The material of the article considers the production of cellulose from broadleaf cattail by the nitrate method as one of the ways to process spent cattail after a cycle of filtration of water bodies in order to provide a potential effective natural biofilter in an economical way of disposal. Morphological differences of plants in composition, structure, packing density, conformation of molecules do not allow equally effective application of the same methods of material processing, which requires adjustment of the process conditions. For the nitric acid method, the process conditions were optimized in terms of the time parameters of soaking in solutions of nitric acid and alkali for its application to broadleaf cattail with the highest yield of the target product. The isolated cellulose by the nitric acid method satisfies the basic requirements for the content of α-cellulose and residual lignin to obtain a wide range of further products, including the synthesis of cellulose nitrates for the defense industry with the possibility of selecting processing conditions to obtain the required composition. Also, broadleaf cattail proved to be a relatively capacious source of holocellulose. The paper presents the mass fraction of holocellulose in samples subjected to different processing time parameters, the content of α-cellulose and residual lignin in holocellulose from broadleaf cattail.