Production Submodel Research Articles

Energy optimization of a wastewater treatment plant based on energy audit data: small investment with high return.

Ambitious energy targets in the 2020 European climate and energy package have encouraged many stakeholders to explore and implement measures improving the energy efficiency of water and wastewater treatment facilities. Model-based process optimization can improve the energy efficiency of wastewater treatment plants (WWTP) with modest investment and a short payback period. However, such methods are not widely practiced due to the labor-intensive workload required for monitoring and data collection processes. This study offers a multi-step simulation-based methodology to evaluate and optimize the energy consumption of the largest Italian WWTP using limited, preliminary energy audit data. An integrated modeling platform linking wastewater treatment processes, energy demand, and production sub-models is developed. The model is calibrated using a stepwise procedure based on available data. Further, a scenario-based optimization approach is proposed to obtain the non-dominated and optimized performance of the WWTP. The results confirmed that up to 5000MWh annual energy saving in addition to improved effluent quality could be achieved in the studied case through operational changes only.

A system dynamics approach to biodiesel fund management in Indonesia

Indonesia, which is strongly supportive of renewable energy development, meets its obligation to extensively employ renewable energy through the mandatory mixing of palm oil-derived biodiesel with diesel, a fossil fuel. The success of the program is inseparable from the government's provision of incentives which involve paying the difference in cost between biodiesel and diesel through export taxes collected from producers of palm oil and its derivatives. However, the increase in the government's mandatory target is not in line with the higher revenue obtained with the result that under current policy conditions the government will not be able to provide incentives for the B30 plan during the next few years. This research presents a dynamic simulation modelling of Indonesia's biodiesel program and analyzes the policy applied in support of that program. A system dynamics model is proposed to enable decision-makers to understand the relationship between the different variables inherent in the system and the impact of different variables on the continuity of the biodiesel program in Indonesia. The system dynamics model consists of three sub-models: the palm oil production sub-model, the biodiesel demand and production sub-model and the biodiesel fund management sub-model. Fund management of biodiesel is specifically discussed, starting with the source of funds, through fund management, to the impact of fund availability on the biodiesel mandatory program's continuity. The simulation shows that increasing palm oil production capacity is insufficient in order to ensure the success of the government program. Simulations indicate that the biodiesel program will operate optimally if two prerequisites are satisfied; first, that the levy policy adopted is not dependent upon crude palm oil (CPO) prices and, second, that the provision of incentives is limited solely to those required by the prevailing Public Service Obligation (PSO).

A decision support model for individual tree stump harvesting options based on criteria for economic return and environmental protection

ABSTRACTBased on principles of multi-criteria analysis techniques, a model (MAPStump-E) for decision support on stump harvesting at stand level was developed. The model applies the concept that each stump can be attributed production values (economic) and environmental values (here soil protection and water quality). Individual tree stump information was incorporated directly from the production reports of harvesters and combined with high-resolution Geographical Information System data on topography and soil type to create a production submodel and a soil and water vulnerability submodel (SWM). To test the model, it was applied to a 45-ha study forest in south-central Sweden and the outcome of nine scenarios with varying bioenergy prices and environmental protection levels was examined. Combined analysis of the effects of production and environmental criteria on total dry mass of harvestable stumps at the study site showed that biomass prices had a stronger influence than environmental criteria. Conflict stumps were defined as stumps suitable for harvest based on production criteria, but unsuitable based on soil and water protection criteria. In a “medium” price scenario, the proportion of conflict stumps at the study site ranged from 6% to 18%, depending on protection level set.

