The idea was inspired by @Darko Savić's filter feedeers session.
In this paper, scientists reported high doses of heavy metals in mussels and veined rapa. They stated that "the highest average concentrations in mussels were those of cadmium (0.280 mg/kg), followed by lead (0.251 mg/kg) and mercury (0.017 mg/kg). Veined rapa whelks also showed highest levels of cadmium (1.113 mg/kg), followed by lead (0.045 mg/kg) and mercury (0.034 mg/kg)". With current prices of cadmium, lead and mercury being 2.3 $ , 2326.50 $ and 2000 $ , I smell a heavy metal business.
I would finance the start-up company from the state nature conservation funds or EU green initiatives and grow huge mussels and veined rapa farm and research center. I would consider other organisms, too, after some experiments on filter and accumulation efficiency. Certain genetical modifications could be done to increase the accumulation rate of heavy metals.
When a huge amount of organisms would be produced, I would open pop-up farms in the world's most polluted rivers, where I measure the highest levels of heavy metals. The filtering organisms would actively accumulate heavy metals and clean rivers.
The mussels would then be returned to the core facility and heavy metals would be extracted. The extraction would happen in special pools with small nanocarriers dissolved in water. The water with nanocarriers would "collect" the metals while mussels actively filter it.
Let's say each kilogram of mussels could yield 0.045 mg of lead. That means one ton of mussels yield 45 mg of lead and 20 tons yield almost one gram of lead, which can be sold for 2.327 $. Having a 1000 ton farm can yield 2326.50 $. Now it's only the question of how long does it take to accumulate 0.045 mgs.
If combined with more heavy metals being extracted, on many locations, in large quantities and filtering capabilities enhanced with genetic engineering, this business could grow and sustain itself. Since it would clean rivers, it could continue to receive inexhaustible finances from the government and non-profit agencies, through various projects.