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Article

Advanced Treatment of the Municipal Wastewater by Lab-Scale Hybrid Ultrafiltration

by
Tijana Marjanović
1,
Minja Bogunović
1,*,
Slaven Tenodi
1,
Vesna Vasić
2,
Djurdja Kerkez
1,
Jelena Prodanović
2 and
Ivana Ivančev-Tumbas
1
1
Department of Chemistry, Biochemistry and Environmental Protection, Faculty of Sciences, University of Novi Sad, Trg Dositeja Obradovića 3, 21000 Novi Sad, Serbia
2
Faculty of Technology Novi Sad, University of Novi Sad, Bulevar Cara Lazara 1, 21000 Novi Sad, Serbia
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(12), 9519; https://doi.org/10.3390/su15129519
Submission received: 29 March 2023 / Revised: 12 May 2023 / Accepted: 5 June 2023 / Published: 14 June 2023
(This article belongs to the Special Issue Water, Wastewater Treatment, and Sustainable Development)
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Abstract

:
In this study, hybrid ultrafiltration which involves adsorption onto activated carbon and/or coagulation was tested for the removal of ibuprofen, caffeine and diclofenac from the municipal wastewater treatment plant effluent (c0 = 2–3 µg/L). Ultrafiltration was tested in combination with powdered activated carbon dose of 5 mg/L separately or with coagulants (FeCl3, dose 4 mg Fe (III)/L and, natural coagulant isolated from bean seeds, dose 33 µL/L). In addition to the removal of organic micropollutants, the removal of As, Cr, Cu and Zn was also tested (c0~100 µg/L). The research was conducted on a laboratory pilot plant (capacity 30 L/h, in-out dead-end filtration, flux of 80 L/m2h). The best results were obtained for caffeine when adsorption on PAC is combined with a FeCl3 (removal efficiency 42–87%). The addition of a natural coagulant did not show benefits for the removal of organic micropollutants compared to the other tested processes, but both coagulants had similar effects on the content of metals and As Hybrid membrane processes proved to be the most efficient for Zn (44–87%) and Cr (33–87%) removal. The lowest efficiency was determined for As (˂19%). Ultrafiltration with PAC and coagulants removed 5–33% of effluent organic matter, depending on the type of coagulant; 57–87% of total nitrogen and PAC/FeCl3/UF was also partially effective for removing total phosphorus (11–39%).

