Floating Treatment Wetlands Research Articles

Floating treatment wetlands for the bioremediation of oil spills: A review.

Conventional oil spill recovery may cause significant damage to shoreline habitats during the removal of oiled material and from human and equipment interaction. In addition, these methods are costly and can leave a significant amount of residual oil in the environment. Biological remediation strategies may be a less invasive option for recovering oil from sensitive regions, with potential to increase recovery. Floating treatment wetlands are a growing area of interest for biodegradation of oil facilitated by plant-bacterial partnerships. Plants are able to stimulate microbial colonization in the rhizosphere, creating greater opportunity for contaminant interaction and degradation. A literature review analysis revealed thirteen articles researching this topic, and found that floating treatment wetlands have high potential to degrade oil contaminants. In some instances, plants and inoculated bacteria exhibited the highest degradation potential, however, plants alone had higher degradation potential than bacteria alone. Research is needed to explore how floating treatment wetlands perform in field-based trials and under variable environmental conditions.

A biological and chemical approach to restoring water quality: A case study in an urban eutrophic pond.

Efforts to improve water quality of eutrophic ponds often involve implementing changes to watershed management practices to reduce external nutrient loads. While this is required for long-term recovery and prevention, eutrophic conditions are often sustained through the recycling of internal nutrients already present within the waterbody. In particular, internal phosphorus bound to organic material and adsorbed to sediment has the potential to delay lake recovery for decades. Thus, pond and watershed management techniques are needed that not only reduce external nutrient loading but also mitigate the effects of internal nutrients already present. Therefore, our objective was to demonstrate a biological and chemical approach to remove and sequester nutrients present and entering an urban retention pond. A novel biological and chemical management technique was designed by constructing a 37m2 (6.1m×6.1m) floating treatment wetland coupled with a slow-release lanthanum composite inserted inside an airlift pump. The floating treatment wetland promoted microbial denitrification and plant uptake of nitrogen and phosphorus, while the airlift pump slowly released lanthanum to the water column over the growing season to reduce soluble reactive phosphorus. The design was tested at the microcosm and field scales, where nitrate-N and phosphate-P removal from the water column was significant (α=0.05) at the microcosm scale and observed at the field scale. Two seasons of field sampling showed both nitrate-N and phosphate-P concentrations were reduced from 50μgL-1 in 2020 to <10μgL-1 in 2021. Load calculations of incoming nitrate-N and phosphate-P entering the retention pond from the surrounding watershed indicate the presented biological-chemical treatment is sustainable and will minimize the effects of nutrient loading from nonpoint source pollution.

Developing a Novel Alum Sludge-Based Floating Treatment Wetland for Natural Water Restoration

Novel alum sludge-based floating treatment wetland (FTW) was developed to enhance the purification performances of natural water bodies, i.e., rivers, lakes, and ponds. Polyurethane was applied to foam the lightweight alum sludge based-substrate (PU-AL) of FTW through the response surface method. Three FTWs configurations were created for a half-year lab-scale operation, and the PU-AL FTW presents the greatest purification performance in the removal rate of chemical oxygen demand (COD) of 62.58 ± 6.65%, total nitrogen (TN) of 53.31 ± 4.65%, and total phosphorus (TP) of 45.39 ± 4.69%. PU-AL substrate could enhance the nutrient removal performance of existing FTW by providing a proper media for microbial and plants’ growth. This study provides a good solution and showcase not only from a natural water restoration point of view but also from the waterworks sludge management view for a better understanding of FTWs and good applications in engineering practice.

Nitrogen removal and microbial mechanisms in a novel tubular bioreactor-enhanced floating treatment wetland for the treatment of high nitrate river water.

