Surface Nitrification Research Articles

Effect of different bypass rates and unit area ratio in hybrid constructed wetlands.

This study presents the performance of a hybrid constructed wetland (Bp(VF + HF)2:1) system which consists of an unsaturated vertical flow (VF) unit followed by a saturated down-flow unit simulating horizontal flow (HF) with HF/VF area ratio of 0.5 and influent bypass to the HF unit. Treating synthetic wastewater simulating municipal wastewater, optimum total nitrogen (TN) removal (57%) was reached at 39% bypass and surface loading rate (SLR) of 33g BOD5/m2day and 9.7g TN/m2day (overall system). On the other hand, treating actual municipal wastewater, the system reached 63% TN removal at 30% bypass and SLR of 18g BOD5/m2day and 4.7g TN/m2day. Surface removal rates reached 5.5 and 3.0g TN/m2day for synthetic and municipal wastewater. Surface nitrification rate in the VF unit was in the range of 5.0-7.4 and 3.6-3.8gN/m2day for synthetic and municipal wastewater, respectively, indicating a large effect of wastewater characteristics on the nitrification process. Infiltration rate in the VF unit remained high and far from clogging risk. Overall greenhouse gas emissions were 0.11 (N2O) and 0.41 (CH4) g/m2day which corresponded to emissions factors (relative to total organic carbon and TN influent) of 0.7% (N2O) and 3.6% (CH4). Compared with a similar system with a different HF/VF area ratio of 2.0, organic matter and nitrogen removal efficiency was similar, but surface removal rates were about 3 times higher.

Iron requirements and uptake strategies of the globally abundant marine ammonia-oxidising archaeon, Nitrosopumilus maritimus SCM1

Ammonia-oxidising archaea (AOA) mediate the rate-limiting step of nitrification, the central component of the marine nitrogen cycle that converts ammonia to nitrite then nitrate. Competition with phytoplankton for ammonium and light inhibition are considered to restrict AOA activity to below the photic zone, but observations of surface nitrification now demand a further understanding of the factors driving AOA distribution and activity. Pico- to nanomolar concentrations of iron (Fe) limit the growth of microorganisms in a significant portion of the world’s surface oceans, yet there is no examination of the role of Fe in AOA growth despite the process of ammonia oxidation being considered to rely on the micronutrient. Here we investigate the Fe requirements and Fe uptake strategies of the Nitrosopumilus maritimus strain SCM1, a strain representative of globally abundant marine AOA. Using trace metal clean culturing techniques, we found that N. maritimus growth is determined by Fe availability, displaying a free inorganic Fe (Fe′) half saturation constant 1–2 orders of magnitude greater for cell growth than numerous marine phytoplankton and heterotrophic bacterial species driven by a reduced affinity for Fe′. In addition, we discovered that whilst unable to produce siderophores to enhance access to Fe, N. maritimus is able to use the exogenous siderophore desferrioxamine B (DFB), likely through a reductive uptake pathway analogous to that demonstrated in phytoplankton. Our work suggests AOA growth in surface waters may be Fe limited and advances our understanding of AOA physiology on the cellular and mechanistic levels with implications for ecosystem dynamics and the biogeochemical N-cycle.

Hydroxylamine as a Potential Indicator of Nitrification in the Open Ocean

AbstractHydroxylamine (NH2OH), a short‐lived intermediate in the nitrogen cycle, is a potential precursor of nitrous oxide (N2O) in the ocean. However, measurements of NH2OH in the ocean are sparse. Here we present a data set of depth profiles of NH2OH from the equatorial Atlantic Ocean and the eastern tropical South Pacific and compare it to N2O, nitrate, and nitrite profiles under varying oxygen conditions. The presence of NH2OH in surface waters points toward surface nitrification in the upper 100 m. Overall, we found a ratio of 1:3 between NH2OH and N2O in open ocean areas when oxygen concentrations were >50 μmol/L. In the equatorial Atlantic Ocean and the open ocean eastern tropical South Pacific, where nitrification is the dominant N2O production pathway, stepwise multiple regressions demonstrated that N2O, NH2OH, and nitrate concentrations were highly correlated, suggesting that NH2OH is a potential indicator for nitrification.

