Feasibility of membrane bioreactors for the removal of pharmaceutical and personal care products present in sewage

Abstract

The Water Framework Directive (2000/60/EC) promotes a long-term progressive reduction of contaminant discharges to the aquatic environment in sewage. It is well-known that inputs of metals and organic contaminants to the urban wastewater system occur from different sources (domestic, commercial and urban runoff) and available literature has quantified their extent and importance. During the last years significant progress has occurred in eliminating the input of pollutants from these sources as reflected in the significant reductions reported for potentially toxic elements concentrations in sewage sludge and surface waters. The list of pollutants of interest comprises substances such as metals (cadmium, chromium, copper) and organic compounds (PAHs, PCBs) but nowadays, a new generation of contaminants is being detected in different water compartments at significantly lower concentration levels, for that reason considered as micropollutants. 20 years ago, the development of the low pressure/submerged filtration systems boosted the development of membrane bioreactor (MBR) technologies for treating municipal or industrial wastewater. These systems combine the unit operations of biological treatment, secondary clarification and filtration into a single process, producing a high quality effluent suitable for any discharge and reuse purposes and are being now accepted as a technology of choice, widely applied in different regions of the world. Apparently, MBR advantages might help to mitigate the continuous release of micropollutants into the aquatic environment, also considering their current market size and growth projections. MBRs are especially important in Japan, with 66% of the total of installations. 98% of these plants work with aerobic biological processes, and 55% of the systems are equipped with submerged membrane modules. Presently, more than 800 installations are in operation only in Europe, and many more are under construction. Only in Spain, the number of sewage treatment plants implementing MBR technology has been multiplied by 4 from 2002 to 2005. Despite the high quality of the effluent produced after MBR treatment, different factors have traditionally impaired the competitiveness of these systems. The most relevant was the high operational costs associated to the energy demand and membrane cleaning/replacement procedures. Thanks to many researching efforts throughout the last years, the main drifters for the quick widespread of the MBR technology have been the reduction in modules prices and an increased energy efficiency, mainly due to improvements in the design and operation practices. Pharmaceuticals on their own (from the Latin pharmaceuticus and the Greek pharmakeutikos), also known as pharmaceutically active compounds (PhACs), have been defined as chemical substances that are used for diagnosis, treatment (cure/mitigation), or for prevention of diseases. This definition covers both prescription and over-the-counter drugs. Personal Care Products such as soaps, perfumes, disinfectants and sunscreen agents are used to alter or improve physiological or physical status. All these substances are largely consumed in modern societies and during the last decade several studies have reported their worldwide occurrence in different environmental compartments (surface waters, groundwaters, soils, sediments, etc.). PPCPs have been detected in extremely low concentrations, ranging from the ng/L to the low µg/L level. Only thanks to the recent developments in analytical techniques, particularly the gas/mass chromatography (GC/MS) and liquid/mass chromatography (LC/MS), the presence of a wide number of these substances has been completely proven, and several routes into the aquatic environment have been identified: ¿Unmetabolized fractions of pharmaceuticals consumed by humans as well as their metabolites entering raw sewage via urine and faeces and by improper disposal over the toilets ¿Drugs of veterinary use and metabolites reach soils after excreta, eventually finishing in groundwaters and aquifers. ¿Application of sewage sludge as a fertilizer represents and additional entry route into the environment. This route is mainly followed in the case of compounds which have tendency to be associated with the solid fraction of sewage. ¿Hospital wastewater, which usually contains higher concentrations of specific pharmaceuticals such as antibiotics, anti-cancer agents or iodinated contrast media. ¿Personal care products and their ingredients which are discharged after their use in wastewater. Therefore, most of PPCPs are released as original compounds and/or metabolites, entering the wastewater treatment plants where they undergo different fate as a funcion of their physical-chemical properties and biodegradability: 1. The substance will be ultimately mineralized to carbon dioxide and water. This concerns very few compounds, e.g. aspirin 2. The lipophilic and/or not readily degradable compound will partially remain in the particulate phase following a sorption mechanism. 3. The substance will be total or partially degraded during the biological treatment step, most probably following co-metabolic pathways (due to their low concentrations). 4. Most recalcitrant PPCPs will remain unchanged after the different stages of the treatment process, passing the wastewater treatment plant and ending up in the receiving waters, eventually becoming pseudo-persistent because their elimination/transformation rates are usually countered by their constant replenishment. 