Does rice straw application reduce N2O emissions from surface flow constructed wetlands for swine wastewater treatment?

Current progress on manganese in constructed wetlands: Bibliometrics, effects on wastewater treatment, and plant uptake

Constructed wetlands (CWs) are a pollutant treatment design inspired by natural wetlands and are widely utilized for the removal of common pollutants. The research focus lies in the circulation of manganese (Mn) in the environment to enhance pollutant removal within CWs. This paper provides a comprehensive review of recent advancements in understanding the role and effects of Mn in chemical weapons, based on literature retrieval from 2002 to 2021. Ecological risk assessment and heavy metals within CWs emerge as current areas of research interest. Mn sources within CWs primarily include natural deposition, heavy metal wastewater, and intentional addition. The cycling between Mn(II) and Mn(IV) facilitates enhanced wastewater treatment within CWs. Moreover, employing a Mn matrix proves effective in reducing ammonia nitrogen wastewater, organic pollutants, as well as heavy metals such as Cd and Pb, thereby addressing complex pollution challenges practically. To comprehensively analyze influencing factors on the system's performance, both internal factors (biological species, design parameters, pH levels, etc.) and external factors (seasonal climate variations, precipitation patterns, ultraviolet radiation exposure, etc.) were discussed. Among these factors, microorganisms, pollutants, and temperature are the most important influencing factors, which emphasizes the importance of these factors for wetland operation. Lastly, this paper delves into plant absorption of Mn along with coping strategies employed by plants when faced with Mn poisoning or deficiency scenarios. When utilizing Mn for the regulation of constructed wetlands, it is crucial to consider the tolerance levels of associated plant species. Furthermore, the study predicts future research hotspots encompass high-efficiency catalysis techniques, matrix-filling approaches, and preparation of resource utilization methods involving Mn nanomaterials.

Assessment of agriwaste derived substrates to grow ornamental plants for constructed wetland

Burning of surplus residues in agricultural fields is a common practice in many countries of the world. This practice adds emissions into the atmosphere and results in the loss of essential plant nutrients, hence, there is a need for developing technologies for the sustainable management of agri-residues. Constructed wetlands offer excellent nature-based, low-cost green technologies for the treatment of wastewater using surplus agricultural residues as wetland substrates to grow ornamental plants as a source of income. This study was conducted to investigate the use of agricultural residues and biochar as substrates to grow ornamental plants in constructed wetlands. Four ornamental plants (Canna Indica, Gerbera jamesonii, Liliumwallichianum, and Tagetes erecta) were grown in six different substrate combinations for 120 days. Data on plant growth parameters were collected for each plant and compared to select the best substrate combination. Canna Indica and Lilium wallichianum resulted in significantly higher growth and nutrient uptake (P<0.001) with the substrate of 15% rice straw, 80% soil, 5% biochar (T4), and 25% sugarcane bagasse, 70% soil, 5% biochar (T5) compared to other plants. The result concluded that agricultural waste-derived substrates are viable alternatives having fertilizing effects with the potential for nutrient recovery. The present study provides an alternative approach to utilize agricultural waste sustainably to grow ornamental plants in the constructed wetland which reduces the overall cost of the wetland unit making it more cost-efficient.

Enhanced denitrification of dispersed swine wastewater using Ca(OH)2-pretreated rice straw as a solid carbon source

In a pilot-scale packed bed reactor, the denitrification performance and microbial community structure of the dispersed swine wastewater treatment using calcium hydroxide (Ca(OH)₂) pretreated rice straw as a carbon source were investigated. In a Ca(OH)₂-pretreated rice straw supported denitrification system (Ca(OH)₂-RS), the removal efficiency of NO₃^--N was 96.39% at an influent NO₃^--N load of 0.04 kg/(m³•d). Meanwhile, there was no obvious accumulation of NO₂^--N or chemical oxygen demand (COD) in the effluent of Ca(OH)₂-RS. The contents of soluble microbial byproduct-like substances and tryptophan-like substances in the effluent of Ca(OH)₂-RS were reduced by 46.2% and 43.4%, respectively, compared with the influent. Overall, the Ca(OH)₂-pretreated rice straw system had a strong resistance to fluctuations in water quality conditions, such as influent NO₃^--N and COD concentrations. According to the microbial assay results, the Ca(OH)₂ pretreatment enriched more denitrifying bacteria. Among them, Proteobacteria (42.33%) and Bacteroidetes (35.28%) were the dominant bacteria. Moreover, the main denitrifying functional bacteria, generanorank_f_Saprospiraceae (13.32%), norank_f_Porphyromonadaceae (4.22%), and Flavobacterium (3.25%), were enriched in Ca(OH)₂-RS. This suggested that using Ca(OH)₂-pretreated rice straw as a carbon source was a stable and efficient technology to enhance the denitrification performance of dispersed swine wastewater.

