osmosis is a water treatment process that forces water through
an extremely fine membrane to remove dissolved minerals.
Purified water passes through the membrane and collects in
a storage container. Most of the dissolved minerals in the
water cannot pass through the membrane and are flushed away
as waste. Small household reverse osmosis systems flush from
three to twenty litres of water to waste for every litre
of treated water. This flushing helps maintain treated water
quality and prevent fouling of the membrane.
Reverse osmosis competes directly in the marketplace with
distillation. Both can provide small amounts of high quality
water for drinking, automatic humidifiers, and watering plants.
The diagram below shows the makeup of a typical household
reverse osmosis unit.
Figure 1. Reverse osmosis mechanics - water passes first
through a sediment filter, which culls coarse solids that
could plug up the reverse osmosis membrane. Water next follows
the spiral winding of the membrane. Contaminated water leaves
the system and goes down the drain; treated water moves on
to a holding tank. When water is drawn from the tank, it
flows through a carbon filter which removes organic chemicals,
then to a spigot.
Small household reverse osmosis units operating on household
water pressure will produce one to five gallons per day.
Many reverse osmosis units are manufacturer rated at 80 psi
supply pressure. Typical private water system pressure is
closer to 30 to 40 psi, so water production will often be
about half the manufacturer's rated capacity.
Total Dissolved Solids in the Water
Higher pressure is required for reverse osmosis to remove
higher levels of total dissolved solids. Many household
reverse osmosis units operate on 25 to 50 psi pressure.
These are only suitable for a maximum of 1,500 to 2,000
ppm total dissolved solids. If reverse osmosis is to be
used to treat water that is higher in total dissolved solids
than this, a booster pump will be required.
Water pressure affects both quality and quantity of the treated
water which is produced. Basically, the higher the pressure,
the more treated water produced and the better the quality
The optimum water temperature for most reverse osmosis membranes
is 25°C. As the temperature drops to 5°C the capacity
of the reverse osmosis unit will be reduced to less than
one half. Long, small-diameter water feed lines will allow
the water to warm up to room temperature (20°C) before
reaching the membrane. This will increase treated water
Reverse osmosis membranes can be fouled and clogged by bacterial
slimes, hard water scale, iron, and silt. Avoid or treat
bacteria contaminated water. Soften water that is harder
than 50 ppm (3 grains per U.S. gallon). Filter any iron
or sediment from the water. Some membranes can be disinfected
using chlorine, formaldehyde, iodine, or peracetic acid.
Check with the supplier about the recommended disinfection
products and procedures for a particular unit.
Maintenance of Reverse Osmosis Units
Where pretreatment is required this pretreatment equipment
is critical. Ensure that pretreatment equipment is working
Change prefilter and postfilter cartridges regularly. These
filters can become "bacteria farms" and contaminate
Check the product water quality regularly. Dealers have
test equipment, a total dissolved solids check only takes
a few seconds. Some units have built in testers.
If the proper membrane is used, and pretreatment is adequate,
a good quality reverse osmosis membrane should be expected
to last from two to five years.
Keep chlorinated water out of most reverse osmosis units,
particularly those with thin film composite (TFC) and polyamide
membranes. Activated carbon prefilters are used to remove
chlorine from the water before is hits the membrane.
An increase of decrease in the amount of water produced
usually indicates trouble with the membrane. Have the product
water quality checked by the equipment supplier.
Substances That Will Go Through a Reverse Osmosis Membrane
Volatile gases such as oxygen and hydrogen sulfide will
pass through reverse osmosis membranes. Some organic substances
with low molecular weight can also pass through the membrane.
Some of these organic substances are suspected of being cancer
causing; fortunately these substances are rarely found in
Alberta water. The activated carbon filters used in most
reverse osmosis units can remove most organic substances
anyway. Bacteria can also "grow through" reverse
osmosis membranes leading to possible bacterial growth in
the postfilter and water storage tank.
- Small household reverse osmosis systems will produce
one to five gallons of water per day.
- If total dissolved solids
is greater than 2,000 ppm, or more than five gallons
of water per day is required, a booster pump will be required.
osmosis will remove 85 per cent to 98 per cent of total
dissolved solids from water.
- Raw water quality must be within
the limits specified for the membrane used for a particular
- Many reverse osmosis membranes do not remove nitrates
- Thin film composite membranes are usually the
best alternative for rural water in Alberta.
- Thin film composite
membranes are durable but can tolerate no chlorine.
such as hydrogen sulfide (rotten egg odor), will pas
through reverse osmosis membranes.
- Maintenance of pretreatment equipment
- Manufacturer's water production ratings are
based on ideal conditions of temperature, pressure,
and low total dissolved solids. Under private water system
conditions expect approximately half of the production
that the sales literature states.
- Small reverse osmosis units waste from
three to twenty litres of water for each one litre
of water produced. Many units are now available with a
shutoff to turn off the inlet water supply when the storage
tank is full.