Subsurface Wastewater Infiltration System Research Articles

Nitrogen removal performance of improved subsurface wastewater infiltration system under various influent carbon-nitrogen ratios.

Subsurface wastewater infiltration system (SWIS) has been recognized as a simple operation and environmentally friendly technology for wastewater purification. However, effectively removing nitrogen (N) remains a challenge, hindering the widespread application of SWIS. In this study, zero-valent iron (ZVI) and porous mineral material (PMM) were applied in SWIS to improve the soil matrix. Our results suggested that the addition of ZVI and PMM could simultaneously enhance N removal efficiency and reduce nitrous oxide emissions. This could be attributed to the abundant electrons generated by ZVI alleviating the electronic limitation of denitrification and the porous structure of PMM providing solid phase support for microbial growth. In addition, the abundance of microbial functional genes increased in modified SWIS, which could further explain the higher pollutant removal efficiency. Overall, this study provides new insights into the mitigation of wastewater pollution and greenhouse gas emissions in SWIS. PRACTITIONER POINTS: ZVI and PMM can adapt to different C loads and enhance pollutant removal efficiency in SWIS. Increasing C-N ratios positively affected the nitrate removal performance and negatively affected ammonium removal performance in SWIS. The amending soil matrix promoted the reduction of the N2 O to N2 and greenhouse gas emissions were well controlled. The abundance of microbial functional genes increased with the improvement of the soil matrix.

Transformation mechanisms of ammonium and nitrate in subsurface wastewater infiltration system: Implication for reducing greenhouse gas emissions

Subsurface wastewater infiltration system (SWIS) has been recognized as a cost-effective and environmentally friendly tool for wastewater treatment. However, there is a lack of knowledge on the transformation processes of nitrogen (N), hindering the improvement of the N removal efficiency in SWIS. Here, the migration and transformation mechanisms of ammonium (NH4+-N) and nitrate (NO3+-N) over 10 days were explored by 15N labeling technique. Over the study period, 49% of the added 15NH4+-N remained in the soil, 29% was removed via gaseous N emissions, and 14% was leaked with the effluent in the SWIS. In contrast, only 11% of the added 15NO3−-N remained in the soil while 65% of the added 15NO3−-N was removed via gaseous N emissions, and 12% with the effluent in the SWIS. The main pathway for N2O emission was denitrification (52-70%) followed by nitrification (15-28%) and co-denitrification (9-20%). Denitrification was also the predominant pathway for N loss as N2, accounting for 88-96% of the N2 emission. The dominant biological transformation processes were different at divergent soil depths, corresponding to nitrification zone and denitrification zone along the longitudinal continuum in SWIS, which was confirmed by the expression patterns of microbial gene abundance. Overall, our findings reveal the mechanism of N transformation in SWIS and provide a theoretical basis for establishing a pollutant management strategy and reducing greenhouse gas emissions from domestic wastewater treatment.

Effect of Fulvic Acid on the Denitrification in Deep Subsurface Wastewater Infiltration System

This work aims to explore the impact of fulvic acid (FA) on denitrification within the purification process of sewage in the deep subsurface wastewater infiltration system (DSWIS). In the system, an organic glass column (height = 2.40 m; radius = 0.30 m) was filled with several layers of soil. Simulated domestic wastewater and extracted FA from landfill leachate were used in the experiments. It was found that before and after the addition of FA, COD, and NH4+-N were efficiently removed when a hydraulic load was 8 cm·d-1. Moreover, after FA addition, the removal efficiency of TN was enhanced from 67.74% to 78.01%. Organic matter transformation analysis indicated that in the under part, the shortage of carbon sources limited the denitrification prior to FA addition, resulting in a low TN removal efficiency. However, after adding FA, more FA-like substances were transferred into protein-like matters than before the addition of FA, which has helped produce more easily biodegradable organics for denitrification. So, the addition of FA could enhance the denitrification process in the system of DSWIS.

