Department Environmental Chemistry

Micropollutants (MP) are only partly removed from municipal wastewater by nutrient removal plants and are seen increasingly as a threat to aquatic ecosystems and to the safety of drinking water resources. The addition of powder activated carbon (PAC) is a promising technology to complement municipal nutrient removal plants in order to achieve a significant reduction of MPs and ecotoxicity in receiving waters. This paper presents the salient outcomes of pilot- and full-scale applications of PAC addition in different flow schemes for micropollutant removal in municipal wastewater treatment plants (WWTPs). The sorption efficiency of PAC is reduced with increasing dissolved organic carbon (DOC). Adequate treatment of secondary effluent with 5–10 g DOC m⁻³ requires 10–20 g PAC m⁻³ of effluent. Counter-current use of PAC by recycling waste PAC from post-treatment in a contact tank with an additional clarifier to the biology tank improved the overall MP removal by 10 to 50% compared with effluent PAC application alone. A dosage of 15 g PAC m⁻³ to a full-scale flocculation sand filtration system and recycling the backwash water to the biology tank showed similar MP elimination. Due to an adequate mixing regime and the addition of adapted flocculants, a good retention of the fine fraction of the PAC in the deep-bed filter were observed (1–3 g TSS m⁻³; TSS: total suspended solids). With double use of PAC, only half of the PAC was required to reach MP removal efficiencies similar to the direct single dosage of PAC to the biology tank. Overall, the application of PAC in WWTPs seems to be an adequate and feasible technology for efficient MP elimination (>80%) from wastewater comparable with post ozonation.

The kinetics of oxidation and disinfection processes during ozonation in a full-scale reactor treating secondary wastewater effluent were investigated for seven ozone doses ranging from 0.21 to 1.24 g O₃ g⁻¹ dissolved organic carbon (DOC). Substances reacting fast with ozone, such as diclofenac or carbamazepine (k_P,O3 > 10⁴ M⁻¹), were eliminated within the gas bubble column, except for the lowest ozone dose of 0.21 g O₃ g⁻¹ DOC. For this low dose, this could be attributed to short-circuiting within the reactor. Substances with lower ozone reactivity (k_P,O3 < 10⁴ M⁻¹ s⁻¹) were only fully eliminated for higher ozone doses.
The predictions of micropollutant oxidation based on coupling reactor hydraulics with ozone chemistry and reaction kinetics were up to a factor of 2.5 higher than full-scale measurements. Monte Carlo simulations showed that the observed differences were higher than model uncertainties. The overestimation of micropollutant oxidation was attributed to a protection of micropollutants from ozone attack by the interaction with aquatic colloids. Laboratory-scale batch experiments using wastewater from the same full-scale treatment plant could predict the oxidation of slowly-reacting micropollutants on the full-scale level within a factor of 1.5. The Rct value, the experimentally determined ratio of the concentrations of hydroxyl radicals and ozone, was identified as a major contribution to this difference.
An increase in the formation of bromate, a potential human carcinogen, was observed with increasing ozone doses. The final concentration for the highest ozone dose of 1.24 g O₃ g⁻¹ DOC was 7.5 μg L⁻¹, which is below the drinking water standard of 10 μg L⁻¹. N-Nitrosodimethylamine (NDMA) formation of up to 15 ng L⁻¹ was observed in the first compartment of the reactor, followed by a slight elimination during sand filtration. Assimilable organic carbon (AOC) increased up to 740 μg AOC L⁻¹, with no clear trend when correlated to the ozone dose, and decreased by up to 50% during post-sand filtration. The disinfection capacity of the ozone reactor was assessed to be 1–4.5 log units in terms of total cell counts (TCC) and 0.5 to 2.5 log units for Escherichia coli (E. coli). Regrowth of up to 2.5 log units during sand filtration was observed for TCC while no regrowth occurred for E. coli. E. coli inactivation could not be accurately predicted by the model approach, most likely due to shielding of E. coli by flocs.

