Settling Velocities of Small Microplastic Fragments and Fibers.

There is only sparse empirical data on the settling velocity of small, nonbuoyant microplastics thus far, although it is an important parameter governing their vertical transport within aquatic environments. This study reports the settling velocities of 4031 exemplary microplastic particles. Focusing on the environmentally most prevalent particle shapes, irregular microplastic fragments of four different polymer types (9-289 µm) and five discrete length fractions (50-600 µm) of common nylon and polyester fibers are investigated, respectively. All settling experiments are carried out in quiescent water by using a specialized optical imaging setup. The method has been previously validated in order to minimize disruptive factors, e.g., thermal convection or particle interactions, and thus enable the precise measurements of the velocities of individual microplastic particles (0.003-9.094 mm/s). Based on the obtained data, ten existing models for predicting a particle's terminal settling velocity are assessed. It is concluded that models, which were specifically deduced from empirical data on larger microplastics, fail to provide accurate predictions for small microplastics. Instead, a different approach is highlighted as a viable option for computing settling velocities across the microplastics continuum in terms of size, density, and shape.

Efficacy of point source legislation quantified for a 64-year river water quality trajectory of phosphorus loading.

Changes in river water quality often follow typical trajectories characterized by sequential phases of degradation and recovery induced by management measures, typically achieved with combinations of legislative and technological interventions. However, the key question about the effectiveness of the different types of legal interventions - source control, use-related, and end-of-pipe - remains poorly understood. With the case of phosphorus (P), which is a valuable indicator of surface water quality and a widespread target of legislation at various governance levels in order to control eutrophication of water bodies, we quantified the relation between point source loading of P and resulting river water quality for a multidecadal trajectory of the river Ruhr (Germany). In particular, we analysed P-related legislation targeting point source pollution enforced at EU, national, state, and local level and linked their development with measured total phosphorus (TP) concentrations in the river Ruhr (Germany). To this end, we combined archival data with information in the literature and conducted interviews with contemporary witnesses to describe and quantify the efficacy of each legislative approach. Although not specifically targeted at P reduction, end-of-pipe measures (sewer systems and wastewater treatment plants (WWTP)) reduced TP inputs to surface waters already in the 1960s and 1970s, curbing TP inputs to the Ruhr by 38% in 1980. The first targeted source control legislation - the banning of phosphates in textile detergents in 1981 - effectively reduced TP concentrations in WWTP influents by 36% since 1990. In combination with stronger end-of-pipe legislation focusing on P elimination in WWTP since the 1990s, TP concentrations in WWTP effluents were reduced by 86% at the end of the 1990s and by 92% in 2021. Complete and successful source control for textile detergents made use-related legislation redundant. Our study demonstrates that source control measures should be prioritized, because they are the fastest way to curb emissions. These findings provide insights that can inform efficient decision-making regarding water quality in a trajectory perspective of hierarchical governance and technological needs, as well as effective policy-making and management for other pollutants requiring control from point sources.

Influence of low oxygen concentrations on biological transformations of trace organic chemicals in sand filter systems.

Managed aquifer recharge systems for drinking water reclamation are challenged by trace organic chemicals (TOrCs) since some of them are poorly retained. Although a lot of research has been done to investigate biological transformation of TOrCs in sand filter systems, there are still uncertainties to predict the removal. A laboratory column system with two different filter sands was set up to test TOrC transformation, the influence of low oxygen concentrations as well as the adaptation and influence of spiked TOrC influent concentrations. Bioactivity was quantified with the fluorescence tracer resazurin. In the experiment, a low elimination performance in the first column segment, defined as lag zone, was observed, implying incomplete adaptation or inhibiting co-factors. To assess these lag zones and to determine the dissipation time DT50 for 50% removal, a modified Gompertz model was applied. For acesulfame, formylaminoantipyrine, gabapentin, sulfamethoxazole, and valsartan acid DT50 of less than 10 h were observed, even when influent oxygen concentrations decreased to 0.5 mg/L. In general, TOrC transformations in technical sand with lower bioactivity and especially valsartan acid transformation responded very sensitive to low influent oxygen concentrations of 0.5 mg/L. However, in well adapted sand originating from soil aquifer treatment (SAT) with sufficient bioactivity, TOrC removal was hardly affected by such suboxic conditions. Furthermore, increasing the influent concentrations of TOrCs to 10 µg/L was found to promote adaptation especially for acesulfame and sulfamethoxazole. Benzotriazole, carbamazepine, diclofenac and venlafaxine were recalcitrant under the applied experimental conditions.

