Oxygen flux implications of observed nitrogen removal rates in subsurface-flow treatment wetlands.
Nitrification, an oxygen-requiring microbial process, is generally considered the rate-limiting step for N removal in subsurface-flow constructed wetlands treating organic wastewaters. We used a simplified model of sequential N transformations and sinks to infer required rates of oxygen supply at 5 stages along experimental wetland mesocosms supplied with four different organic wastewaters with contrasting ratios of COD: N and forms of N. Mass balances of water-borne organic, ammoniacal and nitrate N, and plant and sediment N uptake showed average net rates of N mineralisation ranging from 0.22-0.53 g m(-2) d(-1), nitrification 0.56-2.15 g m(-2) d(-1), denitrification 0.47-1.99 g m(-2) d(-1) (60-84% of measured N removal) and plant assimilation 0.28-0.47 g m(-2) d(-1). The nitrogenous oxygen demand (NOD) required to support the observed nitrification rates alone was high compared to expected fluxes from surficial and plant-mediated oxygen transfer. In the presence of high levels of degradable organic matter (COD removal rates up to 66 g m(-2) d(-1)), heterotrophs with significantly higher oxygen affinities and energy yields are expected to outcompete nitrifiers for available oxygen. Problems with commonly held assumptions on the nature of coupled nitrification-denitrification in treatment wetlands are discussed.
Tanner
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CC
National Institute of Water and Atmospheric Research, PO Box 11-115, Hamilton, New Zealand. c.tanner@niwa.co.nz
Kadlec
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RH
D001673:Biodegradation, Environmental; D017753:Ecosystem; D009584:Nitrogen / Q000737:chemistry / Q000302:isolation & purification* / Q000378:metabolism; D009930:Organic Chemicals / Q000378:metabolism; D010100:Oxygen / Q000032:analysis*; D014865:Waste Disposal, Fluid / Q000379:methods*; D014872:Water Movements; D018508:Water Purification / Q000379:methods*
D009930:Organic Chemicals; D009584:Nitrogen; D010100:Oxygen