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Environ Eng Res > Volume 28(5); 2023 > Article
Phan, Pham, Duong, and Nguyen: Novel carrier for seafood wastewater treatment using moving bed biofilm reactor system


In this work, seafood wastewater was treated by using MBBR reactor over a novel carrier (PUF-PVA gel) derived from the combination of a porous (PUF) and hydrophilic surface (PVA gel). A freezing-thawing method was used for the preparation of the novel PUF-PVA gel which is reported for the first time. Experimental results indicated that PUF-PVA gel carrier which possesses an excellent textural structure (specific surface area of 3.4 m2/g, pore size in the range of 10 – 40 μm and hydrophilic surface) accounts for faster water immersion and better microbial adhesion, resulting in the biofilm content attached at the start-up stage 2.4 times higher relative to conventional PUF carrier alone. The MBBR system can be operated stably at a hydraulic retention time (HRT) of 7 hours in which the organic loading rate ranged from 2.5 kg COD/m3.day to 5.1 kg COD/m3.day. When operated at the organic loading rate of 2.5 kg COD/m3.day, the maximum COD and the nitrogen removal efficiency of MBBR based on the PUF-PVA gel carrier were 92 ± 0.6% and 89 ± 3.4%, respectively, which is higher than that of the PUF carrier.

1. Introduction

The aquaculture industry has been intensely developed in recent decades to satisfy the increasing consumption of aquatic products, particularly seafood. In Vietnam, the seafood processing industry is an important economic sector with various Vietnamese seafood products exported to many countries and territories including the United States, South Korea, and European Union [1]. Half of the seafood produced in Vietnam is derived from aquaculture and the expansion in the aquaculture sector contributes to the development of the Vietnamese economy by growing the labor force, famine elimination, and poverty [2]. Nonetheless, the rapid growth of the fishery industry in Vietnam is posing stress on natural resources and the environment. Indeed, wastewater from the aquaculture industry was often drained without proper treatment, leading to severe problems in the nearby water environment and odor pollution due to the decomposition of organic matter [3, 4]. Noticeably, wastewater from seafood manufacturing contains a high concentration of dissolved organic carbonaceous compounds such as biochemical oxygen demand (BOD) and chemical oxygen demand (COD) as well as nutrients including nitrogen and phosphorus [57]. These substances cause harmful algal blooms and damage the natural aquatic ecosystem [8].
Conventional treatments for seafood processing wastewater include filtration, chemical precipitation, flotation, wetland, and adsorption [912]. These technologies, however, display low removal efficiency, high energy consumption, and limitation to scaled up [13]. Until now, the most crucial need is to upgrade existing wastewater treatment plants with little additional costs and high efficiency [14, 15]. Moving bed biofilm reactor (MBBR) is among emerging technologies that can satisfy such criteria. MBBR has proved its high efficiency and reliable biological technology for carbon and nitrogen removal by combing suspended growth and attached growth processes [16, 17]. By adding free-floating biofilm carriers into the aeration tank, the MBBR system has many advantages such as low space requirement, simple construction, low sludge generation enhancing oxygen transfer, and a large surface area for colonization and high specific biomass activity [1821]. In the MBBR system, the biofilm carriers play an important role in determining the overall efficiency and performance of treating wastewater [19, 22]. Biofilm carriers for MBBR can be classified by non-porous or porous carriers such as activated carbon particles [23], fireclay [24], natural materials [25], and