Anaerobic digestion wastewater decolorization by H₂O₂-enhanced electro-Fenton coagulation following nutrients recovery via acid tolerant and protein-rich Chlorella production

Farmland wastewater such as swine waste effluent contains rich nutrients needed for the production of microalgae. However, the dark color and high turbidity in the effluent hinder light penetration and thus the photosynthesis of the microalgae. In this study, the H₂O₂-enhanced electro-Fenton coagulation (EFC) was employed to remove chemical oxygen demand (COD), total suspended solid (TSS), color, and phosphorus effectively in anaerobic digested (AD) swine wastewater. During the enhanced EFC process, lower pH (pH 3), lower temperature (10 °C), higher H₂O₂ concentration (>200 mg·L⁻¹) and higher wastewater dilution factors (>2) all had positive effects on pollutant removal. After EFC pretreatment, the penetration of light into the medium was improved and the photosynthesis of Chlorella sp. was effectively increased. The initial N:P ratio in Chlorella cultures was adjusted to various levels (8:1 ~ 32:1) by KH₂PO₄ addition. When the N:P ratio was 8:1, the maximum biomass accumulation (2.07 g·L⁻¹), maximum protein content (54.1%) and protein productivity (62.2 mg·L⁻¹·d⁻¹) were obtained. In the process of Chlorella growth, due to the rapid consumption of ammonia, the pH of wastewater reached as low as 4, but the Chlorella still survived, showing that this strain of Chlorella was strongly acid-tolerant. This study showed the H₂O₂-enhanced EFC as an effective means of pretreatment of anaerobic effluent for aqueous culture of protein-rich Chlorella, which demonstrated to be an effective combination for nutrients recycling, and favorable to high value-added biomass production.