I want to design a 50 m3/day RO plant for treating brackish well water with about 3000 ppm feed TDS. The system recovery should be more than 70% and the product water TDS should be less than 300 ppm. It seems that recirculation loop (Concentrate flow to feed flow) have to be used to eliminate the maximum element recovery warning. However, I don’t know how much concentrate flow can be practically recycled and added to the feed flow. Regardless of negative impact on the permeate quality, I think that higher recirculation flow can increase the membrane life time by reducing element recovery. Is there any design guideline? Assume that there is not any limit in pump and pipe size. I sent ROSA software detail report for my design.
Technically there is no upper limit for recirculation rate but for the following factors:
If you increase recirculation rate it may be difficult to reach a steady state as the concentration increase of the feed may lead to more osmotic pressure less permeate and consequently poor product quality. This may require constant change in the recirculation rate.
The upper limit in recirculation flow rate will be defined by the maximum operating pressure of your membranes. In other words, as the recirculation flow rate is increased, so does the pressure that needs to be exerted on feed flow, according to the Van't Hoff law. In the case of the wastewater processing unit that I was formerly responsible for, the recirculation flow rate had been adjusted to be 40% of concentrate flow, while actual feed pressure ranged within 16-20.5 bar. The particular system included FILMTEC membranes as well (BW30-400-FR), so, if you need more details, you can read the publication that I have uploaded in Research Gate about this issue ( PILOT-SCALE STUDY OF ADVANCED WASTEWATER TREATMENT PROCESSES BY EYATH SA).
Technically there is no upper limit for recirculation rate but for the following factors:
If you increase recirculation rate it may be difficult to reach a steady state as the concentration increase of the feed may lead to more osmotic pressure less permeate and consequently poor product quality. This may require constant change in the recirculation rate.
Recirculation of the retentate can be used in RO and NF technologies to mitigate concentration polarization. In particular, retentate recirculation might be necessary when operating a stage at higher water recovery since an increased permeation flux causes increased concentration polarization. Note that solute and overall material balances over a stage for fixed feed, retentate, and permeate composition for a conrol volume that includes the retentate recirculation loop indicate that retentate recirculation will not change any of the aforementioned concentrations. Hence, retentate recirculation will not change the osmotic pressure differential across the membrane since the osmotic pressure differential is determined by the retentate and permeate concentrations that remain unchanged. However, retentate recirculation will increase the pressure drop on the retentate side. This means that the feed pressure to a stage in which retentate recycle is used will have to be increased, which increases the pumping costs and thereby the specific energy consumption. Retentate recycling can help, but has to be optimized based on the module design and operating costs.
A further comment on using retentate recycle. As I mentioned in my earlier comment, one problem with using retentate recycle is that the pressure drop on the retentate side will increase. This can be mitigated somewhat by using short, fat spiral wound modules such as the commercially available 18-inch diameter modules.
If you increase circulation flow rate it may be difficult to reach a steady state as concentration increase of the feed water may lead to more osmotic pressure less permeate and consequently poor product quality of water. this may require constant change in the recirculation rate. Product flow rate and quality of product water destroyed.
Dear all
Thank you so much for your responses. As described by William increase the recirculation flow rate doesn’t increase the concentrate osmotic pressure when the entire system recovery is constant. However, I think controlling the system hydrodynamic can be difficult in high recirculation flow rate. I think the feed and high pressure pumps sizing for this condition is so important to reach steady state conditions.
However, I think the membrane life time can be increased because each element recovery reduced in high recirculation flow rate.
Best regards.
Dear all
Thank you so much for your responses. As described by William increase the recirculation flow rate doesn’t increase the concentrate osmotic pressure when the entire system recovery is constant. However, I think controlling the system hydrodynamic can be difficult in high recirculation flow rate. I think the feed and high pressure pumps sizing for this condition is so important to reach steady state conditions.
However, I think the membrane life time can be increased because each element recovery reduced in high recirculation flow rate.
Best regards.
YOU can use ROSA software to find optimal design suitable for your requirements.
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