Effluent Recirculation Research Articles

Acid Mine Drainage Treatment with Organic Waste in Constructed Wetlands: Effluent Recirculation

Acid Mine Drainage Treatment with Organic Waste in Constructed Wetlands: Effluent Recirculation

Recent advances and prospects of constructed wetlands in cold climates: a review from 2013 to 2023.

Constructed wetland (CW), a promising, environmentally responsible, and effective green ecological treatment technology, is actively involved in the treatment of various forms of wastewater. Low temperatures will, however, lead to issues including plant dormancy, decreased microbial activity, and ice formation in CWs, which will influence how well CWs process wastewater. Applying CWs successfully and continuously in cold areas is extremely difficult. Therefore, it is crucial to find solutions for the pressing issue of increasing the CWs' ability to process wastewater at low temperatures. This review focuses on the effect of cold climate on CWs (plants, substrates, microorganisms, removal effect of pollutants). It meticulously outlines current strategies to enhance CWs' performance under low-temperature conditions, including modifications for the improvement and optimization of the internal components (i.e., plant and substrate selection, bio-augmentation) and enhancement of the external operation conditions of CWs (such as process combination, effluent recirculation, aeration, heat preservation, and operation parameter optimization). Finally, future perspectives on potential research directions and technological innovations that could strengthen CWs' performance in cold climates are prospected. This review aims to contribute valuable insights into the operation strategies, widespread implementation, and subsequent study of CWs in colder climate regions.

Assigning strategic COD flow to enhance denitrification coupled with anaerobic digestion and anammox for systematic upgradation of advanced nitrogen removal from food waste digestate

Anammox is a low-carbon and efficient technology for treating high-strength ammonium in food waste digestate (FWD). However, achieving superior low-nitrogen effluent levels to meet strict discharge standards remains challenging. This study proposes an anammox-based process that integrates simultaneous denitrification and methanogenesis (SDM), partial nitrification (PN), and simultaneous anammox and denitrification (SAD) to realize advanced nitrogen removal from FWD by strategically utilizing influent organic matter. The SAD process achieved an inorganic nitrogen removal efficiency of 94.0 %, with the anammox reaction contributing 96.0 %. Residual nitrate in the SAD effluent was recirculated into SDM, maintaining a removal efficiency of 95 %–97 %. Profiling wastewater and material flows in the integrated process showed a 97.2 % nitrogen removal efficiency, with effluent nitrogen levels meeting discharge standards (<70 mg/L for municipal sewer systems). 3D-fluorescence analysis indicated that quasi-humic acid was effectively removed through SDM. Soluble microbial by-products of FWD served as carbon source for the denitrifying bacteria in SAD. Controlling influent bypass flow and effluent recirculation allowed for strategic allocation of chemical oxygen demand (COD) flow, achieving a total removal efficiency of 71.8 %. SDM removed 45.8 % COD, followed by PN at 13.4 % and SAD at 12.6 %. Compared with those of conventional nitrification and denitrification process, the economic cost, greenhouse gas emission, and total environmental impact of this integrated process were reduced by 85.8 %, 35.8 %, and 84.1 %, respectively. This study provides new insights into the application of anammox-based process for attaining advanced nitrogen removal while minimizing environmental impact.

Horizontal flow aerated constructed wetlands for municipal wastewater treatment: The influence of bed depth

