Population growth and climate change has a huge impact on water availability. To ensure a secure water supply, water-reuse concepts and its implementation are gaining more and more importance. Additionally, water saving potentials to optimize the drinking and water reuse availability have to be considered. However, limited spatial planning opportunities and missing regulation to provide treated wastewater according to the “fit-for-purpose” principle are often hindering its application. Some countries, such as the USA or Singapore, have been leading the way for decades in implementing water-reuse concepts and in treating wastewater for potable and non-potable reuse. The wastewater treatment technologies are currently providing solutions for an adequate provision of reclaimed water. Consequently, the opportunities for water reuse are given, but the challenge is largely in the implementation, which becomes necessary in water-scarce regions. This perspective is thus presenting the current possibilities and challenges of wastewater reuse with respect to existing examples of implementations but also shows the need for action in the future. The relevance of this topic is also underlined in particular by the Sustainable Development Goals (SDG), especially Goal 6 which is related to “Ensure availability and sustainable management of water and sanitation for all”.

Aeration is an energy-intensive process of aerobic biological treatment in wastewater treatment plants (WWTP). Two-stage processes enable energy-efficient operation, but oxygen transfer has not been studied in depth before. In this study, α-factors were determined with long-term ex situ steady-state off-gas measurements in pilot-scale test reactors (5.8 m height, 8.3 m³) coupled to full-scale activated sludge basins. A two-stage WWTP with more than 1 Mio population equivalent was studied over 13 months including rain and dry weather conditions. Operating data, surfactant concentrations throughout the two-stage process, and the effect of reverse flexing on pressure loss of diffusers were examined. The values of α_mean, α_min, and α_max for design load cases of aeration systems were determined as 0.45, 0.33, and 0.54 in the first high-rate carbon removal stage and as 0.80, 0.69, and 0.91 in the second nitrification stage, respectively. The first stage is characterized by a distinct diurnal variation and decrease in α-factor during stormwater treatment. Surfactants and the majority of the total organic carbon (TOC) load are effectively removed in the first stage; hence, α-factors in the second stage are higher and have a more consistent diurnal pattern. Proposed α-factors enable more accurate aeration system design of two-stage WWTPs. Fouling-induced diffuser pressure loss can be restored effectively with reverse flexing in both treatment stages.

Performance of aeration systems in wastewater treatment plants (WWTP) under process conditions can be monitored with off-gas tests. The ex situ off-gas method transfers activated sludge from an adjacent aeration tank into aerated columns to determine oxygen transfer parameters (e.g., the α-factor). This method is an alternative to in situ off-gas testing with hoods at the tank surface; however, its application and measurement uncertainty have not been examined yet. We outline our experience from long-term off-gas testing with two pilot-scale test reactors (8.3 m³ volume). Global variance-based sensitivity analysis using Sobol’ indices revealed oxygen concentration in off-gas and dissolved oxygen as the most important input quantities to determine α-factors accurately. Measurement uncertainty of other instruments was negligible. These findings are transferable to in situ off-gas hoods because the methods are similar. Random measurement error of α-factors was estimated with uncertainty analysis and comparison measurements to a relative standard deviation of about ± 2.8% for our ex situ pilot setup. Diffuser fouling, biofilm growth, or sensor drift caused systematic errors avoidable by maintenance. Additional mixing of bubble column due to sludge inflow into ex situ tanks led to a systematic overestimation of α-factors at lower airflow rates. Hence, the ex situ off-gas method is not suitable to determine α-factors for the design of aeration systems but offers unique possibilities for research of oxygen transfer dynamics and development of aeration equipment because ex situ columns can be operated independently from a full-scale activated sludge tank.

Biological coatings on EPDM-membranes are a problem on many large wastewater treatment plants, as the oxygen supply of the micro-organisms is no longer guaranteed. Investigations prove that the pressure loss and the Shore A-hardness of the EPDM-membranes increase while on the other hand their softener content decreases accordingly. The detected coatings on the membrane surfaces and in the slits or holes of the membranes show extra-cellular organic substances (EPS), which, compared with fibrillar/filamented EPS usually found on surfaces in wastewater treatment plants, are viscous to a much greater extent. As, besides primary organic parts (carbon), the coatings on the membranes as well as in the slits or holes also consist of inorganic constituents (magnesium, silicon, and others), the authors assume that, the separating agent (and also inactive filler) talcum (magnesium silicate), used when producing the membranes, supports at least a first beginning of the coating. Superfine dust constituents and fibres, input via the compressed air, will build up inside the coating and consequently lead to a gradual clogging of the holes or slits. Besides chemical cleaning measures, the exchange of the EPDM-membranes against membranes of silicone would also be a possible measure to solve this problem. The market will decide, if, in the future, a cleaning or an exchange of the EPDM-membranes against membranes of silicone will be applied, but it has to be considered that the loss of softener is irreversible.

With the continuous restructuring of the water market due to liberalisation, privatisation and internationalisation processes, the requirements on waste water disposal companies have grown. Increasing competition requires a target-oriented and clearly structured procedure. At the same time it is necessary to meet the environment-relevant legal requirements and to design the processes to be environment-oriented. The implementation of risk management and the integration of such a management instrument in an existing system in addition to the use of modern technologies and procedures can help to make the operation of the waste water treatment safer and consequently strengthen market position. The risk management process consists of three phases, risk identification, risk analysis/risk assessment and risk handling, which are based on each other, as well as of the risk managing. To achieve an identification of the risks as complete as possible, a subdivision of the kind of risks (e.g. legal, financial, market, operational) is suggested. One possibility to assess risks is the portfolio method which offers clear representation. It allows a division of the risks into classes showing which areas need handling. The determination of the appropriate measures to handle a risk (e.g. avoidance, reduction, shift) is included in the concluding third phase. Different strategies can be applied here. On the one hand, the cause-oriented strategy, aiming at preventive measures which aim to reduce the probability of occurrence of a risk (e.g. creation of redundancy, systems with low susceptibility to malfunction). On the other hand, the effect-oriented strategy, aiming to minimise the level of damage in case of an undesired occurrence (e.g. use of alarm systems, insurance cover).