Rain Water Harvesting Rainwater Harvesting Rainwater harvesting in broad sense can be defined as the collection of runoff for human consumption. There are various techniques for collection process such as the collection from the rooftops and the land surface as well as within watercourses. The water thus collected is often used for different purposes including drinking, cooking, washing, bathing, watering cattle and agriculture. In coastal belt and hilly region region of Bangladesh Bangladesh,, there is scarcity of suitable water sources. The coastal areas suffer from high salinity in surface and ground waters and the hilly areas suffer from absence of surface and groundwater sources for the development of dependable water supply systems. The average annual rainfall in the coastal and hilly hilly regions regions is more more than 3mm, 3mm, against an average average rainfall rainfall of about !! mm in Bangladesh. The collection and storage of rainwater may be an alternate option of water supply in these areas. Rainwater harvesting is also a potential water supply option in the acute arsenic affected areas of Bangladesh. There are two main constraints in development of a completely rainwater based water supply system"
#vailability of suitable catchment area The need for larger storage tank
# large catchment for rainwater collection is needed if the designed water supply is based on rainwater. #gain larger storage reservoir is required to encounter unequal distribution of rainfall throughout the year. The advantages and disadvantages of rainwater harvesting over the other water supply scheme are given in Table $. Table 1: Advantages and Disadvantages of Rainwater Harvesting
Advantages The quality of rainwater is comparatively good% The system is independent and therefore suitable for scattered settlements% &ocal materials and craftsmanship can be used in construction of rainwater harvesting system% 'o energy energy cost cost is needed needed to run the system% system% (asy maintenance by the owner ) user% The system can be located very close to the consumption points.
Disadvantages The initial cost may prevent a family to install a rainwater r ainwater harvesting system% The water availability is limited by the rainfall intensity and available roof area% *ineral+free *ineral+free rainwater has a flat taste, which may not be liked by many% *ineral+free water may cause nutrition deficiencies in people who are already on mineral deficient diet% The poorer section of people may not have roofs suitable for rainwater harvesting.
Rainwater Availability Rainwater is available in adequate quantities in Bangladesh. rom the spatial distribution of rainfall in Bangladesh it is observed that relatively higher rainfall occurs in the eastern part of the country and the highest rainfall occurs in the north+eastern region and eastern part of the coastal area. The lowest rainfall, less than $-mm, occurs in the western part of Bangladesh. In the coastal and hilly areas, with a greater fresh water source problem, rainfall is higher which is favourable for rainwater harvesting.
rom the last ten+year ten+year mean rainfall intensity record, record, it appears that the average rainfall in the country during $/0+1 varied from $- to !/mm, i.e. $.- to !./ m 3 of rainwater was available per m ! of catchment area each year for development of a rainwater based water supply system. 2owever, there are some losses in the collection system. The available rainwater can be estimated by the equation $" 4 5 I # ................. .......... ...........6$7 ....6$7 $
8here, is the total quantity of rainwater available in m 3)year, 5 is the coefficient of available runoff, I is the rainfall intensity in m)year and # is the catchment area in m !. Rainwater Catchment
Rooftop catchment The catchment area for rainwater collection is usually the roof, which is connected with a gutter system to lead rainwater to a storage tank. igure $ shows a schematic diagram of the rooftop collection system.
>utter ?torage tank
Figure 1: Roofto Catchment !ystem
Rainwater can be collected from any types of roof but concrete, tiles and metal roofs give cleanest water. Roof made of asbestos or painted with lead based paints should be avoided. The 5.I. sheet roofs commonly used in Bangladesh perform well as catchment areas. # thatched roof can also be used as catchment area by covering it with polyethylene but it requires good skills to guide water to the storage tank. In coastal areas, cloths fi9ed at four corners with a pitcher underneath are used during rainfall for rainwater collection. But the poorer section of the people in our country have smaller si:e thatched roof or no roof at all, which can be used as catchment for rainwater collection. The minimum catchment area #, required for the collection of rainwater for ' number of people supplied with q litres per capita per day 6lpcd7 of water can be derived from equation !" # 4 .31- q ')65 I7 .....................6!7 #bout !-; of the rainwater may be assumed to be lost by evaporation and by washing the catchment area. The catchment area is usually washed using first rain, which produces inferior quality rainwater. or an average annual rainfall of !.< m)yr, and a coefficient of runoff of .0-, equation ! can be written in the following form" # 4 .!3 q ' .....................637 !torage tan": The unequal distribution of rainfall over the year requires storage of rainwater during the rainy season for use in the dry season. The minimum volume of the storage rainwater tank =, required for can be computed by the equation <"
= 4 .31- f q ' .....................6<7 8here f is the fraction of total available rainwater required to be stored for consumption at a constant rate throughout the year. #uality uality of rainwater is good, but it does not mean that rainwater is completely free from contamination. 8ind blown dirt and bird droppings contribute some pollution. The rainwater lacks mineral salts including fluorides and calcium salts whose presence in water supply is considered essential in appropriate proportions. The !
mineral salts in natural ground and surface waters sometimes impart pleasing taste to water. If the storage tank is completely covered and the organic debris are prevented from entering into the tank by means of suitable trainer or filter, any bacteria or parasites carried with the flowing rainwater will tend to die off. Thus rainwater drawn from clean tank several days after the first rainfall will be of better bacteriological quality than fresh rainwater. Rainwater collected from roof after long dry period may carry noticeable amounts of debris arising from accumulated dust, leaves and bird or animal droppings on the roof and gutter. It is recommended that water running off the roof during first $+! minutes of rainfall should be discarded. 5leaning of the roof and gutter at the beginning of rainy season and their regular maintenance, are very important to ensure better quality of rainwater. The storage tank requires cleaning at least once in a year. (9.$" 5alculate the rainwater available for a family having a roof area of !m ! in the central region of Bangladesh, where rainfall intensity is !. m per year. #ssume a runoff coefficient of .0-. ?oln." uantity of rainwater available ,
4 5 I#
@ here, 5 4 .0-% I 4 !. m)yr% # 4 !m !A
4 .0- 9 !. 9 ! 4 3m 3)yr.
#ns.
(9.!" The average rainfall intensity in Bangladesh is !.< m)yr and the runoff coefficient is .0. 5alculate the minimum catchment area required for a family of 0 persons to be supplied with $- lpcd of water. ?oln.." The catchment area is,
# 4 .31- q ')65 I7
@here, 5 4 .0% I 4 !.< m)yr% ' 4 0% q 4 $-lpcdA
# 4 .31- 9 $- 9 0)6.0 9 !.<7 4 !!./ m 3
#ns.
(9.3" 5alculate the minimum capacity of the storage tank required for a family of / persons to be supplied with $ lpcd of rainwater. The yearly rainfall intensity is !.- m and the rainfall duration is such that at least 3-; of the rainwater must be stored for uninterrupted water supply throughout the year. #lso calculate the minimum catchment area required when the coefficient of runoff is .0. ?oln." ?torage volume required
= 4 .31- f q '
@here, f 4 .3-% ' 4 /% q 4 $ lpcdA
= 4 .31- 9 .3- 9 $ 9 / 4 $.!! m 3 5atchment area, # 4 .31- q ')65 I7
#ns.
@here, 5 4 .0% I 4 !.- m)yr% ' 4 /% q 4 $ lpcdA
# 4 .31- 9 $ 9 / 9 )6.0 9 !.-7 4 !.!)$.0- 4 $1.1 m !
#ns.
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