CHAPTER 2 DESCRIPTION OF OPERATION
Additional operation, maintenance and parts Information may be found
In the following technical manuals which cover the 150,000 Gallon Per
Day (GPD) Reverse Osmosis Water Purification Unit (ROWPU):
TM 10-4610-229-10, Operator's Manual for 150,000 GPD
TM 10-4610-229-24, Unit, Direct and General Support
Maintenance Manual for 150,000 GPD ROWPU
TM 10-4610-229-24P, Repair Parts and Special Tools List for
150,000 GPD ROWPU
2-1 RO process. The RO process separates dean water from salt water or brackish water. During the natural osmosis
process, pure water and saltwater can be separated by a semipermeable membrane in a container at atmospheric
pressure. Because of the difference In salt concentration, pure water will naturally diffuse through the membrane and
raise the water level In the saltwater side as though pressure were being applied to it The effective driving force causing
this flow Is called osmotic pressure. The magnitude of osmotic pressure depends upon the concentration of dissolved
solids in the saltwater and the temperature of the water The greater the concentration of salt In the saltwater and the
higher the water temperature, the higher the osmotic pressure. To reverse the natural osmosis process, therefore,
pressure is applied to the saltwater side (Figure 2-1). When the applied pressure is greater than the osmotic pressure,
purified water diffuses through the semipermeable membrane from the saltwater side to the freshwater side Thus the
term reverse osmosis. To use this principle, feedwater is cleaned by several types of filters and pumped under pressure
across semipermeable membranes in the RO block assembly. The resulting purified water, called product water, flows to
the storage tanks. The water on the outside of the membrane that now has a higher salt concentration, called brine
concentrate, is discharged overboard.
2-2 ROWPU system operation. As shown in Figure NO TAG, seawater (feedwater) to be processed by the ROWPU's
flows from seawater filters (part of the seawater system) to the pretreatment skid A flow rate Indicator on top of the
pretreatment skid monitors the feedwater's degree of cloudiness and flow rate. In addition, incoming water temperature
may be read on a gauge (T1) located In the line on the supply side of the pretreatment skid three-way valves.
The coagulant pump adds coagulant (Hydrapol-50) to the seawater before it enters the seawater filter Seawater
containing coagulant then flows through the seawater filter to three media filters .The coagulant helps the media filters to
remove fine partides and colloids (clouds of fine particles suspended In water). Normally, a dose of 1.0 parts per million
(ppm) is adequate Dosage can be increased, however, if a seawater sample taken at valve R08 contains more Impurities
than usual Dosage Is increased by manually adjusting metering pump stroke length and speed.
In the media filters, seawater flows from the top downward Fine particles and colloids are thus removed from the
seawater so It is suitable for processing by the RO block assembly.
Scale Inhibitor (Hydrapol-100) is added to the seawater as It Is discharged from the media filters and collected into a
single stream. Scale inhibitor limits formation of scale on the RO block membranes . A dose of 40 ppm is added to the
seawater by a small, diaphragm type, positive displacement pump similar to the coagulant metering pump. If necessary,
however, dosage can be manually adjusted by changing metering pump stroke length and speed.
Seawater containing scale inhibitor then flows through the cartridge filter assembly on pretreatment skid. The cartridge
filter assembly removes any particles not removed by the media filters that would be harmful to the HP pump. This
filtered seawater then flows to the HP pump where pressure is increased to the 835 psi (maximum) required for reverse
osmosis, processing in the RO block.