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United Nations Environment Programme
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Newsletter and Technical Publications

<Sourcebook of Alternative Technologies for Freshwater Augumentation
in Some Countries in Asia>

3. FRESHWATER AUGMENTATION

3.1 General Rainwater Harvesting Technologies (1)

Rainwater harvesting, in its broadest sense, can be defined as the collection of runoff for human use. The collection processes involve various techniques such as the collection of water from rooftops and the land surface, as well as within water courses. These techniques are widely used in Asia both for meeting drinking water supply needs and for irrigation purposes.

Technical Description

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.

  • Catchment Areas

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. Figure 1 shows a schematic of a rooftop collection system.

Figure 1

Figure 1. Rooftop Catchment System.

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 (Figure 2). 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.

Figure 2

Figure 2. Ground Catchment System.

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.

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).

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).

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.

Figure 3

Figure 3. Rock Catchment System.

Rock catchments systems: The presence of massive rock outcrops provides suitable catchment surfaces for freshwater augmentation (Figure 3). In these systems, runoff is channelled along stone and cement gutters, constructed on the rock surface, to reservoirs contained by concrete dams. The collected water then can be transported through a gravity fed pipe network to household standpipes.

  • Collection Devices

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.

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.

The design considerations vary according to the type of tank and various other factors have to be considered while designing the rainwater tanks (Latham and Gould, 1986; Gould, 1992) which are:

- A solid, secure cover to keep out insects, dirt and sunlight which will act to prevent the growth of algae inside the tank. - A coarse inlet filter for excluding coarse debris, dirt, leaves, and other solid materials.
- An overflow pipe. - A manhole, sump and drain for cleaning.
- An extraction system that doesn't contaminate the water (e.g., a tap or pump).
- A lock on the tap. - A soakaway to prevent spilled water from forming puddles near the tank.
- A maximum height of 2 m to limit the water pressure acting on the container to minimize burst tanks.
- A device to indicate the level of water in the tank.
- A sediment trap, tipping bucket or other fouled flush mechanism.
- A second, clear water storage tank if the rainwater has to be subjected to some form of water treatment, such as desalination using a density stratification process in the first tank.

  • 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.

Figure 4

Figure 4. Typical Conveyance System

A simple and effective method of diverting rainwater without the need for supervision is depicted in Figure 4. 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.

Figure 5

Figure 5. Typical Distribution System

In order to safely fill a rainwater storage tank, it is necessary to make sure that excess water can overflow, and that blockages in the pipes or dirt in the water do not cause damage or contamination of the water supply. The design of the funnel system, with the drain-pipe being larger than the rainwater tank feed-pipe, helps to ensure that the water supply is protected by allowing excess water to bypass the storage tank. A modification of this design is shown in Figure 5, which illustrates a simple overflow/bypass system. In this system, it also is possible to fill the tank from a municipal drinking water source, so that even during a prolonged drought the tank can be kept full. Care should be taken, however, to ensure that rainwater does not enter the drinking water distribution system.

Calculating the Amount Available: When using rainwater for water supply purposes, it is important to recognize the fact that the supply is not constant throughout the year and plan an adequately-sized storage system to provide water during dry periods. A knowledge of the rainfall quantity and seasonality, the area of the collection area and volume of the storage container, and quantity and period of use during which water is required for water supply purposes is critical. For example, in Tokyo, the average annual rainfall is 1 800 mm, and, assuming that the effective collection area of a house is equal to its roof area, the typical collection area is about 100 m2. Thus, the average annual volume of rainwater falling on the roof may be calculated as the product of the collection area, 100 m2, and rainfall amount, 1 800 mm, or 180 m3. However, in practice, this volume can never be achieved since a portion of the rainwater evaporates from the rooftop and a portion, including the first flush, may be lost to the drainage system. Additional rainwater volume may be lost as overflow from the storage container if the storage tank is of insufficient volume to contain the entire volume of runoff. Thus, the net usable or available amount of rainwater from a tiled roof would be approximately 70% to 80% of the gross volume of rainfall, or about 130 m3 to 140 m3 if the water container is big enough to hold that quantity of rainwater available. Such a volume would be sufficient to save a significant amount of freshwater and money.

Estimation of the Required Volume of Water: The individual daily rate of water consumption per person tends to be variable and may be difficult to calculate. Statistics vary from 130 l to 175 l per person per day in developing countries. Of this volume, at least half is used for purposes for which water of a lesser quality would suffice. Indicative volumes are shown in Table 9, which summarizes the volumes of water used for household purposes, and indicates possibilities for the use of rainwater to supplement a municipal supply. Table 9 clearly shows that approximately 80 to 95 l of the average daily volume of water consumed per person could be provided by the use of rainwater.

TABLE 9. Typical Per Capita Volume of Daily Water Consumption

Table 9

Indicative Rate of Water Use

1 flushing of toilet: 9 l
1 washing machine load: 60 l
1 bathtub full: 140 l
1 shower: 40 l

 

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