Role Of Hydraulic Retention Time Research Articles

Deciphering the role of hydraulic retention time (HRT) in integrated fixed-film activated sludge (IFAS) system under aniline stress: Effects on microbial succession and communication

Integrated fixed-film activated sludge (IFAS) system offered an energy-efficient option for nitrogen removal from aniline wastewater. The regulation of hydraulic retention time (HRT) has been reported to affect the fate of functional microbial communities with different generation cycles, but the role of HRT in IFAS under aniline stress has not been sufficiently revealed. Herein, it was observed that shortening the HRT altered the mechanisms underlying the adaptation of a continuous flow micro-aerated IFAS system to aniline stress, thereby negligibly impacting the removal of aniline. The total nitrogen removal rate in Y2 (HRT = 0.5 day) was 40.28 % higher than Y1 (1 day), and 23.7 % higher than Y3 (0.25 day). This could be attributed to Y2 having an ample supply of electron donors for denitrification compared to Y1, as well as showing weaker stress to inhibit nitrification compared to Y3. Furthermore, with the progressive intensification of stress induced by HRT reduction, microorganisms gradually enhanced their adaptation to aniline. This led to an increased migration of microorganisms towards the inner layers of the biofilm, resulting in enhanced functional complementarity between the sludge and biofilm in Y2. Also, there was a distinct development of autotrophic denitrification, with arenimonas predominant in Y1, and Thauera predominant in Y2 and Y3. Moreover, Y2 possessed a high abundance of functional genes related to the production and reception of quorum sensing signal molecules, which may enhance the electron transport-dependent nitrogen metabolism reaction.

Characterizing the role of hydraulic retention time on nitrate removal indices in denitrifying bioreactors by nonlinear models

Denitrifying bioreactors (DNBRs) are a sustainable and cost-effective practice commonly used at the edge of fields to reduce nitrate from agricultural runoff. The hydraulic retention time (HRT) is a crucial variable that affects nitrate removal rate (NRR, g N m−3 d−1), nitrate removal efficiency (NRE, %), and nitrate concentration reduction per length (Nrd, mg N L−1 m−1). In this study, two nonlinear models, the developed Michaelis-Menten (MM) model and the Mitscherlich (MT) model, were developed to characterize the relationship between nitrate removal indices (NRR, NRE, and Nrd) and HRT. This study first utilizes nonlinear models to quantitatively understand the relationship between NRR, NRE, Nrd, and HRT. To verify the models, eight experiments were conducted under different conditions, including different scales (laboratory and field), media (woodchip, woodchip+biochar, woodchip+silage leachate, woodchip+biochar+silage leachate), and influent nitrate concentrations (6.8–70 mg N L−1). The results showed that the MT model outperformed the MM model and MT could accurately characterize the nitrate removal changes with HRT and provide the optimal HRT (HRTO). Overall, the model could be beneficial for designers and practitioners to optimize nitrate removal.

Hydraulic Retention Time as an Operational Tool for the Production of Short-Chain Carboxylates via Anaerobic Fermentation of Carbohydrate-Rich Waste.

The carboxylate platform is a sustainable and cost-effective way to valorize wastes into biochemicals that replace those of fossil origin. Short-chain fatty acids (SCFAs) are intermediates generated during anaerobic fermentation (AF) and are considered high-value-added biochemicals among carboxylates. This investigation aimed to produce SCFAs through the AF of sugar beet molasses at 25 °C and semi-continuous feeding mode in completely stirred tank reactors. A particular focus was devoted to the role of hydraulic retention time (HRT) variation in SCFAs production and distribution profile. The highest SCFAs concentration (44.1 ± 2.3 gCOD/L) was reached at the HRT of 30 days. Caproic acid accounted for 32.5-35.5% (COD-concentration basis) at the long HRTs of 20 and 30 days due to the carbon chain elongation of shorter carboxylic acids. The findings of this study proved that HRT could be used to steer the anaerobic process toward the targeted SCFAs for specific uses. Furthermore, the successful operation at low-temperature conditions (i.e., 25 °C) makes the process economically promising.

