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Article

Membrane Foulant Removal by Ozone-Biocarrier Pretreatment Technology for Industrial Wastewater Reclamation

by
Ting-Ting Chang
1,2,†,
Sheng-Yi Chiu
1,*,†,
Chun-Chi Lee
1,
Yuan-Liang Tai
1,
Guan-You Lin
1,
Chun-Hsi Lai
3 and
Po-Yu Chen
2
1
Water Technology Division, Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 300044, Taiwan
2
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300044, Taiwan
3
Department of Environmental Engineering, National Cheng Kung University, Tainan 701401, Taiwan
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Water 2025, 17(2), 272; https://doi.org/10.3390/w17020272
Submission received: 23 November 2024 / Revised: 10 January 2025 / Accepted: 17 January 2025 / Published: 19 January 2025
(This article belongs to the Topic Advanced Oxidation Processes for Wastewater Purification)
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Abstract

:
During wastewater reclamation, organic matter is considered the dominant foulant that shortens the lifetime of ultrafiltration (UF) membranes during operation. Additionally, the mineralization efficiency of organic matter in secondary effluent is typically low due to nonbiodegradable carbon sources. Herein, a combination of ozone and a porous biocarrier reactor was applied as a novel pretreatment system to enhance organic matter removal in the effluent in a lab-scale evaluation and pilot test. The results indicated that 70% of the biopolymer was removed, and the chemical oxygen demand (COD) removal efficiency was 1.8 times higher in this combined process than in the process with a porous biocarrier alone. The UF flux increased by 16% after the combined ozonation and porous biocarrier pretreatment process compared with the process with no pretreatment. Interestingly, the genus Flavobacterium (15.59%), containing biopolymer-degrading bacteria, was observed only in the combined ozone plus porous biocarrier process. Moreover, the results show that biopolymers can be removed through the combined ozone and porous biocarrier process due to partial ozone degradation, confirming that this combined process is one of the better pretreatment procedures for organic matter removal and improves the flux of UF during the wastewater reclamation process.

1. Introduction

With extensive regulatory efforts aiming to mitigate water scarcity problems, secondary effluent from wastewater treatment plants has become an alternative water resource due to its stable water supply and acceptable water quality that meets the effluent discharge standards. Membrane filtration is considered a highly promising process for wastewater reclamation due to its compactness, easy operation, and high pollutant removal rate [1]. Recently, low-pressure membranes, including microfiltration (MF) and UF membranes, have experienced significant growth in drinking water purification and industrial wastewater reclamation. The UF process has been widely used to remove contaminants from industrial wastewater at a relatively low cost. However, despite various advantages of UF technology and substantial progress in module design, the performance of UF membranes in wastewater treatment and reclamation is still hindered by membrane fouling, which could lead to higher operating costs and a shorter membrane lifespan [2].
The secondary effluent from wastewater treatment plants contains complex organic compounds, including natural organic matter (NOM) and soluble microbial products (SMPs), generated during biological treatment processes [3]. Studies in the literature reported that high-molecular-weight organic matter, such as hydrophilic SMPs and protein-like extracellular matter, which belongs to biopolymers, caused a dramatic flux decline in membrane processes [4,5,6]. Zheng et al. [7] reported that the reversibility of the membrane was inversely related to the biopolymer concentration in the effluent water. These studies showed that biopolymers could be the major foulants that cause flux decline in ultrafiltration. Therefore, determining how to remove these organic substances that may cause serious film fouling at a reasonable cost should be a priority in the development and application of pretreatment technology for wastewater reclamation. In the pretreatment processes, preoxidation of secondary effluent by ozone shows promising potential in preventing or reducing organic fouling during membrane operation [5]. Lee et al. [8] investigated the membrane permeability and fouling mechanism by preozonation of wastewater treatment plant effluent and observed a 50% reduction in the thickness of the fouling layer on the membrane. Barry et al. [9] also reported that preozonation altered the size distribution of effluent organic matter (EfOM) by partial oxidation of high-molecular-weight (MW) organic macromolecules, and a reduction in UF membrane flux decline was observed. Ikehata et al. [10] concluded that partial degradation of organic compounds could greatly enhance their biodegradability, and this approach is preferable for the pretreatment of wastewater containing refractory organic matter prior to biological treatment.
Therefore, in this research, an attempt was made to use the combined preozonation and biological treatment process with a porous biocarrier made of polyurethane (PU) to remove organics and potential fouling materials in wastewater before UF to evaluate the effects on the performance of peroxidation prior to UF. The objectives of this study are to (1) determine the fouling potential and major foulants in secondary effluent via both a lab pilot test and pilot field verification unit; (2) investigate the efficiency of the biological pretreatment process with or without preozonation in the removal of organic foulants prior to UF; and (3) identify major species of microorganisms associated with the degradation of biopolymers in a porous biocarrier reactor.

