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Article

Influence of Filler Types on the Treatment of Rural Domestic Wastewater in a Biological Trickling Filter: Simultaneous Nitrogen and Phosphorus Removal Performance, Microbial Community, and Metabolic Functions

by
Yuxin Geng
1,
Zhengwei Pan
2,
Liangang Hou
2,
Jiarui Li
1,
Mingchao Wang
2,
Tianhao Shi
2,3,
Dongyue Li
1 and
Jun Li
1,*
1
National Engineering Laboratory for Advanced Municipal Wastewater Treatment and Reuse Technology, Beijing University of Technology, Beijing 100124, China
2
China Construction First Group Construction & Development Co., Ltd., Beijing 100102, China
3
China Construction First Group Corporation Limited, Beijing 100089, China
*
Author to whom correspondence should be addressed.
Water 2024, 16(23), 3343; https://doi.org/10.3390/w16233343
Submission received: 21 October 2024 / Revised: 16 November 2024 / Accepted: 19 November 2024 / Published: 21 November 2024
(This article belongs to the Special Issue Sewage Disposal, Biological Processes and Resource Utilization of Sewage Sludge)
Browse Figures
Figure 1
<p>Schematic diagram of biological trickling filter system.</p> "> Figure 2
<p>(<b>a</b>) Physical and SEM images of the three fillers. (<b>b</b>) Biofilm microscopy.</p> "> Figure 3
<p>XRD patterns of fillers. (<b>a</b>) ceramic granule; (<b>b</b>) zeolite; and (<b>c</b>) sponge.</p> "> Figure 4
<p>Performance of simultaneous nitrogen and phosphorus removal; (<b>a</b>,<b>c</b>,<b>e</b>,<b>g</b>) are the inlet and outlet concentrations of COD, NH<sub>4</sub><sup>+</sup>-N, TN, and TP, respectively, and (<b>b</b>,<b>d</b>,<b>f</b>,<b>h</b>) COD, NH<sub>4</sub><sup>+</sup>-N, TN, and TP removal rate, respectively.</p> "> Figure 5
<p>Microbial community structure; (<b>a</b>) phylum level and (<b>b</b>) genus level.</p> "> Figure 6
<p>KEGG pathway analysis in different reactors. (<b>a</b>) Primary classification; (<b>b</b>) secondary classification; (<b>c</b>) third-level classification.</p> "> Figure 7
<p>KEGG prediction of potentially functional genes and enzymes in metabolic pathways of nitrogen metabolism (<b>a</b>), phosphorus metabolism (<b>b</b>), and TCA cycle (<b>c</b>).</p> ">
Versions Notes

Abstract

:
Biological trickling filtration (BTF) has the advantages of simple operation, low energy consumption, and low sludge production, and its application in the treatment of domestic wastewater in rural areas has been widely discussed. In this study, ceramic granule (R1), zeolite (R2), and sponge (R3), three typical nitrogen and phosphorus removal fillers, were selected to investigate the differences in the removal performance of COD, nitrogen, and phosphorus in BTF, analyze the characteristics of the fillers and biofilm, and determine the performance of simultaneous nitrogen and phosphorus removal. The results show that among the three fillers, zeolite has the larger specific surface area and roughness and has the best treatment effect on the adhesion performance of sewage and biofilm. The richness and diversity of the microbial community are higher, and the system is more stable, with a COD removal rate of 77.10 ± 8.67% and an NH4+-N removal rate of 75.20 ± 6.64%. The TP removal rate was 22.04 ± 10.27%. The surface of ceramic particles showed a regular cluster structure with a loose distribution. The removal rate of COD was 78.49 ± 6.92%, the concentration of NH4+-N in the effluent was 27.95 ± 8.23 mg/L, and the removal rate of TP was 38.83 ± 12.14%. As a polymer composite material, the sponge has large internal pores and a smooth surface, which is not conducive to biofilm adhesion. Therefore, the removal rate of nitrogen and phosphorus in sewage is poor; the removal rate of COD is 75.94 ± 6.98%, NH4+-N is 27.89 ± 21.06%, and the removal rate of TP is 14.07 ± 11.76%. Compared with the metabolic function of genes, zeolites have a more stable enzyme digestion ability than the other two fillers, and the genes related to the nitrification process (amo, hao, nxr, etc.) and functional genes encoding key enzymes related to the TCA cycle are relatively abundant.

