Water Treatment Residuals Research Articles

Effective removal of fluoride and lead ions from aqueous solutions using water treatment residue and rice straw co-pyrolysis biochar composites

The coexistence of fluoride and lead in the waste water often leads to compound pollution, and finding effective and cost-effective solutions remains a significant issue. In this study, water treatment residues biochar (WTR-BC) was synthesized for the first time through the co-pyrolysis of water treatment residues and agricultural wastes to remove fluoride and lead. The results indicated that WTR-BC demonstrated remarkable efficacy in adsorbing fluoride and lead, achieving a maximum capacity of 15.2 mg/g and 83.8 mg/g, respectively. The adsorption mechanism adhered to a pseudo-second-order kinetic model, which illustrated that it was primarily driven by chemisorption. Moreover, X-ray photoelectron spectroscopy analysis indicated the formation of new complexes, F-Al and F-C with F on the WTR-BC surface, and X-ray absorption fine structure spectroscopy showed that the lead in the WTR-BC samples mainly existed as lead ferrihydrite (64.7%) and Pb(C2H3O2)2 (35.3%). These results revealed that the binding of lead and fluorides to the WTR-BC surface was attributed to Fe-O-Pb, C-O-Pb-O-C, and Al-F bond structures. The successful preparation of WTR-BC provides a new approach for the treatment of complex fluoride and lead pollution and holds promise as a novel material for environmental remediation. Furthermore, it broadens the scope of reuse possibilities for both WTR and agricultural wastes.

Characterizing PFAS concentrations in drinking water treatment residuals

AbstractWhile drinking water treatment plants (DWTPs) are not considered a source of per‐ and polyfluoroalkyl substances (PFAS), PFAS concentrate in treatment residuals relative to their source water concentrations. Regulatory actions considered for PFAS‐impacted residuals could affect the cost and viability of conventional residual management practices. This study estimated the annual quantity of residuals generated in the United States and presents a framework for understanding how PFAS may concentrate in these residual streams. Findings of this work indicate that PFAS may substantially impact DWTP residuals management, especially coagulation and softening solids, at concentration factors greater than 100 and spent adsorbents at PFAS concentration factors greater than 10,000. If potential regulatory actions were to apply to coagulation and softening residuals, those regulations must consider impacts on disposal of more than 420,000,000 wet tons of at‐risk DWTP residuals which are generated annually.

Improving crop growing conditions with water treatment residual and compost co-amendments: Soil-water dynamics.

Land application of water treatment residual (WTR) in combination with phosphate-rich organic wastes, like compost or sewage sludge, in nutrient-poor soils was previously shown to promote crop growth. This WTR diversion from landfill to agriculture supports local and international mandates for waste circularity. Although soil-water dynamics-like saturated hydraulic conductivity, water retention, and hydrophobicity-are well-defined for compost and somewhat defined for WTR (except for hydrophobicity), the impacts of co-amending sandy soils with both are not well-defined. In laboratory analyses, co-amendment had an intermediate effect between individual amendments on the hydrophobic sandy soils, increasing water retention by 27% (WTR and compost both increased water retention), decreasing hydrophobicity by increasing hydraulic conductivity twofold (WTR and compost both decreased hydrophobicity), and having no effect on saturated hydraulic conductivity (decreased by WTR and increased by compost). With two positive effects and one "no effect" on soil-water dynamics in laboratory trials, the co-amendment was expected to buffer both crop water use efficiency (WUE) and nutrient availability under drought stress, for Swiss chard (Beta vulgaris L. var. cicla), co-investigated in a multifactorial pot trial. Soil nutrients, particularly phosphate, were shown more critical than soil-water dynamics to improve crop WUE. Thus, co-amended soils have significantly higher crop biomass and WUE than sandy soils. Phosphate-rich organic co-amendment is necessary for crop nutrient sufficiency and thus drought resilience in sandy soils amended with WTR. Thus, pairing wastes to soils for optimum fertility is a critical consideration in waste land application for both biomass and drought resilience.

