What occurs in the biomass layer within a leach field?

• A. Roots uptake water
• B. Bacteria in the biomass feed on the effluent, providing treatment
• C. Chemical oxidation by oxygen
• D. Sedimentation in the layer

Answer: (B)

In the biomass layer of a leach field, microbial life does the heavy lifting. Bacteria form a biofilm on the infiltration media and actively feed on the organic matter in the effluent as it percolates through the bed. This microbial metabolism breaks down contaminants, reducing the organic load (BOD) and polishing the water before it moves deeper into the soil. Oxygen availability in the deeper parts of the leach field is limited, so chemical oxidation by oxygen isn’t the primary mechanism here; the treatment mainly comes from the biological activity of the biomass. Roots taking up water can happen, but it’s not the primary function of the biomass layer, and solids tend to be handled earlier in the system rather than by sedimentation in this layer. So the key idea is that bacteria in the biomass feed on the effluent, providing treatment.