A Phase I bunker is a walled concrete box with an aerated floor. A fan pushes air into a space under the floor, called the plenum, and the air rises through slots into the compost above. A timer runs the fan in cycles so the mass stays aerobic without being cooled too far.
The gain is time and control. DMR completes Phase I in a bunker in 6 to 8 days against 10 to 12 outdoors, and the whole indoor route runs 10 to 12 days. MIDH treats the Phase I bunker as one line inside a compost making unit costed at 30 lakh rupees.