A bioremediation model was developed to describe the projected
rates of metal bioremediation from the AD. The model was
based on empirical data collected during this study and some assumptions
regarding the amount of AW produced at Tarong each
year. A model was developed to test the scenario that the existing
200 ha AD was converted to a series of bioremediation ponds with
the same parabolic profile as that used in our demonstration study.
This scenario was chosen for two reasons. First, it is assumed that
the area of land available to support a bioremediation technology
will not exceed the area dedicated to the existing management
strategy (i.e. onsite retention). Second, life-cycle analyses of utilityconnected
algal cultivation suggest that there are diminishing
returns in C capture as the facility increases in size due to the energetic
costs of pumping flue gas (Rickman et al., 2013). Consequently,
100e200 ha facilities have the greatest C capture potential
(Rickman et al., 2013).
The bioremediation model was developed in three stages;
calculation of the standing stock of metals currently in the AD,
estimation of the annual emissions of metals from new coal combustion,
and calculation of the mass of metals removed from the AD
each year in the harvested Oedogonium. In this way, the predictive
model defines bioremediation as metals that have been sequestered
within algal cells or passively bound to the surface of algal
cells, and then subsequently removed from the system during the
harvesting of biomass. Metals that precipitate in the culture ponds
are not captured in this model and are considered to have remained
in the system for uptake in subsequent cultivation cycles. The
“standing stock” (kg) of each of the 8 ANZECC-listed elements was
estimated in the AD as the product of the mean concentration of
each element in AW(i.e. the “initial” concentrations in Table 1) and
the capacity of the AD (46,000 ML). This is a conservative estimate
as it assumes the AD is at capacity. Second, the annual emissions of
metals from the power station were estimated by multiplying the
known concentrations of each element in AW (Table S1) by the
annual emissions of new AW. No data on AWusage were available
so an annual usage of 0.35 billion L has been assumed on the basis
of a current capacity of 700MW(Smart and Aspinall, 2009). For the
purposes of this model metal emissions are defined as the concentration
of metal in AWand the model considers only metals that
leach from the ash into the AW.
The annual sequestration of metals by Oedogonium was calculated
by multiplying the concentration of each element in Oedogonium
biomass (mg kg1) by the biomass productivity across
200 ha of cultivation. The metal sequestration component of the
model has two key assumptions. First, the elemental profile of the
biomass is consistent with that reported in this study (Table 1). This
is reasonable as the current study has shown that the elemental
profile of the biomass does not vary when productivity ranges from
2.8 to 8.2 g DWm2 d1. The model assumes a mean productivity of
5.62 g DW m2 d1, with 95% confidence intervals of
1.49 g DW m2 d1 on the basis of our empirical data. The second
key assumption is that the bioremediation rates of metals are
consistent through time despite the reduction in metal concentrations
in AW that will occur as metals are sequestered. The
bioremediation model was calculated from these inputs and
plotted through time according to the formula: