Removal Performances of Turbidity, Organics, and ${\mathrm{NH}}_4^{\mathrm{+}}$-N in a Modified Settling Tank with Rotating Biological Discs Used for Enhancing Drinking Water Purification

Abstract

In this study, a modified horizontal settling tank with rotating biological discs was developed to treat slightly polluted surface water, and its performance on the simultaneous removal of turbidity, organics, and ${\mathrm{NH}}_4^{\mathrm{+}}$-N was investigated on a lab scale. Results show that the effluent quality of the modified settling tank is stable in more than two months of continuous operation. At a hydraulic retention time (HRT) of 2.0 h, 73.65 ± 5.15% turbidity, 53.98 ± 5.17% TOC, and 77.01 ± 10.02%, ${\mathrm{NH}}_4^{\mathrm{+}}$-N could be removed by the modified settling tank with an average of 1.96 NTU turbidity, 1.98 mg/L TOC, and 0.46 mg/L ${\mathrm{NH}}_4^{\mathrm{+}}$-N residue in the effluent. Due to the improvement in DO supply, higher removal efficiencies of both organics and ${\mathrm{NH}}_4^{\mathrm{+}}$-N were achieved with increased disc rotating speed (r < 4 r/min). Further study showed that the genus Hyphomicrobium dominant on the posterior discs and the genus Nitrospira dominant on the anterior discs mainly contributed to the enhanced bio-oxidation of organics and ${\mathrm{NH}}_4^{\mathrm{+}}$-N, respectively.

1. Introduction

Water safety is crucial to human beings. Large amounts of untreated or inadequately treated wastewater discharged into the natural environment lead to numerous water sources with high organics and ${\mathrm{NH}}_4^{\mathrm{+}}$-N contents, which pose a direct threat to the drinking water quality [1,2]. It was reported that the presence of organics decreases the removal efficiency of colloids in coagulation [3], causes an increase in odor and taste in the treated drinking water [4], as well as acts as a substrate for bacterial re-growth in drinking water supply networks [5]. In addition, dissolved organics are the major precursors of disinfection by-products (DBPs) [6], which produce long-term health effects on human beings [7,8]. For this reason, the content of drinking water TOC at home and abroad is used to establish the corresponding limit value, and both China and Nigeria for drinking water set the maximum limit of TOC at 5 mg/L [9,10]. To ensure that the amount of disinfection by-products is controlled at acceptable levels, the Jordanian drinking water standard/WHO limit for TOC in drinking water is 2 mg/L [11].

Similarly, the presence of ${\mathrm{NH}}_4^{\mathrm{+}}$-N also causes odor and taste problems, the formation of DBPs, and the re-growth of bacteria in treated drinking water [12]. The World Health Organization set 1.5 mg ${\mathrm{NH}}_4^{\mathrm{+}}$/L and 35 mg ${\mathrm{NH}}_4^{\mathrm{+}}$/L as the odor and taste thresholds, respectively [13]. Health Canada [14] recommended the content of ${\mathrm{NH}}_4^{\mathrm{+}}$-N entering water distribution networks below 0.1 mg/L. The maximum limit of ${\mathrm{NH}}_4^{\mathrm{+}}$-N set by both China and the European Community for drinking water was 0.5 mg/L [15]. Thus, there is a need for efficient and economical methods to remove both ${\mathrm{NH}}_4^{\mathrm{+}}$-N and organics in the source water to guarantee the quality of the treated drinking water.

The purification process widely used in most water plants includes coagulation, sedimentation, filtration, and disinfection units, which is effective in the removal of turbidity-causing materials including suspended solids and colloids but has limited capacity for the removal of soluble pollutants [16,17]. The most widely used stratified sedimentation tank in large water plants [18], for example, has a removal rate of 98% for turbidity but its removal rate for organic matter and ammonia nitrogen is only about 50% [19,20]. To enhance the removal of organics and ${\mathrm{NH}}_4^{\mathrm{+}}$-N, advanced oxidation processes such as UV/O₃ and UV/H₂O₂ have been used in drinking water purification since the 1980s. Although they are efficient for most organic components such as chloromethane, carbon tetrachloride, o-dichlorobenzene, and p-dichlorobenzene [21], the mineralization ratio of humic acids is limited and both the energy and reagent costs are also high [22].

