This may be economical for short missions of spacecraft, but it is not the most viable solution when dealing with the life support systems of a long-term craft (such as a generation ship) or a settlement.
The aim of CELSS is to create a regenerative environment that can support and maintain human life via agricultural means.
The drawback to this method lies in the fact that upon depletion the tanks would have to be refilled; the scrubbers would also require replacement after they become ineffective.
[citation needed] Early space-flight had travelers either ejecting their wastes into space or storing it for a return trip.
CELSS studied means of breaking down human wastes and, if possible, integrating the processed products back into the ecology.
But if the otherwise closed system is allowed to accept high-temperature radiant energy from an external source (e.g., sunlight) and to reject low-temperature waste heat to deep space, it can continue indefinitely.