The influence of bed depth on the performance of aerated horizontal constructed wetlands was investigated at the pilot plant scale. Two horizontal flow subsurface constructed wetlands (HF) intensified units of different bed depth (HF1: 0.90 m and HF2: 0.55 m, 0.8 m and 0.5 m water level, respectively) were fitted with forced aeration, while a third one (HFc, 0.55 m bed depth, 0.5 m water level) was used as control and not aerated. The three HF units were operated in parallel, receiving the same municipal wastewater pre-treated in a hydrolytic up-flow sludge blanket anaerobic digester. Applied surface loading rates (SLR) ranged from 20 to 80 g biochemical oxygen demand (BOD5)/m2·d and from 3.7 to 6.7 g total nitrogen (TN)/m2·d, while it ranges from 6 to 23 g BOD5/m2·d and from 1.1 to 1.7 g TN/m2·d in the control unit. Removal of total suspended solids (TSS) and BOD5 was usually close to a 100 % in all units, whilst chemical oxygen demand (COD) removal was higher for the HF1 unit (97 % on average, range of 96–99 %) than for HF2 (92 %, 82–98 %) and HFc (94 %, 86–99 %). TN removal reached on average 33 % (16–43 %) in HFc, 37 % (10–76 %) in HF2 and 51 % (21–79 %) in HF1. High TN removal required a longer aeration time for nitrification and higher effluent recirculation ratio to enhance denitrification. The results indicate that artificial aeration and a high bed depth allows to increase the SLR by a factor of 4 in HF1 but only by a factor of 2 in HF2.

Enhancing pharmaceutical removal in a full-scale constructed wetland with effluent recirculation

Emerging organic micropollutants pose a threat to aquatic environments even at trace levels. Among the many different groups of emerging pollutants, pharmaceutical residues are of special concern because of their toxicity and long-term effects on biota. Natural wastewater treatment systems are effective at eliminating pharmaceuticals, but removals are usually incomplete. In this regard, effluent recirculation can help to improve the removal of pharmaceuticals in natural wastewater treatment systems with the goal of producing a better-quality effluent for reuse. This is particularly interesting for water-scarce regions such as semi-arid islands of this study. The obtained results provide evidence that effluent recirculation can significantly improve the removal of pharmaceuticals in natural wastewater treatment systems. Of the 11 compounds studied, the highest concentrations and detection frequencies (in decreasing order) were those of caffeine, paraxanthine, nicotine, ibuprofen, naproxen and atenolol. The average removals increased from 87.5% (no recirculation) to 97% (100% recirculation ratio), and these results are associated with an improved ammonium and total N removal with 100% recirculation. To the knowledge of the authors, this is the first study showing the positive effect of effluent recirculation on the removal of pharmaceuticals in a full-scale natural wastewater treatment system.

Analysing the mechanism of food waste anaerobic digestion enhanced by iron oxide in a continuous two-stage process

The food waste (FW) digestion performance can be enhanced by introducing iron oxide (IO) into digesters. However, the role of IO in continuous two-stage digesters in enhancing the FW anaerobic digestion remains unclear. In this study, the effect of IO on the bioenergy recovery from a two-stage digestion process was investigated. The bioenergy recovery was significantly increased by up to 208.43 % with IO addition. The activities of dehydrogenase, α-amylase, and protease increase by 0.82–1.44, 7.24–14.56 and 7.97–20.45 times, respectively, as compared with that of the blank. With IO addition, the metabolic pathway in hydrolytic–acidogenic (HA) reactor shifted from lactic acid fermentation to butyric fermentation, which promoted stable methane production in methanogenic (MG) reactor. The activity of coenzyme F420 increased by 19.19–39.01 times, indicating that IO facilitated FW digestion by promoting hydrogenotrophic methanogenesis. The enhancement in the enzyme activity was attributable to the Fe2+ generated by dissimilatory iron reduction. According to the microbial analysis, IO enhanced interspecies hydrogen transfer between Methanobacterium and Syntrophomonas. Furthermore, IO improved direct interspecies electron transfer between Geobacter sulfurreducens and Methanosarcina. The effluent recirculation strategy greatly facilitated the hydrolysis and acidification of FW, which was critical for improving the two-stage process performance.