The combination of ozonation and activated carbon filtration is commonly used to remove organics in drinking water purification. It has been proven that most of the oxidation products formed in ozonation can be removed through activated carbon adsorption [23]. However, the adsorption capacity of activated carbon would be consumed in a few days, and risks associated with exposure to organics exist if it is not regenerated in time [24]. To extend the generation period of activated carbon, the growth of bio-film on its surface was enhanced to form biological activated carbon (BAC). It was reported that even after 42 days of a continuous run, the BAC filter could maintain a consistent organics removal efficiency [25]. However, the cost of O₃-BAC is still high [26] and its application is generally on the removal of organic pollutants, with less focus on the removal of ${\mathrm{NH}}_4^{\mathrm{+}}$-N [12]. In contrast, the biological carousel technology has a high removal effect for both organic matter and ${\mathrm{NH}}_4^{\mathrm{+}}$-N in water [27,28,29], in addition to its advantages of high shock load tolerance, low sludge volume, low power consumption, and easy maintenance and management [30,31]; this technology has been applied as a pretreatment method in China, Japan, and the United States [29,32,33,34]. To improve the removal effect of organic matter and ${\mathrm{NH}}_4^{\mathrm{+}}$-N by conventional water treatment methods, coupling with biological carousel technology has become the focus of current research. Shi [35] developed the coupling device of an anoxic tank-waterwheel dual-side-drive type rotating biological contactor to remove more than 90% of organic matter and ${\mathrm{NH}}_4^{\mathrm{+}}$-N, and the effluent quality can stably reach the first-grade level A of GB 18918-2002. Talalaj [36] studied the removal of 99% of COD and ${\mathrm{NH}}_4^{\mathrm{+}}$-N from waste leachate using a reverse osmosis process with a rotating biological contactor. Keluskar [37] used a combination of a rotating biological contactor and a moving bed biofilm reactor for the treatment of surimi processing wastewater, which was superior to the conventional reactor for both organic matter and ${\mathrm{NH}}_4^{\mathrm{+}}$-N.

In this study, the horizontal flow settling tank was modified with rotating biological discs to achieve simultaneous removal of turbidity, organics, and ${\mathrm{NH}}_4^{\mathrm{+}}$-N. The performances of the modified settling tank were systemically evaluated under varied organic loading rates (OLR), hydraulic retention times (HRT), and disc rotating speed conditions; the succession of the dominant microbial community in the direction along the water flow path was investigated by high-throughput sequencing. These findings illustrate the advantages of combining the functions of the horizontal flow settling tank and rotating biological discs and provide a possible upgrading choice for traditional plants in treating organic and ${\mathrm{NH}}_4^{\mathrm{+}}$-N polluted source waters.

2. Materials and Methods

2.1. Raw Water Used in the Tests

The raw water was sampled from the Heihe reservoir which is the major water source of Xi’an. The pH, turbidity, TOC, and ${\mathrm{NH}}_4^{\mathrm{+}}$-N contents in the raw water varied in a range of 7.1–7.5, 6.7–8.2 NTU, 3.8–4.7 mg/L, and 1.9–2.5 mg/L, respectively (Table S1). To simulate the actual drinking water purification process, the influent of the modified settling tank was pre-coagulated with poly aluminium chloride (PACl) at a dosage of 5.0 mg/L. The coagulation process involved a rapid mixing at 200 rpm for 1.0 min and followed by a 15 min flocculation at 50 rpm. To ensure the stability of the Black River raw water quality before entering the reaction system, the collected river water needs to be stored at 4 °C for 0–14 days and stabilized to room temperature (if stored) before being fed into the coagulation–sedimentation test system. An existing study demonstrated that raw water stored at 4 °C for 14 days has negligible changes in physicochemical and biological qualities [38].