Production of safe cyanobacterial biomass for animal feed using wastewater and drinking water treatment residuals

The growing interest in microalgae and cyanobacteria biomass as an alternative to traditional animal feed is hindered by high production costs. Using wastewater (WW) as a cultivation medium could offer a solution, but this approach risks introducing harmful substances into the biomass, leading to significant safety concerns. In this study, we addressed these challenges by selectively extracting nitrates and phosphates from WW using drinking water treatment residuals (DWTR) and chitosan. This method achieved peak adsorption capacities of 4.4 mg/g for nitrate and 6.1 mg/g for phosphate with a 2.5 wt% chitosan blend combined with DWTR-nitrogen. Subsequently, these extracted nutrients were employed to cultivate Spirulina platensis, yielding a biomass productivity rate of 0.15 g/L/d, which is comparable to rates achieved with commercial nutrients. By substituting commercial nutrients with nitrate and phosphate from WW, we can achieve a 18 % reduction in the culture medium cost. While the cultivated biomass was initially nitrogen-deficient due to low nitrate levels, it proved to be protein-rich, accounting for 50 % of its dry weight, and contained a high concentration of free amino acids (1260 mg/g), encompassing all essential amino acids. Both in vitro and in vivo toxicity tests affirmed the biomass's safety for use as an animal feed component. Future research should aim to enhance the economic feasibility of this alternative feed source by developing efficient adsorbents, utilizing cost-effective reagents, and implementing nutrient reuse strategies in spent mediums.

Study the drinking water treatment residue (DWTR) based on constructed wetlands with different plants: removal performance and adsorption mechanism.

To investigate the influence of plant types on the pollution removal efficiency of drinking water treatment residue (DWTR) based constructed wetlands, a pilot-scale experiment was constructed, and the adsorption mechanisms for phosphate (PO43--P) were investigated. The results demonstrated that regardless of plant types, the DWTRs-based constructed wetlands exhibited a high removal efficiency for PO43--P, but the contribution of plants in improving the adsorption rate was relatively negligible. Additionally, the Langmuir isotherm model can describe the adsorption behaviors well, which exhibited high adsorption capabilities of 162.11 mg/g. Furthermore, the pseudo-secondorder model can describe the adsorption kinetics well, demonstrating chemical processes were the main force. This experimental study can provide theoretical guidance to enhance our understanding of the pollutant removal mechanisms in DWTRs-based constructed wetlands.

Re-utilization of drinking water treatment residuals (DWTR): a review focused on the adsorption of inorganic and organic contaminants in wastewater and soil

Drinking water treatment residuals (DWTRs) show versatile adsorption properties for the removal of contaminants from wastewater and soil. Most adsorption studies are based on synthetic well-controlled solutions instead of real wastewater samples.

An XAS study of Hg(II) sorption to Al-based drinking water treatment residuals

Drinking water treatment residuals (DWTRs) are produced from the coagulation and flocculation processes in conventional drinking water treatment. The abundant metal oxide content of these materials resulting from the use of coagulants, like alum and ferric chloride, has driven strong research interest into the reuse of DWTRs as sorptive materials. Using a suite of aluminum-based DWTRs, we provide new insights into Hg(II) sorption mechanisms. Experiments performed at circum-neutral pH show that sorption capacities are related to the amount of organic carbon/matter present in DWTRs. We found that carbon rich samples can scavenge about 9000 mg/kg of Hg, in contrast to 2000 mg/kg for lime based DWTRs. X-ray absorption spectroscopy (XAS) at the Hg L3 edge further characterizes mercury coordination. X-ray absorption near edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) results point to a partial association of mercury with sulfur at low mass loadings, transitioning to a full association with oxygen/carbon at higher concentrations of sorbed Hg(II) and in DWTRs with limited sulfur content. These results suggest that sorption of Hg(II) is primarily controlled by the carbon/organic matter fraction of DWTRs, but not by the coagulants.

