To first consideration, earthenware may not seem to be the best media for water purification. What we know about the material is that it has lots of pores but these are only poorly connected. As conventionally processed and fabricated, red firing clays do not have good permeability. Other materials appear better suited to the task, for example, diatomite. But in considering a widely duplicable production methodology for the poor, those most vulnerable to the water born, gastro intestinal illnesses, dysentery, cholera, typhoid, etc.
Sustainability is largely about using local resources.
For this reason there is an opportunity not only from the perspective of the user, but for a widely duplicable fabrication process, from the perspective of the producer. The model should be a good one with respect to low cost, easiness of use and effectiveness, but it is the low startup cost of production that will encourage the entrepreneur. Expect the factory to lack amenities like electricity and running water. The pavement may be earth. This would be an example for micro-scale production, irrespective of economy of scale.