Threshold treatment

Calcium carbonate (calcite) scale formation in hard water can be prevented by the addition of a small amount of soluble polyphosphate in a process known as threshold treatment. The polyphosphate sorbs to the face of the calcite nuclei and further growth is blocked. Polyphosphates can also inhibit the corrosion of metals by the sorption of the phosphate onto a thin calcite film that deposits onto the metal surface. When the polyphosphate is present, a protective anodic polarization results. 

When the treated water pH exceeds 9.4 to 9.6, the use of carbon dioxide to recarbonate the water is recommended. Alternatives include the use of sulfuric acid dosing, or a suitable chemical additive such as a threshold treatment or a calcium carbonate dispersant. 

USE Water softeners and detergents. For leather tanning, dyeing, laundry work, textile processing for the "threshold treatment of softening industrial water supplies. 

Sodium hexametaphosphate (sometimes called Graham s salt) was first used in water treatment to introduce phosphate into boilers without producing precipitation in the feed lines. It was used in the preparation with 14 parts to 1 part of calcium and was essentially a softening process. Because of the mode of action and the extremely small amounts of the additive required, the technique was called "threshold treatment". An alternative name is sequestration. 

Sodium hexametaphosphate is one of a series of chain polymers termed polyphosphates, and threshold treatment using various members of the series has found extensive use in controlling calcium carbonate scale formation. Veale [1984] states that typically 5 mg/l of polyphosphate will inhibit the precipitation of 500 mg/l of CaCOy To control scale in sea water evaporation an additive mixture could include sodium tripolyphosphate, lignin sulphonates and an antifoaming agent. 

Some organophosphorous compounds used in threshold treatment... 

As described in Chapter 8 robust crystal stmctures can be produced under certain conditions, but that crystal structure and growth may be affected by the presence of impurities. The effect is utilised in scale control by the use of additives. Unlike threshold treatment the use of crystal modifiers does not prevent... 

The beneficial effects of threshold treatment tends to breakdown when the hardness level is of the order of 350 - 400 mg/l and above these levels uncontrolled and rapid precipitation may occur [Harris and Marshall 1981], To bring back effectiveness it might be necessary to add a mineral acid to reduce partially, the alkalinity of the water. In cooling water systems, reducing the number of cycles the water makes through the system, may restore the effectiveness of the threshold agent. 

THRESHOLD TREATMENT - Chemical treatment, used to prevent scale formation, which acts to hold hardness in solution at the threshold of precipitation. 

An important application of soluble oligopolyphosphates is in the threshold treatment of hard water to prevent blockages of pipes arising from the growth of calcite crystals. About one part per million, when added to hard water, will inhibit the precipitation of calcite. This is believed to be due to the absorption of phosphate anions on to the surface of the calcite nuclei, which then prevents further crystal growth. The flexibility and dimensions of the polyphosphate chain allow a good fit to be made on the pattern of ions on the exposed calcite face (12.31) [3]. ... 

However, research shows that the most important aspect of the chloride removal process is the generation of hydroxyl ions, the rebuilding of the protective passive film and the removal of the chlorides immediately around the rebar. SHRP research showed that even with a modest total charge passed and only 50 to 80% chloride removal, and with chloride levels still above the corrosion threshold, treatment will give a very low corrosion rate and very passive half cell potentials which last more than five years without reactivation (Bennett and Schue, 1993), If this is true then we should perhaps call the process electrochemical chloride mitigation and avoid requirements to remove more than 90% of the chloride, as this may not be necessary. 

However, even such minor additions as the "threshold" treatment with 1 to 2 ppm sodium hexametaphosphate, for example, would be unacceptable to modern environmental standards. Many coimtries, states, or provinces now forbid the return to source even when the concentrations of natural constituents were higher in the intake water than in the return water. In some areas, thermal pollution is forbidden since the discharge of the same water at a higher temperature than the inlet temperature may be harmful to certain marine species (e.g., oyster beds). 

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