In recent years, many scientific studies have focused their efforts on quantifying the different types of pollutants that are not removed in wastewater treatment plants. Compounds of emerging concern (CECs) have been detected in different natural environments. The presence of these compounds in wastewater is not new, but they may have consequences in the future. These compounds reach the natural environment through various routes, such as wastewater. This review focuses on the study of tertiary treatment with advanced oxidation processes (AOPs) for the degradation of CECs. The main objective of the different existing AOPs applied to the treatment of wastewater is the degradation of pollutants that are not eliminated by means of traditional wastewater treatment.

The increase in industry and population, together with the need for wastewater reuse, makes it necessary to implement new technologies in the circular economy framework. The aim of this research was to evaluate the quality of the effluent of an algae membrane photobioreactor for the treatment of the effluent of an urban wastewater treatment plant, to characterise the ultrafiltration membranes, to study the effectiveness of a proposed cleaning protocol, and to analyse the performance of the photobioreactor. The photobioreactor operated under two days of hydraulic retention times feed with the effluent from the Los Vados wastewater treatment plant (WWTP) (Granada, Spain). The microalgae community in the photobioreactor grew according to the pseudo-second-order model. The effluent obtained could be reused for different uses of diverse quality with the removal of total nitrogen and phosphorus of 56.3% and 64.27%, respectively. The fouling of the polyvinylidene difluoride ultrafiltration membrane after 80 days of operation was slight, increasing the total membrane resistance by approximately 22%. Moreover, the higher temperature of the medium was, the lower intrinsic resistance of the membrane. A total of 100% recovery of the membrane was obtained in the two-phase cleaning protocol, with 42% and 58%, respectively.

The growing demand for water by the population and industry, as well as water scarcity due to climate change, has created a need to reuse treated water for agricultural purposes. In this context, the European Union, through its Regulation (EU) 2020/741, establishes minimum requirements for wastewater reuse, specifying that reuse for agricultural purposes can help to promote the circular economy and reduce the need for fertiliser use by setting high-quality standards. The aim of this article is to study whether the treated water from a pilot plant with membrane bioreactor technology operating with real urban wastewater from the city of Granada (Spain) satisfies the quality standards required for its reuse for agricultural purposes, as well as assessing the use of other resources produced during wastewater treatment, such as biogas and biostabilised sludge. This plant works in four cycles of operation at two different hydraulic retention times (6 and 12 h) and different concentrations of mixed liquor (2429–6696 mg/L). The pilot plant consists of a membrane bioreactor where there are four ultrafiltration membranes working in continuous operation and a sludge treatment line working in discontinuous mode. Subsequently, a tertiary treatment of advanced oxidation process was applied to the treated water for a time of 30 min, with different concentrations of oxidant. The results showed that the effluent has sufficient quality to be used in agriculture, complying with the characteristics established in the European legislation. Furthermore, the biostabilised sludge and biogas can be potentially reusable.

The European Union, as delineated in Regulation (EU) 2020/741, sets forth minimum criteria for the reuse of wastewater. Directive 86/278/CEE sets the regulations for the reuse of sewage sludge in agriculture. This study aimed to investigate the treated water derived from a pilot plant situated in Granada, Spain, that utilizes membrane bioreactor technology to process real urban wastewater with the quality standards necessary for agricultural reuse. Additionally, the study evaluated the utilization potential of other resources generated during wastewater treatment, including biogas and biostabilized sludge. The pilot plant incorporated a membrane bioreactor featuring four ultrafiltration membranes operating continuously alongside a sludge treatment line operating in batch mode. The pilot plant operated during four cycles, each with distinct hydraulic retention times (6 h and 12 h) and variable mixed liquor-suspended solids concentrations (ranging from 2688 mg L^-1 to 7542 mg L^-1). During these cycles, the plant was doped with increasing concentrations of emerging contamination compounds (diclofenac, ibuprofen, and erythromycin) to test their effect on the resources derived from the treatment. Subsequently, a tertiary treatment involving an advanced oxidation process was applied to the different water lines, which left the wastewater treatment plant for a period of 30 min and utilized varying concentrations of oxidant. The results indicate that the effluent obtained meets the required quality standards for agricultural use. Therefore, there is potential to use this waste as a resource, which is in line with the principles of the circular economy. Furthermore, the other resources generated during the treatment process, such as the biogas produced during the digestion process and the biostabilized sludge, have the potential to be used as resources according to the circular economy indicators.

The occurrence of emerging organic contaminants, such as pharmaceuticals, is a growing global concern. In this research, for a membrane bioreactor (MBR) laboratory plant operating at a hydraulic retention time (HRT) of 24 h, fed with real urban wastewater, the heterotrophic biomass behaviour was analysed for two concentrations of erythromycin, ibuprofen, and diclofenac. The concentrations studied for the first phase were erythromycin 0.576 mg L⁻¹, ibuprofen 0.056 mg L⁻¹, and diclofenac 0.948 mg L⁻¹. For Phase 2, the concentrations were increased to erythromycin 1.440 mg L⁻¹, ibuprofen 0.140 mg L⁻¹, and diclofenac 2.370 mg L⁻¹. Heterotrophic biomass was affected and inhibited by the presence of pharmaceutical compounds in both phases. The system response to low concentrations of pharmaceutical compounds occurred in the initial phase of plant doping. Under these operating conditions, there was a gradual decrease in the concentration of mixed liquor suspended solids and the removal of chemical oxygen demand of the system, as it was not able to absorb the effect produced by the pharmaceutical compounds added in both phases.

Currently, there is a growing concern about water scarcity. The rising demand for wastewater treatment systems that facilitate the reuse of wastewater has resulted in a focus on the use of microalgae in sustainable treatments. These methods not only eliminate nutrients from the wastewater but also produce biomass that can be used to obtain high-value products. This study aimed to observe the effect of different hydraulic retention times (HRTs) and treated urban wastewater (TUWW) percentages on the growth of microalgae biomass and nutrient consumption in membrane photobioreactors. Microalgae biomass growth increases with HRT regardless of the percentage of TUWW. Biomass concentration stabilises at between 40% and 60% TUWW but significantly increases when 100% TUWW is used, resulting in the highest biomass concentrations. As HRT increases, ammonium and total nitrogen consumption also rise. A positive trend in ammonium consumption was observed with increasing TUWW, reaching its peak with 100% TUWW. The optimal conditions for biomass growth and nutrient removal are achieved with a 7-day HRT and 100% TUWW as influent, which was confirmed as optimal with the response surface methodology.