I have not read the whole thread but looking at this study:
https://www.researchgate.net/publication/237189238_The_Effectiveness_of_Slow_Sand_Filters_to_Treat_Canadian_Rural_Prairie_Water it shows that slow sand filters on their own remove virtually all viruses and for bacteria it lies between 95-100%. Not sure why there is a range, maybe it is due to different implementations filtering differently. But those numbers look much better than the ones initially posted.
Also on ceramic filters, I know there is this organisation, potters for peace:
https://www.pottersforpeace.org/ that teach how to make your own ceramic filter, so it does seem possible to make this without outside dependency.
Also another interesting filter mechanism I have read of is the sari cloth filter, where 4 layers of a specific cloth are very good at filtering water. Maybe not something one wants by itself, but might be interesting in combination with other filters.
And one other more experimental one, luffa sponge (luffa aegyptiaca) filter, see e.g. here:
https://www.aijcrnet.com/journals/Vol_3_No_3_March_2013/11.pdf
So I do feel there is a lot of potential for
local, electricity-free filtering systems, just that there is not that much resources to be found yet. But I suppose as with so many things as long as
energy is cheap, it is much easier to make money out of non-local systems that do use electricity.
Another interesting one I just found that talks about slow-sand filters, as well as
biochar filters as well as biochar augmented sand biofilters:
https://www.nature.com/articles/s41598-020-57981-0 In these tests it seems that biochar and biochar-sand filters worked best.
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