Biodigestion of easily-acidifying cheese whey in a sequential fermentative-methanogenic process: Strategies for boosting energy production and minimizing alkalinization costs

The biodigestion of cheese whey towards enhanced bioenergy production still challenges researchers, considering incipient biohydrogen (bioH2) production during fermentation and the great dependence on an efficient (usually high-cost) alkalinization strategy to achieve high and stable methane production. Using a sequential fermentative-methanogenic process in fixed-film reactors, this study assessed strategies to boost bioH2 production (pH control at ca. 5.5 + prevention of biomass accumulation) and achieve a low-cost stable methanogenic process (replacing NaHCO3 dosing by effluent recirculation and further eliminating any alkalinization approaches). Maintaining fermentation pH values between 5.0 and 5.5 favored bioH2 and butyrate production, but the enhanced establishment of homoacetogenesis (accounting for 70–100 % of the acetate produced) associated with the selective removal of hydrogen-producing bacteria in biomass discharges resulted in a marked unstable hydrogenogenic activity. Processing fermented cheese whey without applying any alkalinization strategy in methanogenesis was further demonstrated to be technically feasible at an OLR of 5.0 kg COD m−3 d−1. COD removal (70–75 %) and the methane yield (150–170 NmL CH4 g−1COD) were equivalent regardless of the alkalinization (NaHCO3 dosing or effluent recirculation) or non-alkalinization approach. However, the alkalinization-derived production cost of methane was reduced from 1.55 USD Nm−3CH4 to zero when fully removing NaHCO3.

Digestate recirculation rate optimization for the enhancement of hydrogen production: The case of disposable nappies and fruit/vegetable waste valorization in a mesophilic two-stage anaerobic digestion system

After the anaerobic digestion involvement in waste management, several attempts have been made for the biogas yields enhancement. The implementation of methanogenic effluent (digestate) recirculation has been proposed as an alternative for the pH adjustment in two-stage reactors, leading to decreased alkali solution requirements, which could subsequently reduce operational costs. In the current study, a mixture of 40–60% (w/w) fruits/vegetable waste-disposable nappies hydrolysate was valorized through the anaerobic digestion process. The digestate recirculation rate (RR) optimization took place in a continuous acidogenic reactor, followed by the two-stage system operation under various experimental conditions. Particularly, the RRs of 0-0.25-0.5-0.75-1 were tested, indicating up to 208% H2 production increase (RR0.25). The single- and two-stage configurations comparison followed, under the hydraulic retention time (HRT) of 25 d, while the two-stage operation was evaluated for pH adjustment (5.5) not only with alkali solution but also with the applied RR0.5 (reduced alkali solution requirements compared to RR0.25). During the two-stage system operation under RR, additional scenarios were investigated regarding the HRTmethanogenic reduction and the pH adjustment, while using exclusively digestate. Although the RR implementation seems promising, the digestate alkalinity content can significantly determine the system's efficiency.

Anaerobic and microaerobic biodegradation of benzene: Effect of important intermediates

The effect of important intermediates on the anaerobic and microaerobic benzene biodegradation was investigated. An up-flow anaerobic/microaerobic sludge blanket reactor was operated at a hydraulic retention time (HRT) of 24 h and fed with water contaminated with benzene (∼4.2 mg L−1) and ethanol (1 g COD L−1). Initially, reactor operating parameters such as variation of applied flow rates, microaeration application points, and effluent recirculation were studied to optimize microaeration. Secondly, the interference of benzene degradation intermediates (phenol, benzoate, and toluene) was investigated. The addition of low concentrations of oxygen (0.5–2 mL air min−1) ensured high benzene removals (∼76–86 %), with the best obtained for a flow rate of 1 mL of air min−1, for which there was a d% removal increase compared to the anaerobic stage. Although the contact time between the air bubbles was short, the best application point was close to the feed. Effluent recirculation was necessary to maintain good mass transfer and high benzene removal efficiencies. The loss by stripping was considered negligible, even at the microaerobic stages, proving the biological removal of this compound. Finally, the intermediates added to the contaminated water reduced benzene removal, with benzoate being the compound that most negatively influenced the benzene removal process, even under microaerobic conditions. Therefore, it is a key intermediate in the benzene anaerobic and microaerobic biodegradation process.

