An exergoeconomic-based method for forming water quality pyramid and assessing the sustainability of recycling processes – A greenhouse case study

Abstract

Recycling is one of the main ways of reducing the consumption of resources (reducing the exergy loss of systems). The economic indices (NPV, IRR, etc.) cannot be used to comprehensively assess the sustainability of the recycling process. For overcoming this challenge, the assessment function of “the saved exergoeconomic cost of production through recycling (ΔExC)“ was developed. For investing in line with sustainability, the high-priority flow for recycling must be distinguished from other flows in the system. Water Quality Pyramid (WQP) was presented based on the unit exergoeconomiccost index (c) to prioritize the flows in the system. The developed method was used to assess the sustainablity of the recycling process in a hydroponic greenhouse producing 725,000 roses cut-flowers in one year, by comparing the two processes for recycling the wastewater of the system to obtain the flows from the top of the greenhouse’s WQP, including the nutritious feed water and desalinated water flows with the c indices of 1168 and 25 USD/GJ. Results show that recycling the drainage of the feed water (Closed-Fertigation process) with the ΔExC value of +2172 USD/a is the sustainable choice, while the ΔExC value for the second flow is negative (−964 USD/a).