transversal to eukaryotic and prokaryotic organisms, including phototrophs (e.g.,
microalgae, purple bacteria) and heterotrophs, like the well-studied polyphosphate
accumulating organisms (PAO) [47, 58]. Enhanced biological phosphorus removal
(EBPR) is the most common process used in conventional WWTP to remove and
recover phosphorus from wastewaters. The process relies on PAOs that take up
phosphate in excess of their requirements, under sequential anaerobic and aerobic
periods [63]. In the anaerobic period, PAOs are able to hydrolyze intracellularly
stored Poly-P and glycogen, producing energy and reducing power to take up
volatile fatty acids from the wastewater and converting them into intracellular
polyhydroxyalkanoates (PHAs). In the aerobic period, PAOs oxidize the PHA,
obtaining energy for growth, and to take up phosphorous, replenishing glycogen
and Poly-P pools. The amount of phosphorus taken up aerobically exceeds the
phosphorus released anaerobically, resulting in a net P removal from wastewater
that becomes concentrated in the sludge [64]. The sludge can then be digested and
the phosphorus recovered through chemical precipitation as struvite [58]. As a
process that requires intensive aeration during the aerobic phase, EBPR systems
also face the same constrains of other aerated systems (e.g., aeration costs and
carbon emissions). These can be overcome by combining PAOs with microalgae in
the new Photo-EBPR technology.
4.1 Phosphate Recovery: The Case of Photo-EBPR
In recent years, an innovative technology that uses a PAO–microalgae consortium
to recover phosphorus via a photosynthetic enhanced biological phosphorous
removal system (photo-EBPR) was demonstrated by [31]. The photo-EBPR system
was proposed with the objective of treating wastewaters with low COD/P ratios
while eliminating the aeration requirements of EBPR systems. The photo-EBPR
utilizes a consortium culture, composed of heterotrophic bacteria (PAOs) and
photosynthetic organisms (e.g., microalgae and cyanobacteria) where the latest use
light, consume CO 2 , and produce the oxygen necessary for the system operation. Its
configuration is based on sequential dark and light cycles (Fig. 3).
During the dark phase, where no oxygen production by photosynthetic organisms occurs, the system is anaerobic and PAOs take up VFAs and convert them into
PHA, using the energy from poly-P and glycogen hydrolysis. In the light phase,
photosynthetic organisms use the light to produce ATP while generating oxygen,
which can be used by PAOs, to oxidize PHA and obtain ATP for P uptake [31]. The
amount of P that is taken up in the light phase, both by PAOs and photosynthetic
microorganisms, is higher than the P released in the dark phase, contributing to an
effluent with low P concentrations. In the dark idle period, the biomass is settled,
the supernatant is discharged, and anaerobic conditions are achieved, which is
essential for the beginning of the next photo-EBPR cycle [31, 32]. Since the
consortia self-aggregates in microalgae–PAOs biomass flocs, a good settling
capacity is obtained and a clear supernatant is generated [65, 66]. The sludge
produced during a photo-EBPR process can be directly used as fertilizer since
424
J. C. Fradinho et al.
microalgae, purple bacteria) and heterotrophs, like the well-studied polyphosphate
accumulating organisms (PAO) [47, 58]. Enhanced biological phosphorus removal
(EBPR) is the most common process used in conventional WWTP to remove and
recover phosphorus from wastewaters. The process relies on PAOs that take up
phosphate in excess of their requirements, under sequential anaerobic and aerobic
periods [63]. In the anaerobic period, PAOs are able to hydrolyze intracellularly
stored Poly-P and glycogen, producing energy and reducing power to take up
volatile fatty acids from the wastewater and converting them into intracellular
polyhydroxyalkanoates (PHAs). In the aerobic period, PAOs oxidize the PHA,
obtaining energy for growth, and to take up phosphorous, replenishing glycogen
and Poly-P pools. The amount of phosphorus taken up aerobically exceeds the
phosphorus released anaerobically, resulting in a net P removal from wastewater
that becomes concentrated in the sludge [64]. The sludge can then be digested and
the phosphorus recovered through chemical precipitation as struvite [58]. As a
process that requires intensive aeration during the aerobic phase, EBPR systems
also face the same constrains of other aerated systems (e.g., aeration costs and
carbon emissions). These can be overcome by combining PAOs with microalgae in
the new Photo-EBPR technology.
4.1 Phosphate Recovery: The Case of Photo-EBPR
In recent years, an innovative technology that uses a PAO–microalgae consortium
to recover phosphorus via a photosynthetic enhanced biological phosphorous
removal system (photo-EBPR) was demonstrated by [31]. The photo-EBPR system
was proposed with the objective of treating wastewaters with low COD/P ratios
while eliminating the aeration requirements of EBPR systems. The photo-EBPR
utilizes a consortium culture, composed of heterotrophic bacteria (PAOs) and
photosynthetic organisms (e.g., microalgae and cyanobacteria) where the latest use
light, consume CO 2 , and produce the oxygen necessary for the system operation. Its
configuration is based on sequential dark and light cycles (Fig. 3).
During the dark phase, where no oxygen production by photosynthetic organisms occurs, the system is anaerobic and PAOs take up VFAs and convert them into
PHA, using the energy from poly-P and glycogen hydrolysis. In the light phase,
photosynthetic organisms use the light to produce ATP while generating oxygen,
which can be used by PAOs, to oxidize PHA and obtain ATP for P uptake [31]. The
amount of P that is taken up in the light phase, both by PAOs and photosynthetic
microorganisms, is higher than the P released in the dark phase, contributing to an
effluent with low P concentrations. In the dark idle period, the biomass is settled,
the supernatant is discharged, and anaerobic conditions are achieved, which is
essential for the beginning of the next photo-EBPR cycle [31, 32]. Since the
consortia self-aggregates in microalgae–PAOs biomass flocs, a good settling
capacity is obtained and a clear supernatant is generated [65, 66]. The sludge
produced during a photo-EBPR process can be directly used as fertilizer since
424
J. C. Fradinho et al.