1.0 ROOF WATER HARVESTING WITH ABOVE GROUND TANK

TRAINING NOTES BY PROF. BANCY MATI 1.1 What is a roof catchment? A roof becomes a catchment when it is used for harvesting rainwater. Then it can be called a “Roof catchment”. Roofs are the most common types of catchment used for harvesting rainfall. Rainwater harvesting from impervious roof made of corrugated iron sheets corrugated plastic and clay tiles is a popular method for providing portable water directly from rainfall. The system provides water at home, is affordable, easy to practice regardless of physical or climatic conditions and can be designed to suit different conditions (available finances, roof area, family size, rainfall or roof area). In most cases, roof catchment systems provide water that can be used for domestic purposes. However, roof runoff harvesting is also used for agricultural purposes including micro-irrigation of kitchen gardens, watering livestock, and for bee keeping projects. The tank size is dependent on the rainfall regime, the water demand and roof area available. Institutions such as schools, offices, churches and other such buildings have large roofs which can be used to harvest larger quantities of water (Figure 1.1). They however need good management to regulate water abstraction rates as the water will be used by many people. Wherever possible, roofs of individual households are preferable to communal systems.

Figure 1.1 (a) Rainwater harvesting tank for household level (photos by Bancy Mati)

(b) RWH tanks for communal use

Roof water harvesting is particularly attractive where the main alternatives are surface water sources are unavailable and groundwater is either difficult to secure or has been rendered unusable by fluoride, salinity or arsenic. Also where management of shared point sources has proved unsuitable and delivery of water is a particular burden on household members or where householders are prepared to invest in water convenience.

1.2 Advantages and limitations of roof water harvesting systems

Advantages Roof catchments have some advantages over ground and rock catchments.

When buildings with impervious roofs are already in place, the catchment area is effectively available free of charge.

They have high runoff coefficients (normally 0.7-0.9), since they are built to fully protect houses from rains;

They are relatively clean and thus provide safer water, Surface tanks are relatively smaller, they are affordable for household water harvesting, They normally supply water at the point of consumption, while the water from other

catchments needs to be transported or piped. Another benefit of surface tanks (compared to sub-surface tanks) is that water can be

extracted easily through a tap just at the base of the tank. Placing it on a stand or base elevates the tank, so that the water can be piped by gravity to where it is required. In addition, construction of such water tanks makes use of locally available materials and local artisans, thus creating employment.

The storage provided by a tank provides households with security against short-term failure of alternative water sources.

Since the structure is family owned, maintenance is usually very good and no water conflicts occur.

Certain tank types such as plastic or canvas are portable, and can be transported to remote areas where they are fixed at site.

Limitations Despite its advantages, domestic roof water harvesting remains a niche technology and, when considered at all, is usually only considered when all other options have been eliminated. This is because roof water harvesting has a number of limitations. They include:

Roof water harvesting may be inadequate as a stand-alone water supply solution unless in the most water-stressed situations.

The tank capacity necessary to bridge a long dry season would be large and this can be prohibitively expensive.

Surface tanks are relatively expensive when compared with subsurface storage tanks; and They require space in the home compound, and this may be a problem in urban areas Water quality still requires some treatment especially to remove biological pathogens Not suited to areas having air pollution e.g. cities.

1.3 Types of surface tanks Roof water harvesting systems can be typified according to shape, size, construction material, function and cost. Tanks can be made rectangular, spherical, hemi-spherical, cylindrical or shapeless as in the case of collapsible canvass or polythene tanks. In terms of construction material, there are many types including ferrocement, metal, plastic, bricks, interlocking blocks, compressed-soil blocks and concrete. These are described here as follows

1.3.1 Ferrocement jars and tanks The term, ferrocement, means “reinforced mortar” because crushed stones and hardcore stones are not used in ferrocement. The main advantage of wire reinforced mortar (ferrocement) over conventional reinforced concrete tanks is its ability to resist shrinkage cracking during curing (due to the woven reinforcement chicken mesh), and its resistance to severe cracking under tensile load. It also needs only one set of forms for construction when the mortar is applied by hand to one side; pouring concrete into two closely spaced shutters in the conventional method is a highly skilled and difficult task. Ferrocement tanks use much less material (figure 1.2) than the conventional concrete and stone masonry tanks, and when combined with locally available material, they can be quite cost-effective.

