Bioaugmentation as a tool for improving the start-up and stability of a pilot-scale partial nitrification biofilm airlift reactor

The effectiveness of bioaugmentation in the improvement of the start-up of a biofilm airlift reactor to perform partial nitrification was investigated. Two identical biofilm airlift reactors were inoculated. The non-bioaugmented reactor (NB-reactor) was inoculated with conventional activated sludge, whereas the bioaugmented reactor (B-reactor) was seeded with the same conventional activated sludge but bioaugmented with nitrifying activated sludge from a pilot plant performing full nitritation under stable conditions (100% oxidation of influent ammonium to nitrite). The fraction of specialized nitrifying activated sludge in the inoculum of the B-reactor was only 6% (measured as dry matter). To simplify comparison of the results, operational parameters were equivalent for both reactors. Partial nitrification was achieved significantly faster in the B-reactor, showing a very stable operation. The results obtained by fluorescence in situ hybridization assays showed that the specialized nitrifying biomass added to the B-reactor remained in the biofilm throughout the start-up period.     

High reproducibility of ammonia-oxidizing bacterial communities in parallel sequential batch reactors

Aims:  To investigate whether the ammonia-oxidizing bacterial (AOB) communities of replicate nitrifying bioreactors (i) co-evolve or diverge over time and (ii) are stable or dynamic during periods of complete nitrification.
Methods and Results:  Three sequential batch reactors (SBR) were inoculated with sludge from a municipal wastewater treatment plant, fed with ammonium-enriched tap water and operated in parallel for 134 days. Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) demonstrated co-evolvement of the AOB communities over time. During start-up, temporary decreases in nitrification were noticed, and the AOB community rate of change values (Δ _{t (week)}) were medium to high (12–22%). During the adjacent period of complete nitrification, low AOB community dynamics were observed (Δ _{t (week)} < 5%). Further pragmatic processing of the DGGE profiles revealed a high range-weighted richness and a medium functional organization of the AOB communities.
Conclusions:  After a start-up period, high functional stability and low dynamics of the AOB communities were observed. Deterministic rather than stochastic driving forces led to AOB community co-evolvement in the replicate SBR.
Significance and Impact of the Study:  Replicates in identical set-ups are reproducible, and pragmatic processing of DGGE patterns is a straightforward tool to score and compare the functionality of the bacterial communities.     

短程硝化启动运行中功能菌群变化研究

【目的】短程硝化-厌氧氨氧化是可实现的最短生物脱氮工艺,短程硝化是实现该工艺的重要环节和必要条件。【方法】采用序批式反应器(SBR)来实现短程硝化过程的启动和稳定运行,并对该过程中的相关功能菌群变化进行检测分析。【结果】通过控制低DO浓度(<1 mg/L)和逐步提高氨氮进水负荷,可抑制氨氧化细菌(NOB)菌群增殖并促进亚硝酸氧化菌(AOB)菌群规模显著扩大,实现短程硝化过程的启动和稳定运行。在氨氮进水负荷为0.055 kg/(m3.d)时,平均氨氮去除容积负荷和污泥负荷可达到0.043kg/(m3.d)和0.16 kg/(kg.d),平均亚硝酸盐积累率可达到83.4%。在短程硝化启动和稳定运行过程中,NOB菌群密度从2.0×105CFU/mL降至1.5×104CFU/mL,相对丰度从5.51%降至2.14%;AOB菌群密度从4.5×104CFU/mL增加至1.5×107CFU/mL,相对丰度从0.18%增加至7.25%。【结论】AOB菌群规模的扩大是实现短程硝化和氨氮去除能力提高的主要原因,同时较高的进水氨氮浓度和负荷也会造成亚硝化活性的抑制。     

Determination of growth and coupled nitrification/denitrification by immobilized Thiosphaera pantotropha using measurement and modeling of oxygen profiles

