Process Theory

Lime addition to sludge reduces levels of odor-causing microorganisms and pathogens by creating a high-pH environment that is hostile to biological activity and vector attraction. Gases containing nitrogen and sulfur that are evolved during anaerobic decomposition of organic matter are the principal sources of odor in sludge. When lime is added, the microorganisms involved in this decomposition are strongly inhibited or destroyed in the highly alkaline environment. Similarly, pathogens and other microorganisms are inactivated or destroyed by lime addition.

In addition to raising the pH, lime also affects the chemical and physical characteristics of sludge. Although the complex chemical reactions between lime and sludge are not well understood to date, it is likely that complex molecules of hydrolysis, saponification, and acid neutralization split in the high-pH environment. The following equations, simplified for illustrative purposes, show the types of reactions that may occur (WEF, 1998):

Reactions with inorganic constituents:

Calcium: Ca2+ + 2HCO- + CaO ^ 2CaCO3 + H2O Phosphorus: 2PO43- + 6H+ + 3CaO ^ Ca3 (PO4 )2 + 3H2O Carbon dioxide: CO2 + CaO ^ CaCO3

Reactions with organic constituents:

Initially, lime raises the pH of sludge. Then reactions occur such as those in the equations above. If sufficient lime is not added, the pH decreases as these reactions take place. Subsequently, during sludge storage, biological activity resumes creating odor problems as well as possible regrowth of pathogenic organisms. Therefore, excess lime is required. The change in pH during storage of primary sludge using different lime dosages is illustrated in Figure 6.1. Excess lime in the range of 5 to 15% of the amount necessary for initial pH elevation is required to maintain the elevated pH because of slow reactions that continue to occur between lime and both atmospheric carbon dioxide and sludge solids.

In addition to the reactions above and raising the pH, hydration of calcium oxide generates heat. One mole (56 g) of calcium oxide generates 65 kJ of heat, as shown in the following equation:

One kilogram of chemically pure lime (100% CaO) produces 1152 kJ of heat and requires 320 g of water. The heat in Kilojoules produced by the hydration of lime, taking its activity into consideration, is a- 8

Figure 6.1 Changes in pH during storage of primary sludge using different lime dosages.

Days of Storage

Figure 6.1 Changes in pH during storage of primary sludge using different lime dosages.

where

Qr = heat produced, kJ A = lime activity, % Ml = mass lime, kg

The amount of heat in Kilojoules that is required to heat sludge by lime may be determined by the formula

Msl = mass of sludge, kg

CSL = specific heat of sludge, kJ/kg°C

AT = difference between initial temperature of sludge and final raised temperature, °C

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