How the subsurface wastewater infiltration systems with iron‑carbon micro-electrolytic substrate achieve high pollutants removal: A metabolomic view

Being an ecological engineering water treatment technology, subsurface wastewater infiltration system (SWIS) combines ecological principles with anaerobic and aerobic processes. Iron‑carbon micro-electrolysis (ICME) is a rapidly developing SWIS substrate type in recent years. In this study, microbial metabolomics was introduced into SWIS aiming to reveal the “black box” of enhanced nitrogen removal in SWIS assisted by ICME (namely ICMESWIS). Ultra performance liquid chromatography-mass spectrometry (UPLC-MS), principal component analysis (PCA), partial least squares discriminant analysis (PLS-DA) and univariate analysis were carried out to classify the samples from SWIS and ICMESWIS. The results showed that compared with SWIS, 28.5 ± 2.6 %, 33.7 ± 1.3 % and 19.3 ± 0.4 % improvements were found for NO3−-N, NH4+-N and NO2−-N removals in ICMESWIS. According to the model's variable importance factor and fold change >2, metabolites with large differences were screened out. Metabolites, such as 1-(4-nitrophenyl)piperidine (ESI-) and 2-amino-1,3,4-octadecanetriol (ESI-) presented higher abundance in ICMESWIS, in consistent with the higher degradation of pollutants. The Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that nitrification, nitrate reduction, dissimilatory nitrate reduction, denitrification, anaerobic ammonia oxidation (anammox) are the main nitrogen removal pathways in ICMESWIS.

How do aeration mode and influent carbon/nitrogen ratio affect pollutant removal, gas emission, functional genes and bacterial community in subsurface wastewater infiltration systems?

Abstract This study investigated the influences of aeration mode and influent carbon/nitrogen ratio on matrix oxygen concentration, pollutant removal, greenhouse gas emission, functional gene abundances and bacterial community in subsurface wastewater infiltration systems (SWISs). Intermittent or continuous aeration enhanced oxygen supply at 0.6 m depth in the matrix, which improved organics removal, nitrogen removal, the abundances of bacterial 16S rRNA, amoA, nxrA, narG, napA, nirK, nirS, norB, nosZ genes, bacterial community Alpha diversity, the relative abundances of Actinobacteria at 0.6 m depth, the relative abundances of Chloroflexi, Gemmatimonadetes, Bacteroidetes and Firmicutes at 0.9 and 1.2 m depth and reduced CH4 and N2O conversion efficiencies, the abundance of mcrA gene with carbon/nitrogen ratio of 12 and 16 compared with non-aeration. Increased carbon/nitrogen ratio resulted in higher TN removal efficiencies and lower CH4 and N2O conversion efficiencies in aeration SWISs than those in non-aeration SWIS. Intermittent aeration SWIS obtained high removal efficiencies of 83.2, 85.4 and 90.8% for TN, NH4+ -N and COD and low conversion efficiency of 0.21 and 0.65% for N2O and CH4 with optimal carbon/nitrogen ratio of 12. However, high TN (82.6%), NH4+ -N (84.9%) and COD (92.2%) removal efficiencies and low CH4 (0.67%) and N2O (0.23%) conversion efficiencies were achieved in continuous aeration SWIS with carbon/nitrogen ratio of 16.

How does microorganism in different zones cooperatively promote N2O emissions from SWIS during freeze-thaw?

Subsurface wastewater infiltration systems (SWIS) are environmentally-friendly technologies for domestic wastewater treatment, where pollutants are removed by physical, chemical and biological reactions. However, SWIS also produce nitrous oxide (N2O), a potent greenhouse gas. Distribution of dissolved oxygen and nitrogen in SWIS determines denitrification process, which affects microbial activity and N2O release degree in different layers of system. Top layer of SWIS substrate is exposed to environmental factors such as freeze-thaw (FT), which changes microbial community structure in different substrates. Exact mechanisms of microbial-mediated N2O emissions in SWIS are still unclear despite extensive research. Therefore, this study simulated FT process using in-situ SWIS, to investigate how FT disturbance affects microbial community structure and N2O release in SWIS profiles. Results showed that after the ninth freeze-thaw cycle, FT stimulated anaerobic bacteria activities such as Euryarchaeota, accounting for 78.4 % of total Euryarchaeota population in middle (60 cm) and 33.97 % in the lower layer. Under low oxygen conditions, NO2−-N accumulation in middle and lower layers provided a sufficient nitrogen source for Euryarchaeota. Canonical correlation analysis (CCA) showed Euryarchaeota was significantly correlated with N2O emissions in middle and lower layers during FT, contributing 31.68 %–32.01 % and 61.78 %–65.15 %, respectively. These results suggested that FT disturbance enhanced denitrification by anaerobic bacteria in middle and lower layers of SWIS, significantly increasing N2O emissions. However, specific pathways and mechanisms of N2O production by Euryarchaeota remain to be elucidated in future studies.

