Domain: Biochemical Engineering / Life Support Systems (LSS)
Persona: Senior Life Support Systems Engineer (Aerospace & Sub-Surface Environments)
Step 2: Summarize (Strict Objectivity)
Abstract:
This experimental report documents the performance of a 200-gallon algal photobioreactor (PBR) system designed to serve as a biological carbon dioxide (CO2) scrubber and oxygen (O2) generator for human life support in a sealed environment. The study investigates the variables affecting gas exchange efficiency, including biomass density, metabolite accumulation, pH stabilization, and light intensity. Initial tests utilizing Chlorella vulgaris demonstrated significant CO2 reduction capabilities compared to water-only controls; however, the system faced challenges with metabolic load equilibrium and culture stability. Final phase testing achieved a maximum habitation duration of 7 hours and 5 minutes before exceeding safety thresholds (10,000 ppm CO2), suggesting that while biological LSS is viable, the current volumetric scale requires a 200%–300% increase to achieve indefinite atmospheric steady-state for a single human subject.
Experimental Analysis and Key Takeaways:
0:00 Photobioreactor (PBR) Configuration: The system consists of four custom-built PBRs containing 200 gallons of algae. The primary objective is to maintain a breathable atmosphere by utilizing photosynthesis to convert CO2 into O2.
1:43 Biomass Optimization and Metabolites: High-density cultures reach a growth plateau where secondary metabolites (waste products) inhibit further expansion. Optimization requires "restarting" the culture to ensure nutrient balance and prevent overcrowding.
3:51 Water Absorption Control Test: A control experiment was conducted to determine if the water volume acted as a CO2 sink. Results indicated that 200 gallons of plain water provided negligible CO2 scrubbing, confirming that observed gas exchange is primarily driven by algal biological activity.
4:46 Infrastructure Upgrades: To increase photosynthetic efficiency, light output was doubled to 400 watts. Gas exchange surface area was maximized using Basswood diffusers, which utilize fine-grain wood structures to produce micro-bubbles for superior CO2-to-liquid transfer.
6:55 Bacterial Respiration Risk: Maintaining culture sterility is critical. Bacterial contamination can lead to competitive respiration, where bacteria consume O2 and release CO2, counteracting the algae’s LSS function.
12:55 Initial Chlorella Test Failures: The first major test reached 10,000 ppm CO2 within 5 hours. The CO2 rise was linear, indicating the PBRs were under-performing or being overwhelmed by the subject's metabolic output.
17:28 Culture Health and Outgassing: Observations suggested the algae might have been respiring (releasing CO2) or dying due to heat stress from pump equipment, emphasizing the need for active thermal management in biological LSS.
23:17 pH and Acidification Variables: CO2 absorption lowers water pH, potentially making the environment too acidic for optimal algal performance. Digital pH monitoring and the addition of buffer solutions were implemented to maintain a target pH of 7.5.
31:43 Species Transition Challenges: An attempt to pivot to Spirulina failed due to culture dilution issues and contamination during a 2-liter-to-200-gallon scale-up, highlighting the fragility of large-scale monocultures.
34:31 Final Optimized Test Results: Using a refreshed Chlorella culture and stabilized pH, the CO2 accumulation rate was significantly reduced to ~4–5 ppm per minute initially. The time to reach 5,000 ppm was extended to 4 hours and 45 minutes, more than double the duration of the water control.
40:30 Oxygen vs. CO2 Dissociation: In final testing, O2 depletion and CO2 accumulation occurred at nearly identical rates relative to safety limits, reaching 19.5% O2 and 10,000 ppm CO2 simultaneously.
42:01 Scale Requirements for Equilibrium: The study concluded with a final record of 7 hours and 5 minutes. Data projections suggest that 400 to 600 gallons of algae would be required to achieve a net-zero atmospheric exchange for one human subject.