Wastewater Treatment Terminology: Flocculation
They are mainly divided into two major categories: the iron-based series and the aluminium-based series, and of course also include their associated high-polymer series. There are many varieties of flocculants, whose common characteristic is the ability to aggregate and link suspended particles in a solution to form large flocculent clumps or masses.
Flocculants commonly seen in water-treatment engineering include: aluminium sulfate (alum), polyaluminium sulfate (poly aluminium sulfate), tannin extract, etc.
Properties: grey-white powder or orthorhombic crystalline powder, free-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. Hydrolyses slowly in aqueous solution. Relative density (d18) 3.097. Decomposes at 480 C. Commercial product usually contains about 20% water and is pale yellow. Some contains 9 molecules of water of crystallisation. Relative density 2.1. Loses 7 molecules of water of crystallisation at 175 C.
Uses: 1. For silver analysis and sugar determination; used as a dye, ink, water purifier, for aluminium engraving, disinfection, polymerisation catalyst, etc. 2. Analytical reagent, sugar determination, iron catalyst, mordant, water-purification agent, pigment and pharmaceutical manufacture. 3. In water treatment, used as a coagulant for water purification and a treatment agent for sludge. 4. Used as a mordant and coagulant for industrial wastewater, and also in pigments. 5. In medicine, ferric sulfate is used as an astringent and haemostatic. 6. Used in electrolytes for zinc-nickel-iron alloy and zinc-iron-cobalt alloy plating.
Nature: extremely soluble in water. Aluminium sulfate cannot dissolve in pure sulfuric acid (it merely coexists); in sulfuric acid solution it dissolves in water together with the acid, so the solubility of aluminium sulfate in sulfuric acid is the same as its solubility in water. At room temperature it crystallises out containing 18 molecules of water of crystallisation, i.e. octadecahydrate aluminium sulfate, which is mostly what is produced industrially. It contains 51.3% anhydrous aluminium sulfate and will not self-dissolve even at 100 C (dissolving in its own water of crystallisation). It does not easily weather and lose water of crystallisation, is relatively stable, loses water on heating and decomposes into alumina and sulfur oxides at high temperature. Heating to 770 C begins decomposition into alumina, sulfur trioxide, sulfur dioxide and water vapour. Soluble in water, acids and alkalis, insoluble in ethanol. Aqueous solution is acidic. Hydrolyses to form aluminium hydroxide. Prolonged boiling of the aqueous solution can form basic aluminium sulfate. Industrial product is grey-white flaky, granular or lumpy; with a light green tint due to low-iron salts, and the surface yellows due to oxidation of the low-valent iron salts. Crude product is a grey-white fine-crystalline porous mass. Non-toxic, but dust can irritate the eyes.
Functions: 1. In the paper industry, used as a paper-sizing agent to enhance the water resistance and impermeability of paper; 2. After dissolving in water, it causes fine suspended particles and natural colloidal particles in water to coagulate into large flocculent masses and thus be removed from water, so it is used as a coagulant for water supply and wastewater; 3. Used as a turbid-water purifier, and also as a precipitant, colour-fixing agent and filler. In cosmetics it is used as a raw material for antiperspirants (astringent); 4. In the fire-fighting industry, together with baking soda and a foaming agent it forms foam fire-extinguishing agent; 5. Analytical reagent, mordant, tanning agent, oil decolouriser, wood preservative; 6. Stabiliser for albumin pasteurisation (including liquid or frozen whole egg, egg white or egg yolk); 7. Can be used as a raw material for manufacturing artificial gemstones and high-grade ammonium alum and other aluminates; 8. In the fuel industry, used as a precipitant in the production of chrome yellow and lake dyes, while also acting as a colour-fixing and filling agent.
Polyferric sulfate is a pale-yellow amorphous powdered solid, extremely soluble in water; a 10% (by weight) aqueous solution is a red-brown transparent solution and is hygroscopic. Polyferric sulfate is widely used for the purification treatment of drinking water, industrial water, various industrial wastewaters, municipal sewage, sludge dewatering, etc.
Usage method and precautions: because raw-water properties vary, site commissioning or beaker tests should be carried out according to different situations to obtain the best operating conditions and optimum dosage for the best treatment effect.
1. Before use, put this product into the alum-dissolving tank at a certain concentration (10-30%) and add tap water, stirring to allow full hydrolysis; let stand until it becomes a red-brown liquid, then dilute with water to the required concentration before dosing for coagulation. Waterworks may also prepare a 2-5% solution for direct dosing; for industrial-wastewater treatment prepare a 5-10% solution for direct dosing.
2. Determination of dosage: depending on raw-water properties, it can be determined by production commissioning or beaker tests based on the appropriate amount for floc formation; waterworks may use the dosage of other agents previously used as a reference. Under the same conditions, the dosage of this product is roughly equivalent to that of solid polyaluminium chloride, and is 1/3-1/4 of the dosage of solid aluminium sulfate. If a liquid product was originally used, it can be determined by calculation based on the corresponding agent concentration. It is roughly set at a weight ratio of 1:3.
