Effect of activated carbon on the removal of organic micropollutants in activated sludge and membrane bioreactors

Resumo

This Doctoral Thesis is focused on the removal of organic micropollutants, such as Pharmaceutical and Personal Care Products (PPCPs) present in sewage in concentrations from ng L-1 to µg L-1. In the last two decades the knowledge about the occurrence of these compounds in the environment has increased, due to the advance in analytical techniques based on chromatography with Mass Spectrometry (LC-MS or GC-MS) that allow the detection and quantification of compounds at trace levels. Most of the PPCPs are biologically active compounds that have been designed to alter biological systems. Therefore, the lack of information about their potential effects on the environment and human health is one of the reasons to study and evaluate these compounds. The principal sources of PPCPs in the environment are either industrial, agricultural, medical manufactured or common household practices (use of cosmetics, detergents, etc.). Moreover, pharmaceuticals which are not completely metabolised in the human body are excreted with the urine and feces and can reach urban wastewater. Additionally, other significant sources of pharmaceutical discharge in the environment are the effluents from hospitals and leachates generated from municipal landfills. Over the last decade it was shown that conventional treatment plants are not effectives to remove many of PPCPs present in wastewater and also that removal efficiencies depends on configuration and operation mode of each plant. Many of these compounds are persistent to biodegradation and their presence in the environment has been detected a long time. The main removal mechanisms of PPCPs observed in treatment plants are: biological degradation, sorption onto sludge, hydrolysis of conjugates (that can form again the parent compound) and volatilisation or stripping. The importance of the different mechanisms depends on a range of factors, such as weather conditions, microbial activity or the nature of the pollutant. According to different projects carried out in the research group at the University of Santiago de Compostela, it was concluded that applying aerobic and anoxic environments allows high removal efficiencies of some compounds with low susceptibility to biodegradation. Besides, sludge retention time exerts a significant effect on the achievement of a higher biodegradation for some compounds. In this sense, the use of Submerged Membrane Bioreactors (MBRs) working at high sludge retention combined with the physical effect of the membrane filtration was studied. In addition, other configurations were tested for PPCPs removal such as Hybrid Membrane Biological Reactor (HMBR), which used biomass both in suspension and a biofilm growing on the surface of biocarrier. This system increases the availability of biomass and capacity of biodegradation without a significant increase in sludge production. Other alternatives include the use of tertiary treatment such as oxidation with ozone, which has proven to be highly effective in the removal of certain PPCPs and the use of coagulants and flocculation processes and activated carbon either in granular (GAC) or powdered (PAC) form. However, there are reports showing that the efficiency of pharmaceutical compounds removal by the activated carbon can decrease in presence of organic matter due to the competition between macro and micropollutants to the active centers of the adsorbent. This Doctoral Thesis aims at studying removal of PPCPs from sewage, mainly those with a more persistent behaviour, under different operational conditions and operational strategies applied in the conventional activated sludge systems. The list of PPCPs included in this research comprises pharmaceuticals from five different therapeutic classes (antibiotics, anti-depressants, anti-inflammatory drugs, tranquilizers and anti-epileptics), the synthetic hormone ethinylestradiol (EE2) and three polycyclic musk fragrances. The specific objectives of this thesis were: - To study the adsorption process of selected PPCPs either onto sludge or onto activated carbon. Batch adsorption isotherms were determined considering different environmental conditions. In this way, a three-compartment matrix model was studied for a better understanding of the adsorption process. Mass balances were applied taking into account PPCPs concentrations both in liquid and solid phases (activated carbon and sludge). - To evaluate the removal of selected Pharmaceutical and Personal Care Products spiked in a synthetic feeding simulating the typical composition of medium-strength sewage, in a Hybrid Aerobic System (HAS), with biofilm support, and to compare their results with the obtained in a Conventional Activated Sludge (CAS) system. - To evaluate the effect of Granular Activated Carbon (GAC) in the aeration tank of Conventional Activated Sludge (CAS) reactors, studying also the influence of the effect of coagulants such as Ferric Chloride (FeCl3) for removal of PPCPs. - To assess the removal of selected pharmaceutical micropollutants present in high-strength synthetic sewage, using a Sequential Membrane Batch Reactor (SMBR) with a microfiltration membrane. Moreover, the addition of PAC directly into the aeration tank was also assessed as a tool to enhance the removal of the most recalcitrant compounds. The experimental results and discussions are detailed in six chapters: In chapter 1, an updated revision of the literature referring to the studies on PPCPs removal from wastewater is presented. Special attention has been paid to the use of new strategies such as membrane bioreactors, hybrid systems (biofilm + suspended biomass) and the use of additives such as activated carbon either in powdered (PAC) or granular (GAC) form in the aeration tank. In addition, a revision on the evolution of activated sludge treatment plants is studied. Moreover, a discussion about the problems due to the occurrence of organic micropollutants in the environment, their