Within the framework of the “Micropoll Strategy” project of the Swiss Federal Environmental Agency, the suitability of ozonation as an additional treatment stage for the removal of organic trace elements was tested at the Regensdorf wastewater treatment plant. With ozone doses of 0.60 g O₃/g DOC already, a large portion of the organic trace elements could no longer be detected in the plant run-off. Only for a few, very persistent compounds (like iodized radio-opaque substances or atrazine), the removal rates amounted to only about 50 percent, even with high ozone doses (1.16 g O₃ /g DOC). Eco-toxicological methods were used to prove that a clear reduction in summary effects (like e.g. oestrogen effects or the effect of insecticides) took place. In two tests, negative effects were measured directly after ozonation, but they disappeared again after the final sand filter. Oxidation by-products (such as assimilable organic carbon or N-Nitrosodimethylamine [NDMA]) were also reduced through sand filtration. This underlines the importance of a biologically active stage downstream of the ozonation stage, for example a sand filtration stage. The pilot operation also showed that an ozonation stage at a municipal wastewater treatment plant is technically feasible and economically reasonable.

Background, aim, and scope Aquatic microcontaminants (MCs) comprise diverse chemical classes, such as pesticides, biocides, pharmaceuticals, consumer products, and industrial chemicals. For water pollution control and the evaluation of water protection measures, it is crucial to screen for MCs. However, the selection and prioritization of which MCs to screen for is rather difficult and complex. Existing methods usually are strongly limited because of a lack of screening regulations or unavailability of required data.
Method and models Here, we present a simple exposure-based methodology that provides a systematic overview of a broad range of MCs according to their potential to occur in the water phase of surface waters. The method requires input of publicly available data only. Missing data are estimated with quantitative structure–property relationships. The presented substance categorization methodology is based on the chemicals’ distribution behavior between different environmental media, degradation data, and input dynamics.
Results Seven different exposure categories are distinguished based on different compound properties and input dynamics. Ranking the defined exposure categories based on a chemical’s potential to occur in the water phase of surface waters, exposure categories I and II contain chemicals with a very high potential, categories III and IV contain chemicals with a high potential, and categories V and VI contain chemicals with a moderate to low potential. Chemicals in category VII are not evaluated because of a lack of data. We illustrate and evaluate the methodology on the example of MCs in Swiss surface waters. Furthermore, a categorized list containing potentially water-relevant chemicals is provided.
Discussion Chemicals of categories I and III continuously enter surface waters and are thus likely to show relatively steady concentrations. Therefore, they are best suited for water monitoring programs requiring a relatively low sampling effort. Chemicals in categories II and IV have complex input dynamics. They are consequently more difficult to monitor. However, they should be considered if an overall picture is needed that includes contaminants from diffuse sources.
Conclusions The presented methodology supports compound selection for (a) water quality guidance, (b) monitoring programs, and (c) further research on the chemical’s ecotoxicology. The results from the developed categorization procedure are supported by data on consumption and observed concentrations in Swiss surface waters. The presented methodology is a tool to preselect potential hazardous substances based on exposure-based criteria for policy guidance and monitoring programs and a first important step for a detailed risk assessment for potential microcontaminants.

