Microalgal treatment of the effluent from the hydrothermal carbonization of microalgal biomass

The integration of microalgae in the municipal wastewater treatment scheme could certainly be an interesting option to reduce the carbon footprint of the process. The obtained microalgae could be valorized through hydrothermal carbonization (HTC), producing a sorbent material to be used as an alternative to activated carbon. However, HTC of microalgae does produce potentially harmful byproducts. The present work aims to investigate the possibility of using the effluent derived from the hydrothermal carbonization of microalgae (HTC-LF), performed to produce microalgal-based carbon encapsulated iron nanoparticles (ME-nFe), as a growth medium for the microalgae. Since this raw effluent was proved to be toxic for Allivibrio fisheri (EC50 = 1.8 %), the microalgal activity was studied also for detoxification. A 20 % dilution of the HTC-LF, using municipal centrate as the diluent, was proved to be suitable for microalgal growth without affecting the photochemical efficiency. In a continuous test, a community made of Chlorella spp. and Euglena gracilis was able to grow with and without CO2 addition, with average productivity of 62 +/- 5 and 43 +/- 8 mg L-1 d(-1), respectively. The treatment led to a 60 % COD removal, suggesting that the HTC-LF was not harmful to the microalgae-bacteria consortia, which was also able to decrease its toxicity. Environmental implications: The liquid byproduct of the hydrothermal carbonization (HTC-LF) is a known toxic for the water compartment. The possibility to exploit its nutrient content for microalgal growth was already proved by different studies using strong dilution with synthetic medium to perform batch cultivations. Here, the HTC-LF was diluted with the blackwater from biosolid dewatering, without adding any microelements or main nutrients to perform a continuous cultivation. A microalgal community was grown, consuming nutrients and the COD, lowering the toxicity. The obtained biomass was used to produce second-generation nano -particles, designing our process as a cycle in an eco-sustainable way.