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Article

Use of Ultrafiltration Membranes as Tertiary/Quaternary Treatment for Wastewater Reclamation in Municipal WWTPs

by
Á. Sabina Acebrón
,
Julio Revert-Vercher
,
Pau Sanchis-Perucho
*,
Luis Borrás
and
Aurora Seco
CALAGUA—Unitat Mixta UV-UPV, Departament d’Enginyeria Química, Universitat de València, Avinguda de la Universitat s/n, Burjassot, 46100 Valencia, Spain
*
Author to whom correspondence should be addressed.
Water 2025, 17(24), 3453; https://doi.org/10.3390/w17243453
Submission received: 30 October 2025 / Revised: 28 November 2025 / Accepted: 29 November 2025 / Published: 5 December 2025
(This article belongs to the Special Issue New Advances in Membrane Separation Technology for Water Pollution Control and Membrane Fouling Mitigation)
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Abstract

This work assesses the viability of ultrafiltration (UF) membranes as a substitution for classic tertiary technologies for municipal wastewater (MWW) treatment. UF membranes can offer efficient MWW filtration, meeting quality standards regarding solids, bacteria, viruses and emerging pollutants, such as microplastics. All of these make UF not only an attractive competitor regarding tertiary treatments but also a potential quaternary treatment according to the latest legislation. Indeed, the achieved permeate quality meets the more stringent parameters for water reuse in agriculture according to the European standard (A-type water). The UF membrane’s feasibility when used as an MWW tertiary/quaternary treatment was assessed in a semi-industrial plant with commercially available industrial membrane modules under different operating conditions: (1) transmembrane flux, (2) air sparging intensity and filtration/relaxation periodicities, (3) the concentration of solids reached in the membrane tank and (4) the efficacy of chemically enhanced backwashing (CEB) to mitigate fouling. Increasing the air intensity (around 0.25 m3 m−2 h−1), increasing the solids concentration (3–4 g L−1) and using acid chemicals for backwashing at low concentrations but high periodicities (about 25–50 ppm of HCl/citric acid at a pH of 2.5 once or twice every 15 days) displayed great effectiveness in minimizing fouling, which was found to be mainly reversible. Thanks to the stablished conditions, semi-industrial UF membrane filtration was possible for more than 30 days when operating at relatively high transmembrane fluxes (21.5 LMH), achieving an average transmembrane pressure of around 120 mbar with an extremely low fouling growth rate of 0.024 mbar d−1.

1. Introduction

Water scarcity is one of the greatest challenges nowadays. According to [1], 40% of the entire global land area is classified as arid, semi-arid or dry sub-humid, resulting in a significant lack of quality water sources in numerous countries. In addition to this, the energy crisis is also a prominent problem to solve in the coming years [2]. Other essential resources, like phosphorus for agriculture, are also nearing depletion as population growth and food demands continue to rise, indicating potential shortages in the medium term [3]. All these problems, which are, in several cases, interconnected with each other, demand a new approach to human interaction with the environment based on circular economy models [4]. Within this scenario, municipal wastewater (MWW) treatment is now regarded as a potential pathway to help in tackling these increasing problems, since it can be an important source of reclaimed water to meet water demands in certain areas, with its quality being the main concern. Unfortunately, depurated water after conventional tertiary treatment technologies in wastewater treatment plants (WWTPs) presents meaningful quality issues when aiming to reuse it. Indeed, these tertiary treatments are not sufficient to effectively remove many pollutants of emerging concern, such as pharmaceuticals, personal care products, plasticizers, surfactants, nanomaterials and pesticides [5]. This could thus lead to negative effects on human and environmental health if this recycled water is freely used, strongly limiting its applicability. Furthermore, these treatments also display important operating limitations due to the several steps and/or significant investments/operation costs required to ensure the production of water of sufficient quality (e.g., chemicals and/or energy demands) [6]. With regulators aware of this situation, new policies are emerging to avoid the possible negative impacts of low-quality recycled water while promoting greener and more energy-efficient treatments. For instance, Regulation (UE) 2020/741, approved in 2020, aims to stablish minimum requirements for the quality and control of reclaimed water.
Different technologies have been studied over the past few decades to meet the requirements imposed on recycled water, such as advanced oxidation processes, ultraviolet radiation or membrane bioreactors. Specifically, treatment schemes based on membrane technology have been proposed as potential alternatives to improve MWW treatment. The significant reductions in space demands, energy requirements and environmental impacts are some of the main advantages that the use of membranes offers [7], as well as great flexibility thanks to its modularity. According to [8], membrane technology has the potential to bridge the economic and sustainability gap in numerous applications, alongside possibilities of low or no chemical usage, environmental friendliness and easy accessibility to many industries. Due to all the above, the use of membrane systems for urban and industrial wastewater treatment has experienced a clear increase during the last two decades. This is especially true for ultrafiltration (UF) membranes. UF membranes are classified as porous membranes with a pore size range of around 100–1 nm [9]. Due to their average pore dimensions, they can retain suspended solids and bacteria during filtration and can also partially remove certain viruses [10]. Additionally, UF membranes have also shown good retention for different emerging contaminants, such as microplastics [11] or polycyclic aromatic hydrocarbons [12], making them an excellent alternative to produce high-quality reclaimed water. UF also presents significant advantages compared to other membrane processes such as nanofiltration (NF) and reverse osmosis (RO), primarily due to its operation at substantially lower transmembrane pressures. This lower-pressure operation results in reduced specific energy consumption and consequently minimizes operational costs. Recent technological advances have also boosted the membrane market to produce more compact and efficient membranes (i.e., materials with lower resistance to filtration, lower fouling propensities, etc.) at lower prices and with longer lifespans [13], with even more improvements in all these aspects expected in the next few years.
Despite all the above-cited benefits, membrane fouling is still a major issue constraining membrane systems’ implementation in MWW treatment. Indeed, it entails significant increases in the process’ energy demands that restrict the technical and economic viability of multiple membrane-based schemes available. Fouling phenomena can be generally differentiated as reversible and irreversible. The reversible one is generally caused by particle deposition on the membrane surface during filtration in proportion to the applied flux, while irreversible fouling is usually attributed to the deposition of colloids or soluble substances inside the membrane pores, therefore promoting pore narrowing and/or blocking [14]. Since membranes are usually coupled to biological treatments (e.g., aerobic membrane bioreactors (AMBRs) or anaerobic membrane bioreactors (AnMBRs)), the significant amounts of solids achieved in these systems usually play an important role in the reversible fouling occurring in the process. Furthermore, when filtering streams from biological treatments (such activated sludge), the presence of extracellular polymeric substances (EPSs), including soluble microbial products (SMPs), can result in significant membrane fouling, leading to increases in the operating transmembrane pressure (TMP) and, consequently, the overall process operating costs [15]. Thanks to almost completely eliminating suspended solids in the treated stream, reversible fouling can be minimized by reducing the solids concentration in the membrane module. Moreover, since active organisms are completely removed from the treated stream, EPSs in contact with the membrane can be minimized, only needing to deal with its remaining soluble fraction (i.e., SMPs). These conditions could allow the development of highly compact membrane systems with high treatment water fluxes at low operating and capital costs. However, the membrane material and/or other characteristics of the filtered water could influence fouling development. Regarding the first point, no significant fouling differences have been reported when using different membrane materials during the direct filtration of municipal wastewater [16]. However, the water characteristics could strongly influence fouling development, being an important point to take into consideration.
Overall, it is important to develop multiple strategies to combat fouling on an ongoing basis. This is particularly critical in the case of irreversible fouling because, once membrane permeability is impaired by this type of fouling, chemical cleaning becomes necessary to restore it. Such cleaning requires halting the process for several hours, and the use of chemicals significantly shortens the membrane’s lifespan [16]. Physical cleaning methods rely primarily on mechanical forces that continually remove foulants from the membrane surface. As a result, these techniques mainly target reversible fouling mitigation [14], and their effectiveness declines over time as irreversible fouling develops. In this particular case, air scouring and backwashing are some of the most common methods applied in membrane systems treating MWW [17], having shown good results in numerous membrane bioreactors. On the other hand, aiming to control the irreversible fouling propensity, several strategies have been studied in the last few years, such as chemically enhanced backwashing (CEB). This approach involves employing a traditional backwashing method while introducing a moderate concentration of acid, alkali or an oxidant agent. The selection of the chemical is determined by the membrane material and/or the nature of the present fouling. Despite the extensive research available on the use of UF membranes for primary and secondary treatment, little is known about its suitability as a tertiary and quaternary treatment. Most studies have focused on assessing filtration performance under high filtration fluxes [14], focusing on reducing the required membrane area as much as possible. However, the combined effects of key operational parameters on fouling development require deeper evaluation, especially when using commercial membranes to filter real wastewaters. Therefore, the results obtained in this work provide novel and practical insights into operational strategies capable of significantly improving the filtration performance and enhancing the long-term viability of UF as an advanced quaternary treatment.
This work’s aim was therefore to evaluate the feasibility of applying UF membranes to perform tertiary/quaternary treatment in municipal wastewater treatment plants (MWWTPs). A semi-industrial filtration plant was used to carry out this study, assessing membrane fouling occurring in the system when filtering the supernatant of a full-scale MWWTP’s conventional secondary settler. Different operating conditions were studied to control fouling at the lowest possible energy demands and improve the viability of the process, determining the most suitable ones to perform secondary supernatant filtration. The operating conditions evaluated were (1) transmembrane flux, (2) the applied specific air demand (SAD), (3) the filtration/relaxation (F/R) ratio and backwashing periodicity, (4) the concentration of solids in the membrane tank and (5) the application of CEB.

