An ASM/ADM model interface for dynamic plant-wide simulation

Mathematical modelling has proven to be very useful in process design, operation and optimisation. A recent trend in WWTP modelling is to include the different subunits in so-called plant-wide models rather than focusing on parts of the entire process. One example of a typical plant-wide model is the coupling of an upstream activated sludge plant (including primary settler, and secondary clarifier) to an anaerobic digester for sludge digestion. One of the key challenges when coupling these processes has been the definition of an interface between the well accepted activated sludge model (ASM1) and anaerobic digestion model (ADM1). Current characterisation and interface models have key limitations, the most critical of which is the over-use of X_c (or lumped complex) variable as a main input to the ADM1. Over-use of X_c does not allow for variation of degradability, carbon oxidation state or nitrogen content. In addition, achieving a target influent pH through the proper definition of the ionic system can be difficult. In this paper, we define an interface and characterisation model that maps degradable components directly to carbohydrates, proteins and lipids (and their soluble analogues), as well as organic acids, rather than using X_c. While this interface has been designed for use with the Benchmark Simulation Model No. 2 (BSM2), it is widely applicable to ADM1 input characterisation in general. We have demonstrated the model both hypothetically (BSM2), and practically on a full-scale anaerobic digester treating sewage sludge.     

Arsenic Removal from Contaminated Groundwater by Zero Valent Iron: a Mechanistic and Long-Term Performance Study

The present study investigated the application of zero valent iron to remediate the arsenic in naturally contaminated groundwater. A performance evaluation was conducted in the laboratory on groundwater contaminated with artificial arsenic using sodium arsenate (Na₂HAsO₄.7H₂O) to simulate the arsenic concentration in the groundwater. Batch and column experiments were performed to evaluate the arsenic removal capacity by zero valent iron and the removal mechanism. The flow rate (up-flow mode) was maintained for 180 days in each column. The results from both the batch and the column experiments showed that more than 99% of the arsenic was removed successfully. In the column experiments, the arsenic was efficiently removed and the arsenic concentration in the treated water decreased to below the limit of 10 μg /L (WHO's standard) even when the columns were packed with only 25% ZVI by volume. We used SEM and XRD to characterize the surface morphology and the corrosion layer which formed on pristine ZVI and arsenic-treated ZVI to elucidate the arsenic removal mechanism. XRD and SEM results revealed that ZVI gradually converted to a magnetite/maghemite corrosion product mixed with lepidocrocite. Adsorption followed by co-precipitation was an important pathway to removing the arsenic by ZVI. Our results suggest that ZVI, combined with sand, is a suitable candidate for the ex-situ treatment of groundwater in the neutral pH range and in the presence of dissolved oxygen.     

Modeling anaerobic digestion of aquatic plants by rumen cultures: Cattail as an example

Despite of the significance of the anaerobic digestion of lignocellulosic materials, only a limited number of studies have been carried out to evaluate the lignocellulosic digestion kinetics, and information about the modeling of this process is limited. In this work, a mathematical model, based on the Anaerobic Digestion Model No.1 (ADM1), was developed to describe the anaerobic conversion of lignocellulose-rich aquatic plants, with cattail as an example, by rumen microbes. Cattail was fractionated into slowly hydrolysable fraction (SHF), readily hydrolysable fraction (RHF) and inert fraction in the model. The SHF was hydrolyzed by rumen microbes and resulted in the production of RHF. The SHF and RHF had different hydrolysis rates but both with surface-limiting kinetics. The rumen microbial population diversity, including the cattail-, butyrate-, acetate- and H₂-degraders, was all incorporated in the model structure. Experiments were carried out to identify the parameters and to calibrate and validate this model. The simulation results match the experimental data, implying that the fractionation of cattail into two biodegradation parts, i.e., SHF and RHF, and modeling their hydrolysis rate with a surface-limiting kinetics were appropriate. The model was capable of simulating the anaerobic biodegradation of cattail by the rumen cultures.     