CASE STUDY ON NITRIFICATION RATES IN A NON-STIRRED MEMBRANE-AERATED BIOFILM REACTOR OPERATED UNDER LAMINAR REGIME

A lab-scale non-stirred membrane-aerated biofilm reactor with a volume of 14.2 L was operated under laminar flow regime with inorganic synthetic wastewater to assess tertiary nitrification rates. Nitrifying counter-diffusive biofilm grown over microporous polypropylene tubular membranes supplied with atmospheric air at low pressure (3.45 kPa). The reactor was operated at very low water-velocities (in cm/s): from 2.3×10-4 to 2.3 × 10-3. In all runs, the influent ammonium concentration was kept constant ( 26 mg-N/L). By changing the volumetric inflow, the membrane-aerated biofilm (MAB) was tested at seven different ammonium applied loads (range: 0.57 - 6.04 g-N/m2 d). The corresponding ammonium removal rates ranged 0.56 to 3.02 g-N/(m2 d). The percentage of biological ammonium removal presented a range from 50% to 98%. As expected, due to passive experimental dynamic conditions to achieve an ammonium removal percentage greater than 80% a hydraulic retention time of 19 h was required. Clean membrane oxygen transfer rate was evaluated. The nitrifying MAB was found to enhance the oxygen transfer across the membrane when the surface nitrification rates were high. A significant drop in electrical conductivity that is proportional to the ammonium removed was observed. Thus, conductivity measurement can be used as a simple control method of ammonium removal extent. Effluent suspended solids were not detected, therefore a sedimentation process was unnecessary.

CASE STUDY ON NITRIFICATION RATES IN A NON-STIRRED MEMBRANE-AERATED BIOFILM REACTOR OPERATED UNDER LAMINAR REGIME

A lab-scale non-stirred membrane-aerated biofilm reactor with a volume of 14.2 L was operated under laminar flow regime with inorganic synthetic wastewater to assess tertiary nitrification rates. Nitrifying counter-diffusive biofilm grown over microporous polypropylene tubular membranes supplied with atmospheric air at low pressure (3.45 kPa). The reactor was operated at very low water-velocities (in cm/s): from 2.3×10-4 to 2.3 × 10-3. In all runs, the influent ammonium concentration was kept constant ( 26 mg-N/L). By changing the volumetric inflow, the membrane-aerated biofilm (MAB) was tested at seven different ammonium applied loads (range: 0.57 - 6.04 g-N/m2 d). The corresponding ammonium removal rates ranged 0.56 to 3.02 g-N/(m2 d). The percentage of biological ammonium removal presented a range from 50% to 98%. As expected, due to passive experimental dynamic conditions to achieve an ammonium removal percentage greater than 80% a hydraulic retention time of 19 h was required. Clean membrane oxygen transfer rate was evaluated. The nitrifying MAB was found to enhance the oxygen transfer across the membrane when the surface nitrification rates were high. A significant drop in electrical conductivity that is proportional to the ammonium removed was observed. Thus, conductivity measurement can be used as a simple control method of ammonium removal extent. Effluent suspended solids were not detected, therefore a sedimentation process was unnecessary.

Ambient nitrate switches the ammonium consumption pathway in the euphotic ocean

Phytoplankton assimilation and microbial oxidation of ammonium are two critical conversion pathways in the marine nitrogen cycle. The underlying regulatory mechanisms of these two competing processes remain unclear. Here we show that ambient nitrate acts as a key variable to bifurcate ammonium flow through assimilation or oxidation, and the depth of the nitracline represents a robust spatial boundary between ammonium assimilators and oxidizers in the stratified ocean. Profiles of ammonium utilization show that phytoplankton assemblages in nitrate-depleted regimes have higher ammonium affinity than nitrifiers. In nitrate replete conditions, by contrast, phytoplankton reduce their ammonium reliance and thus enhance the success of nitrifiers. This finding helps to explain existing discrepancies in the understanding of light inhibition of surface nitrification in the global ocean, and provides further insights into the spatial linkages between oceanic nitrification and new production.

Effect of step-feeding on the performance of lab-scale columns simulating vertical flow-horizontal flow constructed wetlands.

The effect of step-feeding (untreated wastewater by-pass) on the performance of lab-scale columns simulating a hybrid vertical flow (VF)-horizontal flow (HF) constructed wetland (CW) system was studied. Step-feeding strategies have been adopted in several kinds of CW, but this is the first report about the use of step-feeding in VF+HF hybrid systems treating domestic wastewater. Applied loading rates were 7-11gBOD5/m2day and 2.1-3.4gTN/m2day (overall system). Removal efficiency reached 98% TSS and COD and 99% BOD5 on average, whilst a 50% by-pass improved TN removal from 31 to 50%. Maximum surface nitrification rate (5.5gN/m2day) was obtained in VF unit, whilst maximum denitrification rate (1.8gN/m2day) was observed in HF unit. Referred to the overall system, maximum surface nitrification and denitrification rates were 2.2 and 1.6gN/m2day, respectively. However, potential nitrifying and denitrifying activities (batch assays) were 15.0 and 58.9gN/m2day, respectively. Even at 50% by-pass, operational conditions in HF unit (dissolved oxygen, redox, COD/TN ratio) were not suitable enough for denitrification. However, methane emissions were not observed and nitrous oxide emissions were relatively low.