possible toxic effects over aquatic organisms and their sources and fates in water are presented. Finally, a list of the compounds used in this Thesis including, their physico-chemical characteristics and their main removal mechanisms in wastewater treatment plants are described. In chapter 2, materials and methods employed in the experimental work in subsequent chapters are explained: Protocols for analysis in aqueous and solid phase, physical and microbiological characteristics of the biomass are included. Microbial populations typically found in biological wastewater treatment were followed by Fluorescence In Situ Hybridization (FISH). Also, an extensive table describing the gene probes applied for the detection of bacteria and a phylogenetic tree showing the application of the probes are indicated. Finally, the analytical methods for PPCPs determination are detailed. The organic micropollutants employed in this work including polycyclic musk fragrances (galaxolide, tonalide and celestolide), anti-epileptic (carbamazepine), tranquilizer (diazepam), anti-inflammatory drugs (ibuprofen, naproxen and diclofenac), anti-depressant (fluoxetine), estrogen (17B-ethinylestradiol) and antibiotics (roxythromycin, trimethroprim and erythromicyn). In chapter 3, a model of micropollutant distribution in an activated sludge matrix is studied. This model considers the sludge as a three compartment matrix, in which the pollutant remains in a free state, sorbed to particles or sorbed to dissolved and colloidal matter in the matrix. This approach permits to gain a better knowledge about the affinity of the pollutant for any of these phases, through the determination of two equilibrium constants: Kpart and KDCM. The model has been applied to study the distribution of the synthetic hormone ethinylestradiol (EE2) using Dissolved and Colloidal Matter (DCM) in the range from 1.5 to 5 mg L-1. The experimental results allow concluding that the presence of DCM did not influence significantly EE2 distribution, being most of ethinylestradiol associated to particulate matter. This work was carried out in the Laboratory of Environmental Biotechnology (LBE), a research unit of the French National Institute for Agricultural Research (INRA) in Narbonne, France. In Chapter 4, the influence of the biofilm developed in a laboratory-scale hybrid system on removal of PPCPs is presented. The results obtained using this system are compared with the results obtained in a conventional activate sludge system. The Hybrid Aerobic System (HAS) includes a synthetic support (Kaldnes) in the aeration tank. It is expected that microorganisms requiring higher sludge ages, such as nitrifying biomass, develop onto the support surface while other microorganisms remain as free biomass. The results show no significant effects on the more persistent compounds removals, such as carbamazepine, diazepam and diclofenac, probably due to the limited development of biofilm on the support. However, for other compounds, such as ibuprofen, naproxen and musk fragrances removals up to 90 % were achieved in HAS reactor. Despite of the short operation time of HAS reactor, it seems that the developed biofilm permitted to enhance the removal of some micropollutants. In chapter 5, removal of PPCPs in three activated sludge reactors was studied, assessing the effect of adding a coagulant (ferric chloride) or granular activated carbon (GAC) to the aeration tank. The results indicate that the influence of coagulant addition on PPCPs was negligible, while the addition of GAC enhanced the removal up to 40% for recalcitrant compounds such as carbamazepine and diazepam and 85% for diclofenac. The best removal efficiencies were achieved at higher GAC concentrations. In addition, batch experiments were carried out in order to determine the effect of activated carbon on affinity and further removal of PPCPs. According to these experiments, the results show that the higher GAC affinity is to compounds with recalcitrant characteristics, such as the anti-epileptic carbamazepine, the tranquilizer diazepam, and the anti-inflammatory diclofenac, whereas compounds with lipophilic characteristics like musk fragrances (galaxolide, tonalide and celestolide) have low affinity. The results suggest the addition of activated carbon in the conventional activated sludge reactors as a strategy to increase removal of some recalcitrant PPCPs. Chapter 6 emphasizes the effect of powdered activated carbon (PAC) on PPCPs removal in Sequential Membrane Biological Reactor (SMBR). A single dose of PAC was added to the aeration tank. This system proved to achieve not only high reduction of organic matter (>95%) and macronutrients such as N-NH4+ and P PO4-3 (>80%), but also a high removal efficiencies (90-99%) of PPCPs, that were in trace levels such as anti-inflammatory (i.e. ibuprofen, naproxen and diclofenac) and anti-depressants (fluoxetine). Furthermore, the addition of PAC to the reactor permitted the elimination of persistent compounds such as carbamazepine, diazepam, diclofenac and trimethoprim, which removal efficiencies increased from 10 to 95-99%. Also, in the case of compounds with moderate eliminations rates (40 to 70%) (such as roxithromycin, erythromycin and fluoxetine) the addition of PAC made possible to achieve up to a 98% removal. However, the effect of powdered activated carbon on the removal of some compounds such a musk fragrances and the anti-inflammatory drugs, ibuprofen and naproxen was negligible, due to the fact that absorption and/or biodegradation are the main removal mechanisms for these compounds. According to the research carried out in this Thesis, valuable information on pharmaceutical and cosmetic compounds removals is reported, particularly of those compounds with persistent characteristics in sewage. Moreover, the implementation of different strategies such as activated carbon either in powdered (PAC) or granular (GAC) form in the aeration tanks of conventional activated sludge systems are highlighted to enhance the performance of the system and the PPCPs removal.