PVS Technologies Technical Bulletin

　　　　　It is a very difficult to looking for hygienic drinking water in the global cities before the nineteenth century.

　　　　　At that time, the drinking water sources are mixed with human waste, water frequently causes disease and death. the middle of the nineteenth century, to realize people's health are associated with drinking water which is clean or not, so, in order to ensure the health of people, began to try to create a water purification method. In the 20th century, water treatment methods of the United States, Canada, the Nordic countries have been perfect, water quality is good, and the method is successful standardized.

　　　　　The United States congress passed <<the safe drinking water act>> in 1974,. This act was revised in 1986, as a result, drinking water quality standards and water treatment method got fundamental improvement. Water treatment workers realize that originally effective water treatment methods have not met the requirements of the modern. All operation of waterworks need to be revaluated. in order to meet the new requirements of water quality, correct selection of coagulant is a key problem, ferric chloride became the center topics of discussion.

       　For drinking water treatment industry, ferric chloride is not a new thing, the United States has this coagulant since the 1930 s. however, in recent 15 years, to deal with drinking water with ferric chloride is the consensus of everyone, because the efficiency of treated water is good, the coagulant has good applicability, treated water quality is good. Since 1986, either the muddy water purification, or to eliminate　the color of the raw water, natural organic matter and arsenic, using ferric chloride has become a trend as a coagulant.

       　Ferric chloride is an important water treatment agent. It is a aqueous solution, produced by chlorine oxidation of ferrous chloride. Its prominent feature is the quality purity, ferric content is high. Its real features are that it not only has the function of coagulant to remove impurities in water, and has flocculation function of coagulant aid, so it has the versatility.

       　Ferric chloride react with hydrogen sulfide (H2S), phosphate (PO4), arsenate (AsO4), and hydroxyl basicity (OH) which in water to generate sediment. In drinking water treatment, however, the main purpose of ferric chloride is that products generated by ferric chloride reaction with hydroxyl basicity have the effect of flocculant and coagulant aid.

　　　　　Ferric chloride react with hydroxide alkalinity to generate a variety of hydrolysate in water, then combined into Fe(OH)3. The hydrolysate with a lot of positive charge able to neutralize the negative charge on the colloidal particles, and combine with negatively charged particles and Ferric hydroxide. Due to the combining capacity, ferric chloride has flocculating capacity and form alum flowers.

         Hydrolysis products of ferric chloride (ferric hydroxide) is different from hydrolysis products of ferric sulfate, aluminum sulfate, and sulphate etc, in terms of physical properties, alum flowers particles of ferric chloride has strong discreteness, more denser, with more positive charge. On the contrary, alum flowers particles generated by hydrolysis of ferric sulfate, aluminum sulfate has weak discreteness, like a loose wool cloth with soft nap or floating clouds. Obviously, this situation is caused by the combination of hydrolysis product different form. This difference leads to differences of characteristics and functions from ferric chloride and sulfate coagulants. Water plant want to obtain the same water treatment effect, ferric chloride dosing quantity reduces 30% compared with aluminum sulfate (according to no water content weight).

         The alum flowers of ferric chloride are discrete and dense, so precipitation is fast, and is well in low temperature water. This dense alum flowers with more positive charge, strong combine with colloidal particles in water. Due to the ratio that the charge on the ferric chloride hydrolysis products compare with its mass is large, so its effect and adsorption ability against organic matter of the emulsion and half emulsion in water are strong (such as oil, fat and other natural and synthetic organic compounds).so ferric chloride has strong ability to remove total organic carbon and precursors of disinfection by-products in water.

         Alum flowers generated by ferric chloride are heavy, has a benefit for waterworks, coagulant precipitation sludge volume of ferric chloride is only 1/3 to 2/3 of the sludge volume of sulfate coagulant sedimentation, and easy to dehydration. So, although the molecular weight of ferric chloride is greater than three aluminum hydroxide, but it doesn’t produces more sludge. Instead, because the sludge quantity is calculated in the wet sludge, so the weight of ferric chloride sludge is less, this way, the fee for handling of dry sludge dehydration is less.

         Another characteristic of ferric chloride is that it can form alum flowers in a wide pH range, compared with three aluminum hydroxide, the solubility of three iron hydroxide is very low. Ferric chloride coagulant with these features, so its applicable pH range is very wide, and does not happen that a lot of ferric was took away from the clarification process by treated water to caused lag precipitation phenomenon. this point has a very important significance in the coagulation process to improve water pH to inhibit the corrosion of tap water. Lower limit value of ferric chloride applicable pH range is small, it is also very important to improve the scope of application of coagulant. Although there is no formal data for ferric chloride used as filter aid, but a lot of running data show whether the sand filter is slow filter or filter, after using ferric chloride coagulant can greatly improve the effect of filter to turbidity. there are a number of recent reports, ferric which cover on the sand can be used to remove manganese in the water.