Abstract
Sediment microbes have a great potential to transform reactive N to harmless N2, thus decreasing wastewater nitrogen load into aquatic ecosystems. Here, we examined if spatial allocation of the wastewater discharge by a specially constructed sediment diffuser pipe system enhanced the microbial nitrate reduction processes. Full-scale experiments were set on two Finnish lake sites, Keuruu and Petäjävesi, and effects on the nitrate removal processes were studied using the stable isotope pairing technique. All nitrate reduction rates followed nitrate concentrations, being highest at the wastewater-influenced sampling points. Complete denitrification with N2 as an end-product was the main nitrate reduction process, indicating that the high nitrate and organic matter concentrations of wastewater did not promote nitrous oxide (N2O) production (truncated denitrification) or ammonification (dissimilatory nitrate reduction to ammonium; DNRA). Using 3D simulation, we demonstrated that the sediment diffusion method enhanced the contact time and amount of wastewater near the sediment surface especially in spring and in autumn, altering organic matter concentration and oxygen levels, and increasing the denitrification capacity of the sediment. We estimated that natural denitrification potentially removed 3–10% of discharged wastewater nitrate in the 33 ha study area of Keuruu, and the sediment diffusion method increased this areal denitrification capacity on average 45%. Overall, our results indicate that sediment diffusion method can supplement wastewater treatment plant (WWTP) nitrate removal without enhancing alternative harmful processes.
Highlights
Wastewater effluents are important point sources of reactive nitrogen (N), significantly altering the biogeochemistry of the receiving aquatic ecosystems (Carey and Migliaccio, 2009)
At the wastewaterinfluenced sampling points in both Keuruu and Peta€ja€vesi, NOxÀ concentrations were always higher than NH4, except in winter 2015 in Keuruu, when the nitrification process collapsed in wastewater treatment plant (WWTP) (Table 1)
We have previously shown that wastewater shapes the sediment microbial community composition significantly by bringing in WWTP microbes and modifying habitat characteristics (Saarenheimo et al, 2017), so it is likely that by altering oxygen and organic matter concentrations and quality, wastewater favored certain microbes, which directly or indirectly contributed to denitrification
Summary
Wastewater effluents are important point sources of reactive nitrogen (N), significantly altering the biogeochemistry of the receiving aquatic ecosystems (Carey and Migliaccio, 2009). Recent studies highlight the importance of efficient N removal in wastewater treatment plants (WWTPs; Lofton et al, 2007; Lee et al, 2016). The most common wastewater treatment standard in Europe, North America and Australia is the secondary treatment (Morris et al, 2017), where activated sludge is used to remove organic material and convert incoming ammonium (NH4þ) to nitrate (NO3À; Carey and Migliaccio, 2009). A more efficient N removal at the WWTPs may promote emissions of greenhouse gases, e.g. nitrous oxide (N2O; Hauck et al, 2016)
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