Heavy Metals

The Effects of Heavy Metals on the Activated Sludge Process




  1. Toxicants (e.g., hydrocarbons, heavy metals) inhibit chemotaxis by blocking chemoreceptors, thus affecting food detection by motile bacteria as well as predator–prey relationships in aquatic environments.

  2. The most toxic compounds to nitrifiers are cyanide, thiourea, phenol, anilines, and heavy metals (silver, mercury, nickel, chromium, copper, and zinc).

  3. Multiple-antibiotic resistance is furthermore associated with resistance to heavy metals.





Some metals (e.g., nickel, cobalt, and molybdenum) at trace concentrations may stimulate methanogens.



However:



Some organisms found within the wastewater system have the ability to concentrate, accumulate, or precipitate metals, thus allowing the recovery of elements of economic importance. Many bacteria are known to concentrate potassium, magnesium, manganese, iron, calcium, nickel, and cobalt. Other bacteria produce complexing agents which selectively extract metals from dilute solutions. Algae concentrate silica and green-brown algae and fungi concentrate zinc and other heavy metals. Mosses and higher plants concentrate mercury, nickel, zinc, uranium, cesium, and strontium.



Sulfur reducing bacteria of the Desulfovibrio, Desulfotomachulum, Besulfobacter, Desulfococcus, Desulfonema, and Desulfosarcina are especially adept at metal removal from water by producing hydrogen sulfide which precipitate these metals. The constituent members of these groups embrace a wide range of salinity or osmotic pressure, temperature, hydrostatic pressure, pH, Eh, and other environmental conditions, and have thus been used in mine wastewater cleanup. Settling ponds inoculated with these sulfate reducing bacteria and sulfur oxidizing thiobacilli, and algae have been effective in lowering the concentration of uranium, selenium, and molybdenum in wastewater effluent.