overall reduction of CO 2 emissions from organic matter oxidation [27]. In
addition, microalgae can also directly uptake or transform water contaminants,
efficiently removing large amounts of nitrogen and phosphorus and some
organics, through chemoheterotrophic and/or mixotrophic assimilation [42, 45].
The biomass resultant from this synergistic process can be recovered and
directed to different applications (e.g., biofuel production, fertilizers, and animal
feed), depending on the source/type of treated wastewater and on national regulations for the reuse of the biomass [42].
The development of a microalgae–bacterial consortia can occur naturally
through wastewater-borne microorganisms or can be artificially created through
the inoculation of selected species to address particular wastewater treatment
requirements [29]. Overall, the combination of microorganisms with different
metabolic capacities results in robust biological systems that remove a multitude
of different compounds with increased removal efficiencies while being able to
respond to environmental and nutrient loads fluctuations [29, 42].
Wastewater treatment with microalgae–bacteria consortia can occur in facilities
with different configurations, mostly using open systems, either employing
suspended or immobilized methods. The operation of closed systems for large
wastewater treatment is not a current practice due to the high investments and
maintenance costs of photo-bioreactors [30, 42], while open systems are relatively cheap to build, can be easily scaled-up and operated. Among the open
systems, the most used are the waste stabilization ponds (WSP) and open
raceways ponds (ORP) [46]. WSP are indicated for wastewater treatment in
small communities where land is inexpensive, offering effective treatment in
terms of organic carbon and pathogen removal. Wastewater is treated in a
combination of natural and physical processes without employing mixing or
aeration systems which results in very low operation costs [47]. To compensate
the slow removal of pollutants, WSP are typically operated with high hydraulic
retention times (HRT) of 10–40 days [27]. ORP are alternative systems which
are operated with shallow depths (0.15–0.3 m) and the culture is mixed and
circulated around the raceway track by paddlewheels [30]. This increases
removal efficiencies and as such, lower HRT can be applied (3–10 days) [27,
42]. Such ponds, commonly referred as high rate algae pond (HRAP), have been
successfully implemented in several locations with demo-scale installations
treating different types of wastewaters [48]. However, the short HRT of HRAP
may occasionally limit the nitrogen removal from the wastewaters via
nitrification/denitrification processes and the poor sedimentation capacity of the
consortia can result in a total solid concentration in the effluent that does not
fulfill the discharge limits into water bodies [45]. In this last case, promoting the
selection of self-aggregating consortia or implementing immobilization methods
that entrap the microorganisms in polymeric matrixes or support their attachment
to surfaces, creating a biofilm, may be possible solutions [29, 42].
New Phototrophic Factories for Resource Recovery
421
addition, microalgae can also directly uptake or transform water contaminants,
efficiently removing large amounts of nitrogen and phosphorus and some
organics, through chemoheterotrophic and/or mixotrophic assimilation [42, 45].
The biomass resultant from this synergistic process can be recovered and
directed to different applications (e.g., biofuel production, fertilizers, and animal
feed), depending on the source/type of treated wastewater and on national regulations for the reuse of the biomass [42].
The development of a microalgae–bacterial consortia can occur naturally
through wastewater-borne microorganisms or can be artificially created through
the inoculation of selected species to address particular wastewater treatment
requirements [29]. Overall, the combination of microorganisms with different
metabolic capacities results in robust biological systems that remove a multitude
of different compounds with increased removal efficiencies while being able to
respond to environmental and nutrient loads fluctuations [29, 42].
Wastewater treatment with microalgae–bacteria consortia can occur in facilities
with different configurations, mostly using open systems, either employing
suspended or immobilized methods. The operation of closed systems for large
wastewater treatment is not a current practice due to the high investments and
maintenance costs of photo-bioreactors [30, 42], while open systems are relatively cheap to build, can be easily scaled-up and operated. Among the open
systems, the most used are the waste stabilization ponds (WSP) and open
raceways ponds (ORP) [46]. WSP are indicated for wastewater treatment in
small communities where land is inexpensive, offering effective treatment in
terms of organic carbon and pathogen removal. Wastewater is treated in a
combination of natural and physical processes without employing mixing or
aeration systems which results in very low operation costs [47]. To compensate
the slow removal of pollutants, WSP are typically operated with high hydraulic
retention times (HRT) of 10–40 days [27]. ORP are alternative systems which
are operated with shallow depths (0.15–0.3 m) and the culture is mixed and
circulated around the raceway track by paddlewheels [30]. This increases
removal efficiencies and as such, lower HRT can be applied (3–10 days) [27,
42]. Such ponds, commonly referred as high rate algae pond (HRAP), have been
successfully implemented in several locations with demo-scale installations
treating different types of wastewaters [48]. However, the short HRT of HRAP
may occasionally limit the nitrogen removal from the wastewaters via
nitrification/denitrification processes and the poor sedimentation capacity of the
consortia can result in a total solid concentration in the effluent that does not
fulfill the discharge limits into water bodies [45]. In this last case, promoting the
selection of self-aggregating consortia or implementing immobilization methods
that entrap the microorganisms in polymeric matrixes or support their attachment
to surfaces, creating a biofilm, may be possible solutions [29, 42].
New Phototrophic Factories for Resource Recovery
421