2. Materials and Methods

2.1. Semi-Industrial UF Plant

The demonstration-scale UF plant used in this study mainly consisted of a membrane tank (UF, 587 L working volume) fitted with an industrial and commercially available submerged hollow-fiber UF membrane system (PULSION® Koch, 0.03 μm pore size, 43.5 m2 total filtration area). The membrane module was equipped with one screw pump (EcoMoineau™, PCM, Levallois-Perret, France) for vacuum filtration, using another one (NEMO Netzsch, Selb, Germany) for the continuous feeding of the membrane tank with fresh water (supernatant of secondary settler). A regulation tank (RT, 126 L working volume) was used to store and homogenize the influent, while a clean-in-place tank (CIP, 126 L working volume) was installed to store the generated permeate and allow integrated sampling and membrane backwashing when necessary. Air-assisted scouring was used to mitigate fouling during filtration, using a blower (GBH7, Elmo Rietschle, Bad Neustadt, Germany) to inject air from below the membrane tank. One additional mixing pump (EcoMoineau™, PCM, Levallois-Perret, France) continuously recirculated the contents of the membrane tank to ensure its complete mixture and avoid solids stratification. The operational sequence, illustrated in Figure 1, was as follows: the influent was homogenized in the RT and then continuously fed into the UF device by a pump (SP-2). A separate screw pump (SP-3) generated a vacuum to draw the permeate from the lumen (inside the hollow fibers) out to the CIP. The blower (B) injected air from below the submerged membrane module, while the mixing pump (MP) continuously recirculated the mixed liquor from the bottom to the top of the UF device. The semi-industrial plant was continuously fed with municipal wastewater (MWW) from the “Conca del Carraixet” full-scale MWW treatment facility (Alboraya, Spain). The influent source used in this work was the supernatant of the secondary settler. Figure 1 shows a flow diagram of the system, while the characterization of the influent fed to the membrane module and its elimination capacity concerning each analyzed pollutant can be found in Table 4.

2.2. Instrumentation, Automation and Control

The semi-industrial plant was constructed with a high level of automation. Numerous online sensors and automatic devices were installed to allow the full monitoring and control of the system. The online instrumentation included the following: a three-level sensor (Cerabar PMP11, Endress + Hauser) installed in the TR (LIT-1), MT (LIT-2) and CIP (LIT-3) tanks; one air flow meter (FIT-1) to regulate the air injected into the membrane tank; one pressure sensor (IP65, Druck, PIT-1) to monitor the TMP in the membrane tank during filtration; and two liquid flow meters (Promag, Endress+ Hauser) to control the inlet (influent fed to the MT, FIT-2) and outlet (permeate generated during filtration, FIT-3) flow rates in the MT.
The plant also included several actuators: four frequency converters (SINAMICS G120C, Siemens)—two for adjusting the liquid pump flow rates, one to control the flow rate of the mixing pump and one to control the flow rate of the air injected into the membrane tank; an automatic needle valve to precisely control the air flow injected into the membrane tank (VC-1); and two solenoid valves (on/off control valves) to avoid undesired water flows during the relaxation and backwash phases (EV-1 and EV-2). In addition, eight manual valves (V-1 to V-8) were installed along the water line to allow punctual sampling and manual operations. All instrumentation was connected to a programmable logic controller (PLC) to allow comprehensive multi-parameter management and data collection. The PLC communicated with a PC via an Ethernet network, where a SCADA system centralized all collected data and supported system monitoring and operation. Because of the substantial quantities of sensors and actuators that were installed, the control script relied on multiple control loops. These loops included classic PID and on/off controllers that were specifically designed to act on the primary operational parameters (such as the liquid and air flow rates, TMP control and tank level measurements, among others) to achieve the predetermined set points.