Modelling cattle population as lifetime trajectories driven by management options: A way to better integrate beef and milk production in emissions assessment

Methods to assess the consequences of mitigation scenarios on greenhouse gas (GHG) emissions in the cattle sector have been widely developed. A crucial issue of these methods is handling the co-products in emissions allocation. In the cattle sector, milk and beef production systems are closely interconnected because of the meat obtained from dairy culled cows and fattened surplus calves. To fully evaluate a change in the dairy sector, the change in the meat production induced should also be taken into account. With this objective, a national cattle population model was developed, based on three sub-models. The herd functioning sub-model represents the bunch of cattle trajectories generated by reproductive and finishing processes depending on breeds, production levels and finishing types. Assuming a steady state herd, the demographics sub-model represents the number of animals in age groups depending on trajectories length. The production sub-model represents the number of animals slaughtered or exported, the quantity of carcasses, milk and direct GHG emissions. The model was calibrated on French data. The simulation of a reference scenario reflecting the French situation in 2010 confirmed that the model allowed linking cattle trajectories, demographics and production of milk, meat and direct GHG emissions. Scenarios for the dairy sector were simulated to illustrate the potential of such modelling approach to evaluate direct GHG emission at the national level. The results confirmed that the interconnection between dairy and beef sectors can modify the benefit of strategies such as dairy intensification. Further, the results also demonstrated that the compensation between beef and dairy cattle to achieve production objectives lead to a change in the type of carcass produced due to differences between breeds and finishing strategies. Accounting for such differences allows deepening the evaluation of the consequences of GHG mitigation options.

An Integrated Model for Production and Distribution Planning of Perishable Products with Inventory and Routing Considerations

In many conventional supply chains, production planning and distribution planning are treated separately. However, it is now demonstrated that they are mutually related problems that must be tackled in an integrated way. Hence, in this paper a new integrated production and distribution planning model for perishable products is formulated. The proposed model considers a supply chain network consisting of a production facility and multiple distribution centers. The facility produces a single perishable product that is storable only for predetermined periods. A homogenous fleet of vehicles is responsible for delivering the product from facility to distribution centers. The decisions to be made are the production quantities, the distribution centers that must be visited, and the quantities to be delivered to them. The objective is to minimize the total cost, where the trip minimization is considered simultaneously. As the proposed formulation is computationally complex, a heuristic method is developed to tackle the problem. In the developed method, the problem is divided into production submodel and distribution submodel. The production submodel is solved using LINGO, and a particle swarm heuristic is developed to tackle distribution submodel. Efficiency of the algorithm is proved through a number of randomly generated test problems.

Evaluation of predictions of volatile fatty acid production rates by the Molly cow model

Predicting ruminal volatile fatty acid (VFA) production is important, as VFA are an energy source to the animal, affect nutrient partitioning, and dictate methane production. The VFA production submodel in the Molly cow model was evaluated using data from 8 publications that reported VFA production rates for cattle. Evaluations were conducted with ruminal water balance predictions enabled and the ruminal VFA stoichiometry coefficients set to “mixed” for all diets, or “mixed” when forage represented between 20 and 80% of the diet, “concentrate” when <20% forage, or “forage” when >80% forage. Prediction errors were relatively insensitive to changes in VFA coefficients by diet type. Root mean square prediction errors (RMSPE) were 63, 63, and 49% for acetate, propionate, and butyrate production rates, respectively. A large proportion of the error was slope bias for acetate and butyrate, and a modest proportion for propionate. Because interconversions between acetate and propionate represent approximately 15% of the variation in net production rates, lack of such consideration in the model may contribute to the substantial model prediction errors. The potential of using thermodynamic equations to predict interconversions was assessed using observed ruminal pH and VFA concentrations from 2 studies and assuming constant hydrogen pressure and concentrations of CO2, H2O, adenosine diphosphate, ATP, and inorganic P. Rate constants for conversion of acetate to propionate and propionate to acetate were derived independently from the control treatments and used to predict the fluxes for the other treatment. The observed changes in VFA concentrations and pH explained the observed changes in conversion of acetate to propionate, but overpredicted the change in the propionate to acetate flux in one study. When applied to the other study, the equations predicted the increase in propionate to acetate flux, but failed to predict the observed reduction in acetate to propionate flux. The inability to predict responses accurately may be due to a lack of data for controlling factors other than pH and VFA concentrations.

