Alternative Sand abstraction technologies.

Five more alternative technologies have been introduced in Dabane trust's programmes. These have been devised in a in order to cater for those areas that cannot accommodate the well point technology. The alternative technologies include:

1. Caissons
2. Sand dams
3. Tube wells
4. Bio-sand filters
5. Underground tanks

Caissons

Caissons as typically larger structures than well-points or infiltration galleries which are installed directly into the riverbed or into alluvial riverbanks where there is low permeability. Lower sections of the caissons are permeable so that water flows from the sediment into the caisson and eventually creates a graded filter around the base of the caisson.

A caisson can be considered to be a well shaft that does not reach the surface. It is generally covered by a slab with a narrow diameter connection either vertically to the surface or horizontally to a sump on the riverbank. Conditions necessary for Caisson Construction include fine aeolian sands, low permeability and low porosity.

Sand dams

The type of riverbeds that are most convenient for sand dam construction have hilly and stony catchments that produce coarse sand where up to 350litres of water can be extracted from 1litre cubic metres of sand. Such rivers are suitable for sand abstraction projects. The main purpose of sand dam is to increase the volume of sand and water in riverbeds by putting a barrier (dam wall) on a river channel.

Listed below are the conditions that are suitable for sand dam projects:

• Two high riverbanks to enable the wing walls to keep over-flowing flood water within the spillway and not flowing over the riverbanks
• Dam walls should never be built on fractured rocks or large boulders because such walls cannot be made water tight. Water will always seep out between the boulders.
• Riverbeds with fine textured sand originating from flat land are also unsuitable for sand dams because less than 5% of the stored in the voids between the sand particles can be extracted.
• Water- indicating vegetation, that requires water all year round, should be growing on the banks where the reservoir will be located as proof of the riverbed capacity to store water.

Bio- sand filter tank

This can be defined as an innovation on traditional slow sand water filters, specifically designed for intermittent or household use. The filter container may be made of concrete or plastic and is filled with layers of specially selected and prepared sand and gravel. The filter is a simple container that encloses layers of sand and gravel, which trap and eliminate sediments, pathogens and other impurities from the water.

The bio- sand filer tank operates in a way that contaminants are removed to ensure maximum possible treatment efficiency.

Bio-sand filter tanks that are used to purify water in the rural areas...







Rainwater Harvesting Tanks

These are rainwater catchment systems, which can seldom compete directly with conventional municipal supplies in terms of cost, reliability and quality. Although the cost of installing rainwater catchment tanks is high, the catchment and delivery systems (gutters, downpipes, etc) are normally already present in the urban context.

Roof tops can act as catchment areas but some key points should be considered when selecting a site for a tank. They include:

• Tanks should never be located near toilets or pit latrines.
• Be located well away from any waste-disposal facility or other source of pollution or contamination which may threaten the quality of the stored water.
• Carefully sited with respect to the possible damaging effect of soil erosion.
• Sites where substantial surface runoff may occur in storms should be avoided.
• Tanks should be located so they can collect water from as large a roof area as possible e.g. between two buildings.
• Tank should not block walkways.
• Water tanks should be located at a minimum distance of 900mm from the wall of their roof catchments.

Water from sand rivers

A book has been published on sand-abstraction and issues related to it.
Issues covered in this edition include:

• sand-rivers
• site identification and selection
• abstraction methodologies
• installation technology
• pump technology
• pumpselection

all this and more from the book, Water from sand rivers...

by Hussey S,W. WEDC , Loughborough university(2009)

get yourself a copy: resourcecentre@dabane.org

Dabane-water workshops

Sand Abstraction technology

Dabane Trust has pioneered a simple, low cost water abstraction system that is suitable to use in arid and semi-arid areas. In these areas the rivers flow only for a few days in each rainy season. However many of them have a significant depth of sand which can retain water in sufficient quantities to provide year round water. Because the water is held in the sand it is naturally filtered and is thus clear, fresh that is clean enough for even safe domestic use. Over more than fifteen years Dabane Trust has developed simple hand pump systems to draw water from these ostensibly dry rivers, not only for household use but also in sufficient quantity for livestock water and for small-scale irrigation of basic vegetables.

