water purification required for human being and other live species and also for agricultural purposes

Water purification

Purification of water mainly depends on the purpose of use. The process generally implemented according to the nature of impurities and where to use this water that is for drinking or washing or to be used in any industrial chemical processes.

Generally, Water impurities can be divided into three categories as under:

·         a- Solid impurities that are suspended solids

·         b- Dissolved impurities

·         c- Bacterial impurities

All the processes of purification designed mostly include these three objectives. 

Let us discuss each in detail

1- Solid or suspended impurities are generally dust particles, silt, fibers, and debris. These things make the water looking dirty. All of these things can be easily removed by filtration. The quality of filtration also depends on the purpose were to use this water like if it is for drinking purposes it can go through 5-micron filter at least, one micron or even less being used in bottled water.

In desalination systems except RO fine filtration is not required only have to remove debris, sand, silt and some dead marine life if sea water being used as feed water. When seawater is used as feed for the desalination plant, the very first thing is the screening system. Seawater direct flow or by pumps passing through screens removing weeds and other marine waste settled in settling tanks where all heavy particles and dust etc. settled down and water is taken from the upper level drawn by pumps to feed the desalination system for further process. Whereas in RO system fine filtration up 5 microns is required to protect membrane pores not to be blocked. So there are several stages being designed for the filtration for RO feed, that is the bag filters, fine filters and then finally the 5-micron cartridge filters. This is a suitable for feed to enter into membrane system. Further to confirm there is a test called Silt Density Index (SDI) to check the feed quality and the check of our filtration system working efficiently or not, giving a clear indication to change or clean the bag filter, fine filters or replace cartridge filers to get the better results again.

In desalination processes like RO, MSF or another system usually before the filtration some chemicals also added like acid to adjust the pH as required and for disinfection, anti-scaling and anti-corrosion and some time some time for coagulation some other chemicals being used for safety and efficiency of the systems. 

1.    There several processes to remove or reduce the dissolved impurities. For general use like drinking, cooking washing desalination commonly required by either process as seems to be more economical and convenient utilizing the local sources and environmentally safe. These process can be listed as:

2.1                        Thermal processes

2.1.1         Multi-Stage Flash Distillation

2.1.2         Multi Effect Distillation

2.1.3         Vapor Compression

2.1.4         Solar Distillation

2.2.1         RO Membrane process

2.3.1         Electro-dialysis

Introduction

Desalination is simply a process to remove salts from seawater or well water or any underground water which may not be good in taste due to excessive salts. Generally, well water or brackish water has salts from 1000 to 11000 ppm TDS while seawater contains 35000 ppm TDS or more in which the major constituent is sodium chloride.

Generally any desalination system thermal or membrane process, consist of 3 parts, one is the pre-treatment system, second is the main desalination system and the 3^rd is the post-treatment system which is just to adjust the quality as per requirement either drinking or any other industrial usage.

Several thermal processes being used like multistage-Flash distillation (MSF), Multi-effect distillation (MED or ME) and the Vapor Compression (VC) processes. These processes are mostly used for seawater and MSF system is the most widely used process in the world.

Membrane systems include reverse osmosis (RO) can be used for brackish water and seawater

and electrodialysis (ED) can be used for brackish water only. 

In the distillation process, the product water TDS is around 20 ppm TDS while in membrane process it can be designed to achieve 100 to 500 ppm TDS. For drinking water, the quality must be adjusted to achieve quality as according to WHO (World Health Organization) standards.