Antibiotic removal from swine farming wastewater by anaerobic membrane bioreactor: Role of hydraulic retention time

This study investigated the impact of hydraulic retention time (HRT) on the removal of veterinary antibiotics by an anaerobic membrane bioreactor (AnMBR) for swine wastewater treatment. Ten commonly used antibiotics in the group of tetracyclines, fluoroquinolones, and sulfonamides were evaluated. Results show that a wide variation from 21 to 99% was observed for antibiotic removal by AnMBR regardless of the applied HRT. The methane yield increased from 0.22 L/g COD to 0.28 L/g COD as the operational HRT was prolonged from 24 to 48 h for AnMBR. Doubling the HRT value enhanced removal of most antibiotics and methane yield by 2–30% and 22–47%, respectively. The enhanced antibiotic removal in response to HRT prolongation could be positively correlated to the enrichment of the genus unidentified_Synergistaceae. Moreover, prolonging HRT increased the relative abundance of the genera unidentified_Synergistaceae, Geobacter and Methanobrevibacter, which could strengthen hydrogenotrophic methanogenesis to enhance methane yield. In addition, HRT increase reduced the ratio of protein to polysaccharide in both extracellular polymeric substances and soluble microbial products, thereby, alleviating membrane fouling in AnMBR operation.

The application of post-denitrification fixed biofilm reactor for polishing secondary effluent: Nitrate removal, soluble microbial products and micropollutants biotransformation

This study was conducted to comprehensively evaluate the role of hydraulic retention time (HRT) on simultaneous removal of nitrate and micropollutants (MPs) in secondary effluent from wastewater treatment plants in post-denitrification fixed biofilm reactor (PDFBR). Results showed that PDFBR was favorable for nitrate removal (above 94%). Prolonged HRT promoted the bio-utilization of nonaromatic soluble microbial products with low molecular weight and reduced biomass production. MPs was partially removed in PDFBRs (below 48%). Microbial diversity increased along the extending of HRT and thus partially enhanced MPs removal. Batch experiments showed that changing HRT had no direct impact on the biodegradation rates of the selected MPs. Correlation analysis revealed that Dechloromonas, Terrimonas, and Phreatobacter were reasonable for simultaneous removal of MPs and nitrate. The abundance of nosZ gene had a rapid decrease under extreme HRT. This study provides insights into polishing nitrate and MPs from secondary effluent in a denitrifying biofilm system.

Metagenomic analysis of microbial community and metabolic pathway of simultaneous sulfide and nitrite removal process exposed to divergent hydraulic retention times

The role of hydraulic retention time (HRT) on S0 production was assessed through metagenomics analyses. Considering comprehensive performance for the tested HRTs (0.25–13.33 h), the optimal HRT was 1 h, while respective sulfide and nitrite loading rate could reach 6.84 kg S/(m3·d) and 1.95 kg N/(m3·d), and total S0 yield was 0.36 kg S/(kg (VSS)·d). Bacterial community richness decreased along the shortening of HRT. Microbacterium, Sulfurimonas, Sulfurovum, Paracoccus and Thauera were highly abundant bacteria. During sulfur metabolism, high expression of sqr gene was the main reason of maintaining high desulfurization load, while lacking soxB caused the continuous increase of S0. Regarding nitrogen metabolism, the rapid decrease of nitrite transporter prevented nitrite to enter in cells, which caused a rapid decrease of nitrite removal under extreme HRT. Adjusting HRT is an effective way to enhance S0 production for the application of the simultaneous sulfide and nitrite removal process.

Role of hydraulic retention time in enhancing bioenergy generation from petrochemical wastewater

The focus of this work was to improve the anaerobic co-digestion yield produced by the optimum hydraulic retention time of petrochemical wastewater and activated manure.Hydraulic retention time appears to be the principal operating parameter, which can affect bacteriological ecology and other features in a reactor operating system. The anaerobic digestion of petrochemical wastewater has been widely studied, and commercial applications have already been recognized. However, the optimization of the hydraulic retention time has not been studied to date.The influence of four hydraulic retention times (12, 9, 6 and 3 days) on biogas and methane generation was investigated. Equal amounts of petrochemical wastewater and activated manure were mixed in a continuously stirred tank reactor. The mixture was run at a mesophilic state (37 °C). At a hydraulic retention time of 9 days, methane generation was 0.99 ± 0.12 LLr−1d−1 with a maximum yield of 0.34 ± 0.02 L g−1 CODremoved−1. The chemical oxygen demand removal efficiency was 98.5 ± 0.5%. The optimum hydraulic retention time (of the three tested values) was determined to be 9 days. The scientific contribution of this anaerobic co-digestion technology with optimized HRT may play a role in degrading the petrochemical wastewater and developing an energy-efficient strategy for waste management.

