Thank-you for your question. My view is that a participatory approach is one of the best ways to ensure the sustainability of a water/sanitation project.
There is a risk of NGOs investing in competing projects, and communication between different NGOs, and between NGOs and beneficiaries is a way of fighting this risk. Also, NGOs must be cautious about other groups who may be affected by the project. For instance, what is in the interests of people such as NGO employees, or donors, may go against the interests of the intended beneficiaries of the project.
I think the beneficiaries and co-sponsors (as opposed to the donor) should own (where possible), or at least feel they own a project. Otherwise, there is a risk that if the donor stops funding, the project can fall into disrepair, not just due to the financial reasons but also due to a lack of coordination, and a lack of clear roles for people. If members of the community can be given the responsibilities for maintenance of the system at the start of the project, the project will be less vulnerable to problems such as these.
I think this is particularly important in water and sanitation, where systems are often heavily reliant on specific technical parts, such as pumps and filters, which may be difficult and expensive to get hold of if the NGO doesn’t assist the community by helping to assign roles and informing people where they would get these parts, for instance.
As for adoption percentage, I think that this is an important measure of the success of projects, but in order to be successful in the long run, projects must be sustainable, and I think the participatory approach helps with this.
These are my views, I would be interest to hear other people's.