In most studies performed to consider the environmental burden of microalgal biodiesel, the key contributors to the algal cultivation process have been identi­fied across raceway ponds and closed photobioreactors. Across all reactor configu­rations, the major contributions to environmental burden in terms of net energy ratio, abiotic depletion, and GHGs were incurred from the energy requirement for mass transfer and mixing in the reactor, as well as the energy requirement asso­ciated with the provision of combined nitrogen for cell growth (Lardon et al., 2009; Batan et al., 2010; Stephenson et al., 2010; Richardson et al., 2012b). These requirements are sensitive to algal biomass concentration, lipid content, and algal productivity. Algal productivity and concentration were the most influential (e. g., Stephenson et al., 2010; Razon and Tan, 2011), owing to their impact on system volume, influencing both energy input for mixing and pumping, and the amounts of nutrients required, as previously demonstrated in other microbial systems (Harding et al., 2008; Harding et al., 2012).

Studies by Jorquera et al. (2010), Stephenson et al. (2010), and Richardson et al. (2012b) compared selected types of reactors. Jorquera et al. (2010) compared horizontal tubular reactors, flat-plate reactors, and raceway ponds using a basis of 100 tons algal biomass per year. The closed photobioreactors provided higher biomass concentrations, and higher volumetric and areal productivities than ponds. Correlated with this, the raceway ponds had an increased land requirement. However, their energetic requirements were significantly lower than the closed photobioreac­tors. Under their operating conditions, the NER of the horizontal tubular reactor illustrated that it was not feasible in terms of either oil or biomass production (NER of 0.07 and 0.20, respectively). The NER of the flat-plate reactor was 54% of that for the open raceway for oil production (NER of 1.65 and 3.05, respectively) and for production of algal biomass (NER of 4.51 and 8.34, respectively).

Stephenson et al. (2010) compared the performance of the integrated algal biodiesel process, from cradle to combustion, using the open raceway and tubular airlift photobioreactor and a two-stage cultivation method to maximize lipid for­mation under nitrogen starvation in the second stage. In their system, the reactor was the dominant contributor to energy consumption. The design of the tubular airlift photobioreactor required energy input an order of magnitude greater than the raceway on an energy equivalence basis, despite its higher productivity. While 85% of the energy requirement of the tubular reactor was attributed to operation and the remainder to reactor manufacture, the latter exceeded the total GWP of the raceway system.

Similarly, Richardson et al. (2012b) compared the performance of the raceway, horizontal tubular reactor, and airlift tubular reactor as a component of the algal bio­refinery producing biodiesel and biogas. Their comparison was made using literature data for Phaeodactylum tricornutum, extensively studied in the Aquatic Species Programme (Sheehan et al., 1998). Considering an integrated biorefinery system with combined biogas production, the relative NER values for these photobioreac­tors compared to the raceway were 64% and 8%, respectively. Under the operating conditions selected, the NER of the airlift reactor was unacceptable, owing to the low productivity achieved relative to the energy input. The airlift reactor can be operated at much reduced gas flow rates and concomitant reduction in energy input without compromising productivity (data not shown), indicating the need to make these comparisons using optimized performance data relevant to commercial-scale operation. In all cases, the reactor energy requirement dominated that of the process, with that of the horizontal tubular reactor being some twofold that of the raceway per unit biodiesel. Owing to the lower biomass and oil concentrations achieved in the raceway reactor, this advantage of reduced reactor energy was partially offset by the greater pumping energy required for the larger volume processed from the raceway (2.2-fold); the pumping energy within the raceway biorefinery is a quarter of the reactor energy requirement. Extending the analysis beyond energy, the acidification and eutrophication impacts of the horizontal tubular reactor were 61% and 73%, respectively, of the raceway system under the standard operating conditions selected. The GWP was negative for the raceway system compared to a positive value of 60% for the horizontal tubular reactor.

Recognizing the increased energy requirement of traditional photobioreactors for mixing and mass transfer as well as manufacture, Batan et al. (2010) assessed the sparged polyethylene photobioreactor bags. While they report positive NER values for these systems, agreement in the literature with respect to the feasibility of the airlift system for biofuel production has not been found and further assessment is required.

Razon and Tan (2011) assessed the combined production of biodiesel and biogas using Haematococcuspluvialis. While low biomass concentrations and productivity resulted in a negative NER, the energy requirement of the flat-plate bioreactor used for intermittent inoculum supply exceeded that of the raceway system used for large — scale production by some twofold, thus confirming the much higher energy demand per unit biofuel in closed reactor systems.

Stephenson et al. (2010) disaggregated the contributions to the reactor energy and GWP in the tubular airlift reactor and raceway pond. These are shown relatively in Figure 9.2 (see color insert), where the total fossil energy requirements estimated for cultivation in the tubular airlift reactor and raceway under standard conditions were approximately 230 and 29 GJ per tonne biodiesel formed, respectively. The corresponding GWPs were 13,550 and 1,900 kg CO2 per tonne biodiesel, respec­tively. In addition to the magnitude, the relative values illustrate that electrical energy for mixing and mass transfer dominates the reactor energy and related GWP in the tubular reactor. The lower mixing energy in the raceway results in the energy for com­bined nitrogen provision being significant. Furthermore, the much larger raceways required, owing to lower productivities achieved, lead to the construction compo­nents making a more dominant contribution, especially the PVC liners of the ponds.

■ N fertilizer

■ P fertilizer

■ Electrical power ■ Perspex tubing

■ N fertilizer

■ P fertilizer

■ Electrical power

■ Concrete walls

■ Steel paddle wheel

■ PVC lining

■ Water supply & treatment

FIGURE 9.2 (See color insert.) The relative contribution of fossil energy (left) and GWP (right) to the total requirements for microalgal biodiesel production using a tubular airlift reactor (upper) and a raceway (lower). From the LCA of C. vulgaris conducted by Stephenson et al. (2010) under standard conditions. The total fossil energy requirements of 230 and 29 GJ and GWP of 13,550 and 1,900 kg CO2 per tonne biodiesel formed were estimated for the tubular reactor and raceway, respectively.