Phosphorus Removal Characteristics Research Articles

Genomics and metabolic characteristics of simultaneous heterotrophic nitrification aerobic denitrification and aerobic phosphorus removal by Acinetobacter indicus CZH-5

This study provides for the first time a systematic understanding of Acinetobacter indicus CZH-5 performance, metabolic pathway and genomic characteristics for aerobic nitrogen (N) and phosphorus (P) removal. Acinetobacter indicus CZH-5 showed promising performance in heterotrophic nitrification aerobic denitrification and aerobic phosphorus removal. Under optimal conditions, the maximum ammonia-N, total nitrogen and orthophosphate-P removal efficiencies were 90.17%, 86.33%, and 99.89%, respectively. The wide tolerance range suggests the strong environmental adaptability of the bacteria. The complete genome of this strain was reconstructed. Whole genome annotation was used to re-construct the N and P metabolic pathways, and related intracellular substance metabolic pathways were proposed. The transcription levels of related functional genes and enzyme activities further confirmed these metabolic mechanisms. N removal was achieved via the nitrification–denitrification pathway. Furthermore, CZH-5 exhibited significant aerobic P uptake, with phosphate diesters as the main species of intracellular P.

Unraveling microbial characteristics of simultaneous nitrification, denitrification and phosphorus removal in a membrane-aerated biofilm reactor

This study describes the simultaneous removal of carbon, ammonium, and phosphate from domestic wastewater by a membrane-aerated biofilm reactor (MABR) which was operated for 360 days. During the operation, the maximum removal efficiencies of chemical oxygen demand (COD), total nitrogen (TN) and total phosphorus (TP) reached 93.1%, 83.98%, and 96.41%, respectively. Statistical analysis showed that the MABR could potentially treat wastewater with a high ammonium concentration and a relatively low C/N ratio. Dissolved oxygen and multiple pollutants, including ammonium, carbon, phosphate, and sulfate, shaped the structure of the microbial community in the MABR. High throughput sequencing uncovered the crucial microbiome in ammonium transformation in MABR. Phylogenetic analysis of the ammonia monooxygenase (amoA) genes revealed an important role for comammox Nitrospira in the nitrification process. Diverse novel phosphate-accumulating organisms (Thauera, Bacillus, and Pseudomonas) and sulfur-oxidizing bacteria (Thiobacillus, Thiothrix and Sulfurimonas) were potentially involved in denitrification in MABR. The results from this study suggested that MABR could be a feasible system for the simultaneous removal of nitrogen, carbon, phosphorus, and sulfur from sewage water.

Characteristics of Tetracycline Degradation Coupled Simultaneous Nitrification-Denitrification and Phosphorus Removal in Aquaculture Wastewater

Although residual tetracycline, NH4 +-N, and P in aquaculture wastewater have been frequently detected, their simultaneous removal has received little attention. The simultaneous and efficient removal of tetracycline, NH4 +-N, COD, and P in aquaculture wastewater was achieved with a laboratory-scale moving bed biofilm reactor (MBBR) under different influent C/N ratios. The results suggested that tetracycline could be degraded efficiently without using an additional organic carbon source. Most of the tetracycline (>80%), NH4 +-N (>73%), TN (>68%), TP (>59%), and COD (>78%) were synchronously removed under the higher C/N ratios. The electron transport system activity (ETSA), dissolved organic matter (DOM), and extracellular polymeric substances (EPS) were evaluated for biofilm performance during aquaculture wastewater remediation. The results indicated that the protein (PN) and polysaccharide (PS) concentrations, and fluorescence intensities were significantly different because the influent C/N ratios changed the biofilm activity. The ETSA values were lowest when tetracycline was the sole carbon source, which suggested that tetracycline affected the nitrification activity of biofilms. Sphingobacteriia, Betaproteobacteria, and Alphaproteobacteria were identified as the dominant bacterial classes. This study provides a new strategy for the removal of tetracycline and nutrients from aquaculture wastewater.

