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Membrane Technologies for Point-of-Use and Point-of-Entry Applications 621
The booster pump is installed with the major functions in raising and maintaining the water
pressure at the optimum level to ensure the highest rejection rate and maximum water
production.
Pretreatment: A pretreatment unit is required for RO in POU/POE system to obtain the
desired quantity or quality of water. Most reverse osmosis units require pretreatment to remove
all manganese, iron, hardness minerals and the TDS. High levels of water hardness tend to
reduce membrane efficacy and result in more frequent replacement of the RO membrane. Also,
high levels of iron, manganese, and aluminum can also cause membrane fouling. Thus, many
treatment methods such as filter and activated carbon can be used for this purpose.
Filtration is a widely used pretreatment unit for RO in POU/POE system to remove tiny
particles of suspended dirt and sediment to prevent membrane fouling. The prefilter is
sometimes referred to as a sediment filter. It removes small suspended particles to extend
the life of the membrane. Fine grain filtration may be required if suspended solids are a
problem. It is important to select a unit which is capable of producing a sufficient amount of
treated water. Additionally, the activated carbon prefilter can be installed. This process can
remove organic materials and any dissolved gases not treated by the RO membrane.
Generally, the upper TDS required for an RO unit is limited to approximately 1,500 mg/L.
In addition, some membrane units are damaged by chlorine, and others by bacterial growth.
In this situation, a carbon prefilter is also recommended. A GAC prefilter is typically used to
remove chlorine, which can damage some types of RO membranes. When choosing a
pretreatment unit for RO process, the characteristic of feed water has to be considered to
determine the appropriate pretreatment method, and the upper limit for TDS and other
contaminants has to be identified to operate the pretreatment unit properly.
Reverse osmosis membrane unit: The central part of the reverse osmosis system is the
module, which is a pressurized container that houses the semipermeable membrane. Several
kinds of reverse osmosis membranes are available. The most common materials are cellulose
acetate or polyamide resins. Mixtures or variations of these materials are also used. Each
product has certain advantages and limitations and these needs to be considered carefully (see
Sect. 3.3 Types and configuration of reverse osmosis for more details). In general, the time
needed to produce 1 gallon of water ranges from 2 to 7 h. Typically, the capacity of RO
membranes for home systems is rated in the range of 24–100 gallons per day. This treated
water comes out much slower than water from a regular tap, so a tank is used to store the
treated water. This treated water is often used only for drinking and cooking.
Posttreatment unit: The major function of a posttreatment unit is to remove any desirable
taste and odor as well as any residual organics from the treated water. The typical unit of
posttreatment is a carbon filter. A post-carbon filter removes test and odor to give water a final
polish prior to delivery. However, if a carbon filter is used as a part of the prefiltration step,
postfiltration is normally eliminated. A standard carbon filter positioned after the storage tank
removes compounds that cause unpleasant taste and odors, including those from the tank or
plastic tubing, just before water is dispensed.
An additional prolonged contact carbon filter (PCCF) can be installed to remove high levels
of organic chemicals such as trihalomethanes, volatile organic chemicals, and chloramines.
This unit is normally placed between the RO membrane and the storage tank. Combining an