In this section we provide an overview of experimental research reported in the literature which have employed the Contois growth model. We provide specific (and
many) instances where the Contois model has proved to be the best choice for fitting experimental data when compared with alternate models like Monod. Specifically, in section3.5.1, applications of this model in the processing of industrial wastewater are discussed. Other types of applications are discussed in section 3.5.2.
3.5.1 Industrial wastewater
The Contois growth model has been applied to both aerobic and anaerobic indus-trial wastewater treatment processes in recent years. It has been shown that the Contois model is suitable for fitting various experimental data from a broad range of organic materials [133–135].
Beltran et al. [136] presented a study of the oxidation treatment of black olive wastewater which was performed by an aerobic biological degradation process and biological ozonation. In the aerobic biological treatment, the Contois kinetic was used to describe the specific decomposition rate. It was found that the Contois model fits the experimental data very well which confirmed that the Contois model is suitable for the present experimental system.
Bhattacharya and Pham [137] studied how Monod and Contois kinetics fit experi-mental data from the anaerobic treatment of cow dung digesters. The result of the comparison indicated that the Contois kinetic model is more suitable for fitting the experimental data of cow dung digesters than the Monod kinetic model.
Hu et al. [39] investigated the process kinetics of the anaerobic digestion of ice-cream wastewater using two kinetic models, Monod and Contois. It was found that the root mean square for the Contois kinetic is much greater than that of the Monod kinetic.
Thus, the comparisons of experimental data and predicted values obtained from Monod and Contois models suggested that the Contois model is more appropriate than the Monod model for modeling the process kinetics of the anaerobic digestion of ice-cream wastewater, i.e., predicting the performance of the anaerobic digester reactor, with the highest correlation coefficient of 0.918. Both the performance of the anaerobic digester reactor and microbial growth were predicted better by the Contois model than by the Monod model. This was because the Contois kinetic model considered the effect of influent substrate in making its prediction [39].
Hu et al. [42] also studied the process kinetics for the anaerobic digestion of sulphate-rich wastewater using models based on Monod and Contois kinetics at continuously stirred tank reactor. The results of the kinetic studies indicated that the Contois model predicted the kinetic reactions of the process very well, showing good agree-ment with the experiagree-mental data and having a greater correlation coefficient of 0.989 than Monod model. Both the performance of the anaerobic digester reactor and the microbial growth were predicted better (very well) by the Contois kinetic model than by the Monod kinetic model. Hu et al. [42] suggested that the reason for the good prediction of Contois kinetic model was due to the attachment of biomass at the walls of the reactor which can provide a source of inoculum.
Isik and and Sponza [41] used several models to study the process kinetics of the anaerobic treatment of textile wastewater in a lab-scale upflow anaerobic sludge blanket reactor, including Monod, Contois, Grau second order, modified Stover-Kincannon, and first order kinetic. The results of their kinetic studies showed that the Contois kinetic model, with a correlation coefficient of 0.967, is more
appropri-ate than the Monod and first order kinetic models for describing microbial kinetics.
The results further emphasized that Contois model is the best model and is more suitable for predicting the performance of anaerobic digester reactors compared to the other applied models with a correlation coefficient of 0.97. Krzystek et al. [138]
showed that the Contois kinetic model had good agreement with experimental data from the aerobic biodegradation of solid municipal organic waste with a correlation coefficient of 0.985.
Moosa et al. [139] investigated the kinetics of the anaerobic reduction of sulphate using several kinetic expressions including the Monod, Chen & Hashimoto and Con-tois models at continuous bioreactors. Among the models that were tested, the Contois kinetic model provided the best agreement with the experimental data of the anaerobic reduction of sulphate for describing the bacterial specific growth rate on sulphate concentration with the highest accuracy. This process has several ap-plications such as the cleaning of sulphate containing industrial effluents and in the cleaning of acid mine drainage. In their study, the kinetic coefficients of the Contois model were determined for different sulphate concentrations in the influent. It was found that the death rate coefficient is not affected by variations in initial concen-tration of sulphate, and it remained constant.However, as the initial concenconcen-tration of sulphate increased, the saturation constant (Ks) increased significantly while the change in the value of the maximum specific growth rate (µmax) was insignificant.
These values are stated in table (3.1).
Sun et al. [140] studied the biological treatment of pharmaceutical wastewater by a functional strain Xhhh employing three ions, Mn+2, Cu+2 and Zn+2. Three different
biodegradation kinetic models, Tessier, Monod and Contois models, were used to simulate the process. Each of the models gave a good fit to the experimental data.