Biogas Production and Metagenomic Analysis in a New Hybrid Anaerobic Labyrinth-Flow Bioreactor Treating Dairy Wastewater

Increasing worldwide milk manufacturing and dairy processing resulted in producing more effluents, and thus effective management of wastewater is now the most important issue. This study used a new design of a pilot plant-scale hybrid anaerobic labyrinth-flow bioreactor (AL-FB) to increase the efficiency of anaerobic biodegradation and biogas productivity and improve anaerobic microflora performance. In addition, effluent recirculation was used to boost the treatment of dairy wastewater. Metagenomic analyses of the anaerobic microbial community were performed. It was found that an organic loading rate (OLR) of 4.0–8.0 g COD/L·d contributed to the highest CH4 yield of 0.18 ± 0.01–0.23 ± 0.02 L CH4/g COD removed, which corresponded to a high COD removal of 87.5 ± 2.8–94.1 ± 1.3%. The evenest distribution of the microorganisms’ phyla determined the highest biogas production. In all tested samples, Bacteroidetes and Firmicutes abundance was the highest, and Archaea accounted for about 4%. Metagenomic studies showed that methane was mainly produced in acetoclastic methanogenesis; however, higher OLRs were more favorable for enhanced hydrogenotrophic methanogenesis. Effluent recirculation enhanced the overall treatment. Thus, at OLR of 10.0 g COD/L·d, the highest COD removal was 89.2 ± 0.4%, and methane production yield achieved 0.20 ± 0.01 L CH4/g COD removed, which was higher by 25% compared to the achievements without recirculation.

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.

Iodine Removal from Carbonate-Containing Alkaline Liquids Using Strong Base Resins, Hybrid Resins, and Silver Precipitation

The ability of several material types to remove aqueous iodine from a mildly alkaline, carbonate-rich nuclear waste stream was evaluated: strong base anion exchange resins (SBARs), hybrid resins, Ag-containing materials, and Bi-containing hybrid resins. A combination of batch testing and flow-through column testing was used in the evaluation. In batch testing, hybrid resins CHM-20, SIR-110-CE, and RTBI were shown to have high efficiency for the removal of both iodide and iodate simultaneously, while Ag-containing materials and SBAR demonstrated high capacity for iodide removal. One example of each material type (CHM-20, A532E, and Ionex 400) was further evaluated for their sorption isotherms and column performance. The Langmuir isotherm, or a Langmuir–Freundlich hybrid isotherm, best described the sorption of iodide to the CHM-20 hybrid resin and Purolite A532E. The Freundlich isotherm best described the uptake of iodate to CHM-20 and A532E and for both iodide and iodate to Ag-containing Ionex 400. In column testing, Purolite A532E had exceptional performance for overall iodide removal. However, with the capacity demonstrated, the A532E resin would exceed class C waste classification before breakthrough initiated, and column change-outs in processing would be dictated by eventual waste classification, not breakthrough. Ionex 400, a Ag-zeolite, was observed to degrade over time under mild alkaline column conditions, whereas the hybrid CHM-20 was limited in the single pass through design and would be best suited for applications where iodide and iodate are present and recirculation of the column effluent is feasible. This work highlights the feasibility of commercially available materials to separate radioiodine from liquid environments.

Extraction of Heavy Metals from Soil Affected by Landfill Leachate through Constructed Wetlands: A Phytoremediation Approach to Rejuvenating the Contaminated Environment