2.2. Modified Horizontal Flow Settling Tank Setup and Operation

The structure of the lab-scale equipment is shown in Figure 1, which comprises a horizontal flow settling tank with enhanced biological oxidation function. The sedimentation tank is made of transparent Plexiglas and the effective volume of the sedimentation tank is designed to be 5.3 L (length = 400 mm and width = 120 mm) according to the requirement of 3~5 aspect ratio of the stratified sedimentation tank in the water supply and drainage design manual [39]. The technical specification for integrated rotating biological contactor sewage treatment device requires that the depth of submergence of the bio-rotor in the tank is 30–40% of the diameter, and the diameter of the bio-rotor of this device is 100 mm so that the area submerged by the water body accounts for about 40% of the area of the disk. [40] The rotating biological discs consisted of a series of closely spaced, parallel rigid polyvinyl chloride discs, 2 mm thick, mounted on a rotating shaft, and installed in the upper part of the sedimentation tank. With reference to the water flow pattern of advective sedimentation, horizontal flow was chosen as the flow pattern for this device.

In the settling tank, about 95% of the discs’ surface area is alternately submerged in the pre-coagulated water and then exposed to the atmosphere. Consequently, the organics and ${\mathrm{NH}}_4^{\mathrm{+}}$-N are biodegraded by microorganisms on the discs. Turbidity-causing materials, including the coagulated flocs and detached biofilms, are removed in the settling area. The treated water is collected by triangular weirs at the rear of the tank and the settled sludge is periodically discharged through perforated pipes.

2.3. Formation of Biofilm on the Discs

In the start-up stage of the modified settling tank, certain amounts of 52 mg/L humic acid were added to the raw water to maintain a 20 mg/L of TOC in the first 15 days and 10 mg/L of TOC in the second 15 days. The temperature, pH, and HRT of the pre-coagulated raw water in the settling tank were maintained at 20–25 °C, 7.0, and 2 h, respectively. After 30 days of operation at a disk rotating speed of 3.0 r/min, a yellow-brown biofilm layer formed on the surface of the disc plates. Then, the system was kept running with the raw water till the removal efficiencies of both organics and ${\mathrm{NH}}_4^{\mathrm{+}}$-N reached stability after approximately 10 days. The final obtained biofilm, with a deep yellow-brown color and a thickness of 3–4 mm, was rich in cocci, bacilli, and filamentous bacteria and a small number of epigenetic organisms such as bell worms and rotifers (Figures S1 and S2).

2.4. Performance of the Modified Horizontal Flow Settling Tank

To evaluate the impacts of HRT (1.0–3.5 d), OLR (0.10–0.45 g/(m²∙d)), and disc rotating speed (1.0–9.0 r/min) on the turbidity, TOC, and ${\mathrm{NH}}_4^{\mathrm{+}}$-N removal performances of the modified settling tank, single factor experiments were conducted. The content of organics was adjusted by 52 mg/L humic acid (Guangfu Instrument, China). When one of the parameters was made as a variable, the others were maintained constant. Upon the system reaching stability again in 10 days, water samples were collected from both the influent and effluent once a day for 5 days continuously. The concentrations of turbidity, organics, and ${\mathrm{NH}}_4^{\mathrm{+}}$-N of 5 observations were then averaged.

2.5. Water Quality Analysis Method

The pH and temperature were measured in situ using a pH meter (Model HQ30D, Hach, Loveland, CO, USA) and a thermometer (Model TTM1-JM-6200IM, Yuan-Da Technology Corporation, Binzhou, China), respectively. A turbidity meter (HI93703-11, Hanna, Italy) was applied to analyze the turbidity of the test water. Concentrations of organics and ${\mathrm{NH}}_4^{\mathrm{+}}$-N were analyzed after filtration through a 0.45 μm Millipore filter. Organic content was assessed by the total organic carbon (TOC), which was determined as the difference between total carbon and total inorganic carbon measurement values using an organic carbon analyzer (Model TOC-5000A, Shimadzu, Kyoto, Japan) calibrated with 5 mg/L, 10 mg/L, and 20 mg/L (calculated as carbon) potassium hydrogen phthalate standard solutions before each run. ${\mathrm{NH}}_4^{\mathrm{+}}$-N was determined using Nessler’s reagent spectrophotometry (iodine mercury method).

