A circular economy approach for sustainable biohydrogen, activated carbon and biofertilizers in a microalgae-based wastewater treatment

This study presents an innovative, multi-step strategy for hydrogen production, bioenergy generation, and resource recovery from domestic wastewater. In the first stage, bicarbonate-supplemented microalgae were cultivated outdoors to enhance carbohydrate content without inhibiting growth. This process achieved up to 62 % nitrogen removal and complete depletion of dissolved phosphorus, with carbohydrate yields up to 30 %. The harvested algal biomass was then chemically hydrolyzed using hydrochloric acid (HCl) and sulfuric acid (H 2 SO 4 ), producing both soluble sugars and solid residues. The solid residues were pyrolyzed into biochar and subsequently converted into activated carbon. Hydrolysis with dilute sulfuric acid produced amorphous carbon with a distorted graphitic structure and sulphur-related peaks, while hydrochloric acid treatment yielded amorphous carbon with a more defined graphitic structure. In contrast, sulfuric acid hydrolysis led to the formation of a crystalline, sulphur-containing material. These structural differences suggest potential applications in environmental treatment and energy storage. The soluble sugars obtained through sulfuric acid hydrolysis were subjected to dark fermentation, resulting in hydrogen production of 215 ± 36 mL/g hexose, primarily via the acetic acid pathway. The digestate from dark fermentation was further assessed for its agricultural value. The liquid effluent, containing 9405.17 ± 83.53 mg/L of available nitrogen (as N–NH 4 + and N–NO 3 - ), supported tomato plant growth over a 21-day cultivation period, indicating its potential as a fertilizer. However, microbial management is recommended to ensure plant health. Overall, this research demonstrates the potential of integrating dark fermentation into wastewater valorization systems, extending its role beyond pollutant degradation to sustainable energy and agricultural applications. • Microalgae cultivated in wastewater boosts bioH 2 production in a multi-stage process. • Bicarbonate addition keeps microalgal carbohydrates high without growth loss. • Waste from hydrolysis was repurposed into biochar for sustainable reuse. • Dark fermentation of sugars yields up to 215 ± 36 mL H 2 /g hexose. • Fermentation effluents support tomato growth but need microbial management.