Study on the effectiveness of an independent biological phosphorus removal system based on immobilized biological fillers nitrogen removal system in municipal wastewater

This study focused on the problem that simultaneous nitrogen and phosphorus removal cannot be achieved in current urban sewage treatment systems. To address this, a pilot system was developed to first use immobilized biologically active fillers to remove nitrogen, and then use activated sludge to independently remove phosphorus. The goal was to maximize the effectiveness of biological phosphorus removal. This study mainly aimed at analyzing the post-independent phosphorus removal characteristics and mechanisms; the composition of the phosphorus removing bacteria; and the contribution rate of the system. The effluent PO43--P of the system remained stable at 0.15–0.2 mgL−1, with the phosphorus removal anaerobic tank maintaining a "micro-release" state. When the hydraulic retention time (HRT) of the aerobic phosphorus removal tank was reduced to 1 h, the contribution rate of the aerobic phosphorus removal bacteria decreased from 28.42% to 16.68%; the contribution rate of the denitrifying phosphorus removal bacteria increased from 72.58% to 83.32%; and the abundance of the dominant bacteria Dechloromonas and Thaurea significantly increased. The phosphorus removal system provides a new approach for deep phosphorus removal in main municipal wastewater systems.

Simultaneous Domestication of Short-cut Nitrification Denitrifying Phosphorus Removal Granules

In this experiment, three replicated SBR reactors were operated using asynchronous acclimation of the phased method (A/O-A/O/A), simultaneous domestication of continuous aeration by A/O/A, and simultaneous domestication of intermittent aeration by A/O/A. Using artificial water distribution as the influent substrate, flocculent sludge was inoculated and granulated by hydraulic selection. The domestication and nitrogen and phosphorus removal characteristics of shortcut nitrification denitrifying phosphorus removal granules under different operation modes were assessed. The results show that simultaneous domestication of intermittent aeration by A/O/A has the most efficient under the combination of short aeration time (140 min) and low aeration strength[3.5 L·(h·L)-1]. The average removal rates of carbon, nitrogen, and phosphorus were 90.74%, 91.15%, and 95.66%, respectively, which could achieve synchronous removal during later stable operation. The particle size was 895 μm, and the particles were small but uniformly dense in microscope observations. The f value (MLVSS/MLSS) was kept stable at 0.8-0.85 and sludge had a high biomass due to the alternate aerobic/anoxic operation with intermittent aeration. This supported anoxic heterotrophic bacteria at the core of the particles, which was conducive to the stability of the granular sludge structure. Batch experiments showed that the specific ammonia-oxidation rate of the simultaneous domestication of intermittent aeration by A/O/A system was 3.38 mg·(g·h)-1, and denitrifying phosphate accumulating organisms (DPAOs) able to utilize nitrite as electron acceptor accounted for 65.46%. This was more conducive to the simultaneous domestication and enrichment of ammonia-oxidizing bacteria (AOB) and NO2--type DPAOs, ensuring a stable treatment effect.

Denitrifying phosphorus removal and microbial community characteristics of two-sludge DEPHANOX system: Effects of COD/TP ratio

Two Lab-scale ANOXic DEPHosphation (DEPHANOX) bioreactors, in the continuous flow (DEPHANOX-CFR) and sequencing batch reactor (DEPHANOX-SBR) modes, were successfully operated and compared with varying influent C/P ratios. A decrease of influent chemical oxygen demand (COD)/total phosphorus (TP) feeding ratio from 250/4 to 250/14 generally did not significantly affect the nutrient removal performances. However, the overall microbial community structure and composition significantly shifted in the both two DEPHANOX bioreactors. Quantitative polymerase chain reaction results revealed that denitrifying phosphate-accumulating organisms (DPAOs) dominated and accounted for around 78.8 % of phosphorus accumulating organisms (PAOs). High-throughput sequencing results further suggested that the most three prominent phyla identified in the DEPHANOX system under different COD/TP ratio conditions included Proteobacteria (38.8–68.7 %), Bacteroidetes (10.3–12.3 %) and Chloroflexi (12.2−1.9 %). In addition, Dechloromonas and Pseudomonas spp. were identified as the dominant DPAOs. Furthermore, PICRUSt prediction analysis suggested that functional genes related to diverse metabolism pathways, including tricarboxylic acid cycle, electron transport, and nitrogen metabolism, were predicted to be actively involved with simultaneous phosphorus and nitrogen (SPN) removal. This study optimized the influent constituent characteristics and operating conditions of the DEPHANOX system, and provided novel insights into key metabolism functions of DPAOs in the DEPHANOX system.

Identification of Genome Sequences of Polyphosphate-Accumulating Organisms by Machine Learning.

