The term “heavy metal” is not altogether clearly defined, but in the case of water pollution, these are metals such as arsenic, cadmium, iron, cobalt, chromium, copper, manganese, mercury, molybdenum, nickel, lead, selenium, vanadium and zinc. While heavy metals do tend to have a high atomic mass, and so are heavy in that sense, toxicity seems to be a further defining factor as to what constitutes a heavy metal and what does not.
|Municipal treatment plants are generally ill-equipped |
to cope with significant heavy metal pollution, both
in terms of removal and safe sludge disposal.
Heavy metals occur in the earth’s geological structures, and can therefore enter water resources through natural processes. For example, heavy rains or flowing water can leach heavy metals out of geological formations. Such processes are exacerbated when this geology is disturbed by economic activities such as mining. These processes expose the mined-out area to water and air, and can lead to consequences such as acid mine drainage (AMD). The low pH conditions associated with AMD mobilise heavy metals, including radionuclides where these are present. Mineral processing operations can also generate significant heavy metal pollution, both from direct extraction processes (which typically entail size reduction - greatly increasing the surface area for mass transfer - and generate effluents) as well as through leaching from ore and tailings stockpiles.
While mining activity poses significant risks for heavy metal pollution, this sector is not the only culprit in the industrial sector. Many industrial processes can generate heavy metal pollution, and in a large number of ways. Clearly, some industries will be more likely to pollute than others. Hence the electroplating industry, which can produce large volumes of metal-rich effluents, will naturally be a more likely polluter than the food processing industry, for example. This is not to say that players in this industry will necessarily pollute, and it is in fact in the electroplating industry’s best economic interests to minimise metal discharges, since these are inversely proportional to resource efficiency. Reducing losses by minimising drag-out from plating baths leads to reduced metal discharges, for example. The lead-acid battery manufacturing industry is another example of an industry which can generate metal-rich effluents as well as airborne lead pollution which can subsequently be deposited in surface water resources (and of course on land). So clearly, where an industry uses heavy metals as key input materials, pollution risks increase.
An example of a large non-point source of heavy metal pollution is coal-fired power generation, which can contaminate water resources through aerial deposition of mercury emitted from boiler flues. Technologies such as wet scrubbing are available to remove much of this mercury, but of course the effluents produced have to be safely handled to prevent subsequent pollution. Some of these processes have the primary goal of removing sulphur dioxide, with heavy metal removal a welcome by-product of the scrubbing process. The industry also generates large amounts of ash which itself contains heavy metals, including uranium.
The importance of minimising heavy metal pollution for industrial organisations extends beyond simple compliance. The impacts of heavy metal pollution on living organisms are very serious. Heavy metals are bio-accumulative, toxic at high concentrations, have neurological impacts, and some are carcinogenic. They can also interfere with chemical processes by poisoning chemical catalysts and can impact on biochemical processes by interfering with enzyme action. There are hence serious environmental, economic and social impacts associated with heavy metal pollution.
As always, a detailed risk assessment, which must include include quantitative measurement, is recommended to help you to understand your heavy metal pollution baseline. Where problems are identified, the solutions you choose should focus on the source of the pollution as far as possible, in line with Cleaner Production principles. End-of-pipe treatment methods are unavoidable in many circumstances, but where they are employed, care should be taken to dispose of resultant concentrated metal wastes safely. For example, lime treatments, which raise pH and precipitate metals, produce concentrated wastes (sludge) requiring safe disposal, as do processes such as reverse osmosis (retentate). Take care then that you are not just moving the problem around through poor management of these wastes, which generally require disposal at certified hazardous waste installations.