Abstract
Subsurface wastewater infiltration systems (SWIS) are one of the important sources of nitrous oxide (N2O) production; understanding the biological processes and contributions of N2O will help control the amount of N2O produced. To quantitatively reveal the contribution of nitrification and denitrifiaction processes, 8 g potassium nitrate with 99.99 atom % 15N (i.e. 15N accounts for 99.99% of the total N) was dissolved in the influent (concentration: 3.3 g/L). Results showed that nitrification released more N2O within 0–12 h, accounting for 79.6 ± 2.4%. The denitrification process accounted for 88.5 ± 1.3% for N2O generation after the 12th hour. Thus, in order to effectively control the release of N2O, the denitrification process should be given more attention. The maximum release rate of N2O was 8.45 ± 0.8 mg/m2·h, which occurred near the end of the first wetting-drying cycle. Since then, peaks appeared periodically, mostly in the “rest” periods.
Highlights
Subsurface wastewater infiltration systems (SWIS) offer a promising opportunity to transform municipal wastewater into renewable energy with significantly reduced energy consumption (Pan et al, 2016a; Jiang et al, 2017)
Each column was filled with farmland soil, mixed matrix and gravel in sequence (Li et al, 2018)
Mixed matrix was composed of fine sand (0.1–1.0 mm in diameter), coal slag (0.1–0.5 mm in particle size) and farmland soil mixed in volume ratio of 1:2:7
Summary
Subsurface wastewater infiltration systems (SWIS) offer a promising opportunity to transform municipal wastewater into renewable energy with significantly reduced energy consumption (Pan et al, 2016a; Jiang et al, 2017). The SWIS process is a soil-based sewage treatment technology. Purification mechanisms involve biological, physical and chemical processes (Li et al, 2017b). The inlet pipe is usually set at a depth of 40–50 cm. The sewage infiltrates the 60–70 cm support layer under the action of gravity, and climbs up to a depth of 20–25 cm driven by capillary force. When gravity is greater than the capillary force, sewage infiltrates into the bottom and is collected. According to the literature (Zhang et al, 2015), aerobic, anoxic and anaerobic states are sequentially dominant from top to bottom, the organic matter can be degraded through different biochemical reactions specific to each of those zones(Lloréns et al, 2011)
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