The removal efficiency for 220 micropollutants was studied at the scale of a municipal wastewater treatment plant (WWTP) upgraded with post-ozonation followed by sand filtration. During post-ozonation, compounds with activated aromatic moieties, amine functions, or double bonds such as sulfamethoxazole, diclofenac, or carbamazepine with second-order rate constants for the reaction with ozone >10⁴ M⁻¹ s⁻¹ at pH 7 (fast-reacting) were eliminated to concentrations below the detection limit for an ozone dose of 0.47 g O₃ g⁻¹ dissolved organic carbon (DOC). Compounds more resistant to oxidation by ozone such as atenolol and benzotriazole were increasingly eliminated with increasing ozone doses, resulting in >85% removal for a medium ozone dose (0.6 g O₃ g⁻¹ DOC). Only a few micropollutants such as some X-ray contrast media and triazine herbicides with second-order rate constants <10² M⁻¹ s⁻¹ (slowly reacting) persisted to a large extent. With a medium ozone dose, only 11 micropollutants of 55 detected in the secondary effluent were found at >100 ng L⁻¹. The combination of reaction kinetics and reactor hydraulics, based on laboratory- and full-scale data, enabled a quantification of the results by model calculations. This conceptual approach allows a direct upscaling from laboratory- to full-scale systems and can be applied to other similar systems. The carcinogenic by-products N-nitrosodimethylamine (NDMA) (≤14 ng L⁻¹) and bromate (<10 μg L⁻¹) were produced during ozonation, however their concentrations were below or in the range of the drinking water standards. Furthermore, it could be demonstrated that biological sand filtration is an efficient additional barrier for the elimination of biodegradable compounds formed during ozonation such as NDMA. The energy requirement for the additional post-ozonation step is about 0.035 kWh m⁻³, which corresponds to 12% of a typical medium-sized nutrient removal plant (5 g DOC m⁻³).

A model based on graph theory was developed to efficiently evaluate the impact of the effluent from 742 wastewater treatment plants (WWTPs) on micropollutant loading throughout all river catchments in Switzerland. Model results agree well with measured loads for 12 compounds in river water samples, revealing mean predictive accuracy factors between 0.8 and 3.4. Subsequently, pollutant concentrations were predicted for river sections downstream from 543 WWTPs where hydrological information was available, and compared with recent recommendations for water quality criteria. At base flow conditions, carbamazepine concentrations (parent compound only) are ubiquitously below a water quality criterion of 0.5 μg L⁻¹. In contrast, the sum of diclofenac and its metabolites is expected to exceed the corresponding water quality criterion of 0.1 μg L⁻¹ in 224 river sections. If diclofenac cannot be eliminated at the source, the model suggests a directed upgrade of 173 WWTPs to meet the condition that concentrations are never to exceed this water quality criterion.

Micropollutants in Wastewater Treatment Plants. Technologies to Eliminate Organic Trace Contaminants. Today’s mechanical-biological treatment plants only inadequately remove organic micropollutants. Advanced technologies are necessary such as ozonation or adsorption on powdered active carbon (PAC) in order to increase removal capacity. These two techniques are being evaluated in various tests on the laboratory level, but also on a large scale. Both ozonation and PA C adsorption are suitable for reducing the emission of organic trace elements from settlement drainage systems into the various bodies of water. This article presents the current knowledge of both technologies for application in municipal wastewater treatment systems.

Micropollutants (MP) from municipal wastewater are frequently detected in surface waters and occur in ecotoxicological relevant concentrations. Therefore a broadly accepted method for the assessment of MP is needed. Here, we propose a procedure for the assessment of MP from municipal wastewater. The suggested method comprises (1) a substance list with relevant MP, (2) toxicologically based quality criteria, (3) an approach for the identification of potentially polluted sites, (4) a sampling strategy that considers the input-dynamics of chemicals and (5) a scheme to rate the water quality concerning MP. In the proposed concept the assessment of continuous entering substances is focused.

Recently, advanced treatment steps for the elimination of organic micropollutants in municipal wastewater are investigated intensively in largescale experiments. One of the key issues is to assess the elimination performance of different advanced treatments routinely in practice. In this article, a concept for the assessment of the elimination of organic micropollutants in municipal wastewater treatment plants using five indicatorsubstances is presented. The selected indicatorsubstances represent larger groups of chemicals, occur in surface waters across Switzerland and are quantifiable with established analytical methods.

Because of improved analytic technologies, organic micropullutants from urban drainage like pharmaceuticals, biocides and cosmetics have been increasingly detected in Svviss Waters in the last few years. The results arise questions about possible impacts on the aquatic environment and on people consuming drinking Water and foods. The article gives an overview of present measured data of persistent substances deriving from waste Water treatment plants in Swiss Waters. There is a need for further analysis, particularly with regard to the dynamics of the load and the occurrence of possible transformation products.