Nitrogen and phosphorus removal comparison between periphyton on artificial substrates and plant-periphyton complex in floating treatment wetlands

Enhancing nitrogen removal through macrophyte harvest and installation of woodchips-based floating beds in surface-flow constructed wetlands

Wetland management maintains nitrogen (N) removal capacity in mature and overgrown constructed wetlands (CWs). We evaluated whether CW management by macrophyte harvesting, and subsequent installation of woodchips-based floating beds (WFBs) planted with Glyceria maxima and Filipendula ulmaria improved N removal. In sixteen heavily overgrown experimental CWs, we applied four treatments: i) only macrophyte harvesting, ii) 5% of the harvested-CW surface covered with WFBs, iii) 20% WFBs cover, and iv) a control treatment (heavily overgrown). N removal was determined in all wetlands at nine occasions. Plant biomass accrual, N assimilation, and denitrification genes nirS, nirK, nosZI and nosZII on plant roots and woodchips from WFBs were estimated. Macrophyte harvesting improved N removal of heavily overgrown CWs, whereas subsequent WFB installation only sometimes improved N removal. Mean N removal efficiencies (± standard deviation) overall were 41 ± 15 %, 45 ± 20 %, 46 ± 16 % and 27 ± 8.3 % for treatments i to iv, respectively. Relative biomass production, root length and root surface area for G.maxima (mean ± standard deviation: 234 ± 114 %, 40 ± 6.5 cm, 6308 ± 1059 cm2g-1, respectively) were higher than those for F. ulmaria (63 ± 86 %, 28 ± 12 cm, 3131 ± 535 cm2g-1, respectively) whereas biomass N assimilation was higher for F. ulmaria (1.8 ± 0.9 gNm-2 of WFB) than for G. maxima (1.3 ± 0.5 gNm-2 of WFB). Denitrification gene abundance was higher on plant roots than on woodchips while G. maxima hosted higher root denitrification gene abundance than F. ulmaria. We conclude that macrophyte harvesting improves N removal in heavily overgrown CWs. WFBs installation has the potential to support plant growth and denitrification in surface-flow constructed wetlands. Further studies need to evaluate the long-term effects of macrophyte harvesting and WFB installation on N removal in CWs.

Effects of herbicides and fertilization on biofilms of Pampean lotic systems: A microcosm study

We experimentally assessed the impact of the application of herbicides and fertilizers derived from agricultural activity through the individual and simultaneous addition of glyphosate, atrazine, and nutrients (nitrogen 'N' and phosphorus 'P') on the biofilm community and their resilience when the experimental factors were removed. We hypothesize that i) the presence of agrochemicals negatively affects the biofilm community leading to the simplification of the community structure; ii) the individual or simultaneous addition of herbicides and nutrients produces differential responses in the biofilm; and iii) the degree of biofilm recovery differs according to the treatment applied. Environmentally relevant concentrations of glyphosate (0.7 mgL-1), atrazine (44 μgL-1), phosphorus (1 mg P L-1 [KH2PO4]), and nitrogen (3 mg N L-1[NaNO3]) were used. Chlorophyll a, ash-free dry weight, abundance of main biofilm groups and nutrient contents in biofilm were analyzed. At initial exposure time, all treatments were dominated by Cyanobacteria; through the exposure period, it was observed a progressive replacement by Bacillariophyceae. This replacement occurred on day 3 for the control and was differentially delayed in all herbicides and/or nutrient treatments in which the abundance of cyanobacteria remains significant yet in T5. A significant correlation was observed between the abundance of cyanobacteria and the concentration of atrazine, suggesting that this group is less sensitive than diatoms. The presence of agrochemicals exerted differential effects on the different algal groups. Herbicides contributed to phosphorus and nitrogen inputs. The most frequently observed interactions between experimental factors (nutrients and herbicides) was additivity excepting for species richness (antagonistic effect). In the final recovery time, no significant differences were found between the treatments and the control in most of the evaluated parameters, evincing the resilience of the community.

Proper C/N ratio enhances the effect of plant diversity on nitrogen removal and greenhouse effect mitigation in floating constructed wetlands

Treating wastewater with low carbon-to-nitrogen (C/N) ratios by constructed wetlands (CWs) is still problematic. Adding chemicals is costly and may cause secondary pollution. Configuring plant diversity in substrate-based CWs has been found to be a better way to treat low-C/N wastewater, but wastewater treatment in floating CWs needs to be studied. In this study, wastewater with C/N ratios of 5 and 10 were set in simulated floating CWs, and 9 combinations with plant species richness (SR) of 1, 3, and 4 were configured. The results showed that (1) increasing SR improved the total N mass removal (NMR) by 29% at a C/N ratio of 5 but not 10; (2) the presence of Oenanthe javanica in the microcosms increased the NMR by 13% and 20% with C/N ratios of 5 and 10, respectively; (3) increasing SR mitigated the net global warming potential (GWP) by 120% at a C/N ratio of 5 but not 10; and (4) a Hemerocallis fulva × O. javanica × Echinodorus parviflours × Iris hybrids mixture resulted in a high NMR and low net GWP. In summary, assembling plant diversity in floating CWs is an efficient and clean measure during the treatment of wastewater with a C/N ratio of 5.

