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To effectively remove N and P from eutrophic water, the Phragmites australis after phytoremediation was harvested for preparation of modified biochar. The MgCl 2 -modified biochar (MPB) was successfully synthesized at 600 °C under N 2 circumstance. The physiochemical characteristics, the adsorption capacity for N and P in the simulated solution, and their adsorption mechanism of MPB were then determined, followed by the treatment of eutrophic water of Tai lake and its inflow river from agricultural source. The results demonstrated that the MPB presented high adsorption capacity to both simulated NH 4 -N and PO 4 -P with the maximum adsorption capacity exceeding 30 and 100 mg g −1 , respectively. The entire ammonium adsorption process could be described by a pseudo-second-order kinetic model whereas the phosphate adsorption process could be divided into three phases, as described by both intra-particle diffusion model and the pseudo-first-order kinetic. It was further found that the dominant mechanism for ammonium adsorption was Mg 2+ exchange instead of functional groups and surface areas and the Mg-P precipitation was the main mechanism for phosphate adsorption. The MPB also showed high removal ratio of practical TP which reached nearly 90% for both the water in Tai lake and its agricultural source. It suggested that MPB based on harvested P. australis was a promising composite for eutrophic water treatment and it could deliver multiple benefits. Graphic abstract

This study involved a comprehensive renovation of fish ponds to improve the water quality of a eutrophic river in Dongguan City. The abandoned fish ponds were transformed into three different types of stabilization ponds: facultative, aerated biological, and submerged plant stabilization ponds. The water of the eutrophic section of the river was pumped into the facultative stabilization pond and discharged into the Haizai River through an aerated biological pond and a submerged plant pond. In the aerated biological pond, secondary treatment was carried out using plant zoning and artificial floating island aeration system. The submerged plant pond used fountain-type aeration and an underwater forest for tertiary treatment. After four months of monitoring the water quality of the stabilization pond and the river, the ammonia nitrogen (NH3-N), total phosphorus (TP), and chemical oxygen demand (CODCr) levels in the raw sewage reduced from 6.53 mg/L to 1.13 mg/L, 1.76 mg/L to 0.29 mg/L, and 63 mg/L to 22 mg/L, respectively; the transparency of water increased to 45 cm, and dissolved oxygen (DO) level increased to 5.32 mg/L. This study provides a reference for the ex-situ treatment of urban eutrophic waterbodies.

Ecological floating bed (EFB) has been widely used to remove nitrogen and phosphorus from eutrophic water. However, its effects on nitrogen and phosphorus removal are different in various studies. Presently it has not been systematically clear what factors produce effects on EFB removing nitrogen and phosphorus from eutrophic water. In this study, we performed a meta-analysis of 169 articles to discuss the effects of EFB characteristics and experimental conditions on EFB removing nitrogen and phosphorus. Results showed that EFB generally decreased nitrogen and phosphorus concentrations in eutrophic water regardless of EFB characteristics and experimental conditions. EFB showed better effects on simultaneously removing TN, NH 4 + -N, and TP when it had one of the characteristics: constructed by monocots, 2–3 plant species, an area of 1.1–3.0 m 2 , and the coverage of 21%-40%. However, NO 3 − -N removal by EFB was complicated due to the effects of nitrification and denitrification. Moreover, EFB plant density also showed different effects on nitrogen and phosphorus removal. Experimental conditions produced evident effects on EFB removing nitrogen and phosphorus, and it showed better effects under one of the conditions: water temperature of 16–25℃, experimental duration of 31–60 days, long hydraulic retention time, and aeration. This study indicates that EFB can significantly remove nitrogen and phosphorus from eutrophic water, and it is an effective technology to control water eutrophication, but the effects of EFB characteristics and environmental conditions on EFB function should be considered in application.