2.3. Operation and Experimental Plan

The semi-industrial plant operated continuously, performing consecutive filtration/relaxation (F/R) cycles. During relaxation, filtration was temporarily halted, without applying flow in any direction (neither permeate nor backwash). From this relaxation, the layer of solids, colloids and organic matter that had compacted on the surface of the membrane lost cohesion. Part of the reversible fouling was removed. As part of the fouling was removed, the TMP decreased in the next filtration cycle. Permeate backwashing (BW) was performed every “n” F/R cycles for fouling mitigation, with “n” adjusted to between 5 and 20 F/R cycles depending on the experiment (see Table 1). Filtration was performed at a constant flux for each experiment; the increment in the transmembrane pressure (TMP) during filtration was used as an indicator of the fouling occurring in the membrane. The membrane tank was operated under an infinite SRT during all experiments (i.e., no purge was performed except for sludge sampling, which was considered negligible compared to the total tank volume). This allowed an increase in the total suspended solids (TSS) concentration in the membrane tank from the initial 0.03 g L−1 in the secondary supernatant to 3–4 g L−1 after 9 months of filtration, similar to those achieved in the WWTP’s secondary sludge. The transmembrane pressure was monitored throughout the entire filtration period, without excluding any concentration period.
Different operating conditions and fouling control strategies were evaluated during continuous filtration, aiming to minimize fouling (see Table 1). The effect of the permeate flux (J) was evaluated first in order to establish appropriate values concerning the treated MWW, ranging from 9.6 to 17.6 LMH (test block B1). Air sparging intensity and backwashing were evaluated as potential fouling control strategies, assessing the effects of two different SADs (0.1 and 0.25 Nm3 m−2 h−1, test block B2) and two different F/R times and backwashing periodicities (test block B3). The effect of the TSS concentration achieved in the membrane tank on membrane fouling during continuous filtration was also assessed by replicating the operating conditions of a former experiment after incrementing the operating TSS concentration from 0.4 to 3 g L−1 (test block B4). On the other hand, experimental blocks B5 to B8 were conducted to study the possibility of recovering the membrane permeability under different CEB conditions. In this regard, the potential benefits in terms of fouling mitigation of applying CEB were evaluated under different conditions: (1) two reagents (citric acid and sodium hypochlorite), (2) five chemical concentrations (from 50 to 400 ppm), (3) effectivity improvement in the citric acid by reducing the pH to 2.5 with HCl and (4) three different backwashing intensities. More information concerning the CEB protocol can be found in Section 3.2. Finally, experiment B9 was performed, where a set of operating conditions was selected according to the results obtained in the previous experiments to test their suitability in the medium term. These last blocks of experiments are shown in Table 3.

2.4. Analytical Methods, Calculations and CEB Protocol

Samples of the secondary settle, the membrane tank and the produced permeate were collected twice per week to assess the system’s retention of solids and organic matter (OM). Concentrations of total and suspended solids (TS and TSS), as well as the total chemical oxygen demand (COD), were measured following standard methods [18]. To obtain the soluble fraction of each sample, 0.45 µm glass fiber membrane filters (Millipore, Merck, Darmstadt, Germany) were used. NH4+ and PO43− concentrations were measured from this soluble fraction according to standard methods [18]. The concentrations of soluble microbial products (SMPs) and the filterability of the membrane tank mixed liquor were measured weekly to confirm that they remained stable throughout the experiments. Proteins and carbohydrates were considered the main components of SMPs; their concentrations were analyzed using a commercial total protein kit (Micro Lowry Peterson’s Modification, Sigma-Aldrich, St. Louis, MO, USA) and the Dubois method [19], respectively. Sludge filterability was evaluated using the time-to-filter (TTF) method, following the protocol outlined in the standard methods [18]. Regarding pathogens, the identification of E. coli was carried out according to the UNE-EN ISO 8199 and 9308-1 methodology, Legionella spp. by means of the UNE-EN ISO 11731 methodology and intestinal nematodes by means of the modified Bailenger method. DNA isolation was carried out by using the instructions specified in the kit used (E.Z.N.A.® Soil DNA Kit, Omega Bio-tek, Norcross, GA, USA), analyzing both the effluent of the secondary settler and the effluent of the UF membrane. Once the DNA was extracted, the solution obtained was analyzed using a Thermo Scientific NanoDrop 2000 spectrophotometer (Thermo Scientific, Waltham, MA, USA) to obtain the DNA concentrations of the samples and to check their purity by performing absorption measurements at 230, 260 and 280 nm. The particle size distribution of the influent was characterized by means of a laser granularity distribution analyzer (Mastersizer 2000, Malvern, Worcestershire, UK; detector range of 0.01 to 1000 µm).
The 20 °C-normalized membrane permeability (K20) was determined using the temperature-normalized flux (J20) and the daily average transmembrane pressure (TMPave) measured during filtration, according to the methodology exposed in [18]. The effectiveness of CEB was evaluated by assessing the recovery of membrane permeability once it was applied, calculated as the difference in the average K20 from three filtration cycles before and after CEB. The cost of chemicals was estimated based on their market prices and data from other works, establishing the following: EUR 1.62 per L for HCl at a volumetric concentration of 35% and EUR 0.75–5.20 per kg for citric acid [20,21]. A CEB cleaning protocol was carried out to determine the main fouling source and the chemical cleaning effectiveness of each reagent used. It consisted of performing CEB whenever the TMP dropped below −400 mbar. This cleaning was conducted at a backwashing flux of 9.2 LMH with a duration of 5 min and was performed under different concentrations (50, 100 and 400 ppm) of citric acid and sodium hypochlorite, aiming to determine the best reagent and chemical concentration (see Table 3). Once the better reagent was determined, a battery of tests with increasing concentrations (50, 100, 200, 300 and 400 ppm) was carried out to evaluate the chemical concentration effect on the permeability recovery effectiveness. All cited concentrations were referenced to the final concentrations of each chemical reached in the membrane module after concluding the CEB, with the chemicals prepared in the CIP tank and mixed with the produced reclaimed water whenever CEB was programmed.

