Kinetic modelling of nitrogen and organics removal in vertical and horizontal flow wetlands

This paper provides a comparative evaluation of the kinetic models that were developed to describe the biodegradation of nitrogen and organics removal in wetland systems. Reaction kinetics that were considered in the model development included first order kinetics, Monod and multiple Monod kinetics; these kinetics were combined with continuous-stirred tank reactor (CSTR) or plug flow pattern to produce equations to link inlet and outlet concentrations of each key pollutants across a single wetland. Using three statistical parameters, a critical evaluation of five potential models was made for vertical and horizontal flow wetlands. The results recommended the models that were developed based on Monod models, for predicting the removal of nitrogen and organics in a vertical and horizontal flow wetland system. No clear correlation was observed between influent BOD/COD values and kinetic coefficients of BOD₅ in VF and HF wetlands, illustrating that the removal of biodegradable organics was insensitive to the nature of organic matter. Higher effluent COD/TN values coincided with greater denitrification kinetic coefficients, signifying the dependency of denitrification on the availability of COD in VF wetland systems. In contrast, the trend was opposite in HF wetlands, indicating that availability of NO₃-N was the main limiting step for nitrogen removal. Overall, the results suggested the possible application of the developed alternative predictive models, for understanding the complex biodegradation routes of nitrogen and organics removal in VF and HF wetland systems.     

改进型氧化沟的水力流态特性及其脱氮除磷效果

氧化沟前端增设厌氧区,循环廊道内布置缺氧区、好氧区.为了保证缺氧区、好氧区内溶解氧浓度的合理分布,于循环廊道内增设插板,并通过试验对比了增设插板前、后循环廊道内混合液的水力流态特性及系统的脱氮除磷效果.试验结果表明,增设插板后循环廊道内混合液的流态更趋于推流,有助于提高系统的脱氮除磷性能;增设插板前、后系统对COD、NH3-N的去除效果没有明显变化,对二者的平均去除率均在95%以上,但对TN和TP的平均去除率分别由40%和57%提高至70%和75%,增设插板后出水TN<15 mg/L、TP为2 mg/L左右.     

Dispersion in the inclined plane model of trickling filters

In this paper an attempt has been made to set up a relationship between the removal efficiency and the hydraulic regimes of biological filters. For this purpose an equation has been first derived to determine the dispersion number. Using this equation, effluent concentration has been calculated by means of dispersed plug flow model in the range of channelized flow, and for large hydraulic loadings. Around the minimum wetting rate, the removal efficiencies are computed with equation of complete mixing. The application of this theory to the experimental results reported in literature has also been shown and the ranges of validity of dispersed plug flows and mixflows are determined. It has been demonstrated that the predicted concentrations are in good agreement with the measured values.     

前置反硝化BAF工艺处理景观水体

为了提高曝气生物滤池(BAF)对景观水体的脱氮效果,开发了前置反硝化BAF工艺,考察了其对景观水体的处理效果及影响因素,并与传统BAF工艺作比较。结果表明,前置反硝化BAF工艺对TN和浊度的去除效果明显优于传统BAF工艺,前者对TN和浊度的平均去除率分别为45%和88%,而后者的分别为30%和47%;两种工艺对COD和氨氮的去除效果相近,但前置反硝化BAF工艺的出水水质更稳定;对于前置反硝化BAF工艺,当水力负荷为1.42~4.95 m/h时,对COD、氨氮和TN的去除率均随水力负荷的增加而降低;在水力负荷为2.12 m/h的条件下,回流比对COD和氨氮的去除效果影响不大,但对TN的去除率随回流比的增加呈先升高后降低的趋势,最佳回流比为1.5∶1;另外,对TN的去除率随进水C/N值的增加而升高,当C/N6时,系统的脱氮效果较好。     

水力负荷对木炭填料生物滤池处理污水的影响

以木炭作为曝气生物滤池的填料,并与陶粒填料作对比,考察了其在不同水力负荷下对污水的处理效能.结果表明,改变水力负荷对两个反应器的出水COD影响不大,但对其脱氮除磷效果的影响较显著.木炭滤池对氨氮和TN的去除效果要好于陶粒滤池的,当水力负荷为4m~3/(m~3·d)时,两反应器的脱氮效果最佳,此时木炭和陶粒滤池的出水氨氮浓度分别为4.7、7.7mg/L(去除率分别为80%和65%).综合考虑木炭滤池对COD、NH_4~+-N、TN和TP的去除效果,确定其最佳水力负荷为4 m~3/(m~3·d).     

