Study on rural sewage quality. The influent pollutants concentrations of the 63 rural sewage treatment stations are shown in Fig. 1. The average influent concentrations over the year of COD, BOD5, NH4+-N, TN, TP and SS of rural sewage treatment stations in northern Shaanxi were 212.60, 85.94, 34.59, 39.19, 4.34 and 323.03 mg/L respectively. The average influent concentrations of COD, BOD5, NH4+-N, TN, TP and SS in central Shaanxi were 171.10, 69.77, 28.26, 32.63, 3.49 and 207.48 mg/L, and the average influent concentrations in southern Shaanxi were 100.23, 38.22, 19.26, 23.69, 2.03 and 149.71 mg/L. The comparison of rural sewage quality in Shaanxi Province with that in other districts of China is shown in Table 2 of the Supplementary Material.
Northern Shaanxi is located in the arid and semi-arid areas of northwest China, where serious soil and water loss, barren soil and a generally poor ecological environment occurs. This leads to high sand content in surface drainage and rural sewage. In addition, due to the relatively under developed economy in this region, the quality of sewage pipeline planning and construction is low, resulting river water and industrial sewage entering pipelines prior to reaching the sewage treatment stations. This may account for the high concentrations of sewage pollutants in villages and towns in northern Shaanxi.
Central Shaanxi is located in the plain area, where soil and hydrological conditions are relatively better. In addition, the quality of the planning and construction of the sewerage pipe networks in the area is high, thus the mixing of other sewage and surface drainage into the pipes is rare. Therefore, the concentration of pollutants in the sewage of villages and towns entering treatment stations in this region is consistent with that of pollutants in typical rural sewage (Guo et al., 2014).
Southern Shaanxi is located in a mountainous area with numerous river systems, abundant rainfall and water resources. In addition, due to the less developed regional economy and the serious limitations caused by the topography of the region, the construction of rural sewage pipelines in this region has lagged behind others, and the hence the water quality of the treatment station influent was low, with surface water often imported into the pipelines. Hence, the dilution by rainwater and surface water is the main reason for the low pollutant concentrations in rural sewage in southern Shaanxi.
The B/D (BOD5/COD), C/N (COD/TN) and C/P (COD/TP) ratios of rural sewage in Shaanxi are presented in Table 1. The B/D of rural sewage in Shaanxi were around 0.40, which suggest that rural sewage in Shaanxi has good biochemical characteristics, and is therefore appropriate for biological wastewater treatment (Testolin et al., 2020). However, the C/N and C/P ratios in Shaanxi were lower than the theoretical values required for nutrient (nitrogen and phosphorus) removal in BNR. For a good performance of nitrogen removal, wastewater COD/TN ration higher than 10 were recommended18,19. However, nowadays domestic wastewater in many rural districts of China has low C/N ratio, sometimes lower than 5, resulting in poor total nitrogen removal (below 50%) due to the lack of sufficient carbon sources in the denitrification process. Therefore, rural sewage in Shaanxi is characterized by low carbon source content, which has a serious negative impact on the efficient operation of BNR system.
Table 1
The BOD5/COD, COD/TN and COD/TP ratios in rural sewage in Shaanxi Province, China.
| Item | Northern Shaanxi | Central Shaanxi | Southern Shaanxi |
| BOD5/COD | 0.40 | 0.38 | 0.41 |
| COD/TN | 5.42 | 5.24 | 4.23 |
| COD/TP | 48.99 | 49.03 | 49.37 |
Study on fluctuation and discontinuity characteristic of rural sewage. The daily fluctuation characterization of rural sewage for northern, central and southern regions in Shaanxi province, China is shown in Fig. 2. The daily inflow quantity of the village sewage treatment stations (20 ~ 200 t/d) has an obvious fluctuation rhythm (Fig. 2A), with the peak inflow concentrated in three time periods: 08:00 in the morning, 13:00 at midday and 18:00 in the afternoon. The average daily influent loads of village sewage treatment stations in northern Shaanxi, central Shaanxi and southern Shaanxi are 33%, 29% and 22%, respectively. As shown in Fig. 2B, the daily inflow quantity of the town sewage treatment stations (240 ~ 2000 t/d) has an obvious fluctuation as well, with the peak inflow also concentrated in the same three time periods. The average daily influent quantity load of town sewage treatment stations in northern Shaanxi, central Shaanxi and southern Shaanxi are 48%, 45% and 39%, respectively. Compared with village sewage treatment station peak flow times, the peak inflows of town sewage treatment stations have obvious lag times, which is most likely related to long sewage transportation distances in towns compared to those in villages.
