My immediate reaction to this question is to focus on context. In our work, we find that above all the local environment and existing stakeholders in each distinct country we work in (11 internationally for iDE) are primary determinants for the degree to which the private sector can play a significant in a) design/R&D, b) demonstration/piloting, and c) scalable rollout and diffusion of WASH solutions developed.
For example, in Bangladesh we have a very mature and robust private sector infrastructure, with large corporations leveraging distributions networks in WASH-related products as large as 3,500 independent retailers across all 64 districts of the country. Accordingly, these conditions provide iDE's WASH programming with the opportunity to focus its technical strateguy on long-term engagements with such firms, to co-create and fully transfer the production, distribution and marketing of WASH products and services through these robust and large-scale networks. The SaTo Pan developed by American Standard Brands in collaboration with iDE and RFL Plastics Ltd. is a great example of this - RFL's incredible market reach enabled the diffusion of this innovative and highly customer-oriented product at scale relatively rapidly.
In other countries with less mature market infrastructure, iDE focuses on models oriented more toward social enterprise, directly catalyzing the market to develop and utilizing that demonstration effect from its impacts to encourage crowding-in for maximum scale, efficiency and consumer satisfaction. The trick is to comprehensively analyze the national context in terms of the private sector's overall strengths, capacities, and reach to BoP consumers.