Water is one of the most essential elements of life. The water shortage is becoming a lurid issue in many regions, with over a billion people without passable water for drinking purposes. The leachate from landfill sites is a major problem and poses a threat to aquatic ecosystems and public health. To overcome this situation, either to remove contaminants or to reduce the amount of contamination, constructed wetlands using phytoremediation can be considered the best solution. This green low-cost technology uses plants to remove heavy metals from soil and water. The objective of this report is to study the removal of specific heavy metals such as Zinc (Zn), Nickel (Ni), Chromium (Cr), Cadmium (Cd), Iron (Fe) and Lead (Pb) from landfill leachate by using two laboratory scaled wetlands. These wetlands were filled with soil and planted with Typha Latifolia. One system was operated without recirculation and the other with effluent recirculation with an interval of one day. The influent and effluent physicochemical parameters were analyzed after 30 days and the concentrations of the heavy metals were observed. The wide variation is seen in the case of Nickel, Lead, Chromium, Cadmium, Zinc and Iron. The percentage of removal with recirculation and without recirculation is 100% for Cadmium and Iron, in the case of Nickel, Lead, Chromium and Zinc the percentage difference between recirculation and without recirculation was found to be 1.6, 2.4, 0, 0. Since the removal efficiency for Cadmium and Iron is predominant this study indicates that this technology gives good removal of heavy metals and has a scope for its effective analysis since it has low working and conservation costs; it is comparatively a step toward a sustainable economy.

Biochar regulates enzymes activity and interspecies electron transfer to promote bioenergy recovery from a continuous two-stage food waste anaerobic digestion process

The influence of biochar on the performance of food waste anaerobic digestion in a continuous two-stage process was examined. Biochar was added to a methanogenic (MG) reactor, with 50% of its effluent recirculated to a hydrolytic–acidogenic (HA) reactor. The addition of biochar in the MG reactor simultaneously enhanced the performance of both reactors, and the energy recovery efficiency increased by 25.01–70.99%. Biochar improved the activities of protease and α-amylase in the HA and MG reactors by 1.08–2.23 and 0.46–1.21, and 13.65–15.76 and 8.73–13.64 times, respectively. The enhancement of these hydrolytic enzymes' activity in the HA reactor was correlated to the effluent recirculation from the MG reactor, contributing to the improved hydrolysis of food waste. Furthermore, the activity of coenzyme F420 in the MG reactor increased by 11.47%, 18.81%, and 27.03%, respectively, indicating that biochar can promote hydrogenotrophic methanogenesis. Microbial analysis revealed that the presence of biochar promoted interspecies hydrogen transfer between Syntrophomonas and hydrogenotrophic methanogens. Additionally, biochar triggered direct interspecies electron transfer between Geobacter sulfurreducens and acetotrophic methanogens, enabling effective methanogenesis.

Anaerobic digestion of vinasse and water treatment plant sludge increases methane production and stability of UASB reactors

Studies are still needed to increase the stability and efficiency of methane production from vinasse. Therefore, operations strategies, such as the anaerobic digestion with one or more wastes and adding micronutrients, especially iron, become attractive. The performance of two treatment systems, each one composed of two UASB reactors in series, operated under mesophilic (R1M and R2M) and thermophilic (R1T and R2T) temperature conditions, was evaluated in the anaerobic digestion of vinasse (ADV). First, the reactors were operated with the effluent recirculation and increasing organic loading rate (OLR) up to 20 g CODtotal L−1d−1 in the R1M and R1T. Then, the anaerobic digestion of vinasse and water treatment plant (WTP) sludge (ADVS) was performed in the proportions of 25:75 to 50:50 (% v/v) in both systems. In the ADV, applying the highest OLR, the mesophilic and thermophilic reactors instabilities happened. The ADVS of over 35% of WTP sludge promoted the recovery of the mesophilic and thermophilic UASB reactors with significantly reduced total volatile acids and increased alkalinity and biogas production. The higher average values of the volumetric methane production (VMP) occurred in the ADVS at 50% of WTP; in the R1M and R1T, they were 3.23 and 3.00 L CH4 L−1d−1, respectively. In the ADV, the thermophilic system presented higher VMP concerning the mesophilic for OLR up to 15 g CODtotal L−1d−1. For higher OLR, the mesophilic system showed better carrying capacity and stability. The ADVS with above 35% of WTP sludge promoted similar benefits in the two systems, with no significant differences in CODtotal removal and VMP. Therefore, adding iron by WTP sludge in ADVS improves methane production and increases the stability of UASB reactors under mesophilic and thermophilic conditions.

