Concentrated Stream Research Articles

Conversion of C4F8 via plasma catalysis over Al2O3/Zr/SO4-2 catalyst: Effects of H2O(g)

Reducing the emission of octafluorocyclobutane (C4F8) which has a high global warming potential (GWP) of 10,300 is essential. This study investigated the effectiveness of C4F8 removal achieved with plasma catalysis system. Effects of Ar content in the gas stream and inlet C4F8 concentration on C4F8 conversion efficiency were evaluated with two types of catalysts including Al2O3 (A) and Al2O3 /ZrO2/SO42-(AZS). Results demonstrated a linear correlation between Ar content with C4F8 conversion, and energy efficiency (EE). Lissajous figure analysis coupled with BOLSIG + simulations revealed that the AZS catalyst applied increased the mean electron energy (MEE) to 2.68 eV and modified the electron energy distribution function (EEDF), likely contributing to its superior performance. At a C4F8 concentration of 400 ppm, the AZS catalyst coupled with plasma achieved 80 % conversion with an EE of 1.85 g/kWh. Interestingly, at an optimal inlet C4F8 concentration of 5,000 ppm, the highest EE (11 g/kWh) was obtained with a C4F8 conversion efficiency of 18 %. The addition of H2O(g) as a reactant resulted in complete C4F8 conversion when AZS catalyst was applied. The optimal plasma-catalytic conditions determined through C4F8 conversion, EE and CO2 equivalent (CO2e) assessments, yielded a CO2e emission (CO2e) to CO2 reduction (CO2r) ratio of 1:9.09. Product analysis revealed the formation of CO2, CO, and HF during C4F8 conversion with AZS catalyst in the presence of H2O(g). The system was operated at 12 kV for the gas stream containing 50 % Ar and N2 as carrier gas at a flow rate of 100 mL/min. These findings highlight the potential of plasma catalysis for the effective abatement of C4F8, offering a promising strategy for mitigating its environmental impact.

Risk assessment and quantification of elemental concentrations in river and stream in Nigeria

The rising prevalence of elements in surface water is becoming a major global environmental problem due to their resistance to change and capacity to accumulate in organisms. Humans are causing significant heavy metal contamination in the aquatic environment, which is killing aquatic creatures. The purpose of this study is to determine the elemental composition of surface water in Ogun State. Water samples were collected from a river that flows alongside Liberty Estate in Ewupe, Ado-Odo Ota Local Government Area, and a stream that runs alongside Pacific Estate in Iganmode. The element composition of water samples was determined using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). Several components were found to exceed the limitations imposed by the World Health Organisation (WHO) and the Environmental Protection Agency (EPA). In River I, sodium (Na) had the highest apparent concentration, whereas titanium (Ti) had the lowest average concentration. In River II, silicon (Si) had the highest apparent concentration, whereas chromium (Cr) had the lowest average concentration. Titanium (Ti) had the lowest mean concentration value, whereas sodium (Na) appeared to have the highest concentration in stream I. Stream II has the highest concentration of sodium (Na), while Uranium (U) has the lowest average concentration value. Nonetheless, the incremental lifetime cancer risk (ILCR) values for adults and children who drink river and stream water are similar, ranging from 10–9 to 10–8. This implies a high carcinogenic risk (CR) linked with drinking water from both sources. Children who had access to both stream and river water demonstrated higher risk levels than adults. Routine studies of the state's streams and rivers should be conducted to avoid the subtle impact that may emanate from the water bodies.

Using environmental DNA to assess the response of steelhead/Rainbow Trout and Coastrange Sculpin populations to postfire debris flows in coastal streams of Big Sur, California