Arbuscular mycorrhizal fungi enhanced the drinking water treatment residue-based vertical flow constructed wetlands on the purification of arsenic-containing wastewater

Arsenic (As) is a toxic metalloid that poses a potential risk to the environment and human health. In this study, drinking water treatment residue (DWTR) and ceramsite-based vertical flow constructed wetlands (VFCWs) were built to purify As-containing wastewater. As a method of bioaugmentation, arbuscular mycorrhizal fungi (AMF) was inoculated to Pteris vittata roots to enhance the As removal of the VFCWs. The results showed that the As removal rates reached 87.82–94.29% (DWTR) and 33.28–58.66% (ceramsite). DWTR and P. vittata contributed 64.33–72.07% and 7.57–29% to the removal of As, while AMF inoculation intensified the As accumulation effect of P. vittata. Proteobacteria, the main As3+ oxidizing bacteria in the aquatic systems, dominated the microbial community, occupying 72.41 ± 7.76%. AMF inoculation increased As-related functional genes abundance in DWTR-based wetlands and provided a reliable means of arsenic resistance in wetlands. These findings indicated that the DWTR-based VFCWs with AMF inoculated P. vittata had a great purification effect on As-containing wastewater, providing a theoretical basis for the application of DWTR and AMF for As removal in constructed wetlands.

Per- and polyfluoroalkyl substances in water treatment residuals: Occurrence and desorption.

Per- and polyfluoroalkyl substances (PFAS) in surface and ground waters supplying municipal drinking water are a growing concern. However, PFAS concentrations in water treatment residuals (WTRs)-a solid by-product of water treatment-have yet to be explored. In a first of its kind assessment, we examine PFAS occurrence in seven calcium (Ca)-, iron-, and aluminum-based drinking water treatment residuals (DWTRs) and one wastewater effluent treatment residual (WWETR) produced using aluminum chlorohydrate (ACH). Only perfluoroalkyl acids (PFAAs) were detected, with total PFAA concentrations in the seven DWTRs produced from naturally recharged water sources ranging from 0 to ∼3.3μg kg-1 ; no PFAS were detected in either of the Ca-DWTRs. The ACH-WWETR contained the highest number and concentration of PFAAs (34μg kg-1 ). Desorption of resident PFAAs from the WTRs was negligible for the carboxylates (PFCAs). Some desorption of the sulfonates (PFSAs) was detected, particularly for PFOS which had the highest concentration among all resident PFAAs. The ACH-WWETR was further evaluated for its potential to attenuate additional PFAAs (3500μg mL-1 total PFAAs) in a biosolid-derived porewater matrix. Sorption was highest for long-chain PFAAs and subsequent desorption of the adsorbed PFAAs ranged from 0% to no more than 26%, with the WWETR mass added strongly affecting both PFSA and PFCA sorption/desorption. These findings suggest that WTRs, if introduced into the environment, are unlikely to be a major source of PFAS. Also, the use of particular WTRs as amendments may provide a beneficial reduction in PFAS mobility.

Yttrium-modified drinking water treatment residue for efficient phosphorus removal: efficacy, mechanism, and reproducibility.

The excessive presence of phosphate can cause eutrophication in water bodies. Yttrium has an extremely high affinity for phosphorus and is capable of forming stable complexes at low concentrations. Moreover, limitations in the resourcefulness of drinking water treatment residues were observed. In this study, a highly efficient phosphorus removal adsorbent (RJDWTR@Y) was prepared by calcination-alkali leaching-yttrium-loaded composite modification employing domestic drinking water treatment residue as raw material. And the effects of multiple factors on phosphate adsorption by RJDWTR@Y were examined. The results illustrated that the maximum adsorption capacity of the RJDWTR@Y for phosphate was 319.76 mg/g, with the chemical reaction of the multilayer as the predominant adsorption process. The adsorption mechanism is electrostatic gravitational force and the inner sphere complexation effect. RJDWTR@Y was effective against interference even at high concentrations of the coexisting anion. After five cycles, the desorption efficiency of phosphate was 75.11%. Filling the fixed bed with the material can efficiently remove phosphorus from the flowing liquid. The synthesis of RJDWTR@Y and the results of the study indicated that it has good application prospects. In addition to efficiently removing phosphorus, it can also recycle waste and achieve sustainability.

