Effects of protozoa on carbon mineralization in activated sludge

Positive effects of protozoa on carbon mineralization by bacteria are considered here specifically in the context of waste-water treatment. The following effects are reviewed: (1) changes in the C:N:P ratio of the substrate; (2) excretion of growth factors; and (3) grazing. Depending on the composition of the substrate, grazing can stimulate biological breakdown of carbon. Substrate with a high carbon-nutrient ratio permits mineralization increase due to grazing, while mineralization is not enhanced when the substrate has a low carbon-nutrient ratio. The effects of growth factors and grazing depend on several processes. The availability of bacteria for suspension feeders might be under-estimated. A study concerning the exchange between bacteria in the suspended liquor and bacteria growing in a floc or biofilm is highlighted.