Figure 1.2 (a) Ferrocement tank under construction (photo by Bancy Mati)

(b) Newly constructed ferrocement tank (photo courtesy of ERHA, 2007)

1.3.2 Brick and block tanks Surface water tanks can be constructed using burnt bricks or blocks made out of compressed soil, concrete, quarry stone or rubble stone or other locally available materials. There are a range designs and construction methods developed for brick and block tanks. The tanks are built of bricks or blocks reinforced with barbed wire, wrapped tightly as spiral around the exterior tank walls. There is heavier reinforcement at the bottom of the tank as the deeper part of the tank is subjected to higher water pressure. These tanks, like all other water tanks, are normally cylindrical in shape. They should be reinforced adequately and plastered well. These are necessary precautions; otherwise the tanks are liable to cracks that are difficult to repair.

Figure 1.3 Water tank being built of (a) burnt bricks, and (b) compressed soil block (source: Danida, 2007)

1.3.3 Metal tanks Galvanised metals are widely used for water tanks with some success. Their durability depends upon the quality and thickness of the metal, protection provided (e.g. protective paint), and the level of exposure to saline or acidic water or atmospheric moisture. Also, old oil drums can be converted into RWH tanks albeit they may not have taps, and the water has to be scooped out. The main limitation with metal tanks is that they are prone to corrosion and thus are not recommended for hot, humid areas such as near the coast.

1.3.5 Plastic tanks Moulded plastic tanks are commercially available and since they are designed for drinking water collection, they offer a quick solution to roof RWH. Plastic tanks are portable hence good for households that keep moving. Another advantage is that they are light and flexible, and can thus be easily transported. They are also durable than and relatively reliable, as they do not corrode. Plastic tanks up to 10 m3 are common, but tanks up to 25 m3 can be produced and transported. However, plastic tanks are relatively expensive.

1.3.6 Ready-made tanks Ready-made tanks are factory produced and sold to users commercially. The more common types of ready-made tanks are made of plastic, metal and ferrocement (figure 1.4). The advantages of ready-made tanks compared to constructed ones include:

They can quickly be erected at site within days; Since ready-made tanks are centrally produced in large quantities, have economies of

scale and quality control, and thus can make them more durable and cost effective; and The tanks do not require skilled labor, nor much of the construction material and

equipment at the site to be installed. The main disadvantage with ready-made tanks includes the fact that they are more expensive and the cost of their transport can be high.

Fire 1.4 (a) Portable plastic tank (a) Portable collapsible canvas tank (photos by Bancy Mati)

1.3.7 Other tank types There are tanks made of fibreglass, but are expensive when compared to tanks built on site. There are also tanks made of wood, bamboo or sisal fibre. These used to be promoted as appropriate technology but most of them failed due as the fibres rotted with time. They are

mentioned here not to promote them, but to discourage their replication as they are generally not durable.

1.4 Design of roof water harvesting systems The design of roof water harvesting systems considers all the components as well as the water demand, and capacity of the rainfall and roof area to provide this water. The main components of a roof RWH include the following: 1.4.1 Components of roof water harvesting systems

Roof water harvesting systems comprise six basic components irrespective of the size of the system (figure 1.5). These are:

(i) Catchment area/roof: The surface upon which the rain falls; the roof has to be appropriately sloped preferably towards the direction of storage and recharge.

(ii) Gutters and downpipes: The transport channels from catchment surface to storage; these have to be designed depending on site, rainfall characteristics and roof characteristics.

(iii) Leaf screens and roof washers: The systems that remove contaminants and debris; a first rain separator has to be put in place to divert and manage the first 2.5 mm of rain.

(iv) Storage tank, cisterns or sumps, where collected rain-water is safely stored or recharging the ground water through open wells, bore wells or percolation pits,

(v) Water abstraction system by gravity such as tap (vi) Water treatment: Filters to remove solids and organic material and equipment, and

additives to settle, filter, and disinfect. (vii) Other technical auxiliary structures such as tank cover, overflow pipe, soak away pit for

the safe disposal of waste water and a manhole that allows entry into the tank for cleaning and maintenance or repair purposes.