Effects of drinking water treatment residual amendments to biosolids on plant uptake of per- and polyfluoroalkyl substances.

Drinking water treatment residuals (DWTRs), solid by-products of drinking water treatment, are dominated by calcium (Ca), iron (Fe), or aluminum (Al), depending on the coagulant used. DWTRs are often landfilled, but current research is exploring options for beneficial reuse. Previous studies have shown that Al- and Fe-rich materials have potential to reduce the mobility of per- and polyfluoroalkyl substances (PFAS). Here, we investigated how amending biosolids with 5%wt/wt DWTRs affected plant bioavailable PFAS in two different simulated scenarios: (1) agricultural scenario with Solanum lycopersicum (tomato) grown in soil amended with an agronomically relevant rate of DWTR-amended biosolids (0.9% w/w, resulting in 0.045% w/w DWTR in the biosolids-amended soil) and (2) mine reclamation scenario examining PFAS uptake by Lolium perenne (perennial ryegrass) grown in soil that received DWTR-amended biosolids amendment at a rate consistent with the mine remediation (13% w/w, resulting in 0.65% w/w DWTR in the biosolids-amended soil). Amending biosolids with Ca-DWTR significantly reduced perfluorobutanoic acid (PFBA) uptake in ryegrass and perfluorohexanoic acid uptake in tomatoes, possibly due to DWTR-induced pH elevation, while Fe-DWTR amendment reduced PFBA bioaccumulation in ryegrass. The Al-DWTR did not induce a significant reduction in accumulated PFAS compared to controls. Although the reasons for this finding are unclear, the relatively low PFAS concentrations in the biosolids and relatively high Al content in the biosolids and soil may be partially responsible.

Mitigation of alkali-silica reaction in blast-furnace slag-based alkaline activated material through incorporation of alum water treatment residue

Alkaline-activated materials (AAMs) have been developed as an alternative binder for cement in an effort to reduce the carbon footprint. However, due to the high alkaline contents in AAMs, these materials, particularly ground granulated blast-furnace slag (GGBS)-based AAM, may have a high potential of detrimental alkali-silica reaction (ASR) when amorphous silica-rich aggregates are used. This study utilised alum water treatment residue (AWTR), a waste from drinking water treatment, as an additional precursor in slag-based AAM to mitigate ASR in mortars with high reactive glass aggregate. The results showed that incorporating 40% AWTR in the precursor improved the compressive strength from 47 to 64 MPa and reduced ASR-induced expansion from 0.9% to 0.2% after 28 days of the accelerated test. The characteristics of the ASR gels were analysed using backscattered electron images coupled with energy-dispersive x-ray spectroscopy. Microscopic observations revealed that the ASR products formed in two stages. In the earlier stage, amorphous ASR gels precipitated surrounding the aggregates, exerting pressure on them and the matrix, resulting in initial cracking. In the latter stage, ASR products extruded into the open cracks, and crystalline products were formed, causing additional damage to the matrix. The quantitative elemental analysis demonstrated that incorporating AWTR could slow down silica dissolution by forming an Al layer in the aggregate surface, prevent the ASR gel calcification and reduce the growth of the harmful crystalline ASR products.

Sediment resuspension causes horizontal variations in the distributions of phosphorus (P) and P-inactivating materials with differing P immobilization in different sediment planes.