Effects of the Antidepressant Amitriptyline on Juvenile Brown Trout and Their Modulation by Microplastics.

Pharmaceuticals such as antidepressants are designed to be bioactive at low concentrations. According to their mode of action, they can also influence non-target organisms due to the phylogenetic conservation of molecular targets. In addition to the pollution by environmental chemicals, the topic of microplastics (MP) in the aquatic environment came into the focus of scientific and public interest. The aim of the present study was to investigate the influence of the antidepressant amitriptyline in the presence and absence of irregularly shaped polystyrene MP as well as the effects of MP alone on juvenile brown trout (Salmo trutta f. fario). Fish were exposed to different concentrations of amitriptyline (nominal concentrations between 1 and 1000 µg/L) and two concentrations of MP (104 and 105 particles/L; <50 µm) for three weeks. Tissue cortisol concentration, oxidative stress, and the activity of two carboxylesterases and of acetylcholinesterase were assessed. Furthermore, the swimming behavior was analyzed in situations with different stress levels. Exposure to amitriptyline altered the behavior and increased the activity of acetylcholinesterase. Moreover, nominal amitriptyline concentrations above 300 µg/L caused severe acute adverse effects in fish. MP alone did not affect any of the investigated endpoints. Co-exposure caused largely similar effects such as the exposure to solely amitriptyline. However, the effect of amitriptyline on the swimming behavior during the experiment was alleviated by the higher MP concentration.

Perfluoroalkyl substances (PFAS) adsorption in drinking water by granular activated carbon: Influence of activated carbon and PFAS characteristics.

Perfluoroalkyl substances (PFAS) persistence in the environment leads to their presence in drinking water, that is of high concern due to their potential human health risk. Adsorption onto activated carbon (AC) has been identified as an effective technique to remove PFAS. Adsorption isotherms and breakthrough curves, determined by rapid small-scale column tests (RSSCTs), were studied for eight PFAS and four granular ACs, characterized by different origins, porosities and numbers of reactivation cycles. Both batch and RSSCT results highlighted the strong interaction of AC and PFAS characteristics in adsorption capacity. The most important factor affecting AC performance is the surface charge: a positively-charged AC showed higher adsorption capacities with greater Freundlich constants (KF) and later 50% breakthroughs compared to the AC with neutral surface. Among the positively-charged ACs, a microporous AC demonstrated higher adsorption capacities for hydrophilic and marginally hydrophobic PFAS, while the mesoporous AC performed better for more hydrophobic PFAS, possibly due to lower pore blockage by organic matter. These results were confirmed at full-scale through a one-year monitoring campaign, in which samples were collected at the inlets and outlets of GAC systems in 17 drinking water treatment plants spread in a wide urban area, where the four analyzed ACs are used.

Simulating Effluent Organic Matter Competition in Micropollutant Adsorption onto Activated Carbon Using a Surrogate Competitor.

Adsorption onto activated carbon is a promising option for removing organic micropollutants (OMPs) from wastewater treatment plant (WWTP) effluents. The heterogeneity of activated carbons and adsorption competition between OMPs and adsorbable compounds of the effluent organic matter (EfOM) complicate the prediction of OMP removals. Thus, reliable and simple test systems are desirable. For this study, batch experiments with powdered activated carbon (PAC) were carried out to examine methyl orange (MO) as a selected surrogate competitor to simulate EfOM adsorption competition. MO solutions were spiked with carbamazepine (CBZ) as an indicator substance for well-adsorbing OMPs. On the basis of CBZ adsorption isotherms in WWTP effluents, MO concentrations for batch test solutions with identical adsorption competition toward CBZ were calculated. The calculations were performed according to an empirical model of CBZ adsorption in the presence of MO, since predictions employing the ideal adsorbed solution theory (IAST) proved to be inaccurate. Comparative batch tests with five different PACs were conducted with WWTP effluent and respective MO batch test solutions. Except for one PAC, the achieved CBZ removals were very similar in WWTP effluent and the test solution. Additionally, a universal correlation between MO and CBZ removals was found for four PACs.