2. Materials and Methods

2.1. Equipment Setup for Lab Testing System

The secondary effluent was collected from the Linhai Industrial Park wastewater treatment plant (Kaohsiung, Taiwan) for the evaluation of pretreatment. A lab-scale pretreatment unit including an ozonation unit and a porous medium bioreactor was set up for pretreatment process evaluation, and the process flow chart is shown in Figure S1. Ozone was produced by a Sumitomo SGA-01A-PSA4 ozone generator (Tokyo, Japan) with a maximum output of 27 g/h, and the ozone dose was increased from 0.5 to 1.0 mg O3/mg dissolved organic carbon (DOC) for partial oxidation of the residual organics in the secondary effluent. For the porous biocarrier, a PU foam biocarrier patented by Industrial Technology Research Institute (ITRI) was used as the biological medium, with approximately 80% of the bioreactor volume filled with porous biocarrier. The hydraulic retention time (HRT) through the bioreactor was 4 h, and the COD loading was 0.2 kg/m3-day. The inoculum of the activated sludge in the biological process was obtained from the wastewater treatment in ITRI campus. The lab-scale pretreatment test was performed for 3 months.

2.2. Equipment Setup for Pilot Testing System

The pilot system was established at the Linhai Industrial Park wastewater treatment plant (Kaohsiung, Taiwan). The secondary effluent was collected and treated by the wastewater reclamation process pilot system. The wastewater reclamation process pilot system was composed of an ozone oxidation reaction tank, a biocarrier process reaction tank, and UF membrane filtration units. A set of hollow fiber UF modules was used for continuous monitoring of the effect of pretreatment processes on the membrane flux performance. In the pilot system, the hollow fiber membrane in the UF module was made of polyvinylidene difluoride (PVDF) with an area of 45 m2 and capacity of 112 m3 per day (CMD). The average pore size was between 0.38 and 0.39 μm. The internal structure of the UF membrane was characterized by a Thermo Scientific Helios G4 Plasma FIB (PFIB) DualBeam (focused ion beam scanning electron microscope, or FIB-SEM) (Thermo Scientific, Waltham, MA, USA). A SEM image of the UF membrane is shown in Figure S2.

2.3. Water Quality Analysis

Effluent samples from the pretreatment system were collected once weekly, and DOC, NH4+, COD, and biological oxygen demand (BOD) were measured, as shown in Table 1. All water samples were filtered through a 0.22 μm filter before analysis. DOC analysis was conducted using a Sievers 900 TOC analyzer (Suez, Paris, France), and NH4+ was measured by Dionex ion chromatography (Thermo Scientific, MA, USA).

2.4. Excitation–Emission Matrix (EEM) Fluorescence Spectroscopy

The EEM scans of each sample were conducted using a fluorescence spectrophotometer (Model: F-7000 FL Spectrophotometer, Hitachi, Tokyo, Japan). Samples were placed in 1 cm quartz cuvettes. EEMs were generated by scanning over excitation wavelengths of 200 to 450 nm at intervals of 5 nm and emission wavelengths of 250 to 550 nm at intervals of 5 nm. The excitation slit width was set at 2.5 nm, and the emission slit width was set at 5 nm. The photomultiplier tube lamp voltage was 700 V, and the scanning speed was set to 30,000 nm/min for all measurements.

2.5. Assimilable Organic Carbon (AOC)

Water samples were filtered through a 0.22 μm membrane (Millipore, Darmstadt, Germany) to remove particulate matter and bacteria. AOC was determined with Pseudomonas fluorescens strain P17 (ATCC 49642). The strains were stored at −80 °C in culture medium with 25% glycerol. P17 cells were grown in a medium containing sodium acetate solution (2000 μg/L acetate-C) at 25 °C for 7 days prior to use. Cell concentrations of the stock solution were determined by plate count on R2A agar (25 °C, 3 days). The BacTiter-GloTM microbial Cell Viability Assay (G8231, Promega Corporation, Fitchburg, WI, USA) and Synergy HTX Multi-Mode Reader (Biotek Instruments, Winooski, VT, USA) were used to measure adenosine triphosphate (ATP) luminescence (relative light unit, RLU). The protocol recommends a volume of reagent equal to the volume of cell culture present in each well (100 μL reagent to 100 μL of sample for the 96-well plate) and incubating the mixture at room temperature for 5 min. The internal standard was composed of distilled water with sodium acetate. The water samples were supplemented with K2HPO4 (7 mg/L), KH2PO4 (3 mg/L), MgSO4·7H2O (0.1 mg/L), (NH4)2SO4 (1 mg/L), NaCl (0.1 mg/L), and FeSO4·7H2O (1.8 μg/L). Duplicate centrifuge tubes contained indigenous microbes cultured at 10,000 cells/mL density with acetate-C concentrations of 0, 100, 200, 500, 1000, and 2000 μg/L at 25 °C for 3 days. Samples were taken at various intervals and used for ATP analysis.

