A Tool to Help Meet Stricter Standards and Improve Operations
As water treatment plants strive to meet stricter standards, many are turning to aluminum chlorohydrate (ACH) to improve finished water quality, control costs and enhance efficiency. This specialized coagulant also can help reduce chemical solids and alkali usage while helping plant operations.
A Tool to Help Meet Stricter Standards and Improve Operations
Turbidity goals under the EPA Partnership for Safe Water are a good example of basic changes in the water community during the 90s. These goals require less than 2 NTU turbidity for settled water and less than 0.1 NTU from each filter at all times. Plants initially increased alum or ferric dosage in striving to attain these levels, but this strategy often fell short and led to higher cost, especially for sludge disposal and alkali demand. Many plants then found they could gain the results they needed at reasonable cost with enhanced inorganic coagulants like ACH.
Bridging the gap
ACH combines many of the best features of inorganic and organic coagulants. In a very real sense, it bridges the performance gap between organic polymers and traditional metal-based coagulants like alum and iron salts.
Bridging the gap
ACH offers a number of features that set it apart from polyaluminum chloride (PACl) and alum. It is the most concentrated soluble aluminum-based coagulant available and has the highest basicity. Basicity describes the amount of hydroxide (or OH- groups) associated with the aluminum aggregates. It is defined by the equation: Basicity = [OH]/(3 x [Al]). High basicity products have a higher positive charge and are more efficient in coagulating negatively charged contaminants.
Bridging the gap
ACH contains 23 percent to 24 percent Al2O3, or more than 12 percent aluminum metal, and has a basicity of 82 percent to 85 percent.
Bridging the gap
By contrast, alum has 8.3 percent Al2O3 on a liquid basis and no basicity, while PACl usually has about 10 percent Al2O3 and a basicity of about 50 percent. Since ACH contains more metal, its dosage often is half that needed for PACl and about one-third that for liquid alum.
Bridging the gap
This dosage decrease with ACH is also due to its charge characteristics. Some of the aluminum metal in ACH is more cationic (more positively charged) than that in alum or PACl. The increased charge lets ACH work at lower metal dosages, further lowering chemical solids generation and aluminum residual levels in finished water.
Bridging the gap
ACH also needs less alkalinity (OH- ions) in the raw water than PACl or alum, because of its high basicity, i.e. it has a high OH content since its theoretical formula is Al2(OH)5Cl. As a result, ACH, like organic polymers, can be used in both high- and low-alkalinity waters.
Bridging the gap
The following points illustrate some of the properties and uses of ACH.
Bridging the gap
- ACH forms a stronger, denser floc that settles faster than the fluffier flocs of traditional, inorganic coagulants. This is especially important in winter when water is colder and more viscous. Many plants encounter problems with alum at such times as hydrolysis reactions slow and less dense floc carries over to the filters. One plant using alum had severe postfilter precipitation problems in cold water. Although turbidity was 0.22 NTU just after the filters, turbidity at the plant outlet, after water had passed through the clearwell, was 0.46 NTU. The problem was eliminated with ACH, which reduced filtered and finished water turbidity below 0.1 NTU.
Bridging the gap
- ACH helps treat settled waters in conventional plants as a filter aid, an application that is similar to that in direct filtration plants. Use of ACH also can benefit plants using organic polymer filter aids. Problems with organic polymers arise because they do not penetrate the filter bed deeply and can stick to the upper surface, leading to filter fouling, mud balling and shorter run lengths.
Bridging the gap
- Where chemical solids disposal is more expensive, treatment facilities can use ACH to reduce solids. One plant using 75 ppm of liquid ferric sulfate had generated 144 lb. of chemical solids/MG. After switching to ACH, it reduced chemical solids to 73 lbs./MG. In addition, the solids generated by ACH dewatered more efficiently, further decreasing handling and disposal costs.
Bridging the gap
- ACH removes particles efficiently because its cationic charge density is higher than that of other inorganic coagulants. As an enhanced coagulant, it generally removes colloids and particles less than 10m, a size range that encompasses bacteria and parasites, and is the most troublesome portion of the fine fraction.
