**Comparison between a Conventional Anti-Biofouling Compound and a Novel Modified Low-Fouling Polyethersulfone Ultrafiltration Membrane: Bacterial Anti-Attachment, Water Quality and Productivity**

**Norhan Nady 1,\* , Noha Salem <sup>2</sup> , Ranya Amer <sup>3</sup> , Ahmed El-Shazly <sup>4</sup> , Sherif H. Kandil <sup>5</sup> and Mohamed Salah El-Din Hassouna <sup>2</sup>**


Received: 12 August 2020; Accepted: 4 September 2020; Published: 10 September 2020

**Abstract:** In this work, the efficiency of a conventional chlorination pretreatment is compared with a novel modified low-fouling polyethersulfone (PES) ultrafiltration (UF) membrane, in terms of bacteria attachment and membrane biofouling reduction. This study highlights the use of membrane modification as an effective strategy to reduce bacterial attachment, which is the initial step of biofilm formation, rather than using antimicrobial agents that can enhance bacterial regrowth. The obtained results revealed that the filtration of pretreated, inoculated seawater using the modified PES UF membrane without the pre-chlorination step maintained the highest initial flux (3.27 ± 0.13 m<sup>3</sup> · m−<sup>2</sup> · h −1 ) in the membrane, as well as having one and a half times higher water productivity than the unmodified membrane. The highest removal of bacterial cells was achieved by the modified membrane without chlorination, in which about 12.07 × 10<sup>4</sup> and 8.9 × 10<sup>4</sup> colony-forming unit (CFU) m−<sup>2</sup> bacterial cells were retained on the unmodified and modified membrane surfaces, respectively, while 29.4 × 10<sup>6</sup> and 0.42 × 10<sup>6</sup> CFU mL−<sup>1</sup> reached the filtrate for the unmodified and modified membranes, respectively. The use of chlorine disinfectant resulted in significant bacterial regrowth.

**Keywords:** biofouling; ultrafiltration; polyethersulfone; chlorine; membrane modification; low-fouling surface

### **1. Introduction**

Proper pretreatment of the feed seawater for reverse osmosis (RO) helps to reduce microorganisms, thus protecting the RO membranes from fouling [1–3]. The ultrafiltration (UF) process is used as a pretreatment step; it serves as a barrier to remove components with a pore size larger than 100 nm, such as fine colloidal particles, bacteria, viruses, and larger molecules such as proteins [4–6]. The UF process has proved to be an effective alternative to conventional technologies in terms of both cost effectiveness and energy efficiency [2,7,8]. Moreover, according to environmental concerns, it is a good

choice as it provides the production of higher quality brine with low levels of toxic chemicals and contaminants compared to conventional pretreatment technologies [3,9].

Generally, membrane systems are prone to several types of fouling depending on the type of foulant itself, e.g., inorganic or scaling fouling, particulate and colloidal fouling, organic fouling and biological fouling [10–13]. However, biological fouling is the most difficult to control in seawater desalination [13]. Biofilm formation consists of three major phases; induction, logarithmical growth, and plateau phases. The induction phase is the phase in which biofouling starts, with bacterial attachment to the membrane surface by weak physicochemical interactions [11,14,15]. The second phase is the logarithmical growth phase of the attached microorganisms. This phase is associated with extracellular polymeric substance (EPS) secretion and biofilm development [11,14]. The third phase is the plateau, where biofilm growth is limited by fluid shear forces. This phase is the detachment process, as bacteria tend to leave the biofilm for another part of the membrane surface due to an increase in population density and a lack of nutrients [11,12]. This stage of biofouling is more difficult control compared to earlier stages and is mainly affected by nutrients, bacterial growth rate, the mechanical stability of the biofilm, and the effective shear forces [13–15]. Generally, the rapid flux decline occurs at the early stage of biofilm formation due to the initial attachment and growth of microorganisms, followed by a gradual decay by the establishment of an equilibrium condition between the growth of biofilm, EPS production, and the detachment of cells [11–15].

There are three major strategies commonly used to control the biofouling phenomenon in membrane-based processes: chemical and physical pretreatment of the feed water to reduce nutrient availability [4,16–18], biocide usage for metabolic inactivation [17,19–21], and membrane modification to make the membrane less prone to biofouling [22–25]. The conventional method of dealing with the incidence of biofouling is to treat feed water with a biocide or disinfectant [18]. Chlorine remains the most commonly used disinfectant because of its availability, reasonable cost, and effectiveness [26–28]. Furthermore, the efficiency of chlorine can be improved with the use of coagulants to remove the suspended materials [29]. However, there are limitations to chlorine usage including the need for controlling pH, turbidity, and contact time [18,26]; also, hypochlorous acid, which is formed at lower pH values, is highly reactive and corrosive [30].

Most of the used membranes are very susceptible to chlorine degradation. Therefore, chlorination has to be followed by dechlorination in the pretreatment strategy when membranes are used for water treatment. It was reviewed that the oxidative nature of hypochlorite may have detrimental effects on polyethersulfone (PES) membranes such as high protein retention [27,30,31], polymer chain breakage, and consequent expansion of the membrane pore size [27,32–34], changes in membrane surface charge (hydrophilicity/hydrophobicity) [31,33,34], and the deterioration of the membrane's mechanical strength [27,33].

Many bacteria can develop resistance against chlorine [28,35,36]. In addition, the removal of 99.9% of bacteria may not be sufficient to prevent their regrowth, as the surviving cells can multiply at the expense of biodegradable substances [37]. Moreover, the inactive biomass left in feed water after chlorination serves as a rich nutrient, resulting in a rapid bacterial growth rate [20,38]. Chlorine can also promote microbial regrowth by breaking down humic acids and producing assimilable organic carbon (AOC), which may act as a supportive nutrient for chlorine-resistant bacteria [18].

On the other hand, chlorine has environmental implications as it reacts with the organic matter of the feed water and produces various disinfection byproducts (DBPs) [39,40]. The types and concentrations of these DBPs depend on several factors such as the type and amount of disinfectant used, contact time, organic and inorganic contents, temperature, turbidity, and pH [13]. DBPs pose potential risks to human health and aquatic ecosystems when they are discharged in brine. Mediterranean seawater is particularly problematic as it usually contains high concentrations of bromide, which raise the risk of the formation of brominated DBPs that are more carcinogenic or mutagenic than their chlorinated analogs [41].

Another strategy used for fouling mitigation is the surface modification of ready-made membranes to acquire an effective anti-biofouling property [42,43]. PES is widely used for the preparation of UF membranes due to its excellent chemical resistance [44], good thermal stability, and mechanical properties [45]. PES membranes also show high flux and have a reasonable cost compared to other membrane materials. However, they are relatively hydrophobic, and their surfaces adsorb the components of the used fluid, which make them more susceptible to fouling [46]. Surface modifications of PES membranes are one of the current trends to control membrane fouling; they increase membrane surface hydrophilicity and consequently reduce the adsorption or adhesion of the different substances in feed water [47]. Surface modifications of PES membranes can be carried out in many ways, such as coating, blending, compositing, or grafting [24]. Several techniques can be used to initiate the grafting process, including chemical, photochemical, and high-energy radiation initiators [23,24,42,48], as well as enzymatic techniques [24].

Laccases are a group of oxidative enzymes whose exploitation as biocatalysts in the modification (grafting) of poly(ethersulfone) (PES) membranes represent a successful example of an environmentally friendly modification of PES membranes [49]. Phenols and aromatic or aliphatic amines are suitable substrates for laccase enzymes. Laccase-catalyzed reactions are proceeded by the monoelectronic oxidation of the substrate molecules to the corresponding reactive radicals that can then produce dimers, oligomers, and/or polymers [50,51].

Recently, a PES membrane was modified by the surface grafting of a brush-like hydrophilic polymer layer. This was achieved by enzyme-catalyzed grafting of an amine-bearing modifier, 3-aminophenol (3-AP), to obtain more hydrophilic PES membranes due to the presence of amine groups on the membrane surface. This method is known for its mildness and eco-friendliness as it can be carried out at room temperature, and uses only air as a source of oxygen and aqueous reaction medium, while no harsh chemicals are needed [47].

This study highlights an effective strategy to reduce biofouling in seawater desalination. It compares the effectiveness of membrane modification to reduce bacterial attachment, which is the initial step of biofilm formation, and the traditional strategy of using antimicrobial agents to kill bacteria cells in the seawater feed stream. The main aim of this study is to compare the efficiency of a conventional chlorination pretreatment step for feed water (seawater) with that of a modified PES UF membrane with brush-like oligomers of poly (3-AP) on its surface in terms of the UF membrane biofouling reduction, as well as comparing the environmental impacts of both strategies in terms of membrane performance and filtrate water quality. To the best of our knowledge, this is the first application study that compares the effect of membrane surface modification on the biofouling phenomenon compared to the traditional strategy of using antimicrobial agents that to mitigate biofouling in the membrane-based desalination process.

### **2. Materials and Methods**

### *2.1. Materials*

3-aminophenol (3-AP, C6H7NO), dichloromethane (DCM, CH2Cl2), sodium acetate (anhydrous, C2H3NaO2), acetic acid (C2H4O2), catechol (C6H6O2), and sodium thiosulfate pentahydrate (Na2S2O3·5H2O) were obtained from Sigma-Aldrich (Germany). All of them were at least 98% purity. A flat sheet of polyethersulfone (PES; 0.03 µm pore size) was purchased from Sterlitech (USA). Laccase from *Trametes versicolor* (>0.5 U·mg−<sup>1</sup> ) was obtained from Fluka (Germany). Sodium hypochlorite solution (NaOCl, available chlorine 4%–5%) was purchased from Alpha Chemika (India). Sodium bisulfite (a mixture of NaHSO<sup>3</sup> and Na2S2O<sup>3</sup> powder) was obtained from Acros Organics (Belgium). Ethanol (analytical reagent grade) and *N,N*-diethyl-*p*-phenylenediamine 4 (DPD4) Palintest were purchased from Fisher (United Kingdom). Ferric chloride (FeCl3, anhydrous) was obtained from Oxford Laboratory (India). Luria–Bertani (LB) agar (Lennox) was obtained from Conda (Spain). Yeast extract was obtained from Bio Basic (Canada Inc., Canada). Peptone water

medium (peptone 5.0, tryptone 5.0, sodium chloride 5.0) was purchased from Lab a Neogen Company (United Kingdom). Sodium phosphate monobasic, disodium hydrogen phosphate-2-hydrate, and potassium iodide (KI) were obtained from Riedel-de Haën (Germany). Soluble starch was purchased from Daejung (Korea).