2.6. Microbial Community Analysis

An optical microscope (Nikon 90i, Tokyo, Japan) was used to observe the microorganism growing status on the bio-discs. To analyze the microbial community structure, disc bio-film samples were collected from the anterior, middle, and posterior of the settling tank and stored at −20 °C for DNA extraction. Total genomic DNA from the samples was extracted using the CTAB/SDS method. Subsequently, the integrity of the DNA was monitored by agarose gel electrophoresis. DNA concentrations and purities were quantified with a NanoDrop^® 2000 spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA). The following primers were applied for 16S rRNA gene fragments amplifying: 338F (5′-ACTCCTACGGGAGGCAGCA-3′) and 806R (5′-GGACTACHVGGGTWTCTAAT-3′). The polymerase chain reaction (PCR) was carried out by an ABI GeneAmp^® 9700 thermal cycler (Applied Biosystems, Waltham, MA, USA) with TransStart^® FastPfu DNA Polymerase. The PCR products of the three samples were sent for sequencing by IlluminaHiSeq2500 platform (Illumina, San Diego, CA, USA). The operational taxonomic unit (OTU) was performed by Uparse software (Uparse v7. 0.1001, http://drive5.com/uparse/) with 97% similarity cut-off values. The rarefaction curves and diversity indices (Chao and Shannon) were calculated using QIIME (version 1.7.0).

2.7. Data Analysis

Origin 10.0 software was mainly applied for data analysis. Statistically significant differences between samples were carried out by SPSS 18.0 (SPSS Inc., Chicago, IL, USA), and the differences were significant when p < 0.05.

3. Results and Discussion

3.1. Pollutant Removal Performances of the Modified Settling Tank

The modified settling tank was continuously operated from 15 March to 21 May after a 40-day start-up stage, as described in Section 2.3. Figure 2 shows the removal efficiencies of turbidity, TOC, and ${\mathrm{NH}}_4^{\mathrm{+}}$-N with operation time when 7.5 ± 0.5 NTU turbidity, 4.3 ± 0.3 mg/L TOC, and 2.0 ± 0.2 mg/L ${\mathrm{NH}}_4^{\mathrm{+}}$-N were maintained in the influent. Results showed that significant removal of turbidity was achieved in the modified settling tank. The residual turbidity in the treated water ranged between 1.71 and 2.37 NTU, corresponding to a removal efficiency of 68.5%–78.8%, which was close to the reported turbidity-removing capacity of the horizontal settling tank [41]. This observation indicated that the addition of rotating biological discs had little effect on the settling function of the settling tank. Moreover, the removal efficiency of turbidity increased slightly with the increase in operation time, which might be attributed to the increase in water temperature, and the promoted aggregation and sedimentation of the particles [42].

Unlike domestic wastewater, both the content and biodegradability of the organics in natural water are much lower, which limited the growth of microorganisms. However, the average removal efficiency of TOC obtained in the modified settling tank is still as high as 53% (Figure 2b). The organic content decreased notably from 4.0 to 4.6 mg/L in the influent to 1.8–2.2 mg/L in the effluent, which would effectively inhibit the formation of DBPs, bacteria re-growth, and disinfectants consumption in the subsequent disinfection and water distribution processes [43,44]. The organic carbon removal data also proved that the formation of a stable active biofilm on the rotating discs is possible under an extremely low OLR condition.

Compared with turbidity and TOC, the removal efficiency of ${\mathrm{NH}}_4^{\mathrm{+}}$-N increased notably with operation time, which might be contributed to the inhibited activity of ammonia-oxidizing bacteria and autotrophic nitrifying bacteria at low temperatures [45,46]. From 15 March to 30 March, the water temperature ranged between 8 and 15 °C, corresponding to 70% of ${\mathrm{NH}}_4^{\mathrm{+}}$-N removal in the modified settling tank (Figure 2c). From 30 March, the water temperature increased to above 15 °C and the removal efficiency of ${\mathrm{NH}}_4^{\mathrm{+}}$-N increased remarkably to approximately 80% on 5 April. Besides the biological enzyme activity, an increase in the Henry constant at high temperatures would also increase the removal of ${\mathrm{NH}}_4^{\mathrm{+}}$-N by promoting its transfer from the aqueous phase to a gaseous phase [47].