Effect of organic loading rate and effluent recirculation on biogas production of desulfated skim latex serum using up-flow anaerobic sludge blanket reactor

High sulfate contents in skim latex serum (SLS) can be reduced by rubber wood ash (RWA). Subsequently, the desulfated skim latex serum (DSLS) can be further anaerobically treated more effectively with the accompanying generated biomethane. In this study, DSLS was treated using an up-flow anaerobic sludge blanket (UASB) reactor operated at 10-day HRT and under mesophilic (37 °C) conditions. The effect of organic loading rates (OLR) at 0.89, 1.79 and 3.57 g-COD/L-reactor∙d on DSLS biodegradability was investigated in Phase I-IV using NaHCO3 as an external buffering agent. Maximum methane production yield of 226.35 mL-CH4/g-CODadded corresponding to 403.25 mL-CH4/L reactor·d was achieved at the suitable OLR of 1.79 g-COD/L-reactor∙d. UASB effluent recirculation which was then applied to replace the NaHCO3. It was found that with 53% effluent recirculation similar to an OLR of 2.01 g-COD/L-reactor∙d, an average of 185.70 mL-CH4/g-CODadded corresponding to 371.40 mL/L reactor·d of methane production was reached. The dominant bacteria in UASB reactor were members of Proteobacteria, Bacteroidota, Firmicutes, and Desulfobacterota phyla. Meanwhile, the archaeal community was majorly dominated by the genera Methanosaeta sp. and Methanomethylovorans sp. The study clearly indicates the capabilities of UASB reactor with effluent recirculation to treat DSLS anaerobically.

Evaluating granulometry and metal content in sludge from UASB reactors treating sugarcane vinasse

Few studies have been carried out on sludge blankets in treatment systems. However, the sludge blanket in a UASB system is the area where microorganisms responsible for converting organic matter into biogas are concentrated. Considering its importance, the present research aimed to study the sludge blanket of three UASB reactors used in the treatment of sugarcane vinasse (pilot scale). The main difference between them was the applied effluent recirculation rate (100 %, 200 %, and 400 %). The research objective was to analyze the sludge blanket of these reactors: to determine the granulometry; the concentration of zinc, lead, cadmium, nickel, iron, manganese, copper, total chromium, calcium, magnesium, sodium, and potassium; and the organic and inorganic contents, aiming to identify the physical-chemical alterations in the sludge caused during the operational period. A reduction in the granular diameter was observed in all systems up to the range between 0.4 and 1 mm, provided by the increase in the hydraulic load, the organic loading rate and biogas production, and the increase in the mineral fraction in the sludge blanket, associated with the metal contribution of vinasse to the sludge blanket. Metal accumulation such as magnesium, sodium, calcium, and potassium was not detrimental to the methanogenic process due to the strategy of adapting biomass to vinasse, by gradually increasing the vinasse load, and the effects of antagonisms between magnesium and sodium and between potassium and sodium, which reduced its toxic effects.

Valorization of cannabis waste via hydrothermal carbonization: solid fuel production and characterization.

Hydrothermal carbonization (HTC) was employed to convert cannabis waste into valuable solid fuel (hydrochar) under different operating conditions, including reaction temperature (170-230°C), biomass-water ratio (1:10-1:20), and residence time of 60min. The produced hydrochar was examined for their fuel properties including calorific value (HHV), proximate and ultimate analysis, thermal stability and combustion behavior, etc. The results revealed higher HTC temperature led to a higher degree of carbonization, which is beneficial for increasing carbon content and HHV of the hydrochar. The HHV of the hydrochar improved significantly up to 24.65MJ/kg after the HTC compared to 17.50MJ/kg for cannabis waste. The energy yield of hydrochar from the HTC process was in a range of 70.41-82.23%. The optimal HTC condition was observed at 230°C and a biomass-water ratio of 1:10, producing high-quality hydrochar with 24.24MJ/kg HHV and 72.28% energy yield. The hydrochar had similar fuel characteristics to lignite coal with significantly lower ash content. Additionally, recirculation of liquid effluent showed a positive influence on the HHV of hydrochar besides minimizing the release of wastewater from the HTC process. The study revealed that HTC is a promising technique for valorization of cannabis waste into high-value solid fuel, which can be potentially an alternative to coal.

