Views: 589 Author: Site Editor Publish Time: 2024-12-16 Origin: Site
The pH value is an essential measurement item in sewage. The pH value is carried out in daily sewage testing projects as a routine test. In the operation and management of sewage plants, the pH measurement of sewage is not only a factor in monitoring the quality of wastewater but also affects the living environment of microorganisms in the activated sludge. Significant changes in the pH value of sewage are also indicators for judging whether the sewage is polluted or if there are other environmental factors.
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2. How can pH affect wastewater treatment?
2.1 Effectiveness of Treatment Processes
2.2 Effect of pH value on physical and chemical methods
2.3 Effect of pH value on chemical method
2.4 Effect of pH value on biological treatment
2.4.1 Effect of pH on aerobic biological treatment
2.4.2 Effect of pH on anaerobic microorganisms
3. How is pH measured and controlled in wastewater treatment plants?
pH, also known as the hydrogen ion activity index, is a scale of hydrogen ion activity in a solution, which is also a measure of the acidity or alkalinity of a solution in the usual sense. The closer the pH value is to 0, the more acidic the solution is. Conversely, the closer the pH value is to 14, the more alkaline the solution is. At room temperature, a solution with a pH of 7 is neutral.
Standard sewage treatment methods include physical, chemical, and biological processes. Many chemical reactions must be carried out in these treatment methods at a specific pH value; otherwise, the desired product cannot be obtained.
The control of indicators in the sewage treatment process is of great significance, which is mainly reflected in the following aspects:
Ensure that the effluent water quality meets the standards: Ensure that the treated sewage meets the national and local discharge standards to avoid pollution of the receiving water body.
Maintain the stable operation of the sewage treatment system: Reasonable control of various indicators helps optimize the treatment process and maintain the activity of microorganisms and the standard working state of the treatment facilities.
Save costs and resources: Through precise control of indicators, the treatment efficiency can be improved, the use of chemicals and energy consumption can be reduced, and the operating costs can be reduced.
Promote sustainable development: Effective sewage treatment indicator control helps to recycle water resources and alleviate the pressure of water shortage.
Protect public health: Prevent pathogens, heavy metals, and other harmful substances in sewage from threatening human health. Sewage treated with effective indicator control can reduce the risk of disease transmission.
The pH level of wastewater significantly impacts various treatment processes in several ways:
Coagulation and Flocculation: The efficiency of coagulation and flocculation processes, which involve aggregating particles to form larger flocs that can be easily removed, is highly dependent on pH. Optimal pH levels ensure the effectiveness of coagulants and flocculants, leading to better removal of suspended solids and other pollutants.
Sedimentation and Filtration: Proper pH control is essential for the sedimentation and filtration processes. It affects the settling rate of particles and the filtration efficiency, ensuring that the treated water meets the required standards.
The physical and chemical methods used for sewage treatment include coagulation, flotation, adsorption, magnetic adsorption, electrochemical processes, etc., among which flotation and coagulation sedimentation are more commonly used.
Coagulation sedimentation is a separation technology for insoluble pollutants. It refers to a water treatment method that makes colloids and delicate suspended matter in wastewater condense into floccules under the action of coagulants and then separate and remove them.
Coagulation sedimentation is widely used in water supply and wastewater treatment. It can reduce the sensory water quality indicators such as turbidity and color of raw water, but it can also remove various toxic and harmful pollutants.
There are two major types of coagulants for wastewater treatment: inorganic metal salts and organic polymers. The former mainly includes high-valent metal salts such as iron and aluminum, which can be divided into ordinary iron, aluminum salts, and alkaline polymer salts; the latter is divided into two types: artificial synthesis and natural. The leading equipment used in the coagulation method includes a mixing tank for mixing coagulants and raw water, a reaction tank for the reaction process, and a sedimentation tank for separating water and floccules.
The influence of water pH on coagulation varies depending on the type of coagulant. When aluminum sulfate is used to remove turbidity in water, the optimal pH range is between 6.5 and 7.5; when used to remove color, the pH range is between 4.5 and 5; when trivalent iron salts are used, the optimal pH range is between 6.0 and 8.4, which is wider than aluminum sulfate. For example, when ferrous sulfate is used, Fe3+ can only be quickly formed when the pH is more significant than 8.5 and there is enough dissolved oxygen in the water, which makes the equipment and operation more complicated. For this reason, the chlorination oxidation method is often used.