The role of hydraulic retention time on controlling methanogenesis and homoacetogenesis in biohydrogen production using upflow anaerobic sludge blanket (UASB) reactor and packed bed reactor (PBR)

Methanogenesis and homoacetogenesis are two notorious hydrogen-consuming reactions during dark fermentation for biohydrogen production. The focus of this study was on the role of hydraulic retention time (HRT) to control methanogenesis and homoacetogenesis in an upflow anaerobic sludge blanket (UASB) reactor and a packed bed reactor (PBR). The HRT was changed from 24 to 4 h and 24 to 2 h in the UASB and PBR, respectively. A maximal hydrogen yield of 1.47 mol/mol glucoseadded with a high hydrogen production rate of 4.38 L/L/d was achieved at 8 h HRT in UASB. In comparison, a maximal hydrogen yield of 0.89 mol/mol glucoseadded with a high hydrogen production rate of 10.66 L/L/d was achieved at 2 h in PBR. With the reduction of the HRT, the volumic hydrogen consumption due to methanogenesis in the UASB was decreased from 12.1 to 3.1%. As for PBR, the value was reduced from 66.9 to 31.4%. Homoacetogenesis in the UASB and PBR was dramatically suppressed when the HRT was decreased to 8 and 4 h, respectively. However, these hydrogen-consuming microbes cannot be completely removed. Microbial diversity analysis using Illumina MiSeq sequencing revealed the existence of Clostridium ljungdahlii, a homoacetogen, in UASB and PBR at low HRT. In addition, the low HRT reduced relative abundance of Clostridiaceae and accelerated the proliferation of lactic acid producers and ethanol producers in the UASB and PBR, which were mainly from the families Ruminococcaceae and Leuconostocaceae.

Start-up of moving bed biofilm reactors for deammonification: the role of hydraulic retention time, alkalinity and oxygen supply

Aerobic and anaerobic ammonium oxidation can be combined in a completely mixed moving bed biofilm reactor, allowing for single-stage ammonium removal from wastewater with low COD/N ratio unsuitable for conventional nitrification/denitrification processes (‘deammonification’). Mandatory preconditions are: (a) a low hydraulic retention time to wash out suspended cells competing with mass transfer limited biofilm cells for alkalinity as limiting substrate; and (b) an oxygen flux adapted to the surface loading rate to prevent complete nitrification to nitrate. pH control or ‘NH3 inhibition’ of nitrite oxidation are neither useful nor necessary. By this strategy, oxygen limited biofilms with simultaneous presence of NH4-N and NO2-N were enriched, which allowed for growth of anaerobic ammonium oxidizers. It could be demonstrated that a deammonifying reactor can be purposefully started up within a reasonable span of time and without prior inoculation, if this explicitly described strategy is applied. Depending on surface loading and air flow rate, N removal rates of 4–5 g N/m2 d could be achieved at DO concentrations between 1.0 and 4.0 mg/l.

Role of hydraulic retention time and granular medium in microbial removal in tertiary treatment reed beds

The main objective of this paper is to evaluate the role of hydraulic retention time (HRT) and granular medium in faecal coliform (FC) and somatic coliphage (SC) removal in tertiary reed beds. Experiments were carried out in a pilot plant with four parallel reed beds (horizontal subsurface flow constructed wetlands), each one containing a different type of granular medium. This pilot plant is located in a wastewater treatment plant in Montcada i Reixac, near Barcelona, in northeastern Spain. The microbial inactivation ratios obtained in the different beds are compared as a function of three selected HRTs. Secondary effluent from the wastewater treatment plant was used as the influent of the pilot system. The microbial inactivation ratio ranged between 0.1 and 2.7 log-units for FC and from 0.5 to 1.7 log-units for SC in beds with coarser granular material (5–25 mm), while it ranged between 0.7 and 3.4 log-units for FC and from 0.9 to 2.6 log-units for SC in the bed with finer material (2–13 mm). HRT and granular medium are both key factors in microbial removal in the tertiary reed beds. The microbial inactivation ratio rises as the HRT increases until it reaches a saturation value (in general at an HRT of 3 days). The value of the microbial inactivation ratio at the saturation level depends on the granular medium contained in the bed. The specific surface area necessary to reach 2–3 log-units of FC and SC is approximately 3 m 2/person-equivalent.

