Generally, centralized biological processes are of great importance for water pollution control and have been well developed to be used in municipal wastewater treatment plants. Due to the dispersed population, poor wastewater collection, and weaker economy in rural areas, they are still not suitable for rural areas. Based on the concept of “wastewater to energy and resources”, and to meet the demand of wastewater deodorization, we explore a bioecological process comprised of three-stage anaerobic wool-felt filter reactor (AWFR), an anoxic filter (ANF), four-stage water-dropping-self-rotating biological contactor (WDSRBC) and plant type construct wetland (PCW) to treat rural decentralized domestic wastewater. The advantages of the process may display, (1) The AWFR units are used for the conversion of organic matter to methane at ambient temperature without heating, (2) A low-enery ANF-WDSRBC system is explored to denitrification and odor removal in the ANF unit via WDSRBC effluent reflux. (3) The nutrients, such as N and P, are utilized by the ecological PCW unit via planting vegetables and food crop. The main results of this study are summaried as follows:
Firstly, in order to optimize the structure of AWFR, we used lab-scale AWFR to assess the effects of carrier filling ratio on the performances of AWFR under start-up conditions. The study showed with an increase of hydraulic retention time (HRT), the removal efficiencies of COD and SS gradually increased, whereas the VFA of the effluent decreased. Appropriately increasing carrier filling ratio in the system was beneficial to shorten the start-up time and stabilize the operation performance. The performance of AWFR40% was slightly lower than the AWFR55% systems. Considering the proper carrier filling ratio chosen to minimize clogging in the system, we recommend carrier filling ratio of wool-felt in the system is 40%.
Secondly, the pilot-scale AWFR was design to investigate the effect of HRT on the performance of AWFR in different seasons. In order to evaluate the annual performance of the system at ambient temperature, we used mass balance to analysis the pathway of organic matter removal. Meanwhile, the techniques of SEM and high-throughput sequencing were used to comprehensively analyze of the relationship between the structure of microbial community and the performance of the system. It found that COD removal efficiency increased with the extension of HRT, whereas the VFA of the effluent decreased. The system required long HRT to to stabilize the performance due to a decrease of seasonal temperature. The annual performance of the system showed that COD removal efficiency and daily gas production increased significantly with an increase of seasonal temperature and a decrease of HRT, whereas the VFA of the effluent decreased. In summer, COD removal efficiency increased to 76 ± 7.2%, while daily gas production reached a peak value of 10.7L/d. Based on COD mass balance, we found that approximately 43.5% -52.5% of influent COD was converted to methane.
Throughout the whole operation, microbial communities maintained high diversity to convert organic matter to methane. Bacillus, which contributed to the fermentation, was always the most abundant genus, whereas the genera Methanosaeta, Methanobacterium, and Methanolinea were the predominant methanogens. Seasonal temperature and HRT had a strong impact on the relative abundance of methanogens. In order to keep the system achieve a new equilibrium, the most predominant methanogens shifted from acetoclastic methanogens in summer to the hydrogenotrophic methanogens in winter.
Thirdly, as a modified rotating biological contactor, the structure of WDSRBC units directly affects the ANF-WDSRBC denitrification and deodorization. Due to water-dropping gravity rotation of the WDSRBC, water-dropping height, HRT and influent ammonium concentration were key factors of the system performance. In order to optimize the performance of ammonium removal efficiency in the WDSRBC system, response surface methodology (RSM) based on Box-Behnken Design (BBD) was used to assess the effects of the three parameters and their interactions. The results showed that water-dropping height, HRT, influent ammonium concentration, the interaction of water-dropping height and HRT, the interaction of HRT and influent ammonium concentration were significant parameters. The modified model was as follow:R=87.86+2.37X1+5.30 X2-4.25X3+1.19 X1 X2-1.81 X2 X3-2.52X12-4.62X22-2.11X32. The removal efficiency of ammonium of 80% was achieved, when the system was operated with the influent ammonium concentration ranged from 20-50 mg/L, water-dropping height 0.6 m, and an HRT of 2.2 h.
In order to achieve denitrification and deodorization in the ANF-WDSRBC system, WDSRBC effluent was recycled to ANF to utilize nitrate and dissolved oxygen(DO). The study showed that the odor threshold (TON) gradually decreased with the increase of reflux, HRT, DO and nitrate loading rate. However, TN removal efficiency increased at first followed by a decline as reflux ratio increased. Increasing HRT and nitrate loading rate were beneficial to TN removal, whereas high DO concentration decreased TN removal owing to inhibition of denitrification. To balance the performance of deodorization and nitrogen removal, the system was used to treat the effluent of AWFR under continuous-flow operation with a total HRT of 8.8 h and a reflux ratio of 100%. The average concentrations of COD, NH4+–N, and TN effluent were 35.0mg/L,3.7mg/L and 17.1mg/L, respectively. The effluent TON was 16.2, which reached the level-two emission standard in the “Emission standards for odor pollutants" (GB 14554-93).
Based on the High-thoughtput sequencing, the results demonstrated that microbial community in different units of the system maintained diversity, The genera Longilinea, Meniscus, and Bellilinea in the ANF unit were the main genera to participate in organic matter degradation, whereas Thiobacillus and Thauera contributed considerably to denitrification, odor and nitrogen removal. In the WDSRBC units the genera Nitrosomonas, Nitrosospira, Bacillus, and Nitrospira played vital roles in nitrification.
Finally, the study was to investigate the performance of PCW system to utilize nitrogen and phosphorus via planting water spinach, rice, water fennel and wheat. It found that the removal efficiencies of TN and TP increased with hydraulic loading rate (HLR) decreased. Lower HLR was beneficial to improve the production of plants in system.When the PCW system was operated at an HLR of 0.2 m3/(m2?d), the contributions of water spinach and rice of the PCW system to utilized of nitrogen and phosphorus were 18.2% and 31.9%, respectively, that was higher than the contributions of water fennel and wheat( TN 15.1% , TP 23.4%, respectively).