1. Introduction

Certain organic micropollutants (OMP), primarily drugs and their metabolites, are still detected in wastewater effluents, regardless of the possibility of good removal by biological treatment. Table S1 shows the detected drug concentrations in municipal wastewater before and after treatment in a number of countries for which the effects of municipal wastewater treatment plants (WWTP) were analyzed. For the majority of OMPs shown in Table S1, content is not regulated by the legislation related to wastewater and surface water. In addition to OMP, inorganic micropollutants are also detected in municipal wastewater. A recent review of the literature showed that concentrations of arsenic (As), chromium (Cr), copper (Cu) and zinc (Zn) in raw municipal wastewater and municipal wastewater treatment plants effluent range from several μg/L to mg/L [1]. The concentration of zinc is particularly high, up to 325 µg/L [2]. The Directive of the European Parliament and the Council [3], which replaces the Water Framework Directive [4] and the Priority Substances Directive [5], defines environmental quality standards (EQS) for some of the metals (Cd, Ni, Hg and Pb), while the others (As, Bi, Co, Cr, Cu, Mn, Mo and Zn) are regulated by appropriate national legislation [6]. On the other side, EU Regulation (2020/741) defines minimum regulation for water reuse [7]. However, no specific values are defined for organic and inorganic micropollutants so far. Those pollutants are considered in risk assessment management. Salgot et al. (2007) published chemical limits for reclaimed water reuse where for As, Cr, Cu and Zn ranges for different kinds of reuse are 0.02–0.1 mg/L, 0.01–0.1 mg/L, 0.2–1.0 mg/l and 0.5–2.0 mg/L, respectively. At the same time, the value proposed for pharmaceuticals is 0.0001 mg/L [8].
In recent years, research on advanced municipal wastewater treatment has been intensified due to the need for final polishing of the wastewater treatment effluents and removal of various micropollutants, primarily organic. For this purpose, adsorption on powdered activated carbon (PAC) was applied separately, or in combination with ultrafiltration (UF) and coagulation [9,10,11,12]. A list of references related to the application of hybrid ultrafiltration processes for these purposes is given in Table S2. On the other hand, a review of the literature related to the removal of metals from water indicates that the experiments are mostly carried out at high initial concentrations which are not detected in the real municipal wastewater treatment effluents (most often in mg/L) [1]. The results of the work of Marjanović et al. [1] show that it is possible to partially remove Zn and As from real WWTP effluent even at their low concentrations of about 100 µg/L if a natural coagulant isolated from bean seeds or kaolinite is applied. In the case of natural coagulant application, Zn and As removals were 23–52% and 32–39%, respectively, while kaolinite removed 36–48% of As. The same processes were tested for the removal of Cu and Cr but did not prove to be effective. In the same paper, it was confirmed that the hybrid process of coagulation/adsorption on kaolinite did not show any effect compared to separate coagulation or adsorption. Natural coagulants are attractive in research since they do not have a negative impact on the environment and human health and produce a smaller amount of biodegradable sludge which can be safely disposed of in accordance with local regulations.
Hybrid membrane processes in addition to the efficient removal of microorganisms without disinfection byproduct formation are simultaneously effective for the removal of organic micropollutants from WWTP effluent (see Table S2). This makes this process potentially attractive for water reuse and thus sustainable water management. The efficiency directly depends on the conditions under which it is performed and the properties of the process materials used (for example PAC particle size), the method of PAC dosing (single pulse and continuous) [13,14], and the method of coagulant application [12]. A process combining powdered activated carbon and ultrafiltration (PAC/UF) has been shown to be more effective in removing OMPs than a process combining coagulation and ultrafiltration [12,13,14,15]. However, when coagulation and the PAC/UF process were combined [12], coagulation was observed to contribute to the efficiency in terms of OMP removal. Both coagulants containing iron [10,11] and aluminium are used in research [12]. Ma et al. (2023) recently discussed perspectives in membrane life cycle management explaining that today total wastewater treatment capacity of membrane bioreactors (>100.000 m3/ day) equipped with UF modules exceeded 11 mil m3/ day [16]. Trends in research and development show further costs reduction and great research efforts are invested in fouling prevention. Common commercial organic polymer membranes used in drinking water treatment hardly exceed 10 years of operation, while in wastewater treatment this period is usually less than five years [16]. Pistocchi et al. (2022) cited costs for two different hybrid UF installations in the range of 0.09–0.021 eur/m3 depending on the size [17]. However, it should be noted that the costs depend on process type and configuration and are site-specific. Data related to several case studies can be found in Panglisch et al., (2021) [18].
The aim of this preliminary case study was to examine the effectiveness of the PAC/UF process in local wastewater treatment plant effluent with and without the addition of coagulant for the removal of three, globally very frequently detected drugs in wastewater (ibuprofen, caffeine and diclofenac) at real concentrations level (c0 = 2–3 µg/L). Usually, they can be found in WWTP effluents in concentrations from a few ng/L to several tens, hundreds, or even thousands ng/L (Table S1). In addition to the frequently tested and already used coagulant ferric chloride, in the same dose as in literature (4 mg Fe/L), the goal was to test for the first time an innovative natural coagulant isolated from bean seeds [1]. Additionally, the effect on the removal of As, Cr, Cu and Zn in real concentration conditions (c0 around 100 µg/L) was investigated. Lowenberg et al. (2014) tested the in/out and out/in PAC/UF process for the removal of drugs (among which is diclofenac) from wastewater combined with coagulation (4 mg Fe3+/L) [11]. At the PAC dose of 20 mg/L, process efficiency was in the range of 60–95% (depending on the substance). This was lower removal efficiency compared to Saravia et al., (2008) which can be explained by the water matrix difference [19]. Margot et al. (2013) investigated the removal of pesticides, drugs and cosmetics from wastewater using also PAC/UF process in combination with FeCl3 [10]. IB, CF and DCF removal were 83 ± 7%, 65%, and 69 ± 19%, respectively, with a PAC concentration between 10 mg/L and 20 mg/L (median 12 mg/L) [10]. Bogunović et al. (2021) reported high removals of CF from wastewater treatment plant effluent in subsequent cycle composite samples (>99.9%) and DCF (>90%), but at higher concentrations of PAC (near 20 mg/L) and lower initial concentrations of OMP (126.5 ng/L and 54.1 ng/L for CF and DCF, respectively) [9]. To the best of our knowledge, no data have been published so far on the removal of metals and As by such a hybrid ultrafiltration.