The importance of controlling internal phosphorus (P) pollution in lakes has been recognized by scientists, and the application of P-inactivating materials to immobilize sediment P is often considered. However, sediment resuspension, a typical physical process occurring in lakes, has been demonstrated to increase the uncertainty of immobilization. In this study, we explored the characteristics of P immobilization in the horizontal direction under the effects of resuspension using annular flume tests based on drinking water treatment residuals (DWTR). The results showed that resuspension caused the mobile P and bioavailable P to be heterogeneously distributed in sediment planes after DWTR addition, resulting in varying P immobilization efficiencies at different depths. In particular, the coefficient of variation was 14.2-24.5% for mobile P horizontally distributed in the planes, resulting in a range of mobile P decreasing efficiencies at 24.0-47.8%. Further analysis indicated that variations in horizontal distribution were typically due to the varied migration of particles of different sizes. Specifically, P immobilization in sediment planes at different depths was regulated by promoting the migration of <8μm DWTR after relatively low-intensity disturbance (in surface 0-1cm sediment). After relatively high-intensity disturbance (in the whole 0-3cm sediment), immobilization in the horizontal direction was regulated by coupling the migration of >63μm DWTR (to the bottom) with the mixing of <8μm DWTR in the sediment plane at different depths. The varying horizontal distributions of total P, resulting from the migration of 16-32μm sediment, could enhance the heterogeneities of the P immobilization. Thus, the particle size of materials and lake background conditions, for example, the hydrodynamic characteristics and P distributions in differently sized sediments, should be used as key bases to select or develop P-inactivating materials to design proper remediation strategies for controlling internal P pollution in lakes.

Do drinking water treatment residuals underperform in the presence of compost in stormwater media filters?

Drinking water treatment residuals (WTR), a waste-derived product, are often recommended to use as an amendment in stormwater biofilters to enhance their capacity to remove phosphate and microbial pollutants. However, their efficacy has been assumed to remain high in the presence of compost, one of the most common amendments used in biofilters. This study tests the validity of that assumption by comparing the removal capacities of WTR-amended biofilters with and without the presence of compost. Our results show that amending sand with WTR increased E. coli removal by at least 1-log, but the addition of compost in the sand-WTR media lowered the removal capacity by 13 %. Similarly, the addition of WTR to sand improved phosphate removal to nearly 1177 %, but the removal decreased slightly by 8 % when adding compost to the media. The results confirmed that dissolved organic carbon (DOC) leached from the compost could compete for adsorption sites for bacteria and phosphate, thereby lowering WTR's adsorption capacity based on the amount of DOC adsorbed on WTR. Collectively, these results indicate that the stormwater treatment industry should avoid mixing compost with WTR to get the maximum benefits of WTR for bacterial removal and improve the performance lifetime of WTR-amended biofilters.

PH-Dependent Leaching Characteristics of Steel Slag Mitigated with Water Treatment Residual

Abstract Steel slag (S) is a by-product of the steel industry with an annual global production of 280 MMT. A significant amount of the S is landfilled, causing economic loss and waste of valuable land space. S is deemed to have great potential as an aggregate because of its mechanical properties; however, the high swelling potential and alkalinity caused by large amounts of free calcium, hence the risk for metal leaching, limit its utilization; thus, a pretreatment process is needed. Amending the S with alum-based water treatment residual (WTR), which may form insoluble complexes with the free calcium, is expected to yield successful results, yet the leaching behavior and controlling mechanisms need to be determined. This study aims to evaluate the pH-dependent leaching characteristics of the pure and WTR-treated S as well as determine the dominant oxidation states of the trace metals and the controlling mechanisms. Acid neutralization capacity and pHstat leaching tests were performed on S–WTR mixtures with WTR contents varying from 0 to 100 % at a pH range of 2 to 14. The tests showed that leaching of all metals is significantly affected by pH and WTR content. Aluminum and copper exhibit an amphoteric behavior in which minimum leaching is observed at the neutral pH values and the metal concentrations are elevated at extreme acidic and alkaline pHs. On the other hand, cadmium, magnesium, calcium, and barium show a cationic leaching pattern. Furthermore, geochemical analyses were undertaken via Visual MINTEQ to determine predominant oxidation states of the metals with a redox potential and the speciation within the aqueous solutions, which showed that most of the releases from the elements were solubility controlled.

