You can see a video of his process below. The experimentation started with fish tank clarifier, which is — apparently — mostly alum. Alum’s been used to treat wastewater for a long time.

but the term 'alum' only applies to aluminium potassium sulphate (reviewed in Ref. 46) (see the table). Alum was used by Alexander Glenny and captures antigen by precipitation when the solution is ...