Digesters at water resource recovery facilities (WRRFs) operating at different temperatures within the mesophilic and thermophilic temperature range is a flexibilization concept to contribute to heat management. Four 25 L digesters were fed with sewage sludge from a municipal WRRF and were operated at 37, 43, 47 and 53 °C, respectively, to describe changes in the overall process performance and the microbiota. Specific methane yield and COD degradation rates were the highest at 47 °C, only being up to 7% higher compared with at 37 °C. The increase in pH and concentrations of NH₄-N and PO₄-P above 43 °C were statistically significant. The effect on the microbial community was strong, indicating both a constant specialization towards thermophilic organisms as well as a change from acetoclastic to hydrogenotrophic/methylotrophic methanogenesis. The influence of temperature on process-engineering and physicochemical aspects was rather small compared with the changes in the microbiota.

Phosphorus (P) is an essential element to all living beings but also a finite resource. P-related problems center around broken P cycles from local to global scales. This paper presents outcomes from the 9th International Phosphorus Workshop (IPW9) held 2019 on how to move towards a sustainable P management. It is based on two sequential discussion rounds with all participants. Important progress was reported regarding the awareness of P as finite mineable resource, technologies to recycle P, and legislation towards a circular P economy. Yet, critical deficits were identified such as how to handle legacy P, how climate change may affect ecosystem P cycling, or working business models to up-scale existing recycling models. Workshop participants argued for more transdisciplinary networks to narrow a perceived science-practice/policy gap. While this gap may be smaller in reality as illustrated with a Swiss example, we formulate recommendations how to bridge this gap more effectively.

To combat new threats to critical infrastructure, the European Union enacted new regulations for their member states. With the directive on measures for a high common level of cybersecurity (NIS2) and the directive on critical infrastructure resilience (EU RCE), EU member states must identify critical infrastructure (CIs) and enable measures to reduce the risk of default in stress situations. The topic of resilience in urban water systems has already been of interest in previous research. However, there are still open questions. As it is a multidisciplinary term, understanding resilience and its adaptation into management systems is not an easy task for practitioners. This study will provide an overview of resilience within the framework of municipal wastewater treatment plants (WWTP) and show the current situation of existing implementation of safety and security regulations, taking Germany as an example. One of the main requirements of the EU RCE is a risk assessment (RA) for CIs. Until now, risk analysis for WWTP in research was mostly carried out for individual WWTP. By applying guidelines from the drinking water sector, this paper shows a possible methodology for a risk analysis. This paper aims to support practitioners by forming a common understanding of resilience and risk as well as providing an example for a risk analysis.

Co-substrates can increase gas production in a digester significantly. The characteristic properties of substrates, depending on the amounts added, influence the processes in the digester reactor. As a consequence, they can have an impact on the buffer capacity, pH value, C:N ratio, dewaterability of the digested sludge and introduce contaminants to the digester among others. In the future, a discontinuous digester feeding could contribute to the demand-driven energy supply by WRRFs. Due to the increasing instability caused by fluctuating organic load, higher demands are placed on the selection of co-substrates. This study examined to what extent flotation sludge from dairy companies is suitable for a sustainable co-digestion. In addition, it should be evaluated whether flotation sludge is applicable for demand-driven feeding strategies. It was shown that flotation sludge has positive effects on the reactor as well as a significant increase in biogas production and a high degree of degradation of at least 80%. Even at high organic loads pH remained at a high level at around 7.5 due to the high alkalinity of the substrate. Nonetheless, addition of more than 20 w-% flotation sludge lead to a significant decrease of the dewaterability of the digested sludge.

Anaerobic digestion, including the gas storage and the CHP unit plays a key role in energy management of water ressource recovery facilities (WRRF). By demand-driven feeding management, a higher degree of utilization of the gas storage tanks can be achieved, which means that a greater decoupling of energy demand and energy generation is taking place. In this context, the predictability of the produced gas volume plays an important role. In this study, a concept was suggested to forecast gas production rates without the need for any analytical substrate characterization. Substrates lead to specific patterns in the course of the gas production rates, which can be fitted using Gaussian functions. As the parameters describing the Gaussian functions have a good correlation to the amount of substrate fed to the digester a prediction of the rates is possible. Within this study a coefficient of determination 82.9% over a period of 5 days was achieved.