AbstractObjectiveDebris flows are among the most extreme disturbances to streams and are predicted to become more frequent under climate change. We assessed the response of steelhead Oncorhynchus mykiss (anadromous Rainbow Trout)/Rainbow Trout (hereafter, collectively referred to as O. mykiss) and Coastrange Sculpin Cottus aleuticus populations to major postfire debris flows in two small coastal basins of California using noninvasive environmental DNA (eDNA) sampling.MethodsWe analyzed water samples from disturbed reaches for eDNA in the first two summers after debris flows. In Big Creek, all Coastrange Sculpin habitat was disturbed by debris flows, but a major tributary occupied by O. mykiss was not affected. In Mill Creek, all available habitat for both species was disturbed. We also sampled two unburned basins as undisturbed reference streams.ResultIn Big Creek, O. mykiss eDNA was detected in all water samples during both years, and concentrations in some samples approached the lowest concentrations in the reference streams. Coastrange Sculpin eDNA was detected only in a single water sample in the first year and was not detected in the second year. In Mill Creek, neither species was detected in the first year, but during the second year, O. mykiss eDNA was detected at very low concentrations in 40% of samples and Coastrange Sculpin eDNA was detected in a single sample.ConclusionOur results indicate that O. mykiss and Coastrange Sculpins either survived in or quickly reoccupied the disturbed reaches from within the basins. However, the detection rates for cases in which all habitat for a species in a basin was affected by debris flows indicate that abundances were very low, suggesting that persistence may be uncertain unless there is successful reproduction or immigration. Finally, eDNA appears to be effective for monitoring sensitive fish populations after a major disturbance; however, detection rates in individual samples may be low and require appropriate sampling designs to achieve the desired detection probability.

Deep learning models for predicting plant uptake of emerging contaminants by including the role of plant macromolecular compositions

Deep learning models can predict uptake of emerging contaminants in plants with improved accuracy because they leverage advanced data-driven approaches to capture non-linear relationships that traditional models struggle to address. Traditional models suffer from low accuracy in predicting transpiration stream concentration factor (TSCF) and root concentration factor (RCF). This study applied deep neural networks (DNN), recurrent neural networks (RNN), and long short-term memory (LSTM) to enhance the accuracy of predictive models for TSCF and RCF. The three models used nine chemical properties and two plant root macromolecular compositions for predicting TSCF and RCF. The results indicated that deep learning models predict TSCF and RCF with improved accuracy compared to mechanistic models. The coefficient of determination (R2) for the DNN, RNN, and LSTM models in predicting TSCF was 0.62, 0.67, and 0.56, respectively. The corresponding mean squared error (MSE) on the test set for the models was 0.055, 0.035, and 0.060, respectively. The R2 for the DNN, RNN, and LSTM models in predicting RCF was 0.90, 0.91, and 0.84, respectively. The corresponding MSE for the models was 0.124, 0.071, and 0.126, respectively. The results of feature extraction using extreme gradient boosting underlined the importance of lipophilicity and root lipid fraction.

Process design and optimization of membrane-based CO2 capture process with experimental performance data for a steam methane reforming hydrogen plant and a coal-fired power plant

Membrane-based separation is one of the promising next-generation carbon capture technologies. High degrees of freedom in process design for multi-stage membrane networks, make it difficult to fully understand the impact of network configuration on the economics of capture and separation performance. Furthermore, the optimality in process design is heavily influenced by the levels of membrane performance and the characteristics of feed gas. Membrane model utilized for the study is validated and tuned with 183 experimental data measured at various operating conditions, having 2.45 % of overall prediction error. The superstructure-based framework for the optimization of multi-stage membrane networks is used, with which design interactions between network configurations, flue gas characteristics and membrane performance are systematically investigated. Techno-economic analysis is embedded in the optimization framework, which performs rigorous trade-off between capital and operating costs, as well as allows simultaneous determination of optimal process network configuration and operating variables. Flue gas streams emitted from a coal-fired power plant and a steam methane reforming hydrogen production plant are selected as CO2 capture sources for the analysis of flue gas characteristics. Case study shows capture cost ranged from 30.2 $/tCO2 to 79.9 $/tCO2, depending on the network configuration and design specifications, and clearly improves our techno-economic understanding on the network configuration of membrane capture process. In addition, cryogenic-hybridized membrane capture process is examined and the optimal level of enriched CO2 stream concentration between membrane capture and cryogenic distillation processes is determined. Study suggests how network configuration of membrane capture process can impact capture cost.