How to dose powdered activated carbon in deep bed filtration for efficient micropollutant removal.

Direct addition of powdered activated carbon (PAC) to the inlet of a deep bed filter represents an energy- and space-saving option to remove organic micropollutants (OMPs) during advanced wastewater treatment or drinking water purification. In this lab-scale study, continuous dosing, preconditioning a filter with PAC and combinations thereof were investigated as possible dosing modes with respect to OMP adsorption efficiency. Continuous dosing resulted in decreasing effluent concentrations with increasing filter runtime due to adsorption onto accumulating PAC in the filter bed. Approximately constant removal levels were achieved at longer filter runtimes, which were mainly determined by the dose of fresh PAC, rather than the total PAC amount embedded. The highest effluent concentrations were observed during the initial filtration stage. Meanwhile, preconditioning led to complete OMP adsorption at the beginning of filtration and subsequent gradual OMP breakthrough. PAC distribution in the pumice filter was determined by the loss on ignition of PAC and pumice and was shown to be relevant for adsorption efficiency. Preconditioning with turbulent upflow led to a homogenous PAC distribution and improved OMP adsorption significantly. Combining partial preconditioning and continuous dosing led to low initial effluent concentrations, but ultimately achieved concentrations similar to filter runs without preconditioning. Furthermore, a dosing stop prior to the end of filtration was suitable to increase PAC efficiency without affecting overall OMP removals.

Degassing, gas retention and release in Fe(0) permeable reactive barriers.

Corrosion of Fe(0) has been successfully utilized for the reductive treatment of multiple contaminants. Under anaerobic conditions, concurrent corrosion leads to the generation of hydrogen and its liberation as a gas. Gas bubbles are mobile or trapped within the irregular pore structure leading to a reduction of the water filled pore volume and thus decreased residence time and permeability (gas clogging). With regard to the contaminant transport to the reactive site, the estimation of surface properties of the reactive material indicated that individual gas bubbles only occupied minor contact areas of the reactive surface. Quantification of gas entrapment by both gravimetrical and tracer investigations revealed that development of preferential flow paths was not significant. A novel continuous gravimetrical method was implemented to record variations in gas entrapment and gas bubble releases from the reactive filling. Variation of grain size fractions revealed that the pore geometry had a significant impact on gas release. Large pores led to the release of comparably large gas amounts while smaller volumes were released from finer pores with a higher frequency. Relevant processes are explained with a simplified pictorial sequence that incorporates relevant mechanisms.

Pilot-scale investigation of drinking water ultrafiltration membrane fouling rates using advanced data analysis techniques.

A pilot-scale investigation of the performance of biofiltration as a pre-treatment to ultrafiltration for drinking water treatment was conducted between 2008 and 2010. The objective of this study was to further understand the fouling behaviour of ultrafiltration at pilot scale and assess the utility of different foulant monitoring tools. Various fractions of natural organic matter (NOM) and colloidal/particulate matter of raw water, biofilter effluents, and membrane permeate were characterized by employing two advanced NOM characterization techniques: liquid chromatography - organic carbon detection (LC-OCD) and fluorescence excitation-emission matrices (FEEM) combined with principal component analysis (PCA). A framework of fouling rate quantification and classification was also developed and utilized in this study. In cases such as the present one where raw water quality and therefore fouling potential vary substantially, such classification can be considered essential for proper data interpretation. The individual and combined contributions of various NOM fractions and colloidal/particulate matter to hydraulically reversible and irreversible fouling were investigated using various multivariate statistical analysis techniques. Protein-like substances and biopolymers were identified as major contributors to both reversible and irreversible fouling, whereas colloidal/particulate matter can alleviate the extent of irreversible fouling. Humic-like substances contributed little to either reversible or irreversible fouling at low level fouling rates. The complementary nature of FEEM-PCA and LC-OCD for assessing the fouling potential of complex water matrices was also illustrated by this pilot-scale study.

Influence of hydronium, sulfate, chloride and other non-carbonate ions on hydrogen generation by anaerobic corrosion of granular cast iron.