materials supported on polymers including polyethylene cylinder [26], polypropylene [27], poly(vinyl chloride) [28], polyurethane foam [29], poly(vinyl alcohol) gel [30]. Among mentioned carriers, non-porous carriers such as polyethylene cylinders (Kalnes types) possess a low specific surface area (about 300–500 m2/m3) and are prone to the detachment of biofilm given their smooth surface [31]. In contrast, the porous carriers with high porosity and large surface area (up to 900 m2/m3) can improve biofilm accumulation and protect the biofilm from fluid shearing and collision by providing sheltered anchoring points [32, 33]. The interaction pathways of microorganisms in the porous structure of carriers have been well discussed in the literature [3236]. Typically, the microorganisms are retained on a porous carrier in two different forms: (i) growing on the skeleton surface, and (ii) trapping in the pores of the carrier. Among polymer-based carriers, polyurethane foam (PUF) has been widely used as an effective carrier for MBBR given its high porosity, flexibility, interconnected pores, low cost, and less harmful to the environment [37, 38]. Despite such advantages, PUF carriers have low adherence to the microorganisms [39] and are inclined to hydrolyzation in the wastewater environment, thereby, reducing its stability under continuous operations in harsh environments i.e., aerotank [40]. The susceptibility of PUF to hydrolysis originated from the ester bonds in the polyester polyols and the urea bonds formed during the foaming process [41]. Moreover, the smooth skeleton surface of PUF also leads to biofilm detachment during treatment reducing removal efficiency [42]. Researchers also indicated that bacterial cells attached to the PUF foam surface and micro granules were only mechanically retained in its pores [21]. In the meanwhile, polyvinyl alcohol (PVA) gel has demonstrated its efficiency for the growth of microorganisms from wastewater [43, 44]. The PVA gel is a spherical bead with a diameter of around 4 mm and interconnected pores with a diameter of 4–20 mm, allowing the cultivation of microorganisms in a sheltered mode to reduce the detachment of biomass [45]. The higher specific surface area (about 2500 m2/m3) and porosity of PVA gel favor better permeability of oxygen and nutrients to the microorganisms colonized inside the beads [46]. Such promising properties lead to high treatment stability of PVA gel under variable loadings. Furthermore, due to the hydrophilic nature of PVA gel and biofilm, microorganisms in wastewater can be adhered better to its surface reducing their flaking. The PVA gel beads are also more stable in aerotanks due to less susceptible to hydrolysis [47]. The combination between PUF and PVA is expected to provide a promising carrier with excellent treatment efficiency and stability. To the best of our knowledge, none of the studies reported on using such PUF-PVA gel carrier for MBBR system for seafood wastewater treatment are available in the literature.
This work herein reports for the first time a novel carrier (PUF-PVA gel) with a porous structure for the treatment of seafood processing wastewater. The PUF-PVA gel carrier possesses an excellent porous structure with combined advantages of PUF and PVA individuals. Such a high surface area of PUF-PVA carrier enhances the attachment and growth of the microorganisms significantly. The start-up time of the MBBR system is considerably shortened with a high-achieved COD, total nitrogen (TN), and total phosphorous (TP) removal efficiency. Further, the presence of the hydrophilic nature of PVA gel improves the attachment ability of microorganisms on the carrier. The findings from this work unfold promising routes to develop novel hybrid carriers for MBBR to treat wastewater not only limited to seafood processing but also other industrial operations in sustainable and economical ways.

