sanitation technology toward a bioproduct recovery industry and a recycled
water supplier. This is a paradigm shift from the general conception of
wastewater treatment, where carbon and reactive nitrogen are removed from
water to the atmosphere as CO 2 and N 2 , into the more recent concept of resource
recovery [26, 36]. Biogas, biopolymers, irrigation water, and biofertilizers are
examples of possible products that can be obtained from converting Waste Water
Treatment Plants (WWTP) into the new “water factories” [37]. Nonetheless, the
sustainability of wastewater treatment still continues to strongly depend on the
adequate selection of a particular treatment methodology, determined through
multicriteria decision-making techniques that take into consideration economic,
socio-cultural, and environmental objectives as a whole [38]. With a multitude of
different factors impacting on the technology selection, the implementation of
phototrophic treatment processes has risen since it targets a decrease of capital,
operational and maintenance costs, a simplification of the treatment process, a
high removal efficiency of nutrients and organic pollutants, and associated high
carbon and nutrient capture [27, 39, 40]. However, it is also clear that as a light
dependent technology, local climate (sunlight availability) and land requirement
(sufficient space to accommodate the facilities) will naturally impact the global
application of the process. Nevertheless, the last decade has witnessed an
increasing number of studies proposing phototrophic systems for wastewater
treatment and resource recovery using microalgae–bacterial-based processes [29,
41, 42], or also, the more recently developed anoxygenic purple phototrophic
bacteria (PPB)-based treatment [26, 43, 44]. These systems exhibit different
treatment efficiencies and should be used according to the wastewater treatment
requirement. All together, these new technologies either applied to domestic,
industrial, or livestock wastewaters, from laboratory up to full-scale treatment,
are contributing to define local and global water management strategies that
assure proper sanitation to populations while preserving the environment.
3.1 Microalgae–Bacterial Processes
Wastewaters are polluted liquid streams which result from domestic, agricultural,
and industrial activities. Its composition may contain variable concentrations of
organics and nutrients, such as nitrogen and phosphorous, as well as pollutants
like xenobiotics, heavy metals and microplastics [29, 42]. Conventional
wastewater treatment usually employs aerobic activated sludge, which requires
intensive aeration (accounts for *50% of the total energy consumption) and
presents low-carbon capture efficiency with the organics being partially dissipated to the atmosphere as CO 2 [27, 44]. The combination of microalgae and
heterotrophic bacteria in microalgae–bacterial processes is a cost effective and
sustainable alternative that can overcome activated sludge constrains. Microalgae provide natural and free oxygenation to the heterotrophic bacteria which in
turn produce CO 2 that is used for microalgae photosynthesis, leading to an
420
J. C. Fradinho et al.
water supplier. This is a paradigm shift from the general conception of
wastewater treatment, where carbon and reactive nitrogen are removed from
water to the atmosphere as CO 2 and N 2 , into the more recent concept of resource
recovery [26, 36]. Biogas, biopolymers, irrigation water, and biofertilizers are
examples of possible products that can be obtained from converting Waste Water
Treatment Plants (WWTP) into the new “water factories” [37]. Nonetheless, the
sustainability of wastewater treatment still continues to strongly depend on the
adequate selection of a particular treatment methodology, determined through
multicriteria decision-making techniques that take into consideration economic,
socio-cultural, and environmental objectives as a whole [38]. With a multitude of
different factors impacting on the technology selection, the implementation of
phototrophic treatment processes has risen since it targets a decrease of capital,
operational and maintenance costs, a simplification of the treatment process, a
high removal efficiency of nutrients and organic pollutants, and associated high
carbon and nutrient capture [27, 39, 40]. However, it is also clear that as a light
dependent technology, local climate (sunlight availability) and land requirement
(sufficient space to accommodate the facilities) will naturally impact the global
application of the process. Nevertheless, the last decade has witnessed an
increasing number of studies proposing phototrophic systems for wastewater
treatment and resource recovery using microalgae–bacterial-based processes [29,
41, 42], or also, the more recently developed anoxygenic purple phototrophic
bacteria (PPB)-based treatment [26, 43, 44]. These systems exhibit different
treatment efficiencies and should be used according to the wastewater treatment
requirement. All together, these new technologies either applied to domestic,
industrial, or livestock wastewaters, from laboratory up to full-scale treatment,
are contributing to define local and global water management strategies that
assure proper sanitation to populations while preserving the environment.
3.1 Microalgae–Bacterial Processes
Wastewaters are polluted liquid streams which result from domestic, agricultural,
and industrial activities. Its composition may contain variable concentrations of
organics and nutrients, such as nitrogen and phosphorous, as well as pollutants
like xenobiotics, heavy metals and microplastics [29, 42]. Conventional
wastewater treatment usually employs aerobic activated sludge, which requires
intensive aeration (accounts for *50% of the total energy consumption) and
presents low-carbon capture efficiency with the organics being partially dissipated to the atmosphere as CO 2 [27, 44]. The combination of microalgae and
heterotrophic bacteria in microalgae–bacterial processes is a cost effective and
sustainable alternative that can overcome activated sludge constrains. Microalgae provide natural and free oxygenation to the heterotrophic bacteria which in
turn produce CO 2 that is used for microalgae photosynthesis, leading to an
420
J. C. Fradinho et al.