However, the Contois kinetic model was found to be the most appropriate for de-scribing the microbial growth of Xhhh growth with the highest correlation coefficient of 0.984. Similarly, using three kinetic models, i.e., the Monod, Contois and Chen
& Hashimoto kinetic models, Abdurahman et al. [141,142] reviewed the treatment of palm oil mill effluent using membrane anaerobic system. In both studies, they found that the Contois model provided an excellent fit with experimental data with correlation coefficients of 0.962 and 0.997, respectively. In the latter case the Contois model gave the highest value for correlation coefficient. Poh et al. [143] cultivated a thermophilic mixed culture for treating the palm oil mill effluent at thermophilic conditions using a continuous stirred tank reactor. In this study, the Contois model provided a good fit with the experimental data with R2 values ranging between 0.82 and 0.97 depending on the effluent concentration.
Pinna et al. [144] investigated caffeic aside biodegradation using a mixed culture in a batch reactor. The Contois model, with an R-squared value of 0.851, was found to give a minor improvement in fitting experimental data over the Monod model, which had an R-squared value of 0.848. Zhang et al. [145] developed a kinetic model to describe simultaneous saccharification and co-fermentation of paper sludge using the xylose-utilizing yeast Saccharomyces cerevisiae RWB222 and the commercial cellu-lase preparation Spezyme CP. Their results showed that the Contois model gave the best description of the specific rate of growth on xylose.
Paulo et al. [146] used a bioreactor to treat mining influenced water that had been
discharged from abandoned mining sites. The Contois model and the first-order model were used to simulate the decomposition process. They compared their re-sults with the predictions based on the Contois and the first-order models to conclude that the Contois model stayed closer to the experimental data. Emerald et al. [147]
investigated the kinetics of the activated sludge process treating synthesized dairy wastewater using four kinetic models, i.e. the Monod, Moser, Contois and Chen &
Hashimoto models, at an organic loading rate of 1200 mg L−1. Among the tested models, the Contois kinetic model, with a correlation coefficient of 0.95, was found to be an appropriate model and the best kinetic model for describing the kinetic reactions of the activated sludge process due to fitting the data very well.
Karim et al. [148] proposed a new model, including the Contois kinetic model and an endogenous decay model to predict the methane production rate from the anaerobic digestion of cow manure in bench-scale gas-lift digesters. The proposed model and two other well-known kinetic models, the Chen & Hashimoto [149] and Hill [150]
models, were used to fit the experimental data obtained for methane production rate. It was found that the fitting of the Chen & Hashimoto and Hill models with the experimental data was poor with the values of the correlation coefficient being 0.86 and 0.51 respectively whereas their new model featuring Contois kinetics fit-ted the experimental data of observed methane production rate with a correlation coefficient of 0.99. Thus, the comparisons of experimental values with the results produced by the three models showed that the proposed model that included Con-tois model is more suitable for modeling the methane production rate.
The first-order kinetic model has been traditionally used to simulate hydrolysis
re-action in anaerobic digestion which is independent of the hydrolytic microorganism concentrations. However, some researchers have shown that the hydrolysis step de-pends on the microorganism concentration and activity [151]. The first-order kinetic may not describe the hydrolysis steps accurately [135] due to the complexity of the multi-step process involved in the hydrolysis of carbohydrase, proteins and lipids hydrolysis while the Contois model was found to fit the experimental data very well [135]. This result has been supported by several studies in which the Contois model was used to illustrate the hydrolysis of particulate wastes [116–118,152].
Ramirez et al. [116] developed anaerobic digestion model No.1 (ADM1) to describe the thermophilic anaerobic digestion of thermally pre-treated waste activated sludge using a batch reactor that involved three disintegration biochemical parameters, nine hydrolytic biochemical parameters and four stoichiometric parameter values. The Contois model was used to simulate the disintegration and hydrolysis processes. It was found that the Contois model gave a better fit to the experimental data than the standard ADM1 model which uses a first-order kinetic expression. Vavilin et al. [118]
discussed four kinetic models, including Monod, first-order, two-phase and Contois kinetics, for the hydrolysis of particulate organic material in anaerobic digestion.
