Combining energy recovery and leakage reduction in water distribution networks

Abstract

Energy consumption in water supply and distribution networks represents 7% of the world’s consumption of energy. According to the Key World Energy Statistics published by the International Energy Agency, the water sector accounted for 820 TWh of global electricity consumption in 2014. This amounts to 70 million tons of oil equivalent (MTOE). These Key World Energy Statistics summarise the average electricity consumption for all processes within the potable water supply chain, from abstraction through treatment to distribution. Incorporating these global averages, and by using average water consumption for South African metros as reported by the Department of Water and Sanitation and the Eskom average electricity rate, we can approximate the average annual electrical cost for water treatment and distribution of a metropolitan municipality with a population of 3 million to R166 million. According to the GreenCape Market Intelligence Report on Water for 2017, South African municipalities currently use about 4 500 million m³/year of water, of which 37% is non-revenue water. On this basis, on average, a metropolitan municipality with a population of 3 million loses around 180 million m³ of potable water per year, which amounts to a cost of roughly R60 million per year on the electrical cost for the treatment and distribution of nonrevenue water. This R60 million cost could be recovered to some extent through energy recovery using hydro turbines or energy recovery turbines (ERTs). Subsequent savings on water losses and the overall energy cost of the system are also present, but should be quantified on a site-specific level, as these vary according to location and system configuration. The energy recovery benefit figures change with various factors, such as the topography, treatment processes and distribution lengths. The City of Tshwane has a population of roughly 2.9 million, but its energy cost of water would be higher due to the higher pumping cost of water procured through Rand Water and pumped from the lower-lying Vaal Dam.