3. Results and Discussion

3.1. Effects of Transmembrane Flux on Membrane Fouling

Since membrane acquisition costs represents the main bottleneck in numerous emerging membrane-based treatment schemes [21], operating at elevated transmembrane fluxes would be necessary to prove the feasibility of UF membranes as a potential MWW tertiary/quaternary treatment alternative. Unfortunately, severe fouling was observed when operating at relatively high fluxes (about 15–17 LMH), forcing the halting of filtration due to the elevated TMPs reached (see Figure 2). Indeed, even when operating at medium fluxes (about 9 LMH), filtration was only possible for less than a month, demonstrating the important fouling issues in filtering this depurated MWW (i.e., supernatant of the secondary settler). The observed fouling was more severe than anticipated since a significant solids concentration was not reached in the membrane module during this period (TSS concentration lower than 0.2 g L−1). Furthermore, the particle size distribution of the membrane module’s content showed that a significant amount of small-sized particles was not present in the filtered bulk (see Figure S1 in the Supplementary Materials), thereby reducing possible pore narrowing and/or blocking issues due to the accumulation of colloids in the membrane pores. Potential fouling sources were thus reduced, in general terms, to two: gel layer formation by viscous soluble compounds, such as remnant SMPs, and membrane scaling.
On the one hand, SMPs are identified as one of most relevant foulants to affect the increment in fouling when filtering raw water [22,23], as well as strongly controlling the sludge filterability [24]. Since the supernatant of a secondary settler was filtered in this work, SMPs would be equivalent to those observed in AMBRs, sharing their fouling issues. Indeed, the SMP concentration in the membrane bulk rose to about 110 mg COD L−1 during the filtration period, similar to those reported in AMBRs [25]. Fortunately, since AMBRs have been extensively studied, potential fouling control strategies focused on mitigating SMP fouling could be employed to improve the presented alternative. On the other hand, [26] found that the development of the cake layer is mainly related to phosphorus precipitation. In fact, significant phosphate precipitation is reported in different MWW treatment filtration processes (see, for instance, [27]). In addition, the low TS retention obtained in this work (see Table 3) indicates the large proportions of salts and small-sized colloids (lower than 0.01 µm) present in the treated influent with respect to all of the solids fed to the membrane module. Inorganic salt precipitation would therefore be another important fouling source to control when using UF membranes as tertiary/quaternary treatments. These unexpected fouling sources may thus represent an important limitation for the proposed alternative. Fortunately, an important fraction of the fouling observed in this work was identified as reversible (see Figure S2 in the Supplementary Materials), with classic fouling control strategies applied in membrane systems, such as air scouring and backwashing, able to properly control it.

3.2. Fouling Mitigation by Physical Methods

Figure 3 shows an example of the reversible fouling’s evolution as filtration advanced in the short term (4 min). Since a great increment in the TMP was observed both during each single filtration cycle and during consecutive F/R cycles, increasing to about 18.7–25.2 mbar min−1 when operating at a transmembrane flux of 17.5–14.7 LMH, its reduction to lower values was imperative to achieve feasible operations. Firstly, the effectiveness of increasing the air scouring intensity was evaluated (see Table 1). A significant improvement was observed when raising the SAD from 0.1 to 0.25 Nm3 m−2 h−1, reducing the TMP rate during each filtration cycle from 64.4 to 31.9 mbar day−1 at a transmembrane flux of 14.7 LMH (see Figure 4). In fact, filtration was possible for 14.6 days of continuous operation thanks to this action, more than doubling its extension from the original 6.2 days when operating at the reduced SAD.
Secondly, the F/R time extension and the backwashing periodicity were also evaluated to minimize reversible fouling evolution. A sharp reduction in the fouling growth rate was also accomplished by reducing the filtration time period from 300 to 150 s and increasing the backwashing periodicity from 10 to 5, dramatically extending filtration from the former 15 days to more than 20 (see Figure 4). Indeed, the fouling growth rate was significantly reduced from 31.9 to 3.7 mbar day−1, almost completely mitigating reversible fouling during each filtration cycle (see Figure S3). However, it is important to take into account that the latter strategy also implies a reduction the net flux of filtered water, with part of the fouling prevented due to the reduction in the amount of pollutants reaching the membranes. Furthermore, a slight increment in the membrane area would also be necessary despite operating at the same flux (14.7 LMH in this case), since lower water productivity could be reached in this case. Nevertheless, the fouling growth rate reductions achieved are still important when considering the daily water treated. Water production was only reduced by 6.7% (from 14.9 to 13.9 m3 day−1), while the fouling growth rate was diminished from 4.8 to 1.7 mbar per m3 of treated water, when more suitable F/R times and backwashing periodicities were applied (i.e., conditions established in experimentsB4.1-B4.2). The application of these conditions was thus considered more suitable to enhance the viability of the proposed strategy, despite increasing both the energy requirements of the process and the membrane area demand due to the increment in the SAD and the reduction in the net transmembrane flux.
The SAD and filtration cycle time period in this work were initially based on other filtration systems treating MWW without biological activity (i.e., direct filtration of raw MWW or the supernatant of the primary settler), where reversible fouling played a negligible role (more information can be found in [16]), expecting, in this case, similar behavior. However, as aforementioned, fouling in this system was found to be more similar to that observed in AMBRs. Due to the low solids concentrations in the membrane bulk during all experiments (TSS concentration below 0.1 g L−1), colloids and soluble viscous substances probably played a dominant role in the observed reversible fouling. SMPs could be attached to the membrane surface and pores, increasing the filtration resistance or promoting the formation of a thick cake layer on the membrane surface by strongly adhering to particles. Increasing the SAD could thus prevent particle deposition and attachment to the membrane by inducing shear stress and fluctuations in the liquid layer near the membrane surface. Similarly, a reduction in the filtration time could help to reduce the amounts of materials interacting with the membrane during each cycle, preventing the formation of denser and thicker cake layers, which may result in more prominent reversible fouling that may evolve to irreversible fouling as more compression is applied. Certainly, when filtering raw MWW and the supernatant of the primary settler [28], it was found that denser cake layers could be formed when filtering influents with smaller particle sizes, with the filtration resistance increased in these situations. Given the small range of particles present in the secondary settler supernatant (see Figure S1 in the Supplementary Materials), similar behavior could be expected in this case. Thus, shorter filtration times would prevent further cake layer development, while longer relaxation times and more frequent backwashing would contribute to more membrane particle detachment.
Finally, the effect of increasing the operating solids concentration in the membrane tank on fouling evolution was evaluated (experiment B4). Strong fouling mitigation was achieved by increasing the TSS concentration from about 0.03–0.1 to 3–4 g L−1, reducing the fouling growth rate from the initial 100.5 in experiment B1 to 0.3 mbar day−1 (see Table 2). Indeed, this strategy proved to be highly efficient in controlling fouling evolution in this system, allowing it to operate for several weeks without a significant fouling increment, even when applying similar operating conditions to those used in the first set of experiments (i.e., a low SAD of 0.1 Nm3 m−2 h−1, long filtration time periods of 300 s and a low backwashing periodicity of one every 20 F/R cycles; see Table 2).
These results are in line with those obtained in other studies filtering raw MWW and the primary settler supernatant [16,29]. As reported in these works, increasing the TSS concentration of the bulk could result in beneficial effects regarding fouling mitigation by capturing soluble compounds and colloids and avoiding their interaction with the membrane [16,29]. In fact, the SMP/TSS ratio of the filtered sludge significantly decreases when increasing the TSS concentration in the bulk (see Table 2), with the increment in the particle number raising the probability of collisions among them and reducing the amounts of free colloids and other sticky substances. In addition, the relatively high filterability of the filtered sludge was observed in this study despite increasing the TSS concentration (see Table 2). It was more similar to that observed in biological sludges, such as activated sludge, rather than that found in raw MWW filtration [24]. These results suggest that the treated sludge still presents sufficient substances from the previous activated sludge treatment, such as the residual SMP concentration detected in the bulk, to flocculate suspended materials, thereby improving the sludge filterability as in activated sludge processes [24]. However, it may lack sufficient active materials to properly flocculate the smaller-sized free colloids, therefore not being able to prevent the quick irreversible fouling observed in this study when operating at low TSS concentrations, which was more similar to that observed when filtering raw MWW than when filtering activated sludge [16,28,29,30]. In this regard, it is important to take into account that the TTF analysis was performed with 0.45 µm filters, as is stablished in the standard methods [18], while the operated membrane had an average pore size of 0.03 µm. This significant difference between the filters’ pore sizes could have been the source of the observed discrepancies, with the smaller particles more severely hindering the operated membrane than the lab filters. Thus, despite the discrepancies observed in the reversible and irreversible fouling evolution of the treated influent (supernatant of the secondary settler) and that of direct membrane filtration (DMF), some of the fouling mechanisms dominating the latter may strongly affect the former under some circumstances. Fouling when filtering the secondary settler supernatant could therefore be considered halfway between physical processes (i.e., DMF) and biological sludge (i.e., MBR), albeit closer to the latter than to the former according to the results obtained in this study. Further research is required to properly determine the mechanisms dominating fouling in this type of influent and to determine the utility of the TTF method to estimate the filterability of these types of samples.

