One of the key elements in the transformation towards a circular economy (CE) is providing more sustainable practices for resources and waste management. Improvement actions focused on transformation towards a CE should be targeted at all groups of materials and waste. As water is essential for human survival and well-being and plays a significant role in sustainable development (SD), the actions related to the reuse of water and the recovery of raw materials from wastewater and other water-based waste should be taken. The paper presents a proposition for a new CE model framework in the water and wastewater sector, which includes the six following actions:reduction—prevent wastewater generation in the first place by the reduction of water usage and pollution reduction at source; reclamation (removal)—an application of effective technologies for the removal of pollutants from water and wastewater; reuse—reuse of wastewater as an alternative source of water supply (non-potable usage), recycling—recovery of water from wastewater for potable usage; recovery—recovery of resources such as nutrients and energy from water-based waste, and rethink—rethinking how to use resources to create a sustainable economy, which is `free` of waste and emissions. The novelty of the proposed CE model framework is that it presents possible ways of implementing CE principles in the water and wastewater sector, with a strong emphasis not only technological but also organisational and societal changes. Application of the proposed model may help to further transform the European economy to the CE model. Moreover, the indicated model can be significant tool supporting an assessment of local or regional progress towards CE in the water and wastewater sector and further environmental management and planning.

Despite the implementation of strict legal standards concerning nutrient loads within wastewater discharges in all European Union (EU) Member States it was not possible to achieve good ecological and chemical water status by 2015 in all EU countries. The main reasons for this situation are the imperfections of the legislation tools regarding the standardization of wastewater quality and the methodology of determining the conditions for wastewater introduction into receivers. The study aims to review and analyze the currently existing in various countries legal regulations setting the standards for wastewater discharged into receivers, which were intended for surface water protection and eutrophication mitigation. Besides the EU effluent standards, the regional and national regulations in chosen EU Member States (e.g., Germany, Sweden, and Denmark) have been reviewed. Moreover, the Helsinki Commission recommendations for signatory countries within the Baltic Sea catchment and the approaches for wastewater quality standardization in non-EU countries (e.g., Russia, Belarus, Switzerland, China, USA, Canada, and Dubai) were assessed. The analysis of the reviewed legal regulations allowed to diversify the methodological approaches for setting effluent quality standards in different regions and countries and to assess the effectiveness of existing legal tools in the field of eutrophication mitigation with the consideration of the environmental and economic reasonability. The results suggest that the receiver-oriented policies used among others in Switzerland and China are the most reasonable in terms of eutrophication mitigation.

One of the main factors of the increased eutrophication level of surface waters is the high anthropogenic loads of biogenic substances discharged into water bodies. Municipal wastewaters, containing large amounts of nitrogen and phosphorus play one of the key roles in the acceleration of eutrophication intensity. The main direction in the prevention of eutrophication caused by wastewater discharge has become the reduction of nutrient loads introduced to wastewater receivers in accordance with strict legal requirements achievable only in advanced technologies. The treated wastewater quality standards are actually developed for total nitrogen and total phosphorus content, disregarding the fact that eutrophication potential of treated wastewater is determined by the content of non-organic nutrient forms directly bioavailable for water vegetation. That is why the currently used energy-consuming and expensive technologies do not always guarantee effective protection against eutrophication and its consequences. The goal of the study was to analyze the most widely used wastewater treatment technologies for enhanced biological nutrients removal in treated wastewater eutrophication potential. For this purpose, an analysis of the operation of 18 wastewater treatment plants based on different technologies in Finland, Canada, Poland, Russia and the United States was realized. The analysis concluded that the eutrophication potential of treated wastewater to a large extent is conditioned by the applied technology. The results of the research concluded that the eutrophication potential can serve an important criterion for decision-making regarding the proper selection of wastewater treatment technologies aimed at eutrophication mitigation.

Mitigation of eutrophication, intensified by excessive nutrient load discharge in wastewaters regulated by restrictive legal requirements, remains one of today’s most important global problems. Despite implementation of the Water Framework Directive, the Urban Wastewater Directive and the HELCOM recommendations, the actual condition of surface water is still not satisfactory. In response to the above, the study presents an alternative approach for surface water protection against eutrophication based on the selection of appropriate nutrient removal technologies. An activated sludge model simulation was used to enable the identification of environmentally justified nutrient removal systems with lowest eutrophication potential of treated wastewater conditioned by bioavailable nutrient forms content. Based on the outcome of the study, the 3-stage Bardenpho system was identified as the most efficient for bioavailable phosphorus removal, while the Johannesburg system proved to have the highest efficiency for bioavailable nitrogen removal. The proposed eutrophication mitigation approach underlines the need for a reconsideration of current legal regulations which ignore nutrient bioavailability and key eutrophication limiting factors.

The study is an attempt to assess the pollution impact on the aquatic ecosystem related with an emergency discharge of untreated municipal wastewater from the “Czajka” wastewater treatment plant (WWTP) in Warsaw. The present case study is based on the analysis of available monitoring data for chemical oxygen demand (COD), total nitrogen (TN), ammonia nitrogen (N-NH₄), and total phosphorus (TP) in the effluent mixing zone (MZ), the stretch of the river and before the discharge point. Data analysis was supported by a basic statistical analysis based on Pearson’s correlation coefficient. The results proved the importance of efficient and reliable nutrient removal technologies used in modern WWTPs. A statistically significant correlation was achieved between the COD (r = 0.567) and TP (r = 0.714) discharged loads and their concentration in the MZ. However, no significant correlation has been identified with TN and N-NH₄. Furthermore, the dissolved oxygen (DO) deficits in the MZ were observed within 7 days of the discharge period resulting in an average DO concentration decrease from 8.4 to 7.1 mgO₂/L. The river stretch has not been affected by DO deficits while the average observed DO concentration 30 km behind the discharge point was 9.1 mgO₂/L. The analysis results present the pollutants assimilation capacity of a river ecosystem and its real reaction to sudden excessive nutrient loads discharge.