Figure 1.5 Components of a roof water harvesting system (adapted from DTU 1998)

Roof surface

Galvanized all make good roof catchment surfaces (Figure xxx). Roof water harvesting systems can be constructed from a wide range of materials metal, wood, plastic, fiberglass, bricks, interlocking blocks, compressed-soil, rubble stone blocks, ferrocement and concrete. Flat cement or felt-covered roofs can also be used provided they are clean. Corrugated iron is now widely used as a roofing material in much of Africa. Thatched roofs can make good catchments when certain palms are tightly thatched. Most palms and all grasses, however, do not produce thatch suitable for high quality rainwater collection, since they discolor the water and make it less palatable and attractive for domestic purposes. The materials used to make of the roof should be non-toxic in nature. Roof surfaces should be smooth, hard and dense since they are easier to clean and are less likely to be damaged and release materials/ fibres into the water. Roof painting is not advisable since most paints contain toxic substances and may peel off. No overhanging trees should be left near the roof. The nesting of birds on the roof should be prevented. All gutter ends should be fitted with a wire mesh screen to keep out leaves, dust and dirt. A first-flush rainfall capacity, such as detachable down pipe section, should be installed. A hygienic soak away channel should be built at water outlets and a screened overflow pipe should be provided. The storage tank should have a tight fitting roof that excludes light, a manhole cover and a flushing pipe at the base of the tank (for standing tanks). There should be a reliable sanitary extraction device such as a gravity tap or a hand pump to avoid contamination of the water in the tank. There should be no possibility of contaminated wastewater flowing into the tank (especially for tanks installed at ground level). Water from other sources, unless it is reliable source, should not be emptied into the tank through pipe connections or the manhole cover.

1.4.2 Design volume of a water tank

Rooftop water harvesting systems can provide good quality potable water if the design features outlined below are taken into account. Water Availability

Since the available roof area is usually limited, the system is used to meet water requirements during the dry season, which varies with the weather patterns of an area. Rainfall data is used to design a volume of storage equivalent to a given return period. The more the seasonal rainfall, the larger the tank volume. Estimating the Size of the Required Systems

In actual field conditions, the size of the collector and storage system is dictated by the available roof area and the rainfall. The water harvested from the available roof area, therefore, is more or less fixed and has to be judiciously used. In rare cases we have the real option of building enough roof area to meet the predetermined per capita requirement of a given family or community.

The size of the catchment area and tank should be enough to supply sufficient water for the users during the dry period. It is important to have rainfall data and to know the average length of the dry season and the average water use. Thus, assuming a full tank at the beginning of the dry season (end of rainy season), the volume of the tank can be calculated by the following formula:

V = (t × n × q) + et

Where, V = Volume of tank (litres) t = Length of the dry season (days) n = Number of people using the tank q = Consumption per capita per day (litres) et = Evaporation loss during the dry period

Since evaporation from a closed storage tank is negligible, the evaporation loss (ET) can be ignored (=zero). Experience shows that with the water storage tanks next to their houses, people use between 20 to 40 litres of water per person per day (lpd). However, this may rise in time as people relax their water use habits because of easy access. This contrasts with a maximum of l0 lpd consumption levels under similar environments with people fetching water from distant sources. Together with the community/ family, a decision must be taken on how the water will be used or what affordable service level can be provided.

1.4.3 Selection of appropriate storage tank designs

Surface tanks The rainwater storage tank can be constructed above the ground – called a surface tank or it can be underground or semi-underground. In this chapter, only above ground tanks are described. A good RWH tank should be watertight, durable, affordable and not contaminate the water in any way. Surface tanks are used in elevated catchment surfaces, which are by and large roofs.

Materials used for tank construction

Surface tanks can be constructed from metal, wood, plastic, fiberglass, brick, interlocking blocks, compressed-soil or rubble stone blocks, ferrocement, or concrete. The selection of the material depends on local situations For example; metals are not suited to saline areas, plastic to strong light areas, and wood to termite areas. The comparative costs of locally available materials should be a key factor in choosing the most appropriate tank.

Table 1: Recommended concrete mix, thickness and reinforcement for RWH tanks Tank component

Reinforcement Mortar/concrete Finishing coat/plastering

Ratio Thick. Ratio Thick

Foundation Weld-mesh Steel bars, 6-12 mm

diameter

1:3:4 10 cm 1:3 + Nil 2 cm

Wall of Ferro cement

Weld mesh gauge 8 Chicken mesh Galvanized wire 3-4

mm diameter

1:3:3 5 cm Nil 1 cm

Wall of burnt bricks

Chicken mesh 1:3:1/3 lime for mortar joints

2 cm (mortar joints)

1:3 + Nil 4 cm

Wall of rubble stone

Chicken mesh 1:3 for mortar joints

2-3 cm mortar joints

1:3 + Nil 5 cm

Wall of reinforced concrete

Steel bars 8 &10 mm diameter

1:2:3 7 cm 1:3 + Nil 3 cm

N.B: Nil is cement slurry made from mixing cement and water, which can be used as a substitute for waterproof cement. (source, Nega, H. 2006)

Tank size Generally, surface tanks may vary in size from 1 m3 to more than 40 m3 for households and up to 200 m3 or more for schools and such institutions. The selection of an actual size depends on various factors such as the amount and distribution of rainfall over the year, available roof area, household demand, and the presence of other supply sources.