Estimation of nitrous oxide emissions from rice paddy fields using the DNDC model: a case study of South Korea

The Denitrification-Decomposition (DNDC)-Rice is a mechanistic model which is widely used for the simulation and estimation of greenhouse gas emissions [nitrous oxide (N₂O)] from soils under rice cultivation. N₂O emissions from paddy fields in South Korea are of high importance for their cumulative effect on climate. The objective of this study was to estimate the N₂O emissions and biogeochemical factors involved in N₂O emissions such as ammonium (NH₄⁺) and nitrate (NO₃^-) using the DNDC model in the rice-growing regions of South Korea. N₂O emission was observed at every application of fertilizer and during end-season drainage at different rice-growing regions in South Korea. Maximum NH₄⁺ and NO₃^- were observed at 0-10 cm depth of soil. NH₄⁺ increased at each fertilizer application and no change in NO₃^- was observed during flooding. NH₄⁺ decreased and NO₃^- increased simultaneously at end-season drainage. Minimum and maximum cumulative N₂O emissions were observed at Chungcheongbuk-do and Jeju-do regions of South Korea, respectively. The simulated average cumulative N₂O emission in rice paddies of South Korea was 1.37 kg N₂O-N ha^-1 season^-1. This study will help in calculating the total nitrogen emissions from agriculture land of South Korea and the World.

Effects of reducing nitrogen application and adding straw on N2O emission and soil nitrogen leaching of tomato in greenhouse

Excessive input of nitrogen(N) fertilizer and improper selection of fertilizer types in the greenhouse vegetable production process will lead to a large amount of N loss. In order to relieve the environmental pollution caused by N loss, a planting experiment was carried out in a solar greenhouse in Shouguang, Shandong, China, to investigate the effects of N-reducing fertilizer and straw application on greenhouse vegetable yield and soil N loss, and to explore the fate of N after fertilizer application using the ¹⁵N isotope tracing technique. The experiment was planted for two seasons from July 2017 to June 2018 with four treatments: control (CK), conventional fertilizer (CN), reduced N topdressing (SN), and reduced N topdressing + straw (SNS). The results indicated that N reduction fertilizer and straw application resulted in a 35.25%-35.49% reduction in total N₂O emissions and 15.76%-41.77% reduction in mineral N leaching losses. ¹⁵N isotopes as tracers showed that the maximum abundance in N₂O was reduced by 58.5% and 55.68% for SN and SNS, respectively, and cumulative N₂O emissions were reduced by 80.44% and 81.67%, respectively, and mineral N leaching was reduced by 74.4% and 70.48%, respectively, after fertilization compared to CN treatment. There was no significant difference in tomato yield between the three fertilizer treatments in the two growing seasons. Therefore, in greenhouse vegetable production, the amount of N fertilizer was reduced by 40.7% and the addition of straw reduced N₂O emissions and N leaching without affecting tomato yields.

Effects of water level on nitrous oxide emissions from vegetated ditches

Nitrous oxide (N₂O) is considered a powerful greenhouse gas. Vegetated ditches are an important source of N₂O emissions in the agricultural systems. However, few studies have examined on the relationship between N₂O emissions and the water level in vegetated ditches. To investigate the effect of water level on the N₂O emissions, three pilot-scale ditches vegetated with Myriophyllum aquaticum were constructed with low (LW), medium (MW), and high (HW) water levels. The examined results indicated that the M. aquaticum ditches decreased N₂O emissions by 38.4% and 67.9% in MW and HW, respectively, as compared to the LW ditch. In addition, the N₂O emission factor decreased with increasing water level in the order of: LW (0.18%) > MW (0.11%) > HW (0.06%). The MW and HW ditches reduced the N₂O emissions by controlling the sediment nitrogen contents, in which the ammonia nitrogen increased with increasing the level of water, while nitrate nitrogen decreased with increasing the level of water. The increase in the level of water significantly reduced the gene abundance of ammonia-oxidizing archaea (AOA) (p < 0.05), thereby reducing the N₂O emissions in the MW and HW conditions due to the positive correlation between N₂O emissions and AOA gene abundances. The unclassified_k_norank_d_Bacteria was the dominant denitrifying bacterial genus observed in the M. aquaticum ditches, and its highly relative abundance yielded low N₂O emissions in the HW ditch. These findings indicate that reducing N₂O emissions may be achieved by controlling the water level in vegetated ditches.

