Researchers from the School of Chemical and Biomolecular Engineering at the Universiy of Sydney (Sydney, Australia; www.sydney.edu.au), led by Alejandro Montoya, have developed an electrochemical oxidation process to clear up wastewater, which is heavily contaminated with organic and inorganic species during a biofuel production process, using naturally abundant microalgae. The electrochemical oxidation process uses a boron-doped diamond anode to improve the quality of the aqueous-phase coproduct from the hydrothermal liquefaction (HTL-ACP) of microalgae Chlorella sp.
According to the researchers, the oxidation process produced an outstanding decrease of the chemical oxygen demand (COD) — ranging from 11 to 185 g/L with a median value of 65 g/L — including the decomposition of organic nitrogen and the discoloration of an original brown HTL-ACP. The nitrogen remains largely as ammonia/ammonium and nitrates. The deamination of nitrogenous organic compounds with subsequent accumulation of NH3/NH4– in the HTL-ACP and the partial oxidation to N2 take place more easily at lower current density, while the complete oxidation to NO3– is more pronounced at higher current densities.
The HTL-ACP contains between 25 and 50% of the carbon from the feedstock, as well as nitrogen, phosphorus, and a variety of ions. The total nitrogen ranges from 2.3 to 13.6 g/L with ammonia comprising 20 to 50% of total nitrogen content and the remaining appearing mainly as toxic N-heterocyclic organic compounds.
The researchers say the high variability of COD and total nitrogen are associated with the diverse combinations of HTL process conditions, such as biomass composition, feed slurry concentration, temperature, pressure and residence time. These conditions define the yield and quality of biocrude and HTL-ACP properties.