An anaerobic two-layer permeable reactive biobarrier for the remediation of nitrate-contaminated groundwater

In this paper, an anaerobic two-layer permeable reactive biobarrier system consisting of an oxygen-capturing layer followed by a biodegradation layer was designed firstly for evaluating the remediation effectiveness of nitrate-contaminated groundwater. The first layer filling with granular oxygen-capturing materials is used to capture dissolved oxygen (DO) in groundwater in order to create an anaerobic condition for the microbial denitrification. Furthermore, it can also provide nutrition, such as carbon and phosphorus, for the normal metabolism of immobilized denitrifying bacteria filled in the second layer. The second layer using granular activated carbon as microbial carrier is able to biodegrade nitrate entering the barrier system. Batch experiments were conducted to identify the effect of DO on microbial denitrification, oxygen-capturing performance of zero valent iron (ZVI) powder and the characteristics of the prepared oxygen-capturing materials used to stimulate growth of denitrifying bacteria. A laboratory-scale experiment using two continuous upflow stainless-steel columns was then performed to evaluate the feasibility of this designed system. The first column was filled with granular oxygen-capturing materials prepared by ZVI powder, sodium citrate as well as other inorganic salts, etc. The second column was filled with activated carbon immobilizing denitrifying microbial consortium. Simulated nitrate-contaminated groundwater (40 mg NO₃–N/L, pH 7.0) with 6 mg/L of DO content was pumped into this system at a flow rate of 235 mL/d. Samples from the second column were analyzed for nitrate and its major degradation byproduct. Results showed that nitrate could be removed more than 94%, and its metabolic intermediate, nitrite, could also be biodegraded further in this passive system. Further study is necessary in order to evaluate performance of its field application.     

Effects of geochemical constituents on the zero‐valent iron reductive removal of nitrobenzene in groundwater

Nitrobenzene (NB) was chosen as the model contaminant, and batches lab scale experiments were conducted to investigate the effects of geochemical compositions on reductive removal of nitrobenzene by zero‐valent iron (ZVI) in groundwater. Experimental results showed that the kinetics of nitrobenzene reduction by ZVI was a pseudo‐first order with an observed rate constant, k_obs, of 3.67 × 10^?4/s; the calculated half‐life of nitrobenzene was 1.89 × 10³/s. The geochemical constituents of groundwater have significant influences on ZVI reactivity and nitrobenzene reduction. The presence of high concentration of nitrate, carbonate, sulphate and hardness had detrimental effects on reduction of nitrobenzene and formation of aniline; chloride had a slight positive effect on the nitrobenzene reduction and the formation of aniline. Bicarbonate enhanced the ZVI reactivity initially at lower concentration and inhibited the nitrobenzene reduction at high concentration. Therefore, the performance and reactivity of ZVI were found to be strongly affected by the geochemical constituents of groundwater.     

Impact of nano zero valent iron (NZVI) on methanogenic activity and population dynamics in anaerobic digestion

Nano zero valent iron (NZVI), although being increasingly used for environmental remediation, has potential negative impact on methanogenesis in anaerobic digestion. In this study, NZVI (average size = 55 ± 11 nm) showed inhibition of methanogenesis due to its disruption of cell integrity. The inhibition was coincident with the fast hydrogen production and accumulation due to NZVI dissolution under anaerobic conditions. At the concentrations of 1 mM and above, NZVI reduced methane production by more than 20%. At the concentration of 30 mM, NZVI led to a significant increase in soluble COD (an indication of cell disruption) and volatile fatty acids in the mixed liquor along with an accumulation of H₂, resulting in a reduction of methane production by 69% (±4% [standard deviation]). By adding a specific methanogenesis inhibitor-sodium 2-bromoethanesulfonate (BES) to the anaerobic sludge containing 30 mM NZVI, the amount of H₂ produced was only 79% (±1%) of that with heat-killed sludge, indicating the occurrence of bacterially controlled hydrogen utilization processes. Quantitative PCR data was in accordance with the result of methanogenesis inhibition, as the level of methanogenic population (dominated by Methanosaeta) in the presence of 30 mM NZVI decreased significantly compared to that of the control. On the contrary, ZVI powder (average size <212 μm) at the same concentration (30 mM) increased methane production presumably due to hydrogenotrophic methanogenesis of hydrogen gas that was slowly released from the NZVI powder. While it is a known fact that NZVI disrupts cell membranes, which inhibited methanogenesis described herein, the results suggest that the rapid hydrogen production due to NZVI dissolution also contribute to methanogenesis inhibition and lead to bacterially controlled hydrogenotrophic processes.     