### *2.2. Methods*

### 2.2.1. Laccase Activity

Laccase activity was determined using catechol as a substrate, as previously described [46]. Briefly, the assay mixture contained 0.33 mL of 10 mM catechol, and 2.67 mL of 0.1 M sodium acetate buffer (pH 5), with 0.025 UmL−<sup>1</sup> laccase. Catechol oxidation was monitored by following the increase in absorbance at 400 nm (ε = 26,000 M−<sup>1</sup> ·cm−<sup>1</sup> ) with a reaction time of 20 min. One unit of laccase activity is defined as the amount of enzyme required to oxidize 1 µmol of catechol per minute at 25 ◦C. − ε − −

### 2.2.2. Modification of PES Membrane Surfaces

Flat rectangular sheets of commercial PES membrane (200 × 200 mm, 0.03 µm pore size, (Sterlitech, USA) were cut into circles of 4.5 cm diameter to fit in a 50-mL Amicon filtration cell. The membrane modification was carried out as previously described [47,52]. The membrane circles were immersed in 40 mL of 0.1 M sodium acetate buffer (pH 5) containing equal volumes of 15 mM 3-AP and laccase enzyme (0.5 U·mL−<sup>1</sup> ). Air was bubbled through the solution for the purpose of good mixing and as a source of oxygen for the enzyme catalytic cycle (i.e., enzyme reactivation). The reaction was carried out for 30 min at room temperature (23 ± 2 ◦C). After completing the modification, the membrane circles were washed first by spraying with deionized water, followed by dipping in freshly boiled deionized water (95 ± 2 ◦C), and they were subsequently dried for 24 h in a desiccator.

### 2.2.3. Seawater Sampling

This study was conducted using Mediterranean seawater from the El Max region of west Alexandria, Egypt, in February 2018. Total dissolved salts (TDS), turbidity, and calcium content were measured immediately after sampling using standard methods [53]. The purpose of using seawater was to maintain the natural composition of the feed water used in the experiments. The feed seawater was stored at a controlled room temperature (20 ± 2 ◦C). Figure 1 shows a schematic diagram of the experimental steps, as described in the following sections.

**Figure 1.** A schematic diagram of the experimental steps.

### 2.2.4. Preparation of the Feed Seawater

### Pretreatment of the Feed Seawater by Coagulation

The collected seawater was pretreated to reduce the suspended matter in order to prevent membrane blockage by other types of fouling rather than biofouling, which was being investigated. Coagulation was carried out using ferric chloride (FeCl3) by the conventional standard jar test. A stock solution of FeCl<sup>3</sup> (1000 mg·L −1 ) was prepared, 200 mL samples of seawater were placed in 250 mL beakers, and different concentrations of ferric chloride (2, 4, 6, 8, 10, 15, and 20 mg L−<sup>1</sup> ) were added into the beakers. Samples were stirred for 1 min at 100 rpm followed by 20 min of slow mixing at 30 rpm [54]. Residual turbidity was determined as an indicator of performance, and the optimum dose of the coagulant was identified. The optimum dose of the FeCl<sup>3</sup> was added to seawater and then kept for a week for sedimentation, and subsequently, the supernatant of clear water was taken.

### Seeding of Seawater with Bacterial Load

The pretreated seawater was inoculated with various bacterial strains that are actually present in seawater in order to investigate the biofouling phenomenon within a relatively short time. Inoculum was prepared by adding 1 mL of seawater into 30 mL sterile Luria–Bertani (LB) broth (consisting of 0.5% tryptone, 0.5% peptone, 0.5% yeast extract, and 0.5% NaCl). The tubes were incubated in a shaker incubator at 30 ◦C and 150 rpm for five days. Then, the pretreated seawater was inoculated with 1.5% of this bacterial suspension (OD<sup>680</sup> = 1.8) immediately before the UF experiment.

### Disinfection of the Feed Seawater

The disinfection of the seawater was carried out using chlorine in the form of sodium hypochlorite (NaOCl) solution with 4%–5% available chlorine. Practically, only 3% of the available chlorine was determined by standardization using 0.01 N sodium thiosulfate (Na2S2O3) solutions. The standard method of iodometric titration [55] was used: 200 mL of chlorine solution was placed in a conical flask and 5 mL of glacial acetic acid was added to reduce the pH to between 3.0 and 4.0, followed by adding 1 g of potassium iodide, and the solution was mixed well. The potassium iodide solution was titrated against 0.01 N sodium thiosulfate solution until the yellow color of the liberated iodine almost faded away. Then, 1 mL of 1% starch solution indicator was added, producing a blue color, followed by titration again against 0.01 N sodium thiosulfate solution till the blue color disappeared; the total volume of titrant was measured, and total chlorine was determined in mg·L −1 from Equation (1):

$$\text{Residual chloride} = \text{A} \times \text{N} \times 35.45 \times 1000 \text{/mL sample taken} \tag{1}$$

where A is the mL of titrant for the sample and N is the normality of sodium thiosulfate.

In this work, a stock solution of sodium hypochlorite (1000 mg L−<sup>1</sup> ) was freshly prepared. A chlorine dose of 6 mg L−<sup>1</sup> , which was prepared from this stock solution, was added to the inoculated pretreated feed water (pH 6) and was kept for 90 min contact time in the dark at room temperature (23 ± 3 ◦C). Then, the free residual chlorine was measured using the standard DPD colorimetric method.

### Dechlorination Process

Sodium bisulfite was added as a dechlorinating agent for the removal of any residual chlorine before UF to protect the membrane from deterioration by chlorine; a stock solution of sodium bisulfite (100 mg L−<sup>1</sup> ) was prepared using sterile distilled water. Excess sodium bisulfite was used to confirm the complete removal of chlorine (each 5 mg L−<sup>1</sup> of sodium bisulfite was added to remove 1 mg L−<sup>1</sup> of residual chlorine), kept for 30 min contact time at a room temperature of 23 ± 2 ◦C. Then, residual free chlorine was measured using the DPD colorimetric method; a chlorine standard curve was established by the preparation of different known concentrations of chlorine (0.05, 0.2, 0.5, 0.8, 1, 2, and 3 mg L−<sup>1</sup> ) in 10 mL sterilized distilled water. The concentration of residual chlorine was measured by adding

DPD tablets into 10 mL of solution and shaking for 2 min. The intensity of the produced red color was measured using a Vis-spectrophotometer at 515 nm wavelength, and the concentration of residual chlorine was determined in mg L−<sup>1</sup> using the prepared chlorine standard curve.

### 2.2.5. Ultrafiltration (UF)

A dead-end stirred filtration cell (Millipore, Amicon Model 8050, 13.4 cm<sup>2</sup> active filtration area) was used at a constant pressure of 1 bar and 200 rpm stirring at 23 ± 2 ◦C. Unmodified and modified PES membranes were used in the filtration of different conditions of feed seawater. Two different feed seawater types, in terms of pretreatment steps, were used: one pretreated by coagulation only, without the chlorination step, and the other pretreated by coagulation followed by chlorination and dechlorination steps. Then, the pretreated feed seawater samples were seeded with a bacterial suspension and filtered for 9 h over three days (3 h/day) of filtration time. UF was also carried out for the pretreated feed seawater without bacterial loading for 2.5 h of filtration time in order to investigate the membrane permeability in the absence of bacterial cells.

First, the filtration cell was immersed in 70% ethanol overnight, and then it was washed three times with sterilized deionized water to remove any traces of ethanol. The membranes were also washed three times with sterilized deionized water before the experiment, and then UF was carried out. At the end of each day, the membrane was placed in 50 mL of initial pretreated seawater without bacterial suspension to avoid any damage caused by drying. The experiment for each tested condition was performed three times; a new membrane was used in each experimental assay. The microbiological results were normalized and their average was taken.

### Water Flux

The pure water flux was determined at the start and end of each day using Equation (2):

$$\mathbf{J}\_{\mathbf{w}} = \frac{\mathbf{Q}}{\Delta t \cdot \mathbf{A}} \tag{2}$$

where J<sup>w</sup> = water flux (m<sup>3</sup> ·m−<sup>2</sup> ·s −1 ), *Q* is the volume of permeate collected (m<sup>3</sup> ), ∆*t* is the sampling time (s), and *A* is the membrane area (m<sup>2</sup> ) [47].

### Water Productivity

The volume of the output of filtrate from the membrane was determined in m<sup>3</sup> ·h <sup>−</sup><sup>1</sup> at the end of each day for three days.

### 2.2.6. Bacterial Count

Bacterial growth was counted in the initial feed seawater and the filtrate produced each day as well as the filtrate mixture produced. Standard plate counts were used by plating 100 µL of suitable serially diluted bacterial suspension in phosphate buffer solution (PBS) pH 7, ranging from 10−<sup>1</sup> to 10−<sup>5</sup> , on LB agar in three replicates, followed by incubation at 30 ◦C overnight. The number of separate colonies was recorded as a colony-forming unit (CFU) mL−<sup>1</sup> [56].

