Effect of zero-valent iron and granular activated carbon on nutrient removal and community assembly of photogranules treating low-strength wastewater

Traditional wastewater treatment processes with high energy consumption and greenhouse gas emissions are not suitable for rural areas with low sewage strength and wide distribution. In this study, a microalgae-bacteria synergistic photogranules system was developed under the impetus of green chemical additives to address these challenges. The results showed that zero-valent iron (ZVI) or granular activated carbon (GAC) addition made successful photogranulation treating low-strength wastewater with excellent settleability and stability performance (settling velocity: 14–22 m h −1 ; integrity coefficient: 0.81–6.62%), while systems without light or additives failed due to the bio-granules disintegration caused by the overgrowth of predators or phototrophic species. A better nutrient removal performance (TN < 15 mg L −1 , TP < 0.4 mg L −1 ) was observed in photogranules systems, and stoichiometric and biological analysis found that the divisions of nitrogen removal by microalgae and bacteria were different for photogranules between GAC and ZVI additions. As a physical enhancer, GAC can be used as the nucleus of photogranules regenerating after granules disintegration rather than affecting the community succession process. However, ZVI addition strengthened the sedimentation ability and stability of photogranules through chemical and biological effects, focusing on enhancing bacterial community diversity, enriching biofilm formation bacteria and inhibiting the overgrowth of filamentous cyanobacteria. Notably, the photogranules process with ZVI addition could be operated under non-aeration conditions without compromising removal efficiency. There existed an ideal distribution of microalgae and bacterial functional species in the photogranules, which seemed to be essential for its self-sustained synergistic symbiosis and stability. Consequently, this work might provide engineering alternatives for realizing carbon neutrality and environmental sustainability of the decentralized wastewater treatment process for low-strength wastewater in rural areas. • Microalgae-bacteria synergistic cooperation improved TN and TP removal efficiency. • ZVI or GAC addition achieved photogranulation for low-strength wastewater treatment. • ZVI enhanced bacteria diversity and inhibited filamentous cyanobacteria overgrowth. • Photogranules realized non-aerated operation without compromising removal effect.