An evaluation of substrate degradation patterns in the composting process. Part 2: temperature-corrected profiles

In this paper, the patterns of 44 substrate degradation profiles obtained from the composting literature are examined following their correction to a constant temperature of 40 degrees C, using a new procedure presented in this work. The applicability of a single exponential model, a double exponential model and a non-logarithmic Gompertz model in describing their behaviour is then evaluated. Multi-phase profiles were most commonly seen, with convex shapes observed in only a relatively small proportion of the profiles. Convex shapes were also embedded within other profiles, either preceeded by a lag phase, or followed by non-convex behaviour. Sigmoidal patterns were relatively rare. Of the temperature-corrected data sets examined, 33 were found to be either not well modelled by, or inappropriate for, any of the above models. Two fits rated as good were obtained when using the single exponential model, and one fit rated as excellent, plus one fit rated as good, were obtained when using the double exponential model. A single fit rated as excellent was found when using the non-logarithmic Gompertz model. The lag phase, which was observed in many data sets, was successfully modelled using the non-logarithmic Gompertz function where excellent and good fits were obtained, but as expected this phase of the profile could not be modelled by either the single or double exponential functions. When the lag phase or post-convex curve data was removed from 20 data sets, use of the single exponential function resulted in three fits rated as excellent and two rated as good. When a double exponential model was applied to these data sets, three fits rated as good were obtained, whilst application of the modified Gompertz model gave one fit rated as good. The remainder of the fits were rated as moderate to fair. It is concluded that the evidence supporting the use of the single exponential model, the double exponential model or the non-logarithmic Gompertz model to describe full substrate degradation profiles in composting following their adjustment for temperature effects is limited. Further work is suggested in order to investigate the nature of those profiles which were not well modelled, to more precisely ascertain the cardinal temperatures for composting used in the function of Rosso et al. (1993) [Rosso, L., Lobry, J.R., Flandrois, J.P., 1993. An unexpected correlation between cardinal temperatures of microbial growth highlighted by a new model. J. Theor. Biol 162, 447-463.], which was employed in the present temperature correction procedure, and to incorporate correction for varying moisture and oxygen concentrations.     

Predicting biodegradable volatile solids degradation profiles in the composting process

This paper presents a new method for the prediction of the pattern of biodegradable volatile solids (BVS) degradation in the composting process. The procedure is based on a re-arrangement of the heat balance around a composting system to numerically solve for the rate of BVS carbon (BVS-C) disappearance. Input data for the model was obtained from composting experiments conducted in a laboratory-scale, constant temperature difference (CTD) reactor simulating a section of an aerated static pile, and using a simulated feedstock comprising ostrich feed, shredded paper, finished compost and woodchips. These experiments also provided validation data in the form of exit gas CO(2) carbon (CO(2)-C) profiles. The model successfully predicted the generic shape of experimental substrate degradation profiles obtained from CO(2) measurements, but under the conditions and assumptions of the experiment, the profiles were quantitatively different, giving an over-estimate of BVS-C. Both measured CO(2)-C and predicted BVS-C profiles were moderately to well fitted by a single exponential function, with replicated rate coefficient values of 0.08 and 0.09 d(-1), and 0.06 and 0.07 d(-1), respectively. In order to explore the underlying shape of the profiles, measured and predicted data at varying temperature were corrected to a constant temperature of 40 degrees C, using the temperature correction function of Rosso et al. [Rosso, L., Lobry, J.R., and Flandrois, J.P., 1993. An unexpected correlation between cardinal temperatures of microbial growth highlighted by a new model. Journal of Theoretical Biology, 162, 447-463], with cardinal temperatures of 5, 59 and 85 degrees C. Multi-phase profiles were generated for both the measured CO(2)-C and the predicted BVS-C data in this case. However, when alternative cardinal temperatures of 5, 55 and 80 degrees C, or 5, 50 and 80 degrees C, were used, the predicted profiles assumed an exponential shape, and excellent fits were obtained using a double exponential function. These findings support the argument that a substrate degradation curve generated under laboratory conditions at 40 degrees C, would, given correct cardinal temperatures, generate a correct substrate degradation profile under varying temperature conditions and that this in turn would enable an accurate and precise prediction of the temperature profile, using a heat and mass balance approach. In order to realise this prospect, it is proposed that further work to obtain experimental data under completely mixed conditions, more accurately estimate the overall heat transfer coefficient and obtain correct values for the cardinal temperatures used in the temperature correction function, is required.     

