Elimination of Micropollutants during Post-Treatment of HospitalWastewater with Powdered Activated Carbon, Ozone, and UVLubomira Kovalova,† Hansruedi Siegrist,† Urs von Gunten,†,‡ Jakob Eugster,† Martina Hagenbuch,†
Anita Wittmer,† Ruedi Moser,§ and Christa S. McArdell*,†
†Eawag, Swiss Federal Institute of Aquatic Science and Technology, CH-8600 Duebendorf, Switzerland‡School of Architecture, Civil and Environmental Engineering (ENAC), Ecole Polytechnique Federale de Lausanne (EPFL),CH-1015 Lausanne, Switzerland§Hunziker Betatech AG, CH-8411 Winterthur, Switzerland
*S Supporting Information
ABSTRACT: A pilot-scale hospital wastewater treatment plant consisting ofa primary clarifier, membrane bioreactor, and five post-treatmenttechnologies including ozone (O3), O3/H2O2, powdered activated carbon(PAC), and low pressure UV light with and without TiO2 was operated totest the elimination efficiencies for 56 micropollutants. The extent of theelimination of the selected micropollutants (pharmaceuticals, metabolitesand industrial chemicals) was successfully correlated to physical-chemicalproperties or molecular structure. By mass loading, 95% of all measuredmicropollutants in the biologically treated hospital wastewater feeding thepost-treatments consisted of iodinated contrast media (ICM). Theelimination of ICM by the tested post-treatment technologies was 50−65% when using 1.08 g O3/gDOC, 23 mg/L PAC, or a UV dose of 2400 J/m2 (254 nm). For the total load of analyzed pharmaceuticals and metabolitesexcluding ICM the elimination by ozonation, PAC, and UV at the same conditions was 90%, 86%, and 33%, respectively. Thus,the majority of analyzed substances can be efficiently eliminated by ozonation (which also provides disinfection) or PAC (whichprovides micropollutants removal, not only transformation). Some micropollutants recalcitrant to those two post-treatments,such as the ICM diatrizoate, can be substantially removed only by high doses of UV (96% at 7200 J/m2). The tested combinedtreatments (O3/H2O2 and UV/TiO2) did not improve the elimination compared to the single treatments (O3 and UV).
1. INTRODUCTION
Hospital wastewater contains pharmaceuticals and disinfec-tants1−4 as well as pathogens and antibiotic resistant bacteria5−7
in high concentrations. Some of the environmentally relevantgroups of pharmaceuticals used in relatively high amounts inhospitals are antibiotics which contribute to the spread ofantibiotic resistance,2 cytostatics which are potentially eco-toxic,8−10 and X-ray contrast media which are used in highquantities,11,12 but many more pharmaceuticals are present forwhich not sufficient information is available for a riskassessment.Treatment of the hospital wastewater at the source has
advantages of avoiding dilution due to mixing with themunicipal sewage, avoiding losses into environment due toleaks and combined sewer overflows, and preventing pathogensand antibiotic multiresistant bacteria to enter the environ-ment.11,13,14 Biological treatment of hospital wastewater on siteshowed to be feasible and efficient to remove dissolved organiccarbon (DOC) and chemical oxygen demand (COD)significantly to >90% as well as nitrogen (>99% N-eliminationby denitrification).11,15 Nevertheless, any biological treatment(conventional activated sludge or membrane bioreactor, MBR)
is typically insufficient to eliminate the majority of micro-pollutants.11,16,17 Therefore, additional oxidative or otherphysical-chemical post-treatments are necessary. Good resultsin municipal wastewater treatment were obtained withozone,18−22 activated carbon,23−26 UV/H2O2,
27,28 and nano-filtration or reverse osmosis.29−33
Five post-treatment technologies, namely, ozonation (andO3/H2O2), UV in the absence and presence of TiO2 andpowdered activated carbon (PAC) were selected in this studyto estimate the efficiency for hospital wastewater treatment. 56target analytes including pharmaceuticals, human metabolites,and industrial chemicals were chosen as elucidated in our earlierpublication on their behavior during treatment with MBR. 11
The main objectives of our study were to (1) determineelimination efficiencies for 56 organic micropollutants by theselected post-treatment technologies, treating an effluent of anMBR fed with raw hospital wastewater, and compare them to
Received: February 14, 2013Revised: June 7, 2013Accepted: June 11, 2013Published: June 11, 2013
municipal wastewater treatment; to (2) compare and predictthe elimination efficiency of the selected micropollutants by theselected post-treatment technologies; and to (3) evaluatefeasibility, energy consumption, and costs.
