Threshold agents

Sand filters are used in most municipal water companies to remove parasites (such as liver flukes) from drinking water. Many Asians are infected with liver flukes that attack their livers and are very difficult to eliminate from a patient once they are infected. Untreated, a sand filter will cement grains of sand together in a short time as a result of calcium carbonate and other insolubles cementing grains of sand. Once a filter is cemented it is practically useless. But if a water company includes about 100 to 200 parts per million polyphosphate in its potable water, sand filters are effective for several months before their sand must be replaced. It is an area where eutrophication of our water resources has never been an issue. 

---

External coil problems White calcium carbonate CaC03 Phosphate scale from threshold agents. Blue/green/black copper phosphate trihydrate Cu3(P04)2 3H20 Dirt pockets due to low level of coil. BW sludgei settles on external fins. Gasket weeping from inhibitor. Color stains from inhibitor dyes. 

Phosphate is sometimes present in MU water sources (say, 1-2 ppm or more) usually as a result of field and factory run-off or from the deliberate addition as a city water threshold agent to prevent corrosion and deposition in the mains. The steady growth in the reuse of secondary water sources such as municipal and industrial waste waters means that phosphate is increasingly likely to be present in MU. If the phosphate remains undetected, it likely will scale and foul FW lines by forming amorphous calcium orthophosphate [tricalcium phosphate Caj(P04)2] sludge before it reaches the boiler section. 

As stated at the outset of the section on threshold agents, risk is characterized (Step 4) by deriving what is sometimes called a hazard index, the ratio of known or expected doses incurred by the human population (Du) to the RfD, TDI, ADI, or MRL. A hazard index exceeding 1.0 suggests a risk that is, it can be taken to mean that some members of the population are exposed at levels exceeding the estimated population threshold. It is an unquantified risk, in two senses. First, although the RfD (or other estimates of safe dose) is considered... 

Regulators in the United States and in Europe, and other public health institutes, appear to have found UF refinements such as those proposed by Renwick to be valuable additions to the risk assessment model for threshold agents, and we should not be surprised to see many examples in the future in which data are judged sufficient to replace the default UFs in just the way we have described. 

In fact this same approach had been widely accepted, at least implicitly, prior to the FDA s actions. The methods long in use to establish ADIs for threshold agents cannot, as we have seen, guarantee absolute safety, even though they may appear to do so, and this conclusion holds for the more refined, but similar, method we have described for establishing RfDs and other health protective limits. 

The FDA also regulates food additives - substances, such as antioxidants, emulsifiers and non-nutritive sweeteners, that are intentionally and directly added to food to achieve some desired technical quality in the food. As noted, the Delaney clause prohibits the deliberate addition to food of any amount of a carcinogen. These additives, if they are threshold agents (not carcinogenic), can be allowed as long as the human intake does not exceed a well-documented ADI. Those who would seek approval for an additive need to supply the FDA with all of the toxicity information needed to establish a reliable ADI, and all of the product-use data that would permit the agency to assure itself that the ADI will not be exceeded when the additive is used. 

Early antisealants used sodium hexametaphosphate (SHMP) as a threshold agent to inhibit the growth of calcium carbonate and sulfate-based scales.6 Most antisealants on the market today contain sulfonate, phosphate, or carboxylic acid functional groups. Perhaps the most effective antisealants today contain and blend of polyacrylic acid (PAA) and phosphoric acid or polyacrylate and a hydroyethylidene diphosphonate (HEDP).12 The polyacrylate-HEDP blends also claim to have good dispersion qualities toward silts and clays.12 Some new inhibitors include a chelant and disper-ant to keep suspended solids such as iron and manganese oxides in solution. These newer antisealants are generally more effective than SHMP for a variety of potential scales.6... 

Threshold agent risks - the acceptable daily intake... 

The conventional chemicals used for the prevention or limitation of scale formation (the deposition of inverse solubility salts) include threshold agents, crystal modifiers, dispersants, and surfactants. 

Threshold agents, in relatively small amounts, have the ability to inhibit or retard the precipitation of alkaline salts and scaling compounds. Even if the precipitation is not prevented, the action of the chemical additives often produces a softer sludge-like precipitate, which is more easily removed from the heat-transfer surface. Polyphosphate chain polymers such as hexa-metaphosphate have been used to control CaCOs... 

More conventional chemical treatments (sometimes referred to as inhibition) use one or more of four groups of additives that includes threshold agents, crystal modifiers, dispersants and surfactants. 

Threshold agents have the ability, when added in very small concentrations, to inhibit the precipitation of alkaline salts and other scaling compounds on a non-stoichiometric basis. Reductions in the potential for precipitation of calcium salts, iron and manganese can be achieved. In certain cases the action of the additive includes the formation of a sludge rather than a scale. Although still a potential foulant, the sludge is more easily removed from the system. Sequestering additives... 

These authors demonstrate the effectiveness of a threshold agent (aceto diphosphonic acid) and a crystal modifying additive (polymaleic acid) Fig. 14.4 shows the plot of fouling resistance with time and it will be evident that the asymptotic fouling resistance has been reached in each example. The fouling resistance in the presence of both these additives is significantly less than that obtained with no inhibitor present. 

It is of interest to gauge performance of additives against the Ryznar Index for the water. Fig. 14.7 shows the effect of the threshold additive acetodiphosphonic acid and the crystal modifier polymaleic acid at different Ryznar indices. As would be expected from the definition of the Ryznar Index, the scaling is increased as the index is reduced when there is no additive present. In the presence of the threshold agent the rate of scaling is also reduced but below a Ryznar Index of about 4.5, the threshold effect is exceeded and the scaling rate rapidly rises. 

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. 

Sodiumhexametaphosphate (SHMP) is a threshold agent derived from the dehydration of orthophosphoric acid or its sodium salt. It is used to inhibit the formation of calcium carbonate and metalHc sulphate scale. It is mosdy widely used because it offers good inhibition at a low cost. Depending on the concentration of calcium and sulphate, and depending on the CF (Equation 2.31), the dosage is in the range of 2-5 ppm. SHMP can prevent calcium sulphate precipitation up to 150% of the saturation limit. Organopho-sphonates are an improvement over SHMP since they are more resistant to hydrolysis, but are more expensive. They offer scale inhibition and dispersion abihty similar to SHMP [14,40,41]. 

Yet another variable that cannot be ignored is the capacity and shape of a crystallizer. Heat transfer from a system will depend upon the quantity of melt being crystallized, the surface-to-volume ratio, the temperature gradient between the melt and its environment, the heat of crystallization of the crystals, and the viscosity of the melt, to mention but a few that could be considered. Impurities leached from a container can also have great influence on the behavior of a system. Some substances can act as threshold agents in melts, just as polyphosphates act as threshold agents in aqueous solutions, preventing the crystallization of scale in boilers, sewers, etc. 

The future of phosphate fibers should be very bright when the patents on the composition expire in 1999. In the author s mind there has never been a safety issue with phosphate fibers or any other inorganic phosphate that does not contain a toxic cationic or anionic function. Phosphates such as sodium, potassium, calcium, magnesium, ammonium, and hydrogen have never caused a problem of any type with perhaps the exception of renal stones. In this case it may be more of a condition than the cause of a condition. Urine is known to be saturated with respect to calcium phosphates. Some threshold agent or other mechanism that is poorly understood keeps kidneys from becoming completely calcified even in the healthiest of persons. 

---