GISCOD: General Integrated Solid Waste Co-Digestion model

This paper views waste as a resource and anaerobic digestion (AD) as an established biological process for waste treatment, methane production and energy generation. A powerful simulation tool was developed for the optimization and the assessment of co-digestion of any combination of solid waste streams. Optimization was aimed to determine the optimal ratio between different waste streams and hydraulic retention time by changing the digester feed rates to maximize the biogas production rate. Different model nodes based on the ADM1 were integrated and implemented on the Matlab-Simulink^® simulation platform. Transformer model nodes were developed to generate detailed input for ADM1, estimating the particulate waste fractions of carbohydrates, proteins, lipids and inerts. Hydrolysis nodes were modeled separately for each waste stream. The fluxes from the hydrolysis nodes were combined and generated a detailed input vector to the ADM1. The integrated model was applied to a co-digestion case study of diluted dairy manure and kitchen wastes. The integrated model demonstrated reliable results in terms of calibration and optimization of this case study. The hydrolysis kinetics were calibrated for each waste fraction, and led to accurate simulation results of the process and prediction of the biogas production. The optimization simulated 200,000 days of virtual experimental time in 8 h and determined the feedstock ratio and retention time to set the digester operation for maximum biogas production rate.     

Resilient modulus prediction of soft low-plasticity Piedmont residual soil using dynamic cone penetrometer

Dynamic cone penetrometer (DCP) has been used for decades to estimate the shear strength and stiffness properties of the subgrade soils. There are several empirical correlations in the literature to predict the resilient modulus values at only a specific stress state from DCP data, corresponding to the predefined thicknesses of pavement layers (a 50 mm asphalt wearing course, a 100 mm asphalt binder course and a 200 mm aggregate base course). In this study, field-measured DCP data were utilized to estimate the resilient modulus of low-plasticity subgrade Piedmont residual soil. Piedmont residual soils are in-place weathered soils from igneous and metamorphic rocks, as opposed to transported or compacted soils. Hence the existing empirical correlations might not be applicable for these soils. An experimental program was conducted incorporating field DCP and laboratory resilient modulus tests on “undisturbed” soil specimens. The DCP tests were carried out at various locations in four test sections to evaluate subgrade stiffness variation laterally and with depth. Laboratory resilient modulus test results were analyzed in the context of the mechanistic-empirical pavement design guide (MEPDG) recommended universal constitutive model. A new approach for predicting the resilient modulus from DCP by estimating MEPDG constitutive model coefficients (k₁, k₂ and k₃) was developed through statistical analyses. The new model is capable of not only taking into account the in situ soil condition on the basis of field measurements, but also representing the resilient modulus at any stress state which addresses a limitation with existing empirical DCP models and its applicability for a specific case. Validation of the model is demonstrated by using data that were not used for model development, as well as data reported in the literature.     

Modelling the energy balance of an anaerobic digester fed with cattle manure and renewable energy crops

Knowledge of the net energy production of anaerobic fermenters is important for reliable modelling of the efficiency of anaerobic digestion processes. By using the Anaerobic Digestion Model No. 1 (ADM1) the simulation of biogas production and composition is possible. This paper shows the application and modification of ADM1 to simulate energy production of the digestion of cattle manure and renewable energy crops. The paper additionally presents an energy balance model, which enables the dynamic calculation of the net energy production. The model was applied to a pilot-scale biogas reactor. It was found in a simulation study that a continuous feeding and splitting of the reactor feed into smaller heaps do not generally have a positive effect on the net energy yield. The simulation study showed that the ratio of co-substrate to liquid manure in the inflow determines the net energy production when the inflow load is split into smaller heaps. Mathematical equations are presented to calculate the increase of biogas and methane yield for the digestion of liquid manure and lipids for different feeding intervals. Calculations of different kinds of energy losses for the pilot-scale digester showed high dynamic variations, demonstrating the significance of using a dynamic energy balance model.     

Applying an electric field in a built-in zero valent iron--anaerobic reactor for enhancement of sludge granulation

A zero valent iron (ZVI) bed with a pair of electrodes was installed in an upflow anaerobic sludge blanket (UASB) reactor to create an enhanced condition to increase the rate of anaerobic granulation. The effects of an electric field and ZVI on granulation were investigated in three UASB reactors operated in parallel: an electric field enhanced ZVI-UASB reactor (reactor R1), a ZVI-UASB reactor (reactor R2) and a common UASB reactor (reactor R3). When a voltage of 1.4 V was supplied to reactor R1, COD removal dramatically increased from 60.3% to 90.7% over the following four days, while the mean granule size rapidly grew from 151.4 μm to 695.1 μm over the following 38 days. Comparatively, COD removal was lower and the increase in granule size was slower in the other two reactors (in the order: R1 > R2 > R3). The electric field caused the ZVI to more effectively buffer acidity and maintain a relatively low oxidation-reduction potential in the reactor. In addition, the electric field resulted in a significant increase in ferrous ion leaching and extracellular polymeric substances (EPS) production. These changes benefited methanogenesis and granulation. Scanning electron microscopy (SEM) images showed that different microorganisms were dominant in the external and internal layers of the reactor R1 granules. Additionally, fluorescence in situ hybridization (FISH) analysis indicated that the relative abundance of methanogens in reactor R1 was significantly greater than in the other two reactors. Taken together, these results suggested that the use of ZVI combined with an electric field in an UASB reactor could effectively enhance the sludge granulation.     

