Rainwater harvesting systems (RWHs) in buildings are benefits in alleviating urban water scarcity. However, climate change and changes in building water demand affect the expected performance of RWHs over their life cycle. Previous studies have only focused on the adaptability of RWHs to climate change without considering the changes in building water demand. To address this issue, this study explores the impact of combined changes in precipitation patterns and domestic water demand on the water-saving efficiency of RWHs over their 25-year life cycle based on historical observations. Three cities in Japan were selected as case studies. The results indicate that during 25-year life cycle of RWHs, precipitation in different cities in Japan shows an increasing trend, while domestic water demand shows a decreasing trend. Such trend resulted in a change of -2.90–4.02% in the water-saving efficiency of the established RWHs and a change of -2.90–3.82% in the theoretical optimal water-saving efficiency, and these effects were greater in RWHs in buildings with high non-potable water demands. Therefore, RWHs in buildings with low non-potable water demand can appropriately reduce the rainwater tanks, while RWHs in buildings with high non-potable water demand must increase the rainwater tank size to eliminate this adverse effect. Furthermore, both reductions in domestic water demand and increases in precipitation have a positive effect on the water-saving efficiency of RWHs. However, when both domestic water demand and precipitation increase or decrease simultaneously, the change in water-saving efficiency of RWHs is related to the non-potable water demand of the building. This study can provide data support and theoretical evidence for the further implementation of RWHs under climate change, and can help stakeholders to optimize and update RWHs to ensure system feasibility.

High-rate algal ponds (HRAPs) are a highly promoted wastewater treatment system that uses sunlight as an energy source to provide the oxygen needed in the system through photosynthesis and has a high nutrient and organic matter removal capacity. In addition, the microalgae in the system can use wastewater as a growth substrate to produce valuable bioproducts, biomaterials, and bioenergy, so it is receiving more and more attention. This review uses bibliometric analysis to explore current research hotspots and future research trends in this emerging technology. By analyzing research papers related to HRAPs published in the Web of Science (WOS) from 1987 to 2021 based on the co-occurrence and clustering of keywords, it shows that the research hotspots of HRAPs are mainly focused on wastewater treatment, nutrient removal, microalgal biomass, biofuel, and biogas upgrading. In the future, in-depth research will continue to be added on the contribution of HRAPs to environmental sustainability, including E. coli removal, biogas upgrading and oxygen removal, treatment of aquaculture wastewater, purple phototrophic bacteria, aqueous biorefineries, and biorefineries. The results assist scholars in systematically understanding the current research status, research frontiers, and future trends of HRAPs from a macro perspective.

A novel multi-component biological filler, consisting of residual activated sludge, clay, humus, silicon compounds, fly ash, and humic acid organic fertilizer, is described here. The aim of the study was to enhance and improve the activity of activated sludge in a biological wastewater treatment process. Following the addition of the filler to a sequencing batch reactor, significant effects on sludge settling properties, sludge yield, sludge dewatering performance, and sludge odor were observed. To further optimize the components of the novel multi-component composite bioactive filler and to improve the activity of the sludge, its composition and microstructure were studied. The biological filler was characterized by different techniques, i.e. field emission scanning electron microscope (FESEM), energy dispersive spectrometer (EDS), and Fourier transform infrared spectroscopy (FTIR). Through measurements of the composition of organic matter and humus, it was found that the content of each component remained unchanged over a six-month period. The FESEM results showed that the surface of the filler was rough and porous, while the EDS results revealed that the main elements in the filler were O, Si, and C, and the FTIR results indicated that the main functional groups of the absorption peak were –COOH, , SiO_2, and –C=O. The elements contained within these functional groups (O, Si, and C) were the same as those identified by the EDS.