1、外文翻译Coagulation and flocculation14.1IntroductionCoagulation and flocculation provide the water treat-ment process by which finely divided suspended and colloidal matter in the water is made to agglomerate and form flocs. This enables their removal by sedi-mentation, dissolved air flotation or filtra
2、tion. Colloidal particles (colloids) are midway in size 1 between dissolved solids and suspended matter. Colloids are kept in suspension (stabilised) by electrostatic repulsion and hydration.Electrostatic repulsion occurs because colloids usually have a surface charge due to the presence of a double
3、 layer of ions around each particle. Thus, the colloid has an electric charge, mostly a negative one. Hydration is the reaction of particles at their surface with the surrounding water. The resulting particle-water agglomerates have a specific gravity, which differs little from that of water itself.
4、The substances that frequently are to be removed by coagulation and flocculation are those that cause turbidity and colour. Surface waters in tropical countries often are turbid and contain colouring material. Turbidity may result from soil erosion, algal growth or animal/vegetable debris carried by
5、 surface run-off. Substances leached from decomposed organic matter, leaves, or soil such as peat may impart colour. Both turbidity and colour are mostly present as colloidal particles.The electrostatic repulsion between colloidal particles effectively cancels out the electronic attraction forces (V
6、an der Waals forces) that would attach the particles together. Certain chemicals (called coagulating agents, coagulants) have the capacity to compress the double layer of ions around the colloidal particles. They reduce the range of the electrostatic repulsion, and thus enable the particles to flocc
7、ulate, i.e. to form flocs. These flocs can grow to a sufficient size and specific weight to allow their removal by settling, flotation or filtration.Generally water treatment processes involving the use of chemicals are not so suitable for small community water supplies. They should be avoided whene
8、ver possible. Chemical coagulation and flocculation should only be used when the needed treatment result cannot be achieved with another treatment process using no chemicals. If the turbidity and colour of the raw water are not much higher than is permissible for drinking water, it should be possibl
9、e to avoid chemical coagulation in the treatment of the water. A process such as slows and filtration or multi-stage filtration would serve both to reduce the turbidity and colour to acceptable levels, and to improve the other water quality characteristics, in a single unit.A roughing filter can ser
10、ve to reduce the turbidity load on the slow sand filter, if necessary.14.2CoagulantsAlum (Al2(SO4)3.nH20) where n=14, 16, or 18, depends on the form of alum supplied. This may be in liquid solution, broken crystalline granules 2-5 cm size (kibbled) or crystalline blocks. It is by far the most widely
11、 used coagulant. Iron salts (e.g. ferric chloride (FeCl3), orferric sulphate (Fe2(SO4)3.9H2O) can be used as well and in some instances have advantages over alum. A significant advantage of iron salts over aluminium is the broader pH2 range for good coagulation. Thus, in the treatment of soft colour
12、ed waters where colour removal is best obtained at low pHs, iron salts may be preferred as coagulants. Ironsalts should also be considered for coagulation at high pHs, since ferric hydroxide is highly insoluble in contrast to aluminium salts, which form soluble aluminate ions at high pHs. Sodium alu
13、minate is mostly used for coagulation at medium pHs. Synthetic organic polyelectrolytes have become available as coagulants but are generally not economical for small water supply systems, nor are they readily available.The use of extracts from the crushed seeds of certain tropical trees is describe
14、d in detail in the next section, 14.3.Coagulants such as soluble aluminium and iron salts react with the alkalinity of the water, and hydrolyse in it. For example, alum reacts to form aluminium-hydroxide floc,Al(OH)3, a gelatinous precipitate. The required alkalinity may be naturally present in the
15、water or it has to be added through dosage of lime, Ca(0H)2or sodium carbonate,Na2CO3(also called soda ash).For good coagulation the optimal dose of coagulant should be fed into the water and quickly and properly mixed with it. The optimal dose will vary depending upon the nature of the raw water an
16、d its overall composition. It is not possible to compute the optimal coagulant dose for a particular raw water. A laboratory experiment called the jar test is generally used for the periodic determination of the optimal dose.The jar test may be briefly described as follows:A series of samples of water are placed on a special multiple stirrer and the samples,typically 800 ml in 1-litre beakers (or jars), are dosed with a range of coagulant, e.