1. Introduction

At present, domestic wastewater in the village and town areas is characterized by scattered spatial distribution, intermittent temporal distribution, and low concentration [1]. However, the treatment of village and township wastewater is mainly divided into three categories: ecological treatment process (artificial wetland, land infiltration, and stabilization ponds, etc.), biological treatment process (A2/O process, etc.), and combined process (improved multi-stage A/O membrane bioreactor (MBR), etc.), which have the problems of poor applicability of technology, large area and energy consumption of structures, and high infrastructural and operational costs and management requirements [2]. Compared with traditional sewage treatment facilities with high energy consumption and complex structure, a biological trickling filter (BTF) is a low-load biofilter that has a simple structure, stable performance, a small footprint, low operation and maintenance costs, and low sludge production [3,4]. BTF is suitable for intermittent, low-concentration sewage treatment in villages and towns.
BTF is a biological treatment technology based on the biofilm method. The structure is roughly divided into three parts: water distribution equipment, pool body, filler, and drainage system. The shape of the pool body is mostly rectangular and circular [5]. The principle of BTF purification of sewage is that sewage is continuously and evenly sprayed on the surface of the upper layer of filler by the water distributor located above the tank body. When sewage flows through the filler, part of the pollutants in the sewage are removed under the action of adsorption and retention of the filler [6]. At the same time, in the appropriate environment provided by the BTF, microorganisms using organic matter in the sewage began to multiply and gradually formed a biofilm on the surface of the filler [7], and the other part of contaminants was ingested and utilized by microorganisms on the surface of the filler so that the sewage was purified. The main body of BTF is the tank and filler. The biggest difference between BTF and an ordinary biofilter is the use of multi-layer filler. On the one hand, it can achieve the purpose of uniform distribution of water in the filter; on the other hand, it can make use of the characteristics of different fillers to improve the biomass of the tank and the ability of nitrogen and phosphorus removal. Most BTFs use natural ventilation to supply oxygen to the filter during operation, which provides a suitable living space for aerobic microorganisms [8,9].
Therefore, the filler is the core part of the biological trickling filter treatment technology, which directly affects the treatment effect, investment cost, and operation cost, and is the key factor restricting the removal efficiency of the biological trickling filter [10,11,12]. It is generally believed that a good filler has the following characteristics: (1) larger specific surface area, which can fully provide the film area and phase contact area; (2) surface roughness, wettability, and biological activity are high, making it easy for microorganisms to grow on the film; (3) larger void rate, so that the fluid penetration resistance and pressure drop are small and energy consumption is low; (4) lightweight and has a certain degree of mechanical strength and biological stability; and (5) the material is easily available, cheap, and abundant. Few fillers can meet all the above characteristics at the same time; usually, the carrier filler is always selected according to the process design and actual needs. There are many types of filter media for BTF; zeolite (natural and artificial), quartz sand, ceramic granule, activated carbon, and organic polymer filter media (such as polypropylene, PVC, polypropylene, etc.) are commonly used [13]. Among them, zeolites and ceramic granules are commonly used as inorganic fillers [14], but few studies have focused on the use of zeolites for the simultaneous removal of nitrogen and phosphorus from wastewater. The organic polymer filter material has a relatively large surface area and small particle size and has a better interception effect on suspended solids, and a recent study has shown that sponges effectively retain slow-growing nitrifying bacteria (1.13–7.49 × 107 copies/mL) and generally have high microbial diversity on their surfaces and within them [15]. However, the effects of filler types on the performance of nitrogen and phosphorus removal, microbial community, and metabolic function of biological trickling filters for treating village and township domestic wastewater are not clear at present.
Therefore, in this study, three typical BTF packings (ceramic granule, zeolite, and sponge) were selected to investigate the effect of packing types on the performance of nitrogen and phosphorus removal, microbial community, and metabolic function of BTF for treating village and town domestic wastewater, and the main research contents were as follows: (1) to characterize the morphology, physical, and chemical properties of different fillings and their biofilms; (2) to analyze the differences in the performance of synchronous nitrogen and phosphorus removal of the reactor with different fillings; (3) to analyze the structural composition of microbial communities on the surface; and (4) to analyze the differences in microbial metabolic functions. This study will help to promote the practical application of biological trickling filters for the treatment of domestic wastewater in villages and towns.

2. Materials and Methods

2.1. Experimental Materials and Devices

In this study, ceramic granules, zeolites, and sponges were selected as the test fillers for the lab-scale reactor of the BTF. The test set-up (Figure 1) consisted of an inlet bucket, inlet peristaltic pumps, three parallel lab-scale reactors of the BTF, and a catchment tank named R1 (packing for ceramic granules), R2 (packing for zeolites), and R3 (packing for sponges) from left to right, respectively. Each BTF reactor has 5 layers, the size of each layer is 15.0 cm × 11.0 cm × 6.7 cm, filler filling height of 4.5 cm, the actual working volume of 3.71 L, filler above the setting of the 2.2 cm-thick gap and direct contact with the air, the use of natural ventilation of the system for the supply of oxygen, set between each layer of the ventilating holes, a total of 10. The device adopts the form of a single-stage parallel connection; three parallel BTF devices were filled with ceramic granules, zeolite, and sponge; the top of the device set up a water distributor to ensure continuous and uniform water intake; and the bottom of the device set up a catchment basin to collect the water after treatment by the BTF filler. The hydraulic retention time is designed to be 11 h.

2.2. Experimental Water Quality

Artificially simulated rural domestic wastewater was used for the test influent. Sodium acetate (C2H3NaO2), ammonium chloride (NH4Cl), and potassium dihydrogen phosphate (KH2PO4) were the only carbon, nitrogen, and phosphorus sources. The final chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP) in the influent water were 193.12 ± 31.70, 42.55 ± 2.65, and 3.80 ± 0.89 mg/L, respectively. The synthetic wastewater contained 1 mL/L of trace element solution, according to Li et al. [16].

2.3. Characterization of Filler and Biofilm

On the 10th and 30th days of the experiment, 20 grains of packing with good biofilm attachment were taken from each of the three BTF cell reactors, placed in dry and clean conical flasks, added with 50 mL of distilled water, and then placed in a thermostatic oscillator to oscillate until the biofilm on the surface of the packing was completely detached in the water, and then stopped oscillating, and then a biofilm sample was formed after removing the packing. Surface and biofilm were observed using an optical microscope (Olympus BX51, Tokyo, Japan).
The packing porosity was determined by the mercury pressure method, and the specific surface area was determined by BET-specific surface area measurement. The surface microstructure of the filler was observed by a field emission scanning electron microscope (Hitachi, S4800, Tokyo, Japan) to analyze the surface morphology and roughness. X-ray diffraction (XRD) analysis was used to determine the metal elements of the fillers, and the specific steps of XRD analysis were as follows: the dry fillers were powdered, and the samples were scanned by Cu-target radiation (40 kV, 40 mA) with a start angle of 5°, an end angle of 80°, a step of 0.02°, and a scanning speed of 6° m−1.

