Info

250 500 1000 2500 5000 10000 25000

Bicarbonate Alkalinity - mg/L as CaCO,

Figure 13.20 Effcct of pH on the relationship between the bicarbonate alkalinity of the liquid phase and the carbon dioxide content of the gas phase in an anaerobic process. (From G. F. Parkin and W. F. Owen, Fundamentals of anaerobic digestion of wastewater sludges. Journal of the Environmental Engineering Division, ASCE 112:867-920, 1986. Copyright © American Society of Civil Engineers; reprinted with permission.)

250 500 1000 2500 5000 10000 25000

Figure 13.20 Effcct of pH on the relationship between the bicarbonate alkalinity of the liquid phase and the carbon dioxide content of the gas phase in an anaerobic process. (From G. F. Parkin and W. F. Owen, Fundamentals of anaerobic digestion of wastewater sludges. Journal of the Environmental Engineering Division, ASCE 112:867-920, 1986. Copyright © American Society of Civil Engineers; reprinted with permission.)

Adverse pH conditions can be corrected by the addition of appropriate chemicals, but, care must be exercised in their selection because of the complex interactions that can occur and the potential for adding toxicants. Commonly used chemicals include sodium bicarbonate, sodium carbonate, lime, sodium or potassium hydroxide, and ammonia.

Sodium bicarbonate is preferred for pH adjustment because its impact is longer lasting and its toxicity potential is low. It adjusts the pH by the direct addition of bicarbonate ions which, as illustrated in Figure 13.20, will result in a direct increase in the pH without affecting the carbon dioxide content of the gas space.

The addition of hydroxide ions by adding lime, sodium hydroxide, or potassium hydroxide, adjusts the pH because the hydroxide ion reacts with carbon dioxide to form bicarbonate alkalinity. Using lime as the example pH adjustment chemical gives:

This reaction is accompanied by a decrease in the carbon dioxide content of the gas space, which further contributes to the rise in the bioreactor pH. Unfortunately, further production of carbon dioxide by the microorganisms in the process will restore the original gas space carbon dioxide content and reduce the pH.

The use of carbonate based chemicals reduces the magnitude of the pH variation, as follows:

Comparison to Eq. 13.10 illustrates that only one mole of carbon dioxide is required to produce two moles of bicarbonate from carbonate while two moles of carbon dioxide are required to produce two moles of bicarbonate from hydroxide. Thus, when carbonate-based chemicals are used for pH adjustment, the immediate consumption of carbon dioxide from the gas space is one-half of that when hydroxide-based chemicals are used.

These changes in pH are illustrated in Figure 13.21. Consider an initial condition represented by a gas phase carbon dioxide content of 40% and a bicarbonate alkalinity of 500 mg/L as CaCO, (point 1), which corresponds to a pH of about 6.3. The addition of sufficient sodium bicarbonate to elevate the bioreactor bicarbonate alkalinity to 2,100 mg/L as CaCO, would directly increase the bioreactor pH to about 6.9 (point 2). The addition of an equivalent amount of hydroxide-based chemical will result in not only an increase in the bicarbonate alkalinity to 2,100 mg/L as CaCO„ but also in an immediate decrease in the carbon dioxide content of the gas space as it is removed to produce the bicarbonate (Eq. 13.10). The actual decrease in the carbon dioxide content of the gas will depend on the relative gas and liquid volumes in the bioreactor. If the requirement for carbon dioxide is large relative to the amount available, a negative pressure can be created, causing air to be drawn into the gas space, creating an explosive mixture of methane and oxygen. Furthermore, under extreme conditions, removal of carbon dioxide can cause a sufficiently strong negative pressure to collapse the structure. However, for the purposes of this example a decrease to 10% is assumed, and no other adverse consequences are experienced. This results in a pH of approximately 7.5 immediately after addition of the chemical (point 3). However, as additional carbon dioxide is produced by the biomass, the carbon dioxide content of the gas space will increase to its equilibrium

0 0

Post a comment