Antiscalants

Pretreatment (e.g., filters, softeners, or antiscalents) and posttreatment (permeate flush and clean-in-place systems)... 

Acid addition is commonly used to convert bicarbonates into the more soluble sulfate salts to reduce the alkalinity of the RO RW, which in turn modifies the brine reject water LSI. Sometimes it is required to maintain the pH level within membrane limits. Additionally, it may be used in conjunction with a reduced dosage of antiscalent chemical to reduce the overall chemical treatment costs. 

Duty Typically, organic antiscalents supplied for RO applications are... 

A premium antiscalent product in this group is Flocon 100 from Great Lakes Chemical Corporation (originally the brand was owned by Pfizer, then Ciba-Geigy, then FMC). Flocon 100 is a 35% w/w, 2,000 MW acrylic acid polymer. An alternate and much more concentrated product is Good-Rite K-752, a 62.5% w/w, 2,100 MW acrylic acid polymer from Noveon, Inc. (formerly BF Goodrich Company). 

A premium antiscalent product in this group is Belros 285 from Great Lakes Chemical Corporation (originally the brand was owned by Ciba-Geigy, then FMC). Belros 285 is a 35% w/w, neutralized PMA-based product. 

Deposit control agents (DCAs) or antiscalents to inhibit the deposition of CaC03 and other alkaline earth salts. 

PAAs with a MW of approximately 2,000 to 3,000 are useful as general-purpose antiscalents. Examples include Polacryl A40-43, Good-Rite K-752, Acumer 1000, Aquatreat AR-900, and Sokalan PA20. 

PMA is useful as a calcium sulfate inhibitor and more recently has been promoted as an antiscalent for control of high-silica waters, countering newer silica control terpolymers (such as Acumer 5000). 

Typically, organic antiscalents supplied for RO applications are 35% w/v active materials and dosed at 2.5 to 6 ppm chemical product (dependent on the composition of the source water analysis). Some chemical products are available at up to 50% w/v activity, but they require a proportionally lower dose rate. Higher strength chemicals are often extremely viscous, which may present pumping problems. 

In addition, modem chemical polymer treatments tend to have wider applications than before, so, for example, many deposit control agents are also antifoulants as well as antiscalents, which only serves to confuse the issue. 

Polyacrylates with MW of 1500 to 2000 tend to be useful as CaCO antiscalents under severe operating conditions. 

Polyacrylates with MW of 2000 to 3000 tend to be useful as general-purpose antiscalents and can often operate in high calcium or brine waters. 

PMA homopolymer is also available as a neutralized salt and in several grades, often with precise molecular weight distributions, for special applications such as antiscalent duty in seawater distillation and sugar evaporator processes. Maleic anhydride chemistry has also been successfully developed to provide functional components in copolymers [examples are acrylic acid, maleic anhydride (AA/MA) and sulfonated styrene, maleic anhydride (SS/MA)] and terpolymers [example is maleic anhydride, ethyl acrylate, vinyl acrylate (MA/EA/VA)]. 

The test kit is based on immunoassay techniques and the method takes about 10 minutes to provide an analysis. A unique tag, permanently attached to the polymer, changes the color of special test strips exposed to ppm levels of polymer. The strips indicate the amount of tagged polymer that is present, without interference from other additives and contaminants. This technology is not currently available for continuous inhibitor detection, but given the importance of AH Organic Programs, there is little doubt that further developments will take place. The Water Additives Division of Great Lakes Chemical Corp. have also recently introduced a similar simple and accurate immunoassay test for the detection of Belclene 200 antiscalent. 

The duration of each run is normally 6 to 8 hours, although several longer runs of up to 50 hours duration have been made with effective antiscale additives. At the end of a run, the feed water is turned off and tap water is allowed to flow through the evaporator while evaporation is still proceeding. Steam is then turned off and the apparatus allowed to cool, thus ensuring that the evaporator tube is never allowed to run dry while still hot. The tube is removed when cool and cut in half, lengthwise, while still wet, to enable the scale to be examined. 

A new additive, low molecular weight polyacrylic acid, has shown promise as an antiscale additive. It is suggested that scale prevention in this case is due to protection of the surface by a polymeric film, which alternately breaks away from the surface and regenerates, resulting in only slight resistance to heat flow. 

Phosphate (P04 ) forms a low solubility salt with calcium. Water containing phosphate generally requires the addition of an antiscalant. 

For RO applications, a positive LSI or SDSI indicates that the influent water has a tendency to form calcium carbonate scale. In these cases, pretreatment in the form of softening (either with lime or ion exchange), or via the use of antisCalants and/or acid is required. 

Precipitation of bicarbonates slight but significant of the quality of the antiscale treatment. It is measured by the M alk whose increase does nor match the rise in chlorides. 

Stabilization processes with scale inhibitors therefore lead to control of the formation of the protective calcium carbonate layer. Antiscale-dispersing agents can in fact prevent unwanted deposits effectively. But the calcium carbonate protective film can be very... 

In an attempt to reduce corrosion rates, additional elements have been included in antiscale formulations. 

Zinc is a cathodic corrosion inhibitor. At usual antiscale concentrations and routine free pH values of>8.5, the zinc content in the system will remain lower than 1 ragT, whatever the amount added (too much excess zinc can give rise to extra sludge). 

The stabilization process with antiscale-dispersant inhibitors will therefore be used. 

Here it is recommended to remove the iron from the water by aeration-filtration possibly followed by neutralization of the excess CO2 and then by antiscale-dispersant conditioning. 

Possible use of antiscale dispersants (polyphosphates, phosphonates, acrylates). 

Average price for an antisealant in 2014 nms about 4.00 per pound. At a dosage of 10.2 ppm, the dally cost for antisealant is about 72.00 or about 2,200 per month. With acid feed, the antisealant requirement is about 905 per month. Sulfuric acid, at about 700 per ton, and 3.4 ppm feed rate, would add about 64 per month. Monthly total for the antisealant only option is about 2,400, while that for the acid/antiscalant option is about 970. 

For this high-hardness case, the softener and antisealant options appear to be about the same when only antisealant (no acid) is used. When acid is feed in conjunction with the antisealant, the lower cost option is the chemical one when comparing cost of salt versus cost of antiscalant/acid. Note that any local variation in cost of either salt or antisealant can change the results. What is not accounted for is the affect a softener has on the ability to remove suspended solids and SDI out of feed water, such as any iron or manganese that may be in the well water. If the water were to contain soluble iron and manganese, the softener would remove these species... 

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