Rainwater harvesting is a technology used for
collecting and storing rainwater from rooftops, the land surface or rock
catchments using simple techniques such as jars and pots as well as more complex
techniques such as underground check dams. The techniques usually found in Asia
and Africa arise from practices employed by ancient civilizations within these
regions and still serve as a major source of drinking water supply in rural
areas. Commonly used systems are constructed of three principal components;
namely, the catchment area, the collection device, and the conveyance system.
A) Catchment Areas
I.
Rooftop
catchments: In the most
basic form of this technology, rainwater is collected in simple vessels at the
edge of the roof. Variations on this basic approach include collection of
rainwater in gutters which drain to the collection vessel through down-pipes
constructed for this purpose, and/or the diversion of rainwater from the
gutters to containers for settling particulates before being conveyed to the
storage container for the domestic use. As the rooftop is the main catchment
area, the amount and quality of rainwater collected depends on the area and
type of roofing material. Reasonably pure rainwater can be collected from roofs
constructed with galvanized corrugated iron, aluminium or asbestos cement
sheets, tiles and slates, although thatched roofs tied with bamboo gutters and
laid in proper slopes can produce almost the same amount of runoff less
expensively (Gould, 1992). However, the bamboo roofs are least suitable because
of possible health hazards. Similarly, roofs with metallic paint or other
coatings are not recommended as they may impart tastes or colour to the
collected water. Roof catchments should also be cleaned regularly to remove
dust, leaves and bird droppings so as to maintain the quality of the product
water.
II.
Land
surface catchments:
Rainwater harvesting using ground
or land surface catchment areas is less complex way of collecting rainwater. It
involves improving runoff capacity of the land surface through various
techniques including collection of runoff with drain pipes and storage of
collected water. Compared to rooftop catchment techniques, ground catchment
techniques provide more opportunity for collecting water from a larger surface
area. By retaining the flows (including flood flows) of small creeks and
streams in small storage reservoirs (on surface or underground) created by low
cost (e.g., earthen) dams, this technology can meet water demands during dry
periods. There is a possibility of high rates of water loss due to infiltration
into the ground, and, because of the often marginal quality of the water
collected, this technique is mainly suitable for storing water for agricultural
purposes. Various techniques available for increasing the runoff within ground
catchment areas involve: i) clearing or altering vegetation cover, ii)
increasing the land slope with artificial ground cover, and iii) reducing soil
permeability by the soil compaction and application of chemicals .
III.
Clearing
or altering vegetation cover: Clearing
vegetation from the ground can increase surface runoff but also can induce more
soil erosion. Use of dense vegetation cover such as grass is usually suggested
as it helps to both maintain an high rate of runoff and minimize soil erosion.
IV.
Increasing
slope: Steeper slopes can allow rapid runoff of
rainfall to the collector. However, the rate of runoff has to be controlled to
minimise soil erosion from the catchment field. Use of plastic sheets, asphalt
or tiles along with slope can further increase efficiency by reducing both
evaporative losses and soil erosion. The use of flat sheets of galvanized iron
with timber frames to prevent corrosion was recommended and constructed in the
State of Victoria, Australia, about 65 years ago (Kenyon, 1929; cited in UNEP,
1982).
V.
Soil
compaction by physical means: This
involves smoothing and compacting of soil surface using equipment such as
graders and rollers. To increase the surface runoff and minimize soil erosion
rates, conservation bench terraces are constructed along a slope perpendicular
to runoff flow. The bench terraces are separated by the sloping collectors and
provision is made for distributing the runoff evenly across the field strips as
sheet flow. Excess flows are routed to a lower collector and stored (UNEP,
1982).
VI.
Soil
compaction by chemical treatments: In addition to clearing, shaping and compacting
a catchment area, chemical applications with such soil treatments as sodium can
significantly reduce the soil permeability. Use of aqueous solutions of a
silicone-water repellent is another technique for enhancing soil compaction
technologies. Though soil permeability can be reduced through chemical
treatments, soil compaction can induce greater rates of soil erosion and may be
expensive. Use of sodium-based chemicals may increase the salt content in the
collected water, which may not be suitable both for drinking and irrigation
purposes.
B) Collection Devices
I.
Storage
tanks: Storage
tanks for collecting rainwater harvested using guttering may be either above or
below the ground. Precautions required in the use of storage tanks include provision
of an adequate enclosure to minimise contamination from human, animal or other
environmental contaminants, and a tight cover to prevent algal growth and the
breeding of mosquitos. Open containers are not recommended for collecting water
for drinking purposes. Various types of rainwater storage facilities can be
found in practice. Among them are cylindrical ferrocement tanks and mortar
jars. The ferrocement tank consists of a lightly reinforced concrete base on
which is erected a circular vertical cylinder with a 10 mm steel base. This
cylinder is further wrapped in two layers of light wire mesh to form the frame
of the tank. Mortar jars are large jar shaped vessels constructed from wire
reinforced mortar. The storage capacity needed should be calculated to take
into consideration the length of any dry spells, the amount of rainfall, and
the per capita water consumption rate. In most of the Asian countries, the
winter months are dry, sometimes for weeks on end, and the annual average
rainfall can occur within just a few days. In such circumstances, the storage
capacity should be large enough to cover the demands of two to three weeks. For
example, a three person household should have a minimum capacity of 3 (Persons)
x 90 (l) x 20 (days) = 5 400 l.