Runoff coefficient of Roofs

As with any catchment, a roof has a runoff coefficient which depends on the material used and its condition. Roof catchments have runoff coefficients exceeding 90%. This is because water losses from pitched metal roofs, concrete or asphalt roofs average less than 10% of total rainfall. However, a smooth, clean, and more impervious roofing material contributes to better water quality and greater quantity. Losses can also occur in the gutters and in storage. Regardless of roofing material, it is realistic to assume loss on annual rainfall to be up to 25%. These losses are due to several factors: the roofing material texture which slows down the flow; evaporation; and inefficiencies in the collection process. Tank shape

Tanks can be bought or constructed at site. They are available in many shapes and sizes. Rectangular and square shaped tanks always tend to crack. It is therefore important that tanks are built spherical, hemispherical or cylindrical in shape. This is because these shapes distribute the load due to internal and external pressures uniformly on the walls of the tank, reducing uneven tension and thus possibility of cracking. They also minimize the use of construction material owing to their shape.

Cylindrical tanks are the most popular as they optimize volume per material used, and are relatively easy to design and construct. But sspherical/hemispherical shapes perform even better than cylindrical tanks because they are curved both vertically and horizontally obtaining the maximum strength unlike the cylindrical shaped tanks that are curved only horizontally. However, the construction of particularly larger spherical surface tanks is very difficult and expensive; and also requires complex shaped formwork. The compromise regarding shapes is that smaller surface tanks are built in spherical shapes, with the larger ones made cylindrical. The more popular one is the small Thai jars (Figure 1.6), which have spherical shapes.

Figure-1.6 (a) Spherical tank (Thai jar) for roof RWH (Source: Gould and Peterson, 1999

(b) Water jar for household drinking water

(photo courtesy of Mary Kakinda)

Siting of RWH tanks

Tanks used for roof water harvesting should be located adjacent or as close as possible to the building/roof. They should be situated at places where they collect rainwater from a large roof area; this may mean for example placing them between two buildings (figure xxx). Location of tank should also allow for minimum guttering. For roofs having a single peak guttering is best located at the end of the house where both gutters can access it. having a single slope with no peak, the tank is located midway of the roof edge. For tanks utilizing multiple buildings for roof area, the tank can be situated at the centre (figure 1.7). Unless there are certain constraints/restrictions on the use of space, tanks are best located where guttering requirements are minimum. As can be seen in Figure 1.7, if In both cases the tank should be located as close to the house as practicable, taking into account the damage this may cause to the foundation of the house and the convenience of tank construction. In this regard, there should be a minimum of 90 cm between the wall of the house and the tanks. If longer however, downgutters will need support in the middle as well.

Figure 1.7 (a) Tank located midway at one side of the building (Source: Gould and Peterson, 1999)

(b) Tank located at the confluence of two buildings (photo by Bancy Mati)

Tanks should be located away from toilets/latrines, waste disposal facilities, and other such water polluting facilities. They should not be placed at locations where their foundation may be damaged by erosion. They should be situated at sites where they can conveniently be accessed by users; and finally they should not block walkways. The tank, in addition to preventing the entry of debris and silt, must also be secure from small animals such as lizards and insects. It is essential that gutters are fitted properly with a constant gentle slope to lead water to the tank and prevent blockages. In practice, the efficiency of many roof RWH systems are greatly reduced because gutters have been poorly installed and only a fraction of the roof area is utilized.

1.4.4 Auxiliary structures Roof Gutters A gutter is the conduit that collects water from the roof and directs it, through a down-pipe into the tank. Thus, the gutter system is necessary for a roof the RWH system to operate efficiently and effectively. Gutters are reasonably affordable as and can be made locally by folding sheet metal. Size of gutter Gutters should neither be too small nor too large. If too large they become expensive; if too small they cannot be able to accommodate heavy storms and avoid overflow. Gutters are governed by the principles of open channel hydraulics, and thus their water conveyance capacity depends on gutter cross sectional area, shape, roughness and slope. Rainfall intensity and roof area also need to be considered in determining appropriate gutter sizes. As a general guide, at least 1 cm2 gutter cross sectional area is needed for every 1 m2 roof area. Gutter shape Semi-circular gutters are the most efficient at conveying water, as they have the largest cross sectional area for a given perimeter. Trapezium gutters with all three sides equal, although lower

