Environmental Engineering Reference
In-Depth Information
alternatives like combustion, composting or animal feed production to
recover this value. If aerial biomass is exported from the system for animal
feed production, for example, these nutrients need to be compensated for by
the use of additional artificial fertilizer, which implies extra costs that need
to be covered by extra income from animal feed sales. If residues are
composted, only a fraction of the nitrogen is recovered in the final product
(35%). Similarly, if by-products are left in the field to decompose, nutrients
are only partially incorporated in the soil for the next cropping season.
When combustion is performed, nitrogen is lost but phosphorus and
potassium can be partially recovered in the ashes depending on the
temperature of the operation used. The advantage of AD to recover
nutrients from industrial effluents is especially evident in the case of
sugarcane, whereas in the case of oil palm and cassava, the flows from the
digestion of aerial biomass are especially important.
The recovery of water via AD represents not only an advantage but a
necessity, given the organic load in effluents from biofuel production and
the water consumption of crops and industrial processes. Digestion of the
whole crop represents very important water savings since digestion can be
performed at high solids content, in contrast to the ethanol and biodiesel
industries which require large amounts of water. For the studied systems,
advantages in terms of water savings from current biofuel producing
systems to full AD biomass conversion fluctuate between 1292 and
8789 kton yr
1
for oil palm and sugarcane respectively, i.e. cascade D as
compared with cascade C. These values could be expressed in energy terms if
considering the energy used in producing fresh water for the purposes of
irrigation in the studied systems and added to the overall energy balance.
This extra step was not performed in this specific exercise.
From the presented analysis it is clear that AD of by-products from the
Colombian biofuel industry can provide substantial land savings as well as
significant advantages in terms of water and nutrient recovery. It has also
been shown that AD of the full crop can provide similar net energy output
as systems producing biofuels and digesting residues, although the quality of
the different outputs is different in terms of energy density. Some of the
current biofuel systems seem inefficient from the perspective of energy use if
considering by-products are exported from the system. However, the
ultimate desirability of implementing AD systems to add value to by-
products depends on the alternative uses they are actually receiving and the
market trends. Since, in Colombia, the infrastructure for delivering natural
gas exists for both vehicular and the domestic/industrial market, the
feasibility of promoting biogas as an energy alternative will depend on its
economic competitiveness with other energy carriers within those markets.
Furthermore, the incentives given for the extra benefits provided by the
technology (i.e. nutrient provision and water use reductions) can be of
Search WWH ::
Custom Search