PHYSICAL PRETREATMENT OF BIOMASS - Biomass as a Sustainable Energy Source for the Future - page 240

centrifugation currently is the preferred method for harvesting due to the ease of
cleanup and sterilization.

8.2.4.2 Macroalgae Although presently it is possible to grow macroalgae in open
ponds on shore, this is still uncommon. Offshore cultivation is much more interesting
for macroalgae. Macroalgae can grow either attached to a solid or free-floating in
water. Four different systems for cultivation can be distinguished (Reith et al., 2005):

1. Longlines
2. Ladders
3. Grids
4. Rings

A combination with power-producing wind parks is a highly attractive option as this
forms a combined renewable energy supply system that requires no land.
The ladder system is a variation of the longline system in which multiple longlines
are placed on top of each other. The advantages of these systems are the low capital
costs and simple installation. Disadvantages are the mechanical damages that occur
due to rough conditions prevailing at sea and difficulties in controlling process
parameters such as temperature, nutrients, and light.
The grid system or net-style system is one in which algal spores are seeded onto
nets that are then fixed to some kind of support system. At low tide, the nets are above
the water line and are thus exposed to the atmosphere. This tidal variation enhances
growth conditions and inhibits fouling by other organisms.
The ring system is assembled and inoculated on shore after which it is transported
out to sea. The ring can be applied at depths of 5 8 m; a further refinement of the

system still has to occur. The ring is transported back onto shore for harvesting.
When the macroalgae are attached to a solid, it is necessary to cut them, whereas
free-floating algae can simply be collected. The most dominant harvesting practice is
the collection of the total biomass of each plant, while a much rarer technique is
cutting the plant above the holdfast to enable regeneration. The different techniques
involved in macroalgae harvesting are discussed in the following.
The first distinguishing factor in harvesting is that it can be done either manually or
using mechanical systems. Manual harvesting has been used since the preindustrial
age, and the equipment involved is quite limited and may consist of a diving
apparatus, knives, sacks for algae collection, and possibly boats for shore access.
Mechanical systems should ensure sustainable life and minimal damage to the
ecosystem. They can be applied using specially equipped boats. Four categories of
mechanical systems that are looked into for the collection of macroalgae are:

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1. Drag rake
2. Cutter blade
3. Suction harvester
4. Scoubidou