Nitrogen removal performance in roadside stormwater bioretention cells amended with drinking water treatment residuals.

Bioretention cells, a type of green stormwater infrastructure, have been shown to reduce runoff volumes and remove a variety of pollutants. The ability of bioretention cells to remove nitrogen and phosphorus, however, is variable, and bioretention soil media can act as a net exporter of nutrients. This is concerning as excess loading of nitrogen and phosphorus can lead to eutrophication of surface waters, which green stormwater infrastructure is intended to ameliorate. Drinking water treatment residuals (DWTR), metal (hydr)oxide-rich by-products of the drinking water treatment process, have been studied as an amendment to bioretention soil media due to their high phosphorus sorption capacity. However, very few studies have specifically addressed the effects that DWTRs may have on nitrogen removal performance within bioretention cells. Here, we investigated the effects of DWTR amendment on nitrogen removal in bioretention cells treating stormwater in a roadside setting. We tested the capacity of three different DWTRs to either retain or leach dissolved inorganic nitrogen in the laboratory and also conducted a full-scale field experiment where DWTR-amended bioretention cells and experimental controls were monitored for influent and effluent nitrogen concentrations over two field seasons. We found that DWTRs alone exhibit some capacity to leach nitrate and ammonium, but when integrated into sand- and compost-based bioretention soil media, DWTRs have little to no effect on the removal of nitrogen in bioretention cells. These results suggest that DWTRs can be used in bioretention media for enhanced phosphorus retention without the risk of contributing to nitrogen export in bioretention effluent.

Phosphorus migration from sediment to phosphorus-inactivating material: A key process neglected by common phosphorus immobilization assessments for lake geoengineering

Various phosphorus (P)-inactivating materials with a strong capability of immobilizing P in sediment have been developed for lake geoengineering purposes to control internal P pollution. However, unsatisfactory applications have raised concerns about the reliability of the method. This study hypothesized that P migration from sediment to material is a key process regulating the immobilization, which is often neglected by common assessment procedures that assume that the material is closely in contact with sediment (e.g., as mixtures). To verify this hypothesis, 90-day incubation tests were conducted using drinking water treatment residue (DWTR). The results showed that the soluble P in the overlying water of sediment–DWTR mixtures and the mobile P in the mixtures were substantially reduced from the initial period and remained low during the whole incubation tests. However, assessment based on separated samples indicated a gradual P migration from sediment to DWTR for immobilization. Even after 90 days of incubation, mobile P still accounted for ∼5.33% of total P in the separated sediment. Further analysis suggested that using mixtures of sediment with DWTR accelerated P migration during the assessment, leading to a faster P immobilization assessment. Considering the relatively low levels of mobile P in the separated DWTR during incubation, the gradual decrease in mobile P in the separated sediment indicates that sediment P release regulates P immobilization efficiency. Therefore, designing a proper strategy to ensure sufficient time for the material to remain in close contact with the target sediment is critical to reducing uncertainties in lake geoengineering.

A circular economy approach to drinking water treatment residue management in a catchment impacted by historic metal mines

Drinking water treatment residues (DWTR) from mining areas which remove and contain potentially toxic elements (PTE) could still potentially be used as a soil amendment to restore contaminated sites in the same catchment, thus eliminating waste and reducing the chemical and physical mobility of the pollutants. To assess this restorative and regenerative approach to DWTR management, field and pot trials were established with soils from a historic Pb–Zn mine site in the North East of England, amended with either local DWTR or the nearest available municipal green waste compost (GWC). Soils from the mine site were found to have very low levels of nutrients and very high levels of PTE (Pb and Zn > 13, 000 mg/kg). The perennial grass species Phalaris arundinacea, known for many ecosystem service benefits including soil stabilization, was used throughout this study. The application of the BCR sequential extraction to soils amended with the DWTR in the pot trials found a significant decrease in the bioavailability of Pb and Cu (p < 0.05) after plant growth when compared with an unamended control. The field trial involved 648 pre-grown grass plants planted-out into mine soils amended with either DWTR, GWC or a mixture (MIX) of the two, all at rates of 25–30% w/w. Both amendments and the MIX had significant positive effects on biomass production compared to the unamended control in the following order GWC > MIX > DWTR (p < 0.05). Results of the elemental analysis of biomass from the field trial were generally ambiguous and did not reflect the decreased bioavailability noted in the pot trials using the BCR procedure. Pot trials, however, showed increases in plant growth and decreases in concentrations of Cr, Cu, Pb and Zn in above ground biomass following the application of both amendments. Further work should involve the testing of a mixture of DWTR and other soil amendments to enhance plant growth. The success of these trials should provide confidence for those working in drinking water treatment and catchment management to reuse the waste residues in a circular economy and a sustainable way that could improve water quality over time.

