Utilizing Parachlorella microalgae and Arthrospira cyanobacteria for tertiary wastewater treatment and biomass valorization as raw material for biopolymer production

The use of treated municipal effluents for microalgae cultivation provides an alternative to tertiary treatment in limited-nutrient removal wastewater treatment plants. Therefore, the present study aimed to investigate microalgae cultivation (Arthrospira platensis and Parachlorella kessleri) in secondary-treated wastewater and the reuse of biomass to produce biopolymers. The microalgae were cultivated in effluents from two wastewater treatment plants operating with different technologies-a hybrid-based treatment plant involving a trickling biofilter, dissolved air flotation and disinfection with free chlorination and a biological-based treatment plant combining biological processes (up-flow anaerobic sludge blanket reactor and stabilization ponds)-in two cities in southern Brazil. First, the microorganism growth in the Zarrouk medium and the effluents of the two wastewater treatment plants were studied for ten days. Further, the use of the microalgae for the simultaneous removal of organic waste, nitrogen and phosphorus from the wastewater was assessed. The two species exhibited slow growth in the wastewater from the hybrid-based treatment plant. However, Parachlorella kessleri exhibited better growth, especially in the biological-based wastewater, with a productivity of 6.12 g L-1. The nitrogen and phosphorus removal were 81 and 93%, respectively. No heavy metals were detected in biomass by X-ray photoelectron spectroscopy analysis. The results suggested that biomass processing should be performed at a maximum temperature of 170 degrees C to avoid the degradation of the biopolymers. This study proved that it is possible to use microalgae in the treatment of biologically treated domestic wastewater and reuse the biomass to produce biopolymers.Graphical abstractMicroalgae cultivation in secondary treated wastewater presents a viable alternative to conventional tertiary treatment methods. This approach serves as an effective mechanism for regulating the release of nitrogen and phosphorus into the environment. The process achieved removal efficiencies of over 81% for total nitrogen and 93% for total phosphorus. Additionally, the resulting algal biomass demonstrated potential for use in plastic production.