Hydrothermal Carbonization of Microalgae-Fungal Pellets: Removal of Nutrients from the Aqueous Phase Fungi and Microalgae Cultivation

Hydrothermal carbonization (HTC) has a unique advantage over conventional processes to convert wet biomass into a solid fuel without the need to dewater and dry the biomass. A byproduct of the HTC process is the aqueous phase (AP), rich in organic and inorganic compounds. The AP, if discharged without proper treatment, can cause environmental pollution. In this study, mono-, co-, and stepwise-culture of fungi (Penicillium sp.) and microalgae (Chlorella sp.) on AP from HTC of the same fungi-microalgae strains was investigated. The results showed that the stepwise culture for six days (first fungi and then microalgae cultivation) in 5% AP achieved a biomass yield of 0.63 g/L and the best performance for the removal of TN (32.91%), TP (95.30%), chemical oxygen demand (64.38%), NH4–N (26.97%), and organic components, probably due to the strong detoxification capability of the fungi, which improved microalgae growth. The microalgae and fungi biomass harvested from 5% AP can be sustainable feedstocks for fuel production. This study demonstrated that utilizing the AP for the cultivation of fungi and microalgae has the potential to form the semi-closed loop energy system in the biomass HTC approach.