A steady-state model for the rotating biological disc reactor

A steady-state model for substrate removal in a rotating biological disc reactor is presented. The model considers the consumption of substrate by micro-organisms in the biofilm attached to the rotating disc, and mass transfer from the attached liquid film to the biofilm. A mass balance on substrate over the liquid in the trough provides an expression for effluent substrate in terms of the microbial kinetic constants, the geometry of the system and the operating conditions. In order to simplify the solution of the equations in the model, first order kinetics are assumed for the rate of microbial growth and substrate utilization. This simplified model can be solved, for example, by using a programmable calculator. The model predicts that the fractional removal of substrate per stage is strongly dependent on the hydraulic loading rate per unit disc area but independent of feed substrate concentration. It predicts constant removal independent of disc size provided the hydraulic loading per unit area is kept constant. The rotational speed has only a slight predicted effect on the rate of substrate removal. The predictions of the model are compared with published data from the literature for both domestic wastewater and some industrial wastes. Trends observed in the field are predicted qualitatively by the model.     

Non-steady-state modelling of faecal coliform removal in deep tertiary lagoons

In Noirmoutier, a French island off the Atlantic coast, secondary effluents flow into a series of four lagoons, 1.4-2.8 m deep, and are reused for agricultural irrigation. The excess water is disposed of to the sea. The aim of this study was to provide a model capable of predicting the microbiological quality of the water pumped for irrigation or discharged to the sea. Meteorological variables, flow rates, physical-chemical characteristics and faecal coliform (FC) contents were monitored for a year and a half. The hydraulic pattern of each lagoon was assumed to be that of completely mixed reactor because of the calculated dispersion numbers and the wind mixing effect. Coliform decay was assumed to follow first order kinetics in each lagoon. Die-off coefficients were calculated in each lagoon using a non-steady-state model. The main bacterial removal mechanism was shown to be solar irradiation. Empirical equations were established to calculate die-off coefficients as a function of received solar energy and temperature. FC die-off rates were higher in the first lagoon and then decreased successively in those following. FC numbers in the different lagoons were predicted with reasonable accuracy in spite of high variation in inlet water quality. The model will facilitate the prediction of water quality under various climatic conditions and different water reuse scenarios and will help to optimise reclamation and storage facilities.     

人工湿地用于污水深度处理的反应动力学

比较3种不同形式人工湿地(潜流、表面流和组合流)对城市污水厂二级生物处理出水中污染物的去除效果,并采用一级反应动力学模型进行模拟.结果表明,潜流湿地对有机物、总氮、总磷的去除效率高于其它2种湿地,其标准温度下反应动力学常数( KA20)分别为0.29、0.20和0.28 m/d;表流湿地对氨氮的去除效率最高,其KA20值为0.12 m/d.温度变化对3种人工湿地中有机物和总磷去除的影响不明显,对氨氮和总氮的去除有明显影响,尤其对表面流湿地影响最为显著.3种人工湿地对污染物的去除效率均随着水力负荷的增大而显著下降,污染物的面积去除量将随着面积负荷的不断提高逐渐趋于定值,因此人工湿地宜在低负荷工况下运行.     

A biofiltration model for tertiary nitrification of municipal wastewaters

The main objective of this work concerns the evaluation of the biological aerated filtration model found in GPS-X^®, which had never been evaluated with adequate data. This model is interesting since it integrates the physical and biological phenomena involved during filtration with a low complexity of use. The validation of the model parameters combines experimental and theoretical approaches. Experimental data were recorded at a semi-industrial pilot scale submerged biofilter operated at a tertiary nitrification stage, receiving the effluent of a medium loaded activated sludge process for municipal wastewater. Also, several protocols were regularly applied to characterize the biofilm and the nitrogen removal performances: dry density and thickness of biofilm, nitrification rates and corresponding quantity of autotrophic biomass accumulated inside the filtering media, quantity of extracted autotrophic bacteria in the backwash water, nitrification capacity along the biofilter, as well as nitrogen compounds in the effluent. For short-term dynamic conditions, a set of reliable parameter values has been used to predict nitrogen removal for different data sets. For long-term dynamic periods, the need to adapt some of the parameters from one set of data to another is demonstrated. It is shown that the hydraulic loading rate and the backwashing frequency are the main parameters responsible for these modifications.     

水力停留时间对BAF除污性能的影响

研究了水力停留时间(HRT)对曝气生物滤池(BAF)除污效果的影响。结果表明:HRT对BAF处理效果的影响较大,当HRT为0.63 h时,出水浊度、COD、氨氮和总氮浓度均较高,BAF的除污效果较差;当HRT为0.83 h时,出水COD浓度可降至50 mg/L以下,去除率可达到85.87%;当HRT为1.0 h时,BAF对浊度、氨氮和总氮均有较好的去除效果,去除率分别为95.98%7、7.08%4、0.09%,出水浊度<4 NTU、氨氮<8 mg/L、总氮<35 mg/L。     

Design and evaluation of hydraulic baffled-channel PAC contactor for taste and odor removal from drinking water supplies