In the field of sewage treatment, the identification of polyphosphate-accumulating organisms (PAOs) usually relies on biological experiments. However, biological experiments are not only complicated and time-consuming, but also costly. In recent years, machine learning has been widely used in many fields, but it is seldom used in the water treatment. The present work presented a high accuracy support vector machine (SVM) algorithm to realize the rapid identification and prediction of PAOs. We obtained 6,318 genome sequences of microorganisms from the publicly available microbial genome database for comparative analysis (MBGD). Minimap2 was used to compare the genomes of the obtained microorganisms in pairs, and read the overlap. The SVM model was established using the similarity of the genome sequences. In this SVM model, the average accuracy is 0.9628 ± 0.019 with 10-fold cross-validation. By predicting 2,652 microorganisms, 22 potential PAOs were obtained. Through the analysis of the predicted potential PAOs, most of them could be indirectly verified their phosphorus removal characteristics from previous reports. The SVM model we built shows high prediction accuracy and good stability.

도금폐수 처리공정 중 BPC 단위공정 내 ORP 및 pH 운전조건별 도금폐수의 Ni 및 T-P 제거특성

도금폐수 처리공정 중 BPC 단위공정 내 ORP 및 pH 운전조건별 도금폐수의 Ni 및 T-P 제거특성

Roles of nitrate recycling ratio in the A2/O - MBBR denitrifying phosphorus removal system for high-efficient wastewater treatment: Performance comparison, nutrient mechanism and potential evaluation

The long-term effect of nitrate recycling ratios (R = 100%–500%) on the denitrifying phosphorus removal (DPR) characteristics was studied in a novel two-sludge system, which coupled Anaerobic Anoxic Oxic (A2/O) with Moving Bed Biofilm Reactor (MBBR) for simultaneous nitrogen (N) and phosphorus (P) removals. During the 220 days’ operation, effluent COD (30.87–45.15 mg/L) can meet the discharge standard completely, but N and P removals were significantly affected by the R-value, including CODintra removal efficiency (CODintra-Re: 56.09–85.98%), TN removal (TN-Re: 52.06–80.50%), anaerobic PO43− release (PO43--An: 10.66–29.02 mg/L) and oxic PO43− absorption (PO43--O: 2.22–6.26 mg/L). Meanwhile, N and P displayed close correlation with the ΔPO43−/ΔNO3- ratio of 4.20–4.41 at R = 300%–400%, resulting in the high-efficient anoxic poly-β-hydroxyalkanoates (PHA) utilization (ΔPHAA: 64.88 mgCOD/gVSS). Based on the stoichiometry methodology, at R of 300%–400%, the percentages of phosphorus accumulation organisms (PAOs) and glycogen accumulating organisms (GAOs) contributed to ΔPHAAn (ΔGlyAn) were 71.7%, 28.3% (61.3%, 38.7%) in the anaerobic stage, respectively, while N denitrification rate (NDRA: 3.91–3.93 mg N/(gVSS·h)) and P uptake rate (PURA: 3.76–3.90 mg P/(gVSS·h)) reached the peak, suggesting superior DPR performance with higher contribution of denitrifying PAOs (DPAOs) (70%) than denitrifying GAOs (DGAOs) (30%) in the anoxic stage. Microbial community analysis showed that Accumulibacter (27.66–30.01%) was more enriched than Competibacter (13.41–14.34%) and was responsible for the improved C, N, P removals and DPR characteristics. For optimizing operation, the combined effect of nitrate recycling ratio with other process parameters especially economic evaluation should be considered.

Enhanced Phosphorus Removal from Wastewater Using RSPRC and a Novel Reactor

Fly ash and steel slag both have a good adsorption performance and many researchers have mixed the two to make effective adsorbents. Based on previous knowledge, activated clay is added in this study. In order to deep dephosphorize wastewater, two different industrial wastes (steel slag, fly ash) are blended into activated clay as adsorption substrates, supplemented with a binder and foaming agent to prepare a Residue and Soil Phosphorus Removal Composite (RSPRC). This is prepared to carry out experimental research on the decolorization effect and phosphorus removal characteristics of RSPRC. Meanwhile, a self-developed concentric circular diversion wall adsorption reactor is implemented to study the effect of phosphorus removal. It is found that the addition of activated clay can significantly improve the phosphorus removal performance. The results suggest that the phosphorus concentration in the effluent from the reactor can be stably reduced to below 0.10 mg/L. The concentric circular diversion wall adsorption reactor and RSPRC will have broad application prospects in phosphorus removal.

