Control of denitrification in a septage-treating artificial wetland: the dual role of particulate organic carbon

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=8.4 d) into recalcitrant waste solids (t1/2=16.7 d) and highly labile supernatant (t1/2=5.0 d), allowing this reactive fraction to be further degraded during treatment in aerobic wetland tanks until a less labile material (t1/2=7.3 d) 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 (2.5 g 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 (0.15 mg 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.