2.6. Characterization of Molecular Weight Distribution

Characterization of the apparent molecular weight (AMW) distribution of water samples was conducted using high-performance liquid chromatography (HPLC, LC-20ATV, Shimadzu, Kyoto, Japan). Size exclusion chromatography (SEC) was conducted with sequential online detectors consisting of an ultraviolet detector (UVD) (254 nm, SPD-20A, UV–vis detector, Shimadzu, Kyoto, Japan) and organic carbon detector (OCD) (modified Sievers Total Organic Carbon Analyzer 900Turbo, GE Water &Process Technologies, Trevose, PA, USA). The chromatographic column (20 (I.D.) × 200 (L) mm TSK HW-50S, Tosoh, Tokyo, Japan) utilized a weak cation exchange resin based on polymethacrylate. The eluent system consisted of phosphate buffer (2.4 mM NaH2PO4 + 1.6 mM Na2HPO4; pH 6.8) containing sodium sulfate (25 mM) to achieve an ionic strength of 100 mM. The eluent flow rate was 0.5 mL/min, and the sample volume was 2 mL. PEG standard was used to determine the AMW. Prior to chromatographic separation, samples were made particle-free by being passed through a 0.45 μm filter (Advantec, Tokyo, Japan). Chromatograms were analyzed using a peak-fitting technique to resolve the overlapping peaks and to determine the area under each peak. PeakFit (Version 4.12, Systat Software Inc., San Jose, CA, USA), a commercially available software package, was used for this analysis. The procedure was adapted from the method described by Chow’s research [11].

2.7. Lab Testing of Pretreatment Processes for UF Membrane Filtration

The influence of different pretreatment processes (porous biocarrier only, combined ozonation and porous medium biological process) on subsequent thin-film systems was evaluated. The water sample was filtered through a 0.45 µm membrane (Advantec, Tokyo, Japan) before the UF filtration test. Flat-sheet polyethersulfone (PES) UF membranes (NADIR@UP150, Wiesbaden, Germany) have a molecular weight cutoff of 150 kDa. After trimming the UF membranes to a suitable size, they were soaked in ultrapure water for 24 h. Flux tests were then performed on each membrane with ultrapure water, and the membranes with a measured flux difference within 5% were selected. After the screening procedure, the selected UF membrane was soaked in ultrapure water and stored in a refrigerator at 4 °C, and subsequent filtration tests were conducted within 3 days. The UF experiments were conducted in a filtration cell (Amicon 8200, (Millipore, Darmstadt, Germany) at room temperature (24 ± 1 °C). A constant pressure of 14 psi was applied to drive the water sample through the membrane in dead-end filtration mode. The volume of filtrate was measured using an electronic balance and automatically recorded every 30 s. The filtration experiment was stopped after 500 mL of permeate was produced. The initial flux, Jo, is the average permeate flux calculated by filtration of 200 mL ultrapure water. The apparent flux, J, is monitored by present flux. We used J/Jo to represent the decline ratio of UF flux.