2.4. Water Quality Analyses

The mixed liquor samples were filtered through ø12 cm filter papers with a pore size of 0.45 μm before analysis. NH4+-N, NO2−-N, NO3−-N, TP, and COD were analyzed according to standard methods [17].

2.5. Analysis of Microbial Community Structure and Metabolic Functions

At the end of the experiment, biofilm samples were taken from the three reactors for molecular biology analysis. Total DNA was extracted using the E.Z.N.A.® Tissue DNA kit (Omega Bio-tek, Norcross, GA, USA). The V3 and V4 regions of the 16S rDNA gene were then amplified by PCR using the bacterial primers 338F 5′-ACTCCTACGGGAGGCAGCA-3′ and 907R 5′-GGACACHVGGGTWTCTAAT-3′. The DNA sequences were evaluated on an Illumina MiSeq platform by Guangdong Magigene Technology Co., Ltd. (Guangzhou, China). Kyoto Encyclopedia of Genes and Genomes (KEGG) annotation was performed using PICRUSt2 software (V2.3.0) to obtain the corresponding abundance information. Based on the pathways of the tricarboxylic acid (TCA) cycle, nitrogen metabolism, and phosphorus metabolism, metabolic analyses were carried out using KEGG data of relevant functional genes and enzymes [16].

3. Results and Discussion

3.1. Characterization of Filler and Biofilm

3.1.1. Morphological Characteristics of Filler and Biofilm

The results of scanning electron microscopy (Figure 2a) of the three fillers show that the surfaces of ceramic granules and zeolites are rougher compared to sponges. The surface of the ceramic granules showed a more regular agglomerated structure with a loose distribution, while the surface of the zeolite was a dense multilayered lamellar structure. Compared with ceramic granules, zeolite surface structure is more convex and irregular, roughness is greater, and it is easier for microorganisms to attach and grow. The sponge, as a kind of polymer composite gel, has a single composition, a regular porous structure inside, larger pores, and a smoother surface. As a carrier for carrying biofilm, the filler with higher surface roughness and a larger specific surface area should be preferred, which is conducive to microbial adhesion to the filler surface, so that the biofilm is not easy to dislodge [15].
The results of biofilm microscopy before and after hanging the membrane on the filler are shown in Figure 2b. At the beginning of the membrane hanging, the changes on the surface of the filler were not obvious; with the prolongation of the experimental time, white biofilm appeared on the surface of the filler in all three BTFs, and after the end of the membrane hanging, a large number of bacterial colloid clusters could be observed in the biofilm bacterial liquid oscillating down from the surface of the filler, and other indicator organisms such as nematodes, rotifers, paramecia, and pipeworm were also observed. Bacterial colloid refers to the fusion of pod membrane substances of many bacteria into clusters containing many bacteria, which can prevent the bacteria from being phagocytosed and, at the same time, enhance the resistance of bacteria to adverse environments and have strong adsorption capacity and decomposition of organic matter [2,14]. The large number of protozoa observed in the biofilm bacterial liquid indicates that the biofilm is more stable and the surface of the packing has been hung successfully. Comparing the microorganisms on the surface of the filler of the three groups of reactors, on the 10th day of the operation of the system, it can be clearly seen that the biofilm on the surface of the zeolite has a higher content of bacterial colloid, indicating that at this time, a certain amount of microorganisms have already been attached to the surface of the filler, and the detection of surface biofilm on the 30th day found that the biofilm sample of the zeolite contains a larger number of protozoa, indicating that at this time, there is a higher diversity of microorganisms in the biofilm using the zeolite as a carrier [18], the biofilm system is more complex and stable [19].

3.1.2. Porosity and Specific Surface Area of Fillers

The porosity and specific surface area of the three fillers are shown in Table 1. As inorganic fillers, ceramic granules and zeolite have similar porosity and specific surface area, and the porosity of zeolite is 52.90%, which is slightly larger than that of ceramic granules at 41.75%. The specific surface area of ceramic granules was 21.30 (m2·g−1), which was slightly larger than that of zeolite at 17.43 (m2·g−1), in contrast to sponge, which had a high porosity of 93.10% but a smaller specific surface area of 0.18 (m2·g−1). The trends of the porosity and specific surface area data of the three fillers were determined by the scanning electron microscopy results in 3.1.1. The sponge, as a kind of polymer material, has a large internal pore space and a smooth surface, while the zeolite and ceramic granules have a large specific surface area, which is conducive to the adsorption of the previous period, and the filler has a larger contact area with the wastewater with the same mass, which provides more nutrients for the microorganisms and additionally provides adequate space for the microbial attachment, thus facilitating the adhesion of the biofilm. In addition, it provides enough space for microbial attachment, which is conducive to the adhesion of biofilm and the nitrification process [20].

