Nitrogen removal in wastewater

The oxygen dynamics in the rhizosphere of Zostera marina was studied by use of planar optodes. Oxygen leakage to the rhizosphere was restricted to the root tip and extended only up to ~8 mm up along the root. The oxic sediment volume around the roots increased linearly with irradiance in the interval of 0-250 µmol photons m-2 s-1, but the leakage rate saturated at the maximum irradiance of 500 µmol photons m-2 s-1. Oxygen leakage decreased by ;60% from light to darkness and Z. marina was able to maintain an oxic zone around the root tip even in darkness as long as oxygen in the overlying water was at 100% air saturation (280 µmol L-1). O2 leakage from the root tips stopped at 25% air saturation (70 µmol L-1) and the oxic microniche rapidly disappeared. Increasing the oxygen concentration above 100% air saturation induced oxygen leakage from zones that otherwise appeared impermeable to oxygen. The roots on average grew by 8.7 mm d-1, and a series of O2 images documented the high spatial and temporal dynamics of the oxic microniches around the root tips. The estimated total oxygen release to the rhizosphere of Z. marina beds was 2.3 mmol m-2 d-1, which only corresponded to 12% of the diffusive oxygen uptake at the primary sediment-water interface. Rhizospheres of seagrass are thus probably of minor importance for total benthic O2 uptake rates.

Anaerobic sewer biofilm is a composite of many different microbial populations, including sulfate reducing bacteria (SRB), methanogens and heterotrophic bacteria. Nitrate addition to sewers in an attempt to control hydrogen sulfide concentrations affects the behaviour of these populations, which in turn impacts on wastewater characteristics. Experiments were carried out on a laboratory reactor system simulating a rising main to determine the impact of nitrate addition on the microbial activities of anaerobic sewer biofilm. Nitrate was added to the start of the rising main during sewage pump cycles at a concentration of 30 mg-N L-1 for over 5 months. While it reduced sulfide levels at the outlet of the system by 66%, nitrate was not toxic or inhibitory to SRB activity and did not affect the dominant SRB populations in the biofilm. Long-term nitrate addition in fact stimulated additional SRB activity in downstream biofilm. Nitrate addition also stimulated the activity of nitrate reducing, sulfide oxidizing bacteria that appeared to be primarily responsible for the prevention of sulfide build up in the wastewater in the presence of nitrate. A short adaptation period of three to four nitrate exposure events (approximately 10 h) was required to stimulate biological sulfide oxidation, beyond which no sulfide accumulation was observed under anoxic conditions. Nitrate addition effectively controlled methane concentrations in the wastewater. The nitrate uptake rate of the biofilm increased with repeated exposure to nitrate, which in turn increased the consumption of biodegradable COD in the wastewater. These results provide a comprehensive understanding of the impact of nitrate addition on wastewater composition and sewer biofilm microbial activities, which will facilitate optimization of nitrate dosing for effective sulfide control in rising main sewers.