Simultaneous Short-Cut Nitrification-Denitrification Phosphorus Removal Granules Induced by Phosphorus Removal Granules

This paper investigated domestic sewage with a low C/N ratio. Mature phosphorus removal granules were inoculated to cultivate granules with a simultaneous short-cut nitrification and denitrification function. The characteristics of nitrogen and phosphorus removal of this process were analyzed. Results show that AOB can be enriched by prolonging the sludge age for 30 days with an aeration intensity of 5 L·(h·L)-1 and shorter aeration time (140 min), whereas the simultaneous nitrification and denitrification ability could not be improved. The nitrogen loss increased at the aerobic time when aeration intensity was reduced by 3.5 L·(h·L)-1 and aeration time was prolonged by 200 min. The aeration time was further optimized to restrain the transformation of NO2- to NO3-, and finally the effluent of TP < 0.5 mg·L-1 and TN < 15 mg·L-1. During the process of the system function transformation from phosphorus removal to nitrogen and phosphorus removal, the phosphorus release decreased, however PAOs still played a dominant role (60%) in the process of internal carbon storage. Batch experiments showed that DPAOs that can utilize nitrite as an electron acceptor accounts for 52.43% in the total PAOS, which alleviated the pressure of the carbon source and improved the simultaneous nitrogen and phosphorus removal.

Comparative analysis of the phosphorus removal characteristics between two-stage and single-stage SBR processes for phosphorus and nitrogen removal

Comparative analysis of the phosphorus removal characteristics between two-stage and single-stage SBR processes for phosphorus and nitrogen removal

Utilisation of polarity inversion for phosphorus recovery in electrochemical precipitation with anaerobic digestion effluent

Phosphorus is a non-replaceable and limited resource, whose future drought is a matter of concern. Its recovery from wastewater has gained attention as a method of making the recovered phosphorus available for beneficial use. This study applied electrolysis with a platinum-coated titanium electrode to an actual anaerobic digestion effluent and investigated the phosphorus removal and precipitation characteristics with various current values. The separation of the phosphorus precipitate from the sludge residue and the usefulness of the polarity inversion to recover the phosphorus precipitate were evaluated. The availability of the recovered precipitate as a fertiliser was also demonstrated. The anaerobic digestion effluent after the centrifugal dehydration process with a coagulant was collected from a wastewater treatment plant. Phosphorus was precipitated as a form of calcium phosphate, including hydroxyapatite, which was confirmed by X-ray diffraction. The generated gases from the electrodes during the electrolysis floated and condensed the sludge residue, leading to successful separation of the phosphorus precipitate from the sludge residue. The polarity inversion successfully detached the phosphorus precipitate from the electrode, and at least 3 min polarity inversion was enough to detach the whole phosphorus precipitate. The submerging phosphorus precipitate contained abundant phosphorus (12.6%), in which citric-acid-soluble phosphorus accounted for 94.4%, and the utilisation of the precipitate as a slow-release phosphorus fertiliser was suggested.

Pilot Study on Start-up and Stable Operation at Low Temperature Based on Denitrifying Phosphorus Removal

In order to understand the denitrifying and phosphorus removal characteristics of denitrifying phosphate-accumulating organisms (DPAOs), a pilot scale (20 m3) denitrifying and phosphorus removal experiment was carried out using a modified University of Cape Town (UCT) process at low temperatures of (6-16)℃. The test results show that at such temperatures, the hydraulic retention time (HRT) is 20 h and the solids retention time (SRT) is 35 days, and the modified UCT process can start up successfully and run steadily. When running steadily, the system can maintain nitrogen and phosphorus removal rates of 60%±5% and 80%±5%, respectively. The effluent concentrations of chemical oxygen demand (COD), ammonium nitrogen (NH4+-N), total nitrogen (TN), and total phosphorus (TP) were 20, 5, 11, and 0.5 mg·L-1, respectively, which meet the first A emission standard of "Pollutant Discharge Standard for Urban Sewage Treatment Plant" (GB 18918-2002). In order to further investigate the characteristics of nitrogen and phosphorus removal in the system, the reflux ratio from the aerobic tank to the anoxic tank was increased to 150%. After the system was stabilized, it obtained higher nitrogen and phosphorus removal rates of 80%±10% and 90%±5%, respectively. Among them, denitrifying phosphorus removal in the anoxic tank accounted for 80%±4% of the total biological phosphorus removal. The average effluent concentrations of COD, NH4+-N, TN, and TP were 19.55, 0.1, 7.8, and 0.15 mg·L-1, respectively, which meet the Beijing Standard A discharge standard.