5. Depending on the flow of air getting in contact with wastewater, type of aireation and Henry coefficient, a fraction of a compound might be stripped with the off-gas in the aeration tank. At this time, little information exists regarding the human health impacts of these substances since toxicity studies for pharmaceuticals mostly come from hypersensitivity, overdose and abuse effects which require concentrations clearly higher than the typically measured in the aquatic environment. Regarding the public health, it is also important to highlight that only in the worst-case scenario PPCPs are found in drinking water, mainly due to the efficiency of the drinking water treatment plants. PPCPs are rarely found even in the low ng/L range in drinking waters and hence, harmful effects derived from its consumption are not expected. On the contrary, aquatic ecosystems are subjected to a constant input of these substances, which arises concerns due to the possibility that stationary concentrations might be achieved in particularly sensitive areas. Increasing evidence suggests that chronic exposure to biologically active substances might be hazardous, in despite of the low concentrations in which they occur. Moreover, complex mixtures of these substances are usually found (until the date, more than 150 PPCPs have been identified in different water compartments), that could give place to synergistic effects. The presence of, for example, steroids and other Endocrine Disrupting Compounds (EDCs) has been linked to reproductive malfunction or feminization of fishes. Another well-known example, consequence of the abuse in the use of antibiotics, is the development or proliferation of resistant strains of bacteria, being also of relevance in this case the contribution of antibiotics from stockbreeding activities. Moreover, cumulative effects on the metabolism of non-target organisms should also be considered. The factors mentioned above constitute the reason why currently there are no specific regulations establishing the maximum levels of concentrations for these substances at the outlet of sewage treatment plants and therefore, PPCPs are considered as emerging contaminants: Pollutants not currently included in programs of routine monitoring of quality of the waters, although they can be candidates for future regulation depending of the research concerning his ecotoxicity, potential hazardous effects for the health, public perception and of the data about their presence in different environmental compartments. (6th EU Framework Programme project NORMAN). Major therapeutic groups of PPCPs commonly detected in wastewater treatment plant effluents are antibiotics, antiepileptic, tranquilizers, anti-inflammatories, X-ray contrast media, contraceptives, musk fragrances and several cosmetic ingredients. In this work, the selection of a representative group of PPCPs was based on the following criteria: a wide range of substances found at measurable levels in STP effluents, substances commonly prescribed belonging to different therapeutic groups, substances comprising different physical-chemical properties and therefore behaviour/fate throughout sewage treatment processes, and availability of reliable analytical methods to detect them in complex matrices such as wastewater. Conventional water treatment processes are designed for the removal of organic matter and, in some cases, nitrogenous compounds. Such treatment technologies cannot fully and systematically remove many PPCPs, mainly due to their poor biodegradability. MBRs, which are in fact a modification of the CAS process, allow a major flexibility for the operation of the biological process. There are three relevant characteristics of the MBR technology that are of particular interest for the elimination of different organic micropollutants, particularly those of moderate biodegradability: ¿ MBRs permit to control the Sludge Retention Time (SRT). Previous works in this line suggest that this parameter exert a significant influence in the adaptation of the microorganisms to a continuous input of PPCPs. Therefore longer SRTs will increase the capacity of the biomass to remove recalcitrant substances. ¿ It is possible to work with high concentrations of biomass, which allows to enhance the biological treatment within a reduced space. Additionally, developed biomass during a MBR process present some differences in terms of physical properties compared with biomass developed in conventional systems. For example, a higher surface area of MLSS, directly related to the floc-structure, which probably might increase some enzymatic activities. ¿ Considering the high quality of the final effluent suitable for reuse purposes in many cases, a further post-treatment (for example, nanofiltration, ozonation process or filtration through granular activated carbon columns) might be more efficient, due to the lack of substances that could interfere in such processes (organic matter, colloids, suspended solids,etc.). Therefore, the aim of this doctoral thesis was the evaluation of the MBR technology for eliminating a specific category of organic micropollutants: Pharmaceutical and Personal Care Products (PPCPs). The selection of substances of interest comprised 11 pharmaceutically active compounds from five therapeutic classes (anti-inflammatory drugs, antibiotics, anti-depressants, tranquilizers and anti-epileptics), 3 polycyclic musk fragrances characterised for their wide usage in detergents, soaps and perfumes, 2 natural estrogens and a synthetic hormone, considered endocrine disruptors. The first approach of this work (Chapter 1) consisted of a literature overview explaining