Nitrogen removal performance and microbiological characteristics for the landfill leachate treatment in a three-stage vertical flow constructed wetlands system

Landfill leachate (LL) has the characteristics of high NH 4 + -N and low C/N ratio, increasing the cost and difficulty of efficient treatment. Constructed wetlands system (CWs) has proven to be feasible for the treatment of LL. A constructed three-stage series of vertical flow CWs (termed CW1, CW2, and CW3) based on effluent recirculation (T-VF) was used to treat LL, and nitrogen removal effect and microbiological characteristics was investigated. The results show that the nitrogen removal efficiency showed a increased trend from CW1 to CW3. The average removal efficiencies of TN, NH 3 + -N, and NO 3 − -N in T-VF were 91.43%, 94.19%, and 98.11%, respectively, which showed good nitrogen removal performance. Furthermore, application of the combined CW1, CW2 and CW3 increased diversity of substrate bacterial communities and decreased the relative abundances of the dominant nitrifying(e.g. Nitrosomonas , Nitrobacter ) and heterotrophic denitrifying bacteria (e.g. Thauera ). The relative abundances of autotrophic denitrification (e.g. Thiobacillus ) increased. Pathway of nitrogen metabolism and functional genes analysis showed that the abundance of enzymes associated with the denitrification and dissimilatory nitrate reduction increased and that the abundance of enzymes associated with the nitrification decreased. Application of the combined CW1, CW2 and CW3 could be respectively formed corresponding nitrogen removal microbial community, influent quality and pathways of nitrogen to enhance the nitrogen removal effect of T-VF to treat LL. • The removal rate of nitrogen was over 90% in vertical flow constructed wetlands. • Nitrosomonas and Nitrobacter were the dominant nitrification bacteria. • Denitrifying bacteria Thauera , Thiobacillus , and Lautropia predominated.

Biogas purification and ammonia load reduction in sewage treatment by two-stage down-flow hanging sponge reactor

In this study, a two-stage down-flow hanging sponge (TSDHS) reactor was used as biotrickling filter for biogas desulfurization by utilizing the anaerobic digester supernatant (ADS) of sewage sludge of an activated sludge process (ASP). The reactor comprises a closed-type first-stage down-flow hanging sponge (1st DHS) and an open-type second-stage down-flow hanging sponge (2nd DHS) reactors. In the 1st DHS, hydrogen sulfide in biogas was dissolved into the ADS, and then it was oxidized into elemental sulfur and sulfate by microbe using dissolved oxygen and nitrite in the ADS. More than 99.9 % of hydrogen sulfide was removed within 400 s of empty bed residence time, and >50 % of removed hydrogen sulfide was oxidized into elemental sulfur and accumulated at the surface of the sponge carrier in the 1st DHS. The 1st DHS effluent was fed into the 2nd DHS for nitrogen removal via nitrification and sulfur-based denitrification with the recirculation of the 2nd DHS effluent under nonaeration condition. In the 2nd DHS, 36.8 % of ammonia and 5.3 % of total inorganic nitrogen were removed. Sulfurimonas and Halothiobacillus were increased and contributed to the sulfur-based denitrification as well as the accumulation of elemental sulfur in the 1st DHS, respectively. In the 2nd DHS, Nitrosococcus, Nitrobacter, and Sulfuritalea were considered as the contributors of nitrogen removal via nitrification and sulfur-based denitrification. Further, this study shows that a TSDHS reactor can achieve not only desulfurization of biogas in the 1st DHS but also a 3.5 %–15 % reduction of the ammonia load in the 2nd DHS by effective utilization of the ADS during sewage treatment, assuming that the ADS is returned to the ASP.