3.3. Fouling Mitigation by CEB

Blocks 5 to 9 of the experiments were focused on determining the membrane permeability recovery after CEB conditions, as established in Section 2.4. Firstly, the effectivity of citric acid and NaOCl as CEB reagents was evaluated, aiming to determine the main source of fouling in the system (organic or inorganic). Citric acid was shown to be significantly more effective when dosed at same concentrations as NaOCl (50, 100 and 400 mg L−1), practically tripling the membrane permeability recovered after CEB (see Table 3). Membrane scaling was therefore identified as the major irreversible fouling issue in this work, with organic matter having a greater influence on the observed reversible fouling, as discussed above. Secondly, the effect of increasing the citric acid concentration (from 50 to 400 mg L−1) during CEB was evaluated. Membrane permeability recovery consistently increased when raising the citric acid concentration during CEB, achieving recoveries from 0.63 to 23.00 LMH bar-1 (see Table 3). Their correlation was furthermore found to be linear (see Figure S4 in the Supplementary Materials), indicating that the reagent’s effectiveness does not decrease despite increasing the chemical’s concentration, at the same relatively low contact time between the cleaning solution and the membrane. Given that comparable normalized permeability recoveries were achieved regardless of the CEB concentration used (around 0.19 LMH bar−1 per gram of citric acid), the adjustment of the reagent dosage and its frequency could be tailored to operational requirements (such as a high fouling growth rate, reclaimed water productivity, etc.) or other specific needs.
Since citric acid showed substantial permeability recovery, experiments B7.1 to B7.3 were performed, aiming to enhance the action of this chemical. According to the membrane supplier, traditional membrane cleaning can be enhanced by lowering the pH of the acid solution to 2.5. The pH of the CEB solution prepared in the CIP tank was thus adjusted to this value by using HCl in this set of experiments, following the same experimental protocol as in the previous experiments (see Table 3). Meaningful improvements were observed in permeability recovery thanks to the pH reduction, with the CEB effectivity enhanced 6-, 1.3- and 1.1-fold when dosing 50, 100 and 200 mg L−1 of citric acid, respectively (see Table 3). Following the remarkable outcomes observed upon incorporating HCl into the CEB solution, especially at low citric acid concentrations, an experiment solely utilizing this reagent (labeled exp. B8) was conducted to isolate the effects of the pH on membrane cleaning. The result was membrane permeability recovery of 4.24 LMH bar−1 in this particular case, not overcoming the effectivity obtained when dosing citric acid at a similar concentration (see Table 3). This was unexpected, as acid cleanings are generally employed to eliminate inorganic fouling (i.e., scaling). The reduction in pH facilitates the resolubilization of salts precipitated on the membrane surface or within its pores, thereby removing them [13]. However, the differing effectiveness of each acid solution suggests that the pH was not the only factor involved in fouling removal in this case, but some chemical properties of citric acid may also have contributed to degrading and/or removing other fouling substances attached to the membrane. Further analyses are required to thoroughly investigate this performance difference between the two acid reagents in relation to membrane fouling. Thus, although HCl may be acquired at a slightly lower market price than citric acid, the latter was considered a better option for CEB due to its higher effectiveness. Certainly, when considering the cost of each permeability unit recovered by each reagent, CEB with HCl presents around EUR 0.035 per LMH bar−1 recovered, while citric acid can reduce this cost to about EUR 0.027–0.004 per LMH bar−1 recovered, according to the results obtained in this study. In addition, since pH levels below 2.5 are not approved for the commercial membranes employed in this study [31], fouling control by isolated HCl CEB could be extremely limited. Nevertheless, the use of low concentrations of HCl combined with citric acid seemed to significantly boost the acid’s activity and improve permeability recovery. The non-linearity observed in permeability recovery when increasing the citric acid concentration in the pH 2.5 CEB experiments, with a clear decay in the reagent’s effectivity (see Figure S5 in the Supplementary Materials), indicates that HCl meaningfully contributed to fouling cleaning. The decrease in the HCl concentration during CEB in this set of experiments, caused by the reduction in pH due to the increase in the citric acid concentration, was considered the main reason for the observed decay in permeability recovery (see Table 3 and Figure S5). Consequently, the use of both reagents together during CEB was considered as the most attractive option to improve permeability recovery while reducing the CEB cost, achieving, in this study, the lowest CEB cost when both reagents were added together at low concentrations (about EUR 0.017–0.003 per LMH bar−1 recovered when dosing 50 mg L−1 of citric acid at a pH of 2.5 with HCl).
Finally, to analyze whether the contact time between the reagents and the membrane influences cleaning, one of the tests (exp. B6.1) was repeated, modifying, in this case, the backwashing time. A reduction (exp. B9.1) and an increment (exp. B9.2) in this time were evaluated (from 5 to 2.5 and 10 min, respectively), maintaining nonetheless the effective chemical concentrations achieved in the membrane module so as to solely evaluate the effectiveness of the contact time. As Table 3 shows, no meaningful differences were observed in these tests, thus indicating that the contact time was high enough in all performed experiments and that it could even be reduced by half without reducing the effectivity. However, this was only demonstrated for low concentrations of citric acid and HCl (see Table 3). The optimal backwashing time, depending on the concentration of reagents during CEB, requires a thorough evaluation depending on the specific case. Periodic CEB (once or twice every 15 days of filtration) with low reagent concentrations (about 25–50 ppm of citric acid reached in the membrane tank) at a pH of 2.5 with HCl is proposed to control irreversible membrane fouling based on the results obtained in this study. Furthermore, regular NaOCl CEB (once every one or two months) could also be suggested to prevent organic irreversible fouling.