Lake restoration practices based on reducing fish predation and promoting the dominance of large-bodied Daphnia grazers (i.e., biomanipulation) have been the focus of much debate due to inconsistent success in suppressing harmful cyanobacterial blooms. While most studies have explored effects of large-bodied Daphnia on cyanobacterial growth at the community level and/or on few dominant species, predictions of such restoration practices demand further understanding on taxa-specific responses in diverse cyanobacterial communities. In order to address these questions, we conducted three grazing experiments during summer in a eutrophic lake where the natural phytoplankton community was exposed to an increasing gradient in biomass of the large-bodied Daphnia magna. This allowed evaluating taxa-specific responses of cyanobacteria to Daphnia grazing throughout the growing season in a desired biomanipulation scenario with limited fish predation. Total cyanobacterial and phytoplankton biomasses responded negatively to Daphnia grazing both in early and late summer, regardless of different cyanobacterial densities. Large-bodied Daphnia were capable of suppressing the abundance of Aphanizomenon, Dolichospermum, Microcystis and Planktothrix bloom-forming cyanobacteria. However, the growth of the filamentous Dolichospermum crassum was positively affected by grazing during a period when this cyanobacterium dominated the community. The eutrophic lake was subjected to biomanipulation since 2005 and nineteen years of lake monitoring data (1996-2014) revealed that reducing fish predation increased the mean abundance (50%) and body-size (20%) of Daphnia, as well as suppressed the total amount of nutrients and the growth of the dominant cyanobacterial taxa, Microcystis and Planktothrix. Altogether our results suggest that lake restoration practices solely based on grazer control by large-bodied Daphnia can be effective, but may not be sufficient to control the overgrowth of all cyanobacterial diversity. Although controlling harmful cyanobacterial blooms should preferably include other measures, such as nutrient reductions, our experimental assessment of taxa-specific cyanobacterial responses to large-bodied Daphnia and long-term monitoring data highlights the potential of such biomanipulations to enhance the ecological and societal value of eutrophic water bodies.

Eutrophication of water bodies significantly accelerates water quality degradation, leading to the decline of aquatic organisms. To evaluate the synergistic restoration effects of submerged macrophyte Hydrilla verticillata and filter-feeding bivalve Anodonta woodiana on hypereutrophic water, a 40-day mesocosm simulation experiment in hypereutrophic aquatic ecosystems was conducted by setting up four treatments: control group (CK), A. woodiana group (Aw), H. verticillata group (Hv), and combined H. verticillata + A. woodiana group (HA). The results indicated that the combined application of H. verticillata and A. woodiana significantly reduced total phosphorus (TP), chlorophyll a (Chl a) concentration, and turbidity in the water, with removal rates reaching 58.3%, 60.6%, and 85.4%, respectively. The introduction of A. woodiana substantially altered the algal community composition. At the end of the experiment, the average proportion of cyanobacteria in the CK and Hv groups was 55.6%, whereas in the Aw and HA groups it decreased to 36.0%. Both total phosphorus and water-soluble phosphorus contents in H. verticillata tissues were significantly lower in HA compared to Hv, indicating that the combined treatment could reduce the risk of internal phosphorus release after H. verticillata senescence. These findings collectively demonstrate that the combination of H. verticillata and A. woodiana represents an efficient and environmentally friendly ecological restoration technology of eutrophic waters.

Aluminum-based P-inactivation agent (Al-PIA) is a highly effective material for phosphorus removal, which has great potential to restore eutrophic water bodies. In this study, active capping plate (ACP) was formed by mixing Al-PIA with cement and fly ash. The scour resistance coefficient of the prepared ACP was 98.01 % and the water permeability coefficient was 0.139 cm/s, which meet the requirements of strength and water permeability. Exogenous phosphorus was used as an input in the indoor dynamic experiment to examine the effectiveness of ACP capping to control phosphorus in sediment-water system. The indoor dynamic experiment lasted for 110 days. After experiment, the effect of dissolved oxygen (DO) and pH was explored, the change of phosphorus concentration in the overlying water was examined, and the changes of phosphorus in ACP and sediments were also analyzed. After ACP capping, the pH eventually maintained below 8 and was slightly higher than that of the control system. The DO concentration initially decreased, then increased, and finally remained in the range of 7 to 8 mg/L. Compared to the control system, the average reduction rates of total phosphorus (TP), dissolve total phosphorus (DTP) and dissolve inorganic phosphorus (DIP) in the overlying water were 90.57%, 91.69%, and 92.80%, respectively. ACP promotes the conversion of unstable phosphorus in sediments to stable phosphorus, and calcium-bound phosphorus (Ca-P) is the main form of phosphorus fixed by ACP. In addition, ACP was found to have a limited effect on reducing the risk of release of biologically available phosphorus from sediments.

