title: Control of denitrification in a septage-treating artificial wetland: the dual role of particulate organic carbon.
abstract: We examined the factors controlling organic carbon (C) cycling and its control of nitrogen (N) removal via denitrification in an aerated artificial wetland treating highly concentrated wastewater to nutrient-removal standards. Processing of organic material by the septage-treating wetland affected the biological reactivity (half-life, or t1/2) of organic C pools through microbial degradation and gravity fractionation of the influent septage. Primary sedimentation fractionated the initial septage material (t1/2 = 8.4d) into recalcitrant waste solids (t1/2 = 16.7d) and highly labile supernatant (t1/2 = 5.0d), allowing this reactive fraction to be further degraded during treatment in aerobic wetland tanks until a less labile material (t1/2 = 7.3d) remained. Organic C contributions from in situ fixation by nitrifying bacteria or algae in these tanks were small, about 1% of the C degradation rate. In the aerated tanks, denitrification was correlated with particulate organic C loading rates, although the average C required (0.35 mg C L(-1)h(-1)) to support denitrification was only 12% of the total C respiration rate (2.9 mg C L(-1)h(-1)). Additions of plant litter (2.5g C L(-1)) to the aerated tanks under normal operating conditions doubled denitrification rates to 0.58 mg N L(-1)h(-1), and reduced effluent nitrate levels by half, from 12.7 to 6.4 mg N L(-1). However, C degradation within the plant litter (0.15mg C L(-1)h(-1)) was sufficient to have accounted for only 35% of the additional denitrification. Evidence from laboratory and full-scale plant litter additions as well as process monitoring indicates that the stimulation of denitrification is due to the respiration-driven formation of anaerobic microsites within particulate organic C. In this aerated highly C-loaded septage-treating wetland, anaerobic microsite, rather than C substrate availability limits denitrification.
authors.affiliation: Department of Biology, Woods Hole Oceanographic Institution, MA 02543, USA. rhamersley@umassd.edu
authors.forename: M Robert
authors.initials: MR
authors.lastname: Hamersley
authors.forename: Brian L
authors.initials: BL
authors.lastname: Howes
mesh_terms: D000693:Anaerobiosis; D001419:Bacteria / Q000378:metabolism; D018533:Biomass; D002244:Carbon / Q000378:metabolism*; D019069:Cell Respiration; D056890:Eukaryota / Q000378:metabolism; D008962:Models, Theoretical; D009566:Nitrates / Q000378:metabolism*; D009584:Nitrogen / Q000378:metabolism*; D009930:Organic Chemicals / Q000378:metabolism*; D010944:Plants / Q000378:metabolism; D014874:Water Pollutants, Chemical / Q000378:metabolism; D018508:Water Purification
chemical_list: D009566:Nitrates; D009930:Organic Chemicals; D014874:Water Pollutants, Chemical; D002244:Carbon; D009584:Nitrogen