The use of algae for wastewater treatment is a technology that, although dating back more than 50 years, has not been consolidated within the main technologies of this sector. The main reason is that the treatment objectives for these waters in the past have been significantly different from those of today, where energy efficiency as well as nutrient and CO2 recycling have gained widespread importance in recent years. Only recently, this approach is gaining importance and has currently been evaluated at DEMO scale for freshwater effluents and two system configurations have been studied: photobioreactors (PBR), i.e. closed reactors made of glass or plastic tubes, and high rate algae ponds (HRAP), i.e. shallow open channel ponds. Of these two configurations, HRAPs represent the better option for the treatment of large wastewater effluents, as they have higher capacities and lower costs than PBRs.
Microalgae represent an attractive feedstock that can play an important role, providing an efficient, sustainable and abundant resource that can help replace fossil-based products and processes, contributing to the role of the bioeconomy, and achieving commercial applications in the food, animal feed, fertilizer, cosmetics and pharmaceutical sectors, among others. However, microalgae cultivation and harvesting remain costly with low biomass production rates. In addition, downstream processes are not designed to recover and valorize the multiple components of microalgae, representing a process bottleneck with notable costs.
The EU-funded Saltgae Project (H2020-Water-2015 no. 689785) coordinated by Funditec has studied this issue, including innovations in treatment, harvesting and subsequent valorization of algal biomass in 3 demonstration facilities using wastewater from the food industry, located in Italy, Slovenia and Israel.
To optimize the absorption of organic matter and nutrients (N&P) from the effluent to be treated, the composition of the microbial community present in synergy with the algal community was controlled: bacteria were responsible for the digestion of organic matter, while algae removed most of the N and P nutrients from the water. In this way, lower energy requirements can be achieved, reaching 80% less energy consumption than traditional wastewater treatments. This is because the algae provide the aerobic bacteria with the necessary oxygen through photosynthesis, thus removing the need for aeration (which represents 30-70% of the total energy cost of wastewater treatment plants). It also contributed to the reduction of greenhouse gas emissions as algae consume CO2 for growth and higher value-added by-products were produced from the harvesting of algal biomass. Not least, the wastewater provided the necessary nutrients (N, P) for algae production, thus reducing cultivation costs. The feasibility of producing more than 30 tons/hectare.year of algae by-product without investing in freshwater and nutrients has been demonstrated. Tests on the use of HRAP for the treatment of fresh wastewater have proven to be satisfactory, with BOD, N and P removal results above 80 %.
