic matter, and fungi also have the ability to decompose organic matter. Protozoa mainly feed on free bacteria, which further purifies water quality, while metazoa mainly feed on protozoa.
The biophase of filamentous fungi and protozoa and metazoa can be observed through an optical microscope. The quality of sludge and the quality of treated water can be judged by observing and distinguishing their species and quantity. Therefore, protozoa and metazoa are called indicator organisms in the activated sludge system. In addition to the macroscopic indicators of activated sludge, the microscopic biological indicators of sludge, that is, the biophase of sludge, can be observed using an ordinary optical microscope.
The biological phase observation includes two parts: one is to observe the number and type changes of indicator organisms such as protozoa and metazoa. There are different indicator organisms in activated sludge of different qualities. The quality of activated sludge can be indirectly evaluated by observing the indicator organisms. The other part is to observe the number of filamentous bacteria in activated sludge. The amount of filamentous bacteria in activated sludge of different qualities is different. The quality of activated sludge can also be indirectly reflected by measuring the number of filamentous bacteria.
1. Observation of indicator organisms: For a specific sewage treatment system, when the activated sludge system operates normally, its biological phase is basically stable. If there is a change, it means that the quality of activated sludge has changed, and further observation and treatment measures should be taken. There are many types of microorganisms, and their naming methods are also very complicated. From a practical point of view, operators should be proficient in the most common micro-indicator organisms in activated sludge: amoeba, flagellate, paramecium, bell worm, nematode, etc. Whether one or several of these microorganisms are dominant and how much is the proportion will depend on the operating status of the process.
2. Observation of filamentous bacteria: In the activated sludge system, the fewer filamentous bacteria, the better, because filamentous bacteria play a skeleton role in the sludge flocs. Observing the number, length, and abundance of filamentous bacteria under a microscope can directly reflect the operation of the process. It should be added that biological phase observation is only a qualitative method. It can only be used as a supplement to physical and chemical methods during operation, and cannot be used as the main process detection method. It is necessary to pay attention to accumulating data in continuous practice and summarize the laws of biological phase changes.
MLSS, MLVSS, SRT and other sludge physical and chemical indicators
1.SV30 (sludge settling ratio): The sludge settling ratio refers to the ratio of the volume of settled sludge to the mixed liquid in the aeration tank in a 100ml measuring cylinder after standing for 30 minutes, generally expressed as SV30.
SV30 is an indicator to measure the settling performance and concentration performance of activated sludge. For a certain concentration of activated sludge, the smaller the SV30, the better its settling performance and concentration performance. The MLSS concentration of normal activated sludge is 1500~4000mg/L. SV30 is generally in the range of 15%~30%.
2. SVI30 (volume index of sludge): The volume index of sludge refers to the volume occupied by 1g of suspended solids in the aeration tank mixed liquid in a 100ml measuring cylinder after standing for 30 minutes. It is usually expressed by SVI30, with the unit of ml/g. The following relationship exists between SVI30 and SV30: SVI30= SV30/MLSS×100. The settling ratio SV is related to the concentration of sludge. For sludge with the same settling performance, when the MLSS is larger, the SV is also larger; when the MLSS of the mixed liquid in the aeration tank changes greatly, the SV value cannot be compared with the historical data, and the reflected sludge situation is distorted. The purpose of measuring SV or SVI is to reflect the settling and concentration status of sludge in the secondary sedimentation tank.
SVI is both an indicator for measuring the settling performance of sludge and an indicator for measuring the adsorption performance of sludge. Generally speaking, the larger the SVI value, the worse the settling performance, but the better the adsorption performance; conversely, the smaller the SVI, the better the settling performance, but the worse the adsorption performance. In traditional activated sludge processes, it is generally believed that the SVI value of about 100 has the best overall effect. Too large or too small is not conducive to improving the quality of effluent.