2. Materials and Methods

2.1. Materials

PVA 217 with a polymerization degree of 1725, hydrolysis degree of 87–89%, sodium acetate content of 1.04%, and ash content of about 0.4% was purchased from Kuraray (Japan). Commercial PUF sheets were purchased from local markets and then cut into cubic of 10×10×10 mm in size. Analytical chemicals were purchased from Sigma Aldrich.

2.2. Preparation of PUF-PVA Gel Carrier

PVA 217 has a low hydrolysis degree, which is not favorable for gel formation, therefore, it was hydrolyzed before impregnating for PUF. The PUF cubics of 10×10×10 mm were immersed in an excess of hydrolyzed PVA solution. To ensure that the PVA solution could penetrate deeply into the porous structure, the PUF cubics were manually squeezed and then stabilized for 1 day to remove air bubbles. After that, PUF cubics soaked with PVA solution were drained naturally on a plastic basket and then poured into a stainless-steel tray to perform the freezing-thawing process. The PUF cubics were frozen at −20 °C for 24 hours in a refrigerator and thawed at room temperature for 12h. The freeze-thaw cycle was performed three times to obtain a strong gel.

2.3. Seafood Wastewater Treatment

Seafood wastewater samples were collected from the equalization tank of the wastewater treatment system of the Company Limited Ha Long Canned Food, Danang, Vietnam, and diluted with tap water. The characteristics of experimental wastewater are shown in Table 1.
The BOD5:N:P ratio of 100:39:3 in experimental wastewater is favorable for biological treatment. Experiments were conducted in a pilot-scale MBBR reactor consisting of two reactors (R1 & R2), connected in series, followed by a sedimentation tank of 20 L volume, as shown in Fig. S1. The separated sludge in a sedimentation tank was returned to the R1 reactor to maintain biomass in appropriate proportion. The R1 reactor of 40 L was filled with carriers with a packing ratio of 10% in volume and operated as MBBR. Wastewater was continuously fed to this reactor by using a peristaltic pump and its outlet was directed to the R2 reactor. Stainless steel mesh with mesh size of 6 was arranged to retain the carriers inside the R1 reactor. The air was introduced through a distributor on the bottom of the reactor to provide dissolved oxygen and keep the carriers in suspension. The dissolved oxygen (DO) concentration was maintained at 2–4 mg/L. The pH of the reactors was controlled through the control box and the pH sensor. The R2 reactor of 40 L was operated simply as a conventional activated sludge process. The excess sludge was discharged from the bottom of the clarifier daily. The activated sludge concentration collected from the Company Limited Ha Long Canned Food was used for experiments of 2.5 g/L of mixed liquor suspended solids (MLSS). The hydraulic retention time (HRT) is fixed at 14 hours and 7 hours in the start-up stage and in the experimental phases, respectively.
Previous studies have shown that the time of 5–7 days for the start-up period is sufficient to form a biofilm on carriers [16], [17]. In this study, the start-up time was set to 15 days with a low influent COD concentration of about 540 mg/L, to secure the stable attachment and growth of biofilm on carriers before conducting further experiments. The stable state was defined as the period in which the effluent quality was relatively constant at a constant loading and that was monitored by the COD value. The operational conditions of the pilot are shown in Table 2. Treatment performance was monitored by following COD, total suspended solids (TSS), total nitrogen (TN), and total phosphorus (TP) in the influent and effluent two times a day.

2.4. Analytic Methods

Samples were taken from the raw wastewater tank and from the effluent of the sedimentation tank. Parameters such as chemical oxygen demand (COD), total suspended solids (TSS), total nitrogen (TN), total phosphorus (TP), mixed liquor suspended solids (MLSS), and mixed liquor volatile suspended solids (MLVSS) were analyzed according to “Standard Methods for the Examination of Water and Wastewater” [48]. Whereby, sludge volume index (SVI 30) was determined by the settleability of 1000mL of active sludge for 30 min [4850]. Each experiment was repeated at least three times. The flow rate was determined daily by visual measurement with a cylinder and timer. To determine the biomass trapped in the pores of carriers, a sample of carrier elements from the reactor was sliced, mechanically stirred (OniLAB Electric Overhead) at 500 rpm with distilled water, and then filter on a mesh. The filtrate containing biomass is dried at 105 °C and then weighed. The sliced carriers were dried at 80 °C for 24 hours and then weighed. The difference in weight compared to an average “zero” weight of the fresh carriers was the biofilm amount attached to the carriers. The attached biofilm concentration can be calculated based on the ratio of biofilm weight and carrier volume.