Investigating the effect of typical antibiotics on denitrification rate and microbial population of subsurface wastewater infiltration system

Investigating the effect of typical antibiotics on denitrification rate and microbial population of subsurface wastewater infiltration system

Effects of freeze-thaw intensities on N2O release from subsurface wastewater infiltration system

Subsurface wastewater infiltration system (SWIS) is a low-investment and cost-effective wastewater treatment technology. Nitrous oxide (N2O), an important greenhouse and ozone-depleting gas, is a major by-product of biological denitrification processes within SWIS. Although SWIS has been widely applied in varying zones, until now, whether and how periodical freeze-thaw cycles (FTCs) occurring in aerobic layers affect N2O release still remain unclear? In this study, two runs were arranged, which covered two freeze-thaw (FT) intensities of − 5 °C/5 °C and − 15 °C/5 °C, respectively. An in-situ simulation system of SWIS was carried out to monitor N2O release and microbial community structure in different profiles. The result revealed that raising FT intensity decreased the rate of nitrogen removal of SWIS. Average removal rate of TN under strong FTCs was 88.65–88.97%, which was 1.32–2.47% lower than that under weak FTCs conditions. In addition, a positive correlation was found between FT numbers and N2O release. The contribution of anaerobic zone increased from 29.24% to 75.16% with extending the FT cycles from 1 to 9. High-throughput sequencing results showed that different FT intensities had distinct effects on microbial communities. Euryarchaeota abundance increased significantly under strong FT treatment, while Proteobacteria abundance sharply decreased from 49.5% to 7.6% at day 27 of stable operation. PCA analysis shows that bacterial community composition of samples treated with weak FT and CK is similar. While significant differences in sample composition were found between strong and weak FT treatments, which was the main reason resulting in different N2O release patterns.

Effect of increased carbon load on denitrification efficiency and nitrate isotope enrichment factors in subsurface wastewater infiltration system.

Denitrification plays a dominant role in nitrate removal in subsurface wastewater infiltration system (SWIS). However, the effect of increased carbon (C) load on denitrification efficiency in the SWIS remain unclear. In this study, we used analyses of stable isotopes of nitrogen (N) and oxygen (O) in nitrate to investigate the N and O isotope enrichment factors (15 ε and 18 ε) and quantified N losses via denitrification in SWIS. The results demonstrated that an increase in C loads positively affected the pollutant removal performance of SWIS. The natural abundance of 15 N and 18 O increased with decreasing nitrate concentration from 12.5 to 7.3 mg/L, accompanied by increased 15 ε and 18 ε from -8.7‰ to -10.6‰ and -5.9‰ to -8.2‰, respectively, as the C load increased from 18 to 36 g/(m2 d). The contribution of denitrification to nitrate removal was 62%, 71%, and 77% when C loads were 18, 27, and 36 g/(m2 d), respectively, indicating that increased C loads could improve the nitrate removal through denitrification in SWIS. PRACTITIONER POINTS: Increasing C loads positively affected the nitrate removal performance of SWIS. N and O isotope enrichment factors of nitrate increased with the enhancement of influent C load. A C load of 36 g/(m2 d) is recommended in SWIS to improve the N removal performance and denitrification efficiency.

