Abstract: A biodegradable substrate in sheet form which acts as a source of carbon and hydrogen in oxygen-poor water in biological water-treatment stages. The substrate is covered with at least one layer of a population of spontaneously growing denitrifiers. The substrate is a spun-bonded non-woven fabric weighing 10 to 1500 g/m.sup.2. It has a prescribed effective area that results in an ideal rate of denitrification per clarification stage at a prescribed rate of flow per area and time. The fabric is comprised of at least 50% continuous poly-.epsilon.-caprolactone filament with a mean molecular weight of 20,000 to 70,000. The individual filaments adhere to each other at their intersections.