如何选择絮凝剂?

如何选择絮凝剂

絮凝剂一般有无机絮凝剂和有机絮凝剂，废水处理中如何选择絮凝剂要根据具体行业的废水的特性来选择，同时还要看在哪个环节添加絮凝剂，做何用途。

       絮凝剂的选择主要取决于水中胶体和悬浮物的性质及浓度。如果水中污染物主要呈胶体状态，则应首选无机絮凝剂使其脱稳凝聚，如果絮体细小，则需要投加高分子絮凝剂或配合使用活化硅胶等助凝剂。很多情况下，将无机絮凝剂与高分子絮凝剂联合使用，可明显提高混凝效果，扩大应用范围。对于高分子而言，链状分子上所带电荷量越大，电荷密度越高，链越能充分伸展，吸附架桥的作用范围也就越大，混凝效果会越好。一般选择无机絮凝剂时要考虑废水的成分及PH等，然后选择最适合的一种。在选择有机絮凝剂时，主要是看要用到阴离子聚丙烯酰胺、阳离子聚丙烯酰胺还是非离子聚丙烯酰胺。阴离子聚丙烯酰胺依据水解度不同一般分弱阴、中阴、强阴。阳离子的选择一般用在污泥脱水方面，阳离子聚丙烯酰胺的选型很重要，城市污水处理厂一般用到中强阳离子聚丙烯酰胺，造纸、印染厂污泥脱水一般选择弱阳离子，医药废水一般选强阳离子等等。

根据以下几方面来正确选择絮凝剂：
       1. 絮凝剂的分子结构：絮凝剂的分子结构的选择取决于所要求的脱水性能。阳离子絮凝剂的分子结构如下：线形结构：当选择了正确的分子量时，利用线形结构的絮凝剂进行脱水处理时，絮凝剂的用量小，并且能够达到良好的脱水性能。支状结构：用量中等，可以获得较好的脱水性能。交联网状結构： 使用量较大，但可以获得特殊的滤水性能和抗剪切强度。

       2. 絮凝剂的分子量：分子量的选择取决于脱水处理所用设备的类型，同时分子量也表示了聚合物链的长度。对于离心机式的脱水处理设备：聚合物的分子量越大越好，因为在进行离心脱水处理工程中，絮团将受到一个很大的剪切力的作用。对于过滤式的脱水处理设备：选择分子量较低到中等分子量之间的絮凝剂就可以满足要求，同时可以得到一个良好的虑水功能。

      3. 根据污水中颗粒的类型来选择絮凝剂带电性。一般情况下，大家都会利用带负电的絮凝剂来捕捉无机类污水里的颗粒类东西，用带正电的絮凝剂处理有机物颗粒，当然想要选择最佳絮凝剂最后还是要通过小试和上机实验来确定。

 4. 絮凝剂带电电荷分布密度（离子度）：絮凝剂带电电荷分布的密度表示，在絮凝剂使用量最少的情况下，获得最佳的絮凝效果所需要絮凝剂所带的正电荷或者负电荷的数量。电荷分布密度与污泥类型相关，市政污泥的电荷分布密度通常是污泥中有机物含量的函数，而有机物含量通常又与挥发份的含量有关，挥发份含量越高，则需要带电量越高的絮凝剂。

       每一种废水都有它自己独有的特性，非离子聚丙烯酰胺主要是在弱酸性条件下使用，印染厂用到非离子PAM的比较多。絮凝剂的选型都要根据试验之后才能确定，在试验中先确定大致加药量，观察絮凝沉淀速度，核算处理成本，选择经济、适用的絮凝剂。

How to choose flocculant

Normally,the flocculant can be divided into inorganic and organic flocculant.we should choose the right flocculant according to the characteristics of  the wastewater.meanwhile,we should know which prosess the flocculant will be added and what it is used for.

How to choose flocculant depends on the concerntration of the colloid and suspended matter in the water.if the pollutants show colloidal state,we should choose inorganic flocculant to make it destablization and flocculation,if the floc is small,we need to add high molecular flocculant or can be used  in conjuction with activated silica gel as coagulant aid。It can obviously improves the effect of coagulation and expand the application range if the inorganic floccluant be used in concert with the organic flocculant.for high molecular,for the high molecular, the charge on the chain-shaped molecular is greater ,the charge is higher and the chain can be extend fully,the effective range of absorption and bridging is also bigger,the flocculent effect is better.we should consider the composition and PH value of the wastewater when choosing the inorganic flocculant ,and then choose the optimum model.we should consider if it is anionic PAM or cationic PAM or nonionic PAM when choosing the organic flocculant.the anionic PAM can be divided to weak ,medium and strong anionic PAM according to the different hydrolysis degree.the cationic PAM mainly used for sludge dewatering,the selection test is very important,for  municipal sewage treatment,we often use cationic PAM with medium or strong ionicity.the cationic PAM with weak ionicity often be used for the sludge dewater  of  papermaking and dyeing plant.the cationic PAM with strong ionicity often be used for medical wastewater treatment.

Choosing the right flocculant according to the following the aspects:

1.the molecular structure:the molecular structure of flocculant depends on the dewatering performance be required.the structure of cationic flocculant as follows:

linear structure:after choosing the right molecular weight,the dosage is small and the dewatering performance is better if we make the dewatering treatment using linear flocculant.

Dendritic structure:medium dosage,can obtain better dewatering performance.

corss-linked structure:big dosage but can get special water filtering and and shear resistance performance

2.the molecular weight:: The choice of molecular weight depends on the types of dewatering equipment, and molecular weight are also said the length of the polymer chain. For centrifuge equipment , the higher of the molecular weight the  better of the application result, because in centrifugal dewatering engineering, floc will be subject to a large shear force. For the filter type equipment:,the lower molecular weight to the medium molecular weight flocculant meet the requirement, meanwhile,we  can get a good  water filtering performance..
      3．Choosing the flocculant charged according to the type of particles in waste water. In general, we will use the flocculant with negatively charged  to capture the particle in inorganic sewage , treat organic particles with positively charged flocculant , of course, we have to confirm the the best flocculant throught simple test and pratice on machine.

4. Flocculant charge density (ionic degree): that mean the quantity that the flocculant will be positive charge or negative charge to get the best flocculent result if the caseof minimum quantity. the charge density is related to the type of sludge, municipal sludge is often a function of the content of organic matter in the sludge,the organic content is related to the content of Volatile components.the higher of the Volatile content,the higher charge of the flocculant we need.

Each kind of waste water has its own unique characteristics, nonionic polyacrylamide is mainly used in acidic condition, printing and dyeing mill often use nonionic PAM. We can only confirm the final model according to the selection test .we can confirm the economic and optimum flocculant according to the dosage , flocculent and sedimentation result and the cost.