Introduction
Rainwater harvesting (RWH) is a simple technique that offers many benefits. It can be done very low-tech, doesn’t
cost much and is applicable at small-scale with a minimum of specific
expertise or knowledge; or in more sophisticated systems at large-scale
(e.g. a whole housing area).The most common technique in urban areas
(besides storm water management) is rooftop rainwater harvesting: rainwater is collected on the roof and transported with gutters to a storage reservoir, where it provides water at the point of consumption or is used for groundwater recharge (see also surface and subsurface artificial groundwater recharge). Collected rainwater can supplement other water sources when they become scarce or are of low quality like brackish groundwater or polluted surface water in the rainy season. It also provides a good alternative and replacement in times of drought or when the water table
drops and wells go dry. The technology is flexible and adaptable to a
very wide variety of conditions. It is used in the richest and the
poorest societies, as well as in the wettest and the driest regions on
our planet (HATUM & WORM 2006).
Basic Design Principles
Rooftop rainwater harvesting system.Source: CPREEC (Editor) (n.y.)
Each rainwater harvesting system consists of at least the following components (INFONET-BIOVISION 2010):
- Rainfall
- A catchment area or roof surface to collect rainwater.
- Delivery systems (gutters) to transport the water from the roof or collection surface to the storage reservoir.
- Storage reservoirs or tanks to store the water until it is used.
- An extraction device (depending on the location of the tank - may be a tap, rope and bucket, or a pump (HATUM & WORM 2006); or a infiltration device in the case the collected water is used for well or groundwater recharge (see also surface or subsurface artificial groundwater recharge)
Additionally there are a wide variety of systems available for treating water either before, during and/or after storage (e.g. biosand filter, SODIS, chlorination; or in general HWTS).
Process diagram of a drinking water RWH system.Source: THOMAS & MARTINSON (2007)
Illustration of water flow scheme of a RTRWH system. Basic components: roof, gutters, first flush device (first rain separator), rain barrel with filter and tap and recharge well. Source: RAINWATERCLUB (Editor) (n.y.)
Rainfall
Table 1: Average Annual Rainfall in different regions. Source: HATUM & WORM (2006)
The
rainfall pattern over the year plays a key role in determining whether
RWH can compete with other water supply systems. Tropical climates with
short (one to four month) dry seasons and multiple high-intensity
rainstorms provide the most suitable conditions for water harvesting. In
addition, rainwater harvesting may also be valuable in wet tropical climates (e.g. Bangladesh), where the water quality of surface water
may vary greatly throughout the year. As a general rule, rainfall
should be over 50 mm/month for at least half a year or 300 mm/year
(unless other sources are extremely scarce) to make RWH environmentally
feasible (HATUM & WORM 2006). In the following table, some examples
are given for annual rainfall in different regions (HATUM & WORM
2006).
Catchment Area
To be ‘suitable’ the roof should be made of some hard material that does not absorb the rain or pollute the run-off.
Thus, tiles, metal sheets and most plastics are suitable, while grass
and palm-leaf roofs are generally not suitable (THOMAS & MARTINSON
2007).
Delivery System
A variety of guttering types. Source: HATUM & WORM (2006)
The
delivery system from rural rooftop catchment usually consists of
gutters hanging from the sides of the roof sloping towards a down pipe
and tank. Guttering is used to transport rainwater from the roof to the
storage vessel. Guttering comes in a wide variety of shapes and forms,
ranging from the factory made PVC type similar as the pipes used in water distribution systems)
to home made guttering using bamboo or folded metal sheet. Guttering is
usually fixed to the building just below the roof and catches the water
as it falls from the roof (HATUM & WORM 2006).
Example of a first flush device
(white, vertical PVC pipe, left). Illustration of the working principle
of the device (right). Source: DOLMAN & LUNDQUIST (2008)
Left:
this filter (developed by WISY) fits into a vertical down pipe and acts
as both filter and first-flush system. Right: filter cartridge of Pop-up-filter (developed by KSCST) acts as a first-flush separator. Source: CSE (n.y.), KSCST (n.y.)
A coarse filter, preferably made of nylon or a fine mesh, can also be used to remove dirt and debris before the water enters the tank (HATUM & WORM 2006).
Storage Tanks
RTRWH in Urban Areas using a Plastic Tank. Source: VISHWANATH (n.y.)