A novel tubular bioreactor-enhanced floating treatment wetland (TB-EFTW) was developed for the in situ treatment of high nitrate river water. When compared with the enhanced floating treatment wetland (EFTW), the TB-EFTW system achieved 30% higher total nitrogen removal efficiency. Further, the average TN level of the TB-EFTW effluent was below the Grade IV requirement (1.5mg/L) specified in Chinese standard (GB3838-2002). Microbial analysis revealed that both aerobic and anoxic denitrifying bacteria coexisted in the new system. The relative abundance of aerobic and anoxic denitrifiers were 42.69% and 22% at the middle and end of the tubular bioreactor (TB), respectively. It is reasonable to assume that effective nitrogen removal can mainly be attributed to the addition of solid carbon source and the spatial difference in DO distribution (oxic-anoxic areas in sequence) inside the TB. The initial investment cost and operating costs associated with the TB-EFTW system are approximately 14,000 and 3500 yuan per 1000 m3 river water, respectively. Considering its low cost, minimal maintenance requirements, and effective nitrogen removal, this newly developed system can be regarded as a promising technology for treating high nitrate river water. PRACTITIONER POINTS: A novel TB-EFTW system was developed to upgrade traditional in situ treatment techniques. The TB-EFTW could achieve 30% higher nitrogen removal efficiency than EFTWs. Both aerobic and anoxic denitrifying bacteria coexisted in the system. The system shows better technical and economic performance compared with routine techniques.

Flow distribution and mass removal in floating treatment wetlands arranged in series and spanning the channel width

Floating treatment wetlands (FTWs) use plants’ roots for water quality improvement. The plants are supported by a buoyant structure deployed at the water surface. The roots form a porous zone beneath the structure and remove pollutants carried in suspension through filtering, absorption and uptake. This paper used CFD simulation to model FTWs arranged in series and spanning the channel width and to study the effects of root length and spacing between FTWs on flow distribution and mass removal. The root zone was modelled as a porous media, and removal was computed using first-order decay, for which a range of removal constants was tested. Longer roots increased the reactive volume of the root zone, which increased the fraction of pollutant inflow entering the FTWs. Increasing the distance between FTWs allowed greater mixing between water that went through and beneath the upstream FTW. This increased the concentration entering each FTW, which enhanced mass removal per FTW. However, a larger distance between FTWs reduced the number of FTWs in the channel, reducing the reactive volume. In the tradeoff between mixing and reactive volume, the reactive volume was more important, such that total removal in the channel increased with longer roots and more units of FTW (shorter gap distance). However, removing the gap entirely was detrimental, as FTWs in series removed more mass than a continuous FTW of same volume. This study points to two design recommendations for FTWs in series. First, if resources for building FTWs are not limiting, but the channel length is, it is preferable to prioritize higher reactive volume (shorter gap distance) to achieve maximum removal per channel length. Second, if resources for FTWs are limiting, but channel length is not, it is better to place the FTWs with a longer gap distance, preferably along enough to allow mixing over the full depth between FTWs, as this will achieve maximum removal per FTW.

Nutrient Removal by Floating Treatment Wetlands Under Different Spatial Arrangement Modes: a Field Study.

Floating treatment wetlands (FTWs) are a management method to improve urban rivers, but most studies have been carried out at laboratory, micro, and meso levels, so it is necessary to study full-scale FTWs as a method to improve urban water bodies. In this experiment, the purification effects of water temperature (WT), dissolved oxygen (DO), ammonia nitrogen (NH4+-N), nitrate nitrogen (NO3--N), total phosphorus (TP), chemical oxygen demand (CODMn), and chlorophyll-a (Chl-a) under staggered arrangement (SA) and centered arrangement (CA) were evaluated. It was found that the DO concentration and removal rate of CODMn, Chl-a, and TP in the SA were significantly higher than those in the CA in months with heavy rainfall. However, interestingly, for TP, August showed the opposite trend. The removal rates of NH4+-N and NO3--N were significantly different throughout the test period. The biomass growth values of shoots and roots of plants in the FTWs were 0.40 ± 0.03 kg/m2 and 1.38 ± 0.07 kg/m2 in the SA and 0.32 ± 0.07 kg/m2 and 1.26 ± 0.30 kg/m2 in the CA. The increments of N absorbed by plants in the SA and CA were 7.08 ± 0.49 g/kg and 6.83 ± 0.07 g/kg, respectively, and the increments of P were 0.57 ± 0.02 g/kg and 0.32 ± 0.07 g/kg, respectively, which indicated that the growth status of plants in the FTWs in the SA was slightly better than that in the CA. In summary, the hybrid-constructed FTWs of both arrangements can effectively improve the water quality of urban rivers, and the effect of the SA was greater than that of the CA. The purification effect of in situ tests under different arrangement modes of hybrid-constructed FTWs was evaluated, which provides guidance and support for the field layout of FTWs in rivers in the future.