3. Results and Discussion

3.1. Comparison of the Properties of the PUF-PVA Gel with PUF

It can be seen from digital photos of PUF and PUF-PVA gel carriers (Fig. 1a and 1b) that the shape of the PUF-PVA gel carrier (Fig. 1b) can be easily shaped because it has the shape of PUF (Fig. 1a). The PVA content of about 41 wt% was determined in the PUF-PVA gel carrier. The amount of water stored was estimated at 91% corresponding to a solids content of about 9%. This value is similar to that of the commercial PVA gel from Kuraray Company (Japanese), about 10% [23]. The apparent density of PUF was determined by the mass to volume ratio of the foam and had a value of 21 ± 0.21 g/L. Typically, the foams used in the automotive industry e.g. polyether and polyester have apparent density of 13.5 to 38.0 g/L [51]. In general, the density of the PUF was strongly dependent on the initial materials used and the foaming process. From the SEM image in Fig. 1a1, it can be seen that the commercial PUF used in this study has an open cell structure. The individual pores, which are of different shapes and sizes, were mostly composed of pentagonal walls. For such a structure, the liquid can fill the pores, then the actual density of polyurethane (material without voids) will decide its ability to sink or float. F. Saint-Michel et al. [52] estimated the actual density of PU by calculation was 1180 g/L. However, when the PUF was filled with water in the structure it still floats on the water, this could be due to the air bubbles trapped in the foam frame or the existence of several closed-cell pores. For that reason, the density of the PUF-PVA gel carrier determined by hydrostatic weighing was 1015 g/L. The pore size of PUF ranges from 300–500 mm (Fig. 1b1), which is significantly larger compared to the size of bacteria (0.5 – 5 mm) [53]. The combination with PVA gel can create a hybrid structure between two materials with different pore sizes, and that can increase the specific surface area leading to an increase in wastewater treatment efficiency.
The SEM image at 30× magnification (Fig. 2a) confirms the uniform distribution of PVA gel in the porous structure of PUF. The PVA gel was divided into blocks of about 300–500 mm in size, which are interconnected in the structure of PUF. The specific surface area of PUF determined by the nitrogen adsorption isotherm according to the Brunauer-Emmelt-Teller (BET) method was 0.67 m2/g. After the impregnation with PVA gel, the specific surface area increased by 5 times to reach the value of 3.4 m2/g. The distribution of the pore size in the PVA gel region was determined by using Image J software on SEM images at 300× magnification (Fig. 2b). The results indicated that the pore size of the PVA gel region is mainly in the range of 10–40 μm. This size range is similar to the commercial PVA gel of Kuraray (Japan).

3.2. Enhanced Biofilm Attachment of PUF-PVA Gel Compared to PUF in the Start-up Stage

The variation of influent and effluent COD concentrations and the removal efficiencies of the MBBR using PUF and PUF-PVA gel as a carrier were continuously monitored during the start-up stage. As shown in Fig. 3, the results of the first 7 days of operation at HRT of 14 hours corresponding to an organic loading of about 0.930 kg COD/m3.day, the difference in COD removal efficiency between the two types of carriers is not obvious. This might be due to the acclimatization of microorganisms in the startup phase. After the 7th day, the COD removal efficiencies are more stable and increase slightly at the end of the start-up stage indicating that the start-up period of 15 days is enough for bacteria to adhere to the carriers, grow, and play their role. The COD removal efficiency of the PUF-PVA gel carrier in the last days of the start-up stage was stable at 88% which is 5% higher than that of the PUF carrier. The higher performance of the PUF-PVA gel carrier can be explained by its hydrophilic nature and higher density, which make it easier to immediately submerged in water and interact with the activated sludge, resulting in a shortened time of start-up stage. Meanwhile, the PUF carrier remained floating on the surface for the first two days. Ewa Dacewicz et al. [54] have reported that unused PUFs can float on water for more than six days under undisturbed conditions.
As shown in Fig. S2, the attachment and proliferation of microorganisms on carriers can be observed through the color change of material from the initial white to yellow-brown after 15 days of operation in the start-up stage. The PUF carrier is more similar in color to the activated sludge than the PUF-PVA gel carrier. This may be due to the PUF carrier having large pores, so the suspended activated sludge particles conveniently enter and get trapped between the pores. Researchers also indicated the presence of bacterial aggregates physically trapped in the cavities of PUF [55].
Fig. 4 shows the SEM images of the carriers after 15 days of the start-up stage at different magnifications. At the low magnification i.e., 30x, the surface of PUF carrier displayed a negligible deterioration (Fig. 4a). At high magnification (200x) (Fig. 4b), the biomass arrays had loosened the adhesion and seemed to be separated from the porous skeleton. These biomass arrays can come from either trapped suspended activated sludge particles or from biofilms that have been peeled off from the carrier. The detachment of the biofilm was explained by the smooth surface of the skeleton and the poor hydrophilic nature of PUF. In contrast, strong adhesion of biofilm on the PUF-PVA carrier surface can be observed at all magnifications (Fig. 4c and 4d). In addition, although the total sludge concentration on the PUF carrier of 0.86 g/L includes the trapped biomass in the pores and the attached biofilm on the skeleton was 4% higher than that on the PUF-PVA gel carrier; nonetheless, its attached biofilm concentration was 2.4 times lower than that of the PUF-PVA gel. The presence of more hydrophilic groups on the PUF-PVA gel carrier favors biofilm attachment and growth. This conclusion is in good agreement with other researchers [54, 56].