3.1.3. Crystal Structure and Composition of Fillers

The crystal structure of the material was analyzed using an X-ray diffractometer. The XRD patterns obtained were qualitatively analyzed using MDI Jade 9.0 software, and the diffraction patterns of different fillers are shown in Figure 3. From Figure 3, it can be seen that the main phase of the ceramic granule filler is SiO2, and the secondary phase is AlPO4(H2O)2; the main phase of the zeolite filler is Ca4.52Al9.04Si26.96O72(H2O)13.4, and the secondary phase is C50H78Fe2N4O2S4; the main phase of sponge filler is C66H84Sn2. Stable SiO2 is the main component of ceramic granules, in addition to aluminum ions and other metal elements. The surface roughness of zeolite is large, and the filler composition is mainly calcium-containing aluminosilicate minerals. Lin et al. [21] noted the release of Ca2+ ions from zeolites during ammonia nitrogen adsorption. It has been shown that the adsorption of ammonia nitrogen by zeolite is through the release of Ca2+ ions into solution by the zeolite itself and that the released Ca2+ ions can be removed by forming a precipitate with phosphate [22]. In addition, zeolite also contains iron. Aluminum and iron elements can react with the phosphorus in the sewage and generate precipitation, which is conducive to the adsorption of phosphorus removal [23,24]. The composition of sponge polymer materials is relatively simple, mainly consisting of C66H84Sn2.

3.2. Performance of Simultaneous Nitrogen and Phosphorus Removal

3.2.1. COD

The COD concentration and removal rate of the reactor with different packing materials were shown in Figure 4a,b for 30 days of continuous operation of the BTF. The average influent COD concentration of the reactor was 193.12 mg/L. At the beginning of the system operation, the COD removal rate of the effluent from the reactor rapidly reached more than 60% and up to 85%. After 7 days of operation, the COD concentration of the system effluent was basically lower than 50 mg/L. It was analyzed that the removal of COD by the filler at the initial stage was mainly adsorption, which could adsorb COD in the influent water rapidly. Some studies have shown that the adsorption of COD by the filter media is mainly physical adsorption, which is generated by electrostatic force and capillary force on the surface of the filter media [3]. Since the adsorption of organic pollutants by the packing material reached saturation very quickly, the removal rate showed a small decreasing trend after 12 days of system operation, but the removal rate was still above 60%. Thereafter, the COD concentration of the effluent from the BTF reactor with three types of packing was maintained below 50 mg/L, and the COD removal rate fluctuated between 70% and 90%, indicating that the surface of the packing had been gradually enriched with heterotrophic microorganisms. At the later stage of the long-term stable operation of the BTF reactor, the COD removal rate of the system was basically maintained at more than 70%, in which the highest COD removal rate of zeolite packing could reach 96.49% and the lowest COD concentration of effluent was 6.02 mg/L. It can be shown that the three kinds of fillers of the BTFs have a better performance in organic pollutant removal.
Analyzing the organic pollutant removal ability of different fillers, within 30 days of reactor operation, it is obvious that the removal effect of three fillers on COD has a similar trend, and there is no obvious difference in the organic pollutant removal performance of different fillers, among which zeolite filler is slightly better than the other two fillers, and in the latter 15 days of reactor operation, the effluent COD concentration is stable below 50 mg/L. The reason may be that the surface roughness and specific surface area of zeolite filler are larger, and the contact area with sewage is larger compared with the other two fillers, which can adsorb more organic pollutants [25]. In addition, after the stable operation of the reactor, the rougher surface of zeolite filler is more conducive to microbial adhesion, so it strengthens the microbial ability of organic pollutant degradation [26].