Three common floating bed plants, Eichhornia crassipes, Pistia stratiotes, and Ipomoea aquatica, were selected in the present study to investigate their inhibitory effects on toxic Microcystis aeruginosa. The results showed that all three types of floating-bed plants could considerably inhibit the growth of M. aeruginosa and effectively remove the microcystins (MCs) from water systems, among which, E. crassipes and P. stratiotes were more effective in resisting M. aeruginosa, and the removal rate of the intracellular MCs could be up to 100%. In addition, the roots and leaves of the three plants were enriched with a large number of MCs and demonstrated significant antioxidant responses, as evidenced by the increase in the content of catalase (CAT), glutathione peroxidase (GSH-PX), superoxide dismutase (SOD), and malondialdehyde (MDA) in the roots, stems, and leaves of the plants. Furthermore, this study also showed that Proteobacteria, Bacteroidota, Myxococcota, Verrucomicrobiota, and Actinobacteriota dominated the root microorganisms of the three plants. Moreover, a variety of MC-degrading bacteria, including Sphingomonas, Acinetobacter, Novosphingobium, and Pseudomonas, were found at the genus level, which further provides important basic data for the regulation of eutrophic water bodies and the removal of MCs.

In this article, dynamic simulation experiments have studied the effects of three capping materials, quartz sand (QS), aluminum-based phosphorus-locking agent (Al-PIA), and lanthanum-modified bentonite (LMB) in reducing phosphorus load in eutrophic water bodies. The changes of various forms of phosphorus in Al-PIA and sediment before and after the test were analyzed, and the mechanism of phosphorus migration and transformation in different capping systems was described. The dynamic simulation test lasted 95 days. The results showed that when the initial concentration of total phosphorus (TP) was 3.55 mg/L, the capping strength was 2 kg/m 2 and the hydraulic retention time of water circulation was 0.5 days, indicating that the average reduction rates of TP by LMB, Al-PIA and QS systems were 74.66%, 69.54%, and 3.64%, respectively, compared with the control system. The analysis of variance showed that there were significant differences ( P  < 0.05) in the TP concentration of the overlying water between the LMB, Al-PIA capping system, and the control system. Lanthanum ions in LMB can fix phosphorus. Al-PIA reduces the phosphorus concentration in water by means of ion exchange, adsorption, complexation, etc. LMB and Al-PIA promoted the migration of phosphorus in sediment. Among them, the phosphorus fixed by Al-PIA was mainly in the form of non-apatite inorganic phosphorus (NAIP) in inorganic phosphorus (IP), which can be seen; Al-PIA can effectively reduce the phosphorus load of eutrophic water.

Eutrophication of urban rivers has caused severe environmental problems due to the pollution from point and diffuse sources. Although eutrophication can be alleviated by reducing the input to the river system, fast-treating terminal control technologies, especially under emergent situations, should be developed to reduce risks induced by eutrophication. The present study developed an emergency purification device based on dissolved air flotation (DAF) technology. After equipment commissioning and parameter optimization for applications in the field of engineering, the device was found to effectively remove total phosphorus, chlorophyll a, chemical oxygen demand, and turbidity in water by controlling the coagulant dosage and adjusting the gas-liquid mixing pump parameters. Dissolved air in water could enhance dissolved oxygen, and dissolved oxygen in polluted rivers could be raised from 0.2–2 mg/L to 3–3.5 mg/L. Removal of total nitrogen was poor because the majority of nitrogen contents were dissolved. Finally, DAF has been proven to be a promising technology due to its ease of implementation, low equipment investment requirement, and low operation cost.

The increase of nitrogen and phosphorus causes eutrophication in water bodies. Using submerged plants to decrease the pollution from water bodies is an effective way. In this research, three common submerged plants (Vallisneria natans, Hydrilla verticillata, and Ceratophyllum demersum) and their combinations were used to purify eutrophic water. The control treatment did not contain any plants. The removal effects and dynamic regulations of the three plants with their combinations of nutrients (such as nitrogen and phosphorus) in water were analyzed. All three species and their combinations above could grow in the eutrophic water and efficiently remove aquatic nutrients. All the treatment groups had a higher pollutant removal rate for total nitrogen (TN) and total phosphorus (TP) than that of the blank control. In these treatment groups, treatment F (50 g Vallisneria natans plus 50 g Ceratophyllum demersum) had the highest removal rate of TP at 57.53%; treatment B (100 g Vallisneria natans) had the best removal rate of TN at 92.04 %. Among these plants and their combinations, Vallisneria natans and Ceratophyllum demersum showed better purification ability; the combination of these two submerged plants and the combination of three submerged plants were more applicable for the restoration of eutrophic water.