Bridging the gap
- ACH has the least affect on treated water alkalinity levels of any inorganic coagulant, so it decreases or eliminates pH depression and the corresponding need for alkali. The higher alkalinity also makes the finished water significantly less corrosive. One plant with a low alkalinity water source found that ACH increased settled water alkalinity from 30 ppm to 34 ppm, while alum lowered it to 15 ppm. Another plant using 50 ppm of ferric sulfate for clarification, had to add 46 ppm of soda ash to stabilize the finished water and make it unlikely to promote corrosion. After it switched to ACH for clarification (at a dose of 11 ppm), it dropped soda ash dose to 6 ppm.
Bridging the gap
- ACH is especially adept at handling variable waters in which turbidity, particles, organics and other parameters change rapidly. Such waters can challenge other metal-based coagulants. One plant facing a wide range of water conditions evaluated ferric sulfate and ACH during two comparable large raw water turbidity excursions. Ferric sulfate dosage rose from 80 ppm to 130 ppm, while ACH went from 37 ppm to 45 ppm. ACH gave somewhat lower settled water turbidity and about 60 percent lower filtered water turbidity than ferric sulfate. ACH had a faster response time, because of its smaller dose. Caustic demand was nearly 90 percent less with ACH than with ferric sulfate, improving operations and reducing cost. ACH also cut chemical sludge almost in half during the high turbidity episode when chemical demand was high.
Bridging the gap
- ACH offers substantial advantages for phosphorus removal. As the most concentrated form of aluminum, it takes much less ACH on a volume basis to remove phosphorus. Theoretically, 0.87 Al metal is needed to remove 1 part of phosphorus. This translates to 20 ppm of liquid alum or 7.25 ppm of ACH per ppm of phosphorus. Since coagulants treat waters that have both phosphorus and suspended solids, the one that removes both most effectively will benefit the overall operation most.
Bridging the gap
- ACH may be effective for total organic carbon (TOC) removal at many plants. Even though it does not lower pH as do other metal-based coagulants, it adds high amounts of a highly charged metal that can lower TOC levels.
Bridging the gap
- As a co-coagulant with alum or iron salts, ACH gives plants the option to use a familiar coagulant along with one designed to improve many operating factors. For instance, if a turbidity problem arises or caustic costs are high, operators can cut traditional coagulant dosage significantly by adding ACH. One plant lowered liquid alum from 40 ppm to 20 ppm by adding less than 5 ppm of ACH, which significantly decreased caustic feed rate and chemical solids.
Guidelines for use
Alum and ACH react to form a precipitate. When switching between these coagulants, it is critical to clean tanks, lines and pumps. If ACH is used as a co-coagulant with alum, ACH should be fed through a different line so the two do not mix before addition. Plants also should make sure pumps used with alum can handle the lower volumes involved with ACH.
Guidelines for use
Potable water plants should look for ACH manufacturers that ensure purity with good quality control procedures. Purity is often affected by the raw materials used. For example, ACH made from industrial by-products may contain heavy metals or volatile organic compounds that can appear in the finished water or chemical solids. Almost as important as product quality is the expertise suppliers can make available to help guide water plants as they change their coagulant program.
Conclusion
ACH offers a highly efficient approach to coagulation that provides strong cost and operational efficiencies while producing high water quality. It provides many benefits, including less pH reduction, sludge generation and alkalinity removal than other metal-based coagulants.
Conclusion
As the most versatile of aluminum-based coagulants, it works just as well in Great Lakes water as in Mississippi River water, low or high alkalinity waters, cold and warm waters, and waters that have high particulates or that are highly variable. It also can be used a primary coagulant, co-coagulant, or filter aid.
Conclusion
About the Author: Dr. Karen Ruehl is manager of enhanced coagulants at General Chemical. She oversees all sales and marketing activities for the companys polyaluminum hydroxychloride, alum/polymer and acid/alum products, and supervises its staff of market development engineers.