3.3. Seafood Wastewater Treatment

3.3.1. Carbon removal (COD)

After the start-up period of 15 days, the experiments to evaluate the carbon and nutrient removal efficiency were continued for the next 20 days in phases I and II. At HRT of 7 hours, the variation in the COD concentration of the influent was accompanied by a change in organic loading rate. The carbon removal efficiency was assessed through COD and TSS parameters. The average results obtained from the Physico-chemical analysis are summarized in Table 3. When the system was operated at an organic loading rate of 2.5 kg COD/m3.day, the COD removal efficiency of 92 ± 0.6% was observed in the PUF-PVA gel carrier, which was higher than that for the PUF carrier of 84 ± 1.7%. This result is comparable with the results obtained by Derakhshan, Z., et al [17] and Rajpal et al.[57], who used MBBR systems with PVA gel carriers for different types of wastewater. With a BOD/COD stable ratio of around 0.64 of the influents, the average food to microorganisms (F/M) ratio was 0.66 kgBOD5/kg MLVSS corresponding to the organic loading rate of 2.5 COD/m3.day. This value is outside the normal range of the conventional activated sludge process (0.2 to 0.5 kgBOD5/kg MLVSS) [58]. This shows the potential of MBBR technology in the treatment of seafood wastewater. The greater organic matter removal efficiency of PUF-PVA gel can be attributed to its large specific surface area and hydrophilic nature, which helps micro-organisms to adhere and grow better on the carrier. The effluent COD concentration of PUF-PVA gel carrier is 58 ± 6 mg/L. This value is lower than the upper limit value of column A (< 75 mg/L) in QCVN 11-MT:2015/BTNMT- National technical regulation on the effluent of seafood wastewater. Meanwhile, the effluent COD concentration of the PUF carrier is 114 ± 9.7 mg/L, which only meets the standard of column B of QCVN 11-MT:2015/BTNMT.
The COD removal efficiency decreased with the increase of the organic loading rate from 2.5 kg COD/m3.day to 5.1 kg COD/m3.day corresponding to the F/M ratio of 1.35 kgBOD5/kg MLVSS. This could be due to the sudden increase in the influent organic concentration leading to the F/M ratio being far from the normal range of the conventional activated sludge process. However, it can also be seen that the COD removal efficiency of PUF-PVA gel is always higher than that of PUF. Furthermore, removing TSS also contributes to reducing effluent COD concentration. The effluent TSS increased with the increase of the organic loading rate. The effluent TSS concentration of PUF-PVA gel is lower than that of PUF as shown in Table 3. Similar to TSS, the SVI value increased with the increase in organic loading rate and ranged between 100 mL/g and 200 mL/g, which indicates the lightweight characteristics of the sludge. Schular et al. [59] also indicated a threshold SVI value of 150 mL/g for good settling properties of the activated sludge.

3.3.2. Nutrient removal (TN and TP)

The nutrient removal efficiency was evaluated through two characteristic parameters, TN and TP. The obtained results are summarized in Table 4. Similar to the trend observed in the carbon removal efficiency, the nutrient removal efficiency decreased with an increase in the organic loading rate. The nitrogen removal efficiency of PUF-PVA gel of 89 ± 3.4% was 4% higher than that of PUF at the organic loading rate of 2.5 kg COD/m3.day; however, these two values became almost homogeneous at the organic loading rate of 5.1 kg COD/m3.day. Meanwhile, the difference in phosphorus removal efficiency between PUF-PVA gel and PUF was not well observed from the investigated organic loading rates. Such observations verify the role of PUF-PVA gel carriers in cost-reducing for large-scale applications.

4. Conclusions

A novel carrier, PUF impregnated PVA, with a porous and hydrophilic surface was prepared by the freezing-thawing method of. The formed PUF-PVA gel material with a hybrid structure showed many advantages as a carrier for wastewater treatment. The large specific surface area and hydrophilic surface enhanced the adhesion and growth of microorganisms. The specific surface area and the biofilm content attached to the PUF-PVA gel carrier at the start-up stage increased by 5 times and 2.4 times compared to the PUF, respectively. At the appropriate organic loading rate (2.5 kg COD/m3.day), the COD and nitrogen removal efficiency of MBBR based on the PUF-PVA gel carrier corresponding to 92 ± 0.6% and 89 ± 3.4% were higher than that of the PUF carrier. The findings from this work have opened novel routes to develop an effective carrier that can be used for wastewater treatment with the MBBR system effectively and sustainably.

