Effect Of Effluent Recirculation Research Articles

The Effect of Effluent Recirculation in a Full-Scale Constructed Wetland System

This study deals with the effect of effluent recirculation (ER) on the pollutant removal efficacy of a full-scale, hybrid treatment system composed of a macrophyte pond and a horizontal flow constructed wetland. The average removals of 5-day biochemical oxygen demand (BOD), chemical oxygen demand (COD), total suspended solids (TSS), turbidity, total N (TN), ammonium nitrogen (NH4-N), total phosphorus (TP), sulfates, E. coli and Total coliforms (TC) for the years 2017–2018 (no recirculation), 2019 (50% recirculation) and 2021 (100% recirculation) were compared. Results show a general improvement of the effluent with ER. Removals for 0%, 50% and 100% ER, respectively, were: 59%, 61% and 66% for COD; 90%, 96% and 96% for BOD; 94%, 94% and 99% for TSS; 33%, 40% and 67% for TN; 22%, 30% and 55% for NH4-N; 92%, 98% and 96% for sulfates; 99.6%, 99.7% and 99.9% for E. coli; and 99.5%, 99.7% and 9.9% for TC. No clear effect was observed on the removal of TP and dissolved PO4-P, which were very low. 50% ER improved turbidity removal from 88% to 91%, but 100% ER provided worse results. The removal of NH4-N and TN significantly improved with 100% ER. This indicates that ER can be a simple, economic, and feasible way to upgrade the performance of full-scale natural wastewater treatment systems.

Removal of Pharmaceuticals in a Macrophyte Pond-Constructed Wetland System and the Effect of a Low Effluent Recirculation

Waste stabilization ponds and constructed wetlands (CWs) are effective at eliminating pharmaceutical residues, but removals are not usually complete. Their combination is regarded as an efficient, robust wastewater treatment method, but their efficiency in the removal of pharmaceuticals and the effect of a mild effluent recirculation has not been sufficiently studied in full-scale systems. Effluent recirculation can help to improve performance by increasing hydraulic residence time and, eventually, dissolved oxygen concentration. In this work, the presence of pharmaceuticals in wastewater from a university campus, their removal in a macrophyte pond–CW system, and the effect of effluent recirculation on removal and ecological risk were evaluated. Stimulants (caffeine and nicotine) and non-steroidal anti-inflammatories (naproxen and ibuprofen) were the most detected compounds in the influent and showed the highest concentrations, ranging from 0.5 to 300 µg·L−1. The pond–CW combination showed notable elimination for these compounds, achieving 87% on average. The ecological risk was also reduced by between 5.5 and 12.4 times, but it was still over values that indicates high ecological risk, mainly because of the concentrations of nicotine and ibuprofen. The effect of effluent recirculation was not as high as expected since the removals of caffeine, paraxanthine and naproxen were significantly improved, but those of atenolol and ibuprofen were lower. These results suggest that a higher recirculation ratio should be tested.

Effect of effluent recirculation on nutrients and organics removal performance of hybrid constructed wetlands: Landfill leachate treatment

Landfill leachate treatment is challenging because of complex chemical composition. This study identifies the factors associated with organics and nutrients removal performance improvement in construction or organic materials based four hybrid wetland systems dosed with landfill leachate. The imbalanced pollutant composition ratio of leachate influenced removal pathways in the hybrid wetlands; effluent recirculation (50 and 25% ratios) adjusted leachate composition and improved removal performances. Mean nitrogen (N), phosphorus (P), biochemical oxygen demand (BOD), and chemical oxygen demand (COD) removal percentages in the hybrid wetlands during without, with recirculation periods ranged between 50 and 93, 69–100, 34–89, and 55–76%, respectively. Pollutant removals were achieved through adsorption, microbial (heterotrophic organic removal, ammonia assimilation, nitrification, denitrification) degradation, and plant uptake. Removal rates did not vary substantially among organic or construction materials based first stage VF (vertical flow) wetlands because of leachate composition. Constant water saturation depth variation influenced pollutant removal rates among the second stage wetlands. Shallow water depth HF (horizontal flow) wetlands appeared to be a promising wastewater treatment technology due to the coexistence of unsaturated, saturated bed matrix that supported diverse removal pathways.

