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Mixed culture purple phototrophic bacteria (PPB) is a rapidly emerging technology for resource recovery from wastewaters. PPB biomass can be used as single-cell protein, with a high protein content complemented by value-added components (e.g. pigments and polyhydroxyalkanoates), merging functionalities within a single product. This has the potential to increase the value and impact the economic feasibility, justifying higher capital costs for PPB photobioreactors for real-life applications. Artificial illumination is prohibitively expensive, and naturally illuminated, outdoor units are a critical next step. However, information required for informed technoeconomic assessment of single-cell protein from PPB is still missing and can only be determined in dedicated larger-scale, outdoor systems. Larger scale units are also required to supply feed for larger cohort trials. Although data from microalgae research can be used as starting point to estimate costs, they cannot be translated directly for PPB, as the organisms and metabolic growth are fundamentally different., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Sustainable development is driving a rapid focus shift in the wastewater and organic waste treatment sectors, from a "removal and disposal" approach towards the recovery and reuse of water, energy and materials (e.g. carbon or nutrients). Purple phototrophic bacteria (PPB) are receiving increasing attention due to their capability of growing photoheterotrophically under anaerobic conditions. Using light as energy source, PPB can simultaneously assimilate carbon and nutrients at high efficiencies (with biomass yields close to unity (1 g COD biomass ·g COD removed )), facilitating the maximum recovery of these resources as different value-added products. The effective use of infrared light enables selective PPB enrichment in non-sterile conditions, without competition with other phototrophs such as microalgae if ultraviolet-visible wavelengths are filtered. This review reunites results systematically gathered from over 177 scientific articles, aiming at producing generalized conclusions. The most critical aspects of PPB-based production and valorisation processes are addressed, including: (i) the identification of the main challenges and potentials of different growth strategies, (ii) a critical analysis of the production of value-added compounds, (iii) a comparison of the different value-added products, (iv) insights into the general challenges and opportunities and (v) recommendations for future research and development towards practical implementation. To date, most of the work has not been executed under real-life conditions, relevant for full-scale application. With the savings in wastewater discharge due to removal of organics, nitrogen and phosphorus as an important economic driver, priorities must go to using PPB-enriched cultures and real waste matrices. The costs associated with artificial illumination, followed by centrifugal harvesting/dewatering and drying, are estimated to be 1.9 -1 )), facilitating the maximum recovery of these resources as different value-added products. The effective use of infrared light enables selective PPB enrichment in non-sterile conditions, without competition with other phototrophs such as microalgae if ultraviolet-visible wavelengths are filtered. This review reunites results systematically gathered from over 177 scientific articles, aiming at producing generalized conclusions. The most critical aspects of PPB-based production and valorisation processes are addressed, including: (i) the identification of the main challenges and potentials of different growth strategies, (ii) a critical analysis of the production of value-added compounds, (iii) a comparison of the different value-added products, (iv) insights into the general challenges and opportunities and (v) recommendations for future research and development towards practical implementation. To date, most of the work has not been executed under real-life conditions, relevant for full-scale application. With the savings in wastewater discharge due to removal of organics, nitrogen and phosphorus as an important economic driver, priorities must go to using PPB-enriched cultures and real waste matrices. The costs associated with artificial illumination, followed by centrifugal harvesting/dewatering and drying, are estimated to be 1.9 , 0.3-2.2 and 0.1-0.3 $·kg dry biomass -1 . At present, these costs are likely to exceed revenues. Future research efforts must be carried out outdoors, using sunlight as energy source. The growth of bulk biomass on relatively clean wastewater streams (e.g. from food processing) and its utilization as a protein-rich feed (e.g. to replace fishmeal, 1.5-2.0 $·kg -1 ) appears as a promising valorisation route., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Resource recovery, preferably as high value products, is becoming an integral part of modern wastewater treatment, with conversion to heterotrophic or phototrophic/photosynthetic microbes a key option to minimise dissipation, and maximise recovery. This study compares the treatment capacities of purple phototrophic bacteria (PPB) and microalgae of five agri-industrial wastewaters (pork, poultry, red meat, dairy and sugar) to recover carbon, nitrogen, and phosphorous as a microbial product. The mediators have different advantages, with PPB offering moderate removals (up to 74% COD, 80% NH 4 -N, 55% PO 4 -P) but higher yields (>0.75 gCOD removed gCOD added -1 ). The microalgae tests achieved a better removal outcome (up to 91%COD, 91% NH -1 ). The microalgae tests achieved a better removal outcome (up to 91%COD, 91% NH 4 -N, 73%PO 4 -P), but with poorer quality product, and <30% abundance as algae., (Copyright © 2018 Elsevier Ltd. All rights reserved.)