2. Materials and Methods

The effluent of the municipal wastewater treatment plant, JKP “Vodokanal”, Sombor was used in the research. Taken grab effluent sample was immediately transported to the laboratory, homogenized and used in experiments. Effluent quality is shown in Table 1.
Powdered activated carbon NORIT SAE designed for wastewater treatment, was used. According to the manufacturer’s specifications BET’s surface area is 1150 m2/g and the particle size D50 is 15 µm. As coagulants, ferric chloride (≥99%, Centrochem) and a natural coagulant isolated from beans seeds (nCOA) (Phaseolus vulgaris L.) in the laboratories of the Faculty of Technology in Novi Sad [1] were tested.
Hybrid membrane processes: Ultrafiltration was combined with adsorption onto PAC alone with or without coagulants-FeCl3 or nCOA. The reason for PAC addition is the adsorption of organic pollutants that pass through the ultrafiltration membrane pores (10–50 nm) which are considerably larger than molecules [20]. Their molecular weights are 206.3 g/mol, 194.2 g/mol and 296.1 g/mol for ibuprofen, caffeine and diclofenac, respectively. In addition, the scouring effect of PAC on the membrane is evidenced in literature [19,21] as well as its beneficial effects on coagulation pre-treatment [21] which is a viable option for municipal wastewater treatment [11,18]. To test the effectiveness of hybrid membrane processes for the removal of ibuprofen, caffeine, diclofenac and selected inorganic micropollutants (Cr, Cu, Zn and As), a laboratory in/out ultrafiltration pilot plant (capacity of 30 L/h) was used. A schematic presentation of the plant was published elsewhere [13]. It was equipped with a dizzer® Lab Multibore® 1.5 (polyethersulfone, 0.2 m2 membrane surface, produced by company inge GmbH, Greifenberg, Germany). The module was not brand new, but already used in previous research. In between experiments, it was kept in accordance with the manufacturers’ instructions showing transmembrane pressure values in the acceptable operating range.
The following process types were tested:
-
Hybrid adsorption/ultrafiltration (PAC/UF),
-
Hybrid adsorption/coagulation/ultrafiltration using ferric chloride (PAC/FeCl3/UF) and natural coagulant isolated from bean seeds (PAC/COA/UF).
Before testing the hybrid membrane processes, ultrafiltration (UF) was performed to evaluate the sorption of selected organic/inorganic substances on the membrane surface. All experiments were performed in dead-end mode and at the flux rate of 80 L/m2h with the same module. Three filtration cycles (30 min each) were performed for each process type. Each cycle was followed by 30 s backwashing with permeate (flux 230 L/m2h). After each type of process, disinfection and both base and acid chemical cleaning were performed in accordance with the manufacturer’s guidelines. The feed temperature during the filtration experiments was 18–21 °C. PAC was dosed in-line as a single pulse dose of 5 mg/L (80 mL of PAC suspension with concentration 0.5 g/L was dosed by Ismatec® Reglo-Z-130 pump (Laboratoriumstechnik GmbH, A Unit of IDEX Corporation, Wertheim-Mondfeld, Germany)); the contact time between PAC and water was 12 s. This dose was selected based on our previous research where very good removals were achieved for diclofenac in mg/L range [13]. Coagulants were continuously dosed (coagulation/flocculation time was 7 s) by Stepdose® FEM 03 KT.18RC pump (KNF FLODOS AG, Sursee, Switzerland). Appropriately diluted coagulant (45 mL) was dosed during each cycle with a flow of 1.5 mL/min. The dose of Fe (III) was 4. mg/L. It was selected based on the work of Löwenberg et al. who used it in testing of hybrid ultrafiltration for removal of organic micropollutants from the municipal wastewater treatment plant effluent [11]. The dose of natural coagulant was 33 µL/L. This particular dose was achieved in experimental conditions at the plant as the closest possible to the optimal dose (37.5 µL/L) which was experimentally determined in our previous work for the removal of organic matter in the same wastewater treatment plant effluent [1]. Composite water samples after each cycle were collected and analyzed. Before starting the experiments, the concentration of ibuprofen, caffeine, diclofenac, As and metals in the WWTP effluent was determined (Table 1). After that, the effluent was enriched with a methanol solution of organic micropollutants to achieve a concentration of 2–3 µg/L and a water solution of inorganic micropollutants to achieve a concentration near 100 µg/L. In further data processing, nominal concentrations were used to calculate the process efficiency: 2.20 µg/L for ibuprofen, 2.18 µg/L for caffeine, and 3.08 µg/L for diclofenac. Initial concentrations of As, Cr, Cu and Zn in the enriched WWTP effluent were measured as 116 µg/L, 123 µg/L, 99.0 µg/L and 128 µg/L, respectively.
In addition to testing the efficiency of organic and inorganic micropollutants removal, the removal of organic matter effluent (based on the difference in chemical oxygen demand (COD) [22] before and after the process) and nutrients (total nitrogen and phosphorus) was tracked. Total Kjeldahl nitrogen (TKN) [23] and total phosphorus (TP) [24] were determined. The characteristics of the applied methods are shown in Table S3.
Solid-phase extraction (150 mg, Oasis® HLB, WatersTM, Wexford, Ireland) was used to prepare water samples for the analysis of selected OMPs, and gas chromatography–mass spectrometry (GC/MS) was used for analysis. The detailed procedure of sample preparation and analysis is given in Table S4.
The total content of As, Cr, Cu in the water was determined using Atomic Absorption Spectrophotometry (AAS) with a graphite furnace, on a PerkinElmer Analyst™ 700 instrument, according to the EPA 7010 method [25]. Zn was analyzed using the AAS, flame technique, according to method 7000 b, on a PerkinElmer Analyst™ 700 instrument [26]. The overall characteristics of the method for the selected metals and As are shown in the previous article [1].
Data analysis: OMP concentrations in the WWTP effluent before enrichment and after the applied processes were determined by the standard addition method including internal standards addition for both extraction efficiency and chromatography performance control (details provided in Supplementary Material). The external standard calibration was used for metals and As analysis.