3.2. Effects of Operation Parameters on the Removal of Pollutants

3.2.1. Organic Loading Rates

The effects of OLR on the removal of turbidity, TOC, and ${\mathrm{NH}}_4^{\mathrm{+}}$-N were evaluated at an HRT of 2.0 h and disc rotating speed of 3.0 r/min (Figure 3). The removal of turbidity from water mainly depends on the size, density, and strength of the flocculated particles and the hydraulic condition of the settling tank [48,49]. Therefore, the variation in organic content in the influent would not affect the sedimentation of the particles. The residual turbidity in the effluent was relatively stable, at 1.77 ± 0.16 NTU (Figure 3a). The result also proved that an increase in the organic content in the influent would not cause notable biofilm detachment under all the selected OLR conditions, which was mainly related to the extremely low nutrition condition.

The increase in OLR would promote the growth of heterotrophic bacteria attached to the discs, and thus lead to enhanced removal of organics at a higher OLR, as shown in Figure 3b. With the increase in OLR from 0.1 to 0.4 g/(m²∙d), the removal efficiency of TOC increased notably from 53.39% to 71.40%. The increment of TOC in the effluent (1.30 mg/L) was much lower than that of the influent (5.53 mg/L) indicating that the modified settling tank showed good resistance to shocks of OLR. Compared with organics, OLR exhibited an adverse effect on the removal of ${\mathrm{NH}}_4^{\mathrm{+}}$-N (Figure 3c). The removal efficiency of ${\mathrm{NH}}_4^{\mathrm{+}}$-N decreased slightly from 88% to 82% as the OLR of the modified settling tank increased from 0.1 to 0.45 g/(m²∙d). This observation might be related to the inhabitation effect of heterotrophic bacteria on the growth-autotrophic ammonia-oxidizing bacteria [50].

3.2.2. Hydraulic Retention Time

Figure 4 shows the removal performance of turbidity, organics, and ${\mathrm{NH}}_4^{\mathrm{+}}$-N at different HRT. At a higher HRT, the particles also have more settling time in the settling tank, which facilitates the removal of turbidity. Accordingly, when the HRT of the settling tank increased from 1.0 h to 2.0 h, the removal efficiency of turbidity increased sharply from 35.0% to 76.1%. At HRT > 2.0 h, the increasing rate of turbidity removal efficiency turned slower. At the HRT of 3.5 h, the removal efficiency of turbidity reached as high as 86.3%, corresponding to a 1.31 NTU of turbidity residual in the settled water, which is already close to the limiting value (1.0 NTU) of the treated water after conducting filtration [51]. Similar to the study by Zhu et al. [52], the increase in HRT in this device is beneficial to reduce effluent turbidity but the reduction is limited and the removal rate decreases rapidly with the increase in HRT because HRT is inversely proportional to the turbidity removal rate. Therefore, under the conditions of this study, from the perspective of maintaining a high turbidity removal rate, it is appropriate to choose a smaller HRT in the range of HRT = 2–3.5 h.

Unlike turbidity, the effect of HRT on the removal of organics was limited (Figure 4b). Increasing the HRT of the settling tank from 1.0 h to 1.5 h, the residual concentration of TOC in the effluent decreased slightly from 2.4 mg/L to 1.9 mg/L and the removal efficiency of TOC stabilized at 55% when the HRT was above 1.5 h. This finding indicated that the disc biofilm showed high activity for the degradation of organics and the removal process can be completed in 1.5 h. In addition, an increase in HRT at above 1.5 h could significantly improve the removal efficiency of turbidity, further proving that most of the organics were removed through disc biofilm degradation but not the co-precipitation with suspended particles [53].

Similarly, the removal efficiency of ${\mathrm{NH}}_4^{\mathrm{+}}$-N was also the lowest (20%) at HRT = 1.0 h and increased notably to 83% when the HRT increased to 2.0 h (Figure 4c). Increasing the HRT of the settling tank further to above 2.0 h, the removal efficiency of ${\mathrm{NH}}_4^{\mathrm{+}}$-N was relatively stable. This result indicated that the oxidation of ${\mathrm{NH}}_4^{\mathrm{+}}$-N by the disc biofilm can be completed in 2.0 h, which was longer than the time needed for the degradation of organic pollutants. To guarantee the removal efficiencies of both organics and ${\mathrm{NH}}_4^{\mathrm{+}}$-N, the HRT of the modified settling tank was suggested to be maintained at no less than 2 h.