2.8. Microbial Analysis

For the porous biocarrier system, the biomass produced in the processes was collected, followed by DNA extraction and analysis by next-generation sequencing (NGS). Raw 16S rRNA sequences of 40,132 gene amplicons were generated by Illumina sequencing analysis. After the low-quality sequences were screened, 40,132 high-quality 16S rRNA gene sequences were grouped into 49 bacterial operational taxonomic units (OTUs). For the combined ozonation and porous biocarrier system, 32,860 raw 16S rRNA gene amplicons were generated by Illumina sequencing analysis. After the low-quality sequences were screened, 32,860 high-quality 16S rRNA gene sequences were grouped into 32 bacterial OTUs. Bowtie2 was used to align sequencing reads against the clusters of the V4 sequence. A 97% similarity standard was applied to V4 sequence clusters. To characterize the dominant bacterial genus, we classified the OTUs taxonomically based on their 16S rRNA-encoding gene sequences for each of the bacterial communities developed under the different pretreatment processes. Biocarrier samples on the top (0.75 m from the bottom of the reactor) and middle (0.45 m from the bottom of the reactor) layers of the biological reactor were taken through sampling ports. The Polymerase Chain Reaction (PCR) CCR primers F515 (5′-GTGCCAGCMGCCGCGGTAA-3′) and R806 (5′-GGACTACHVGGGTWTCTAAT-3′) were designed to amplify the V4 domain of bacterial 16S ribosomal DNA. PCR amplification was performed in a 50 μL reaction volume containing 25 μL 2X Taq Master Mix (Thermo Scientific, MA, USA), 0.2 μM of each forward and reverse primer, and 20 ng DNA template. The reaction conditions consisted of an initial temperature of 95 °C for 5 min; followed by 30 cycles of 95 °C for 30 sec, 54 °C for 1 min, and 72 °C for 1 min; and then a final extension of 72 °C for 5 min. Next, the amplified products were checked by 2% agarose gel electrophoresis and were purified using the AMPure XP PCR Purification Kit (Beckman Coulter, San Jose, CA, USA). Quantification was performed by using a Qubit dsDNA HS Assay Kit (Thermo Scientific, MA, USA) on a Qubit 2.0 Fluorometer (Thermo Scientific, MA, USA) according to the manufacturer’s instructions. For V4 library preparation, Illumina adapters were attached to the amplicons using the Illumina TruSeq DNA Sample Preparation v2 Kit (San Diego, CA, USA). Purified libraries were applied for cluster generation and sequencing on the MiSeq system. Sequencing reads from different samples were separated and identified according to specific barcodes at the 5′ end of the sequence (two mismatches allowed). FASTX-Toolkit (https://github.com/agordon/fastx_toolkit (accessed on 22 June 2019)) was employed to process the raw read data files. There were three steps of sequence quality processing: 1. The command was “fastq_quality_filter -Q33 -q 20 -p 70”. “-q 20” means that the minimum quality score to keep is 20. “-p 70” indicates that the minimum percentage of bases must have “-q” quality over or equal to 70%. 2. The command was “fastq_quality_trimmer -t 20 -l 100 -Q33”. “-t 20” bases with lower quality (<20) were trimmed (checking from the end of the sequence). “-l 100” indicates that the minimum acceptable length of the sequence is 100 after trimming the sequence. 3. Sequences were retained if both forward and reverse sequencing reads passed the first and second steps. To generate taxonomy assignments, the collection of 16S rRNA sequences was retrieved from the SILVA ribosomal RNA sequence database. These sequences were extracted by V4 forward and reverse primers. Then, UCLUST was used to create representative sequence clusters over or equal to 97% similarity. Bowtie2 was used to align sequencing reads against the clusters of the V4 sequence. A 97% similarity standard was applied to V4 sequence clusters. 16S rRNA gene sequences of the microbial community have been deposited in GenBank with accession numbers SRR17332969 and SRR17332970.

3. Results and Discussion

3.1. Lab Evaluation of the Pretreatment Process and UF Membrane Filtration

The basic characteristics of the secondary effluent from the Linhai Industrial Park wastewater treatment plant (Linhai WWTP) are summarized in Table 1. The low BOD5/COD ratio, which was roughly 0.16, indicated that the residual organic matter in the secondary effluent of the industrial wastewater mainly consisted of recalcitrant organic compounds.
For the removal of COD, NH4+-N, and NO3−-N by the combined ozonation and porous biocarrier pretreatment process, an average COD of 41.41 mg/L, an average NH4+-N of 14.45 mg/L, an average NO3−-N of 138.69 mg/L, and a DOC of 17.67 mg/L were observed. Compared with the porous biocarrier biological process alone, the combined ozonation and porous biocarrier pretreatment process resulted in slightly increased COD and DOC removal. The NH4+-N removal was also maintained at levels above 50%. Furthermore, based on total nitrification equilibrium, the mole ratio of ammonium nitrogen converted to nitrate nitrogen should be 1. In the nitrogen conversion performance of the combined ozonation and porous biocarrier process, the ratio of ∆NH4+-N to ∆NO3−-N was 0.98, which is very close to the stoichiometric theoretical value. For investigation of the nitrogen conversion, both pretreatment processes—biocarrier only and ozone combined biocarrier processes—showed that the recalcitrant organic compounds would not have an effect on nitrification, even though the ozone combined biocarrier process could convert the recalcitrant organic compounds into biodegradable organic compounds.
In further investigation of the pretreatment in terms of its effect on the change in flux of the UF membrane, the secondary effluent was filtered through the UF membrane with or without the pretreatment. Figure 1 shows a comparison of the decline in the UF flux of wastewater effluent, with the pretreatment of porous biocarrier and ozone combined porous biocarrier. In this experiment, we used J/Jo to represent the decline ratio of UF flux. The results showed that the lowest flux decline was observed with UF after the combined low-dose ozonation and porous biocarrier pretreatment process compared with the porous biocarrier pretreatment alone or no pretreatment. It was confirmed that the UF flux increased by 16% after the combined ozonation and porous biocarrier pretreatment process compared with the process with no pretreatment. Hamid et al. [12] showed that the decolorization of wastewater with an insignificant change in DOC after ozonation provides evidence of the decomposition of the carbon–carbon double bonds in aromatic structures and the partial degradation of high-MW organic substances with the use of an ozone pretreatment process before membrane filtration. In this study, we demonstrated both a small reduction in DOC and a substantial decrease in the high-MW fraction of organic substances with ozone pretreatment coupled with a porous biocarrier process. Our observation also provides supporting evidence that ozonation partially degraded macromolecules in the secondary effluent, and this transformation resulted in an increase in the permeate flux and alleviated the UF pore blocking, as shown in Figure 1. To further understand the cause of flux improvement, secondary effluent treated with different pretreatment processes was also evaluated with the following organic component characterization.

