Anaerobic Digestion

The two types of anaerobic digestion processes are mesophilic and thermo-philic. Mesophilic processes occur in the temperature range 32 to 35°C, whereas thermophilic process requires a temperature range of 50 to 55°C. Figure 9.1 shows the technological scheme for both mesophilic and thermo-philic digestion in an activated sludge treatment plant.

Let us examine the energy required for each process by assuming the following:

Wastewater Sludge Processing, By Izrail S. Turovskiy and P. K. Mathai Copyright © 2006 John Wiley & Sons, Inc.

Figure 9.1 Technical scheme of anaerobic digestion.

• The quantity of gas obtained during digestion is approximately 1.0 m3 for every 1.0 kg of volatile solids (VS) reduced.

• The ratio of raw sludge from primary clarifiers to the thickened waste activated sludge from secondary clarifiers, as calculated by the mass of dry solids, is 1 : 1.

• The solids content in the primary-secondary sludge mixture is 4%.

• Of the 4% solids, 70% is VS, resulting in 2.8% VS as a percentage of the mass of dry solids (70% x 4%).

• The disintegration of VS in the mesophilic process is 40%, and in the thermophilic process it is 50%.

One cubic meter of the primary-secondary sludge mixture contains 40 kg of dry solids (1.0 m3 x 1000 kg/m3 x 4%), and therefore 28 kg (40 kg x 70%) VS. The quantity of gas obtained from the digestion of this 1 m3 of sludge will be 28 x 40% = 11.2 m3 for the mesophilic process and 28 x 50% = 14.0 m3 for the thermophilic process. The gas produced from digestion can be expected to be 60 to 70% methane, 16 to 34% carbon monoxide, and 0.4 to 6% nitrogen, hydrogen, and oxygen. The heat of combustion of the gas will average around 21 million joules/m3 (MJ/m3).

For digestion to occur, it is necessary to provide heat as follows:

mesophilic process: (1.1)(5.67)(33 -16) = 106 MJ/m3 of sludge thermophilic process: (1.1)(5.67)(53 -16) = 230Mj/m3 of sludge where

I.1 = coefficient that takes into account heat loss

5.67 = quantity of heat (in MJ) required to raise the temperature of 1.0 m3 of sludge with steam by 1°C 33 and 53 = temperatures (in °C) of mesophilic and thermophilic processes, respectively 16 = temperature (in °C) of the original sludge mixture prior to digestion

The excess heat released by the combustion of gas produced during the digestion of 1 m3 of sludge is as follows:

mesophilic process: (11.2)(21)-106 = 129Mj/m3 of sludge thermophilic process: (14.0)(21)- 230 = 64Mj/m3 of sludge where

II.2 and 14.0 = quantity of gas (in m3) from the digestion of 1.0 m3 of sludge in the mesophilic and thermophilic processes, respectively

21 = heat of combustion of 1.0 m3 of gas (in MJ/m3)

106 and 230 = energy required to heat 1.0 m3 of sludge (in MJ) for the mesophilic and thermophilic processes, respectively (see above)

Since the digestion of sludge organics creates gas, there is a corresponding decrease in the amount of sludge organics remaining following digestion. The 28 kg of sludge organics (volatile solids) will be decreased by 11.2 kg, to 16.8 kg (28 - 11.2) in the mesophilic process. In the thermophilic process, the quantity of sludge organics will be decreased by 14.0 kg, to 14.0 kg (28 - 14.0).

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