A sponge-like membrane removes more toxins than a typical water filter, and then releases clean water when heated by the sun fastcompany.com/90621594/this-low-cost-water-filter-runs-on-just-sunlight.
aquacycl.com provides bioelectrochemical technologies to address global challenges.
Solarsack.com use sun UV to purify water.
So the sponges could be purified by UV light instead of sun light during night