2. MATERIALS AND METHODS2.1. Wastewater Treatment Setup. A pilot plant was
installed at the cantonal hospital in Baden, Switzerland (346beds, water consumption 673 L per bed and day). First, thewastewater was treated by a MBR (Picatech Huber AG, Kriens,Switzerland), as described elsewhere.11 MBR-permeate waterquality was as follows: 6−8 mg/L DOC, 30 mg/L dissolvedCOD, 3.7 mg NO3−N/L, pH 8.1−8.5, temperature 27−28 °C,30−40 μg/L bromide. The MBR-permeate was collected in a 1m3 tank and disconnected from the MBR for individual post-treatment experiments with activated carbon, ozone, or UV (SI,Figure S3). Experiments were performed in two replicates withdifferent MBR permeate composition. The removal efficiencywas calculated as the average of the two replicate experiments.2.2. Activated Carbon Treatment. Post-treatment of the
MBR-permeate with PAC (Norit SAE Super, surface area 1300m2/g, particle size d5015 μm, pHPZC 9.8, Norit, Amersfoort, TheNetherlands) took place in a 180-L reactor. The constantMBR-permeate flow of 180 L/day resulted in a hydraulicresidence time of 1 day. The effluent of the PAC reactor wasfiltered through a flat sheet ultrafiltration membrane (MartinSystem, siClaro FM 611, pore size 0.04 μm, and membranesurface 6.2 m2). 90 L/day of filtered PAC effluent was removed,while also 90 L/day of loaded PAC in solution was withdrawn(5 L every 80 min) to keep the PAC retention time at 2 days (afew experiments run with 5 days of PAC retention time gavesimilar results). For the required PAC doses of 10, 20, and 40mg/L, the start-up concentration in the PAC reactor was 20,40, and 80 mg/L for a PAC retention time of 2 days. Thedosing frequency of 150 mg PAC from a stock solution of 2 g/L was adjusted to 120, 60, or 30 min, to achieve the threedifferent carbon doses. The reactor was emptied and cleanedbefore switching to the next carbon concentration. The actualPAC concentration in the PAC-reactor as well as temperature,pH, and DOC were regularly measured and recorded (TableS7). The influent of the first replicate (Monday-Friday) wassampled before the start of the experiment and after 2 and 4days (concentrations stayed constant over the time, the averagetaken as influent). The corresponding effluent was sampledafter 2 and 4 days (average taken as effluent). For the secondreplicate (Friday-Monday) the influent was sampled before thestart of the experiment and after 3 days (average taken asinfluent). The corresponding effluent was sampled after 3 days.Equilibrium was found to be reached after 2 days. Campaignswere run with 8 ± 4 mg/L PAC, 23 ± 7 mg/L PAC, and 43 ±14 mg/L PAC.2.3. Ozone Treatment. A counter current bubble column
of 2 m height and 4.5 L volume was used for the post-treatmentof the MBR-permeate with ozone. The ozone was producedfrom oxygen in the air with a Chemodata 1.0 g/Hmax ozonegenerator (Chemonorm AG, Altendorf, Switzerland), with anozone production of 0.1−0.15 g O3/h. The ozone concen-tration was regulated by adjustment of the wastewater flow.The MBR-permeate flows of 12−23 L/h resulted in therequired ozone doses in the column (hydraulic residence timeof 12−23 min). An ozone analyzer BMT 964 (BMTMesstechnik Gmbh, Stahnsdorf, Germany) provided onlinemeasurement. The transferred ozone dose was calculated from
the difference of the applied dose and the ozone in the off-gas(ozone transfer efficiency 66−78%). The three tested trans-ferred ozone doses of 4.2, 5.8, and 7 mg/L resulted in specificozone doses of 0.64, 0.89, and 1.08 gO3/gDOC (Table S8).Additional experiments were conducted with the advancedoxidation process O3/H2O2. For these experiments, the MBR-permeate was pretreated with 1.2 gO3/gDOC, followed byaddition of 2.5 mg/L H2O2 and 0.8 gO3/gDOC (H2O2:O3molar ratio approximately 1:2). In a parallel run, the samepretreated wastewater was treated with 0.8 gO3/gDOC withoutan addition of H2O2.
2.4. UV Treatment. A commercially available setup fromUBE Industries (Model 3 m3/h, UBE corporation Europe, S.A.,Castellon, Spain) with a volume of 3.3 L and a water flow of 0.6m3/h was used for the post-treatment of the MBR-permeate byUV (Table S9). The reaction column was positioned around alow pressure UV lamp (254 nm, 220 V, 100−400 W overallenergy consumption including the 40 W lamp powerconsumption). A nominal fluence rate of 10 mW/cm2 and ahydraulic residence time of 20 s resulted in a nominal fluence of2000 J/m2. The effective fluence rate of UV treatment wascalculated from the observed elimination rates of twocompounds (diclofenac and iopromide) for which fluence-based rate constants are available from the literature34 (SI, ppS21−S22). An effective fluence rate of 4 mW/cm2 wascalculated for the reactor with hospital wastewater (39.8 W/m2 for diclofenac, 40.5 W/m2 for iopromide). The effectivefluence rate is lower by a factor of 2.5 than the nominal fluencerate value (10 mW/cm2), probably due to the absorption oflight by the wastewater in which the optical density is notnegligible. The calculated effective fluence rate corresponds to afluence of 800 J/m2 for a single treatment. Typically 3treatment cycles were applied with a cumulative dose of 2400J/m2.The UV/TiO2 experiments were performed with the same
setup from UBE Industries after inserting four conicalcartridges around the UV lamp consisting of a photocatalyticfiber (titanium-dispersed silica-based fiber with a sinteredanatase-TiO2 layer on the surface35). The column volume ofthis setup was 3.0 L with a hydraulic residence time of 18 s. Thecalculated fluence rate with the photocatalytic fiber was 1.7mW/cm2 (fluence of 306 J/m2 after a single treatment), lessthan half of the values obtained for the reactor without thephotocatalytic fibers.The MBR-permeate was prefiltered by a polypropylene filter
(30 μm, Positech, Switzerland) prior to UV treatment to retainparticles. Further, two prefilters with pore sizes of 25 and 5 μm,located just before the UV reactor, were part of the commercialsetup. Because of the short hydraulic residence time of thewastewater in the UV system, the experiments were conductedas follows. 200 L MBR permeate was treated with UV andcollected into another tank (called “first run”). Then thecollected water was recirculated through the UV reactor for 180min, which is equivalent to 9 cycles, as one cycle of 200 L at aflow rate of 600 L/h takes 20 min. The samples were taken atthe start of each experiment and after 1, 2, 3, 6, and 9 cycles.The removal efficiencies were calculated for the first run andafter 3 and 9 cycles.
