Scale, fresh water

In treating ionic equilibria in aqueous solution, two activity scales have proved especially useful. The first is the traditional infinite dilution activity scale, which is defined in such a way that the activity coefficient yA = A /[A] approaches unity as the solution approaches pure water. One might refer to this scale as the fresh water scale. 

The most important property of the dissolved solids in fresh waters is whether or not they are such as to lead to the deposition of a protective film on the steel that will impede rusting. This is determined mainly by the amount of carbon dioxide dissolved in the water, so that the equilibrium between calcium carbonate, calcium bicarbonate and carbon dioxide, which has been studied by Tillmans and Heublein and others, is of fundamental significance. Since hard waters are more likely to deposit a protective calcareous scale than soft waters, they tend as a class to be less aggressive than these indeed, soft waters can often be rendered less corrosive by the simple expedient of treating them with lime (Section 2.3). 

Those waters in which the carbon dioxide content is in excess of that required as bicarbonate ion to balance the bases present are among the most aggressive of the fresh waters. Hard waters usually, though not invariably, deposit a carbonate scale and are generally not appreciably corrosive to cast iron, corrosion rates of less than 0-02 mm/y being frequently encountered. Water-softening processes do not increase the corrosivity of the water provided that the process does not result in the development of an excess of dissolved carbon dioxide. 

The sediment reservoir (1) represents all phosphorus in particulate form on the Earth s crust that is (1) not in the upper 60 cm of the soil and (2) not mineable. This includes unconsolidated marine and fresh water sediments and all sedimentary, metamorphic and volcanic rocks. The reason for this choice of compartmentalization has already been discussed. In particulate form, P is not readily available for utilization by plants. The upper 60 cm of the soil system represents the portion of the particulate P that can be transported relatively quickly to other reservoirs or solubilized by biological uptake. The sediment reservoir, on the other hand, represents the particulate P that is transported primarily on geologic time scales. 

This process, according to the manufacturer,54 has been developed in such a way that space requirements are kept to a minimum. A BIOPAQ IC reactor is used as the initial step in the treatment process. The name of this anaerobic reactor is derived from the gas-lift driven internal circulation that is generated within a tall, cylindrical vessel. These reactors have been operational in the paper industry since 1996. The second step in the purification process is a mechanically mixed and aerated tank. The aerating injectors can be cleaned in a simple way without the need to empty the aeration tank. Potential scaling materials are combined into removable fine particles. At the same time, the materials that may cause an odor nuisance are oxidized into odorless components. The process can be completed by a third and a fourth step. The third step focuses on suspended solids recovery and removal. The fourth step is an additional water-softening step with lamella separation and continuous sand filters in order to produce fresh water substitute. The benefits claimed by the manufacturer are as follows54 ... 

To assess, forecast, and mitigate the risks of key toxicants on fresh water and marine ecosystems and their biodiversity at a river basin and adjacent marine environment scale focusing on DSSs for the selection of the most efficient management options to prevent effects on biodiversity and to prioritise contamination sources and contaminated sites. 

Reverse osmosis is now extensively used to reduce salt concentrations in brackish waters and to treat industrial waste water, for example, from pulp mills. Reverse osmosis has also proved economical (the cost can be as low as about 1 per 1000 liters) for large-scale desalination of seawater, a proposition of major interest in the Middle East, where almost all potable water is now obtained by various means from seawater or from brackish wells. Thus, at Ras Abu Janjur, Bahrain, a reverse osmosis plant converts brackish feedwater containing 19,000 ppm dissolved solids to potable water with 260 ppm dissolved solids at a rate of over 55,000 m3 per day, with an electricity consumption of 4.8 kilowatt hours per cubic meter of product. On a 1000-fold smaller scale, the resort community on Heron Island, Great Barrier Reef, Australia, obtains most of its fresh water from seawater (36,000 ppm dissolved salts) directly by reverse osmosis, at a cost of about 10 per 1000 liters. 

The most common thermometer in the world is the Celsius thermometer, named in honor of the Swedish astronomer Anders Celsius (1701—1744), who first suggested the scale of 100 degrees between the freezing point and boiling point of fresh water. In a Celsius thermometer, the number 0 is assigned to the temperature at which pure water freezes and the number 100 is assigned to the temperature at which it boils (at standard atmospheric pressure), with 100 equal parts called degrees between these two points. 

The infinite dilution activity scale is useful for ionic equilibria in fresh waters, but for equilibria in sea water one gains precision by applying an ionic medium activity scale. Measuring pH in sea water gives less information than total alkalinity and total carbonate. Calculations on redox equilibria are simplified by introducing the master variable pE -----log e. ... 

Although the minimum energy needed for the conversion of saline water to fresh water is the same for any isentropic process, the goal is to achieve this conversion efficiently in a real process, on a large scale, at low cost, and with easily maintained equipment. 

While the reverse solubility curve of calcium sulfate is often the main reason for scale deposition in fresh water boilers and in brackish water distillation, when the sea water is not chemically treated the cause is chemical rather than physical. Sea water contains bicarbonate ion. On heating, the bicarbonate ion reacts with water to form carbonate ion plus carbon dioxide, which tends to be evolved as a gas as shown in the equations... 

Scale Prevention. The scale normally formed on heat transfer surfaces of sea water evaporators consists of calcium carbonate, magnesium hydroxide, and/or calcium sulfate. The first two form as a result of the breakdown of bicarbonate in sea water, which is initially saturated with calcium carbonate. Calcium sulfate scale forms purely as a result of its inverted solubility curve. Sea water is not saturated with calcium sulfate and an economically reasonable amount of fresh water can be recovered from sea water without exceeding saturation with calcium sulfate. However, at the start of this investigation, the solubility of calcium sulfate in sea water was not accurately enough known to tell whether 30, 50, or 80% of the water content could be removed at various temperatures without encountering calcium sulfate scale. 

On much longer time scales, glaciers, ice sheets, and ice caps store fresh water for long-term use in the planets water cycle. Melted water from these sources flows into the oceans and seeps... 

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