In what way does the biomat form over time in a soil absorption system?

The biomat forms over time in a soil absorption system primarily through bacterial growth. This process involves the establishment of a community of microorganisms that develop in the upper layers of the soil where wastewater is treated. As the effluent moves through the soil, bacteria naturally present in the environment decompose organic matter and other contaminants in the wastewater, leading to the formation of a "biomat."

This biomat is essentially a layer of organic material, bacteria, and other microbial life that creates a barrier to flow while simultaneously providing active treatment for the incoming effluent. It plays a crucial role in the filtering and breaking down of pollutants within the wastewater, therefore facilitating the natural purification process that occurs within the soil.

While both sedimentation and chemical reactions can play a role in the overall functioning of a soil absorption system, the primary mechanism directly involved in biomat formation is the activity and growth of bacteria. Evaporation is not a factor in biomat formation, as it pertains more to the loss of water rather than the biological development essential to the formation of this layer.