2.5. Sample Preparation and Analysis of Micro-pollutants. A detailed description of the sample preparationand analysis can be found elsewhere.11 Shortly, samples werefiltered with a 0.7 μm GF/F glass−fiber filter (Whatman,Dassel, Germany) and further through a 0.2 μm regenerated
cellulose filter (Sartorius AG, Gottingen, Germany), spikedwith isotope labeled internal standards and analyzed by onlineSPE-HPLC-MS/MS with a Triple Quadrupol mass spectrom-eter (TSQ Quantum Ultra, Thermo Fisher Scientific). Limits of
quantification are listed in the Supporting Information (SI,
Table S4).2.6. Other Measurements. The adsorbable organic
halogen compounds (AOX) of three grab samples (effluents
of MBR, 43 mg/L PAC, and 1.08 gO3/gDOC) weredetermined coulometrically after incineration according toDIN EN 1485-H14 and DIN EN ISO 9562 (by Bachema AG,Schlieren, Switzerland).Chemical oxygen demand (COD) was measured by a Dr.
Lange test kit (Hach Lange GmbH, Dusseldorf, Germany).Dissolved organic carbon (DOC) was determined using aShimadzu TOC analyzer. Bromate in the samples of high ozonedosages was measured by ion chromatography.36 Forgadolinium and platinum measurements, samples were acidifiedwith 60% nitric acid (1%, v/v) and analyzed by ICP-MS(Thermo Scientific, Element 2).
3. RESULTS AND DISCUSSION3.1. Occurrence and Post-Treatment Elimination
Efficiencies of Target Compounds. From the 67 targetanalytes (SI, Tables S1 and S2), 9 were not detected in the rawhospital wastewater prior to the MBR treatment (SI, Table S5),while cilastatin and paracetamol were removed during MBRtreatment to nondetectable residues.11 The concentrations ofthe remaining 56 micropollutants after MBR treatment arelisted in Table 1, together with elimination efficiencies for thethree post-treatments. Even after biological treatment, concen-trations are very high, e.g. at 3.4 mg/L for the iodinatedcontrast medium (ICM) iopamidol, and for several othercompounds higher than 10 μg/L (other ICM, 4/5 methyl-benzotriazole, benzotriazole, the metabolites of aminopyrine 4-AAA and 4-AA, ciprofloxacin, gabapentin). In total, ICM occurwith a concentration of 4.6 mg/L, from which a C-concentration of 1.2 mg/L was calculated (15−20% fractionof DOC). Elimination efficiencies were evaluated for the threedifferent post-treatments with PAC, ozone, and UV, each withthree different dosages. Most compounds are eliminated wellwith PAC and ozone, while elimination is much lower with UV.Due to low concentration or low frequency of detection, onlypartial information on elimination efficiencies was obtained forbezafibrate, ifosfamide, morphine, oseltamivir, oseltamivircarboxylate, ritonavir, sulfadiazine, thiopental, trimethoprim,and verapamil. Based on the results, we selected the doses 23mg/L, 1.08 gO3/gDOC, and 2400 J/m2 for PAC, ozone, andUV, respectively, for the further discussions in the followingsections, because they showed relatively good abatement ofmicropollutants and are reasonable in the context of cost andother constraints (see section 3.5.2). Elimination efficiencieswith PAC and ozone normalized to DOC compare very wellwith efficiencies in municipal wastewater treatment, as shown inTables S10 and S11 in a literature overview.3.2. Prediction of the Micropollutants’ Elimination
from Physical-Chemical Properties. 3.2.1. Ozone. Thereactivity of micropollutants with ozone at a certain pH(apparent second order rate constant, k′′O3,app) can beestimated based on their molecular structures and pKa valuesand roughly divided into three classes:38 (i) low or no reactivitywith ozone (k′′O3,app < ∼10 M−1 s−1): nitro group, amides, N-phenyl amides; (ii) intermediate reactivity with ozone (∼10 <k′′O3,app < ∼1 × 105 M−1 s−1): primary amines, protonatedsecondary amines, thioethers, anisols, quinones, naphthalenes,nondissociated phenols; and (iii) high reactivity with ozone(k′′O3,app > ∼1 × 105 M−1 s−1): olefins, dissociated phenols(phenolates), anilines, thiophenols, deprotonated thiols, andtertiary amines. The reactivity of ozone-reactive moieties(ORMs) increases or decreases by neighboring electrondonating or electron withdrawing groups, respectively.