3.2.3. Disc Rotating Speed

The rotating biological discs would change the hydraulic condition of the settling tank. An increase in turbulence intensity would normally inhibit the settling of particles and thus affect the removal efficiency of turbidity [54]. Figure 5a shows that the removal efficiency of turbidity is relatively stable (approximately 76%) when the disc rotating speeds are less than 5 r/min. At disc rotating speeds above 5 r/min, however, the removal efficiency of turbidity in the settled water decreased notably, indicating that the turbulence intensity was strong enough to affect the settling of suspended particles and part of the biofilm might be detached from the discs at above 5 r/min. Accordingly, the disc rotating speed should be maintained below 5 r/min to guarantee good hydraulic condition in the modified settling tank and for removal efficiency of turbidity.

The removal efficiencies of both TOC and ${\mathrm{NH}}_4^{\mathrm{+}}$-N increased first and then tended to decrease with the increase in the disc rotating speed (Figure 5b,c). When the disc rotating speed was low, the growth of microorganisms would be inhibited because of the low dissolved oxygen content in the biofilm [55] and thus leading to insufficient removal of TOC and ${\mathrm{NH}}_4^{\mathrm{+}}$-N. Existing studies confirm that hydrodynamic shear plays an important role in biofilm deformation and bacterial shedding [56] and that biofilm shedding causes changes in microbial populations, which in turn affect the nitrification process [57]. Therefore, biofilm shedding from the discs and changes in community structure also affected the removal of TOC and ${\mathrm{NH}}_4^{\mathrm{+}}$-N when the disc rotation speed exceeded 5.0 r/min. Meanwhile, it should be noted that the sensitivity of different microorganisms to the settling tank hydraulic condition change was also different, which might be the major reason that the disc rotating speed showed a more notable effect on the removal of ${\mathrm{NH}}_4^{\mathrm{+}}$-N than organics.

3.3. Microbial Communities on the Bio-Dics

3.3.1. Microbial Diversity

The microorganisms in the disc biofilm are the main contributors to organics and ${\mathrm{NH}}_4^{\mathrm{+}}$-N removal. The microbial communities in the anterior, middle, and posterior parts of the settling tank’s disc plates were examined using high-throughput sequencing of 16S RNA. The results of alpha biodiversity are shown in Figure 6. The coverage values of all samples were more than 0.99, indicating that the probability of gene sequence detection in all samples was very high and the genes of collected samples well covered the microbial population (Figure 6a). The microbial community diversity and richness, represented by the Shannon and Chao indices in Figure 6b,c, respectively, varied remarkably between the middle and posterior disc biofilm samples. The diversity of microorganisms decreased while the microbial richness increased along the direction of water flow. The community diversity and microbial richness were also illustrated in the rarefaction curves (Figure 6d). The curves showed that the sequencing depth for analyzing the diversity of microbial communities was sufficient and the microbial communities became more concentrated along the water flow path. These observations were corroborated by Sequencing coverage, Shannon’s diversity, and the Chao richness estimators.

3.3.2. Structures of Functional Bacteria

The relative abundance of microorganisms attached to the surface of the discs along the water flow was further analyzed. Figure 7a shows the microbial community composition at the phylum level. The most abundant functional bacteria were Proteobacteria (71%), followed by Bacteroidetes (10%), Actinobacteria (6%), WPS-2 (5%), and Nitrospira (4%) in the anterior of the rotating disc. These five dominant bacterial groups varied along the direction of water flow. The relative abundance of Proteobacteria increased notably to 84% in the posterior part of the rotating disc. The notable increase in Proteobacteria indicated its key role in organics removal [58]. The abundance of Bacteroidetes, which also contribute to the removal of organics [59], decreased notably in the middle and posterior of the biological discs.

Figure 7b illustrates the relative abundance of each class belonging to the phylum Proteobacteria. Alphaproteobacteria exhibited an evident increase along the direction of water flow, while Betaproteobacteria decreased significantly. It is reported that Alphaproteobacteria are competitive under low nutrient concentrations and can degrade complex organic compounds, whereas betaproteobacteria are fast-growing and nutrient-loving [60]. Indeed, in the present work, both the organic contents and biodegradability decreased gradually along the water flow path, which led to a decrease in betaproteobacteria. According to Figure 7c, the genus Hyphomicrobium, a member of the family Alphaproteobacteria, was highly enriched on the posterior discs. Feng et al. [61] reported Hyphomicrobium as a heterotrophic bacteria species and mostly contributed to refractory organics removal.