Effect of larval instar and post‐harvest treatments on heavy metals in BSFL and frass reared on commercial food waste streams

SummaryThe use of black soldier fly larvae (BSFL) to valorise different organic waste streams and the subsequent use of resulting larvae as a feedstock ingredient is increasing rapidly in several regions across the globe. The lack of knowledge about several safety issues including chemical contaminants (e.g. heavy metals) seems to affect the upscaling and commercialisation of this product. This study evaluates the safety of the BSFL against chemical contaminants including heavy metals and mycotoxins in both BSFL and frass samples reared with different food waste streams (e.g. soy waste, customised bread‐vegetable diet, food waste mixture, supermarket and childcare centre) from two commercial production facilities. The effect of larval instars and post‐harvest treatments (e.g. blanching and drying) on the safety of the BSFL was also investigated. The concentration of heavy metals was primarily influenced by the concentration in the food waste streams. The concentration was also higher in 6th instar compared to 5th instar larvae. The effect of blanching and drying have a varied effect on the concentration of heavy metals. Mycotoxins were found to be below the limit of quantification for all samples. The outcomes of this study indicated that BSFL grown on food waste streams and the resulting frass is safe against different heavy metals analysed. The findings of this study will assist the commercial BSFL manufacturers with the identification of relevant control points to ensure the chemical safety of their products. Therefore, encourage the use of different food waste streams as feedstock for rearing BSFL.

Inferences based on diatom compositions improve estimates of nutrient concentrations in streams

Nutrient concentrations in streams vary strongly with flow conditions, and routinely gathered field measurements of nutrients reflect this variability. Diatom assemblage composition has been used in previous studies to infer nutrient concentrations, and because diatoms integrate nutrient concentrations over longer periods of time, diatom inferences may be less susceptible to fluctuations in streamflow. We tested this hypothesis by leveraging differences in the flashiness of streams across a large continental data set. More specifically, we tested whether the variabilities of direct measurements and diatom inferences of dissolved phosphorus and nitrate were greater in flashy versus non-flashy streams. We further considered whether models linking landscape predictor variables to nutrient concentrations yielded consistent results across flashy and non-flashy streams. Our analysis indicated that measured nutrient concentrations were more variable in flashy compared to non-flashy streams and that landscape models identified different important predictors of nutrient concentrations when fit using data from flashy vs. non-flashy streams. In contrast, variabilities of diatom-inferred nutrient concentrations were similar among stream types, as were the important predictor variables (e.g., manure application rates for nitrate and number of wet days for dissolved phosphorus). These analyses indicate that use of diatom-inferred nutrient concentrations can potentially improve efforts to quantify stream nutrient concentrations.

Exploring the Spatiotemporal Heterogeneity of Stream Nitrogen Concentrations in a Typical Human‐Activity‐Influenced Headwater Watershed in South China

AbstractStream nitrogen concentrations significantly impact nitrogen loads and greenhouse gas emissions, but their spatiotemporal heterogeneity and human influences remain highly uncertain. This study thoroughly explored the spatiotemporal variations in stream nitrogen concentrations in a typical headwater watershed in South China. Spatially distributed measurements were conducted during 2020–2022, and mathematical modeling was implemented based on incorporating these data. More than 4,400 data points were collected for water temperature and concentrations of ammonium nitrogen (NH4‐N), nitrate nitrogen (NOx‐N), dissolved total nitrogen (DTN), total nitrogen (TN), and dissolved oxygen. Results showed that NOx‐N was the largest component of TN, with average concentrations of 1.20 and 1.66 mg L−1, respectively. The stream N2O concentration could be predicted using NH4‐N and NOx‐N concentrations via the Michaelis‐Menten equation. Significant downstream decreases in NH4‐N, NOx‐N, DTN, and TN concentrations were identified in the largest river in the watershed, and clear spatial differences in these nitrogen concentrations existed among the three main rivers. Clear seasonal and annual variations in stream nitrogen concentrations were observed. NH4‐N, NOx‐N, DTN, and TN concentrations correlated with cumulative precipitation from the preceding 8–12 days, while stream N2O concentrations correlated over 13–20 days. Stream N2O concentrations and emissions averaged 12.77 nmol L−1 and 1.12 nmol m−2 s−1, respectively, and were lower in summer than in other seasons. Upstream tea plantations, villages, and adjacent agricultural lands significantly affected nitrogen concentrations, while overflow dams did not. These findings highlight nitrogen cycle's complexity and the need for high‐resolution data to guide effective watershed management.