An oxygen microsensor in combination with mathematical modeling was used to determine the behavior of immobilized Thiosphaera pantotropha. This organism can convert ammonia completely to nitrogen gas under aerobic conditions (coupled nitrification/denitrification) and denitrifies nitrate at highest rates under anaerobic conditions. Immobilization of T. pantotropha can result in aerobic and anaerobic zones inside the biocatalyst particle which will be advantageous for the conversion of ammonia and nitrate from wastewater. However, information of the effects of immobilization on the physiology of T. pantotropha is necessary for the development of such a system. This article gives the extension of a model developed to describe the behavior of chemostat cultures of T. pantotropha so that it can be used for immobilized cells. The original model was based on metabolic reaction equations. Kinetic and diffusion equations have now been added. Experimental verification was carried out using a stirred tank reactor and a Kluyver flask. After immobilization in agarose, the cells were grown in the particles under continuous culture conditions for 3 days. After 24 h the oxygen penetration depth showed a constant value of 100 mu, indicating that a steady state was reached. Scanning electron micrographs showed that large colonies of cells were present in this 100-mum aerobic layer.From the dynamics of the start-up phase, several parameters were determined from measurements of the oxygen concentration profiles made every few hours. The profiles simulated by the model were fitted to the measured data. The average value for the maximum specific growth rate was 0.52 h(-1), and the maximum oxygen conversion rate was 1.0 mol Cmol(-1) h(-1). The maximum specific acetate uptake rate was 2.0 mol Cmol(-1) h(-1), and the Monod constant for acetate was 2.9 x 10(-2) mol m(-3). The maximum specific nitrification rate was 0.58 x 10(-1) mol Cmol(-1) h(-1), and the amount of oxygen necessary for nitrification was 11% of the total oxygen uptake rate. Most of the kinetic parameters determined for the immobilized cells were in good agreement with those for the suspended cells. Only the maximum specific growth rate was significantly higher, and the maximum specific nitrification rate was some what lower than for suspended cells. The experimental results clearly show that an oxygen microsensor, in combination with mathematical modeling, can successfully be used to elucidate the kinetic behavior of immobilized, oxygen-consuming, cells.     

曝气充氧条件下污染河道氨挥发特性模拟

以污染河道为研究对象,模拟研究污染河道在曝气充氧(底泥曝气,ES组;水曝气,EW组)条件下氨挥发的特性,探讨主要影响因素及其作用过程.研究表明,污染河道水体具有一定氨挥发潜力,在实验室模拟条件下,氨挥发速率平均为2.51mg·m-2·h-1,相当于0.50 kgN· hm-2·d-1;曝气污染河道水体的氨挥发有一定的促进作用,与对照相比(EC组)氨挥发速率和累积氨挥发量存在显著差异(P＜ 0.05);不同曝气方式对氨挥发过程影响不同,氨挥发速率存在显著差异(P＜0.05);至实验结束,EW组的累积挥发量为2809.76 mg/m2,分别是ES组和EC组的1.17和2.25倍;各实验组的氨挥发累积量用一级动力学方程能很好地拟合,根据模型可以预测氨挥发量;同一温度条件下,pH值、铵氮浓度和通气频率是影响氨挥发的主要因素;曝气可以通过增加通气频率和提高水体pH值来促进氨挥发进行;在曝气作用下随着硝化过程的进行对氨挥发有一定的限制作用;曝气条件下,氨挥发作用在硝化过程启动阶段最为明显.     

Achieving nitrite accumulation in a continuous system treating low-strength domestic wastewater: switchover from batch start-up to continuous operation with process control

Although biological nitrogen removal via nitrite is recognized as one of the cost-effective and sustainable biological nitrogen removal processes, nitrite accumulation has proven difficult to achieve in continuous processes treating low-strength nitrogenous wastewater. Partial nitrification to nitrite was achieved and maintained in a lab-scale completely stirred tank reactor (CSTR) treating real domestic wastewater. During the start-up period, sludge with ammonia-oxidizing bacteria (AOB) but no nitrite-oxidizing bacteria (NOB) was obtained by batch operation with aeration time control. The nitrifying sludge with the dominance of AOB was then directly switched into continuous operation. It was demonstrated that partial nitrification to nitrite in the continuous system could be repeatedly and reliably achieved using this start-up strategy. The ratio of dissolved oxygen to ammonium loading rate (DO/ALR) was critical to maintain high ammonium removal efficiency and nitrite accumulation ratio. Over 85% of nitrite accumulation ratio and more than 95% of ammonium removal efficiency were achieved at DO/ALR ratios in an optimal range of 4.0–6.0 mg O₂/g N d, even under the disturbances of ammonium loading rate. Microbial population shift was investigated, and fluorescence in situ hybridization analysis indicated that AOB were the dominant nitrifying bacteria over NOB when stable partial nitrification was established.     