A GLOBAL NOx SUBMODEL FOR PULVERIZED COAL FLAMES AT ELEVATED PRESSURES

ABSTRACT This study formulates a global NOX submodel for deployment in CFD simulations from a database on flames of three diverse coals at pressures to 3.0 MPa for broad ranges of stoichiometric ratio (S.R.). A new reaction scheme was formulated from a sensitivity analysis of simulations based on detailed reaction mechanisms for all tests. It shares many elements in common with commercial submodels, yet it correctly predicts that (1) less coal-N is converted into NO; and (2) HCN persists to higher S.R. for progressively higher pressures. Explicit dependences on O2 concentrations are responsible for the first feature, because the variations in O2 concentrations mimic the ways that the oxyhydroxyl radical pool shrinks at progressively higher pressures, which shifts HCN conversion toward N2 production. The second feature was depicted by resolving the intermediate products of HCN decomposition in the global scheme. Discrepancies surfaced when the new submodel was applied to different coals without re-adjusting rate parameters, which probably reflects a generic limitation of global NOX production submodels for coal combustion.

A process-based model of nitrogen cycling in forest plantations: Part I. Structure, calibration and analysis of the decomposition model

We present a new decomposition model of C and N cycling in forest ecosystems that simulates N mineralisation from decomposing tree litter. It incorporates a mechanistic representation of the role of soil organisms in the N mineralisation-immobilisation turnover process during decomposition. We first calibrate the model using data from decomposition of 14C-labelled cellulose and lignin and 14C-labelled legume material and then calibrate and test it using mass loss and N loss data from decomposing Eucalyptus globulus residues. The model has been linked to the plant production submodel of the G’DAY ecosystem model, which previously used the CENTURY decomposition submodel for simulating C and N cycling. The key differences between this new decomposition model and the previous one, based on the CENTURY model, are: (1) growth of microbial biomass is the process that drives N mineralisation-immobilisation, and microbial succession is simulated; (2) decomposition of litter can be N-limited, depending on soil inorganic N availability relative to N requirements for microbial growth; (3) ‘quality’ of leaf and fine root litter is expressed in terms of biochemically measurable fractions; (4) the N:C ratio of microbial biomass active in decomposing litter is a function of litter quality and N availability; and (5) the N:C ratios of soil organic matter (SOM) pools are not prescribed but are instead simulated output variables defined by litter characteristics and soil inorganic N availability. With these modifications the model is able to provide reasonable estimates of both mass loss and N loss by decomposing E. globulus leaf and branch harvest residues in litterbag experiments. A sensitivity analysis of the decomposition model to selected parameters indicates that parameters regulating the stabilisation of organic C and N, as well as those describing incorporation of soil inorganic N in Young-SOM (biochemical immobilisation of N) are particularly critical for long-term applications of the model. A parameter identifiability analysis demonstrates that simulated short-term C and N loss from decomposing litter is highly sensitive to three model parameters that are identifiable from the E. globulus litterbag data.