Modified ADM1 structure for modelling municipal primary sludge hydrolysis

This study elaborates the rate-limiting steps of particle disintegration/hydrolysis of primary sludge using methane production rate (MPR) curves from multiple batch experiments. Anaerobic batch degradation of fresh primary sludge showed a complex MPR curve marked with two well-defined temporal peaks. The first immediate peak was associated with the degradation of relatively readily hydrolysable substrates, while the second delayed peak was associated with the degradation of large-sized particles. For simulating the second delayed peak, it was necessary to consider a more elaborate particle disintegration/hydrolysis model. Based on the anaerobic respirograms of 17 runs in four datasets and using a substrate characterisation approach similar to activated sludge models (ASMs), the primary sludge was classified into three biodegradable fractions having different kinetics. These are (1) a hydrolysable substrate (X(Settle-I)) showing a degradation typical to slowly biodegradable compounds, (2) a substrate fraction (X(Settle-II)) having a degradation similar to lysis of biomass fraction and (3) a substrate requiring disintegration before hydrolysis (X(Settle-III)) representing the large-sized particles in primary sludge. Based on these results, modifications in the model structure of anaerobic digestion model no. 1 (ADM1) are proposed to improve the modelling of primary sludge solid degradation in anaerobic digesters.     

Modeling microbial diversity in anaerobic digestion through an extended ADM1 model

The anaerobic digestion process comprises a whole network of sequential and parallel reactions, of both biochemical and physicochemical nature. Mathematical models, aiming at understanding and optimization of the anaerobic digestion process, describe these reactions in a structured way, the IWA Anaerobic Digestion Model No. 1 (ADM1) being the most well established example. While these models distinguish between different microorganisms involved in different reactions, to our knowledge they all neglect species diversity between organisms with the same function, i.e. performing the same reaction. Nevertheless, available experimental evidence suggests that the structure and properties of a microbial community may be influenced by process operation and on their turn also determine the reactor functioning. In order to adequately describe these phenomena, mathematical models need to consider the underlying microbial diversity. This is demonstrated in this contribution by extending the ADM1 to describe microbial diversity between organisms of the same functional group. The resulting model has been compared with the traditional ADM1 in describing experimental data of a pilot-scale hybrid Upflow Anaerobic Sludge Filter Bed (UASFB) reactor, as well as in a more detailed simulation study. The presented model is further shown useful in assessing the relationship between reactor performance and microbial community structure in mesophilic CSTRs seeded with slaughterhouse wastewater when facing increasing levels of ammonia.     

ADM1 can be applied to continuous bio-hydrogen production using a variable stoichiometry approach

The IWA Anaerobic Digestion Model No.1 (ADM1) has been extensively used in recent years. However, its application to non-methanogenic systems is limited by the use of constant-stoichiometry to describe product formation from carbohydrate fermentation. This study presents a modification of the ADM1 using a variable stoichiometry approach, derived from experimental information. The biomass and product yields from glucose degradation are assumed to be dynamically depending on the total concentration of undissociated acids in the reactor. Experimental data from an 11 L mesophilic continuous bio-hydrogen reactor fed with 20, 40, 50 and 10 g/L of sucrose, were used to validate the approach. The modified model achieved good predictions of the experimental data, using the standard ADM1 parameter values, without any parameter fitting beyond the implementation of the variable stoichiometry. The modification approach proposed extends the applicability of the ADM1 to non-methanogenic fermentative systems and in particular to continuous bio-hydrogen production.     

