* In case of cracked linings, the following could be tried to salvage the tanks:
**If the crack is only at the base, covering the tank base with clay and compacting it might work. Addition of powdered anthills or lime is said to make this lining more robust. If the cracks are also found in the walls, then rehabilitation or an alternative lining might be a solution. Taking the example of berkeds in Somaliland, many remain unused due to previous poor workmanship, yet rehabilitation is expensive ($8 per m3), requires skill and is not always successful. In some cases it is also not even possible to rehabilitate – some cracked berkeds can be rehabilitated if the original walls were made solidly enough, but otherwise there are many berkeds that can never be rehabilitated. In such cases, plastic linings might be worth trying.
====Tank modifications and maintenance====
* Add a coarse mesh after the silt trap before inlet to prevent large debris from entering the tank.
* Where catchments have low runoff coefficients, this can be increased by modifying the existing surface or creating an artificial surface:
# In China, soils with reasonable infiltration capacity had a runoff coefficient of 2%, which was increased to 20% after the soil was compacted.
# Artificial lining of catchments is a possibility. Various catchment types and their runoff coefficients are: concrete (73-76%), cement-soil mix (33-42%), buried plastic sheet (28-36%).
* Shallow drainage canals can be dug to direct the runoff into the tank.
* Silt intake into sub-surface tanks ideally should be limited – how much silt will accumulate will depend on the area. In China, 80m3 had accumulated in 4 years. Ideas to limit silt include:
# Keeping a good cover of grasses or vegetation in the run-off area.
# Silt trap prior to tank intake. However, experience from Somaliland shows that silt traps (small mini reservoirs prior to main tank) are not very effective. A better method might be to replicate silt traps used in Charco dams where perennial vegetation is grown between small dams in the intake channel to encourage deposition.
# Alternatively where vegetation may not grow due to climate, stones similar to roughing filter can be used on intake to increase sedimentation before water enters the tank. A roughing filter operates through increasing the surface for sedimentation and could be designed into the berked intake where stones of 3 different sizes between 25 and 5mm are used in 3 separate sections. But if it is to function properly its area needs to be designed based on flow rates and inflow water quality. The filtration rate should be calculated by flow (m3/hr) divided by surface area (m2) and then different filtration rates are suitable for different water qualities – this information may be hard to estimate in the field though.
* Create a fence around the tank to prevent children from possibly falling in, and to prevent large vehicles from driving too close and damaging the lining.
* Support the notion of private ownership & management. A fence can be constructed to improve private ownership.
* Fish can be introduced to eat mosquito larvae, while at the same time providing a source of nutrition.
* Support the capacity of the government or private sector to be able to provide (for payment) a tankering scheme to fill tanks during the driest parts of the year.
==Costs==