Reuse of reverse osmosis (RO) plant's concentrate

 

Technical description

 

In many cases, raw water, that is usually groundwater, contains high content of dissolved solids. The use of water of high dissolved solids for textile finishing leads to various problems such as quality issues in the fabric and relatively higher consumption of chemicals during wet processing. This water also cannot be used for steam boilers. Therefore, in most of the textile finishing industries, raw water of high dissolved solids is pretreated through reverse osmosis (RO) plant to minimize its total dissolved solids (TDS) and hardness in order to provide process water at required quality both for wet finishing processes and boiler feed water (see Figure 1).

As being a separation technology, the RO produces the so-called permeate, which is about 70% of the total raw water supplied to the RO plant, as well as the concentrate, which is highly concentrated with the salts with a percentage of 30% respectively. The concentrate is also called reject water. This reject water is often wasted.

The typical quality of RO reject water from one textile finishing industry is given in Table 1.

Table 1: Typical quality of (RO) water

pH

7.56

Total Hardness as CaCO3 (mg/l)

1,475

Total Dissolved Solids-TDS (mg/l)

8,740

Total Iron (mg/l)

0.08

Note: The groundwater TDS of this industry is 3,190 mg/l and pressure at the RO plant is 230 psig or 15.86 barg (two stage RO plant)

Reject water is high in TDS contents which cannot be used for a wet finishing process but can be utilized for general purposes where high quality water is not required. It should be collected and used for general purposes as given below:

  • At color kitchen for washing the floors and chemical drums
  • Showering water at solid fuel fired boilers wet scrubbers
  • Toilets
  • Printing machine screens washing
  • Showering water in the cooling rooms
  • Vehicular washing
  • Coal moisturizing for boilers
  • Water injection at water ejectors of caustic recovery plant (CRP)
  • Showering water at fluff scrubber at singeing machines.

RO reject water rate can also be decreased by increasing number of membranes (stages) of the RO plant.


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