Nitrogen removal in improved subsurface wastewater infiltration system: Mechanism, microbial indicators and the limitation of phosphorus

To enhance the nitrogen removal capacity, scrap iron filings and Si–Al porous clay mineral material (PCMW) was used to improve a subsurface wastewater infiltration system (SWIS). The results showed TN and NH4+-N removal efficiencies of improved SWIS were 20.72% and 5.49% higher than those of the control SWIS, respectively. Based on the response of the removal performance, microbial community and function analysis of 16s rRNA amplicon sequencing results, the amending soil matrix substantially enriched the nitrogen removal bacteria (Rhizobiales_Incertae_Sedis and Gemmatimonadaceae), and significantly improved the activities of key enzymes (Hao, NasAB, NarGHI, NirK, NorBC, NirA and NirBD), particularly at co-occurrence zone of nitrification and denitrification (70–130 cm depth). The amending soil matrix not only extended the growth space of microbes, but also provided additional electrons and carbon sources for denitrifying bacteria by regulating the structure and function of the microbial community. In addition, amending soil matrix could enhance phosphate metabolism genes and phosphate solubilizing microbes in the denitrification zone by increasing the phosphorus source, thus strengthening nitrogen metabolism. Nitrospiraceae, Rhizobiales_Incertae_Sedis and Gemmatimonadaceae related to nitrogen removal and Bacillaceae with phosphate-solubilizing ability could be used as microbial indicators of nitrogen removal in SWISs. The reciprocal action of environmental on microbial characteristics exhibited microbial functional were related to DO, Fe2+, TOC, TP, TN, NH4+-N and NO3−-N. Those could be used as physicochemical and biological indicators for application and monitoring of SWIS. In conclusion, this study provided a low-cost and efficient enhancement approach for the application of SWIS in decentralized domestic sewage treatment, and furnished theoretical support for subsequent applications.

Simultaneous removal of typical antibiotics and nitrogen by SWIS assisted by iron carbon micro-electrolysis

In this study, a combination of subsurface wastewater infiltration system (SWIS) with iron carbon micro-electrolysis (namely ICMESWIS) was used to enhance the simultaneous removal of nitrogen and typical antibiotics (SMX, OTC and HCl-Lev was selected). The results indicated that assisted by 10 % dopant amount of ICME, SMX, OTC, Lev-HCl, NH4+-N, NO3--N and NO2--N removal rates increased by 26.6 %, 19.7 %, 10.2 %, 33.7 %, 28.5 % and 19.3 %, respectively via ICMESWIS than SWIS. The higher TFe release rate, organic matter content and richer microbial populations encouraged ICMESWIS to resist the impact of antibiotics. High-throughput sequencing revealed that electrochemical effect enhanced the microbial community structure, which was the main pathway for nitrogen and antibiotics removal. The total proportions of nitrogen and antibiotics removal relating bacteria were 18.38 % and 46.67 %, 5.49 % and 4.72 % higher than those of SWIS, respectively. Anammox became an additional N removal way in ICMESWIS.

Does the repeated freezing and thawing of the aerobic layer affect the anaerobic release of N2O from SWIS?

Subsurface wastewater infiltration systems (SWIS) is an efficient, economical, and less temperature affected sewage treatment technology. Pollutants are removed by physical, chemical, and biological reactions such as filtration, adsorption, oxidation, and degradation. Under the conditions of limited carbon source and inactivation of nitrous oxide reductase, N2O, an important greenhouse gas, is released from the anaerobic layers of SWIS. However, is N2O release affected by repeated freeze-thawing of the upper aerobic layer? How does microbial population structure and denitrogenate activity in different profiles respond to the freeze-thaw cycle (FTC)? These questions have not yet been revealed. In this study, a SWIS simulator with in-situ regulation of FTC was first constructed. The re-distribution of N2O, microbial composition and denitrogenate activity were analyzed in response to FTC. Furthermore, potential bio-markers were screened and identified. The results revealed that the release of N2O was correlated with FTC, with the anaerobic layer being the main contributor throughout, accounting for 73.32–75.8 % of the total release. The limiting factor for N2O emissions was the NO3−-N concentration in the anaerobic zone, and there were no simple linear communications between total nitrogen and N2O generations. High throughput sequencing results showed the main markers of SWIS were Proteobacteria, Gemmatimonadetes, Firmicutes, Acidobacteria, Actinobacteria, Bacteroidetes, Chloroflexi and Nitrospirae, accounting for 97.4 %–98.1 % of the total relative abundance. A significant positive correlation between Firmicutes and anaerobic release of N2O was observed, where Firmicutes abundance increased from 5 % to 21 % during the experimental cycle, while N2O concentration increased from 2.65 mg·L−1 to 18.88 mg·L−1. The results indicated that Firmicutes was an important biomarker of N2O release under freeze-thaw conditions.