Based on the concept of hydraulic flocculator, a baffled-channel powdered activated carbon (PAC) contactor, placed before the rapid-mixing basin, was designed and evaluated for removal of taste and odor (T&O) in drinking water. PAC adsorption kinetic tests for raw water samples were conducted for selection of design parameters related to contact time and degree of mixing. Within the tested range of velocity gradient (G) from 18 to 83s(-1), mixing had a relatively minor effect on the adsorption kinetics of the PAC. The hydrodynamic characteristics of the pilot-scale horizontally and vertically baffled-channel PAC contactor were investigated by tracer tests. It was found that the plug flow fractions of vertically baffled-channel PAC contactor (vBPC) were higher than those of the horizontally baffled-channel PAC contactor (hBPC) for the same bend width or bend height. However, the hBPC seems to be more appropriate than the vBPC in terms of construction and maintenance. The geosmin and MIB removal rate increased with the number of baffles, PAC dose and contact time increased regardless of bend width in the pilot-scale hBPC. The pair of full-scale hBPCs at Pohang water treatment plant, having a design capacity of 6.5x10(4)m(3)/d with 20min of hydraulic retention time with a safety factor of 2, was designed based on lab- and pilot-scale experimental results. Under a velocity gradient of 20s(-1), the number of baffles to be installed was calculated to be 20 with a space of about 2m between each baffle, resulting in a hydraulic head loss through the contactor of about 0.056m. The successful application of hBPC for T&O removal from drinking water supplies should provide momentum for developing more effective treatment methods.     

螺旋升流式反应器的流态特性及流动模型研究

通过示踪试验以及计算流体力学软件Phoenics3．6对SUFR系统单元反应器的流态特性以及流动模型进行了研究。结果表明，SUFR反应器兼有推流与全混流的特点；SUFR单元反应器内的水流以螺旋方式行进，有助于反应器形成推流特征；SUFR单元反应器内的流动模型可以看作8个单元反应器串联而成，其中推流单元反应器与全混流反应器间隔排列，推流流态的容积占有率约为63％，其推流特征较为明显。     

An analysis of performance models for free water surface wetlands

Although treatment wetlands are intended to attenuate pollutants, reliably predicting their performance remains a challenge because removal processes are often complex, spatially heterogeneous, and incompletely understood. Although initially popular for characterizing wetland performance, plug flow reactor models are problematic because their parameters exhibit correlation with hydraulic loading. One-dimensional advective-dispersive-reactive models may also be inadequate when longitudinal dispersion is non-Fickian as a result of pronounced transverse gradients in velocity (preferential flow). Models that make use of residence time distributions have shown promise in improving wetland performance characterization, however their applicability may be limited by certain inherent assumptions, e.g. that transverse mixing is nil. A recently-developed bicontinuum (mobile-mobile) model that addresses some of these weaknesses may hold promise for improving wetland performance modeling, however this model has yet to be tested against real-world wetland data. This paper examines the state of the science of free water surface wetland hydrodynamics and transport modeling, discusses the strengths and weaknesses of various steady state models, and compares them to each other in terms of each model’s ability to represent data sets from monitored wetlands.     

新型填料曝气生物滤池去除氮化合物的效果

采用聚丙烯腈中空纤维膜段作为上流式曝气生物滤池的填料,就系统启动期间生物膜的形成以及对生活污水中氮化合物的去除规律进行了研究,并考察了稳定运行阶段的气水比、水力停留时间等对硝化和反硝化的影响.试验发现:在载体的外表面进行硝化反应的同时,其内部还发生了反硝化反应;中空纤维膜段的形状结构和表面特点使其适宜作为生物膜的载体.     

BAF工艺处理微污染含铁锰地下水试验研究

采用BAF工艺处理微污染含铁锰地下水,研究了其净化效能和适宜的运行条件.结果表明,在水力负荷为4-5 m3/(m2·h),气水比为3:1-4:1的试验条件下,BAF工艺能有效去除氨氮、锰、铁、CODMn.和浊度.当原水铁含量小于2 mg/L,滤层中部DO在4 mg/L左右时,对氨氮的去除效果最佳.微量Fe2+即可维系滤...     

Grey-box modelling of pollutant loads from a sewer system

Using a compact measuring unit with on-line meters for UV absorption and turbidity, it is possible to determine concentrations of organic load (chemical oxygen demand (COD) and suspended solids (SS)) anywhere in a sewer system. When measurements of the flow are available as well, the pollutant mass flow at the measuring point can be calculated.The measured data are used to estimate different models describing the load of pollutants in the sewer. A comparison of the models shows that a grey-box model is most informative and best in terms measured by the multiple correlation coefficient. The grey-box model is a state-space model, where the state represents the actual amount of deposition in the sewer, and the output from the model is the pollutant mass flow to the wastewater treatment plant (WWTP). The model is formulated by means of stochastic differential equations. Harmonic functions are used to describe the dry weather diurnal load profiles. It is found that the accumulation of deposits in the sewer depends on previous rain events and flows.By means of on-line use of the grey-box models, it is possible to predict the amount of pollutants in a first flush at any time, and hence from the capacity of the plant to decide if and when the available detention basin is to be used for storage of wastewater. The mass flow models comprise an important improvement of the integrated control of sewer and WWTP including control of equalisation basins in the sewer system. Further improvements are expected by the introduction of an additive model where dry weather situations and storm situations are modelled separately before addition to the resulting model.     

