Prediction of organic chemical fates in biological treatment systems

The removal of Polycyclic Aromatic Hydrocarbons in the wastewater treatment process: experimental calculations and model predictions

The removal of PAHs during the wastewater treatment process was examined in an activated sludge mode conventional facility. Concentrations reported are taken from an earlier measuring campaign. Removals of PAHs ranged between 28 and 67% in the primary, <1-61% in the secondary stage, and 37-89% in the whole process. Significant positive relationships were observed for removal efficiencies and the log K(ow) of PAHs in the primary and the log K(H) of PAHs in the secondary stage. Experimental removals were compared to those obtained from the FATE model. In the primary stage, predicted removals were lower than those experimentally calculated while in the secondary stage were higher. Predicted removals were apportioned mainly to sorption with negligible contribution from volatilization and biodegradation. Remarkable consistency between experimental and modeled removal efficiencies (-20-+20%) was observed for almost all PAHs in the whole treatment process.

Persistent organic pollutants (POPs) in the conventional activated sludge treatment process: model predictions against experimental values

The FATE and treatibility estimator (FATE) model, developed by the United States Environmental Protection Agency was used for the prediction of the FATE of 26 persistent organic pollutants (POPs), i.e. 7 PCBs and 19 organochlorine compounds (OCs), during the conventional activated sludge treatment process applied in the waste water treatment plant (WWTP) of Thessaloniki, Greece. The removal rates predicted by the model for the primary and the secondary treatment stages were found to differ substantially from those experimentally measured. When the overall treatment was considered, the differences between measured and model predicted removals were within acceptable limits of confidence. Possible reasons that might cause deviations from experimental values were suggested to be the wastewater content in dissolved organic carbon (DOC), and/or the low concentrations of POPs in untreated wastewater.

The fate of xenobiotic organic compounds in wastewater treatment plants

The effective operation of wastewater treatment plants plays an important role in minimising the release of xenobiotic compounds into the aquatic environment. Considerable effort has been expended in developing models to quantify the overall removal and fate of these compounds in biological treatment plants. A synthesis and modification of these approaches has been made and a generalised fate model for organic compounds in an activated sludge plant is presented. The influence of the different removal mechanisms, such as sorption, volatilisation and advection for chemicals with different physico-chemical properties is investigated and the important role of biotransformation is discussed. The effect of some operating parameters has been found to have an important influence upon the concentration of xenobiotic released in the sludges and final effluent. This may have significance for a wide range of ecotoxic compounds and in particular the class of compounds increasingly recognised as having the potential to disrupt endocrine activity in some aquatic vertebrates.

Treatment of organic water contaminants in oil refinery effluents investigated by using a simulation model

In this paper the wastewater treatment of organic compounds, particularly the water soluble (polar) compounds in refinery effluents, is simulated by addressing biodegradation, adsorption, and stripping/volatilization. The study examines pollutant data from two refineries in Norway and one in the United States. Previously, little attention has been paid to polar compounds. However, they are present in effluents, sometimes in large amounts, and are difficult to characterize and quantify. The study shows that many polar compounds, including the recently introduced methyl-tertiary-butyl-ether, are biodegraded by < 10% in simulated wastewater treatment. The concentrations of some polymers are barely reduced at all, and some metabolites of nonionic surfactants increase in toxicity along the route of degradation, resulting in an end product of the highest toxicity. The study demonstrates that of the substances entering the treatment systems, the residual of selected polar compounds is 38% of the incoming amount, while oil is reduced to 8%. It can be inferred from the study that polar compounds are contaminants, many of which may pose a pollution problem, and that reduction of polar compounds in effluents should be carried out upstream at the source of pollution, rather than downstream by end-of-pipe treatment.

Dynamic response of naphthalene biodegradation in a continuous flow soil slurry reactor

Periodic perturbations were used to evaluate the system stability and robustness of naphthalene biodegradation in a continuous flow stirred tank reactor (CSTR) containing a soil slurry. The experimental design involved perturbing the test system using a sinusoidal input either of naphthalene or non-naphthalene organic carbon at different frequencies during steady state operation of the reactors. The response of the test system was determined by using time series off-gas analysis for naphthalene liquid phase concentration and degradation, total viable cell counts, and gene probe analysis of naphthalene degradative genotype, and by batch mineralization assays. Naphthalene biodegradation rates were very high throughout the experimental run (95 to greater than 99% removed) resulting in very low or undetectable levels of naphthalene in the off-gas and reactor effluent. Attempts to reduce the rate of naphthalene biotransformation by either reducing the reactor temperature from 20 degrees C to 10 degrees C or the dissolved oxygen level (greater than 1 mg/L) were unsuccessful. Significant naphthalene biodegradation was observed at 4 degrees C. While variable, the microbial community as measured by population densities was not significantly affected by temperature changes. In terms of naphthalene biotransformation, the system was able to adapt readily to all perturbations in the reactor.