Molecules without ORMs are typically oxidized by hydroxylradicals (•OH) which are formed as secondary oxidants duringozonation.38,39 In a recent study we showed that it is possible topredict the abatement of ozone-resistant micropollutants bymeasuring •OH exposures.40
This theoretical approach is well applicable to predictelimination. O3 reactivity estimations are described in TableS6 and presented in comparison with the achieved eliminationsin Table S11. All the compounds that showed ≥97% or higherelimination from the hospital wastewater at an ozone dose of1.08 gO3/gDOC contained moieties with high or intermediatereactivity with ozone (Tables S11-A and S11-B). Although therange for intermediate reactivity is relatively large with its 5orders of magnitude, only <20% of the target micropollutantsbelong to this group. For compounds without ORMs, reactivitywith •OH is typically high (second-order rate constant typicallyin the range of 109−1010 M−1 s−1), although the •OHconcentration is very low compared to the ozone concen-tration, and most of the •OH are scavenged by the matrix priorto reacting with micropollutants.38,39
Further to the correlation of the elimination to the molecularstructure, a comparison to known ozonation rate constantsfrom the literature was performed (Figure 1, Table S11). For
compounds for which ozone rate constants were not available(e.g., ritonavir), the reactivity range with ozone (none/low,intermediate and high reactivity) was estimated based on thestructure (Table S6) and indicated in Figure 1 with opensymbols at an arbitrary log k′′O3 within the appropriatereactivity range. The compounds with apparent ozonation rateconstants at pH 8−8.5 >105 M−1 s−1 (carbamazepine,ciprofloxacin, clarithromycin, diclofenac, furosemide, lidocaine,mefenamic acid, ranitidine, sulfamethoxazole, and tramadol)show removal above 97%. As the ozone rate constants decreasea decrease in transformation is observed.
3.2.2. PAC. Adsorption is depicted relative to the Dow of theadsorbates (micropollutants) in Figure 2. As the adsorption to
Figure 1. Elimination of micropollutants during ozonation of hospitalwastewater (1.08 gO3/gDOC, 7 mg/L O3, pH 8.5) versus ozonationrate constants from literature (M−1 s−1, Table S11) or estimatedreactivity at pH 8−8.5 (Table S6) if kO3 is not available. Arrowsrepresent log kO3 < 1 or 10, respectively. See Table 1 formicropollutant numbering. Micropollutants for which quantificationof elimination was limited by LOQ (2, 6, 8, 10, 19, 35, 38, 39, 41, 43,47, 48, 50, 51, 53, and 56) are not included.
activated carbon depends on multiple properties of micro-pollutants and carbon, prediction for adsorption is not asaccurate as for ozonation (Table S10). Interactions between theadsorbent (PAC) and adsorbate are controlled by nonspecificdispersive interactions. In case of charged adsorbates alsoelectrostatic interactions with a charged adsorbent surface canbe important. Properties of an adsorbate that mainly influencethe adsorption process are its octanol−water distributioncoefficient (Kow, or Dow when accounting for acid−basespeciation), pKa, molecular size, aromaticity versus aliphaticity,and presence of specific functional groups. Properties of anadsorbent that mainly influence the adsorption process are itssurface area, pore size and texture, surface chemistry (functionalgroups, point of zero charge pHPZC), and mineral mattercontent.41,42 Despite complex interactions, all compounds inour data set with logDow > 2 were removed completely (Figure2) or to below their LOQs (Table S10−C). Furthermore, foruncharged compounds with logDow ∼ 2 and smaller, adecreasing trend in adsorption efficiency with decreasinglogDow values was observed. A good estimation for unchargedcompounds was possible, as no electrostatic interactions withthe functional groups on the surface of the activated carbonoccur and only dispersive forces are present. As also shown inFigure 2, a simple estimation from logDow values for chargedpolar compounds does not work and leads to an under-estimation of elimination efficiencies for many compounds(e.g., ciprofloxacin, furosemide, atenolol acid). At the workingpH 8.8, the surface of the utilized activated carbon with pHPZC9.8 is predominantly positively charged. This may explain thestrong sorption of some compounds with anionic functionalgroups (e.g., furosemide); others, however, show much lowerelimination (e.g., sulfamethoxazole). The same observation ismade for polar zwitterionic compounds. For polar cationiccompounds, a general statement is difficult since onlycompounds with logDow ∼ 0 and higher were analyzed forwhich a certain adsorption is expected. Although the pHPZC ofthe carbon is known, the actual functional groups responsiblefor electrostatic interaction are not known, and this can be apossible explanation for the varying behavior of individualcharged compounds.
