Membrane Bioreactor (mbr) as an Advanced Wastewater Treatment Technology
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TRICHLUCTHUADAT LIEN 1, TRICHLUCTHUADAT LIEN 1, TRICHLUCTHUADAT LIEN 1, TRICHLUCTHUADAT LIEN 1
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Removal of Hormones Estrogenic substances have been identified and quantified in a wide var- iety of environments associated with industrial and municipal effluents, as well as urban and agricultural runoffs [185–191]. Negative adverse health effects on aquatic organisms which could be attributed to endocrine disrupt- ing compounds (EDCs) are reported by several authors [192, 193]. EDCs are substances that interfere with the hormone system of animals and human be- ings. When absorbed into the body, they either mimic or block hormones and disrupt the body’s normal functions. This disruption can happen through altering normal hormone levels, halting or stimulating their production, or changing the way they travel through the body, thus affecting the functions that these hormones control. According to a description by the European Commission (CEC 1999), an endocrine disruptor is “an exogenous substance or mixture that alters function(s) of endocrine system and consequently causes adverse health effects in an intact organism, or its progeny, or (sub) populations”. Two different classes of substances causing endocrine disrup- tion can be identified: natural substances, including natural sexual hormones (estrogens, progesterone and testosterone) and phytoestrogens (chemicals produced by plants that act like estrogens in animal cells and bodies), xeno- biotic substances, including synthetic hormones as the contraceptive 17α- ethinylestradiole (EE2) as well as man-made chemicals and their by-products (e.g., pesticides, cleaning agents, flame retardants, etc.). Sumpter et al. [194] documented feminization of male and sexually imma- ture fish caused by stimulation of vitellogenin synthesis in male fish by EDCs. Guillette et al. [195] noted changes in gonads of alligators. Several studies confirm that human estrogens, mainly 17β-estradiol (E2), estrone (E1) and 78 J. Radjenovi´c et al. EE2 are responsible for a significant part of the endocrine-disrupting effects seen in the aquatic environment [185, 196]. They are known to have a strong biological impact already at very low concentrations of 0.1–0.5 ng L –1 [197]. Xenoestrogens like nonylphenol (NP) and bisphenol-A (BPA) are not as es- trogenically active as for example E2, but due to their widespread production, they can be found in the environment at much higher concentrations [185]. Human excretion is thought to be the principal source of estrogens and pro- gesterone. Although they undergo various transformations in liver of humans and mammalians (oxidation, deoxydation, hydroxylation and methylation), es- trogens are principally excreted as inactive polar conjugates of glucuronide acid and sulphate [198]. However, these conjugates can be cleaved in WWTP due to the presence of β-glucoronidase and arylsulfatase enzymes of a bacte- rial sludge, and they reform original compounds [186, 199, 200]. Therefore the estrogenicity of the effluent is greatly increased, since the estrogenic potentials of conjugated forms of estrogens are clearly much lower [201]. Since EDCs are suspected to enter rivers, streams, and surface water through the effluents of WWTPs, the elimination of these substances in these treatment plants is of elementary interest. Different processes of varying efficiency are applied. Regarding conventional wastewater treatment, some authors consider that in current European activated sludge treatment plants with a HRT no greater than 14 h, the elimination of estrogens and proges- terons present in the influent is not complete [186, 199, 202–204]. However, results obtained by now are very diverse, and efficiencies over 90% have been reported for E1, E2, and EE2 in various municipal WWTPs [151, 184, 185, 205–208]. Slightly lower removal of E1 is frequently detected in WWTPs, which is probably due to the conversion of E2 to E1 during the treatment and the cleavage of glucuronides [186, 209]. Advanced water-purification tech- niques like UV radiation, ozonization, or activated charcoal may significantly improve the removal of endocrine disrupters, but these techniques are not broadly applied because of their high cost. Membrane bioreactor technology is a possibility to enhance the removal of EDCs. Factors like high sludge ages and low organic loads could not yet be cor- related to improved degradation capacity, although biomass compositions might influence hormone removal in MBRs [210]. Servos et al. [201] found that EE2 was not degraded under non-nitrifying conditions, while nitrifying sludge could oxidize it to more hydrophobic compound. Vader et al. [201] also showed that the EE2 degradation capability of sludge was associated to the nitrifying activity. This phenomenon is probably a reflection of im- proved biological diversity and growth conditions in these systems resulting in increased biological transformations. Synthetic estrogens in general ex- hibit greater resistance to the activated sludge process [211]. Besides aerobic biodegradation, sorption is also hypothesized as one of the main mechanisms for the elimination of hormones in the aquatic en- Membrane Bioreactor (MBR) as an Advanced Wastewater Treatment Technology 79 vironment. However, there is no clear agreement about its relative impor- tance [211]. Although E1 and E2 are relatively water soluble, a significant frac- tion can be associated with organic particles or colloids in the treatment sys- tems, potentially influencing their degradation and ultimate fate [56]. Clara et al. [212] investigated sorption behavior of BPA, E2, and EE2. High adsorp- tion potential to sewage sludge could be observed for these substances, with no saturation levels detected. However, at high pH values typical for WWTPs (due to the application limestone or milk of lime for sludge conditioning) it was observed a release of the adsorbed fractions of all three investigated sub- stances. Several researchers have shown that higher pH values (pH > 11) led to an almost complete desorption of EDCs (ex. BPA), which generally exhibits phenolic character [212, 213]. Also, in the experiments of Reddy et al. [200] acidic pH showed to be the best method in preventing dissociation of the steroid conjugates to free steroids. In a study by Joss et al. [205], the MBR elimination of natural estrogens E1 and E2 was seen to be higher than in the CAS sludge by the factor of 2– 3. Higher removal rates in MBR compared to CAS could be explained by the smaller flock size of MBR sludge: it was measured to be 10–100 µm for MBR flocks and 100–500 µm for CAS [56]. The thickness of the boundary layer is estimated to be 10–20 µm for MBR and 20–30 µm for CAS flocks [205]. Although there is no information available as to where various estrogen degradations take place throughout the flock, on the flock surface (e.g., on the outer biofilm layer) or in the bulk medium (e.g., catalyzed by extracellu- lar enzymes), it is considered that the size of flocks may also contribute to degradation activity of the sludge. Among natural hormones, E2 is considered as highly biodegradable, while EE2 is slowly biodegradable [186, 214]. The removal of E2 was always recorded to be high regardless of pH [177]. In another study, the same authors found pH conditions irrelevant for the sorption of E1, E2, EE2, and BPA [170]. These estrogens do not have hydrophilic functional groups and therefore the decrease in pH has no effect on their water–sludge partition coefficient (K p ) [170]. A linear relationship was found between their log K p and log- arithm of K ow (octanol-water partition coefficient, log K ow ) values, which means that adsorption of compounds increases linearly with an increase in its hydrophobicity [170]. Generally, the micro and ultrafiltration membranes do not display a bar- rier effect to hormones, but compared to conventional secondary and tertiary systems, a high removal can be expected due to full particle retention pro- moting the adsorption onto the sludge flocks. Schäfer et al. [213] showed that tải về 0.95 Mb. Chia sẻ với bạn bè của bạn:
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