Save Our Ocean - The Desalination Dilemma
When we talk about Blue Economy fields in strong growth in 2019, in addition to the deep sea mining (which we talked about last week here), another topic that is always mentioned is that of water desalination. This is a very broad topic and, to deal with it in the best way, we will dedicate to it both this Save Our Ocean and the next. Today we will therefore focus on defining what is desalination while next week we will see how this is developing in the world and what future prospects there are, bringing back, once again, the European Blue Economy report.
Before starting, I remind you that you can find a list of the articles in this series, constantly updated and ordered according to thematic categories, by visiting this index.
The operating scheme of a multi-stage flash watermaker
Let's start by saying that desalination is the process of removing the salt fraction from the sea waters (although it can actually be implemented on any water containing salt). The resulting water is then used both for food use and for industrial use. The equipment used to create this process is called watermaker. Often it is also called water distiller, but this is a wrong definition given by the fact that the old plants resembled the typical distillation columns. However, desalination has nothing in common with distillation.
There are numerous types of desalination plants which we can however group into three macro-groups. It should however be said that the various types are not mutually exclusive and many modern desalination plants have combined systems. However, let's see these types:
- The Evaporative Watermakers are the most traditional ones. They are based on water evaporation and its recovery by condensation. Usually it also generates a rejection with salinity higher than the brackish water of the feeding and, in some cases, the solid part (sodium chloride) is collected in crystalline form after the evaporation of the liquid part. These plants operate at high temperatures (between 40 and 200 °C) and therefore need to be built in special materials. There are at least three different types of evaporative desalination systems (multiple effect, multi-stage flash or Vapor-compression), but they differ only in how the system is structured, the concept of basic operation is always the same
- The Permeation Watermakers, also called reverse osmosis one or membrane one, are a modern solution that is based, precisely, on the use of special semipermeable membranes to carry out the separation process. The characteristics of the water and the rejection obtained are similar to the previous type, but the quantities of both are smaller (the membranes do not allow the treatment of solid phases). This type of plant, therefore, produces less water, but is also less polluting (we will see it better below).
- The Ion Exchange Watermakers are the smaller ones and are based on a chemical process that, using special resins, leads to the removal of salt ions. It allows to obtain, in a single passage, a strongly desalted and pure water while the rejection is composed of the residues from the regeneration of the resins.
The Dubai desalination plant
The Economic Dilemma
The first major mass of criticism that is directed to desalination is given from an economic point of view. Desalination plants cost, cost a lot. The factors that determine this cost are: the capacity and type of structure, the location, the water supply, the labor required, the methods of disposal of the rejection, the controls necessary to verify that everything works perfectly and, above all, the energy used to operate the plant. In general, reverse osmosis watermakers are less expensive than evaporative ones and in recent years the overall cost has been much lower, but we are still far from the possibility of large-scale use
A December 2018 study commissioned by the UN reveals that the more or less sweet water production capacity of desalination plants is approximately 95 billion liters per day. To better understand the situation, we can make a comparison with a study by FAO, the United Nations Food and Agriculture Organization, which estimated to 628,000 liters, the fresh water of which would need a single person today in a year (considering food, industry and various activities). The perfect implementation of desalination would solve many water problems in the world.
But the situation is blocked and the desalted water remains the water of the rich. From the few experimental facilities of the 1960s in the Middle East, we are today at almost 160,000 plants in 177 countries. Two thirds of these, however, are high-income countries, where it is common for desalinated water to be used for non-primary activities. Although they have dropped a lot in the last few years, the costs are still too high for many of the Countries that need it most. Furthermore, the need to make desalination technologies accessible to low and medium-low income countries also comes up against a series of environmental problems.
A drain of brine from a desalination plant
The Environmental Dilemma
Unfortunately, the desalination plants have a big environmental impact that generates numerous problems. Let's see one at a time:
- The production of Brine, the rejection aqueous solution that has a salinity greater than 5%, is the main environmental problem generated by these plants. This brine is toxic as other hazardous industrial waste and should be treated as such. However, management is very expensive and for this reason, in many areas, most of the rejection ends up directly in the oceans (through sewers, surface waters or wells), with DEVASTANT consequences for the marine ecosystem.
- Another significant environmental problem is the acquisition of sea water. This in fact can drag with it fish and molluscs (or their eggs) into the industrial system which are then killed or damaged by heat, physical stress or chemical components. Also in this case the problem can be solved by using beach wells, but their implementation is very expensive and, therefore, often avoided.
- A third major problem is given by the influence that the massive use of desalinated water could have on human health. The desalination process, in fact, removes the iodine from the water and could increase the risk of iodine deficiency disorders. Many studies are being carried out on the subject, especially in Israel, but there are not yet certain results.
- Then there are a whole series of minor secondary environmental problems. The desalination plants need about 25 acres of land and must be built on or near the shoreline. If located inland, they need to install pipes on the ground that connect them to the water and these pipes could have leaks, penetrating and contaminating nearby aquifers. In addition, many desalination plants are very noisy.
Despite all these problems, the recovery of water through desalination is not a process to be discarded and, as we will see next week, it is a sector destined to grow a lot in the coming years. This is because many of the environmental risks listed present solutions (or there are some in development) that are progressively becoming safer and less expensive. Brine, for example, could become a valuable resource for irrigating salt-tolerant species and for aquaculture, as well as for recovering salts, metals and other elements that are part of it. For the time, however, it is evident that the many alternative solutions (water conservation and efficiency, waste water reclamation, capture of storm waters) remain the most advantageous for the man in terms of environmental impact.
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