3.2.2. Nitrogen Removal

Nitrogen mainly exists in the form of NH4+-N in the water body, and the removal effect of ammonia nitrogen can reflect the growth of nitrifying bacteria on the biofilm in the hanging film stage on a macro level, so the ammonia nitrogen removal effect is taken as an investigation index of whether the reactor start-up is successful or not [20]. The inlet and outlet water NH4+-N concentrations and removal rates of the three types of packed BTF reactors during 30 days of continuous operation are shown in Figure 4. The average influent ammonia nitrogen concentration of the reactor was 42.55 mg/L. In the first 5 days of reactor operation, the NH4+-N removal rate of zeolite packing effluent rapidly reached more than 70%, and the effluent NH4+-N concentration was lower than 15 mg/L, while the NH4+-N removal rate of ceramic granule and sponge packings was below 30%. Within the first 10 days of reactor operation, the NH4+-N removal rate of zeolite packing was maintained above 60%. Combined with the removal effect of the system on COD at this stage, the removal rate of the system on the two major pollutants in the wastewater, COD and ammonia nitrogen, was stable above 60%, which can prove that at this time, the zeolite packing in the biological trickling filter reactor has been successfully hung film.
From Figure 4c, the removal rate of ammonia nitrogen in wastewater by zeolite packing was as high as 88% on the first day of reactor operation and then gradually decreased in the following 8 days. Analyze the reason may be due to the initial operation of the filler on the NH4+-N mainly playing a role in adsorption, mainly including physical adsorption and ion exchange, and zeolite as a kind of natural mineral material, the surface of the convex and concave irregularity, the specific surface area is larger, able to ammonia nitrogen adsorption quickly, so the early period of the removal effect of the NH4+-N is better. It can be seen from Figure 4c that during the first 9 days of system operation, the NH4+-N concentration in the effluent of zeolite filler gradually increased, and the removal rate of ammonia nitrogen gradually decreased. At this time, the surface of the filler had not successfully hung film, so the main removal mechanism was adsorption, indicating gradual saturation of adsorption. At the early stage of reactor operation, zeolite adsorbed a large amount of ammonia nitrogen in sewage, and the adsorbed ammonia nitrogen was more easily transferred to the attached biofilm [27], which provided nutrients for the life activities of nitrobacteria, promoting the attachment of nitrobacteria on the surface of zeolite, and was conducive to the formation of biofilm [28]. Before the biofilm formation, the adsorption capacity of zeolite was basically saturated, and the adsorbed ammonia nitrogen was used for the life activities of microorganisms in the biofilm after hanging [29]. In this stage, because the surface roughness of ceramic granules and sponges was lower than that of zeolite, the adsorption capacity of NH4+-N in the feed water was weaker in the initial stage, and the NH4+-N concentration of the effluent water was above 30 mg/L. From the 10th day of reactor operation, zeolite on the removal of NH4+-N in the sewage improved, the filler on the adsorption of pollutants reached saturation, and on the surface of a certain amount of microorganisms enriched in the formation of a thin layer of biofilm, the microbial action removal began. At this time, microorganisms began to attach to the surface of the filler, and nitrobacteria gradually enriched and produced nitrification under aerobic conditions, converting NH4+-N into NO3−-N, and the nitrogen removal effect was better at this time [28]. So, after 10 days of system operation, the effluent NH4+-N concentration declined, and thereafter, until the end of the experiment, the zeolite filler on the influent NH4+-N concentration of 30 mg/L. After that, until the end of the experiment, the removal rate of zeolite filler on NH4+-N in the sewage was stable at more than 70%, and the concentration of NH4+-N in the effluent was less than 15 mg/L. At this stage, the removal ability of ceramic granules and sponges on NH4+-N was obviously weaker than that of zeolite. In the early stage of system operation, the removal rate of ceramic granules on NH4+-N was maintained at about 20%, and the removal effect was improved from the 18th day onwards, and the reason for this may be that the surface of ceramic granules began to enrich microorganisms at this time, but due to the low roughness of the surface [30], which is not conducive to the microorganisms adhering to the surface of the filler, the enrichment of the microorganisms was slower, and the rate of removal increased gradually thereafter, and the removal rate of ceramic granules on the wastewater in the 25th day reached 60%. On the 25th day, the removal rate of ceramic granules on NH4+-N in wastewater reached 60%; this indicates that the film is successfully hung on the surface of the packing material. In contrast, the sponge’s ability to remove NH4+-N in sewage is poor; as an organic polymer material, its internal structure is very regular, and the surface is relatively smooth [20]. The removal rate of NH4+-N in 20 days of system operation is stable at less than 20%, and thereafter the removal rate of NH4+-N rises gradually, and the removal rate reaches 60% on the 26th day, which indicates that the surface of the packing is successful in hanging the film.
The NO2−-N and NO3−-N concentrations and removal rates of the influent and effluent water of the three kinds of packed BTF during 30 days of continuous operation are shown in Figure S1. In the first few days of reactor operation, the content of NO2−-N and NO3−-N in the effluent water of each layer of the system was low, indicating that the BTF mainly removed ammonia and nitrogen from the water by adsorption at the beginning of the experiment, and nitrification accounted for a small proportion. After 10 days of system operation, a small amount of nitrite and nitrate appeared in the effluent water of the zeolite reactor, indicating that microorganisms began to attach on the surface of the packing material, nitrifying bacteria were gradually enriched, and nitrification was produced under aerobic conditions to convert NH4+-N into NO3−-N. At this time, the removal rate of total nitrogen was close to 80%, and the effect of denitrification was good. After 15 days of system operation, ceramic granule and sponge packing reactor system effluent began to accumulate part of the nitrite. Twenty-five days later, the zeolite packing system began to accumulate nitrite in the effluent, indicating that the zeolite surface microbial nitrification and denitrification maintained for a longer period of time, and the biofilm adhesion was stronger. In the first 25 days of operation of the zeolite device, the content of nitrite in the effluent is very low and can be ignored. In the 25–30 days of operation, the concentration of nitrite in the effluent gradually increases, but it is also maintained below 5 mg/L. If the concentration of nitrite continues to increase in the later period, the solution of cleaning the filler or replacing the filler can be adopted. In contrast, more nitrite is produced in the effluent of R1 and R3, and the effluent nitrite reaches more than 5 mg/L on the 20th day, at which time the fillers need to be cleaned or replaced, so the service cycle of ceramic granules and sponges is shorter, and the maintenance cost is higher than that of zeolites. Comparison of TN removal effects of three kinds of filler BTFs (Figure 4e,f): In the early stage, there was no production of nitrite and nitrate in the effluent, and the trend of the TN removal rate was similar to the NH4+-N removal rate. After stable operation of the reactors, a small amount of nitrite and nitrate appeared in the effluent water, and the TN removal rate decreased slightly. However, compared with ceramic granules and sponges, the zeolite reactor has a higher TN removal rate, with an average TN removal rate of more than 70% and an average TN concentration of 12 mg/L in effluent.

3.2.3. Phosphorus Removal

The concentration and removal rate of TP in and out of water within 30 days of stable operation of the bioreactors with three fillers are shown in Figure 4g,h. The average TP concentration in the influent water of the system was 3.80 mg/L. From the experimental data, it can be seen that the removal effect of the three kinds of fillers on phosphorus was relatively unstable, and the removal rate of phosphorus was below 50% during the 30 days of reactor operation. In the first 3 days of reactor operation, the total phosphorus removal rate of ceramic packing was stable at more than 40%, the total phosphorus removal rate of zeolite packing was more than 30%, and the total phosphorus removal rate of sponge packing was about 20%. It is assumed that at this time the filler is mainly used to remove phosphorus in the sewage by adsorption, and the adsorption began to be gradually saturated after the 8th day, and the phosphorus removal rate decreased. After the 14th day, due to the zeolite packing surface enriched with a certain number of microorganisms, the total phosphorus removal rate rose slightly. Microorganisms remove a small portion of phosphorus in the water through their own assimilation; at this time, the system phosphorus removal rate of the basic maintenance was 20~40%. In contrast, the removal rate of ceramic granules for phosphorus in wastewater was better than that of zeolites and sponges, and the TP removal rate could reach up to 67.15% in the 30 days of reactor operation, and the lowest TP concentration in the effluent was 0.728 mg/L, and the TP removal rate of the system was around 40% after stabilization.
Wastewater biological treatment utilizes phosphorus-aggregating bacteria to remove phosphorus from water by releasing phosphorus in excess under anaerobic conditions and absorbing phosphorus under aerobic conditions [31]. Since phosphorus removal requires two stages of anaerobic phosphorus release and aerobic phosphorus uptake, the oxygen supply condition of the biofilter in this experiment is natural ventilation, and the system lacks the anaerobic and aerobic alternating environments suitable for phosphorus release and uptake, so the BTF cannot enrich more phosphorus-polymerizing bacteria, and it only relies on the anion exchange, electrostatic adsorption, and microbial assimilation to remove the phosphorus in the wastewater [32,33], and thus the removal efficiency of TP is low.

