Membrane and Desalination Technologies - TCE Moodle Website

578 J.P. Chen et al.
High pressure
seawater out
with usual energy recovery devices based on turbines. The direct transfer of the fluid pressure
from the concentrate to the fluid pressure in the feed stream with an efficiency of approximately
98% allows for short amortization times for the system itself if installed in new plants
as well as for retrofitting of existing plants (34).
Another energy recovery technology is to convert the hydraulic energy in the reject stream
into rotational energy, in the form of mechanical shaft power, by an ERT. In an RO system,
this rotational mechanical energy must then be transferred into another pumping device that
pressurizes the incoming stream (Fig. 13.9). The efficiency of centrifugal ERT is typically in
the range of 80–88% in converting hydraulic energy into rotational mechanical power which
is converted back to hydraulic energy. Therefore, when the efficiency losses of the pump are
accounted for, the real net energy transfer efficiencies are about 35–75% for these devices
even when operating at their optimum efficiency design points.
4. PRETREATMENT AND CLEANING OF MEMBRANE
4.1. Mechanisms of Membrane Fouling
Pump Rotation ERT
High pressure
reject water in
Fig. 13.9. Simple schematic of energy recovery with energy recovery turbine and pump.
The pretreatment system that is used is highly dependent on the feed water quality.
A complete and exact analysis of source water is thus important for the design of a pretreatment
system and the entire RO system because this often determines the type and size of the
pretreatment. The source of water may contain various concentrations of suspended solids
and dissolved matter. Suspended solids may consist of inorganic particles, colloids, and
biological debris including bacteria and algae. Dissolved matter may consist of highly soluble
salts, such as chlorides, and sparingly soluble salts, such as carbonates, sulfates, and silica.
During operation, the concentration of suspended particles and dissolved ions increases with
a decreasing volume of feed water. The settled suspended particles on the membrane surface
may block feed channels and increase pressure drop across the system. The precipitated
soluble salts from the concentrate stream may form a scale on the membrane surface and thus
decrease water permeability through the RO membranes. The process of forming a deposited
layer on the membrane surface is called membrane fouling. Membrane fouling remains the
most difficult obstacle in membrane operation. The consequence of membrane fouling is
reduction of permeate production rate and/or an increase in solute passage across the
membrane with time. Fouling may also cause increase in energy consumption as transmembrane
pressure can increase substantially because of fouling. In addition, fouling
increases downtime and may shorten membrane life span. There is also a high level of
biological activity resulting from biological treatment.