Kinetics and mechanisms of pH-dependent selenite removal by zero valent iron

The kinetics of Se(IV) removal by zero valent iron (ZVI) open to the air as a function of pH and the involved mechanisms were investigated in this study. The specific rate constants of Se(IV) removal by ZVI decreased from 92.87 to 6.87 L h⁻¹ m⁻² as pH increased from 4.0 to 7.0. The positive correlation between the removal rate of Se(IV) and the generation rate of Fe(II) and the depression of Se(IV) removal in the presence of 1,10-phenanthroline indicated that both ZVI and adsorbed Fe(II) on ZVI surface contributed to the reductive removal of Se(IV). The soft X-ray STXM measurement confirmed the adsorption of Fe(II) on the surface of ZVI and freshly formed ferric (hydr)oxides. Se(IV) was removed by adsorption followed by reduction to Se(0) on ZVI surface at pH 4.0–7.0, as revealed by XANES spectra. A core-shell structure was observed when ZVI reacted with Se(IV)-containing solution for 3 h at pH 6.0. Se(IV) was reduced to Se(0) and co-precipitated with the freshly formed Fe(III), forming the shell surrounding the iron core. After reaction for 24 h, the generated Se(0) was surrounded by multiple layers of Fe(III) oxides/hydroxides. SEM images and XRD patterns revealed that the corrosion products of ZVI at pH 6.0 transformed from amorphous iron hydroxides to lepidocrocite (γ-FeOOH) as reaction proceeded. The final corrosion products of ZVI contained both lepidocrocite and goethite at pH 5.0 while they were X-ray amorphous at pH 4.0 and 7.0.     

Pharmaceutical wastewater degradation: effective and economical treatment using waste-metallic iron shavings

Industrial wastewater generated by a drug manufacturing plant located in Spain was degraded by Fenton oxidation processes, which employ waste-metallic iron shavings as heterogeneous zero-valent iron (ZVI) catalyst and hydrogen peroxide. The effluent comprises a complex mixture of organic substances which are very refractory to common conventional treatments and it is characterized by a low BOD/COD ratio. The stirring speed or the particle size has been found to be the determining factors, greatly influencing the degradation of the organic pollutants present in the wastewater. The influence of the initial hydrogen peroxide concentration has also been evaluated. The optimal conditions for degradation led to total organic carbon (TOC) reductions of up to 60%. The remarkable results of TOC mineralization could also be attributed to the physico-chemical modification of the ZVI during the oxidizing process. This study shows that the ZVI/H₂O₂ system can be considered as an easy, economic and effective alternative solution as a pre-treatment step before biological treatments.     

Modelling of two-stage anaerobic digestion using the IWA Anaerobic Digestion Model No. 1 (ADM1)

The aim of the study presented was to implement a process model to simulate the dynamic behaviour of a pilot-scale process for anaerobic two-stage digestion of sewage sludge. The model implemented was initiated to support experimental investigations of the anaerobic two-stage digestion process. The model concept implemented in the simulation software package MATLAB/Simulink is a derivative of the IWA Anaerobic Digestion Model No.1 (ADM1) that has been developed by the IWA task group for mathematical modelling of anaerobic processes. In the present study the original model concept has been adapted and applied to replicate a two-stage digestion process. Testing procedures, including balance checks and 'benchmarking' tests were carried out to verify the accuracy of the implementation. These combined measures ensured a faultless model implementation without numerical inconsistencies. Parameters for both, the thermophilic and the mesophilic process stage, have been estimated successfully using data from lab-scale experiments described in literature. Due to the high number of parameters in the structured model, it was necessary to develop a customised procedure that limited the range of parameters to be estimated. The accuracy of the optimised parameter sets has been assessed against experimental data from pilot-scale experiments. Under these conditions, the model predicted reasonably well the dynamic behaviour of a two-stage digestion process in pilot scale.     

Lastkollektive für Eisenbahnbrücken aus Messdaten

Load spectra for railway bridges from measurement data – Part 1: systematic investigations into damage equivalent factor λ₁ on single span girders. The rapid progress in the evolution of memory device leads to an increasing data‐gathering. This offers new opportunities to define fatigue loads for railway bridges by systematic evaluations. So far, the definition of load spectra based on predefined trains and traffic mix was the only approach, but now it can be done much more detailed but even more CPU‐intensive by using influence data over a long period of time. This article shows practical results of this new approach for building load spectra by using systematic calculations on single span girders together with short theoretical explanations. The used influence data provided by the Österreichischen Bundesbahnen result in significant differences comparing to standardized values. Part 1 of this article includes exclusively the damage equivalent factor λ₁ while the second part includes the damage equivalent factors λ₂ and λ₃ as well as the threshold value λ_max and a model with respect to traffic trends.