Eliminación de xenobióticos de aguas residuales urbanas mediante biorreactores de membrana sumergida

Abstract

Abstract Lack of access to clean water and safe sanitation has been a limiting factor for development of societies and a basic element for almost all ecosystems. Proper wastewater treatment systems are essential especially in areas where limited water resources coincide with high population densities and the use of non-conventional water resources appears as a necessary option to supplement diminishing fresh water availability. Treated wastewater reuse has been increasingly used for a number of applications, including agricultural, industrial, urban and direct potable purposes. Modern society depends on a large range of organic chemicals and these may ultimately enter urban wastewater. Over the last decades, the occurrence of xenobiotic compounds in the aquatic environment has become a worldwide issue of increasing environmental concern. Xenobiotics compounds group consist of a vast array of anthropogenic as well as natural substances. The removal of some xenobiotic compounds using conventional activated sludge ix process (CASP) is often incomplete and low concentrations of these pollutants (μg/L or ng/L) pass through the treatment plants and enter the receiving water systems. Membrane bioreactor (MBR) is an alternative membrane application in wastewater treatment. This technology combines the biological activated sludge process with solid/liquid physical separation by membrane. MBR technology merges the second and third treatment phases allowing the construction plants capable of producing directly reusable effluent. This technology is expected to enhance the removal of xenobiotics compounds produced by industrial activities and it has been selected for wastewater treatment due to this capacity. The elimination of xenobiotic compounds can occur through various mechanisms in the activated-sludge process, mainly by biodegradation, sorption or volatilization. So, their removal mechanism in MBR can defer with respect to CASP. The MBR system can operate with high SRTs and high HRTs, which may allow the development of more diverse slow-growing microbial communities capable of biodegrading specific organic compounds. A high SRT causes high biomass retention in the activated sludge, so a higher contact between biomass and pollutants is possible, increasing the adsorption of hydrophobic compounds and thereby improving the retention capacity of several pollutants. Given these characteristics, MBR systems offer a good alternative to improve the removal of pollutants such as xenobiotic compounds. The xenobiotic pollutants can be a classical persistent organic pollutant, long considered as a risk to human health and the environment due to their persistence, potential to bioaccumulation and toxicity; or they can be an emerging pollutant, these are chemicals that are not commonly monitored but have the potential to enter the environment and cause adverse ecological and human health effects. Over the last years, the occuurrence of emerging pollutants in treated wastewater has been identified as a significant environment and health concern. The present work is focused on the behaviour and removal efficiency of xenobiotic compounds during the real urban wastewater treatment by an experimental full-scale MBR working at high sludge retention time (SRT) and x high hydraulic retention time (HRT). Two of the main groups of xenobiotics were selected, pharmaceuticals products were chosen from among emerging pollutants and polycyclic aromatic hydrocarbons from among classical pollutants. Given the heterogeneity of these groups, some important compounds were selected due to different reasons: Pharmaceutical pollutants which are resistant to conventional activated sludge treatment therefore they are persistent in the environment, such as carbamazepine (CBZ) and diclofenac (DCF) and their negative effects pose a risk to the environment Pharmaceutical pollutants considered less resistant and more easily degradable compounds being commonly detected in different water bodies due to their abundant and widespread use, and with potential negative effects on the environment, such us ibuprofen (IBU), naproxen (NPX) and ketoprofen (KTP). Polycyclic aromatic hydrocarbons (PAHs) hazardous to ecosystems and a human health risk due to their high toxicity, persistence and widespread distribution. Some of these compounds are considered priority pollutants during the last decades; however they are still commonly detected in different ecosystems and organisms. An experimental facility with two independent full-scale MBRs working in pre-denitrification configuration was used for this work. One membrane reactor was equipped with hydrophilicised micro-filtration flat-sheet membranes (0.4 μm nominal pore size) made of chlorine polyethylene; and the other membrane reactor was equipped with hollow fibre submerged ultrafiltration membranes (0.034 μm nominal pore size) made of polyvinylidenefluoride (PVDF). Both systems were composed of an anoxic bioreactor, aerobic bioreactor and membrane reactor. The facility was fed with pre-treated urban wastewater from Granada Wastewater Treatment Plant (Granada-Sur, Spain) without rubbish, sand and oils. The pre-treated wastewater was fed into xi anoxic reactor. Activated sludge of anoxic reactor was pumped to aerobic reactor which fed membrane tank. The membrane tank concentrate returned to the anoxic reactor and permeate passed to the backwashing tank, the