Do we use coagulant or flocculant in Sand Filters for pre-treatment of RO plant?

As per my little knowledge in water treatment, the coagulant reduces the permeability of the filter bed, consequently, reduces the filtration rate and increases the backwash pump head requirements; coagulants can be used in the process units located upstream of the sand filters, some new systems has been improved by some manufactures like turbo and Acti flow sedimentation system, these systems use the fine sand to improve the suspended solids molecular weight and thus, improving the sedimentation efficiency without using any flocculent.

 we can use it coagulant as fecl3.

You need to have a break tank and settlement tank in this case

I would suggest not to uses any of them in the filter, it will get clogged, however these chemical can be used in the pretreatment prior to sand filter and is already practiced in many water treatment plants.

Yes we use it and the coagulant like ferric chloride

This is due to amount of suspended solids in raw water

are used in combination with coagulants to agglomerate suspended particles for removal by filtration. This technique is used extensively in all types of surface waters to reduce the silt density index (SDI) and minimise membrane fouling. Although organic cationic flocculants are particularly effective, their widespread use in membrane applications is limited because of perceived problems with flocculant fouling at the membrane surface causing irreparable damage. Reference material from some of the major membrane manufacturers on the use of flocculants is given. Autopsies showthat more than50%ofmembranefoulingiscaused byinadequate,deficient or poorly operated pre-treatment systems. The Authors suggest that if the correct chemistry is considered, the addition of an effective flocculant can be simple and safe. The paper discusses the use of cationic flocculants and the fouling process that occur at the membrane surface. The paper explains the types of coagulants and flocculants used and considers cationic flocculants and the way they function. Operational results when using a soluble polyquaternary amine flocculant (Genefloc GPF) developed by Genesys International Limited is presented. Genefloc GPF has been in extensive use for several years and a review of the results clearly shows no detrimental effect on membrane performance. Acase study shows the effectiveness of Genefloc GPF in reducing SDI and increasing particle size prior to filtration. In every case a significant improvement in plant performance is seen with a reduction in cleaning frequency and extended membrane life. New particle counting techniques [1] have assisted us in optimising flocculant dosage rate and monitor on-going performance. Keywords: Reverse osmosis; Membrane; Fouling; Cationic flocculant; Polyamine; Polyquaternary