For the most part, nutrient changes in recirculation systems are transformations or
relocations of nutrients that still have to be dealt with in terms of disposal. Possible
exceptions might be nitrogen, when denitrification is included as part of a system, and
organic matter, which is partially decomposed in biofilters or in solids removal systems in
which solids are at least partially digested. Formulation of feeds that result in reduced
dissolved P excretion (Skonberg et al., 1997; Bureau and Cho, 1999; Sugiura et al., 1999)
suggests that rapid solids removal may be an effective method of capturing and separating
substantial proportions of the P released into the water. This contrasts with the situation
that is faced with N, where release occurs primarily in the dissolved form (ammonia and
urea), requiring the more difficult and expensive treatment alternatives of nitrification and
denitrification.