Mathematical modelling of the composting process: a review

In this paper mathematical models of the composting process are examined and their performance evaluated. Mathematical models of the composting process have been derived from both energy and mass balance considerations, with solutions typically derived in time, and in some cases, spatially. Both lumped and distributed parameter models have been reported, with lumped parameter models presently predominating in the literature. Biological energy production functions within the models included first-order, Monod-type or empirical expressions, and these have predicted volatile solids degradation, oxygen consumption or carbon dioxide production, with heat generation derived using heat quotient factors. Rate coefficient correction functions for temperature, moisture, oxygen and/or free air space have been incorporated in a number of the first-order and Monod-type expressions. The most successful models in predicting temperature profiles were those which incorporated either empirical kinetic expressions for volatile solids degradation or CO2 production, or which utilised a first-order model for volatile solids degradation, with empirical corrections for temperature and moisture variations. Models incorporating Monod-type kinetic expressions were less successful. No models were able to predict maximum, average and peak temperatures to within criteria of 5, 2 and 2 degrees C, respectively, or to predict the times to reach peak temperatures to within 8 h. Limitations included the modelling of forced aeration systems only and the generation of temperature validation data for relatively short time periods in relation to those used in full-scale composting practice. Moisture and solids profiles were well predicted by two models, but oxygen and carbon dioxide profiles were generally poorly modelled. Further research to obtain more extensive substrate degradation data, develop improved first-order biological heat production models, investigate mechanistically-based moisture correction factors, explore the role of moisture tension, investigate model performance over thermophilic composting time periods, provide more information on model sensitivity and incorporate natural ventilation aeration expressions into composting process models, is suggested.     

Modeling of substrate degradation and oxygen consumption in waste composting processes

A multi-component modeling system was developed to simulate substrate degradation and oxygen consumption in waste composting processes. Levels of soluble substrate (Ss), insoluble substrate (Si), active biomass (X), inert material, moisture, temperature, and oxygen concentration were considered as state variables. The relationships among these variables were also incorporated within the modeling framework. Three conversion reactions, including growth of aerobic biomass, decay of aerobic biomass, and solubilisation of insoluble substrate, were considered in the simulation system. The modeling inputs included temperature, moisture, oxygen concentration, and initial conditions of the state variables, while the outputs included oxygen uptake accumulation (OUA), oxygen uptake rate (OUR), Ss, Si, and X for representing the substrate degradation and oxygen consumption status. The effectiveness of the developed model was demonstrated through its application to a case study in a 30L vessel over 200h. Through verification-based composting experiments, it was shown that the modeling solutions were consistent with the experimental results with an acceptable accuracy level. Sensitivity analyses of the model showed that an increased maximum microbial growth rate would result in raised OUA, OUR, Ss, and X levels; a decreased biomass decay rate constant would help enhance the composting process. Moreover, variations in the maximum growth rate would affect the composting process more significantly than those of the biomass decay rate constant.     

Biodegradation of Whey Protein-Based Edible Films

The biodegradability of the edible films made of whey proteins by disulfide cross-linking was investigated. Whey protein concentrate (WPC) and whey protein isolate (WPI) films were subjected to microbial degradation using Pseudomonas aeruginosa and composting burial degradation. Results from the microbial degradation showed that whey protein films could support the growth of P. aeruginosa. The bacterial growth characteristics were well described using the Gompertz model. WPC films degraded faster than WPI films, suggesting that the biodegradability of protein films is associated with the film composition and the extent of covalent cross-linking. WPI films buried in a compost pile began to degrade in two days and became darker over time. More than 80% of total solids were lost in 7 days.     

