Wastewater Glossary: Flocculation
They are mainly divided into two categories: iron-based series and aluminum-based series, and of course also include their polymeric derivatives. There are many types of flocculants, whose common feature is the ability to aggregate suspended particles in a solution into coarse flocs or clumps.
Common flocculants in water-treatment engineering include: aluminum sulfate (alum), polyaluminum sulfate (poly aluminium sulfate), tannin extract, and so on.
Properties: gray-white powder or orthorhombic crystalline powder; flowing pale-yellow powder. Light-sensitive. Easily deliquescent. Dissolves slowly in water but faster when trace ferrous sulfate is present; slightly soluble in ethanol, almost insoluble in acetone and ethyl acetate. Hydrolyzes slowly in aqueous solution. Relative density (d18) 3.097. Decomposes at 480 °C. Commercial product usually contains about 20% water and appears pale yellow; some forms contain 9 molecules of water of crystallization, relative density 2.1, losing 7 molecules of water of crystallization at 175 °C.
Uses: 1) for silver analysis and sugar assay; as a dye, ink, water clarifier, aluminum engraving agent, disinfectant, polymerization catalyst, etc. 2) analytical reagent, sugar assay, iron catalyst, mordant, water-treatment coagulant, pigment, and pharmaceutical. 3) in water treatment as a coagulant for purification and a sludge-treatment agent. 4) used as a mordant and coagulant for industrial wastewater, also in pigments. 5) in medicine ferric sulfate is used as an astringent and hemostatic. 6) used in electrolytes for Zn-Ni-Fe and Zn-Fe-Co alloy plating.
Properties: extremely soluble in water; aluminum sulfate cannot dissolve in pure sulfuric acid (it merely coexists), and in sulfuric acid solution it dissolves in water together with the acid, so its solubility in sulfuric acid equals its solubility in water. At room temperature it crystallizes with 18 molecules of water as aluminum sulfate octadecahydrate, which is mostly what industry produces, containing 51.3% anhydrous aluminum sulfate and not self-dissolving even at 100 °C (it dissolves in its own water of crystallization). It does not easily effloresce and lose water of crystallization, is relatively stable, loses water on heating, and at high temperature decomposes into alumina and sulfur oxides. Heating to 770 °C begins decomposition into alumina, sulfur trioxide, sulfur dioxide, and water vapor. Soluble in water, acids, and alkalis; insoluble in ethanol. Aqueous solution is acidic and hydrolyzes to aluminum hydroxide; prolonged boiling yields basic aluminum sulfate. The industrial product is gray-white flake, granular, or lumpy, pale green due to low-iron salts and yellowing on the surface as ferrous salts are oxidized. The crude product is a gray-white fine crystalline porous mass. Non-toxic, but dust irritates the eyes.
Functions: 1) in the paper industry as a sizing agent to enhance water resistance and impermeability; 2) in water it causes fine particles and natural colloids to coagulate into large flocs and thus be removed, so it is used as a coagulant for water supply and wastewater; 3) as a turbidity-removal agent, also as a precipitant, fixing agent, and filler; in cosmetics as an antiperspirant raw material (astringent); 4) in fire protection, with baking soda and a blowing agent, as a foam fire-extinguishing agent; 5) analytical reagent, mordant, tanning agent, grease decolorizer, and wood preservative; 6) stabilizer for albumin pasteurization (including liquid or frozen whole egg, egg white, or yolk); 7) as a raw material for synthetic gemstones and high-grade ammonium alum and other aluminates; 8) in the fuel industry, as a precipitant in the production of chrome yellow and lake dyes, also serving as a color-fixing and filling agent.
Polyferric sulfate is a pale-yellow amorphous powdery solid, extremely soluble in water; a 10% (by weight) aqueous solution is a red-brown transparent liquid and is hygroscopic. Polyferric sulfate is widely used for purification of drinking water, industrial water, various industrial wastewaters, municipal sewage, and sludge dewatering.
Usage and precautions: because raw water varies, site commissioning or beaker tests should be performed for different conditions to determine the optimal operating conditions and dosage for the best treatment effect.
1) Before use, add the product at a certain concentration (10–30%) to an alum-dissolving tank, inject tap water and stir for full hydrolysis, let stand until a red-brown liquid forms, then dilute with water to the required concentration for dosing. Water plants may also prepare a 2–5% solution for direct dosing; for industrial wastewater treatment prepare a 5–10% solution for direct dosing.
2) Dosage determination: based on raw-water properties, the appropriate amount can be set via production commissioning or beaker tests by observing floc formation; water plants may refer to the dosage of other chemicals previously used. Under the same conditions this product's dosage is roughly equivalent to that of solid polyaluminum chloride and is 1/3–1/4 of that of solid aluminum sulfate. If a liquid product was previously used, calculate accordingly from the corresponding chemical concentration; roughly a weight ratio of 1:3 applies.
