Ionic cross-linking between the SAC component and (a) F₆PBI by imidazole group protonation or (b) N-methylmorpholine after quaternization.

Integrated planning of urban blue–green infrastructures is crucial to strengthen urban environmental quality and mitigate negative climate change-associated effects. It implies, however, increased water demand for irrigation, wherefore greywater (wastewater excluding wastewater from toilets and urinals) can be used, yet it requires handling for safe reuse. One treatment option is the use of constructed wetlands (CW), which have thus far not been broadly applied in inner-city districts due to large area requirements. This work investigates a novel bipartite container-based vertical-flow constructed wetland (VFCW) for the treatment of light greywater (from showers and hand wash basins) and its use as irrigation water for urban facade greenery. The VFCW consists of two compartments with 2.5 m² filter area each, filled with 75 cm zeolite-containing lava sand (0–4 mm) and 75 cm Rhine sand (0–2 mm), respectively. In short, screening has proven to be well suitable for coarse solids removal, so there is no further need to settle light greywater, which reduces overall treatment area and benefits urban application. Treated greywater complied with irrigation standards at all times, yet mixing with rainwater can help reduce salt contents, if applicable. The modular/elevated lava sand VFCW exhibited extensive nitrification, even at extremely low water temperatures, as well as mean effluent concentrations of 6.3 mg/L COD and <0.05 mg/L P_tot, which makes it a very promising treatment option for greywater. All in all, the modular/elevated design promotes urban application of VFCW as a multifunctional blue–green system that can help increase urban resilience.

Separation of wastewater split streams and utilization of the resources contained therein are an important prerequisite for a sustainable wastewater treatment. In areas with existing infrastructures, this leads to transition states during which current systems must continue to operate. A simple mass-flow based method was developed to assess the impacts of an incremental transition (co-digestion of blackwater and sewage sludge) upon plant operation. The results proved that blackwater co-digestion could be easily integrated into existing wastewater infrastructures; however, beyond 8% transition thickening was required due to hydraulic limitations in the digester. Additionally, nitrogen recovery was necessary beyond 35% transition to avoid unfavorable C:N ratios. The investigated concept also led to a power gain of 11 kWh/(PE∙a) due to enhanced biogas production and aeration savings. The developed Excel-based method was considered suitable for evaluating the benefits and probable tipping points for plant operation during transition to source-separated sanitation systems.

Source separation has thus far been addressed mainly within the context of decentralization in new development areas; centralized approaches for resource-oriented sanitation remained, however, largely disregarded. By means of inhabitant-specific load and volume flow balances, based on typical reference values for municipal wastewater in Germany, a stepwise transition towards on-site greywater recycling was investigated for a model wastewater treatment plant (WWTP). Up to 17% transition (separation of greywater from 17% of the total inhabitants), greywater separation was proven to benefit plant operation by reducing energy consumption for aeration. From 17% transition onwards, however, unfavorable carbon to nitrogen ratios (C/N) were reported, as less biodegradable carbon reaches denitrification, thus shifting C/N ratios negatively. Therefore, nitrogen recovery/removal from N-rich sludge sidestreams would be required. At 35% transition, a 50% N recovery from sludge liquor was proven to be sufficient in order to ensure full denitrification; combined with greywater separation, nutrient recovery yielded 14% reduction in power demand for aeration (on the actual state). Additionally, extensive mainstream process changeovers could be avoided by separating N-rich urine alongside greywater from the main wastewater stream. Urine separation was proven to maintain denitrification stability as well as reduce power demand for aeration. The calculations show that, under consideration of specific boundary conditions, existing WWTP can be successfully integrated in transition concepts for resource-oriented sanitation.