Investigation of Floating Treatment Wetland and its Role in Wastewater Treatment of Textile Industry

Investigation of Floating Treatment Wetland and its Role in Wastewater Treatment of Textile Industry

Water Quality Assessment of Urban Ponds and Remediation Proposals

Ponds are a common feature in urban parks to provide aesthetic and recreational functions, but also deliver a wide range of ecosystem services. The objective of this study was to assess the water quality of six urban ponds in the city of Lisbon, Portugal, to determine the factors that influence it and consider remediation measures for them. Besides that, our study aims to deliver data in order to support the best approach for a future monitoring program, toward more strategic and sustainable management. Floating treatment wetlands (FTW) were installed in three of the ponds during the study, where nutrient levels were higher. Water sampling was performed since 2016, with more intensive campaigns in 2020 and 2021. Average pH ranged from 7.9 to 9.0, average Chemical Oxygen Demand ranged from 36 mg/L to 90 mg/L and average Total Suspended Solids ranged from 7 to 93 mg/L. The main factors that contribute to these values were identified as the presence of waterbirds, vegetative debris that falls in ponds, and contamination with sewage. The FTW that were installed in some of the ponds could help to improve the water quality, but additional measures such as removal of bottom sediments and leaves in the fall, may be necessary. It is expected that the assessment of water quality carried out in the urban ponds can contribute to the overall improvement of urban water management.

Developing a 3D Hydrodynamic and Water Quality Model for Floating Treatment Wetlands to Study the Flow Structure and Nutrient Removal Performance of Different Configurations

Floating treatment wetlands (FTWs) are widely used in surface water. The nutrient removal performance depends on both physical processes and chemical/biological transformations in FTWs. However, research describing the coupling processes of hydrodynamic and water quality in the system remains limited. Therefore, a coupled three-dimensional model of hydrodynamic and water quality for FTWs was developed based on the Environmental Fluid Dynamics Code (EFDC). Additional plant drag terms were added to the momentum equations to simulate the suspended canopy effect, and the chemical/biological processes occurring in FTWs were integrated into the original water quality equations simultaneously. The fully calibrated model was used to compare the hydrodynamic characteristics and nutrient removal performance of seven FTW configurations. The modeling results showed that the main stream would turn to the bottom and side of the plant root zone because of the block in FTWs. The differences in the hydrodynamic characteristics among the seven configurations led to a difference in water quality improvement effects. Segmenting a single FTW into a pair of parallel FTWs could achieve the maximum nitrogen and phosphorus mass removal. The results of the study are useful for designing an optimal FTW configuration in surface water.

Textile Wastewater Treatment Through Plant–Microbe Synergism in a Floating Treatment Wetland System

Textile wastewater treatment is a complex process that requires a range of chemicals and generates hazardous sludge. This article evaluates a chemical-free, floating treatment wetlands system for the treatment of textile wastewater containing reactive red and reactive yellow dyes. Two aquatic plants, Eichhornia crassipes Solms and Pistia stratiotes L., were selected and vegetated in the system. The efficacy of the system was enhanced by inoculation of two pollutant-degrading and plant growth-promoting bacteria: Bacillus cereus and Bacillus subtilis. The eight treatments were designed with different combinations of plants and bacteria. The performance of the system was evaluated through key parameters of electrical conductivity, total dissolved solid, total suspended solids, biological oxygen demand, chemical oxygen demand, and dye concentration for the treated wastewater. The best treatment achieved the optimized plant–bacteria synergism and exhibited good reductions in the physicochemical parameters and the values of all the wastewater parameters were within the limits set in the standards of National Environmental Quality Standards and Zero Discharge of Hazardous Chemicals Program.

Assessment of effect on water quality by hybrid constructed floating treatment wetlands in the low flow urban rivers.