Design method and experimental study on a novel self-sustaining internal combustion burner

Peak shaving represents a particular challenge with regard to coal-fired power plants, as methods to achieve stable, high-efficiency, and low-nitrogen-oxide combustion at ultralow loads remain unavailable. In this study, based on analysis of the ignition mechanism, heating mode, and theory of preheating method, we proposed a novel pulverized coal full-time self-sustaining internal combustion burner by combining the processes of plasma ignition, reverse jetting, and multi-stage ignition. A flame stability model for the burner's recirculation zone was established using the temperature change rate of the recirculation zone as a criterion. The ignition position of the stable self-sustained combustion of the pulverized coal gas stream was determined under various working conditions, and the length of the ignition core stage, a component of the burner inside which the pulverized coal gas stream achieves stable self-sustained combustion, was determined based on the calculated maximum ignition position; a self-sustaining internal combustion burner was then designed based on the ascertained length. Subsequently, we established an experimental system for the self-sustaining internal combustion burner and conducted tests in the load range of 25 %–100 %. Notably, the results of the pulverized coal self-sustained ignition position experiment were consistent with the computational results of the model. The self-sustaining internal combustion burner offers a quick start–stop feature, with the pulverized coal able to achieve stable self-sustained combustion after the plasma igniter shuts off. The resultant gas-phase products exiting the burner met the boiler's concentration requirements for enhanced nitrogen reduction, and the burner exhibited good anti-interference properties. The influence of the pulverized coal gas stream velocity and concentration on the self-sustained ignition was expounded. Under the experimental conditions, an optimal concentration range was ascertained for stable ignition of the pulverized coal gas stream, with the ignition position gradually advancing backward with increasing gas stream velocity. Together, our findings highlight the potential of the proposed burner design to support clean and efficient pulverized coal combustion in coal-fired power plants.

Ce anomaly in hemiboreal headwater streams: An indicator of dominant groundwater flow paths through the catchment

The water quality in headwater streams depends on the groundwater origin and its transport pathways before it eventually discharges as surface water. In this case study, we present the Ce anomaly of over 100 hemiboreal headwater streams in Sweden and discuss the potential of the Ce anomaly to define two stream end members as proxies for the stream origin. The data show a relation between topography and the Ce anomaly, with more negative values (−0.8 to −0.4) in hilly catchments with distinct slopes, defined as an oxidized groundwater end member. The second end member is dominated by groundwater discharge from reduced, organic-rich riparian and wetland soils (reduced groundwater) having small Ce anomalies (−0.3 to zero). Element concentrations show a wide range, depending on the end member of the stream. For example, redox elements (Fe, Mn, S and N) show concentrations up to five times in streams with small negative Ce anomalies (reduced groundwater) compared to the concentrations in streams with large negative Ce anomalies. While element concentrations (of the classic redox elements Fe, Mn, S, N) show seasonal variations due to a summer drought period and the resulting reduced conditions, the Ce anomaly is constant, offering an excellent indicator of the dominant groundwater flow paths regardless of seasonal variations.

Distribution and seasonal variations of perfluoroalkyl substances (PFAS) in streams within the Changwon region

This study investigates the distribution and seasonal variations of Poly- and Perfluoroalkyl Substances (PFAS) concentrations in local streams within the Changwon area, aiming to quantify the total PFAS load entering Masan Bay. Using solid-phase extraction (SPE) cartridges for pretreatment, we analyzed 36 types of Perfluorinated carboxylic acids (PFCAs), Perfluorinated sulfonates (PFSAs), and precursor compounds via liquid chromatography-high resolution mass spectrometry (LC-HRMS). The study observed varying concentration levels across different streams, with notable detections of Perfluorohexanoic acid (PFHxA), Perfluorooctanoic acid (PFOA), Perfluorobutanesulfonic acid (PFBS), Perfluoropentanoic acid (PFPeA), and Perfluoroheptanoic acid (PFHpA) at quantifiable levels. Except for the Sogye Stream (SGC), PFCA-based compounds comprised over 50% of the total PFAS detected, predominantly featuring shorter-chain compounds with fewer than eight carbon atoms. Seasonal trends indicated that PFAS concentrations were lowest during the high rainfall period in summer and peaked in winter and spring. The Nam Stream (NC) contributed the highest PFAS inflow into Masan Bay, attributed to its elevated flow rates, while Sum B had high concentrations but lower emissions due to its limited flow. The total annual PFAS discharge was estimated at 1.9 kg. These findings highlight the urgent need for targeted management and comprehensive countermeasures to protect environmental health.