Dabane and Sand-abstraction Trainning

Dabane Water Workshops is involved as a sand-abstraction technical partner in the Department for International Development’s (DFID) Protracted Relief Programme Phase II (PRP II). Dabane Water Workshops is thus tasked with providing training for all PRP II partners involved in sand-abstraction system development. Two partners were trained in first year of PRP II (July 2008 – June 2009), these being World Vision Zimbabwe (WVZ) and Action Contre la Faim (ACF). The remaining partners were then all pencilled for the second year with the theoretical training for the partners to be conducted all at once at the Dabane Water Workshops premises. The training was then conducted from the 24 – 28 July 2008 at the Dabane Water Workshops premises as planned.

Seven organisations were represented bringing the total number of trainees to 24. The organisations were; Concern worldwide, CADEC, Care International, Save the children UK, Action Contre firm and Zimbabwe Red cross services.

During the initial year 2006-2007 of this programme, staff members of four international NGOs were trained in sand-abstraction site selection, well point and hand pump installation. The organisations involved were Care International, World Vision Zimbabwe, Catholic Relief Services (CRS) who nominated the Organisation of Rural Association for Progress (ORAP) and CAFOD who nominated CADEC. Training was provided to teams of three people, with Care sending two teams. 15 field workers in total were trained.

Storage of water in sand rivers

Water storage in sand rivers has several advantages, such as;

1) Evaporation losses are reduced gradually to zero when the water level is 60-100cm or below the sand surface.
2) Livestock and other animals cannot contaminate the water reservoir because it is hidden under a surface of dry sand.
3) Mosquitoes, and insects that carry water-borne diseases, cannot breed in underground water reservoirs.

Water is naturally retained in the sediment of sand river channels although in a dry riverbed this is not always apparent. The water in a sand river is clean and not subject to the same amount of evaporation as an open surface dam. Sand river water supplies can be used to augment the supply of water from underground aquifers and dams, especially in remote rural areas where it is imperative that local communities are able to operate independently and maintain their own water supplies. Sand dams and subsurface dams function like sand rivers as they retain water in the sediment and reduce evaporation.

The Dabane Trust has installed simple handpump technology sand-abstraction systems at some 100 sites in Zimbabwe over a period of 14 years. The original installations put in place in 1992 are still in operation, independently managed by rural community groups.
The picture below shows the layout of a typical small-scale sand abstraction system as developed by Dabane Trust with a single home-made well-point and a flexible connecting pipe to a rower pump on the riverbank. The rower pump is situated no more than 5 metres above the saturated river sediment level and discharges water into a sump. Water is then transferred a greater distance and height by a Joma pump to a water supply point such as a water storage tank in a garden. This can be several hundred metres distant and some 8 metres higher. The Joma pump uses rower pump components in uPVC pipe work with standard pipe fittings mounted in a steel frame.

Sand abstraction as a method of drawing water from sand rivers for domestic, livestock and irrigating small gardens using different systems has proven to be a useful method in the communal areas of zimbabwe.

The power of sand as a water source....

The Dabane Trust has installed simple handpump technology sand-abstraction systems at some 100 sites in Zimbabwe over a period of 14 years. The original installations put in place in 1992 are still in operation, independently managed by rural community groups.

The picture below shows the layout of a typical small-scale sand abstraction system as developed by Dabane Trust with a single home-made well-point and a flexible connecting pipe to a rower pump on the riverbank. The rower pump is situated no more than 5 metres above the saturated river sediment level and discharges water into a sump. Water is then transferred a greater distance and height by a Joma pump to a water supply point such as a water storage tank in a garden. This can be several hundred metres distant and some 8 metres higher. The Joma pump uses rower pump components in uPVC pipe work with standard pipe fittings mounted in a steel frame.

What is sand abstraction

Sand abstraction is a method of drawing water from sand rivers for domestic, livestock and irrigating small gardens using different systems.

Water storage in sand

Sand consists of small stone particles that originate from stones and rocks being broken down by the effects of sunshine, rains and temperature variations.
Voids, which are empty spaces, are always found between sand particles. When dry riverbeds are flooded by rains and flash-floods, the air in the voids is pressed out by the water because it is heavier than air
Coarse textured sand has larger voids that get saturated much quicker than fine sand. Therefore, much more water can be extracted from riverbeds containing coarse sand than from riverbeds with fine sand.
Fine textured sand has tiny voids that get saturated slowly with water. Only about 10% of water can be extracted from the volume of fine sand.
Coarse textured sand has larger voids and is therefore saturated much quicker than fine sand. The volume of water that can be extracted from coarse sand is about 35% of the volume of sand