In this study, the effect of hybrid constructed floating treatment wetlands (FTWs) in a low-flow urban river was evaluated, and the influence radius on dissolved oxygen (DO), ammonia nitrogen (NH4+-N), and total phosphorus (TP) under different flow conditions was quantified. It was found that DO considerably increased and NH4+-N and TP decreased at the outlet after treatment with hybrid constructed FTW. The NH4+-N removal rate continued to increase to 72.03%, whereas that for TP decreased to 63.16% with fluctuation. The cloud picture of the influence radius showed that when DO ≤ 3 mg/L, Rflow ≈ 5 m, Rstill ≈ 3 m, and when DO > 3 mg/L, Rflow = Rstill ≈ 8 m. For NH4+-N under flow conditions, Rflow ≈ 8 m, whereas under still conditions when NH4+-N ≤ 7 mg/L, Rstill ≈ 5 m, and when NH4+-N > 7 mg/L, the concentration changes were not considerable. When TP ≤ 0.4 mg/L, Rflow ≈ 5 m, and when TP > 0.4 mg/L, Rflow ≈ 8 m; while under still conditions, when TP ≤ 0.8 mg/L, Rstill ≈ 5 m, and when TP > 0.8 mg/L, the concentration changes were not remarkable. Various water quality parameters and water flow affected the radius of influence of FTW on DO, NH4+-N, and TP at the spatial level. For the same water quality parameter, the influence radius in the flow state was larger than that in the still state (Rflow ≥ RStill). This study evaluated treatment efficiency of the hybrid constructed FTW in urban rivers and provided guidance and support for FTW deployment in future.

Floating Treatment Wetlands (FTWs) is an Innovative Approach for the Remediation of Petroleum Hydrocarbons-Contaminated Water

Floating Treatment Wetlands (FTWs) is an Innovative Approach for the Remediation of Petroleum Hydrocarbons-Contaminated Water

Metal Removal Kinetics, Bio-Accumulation and Plant Response to Nutrient Availability in Floating Treatment Wetland for Stormwater Treatment

Floating treatment wetland (FTW) is a recent innovation to remove nutrients from stormwater, but little is known about its effectiveness for metal removal. This study aims to test the hypothesis that the metal removal performance of FTWs will be affected by nutrient (NH3-N, NO3-N, and PO4-P) availability in stormwater. Two experiments were carried out in nutrient-deficient tap water, and two experiments were carried out in nutrient-rich lake water using four native Australian plants, namely Carex fascicularis, Juncus kraussii, Eleocharis acuta, and Baumea preissii. Up to 81% Cu and 44.9% Zn removal were achieved by the plants in 16 days in tap water. A reduction in Cu and Zn removal of 28.4–57.3% and 1.0–19.7%, respectively, was observed in lake water compared with tap water for the same duration. The kinetic analysis also confirmed that plant metal uptake rates slowed down in lake water (0.018–0.088 L/mg/day for Cu and 0.005–0.018 L/mg/day for Zn) compared to tap water (0.586–0.825 L/mg/day for Cu and 0.025–0.052 L/mg/day for Zn). A plant tissue analysis revealed that E. acuta and B. preissii bioaccumulated more than 1000 mg/kg of both metals in their tissue, indicating high metal accumulation capacities. To overcome the slower metal uptake rate problem due to nutrient availability, future studies can investigate multi-species plantations with nutrient stripping plants and metal hyper-accumulator plants.

Pontederia sagittata and Cyperus papyrus contribution to carbon storage in floating treatment wetlands established in subtropical urban ponds

Carbon sequestration is considered an ecosystem service of regulation provided by diverse ecosystems, including wetlands. It has been widely evaluated in the soil of natural wetlands while in constructed wetlands, there is scanty information. In Floating Treatment Wetlands (FTW) there is none. Previously, our research group reported the efficient performance of FTW in an urban polluted pond for two years. As a follow up, the aim of this work was to investigate the contribution of Cyperus papyrus and Pontederia sagittata to carbon storage (CS) in four FTW established in eutrophic urban ponds in a subtropical region. Plant growth, productivity, and CS were assessed in the aboveground biomass of C. papyrus and P. sagittata and the belowground biomass (root mix from C. papyrus and P. sagittata), throughout 26 months in 2 FTW with an area of 17.5 m2 (FTW1) and 33 m2 (FTW2) and throughout 19 months in 2 FTW with an area of 25 m2 (FTW3) and 33 m2 (FTW4), respectively. The macrophyte growth depended on various factors, such as the season, the plant species, and the location of the FTW. High relative growth rate values were found for both species (0.125 and 0.142 d−1 for P. sagittata and C. papyrus, respectively), especially during summer and early autumn. The highest values of productivity were 337 ± 125 gdw m−2d−1 for the aboveground biomass of C. papyrus in FTW2, 311 ± 96.90 gdwm−2d−1 for the aboveground of P. sagittata in FTW1, and 270 ± 107 gdw m−2d−1 for the belowground biomass in FTW2. The mean values of CS for P. sagittata found in FTW1 were 1.90 ± 0.94 kg m−2, while for C. papyrus in FTW2 they were 4.09 ± 0.73 kg m−2. The contribution of the belowground biomass to CS was also significant in FTW2 (4.58 ± 0.59 kg m−2).