GRASSLAND TO WOODLAND TRANSITIONS: INTEGRATING CHANGES IN LANDSCAPE STRUCTURE AND BIOGEOCHEMISTRY

In many of the world's drylands, human-induced alteration of grazing and fire regimes over the past century has promoted the replacement of grasses by woody vegetation. Here, we evaluate the magnitude of changes in plant and soil carbon and nitrogen pools in a subtropical landscape undergoing succession from grassland to thorn woodland in southern Texas. Our approach involved linking a process-based ecosystem model to a transition matrix model. Grass and forest production submodels of CENTURY were parameterized with field data collected from herbaceous and wooded landscape elements broadly representative of habitats in global savanna systems. The Markov (transition matrix) model simulated the displacement of grassland communities under land use practices typical of many modern grasslands and savannas (heavy livestock grazing; no fire) and climate events. The modeled landscape was initialized for pre-Anglo-European settlement grassland conditions and then subjected to heavy, continuous livestock grazing and elimination of fire beginning in the mid-1800s. Rates of woody plant encroachment were directed by the Markov model, and the consequences for net primary production and plant and soil C and N pools were tracked by CENTURY. Modeled output of plant and soil organic C were in good agreement with those quantified for present-day patch types, suggesting our reconstructions were reasonable. Results indicated that, in the absence of woody plant encroachment, heavy grazing and fire suppression would have reduced soil organic carbon mass in southern Texas grasslands 17% (clay loam lowlands) to 18% (sandy loam uplands) by the 1990s. Soil and plant carbon stocks in current (mid-1900s) Prosopis woodlands are estimated to exceed those of the pristine grasslands they have replaced by 1.3× and 10×, respectively. Our reconstructions thus suggest that an initial degradation phase induced by heavy livestock grazing was followed by a woody-plant-induced aggradation phase that is still in progress. Under climatic/atmospheric conditions of the past 100 years, future landscapes would equilibrate at soil and plant C densities that would be 3× and 15–24× that of the pristine, presettlement grasslands they have replaced. Replacement of grasslands and savannas by woodlands in this bioclimatic region has thus resulted in significant and ongoing increases in landscape-scale ecosystem carbon stocks in a relatively short (∼100 years) period of time.

A simulation model of Mexican long-nosed bat ( Leptonycterisnivalis) migration

We (1) describe a model that simulates migration of the Mexican long-nosed bat ( Leptonycteris nivalis) based on the flowering phenology (nectar–pollen production) of agaves (Agavaceae) and hypothesized ‘rules’ governing bat movements; (2) evaluate the model by comparing simulated seasonal and spatial patterns of nectar production and consumption, and bat movements and densities, to patterns observed in the field; and (3) use the model to examine various hypotheses concerning factors that control migration. A nectar production sub-model represents the flowering phenology of agaves in terms of the daily availability of nectar within each of four latitudinal intervals between 18 and 29°N. A bat migration sub-model represents the number of bats present within each latitudinal interval each day, with bat movements from one site to another depending on availability of nectar and season. Simulated patterns of nectar consumption are similar to observed patterns of nectar production based on the number of flowering plant species present at different latitudes. Simulated patterns of bat movements are similar to general patterns observed in the field for Leptonycteris curasoae and L. nivalis. Simulated fluctuations of bat density at the southernmost latitude exhibit the same general annual cycle observed at a southern roost of L. curasoae. The seasonal representation of nectar production in the model corresponds well with patterns of nectar production observed over a 2-year period at a site in northern Mexico, although nectar production in the model begins somewhat earlier. Simulations examining factors hypothesized to control migration indicate that predicted migration patterns correspond well with field observations only when model rules assume that both food availability and season limit migration, and that all bats with access to sufficient energy during the correct season migrate.

Application of a dynamic simulation model on the effects of calving season and weaning age on bioeconomic efficiency

A dynamic simulation model, Alberta Beef Production Simulation System (ABPSS), which includes herd inventory, nutrient requirements, forage production, and economic submodels, was used to compare bioeconomic efficiency in spring and fall calvings and different weaning ages (220, 200, 180, 160, and 140 d). Comparisons were made assuming a mature cow weight of 550 kg and a peak milk yield of 8.2 kg d−1. The first day of calving was assumed to be 28 March and 8 September for spring- and fall-calving cows, respectively. Bioeconomic efficiency was measured as the net return per cow (total return minus total cost). Fall calving in Alberta generally results in longer exposure of young calves to extreme cold weather after calving, and therefore total annual DMI and feed cost were higher in the fall-calving season group than in the spring-calving season group. Bioeconomic efficiency improved as weaning age increased from 140 to 220 d in both calving seasons. For weaning age of 200 d or less, spring calving was more efficient than fall calving. However, at a weaning age of 220 d, fall calving had higher bioeconomic efficiency than spring calving, primarily due to higher market prices for fall-born calves. This indicated that interactions of calving season by weaning age was an important factor affecting bioeconomic efficiency. It must be noted that the model was developed based on experimental results and data from the liteature, and due to the unavailability of suitable data the model could not be validated. We suggest that the ABPSS model has the potential for providing a useful decision-making tool for simultaneous consideration of many factors in an integrated system and for evaluating the effects of alternative management strategies on profitability of beef production systems. Key words: Beef cattle, simulation and modelling, production system, calving season, weaning age, bioeconomic efficiency

