The relationship between alkalinity and pH

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Often, people get confused about the relationship between alkalinity and pH. Alkalinity and pH are not the same concept, nor do they have the same meaning; alkalinity indicates buffering capacity, while pH is a direct indication of acidity or alkalinity! One is internal strength, the other is techniques—that’s the difference! 1. What is pH? pH value, also known as the hydrogen ion concentration index or acidity level, is a scale that indicates the activity of hydrogen ions in a solution; it is essentially the measure of how acidic or basic a solution is. “In pH, the “H” stands for hydrogen ions (H+), while there are various explanations for the origin of the “p” symbol; according to chemical terminology, the p is placed in front of a dimensionless quantity to indicate its negative logarithm. The pH value is actually a “logarithmic unit”. Each digit represents a 10-fold change in the acidity of the water. Water with a pH of 5 is 10 times more acidic than water with a pH of 6. At standard temperature and pressure, an aqueous solution with a pH of 7 is neutral; for example, pure water is neutral because the product of the concentrations of hydrogen ions and hydroxide ions produced by the natural ionization of water at standard temperature and pressure is always 1×10-14. The concentration of both ions is 1×10-7 moL. A pH value less than 7 indicates that the concentration of H+ is greater than that of OH-, meaning the solution is acidic, while a pH value greater than 7 indicates that the concentration of H+ is less than that of OH-, meaning the solution is basic. Therefore, the lower the pH value, the stronger the acidity of the solution ； The higher the pH, the stronger the alkalinity of the solution. 2. What is alkalinity? Alkalinity refers to the total amount of substances in water that can neutralize strong acids. Such substances include strong bases, weak bases, and salts of strong bases and weak acids. The alkalinity in natural water is primarily caused by bicarbonate, carbonate, and hydroxide, among which bicarbonate is the main form of alkalinity in water. The main sources of pollution that contribute to alkalinity are wastewater discharged from industries such as papermaking, printing and dyeing, chemicals, and electroplating, as well as the loss of detergents, fertilizers, and pesticides during their use. Alkalinity and acidity are important indicators for assessing water quality and controlling wastewater treatment. Alkalinity is also commonly used to evaluate the buffering capacity of water bodies, as well as the solubility and toxicity of metals in them. In engineering, the term total alkalinity is more commonly used, and it is generally expressed as a concentration value equivalent to that of calcium carbonate. 3. Relationship between pH value and alkalinity: There is no clear corresponding relationship between the two. Water (or solutions) with the same alkalinity do not necessarily have the same pH value. Conversely, water (or solutions) with the same pH value do not necessarily have the same alkalinity. The reason is that the pH value directly reflects the concentration of H+ or OH- in water, whereas alkalinity includes not only OH- but also the concentrations of basic substances such as CO3-2 and HCO3-. For example: NaOH solution with an alkalinity of 0.1 mmol/L, pH=13 ； NH3-H2O solution with an alkalinity of 0.1 mmol/L, pH=11 ； NaHCO3 solution with an alkalinity of 0.1 mmol/L, pH=8.3. Although there is no direct numerical relationship between alkalinity and pH, in practice, the higher the alkalinity, the higher the corresponding pH; the lower the alkalinity, the lower the corresponding pH. A higher alkalinity provides greater buffering capacity for the pH solution, while a lower alkalinity results in weaker buffering capacity for that solution! 4. The impact of pH on wastewater treatment: The pH value affects the charge of the microbial cell membranes in the activated sludge used in wastewater treatment, which in turn influences the activity of enzymes involved in the absorption and metabolism of nutrients by microorganisms ； Change the availability of nutrients and the toxicity of harmful substances in the growth environment. Each microorganism in activated sludge has its optimal pH value and a certain pH range. Enzyme activity is highest within the optimal range; if other conditions are favorable, the growth rate of microorganisms is also highest. The optimal pH for most bacteria, algae, and protozoa is 6.5–7.5, and they can also grow within the pH range of 4–10. Actinomycetes generally thrive in a slightly alkaline environment, with a pH of 7.5–8 being optimal ； Yeast and molds are suitable for acidic environments with a pH of 5-6. 1. The effect of pH value on flocculants: In actual adjustment processes, it is preferable for the pH to be on the alkaline side rather than the acidic side, mainly because an alkaline pH facilitates improved flocculation and precipitation effects in later stages. The pH value of water has a significant impact on the effectiveness of inorganic coagulants; it determines the type of coagulant to be used, the amount to be added, and the efficiency of coagulation and sedimentation. The H+ and OH- ions in water are involved in the hydrolysis reaction of flocculants; therefore, the pH value has a significant impact on the rate of decomposition of the flocculants, as well as on the form and properties of the hydrolysis products. Taking aluminum salts, which achieve coagulation by generating Al(OH)3-charged colloids, as an example, at a pH of 4, Al3+ cannot hydrolyze significantly into Al(OH)3 and exists mainly in the form of Al3+ ions, resulting in extremely poor coagulation effects. When the pH is between 6.5 and 7.5, Al3+ undergoes hydrolysis to form Al(OH)3+ neutral colloids with a high degree of polymerization, resulting in good coagulation effects. When the pH is above 8, Al is hydrolyzed to AlO2-, and the coagulation effect becomes very poor. The alkalinity of water acts as a buffer for the pH value; when the alkalinity is insufficient, agents such as lime should be added to compensate for it. When the pH value of water is too high, acid needs to be added to adjust it to a neutral level. In contrast, polymeric flocculants are less affected by pH value. 