Supplementary Information


This work was supported by The University of Danang, University of Science and Technology, code number of Project: T2021-02-38.


Conflict-of-Interest Statement

The authors declare that they have no conflict of interest.

Author Contributions

T.A.P (Ph.D) conducted all the experiments and wrote the manuscript independently. T.N.P (Ph.D), T.H.D (Ph.D.) and H.M.N (Ph.D.) participated in the coordination of the study and reviewed the manuscript. All authors read and approved the final manuscript.


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Fig. 1
(a) Digital photo and (a1) SEM image of commercial PUF; (b) Digital photo and (b1) SEM image of freeze-dried PUF-PVA gel
Fig. 2
SEM image of PUF-PVA gel at 300× magnification
Fig. 3
Variation of influent and effluent COD concentrations and removal efficiencies in the start-up stage. Inf. COD is denoted of influent COD and Eff. COD is effluent COD
Fig. 4
(a) & (b) SEM image of PUF; and (c) & (d) PUF-PVA gel after 15 days of the start-up stage at different magnification, respectively
Table 1
Characteristics of experimental wastewater
Parameter Unit Range (n=5) QCVN 11-MT:2015/BTNMT * (Colum A) QCVN 11-MT:2015/BTNMT * (Colum B)
BOD5 mg/l 974 ± 51 30 50
COD mg/l 1530 ± 35 75 150
TN mg/l 382 ± 15 30 60
TP mg/l 29 ± 5 10 20
pH - 7.2 ± 0.5 6–9 5,5 – 9
TSS mg/l 115 ± 10 50 100

QCVN 11-MT:2015/BTNMT: National technical regulation on the effluent of seafood wastewater (Ministry of Natural Resources and Environmental of Vietnam).

Table 2
Operation conditions of the treatment experiments
Parameter Starup Phase I Phase II

Days 15 15 10 10 10 10
Influent COD (mg/L) 541±36 543±24 745±21 756±25 1517±112 1502±104
Total working volume of two reactor (m3) 0.08 0.08 0.08 0.08 0.08 0.08
Reactor volume filled carrier of 10% (m3) 0.04 0.04 0.04 0.04 0.04 0.04
Flow (m3.day) 0.1370 0.137 0.270 0.270 0.270 0.270
Orgamic loading (kg COD/m3.day) 0.926 0.930 2.514 2.552 5.120 5.069
HRT (h) 14 14 7 7 7 7
Table 3
Summary of the carbon removal efficiencies of MBBRs using PUF and PUF-PVA gel as a carrier in various organic loading rates (± is standard deviation)
Types of carriers HRT (h) Organic loading (kg COD/m3.day) Parameters


Influent (mg/L) Effluent (mg/L) Removal (%) Effluent (mg/L) mL/g
PUF 7 2.514±0.071 745±21 114±9.7 84±1.7 30±3.1 104±4
7 5.120±0.378 1517±112 492±16.8 68±1.6 66±6.2 132±7

PUF-PVA gel 7 2.552±0.084 756±23 58±6 92±0.6 28±2.1 105±3
7 5.069±0.351 1502±104 418±13.9 72±1.9 53±3.6 124±5
Table 4
Summary of the nutrient removal efficiencies of MBBRs using PUF and PUF-PVA gel as a carrier in various organic loading rates (± is standard deviation)
Carrier HRT (h) Organic loading (kg COD/m3.day) Parameters


Influent (mg/L) Effluent (mg/L) Removal (%) Influent (mg/L) Effluent (mg/L) Removal (%)
PUF 7 2.552±0.084 192±8.1 28.7±3.1 85±2.1 14.7±3.1 3.6±1.6 76±8.8
7 5.120±0.378 389±22.7 86.7±7.2 78±3.1 29.9±4.2 9.3±2.9 69±5.5

PUF-PVA gel 7 2.514±0.071 201±14.4 22±5.3 89±3.4 13.5±2.4 3.8±1.2 72±7.9
7 5.069±0.351 390±13.4 82.2±10.2 79±1.9 29.6±3.5 10.3±1.5 71±2.2
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