3.4. Medium-Term Filtration

A final experiment was proposed to evaluate fouling evolution when applying all of the most suitable conditions established in this work. They consisted of (1) minimizing the filtration time within each filtration/relaxation (F/R) cycle to prevent cake layer compaction and facilitate the removal of reversible fouling by relaxation and air scouring; (2) operating at high concentrations of TSS in the membrane tank to prevent soluble compounds and colloids from reaching the membrane surface; and (3) applying periodic CEB with low concentrations of citric acid and HCl (pH of 2.5) to control inorganic fouling, while minimizing costs and chemical consumption. Therefore, the operating flux was set to 21.5 LMH with an F/R time of 180/40 s. Backwashing with permeate water was performed every 10 F/R cycles, while the SAD was set to 0.2 Nm3 m−2 h−1. The average operating TSS concentration during this experiment was 3.7 g L−1, and CEB with citric acid and HCl at a pH of 2.5 was applied once a week, lasting 5 min. The obtained fouling evolution was dramatically lower than that obtained in all former experiments, although relatively high transmembrane fluxes were applied in this case. In fact, the operating TMP barely increased in the 35 days of continuous filtration applied in this experiment, achieving an extremely low fouling growth rate of 0.024 mbar d−1, indicating that higher transmembrane fluxes could be feasibly used (see Figure 5).
Other authors have also evaluated the use of MF/UF membranes as tertiary treatments for MWW, providing remarkable information. Overall, significantly higher fluxes than those used in this study have been reported, with [14] determining J20 of 60–80 LMH as the most suitable flux to operate the system, while [32] suggested around 30–40 LMH. This strongly contrasts the severe fouling observed in this work, which required us to use a significantly lower flux range of about 10–20 LMH. This indicates how fouling when filtering the supernatant of the secondary settler can differ depending on the MWW treated and/or the different steps contemplated in the MWW treatment facility. Unfortunately, since the cited studies do not provide enough information regarding the SMP content or the particle size distribution of the filtrated water, a suitable comparison to determine the main source of this fouling rate disparity is not possible. Further studies focused on evaluating the characteristics of the filtered water (e.g., TS, SMPs, particle size distribution, soluble nutrients, etc.) and their impacts on the fouling rate and mechanisms are required to elucidate the most convenient operating conditions (e.g., transmembrane flux, fouling control strategies, etc.) depending on the case.
On the other hand, the approach taken in the cited works [14,32] involved reducing the membrane area requirement as much as possible to reduce the capital costs and space demands of this technology. In consequence, the periodicity of the classic chemical cleanings suggested in these works was significantly higher than that obtained in this study (about every 10–50 days in [32] and 26 days in [14] versus the more than 1 year estimated in this work) (see Figure 5). In addition, more intensive fouling control strategies were also applied in the cited works, with [14] recommending CEB periodicities of about one every 3–4 days of filtration with both sulfuric acid and NaOCl, while [32] suggested high SAD intensities (around 0.3–0.6 Nm3 m−2 h−1) to mitigate fouling. In contrast, this study achieved long-term filtration without severe fouling rates while maintaining relatively low SADs. Further research is required to determine the best strategy to develop this alternative, focusing on the results from life cycle cost (LCC) and life cycle analysis (LCA) assessments. Concerning the CEB reagent, [14] determined that CEB with NaOCl had high effectivity in mitigating fouling. These results contrast those obtained in this study, which indicated that basic solutions had a reduced impact in recovering the lost membrane permeability. As aforementioned, this may indicate that fouling could be extremely different depending on the case evaluated. Depending on the main fouling sources and mechanisms, determining suitable operating conditions and effective methods to mitigate fouling in each particular case may thus be key to boosting this technology’s feasibility. It should be noted that, for this study, a commercially available industrial membrane module was employed. Consequently, if full-scale implementation were pursued, an identical module could be utilized, with the number of units simply adjusted to meet the total membrane area requirements. Because real wastewater was treated, the findings presented herein are directly scalable to industrial applications. At full scale, additional modules should be procured to accommodate the plant’s capacity; however, no modifications to the operating conditions or effluent water quality would be expected.

3.5. Reclaimed Water Quality

Table 4 shows the quality of the water produced by the UF membrane when used as a tertiary/quaternary step. As expected, no TSS were detected in the permeate thanks to the pore size of the membrane module used (0.03 µm), also significantly reducing the amounts of COD and BOD from the secondary settler supernatant and thereby enhancing the quality of the produced water. Indeed, this high OM capture capacity of UF has been reported by other authors [28,29], not only including the fraction associated with the particulate material but also including a portion of the soluble one, such as the SMP capture capacity observed in this work. The turbidity of the UF water was also meaningfully improved thanks to the retention of particles and colloids larger than the membrane’s pore size (0.03 µm). However, the used membrane was not able to capture a large proportion of the influent TS, only retaining about 5%. This indicates that almost all TS fed to the membrane tank were colloids under 0.01 µm (see Figure S1 in the Supplementary Materials) or soluble salts, which aligns with the important scaling problems detected during filtration in this work. In fact, when analyzing the nutrient capture capacity of the used UF membrane, significant reductions in the influent PO43− concentrations (about 18%) were observed. Since UF membranes do not have the capacity to retain ions, this phosphate reduction could be due to the precipitation of this compound as multiple salts. Phosphate is identified as one of the major membrane scaling promoters due to the various salts that it can form depending on the other ions present in water and the pH. For instance, it can easily precipitate as struvite, which is very challenging to manage. Similar PO43− losses have been reported in other membrane systems treating MWW, such as AnMBRs [27] or DMF [29], with the cited works also associating phosphate retention with possible PO43− salt precipitation. On the other hand, we also detected the complete retention of the influent NH4+ by the UF membrane. In this case, it was associated with the stripping of NH3 during filtration, as other studies also indicate [29]. The consumption of COD and nutrients through the possible development of aerobic bacteria in the membrane module was also contemplated in this work, given the extremely favorable conditions for their proliferation (large SRTs and continuous air injection in the system). However, due to the relatively short hydraulic retention times used in this work (about 0.6–0.3 h), the impact of aerobic biological activity on soluble nutrients’ consumption was considered negligible. Finally, regarding the membrane module’s capacity to retain pathogens, E. coli, Legionella spp. and intestinal nematodes were analyzed to confirm that the permeate quality was able to reach the indications of Regulation 2020/741 for class A reclaimed water (see Table 4). As expected, the generated permeate was completely free of potential pathogens (viruses and bacteria) since UF was used, as several studies also demonstrate [33,34,35]. The used membrane was even able to effectively eliminate the DNA concentration identified in the influent (see Table 4), reducing it below the limit of detection, i.e., 2.0 ng µL−1, of the used method (Thermo Scientific NanoDrop 2000 spectrophotometer), thus ensuring the quality of the reclaimed water. Nonetheless, despite the extremely positive results obtained in this study regarding DNA removal, other authors have reported significant DNA concentrations in the permeate after UF [36,37]. Thus, further studies considering the methodology used for its identification, as well as identifying the viability of the detected DNA in the permeate, are required to properly assess the disinfection capacity of UF.
According to the results obtained in this work, the produced water could be suitable for fertigation purposes when possible or discharged safely into any water body when its nutrient content has been properly recovered. Indeed, this reclaimed water could also be used to recover the dissolved nutrients in MWW via electroconcentration [38,39,40], microalgae cultures [41,42] or osmosis membranes [43,44], since the absence of solids in the produced permeate would allow the application of any of these methods without important contaminant/fouling issues. On the other hand, whenever nutrient recovery is performed before the tertiary treatment (e.g., during the activated sludge treatment), membrane scaling during this alternative could be significantly reduced thanks to the decrease in the phosphate concentration reaching the membrane. In scenarios where phosphate is reduced before filtration, the application of much higher transmembrane fluxes could also be possible, further enhancing the viability of the proposed alternative. Concerning contaminants of emerging interest, UF membranes have also displayed moderate effectivity, with removal percentages of around 20% for pharmaceutically active compounds [33] and 90% for PFAs [45] and reaching elimination values of around 1.7 log for antibiotic resistance genes [36]. Including UF in any step of MWW treatment (i.e., DMF, AnMBR or as a tertiary/quaternary step) could thus be very beneficial in alleviating the pressure on later treatment steps, such as ozonation in electrocatalysis, which, although effective in eliminating emerging contaminants, are energy-demanding treatments. Further research will be performed in the field to validate some of the above-presented hypotheses, while the filtration process in the long term will also be evaluated.