2. The effect of pH on the nitrification reaction: Nitrifying bacteria are highly sensitive to pH; they exhibit the highest biological activity under neutral or slightly alkaline conditions (with a pH range of 8–9), resulting in a rapid nitrification process. When pH is greater than 9.6 or less than 6.0, the biological activity of nitrifying bacteria is suppressed and tends to cease. When pH > 9.6, although the conversion of NH4+ to NO2— and NO3— still occurs very rapidly, it can be seen from the ionization equilibrium of NH4+ that the concentration of NH3 increases quickly. Since nitrifying bacteria are extremely sensitive to NH3, this affects the rate of nitrification. Under acidic conditions, the nitration rate slows down when pH is less than 7.0; it slows down significantly when pH is less than 6.5, with the nitration rate dropping markedly. When pH < 5.0, the nitrification rate is close to zero. 3. Relationship between pH value and other indicators: (1) Relationship with water quality and quantity: Fluctuations in pH in industrial wastewater are mainly caused by the acids and bases used in production. It is necessary to gradually become familiar with the wastewater characteristics of a particular enterprise over time, and to gain experience in determining whether the water is acidic or alkaline based on physical properties such as color. (2) Relationship with sedimentation ratio: A pH level below 5 or above 10 will have an adverse effect on the system, resulting in slow sedimentation of sludge, turbid supernatant, and even floating sludge flocs on the surface of the liquid. (3) Relationship with sludge concentration (MLSS): A higher sludge concentration results in greater tolerance to pH fluctuations. After being impacted, the sludge discharge rate should be increased to promote the renewal of activated sludge. (4) Relationship with the reflux ratio: Increasing the reflux ratio to dilute the acidity and alkalinity of the feed water is also one of the methods to reduce the impact of pH fluctuations on the system. 5. Calculation of the alkalinity dosage: During the nitrification and denitrification processes, 7.14 g of alkalinity is consumed per gram of ammonia nitrogen oxidized in the nitrification step, while 3.57 g of alkalinity can be recovered during denitrification. Therefore, a total of 3.57 g of alkalinity is required for the entire nitrification and denitrification process. If the alkalinity in the raw water is insufficient, it will cause the pH of the system to drop, leading to obstacles in biochemical processes. Therefore, in order to raise the pH, it is necessary to add alkali! 1. Generally, in the nitrification reaction, 7.14 g of alkalinity is required to nitrate each 1 lg of NH3-N; therefore, the amount of alkalinity needed during nitrification can be calculated using the following formula: Alkalinity = 7.14 × Q × ΔCNH3-N × 10-3 (1) Where: Q is the average daily volume of wastewater entering the filter, in m3/d ； ΔCNH3-N is the difference between the NH3-N concentrations on the inlet and outlet sides of the filter, in mg/L ； 7.14 is the coefficient of alkali requirement for nitrification, kg of alkalinity per kg of NH3-N. 2. For industrial wastewater with a high ammonia nitrogen concentration, it is usually necessary to add alkalinity in order to maintain the pH value in the nitrification reactor between 7.2 and 8.0. The calculation formula is as follows: Alkalinity = K × 7.14 × QΔCNH3-N × 10-3 (2) Where K is the safety factor, typically ranging from 1.2 to 1.3. 3. In actual engineering projects, the calculation of alkalinity should take into account the following factors: the alkalinity in the incoming wastewater, the alkalinity consumed by biological nitrification, the alkalinity generated by the decomposition of BOD5, and the residual alkalinity that needs to be maintained in the mixture. For proper biological nitrification to take place, the following condition must be met: ALKw + ALKc > ALKN + AlKE (3). If the alkalinity is insufficient, soda ash must be added to supply the necessary alkalinity in order to enable proper nitrification. The amount of alkali to be added can be calculated using the following formula: ΔALK = (ALKN + ALKE) – (ALKw + ALKc) Equation (4). Here, ΔALK represents the alkalinity that needs to be added to the system, in mg/L ； ALKN represents the amount of alkali consumed in biological nitrification ； ALKN is generally calculated as 7.14 kg of alkali per kg of NH3-N consumed in nitrification. ALKE represents the amount of alkali that should be maintained in the mixture; it is generally calculated as 50 mg/L of alkalinity (expressed as Na2CO3) remaining in the mixture discharged from the aeration tank. ALKw represents the total alkalinity in raw wastewater ； ALKc is the amount of alkali produced during the breakdown of BOD5 ； ALKc is related to the system’s SRT: when SRT > 20 days, it can be calculated as 0.1 kg of alkali produced per kilogram of BOD5 degraded ； When SRT = 10–20 days, it is 0.05 kg ALK/kg BOD5 ； When SRT＜10d, use 0.01gALK/kgBOD5.