The invention provides inoculants for the generation of a biologically active layer in a water treatment or purification device. In some embodiments, the invention provides inoculants for a biologically active zone in a wastewater treatment system or device, for example in municipal wastewater and sewage treatment. In other aspects, the invention provides inoculants for water purification, for example in municipal drinking water purification or in slow sand filtration. Inoculants of the invention increase the effectiveness of the above systems and devices in providing useable water.