3. Results and Discussion

3.1. Removal of Organic Micropollutants

Based on the results shown in Table 2, one can conclude that caffeine was the most successfully removed among the tested substances (˃80%) by hybrid membrane processes. That was achieved when adsorption onto PAC was combined with ferric chloride (removal efficiency 42–87%). The addition of a natural coagulant did not show benefits for the removal of OMPs compared to the other tested processes. It was proven to be moderately effective in two of three cycles for caffeine (achieved removal efficiency of 47% and 37%) and in one cycle for diclofenac (achieved removal efficiency of 50%), and ibuprofen was not effectively removed. In 30% of cases (11 out of a total of 36 measurements), the concentrations obtained after the hybrid process were higher than the initial concentration to a greater extent than the systematic error of the analytical determination. Such results were discarded as inconclusive.
It can be concluded that the total number of unacceptable results from the first to the third process tested (presented in Table 2) was mostly two cases out of the total of nine per tested process (except for UF when there were three such cases) and mostly in the second filtration cycle, which may indicate potential desorption of OMPs that were removed during the first cycle. In the fourth type of experiment with a natural coagulant, such a situation occurred as many as five times in the I and/or III cycle, which may also indicate the influence of natural organic matter introduced by the coagulant on the eventual desorption of OMP due to the possibility of hydrogen bonding to the soluble components of the coagulant or complex desorption mechanisms. Out of a total of 12 individual measurements for each substance, this happened four times for IB (in PAC/UF, PAC/FeCl3/UF and PAC/nCOA/UF processes), two times for CF (in UF and PAC/nCOA/UF processes) and five times for DCF (in UF, PAC/UF, PAC/FeCl3/UF and PAC/nCOA/UF processes). The hypothesis on desorption should be further tested. Analytical interferences of the process materials when used without membrane filtration were not observed. In the literature, there are known findings of higher concentrations of substances after the process, which can be explained by the presence of compounds in the form of conjugates in the WWTP effluents that can be decomposed and give an increased concentration of the substance after the process. Such potential effects were not further investigated here. Due to the high values of the practical limit of quantitation (PQL, Table S4) the removal of ˃87% in the case of CF and ˃83% in the case of DCF could not be determined.
Ibuprofen was not sorbed in the first ultrafiltration cycle, while 59% was sorbed during the second filtration cycle. It is known that during the ultrafiltration process, a barrier of organic and inorganic constituents of the WWTP effluent is formed on the membrane. The barrier is hydrophobic and negatively charged [27]. Such a layer generally enables the formation of a complex between EfOM and OMP due to hydrogen bonds and electrostatic attraction of polar groups of OMP with phenolic and carboxyl groups of effluent organic matter.). Caffeine was sorbed on the UF membrane by 34% and 68% (I and III filtration cycle), while in the II cycle, a higher concentration was observed in the permeate. Caffeine is a hydrophilic molecule (log D at pH 7.4 is 0.28), but unlike ibuprofen (log D at pH 7.4 is 0.45) and diclofenac (log D at pH 7.4 is 1.37) has no charge. This probably helps the interactions with EfOM which are mostly negatively charged. Neutral molecules might be better adsorbed. The presence of EfOM can have a negative effect on the sorption of OMP during the UF process, as EfOM diffuses into the pores of the membrane material, thus filling the spaces on the membrane and reducing the sorption capacity for OMP [28]. DCF was removed by 17% in the first cycle, it failed to be removed in the second cycle, while in the third cycle, an increased concentration was detected compared to the initial concentration.
The obtained results are in accordance with the literature in which the diverse behavior of IB, CF and DCF during the ultrafiltration of municipal wastewater effluent was confirmed. For example, the work of Bogunović et al. [9] demonstrated high sorption of caffeine (99.9%) and diclofenac (>75%) from WWTP effluent (three ultrafiltration cycles), but at significantly lower initial concentrations (348 ng/L and 410 ng/L for CF and DCF, respectively) in relation to the concentrations in this experiment. The low sorption of caffeine and diclofenac on a different, polyvinylidene fluoride ultrafiltration membrane was proven in the work of Chon et al. [29], also at low initial OMP concentrations (126.5 ng/L and 54.1 ng/L) and was 20% and 45% for CF and DCF, respectively. On the other hand, Sheng et al. [15] observed in their research that there was no sorption of ibuprofen and caffeine on the ultrafiltration membrane, while the sorption of diclofenac was 36%. It can be concluded that, in addition to the quality of the matrix, the sorption is influenced by the initial concentration of the substances as well as the process performance.
In hybrid ultrafiltration, in the case of ibuprofen and diclofenac, the best removal efficiency was determined using the PAC/UF and PAC/FeCl3/UF processes during the I and III filtration cycle, respectively (34 and 52% for IB, and 39 and 54% for DCF using PAC/UF process, 45 and 55% for IB, and 49 and 83% DCF using the PAC/FeCl3/UF process). Removal was not observed for IB and DCF during the II cycle of PAC/UF and PAC/FeCl3/UF, which may indicate the already-mentioned desorption of OMP. Caffeine was relatively efficiently removed (42–87%) during all three cycles using the PAC/UF and PAC/FeCl3/UF processes. Based on the presented results, it cannot be concluded that the influence of the ferric chloride had a significant effect on the removal of OMP. Bogunović et al. [9] achieved higher removals for CF (>99.9% in two filtration cycles) and DCF (>87.8%, I filtration cycle and >99.9% II filtration cycle), but at the higher concentrations of the carbon (about 20 mg/L) and lower initial concentration of OMP (126.5 ng/L and 54.1 ng/L for CF and DCF) from municipal wastewater treatment plant effluent (Table S2). Acero et al. [30] achieved CF and DCF removal efficiencies of 30% and 60%, respectively, using PAC/UF at the lowest tested PAC dose of 10 mg/L. With the increase in the dose of PAC to 50 mg/L, the efficiency of substance removal was higher (˃90%). The higher CF removal efficiency obtained in this work is probably due to different experimental conditions (e.g., lower initial CF concentration of 2.20 µg/L compared to 0.50 mg/L in the work of Acero et al. [30]). Lower removal efficiencies compared to this work were obtained by Shang et al. [15], using a PAC/UF process with a lower dosage of 10 mg PAC/L. The removal efficiency of CF, DCF and IB from raw wastewater was ˂40%. Hoffmann et al. [14] showed that a negative impact of the formed PAC agglomerates in the suspension dosed into water is possible and that it depends on the concentration of the PAC suspension. It is assumed that agglomerates lead to slower adsorption kinetics and inhomogeneous distribution of PAC in the membrane system. In our work, a concentration of PAC suspension of 0.5 g/L was used, and based on the results of Hoffmann et al. [14], there is a still possibility of the influence of the formed agglomerates on the removal of selected substances, which can be interesting for the future research. Löwenberg et al. [11] tested an in/out and out/in PAC/UF process for OMP removal from wastewater combined with coagulation (4 mg Fe (III)/L). At a PAC dose of 20 mg/L, the efficiency of both processes was in the range of 60–95% depending on the substance, among which was diclofenac, which was removed by more than 70%. Recent research indicates that membrane coating with a coagulant (0.5 mg Al (III)/L) and continuous dosing of PAC (30 mg/L) provide operational stability in terms of cake formation on the membrane surface that is easily removed by backwashing and achieve significant OMP removal [12]. Applying fine PAC (~8 μm) at a dose of 30 mg/L achieved a high removal efficiency for most of the selected substances (˃80%), and DCF was removed around 85% [12].
The PAC/nCOA/UF process, which uses a natural coagulant isolated from bean seeds, is an innovative hybrid process that has not been tested in this way until now. This process proved effective in two out of three cycles for caffeine (47% and 37%) and in one cycle for diclofenac (50%). Ibuprofen was not effectively removed in any cycle using the PAC/nCOA/UF process.

