Enhancing biochar structure and removal efficiency of ammonium and microalgae in wastewater treatment through combined biological and thermal treatments

Abstract

This study explored the efficiency of biological pre-treatment combined with thermal treatment to produce biochars, called E-biochars, for the removal of ammonium and microalgae without using chemical pre-treatment. The aim was to develop an environmentally sustainable approach for large-scale biochar production with high efficiency in wastewater treatment. The biological pre-treatment involved incubating the sawdust with Bacillus subtilis for 14 to 20 days and then subjected to pre-treatment at low temperatures (250-350 °C). This process broke down cellulose and altered its lignocellulosic structure, improving the biochar's morphology by leading to a more anisotropic pore structure. The thermal pre-treatment after that will help to open a broader range of pore sizes, spanning from macropores to micro-pores in the 1 μm to 5 μm dimension range. These alterations facilitated the efficient removal of ammonium and microalgae during adsorption experiments. The findings fit the Langmuir and the Freundlich models, according to the isotherm test of NH4+ and microalgae (R2 = 0.92÷0.99). The E-biochar termed EBH20, composted with Bacillus subtilis for 20 days at 350 °C, performed best, with Qmax equal to 40.2 mg/ g NH4+ under neutral conditions (pH = 7). Interestingly, the co-occurrence of ammonium and microalgae enhanced the removal of these contaminants. The interaction between microalgae cells and ammonium facilitated ammonium adsorption onto biochar and microalgae. The column breakthrough curves under NH4+ and NH4+ mixed microalgae adsorption were predicted using the Thomas model with the high agreement (R2 = 0.91÷0.99) for ammonium but for algae between prediction and running experimental column data.