Rapid urbanization and population growth have increased sewage generation, creating major environmental and public health challenges, particularly in regions lacking centralized treatment. Conventional systems are effective but costly and energy-intensive, limiting decentralized deployment. Integrating water hyacinth (Eichhornia crassipes) into hydroponic systems offers a low-cost, nature-based alternative for nutrient and organic removal. This review makes three key contributions: (i) it defines a quantitative design–performance envelope linking hydraulic retention time, plant density, and harvesting frequency to treatment efficiency; (ii) it reframes biomass harvesting as a core process control governing net nitrogen and phosphorus removal and root-zone oxygen dynamics; and (iii) it integrates reactor design, biosecurity, and biomass valorization into a unified framework for decentralized sewage treatment. Synthesis of 220 studies shows that controlled floating hydroponic systems typically achieve 50–90% total nitrogen, 60–95% total phosphorus, and 60–95% BOD removal at 7–30 days HRT, driven by coupled plant uptake, rhizosphere nitrification–denitrification, and biofilm adsorption. Pathogen removal is generally limited to 0.5–2 log reductions for indicator bacteria (total and faecal coliforms/Escherichia coli), indicating that post-treatment polishing (UV, chlorination, maturation ponds, or wetlands) is required depending on the intended reuse or discharge standard. Performance declines below 15°C without greenhouse protection or hybridization with conventional biological units. Key constraints include seasonal metabolic limitations, hydraulic sensitivity to shock loading, invasive escape risks, and the need for standardized protocols for metal-laden biomass management. Proposed solutions include adaptive harvesting regimes, modular plug-flow layouts, hybrid treatment trains, and biochar production to stabilize contaminants and enable carbon sequestration. Positioned between passive wetlands and energy-intensive membrane systems, water hyacinth hydroponics offers moderate land demand, low energy use (0.02–0.1 kWh m⁻³), and circular bioeconomy potential for scalable decentralized sewage treatment.

A 2-acre reedbed system, cultivated with Phragmites australis, was established and utilized to remediate groundwater polluted with chlorinated hydrocarbons at a former industrial site. The reedbed comprised a combination of horizontal and vertical systems over four parallel installations, with a treatment capacity of 305 m ³/day. The mean inlet concentration for the four-line treatment was 112.4 mg/L, which was below the specified inlet concentration of 250 mg/L. From 2019 to 2024, the reedbed system effectively eliminated 1,2-Dichloroethane (1,2-DCA), with average removal rates of 97.7%, 98.8%, 98.5%, and 98.6% for Lines 1 to 4, respectively. The average outlet concentrations of 1,2-DCA were 0.70 mg/L, 0.40 mg/L, 0.42 mg/L, and 0.52 mg/L for Lines 1–4, respectively, resulting in an overall average of 0.51 mg/L. We performed the assessment of natural attenuation by first-order decay kinetics for five groundwater monitoring wells, showing values between 0.0012/year and 0.0036/year (shallow wells), 0.0003/year and 0.0021/year (middle wells), and 0.0003/year and 0.0009/year (deep wells). Here, shallow groundwater showed the highest kinetic rates compared to middle and deep groundwater wells. The results indicated that the reedbed system removed the bulk of contaminants through active biological processes involving plants and microbes, and that natural attenuation further degraded 1,2-DCA in the groundwater profiles. Based on data monitoring from 2019 to 2024, the reduction and degradation results showed good removal efficiency for the reedbed systems, combined with natural attenuation in the groundwater.

Fisheries wastewater contains high levels of suspended solids and organic matter, posing significant environmental risks and necessitating effective and sustainable treatment approaches. This study aims to determine the characteristics of the neem (Azadirachta indica) leaf biocoagulant, assess the interactions among research variables, and optimize its use to reduce total suspended solids (TSS) and chemical oxygen demand (COD) levels in fisheries wastewater. The method used is response surface methodology (RSM), specifically the Box–Behnken Design (BBD), which involves three variables (biocoagulant concentration, fast stirring speed, and sedimentation time) and two responses (TSS and COD removal). Characterization results (Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), and zeta potential) indicated that the biocoagulant contains functional groups such as hydroxyl, carboxyl, and amine, contributing to coagulation–flocculation through adsorption and polymer bridging mechanisms. Statistical analysis confirmed that the developed quadratic models were significant (p-value < 0.05), with high F-values, non-significant lack of fit, and strong coefficients of determination (R² = 0.9111 for TSS and 0.9419 for COD), along with low coefficients of variation (CV < 5%), indicating good model reliability. Although the model generally has a significant effect on the response, the fast stirring speed does not, while the other two factors do. The optimal conditions (based on desirability) were determined to be a biocoagulant concentration of 79.8 mg/L, a fast stirring speed of 100 rpm, and a sedimentation time of 27.5 min. Under these conditions, TSS and COD removals of 88.72% and 79.98%, respectively, were achieved. These findings demonstrate the potential of neem leaf biocoagulant as a sustainable, environmentally friendly alternative to conventional chemical coagulation, supporting cleaner production in aquaculture systems.