3.2. Organic Component Characterization

Based on the results of lab testing of the combined ozonation and porous biocarrier pretreatment process, the ozone dose plays a crucial role in the removal of secondary wastewater EfOM. Therefore, an ozonation test was conducted on the Linhai secondary effluent with different ozone doses based on the O3/DOC ratio, and EEM analysis was used to evaluate the decomposition of EfOM fractions by ozonation.
As shown in Figure 2, with increasing ozone dose, no more than 10% of DOC could be removed through direct ozonation of Linhai secondary effluent even by increasing the dosage to 75 mgO3/mg DOC. It showed that the organic compounds in the effluent could not be completely removed but could be undergoing possible partial bond breaking. Organic substances in the wastewater treated with partial ozonation would result in the effectiveness of ozonation on the degradation of a wide variety of compounds with functional groups or moieties, including carbon–carbon double bonds, amines, and activated aromatic rings. The EEM analysis could provide the information related to functional groups breaking down in aromatic proteins, fulvic acid-like compounds, soluble microbial product-like compounds, and humic acid-like compounds. Figure S3 shows the EEM analysis with different doses of ozonation of water samples from secondary effluent before and after porous biocarrier pretreatment and combined ozonation and porous biocarrier pretreatment. The results showed that ozonation at a dose of 0.5 mg O3/mg DOC resulted in more than 50% removal of DOC compounds, including aromatic protein II (AP), fulvic acid-like (FA), SMP-like, and humic acid-like (HA) fluorescence shown in Table 2. Table S1 shows the excitation and emission wavelength pairs of the main peak of the component compared with the compound descriptions of similar components reported in published studies [13,14,15]. The EEM fluorescence spectra of the water samples of the secondary effluent and the effluent of the porous biocarrier process were similar. However, the EEM fluorescence intensity decreased significantly after the secondary effluent was treated with the combined ozonation and porous biocarrier process. A fluorescence intensity 30% above reduction in protein-like extracellular matter was observed with the ozone doses of 0.2 and 0.5 mg O3/mg TOC. Meng et al. [16] studied the reaction mechanism between ozone and extracellular polymeric substances (EPS) extracted from activated sludge and found that the protein-like extracellular matter concentration decreased sharply after treatment with ozone and that the macromolecular contents broke down into small molecules, including carboxylic acids, aldehydes, and ketones, which are more soluble and readily utilized as substrates by activated sludge. Therefore, through proper control of the ozone dose, disruption of the organic macromolecule structure with oxidation of carbon–carbon double bonds was achieved with minimum or partial mineralization of EfOM.
Organic components in different molecular weight fractions in wastewater were also characterized by SEC-OCD spectra. After the secondary effluent was treated with the combined ozonation and porous biocarrier pretreatment process, the intensity of the biopolymer fraction in the SEC-OCD spectra decreased significantly. This shows that the combined pretreatment process removed biopolymers (>10,000 Da) efficiently.
Table 3 summarizes the pollutant removal efficiency results obtained by two different pretreatment processes in lab testing. The removal efficiencies of the combined ozonation and porous biocarrier process for protein-like substances, biopolymers, and AOC were 32%, 70.2%, and 75%, respectively. In addition, less than 300 µg/L AOC was detected in the effluent from the combined ozonation and porous biocarrier pretreatment process. Based on the research of Huber et al. [17], compounds with molecular weights over 10,000 Da were characterized as biopolymers, including amino sugars, polypeptides, and SMP; compounds with molecular weights between 500 and 1200 Da were characterized as humic hydrophobic substances; and compounds with molecular weights lower than 350 Da were denoted as low-molecular-weight (LMW) neutrals, including alcohols, aldehydes, ketones, and amino acids. Hammes et al. [18] have shown that ozonation of drinking water increased the concentration of LMW organic byproducts, and the compounds were more easily utilizable by microorganisms. In our study, the significant reduction in biopolymers on the SEC-OCD footprint of secondary effluent after combined ozonation and porous biocarrier pretreatment provides evidence that ozonation converted larger molecules of EfOMs into LMW compounds that were more easily utilized by microorganisms in porous biocarrier systems, corresponding to a decrease in biopolymers and a slight increase in LMW compounds.