3.2. Removal of Inorganic Micropollutants

The results of the removal of metalloid As and metals are shown in Table 3. It can be concluded that the investigated processes are the most efficient for the removal of chromium (33–87%) and zinc (44–87%), and the least efficient for the removal of arsenic (since the processes are not efficient up to a maximum efficiency of 19%). When it comes to metal sorption on the ultrafiltration membrane, it was only observed in the case of Cr (28–49%) and Zn (29–42%). Based on the Visual MINTEK-version 3.1 software, the species of As ions and the mentioned metals were identified at pH 8. In the case of arsenic, the dominant form is HAsO42−, in the case of chromium, Cr(OH), in the case of copper, CuOH+, and in the case of zinc, the dominant form is Zn2+.
Removal of arsenic (V)—The best As ion removal was achieved using the PAC/FeCl3/UF process, and it was only 13–19%. Other results after applied processes are not relevant taking into account the systematic error of analytical determination (9–10%). The addition of FeCl3, which improved the efficiency of As removal compared to the PAC/UF process and the PAC/nCOA/UF process may be due to charge neutralization, as it is one of the main mechanisms of coagulation in which flocs are formed. Given that the dominant form of As(V) at pH 8.0 is HAsO42−, it was expected that the PAC/UF process effectively remove As(V), due to the positive charge at the pH 8 (the isoelectric point of the applied PAC is 9.8 [31]).
It can be assumed that the negatively charged centers of the dissolved organic matter of the effluent by competition can prevent the removal of As(V) in the PAC/UF process, whether the carbon is deposited on the surface of the membrane or it is in suspension. The addition of natural coagulant in the PAC/UF process did not prove to be effective for the removal of As, which is not in accordance with the results published in the work of Marjanović et al. [1] where the coagulation activity of the natural coagulant in terms of As removal was 32–39%. Although most of the macromolecules in raw bean extract are negatively charged at pH 8 (isoelectric point (pI) of raw bean extract 3.61 [1]) in coagulant proteins, due to the presence of an amino group, a partial positive charge may be present, which potentially helps removal of H2AsO4−, which was demonstrated in the batch experiments with coagulant in the mentioned work. However, in dynamic membrane filtration conditions, this interaction is clearly not favored, most likely due to possible interactions between the natural coagulant and the membrane surface, which does not lead to floc formation with arsenic.
Removal of chromium (III)—Chromium was sorbed on the UF membrane (28–49%) probably in the form of a complex with organic matter from the effluent of a municipal wastewater treatment plant. Only efficiencies above the systematic error of analytical determination (20–24%) are considered relevant. Since Cr(III) was added to the wastewater samples, it was assumed that the dominant form at pH 8.0 was the neutral form of chromium Cr(OH)3 and that Cr(VI) was not present. A good chromium removal efficiency in the PAC/UF process (33–75%) was observed, which was probably contributed by interactions with EfOM [32]. The slightly higher efficiency in combination with coagulant and PAC/UF (41–87%), where the formation of flocs with EfOM complexes might take place, seems not to be highly relevant since the bias of the measurements was in the range of 20–24%, and relative standard deviation high (23%). Similar efficiency was achieved with the addition of a natural coagulant into the process (from 58% to 76% for all three cycles).
Removal of copper—In the case of Cu ions, ultrafiltration and PAC/UF processes achieve an efficiency lower than the systematic error of analytical determination, which ranges from 20 to 22%. The efficiency of Cu removal (the dominant form of CuOH+) with the addition of FeCl3 was 37–42% in all three cycles, and a possible mechanism is the formation of a Cu complex with EfOM; the formation of flocs was due to the addition of FeCl3 and removal by ultrafiltration. Electrostatic interactions of Cu with natural coagulant had similar effects on Cu removal efficiency compared to ferric chloride (removal efficiency was 28–50%, in three cycles with natural coagulant).