Moreover, at the end of the experiment, the membrane was cut into two identical halves; one half was placed in 30 mL PBS and the bacteria attached on the membrane surface were determined twice; the first bacterial count was determined immediately for the loosely attached bacteria by gently handshaking for a minute, and then the count was determined again for the total bacterial cells attached on the membrane surface using mechanical shaking after incubation overnight in a shaker incubator at 150 rpm, 30 ◦C. The bacterial counts were determined as the CFU m−<sup>2</sup> of the membrane surface.

### 2.2.7. Scanning Electron Microscope Imaging

Both unmodified and modified membrane surfaces, after UF of both pretreated, inoculated feed seawater without the chlorination step and pretreated, inoculated feed seawater with chlorination and dechlorination steps, were imaged using a JeolJsm 6360 LA scanning electron microscope (SEM, JEOL Ltd., Tokyo, Japan). After the experiment, the membranes were cut using a very sharp blade and were preserved in a fixer composed of 0.3% glutaraldehyde, 5% formaldehyde in phosphate buffer (pH 7.2), and serially dehydrated in modified ethyl alcohol [57]. Moreover, the formed layers on both unmodified and modified PES membranes, after UF of pretreated feed seawater with neither the chlorination step nor bacterial loading and after UF of pretreated feed seawater with the chlorination step and without bacterial loading, were imaged. All surfaces were coated with Au before imaging. A voltage of 20 KV and a resolution of 1280 × 960 pixels were used.

### 2.2.8. Atomic Absorption Spectroscopy Analysis

Atomic absorption spectroscopy (Shimaddzu AA-7000, Tokyo, Japan) was used for the analysis of the layers formed on both the unmodified and the modified PES UF membrane surfaces after filtration of the two types of feed seawater, without bacterial loading (feed seawater pretreated without the chlorination step and feed seawater pretreated with chlorination and dechlorination steps), at a constant pressure of 1 bar and 200 rpm, at 23 ± 2 ◦C, for 2.5 h of filtration time.

### **3. Results and Discussion**

### *3.1. Chemical Analysis of the Used Seawater*

A chemical analysis of the seawater showed a high calcium content (483.36 ± 8.92 mg L−<sup>1</sup> ) due to the winter season, as previously reported [58]. The turbidity was determined as 3.17 Nephelometric Turbidity Units (NTU). The total dissolved salts (TDS) concentration was 27.2 ppt (ng/L), which is less than the average for Mediterranean seawater [59] because of the proximity of the El-Mahmoudiyah canal outfall to the sampling point.

### *3.2. Membrane Characterization*

The membrane characterization was previously performed and published [47], and the obtained results are briefly presented as follows: Thermogravimetric Analyses (TGA) show that the rate of decomposition of the backbone of the modified membranes is somewhat slower than that of the blank membrane. As shown at 800 ◦C, only 38 wt% of the modified membrane remained, whereas only 15 wt% remained of the blank membrane. Moreover, Differential Scanning Calorimetry (DSC) analysis revealed that the glass transition temperature of the blank PES membrane was 226 ◦C, and it decreased very slightly upon modification to 224 ◦C. X-ray diffraction (XRD) analysis shows the effect of the amorphous structure of poly(3-AP) on the intensity of the characteristic peak of the blank membrane; it is proposed that the addition of poly(3-AP) may contribute to the increase in the flux of the modified membranes. The tensile strength test of the membranes showed a very slight decrease in the tensile strength of the blank membranes. However, the modified membranes at a high grafting yield showed slightly stronger mechanical properties than the blank membrane. The Raman spectra of the modified membrane confirm the presence of amine groups on the membrane surface. Scanning Probe Microscope (SPM) images show the formation of a brush-like modifying layer of poly (3-AP). Furthermore, the Nuclear Magnetic Resonance (NMR) integration results of the analyzed peaks do not favor a particular structure. The proposed structure of the formed poly(3-AP) layer is shown in Supplementary Figure S1. The water flux of the most modified membranes increased up to 35% relative to the blank (unmodified) membrane, and an up to 90% reduction in protein adsorption was obtained. In general, this modification does not harmfully affect the bulk properties of the original blank membrane.

### *3.3. Pretreatment of the Feed Seawater (Coagulation–Flocculation)*

Coagulation and flocculation are important pretreatment processes for the removal of colloidal particles responsible for the turbidity of seawater [13]. The destabilization of colloidal particles is usually carried out by adding coagulants followed by the clotting of the resulting unstable colloidal particles, which are then removed from water by sedimentation [60]. In this work, coagulation was carried out by the standard jar test using ferric chloride (FeCl3) due to its proven performance as a coagulant in water treatment plants [61]. The addition of FeCl<sup>3</sup> resulted in the rapid removal of turbidity as a result of the neutralization of the negatively charged particles with different cationic species produced from the hydrolysis of ferric chloride in water, leading to the destabilization of such particles and subsequently flocculation (Supplementary Figure S2). Maximum turbidity removal was about 82.6% at 8 mg/L of FeCl<sup>3</sup> (Supplementary Figure S3). However, when high concentrations of FeCl<sup>3</sup> were used, lower turbidity removal was obtained due to competition between the re-conformation rate of negatively charged particle networks and the collision rate of destabilized colloids [61,62].

### *3.4. Ultrafiltration Process*

### 3.4.1. Water Flux

### Pretreated Feed Seawater without Bacterial Loading

A UF experiment was carried out using pretreated seawater with neither bacterial loading nor the pre-chlorination step in order to investigate the effect of other seawater contents, which can affect membrane performance. As shown in Figure 2, the water flux reduced over time for both unmodified and modified membranes to less than half its initial value (unmodified, 57.5% reduction; modified, 62.9% reduction). Observation of the membrane before and after filtration showed the formation of a colored layer that precipitated on the membrane surface (Figure 3) (unmodified PES, c; modified PES, d). SEM images of this formed layer showed salt precipitation. The presence of calcium was proposed due to its high content in the raw feed seawater (483.36 ± 8.92 mg L−<sup>1</sup> ) and was confirmed by atomic absorption analysis. However, there may be other salts/dissolved materials that were precipitated on the membrane surface. It should be noted that, in desalination plants, the inlet feed is usually diluted to reduce the water salinity to around 15,000–20,000 ppm to minimize salt precipitation on the membrane surface. Calcium ions have a negative impact on the membrane flux by altering the surface chemistry through interaction with foulant molecules, such as natural organic materials (NOM) [11,63,64]. Calcium can also link two negatively charged functional groups together to form intermolecular complexes; when the linkage happens between two humic acid molecules, a gel layer of macromolecules can be formed through this intermolecular bridging, and it becomes more compact and cohesive by the cross-linking effect of calcium [64]. As shown in Figure 4, scanning electron microscope images showed that most of the formed layers appeared as separate crystals on the modified membranes, whereas they appeared as a packed gel layer on the unmodified PES membrane. The pre-chlorination step of the feed seawater did not make a significant change to the general performance of both unmodified and modified membranes (i.e., only a fluctuation up to 6%).

### Pretreated Feed Seawater with Bacterial Loading

The bacterial count of the seawater sample was determined as 1600 CFU mL−<sup>1</sup> . In order to investigate the biofouling phenomenon within a relatively short time, the pretreated seawater was inoculated with a bacterial load of various bacteria strains that are naturally present in seawater. The seawater was seeded by a 1.5% bacterial suspension of OD<sup>680</sup> = 1.8 and was filtered using a dead-end stirred filtration cell at a constant pressure of 1 bar at 23 ± 2 ◦C and 200 rpm stirring for 9 h (3 h × 3 days) filtration time. PES membranes of 0.03 µm pore size were used in the filtration of pretreated (i.e., by the coagulation step as described in the previous section) inoculated feed seawater, with or without the pre-chlorination step. The biofouling phenomenon and biofilm formation on

the PES membrane surface with the consequential effect on membrane performance was studied. Membrane performance was evaluated by determining: (1) the initial and final flux of the membrane, (2) water productivity (volume of filtrate), (3) bacterial count of the filtrate on each day, (4) bacterial count of the filtrate mixture produced over three days, (5) bacterial count on the membrane surface and (6) SEM imaging of the membrane surface.

**Figure 2.** The flux of pretreated seawater with neither bacterial load nor the chlorination step. Three samples were tested for both the unmodified and modified membranes. Reference conditions: pressure of 1 bar, 23 ± 2 ◦C at 200 rpm stirring for 2.5 h filtration time. Unmodified membrane (black filled circle) and modified membrane (black unfilled diamond).

**Figure 3.** Photos of unmodified membrane before (**a**) and after (**c**) filtration of pretreated seawater without bacterial loading, and modified membrane before (**b**) and after (**d**) filtration of pretreated seawater without bacterial loading. Photo of unmodified membrane after filtration of pretreated, inoculated seawater without chlorination (**e**) or with the chlorination (**g**) pretreatment step, and photo of modified membrane after filtration of pretreated, inoculated seawater without chlorination (**f**) or with the chlorination (**h**) pretreatment step. Reference conditions: 1 bar, 23 ± 2 ◦C and 200 rpm stirring for 2.5 h filtration time.

**Figure 4.** SEM images of unmodified ((**a**), ×1000) and modified ((**b**), ×1000) polyethersulfone (PES) membranes after filtration of pretreated seawater with neither bacterial loading nor the chlorination step, and modified ((**c**), ×2000) PES membranes after filtration of pretreated seawater without bacterial loading but after the chlorination step. Reference conditions: 1 bar, 23 ± 2 ◦C and 200 rpm stirring for 2.5 h filtration time.