Magnetohydrodynamics Marangoni boundary-layer copper/water nanofluid flow driven by surface temperature gradient over a rotating disk

Marangoni driving problem is very important in many practical science engineering, such as semiconductor industry, crystal growth, aerospace, material synthesis. Consider these applications, this work we investigate Marangoni boundary-layer Copper/Water nanofluid driven by the surface temperature gradient over a rotating disk in the presence of magnetic field. Buongiorno model of nanofluids, which contains two important terms, thermophoresis and Brownian motion, are taken into account. Rotating disk model is established, also a suitable Kármán transformation and the multi-shooting technique are applied. Graphical discussion include free stream concentration, rotating speed, magnetic field, Marangoni driving, thermophoresis, Brownian diffusion, Prandtl number, and Schmidt number. The results show that rotating speed inertia and Marangoni driving force tend to reduce temperature/concentration and thickness of thermal/mass boundary layer, while free stream concentration and magnetic field raise up. Meanwhile, Prandtl number tends to reduce temperature, while thermophoresis and Brownian diffusion raise up, and Schmidt number has little influence on temperature, and Schmidt number and Brownian diffusion tend to reduce concentration, while thermophoresis raises up, and Prandtl number has an important influence on concentration. For most selected values of physical parameters, as the nanofluid far away from the disk, the local temperature decreases directly, and the local concentration firstly increases to a peak and then decreases to zero.

Separation and concentration of CO2 from air using a humidity-driven molten-carbonate membrane

Separation processes are substantially more difficult when the species to be separated is highly dilute. To perform any dilute separation, thermodynamic and kinetic limitations must be overcome. Here we report a molten-carbonate membrane that can ‘pump’ CO2 from a 400 ppm input stream (representative of air) to an output stream with a higher concentration of CO2, by exploiting ambient energy in the form of a humidity difference. The substantial H2O concentration difference across the membrane drives CO2 permeation ‘uphill’ against its own concentration difference, analogous to active transport in biological membranes. The introduction of this H2O concentration difference also results in a kinetic enhancement that boosts the CO2 flux by an order of magnitude even as the CO2 input stream concentration is decreased by three orders of magnitude from 50% to 400 ppm. Computational modelling shows that this enhancement is due to the H2O-mediated formation of carriers within the molten salt that facilitate rapid CO2 transport.

The influence of burn severity on dissolved organic carbon concentrations across a stream network differs based on seasonal wetness conditions

Abstract. Large, high-severity wildfires in many regions across the globe have increased concerns about their impacts on carbon cycling in watersheds. Altered sources of carbon and changes in catchment hydrology after wildfire can lead to shifts in dissolved organic carbon (DOC) concentrations in streams, which can have negative impacts on aquatic ecosystem health and downstream drinking-water treatment. Despite its importance, post-fire DOC responses remain relatively unconstrained in the literature, and we lack critical knowledge of how burn severity, landscape elements, and climate interact to affect DOC concentrations. To improve our understanding of the impact of burn severity on DOC concentrations, we measured DOC at 129 sites across a stream network extending upstream, within, and downstream of a large, high-severity wildfire in Oregon, USA. We collected samples across the study sub-basin during four distinct seasonal wetness conditions. We used our high-spatial-resolution data to develop spatial stream network (SSN) models to predict DOC across the stream network and to improve our understanding of the controls on DOC concentrations. Spatially, we found no obvious wildfire signal – instead, we observed a pattern of increasing DOC concentrations from the high-elevation headwaters to the sub-basin outlet, while the mainstem maintained consistently low DOC concentrations. This suggests that effects from large wildfires may be “averaged” out at higher stream orders and larger spatial scales. When we grouped DOC concentrations by burn severity group, we observed a significant decrease in the variability of DOC concentrations in the moderate and high burn severity sub-catchments. However, our SSN models were able to predict decreases in DOC concentrations with increases in burn severity across the stream network. Decreases in DOC concentrations were also highly variable across seasonal wetness conditions, with the greatest (−1.40 to −1.64 mg L−1) decrease occurring in the high-severity group during the wetting season. Additionally, our models indicated that in all seasons, baseflow index was more influential in predicting DOC concentrations than burn severity was, indicating that groundwater discharge can obscure the impacts of wildfire in a stream network. Overall, our results suggested that landscape characteristics can regulate the DOC response to wildfire. Moreover, our results also indicated that the seasonal timing of sampling can influence the observed response of DOC concentrations to wildfire.

