Zebra Mussels

In 1998, a ship that was previously in the Black Sea, near Russia, was in the Great Lakes of the US. It dumped its ballast and introduced a new animal into the lakes. This animal was the filter feeding Zebra Mussel, and both good and bad things happened because of this.
On the good side, the lakes were quite murky and polluted, and the mussels cleaned every drop of lake water. Within only a few years, lake visibility improved dramatically. Before 1998, one would be an idiot to drink the lake's water, now it is the best source of water in the United States for its area. Also, unlike many invasive species, there are native animals that love to eat the mussels. Perch, Roaches, and Crawfish love them. Crawfish eat over a thousand a day if given the opportunity.
The bad news is that they are encrusting every last surface of the lake, including boats, pumps, and devices, and they did not only eat the waste, but all filter-food available to them. This has starved out a number of native species. Also, they have bioaccumulated the lake's waste into themselves. This means that eating them, or anything that has eaten them, is unsafe.
While Zebra Mussels are, if clean, edible to humans, one should not eat any of the existing ones. They have all the PCBs and other pollution that all cities between Detroit and Chicago dumped lodged in their bodies.
There are two things I would suggest doing with this information. One is to harvest every Zebra Mussel currently in the lake, but discard the bodies into a toxic waste dump. I estimate there to be 6,432,215,961,600,000 mussels in the lake at most, so we go through a regular process of scraping mussels off available surfaces, counting and discarding until we have harvested this many. At that point, further mussels can be assumed to be safe. We can harvest them for cheap food, introduce crawfish that will eat them up and then promptly become Cajun food.
The other is to build a device that pumps water through mussel-filled chambers over and over. After about 10 or so cycles, the water will be drinkable. Periodically it will be necessary to remove the mussels and treat their remains for pollution, and because they will reproduce until they clog the chambers.
This device will use less power than traditional filters, and the filtration parts are 100% biodegradable. In addition, the filtration parts are 100% natural. In fact, the filtration will reproduce itself, making this machine partially self-assembling. Depending on how one circulates the water, it could even be off-the-grid, resulting in zero environmental impact. It scales easily up to a city water treatment, or down to a single-person use. Muhahahaha.
PETA will complain. Muhahahahahahahaha.