Enhancing nitrogen removal from anaerobically-digested swine wastewater through integration of Myriophyllum aquaticum and free nitrous acid-based technology in a constructed wetland

Despite of low operation costs and convenient maintenance, the application of natural systems for swine wastewater treatment has been limited by large construction area and unsatisfactory effluent quality. Introducing ammonium high uptake aquatic plants and shifting nitrogen removal pathway from nitrate to nitrite in constructed wetlands (CWs) has been regarded as promising approach to promote their performances. This study aimed to establish nitrite pathway and enhance N removal via free nitrous acid (FNA)-sediment treatment and Myriophyllum aquaticum vegetation in the CWs treating anaerobically digested swine wastewater. Nitrite pathway was successfully and stably achieved in the M. aquaticum CW with FNA-treated sediment. The overall removal efficiencies of ammonium nitrogen and total nitrogen were 42.3 ± 10.2% and 37.7 ± 9.3% in the planted CWs with FNA-treated sediment, which were 76.3% and 65.4% higher than those in the conventional oxidation pond system, respectively. Microbial community analysis (qPCR and metagenomics) suggested that the nitrite pathway established through FNA-sediment treatment was based on the inactivation of nitrite oxidizing bacteria (lower nxrA gene abundance) and the reduction of relative abundances of NOB (especially Nitrobacter and Nitrospira). During the denitrification processes, the integration of M. aquaticum vegetation with FNA-sediment treatment can lower the nitrate reduction by decreasing narG gene abundances and decreasing the relative abundances of napA affiliated bacteria (especially Bradyrhizobium), while strengthening reduction of nitrite and nitrous oxide by increasing nirK and nosZ gene abundances and enriching the corresponding affiliated microbial taxa, Mycobacterium and Bacillus, respectively. Our findings suggest that applying FNA-based technology in CW systems is technically and economically feasible, which holds promise for upgrading current CW systems treating swine wastewater to meet future water quality requirements.

A comparison of the mechanisms and performances of Acorus calamus, Pontederia cordata and Alisma plantagoaquatica in removing nitrogen from farmland wastewater

This study examined the performances of Acorus calamus, Pontederia cordata, and Alisma plantagoaquatica in removing nitrogen (N) from farmland wastewater. P. cordata showed the fastest rate of N removal, followed by A. plantagoaquatica, whereas that of A. calamus was slowest. P. cordata and A. plantagoaquatica achieving a greater rate of TN reduction in soil than that by A. calamus. A. plantagoaquatica demonstrated the highest N adsorption capacity, 32.6% and 392.1% higher than that of P. cordata and A. calamus, respectively. The higher potential nitrification and denitrification rate, and abundance of functional genes in the P. cordata microcosm resulted in a stronger process of nitrification-denitrification, which accounted for 65.99% of TN loss. Plant uptake and nitrification-denitrification were responsible for 50.06% and 49.94% of TN removed within the A. plantagoaquatica. Nitrification-denitrification accounted for 86.35% of TN loss in A. calamus. These findings helped to insight into N removal mechanisms in different plants.

Floating treatment wetlands in domestic wastewater treatment as a decentralized sanitation alternative

Floating treatment wetlands (FTW) are technologies that have stood out for their efficiency, ease of installation and maintenance. They consist of macrophytes emerging in a floating structure that keep the plant roots in direct contact with the effluent regardless of the water flow variation over time, allowing the removal of pollutants by various processes. The application of FTWs for the treatment of domestic wastewater has the advantage of low costs in terms of removing nutrients and at the same time reducing the cost of maintenance and energy consumption when compared to the conventional centralized treatment of effluent. The lack of wastewater treatment in areas distant from urban centers is even more limited, mainly due to the high cost of tubing and pumps for the effluent to reach the treatment plants. Therefore, the objective of this study was to research FTW systems applied to the decentralized treatment of domestic wastewater. First, a bibliometric analysis was conducted comparing the main issues involving FTW, and the challenges regarding the integration of FTW and domestic wastewater treatment systems. The feasibility of the floating system as a decentralized treatment approach were discussed, as well as the removal of nutrients in domestic wastewater, which was the most covered topic by researchers who developed studies in the area. In addition, other technologies are being integrated into the phytoremediation systems seeking to improve the quality of the treated effluent and assessing the potential reuse in the homes where they are generated and treated, determining the costs and space requirements for the entire process. There is a large research gap regarding the treatment of domestic wastewater by FTW in decentralized systems, mainly in terms of operation, cost assessment and reuse Therefore, further investigations in order to better understand the performance of the process and the reactions that occur with physical, chemical and microbiological removal mechanisms are still necessary.

Characterization of dissolved organic matter and carbon release from wetland plants for enhanced nitrogen removal in constructed wetlands for low C-N wastewater treatment

The effects of pretreatment methods on the structure of functional groups and denitrification promotion capacity of solid carbon sources derived from reeds and cattails were elucidated. Alkaline treatment improved the relative content of carbon in the plant tissues, as well as prolonged the high denitrification rate of 0.40 mg/(L·h) from 6 days up to circa 28 days. Moreover, alkaline-heated cattails (ALH-C) showed high denitrification promotion capacity, and increased the removal rate of TN, NO₃^--N and NH₄⁺-N in the CW by 24.41%, 31.80% and 8.80%, respectively. Furthermore, the quantity, quality and migration of dissolved organic matter (DOM) released from ALH-C in CW analyzed via fluorescence excitation-emission matrix spectrophotometry showed mainly humic acid-like, tyrosine-like, and tryptophan-like components. These DOM components were highly bioavailable and had minimal effects on COD removal. These results provide insights into the preparation and environmental applications of plant carbon sources.

