Harvesting of Microalgal Biomass - Biotechnological Applications of Microalgae - page 85

less energy-intensive harvesting technologies to make microalgae cultivation cost
effective and competitive for renewable bioenergy production. Thus far, no single
harvesting technique can be universally applied to algae cultivation systems, and
a combination of different techniques could be applied in a specific sequence to
achieve maximum biomass concentration with minimum energy usage. Moreover,
there could be considerable costs and energy savings in custom-designed, multi-
stage harvesting techniques for algal farms, in which a variety of harvesting
techniques are arranged in a specific sequence based on culture chemistry, and the
specific characteristics of each technique and its energy requirements. Such systems
can achieve dewatering of pond water to either 5%, or 10% to 20% solids at the least
energy input and cost. In an open pond system, dominant algal species could range
from small unicellular to large colonial or filamentous species. In such cases, TFF
and other filtration techniques could be used as the first stage to remove filamentous
and auto-flocculated algae, followed by chemical flocculation, sedimentation, and/
or flotation to produce algal slurries with 1% to 5% solids, which could be either
directly subject to the AD process if biogas is the final biofuel or subject to cen-
trifugation to achieve >20% solids. Centrifuging 5% algal slurry would reduce the
energy and cost requirements for this technique by 100 times, as opposed to direct
centrifugation of otherwise very dilute algae culture (~0.05% solids). Similarly, the
pond water temperature, alkalinity, and pH may also vary during different climatic
conditions throughout the year and even during different times of the day, thus
impacting the ionic strength, salts solubility, and eventually the biomass auto-
flocculation properties. Auto-flocculation could be the lowest-cost, lowest-energy
dewatering process by far, at one tenth those of membrane filtration and polymer-
based dewatering.
6.10 CONCLUSION
There are several biomass harvesting techniques available for the recovery of algae
from culture broth. However, no individual technique can be applied ubiquitously
due to technical and economical limitations. Gravity sedimentation is relatively less
energy intensive but the slow sedimentation rates of algae may negatively impact
production economics. Centrifugation is very efficient, but the large energy require-
ments for the centrifuge clearly eliminate it as an option for direct harvesting of a
low-value energy crop. However, if used as a second-stage process for harvesting
5% algal slurries to higher solids concentrations, this could significantly reduce the
energy and cost requirements. Membrane filtration is an efficient harvesting option;
however, field experience on algae farms would be required to verify the lifetime and
maintenance costs of filter elements. Dissolved air flotation would be an expensive
option in terms of cost and energy. Polymer flocculation is also efficient but energy
intensive for dewatering, while electro-flocculation is cost effective with low energy
usage. Auto-flocculation is the lowest cost, lowest energy dewatering process by far.
It is recommended to apply custom-designed multi-stage harvesting techniques for
algal harvesting in which a variety of harvesting technologies are organized in some
sequence to achieve the highest efficiency and lowest cost.