As shown in Figure 5, on the first day of filtration of the pretreated, inoculated feed seawater, without the chlorination step, the initial flux of the unmodified membrane was significantly reduced, and only about 43% of its value was maintained relative to seawater without bacterial loading. Meanwhile, the modified membrane was able to maintain about 76.3% of its initial flux relative to seawater without bacterial loading. The modified membrane had a higher initial flux compared to the unmodified one, which can be attributed to the presence of free polar groups of brush-like oligomers formed on the membrane surface, as previously presented and shown in Supplementary Figure S1 [47], which increased both its hydrophilicity (the static water contact angle of the unmodified and modified PES are 75.9 ± 2 ◦ and 41.2 ± 1.7◦ , respectively) and the repellence of bacteria. Both effects can facilitate water permeation. The flux declined rapidly to reach about 1% of its initial flux for both unmodified and modified membranes by the end of the first day (3 h filtration). This rapid flux decline can be correlated with two main effects.

The first effect is concentration polarization, which resulted from the accumulation of larger solutes (such as calcium crystals, as illustrated in the previous section) that were rejected and retained at the membrane surface. As they could not diffuse back to the bulk solution, they caused a concentration gradient above the membrane surface and created an osmotic back pressure that reduced the effective transmembrane pressure of the system [11,64].

The second factor responsible for the rapid flux decline was the early attachment and proliferation of bacterial cells maintained on the membrane surface [11,17,65,66]. Bacterial cells colonized the membrane through the reversible and irreversible attachment of bacteria's surface via electrostatic and hydrophobic interactions [17] as the first step of biofilm formation. Moreover, depositions of bacterial cells on the membrane surface formed hydraulic resistance, which resulted in additional concentration polarization as bacterial cells affected the porosity and pore size distribution on the membrane surface, resulting in the precipitation of salts within membrane pores [20,65].

On the other hand, when the pretreated, inoculated seawater was exposed to chlorination and dechlorination pretreatment steps, it was observed that the chlorination did not cause evident changes in the initial flux of the unmodified membrane, as it decreased to about 53% of its value relative to the feed seawater without bacterial loading. Meanwhile, the chlorination step resulted in a reduction in the modified membrane flux to about 36.7% of its value relative to the case of using feed seawater without bacterial loading, and an even greater reduction compared to the pretreated, inoculated seawater without chlorination (45.8%). There is no obvious explanation for the effect of chlorination on the modified layer; however, SEM images of the salt layer formed on the membrane surface, when UF was carried out using pretreated seawater without bacterial loading, showed a difference in the shape of the formed layer in the presence or absence of chlorine. When the feed water was pretreated without the chlorination step, a continuous gel layer was formed on the unmodified membrane surface (Figure 4a), while clearly separated crystals were formed on the modified membrane surface (Figure 4b). However, when chlorine was used, the salt layer formed on the modified membrane (Figure 4c) was similar to that

formed on the unmodified membrane. The significant flux reduction in the modified membrane after the filtration of pretreated, inoculated seawater with chlorination and de-chlorination pretreatment steps may be explained by the presence of dead biomass in the feed water which represented a high content of NOM, which could interact with calcium ions and form a thick, packed gel layer that affects membrane permeability. Furthermore, this condensed gel layer could be easily consumed by bacterial cells resulting in a higher bacterial growth rate. However, the effect of the chlorination–dechlorination pretreatment steps on the modified membrane structure and the mechanisms by which these two different layers were formed in the presence or the absence of chlorine require further investigation. The very low flux observed on the second and third days of filtration was a common trend for both the unmodified and modified PES membranes in the two cases of pretreated, inoculated chlorinated or non-chlorinated seawater. However, the rate of flux decline was more gradual, which can be related to the equilibrium condition between biofilm growth, EPS production, and biofilm loss (cell detachment) caused by hydrodynamic shear at the solution–biofilm interface [17,66].

**Figure 5.** Initial and final flux of both the unmodified and modified PES membranes in two cases: bacterial loaded feed seawater without chlorination (**a**) or with the chlorination (**b**) pretreatment step. Reference conditions: pressure of 1 bar at 23 ± 2 ◦C and 200 rpm stirring for 9 h (3 h × 3 days) filtration time.

### 3.4.2. Water Productivity

The filtrate volume per unit of time is expressed as water productivity (m<sup>3</sup> ·h −1 ), as shown in Figure 6. The highest productivity was recorded on the first day for the different testing conditions. Meanwhile, the productivity was greatly decreased on the second and third days of filtration (about 73 and 84%, respectively) as a consequence of flux decline. The largest volume of the filtrate was produced when the modified membrane was used to filter pretreated, inoculated seawater, without the chlorination step (water productivity was almost one and a half times the water productivity of the unmodified membrane). When chlorine was used, the productivity of the modified PES membrane was reduced due to flux decline, as mentioned in the previous section. −

**Figure 6.** Water productivity (m<sup>3</sup> h −1 ) of both the unmodified and modified PES membranes in two cases: bacterial loaded feed seawater without chlorination (**a**) or with the chlorination (**b**) pretreatment step. Reference conditions: pressure of 1 bar at 23 ± 2 ◦C and 200 rpm stirring for 9 h (3 h × 3 days) filtration time.

### 3.4.3. Bacterial Counts

Bacterial Counts in the Pretreated Inoculated Feed Seawater over Three Days

Chlorine is usually added to control bacterial growth in most water treatment/desalination plants; the effect of the chlorination step on the feed water was investigated by counting the bacterial cells in

pretreated, inoculated feed seawater over three days. The feed seawater used for the experiments was freshly prepared on the first day and then kept at 4 ◦C overnight to be used on the second and third days. With the chlorination pretreatment step for feed water, the rate of bacterial growth increased rapidly over the three days relative to the bacterial growth of feed water that was not chlorinated, as shown in Figure 7. This high growth rate may be attributed to the presence of a rich nutrient supply of inactive biomass (dead bacteria) in the feed water [20,38]. This means that chlorine is not the optimum choice to control bacterial growth, even if it is efficient to remove most of the bacteria, as the surviving bacteria can undergo a rapid regrowth.

**Figure 7.** Bacterial count in pretreated, inoculated chlorinated and non-chlorinated feed seawater.

Bacterial Counts in the Filtrate Water over Three Days

− − − − Since UF is commonly used to remove fine colloidal particles, bacteria, viruses and large molecules such as proteins [5], both unmodified and modified membranes showed a high bacterial removal efficiency under the different testing conditions. On the first day of filtration of the pretreated, inoculated feed seawater (Figure 8), about 99.8% of total bacterial cells were removed by the unmodified membrane (retained on the membrane), while about 0.2% were permeated with the filtrate (2.575 × 10<sup>4</sup> CFU mL−<sup>1</sup> ). The highest removal of bacterial cells was achieved by the modified membrane, in which about 99.99% of total feed bacterial cells were retained on the membrane surface, while only 0.01% of feed bacteria (0.318 × 10<sup>4</sup> CFU mL−<sup>1</sup> ) reached the filtrate. This high bacterial rejection confirms the antifouling ability of such a modification, as illustrated in a previous work [47]. The antifouling mechanism of the modified membrane is based on steric hindrance and the osmotic effect of the hydrated brush-like polymer layer, which keeps bacterial cells as well as macromolecules (nutrients for bacteria) at a distance from the membrane surface [50]. On the second day of filtration, the bacterial cells on the membrane surface began to metabolize and secrete extracellular polymeric substances as the first step of biofilm formation [11,14,15,17]. For this, the filtrate on the second day recorded the highest bacterial count for both the unmodified membrane (355 × 10<sup>4</sup> CFU mL−<sup>1</sup> ) and modified membrane (2.41 × 10<sup>4</sup> CFU mL−<sup>1</sup> ); however, the counted bacterial cells in the filtrate of the modified membrane were much lower than those in the filtrate of the unmodified membrane.

**Figure 8.** Bacterial count of the ultrafiltration (UF) filtrate for pretreated, inoculated feed seawater without chlorination (**a**) or with the chlorination (**b**) pretreatment step. Reference conditions: 1 bar, 23 ± 2 ◦C and 200 rpm stirring for 9 h (3 h × 3 days) filtration time.

This phenomenon can be attributed to the logarithmical growth phase. Meanwhile, bacterial growth may be promoted by the presence of calcium salts. Calcium not only causes flux decline, as discussed before, but also plays a vital role in bacterial biofilm formation. The calcium ion was reported as a universal messenger, transmitting signals from the cell surface to the interior of the cell [67,68]. Calcium signaling is regulated by calmodulin, which is a calcium-modulating protein that controls cell proliferation, programmed cell death, and autophagy [69]. Moreover, calcium was assigned in specific and non-specific interactions between cells and the localized surface, in which calcium-binding proteins are often involved in bacterial adhesion to a surface. This binding is important for cell–cell aggregation. In addition, calcium is also recorded as an ionic cross-bridging molecule for negatively charged bacterial polysaccharides [68].

On the third day of filtration, the bacterial count recorded in the filtrate of the unmodified membrane was lower than that on the second day. This may be explained as the bacterial growth reaching the plateau phase, in which the biofilm growth phase was limited by the "detachment process" of the fluid shear forces. This phase may be attributed to the increase in population density and the lack of nutrients in the biofilm, and because the bacterial attachment to the membrane is limited [11,14,15]. Another reason for the inability of bacteria to reach the filtrate of the unmodified membrane is the complete blockage of most membrane pores. Regarding this, the bacterial cells were forced to settle on the membrane surface. Meanwhile, in the case of the modified membrane, the bacterial counts of the filtrate on the second and third days were approximately the same. This can be explained by the incomplete blockage of membrane pores. The modified membrane had available spaces for bacterial attachment. This observation was confirmed by SEM images, as will be discussed in the following section.