Removal of zinc (II)—Values greater than 20% are relevant considering the systematic error of the analytical determination (9–20%). Positively charged Zn (II) was efficiently sorbed on the ultrafiltration material (29–42%). The efficiency of the PAC/UF process in all three cycles was 44–50% (not improved compared to ultrafiltration alone), and the addition of FeCl3 improved the process (78–87%, for three filtration cycles). The natural coagulant, which is partially negatively charged, also improved the removal efficiency of the positive form of Zn, and the possible mechanism is the electrostatic interaction (59–71% for three filtration cycles).

3.3. Removal of Effluent Organic Matter and Nutrients

Figure 1 shows the efficiency of hybrid membrane processes for the removal of organic matter in the effluent (EfOM), which was expressed as chemical oxygen demand (COD).
During ultrafiltration, it was possible to remove 11–16% of effluent organic matter (expressed as COD), which is in accordance with the findings of Chon et al. [29] and Acero et al. [30] who achieved the removal of 10% and 24% of EfOM from municipal wastewater treatment plant effluent by ultrafiltration. PAC addition did not improve the removal of organic matter in municipal wastewater treatment plant effluent (Figure 1). This was also observed in the work of Acero et al. [30], using the PAC/UF process at doses of 10 and 20 mg PAC/L where the removal efficiency of EfOM, expressed as COD value, did not change significantly (dose 10 mg/L, removal efficiency 34%, dose 20 mg/L, removal efficiency 37%), compared to a separate UF process (24%). The addition of FeCl3 coagulants had a positive effect on the retention of effluent organic matter, which was especially pronounced in the first filtration cycle (Figure 1). The efficiency achieved using PAC/FeCl3/UF was 13–33%, but showed a decreasing trend during multiple cycles, while with natural coagulant it was 5–23% during three filtration cycles. The positive effect of coagulant addition was also observed by Acero et al. [30], the Fe(III)/UF combination showed significant efficiencies (44%) compared to UF alone (24%), and the Fe(III)/PAC/UF combination showed high EfOM removal (88%), due to the fact that high doses of Fe(III) 130 mg/L and PAC dose 600 mg/L were used in their work.
Figure 2 shows the results of hybrid membrane processes for total Kjeldahl nitrogen (TKN) removal. Based on the results presented in Figure 2, it can be concluded that by applying ultrafiltration, the efficiency of TKN removal in different cycles varies from none to 45%. In the work of Chon et al. [29] and Acero et al. [30] using ultrafiltration to remove nitrogen compounds, less than 10% of total N and nitrates were removed. In the PAC/UF process in our work, a decrease from 32% to 12% was observed during three cycles (Figure 2), while in the work of Acero et al. [30] PAC/UF, 29% TKN removal was achieved, independent of the PAC dose (10, 20 or 50 mg/L). In our work, a very high efficiency of hybrid processes using a coagulant (57–87%), especially natural coagulant (74–87%), was observed, while the opposite was observed in the work of Acero et al. [30] The addition of Fe(III) had no positive effect; moreover, the efficiency of the Fe(III)/PAC/UF process was only 16%, at a high Fe(III) dose of 130 mg/L and a PAC dose of 600 mg/L [30].
Among the all tested processes, only the process in which the coagulant FeCl3 is applied together with PAC and ultrafiltration (PAC/FeCl3/UF) showed an efficiency of 11–39% for the removal of phosphorus compounds. A trend of increasing removal over a series of cycles was also observed, potentially due to the formation of effluent organic matter cakes. Acero et al. [30] also showed for this type of process, very efficient removal of total phosphorus (88%), but at significantly higher doses of process materials (Fe(III) of 130 mg/L and PAC dose of 600 mg/L) and at a lower initial concentration of total phosphorus (0.173 mg/L) in comparison to this work. Chon et al. [29] removed 30–40% of phosphorus compounds from the effluent of a municipal wastewater treatment plant by ultrafiltration, which is not in accordance with the results of this work, where ultrafiltration did not give any results. Acero et al. [30] removed about 8.5% of total P by ultrafiltration, the addition of PAC at different doses (10, 20 and 50 mg/L) did not significantly affect the removal of total phosphorus, the efficiency was 12.5, 17.9 and 29.5%, respectively.