The use of plants as biocoagulants in water/wastewater treatment is currently emerging. This review article explores the potential of each plant's part functioning as a biocoagulant for pollutant removal. Bibliometric analysis was employed to analyze the development of research in natural and biocoagulants, while descriptive analysis was used to clearly juxtapose the performance of each plant's part in treating water/wastewater. Bibliometric findings reveal a high increment in the publication of natural coagulants in the year 2016. The keywords of flocculation, pH, turbidity, and water purification are mentioned to be the closest node related to the coagulation. Comparison between plant parts showed that research on seeds is dominating the previous literature (26.31 %), followed by leaves (10.89 %) and peels (5.75 %). Overall performance analysis showed that plant biocoagulants are superior in removing turbidity (median 83.45 %), while the performance of removing total suspended solids, chemical oxygen demand, and biological oxygen demand are also considerably good (mean 68.38 %, 71.36 %, 67.16 %, respectively). The seeds and other parts of the plants showed the highest removal of turbidity among other parts (mean of removal 90 % and median >90 %). Other parts of the plants are composed mostly of plant extracts, including mucilage (18.47 %), gum (9.67 %), starch (8.36 %), etc. (10.37 %). Overall, the effectiveness of plant biocoagulants in removing pollutants varies compared to that of commercially available coagulants. The current development of biocoagulants indicates that research is currently in the integration and hybridization stage. Future approaches are suggested to focus on upscaling the treatment to an industrial scale, simplifying the extraction procedures, and conducting species-specific analysis to enhance and polish the current knowledge of plant bicocoagulants in water/wastewater treatment.

Hexavalent chromium is one of the toxic metals in water pollution. This study is aimed at analyzing the involvement of chromium reductase and biosorption potential in chromium-resistant species of Bacillus cereus. A total of 10 % (v/v) of B. cereus biomass was inoculated into a 90 mL chromium-contaminated solution with an initial concentration of 60 mg/L. Biomass digestion was carried out every day for a 5-day treatment period for chromium content analysis, while biomass characterization was carried out at the end of the treatment period, comparing the exposed vs. non-exposed bacteria. Results indicated that the highest chromium removal (16.12 ± 0.63 %) was obtained on day 3, while the maximum biosorption capacity was obtained on day 1, reaching 0.461 ± 0.02 mg Cr/g dry cell of biomass. XRD showed the crystalline structure of the bacteria cell after being exposed to chromium, suggesting that interactions between polysaccharides and proteins in the membrane may occur during the treatment. In addition, FT-IR spectra also showed decreasing peaks and the involvement of hydroxyl, carboxyl, carbonyl, and nitroxide groups during the treatment. SEM-EDX results indicated that bacteria are experiencing cell structure alteration with more intense chromium spectra on the surface, while TEM images showed endospore formation by B. cereus under adverse environmental conditions. This study suggested that the removal of hexavalent chromium by B. cereus might be dominant via biosorption (translocated into cell biomass).

Proteins and carbohydrates are both major biodegradable fractions in wastewater. Complexation with coexisting compounds, such as iron (Fe) and humic acids (HA), which are both commonly present in wastewater, could influence the different degradation rates of proteins and carbohydrates. Depending on the redox conditions, Fe exists as Fe(II) or Fe(III), with differing binding affinities and chemical behaviour. This research aims to systematically assess the complex interaction between Fe, protein, and HA compounds under aerobic and anaerobic conditions. The results showed that the addition of Fe(III) and HA to a protein solution inhibited its hydrolysis rate by more than 90 % under aerobic conditions. In contrast, interactions between the same compounds and carbohydrates were much weaker and had a minimal effect on hydrolysis rates. Complexation with Fe, proteins, and HA was indicated by increased molecular sizes and reduced concentrations of free iron, protein, and HA. FTIR results showed that Fe(III) formed complexes with proteins and HA through electrostatic and coordination bonds involving various functional groups. Anaerobic reduction of Fe(III) to Fe(II) by hydrazine resulted in weaker binding and the formation of smaller, less stable protein–humic acid complexes. These findings suggested that modulating Fe complexation under alternating aerobic and anaerobic conditions, such as those found in redox-cycling wastewater treatment, can be a promising strategy to enhance protein degradation.