3.3. Microbial Communities

3.3.1. Microbial Abundance and Diversity

The abundance and diversity of microbial communities in different reactors are shown in Table 2, with a coverage of 1, indicating that the sequencing depth was accurate enough to detect microbial species. From the data, it can be seen that the Chao1 index, ACE index, and Shannon index of ceramic, zeolite, and sponge increased sequentially. This indicates that the increase in porosity improves microbial diversity and abundance; zeolites and sponges have higher porosity and can attach more microorganisms to their surfaces [34], and zeolites have a larger specific surface area and greater surface roughness, which is more favorable for biofilm attachment in comparison. However, it has been shown the microbial community richness is too high, which may lead to fierce competition of functional flora; multiple dominant flora compete for nutrients in the system, resulting in an uneven distribution of community abundance [35]; the microbial community richness is too low, indicating that the dominant microorganisms account for a smaller proportion of microorganisms; and the microbial community structure complexity is low, which may lead to the unstable system biological action, which is not conducive to the removal of pollutants in the wastewater.

3.3.2. Microbial Community Structure

As can be seen in Figure 5a, the microbial community structure in biofilms on different filler surfaces was slightly different at the phylum level. The dominant phylum of microorganisms on the surface of ceramic filler was Proteobacteria, Bacteroidetes, and Chloroflexi, and the dominant phylum of microorganisms on the surface of zeolite filler was Epsilonbacteraeota, Proteobacteria, Bacteroidetes, and Chloroflexi, and the dominant phylum of microorganisms on the surface of sponge filler was Bacteroidetes. The microbial dominant phylum on the surface of sponge packing was Bacteroidetes, Proteobacteria, and Firmicutes. The role of Proteobacteria and Chloroflexi in nitrogen and phosphorus removal has been previously reported in different studies [36,37]. Chloroflexi is rich in a variety of heterotrophic bacteria, and most of them have denitrification ability and can participate in the nitrogen cycle. Proteobacteria and Bacteroidetes contain a variety of heterotrophic and nitrite-oxidizing bacteria, which play an important role in nitrogen removal and have a denitrification function [38]. The abundant lipopolysaccharides present on the surface of the bacteria are favorable for microbial adhesion and proliferation. Bacteroidetes are organic matter-degrading bacteria, especially for high molecular weight substances with good degradation [39]. Bacteroidetes mainly exist in low-oxygen or anoxic environments and are involved in denitrification. Acidobacteriota can survive in a large range of dissolved oxygen and hydrolyze organic matter, which plays a key role in wastewater treatment [2].
At the genus level (Figure 5b), the dominant genera of bacteria in the biofilm on the surface of ceramic filler were Herpetosiphon, Zoogloea, Arcobacter, and Cloacibacterium, of which Herpetosiphon is a heterotrophic bacterium that can degrade organic pollutants in wastewater, and Zoogloea is a filamentous denitrifying bacterium. The dominant genus of zeolite surface biofilm, Arcobacter, is a facultative anaerobic bacterium with high nitrogen removal efficiency at low C/N ratios and low temperatures [36]. In addition, the dominant genus on the zeolite surface contains Acinetobacter, which contains aerobic denitrifying bacteria and heterotrophic bacteria that can remove some NO3−-N and organic pollutants from wastewater. Lentimicrobium and Azoarcus are the dominant species of spongy packing surface biofilm, and Azoarcus is a bacterium with a denitrification function [40].
The presence of the above genera indicates that the system has good synchronous nitrification and denitrification regardless of the filler, but comparing the differences in the relative abundance of microorganisms on the surface of different fillers, it is obvious that the relative abundance of genera related to nitrification and denitrification on the surface of the zeolite filler is higher and it contains a certain amount of aerobic denitrifying bacteria, which compensates for the lack of anoxic and anaerobic environments under the conditions of natural ventilation. Therefore, compared with ceramic and sponge, the zeolite packing BTF system has better nitrogen removal performance. In addition, a small amount of Dechloromonas, i.e., phosphorus aggregating bacteria, was detected in the biofilm on the surface of different fillers, so the system was able to remove a very small amount of phosphorus, but the lack of alternating anaerobic and aerobic environments was not conducive to the enrichment of the phosphorus aggregating bacteria [41]. In addition, the system was capable of removing organic pollutants due to the high abundance of heterotrophic bacterial genera contained in the biofilms of all three fillers.