than the semicircular, also have higher cross sectional area when compared with other gutter shapes. Square shaped gutters are also used to convey runoff water. For large roofs such as, those of schools V-shaped gutters, with splash-guard and steeper than 1% gradient, are used to handle the large flows without losses due to overflow. Gutter material There are various material used for gutters. These include galvanized metal sheets, aluminium, and plastic. However, the use of galvanized metals is recommended due to strength or cost or both reasons. Installation of gutters When installing gutters, they must be placed 3 cm inward from the roof edge in order to catch the back drop of water from light showers. Placing tanks at the end of long roofed buildings such as schools, requires large gutters. If instead, they are placed in the middle, they allow smaller gutters which can be easily and cheaply installed. Gutter brackets must be strong enough so as not to bend under the added weight of water, and pressure from wind. The distance between brackets should not normally exceed 100 cm for square and triangular gutters made of gauge 26 or thicker metal sheets. Causes of gutter failure The gutter is a very important component of the roof catchment system because its failure could mean failure to collect any rain water. Reasons for gutter failure include; inadequate capacity hence poorly intercepting runoff properly, overflows, leaks, sagging, twisted, clogging with debris if not cleaned, improper sloping of the gutter, covering only part of the roof area, and poor repairs and maintenance.

Splash-guards

A splash-guard is a long strip of sheet metal which is bent at an angle and hung over the edge of the roof by 2-3 cm to ensure that all runoff enters the gutter. The vertical leap of the splash guard intercepts the overshooting runoff and directs it straight into the gutter. Figure 1.8 shows how this works. The splash guard increases runoff coefficient significantly and also solves the problems of uneven roofs, and roofs with no facial board. It is easy to manufacture and install splash-guards. The triangular gutter can also function as downpipe, by continuing it from the roof straight to the tank inlet.

Figure 1.8 A sketch showing V shaped gutter system with splash-guard (Source: Gould and Peterson,1999)

Normally, the splash-guard is first fixed, and then the gutter is fixed above it using a system of wires. The gutter hangers are made by bending 3 mm GI wire into a triangular or any gutter shape; the gutter can then be mounted onto these hangers. A triangular shaped gutter, although not as efficient hydraulically as the other shapes, has the advantage that it is more stable, as it is fitted into the triangular shaped 3 mm wires that do not buckle easily under the weight of water during heavy rainfall events. The adjustable wires enable an even gradient of about 1%, by keeping the gutter suspended in the correct position under the eve. Downpipes

Downpipes are normally the piping or conduits that bring the water down from the roof into the tank. In some cases, it is replaced by the gutter channel which is extended to convey water to the tanks called a downgutter. Both the gutters and downgutters should have similar shapes and dimensions, although the down gutters have steeper slopes. This arrangement avoids water wastage, because it does not reduce the speed of the water, and debris are easily flushed away at the slopping screen cover of the tank without causing blockage. Downpipes may not be truly vertical. They may be designed with a smaller cross sectional area as the water flows faster downward. Typical sizes of downpipes for various roof catchment areas are shown in table 2. Table-2 Recommended Downpipe Sizes for Various Roof Areas Roof area (m2) 13 17 21 25 29 34 40 46 54 66 Downpipe size diameter (mm)

20 25 25 32 32 32 40 40 40 40

Source: ERHA RWH manual

A downpipe/gutter should normally enter a tank at the highest point of the tank/dome to maximize volume. To exclude leaves and debris from entering tanks, a coarse 5 mm or smaller wire mesh may be placed over the top of a downpipe inlet, at the end of the gutter. This should be regularly cleaned of any debris that accumulates. A better alternative is however, the use of a self cleaning tank inlet or guttersnipe to be such as a foul flush system. Abstraction devices normally include water taps, pumps or rope and bucket. Water Taps

Most surface water tanks utilize taps. Water is delivered to the taps through draw-off pipes laid through foundations. Where the base of the tank is at ground level, a small pit is excavated to facilitate gravity flow and the tap can be at the lowest level of the tank. This also allows space for the water collection container below tap level. Taps should be of good quality, durable and self closing particularly where used communally. It would also be useful if tap stations are locked. A lockable manhole can be provided over the tap station to prevent misuse of the water in the tank. Tank overflow

Tanks may overflow for various reasons, including heavier rains occurring in certain years or due to the under design of tanks. In such situations the overflowing water needs to be safely disposed of, with the help of overflow pipes. Overflow pipes are however sometimes wrongly placed below the top of the tank reducing storage capacity. It is therefore necessary that they are placed on top of the tank in a manner that maximum storage is possible. If tank roof is flat and made of reinforced concrete, the overflow pipe can be concreted in to it. If the roof is dome, it can be kept at the level of the tank inlet, alternatively the tank inlet can be used as the over flow. The hole for the overflow pipe is cut out of the mortar while it is still fresh, before it has set. The use of plastic pipes is not suitable, because they are not rigid enough.

