Introduction

Desalinization of seawater is an essential process for the support of human communities in many places around the world. Seawater has a salt content of around 35,000 parts per million (ppm) depending on the location and circumstances: to produce the equivalent of freshwater (with around 1000 ppm (AMS, 2014) therefore requires the removal of over 97 per cent of the salt content. The main purpose of desalinization is to produce water for drinking, sanitation and irrigation. The process can also be used to generate ultra-pure water for certain industrial processes. This chapter reviews the scale of desalinization, the processes involved and its social and economic benefits. Issues relating to discharges from desalinization plants are considered in Chapter 20 (Coastal, riverine and atmospheric inputs from land).

Nature, location and magnitude of desalinization

As Figure 28.1 shows, desalinization capacity has grown rapidly over the past half-century. About 16,000 desalinization plants were built worldwide between 1965 and 2011. About 3,800 of these plants are thought to be currently out of service or decommissioned. The current operational capacity is estimated to be about 65,200 megalitres per day (65,200,000 cubic metres per day (m3/d) – in comparison, the public water supply of New York City, United States of America, delivers in total about 3,800 megalitres per day) (GWI, 2015; NYCEP, 2014).

Historically, human settlements have tended to grow up where freshwater was available, and their growth has been conditioned by freshwater availability and the possibilities of bringing it to serve the settlement. As long ago as 312 BCE, the Romans had had to build a 16.4-kilometre aqueduct to bring water to Rome in order to avoid this constraint (Frontinus). Desalinization represents an alternative technology for avoiding this constraint on the growth of human settlements in areas with very limited availability of freshwater. That capability, however, comes at the price of considerable capital investment and energy consumption. Gleick et al. (2009) give an overview of the worldwide distribution of desalinization capacity in 2009.

The nature of the industry, however, varies in many ways between the different regions, particularly in respect of the technology used: the Middle East has relied more on thermal processes, while the United States has relied more on membrane processes. Thermal processes (mainly Multi-Stage-Flash (MSF) and Multiple-Effect-Distillation (MED)) evaporate the water and then re-condense it.