Comparative study of the nitrification characteristics of two different nitrifier immobilization methods

The research investigated the nitrification characteristics of two different immobilization methods: nitrifier encapsulation in polyethylene glycol (PEG) gel pellets and nitrifier biofilm attachment on elastic plastic filler. The two carriers were placed in identical reactors. They reached a maximum nitrification rate of 39 and 25 mgN/L·h 30 days after start-up. The results showed that the nitrification efficiency in the PEG reactor was higher than in the biofilm reactor under the same conditions. Variations in temperature decreased the nitrification rate by approximately 55% in the PEG reactor from 28 to 8°C, while 74.2% in the biofilm reactor. When the COD loading rate was increased to 0.8 kg/m³ day, the nitrification efficiency in the biofilm reactor dropped sharply to 23%, and that of PEG reactor remained over 80%. PEG pellets with a high nitrification rate under all conditions showed promise as an immobilization medium, and are likely to be utilized in the nitrification of high-strength ammonia and COD wastewater during long-term operation.     

Experimental measurement of sediment nitrification and denitrification in Hamilton Harbour,Canada

This research examines the role of sediment nitrification and denitrification in the nitrogen cycle of Hamilton Harbour. The Harbour is subject to large ammonia and carbon loadings from a waste-water treatment plant and from steel industries. Spring ammonia concentrations rapidly decrease from 4.5 to 0.5 mg 1⁻¹, while spring nitrate concentrations increase from 1 to 2 mg l⁻¹, by mid-summer. A three-layer sediment model was developed. The first layer is aerobic; in it, oxidation of organics and nitrification occurs. The second layer is for denitrification, and the third layer is for anaerobic processes. Ammonia sources for nitrification include diffusion from the water column, sources associated with the oxidation of organics, sources from denitrification and from anaerobic processes. Diffusion of oxygen, ammonia and nitrate across the sediment-water interface occurs. Temperature effects are modelled using the Arrhenius concept. A combination of zero-order kinetics for nitrate or ammonia consumption with diffusion results in a half-order reaction, with respect to the water column loss rate to sediments. From experimental measurement, the rate of nitrification is 200 mg N 1⁻¹ sediment per day, while that of denitrification is 85 mg N 1–1 sediment per day at 20 °C. The Arrhenius activation energy is estimated as 15 000 cal/ mole-K and 17 000 cal/ mole-K for nitrification and denitrification, respectively, between 10 °C and 20 °C. Calculations of the flux of ammonia with the sediments, using the biofilm model, compare favourably with experimental observations. The ammonia flux from the water column is estimated to account for 20% of the observed decrease in water column stocks of ammonia, while the nitrate flux from the water column is estimated to account for 25% of the total nitrogen produced by the sediments.     

Formation of nitrifying biofilms on small suspended particles in airlift reactors

For a stable and reliable operation of a BAS-reactor a high, active biomass concentration is required with mainly biofilm-covered carriers. The effect of reactor conditions on the formation of nitrifying biofilms in BAS-reactors was investigated in this article. A start-up strategy to obtain predominantly biofilm-covered carriers, based on the balancing of detachment and a biomass production per carrier surface area, proved tp be very successful. The amount of biomass and the fraction of covered carrier were high and development of nitrification activity was fast, leading to a volumetric conversion of 5 kg(N) . m(-3) . d(-1) at a hydraulic retention time of 1h. A 1-week, continuous inoculation with suspended purely nitrifying microorganisms resulted in a swift start-up compared with batch addition of a small number of biofilms with some nitrification activity. The development of nitrifying biofilms was very similar to the formation of heterotrophic biofilms. In contrast to heterotrophic bio-films, the diameter of nitrifying biofilms increased during start-up. The detachment rate from nitrifying biofilms decreased with lower concentrations of bare carrier, in a fashion comparable with heterotrophic biofilms, but the nitrifying biofilms were much more robust and resistant. Standard diffusion theory combined with reaction kinetics are capable of predicting the activity and conversion of biofilms on small suspended particles. (c) 1995 John Wiley & Sons Inc.     