Application of a simplified mathematical model to estimate the effect of forced aeration on composting in a closed system

The aeration rate is a key process control parameter in the forced aeration composting process because it greatly affects different physico-chemical parameters such as temperature and moisture content, and indirectly influences the biological degradation rate. In this study, the effect of a constant airflow rate on vertical temperature distribution and organic waste degradation in the composting mass is analyzed using a previously developed mathematical model of the composting process. The model was applied to analyze the effect of two different ambient conditions, namely, hot and cold ambient condition, and four different airflow rates such as 1.5, 3.0, 4.5, and 6.0m(3)m(-2)h(-1), respectively, on the temperature distribution and organic waste degradation in a given waste mixture. The typical waste mixture had 59% moisture content and 96% volatile solids, however, the proportion could be varied as required. The results suggested that the model could be efficiently used to analyze composting under variable ambient and operating conditions. A lower airflow rate around 1.5-3.0m(3)m(-2)h(-1) was found to be suitable for cold ambient condition while a higher airflow rate around 4.5-6.0m(3)m(-2)h(-1) was preferable for hot ambient condition. The engineered way of application of this model is flexible which allows the changes in any input parameters within the realistic range. It can be widely used for conceptual process design, studies on the effect of ambient conditions, optimization studies in existing composting plants, and process control.     

Effect of the composting substrate on biodegradation of solid materials under controlled composting conditions

Biodegradability under composting conditions is assessed by test methods, such as ASTM D 5338-92, based on the measurement of CO₂ released by test materials when mixed with mature compost and maintained in a controlled composting environment. However, in real composting, biodegradation occurs in fresh waste. To clarify this point, the biodegradation of paper and of a starch-based biodegradable thermoplastic material, Mater-Bi ZI01U, was followed by measuring the weight loss of samples introduced either into a mature compost or into a synthetic waste. The weight loss in mature compost was higher at the beginning but tended to decrease; in synthetic waste a first lag phase was followed by an exponential phase. Complete degradation of paper was noticed simultaneously in the two substrates (after 25 days). The bulkier Mater-Bi samples were fully degraded after 20 days in fresh waste, but after 45 days in mature compost. Therefore, the test methods using mature compost as a substrate can possibly underestimate the biodegradation rate occurring in fresh waste, i.e., in real composting plants, and have to be considered as conservative test methods. The test procedure described in this paper seems very suitable as a screening method to verify the compostability of plastic materials in a composting environment.     

Open windrow composting of polymers: an investigation into the operational issues of composting polyethylene (PE)

This paper investigates the operational issues surrounding the open windrow composting of degradable polyethylene sacks. Areas for consideration were the impact of degradable polyethylene sacks on the composting process, the quality of the finished compost product, and how the use of sacks influenced the on-site processing. These factors were investigated through determining the amount of polymer residue and chemical contaminants in the finished compost product and the daily monitoring of windrow temperature profiles. Site and practical handling considerations of accepting an organic waste contained within PE sacks are also discussed. Statistical analysis of the windrow temperature profiles has led to the development of a model that can help to predict the expected trends in the temperature profiles of open compost windrows where the organic waste is kerbside collected using a degradable PE sack.     

Physical modelling of the composting environment: a review. Part 2: Simulation performance

