You have not mentioned where the boiler is used  so that some solution can be discussed.

Thank you M. Elangovan Sundaram for your answer.

The boiler is fed by the permeate to produce steam

Dear Mustapha,

Pretreated well water is used for RO filtration and you like to increase the recovery of the RO. It is a nice and intriquing question. It is difficult to determine critical factor(s) for increasing water production without more background knowledge. What is the most likely problem or fouling risk (scaling, what is the experience with the plant)? How is the plant configured and operated? Is an antiscalant dosed? Could the recovery be increased by elevating the feed pressure?Best regards,

Hans

I agree with Dr. Vrouwenvelder

ROSA software (DOW - Filmtec company) is available on the web and can be used to estimate the maximum possible recovery can be achieved in your plant with/without adding 2nd RO stage. Chemical analysis of water out of the softner is needed to do the calculations.

Thank you for your answers Dr. Vrouwenvelder  & Dr.  Abbas;

Reverse osmosis of 1600 liters / hour.

To inhibit biofilm formation, chlorine is added (5 liters / hour), then added the sequestrant (SEKO) (5 liters / hour) and dechlorinating (SEKO) (5 liters / hour) to avoid fouling and scaling of the membrane.

what are the parameters necessary to recycle the concentrate without causing problems to the membrane? 

Generally, proper pretreatment on RO feed water and have one pass two stage RO arrangement design could be helpful in increasing RO recovery rate.  The point is to have the complete RO feed water characteristics.  In the RO design, if you do want to increase the RO production rate, the point in RO pretreatment design is to remove some species and then the RO fouling/scaling potential could be minimized.

With seawater desalination the limiting factor is the osmotic pressure. With 50% recovery the osmotic pressure in the last element is about 60 bar (=2*30bar). So you need 70 bar to produce enough water in the last element. So for higher recoveries you need even higher pressures.

For brackish water this is in most cases different: the limiting factor for the recovery  is scaling. We have brackish water installation with a 2-1 staging with about 75-85% recovery with the use of anti-scalant.

Dear Mustapha,

As I found recovery of your mentioned RO plant is about 55% (100-45). The recovery factor is dependent to the RO feed water quality and can be about 30 - 40% for seawater and 70-85% for brackish water. Well near the beach, desert or forest has very different quality which explained by TDS (Total dissolved solid), SDI (Silt density index), solute analysis and etc. However, the quality of brine flow is required to design of a new stage.

Please find the attached file which is a good guideline to design a RO plant for different feed water conditions.

Best regards 

If tap water is used, for a new RO, I suggest to consider selecting a more economic device: the reduction in the reject water stream (down to 25%) is achieved by recirculating the reject water at high speed over the RO membranes, avoiding membrane plugging and foiling. If a double RO is used, the reject water of the second RO can be delivered to the inlet of the first one.

Mustafa many of the answers made before are points to consider, but particularly you must put focus in the chemical profile, if you are giving a pretreatment, then ypu must analysis which is the behaviour of the chemical profile since the inlet (well water) to the inlet in the osmosis and the output of your concentrate. Check the percentage of recuperation of the water and verify if you are able to make and injection to the inlet of the RO (you will do a kind of dilution) and you will not force so much the process if it is calculated to suport the quantity of salts you could inject (many times you can recover from 5 to 15 % of the rejected water), but I insist you need to know the exact chemical profile because if the hardeness is high or you have some contaminants in water, like a high quantity of sulphur or manganese is probable you cannot recover many of the water, You can consider too have an strongest pretreatment and recover part of the RO rejected water. The key control parameter in the operation is the electrical conductivity. And do not forget check with frequency the chemical profile which, if you have standardized parameters in the RO, is the key issue.I hope this little help will be useful.

Better pre-treatment and recycling and using 2-3 stages to minimize the loss of water treated by reverse osmosis.

Hi Mustapha,

Reducing the concentrate flow can be done by a control valve. The reduced concentrate flow will increase the salt concentration and reduce your flux.  Depending on the salt concentration, you can offset this effect by increasing the operating pressure (as long as you stay within the safe operating allowance for the system).  That shoud allow you to keep more of the water as permeate.

Pietro

According to your recent condition, the recovery of your system is 55%. Recovery of reverse osmosis can be increased up to 75% by optimizing array (filtration stage) configuration. You can optimize the system design (using RO design software provided by manufacturer) by maximizing system recovery while maintaining acceptable value of permeate conductivity.

RO reject water is of excellent quality, a shame to dump it to the drain.Connor et al; (Nephron Clin Pract 2010.116 C200-206) reported about their more than 10 years positive experience how to beneficially collect their RO reject water into a reservoir, with full automatic service and delivery to their hospital toilets. The authors reported that this resulted into a potentially annual saving of 12000 Euros.