소성 굴 패각을 이용한 인 제거 및 반응결정물의 특성

소성 굴 패각을 이용한 인 제거 및 반응결정물의 특성

Kinetics and Thermodynamics of Efficient Phosphorus Removal by a Composite Fiber

In the current study, we investigated the kinetics and thermodynamics of phosphorus removal by zeolite/steel slag/fly ash/basalt (ZSFB) composite fiber. Kinetic analysis indicated that the adsorption of phosphorus is best fitted with the pseudo-second-order model. The maximum adsorption capacity of the fiber calculated by the Langmuir model was found to be 4.18 mg/g and the partition coefficient was 1.49 mg/g/µM. Thermodynamics results revealed that the Gibbs free energy ΔG0 of the composite fiber was negative, indicating that the adsorption is a spontaneous process; the standard enthalpy of reaction ΔH0 was positive, indicating that the adsorption is endothermic. Adsorption under different influencing factors and desorption experiments were conducted to investigate the phosphorus removal characteristics of ZSFB composite fiber. Dynamic adsorption and the phosphorus removal experiment were also conducted in a fixed-bed reactor to study factors affecting the time of breakthrough. Results indicate that the performance of ZSFB composite fiber was not relatively outstanding compared with nanomaterials and magnetic composites. However, its performance has already met the class A demands of “discharge standard of pollutants for municipal wastewater treatment plant” (GB18918-2002) which means it can be applied to remove phosphorus from real wastewater in a cost-effective way with low-priced raw materials.

Start-up and Stable Operation of ABR-MBR Denitrifying Phosphorus Removal Process

The nitrogen and phosphorus removal characteristics during the start-up and the long-term operational stability of an anaerobic/anoxic (A/A) ABR coupled aerobic MBR system treating low C/N domestic wastewater were investigated. The results showed that the denitrifying phosphorus bacteria (DPBs) were successfully enriched within 46 d by controlling the nitrate recycling ratio (increasing from 150% to 300%), with a temperature of 30℃±2℃, volume loading rate of 0.8 kg·(m3·d)-1 and sludge reflux ratio of 80% in the ABR, sludge retention time (SRT) in the denitrifying phosphorus removal functional area of 25 d, and the dissolved oxygen (DO) of 1-2 mg·L-1 in the MBR. The net phosphorus release and phosphorus uptake of DPBs reached 20.56 mg·L-1and 27.74 mg·L-1, respectively. Batch tests demonstrated that about 84.8% of phosphorus-accumulating organisms (PAOs) could use NO3--N as an electron acceptor for denitrifying phosphorus removal. After 50 d of stable operation after the successful system start-up, the average removal rates of COD, NH4+-N, TN, and PO43--P were 91.8%, 99.0%, 71.5%, and 94.2%, respectively. The results also suggested that 0.83 mg·L-1NO3--N was consumed per 1 mg·L-1 PO43--P removed during the denitrifying phosphorus removal, indicating that the simultaneous nitrogen and phosphorus removal was achieved in the ABR-MBR system.

Operating Characteristics of a DPR-SNED System Treating Low C/N Municipal Wastewater and Nitrate-containing Sewage