There are a number of different methods used for sizing the tank. These methods vary in complexity and sophistication. PRACTICAL ACTION (2008) gives an overview over three different methods. Some are readily carried out by relatively inexperienced, first-time practitioners, while others require computer software and trained engineers who understand how to use this software. The storage requirement will be determined by a number of interrelated factors, which include: local rainfall data and weather patterns, size of roof, runoff coefficient (depending on roof material and slope) and user numbers and consumption rates.
In reality the cost of the tank materials will often govern the choice of tank size. In other cases, such as large RWH programmes, standard sizes of tank are used regardless of consumption patterns, roof size or number of individual users (although the tank size will, hopefully, be based on local averages) (PRACTICAL ACTION 2008).
Infiltration
Collected water can also be used for replenishing a well or the aquifer (see also surface or subsurface artificial groundwater recharge). In a case study of SHRESTHA (2010), excess rainwater during the rainy season is used to recharge a dug well, as well as the groundwater. In this case recharging the groundwater even improved the water quality in the dug well.User Behaviour
Depending on the user behaviour the storage and treatment (water quality) infrastructure is probably different. In some parts of the world, RWH is only used to collect enough water during a storm to save a trip or two to the main water source (open well or pump). In this case only a small storage container is required. In arid areas, however, people strive to create sufficient catchment surface area and storage capacity to provide enough water to meet all the needs of the users (HATUM & WORM 2006).Four types of user regimes can be discerned:
Occasional - Water is stored for only a few days in a small container. This is suitable when there is a uniform rainfall pattern and very few days without rain and there is a reliable alternative water source nearby.
Intermittent - There is one long rainy season when all water demands are met by rainwater, however, during the dry season water is collected from non-rainwater sources. RWH can then be used to bridge the dry period with the stored water when other sources are dry.
Partial - Rainwater is used throughout the year but the ‘harvest’ is not sufficient for all domestic demands. For instance, rainwater is used for drinking and cooking, while for other domestic uses (e.g. bathing and laundry) water from other sources is used.
Full - Only rainwater is used throughout the year for all domestic purposes. In such cases, there is usually no alternative water source other than rainwater, and the available water should be well managed, with enough storage capacity to bridge the dry period.
Cost Considerations
Run-off
from a roof can be directed with little more than a split pipe or piece
of bamboo into an old oil drum (provided that it is clean) placed near
the roof. The water storage tank or reservoir usually represents the biggest capital investment element of small-scale rooftop urban rainwater harvesting
system and therefore require careful design to provide optimal storage
capacity while keeping the cost as low as possible. Installing a water
harvesting system at household level can cost anywhere from USD 100 up
to USD 1000. It is difficult to make an exact estimate of cost because
it varies widely depending on the availability of existing structures,
like rooftop surface, pipes and tanks and other materials that can be
modified for a water harvesting structure. Expensive systems with large
tanks deliver more water than cheaper systems with small tanks (THOMAS
& MARTINSON 2007).
Health Aspects
Rainwater itself is
of excellent quality, only surpassed by distilled water – it has very
little contamination, even in urban or industrial areas, so it is clear,
soft and tastes good. Contaminants can however be introduced into the
system after the water has fallen onto a surface (THOMAS & MARTINSON
2007).
Firstly, there is the issue of bacteriological water quality. Rainwater can become contaminated by pathogenic bacteria (e.g. form animal or human faeces) entering the tank from the catchment area.
It is advised that the catchment surface always be kept very clean.
Rainwater tanks should be designed to protect the water from
contamination by leaves, dust, insects, vermin, and other industrial or
agricultural pollutants. Tanks should be sited away from trees, with
good fitting lids and kept in good condition. Incoming water should be
filtered or screened, or allowed to settle to take out foreign matter.
