4. Discussion
According to the analytical results, despite the fact that
almost all vegetable by-products are currently stored for several
days in containers outdoors without any refrigeration, almost all
undesirable substances are present below the maximum permitted
limits. However, as high nitrite levels are found in coffee and wine
by-products, nitrite levels in these by-products must be monitored.
Despite this, it can be concluded that it is feasible to use these vegetable
by-products as raw materials for animal feed provided that
they are present below the maximum permitted limit.
In addition, it has been concluded, together with the EPEA Technical
Committee, that there are no important differences in the
composition of the different by-products, thus meaning that they
can be managed as a single by-product, although their high content
as regards the recommended percentage of the most important
nutritional parameters, such as protein, limits their inclusion infeed formulations to between 3% and 6%. Moreover, in general, the
water content of the vegetable waste studied herein is too high for
inclusion in feed formulations without prior treatment; therefore a
drying process must be developed to reduce this variable.
With regard to the drying process, all the different vegetable
meal prototypes produced in this study fulfil the nutritional
requirements of animal feed manufacturing companies and legal
requirements with respect to undesirable substances. As such,
and regardless of economic aspects, all technologies tested are useful
for drying vegetable waste for subsequent use as a raw material
for animal feed.
The action plan proposed was technically validated by conducting
a pilot-scale demonstration trial, thus allowing us to conclude
that, from a technical point of view, it is possible to recover
vegetable by-products.
According to the environmental improvement assessment,
landfill is the option with the greatest impact from a climate
change and aquatic eutrophication point of view due to emissions
to air and water from the decomposition of bio waste, which is in
accordance with previous studies: Cherubini, Bargigli, & Ulgiati,
2009 or Güereca, Gassó, Baldasano, & Jiménez-Guerrero, 2006.
These emissions are abundant in greenhouse gases such as
methane and some nitrogen compounds, which have a greenhouse
potential 24 times higher than that of CO2. Furthermore, the alternative
of anaerobic digestion has the highest impact on human
toxicity as our analysis showed that the release of metals from
the sludge resulting from digestion is a major problem for human
toxicity. Finally, incineration has a marked impact on water depletion
due to the need to use water to stabilize the ash after burning.
Even when considering the avoided products, the recovery of byproducts
such as vegetable meals has a high impact on abiotic
resource depletion due to the high energy demands of the drying
process. In this regard, it should be noted that this impact likely
reflects the dependence on fossil resources in the Spanish electricity
mix. Furthermore, the data entered into the tool are not likely to
be representative of a large-scale operating environment since
they are obtained from tests on a pilot scale. As such, this value
could be reduced significantly if drying data that take efficiency
ratios into account are used. Moreover, as pointed out by
Bernstad and Jansen (2012) there are a lot of variables (regional
and global) involved in the food waste management system and
it is difficult to establish a comparison between different studies
and management options since the guidelines for the correct
measurements are still under development.