3. MLSS (mixed liquor suspended solids concentration): refers to the amount of suspended solids in the mixed liquor after sewage and activated sludge are mixed in the aeration tank, expressed in MLSS, and the unit is mg/L. It approximately represents the concentration of active microorganisms in the aeration tank and is an important parameter for operation and management.
4. MLVSS (mixed liquor volatile suspended solids concentration): refers to the content of organic matter in suspended solids in the mixed liquor, expressed in MLVSS, which can more accurately represent the number of activated sludge microorganisms than MLSS.
5. SRT (sludge age or average cell residence time): It is the average residence time of activated sludge in the entire system, generally expressed as SRT:
SRT = total amount of activated sludge in the activated sludge system / amount of activated sludge discharged from the system every day = (Ma + Mc + MR) / (Mw + Me) Where Ma is the amount of activated sludge in the aeration tank; Mc is the amount of sludge in the secondary sedimentation tank; MR is the amount of sludge in the return system; Mw is the amount of residual sludge discharged every day; Me is the amount of sludge carried away by the effluent of the secondary sedimentation tank every day.
Nutrients
Nutrients play a vital role in the biochemical treatment of industrial wastewater. According to their cell composition and metabolic properties, biologically cultured microorganisms need a certain amount of nutrients during their growth and reproduction, mainly nitrogen and phosphorus. Therefore, during the biological culture of industrial wastewater, nutrients need to be added regularly to ensure that there is enough nitrogen and phosphorus in the wastewater.
BOD: N: P = 100: 5: 1, which is the ratio in aerobic biochemical systems. In aerobic biochemical culture, the lack of nitrogen will lead to the production of filamentous or dispersed microbial communities, making their sedimentation performance poor. In addition, the lack of nitrogen makes it difficult for new cells to form, while old cells continue to remove BOD substances. As a result, microorganisms excrete excessive byproducts outside the cell wall - fluffy flocs, which have poor sedimentation performance. According to experience, 5kg of nitrogen and 1kg of phosphorus are required for every 100kg of BOD removed from the wastewater.
Under many conditions, nitrogen is added to the wastewater in the form of ammonia and phosphorus in the form of phosphoric acid. Bacteria need nitrogen to produce proteins and phosphorus to produce enzymes that decompose organic matter in wastewater. Generally, bacteria can easily utilize ammonia nitrogen. When treating industrial wastewater, if the nitrogen content of the wastewater is low and cannot meet the needs of microorganisms, it is necessary to add nitrogen nutrients, such as urea, ammonium sulfate, manure, etc. Among microorganisms, bacteria have a higher demand for phosphorus. Industrial wastewater generally needs to be supplemented with phosphorus elements, such as potassium phosphate, sodium phosphate, etc.
Dissolved oxygen (DO)
The test of DO plays an important role in the biochemical treatment of wastewater. Various biochemical reactions have high requirements for dissolved oxygen concentration. During the reaction process, the dissolved oxygen concentration in the wastewater must be strictly controlled to ensure that the microorganisms have the highest activity and the biochemical treatment achieves the best treatment effect.
Dissolved oxygen is an important factor affecting the effect of biochemical treatment. In aerobic biological treatment, if the dissolved oxygen is insufficient, the activity of aerobic microorganisms will be affected due to insufficient oxygen, and the metabolic capacity will be reduced. At the same time, microorganisms with lower requirements for dissolved oxygen will emerge, affecting the normal biochemical reaction process and causing the treatment efficiency to decrease. The dissolved oxygen of aerobic biological treatment is generally 2~4mg/L. In this case, the structure of activated sludge or biofilm is normal, and the sedimentation and flocculation performance are good. Excessive oxygen supply will waste energy, and the metabolic activity will be enhanced. Insufficient nutrient supply will cause microorganisms to lack nutrition, causing sludge aging and loose structure. Therefore, in the process of wastewater biochemical treatment, dissolved oxygen should be tested frequently to ensure that the dissolved oxygen concentration in the aeration tank is controlled at a reasonable level, to ensure the normal growth of aerobic microorganisms, and to achieve better treatment effects.