Effect of intermittent operation and shunt wastewater on pollutant removal and microbial community changes in subsurface wastewater infiltration system

Subsurface wastewater infiltration system (SWIS) is widely used in the treatment of rural domestic sewage. However, the nitrogen removal effect is unsatisfactory. In this study, laboratory-scale SWIS was built, and the nitrogen removal capacity of the system was optimized by a combination of intermittent operation and the shunt method. The optimal total nitrogen (TN) removal rate (56.53%) is obtained when the wet-dry ratio (Rwd) is 1:2 and the shunt ratio is 3:1. The optimal ammonia nitrogen (NH4+-N) removal rate (54.97%) is obtained when Rwd is 1:3 and shunt ratio is 2:1. In addition, SWIS achieved a good COD removal rate (>75%) and TP removal rate (>95%) during the whole operation stage. Proteobacteria, Acidobateria and Bacteroidetes are high abundance phyla in SWIS. Bacteroidetes, Chloroflexi and Nitrospirae associated with nitrogen removal are abundant at the shunt ratio of 2:1 and 3:1. At the genus level, Bosea, Bradyrhizobium and Phenylobacterium were functional genera associated with denitrification, enriching the substratum of SWIS at shunt ratios of 1:1, 2:1 and 3:1. Nitrogen (N) forms and ORP analysis showed that under the appropriate shunt ratio and Rwd, the SWIS matrix has a suitable redox microenvironment, and thus creates a well-defined nitrification area and denitrification area. This research provides a reliable theoretical basis for the construction and operation of SWIS in rural areas.

Evaluation of the removal performance in long-term operation of bioaugmented subsurface wastewater infiltration systems under a high hydraulic loading rate

To enhance the pollutant removal performance of a subsurface wastewater infiltration system (SWIS) used in decentralized treatment of domestic sewage, two nitrogen-removing strains were isolated from a SWIS with high removal efficiency and inoculated into another SWIS. Three SWISs were analyzed: a sterilized SWIS (system 1), a control SWIS (system 2), and a bioaugmented SWIS (system 3). Under the conditions of 0.28 m3/m2 ⋅ d hydraulic loading rate and 716 d running time, the removal capacities of COD, NH4+-N, TN and TP of system 3 were 167.19 ± 14.21 mg/L ⋅ d, 23.85 ± 0.74 mg/L ⋅ d, 25.21 ± 0.96 mg/L ⋅ d and 6.46 ± 0.06 mg/L ⋅ d, respectively. Correspondingly, the removal efficiencies of COD, NH4+-N and TN were 89.63%, 92.69%, and 86.95%, respectively, which were 6.41%, 6.45%, and 11.48% higher than those of system 2 and 63.01%, 55.15%, and 51.27% higher than those of system 1. The concentrations of COD, NH4+-N, TN and TP of system 3 stabilized at depths of 130 cm, 110 cm, 150 cm, and 130 cm, respectively. Additionally, both the number of nitrifying/denitrifying bacteria and the nitrate/nitrite reductase activity (NRA/NIRA) showed that the nitrogen removal zone was 90–130 cm deep. The coefficients of determination (R2) between NO3−-N concentrations and the number of denitrifying bacteria/NR activities in system 3 showed the greatest correlation, which were 0.97 at 150 cm and 0.92 at 90 cm. Bioaugmentation by the re-inoculation of strains isolated from a SWIS into new SWIS could be an ideal method to improve the performance and service life of SWISs.

