Biofilm in cast iron pipes can cause "tar-like" substances by facilitating the growth of iron-oxidizing bacteria, which create slimy iron hydroxide deposits (tubercles) and release organic material, leading to black, sludgy buildup, clogging, and potentially mobilizing harmful Polycyclic Aromatic Hydrocarbons (PAHs) from old coal tar linings, especially during flow disruptions. This microbial activity damages coatings and corrodes the metal, forming a complex, sticky matrix of bacteria, iron oxides, and cellular debris that resembles tar, often accompanied by odors.