Heavy metals are recognized as very toxic and extremely persistent species that accumulate in the environment.
The Big Three among the toxic heavy metals are Lead, Mercury and Cadmium. 
All industrial activities  that deal with metals release some quantities of these metals into the environment.
Priority clients are large industries including mining and ore processing, electroplating and coal-powered electricity generation.
There is some natural occurrence of low-level Arsenic in drinking water, resulting in widespread poisoning of large population segments. The clients for arsenic removal are mainly local governments and/or international health authorities and foreign aid agencies.

Arsenic problem details: WHO web-site (click)

  Nowadays, everything seems to be metal plated.  There are more than 15,000 metal-plating and electroplating operations in the USA alone.  This has been an extremely fast growing industry.  Complicated procedures involving often huge vats of metal bearing solutions are being used. When the metal is plated out of the solution, significant concentrations of different heavy metals  are still eventually disposed of in the wastewater, despite recycling trends. These toxic metals need to be removed and, optimally, recycled.

  Deep pits are created by surface mining during which highly contaminated water is constantly pumped out. This effluent contains metals dissolved through natural microbial activity and chemical leaching.  Called Acid Mine Drainage (AMD) it creates an environmental disaster wherever it flows - untreated.
Note the population centre close by.
There are thousands of pits like this one around the globe and mining and ore-processing industries of all kinds appear as priority clients for water treatment.

The problem with AMD is that there may be many uncontrolled sources where it is seeping to the surface from different kinds of mining activities. It is extremely difficult to contain.

Biosorption process offers a cost-effective alternative for treating metal-laden effluents from mining activities.

  The same mining pit 30 years later - the ore was exhausted and the mining site abandoned.  When the water pumping ceased, the water level in the pit has been steadily raising. By now this pit contains 20 billion gallons of acidic water (pH 2.5) that is so toxic with dissolved heavy metals that a flock of geese landing on it died within days. 
The steadily raising water will soon be reaching the water table level and will start spreading the poison even through underground aquifers ...
                     

Coal-burning power generation stations consume huge amounts of coal - easily 25 tons (!) every minute - that is a whole train every day. The massive and slowly moving coal heaps are constantly sprayed with water - creating perfect acidic conditions for trace amounts of metals to leach out. With the huge amount of coal throughput, tons of toxic heavy metals per year end up in the collected drain water that has to be treated. 
Apart from this large contamination source, there are many other points within a power-plant where metals originate - other major sources are the boiler 'blow-down' and the flue-gas scrub water.
All in all, the power-generating plant is one huge source of toxic heavy metal release principally due to its enormous size of operation. 

Power plant air pollution is a conspicuous aspect of its operation. While sulphur emissions are extremely important, traces of Mercury escape in power plant exhaust.  Falling hundreds of km away in rain and snow, toxic mercury builds up in the environment and accumulates particularly in fish. Negative health impacts are well known.

PRECIPITATION  
Conventional precipitation methods of removing heavy metals rely on formation of insoluble precipitates using chemical reagents. 
The sludges formed must be separated by settling or filtration. 
There are serious drawbacks: 
● The time for precipitation is long.
●  Insoluble metal hydroxides or sulfides may  be difficult to remove.
●  Resulting sludges have to be concentrated prior to their disposal.
●  Sludge disposal represents an enormous problem : 
●  Toxic metal sludges are invariably classified as ‘hazardous substances’ .
●  Recovery of metals from sludges is usually not feasible.

Major problem:  Precipitation/separation approach has difficulties in meeting progressively more stringent wastewater criteria.  It generates difficult-to-handle toxic sludges.

ION EXCHANGE 
A contemporary  high-performance process using synthetic resins usually packed into a flow-through column.  The resin binds metals from solution until it is saturated. 
Then it the column must be taken out of operation and the desorption-washing procedure follows directly in the column.  As the resin gets regenerated for another use it releases the metal in the concentrated form in the wash solution. 
Deployment of expensive ion exchange resins for large volumes of contaminated wastewater has been economically rather unsustainable and, as a result, application of this process for wastewater treatment is more often considered not feasible.

Biosorbents are natural and cheap ion exchangers which operate on exactly the same principle, using the same standard process equipment:  columns, pumps, valves and pipes.  Biosorption is likely to open the 'hidden' wastewater treatment markets considered feasibly inaccessible.

REVERSE OSMOSIS 
 The process uses membrane stacks and high pressure.  The metal gets concentrated on one side of the membrane, while purified water ends up on the other.  Membranes are very expensive and easily foul up.  Elevated pressure is also an operating drawback. 
This high-performance system is an expensive alternative treatment method not feasible for wastewater treatment in most cases.
ELECTRO-WINNING  METAL RECOVERY:
Metal recovery is possible only from concentrated enough solutions. 
From preconcentrated solutions metals are recovered by  "electro-winning" processes. 
Metals are plated from the solution by electric current applied through  special electrodes accumulating purified metals.
Electro-winning is a standard conventional technology used  'for profit'  by specialized operators who re-sell the recovered metals. 
They collect suitably pre-concentrated metal solutions for this type of  re-processing coupled with the metal resale.
 

ENVIRONMENTAL vs RECOVERY CONCERNS:  
Metal-laden waste solutions usually contain metal(s) at concentrations too low for recovery that is thus not feasible without an effective pre-concentration step.  The wastewater toxic metal content, however, is high enough to pose a serious environmental hazard.  Biosorption treatment results in low volumes of high-concentration wash solutions suitable for subsequent metal recovery.