3) During use, pump the prepared solution into a metering tank and dose it to coagulate with the raw water via the metering device.
4) Generally prepare on the day of use; prepare with tap water; slight sediment is normal.
5) Pay attention to the hydraulic conditions and floc formation in the three stages of coagulation. (1) Coagulation stage: the rapid mixing of dosed chemical with raw water in the coagulation tank to form fine flocs in a very short time; the water becomes more turbid and requires intense turbulence. In beaker tests, stir rapidly (250–300 rpm) for 10–30 s, generally no more than 2 min. (2) Flocculation stage: flocs grow and coarsen, requiring moderate turbulence and sufficient residence time (10–15 min); later, large amounts of flocs gather and slowly settle, forming a clear surface layer. In beaker tests, stir at about 150 rpm for ~6 min, then at 60 rpm for ~4 min until suspended. (3) Settling stage: floc sedimentation in the settling tank, requiring slow flow; to improve efficiency, inclined-tube (or plate) settlers are generally used (dissolved-air flotation for floc separation is best), where many coarse flocs are blocked by the tube/plate walls and deposited at the bottom, the top water is clear, and the remaining small, low-density flocs slowly descend while continuing to collide and grow until the residual turbidity stabilizes. Beaker tests: slowly stir at 20–30 rpm for 5 min, then still-settle for 10 min and measure residual turbidity.
6) Enhanced filtration: mainly the rational selection of filter-bed structure and filter aids to raise filter removal efficiency; an important measure for improving water quality.
7) This product is used in environmental protection and industrial wastewater treatment with methods largely the same as in water plants; for raw water with high color, high COD, and BOD, aided by auxiliary agents it works very well.
8) For enterprises using chemical coagulation, existing equipment needs no major modification; only an alum-dissolving tank need be added to use this product.
9) This product must be stored in a dry, moisture-proof, and heat-avoiding place (< 80 °C); do not damage the package; the product can be stored long term.
10) This product must be dissolved before use; dissolution and dosing equipment should be made of corrosion-resistant materials.
Flocculation—from a process perspective—is the process by which floc particles aggregate into large flocs and settle through adsorption, cross-linking, and enmeshment.
Flocculant—from a chemical perspective—is an agent that causes colloids and suspended particles to coagulate and flocculate, and is therefore also called a coagulant.
Coagulation—from a mechanism perspective—is the process by which compression of the electric double layer of colloids and dispersed systems, destruction of the ζ-potential, and charge neutralization destabilize and aggregate them into floc particles.
Coagulation (overall process)—destabilization (凝聚) and flocculation (絮凝) together are collectively called 混凝, i.e., the complete coagulation process.
Inorganic flocculants mainly include inorganic low-molecular flocculants and inorganic polymeric flocculants. The most applied and studied are inorganic polymeric flocculants, chiefly iron-based and aluminum-based inorganic polymeric flocculants. The most used inorganic polymeric flocculants are polyferric sulfate (PFS) and polyaluminum chloride (PAC).
Polyaluminum ferric sulfate is prepared from ferrous sulfate as raw material and sodium nitrate as catalyst, and works well on oily wastewater. Polyferric aluminum chloride sulfate is prepared from bauxite, active calcium silicate, hydrochloric acid, sulfuric acid, etc., combining the excellent properties of both polyferric and polyaluminum types, and treats oily wastewater better than polyaluminum chloride. Composite inorganic polymeric flocculants not only combine the excellent properties of polyaluminum and polyferric inorganic polymeric flocculants, but also feature a higher degree of polymerization and higher basicity.
Composite inorganic polymeric flocculants are a class of inorganic flocculants, mainly including polyaluminum ferric chloride (PAFC), polyaluminum ferric sulfate (PAFS), and polyaluminum ferric sulfate chloride (PAF-CS). PAFC is an aluminum-dominated, iron-assisted composite inorganic polymeric flocculant that combines the dual characteristics of aluminum and iron salts, and its coagulation performance usually surpasses that of polyaluminum chloride and ferric chloride. PAFC treating oilfield produced water achieves oil-removal and suspended-solid removal rates above about 90%, meeting oilfield reinjection requirements.
Polyferric sulfate has the advantages of strong flocculation, fast settling, wide application range, low corrosiveness to metal equipment, and low secondary pollution, but it produces more sludge and has a complex process and higher cost; polyaluminum chloride treating oily sewage has markedly better flocculation than other aluminum salts and usually better than polyferric flocculants, mainly showing lower dosage and less sludge, but its drawback is slower flocculation-settling speed, which can easily cause secondary pollution.