水分对林地和草地土壤氮初级转化速率的影响

水分含量是与土壤氮转化相关微生物活性的重要影响因素。本研究以黑龙江省北安市的草地和林地土壤为对象,通过室内培养试验,利用¹⁵N同位素标记技术和FLUAZ数值优化模型研究60%和100%田间持水量(WHC)条件下土壤氮初级矿化速率、初级固定速率、初级硝化速率和初级反硝化速率,以探讨土壤氮初级转化速率对水分含量变化的响应,阐明不同水分条件下土壤中氮的产生、消耗、保存机制及其生态环境效应。结果表明: 土壤水分变化不影响草地和林地土壤氮初级矿化速率和铵态氮固定速率,水分含量由60% WHC增加至100% WHC后显著增加了林地土壤的初级硝化速率,但对草地土壤的初级硝化速率没有显著影响。60% WHC条件下草地和林地土壤的初级反硝化速率可以忽略不计,水分含量增加至100% WHC后土壤初级反硝化速率显著提高,且草地土壤的初级反硝化速率显著低于林地土壤。100% WHC条件下林地土壤初级硝化速率与铵态氮固定速率比值(gn/ia)和N₂O排放量均显著高于60% WHC;100% WHC条件下草地土壤的N₂O排放量显著高于60% WHC,但两个水分条件下的gn/ia值无显著差异。表明短期内水分含量的增加可能会增加草地和林地土壤氮转化的负面环境效应,且对林地土壤的影响尤为显著。     

Kinetics of net nitrification associated with soil aggregates under conventional and no-tillage in a subtropical rice soil

Tillage effects on soil nitrification kinetics at the aggregate scale were studied for a subtropical rice soil. Soil samples were separated into large aggregates (>2.0 mm), macro-aggregates (2.0–0.25 mm), micro-aggregates (0.25–0.053 mm) and silt + clay fractions (<0.053 mm) by wet-sieving. The net nitrification process was simulated by a zero- and first kinetics model. Conventional tillage (CT) increased the proportion of the silt + clay fraction by 60% and decreased large-aggregates by 35% compared to ridge with no-till (RNT). Regression analysis showed that the time-dependent kinetics of net nitrification were best fitted by a zero-order model for the large-aggregates and silt + clay fraction but a first-order kinetic model for macro- and microaggregates and whole soil, regardless of tillage regime. Both potential nitrification rates (V _p ) and net nitrification rates (V _a ) were higher for macroaggregates than microaggregates. The potential nitrification (N _p ) for whole soil under RNT was 38.7% higher than CT. The V _p and V _a for whole soil was 88.5% and 64.7% higher under RNT than CT, respectively. Although nitrification was stimulated under RNT, the kinetics model of nitrification was not affected by tillage. This inferred that the interaction between substrates and enzymes involved in nitrification associated with aggregates was not altered by tillage. For this soil, nitrifying microorganisms were mainly associated with macro- and microaggregates rather than large-aggregates and silt + clay fractions.     

Influence of dissolved oxygen concentration on nitrite accumulation in a biofilm airlift suspension reactor

The biofilm airlift suspension (BAS) reactor can treat wastewater at a high volumetric loading rate combined with a low sludge loading. Two BAS reactors were operated, with an ammonium load of 5 kg N/(m(3) d), in order to study the influence of biomass and oxygen concentration on the nitrification process. After start-up the nitrifying biomass in the reactors gradually increased up to 30 g VSS/L. Due to this increased biomass concentration the gas-liquid mass transfer coefficient was negatively influenced. The resulting gradual decrease in dissolved oxygen concentration (over a 2-month period) was associated with a concomitantly nitrite build-up. Short term experiments showed a similar relation between dissolved oxygen concentration (DO) and nitrite accumulation. It was possible to obtain full ammonium conversion with approximately 50% nitrate and 50% nitrite in the effluent. The facts that (i) nitrite build up occurred only when DO dropped, (ii) the nitrite formation was stable over long periods, and (iii) fully depending on DO levels in short term experiments, led to the conclusion that it was not affected by microbial adaptations but associated with intrinsic characteristics of the microbial growth system. A simple biofilm model based on the often reported difference of oxygen affinity between ammonium and nitrite oxydizers was capable of adequately describing the phenomena.Measurements of biomass density and concentration are critical for the interpretation of the results, but highly sensitive to sampling procedures. Therefore we have developed an independent method, based on the residence time of Dextran Blue, to check the experimental methods. There was a good agreement between procedures.The relation between biomass concentration, oxygen mass transfer rate and nitrification in a BAS reactor is discussed. (c) 1997 John Wiley & Sons, Inc.     