Effects of changes in precipitation patterns and stream flow on stream DOC and [formula omitted] concentrations in a headwater catchment in a subtropical region of southern China

Study regionThe Guantian headwater watershed is located in the subtropical region of South China. Study focusThis study examined how changes in precipitation and consequent shifts in streamflow affect dissolved organic carbon (DOC) and nitrate (NO3−) concentrations in a headwater stream. New hydrological insights for the regionThe results indicate that stream DOC and NO3− concentrations decrease from spring to winter (0.16 mg/L/month for DOC and 0.18 mg/L/month for NO3−) and have a generally positive relationship with streamflow (p < 0.05). Changes in precipitation-concentration, precipitation-amount and consequent shifts in streamflow remarkably affect stream DOC and NO3− concentrations. When precipitation events are more heavily concentrated (when precipitation concentration index (PCI) changed from 12.81 to 18.33) from May to July, the stream DOC and NO3− concentrations tend to increase from May to July but decrease before and after this period. In addition, an increase in precipitation-amount tends to increase stream DOC and NO3− concentrations before the occurrence of peak flow and in the dry period from September to December, but decreases them in the wet period from June to September. When the precipitation-amount decreases, the trend of the change in concentration is exactly the opposite. These findings suggest that stream DOC and NO3− concentrations in the headwater stream are highly chemodynamic: they are transport-limited before the occurrence of high flow and in the dry period from September to December, while they are supply-limited in the June-September period.

Long-term Changes in Sulfate and Nitrate Concentrations in Streams in Western Japan Between 1986 and 2023 in Response to Changes in Sulfur and Nitrogen Deposition from the Atmosphere

Long-term Changes in Sulfate and Nitrate Concentrations in Streams in Western Japan Between 1986 and 2023 in Response to Changes in Sulfur and Nitrogen Deposition from the Atmosphere

Ecological regulation of chemical weathering recorded in rivers

We provide a new generalized framework accounting for the effects of water and nutrient uptake by roots in the weathering zone on broad patterns of solute concentration - discharge relationships in rivers. As plants withdraw water from the subsurface, they decrease the area-normalized fluid drainage rate. This results in a discharge flux to springs and streams which is a function of water use by plants, leading to ecosystem regulation of the characteristic timescales of chemical weathering reactions and fluid flow rate. Our predictive framework quantifies the pore water and river solute chemistry generated from water-rock interactions, regulated by the effects of vegetation on water flow rates. To this we add the capacity for vegetation to enrich, passively admit, or discriminate against a given dissolved solute based on ecophysiological needs, thus further modifying the local concentration of a given chemical species in solution across a weathering profile. This uptake flux is not lost to the atmosphere in the same manner as water is removed from the subsurface via evapotranspiration. Rather, these extracted chemical species become incorporated into the topsoil as litter which may be partially or fully resolubilized, creating an upper boundary condition which depends on the uptake and recycling characteristics of the ecosystem. These processes are united through a new solution to the advection-reaction equation which allows vegetation to regulate broad patterns of stream concentration - discharge relationships. We show that ecosystems are capable of both accelerating and impeding water-rock reaction rates across the weathering profile, ultimately leading to both thermodynamic and ecological controls on the baseflow concentration of rock-derived solutes in rivers.

Phosphorus speciation in sewage sludge and their ashes after incineration as a function of treatment processes.

