Phytotechnology and Photosynthesis 167Figure 7.6 Schematic ACSACS
was passed into the bottom of a bioreactor vessel, containing the plastic filter
elements in water, and allowed to bubble up to the surface through the algal
strands as shown in Figure 7.6.
Again, this approach to carbon sequestration was based on enhanced intra-
reactor photosynthesis, the excess algal biomass being harvested to ensure the
ongoing viability of the system, with the intention of linking it into a compost-
ing operation to achieve the long-term carbon lock-up desired. The ACSACS
though performing well at both bench and small pilot scale, never attained indus-
trial adoption though remaining an interesting possible adjunct to the increasing
demand for methane flaring or utilisation at landfills.
A similar idea emerged again recently, with a system being developed by
Ohio University, which, in a perfect example of selecting an organism from an
extreme environment to match the demands of a particular manmade situation,
utilises thermophilic algae from hot springs in Yellowstone National Park. In
this process, which has received a $1 million grant from the US Department of
Energy, smoke from power stations is diverted through water to permit some
of the CO 2 to be absorbed and the hot, carbonated water produced then flows
through an algal filter formed on vertical nylon screens.
This design, which is essentially similar to the earlier ACSACS, enables the
largest possible algal population per unit volume to be packed into the filter
unit, though like the previously described HRAP, light is a limiting factor, since
direct sunlight will only penetrate through a few feet of such an arrangement.
However, it is claimed that these carbon biofilters could remove up to 20% of
the carbon dioxide, which would, of course, otherwise be released to atmosphere.
This makes solving the problem something of a priority. One solution involves
the use of a centralised light collector, connected to a series of fibre-optic cables