Sustainable, -adj. (following definitions for verb "sustain"

1. to support, hold, or bear up from below; bear the weight of, as a structure.
2. to bear (a burden, charge, etc.).
3. to undergo, experience, or suffer (injury, loss, etc.); endure without giving way or yielding.
4. to keep (a person, the mind, the spirits, etc.) from giving way, as under trial or affliction.
5. to keep up or keep going, as an action or process: to sustain a conversation.
6. to supply with food, drink, and other necessities of life.
7. to provide for (an institution or the like) by furnishing means or funds.

* Excerpted from the Random House Webster's Unabridged Dictionary 3.0.

Wordsmyth adds:
2. to keep in existence.
Example: Constant effort sustained the project.