Silvery Fishy Story

Many years back I'd read an Article in Newstime (a local paper).
It talked about how people in West Bengal had come up with a novel way
to make use of Sewage using simple techniques.

They would make use of wasteland/barren land.
Initially bunds are created and sewage water is flooded into this.
The water is allowed to rest for a month or so.
Gravity leads to settling of the particulate matter which forms a layer over the barren land.
Exposure to Sunlight with its UV rays leads to sterilisation of the waters.
Once the waters become clear, fish eggs are put into them.

The growth of algae and insects due to nutrient rich water leads to fat fish growing.
Soon they are harvested and sold off at the market.
The layer of sediment broken down due to oxidation and bacteria makes
the barren land fertile over a few repetitions. This can later be used for farming.

I don't clearly recollect the articles take on the presence of Heavy Metals and pollutants in the sewage. What happens to those? Or is the sewage not having these things. I wonder...