Environmental Engineering Reference
In-Depth Information
27.2.2 Challenges to Desalination Processes
Inherent irreversibilities present in real systems typically drive the energy requirements
higher than the theoretical minima. Some of these irreversibilities relate to the presence
and subsequent removal of organics and particulates and varying quality (such as pH and
salinity) of source waters. Others relate to operation of mechanical and electrical equipment
at varying energy eficiencies. The varying source water content can also lead to scale for-
mation and deposition or membrane fouling in desalination plants [16]. To mitigate these
problems, desalination plants often employ extensive pretreatment steps involving chemi-
cal treatment processes, including precipitation, locculation, lime softening, ion-exchange
columns, or mechanical processes such as aeration and sedimentation. For example, to
minimize scaling, pretreatment of feedwater by introducing an acid followed by CO
2
degassing has shown to be an effective method for preventing alkaline scale formation
[16]. Antiscalants are particularly popular because of their effectiveness at low concentra-
tions, consequently reducing the overall chemical load. The chief chemical families from
which antiscalants have been developed from are condensed polyphosphates, organo-
phosphonates, and polyelectrolytes. Of these three classes of compounds, polyphosphates
are the most economical while effectively retarding scale formation and offering corrosion
protection [16]. Organophosphonates are suitable for a wider range of operating pH and
temperature conditions than polyphosphates [16]. The main consequence of all pretreat-
ment processes is that all of these involve increased energy consumption and material
costs regardless of the speciic desalination method employed. However, other concerns
exist with the use of antiscalants. In areas where polyphosphates are used, eutrophication
has been observed owing to the potential of the polyphosphate to be converted to ortho-
phosphate, which is a major nutrient for primary producers such as algae. Phosphonates
and polycarbonic acids are of some concern because of their chemical stability, giving
them a long residence time in water, with the potential to disrupt natural processes due
to their dispersion and complexing of magnesium and calcium ions. However, the overall
toxicity of antiscalants to aquatic life is low [17].
27.2.3 Common Separation Methods
Desalination dates back to the fourth century when Greek sailors used evaporation
aboard ships to produce freshwater, while membrane-based water puriication including
desalination became popular after World War II [18]. Approximately 86% of the available
desalination production capacity employs either RO or multistage lash (MSF) distillation
processes for freshwater production from either brackish water or seawater [7]. Other main
technologies include multieffect distillation (MED), vapor compression (VC), and electro-
dialysis [19]. The primary energy requirement for MSF, MED, and VC is in the form of
thermal energy, while RO requires primarily mechanical energy for pumping water, and
electrodialysis requires primarily electrical energy. Other methods such as solar distilla-
tion, freezing, gas hydrate processes, membrane distillation, humidiication-dehumidi-
ication processes, forward osmosis, capacitive deionization, and ion exchange are also
used for desalination; however, current technology levels for these processes ind limited
or niche use and are not yet commercially viable on a worldwide scale for widespread
implementation.
Table 27.2 summarizes energy consumption of commonly used desalination methods.
Comparing energy consumption, membrane separations require the lowest energy input,
by at least a factor of 6 compared with the thermal methods.