When chlorine was applied in the pretreatment, it removed about 99.6% of the total bacterial cells in the feed water (Figure 9). On the first day of filtration, the unmodified membrane removed about 99.3% of the bacterial cells that remained in the feed water after the chlorination step, and only 0.7% of bacterial cells reached the filtrate. Bacterial counts in the filtrate increased by the second and third days of filtration to reach about 306.5 × 10<sup>3</sup> CFU mL−<sup>1</sup> by the end of the third day. This was attributed to the growth of bacterial cells on the membrane surface due to dead biomass and calcium ions, as mentioned before. Although the modified membrane showed a significant flux decline in the presence of chlorine, as illustrated in the previous section, it was efficient in removing most of the total bacteria remaining in the feed seawater after the chlorination step over the three days of filtration. In addition, the modified membrane showed the lowest bacterial counts recorded for filtrate mixtures of both chlorinated and non-chlorinated feed seawater. −

**Figure 9.** Bacterial count of the filtrate mixture produced after three days of filtration using pretreated, inoculated feed seawater without chlorination (**a**) or with the chlorination (**b**) pretreatment step. Reference conditions: 1 bar, 23 ± 2 ◦C, and 200 rpm stirring for 9 h (3 h × 3 days) filtration time.

### Bacterial Count on the Membrane Surface

As shown in Figures 10 and 11, the bacteria that were removed from the modified membrane surface after the filtration of inoculated feed seawater, pretreated with or without chlorine, after gentle handshaking for 1 min, were more than those removed from the unmodified membrane. Meanwhile, the total bacterial cells removed from the unmodified membrane surface after mechanical shaking for 24 h were more than those removed from the modified membrane surface. This can be explained as a looser attachment of cells to the modified membrane compared to the unmodified membrane. This, in fact, confirms the antifouling effect of the modified membrane as it can keep bacteria at a distance from the membrane surface [47]. Meanwhile, the bacteria on the unmodified membrane were more closely attached as it is more hydrophobic and hence more favorable for bacterial attachment [17]. This was shown by the thick layer of biofilm and EPS secretion, as evidenced by SEM images. Based on this result, we can say that, after routine washing, the modified membrane can retain its normal flux and performance.

**Feed water without chlorination step (a)**

**Figure 10.** Bacterial count of loosely attached bacteria on the surface of unmodified and modified PES membranes after three days of filtration under different conditions of feed water: feed water pretreated without the chlorination step (**a**), and feed water pretreated with the chlorination step (**b**) using hand shaking for 1 min. Reference conditions: 1 bar, 23 ± 3 ◦C at 200 rpm stirring for 9 h (3 h × 3 days) filtration time.

**Figure 11.** Bacterial count of total bacteria attached on the surface of the unmodified and modified PES membranes after three days of filtration under different conditions of feed water: feed water pretreated without the chlorination step (**a**), and feed water pretreated with the chlorination step (**b**) using mechanical shaking for 24 h. Reference conditions: 1 bar, 23 ± 2 ◦C at 200 rpm stirring for 9 h (3 h × 3 days) filtration time.

Figure 12a,b show SEM images for unmodified PES membrane and modified PES membrane before filtration, respectively. For the feed water pretreated without chlorination, SEM images shown in Figure 12c,d show the formation of a thick layer of biofilm with EPS secretion and the complete blockage of most of the unmodified membrane pores. Meanwhile, the layer of biofilm formed on the modified membrane surface was not as thick as on the unmodified membrane and, clearly, the pores were not completely blocked. On the other hand, in the case of the chlorine disinfection step, the SEM image in Figure 12e shows the presence of bacteria on the unmodified membrane surface in aggregations at the start of biofilm formation, while, on the modified membrane surface, bacteria did not form aggregations. An SEM image (shown in Figure 12f) of the modified membrane surface under both conditions (i.e., chlorinated or non-chlorinated feed seawater) showed that bacteria were more loosely attached, as discussed before.

**Figure 12.** SEM images for unmodified PES membrane (**a**) and modified PES membrane (**b**) before filtration, respectively. SEM images of unmodified PES membrane (**c**) and modified PES membrane (**d**) after filtration of pretreated, inoculated feed seawater without the chlorination step, respectively. SEM images of unmodified PES membrane (**e**) and modified PES membrane (**f**) after filtration of pretreated, inoculated feed seawater pretreated with the chlorination step, respectively. Reference conditions: 1 bar, 23 ± 2 ◦C at 200 rpm stirring for 9 h (3 h × 3 days) filtration time. SEM images were taken at 20,000× magnification, and the scale bar is 1 µm.

### **4. Conclusions**

The filtration of pretreated, inoculated seawater using a modified PES UF membrane without the pre-chlorination step maintained the initial flux of the membrane as well as the largest permeated volume (productivity). The modified membrane was able to reject bacteria from the membrane surface in both the presence or absence of chlorine disinfectant. The addition of chlorine generally resulted in a cleaner membrane; however, its usage in conjunction with the modified membrane resulted in a significant reduction in the membrane flux. Furthermore, bacterial counts of chlorinated feedwater over three days of filtration reflected enhanced bacterial regrowth. SEM images showed a looser attachment of bacteria on the modified membrane surface.

In general, the modified PES membrane with a brush-like oligomer of the 3-AP modifier shows a higher membrane performance in terms of improving the quality and productivity of the filtrate as well as reducing the bacterial attachment onto the membrane's surface. Both the steric hindrance and the osmotic effect of the hydrated brush-like polymer layer keep bacteria at a distance from the membrane surface, which facilitates their removal by routine membrane-washing procedures. On the other hand, the use of chlorine disinfectant in the pretreatment of feed water prior to UF had no evident effect; it resulted in a further reduction in both the quality and water productivity of the membrane compared to the modified one. Moreover, significant bacterial regrowth was enhanced by chlorine usage.

Depending on the obtained results from this study, many points still require further investigation to understand the effect of the membrane structure on the biofouling phenomenon. More studies are needed to investigate the effect of chlorination–dechlorination steps on the structure of the modifying layer. Moreover, the interaction between the modifying layer and the bacterial cells and its effect on biofilm formation require further in-depth studies.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2077-0375/10/9/227/s1, Figure S1: Schematic representation of four possible chemical structure(s) of the PES surface after modification with 3-aminophenol (3-AP), containing O-linked and N-linked structures [47]. Figure S2: Photos of seawater (a) before coagulant addition, (b) after sedimentation, and (c) after pretreatment. Figure S3: Residual turbidity as a function of coagulant (FeCl<sup>3</sup> ) concentration (mg·L −1 seawater).

**Author Contributions:** Conceptualization, M.S.E.-D.H., N.N., and R.A.; methodology, M.S.E.-D.H., N.N., R.A., and N.S.; formal analysis, N.N., R.A., N.S., and M.S.E.-D.H.; investigation, M.S.E.-D.H., S.H.K., R.A., N.N., and N.S.; resources, M.S.E.-D.H., R.A., A.E.-S., and N.N.; data curation, N.N., R.A., M.S.E.-D.H., and N.S.; writing—N.N., N.S., and M.S.E.-D.H.; original draft preparation, N.N. and N.S.; writing—review and editing, N.N., M.S.E.-D.H., and S.H.K.; supervision, N.N., R.A., and M.S.E.-D.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Article* **Technical and Economic Evaluation of WWTP Renovation Based on Applying Ultrafiltration Membrane**

#### **He Bai <sup>1</sup> , Yakai Lin 1,2,\*, Hongbin Qu <sup>1</sup> , Jinglong Zhang <sup>1</sup> , Xiaohong Zheng <sup>1</sup> and Yuanhui Tang 3,\***


Received: 6 July 2020; Accepted: 4 August 2020; Published: 7 August 2020

**Abstract:** Nowadays, the standards of discharging are gradually becoming stricter, since much attention has been paid to the protection of natural water resources around the world. Therefore, it is urgent to upgrade the existing wastewater treatment plant (WWTP), to improve the effluent quality, and reduce the discharged pollutants to the natural environment. In this paper, taking the "Liaocheng UESH (UE Envirotech) WWTP in Shandong province of China" as an example, the existing problems and the detailed measures for a renovation were systemically discussed by technical and economic evaluation, before and after the renovation. During the renovation, the ultrafiltration membrane was added as the final stage of the designed process route, while upgrading the operation conditions of biochemical process at the same time. After the renovation, the removal rates of chemical oxygen demand (CODcr), biochemical oxygen demand (BOD5), total phosphorus (TP) and other major pollutants were improved greatly, and the results fully achieved the standards of surface water class IV. The ultrafiltration system performs a stable permeability around 1.5 LMH/kPa. Besides, the economic performance of the renovation was evaluated via the net present value (NPV) method. The result reveals that the NPV of the renovation of the WWTP within the 20 year life cycle is CNY 72.51 million and the overall investment cost can be recovered within the fourth year after the reoperation of the plant. This research does not only indicate that it is feasible to take an ultrafiltration membrane as the main technology, both from technical and economic perspectives, while upgrading the biochemical process section in the meantime, but also provides a new strategy for the renovation of existing WWTPs to achieve more stringent emission standards.

**Keywords:** WWTP; renovation and upgrading; ultrafiltration membrane; net present value

### **1. Introduction**

Pollution to natural water resource is a worldwide emergent and critical problem. It is definite that the severe deterioration of water bodies has both short- and long-term negative effects to human and environmental health [1]. In particular, chemicals and microbial contaminants in treated wastewater would cause public health concerns [2]. In recent years, wastewater discharging has gained increasing attention in many countries, due to reasons of ensuring water security and developing effective strategies for the sustainable utilization of water resources. Some developed countries have formulated different policies or standards on the discharge of wastewater treatment plant (WWTP) [3]. The point

source emission standard of the United States, which has experienced a shift in policy direction from "technologically based" to "water bodies-based" in recent years, requires the limit of water discharged to sensitive water body of total nitrogen (TN) < 3 mg/L, total phosphorus (TP) < 0.1 mg/L [3,4]. The Japanese government also proposed a special emission standard limits for Osaka Bay of BOD<sup>5</sup> < 8 mg/L, TN < 8 mg/L and TP < 0.8 mg/L.

