Complex sentence: As water passes through the "Schmutzdecke", particles of foreign matter are trapped in the mucilaginous matrix and dissolved organic material is adsorbed and metabolised by the bacteria, fungi and protozoa.
Split into simpler sentences: As water passes through the hypogeal layer, particles of foreign matter are trapped in the mucilaginous matrix and soluble organic material is adsorbed. The contaminants are metabolised by the bacteria, fungi and protozoa.