text: The biofilm that develops in a trickling filter may become several millimetres thick and is typically a gelatinous matrix that may contain many species of bacteria, ciliates and amoeboid protozoa, annelids, round worms, insect larvae, other microfauna. (If annelids are abundant, the filter may be considered a  vermifilter.) This is very different from many other biofilms, which may be less than 1 mm thick. Within the biofilm, both aerobic and anaerobic zones can exist supporting both oxidative and reductive biological processes. At certain times of year, especially in the spring, rapid growth of organisms in the film may cause the film to be too thick and it may slough off in patches leading to the "spring slough".