Bioremediation of hydrocarbons in soil is a highly complex process, involving a multiplicity of physical, chemical and biological phenomena. Therefore, it is extremely difficult to control and boost the bioremediation of these systems after an oil spill. A mathematical model was developed to assist in the prediction and decision-making regarding the in situ bioremediation of hydrocarbon-contaminated soils. The model considered the most relevant processes involved in the mass transfer and biodegradation of alkanes over time and along the depth of a flooded soil column. Aliphatic hydrocarbons were chosen since they are less water soluble than aromatics and account for 50–90% of the hydrocarbon fraction in several petroleum products. The effect of adding oxygen, nitrate, iron (III) or sulfate as electron acceptors was then simulated (bioremediation scenarios). Additionally, and to feed the model, batch assays were performed to obtain experimental data on hydrocarbon adsorption to soil particles (more than 60% of hydrocarbons tends to be adsorbed to soil particles), as well as hydrocarbon biodegradation rates in the presence of nitrate (0.114 d−1) and oxygen (0.587 d−1). The model indicates that saturated hydrocarbon removal occurs mainly with adsorption/desorption and transport processes in the upper layers of soil due to methanogenic biodegradation in deeper layers, since the other microbial processes are soon limited by the lack of electron acceptors. Simulation results show that higher initial electron acceptor concentrations led to higher hydrocarbon removal, confirming that the model is performing in accordance with the expected. Close to the surface (at 0.1 m depth), all scenarios predicted more than 83% hydrocarbon removal after two years of simulation. Soil re-aeration results in faster hydrocarbon removal (more than 20% after one year) and surfactants addition (around 15% after one year) may also accelerate soil bioremediation. With this model, the simultaneous contributions of the various physicochemical and biological processes are integrated, facilitating the simulation and comparison of different bioremediation scenarios. Therefore, it represents a useful support tool for the management of contaminated sites.
A mixture of skim milk and sodium oleate was fed to an upflow sludge bed reactor operated in cycles. Each cycle had a feeding phase under continuous operation and a reaction phase in batch. Five cycles were performed with organic loading rates applied during feeding phases varying between 4.4 and 8 kg COD.m.d and a constant hydraulic retention time of 1.6 days. In the first two cycles, 70% of the methane-COD was produced in the reaction batch phase, whereas from the third to the fifth cycles, biogas production in the reaction phase was less than 3% of total production. Overall methane yields increased steadily, from 0.67 to 0.91 kg COD-CH4.kg COD removed. LCFA accumulated into the sludge in the first two cycles, being palmitate and stearate the dominant intermediates quantified. In the subsequent cycles no LCFA were detected in the solid or liquid phases. The specific methanogenic activity in the presence of acetate and H2/CO2 increased significantly along the operation, particularly between time zero and the end of the third cycle. These results show that a discontinuous operation promoted the development of an active anaerobic community able to efficiently convert a continuous organic load of 8.2 kg COD.m.d, from which 50% was oleate.
For many years the focus of lipids/long-chain fatty-acids (LCFA) wastewater treatment was on technological and process developments. More recently, promising results on the anaerobic treatment of LCFA-containing wastewaters widened the attention to the microbiology aspects as well. In anaerobic bioreactors, LCFA can be β-oxidized to acetate and H2 by acetogenic bacteria, in obligatory syntrophy with methanogens. Presently, 14 species have been described that grow on fatty-acids in syntrophy with methanogens, all belonging to the families Syntrophomonadaceae and Syntrophaceae. Among these, only 4 species are able to degrade monoand/or polyunsaturated LCFA. The reason why the degradation of unsaturated LCFA is not more widespread remains unknown. Early studies suggested that degradation of unsaturated LCFA requires complete chain saturation prior to β-oxidation. Unsaturated LCFA, such as linoleate (C18:2) and oleate (C18:1), would be metabolized through a hydrogenation step yielding stearate (C18:0), then entering the β-oxidation cycle. However, this theory is inconsistent with the observed accumulation of palmitate (C16:0) in continuous bioreactors fed with oleate.
Background: Metabolic pathways for the degradation of unsaturated long-chain fatty acids (LCFA) in anaerobic bioreactors have not been extensively studied 1 . A two-step mechanism was proposed for oleate (C18:1) degradation: oleate biohydrogenation to stearate (C18:0) followed by b-oxidation to acetate 2 . However, accumulation of palmitate (C16:0) in bioreactors fed with oleate has been observed 3 , differing from the described mechanism. Complete b-oxidation of LCFA is only possible in syntrophy, when the H2 concentration is kept low by H2-consuming archaea. Conversion of oleate to palmitate might be an alternative mechanism for unsaturated-LCFA degradation.
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