Rainwater Harvesting (Urban)

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).

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).

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).

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).

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).

Debris, dirt, dust and droppings will collect on the roof of a building or other collection area. When the first rains arrive, this unwanted matter would be washed into the tank. This will cause contamination of the water and the quality will be reduced. Many RWH systems therefore incorporate a system for diverting this ‘first flush’ water so that it does not enter the tank. These systems are called first flush devices. Further information on first flush devices is provided in DOLMAN & LUNDQUIST (2008) and PRACTICAL ACTION (2008).

The simpler ideas are based on a manually operated arrangement whereby the inlet pipe is moved away from the tank inlet and then replaced again once the initial first flush has been diverted. This method has obvious drawbacks in that there has to be a person present who will remember to move the pipe. Other, more sophisticated methods provide a much more elegant means of rejecting the first flush water, (described in PRACTICAL ACTION (2008), training material). But practitioners often recommend that very simple, easily maintained systems be used, as these are more likely to be repaired if failure occurs (PRACTICAL ACTION 2008).

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).

There are almost unlimited options for storing rainwater. Common vessels used for very small-scale water storage in developing countries include plastic bowls and buckets, jerry cans, clay or ceramic jars, cement jars, old oil drums, empty food containers, etc. For storing larger quantities of water, the system will usually require a tank above or below ground. These can vary in size from a cubic metre (1000 litres) up to hundreds of cubic metres for large projects (PRACTICAL ACTION 2008). For domestic systems volumes are typically up to a maximum of 20 or 30 cubic metres (PRACTICAL ACTION 2008). Surface tanks are most common for roof collection. Materials for surface tanks include metal, wood, plastic, fibreglass, brick, inter-locking blocks, compressed soil or rubble-stone blocks, ferro-cement and reinforced concrete. The choice of material depends on local availability and affordability. The material and design for the walls of sub-surface tanks or cisterns must be able to resist the soil and soil water pressures from outside when the tank is empty. Tree roots can damage the structure below ground. Careful location of the tank is therefore important (HATUM & WORM 2006).

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).

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.

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.

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).

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).