It is a flocculant, and very commonly used in sewage treatment facilities.

Since the project I am on is recyling waste water into potable water to replenish the depleted reservoirs here, I am finding that there are things in the water that you really want to filter and/or destroy. There's an awful lot of treated sewage runoff even from contremporary sewage treatment plants entering our freshwater system, not to mention what's already in the reservoirs, and microfiltration/ionization is probably the most effective treatment method. In fact, to turn sewage water (or any open source water for that matter)into drinkable water again, you have to remove microbials, a whole host of organic and inorganic contaminants, including endochrine products like hormones, then add back chemicals to prohibt microbial re-growth, then store in containers that the water won't react with.

Flocculation does nothing to purify the water, it only removes suspended solids. Our current process includes a flocculation system, then a microfilter, then a three pass reverse osmosis system, chemical precipitation/extraction (to remove inorganics in solution and miscible VOCs), UV irradiation, Ion Exchange to further reduce the mineral load, then Sodium Hypochlorite for short term storage before transport. The plant will produce 100 mega liters per day maximum, and will cost around $300 million to build. It has to be completed and operating by the end of 2008 to augment the supply from the reservoirs or else the government here will have to implement overland transport from somehwere else then. The funny thing is the water our plant will produce is ten times purer than what is coming out of the reservoir now. Much of Australia is in the worst drought they've had in a thousand years, and the reservoirs are below 25% of their normal levels for this time of year. It is no longer a question of if they will run out of water, but when.

Bear in mind, too, that the supply of water from the reservoirs is also treated and processed considerably. It may not smell, taste or look pristine, but it will remain safe for consumption until they run out altogether.