The coagulation effect of polymer coagulants, especially organic polymer coagulants, is less affected by pH. It can be seen from the hydrolysis reaction formula of aluminum salts and iron salts that the continuous generation of H+ during the hydrolysis process will inevitably reduce the pH value of water. Alkaline substances should be present to neutralize the pH value within the optimal range.
When the alkalinity in the raw water is sufficient, it will not affect the coagulation effect. Still, when the alkalinity in the raw water is insufficient, or the amount of coagulant added is large, the pH value of the water will drop significantly, affecting the coagulation effect. At this time, lime or sodium bicarbonate should be added.
In oilfield wastewater treatment, chemical methods are mainly used for deep treatment of oily wastewater, including chemical demulsification, chemical oxidation (air oxidation, ozone oxidation, chlorine oxidation, hydrogen peroxide oxidation, Fenton reagent oxidation, KMn04 oxidation, K2FeO4 oxidation), photochemical oxidation, etc. Currently, the most researched and applied methods are Fenton reagent oxidation and K2FeO4 oxidation.
Fenton reagent oxidation iusesH2O2 and FeSO4 mixed in proportion to obtain a highly oxidizing agent to treat oily wastewater, which has the dual effects of oxidation and coagulation. According to the results of experiments and practical applications, the treatment effect is believed to be best when pH≤3. The COD removal rate is more than 90%, and the average decolorization rate is more than 95%.
Relevant researchers applied the Fenton method to treat polymer-containing oilfield wastewater and observed the effect of the initial pH value on the removal rate of polyacrylamide. As shown in Table 1, the removal rate is the highest when pH=3.0, and pH values that are too high or too low cannot achieve the ideal treatment effect.
| Initial pH value of the system | 2.0 | 3.0 | 3.5 | 4.5 | 5.9 |
| PAM residual rate /% | 62 | 5 | 30 | 70 | 75 |
Table 1 Effect of initial pH value on the removal of polyacrylamide by Fenton's reagent
K2FeO4 oxidation is an efficient water treatment method developed in recent years. The standard electrode potential of K2FeO4 is 1.90 V, which is more oxidizing than potassium permanganate, etc. It can remove organic pollutants and heavy metals in water and decolorize and deodorize. Chen Ying et al. used potassium ferrate to treat polymer-containing oilfield wastewater and observed the effect of pH on the treatment effect. The oxidation degradation rate was high under low pH conditions, but the degradation rate would decrease when the pH value was less than 2. Generally, the pH value was controlled at about 3 to obtain a good degradation effect.
Biological treatment of wastewater is divided into aerobic biological treatment and anaerobic biological treatment. Anaerobic biological treatment of wastewater refers to the process of decomposing various complex organic substances in sewage into methane, carbon dioxide, and other substances through the action of anaerobic microorganisms (including facultative anaerobic microorganisms) under the condition of no molecular oxygen, which is also called anaerobic digestion. Aerobic biological treatment uses the metabolism of aerobicorganismss under aerobic conditions to oxidize and decompose various complex organic substances in wastewater into carbon dioxide and water.
The factors that affect aerobic biological treatment are mainly nutrients, temperature, pH, dissolved oxygen in water, poisons and the nature of organic matter in wastewater. Generally, the most suitable pH for aerobic microorganisms is between 6.5 and 9; at pH 6.5, fungi will dominate and cause sludge expansion. At this time, lime addition, water intake control and aeration reduction can be adopted to inhibit sludge expansion.
The anaerobic digestion process is divided into three consecutive stages: hydrolysis and acidification, hydrogen and acetic acid production, and methanogenesis. The first stage is the hydrolysis and acidification stage. Complex macromolecules and insoluble organic matter are first hydrolyzed into small molecules under the action of extracellular enzymes. Soluble organic matter penetrates the cell body and decomposes to produce volatile organic acids, alcohols, aldehydes, etc.
The second stage is the hydrogen and acetic acid production stage. Under the action of hydrogen and acetic acid production bacteria, the organic acids produced in the first stage decomposed and converted into acetic acid and H2, carbonic acid, and new cell substances.
The third stage is methanogenesis, in which methanogenic bacteria convert acetic acid, acetate, CO2, and H2 into methane, carbon dioxide, and new cell substances.