Decentralized Domestic Sewage Treatment Using an Integrated Multi-Soil-Layering and Subsurface Wastewater Infiltration System

In this study, an integrated multi-soil-layering and subsurface wastewater infiltration (MSL-SWI) system was developed for decentralized domestic sewage treatment under high hydraulic loading rates (HLRs). To improve sustainable nitrogen removal, the influence of intermittent operation and shunt distributing wastewater on the performance of MSL-SWI systems was investigated. The optimal performance—with removal efficiencies of 93.41% for chemical oxygen demand, 97.91% for total phosphorus, 74.02% for ammonia nitrogen, and 73.56% for total nitrogen—was achieved using both intermittent operation and shunt distributing wastewater under an HLR of 0.3 m3 m−2 d−1. The activity of microbial nitrogen functional genes (i.e., amoA, nirK, nirS, nosZ, and anammox 16S rRNA) and their relationships with nitrogen transformation rates were further analyzed in different layers of the system. The results imply that nitrification and anaerobic ammonium oxidation in the MSL section coupled with nitrification and denitrification in the SWI section contribute to main the mechanisms of sustainable nitrogen removal. In summary, MSL-SWI systems not only operate with high efficiency under high HLRs, but the contaminant removal is also stable and sustainable, which are promising properties for domestic sewage treatment in areas where land resources are limited.

Accumulation and characteristics of fluorescent dissolved organic matter in loess soil-based subsurface wastewater infiltration system with aeration and biochar addition.

Subsurface wastewater infiltration systems (SWISs) have been widely used to treat rural domestic sewage. However, the low nitrogen removal and severe clogging problem always restrict the sustainability of SWISs for wastewater treatment. This study investigated the effects of aeration and biochar on the accumulation of nutrients and dissolved organic matter (DOM) in the substrate of loess soil-based SWISs for understanding the accumulation characteristics of DOM and the enhanced decontamination mechanism. The results showed that biochar addition could not improve the accumulation of nitrogen and phosphorus in the substrate, but could enhance denitrification (22%) via providing sufficient carbon for microorganisms. Moreover, the accumulation of organic matter in the substrate was also greatly affected. The DOM concentration of System D in the 40-60cm layer reached 85.76mgL-1, which indicated that biochar could release abundant DOM. Substrate DOM mainly contained humic acid-like and tryptophan-like substances. Moreover, the refractory macromolecular DOM components with high aromaticity and humification were found in the substrate below 60cm of systems with biochar addition. This may be related to the DOM released by biochar and the extracellular polymeric substance (EPS) produced by microorganisms. It may affect the sustainability of the substrate to a certain extent, but fortunately that intermittent aeration could reduce this adverse effect. This research could provide new insights for preventing clogging and useful guidance for improving wastewater treatment performance in SWISs.

Oxygen Consumption in Two Subsurface Wastewater Infiltration Systems under Continuous Operation Mode

In this work, an innovative arrangement of a vertical subsurface flow wastewater infiltration system (SWIS) was studied. The principal objective of this study was to evaluate the oxygen transfer rate (OTR) in two different pilot-scale arrangements of an SWIS. The two pilot plants were composed of four filter beds in series, one with a vertical arrangement of the beds (one over the other) and the other with a horizontal arrangement of the beds (one next to the other). Furthermore, two kinetic models were applied for correlating the COD and NH4+-N concentrations at the inlet and outlet of each treatment step in both pilot plants. The fitting of experimental data to the models allowed the calculation of the areal rate constants. The OTR values obtained were 54.69 g m−2 h−1 and 28.84 g m−2 h−1 for horizontal and vertical arrangement, respectively. These values were considerably higher than those obtained by other authors. The plug flow model describes the behaviour of these SWISs better, and the best fits were achieved for the vertical arrangement. The areal rate constant values obtained in this study were higher than those reported in the bibliography, which indicates a great removal efficiency and therefore lower surface area needed for the treatment.

The cotreatment of old landfill leachate and domestic sewage in rural areas by deep subsurface wastewater infiltration system (SWIS): Performance and bacterial community☆.