4. Conclusions

UF proved to be an attractive alternative to carry out the tertiary/quaternary treatment of MWW. The main findings were as follows:
  • Fouling when filtering the secondary settler supernatant of a full-scale MWWTP was shown to be more severe than anticipated, importantly hindering the operating flux and requiring its reduction to about 10 LMH in this case in order to prolong filtration for more than one month without chemical cleaning. Fortunately, air sparging and classical backwashing showed great effectivity in mitigating this fouling, which was mainly reversible in the short term. An SAD of 0.2 Nm3 m−2 h−1 with prompt permeate backwashing every 10 F/R cycles was determined as an effective strategy to improve the filtration flux from the original 10 LMH to about 15, without a significant increase in the fouling rate.
  • Increasing the TSS concentration in the membrane tank during continuous filtration was also found to meaningfully mitigate fouling, allowing us to increase the operating flux to about 17.5 LMH at low fouling growth rates (around 0.3 mbar d−1).
  • The use of CEB also displayed great results in controlling fouling, determining acid solutions as the best option in this case. Specifically, the use of periodic CEB every 15 days at low concentrations of citric acid (about 50 mg L−1 reached in the membrane tank) at a pH of 2.5 with HCl was determined as the most suitable option in this study.
  • Filtration was possible for more than 35 days at a J20 value of 21.5 LMH, with a reduced fouling growth rate of about 0.024 mbar day−1, thanks to all the fouling control strategies evaluated in this work. Thus, continuous filtration could be possible for more than 1 year without requiring the process to be halted to conduct conventional chemical cleanings.
  • The permeate obtained showed great quality, with complete removal of the TSS, turbidity of 0.49 ± 0.22 NTU, COD and BOD5 of 8.40 ± 3.32 and 2.20 ± 0.95 mg L−1, respectively, and the absence of the pathogens established in the European regulations (i.e., E. coli, Legionella spp. and intestinal nematodes), with no detection of DNA residues. The generated permeate therefore satisfied the indications of Regulation 2020/741 for class A reclaimed waters, being highly attractive for fertigation usage due to the presence of soluble nutrients.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/w17243453/s1. Figure S1: Particle size distribution of the influent fed to the membrane tank (supernatant of the secondary settler). Figure S2: Example of reversible fouling evolution during each filtration cycle. Dots represent the obtained experimental data during two F/R cycles, while the dotted line represents the fouling baseline at the start of each corresponding cycle. Figure S3: Evolution of the TMP membrane when reducing the filtration time and backwashing periodicity: (A) TMP evolution in the medium term and (B) TMP evolution in one unique filtration cycle. Note that, in (B), all data were obtained after backwashing, when the membrane presented a similar fouling state. Figure S4: Comparison between acid chemicals used for CEB. Figure S5. Effects of adding HCl to reduce the pH during the membrane permeability recovery experiments.

Author Contributions

Conceptualization, L.B., A.S. and P.S.-P.; methodology, J.R.-V., Á.S.A. and P.S.-P.; formal analysis, J.R.-V. and Á.S.A.; investigation, J.R.-V., Á.S.A. and P.S.-P.; resources, L.B. and A.S.; data curation, J.R.-V., Á.S.A. and P.S.-P.; writing—original draft preparation, J.R.-V., Á.S.A. and P.S.-P.; writing—review and editing, L.B., A.S. and P.S.-P.; supervision, L.B., A.S. and P.S.-P.; project administration, L.B. and A.S.; funding acquisition, L.B. and A.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research work was funded by the Generalitat Valenciana through the project PROMETEO/2021/085.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors are grateful to the EPSAR (Entidad Pública de Saneamiento de Aguas de la Comunitat Valenciana) for its support of this work.

Conflicts of Interest

The authors declare no conflicts of interest.