the different removal mechanisms PPCPs undergo during sewage treatment and the different operational parameters and factors which influence the degree of removal achieved. Besides, a broad description of the substances studied is provided, detailing their most relevant physical-chemical properties which influence their fate and behaviour along sewage treatment. The chapter finishes with an analysis of the influence of different sewage treatment technologies available, mainly focused in the comparison of results achieved with the Conventional Activated Sludge (CAS) process and MBRs. In Chapter 2, the materials and experimental methods employed to carry out the experimental work of this doctoral thesis are described. Firstly, the methods employed to analyse the properties of the liquid phase and conventional parameters (organic matter, nitrogen, temperature, solids content, pH and dissolved oxygen content) used for wastewater and sludge characterization are detailed. They are followed by the techniques for the analytical determination of PPCPs in liquid and solid samples. Chapter 3 consisted in a preliminary study about the occurrence of PPCPs in municipal wastewater and their fate and behaviour along the different units of sewage treatment. This research was carried out in a wastewater treatment plant placed in Northwest England, where a fully instrumented pilot-plant was operated at its premises. Usually, CAS units consists of a pretreatment step for grit, fat and grease removal followed by primary treatment, where most suspended solids are eliminated as primary sludge. During both steps, a fraction of PPCPs is expected to be removed by sorption onto the particulate phase. Then, secondary treatment where the elimination of some PPCPs is achieved following two different mechanism: biological degradation and/or sorption onto secondary sludge. Finally, the final effluent is obtained after secondary settling step. In the studied system the processes of organic matter and ammonia removal take place simultaneously and it truly represents the technology more commonly used in sewage treatment plants. In order to get more insight into the mechanisms responsible for the PPCPs elimination throughout the different treatment units of the pilot-plant, mass balances for each quantified PPCP were calculated. The methodology consisted of a two-days sampling campaign where liquid samples at the inlet and outlet of each one of the considered units were immediately processed after collection. Although no solid samples were collected, the amount of PPCPs which might be present on this phase was estimated with distribution coefficients from the literature. The most frequently detected PPCPs were anti-inflammatory drugs and musk fragrances. The mass balances permitted to calculate the degree of elimination achieved in each one of the units of the plant, which was useful in order to elucidate their behaviour along sewage treatment. Additionally, the plant treated a stream of returning liquors from the sludge centrifuge unit, which was also sampled and therefore considered in the PPCPs mass balances, since substantial concentrations of them were also detected in the mentioned stream. Considering the results obtained, the daily output of PPCPs that a medium-sized STP might release into the aquatic environment was calculated. For example, in the case of diclofenac, one of the most recalcitrant substances considered in this study, a daily release of 1.5 kg/d was estimated only considering the liquid phase. In Chapter 4 the fate of selected PPCPs during MBR treatment was aimed to be assessed. For this purpose, a MBR was operated indoors, at the premises of the School of Engineering (University of Santiago de Compostela). Feeding consisted on a synthetic influent which reproduced the typical characteristics of a medium strenght municipal wastewater. In this chapter, the solid-phase was not considered, since solids content in the feeding was negligible and therefore, the elimination of each one of the considered PPCPs could be calculated in abcense of these data. The highest transformation (>90%) was determined for the anti-inflammatories IBP and NPX whereas CBZ, DCF and DZP were poorly removed. Surprisingly, musk fragrances elimination was only moderate (50-60%) compared with results achieved in conventional systems (>80%). Antibiotics showed a different fate. For example, SMX removal was intermediate, TMP was recalcitrant (<20%) whereas ROX and ERY were efficiently removed. These results showed slightly improved removals comparing with CAS systems with the exception of musk fragrances. However, a deeper study about the influence of the different operational parameters was not considered at this stage of the research. Another relevant aspect to improve was the composition of the feeding, which directly influenced the developed MBR biomass. Therefore, the pilot-plant was set outdoors at the premises of a full-scale sewage treatment plant, and was fed with settled sewage (Chapter 5) for an extended period of operation. The varying parameters that were studied at this stage of the research were the Mixed Liquor Solids Concentration (MLSS), the temperature of the biomass and the adaptation of the microorganisms to a continuous input of the selected PPCPs. Differences in the behaviour and fate were observed depending on the substance considered. For example, sulfamethoxazole removal was moderate (50-75%) and particularly influenced by the mixed liquor suspended solids (MLSS) concentration. The elimination of other antibiotics strongly increased during the operation of the MBR, probably due to biomass adaptation. Operating