In this work, two deep subsurface wastewater infiltration systems (SWISs) were constructed and fed with domestic sewage (control system, S1) and mixed wastewater consisting of old landfill leachate and domestic sewage (experimental system, S2). S1 and S2 exhibited favorable removal efficiencies, with TP (98.8%, 98.7%), COD (87.6%, 86.9%), NH4+-N (99.8%, 99.9%) and TN (99.2%, 98.9%). Even when increasing the pollutant load in S2 by adding old landfill leachate, the almost complete removal performance could be maintained in terms of low effluent concentrations and even increased in terms of load removal capabilities, which included COD (19.4, 25.9g∙m-2·d-1), NH4+-N (8.2, 19.9g∙m-2·d-1), TN (8.9, 20.6g∙m-2·d-1). To investigate the transformation of dissolved organic matter along depth, Three-Dimensional Excitation Emission Matrix fluorescence spectroscopy combined with Fluorescence Regional Integration analysis was applied. The results showed that PⅠ,n and PⅡ,n (the proportions of biodegradable fractions) increased gradually from 6.59% to 21.8% at S2_20 to 10.8% and 27.7% at S2_110, but PⅢ,n and PⅤ,n (the proportions of refractory organics) declined from 23.1% to 27.8% at S2_20 to 21.1% and 16.4% at S2_110, respectively. In addition, high-throughput sequencing technology was employed to observe the bacterial community at different depths, and the predicted functional potential of the bacterial community was analyzed by PICRUSt. The results showed that the genera Flavobacterium, Pseudomonas, Vogesella, Acinetobacter and Aquabacterium might be responsible for refractory organic degradation and that their products might serve as the carbon source for denitrifiers to achieve simultaneous nitrate and refractory organic removal. PICRUSt further demonstrated that there was a mutual response between refractory organic degradation and denitrification. Overall, the combined treatment of domestic sewage and old leachate in rural areas by SWIS is a promising approach to achieve comprehensive treatment.

Domestic wastewater treatment for single household via novel subsurface wastewater infiltration systems (SWISs) with NiiMi process: Performance and microbial community

Subsurface wastewater infiltration systems (SWISs) were widely used in decentralized domestic wastewater treatment. However, their clogging risk and unstable performance remain pressuring issues. In this study, novel SWISs with NiiMi process have been developed in the treatment of domestic wastewater. Moreover, the impacts of substrates and operating modes on removal performance of SWISs were systemically evaluated. The results indicated that optimal removal performance was achieved in Sandy soil system with intermittent time of 6h and RWD of 1.0. The removal efficiencies of chemical oxygen demand (COD), total phosphorus (TP), and NH4+–N of best-performing Sandy soil system reached 94.81%, 97.25% and 75.02%, respectively. The average removal rete of COD, TP, and NH4+–N of Sandy soil system reached 35.80, 0.74, and 2.79 gm−2d−1, respectively. The effluent of Sandy soil system has satisfied the limits of The Reuse of Urban Recycling Water (GB/T 18921–2002). High-throughput sequencing indicated that the percentage of denitrifiers in Krasnozem system, Brown soil system, and Sandy soil system reached 58.95%, 64.79%, and 25.31%, respectively. Pseudomonas, Anaerolineaceae-uncultured and Bacillus were predominant functional genera in carbon and nitrogen cycling of SWISs. Overall, the Sandy soil system with NiiMi process is feasible for single household in the treatment and reclamation of domestic wastewater.

Contribution of co-denitrification to nitrous oxide production from subsurface wastewater infiltration system

Sewage treatment is one of the important sources of nitrous oxide (N2O) production, understanding its microbial process is fundamental for reducing N2O emissions. In this study, the stable isotope tracer method was used to reveal the contribution of co-denitrification to the release of N2O in addition to nitrification and denitrification in subsurface wastewater infiltration system (SWIS). The effects of nitrate addition modes were also explored. The results indicated that co-denitrification, significantly affected by dissolved oxygen, pH and potential denitrification activity, had to be taken into account in N2O formation. When the addition of nitrate nitrogen changed from one-time to intermittent, the contribution of co-denitrification increased from (1.87 ± 0.3) % to (34.2 ± 3.8) %. As a result, N2O emission and conversion rate increased from (5.17 ± 1.4) × 10−3 mg/m2·h to (6.77 ± 1.7) × 10−3 mg/m2·h and from (0.17 ± 0.02) % to (0.232 ± 0.01) %, respectively. The results also suggested that, given the increasing scale of SWIS applications, more attention should be paid to the nitrogen cycle in the system. To effectively mitigate the generation and conversion of N2O, measures to reduce nitrate accumulation need to be taken.