Cut-off durations of rural sewage treatment stations were investigated in Fig. 3, which shows the cut-off duration in one day of rural sewage for Shaanxi. The average cut-off durations of village sewage treatment plants in northern, central and southern Shaanxi were 10 h/d, 14 h/d and 16 h/d, respectively. The average cut-off durations for town sewage treatment plants in northern, central and southern Shaanxi were 6 h/d, 8 h/d and 11 h/d, respectively. Station cut-offs were mostly concentrated at night (8pm to 8am) for both village and town stations. Hence most rural sewage treatment stations were in a condition of discontinuous influent, which would cause a huge obstacle to the stable operation of BNR system.
Study on pollutant removal performance of rural sewage treatment stations. According to the design scale, the 63 villages and towns sewage treatment stations sampled in this study can be divided into two sections: <200 t/d (village-level) and 240 t/d to 2000 t/d (town-level) respectively. The nutrient removal rate of each village and town sewage treatment station are shown in Fig. 4. It can be seen that the average removal rates of COD, NH4+-N, TN, TP and SS at the village-level sewage treatment stations were 50.0 ± 29.2%, 46.0 ± 26.1%, 38.5 ± 24.9%, 38.3 ± 23.8% and 47.1 ± 26.9%, respectively, and the corresponding values at the town-level sewage treatment stations were 67.0 ± 20.7%, 75.3 ± 14.7%, 53.4 ± 18.9%, 51.6 ± 21.0% and 71.7 ± 12.0%, respectively. The removal efficiency of pollutants in rural sewage treatment stations with different processes is shown in Fig. 2 of the Supplementary Material.
Compared with village-level sewage treatment stations, town-level sewage treatment stations had better pollutant removal efficiencies, which is most likely related to the difference in operation and maintenance modes between village and town-level sewage treatment stations. Town-level sewage treatment stations usually employ operation and maintenance personnel on-site, which can deal with abnormal situations in the daily operation process in a timely way. However, village-level sewage treatment stations usually adopt operation and maintenance modes using regular inspection by off-site staff, thus making it difficult to deal with abnormal situations in time, hence reducing the energy efficiency of pollutant removal.
The A2/O and MBR processes were the main processes in the sampling survey of rural sewage treatment stations in this study. These two mainstream activated sludge wastewater treatment processes can usually remove more than 80–90% of conventional pollutants under normal influent conditions20–22. Figure 4 shows the pollutant removal efficiencies for the rural sewage treatment stations observed in this study. The average removal efficiency of conventional pollutants in the surveyed rural sewage treatment stations was only between 38 ± 24% and 75 ± 15%. This was probably due to the intermittent nature of sewage flow in rural areas.
The relationship between nutrient removal rate of rural wastewater treatment stations and the cut-off duration is shown in Fig. 5. The results show that the removal rates of COD, NH4+-N, TN and TP at the 63 rural sewage treatment stations showed a negative correlation with the cut-off durations, i.e., the removal rates of all four pollutant indexes all decreased with increasing cut-off duration.
The direct impact of influent discontinuous on BNR system is that the substrate which microorganisms depend on for survival is insufficient, which makes the functional bacteria in a state of starvation for a long time. Normally under this condition, bacteria become less active by tweaking their metabolism to reduce their need for energy. In addition, as an independent mechanism, the bacteria may also switch on programmed cell death (PCD), a genetically programmed process of cell self-destruction that is triggered only under extreme starvation conditions23. The main purpose of this process is maintaining partial population activity to avoid losing an advantage in competition with other microbial populations, and this process will lead to cell death23.
The direct impact to a BNR system brought by either the activity reduction of functional bacteria or cell death is the reduction of the pollutants removal efficiency, which is the main internal reason why sewage treatment stations using this process cannot operate normally under conditions of intermittent sewage inflow from villages and towns.
Challenges and Recommendations. Rural sewage treatment in China is no longer as underfunded as it used to be, and various provinces and cities have introduced reasonable local standards for their rural sewage discharge requirements. The operation and maintenance modes of rural sewage treatment facilities are gradually changing from unmanned management or government-independent management to third party professional management. With the advent of the Internet of Things, the management of rural sewage treatment facilities is gradually becoming digital. In addition, China's No. 1 Central Document of 2021 introduced a five-year action plan to improve the rural living environment, including requirements for the improvement of rural water environment governance. Therefore, China's rural sewage treatment facilities need to improve, especially with respect to the long-term stability of pollutant removal.