Domestic wastewater discharge is the major source of pollution in Malaysia. Phytoremediation under hydroponic conditions was initiated to treat domestic wastewater and, at the same time, to resolve the space limitation issue by installing a hydroponic system in vertical space at the site. Water hyacinth (WH) was selected in this study to identify its performance of water hyacinth in removing nutrients in raw sewage under batch operation. In the batch experiment, the ratio of COD _initial/plant _initial was identified, and SPSS ANOVA analysis shows that the number of plant size factors was not statistically different in this study. Therefore, four WH, each with an initial weight of 60 ± 20 g, were recommended for this study. Throughout the 10 days of the batch experiment, the average of COD, BOD, TSS, TP, NH4, and color removal was 73%, 73%, 86%, 79%, 77%, and 54%, respectively. The WH biomass weight increased by an average of 78%. The plants have also improved the DO level from 0.24 mg/L to 4.88 mg/L. However, the pH of effluent decreased from pH 7.05 to pH 4.88 below the sewage Standard B discharge limit of pH 9–pH 5.50. Four WH plant groups were recommended for future study, as the COD removal among the other plant groups is not a statistically significant difference (p < 0.05). Furthermore, the lower plant biomass is preferable for the high pollutant removal performance due to the fact that it can reduce the maintenance and operating costs.

Microalgae-based wastewater treatment is an alternative to physico-chemical and bacteria-based technologies. Microalgae-based wastewater treatment showed enormous potential, not only exhibiting excellent pollutant removal efficiencies but also unlimited opportunities for resource recovery. Despite its promising future, the question of selecting autotrophy or heterotrophy regimes for optimal organic pollutant removal remains. This current work juxtaposes the performance of autotrophic and heterotrophic cultures in treating organic-rich wastewater to shed light on the unsolved puzzle. This review paper details the autotrophy and heterotrophy growth regimes for microalgae, as well as highlights the source of organic-rich wastewater and its characteristics. A clear comparison between both regimes was then discussed with recent references. Heterotrophic cultures showed better parameter removal performances, especially carbon-related and N-related compounds, while the removal of P-related compounds is considerably similar. Heterotrophic regimes also resulted in higher biomass yield with higher P content as compared to autotrophy. Despite their superiority, heterotrophic regimes continuously require additional carbon sources, posing a cost-related limitation. In contrast, autotrophic culture has an added value of carbon sequestration, making it beneficial for climate mitigation and lowering operational costs. Future research should concentrate on techno-economic and cost-benefit analyses to further refine the currently discussed topic.

Heavy metal and microplastic pollutions are prevalent in freshwater ecosystems, with many freshwater bodies being contaminated by one or both of these pollutants. Recent studies reported extreme detections of Cd, Pb and Zn, high concentrations of Cr, Pb and Cu and microplastics acting as vectors of pollutants, including heavy metals. Mayflies can serve as bioindicators of heavy metal contamination in freshwater ecosystems because changes in their community structure, physiology, and behaviour can reflect and help predict the concentrations of metals in these environments. This review discusses the ecological alterations induced by tissue metal concentration in mayflies and other macroinvertebrates. As sensitive taxa to heavy metal contamination, mayflies can reflect the impacts of this pollution through their ethology and relationship to the substrate, highlighting issues such as eutrophication, alterations in community structure, inhibitory effects and sediment toxicity. Mayflies are also highly affected by microplastic exposure, which leads to ingestion, bioaccumulation, biomagnification, habitat and community alteration, behavioural changes, physiology alteration and toxicity. Mayflies bioindication metrics for assessing the impact of heavy metals and microplastics include the examination of community alteration, functional feeding behaviour, molecular structure, dietary and toxicity impacts, bioaccumulation and biomagnification and biomarkers. Current challenges for the utilization of mayflies as bioindicators include temporal variations in sensitivity, lack of universally recognised protocols and need for standardised protocols for microplastic analysis. Additionally, the applicability of mayflies as bioindicators may vary across different ecosystems, emphasising the need for selecting suitable indicators that align with the unique characteristics of the ecosystem.

In this study, biosorption potential of nine epiphytic bacteria isolated from the rhizosphere of Lepironia articulata and Scirpus grossus were assessed. Identification of the isolated epiphytic rhizobacteria using 16S rRNA analysis showed species belonging to the four genera of Bacillus, Enterobacter, Aeromonas, and Chromobacterium. Batch biosorption studies were carried out to assess the capacity of the isolated bacteria to act as Pb and Cu biosorbents. Different initial concentrations of the two heavy metals (50, 100, 200, 300, and 400 ppm) were used to determine the ability of the biosorbent to reach a tolerance level and then calculate the percentage of biosorption with respect to 0.1 g dry weight. Initial concentration of Pb and Cu exposed showed that the isolated bacteria have high tolerance up to 400 ppm. Bacteria prefer Pb ions over Cu, which is indicated by higher removal of Pb in all tested reactors. Bacillus sp. (coded Sc1) showed the highest biosorption capacity with 100% Pb and 97% Cu removal.