Temperature

Temperature has an important effect on bacterial growth rates and, accordingly, changes the relationship between SRT and digester performance. Figure 5.3 illustrates the effect of temperature on methane production and volatile solids reduction. The figure shows that stabilization is slowed at lower temperatures, with 20°C (68°F) appearing to be the minimum temperature at

10

Bench-scale digestion of primary sludge at 96°F (35°C)

Bench-scale digestion of primary sludge at 96°F (35°C)

Solids retention time (SRT), days

Figure 5.2 Effects of SRT on the relative breakdown of degradable components and methane production. (From U.S. EPA, 1979.)

Solids retention time (SRT), days

Figure 5.2 Effects of SRT on the relative breakdown of degradable components and methane production. (From U.S. EPA, 1979.)

which sludge stabilization can be accomplished with a practical SRT. Most anaerobic digesters are designed to operate in the mesophilic temperature range 30 to 38°C (85 to 100°F), 35°C (95°F) being the most common. Some digesters are designed to operate in the thermophilic range 50 to 57°C (122 to 135°F).

It is important that a stable operating temperature be maintained in the digester. Sharp and frequent fluctuations in temperature affect the bacteria, especially methanogens. Process failure can occur at temperature changes

11.8

iE 5

Raw Sludge

11.8 g/L Degradable 6.6 g/L Nondegradable 18.4 TOTAL

Raw Sludge

11.8 g/L Degradable 6.6 g/L Nondegradable 18.4 TOTAL

Degradable Volatile Solids

11.8

iE 5

10 20 30 40

Soids Retention Time (SRT), days

Figure 5.3 Effects of temperature and SRT on methane production and volatile solids breakdown. (From U.S. EPA, 1979.)

Degradable Volatile Solids

10 20 30 40

Soids Retention Time (SRT), days

Figure 5.3 Effects of temperature and SRT on methane production and volatile solids breakdown. (From U.S. EPA, 1979.)

greater than 1°C/d. Changes in digester temperature greater than 0.6°C/d should be avoided.

5.2.3 pH and Alkalinity

Methane-producing bacteria are extremely sensitive to pH. Optimum pH for methane formers is in the range 6.8 to 7.2. Volatile acids produced in the acid-forming phase tend to reduce the pH. The reduction is normally countered by methane formers, which also produce alkalinity in the form of carbon dioxide, ammonia, and bicarbonate.

Bicarbonate Alkalinity as CaC03, mg/L

Figure 5.4 Relationship between pH and bicarbonate concentration at 35°C. (From U.S. EPA, 1979.)

Bicarbonate Alkalinity as CaC03, mg/L

Figure 5.4 Relationship between pH and bicarbonate concentration at 35°C. (From U.S. EPA, 1979.)

In the anaerobic digestion process, the carbon dioxide-bicarbonate relationship is very important. Figure 5.4 shows that the system pH is controlled by the carbon dioxide concentration of the gas phase and the bicarbonate alkalinity of the liquid phase. If the proportion of CO2 in the gas phase remains the same, addition of bicarbonate alkalinity will increase digester pH.

The best way to increase pH and buffering capacity in a digester is by the addition of sodium bicarbonate. Lime will also increase bicarbonate alkalinity but may react with bicarbonate to form insoluble calcium carbonate, which promotes scale formation.

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