Nitrogen cycling in the Southern Ocean Kerguelen Plateau area: evidence for significant surface nitrification from nitrate isotopic compositions

Abstract. This paper presents whole water column data for nitrate N, O isotopic composition for the Kerguelen Plateau area and the basin extending east of Heard Island, aiming at understanding the N-cycling in this naturally iron fertilized area that is characterized by large re-current phytoplankton blooms. The KEOPS 2 expedition (October–November 2011) took place in spring season and complements knowledge gathered during an earlier summer expedition to the same area (KEOPS 1, February–March 2005). As noted by others a remarkable condition of the system is the moderate consumption of nitrate over the season (nitrate remains >20 μM) while silicic acid becomes depleted, suggesting significant recycling of nitrogen. Nitrate isotopic signatures in the upper water column do mimic this condition, with surprising overlap of spring and summer regressions of δ18ONO3 vs. δ15NNO3 isotopic compositions. These regressions obey rather closely the 18ϵ/15ϵ discrimination expected for nitrate uptake (18ϵ/15ϵ = 1), but regression slopes as large as 1.6 were observed for the mixed layer above the Kerguelen Plateau. A preliminarily mass balance calculation for the early bloom period points toward significant nitrification occurring in the mixed layer and which may be equivalent to up to 47% of nitrate uptake above the Kerguelen Plateau. A further finding concerns deep ocean low δ18ONO3 values (<2‰) underlying high chlorophyll waters at the Polar Front Zone and which cannot be explained by remineralization and nitrification of the local particulate nitrogen flux, which is too small in magnitude. However, the studied area is characterized by a complex recirculation pattern that would keep deep waters in the area and could impose a seasonally integrated signature of surface water processes on the deep waters.

Surface nitrification: A major uncertainty in marine N2O emissions

AbstractThe ocean is responsible for up to a third of total global nitrous oxide (N2O) emissions, but uncertainties in emission rates of this potent greenhouse gas are high (>100%). Here we use a marine biogeochemical model to assess six major uncertainties in estimates of N2O production, thereby providing guidance in how future studies may most effectively reduce uncertainties in current and future marine N2O emissions. Potential surface N2O production from nitrification causes the largest uncertainty in N2O emissions (estimated up to ~1.6 Tg N yr−1 or 48% of modeled values), followed by the unknown oxygen concentration at which N2O production switches to N2O consumption (0.8 Tg N yr−1 or 24% of modeled values). Other uncertainties are minor, cumulatively changing regional emissions by <15%. If production of N2O by surface nitrification could be ruled out in future studies, uncertainties in marine N2O emissions would be halved.

Effects of Surface Nitrification on Thermal Conductivity of Modified Aluminum Oxide Nanofibers-Reinforced Epoxy Matrix Nanocomposites

Aluminum oxide () nanofibers were treated thermally under an ammonia () gas stream balanced by nitrogen to form a thin aluminum nitride (AlN) layer on the nanofibers, resulting in the enhancement of thermal conductivity of /epoxy nanocomposites. The micro-structural and morphological properties of the -assisted thermally-treated nanofibers were characterized by X-ray diffraction (XRD) and atomic force microscopy (AEM), respectively. The surface characteristics and pore structures were observed by X-ray photoelectron spectroscopy (XPS), Zeta-potential and /77 K isothermal adsorptions. From the results, the formation of AlN on nanofibers was confirmed by XRD and XPS. The thermal conductivity (TC) of the modified nanofibers/epoxy composites increased with increasing treated temperatures. On the other hand, the severely treated /epoxy composites showed a decrease in TC, resulting from a decrease in the probability of heat-transfer networks between the filler and matrix in this system due to the aggregation of nanofiber fillers.

Membrane-attached biofilm as a mean to facilitate nitrification in activated sludge process and its effect on the microfaunal population

A membrane-attached biofilm (MAB) was incorporated into a conventional activated sludge (AS) process to facilitate the nitrification. The simultaneous carbon removal and nitrification of the MAB incorporated AS (AS + MAB) process was compared with that of the AS process. The total organic carbon (TOC) loading was varied from 0.43 to 0.68 g/(L d), and the corresponding total nitrogen (T-N) loading was varied from 0.15 to 0.24 g/(L d) with keeping the C/N ratio at 2.85. The TOC removal efficiencies were above 95% for both processes within the range of TOC loading tested. The results showed the excellent carbon removal in both processes. The volumetric nitrification rate of AS + MAB process was about 0.14 g/(L d) at the T-N loading of 0.21 g/(L d), and was about seven times higher than that of AS process at the loading of 0.22 g/(L d). The results clearly demonstrated that the MAB facilitated the nitrification in the AS process. The maximum surface nitrification rate was about 8.4 g/(m 2 d), which was based on the surface area of the biofilm module. This value was almost comparable to that of nitrifying MAB under the same T-N loading, indicating little carbon inhibition against the nitrification in the AS + MAB process. On the other hand, the presence of MAB enabled the growth of slow-growing microorganisms not only nitrifying bacteria but also microfauna. The enumeration of microfauna in bulk suspension was carried out for both processes during the treatment. The abundance in rotifers, in addition to the appearance of oligochaetes, in the AS + MAB process suggested the less sludge generation due to the enlarged food-chain. The incorporation of MAB into AS process enabled the simultaneous nitrification and carbon removal, and had high potential in less sludge generation.