Figure 1.9(a) Overflow pipe concreted into a flat tank roof slab (source: Gould and Peterson, 1999)

(b) RWH tank with overflow pipe (photo by Bancy Mati)

An overflow pipe should be placed vertically over the tap stand, for the water to fall on to the concreted tap stand excavation, from where it is drained/led to the soak away pit, without ponding on the ground, and causing erosion and damage to the structure. It is also necessary to note that the opening should be covered by a screen to prevent small animals and insects such as mosquitoes from entering into the tank. As a matter of fact any opening that leads small animals/debris into the tank should be provided with screen cover. Man hole and internal ladder It is necessary to get into the tank once in a while, in order to be able to clean and maintain/repair it from the inside. Access into the tank is possible through a manhole which should be kept near the highest point. Manhole covers can be made of concrete or sheet metals. A tight fit is important to prevent small animals and insects from entering. It would also be necessary to provide internal ladders to facilitate access into the tanks. They are included along with the design of the central pillar supporting the roof of tanks. They can be made of 10 mm PVC packed with concrete and five 50 cm length, 20 mm reinforcement bars.

1.5 Tank construction

The construction of tank will depend on type of tank, site conditions, materials being used design considerations presented above. An example of construction of a brick tank is given here below, with illustrations (Figures 1.9 a-h). Tanks constructed from bricks Burnt bricks or blocks may be used to construct tanks. Financial capability, availability of materials to make bricks, the required durability and strength of the tank and its quality will dictate the type of bricks to be used. If big tanks are to be constructed then a high expertise is needed. Measurements given in the following instructions are for the 10 m cubic tank which is equal to 10,000 litres. The following are the illustrated steps to follow: The tank should be built near the house for easy water collection from the roof. The tank must be located such that water from different parts of the house (depending on roof design) can be easily directed into the tank. The chosen site should be free of tree as they can expand and weaken the tank through development of cracks.

A piece of pipe or a round metal rod measuring 2.5 metres should be installed by hammering in the middle of the construction site and tied with a long piece of rope. A wooden peg should be tied to the rope leaving a length of 160cm. Draw a circle on the surface by using the peg. The tank foundation is built within the circle (Figure 1.9a).

Dig and remove the soil in the circle leaving the pipe/ metal rod in place at the centre. Dig to a depth of 15cm ensuring that the bottom is leveled out. In case of swelling clays, dig to remove problem soil layer (Figure 1.9b).

The foundation must be at least 7cm thick. Wire mesh and the outlet pipe should be placed on the foundation as explained under the section on construction of concrete tanks. Concrete of the ratio 1:3:4 (cement: sand: aggregate) should be used to construct the foundation (Figure 1.9.c).

Add a 6cm layer of concrete (on the wire mesh), compact and make sure that the surface is level by using a piece of flat wood. Leave the surface rough by scratching it with a wooden peg or a stick. Cover the foundation concrete with damp old clothing or sisal bags to maintain wet (Figure 1.9d).

Next day bricks should be arranged 15cm from the edge of the foundation. They should be arranged to achieve uniform spacing before bonding them with mortar (Figure 1.9e).

Fit a loose wire ring around the pipe at the centre by introducing it from above. It should be such that it can rotate freely on the pipe. Tie a 160cm long rope on the ring. Make a knot on the rope so as to get 145cm from the pipe at the centre. By rotating the rope, the knot will show the correct location of the outer edge of each brick in the tank wall. Pour water on bricks after they have been properly laid in place. Bricks should be lifted in turn, one after the other, and mortar placed on the foundation before returning them. Ensure that the bricks are level both horizontally and vertically by using a builder’s level and adjust if necessary. Use a knot on the rope so as to a certain the outer edge of each brick in the first course. The space left between the bricks should be roughly the same size and the pipe at the middle should be straight all the time.

For the second brick course and others to follow, the space left between the bricks should be at the centre of the brick below.