The anaerobic method has stricter requirements on environmental conditions than the aerobic method. Essential factors control the efficiency of anaerobic treatment: one is the crucial factors, including microbial biomass (sludge concentration), nutrient ratio, mixed contact conditions, organic load, etc.; the other is environmental factors, such as temperature, pH value, redox potential, toxic substances, etc.
Methanogenic bacteria are the main microorganisms that determine the efficiency and success of anaerobic digestion. The methanogenic stage is the rate-limiting step of the anaerobic process. Each organism can be active within a specific pH range. Acidogenic bacteria are not as sensitive to pH as methanogenic bacteria, and their suitable pH range is more expansive, between 4.5 and 8.0.
Methanogenic bacteria require the pH value of the environmental medium to be near neutral, with the most suitable pH value being 7.0 to 7.2. Ph 6.6 to 7.4 being more appropriate In the application of anaerobic wastewater treatment since acetogenesis and methanogenesis are mainly carried out in the same structure, to maintain balance and avoid excessive acid accumulation, the pH value in the reactor is often kept within the range of 6.5 to 7.5 (preferably 6.8 to 7.2).
Effect of pH on the activity of methanogenic bacteria The most suitable pH range for the growth of methanogenic bacteria is about 6.8 to 7.2. The development and reproduction will be significantly affected if the pH value is lower than six or above 8.
pH is measured and controlled in wastewater treatment plants through a combination of automated systems, sensors, and chemical additives. Here are the key methods and technologies used:
pH Sensors: Specialized pH sensors continuously monitor wastewater's pH levels. These sensors are typically installed at various points in the treatment process to provide real-time data.
Automatic pH Measurement Systems: Systems like the Liquiline Control CDC90 from Endress+Hauser are used for automatic pH measurement. These systems can automatically clean the sensors to maintain accuracy and reliability over time.
PLC-Based Automatic Control Systems: Programmable Logic Controllers (PLCs) automate the pH control process. These systems can adjust the addition of acidic or alkaline chemicals based on real-time pH measurements to maintain the desired pH level.
Four pH Limits: A standard method involves setting four pH limits (acidic switch limit, alkaline switch limit, etc.) to trigger the addition of neutralizing agents when the pH deviates from the set range.
Fuzzy Logic and PID Controllers: Advanced control systems, such as fuzzy logic and Proportional-Integral-Derivative (PID) controllers, are used to improve the precision and stability of pH control. These systems can adapt to pH neutralization processes' non-linear and time-varying nature.
Fuzzy Adaptive PID Control: This method combines fuzzy logic with PID control to enhance pH control systems' response time and accuracy.
3.2.1 Chemical Additives: Various chemicals are used to adjust the pH of wastewater. Common acidic additives include sulfuric acid and carbon dioxide, while alkaline additives include sodium hydroxide and lime.
Sulfuric Acid: Widely used due to its effectiveness and availability. However, it can cause corrosion and calcium tie-up if not appropriately managed.
Carbon Dioxide: A safer alternative to sulfuric acid, CO2 is less corrosive and produces no harmful byproducts. It is increasingly used for pH adjustment in wastewater treatment.
3.2.2 Mechanical Processes: Some treatment plants use reverse osmosis and electrocoagulation methods to control pH without chemical additives. These methods filter out substances that affect pH levels.
Reverse Osmosis: Removes contaminants that can alter pH levels, providing a more stable pH environment.
Electrocoagulation: Uses an electric current to bind contaminants, making them easier to filter out and thus controlling pH levels indirectly.
3.2.3 Batch vs. Continuous Processing: Wastewater treatment plants can choose between batch and continuous processing methods to control pH.
Batch Processing: Treats water in controlled batches, adjusting pH levels in each batch before release.
Continuous Processing: Maintains a constant water flow through treatment tanks, continuously adjusting pH levels as new water enters the system.
pH in wastewater treatment plants is measured using specialized pH sensors and controlled through automated systems like PLCs, advanced control algorithms (fuzzy logic and PID), and the addition of chemical additives such as sulfuric acid and carbon dioxide. Mechanical processes like reverse osmosis and electrocoagulation are also used to control pH without chemicals. The choice between batch and continuous processing further influences how pH levels are managed in the treatment process.