Microbial genome (Illumina MiSeq) sequencing of drinking water treatment residuals to evaluate compatibility with environmental applications

The clarification of drinking water leads to the production of large quantities of water treatment residuals (WTRs). DNA was extracted from six WTR samples collected from water treatment plants within the UK to compare their bacterial communities and examine whether factors such as coagulant usage (aluminium versus iron salt), the type of water source (reservoir or river), or leachable chemical composition influence these communities. Bacterial 16S variable region 4 (V4) was amplified and sequenced using Illumina MiSeq sequencing. The most abundant phyla in WTR samples were Proteobacteria, Actinobacteria, Bacteroidetes, Acidobacteria, and Firmicutes, collectively representing 92.77–97.8% of the total bacterial sequences. Statistical analysis of microbial profiles indicated that water source played a significant role in microbial community structure, diversity, and richness, however coagulant type did not. PERMANOVA analysis showed that no single chemical variable (pH, organic matter, or extractable element concentration) influenced microbial composition significantly; however, canonical correspondence analysis of WTR microbiomes yielded a model using all these variables that could be used to explain variations in microbial community structures of WTRs (p < 0.05). No common, potentially toxic cyanobacteria, or related pathogens of concern were found. Analysis with PICRUSt showed that WTRs all had similar predicted microbial functional profiles. Overall, the results indicate that WTRs analysed in this study are unlikely to pose any threat to soil microbial community structure when applied to land as a soil conditioner or enhancer and may help to enhance the soil microbial community.

Organic matter from water treatment residuals stabilizes amorphous iron to maintain phosphorus adsorption by iron hydroxides

Functional materials enriched with amorphous iron are commonly used for environmental remediation. However, their performance is often susceptible to aging and reductive dissolution effects. Amorphous iron in drinking water treatment residuals (DWTRs) primarily exists in organic matter-bound forms, possessing a stable phosphorus adsorption capability. In this study, in-depth investigations were conducted to evaluate the effect of organic matter extracted from DWTRs on amorphous iron stability in iron hydroxides by using incubation tests for 180 days. The results showed that organic matter from different DWTRs exhibited similar FTIR and XPS patterns; whereas, the solid-state carbon-13 NMR analysis and the carbon/nitrogen atom ratios indicated variations in structures and origins. Notably, organic matter addition increased the 0.01 and 0.005 M-oxalic acid-ammonium oxalate extractable iron, especially the later one which was greater than the control by approximately 24 times, inhibiting iron aging in iron hydroxides. The addition also promoted the transformation of iron from acid-soluble and reducible to oxidizable and non-European Community Bureau of Reference method-extractable forms, reducing the iron release potential. This effect was intensified when organic matter contained relatively low levels of carboxylic carbon and nitrogen/oxygen-alkyl carbon while high levels of carbon=oxygen/ketone carbon. The inhibited iron aging further led to stronger phosphorus adsorption capabilities of iron hydroxides, and the reduced iron release potential caused the adsorbed phosphorus to exist in more stable forms. Therefore, organic matter from DWTRs stabilized amorphous iron to maintain phosphorus adsorption capabilities of iron hydroxides. These mechanisms show potential for application in the development of iron-based materials.