Developing countries are facing a more serious deterioration situation of natural water bodies with the development of their economy and the improvement of industrialization. Many countries have taken steps to limit the emission of pollutants [5]. Taking China as an example, only chemical oxygen demand (CODcr), biochemical oxygen demand (BOD5), suspended solids (SS) and other major pollutants were taken into consideration for the emission standards in the early years. Later, the nutrient salts, nitrogen and phosphorus were added in the emission standard list that needed to be controlled as conventional indicators. Currently, the Chinese government has further restricted nutrient emissions and many local governments have issued even stricter local emission standards according to the natural environment in different regions [6]. Hence, it is needed to develop simple, reasonable and acceptable treatment strategies for the renovation of existing wastewater plants in response to increasingly stringent emission standards. It will reduce the discharge of pollutants into natural water bodies; and eliminate negative impacts on the environment and human health, which would be in accordance with the national and international water quality regulations and guidelines.

Since normally urban wastewaters are only treated by conventional activated sludge systems without further treatment [7], the effluents quality can no longer meet the requirements of current emission standards, especially the indicators of SS and nutritive salts due to a lack of technical process as well as equipment [6,7]. Because of its potential economic and environmental benefits, the renovation of existing WWTP to fix water deterioration is regarded as one of the best options for protecting natural water bodies and developing sustainable water management strategies. However, the renovation is a difficult decision for many plant managers, because the renovation of a WWTP is directly related to the total construction costs, operating costs, treatment effect, the floor area, the convenience of management and other key issues. The selection of the technology and the full use of existing facilities are important for the renovation. If only simply modifying the existing system but not adding new technology, no remarkable success would be achieved in significantly decreasing the concentrations of major pollutants, to the levels stated in the restricted criteria corresponding to the water discharged to the natural environment. However, there are numerous problems, such as the footprint, time limit and the difficulty in estimating the expected benefit. Moreover, it is the fate that the WWTP effluents flow to densely populated urban cities paradoxically in order to protect the local natural environment. Therefore, there is a growing need for the development of treatment renovation methodologies by considering the cost-effective and technical benefits.

Today, the implementation of ultrafiltration membrane in wastewater reclamation and reuse has become more attractive, since ultrafiltration membrane separation ensures a higher removal rate of particles, bacteria and large molecular weight organic matters as well as reducing chemical usage and better on-stream time [8,9]. However, there are few applications incorporating the biochemical system for the pretreatment that could keep the inlet water quality of ultrafiltration membrane stable. In addition, there is also a lack of systematic experience in the selection of ultrafiltration membrane products. Therefore, although ultrafiltration membrane is receiving more and more attention in recent years, however, some people worry that the investment is too large to be recovered, or about the rapid contamination of the membrane, and others do not know how to systematically evaluate the renovation methodologies. The study of Al Aani et al. shows that fouling (27%), modelling (17%) and wastewater reuse (12%) were the dominant research topics for the ultrafiltration membrane, however, there is very little research on ultrafiltration membrane for the renovation of existing WWTP [10]. Therefore, it results in a lack of evidence for the WWTP manager to follow about the ultrafiltration membrane and the upgrading of pretreatment, either in a technical or an economic aspect. [11]. Moreover, the incomplete or insufficient economic analyses of options by ultrafiltration membrane processes for

wastewater discharge do not allow to balance or accurately evaluate the disparity among the benefits brought by the increase in water price after the renovation and overall investment cost of the whole plant renovation [12].

Considering the current situation summarized above, taking the Liaocheng UESH WWTP in Shandong province of China as an example, this work introduces a renovation process route with ultrafiltration as the main technology and analyzes the existing problems and the specific measures for the renovation. Then, based on continuously monitoring the operation data of ultrafiltration performances, the actual renovation effects and economic feasibilities of membrane treatments were studied using the net present value method. By way of technical and economic perspective, the viability of the renovation methodologies that take ultrafiltration as the main technology and combines with the upgrading of conventional activated sludge systems in the production of discharged water from urban WWTP effluents was elaborately evaluated, in order to provide a new direction for the renovation of existing WWTP to accommodate more stringent emission standards.

### **2. Background**

The UESH WWTP is located in the Liaocheng Economic Development Zone (Liaocheng, China) and it was put into operation in May 2009 with a designed capacity of 30,000 m<sup>3</sup> /d. The plant mainly treats the domestic sewage in the economic development zone and a part of industrial sewage from enterprises. The conventional method of A/A/O (Anaerobic-Anoxic-Oxic) technology with steps in anaerobic, anoxic and aerobic was adopted as the main part of the treatment process, supplemented by biological phosphorus removal methods to achieve the purpose of nitrogen and phosphorus removal. The effluent quality met the primary-level A-class standard of "Pollutant Discharge Standards of Urban Sewage Treatment Plants" (GB 18918-2002) that is shown in Table 1, and then the treated wastewater could be discharged into the Haihe River (Liaocheng, China), the local natural water body.



However, with an increase in effort from the Chinese government for the protection of natural water bodies, various policies have been formulated and promulgated, and the discharge standards implemented by the sewage plant could no longer meet the needs of protecting the local natural water environment. Both the "Action Plan for Water Pollution Prevention and Control" that was formulated and promulgated by the State Council in 2015 and the "Work Plan of Liaocheng City Water Pollution Prevention and Control in 2017" that was formulated and issued by the Liaocheng People's Government clearly require that the water quality of the Haihe River, to which the effluent is discharged from the plant, should reach the Class IV of "Quality Standard of Surface Water" [13], which is shown in Table 2. Therefore, it was urgent for the plant to carry out some renovation in order to reduce the pollution load and protect the local natural environment.


**Table 2.** Water quality standards of surface water.

### *2.1. Plant Current Condition*

The plant is running at a full capacity of 30,000 m<sup>3</sup> /d, and the flow diagram of the treatment process is shown in Figure 1. The sewage was pre-treated by a grilles and grit chamber, and then treated by A/A/O conventionally activated sludge method. The effluent from the secondary sedimentation tank was treated by a rotary filter, and then discharged into the receiving water body after disinfection. The concentrated and dehydrated sludge would be transported to a company specializing in sludge disposal for further treatment. The main pollutant indicators of the influent and effluent from the Liaocheng UESH WWTP before the renovation are shown in Table 3.

**Figure 1.** Treatment process of the plant before renovation.

As listed in Table 3, the variation range of organic matter content was large, the BOD<sup>5</sup> of the influent was around 40–80 mg/L, while the CODCr fluctuated between 130 mg/L and 250 mg/L, and the suspended solid was about 100 mg/L. The concentration of nutrient salts was relatively stable, ammonia nitrogen was at about 15 mg/L, and the total phosphorus fluctuated around 3 mg/L. Besides, according to the description of a field operator, the sludge concentration in the aerobic tank was relatively high, with an MLSS (Mixed Liquor Suspended Solid) value at 8000–11,000 mg/L and MLVSS (Mixed Liquor Volatile Suspended Solid) at 3400–4400 mg/L, and the MLVSS was only about 40% and the sludge load was only 0.02 kg BOD5/(MLVSS.d), which signified the poor sludge sedimentation ability of the previous treatment process. Therefore, the biochemical properties of the water also needed to be improved.

In addition, due to the long-term full capacity or even overload operation of the plant, there also existed a serious loss of facilities, equipment aging and other problems, such as the bad performance of the aeration distribution system that was caused by the corrosion of the pipelines, which resulted in the uneven mixing of sludge and water in the aerobic tank and an unsatisfactory flow state. The failure of the propeller affected the uneven mixing of sludge and water even further. Besides, the rotary filter was easily fouled and the inefficient backwash made the SS of the effluent unstable.

As shown in Table 3, although the effluent quality could meet the primary-level A-class standard of "Pollutant Discharge Standards of Urban Sewage Treatment Plants", however, except for the ammonia nitrogen, the concentration of other main pollutant indicators, especially the total phosphorus and suspended solid, were still much higher than the requirement of the Class IV standard listed in Table 2. In order to improve the effluent quality of the plant, it was necessary to carry out the plant renovation as soon as possible, to reduce the concentration of the main indicators and gradually restore the regional water ecological function.


**Table 3.** Main pollutant indicators of the influent and effluent from the wastewater treatment plant (WWTP) before renovation \*.

\* The data came from the semi-annual water report of the plant in 2017.

### *2.2. Selection of Renovation Methodologies*

The new standard requires the concentration of total nitrogen (TN), total phosphorus (TP) and suspended solid (SS) of the effluent to be much lower. The removal of dissolved substances still needs to be treated by biochemical process, while the removal of suspended solid needed to be further treated. The effluent SS was about 10 mg/L, and it was difficult to achieve the corresponding standards of 6 mg/L only by the rotary filter due to its easily fouling rate and inefficient backwash. Considering the possible high SS that was caused by chemical dosage in the biochemical process after the renovation, a new treatment process needed to be added in order to keep the effluent quality stable. The newly added treatment process should reach the characteristics of small footprint and compact structure because of the limitation of land area, with only 720 m<sup>2</sup> available, which became one of the major difficulties for the renovation. In addition, features such as the high degree of automation and stable effluent water quality also needed to be taken into consideration. The optional processes included the ultrafiltration membrane, sand filtration combined with ozone and other processes, while the ultrafiltration has obvious advantages due to the operational safety and land saving.