conditions did not influence the elimination of hormones, ibuprofen and naproxen, which were almost completely eliminated (90-99%). Similarly, the removal of carbamazepine, diazepam and diclofenac was not influenced by the operating conditions although their elimination was incomplete (20-50%). Elimination of fragrances varied significantly between operational periods: low eliminations were observed in the winter period whereas eliminations up to 70% were measured during samplings carried out in warmer periods. Sludge age, temperature and physical-chemical characteristics of the MBR sludge might exert influence on the observed eliminations. In Chapter 6, a direct comparison between MBR and CAS systems was carried out by parallel operation of the MBR studied in the previous chapter and a lab-scale activated sludge unit. Different parameters and conditions such as temperature, pH, MLSS, HRT and SRT were maintained at similar values in both systems. Additionally, the influence of HRT and SRT on PPCPs removal was checked in both systems. HRT influences the contact time of the soluble components of the PPCPs in STPs, affecting the biological activity of the activated sludge. Its decrease has been shown to adversely affect the overall quality of treatment. Hence, higher HRT may be preferable for more effective elimination of micropollutants in STPs. The SRT is considered a critical operational parameter commonly used for STPs design and can be optimised during secondary wastewater treatment in order to achieve better elimination of micropollutants. Longer SRTs allow the growth of slowly growing bacteria, subsequently leading to the formation of diverse ecology of microorganisms with wider spectrum of physiological and adaptation characteristics. The main differences between the CAS and MBR systems, in terms of PPCPs removal, were found for compounds which concentrations in solid-phase were higher such as musk fragrances or antidepressants. Distribution coefficients for each one of the considered PPCPs were calculated for the two sludges. Although the differences were low, the CAS sludge had slighty higher coefficients for many substances. In general, the performance of the CAS for the elimination of compounds sorpted onto the solid phase was remarkably higher. Interestingly, the effect of the sludge purges performed to control the SRT was related to the performance of both systems since, after intensive period of purges, the overall elimination efficiencies of both bioreactors increased, and this effect was more marked in the MBR. Another influencing parameter was the SRT. After decreasing this parameter to values below 10 d, the elimination of many substances such as antibiotics was severely reduced. In this specific case, the MBR performance was superior compared to CAS. However, the main conclusion of this chapter is that the differences between both technologies eliminating recalcitrant substances are low, and the upgrade of a conventional treatment plant to MBR is not justified merely in terms of micropolutants removal. Chapter 7 considered a different aspect of the membrane bioreactors technology: the effect of the membrane filtration step. Therefore, 3 side-stream modules coupled to a MBR and a submerged hollow fiber membrane were simultaneously tested during three sampling periods. Some of the studied PPCPs were already present in sewage from Cranfield University (UK). However, in order to work with substances representing a broader range of physical-chemical properties, 5 more PPCPs were spiked into the MBR mixed liquor. Additionally, the biological performance of the system was also tested, considering the operational parameters (pH, temperature and HRT) which varied during the different sampling campaigns. The highest transformation was achieved for ibuprofen (>98%) and naproxen (75 and 91%). On the contrary, carbamazepine elimination was poor (36 and 47%). Different fate was observed depending on the sampling period in the case of diclofenac, sulfamethoxazole and erythromycin since their elimination steadily increased. Therefore, a combination of acidic pH values (measured during the third sampling period), warm temperatures and a prolonged period of operation with a continuous input of PPCPs seemed to be the optimum conditions to maximize PPCPs removal. However, a complete depletion of the micropollutant content in sewage was never achieved. Analysis of the mixed liquor supernatant showed lower concentrations of galaxolide and diclofenac compared with the produced permeates. Therefore, additional data gathered from the operation of the MBR operated in Silvouta was used in order to confirm this behaviour. MBR performance removing PPCPs from the liquid phase was not dependant on membrane material or configuration at any extent, whereas in the case of musk fragrances and diclofenac, the filtration step seemed to contribute to increase their concentration in permeates thus reducing their overall elimination from the liquid phase. With the different works carried out and reported along the present doctoral thesis, the knowledge about some of the key aspects of the use of MBRs for PPCPs removal has been considerably enhanced. Therefore it is considered that the obtained knowledge, summarised along the present section, will make easier the decision of implementing MBR or CAS processes in order to treat different types of wastewaters, always considering their micropollutants content. In case a MBR is the technology of choice, this research also permits to decide the optimum parameters and operational strategies in order to maximize their efficiency in terms of PPCPs removal.