Hybrid support vector regression and crow search algorithm for modeling and multiobjective optimization of microalgae-based wastewater treatment.

Microalgae-based wastewater treatment (and biomass production) is an environmentally benign and energetically efficient technique as compared to traditional practices. The present study is focused on optimization of the major treatment variables such as temperature, light-dark cycle (LD), and nitrogen (N)-to-phosphate (P) ratio (N/P) for the elimination of N and P from tertiary municipal wastewater utilizing Chlorella kessleri microalgae species. In this regard, a hybrid support vector regression (SVR) technique integrated with the crow search algorithm has been applied as a novel modeling/optimization tool. The SVR models were formulated using the experimental data, which were furnished according to the response surface methodology with Box-Behnken Design. Various statistical indicators, including mean absolute percentage error, Taylor diagram, and fractional bias, confirmed the superior performance of SVR models as compared to the response surface methodology (RSM) and generalized linear model (GLM). Finally, the best SVR model was hybridized with the crow search algorithm for single/multi-objective optimizations to acquire the global optimal treatment conditions for maximum N and P removal efficiencies. The best-operating conditions were found to be 29.3°C, 24/0 h/h of LD, and 6:1 of N/P, with N and P elimination efficiencies of 99.97 and 93.48%, respectively. The optimized values were further confirmed by new experimental data.

Kinetics of Oxidative Cracking of n-Hexane to Light Olefins using Lattice Oxygen of a VOx /SrO-γAl2 O3 Catalyst.

The kinetics of oxidative cracking of n-hexane to light olefins using the lattice oxygen of VOx /SrO-γAl2 O3 catalysts has been investigated. Kinetic experiments were conducted in a CREC Riser Simulator (CERC: Chemical Reactor Engineering Center), which mimics fluidized bed reactors. The catalyst's performance is partly attributed to the moderate interaction between active VOx species and the SrO-γAl2 O3 support. This moderate interaction serves to control the release of lattice oxygen to curtail deep oxidation. The incorporation of basic SrO component in the support also helped to moderate the catalyst's acidity to checkmate excessive cracking. Langmuir-Hinshelwood model was applied to formulate the rate equations. The intrinsic kinetic parameters were obtained by fitting the experimental data to the kinetic model using a nonlinear regression algorithm at a 95% confidence interval, implemented in MATLAB. n-Hexane transforms to olefins at a specific reaction rate of 1.33 mol/gcat.s and activation energy of 119.2 kJ/mol. These values when compared with other duplets (i. e., ki° and EA ) for paraffins to olefins, show that indeed olefins are stable products of the oxidative conversion of n-hexane over VOx /SrO-γAl2 O3 under a fluidized bed condition. Values of activation energy for all COx formation routes indicate that intermediate paraffins are likely to be cracked to form CH4 than to be converted directly to COx . On the other hand, olefins may transform partly, and directly to COx (E9 =9.65 kJ/mol) than to form CH4 (E8 =89.1 kJ/mol) in the presence of excess lattice oxygen. Overall, olefins appear to be stable to deep oxidation due to the role of SrO in controlling the amount of lattice oxygen of the catalyst at the reaction temperature.

Biological CO2 fixation using Chlorella vulgaris and its thermal characteristics through thermogravimetric analysis.

The present research is focused on cultivation of microalgae strain Chlorella vulgaris for bio-fixation of CO2 coupled with biomass production. In this regard, a single semi-batch vertical tubular photobioreactor and four similar photobioreactors in series have been employed. The concentration of CO2 in the feed stream was varied from 2 to 12 % (v/v) by adjusting CO2 to air ratio. The amount of CO2 capture and algae growth were monitored by measuring decrease of CO2 concentration in the gas phase, microalgal cell density, and algal biomass production rate. The results show that 4 % CO2 gives maximum amount of biomass (0.9 g L(-1)) and productivity (0.118 g L(-1) day(-1)) of C. vulgaris in a single reactor. In series reactors, average productivity per reactor found to be 0.078 g L(-1) day(-1). The maximum CO2 uptake for single reactor also found with 4 % CO2, and it is around 0.2 g L(-1) day(-1). In series reactors, average CO2 uptake is 0.13 g L(-1) day(-1) per reactor. TOC analysis shows that the carbon content of the produced biomass is around 40.67 % of total weight. The thermochemical characteristics of the cultivated C. vulgaris samples were analyzed in the presence of air. All samples burn above 200 °C and the combustion rate become faster at around 600 °C. Almost 98 wt% of the produced biomass is combustible in this range.

Effects of CO2 Concentration and pH on Mixotrophic Growth of Nannochloropsis oculata.

This communication reports an experimental investigation of integrated CO2 bio-conversion, wastewater treatment, and biomass production by microalgae cultivation. In this regard, the effects of CO2 concentrations on mixotrophic growth kinetics of a microalgae strain (Nannochloropsis oculata) are conducted in a semi-batch photobioreactor. The concentration of CO2 in the feed stream is varied from 4 to 12 mol% by adjusting CO2-to-air ratio. The variation of pH of the synthetic wastewater culture media and nutrient uptake by the microalgae are also monitored. The experimental evaluation shows that 8 % CO2 gives the highest growth rate of N. oculata with a productivity of 0.088 g L(-1) day(-1). Under the studied conditions, the pH value of the culture media between 5.5 and 6.5 is favorable for the growth of N. oculata in mixotrophic condition. Among the nutrients available in the culture media, percentage of ammonia removal is found to be the highest (98.9 %) as compared that of other compounds such as nitrate (88.2 %) and phosphate (18.9 %). The thermochemical characteristics of the cultivated microalgae are assessed by thermogravimetric analysis in presence of air. The produced microalgae is thermally stable up to 200 °C. Following that, the microalgae biomass is sharply decomposed within 600 °C.

Apparent kinetics of high temperature oxidative decomposition of microalgal biomass.

The oxidative thermal characteristics of two microalgae species biomass Nannochloropsis oculta and Chlorella vulgaris have been investigated. The apparent kinetic parameters for the microalgal biomass oxidation process are estimated by fitting the experimental data to the nth order rate model. Also, the iso-conversional methods Kissinger-Akahira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO) were used to evaluate the apparent activation energy. The results indicate that biomass of different microalgae strains exhibit different thermal behavior and characteristics. In addition, growth parameters and medium composition can affect the biomass productivity and composition. This would have significant impact on the thermal decomposition trend of the biomass. The kinetic modeling of the oxidation reaction with direct model fitting method shows good prediction to the experimental data. The apparent activation energies estimated by KAS and FWO methods for N. oculta were 149.2 and 151.8kJ/mol, respectively, while for C. vulgaris were 214.4 and 213.4kJ/mol, respectively.