The membrane filtration system is a pressure-driven separation process, in which particles and impurities between 0.02 and 0.1 µm in diameter can be intercepted through the micro pores distributed on the membrane surface, which can effectively remove water floc, bacteria and macromolecular organic matter [14]. The ultrafiltration membrane among the relatively mature technologies from recent years, especially after entering the 21st century, which has rapidly developed into a utility engineering technology, which is widely used in various fields of water treatment and become more

competitive compared with traditional technologies, because of the scale production of membrane materials, the integration of membrane modules, the popularization of membrane manufacture and the affordable prices [15]. Not only all bacteria and suspended solids are trapped by the efficient intercept of the ultrafiltration membrane, but also the CODcr, the total phosphorus and total nitrogen carried by a suspended substance, which realized the further protection of effluent quality after biochemical treatment. Additionally, some refractory macromolecular organic matter can be retained and returned to the biochemical tank by backwashing, in order to prolong its residence time and maximize its degradation.

After a certain period of operation of the ultrafiltration membrane, the retained pollutants will be accumulated on the membrane surface and formed into a filter cake layer that would reduce the membrane flux [14–16]. Therefore, it is necessary to maintain good inlet water quality to protect the stable operation of the ultrafiltration membrane, prolong the cleaning cycle and increase its service life. As a result, the pretreatment facilities also needed to be upgraded.

Through the analysis of the main pollutant of inlet water and production requirements, the effluent CODcr was about 37 mg/L, so it is necessary to maintain a good biochemical performance, to prevent the membrane from fouling, and at the same time, the ultrafiltration membrane could help to further reduce the concentration of CODcr. Besides, the high concentration of ammonia nitrogen (NH3–N) and the total nitrogen (TN), as well as the low carbon source in the raw water, were not conducive to denitrification and could affect the performance of phosphorus removal at the same time. Moreover, there was a risk of effluent short circuit on the existing complete mixing of activated sludge that would easily result in sludge swelling, which can also influence the performance of denitrification and phosphorus removal. How to improve the efficiency of denitrification and phosphorus removal in the A/A/O process was one of the technical difficulties for the upgrading of biochemical treatment.

The influent BOD5/NH3–N was about 3.5 while the NH3–N/CODcr was about 0.09, showing a lack of carbon source in the biochemical treatment process, which previously resulted in the relatively unstable level of effluent TN content [17]. In addition, as the ratio of BOD<sup>5</sup> to TP was about 20, which meant that the biological phosphorus removal process can be adopted [18], but in order to improve the removal rate of total phosphorus, so as to meet the stricter phosphorus removal target, not only was there a need to optimize the biochemical treatment process, but also to carry out an auxiliary chemical phosphorus removal system.

The renovation was required to tap the potential of the existing facilities, and new technology needed to be added at the same time to make the effluent water quality fully up to the standard. Furthermore, reducing the investment and operation cost as much as possible, as well as achieving the convenient operation and management of the WWTP should also be taken into consideration. Therefore, the final renovation methodology of the plant was determined to be the upgrading of the existing biochemical treatment process, the addition of a chemical phosphorus removal system and an ultrafiltration membrane treatment process. The treatment process after renovation is shown in Figure 2.

**Figure 2.** Treatment process of the plant before renovation.

### *2.3. Implementation of Renovation*

### 2.3.1. Upgrading of A/A/O Treatment Process

Considering the existing problems of uneven mixing, short flow and serious sludge accumulation at the bottom of the biochemical tank, which resulted in the unstable performance of the effluent quality, following specific renovation measures should be carried out while keeping the original structure. Giving the returned sludge inlet the same position as the feed water inlet in the anaerobic tank, and optimizing the arrangement of mixer at the same time, allows the water and sludge to fully mix with the kinetic energy during the water feed, meanwhile, it also enables to make full use of the anaerobic tank volume during the mixture. The anoxic tank has the same problems as the anaerobic tank. Similarly, by adjusting the position of the returned sludge inlet port and optimizing the arrangement of mixer, the mixed logistics state of the sludge and water in the anoxic tank can be improved, so as to improve the treatment efficiency of the anoxic zone.

In order to solve the problem of insufficient carbon source in the influent, a carbon source feeding device is set near the anoxic tank to periodically and quantitatively transport a proper amount of high concentration and high biodegradability sewage and sodium acetate, after accounting as a supplementary carbon source.

There used to be a short flow problem for the design of internal reflux, which was that the water could flow from the inlet port of the aerobic tank directly into the anaerobic tank. Therefore, a separation wall was added to block the short flow. In addition, the aeration in the reflux water from the aerobic tank to the anaerobic tank is greatly reduced, to avoid affecting the performance of the anaerobic zone.

### 2.3.2. Upgrading of Phosphorus Removal System

The original biochemical phosphorus removal process is still used. To increase the phosphorus removal rate, chemical agents are added to make phosphorus form into insoluble substances, so as to be discharged together with the residual sludge [19]. Hence, a new chemical phosphorus removal and dosing device was redesigned in the renovation. The new liquid phosphorus removal agent, whose main component is ferric sulfate, can be used continuously and automatically.

### 2.3.3. Addition of Ultrafiltration Membrane

The ultrafiltration system includes the ultrafiltration membrane and membrane frame, inlet water pump, backwash system, chemical cleaning system, pipe valve, compressed air system, instrument and automatic control system, among which the core part is the ultrafiltration membrane. The SMT600 series of the pressurized ultrafiltration membrane were selected for the plant renovation. PVDF hollow fiber ultrafiltration membrane is generally produced by thermally induced phase separation (TIPS) process or non-solvent induced phase separation process (NIPS). Compared with NIPS membranes, TIPS membranes, which are prepared driven by a temperature change, have many advantages such as an easily controllable structure, stable membrane quality, narrow micro pore distribution and symmetric structure [20,21]. Moreover, the characteristics of PVDF raw materials can be maintained during manufacturing, which makes the membranes possess higher mechanical strength and chemical resistance. Therefore, the ultrafiltration membranes fabricated by the TIPS method can tolerate a high concentration of soaking and cleaning. Moreover, the advantages of the TIPS membrane can minimize the number of membrane modules due to higher flux and good recoverability, so as to save the floor space and prolong the service life [21]. Table 4 describes the technical parameters of the ultrafiltration membranes used in this renovation.


**Table 4.** Main pollutant indicators of the influent and the effluent from WWTP before renovation.

### **3. Evaluation Method**

### *3.1. Technical Analysis*

The purpose of the renovation of the Liaocheng UESH WWTP was to improve the removal rate of the main treatment indicators in the water to achieve a level Class IV of "Water Quality Standard of Surface Water", so as to protect the receiving natural water body. Both the old standard and the new standard did not make clear provisions on calcium, magnesium and carbonate content for the wastewater that discharged into natural water bodies, as new standards mainly made more stringent requirements on COD, nitrogen, phosphorus and other nutritive salts. Moreover, since there is no reverse osmosis after the UF (Ultrafiltration) process, thus the scaling problem does not need to be taken into consideration, hence, the concentration of these ions is not measured by the plant. As the traditional municipal sewage, the content of these ions is not high, which is acceptable for the operation of the UF membrane. Therefore, this paper focuses on the concentration of the main pollutant in the effluent and the change of the removal rate of before and after the renovation as a part of the technical evaluation.

On the other hand, ultrafiltration membrane permeability will be used to investigate the effect of biochemical process upgrading and the stability of the whole process after renovation. One of the characteristics of ultrafiltration membrane fouling is the increase in transmembrane pressure difference (TMP) and the drop of permeability [22]. This paper focuses on the changes of the membrane permeability, which is usually expressed as the flowrate per hour per square meter of membrane area under unit pressure. The influence of pretreatment on the membrane fouling rate as well as the cleaning effect and recovery performance can be evaluated by the continuous monitoring of membrane permeability.

### *3.2. Economical Analysis*

The net present value method of the dynamic evaluation index in engineering economics is used for economic evaluation in this paper. The economic evaluation index is divided into static and dynamic, where static evaluation means that the time value of the fund will be not taken into account and the compound interest will be not calculated when calculating the benefits and costs of the scheme, while they will be taken into account by the dynamic evaluation, of which the calculation process is based on the equivalent basic conversion formula, which includes the net present value [23].

The total cost of the system in its whole life cycle is the sum of the construction, operation, maintenance and energy costs. However, since the changes in the time value of money, the project costs occurring at different points in the asset life cycle cannot be compared or simply added together. They must be discounted to their present value. Appropriate formula for the net present value is as follows [24]:

$$\text{NPV} = \sum \frac{(\mathbb{C}\_I - \mathbb{C}\_O)}{(1+i)^t} \tag{1}$$

where, NPV = net present value; *C<sup>I</sup>* = cash inflows; *C<sup>0</sup>* = cash outflows; *i* = discount rate in decimals; *t* = time period.

The result of the NPV method is more realistic because it takes the time value of money into account and it also considers the risk inherent in making projections about the future. Hence this method is useful in the rational arrangement and financial management of the future costs and activities of the WWTP.

### **4. Results and Discussion**

### *4.1. Technical Results*

Table 5 shows the concentration of a major pollutant in the influent and effluent as well as the removal rate after the plant renovation. Figure 3 shows the improvement of removal rate of major pollutants before and after the plant renovation. The data were collected from the semi-annual water quality analysis report of the plant in 2019.


**Table 5.** Main pollutant indicators of the influent and effluent from the WWTP after renovation.

**Figure 3.** Removal rate of the major pollutants before and after the plant renovation, as well as the comparison of effluent water quality with the new discharge standards.

It can be seen from Figure 3, that the removal rates of CODcr, BOD5, TN, TP and SS were significantly improved after the renovation, while maintaining the removal rate of ammonia nitrogen in the original level. The concentrations of major pollutants in the effluent are much lower, which totally meet the requirement of the new discharge standard. Among them, the water and sludge can be more fully mixed due to the adjustment of the water inlet port and the sludge inlet port as well as the improvement of the thruster, and the hydraulics flow pattern of the reflux between each biochemical tank is also improved, so as to avoid the formation of dead sludge accumulated in the tank [25]. The removal rate of BOD<sup>5</sup> is increased to 94% after the renovation, which is better than 91% of that in the original process. In addition, with the interception of a certain amount of macromolecular organic matter by the ultrafiltration membrane, the CODcr removal rate was greatly increased from 78% to 91%.