The role of HRT and low temperature on the acid-phase anaerobic digestion of municipal and industrial wastewaters

Two identical 3l completely mixed reactors with solids recycling capabilities were used to investigate the effects of hydraulic retention time (HRT) and low temperatures on volatile fatty acid (VFA) production. One reactor was fed with a 1:1 ratio of diluted primary sludge and a starch-rich industrial wastewater, while the other was fed with diluted primary sludge alone. The VFA and soluble COD concentrations and specific production rates reached their highest values at 30 h HRT and at 25°C. Further increase in HRT (at 25°C) or decrease in temperature (at an HRT of 30 h) resulted in lower amounts of VFA and COD produced. All parameters related to VFA and COD production were significantly higher in the industrial–municipal reactor than in the municipal-only reactor. The VFA:COD ratios were very high, with values ranging from about 0.8 to 1.0 indicating that hydrolysis was the rate-limiting step. Degradation of proteins (measured by ammonia production) was inhibited by the starch-rich wastewater in the industrial–municipal reactor, while no evidence of inhibition was found in the municipal-only reactor. This study revealed that VFA production was feasible at low temperatures (down to 8°C), particularly in the presence of the industrial waste. Ultimately, the amount VFA produced was adequate, in most cases, to support subsequent biological nutrient removal (BNR) processes.

Performance of anaerobic contact filter in series for treating distillery spentwash

Investigations were carried out by using rigid polyurethane foam as a packing material in the anaerobic contact filter (series) to treat distillery spentwash. The effect of hydraulic retention time (HRT) in treatment efficiency of reactor (I) and (II) was evaluated at different initial substrate concentrations ranging from 1500 mg/l to 19,000 mg/l. The effect of toxic parameters such as sulphate present in the distillery spentwash and the corresponding parameters such as total sulphide and un-ionized hydrogen sulphide generated during digestion of wastewater were evaluated to assess the reactor performance. The results showed that at 4 d HRT the overall COD removal percent ranged from 98% to 73% for an influent COD of 1500 mg/l to 19,000 mg/l. The overall performance of COD removal percent in reactor (I) and (II) at 2, 3 and 4 d HRT's were investigated. At 3 d HRT the reactor (II) showed a higher COD removal percent when compared to reactor (I), which clearly shows the role of hydraulic retention time in degradation of the organic matter present in the wastewater above an influent COD concentration of 5000 mg/l.

A study on the role of hydraulic retention time in eutrophication of the Asahi River Dam reservoir

The Asahi River Dam reservoir is one of the important freshwater resources for Okayama City, Japan. The eutrophication of the reservoir has been warned of since the 1980s. In this study, we discuss the relationship between hydraulic retention time and increase of phytoplankton, and the influence of wind-driven currents on the spatial distribution of phytoplankton based on the observations and numerical simulations. Observations were carried out from 1993 to 1995. The numerical simulation of hydraulics in the reservoir was carried out by applying an orthogonal curvilinear finite difference method. Judging from phosphorus and nitrogen concentrations, the water quality of the Asahi River Dam reservoir is usually eutrophic. However, high concentrations of chlorophyll-a are not always observed. The observed dependence of chlorophyll-a concentration on hydraulic retention time is reproduced fairly well by a simple relationship derived from the balance of phytoplankton based on the assumption of complete mixing. The hydraulic retention time is a limiting factor of an increase in phytoplankton in the Asahi River Dam reservoir. It takes a retention time of 2 weeks for the sufficient increase of phytoplankton. The results obtained by the simulations show that the wind-driven currents play an important role in the spatial distribution of phytoplankton.

A Study on the role of hydraulic retention time in eutrophication of the asahi river dam reservoir

The Asahi River Dam reservoir is one of the important freshwater resources for Okayama City, Japan. The eutrophication of the reservoir has been warned of since the 1980s. In this study, we discuss the relationship between hydraulic retention time and increase of phytoplankton, and the influence of wind-driven currents on the spatial distribution of phytoplankton based on the observations and numerical simulations. Observations were carried out from 1993 to 1995. The numerical simulation of hydraulics in the reservoir was carried out by applying an orthogonal curvilinear finite difference method. Judging from phosphorus and nitrogen concentrations, the water quality of the Asahi River Dam reservoir is usually eutrophic. However, high concentrations of chlorophyll-a are not always observed. The observed dependence of chlorophyll-a concentration on hydraulic retention time is reproduced fairly well by a simple relationship derived from the balance of phytoplankton based on the assumption of complete mixing. The hydraulic retention time is a limiting factor of an increase in phytoplankton in the Asahi River Dam reservoir. It takes a retention time of 2 weeks for the sufficient increase of phytoplankton. The results obtained by the simulations show that the wind-driven currents play an important role in the spatial distribution of phytoplankton.