Water, which is relatively clean on entry to the tank, will usually
improve in quality if allowed to sit for some time inside the tank. Bacteria
entering the tank will die off rapidly if the water is relatively
clean. Algae will grow inside a tank if sufficient sunlight is available
for photosynthesis. Keeping a tank dark and sited in a shady spot will prevent algae growth and also keep the water cool. As mentioned above, first flush devices
help to prevent the dirty ‘first flush’ water from entering the storage
tank. The area surrounding a RWH should be kept in good sanitary
condition, fenced off to prevent animals fouling
the area or children playing around the tank. Any pools of water
gathering around the tank should be drained and filled (PRACTICAL ACTION
2008).Secondly, there is a need to prevent insect vectors from breeding inside the tank. In areas where malaria is present, providing water tanks without any care for preventing insect breeding can cause more problems than it solves. All tanks should be sealed to prevent insects from entering. Mosquito proof screens should be fitted to all openings (PRACTICAL ACTION 2008).
| Working Principle | Rainwater collected on the rooftop is transported with gutters to a storage reservoir. There is a wide variety of systems available for RWH systems as well as for treating water before, during and after storage, which helps to prevent water from contamination. |
| Capacity/Adequacy | The supply is limited by the amount of rainfall and the size of the catchment area and storage reservoir (HATUM & WORM 2006). Storage reservoirs can vary in size from a cubic metre up to hundreds of cubic metres for large projects, but typically up to a maximum of 20 or 30 cubic metres for a domestic system (PRACTICAL ACTION 2008). |
| Performance | Rainwater is generally better quality than other available or traditional water sources (groundwater may be unusable due to fluoride, salinity or arsenic; HATUM & WORM 2006). |
| Costs | 100 to 1000 USD depending on material, storage size and technology. |
| Self-help Compatibility | Depending on the scale, construction of RWH systems can be very simple and local people can easily be trained to build these themselves. This reduces costs and encourages more participation, ownership and sustainability at community level (HATUM & WORM 2006). |
| O&M | Proper operation and regular maintenance is a very important factor that is often neglected. Regular inspection, cleaning, and occasional repairs are essential for the success of a system (HATUM & WORM 2006). |
| Reliability | If well constructed and maintained drinking water in good quality is available. |
| Main strength | It provides water, which otherwise would have been lost, at the point of consumption (HATUM & WORM 2006). |
| Main weakness | Limited supply: The supply is limited by the amount of rainfall and the size of the catchment area and storage reservoir (HATUM & WORM 2006). |
Applicability
RTRWH in urban areas can be implemented everywhere from a
single household to community level (SHRESTHA 2010): the technology is
flexible and adaptable to a very wide variety of conditions. It is used
in the richest and the poorest societies, as well as in the wettest and
the driest regions on our planet. Collected rainwater can supplement other water sources when they become scarce or are of low quality like brackish groundwater or polluted surface water in the rainy season. It also provides a good alternative and replacement in times of drought or when the water table drops and wells go dry. (HATUM & WORM 2006).Advantages
- Local people can easily be trained to build RWH systems themselves. This reduces costs and encourages more participation, ownership and sustainability at community level (HATUM & WORM 2006)
- Rainwater is better than other available or traditional sources (groundwater may be unusable due to fluoride, salinity or arsenic) (HATUM & WORM 2006)
- Costs for buying water and time to extract from the city water supply can be saved (SHRESTHA 2010)
- It provides water at the point of consumption (HATUM & WORM 2006)
- Not affected by local geology or topography (HATUM & WORM 2006)
- Almost all roofing material is acceptable for collecting water for household purposes (HATUM & WORM 2006)
- Rooftop RWH reduces the amount of rainwater going into sewers, drains and may reduce flooding and clogging of water channels and uptakes (WATERAID 2008)
Disadvantages
- Limited by the amount of rainfall and the size of the catchment area and storage reservoir (HATUM & WORM 2006)
- Supply is sensitive to droughts: Occurrence of long dry spells and droughts can cause water supply problems (HATUM & WORM 2006)
- The cost of rainwater catchment systems is almost fully incurred during initial construction (HATUM & WORM 2006)
- Proper operation and regular maintenance is a very important factor that is often neglected (HATUM & WORM 2006)
- Rainwater quality may be affected by air pollution, animal or bird droppings, insects, dirt and organic matter (HATUM & WORM 2006)
References
WAN (Editor) (2008): Nepal’s Experiences in Community-Based Water Resource Management. (= Fieldwork paper). Water Aid Nepal (WAN) and End Water Poverty. URL [Accessed: 30.03.2010].
Thanks
http://www.sswm.info/category/implementation-tools/water-sources/hardware/precipitation-harvesting/rainwater-harvesting-u
Thanks
http://www.sswm.info/category/implementation-tools/water-sources/hardware/precipitation-harvesting/rainwater-harvesting-u