When the wall attains a height of 200cm a PVC pipe with a diameter of 10cm and a height of 185cm should be inserted to the pipe at the middle of the tank so as to create a king pin to support the roof. Round iron rods with a diameter of 12mm should be placed in the PVC pipe mentioned above (Figure 1.9f).

The PVC pipe should then be filled with a 1:3:3 concrete and then left for some days to harden. Wetting the concrete within the PVC pipe is important. Nail a barbed wire around the wall of the tank at 5cm intervals in the lower half and 10cm intervals in the upper half.

Make a circular formwork the size of the inside diameter the tank on which to pour a concrete mixture of 1:3:3 after placing long iron rods (10mm diameter) pieces of same length as outside diameter of the tank (Figure 1.9g).

Plaster the outer tank wall using cement: sand mixture of 1:4. The plaster should be at least 2cm thick. Inside the tank, the plaster should be of 1:3 –cement: sand ratio. Cement slurry should be applied to the plaster inside the tank before it dries so as to make it water proof. In order to reduce the costs, a tank roof can be constructed by using timber and corrugated iron sheet in the same way as for a house roof (Figure 1.9h). Concrete mixing and making

When cement based products are made in developing countries they are usually hand mixed. Due to the problems caused by the high ambient temperature, such as rapid setting due to water loss and in turn reduced workability there is a tendency to make a relatively wet mix. There is also sometimes a lack of knowledge about the correct consistency of cement. As stated previously, for the optimum strength of the cement, a water cement ratio of 0.4 is required. With increased mixing time, the strength of the cement paste slightly increases. To improve uniformity, all the dry materials are thoroughly mixed and then wet mixed properly. Applying and curing

The prepared mortar or concrete should be applied within a maximum of half an hour of mixing. Cement starts losing its strength half an hour after it has been mixed with water. Curing is done

by covering the tank and the wall with polythene sheet or plastic sacks which must be properly secured wrapping sisal strings all around (figure 1.10). Water is poured between the tank and the sheets morning and evening for three weeks. All other concrete and plaster works should also be cured for a similar period. Cure cements work properly by keeping it moist and under shade for at least three weeks after its application. If this is not done properly, the strength and water proof properties of the concrete and mortar are considerably reduced. For example, if the curing process is stopped after seven days, the cement will have only about 60% of its potential compressive strength.

Figure 1.10 (a) Relationship between compressive strength and curing period of mortar, and (b) tank is being cured under polythene sheeting for 3 weeks (source: Danida, 2007)

The use of lime

A small amount of lime added to mortar helps it to be more workable, and final plaster to be more water proof. The other advantage of lime is that it is cheaper than cement and thus help in reducing costs.

1.6 Hygiene and care of roof water catchment systems When careful construction and maintenance of roof catchment tanks is done, the system can provide portable rainwater clean enough to drink. The design of the system should incorporate hygiene and safety features to prevent entry of dirt into the tank (figure 1.11). To achieve this, certain design features must be incorporated and other criteria met. These include: Immediately following heavy rainfall the quality of water in the tank may be lowered due to

any debris washed into the tank or stirred up from the bottom, which may take some time to settle out. It is appropriate therefore to avoid drinking water directly from the tank for a few days.

A clean impervious roof made from non-toxic material is essential. Lead roofs should be avoided, as should any covered with lead-based paint.

The roof surface should be smooth and any moss, lichen or other vegetation removed including branches from over-hanging trees since these provide sanctuary for birds and access for rodents and other animals to the catchment surface where defecation could contaminate the rainwater runoff.

Taps or draw-off pipes on roof tanks should be at least 5cm above the tank roof (more if debris accumulation rates are high); this allows any debris entering the tank to settle on the

bottom where provided it remains undisturbed it should not adversely affect water quality. Alternatively if available and affordable a floating filter outlet can be used.

A coarse filter and/or foul-flush device should intercept water before it enters the tank to remove dart and debris.

Wire or nylon mesh should cover all inlets to prevent any insects, frogs, toads, snakes, small mammals or birds entering the tank.

If birds persist in perching on the catchment bird scaring and other physical measures (or the services of a cat) may be required

The tank must be covered and all light must be excluded to prevent the growth of algae and microorganisms.

Tanks, gutters, screens and all system components should be inspected and cleaned annually, if possible. A tank floor sloping towards a sump and washout pipe can greatly aid tank cleaning. A well-fitting manhole to allow access is essential.

Water should not, if possible be consumed directly from the tank without treatment for the first few days following major rainfall.

Water from other sources should not be mixed with that in the tank.

