Does influent C/N ratio affect pollutant removal and greenhouse gas emission in wastewater ecological soil infiltration systems with/without intermittent aeration?

Wastewater ecological soil infiltration system (WESIS) is a land treatment technology for decentralized wastewater treatment that has been applied all over the world. In this study, the pollutant removal, emission of greenhouse gases (GHGs) and functional gene abundances with different influent C/N ratios were evaluated in WESISs with/without intermittent aeration. Intermittent aeration and influent C/N ratio affect pollutant removal and GHG emission. Increased influent C/N ratio led to high total nitrogen (TN) removal, low CH4 and N2O emission in the aerated WESIS, which was different from the non-aerated WESIS. High average removal efficiencies of chemical oxygen demand (COD) (94.8%), NH4 +-N (95.1%), TN (91.2%), total phosphorus (TP) (91.1%) and low emission rates for CH4 (27.2 mg/(m2 d)) and N2O (10.5 mg/(m2 d)) were achieved with an influent C/N ratio of 12:1 in the aerated WESIS. Intermittent aeration enhanced the abundances of bacterial 16S rRNA, amoA, nxrA, narG, napA, nirK, nirS, qnorB, nosZ genes and decreased the abundances of the mcrA gene, which are involved in pollutant removal and GHG emission. Intermittent aeration would be an effective alternative to achieving high pollutant removal and low CH4 and N2O emission in high influent C/N ratio wastewater treatment.

Effect of COD/N Ratio on Removal Performances in Two Subsurface Wastewater Infiltration Systems.

Dissolved oxygen (DO), were investigated. Aerobic conditions were effectively developed in 50 cm depth of the matrix and anoxic or anaerobic conditions were not changed in 80 and 110 cm depth by intermittent aeration, which encouraged nitrification. Increased influent COD/N ratio led to lower COD and nitrogen removal in conventional SWISs. Sufficient carbon source in high COD/N ratio influent promoted denitrification with intermittent aeration. High removal rates of COD (95.68 ± 0.21%), TP (92.02 ± 0.28%), -N (99.33 ± 0.05%), and - (89.65 ± 0.6%) were obtained with influent COD/N ratio of 12 in aerated SWISs. Under the COD/N ratio of 12 and 18, intermittent aeration boosted the growth and reproduction of nitrifying bacteria and denitrifying bacteria. Meanwhile, nitrate and nitrite reductase activities with intermittent aeration were higher than that without aeration in 80 and 110 cm depths.

Effects of urea and (NH4)2SO4 on nitrification and acidification of Ultisols from southern China.

The mechanisms for the effects of ammonium-based fertilizers on soil acidification in subtropical regions are not well understood. Two Ultisols collected from cropland and a tea garden in Anhui and Jiangxi Provinces in subtropical southern China, respectively, were used to study the effects of urea and (NH4)2SO4 on the nitrification and acidification of soils with incubation experiments. Nitrification occurred at very low pH with no N fertilizer added and led to lowering of the soil pH by 0.53 and 0.30 units for the soils from Jiangxi and Anhui, respectively. Addition of urea accelerated nitrification and soil acidification in both Ultisols; while nitrification was inhibited by the addition of (NH4)2SO4, and greater input of (NH4)2SO4 led to greater inhibition of nitrification. Ammonia-oxidizing bacteria (AOB) played an important role in nitrification in cropland soil under acidic conditions. Addition of urea increased the soil pH at the early stages of incubation due to hydrolysis and stimulated the increase in the AOB population, and thus accelerated nitrification and soil acidification. At the end of incubation, the pH of Ultisol from Jiangxi had decreased by 1.25, 1.54 and 1.84 units compared to maximum values for the treatments with 150, 300 and 400 mg/kg of urea-N added, respectively; the corresponding figures were 0.95, 1.25 and 1.69 for the Ultisol from Anhui. However, addition of (N-H4)2SO4 inhibited the increase in the AOB population and thus inhibited nitrification and soil acidification. Soil pH for the treatments with 300 and 400 mg/kg of (NH4)2SO4-N remained almost constant during the incubation. AOB played an important role in nitrification of the cropland soil under acidic conditions. Addition of urea stimulated the increase in the AOB population and thus accelerated nitrification and soil acidification; while addition of (NH4)2SO4 inhibited the increase in the AOB population and thus inhibited nitrification.

Effects of Exotic Spartina alterniflora Invasion on Soil Phosphorus and Carbon Pools and Associated Soil Microbial Community Composition in Coastal Wetlands.

Soil microorganisms can be altered by plant invasion into wetland ecosystems and comprise an important linkage between phosphorus (P) availability and soil carbon (C) chemistry; however, the intrinsic mechanisms of P and C transformation associated with microbial community and function are poorly understood in coastal wetland. In this study, we used a sequential fractionation method and 13C nuclear magnetic resonance (NMR) spectroscopy to capture the changes in soil P pools and C chemical composition with bare flats (BF), native Phragmites australis(PA), and invasive Spartina alterniflora(SA), respectively. The responses of the soil microbial community using phospholipid fatty acid (PLFA) profiling and function indicated by nine enzyme activities associated with C, nitrogen (N), and P cycles were also investigated. Compared to PA and BF, SA invasion significantly (P < 0.05) changed P pools and mainly increased the available P by 17.5 and 37.0%, respectively. The presence of the plants (SA and PA) significantly (P < 0.05) altered the soil C chemical composition mainly by affecting the aliphatic functional groups, resulting in a lower alkyl C/O-alkyl C ratio value. Compared to BF and SA, PA significantly (P < 0.05) increased arbuscular mycorrhizal fungi (AMF) abundance. Soil enzyme activity, especially for the P and C cycle enzymes, was also affected by plant species with the highest geometric mean enzyme and hydrolase activity for the PA zone. We also found that soil C compositions and P pools were associated with microbial community structure and enzyme activity, respectively. However, little interaction between C and P was found on either soil microbial composition or soil enzyme activity variation. Further, microbial community composition was tightly correlated with the soil P compared to soil C chemistry, while enzyme activity showed more response with soil C chemistry compared to soil P pool changes.

Nitrogen removal and N2O emission in subsurface wastewater infiltration systems with/without intermittent aeration under different organic loading rates.

Nitrogen removal, N2O emission and nitrogen removal functional gene abundances in intermittent aerated (IA) and non-aerated (NA) subsurface wastewater infiltration systems (SWISs) under different organic loading rates (OLRs) were investigated. Aeration successfully created aerobic condition at 50cm depth and did not change anoxic or anaerobic condition at 80 and 110cm depths under OLR of 5.3, 10.9 and 16.5g BOD/(m2d). Meanwhile, aeration enhanced COD, NH4+-N, TN removal and the enrichment of nitrogen removal functional genes (amoA, nxrA, napA, narG, nirS, nirK, qnorB and nosZ) compared to NA SWIS. High COD removal rate of 91.6%, TN removal rate of 85.9% and low N2O emission rate of 32.4mg/(m2d) were obtained in IA SWIS under OLR of 16.5g BOD/(m2d). Intermittent aeration is a sensible strategy to achieve high OLR, low N2O emission, satisfactory organic matter and nitrogen removal performance for SWISs.