The metabolic weight of nearly a thousand people had become a physical burden on Ashfall's lower strata. Until now, biological waste had been managed through simple limestone sumps and localized composting for the aquaculture vats. But at this scale, the accumulation was becoming a source of toxic seepage and a drain on the mountain's nutrient efficiency. Kael knew that for the population to continue its steady climb, the barony had to stop disposing of its waste and start processing it as a strategic asset. He initiated the construction of the Digester—a Tier 11 industrial complex designed to close the ecological loop.
The technical core of the system was the Anaerobic Reactor. Kael engineered a series of massive, airtight iron cylinders buried in the warmest part of the mountain, near the geothermal bore. All biological waste—from the Green Ring's trimmings to the residential sewage—was channeled into these cylinders. Inside, Kael introduced a specific culture of methanogenic bacteria harvested from the deep northern bogs. In the absence of oxygen and the presence of geothermal heat, these bacteria broke down the waste into two vital products: methane gas and high-purity liquid fertilizer.
"We aren't just cleaning the mountain, Elms," Kael said, monitoring the pressure gauges on the gas-collection dome. "We are mining the city itself. This methane will replace a third of the peat we used to burn in the foundry, and the 'Digestate' will double the yield of our grain terraces. It's the ultimate economy—nothing leaves the stone."
The grit of the construction was the plumbing. To connect the entire mountain to the Digester, the Tier 0 crews and the newest Aspirants had to lay miles of gravity-fed ceramic pipes beneath the existing residential floors. This required "Precision Trenching"—carving narrow channels into the limestone without disturbing the steam-lines or the acoustic pipes. The air in the lower construction zones was thick with the smell of wet clay and the sharp, vinegar-like tang of the early fermentation process.
Socially, the Digester was the least glamorous but most essential project in Ashfall. Kael created the "Resource Recovery Tier," a group of laborers responsible for the sorting and "Feeding" of the reactor. To maintain the health of the bacteria, the input had to be balanced; too much citrus waste or too much antiseptic from the Bio-Foundry could "kill" the tank. These workers became the mountain's first environmental engineers, monitoring the pH levels of the slurry with litmus-strips made from fermented bog-berry juice.
A technical failure occurred during the first "Gas-Tap." A small amount of oxygen had leaked into the primary collection dome during the final sealing, creating a volatile mixture. When the first burner in the foundry was lit, a "Flash-Back" traveled through the pipe, causing a localized explosion in the Digester's vent-stack. The heavy iron dome buckled, and for an hour, raw methane leaked into the Tier 11 corridors.
Kael utilized the "Hydraulic Seal" bypass. He ordered the pumps to flood the vent-stack with water, creating a liquid barrier that blocked the gas flow and prevented further explosions. He then redesigned the system with a "Flame-Arrestor"—a series of fine copper meshes in the gas line that would dissipate heat and stop a flame from traveling backward into the tank.
The engineering of the Digester allowed the mountain to become truly self-sufficient. As the first blue flames of methane began to roar in the foundry furnaces and the grain in the Green Ring deepened in color from the new fertilizer, the citizens realized that their very presence was part of the mountain's power.
The circle is closed, Elms, Kael said, looking at the steady pressure in the methane tanks. We can feed more people because we can process more waste. We are no longer consuming our environment; we are becoming it.
But as the population reached the threshold of a true city, the "Information Citizens" reported a new kind of signal on the Seismic Mirror. It wasn't the rhythmic march of a legion or the clink of a shovel. It was a faint, high-frequency "Ping" that occurred at sunset every day, originating from the northern peaks.
Kael looked at the white-washed wall. "Vane hasn't left. He's shifted his strategy. He's not trying to dig in anymore; he's trying to 'Signal' something out. He's using an optical flash-code from the heights. He's calling for an Imperial Surveyor."