Internal Gas Exchange Photobioreactor

Abstract

Two eight-tube internal gas exchange photobioreactors were deployed on an inclined platform at an ocean-front research facility in Honolulu, Hawaii. The photobioreactors consisted of 3.8-cm inside diameter, 20-m long tubes joined at the bottom to a manifold aerator and at the top to a degassing reservoir, with a total volume of 230 L and a nominal culture volume of 200 L. The eight-tube bioreactors were operated with the two outside tubes unaerated, thus establishing a recirculating flow through the bioreactors. Arthrospira (Spirulina) sp. was cultured using Zarrouk’s medium. Cultures were monitored daily by analyzing samples for dry weights and optical cell densities. An automated system recorded incident light, culture temperature, pH, and dissolved oxygen concentrations. Carbon dioxide was supplied once a day by bubbling pure CO₂ through the culture. As needed, cultures were cooled by a sprinkler system. Cultures were inoculated from laboratory stocks into a single-tube outdoor bioreactor at a density of less than 0.2 g/L, and maintained under 80% shade cloth to prevent photooxidation. Initial instantaneous growth rates ranged from 0.3 to 0.5 per day with final densities up to 2.5 g/L. An initial trial with a dilution of 30% per day under a 50% shade cloth demonstrated a productivity of about 1 g/L/day over a six-day period and an average of 0.7 g/L/day for a 19-day period. The utility of tubular photobioreactors for the development of a biophotolysis process is being explored further.