4. Conclusions

In the applied membrane and hybrid membrane processes, the removal efficiency of ibuprofen, caffeine and diclofenac fluctuated. Caffeine was best removed in the process when the PAC adsorption is combined with ferric chloride (42–87%). The addition of a natural coagulant did not improve the removal of organic micropollutants compared to the other tested processes. Both coagulants had similar effects on metal and metalloid As content. For the removal of inorganic micropollutants, hybrid membrane processes proved to be the most effective for zinc removal (44–50% for the PAC/UF process, 78–87% for the PAC/FeCl3/UF process and 59–71% for the PAC/nCOA/UF process) and chromium removal (33–75% for the PAC/UF process, 41–87% for the PAC/FeCl3/UF process and 58–76% for the PAC/nCOA/UF process). The efficiency of arsenic ion removal was insignificant and it was only achieved with the application of PAC/FeCl3/UF (13–19%). The removal of copper ions was possible only in the presence of coagulants which showed similar effects, regardless of their nature.
The applied hybrid membrane processes can be partially effective for the removal of nutrients. The efficiency of effluent organic matter removal using PAC/FeCl3/UF was 13–33%, while with natural coagulant it was 5–23%. The separate PAC/UF did not show good performance (<10%). Up to 45% of total Kjeldahl nitrogen (TKN) could be removed by ultrafiltration. In the PAC/UF process, a drop in removal efficiency for TKN was observed from 32% to 12% during three cycles, and very high efficiency was shown by hybrid membrane processes with coagulants (57–87%), especially natural coagulant (74–87%). Only PAC/FeCl3/UF was partially effective for total phosphorus removal (11–39%).
Results confirmed that hybrid ultrafiltration has the potential for advanced wastewater treatment. Due to the ever-increasing requirements regarding the quality of WWTP effluent, it is expected that the tested processes can obtain importance in the future, either as separate treatments or as an upgrade of the existing treatment trains. In order to check the full potential for the application in water reuse further tests must include long-term experiments including optimization (e.g., both related to carbon and coagulant dose, way of dosing, contact time, influence on ultrafiltration performance, etc.) and cost estimation. The use of biodegradable process materials such as natural coagulants might be an important contribution to safe sludge disposal.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su15129519/s1, Table S1: Occurrence of ibuprofen, caffeine and diclofenac in wastewater treatment plants. Table S2: Literature review related to the application of hybrid ultrafiltration processes for the removal of pollutants from municipal wastewater. Table S3: Method characteristics for the analysis of total N and P. Table S4: Method characteristics. References [33,34,35,36,37,38,39,40,41,42,43] are cited in the supplementary materials.

Author Contributions

Conceptualization—M.B., I.I.-T. and J.P.; Methodology—M.B., T.M. and I.I.-T.; Validation, M.B. and T.M.; Formal analysis, T.M., S.T., Dj.K. and V.V.; Investigation, M.B. and T.M.; Resources, J.P.; Data curation, T.M.; Writing—original draft preparation, T.M.; Writing—review and editing, M.B., S.T., Dj.K., V.V., J.P. and I.I.-T.; Visualization, T.M.; Supervision, I.I.-T.; Project administration, I.I.-T.; Funding acquisition, I.I.-T. All authors have read and agreed to the published version of the manuscript.