3.3. Analysis of Microbial Community Diversity

To gain a deeper understanding of how microbial communities relate to biopolymer removal, we used NGS to investigate the composition of the microbial community associated with each pretreatment process. The results of an analysis of the diversity of the microbial community resulting from the two pretreatments are shown in Figure 3. The major species in the bacterial communities developed in the porous biocarrier system were identified as the nitrite-oxidizing bacteria Nitrospira (73.92%), with some ammonia-oxidizing bacteria Nitrosovibrio (6.06%) and Nitrosomonas (5.82%), whereas the relative abundance of Geobacter (3.13%) was low. The combined ozonation and porous biocarrier system supported bacterial communities composed mainly of nitrite-oxidizing Nitrospira bacteria (64.21%), with some interesting biopolymer-degrading Flavobacterium bacteria (15.59%), whereas the relative abundance of ammonia-oxidizing Nitrosomonas (5.74%), Dokdonella (5.24%), and Geobacter (4.15%) bacteria was low.
In the porous biocarrier process, 44.1% of ammonia was removed through the pretreatment process. In fact, Nitrospira defluvii (50.6%) and Nitrospira sp. (21.07%), known as nitrite-oxidizing bacteria, were found to be the dominant species in the porous biocarrier system. In addition, several different ammonium-oxidizing bacteria, including Nitrosovibrio sp. (6.06%) and Nitrosomonas nitrosa (3.25%), were identified in the porous biocarrier system. Uncultured Geobacter sp. (3.13%), an anaerobic bacterium that usually plays an important role in the bioremediation of groundwater, was also found in the porous carriers. For the combined ozonation and porous biocarrier process, with more than 50% of the average ammonium removal rate, the dominant biofilm bacteria found in the biocarrier medium included Nitrospira defluvii (44.57%), Nitrospira sp. (16.35%), and Nitrospira marina (3.28%), known as nitrite-oxidizing bacteria.
According to the results of microbial community analysis, several different bacteria in the combined ozonation and porous biocarrier process were identified. Dokdonella immobilis (5.24%) has also been isolated from activated sludge in a sequencing batch reactor used for the treatment of triphenylmethane dye effluent by [19]. Interestingly, Flavobacterium sp. (15.59%), which are known as biopolymer-degrading bacteria, were found only in the combined ozonation and porous biocarrier process. In earlier studies, [20] found that the classes Cytophagia-Flavobacteria seemed to play an important role in the degradation of biopolymers in marine and freshwater environments, and according to a whole-genome sequencing study, the biopolymer-degrading bacterium Flavobacterium johnsoniae has been increasingly used as a model microorganism to identify genes involved in biopolymer utilization processes [21]. A recent study by Sack et al. [22] showed the high affinity of Flavobacterium johnsoniae strain A3 for 5–6 kDa linear laminarin in oligotrophic conditions and concluded that strain A3 is involved in the bacterial utilization of biopolymers at low concentrations in oligotrophic freshwater environments. Therefore, the microbial community of the class Flavobacteria found in the ozone/porous biocarrier process could play a central role in the degradation of biopolymers in secondary effluent. The low-dose ozonation might have transformed the larger-molecule compounds into smaller-molecule biopolymers (ex., laminarin), which could be more easily utilized by Flavobacterium sp. This result highlights the promising use of the combined ozonation and porous biocarrier process for the effective reduction of secondary effluent biopolymers to alleviate membrane fouling of UF.

3.4. Pilot Testing of the Combined Ozonation and Porous Biocarrier Pretreatment Process

In the laboratory test, we observed that the decline in the flux of UF membrane filtration was effectively reduced by the combined low-dose ozonation and porous biocarrier pretreatment process. Subsequently, on-site pilot testing with 200 CMD pretreatment capacity was conducted to study the effectiveness of pretreatment processes on UF membrane filtration. Three sets of pretreatment processes were tested. The first and second sets of tests were separate tests of ozonation and porous biocarrier pretreatment processes individually, and finally, the combined ozonation and porous biocarrier pretreatment process was tested with an ozone dose of 0.08 g O3/g COD as preozonation. The results of the pilot study are shown in Table 4. When secondary effluent was directly fed into the porous biocarrier reactor without any pretreatment, a 13% DOC removal was observed. On the other hand, the DOC concentration did not change substantially after the secondary effluent was treated with ozone, but the DOC removal was significantly increased to 28% when ozonation was coupled with the porous biocarrier treatment process. The result suggests that preozonation might have changed the size and structure of EfOMs and thus increased the biodegradability of EfOMs by transforming larger organic molecules into smaller molecules, which were more readily decomposed by the following biological treatment process. From the measurement of the BOD5 (Table 4), it was found that the raw water BOD5 increased from 8.2 mg/L to 10.8 mg/L after ozone treatment. This provides additional evidence that ozone could convert refractory organic matter into more biodegradable forms, resulting in an increase in EfOM removal by the combined pretreatment process. Notably, the porous biocarrier process demonstrated a significant BOD5 removal of 50% without ozone pretreatment of wastewater.
In the pilot test, a set of hollow fiber UF modules was used for the study of the effect of pretreatment processes on the membrane flux performance. Figure 4 shows the change in the flux of the UF system operating in constant pressure mode with two types of pretreatment processes. When the wastewater treatment plant secondary effluent was treated only by the porous biocarrier process, a significant decline in the flux was observed after approximately 10 tons of permeate production. However, when porous biocarrier pretreatment was coupled with preozonation, a slight decline in flux was observed only after 26 tons of permeate production, with only 1% decline. The results indeed showed that the flux decline due to membrane fouling was significantly reduced by the combined ozonation and porous biocarrier system. Table 4 also shows that the concentration of biopolymer, which resulted in membrane fouling, as reported by previous researchers [7,15], decreased significantly from 0.87 ppm to 0.52 ppm after the combined ozonation and porous biocarrier pretreatment process. A decrease in DOC concentration from 9.56 ppm to 6.86 ppm was also observed. Wang et al. [23] investigated the impacts of preozonation on membrane fouling for secondary effluent reclamation and concluded that the decomposition of macromolecules such as biopolymers into smaller fragments at an ozone dose of 0.9–2.0 mg O3/mg DOC increased the free energy of cohesion of EfOM and led to a decrease in attraction and an increase in repulsion between foulants and the hydrophobic PES membrane surface, which might alleviate the decline in flux for PES UF membrane filtration. Our pilot test results demonstrated that the use of ozone in secondary effluent treatment before the porous biocarrier process effectively decomposed fouling-causing biopolymers, delayed the development of UF membrane fouling, and thereby significantly increased permeate production. The reduction in the biopolymer concentration in the secondary effluent verifies that ozone as a pretreatment procedure effectively degraded macromolecule biopolymers into smaller molecules that could more easily be utilized by microorganisms in the subsequent porous biocarrier process and mitigate the fouling potential of UF membranes.
In the pilot testing, COD mineralization will relate to biomass increase and CO2 production as well as DOC removal. However, in this research, we did not conduct the long-term experiment. We tried to use the lab testing evaluation and pilot testing to confirm the effects of the ozone-combined biocarrier process on UF water reclamation. A long-term experiment will be conducted to formulate equations for the biological process and perform a long-term analysis of UF module flux in our future research.