Structure of a dynamic simulation model for beef cattle production systems

A dynamic deterministic model for simulating beef cattle production systems is developed to evaluate the effects of production traits and management strategies on the bioeconomic efficiency of beef production systems. The model, named Alberta Beef Production Simulation System (ABPSS), is composed of four major submodels: herd inventory, nutrient requirement, forage production, and economic submodels. The herd inventory submodel is used to simulate population dynamics and feed requirements in the herd. The nutrient requirements submodel is mainly based on the 1996 version of the National Research Council (NRC). It is used to evaluate nutrients and feed requirements for calves and cows depending on their physiological status (maintenance, growth, lactation and gestation) and the climatic condition. The forage production sub-model is used to predict forage growth rate, cattle grazing rate, available forage biomass and total hectares required for grazing. The economic submodel measures bioeconomic efficiency, as net return per cow, by subtracting total cost from total return. The nutrient requirements predicted by ABPSS were compared with those recommended by the NRC for testing. The results that were predicted by the NRC model and ABPSS model were similar, as expected. Sensitivity analyses showed that cow mature weight, milk production, calf weaning weight and feed prices were the most critical input parameters in the model. It must be noted that the model was developed based on available experimental results and data from the literature and, due to the unavailability of a suitable data set, the model could not be validated. We suggest that the ABPSS has the potential for providing a useful method for simultaneous consideration of many factors in an integrated system, which could be helpful to beef cattle extension specialists and cow-calf production managers for assessing the potential effects of different management and selection strategies on bioeconomic efficiency. Key words: Beef cattle, simulation and modelling, production system, optimization

A Unified Approach to the Modelling of the Venice Lagoon – Adriatic Sea Ecosystem

In this paper, a first attempt at analysing the macronutrients' mass balance between the north Adriatic coastal sea and the lagoon of Venice by means of a nested and coupled model is presented. The hydrodynamic part of the model simulates the evolution of the sea-surface elevation and of the three-dimensional field of velocity, temperature, salinity and density. Tides, winds, river discharges, thermal and evaporative fluxes are included as forcings. Two models are nested in order to handle the correct spatial scales. The first one, with a resolution of about 10km, is able to describe the basin and sub-basin scale hydrodynamical features; the second one, with a resolution of 1·2km, describes the interactions between the open sea and the lagoon. This last circulation model has been coupled with a simple primary production submodel, in order to investigate the short-term dynamic of the ecosystem during spring time. Results show that, in some instances, the primary production can be sustained by macronutrients' fluxes coming from the coastal area.

Modelling nitrogen turnover and production in an irrigated short-rotation forest

Biomass production and the amount of nitrogen in an established short-rotation forest were simulated on a daily basis, using a model based on leaf photosynthesis, allocation of assimilates within the plant, plant uptake of nitrogen and supply of nitrogen through decomposition of organic matter. The model was developed for willow stands with 1-year-old shoots and has, except for the areal leaf weight (the ratio between leaf biomass and leaf area), only climatic factors and fertilization rate as input variables. The model was constructed by combining two submodels, a biomass production submodel and a nitrogen turnover submodel. The model is valid for horizontally uniform stands in which growth is not limited by shortage of water or nutrients other than nitrogen. The simulated stem, leaf and leaf fall dry matter, as well as the stem nitrogen content of 1-year-old shoots of a Salix viminalis stand in southern Sweden, agreed well with measured data. The canopy nitrogen content was overestimated. This was probably caused by inaccurate estimates of the mineralization rate of the litter where the role of large soil animals, such as earthworms, was difficult to evaluate. A sensitivity test showed that stem production is most affected by modifications of the “growth parameters”. Only the sensitivities to the parameters of the leaf area development are of equal significance. Stem production is most sensitive to changes in the light extinction coefficient, while the plant nitrogen content is most influenced by the initial amount of litter. If the nitrogen supply to the plant is decreased, production becomes more sensitive to variations in the “decomposition parameters”. Different approaches to nitrogen partitioning within the plant showed that leaves have a lower priority for nitrogen than the roots. Nitrogen uptake by the canopy after fertilization can be 30% higher than that needed to supply the newly formed leaves with “maximum” nitrogen content.