Simulating oligotrophication in a eutrophic shallow lake to assess the effect of periphyton bioreactor on phytoplankton and epipelon

We evaluated the effects of a periphyton bioreactor on phytoplankton by experimentally simulating oligotrophication in a shallow eutrophic system. The experiment had two 50% diluted treatments with and without a periphyton bioreactor. Sampling was performed on days 6, 9, 12, 15, and 20 of the experimental period. The periphyton bioreactor accumulated biomass (chlorophyll-a, AFDM) and TP during the experimental period. Despite the biomass and TP loss due to periphyton detachment from the substrate after community reaching the algal biomass peak, the gains exceeded the losses, and the net rate was positive for all attributes in the bioreactor. Based on the average, our findings suggest that periphyton bioreactors negatively affected the phytoplankton total biovolume. Cyanobacteria were the most abundant phytoplankton group. However, the periphyton bioreactor caused the biomass loss of the Raphidiopsis raciborskii in phytoplankton. Our results suggest that bioreactor influenced the phytoplankton structure, reducing cyanobacterial biomass, especially Raphidiopsis raciborskii. However, the bioreactor did not reflect a significant increase in the epipelon biomass during the experimental period. We conclude that the periphyton bioreactor has the potential to assist in the maintenance of restored shallow lakes and reservoirs, especially in controlling phytoplankton growth.

Nitrogen transformation in slightly polluted surface water by a novel biofilm reactor: Long-term performance and microbial population characteristics

This study proposes a modular floating biofilm reactor (MFBR) for in situ nitrogen removal from slightly polluted water in rivers using enriched indigenous microorganisms. Its main structure is a 60 cm × 60 cm × 90 cm rectangular reactor filled with hackettens. After a 96-day startup, the removal efficiencies of ammonia-N and total N (TN) reached 80% and 25%, respectively, with a hydraulic retention time (HRT) of 10 h, whereas those in a control reactor (without biofilm) were only 4.9% and 0.2%, respectively. The influences of HRT and dissolved oxygen (DO) were also investigated. As a key factor, HRT significantly affected the removal efficiencies of ammonia-N and TN. When HRT was close to the actual value for a river studied (2.4 min), the removal efficiencies of ammonia-N and TN were only 8.7% and 3.1%, respectively. Aeration increased the concentration of DO in water, which enhanced nitrification but inhibited denitrification. When HRT was 2.4 min, aeration intensity was 20 L/min; the ammonia-N and TN removal rates were 9.5 g/(m²·d) and 11.3 g/(m²·d), respectively. The results of microbial community analysis indicated that the microorganisms forming the biofilm were indigenous bacteria. The findings demonstrated a concept-proof of MFBR, which may be evaluated in scaling up investigation for developing a new methodology for nitrogen removal from slightly polluted surface water in plain river networks.

Performance of basalt fiber-periphyton in deep-level nutrient removal: A study concerned periphyton cultivation, characterization and application

Nutrients in centralized discharge area of treated sewage can cause high ecological risks to aquatic systems, thus a deep-level nutrient removal is necessary. Recently, periphyton has attracted increasing interests for its excellent performance in nutrient removal. In this study, the suitability and durability of basalt fiber (BF) as a new green carrier of periphyton was evaluated, and development process of basalt fiber-periphtyon (BFP) was tracked with bacterial community succession and physiological indicators. Then, well-developed BFP was applied to deeply purify water containing the same concentration of nutrient as the treated sewage. Results showed the periphyton could adapt to BF and formed in large quantities. In addition, the tensile strength of BF after being used as a carrier was still strong. Bacterial community and physiological indicators indicated that BFP was well developed in 40-50 days. LEfSE and random forest analysis revealed that Deinococcus-Deinococci, Spartobacteria and Chlamydiia at class-level, Rhizobiales and Rhodobacterales at order-level were the biomarkers for development of BFP. Moreover, application results showed BFP efficiently removed nitrogen and phosphorus from water and promoted the transformation of ammonia to nitrate. The concentration of ammonia and phosphorus severely decreased from 4.90 ± 0.11 mg/L to 0.51 ± 0.20 mg/L, from 0.66 ± 0.016 mg/L to 0.023 ± 0.013 mg/L, respectively. The efficient nutrient removal was attributed to accumulation of nitrogen and phosphorus metabolism related organisms in BFP as well as favorable water physic-chemical conditions created by BFP. These results suggest that BF is a suitable and durable green carrier of periphyton, and BFP could efficiently reduce ecological risk to aquatic systems receiving treated sewage.

