Experimental study on building wastewater treatment and water production performance of droplet falling film evaporation enhanced by nano-structured surfaces

Addressing water scarcity in the Yellow River Basin and the need for building wastewater reuse, this study proposed a novel treatment system based on droplet falling film evaporation on nano-structured surfaces. The system utilized low-grade thermal energy from building-integrated photovoltaic/thermal (PV/T) units to drive a multi-effect falling film evaporation process, enabling low-carbon wastewater treatment for school office buildings. Experiments were conducted to examine the effects of the tube contact angles (smooth tube, nano‑structured tubes A and B), Reynolds number (Re), and steam temperature on the water production capacity. The results demonstrated that the system effectively reduced the chemical oxygen demand (COD) from 512 mg·L−1 to 8.1 mg·L−1. Chloride ions and ammonia nitrogen were below the detection limit, and the produced water met the national standard for urban miscellaneous water reuse. Under droplet falling film flow with Re < 400, nano-structured surfaces significantly enhanced heat transfer due to their high wettability. At Re = 60, the water production rate using nano-structured tube B was 1.65~1.73 times that of the smooth tube. Additionally, higher steam temperatures reduced wastewater viscosity, promoting liquid film spreading and improving evaporation efficiency. This research provides a feasible pathway for low-carbon recycling of building wastewater.