According to Figure 7c, Nitrospira belonging to the phylum Nitrospirae was dominant on the anterior discs. In oligotrophic waters, such as drinking water treatment plants, Nitrospira is always found to be abundant [62]. Conventionally, the oxidation of ammonium nitrogen to nitrate was considered to be conducted by both ammonia-oxidizing bacteria (Nitrosomonas) and nitrite-oxidizing bacteria (Nitrospira) [63]. Recently, it was reported that Nitrospira could also carry out complete ammonia oxidation [64]. In the meanwhile, Rhodobacter, Zoogloea, Methylibium, Acidovorax, and Methylotenera on the anterior discs, Mesorhizobium on the middle discs, and Hyphomicrobium on the posterior discs mainly contributed to the reduction of nitrate ions into nitrogen gas [65,66]. The above phenomenon suggests that the presence of nitrifying and denitrifying bacteria at different locations on the rotating biological contactor can achieve efficient removal of ${\mathrm{NH}}_4^{\mathrm{+}}$-N at very low nutrient levels. To further investigate the fate of nitrogen in the system, we investigated the levels and removal of ${\mathrm{NH}}_4^{\mathrm{+}}$-N, total nitrogen, and NO₃⁻-N in the influent and effluent of the system under different organic loading conditions (Figure S3). When the influent ${\mathrm{NH}}_4^{\mathrm{+}}$-N concentration was maintained in the range of 0.63~0.71 mg/L and total nitrogen was maintained in the range of 1.5–2.5 mg/L, the concentration of ${\mathrm{NH}}_4^{\mathrm{+}}$-N in the treated effluent was always below the detection limit with the increase in organic load and the average removal rate was above 98%, which indicated that the microorganisms in the carousel had a good nitrification performance on ${\mathrm{NH}}_4^{\mathrm{+}}$-N in the water. In addition, the NO₃⁻-N in the treated water increased significantly, and the proportion of NO₃⁻-N in the final effluent rose to more than 76% of the total nitrogen. The above phenomenon shows that under different organic loads, the ${\mathrm{NH}}_4^{\mathrm{+}}$-N in water was mainly converted to NO₃⁻-N after treatment by the system, and the denitrification process was inhibited by the poor reducing environment of the system (high dissolved oxygen, which is not conducive to denitrification), resulting in an increase in NO₃⁻-N in the effluent.

4. Conclusions

With the discharge of inadequately treated and even untreated domestic wastewater, many water sources in China are facing, to different extents, pollution problems. To enhance the removal of both organics and ${\mathrm{NH}}_4^{\mathrm{+}}$-N in traditional water plants, rotating biological discs were added in the upper part of the horizontal flow settling tank. Results showed that the introduction of the rotating biological discs did not have obviously adverse impacts on the turbidity-removing function of the settling tank. The residual turbidity in the settled water stabilized in the range of 1.71–2.37 NTU. When the modified settling tank was operated at an HRT of 2 h, 49–59% of the organics and 67–87% of the ${\mathrm{NH}}_4^{\mathrm{+}}$-N could be removed. The microorganisms attached to the disc biofilm, especially the Hyphomicrobium genus on the posterior discs and the Nitrospira genus on the anterior discs, mainly contributed to the bio-oxidation of organics and ${\mathrm{NH}}_4^{\mathrm{+}}$-N, respectively. An increase in OLR promoted the growth of heterotrophic bacteria but inhibited the growth of autotrophic ammonia-oxidizing bacteria, and thus led to an opposite variation trend on the removal efficiencies of organics and ${\mathrm{NH}}_4^{\mathrm{+}}$-N. To prevent detachment of the biofilm, the disc rotating speed is suggested to be below 5 r/min. Although the above findings proved the possibility of combining the functions of a horizontal flow settling tank and rotating biological discs in drinking water purification, the use of a bio-disc might increase the number of bacteria in the settling effluent and decrease the following disinfection efficiency. Further, both the size and bio-disc number optimization of the modified tank were not conducted either, which should be considered in future work. Notwithstanding its limitations, this study does provide a possible upgrade choice for traditional plants used in treating organic- and ${\mathrm{NH}}_4^{\mathrm{+}}$-N-polluted source waters.