Worldwide, 844 million people don’t have access to safe water. That means 1 in 9 people have no access to safe and clean drinking water. Increasing access to safe water will help improve health by preventing the spread of infectious illnesses. That means a decrease in mortality rates. Additionally, cases of physical injuries resulting from carrying heavy weights of water over long distances will reduce.
Dirty water may contain chemicals, pathogens or other contaminants which need to be removed to make it safe to drink. To make dirty water drinkable, remove the impurities. To achieve this, remove small particles out of the water by forcing it through a filter or semi-permeable membrane (reverse osmosis).
With pore size of about 0.0001 micron, reverse osmosis (RO) systems are very effective in removing impurities. RO effectively removes protozoa, bacteria, viruses and common chemical contaminants.
How Reverse Osmosis Works
- Feed water (salt water or brackish water) is forced across a semi-permeable membrane leaving behind almost all the dissolved salts in the concentrate stream. The water that passes through the semi-permeable membrane is called desalinated water, de-ionized water or de-mineralized water. Water that did not move across the semi-permeable membrane is known as the concentrate or the reject stream and is full of contaminants.
- The contaminated water is either sent to the drain or is recycled to conserve water. The water that was allowed to pass through the semi-permeable membrane is known as permeate or product water. This water usually has approximately 95% to 99% of soluble salts extracted from it.
Below is a simple, step by step, explanation of how this process works to obtain drinkable water from dirty water.
Figure 1 A simple illustration of reversed osmosis (RO) system
A – Applied pressure
B – Salt water in (feed water)
C – Contaminants
D – Semi permeable membrane
E – Clean water out
F – Distribution
To purify seawater or brackish water by reversed osmosis:
- First, apply hydraulic pressure to the feed inlet side.
The high-pressure pumps and the control valves generate pressure required for reversed osmosis in addition to controlling the flow in the system. A cold water line valve is fitted on the inlet water supply line to regulate the quantity of water from the source, entering the pre-filtration chambers.
- Next, allow contaminated water to enter the reverse osmosis system through the pre-filters.
The role of pre-filters is to protect the reverse osmosis membranes from destruction by sediments (sand and silt) or chlorine. The commonly used pre-filters are either sediment or charcoal pre-filters. The charcoal pre-filters are preferred in removing chlorine and other oxidizing compounds.
Water from the pre-filters is directed into the reverse osmosis membrane. The RO membrane is the most vital part of the RO system. The RO membrane eliminates various contaminants. The commonly used RO membrane is the spiral-wound. The cellulose tri-acetate (CTA) membrane is somehow chlorine-tolerant while the thin film composite (TFC) and thin film material (TFM) membranes are not.
- From the RO membrane, the water runs into a pressurized tank for storage.
From the storage tank, water flows through a post-filter(s) to the clean water outlet. The post filters contain activated carbon which removes bad tastes and smells.
The RO system has an inbuilt automatic shut-off valve. To save water, the shut-off valves automatically close when the storage tank is full. This prevents water from running into the membrane.
When water is released from the storage tank, pressure in the tank lowers. This, in turn, allows the shut-off valves to open, allowing drinking water to flow into the RO membrane and dirty water to the RO drain.
A check valve is positioned at the outlet end of the RO membrane housing. The check valve prevents treated water from flowing back to the storage tank hence avoiding damage to the RO membranes.
A flow restrictor located in the RO drain line regulates water passing through the RO membrane. The flow restrictor maintains a rate of flow required to get the best quality drinking water; depending on the quality and capacity of the RO membrane. Additionally, the restrictor maintains pressure on the inlet side of the RO membrane.
If the flow restrictors were absent, the pressure within the membrane chamber would be inconsistent. Therefore, very little clean water will be obtained as incoming water will take the path of least resistance, and flow down the drain line.
Purified water flows from the RO membranes to an overhead storage tank. The capacity of the RO storage tank varies depending on the capacity of the RO membranes, the volume of water, among other factors.
Typically, an under-counter RO tank measures a 6-inch radius and is 15-inch high. The RO faucet is the valve that controls the reverse osmosis unit or the RO tank outlet flow. Usually, the reverse osmosis unit has its own faucet fitted on the kitchen sink.
The drain line runs from the outlet end of the RO membrane housing to the drain. This discharges wastewater, filled with the high concentration of contaminants that have been filtered out by the RO membrane.
Access to drinkable water has a great effect on controlling the spread of water-borne illnesses and chemical-induced deaths. Reverse osmosis efficiently treats surface and groundwater for large and small-scale applications. Just like any other treatment systems, RO systems are susceptive to wear.
However, with the correct system design, and maintenance schedules, your RO system will provide many years of high-quality water.