3. In use, pump the above-prepared agent solution into the measuring tank and dose it by metering to coagulate with the raw water.
4. Generally, prepare on the day of use and use on the same day; tap water is needed for preparation, and a slight precipitate is normal.
5. Pay attention to the hydraulic conditions and floc-formation state in the three stages of the coagulation process. (1) Coagulation stage: the process in which the agent is injected into the coagulation basin and rapidly coagulates with the raw water to form fine flocs in a very short time; the water becomes more turbid, requiring intense turbulence. In beaker tests, stir rapidly (250-300 rpm) for 10-30 s, generally not exceeding 2 min. (2) Flocculation stage: the process in which flocs grow and coarsen, requiring moderate turbulence and sufficient retention time (10-15 min); toward the later stage a large number of flocs can be observed gathering and slowly sinking to form a clear surface layer. Beaker test: first stir at 150 rpm for about 6 min, then at 60 rpm for about 4 min until a suspended state is reached. (3) Settling stage: the settling of flocs carried out in the settling basin, requiring slow water flow; to improve efficiency inclined-tube (plate) settling basins are generally used (preferably flotation to separate the flocs), where large coarse flocs are blocked by the inclined tubes (plates) and deposited at the bottom, the upper water being clarified; the remaining small, low-density flocs slowly descend while continuing to collide and grow, and toward the later stage the residual turbidity is essentially unchanged. Beaker test: slowly stir at 20-30 rpm for 5 min, then let settle statically for 10 min and measure the residual turbidity.
9. This product must be stored in a dry, moisture-proof and heat-avoiding place (< 80 C); do not damage the packaging, and the product can be stored for a long time.
7. This product, when applied to environmental and industrial wastewater treatment, is used in much the same way as in waterworks; for raw water with high colour, high COD and BOD, the effect is excellent with the aid of auxiliary agents.
8. For enterprises adopting the chemical coagulation method, the original equipment needs no major modification; this product can be used simply by adding an alum-dissolving tank.
6. Enhanced filtration mainly involves reasonably selecting the filter-layer structure and filter aids to raise the removal rate of the filter; it is an important measure for improving water quality.
10. This product must be dissolved before use; the dissolving equipment and dosing facilities should be made of corrosion-resistant materials.
Flocculation — from the process perspective, refers to the process by which floc particles aggregate into large flocs and settle through adsorption, cross-linking and sweep (network) capture.
Flocculant — from the chemical perspective, is an agent that causes colloids and suspended particles to coagulate and flocculate, and is therefore also called a coagulant.
Coagulation — from the mechanism perspective, refers to the process by which the double electric layer of colloids and dispersed systems is compressed, the zeta-potential is destroyed, electrical neutralisation occurs, and the system is destabilised and aggregated into floc particles.
Coagulation (in the broad sense) — coagulation and flocculation are collectively called coagulation.
Inorganic flocculants mainly include inorganic low-molecular flocculants and inorganic high-molecular (polymer) flocculants. The more widely applied and studied are inorganic polymer flocculants, mainly ferric-polymer and aluminium-polymer inorganic flocculants. Among inorganic polymer flocculants, polyferric sulfate (PFS) and polyaluminium chloride (PAC) are the most used.
Polyferric sulfate has the advantages of strong flocculating ability, fast settling, wide application range, low corrosiveness to metal equipment and low secondary pollution, but it produces a larger amount of sludge and has a complex production process and higher cost; polyaluminium chloride, when treating oily wastewater, has a markedly better flocculating effect than other aluminium salts and is usually better than ferric-polymer flocculants as well, mainly showing a smaller dosage and less sludge, but its drawback is slower flocculation and settling speed, which can easily cause secondary pollution.
Composite inorganic polymer flocculants are a class of inorganic flocculants, mainly including polyaluminium ferric chloride (PAFC), polyferric aluminium sulfate (PAFS) and polyaluminium ferric sulfate chloride (PAF-CS), etc. Polymeric aluminium ferric chloride is a composite inorganic polymer flocculant with aluminium salts as the main component and iron salts as the auxiliary, combining the dual characteristics of aluminium-salt and iron-salt flocculants; its coagulation performance is usually better than that of polyaluminium chloride and ferric chloride. The polyaluminium ferric chloride flocculant, when treating oilfield oily wastewater, achieves oil-removal and suspended-solids-removal rates of about 90% or above, meeting the requirements for oilfield reinjection water.
Polyferric aluminium sulfate is prepared from ferrous sulfate as raw material and sodium nitrate as catalyst, and has a good treatment effect on oily wastewater. Polyferric aluminium sulfate chloride is prepared from bauxite, active calcium silicate, hydrochloric acid, sulfuric acid, etc., combining the excellent properties of both ferric and aluminium polymers; this flocculant treats oily wastewater better than polyaluminium chloride. Composite inorganic polymer flocculants not only combine the excellent properties of both aluminium-polymer and ferric-polymer inorganic polymer flocculants, but also feature a higher degree of polymerisation and higher basicity.