Funding

This research received financial support of the Ministry of Education, Science and Technological Development of the Republic of Serbia (Grant No. 451-03-47/2023-01/200125).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors gratefully acknowledge the financial support of the Ministry of Science, Technological Development and Innovation of the Republic of Serbia (Grant No. 451-03-47/2023-01/200125). The work of student Tijana Marjanović (No. 2338) was supported by the Ministry of Education, Science and Technological development. Authors acknowledge the support of the Public Utility Company “Vodokanal”, Sombor, for taking WWTP effluent samples.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Removal efficiency of effluent organic matter using hybrid membrane processes. Note: * no removal achieved.
Figure 1. Removal efficiency of effluent organic matter using hybrid membrane processes. Note: * no removal achieved.
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Figure 2. Total Kjeldahl nitrogen (TKN) removal efficiency using hybrid membrane processes. Note: * no removal achieved.
Figure 2. Total Kjeldahl nitrogen (TKN) removal efficiency using hybrid membrane processes. Note: * no removal achieved.
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Table
Table 1. Effluent sample quality.
Table 1. Effluent sample quality.
ParameterUnitResult
pH-8.20 **
COD amg O2/L38.3 **
Total nitrogenmg N/L14.3 *
Total phosphorusmg P/L1.91 **
Arsenicµg/L13.0 **
Chromiumµg/L30.0 **
Copperµg/L3.30 **
Zincµg/L24.0 **
Ibuprofenµg/L0.02 **
Caffeineµg/L0.06 **
Diclofenacµg/L1.02 **
a Chemical oxygen demand (COD); * Analyses performed in the laboratory of JKP “Vodokanal”, Sombor, ** Analyses performed in the laboratory at the Faculty of Sciences in Novi Sad.
Table
Table 2. Test results of hybrid membrane processes.
Table 2. Test results of hybrid membrane processes.
Organic MicropollutantIBCFDCF
CycleIIIIIIIIIIIIIIIIII
%%%%%%%%%
UF−1859−2134- *6817**- *
PAC/UF
5 mg/L		52- *34586587 139- *54
PAC/FeCl3/UF
5 mg/L PAC/4 mg/L Fe (III)		55-*45874287 149- *83 1
PAC/nCOA/UF 2
5 mg/L PAC/33µL/L nCOA		- *−13- *4737- *- *50- *
* Result rejected because the concentration value after treatment is significantly higher than the input concentration, to a greater extent than the systematic error of the analytical determination; ** The analysis failed since the concentration in the sample with the standard addition was lower in comparison with the sample without the standard addition; 1 The concentration of substances after the applied process is lower than the PQL of the method, the value of PQL/2 was applied; 2 Processes in which a natural coagulant isolated from bean seeds is used.
Table
Table 3. The removal efficiency of As and selected metals by hybrid membrane processes.
Table 3. The removal efficiency of As and selected metals by hybrid membrane processes.
ProcessThe Dominant Form of Inorganic Micropollutant at pH 8Analytical Method Bias %Removal %Standard Deviation %
I CycleII CycleIII Cycle
UFZn2+9–204232297
Cr(OH)320–2447284912
CuOH+20–221515182
HAsO42−9–10−3.0−4.00.02
PAC/UF
5 mg/L PAC		Zn2+9–205044443
Cr(OH)320–2475336322
CuOH+20–221913193
HAsO42−9–106.0−1.0−4.05
PAC/FeCl3/UF
5 mg/L PAC/4 mg/L Fe(III)		Zn2+9–207884875
Cr(OH)320–2441876623
CuOH+20–224042373
HAsO42−9–101918133
PAC/nCOA/UF
5 mg/L PAC/33 µL/L nCOA		Zn2+9–206571596
Cr(OH)320–247158769
CuOH+20–2228315012
HAsO42−9–100.0−4.0−22 *3
PAC/nCOA/UF-Process in which a natural coagulant is applied; * Removal efficiency was not achieved, and the result indicates a higher concentration after treatment, because the deviation (−22) is greater than the systematic error of the analytical determination (up to 10%).
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Marjanović, T.; Bogunović, M.; Tenodi, S.; Vasić, V.; Kerkez, D.; Prodanović, J.; Ivančev-Tumbas, I. Advanced Treatment of the Municipal Wastewater by Lab-Scale Hybrid Ultrafiltration. Sustainability 2023, 15, 9519. https://doi.org/10.3390/su15129519

AMA Style

Marjanović T, Bogunović M, Tenodi S, Vasić V, Kerkez D, Prodanović J, Ivančev-Tumbas I. Advanced Treatment of the Municipal Wastewater by Lab-Scale Hybrid Ultrafiltration. Sustainability. 2023; 15(12):9519. https://doi.org/10.3390/su15129519

Chicago/Turabian Style

Marjanović, Tijana, Minja Bogunović, Slaven Tenodi, Vesna Vasić, Djurdja Kerkez, Jelena Prodanović, and Ivana Ivančev-Tumbas. 2023. "Advanced Treatment of the Municipal Wastewater by Lab-Scale Hybrid Ultrafiltration" Sustainability 15, no. 12: 9519. https://doi.org/10.3390/su15129519

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