Chemical agents also have a positive effect on the removal of total phosphorus. Usually the biological phosphorus removal method cannot achieve the ideal effect because it is very sensitive to temperature, water salinity and other aspects. After the additional chemical phosphorus removal method, the phosphorus is changed into insoluble phosphate precipitation form by adding iron salt phosphorus removal agent. On the one hand, iron combines with phosphoric acid, and on the other hand, its hydrolysates can form Fe(OH)<sup>3</sup> and other complexes, which make the original colloids in the water destabilized by adsorption bridging and net capture and sweep, so as to be flocculated and precipitated, which is much easier after it is combined into macromolecules [26]. Compared with the biological phosphorus removal method only, the total phosphorus removal rate was increased from 85% to 93% by adding the chemical phosphorus removal system.

The ultrafiltration membranes, which are the key technology of the renovation, have a significant interception rate for suspended solid, and then the concentration of SS in the effluent is almost impossible to be detected, which is far lower than the required 6 mg/L. Meanwhile, the upgrading of the biochemical treatment also benefits the operation performance of the ultrafiltration membrane. Figure 4 describes the permeability trend of the ultrafiltration membrane within three months after the renovation.

**Figure 4.** Permeability trend of the ultrafiltration membrane within three months after the renovation.

The newly added ultrafiltration membrane was divided into four sets, each set with 80 membrane modules, each of which could operate independently. As shown in Figure 4, the ultrafiltration membrane permeability of the four sets remains basically stable in the operation for three consecutive months. In late October to early November of 2019, there was an impact dosage of the chemical phosphor removal agent, which resulted in the rapid fouling of the membrane and decline of the permeability [27]. However, it was returned to the initial level after one time of chemical cleaning, which certified a good recoverable performance of the membranes. The permeability of the membrane system was basically maintained at about 1.5 LMH/kPa, which was in the higher level compared with the other ultrafiltration membrane on the market [28].

### *4.2. Economical Results*

The NPV method was used to evaluate the economy performance of the renovation. The cash inflow is the sewage treatment fee charged by the plant after the renovation, while the cash outflow includes the initial investment cost, operation and maintenance cost [29]. The cost of operation and maintenance mainly include the cost of phosphorus removal agent, the cost of carbon source

supplement and the ultrafiltration membrane cleaning agent, power consumption and labor cost of the new equipment, while the data are collected from the plant. The depreciation cost of the ultrafiltration membrane was calculated based on the warranty period given by the membrane manufacturer. The details are as follows:

The total investment cost of this renovation was CNY 25.626 million.

The power consumption mainly included the consumption of ultrafiltration feed pump, backwash pump, metering pump and other power equipment, as well as the power consumption of the newly added lighting and control device. It was calculated that the additional power consumption of the project was 2.199 million KWH per year.

The chemical consumption mainly comes from the chemical phosphor removal, the supplemented carbon source and the cleaning of the ultrafiltration membrane. The main agents that were newly added were sodium acetate, ferric sulfate new agents, sodium hypochlorite, hydrochloric acid and sodium hydroxide. According to the design value, the main chemical consumption was shown in Table 6. According to the three-month operation, the actual consumption was lower than the design value. However, considering uncertain factors such as water quality fluctuation in the future, the design value will be used for the calculation of economic evaluation.


**Table 6.** Additional chemical consumption after renovation.

The depreciation cost of the ultrafiltration membrane was calculated according to the six year warranty period given by the manufacturer, and the local electricity price, labor cost and pharmaceutical price were calculated according to the local average price of the last two years. The annual maintenance cost was calculated at 2% of the investment cost.

The unit sewage treatment fee charged by the water plant was CNY 2.76/ton after the renovation, and the water treatment fee per ton was CNY 1.58 higher than the original CNY 1.18 before the renovation. Thus, the CNY 1.58/ton will be used as the calculation basis for cash the inflow during the economic evaluation.

In addition, the base year for the NPV calculation WAs 2019, which WAs the commissioning stage after the renovation. The NPV analysis requires a discount rate calculated using interest rates and inflation rates [30]. Interest rates and inflation are based on the historical data of the past 25 years. The average interest rate is calculated as 14% and the average inflation rate is 8% after the average value is collected, hence, the calculated discount rate is 5.55%. All capital inflows and outflows were converted into the present value of the base year in 2019, and then added by the NPV method to obtain the cost of different calculation life cycle. All the calculations were completed using MS Office Excel, and the results are shown in Figure 5.

Due to the improvement of water quality after the renovation, the cost of sewage treatment fee was greatly increased. As shown in Figure 5, the net present value within the 20 year life cycle is CNY 72.51 million, and the overall cost can be recovered in the fourth year after the renovation, which brings considerable economic benefits to the plant.

In addition, it can be seen that one of the variables of this renovation methodology was the selection of ultrafiltration membrane. There are different kinds of ultrafiltration membranes with different materials, different performance, operation stabilities and life cycles on the market, which directly affect the economic benefits of water plants by the chemical consumption, membrane depreciation cost and other factors [31]. This paper takes the service life of the ultrafiltration membrane as an example to study the influence of different membrane replacement cycles on the NPV result, as shown in Figure 6.

**Figure 6.** NPV calculation results under the different membrane life cycles.

Figure 6 shows the NPV result of the water plant from 5 to 20 years with the different ultrafiltration membrane service life cycles. It can be seen that different ultrafiltration membranes replacement cycles have a direct impact on the NPV value of the whole life cycle of the plant. When the service life of the membrane is less than 3 years, the influence is much more significant. Ultrafiltration membrane species also affect chemical consumption, power consumption and other factors, and then the selection of ultrafiltration membrane is important in the renovation based on the process route introduced in this paper. The TIPS ultrafiltration membrane was used in the Liaocheng USEH WWTP, which could benefit the water plant continuously due to its good chemical resistance and recoverability.

### *4.3. Challenges and Future Research Orientations*

It is obvious that the ultrafiltration membrane will play an increasingly important role in wastewater treatment plant renovation [10]. However, while the ultrafiltration membrane technology is constantly innovating, the research on the mechanism of membrane fouling by specific pollutants is insufficient, moreover, the market supervision is also deficient, which is manifested in the following aspects. Firstly, there is a lack of an integrated outline and systematic standard for ultrafiltration membrane evaluation in the technical and economic dimensions; in addition, there is also no widely accepted operation standards, which result in a lack of evidence for the WWTP to follow during management [10,32,33]. Therefore, it can be predicted that the future research orientation will be more inclined to build the evaluation system of ultrafiltration membrane, and to determine the weight of each indicator, in order to propose a comprehensive and systematic outline of technical and economic dimensions. Additionally, membrane fouling, shrinkage, cycles of operation, and regeneration prospects also need be further studied in the future. In a wastewater treatment process, the physically irreversible fouling of ultrafiltration membranes is severe and inevitable, the permeability loss restricts the application of ultrafiltration for wastewater treatment, and it will also reduce the service life of the membrane and increase the cost of membrane replacement [32–34]. The key issue to solve the fouling problem is to understand the fouling mechanism and cleaning efficiency of a specific pollutant, and find an effective way to regenerate the membrane [35–37]. In addition, how to select and upgrade pretreatments, which can represent important savings in the operational costs related to the membrane's cleaning procedures and maintenance, will also be one of the future research orientations [38].

However, due to the limitation of the site spaces and time, no other membrane products were performed in this project. Therefore, by reading a large number of studies, the author compared the application of different ultrafiltration membranes in other projects, especially in a municipal wastewater field, and summarized the relevant factors affecting the performance of the ultrafiltration membrane [39]. Meanwhile, we proposed an integrated outline for the evaluation of ultrafiltration membrane-based renovation methodologies of the technical and economic dimensions, which are presented based on the actual Chinese market situation, as shown in Tables 7 and 8 [40,41], which can be regarded as a reference for the establishment of ultrafiltration membrane evaluation systems in the future.


**Table 7.** Technique indexes of the ultrafiltration membrane system-based WWTP renovation methodology evaluation.


**Table 8.** Economic indexes of the ultrafiltration membrane system-based WWTP renovation methodology evaluation.

### **5. Conclusions**

In this work, taking the Liaocheng UESH WWTP as an example, this research proves that from the technical perspective, it is a feasible scheme to take the ultrafiltration membrane as the main technology and upgrade the biochemical process section in the meantime. Due to the high efficiency of the ultrafiltration membrane interception characteristics, the main pollutant in the effluent after the renovation could totally meet the Class IV requirement of "Water Quality Standard of Surface Water". At the same time, the upgrading of the biochemical treatment can also reduce the fouling rate of the ultrafiltration membrane and keep a stable operation status, and thus bring a beneficial impact on the local natural environment. Economic performances evaluated by the NPV method have clearly demonstrated that based on the operational perspective, the ultrafiltration membrane represents a highly competitive technological solution. Thus, we anticipate that the ultrafiltration membrane would play an important role in the renovation of WWTPs. Meanwhile, systematic evaluation systems and research on the fouling mechanism of the ultrafiltration membrane will be the emphases of future research and development.

**Author Contributions:** Conceptualization: Y.L.; data curation: H.B. and H.Q.; investigation: X.Z.; project administration: J.Z.; writing—original draft: H.B.; writing—review and editing: Y.L. and Y.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was financially supported by the Open Project of the State Key Laboratory of Chemical Engineering (no. SKL-ChE-19A02).

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


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