Removal of nutrients from domestic wastewater using constructed wetlands: assessment of suitable environmental and operational conditions

Constructed wetlands (CWs) offer an eco-friendly wastewater treatment technology primarily for decentralized locations. They support a dense growth of macrophytes which help in the reduction of water velocity, development of conducive microenvironments and provide adherence sites for microorganisms to develop biofilms. Several environmental and operational parameters are crucial for the efficient working of CWs wherein, suitable pH, temperature and dissolved oxygen (DO) are more significant. The performance of CWs has been enhanced significantly through effluent recirculation and artificial aeration. Removal of phosphorus proceeds via adsorption within media material, sedimentation, cation exchange, precipitation and uptake by macrophytes and the removal of nitrogen occurs mainly by microbial communities and macrophytic uptake. In colder climates, the bioaugmentation of microbial communities is required to increase the treatment efficiency. Various previous research findings showed several aspects of CWs that could potentially affect removal efficiency of the nutrients from the wastewater. However, maintaining suitable environmental and operational conditions for the effective operation of CWs remains a challenge. Therefore, the objectives of this review-based study are to provide the most favorable environmental conditions for the effective operation of CWs. Recent developments in operational and working parameters of CWs focusing on the selection of macrophytes and substrate material, carbon source, feeding mode, hydraulic loading rates and retention times are discussed. The study also provides effect of effluent recirculation, bioaugmentation of microbes, suitable C/N ratio and artificial aeration for the domestic wastewater treatment.

Blackwater biomethane recovery using a thermophilic upflow anaerobic sludge blanket reactor: Impacts of effluent recirculation on reactor performance

Thermophilic anaerobic digestion is a promising process for high-solid blackwater (BW) treatment due to improved hydrolysis rates, high methanogenesis efficiency, and pathogen removal, when compared with mesophilic treatment. In the present work, the effects of effluent recirculation (i.e., mixing) on thermophilic blackwater treatment were studied. A laboratory-scale thermophilic upflow anaerobic sludge blanket reactor was operated with and without effluent recirculation. The methanogenesis efficiency of the BW treatment increased from 45.0 ± 2.9% when effluent recirculation was applied to 56.7 ± 5.5% without effluent recirculation. Without effluent recirculation, the COD accumulation in the bioreactor was reduced from 17.2 to 3.8% and the effluent volatile fatty acids (VFA) concentration was reduced from 0.64 ± 0.18 to 0.15 ± 0.10 g/L. Further, both acetoclastic and hydrogenotrophic methanogenic activity increased from 101.3 ± 10.8 and 93.9 ± 6.1 to 120.4 ± 9.4 and 118.2 ± 13.2 mg CH4-COD/(gVSS⋅d), respectively, after effluent recirculation was discontinued. The predominant methanogens changed from Methanothermobacter (67%) with effluent recirculation to Methanosarcina (62%) without effluent recirculation. As compared to the effluent recirculation conditions, the enhanced biomethane recovery and treatment performance without effluent recirculation can be attributed to the close proximity of bacteria and archaea groups and the reduced VFA accumulation. Predicted functional gene comparison showed higher prevalence of function for intermediate metabolite transportation (transporters, ATP-binding cassette (ABC) transporters, and two-component system) after discontinuing effluent circulation, which contributed to improved syntrophic propionate oxidation and syntrophic acetate oxidization and enhanced hydrogenotrophic methanogenesis.

Effluent recirculation enables near-complete oxidation of organics during supercritical water oxidation at mild conditions: A proof of principle

This work presents a continuous set-up for SCWO, which was operated at mild conditions (380 °C, 25 MPa, oxidant equivalence ratio of 2.0 and residence time of 26 s) to oxidize cellulose, lignin, and acetic acid as model compounds. The aim was to oxidize different organics consecutively to near completion in the same mild reaction conditions and set-up. These conditions can overcome some drawbacks associated to SCWO. To combine near complete oxidation with the applied mild process conditions, aqueous effluent from SCWO, containing intermediates from incomplete oxidation, was recycled for consecutive oxidation. Meanwhile, fresh feedstock was continuously fed to retain the process capacity. Upon recycling the aqueous effluent three to four times, depending on the feedstock, the oxidation efficiency increased from 63.9%, 45.3% and 28.3% in a single pass for cellulose, lignin, and acetic acid, respectively, to near 100%. The principle of effluent recirculation should allow a compact set-up to perform almost complete oxidation of different organics at mild conditions. The principles and effects of effluent recirculation are outlined, as well as practical consequences and perspectives of this novel principle to SCWO.

Enhancement of total nitrogen removal through effluent recirculation and fate of PPCPs in a hybrid constructed wetland system treating urban wastewater.