Effect of low dissolved oxygen on simultaneous nitrification and denitrification in a membrane bioreactor treating black water

Effect of low dissolved oxygen on simultaneous nitrification and denitrification was evaluated in a membrane bioreactor treating black water. A fully aerobic membrane bioreactor was operated at a sludge age of 60 days under three low dissolved oxygen (DO) levels below 0.5mg/L. It sustained effective simultaneous nitrification/denitrification for the entire observation period. Nitrification was incomplete due to adverse effects of a number of factors such as low DO level, SMPs inhibition, alkalinity limitation, etc. DO impact was more significant on denitrification: Nitrate was fully removed at low DO level but the removal was gradually reduced as DO was increased to 0.5mg/L. Nitrogen removal remained optimal within the DO range of 0.15-0.35 mg/L. Experimental results were calibrated and simulated by model evaluation with the same model coefficients. The model defined improved mass transfer with lower affinity coefficients for oxygen and nitrate as compared to conventional activated sludge.     

Seasonal distribution of nitrifying bacteria and rates of nitrification in coastal marine sediments

The distribution of nitrification potential (NP) with depth in sediment and season was investigated in a shallow sandy sediment (0.5 m water) and a deeper muddy sediment (17m water). In both sediments, nitrifying bacteria were present in the anoxic strata (oxygen penetration was 5 mm below the surface). The NP at 6–8 cm depth in the sediment was 50% and 10% of the surface NP at the sandy and muddy sediment, respectively. It is suggested that bioturbation and physical disturbance of the sediment were the most likely reasons for this distribution. The NP increased as sediment temperature decreased. This effect was less marked in the muddy sediment. It is concluded that during the summer, the numbers or specific activity of nitrifying bacteria diminished for the following reasons: There was decreased O₂ penetration into the sediment and increased competition for O₂ by heterotrophs; there was increased competition for NH₄ ⁺ and there was inhibition by H₂S. These effects counteracted the potentially higher growth rates and increased rates of NH₄ ⁺ production at the elevated summer temperatures. The potential nitrification rates in the upper 1 cm, which were measured at 22°C, were converted to calculated rates at the in situ temperature (Q₁₀=2.5) and in situ oxygen penetration. These calculated rates were shown to closely resemble the measured in situ rates of nitrification. The relationship between the in situ rates of nitrification and the nitrification potential is discussed.     

The potential for nitrification and nitrate uptake in the rhizosphere of wetland plants: a modelling study

BACKGROUND AND AIMS: It has recently found that lowland rice grown hydroponically is exceptionally efficient in absorbing NO3-, raising the possibility that rice and other wetland plants growing in flooded soil may absorb significant amounts of NO3- formed by nitrification of NH4+ in the rhizosphere. This is important because (a) this NO3- is otherwise lost through denitrification in the soil bulk; and (b) plant growth and yield are generally improved when plants absorb their nitrogen as a mixture of NO3- and NH4+ compared with growth on either N source on its own. A mathematical model is developed here with which to assess the extent of NO3- absorption from the rhizosphere by wetland plants growing in flooded soil, considering the important plant and soil processes operating. METHODS: The model considers rates of O2 transport away from an individual root and simultaneous O2 consumption in microbial and non-microbial processes; transport of NH4+ towards the root and its consumption in nitrification and uptake at the root surface; and transport of NO3- formed from NH4+ towards the root and its consumption in denitrification and uptake by the root. The sensitivity of the model's predictions to its input parameters is tested over the range of conditions in which wetland plants grow. KEY RESULTS: The model calculations show that substantial quantities of NO3- can be produced in the rhizosphere of wetland plants through nitrification and taken up by the roots under field conditions. The rates of NO3- uptake can be comparable with those of NH4+. The model also shows that rates of denitrification and subsequent loss of N from the soil remain small even where NO3- production and uptake are considerable. CONCLUSIONS: Nitrate uptake by wetland plants may be far more important than thought hitherto. This has implications for managing wetland soils and water, as discussed in this paper.     

A metabolic model of the biological phosphorus removal process: II. Validation during start-up conditions

A metabolic model of the biological phosphorus removal process has been developed and validated previously for complex conversions during the process under anaerobic and aerobic conditions at different growth rates in sequencing batch reactors in steady state. For additional validation of the metabolic model, the model was applied to the dynamic conditions which occur during the start-up phase of the biological P removal in the presence and absence of non-polyP heterotrophic microorganisms. In a laboratory scale sequencing batch reactor, experiments were performed to examine the enrichment of the population with polyphosphate organisms during the start-up and the subsequent shift from non-polyP, heterotrophic organisms to polyP organisms in the sludge. The effect of different influent loading patterns for acetate and phosphate was studied. In these experiments, the maximal growth rate of the polyP organisms and the behavior of the internal storage compounds could be derived. The metabolic model was capable of describing the experimental results, without the need to adjust the kinetic or stoichiometric parameters obtained under steady state conditions. (c) 1995 John Wiley & Sons, Inc.     

