in PAOs, activated sludge is widely available and a photo-EBPR system can be
readily implemented in WWTP that currently do not perform EBPR and where
there is no direct access to a sludge enriched in PAOs [32].
Besides the phosphorous recovery, another feature that can be explored in
photo-EBPR systems is their capacity for nitrogen removal. The operation with
sludge retention times over 10 days and the cyclic variations of oxygen levels and
light allows the development of a multitude of microorganisms with complementary
metabolic pathways. Since the measured oxygen levels are always very low during
the light phase [31, 32], and some PAOs are capable of using nitrates as electron
acceptors in anoxic conditions (denitrifying PAOS—dPAOs) [75–77], it is expected
that the photo-EBPR could be operated in sequential dark (anaerobic)—light
(aerobic)—dark (anoxic) cycles. During the light aerobic phase, ammonia could be
removed by the combination of assimilation processes and nitrification by aerobic
autotrophic nitrifiers. During the dark anoxic phase, nitrates could be removed by
dPAOs. Future research will ascertain on the photo-EBPR potential for the dual
capacity removal of P and N.
Summing-up, the main advantage of the photo-EBPR, in relation to the conventional EBPR, is the independence from external aeration, since the oxygen
needs are covered by microalgae that produce the O 2 used by PAOs, substantially
reducing the process operation costs [31]. Furthermore, photo-EBPR systems
require a lower amount of COD in the wastewater to remove phosphorous since the
microalgae also contribute for P uptake while using CO 2 as carbon source. The high
quantity of CO 2 that microalgae need to satisfy the O 2 requirements of aerobic
bacteria [78] is a further feature of the photo-EBPR that contributes to CO 2
sequestration, reducing the concentration of greenhouse gases in the atmosphere
[31]. Finally, the P-rich microalgae–bacterial sludge can be used as a slow-release
fertilizer or directed for phosphorus recovery.
On the other hand, the photo-EBPR systems require land space and sunlight,
being fit for implementation in areas with a good solar distribution along the year
and warm temperatures [66]. Nevertheless, the system can be used in large city
surroundings, as well as small towns and villages in rural areas where space is not a
constraint for WWTPs and where wastewater is sometimes treated in WSP or ORP
using algal–bacterial consortia. The photo-EBPR system can be an appropriate
solution for these cases as it optimizes phosphorous removal in combination with
low-cost operation (no aeration needed) and low maintenance requirements [32].
On a future note, the fact that current photo-EBPR systems show good P
removal capacities (P net removal *34 mg P/L) for wastewaters with low COD/P
ratios (phosphate concentration in the influent *60 mg P/L) [31, 32] indicates the
possibility of full P removal when treating domestic wastewater, which typically
present lower phosphate concentrations (<10–20 mg P/L). Furthermore, the presence of low oxygen levels during the cycles opens up the possibility of developing
the photo-EBPR system for future simultaneous P and N removal.
New Phototrophic Factories for Resource Recovery
427
readily implemented in WWTP that currently do not perform EBPR and where
there is no direct access to a sludge enriched in PAOs [32].
Besides the phosphorous recovery, another feature that can be explored in
photo-EBPR systems is their capacity for nitrogen removal. The operation with
sludge retention times over 10 days and the cyclic variations of oxygen levels and
light allows the development of a multitude of microorganisms with complementary
metabolic pathways. Since the measured oxygen levels are always very low during
the light phase [31, 32], and some PAOs are capable of using nitrates as electron
acceptors in anoxic conditions (denitrifying PAOS—dPAOs) [75–77], it is expected
that the photo-EBPR could be operated in sequential dark (anaerobic)—light
(aerobic)—dark (anoxic) cycles. During the light aerobic phase, ammonia could be
removed by the combination of assimilation processes and nitrification by aerobic
autotrophic nitrifiers. During the dark anoxic phase, nitrates could be removed by
dPAOs. Future research will ascertain on the photo-EBPR potential for the dual
capacity removal of P and N.
Summing-up, the main advantage of the photo-EBPR, in relation to the conventional EBPR, is the independence from external aeration, since the oxygen
needs are covered by microalgae that produce the O 2 used by PAOs, substantially
reducing the process operation costs [31]. Furthermore, photo-EBPR systems
require a lower amount of COD in the wastewater to remove phosphorous since the
microalgae also contribute for P uptake while using CO 2 as carbon source. The high
quantity of CO 2 that microalgae need to satisfy the O 2 requirements of aerobic
bacteria [78] is a further feature of the photo-EBPR that contributes to CO 2
sequestration, reducing the concentration of greenhouse gases in the atmosphere
[31]. Finally, the P-rich microalgae–bacterial sludge can be used as a slow-release
fertilizer or directed for phosphorus recovery.
On the other hand, the photo-EBPR systems require land space and sunlight,
being fit for implementation in areas with a good solar distribution along the year
and warm temperatures [66]. Nevertheless, the system can be used in large city
surroundings, as well as small towns and villages in rural areas where space is not a
constraint for WWTPs and where wastewater is sometimes treated in WSP or ORP
using algal–bacterial consortia. The photo-EBPR system can be an appropriate
solution for these cases as it optimizes phosphorous removal in combination with
low-cost operation (no aeration needed) and low maintenance requirements [32].
On a future note, the fact that current photo-EBPR systems show good P
removal capacities (P net removal *34 mg P/L) for wastewaters with low COD/P
ratios (phosphate concentration in the influent *60 mg P/L) [31, 32] indicates the
possibility of full P removal when treating domestic wastewater, which typically
present lower phosphate concentrations (<10–20 mg P/L). Furthermore, the presence of low oxygen levels during the cycles opens up the possibility of developing
the photo-EBPR system for future simultaneous P and N removal.
New Phototrophic Factories for Resource Recovery
427