This paper reviews previously published heat balance data for experimental and full-scale composting reactors, and then presents an evaluation of the simulation performance of laboratory and pilot-scale reactors, using both quantitative and qualitative temperature profile characteristics. The review indicates that laboratory-scale reactors have typically demonstrated markedly different heat balance behaviour in comparison to full-scale systems, with ventilative heat losses of 36-67%, and 70-95% of the total flux, respectively. Similarly, conductive/convective/radiative (CCR) heat losses from laboratory reactors have been reported at 33-62% of the total flux, whereas CCR losses from full-scale composting systems have ranged from 3% to 15% of the total. Full-scale windrow temperature-time profiles from the literature were characterised by the present authors. Areas bounded by the curve and a 40 degrees C baseline (A(40)) exceeded 624 degrees C. days, areas bounded by the curve and a 55 degrees C baseline (A(55)) exceeded 60 degrees C days, and times at 40 and 55 degrees C were >46 days and >24 days, respectively, over periods of 50-74 days. For forced aeration systems at full scale, values of A(40) exceeded 224 degrees C days, values of A(55) exceeded 26 degrees C days, and times at 40 and 55 degrees C were >14 days and >10 days, respectively, over periods of 15-35 days. Values of these four parameters for laboratory-scale reactors were typically considerably lower than for the full-scale systems, although temperature shape characteristics were often similar to those in full-scale profiles. Evaluation of laboratory-, pilot- and full-scale profiles from systems treating the same substrate showed that a laboratory-scale reactor and two pilot-scale reactors operated at comparatively high aeration rates poorly simulated full-scale temperature profiles. However, the curves from two moderately insulated, self-heating, pilot-scale reactors operated at relatively low aeration rates appeared to closely replicate full-scale temperature profiles. The importance of controlling aeration rates and CCR losses is discussed and further work suggested in order to investigate the links between simulation of the composting environment and process performance.     

Evaluation of laboratory-scale in-vessel co-composting of tobacco and apple waste

Efficient composting process requires set of adequate parameters among which physical–chemical properties of the composting substrate play the key-role. Combining different types of biodegradable solid waste it is possible to obtain a substrate eligible to microorganisms in the composting process. In this work the composting of apple and tobacco solid waste mixture (1:7, dry weight) was explored. The aim of the work was to investigate an efficiency of biodegradation of the given mixture and to characterize incurred raw compost. Composting was conducted in 24 L thermally insulated column reactor at airflow rate of 1.1 L min^?1. During 22 days several parameters were closely monitored: temperature and mass of the substrate, volatile solids content, C/N ratio and pH-value of the mixture and oxygen consumption. The composting of the apple and tobacco waste resulted with high degradation of the volatile solids (53.1%). During the experiment 1.76 kg of oxygen was consumed and the C/N ratio of the product was 11.6. The obtained temperature curve was almost a “mirror image” of the oxygen concentration curve while the peak values of the temperature were occurred 9.5 h after the peak oxygen consumption.     

Composting kinetics in full-scale mechanical–biological treatment plants

This study focuses on the investigation of the kinetics of municipal solid waste composting in three full-scale mechanical–biological treatment (MBT) plants. The aims were to test a kinetic model based on volatile solids (VS) content change for describing the composting process in MBT plants, and to identify the model parameters that affected the estimation of the reaction rate constant most. To achieve this, VS content and several environmental conditions, namely temperature, moisture content, oxygen concentration and total bulk density were monitored throughout the composting process. Experimental data was fitted with a first-order kinetic model, and a rate constant (k) characteristic of composting under optimum environmental conditions was obtained. The kinetic model satisfactorily described the experimental data for the three MBT plants. k values ranged from 0.043 ± 0.002 d^?1 to 0.082 ± 0.011 d^?1. Sensitivity analysis showed that the model parameters that most affected the estimation of k were the initial biodegradable volatile solids content, the maximum temperature for biodegradation and the optimum moisture content. In conclusion, we show for the first time that full-scale MBT plants can be successfully modelled with a composting kinetic model.     

Contribution of the lignocellulosic fraction of two-phase olive-mill wastes to the degradation and humification of the organic matter during composting

One of the main disadvantages in the composting of two-phase olive mill wastes (TPOMW) is the long time required for its transformation (up to 40 weeks). The aim of this work was to evaluate the relationship between the degradation of the lignocellulosic fraction of TPOMW and the organic matter (OM) mineralisation rate in four composting piles prepared with different bulking agents and N-sources used to enhance OM degradation. The kinetics of degradation of the lignocellulosic fraction was compared to conventional maturation and stability indices to evaluate its impact on the duration of the composting process. The composition of bulking agents mainly affected the water-soluble fraction which influenced the OM degradation rate (linear or exponential OM degradation pattern) at early stages of the composting process but it neither modified the duration of the process (between 34 and 36 weeks) nor the total OM degradation underwent by the piles. The high initial mineral N availability was a key factor to significantly enhanced microbial activity. The mixture with urea as N-source registered the most efficient degradation of hemicellulose, cellulose and lignin, reducing the thermophilic phase and the total duration of TPOMW composting.     