4. Conclusions

The process combining low-dose ozone with advanced porous biocarrier treatment as the pretreatment process for industrial secondary effluent reclamation can effectively slow membrane fouling in the UF process. The results indicated that 70% of the biopolymers were removed, and the COD removal efficiency was 1.8 times higher in this combined process than in the process with a porous biocarrier alone. Compared with the process with no pretreatment, the UF flux increased by 16% after the combined ozonation and porous biocarrier pretreatment process. In addition, species in the genus Flavobacterium (15.59%), which are known as biopolymer-degrading bacteria, were observed only in the ozone-combined porous biocarrier process. The results show that biopolymers can be removed through the ozone-combined porous biocarrier process due to partial ozone degradation, confirming that this combined process is one of the better pretreatment procedures for organic matter removal.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w17020272/s1, Figure S1: Flow chart of the pretreatment system for secondary effluent from the Linhai industrial park wastewater treatment plant; Figure S2: UF characterization by SEM. The average pore size of top layer and bottom layer of UF is 0.39 μm and 0.38 μm; Figure S3: Typical excitation-emission matrix (EEM) fluorescence spectra. (a) Wastewater effluent; (b) porous biocarrier; (c) ozone+porous biocarrier. Table S1: The spectral characteristics of the component identified in this study and comparisons with previously identified components.

Author Contributions

T.-T.C.: Writing—original draft, data collection and curation, S.-Y.C.: Writing—original draft, data collection and curation, C.-C.L.: Data collection and curation, Y.-L.T.: Conceptualization, Funding acquisition, G.-Y.L.: Conceptualization, Funding acquisition, C.-H.L.: Data collection and curation, P.-Y.C.: Review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was founded by the Ministry of Economic Affairs, A+ Industrial Innovative RD Program and the Ministry of Science and Technology (MOST 109-2221-E-007-058, 110-2221-E-007-054 and 110-2731-M-007-001).

Data Availability Statement

Data is unavailable due to privacy.

Acknowledgments

The authors would like to thank the Ministry of Economic Affairs, A+ Industrial Innovative RD Program and the Ministry of Science and Technology for supporting this research.

Conflicts of Interest

All authors declare that there are no conflicts of interest. The authors report no commercial or proprietary interest in any product or concept discussed in this article.