3.2.3. UV. No quantitative structure activity relationship(QSAR) to the fluence-based photolysis rate constants ofhomologous compounds is known. Therefore, a prediction ofthe elimination efficiency by UV from molecular structures orphysical chemical properties is currently not possible.Elimination of a contaminant by photolysis was shown to bepredictable only when experimentally determined fluence-basedrate constants are available.34
3.3. Elimination Efficiencies of Different Classes ofAnalytes. 3.3.1. Iodinated X-ray Contrast Media. Within thegroup of ICM, different elimination behavior of unchargedICM (iomeprol, iopamidol, and iopromide) compared to thecharged diatrizoate and ioxitalamic acid were observed.Adsorption of ICM to PAC is expected to be low (low logKowvalues, SI, Table S6). Charged ICM are even more polar(logDow values in Table S3) and are eliminated significantly lessthan uncharged ICM. For ICM, elimination by ozonation iscomparable or less efficient than by adsorption (Figure 3 shows
a comparison of the removal efficiencies of ozonation andPAC). ICM have second-order rate constants <1 M−1 s−1, andelimination is dominated by oxidation by •OH. 39 Concen-trations in the μg/L to low mg/L range could still be found inthe effluent. Removal by UV for uncharged ICM is relativlyhigh, comparable to removal by high PAC doses. For the twocharged ICM, only UV proved to be efficient, at theinvestigated doses for each treatment. The maximum testedUV dose (9 cycles, fluence 7200 J/m2), though energeticallyvery demanding, can remove around 95% of diatrizoate andioxitalamic acid, although the remaining concentrations in theUV effluent of up to 20 μg/L are still considerable.
3.3.2. Corrosion Inhibitors. Methylbenzotriazole and benzo-triazole are relatively well eliminated by adsorption to PAC aswell as by ozonation. Nevertheless, concentrations of 200−1000 ng/L still remain even after application of the highesttested doses due to high inflow concentrations of thesecompounds. Both compounds are moderately polar withmethylbenzotriazole slightly more hydrophobic than benzo-triazole (SI, Tables S3 and S6), which resulted in 97 and 94%elimination at the highest tested PAC dose for the twocompounds, respectively. Ozone rate constants of the twostructurally similar compounds are in the intermediate range,with a higher value for methylbenzotriazole (104 M−1 s−1 for
Figure 2. Removal of micropollutants from hospital wastewater (23 ±7 mg/L PAC, pH 8.8) as a function of logDow at pH 9 (Table S3). SeeTable 1 for micropollutant numbering. Micropollutants for whichquantification of the elimination was limited by LOQ (6, 10, 19, 24,25, 35, 38, 43, 46, 51, 53, and 56) are not included.
Figure 3. Comparison of micropollutants’ elimination by ozone andPAC. Ozone dose: 7 mg/L (1.08 gO3/gDOC, pH 8.5); PAC dose: 23± 7 mg/L, pH 8.8. See Table 1 for micropollutant numbering.
the anion,38 pKa 8.9) due to the presence of an electrondonating methyl group that increases the reactivity comparedto benzotriazole (2650 M−1 s−1 for the anion,38 pKa 8.6). Nosignificant elimination by UV was observed up to an applieddose of 7200 J/m2.3.3.3. Metamizole Metabolites. Four human metabolites of
the analgesic metamizole (aminopyrine) were abundant in theMBR-treated hospital wastewater: 4-methylaminoantipyrine (4-MAA, hydrolysis product of metamizole), 4-aminoantipyrine(4-AA, second generation TP), 4-acetamidoantipyrine (4-AAA,third generation TP), and 4-formylaminoantipyrine (4-FAA,third generation TP). These moderately hydrophilic pyrazolonederivates are well removed by ozonation due to an olefin group,which has a high ozone reactivity. Only partial eliminationcould be achieved by adsorption to PAC and UV treatment.This resulted in effluent concentrations between ∼500 and3000 ng/L after treatment with PAC and up to above 20 μg/Lfor 4-AA after the treatment with the highest tested UV dose(7200 J/m2).3.3.4. Cyclophosphamide. Cyclophosphamide is a polar
uncharged cytostatic with a similar persistence to PAC andozone treatment as uncharged ICM. The concentration ofcyclophosphamide is about 3 orders of magnitude lower thanthe concentrations of ICM; however, its toxicity is considerablyhigher. With a logKow value ∼0, no active sites for ozone attack,and no photochemical removal, the resulting final concen-tration of cyclophosphamide is often >100 ng/L, despite itsrelatively low influent concentrations. The structurally relatedcytostatic ifosfamide shows similar removal trends, although itwas rarely detected in the wastewater.3.3.5. Gabapentin and Levetiracetam. The two structurally
unrelated compounds are both antiepileptic preparationsoccurring in raw hospital wastewaters in μg/L levels.11
Gabapentin is a very polar carboxylic acid (logDow −1.6 atpH 9) that shows only poor adsorption. At pH 8−8.5 itcontains a protonated primary amine, which shows inter-mediate reactivity with ozone. Levetiracetam is a polar anduncharged compound with a higher log Dow than gabapentin,which results in a slightly higher PAC adsorption compared togabapentin (Figure 3). Levetiracetam has no ozone-reactivemoieties (two amides), which leads to only a partial removaldue to reaction with •OH. Therefore, the two antiepileptics arenot removed satisfactorily by any of the tested post-treatmenttechnologies. The remaining concentrations are >100 ng/L andin the case of gabapentin, μg/L levels (Table 1).3.3.6. Fluconazole, Metronidazole, and Sulfonamides.