The obtained results showed that using the Contois kinetic for the hydrolysis ki-netics of swine waste, sewage sludge, cattle manure and cellulose provided excellent fits to the experimental data. Gawande et al. [153] presented the development of a generalized biochemical process model. The hydrolysis of particulate matter was modeled using Contois kinetics which also described the microbiologically me-diated reaction. Gawande et al. [152] used their model to simulate the anaerobic
reduction of municipal solid waste. The use of Contois kinetics was found to give an excellent prediction in the reduction of solid concentrations. Myint and Nir-malakhandan [117] compared three models, i.e., the first-order, the second-order and the surface-limiting reaction models (also known as Contois kinetic model) for simulating hydrolysis-acidogenesis in the digestion of cattle manure residues using a batch reactor. They found that the Contois kinetic model fit the experimental data very well, with a correlation coefficient of 0.914, which was better than the other applied models. Therefore, the results discussed above provide convincing evidence that the substrate-microorganism ratio (S/X) used in the Contois model could be a better limiting factor in the hydrolysis of particulate substrate, rather than the substrate concentration (S) as modeled by the first-order reaction model.
Several researchers have conducted theoretical studies of a continuous flow bioreactor model using Contois kinetics [39,41,118,137,139,154]. One should note that at high feed substrate concentrations, the growth rate as a function of residual substrate concentration has been predicted successfully by the Contois kinetic model [31].
3.5.2 Other applications of the Contois model
In this section we review other experimental applications of the Contois model.
Hidaka et al. [155] developed a model to describe the lactate fermentation char-acteristics of B. Coagulans in a batch reactor. The model included the inhibitory effects of the substrate, lactate (product), NaCl, and bacterial growth. The Contois kinetic model was used to simulate the degradation of particulate carbohydrates.
Their results indicated that the Contois model is more suitable for simulating the
hydrolysis of particulate kitchen garbage than first order reaction. This result is in agreement with other studies that used the Contois model to describe the anaerobic hydrolysis of particulate organic wastes [116,156].
Several studies have indicated that the Contois equation can be used to describe fungal growth kinetics [157]. Zhou et al. [158] studied the growth kinetics of Rhizo-pus nigricans fungal on glucose using the Contois model. The results showed that the Contois model is useful for simulating the kinetics of cell growth due to the high value of the correlation coefficient of 0.99.
Mazutti et al. [159] developed a phenomenological model containing 19 kinetic mod-els for microbial growth to simulate inulinase production in a batch reactor by the yeast Kluyveromyces marxianus NRRL Y − 7571, employing a medium containing agroindustrial residues as the substrate. The results of this study showed that the Contois model was the most appropriate model for use in representing inulinase production.
Hernalsteens and Maugeri [160] investigated the Rhodotorula red yeast strains which produce extracellular enzyme. The growth kinetics of the microorganisms was de-scribed by the Contois model. It was found that the Contois model gave a good agreement with the experimental data, with a high correlation coefficient of 0.95 while the classic Monod model did not fit this process.
The Contois model has been used to simulate the algae growth [161] and to model biological reactions in composting [162,163]. In addition, it has been receiving in-creasing attention in ecology [164–166].
Table 3.1: Kinetic constants obtained using the Contois kinetic
Substrate Initial concen-tration
µmax Ks α Kd R2 Reference
Ice-cream wastewater 0.9297 day−1 0.4818 (g COD)(g VSS)−1
Textile wastewater 0.105 day−1 0.465
(mg COD)(mg VSS)−1 0.125
(mg VSS)(mg COD)−1
0.0065 day−1 0.967 Isik and Sponza [41]
Inulinase production 0.133 h−1 6.293g L−1 0.154(-) - 2.503(SSR)best
(Monod=2.521)
Extracellular enzyme 0.8 h−1 22.4 g L−1 0.5 g g−1 - 0.95 Hernalsteens and
Maugeri [160]
Note: 1-Before and after optimization with Mn2, Cu2and Zn2.
Analysis of single reactor with recycle
4.1 Introduction
In this chapter, the treatment of industrial wastewater in single reactor with recycle is studied. A very similar model was investigated earlier by Nelson et al [101] for a slightly different reactor configuration. In this earlier study the reactor membrane was assumed to be placed after the settling unit. In this chapter, a more natural configuration is used in which the reactor membrane is placed before the settling unit. These two models are identical when there is no membrane present. However, even for this case we present several new results which did not appear in [101].
Figure (4.1) shows a bioreactor with recycle. In such a reactor, the effluent emerges from the reactor and flows into the settling unit. Microorganisms settle to the bottom of the tank, and they are recycled into the reactor vessel. The remaining microorganisms, the substrate, and the reaction products flow out the reactor. One
46
advantage of using a setting unit is that it increases the concentration of the mi-croorganisms inside the reactor, thereby increasing the efficiency of the process.
The objectives of the current chapter are:
1. to provide a more detailed investigation of the model.
2. to investigate the conditions that minimize the concentrations of the pollutants in the effluent.