5.4 Alternative Water Resources: Desalination

5.4.5 Power Consumption

The majority of large-scale water treatment systems require power for their operations. Desalination processes, as opposed to other methods of water treatment, have significantly higher power requirements. The development of Reverse Osmosis (RO) technology has made desalination viable as a municipal water supply largely because of the increased efficiency this technology offers over other systems. Thermal technologies are energy-intensive and even MSF, the most efficient of thermal technologies, uses significantly more energy than RO to desalinate typical seawater (Wangnick 2004). Table 3 presents a range of published energy consumption values associated with RO systems. The associated costs of powering RO desalination plants can be determined by applying these values to the existing and anticipated market rate of power.

Table 3: Reverse Osmosis Electrical Consumption
^a. Cooley 2006. ^b. Marin Municipal Water District 2006. ^c. Energy Recovery Inc. 2006.

Electrical energy use can represent up to 44% of the cost of water derived from an RO system, so any gained efficiency in energy used can reduce the cost to the end user (Cooley et al. 2006). Energy recovery systems can increase the efficiency of an RO plant by up to 57%. Existing energy recovery technologies include turbines and wastewater pressure exchangers. Both systems work by recapturing a portion of the energy used in the RO process by harnessing the pressure of the wastewater (brine) and transferring it to the energy input requirements of the production stream. Figure 8 below depicts the general process of a pressure exchanging system.

Figure 8: Pressure Exchanger Schematic

Pressure exchangers have been used with success in the reverse osmosis process to reduce energy demands. The development of this technology is ongoing with many manufacturers advertising high levels of recovery. With energy recovery systems, manufacturers have been able to operate RO systems consuming 1.6 kWhr/m³ (Energy Recovery, Inc. 2006). This is less than half the energy consumption required to typically desalinate Pacific Ocean water and approaching the theoretical energy value of 0.8 kWhr/m³.

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