The relevance of microalgae–bacterial processes for wastewater treatment and
resource recovery is reflected on the numerous technologies that are continuously proposed every year. The reduction of energy consumption and the
decrease of CO 2 emissions while maintaining high removal efficiencies of pollutants are the most attractive features, but further technology development is
required, especially targeting the reduction of land requirements and validating
the proposed processes at larger scales for different types of wastewater.
3.2 Purple Phototrophic Bacteria Processes
Phototrophic wastewater treatment technologies have been proposed as means of
decreasing operational costs (e.g., energy) and promoting higher carbon and
nutrient recovery. As mentioned in Sect. 3.1, microalgae–bacteria consortia
promote an overall reduction of CO 2 emissions that originate from organic
matter oxidation due to CO 2 fixation by microalgae. However, reactive nitrogen
species may still be lost to the atmosphere. Indeed, in aerobic environments, such
as the ones found in conventional activated sludge and microalgae–bacterial
systems, ammonia can be removed by aerobic nitrification, and if anoxia occurs,
nitrites/nitrates can be further denitrified and removed from the liquid as gaseous
nitrogen species, such as N 2 O (greenhouse gas) and N 2 [36, 49]. Anoxygenic
PPB-based processes were proposed for wastewater treatment in illuminated
anaerobic environments to promote high removal and recovery of carbon,
nitrogen, and phosphorous trough assimilative and accumulative processes [40,
43, 50]. These systems have seen an increased development in the last decade,
exploring the unique features of PPB. PPB are well adapted for photoheterotrophy, being capable of direct treatment of high organic load wastewater
[40]. The diversified metabolism of PPB, with some organisms like Rhodobacter
spp. and Rhodopseudomonas spp. being reported as capable of
degrading/remediating hazardous compounds, allows the employment of PPB
systems for toxic wastewater treatment containing pharmaceuticals [51],
emerging pollutants [52], heavy metals, oils or dyes [44, 53]. On the other hand,
treatment of non-toxic wastewaters allows the extraction of high value-added
compounds from the biomass, such as pigments, coenzyme Q-10, polyhydroxyalkanoates [40, 54, 55], or the whole biomass utilization as a protein rich food
additive for animal feed [56]. As in other phototrophic technologies, the recovery
of energy or high valuable compounds along with wastewater treatment is
fundamental for the economic feasibility of PPB processes and their implementation at full scale. Currently, PPB technologies are still in an early stage of
development with the first studies occurring in outdoor demo facilities [57]. New
technology improvements are expected during the next years.
422
J. C. Fradinho et al.
resource recovery is reflected on the numerous technologies that are continuously proposed every year. The reduction of energy consumption and the
decrease of CO 2 emissions while maintaining high removal efficiencies of pollutants are the most attractive features, but further technology development is
required, especially targeting the reduction of land requirements and validating
the proposed processes at larger scales for different types of wastewater.
3.2 Purple Phototrophic Bacteria Processes
Phototrophic wastewater treatment technologies have been proposed as means of
decreasing operational costs (e.g., energy) and promoting higher carbon and
nutrient recovery. As mentioned in Sect. 3.1, microalgae–bacteria consortia
promote an overall reduction of CO 2 emissions that originate from organic
matter oxidation due to CO 2 fixation by microalgae. However, reactive nitrogen
species may still be lost to the atmosphere. Indeed, in aerobic environments, such
as the ones found in conventional activated sludge and microalgae–bacterial
systems, ammonia can be removed by aerobic nitrification, and if anoxia occurs,
nitrites/nitrates can be further denitrified and removed from the liquid as gaseous
nitrogen species, such as N 2 O (greenhouse gas) and N 2 [36, 49]. Anoxygenic
PPB-based processes were proposed for wastewater treatment in illuminated
anaerobic environments to promote high removal and recovery of carbon,
nitrogen, and phosphorous trough assimilative and accumulative processes [40,
43, 50]. These systems have seen an increased development in the last decade,
exploring the unique features of PPB. PPB are well adapted for photoheterotrophy, being capable of direct treatment of high organic load wastewater
[40]. The diversified metabolism of PPB, with some organisms like Rhodobacter
spp. and Rhodopseudomonas spp. being reported as capable of
degrading/remediating hazardous compounds, allows the employment of PPB
systems for toxic wastewater treatment containing pharmaceuticals [51],
emerging pollutants [52], heavy metals, oils or dyes [44, 53]. On the other hand,
treatment of non-toxic wastewaters allows the extraction of high value-added
compounds from the biomass, such as pigments, coenzyme Q-10, polyhydroxyalkanoates [40, 54, 55], or the whole biomass utilization as a protein rich food
additive for animal feed [56]. As in other phototrophic technologies, the recovery
of energy or high valuable compounds along with wastewater treatment is
fundamental for the economic feasibility of PPB processes and their implementation at full scale. Currently, PPB technologies are still in an early stage of
development with the first studies occurring in outdoor demo facilities [57]. New
technology improvements are expected during the next years.
422
J. C. Fradinho et al.