are you still interested

I am interested

take a look on u tube at electroplating barrels on u tube then look at a gear generator on the net making wooden gears

Internal -> wastewater goes into the drum and is filtered through the surface to the outside of the drum, as you can see here. Solids are captured inside the drum and guided to a solids bin. External -> wastewater goes onto the surface of the drum; the water penetrates through the mesh and is captured internally inside the drum; the solids stay on the exterior surface of the drum and are removed by a scraper or a spray bar or both. I'm not the best person to ask which is better for suspended solids, though. I think they're both fine; the issue is typically around which way do you want to dispose of the solids, and how compact do you want to make the system.