Quality
of stored water
To prevent leaves and debris from entering the system, mesh
filters should be provided at the mouth of the drainpipe. Further,
a first-flush device should be provided in the conduit before
it connects to the storage container.
If the stored water is to be used for drinking purposes, a sand
filter should also be provided. Methods to protect rainwater
quality include appropriate system design, sound operation and
maintenance and use of first flush devices and treatment. Treatment
is mainly appropriate as a remedial action if contamination
is expected. First flush devices can be effective in reducing
levels of contamination if properly maintained. Good system
design, operation and maintenance are generally the simplest
and most effective means of protecting water quality.
a. System design: The best
initial step to protecting water quality is to ensure good system
design. Water quality will generally improve during storage
provided sunlight and living organisms are excluded from the
tank and fresh inflows do not stir up any sediment. The design
should include
- Clean impervious roof made from smooth, clean non-toxic
material. Over hanging branches above the catchment surface
should be removed
- Taps or draw-off pipes on tanks should be atleast five
centimeters above the tank floor (more if debris accumulation
rates are high). A tank floor sloping towards the sump can
greatly aid tank cleaning as will a well-fitting access
manhole.
- Wire or nylon mesh should cover all inlets to prevent
any insects and other creatures from entering the tank.
The tank must be covered and all light excluded to prevent
growth of algae and other organisms. The grill at the terrace
outlet for rainwater arrests most of the debris carried
by the water from the rooftop like leaves, plastic bags
and paper pieces.
- A coarse filter
and/or foul flush
device should be fitted to intercept water before
it enters the tank for removing leaves and other debris.
b. Operation and maintenance:
Proper operation and maintenance of rainwater harvesting systems
helps to protect water quality in several ways. Regular inspection
and cleaning of catchment, gutters, filters and tanks reduce
the likelihood of contamination. Water from other sources should
not be mixed with that in the tank.
c. Treatment: Treatment of stored
rainwater only makes sense if it is done properly and if hygienic
collection and use of the water will ensure it does not suffer
from re-contamination. There are several types of treatment
possible, the most common being chlorination, boiling, filtration
and exposure to ultraviolet or natural sunlight.
i) Chlorination: Chlorination
is most appropriately used to treat rainwater if contamination
is suspected due to the rainwater being coloured or smelling
bad. It should only be done if the rainwater is the sole source
of supply and the tank should first be thoroughly inspected
to try to ascertain the cause of any contamination.
Chlorination is done with stabilised bleaching powder (calcium
hypochlorite - CaOCl2) which is a mixture of chlorine and
lime. Chlorination can kill all types of bacteria and make
water safe for drinking purposes. About 1 gm (approximately
1/4 tea spoon) of bleaching powder is sufficient to treat
200 litres of water.
ii) Chlorine tablets:
Chlorine tablets are easily available in the market.
One tablet of 0.5 g is enough to disinfect 20 litres (a bucketful)
of water.
iii) Boiling: Boiling
is a very effective method of purification and very simple
to carry out. Boiling water for 10 to 20 minutes is enough
to remove all biological contaminants.
iv) Direct sunlight:
This can also be used to kill many of the harmful
bacteria in water by exposing it in clear glass or plastic
bottles for several hours. Although feasible in some circumstances,
the water must be clear, the weather fine and the water cooled
overnight before consumption.
SODIS: Solar disinfection
method uses sun's ultra-violet (UV) radiation to improve the
microbiological quality of drinking water. It has been proven
that synergies induced by radiation and thermal treatment
have a significant effect on the die-off rate of microorganisms.
The processes involved are indicated in the illustration.
Functioning of Sodis depends on
the following factors:
1) Weather and climate: Sodis requires sun's radiation
and temperature to purify drinking water. The container needs
to be exposed to direct sunlight for about six hours. If the
temperature raises above 50 degrees Celsius, the disinfection
process is three times faster. If weather conditions are not
optimal the efficiency of the disinfection process can be
increased by using half-blackened plastic bottles, which achieve
approximately 5 degrees C higher water temperatures than fully
transparent, non painted bottles. Placing the plastic bottles
on a black corrugated zinc sheet would also help in achieving
disinfection
- Water Turbidity: Suspended particles in the water reduce
the penetration of solar radiation into the water and protect
microorganisms from being irradiated. Sodis requires relatively
clearer water with a turbidity ofless than 30 NTU (Naphelometric
Turbidity Unit - naphelometer is a modern commercial instrument
used to measure turbidity)
- Material: Various types of transparent plastic materials
are good transmitters of light in the UV and visible range
of the solar spectrum. Plastic bottles made of PET (PolyEthylene
Terephtalate) are preferred because they contain less UV-
stabilizers than PVC (Poly Vinyl Chloride) bottles.
- Shape of containers: More the area of bottle exposed
to sunlight, more would be efficiency in achieving disaffection.
- Oxygen: Sodis is more efficient in water containing high
levels of oxygen. Shaking a water container which is about
3/4th full for about 20 seconds before they are filled completely
could increase oxygen levels. On reacting with this water,
sunlight produces highly reactive forms of oxygen (oxygen
free radicals and hydrogen peroxides). These reactive forms
of oxygen kill microorganisms
Although Sodis can neither treat turbid waters nor change
the chemical quality of water, this method is ideal to disinfect
small quantities of water used for consumption.
For more details:
www.sodis.ch
Tips to ensure quality of harvested
rain
It is extremely important to maintain the rainwater harvesting
systems regularly for high quality performance. Following aspects
should be taken care of:
- Just before the arrival of monsoon, the rooftop/catchmet
area has to be cleaned properly.
- The roof outlet on the terrace should be covered with
a mesh to prevent entry of leafs or other solid waste into
the system.
- The filter materials have to be either replaced or washed
properly before the monsoon.
- The diversion valve has to be opened for the first 5
to 10 minutes of rain to dispose off the polluted first
flush.
- All polluted water should be taken away from the recharge
structures.
- The depth of bores (of recharge structures) shall be
finalised depending on the actual site condition
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