. Advantage of WET digestion is Mixing mainly with gas injection Simple process design No heating, insulation only against soil All of the above

Analyzing the Options

1. Mixing mainly with gas injection: Wet digestion processes often utilize gas injection as an effective means of mixing, which can enhance the efficiency of the digestion process.

2. Simple process design: Wet digestion tends to have a simpler process design compared to other digestion methods. The equipment and setup required are usually less complex, which can reduce costs and improve operational efficiency.

3. No heating, insulation only against soil: Wet digestion does not typically require extensive heating, as it often relies on the natural microbial activity present in the substrate. Insulation against soil can be relatively straightforward and does not require elaborate systems.

4. All of the above: Considering that each of the above statements offers a unique advantage, this option suggests that all the benefits discussed contribute to the overall advantages of wet digestion.

Final Answer

The most comprehensive answer is All of the above. Each statement highlights a significant advantage of wet digestion processes, demonstrating that they are effective, simple to implement, and do not require complex heating systems.