Thanks a lot guys. So after ...

Thanks a lot guys.

So after reading all the answers I feel I should be able to solve the issue. Now let me answer some of people's questions about fault in measuring the TDS and flow inputs.

Well I have checked these TDS with correct TDS meters.

Well most important thing is that wastage flux TDS is decreased (fom 4000 to 3200 mg/l) while the inlet water TDS is increased from 2300 to 2800 while treated output water TDS from 80 to 180 mg/l. This can only happen when some amount of inlet water is directly purring to the wastage flux water and in my that could be due to broken/damage 'O' ring between 2 membranes. And almost all the answers I got are emphasizing to the same fact so I believe broken O ring would be a common issue.

Also I would mention one more point here is that we did membrane wash with 'Namak ka acid' (a diluted acid which is mainly concentrated with salt) and that might be the cause of O seal ring damage. 

This 'Namak ka acid' was recommended by the guys who commissioned the plant and to be more precise they have used it before 1.5 year back (the plant was commissioned around 2.3 year back) in the same plant when we reported low treated water output flow issue to them.

So I need one suggestion now, should I check the O ring by my own or should it be checked by some mechanic? I am not sure how much easy/difficult it would be to re-inserting the membranes inside the housing.

Regards,

Dharmendra