3.4. Analysis of Microbial Metabolic Functions

3.4.1. Classification of KEGG Functional Categories

Potential functions of the microbial community were explored at three levels to elucidate the effect of packing on microbial metabolic pathways. As shown in Figure 6a, level 1 analysis focused on the functions of microbial communities, which were categorized into six groups: metabolism (79.05–80.33%), genetic information processing (11.97–12.27%), cellular processes (4.90–5.65%), environmental information processing (2.11–2.45%), organismal systems (0.36–0.38%), and human diseases (0.29–0.31%). Notably, metabolic functions accounted for the highest percentage.
As shown in Figure 6b, the level 2 analysis focused on functional pathways related to biofilms and microorganisms on the filler surface. Ten metabolic pathways were identified in this study, of which amino acid metabolism (12.47–12.74%) accounted for the highest proportion, followed by the metabolism of cofactors and vitamins (12.30–13.10%) and carbohydrate metabolism (11.29–12.40%). The metabolism of cofactors and vitamins reflects cellular activity and may be related to EPS secretion and biosynthesis [42]. The highest abundance of genes related to the metabolism of cofactors and vitamins was found in zeolite surface biofilms. Amino acid and carbohydrate metabolism reflected the ability of bacteria to utilize organic matter and nitrogen to support life activities and cell synthesis [16]. The abundance of genes related to amino acid and carbohydrate metabolism in the biofilm on the ceramic granule, sponge, and zeolite surfaces decreased sequentially, but the overall difference was not significant, which may be the reason for the high COD removal efficiencies of all three types of fillers.
As shown in Figure 6c, the level 3 analysis focused on the effect of the three fillers on the carbohydrate metabolism of the biofilm. The citric acid cycle (TCA cycle) is a fundamental part of microbial metabolism involving some essential coenzymes such as nicotinamide adenine dinucleotide (NADH) and acetyl coenzyme A [43]. The TCA cycle increased in the biofilm in the order of the ceramic granule, zeolite, and sponge surfaces, while the trend was the opposite for pyruvate metabolism and C5-branched dibasic acid metabolism. It indicates that zeolite has a more stable enzymatic digestion capacity compared to the other two fillers.

3.4.2. Analysis of Functional Metabolic Pathways

To further elucidate the effects of the three fillers on microbial metabolic functions, we evaluated the pathways involved in nitrogen metabolism, phosphorus metabolism, and the TCA cycle (Figure 7). In terms of nitrogen metabolic pathways, genes related to nitrification and denitrification processes were mainly investigated [35]. R2 had the highest NH4+-N removal rate as shown in Figure 7a, and the relative abundance of genes related to the nitrification process (amo, hao, nxr, etc.) was the highest in R2, suggesting that zeolite enhanced microbial metabolism of the nitrification process. The relative abundance of genes related to the denitrification process (nap, nir, nor, nos, etc.) in R2 was lower than that of R1 and R3, which was consistent with the fact that the effluent water of R2 still had a high NO3−-N concentration. However, R2 was still the highest in terms of TN removal because it had the strongest nitrification metabolism.
In terms of phosphorus metabolism (Figure 7b), functional genes related to phosphate metabolism mainly included ppk1, ppx, and adk, but their relative abundance was low in all three groups of reactors, consistent with their poor TP removal. This was mainly related to the fact that A. polyphosphoria needed to be enriched in an alternating anaerobic and aerobic manner and could not be effectively enriched under the conditions of natural ventilation in the BTF. The poor biological removal performance of the system for TP indicates that the system mainly relies on the physicochemical adsorption effect of the filler itself for removal. The abundance of functional genes related to phosphorus metabolism was the lowest in zeolite, but its TP removal was not the worst, which indicated that zeolite has a relatively good adsorption capacity for phosphate, and after some modification treatments [14], zeolite has an excellent removal potential for TP.
The TCA cycle is an important pathway for carbohydrate metabolism and microbial energy sources. Figure 7c shows the main metabolic pathways and the key enzymes involved. The relative abundance of functional genes encoding the relevant key enzymes in R2 was all relatively high, which provided a more adequate energy supply for biofilm formation and the nitrogen and phosphorus metabolism processes [16].

4. Conclusions

In this study, we compared the filler and biofilm properties, synchronous nitrogen and phosphorus removal performance, microbial community structure on the filler surface, and microbial metabolism function differences among three types of BTF carriers, namely ceramic, zeolite, and sponge. The following results were obtained:
(1) Among the three fillers, zeolite has the highest porosity and roughness, which is conducive to the adsorption of pollutants in the early stage and the adhesion of biofilm in the later stage. Moreover, zeolite also contains iron that can adsorb and precipitate phosphorus in sewage. The porosity of ceramic granule is second only to zeolite, while sponge as a polymer composite material, the composition is relatively simple; there is no Fe3+, Al3+, Ca2+, and other metal ions that can adsorb and precipitate with phosphate, and the internal pores are large and the surface is smooth, which is not conducive to biofilm adhesion.
(2) Zeolite has the best removal effect on organic matter and nitrogen in sewage, while ceramic granules have the best removal effect on phosphorus in sewage. However, sponges have poor removal effects on nitrogen and phosphorus in sewage. On the 10th day of operation, the zeolites packing in the BTF were successfully attached to the membrane, and the membrane hanging time of zeolites was shorter than that of ceramic granule and sponge.
(3) Zeolite has a high microbial community richness and diversity, and the system is relatively stable. The bacteria related to nitrification and denitrification on the surface are relatively abundant and contain a small amount of phosphorus-accumulating bacteria.
(4) Compared with the other two fillers, zeolite is more stable in terms of enzymatic digestion, and the relative abundance of genes related to the nitrification process (amo, hao, nxr, etc.) and functional genes encoding related key enzymes in the TCA cycle are higher, indicating that zeolite enhances the metabolism of the microbial nitrification process.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w16233343/s1, Figure S1. Influent and effluent concentrations of NO2−-N (a) and NO3−-N (b).

