RESEARCH ARTICLE Mechanistic insight into the biofilm formation and process performance of a passive aeration ditch (PAD) for decentralized wastewater treatment Jibin Li 1* , Jinxing Ma 1,2* , Li Sun 1 , Xin Liu 3 , Huaiyu Liao 1 , Di He (✉) 1,2 1 Key Laboratory for City Cluster Environmental Safety and Green Development of the Ministry of Education, Institute of Environmental and Ecological Engineering, Guangdong University of Technology, Guangzhou 510006, China 2 Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou 511458, China 3 School of Environmental Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, China 1 Introduction In the past decades, there has been a growing attention on the management of the decentralized wastewater, which is widely distributed in rural areas in developing countries due to the lack of wastewater collection systems. The discharge of the unpolished decentralized wastewater into the aquatic environment has caused many ecological problems and posed a threat to human health in rural areas (Massoud et al., 2009; Guo et al., 2014; Singh et al., 2015; Zhang et al., 2020). In coastal communities, the character of the decentralized wastewater is more compli- cated. In the addition to the typically high concentrations ✉ Corresponding author E-mail: di.he@gdut.edu.cn * These authors contribute equally to this work. Front. Environ. Sci. Eng. 2022, 16(7): 86 https://doi.org/10.1007/s11783-021-1494-3 HIGHLIGHTS • A Passive Aeration Ditch was developed to treat decentralized wastewater. • A model was developed to describe the process performance. • A high C/N ratio facilitates microbial growth but nitrification deteriorates. • A high salinity decreases both organic and nitrogen contaminants removal. ARTICLE INFO Article history: Received 14 July 2021 Revised 23 September 2021 Accepted 30 September 2021 Available online 8 November 2021 Keywords: Decentralized wastewater Passive aeration ditch Biofilm formation C/N ratio Salinity Model simulation GRAPHIC ABSTRACT ABSTRACT Decentralized wastewater containing elevated salinity is an emerging threat to the local environment and sanitation in remote coastal communities. Regarding the cost and treatment efficiencies, we propose a passive aeration ditch (PAD) using non-woven polyester fabric as a feasible bubbleless aerator and biofilm carrier for wastewater treatment. Consideration has been first given to PAD’s efficacy in treating saline decentralized wastewater, and then to the impact of chemical oxygen demand-to-nitrogen (C/N) ratio and salinity on biofilm formation. A multispecies model incorporating the salinity effect has been developed to depict the system performance and predict the microbial community. Results showed that the PAD system had great capacity for pollutants removal. The biofilm thickness increased at a higher C/N ratio because of the boost of aerobic heterotrophs and denitrifying bacteria, which consequently improved the COD and total nitrogen removal. However, this led to the deterioration of ammonia removal. Moreover, while a higher salinity benefited the biofilm growth, the contaminant removal efficiencies decreased because the salinity inhibited the activity of aerobic heterotrophs and reduced the abundance of nitrifying bacteria inside the biofilm. Based on the model simulation, feed water with salinity below 2% and C/N ratio in a range of 1 to 3 forms a biofilm that can reach relatively high organic matter and ammonia removal. These findings not only show the feasibility of PAD in treatment of saline decentralized wastewater, but also offer a systematic strategy to predict and optimize the process performance. © Higher Education Press 2021