Actually, I know that a similar idea has been used before in late 19th century, earlier in the 20th century, except it was much cruder. Basically a section of the river was marked off by nets and people could swim in them. I know some resorts still use a similar method, but again the filtering is only for very large particles.

The hard part is the filtering. Developing an effective filtering method for such a large body of water would be an impressive achievement.