Horizontal sub surface flow Constructed Wetlands coupled with tubesettler for hospital wastewater treatment

Hospital wastewater are a lurking threat to environment and human health security for any given moment of time owing to its complexity and high vulnerability to cause disease outbreak. Though there are a number of treatment process for wastewater., there is a high need for employing cost-efficient and sustainable method of treatment. Hence a pilot scale horizontal surface flow Constructed Wetland (HSFCW) coupled with Tubesettler was installed at New Delhi, India (February to may 2019). This study reports comparative pollutants removal from hospital wastewater using Constructed Wetlands and associated tubesettler dosed with Hospital wastewater. A pilot scale CW system was used for treating 10m³/day of hospital wastewater. The system was tested for 3 Months to evaluate its performance for removing pollutants from the wastewater. The HSFCW coupled with tubesettler achieved over all removal efficiency of 94% (COD), MLSS (97%), TSS (98%), BOD₅ (96%), Phosphate (79%). However, process of nitrification was not observed and accumulation of Nitrate up to 197% was observed. The study concluded that it may be due to the presence of pharmaceuticals and other elements present in hospital wastewater. This conclusion was based on the fact that Alkalinity increased by 52% in effluent and pH value also exhibited an average increase of 12%. Further research studies are required to investigate effect of pharmaceutical originating from hospital on treatment efficiency, to incorporate anaerobic setup to complete denitrification-nitrification process and also to determine efficiency of thermophilic, mesophilic, and psychrophilic bacteria with respect to climate and temperature.

Nutrients release and greenhouse gas emission during decomposition of Myriophyllum aquaticum in a sediment-water system

Aquatic macrophytes play a significant role in nutrients removal in constructed wetlands, yet nutrients could be re-released due to plant debris decomposition. In this study, Myriophyllum aquaticum was used as a model plant debris and three debris biomass levels of 3 g, 9 g dry biomass, and 20 g fresh biomass (D3, D9, and F20, respectively) were used to simulate 120-d plant debris decomposition in a sediment-water system. The biomass first-order decomposition rate constants of D3, D9, and F20 treatments were 0.0058, 0.0117, and 0.0201 d^-1, respectively with no significant difference of decomposition rate among three mass groups (p > 0.05). Plant debris decomposition decreased nitrate and total nitrogen concentrations but increased ammonium, organic nitrogen, and dissolved organic carbon (DOC) concentrations in overlying water. The parallel factor analysis confirms that three components of DOC in overlying water changed over decomposition time. Emission fluxes of methane and nitrous oxide in the plant debris treatments were several to thousands of times higher than the control group within the initial 0-45 d, which was mainly attributed to DOC released from the plant debris. Plant debris decomposition can affect the gas emission fluxes for relatively shorter time (30-60 d) than water quality (>120 d). The 16S rRNA, nirK, nirS and hazA gene abundance increased in the early stage for plant debris treatments, and then decreased to the end of 120-d incubation time while ammonia monooxygenase α-subunit A gene abundance of ammonia-oxidizing archaea and bacteria had no large variations during the entire decay time compared with no plant debris treatment. The results demonstrate that decomposition of M. aquaticum debris could affect greenhouse gas emission fluxes and microbial gene abundance in the sediment-water system besides overlying water quality.

Nitrous oxide emissions from pilot scale three-stage constructed wetlands with variable nitrogen loading

Pilot scale three-stage surface flow constructed wetlands (SFCWs) were constructed to study nitrous oxide (N₂O) emissions from swine wastewater with different nitrogen levels. The SFCWs had mean total nitrogen (TN) removal efficiencies and removal rates of 84.6-97.1% and 0.6-2.4 g N m^-2 d^-1 respectively. The N₂O emissions and nitrate nitrogen (NO₃^--N) concentration both peaked at a TN value of approximately 100 mg N L^-1. N₂O emissions had a positive correlation with NO₃^--N concentration (p < 0.001). This correlation suggests that the effect of TN loading on N₂O emissions may be related to NO₃^--N in aquatic environment. Significant correlation was observed between N₂O emission and the gene abundance of N₂O reductase (nosZ; p < 0.05). The general linear model revealed that TN loading affected nosZ gene abundance. These results suggest that pollution loading should be considered to balance nitrogen removal and N₂O emissions when designing constructed wetlands.