Optimizing floating treatment wetland and retention pond design through random forest: A meta-analysis of influential variables

Floating treatment wetlands (FTWs), artificial systems constructed from buoyant mats and planted with emergent macrophytes, represent a potential retrofit to enhance the dissolved nutrient removal performance of existing retention ponds. Treatment occurs as water flows through the dense network of roots suspended in the water column, providing opportunities for pollutants to be removed via filtration, sedimentation, plant uptake, and adsorption to biofilms in the root zone. Despite several recent review articles summarizing the growing body of research on FTWs, FTW design guidance and strategies to optimize their contributions to pollutant removal from stormwater are lacking, due in part to a lack of statistical analysis on FTW performance at the field scale. A meta-analysis of eight international FTW studies was performed to investigate the influence of retention pond, catchment, and FTW design characteristics on effluent concentrations of nutrients and total suspended solids (TSS). Random forest regression, a tree-based machine learning approach, was used to model complex interactions between a suite of predictor variables to identify design strategies for both retention ponds and FTWs to enhance treatment of nutrient and sediment. Results indicate that pond design features, especially loading ratio and pond depth (which should be limited to 200:1 and 1.75 m, respectively), are most influential to effluent water quality, while the benefits of FTWs were limited to improving mitigation of phosphorus species and TSS which was primarily influenced by FTW coverage and planting density. Findings from this work inform wet retention pond and FTW design, as well as guidance on scenarios where FTW implementation is most appropriate, to improve dissolved nutrient and sediment removal in urban runoff.

Operational parameters optimization for remediation of crude oil-polluted water in floating treatment wetlands using response surface methodology

The application of floating treatment wetlands (FTWs) is an innovative nature-based solution for the remediation of polluted water. The rational improvement of water treatment via FTWs is typically based on multifactorial experiments which are labor-intensive and time-consuming. Here, we used the response surface methodology (RSM) for the optimization of FTW’s operational parameters for the remediation of water polluted by crude oil. The central composite design (CCD) of RSM was used to generate the experimental layout for testing the effect of the variables hydrocarbon, nutrient, and surfactant concentrations, aeration, and retention time on the hydrocarbon removal in 50 different FTW test systems planted with the common reed, Phragmites australis. The results from these FTW were used to formulate a mathematical model in which the computational data strongly correlated with the experimental results. The operational parameters were further optimized via modeling prediction plus experimental validation in test FTW systems. In the FTW with optimized parameters, there was a 95% attenuation of the hydrocarbon concentration, which was very close to the 98% attenuation predicted by the model. The cost-effectiveness ratio showed a reduction of the treatment cost up to $0.048/liter of wastewater. The approach showed that RSM is a useful strategy for designing FTW experiments and optimizing operational parameters.

Effects of Enrofloxacin on Nutrient Removal by a Floating Treatment Wetland Planted with Iris pseudacorus: Response and Resilience of Rhizosphere Microbial Communities

Constructed wetlands (CWs), including floating treatment wetlands (FTWs), possess great potential for treating excessive nutrients in surface waters, where, however, the ubiquitous presence of antibiotics, e.g., enrofloxacin (ENR), is threatening the performance of CWs. In developing a more efficient and resilient system, we explored the responses of the FTW to ENR, using tank 1, repeatedly exposed to ENR, and tank 2 as control. Plant growth and nutrient uptake were remarkably enhanced in tank 1, and similar phosphorus removal rates (86~89% of the total added P) were obtained for both tanks over the experimental period. Contrarily, ENR apparently inhibited N removal by tank 1 (35.1%), compared to 40.4% for tank 2. As ENR rapidly decreased by an average of 71.6% within a week after each addition, tank 1 took only 4 weeks to adapt and return to a similar state compared to that of tank 2. This might be because of the recovery of microbial communities, particularly denitrifying and antibiotic-resistance genes containing bacteria, such as Actinobacteria, Patescibacteria, Acidovorax and Pseudomonas. After three ENR exposures over six weeks, no significant differences in the nutrient removal and microbial communities were found between both tanks, suggesting the great resilience of the FTW to ENR.