Influence of geometrical and operational parameters on the axial dispersion in an aerated channel reactor

Residence time distribution experiments have been performed on an activated sludge 3000 m³ channel reactor aerated by gas diffusion (for different liquid flowrates under constant aeration rate and constant water depth) and on a bench-scale channel reactor aerated from the bottom (for different liquid and gas flowrates and water depths) in order to characterize their hydrodynamics. Both units can be modeled as plug flow reactors with axial dispersion. A general correlation has been obtained to predict the axial dispersion coefficient as a function of the gas and liquid velocities and the geometrical parameters of the full-scale and bench-scale reactors. Finally, to facilitate the simulation of biological reactions in transient state, an equivalent model based on tanks-in-series with variable back-mixing flowrate is proposed.     

Impact of water boundary layer diffusion on the nitrification rate of submerged biofilter elements from a recirculating aquaculture system

Total ammonia nitrogen (TAN) removal by microbial nitrification is an essential process in recirculating aquaculture systems (RAS). In order to protect the aquatic environment and fish health, it is important to be able to predict the nitrification rates in RAS's. The aim of this study was to determine the impact of hydraulic film diffusion on the nitrification rate in a submerged biofilter. Using an experimental batch reactor setup with recirculation, active nitrifying biofilter units from a RAS were exposed to a range of hydraulic flow velocities. Corresponding nitrification rates were measured following ammonium chloride, NH₄Cl, spikes and the impact of hydraulic film diffusion was quantified.The nitrification performance of the tested biofilter could be significantly increased by increasing the hydraulic flow velocity in the filter. Area based first order nitrification rate constants ranged from 0.065 m d⁻¹ to 0.192 m d⁻¹ for flow velocities between 2.5 m h⁻¹ and 40 m h⁻¹ (18 °C). This study documents that hydraulic film diffusion may have a significant impact on the nitrification rate in fixed film biofilters with geometry and hydraulic flows corresponding to our experimental RAS biofilters. The results may thus have practical implications in relation to the design, operational strategy of RAS biofilters and how to optimize TAN removal in fixed film biofilter systems.     

Metabolic modelling of full-scale biological nitrogen and phosphorus removing wwtp's.

This paper evaluates the experiences with modelling full-scale biological phosphorus and nitrogen removing wastewater treatment plants. For the simulation, we used a metabolic phosphorus model integrated in ASM2d, further referred to as TUDP-model. It was found that the metabolic model for bio-P removal can be applied for modelling full-scale wwtp's, without extensive parameter adjustments. A stepwise modelling approach was proposed. Only three specific parameters were calibrated. Two parameters, the inert fraction in the influent and the actual anoxic sludge fraction will need calibration for all systems. Parameter sensitivity analyses showed that the sensitivity of operational data, often considered as known input data, is high. The model kinetics were found less sensitive. This will in general be the case for all low-loaded wwtp's recorded in their pseudo-steady state. Based on mass balance calculations, operational data and measurements were evaluated. Since all terms on a phosphorus balance can be measured, errors in operational data (e.g. SRT and flow rates) become very apparent in P-removal models (ASM2d, TUDP). We suggest using the P-balance in general for wwtp modelling, as a check on data consistency. This study showed that considering operational data per definition as known input data is not justified. Therefore, operational data should be evaluated, or considered in model calibration over the use of kinetic and stoichiometric parameters.     

Removal of organic matter in stormwater ponds: a plug-flow model generalisation from waste stabilisation ponds to shallow rivers

ABSTRACT

This study investigates the effect of flow conditions on the organic matter concentration, removal efficiency, and reaction kinetics in a stormwater pond in Fortaleza, Brazil. As a result of unauthorized sewage discharges, BOD and COD concentrations were similar to those of combined sewer systems. The concentrations remained roughly the same during the rainy season, which was attributed to sewage network overflow. Removal efficiencies ranged from 70–90%, similar to primary facultative ponds. Fitting different hydraulic models to the field data, it was possible to obtain BOD and COD removal rates that could be described as functions of the Reynolds number. The best fit was achieved considering the plug-flow assumption, and a general BOD model including data reported in the literature for waste stabilisation ponds, stormwater ponds and shallow rivers was derived. Lastly, simulations with this general model were performed to assess the impact of remediation measures on the studied pond.