Evolution of organic matter during composting of different organic wastes assessed by CPMAS 13C NMR spectroscopy

In this paper, the evolution of organic matter (OM) during composting of different mixtures of various organic wastes was assessed by means of chemical analyses and CPMAS ¹³C NMR spectroscopy measured during composting. The trends of temperatures and C/N ratios supported the correct evolution of the processes. The CPMAS ¹³C NMR spectra of all composting substrates indicated a reduction in carbohydrates and an increase in aromatic, phenolic, carboxylic and carbonylic C which suggested a preference by microorganisms for easily degradable C molecules. The presence of hardly degradable pine needles in one of the substrates accounted for the lowest increase in alkyl C and the lowest reduction in carbohydrates and carboxyl C as opposite to another substrate characterized by the presence of a highly degradable material such as spent yeast from beer production, which showed the highest increase of the alkyl C/O-alkyl C ratio. The highest increase of COOH deriving by the oxidative degradation of cellulose was shown by a substrate composed by about 50% of plant residues. The smallest increases in alkyl C/O-alkyl C ratio and in polysaccharides were associated to the degradation of proteins and lipids which are major components of sewage sludge. Results obtained were related to the different composition of fresh organic substrates and provided evidence of different OM evolution patterns as a function of the initial substrate composition.     

Composting of vegetable waste.

The effects of the aeration, seeding, and agitation on the composting of vegetable waste were studied in a laboratory-scale reactor. Experimental results showed that the final product at the end of a 4-day composting period met multiple maturity indices suggested by many researchers. The evolution of carbon dioxide during the composting process could be modelled with a modified Gompertz equation that described the bacteria growth successfully. Multivariate regression analysis was used to study the effects of operating parameters on the carbon conversion. The response surface contour plots were constructed using the regression equation for the examination of the dependence of carbon conversion on operating parameters. The maximum carbon conversion of 14.54% was obtained when the percentage of seeding was set at 14.5%, the air suction rate was set at 2.6 L kg(-1) dry-solid min(-1), and the agitator operated half of the time, alternating on and off for every 5 min. Future work will focus on the application of the data and the experience gained in this work to composters of pilot and semi-commercial scales.     

Influence of aeration rate and biodegradability fractionation on composting kinetics

The influences of aeration rate and biodegradability fractionation on biodegradation kinetics during composting were studied. The first step was the design of a suitable lab-reactor that enabled the simulation of composting. The second step comprised of composting trials of six blends of sludge (originating from a food processing effluent) with wood chips using aeration rates of 1.69, 3.62, 3.25, 8.48, 11.98 and 16.63 L/h/kg DM of mixture. Biodegradation was evaluated by respiration measurements and from the analysis of the substrate (dry matter, organic matter, total carbon and chemical oxygen demand removal). Continuous measurement of oxygen consumption was coupled with the analysis of initial substrate and composted product for chemical oxygen demand (in the soluble and non-soluble fractions), which enabled an evaluation of the organic matter biodegradability. Oxygen requirements to remove both the easily and slowly biodegradable fractions were determined. Dividing the substrate into different parts according to biodegradability allowed explanation of the influence of aeration rate on stabilization kinetics. Considering that the biodegradation kinetics were of the first-order, the kinetic constants of the easily and slowly biodegradable fractions were calculated as a function of temperature. The methodology presented here allows the comparison of organic wastes in terms of their content of easily and slowly biodegradable fractions and the respective biodegradation kinetics.     

