Monday, November 7, 2011

Thoughts on Freshwater Salinity

"Salinisation" of freshwater resources refers to an increase in the content of inorganic salts, measured as Total Dissolved Solids (TDS) in ppm, or expressed as a conductivity value (mS/m). Natural freshwater will always contain some level of dissolved inorganic species, but it is when concentrations of these constituents become elevated that water users experience problems.
Elevated salinity imposes costs on industrial water users in a number of ways, just a few examples of which are:
·         Scaling of heat transfer surfaces;
·         Reduced cycles of concentration for cooling towers and boilers;
·         Increased treatment costs for applications such as boiler feed water production and other processes requiring soft water;
·         Interference with biochemical and chemical processes;
And it’s not only industrial users who are impacted. Farmers suffer from reduced crop yields and domestic consumers experience problems with household appliances such as irons and kettles. Elevated salinity levels also impact on aquatic fauna and flora and can modify the receiving environment significantly. What then are the causes of elevated salinity, and what can we do about the problem?
Natural fluctuations in salinity occur due to interactions between water and natural geological formations, for example when an aquifer overflows and discharges into a watercourse, or when rainfall leaches salts out of rock formations and seepage into river systems occurs. Hence certain geographical areas are prone to having periods in which salinity levels are higher than elsewhere. Water management authorities often employ strategies such as releases of water from fresher sources to dilute the salinity to ensure that users receive water of acceptable quality, though this may not always be possible.
Agricultural activity is a further cause of salinity increase, with the main issue here being the leaching of salts from geological formations as well as the leaching of inorganic fertiliser components from soil, both as a result of irrigation.  Land use practices such as the manner in which fields are ploughed can exacerbate the situation and also magnify the impacts of the interactions between local geology and precipitation. More efficient irrigation techniques help to reduce return flows to local watercourses and reduce the quantity of salts leached from underlying rock. Fertiliser and land use practices should therefore also take salinisation impacts into account.
Industries and mines can contribute significantly to salinisation. For example, inorganic chemicals such as acidic and basic solutions used for cleaning in the food industry increase effluent salinity, as do cooling tower blow downs. The pulp and paper industry, with its use of inorganic chemicals for processes such as bleaching and causticising is another of many industries which can impact on the salinity of local water resources. In the case of mines, mine water is in close contact with geological formations, and mineral processing produces saline effluents by its nature, due to the size reduction of the ore and consequent intimate contact with water typical in the industry, not to mention the inorganic chemicals employed. Heavy water-intensive industries such as power generation, iron and steel production and mining also tend to make extensive use of un-lined dams, from which water seeps through rock formations and increases in salinity, ultimately emerging and finding its way into local rivers. The discharge of cooling tower blow downs into such dams means that in some instances the water is saline to begin with.  Phenomena such as acid rain can also increase the salinity of surface and groundwater. Acid rain occurs when sulphur dioxide and nitrous oxides arising from the combustion of fossil fuels like coal are contacted with water in the atmosphere, forming acidic precipitation.
The processes required to reduce the salinity impacts of industrial concerns are expensive, and will themselves produce waste streams of a saline nature that will have to be safely disposed of. A hazardous waste storage facility is generally required. The integrity of such facilities must of necessity be of the highest standard, but one has to wonder if this is really a sustainable solution.
So we see that salinity has many impacts, and that water users that contribute towards salinity problems are also impacted upon by salinity problems, though these impacts are generally a result of the activities of those upstream of them. We come across many industries that do not measure the salinity of their incoming water or assess the impact they are having on the salinity of receiving watercourses. With measurement of salinity being such a simple matter, make a point of understanding the cost of salinity to your organisation and your contribution to the salinisation of downstream water resources, and consequent impacts for local communities and other users. Pay careful attention also to the nature of the salinity you produce. Often with industrial processes, the chemicals associated with salinity (e.g. heavy metals and/or persistent organic pollutants) are more harmful than the salinity itself.