membrane reactor was also aerated by a blower to remove solids from the membrane and to control fouling. A constant sludge purge was carried out form the aerobic reactor. Influent, effluent, purge flow, and operational parameters such as dissolved oxygen (DO), pH, temperature, and transmembrane pressure (TMP) were continuously monitored. All influent and effluent samples were analysed for total (TSS) and volatile (VSS) suspended solids, biochemical oxygen demand at five days (BOD5), chemical oxygen demand (COD), NH4+, NO3-, NO2- and total nitrogen. Activated sludge samples were analysed for TSS and VSS. The total concentrations of the selected xenobiotic compounds were determined in influent, effluent, purge, and activated sludge samples in order to determine the mass balance of each substance in the experimental MBR. Respirometric assays were made in the absence and presence of different xenobiotic compounds in order to evaluate their influence in the bacterial activity. Data were supplemented by microbiological analyses to determine the presence of culturable microorganisms capable of degrading the selected pollutants. During the experimental period with PAHs, the operational HRT was 35 h and the SRT were 12 and 25 d. While operational HRT was 34 h for the works with pharmaceutical products and the SRT was 37 d, except during the CBZ assays when SRT reached 40 d. The CBZ concentration assayed was higher than in the usual urban wastewater and negative effects were detected in the MBR bacterial community during the initial period of dosing. However, the effects were not permanent and the biomass spiked with CBZ had behaviour similar to that of the biomass without CBZ after a few hours. During and after the experimental process, CBZ did not significantly affect the efficiency of the MBR system, and the quality of the effluent xii was not affected by the dosing of CBZ in terms of COD and nitrogen removal. Biodegradation was not detected during the MBR treatment. The system showed an inefficient elimination of CBZ (less than 10%) with a high concentration in the effluent. The small percentage of CBZ removal was associated with the sludge retention and eliminated by the purge. The MBR system showed high efficiency eliminating IBU, NPX, and KTP from urban wastewater, with performance levels higher than 95%, whose main transformation occurred in the aerobic reactor with a low contribution from the anoxic reactor. The system reached a biodegradable organic matter removal higher than 99.5 % and worked with complete nitrification, also achieving an effective retention of the unbiodegradable organic matter due to recirculation. Biodegradation/biotransformation is the main mechanism involved in IBU, NPX and KTP removal by MBR. DCF removal was low with negative removal yields for several samplings. Both removal and increase transformation of DCF also occurred in the aerobic reactor, this not being observed in the anoxic one. DCF tends to accumulate in the system and to be recirculated. Thus, during the sampling in which DCF influent concentration decreases, removal yields turn negative. The increase of DCF concentration in the aerobic bioreactor also contributes to the negative removal yields. A similar behaviour was observed for CBZ. The elimination values for the NSAIDs assayed depended on the greater or lesser capacity of biotransformation on the part of the microorganisms of the activated sludge, being more significant for co-metabolisable compounds, so that the final concentrations of these compounds in the effluents will depend both on their concentration in the influent as well as their biodegradability. xiii Bench-scale experiments with experimental MBR activated sludge showed a high removal capacity for the selected PAHs. They reveal that PAHs removal is mainly due to sorption and air stripping, however the volatilization and biodegradation present a questionable insignificant contribution. Toxicity by PAHs during MBR treatment cannot be expected due to the low bio-available for the microorganisms mainly as a result of the high removal by air stripping. A second period of assays with PAHs was carried out. The fate and removal of phenanthrene (Phen), fluoranthene (F) and pyrene (Py) in urban wastewater treatment by membrane bioreactor (MBR) with low influent polycyclic aromatic hydrocarbons (PAHs) concentration were studied. All effluent samples presented concentrations of PAHs, with removal levels of 91% and 92% for F and Py respectively, while for Phen performance did not surpass 82%. Levels are lower for compounds with greater solubility in water, which are more easily carried away in the effluent. In spite of the high hydrophobicity of the tested compounds, their accumulation in the biomass was scarce and the sludge presented a low PAH concentration. The experiments reveal that PAHs removal is mainly due to air stripping, which reduces the accumulation of PAHs in the biomass and limits the establishment of microorganisms capable of biodegrading or biotransforming these compounds, most of which are released directly into the atmosphere. Biodegradation and adsorption make an insignificant contribution. Therefore, removal efficiencies of xenobiotic compounds during the real urban wastewater treatment by an experimental full-scale MBR working at high sludge retention time (SRT) and high hydraulic retention time (HRT) are diverse. Different physicochemical properties of the xenobiotic compounds and MBR operating conditions determine the main removal mechanisms. xiv