The significance of nitrification for oceanic new production

The flux of organic material sinking to depth is a major control on the inventory of carbon in the ocean. To first order, the oceanic system is at equilibrium such that what goes down must come up. Because the export flux is difficult to measure directly, it is routinely estimated indirectly by quantifying the amount of phytoplankton growth, or primary production, fuelled by the upward flux of nitrate. To do so it is necessary to take into account other sources of biologically available nitrogen. However, the generation of nitrate by nitrification in surface waters has only recently received attention. Here we perform the first synthesis of open-ocean measurements of the specific rate of surface nitrification and use these to configure a global biogeochemical model to quantify the global role of nitrification. We show that for much of the world ocean a substantial fraction of the nitrate taken up is generated through recent nitrification near the surface. At the global scale, nitrification accounts for about half of the nitrate consumed by growing phytoplankton. A consequence is that many previous attempts to quantify marine carbon export, particularly those based on inappropriate use of the f-ratio (a measure of the efficiency of the 'biological pump'), are significant overestimates.

Boosting nitrification by membrane-attached biofilm

Nitrification is a key step for reliable biological nitrogen removal. In order to enhance nitrification in the activated sludge (AS) process, membrane-attached biofilm (MAB) was incorporated in a conventional activated sludge tank. Simultaneous organic carbon removal and nitrification of the MAB incorporated activated sludge (AS + MAB) process was investigated with continuous wastewater treatment. The effluent TOC concentration of AS and the AS + MAB processes were about 6.3 mg/L and 7.9 mg/L, respectively. The TOC removal efficiency of both AS and AS + MAB were above 95% during the wastewater treatment, indicating excellent organic carbon removal performance in both processes. Little nitrification occurred in the AS process. On the contrary, successful nitrification was obtained with the AS + MAB process with nitrification efficiency of about 90%. The volumetric and surface nitrification rates were about 0.14 g/Ld and 6.5 g/m2d, respectively. The results clearly demonstrated that nitrification in the conventional AS process was boosted by MAB. Furthermore, the microfaunal population in the AS + MAB process was different from that in the AS process. The high concentration of rotifers in the AS + MAB process was expected to decrease the generation of excess sludge in the process.

Nitrification of synthetic wastewater in a cross flow medium trickling filter

The nitrification performance of a synthetic wastewater was investigated in a cross flow medium trickling filter. The attachment of biomass on packing medium was studied. The reactor was operated continuously for three different hydraulic loading rates 5, 9 and 13␣m3/m2 · d under various influent ammonium concentrations. The maximum specific surface nitrification rate was about 1.21 g/m2 · d. There was a good agreement between actual and predicted alkalinity consumption. The ammonium conversion along the filter depth was also investigated. The external surface of the nitrifying biofilm was captured by scanning electron microscope. The results are found to be satisfactory.

Silicon surface nitrification using hydrogen and ammonia by hybrid excitation

AbstractA hybrid‐excitation technique using plasma excitation and photoexcitation in chemical vapor phase reaction has been developed. Using this technique, the silicon surface was nitrified in a mixture of ammonia (NH3) and hydrogen (H2) gases. First, a (100) Si substrate heated at 180°C was illuminated by ultraviolet ray (wavelengths: 184.9 nm and 253.7 nm) under NH3 atmosphere. At the same time, hydrogen radicals generated by decomposition of H2 gas in an RF (13.56 MHZ) inductive discharge tube were supplied to the substrate. Auger analysis indicated that the (100) Si surface up to about 1 nm in depth was nitrified and that the hybrid excitation was based on the synergy effect of photoexcitation and hydrogen radical excitation.Next, staggered‐structure thin‐film transistors were fabricated using amorphous Si whose surfaces nitrified by the foregoing method and the conventional plasma excitation method. It was found that when the hybrid‐excitation method was used, the field effect mobility was highest. In the gas analysis of hydrogen radical excitation, the presence of hydrazine was confirmed. Using this observation, the direct nitrification mechanism in the hybrid excitation was assumed.