According to the results of rural sewage discharge characteristics in Shaanxi Province, China in this study, the following challenges need to be considered in order to optimize rural sewage treatment:
1) Rural sewage usually only contains domestic sewage with low concentration of organic pollutants, thus the concentration of BOD5 is low, which leads to insufficient carbon in the influent of sewage treatment facilities. This will seriously reduce the efficiency of nitrogen and phosphorus removal in BNR systems. 2) Since residents in rural areas use little water and usually only produce sewage at specific times of the day, the rural sewage treatment station operated without influent for most of the time. This has a huge impact on the stable operation of BNR systems.
In view of the above challenges, this paper proposes the BEICT system for rural village and town sewage treatment stations in China:
In view of the intermittent supply of sewage in rural areas, the sewage treated by BNR systems can be mixed with sludge fermentation broth in specified proportions. The mixed liquor can be stored and used during influent discontinuation periods, so that the BNR system can undergo continuous operation using internal circulation. Using these ideas, Fig. 8 suggests the BEICT mode for rural sewage. As shown in the Fig. 8, part of the tail water produced by the bioreactor during the daytime is collected into the mixing tank. Meanwhile, the excess sludge discharged by the bioreactor is anaerobic fermented. The VFAs in the fermentation tank are accumulated through process regulation, and the supernatant of the fermentation liquid is discharged into the mixing tank and mixed with the tail water. The mixed liquor contains a large amount of VFAs, so it is more conducive to the growth and metabolism of microorganisms in the BNR system. When the carbon source of the bioreactor is insufficient or there is no wastewater treatment at nighttime, the mixed liquor in the mixing tank can be recycled to realize the supplement of carbon and water source.
Operation effect of the BEICT bioreactor. The pollutant removal efficiency of the bioreactor is shown in Fig. 6. As can be seen from Fig. 6A and 6B, there is little difference in the removal effect of the system on COD and NH4+-N in stage I and II. The average removal rates of COD and NH4+-N in the whole experiment were 88.42% and 96.55%, respectively. The corresponding average effluent concentrations were 31.24 mg/L and 0.74 mg/L, respectively. The results showed that the influent mode of the mixture of recharge tail water and sludge fermentation liquid during cut-off period did not affect the removal performance of COD and NH4+-N in the BEICT system. In the whole process of the experiment, the BEICT system maintained an efficient and stable removal efficiency of COD and NH4+-N, and the corresponding average effluent concentration could meet the national water quality standard limits.
As can be seen from Fig. 6C and 6D, the removal efficiency of TN and TP in stage II showed an obvious increasing trend compared with stage I. The average TN removal rates of the system in stage I and Stage II were 68.11% and 77.42%, respectively, and the corresponding average effluent concentration was 16.14 mg/L and 12.46 mg/L. The average TP removal rates of the system in stage I and Stage II were 79.63% and 89.69%, respectively, and the corresponding average effluent concentration was 1.14 mg/L and 0.65 mg/L. In conclusion, the influent mode of the mixture of recharge tail water and sludge fermentation liquid during cut-off period can improve the removal performance of TN and TP in BEICT system.
The variation of MLSS, MLVSS and SVI of activated sludge in the aerobic zone of the BEICT system are shown in Fig. 7. It can be seen that the activated sludge concentration in the system was relatively stable, and the average MLSS, MLVSS and SVI of the activated sludge in the whole experiment process were 3702 mg/L, 2943 mg/L and 124 mL/g, respectively. Therefore, the activated sludge in the system maintained good activity and sludge settling performance, and there was no sludge bulking phenomenon. The average MLVSS/MLSS of stage I and II were 0.63 and 0.72, respectively, and the average MLVSS/MLSS of activated sludge in the system was increased in stage II, which further indicated that the sludge activity was improved in stage II. In addition, the SVI of activated sludge in the system increased in stage II, and the average SVI of stage I and II were 104 mL/g and 128 mL/g, respectively. Therefore, the influent mode of the mixture of recharge tail water and sludge fermentation liquid during cut-off period will worsen the sedimentation performance of activated sludge, but it will not adversely affect the sludge activity and pollutant removal performance of the system.