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Water 17 03453 g001aWater 17 03453 g001b
Figure 1. Flow diagram of the UF membrane system.
Figure 1. Flow diagram of the UF membrane system.
Water 17 03453 g001aWater 17 03453 g001b
Water 17 03453 g002
Figure 2. Evolution of the TMP during continuous filtration for each transmembrane flux evaluated (test block B1). Constant operating conditions for these tests were SAD = 0.1 Nm3 m−2 h−1, TSS = 0.4 g L−1, (F/R)n:BW = (300/60)n:60 and n = 10. Dots represent the obtained experimental data, while the continuous line represents the maximum TMP imposed to protect the membrane’s integrity.
Figure 2. Evolution of the TMP during continuous filtration for each transmembrane flux evaluated (test block B1). Constant operating conditions for these tests were SAD = 0.1 Nm3 m−2 h−1, TSS = 0.4 g L−1, (F/R)n:BW = (300/60)n:60 and n = 10. Dots represent the obtained experimental data, while the continuous line represents the maximum TMP imposed to protect the membrane’s integrity.
Water 17 03453 g002
Water 17 03453 g003
Figure 3. Example of short-term TMP variation concerning the transmembrane flux applied. Note that all data were obtained after backwashing, when the membrane presented a similar fouling state.
Figure 3. Example of short-term TMP variation concerning the transmembrane flux applied. Note that all data were obtained after backwashing, when the membrane presented a similar fouling state.
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Water 17 03453 g004
Figure 4. Evolution of membrane fouling when raising the SAD from 0.1 to 0.25 Nm3 m−2 h−1 and modifying the (F/R)n from (300/60)10 to (150/60)5 The horizontal grey line marks the maximum TMP threshold (~550 mbar), indicating the end of the operation cycle. Note that all the shown data were obtained at an average transmembrane flux of 14.7 LMH.
Figure 4. Evolution of membrane fouling when raising the SAD from 0.1 to 0.25 Nm3 m−2 h−1 and modifying the (F/R)n from (300/60)10 to (150/60)5 The horizontal grey line marks the maximum TMP threshold (~550 mbar), indicating the end of the operation cycle. Note that all the shown data were obtained at an average transmembrane flux of 14.7 LMH.
Water 17 03453 g004
Water 17 03453 g005
Figure 5. TMP evolution when applying together all the most suitable conditions established in this work. Note that the operating flux was set to around 21.5 LMH in this final test. Dots represent experimental data, while the solid line represents their linear adjustment to estimate the average fouling growth rate during the experimental period.
Figure 5. TMP evolution when applying together all the most suitable conditions established in this work. Note that the operating flux was set to around 21.5 LMH in this final test. Dots represent experimental data, while the solid line represents their linear adjustment to estimate the average fouling growth rate during the experimental period.
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Table 1. Experimental plan.
Table 1. Experimental plan.
Test BlockExperimentJ20
(LMH)		SAD
(Nm3 m−2 h−1)		Backwash Flux
(LMH)		TSS
(g L−1)		(F/R)n:BW
(s)		F/R Cycles
Before Backwash (n)		Treated Flow
(m3 d−1)
B1B1.117.60.106.90.4(300/60)n:601014.9
B1.216.614.1
B1.314.612.4
B1.49.68.1
B2B2.114.60.256.90.4(300/60)n:601012.4
B2.214.60.1012.4
B3B3.114.60.106.90.4(300/60)n:601012.4
B3.214.6(150/30)n:60511.5
B4B4.114.60.106.93(300/60)n:602012.5
B4.217.60.25(150/30)n:60513.9
Table 2. Effects of solids increase in the membrane tank.
Table 2. Effects of solids increase in the membrane tank.
Test BlockB1B4
ExperimentB1.1B4.1B4.2
Operation period
(d)		378
J20
(LMH)		17.620.017.6
SAD
(Nm3 m−2 h−1)		0.10.10.1
Backwash flux
(LMH)		6.96.96.9
TSS
(g L−1)		0.43.03.0
TTF
(min−1)		0.311.141.12
SMP
(mg COD L−1)		128.3102.2105.1
SMP/TSS ratio
(mg COD g TSS−1)		320.834.035.0
(F/R)n:BW
(s)		(300/60)n:60(300/60)n:60(150/30)n:60
F/R cycles before backwash (n)10205
Treated flow
(m3 d−1)		14.912.513.5
Δ TMP
(mbar d−1)		100.56.90.3
Table 3. Membrane permeability recovery by CEB.
Table 3. Membrane permeability recovery by CEB.
Test BlockExp.Reagent DosedReagent Concentration (mg L−1) *pH of Solution **CEB Time (min)Recovered Permeability (LMH bar−1)
B5B5.1Citric acid503.050.6
B5.21002.97.5
B5.32002.710.3
B5.43002.615.0
B5.54002.623.0
B6B6.1NaOCl509.550.0
B6.21009.72.8
B6.340010.07.4
B7B7.1Citric acid + HCl ***502.553.6
B7.21009.9
B7.320011.2
B8B8.1HCl1152.554.2
B9B9.1Citric acid + HCl ***502.52.54.0
B9.2104.8
Notes: * Final concentration of each reagent reached in the membrane tank after CEB. ** pH level measured in the CIP tank. *** pH was almost at the set point (2.5) with citric alone, and the amount of HCl added was very low, around 0.08–0.03 mg L−1, as the citric acid concentration increased.
Table 4. Mean values of parameters measured in streams involved in the UF process and percentages of elimination.
Table 4. Mean values of parameters measured in streams involved in the UF process and percentages of elimination.
ParameterInfluent
( x ¯ ± S D ) 		Effluent
( x ¯ ± S D ) 		Membrane Retention Capacity (%)
TSS (mg SS L−1)23.0 ± 12.00100.0
TS (mg SS L−1)1185 ± 52.01130 ± 52.04.65
Turbidity (NTU)5.7 ± 3.50.5 ± 0.292.4
COD (mg L−1)19.3 ± 12.58.4 ± 3.356.5
BOD5 (mg L−1)5.5 ± 1.22.2 ± 0.960.1
NH4+ (mgN L−1)11.1 ± 3.60.3 ± 0.397.1
PO43− (mgP L−1)8.1 ± 2.66.7 ± 3.318.0
E. coli (UFC 100 mL−1)21.1 ± 17.70100
Legionella spp. (UFC L−1)00-
Intestinal nematodes (eggs L−1)00-
DNA concentration
(ng µL−1)		3.3 ± 1.5<l.d. *100.0
Note: * <l.d. = below limit of detection.
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Acebrón, Á.S.; Revert-Vercher, J.; Sanchis-Perucho, P.; Borrás, L.; Seco, A. Use of Ultrafiltration Membranes as Tertiary/Quaternary Treatment for Wastewater Reclamation in Municipal WWTPs. Water 2025, 17, 3453. https://doi.org/10.3390/w17243453

AMA Style

Acebrón ÁS, Revert-Vercher J, Sanchis-Perucho P, Borrás L, Seco A. Use of Ultrafiltration Membranes as Tertiary/Quaternary Treatment for Wastewater Reclamation in Municipal WWTPs. Water. 2025; 17(24):3453. https://doi.org/10.3390/w17243453

Chicago/Turabian Style

Acebrón, Á. Sabina, Julio Revert-Vercher, Pau Sanchis-Perucho, Luis Borrás, and Aurora Seco. 2025. "Use of Ultrafiltration Membranes as Tertiary/Quaternary Treatment for Wastewater Reclamation in Municipal WWTPs" Water 17, no. 24: 3453. https://doi.org/10.3390/w17243453

APA Style

Acebrón, Á. S., Revert-Vercher, J., Sanchis-Perucho, P., Borrás, L., & Seco, A. (2025). Use of Ultrafiltration Membranes as Tertiary/Quaternary Treatment for Wastewater Reclamation in Municipal WWTPs. Water, 17(24), 3453. https://doi.org/10.3390/w17243453

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