ARMY TM 10-4610-309-14
MARINE CORPS TM 10802A-14/1
TACTICAL WATER PURIFICATION SYSTEM (TWPS)
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DESCRIPTION AND THEORY OF OPERATION
THEORY OF OPERATION
The total time off-line for backwash and rewet is 3 minutes. During this time water continues to be drawn
from the RO feed tank and processed through the rest of the TWPS to provide a continuous flow of
product water. Since filtrate is temporarily stopped from flowing to the RO feed tank during backwash,
the RO feed tank level drops to approximately 1/4 full during this time. The MF feed tank level will drop
approximately 125 gallons during the shell sweep stage and will be low by approximately 90 gallons after
backwash is completed. If Flow Control Valve V-107 is properly adjusted, the raw water flow will replace
this loss by the next backwash. The MF feed tank holds 1000 gallons full and is normally kept between
half and three quarters full by adjusting the raw water flow.
The blow-back stage results in a sudden and very high velocity surge in the waste out piping and hose,
which must be allowed to pass easily. The red-banded, waste out hose is a 6-in. discharge hose, large
enough that it will not be fill with water and can expand to pass the blow-back air. This hose should be
set to slope steadily back to the water source.
The backwash removes all of the loose dirt. However, some material sticks or gets trapped inside the
pores of the fibers. As a result, the pressure drop (TMP) across the fibers increases over time. To
measure this, pressure transmitters PT-101 and PT-102, measure the MF feed water and filtrate
pressures. The pressure drop, trans-membrane pressure (TMP) is calculated by the PLC and displayed
at the OCP. When it reaches a set maximum, a high TMP alarm indicates the need to schedule cleaning
within the next 40 hours. Typically, the interval between cleanings will be at least 300 hours of operation
and could be as long as 1000 hours.
The filtrate typically is very low in suspended solids content and is free of micro-organisms. Broken
fibers, however, may allow a flow of raw water directly into the filtrate. A high number of broken fibers will
lead to enough material in the filtrate to foul the RO elements with dirt and bacterial slime. Broken fibers
can be detected by a pressure hold (integrity) test, which is conducted automatically after operator
selection. A test fail light indicates that the leakage detected is too high. The alarm is set well below the
point where serious RO fouling would occur. The air test is conducted at the beginning and end of each
mission to indicate fiber condition and the need for scheduled maintenance. The distribution of broken
fibers among the 12 elements is determined by using a sound probe leak detector to determine the point
of leakage. Defective elements can be individually replaced and may also be individually isolated by
closing the individual module filtrate valves.
REVERSE OSMOSIS (RO) SYSTEM
The RO process is pressure driven. Pressure moves water through the pores of the RO membranes
while most of the salts are rejected by molecular level electrical charges in the pores. Only a portion of
the feed water is forced through the membranes, becoming product water. The water that does not pass
through the membranes contains concentrated salts and is discharged as reject. Increasing the pressure
increases the product water flow. Feed water with a higher salt content requires higher pressure to force
water through the RO membranes and produce water than feed water with a lower salt content. For
example, low salt river water may require only 300 pounds per square inch gauge (psig) to produce 25
gpm, while normal seawater may require 800 psig to produce 20 gpm. The maximum pressure available
is 1200 psig. Colder feed water also requires higher pressure than warmer feed water. Fouling,
biological films, and salt scale also restrict water flow through the membrane and force operation at
higher than normal pressures.
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