12.1. Hyperconcentrated cultures
Hyperconcentrated cultures are cultures with an algal biomass
>1.5 g l1. On a small-scale, experiments with hyperconcentrated
cultures have shown that these can accelerate the removal
of nutrients compared to normal cultures. Algae for
such experiments are concentrated by flocculation and settling
using a flocculent such as chitosan (Lavoie and De la Nou¨ e,
1983; Morales et al., 1985). Cell concentrations of up to
1.9 g dry weight l1 have been obtained for Oscillatoria sp.
grown on sewage sludge (Hashimoto and Furukawa, 1989).
Working with Scenedesmus obliquus cultures have shown
that great nitrogen removal was greatly accelerated for 1.9 g
dry weight l1 cultures compared to normal density cultures
of 0.5 g dry weight l1 (Lavoie and De la Nou¨ e, 1985). They
have also demonstrated that the rate of removal of ammonium
and phosphorous in these hyperconcentrated cultures was proportional
to algal concentration and independent of the obvious
light limitation due to self-shading. Although this work
has been carried out only on a small scale so far, the use of
such hyperconcentrated cultures would require smaller pond
areas, or would permit a reduced residence time, both of which
have potential advantages. The engineering and economic
feasibility of such systems on a large-scale remains to be
determined.