Water treatment plant

 

Water treatment plant (WTP)

WTP is the abbreviation of ‘Water treatment plant’ . Water treatment systems provide for renovation, recovery, and reuse of water heretofore discarded into streams. So,water recycling is defined as putting water back into processes in which it was first used after desirable materials have been removed.
 

Types of WTP:

Four different recycling methods are available and provide for the removal of solid materials. These methods are:
1) Evaporation
2) Microfiltration
3) Ultra filtration
4) Reverse Osmosis

Evaporation:
Evaporation is a process in which waste water is brought to its boiling point and pure water is vaporized and then collected for power production or condensed for recycling as hot water. The result is that solids are concentrated, process materials are recovered and hot water is produced. However, material recovery is not always possible but waste materials handling is made easier by reducing the volume.

Principle of Operation:

Wastewater treatment evaporators are generally heated by steam condensing on metallic, cylindrical tubes. As stream condenses on the outside of the tube, the heat of condensation causes water flowing on the inside of the cylinder to be evaporated. The efficiency of an evaporator is directly related to its heat transfer rate through the metal tubes to the heating surface.
This rate of heat transfer is the product of three factors, which are the heat transfer co-efficient the heating surface area, and the overall change in temperature between the waste and the condensing system.

Application of evaporation:

Very pure water for recycling can be obtained using the recovery method because it is a distillation process. However, for textile waste water treatment, the major factor to be considered when selecting evaporation is its energy cost.

Figure:- Evaporation System

The value of hot water in energy cost currently is estimated to be $3/1000 gal, the energy cost of pure water is $25/1000 gal. Therefore, the evaporation process is economical only when the concentrate is reusable and its value outweighs the energy cost of evaporation. 

Microfiltration:
Microfiltration (MF) is a physical separation process in which suspended solids, ranging in size from 0.02 to 10 and larger, are removed from waste systems. These suspended particles may be either organic or inorganic material. For a perspective of the size of particles with which microfiltration is concerned.
The performance of filtering material in microfiltration is based on three factors. These are the size of the particles it is able to retain, the permeability or filtration rate, and the time lapse before the filter is replaced or cleaned. 

Principle of Operation:
The total resistance to flow consists of two parts . The first is the resistance to flow contributed by the filter medium . The second resistance to flow is the cake of accumulated particles on the upstream surface of the filter medium or within the pores.
Application of Microfiltration:
A type of microfiltration recently introduced is cross flow microfiltration. This involves
flowing the waste feed stream parallel to the suface of the filter to prevent concentration build up. Higher capital and operating costs are incurred using this method due to the need for pumps for circulating the waste stream over the surface of the microfilter medium, but usually higher fluxes and minimum filter replacements can be obtained.

Ultra filtration:
Ultrafiltration is a pressure driven separation process involving the filtering of waste streams by microporous membranes capable of removing macromolecules in solution as well as collides and suspended solids. In this process the feed solution is pumped past the membrane at high velocity at cross flow fashion.
The objective of UF is to separate the feed stream into two strams, one containing the purified nwater or permeatr, and the other containg a high concentrationof separated impurities called concentrate. Water, containing impurities smaller than the membrane pore size, is forced through the membrane and becomes the permeate.

Application of Ultra filtration:
The main application for Ultra filtration so far has been area where valuable components of waste streams can be recovered. In textile wet processing, UF has been used to concentrate and recover poly vinyl alcohol sizing from the desize waste stream .

Reverse Osmosis:
Reverse Osmosis is a pressure driven separation process involving the filtering of waste of stream by membranes capable of separation substantial fraction of dissolved inorganic and organic impurities as well as the larger colloidal and suspended material. In reverse Osmosis a feed stream under under pressure is passed across the surface of membrane. Osmosis is the spontaneous passage of water from a dilute solution to a more concentrated solution across an ideal semipermiable membrane that allows the passage
Of water but not dissolved colloidal or suspended materials. The transfer of water is continued until the pressure in the concentrated is sufficiently large preventing further transfer of water, when osmotic equilibrium is reached. When pressure is applied to the concentrated solution greater than the osmotic pressure differential, water flows through the membrane from the concentrated solution to the dilute solution. Because this flow is reversed of that encountered in that normal osmotic process, it is named reverse osmosis.

Principle Of Operation: 

Pretreated water is pumped into the membrane housings along the membrane surface. Pure water is permitted to pass through the membrane while ionic, organic, colloidal and bacterial contaminants are swept away in a concentrated solution. Consequently, a reverse osmosis system always creates two continuous exit streams: pure water and brine. Normally 50 to 75 percent of the feed water can be recovered as pure water.As an example caustic (NaOH) is recovered from the mercerizing process .As fabric passes through the washing solutions, hot wash water flows in the opposite direction. As the water flows , the concentration of caustic in solution increases. At the exit end of the water flow , water contains two valuable products , high temperature water and caustic. The water is then diverted to a heat exchanger and transfer heat to cold water which in turn can be introduced into other processes.

 

Application of Reverse Osmosis:

 As an example caustic (NaOH) is recovered from the mercerizing process .As fabric passes through the washing solutions, hot wash water flows in the opposite direction. As the water flows , the concentration of caustic in solution increases. At the exit end of the water flow , water contains two valuable products , high temperature water and caustic. The water is then diverted to a heat exchanger and transfer heat to cold water which in turn can be introduced into other processes.