Evaluation of phosphorus removal in floating treatment wetlands: New insights in non-reactive phosphorus

Excess phosphorus (P) in surface runoff has significant deleterious impacts on water quality through eutrophication. Commonly, P is transported via non-point pollution and the proportion of easily plant-available reactive P (RP) among other P forms may vary significantly. Non-reactive P (NRP) can potentially contribute to the eutrophication of waterbodies, however the cleavage into bio-available P forms and eventually their biological uptake remains uncertain. This holds also true for floating treatment wetlands (FTWs) which became established as nutrient mitigation measures for surface waters in recent years. However, little information is available about the conversion and removal of NRP in FTWs. In this study, the conversion and removal of different forms of P in FTWs were investigated. Experiments were operated in batch mode and treatments consisted of (1) two concentration levels: a high P concentration of 3.0 mg/L and a low P concentration of 1.0 mg/L, and (2) four mesocosm treatments: (a) artificial roots only, (b) substrates only, (c) plants only, (d) plants and substrates. The results showed that RP removal mainly depended on sedimentation, substrate sorption, and biological assimilation. The removal of NRP mainly depended on hydrolysis, microbial-mediated conversion, and biological absorption. The combination of plant and substrate provided stable and efficient phosphorus removal performance in high P conditions, while plants were important for P removal in low P conditions. Living plants were indispensable and greatly affected the performance of FTWs. The specific enrichment and culling of microorganisms by plants resulted in the formation of specific rhizosphere microbial communities and promoted the removal of NRP. Pseudomonas, Enterobacter, Acidovorax might be responsible for P mineralization in the FTWs. Comprehensive analysis indicated that the conversion and removal pathways of P in the FTWs were not mutually independent, and the plant-microbe-substrate interactions cannot be underestimated.

Viability assessment for the use of floating treatment wetlands as alternative production and remediation systems for nursery and greenhouse operations

Floating treatment wetlands (FTWs) are deployed in nursery and greenhouse water retention reservoirs to help manage and mitigate nutrient contaminants within irrigation return flow. One management issue for FTWs is the need for plant harvest to avoid releasing nutrients back into the water column when plant tissues begin to senesce. Some researchers recommend harvesting the entire plant to prevent nutrient release. Resale of plants harvested from FTWs is one possible use of this live biomass by the nursery and greenhouse industry. To determine if nursery and greenhouse operations could use FTWs to clean water and simultaneously produce saleable plants, the objectives of this study were to (1) quantify the survival and aesthetics of plants after transplant from a FTW and (2) compare the time to market between traditional (containerized) and FTW production systems. Overall, plants harvested from FTWs were resalable. Plants transplanted from FTWs had higher survival and aesthetic ratings (scale of 1–5, 5 = highest quality) when transplanted to containers (4.26) compared to directly into the ground (2.81). The growth of Panicum virgatum and Canna × generalis ‘Firebird’ in FTW systems was two weeks slower than that of plants grown in traditional production areas. Sustainability of FTW systems combined with the capacity to produce salable plants that will return a profit improves adoption feasibility for production horticulture.

REMEDIATION OF NUTRIENTS FROM MUNICIPAL WASTEWATER USING GERMAN GRASS-AIDED FLOATING TREATMENT WETLAND

The potential of using German grass (Echinochloa polystachya) in FTWs (Floating Treatment Wetlands) to treat municipal wastewater was investigated in this study. This mesocosm-based study was conducted over a 7-day Hydraulic Retention Period (HRT) with a total of 28 experimental days following a Randomized Complete Block Design (RCBD) with four treatments and three replicates. To evaluate system efficiency, mean temperature, PH, total dissolved solids (TDS), and electrical conductivity (EC) were also measured along with nitrate and phosphate concentrations. On day 28, the nitrate removal rate obtained with Pickerel weed was 88.65 % while German grass showed 86.56 % of remediation. On the other hand, while Pickerelweed removed 9.5 % of phosphate, German grass removed 7.47 %. The study demonstrated that the use of German grass in FTWs can be highly effective in the removal of nitrate ions from wastewater, while the removal of phosphate ions was minimal.