The effect of effluent recirculation on the removal of total nitrogen (TN) and eight pharmaceuticals and personal care products (PPCPs) was evaluated during 9months in an experimental hybrid constructed wetland (CW) system applied in the treatment of urban wastewater. An Imhoff tank was followed by three stages of CWs (two 1.5-m2 vertical subsurface flow (VF) beds alternating feed-rest cycles, a 2-m2 horizontal (HF) and a 2-m2 free water surface (FWS) wetland in series). A fraction of the final effluent was recycled back to the Imhoff tank with a recirculation rate of 50% (hydraulic loading rate=0.37md-1). The system's performance varied throughout the study. In Period I (summer) consistently high load removal efficiencies of TN (89±5%) and a removal rate of 6.6±1.4gTNm-2d-1 were exhibited. In Period II (fall), the poor performance of the FWS during the senescence of macrophytes caused a large increase in organic matter, solids and nutrient concentrations, drastically deteriorating water quality. The determination of PPCPs was conducted during this period. Recalcitrant compounds, namely sulfamethoxazole, carbamazapine, TCEP and sucralose were negligibly removed in all CWs. However, noteworthy was the ≈30% removal of sucralose in the VF wetland. Caffeine (80%) and fluoxetine (27%) showed similar elimination rates in both VF and HF units, whereas trimethoprim and DEET were significantly better removed in the VF than in the HF. The concentration of the four latter compounds showed a severe increase in the FWS, indicating possible desorption from the sediment/biomass during adverse conditions. Harvesting of the aboveground biomass in this unit returned the system's performance back to normality (Period III), achieving 77±7% TN removal despite the winter season, proving effluent recirculation as an effective strategy for TN removal in hybrid CW systems when stringent restrictions are in place.

Biomethanation of vegetable market waste in an anaerobic baffled reactor: Effect of effluent recirculation and carbon mass balance analysis

In the present study, feasibility of biomethanation of vegetable market waste in a 4-chambered anaerobic baffled reactor (ABR) was investigated at 30d hydraulic retention time and organic loading rate of 0.5gVS/L/d for one year. Indicators of process stability viz., butyrate/acetate and propionate/acetate ratios were consistent with phase separation in the different chambers, which remained unaltered even during recirculation of effluent. Chemical oxygen demand (COD) and volatile solids (VS) removal efficiencies were observed to be consistently high (above 90%). Corresponding biogas and methane yields of 0.7–0.8L/g VS added/d and 0.42–52L/g VS added/d respectively were among the highest reported in case of AD of vegetable waste in an ABR. Process efficiency of the ABR for vegetable waste methanation, which is indicated by carbon recovery factor showed that, nearly 96.7% of the input carbon considered for mass balance was accounted for in the product.

Comparison of nutrient removal and bacterial communities between natural zeolite-based and volcanic rock-based vertical flow constructed wetlands treating piggery wastewater

This study designed two vertical flow constructed wetlands (VFCW) as a secondary treatment for piggery wastewater. The effect of effluent recirculation and substrate type on treatment efficiency of constructed wetlands on piggery waster was firstly investigated. Recirculation considerably improved the removal of NH4-N, total nitrogen (TN), and total phosphorus (TP), but not that of chemical oxygen demand (COD). The natural zeolite-based VFCW performed better than volcanic rock-based VFCW for all four parameters. At the highest recirculation ratio, the average treatment efficiencies of natural zeolite-based VFCW reached 50.7, 91.6, 48.3, and 80.7% for COD, NH4-N, TN, and TP, respectively. The diversity and composition of total bacteria and ammonia-oxidizing bacteria (AOB) in the bioflim of natural zeolite and volcanic rock were then determined. Both total bacteria and AOB in biofilm of natural zeolite were more diverse than those in biofilm of volcanic rock. Phylogenetic analyses further revealed two points: first, genera Alcaligenes and Comamonas dominated in the β-proteobacteria of natural zeolite and volcanic rock, respectively, and second, genera Nitrosospira and Nitrosomonas dominated in AOB of natural zeolite and volcanic rock, respectively. Our results suggest that natural zeolite-based VFCW with effluent recirculation is a good choice for piggery wastewater treatment. In addition, substrate type not only influences nutrients removal, but also plays a part on shaping microbial communities in constructed wetlands.

Effluent recirculation to improve perchlorate reduction in a fixed biofilm reactor

The effect of effluent recirculation on perchlorate reduction in a nominally plug-flow fixed biofilm reactor was studied in two cases: influent concentrations of 10 and 400 microg/L at low hydraulic loading rates (1.9 and 37.5 m(3)/m(2)/day without and with recirculation, respectively) and after a step increase in perchlorate concentration to 1,000 microg/L at the higher hydraulic loading rate (5 and 100 m(3)/m(2)/day without and with recirculation, respectively). Complete perchlorate reduction was sustained for influent concentrations of 400 and 10 microg/L in both flow regimes at the lower hydraulic loading rates. Reactor tracer profiles showed that biofilm diffusion had a more significant effect on mass transfer in the plug flow reactor compared with recirculation. The recirculation bioreactor acclimated more rapidly to increased hydraulic and perchlorate mass loading rates with significantly lower effluent perchlorate compared to the plug flow reactor: 16 microg/L versus 46 microg/L, respectively, although complete perchlorate removal was not achieved in either flow regime after 21 days acclimation to the higher loading. Total biofilm mass was more uniformly distributed in the recirculation reactor which may have contributed to better performance under increased perchlorate loading.