Anti-infectives discussed here in detail do not include antiviralsor antibiotics which are either not frequently detected in thepost-treatment influent (e.g., ritonavir, roxithromycin) or arewell removed by both PAC and ozonation (e.g., ciprofloxacin,clarithromycin). The moderately hydrophilic antimycoticfluconazole, a triazole derivative, has no active sites for ozoneattack and therefore exhibits similar elimination efficiency byozonation as ICM due to only •OH oxidation. Elimination byadsorption to PAC is better, 95% at a PAC dose of 23 mg/L(Figure 3). Metronidazole is polar and uncharged, similar tolevetiracetam, which resulted in a comparable eliminationefficiency (∼75%) with the highest PAC dose of 43 mg/L. Itcontains a nitro group, which deactivates the molecule forozone attack, resulting in elimination efficiencies comparable tocompounds which were eliminated only by •OH. No significantelimination by UV was observed for fluconazole andmetronidazole. The sulfonamides sulfadiazine and sulfapyridine
were found in the influent of the post-treatments only in lowconcentrations. Sulfamethoxazole was always detected in therange of 1−10 μg/L in the influent, and mostly above 100 ng/Lin the effluent of the three post-treatments. The threesulfonamides differ in their pKa values and slightly in polarity(SI, Table S3), which influences their adsorption behavior. Atthe working pH 8.8, the least charged and at the same time themost hydrophobic of the three − sulfapyridine − shows thehighest elimination efficiency by PAC (Table 1). Due to theaniline functional group, the three sulfonamides react very fastwith ozone.20 As most of the attack occurs at the aniline ring,N4-acetylated metabolites of sulfonamides, e.g. N4-acetylsulfa-methoxazole, exhibit lower reactivity with ozone due todeactivation due to the presence of the N-acetyl instead of aNH2 functional group. The three sulfonamides are also to alower extent sensitive to UV treatment.
3.3.7. Gadolinium and Platinum. Additionally to organicmicropollutants, the two metals were measured. They occur inraw wastewater of the studied hospital mainly due togadolinium complexes used as magnetic resonance imaging(MRI) contrast media and platinum containing cytostatics (seeSI, Figures S1 and S2), in average concentrations of 13.4 μg/Land 220 ng/L, respectively. After the MBR treatment, averageconcentrations of 12.9 μg/L for Gd and 130 ng/L for Pt werefound. Platinum-containing cytostatics were previously re-ported to sorb to 60% to activated sludge of a hospitalwastewater.43 Metals are principally not removed by ozonationor photolysis. The results for the PAC dose of 10 mg/L and theozone dose of 0.64 gO3/gDOC show 47% elimination ofplatinum by PAC but no significant elimination neither ofplatinum by ozonation nor of gadolinium by any of thosetreatments. Gadolinium-containing contrast media are there-fore very stable against oxidation and are not adsorbing to PAC.
3.4. Options To Enhance the Post-Treatment Steps.3.4.1. Ozone. Further experiments were performed by addinghydrogen peroxide. This accelerates ozone decomposition andpartially increases the •OH exposure in the system.38 Thus, theelimination of micropollutants with low reactivity with ozonemay be enhanced.44 To minimize the consumption of ozone bythe background organic matter, the MBR-permeate waspretreated with ozone prior to the O3/H2O2 experiment.Results from this first treatment with 1.2 gO3/gDOC agreedwell with the previously shown data. Micropollutant eliminationafter the post-ozonation with hydrogen peroxide was notsignificantly improved (±10%) compared to ozone alone (SI,Figure S4). This is in agreement with previous studies, whereonly little improvement was found for O3/H2O2 compared toO3 alone, especially in waters with high DOC.44
3.4.2. PAC. Elimination efficiencies of the PAC process couldpotentially be improved if it is combined with nanofiltration(instead of ultrafiltration), which would additionally retainmicropollutants by the membrane.45 However, due to its highmicropollutant concentration, the nanofiltration concentratehas to be treated or otherwise taken care of.46 Alternatively,loaded activated carbon from PAC post-treatment can bepumped to the MBR for recycling and thereby significantlyincrease the elimination in the bioreactor.23 No additionalexperiments on any of those processes were performed in thisstudy.
3.4.3. UV. The photocatalytic treatment of hospital waste-water MBR permeate by UV with a TiO2 photocatalyst showedlower elimination than UV treatment alone (SI, Table S12).With the photocatalytic TiO2 fibers, there might be an
additional indirect phototransformation; however, the screen-ing and absorption of the light by the fibers proved to be higherthan the possible indirect phototransformation. The calculatedfluence rate with the photocatalytic fiber was only 1.7 mW/cm2,less than half of the value for the reactor without thephotocatalytic fibers (4.0 mW/cm2) (SI, pp S21−S22).Heterogeneous advanced oxidation by UV/TiO2 involvesdirect and indirect photolysis. A micropollutant is adsorbedand attacked by •OH formed on the TiO2 surface.Unfortunately, the quantum yield for the •OHTiO2 formationis very low, and the whole process could suffer from adisadvantage of this inefficient step.47 Another process that isknown to significantly improve UV treatment of micro-pollutants is addition of hydrogen peroxide. It was shownbefore that the elimination of atenolol, carbamazepine,primidone, and trimethoprim can be significantly improvedby the addition of hydrogen peroxide.27 Using a low pressureUV lamp at 700 J/m2, the elimination of trimethoprim withaddition of 20 mg/L H2O2 was enhanced from <10% to 92%.Good elimination was also shown for sulfamethoxazole,clarithromycin, and lidocaine using a low pressure UV lampwith addition of H2O2.