Author Contributions

Conceptualization, Y.G., Z.P., D.L. and J.L. (Jun Li); methodology, Y.G., Z.P., J.L. (Jiarui Li), T.S. and J.L. (Jun Li); software, Y.G. and J.L. (Jiarui Li); validation, Z.P.; formal analysis, Z.P., L.H. and M.W.; investigation, Y.G., L.H. and T.S.; resources, Z.P.; data curation, Y.G., L.H., M.W. and D.L.; writing—original draft, Y.G. and J.L. (Jiarui Li); writing—review and editing, L.H. and J.L. (Jun Li); visualization, Y.G. and J.L. (Jiarui Li); supervision, M.W., D.L. and J.L. (Jun Li); funding acquisition, J.L. (Jun Li); project administration, T.S. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially aided by the Beijing Natural Science Foundation (8222039) and the science and technology R&D plan of China Construction First Group Co., Ltd. (KJYF-2021-14).

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

Zhengwei Pan, Liangang Hou, Mingchao Wang, and Tianhao Shi were employed by China Construction First Group Construction & Development Co., Ltd. Tianhao Shi was also employed by China Construction First Group Corporation Limited. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Water 16 03343 g001
Figure 1. Schematic diagram of biological trickling filter system.
Figure 1. Schematic diagram of biological trickling filter system.
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Figure 2. (a) Physical and SEM images of the three fillers. (b) Biofilm microscopy.
Figure 2. (a) Physical and SEM images of the three fillers. (b) Biofilm microscopy.
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Figure 3. XRD patterns of fillers. (a) ceramic granule; (b) zeolite; and (c) sponge.
Figure 3. XRD patterns of fillers. (a) ceramic granule; (b) zeolite; and (c) sponge.
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Figure 4. Performance of simultaneous nitrogen and phosphorus removal; (a,c,e,g) are the inlet and outlet concentrations of COD, NH4+-N, TN, and TP, respectively, and (b,d,f,h) COD, NH4+-N, TN, and TP removal rate, respectively.
Figure 4. Performance of simultaneous nitrogen and phosphorus removal; (a,c,e,g) are the inlet and outlet concentrations of COD, NH4+-N, TN, and TP, respectively, and (b,d,f,h) COD, NH4+-N, TN, and TP removal rate, respectively.
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Figure 5. Microbial community structure; (a) phylum level and (b) genus level.
Figure 5. Microbial community structure; (a) phylum level and (b) genus level.
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Figure 6. KEGG pathway analysis in different reactors. (a) Primary classification; (b) secondary classification; (c) third-level classification.
Figure 6. KEGG pathway analysis in different reactors. (a) Primary classification; (b) secondary classification; (c) third-level classification.
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Figure 7. KEGG prediction of potentially functional genes and enzymes in metabolic pathways of nitrogen metabolism (a), phosphorus metabolism (b), and TCA cycle (c).
Figure 7. KEGG prediction of potentially functional genes and enzymes in metabolic pathways of nitrogen metabolism (a), phosphorus metabolism (b), and TCA cycle (c).
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Table 1. Porosity and specific surface area of fillers.
Table 1. Porosity and specific surface area of fillers.
Filler NameGrain Size/mmPorosity/%Specific Surface Area/(m2·g−1)
Ceramic3~641.7521.30
Zeolite3~552.9017.43
Sponge18~2093.100.18
Table 2. Microbial abundance and diversity.
Table 2. Microbial abundance and diversity.
SamplesCommunity RichnessCommunity DiversityCoverage
ACEChao1Shannon
R1428.36397.12.661
R2494.34474.13.261
R3583.84563.141
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Geng, Y.; Pan, Z.; Hou, L.; Li, J.; Wang, M.; Shi, T.; Li, D.; Li, J. Influence of Filler Types on the Treatment of Rural Domestic Wastewater in a Biological Trickling Filter: Simultaneous Nitrogen and Phosphorus Removal Performance, Microbial Community, and Metabolic Functions. Water 2024, 16, 3343. https://doi.org/10.3390/w16233343

AMA Style

Geng Y, Pan Z, Hou L, Li J, Wang M, Shi T, Li D, Li J. Influence of Filler Types on the Treatment of Rural Domestic Wastewater in a Biological Trickling Filter: Simultaneous Nitrogen and Phosphorus Removal Performance, Microbial Community, and Metabolic Functions. Water. 2024; 16(23):3343. https://doi.org/10.3390/w16233343

Chicago/Turabian Style

Geng, Yuxin, Zhengwei Pan, Liangang Hou, Jiarui Li, Mingchao Wang, Tianhao Shi, Dongyue Li, and Jun Li. 2024. "Influence of Filler Types on the Treatment of Rural Domestic Wastewater in a Biological Trickling Filter: Simultaneous Nitrogen and Phosphorus Removal Performance, Microbial Community, and Metabolic Functions" Water 16, no. 23: 3343. https://doi.org/10.3390/w16233343

APA Style

Geng, Y., Pan, Z., Hou, L., Li, J., Wang, M., Shi, T., Li, D., & Li, J. (2024). Influence of Filler Types on the Treatment of Rural Domestic Wastewater in a Biological Trickling Filter: Simultaneous Nitrogen and Phosphorus Removal Performance, Microbial Community, and Metabolic Functions. Water, 16(23), 3343. https://doi.org/10.3390/w16233343

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