II.
Rainfall
water containers: As an
alternative to storage tanks, battery tanks (i.e., interconnected tanks) made
of pottery, ferrocement, or polyethylene may be suitable. The polyethylene
tanks are compact but have a large storage capacity (ca. 1 000 to 2 000 l), are
easy to clean and have many openings which can be fitted with fittings for
connecting pipes. In Asia, jars made of earthen materials or ferrocement tanks
are commonly used. During the 1980s, the use of rainwater catchment
technologies, especially roof catchment systems, expanded rapidly in a number
of regions, including Thailand where more than ten million 2 m3 ferrocement
rainwater jars were built and many tens of thousands of larger ferrocement
tanks were constructed between 1991 and 1993. Early problems with the jar
design were quickly addressed by including a metal cover using readily
available, standard brass fixtures. The immense success of the jar programme
springs from the fact that the technology met a real need, was affordable, and
invited community participation. The programme also captured the imagination
and support of not only the citizens, but also of government at both local and
national levels as well as community based organizations, small-scale
enterprises and donor agencies. The introduction and rapid promotion of Bamboo
reinforced tanks, however, was less successful because the bamboo was attacked
by termites, bacteria and fungus. More than 50 000 tanks were built between
1986 and 1993 (mainly in Thailand and Indonesia) before a number started to
fail, and, by the late 1980s, the bamboo reinforced tank design, which had
promised to provide an excellent low-cost alternative to ferrocement tanks, had
to be abandoned.
C) Conveyance systems .
Conveyance systems are required to
transfer the rainwater collected on the rooftops to the storage tanks. This is
usually accomplished by making connections to one or more down-pipes connected
to the rooftop gutters. When selecting a conveyance system, consideration
should be given to the fact that, when it first starts to rain, dirt and debris
from the rooftop and gutters will be washed into the down-pipe. Thus, the
relatively clean water will only be available some time later in the storm.
There are several possible choices to selectively collect clean water for the
storage tanks. The most common is the down-pipe flap. With this flap it is
possible to direct the first flush of water flow through the down-pipe, while
later rainfall is diverted into a storage tank. When it starts to rain, the
flap is left in the closed position, directing water to the down-pipe, and,
later, opened when relatively clean water can be collected. A great
disadvantage of using this type of conveyance control system is the necessity
to observe the runoff quality and manually operate the flap. An alternative
approach would be to automate the opening of the flap as described below.
A
funnel-shaped insert is integrated into the down-pipe system. Because the upper
edge of the funnel is not in direct contact with the sides of the down-pipe,
and a small gap exists between the down-pipe walls and the funnel, water is
free to flow both around the funnel and through the funnel. When it first
starts to rain, the volume of water passing down the pipe is small, and the
*dirty* water runs down the walls of the pipe, around the funnel and is
discharged to the ground as is normally the case with rainwater guttering.
However, as the rainfall continues, the volume of water increases and *clean*
water fills the down-pipe. At this higher volume, the funnel collects the clean
water and redirects it to a storage tank. The pipes used for the collection of
rainwater, wherever possible, should be made of plastic, PVC or other inert
substance, as the pH of rainwater can be low (acidic) and could cause
corrosion, and mobilization of metals, in metal pipes.
There is always some skepticism regarding Roof
Top Rainwater harvesting since doubts are raised that rainwater may contaminate
groundwater. There is remote possibility of this fear coming true if proper
filter mechanism is not adopted. Secondly all care must be taken to see that
underground sewer drains are not punctured and no leakage is taking place in
close vicinity. Filters are used fro treatment of water to effectively remove
turbidity, color and microorganisms. After first flushing of rainfall, water
should pass through filters. There are different types of filters in practice,
but basic function is to purify water.
Sand Gravel Filter
These
are commonly used filters, constructed by brick masonry and filleted by
pebbles, gravel, and sand as shown in the figure. Each layer should be
separated by wire mesh.
Charcoal filter
Charcoal
filter can be made in-situ or in a drum. Pebbles, gravel, sand and charcoal as
shown in the figure should fill the drum or chamber. Each layer should be
separated by wire mesh. Thin layer of charcoal is used to absorb odor if any.
PVC- Pipe filter
This filter can be made by PVC pipe of 1 to 1.20 m length; Diameter of pipe depends on the area of roof. Six inches dia. pipe is enough for a 1500 Sq. Ft. roof and 8 inches dia. pipe should be used for roofs more then 1500 Sq. Ft. Pipe is divided into three compartments by wire mesh. Each component should be filled with gravel and sand alternatively as shown in the figure. A layer of charcoal could also be inserted between two layers. Both ends of filter should have reduce of required size to connect inlet and outlet. This filter could be placed horizontally or vertically in the system.
Sponge Filter
It
is a simple filter made from PVC drum having a layer of sponge in the middle of
drum. It is the easiest and cheapest form filter, suitable for residential
units