Recent developments and applications of floating treatment wetlands for treating different source waters: a review

Most water bodies around the world suffer from pollution to varying degrees. Floating treatment wetlands (FTWs) are a simple and efficient ecological treatment technology and have been widely studied and applied as a sustainable solution for different source waters. Based on the analysis of abundant literature in the last ten years, this paper systematically reviews the history and the latest development of FTWs. Meanwhile, the treatment performance and pollutant removal mechanisms of FTWs on the natural water, stormwater, domestic wastewater, industrial wastewater, and agricultural runoff are analyzed. In particular, very interesting information is provided, such as water depth, water surface coverage, the ratio of dissolved to total phosphorous (DRP/TP), the ratio of nitrogen to phosphorous (N/P), BOD/COD ratio, and its effects on the efficiency and removal mechanisms of FTWs. This information will provide useful references and guidance for optimizing the design of FTW and pollutant treatment efficiency of different source waters. This paper also provides an objective review of the limitations of FTWs. Subsequently, the enhancements of FTW technology which are recognized to be effective, including aeration, adding functional fillers or obligate degrading bacteria, and construction of hybrid FTWs, are summarized and recommendations are made for further research.

How to enhance the purification performance of traditional floating treatment wetlands (FTWs) at low temperatures: Strengthening strategies

Pollution of freshwaters poses a major threat to water quality and human health and thus, nutrients have been targeted for mitigation. One such control measure is floating treatment wetlands (FTWs), which are designed to employ vigorous macrophytes above the water surface and extensive plant root system below the water surface to increase plant uptake of nutrients. The efficacy of FTWs in purifying different water systems has been widely studied and reviewed, but most studies have been performed in warm periods when FTW macrophytes are actively growing. In low-temperature conditions, the metabolic processes of macrophytes and microbial activity are usually weakened or reduced by the winter months and are not actively assimilating pollutants. These circumstances hamper the purification ability of FTWs to perform as designed. Furthermore, decayed macrophytes could release pollutants into the water column. Hence, this paper aimed to systematically summarize strategies for use of enhanced FTWs in eutrophic water improvement at low temperature and identify future directions to be addressed in intensifying FTW performance in low-temperature conditions. Low-temperature FTW show variable nutrient removal efficiencies ranging from 22% to 98%. Current amendments to enhance FTW purification performance, ranging from direct strategies for internal components to indirect enhancement of external operation environments encourage the FTW efficacy to some extent. However, the sustainability and sufficiency of water purification efficiency remain a great challenge. Keeping in mind the need for optimizing the FTW components and dealing with high organic and inorganic chemicals, future research should be carried out at the large field-scale and focus on macrophyte- benthos- microorganism synergistic enhancement, breeding of cold-tolerant macrophytes, and combination of FTWs with many strategies, as well as rational design and operational approaches under cold conditions.

Development and evaluation of a process-based model to assess nutrient removal in floating treatment wetlands

Modelling is a useful tool for comprehensively describing the processes occurring in floating treatment wetlands (FTWs). However, temperature effects and phosphorus dynamics are not considered in the current FTW models. Therefore, a process-based model comprised of a plant growth submodel, a nitrogen dynamic submodel and a phosphorus dynamic submodel was developed to understand the complicated processes occurring in FTWs. The model was fully calibrated using a mesocosm FTW system operated for 168 days. Global sensitivity analysis revealed that nitrogen removal performance was predominantly sensitive to parameters representing plant characteristics and microbial activity. Because of the high concentration of organic matter, mineralization and sedimentation played important roles in nitrogen and phosphorus removal. In addition, the coprecipitation rate of phosphate also had a significant influence on phosphorus removal performance. When further investigation was applied to understand the behavior of the model, the ratio of nitrogen to phosphorus in plant tissue was found to be an indicator of the nutrient limitation in the water column. Furthermore, the model illustrated that both FTW operating conditions and plant characteristic parameters exerted an important influence on nitrogen removal and plant uptake contribution. Therefore, the selection of appropriate operating conditions and plant species can achieve high nutrients removal and make effective use of plants in FTWs. The model provides a useful tool for assessing the nutrients removal performance of FTWs and for evaluating strategies for them in design and operation.