28
3.5. Overall Comparison of the Three Post-TreatmentTechnologies. 3.5.1. DOC Removal. The difference betweenthe tested technologies in removal of dissolved organic carbon(DOC) is very significant. A decrease of 60% in DOC levelswas observed after the treatment of MBR permeate by 23 mgPAC/L (SI, Table S7). The highest tested dose of 43 mg PAC/L removes around 70% of DOC and improves the wastewaterquality of the MBR permeate from 6.3 mg C/L to 2.0 mg C/L.The decrease in DOC levels after ozonation and UV treatmentis less than 15% at any tested conditions (SI, Tables S8 and S9).While PAC removes DOC efficiently by adsorption, mineral-
ization of dissolved organic matter during ozonation and UVtreatment is small.
3.5.2. Organic Micropollutants. For this comparison, arealistic dose was selected for each treatment: 7 mg/L ozone(1.08 gO3/gDOC ozone; higher doses could produce an excessof bromate); 23 mg/L PAC (upper margin of range commonlyused at full-scale without recycling to the biological reactor 23);and 3 cycles for UV treatment (2400 J/m2; a higher fluence isunrealistic due to high energy consumption, see section 3.5.4).An evaluation of the three tested technologies with regard tomicropollutant removal is summarized in Table 2. Forcomparison, the overall elimination efficiencies of the combinedtreatments with MBR are also shown. Removal efficiencies forthe sum of the load of all target analytes (52−63%) are close toICM removal efficiencies (50−65%) for the three single post-treatments, because the MBR-permeate before the post-treatment consists of over 95% of ICM by load. Thus, it ismore meaningful to evaluate elimination efficiencies insubgroups: 1) iodinated X-ray contrast media (ICM); 2)pharmaceuticals and metabolites excluding ICM; and 3)industrial chemicals.As discussed in 3.3.1, ICM are very difficult to remove by any
of the three tested technologies, although adsorption to PAC(61%) and UV treatment with very high doses (65%) show aslightly better performance than ozonation (50%). Analternative and more efficient solution to remove ICM isprobably by collecting and incinerating the patients’ urine inhospitals and from outpatients.For the subgroup of pharmaceuticals and metabolites
excluding ICM, performance of ozonation (90%) is slightlybetter compared to PAC (86%), while UV (33%) is notsatisfactory. A similar trend and an even stronger differencebetween ozonation (98%), PAC (91%), and UV (6%) is
Table 2. Comparison of MBR and the Three Post-Treatment Technologies (O3, PAC, UV) for Groups of Micropollutants:Elimination of Micropollutant Loads
single treatmentscombined treatments (same doses as for
observed for the subgroup of industrial chemicals. Micro-pollutants of those two subgroups, containing the majority ofanalyzed substances, can thus be treated most efficiently byozonation or PAC. Most of the target micropollutants havesimilar elimination efficiencies by ozonation and PAC (Figure3). Few exceptions are gabapentin, oseltamivir carboxylate,ritalinic acid, sulfadiazine, and sulfamethoxazole − for whichozonation is more efficient, or fluconazole, iopromide,levetiracetam, metronidazole, 4-MAA, and 4-AAA − forwhich PAC is more efficient. The anionic ICM diatrizoateand ioxitalamic acid show unsatisfactory removal by eitherozone or PAC and were the only compounds for which directphototransformation by UV at high doses of 2400 J/m2 turnedout to be most efficient.3.5.3. Energy Consumption. For the analysis of energy
consumption of the different post-treatments, calculations formunicipal wastewater treatment can be used. For ozonation, anenergy consumption of 0.02 kWh/g ozone is needed for thegeneration of ozone from oxygen, including the production andtransport of oxygen.19,48 When using 5−10 mg/L ozone, thetotal energy consumption is 0.1−0.2 kWh/m3. Energyconsumption at the municipal wastewater treatment plantwould rise by about 20−40% when using ozonation. For PACtreatment, the energy consumption at the plant is only around0.05 kWh/m3, so lower than for ozonation.48,49 However, theproduction of PAC is energy intensive, which is reflected in thehigh costs of PAC (2 €/kg PAC). The UV reactor has a totalenergy consumption of 100−400 W, depending on theselection of the pump, including a lamp power consumptionof 40 W. In a single treatment with a flow rate of 1000 L/h theenergy consumption of the photocatalytic reactor is 0.1−0.4kWh/m3 and with nine cycles 0.9−3.6 kWh/m3. Therefore, theenergy consumption in a single UV treatment is in a similarrange as with 7 mg/L ozone (0.12 kWh/m3), however, withmuch lower elimination efficiencies. Even with nine cycles theelimination by UV is lower than with ozone for mostcompounds except for ICM. UV/H2O2 was found to efficientlyoxidize micropollutants, but energy requirements weresubstantially higher compared to ozonation.28,50
3.5.4. Cost. The treatment of hospital wastewater with ozoneor PAC following an MBR would cost 2.90 or 3.20 CHF (2.40or 2.70 €) per m3, respectively (details of the calculation aregiven in Tables S14−S16). In Switzerland, municipal waste-water treatment costs about 2 CHF (1.70 €) per m3 wastewater(median), and with an additional treatment with ozone or PACcosts are estimated to be around 2.20 CHF (1.80 €) per m3.The costs per m3 wastewater for the treatment of hospitalwastewater (i.e., requiring biological treatment followed byeither ozonation or PAC) is slightly higher than for thetreatment of the whole wastewater stream at the centralmunicipal wastewater plant with the same technology. If thehospital has a separate treatment and discharges the treatedwastewater directly to the receiving water, the investment costfor the municipal wastewater treatment will hardly change; onlythe operation cost is reduced, which is in the range of 1−1.50CHF (0.80−1.20 €) per m3 (for BOD, nutrient andmicropollutant removal, depending on the size of the plant).The hospital has the advantage of reusing the treatedwastewater for gardening, which would then additionallyreduce the drinking water cost. Additionally, there is no lossof raw hospital wastewater by combined sewer overflows andexfiltration from sewer systems.