Simulation of substrate degradation in composting of sewage sludge

To simulate the substrate degradation kinetics of the composting process, this paper develops a mathematical model with a first-order reaction assumption and heat/mass balance equations. A pilot-scale composting test with a mixture of sewage sludge and wheat straw was conducted in an insulated reactor. The BVS (biodegradable volatile solids) degradation process, matrix mass, MC (moisture content), DM (dry matter) and VS (volatile solid) were simulated numerically by the model and experimental data. The numerical simulation offered a method for simulating k (the first-order rate constant) and estimating k₂₀ (the first-order rate constant at 20 °C). After comparison with experimental values, the relative error of the simulation value of the mass of the compost at maturity was 0.22%, MC 2.9%, DM 4.9% and VS 5.2%, which mean that the simulation is a good fit. The k of sewage sludge was simulated, and k₂₀, k_20s (first-order rate coefficient of slow fraction of BVS at 20 °C) of the sewage sludge were estimated as 0.082 and 0.015 d^?1, respectively.     

Prediction of temperature and thermal inertia effect in the maturation stage and stockpiling of a large composting mass

A macroscopic non-steady state energy balance was developed and solved for a composting pile of source-selected organic fraction of municipal solid waste during the maturation stage (13,500 kg of compost). Simulated temperature profiles correlated well with temperature experimental data (ranging from 50 to 70 degrees C) obtained during the maturation process for more than 50 days at full scale. Thermal inertia effect usually found in composting plants and associated to the stockpiling of large composting masses could be predicted by means of this simplified energy balance, which takes into account terms of convective, conductive and radiation heat dissipation. Heat losses in a large composting mass are not significant due to the similar temperatures found at the surroundings and at the surface of the pile (ranging from 15 to 40 degrees C). In contrast, thermophilic temperature in the core of the pile was maintained during the whole maturation process. Heat generation was estimated with the static respiration index, a parameter that is typically used to monitor the biological activity and stability of composting processes. In this study, the static respiration index is presented as a parameter to estimate the metabolic heat that can be generated according to the biodegradable organic matter content of a compost sample, which can be useful in predicting the temperature of the composting process.     

Optimal bulking agent particle size and usage for heat retention and disinfection in domestic wastewater sludge composting

Composting of two types of sludge produced in wastewater treatment plants, raw sludge (RS) and anaerobically digested sludge (ADS), has been systematically studied by means of the experimental design technique. The results have been analyzed using a full factorial experimental design in order to determine the optimal conditions for composting such sludges in terms of bulking agent particle size and bulking agent:sludge volumetric ratio, two of the key parameters to ensure an optimal performance of the composting process. The objective function selected was a simulated death kinetics of Salmonella, which was chosen as a model pathogen microorganism to represent the disinfection of the material. For both types of sludge, optimal values were found at 5mm bulking agent particle size and 1:1 bulking agent:sludge volumetric ratio when a Gaussian function was fitted to the experimental data. Pilot scale experiments using optimal values obtained were successfully undertaken and confirmed a full disinfection of the sludge by means of the composting process. A mathematical model to simulate the disinfection performance of a composting material is presented. The model can be applied to simulate the disinfection performance of a given pathogen.     

Characterizing effects of uncertainties in MSW composting process through a coupled fuzzy vertex and factorial-analysis approach

A coupled fuzzy vertex and factorial-analysis approach was developed in this study for systematically characterizing effects of uncertainties in a municipal solid waste composting process. A comprehensive composting process model was also embedded into the system framework and used to address substrate decomposition and biomass growth, as well as the interactions between moisture contents, temperatures, and oxygen concentrations. The applicability of the proposed method was verified through a custom-made pilot-scale composting system. Results from fuzzy simulation indicated that the fuzzy vertex method could effectively communicate implicit knowledge into dynamic simulations and thus provide valuable information for enhancing composting process control under uncertainty. The factorial analysis was effective in quantifying the proportion to which the uncertainty of each single or interactive effect of model parameters contributes to the overall uncertainty of the system outcomes. Thus, sensitive parameters that may lead to errors or unreasonable predictions can be determined. The proposed study system could not only be used in characterizing combined effects of uncertainties for composting processes, but was also applicable to many other environmental modelling systems.