2 Estimated Energy Balances
The set of relatively well-documented peer-reviewed estimates of energy balances
for biodiesel derived from microalgae grown in open ponds is summarized in
Table 1.
This table shows that in none of the studies considered here estimated energy
balances met the criterion of a factor 5–8. As furthermore can be seen in Table 1,
the variability of published energy balances for microalgal biodiesel is large (also:
Quinn et al. 2014). In the following, several important causes of this variability are
discussed. These regard assumptions about biomass yield and allocation and the
combinations with wastewater treatment and power plants fueled by carbonaceous
substances. Thereafter, the handling of uncertainties is considered.
3 Combining Microalgal Biodiesel Production
with Wastewater Treatment
In studies considered here, relatively good (high) energy balances tend to be
obtained when wastewater containing microalgal nutrients is used for cultivation.
So far, the use of wastewater for microalgae cultivation has only been subject to
small-scale investigations (Laurens et al. 2017). In the life cycle assessments
considered here, the use of wastewater containing nutrients has been handled in a
variety of ways. Firstly, it can be assumed that the cultivation of microalgae substitutes for conventional wastewater treatment. Based on this assumption, the input
energy for conventional wastewater treatment is then subtracted from the
cradle-to-gate microalgal biodiesel production (e.g., Clarens et al. 2010; Mu et al.
2014; Chowdhury and Franchetti 2017). Such subtraction does not take account of
the limitations to algal purification of wastewater. For instance, wastewater tends to
contain relatively large organic molecules, which cannot be metabolized by currently applied microalgae (Perez-Garcia et al. 2011). Furthermore, wastewater may
well contain pathogens, predatory zooplankton, and microorganisms that might
outcompete microalgae (Pittman et al. 2011; Cai et al. 2013). For this reason,
wastewater may need treatment (e.g., sterilization) to minimize infection risk.
Energy needed for such treatment has not been included in the life cycle assessments considered here.
In addition, or alternatively, to energy credits associated with the substitution of
conventional wastewater treatment, it can be assumed that the nutrients in
wastewater substitute for the inputs of synthetic nutrients and that the energy input
of these nutrients present in wastewater can be valued at zero. It can be assumed too
that the treatment of wastewater from microalgal biodiesel production is not within
the system boundaries of life cycle assessment. Exceptions regarding the latter
174
L. Reijnders
The set of relatively well-documented peer-reviewed estimates of energy balances
for biodiesel derived from microalgae grown in open ponds is summarized in
Table 1.
This table shows that in none of the studies considered here estimated energy
balances met the criterion of a factor 5–8. As furthermore can be seen in Table 1,
the variability of published energy balances for microalgal biodiesel is large (also:
Quinn et al. 2014). In the following, several important causes of this variability are
discussed. These regard assumptions about biomass yield and allocation and the
combinations with wastewater treatment and power plants fueled by carbonaceous
substances. Thereafter, the handling of uncertainties is considered.
3 Combining Microalgal Biodiesel Production
with Wastewater Treatment
In studies considered here, relatively good (high) energy balances tend to be
obtained when wastewater containing microalgal nutrients is used for cultivation.
So far, the use of wastewater for microalgae cultivation has only been subject to
small-scale investigations (Laurens et al. 2017). In the life cycle assessments
considered here, the use of wastewater containing nutrients has been handled in a
variety of ways. Firstly, it can be assumed that the cultivation of microalgae substitutes for conventional wastewater treatment. Based on this assumption, the input
energy for conventional wastewater treatment is then subtracted from the
cradle-to-gate microalgal biodiesel production (e.g., Clarens et al. 2010; Mu et al.
2014; Chowdhury and Franchetti 2017). Such subtraction does not take account of
the limitations to algal purification of wastewater. For instance, wastewater tends to
contain relatively large organic molecules, which cannot be metabolized by currently applied microalgae (Perez-Garcia et al. 2011). Furthermore, wastewater may
well contain pathogens, predatory zooplankton, and microorganisms that might
outcompete microalgae (Pittman et al. 2011; Cai et al. 2013). For this reason,
wastewater may need treatment (e.g., sterilization) to minimize infection risk.
Energy needed for such treatment has not been included in the life cycle assessments considered here.
In addition, or alternatively, to energy credits associated with the substitution of
conventional wastewater treatment, it can be assumed that the nutrients in
wastewater substitute for the inputs of synthetic nutrients and that the energy input
of these nutrients present in wastewater can be valued at zero. It can be assumed too
that the treatment of wastewater from microalgal biodiesel production is not within
the system boundaries of life cycle assessment. Exceptions regarding the latter
174
L. Reijnders