Sisal/Agave - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 341

Table 15.4 Digestion conditions for a reactor that treats vinasse and produces methane a
byproduct
Stage Conditions Products

Acclimation 3:7 dilution of vinasse to water Acclimation achieved when chemical oxygen

demand is reduced and 50 % methane is

>

pH 7.4

generated

Dilution
tank

pH 6.5-7.0 Mixture of vinasse and water with concentra-

tion gradually increased of time for start-up
phase

Start-up 40 % volume from acclimated

anaerobic sludge

80 % of chemical oxygen demand removed

3:2 ratio of total suspended
solids to volatile suspended
solids

Alkalinity achieved

N additions to achieve anaero-
bic conditions

Accumulation of volatile fatty acids reduced

pH 7.4
35 C

Processing Conditions in start-up stage

stabilized

Alkaline solution

Raw vinasse inputs Volatile fatty acids eliminated
Consistent supply of CH 4
Reprinted from Ref. [ 45 ]. With permission from Springer Verlag

added until raw vinasse can be added without dilution in water. The result is a
constant supply of methane that is produced from an otherwise problematic waste.
Following methods used for sugarcane ethanol, treated vinasse could then be
returned to the field with a much reduced BOD, yet retaining the nutrients
(including N, P and K) that were present in the harvested biomass [ 47 ]. Continuous
fermentation to methane would in theory be more efficient and require smaller
fermentors, but has to operate without contamination by unwanted microbes.
However, progress has been made in optimizing semicontinuous fermentation of
Agave at a pilot scale [ 48 ].

Market Challenges/Barriers to Commercialization

A major obstacle to large-scale production of Agave spp. for bioenergy is the capital
cost of manual labor to manage and harvest the crop [ 23 ]. The harvesting of the
plants requires a practiced hand to wield a “coa de jima,” a sickle-like instrument,
cutting individual leaves. Harvesting of offsets, weeding, and pest control also incur
substantial manual labor costs. Together with other production costs, the intensive
labor requirements amount to a greater cost (per unit volume) to produce ethanol
from Agave feedstock in Mexico than that of corn ethanol in the USA or sugarcane
ethanol in Brazil [ 23 ]. For fiber Agave crops, a few leaves are removed from each