## 1 Introduction  
Water, land, and energy are important resources for the rapid growth and development of the global economy. However, in recent years, many developing countries face challenges and pressures of water and land pollution, and energy shortages. Particularly in rural areas of China, water pollution problems are increasingly aggravated due to the direct discharge of a large amount of untreated domestic wastewater [1, 2]. Although centralized biological processes have been well developed to be used in urban municipal wastewater treatment plants, they are not suitable for rural areas owing to the dispersed population, poor wastewater collection, and weaker economy in these areas. In addition, a concept of wastewater-to-resource is receiving increased attention by researchers and engineers working on wastewater treatment technology development [3]. Therefore, it is highly desirable to select a sustainable, robust and cost-effective process for the treatment of decentralized domestic wastewater in developing countries [4]. Simultaneous energy recovery and sustainable wastewater treatment make the application of anaerobic biotechnology in decentralized domestic wastewater treatment interesting [5, 6, 7]. Upflow anaerobic sludge blanket reactors (UASB) [8, 9], anaerobic baffled reactor (ABR) [10], and anaerobic membrane bioreactor (AnMBR) [7, 11, 12] are commonly used to treat domestic wastewater. Despite the effective chemical oxygen demand (COD) removal by these anaerobic biotechnologies, some challenges still exist, such as complex three-phase separator and long solids' retention time of UASB,membrane fouling and high energy costs of AnMBR, and high land footprint of ABR. Compared to these high-rate anaerobic reactors, the anaerobic filters (AFs) have drawn attention because of the following advantages [13; 14; 15]: (1) simple design configuration with low capital and operating costs, (2) excellent capability of high biomass retention with carriers, (3) stable operation, (4) greater tolerance to hydraulic loading rate and organic loading rate, and (5) low footprint. The potential of using AFs for treating wastewater have been well developed not only for industrial wastewater, but also for domestic wastewater [5; 16; 17; 18]. However, most of these studies focused on the effects of carriers, hydraulic retention time (HRT), and organic loading rate on the performance of AFs, and there were a few studies that focused on the effects of seasonal temperature [5; 18]. Previous studies have demonstrated that anaerobic biotechnology strongly depends on operation temperature, and AFs are commonly operated under mesophilic conditions [5; 19]. Nevertheless, considering the energy consumption and capital expenditure, it is economically unviable to heat anaerobic systems in decentralized domestic wastewater treatment of rural areas. Therefore, the application of AFs at seasonal temperature is more useful due to less energy demand. In context to China, this study is inevitable as the seasonal temperature varied throughout the whole year. Additionally, carriers are an important component of AFs, which determine the biomass retention capacity and the performance of the system. Therefore, the choices of appropriate carriers play an important role in AF system performance, particularly in the rural area of developing countries. A variety of natural materials, including zeolite [20], ceramic [21], rock [5; 22] and coconut shells [23] have been adopted as biofilm carriers. Even though these natural materials are low-cost, they are susceptible to clogging and require significant operation attention due to biofilm growth [22]. Wool felt is a class of natural porous materials that is a common waste product of paper making factories in China. Compared to solid materials, wool felt is a class of soft filler with a large specific surface area of about 950 m\({}^{2}\)/g that avoids clogging. Wool felt may be a viable option for AFs due to low cost and a large specific surface.