Phosphorus (P) is a key component in agricultural fertilizers, but it is also a scarce resource, why its recycling has been thoroughly investigated and one promising resources is sewage sludge. Because of stricter regulations in terms of sludge disposal, thermal treatment (e.g. incineration) has become an attractive option. The incineration process alters the chemical speciation of P in favour to calcium-associated (apatite, apatite phosphorus (AP)) species, which is preferred for P recovery. In order to achieve qualitatively transformation, it is important to identify limiting or promoting factors. This study reports on the impact of iron, aluminium and calcium on the transformation of iron- and aluminium-phosphate (NAIP) to AP species, assessed by studying sludge and ash from 10 municipal wastewater treatment plants in Sweden. The effect of iron and aluminium added in the treatment processes was also evaluated. The obtained results show that high calcium concentration favours formation of AP species in both sludge and ashes, whereas high concentration of iron and aluminium favours formation of NAIP species in the sludge. The transformation from NAIP to AP species is hampered by aluminium, irrespectively of its origin, whereas no such correlations could be seen for iron. Therefore, in order to enable efficient P recovery from sewage sludge ash, the amount of aluminium added in the treatment process, as well as its concentration in influent streams to the treatment plants, must be limited.

Causal inference approaches reveal both positive and negative unintended effects of agricultural and urban management practices on instream biological condition

Agricultural and urban management practices (MPs) are primarily designed and implemented to reduce nutrient and sediment concentrations in streams. However, there is growing interest in determining if MPs produce any unintended positive effects, or co-benefits, to instream biological and habitat conditions. Identifying co-benefits is challenging though because of confounding variables (i.e., those that affect both where MPs are applied and stream biota), which can be accounted for in novel causal inference approaches. Here, we used two causal inference approaches, propensity score matching (PSM) and Bayesian network learning (BNL), to identify potential MP co-benefits in the Chesapeake Bay watershed portion of Maryland, USA. Specifically, we examined how MPs may modify instream conditions that impact fish and macroinvertebrate indices of biotic integrity (IBI) and functional and taxonomic endpoints. We found evidence of positive unintended effects of MPs for both benthic macroinvertebrates and fish indicated by higher IBI scores and specific endpoints like the number of scraper macroinvertebrate taxa and lithophilic spawning fish taxa in a subset of regions. However, our results also suggest MPs have negative unintended effects, especially on sensitive benthic macroinvertebrate taxa and key instream habitat and water quality metrics like specific conductivity. Overall, our results suggest MPs offer co-benefits in some regions and catchments with largely degraded conditions but can have negative unintended effects in some regions, especially in catchments with good biological conditions. We suggest the number and types of MPs drove these mixed results and highlight carefully designed MP implementation that incorporates instream biological data at the catchment scale could facilitate co-benefits to instream biological conditions. Our study underscores the need for more research on identifying effects of individual MP types on instream biological and habitat conditions.

Multiple stressors alter greenhouse gas concentrations in streams through local and distal processes.

Streams are significant contributors of greenhouse gases (GHG) to the atmosphere, and the increasing number of stressors degrading freshwaters may exacerbate this process, posing a threat to climatic stability. However, it is unclear whether the influence of multiple stressors on GHG concentrations in streams results from increases of in-situ metabolism (i.e., local processes) or from changes in upstream and terrestrial GHG production (i.e., distal processes). Here, we hypothesize that the mechanisms controlling multiple stressor effects vary between carbon dioxide (CO2) and methane (CH4), with the latter being more influenced by changes in local stream metabolism, and the former mainly responding to distal processes. To test this hypothesis, we measured stream metabolism and the concentrations of CO2 (pCO2) and CH4 (pCH4) in 50 stream sites that encompass gradients of nutrient enrichment, oxygen depletion, thermal stress, riparian degradation and discharge. Our results indicate that these stressors had additive effects on stream metabolism and GHG concentrations, with stressor interactions explaining limited variance. Nutrient enrichment was associated with higher stream heterotrophy and pCO2, whereas pCH4 increased with oxygen depletion and water temperature. Discharge was positively linked to primary production, respiration and heterotrophy but correlated negatively with pCO2. Our models indicate that CO2-equivalent concentrations can more than double in streams that experience high nutrient enrichment and oxygen depletion, compared to those with oligotrophic and oxic conditions. Structural equation models revealed that the effects of nutrient enrichment and discharge on pCO2 were related to distal processes rather than local metabolism. In contrast, pCH4 responses to nutrient enrichment, discharge and temperature were related to both local metabolism and distal processes. Collectively, our study illustrates potential climatic feedbacks resulting from freshwater degradation and provides insight into the processes mediating stressor impacts on the production of GHG in streams.