Computer simulation of changes in soil mineral nitrogen and crop nitrogen during autumn, winter and spring

summaryThe computer model described simulates changes in soil mineral nitrogen and crop uptake of nitrogen by computing on a daily basis the amounts of N leached, mineralized, nitrified and taken up by the crop. Denitrification is not included at present. The leaching submodel divides the soil into layers, each of which contains mobile and immobile water. It needs points from the soil moisture characteristic, measured directly or derived from soil survey data; it also needs daily rainfall and evaporation. The mineralization and nitrification submodel assumes pseudo-zero order kinetics and depends on the net mineralization rate in the topsoil and the daily soil temperature and moisture content, the latter being computed in the leaching submodel. The crop N uptake and dry-matter production submodel is a simple function driven by degree days of soil temperature and needs in addition only the sowing date and the date the soil returns to field capacity, the latter again being computed in the leaching submodel. A sensitivity analysis was made, showing the effects of 30% changes in the input variables on the simulated amounts of soil mineral N and crop N present in spring when decisions on N fertilizer rates have to be made. Soil mineral N was influenced most by changes in rainfall, soil water content, mineralization rate and soil temperature, whilst crop N was affected most by changes in soil temperature, rainfall and sowing date. The model has so far been applied only to winter wheat growing through autumn, winter and spring but it should be adaptable to other crops and to a full season.The model was validated by comparing its simulations with measurements of soil mineral N, dry matter and the amounts of N taken up by winter wheat in experiments made at seven sites during 5 years. The simulations were assessed graphically and with the aid of several statistical summaries of the goodness of fit. The agreement was generally very good; over all years 72% of all simulations of soil mineral N to 90 cm depth were within 20 kg N/ha of the soil measurements; also 78% of the simulations of crop nitrogen uptake were within 15 kg N/ha and 63% of the simulated yields of dry matter were within 25 g/m2 of the amounts measured. All correlation coefficients were large, positive, and highly significant, and on average no statistically significant differences were found between simulation and measurement either for soil mineral N or for crop N uptake.

The population ecology of Datura ferox in soybean crops. A simulation approach incorporating seed dispersal

A numerical model for simulating the population dynamics of Datura ferox L. (chamico, chinese thronapple) has been built based on previously reported data. In the model, a soybean field is divided into 0.7×0.7-m conceptual modules. A seed production sub-model simulates the annual seed output of each module, and a seed dispersal sub-model simulates the distribution of these propagules within the field in accordance to a specified dispersal pattern. Different model scenarios were generated by varying the proportion of seeds lost from the soil bank, the annual recruitment, the seedling mortality and the proportion of seeds exported from the field during crop harvest. The results obtained by simulation suggest that (1) seed dispersal due to crop harvesting tends to produce an exponential growth of weed seed production, (2) limited or no success could be attained in the control of D. ferox using procedures that kill the seedlings efficiently if combine harvesters are not adjusted so as to maximize the proportion of weed seeds that are exported from the field and (3) if cleaning debris continues to be returned to the ground during crop harvesting, the improvement in the efficiency of the grain/weed separating mechanisms does not provide an effective long-term strategy to avoid grain contamination problems.