The energy-water nexus in campuses

Water is a critical index of an organization's sustainability. Since water reuse consumes energy, water management requires careful analysis of energy implications. To this end, we study the energy-water nexus in a multi-building campus with a water delivery network that spans multiple grades (such as potable, reclaimed sewage, etc). Using data collected over several months, we answer these questions: (i) What are the trade-offs between the external water footprint of a campus and its internal energy footprint of water? (ii) Are improvements in either footprint realizable in practice? (iii) Does reducing the consumption of one water grade have more impact on the energy consumption than other water grades? (iv) Does rainwater harvesting help reduce a facility's energy footprint? We construct a multi-grade logical flow network with a per-link cost model for energy derived from the measured data. Under the constraint that demands are always met using the existing supplies, we optimize this flow-network for individually minimizing internal energy consumption of water and external water intake. Our study reveals the following: (i) minimizing external water footprint does not correspond to minimizing the internal energy footprint of water; (ii) demand reduction of different water grades impact the energy and water footprints differently; Contrary to intuition, reduction in second grade water demand yields highest reduction in water footprint while reduction in first grade water demand yields higher reduction in energy; (iii) Rainwater harvesting (RWH) can significantly reduce the energy footprint of a campus water network with sewage re-use. Our results show a potential for improving the operating condition of the campus's water network that can reduce the energy consumption by nearly 56 MWh (10.5%) and 99.6 MWh (18%) annually without and with RWH respectively.