If no forces except gravity were at work, the world's water would settle into the ocean basins and remain there. The land surfaces would become lifeless deserts. Water, however, does not stagnate in the oceans. It is continually evaporated from the oceans and other bodies of water by the heat of the sun and blown by the winds across sea and land. Thus an immense amount of water is always suspended in the atmosphere, some of the water vapor forms clouds. When such clouds accumulate more water vapor than they could hold, the water is returned to the land as rain. This process of moving water out of the oceans, into the atmosphere, and back to the land and oceans is called the water cycle. The image below illustrates the stages of the water cycle. Although Grenada does not store water in ice and snow, it is still a very important part of many countries' water cycle and should not be discounted.
Click on the links below to view a movie of the water cycle.
The movies are created by NASA Water and Energy Study.
Movie Without Text (25Mb) - Movie With Text (45Mb)
A fast internet connection is needed to view the movies properly.
Choosing a water source
Before water can be properly treated it must be collected from a surface or underground source. When a choice has to be made between sources, the quality of the water at the source and the quantity it produces must be considered. Once a source has been choosen, measures should be taken to protect it form contamination. These considerations must be taken into account to ensure that the chosen water source can satisfactorily meet the need for water in the area.Sources of Water
Surface sources of water are formed from runoff of rain water. Ground water sources are found below the surface of the ground, and are formed when water is absorbed into the ground. Wells or boreholes are dug to extract ground water. Desalination plants are also constructed to directly prepare sea water for consumer use.
In Grenada, NAWASA use the following sources to provide water to the nation:
- Surface :- Rivers, streams, springs and lakes.
- Ground :- Deepwells (Bore holes)
- Desalination plants
Rain falling over mountains and collecting at rivers and streams
Illustration of a typical bore hole station
Woburn desalination plant
Surface water is damned at areas where there is the least amount of contamination
Annadale and Les Advocate Dams
The main purpose of water treatment is to remove the impurities and improve the colour, taste and smell of the raw or untreated water.
The process of water treatment is divided into four main processes. They are Screening
Screening is the process of using grates or mesh to prevent large objects (from a few inches to feet) such as branches, boulders and other objects of similar size from reaching the treatment plant, and is usually done at the dam or intake to the plant. Sedimentation
Alum is added and mixed with the raw water in a large tank or basin. The chemicals cling to the impurities in the water (coagulation), forming larger and heavier particles (flocculation). These particles called floc then sink to the bottom of the basin and collect to form sludge which is later removed.
Lime and other chemicals are added to improve the quality of the water.Filtration
Smaller particles are removed by passing the water through beds of sand and gravel called filters. This process is called filtration. After filtration all the particles should have been removed leaving only impurities such as micro-organisms.Chlorination
Chorination is the process by which the filtered water is disinfected with chlorine to kill all disease-causing germs. After treatment the water should be safe, potable and pure.
In our country, ground water does not usually require and great deal of treatment. This type of water is often filtered naturally as it passes through the ground. Chlorine is added to protect consumers from bacteria.
Water is transported from source to treatment plants. After treatment water is stored at reservoirs until further distribution is needed
Woburn storage tank being constructed
Distribution of water
Treated water is distributed to our homes, offices, schools, factories and other places by a network of pipes, pumps, storage tanks or reservoirs and valves. Such a network makes up the Water Distribution System.
The water which leaves a treatment plant or well is pumped or allowed to flow by gravity. In our country the water distribution system is largely dependent on gravitational forces. The water supply of a few areas are dependent on the public electricity supply.
The service lines which carry water into the homes of consumers are usually 12mm (1/2") or 20mm (3/4") in size. Distribution mains are larger and actual sizes vary depending on the water demands of a particular area.
In the case of storage tanks and reservoirs, these are carefully located to store water during periods of low use (such as duringthe night) to be supplied at peak periods of high use (such as during the day).
Valves control the passage of water at points along a main. By opening or closing a valve the flow of water in a main can be increased, decreased or stopped.
Distribution of water
COLLECTION, TREATMENT AND DISPOSAL OF WASTEWATER The Grand Anse Sewer System
The Grand Anse Sewer System comprises some 15 miles of force main and gravity collection lines, 4 pump stations and a marine outfall 1145 feet long. The collection, which includes all grey water (baths and sinks), begins just about Defreitas Cottages and collects all of the sewage from residences, commercial and industrial establishments in the Falege, North Grand Anse, Comerhogne and Frequente areas. It is designed to handle all the future expansion in the entire South St. George's area.
Septic Tank SystemThe first component in the system is a septic tank that uses natural processes to treat the wastewater generated in your home. The second component is a drainfield or subsurface infiltration field that recycles the treated materials. The system accepts both "blackwater" (toilet wastes) and "greywater" (wastes from the kitchen sink, bath and showers, laundry, etc.) Water that should not be discharged to the system includes water from foundation or footing drains, roof gutters and other "clear" water. The Septic Tank
The septic tank provides the first step in treatment. Its primary purpose is to protect the drainfield or other system components from becoming clogged by solids suspended in the wastewater. The wastewater discharged from the home goes directly into the tank where it is retained for a day or more. During the time it is in the tank, the heavier solids settle to the bottom to form a sludge layer. The lighter solids, greases and oils float to the top to form a scum layer.
In addition to acting as a sedimentation chamber, and providing storage for the sludge and scum, the septic tank also digests or breaks down the waste solids. Anaerobic and facultative micro-organisms that thrive without oxygen feed on the solids to reduce the volume of sludge and scum. In the process, carbon dioxide, hydrogen sulfide and other gases are produced which must be vented from the tank through the plumbing vent on the roof. Only about 40% of the sludge and scum volume can be reduced in this manner, however, so the tank must be pumped regularly to remove the accumulated solids. If not done the tank will fill with sludge and the solids will be washed out into the drainfield where they will quickly clog the soil.The Drainfield
The drainfield provides final treatment of the wastewater and disposes the treated water through groundwater recharge. The drainfield is typically built as a series of trenches or as one larger bed, and is usually 1 to 3 feet below ground level. The drainfield must be constructed in permeable soils, have a level bottom, and be 2 or more feet above the groundwater table. While there are many types of drainfield systems, we will describe here a conventional gravel and pipe system.
The excavated trench or bed is filled with 6 to 12 inches of gravel. The gravel exposes a soil infiltrative surface and provides storage for the wastewater, A perforated pipe is laid over the gravel to distribute the partially treated liquid, called effluent, from the septic tank over the bottom of the drainfield. The gravel and pipe is covered with synthetic fabric to help keep soil particles out of the system and the area backfilled with soil to cover the system.
The Septic tank effluent is allowed to flow to the drainfield by gravity or is dosed by pump or siphon. The effluent enters the soil and is treated as it percolates to the groundwater. The soil acts as biological filter to remove nearly all harmful substances including disease-causing bacteria and viruses, toxic organics and other undesirable wastewater constituents remaining in the septic tank effluent.
Drainfields other than those described above can be used such as at-grades, mounds, and drip distribution.