Effect of submergence on subsequent nitrification in a wetland cultivated rice soil

Summary Soil samples from the surface (0–0.5 cm) and subsurface (0.5–5.0 cm) in a wetland field cultivated with rice and submerged for different periods up to 6 weeks were incubated for 2 weeks in the laboratory under flooded or unflooded condition, with added NH₄ ⁺–N. The ammonium and nitrate-N of the incubated-soils indicate that in the surface soil (a) nitrification is retarded when submerged for 4 weeks or more (b) nitrification did not resume to former levels within 2 weeks after air drying for a period of 1 week. In the subsurface soil, submergence for 2 weeks caused a retardation of nitrification but longer submergence did not reduce nitrification any further.During submergence, redox potential at 2 mm remained at relatively high values but began to decline 30 days after submergence. At 5 cm, Eh indicated reduced conditions from the time of submergence.     

Neem (Azadirachta indica) seed kernel powder retards urease and nitrification activities in different soils at contrasting moisture and temperature regimes

A laboratory experiment was conducted to examine the potentiality of a natural resource neem (Azadirachta indica) seed kernel powder (NSKP) to reduce the urease and nitrification activities in different soils (viz., normal, acid, and sodic) at contrasting moisture (1:1 soil to water and field capacity) and temperature regimes (10 degrees C and 37 degrees C). Results have revealed that application of NSKP with urea did not exhibit any urease inhibitory property in normal and sodic soils, but in acid soil it had maintained higher concentration of urea than the urea alone treated samples for two weeks after application. At 37 degrees C and under field capacity moisture level, urea hydrolysis was more rapid than at 10 degrees C and under waterlogged (1:1) conditions. The NSKP has showed variable effects (4-28%) to inhibit nitrification during 7-21 days after application, depending upon the soil types, temperature and moisture regimes. The nitrification activity was significantly low in acid soil followed by normal and sodic soils. The present study suggests that NSKP has the potential to retard the urease activity in acid soil, and nitrification in all the soils, and thus it may be used along with urea for the better use of applied -N.     

Spatial pattern of soil nitrogen availability and its relationship to stand structure in a coniferous-broadleaved mixed forest with a dense dwarf bamboo understory in northern Japan

Natural disturbances create spatial patterns of the ecosystem processes and functions in natural forests. However, how dynamics and the spatial structure of forests relate to soil nitrogen dynamics is not well understood. We examined the spatial relationship between the distributions of canopy and understory species, and soil nitrogen dynamics in a natural coniferous-broadleaved mixed forest with a dense understory of Sasa dwarf bamboo in northern Japan. The O horizon was thick where coniferous litter predominated, and it was thin where broadleaved litter predominated. The soil water content was low in areas with a thick O horizon and a high abundance of coniferous trees. The soil nitrate content was low where the soil water content was low, and the soil nitrate content increased linearly with increasing net nitrification potential. These results suggest that the soil nitrate content under the coniferous canopy was lower because of the low nitrification potential of soil microbes in soils with low water contents. The soil nitrate content and nitrification potential were higher in the canopy gap than under the canopy. Our results suggest that forest structure, specifically the thickness of the forest floor, significantly affects the spatial pattern of the soil water content, thereby creating a spatial pattern of soil nitrogen availability at a relatively small scale with flat topography. The higher nitrification potential under the canopy gap could pose a long-term risk of nitrate leaching because of the suppression of the natural regeneration of canopy species by dense Sasa dwarf bamboo in this forest ecosystem.     

Effect of sludge age on simultaneous nitrification and denitrification in membrane bioreactor

This study evaluated the effect of sludge age on simultaneous nitrification and denitrification in a membrane bioreactor treating black water. A membrane bioreactor with no separate anoxic volume was operated at a sludge age of 20 days under low dissolved oxygen concentration of 0.1-0.2 mg/L. Its performance was compared with the period when the sludge age was adjusted to 60 days. Floc size distribution, apparent viscosity, and nitrogen removal differed significantly, together with different biomass concentrations: nitrification was reduced to 40% while denitrification was almost complete. Modelling indicated that both nitrification and denitrification kinetics varied as a function of the sludge age. Calibrated values of half saturation coefficients were reduced when the sludge age was lowered to 20 days. Model simulation confirmed the validity of variable process kinetics for nitrogen removal, specifically set by the selected sludge age.