Difference between revisions of "Classical ferrocement tank"

Revision as of 06:22, 13 April 2012

Conclusion of Acul des Pins Ferro-Cement Tank Project – Haiti

Ferrocement water tanks are made of steel mesh and wire, covered on the inside and outside with a thin layer of cement-and-sand mortar. The walls may be as thin as 2.5 cm. The tanks can be used for individual households or for whole communities, and they provide a relatively inexpensive and easy-to-maintain storage method. To avoid bending forces in the material, most ferrocement tanks have curved walls, in the form of a cylinder, a globe or an egg. Compared to concrete reservoirs, ferrocement tanks are relatively light and flexible. To protect the water from contamination, the tank is covered with a lid or a roof that can be made of various materials, but is usually ferrocement. In this case, an aeration pipe with a screen is needed to allow fresh air to circulate in the tank, while keeping out rodents and insects. A manhole in the roof gives access to the tank for cleaning and repairs. Water flows into the reservoir through an inlet pipe, which is normally above the water level.

Suitable conditions

• Avoid making sub-surface ferrocement or anthill/lime/cement tanks in unstable soil.
• Do not site tanks near big trees whose roots might crack the walls.
• Do not site tanks where heavy vehicles will pass close to tank wall.
• Do not site sub-surface tanks in areas of high water tables to reduce risk of flotation.

Construction, operations and maintenance

Range of volume: From 1 m3 to over 80 m3. Range of depth: Usually, between 1.5–3.0 m.

Maintenance

Operation consists of opening and closing the valves, and managing a chlorinator, if provided. A ferrocement tank that is well-designed and well-built needs very little maintenance. The surroundings, including the drain, must be kept clean on a regular basis; every two months, the valves must be opened and closed to prevent them sticking, and the screens must be checked. Occasionally the fence, a screen or tap may need repair. Every six months, or when contamination is suspected, the reservoir must be drained, de-silted, cleaned with a brush, and disinfected with chlorine. Any leaks have to be repaired immediately. Repair involves some special techniques using wire and mesh, cement, sand and water, but they are easy to learn.

Costs

• Ferrocement tanks: US$26 - $50 per m3 storage depending on size (e.g. 11m3 for US$550, 46m3 for US$1,200). Experience from the RAIN Foundation seems to indicate a cost of between 40-100 Euro per m3 storage for ferrocement tanks (including everything like materials, transport and labour). Partial below-ground ferrocement tanks have also been made where part of the structure is underground and part is above ground - US$24.5 per m3 storage (e.g. 10.8m3 for US$265).
• In Kenya in 1993, a 20 m3 tank with a roof cost US$ 420 (US$ 21 per m3). In the South Pacific Islands in 1994, 5.5–12 m3 tanks cost an average of US$ 50 per m3.

To save on costs
Reduce the size of structures, which increases the cost per m3 storage but more manageable to construct in terms of cash flow, and easier to cover. This way, tanks are more affordable to families, and more tanks can be added in subsequent years, thus spreading out costs.

Access to finance is a main obstacle to promotion of rainwater harvesting for households, and is important so that users can replicate the technology – however, so far there are few examples on a global level with micro-credit for rainwater harvesting. An initiative is underway in Nepal to trial the promotion of rainwater harvesting tanks through a combination of subsidy and credit from micro-finance institutions (MFIs), mainly targeting women, where gradually over time the subsidy will be replaced by credit completely where rainwater harvesting systems will hopefully be one of the credit products for MFIs. Lessons learned from micro-credit for biogas plants in Nepal show that the system can work when initially mixed with government subsidy – total repayment period ranges from 5 – 7 years and interest rates vary between 11.5 – 16%.

Field experiences

Reference manuals, videos, and links

• Large Ferro-Cement Water Tank: Design Parameters and Construction Details. UNHCR, July 2006.
• Video: Storage tanks: ferrocement tank. How to build a ferrocement tank (English and Spanish).

Acknowledgements