accumulation [102]. In permanent feast conditions, microalgae were outcompeted
and the culture become highly enriched in PPB. The culture favored carbon
recovery as biomass in the selector reactor (0.64 Cmol X/Cmol S of growth yield)
and thus, presented low PHB values (3–5% PHA). Yet, when exposed to higher
light intensities in accumulator reactors (intensities achievable under natural
sun-light illumination), the culture substantially increased its metabolic rates,
doubling the carbon uptake rate and increasing 15 times the PHB storage rate,
which resulted in a PHB content of 60%, the maximum ever recorded for a PMC. In
this case, carbon recovery as PHB was favored with a storage yield of 0.67 Cmol
PHB/Cmol Acetate. Globally, the carbon mass balance indicated an 80% carbon
recovery as biomass and PHB. An interesting parallel finding was the high uptake
rate of phosphate observed during the accumulation tests, with 95% of the removed
phosphate being stored in the first hours of the tests, likely as polyphosphate [102].
These results indicate that selecting PMCs under the permanent presence of carbon
and exposing them to high light availabilities enables a high carbon and phosphorous recovery, partly through assimilation into the cell biomass, but mostly
through accumulation as internal PHB and polyphosphate polymers.
Under light optimized conditions in the accumulator reactors, it was possible to
further increase PHA production rates up to 2.21 ± 0.07 Cmol PHB/Cmol X d [55].
This is less than a threefold difference in comparison to acetate-fed aerobic systems,
which approaches the storage performances of phototrophic and aerobic systems.
Also, high PHB storage yield on acetate was again registered (0.83 ± 0.07 Cmol
PHB/Cmol Acetate) in contrast with the lower yields observed in aerobic systems
(0.4–0.6 Cmol PHB/Cmol Acetate) [55]. When the PMC was fed with real fermented cheese whey containing lactate, ethanol, and VFAs (C2 to C5), the PPB
culture showed its robustness, adapting well to the new feedstock and achieving
20% PHA content with 12% HV content. These values are even more relevant
seeing that all the tests were conducted with light intensities that can be naturally
obtained outdoors, indicating the possibility of system operation under direct
sunlight illumination [55].
Taking all these results into consideration, both the selection of phototrophic
mixed cultures through feast and famine conditions (targeting PHA storage as
carbon reserve) or through the permanent presence of carbon (targeting PHA
storage as electron sink) can be potential strategies for PHA production from
VFA-rich streams under natural outdoor sunlight conditions. However, the establishment of PMC systems as sustainable PHA producing technologies requires the
technology transfer from laboratory to demo-scale and technology validation with
on-site studies. These studies have occurred under the framework of the European
Project INCOVER [103] in collaboration with the company FCC AQUALIA,
Spain. The technology initially developed in 6L laboratory reactors, simulating
outdoor light and temperature conditions, was transferred to two demo-scale ponds
of *10m
3 each (Fig. 6). These purple phototrophic bacteria ponds (PPBPonds)
were illuminated by sunlight and fed with locally fermented domestic wastewater
mixed with molasses for PHA production. Preliminary results indicated the culture
capacity of producing up to 1.3 kg PHA Pond
−1 d
−1 during spring time
432
J. C. Fradinho et al.
and the culture become highly enriched in PPB. The culture favored carbon
recovery as biomass in the selector reactor (0.64 Cmol X/Cmol S of growth yield)
and thus, presented low PHB values (3–5% PHA). Yet, when exposed to higher
light intensities in accumulator reactors (intensities achievable under natural
sun-light illumination), the culture substantially increased its metabolic rates,
doubling the carbon uptake rate and increasing 15 times the PHB storage rate,
which resulted in a PHB content of 60%, the maximum ever recorded for a PMC. In
this case, carbon recovery as PHB was favored with a storage yield of 0.67 Cmol
PHB/Cmol Acetate. Globally, the carbon mass balance indicated an 80% carbon
recovery as biomass and PHB. An interesting parallel finding was the high uptake
rate of phosphate observed during the accumulation tests, with 95% of the removed
phosphate being stored in the first hours of the tests, likely as polyphosphate [102].
These results indicate that selecting PMCs under the permanent presence of carbon
and exposing them to high light availabilities enables a high carbon and phosphorous recovery, partly through assimilation into the cell biomass, but mostly
through accumulation as internal PHB and polyphosphate polymers.
Under light optimized conditions in the accumulator reactors, it was possible to
further increase PHA production rates up to 2.21 ± 0.07 Cmol PHB/Cmol X d [55].
This is less than a threefold difference in comparison to acetate-fed aerobic systems,
which approaches the storage performances of phototrophic and aerobic systems.
Also, high PHB storage yield on acetate was again registered (0.83 ± 0.07 Cmol
PHB/Cmol Acetate) in contrast with the lower yields observed in aerobic systems
(0.4–0.6 Cmol PHB/Cmol Acetate) [55]. When the PMC was fed with real fermented cheese whey containing lactate, ethanol, and VFAs (C2 to C5), the PPB
culture showed its robustness, adapting well to the new feedstock and achieving
20% PHA content with 12% HV content. These values are even more relevant
seeing that all the tests were conducted with light intensities that can be naturally
obtained outdoors, indicating the possibility of system operation under direct
sunlight illumination [55].
Taking all these results into consideration, both the selection of phototrophic
mixed cultures through feast and famine conditions (targeting PHA storage as
carbon reserve) or through the permanent presence of carbon (targeting PHA
storage as electron sink) can be potential strategies for PHA production from
VFA-rich streams under natural outdoor sunlight conditions. However, the establishment of PMC systems as sustainable PHA producing technologies requires the
technology transfer from laboratory to demo-scale and technology validation with
on-site studies. These studies have occurred under the framework of the European
Project INCOVER [103] in collaboration with the company FCC AQUALIA,
Spain. The technology initially developed in 6L laboratory reactors, simulating
outdoor light and temperature conditions, was transferred to two demo-scale ponds
of *10m
3 each (Fig. 6). These purple phototrophic bacteria ponds (PPBPonds)
were illuminated by sunlight and fed with locally fermented domestic wastewater
mixed with molasses for PHA production. Preliminary results indicated the culture
capacity of producing up to 1.3 kg PHA Pond
−1 d
−1 during spring time
432
J. C. Fradinho et al.