Figure 1.11: Roof RWH system incorporating a separator and filtering mechanisms (adapted from SWALIM, 2007)

Leaf screening

At the roof catchment level, large organic matter such as leaves deposited on the roof surfaces by wind can be removed using leaf screens. These are placed at the lower edges of the roofs and extended over the gutters. During initial rainfall events, runoff water flows into the gutters carrying with it minute particles. The leaves cannot enter the fine mesh covering the gutters and will thus go over the gutters and fall to the ground. Leaf screening can also be achieved along down pipes. A section of the pipe is cut in a slanting manner and a fine mesh placed over it. Runoff flowing on the pipe will pass through the mesh leaving behind the large organic matter such as leaves, which will slide to the ground.

Foul-flush and filter systems

Although not absolutely essential for the provision of portable water in most circumstances when effectively operated and maintained foul flush and filter systems can significantly improve the quality of roof runoff. If poorly operated and maintained, however, such systems may result in the loss of rainwater runoff through unnecessary diversion or overflow and even the contamination of the supply. Manual foul flush mechanisms

This is a plastic by-pass mechanism which is initially positioned to allow foul flush to flow away as wastewater. After about five minutes of a rainfall event, it is manually changed so that clean water is re-routed to the tank (figure 1.12). The disadvantage of this system is that the operator has to be around to re-position the foul flush gadget so that it receives clean water after a rainfall event.

Figure 1.12: Manual foul flush mechanism (photo courtesy of SWALIM, 2007)

Filtration A number of filtration mechanisms are in use gravity, pressure and sand- based filters. Sand-based filters remove turbidity (suspended particles), colour and even microorganisms. Depending on availability of different materials, layers of sand are arranged as shown in figure 1.13. A more sophisticated filter system can be installed using slotted plastic cover; a separator and filtering mechanisms (figure 3.14). Before the tank is put into use it should be thoroughly cleaned and disinfected with high dosage of chlorine. Since the water shall remain stored for quite a long time, periodical disinfection of stored water is essential to prevent growth of pathogenic bacteria.

Figure 1.13: Sand and gravel- based water filters (Adapted from SWALIM, 2007)

Top left Top right

Bottom left Bottom right

Figure 1.14 Use of separator and filtering mechanisms to treat roof runoff water (Source: SWALIM, 2007)

The alternative to the use of separator extension pipe is the use of a floating ball technique. In this technique, initial runoff flows into a container with conical ends. The container has a ball that floats and rises with the filling water. The advantage of this method is that the operator doesn’t have to be around during a rainstorm. He can remove the foul flush after the rainfall event. Disinfection Disinfection consists of boiling water and using chemicals and ultra-violet light. Boiling water for 10 to 20 minutes is sufficient to remove all biological contaminants. Chlorination is done using calcium hypochlorite (CaOCl2), known as bleaching powder, at a mix of 1 g per 200 litres of water to kill all types of bacteria. Alternatively, chlorine tablets can be used at a mix of 0.5 g to treat 200 litres of water. At the collection conveyance stage, physical treatment through leaf screening is done. This is followed by a physical and biological stage especially when automatic foul flush gadgets are used together with sand. Finally, at the storage and utilization stage, water must be boiled and chemicals applied to kill harmful micro-organisms.

1.7 Operation and Maintenance

Roof top catchment tanks, like all water supply systems, demand periodic management and maintenance to ensure a reliable and high quality water supply. Regular maintenance is critical to any dependable water harvesting system. If the various components of the system are not regularly cleaned, water use is not properly managed, problems are not identified or necessary repairs not performed, the roof catchment system will cease to provide reliable, good quality supplies. Maintenance and management requirements gives a basis for monitoring checks.

During the rainy season, the whole system (roof catchment, gutters, pipes, screens, first-flush and overflow) should be checked before and after each rain and preferably cleaned after every dry period exceeding a month.

At the end of the dry season and just before the first shower of rain is anticipated, the storage tank should be scrubbed and flushed of all sediment and debris (the tank should be re-filled afterwards with a few centimetres of clean water to prevent cracking). Ensure

timely service (before the first rains are due) of all tank features, including replacement of all worn screens and servicing of the outlet tap or hand pump.

It is important to ensure that the gutters and downpipes are free of debris. This is done through periodically clean and/or repairs to prevent corrosion.

Water Use Management

Control over the quantity of water abstracted from the tank is important to optimize water use. Water use should be managed so that the supply is sufficient to last through the dry season. Failure to do so will mean exhausting all the stored water. On the other hand, underutilization of the water source due to severe rationing may leave the user dissatisfied with the level of the service provided.