In order to realize the simultaneous treatment of low C/N municipal wastewater and high nitrate wastewater, a sequencing batch reactor (SBR), inoculated with activated sludge, was used to initiate the denitrifying phosphorus removal coupled with simultaneous nitrification and endogenous denitrification (DPR-SNED). The anaerobic/anoxic/hypoxic durations and dissolved oxygen (DO) concentration were appropriately controlled, and the nitrogen and phosphorus removal characteristics were examined. The experimental results demonstrated that, in the anaerobic/hypoxia operation mode, with an anaerobic duration of 3 h and DO concentration of 0.5-1.0 mg·L-1, the simultaneous nitrification of phosphorus removal (SNEDPR) system successfully began in 60 d. The effluent PO43--P concentration was below 0.5 mg·L-1, the nutrient and COD removal efficiencies were stably maintained above 90% and 80%, respectively, and the SNED efficiency and CODins efficiency reached 70% and 95%, respectively. When the operation mode was anaerobic/anoxic/hypoxic and nitrate-containing sewage was added at the beginning of the anoxic stage, DPR-SNED was achieved with the effluent PO43--P concentration<0.5 mg·L-1, nutrient and COD removal efficiencies above 88% and 90%, respectively, and SNED efficiency and CODins efficiency maintained at 62% and 90%, respectively. After the successful initiation of DPR-SNED, enhanced intracellular carbons storage was achieved by phosphorus-and glycogen-accumulating organisms using the limited carbons in raw municipal wastewater to provide sufficient carbon sources for subsequent nutrient removal. In addition, the endogenous partial denitrification ensured the efficient nitrogen removal performance of the DPR-SNED system at low C/N conditions (average 4).

Phosphorus and ammonium removal characteristics from aqueous solutions by a newly isolated plant growth-promoting bacterium

ABSTRACT An indigenous plant growth-promoting bacterium isolated from Peganum Harmala rhizosphere in the arid ecosystem was found to solubilize and accumulate phosphates. This isolate was identified as Pseudomonas sp. (PHR6) by partial 16S rRNA gene sequence analysis. Controlled batch experiments on nutrients removal by this isolate in mineral medium showed relatively high efficiencies after 24 h of aerobic incubation with average values of 117.59 and 335.38 mg gVSS−1 for phosphorus (P-PO4) and nitrogen (N-NH4), respectively. Furthermore, the strain performed heterotrophic nitrification ranging from 48.81% to 84.24% of the total removed nitrogen. On the other hand, the experimental results showed that a short idle period (24 h) significantly enhanced P accumulation (up to 95%) and N assimilation (up to 50%) of the total removed amounts. However, long idle period (20 days) revealed firstly aerobic phosphorous release phase succeeded by another removal one within 24 h of incubation. Overall, the idle treatment enhances P removal efficiency from the mineral liquid medium without significant effects on N-NH4 removal performance. The isolated strain showed also significant nutrient removal ability from synthetic wastewater providing an accumulated fraction of 98% from the total removed phosphorus amount. This study highlights the potential contribution of the selected rhizobacterium PHR6 to both environmental nutrient recycling and pollution control especially regarding phosphorus.

Effect of Different Sludge Retention Time (SRT) Operations on the Nutrient Removal Characteristics of a SNEDPR System

This study focuses on the nitrogen (N) and phosphorus (P) removal characteristics in a simultaneous nitrification endogenous denitrification and phosphorus removal (SNEDPR) system operating at different sludge retention time (SRT). Four extended anaerobic/low aerobic (dissolved oxygen:0.5-1.0 mg·L-1)-operated sequencing batch reactors (SBRs) fed with municipal sewage were studied at different SRT of 5, 10, 15, and 25 d. The experimental results show that a shorter SRT at an SRT ≥ 10 d enhances the competitive advantage of PAOs in the system and an efficient phosphorus removal performance of the SNEDPR system was achieved at a SRT of 10 d and 15 d. Especially at an SRT of 10 d; the average PPAOs, An was 68.4%, the PRA and PUA reached 31.9 and 34.3 mg·L-1, respectively. The nitrification performance of the system was not affected by SRT changes. The most efficient nitrogen removal performance was achieved at a SRT of 15 d, with a high average TN removal and SNED efficiencies reaching 89.6% and 71.8%, respectively. At a SRT ≥ 10 d, the COD removal performance of the SNEDPR system was also not affected by SRT changes. The COD removal efficiencies were higher than 78%. However, when the SRT was shortened to 5 d, the C, N, and P performances of the system worsened due to the loss of biomass; the SNED and PO43--P removal efficiencies were as low as 5.7% and 0.5%, respectively. In addition, at an SRT=15 d, the sludge-settling performance of the system was the best. The SV and SVI were 20% and 64 mL·g-1, respectively, and the sludge concentration increased with the extension of the SRT. Under long SRT (25 d) operation, the system showed a good resistance to shock loads, but the sedimentation performance of the sludge deteriorated.