Sisal/Agave - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 340

[ 42 ]. Rapid recent advances in breeding and engineering more efficient and tolerant
industrial strains of yeast for other sugar fermentations, enabled by genomics,
directed evolution and metabolic engineering suggest that large gains in efficiency
are likely in the future [ 43 ].

Residue Processing

There are several uses for byproducts from Agave conversion to ethanol. The
bagasse materials, referenced in the potential hydrolysis and fermentation processes
above, are a product of distillation after the processing of the Agave pi˜a. The solids
that are left after processing the pi˜a could be converted to ethanol using the
delignification process previously described or it can be used in combustion for
co-powering the conversion facilities. The solid wastes from the cellulosic conver-
sion can then be returned to the agricultural fields as a fertilizer. There are also
liquid wastes, called vinasse, that are toxic but as in the case of sugarcane ethanol
vinasse can be made useable by wastewater treatment. Such treatment can include
methanogenesis to yield methane [ 44 ] that could be readily used to provide
combined heat and power (CHP) for the processing plant.
Vinasse from the processing of pi˜as is often returned to fields without treat-
ment, which poses a risk of contamination to groundwater. Vinasse is very acidic
(pH of 3.35) with high biological and chemical oxygen demands (BOD and COD),
suspended solids, and volatile compounds [ 45 ]. Methanogenesis provides an ideal
way of recovering energy from this organic waste slurry since as an insoluble gas, it
requires no energy loss in dewatering. Simultaneously, it lowers the BOD and
COD, which are the main causes of toxicity when the slurry is released to water
courses or spread onto the land. However, use of plant residues in general from
ethanol production is relatively new and has only recently attracted more intensive
scientific analysis. A major limitation is that the methanogens can only produce
methane from just two substrates, CO 2 and acetate. They therefore require other
microbes to digest the material to these substrates and provide the energy sources to
drive reduction to methane. Efficiency of methane production could be improved by
understanding how to optimize the community of organisms needed for conversion
and by engineering the methanogens so that

they may interact with more

substrates [ 46 ].
A stable digestion system that converts Agave vinasse into methane has been
demonstrated, although it is not typically used. In a 6 l experimental digester,
Mendez-Acosta et al. [ 45 ] were able to produce 14 l d 1 of biogas comprising
65 %methane under stable reaction conditions. This process is scalable and has low
energy inputs because reactor temperature is maintained at 35 C. Digestion reactor
conditions are detailed in Table 15.4 [ 45 ]. An acclimation period of 50 days was
used to initiate this process, although it is likely that acclimation can be achieved in
a shorter amount of time. The acclimation period is followed by a start-up phase
with controlled dilutions of vinasse; higher concentrations of vinasse are gradually