Effects of effluent recirculation in vertical-flow constructed wetland on treatment efficiency of livestock wastewater

Enhancing the treatment efficiency of livestock wastewater by effluent recirculation is investigated in a pilot-scale vertical-flow constructed wetland. The wetland system is composed of downflow and upflow stages, on which narrow-leaf Phragmites communis and common reed Phragmites typhia are planted, respectively; each stage has a dimension of 4 m(2) (2 m x 2 m). Wastewater from the facultative pond is fed into the system intermittently at a flow rate of 0.4 m(3)/d. Recirculation rates of 0, 25%, 50%0, 100% and 150% are adopted to evaluate the effect of the recirculation rate on pollutants removal. This shows that with effluent recirculation the average removal efficiencies of NH4-N, BOD5 and SS obviously increase to 61.7%, 81.3%, and 77.1%, respectively, in comparison with the values of 35.6%o, 50.2%, and 49.3% without effluent recirculation. But the improvement of TP removal is slight, only from 42.3% to 48.9%. The variations of NH4-N, DO and oxidation-reduction potential (ORP) of inflow and outflow reveal that the adoption of effluent recirculation is beneficial to the formation of oxide environment in wetland. The exponential relationships with excellent correlation coefficients (R(2) > 0.93) are found between the removal rates of NH4-N and BOD5 and the recirculation rates. With recirculation the pH value of the outflow decreases as the alkalinity is consumed by the gradually enhanced nitrification process. When recirculation rate is kept constant at 100%, the ambient temperature appears to affect NH4-N removal, but does not have significant influence on BOD5 removal.

Intermittent vs continuous operation of upflow anaerobic sludge bed reactors for dairy wastewater and related microbial changes

This work compares continuous vs intermittent UASB reactors inoculated with flocculent sludge for the treatment of dairy effluents. The effects of effluent recirculation on the performance of intermittent reactors were assessed as well as the differences in specific methanogenic activity (SMA) with different substrates for the biomass from continuous and intermittent UASB reactors. Compared to the continuous operation the intermittent operation resulted in higher methanization of the removed COD (64-78% and 65-88%, respectively) whilst the effluent recirculation presented beneficial effects when applied during the stabilization period and was clearly detrimental when applied during the feed period of the intermittent operation. The SMA tests showed that the intermittent operation causes a shift in the microbial populations towards a better adaptation for the degradation of complex substrates confirmed by the meaningfull contribution of methane production through a pathway other than acetoclastic methanogenesis observed in the biomass taken from intermittent UASB reactors.

Effect of effluent recirculation on the performance of a reed bed system treating agricultural wastewater

Effluent recirculation was investigated in a vertical flow reed bed system treating an agricultural wastewater. A comparison was made for the efficacy of this system before and after recirculation was adopted. Recirculation considerably improved the removal of BOD 5, COD, suspended solids (SS) and NH 4-N. The most significant improvement was made for NH 4-N, and nitrification of NH 4-N into NO 2-N and NO 3-N only took place when recirculation was employed. The removal of PO 4-P, however, was only slightly improved by recirculation. Generally, the removal rate of NH 4-N increased proportionally with its loading onto the system. With recirculation the pH value of the wastewater gradually decreased as a result of alkalinity being consumed during the nitrification process, whereas without recirculation the pH remained virtually unchanged. When recirculation was employed, the ambient temperature did not have a significant influence on the removal of BOD 5 but appeared to affect NH 4-N removal.

An empirical model for predicting the organic concentration of anaerobic filter effluents

The results of experiments in treating domestic wastewater and soybean processing waste‐water in two laboratory‐scale anaerobic filters are described. An empirical model, with inlet chemical oxygen demand (COD) concentration Si and hydraulic retention time (HRT) as two independent variables to predict the outlet COD concentration of the anaerobic filters, was found to adequately describe the anaerobic filter systems. Good agreement was obtained between the prediction using the current model and the experimental results of pilot‐ and full‐scale anaerobic filters for the treatment of soybean processing wastewater. The current model, which is able to show the predominant parameter (Si or HRT) determining the outlet organic concentration, is a modification of Young and McCarty's (1969) model. The current model also implies the possible effect of effluent recirculation on the performance of an anaerobic filter.

The effect of effluent recirculation on the performance of a two stage anaerobic process

The effect of effluent recirculation on the performance of a two stage anaerobic process

Effect of effluent recirculation on start-up and steady state operation of the downflow stationary fixed film (DSFF) reactor

In this study, the effect of effluent recirculation on the start-up and steady state operation of the DSFF is reported. Recirculation had little effect on start-up; however, the maximum loading rate which could be applied to the reactor was higher at higher rates of effluent recirculation.