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Water 17 00272 g001
Figure 1. Comparison of decline in flux of wastewater effluent, porous biocarrier, and ozone combined porous biocarrier. The closed circles, open triangles, and open squares indicate the wastewater effluent, porous biocarrier, and ozone combined porous biocarrier, respectively.
Figure 1. Comparison of decline in flux of wastewater effluent, porous biocarrier, and ozone combined porous biocarrier. The closed circles, open triangles, and open squares indicate the wastewater effluent, porous biocarrier, and ozone combined porous biocarrier, respectively.
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Figure 2. Effect of ozonation on DOC removal from Linhai Industrial Park secondary effluent.
Figure 2. Effect of ozonation on DOC removal from Linhai Industrial Park secondary effluent.
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Figure 3. Results of NGS analysis showing the diversity of microbial communities in biomass samples from the biological porous biocarrier only process and the ozone combined with porous biocarrier process.
Figure 3. Results of NGS analysis showing the diversity of microbial communities in biomass samples from the biological porous biocarrier only process and the ozone combined with porous biocarrier process.
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Figure 4. Comparison of decline in flux with porous biocarrier and ozone-combined porous biocarrier. The open triangles and open squares indicate porous biocarriers and ozone-combined porous biocarriers.
Figure 4. Comparison of decline in flux with porous biocarrier and ozone-combined porous biocarrier. The open triangles and open squares indicate porous biocarriers and ozone-combined porous biocarriers.
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Table 1. Water sampling results for secondary effluent before and after pretreatment processes in lab testing.
Table 1. Water sampling results for secondary effluent before and after pretreatment processes in lab testing.
ProcessSecondary EffluentPorous Biocarrier Reactor EffluentOzonation + Porous Biocarrier Reactor Effluent
COD (mg/L)46.45 ± 17.9543.85 ± 18.5741.41 ± 16.39
DOC (mg/L)21.44 ± 10.9018.83 ± 7.8617.67 ± 6.70
BOD5 (mg/L)7.54 ± 1.99n.a. *n.a. *
NH4+-N(mg/L)32.84 ± 17.5314.83 ± 9.6814.45 ± 9.89
NO3−-N(mg/L)122.47 ± 32.26139.38 ± 27.39138.69 ± 26.91
∆NH4+/∆NO3−n.a. *0.930.98
Note: * n.a. represents not analyzed.
Table 2. The removal of fluorescence: EEM fluorescence results with different O3/DOC ratios.
Table 2. The removal of fluorescence: EEM fluorescence results with different O3/DOC ratios.
Ozone Dose (mg O3/mg TOC)
Fluorescence Intensity Removal (%)0.20.51
API8.32971
APII285578
FA305269
SMP336782
HA356379
Table 3. Results for SEC-OCD and AOC in secondary effluent from different pretreatment processes in lab testing.
Table 3. Results for SEC-OCD and AOC in secondary effluent from different pretreatment processes in lab testing.
Secondary EffluentPorous Biocarrier Reactor EffluentOzonation + Porous Biocarrier Reactor Effluent
Concentration of DOC, in molecular weight>10,000 Da (μg-C/L)0.2850.5610.085
500–1200 Da (μg-C/L)6.747.627.67
<350 Da (μg-C/L)1.903.352.24
Protein-like substance removal (%)n.a. *3.832.0
Biopolymer removal (%)n.a. *−96.970.2
AOC (μg/L)1196.11 ± 186.932512.36 ± 105.97253.82 ± 31.15
Note: * n.a. represents not analyzed.
Table 4. Results for secondary effluent before and after pretreatment processes in pilot testing.
Table 4. Results for secondary effluent before and after pretreatment processes in pilot testing.
ProcessWastewater EffluentOzonePorous BiocarrierOzone + Porous Biocarrier
DOC (mg/L)9.59.78.26.8
BOD5 (mg/L)8.210.84.14.0
Biopolymer (mg/L)0.870.89n.a. *0.52
Concentration of DOC, in molecular weight>10,000 Da (μg-C/L)0.870.89n.a. *0.52
500–1200 Da (μg-C/L)6.496.64n.a. *5.01
<350 Da (μg-C/L)2.202.13n.a. *1.33
Note: * n.a. represents not analyzed.
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MDPI and ACS Style

Chang, T.-T.; Chiu, S.-Y.; Lee, C.-C.; Tai, Y.-L.; Lin, G.-Y.; Lai, C.-H.; Chen, P.-Y. Membrane Foulant Removal by Ozone-Biocarrier Pretreatment Technology for Industrial Wastewater Reclamation. Water 2025, 17, 272. https://doi.org/10.3390/w17020272

AMA Style

Chang T-T, Chiu S-Y, Lee C-C, Tai Y-L, Lin G-Y, Lai C-H, Chen P-Y. Membrane Foulant Removal by Ozone-Biocarrier Pretreatment Technology for Industrial Wastewater Reclamation. Water. 2025; 17(2):272. https://doi.org/10.3390/w17020272

Chicago/Turabian Style

Chang, Ting-Ting, Sheng-Yi Chiu, Chun-Chi Lee, Yuan-Liang Tai, Guan-You Lin, Chun-Hsi Lai, and Po-Yu Chen. 2025. "Membrane Foulant Removal by Ozone-Biocarrier Pretreatment Technology for Industrial Wastewater Reclamation" Water 17, no. 2: 272. https://doi.org/10.3390/w17020272

APA Style

Chang, T.-T., Chiu, S.-Y., Lee, C.-C., Tai, Y.-L., Lin, G.-Y., Lai, C.-H., & Chen, P.-Y. (2025). Membrane Foulant Removal by Ozone-Biocarrier Pretreatment Technology for Industrial Wastewater Reclamation. Water, 17(2), 272. https://doi.org/10.3390/w17020272

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