Permeable reactive barriers are successfully applied for the removal of various contaminants. The concomitant reduction of hydrogen ions and the subsequent formation of hydrogen gas by anaerobic corrosion lead to decreased pore volume filled with water and thus residence times, so called gas clogging. Long term column experiments were conducted to elucidate the impact of ubiquitous water constituents on the formation of hydrogen gas and potential passivation due to corrosion products. The collected gas volumes revealed a relation to the hydronium concentration (pH) but were only slightly increased in the presence of chloride and sulfate and not significantly influenced in the presence of phosphate, silicate, humic acid and ammonium compared to deionized water. Significant gas volumes within the reactive filling were verified by gravimetry. The presence of nitrate completely eliminated hydrogen formation by competition for electrons. Solid phase analyses revealed that neither chloride nor sulfate was incorporated in corrosion products in concentrations above 0.1 weight percent, and they did not alter the formation of mainly magnetite in comparison to deionized water.

Investigating dominant processes in ZVI permeable reactive barriers using reactive transport modeling.

The reactive and hydraulic efficacy of zero valent iron permeable reactive barriers (ZVI PRBs) is strongly affected by geochemical composition of the groundwater treated. An enhanced version of the geochemical simulation code MIN3P was applied to simulate dominating processes in chlorinated hydrocarbons (CHCs) treating ZVI PRBs including geochemical dependency of ZVI reactivity, gas phase formation and a basic formulation of degassing. Results of target oriented column experiments with distinct chemical conditions (carbonate, calcium, sulfate, CHCs) were simulated to parameterize the model. The simulations demonstrate the initial enhancement of anaerobic iron corrosion due to carbonate and long term inhibition by precipitates (chukanovite, siderite, iron sulfide). Calcium was shown to enhance long term corrosion due to competition for carbonate between siderite, chukanovite, and aragonite, with less inhibition of iron corrosion by the needle like aragonite crystals. Application of the parameterized model to a field site (Bernau, Germany) demonstrated that temporarily enhanced groundwater carbonate concentrations caused an increase in gas phase formation due to the acceleration of anaerobic iron corrosion.

Influence of dissolved inorganic carbon and calcium on gas formation and accumulation in iron permeable reactive barriers.

Uncertainties in long-term reactivity and gas accumulation in Fe(0) permeable reactive barriers still hinder a broad application of this groundwater remediation technology. In this study long-term column experiments were conducted under varying geochemical conditions. Generation of hydrogen by anaerobic corrosion in Fe(0) reactive filters was mainly influenced by the mass flux of dissolved inorganic carbon. Both increased concentrations and volume flows led to a substantial rise in gas generation but only to slight differences of gas accumulation within the pores of the reactive filter. Comparisons of columns with different lengths showed higher averaged corrosion rates in the shorter and lower corrosion rates in the longer columns. Calcium in conjunction with dissolved inorganic carbon formed compact and localized aragonite minerals, while in the absence of calcium chukanovite dominated, which covered and passivated the reactive surface to a higher extent. Magnetite was the major crystalline corrosion product in the absence of carbonate and no decline in long term corrosion rates was observed within up to 700 days of operation. Total gas yields of columns were restricted by passivation and approached a volume of approximately 13.5 mL/g granulated cast iron.

Impact of coagulation and adsorption on DOC fractions of secondary effluent and resulting fouling behaviour in ultrafiltration.

Membrane fouling by macromolecular dissolved organic compounds is still a fundamental drawback in low-pressure membrane filtration of secondary effluent. In this study, pre-treatment of secondary effluent by coagulation and/or adsorption was investigated in terms of removal of different dissolved organic carbon (DOC) fractions, especially macromolecular substances. DOC fractionation has been characterised by size exclusion chromatography. Adsorption tests using four commercially available activated carbons yielded a removal of small as well as larger organic compounds, revealing differences in the affinity towards macromolecules depending on the type of applied activated carbon. By contrast, coagulation removed predominantly larger molecules, i.e., biopolymers and humic substances. In terms of DOC reduction, the coagulant ferric chloride was superior to aluminium chloride. A combination of coagulation and adsorption resulted in the addition of individual removal efficiencies, suggesting that different fractions of organic compounds were involved in each of the processes. After removal of macromolecular organic compounds either by coagulation or by adsorption, a significant reduction of membrane fouling was observed in tests using two different types of ultrafiltration flat-sheet membranes in 20-h cross-flow filtration tests.