3.5.5. Other Important Aspects. The main advantage ofPAC over ozonation and UV is the “true removal” of chemicalsfrom the wastewater by adsorption to carbon (that can beincinerated after use), rather than transformation to oxidationor photodegradation products. Those transformation productsare often unknown and have a potential to also haveecotoxicological effects.51,52 It is recommended that afteroxidative treatments a sand filter (biofilter) is installed toeliminate well degradable transformation products. It wasshown that sand filtration is an effective barrier to toxicoxidation byproducts originating mainly from the matrix.53,54
Furthermore, ozonation can produce bromate, which is formedin bromide-containing waters and is the ozonation byproduct ofmajor concern.55 In our study, the bromide level in the MBR-permeate was 30−40 μg/L, representative for municipalwastewater treatment plants in Switzerland. Bromate wasnever found in quantifiable concentrations above 1 μg/L evenfor a high ozone dose of 1.08 g ozone/g DOC (7 mg/L ozone),thus it is well below the drinking water standard of 10 μg/L.56
A value to consider to fulfill requirements of municipalwastewater treatment plant effluents is AOX (adsorbableorganic halogen compounds). AOX typically consists ofcompounds formed during chlorination, which are potentiallytoxic. Hospital wastewater, however, is special due to the highconcentrations of nontoxic X-ray contrast media, which containiodine. Organic iodine compounds may account for about 50%of AOX input into municipal wastewater.57 The AOX value,measured in this study only once (SI, Table S13), wasdecreased from 0.56 mg/L in the MBR-permeate better byPAC (around 70%) than by ozonation (40%). The AOX in thefinal effluent after post-treatments, however, can be aboverequirements for wastewater treatment plant effluent (require-ment for Switzerland: 0.08 mg/L).58
Another important issue, specific for hospital wastewaters, ispreventing input of pathogenic or (multi)resistant bacteria andother potentially dangerous microorganisms and parasite eggsfurther into the environment. Even though the MBR is a goodbarrier, a further disinfection step is advisable. For inactivationof pathogens and possibly removal of antibiotic resistance, UVor ozonation is more efficient compared to PAC.
■ ASSOCIATED CONTENT*S Supporting InformationAnalyzed micropollutants, analytical method, gadolinium andplatinum measurements, parameters of the post-treatmentplants, more information on elimination of micropollutantsand comparison with literature, AOX, cost evaluation (TablesS1−S16, Figures S1−S4). This material is available free ofcharge via the Internet at http://pubs.acs.org.
■ ACKNOWLEDGMENTSWe thank the collaborators of the project from Eawag S.Canonica, Y. Lee, R. Fankhauser, E. Salhi, and D. Kistler. Weacknowledge the great support of the Cantonal hospital ofBaden, especially A. Eggmann and H. Wernli. Hoffmann-La
Roche Ltd. (Basel, Switzerland) is acknowledged for supplyingoseltamivir reference standard and Chemonorm AG (Altendorf,Switzerland) for supplying the ozone generator. We thank Prof.A. Reller (University of Augsburg, Germany) for funding thestudies on the UV/TiO2 reactor from a project with theSchwyzer-foundation. The work was further funded by theSwiss State Secretariat for Education and Research (SER)/COST within COST Action 636, the EU project Neptune(contract no. 036845, SUSTDEV-2005-3.II.3.2) within theEnergy, Global Change and Ecosystems Programme of theSixth Framework (FP6-2005-Global-4), the Swiss FederalOffice for the Environment (FOEN), the Swiss Cantons AG,BE, BL, GE, SG, SH, SO, SZ, TG, VD, ZH, and the FederalOffice for Spatial Development (ARE), cofinanced by thebudget 2 of NRP in the framework of the InterReg IVB NorthWest Europe project PILLS.
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